GaN Single Crystals Research Articles

Dislocation evolution in anisotropic deformation of GaN under nanoindentation

The exceptional performance of GaN semiconductors in lasers, wireless communication, and energy storage systems makes them crucial for future multi-functional devices. However, during the polishing of GaN wafers, abrasive particles can induce subsurface damage, compromising device performance. This study investigates dislocation loops in GaN single crystal to understand dislocation nucleation and glide under external stress. Using nanoindentation for compressive stress, we confirmed multiple slip system activation via transmission electron microscopy after pop-in. We also performed molecular dynamics to simulate the nucleation and multiplication of U-shaped dislocation loops. Furthermore, we developed a theoretical model using Peierls–Nabarro stress to quantify GaN's critical shear stress. Raman spectroscopy was also used to analyze shear stress on U-shaped loops, supporting our model. This study provides insights into GaN dislocation dynamics under mechanical stress, aiding in wafer defect evaluation during machining and offering guidance for dislocation evolution.

Liquid phase epitaxy of GaN films on sapphire substrates under an atmospheric pressure nitrogen ambience

GaN films were grown on sapphire substrates using liquid phase epitaxy under an atmospheric pressure nitrogen ambience, employing molten Ga and Fe3N as a source mixture. Single-crystal GaN (0001) films were successfully grown on sapphire (0001) substrates within a growth temperature (T g) range of 750 °C–900 °C. When varying the Fe3N concentration in the range of 0.05–3 mol%, lower iron nitride resulted in high crystallinity of GaN (0001) films. The incorporation of iron atoms in GaN can negatively impact crystal quality. Parameterizing T g at a concentration of 0.1 mol% Fe3N showed that higher T g led to a reduction in the peak width of GaN (0002) X-ray rocking curves. However, at 3 mol%, elevating T g resulted in the degradation of the crystallinity of GaN. This degradation may be attributed to the increased solubility of iron atoms in GaN with increasing T g.

Quasi van der Waals Epitaxy of Single Crystalline GaN on Amorphous SiO2/Si(100) for Monolithic Optoelectronic Integration.

The realization of high quality (0001) GaN on Si(100) is paramount importance for the monolithic integration of Si-based integrated circuits and GaN-enabled optoelectronic devices. Nevertheless, thorny issues including large thermal mismatch and distinct crystal symmetries typically bring about uncontrollable polycrystalline GaN formation with considerable surface roughness on standard Si(100). Here a breakthrough of high-quality single-crystalline GaN film on polycrystalline SiO2/Si(100) is presented by quasi van der Waals epitaxy and fabricate the monolithically integrated photonic chips. The in-plane orientation of epilayer is aligned throughout a slip and rotation of high density AlN nuclei due to weak interfacial forces, while the out-of-plane orientation of GaN can be guided by multi-step growth on transfer-free graphene. For the first time, the monolithic integration of light-emitting diode (LED) and photodetector (PD) devices are accomplished on CMOS-compatible SiO2/Si(100). Remarkably, the self-powered PD affords a rapid response below 250µs under adjacent LED radiation, demonstrating the responsivity and detectivity of 2.01×105 A/W and 4.64×1013Jones, respectively. This work breaks a bottleneck of synthesizing large area single-crystal GaN on Si(100), which is anticipated to motivate the disruptive developments in Si-integrated optoelectronic devices.

Investigation of cluster magnetorheological electro-Fenton composite polishing process for single-crystal GaN wafer based on BBD experimental method

A cluster magnetorheological (MR) electro-Fenton composite polishing technique was proposed in this work, which can realize high efficiency, ultra-smooth and damage-free of GaN wafer by the synergistic effect of electro-Fenton reaction and flexible mechanical removal of MR polishing. The key parameters of electro-Fenton were optimized through methyl orange degradation experiments based on BBD experimental method. The results showed that the decolorization rate had a strong dependence on H2O2 concentration, Fe–C concentration and pH value, where the decolorization rate had the maximum value when the H2O2 concentration of 5 wt%, Fe–C concentration of 3 wt% and pH value of 3. Compared with the Fenton reaction, the decolorization and REDOX potential of methyl orange solution were significantly improved in the electro-Fenton reaction. Furthermore, the process parameters of the cluster MR electro-Fenton composite polishing were optimized to obtain the best polishing result, which was realized under the conditions of 3 wt% diamond (grain size: 0.5 µm), a polishing gap of 0.9 mm and a polishing time of 60 min. The novel method achieved a material removal rate of 10.79 μm h−1, which was much higher than that of the conventional technique. In addition, an ultra-smooth and damage-free surface with a roughness of 1.29 nm Ra was obtained.

Studying the effect of temperature and pressure on GaN crystals via the Na-flux method

GaN single crystals (15 × 15 mm) were epitaxially grown using the Na-flux method, and substrate self-peeling was achieved. The effect of temperature and pressure on the growth rate and the quality of GaN single crystals are investigated.

Epitaxy Single Crystal GaN on AlN Prepared by Plasma-enhanced Atomic Layer Deposition

Epitaxy Single Crystal GaN on AlN Prepared by Plasma-enhanced Atomic Layer Deposition

Effects of Ion Irradiation and Temperature on Mechanical Properties of GaN Single Crystals under Nanoindentation.

Understanding the impact of irradiation and temperature on the mechanical properties of GaN single crystals holds significant relevance for rational designs and applications of GaN-based transistors, lasers, and sensors. This study systematically investigates the influence of C-ion irradiation and temperature on pop-in events, hardness, Young's modulus, and fracture behavior of GaN single crystals through nanoindentation experiments. In comparison with unirradiated GaN samples, the pop-in phenomenon for ion-irradiated GaN samples is associated with a larger critical indentation load, which decreases with increasing temperature. Both unirradiated and ion-irradiated GaN samples exhibit a decline in hardness with increasing indentation depth, while Young's moduli do not exhibit a clear size effect. In addition, intrinsic hardness displays an inverse relationship with temperature, and ion-irradiated GaN single crystals exhibit greater intrinsic hardness than their unirradiated counterparts. Our analysis further underscores the significance of Peierls stress during indentation, with this stress decreasing as temperature rises. Examinations of optical micrographs of indentation-induced fractures demonstrate an irradiation embrittlement effect. This work provides valuable insights into the mechanical behavior of GaN single crystals under varying irradiation and temperature conditions.

Simultaneous Growth Strategy of High-Optical-Efficiency GaN NWs on a Wide Range of Substrates by Pulsed Laser Deposition.

Here, we explore a catalyst-free single-step growth strategy that results in high-quality self-assembled single-crystal vertical GaN nanowires (NWs) grown on a wide range of common and novel substrates (including GaN, Ga2O3, and monolayer two-dimensional (2D) transition-metal dichalcogenide (TMD)) within the same chamber and thus under identical conditions by pulsed laser deposition. High-resolution transmission electron microscopy and scanning transmission electron microscopy (HR-STEM) and grazing incidence X-ray diffraction measurements confirm the single-crystalline nature of the obtained NWs, whereas advanced optical and cathodoluminescence measurements provide evidence of their high optical quality. Further analyses reveal that the growth is initiated by an in situ polycrystalline layer formed between the NWs and substrates during growth, while as its thickness increases, the growth mode transforms into single-crystalline NW nucleation. HR-STEM and corresponding energy-dispersive X-ray compositional analyses indicate possible growth mechanisms. All samples exhibit strong band edge UV emission (with a negligible defect band) dominated by radiative recombination with a high optical efficiency (∼65%). As all NWs have similar structural and optical qualities irrespective of the substrate used, this strategy will open new horizons for developing III-nitride-based devices.

Research on the status of GaN single crystal growth

GaN is a third-generation semiconductor. As an ideal material of the new generation semiconductor today, it has many features such as high saturated electron mobility and wide band. The methods of GaN growth developed rapidly recently. We reviewed recent artiles and then we collect the various of methods about the GaN single crystal growth, after that we compare the difference among these method. This paper mainly talks about three methods of growing GaN single crystal: The Ammonothermal method, the Sodium-flux growth method, and the Halide vapor phase epitaxy (HVPE), and we collect both of the advantages and disadvantages. By putting all characteristics of each methods together we found that combining the HVPE method and Ammonothermal method together can connect the advantages together and avoid some of these disadvantages. We can use the HVPE method to grow the GaN seed and expand the crystal using the Ammonothermal method may be a good choice to grow the GaN single crystal at a low cost. Because if we start with Ammonothermal method, it is easy to form the polycrystalline and if we use the HVPE method to grow large crytal, it is too expensive to do so. Therefore, combining two methods can avoid the disadantages in each other and save the cost when growing.

Ohmic Contact to n-GaN Using RT-Sputtered GaN:O.

One of the key issues in GaN-based devices is the resistivity and technology of ohmic contacts to n-type GaN. This work presents, for the first time, effective intentional oxygen doping of sputtered GaN films to obtain highly conductive n+-GaN:O films. We have developed a novel and simple method to obtain these films. The method is based on the room temperature magnetron sputtering of a single crystal bulk GaN target doped with oxygen. The n+-GaN:O films exhibit a polycrystalline structure with a crack-free surface and a free electron concentration of 7.4 × 1018 cm3. Ohmic contact to GaN:Si with n+-GaN:O sub-contact layer achieves specific contact resistance of the order of 10-5 Ωcm2 after thermal treatment. The obtained results are very promising for the development of the technology of a whole new class of ohmic contacts to n-GaN.

Study of corrosion rate control mechanism based on magnetorheological electro-Fenton composite polishing of single-crystal GaN wafers

Study of corrosion rate control mechanism based on magnetorheological electro-Fenton composite polishing of single-crystal GaN wafers

Investigation of Proton Irradiation-Induced Dynamic Characteristics on High-Performance GaN/AlGaN/GaN Schottky Barrier Diodes

Abstract In this paper, dynamic characteristics of the single-crystal GaN-passivated lateral AlGaN/GaN Schottky barrier diodes (SBDs) treated with proton irradiation were investigated. Radiation-induced changes including idealized Schottky interface and slightly degraded on-resistance (RON) were observed with 10-MeV proton irradiation at fluences of 1014 cm-2. Benefited from the existing negative polarization charges induced at GaN/AlGaN interface, the dynamic on resistance (RON,dyn) showed negligible degradation after a 1000-s-long forward current stress of 50 mA for devices irradiated or not. Furthermore, the normalized RON,dyn increased by only 14% than the initial case after a 100-s-long bias of -600 V for the irradiated devices. The high-performance lateral AlGaN/GaN SBDs with tungsten as anode metal and in-situ single-crystal GaN as passivation layer show a great potential in the harsh radiation environment of space.

Surface states passivation in GaN single crystal by ruthenium solution

GaN single crystal samples were cleaned and passivated with ruthenium solution. Photoluminescence (PL) and scanning tunneling spectroscopy (STS) were used to characterize the passivated surface. PL study showed an effective increase in band edge emission after passivation. I–V (current–voltage) and dI/dV (differential conductance) spectra measurements of GaN single crystal samples using ambient STS revealed the variation in the density of states (local), shifting of Fermi-level position, and onset/offset of valence and conduction bands. We found a significant change in I–V and dI/dV measurements after surface treatment, which means modification in surface electronic properties. The ruthenium solvent passivates the surface states, converting the surface into a highly ordered and air oxidation-resistant state. Finally, Ni/GaN Schottky diodes were fabricated to demonstrate improved device characteristics after passivation, which was a direct indication of improved GaN interface due to ruthenium passivation.

Progress in GaN Single Crystals: HVPE Growth and Doping

Progress in GaN Single Crystals: HVPE Growth and Doping

ReS2 on GaN Photodetector Using H+ Ion-Cut Technology.

The wafer-scale single-crystal GaN film was transferred from a commercial bulk GaN wafer onto a Si (100) substrate by combining ion-cut and surface-activated bonding. Well-defined, uniformly thick, and large-scale wafer size ReS2 multilayers were grown on the GaN substrate. Finally, ReS2 photodetectors were fabricated on GaN and sapphire substrates, respectively, and their performances were compared. Due to the polarization effect of GaN, the ReS2/GaN photodetector showed better performance. The ReS2/GaN photodetector has a responsivity of 40.12 A/W, while ReS2/sapphire has a responsivity of 0.17 A/W. In addition, the ReS2/GaN photodetector properties have reached an excellent reasonable level, including a photoconductive gain of 447.30, noise-equivalent power of 1.80 × 10-14 W/Hz1/2, and detectivity of 1.21 × 1010 Jones. This study expands the way to enhance the performance of ReS2 photodetectors.

Molecular dynamics simulation of laser assisted grinding of GaN crystals

Gallium nitride crystal is a typical difficult-to-machine material due to its distinct anisotropy, high brittleness, and high hardness. The molecular dynamics simulations of traditional grinding and laser assisted grinding of GaN single crystals with a single grit were performed, and the influences of the laser power density on grinding force, stress distribution, material damage mechanism, subsurface damage depth, and abrasive wear were systematically studied. The results demonstrated that dislocations, stacking faults, hexagonal-to-cubic phase transition, and amorphous transition were generated during both traditional grinding and laser assisted grinding processes. Compared with the traditional grinding, laser assisted grinding with an appropriate laser power density reduced the grinding force, stress distribution, phase transition percentage, dislocation loop length, subsurface damage depth, and wear damage of the abrasive. However, excessive laser power densities caused deeper subsurface damage depth and severer amorphous damage for the rake face of the abrasive particle, which seriously deteriorated the integrity of the ground surface and subsurface. The results not only enhance the understanding of material removal and damages under the coupling actions of the laser and abrasive machining, but also provide a theoretical basis for parameter optimization during the machining of GaN single crystals.

X-ray analyses on the variation of the surface and structure properties of GaN and AlN single crystals subject to femtosecond laser irradiation

X-ray analyses on the variation of the surface and structure properties of GaN and AlN single crystals subject to femtosecond laser irradiation

Probing carrier concentration of doped GaN single crystals from LO phonon-plasmon coupled modes

Raman and infrared reflectance spectroscopy were used to study GaN single crystals with different doping types and carrier concentrations. In different samples, the characteristic spectra related to LO phonon-plasmon coupled (LOPC) mode are observed as changing regularly with carrier concentration. Furthermore, to obtain the carrier concentration and mobility of GaN crystals, Raman and infrared reflectance spectra with LOPC modes were fitted. The carrier concentration obtained agrees well with that obtained from the Hall measurement, which confirms that these non-contact and non-destructive spectroscopy methods with high spatial resolution are reliable to probe the carrier concentration of doped GaN single crystals.

Mechanism of current-collapse free for lateral GaN Schottky barrier diodes utilizing polarization-induced hole injection

In this Letter, a mechanism of current-collapse free for high-performance lateral AlGaN/GaN Schottky barrier diodes (SBDs) with a thick single-crystal GaN cap layer is investigated. A high breakdown voltage of 1.74 kV and a low specific on-resistance (RON,sp) of 5.04 mΩ cm2 are achieved for the fabricated SBD with a spatial distance (LAC) of 15 μm. Benefiting from the optimized groove anode technique, including a low-damaged etching process and a post-anode-annealing process, a low leakage current of 87 nA/mm at −500 V as well as a low turn-on voltage of 0.8 V defined at an anode current density of 1 mA/mm are obtained. Holes induced by polarization effects at the GaN/AlGaN interface inject to the anode edge at the ON state, and the negatively charged centers induced by a groove etching process are neutralized, which lead to the suppressed tunneling current (ITU) and idealized ideality factor (η). The forward current density decreases due to the suppressed ITU at the barrier-limited region. When the devices are biased at the OFF state, holes around cathodes inject to the buffer layer, which suppress hole emission at the buffer layer, and current collapse is greatly suppressed even after a 300-s-long bias test at −600 V.

S2-semipolar GaN grown by HVPE on a non-polar m-plane sapphire: Features of growth and structural, morphological, and optical properties

In our study, we have analysed the effect of a non-polar m-plane of sapphire substrate on the features of HVPE growth as well as the structural, morphological, optical properties, and Raman scattering of the grown single crystal GaN epitaxial film.We found that the chosen technological approach makes it possible to obtain samples of structurally high-quality semipolar gallium nitride with an S2 orientation (11‾22) on the m-sapphire. The S2 plane of (11‾22) GaN has a polar reversal of φ ∼ 75° relative to the non-polar (10‾10) m-plane of sapphire, as well as an azimuthal tilt. Under the selected conditions of epitaxial growth, the GaN film has the morphology of a stepped terrace formed during the growth of mesa-like crystal islands, and their coalescence determined the observed surface topography. Based on the data analysis obtained by complex structural-spectroscopic methods, the level of residual biaxial stresses and refractive index dispersion were determined, testifying to the high structural and optical quality of the epitaxial film of semipolar GaN. An analysis of Raman spectra revealed the presence of optical anisotropy depending on the polar angle when the heterostructure is rotated around the m axis of the sapphire substrate.Optimisation of the technological methodology may in future lead to a promising approach for the growth of quality GaN structures on large area m-sapphire substrates as optoelectronic devices based on these layers will have excellent characteristics.