A M Beltrán
email: beltran@cemes.fr

S Schamm-Chardon

V Mortet

E Bedel-Pereira

F Cristiano

C Strenger

A J Bauer

Compositional characterization of SiC-SiO 2 interfaces in MOSFETs

Keywords: 4H-SiC MOSFETs, STEM-EELS, SiC/SiO 2 interface

Silicon carbide (SiC) is extendedly studied for its use in power electronic devices working at particular conditions thanks to its advantageous physical properties. Among the different polytypes, 4H-SiC is the most attractive due to its largest bandgap, high electron bulk mobility and availability. Nevertheless, the development of metal oxide semiconductor field effect transistors (MOSFETs) based on this material is limited because of the mobility degradation such as the low carrier mobility in the inversion layer [1,2]. Lately, it has been proposed that bulk traps in SiC reduce electron mobility and they are related to carbon (C) clusters (0.10-0.15 nm) at the SiC-SiO 2 interface [3] or C-enrichment over few to few tens nm across the SiC-SiO 2 interface, as it has been observed by Electron Energy Loss Spectroscopy (EELS) [4].

In the context of the MobiSiC project (Mobility engineering for SiC devices) [5], we study 4H-SiC MOSFETs with the aim to get more insight in the C distribution and nature across the SiC-SiO 2 interface and to correlate the results with electron mobility measurements. Investigations are based on the combination of structural and compositional analyses carried out by high resolution transmission electron microscopy (HRTEM) and spatially resolved EELS.

Two sets of samples with n-channel planar MOSFETs are studied. The first set (pepi) and the second (p-imp) were fabricated on p-and n-type 4°-off 4H-SiC (0001) Siface epilayers, respectively. The substrates were provided by CREE Inc. The gate oxides were grown by oxidation in N 2 O atmosphere at 1553 K and subsequently annealed at the same temperature for 30 min under N 2 ambient. Further details about the sample fabrication process and the mobility results can be found in references [6,[START_REF] Strenger | Proceeding to the ICSCRM 2011[END_REF].

TEM investigations have been performed on a field emission FEI Tecnai™ F20 microscope operating at 200 kV. This microscope is equipped with a corrector for spherical aberration dedicated for the direct observation of atomic structures at interfaces with substantially reduced contrast delocalization in the images. It allows us the analysis of defects in the SiC side and eventual C clusters in the amorphous SiO 2 .

Local EELS studies were conducted with a scanning stage (STEM) allowing a focused one nanometre-sized probe to be scanned over the sample area of interest and an imaging filter (Gatan GIF TRIDIEM) used as a spectrometer. From this, elemental profiles across the SiC/SiO 2 interface have been acquired. TEM-lamellas have been prepared by focused ion beam (FIB).

Additionally to the common difficulties to prepare TEM-lamellas, especially by FIB, these samples are even more difficult since the SiC is harder than SiO 2 , so a gradient of thickness around 30% is observed at the interface whatever the process used for the fabrication of the sample (p-epi and p-imp). Structural investigations by HRTEM but also by weak beam electron diffraction reveals no defects in the SiC except those related to the dopant implantation and also no extended C cluster in the SiO 2 . From elemental profiles, we do not see C over-stoichiometry across the interface but rather an area of smoothly compositional variation that we defined as transition layer (TL). The only difference between both studied samples is the width of the TL, being larger for the p-epi than for the p-imp. This result can be related to the better mobility of the pimp sample than the p-epi [6,[START_REF] Strenger | Proceeding to the ICSCRM 2011[END_REF].

To complete these studies, additional chemical and structural investigations are performing such as atom probe tomography and strain studies.

The detailed analyses of the compositional distribution at the interface help us to understand the mobility results and work on its improvement in order to achieve the best design for MOSFETs with sufficiently high mobility to build more energy efficiency devices.


A M Beltrán
email: beltran@cemes.fr

A M Sánchez

F De La Peña

T Ben

D L Sales

A G Taboada

J M Ripalda

M Varela

S J Pennycook

S I Molina

EELS analysis of In x Ga 1-x As y Sb 1-y nanostructures

Keywords: EELS, nanostructures, InAs-GaAs-GaSb

Semiconductor nanostructures, such as quantum dots (QDs) are widely studied for a number of opto-electronic applications. Several approaches have been studied to achieve long wavelength emission values. Among them, GaSb incorporation to InAs/GaAs QDs is considered an important advance for improving the opto-electronic properties due to staggered band alignment of the GaSb/InAs systems [1]. In spite of the reported miscibility gap for quaternary alloys of GaSb and InAs in the absence of strain, the formation of ternary and quaternary alloys from the intermixing of the binary compounds is very likely when strain is present.

Since opto-electronic properties depend on the structural quality and compositional distribution, the characterization of the heterostructures is important in order to establish a relationship between real composition and structure and the achieved properties. The characterization techniques available in the electron microscope are very well suited to help to relating the opto-electronic properties of the QDs to their structure at the atomic level. In effect, the size, distribution, crystalline quality and composition of the nanostructures can be studied by high angle annular dark field imaging (HAADF), electron diffraction and electron energy loss spectroscopy (EELS), respectively.

Although different approaches to introduce Sb in InAs/GaAs QDs heterostructures have been analysed with the aim to identify the key parameters that enhance the opto-electronic properties of the nanostructures, this work is focused on the configuration which results in the best photoluminescence (PL) results.

All the heterostructures have been grown by molecular beam epitaxy (MBE), following similar conditions (details about growth conditions can be found in [2]). Over a GaAs (001) substrate, InAs has been grown to form a wetting layer (WL). The formed QDs were capped by a GaSb layer (sample S0), a GaAs layer of different thickness previous to the GaSb layer (S3, S6 and S12, of 3, 6 and 12 monolayers, ML, of thickness, respectively) or irradiated by a Sb flux before the deposition of 3 ML of GaAs and 2.2 ML GaSb (sample S). All of them were finally capped by a GaAs layer. EEL spectra were acquired at the SuperSTEM laboratory (UK) and ORNL (USA). For the compositional analysis, core loss and/or low EELS is frequently used. The studied systems are difficult to analyse conventionally in the core loss region because of the overlapping of the edges. For this purpose, Hyperspy software has been used over these edges. It is based on a combination of principal components analysis and blind source separations methods [3,4].

Focusing on the sample which shows the better PL emission, sample S [2], concerning the structural characterization, a lower defect density is observed for this sample, and the height of the QDs is slightly larger than the other samples [5,6]. Hence the size of the QDs and the crystalline quality contribute to improve the PL intensity value.

Compositional analysis by EELS in combination with the Hyperspy software of sample S, shows that In is mainly located inside the QD, although it is also detected outside due to the well-known segregation phenomenon very common in InAs-GaAs systems. Whereas, Sb is also observed inside QD but seems to cover the core of the QDs (Figure 1). There is no evidence of the GaAs and GaSb layers which has been nominally grown over the QDs. Instead a continuous composition is observed as a consequence of the intermixing phenomena between elements involving GaAs-GaSb and InAs, as well as the Sb from the flux. Ternaries and quaternary alloys are formed from these elements. Ga and As are detected in all the analysed area, since they are mainly in the substrate and top-capped layer and they are also inside the QD due to the segregation process.

Due to the projection effects we do not exactly know if we have analysed a completed QD or just a portion but from the results we are able to estimate the concentration and confirm the presence of Sb inside the QD in very low proportion.

Comparing to the other studied approaches, in the case of sample S0, Sb has been also detected inside the QDs, although in a higher concentration [7]. Samples S3, S6 and S12 were grown with the aim to avoid the formation of the quaternary alloy by the growth of an intermediary GaAs layer. Nevertheless, EELS analyses have also revealed the formation of ternary and quaternary alloys as in the previous samples. We observe a different behaviour depending on the thickness of the GaAs intermediate layer. Up to 6 ML two InGaAs intermediate layer while for 12 ML of GaAs layer, all In is segregated and only the formation of a quaternary alloy In x Ga 1-x As y Sb 1-y is observed In both cases; the Sb content is lower than 5% [8].

In conclusion, Sb irradiation of InAs QDs grown by MBE on GaAs and further GaSb/GaAs capping results in a huge increase of the room temperature PL signal at about 1.3 µm, thanks to the good crystalline quality of the heterostructure and the compositional distribution even with the formation of an In x Ga 1-x As y Sb 1-y alloy inside the QD.

Figure 1 .

 1 Figure 1. a) HAADF image of sample S and b) compositional map of elemental distribution obtained for In and Sb (C).


C Castro
email: celia.castro@cemes.fr

A Andreozzi

G Benassayag

A M Beltran

G Seguini

M Perego

S Schamm

EFTEM studies on the localization of silicon nanocrystals embedded in SiO 2 for nano-devices

Keywords: EFTEM, non-volatile memories, self-assembled block-copolymer, e-beam evaporation, ULE-IBS

Silicon nanocrystals (Si Ncs) based devices attract a strong interest thanks to their potential application in microelectronic and opto-electronic domains [START_REF] Koshida | Device Applications of Silicon Nanocrystals and Nanostructures[END_REF]. Si Ncs help to overcome downscaling limitations of the standard floating gate used in non-volatile memory devices [START_REF]ITRS Home[END_REF][START_REF] Tiwari | International Electron Devices Meeting[END_REF]. In particular, the use of a multi-node charge storage provided by a Si Ncs array is beneficial to get improved retention and programming at lower voltage [START_REF] Yamazaki | [END_REF]. Because a strong correlation exists between structural characteristics and electrical properties of nanomaterials, an accurate control of the synthesis process in relation with the Ncs characteristics such as size, density and organization is necessary for their development.

2D array of Si Ncs embedded in thin SiO2 layer with controlled size, density and depth have been demonstrated for different techniques like CVD [5], MBE and Ion Beam Synthesis (IBS) [6]. In particular, in the past decade, operating non-volatile memory and optoelectronic devices have been fabricated by the Ultra-low energy (ULE)-IBS followed by thermal annealing [6,7]. Downscaling, i.e. reducing the effect of fluctuations while the number of Ncs involved in the device becomes small requires that the localization of the Ncs within the plane should be precisely controlled [8].

For this purpose, we investigated for the first time the capabilities of combining Si NCs fabrication methods like e-beam evaporation and ULE-IBS with block copolymers nanostructured masks (BC) [9]. These masks are 21 nm thick polystyrene films with a hexagonal array of vertical pores of diameter 17 nm. The mean distance between the pores is 33 nm (Fig. 1c andd) [9]. For the e-beam technique, Si or SiO are evaporated though the BC mask and after the removal of the BC, the Si dots are capped by a 10 nm SiO2 layer. For ULE-IBS 1 keV Si + implantations with a fluence of 1x10 16 cm -2 are realized through the BC mask synthesized on a 15 nm thick SiO2 layer thermally grown on a (100) Si p-type wafers. In both cases a final thermal treatment under N2 is added to form the Si Ncs.

Energy filtered transmition electron microscopy of the Si plasmon signal (EFTEM) was used to visualize the position of the nanoparticles within the plane they form and therefore to determine precisely their characteristics (size, morphology, density and distribution). This chemical method allows the imaging of all the Ncs independently of their crystalline state [10]. Plan-view (Fig. 1a ande) and cross-section (Fig. 1b) configurations were considered.

Thanks to the quantitative analysis of the filtered images, we propose to demonstrate our ability to organize Si Ncs at the nanometer level (Fig. 1e and Fig. 2). We will also show how the Si Ncs fabrication method influences the organization of the Ncs within the hexagonal array. In particular, Si and SiO e-beam evaporations will be compared. Moreover, fluence and size of the BC pores will be considered for the case of the ULE-IBS method.

The association of ULE-IBS or e-beam evaporation with BC template is a low cost and high throughput approach for in plane organisation over large areas of Si Ncs embedded in SiO2 layer. Moreover, quantitative data provided by EFTEM analyses combined with other indirect analyses like electrical measurements is a powerful tool to support the development of future devices but also the validation of models [11][12].

Figure 1 .

 1 Figure 1. EFTEM images of ULE-IBS prepared samples without BC mask a) plan-view and b) cross-section and through the BC mask e) plan-view. c) SEM image of the BC mask and d) the corresponding pore diameter distribution. f) HRTEM image of crystalline nanoparticles.

Figure 2 .

 2 Figure 2. EFTEM image of a ULE-IBS prepared sample through the BC a) plan-view, b) binarisation of image a) for image analysis, c) Characteristics of the Ncs population deduced from the image analysis. Roundness = 4Pi*Area/Perimeter2 .


Koen Stroeken

Pieter Remes

Petra De Koker

Kristien Michielsen

Anke Van Vossole

HIV among out-of-school youth in Eastern and Southern Africa: a review

Keywords: Review, HIV/ AIDS, Out-of-school youth, Sub-Saharan Africa, Risky sexual behavior

published or not. The documents may come   L'archive ouverte pluridisciplinaire

Given the established role of behavioral change and education in countering the HIV epidemic in Sub-Saharan Africa [START_REF] Gillespie | Investigating the empirical evidence for understanding vulnerability and the associations between poverty, HIV infection and AIDS impact[END_REF], disregarding the out-of-school category is no longer an option.

(INSERT figure 1)

Sub-Saharan Africa has the highest proportion of children out of school with 26% of the primary school-age population; girls are more often out of school than boys (UNESCO 2009). There are positive trends though. Eastern Africa is catching up with southern Africa, after national policies dropped user-fees at the turn of the century (figures 1 and 2). Since about 1996 Sub-Saharan Africa has witnessed a reversal of association between educational attainment and risk of HIV infection, from hazardous to protective [START_REF] Michelo | Steep HIV prevalence declines among young people in selected Zambian communities: population-based observations (1995 -2003)[END_REF], Hargreaves et al. 2008a). Out-of-school youth are the main category failing to benefit from this reversal. Therefore, a systematic review of research relating out-of-school youth and HIV seems timely. Our review covers the last ten years and concentrates on the region most affected by HIV: East and Southern Africa.

(INSERT figure 2)

The association between educational attainment and less RSB is significant, although not uniform across the region. In two thirds of 18 sub-Saharan African countries education is associated with more condom use at last intercourse [START_REF] Bankole | Condom use for preventing STI/HIV and unintended pregnancy among young men in Sub-Saharan Africa[END_REF]. Education increases health literacy [START_REF] Bogale | Condom-use among low-literate, rural females in Ethiopia: the role of vulnerability to HIV infection, condom attitude and self efficacy[END_REF]) and can delay sexual debut [START_REF] Gupta | Sexual initiation among adolescent girls and boys: trends and differentials in sub-Saharan Africa[END_REF], which is important because of the role in Southern Africa's generalized HIV epidemic of early sexual debut combined with high levels of partner concurrency [START_REF] Dixon-Mueller | Starting young: sexual initiation and HIV prevention in early adolescence[END_REF][START_REF] Kenyon | A network-level explanation for the differences in HIV prevalence in South Africa's racial groups[END_REF]. Partners of 3 years older or more significantly increase the risk of HIV infection [START_REF] Katz | Why has HIV stabilized in South Africa, yet not declined further? Age and sexual behavior patterns among youth[END_REF]Low-Beer 2008, Chapman et al. 2010). One factor is the age-disparity hampering negotiation of condom use [START_REF] Longfield | Relationships between older men and younger women: implications for STIs/HIV in Kenya[END_REF][START_REF] Clark | Protecting young women from HIV/AIDS: the case against child and adolescent marriage[END_REF] as well as hampering STI/HIV risk perception (Clark 2004[START_REF] Clark | Relationship transitions among youth in urban Kenya[END_REF].

Hence, termination of a girl's school career due to early marriage deprives her not only of health literacy but also of the school's safer, age-near sex network. The first thing that strikes is the lack of standardization in defining youth. Four out of 16 studies opted for teenagers: the age group 10 to 19 years, or 10 to 18 years. This seems advisable so as not to exclude any sexually active youth. Most (6 out of 16) however opted for the groups 13 to 18 years, 13

to 19 years or 12 to 21 years. The rest chose an even older age group: 14 to 21 years, 14 to 25 years, or 15 to 24 years. We may be witnessing a gradual consensus in the field though, because the prevailing trend, since the Zambian study in 2005, is towards the age group of 10 to 19 years, which includes teenagers and excludes older adolescents who are commonly out-of-school anyway.

A second remarkable fact is the dearth of data on HIV serostatus and/or STI infection among out-ofschool youth. Only two studies report on it. In a 2001 sample of 1003 female and 916 male youths in the South African rural province of Limpopo, Hargreaves et al. (2008b) found that the chance of being HIV positive was about 20% higher for non-students (more exactly, between 0.06 and 0.71 times higher, confidence interval [CI]of 95%). After adjusting for age the association was no longer significant for women. A urine PCR of 522 adolescents from Addis Ababa showed that Chlamydia trachomatis and Neisseria gonorrhoeae were significantly (p<0.05) more common among out-ofschool than in-school youth (Taffa et al. 2002a).

The dominant theme of research is risky sexual behavior (8 studies) and its counterpart, condom use report sexual activity in the past year (p < 0.001). [START_REF] Taffa | Reproductive health perceptions, beliefs and sexual risk-taking among youth in Addis Ababa, Ethiopia[END_REF] confirm that sexual activity of out-of-school girls in Addis Ababa is higher in comparison to in-school urban girls (between 1.8 and 4.5, CI 95%). In rural South Africa this is true for out-of-school boys (Hargreaves et al. 2008b).

Secondly, more concurrent or multiple partners are found in the out-of-school category in rural South Africa, for both sexes aged between 14 and 25 (Hargreaves et al. 2008b). A smaller sample of 561 respondents in urban Ethiopia (Addis Ababa) yielded this significant association for young women only [START_REF] Taffa | Reproductive health perceptions, beliefs and sexual risk-taking among youth in Addis Ababa, Ethiopia[END_REF].

These associations contrast with a recent, more comprehensive cross-sectional study by [START_REF] Masatu | Predictors of risky sexual behavior among adolescents in Tanzania[END_REF] among 3495 boys and girls aged between 10 and 19, of whom 1000 were out-of-school. The data showed less concurrency or multiple partners for out-of-school boys in Tanzania (OR= 3.0; 95%CI= 2.2-4.1). Unprotected sex and multiple partners were more likely with a factor 2.3 among Christians than among non-Christians. The authors however note that the three villages of the sample were selected for high HIV prevalence and that the local schools may have sexually risky peerpressure cultures.

Third, transactional sex can be a way to gain material support, affirm self-worth, achieve social goals, or increase longer-term life chances. Focus group discussions in Uganda revealed that rural out-ofschool young women find material need a more appropriate motive for sex than pleasure (Nobelius et al. 2010b). In quantitative research among 628 out-of-school youth aged between 15 and 24 in Bahir town, [START_REF] Alemu | Factors predisposing out-of-school youths to HIV/AIDS-related risky sexual behavior in northwest Ethiopia[END_REF] observed that the probability of having sex with either a commercial or a non-regular sexual partner rose with a factor ranging between 1.83 and 4.23 (CI 95%) for out-ofschool adolescents who drink alcohol at least once a week (in comparison to out-of-school youth reporting no alcohol intake). The probability of such sex was also 6 times higher for those chewing khat. The sample did not contain in-school youth for comparison though.

Fourth, in the same study age-mixing often went hand in hand with transactional sex. Junior wives in polygamous marriages engage in cross-generational sex with all the risks attached and suffer from reduced negotiation power. Besides having more concurrent or multiple partners, out-of-school girls in rural South Africa engage more in age-mixing than those in school (Hargreaves et al. 2008b; for partners of three years older: 95% CI 0.37 to 0.92).

Fifth, a large survey comparing in-school and out-of-school youth in South Africa has established that secondary school attendance decreases the lifetime number of partners [START_REF] Zambuko | Sexual risk behavior among the youth in the era of HIV/AIDS in South Africa[END_REF].

Female students are less likely to have partners more than three years older than themselves, to have had sex more than five times with a partner and to have had unprotected intercourse during the past year.

Sixth, in the same survey inconsistent condom use was mostly found in South Africa among out-ofschool girls, and more so among those who were never pregnant, hence had less chance of previous Condom use requires, among others, a certain degree of HIV/AIDS information, more generally termed health literacy, which can be obtained in school. Self-efficacy in SRH planning is an educational outcome often lacking among low-literate rural young women in Ethiopia (Taffa et al. 2002b[START_REF] Taffa | Reproductive health perceptions, beliefs and sexual risk-taking among youth in Addis Ababa, Ethiopia[END_REF]. HIV knowledge is relatively low among out-of-school youth, both in Tanzania [START_REF] Bastien | Out-of-school and "at risk"? Socio-demographic characteristics, AIDS knowledge and risk perception among young people in northern Tanzania[END_REF]) and in Uganda [START_REF] Ndyanabangi | Reproductive health behavior among in-school and out-of-school youth in Kabarole District, Uganda[END_REF]). In a sample of 1007 urban and rural out-of-school youth (aged 13-18) [START_REF] Bastien | Out-of-school and "at risk"? Socio-demographic characteristics, AIDS knowledge and risk perception among young people in northern Tanzania[END_REF] found that boys in urban areas and with higher education are more knowledgeable about HIV/AIDS than their rural, less educated counterparts. The sample did not contain in-school youth to extend the comparison. [START_REF] Bastien | Access, agency and ambiguity: communication about AIDS among young people in northern Tanzania[END_REF] qualitative study based on 78 IDI described the limited exposure among out-of-school youth to HIV information and their less frequent communication about the topic with peers. [START_REF] Francis | Between the cracks : out-of-school youth and discourses of HIV / AIDS[END_REF] more HIV prevalence (Shisana et al. 2009: 36). 1 We advise that future studies on out-of-school youth differentiate their data according to these categories.

Thirdly, the association of out-of-school youth with HIV is unclear as to the direction of causality. No study as yet could exclude that the association stems from HIV infected youth actually leaving school after feeling stigmatized because of their HIV status and/or their RSB. Research among out-of-school cohorts who began to experience symptoms sufficiently long after their schooling period is necessary.

Longitudinal cohort studies could reveal whether schools form relatively isolated communities, where risky sexual behavior is epidemiologically less problematic than in the open, densely connected network outside of school. This effect of the sex network is hard to discern through RSB research with its individual-behavioral slant.

The longitudinal cohort study of [START_REF] Bärnighausen | The socioeconomic determinants of HIV incidence: evidence from a longitudinal, population-based study in rural South Africa[END_REF] in rural KwaZulu-Natal has summed up the SRH impact of educational attainment in their by now widely cited conclusion that HIV risk decreases with 7% for each additional year of educational attainment, holding age and sex constant. Can it be generalized? The Demographic and Health Surveys (DHS) of countries neighboring KwaZulu-Natal all exhibit a steady decrease of HIV prevalence as educational attainment advances from the level of lower (or incomplete) primary school to the level of secondary school or more (see country databases at www.measuredhs.com): for women from 33.8 to 30.5 in Swaziland (2006-07), 28.7 to 23.8 in Lesotho (2009), 22.4 to 15.8 in Zimbabwe (2005-06). However, in other southern African countries HIV 1 The association is admittedly hard to ascertain since symptoms of HIV infection can be subsumed under disability due to its broad definition. Respondents with disabilities also score low on HIV status awareness (Shisana et al. 2009: 4, 51). (2009( ), 13.2 to 15.1 in Malawi (2004)). Future research could determine whether the hazard of the higher educated follows from finding employment after school that expands the sex network. Out-ofschool status might be a stronger SRH discriminant than educational attainment for revealing the respodent's current network. Hence, longitudinal cohort studies can complement RSB research but their significance should be cross-regionally verified.

A final issue is the unexpected higher RSB in rural Tanzanian schools, which [START_REF] Masatu | Predictors of risky sexual behavior among adolescents in Tanzania[END_REF] attribute to peer-pressure. We would rather point to the out-of-school sex network in rural Tanzania where cultivation is demanding and elicits behavioral restraint rather than opportunity for partnering, yet where any globalizing process (e.g., through infrastructure or mining) opening the farmer's protective network can exact a high price for health illiteracy. The widely advertised decline of HIV prevalence rates in Southern and Eastern Africa threatens to obscure where the decline takes place:

in the educated urban areas reached by interventions. Prevalence trends among antenatal care attendees reveal a slight to significant rise currently in the rural areas, among others in Malawi, Swaziland, Uganda, Burundi, and Angola (UNAIDS 2010: 17). As the young villagers' sex network opens up we should expect the HIV epidemic to go fully rural, and school to play the protective role.

Conclusion

Out-of-school youth are a particularly vulnerable category, kept under the radar of epidemiological research. Their RSB is more pronounced. Their health literacy is under par. Our review pointed to the importance of (1) scaling up SRH-related research on out-of-school adolescents, including basic biomarker information such as serostatus, (2) standardizing the definition and age-group of this category, (3) acknowledging the proper SRH profile and corresponding needs of out-of-school youth, (4) differentiating between urban and rural risk groups, (5) more longitudinal cohort analyses of the sex networks to end speculations about the causal direction of the established associations, (6) a sustained effort in verifying the cross-regional significance of the associations. Potentially relevant unique titles screened: n=1602

Studies included in synthesis: n=295

Full manuscript assessed: n=440

Excluded:

not in Eastern of Southern Africa: 300 -More STI infections among out-of-school youth in urban Ethiopia (Taffa 2002); more HIV infection among non-students in rural South Africa (Hargreaves 2008).

-
-Less HIV knowledge among out-of-school youth both in Tanzania [START_REF] Bastien | Out-of-school and "at risk"? Socio-demographic characteristics, AIDS knowledge and risk perception among young people in northern Tanzania[END_REF][START_REF] Bastien | Access, agency and ambiguity: communication about AIDS among young people in northern Tanzania[END_REF]) and in Uganda [START_REF] Ndyanabangi | Reproductive health behavior among in-school and out-of-school youth in Kabarole District, Uganda[END_REF] -Less condom use among out-of-school young women in South Africa, both rural (Hargreaves 2008) and urban [START_REF] Zambuko | Sexual risk behavior among the youth in the era of HIV/AIDS in South Africa[END_REF]. Less condom use among out-of-school young men and women in Uganda [START_REF] Ndyanabangi | Reproductive health behavior among in-school and out-of-school youth in Kabarole District, Uganda[END_REF]) and in urban Ethiopia [START_REF] Alemu | Factors predisposing out-of-school youths to HIV/AIDS-related risky sexual behavior in northwest Ethiopia[END_REF][START_REF] Taffa | Reproductive health perceptions, beliefs and sexual risk-taking among youth in Addis Ababa, Ethiopia[END_REF].

-More risky sexual behavior among out-of-school youth in Uganda [START_REF] Ndyanabangi | Reproductive health behavior among in-school and out-of-school youth in Kabarole District, Uganda[END_REF], Nobelius 2010a), in urban Kenya (Khasakhala 2008), in Ethiopia, especially by women (Erulkar 2009) and in South Africa, especially by men [START_REF] Zambuko | Sexual risk behavior among the youth in the era of HIV/AIDS in South Africa[END_REF] -Earlier sexual debut among out-of-school youth in Uganda [START_REF] Ndyanabangi | Reproductive health behavior among in-school and out-of-school youth in Kabarole District, Uganda[END_REF], Nobelius 2010a) -More sexual activity among out-of-school youth in Uganda [START_REF] Ndyanabangi | Reproductive health behavior among in-school and out-of-school youth in Kabarole District, Uganda[END_REF], among out-of-school girls in urban Ethiopia (Taffa 2002) and out-of-school boys in rural South Africa (Hargreaves 2008) -More concurrent or multiple partners among out-of-school young women in urban Ethiopia [START_REF] Taffa | Reproductive health perceptions, beliefs and sexual risk-taking among youth in Addis Ababa, Ethiopia[END_REF]) and for both sexes in rural South Africa (Hargreaves 2008), but less concurrency or multiple partners for out-ofschool boys in Tanzania [START_REF] Masatu | Predictors of risky sexual behavior among adolescents in Tanzania[END_REF]).

-More age mixing among out-of-school girls in rural South Africa (Hargreaves 2008) -More transactional sex among out-of-school men in urban Ethiopia [START_REF] Alemu | Factors predisposing out-of-school youths to HIV/AIDS-related risky sexual behavior in northwest Ethiopia[END_REF] -More sexual abuse experienced by out-of-school young women both in urban Ethiopia (Erulkar 2009) and in Zambia (Slonim-Nevo 2005).

  -school adolescents are an upcoming population of research in the field of sexual and reproductive health (SRH). Most studies and interventions have targeted in-school youth: they are easier to reach, making surveys and interventions cheaper and less complex in terms of logistics.

(

  7 studies), which usually feature together. HIV/AIDS research traditionally identifies the following eight practices as indicators of risk of infection. In most studies risky sexual behavior is determined by probing a number of these practices.First,[START_REF] Ndyanabangi | Reproductive health behavior among in-school and out-of-school youth in Kabarole District, Uganda[END_REF] have established in a cross-sectional study in Uganda (n= 556) that sexual debut is earlier among out-of-school than in-school youth. The former are also more likely to

  health services and their information. Unmarried out-of-school young men tended towards unprotected sex in case they had never impregnated a woman nor engaged in transactional sex. An exacerbating factor was having peers who are indifferent or negative towards condom use. Our review's cross-sectional studies reveal that out-of-school youth resort less to condoms during casual sex. The seminal study in Zambia by Slonim-Nevo and Mukuka (2005) of randomly selected subsamples containing both rural and urban, out-of-school and in-school, male and female youth aged 10 to 19 (n= 3360) pointed to a higher likelihood (p < 0.01) among the out-of-school categories to have unprotected vaginal and anal sex, as well as to be sexually active, combine alcohol and sex, trade sex for money or food, and to have had an STI. Higher risk tendencies were encountered among older and married, poorer and rural out-of-school adolescents. Significantly less condom use among out-of-school youth has been confirmed for Uganda (p < 0.001;[START_REF] Ndyanabangi | Reproductive health behavior among in-school and out-of-school youth in Kabarole District, Uganda[END_REF]. Little over a third of out-of-school young Ethiopians who had sex in exchange for money consistently used a condom[START_REF] Alemu | Factors predisposing out-of-school youths to HIV/AIDS-related risky sexual behavior in northwest Ethiopia[END_REF]).

  rise with education from 15.8 to 17.4 in Zambia (2007), 14.4 to 15.0 in Mozambique

Figure 1 :Figure 2 :

 12 Figure 1: Percentages of primary-school-aged children out-of-school: East Africa (UNESCO -UIS online database 2010)

  The methodology of this review is based upon the PRISMA statement(Moher et al. 2009). Study eligibility and extraction procedures were specified in a systematic review protocol applying the following criteria of inclusion:• Location: the article must report on Eastern and Southern African countries. Countries included are: Eritrea, Ethiopia, Uganda, Kenya, the United Republic of Tanzania, Burundi, Rwanda, Target population: the article must report on young people aged 10 to 24 years and include out-ofschool youth. We specified additional categories related to out-of-school youth. Firstly, mainstream groups such as pregnant girls and married adolescents, rural boys and girls. Secondly socially marginalized youth, such as street children, orphans and vulnerable children in communities highly affected by HIV, adolescents involved in commercial sex work, child soldiers,

	III. Results
	Since 2007 an increase of publications can be noted on HIV among Eastern and Southern African out-
	of-school youth. The previous ten years were meager. Ethiopia, South Africa, the United Republic of
	Tanzania and Uganda have benefited more than once from such research; Kenya and Zambia only
	Democratic Republic of the Congo, Malawi, Zambia, Mozambique, Angola, Botswana, Lesotho, once. The category has not been put on the research agenda elsewhere. The next two tables
	Namibia, Swaziland, South Africa, Zimbabwe, Comoro, Madagascar, Seychelles, Mauritius; schematize the various studies included in this review. Study design and results are specified.
	F o r P e e r R e F o r e r R e Qualitative studies are in table 2 (Q= qualitative, IDI= in-depth interviews, FGD= Focus Group Discussions). When articles report on the same study, the table of results only includes the earliest publication (the dates of the later publications are between brackets). • To remedy any gaps, four authors (INSERT table 1) P e (INSERT table 2)
	v i e of key articles were contacted to suggest additional material. Three authors responded, of whom one suggested two additional papers. v i e
	w An electronic data extraction sheet was piloted by two authors and refined thereafter. Data extraction was done independently by the five authors. Of the 2216 articles, including 614 duplicates, w
	O n l y inclusion criteria were met by 283 descriptive articles and 157 intervention studies. This review considers only the descriptive articles. After assessing the full text, we remained with 19 descriptive studies dealing explicitly with HIV among out-of-school youth in Eastern and Southern Africa (11 from O n l PubMed and 6 from ISI Web of Science). After deducting publications concerning the same study we were left with 16 studies forming the key literature for our review. Twelve are cross-sectional studies y
	of random population samples. Four are qualitative studies, based either on focus group discussions
	The well-studied variable of educational
	or on in-depth interviews.
	attainment cannot register this second protective effect. A variable determining whether one is in or
	out of school can.
	(INSERT figure 3)
	II. Methods
	3

migrants, refugee children, drug users and gang members;

• Type of study: the article should report on the status of HIV/STI among youth or on HIV prevention or SRH promotion programs;

• Time limitation: articles should be published between January 2000 and August 2010.

With the corresponding word string, articles were searched in online databases PubMed, ISI Web of Science and OVIDsp. Bibliographic sections of the selected articles were searched as well as 'grey literature' on websites of major international health organizations.

  recommendation denies the out-of-school category adapted SRH interventions. The lure of pastoralist and agricultural lifestyles should not be underestimated, nor their incompatibility with school curricula and attendance. Civic organizations and the parental home can be alternatives for the school's protective role (see[START_REF] Harrison | Young people's sexual partnerships in KwaZulu-Natal, South Africa: patterns, contextual influences and HIV risk[END_REF]. Local bars and media channels such as radio can be used to reach this population (UNFPA 2011).Secondly, well-targeted interventions require more differentiated descriptions than the conventional one of UNESCO reports considering children to be out-of-school if they had no exposure to school during the school year in question. Children may be officially enrolled in schools yet not attending due to agricultural work, sickness, teacher absence, deaths in the family, the need to care for someone ill at home, or lack of means to cover school-related expenses[START_REF] Burke | Why children aren't attending school: the case of rural Tanzania[END_REF]. SRH profiles will differ between urban and rural out-of-school youth, between street children (whose out-of-school status is rarely mentioned by studies, see UNFPA 2011) and children orphaned by AIDS (who attend school only slightly less, except in Mozambique and Ethiopia, see UNAIDS 2010). Finally, in lowincome countries out-of-school status can be up to sixty percentage points higher among persons with disabilities (UNESCO 2009: 82). In South Africa at least, disability is associated with almost 50%

	F o r
	P
	e
	e r
	R
	e
	v i e
	w
	O n l
	y

paint a similar picture for out-of-school youth in KwaZulu-Natal, South Africa, where the absence of a structured learning environment such as a school prevents these adolescents from breaking with entrenched notions of gender, sexuality and low self-esteem.

(INSERT table 3) IV. Discussion

From the results presented above we can only infer that young people in school exhibit individual behavior that is sexually less risky. The logical recommendation is to have youth stay longer in school, beyond the age of sexual maturation. There are a number of caveats though. First, such

Table 1 :

 1 Overview of key descriptive literature: Cross-sectional studies

	not among out-of-school
	youth: 540
	-not reporting in HIV/SRH
	status or on HIV
	prevention/SRH
	promotion: 322
	Excluded: 180
	Records identified through a
	targeted search of other
	sources:
	Google Scholar=1, the screening
	of reference lists=1, and
	websites of international
	organizations (grey
	literature)=31
	Records identified through
	key authors: n=2

Table 2 :

 2 Overview of descriptive literature: Qualitative studies

	Country	First author	Study	Age	Sample	Main themes	Other themes Main factors	Other factors
	Year		population	group	size
	Uganda	Nobelius,	Rural out-of-	13-19	31	Risky sexual	Culture (peer	Parental
	2010a	AM	school youth		(q: FGD)	behavior	pressure),	assistance
						(sexual debut)	gender
	2010b	Nobelius,	Rural out-of-	13-19	31	Transactional	Culture, gender
		AM	school youth		(q:FGD)	sex
	Tanzania	Bastien, S	In-and out-	13-18	78	HIV/AIDS	Being out-of-	Culture (social
	2009		of-school		(q: IDI)	knowledge	school	network)
			youth in			(health literacy)
			Kilimanjaro
	South	Francis, D	Out-of-	10-18	32	Risky sexual	Culture	Socio-economic
	Africa		school youth		(q: IDI)	behavior,	(tradition and	(poverty),
	2008					condom use	peer), gender	education level,
							relations	sense of agency

Table 3 : Significant associations for out-of-school youth

 3

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Acknowledgments

Thanks are due to UNFPA for sponsoring the review and to ICRH (UGent) for facilitating it. We are indebted to an anonymous referee for one invaluable reference.