S. Ruffenach-Clur

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.

Raman study of resonance effects in Ga 1−x Al x N solid solutions

The photoluminescence and Raman spectra of several Ga 1−x Al x N layers (0 ≤ x ≤ 0.86) grown on sapphire substrates by metal-organic vapor phase epitaxy have been recorded at room temperature, under an excitation at 244 nm. Using the photoluminescence spectra, the variation of the band gap of these alloys can be followed only up to x = 0.5. From resonant Raman scattering, it can be deduced that the band gap energy of the solid solution for x very close to 0.7 corresponds to the incident photon energy (5.08 eV). This result is confirmed by a detailed comparison of the present work with previous experimental data on the A 1 (LO) phonon peak position, obtained under visible excitation.

MOCVD Growth of Cubic Gallium Nitride: Effect of V/III Ratio

We have grown cubic GaN on GaAs(001) substrates by MOCVD using a double step process with a buffer deposition at 400 °C. The purpose of the buffer deposited at very low temperature is to prevent GaAs decomposition at growth temperature. We have focused on the effect of the NH3/TEGa molar ratio on the growth of this metastable phase. We have assessed the crystalline quality using X-ray diffraction in the θ–2θ mode, and have used low temperature (2 K) PL for evaluating the optical properties of the GaN films. We found an optimum V/III ratio of 1250 at the growth temperature of 800 °C.

Role of the V:III precursor ratio on exciton dynamics in GaN MOCVD epilayers

Abstract Time-resolved photoluminescence spectroscopy is performed on strained epilayers of GaN, grown by MOVPE on Al2O3 substrates. As the V/III precursor ratio R is increased, the decay time of the donor–accepter pair (DAP) recombinations at ∼3.22 and ∼3.29 eV is changed from ∼1 ns to much more than 20 ns. Coherently with this change, the XA and XB free excitons and a donor-bound exciton DoX lying 5.7 meV below XA are subject to significant variations. For R∼8000, DAP transitions are fast and the DoX recombination dominates the spectrum at T=8 K, with decay times of ∼35–45 ps, while free excitons decay on a ∼25 ps time scale. For R∼10000, the DAP transitions are very slow and all excitonic recombinations become faster: times of up to ∼280 ps are obtained for DoX and 35 ps for the XA line, which now dominates the time-integrated spectrum. Exciton dynamics in GaN epilayers can thus be controlled by the V/III precursor ratio, which affects both the density of vacancies and extrinsic impurity incorporation.

Comparison of Luminescence and Physical Morphologies of GaN Epilayers

In this paper we examine a series of four GaN epilayers grown by MOVPE on sapphire substrates with different AlN buffer layer thicknesses. We examine the effect of the buffer layer thickness on the physical and optical properties of the samples via optical microscopy, cathodoluminescence imaging and photoluminescence and cathodoluminescence spectroscopy. While the morphological and optical properties of all the films (excepting that with the thinnest buffer layer of 30 nm) are good, i.e., the films are smooth and the luminescence is dominated by excitonic luminescence, a number of circular island like features are observed in all the films whose density decrease with increasing buffer layer thickness. A large circular island present on the sample with the thinnest buffer layer and surrounded by cracks in the directions, displays some interesting acceptor related luminescence.

X-Ray Reciprocal space mapping studies of strained GaN/AlGaN quantum wells

In this work, we report on the systematic X-ray reciprocal space mapping of a series of GaN/AlGaN samples, with different Al content and well thickness. For coherently grown samples, we present a calculation which allow us to precisely determine the strain state and Al content in the samples in one diffraction experiment. In our samples, both GaN and AlGaN are strained, and we discuss the effect of these strains on the band structure of GaN, which is probed by low temperature reflectivity and correlates perfectly our x-ray results.

Investigation of GaN epilayers growth mechanisms using in-situ reflectance in MOCVD

GaN is a wide gap semiconductor which is now used to produce blue and green light emitting diodes, blue laser diodes and which has numerous other potential applications, like high frequency HEMT transistors, UV sensors, etc. A complicated two step process, using a low temperature buffer layer, subsequently annealed and followed by the deposition of the monocrystalline semiconductor was developed, and due to the excellent results obtained following this method, a rush towards applications resulted. There is now a need to investigate in more detail the growth mechanisms, and the influence of the growth parameters, in order to ensure a better reproducibility of the results. In this paper, we report an investigation of the growth mechanisms and the influence of the growth parameters using in-situ reflectance experiments. The reflectance measurements allow us to follow the growth rates, and the changes in the surface morphology (transitions between islands growth and 2D growth). Additional exsitu measurements (AFM ) were performed at different stages of the growth process to ensure additional information. As a result, we demonstrate that the recrystallization of the low temperature buffer layer is a critical step, which is drastically influenced by the composition of the annealing atmosphere (amount of ammonia present in the gas phase), while the deposition temperature and buffer thickness have a moderate effect. We will discuss here the growth mechanisms which may be involved to explain such a behavior.