Gallium Nitride Films Research Articles

AlN as interlayer for effective thermal dissipation from gallium nitride to CVD diamond using nanocrystalline diamond seeding

AbstractWith the increasing power density achieved in gallium nitride (GaN) electronic devices, the thermal dissipation becomes a key issue that restricts their ultimate performances. However, the effective thermal boundary resistance (TBReff) between GaN and their heat spreader usually dominates the heat concentration. Here we introduce a super‐thin AlN interlayer with nanocrystalline diamond (NCD) seeding as the nucleation for the polycrystalline diamond (PCD) film growth on the GaN films. A thermal conductivity approaching 250 W/mK for the 1.2 μm‐thick PCD film is obtained. The TBReff between GaN and PCD films is estimated to be 5 m2K/GW, which is much smaller than that of the typical SiNx interlayer. Since AlN can be deposited simultaneously with the device structure, this work is promising to achieve the full potential of using diamond as the heat spreader for GaN‐based transistors.

High-quality GaN thin film deposition at low temperature by ECR plasma-assisted sputter deposition method and its dependence of sapphire substrate misorientation angle

Gallium nitride (GaN) thin films were deposited by electron cyclotron resonance (ECR) plasma-assisted sputtering, which combines GaN-magnetron sputtering with argon and nitrogen plasma assistance using an ECR high-density plasma. GaN films on the misorientation-angle-0.0° (just) sapphire substrate showed very good crystallinity with a GaN(0002) rocking curve (XRC) full width at half maximum (FWHM) of 0.042° and epitaxial growth confirmed by φ-scan measurements at a low heating temperature of 350 °C. However, the GaN thin film had a rough surface with circular grains about 100 nm in diameter and a surface root-mean-square height (Sq) of 1.21 nm. Therefore, the misorientation angle of the sapphire substrate was varied from 0.2° to 10.0°. As a result, the grains observed on the just substrate disappeared at 0.5°. The film had Sq: 0.33 nm, and the FWHM of the XRC of GaN(0002) was 0.066°, indicating improved surface flatness while maintaining crystallinity. This is considered to be due to the step flow, which promotes ECR plasma-assisted diffusion on the terrace even at the low temperature of 350 °C. The polarity of the GaN thin film was analyzed by time-of-flight atomic scattering surface analysis and found to be N-polar on all substrates.

Counterbalancing effects of bowing in gallium nitride templates by epitaxial growth on pre-strained sapphire substrates

To minimize bowing in a gallium nitride (GaN) template, which is caused by differences in the properties of the GaN film and the sapphire substrate, we studied how to offset bowing by directly growing GaN using the hydride vapor phase epitaxy (HVPE) method on the frontside of a pre-strained sapphire substrate prepared using a backside grinding process. The relationships between the respective thicknesses and stresses of the undamaged sapphire, damaged sapphire, and GaN layers were analyzed by deriving the bow formula, which is a modification of Freund's equation, and the theoretical formula-derived results were compared with experimentally determined values. The bow of the GaN template was measured using a micrometer, and the residual stresses in the GaN and sapphire, carrier mobilities, and GaN crystal qualities were measured using a micro-Raman spectrophotometer. The results confirmed the annealing effect on the damaged sapphire layer, and the introduction of pre-strained GaN templates facilitated easier bow control compared to that afforded by a conventional GaN template. Additionally, we identified trends and optimal conditions for changes in residual stress, carrier mobility, and crystal quality in GaN and the damaged sapphire layer based on layer thickness and stress variations. We explain the mechanisms responsible for changes in bow, stress, carrier mobility, and crystal quality of the GaN template.

Physical Investigations of GaN/Porous Silicon at Different Laser Wavelengths

In this study, we prepared PSi using the laser-assisted electrochemical etching method and deposited Gallium nitride (GaN) on the PSi substrate using the pulsed laser technique at different pulsed laser wavelengths (1064, 532, and 355 nm). We investigated the optical, structural, topographical, and morphological properties of Gallium Nitride on the substrate PSi by various pulsed laser wavelengths. The X-Ray Diffraction (XRD) analysis revealed that gallium nitride on PSi was polycrystalline and hexagonal, cubic structural at 532 nm, with a high peak intensity and crystallite size at 2θ=36.96°and 57.80 related to (101) and (110) planes, respectively, while the c-GaN phase is observed at 2θ = 25.43 degrees and is reflected from the (200) plane. PL shows two emission peaks were observed for the GaN film (430,345,413 nm) and the PSi substrate (867,891,876 nm), and the energy gap increased as the wavelength decreased. The field emission scanning electron microscopy (FESEM) pictures revealed that the synthetic sample had an average size of 24.45, 23.91, and 21.30 nm, and the nanoparticles appeared spherical and similar to cauliflower. The atomic force microscopy (AFM) results showed that the average roughness was 7.62, 10.64, and 13.62 nm, respectively, and it was observed the root mean square increased as a result of the uniform distribution of high-quality crystals and the excellent quality of the crystal structure. The UV-Visible (UV) results showed that the transmission decreased with a decrease in wavelength, and the absorption was inversely proportional to the transmission.

High‐Quality Nonpolar a‐Plane AlGaN Film Grown on Si‐Doped AlN Template by Metal Organic Chemical Vapor Deposition

High crystalline quality and flat a‐plane aluminum gallium nitride (AlGaN) films are obtained on Si‐doped AlN templates with a moderate silane (SiH4) flow rate by metal‐organic chemical vapor deposition (MOCVD). The effects of the SiH4 flow rate on the surface morphology, crystalline quality, stress state, and optical property of a‐plane AlN templates and AlGaN films are comprehensively investigated. As the SiH4 flow rate increases from 0 to 7.0 nmol min−1, the full width at half maximum of X‐ray rocking curve values along [0001]/[1–100] directions for the AlGaN films are monotonically reduced to 1124/1143 arcsec. Meanwhile, the surface root mean square roughness value is decreased to 0.88 nm. These achievements are attributed to the suppression of the anisotropy degrees and in‐plane stress of AlN templates. In addition, an excess SiH4 flow rate leads to deteriorated surface morphologies and increased basal plane stacking fault (BPSF) densities for both AlN and AlGaN films. This work suggests that doping AlN layers with an appropriate SiH4 flow rate is a promising route to obtain high‐quality a‐plane AlGaN films for efficient nonpolar deep ultraviolet (DUV) devices.

Optical characterizations of GaN/MoS2 van der Waals heterojunctions with different band alignments

The integration of gallium nitride (GaN) with 2D molybdenum disulfide (MoS2) to form GaN/MoS2 semiconductor heterojunctions will have high potential applications in novel electronics and optoelectronics. In this study, GaN thin films were grown on pulse-laser-deposited MoS2 layer by plasma-assisted MBE at different substrate temperatures (500 °C, 600 °C and 700 °C, respectively). The energy-band alignments of GaN/MoS2 semiconductor heterostructures were analyzed by X-ray photoelectron spectroscopy. The epitaxial growth conditions of GaN films influenced the band alignment of GaN/MoS2 heterojunction. Type-I heterostructure, a straddling relation between narrow-bandgap MoS2 and wide-bandgap GaN, was observed at the optimized growth temperature of 600 °C. At the same time, photoluminescence (PL) and photoreflectance spectroscopies were employed to analyze the optical properties of MoS2 and GaN/MoS2 heterostructures. The PL and exciton energy transition of 2D MoS2 layer can be enhanced by the capping layer GaN, especially for type-I band alignment structure.

Fabrication of gallium nitride waveguide resonators by high-power impulse magnetron sputtering at room temperature

In this work, we demonstrate gallium nitride (GaN) waveguide resonators by sputtering amorphous GaN films on the silicon-based substrate. With the aid of high-power impulse magnetron sputtering (HiPIMS), high-quality, high-deposition-rate, and high-flatness GaN films can be deposited directly onto the silicon substrate with a 4 μm buried oxide layer at room temperature. Waveguide resonators with a quality factor of up to 4 × 104 are demonstrated, and closely critical coupling is achieved at a 0.2 μm gap by optimizing the gap sizes, showing a high extinction ratio of waveguide resonators at ≈24 dB. The fabrication process of HiPIMS-GaN waveguide resonators utilizes CMOS-compatible techniques and operates at a low thermal budget. Compared to conventional GaN films fabricated using metal-organic chemical vapor deposition, this study offers the potential to produce low-cost GaN waveguides on amorphous substrates and realize integrated GaN photonics in optical communication, nonlinear photonics, and quantum photonics by high-quality HiPIMS films.

Thermal Atomic Layer Deposition of Gallium Nitride Films Using Tris(dimethylamido)Gallium and Ammonia

Gallium nitride (GaN) is a III-V semiconductor material with a wide and direct bandgap, relatively high carrier mobility, and high breakdown voltage. GaN is suitable for applications in optoelectronic devices and power devices, including light-emitting diodes (LEDs) and high electron mobility transistors (HEMTs). Typically, epitaxial GaN layers are grown by metal-organic chemical vapor deposition (MOCVD) at high deposition temperatures. However, low-temperature growth of GaN is necessary for temperature-sensitive devices and substrates. Atomic layer deposition (ALD) is a technique that offers unique advantages, such as low process temperature, excellent conformality, and atomic-level thickness control.In this study, we investigated the thermal ALD process of GaN at a low temperature of 200°C. Tris(dimethylamido)gallium (TDMAGa) was selected as the Ga precursor because tris(dimethylamido)titanium and tris(dimethylamido)aluminum were successfully adopted in the low-temperature ALD process of TiN and AlN thin films [1]. The growth and properties of ALD GaN films were investigated. ALD GaN films exhibited a self-limiting reaction with a deposition rate of 1.34 Å/cycle at 200°C, with a refractive index of 2.07. The saturation times for TDMAGa and NH3 pulses were 5 s and 30 s, respectively. Deposition at 250°C showed a deposition rate higher than 3 Å/cycle due to the thermal decomposition of TDMAGa. The ALD GaN film deposited at 200°C was amorphous and nitrogen deficient. To improve the crystallinity and composition, the as-deposited GaN films were annealed in an NH3 atmosphere in the deposition chamber without air exposure. The effects of annealing on the crystallinity and composition of the films were investigated. Acknowledgments This work was supported by Samsung Display Co., Ltd.

Effect of proton irradiation on the cathodoluminescence of gallium nitride films

In this work, we studied the effect of 350 keV proton irradiation with two different fluences of 1013 cm−2 and 1015 cm−2 on the cathodoluminescence (CL) spectrum of GaN. Three lines were observed in the CL spectra of virgin and proton pre-irradiated samples - band edge line (3.4 eV), blue line (2.8 eV) and yellow line (2.15 eV). A strong decrease in the CL intensity for samples pre-irradiated by protons, especially at high proton fluence of 1015 cm−2, is shown. The evolution of the cathodoluminescence spectra during electron irradiation demonstrates a further decrease in all line intensities. It was shown that virgin and proton pre-irradiated GaN samples were not charged under electron irradiation. The cathodoluminescence signal drop can be caused by a decrease in the concentration of initial luminescence centers associated with intrinsic and impurity defects due to their transformation under proton bombardment and electron-stimulated diffusion implanted H+ ions.

GaN film optical nonlinearity: wavelength dependent refractive index for All-Optical switching application

Gallium Nitride (GaN) is fast becoming a potential replacement for silicon in complex electronic and photonics circuitries and systems. At higher optical intensity, optical effects such as non-linear effects would become non-trivial. To this end, we investigated nonlinear optical properties of metal–organic chemical vapor deposited GaN. Specifically, continuous-wave laser Z-scan techniques were employed to obtain nonlinear refractive index, n2, of a 1-µm thin layer on a c-plane sapphire substrate. FESEM, PL, XRD and EDS analysis confirmed the good quality growth of GaN thin film. We detected a reversal of polarity of the n2 value at 637 nm and 405 nm of CW laser excitation and femtosecond laser pulse at 808 nm. This is substantiated by theoretical calculation of GaN’s n2 showing sign switchover at wavelength shorter than 420 nm. This presents a new all-optical switching scheme based on the switchover of wavelength dependent n2. These findings point to opportunities for GaN-based integrated photonics for nonlinear and quantum photonics applications.

Effect of AlN/GaN supercycle ratio on properties of AlxGa1−xN films using super-cycle plasma enhanced atomic layer deposition

In this study, aluminum gallium nitride (AlGaN) films are prepared by supercycle plasma enhanced vapor deposition (PEALD) with trimethylgallium, trimethylaluminum, and NH3 plasma. The ratio of aluminum nitride (AlN) to gallium nitride (GaN) PEALD cycle number is varied to investigated its effect on film properties. The experimental results show that the growth per cycle is lower than that of the mixing rule prediction, and the Ga content dramatically drops with increasing the AlN cycle ratio. It is implied that AlN surface partly hinders the growth of GaN during supercycle deposition. The Ga:Al elemental ratio is found to be 1:1 at the AlN cycle ratio of 20%, beyond which the film transforms from Ga-rich to Ga-poor and is accompanied with increased Al-Al metallic bonds. The film with the 20% AlN cycle ratio also exhibits the least aggregated surface morphology. The elemental depth profile investigated using secondary ion mass spectroscopy indicates an uniform distribution of Ga, Al and N. The bandgap of supercycle PEALD AlGaN films can be tuned when the AlN cycle ratio is less than 20%. Moreover, the AlGaN film with 20% AlN cycle ratio shows a photoluminescence peak centered at 277 nm, agreeing with the bandgap of the film.

Preparation of GaN/Porous silicon heterojunction photodetector by laser deposition technique

In this work, gallium nitride (GaN) thin film was deposited on porous silicon (PSi) substrate via a pulsed laser deposition route with a 355 nm laser wavelength, 900 mJ of laser energy, and various substrate temperatures raging from 200 to 400 °C. The structural and optical properties of GaN films as a function of substrate temperature are investigate. XRD studies reveal that the GaN films deposited on porous silicon are nanocrystalline with a hexagonal wurtzite structure along (002) plane. The photoluminescence emission peaks of the GaN/PSi prepared at 300 °C substrate temperature are located at 368 nm and 728 nm corresponding to energy gap of 3.36 eV and 1.7 eV, respectively. The GaN/PSi heterojunction photodetector prepared at 300 °C exhibits the maximum performance, with a responsivity of 29.03 AW−1, detectivity of 8.6 × 1012 Jones, and an external quantum efficiency of 97.2% at 370 nm. Similarly, at 575 nm, the responsivity is 19.86 AW−1, detectivity is 8.9 × 1012 Jones, and the external quantum efficiency is 50.89%. Furthermore, the photodetector prepared at a temperature of 300 °C demonstrates a switching characteristic where the rise time and fall time are measured to be 363 and 711 μs, respectively.

Deep ultraviolet–visible highly responsivity self-powered photodetector based on β-Ga2O3/GaN heterostructure

We have reported unique β-Ga2O3 nanoscale-strips (β-Ga2O3-NSS) morphology on epitaxial Gallium nitride film by thermal oxidation and fabricated heterostructure based self-powered photodetector (PD) operable from 230 nm to 800 nm (UVC to Vis) broad spectral range. The device demonstrates an ultrahigh responsivity of 5.8 × 103 mAW−1 under 266 nm illuminations at zero bias. The developed broadband photodetector at zero applied bias exhibits excellent responsivity of 2600, 56, and 450 mAW−1 under light illuminations of 355 nm, 455 nm, and 532 nm, respectively. In addition, the device also shows very high responsivity (1.6 × 105 mAW−1) with low noise equivalent power (10−13 WHz−1/2) and excellent external quantum efficiency (104%) under the bias of 5 V at 266 nm. The outstanding performance of the developed device is attributed to the light-trapping geometry of the β-Ga2O3-nanoscale-strips. The simple architecture of the device opens up new avenues for the fabrication of the next generation of Ga2O3-based broadband photodetection devices.

HiPIMS-grown AlN buffer for threading dislocation reduction in DC-magnetron sputtered GaN epifilm on sapphire substrate

Gallium nitride (GaN) epitaxial films on sapphire (Al2O3) substrates have been grown using reactive magnetron sputter epitaxy with a liquid Ga target. Threading dislocations density (TDD) of sputtered GaN films was reduced by using an inserted high-quality aluminum nitride (AlN) buffer layer grown by reactive high power impulse magnetron sputtering (R-HiPIMS) in a gas mixture of Ar and N2. After optimizing the Ar/N2 pressure ratio and deposition power, a high-quality AlN film exhibiting a narrow full-width at half-maximum (FWHM) value of the double-crystal x-ray rocking curve (DCXRC) of the AlN(0002) peak of 0.086° was obtained by R-HiPIMS. The mechanism giving rise the observed quality improvement is attributed to the enhancement of kinetic energy of the adatoms in the deposition process when operated in a transition mode. With the inserted HiPIMS-AlN as a buffer layer for direct current magnetron sputtering (DCMS) GaN growth, the FWHM values of GaN(0002) and (10 1‾ 1) XRC decrease from 0.321° to 0.087° and from 0.596° to 0.562°, compared to the direct growth of GaN on sapphire, respectively. An order of magnitude reduction from 2.7 × 109 cm−2 to 2.0 × 108 cm−2 of screw-type TDD calculated from the FWHM of the XRC data using the inserted HiPIMS-AlN buffer layer demonstrates the improvement of crystal quality of GaN. The result of TDD reduction using the HiPIMS-AlN buffer was also verified by weak beam dark-field (WBDF) cross-sectional transmission electron microscopy (TEM).

Etching conditions effect on anisotropic properties of sapphire

By patterning sapphire substrate (PSS) and then growing gallium nitride (GaN) film through wet etching process, the generation of dislocation and lattice defects of LED substrate can be significantly reduced, and the luminous efficiency and stability of the prepared led beads can be greatly improved. Due to the current anisotropic etching mechanism of sapphire has not been fully revealed, the etching evolution process and structure morphology cannot be accurately predicted and controlled, and ultimately it is difficult to ensure the structural quality of the patterned substrate. In this study, the etching rates of all crystal planes of sapphire in 98%H2SO4:85%H3PO4 = 3:1 etching solution were obtained through experiments, and the microstructure etching process and surface morphology were analyzed in detail. In addition, the influence of etching conditions on sapphire etching microstructure and surface morphology was also analyzed through experiments. The experimental results show that: 1) increasing the temperature can speed up the etching reaction, but it will make the surface quality worse; 2) Phosphoric acid as an etching buffer can improve the quality of etched structural planes; 3) The optimal etching rate and surface quality can be obtained by etching in 98%H2SO4:85%H3PO4 = 3:1 solution.

Improved electrical contact property of Si-doped GaN thin films deposited by PEALD with various growth cycle ratio of SiNx and GaN

Gallium Nitride (GaN) is becoming increasingly attractive due to its advantages in efficiency, switching speed, and high temperature operation. Although the performance of GaN in light-emitting devices and power electronics has been significantly improved, the need to reduce the power loss of GaN-based devices still exists. In this paper, Si derived from the SiNx sub-cycle in plasma-enhanced atomic layer deposition (PEALD) was used as a dopant to improve the ohmic contact resistance of GaN. The doping concentration of Si ranging from 0% to 33 at% were obtained by varying the SiNx sub-cycle ratio from 0% to 50%. A 13.56 at% Si-doped GaN film with moderate SiN bonding deposited at a SiNx sub-cycle ratio of 20% exhibits the highest carrier concentration of 1.29 × 1018 cm−3 and the lowest resistivity of 0.78 Ω·cm. The transmission line model (TLM) measurements show that the as-deposited Si-doped GaN has a specific contact resistance of 4.77 × 101 Ω·cm2, which decreases to 8.15 × 10−4 Ω·cm2 after annealing at 500 °C.

Comparative Analysis of Optoelectrical Performance in Laser Lift-Off Process for GaN-Based Green Micro-LED Arrays.

This work explores the pivotal role of laser lift-off (LLO) as a vital production process in facilitating the integration of Micro-LEDs into display modules. We specifically investigate the LLO process applied to high-performance gallium nitride (GaN)-based green Micro-LED arrays, featuring a pixel size of 20 × 38 μm on a patterned sapphire substrate (PSS). Scanning electron microscopy (SEM) observations demonstrate the preservation of the GaN film and sapphire substrate, with no discernible damage. We conduct a comprehensive analysis of the optoelectrical properties of the Micro-LEDs both before and after the LLO process, revealing significant enhancements in light output power (LOP) and external quantum efficiency (EQE). These improvements are attributed to more effective light extraction from the remaining patterns on the GaN backside surface. Furthermore, we examine the electroluminescence spectra of the Micro-LEDs under varying current conditions, revealing a slight change in peak wavelength and an approximate 10% decrease in the full width at half maximum (FWHM), indicating improved color purity. The current-voltage (I-V) curves obtained demonstrate the unchanged forward voltage at 2.17 V after the LLO process. Our findings emphasize the efficacy of LLO in optimizing the performance and color quality of Micro-LEDs, showcasing their potential for seamless integration into advanced display technologies.

Lattice Distortion and Piezoelectricity Enhancement in GaN by Alloying with Group III and Rare‐Earth Elements: A Comparative Experimental Study

The lattice of wurtzite nitrides such as gallium nitride (GaN) and aluminum nitride (AlN) is distorted by alloying with scandium (Sc). This lattice distortion, which entails a decrease in the lattice constant ratio c/a, leads to a significant improvement in piezoelectricity and the appearance of ferroelectricity, which are attractive properties for material applications. However, systematic studies on the effect of different alloying elements on the lattice distortion of wurtzite nitrides are scarce and restricted to theoretical investigations. Herein, the experimental investigation of the lattice distortion and its influence on the piezoelectricity of cosputtering‐prepared thin films of GaN alloyed with group III and rare‐earth elements other than Sc, that is, yttrium, dysprosium, and ytterbium is described. All the investigated elements greatly reduce the lattice constant ratio of GaN from 1.63 to 1.61 and cause a twofold increase in the piezoelectric charge coefficient. Therefore, it is indicated in the experimental results that alloying with elements other than Sc also results in the distortion of the crystal lattice and the enhancement of the piezoelectric properties of GaN. Furthermore, regardless of the particular alloying element used, a correlation between lattice distortion and piezoelectricity is experimentally demonstrated.

Decoupling thermal effects in GaN photodetectors for accurate measurement of ultraviolet intensity using deep neural network

Precise and reliable monitoring systems under various harsh conditions are important to realize an overall future automation system for diverse industries. In this study, a facile fabrication method for a gallium nitride (GaN)-based ultraviolet (UV) photodetector was developed as a system-on-chip decoupling system, and a novel decoupling system was proposed to separate the thermal effects from the GaN-based sensor using a deep neural network (DNN)-based regression model. To predict the true values of UV intensity, the currents of both UV- and thermal-responsive GaN film and thermal-responsive copper film were used as input features without additional measurement devices. In addition, the performance of the DNN regression model was evaluated using the mean absolute percentage error. Consequently, the proposed model showed a prediction accuracy of ∼97.86% with two input features. Furthermore, the thermal decoupling method using the proposed sensor was successfully demonstrated, even when the thermal conditions changed significantly. These results support a novel decoupling method for reliable real-time monitoring systems for various harsh environmental industries, such as aerospace, ocean, subterranean, power plants, and combustion engines.

Study on the Structure of GaN films deposited on MoS2/Sapphire via Plasma-Assisted Molecular Beam Epitaxy

The gallium nitride (GaN) films were grown on molybdenum disulfide (MoS2) layers via plasma-assisted molecular beam epitaxy (PA-MBE). The heterostructures of the GaN film were studied using reflection high-energy electron diffraction (RHEED) and HR-XRD. The heterostructures of GaN/MoS2/sapphire were revealed through cross-sectional transmission electron microscopy (TEM). The surface texture of the GaN films was analyzed using FE-SEM. Single-crystal heterostructure GaN films can be obtained on 2D MoS2/c-sapphire. The RHEED demonstrated spot patterns with high intensity showing the single crystal structure constructed in the GaN films. The GaN films on the surface exhibited a hexagonal structure. TEM images taken perpendicular to the surface revealed that, even after 60 minutes of epitaxial growth, the thickness of the GaN films remained consistent at approximately 4 nm. However, the 2D MoS2 layer was not observable in the images due to harm incurred during heteroepitaxial growth. Based on the surface structure, it was found that GaN films were successfully grown on the MoS2 layers using the PA-MBE system.