Hua Xuemei

Chemical mechanical polishing of freestanding GaN substrates

Chemical mechanical polishing (CMP) has been used to produce smooth and scratch-free surfaces for GaN. In the aqueous solution of KOH, GaN is subjected to etching. At the same time, all surface irregularities, including etch pyramids, roughness after mechanical polishing and so on will be removed by a polishing pad. The experiments had been performed under the condition of different abrasive particle sizes of the polishing pad. Also the polishing results for different polishing times are analyzed, and chemical mechanical polishing resulted in an average root mean square (RMS) surface roughness of 0.565 nm, as measured by atomic force microscopy.

Influence of Dry Etching Damage on the Internal Quantum Efficiency of Nanorod InGaN/GaN Multiple Quantum Wells

The influence of dry etching damage on the internal quantum efficiency of InGaN/GaN nanorod multiple quantum wells (MQWs) is studied. The samples were etched by inductively coupled plasma (ICP) etching via a self-assembled nickel nanomask, and examined by room-temperature photoluminescence measurement. The key parameters in the etching process are rf power and ICP power. The internal quantum efficiency of nanorod MQWs shows a 5.6 times decrease substantially with the rf power increasing from 3 W to 100 W. However, it is slightly influenced by the ICP power, which shows 30% variation over a wide ICP power range between 30 W and 600 W. Under the optimized etching condition, the internal quantum efficiency of nanorod MQWs can be 40% that of the as-grown MQW sample, and the external quantum efficiency of nanorod MQWs can be about 4 times that of the as-grown one.

Roles of V/III ratio and mixture degree in GaN growth: CFD and MD simulation study

To understand the mechanism of Gallium nitride (GaN) film growth is of great importance for their potential applications. In this paper, we investigate the growth behavior of the GaN film by combining computational fluid dynamics (CFD) and molecular dynamics (MD) simulations. Both of the simulations show that V/III mixture degree can have important impacts on the deposition behavior, and it is found that the more uniform the mixture is, the better the growth is. Besides, by using MD simulations, we illustrate the whole process of the GaN growth. Furthermore, we also find that the V/III ratio can affect the final roughness of the GaN film. When the V/III ratio is high, the surface of final GaN film is smooth. The present study provides insights into GaN growth from the macroscopic and microscopic views, which may provide some suggestions on better experimental GaN preparation.

Enhanced Light Output of InGaN-Based Light Emitting Diodes with Roughed p-Type GaN Surface by Using Ni Nanoporous Template

Roughened surfaces of light-emitting diodes (LEDs) provide substantial improvement in light extraction efficiency. By preparing the self-assemble nanoporous Ni template through rapid annealing of a thin Ni film, followed by a low damage dry etching process, a p-side-up LED with a roughened surface has been fabricated. Compared to a conventional LED with plane surface, the light output of LEDs with nanoporous p-GaN surface increases up to 71% and 36% at applied currents of 1 mA and 20 mA, respectively. Meanwhile, the electrical characteristics are not degraded obviously after surface roughening.

GaN hexagonal pyramids formed by a photo-assisted chemical etching method

A series of experiments were conducted to systematically study the effects of etching conditions on GaN by a convenient photo-assisted chemical (PAC) etching method. The solution concentration has an evident influence on the surface morphology of GaN and the optimal solution concentrations for GaN hexagonal pyramids have been identified. GaN with hexagonal pyramids have higher crystal quality and tensile strain relaxation compared with as-grown GaN. A detailed analysis about evolution of the size, density and optical property of GaN hexagonal pyramids is described as a function of light intensity. The intensity of photoluminescence spectra of GaN etched with hexagonal pyramids significantly increases compared to that of as-grown GaN due to multiple scattering events, high quality GaN with pyramids and the Bragg effect.

High-Efficiency InGaN/GaN Nanorod Arrays by Temperature Dependent Photoluminescence

We report on the photoluminescent characteristics of InGaN/GaN multiple quantum well (MQW) nanorod arrays with high internal quantum efficiency. The InGaN/GaN MQWs are grown by metalorganic chemical vapor deposition on c-plane sapphire substrates, and then the MQW nanorod arrays are fabricated by using inductively coupled plasma etching with self-assembled Ni nanoparticle mask with low-damage etching technique. The typical diameter of the nanorods is from 200 nm to 300 nm and the length is around 800 nm, which almost is dislocation free. At room temperature, an enhancement of 3.1 times in total integrated photoluminescence intensity is achieved from the MQW nanorod arrays, in comparison to that of the as-grown MQW structure. Based on the temperature-dependent photoluminescence measurements, the internal quantum efficiency of the nanorod structure is 59.2%, i.e., 1.75 times of as-grown MQW structure (33.8%). Therefore, the nanorod structure with a significant reduction of defects can be a very promising candidate for highly efficient light emitting devices.

The Formation and Characterization of GaN Hexagonal Pyramids

GaN with hexagonal pyramids is fabricated using the photo-assisted electroless chemical etching method. Defective areas of the GaN substrate are selectively etched in a mixed solution of KOH and K2S2O8 under ultraviolet illumination, producing submicron-sized pyramids. Hexagonal pyramids on the etched GaN with well-defined {101?1?} facets and very sharp tips are formed. High-resolution x-ray diffraction shows that etched GaN with pyramids has a higher crystal quality, and micro-Raman spectra reveal a tensile stress relaxation in GaN with pyramids compared with normal GaN. The cathodoluminescence intensity of GaN after etching is significantly increased by three times, which is attributed to the reduction in the internal reflection, high-quality GaN with pyramids and the Bragg effect.

Growth and properties of wide spectral white light emitting diodes

Wide spectral white light emitting diodes have been designed and grown on a sapphire substrate by using a metal-organic chemical vapor deposition system. Three quantum wells with blue-light-emitting, green-light-emitting and red-light-emitting structures were grown according to the design. The surface morphology of the film was observed by using atomic force microscopy. The films were characterized by their photoluminescence measurements. X-ray diffraction θ/2θ scan spectroscopy was carried out on the multi-quantum wells. The secondary fringes of the symmetric ω/2θ X-ray diffraction scan peaks indicate that the thicknesses and the alloy compositions of the individual quantum wells are repeatable throughout the active region. The room temperature photoluminescence spectra of the structures indicate that the white light emission of the multi-quantum wells is obtained. The light spectrum covers 400–700 nm, which is almost the whole visible light spectrum.

Synthesis and Growth Mechanism: A Novel Fishing Rod-Shaped GaN Nanorods

A novel fishing rod-shaped GaN nanorod is successfully fabricated through a new method by using the two-step growth technology. This growth method is applicable to continuous synthesis and is able to produce a large number of single-crystalline GaN nanorods with a relatively high purity and at a low cost. X-ray diffraction, scanning electron microscopy and high-resolution transmission electron microscopy are used to characterize the as-synthesized nanorods. The results show that most of the nanorods consist of a main rod and a top curved thread. It is single-crystal GaN with hexagonal wurtzite structure. The representative photoluminescence spectrum at room temperature exhibits a strong UV light emission band centered at 370.8 nm. Furthermore, a possible two-stage growth mechanism of the fishing rod-shaped GaN nanorod is also briefly discussed.

Growth and Properties of Blue and Amber Complex Light Emitting InGaN/GaN Multi-Quantum Wells

Blue-red complex light emitting InGaN/GaN multi-quantum well (MQW) structures are fabricated by metal organic chemical vapor deposition (MOCVD). The structures are grown on a 2-inch diameter (0001) oriented (c-face) sapphire substrate, which consists of an approximately 2-μm-thick GaN template and a five-period layer consisting of a 4.9-nm-thick In0.18Ga0.82N well layer and a GaN barrier layer. The surface morphology of the MQW structures is observed by an atomic force microscope (AFM), which indicates the presence of islands of several tens of nanometers in height on the surface. The high resolution x-ray diffraction (XRD) θ/2θ scan is carried out on the symmetric (0002) of the InGaN/GaN MQW structures. At least four order satellite peaks presented in the XRD spectrum indicate that the thickness and alloy compositions of the individual quantum wells are repeatable throughout the active region. Besides the 364 nm GaN band edge emission, two main emissions of blue and amber light from these MQWs are found, which possibly originate from the carrier recombinations in the InGaN/GaN QWs and InGaN quasi-quantum dots embedded in the QWs.