Effects of Si-doping on structural and electrical characteristics of polar,semi-polar,and non-polar AlGaN epi-layers

The effects of Si-doping on the structural and electrical properties of the wurtzite AlGaN epi-layers grown on polar, semi-polar, and non-polar sapphire substrates by metal organic chemical vapor deposition (MOCVD) were studied with X-ray diffraction, scanning electron microscopy, and hall effect measurement. The characterization results showed that both the surface morphology and the crystal quality of the polar AlGaN samples grown on a-plane sapphire substrates was improved with increasing Si concentration due to the Si-induced increase in dislocation movement. It was also found that the folds on the surfaces of the semi-polar and non-polar AlGaN samples grown on m- and r-plane sapphire substrates, respectively were significantly reduced in consequence of the growth suppression along c direction by Si-doping. Moreover, owing to the enhanced crystal quality, an increase in both the mobility and the carrier density for the polar AlGaN samples grown on a-plane sapphire substrates was achieved as the Si-doping level was increased. In addition, a relatively high electron concentration was obtained from the undoped semi-polar AlGaN samples grown on m-plane sapphire substrate, which is helpful to fabricate high quality semi-polar AlGaN-based ultra-violet light emitting diodes (UV-LEDs).     

Effect of Nitridation on the Regrowth Interface of AlGaN/GaN Structures Grown by Molecular Beam Epitaxy on GaN Templates

AlGaN/GaN structures were regrown on GaN templates using plasma- assisted molecular beam epitaxy (PA-MBE). Prior to the regrowth, nitridation was performed using nitrogen plasma in the MBE chamber for different durations (0?min to 30?min). Direct-current measurements on high-electron-mobility transistor devices showed that good pinch-off characteristics and good interdevice isolation were achieved for samples prepared with a 30-min nitridation process. Current?Cvoltage measurements on Schottky barrier diodes also revealed that, for samples prepared without nitridation, the reverse-bias gate leakage current was approximately two orders of magnitudes larger than that of samples prepared with a 30-min nitridation process. The improvement in the electrical properties is a result of contaminant removal at the regrowth interface which may be induced by the etching effect of nitridation.     

AlGaN中Mg掺杂对蓝光发光二极管光电性质的影响

采用金属有机化学气相沉积技术生长了GaN基多量子阱(MQW)蓝光发光二极管外延片,并采用高分辨率X射线衍射仪(HRXRD)和光致光谱仪(PL)表征晶体质量和光学性能,其他的光电性能由制成芯片后测试获得,目的是研究外延片p型AlGaN电子阻挡层Mg掺杂的优化条件.结果表明,在生长p型AlGaN电子阻挡层的Cp2Mg流量为300 cm3/min时,蓝光发光二极管获得最小正向电压VF,而且在此掺杂流量下的多量子阱蓝光发光二极管芯片发光强度明显高于其他流量的样品.因此可以通过优化AlGaN电子阻挡层的掺杂浓度,来显著提高多量子阱蓝光发光二极管的电学性能和光学性能.     

氮化对蓝宝石衬底上AlGaN/GaN异质结

研究了不同氮化条件对蓝宝石衬底上生长的AlGaN/GaN异质结材料特性的影响。研究表明,随着氮化过程NH₃流量的增大,GaN外延层的晶体质量得到了改善,GaN内应力释放,但是AlGaN/GaN异质结构中二维电子气的迁移率有所恶化。讨论了上述现象出现的原因。     

Diversity of Properties of Device Structures Based on Group-III Nitrides,Related to Modification of the Fractal-Percolation System

A fractal-percolation system that includes extended defects and random fluctuations in the alloy composition is formed during the growth of device structures based on Group-III nitrides. It is established that the specific features of this system are determined not only by the growth conditions. It is shown that the diversity of the electrical and optical properties of InGaN/GaN LEDs (light-emitting diodes) emitting at wavelengths of 450–460 and 519–530 nm, as well as that of the electrical properties of AlGaN/GaN HEMT (high-electron-mobility transistor) structures, is due to modification of the properties of the fractal-percolation system both during the growth process and under the action of the injection current and irradiation. The influence exerted by these specific features on the service life of light-emitting devices and on the reliability of AlGaN/GaN HEMT structures is discussed.

     

Mobility spectrum analysis of anisotropic electron transport in N-polar GaN/AlGaN heterostructures on vicinal sapphire substrates

In this work, we present results of a study of anisotropic two-dimensional electron gas (2DEG) transport in N-polar GaN/AlGaN heterostructures grown on slightly mis-oriented sapphire substrates. High-resolution mobility spectrum analysis of magnetic-field dependent Hall-effect and resistivity indicate an isotropic 2DEG sheet carrier density, yet significant anisotropy was observed in carrier mobility. A single electron species with a narrow mobility distribution was found to be responsible for conduction parallel to the multi-atomic steps resulting from growth on the vicinal substrates; whilst in the perpendicular direction two distinct electrons peaks are evident at T ? 150 K, which merge near room temperature. The linewidth of the mobility distributions for transport in the perpendicular direction was found to be significantly broader than that of the single electron in the parallel direction. The broader mobility distribution and the lower average mobility for the 2DEG in the perpendicular direction are attributable to interface roughness scattering associated with the GaN/AlGaN interfacial steps.     

Effect of Annealing on the Characteristics of Pd/Au Contacts to p-Type GaN/Al0.45Ga0.55N

The effect of annealing on the characteristics of Pd/Au contacts to p-type GaN/Al_0.45Ga_0.55N was investigated. The electrical characteristics of Pd/Au contacts and a p-GaN/AlGaN sample were measured after annealing at different temperatures from 550°C to 850°C. Changes in the surface electrical characteristic of p-GaN/AlGaN material were observed after each annealing step. It is indicated that the surface electrical characteristic of p-GaN/AlGaN material plays an important role in the current transport through Pd/Au pads. A possible reason for those changes is impurity contamination, most likely oxygen and carbon contamination introduced from processing. The microstructure of Pd on p-GaN/AlGaN was investigated by glancing-incidence x-ray diffraction (GXRD). The results of GXRD show that the AlPd₂ and GaPd₂ phases were formed at the interface after annealing, and the formation of these phases could be effective for forming ohmic contacts on p-GaN/AlGaN.     

基于选择性湿法粗化方法制作n型欧姆接触的垂直结构发光二极管的电学性能研究

低阻和高稳定性的氮极性面N型欧姆接触对于GaN基垂直结构发光二极管尤为重要。研究发现,相对于未湿法粗化N型GaN面,湿法粗化N型GaN面的欧姆接触电阻较大,且最终制成的发光二极管具有较高的开启电压和较低的反向漏电。本文提出了一种选择性湿法粗化方法制作氮极性面N型GaN欧姆接触,基于此方法最终制成的垂直结构发光二极管具有更低的开启电压和反向漏电,并且输出光功率略有提升。进一步的老化实验表明该法制作的欧姆接触具有良好的稳定性。     

Polarity dependent structure and optical properties of freestanding GaN layers grown by hydride vapor phase epitaxy

GaN freestanding substrate was obtained by hydride vapor phase epitaxy directly on sapphire with porous network interlayer. The morphologies of Ga-face and N-face of freestanding GaN substrate were analyzed by a variety of characterization techniques before and after etching in boiled KOH for 1 min. The obtained characteristics of unetched GaN are strongly dependent on the growth polarity. The N-polar GaN layer has high free electron, impurity and point defect concentrations. In the layers grown on the (0 0 0 1) Ga-polar side, these concentrations are very low. After etching, the Ga-polar GaN has identical properties to those of the unetched Ga-polar GaN layer. But the etched N-polar GaN has significant difference with unetched N-polar GaN layer in structure and optical properties. The etched N-polar GaN has a smaller (0 0 0 2) DCXRD width (646″) than the unetched N-polar GaN (1351″). The optical quality of etched N-polar GaN is comparable with that of Ga-polar GaN, and the FWHMs of the D⁰X line of Ga-face and etched N-face are 9.3 and 12.8 meV, respectively. The LPP mode in the Raman spectra and FERB peak in PL spectra were used to analyze the free carrier concentration of two sides of etched and unetched freestanding GaN substrate.     

A study on the improved performances of OLED using CMP process parameters determined by DOE method

In order to study the chemical-mechanical polishing (CMP) characteristics of indium-tin oxide (ITO) thin film with a sufficient removal amount and a good planarity, the optimal CMP process conditions were determined by using a design of experiment (DOE) approach. The electrical and optical properties, such as current-voltage (I-V) curve and photoluminescence spectrum, were discussed in order to evaluate the possibility of the CMP application for an organic light emitting diode (OLED) device using an ITO film. The electrical I-V characteristics and optical properties of ITO thin film were improved after the CMP process using optimized process conditions compared to that of as-deposited thin film before the CMP process.     

含有Al组分阶变AlGaN过渡层的Si基AlGaN/GaN HEMT

采用一个AlN缓冲层和两个Al组分阶变的AlGaN过渡层作为中间层,在76.2mm Si衬底上外延生长出1.7μm厚无裂纹AlGaN/GaN异质结材料,利用原子力显微镜、X射线衍射、Hall效应测量和CV测量等手段对材料的结构特性和电学性能进行了表征。材料表面平整光滑,晶体质量和电学性能良好,2DEG面密度为1.12×1013cm-2,迁移率为1 208cm2/(V.s)。由该材料研制的栅长为1μm的AlGaN/GaN HEMT器件,电流增益截止频率fT达到10.4GHz,这些结果表明组分阶变AlGaN过渡层技术可用于实现高性能Si基GaN HEMT。     

Effect of rapid thermally nitrided titanium films contacting silicided and nonsilicided junctions

The effect of rapid thermally nitrided titanium films contacting silicided (titanium disilicided) and nonsilicided junctions has been studied in the temperature range of 800 to 900°C. The rapid thermal nitridation of titanium films used as diffusion barriers between aluminum and silicon, has a major impact on shallow junction complementary metal oxide semiconductor technologies. During the process of rapid thermal nitridation, the dopants in the junctions undergo a redistribution and affect the electrical properties of shallow junction structures. This work focuses on using novel contact resistance structures to measure the variation in electrical parameters for rapid thermally nitrided titanium films annealed at different temperatures. The self-aligned silicide (salicide) junctions in this study were formed using rapid thermally annealed titanium films. Electrical contact resistance testers were used to measure the interface contact resistance between the salicide and silicon, as well as between the metal and the salicide. The results show that the interface contact resistance to the p⁻ diffused salicided junctions increases with rapid thermal nitridation of the additional titanium film, whereas the interface contact resistance to the n⁻ diffused salicided junction shows a decrease. Further, as a function of the rapid thermal annealing temperature (for fixed titanium thickness), the nonsalicided diffusions show an increase in the interface contact resistance. The boron profiles at the TiSi₂/Si interface obtained using secondary ion mass spectroscopy show an excellent qualitative agreement with the electrical results for each of the conditions discussed. The films were also characterized using Rutherford back-scattering spectrometry and transmission electron microscopy and the results show good agreement with the measured variation in electrical parameters. These results also show that as the anneal temperature is increased, the TiN thickness increases, further the change in the silicide/silicon interface position with the nitridation of the additional titanium layer was verified. This work was carried out when the author was working at AT&T Bell Labs     

Structural and electric properties of AlN substrates used for LED Heterostructures’ growth

The structural characteristics and electrical properties of bulk aluminum nitride crystals grown by sublimation and used as substrates for light emitting diode (LED) structures and AlGaN/GaN field effect transistors were studied. The crystalline perfection was assessed by selective chemical etching and by X-ray diffraction techniques. Electrical and optical properties were investigated using the temperature dependence of conductivity, admittance spectroscopy, high-temperature/low-frequency capacitance voltage measurements and by photoinduced transient current spectroscopy (PICTS), microcathodoluminescence (MCL) spectra and MCL imaging techniques. It was established that the studied samples were single crystals with a large grain substructure, with characteristic grain size of several hundred microns and a dislocation density of 10²–10⁴ cm^?2 inside the grains. The electrical characteristics of the crystals were governed by the compensation of residual donors with a level near E _c—0.3 eV by deep centers with activation energy of 0.7 eV, both centers manifesting themselves in the temperature dependence of conductivity and in admittance spectra. In addition, deep centers responsible for the luminescence band with the peak energy of 3.3 eV and associated with low-angle grain boundaries were also observed.     

p-AlGaN/GaN超晶格做p型层350 nm紫外AlGaN基LED

使用p-AlGaN/p-GaN SPSLs作为LED的p型层,在蓝宝石衬底上生长出发光波长为350 nm的AlGaN基紫外LED。[JP+1]由于AlGaN/GaN超晶格的极化效应,使得Mg受主的电离能降低,大幅提高了器件的光学和电学性能。在工作电流为350 mA下发光亮度达到了22.66 mW,相应的工作电压为3.75 V,LEDs的光功率满足了实际应用需求。     

RF-MBE生长的高Al势垒层AlGaN/GaN HEMT结构

采用RF-MBE技术,在蓝宝石衬底上生长了高Al组分势垒层AlGaN/GaN HEMT结构.用三晶X射线衍射分析得到AlGaN势垒层的Al组分约为43%,异质结构晶体质量较高,界面比较光滑.变温霍尔测量显示此结构具有良好的电学性能,室温时电子迁移率和电子浓度分别高达1246cm2/(V·s)和1.429×1013cm-2,二者的乘积为1.8×1016V-1·s-1.用此材料研制的器件,直流特性得到了提高,最大漏极输出电流为1.0A/mm,非本征跨导为218mS/mm.结果表明,提高AlGaN势垒层Al的组分有助于提高AlGaN/GaN HEMT结构材料的电学性能和器件性能.     

3英寸Si基碲镉汞分子束外延工艺研究

随着红外焦平面阵列规模的扩大,由于尺寸和成本的限制,传统晶格匹配的碲锌镉衬底逐渐成为碲镉汞红外焦平面探测器发展的瓶颈,大尺寸、低成本硅基碲镉汞材料应运而生。本文采用分子束外延工艺生长获得了3 in Si基中波碲镉汞薄膜材料,通过采用金相显微镜、傅里叶红外光谱仪、双晶X射线衍射仪、湿化学腐蚀位错密度(EPD)法、Hall测试系统等检测手段对Si基中波碲镉汞分子束外延薄膜材料进行表面、光学、结构和电学性能表征,并采用标准平面器件工艺制备中波640×512焦平面探测阵列进行材料验证,结果表明该材料性能与国际先进水平相当。     

Conductivity-type conversion in multiple-implant/multiple-annealSOI

Results of an investigation of the electrical properties of superficial silicon and epitaxial capping layers grown on multiple-implant/anneal SIMOX and zone melt recrystallization substrates are presented. An unexpected SIMOX conductivity type change (from n to p) was observed in the SIMOX superficial layer, as well as in subsequently grown epi-layers. It is believed that the conductivity-type change is related to the presence of a process-induced acceptor impurity or an impurity (oxygen)-vacancy complex     

Effective improvement of high-k Hf-silicate/silicon interface with thermal nitridation

The electrical properties of polysilicon gate MOS capacitors with hafnium silicate (HfSiO) dielectric, with and without NH/sub 3/ nitridation, were investigated. The results show that with NH/sub 3/ nitridation prior to deposition of HfSiO can effectively tune the flatband voltage close to that of conventional oxide and significantly improve the leakage properties over SiO/sub 2/ (three orders reduction). Furthermore, the excellent interface quality has been evidenced by the result of immunity against soft breakdown with NH/sub 3/ nitridation.     

Al_2O_3/AlGaN/GaN MOSH结构的制备和性能研究

采用激光脉冲沉积法(PLD)在AlGaN/GaN半导体异质结构衬底上沉积Al2O3栅介质层,并对该异质结构的电学性能进行研究。结果表明,Al2O3栅介质层改善了异质结构的界面质量,增强了器件结构的抗击穿电场强度。研究了沉积氧分压对异质结构性能的影响,电流-电压(I-V)测试结果表明,适当氧分压(0.1 Pa)有利于降低栅漏电流。Hall测量和电容-电压(C-V)模拟结果表明,不同的氧分压会改变Al2O3/AlGaN界面处的正电荷密度,从而改变半导体内的二维电子气(2DEG)密度。     

Investigations of GaN growth on the sapphire substrate by MOCVD method with different AlN buffer deposition temperatures

The effects of different AlN buffer deposition temperatures on the GaN material properties grown on sapphire substrate was investigated. At relatively higher AlN buffer growth temperature, the surface morphology of subsequent grown GaN layer was decorated with island-like structure and revealed the mixed-polarity characteristics. In addition, the density of screw TD and leakage current in the GaN film was also increased. The occurrence of mixed-polarity GaN material result could be from unintentional nitridation of the sapphire substrate by ammonia (NH₃) precursor at the beginning of the AlN buffer layer growth. By using two-step temperature growth process for the buffer layer, the unintentional nitridation could be effectively suppressed. The GaN film grown on this buffer layer exhibited a smooth surface, single polarity, high crystalline quality and high resistivity. AlGaN/GaN high electron-mobility transistor (HEMT) devices were also successfully fabricated by using the two-step AlN buffer layer.