Interface states and impurities in MOS structures with very thin tunneling barriers

Electron tunneling spectroscopy was used to investigate MOS junctions with very thin silicon oxide or silicon oxynitride layers (2–5 nm) as tunneling barriers. For the tunneling measurements at 4.2 K highly degenerate P-doped (3×10²⁰ cm^-3) Si substrates, oxidized in dry oxygen at 600°C were used. Silicon oxynitride layers were prepared by plasma nitridation in an NH₃ discharge. As gate electrodes evaporated films of Al, Au or Pb were utilized. Changes in the tunneling conductivity were attributed to changes in the density of interface states, caused by hydrogen annealing or by high field stress. The results indicate a correlation between the generation of interface states and the removal of Si-H configurations. Vibrational modes of phonons, dopants and impurities were detected by inelastic electron tunneling spectroscopy.     

电离辐射对部分耗尽绝缘体上硅器件低频噪声特性的影响

本文针对辐射前后部分耗尽结构绝缘体上硅(SOI)器件的电学特性与低频噪声特性开展试验研究. 受辐射诱生埋氧化层固定电荷与界面态的影响, 当辐射总剂量达到1 M rad(Si) (1 rad = 10^-2 Gy)条件下, SOI器件背栅阈值电压从44.72 V 减小至12.88 V、表面电子有效迁移率从473.7 cm²/V·s降低至419.8 cm²/V· s、亚阈斜率从2.47 V/dec增加至3.93 V/dec; 基于辐射前后亚阈斜率及阈值电压的变化, 可提取得到辐射诱生界面态与氧化层固定电荷密度分别为5.33×10¹¹ cm^{- 2}与2.36×10¹² cm^-2. 受辐射在埋氧化层-硅界面处诱生边界陷阱、氧化层固定电荷与界面态的影响, 辐射后埋氧化层-硅界面处电子被陷阱俘获/释放的行为加剧, 造成SOI 器件背栅平带电压噪声功率谱密度由7×10^{- 10} V²·Hz^-1增加至1.8×10^-9 V² ·Hz^-1; 基于载流子数随机涨落模型可提取得到辐射前后SOI器件埋氧化层界面附近缺陷态密度之和约为1.42×10¹⁷ cm^-3·eV^-1和3.66×10¹⁷ cm^-3·eV^-1. 考虑隧穿削弱因子、隧穿距离与时间常数之间关系, 本文计算得到辐射前后埋氧化层内陷阱电荷密度随空间分布的变化.     

Optical and compositional studies of buried oxide layers in silicon formed by high dose implantation

Samples of standard SIMOX material, dose 2.4×10¹⁸/cm² at 200 keV have been examined in the as-implanted and annealed condition. The samples were prepared by wet-chemical etching with the silicon overlayer being removed first. Samples were then etched to remove the buried oxide, revealing and successively exposing the interface. These samples were then examined by ellipsometry and SIMS with an emphasis on the optical properties and compositional nature of the interfaces. Ellipsometry results indicate that a final layer of the buried oxide with refractive index 2.5 to 3 and thickness 150 to 250 Å is very resistant to the oxide etch. This may be correlated with the compositional profile of the interfacial region obtained by SIMS. The interface between the buried oxide and bulk silicon includes two layers, the first of silicon with SiO₂ precipitates and the second of entirely SiO₂.     

Real-time monitoring of SiO2/Si(1 1 1) interlayer etching by Brewster-angle reflectometry

A transitory etching regime after SiO₂ dissolution and before bulk Si(1 1 1) etching in neutral NH₄F solutions was monitored by in situ Brewster-angle reflectometry (BAR). An observed intermediate increase of the BAR reflectance signal is attributed to a fast dissolution of a stressed/strained interlayer beneath the SiO₂/Si(1 1 1) interface. Similar effects were observed on thin thermal oxides (18.2 nm), grown on float zone silicon, as well as on ultra-thin native oxides (1.2 nm) on Czochralsky silicon. Native oxide covered samples showed an increased surface roughness in the course of interlayer dissolution while the surface is progressively covered with compounds of fluorinated silicon. The etch rate, determined by atomic force microscopy (AFM) and compared to the etch rate of bulk silicon, is increased by a factor of four. In the limit of extended etching, the known low etch rates for silicon in 40% NH₄F are observed. Structural and chemical properties of the interfacial layer were analyzed by synchrotron radiation photoelectron spectroscopy (SRPES) which confirmed the presence of Si^3+/4+ valence states throughout the interlayer and by near open-circuit potential (N-OCP) dark current measurements. As a result, oxide etch rates in NH₄F in the pH-range 7–8 as well as the silicon interlayer depth can be assessed by in situ BAR.     

势垒硅掺杂对GaN基LED极化电场及其光电性能的影响

势垒硅掺杂对InGaN量子阱中的电场及LED器件的光电性能有着重要的影响。采用6×6 K·P方法计算了不同势垒硅掺杂浓度对量子阱中电场的变化,研究表明当势垒硅掺杂浓度>1e¹⁸ cm^-3时,阱垒界面处的电场强度会变大,这主要是由于硅掺杂浓度过高导致量子阱中界面电荷的聚集。进一步发现随着势垒掺杂浓度的升高,总非辐射复合随之增加,其中俄歇复合增加,而肖克莱-霍尔-里德复合随之减少,这是由于点陷阱的增大形成了缺陷能级。电流电压曲线表明势垒掺杂可有效改善GaN基LED的工作电压,这归于掺杂浓度的提高改善了载流子的传输特性。当掺杂浓度为1e¹⁸ cm^-3时,获得了较高的内量子效率,这主要是由于适当的势垒掺杂降低了量子阱中界面电荷的损耗。     

Microstructural analysis of hard amorphous carbon films deposited with high-energy ion beams

Hard amorphous carbon films produced using high-energy (ca. 30 keV) ion beam deposition of CH₃⁺ and CH₄⁺ on silicon wafers, have been investigated by Positron Annihilation Spectroscopy (PAS), the results are correlated with Raman Spectroscopy and Electrical Resistivity measurements. The microstructural modifications of the films as a function of the annealing temperature in the 300–600°C range have been studied. The evolution of the fractions of sp² and sp³ bonds is described and related to the changes of the open volume defect distribution and the graphitization process.     

UHV-MOCVD growth of TiO2 on SiOx/Si(1 1 1): Interfacial properties reflected in the Si 2p photoemission spectra

Metal–organic chemical vapour deposition growth of titanium oxide on moderately pre-oxidised Si(1 1 1) using the titanium(IV) isopropoxide precursor has been studied for two different growth modes, reaction-limited growth at 300 °C and flux-limited growth at 500 °C. The interfacial properties have been characterized by monitoring synchrotron radiation excited Si 2p photoemission spectra. The cross-linking from oxidised Si to bulk Si after TTIP exposure has been found to be very similar to that of SiO_x/Si(1 1 1). However, the results show that the additional oxidation of Si most probably causes a corrugation of the SiO_x/Si interface. Those conclusions are valid for both growth modes. A model is introduced in which the amorphous interface region is described as (TiO₂)_x(SiO₂)_y where x and y changes linearly and continuously over the interface. The model quantifies how (TiO₂)_x(SiO₂)_y mixing changes the relative intensities of the signals from silicon oxide and silicon. The method can be generalised and used for the analyses of other metal-oxides on silicon.     

A quantum chemical study of ZrO2 atomic layer deposition growth reactions on the SiO2 surface

Zirconium oxide (ZrO₂) is one of the leading candidates to replace silicon oxide (SiO₂) as the gate dielectric for future generation metal-oxide-semiconductor (MOS) based nanoelectronic devices. Experimental studies have shown that a 1–3 monolayer SiO₂ film between the high permittivity metal oxide and the substrate silicon is needed to minimize electrical degradation. This study uses density functional theory (DFT) to investigate the initial growth reactions of ZrO₂ on hydroxylated SiO₂ by atomic layer deposition (ALD). The reactants investigated in this study are zirconium tetrachloride (ZrCl₄) and water (H₂O). Exchange reaction mechanisms for the two reaction half-cycles were investigated. For the first half-reaction, reaction of gaseous ZrCl₄ with the hydroxylated SiO₂ surface was studied. Upon adsorption, ZrCl₄ forms a stable intermediate complex with the surface SiO₂–OH^* site, followed by formation of SiO₂–O–Zr–Cl^* surface sites and HCl. For the second half-reaction, reaction of H₂O on SiO₂–O–Zr–Cl^* surface sites was investigated. The reaction pathway is analogous to that of the first half-reaction; water first forms a stable intermediate complex followed by evolution of HCl through combination of a Cl atom from the surface site and an H atom from H₂O. The results reveal that the stable intermediate complexes formed in both half-reactions can lead to a slow film growth rate unless process parameters are adjusted to lower the stability of the complex. The energetics of the two half-reactions are similar to those of ZrO₂ ALD on ZrO₂ and as well as the energetics of ZrO₂ ALD on hydroxylated silicon. The energetics of the growth reactions with two surface hydroxyl sites are also described.     

Luminescence from porous silicon doped with erbium–ytterbium complexes

The study of photoluminescence (PL) from porous silicon (PS) containing complexes of gadolinium oxychloride with Er³⁺- and Er³⁺–Yb³⁺ is reported. The concentration dependencies of PL intensity of PS with Er³⁺ containing complex have been studied. The dependencies have retained the main features that are characteristic of the pure complex for both IR and visible regions of the PL spectra. This allows interpretation of PL processes in complex-containing PS lthrough the concept of multiplication of low-energy electron excitations and cross-relaxation degradation of higher excited states. It has been shown that introducing Yb³⁺ ions into the complex significantly increases the PL intensity. Mechanisms associated with defect formation, the intrinsic conversion of excitation energy within Yb³⁺, and the conversion within Er³⁺ ions followed by transferring of excitation energy to the Yb³⁺ ions has been considered. The PL polarization with excitation in the visible is reported as well.     

硅—二氧化硅界面过渡区的XPS研究

本文报道了用X射线光电子能谱的角度效应研究硅-二氧化硅界面过渡区的结果。样品为(111)取向的硅单晶片上低温(700℃)氧化生成的超薄氧化膜,膜的厚度不大于50?。氧化膜与单晶衬底中Si_2p光电子谱峰之间的化学位移(δ)和强度比(I_0x/I_si)随光电子发射角(θ)的变化明显地偏离理想界面所预期的结果,表明在硅-二氧化硅的界面处存在化学比为SiO_x(0yO_4-y(0≤y≤4)型四面体。比较实验曲线与随机成键模型的计算结果,估计出过渡区的宽度不大于20?,小于Si_2p光电子在二氧化硅中的平均逃逸深度。对改变氧化时间结合Ar⁺刻蚀制得的氧化膜厚度不同的样品所作的测量,得到与角度实验相一致的结果。 关键词：     

氢化非晶硅薄膜晶体管的低频噪声特性

针对氢化非晶硅薄膜晶体管(hydrogenated amorphous silicon thin film transistor,a-Si:H TFT)的低频噪声特性展开实验研究.由测量结果可知,a-Si:H TFT的低频噪声特性遵循1/f~γ(f为频率,γ≈0.92)的变化规律,主要受迁移率随机涨落效应的影响.基于与迁移率涨落相关的载流子数随机涨落模型(?N-?μ模型),在考虑源漏接触电阻、局域态俘获及释放载流子效应等情况时,对器件低频噪声特性随沟道电流的变化进行分析与拟合.基于a-Si:H TFT的亚阈区电流-电压特性提取器件表面能带弯曲量与栅源电压之间的关系,通过沟道电流噪声功率谱密度提取a-Si:H TFT有源层内局域态密度及其分布.实验结果表明:局域态在禁带内随能量呈e指数变化,两种缺陷态在导带底密度分别约为6.31×10~(18)和1.26×10~(18)cm~(-3)·eV~(-1),特征温度分别约为192和290 K,这符合非晶硅层内带尾态密度及其分布特征.最后提取器件的平均Hooge因子,为评价非晶硅材料及其稳定性提供参考.     

XPS study of residual oxide layers on p-GaAs surfaces

The total thickness and composition of a residual oxide layer after chemical etching of p-GaAs:Zn + In has been studied by X-ray photoelectron spectroscopy (XPS). The variation of the Ga to As oxides ratio along the depth has been determined. A concentration correlation of doping isovalent impurity and the dislocation density with the composition of residual oxides is looked for. The total thickness of the residual oxide layer on p- and semi-insulating GaAs is about 5–6 Å. It is found that the Ga₂O₃ quantity in the oxide bulk is greater than the same value of As₂O₃ in highly In-doped samples. In-doping in concentrations over 1.5 × 10¹⁹ cm⁻³ increases the Ga₂O₃ content and the density of the residual oxide. This influence is determined by reducing the dislocation density and changing the point defect environment. The presence of As-rich precipitates on the dislocations and in the matrix decreases the sputtering time and changes the composition of the residual oxide. The correlation between the type of high temperature dislocations revealed by Abrahams-Buiocchi (AB) etching and the oxide layer composition is shown. The results obtained could be used in the first stages of epitaxial growth, metallization and other technological processes of semiconductor device and ICs fabrication.     

Electron paramagnetic resonance of hydrogen in silicon

A review of the investigations by means of electron paramagnetic resonance (EPR) of hydrogen and hydrogen-related defects in crystalline silicon is presented. The main features of the EPR center Si-AA9 (bond-centered hydrogen), which is known as the hydrogenic analogue of the anomalous muonium (Mu^*) in silicon, are discussed. It was found that the process of annealing the AA9 center is characterized by an activation energy, E = 0.48 ± 0.04 eV with a second-order pre-exponential factor, K₀ = (1.25 ± 2.5) × 10^-7 cm³/s.

A detailed investigation by EPR of the defect (Si-AA1), which we identify as the hydrogen-related shallow donor in a positive charge state, is also presented. In particular it is shown that the H-related shallow donor is a helium-like center and its wave function has C_2v symmetry. Moreover, the main features of the series of EPR spectra in silicon characteristic for the implantation of hydrogen are presented.     

High field phenomena in thin plasma nitrided SiO2 films

The effects of high field tunnel electron injection on the electrical properties of Al - thin plasma nitrided SiO₂ films - Si (p-type) structures are studied. Under high field injection, it has been observed that electron trapping, positive charge generation near the Si-SiO₂ interface (slow states) and fast state generation at the Si-SiO₂ interface have taken place. After high temperature N₂ annealing, the nitridation induced electron trap density is considerably decreased. Furthermore, under high field injection the generation rate of both the slow states and the interface states and consequently, the degradation rate of the nitrided oxide films have been also decreased after annealing.     

Hydrogen-release mechanisms in the breakdown of thin SiO2 films

The mechanism of hydrogen release from the anode Si/SiO(2) interface that triggers defect generation and finally the dielectric breakdown of the oxide in metal-oxide-semiconductor structures is investigated. Extensive experimental charge-to-breakdown statistics are used to derive the defect generation efficiency as a function of gate voltage and oxide thickness in wide ranges. The presented results provide strong support to single-electron assisted Si-H bond breakage and discard multiple electron induced incoherent vibrational heating mechanisms.     

载流子选择性接触:高效硅太阳电池的选择

太阳电池可看成由光子吸收层和接触层两个基本单元组成,接触层是高复合活性金属界面和光子吸收层之间的区域.为了进一步提高硅太阳电池的转换效率,关键是降低光子吸收层和接触之间的复合损失.近年来,载流子选择性接触引起了光伏界的研究兴趣,其被认为是接近硅太阳电池效率理论极限的最后的障碍之一.本文分析了三种类型的载流子选择性接触:在光子吸收层与金属界面之间引入薄的重掺杂层,即所谓的发射极或背面场;利用两种材料之间的导带或价带对齐;利用高功函数的金属氧化物与晶硅接触从而在晶硅中感应能带弯曲.基于一维太阳电池模拟软件wx AMPS,模拟了扩散同质结硅太阳电池[结构为(p~+)c-Si/(n)c-Si/(n~+)c-Si]、非晶硅薄膜硅异质结太阳电池[结构为(p~+)a-Si/(i)a-Si/(n)c-Si/(i)a-Si/(n~+)a-Si]和氧化物薄膜硅异质结太阳电池[结构为(n)MoO_x/(n)c-Si/(n)TiO_x]暗态下的能带结构和载流子浓度的空间分布,其中c-Si为晶硅;a-Si为非晶硅;(i),(n)和(p)分别表示本征、n型掺杂和p型掺杂.模拟结果表明:载流子选择性接触的核心是在接触处晶硅表面附近形成载流子浓度空间分布的不对称进而使得电导率的不对称,形成了对电子的高阻和空穴的低阻或者对空穴的高阻和电子的低阻,从而让空穴轻松通过同时阻挡电子,或者让电子轻松通过同时阻挡空穴,形成空穴选择性接触或者电子选择性接触.     

硅锗量子阱结构在硅异质结太阳电池中应用的数值模拟

利用半导体工艺和器件仿真软件silvaco TCAD(Technology Computer Aided Design),模拟研究了采用硅/硅锗合金(silicon/silicon germanium alloy,Si/Si_(1-x)Ge_x)量子阱结构作为吸收层的薄膜晶体硅异质结太阳电池各项性能.模拟结果显示,长波波段光学吸收随锗含量的增加而增加,而开路电压则因Si_(1-x)Ge_x)层带隙的降低而下降.锗含量为0.25时,短路电流密度的增加补偿了开路电压的衰减,效率提升0.2%.氢化非晶硅/晶体硅(a-Si:H/c-Si)界面空穴密度以及Si_(1-x)Ge_x)量子阱的体空穴载流子浓度制约着空穴费米能级的位置,进而影响到开路电压的大小.随着锗含量增加,a-Si:H/c-Si界面缺陷对开压的影响降低,Si_(1-x)Ge_x)量子阱的体缺陷对开压的影响则相应增加.高效率含Si_(1-x)Ge_x)量子阱结构的硅异质结太阳电池的制备需要a-Si:H/c-Si界面缺陷的良好钝化以及高质量Si_(1-x)Ge_x)量子阱的生长.     

A molecular dynamics simulation of ultrathin oxide films onsilicon: Growth by thermal O atoms and sputtering by 100 eV Ar+ ions

Molecular dynamics was applied to study the growth and sputtering of ultrathin oxide films on (100) Si surfaces. A multibody potential which stabilized the Si/SiO₂ interface was used for this purpose. Oxide growth by exposure to O atoms was found to follow Langmuir-type kinetics with unity initial sticking coefficient of O and saturation coverage of around four monolayers, in agreement with experimental data. Sputtering of an ultrathin oxide film on silicon by 100 eV Ar⁺ ions was simulated to study ion-assisted surface cleaning. Ion irradiation was found to promote restructuring of the surface into oxide islands, as observed experimentally. Island formation was accompanied with an increase in surface roughness. The evolution of the surface state with ion dose was predicted quantitatively     

γ射线作用下氧化铪基MOS结构总剂量效应研究

使用原子层淀积方法得到了7.8 nm厚度的HfO₂薄膜并通过直接溅射金属铝电极得到了Al/HfO₂/Si MOS电容结构,测量得到了HfO₂基MOS结构在60Co γ射线辐照前后的电容-电压特性,使用原子力显微镜得到了HfO₂薄膜在辐照前后的表面微观形貌,使用X射线光电子能谱方法测量得到了HfO₂薄膜在辐照前后的化学结构变化。研究发现,使用原子层淀积方法制备的HfO₂薄膜表面质量较高;γ射线辐照在HfO₂栅介质中产生了数量级为1012 cm-2的负的氧化层陷阱电荷;HfO₂薄膜符合化学计量比,介质内部主要的缺陷为氧空位且随着辐照剂量的增加而增加,说明辐照在介质中引入了陷阱从而导致MOS结构性能的退化。     

LaTiO高k栅介质Ge MOS电容电特性及Ti含量优化

采用共反应溅射法将Ti添加到La_2O_3中,制备了LaTiO/Ge金属-氧化物-半导体电容,并就Ti含量对器件电特性的影响进行了仔细研究.由于Ti-基氧化物具有极高的介电常数,LaTiO栅介质能够获得高k值;然而由于界面/近界面缺陷随着Ti含量的升高而增加,添加Ti使界面质量恶化,进而使栅极漏电流增大、器件可靠性降低.因此,为了在器件电特性之间实现协调,对Ti含量进行优化显得尤为重要.就所研究的Ti/La_2O_3比率而言,18.4%的Ti/La_2O_3比率最合适.该比率导致器件呈现出高k值(22.7)、低D_(it)(5.5×10~(11)eV~(-1)·cm~(-2))、可接受的J_g(V_g=1V,J_g=7.1×10~(-3)A·cm~(-2))和良好的器件可靠性.