High-quality GaN Films Research Articles

Direct Growth of Wafer-Scale Self-Separated GaN on Reusable 2D Material Substrates.

Free-standing gallium nitride has been prepared using various methods; however, the removal of the original substrate is still challenging with low success rates. In this work, 2-inch free-standing GaN films are obtained by direct growth on a fluoro phlogopite mica by hydride vapor-phase epitaxy. Depending on the van der Waals (vdW) interaction between GaN and mica, the effect of the significant lattice mismatch is effectively reduced; thus, enabling the production of a high-quality wafer-scale GaN film on mica. The vdW-induced cracks at GaN-mica interface are found to be initiated near the interface so that GaN can easily separate from mica during rapid cooling. Owing to the hydrophilic nature of mica, the residual GaN on the mica can be lifted off by following deionized water treatment, and the mica substrate can be repeatedly used to grow free-standing GaN films. The self-separated GaN films grown on both pristine and used mica substrates are single crystallinity and strain-free. Additionally, a fully functional ultraviolet light-emitting diode is demonstrated to show that the self-separated GaN films are of device quality. The proposed approach achieves epitaxial growth of wafer-scale single-crystalline GaN on 2D materials and provides a new substrate option in the technology of III-V materials.

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.

Pulsed-Mode Metalorganic Vapor-Phase Epitaxy of GaN on Graphene-Coated c-Sapphire for Freestanding GaN Thin Films.

We report the growth of high-quality GaN epitaxial thin films on graphene-coated c-sapphire substrates using pulsed-mode metalorganic vapor-phase epitaxy, together with the fabrication of freestanding GaN films by simple mechanical exfoliation for transferable light-emitting diodes (LEDs). High-quality GaN films grown on the graphene-coated sapphire substrates were easily lifted off by using thermal release tape and transferred onto foreign substrates. Furthermore, we revealed that the pulsed operation of ammonia flow during GaN growth was a critical factor for the fabrication of high-quality freestanding GaN films. These films, exhibiting excellent single crystallinity, were utilized to fabricate transferable GaN LEDs by heteroepitaxially growing InxGa1-xN/GaN multiple quantum wells and a p-GaN layer on the GaN films, showing their potential application in advanced optoelectronic devices.

Evolutionary growth strategy of GaN on (1 1 1) diamond modulated by nano-patterned buffer engineering

GaN-on-diamond wafers are investigated to overcome the heat accumulation issues in GaN-based electronic devices. Until now, growing the high-quality GaN film on the diamond for device manufacture is still a great challenge. In this study, a new strategy of high-quality GaN on (111) diamond was proposed for the realization of high-performance GaN-on-diamond devices. The nano-patterned buffer engineering was utilized to greatly improved the GaN film on (111) diamond. The Al0.78Ga0.22N nano-patterned buffer reformed the morphology, ensuring that c orientation is the primary growth direction. The subsequent overgrowth of Al0.47Ga0.53N broadened the nano pillars and introduced stacking faults, improving the coalescence process and dislocations blockage. In addition, the coalescence process of the Al0.26Ga0.74N layer bent the treading dislocations, inducing the dislocations to annihilate. As a result, we could obtain remarkably high-quality GaN films on (111) diamond. The high-resolution X-ray diffraction rocking curve showed a full width at half maximum (FWHM) value of 770/986 arcsec at (002)/(102), which is the narrowest result reported. To the best of our knowledge, the RC FWHM at (102) of epitaxial GaN on diamond is reported firstly. Overall, this study provided a new strategy for obtaining high-quality GaN films on (111) diamonds for high-performance GaN-on-diamond devices.

Plasma-enhanced atomic layer deposition of crystalline GaN thin films on quartz substrates with sharp interfaces

Polycrystalline hexagonal GaN films were deposited directly on amorphous quartz (fused glass) substrates at 250 °C by plasma-enhanced atomic layer deposition. An atomically sharp GaN/quartz interface is observed from transmission electron microscopy images, which is further demonstrated by x-ray reflectivity measurements. The atomic force microscopy image reveals a smooth surface of GaN. The concentrations of oxygen and carbon impurities in GaN are 6.3 and 0.64%, respectively, according to x-ray photoelectron spectroscopy analysis. The electron mobility measured by Hall is 1.33 cm2 V−1 s−1. The results show that high-quality GaN films are obtained on amorphous quartz substrates, and GaN/quartz can be used as a template for the fabrication of GaN-based devices.

Wafer-scale heteroepitaxy GaN film free of high-density dislocation region with hexagonal 3D serpentine mask

GaN-based III-Nitride compound semiconductors are fundamental materials for high-performance optoelectronic and electronic devices. Low-defect-density substrate has been a major bottleneck in achieving high internal quantum efficiency and high breakdown voltage in those devices. However, a large-scale and uniform low dislocation density GaN film is still hard to obtain on foreign substrates, which is the mainstream economical method in scientific research and industrial production. Here, we present a 3D serpentine mask method of growing high-quality GaN film on foreign substrates without high dislocation density (HDD) areas by designing both sectional and planar shapes of stacked masks. The serpentine channel in the cross-section mechanically blocks the extended dislocations and the hexagonal planar pattern reduces the dimension of coalescence geometry. A wafer-scale GaN epilayer, free of HDD zone, with the overall threading dislocation density of 1.7 × 107 cm−2 estimated by plan-view cathode luminescence, is achieved in the metal-organic chemical vapor deposition system. Then, an array of light-emitting diodes based on this substrate with low forward voltages and high light output powers are fabricated, which introduces a practical method to provide high-quality GaN films for high-performance optoelectronic devices.

High quality GaN films on miscut (1 1 1) diamond substrates through non-c orientation suppression

The GaN films are grown on 4° miscut to (110) and (100) orientation (111) diamond substrates by metal organic chemical vapor deposition respectively. The ordinary (111) diamond substrate is used as reference. The morphology and crystal quality of the three GaN films are characterized by atomic force microscope, scanning electron microscope, Raman spectra, photoluminescence, and X-ray diffraction (XRD). The GaN epilayer on miscut to (110) orientation (111) diamond presents a superior quality with smooth surface, clear A1(LO) peak of Raman shift and XRD rocking curve with full width half maximum of 1357 arcsec, which is a remarkable result reported. Through the analysis, we found that when GaN grows on miscut to (110) orientation (111) diamond substrates, non-c orientation grain would be greatly suppressed so that c orientation buffers easier to grow laterally and merge into thin films. The non-c orientation suppression process greatly improves the quality of GaN film on (111) diamond.

氮掺杂单层石墨烯上无中间层沉积高质量氮化镓

GaN on graphene/Al2O3 substrates grown via van der Waals epitaxy compensates for the deficiencies and defects caused by metal-organic chemical vapor deposition (MOCVD) on substrates with significant mismatches to GaN. However, the absence of dangling bonds on graphene leads to insufficient nucleation sites; hence, a thin layer of AlN or ZnO nanowalls should be deposited on graphene as an intermediate layer. In this work, high-quality GaN crystals with a low biaxial compressive stress of 0.023 GPa and low screw dislocation density of 9.76 × 107 cm−2 were successfully synthesized by MOCVD on nitrogen-doped graphene without a buffer layer. First-principles calculations demonstrated significant improvement in the adsorption energy of the Ga atom on the surface of nitrogen-doped graphene compared with that of pristine graphene, in agreement with the experimental observations of nucleation. In most cases, GaN films were obtained by forming C—Ga—N and N—Ga—N configurations via atomic nitrogen pretreatment on monolayer graphene. Therefore, it is hoped that the efficient method of atomic modulation of high-quality GaN films grown on nitrogen-doped graphene via interface manipulation used in this work will promote the industrial development of innovative semiconductor devices.

Joint effect of miscut r-plane sapphire substrate and different nucleation layers on structural characteristics of non-polar a-plane GaN films

High-quality non-polar a-plane GaN films are achieved with optimized miscut r-plane sapphire substrate and nucleation layers.

N-polar GaN Film Epitaxy on Sapphire Substrate without Intentional Nitridation.

High-temperature nitridation is commonly thought of as a necessary process to obtain N-polar GaN films on a sapphire substrate. In this work, high-quality N-polar GaN films were grown on a vicinal sapphire substrate with a 100 nm high-temperature (HT) AlN buffer layer (high V/III ratio) and without an intentional nitriding process. The smallest X-ray full width at half maximum (FWHM) values of the (002)/(102) plane were 237/337 arcsec. On the contrary, N-polar GaN film with an intentional nitriding process had a lower crystal quality. In addition, we investigated the effect of different substrate treatments 1 min before the high-temperature AlN layer’s growth on the quality of the N-polar GaN films grown on different vicinal sapphire substrates.

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.

PECVD-prepared high-quality GaN films and their photoresponse properties

In this study, the high-quality GaN films are prepared by a simple, green and low-cost plasma enhanced chemical vapor deposition (PECVD) method at 950 ℃, with Ga<sub>2</sub>O<sub>3</sub> and N<sub>2</sub> serving as a gallium source and a nitrogen source, respectively. In order to improve the crystal quality of GaN films and ascertain the photoresponse mechanism of GaN films, the effect of the preparation temperature of GaN buffer layer on the crystal quality and photoelectric properties of GaN thin films are investigated. It is indicated that with the increase of the buffer temperature of GaN films, the crystal quality of GaN films first increases and then decreases, and the highest crystal quality is obtained at 875 ℃. When buffer layer temperature is 875 ℃, the calculated total dislocation density is 9.74 × 10<sup>9</sup> cm<sup>–2</sup>, and the carrier mobility is 0.713 cm<sup>2</sup>·V<sup>–1</sup>·s<sup>–1</sup>. The crystal quality of GaN film after being annealed is improved. The total dislocation density of GaN film decreases to 7.38 × 10<sup>9</sup> cm<sup>–2</sup>, and the carrier mobility increases to 43.5 cm<sup>2</sup>·V<sup>–1</sup>·s<sup>–1</sup>. The UV-Vis absorption spectrum results indicate that the optical band gap of GaN film is 3.35 eV. The scanning electron microscope (SEM) results indicate that GaN film (buffer layer temperature is 875 ℃) has smooth surface and compact structure. The Hall and X-ray photoelectron spectroscopy (XPS) results indicate that there are N vacancies, Ga vacancies or O doping in the GaN film, which act as deep level to capture photogenerated electrons and holes. With the bias increasing, the photoresponsivity of the GaN film photodetector gradually increases and then reaches a saturation value. This is due to the deep levels produced by vacancy or O doping. In addition, photocurrent response and recovery of GaN film are slow, which is also due to the deep levels formed by vacancy or O doping. At 5-V bias, the photoresponsivity of GaN film is 0.2 A/W, rise time is 15.4 s, and fall time is 24 s. Therefore, the high-quality GaN film prepared by the proposed green and low-cost PECVD method present a strong potential application in ultraviolet photodetector. The PECVD method developed by us provides a feasible way of preparing high-quality GaN films, and the understanding of the photoresponse mechanism of GaN films provides a theoretical basis for the wide application of GaN films.

Dislocation density control of GaN epitaxial film and its photodetector

At present, GaN-based ultraviolet photodetectors (UV PDs) is confronted with the challenge of high dark current and low responsivity. In this work, we designed and prepared high-performance GaN-based UV PDs on the sapphire substrates by the combination of low-temperature pulsed laser deposition (LT-PLD) and high-temperature metal-organic chemical vapor deposition growth (HT-MOCVD). The as-grown GaN film shows high-crystalline with edge dislocation density of 6.50 × 107 cm−2 and screw dislocation density of 1.92 × 108 cm−2, respectively. Furthermore, the optimized electrode structure has been well designed to enhance the carrier transport characteristics in GaN-based UV PDs, which improves the performance of GaN-based UV PDs accordingly. Based on the high-quality GaN film and the optimized electrode structure, high-performance GaN-based UV PDs are fabricated, which reveals the very high responsivity and small dark current of 1.4 A/W and 4.8 nA@5V, respectively. These high-performance GaN-based UV PDs shed light on the potential application in UV warning.

Ion beam sputtering-deposited thermally annealed h-BN transferred film for improving GaN crystal quality

In this article, we focus on the effect of high-temperature annealing (HTA) on ion beam-sputtered hexagonal boron nitride (h-BN)/sapphire samples with different thicknesses. Using atomic force microscopy (AFM), Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), ultraviolet–visible (UV–vis) spectroscopy, and other test methods, the changes in the properties of the h-BN material after HTA were compared and discussed in depth. The experimental results showed that HTA can reduce the B/N ratio and carbon in the h-BN film, increasing the film's optical bandgap. In addition, based on the annealed h-BN/sapphire template, high-quality GaN film epitaxy on h-BN was achieved by the metal-organic chemical vapor deposition (MOCVD) method. The method of HTA for h-BN grown by IBSD has huge potential for large size, low cost, and safe production.

High quality GaN-on-SiC with low thermal boundary resistance by employing an ultrathin AlGaN buffer layer

High quality GaN films on SiC with low thermal boundary resistance (TBR) are achieved by employing an ultrathin low Al content AlGaN buffer layer. Compared with the conventional thick AlN buffer layer, the ultrathin buffer layer can not only improve the crystal quality of the subsequent GaN layer but also reduce the TBR at the GaN/SiC interface simultaneously. The ultrathin AlGaN buffer layer is introduced by performing a pretreatment of the SiC substrate with trimethylaluminum followed by the growth of GaN with an enhanced lateral growth rate. The enhanced lateral growth rate contributes to the formation of basal plane stacking faults (BSFs) in the GaN layer, where the BSFs can significantly reduce the threading dislocation density. We reveal underling mechanisms of reducing TBR and dislocation density by the ultrathin buffer layer. We propose this work is of great importance toward the performance improvement and cost reduction of higher power GaN-on-SiC electronics.

Effect of Si Doping on the Performance of GaN Schottky Barrier Ultraviolet Photodetector Grown on Si Substrate

GaN Schottky barrier ultraviolet photodetectors with unintentionally doped GaN and lightly Si-doped n−-GaN absorption layers were successfully fabricated, respectively. The high-quality GaN films on the Si substrate both have a fairly low dislocation density and point defect concentration. More importantly, the effect of Si doping on the performance of the GaN-on-Si Schottky barrier ultraviolet photodetector was studied. It was found that light Si doping in the absorption layer can significantly increase the responsivity under reverse bias, which might be attributed to the persistent photoconductivity that originates from the lowering of the Schottky barrier height. In addition, the devices with unintentionally doped GaN demonstrated a relatively high-speed photo response. We briefly studied the mechanism of changes in Schottky barrier, dark current and the characteristic of response time.

A green, low-cost method to prepare GaN films by plasma enhanced chemical vapor deposition

In this study, the GaN films have been prepared by a green and low-cost plasma enhanced chemical vapor deposition (PECVD)method on Al2O3 substrate, along with Ga2O3 and N2 as gallium source and nitrogen sources, respectively. The results show that the oxygen content in the GaN films is significantly influenced by the reaction temperature and N2 flow rate. The uniform and high crystallinity GaN films were obtained at 950 °C with N2 flow rate 150 sccm, which was also proved by high- resolution transmission electron microscopy (HRTEM) analysis. It is found that the high energy nitrogen plasma and additive graphite play vital role in the growth of the high-quality GaN films; and the graphite, used as reductive agent, avoided the unfavorable effect caused by the hydrogen radicals, thus, contributing to the GaN nucleation. Moreover, the photoresponsivity of the GaN film was observed to be 0.0125 A/W by 365 nm laser. Therefore, the GaN nanofilms prepared by the proposed green and low-cost PECVD method present a strong potential of application in photoelectric devices, such as ultraviolet photodetector, light emitting diodes and epitaxial substrate for the photoelectric materials.

Influence of nitridation on III-nitride films grown on graphene/quartz substrates by plasma-assisted molecular beam epitaxy

Growing III-nitride semiconductors on non-single-crystalline substrates at low temperature is essential to realize low-cost and large-area GaN-based transferable and flexible devices. Two-dimensional (2D) layered materials such as graphene have the similar in-plane lattice arrangements compare with III-nitrides, hence III-nitrides can be grown on various non-single-crystal substrates through van der Waals epitaxy (vdWE) assisted by 2D layered material. Nevertheless, because of the low chemical reactivity of perfect graphene surface, it is hard to obtain high-quality GaN thin film with smooth surface without any artificial treatments at the initial stage of epitaxy growth. In this work, the influence of nitridation process on vdWE grown III-nitride films on graphene/quartz substrates has been investigated by using plasma-assisted molecular beam epitaxy. It is demonstrated that moderate intentional defects can be introduced into the surface of graphene/quartz substrate by N2 plasma treatments in the initial stages of vdWE, which is beneficial to the following nucleation process of III-nitrides. Besides, suitable nitridation conditions are essential in order to obtain high-quality GaN film on graphene/quartz substrates at low growth temperature. Such results can act as paradigms for vdWE of III-nitrides when using other 2D layered materials or non-single-crystalline substrates.

Construction of van der Waals substrates for largely mismatched heteroepitaxy systems using first principles

The high density of defects induced by large strains in largely mismatched heteroepitaxy systems, such as AlN and GaN, because of the large lattice and thermal mismatch between substrates and epilayers is the bottleneck problem. Graphene-assisted van der Waals (vdW) heteroepitaxy offers a new opportunity to resolve this problem. However, it suffers from the difficulty of nucleation. Here we theoretically assess the effects of five 2D materials for vdW heteroepitaxy of AlN and GaN, including graphene, hBN, MoS2, gC3N, and gC3N4, and provide physical insights using first-principle calculations. MoS2 and gC3N exhibit significant potential to overcome the shortcomings of graphene owing to their appropriate binding strengths and Al (or Ga) diffusion barriers. Moreover, the interface behavior between the epilayers and the substrates are carefully analyzed. Our findings are helpful not only for obtaining high-quality AlN and GaN films but also for developing new criterions to discover effective 2D materials for vdW heteroepitaxy.

Interfacial Tailoring for the Suppression of Impurities in GaN by In Situ Plasma Pretreatment via Atomic Layer Deposition.

A method for suppressing impurities in GaN thin films grown via plasma-enhanced atomic deposition (PEALD) through the in situ pretreatment of Si (100) substrate with plasma was developed. This approach leads to a superior GaN/Si (100) interface. After pretreatment, the thickness of the interfacial layer between GaN films and the substrates decreases from 2.0 to 1.6 nm, and the oxygen impurity content at the GaN/Si (100) interface reduces from 34 to 12%. The pretreated GaN films exhibit thinner amorphous transition GaN layer of 5.3 nm in comparison with those nonpretreated of 18.0 nm, which indicates the improvement of crystallinity of GaN. High-quality GaN films with enhanced density are obtained because of the pretreatment. This promising approach is considered to facilitate the growth of high-quality thin films via PEALD.