Epilayer Growth Temperature Research Articles

N-polar GaN evolution on nominally on-axis c-plane sapphire by MOCVD part-I: Growth optimization

The present work is part-I of a study that demonstrates the influence of metalorganic chemical vapor deposition process parameters on the evolution of gallium nitride (GaN) deposited in a two-step growth process on c-plane sapphire substrates with a particular emphasis on nitrogen-polar GaN (N-polar GaN). First, the temperature domains of sapphire nitridation that result in Ga-polar and N-polar GaN epilayers were identified. Then, the structural and morphological evolution of low-temperature GaN nucleation layers were examined at the observed temperature domains of nitridation. Subsequently, a series of experiments were performed to study the influence of Ga and N precursor flow rates and epilayer growth temperature on the surface evolution of N-polar GaN. The observed results were analyzed in terms of thermodynamic and kinetic parameters of GaN growth. This work reports high-surface quality N-polar GaN epilayers without surface hillocks, exhibiting surface roughness about 1.5 nm over 2 × 2 μm2 area and with good crystalline quality on nominally on-axis c-plane sapphire substrates at low epilayer growth temperatures.

Investigation of MOVPE-grown zincblende GaN nucleation layers on 3C-SiC/Si substrates

Cubic zincblende (zb-)GaN nucleation layers (NLs) grown by MOVPE on 3C-SiC/Si substrates were studied to determine their optimal thickness for subsequent zb-GaN epilayer growth. The layers were characterised by atomic force microscopy, X-ray diffraction and scanning transmission electron microscopy. The as-grown NLs, with nominal thicknesses varying from 3 nm to 44 nm, consist of small grains which are elongated in the [1 −1 0] direction, and cover the underlying SiC surface almost entirely. Thermal annealing of the NLs by heating in a H2/NH3 atmosphere to the elevated epilayer growth temperature reduces the substrate coverage of the films that are less than 22 nm thick, due to both material desorption and the ripening of islands. The compressive biaxial in-plane strain of the NLs reduces with increasing NL thickness to the value of relaxed GaN for a thickness of 44 nm. Both the as-grown and annealed NLs are crystalline and have high zincblende phase purity, but contain defects including misfit dislocations and stacking faults. The zb-GaN epilayers grown on the thinnest NLs show an enhanced fraction of the wurtzite phase, most likely formed by nucleation on the exposed substrate surface at elevated temperature, thus dictating the minimum NL thickness for phase-pure zb-GaN epilayer growth.

Effect of epilayer's growth temperature on crystalline quality of InAs 0.6P 0.4/InP grown by two-step growth method

InAs 0.6P 0.4 epilayers grown by low-pressure metalorganic chemical vapor deposition (LP-MOCVD) on InP (1 0 0) substrates were investigated. A two-step growth method, in which low-temperature (450 °C) InAs 0.6P 0.4 buffer layers were introduced into the structure, was employed to relax the mismatch between the InAs 0.6P 0.4 and InP substrate. The effect of epilayer's growth temperature on crystalline quality of InAs 0.6P 0.4 epilayer was studied by X-ray diffraction, scanning electron microscopy, Hall measurements, and photoluminescence spectrum. The characterization results showed that the growth temperature is an important factor for obtaining good quality and property of InAs 0.6P 0.4 epilayers and 530 °C is the optimum epilayer's growth temperature in our experimental conditions.

A study of two-step growth and properties of In 0.82Ga 0.18As on InP

In 0.82Ga 0.18As was grown by LP-MOCVD on InP substrates with the two-step growth technique. It was analyzed that epilayer's growth temperature affected on the crystalline quality, surface morphology, carrier concentration, and mobility of the In 0.82Ga 0.18As, which was characterized by X-ray diffraction, scanning electron microscopy, and Hall measurements. The evaluation of stress in In 0.82Ga 0.18As was made from frequency shift of the GaAs-like LO phonon of the Raman spectrum.

Effect of buffer layer thickness and epilayer growth temperature on crystalline quality of InAs0.9Sb0.1 grown by MOCVD

InAs0.9Sb0.1 epilayers are grown on GaAs (001) substrates by metal organic chemical vapor deposition (MOCVD). In order to relax compressive strain caused by lattice mismatch between InAs0.9Sb0.1 and GaAs, we employ a two-step growth method in which low temperature (430 °C) InAs0.9Sb0.1 buffer layers with different thicknesses are introduced into the structure. Effect of the buffer layer thickness and the epilayer's growth temperature on crystalline quality of the epilayer is investigated, respectively. It is clear that there are strip pyramids paralleling with each other on most surface of the samples. The crystalline quality gets well obviously when the buffer layer thickness change from 0 to 50 nm, but it gets worse when the buffer layer thickness increases to 100 nm. It is also shown that the crystalline quality of the epilayer is improved obviously when the epilayer is grown at 500 °C, and it gets worse when the growth temperature decreases or increases.

Raman and transmission electron microscopy characterization of InN samples grown on GaN/Al2O3 by molecular beam epitaxy

Raman spectroscopy and transmission electron microscopy were employed to study the vibrational properties and the microstructure of epitaxially grown InN films on GaN/Al 2 O 3 templates. The variations of the InN lattice constants, as deduced by electron diffraction analysis, along with the red-shifted E 2 2 mode frequency reveal that InN films exhibit residual tensile stress, strongly dependent on the epilayer growth temperature. Threading dislocations are the dominant structural defects in the films, having a density in the order of 10 9 -10 10 cm -2 . Profile analysis of the E 2 2 Raman peak by means of the Spatial Correlation Model provides useful information concerning the effective mean length for free phonon propagation (L), which is a measure of the structural quality of the samples. In all the studied samples, L monotonically increases with decreasing threading dislocation density of pure screw and mixed type character.

GaN thin films by growth on Ga-rich GaN buffer layers

The influence of the composition of the low-temperature GaN buffer layer on the structural, electrical, and optical properties of the subsequently grown GaN epilayer by molecular-beam epitaxy is investigated. It is found that decreasing the N/Ga flux ratio during the buffer layer growth yields excess Ga in the buffer layer. GaN epilayers grown on the buffer layer with excess Ga showed increased electron Hall mobility and reduced threading dislocation density. The tilt and twist disorders of the epilayer were also significantly reduced. The compressive stress in the epilayers at room temperature increased as the N/Ga flux ratio during the buffer layer growth was reduced. The improved properties of the epilayers are explained by the hypothesis that excess Ga in the buffer layer facilitates stress relaxation at the epilayer growth temperature.

Improved Heteroepitaxial MBE GaN Growth with a Ga Metal Buffer Layer

ABSTRACTWe demonstrate that the use of pure gallium (Ga) as a buffer layer results in improved crystal quality of GaN epilayers grown by plasma-assisted molecular beam epitaxy on c-plane sapphire. The resulting epilayers show electron Hall mobilities as high as 400 cm2/Vs at a background carrier concentration of 4 × 1017 cm−3, an outstanding value for an MBE-grown GaN layer on sapphire. Structural properties are also improved; the asymmetric (101) X-ray rocking curve width is drastically reduced with respect to that of the reference GaN epilayer grown on a low-temperature GaN buffer layer. Nitrided Ga metal layers were investigated for different Ga deposition time. These layers can be regarded as templates for the subsequent Ga main layer growth. It was found that there is an optimum Ga metal layer deposition time for improving the electron mobility in the epilayer. Heating of the Ga metal layer to the epilayer growth temperature under nitrogen plasma is found to be sufficient to produce highly oriented GaN crystals. However, nonuniform surface morphology and incomplete surface coverage were observed after nitridation of comparatively thick Ga metal layers. This is shown to be the reason for the decreasing electron mobility of the epilayers as the Ga metal layer thickness exceeds the optimum value.

Substrate photoluminescence: An aid in the evaluation of pulsed laser evaporation and epitaxial growth of Cd1−xMnxTe epilayers on (111)GaAs

Photoluminescence (PL) emitted from (111)Cd1−xMnxTe/(111)GaAs:Si heterojunctions produced using pulsed laser evaporation and epitaxy (PLEE) has been studied at 11.5 K. The heterojunctions show PL from the substrate E0 and E0+Δ0 gap regions as well as from the epilayer. The substrate E0+Δ0 signal is particularly sensitive to the epilayer growth temperature and is strongest for samples grown on substrates held at Tg=290 °C. The heterojunction PL also includes a component produced by Si dopant atoms in the substrate that undergo clustering changes as the growth temperature is raised. Through comparison with annealing studies of the substrate, it is argued that the PLEE ablated material possesses sufficient kinetic energy to increase the effective temperature above Tg at the growth surface.

Growth temperature and substrate orientation dependences of moving emission and ordering in Ga0.52 In0.48 P

The excitation intensity and temperature dependences of the photoluminescence (PL) from Ga0.52 In0.48 P grown by organometallic vapor phase epitaxy on GaAs substrates have been investigated as a function of the epilayer growth temperature and the substrate misorientation from (100). The energy band gap is dependent on substrate temperature during growth but is effectively independent of substrate misorientation. A strong excitation intensity dependence of the PL is found to be a function of both substrate orientation and growth temperature. We postulate that the strong dependence of PL emission energy on excitation intensity in some samples may result from the sizes, shapes, and orientations of ordered domains.