Y. D. Zheng

Metal–Semiconductor–Metal Ultraviolet Avalanche Photodiodes Fabricated on Bulk GaN Substrate

We report the first demonstration of a GaN-based planar metal-semiconductor-metal (MSM) ultraviolet (UV) avalanche photodiode (APD). The MSM APD with semitransparent interdigitated Schottky electrodes is fabricated on a low-defect-density GaN homoepitaxial layer grown on a bulk GaN substrate by metal-organic chemical vapor deposition. The dislocation density of the GaN homoepilayer characterized by a cathodoluminescence mapping technique is ~ 5 ×106 cm-2. The photodiode exhibits a low dark current density of ~ 1.4 × 10-9 A/cm2 and a high UV-to-visible rejection ratio up to five orders of magnitude under 20-V bias. At high bias, a room-temperature avalanche gain of more than 1100 is achieved under 365-nm UV illumination. The breakdown voltage of the APD shows a positive temperature coefficient of 0.15 V/K, confirming that the high-voltage gain is dominated by the avalanche breakdown mechanism.

Programmable Bidirectional Folding of Metallic Thin Films for 3D Chiral Optical Antennas

3D structures with characteristic lengths ranging from nanometer to micrometer scale often exhibit extraordinary optical properties, and have been becoming an extensively explored field for building new generation nanophotonic devices. Albeit a few methods have been developed for fabricating 3D optical structures, constructing 3D structures with nanometer accuracy, diversified materials, and perfect morphology is an extremely challenging task. This study presents a general 3D nanofabrication technique, the focused ion beam stress induced deformation process, which allows a programmable and accurate bidirectional folding (-70°-+90°) of various metal and dielectric thin films. Using this method, 3D helical optical antennas with different handedness, improved surface smoothness, and tunable geometries are fabricated, and the strong optical rotation effects of single helical antennas are demonstrated.

Ultrasensitive Detection of Phosphate Using Ion-Imprinted Polymer Functionalized AlInN/GaN High Electron Mobility Transistors

In this letter, a first study on phosphate detection based on AlInN/GaN high electron mobility transistors (HEMTs) is presented. The ungated regions of GaN HEMT-based sensors were functionalized with the phosphate ion-imprinted polymer and their sensing behaviors were analyzed by detecting different concentrations of phosphate solutions. The results show that the AlInN/GaN sensor exhibits an ultrasensitive response and a specific recognition to phosphate anion and reaches a detection limit below 0.02 mg/L level, which is much lower than the limited indicator level of 0.1 mg/L for the plankton growth, while the AlInN/GaN sensor shows a higher sensitivity to phosphate anion when compared with the AlGaN/GaN sensor. This ultra-high sensitivity is attributed to the use of thinner barrier layer in the AlInN/GaN heterostructure, which makes 2-D electron gas channel more sensitive to the change of surface charge.

Analysis of 270/290/330-nm AlGaN-Based Deep Ultraviolet Light-Emitting Diodes With Different Al Content in Quantum Wells and Barriers

The optical and electrical properties of 270/290/330-nm AlGaN-based deep ultraviolet (UV) light-emitting diodes (LEDs) with different Al content in quantum wells and barriers have been investigated systematically. Based on the experimental and numerical study, it is observed that the UV LEDs with longer wavelength and lower Al composition in AlGaN multiple quantum wells (MQWs) possess less dislocation density, higher light output power, and external quantum efficiency. Large ideality factors calculated from the I-V curves and simulated energy band profiles indicate that the current in the deep UV LEDs with high Al content is dominated by tunneling mechanism, which is attributed to the resulting potential drop in the active region caused by large polarization field in AlGaN MQWs.

Characteristics of GaN thin films by inductively coupled plasma etching with Cl2/BCl3 and Cl2/Ar

GaN thin films were etched by inductively coupled plasma (ICP). The effects of BCl3 and Ar with different Cl2 fraction are studied and compared. The ICP power and RF power are also altered to investigate the different effects by using Cl2/BCl3 or Cl2/Ar as etching gases. The etch rate and surface morphology of the etched surface are characterized by using surface profiler, scanning electron microscopy and atomic force microscopy. The root-mean-square roughness values are systematically compared. It is found that the etch rates of Cl2/Ar are higher than that of the Cl2/BCl3 in the Cl2 fraction ranging from 10 to 90%. When the ICP power is increased, the RMS roughness of GaN surface after ICP etching shows reverse trend between Cl2/BCl3 and Cl2/Ar gas mixture. The results indicate quite different features using Cl2/BCl3 and Cl2/Ar for GaN ICP etcing under the same conditions.

Investigation of structural and optical anisotropy of m-plane InN films grown on gamma-LiAlO(2)(100) by metal organic chemical vapour deposition

In this paper a systematic investigation of structural and optical anisotropy of m-plane InN film grown on γ -LiAlO2(1 0 0) substrate by metal organic chemical vapour deposition, which is believed to be much more difficult than molecular beam epitaxy, is presented. The InN film showed pure m-plane phase as confirmed by x-ray diffraction ω/2θ scan together with polarized Raman spectroscopy. The epitaxial relationship between the m-plane InN and the substrate was found to be [ 0001 ]InN � [0 1 0]LAO and [1 1 − 20 ]InN � [0 01 ]LAO. The inherent film mosaic anisotropy was studied by x-ray rocking curve analysis. Atomic force microscopy revealed that stripe-like features appear on the surface of the m-plane InN film, which originated from the replication of the surface morphological anisotropy of the substrate. Scanning electron microscopy showed blocky surface structures that were indicative of three-dimensional growth mode, which was related to the anisotropic structural mismatch of the film and the foreign substrate. A small direct band gap of ∼0. 7e V of them-plane InN film was confirmed by low-temperature photoluminescence spectra, which showed evident polarization anisotropy in terms of both intensity and peak energy. Finally, a blue-shift of absorption edge as compared with the photoluminescence peak energy was observed and fully accounted for by the Burstein–Moss effect. (Some figures in this article are in colour only in the electronic version)

Non-polar m-plane GaN film and polarized InGaN/GaN LED grown on LiAlO2 (001) substrates

The non-polar m-plane GaN film and polarized InGaN/GaN light-emitting diode (LED) grown by metal-organic chemical vapor deposition (MOCVD) on LiAlO2(100) substrates were investigated. Firstly, the simulation of excitonic transition energies and polarization effects on band structure of non-polar plane GaN was studied using the k· p Hamiltonian approach. Due to small lattice mismatch between GaN and LiAlO2 substrate, X-ray diffraction (XRD) revealed that the obtained m-plane GaN film has only [11-00] orientation with single-crystalline quality. In addition, anisotropic crystallographic properties and strain were found, which originates from the broken hexagonal symmetry. The anisotropic strain further separates the energy levels of top valence band at Γ point. The energy splitting as 37meV as well as in-plane polarization anisotropy for transitions are found by the polarized photoluminescence spectra at room temperature, which is consistent with our simulation. The fabricated InGaN/GaN LED on LiAlO2(100) emits green polarized light at room temperature. And the polarization degree of the emission reaches up to 60% at the wavelength of 520nm.

The Growth of Si on SiC Complex Substrate by CVD

In this work, the Si layer is deposited on the SiC complex substrate which is composed of Si(111) substrate and 3C-SiC film grown on it. These Si and 3C–SiC films grown under different temperatures in a chemical vapor deposition system are analyzed. The crystalline orientation, the crystalline quality and the conduction type of the films are measured by X-ray diffraction, Raman scattering ,Scanning electron microscope, and 1150 °C is found the optimized temperature for the epitaxial growth of SiC film grown on the carbonized layer. Measurement results also show that the epitaxial layer is n-type 3C-SiC which has the same crystalline orientation with the Si (111) substrate. Si film grown on the SiC complex substrate under the temperature of 690 °C has the best crystalline quality. This film is composed of p-type monocrystal Si and has the same crystalline orientation with the substrate.

An analyzing of anomalous peak in the capacitance-voltage characteristics at Hg/GaN Schottky contact

The frequency dependence of capacitance-voltage (C-V) characteristics of Hg/GaN and Hg/InGaN/GaN Schottky contacts are investigated for 1KHz, 10KHz and 1MHz at room temperature. An anomalous peak in the C-V curves of Hg/GaN sample is observed but no peak for the Hg/InGaN/GaN sample occurs. The interface states, series resistance and minority-carrier injection would be the origin of this anomalous peak. The interface states density is calculated through C-V measurement of high and low frequencies, and a four-element circuit model is proposed to determine the series resistance. In addition, energy bind structures are taken into account. The three aspects above are analyzed for both two samples, and the origin of the anomalous peak is attributed to the series resistance and minority-carrier injection rather than the interface states.

Composition of SiC layer grown on Si(111) substrate analyzed by plan-view energy dispersive spectroscopy

In this work, the composition distribution in SiC films grown on Si(111) using chemical vapor deposition (CVD) method has been measured by the plan-view energy dispersive spectroscopy (EDS). The measuring original EDS data are modified by considering the multilayer structure and the attenuation due to the diffuse reflection at the interface of the voids. The relative error rate of the improved EDS data is reduced by ∼ 45% comparing to the measurement result of X-ray photoelectron spectroscopy. Such a modification shows that the plan-view EDS method can be an effective way for characterizing element content of the SiC/Si structure in semi-quantitative measurement.