189 research outputs found
Optical and Structural Studies of High-quality Bulk-like GaN Grown by HVPE on a MOVPE AlN Buffer Layer
No full textHigh-quality 400 μm thick GaN has been grown by hydride vapour phase epitaxy (HVPE) on (0 0 0 1) sapphire with a 2 μm thick AlN buffer layer. The material’s crystalline quality and homogeneity was verified by x-ray diffraction (XRD), low-temperature photoluminesence (LT-PL) and LT cathodoluminescence. Plan-view transmission electron microscopy images reveal a low dislocation density of ?1.25 × 10^7 cm^(?2). The residual stress of the material was studied by two complementary techniques. LT-PL spectra show the main neutral donor bound exciton line at 3.4720 eV. This line position suggests virtually strain-free material with a high crystalline quality as indicated by the small full width at half maximum value of 0.78 meV. The presence of well resolved A- and B-free excitons in the LT-PL spectra and the absence of a yellow luminescence band prove the high quality of the HVPE-GaN in terms of purity and crystallinity. These findings are consistent with the XRD results, implying the high crystalline quality of the material grown. Hence, the material studied is well suited as a lattice parameter and thermal-expansion-coefficient matched substrate for further homoepitaxy, as needed for high-quality III-nitride device applications. Strain-free homoepitaxy on native substrates is needed to decrease considerably the defect density and in that way an improvement of the device’s performance and lifetime can be achieved
Optical and structural studies of high-quality bulk-like GaN grown by HVPE on a MOVPE AIN buffer layer
No full textHigh-quality 400 mu m thick GaN has been grown by hydride vapour phase epitaxy (HVPE) on (0 0 0 1) sapphire with a 2 Am thick AIN buffer layer. The material's crystalline quality and homogeneity was verified by x-ray diffraction (XRD), low-temperature photoluminesence (LT-PL) and LT cathodoluminescence. Plan-view transmission electron microscopy images reveal a low dislocation density of similar to 1.25 x 10(7) cm(-2). The residual stress of the material was studied by two complementary techniques. LT-PL spectra show the main neutral donor bound exciton line at 3.4720 eV. This line position suggests virtually strain-free material with a high crystalline quality as indicated by the small full width at half maximum value of 0.78 mev The presence of well resolved A- and B-free excitons in the LT-PL spectra and the absence of a yellow luminescence band prove the high quality of the HVPE-GaN in terms of purity and crystallinity. These findings are consistent with the XRD results, implying the high crystalline quality of the material grown. Hence, the material studied is well suited as a lattice parameter and thermal-expansion-coefficient matched substrate for further homoepitaxy, as needed for high-quality Ill-nitride device applications. Strain-free homoepitaxy on native substrates is needed to decrease considerably the defect density and in that way an improvement of the device's performance and lifetime can be achieved
Luminescence and reflectivity studies of undoped, n- and p-doped GaN on (0001) sapphire
No full textGaN grown by three different methods (MOVPE, GSMBE and HVPE) have been studied using temperature (T) dependent reflectivity and photoluminescence (PL). Both non intentionally doped (MOVPE, GSMBE and HVPE), n- and p-doped samples (MOVPE and GSMBE) have been investigated. Reflectivity is used to obtain intrinsic transition energies. These energies vary with the amount of strain in the crystal. Growth parameters influencing this strain state are discussed. Using MOVPE (T-g approximate to 1050 degrees C) and GSMBE (T-g approximate to 800 degrees C), it is possible to grow samples whose low temperature PL spectra are dominated by free and bound excitons and their phonon replica. Intentional n-type doping up to 10(20) cm(-3) is easily achieved with Si. For n much greater than 10(18) cm(-3), the spectra broaden and exhibit a blue shift, attributed to band filling. p-Type doping has been attempted using Mg, C and Ca. Ca doping led to compensated samples. C doping using CCl4 resulted in n-type samples, due to simultaneous oxygen incorporation in the layers; a strong enhancement of the 3.27 eV donor acceptor pair PL is also observed in this case. p-Type doping up to 10(18) cm(-3) has been achieved with Mg. With increasing densities, a deepening of the donor acceptor pair PL energy is observed. For high Mg doping, the spectra are dominated by a blue band in the 2.8 eV range, involving deep electron states. (C) 1997 Elsevier Science S.A.LASP
Coupled longitudinal optic phonon-plasmon modes in p-type GaN
No full textRaman scattering by coupled LO phonon-plasmon modes in p-type GaN layers has been measured for different hole densities up to 3 x 10(18) cm(-3) Both axial and planar coupled modes were clearly observed. The breakdown of usual selection rules for the planar mode is discussed. The lineshape analysis of coupled modes was achieved in the frame of a dielectric model, using electrical data from Hall measurements; in this calculation we used the value of the Faust-Henry coefficient determined from Raman intensity measurements on the LO and TO phonons. A good fit of the experimental data is obtained and gives evidence for hole mass anisotropy. (C) 1998 Elsevier Science Ltd. All rights reserved.LASP
MÖSSBAUER STUDIES ON GRANULAR Fe-SiO2
No full textLes composés granulaires Fe-SiO2 ont été préparés par copulvérisation chathodique en utilisant des cibles segmentées Fe-SiO2, un plasma d'argon (ou argon + H2) à 10-3 torr. Leurs spectres Mössbauer mettent en évidence la transition superparamagnétisme-ferromagnétisme. Cette température de transition dépend de la taille des grains de fer dans la matrice de SiO2. Une analyse fine des spectres Mössbauer montre qu'il existe plusieurs sites superparamagnétiques. Ceci s'interprète par des effets d'interface. Il existe aussi plusieurs sites magnétiques, ceci est dû à une distribution des tailles de grains à l'intérieur du composé. Les résultats de la spectrométrie Mössbauer donnent des détails sur la nature des composés granulaires qui n'avaient jamais été mentionnés auparavant.Granular compounds Fe-SiO2 were prepared by cosputtering using Fe and SiO2 targets, Argon (or Argon + H2) plasma under 10-3 torr. Mössbauer spectra from 4 to 300 K clearly show the transition superparamagnetism-ferromagnetism. The transition temperature depends on the size of the iron grain in the SiO2 matrix. A carefull analysis of the Mössbauer spectra show that several superparamagnetic sites exist. This was interpretated by interface effects. Several magnetic peaks exist too, which are due to a distribution of grains size among the sample. Mössbauer data gives details on the nature of granular compounds which were never seen before
Nature of defects in heavily Te-doped GaAs
No full textHeavily Te-doped GaAs has been studied by 125Te Mössbauer spectroscopy and transmission electron microscopy (TEM). It is shown that for ND-N A > 1019 cm-3, microprecipitates are observed along dislocations loops on TEM micrographs whereas a quadrupole split second site appears in Mössbauer spectra. Both features are consistent with the existence of microprecipitates of gallium telluride, probably Ga2Te3.GaAs fortement dopé en tellure a été étudié par spectroscopie Mössbauer sur 125Te et par microscopie électronique en transmission (TEM). Pour ND-NA > 1019 cm-3 , des microprécipités apparaissent le long des boucles de dislocations sur les microphotographies, tandis qu'un second site Te présentant un couplage quadrupolaire apparait dans les spectres Mössbauer. Ces deux faits peuvent s'expliquer par l'existence de microprécipités de tellurure de gallium, probablement Ga2Te 3
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