c-GaN Layers Research Articles

Growth temperature dependence of cubic GaN step structures and cubic InN dot arrays grown on MgO (001) vicinal substrates

In this paper, we report on the detailed structural features of step-bunched surface formed on cubic (c-) GaN and self-assembled c-InN dot arrays grown on the c-GaN surface, particularly focusing on the growth temperature dependence. Samples were fabricated on MgO (001) vicinal substrates with off-cut angles of 2.0° and 3.5° toward [110] by RF-MBE. Multisteps and terraces with a certain periodicity in the vicinal direction were observed on c-GaN layers. The average terrace width narrowed with lowering the growth temperature and increasing the substrate off-cut angle. The formation of c-InN dots proceeded in the Stranski–Krastanov growth mode with critical thickness of 0.54–0.66 nm. The c-InN dot exhibited a variety of structural features depending on the growth conditions. Positional dot alignment along the multistep edges of the underlayer was observed under some conditions. The degree of alignment was found to be affected by the terrace width on the c-GaN underlayer.

Joint Raman spectroscopy and HRXRD investigation of cubic gallium nitride layers grown on 3C-SiC

Cubic gallium nitride (GaN) films are analyzed with highresolution X-ray diffraction (HRXRD) and Raman spectroscopy. Several cubic GaN layers were grown on 3C-SiC (001) substrate by radio-frequency plasma-assisted molecular beam epitaxy. The layer thickness of the cubic GaN was varied between 75 and 505 nm. The HRXRD analysis reveals a reduction of the full-width at half-maximum (FWHM) of omega scans for growing layer thicknesses, which is caused by a partial compensation of defects. The Raman characterization confirms well-formed c-GaN layers. A more detailed examination of the longitudinal optical mode hints at a correlation of the FWHM of the Raman mode with the dislocation density, which shows the possibility to determine dislocation densities by Ramanspectroscopy on a micrometer scale, which is not possible by HRXRD. Furthermore, this Raman analysis shows that normalized Raman spectra present an alternative way to determine layer thicknesses of thin GaN films.

Low fraction of hexagonal inclusions in thick and bulk cubic GaN layers

We report present a study of the formation of hexagonal inclusions in thick (∼5μm) and bulk (∼50μm) cubic GaN (c-GaN) layers grown on GaAs (001) substrates. Surface analysis of both samples revealed the evidence of hexagonal grains on the surface. However, larger grains were formed in the bulk sample. In the XRD measurement, the signature of the h-GaN in the bulk sample was more visible than the thick sample. This is consistent with the PL measurement, of which the h-GaN peak emission was found to be more intense in the bulk sample with respect to the thick sample. Such observations showed that the fraction of hexagonal inclusions had increased by increasing the thickness of the c-GaN layer. As presented in the Raman spectroscopy, the signals of the h-GaN were dominant, but the transverse optical (TO) and longitudinal optical (LO) modes of c-GaN were still observable at ∼559cm−1 and ∼739cm−1, respectively. Finally, a micro-PL measurement showed that the average content of the hexagonal inclusions in the c-GaN layer up to ∼50μm was less than 22%, suggesting that our samples have a lower hexagonal GaN content than some reported thin c-GaN samples.

Hexagonal (wurtzite) GaN inclusions as a defect in cubic (zinc-blende) GaN

The dependence of the hexagonal fraction with thickness in MBE-grown bulk cubic (c-) GaN epilayer is presented in this paper. A number of c-GaN epilayers with different thicknesses were characterized via PL and XRD measurements. From the PL spectra, the signal due to h-GaN inclusions increases as the thickness of the c-GaN increases. On the contrary, in the XRD diffractogram, c-GaN shows a dominant signal at all thicknesses, and only a weak peak at ∼35° is observed in the diffractogram, implying the existence of a small amount of h-GaN in the c-GaN layer. The best quality of c-GaN is observed in the first 10μm of GaN on the top of GaAs substrate. Even though the hexagonal content increases with the thickness, the average content remains below 20% in c-GaN layers up to 50μm thick. The surface morphology of thick c-GaN is also presented.

In situ growth regime characterization of cubic GaN using reflection high energy electron diffraction

Cubic GaN layers were grown by plasma-assisted molecular beam epitaxy on 3C-SiC (001) substrates. In situ reflection high energy electron diffraction was used to quantitatively determine the Ga coverage of the GaN surface during growth. Using the intensity of the electron beam as a probe, optimum growth conditions of c-GaN were found when a 1 ML Ga coverage is formed at the surface. 1μm thick c-GaN layers had a minimum surface roughness of 2.5nm when a Ga coverage of 1 ML was established during growth. These samples revealed also a minimum full width at half maximum of the (002) rocking curve.

Molecular Beam Epitaxy of Cubic Group III-Nitrides on Free-Standing 3C-SiC Substrates

Cubic GaN, AlxGa1-xN/GaN and InyGa1-yN/GaN multiple quantum well (MQW) layers were grown by plasma assisted molecular beam epitaxy on 200 &m thick free standing 3C-SiC substrates. The influence of the surface roughness of the 3C-SiC substrates and the influence of metal coverage during growth are discussed. Optimum growth conditions of c-III nitrides exist, when a one monolayer Ga coverage is formed at the growing surface. The improvement of the structural properties of cubic III-nitride layers and multilayers grown on 3C-SiC substrates is demonstrated by 1 μm thick c-GaN layers with a minimum x-ray rocking curve width of 16 arcmin, and by c-AlGaN/GaN and c-InGaN/GaN MQWs which showed up to five satellite peaks in X-ray diffraction, respectively.

Effect of growth temperature on polytype transition of GaN from zincblende to wurtzite

We have investigated effect of growth temperature on the polytype conversion of cubic GaN (c-GaN) grown on GaAs (001) substrates by MOVPE. It was found that the polytype transition of GaN from zincblende (cubic) to wurtzite (hexagonal) structures is much dependent on the growth temperature. Transmission electron microscopy (TEM) observations demonstrate that the GaN grown layers have the cubic structure (c-GaN) and contain bands of stacking faults (SFs) parallels to {111} planes. For low growth temperatures (∼ 900 °C), XRD results demonstrate that the GaN grown layers with the cubic phase purity higher than 85% were obtained. No different types of single diffraction spots, indicating the incorporation of single-crystal h-GaN, on the selected area diffraction (SAD) pattern was observed. It is also found that a density of SFs decreases with the distance from the interface of c-GaN/GaAs. On the other hand, GaN layers exhibited a transition from cubic to mixed cubic/hexagonal phase under conditions of increasing growth temperature (∼ 960 °C) as determined using TEM-SAD technique with complementary XRD and PL observations. In addition, the optical characteristics of c-GaN layers are shown to be very sensitive to the presence of the single-crystal h-GaN.

Ni Schottky diodes on cubic GaN

Schottky diodes were fabricated by thermal evaporation of nickel on phase-pure cubic GaN(c-GaN) layers grown by plasma assisted molecular beam epitaxy on freestanding 3C-SiC. Detailed analysis of the I-V characteristics revealed the existence of a thin surface barrier at the Ni-semiconductor interface. Thermal annealing in air at 200°C alters the composition of this thin surface barrier, reduces the leakage current by three orders of magnitude, and increases the breakdown voltage. The dependence of the breakdown voltage on the doping density of the c-GaN layers is in good agreement with calculated values. We obtain a critical electric-field strength of Ecrit≅2.5×106V∕cm for c-GaN.

Photoluminescent and structural properties of GaN thin films obtained by radical-beam gettering epitaxy on porous GaAs (0 0 1)

Cubic GaN (c-GaN) layers were grown on porous GaAs (001) substrate by radical-beam gettring epitaxy. X-ray difraction and photoluminescence measurement indicate that the both single crystalline cubic GaN and hexagonal GaN was successfully grown on the porous GaAs (001) substrate.

Reduction of Planar Defect Density in Laterally Overgrown Cubic-GaN on Patterned GaAs(001) Substrates by MOVPE

The metalorganic vapor phase epitaxy (MOVPE) growth of cubic-GaN (c-GaN) layers has been performed on the [110]-stripe patterned GaAs (001) substrates. The surface morphology of the laterally overgrown layer was much improved with the formation of the flat (311)A surfaces as the growth proceeded. It is shown that the c-GaN layer thicker than 20 μm was grown on GaAs(001) substrates with the stripe direction in [110] for 90 min. Microstructure and extended defect distribution in the laterally overgrown c-GaN films have been analysed by transmission electron microscopy (TEM). It is found that the planar defect (stacking faults and twins) density drastically decreases at the region away from the substrate toward the top of the c-GaN stripe. On the other hand, the dislocations become dominant. The density of the dislocations was found to be lower than 10 8 cm -2 . These results suggest that the lateral overgrowth on [110]-stripe-patterned GaAs (001) substrates via MOVPE is an efficient method for obtaining a low planar defect density c-GaN layer.

Suppression of Hexagonal GaN in Cubic GaN Growth by In Surfactant Effect Combined with the Precise Control of V/III Ratio

An interesting correlation between insertion ratio of hexagonal (h-) GaN phase and surface flatness in the cubic (c-) GaN rf-MBE growth on GaAs substrate has been discussed. Combined with the precise control of V/III ratio, In beam irradiation is a very effective method to improve the surface flatness and to suppress the h-GaN insertion on the {111}B planes during growth. This may be due to the In surfactant effect, and we have successfully controlled the insertion ratio through the improvement of the surface flatness. The experimental results strongly indicate that it is possible to grow high quality c-GaN layers by In beam irradiation combined with precise control of V/III ratio.

Suppression of Hexagonal GaN in Cubic GaN Growth by In Surfactant Effect Combined with the Precise Control of V/III Ratio

An interesting correlation between insertion ratio of hexagonal (h-) GaN phase and surface flatness in the cubic (c-) GaN rf-MBE growth on GaAs substrate has been discussed. Combined with the precise control of V/III ratio, In beam irradiation is a very effective method to improve the surface flatness and to suppress the h-GaN insertion on the {111}B planes during growth. This may be due to the In surfactant effect, and we have successfully controlled the insertion ratio through the improvement of the surface flatness. The experimental results strongly indicate that it is possible to grow high quality c-GaN layers by In beam irradiation combined with precise control of V/III ratio.

Optical Properties of Cubic Gallium Nitride on SiC/Si Pseudo-Substrates

The cubic phase of III-nitrides has received much less attention than the hexagonal structure, because of difficulties inherent to the growth of this meta-stable phase. However, the possibility to avoid the effects of piezoelectric fields in cubic phase GaN grown on non-polar substrates gives new interest for this material. In this work, we present optical properties (photoluminescence and photoreflectance) of cubic GaN layers grown by MBE on c-SiC/Si pseudo-substrates at room and low temperature. Photoluminescence properties of doped (n- and p-type) c-GaN layers and of AlGaN layers are also presented.

Mixing Mechanism of h-GaN in c-GaN Growth on GaAs (001) Substrates

Systematic investigation of the V/III ratio dependence in the mixing of the h-GaN phase to the c-GaN layers grown on GaAs (001) substrates has been performed and a drastical change of the mixing nature in the initial process is reported. The h-GaN mixing nature on the {111} facets during growth strongly depends on the V/III ratio in the initial buffer layer formation. The h-GaN grain-mixing only occurs on {111}A facets under a Ga-rich formation condition, but the h-GaN mixing grains on the {111}B facets gradually appear together with the vanishing of h-GaN grains on the {111}A facets during the annealing process under rf-nitrogen beam irradiation. The mixing processes under N-rich and Ga-rich growth conditions are also discussed. The results indicate that it is possible to grow high quality c-GaN layers by control of the mixing at the initial growth stage through the effective V/III ratio dependence.

Mixing Mechanism of h-GaN in c-GaN Growth on GaAs (001) Substrates

Systematic investigation of the V/III ratio dependence in the mixing of the h-GaN phase to the c-GaN layers grown on GaAs (001) substrates has been performed and a drastical change of the mixing nature in the initial process is reported. The h-GaN mixing nature on the {111} facets during growth strongly depends on the V/III ratio in the initial buffer layer formation. The h-GaN grain-mixing only occurs on {111}A facets under a Ga-rich formation condition, but the h-GaN mixing grains on the {111}B facets gradually appear together with the vanishing of h-GaN grains on the {111}A facets during the annealing process under rf-nitrogen beam irradiation. The mixing processes under N-rich and Ga-rich growth conditions are also discussed. The results indicate that it is possible to grow high quality c-GaN layers by control of the mixing at the initial growth stage through the effective V/III ratio dependence.

MOVPE Growth of High Quality Cubic GaN on GaAs: The Role of Growth Rates

Cubic GaN (c-GaN) layers are grown on GaAs(001) substrates by metalorganic vapor phase epitaxy (MOVPE). To attain high quality c-GaN, we investigate MOVPE growth conditions, particularly focusing on growth rates. As a result, it is found that by increasing the growth rate from 0.35 to 1.05 μm/h, the temperature region where the c-GaN composition reached its maximum (90%) is shifted from (700 to 800) °C towards (850 to 900) °C. This phenomenon is interpreted in terms of the lateral growth rate. It is also revealed that c-GaN with superior optical and electrical properties, that is, excitonic emission and high resistivity, cannot be realized without increasing the growth rate.

Optical gain and stimulated emission of cleaved cubic gallium nitrite

In this letter, we report on the observation of optically excited stimulated emission of c-GaN layers grown by molecular-beam epitaxy (MBE). Stimulated emission was observed at 1.8 K and room temperature. The threshold intensity for excitation of stimulated emission from our MBE-grown c-GaN layers is significantly lower than that reported for c-GaN grown by metalorganic chemical vapor deposition (MOCVD). The experimental data of optical gain and stimulated emission presented in this letter demonstrate that this material has a good potential for the future realization of cleaved cavity blue light-emitting laser diodes.

Relation between GaAs surface morphology and incorporation of hexagonal GaN into cubic GaN

Cubic GaN (c-GaN) layers were grown on GaAs (0 0 1) substrates by metalorganic vapor-phase epitaxy, using dimethylhydrazine as a nitrogen source. For the investigated growth temperature region of 600–820°C, only h-GaN whose c-axis is oriented in the [1 1 1] A or [ 1 ̄ 1 ̄ 1] A direction was incorporated into c-GaN. This orientation anisotropy of h-GaN caused the anisotropy of the width of c-GaN (0 0 2) X-ray diffraction (XRD) peaks. By flattening the GaAs surface to the atomic level, the hexagonal composition was effectively reduced and the anisotropy in the width of XRD was eliminated. The observed result was interpreted in terms of molecular steps appearing on the GaAs surface.

High-Resolution X-Ray Diffraction Analysis of “Devicequality” Cubic GaN Grown on (001)GaAs Substrate Prepared by Atomic-Hydrogen Treatment At “High Temperatures”

AbstractCubic GaN (c-GaN) layers were grown by if-plasma source MBE on (001) GaAs prepared by atomic-hydrogen treatment at “high temperatures”, and the structural properties of the epilayers were investigated by the high-resolution X-ray rocking curve and the reciprocal space mapping measurements. The growth temperature was varied from 620 to 740°C. It was found that single domain “device-quality” c-GaN layers could be grown for the first time; the FWHM of the X-ray rocking curves for the (002) c-GaN could be as small as 70 - 90 arcsec and the inclusion of h-GaN phase in the c-GaN epilayers grown at temperatures above 680°C could be less than 4×10-3.

Optical transitions in cubic GaN investigated by spatially resolved cathodoluminescence

Spatially resolved cathodoluminescence (CL) spectroscopy in connection with scanning electron microscopy performed on cubic (c) GaN between 5 and 300 K reveals that at low temperatures the CL spectra of c-GaN single crystals consist of four well-separated lines. The two lines highest in energy were previously identified as excitonic and donor-acceptor transitions, respectively. Here, we show that the lines lowest in energy are due to an additional free-to-bound transition, involving an impurity different from those related to the donor-acceptor transition, and its phonon replica. The CL spectra of c-GaN layers, while being rather broad, are composed of these four lines. Moreover, at 300 K the spectra of the layers and of the crystals are both dominated by the excitonic transition and closely resemble each other.