CdZnTe像素阵列探测器成像评价模型及实验

考虑载流子陷获效应建立了碲锌镉(CdZnTe)像素阵列探测器感应电荷分布模型,并从非平衡载流子连续方程出发,推导了晶体内部陷获载流子数密度分布,得到了CdZnTe探测器成像调制传递函数评价模型。数值计算结果表明:随入射光子能量的增加,探测器成像质量明显下降;当电子载流子与空穴载流子迁移寿命积范围分别为0.510-3 to 5.010-3 cm2/V,2.010-5 to 7.510-5 cm2/V时,电子载流子感应信号是探测器响应信号的主要来源,而空穴迁移寿命积变化对探测器成像性能的影响有限,所建立模型的载流子收集特性与实际探测器载流子收集特性相符。搭建了40 mm40 mm的CdZnTe成像探测系统,探测并获得了系统预采样调制传递函数。实验结果表明:模型理论值与实验数据相符合,实际CdZnTe晶体中存在的固有深能级缺陷、实验所采用的非单色性X射线源及较大的实际像素间隙是造成理论值与实验结果存在一定偏差的主要原因。     

六方AlN本征缺陷的第一性原理研究

用基于密度泛函理论平面波赝势法首先对六方AlN本征点缺陷(氮空位、铝空位、氮替代铝、铝替代氮、氮间隙、铝间隙)存在时的晶格结构进行优化，得到其稳定结构；然后通过各缺陷形成能的计算可得知其在生长过程中形成的难易程度；最后从态密度的角度对各种本征点缺陷引起的缺陷能级及电子占据情况进行了分析.发现除氮空位外其他本征缺陷在带隙中形成的能级都很深，要得到n型或p型AlN必须要引入外来杂质.计算得到的本征缺陷能级对于分析AlN的一些非带边辐射机理有重要帮助. 关键词： 六方AlN 形成能 缺陷能级 态密度     

有机固体中载流子陷阱的表征

有机固体包括有机分子晶体、非晶态有机固体、有机高聚物等.在有机固体中保持单个分子的特性,如吸收光谱;分子间的范德瓦耳斯相互作用能比较小,即使是单晶体,也容易产生结构欠序和杂质、晶界等缺陷.这些缺陷使电子价带与导带间的禁区中出现局域能级,导带的载流子(电子)或价带的载流子(空穴)会被局域于这些杂质或缺陷的附近,出现电荷存储现象,影响稳态和瞬态电导下的载流子输运,导致暗导的非欧姆性、载流子迁移率的下降、光导衰减时间大于光激发载流子的寿命等.多年来,这些一直被固体的电导和光导研究者所重视[1-6],并称这些局域能级为陷阱.…     

金红石型TiO2点缺陷性质的第一性原理研究

本文运用基于局域密度泛函和赝势的第一性原理方法研究了金红石相TiO2点缺陷的电子性质,结果表明氧空位缺陷使晶体的费米能量升高,在能隙中没有产生杂质能级.钛空位缺陷使晶体的费米能量降低,并在价带顶部产生了一个杂质能级,与价带顶能量相差约0.4 eV.本文还计算了金红石相TiO2在具有氧空位和钛空位点缺陷情况下的键长变化、态密度和电荷布居状况.     

运用第一性原理研究氧化锌电阻阀片中氧空位与掺杂条件下ZnO晶体电学性质

为了比较Nb_2O_5、MnO_2、MgO三种添加剂对氧化锌电阻阀片电学性能影响,在微观层面模拟Nb、Mn、Mg三种元素分别掺杂ZnO完整超晶胞和带有氧空位缺陷的ZnO超晶胞,并运用第一性原理分析掺杂晶胞的特性.本文计算了晶体结构、掺杂形成能、氧空位形成能、能带结构、态密度、载流子迁移率、电导率等.结果表明,掺入Nb原子的掺杂体系晶格体积最大,Mg掺杂体系的形成能最大,稳定性最弱,Nb掺杂氧空位形成能最低,更容易引入氧空位.Nb掺杂的ZnO超晶胞禁带宽度最小,氧空位缺陷增大掺杂晶体的禁带宽度.在相同掺杂浓度和同等条件下,Mn掺杂的晶体电导率最高.     

Al辐照损伤初期的第一性原理研究

采用密度泛函理论框架下的第一性原理平面波赝势方法,对Al辐照损伤初期产生的本征点缺陷和He缺陷进行了研究.通过晶体结构、缺陷形成能和结合能,分析比较了缺陷形成的难易程度及对晶体稳定性的影响,并从态密度、差分电荷密度和电荷布居的角度,分析了其电子机理.结果表明:对于同类型的缺陷,其造成的晶格畸变越大,体系稳定性越低,缺陷形成的难度越大.同类型缺陷形成的难易程度由易到难依次为空位(置换位原子)、八面体间隙原子和四面体间隙原子,但相同位置的本征缺陷的形成难度小于He缺陷.间隙原子容易与空位结合,且Al原子与空位结合的能力强于He原子.间隙Al原子和He原子主要存在于八面体,且缺陷原子引起部分电子向更高能级转移,并导致与其最邻近的Al原子之间的共价作用减弱,从而降低了体系稳定性.间隙Al原子与最邻近的Al原子之间产生了强烈的共价作用,而He原子和最邻近Al原子之间主要为范德瓦耳斯力和较弱的离子键,这是含He缺陷的体系稳定性更低的重要原因.     

Ge掺杂GaN晶体双光子诱导超快载流子动力学的飞秒瞬态吸收光谱研究

本文利用飞秒瞬态吸收光谱技术,在近红外波段对Ge掺杂GaN(GaN:Ge)晶体进行了超快载流子动力学研究.在双光子激发下,瞬态吸收动力学呈现出双指数衰减,其中慢过程寿命随着泵浦光强增加而增加.瞬态吸收响应随着探测波长而单调增强,并在约1050nm处由空穴吸收占据主导.利用简化模型模拟载流子动力学发现,GaN:Ge中碳杂质形成的深受主能级对空穴有很强的俘获能力,并且引起了缺陷发光.在较适中的载流子注入下,n型GaN中的载流子寿命可以通过控制缺陷浓度和载流子浓度来共同调控,使其可应用于发光二极管和光通信等不同的领域.     

金红石型TiO_2中四种点缺陷态研究

利用第一性原理计算方法研究了金红石型TiO_2中四种缺陷的电子态.这四种缺陷包括氧空位(O_v)、钛空位(Ti_V)、钛间隙(Ti_S)以及氧空位O_v与钛间隙态Ti_S共存态.氧空位的存在导致禁带内施主缺陷能级较浅,而深施主能级与Ti间隙态有关.预测了氧空位更倾向于与钛间隙结合,主要通过钛间隙态的3d电子部分转移到近邻近氧空位的部分形成O_V-Ti_S对缺陷.具有O_v、Ti_S或O_V-Ti_S缺陷的体系都出现间隙态,促进体系出现红外吸收.     

PECVD法制备高结晶GaN薄膜及其光电响应性能

采用一种简单、绿色、低成本的等离子增强化学气相沉积(PECVD)法,在950℃下成功制备了高结晶质量的GaN薄膜.为了提高GaN薄膜结晶质量和弄清GaN薄膜光响应机制,研究了GaN缓冲层制备温度对GaN薄膜结晶质量和光电性能的影响.研究表明,随着GaN缓冲层制备温度的增加,GaN薄膜的结晶质量先提高后降低,在缓冲层温度为875℃时,结晶质量最高,此时计算得出的总位错密度为9.74×10~9 cm^-2,载流子迁移率为0.713 cm~2/(V·s).经过退火后,GaN薄膜的总位错密度降低到7.38×10~9 cm^-2,载流子迁移率增大到43.5 cm~2(V·s),此时GaN薄膜光响应度为0.20 A/W,光响应时间为15.4 s,恢复时间为24 s,可应用于紫外光探测器.通过Hall测试和X射线光电子能谱仪分析得出,GaN薄膜内部存在着N空位、Ga空位或O掺杂,它们作为深阱能级束缚和复合光生电子和空穴,使得光响应度与偏压呈抛物线关系;另外,空位和O掺杂形成的深阱能级也是导致GaN薄膜的光电流响应和恢复缓慢的根本原因.     

非线性光电导开关载流子碰撞电离分析

对非线性GaAs光电导开关在锁定期间的电流成丝现象、具有负微分迁移率的速场特性、深能级的陷阱效应、光子的再吸收等因素进行分析,建立了非线性光电导开关锁定期间的连续性方程和电中性方程.基于该方程组,用有限差分法计算了偏压为2 200 V的3.5 mm GaAs∶EL2非线性光电导开关的电流实验数据,得到电流丝内载流子瞬态特性为：载流子浓度约为1017 cm－3,EL2电子陷阱近似饱和;电子电流随锁定时间明显下降,空穴电流基本不变;单位寿命时间载流子雪崩倍增因子的均值约为1.2,其统计起伏随锁定时间增大.     

Relaxor properties and the mechanism of conduction in TlInS<Subscript>2</Subscript> crystals exposed to gamma irradiation

The density of states at the energy levels associated with radiation-induced defects, the localization length of a defect center, and the hopping distance of charge carriers are determined in a TlInS₂ crystal. It is demonstrated that, by varying the dose of gamma irradiation, it is possible to control the dielectric properties of ferroelectrics and to attain a stable relaxor state. In the temperature range of existence of this state, charge carriers execute tunneling from electron levels in the band gap through potential barriers created by an incommensurate superstructure of the TlInS₂ crystal.     

First-principles calculations of F-, Cl-, and N-related defects of amorphous SiO_2 and their impacts on carrier trapping and proton release

The first-principles calculations based on density functional theory are performed to study F-,Cl-,and N-related defects of amorphous SiO2(a-SiO2) and their impacts on carrier trapping and proton release.The possible geometric configurations of the impurity-related defects,the formation energies,the hole or electron trapping of the neutral defects,and the mechanisms to suppress proton diffusion by doping N are investigated.It is demonstrated by the calculations that the impurity atoms can interact with the oxygen vacancies and result in impurity-related defects.The reactions can be utilized to saturate oxygen vacancies that will cause ionization damage to the semiconducting devices.Moreover,the calculated formation energy indicates that the F-or Cl-related oxygen vacancy defect is a deep hole trap,which can trap holes and prevent them from diffusing to the a-SiO2/Si interface.However,three N-related defects,namely N(2)o-H,N(2)o=O,and N(3)o-Vo,tend to act as shallow hole traps to facilitate hole transportation during device operation.The N(2)o and N(3)o configurations can be negatively charged as deep electron traps during the oxide charge buildup after ionization radiation.In addition,the nudged elastic band(NEB) calculations show that four N-related defects,namely N(2)o,N(2)o-H,N(2)o=O,and N(3)o are capable of capturing protons and preventing them from diffusing to and de-passivating the interface.This research reveals the fundamental properties of the F-,Cl-,and N-related defects in amorphous silica and the details of the reactions of the carrier trapping and proton release.The findings help to understand the microscopic mechanisms that alleviate ionization damage of semiconducting devices by doping a-SiO2.     

Double carriers pulse DLTS for the characterization of electron-hole recombination process in GaAsN grown by chemical beam epitaxy

A nitrogen-related electron trap (E1), located approximately 0.33 eV from the conduction band minimum of GaAsN grown by chemical beam epitaxy, was confirmed by investigating the dependence of its density with N concentration. This level exhibits a high capture cross section compared with that of native defects in GaAs. Its density increases significantly with N concentration, persists following post-thermal annealing, and was found to be quasi-uniformly distributed. These results indicate that E1 is a stable defect that is formed during growth to compensate for the tensile strain caused by N. Furthermore, E1 was confirmed to act as a recombination center by comparing its activation energy with that of the recombination current in the depletion region of the alloy. However, this technique cannot characterize the electron−hole (e-h) recombination process. For that, double carrier pulse deep level transient spectroscopy is used to confirm the non-radiative e-h recombination process through E1, to estimate the capture cross section of holes, and to evaluate the energy of multi-phonon emission. Furthermore, a configuration coordinate diagram is modeled based on the physical parameters of E1.     

Tl掺杂对InI禁带宽度和吸收边带影响的第一性原理研究

采用基于密度泛函理论框架下的第一性原理平面波超软赝势方法,建立了未掺杂与不同浓度的Tl原子取代In原子的In1-xTlxI超胞模型,分别对模型进行了几何优化、能带分布、态密度分布和吸收光谱的计算.结果表明:Tl掺杂浓度越小,In1-xTlxI形成能越低,晶体结构越稳定;Tl的掺入使得InI体系导带向高能方向移动,而价带顶位置基本没变,导致禁带宽度变宽,InI吸收光谱出现明显蓝移现象.     

Effect of electron and proton irradiation on recombination centers in GaAsN grown by chemical beam epitaxy

Deep level transient spectroscopy (DLTS) was deployed to study the evolution, upon electron irradiation and hydrogenation of GaAsN grown by chemical beam epitaxy, of the main nitrogen-related nonradiative recombination center (E1), localized at 0.33 eV below the bottom edge of the conduction band of the alloy. On one hand, the electron irradiation was found to enhance the density of E1 depending on the fluence dose. On the other hand, the hydrogenation was found to passivate completely E1. Furthermore, two new lattice defects were only observed in hydrogenated GaAsN films and were suggested to be in relationship with the origin of E1. The first defect was an electron trap at average thermal activation energy of 0.41 eV below the CBM of GaAsN and was identified to be the EL5-type native defect in GaAs, originating from interstitial arsenic (As_i). The second energy level was a hole trap, newly observed at average thermal activation energy of 0.11 eV above the valence band maximum of the alloy and its origin was tentatively suggested to be in relationship with the monohydrogen–nitrogen (N–H) complex. As the possible origin of E1 was tentatively associated with the split interstitial formed from one N atom and one As atom in single V-site [(N–As)_As], we strongly suggested that the new hole trap took place after the dissociation of E1 and the formation of N–H complex.     

Nature of gallium deep centres in lead telluride based semiconductors

Doped with Ga lead telluride was taken as a model object to explain the nature of group-III deep levels in IV-VI semiconductors and to elucidate the vapour phase doping mechanism. For this goal, interaction of various gallium-containing molecules with defect-free crystal as well as with native defects in PbTe was considered. Formation energies for different point defects created in PbTe as a result of interaction the Ga₂Te molecules, Ga₂ dimers and single Ga atoms with a host crystal were calculated using density functional theory. Particularly Ga_Pb and Ga_i together with formation of accompanied self interstitials Pb_i in various charge states were examined. In addition we propose the new type of defects - the impurity complex (2Ga)_Pb which looks like <111>-oriented gallium dumbbell. Calculations suggest the double donor behaviour and DX-like properties of this defect together with extremely low formation energy values. Namely, (2Ga)_Pb centres are preferably formed under Ga₂Te doping while (Ga₂)_Pb+Pb_i ones are formed under Ga₂ or Ga doping. In all cases, formation energies are negative and resulting defect concentration is determined by reaction kinetics only. Mechanisms of the lead vacancy compensation with the vapour phase doping are considered as well.     

Spatial subharmonics in a photorefractive semiconductor

The generation of spatial subharmonics is reported in a photorefractive semiconductor (CdTe:Ge) for the first time to our knowledge. Space charge waves with a narrow spatial spectrum are detected in homogeneously illuminated ac-biased CdTe:Ge sample by observation of a well developed low divergent self-diffracted beam. The mobility-lifetime product of the free carriers and the effective trap concentration are estimated from the threshold ac-field measured for different subharmonics at different grating vectors of the generated grating. Received: 5 April 2000 / Published online: 7 June 2000     

Some considerations on the defect chemistry of ionic solids

The defect chemistry of the solid state deals with the relative densities of different possible types of defects in a crystal as a function of the ambient atmosphere and temperature, and their distribution in the allowed energy levels, and this paper discusses these aspects in some length. Principles of calculating the actual energy levels are then considered in terms of the Born-Mayer theory.     

Specific features of the resonance interaction of a coherent wave with an ensemble of impurity atoms of a photonic crystal

Two-quantum relaxation transitions in an impurity atom that are activated by its resonance interaction with a monochromatic wave lead to hole burning in the distribution of atoms populating the lower energy level over detunings from the resonance. In this case, the levels of an impurity atom lying within the gap or the pseudogap in the density of photon states of a photonic crystal act as a trap. The impurity atoms change to these levels in the process of interaction of a monochromatic wave with the atomic transition that is not influenced by specific spectral features of a photonic crystal.