Thick GaN Films Research Articles

Three-dimensional imaging of threading dislocations in GaN crystals using two-photon excitation photoluminescence

The three-dimensional imaging of threading dislocations in GaN films was demonstrated using two-photon excitation photoluminescence. The threading dislocations were shown as dark lines. The spatial resolutions near the surface were about 0.32 and 3.2 µm for the in-plane and depth directions, respectively. The threading dislocations with a density less than 108 cm−2 were resolved, although the aberration induced by the refractive index mismatch was observed. The decrease in threading dislocation density was clearly observed by increasing the GaN film thickness. This can be considered a novel method for characterizing threading dislocations in GaN films without any destructive preparations.

Defects induced by MeV H+ implantation for exfoliating of free-standing GaN film

High-energy ion slicing is promising to produce the free-standing GaN films with thickness in the range of 10–20 µm, which would promote the mass applications of GaN substrates. In this paper, bulk GaN was implanted by 1.6 MeV H ions with the mean projected range Rp of around 17 μm and the thermal evolution of the H-induced defects was investigated in detail. Due to the migration-coalescence mechanism, the H-induced point defects gather to form the initial cavity defects which grow up via the Ostwald ripening mechanism. The cavity defect distribution is determined by the distributions of the implanted hydrogen and the implantation-induced damages. The area ratio of cavity defects in the center damage band of the 1.6 MeV sample was around 3.4%. Annealing at higher temperature enhances the defect migration and recovery. Larger H ion fluence or higher annealing temperature is required to accomplish the exfoliation of a free-standing GaN thick film.

Anisotropic Fracture Toughness of Bulk GaN

The wurtzite structure of GaN determines its anisotropic mechanical properties, which is significant in the processes of material preparation and application. By using nanoindentation method, the fracture toughnesses along the c‐, a‐, and m‐planes of GaN were measured. Experimental results reveal that the fracture toughnesses along nonpolar m‐ and a‐planes are similar, 1.55 ± 0.13 MPa · m1/2 and 1.59 ± 0.12 MPa · m1/2, respectively, but obviously smaller than the value of c‐plane, about 2.12 ± 0.21 MPa · m1/2. This could be understood by the differences of GaN bond densities and energies among the corresponding planes. Using first‐principles calculations, the fracture toughness of GaN under different stress states was investigated within the framework of density functional theory. The anisotropic property that the fracture toughness for c‐plane GaN is higher than that of a‐ or m‐plane GaN for all cases of stress states, should be the fundamental reason for the great difficulty in complete separation of thick GaN films, which is the key process in manufacturing GaN substrates.

Nondestructive measurement of homoepitaxially grown GaN film thickness with Fourier transform infrared spectroscopy

In vertical devices containing GaN homoepitaxial layers on GaN substrates, the layer thickness is a key parameter that needs to be clarified before starting the device process. We applied Fourier transform infrared spectroscopy (FT-IR) to a homoepitaxially grown GaN film that consisted of an n−-GaN layer. The estimated film thickness from the FT-IR spectrum agreed well with the results of cross-sectional scanning electron microscope cathodoluminescence images. This is the first report of nondestructive film thickness measurements for homoepitaxially grown GaN and indicates the applicability of FT-IR to the nondestructive inspection of vertical GaN power devices.

Halide vapor phase epitaxy of thick GaN films on ScAlMgO4 substrates and their self-separation for fabricating freestanding wafers

Halide vapor phase epitaxy of thick GaN films was demonstrated on ScAlMgO4 (SCAM) substrates, and their self-separation was achieved. The 320-µm-thick GaN film was self-separated from the SCAM substrate during the cooling process after the growth. This separation phenomenon occurred because of both the c-plane cleavability of SCAM and the difference in the thermal-expansion coefficients between GaN and SCAM. The dark-spot densities for the GaN films on the SCAM substrates were approximately 30% lower than those on sapphire substrates. These results indicate that SCAM substrates are promising for fabricating a high-quality freestanding GaN wafer at a low cost.

Separation of thick HVPE-GaN films from GaN templates using nanoporous GaN layers

In this work, we have succeeded in growing an approximate 2-inch self-separated thick GaN wafer by hydride vapor phase epitaxy with an introduction of a sacrificial layer of nanoporous GaN. Such nanoporous GaN layer is invented by using the HVPE growth of thin GaN layer on the spin-coating silica nanosphere layer followed by a hydrofluoric acid etching to the silica nanosphere layer. It has been found that the nanoporous GaN layer, enabling a reduction of stickiness between thick GaN films and the substrates, plays a significant role in the self-separation of thick GaN films during the cooling process. However, the thickness of the nanoporous GaN layer is another key issue to achieve good quality self-separated GaN thick films. In our study, we suggest that the nanoporous GaN layer with a thickness of approximately 150–240 nm can best serve as the sacrificial layer in self-separation process. Raman spectroscopy also indicates the self-separated thick GaN films by using the proposed approach are virtually strain-free.

Optoelectronic Properties and Structural Characterization of GaN Thick Films on Different Substrates through Pulsed Laser Deposition

Approximately 4-μm-thick GaN epitaxial films were directly grown onto a GaN/sapphire template, sapphire, Si(111), and Si(100) substrates by high-temperature pulsed laser deposition (PLD). The influence of the substrate type on the crystalline quality, surface morphology, microstructure, and stress states was investigated by X-ray diffraction (XRD), photoluminescence (PL), atomic force microscopy (AFM), transmission electron microscopy (TEM), and Raman spectroscopy. Raman scattering spectral analysis showed a compressive film stress of −0.468 GPa for the GaN/sapphire template, whereas the GaN films on sapphire, Si(111), and Si(100) exhibited a tensile stress of 0.21, 0.177, and 0.081 GPa, respectively. Comparative analysis indicated the growth of very close to stress-free GaN on the Si(100) substrate due to the highly directional energetic precursor migration on the substrate’s surface and the release of stress in the nucleation of GaN films during growth by the high-temperature (1000 °C) operation of PLD. Moreover, TEM images revealed that no significant GaN meltback (Ga–Si) etching process was found in the GaN/Si sample surface. These results indicate that PLD has great potential for developing stress-free GaN templates on different substrates and using them for further application in optoelectronic devices.

Fe-doped semi-insulating GaN with solid Fe source grown on (110) Si substrates by NH3 molecular beam epitaxy

Iron doped GaN layers were grown on (110) Si substrates by ammonia molecular beam epitaxy (MBE) using solid elemental iron as a source. Specular films with concentrations up to 1×1020cm−3, as determined by secondary ion mass spectroscopy, were grown, unlike a limited incorporation of Fe into GaN by metal-rich rf plasma MBE. The Fe concentration in the film showed an exponential dependence on the inverse of source temperature with an activation energy of 3.4eV, which agrees well to the reported value for the sublimation of Fe. A 1.5µm thick GaN film with a sheet resistance of 1GΩ/sq. was obtained by compensating unintentional residual donors with a small Fe concentration of 1×1017cm−3. X-ray diffraction rocking curves indicated high crystalline quality, very similar to an undoped film, showing that the Fe incorporation required to obtain the semi-insulating film properties did not affect the structural properties of the film. The low-temperature PL spectra of highly resistive and semi-insulating Fe:GaN in the range of 1017~1018cm−3 show dominant exciton emissions and enhanced donor-acceptor-pair (DAP) emissions, implying that Fe ions contribute to the DAP transition between donor levels and Fe-related acceptor levels, possibly compensating the residual donors to achieve the semi-insulating electrical properties.

Numerical design of Metal‐Organic Vapour Phase Epitaxy process for gallium nitride epitaxial growth

The paper presents the results of numerical simulations and experimental measurements of the epitaxial growth of gallium nitride in Metal Organic Vapor Phase Epitaxy within a AIX‐200/4RF‐S reactor. The aim was to develop optimal process conditions for obtaining the most homogeneous crystal layer. Since there are many factors influencing the chemical reactions on the crystal growth area such as: temperature, pressure, gas composition or reactor geometry, it is difficult to design an optimal process. In this study various process pressures and hydrogen volumetric flow rates have been considered. Due to the fact that it is not economically viable to test every combination of possible process conditions experimentally, detailed 3D modeling has been used to get an overview of the influence of process parameters. Numerical simulations increased the understanding of the epitaxial process by calculating the heat and mass transfer distribution during the growth of gallium nitride. Appropriate chemical reactions were included in the numerical model which allowed for the calculation of the growth rate of the substrate. The results obtained have been applied to optimize homogeneity of GaN film thickness and its growth rate.

Incorporation of pervasive impurities on HVPE GaN growth directions

High crystallinity thick films with low free carrier concentration (≤1×1015/cm3) and low compensation are required for many GaN-based electronic device applications. It has been demonstrated that low pressure chemical vapor and molecular beam epitaxy techniques can reproducibility deposit homoepitaxial films with low residual impurity concentrations. However, their typical slow growth rates prevent their utilization for thick film growth. Presently, hydride vapor phase epitaxy is the sole method that can deposit films with residual impurity concentrations ≤5×1016/cm3 at hundreds of microns per hour growth rate. It is crucial to verify if this method can reproducibly deliver thick free-standing GaN films of high crystalline quality with exceptionally low and uniform free carrier concentration. X-ray diffraction, Raman scattering, and low temperature photoluminescence experiments were carried out on a number of samples prepared by dicing a free-standing wafer into several pieces perpendicular and parallel to the major growth directions; namely, c-plane {0001}, a-plane {11−20}, and m-plane {1−100}. SIMS depth profiles were employed to identify and quantify the concentration of the pervasive impurities. Spatial maps of a Raman line sensitive to free-carrier concentration were measured to determine the spatial distribution of the net impurity concentration. The reduced concentration of un-compensated shallow donors was also verified by low temperature electron paramagnetic resonance.

HVPE GaN wafers with improved crystalline and electrical properties

The quest for low cost GaN substrates with optimized crystalline and electrical properties continues to fuel the search for a fast growth method to produce commercial wafers that will allow the fabrication of devices capable of achieving high performance at high power and/or high frequency. Thick films grown by hydride vapor phase epitaxy (HVPE) on Ammono substrates in addition to reproducing the high crystalline quality of those substrates show significant reduction in free carrier concentration. This work presents a detailed spectroscopic, X-ray diffraction, and Raman spectroscopy imaging investigation of thick freestanding HVPE GaN films deposited on HVPE/Ammono-GaN templates. The results demonstrate that they are stress-free, and have a nearly uniform and relatively lower residual background doping, in addition to high crystalline quality. This result is extremely important, because it demonstrates the usefulness of this new type of HVPE-GaN substrate to fabricate highly efficient optoelectronic and electronic devices.

Improvements of epitaxial quality and stress state of GaN grown on SiC by in situ SiNx interlayer

In this study, 4.5 μm thick GaN films with graded AlxGa1−xN/AlN buffer and SiNx interlayer were prepared on 6H–SiC substrates by metal–organic chemical vapor deposition. To determine the effects of SiNx interlayer on epitaxial quality and stress state of GaN films, a series of comparative experiments were carried out by changing the deposition time and the insert location of SiNx interlayer. By optimizing growth conditions of SiNx interlayer, the full width at half maximum values of \( (0002) \) and \( (10\bar{1}2) \) rocking curves of GaN films were improved to 142 and 170 arcsec, respectively. A crack-free GaN film with a small root-mean-squared roughness of 0.21 ± 0.02 nm was achieved. Simultaneously, the reduction in threading dislocation density of GaN films was confirmed by using wet etching method. In addition, stress values in GaN films were investigated by Raman and low-temperature photoluminescence spectra, which indicated that the lower tensile stress in GaN film, the higher the film’s crystallinity.

Modeling the curvature and interface shear stress of GaN-sapphire system

The curvature and interface shear stress of GaN-sapphire system are studied by establishing the mechanical equations based on two main assumptions: (a) the thickness of GaN film can be compared to the thickness of sapphire substrate, and (b) the thickness of GaN film is non-uniform. Our results show that the curvature of GaN-sapphire system is a variable within the whole circular system. The interface shear stress changes direction around at the middle of radius for the circular system, and the curvature have an important effect on the interface shear stress due to the consideration of non-uniform thickness for GaN film.

Investigation of low-temperature cathodoluminescence mechanism of Er-doped GaN thick films by ion implantation

Er ions are implanted into the GaN thick films grown by hydride vapor phase epitaxy. The implantation energy is 200 keV and the implantation doses are 1×1013, 1×1014, 1×1015, and 5×1015 atom/cm2, respectively. The effects of the implantation dose and annealing temperature on the GaN band-edge luminescence are investigated. The cathodoluminescence spectra from 82 to 323 K are measured for 1×1015 atom/cm2-implanted GaN annealed at 1100°C. Luminescence peaks at 356, 362, 376, 390, and 414 nm are observed on the 82 K cathodoluminescence spectrum. When the temperature is increased to 150 K, the intensities of the 356 and 414 nm peaks are nearly unchanged and the 362, 376, and 390 nm peaks disappear. The intensity ratio of 538 nm (H11/22→I15/24) and 559 nm (S3/24→I15/24) is increased with the increase in temperature. We try to shed light on the above interesting phenomena.

Enabling High-Injection Current Light-Emitting Diodes Prepared on 10-$\mu \text{m}$ -Thick GaN Films Grown by Hydride Vapor Phase Epitaxy

In this paper, we successfully introduced the sputtered AlN/patterned sapphire substrate template into hydride vapor phase epitaxy (HVPE) using a two-step growth method. This method simplified the procedure, realized uninterrupted growth, and improved the crystal quality of GaN films. The effects of light-emitting diodes (LEDs) grown on a 10- $\mu \text{m}$ -thick GaN film through HVPE were also compared with those of the LEDs grown on a 2.4- $\mu \text{m}$ -thick GaN film through metal organic chemical vapor deposition (MOCVD). Compared with the LEDs grown on the 2.4- $\mu \text{m}$ -thick GaN film through MOCVD, the LEDs grown on the 10- $\mu \text{m}$ -thick GaN film through HVPE had significantly enhanced light output power (3.24–4.79 mW) and extended saturation current (300–355 mA). These improvements of the LEDs were attributed to the better thermal conductivities and better crystal quality of the 10- $\mu \text{m}$ -thick GaN film through HVPE than those of the 2.4- $\mu \text{m}$ -thick GaN film through MOCVD.

Structure and Strain Properties of GaN Films Grown on Si (111) Substrates with AlxGa1−xN/AlyGa1−yN Superlattices**Supported by the National Natural Science Foundation of China under Grant Nos 61076120 and 61106130, and the Natural Science Foundation and Scientific Support Plan of Jiangsu Province under Grant Nos BK2012516, BK20131072, and BE2012007.

GaN films with an AlxGa1−xN/AlyGa1−yN superlattice (SL) buffer layer are grown on Si(111) substrates by metal-organic chemical vapor deposition (MOCVD). The structure and strain properties of the samples are studied by optical microscopy, Raman spectroscopy, x-ray diffractometry and atomic force microscopy. The results show that the strain status and crystalline quality of the GaN layers are strongly dependent on the difference of the Al composition between AlxGa1−xN barriers and AlyGa1−yN wells in the SLs. With a large Al composition difference, the GaN film tends to generate cracks on the surface due to the severe relaxation of the SLs. Otherwise, when using a small Al composition difference, the crystalline quality of the GaN layer degrades due to the poor function of the SLs in filtering dislocations. Under an optimized condition that the Al composition difference equals 0.1, the crack-free and compressive strained GaN film with an improved crystalline quality is achieved. Therefore, the AlxGa1−xN/AlyGa1−yN SL buffer layer is a promising buffer structure for growing thick GaN films on Si substrates without crack generation.

Effect of Au/SiO2 substrate on the structural and optical properties of gallium nitride grown by CVD

The improvement of the growth of thick GaN films using a fused silica wafer covered with a thin gold layer by chemical vapour deposition at 800 ∘C is reported. In order to compare the surface properties, crystalline quality, micromilling performance and luminescence, the characterization of a GaN film grown on a silicon wafer is presented as well. The different morphologies of the surface observed on the GaN films are compared on each substrate and the resulting microstructures are presented in detail. High resolution TEM images of the GaN films show the main crystallographic planes characterizing these structures. The wurtzite structure was determined for each sample using the substrates of Au/SiO2 and Si (100) from the XRD patterns. Also, the re-deposition effect after ion milling of the GaN films is reported. The performance of ionic beam on the surface of the GaN thick films for the geometries patterning of rectangular, circular and annular with two different ion doses was compared. Cathodoluminescence spectra showed that the top surfaces of the samples emit strong UV emissions peaked at 3·35 and 3·32 eV which are related to the Y 4 and Y 6 transitions.

Study of Dislocation Densities of Thick GaN Films

The dislocation density of GaN thick films has been measured by high-resolution X-ray diffraction. The results show that both the edge dislocations and the screw dislocation reduce with increasing the GaN thickness. And the edge dislocations have a larger fraction of the total dislocation densities, and the densities for the edge dislocation with increasing thickness reduce less in contrast with those for the screw dislocation.

Influence of 3C–SiC/Si (111) template properties on the strain relaxation in thick GaN films

In this work, we study the influence of 3C–SiC/Si (111) template parameters (thickness, roughness and substrate miscut) on the GaN crystal quality and its strain state. For this, structures with an AlN nucleation layer and 1–4µm thick GaN layer have been grown by molecular beam epitaxy in order to select the best templates for the growth of thick GaN structures. Similar GaN structures have been grown directly on silicon for comparisons. The influence of the high silicon doping is confirmed on the enhancement of strain relaxation. Despite this limitation, a 5µm thick crack-free continuous GaN structure (with 1µm silicon doped) has been successfully grown on the best selected template. Furthermore, the growth by metal–organic chemical vapor deposition of structures with AlN and SiN inter-layers and thick continuous GaN layers on Si (111), 3C–SiC/Si (111) and SoPSiC (silicon on polycrystalline silicon carbide) has been achieved in order to show the relative benefit of each approach in terms of layer quality and strain state.

Strain states in GaN films grown on Si(111) and Si(110) substrates using a thin AlN/GaN superlattice interlayer

AbstractWe report the characterization results of the strain states in GaN films grown both on Si(111) and Si(110) substrates by applying a thin AlN/GaN superlattice structure (SLs) as an interlayer (IL). The strain states in the GaN films grown on Si(111) and Si(110) substrates are compared. It is demonstrated that the modulation effect by the SL ILs on the strain state in the GaN film is universal although different behaviours on the Si(111) and Si(110) substrate are observed due to the different lattice symmetry relationship between the substrate and the epilayer. By using the technique, we succeeded in growing thick continuous GaN films (> 3 µm), which are necessary for the high‐power, high‐voltage device applications. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)