Growth Of GaN Films Research Articles

Utilizing Island Growth in Superlattice Buffers for the Realization of Thick GaN‐on‐Si(111) PIN‐Structures for Power Electronics

In this study, the effect of implementing island growth in an AlN/Al0.1Ga0.9N superlattice on the structural properties of vertical GaN‐on‐Si(111) PIN‐structures is investigated. It is demonstrated by scanning electron microscopy (SEM) and atomic force microscopy (AFM) that islands are formed on‐top of V‐pits present in the AlN nucleation layer and that the island coalescence height can be controlled by the growth temperature. Defect selective etching analyses confirm a noteworthy reduction in the threading dislocation density (TDD), which diminishes from 1.2 × 109 cm−2 ± 7.5 × 107 cm−2 to 8.5 × 108 cm−2 ± 7.3 ×107 cm−2 as the island coalescence height increases from ≈160 nm to ≈450 nm, achieved by increasing the growth temperature. Cross‐sectional transmission electron microscopy (TEM) shows that island growth is particularly favorable for the reduction of a‐type dislocations. As a consequence of the significant reduction of a‐type dislocations in the buffer, stress relaxation during the GaN film growth is reduced, which is supported by in situ wafer curvature measurements and high‐resolution X‐ray diffraction (XRD). Owing to the optimized island growth conditions, thick and crack‐free GaN layers on Si(111) substrates are obtained with an absolute wafer bow of <50 μm.

Molecular dynamics simulations and experimental studies on low-temperature growth of GaN

Growth mechanisms of (0001) wurtzite GaN films at low temperature are investigated by molecular dynamics simulations and experiments. The crystallization properties of GaN films deteriorate dramatically at low temperature due to the limited energy available for atomic surface migration and incorporation into the perfect lattice sites. In our simulation, growth interruption stage is periodically introduced and the as-deposited GaN films are treated with energy-carrying argon ions at this stage. The surface atoms located at the weak binding sites can acquire energy from the argon ions for secondary migration and incorporation into the perfect lattice sites. As a result, the crystallization properties of GaN films are significantly improved. GaN films are experimentally grown on sputtered AlN/sapphire substrates at 600 °C via inductively coupled plasma metal organic chemical vapor deposition along with periodic argon plasma treatment. The as-deposited film acquires energy from the plasma, leading to significant improvement of the crystalline properties. The surface morphology of the GaN film demonstrates a noticeable smoothing effect, with an evident increase in grain size from submicron to micron level. Additionally, GaN film with the optimized surface morphology exhibits high c-axis and in-plane orientations, and the full width half maximums of (002) and (102) x-ray diffraction rocking curves are 0.25° and 0.32°, respectively. These results provide effective guidance for the growth of GaN films at low temperature.

High growth rate magnetron sputter epitaxy of GaN using a solid Ga target

Magnetron sputter epitaxy (MSE) is a promising processing route for group-III nitride semiconductors, with the potential to enable high-quality and low cost GaN growth for widespread use. However, fundamental technological hurdles must be overcome to enable the adoption of MSE in industrial production. Here, we present a new UHV-compatible magnetron design with high-performance cooling, enabling high GaN growth rates at high growth temperatures using a solid Ga target. The magnetron is tested with a wide range of process parameters and a stable process is feasible while maintaining the solid state of the Ga target. High GaN growth rates of up to 5 μm/h are achieved at room temperature and a growth rate of 4 μm/h at high temperature, which is one order of magnitude higher compared to MSE with a liquid target. We grow GaN on c-plane sapphire substrates and show the impact of partial pressure ratio and target-to-substrate distance (TSD) on growth rate, film morphology and crystal quality of GaN films with scanning electron microscopy and X-ray diffraction. While the growth rate and film morphology are strongly impacted by the process parameter variation, the crystal quality is further impacted by the overall film thickness. For a 2 μm thick GaN film a full width at half maximum of X-ray rocking curve (ω-FWHM) of GaN 10 1‾ 1 reflection of 0.32° is achieved. We demonstrate a process window for growth of dense and smooth GaN films with high crystal quality using low N2 flow rates and high TSD. By introducing a 20 nm AlN nucleation layer prior to the growth of 390 nm GaN, the ω-FWHM of GaN 0002 reflection of 0.19° is achieved. The epitaxially grown crystalline structure is precisely examined by transmission electron microscopy.

Growth of SiC, AlN, and GaN Films on Silicon Parts of Arbitrary Geometry for Microelectromechanical Applications

Growth of SiC, AlN, and GaN Films on Silicon Parts of Arbitrary Geometry for Microelectromechanical Applications

Enhanced photoresponsivity in Bi2Se3 decorated GaN nanowall network-based photodetectors

Recently, highly responsive photodetectors which are capable of photodetection in a wide range of wavelength spectra are in great demand. To fulfil this, photodetectors formed using heterojunctions of multiple semiconducting materials gain a lot of interest as they cover a broad range of photodetection, and the built-in electric field developed at their interface helps to enhance photoresponse. Here, we have demonstrated a fascinating broadband metal-semiconductor-metal type photodetector (PD) by integrating sputtered Bi2Se3 with the laser molecular beam epitaxy (LMBE) grown GaN film and porous nanowall network on sapphire (0001) substrates. The fabricated photodetectors using a hybrid Bi2Se3/LMBE-GaN nanowall structure exhibit broadband characteristics with a high responsivity of 51.4 and 1.46 A/W when illuminating with the laser of wavelength 355 nm (UV) and 1064 nm (NIR), respectively. The enhanced UV responsivity and good response in the NIR region using Bi2Se3/GaN heterojunction nanostructures suggest futuristic application in dual-wavelength detector devices.

Growth of thick GaN films on 2-inch ScAlMgO4 substrates by halide vapor phase epitaxy

Thick GaN bulks up to 5.2 mm were grown on 2-inch ScAlMgO4 (SAM) substrates by halide vapor phase epitaxy. Free-standing GaN substrates were fabricated by spontaneous separation during the cooling process. The crystalline quality and orientation of GaN for each film thickness were evaluated by X-ray Rocking Curve (XRC) measurements on the (0002) and (10–12) planes. The FWHM of the XRC (0002) and (10–12) diffraction peaks for the 2.9-mm-thick GaN free-standing bulk were 39 and 51 arcsec, respectively, and the dark spot density measured by cathodoluminescence mapping was 1.5 × 106 cm−2. In addition, the radius of curvature of its free-standing GaN bulk was 5.0 m. The impurity concentrations originating from the SAM almost reached detection limits of the SIMS measurements at a GaN film thickness of around 10 µm and more. The crystalline quality of GaN grown on the SAM substrates was comparable to that of GaN grown on sapphire substrates, indicating that the SAM substrate can replace conventional sapphire substrate because of the ease of fabrication of free-standing GaN bulk.

Nonpolar Growth of GaN Films on Polar Sapphire Substrate Using Pulsed Laser Deposition: Investigation of Substrate Temperature Variation on the Quality of Films

The growth of GaN films along nonpolar crystallographic planes, especially s‐plane, on c‐Al2O3 substrate using pulsed laser deposition (PLD) is reported. The role of substrate temperature variation on the structural and morphological properties along this plane is investigated using different techniques. The degree of crystallinity, surface roughness, etc. associated with films are observed to depend strongly on deposition conditions, for example, substrate temperature kept during deposition. Micro‐Raman investigations reveal the presence of an unexpected phonon mode, E1(LO), in the spectra for films deposited at high temperature where prominent crystal growth is observed. The presence of the particular phonon mode observed herewith is due to the nonpolar growth of GaN crystals as well as high density of defects and/or plasmon coupling present in the films. Strong near‐band‐edge emission in the photoluminescence spectra for all specimens shows moderate optical properties of the films. Elemental analysis using X‐ray photoelectron spectra technique confirms the formation of GaN phase for all specimens.

Direct growth of GaN film on ScAlMgO4 substrate by radio-frequency plasma-excited molecular beam epitaxy

ScAlMgO4 (SAM) has attracted attention as a substrate for nitride semiconductor crystal growth owing to its small a-axis lattice mismatch with GaN and InGaN. In this study, we investigated GaN growth on a SAM substrate via radio-frequency plasma-excited molecular beam epitaxy. By optimizing the growth conditions, GaN with the following epitaxial orientation relations (0001)GaN//(0001)SAM and [11–20]GaN//[11–20]SAM was successfully grown directly on the SAM substrate. The atomically flat and abrupt interface of GaN directly grown on the SAM substrate was observed via high-resolution transmission electron microscopy, and uniform GaN growth on a two-inch SAM substrate was also demonstrated.

Thermodynamic Analysis of Group-III-Nitride Alloying with Yttrium by Hybrid Chemical Vapor Deposition.

Group-IIIb-transition-metal-alloyed wurtzite Group-IIIa-nitride (IIIb-IIIa-N) thin films have higher piezoelectric characteristics than binary IIIa-N for a broad range of applications in photonic, electronic, sensing, and energy harvesting systems. We perform theoretical thermodynamic analysis for the deposition and epitaxial growth of Y-alloyed GaN and AlN films by a newly introduced growth technique of hybrid chemical vapor deposition (HybCVD), which can overcome the limitations of the conventional techniques. We investigate the equilibrium vapor pressures in the source zones to determine the dominant precursors of cations for the input of the mixing zone. Then, we study the driving force for the vapor-solid phase reactions of cation precursors in the growth zone to calculate the relationship between the solid composition of YxGa1-xN and YxAl1-xN and the relative amount of input precursors (Y vs. GaCl and AlCl3) in different deposition conditions, such as temperature, V/III precursor input ratio, and H2/inert-gas mixture ratio in the carrier gas. The xY composition in YAlN changes nearly linearly with the input ratio of cation precursors regardless of the growth conditions. However, YGaN composition changes non-linearly and is also substantially affected by the conditions. The thermodynamic analysis provides insight into the chemistry involved in the epitaxial growth of IIIa-IIIb-N by the HybCVD, as well as the information for suitable growth conditions, which will guide the way for ongoing experimental efforts on the improvement of piezoelectricity of the lead-free piezoelectric materials.

Hexagonal GaN nanostructure via electrochemical etching

Gallium nitride GaN thin films were deposited on Si (111) substrates using electrochemical deposition technique at 20 °C. SEM images and EDX results indicated that the growth of GaN films varies with the current density. XRD and Raman analyses showed the presence of hexagonal wurtzite and cubic zinc blende GaN phases with the crystallite size around 18–19 nm. Photoluminescence spectrum showed that the energy gaps of h-GaN/Si (111) and c-GaN/Si (111) were near 3.39 eV and 3.2 eV respectively at 300 K. Raman spectrum indicated the presence of mixed phonon modes of hexagonal and cubic GaN.

Epitaxy of single-crystalline GaN films on novel Ca3Ta(Ga0.5Al0.5)3Si2O14 substrates by metal-organic chemical vapor deposition

The langasite (La3Ga5SiO14, LGS) is a nearly lattice-matched substrate for epitaxy of GaN films. However, it is a challenge to grow GaN films on LGS substrates by metal-organic chemical vapor deposition (MOCVD) because LGS is unstable in aggressive gas atmosphere at high temperature of ∼1000 °C. To overcome the problem, this study chooses another LGS family crystal Ca3Ta(Ga0.5Al0.5)3Si2O14 (CTAGS) which has the same crystal structure as LGS and is more stable in high temperature. The morphology of GaN films at different growth stages are characterized and the results show continuous and smooth GaN films could be obtained after growing for 3 h. Single-crystalline GaN films are determined by transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) measurements. The epitaxial relationship between GaN and CTAGS is GaN(0001)//CTAGS(0001) and GaN(101¯0)//CTAGS(213¯0), the lattice mismatch between GaN and CTAGS is 4.15%. The work is great potential in promoting the development of GaN-based devices.

Influence of the lattice parameter of the AlN buffer layer on the stress state of GaN film grown on (111) Si

GaN films grown on (111) Si substrate with different lattice parameters of the AlN buffer layer by metal–organic chemical vapor deposition are studied. The stress states obtained by different test methods are compared and it is found that the lattice parameter of the AlN buffer layer may have a significant effect on the stress state in the initial stage of subsequent GaN film growth. A larger compressive stress is beneficial to improved surface morphology and crystal quality of GaN film. The results of further orthogonal experiments show that an important factor affecting the lattice parameter is the growth rate of the AlN buffer layer. This work may be helpful for realizing simple GaN-on-Si structures and thus reducing the costs of growth processes.

Influence of Polarities of GaN Substrates on Surface Morphologies and Physical Properties of Epitaxial Mg‐Doped GaN Films Grown by MOCVD

Herein, the influence of free-standing GaN (FS–GaN) substrates with different polarities on the surface morphologies and physical properties of epitaxial Mg-doped GaN films grown by metal–organic chemical vapor deposition (MOCVD) is investigated. For the Mg-doped GaN films grown on polar, semipolar, and nonpolar FS–GaN substrates, their surface morphologies vary greatly, which is attributed to the different surface states and growth modes. All films show high crystalline quality and are basically in a stress-free state. For polar p-GaN, good ohmic contact is achieved with Ni/Au electrode. For semipolar and nonpolar p-GaN, there are obvious contact barriers between the p-GaN and the metal electrodes, and the Mg-doping concentration may be higher than that of the c-plane epitaxial GaN according to the intensity ratio of the band edge peak with respect to the ultraviolet luminescence peak in photoluminescence spectra, which will be investigated in the near future.

Growth of high-quality nitrogen-polar GaN film by two-step high-temperature method

Normally, nitrogen-polar (N-polar) GaN films with better crystalline quality tend to have poor surface morphology, whereas those with good surface morphology suffer from degraded crystalline quality. In this work, we use a two-step high-temperature (HT) growth method to grow N-polar GaN films on vicinal c-plane sapphire substrates by metalorganic chemical vapor deposition, which can effectively solve the irreconcilability between surface morphology and crystalline quality. The GaN film consists of two GaN layers (HT-GaN 1 and HT-GaN 2), which were grown at different growth parameters, respectively. The 300-nm-thick HT-GaN 1 layer was directly grown on the low-temperature GaN buffer layer under the higher V/III ratio and pressure at 1040 °C to reduce dislocation density. The 1.7-μm-thick HT-GaN 2 layer was grown under the lower V/III ratio and pressure to improve surface morphology. Besides, the growth temperature of HT-GaN 2 layer was ramped up to 1070 °C to suppress the yellow-band luminescence. Finally, high-quality N-polar GaN films with smooth surface morphology and high crystalline quality were obtained, for which the full-width at half-maximum values of (0002) and (101¯2) planes X-ray diffraction rocking curves are 131 and 390 arcsec, respectively, the root mean square roughness over an area of 20 × 20 μm2 is 1.88 nm, and the residual compressive stress is as low as 0.025 GPa. It is reasonably believed that this work provides an effective approach to preparing high-quality N-polar GaN films for the application of N-polar nitride devices.

Effect of Heat Treatment on Properties of Gan Films Grown by Mocvd

Effect of Heat Treatment on Properties of Gan Films Grown by Mocvd

Enhancement of optical property and crystal structure for GaN films on 2D MoS2 buffer layer by nitridation treatment

The growth of GaN films on layered 2D-MoS2/c-sapphire substrate by plasma-assisted molecular beam epitaxy system was investigated. Pre-nitridation process and deposition time as growth parameters were employed to exploit the heteroepitaxial growth of GaN films at the temperature of 700 °C. The characterizations of surface condition, surface morphology, chemical composition, optical properties, crystal orientation and quality of GaN epitaxial layers were conducted by reflection high energy electron diffraction (RHEED), scanning electron microscope (SEM), atomic force microscope (AFM), X-ray photoelectron Spectroscopy (XPS), Raman spectroscopy, photoluminescence spectroscopy (PL) and high-resolution X-ray diffraction (HR-XRD), respectively. The pre-nitridation process and growth duration can enhance the optical properties and crystal quality of GaN films on 2D-MoS2/c-plane sapphire. The pre-treatment of MoS2 layer by nitrogen plasma before the growth, providing a wetting layer of nitrogen for GaN growth, can eliminate the oxygen contamination and reduce the defects in the films. Band-to-band emission of Wurtzite GaN with the bandgap of 3.5 eV can be improved by pre-nitridation process, and yellow band emission can be suppressed due to the elimination of defect states in the films. Meanwhile, the crystal orientation and quality can be improved by the pre-nitridation process and longer duration of growth time. This study demonstrates a van der Waals epitaxial GaN films on 2D-MoS2 layer with higher crystal quality by pre-nitridation process, and it provides a potential solution for the future applications of GaN-based devices.

Plasma-Assisted MOCVD Growth of Non-Polar GaN and AlGaN on Si(111) Substrates Utilizing GaN-AlN Buffer Layer

We report the growth of non-polar GaN and AlGaN films on Si(111) substrates by plasma-assisted metal-organic chemical vapor deposition (PA-MOCVD). Low-temperature growth of GaN or AlN was used as a buffer layer to overcome the lattice mismatch and thermal expansion coefficient between GaN and Si(111) and GaN’s poor wetting on Si(111). As grown, the buffer layer is amorphous, and it crystalizes during annealing to the growth temperature and then serves as a template for the growth of GaN or AlGaN. We used scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray diffraction (XRD) characterization to investigate the influence of the buffer layer on crystal structure, orientation, and the morphology of GaN. We found that the GaN buffer layer is superior to the AlN buffer layer. The thickness of the GaN buffer layer played a critical role in the crystal quality and plane orientation and in reducing the cracks during the growth of GaN/Si(111) layers. The optimum GaN buffer layer thickness is around 50 nm, and by using the optimized GaN buffer layer, we investigated the growth of AlGaN with varying Al compositions. The morphology of the AlGaN films is flat and homogenous, with less than 1 nm surface roughness, and has preferred orientation in a-axis.

Structural, Morphological, Vibrational and Optical Properties of GaN Films Grown by Reactive Sputtering: The Effect of RF Power at Low Working Pressure Limit

This work reports the properties of GaN films grown onto c-Si (100) at relatively low substrate temperature (400°C) by reactive magnetron sputtering. The study depicts the effect of working pressure and RF power on the GaN film structural, vibrational and optical properties characterized by X-ray diffraction, atomic force and scanning electron microscopies, Raman spectroscopy and spectroscopic ellipsometry. Unusual low pressure deposition condition (0.40 Pa) was achieved by using a separated argon inlet directed to the Ga target surface, resulting in improved crystalline quality of the films. In this condition, the preferential crystalline orientation, the surface morphology and the optical gap of the GaN films show a strong dependence on the RF power applied to the Ga target, where low RF power (30-60 W) was responsible for increasing the c-axis orientation and the optical gap, while higher RF power (75-90 W) decreased the overall crystal quality and increased the surface roughness.

Effect of Pretreatment of the Silicon Substrate on the Properties of GaN Films Grown by Chloride–Hydride Vapor-Phase Epitaxy

Effect of Pretreatment of the Silicon Substrate on the Properties of GaN Films Grown by Chloride–Hydride Vapor-Phase Epitaxy

Growth of bulk single crystal ScAlMgO4 boules and GaN films on ScAlMgO4 substrates for GaN-based optical devices, high-power and high-frequency transistors

Single crystal growth of ScAlMgO4 boules with 10 mm up to 4 in. (c.a. 101.6 mm) in diameter by Czochralski technique was demonstrated. Some high quality ScAlMgO4 single crystal boules with 2-inches (c.a. 50.8 mm) in diameter were successfully grown without dislocation, which was revealed by X-ray rocking curve and X-ray topography measurements for the c-plane wafers. After a GaN template was formed on a substrate cut out from a grown ScAlMgO4 boule by using metalorganic vapor phase epitaxy, a thick GaN film was epitaxially grown on the template using hydride vapor phase epitaxy (HVPE). At the cooling stage after the growth in the HVPE process, the thick grown GaN film was separated by itself from the ScAlMgO4 substrate due to high cleavability of ScAlMgO4, which makes it possible the reuse of the substrate. The free-standing GaN grown on the reused ScAlMgO4 substrate showed the same quality as a GaN fabricated on a fresh ScAlMgO4 substrate. The reuse of ScAlMgO4 substrates is beneficial for saving the GaN films fabrication cost. The fabrication of a free-standing GaN substrate with 2 in. in diameter and 1 mm in thickness on ScAlMgO4 substrates was demonstrated to be more high-yield than the fabrication on a sapphire substrate. The growth method shown in this study is very promising, and it opens an efficient way to obtain GaN free-standing wafers and to reduce the production cost of GaN wafers.