Shuguang Zhang

Synthesis of homogeneous and high-quality GaN films on Cu(111) substrates by pulsed laser deposition

GaN films were grown on Cu(111) substrates by growing an AlN buffer layer with an in-plane alignment of GaN[11−20]//AlN[11−20]//Cu[1−10] using pulsed laser deposition. It is found that by optimizing the laser rastering program and the epitaxial growth temperature, the thickness homogeneities, surface morphologies and structural properties of the GaN films can be greatly improved. Especially, the as-grown GaN films, grown at 750 °C with the optimized laser rastering program, exhibit excellent thickness uniformity with a root-mean-square (RMS) thickness inhomogeneity of less than 2.8%, and a very smooth and flat surface with a surface RMS roughness of 2.3 nm. The as-grown ~102 nm thick GaN films are almost fully relaxed with an in-plane compressive strain of only ~0.53%. No interfacial layer exists between the AlN buffer layer and the GaN film. Furthermore, with an increase in growth temperature from 550 to 750 °C, the surface morphologies and structural properties of the as-grown ~102 nm thick GaN films are improved significantly. The homogeneous and high-quality GaN films produced offer a broad prospect for future applications of GaN-based devices on Cu substrates.

Deposition of nonpolar m-plane InGaN/GaN multiple quantum wells on LiGaO2(100) substrates

High-quality nonpolar m-plane InGaN/GaN multiple quantum wells (MQWs) have been deposited on LiGaO2(100) substrates by the combination of pulsed laser deposition (PLD) and molecular beam epitaxy (MBE) technologies. This work opens up a new prospect for achieving high-efficiency nonpolar m-plane GaN-based devices.

Plasmon-enhanced ultraviolet photoluminescence from highly ordered ZnO nanorods/graphene hybrid structure decorated with Au nanospheres

Highly ordered and vertically aligned ZnO nanorods using an inverted self-assembled monolayer (SAM) template was fabricated via the hydrothermal process, and few-layer graphene was transferred on the surface of the ZnO nanorod arrays with 20 nm Au nanospheres coverage on the graphene surface. The band edge emission of the ZnO nanorods/graphene hybrid structures with Au nanospheres was increased by approximately three times and the defect-related emission was significantly reduced compared with the pristine ZnO nanorods arrays. The improved PL intensity can be attributed to the resonant coupling between the exciton emission of ZnO nanorods and plasmonic effects of graphene-Au structures. Our results will be promising for designing and fabricating ordered ZnO nanorod-based optical and optoelectronic devices.

Effect of InGaAs interlayer on the properties of GaAs grown on Si (111) substrate by molecular beam epitaxy

High-quality GaAs films have been epitaxially grown on Si (111) substrates by inserting an InxGa1−xAs interlayer with proper In composition by molecular beam epitaxy (MBE). The effect of InxGa1−xAs (0 < x < 0.2) interlayers on the properties of GaAs films grown on Si (111) substrates by MBE has been studied in detailed. Due to the high compressive strain between InGaAs and Si, InGaAs undergoes partial strain relaxation. Unstrained InGaAs has a larger lattice constant than GaAs. Therefore, a thin InGaAs layer with proper In composition may adopt a close lattice constant with that of GaAs, which is beneficial to the growth of high-quality GaAs epilayer on top. It is found that the proper In composition in InxGa1−xAs interlayer of 10% is beneficial to obtaining high-quality GaAs films, which, on the one hand, greatly compensates the misfit stress between GaAs film and Si substrate, and on the other hand, suppresses the formation of multiple twin during the heteroepitaxial growth of GaAs film. However, when t...

Achieving high-quality In0.3Ga0.7As films on GaAs substrates by low-temperature molecular beam epitaxy

The effects of the thickness of the large-mismatched amorphous In0.6Ga0.4As buffer layer on In0.3Ga0.7As epi-films grown on a GaAs substrate have been systematically investigated. The In0.3Ga0.7As films with the In0.6Ga0.4As buffer layer of 0, 1, 2, and 4 nm thickness are grown by low-temperature molecular beam epitaxy (LT-MBE) and are characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). It is found that the degree of relaxation and the crystallinity of the as-grown In0.3Ga0.7As films are strongly affected by the thickness of the amorphous In0.6Ga0.4As buffer layer. The thinner In0.6Ga0.4As buffer layer is not enough to efficiently release the misfit strain between the In0.3Ga0.7As epilayer and the GaAs substrate, while the thicker In0.6Ga0.4As buffer layer is unfavorable to trap the dislocations and prevent them from extending into the In0.3Ga0.7As epi-films. We have demonstrated that the amorphous In0.6Ga0.4As buffer layer with a thickness of 2 nm can advantageously prevent threading and misfit dislocations from propagating into the subsequent In0.3Ga0.7As epilayer and increase the degree of relaxation of the as-grown In0.3Ga0.7As, ultimately leading to a high-quality In0.3Ga0.7As film. Our novel buffer layer technology has triggered a simple but effective approach to grow high-crystallinity In0.3Ga0.7As epitaxial film and is favorable for fabrication of GaAs-based high-efficiency four-junction solar cells.

Effect of Si doping in barriers of InGaN/GaN multiple quantum wells on the performance of green light-emitting diodes

Green InGaN/GaN multiple quantum wells (MQWs) light-emitting diodes (LEDs) are of high In composition in well layers, a condition which results in large lattice mismatch between InGaN well layers and GaN barrier layers and hence boosts up the quantum-confined Stark effect (QCSE). Screening of the polarization electric fields within MQWs can effectively improve the internal quantum efficiency (IQE). This work investigates the effect of Si doping in barrier layers on the performance of green InGaN/GaN MQWs LEDs. It is revealed that a suitable doping density of Si (3.4 × 1016 cm−3) in barrier layers can promote the interfacial abruptness between the barrier and the well layers, reduce the In-composition fluctuation, and effectively screen the polarization electric fields. At the same time, suitable Si doping in barrier layers improves the lateral current spreading, alleviating the current crowding effect which is responsible for local heating and efficiency droop. However, overdoping of Si in barrier layers causes a series of negative effects, such as deteriorated interfacial quality and large current leakage. An effective approach for improving the performance of green LEDs is hence proposed.

Strain relaxation of the In0.53Ga0.47As epi-layer grown on a Si substrate using molecular beam epitaxy

In0.53Ga0.47As films were grown on a Si (111) substrate with two different InxGa1−xAs buffer layers using molecular beam epitaxy (MBE). The effect of buffer layer on the as-grown In0.53Ga0.47As epi-layers was investigated using X-ray diffraction (XRD), reciprocal space mapping (RSM), Raman and transmission electron microscopy (TEM). XRD results showed that the crystalline quality of the as-grown In0.53Ga0.47As epi-layer grown on the Si substrate, using a low-temperature In0.4Ga0.6As buffer layer with in situ annealing, was better than that grown using In0.2Ga0.8As/In0.4Ga0.6As buffer layers. Moreover, the misfit strain between the In0.53Ga0.47As epi-layers and the Si substrate was nearly completely released by inserting a single In0.4Ga0.6As buffer layer grown at 390 °C with in situ annealing at 560 °C. Specifically, the relaxation value of the In0.53Ga0.47As epi-layer with the single In0.4Ga0.6As buffer layer was 97.16%. The lattice mismatch strain of the In0.53Ga0.47As epi-layer was well confined to the In0.4Ga0.6As buffer layer, without being extended to the subsequently grown In0.53Ga0.47As epi-layer compared with its counterpart using the In0.2Ga0.8As/In0.4Ga0.6As buffer layers. The low-temperature In0.4Ga0.6As buffer layer shows a way to realize fully relaxed In0.53Ga0.47As films with a high crystalline quality on the Si substrate.

Growth of InN Nanowires with Uniform Diameter on Si(111) Substrates: Competition Between Migration and Desorption of In Atoms

The effects of the growth parameters on the uniformity and the aspect ratio of InN nanowires grown on Si(111) substrates have been studied systematically, and a modified quasi-equilibrium model is proposed. The growth temperature is of great importance for both the nucleation of the nanowires and the migration of In and N atoms, thus affecting the uniformity of the InN nanowires. In order to improve the uniformity of the InN nanowires, both traditional substrate nitridation and pre-In-droplet deposition have been implemented. It is found that the substrate nitridation is favorable for the nucleation of InN nanowires. However, the initial In atoms adhered to the substrate are insufficient to sustain the uniform growth of the InN nanowires. We have found that the initial In droplet on the substrate is not only advantageous for the nucleation of the InN nanowire, but also favorable for the In atom equilibrium between the initial In droplets and the direct In flux. Therefore, InN nanowires with a uniform aspect ratio and optimal diameter can be achieved. The results reported herein provide meaningful insights to understanding the growth kinetics during the InN nanowires growth, and open up great possibilities of developing high-performance group III-nitride-based devices.

The temperature dependence of atomic incorporation characteristics in growing GaInNAs films

We have systematically studied the temperature dependence of incorporation characteristics of nitrogen (N) and indium (In) in growing GaInNAs films. With the implementation of Monte-Carlo simulation, the low N adsorption energy (−0.10 eV) is demonstrated. To understand the atomic incorporation mechanism, temperature dependence of interactions between Group-III and V elements are subsequently discussed. We find that the In incorporation behaviors rather than that of N are more sensitive to the Tg, which can be experimentally verified by exploring the compositional modulation and structural changes of the GaInNAs films by means of high-resolution X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscope, and secondary ion mass spectroscopy.

Correlations among morphology, composition, and photoelectrochemical water splitting properties of InGaN nanorods grown by molecular beam epitaxy

The mechanism underlying the effect of growth condition on the morphology evolution of InGaN nanorods (NRs) has been systematically investigated. The increased Ga flux enhances both the axial and the radial growth at the growth stage. However, the changed Ga flux influences not only the growth but also the nucleation of InGaN NRs. At the nucleation stage, the increased Ga flux shortens the delay time for NR formation, and prolongs the growth stage for a fixed total growth time. Those two aspects result in the increase of NR diameter and height with the supplied Ga flux. In addition, the continuous nucleation is ended much earlier due to the accelerated saturation of substrate area with the increased Ga flux, resulting in a decreased final NR density. In addition to the morphology evolution with the Ga flux, the composition characteristic of InGaN NRs has been also studied. The In distribution of InGaN NRs depends critically on the NR diameter along the NR growth direction, and the NRs show a morphology-dependent In incorporation. Interestingly, the InGaN NRs discussed here show a radial Stark effect induced by the pinned Fermi level. The radial Stark effect shifts the absorption edge of the InGaN NRs toward longer wavelengths, makes the InGaN NRs attractive for photoelectrochemical water splitting applications. The photoelectrochemical measurements present a significant increase in the photocurrent with the increased total surface area of the InGaN NRs, which is due to the enhanced light absorption effects and the enlarged interfacial area of the semiconductor/electrolyte.