Phase GaN Research Articles

Arsenic incorporation and growth mode of GaNAs grown by low-pressure metal-organic chemical vapor deposition

GaNAs epilayers were grown on GaN/sapphire substrates by low-pressure metal-organic chemical vapor deposition. Tertiarybutylarsine (TBAs) was used as the As source. The As contents in the GaNAs epilayers was measured by secondary ion mass spectrometry, and the surface morphology was studied by atomic force microscopy. The As contents in GaNAs increased drastically with decreasing growth temperature, and became saturated at 5×10 20 cm −3 at 565°C. This As incorporation behavior is attributed to the temperature dependence of NH 3 decomposition and the weaker Ga–As bond. When GaNAs was grown above 565°C, the GaNAs surface was granular with typical granule size of several ten nm. Size and density of the granules depend on the TBAs flow rate. At 530°C, not only a GaN phase but also a GaAs phase was observed by X-ray diffraction with very large islands on a flat surface. It is believed that excessive As incorporation results in the formation of large islands.

High-quality GaN nanowires synthesized using a CVD approach

High-quality GaN nanowires were synthesized on a large-area Si substrate by direct reaction of gallium with ammonia using InCl3 as a catalyst. The morphology and microstructure of the resulting products were characterized using a field-emission scanning electron microscope (SEM), a high-resolution transmission electron microscope, and X-ray diffraction (XRD). XRD and electron diffraction revealed that the nanowires are of a hexagonal GaN phase with the wurtzite structure. The SEM study showed that the nanowires are straight and have a smooth morphology with lengths up to 500 μm. The present results reveal that InCl3 is an optimal catalyst in GaN nanowire production.

Gas-phase nanoparticle formation during AlGaN metalorganic vapor phase epitaxy

Using in situ laser light scattering, we have directly observed the formation of gas-phase nanoparticles during AlN, GaN, and AlGaN metalorganic vapor phase epitaxy. The nanoparticles are sharply distributed in height 6 mm from the surface, in good agreement with a model based on a balance between thermophoretic and convective forces. By measuring the angular dependence of the scattering intensity, we determined that the AlN particle sizes were 35–50 nm, and particle densities were 1–6×108 cm−3, which corresponds to 20%–80% of the input Al being converted into nanoparticles.

Structural and Optical Properties of GaN Fine Particles Prepared from GaOOH

In this work, we give the first report on the synthesis of GaN fine particles from gallium oxide hydroxide (GaOOH) powders and on their structural and photoluminescence (PL) properties. Simple heat treatment of GaOOH in NH3 gas flow leads to the formation of submicron-sized hexagonal GaN powders, even at the low reaction temperature of 800°C, through intermediate conversion to the metastable gallium oxide, α-Ga2O3. X-ray diffraction measurements show that the powders obtained are of pure single phase GaN without any impurity phases or structural defects. However, energy dispersive spectroscopy, Fourier transform infrared, and PL measurements indicate oxygen-related characteristics. The results obtained demonstrate that submicron-sized GaN powders can be synthesized from GaOOH powders. However, there is a need to minimize the oxygen incorporation during the reaction process.

GaN thin film growth on GaAs (0 0 1) by CBE and plasma-assisted MBE

GaN thin films were deposited on GaAs (0 0 1) substrates using chemical beam epitaxy (CBE) and plasma-assisted molecular beam epitaxy (MBE) methods. The effect of in situ substrate cleaning and pre-treatment on the interfacial structure was investigated. Time-of-flight mass spectroscopy of recoiled ions was utilized to measure in situ the surface composition. The microstructure of GaN films was studied with selected area electron diffraction, and both conventional and high-resolution transmission electron microscopy. It was found that simultaneous electron cyclotron resonance (ECR) plasma treatment using an Ar/N 2 mixture increases the yield of a cubic phase in GaN films during CBE growth. Resulting films are less defective than mixed phase GaN films grown after pure N 2 plasma nitridation. In the case of plasma-assisted MBE growth, a low-temperature annealing followed by N 2 ECR nitridation process yields cubic GaN thin films while N 2 ECR nitridation without a sample annealing to 600°C yields hexagonal phase GaN films.

Growth kinetics of AlGaN films by plasma-assisted molecular-beam epitaxy

In this letter, we report on the kinetics of growth of aluminum gallium nitride films by rf plasma-assisted molecular-beam epitaxy on (0001) sapphire substrates at the temperature range of 750–800 °C. The determination of the incorporation probabilities of aluminum and gallium atoms was arrived at by measurements of the growth rate and composition of the films. We find that, in both the nitrogen-rich and group-III-rich growth regimes, the incorporation probability of aluminum is unity for the entire investigated temperature range. On the other hand, the incorporation probability of gallium is constant and equals 0.75 at a substrate temperature of 750 °C only in the nitrogen-rich growth regime. The temperature dependence of the incorporation probability in this regime has an activation energy of 2.88 eV which is consistent with gallium desorption from the surface. In the group-III-rich growth regime, the incorporation probability of gallium decreases monotonically with group-III fluxes due to the competition with aluminum for the available active nitrogen. In this regime, the GaN phase growth rate is determined by the capture probability of the available active nitrogen from the gallium surface adlayer.

Depth-dependent investigation of defects and impurity doping in GaN/sapphire using scanning electron microscopy and cathodoluminescence spectroscopy

Cathodoluminescence (CL) imaging and temperature-dependent cathodoluminescence spectroscopy (CLS) have been used to probe the spatial distribution and energies of electronic defects near GaN/Al2O3 interfaces grown by hydride vapor phase epitaxy (HVPE). Cross sectional secondary electron microscopy imaging, CLS, and CL imaging show systematic variations in defect emissions with a wide range of HVPE GaN/sapphire electronic properties. These data, along with electrochemical capacitance–voltage profiling and secondary ion mass spectrometry provide a consistent picture of near-interface doping by O out-diffusion from Al2O3 into GaN over hundreds of nanometers. Low-temperature CL spectra exhibit a new donor level at 3.447 meV near the interface for such samples, characteristic of O impurities spatially localized to the nanoscale interface. CLS emissions indicate the formation of amorphous Al–N–O complexes at 3.8 eV extending into the Al2O3 near the GaN/sapphire interface. CLS and CL images also reveal emissions due to excitons bound to stacking faults and cubic phase GaN. The temperature dependence of the various optical transitions in the 10–300 K range provides additional information to identify the near interface defects and impurity doping.

Effect of anisotropic current of the order parameter onInxGa1−xNcompositional fluctuation caused by phase separation

We studied details of the anisotropic spatial fluctuation in In compositions numerically as described by an early stage in the time evolution of InGaN phase separation using the cell dynamical system (CDS) approach. We extended the previous isotropic CDS approach to the anisotropic case of current of the order parameter corresponding to the two-dimensional anisotropic diffusion of the constituents. The results of this numerical-based work indicate that the anisotropic current of the order parameter, which consists of a reduction of the current in one direction and a nonreduction in the opposite direction, suppresses the In compositional fluctuation. This suppression arises from the unidirectional current of the order parameter, which relates to the enhancement of anisotropic diffusion of the constituents. Thus we substantiate the homogeneous growth of the InGaN film by using the surface of an off-axis substrate consisting of uniformly spaced steps with high density.

Synthesis and characterization of GaN using gas–solid reactions

Gallium nitride powders were synthesized in a new vertical reactor to maximize contact between gas and solid phases. In the experiments, GaN powders were obtained by two different routes: (1) gas–solid reaction from Ga 2O 3/NH 3 system; (2) gas–solid reaction from Ga 2O 3/3C/NH 3 reaction system. The last system presents a new route to GaN from gas–solid reaction. The powders produced were collected and characterized using X-ray diffraction (XRD). Transmission electron microscopy (TEM) was used to characterize GaN produced from Ga 2O 3/NH 3 system. XRD taken from the powders have shown different results. For the powder produced from Ga 2O 3/NH 3 system, XRD pattern shows presence of GaN and unreacted Ga 2O 3. On the other hand, powder produced from Ga 2O 3/3C/NH 3 system, XRD pattern shows presence of primarily GaN phase. Results show high conversion to GaN, when the solid phase reactant had been a Ga 2O 3 and carbon source mixtures.

Preparation of Phase Pure Cubic Gallium Nitride, c-GaN, by Ammonothermal Conversion of Gallium Imide, {Ga(NH)3/2}n

Gallium imide, {Ga(NH)3/2} n , was sealed in quartz tubes with anhydrous NH3 and NH4X (X=Cl, Br, I) and heated vertically in an autoclave with the hot-zone (bottom) temperature ranging from 300–530°C. With NH4I mineralizer, the imide was converted to primarily the zinc-blende phase of GaN (c-GaN), usually in the form of micron sized or smaller tetrahedra. With NH4Cl mineralizer, hexagonal GaN nanoparticles formed instead, and NH4Br facilitated the conversion of imide to irregular h-GaN nanoparticles at lower temperatures, and larger particles above 500°C. At the higher temperatures, chemical transport took place and GaN crystals grew on the walls at the middle to the top of the tube. Most of the deposited GaN was cubic, usually in a heavily etched triangular prismatic morphology. Several experiments were performed in a Pt lined pressure vessel in order to demonstrate that the synthesis method can be scaled up.

Theoretical Study of RF Breakdown in GaN Wurtzite and Zincblende Phase MESFETs

In this paper, we present a comparison of the RF breakdown behaviors of representative wurtzite and zincblende phase GaN MESFET structures based on a theoretical analysis. The calculations are made using a full band ensemble, Monte Carlo simulation that includes a numerical formulation of the impact ionization transition rate. Calculations of the RF breakdown voltages are presented for submicron MESFET devices made from either wurtzite or zincblende phase GaN. The devices are otherwise identical. It is found that the RF-breakdown voltage of the devices increases with increasing frequency of the applied large signal RF excitation.

Strong Photoluminescence Emission from Polycrystalline GaN Grown on Metal Substrate by NH3 Source MBE

Polycrystalline GaN layers were grown on Mo and electron-beam-deposited Mo/glass substrates with improved surface smoothness by NH 3 source molecular beam epitaxy. X-ray diffraction rocking curves showed preferential GaN(0002) orientation. Strong photoluminescence (PL) emission without yellow luminescence was observed from these polycrystalline GaN layers. For the previously-grown GaN on metal substrates two PL peaks were observed at around 3.48 and 3.27 eV at 77 K, while for the GaN grown in this study the 3.27 eV peak was eliminated. This improvement is considered to be mainly due to the improved surface morphology of the substrates and the origin of the 3.27 eV peak was attributed to the cubic GaN phase. The 3.48 eV peak is the excitonic peak from the hexagonal GaN phase.

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.

Theoretical study of direct-current andradio-frequency breakdown in GaN wurtzite- and zinc-blende-phaseMESFETs (metal-semiconductor field-effect transistors)

In this paper, we present a comparison of the direct-current (DC)and radio-frequency (RF) breakdown behaviours of representativewurtzite- and zinc-blende-phase GaN MESFET structures based on atheoretical analysis. The calculations are made using afull-band ensemble Monte Carlo simulation that includes a numericalformulation of the impact ionization transition rate. Calculationsof both the DC and RF breakdown voltages are presented for submicronMESFET devices made from either wurtzite- or zinc-blende-phase GaN.The devices are otherwise identical. It is found that the DCbreakdown voltage in the wurtzite-phase GaN MESFET is significantlylarger than that in the zinc-blende-phase device. This is due tothe fact that electron heating occurs more rapidly within thezinc-blende phase than the wurtzite phase of GaN. As a consequence,avalanche breakdown occurs at higher applied field strengths andvoltages in the wurtzite phase than in the zinc-blende phase of GaN.It is further found that the RF breakdown voltage of the devicesincreases with increasing frequency of the applied large-signal RFexcitation.

Electronic Defect States Observed by Cathodoluminescence Spectroscopy at GaN/Sapphire Interfaces

Cathodoluminescence (CL) imaging and temperature-dependent cathodoluminescence spectroscopy (CLS) have been used to probe the spatial distribution and energies of electronic defects near GaN/Al2O3 interfaces grown by hydride vapor phase epitaxy (HVPE). Cross sectional secondary electron microscopy imaging, CLS and CL imaging show systematic variations in defect emissions with a wide range of HVPE GaN/sapphire electronic properties. Highly degenerate interface regions give rise to above bandgap emissions due to band filling and free electron recombination. Besides the common donor and acceptor bound exciton, CLS and CL images also reveal emissions due to excitons bound to stacking faults and cubic phase GaN.

Electronic Defect States Observed by Cathodoluminescence Spectroscopy at GaN/Sapphire Interfaces

Cathodoluminescence (CL) imaging and temperature-dependent cathodoluminescence spectroscopy (CLS) have been used to probe the spatial distribution and energies of electronic defects near GaN/Al 2 O 3 interfaces grown by hydride vapor phase epitaxy (HVPE). Cross sectional secondary electron microscopy imaging, CLS and CL imaging show systematic variations in defect emissions with a wide range of HVPE GaN/sapphire electronic properties. Highly degenerate interface regions give rise to above bandgap emissions due to band filling and free electron recombination. Besides the common donor and acceptor bound exciton, CLS and CL images also reveal emissions due to excitons bound to stacking faults and cubic phase GaN.

Polar optical phonon scattering and negative Krömer–Esaki–Tsu differential conductivity in bulk GaN

GaN is being considered as a viable alternative semiconductor for high-power solid-state electronics. This creates a demand for the characterization of the main scattering channel at high electric fields. The dominant scattering mechanism for carriers reaching high energies under the influence of very high electric fields is the polar optical phonon (POP) emission. To highlight the directional variations, we compute POP emission rates along high-symmetry directions for the zinc-blende and wurtzite crystal phases of GaN. Our treatment relies on the empirical pseudopotential energies and wave functions. The scattering rates are efficiently computed using the Lehmann–Taut Brillouin zone integration technique. For both crystal phases, we also consider the negative differential conductivity possibilities associated with the negative effective mass part of the band structure.

Emission studies of InGaN layers and LEDs grown by plasma-assisted MBE

We prepared InGaN layers on GaN/sapphire substrates using rf-MBE. Photoluminescence (PL) from these layers, grown at different temperatures T S, shows that there is a strong tendency of GaN to form a separate phase as T S is increased from 600°C to 650°C. Concomitant with the phase separation, the PL from the InGaN phase broadens, which indicates that indium composition in this phase becomes increasingly non-uniform. Indium compositions measured by Rutherford backscattering (RBS) are consistent with these results. We also observed an increase in PL intensity for InGaN layers grown at higher temperatures. In this paper, we also report on preparing a top-contact InGaN/GaN light emitting diode. The device was operated at 447 nm and had the emission line width of 37 nm with no observable impurity related features. The turn-on voltage was 3.0 V. The output power was 20 μW at 60 mA drive current.

Experimental investigation of inclusion of hexagonal GaN phase-domain by varying nitrogen-beam direction to a 〈1 1 1〉 axis in MBE growth of cubic GaN

Crystalline quality of cubic phase (c-) GaN epilayers on (0 0 1) GaAs has been systematically investigated by varying nitrogen (N-) beam direction in molecular beam epitaxy. In this study, the N-beam was obliquely incident to the surface normal. The N-beam direction during both buffer layer and epilayer depositions could approximate either c-GaN[1 1 1]A or c-GaN [ 1 ̄ 1 1] B. In the c-GaN epilayers grown with different N-beam directions, the c-GaN domain or hexagonal (h-) GaN domain showed the tilt and the mosaicity. From the X-ray rocking curves, the extraordinary c-GaN domains and the h-GaN domains were tilted by influence of oblique incidence of the N-beam. The phase purity of c-GaN domain became high when the N-beam direction was along a c-GaN[1 1 1]A direction during both the buffer layer and the epilayer depositions. In order to understand the difference in crystalline quality with change in the N-beam direction, we took account of stack of the Ga and N precursors on (1 1 1)A (or ( 1 ̄ 1 1) B) c-GaN facet. It is considered that the difference of the crystalline quality result from the polarity of the facets.