Control of LPCVD ZnO growth modes for improved light trapping in thin film silicon solar cells

By controlling the deposition parameters of low pressure chemical vapor deposition (LPCVD) grown ZnO transparent conductive oxide (TCO), we show that it is possible to achieve a switch of crystallographic orientations and to obtain novel surface nanotexture. The new layers are (0 0 0 2) oriented and exhibit light scattering at larger angles than state-of-the-art TCO used in thin film silicon solar cells. This property translates into a strong enhancement of the top amorphous cell current in micromorph tandem devices.     

改善LTO膜均匀性

本文阐述了设备结构与LTO膜均匀性的关系,着重分析了气体分配器和笼罩舟的结构对改善LTO膜均匀性的作用,以及LTO优质膜的制备。     

Current Conduction and Dielectric Behavior of High k-Y2O3 Films Integrated with Si Using Chemical Vapor Deposition as a Gate Dielectric for Metal-Oxide-Semiconductor Devices

Integration of Y₂O₃ high-k thin film over Si as gate dielectric in high performance CMOS and high-density MOS memory storage capacitor devices is described. Y₂O₃ film growth by low-pressure chemical vapor deposition induces interfacial reactions and complex SiO_{2 – x} layer growth. It has a graded structure, in crystalline-SiO₂ form at Y₂O₃ side and amorphous SiO_{2 – x} form at Si side. MIS devices based on Y₂O₃/SiO₂-SiO_{2 – x} composite dielectric integrated with Si show high frequency C-V behavior indicative of inversion to accumulation changes in capacitance. Observed bi-directional hysterisis in C-V is detrimental to the functioning of storage capacitor in memory function. Detailed investigation of this effect led to understanding of gate bias controlled emission of carriers as responsible mechanism. Observed anomalous increase in inversion capacitance at low frequency is attributed to additional charges transferred from SiO_{2 – x}/Si interface states. Leakage current and injected charge carrier transport across bilayer interface is dominated by Poole-Frankel (PF) process at low fields and by Fowler-Nordhiem (FN) at high fields. This investigation provides a greater understanding of the complex nature of integration of Y₂O₃ films.     

Structural characteristics of as-deposited and crystallized mixed-phase silicon films

In this work, we report on the structural characteristics of as-deposited and crystallized mixed-phase silicon films prepared by thermal decomposition of silane in a low pressure chemical vapor deposition reactor. Mixed-phase films consist of crystallites embedded in an amorphous matrix. The size of these crystallites depends upon the surface diffusion length, a parameter quantitatively expressing the potential of adsorbed silicon atoms for surface diffusion. The density of the pre-existing crystallites can be related to the maximum density of critical nuclei, which develops during the deposition of the film. Both variables were quantitatively related to the deposition temperature and rate via physical models reflecting the experimental observations. Values for the parameters associated with these models were extracted by fitting the experimental data to the theoretical equations. Our theoretical analysis is the first to relate quantitatively the structural characteristics of as-deposited mixed-phase films to the prevailing deposition conditions. Mixed-phase films can crystallize in a much shorter time than as-deposited amorphous films, due to the combination of the growth of the pre-existing crystallites and the higher nucleation rate of new crystallites within the amorphous matrix of the mixed-phase film. The crystallization time and final grain size of crystallized mixed-phase films were found to decrease with increasing density of pre-existing crystallites. However, we showed that if composite films are deposited, consisting of a mixed-phase layer and an amorphous layer, the grain size after crystallization could be comparable to that of crystallized as-deposited amorphous films, with the crystallization time of such composite films about threefold shorter. The structure of both as-deposited and crystallized single and composite mixed-phase films was found to be identical for films deposited on both oxidized silicon and Corning Code 1735 glass substrates.     

The effect of geometrical misfit dislocation on formation of microstructure and photoluminescence of Wurtzite GaN/Al2O3 (0001) films grown by low pressure metal-organic chemical vapor deposition

The incoherent GaN/sapphire interface and microstructure of GaN were observed by high resolution transmission electron microscopy. The most mobile 60° mixed-type dislocation is related to a structural metastability of the Wurtzite GaN film. In spite of the same feature of interband absorption, the photoluminescence mechanism is sensitive to deep level. A strong light emission from the bound exciton of Wurtzite GaN at 358 nm was observed in an n-type GaN sample with the GaN buffer layer. The donor–acceptor pair recombination at 380 nm with LO phonon replicas at 390 and 403 nm and the deep level at 559 nm were observed in both an undoped GaN sample with GaN buffer layer and an n-type GaN sample with AlN buffer layer. This optical behavior is sensitive to the Si doping and the type of buffer layer materials. The deep level emission along the dislocation line is suggested by the local band bending model providing the potential barrier of 0.63 eV by the space charge.     

Hetero-epitaxial growth of SiCGe on SiC

SiCGe/SiC heterojunction structure is required in development of SiC optoelectronic devices and light-activated switching devices. We present in this paper a primary attempt to grow the ternary alloy SiCGe on SiC substrates under varied growth conditions in a conventional hot-wall CVD system. SiH₄, GeH₄ and C₃H₈ were employed as silicon, germanium and carbon sources, respectively. The samples were measured by means of SEM, XPS, XRD, optical absorption, etc. Two different growth modes i.e., island growth and porous growth were observed. It has been shown that germanium atoms can be effectively incorporated into the ternary alloys, which makes their optical gaps to be narrowed with increasing Ge content. However, incorporation of Ge induces a heavy lattice mismatch, which limits the film thickness by the formation of mismatch defects. It has been shown that use of a buffer layer can effectively improve the growth quality of the ternary alloy.     

Gate bias-dependent junction characteristics of silicon nanowires suspended between polysilicon electrodes

Abstract

Realistic integration of 1D materials into future nanodevices is limited by the lack of a manipulation process that allows a large number of nanowires to be arranged into an integrated circuit. In this work, we have grown Si nanowire bridges using a thin-film catalyst in a batch process at 200 °C and characterized the produced devices consisting of a p⁺-Si contact electrode, a suspended Si nanochannel, and a polysilicon contact electrode. Both the electrodes and connecting lines are made of Si-based materials by conventional low-pressure chemical vapor deposition. The results indicate that these devices can act as gate-controllable Schottky diodes in integrated nanocircuits.     

4H-SiC MESFET工艺中的高温氧化及介质淀积技术

采用自主开发的4H-SiC高温氧化技术,并结合低压化学气相淀积方法,在器件表面形成较为致密的氧化层,降低了器件的反向泄漏电流,提高了器件的击穿电压,同时也提高了器件的输出功率及功率增益,为器件长期稳定可靠工作奠定了工艺基础。采用此技术后,单胞20 mm左右栅宽器件在2 GHz脉冲条件下(脉冲宽度300μs,占空比10%)输出功率达78 W,比原工艺的器件输出功率提高了20 W以上,功率增益提高了1.5 dB,达到8.9 dB左右,功率附加效率也从23%提升到32%,初步显示了该工艺技术在制备4H-SiC微波功率器件中的优势。     

运用低温Purge降低LPCVD氮化硅工艺的微粒污染

LPCVD制备的氮化硅薄膜具有良好的阶梯覆盖性,但是这种薄膜的应力偏高,在石英工艺腔管使用到8μm后易形成剥落微粒。特别是半导体工艺发展到了亚微米阶段,这一问题对于产品良率的影响也越来越大。文中以垂直式LPCVD设备为研究对象,通过利用晶圆冷却时间,运用降低工艺腔体温度并使用N2Purge的方法来改善微粒状况。通过大量对比实验来得出最优温度设定。运用这种方式不但改善了LPCVD设备制备氮化硅膜的微粒状况,还延长了设备的维护周期,增加设备利用时间。