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Characterization of GaAs conformal layers grown by hydride vapour phase epitaxy on Si substrates by microphotoluminescence cathodoluminescence and microRaman
Journal of Crystal Growth 210 (2000) 198}202
Characterization of GaAs conformal layers grown by hydride vapour phase epitaxy on Si substrates by microphotoluminescence cathodoluminescence and microRaman O. MartmH nez!, M. Avella!, E. de la Puente!, J. JimeH nez!,*, B. GeH rard", E. Gil-Lafon# !Fn& sica de la Materia Condensada, ETS Ingenieros Industriales, 47011 Valladolid, Spain "THOMSON-CSF LCR, Domaine de Corbeville, 91404 Orsay, France #LASMEA UMR CNRS 6602, Universite& Blaise Pascal, Les Ce& zeaux, 63177 Aubie% re, France
Abstract Conformal growth of GaAs on Si consists of the con"ned lateral selective epitaxy of GaAs from GaAs oriented seeds on silicon, the vertical growth being stopped by an overhanging dielectric mask. Low defect GaAs "lms are obtained due to the absence of direct nucleation of the conformal GaAs epilayers on Si, and to the geometrical hindrance of the propagation of dislocations into the growing layer by the capping surface and by the substrate. GaAs conformal layers grown by hydride vapour phase epitaxy (HVPE) were characterised by microphotoluminescence (MPL), cathodoluminescence (CL) and microRaman. The GaAs conformal layers were found of superior quality since their luminescence emission was enhanced by several orders of magnitude with respect to the seeds directly grown on the Si substrate. CL and MPL images revealed in plane modulation of the luminescence emission. This modulation was associated with residual stress. MicroRaman measurements revealed stress distribution and eventually local symmetry breakdown. ( 2000 Elsevier Science B.V. All rights reserved. Keywords: GaAs/Si; Cathodoluminescence; MicroRaman; Photoluminescence imaging; Stress distribution; Conformal growth
1. Introduction The growth of III}V compounds on Si substrates gave rise to great expectation for high-quality devices, since it allows to combine the superior properties of III}V compounds with the large-scale integration of Si [1}4]. However, the large lattice mismatch between Si and GaAs (4%) and the
* Corresponding author. Fax: #34-983-423192. E-mail address: [email protected] (J. JimeH nez)
di!erence in their thermal expansion coe$cients prevent from obtaining high-quality materials. The layers directly grown on silicon present a high density of crystal defects (107 cm~2 dislocations), which make them not suitable for device applications [5]. Di!erent procedures have been carried out in order to reduce such a high concentration of defects [1,6}10]. Among these techniques, conformal growth appears very promising [9}12]. In this procedure, a GaAs sacri"cial bu!er layer is conventionally grown on a silicon wafer and covered by
0022-0248/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 6 7 8 - 8
O. Martn& nez et al. / Journal of Crystal Growth 210 (2000) 198}202
a dielectric cap layer on which oriented stripes are periodically opened (typically 10 lm wide with a pitch of 200 lm). The defective GaAs layer is then selectively underetched so as to leave oriented GaAs seed stripes (typically 60 lm wide). Conformal growth is carried out by using a near-equilibrium growth technique such as hydride vapour phase epitaxy (HVPE). The GaAs conformal growth is initiated on the lateral sides of the GaAs seed stripes and develops inside the cavity formed by the silicon substrate and the overhanging dielectric cap layer. One has to note that the conformal growth technique allows for an independent control of the vertical and lateral extensions of the GaAs "lm as the vertical one is settled by the thickness of the initial GaAs sacri"cial layer. The 603 type threading dislocations initially present in the GaAs seeds cannot propagate far through the GaAs growing layer as they are rapidly blocked either by the cap layer or by the substrate. This constitutes an e$cient geometrical defect-"lter. As a result, GaAs/Si conformal layers exhibit dislocation densities lower than 105 cm~2 [10}12]. These conformal layers were up to 40 lm wide, which make them suitable for integration of devices. We present herein an optical characterisation of (1 0 0) GaAs conformal layers grown on Si using microphotoluminescence (MPL), cathodoluminescence (CL) and microRaman spectroscopy.
2. Experimental and samples Typical GaAs conformal layers, 1.5 lm thick, were grown by HVPE at 7303C on misoriented (23 towards [0 1 1]) (1 0 0) Si substrates. The growth rate is about 8 lm/h. The samples appear as successive GaAs stripes (including a central seed and GaAs conformal layers on each side) separated by bare Si. Both [0 1 1] (as sample labelled A, lateral extension 17 lm) and [0 1 11 ] (as sample B, lateral extension 14 lm) oriented stripes were grown. Photoluminescence was mapped using a Micro PL system. The excitation was done with the 488.0 nm line of an Ar` laser. The maximum of the intrinsic luminescence band (j "875 nm) was .!9 selected on the monochromator (only intrinsic
199
luminescence was mapped), then the sample was moved by means of a high precision X}Y stage. The laser beam was focused onto the sample with a high numerical aperture long working distance microscope objective; the beam size at the focus plane was +1 lm, allowing high spatial resolution. All the luminescence measurements were carried out at room temperature. Cathodoluminescence was measured in the scanning electron microscope (SEM) using an Oxford monoCL system. Both panchromatic and monochromatic images were obtained. MicroRaman spectra were obtained with a Dilor X}Y Raman spectrometer attached to a metallographic microscope. The excitation was done with an Ar` laser through the microscope objective, which also collected the scattered light, thus conforming a nearly backscattering geometry. The nominal spatial resolution, corresponding to the beam diameter at the focal plane, was &0.7 lm for our usual experimental conditions (j"514 nm, 100] objective, NA"0.95).
3. Results and discussion MPL images give a general overview of the characteristics of the conformal layers. The general pattern of monochromatic MPL maps and the panchromatic CL images were similar, which demonstrates that the panchromatic CL images are mainly due to intrinsic luminescence. In Fig. 1 examples of near-band gap monochromatic MPL maps of two undoped samples A (a) and B (b) are given. The "rst observation is that the conformal layers are brighter than the seed layer. The luminescence intensity at the conformal layers is scaled up between two and three orders of magnitude compared to the luminescence intensity at the seed, where the luminescence emission is strongly limited by the high density of crystal defects in the layers directly grown on the Si substrate. The emission pattern was di!erent for the two samples. Modulations of the luminescence intensity were observed either perpendicular (Fig. 1a) or parallel (Fig. 1b) to the seed stripes. The same structures were observed in panchromatic CL images, Fig. 2. Cathodoluminescence spectra were obtained
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O. Martn& nez et al. / Journal of Crystal Growth 210 (2000) 198}202
Fig. 3. Cathodoluminescence intensity and j of sample A at .!9 di!erent points (6}10) shown in Fig. 2.
Fig. 1. Near-band gap monochromatic MPL maps of samples A (a) and B (b).
Fig. 2. Panchromatic CL image of sample A at 80 K. The bright regions correspond to high luminescence.
at di!erent points selected in the CL panchromatic images. Typically, the near-band gap (NBG) luminescence of GaAs/Si consists of several bands between 1.4 and 1.52 eV. The dominant band is
located around 1.483 eV at liquid-nitrogen temperature, which corresponds to the conduction band to heavy hole band (e}hh) transition. It is shifted with respect to the intrinsic emission observed in homoepitaxial GaAs/GaAs epilayers [13]. The conduction band to light hole transition (e}lh) was not observed. The CL spectra of samples A and B show a broad band between 820 nm (1.511 eV) and 860 nm (1.441 eV). This band results probably from the convolution of two or more bands. In sample A, the energy of the maximum ranged from 832 nm (1.489 eV) to 838 nm (1.478 eV). This energy shift was associated with the contrast of the CL images. High brightness areas correspond to the spectra with the maximum at 1.489 eV, while the maximum at 1.478 eV was mostly observed in dark regions. The CL spectra of sample B is slightly red-shifted; the maximum of the CL broad band was found at 837 nm (1.4802 eV). Di!erences between dark and bright regions were also observed in this sample. The bright regions showed spectral broadening in the low-energy side as compared to the spectra obtained in the dark regions. This broadening suggests a contribution to the CL band of additional transitions related to impurities. In general, an anti-correlation between the j and .!9 the luminescence intensity was observed, the higher the luminescence intensity the lower the j (see .!9 Fig. 3). This should mean that the bright areas are compressed with respect to the dark regions. This behaviour suggests the existence of stress distribution, the modulation of which di!ers from one
O. Martn& nez et al. / Journal of Crystal Growth 210 (2000) 198}202
Fig. 4. Frequency of the LO Raman mode in the conformal layers, parallel to the seed in sample A (a), and perpendicular to the seed in sample B (b).
sample to another. The stress distribution pattern is dependent on the seed orientation. As a matter of fact, [0 1 1] (sample A) and [0 1 11 ] (sample B) oriented stripes stand perpendicular and parallel, respectively, to the steps of the vicinal Si substrate. Note that the luminescence intensity variations due to this modulation can get a factor of two contrast; therefore stress cannot account for the reported luminescence intensity variations. Since the luminescence emission measured in MPL maps is intrinsic, the bright/dark contrast is probably associated with the presence of deep levels, that act as non-radiative recombination centers in competition with the e}hh recombination. The most important deep center in GaAs is related to the arsenic antisite, As , which suggests that As atoms G! di!use during growth towards the regions under tensile stress. This mid-gap level severely limits the lifetime of carriers and therefore the intrinsic luminescence intensity. Also, residual dislocations can contribute to non-radiative recombinations. MicroRaman measurements were carried out by scanning the laser beam along the growth axis and
201
along the conformal layers parallel to the seeds. The Raman spectrum of sample A showed a nonnegligible contribution from the symmetry forbidden TO mode, which suggests that the layer orientation is not (1 0 0) [14,15], though a more reliable hypothesis is the existence of faceting on the top surface of the layer, since the presence of the forbidden TO mode is only local. The LO phonon frequency was modulated along the conformal layer (Fig. 4a). This modulation had a period of about 10 lm, which is very close to the #uctuation period reported for MPL and CL images. Besides, the period changed depending on the distance to the seed in full agreement with the luminescence intensity distribution. Sample B did not exhibit the forbidden TO mode, which should mean that the layer orientation is the expected (1 0 0) and faceting does not exist. The Raman parameters (Fig. 4b) obtained transversally to the conformal layer showed a region where the LO phonon peak was shifted to the low frequency (tensile stress). This modulation seems to agree with that observed in this specimen by CL and MPL. Additionally, the LO band was broadened in those regions, which could be associated with some disorder or the presence of free electrons in a concentration slightly above 1016 cm3 [14}16]. Disorder has to be discarded because of the high luminescence emission. Therefore, the most reliable hypothesis seems to be the segregation of impurities that result in a free electron concentration in the range of 1016 cm~3. This is consistent with the enhancement of the band-edge luminescence and the observation of an additional luminescence band related to such impurities. Another important point concerns the fact that the LO phonon frequency measured in this layer was close to the phonon frequency measured on a GaAs wafer, and higher than that measured for sample A. This suggests that tensile stress is partially released in layer B, though regions with tensile stress subsist.
4. Conclusion High-quality GaAs conformal layers were grown by HVPE. MPL and CL images demonstrated
202
O. Martn& nez et al. / Journal of Crystal Growth 210 (2000) 198}202
a high luminescence emission, at least two orders of magnitude over the luminescence emission of the seeds. The modulation of the luminescence intensity was associated with residual stresses. The stress distribution was found to depend on the seed orientation with regard to substrate steps, as atoms were assumed to migrate to regions under tensile stress. The existence of the stress distribution was also detected by microRaman measurements. The orientation of the conformal layer was identi"ed according to the dipolar Raman scattering selection rules.
Acknowledgements This work has been supported by the European Commission in the frame of the BRITE-EURAM project `CONFORMa (contract No. BRPR-CT970512).
References [1] S.F. Fang, K. Adomi, S. Iyer, M. Morkoc7 , H. Zabel, C. Choi, N. Otsuka, J. Appl. Phys. 68 (1990) R31.
[2] H.K. Choi, IEEE Phot. Tech. Lett. 3 (4) (1991) 289. [3] T. Egawa, T. Jimbo, M. Umeno, Jpn. J. Appl. Phys. 32 (1993) 650. [4] A. Lubnow, G.-P. Tang, H.-H. Wehmann, E. Peiner, A. Schlachetzki, Jpn. J. Appl. Phys. 33 (1994) 3628. [5] See for example the articles by di!erent authors in Mater. Res. Soc. Symp. Proc. 67 (91) 116. [6] N. Chand, R. People, F.A. Baiocchi, K.W. Wecht, A.Y. Cho, Appl. Phys. Lett. 49 (1986) 815. [7] C. Choi, N. Otsuka, G. Munns, R. Houdre, H. Morkoc7 , S.L. Zhang, D. Levi, M.V. Klein, Appl. Phys. Lett. 50 (1987) 992. [8] T.W. Kang, Y.D. Woo, T.W. Kim, Thin Solid Films 279 (1996) 14. [9] O. Parrillaud, E. Gil, B. GeH rard, D. Pribat, Appl. Phys. Lett. 68 (1996) 19. [10] D. Pribat, V. Provendier, M. Dupuy, P. Legagneux, C. Collet, Jpn. J. Appl. Phys. 30 (3B) (1991) 431. [11] D. Pribat, B. GeH rard, M. Dupuy, P. Legagneux, Appl. Phys. Lett. 60 (1992) 2144. [12] B. GeH rard, D. Pribat, R. Bisaro, E. Costard, J. Nagle, J. Ricciandi, M. Dupuy, Mater. Res. Soc. Symp. Proc. 280 (1993) 675. [13] W. Stolz, F.E.G. Guimaraes, K. Ploog, J. Appl. Phys. 63 (1988) 492. [14] Y. Huang, P.Y. Yu, M.-N. Charasse, Y. Lo, S. Wang, Appl. Phys. Lett. 51 (1987) 192. [15] B. Roughani, M. Kallergi, J. Aubel, S. Sundaram, J. Appl. Phys. 66 (1989) 4946. [16] O. Page`s, M.A. Renucci, O. Briot, R.L. Aulombard, J. Appl. Phys. 80 (1996) 1128.
Characterization of GaAs conformal layers grown by hydride vapour phase epitaxy on Si substrates by microphotoluminescence cathodoluminescence and microRaman O. MartmH nez!, M. Avella!, E. de la Puente!, J. JimeH nez!,*, B. GeH rard", E. Gil-Lafon# !Fn& sica de la Materia Condensada, ETS Ingenieros Industriales, 47011 Valladolid, Spain "THOMSON-CSF LCR, Domaine de Corbeville, 91404 Orsay, France #LASMEA UMR CNRS 6602, Universite& Blaise Pascal, Les Ce& zeaux, 63177 Aubie% re, France
Abstract Conformal growth of GaAs on Si consists of the con"ned lateral selective epitaxy of GaAs from GaAs oriented seeds on silicon, the vertical growth being stopped by an overhanging dielectric mask. Low defect GaAs "lms are obtained due to the absence of direct nucleation of the conformal GaAs epilayers on Si, and to the geometrical hindrance of the propagation of dislocations into the growing layer by the capping surface and by the substrate. GaAs conformal layers grown by hydride vapour phase epitaxy (HVPE) were characterised by microphotoluminescence (MPL), cathodoluminescence (CL) and microRaman. The GaAs conformal layers were found of superior quality since their luminescence emission was enhanced by several orders of magnitude with respect to the seeds directly grown on the Si substrate. CL and MPL images revealed in plane modulation of the luminescence emission. This modulation was associated with residual stress. MicroRaman measurements revealed stress distribution and eventually local symmetry breakdown. ( 2000 Elsevier Science B.V. All rights reserved. Keywords: GaAs/Si; Cathodoluminescence; MicroRaman; Photoluminescence imaging; Stress distribution; Conformal growth
1. Introduction The growth of III}V compounds on Si substrates gave rise to great expectation for high-quality devices, since it allows to combine the superior properties of III}V compounds with the large-scale integration of Si [1}4]. However, the large lattice mismatch between Si and GaAs (4%) and the
* Corresponding author. Fax: #34-983-423192. E-mail address: [email protected] (J. JimeH nez)
di!erence in their thermal expansion coe$cients prevent from obtaining high-quality materials. The layers directly grown on silicon present a high density of crystal defects (107 cm~2 dislocations), which make them not suitable for device applications [5]. Di!erent procedures have been carried out in order to reduce such a high concentration of defects [1,6}10]. Among these techniques, conformal growth appears very promising [9}12]. In this procedure, a GaAs sacri"cial bu!er layer is conventionally grown on a silicon wafer and covered by
0022-0248/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 6 7 8 - 8
O. Martn& nez et al. / Journal of Crystal Growth 210 (2000) 198}202
a dielectric cap layer on which oriented stripes are periodically opened (typically 10 lm wide with a pitch of 200 lm). The defective GaAs layer is then selectively underetched so as to leave oriented GaAs seed stripes (typically 60 lm wide). Conformal growth is carried out by using a near-equilibrium growth technique such as hydride vapour phase epitaxy (HVPE). The GaAs conformal growth is initiated on the lateral sides of the GaAs seed stripes and develops inside the cavity formed by the silicon substrate and the overhanging dielectric cap layer. One has to note that the conformal growth technique allows for an independent control of the vertical and lateral extensions of the GaAs "lm as the vertical one is settled by the thickness of the initial GaAs sacri"cial layer. The 603 type threading dislocations initially present in the GaAs seeds cannot propagate far through the GaAs growing layer as they are rapidly blocked either by the cap layer or by the substrate. This constitutes an e$cient geometrical defect-"lter. As a result, GaAs/Si conformal layers exhibit dislocation densities lower than 105 cm~2 [10}12]. These conformal layers were up to 40 lm wide, which make them suitable for integration of devices. We present herein an optical characterisation of (1 0 0) GaAs conformal layers grown on Si using microphotoluminescence (MPL), cathodoluminescence (CL) and microRaman spectroscopy.
2. Experimental and samples Typical GaAs conformal layers, 1.5 lm thick, were grown by HVPE at 7303C on misoriented (23 towards [0 1 1]) (1 0 0) Si substrates. The growth rate is about 8 lm/h. The samples appear as successive GaAs stripes (including a central seed and GaAs conformal layers on each side) separated by bare Si. Both [0 1 1] (as sample labelled A, lateral extension 17 lm) and [0 1 11 ] (as sample B, lateral extension 14 lm) oriented stripes were grown. Photoluminescence was mapped using a Micro PL system. The excitation was done with the 488.0 nm line of an Ar` laser. The maximum of the intrinsic luminescence band (j "875 nm) was .!9 selected on the monochromator (only intrinsic
199
luminescence was mapped), then the sample was moved by means of a high precision X}Y stage. The laser beam was focused onto the sample with a high numerical aperture long working distance microscope objective; the beam size at the focus plane was +1 lm, allowing high spatial resolution. All the luminescence measurements were carried out at room temperature. Cathodoluminescence was measured in the scanning electron microscope (SEM) using an Oxford monoCL system. Both panchromatic and monochromatic images were obtained. MicroRaman spectra were obtained with a Dilor X}Y Raman spectrometer attached to a metallographic microscope. The excitation was done with an Ar` laser through the microscope objective, which also collected the scattered light, thus conforming a nearly backscattering geometry. The nominal spatial resolution, corresponding to the beam diameter at the focal plane, was &0.7 lm for our usual experimental conditions (j"514 nm, 100] objective, NA"0.95).
3. Results and discussion MPL images give a general overview of the characteristics of the conformal layers. The general pattern of monochromatic MPL maps and the panchromatic CL images were similar, which demonstrates that the panchromatic CL images are mainly due to intrinsic luminescence. In Fig. 1 examples of near-band gap monochromatic MPL maps of two undoped samples A (a) and B (b) are given. The "rst observation is that the conformal layers are brighter than the seed layer. The luminescence intensity at the conformal layers is scaled up between two and three orders of magnitude compared to the luminescence intensity at the seed, where the luminescence emission is strongly limited by the high density of crystal defects in the layers directly grown on the Si substrate. The emission pattern was di!erent for the two samples. Modulations of the luminescence intensity were observed either perpendicular (Fig. 1a) or parallel (Fig. 1b) to the seed stripes. The same structures were observed in panchromatic CL images, Fig. 2. Cathodoluminescence spectra were obtained
200
O. Martn& nez et al. / Journal of Crystal Growth 210 (2000) 198}202
Fig. 3. Cathodoluminescence intensity and j of sample A at .!9 di!erent points (6}10) shown in Fig. 2.
Fig. 1. Near-band gap monochromatic MPL maps of samples A (a) and B (b).
Fig. 2. Panchromatic CL image of sample A at 80 K. The bright regions correspond to high luminescence.
at di!erent points selected in the CL panchromatic images. Typically, the near-band gap (NBG) luminescence of GaAs/Si consists of several bands between 1.4 and 1.52 eV. The dominant band is
located around 1.483 eV at liquid-nitrogen temperature, which corresponds to the conduction band to heavy hole band (e}hh) transition. It is shifted with respect to the intrinsic emission observed in homoepitaxial GaAs/GaAs epilayers [13]. The conduction band to light hole transition (e}lh) was not observed. The CL spectra of samples A and B show a broad band between 820 nm (1.511 eV) and 860 nm (1.441 eV). This band results probably from the convolution of two or more bands. In sample A, the energy of the maximum ranged from 832 nm (1.489 eV) to 838 nm (1.478 eV). This energy shift was associated with the contrast of the CL images. High brightness areas correspond to the spectra with the maximum at 1.489 eV, while the maximum at 1.478 eV was mostly observed in dark regions. The CL spectra of sample B is slightly red-shifted; the maximum of the CL broad band was found at 837 nm (1.4802 eV). Di!erences between dark and bright regions were also observed in this sample. The bright regions showed spectral broadening in the low-energy side as compared to the spectra obtained in the dark regions. This broadening suggests a contribution to the CL band of additional transitions related to impurities. In general, an anti-correlation between the j and .!9 the luminescence intensity was observed, the higher the luminescence intensity the lower the j (see .!9 Fig. 3). This should mean that the bright areas are compressed with respect to the dark regions. This behaviour suggests the existence of stress distribution, the modulation of which di!ers from one
O. Martn& nez et al. / Journal of Crystal Growth 210 (2000) 198}202
Fig. 4. Frequency of the LO Raman mode in the conformal layers, parallel to the seed in sample A (a), and perpendicular to the seed in sample B (b).
sample to another. The stress distribution pattern is dependent on the seed orientation. As a matter of fact, [0 1 1] (sample A) and [0 1 11 ] (sample B) oriented stripes stand perpendicular and parallel, respectively, to the steps of the vicinal Si substrate. Note that the luminescence intensity variations due to this modulation can get a factor of two contrast; therefore stress cannot account for the reported luminescence intensity variations. Since the luminescence emission measured in MPL maps is intrinsic, the bright/dark contrast is probably associated with the presence of deep levels, that act as non-radiative recombination centers in competition with the e}hh recombination. The most important deep center in GaAs is related to the arsenic antisite, As , which suggests that As atoms G! di!use during growth towards the regions under tensile stress. This mid-gap level severely limits the lifetime of carriers and therefore the intrinsic luminescence intensity. Also, residual dislocations can contribute to non-radiative recombinations. MicroRaman measurements were carried out by scanning the laser beam along the growth axis and
201
along the conformal layers parallel to the seeds. The Raman spectrum of sample A showed a nonnegligible contribution from the symmetry forbidden TO mode, which suggests that the layer orientation is not (1 0 0) [14,15], though a more reliable hypothesis is the existence of faceting on the top surface of the layer, since the presence of the forbidden TO mode is only local. The LO phonon frequency was modulated along the conformal layer (Fig. 4a). This modulation had a period of about 10 lm, which is very close to the #uctuation period reported for MPL and CL images. Besides, the period changed depending on the distance to the seed in full agreement with the luminescence intensity distribution. Sample B did not exhibit the forbidden TO mode, which should mean that the layer orientation is the expected (1 0 0) and faceting does not exist. The Raman parameters (Fig. 4b) obtained transversally to the conformal layer showed a region where the LO phonon peak was shifted to the low frequency (tensile stress). This modulation seems to agree with that observed in this specimen by CL and MPL. Additionally, the LO band was broadened in those regions, which could be associated with some disorder or the presence of free electrons in a concentration slightly above 1016 cm3 [14}16]. Disorder has to be discarded because of the high luminescence emission. Therefore, the most reliable hypothesis seems to be the segregation of impurities that result in a free electron concentration in the range of 1016 cm~3. This is consistent with the enhancement of the band-edge luminescence and the observation of an additional luminescence band related to such impurities. Another important point concerns the fact that the LO phonon frequency measured in this layer was close to the phonon frequency measured on a GaAs wafer, and higher than that measured for sample A. This suggests that tensile stress is partially released in layer B, though regions with tensile stress subsist.
4. Conclusion High-quality GaAs conformal layers were grown by HVPE. MPL and CL images demonstrated
202
O. Martn& nez et al. / Journal of Crystal Growth 210 (2000) 198}202
a high luminescence emission, at least two orders of magnitude over the luminescence emission of the seeds. The modulation of the luminescence intensity was associated with residual stresses. The stress distribution was found to depend on the seed orientation with regard to substrate steps, as atoms were assumed to migrate to regions under tensile stress. The existence of the stress distribution was also detected by microRaman measurements. The orientation of the conformal layer was identi"ed according to the dipolar Raman scattering selection rules.
Acknowledgements This work has been supported by the European Commission in the frame of the BRITE-EURAM project `CONFORMa (contract No. BRPR-CT970512).
References [1] S.F. Fang, K. Adomi, S. Iyer, M. Morkoc7 , H. Zabel, C. Choi, N. Otsuka, J. Appl. Phys. 68 (1990) R31.
[2] H.K. Choi, IEEE Phot. Tech. Lett. 3 (4) (1991) 289. [3] T. Egawa, T. Jimbo, M. Umeno, Jpn. J. Appl. Phys. 32 (1993) 650. [4] A. Lubnow, G.-P. Tang, H.-H. Wehmann, E. Peiner, A. Schlachetzki, Jpn. J. Appl. Phys. 33 (1994) 3628. [5] See for example the articles by di!erent authors in Mater. Res. Soc. Symp. Proc. 67 (91) 116. [6] N. Chand, R. People, F.A. Baiocchi, K.W. Wecht, A.Y. Cho, Appl. Phys. Lett. 49 (1986) 815. [7] C. Choi, N. Otsuka, G. Munns, R. Houdre, H. Morkoc7 , S.L. Zhang, D. Levi, M.V. Klein, Appl. Phys. Lett. 50 (1987) 992. [8] T.W. Kang, Y.D. Woo, T.W. Kim, Thin Solid Films 279 (1996) 14. [9] O. Parrillaud, E. Gil, B. GeH rard, D. Pribat, Appl. Phys. Lett. 68 (1996) 19. [10] D. Pribat, V. Provendier, M. Dupuy, P. Legagneux, C. Collet, Jpn. J. Appl. Phys. 30 (3B) (1991) 431. [11] D. Pribat, B. GeH rard, M. Dupuy, P. Legagneux, Appl. Phys. Lett. 60 (1992) 2144. [12] B. GeH rard, D. Pribat, R. Bisaro, E. Costard, J. Nagle, J. Ricciandi, M. Dupuy, Mater. Res. Soc. Symp. Proc. 280 (1993) 675. [13] W. Stolz, F.E.G. Guimaraes, K. Ploog, J. Appl. Phys. 63 (1988) 492. [14] Y. Huang, P.Y. Yu, M.-N. Charasse, Y. Lo, S. Wang, Appl. Phys. Lett. 51 (1987) 192. [15] B. Roughani, M. Kallergi, J. Aubel, S. Sundaram, J. Appl. Phys. 66 (1989) 4946. [16] O. Page`s, M.A. Renucci, O. Briot, R.L. Aulombard, J. Appl. Phys. 80 (1996) 1128.