探测器用CdZnTe晶体载流子输运过程的研究

发布时间：2018-03-17 01:30

本文选题：碲锌镉　切入点：深能级缺陷　出处：《西北工业大学》2015年博士论文　论文类型：学位论文

【摘要】：化合物半导体碲锌镉(CdZnTe)晶体被认为是最具潜力的室温半导体核辐射探测器材料。然而由于生长过程以及后续器件制备过程的复杂性,在晶体内部形成各种缺陷,对载流子输运过程以及器件性能产生严重影响,制约着高品质CdZnTe成像探测器的发展。本文重点研究CdZnTe晶体中缺陷对载流子输运过程以及器件性能的影响,揭示晶体中点缺陷、扩展缺陷以及辐照损伤诱生缺陷对载流子输运过程影响的微观机制,提出优化探测器性能,尤其是时间响应特性以及抗大剂量照射的方法。研制了一套激光诱导瞬态光电流(LBIC)测试系统,用于分析测试高电阻半导体的载流子迁移率等载流子输运参数。构建了CdZnTe晶体载流子输运特性的基本物理模型,确立了不同条件下激光诱导瞬态光电流谱的解谱分析方法。采用该方法可直接得出表面复合速率、俘获时间、去俘获时间、载流子迁移率、空间电荷密度等系列载流子输运特性参数。采用均匀电场下的解谱法对不同电压下的LBIC图谱进行分析得出,CdZnTe晶体典型电子迁移率约为950 cm2/V-s,对应的电子迁移率寿命积为1.32×10-3cm2/V,载流子发射率与电场的平方根呈线性关系。由此得出,电场对缺陷能级的去俘获效应影响的主导机制为Poole-Frenkel效应,即电场可以有效增强载流子去俘获效应,从而达到高的载流子收集效率。采用光致发光谱(PL)和热激电流谱(TSC)对晶体内部缺陷分布进行了精确测试与分析,结合无接触式微波光电导(MWPCD)、LBIC技术以及能谱响应特性研究了不同缺陷分布对载流子输运过程和器件性能的影响。采用Hornbeck-Haynes模型对MWPCD分析结果表明,MWPCD曲线快降部分主要由体复合作用主导,慢降部分主要由载流子去俘获效应主导。同时发现,A中心以及位错相关的缺陷能级可显著的降低CdZnTe晶体的体复合寿命。Te反位和Te间隙可能是导致去俘获时间增长以及电荷收集不完全的主要缺陷。缺陷能级的去俘获时间小于收集时间的特性有利于提高探测器性能。为了保证成像器件应用时具有足够高的计数率,同时避免引入过高的漏电流,采用亚禁带光照办法进行了CdZnTe晶体缺陷能级占据概率及空间电荷分布的调控,从而实现优化。研究结果表明,亚禁带光照后CdZnTe晶体的电阻率由约1.23×10m Ω·cm下降到约4.22×109Ω·cm,同时I-t曲线的弛豫时间从8.21 s减小到0.93 s。结合改进的SRH模型分析发现,亚禁带光可有效调节缺陷能级的占据概率,使得活性陷阱能级的浓度减低。通过对比亚禁带光照前后LBIC图谱可知,亚禁带光照后晶体内部空间电荷浓度从9.03×109cm-3下降到8.67×108cm-3,从而有效的抑制了载流子俘获效应,使内建场畸变减小。由241Am@59.5keV能谱效应特性测试可知,亚禁带光照有效提高了低电压下探测器的能量分辨率,使得探测器的收集效率提高。根据Hecht公式拟合可知,亚禁带光照后电子迁移率寿命积从8.59×104 cm2/V提高到1.17×10-cm2/V。由于载流子在扩展缺陷周围的输运与点缺陷有很大不同,本文首先建立了扩展缺陷周围的势垒调控俘获模型,并结合α源诱导电流技术,分析扩展缺陷对电子输运过程和能谱响应特性的影响。研究结果表明,扩展缺陷俘获电子后在其周围会产生明显的肖特基型耗尽区,使得内部电场畸变。结合LBIC测试得出包含晶界样品的有效电子迁移率为803 cm2/Vs,明显小于单晶样品910 cm2/Vs的电子迁移率。通过α源诱导电流信号上升时间统计分布得出,晶界会对电子漂移时间产生固有的扰动影响,且无法通过提高电压来抑制。结合α源诱导电流信号幅值统计分布发现,晶界作为载流子的复合区域,在低能端形成第二个峰,最终造成电荷收集不完全,影响探测器响应的均匀性。采用超快脉冲X射线研究了不同剂量、不同电压下CdZnTe探测器的时间响应特性。结果表明,CdZnTe探测器的响应时间约为2.2 ns。通过双能级模型对非平衡载流子弛豫过程进行分析得出,自由载流子浓度随时间变化率不只与缺陷能级的俘获(复合)截面有关,还与缺陷能级的占据概率有关。对不同入射剂量下CdZnTe探测器的时间响应特性分析可知,CdZnTe晶体的极化阈值剂量约为2.78×109 photons mm-2s-1。由于能级占据概率的不同,大入射剂量时非平衡载流子主要通过复合作用快速弛豫到平衡态。而小剂量入射时,由于去俘获效应的影响,其时间响应弛豫过程明显延长。提高电压可有效的避免载流子被俘获以及增强载流子去俘获效应,有利于提高探测器时间响应特性。采用高剂量60Coγ射线为辐照源,研究了CdZnTe晶体中的能量沉积机理以及最终形成的稳定缺陷形式,分析了辐照损伤对载流子输运特性以及器件性能的影响。根据Kinchin-Pease理论,经过一次康普顿散射后,Cd、Zn和Te原子获得的最大能量分别为45.42 eV、78.11 eV和40.01 eV。根据Mott-McKinley-Feshbach经典方程,CdZnTe晶体中康普顿反冲电子与不同晶格原子作用,其移位的截面比为σ(Cd):σ(Zn):σ(Te) =1:3.9:1.12。结合不同剂量照射下的PL结果可知,随着辐照剂量的增大,A中心峰、A0X峰的强度不断增强,DAP峰强度不断减弱。该结果表明60Coγ射线在晶体内部产生大量的VCd。VCd与InCd+结合形成稳定的A中心。由LBIC测试结果可知,辐照损伤产生的缺陷主要通过增强电离杂质散射的作用降低电子的迁移率。结合不同剂量下电子迁移率的变化关系,归结出60Coγ射线对CdZnTe晶体电子迁移率损伤经验模型为1/μ=1/μ0+5.489×104-0.02676。辐照损伤引入的缺陷能级通过减低迁移率以及寿命,使电子的迁移率寿命积从1.36×10-3 cm2/V下降到7.56×10-4 cm2/V。同时,辐照损伤造成探测器的能量分辨率以及电荷收集效率的显著降低。
[Abstract]:Compound semiconductor cadmium zinc telluride (CdZnTe) crystal was considered to be the most promising materials for room temperature nuclear radiation detectors. However, due to the growth process and the complexity of subsequent device fabrication process, the formation of various defects in the crystal, the carrier transport affect the transport process and device performance, restricts the development of high-quality CdZnTe imaging detector. This paper focuses on the defects in CdZnTe crystal transport process and device performance of carrier, reveal the point defects, extended defects and micro mechanism of irradiation damage induced defects in the transport process influence on carrier, optimize the detector performance, especially the time response characteristics and method of high dose irradiation. The development of a laser induced the transient photocurrent (LBIC) test system for analysis and testing of high resistance semiconductor carrier mobility and carrier transport The basic physical parameters. The construction model of CdZnTe crystal carrier transport properties, established the solution method of spectral analysis under different conditions of laser induced transient photocurrent spectrum. Using this method can obtain the surface recombination velocity, capture time, to capture time, carrier mobility, space charge density series carrier transport characteristic parameters. LBIC spectra of different voltages were analyzed using uniform electric field method, CdZnTe crystal typical electron mobility is about 950 cm2/V-s, corresponding to the electron mobility Lifetime product is 1.32 * 10-3cm2/V, the carrier rate and the square root of field emission showed a linear relationship. Thus, the electric field of the defect level to capture the effect of the dominant mechanism for the Poole-Frenkel effect, the electric field can effectively enhance the carrier to capture effect, so as to achieve high carrier collection efficiency. The photoluminescence spectrum (PL) and TSC spectrum (TSC) of internal defect distribution of crystal was accurate measurement and analysis, combined with contactless microwave photoconductivity (MWPCD), LBIC technology and the energy spectrum of different defect distribution transport process and device performance of carrier response. Using Hornbeck-Haynes model for MWPCD analysis the results show that the MWPCD curve is down by some of the main body of the composite leading role, slow down the main part of the carrier to capture the dominant effect. At the same time, the A center and dislocation related defects can significantly reduce the level of CdZnTe crystal body composite life trans.Te and Te clearance may be the main defects lead to capture time and growth the charge collection is not complete. The defect level to capture time less than collection time characteristics is helpful to improve the performance of the detector. In order to ensure the application of imaging devices with high enough count rate, the same Avoid introducing leakage current is too high, the sub bandgap illumination method of CdZnTe crystal defect level occupation probability and spatial regulation of the charge distribution, so as to realize the optimization. The results show that sub bandgap light after the resistivity of CdZnTe crystal is about 1.23 * 10m. Cm down to about 4.22 x 109. Cm. At the same time, the I-t curve of relaxation time is reduced from 8.21 s to 0.93 S. with the improved SRH model analysis showed that the sub bandgap light occupying probability can effectively adjust the defect level, the concentration of active traps reduced. By comparing the sub bandgap light and LBIC spectra, sub bandgap light inside the crystal from space charge concentration 9.03 x 109cm-3 decreased to 8.67 * 108cm-3, which effectively inhibits the carrier trapping effect, the built-in field distortion. By 241Am@59.5keV spectrum effect characteristic test shows that sub bandgap illumination can effectively improve the low voltage Under the energy resolution of the detector, the collection efficiency of the detector is improved. According to the Hecht formula fitting, sub bandgap light electron mobility Lifetime product increased from 104 cm2/V to 8.59 * 1.17 * 10-cm2/V. as carrier in the transport and transport extended defects around the point defect is quite different, this paper extended barrier regulation capture the model around the defects, combined with alpha source induced current technology, analysis of extended defects on the response characteristics and the energy spectrum of the electron transport process. The results show that the extended defects capture electrons around it will produce obvious Schottky type depletion region, the internal electric field distortion. Combined with the LBIC test including grain boundary samples effectively the electron transfer rate of cm2/Vs was 803, significantly less than single crystal samples 910 cm2/Vs electron mobility. The induced current signal rise time distribution obtained by alpha source, grain boundary The disturbing effects inherent to electron drift time, and not to suppress by increasing the voltage. Combined with alpha source induced current amplitude distribution, grain boundary as composite carrier area, formed second peaks in the low end, resulting in incomplete charge collection uniformity, influence of detector response were studied by different doses. Ultrafast X ray pulse response time of CdZnTe detector under different voltage. The results show that the response time of CdZnTe detector is about 2.2 ns. on the nonequilibrium carrier relaxation process analysis by two-level model, the capture rate not only with the defect level changes in the free carrier concentration with time (composite) section, but also on the with probability occupy the defect level. Characteristics analysis of response to different dose of CdZnTe time of the detector, the polarization threshold dose of CdZnTe crystal is about 2.7 8 x 109 photons mm-2s-1. due to different level occupation probability, large incident dose of non equilibrium carriers mainly through the compound effect of fast relaxation to the equilibrium state. The small dose of incidence, due to trapping effect, the response time was significantly prolonged. The relaxation process can effectively improve the voltage to avoid capture and enhanced carrier carrier to capture the effect, can improve the response time of the detector. The high dose of 60Co gamma ray as radiation source, studied the mechanism of energy deposition in CdZnTe crystals and finally formed the stable defect form, analysis of radiation damage in transport properties and device performance of carrier. According to Kinchin-Pease theory, after a Compton scattering Cd, Zn and Te atoms, the maximum energy obtained is 45.42 eV, 78.11 eV and 40.01 eV. according to the Mott-McKinley-Feshbach equation, CdZnTe crystal Compton recoil electron with different lattice atoms, the displacement area ratio (Cd): sigma sigma sigma (Zn): (Te) =1:3.9:1.12. with different doses of irradiation PL results showed that with the increase of irradiation dose, A peak, A0X peak intensity increasing, DAP peak intensity decrease. The results show that the 60Co gamma ray VCd.VCd and InCd+ in a lot of crystals combine to form a stable A center by LBIC. Test results show that the main defects of radiation damage by enhancing the ionized impurity scattering effect reduce the electron mobility. Combined with the relationship between different doses of electron mobility, summed up the 60Co gamma ray migration the damage rate of empirical model is 1/ mu =1/ mu 0+5.489 * 104-0.02676. irradiation defect level introduced by reducing the mobility and lifetime of CdZnTe crystal electron, make the electron mobility Lifetime product from 1.36 * 10-3 cm2/V down to 7.5 At the same time, the energy resolution of the detector and the charge collection efficiency are significantly reduced by radiation damage at the same time of 6 x 10-4 cm2/V..

【学位授予单位】：西北工业大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TN304.2

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