用MOVPE方法外延生长Si掺杂的立方相GaN

用MOVPE方法采取一种两步生长过程生长了未掺杂和Si掺杂的GaN.在生长了一个20nm厚的缓冲层后,外延生长了1μm厚的立方GaN外延层.利用二次离子质谱测定了掺杂的程度.并用X射线衍射和光致发光测量来表征了未掺杂和Si掺杂GaN的结构和光学质量.     

Chemical etchant dependence of surface structure and morphology on GaN(0001) substrates

We studied the procedure of cleaning GaN(0001) substrate surfaces by wet etching and subsequent annealing in ultrahigh vacuum for two different types of freestanding GaN wafers: hydride vapor phase epitaxy (HVPE) crystal and Na flux liquid phase epitaxy (LPE) crystal wafers. A flat surface containing GaN(0001)2 × 2 reconstruction was successfully achieved on both HVPE and LPE surfaces by etching in HF and subsequent annealing at ～ 550 °C but was not achieved by etching in HCl, NaOH, and HNO₃.     

Fabrication of freestanding GaN microstructures using AlN sacrificial layers

The fabrication of freestanding GaN microstructures using AlN sacrificial layers (SLs) is reported. GaN layers were grown by plasma assisted molecular beam epitaxy (PAMBE) on polycrystalline AlN sacrificial layers that have been deposited at 600 °C. Isotropic wet chemical etching of the AlN film released GaN microbridges and – cantilevers. The stress gradient and the compressive stress in the GaN‐film was extracted by analysis of the relation between beam geometry and displacement. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

GaAs(111)衬底上生长的GaN的极性与生长方法和生长条件的关系

研究了在GaAs(111)衬底上生长的六角相GaN的极性的相关关系.在高Ⅴ/Ⅲ比的条件下用MOVPE和MOMBE方法生长的GaN的极性和GaAs衬底的极性一致;在(111)A-Ga表面上的生长层呈现Ga的极性,而在(111)B-As表面上的生长层呈现N的极性.然而,在低的Ⅴ/Ⅲ比,或采用一个AIN中间层的条件下,用HVPE和MOMBE方法在GaAs(111)B表面上生长的GaN呈现出Ga的极性.目前,其原因尚不清楚,但是这些结果表明采用HVPE生长方法或用一高温AlN阻挡层可以得到高质量的GaN.     

Surface morphology of GaN nanorods grown by catalyst-free hydride vapor phase epitaxy

GaN nanorods were grown on c-plane sapphire substrates by using catalyst-free hydride vapor phase epitaxy (HVPE). The effects of substrate temperature, Ga boat temperature, and Ga pretreatment on the surface morphology of GaN nanorods were investigated. From the dependence of a radial and axial growth rate on the substrate temperature, the kinetically limited process was found to be a rate determining step in the growth of GaN nanorods in HVPE. In addition, the activation energy of the growth along the both axial and radial directions were estimated. The dependence of a Ga boat temperature and the Ga pretreatment effect revealed that the density of nanorods were dependent on the flux of Ga species on the substrate.     

GaN substrate and GaN homo-epitaxy for LEDs:Progress and challenges

After a brief review on the progresses in GaN substrates by ammonothermal method and Na-flux method and hydride vapor phase epitaxy(HVPE) technology,our research results of growing GaN thick layer by a gas flow-modulated HVPE,removing the GaN layer through an efficient self-separation process from sapphire substrate,and modifying the uniformity of multiple wafer growth are presented.The effects of surface morphology and defect behaviors on the GaN homo-epitaxial growth on free standing substrate are also discussed,and followed by the advances of LEDs on GaN substrates and prospects of their applications in solid state lighting.     

Properties of GaN on different polarity buffer layers by hydride vapour phase epitaxy

This paper reports on N-, mixed-, and Ga-polarity buffer layers are grown by molecular beam epitaxy (MBE) on sapphire (0001) substrates, with the GaN thicker films grown on the buffer layer with different polarity by hydride vapour epitaxy technique (HVPE). The surface morphology, structural and optical properties of these HVPE-GaN epilayers are characterized by wet chemical etching, scanning electron microscope, x-ray diffraction, and photoluminescence spectrum respectively. It finds that the N-polarity film is unstable against the higher growth temperature and wet chemical etching, while that of GaN polarity one is stable. The results indicate that the crystalline quality of HVPE-GaN epilayers depends on the polarity of buffer layers.     

Freestanding circular GaN grating fabricated by fast-atom beam etching

We report here a top-down process for fabricating a freestanding circular GaN grating. The circular gratings are defined by electron-beam lithography and realized by fast-atom beam (FAB) etching. The silicon substrate below the GaN grating region is completely removed to make the circular grating suspended in space. The optical responses of the fabricated GaN gratings are characterized in reflectance measurements. The polarization-independent responses of circular gratings are experimentally demonstrated, corresponding well with the theoretical prediction. This work represents an important step in combining GaN-based material with freestanding nanostructures.     

Air-gap embedding GaN template for enhanced emission from light-emitting diodes

Selective growth by metal-organic chemical vapor deposition (MOCVD), and electrochemical etching of a heavily Si-doped GaN (n⁺-GaN) interlayer were employed to obtain air-gaps embedded in a u-GaN layer. As confirmed by Raman spectroscopy, the introduction of an n⁺-GaN, which was later etched to obtain air-gaps, also enhanced the strain-compliance of GaN epilayer on sapphire substrate. An enhanced electroluminescence emission was observed from the light-emitting diodes (LEDs) fabricated on the air-gap embedding template. Using theoretical LED simulation, it was discerned that the increase in optical emission from the LED was caused predominantly by the redirection of photons at GaN/air-gap interface. Finite-difference time domain (FDTD) simulation method was employed to understand the mechanism of optical emission enhancement and its spatial variation over the LED surface.     

Reversed pyramids as novel optical micro-cavities

Pyramidal micro-cavities represent a novel promising class of semiconductor optical cavities. In contrast to our previous approach based on pyramids sitting on distributed Bragg reflectors, we investigate reversed freestanding GaAs pyramids. The latter are achievable by a wet-chemical etching process where an AlAs sacrificial layer in the epitaxially grown layer structure is used. In freestanding GaAs pyramids, light is simply confined by total internal reflection at the interface of the high refractive index material GaAs to the surrounding. Due to strong optical confinement within the pyramidal shape, small mode volumes are expected. Quality factors up to 3000 were measured in first structures. However, simulations suggest the possibility of much higher values. Therefore, these freestanding pyramids are promising for an optimized ratio between quality factor and mode volume, which is crucial for quantum-optical applications.     

Characterization of thick HVPE GaN films

The morphology of the top surface for HVPE GaN layers grown on a MOCVD GaN template with a thin LT-AlN interlayer was investigated. This surface is characterized by the formation of numerous hillocks associated with screw dislocations or nanopipes. Their size is large and may reach more than 1 mm. The rocking curves of the 002 and 102 reflections correspond to a relaxed layer. The HREM images of the as-deposited and annealed interlayers show a perfect atomic structure with a very abrupt AlN/HVPE GaN interface. Thus, the deposition of the LT-AlN layer has promoted the growth of an HVPE layer with an excellent crystalline quality.     

Effect of Reactor Pressure on Qualities of GaN Layers Grown by Hydride Vapour Phase Epitaxy

The influence of reactor pressure on GaN layers grown by hydride vapour phase epitaxy （HVPE） is investigated. By decreasing the reactor pressure from0. 7 to 0.5 mm, the GaN layer growth mode changes from the island-like one to the step flow. The improvements in structural and optical properties and surface morphology of GaN layers are observed in the step flow growth mode. The results clearly indicate that the reactor pressure, similarly to the growth temperature, is One of the important parameters to influence the qualities of GaN epilayers grown by HVPE, due to the change of growth mode.     

电致发光的完全悬空超薄硅衬底氮化镓基蓝光LED器件的制备与表征

为提升硅衬底氮化镓基LED(发光二极管)器件的光电性能和出光效率,本文提出了一种利用背后工艺实现的悬空薄膜蓝光LED器件。结合光刻工艺、深反应离子刻蚀和电感耦合等离子体反应离子刻蚀的背后工艺,制备了发光区域和大部分正负电极区域的硅衬底完全掏空,并减薄大部分氮化镓外延层的悬空薄膜LED器件。对悬空薄膜LED器件进行三维形貌表征,发现LED悬空薄膜表面平坦,变形程度小,证明背后工艺很好地解决了氮化镓外延层和硅衬底之间由于应力释放造成的薄膜变形问题。表征了LED器件的电流电压曲线和电致发光光谱等光电特性,对不同结构、不同发光区域尺寸的LED器件进行对比,发现悬空薄膜LED器件的光电性能和出光效率比普通LED器件更优越,且发光区尺寸变化对LED器件性能的影响更明显。在15 V驱动电压下,与普通LED器件相比,发光区直径为80μm的悬空LED器件的电流从4.3 mA提升至23.9 mA。在3 mA电流的驱动下,峰值光强提升了约5倍,而发光区直径为120μm的悬空器件与发光区直径为80μm的悬空器件相比,出光效率提升更为明显。本研究为发展高性能悬空氮化物薄膜LED器件提供了更多可能性。     

刻蚀AlN缓冲层对硅衬底N极性n-GaN表面粗化的影响

研究了等离子体刻蚀AlN缓冲层对硅衬底N极性n-GaN表面粗化行为的影响. 实验结果表明, 表面AlN缓冲层的状态对N极性n-GaN的粗化行为影响很大, 采用等离子体刻蚀去除一部分表面AlN缓冲层即可以有效提高N极性n-GaN在KOH溶液中的粗化效果, AlN缓冲层未经任何刻蚀处理的样品粗化速度过慢, 被刻蚀完全去除AlN缓冲层的样品容易出现粗化过头的现象. 经X射线光电子能谱分析可知, 等离子体刻蚀能够提高样品表面AlN缓冲层Al 2p的电子结合能, 使得样品表面费米能级向导带底靠近, 原子含量测试表明样品表面产生了大量的N空位, N空位提供电子, 使得材料表面费米能级升高, 这降低了KOH溶液和样品表面之间的肖特基势垒, 从而有利于表面粗化的进行. 通过等离子体刻蚀掉表面部分AlN缓冲层, 改善了N极性n-GaN在KOH溶液中的粗化效果, 明显提升了对应发光二级管器件的出光功率.     

Growth and doping of bulk GaN by hydride vapor phase epitaxy

Doping is essential in the growth of bulk GaN substrates,which could help control the electrical properties to meet the requirements of various types of GaN-based devices.The progresses in the growth of undoped,Si-doped,Ge-doped,Fedoped,and highly pure GaN by hydride vapor phase epitaxy(HVPE) are reviewed in this article.The growth technology and precursors of each type of doping are introduced.Besides,the influence of doping on the optical and electrical properties of GaN are presented in detail.Furthermore,the problems caused by doping,as well as the methods to solve them are also discussed.At last,highly pure GaN is briefly introduced,which points out a new way to realize high-purity semi-insulating(HPSI) GaN.     

Wavelength dispersive X-ray analysis and cathodoluminescence techniques for monitoring the chemical removal of AlInN on an N-face GaN surface

This paper shows the selective etching process of an AlInN sacrificial layer, lattice-matched to GaN, on N-face GaN by an aqueous solution of 1,2-diaminoethane. Using the wavelength dispersive X-ray (WDX) spectrometers on an electron probe micro-analyser, together with an optical spectrometer and silicon CCD array added to the light microscope, and sharing the same focus as the electron microscope, cathodoluminescence spectra are collected from exactly the same spot as sampled by the WDX spectrometers. This technique allows the compositional properties of the etched AlInN layer and the optical properties of the semiconductor layers underlying the sacrificial layer to be scrutinised, verifying the etching selectivity and the efficiency of the process.     

Size Control of Porous Silicon Nanoparticles by Electrochemical Perforation Etching

Size‐controlled porous silicon‐based nanoparticles are prepared by pulsed electrochemical etching of single crystal silicon wafers, followed by ultrasonic fracture of the freestanding porous layer. When high‐current density pulses are applied periodically during the porous layer etching process, a porous multilayer results in which porous layers are separated by thin layers of much higher porosity. Ultrasonic fracture selectively cleaves the porous film along these high‐porosity perforations, providing greater size control and improved yields (by 5x) of the resulting porous nanoparticles. The effect of pulse width and repetition rate is systematically studied: tunability of the average nanoparticle size in the range 160–350 nm is demonstrated.     

Effect of nanoporous GaN templates with different pore diameters on the subsequent thick GaN layers by HVPE

A set of GaN films were overgrown by hydride vapor phase epitaxy (HVPE) on nanoporous GaN templates with different pore diameters. These samples have various properties as seen from the measurements of X-ray diffraction (XRD) and photoluminescence (PL). Cross-sectional observations under a scanning electron microscopy (SEM) reveal that the overgrowth mechanism and process are strongly related to the dimension of nanopores, indicating that an optimum diameter exists for the properties of subsequent HVPE–GaN layers. When the diameters of nanopores are less than the optimum value, the pores on top of GaN templates can be left, and the properties of HVPE–GaN films show significant improvement. In contrast, the pores are almost stuffed with HVPE–GaN films, which obviously limit the improvement degree of HVPE–GaN films.     

DLTS study of n-type GaN grown by MOCVD on GaN substrates

Electron traps in n-type GaN layers grown homoepitaxially by MOCVD on free-standing GaN substrates have been characterized using DLTS for vertical Schottky diodes. Two free-standing HVPE GaN substrates (A and B), obtained from two different sources, are used. The Si-doped GaN layers with the thickness of 5 μm are grown on an area of 0.9×0.9 cm² of substrate A and on an area of 1×1 cm² of substrate B. Two traps labeled B1 and B2 are observed with trap B2 dominant in GaN on both substrates. There exist no dislocation-related traps which have been previously observed in MOCVD GaN on sapphire. This might be correlated to the reduction in dislocation density due to the homoepitaxial growth. However, it is found that there is a large variation, more than an order of magnitude, in trap B2 concentration and that the B2 spatial distributions are different between the two substrates used.     

一种外延生长高质量GaN薄膜的新方法

采用化学方法腐蚀c-面蓝宝石衬底，以形成一定的图案；利用LP-MOCVD在经过表面处理的蓝宝石衬底上以及常规c-面蓝宝石衬底上外延生长GaN薄膜.采用高分辨率双晶X射线衍射(DCXRD)、三维视频光学显微镜(OM)、扫描电子显微镜(SEM)和原子力显微镜(AFM)进行分析，结果表明，在经过表面处理形成一定图案的蓝宝石衬底上外延生长的GaN薄膜明显优于在常规蓝宝石衬底上外延生长的GaN薄膜，其(0002)面上的XRD FWHM为208.80弧秒，(1012)面上的为320.76弧秒.同时，此方法也克服了传统 关键词： 表面处理 MOCVD 横向外延生长 GaN薄膜