R Kozakov
email: kozakov@inp-greifswald.de

H Schöpp

St Franke

C Stoll

D Kunz

Modification of light sources for appropriate biological action

Keywords: numbers: 87, 18, Yt, 87, 50, W-

The impact of the non-visual action of light on the design of novel light sources is discussed. Therefore possible modifications of lamps dealing with the spectral tailoring and their action on melatonin suppression in usual life situations are investigated. Results of melatonin suppression by plasma lamps are presented. It was shown that even short-time exposure to usual light levels in working areas has an influence on the melatonin onset.

Introduction

The influence of light on the melatonin secretion in humans was discovered almost 30 years ago (Lewy et al. 1980). Approximately 20 years later the discovery of a novel nonvisual light receptor in human eyes followed (Berson et al. 2002). The new receptor, the so called intrinsically photosensitive retinal ganglion cell (ipRGC) was found to be responsible for the entrainment of human endogenous circadian rhythms with the natural 24 hours light variation. A novel photosensitive pigment melanopsin which is contained in ipRGC was discovered even earlier (Provencio et al. 2000). The action spectrum of melatonin suppression was measured experimentally in works of Brainard and Thapan (Brainard et al. 2001, Thapan et al. 2001). This spectrum is believed to be mainly influenced by the sensitivity spectrum of ipRGCs but other photo receptors seem also to have influence on the non-visual responce (Rea et al. 2005) These results have shown that the light plays a major role in the entrainment of suprachiasmatic nucleus which is considered as master clock of biological rhythms in humans. Many other studies have shown that the disturbance and especially the chronical disturbance (for example at shift workers) of biological rhythms can be a reason for several diseases like cardiovascular dysfunctions (Akerstedt et al. 1984), cancer (Levi 2000) and seasonal affective disorder (SAD) (Avissar et al. 1999). It is therefore important to consider the non-visual effects which arise when artificial light is delivered during the evening and night periods of the day. Furthermore several questions arise for future artificial illumination installations, like: -Which spectra are best suited for evening and morning illumination respectively? -What are the best intensity levels in the morning and in the evening? -Which adverse reactions can occur or may lead to diseases? -Which lamp technology is best suited for biologically efficient illumination -plasma based [fluorescent lamps (FL), high intensity discharges (HID)], incandescent or LED? -What are the methods of investigation to determine the "best"? Many published results explain more or less detailed investigations, but up to now no summarized knowledge is published.

The findings of Brainard and Thapan allowed Gall (Gall & Lapuente 2002) to introduce a so called circadian metric of light which enables one to estimate the biological action of light in terms of melatonin suppression. The Gall model defines an action spectrum s ms (figure 1) based on the measurements of Brainard and Thapan. The s ms function is shifted relative to V λ to the short-wavelength region of the spectrum and has a maximum at approximately 460 nm . The melatonin suppression action factor a ms v is then defined as

a ms v = 580 380 s ms S λ dλ •   780 380 V λ S λ dλ   -1 , (1)
where S λ is the spectral irradiance and V λ is the spectral luminous efficiency. The values of a ms v range for Planckian radiators with colour temperatures (CCT) between 2000 and 13000 K from 0.1 to 1.3. The biologically effective illuminance level can be defined as

E c =a ms v •E, (2)
where E is the usual photopic illuminance. Because the maximum of s ms lies in the shortwavelength region there exists a correlation between CCT and a ms v leading to higher a ms v values for higher CCT values. However this correlation is not unambiguous. Spectra with the same CCT can have different values of a ms v varying in distinct limits (Kozakov et al. 2008).

The non-visual metrics of Gall is a first attempt to quantify the biological action of light. Such a quantification is needed to predict the biological effectiveness of lighting and will help in future the lighting industry and architecture to design biologically "friendly" light sources. It should be noted that the Gall model of non-visual action of light is only an approximation based on the measurements of nocturnal melatonin suppression. Non-visual action of light has many aspects like for example sleep quality, alertness and others. In addition, the timing of light exposure is very important for nonvisual action. The photosensitivity of melatonin suppression is believed to be modulated over the day with lower sensitivity during subjective daytime. However unlike some

Wavelength (nm)

Figure 1. Comparison of spectral luminous efficiency V λ (--) and melatonin suppression action spectrum s ms (----) according to (Gall & Lapuente 2002).

rodent species no "dead zone" in photosensitivity during the day could be identified in humans (Jewett et al. 1997).

To be able to predict the exact effect of lighting on health one needs the adequate model based on the comprehensive understanding of non-visual light action. Such action of light seems to be much more complicated than the Gall model can describe. There exist indications that the polychromatic and monochromatic radiation can have different total effect on melatonin suppression (Figueiro et al. 2006a). Furthermore the duration of light exposure does not contribute linearly to the effective dose (stimulus) obtained by a person (Rimmer et al. 1995). Because melatonin suppression action spectrum (Brainard et al. 2001, Thapan et al. 2001) does not coincide with the spectral sensitivity of ipRGCs (Gamlin et al. 2007) it can be assumed that other receptors in retina contribute also to the non-visual action of light, see for example (Rea et al. 2005).

Nevertheless the current understanding of non-visual action of light is sufficient to define some standard evaluation situation for light sources under test. The German standardisation institute (DIN) holds regular meetings on this topic. Recently a preliminary standard on non-visual action of light sources was published (DIN V 5035-100 2009) where the definition of terms considering the non-visual action is performed.

For a refinement of existing models and a development of new ones extensive experimental investigations of melatonin suppression are necessary.

Most of experimental data on melatonin suppression were obtained in strict laboratory conditions. The applicability of models should be proven also for usual lighting situations. The paper represents an attempt to investigate systematically the influence of lighting conditions on melatonin suppression. For the investigations the commercially available luminaries were taken, equipped with lamps of commercially available wattages and placed in usual application position. Hence the light levels used in current work were not specially adjusted. The measured values of vertical illuminance at cornea lie between 80 and 500 lx. The obtained values coincide with the recommended illuminance levels for work places according to (EN12464-1:2003(EN12464-1: -03 2003)). For example bathrooms (200 lx), offices (300-750 lx) and lecture halls (500 lx). However one should mention that the recommended values are given for the illumination of working surfaces and the real illuminance at cornea can be as much as 5 times smaller (Figueiro et al. 2006b). Thus the illuminance values in the present work describe rather the situations of evening activities in well illuminated areas such as sporting halls, shopping malls, offices etc. than the residential environment where much lower illuminance values can be expected (Figueiro et al. 2006b). However some of the lighting conditions used in the present paper, for example bathroom like illumination with fluorescent lamps and illuminancies about 100 lx, can be considered to be representative for bathroom illumination, especially after the ban of incandescent lamps in some parts of the world.

The results were obtained by tests of plasma light sources on the melatonin suppression during the evening melatonin onset. The commercially available light sources have been investigated and modified for proper support of natural biological rhythms. In the first stage of investigation the commercially available light sources were tested in their effectiveness to suppress melatonin. In the second stage the light sources were modified and again tested for melatonin suppression. Section 2 of this article describes the experimental conditions and the light sources used for illumination. Section 3 deals with modifications of light sources. In section 4 the results are discussed. Section 5 gives some hints for possible modification of light sources which were not investigated in present work. Finally a short summary is given.

Experimental conditions

Measurement paradigm

The cyclic rhythm of melatonin concentration is a robust marker of circadian rhythms. Therefore the investigation of the influence of artificial lighting in several lighting situations on melatonin concentration was performed. Three types of lamps were used for this purpose: the tubular fluorescent lamps (FL), the high intensity discharge lamps (HID) and a novel lamp based on the dielectric barrier discharge technology with Xe filling (Xe-DBD). All these lamps were subject for modifications in terms of their spectral power densities.

The melatonin measurements were performed in a sleep laboratory. In a randomized cross-over design the groups of healthy subjects (usually N=7 to 10 per group, totally 28 persons, age 18-35 years) were exposed to 30 minutes of light starting one hour before habitual bedtime. Starting one week before the tests the subjects were asked to keep constant sleep times. Questionnaires (Pittsburgh Sleep Quality Index) allowed to determine the bed-times which were used to distribute the subjects into the 3 person groups with nearly identical bed-times. No subjects were of extreme chronotype.

Saliva samples were collected every 30 minutes before and after the light exposure and every 10 minutes during the light exposure, including one sample 10 minutes just before the lights-on and one sample 10 minutes just after the lights-off. Samples collection started 2 hours before lights-on and ended 30 minutes after lights-off. Melatonin concentrations were defined by radioimmunoassay with the least detectable dose of ± 0.15 pg/ml.

The usual artificial lighting environments were chosen to investigate the influence of everyday lighting situations on the melatonin suppression. The arrangement of the luminaries was chosen to correspond to usual applications of light sources, like for example bathroom arrangement, office arrangement, hall lighting arrangement. The list of light sources used and their application in the corresponding lighting situations is given in table 1. During light exposure subjects were standing on assigned spots and facing the wall in front of them. Depending on specific luminaire they were looking either at the illuminated wall or at the luminaire itself. The luminance in their field of view was measured with a calibrated luminance measurement camera.

The spectrum of light was recorded with the help of a compact spectroradiometer (EPP-2000, StellarNet) in the range between 300 and 800 nm. The illuminance was measured with the help of a illuminance meter (PocketLux 2, LMT). These measurements were performed at the positions of the eyes of subjects (at cornea). For each tested lighting situation tested a dim-light situation (≤ 1 lx) was also randomly included in test. During a dim-light situation the subjects underwent the usual routine with dim-light. The melatonin suppression percentage was defined as the difference between melatonin variations in dim light situation and tested light situation related to the absolute value of melatonin in dim light condition 1 hour before habitual bedtime (instant of light-on):

S = ∆M DIM -∆M T EST M(BT -1) , (3)
where M(t) -melatonin concentration as function of time, ∆M = M(lights-off) -M(lights-on), BT -habitual bedtime in hours. The choice of this suppression definition is defined by the work of (Thapan et al. 2001) where a similar formulation was used for the determination of the melatonin suppression level. The experimental design in that work was similar to that in our project -30 minutes of light delivered during dim light melatonin onset. The dim and light situations were compared by paired two-tailed t test applied to the melatonin change rate calculated as ∆M/M (BT -1).

Light sources at stage 1

In the first stage of investigation the commercially available lamps were tested. The list of the lamps and their characteristics are given in table 2. The spectra of these lamps are shown in figure 2. The spectrum of HID 70W (NARVA NCT 70W nw) lamp is typical for HID lamps and consists of radiation of metals included in lamp filling. The FL6000 (NARVA 18WT8 860) lamp and Xe-DBD (OSRAM) lamp show typical fluorescent spectra consisting of radiation of phosphors and in the case of FL6000 mercury lines. At the beginning of the stage one there was no Xe-DBD lamp for general illumination available. The lamp for technical illumination purposes was used. Therefore the colour rendering of this lamp as read from table 2 is not optimized for the purposes of general illumination. It should however be noted that this lamp is fluorescent one and by proper combination and choice of phosphors very good colour rendering (up to Ra≥90) can also be obtained.

Light sources at stage 2

For the second stage of investigations modified lamps were manufactured. The list of the lamps and their characteristics are given in table 3. The fluorescent lamps with the CCT=1800 K (FL1800) and CCT=10000 K (FL10000), the Xe-DBD lamp with CCT=1500 K (Xe-DBD 1500), the 250 W HID lamp with additional amount of Indium (HID 250 W).

Light sources modifications

The results of the initial investigations have shown that the investigated lighting conditions are active in the biological sense. An example of group-averaged melatonin course is shown in figure 3. The data in figure 3 show some scattering; for example the dim-light situation seems to be shifted relative to other light situations. Furthermore the curves do not coincide before the time "0". The differences may be ascribed to the intra-individual variations of melatonin course and photic history of subjects.

There are situations where biologically active illumination is undesirable, for example in the evening hours, when the natural onset of melatonin production takes place. During this period of time a short exposure to light, for example in the bathroom during preparation to sleep can cause a significant suppression of melatonin concentration in blood and thus reduce the readiness to sleep. On the other hand there exist situations where one would like to suppress the melatonin in order to raise the alertness and reduce the sleepiness, for example in the morning hours after awakening or for the light treatment of jet-lag. The best solution would be a single lamp with changeable spectrum. The use of "morning" and "evening" lamps would be also a possibility for proper biological lighting. Our project uses the latter option as its objective. In the following possible modifications of lamps under investigations are discussed.

Fluorescent lamps

Both usual tubular FL and the Xe-DBD type lamps belong to the fluorescent kind of lamps. Their spectra are mainly determined by the mixture of phosphors used in the lamp. Therefore it is possible to produce lamps with lower and higher values of a ms v . The other constraints on spectrum modification are the good colour rendering index (Ra > 80) and a small distance to the Planckian locus in the colour space (condition on white light).

The possibility to create a "morning" lamp can be accomplished easier than that of creating an "evening" lamp. As far as the maximum of s ms lies in the short-wavelength region of the visible spectrum the increase of blue phosphor fraction in the mixture of phosphors gives the desirable effect on a ms v . Although attention should be paid to balance the short-wavelength radiation with others to keep good Ra and stay in the vicinity of Planckian locus. Such a modification can be performed in the frame of usual three-phosphor technology. Fig. 4(a) shows an example of the spectrum with the increased short-wavelength fraction of the radiation developed for the tests at stage 2. The CCT of this lamp is equal to 10000 K. Recently lamps with similar CCTs became commercially available.

The "evening" lamp represents a bigger challenge to overcome. To reduce the biological effectiveness of a lamp one has to reduce the radiation fraction in the shortwavelength region of the visible spectrum. On the one hand the elimination of shortwavelength radiation causes a difficulty in creating a white light which is only suitable for general lighting. The way to solve this problem lies in the chromatic adaptation possibility of human eye. Any radiator whose spectrum lies near enough to Planckian locus is perceived as "white". To reduce the biological effectiveness of light one needs to use the lower values of CCT. With the decrease of CCT the short-wavelength fraction of radiation of Planckian radiators and hence their biological activity diminishes. Fig. 4(b) shows the spectrum of such an "evening" lamp developed for the tests at stage 2.

On the other hand the almost complete elimination of short-wavelength light is a difficult task itself for the fluorescent lamps because of mercury spectral lines. By simple combination variation of three phosphors it is impossible to filter out those lines. Usual phosphors are more or less transmittent in that region and in order to perform such filtering a special blue-absorbing phosphor was developed and used in the second stage of investigations (see table 3). The abandonment of mercury is a challenging task for fluorescent lamps because of its very efficient conversion of electrical energy into resonant UV-radiation which is then converted into visible radiation. The presence of short-wavelength radiation is inherently persistent for all mercury based fluorescent lamps. One approach to overcome this problem is the Xe-DBD lamp. It also efficiently converts electric energy to the UV excimer radiation around 172 nm. In typical Xe-DBD discharges the amount of visible radiation is negligible relative to the UV radiation. Therefore they are neutral in the sense of non-visual action and there exists a bigger freedom in choosing the combinations of phosphors because one does not have to account for the Xe contribution in colour mixing procedure. More details on science and technology of gas discharge lamps can be found for example in (van den [START_REF] Van Den Hoek | Lamps Ullmann's Encyclopedia of Industrial Chemistry 6 edn[END_REF], Lapatovich 2009).

HID lamps

The modification of spectra of HID lamps represents a difficult task because the light of HID lamps comes from the high pressure discharge arc directly. The filling composition takes effect on the physical conditions within the lamp like temperature and pressure and therefore on the properties of the visible radiation. Adding the salts of some elements does not necessarily lead to the appearance of their characteristic spectral lines in the spectrum of the lamp. Due to the changes in the physical and chemical composition of the vapours the spectra may change completely, for example by adding a readily 4.

ionizable element suppresses the radiation of elements with higher lying energy levels.

Nevertheless it is possible to perform numerical simulations of the possible modifications of the spectra of HID lamps. By such a simulation the possible positions and intensities of additional spectral lines can be determined which should lead to the desirable changes in biological effectiveness of the lamp radiation. The similar strategy of "morning" and "evening" lamp can also be followed here. Again the objective of "morning" lamp can be reached easier than that of "evening" lamp.

On the one hand the element Indium has a very "convenient" resonant radiation at 451 nm which lies near the maximum of the action spectrum s ms . Therefore by increasing indium amount in the lamp filling one can expect the increase of the biological activity of the lamp in sense of melatonin suppression.

On the other hand there are more options in reducing the biological effectiveness of an "evening" lamp. Not only the reduction of the radiation in the short-wavelength region, but also an increase of radiation in other spectral regions can lead to the desirable reduction of a ms v .

In order to investigate the possible spectral ranges suitable for diminishing the a ms v value, the spectrum of a commercial HID lamp was taken and one additional Gaussian peak was numerically added. The position, amplitude and the width of the peak were variable. The values of those three parameters were scanned within reasonable limits yielding an overview of the possible modifications of the given spectrum by one Gaussian peak. The resulting spectra whose chromaticity coordinates were lying near the Planckian locus were kept. As the result a figure of merit between colour rendering and biological effectiveness was obtained.

The spectra of three commercially available HID lamps: Nachroma (Lamp 1), VEG (Lamp 2) and Gold (Lamp 3) from NARVA G.L.E. were tested. The colorimetric and biologic properties of the lamps are summarized in table 4. The results of simulations in figure 5 show that depending on the initial filling of the lamp different combinations of a ms v and Ra are possible. For the particular lamps the significant increase or decrease in the biological effectiveness can be achieved only by a decrease of the colour rendering. The lamps 1 and 2 show a potential of being "morning" lamps and there exists a possibility of increasing both a ms v and Ra. The lamp 3 which was considered as a candidate for "evening" lamp has a limited ability of decreasing a ms v while still keeping Ra higher than the initial value. The possible aim of modification is shown in figure 5 as open circles with corresponding parameters listed in table 4. For lamps 1 and 2 the maximum of Ra(a ms v ) dependence is chosen as modification aim. Another possibility is the crossing point the curve with line Ra=80 at the right wing of the curve. There even higher values of a ms v are possible at still high enough Ra. For lamp 3 the point was chosen where the a ms v value is approx 10% smaller and Ra value is approx 10% higher than in the original spectrum as the possible aim of modification. Other options for reducing biological effectiveness of this lamp can only be achieved by significant decrease of colour rendereing.

Melatonin suppression by modified lamps

For the second stage of investigation the modified lamps were manufactured and tested for melatonin suppression. Lamps were installed in the usual lighting fixtures as shown in table 1. The list of lamps and modifications together with properties is given in table 3. The lamps of fluorescent type were modified by changing the relative composition of three phosphors -blue, green-yellow and red. The lamps with low CCT additionally contained a phosphor which absorbs the radiation in the short-wavelength visible region. It was possible to obtain an a ms v as high as 1.08 and as low as 0.1. The light sources with CCTs smaller than 2000 K are not perceived as "completely" white anymore. Although the Ra can be around 80 those lamp emit yellowish light. Due to chromatic adaptation their light can still be perceived as almost white and comparable to the candle light. With decreasing CCT the task of keeping high value of Ra becomes more and more difficult. This can be seen from the table 3 -the lamp with CCT=1800 can still have Ra=77 but lamp with CCT=1500 has only Ra=63. It should however be noted that the evaluation of colour rendering properties of the lamps with such a small CCT is not a usual task. In some standards like for example German DIN there exists the lowest limit of CCT=2300 K for the reference light source which is used for calculation of colour rendering index. This fact yields the controversy between DIN (DIN 5033-3 1992) and CIE (CIE 13.3-1995(CIE 13.3- 1995) ) colour rendering evaluation routines. For example the spectrum of Xe-DBD 1500 lamp has a Ra value 63 when calculated with CIE standard and Ra value of 74 when calculated with DIN standard with 2300 K reference illuminant. The values of Ra calculated with both standards are given in table 3. One can see a discrepancy between both evaluation procedures for the lamps with lower correlated colour temperatures.

For the "morning" lamp there exist no difficulty with keeping Ra over 80. The HID lamp used here was the metal halide lamp with additional amount of indium. This allowed to achieve the a ms v value of 0.82.

The results of melatonin suppression are shown in figure 6 according to equation (3). One can see that the measured melatonin suppression correlates well with the biologically effective illuminance level as defined by equation (2). The observed linear dependence can be considered to be a linear part of a logistic one which was used in works of Brainard and Thapan. But it should be noted that Ec values are only a rough estimate of the non-visual stimulus. In general it can be stated that the behaviour of melatonin suppression corresponds to the Gall model predictions -higher Ec values yield higher values of melatonin suppression.

Other light sources

The light sources other than plasma type ones were not covered in the scope of our investigations. In the frame of the melatonin suppression model of Gall it is nevertheless possible to give some recommendations about possible modifications of spectra of LEDs and incandescent lamps.

LED

The RGB type LEDs can be considered as one of the most flexible light sources with changeable biological properties. The variation of the relative intensities of individual blue, green and red LEDs can generally give any value of a ms v . The only constraints are the colour rendering properties and the condition for white light. The topic of optimization of LEDs radiation was already considered earlier, see e. 2002). The result of such optimization is a figure of merit between visual efficiency (V s ) and colour rendering (Ra). This curve shows that for given Ra (for example 80) a maximum achievable V s exists. The knowledge about biological activity of light can be considered as a new constraint on the LED light source. The optimization procedure of spectral composition of LED radiation can be made with the additional condition that the biological action of the light should be equal to that of Planckian radiator with the same colour temperature. In (Kozakov et al. 2008) RGB-spectra have been optimized for maximum visual efficiency V s with respect to the Ra. Set of different CCTs has been investigated. It was shown that additional constraint on the biological effectiveness of light reduces the maximum achievable V s .

Incandescent lamps

Spectra of incandescent lamps are very close to Planckian radiators with CCT values lying between 2700 and 3000 K. Their spectrum contains a certain amount of radiation in the short-wavelength spectral range. The biological action factor calculated by equation ( 1) for an incandescent lamp has the value of 0.36 for CCT=2700 K. A fluorescent lamp of type 827 with CCT=2700 and Ra=80 has an a ms v value of 0.25. Therefore incandescent lamps can have higher biological action than fluorescent lamps at the same CCT and also suppress melatonin. Possible modification of incandescent lamps can be performed either with spectrally selective filters ("morning" lamp) or by reducing the lamp power and thus the temperature of the filament ("evening" lamp).

Summary

An overview of current status of research in the field of the non-visual action of light from the point of view of physics and lighting application was given. The ways of modification of light sources for a proper non-visual action were discussed. The results of the investigation on the influence of plasma based light sources on the suppression of evening melatonin production were presented. The concept of "evening" and "morning" lamps was introduced. Modified "evening" lamps have shown significantly smaller melatonin suppression than commercially available lamps. Some increase in the melatonin suppression have been obtained by the modified "morning" lamps. It was shown that short-time (30 minutes) exposure to lighting conditions as described in the introduction do effectively suppress melatonin. This result allows to define a standard procedure to quantify the biological effectiveness of light sources for future use in lighting applications.

Figure 2 .

 2 Figure 2. Spectra of lamps used in stage 1 tests (see table 2). (a) -HID 70W, (b) -FL6000, (c) -Xe-DBD.

Figure 3 .

 3 Figure 3. Example of the melatonin course during the evening (averaged over N=9 subjects) (a) dim-light (--), HID 70W (----), FL6000 (• • • • • •) and Xe-DBD (-• -).

Figure 4 .

 4 Figure 4. Example of spectra of the modified FL -"morning lamp" with CCT=10000 K (a) and "evening lamp" with CCT=1800 K (b). In both cases the Ra ≥ 80.

Figure 5 .

 5 Figure 5. Simulation of spectra modification by one Gaussian peak. The positions of the unmodified lamps are represented by open diamonds. The possible aims of modification are shown by open circles. The corresponding parameters are given in table 4.

  g.( Žukauskas et al.

Figure 6 .

 6 Figure 6. Comparison of melatonin suppression with initial and modified light sources. FL6000 (♦), FL1800 ( ) and FL10000 ( ) -tubular fluorescent lamps in bathroom fixture; HID 70W ( ) and 250W ( ) -lamp with an additional amount of indium in the filling, ceiling fixture and ceiling floodlight respectively; Xe-DBD (• ) -lamp in bathroom fixture; Xe-DBD 1500 (• ) -lamp in ceiling fixture, office arrangement. Asterisks indicate the statistically significant (α=0.05) difference in the melatonin change rate with respect to dim-light situation.

Table 1 .

 1 List of used light sources and lighting conditions.

	Lighting	FL HID Xe-DBD
	condition
	Bathroom fixture	×	-	×
	Office fixture (ceiling fixtures) -	-	×
	Hall fixture (ceiling floodlight) -	×	-
	Hall fixture (downlight)	-	×	-

Table 2 .

 2 List of lamps used in initial tests. Ra CIE is colour rendering index. E is vertical illuminance corneal, a ms v is the biological action factor, equation (1).

	Lamp	Luminaire	E [lx] Ec	CCT [K] Ra CIE a ms v
	FL6000	Bathroom	130	91	5500	80	0.70
	HID 70W Hall downlight 500	245 4200	65	0.49
	Xe-DBD	Bathroom	500	200 2900	67	0.40

Table 3 .

 3 List of tested modified lamps. Ra CIE and Ra DIN are colour rendering indexes calculated according to CIE and DIN standards respectively (see section 4). E is vertical illuminance.

	Lamp	Type	Modification	E [lx] Ec	CCT [K] Ra CIE Ra DIN a m
	FL1800	evening	combination of phosphors 80	8.8 1800	77	81	0.1
	FL10000	morning combination of phosphors 100	108 10000	80	80	1.0
	HID250 W	morning additional indium	450	369 5500	81	82	0.8
	Xe-DBD 1500 evening	combination of phosphors 100	10	1500	63	74	0.1

Table 4 .

 4 Results of modification of the spectrum of the HID lamp with one additional Guassian peak.

	Lamp	Peak wavelength FWHM Amplitude	CCT (K) Ra CIE a ms v
				(% to max)
	Lamp 1 no peak	-	-	10600	85.1	1.27
	Lamp 2 no peak	-	-	6041	78.3	0.88
	Lamp 3 no peak	-	-	2549	49	0.35
	Lamp 1 490 nm	1 nm	119	12080	93.4	1.39
	Lamp 2 505 nm	1 nm	210	6710	83.5	0.92
	Lamp 3 550 nm	3 nm	80	2910	53.7	0.31

Acknowledgments

This work was supported by German ministry of education and research under the grant BMBF FKZ13N8968-74. The authors wish to thank the companies Trilux, OSRAM, NARVA BEL, NARVA G.L.E., Litec LLL for the fruitful cooperation.


B Jenichen
email: bernd.jenichen@pdi-berlin.de

J Herfort

K Kumakura

A Trampert

Long-range order and thermal stability of thin Co 2 FeSi films on GaAs(111)B

Keywords: numbers: 75.50 Bb, 81.15.Hi, 61.05.J-, 68.35.Ct

Co 2 FeSi/GaAs(111)B hybrid structures are grown by molecular-beam epitaxy and characterized by transmission electron microscopy (TEM) and x-ray diffraction. The lattice matched Co 2 FeSi films grow in a three-dimensional island growth mode at substrate temperatures T S between T S = 100 • C and 425 • C. The structures have a stable interface up to T S = 275 • C. The films contain fully ordered L2 1 and partially ordered B2 phases. The spatial distribution of long-range order in Co 2 FeSi is characterized using a comparison of TEM images taken with superlattice reflections and the corresponding fundamental reflections. The spatial inhomogeneities of long-range order can be explained by local non-stoichiometry due to lateral segregation or stress relaxation without formation of extended defects.

INTRODUCTION

There is an increasing interest in Heusler alloys as candidates for sources of spin injection into semiconductors [1][2][3][4]. The Heusler alloy Co 2 FeSi is a ferromagnetic half-metal with a Curie temperature larger than 1100 K [5]. The lattice parameter matches that of GaAs. Therefore, Co 2 FeSi is a suitable material for spin injection into GaAs-based structures such as for example spin light-emitting diodes (spin-LEDs [6][7][8][9]) and spin field effect transistors [10]. Samples produced by radio frequency magnetron sputtering have shown magnetically dead layers near the interfaces [11]. An epitaxial growth technique like molecular-beam epitaxy (MBE) may overcome these difficulties.

Recent theoretical studies indicate that a {111} interface may be superior to {001} interfaces with respect to the half-metallic properties of Co 2 FeSi [12]. Moreover, reacted compounds sometimes developing during Co 2 FeSi growth on GaAs(001) are bounded by {111} planes [13]. This observation led to the hypothesis that a {111} interface would be more stable. At the same time, reduced diffusion is expected perpendicular to the interface during growth on GaAs(111)B.

X-ray and electron diffraction experiments yield information about structure and longrange order of Heusler alloys and related materials [14][15][16][17][18][19]. In this study MBE grown Co 2 FeSi/GaAs(111)B hybrid structures are investigated by transmission electron microscopy (TEM), x-ray diffraction, secondary ion mass spectometry (SIMS), and atomic force microscopy (AFM) in order to characterize the structural properties and the stability of the ferromagnet/semiconductor (FM/SC) interface and the Co 2 FeSi film. The growth mode of the epitaxial film [20,[START_REF] Tsao | Materials Fundamentals of Molecular Beam Epitaxy[END_REF] is expected to have a fundamental influence on its structural properties. Fe 3 Si is a material very similar to Co 2 FeSi and grows on GaAs in the threedimensional island (Volmer-Weber) growth mode [START_REF] Kaganer | [END_REF]23]. and Co(A,C) sites, respectively [14,15,24]. γ is defined as the number of Co atoms on Fe(B) sites. However, Fe and Co atoms cannot be distinguished by TEM, because they have very similar scattering factors (f Fe ≈ f Co ). In this approximation the second terms of the sums (2) and ( 3) are equal to zero. Then three types of reflections are expected for such a lattice: Fundamental reflections (not sensitive to disorder) and the two kinds of superlattice reflections arising due to long-range order. Fundamental reflections are not influenced by disorder. They are given by the rule H + K + L = 4n, where n is an integer. One example for a fundamental reflection is 444. The structure amplitude is given by

F 444 ∝ (f Si + f Fe + 2f Co ), (1)
where f Si , f Fe , f Co are atomic scattering factors of the respective elements. There are two distinct types of superlattice reflections [14,15]. Reflections with odd H, K, L like the 111 reflection are sensitive to two types of disorder (in the Fe(B) and Co(A,C) sublattices with the corresponding order parameters α and β), the structure amplitude being

F 111 ∝ (1 -2α -β)(f Fe -f Si ) + (γ -β)(f Co -f Fe ). ( 2
)
Reflections satisfying the condition H + K + L = 4n -2 are sensitive only to disorder in the Co(A,C) sublattice, an example is 222 with the structure amplitude

F 222 ∝ (1 -2β)(f Co -f Si ) + (1 -2γ)(f Co -f Fe ). (3)
Compared to the fully ordered L2 1 phase, the B2 phase is obtained by a complete mixing between the Fe and the Si atoms in the Co 2 FeSi lattice. The Co(A,C) sublattice remains fully ordered in this case, β= 0, and an intense (222) reflection is expected, see equation (3).

III. EXPERIMENTAL

The structures were grown by solid-source MBE. The growth procedure on GaAs(001) is described in detail in Refs. [25,26]. In our case GaAs(111)B samples with a 50-nmthick GaAs buffer layer and an As-stabilized, (2×2)-reconstructed surface were grown first on a GaAs(111)B substrate and then transferred to an As-free growth chamber, in which stoichiometric ≈ 15 nm thick Co 2 FeSi films were grown on top for different substrate temperatures T S ranging between T S = 100 • C and 425 • C. (The nominal film thickness was slightly increased to 23 nm only for the maximum growth temperature T S = 425 • C.) Co, Fe, and Si are co-deposited from high-temperature effusion cells. The evaporation rates are controlled by the cell temperatures and are in accordance with the optimized fluxes for the growth of stoichiometric Co 2 FeSi films on GaAs(001) substrates [25]. The growth rate was approximately one monolayer in three minutes. The base pressure was about 1 × 10 -10 Torr.

The growth was monitored by in-situ reflection high energy electron diffraction (RHEED).

The samples were investigated by dark-field and high-resolution (HR) TEM. For that purpose cross-sectional TEM specimens were prepared by mechanical lapping and polishing, followed by argon ion milling according to standard techniques. TEM images were acquired with a JEOL 3010 microscope operating at 300 kV. In addition, X-ray measurements of diffraction and reflectivity were performed in a Panalytical X'Pert diffractometer equipped with a Ge (220) hybrid monochromator using CuKα 1 radiation. Some of the samples were characterized by ex-situ AFM almost immediately after the growth. A Digital Instrument Nanoscope was utilized for this purpose. Depth profiles of the film constituents inside the GaAs buffer layer were determined by SIMS. The Co 2 FeSi layer was removed by selective wet chemical etching prior the SIMS profiling. The SIMS measurements were performed with a CAMECA IMS4F system. The striking difference in intensity of both reflections (the GaAs intensity should be taken into account as a reference for a comparison) can be explained by their different structure factors, see equations ( 1) and ( 2). We estimated the order parameters from x-ray diffraction results by comparison of the integrated intensities of the fundamental and superlattice film reflections [17]. (The substrate reflections were used as an internal reference.) Approximately 25% of the Si atoms left their sublattice (sample grown at T S = 275 • C) and changed positions with the Fe atoms. For such a disorder the Co 2 FeSi (444) reflection is expected to be more than 50 times more intense than the Co 2 FeSi (111) reflection, a value, which seems to be in a reasonable agreement with Fig. 7.

IV. NON-DESTRUCTIVE CHARACTERIZATION

Next we consider the long-range order of a film grown at higher temperature: Figure 9 shows a HRTEM micrograph of a sample grown at T S = 425 interface consists of the B2 phase (fringe contrast). This example demonstrates that the long-range order of the Co 2 FeSi lattice is improved for the samples grown at higher T S .

C. Origin of inhomogeneities

In the micrograph shown in Fig. 10 , we observe well distinguished grains in the film grown at T S = 425 • C originating from island growth of Co 2 FeSi on GaAs(111)B and the coalescence of those islands. Grains of different orientation produce different interference patterns (phase contrast) in the HRTEM micrograph. Additional tilt or twist of the grains modify these patterns. In the micrograph, one of the grains seems darker and exhibits a modulated phase contrast, which is probably caused by a slight tilt or twist of that grain with respect to its neighbors. In other regions, these modulation stripes are found to be perpendicular to the FM/SC interface pointing to a lateral strain in the grains. The inhomogeneity in the HRTEM micrograph shown in Fig. 4 resembles the grain structure of the film grown at T S = 275 • C, whereas Fig. 11 shows the grain structure of a film grown at T S = 100 • C. While the GaAs substrate diffracts homogeneously, we see an inhomogeneous contrast in the micrograph of the film, probably caused by residual strain in the different grains. Such a strain could be caused by a local change in stoichiometry, which is always accompanied by a change in the lattice parameter [25]. A change in local stoichiometry may be due to a lateral segregation or stress relaxation near the step edges of the GaAs surface.

Despite a small misfit between the Co 2 FeSi film and the GaAs buffer layer, the observed formation of grains in the film is evidence for three-dimensional (3D) island growth during the first stage of heteroepitaxy of Co 2 FeSi on GaAs(111)B before a continuous film is formed.

Such island growth is caused by the higher surface tension of the metal compared to the semiconductor substrate. Indeed the surface tension of the metals Fe and Co is relatively high [START_REF] Zangwill | Physics at Surfaces[END_REF]. The surface tension of Co 2 FeSi is similar to that of Fe 3 Si, for which the Volmer-Weber (3D island) growth mode was observed on GaAs(001) earlier using in-situ x-ray diffraction [START_REF] Kaganer | [END_REF]23]. The difference of the surface tension ∆σ 1 of Fe 3 Si and Co 2 FeSi should be small compared to the difference ∆σ 2 of GaAs and Co 2 FeSi (∆σ 1 ∆σ 2 ) leading us to the expectation of an 3D island growth mode of Co 2 FeSi on GaAs(111)B. However, in-situ RHEED measurements did not detect a roughening of the surface indicating relatively large and very flat islands. Our growth rate one monolayer in three minutes is very near to a simulated kinetic optimum of 3D island growth with a resulting RMS surface roughness of less than one monolayer during all stages of the growth, see kinetic Monte Carlo simulations in [START_REF] Kaganer | [END_REF]. Under this condition RHEED would show a smooth surface.

In our case the spatial distribution of the long-range order in Fig. 7 On the GaAs side of the FM/SC interface both reflections reveal contrasts due to lattice strain connected to diffusion into the GaAs substrate.

L2 1 LATTICEFully ordered Co 2

 12 FeSi crystallizes into the face centered L2 1 structure. This structure can be viewed as an fcc lattice with a basis consisting of four atoms A, B, C, and D with coordinates A(0, 0, 0), B(0.25, 0.25, 0.25), C(0.5, 0.5, 0.5), and D(0.75, 0.75, 0.75). In the ordered Co 2 FeSi crystal, Co atoms occupy two sublattices A, C, and Fe is on sublattice B, while Si atoms fill the sublattice D. The degree of disorder can be described by three order parameters α, β, and γ. α and β are fractions of Si atoms occupying the Fe(B)

Figure 1

 1 Figure 1 shows high-resolution x-ray diffraction curves near the (222) diffraction peak of some of the samples investigated. Besides the narrow quasi-forbidden GaAs reflection, the broader Co 2 FeSi maximum is pronounced together with thickness fringes. Those fringes indicate a smooth FM/SC interface and and a high quality surface of the film, similar to the results of x-ray reflectivity measurements. The observation of an intense (222) reflection indicates a perfect epitaxial growth with a Co(A,C) sublattice almost free of Si atoms (β ≈ 0), see equation (3). That means the Co 2 FeSi film shows at least a B2 ordering, i.e. the Si atoms are mixed preferably with the Fe atoms (α > 0). Some modifications of the diffraction curves are found. In the case of T S = 325 • C the thickness fringes are less pronounced due to a degradation of surface and interface quality. For T S = 425 • C a shift of the Co 2 FeSi

Figure 5 (

 5 Figure 5 (a) is a multi-beam cross-section TEM micrograph at lower magnification along the GaAs [2 11 ] zone axis of a Co 2 FeSi film grown on GaAs at T S = 325 • C. A large reacted region (lower edge marked by dotted line) is visible below the Co 2 FeSi/GaAs(111) interface.

Figure 5 (FeSiFigure 6

 56 Figure 5 (b) is a higher magnification of one part of (a) illustrating the degradation of the interface in the immediate vicinity of the precipitate. The precipitates in the samples grown at T S = 325• C are always platelets at the interface. They are detected in the GaAs buffer layer, and their predominant boundary is (111). A typical lateral size of these precipitates is ≈100 nm, whereas their extent perpendicular to the interface is about 10-20 nm. Precipitates in samples grown at T S = 425 • C are even larger and have irregular shapes. The distance between the precipitates is of the order of micrometers. The remaining part of the FM/SC interface between the precipitates is almost perfect and may still contribute to an effective spin injection into the semiconductor device. Earlier observations[9] have shown for a similar system, namely for Co 2 FeSi/(Al,Ga)As spin LEDs grown on GaAs(001) substrates at high T S , a high spin injection efficiency inspite of the presence of precipitates.The morphology of the reacted regions near the (111) interface is clearly different from the appearance of the precipitates found in samples grown on GaAs(001)[13]. In our case the precipitates are larger and more flattened along the (111) interface. Therefore (111) boundaries seem to act as barriers for diffusion-driven precipitate formation.

Co 2 Figure 1 .

 21 Figure 1. High resolution x-ray diffraction curves measured near the GaAs (222) reflection of samples grown at different substrate temperatures T S . The narrow peak is the quasiforbidden GaAs reflection and the broader maximum is the Co 2 FeSi (222) superlattice reflection indicating at least a B2 ordering of all the films. The thickness fringes demonstrate a high quality of the FM/SC interface and surface.

Figure 2 .Figure 3 .

 23 Figure 2. (a) AFM micrograph of the Co 2 FeSi surface grown on GaAs(111)B at T S = 100 • C. (b) Root mean squared (RMS) surface roughness in dependence of the growth temperature of the samples.

Figure 4 .

 4 Figure 4. Cross-section HRTEM micrograph along the GaAs [01 1] zone axis of a sample grown at T S = 275 • C. The FM/SC interface has a roughness of 0.8 nm (see region between dashed lines) compared to those of samples grown at lower T S . Inhomogeneities of the Co 2 FeSi film are visible (marked by full lines).

Figure 5 .

 5 Figure 5. Cross-section multi-beam TEM micrograph along the GaAs [2 11 ] zone axis of a Co 2 FeSi film grown on GaAs at T S = 325 • C. In (a) a large reacted region (lower edge marked by dotted line) is visible below the Co 2 FeSi/GaAs(111) interface . (b) is a magnification of the region near the left end of the precipitate.

Figure 6 .Figure 7 .

 67 Figure 6. Selected area diffraction pattern along the Co 2 FeSi/GaAs [01 1] zone axis of a sample grown at T S = 275 • C. A rectangular pattern of Co 2 FeSi fundamental reflections (indexed, red dashed line) of the L2 1 structure is observed. This rectangular pattern is slightly more pronounced than the hexagonal pattern (black dashed line) of the superlattice maxima in the vicinity.

Figure 8 .

 8 Figure 8. SIMS depth profiles for Co in-diffusion at different growth temperatures of the Co 2 FeSi layers. The GaAs buffer layer starts at a depth of zero and ends at 50 nm, because the Co 2 FeSi was removed before the SIMS measurement by etching.

Figure 9 . 2 Figure 10 .

 9210 Figure 9. Cross-section HRTEM micrograph (Fourier filtered) along the GaAs [01 1] zone axis of a sample grown at T S = 425 • C. Here the dominant L2 1 phase of Co 2 FeSi is observed more far from the interface (inset: simulation), whereas the B2 phase (fringed contrast) is seen near the interface.

Figure 11 .

 11 Figure 11. Cross-section multi-beam TEM micrograph at lower magnification along the GaAs [01 1] zone axis of a sample grown at T S = 100 • C. Different grains can be distinguished due to changing in the micrograph. GaAs substrate diffracts

  • C. Here we see an area with clearly dominant well ordered L2 1 phase in the Co 2 FeSi film (honeycomb-like contrast, inset: image simulation, performed with the program Electron Microscopy Image Simulation, EMS On Line, http://cecm.insa-lyon.fr/CIOL/). Only the region very near to the FM/SC

ACKNOWLEDGEMENT

The authors thank Hans-Peter Schoenherr for his support during the MBE growth, Doreen Steffen for sample preparation, Astrid Pfeiffer for help in the laboratory, Esperanza Luna and Vladimir Kaganer for valuable support and helpful discussion.

which can lead to a relaxation of local strain without formation of extended defects.

VII.


S Schafranek
email: rschafranek@surface.tu-darmstadt.de

Li

B Wu

A Klein

F Chen

R Schafranek

S Li

W B Wu

Energy band alignment between Pb(Zr,Ti)O3 and high and low work function conducting oxidesfrom hole to electron injection

Keywords: numbers: 73, 30, +y 79, 60, Jv 77, 84, Cg

The interface formation between Pb(Zr,Ti)O 3 (PZT) and RuO 2 and between PZT and In 2 O 3 :Sn (ITO), respectively was characterised using in-situ X-ray photoelectron spectroscopy (XPS). No interface reaction was observed for the interfaces studied. The Fermi level position at the interface (Schottky barrier height) is strongly different for the two electrode materials. A Fermi level position of 1.0± 0.1 eV above the valence band maximum (VBM) is observed for the contact between PZT and the high work function oxide RuO 2 . For the contact between PZT and the low work function oxide ITO a Fermi level position of 2.1± 0.2 eV above the VBM is found.

Introduction

Lead zirconate titanate (Pb(Zr,Ti)O 3 , PZT) is one of the most commonly used ferroelectric materials applied e.g. in thin film form as capacitors in non-volatile memories [START_REF] Scott | Ferroelectric memories[END_REF] as well as in bulk form in piezoelectric sensors and actuators [START_REF] Moulson | Electroceramics: Materials, Properties, Applications[END_REF]. Electrical fatigue (reduction of the polarization with increasing switching cycles) limits the reliability of these devices and is therefore a crucial issue [START_REF] Scott | Ferroelectric memories[END_REF][START_REF] Tagantsev | [END_REF]4]. In the case of thin PZT films, leakage current and fatigue properties are dependent on the electrode material used [START_REF] Scott | Ferroelectric memories[END_REF]5,6,7,8]. While for Pt electrodes a pronounced fatigue is observed, the use of oxidic electrodes as RuO 2 , LaSr 0.5 Co 0.5 O 3 or SrRuO 3 leads to less fatigue. The observed fatigue of Pt/PZT/Pt capacitors may be linked to the injection of charge carriers (a) inhibiting the formation of opposite domains or (b) leading to an effective low relative permittivity layer ( [START_REF] Tagantsev | [END_REF] and references within). An alternative explanation for the fatigue improvement observed with oxide electrodes is the redistribution and pile-up of oxygen vacancies at the ferroelectric/metal interface with increasing switching cycles, thus hindering the switching process. Oxidic electrodes are supposed to absorb oxygen vacancies [9,10], leading to less fatigue. However, polarisation fatigue was also observed for n-type conductive electrodes like In 2 O 3 :Sn (ITO) [11,12] and SrSnO 3 :La [13,14]. This points to the fact that the PZT polarisation fatigue is more related to the electronic interface properties, which is also supported by the results of Du and Chen [15]. They observed a stronger fatigue improvement using a p-type Si barrier as compared to a ntype Si barrier. Therefore the injection of electrons, rather than holes, might be related to PZT fatigue.

Although, device properties as fatigue are evidently linked to the interface properties, the interface between PZT and different electrode materials was scarcely investigated. In bulk form PZT shows p-type conductivity due to Pb vacancies [16,17]. However, there has been a dispute in literature whether the current transport in PZT thin films is dominated by electrons or holes [START_REF] Scott | Ferroelectric memories[END_REF]18]. For electronic devices the interface properties play a crucial role as the choice of contact material determines the Fermi level position in the PZT at the interface, which corresponds to the zero field Schottky barrier height for electrons or holes, respectively [START_REF] Sze | Physics of Semiconductor Devices[END_REF][START_REF] Mönch | Semiconductor Surfaces and Interfaces[END_REF]. Due to the low carrier concentration in PZT the contact properties define whether majority electronic carriers in PZT are either electrons or holes. The interface formation between PZT and e.g. Pt was studied using photoelectron spectroscopy [START_REF] Takatani | [END_REF]22,23,24]. Scott et al. have used a device model to derive the Fermi level position at the interface of E F -E VB = 1.6 eV using PZT electron affinity and metal work function [START_REF] Scott | Ferroelectric memories[END_REF]25]. For the PZT/Pt interface we have recently reported a Fermi level position E F -E VB = 1.6± 0.1 eV after Pt deposition, directly derived using in-situ photoelectron spectroscopy. However, the barrier height is depending on the chemical state of the interface. After Pt deposition a reduced PZT/Pt interface was found, as could be derived from the occurence of metallic Pb in the Pb 4f X-ray induced photoelectron spectra of PZT. After an annealing step in O 2 the interface was oxidized and a Fermi level position E F -E VB = 1.1 ± 0.1 eV was found, while after two weeks in ultra high vacuum at room temperature a strong reduction of the PZT/Pt interface and a Fermi level position 2.2± 0.1 eV above the VBM was observed [24]. Taking the band gap of PZT of 3.4 eV [START_REF] Scott | Ferroelectric memories[END_REF] into account, this amounts to a Schottky barrier height for holes of 1.1± 0.1 eV for the oxidized and a Schottky barrier height for electrons of 1.2± 0.1 eV for the reduced PZT/Pt interface. Accordingly, in dependence on the interface state a preference for hole or electron injection is possible at the PZT/Pt interface.

Takatani et al. have also found a shift of the Fermi level towards lower E F -E VB upon annealing in O 2 but did not give absolute values for the Schottky barrier heights [START_REF] Takatani | [END_REF]22]. For the SrTiO 3 /Pt and SnO 2 /Pt interfaces a variation of the Schottky barrier of up to 1 eV, depending on preparation conditions, has also been reported [26,27]. The studied Oxide/Pt interfaces are therefore chemically unstable and are readily oxidized or reduced depending on the preparation and post-deposition treatment conditions [26].

In order to investigate the interface properties between PZT and oxidic electrodes, RuO 2 and ITO were chosen as material candidates with strongly different work functions. RuO 2 is an oxide showing metallic conductivity [28] while ITO is a degenerately doped semiconductor [29]. The former has a high work function of ∼ 6.1 eV [30] while highly doped as-deposited ITO films have a work function of ∼ 4.5 eV [31,32]. In this work the interface formation between PZT and RuO 2 as well as PZT and ITO were studied in-situ using photoelectron spectroscopy. It is shown that the barrier height between PZT and the two studied oxides differs significantly and the electronic transport mechanism in PZT is changed from a preference for hole injection for RuO 2 to a preference for electron injection for ITO.

Experimental

The experiments were performed at the DArmstadt Integrated SYstem for MATerial research (DAISY-MAT), which combines a Physical Electronics PHI 5700 multitechnique surface analysis system with several deposition chambers via an ultrahigh vacuum sample transfer [33]. X-ray photoelectron spectra were recorded using monochromated Al Kα radiation with an overall energy resolution of ∼ 0.4 eV, as determined from the Gaussian broadening of the Fermi edge of a sputter cleaned Ag sample. Binding energies are given with respect to the Fermi level of a sputter cleaned Ag sample with an accuracy of ∼ 50 meV. All spectra were recorded using a takeoff angle of 45 • .

Polycrystalline PZT thin films were prepared at the Hefei National Laboratory for Physical Sciences at the Microscale of the University of Science and Technology of China by pulsed laser deposition using a KrF 248 nm excimer laser with a repetition rate of 10 Hz and an energy density of 1.8 J/cm 2 . The PZT target with a Zr/Ti ratio close to the morphotropic composition (52/48) was made by standard solid state reaction. The distance between the substrate and target during deposition was 5 cm, the substrate temperature 650 • C and the oxygen pressure 30 Pa. Using these parameters, the growth rate for PZT is ∼ 15 nm/min. The PZT thin films were deposited on commercially available platinized Si wafers from Inostek. For the interface formation with ITO a PZT film of 200 nm thickness was used while for the interface formation with RuO 2 a 50 nm thick PZT film was utilized.

ITO and RuO 2 were deposited in one of the deposition chambers of the DAISY-MAT system. ITO was deposited by radio frequency magnetron sputtering from a ceramic target, being composed of 90 wt% In 2 O 3 and 10 wt% SnO 2 . A power density of 1.25 W/cm 2 , a substrate temperature of 400 • C, a pressure of 0.5 Pa, a substrate to target distance of 9 cm, and pure Ar as sputter gas were utilized. With these conditions highly doped ITO films with an electrical resistivity of ∼ 2•10 -4 Ω cm and a work function of ∼ 4.5 eV are obtained [31]. RuO 2 was deposited by reactive direct current magnetron sputtering from a metallic Ru target with a power density of 0.5 W/cm 2 . The substrates were held at room temperature during deposition. A pressure of 1 Pa, a substrate to target distance of 10 cm, and an Ar/O 2 ratio of 92.5/7.5 were used. The deposition conditions were chosen to guarantee the formation of fully oxidized RuO 2 , as verified by photoelectron spectroscopy as reported previously [30].

Results and Discussion

Survey spectra

Figure 1 shows X-ray induced survey spectra of a PZT thin film sample after surface cleaning via heating in O 2 as well as of the ∼ 10 nm thick RuO 2 and ITO thin films deposited during the interface experiments. In the case of the PZT thin film only emissions from Pb, Zr, Ti, and O are present. A C 1s emission due to adsorbed hydrocarbons lying at a binding energy of E B ∼ 285 eV can not be found, showing that the PZT film surfaces are free of contaminations. Also in the course of RuO 2 as well as of ITO deposition there is no carbon present. In the RuO 2 (ITO) survey spectra only emissions of the elements Ru and O (In, Sn and O) are found. In the case of the RuO 2 the Ru 3d doublet, while lying in the vicinity of the C 1s emission, completely describes the measured spectrum at ∼ 280 -290 eV binding energy (see also Fig. 2). of lineshape of the O 1s emission can be explained by the shift from insulating PZT, having a symmetric emission, to metallic RuO 2 with an asymmetric Doniach-Sunjic profile [34]. All cation emissions from PZT exhibit a parallel shift of ∼ 0.8 eV to lower binding energies in the course of deposition. From the unchanged substrate cation emission lineshapes as well as from their parallel shift in the course of RuO 2 deposition an interface reaction between PZT and RuO 2 can be ruled out. At small RuO 2 coverages the Ru 3d 3/2 emission lies at considerable lower binding energies than expected for RuO 2 (binding energy of ∼ 281 eV) while reaching the known value for higher RuO 2 thickness. Such a behavior is frequently observed in this kind of experiments and is typically explained by incomplete screening of the core hole in small metal islands occuring in the initial growth state [35].

Deposition of RuO

Schottky barrier at the PZT/RuO 2 interface

The Schottky barrier is determined by the position of the Fermi level in the PZT at the interface. In the inset of Fig. 3 the valence band spectra of the uncovered PZT film as well as of the 10 nm thick RuO 2 film, grown on the PZT substrate, are shown. The valence band maximum (VBM) of PZT is determined from the linear extrapolation of the leading edge of the valence band spectrum with an uncertainty of ± 0.1 eV. The valence band maximum of the PZT sample lies at 1.7 ± 0.1 eV. In Fig. 3 comparison the difference between core level binding energies and valence band maxima, as obtained from the uncoated substrate, were subtracted. The shifts of the core level binding energies saturate at E F -E VB = 1.0 ± 0.1 eV for RuO 2 thickness > 1 nm. The Schottky barrier height for holes Φ B,h can therefore directly be extracted, amounting to 1.0 ± 0.1 eV for the PZT/RuO 2 interface.

Deposition of ITO on PZT

In Fig. 4 X-ray photoelectron core level emissions of the PZT film are presented in course of ITO deposition. With increasing ITO thickness the PZT cation core emissions are exponentially attenuated, indicating a layer by layer growth of the ITO film. As observed for the PZT/RuO 2 interface, no change of the PZT cation lines is visible with increasing ITO thickness. The O 1s emission shows an increasing asymmetry with larger ITO thickness, which is related to Plasmon excitations of the electron gas of the degenerately doped material [31]. For low ITO coverages up to 0.5 nm all PZT core emissions are shifted in parallel by ∼ 0.4 eV to higher binding energies. After the first ITO deposition step the In 3d binding energy exhibits a parallel shift. Reaching an ITO thickness of ∼ 1 nm all spectra are shifted by ∼ 0.5 eV to lower binding energies. The evolution of the binding energies with icreasing ITO overlayer thickness will be explained in section 3.5. From the unchanged PZT cation and In lineforms, as well as the parallel shift of the PZT cation lines, an interface reaction between PZT and ITO The distance between core level lines and VBM have been subtracted. The Schottky barrier height for holes Φ B,h , corresponding to the distance between Fermi level E F and valence band maximum position E VB , amounts to 1.0± 0.1 eV as determined from the saturation of the core level binding energy shifts. Taking the band gap of PZT of 3.4 eV into account [START_REF] Scott | Ferroelectric memories[END_REF], this leads to a Schottky barrier for electrons of 2.4± 0.1 eV. VBM and CBM refer to the band edges of PZT. In the inset the X-ray photoelectron valence band spectra of the uncovered PZT film as well as of the 10 nm thick RuO 2 film grown on the PZT substrate are shown. can be ruled out.

Band alignment between PZT and ITO

The band alignment between PZT and ITO is determined from the evolution of the PZT and ITO cation emissions with increasing ITO thickness, as presented in Fig. 5. In the inset the valence band spectra of the uncovered PZT sample as well as the 12 nm thick ITO film grown on PZT are shown. The valence band maxima positions are derived from the leading edge of the valence bands and lie at 2.3 ± 0.1 eV and 3.25 ± 0.1 eV for uncoated PZT and thick ITO, respectively. In the ITO valence band spectrum the emission at the low binding energy side of the valence band maximum at ∼ 2 -3 eV binding energy can be explained by partially unsaturated cations ( [31] and references within). The 0.6 eV higher VBM position of the 200 nm thick uncoated PZT film used for the PZT/ITO interface study, as compared to the 50 nm thick PZT film used for the PZT/RuO 2 interface, indicates a slight charging of the PZT. Due to the parallel shift of substrate and overlayer binding energies, the band alignment can be reliably determined from the difference of the valence band maxima of PZT and ITO, which are derived from the respective core level binding energies. The discontinuous evolution of binding energies between 0.5 and 1 nm ITO thickness is explained by compensation of the charging effect: Once the thickness of ITO exceeds a certain limit, it becomes conductive enough to short-circuit the sample surface with the electrically grounded sample holder [START_REF] Klein | Solar Energy Materials and Solar Cells[END_REF]. The compensation of the charging effect is verified by the valence band maximum binding energy of the thick ITO film, which is within the range expected for comparably prepared films [31]. This guarantees that the barrier height for charge injection, which is given by the Fermi level position at the interface, is not affected by charging and can thus directly be derived from Fig. 5.

As presented in Fig. 5 a Fermi level position at the PZT/ITO interface of E F -E VB = 2.1 ± 0.2 eV is deduced for ITO thickness ≥ 1 nm. The Fermi level at the PZT/ITO interface is thus in the upper half of the band gap. Since the PZT core level lines do not completely saturate at larger ITO thickness, a higher margin of error is assumed. Taking the band gap of PZT of 3.4 eV [START_REF] Scott | Ferroelectric memories[END_REF] into account, this results in a Schottky barrier for electrons of 1.3± 0.2 eV. Furthermore, a valence band offset between PZT and ITO of 1.1 ± 0.1 eV is derived with the ITO VBM lying at lower binding energies.

Comparison of barrier heights

The difference of the Fermi level position at the contact between PZT and RuO 2 or ITO of 1.1 eV is almost as large as the difference in work function of RuO 2 and ITO of 1.6 eV. This points to a less strong Fermi level pinning for PZT with oxide electrodes than calculated by Robertson and Chen using charge neutrality levels (CNL) [39]. They The distance between core level lines and VBM has been subtracted. From the Fermi level position ∼ 2.1 eV above the valence band maximum a Schottky barrier for electrons of ∼ 1.3 eV can be derived, using a band gap of 3.4 eV for PZT. In addition the valence band offset between PZT and ITO of ∼ 1.1 eV can be deduced from the graph. VBM and CBM refer to the band edges of PZT. In the inset the X-ray photoelectron valence band spectra of the uncovered PZT film as well as of the 12 nm thick ITO film grown on the PZT substrate are shown. have derived a pinning index of 0.31 for PbTiO 3 and 0.4 for PbZrO 3 , which gives ∼0.4 -0.64 eV difference in barrier height for RuO 2 and ITO. This is considerably less than the experimentally found value of 1.1 eV. It is also astonishing that the Fermi level at the contact between RuO 2 and PZT is found just 1 eV above the valence band maximum. In the case of the (Ba,Sr)TiO 3 /RuO 2 interface the Fermi level lies ∼2.3 eV above the VBM [30]. The charge neutrality levels for SrTiO 3 and PbZrO 3 calculated by Robertson are both at 2.6 eV above the VBM, while the CNL for PbTiO 3 is calculated to lie at 1.9 eV [40]. For an almost equal mixture of PbTiO 3 and PbZrO 3 , as used for this study, a higher Fermi level position would have been expected at the PZT/RuO 2 interface from theory.

Summary and Conclusions

We have investigated the interface formation between PZT and RuO 2 and ITO, respectively using in-situ photoelectron spectroscopy. Stepwise deposition of RuO 2 and ITO were performed via magnetron sputtering on contamination free PZT thin films, being prepared beforehand by PLD on platinized Si wafers. Both studied interfaces are not reactive in contrast to the PZT/Pt interface, where a reduction of PZT is observed upon deposition of Pt [START_REF] Takatani | [END_REF]22,24]. A schematic energy band diagram of the deposited PZT/ITO, together with the PZT/RuO 2 contact, are given in Fig. 6. In case of the PZT/RuO 2 interface a Schottky barrier for holes of 1.0 ± 0.1 eV was found. For the PZT/ITO interface a Schottky barrier for electrons of 1.3 ± 0.2 eV was derived. This shows that using different oxide electrode materials allows for engineering of the contact properties. For PZT a transition from preference for hole injection to a preference for electron injection can be achieved when switching from RuO 2 to ITO electrodes.

Figure 1 .

 1 Figure 1. X-ray photoelectron survey spectra of a PZT thin film after surface cleaning and of in-situ deposited RuO 2 and ITO thin films.

Figure 2 .

 2 Figure 2. X-ray photoelectron core level spectra of the PZT sample recorded in the course of RuO 2 deposition. The RuO 2 thickness is given in nm.

Figure 3 .

 3 Figure 3. Evolution of the PZT core level lines with increasing RuO 2 thickness.The distance between core level lines and VBM have been subtracted. The Schottky barrier height for holes Φ B,h , corresponding to the distance between Fermi level E F and valence band maximum position E VB , amounts to 1.0± 0.1 eV as determined from the saturation of the core level binding energy shifts. Taking the band gap of PZT of 3.4 eV into account[START_REF] Scott | Ferroelectric memories[END_REF], this leads to a Schottky barrier for electrons of 2.4± 0.1 eV. VBM and CBM refer to the band edges of PZT. In the inset the X-ray photoelectron valence band spectra of the uncovered PZT film as well as of the 10 nm thick RuO 2 film grown on the PZT substrate are shown.

Figure 4 .

 4 Figure 4. X-ray photoelectron core level spectra of the PZT sample in the course of ITO deposition. The ITO thickness is given in nm.

Figure 5 .

 5 Figure 5. Evolution of the PZT core level lines with increasing ITO thickness d ITO .The distance between core level lines and VBM has been subtracted. From the Fermi level position ∼ 2.1 eV above the valence band maximum a Schottky barrier for electrons of ∼ 1.3 eV can be derived, using a band gap of 3.4 eV for PZT. In addition the valence band offset between PZT and ITO of ∼ 1.1 eV can be deduced from the graph. VBM and CBM refer to the band edges of PZT. In the inset the X-ray photoelectron valence band spectra of the uncovered PZT film as well as of the 12 nm thick ITO film grown on the PZT substrate are shown.

Figure 6 .

 6 Figure 6. Schematic band diagram of the PZT/ITO contact (left) and the PZT/RuO 2 contact (right). ITO is a degeneratively doped semiconductor having a fundamental band gap of ∼ 2.7 eV [37, 38] with the Fermi level lying in the conduction band, as also confirmed by the valence band maxima position of 3.25 eV. All energies given in electron Volt.

  the evolution of the substrate core level lines during stepwise deposition of RuO 2 are shown. For better

		9.6	Pb 4f	Zr 3d	Ti 2p	O 1s	Ru 3d
		4.8
		2.4
	Intensity [arb.units]	0.2 0.6 1.2
		0.07
		0
		145 141 137	187 184 181	466 461 456	532 530 528	290 286 282
				binding energy [eV]

Acknowledgement

The work was supported by the German Science Foundation within the collaborative research center SFB 595 (Electrical Fatigue of Functional Materials).