R.L. Aulombard

Raman determination of phonon deformation potentials in α-GaN

Abstract The effect of the built-in biaxial stress on the E2 and A1 (LO) q = 0 phonon modes of wurtzite GaN layers deposited by Metal Organic Vapor Phase Epitaxy on (0 0 0 1) direction on sapphire substrates is studied by Raman spectroscopy. Shifts in phonon frequencies are measured, which we correlate to the residual strain fields in the epilayers. Using stress calibration measurements taken from reflectance data, the biaxial pressure coefficients of mode frequencies are determined and used to calculate the corresponding deformation potentials.

Interplay of electrons and phonons in heavily doped GaN epilayers

Raman spectroscopy is used to analyze the effect of electrons on the lattice dynamics of unintentionally heavily doped GaN. The deposition temperature of GaN buffer layers on sapphire substrates is found to have an important influence on the presence of free carriers in GaN layers, evidenced by plasmon coupling to the A1(LO) phonon. Data from infrared measurements are used to calculate the Raman line shape of q=0 coupled A1(LO)-plasmon modes in a dielectric approach and give a good fit of the L−(q=0) component observed in Raman spectra. In particular, the fitting procedure applied to spatially resolved micro-Raman measurements reveals an inhomogeneous concentration of electrons on the scale of hexagonal microcrystallites. Partial screening of phonons with wave vectors differing from the q=0 transfer of incident and scattered photons is invoked to explain LO-like scattering over the whole spectral range of optical phonons, attributed to charge density fluctuations on account of its polarization properties.

Influence of the VIII molar ratio on the structural and electronic properties of MOVPE grown GaN

Abstract Low pressure MOVPE has been used to grow a series of GaN epilayers, deposited onto (0001) sapphire substrates. All the growth parameters have been kept constant, except for the V III molar ratio, which has been changed from sample to sample by changing the ammonia flow rate. The growth rate has been measured vs the V III molar ratio at both low temperature (550°C, growth of the buffer layer) and normal process temperature. A strong dependence of the growth rate vs the V III molar ratio is found at both temperatures. Systematic photoluminescence and reflectivity experiments have been performed at 2 K on all samples. The photoluminescence is dominated by the donor bound exciton and donor acceptor pair are visible on samples grown under the lowest V III molar ratio, indicating higher impurity incorporation. An optimum V III molar ratio of 5000 is proposed, which corresponds to the highest growth rate observed and to the lowest full width at half maximum for the photoluminescence line.

MOVPE growth of zincblende magnesium sulphide

Abstract The successful growth of MgS epitaxial layers with the MOVPE technique is reported. The samples were grown on GaAs substrates in a classical horizontal reactor, using bis(methylcyclopentadienyl) magnesium and H 2 S as precursors. The zincblende structure of MgS layers is evidenced by careful X-ray diffraction analysis. The lattice constant is found to be about 5.66 A. The influence of the growth temperature on the morphology and quality of the layers is studied in detail.

Caracterization of AIN buffer layers on (0001)-sapphire substrates

It is now established that low temperature-grown buffer layers are needed to improve the structural and electronic properties of GaN layers grown on sapphire. Using X-ray diffraction, we have studied the dependency on temperature and annealing time of the recrystallization of AlN buffer layers grown by low pressure MOVPE. The Warren‐Averbach method applied to the broadening and shape analysis of the (0002) and (0004) X-ray diffraction peaks has allowed us to separate grain size distribution from micro-strain effects. The obtained evolution of the relative frequency distribution of the grain sizes with annealing conditions is correlated with atomic force microscopy experiments (dynamic mode). The angular distribution of the c-axis of the grains is determined from X-ray rocking-curves experiments. X-ray reflectometry experiments and a simulation procedure have given us access to the roughness and the chemical composition of the sapphire-buffer layer interface and to the thickness and the roughness of the AlN grown.

Low pressure metalorganic vapour-phase epitaxy growth of ZnTe using triethylamine dimethyl zinc adduct

Abstract We present for the first time a detailed investigation of the growth of ZnTe layers deposited on GaAs substrates by low pressure metalorganic vapour-phase epitaxy using the new triethylamine dimethyl zinc adduct in combination with diisopropyl telluride as zinc and tellurium precursors respectively. The influence of the growth temperature ( T g ), the VI/II molar ratio, and the overall growth pressure ( P ) are studied. The successful growth of ZnTe is demonstrated at temperatures as low as 300°C. For growth temperatures ranging between 300 and 450°C, the growth rate is limited by the kinetics at the growing interface, and we measure an activation energy of 28 kcal/mol. At T g = 375°C, we found a linear variation of the growth rate with the overall growth pressure on the one hand, and with both zinc and telluride molar flows on the other hand. Low temperature reflectance and photoluminescence measurements were performed to characterize all the samples and are used to determine the optimal growth conditions: T g = 350°C, VI/II = 2, with P = 40 Torr. The stress experienced by the epilayers is investigated using the reflectance measurements. The relative intensity of the arsenic related bound exciton emission at 2.367 eV decreases as T g decreases. On the other hand, the I 1 b unknown acceptor related bound exciton emission at 2.356 eV decreases linearly with the growth rate.

Luminescence and absorption of GaN films under high excitation

Abstract The samples studied are thin GaN epilayers grown by metalorganic vapor phase epitaxy (MOVPE). At liquid helium temperature, the linear luminescence, transmission and reflection spectra of the films has been recorded, as well as the nonlinear emission spectra under high UV excitation. By using the variable strip length method, we have measured the gain coefficient as a function of the excitation intensity and the temperature. Pump and probe experiments have been performed in the femtosecond regime, showing crossed two-photon absorption without long lasting response. This allows to envisage the use of GaN for the characterization femtosecond pulses in the blue spectral range.

Optical properties and thermal transport of carriers in (Zn,Cd)Se-ZnSe heterostructures

We report a detailed optical study of ZnSe-based graded index separate confinement heterostructures. These structures were grown by metalorganic vapor phase epitaxy and are composed of either one or two Zn0.79Cd0.21Se central well(s) embedded between two ZnCdSe barriers which cadmium composition varies linearly from 5% near the wells to 0% at the end of the barriers. 2K photoreflectance and reflectivity experiments allow the observation of excitonic transitions involving the third electron and heavy hole confined states. The temperature dependence of the photoluminescence lines under in-well resonant excitation conditions (Eexc = 2.661 eV) shows that the thermal quenching of the photoluminescence line is ruled by nonradiative recombinations on defects localized at the heterointerfaces at low temperature and by the thermal escape of the minority carriers at higher temperatures. Under above-barrier excitation conditions (Eexc=3.814 eV), the temperature dependence of the photoluminescence line from the well shows a strong influence of the mechanism of diffusion of the carriers from the barriers to the well.

Raman characterization of ZnSe/GaAs MOVPE heterostructures

Abstract Zinc selenide epilayers on GaAs (100) were grown by the MOVPE technique. Two combinations of source materials were employed. When zinc alkyls and H2Se were used, n-type materials were obtained at low crystal growth temperature (300°C). When zinc and selenide alkyls were employed, temperatures of 500°C were needed for growth: in this case, by using a double zone reactor technique, temperatures as low as 300°C permit the MOVPE growth of ZnSe on GaAs (100). Raman spectroscopy was used for the characterization of ZnSe/GaAs heterostructures in relation to the growth conditions. The activation of the forbidden TO(Γ) phonon in ZnSe which is observed in some layers is well correlated with the surface morphology of the epilayers. A weakening of the LO peak of GaAs is correlated with the appearance of a broad and relatively intense band near the TO(Γ) phonon frequency in GaAs and is interpreted in terms of a coupled LO phonon-p-type plasmon located at the interface in the GaAs side.

Growth kinetics and structural quality in GaN epitaxy by low pressure MOVPE

GaN epilayers have been grown using low pressure MOVPE (76 Torr) onto (0001) sapphire substrates. Triethylgallium (TEGa) and Ammonia (NH{sub 3}) were used as precursors. The growth rate has been determined versus growth parameters. The growth rate versus NH{sub 3} flow displays a rather unusual behavior, in particular at low growth temperature (buffer layer), where the growth rate decreases strongly when the NH{sub 3} flow increases. A growth mechanism involving a competitive adsorption step onto the surface is proposed, and results in a strong dependence of the growth rate on the V/III ratio. The structural quality of the layers has been assessed by X-ray diffraction versus the growth parameters, and will be reported here. A substrate nitridation step, prior to buffer layer deposition has been introduced in the growth process. The effect of substrate nitridation on the structural and optical properties of the epilayers is studied and they propose an optimized pre-treatment for the MOVPE growth of GaN onto (0001) sapphire substrates.