Fabrication of 4-Inch Nano Patterned Wafer with High Uniformity by Laser Interference Lithography

We report the fabrication of 4-inch nano patterned wafer by two-beam laser interference lithography and analyze the uniformity in detail. The profile of the dots array with a period of 800 nm divided into five regions is characterized by a scanning electron microscope. The average size in each region ranges from 270 nm to 320 nm,and the deviation is almost 4%, which is approaching the applicable value of 3% in the industrial process. We simulate the two-beam laser interference lithography system with MATLAB software and then calculate the distribution of light intensity around the 4 inch area. The experimental data fit very well with the calculated results. Analysis of the experimental data and calculated data indicates that laser beam quality and space filter play important roles in achieving a periodical nanoscale pattern with high uniformity and large area. There is the potential to obtain more practical applications.     

使用AlN/GaN超晶格势垒层生长高Al组分AlGaN/GaN HEMT结构

在蓝宝石衬底上生长了以AlN/GaN超晶格准AlGaN合金作为势垒的HEMT结构材料,并与传统AlGaN合金势垒样品进行了对比.在高Al组分(≥40%)情况下,超晶格势垒样品的表面形貌明显改进,电学性能特别是2DEG面电子浓度也有所改进.对超晶格势垒生长参数进行了初步优化,使得HEMT结构薄层电阻进一步降低,最后获得了251 Ω/□的薄层电阻. 关键词： AlGaN/GaN 结构 AlN/GaN超晶格 二维电子气 高电子迁移率晶体管     

Enhanced Photoluminescence of InGaN/GaN Green Light-Emitting Diodes Grown on Patterned Sapphire Substrate

Green InGaN/GaN based light-emitting diodes （LEDs） are fabricated both on planar and wet-etched patterned sapphire substrates by metalorganic vapour phase epitaxy （MOVPE）. Their photoluminescence （PL） properties of the two samples are studied. The results indicate that the PL integral intensity of the green LED on the patterned substrate is nearly two times of that on the planar one within the whole measured temperature range. The enhanced PL intensity in the green LED on the patterned substrate is shown completely contributed from the extraction efficiency, but not from the internal quantum efficiency. The conclusion is supported by temperature-dependent PL analysis on the two samples, and the mechanisms axe discussed.     

Characteristic improvements of thin film AlGaInP red light emitting diodes on a metallic substrate

We report a type of thin film Al Ga In P red light emitting diode(RLED) on a metallic substrate by electroplating copper(Cu) to eliminate the absorption of Ga As grown substrate.The fabrication of the thin film RLED is presented in detail.Almost no degradations of epilayers properties are observed after this substrate transferred process.Photoluminescence and electroluminescence are measured to investigate the luminous characteristics.The thin film RLED shows a significant enhancement of light output power(LOP) by improving the injection efficiency and light extraction efficiency compared with the reference RLED on the Ga As parent substrate.The LOPs are specifically enhanced by 73.5% and 142% at typical injections of 2 A/cm~2 and 35 A/cm~2 respectively from electroluminescence.Moreover,reduced forward voltages,stable peak wavelengths and full widths at half maximum are obtained with the injected current increasing.These characteristic improvements are due to the Cu substrate with great current spreading and the back reflection by bottom electrodes.The substrate transferred technology based on electroplating provides an optional way to prepare high-performance optoelectronic devices,especially for thin film types.     

Carrier transport in III–V quantum-dot structures for solar cells or photodetectors

According to the well-established light-to-electricity conversion theory,resonant excited carriers in the quantum dots will relax to the ground states and cannot escape from the quantum dots to form photocurrent,which have been observed in quantum dots without a p–n junction at an external bias.Here,we experimentally observed more than 88% of the resonantly excited photo carriers escaping from In As quantum dots embedded in a short-circuited p–n junction to form photocurrent.The phenomenon cannot be explained by thermionic emission,tunneling process,and intermediate-band theories.A new mechanism is suggested that the photo carriers escape directly from the quantum dots to form photocurrent rather than relax to the ground state of quantum dots induced by a p–n junction.The finding is important for understanding the low-dimensional semiconductor physics and applications in solar cells and photodiode detectors.     

A method to extend wavelength into middle-wavelength infrared based on InAsSb/(Al)GaSb interband transition quantum well infrared photodetector

We present a method to extend the operating wavelength of the interband transition quantum well photodetector from an extended short-wavelength infrared region to a middle-wavelength infrared region. In the modified In As Sb quantum well, Ga Sb is replaced with Al Sb/Al Ga Sb, the valence band of the barrier material is lowered, the first restricted energy level is higher than the valence band of the barrier material, the energy band structure forms type-II structure. The photocurrent spectrum manifest that the fabricated photodetector exhibits a response range from 1.9 μm to 3.2 μm with two peaks at 2.18 μm and 3.03 μm at 78 K.     

Characteristics of High In-Content InGaN Alloys Grown by MOCVD

InN and In0.46 Ca0.54N films are grown on sapphire with a CaN buffer by metalorganic chemical vapour deposition （MOCVD）. Both high resolution x-ray diffraction and high resolution transmission electron microscopy results reveal that these films have a hexagonal structure of single crystal. The thin InN film has a high mobility of 4 75 cm^2V^-1s^-1 and that oflno.46 Gao.54N is 163 cm^2 V^-1s^-1. Room-temperat ure photoluminescence measurement of the InN film shows a peak at 0.72eV, confirming that a high quality InN film is fabricated for applications to full spectrum solar cells.     

AlGaN/AlN/GaN结构中二维电子气的输运特性

对使用金属有机物汽相沉积法生长的AlGaN/AlN/GaN结构进行的变温霍尔测量，测量结果指出在AlN/GaN界面处有二维电子气存在且迁移率和浓度在2K时分别达到了1.4×10⁴cm²·V^-1·s^-1和9.3×10¹²cm^-2，且在200K到2K范围内二维电子气的浓度基本不变，变磁场霍尔测量发现只有一种载流子(电子)参与导电.在2K温度下，观察到量子霍尔效应，Shubnikov-de Haas (SdH) 振荡在磁场约为3T时出现，证明了此结构呈现了典型的二维电子气行为.通过实验数据对二维电子气散射过程的半定量分析，推出量子散射时间为0.23ps，比以往报道的AlGaN/GaN结构中的散射时间长，说明引入AlN层可以有效减小合金散射，进一步的推断分析发现低温下以小角度散射占主导地位.     

Direct observation of the carrier transport process in InGaN quantum wells with a pn-junction

A new mechanism of light-to-electricity conversion that uses InGaN/GaN QWs with a p-n junction is reported.According to the well established light-to-electricity conversion theory,quantum wells(QWs) cannot be used in solar cells and photodetectors because the photogenerated carriers in QWs usually relax to ground energy levels,owing to quantum confinement,and cannot form a photocurrent.We observe directly that more than 95% of the photoexcited carriers escape from InGaN/GaN QWs to generate a photocurrent,indicating that the thermionic emission and tunneling processes proposed previously cannot explain carriers escaping from QWs.We show that photoexcited carriers can escape directly from the QWs when the device is under working conditions.Our finding challenges the current theory and demonstrates a new prospect for developing highly efficient solar cells and photodetectors.     

Absorption Enhancement of Silicon Solar Cell in a Positive-Intrinsic-Negative Junction

Absorption coefficient is a physical parameter to describe electromagnetic energy absorption of materials, which is closely related to solar cells and photodetectors. We grow a series of positive-intrinsic-negative(PIN) structures on silicon wafer by a gas source molecule beam epitaxy system and the investigate the absorption coefficient through the photovoltaic processes in detail. It is found that the absorption coefficient is enhanced by one order and can be tuned greatly through the thickness of the intrinsic layer in the PIN structure, which is also demonstrated by the 730-nm-wavelength laser irradiation. These results cannot be explained by the traditional absorption theory.We speculate that there could be some uncovered mechanism in this system, which will inspire us to understand the absorption process further.