分子束外延P-on-N HgCdTe As扩散调控研究

对分子束外延（MBE）生长的原位As掺杂HgCdTe外延材料的热退火造成的As扩散控制进行研究。在较低的退火温度下获得了As扩散长度可控的HgCdTe材料,易于形成符合设计参数的PN结轮廓,为后续新型焦平面器件的研发提供基础。研究发现,在热退火过程中,原位As掺杂HgCdTe的As浓度的大小和纵向分布随着不同的Hg分压而发生改变。并通过理论计算获得了不同Hg分压下的As扩散系数。同时,通过数值模拟对不同As扩散长度的P-on-N器件结构进行了暗电流模拟,验证了As掺杂结深推进工艺的重要性。     

碲镉汞材料中Hg空位、Au、As掺杂的研究进展

碲镉汞材料是制造红外探测器的基础,高性能红外探测器对碲镉汞材料的要求越来越高。为了提升器件性能,必须提高碲镉汞材料的电学性能。而掺杂是一个很好的选择。碲镉汞材料掺杂可以分为n型和p型两种。对于n型掺杂来说,In是一种理想的掺杂剂,其掺杂研究目前已比较成熟。相对而言,p型掺杂研究还不是那么深入。Hg空位、Au、As掺杂均为碲镉汞材料中常见的p型掺杂手段。通过分析和总结近些年的部分相关文献,介绍了碲镉汞材料中Hg空位、Au、As掺杂的研究进展。     

Mercury diffusion in Hg1−xCdxTe

A radiotracer technique has been used to measure both mercury self-diffusion and surface concentration values in bulk and liquid phase epitaxy, LPE, grown Hg_1−xCd_xTe. A high resolution sectioning technique has allowed profiling of thin epitaxial layers in submicron steps. Hg_1-xCd_xTe samples with composition values betweenx _Cd= 0.16 and 0.23 were isothermally annealed in carefully controlled and monitored diffusion con-ditions. Mercury reservoirs containing Hg²⁰³ were used to provide vapour diffusion sources during closed tube isothermal anneals in the temperature range 300° C to 400° C. Evidence has been found which may indicate the presence of two components in the radio-tracer profiles for both bulk and epitaxially grown material. In some cases it was possible to estimate two diffusion coefficients,D ₁andD ₂, from the near surface and deeply penetrating components, respectively. Our results forD ₁andD ₂are compared with other work. For bulk material annealed at 400° C under a saturated mercury pressureD ₁= 2.0 x 10^-12cm²s^-1 andD ₂= 1.1 x 10^-11cm²s⁻¹. Diffusion coefficients at 310° C under saturated mercury pressure, have been measured in bulk and epitaxial material. Close agreement was found between these results with an average value ofD ₁= 1.4 x 10⁻¹³cm²s⁻¹. We believe this to be the first time radiotracer results for epitaxial material have been presented. We have collated diffusion data, as a function of reciprocal temperature, from several workers and suggest there is evidence for a change in the activation energy for mercury diffusion around 350° C. This may be due to a change in the dominant diffusion mechanism.     

As在HgCdTe外延层中的扩散系数

使用二次离子质谱分析(SIMS)方法研究了As在碲镉汞分子柬外延样品中的扩散系数．获得了在240℃．380℃和440℃温度下As在碲镉汞材料中的扩散系数,并发现它与退火过程中Hg的分压有关,且Hg空位对As的扩散有明显的辅助增强作用．研究表明在低温段的240℃／24—48小时的退火中As的扩散非常有限,对样品中As的浓度分布影响不大,而在高温段380℃／16小时和440℃／30分钟退火中,As扩散较为明显,能使原来的PN突变结变缓．综合比较As杂质的电学激活以及As扩散因素,高温段440℃／30分钟的退火条件较理想．     

Annealing experiments in heavily arsenic-doped (Hg,Cd)Te

Arsenic doped molecular beam epitaxy (MBE) (Hg,Cd)Te films were grown on (Cd,Zn)Te substrates. The concentration of arsenic was varied from 5 x 10¹⁸ cm^-3 to 1 x 10²⁰ cm^-3. After the growth, the epitaxial layers were annealed at various partial pressures of Hg within the existence region of (Hg,Cd)Te at temperatures ranging from 400 to 500°C. Hall effect and resistivity measurements were carried out subsequent to the anneals. 77K hole concentration measurements indicate that for concentrations of arsenic <10¹⁹ cm⁻³, most of the arsenic is electrically active acting as acceptors interstitially and/or occupying Te lattice sites at the highest Hg pressures. At lower Hg pressures, particularly at annealing temperatures of 450°C and higher, compensation by arsenic centers acting as donors appears to set in and the hole concentration decreases with decrease in Hg pressure. These results indicate the amphoteric behavior of arsenic and its similarity to the behavior of phosphorus in (Hg,Cd)Te previously inferred by us. A qualitative model which requires the presence of arsenic occupying both interstitial and Te lattice sites along with formation of pairs of arsenic centers is conjectured.     

Status of Te-rich and Hg-rich liquid phase epitaxial technologies for the growth of (Hg,Cd)Te alloys

In this review, we summarize the progress to-date in the technology of Hg_1−xCd_xTe liquid phase epitaxial growth from Hg-rich and Te-rich solutions. Areas of research which need to be pursued to further improve the state of the art in device performance are discussed.     

Effect of cooling procedure after annealing on electrical properties of Cd0.2Hg0.8Te epitaxial films grown by liquid phase epitaxy

Effect of cooling procedure after annealing on the electrical properties of Cd_0.2Hg_0.8Te (CMT) epitaxial films grown by liquid phase epitaxy has been investigated to obtain the CMT films with low carrier concentration of 10¹⁴ cm⁻³ reproducibly. Annealing has been performed at the temperature range from 260 to 350°C for 8 h in a fixed Hg vapor pressure. The quenching and the gradual cooling over a duration of 200 min after annealing have been employed for the cooling procedures. For quenched CMT samples, hole concentration decreases with decreasing anneal temperature and conduction type conversion from p to n is observed at 300°C. For the gradual cooling, all samples show n-type conduction for all annealing temperatures. Electrical properties of annealed layers strongly depend on the cooling procedure. The difference in electrical properties of the annealed CMT between two types of cooling procedure is mainly attributed to the difference in the annihilation of Hg vacancies during cooling procedure. The decrease of Hg vacancies during quenching is negligible, while Hg vacancies are annihilated during gradual cooling by rapid Hg diffusion. The diffusion coefficient of Hg is estimated more than 10⁻⁹ cm²/s and this value is two orders of magnitude larger than that obtained by radiotracer technique.     

原位As掺杂p型碲镉汞薄膜的制备研究

非本征p 型掺杂碲镉汞材料可以有效克服少子寿命偏低等问题,提高长波和甚长波红外焦平面器件的性能。本文重点阐述了As 掺杂实现p型掺杂的基础性原理,以及其制备方法,为p-on-n碲镉汞材料器件研究提供依据。     

Enhanced arsenic diffusion and activation in HgCdTe

Temperature and time dependent Hg-annealing studies for arsenic activation have been carried out on As-doped molecular beam epitaxy HgCdTe eitherin situ or by ion implantation to determine the extent of arsenic activation in the single layer. Enhanced As diffusion and activation in double layer heterostructures have also been investigated to further our understanding of the effects on zero bias resistance-area product (R_oA) and quantum efficiency. The results show that the arsenic activation anneal is limited by Hg self-diffusion into the HgCdTe epilayer. Using this arsenic activation process for eitherin situ doped arsenic or implanted arsenic, high performance p-on-n double layer heterostructure photodiodes have been demonstrated on both mesa and planar device structures.     

Passivation of HgCdTe p-n diode junction by compositionally graded HgCdTe formed by annealing in a Cd/Hg atmosphere

Cadmium telluride (CdTe) is being widely used for passivating the HgCdTe p-n diode junction. Instead of CdTe, we tried a compositionally graded HgCdTe as a passivation layer that was formed by annealing an HgCdTe p-n junction in a Cd/Hg atmosphere. During annealing, Cd diffuses into HgCdTe from the Cd vapor, while Hg diffuses out from HgCdTe, forming compositionally graded HgCdTe at the surface. The Cd mole fraction at the surface was constant regardless of the annealing temperature in the range of 250–350°C. Capacitance versus voltage (C-V) curves for p-type HgCdTe that were passivated with compositionally graded HgCdTe formed by Cd/Hg annealing at 260°C showed a smaller flat-band voltage than the one passivated by thermally deposited CdTe, indicative of the better quality of the passivation. A long-wave infrared (LWIR) HgCdTe p-n junction diode passivated by compositionally graded HgCdTe showed about a one order of magnitude smaller R_dA value than the one passivated by thermally deposited CdTe, confirming the effectiveness of the compositionally graded HgCdTe as a passivant.     

Annealing effect on the P-type carrier concentration in low-temperature processed arsenic-doped HgCdTe

We report the results of annealing effects on the As-doped alloy HgCdTe grown by molecular beam epitaxy (MBE), arsenic (As) diffusion in HgCdTe from Hg-rich solutions at low temperatures, and As ion implantation at room temperature. Hall-effect measurements, secondary ion mass spectrometry and p-on-n test photodiodes were used to characterize the As activation. High As-doping levels (10¹⁷−10¹⁹ cm⁻³) could be obtained using either MBE growth, As diffusion or As ion-implantation. Annealed below 400°C, As doping in HgCdTe shows n-type characteristics, but above 410°C demonstrates that all methods of As doping exhibit p-type characteristics independent of As incorporation techniques. For example, for samples annealed at 436°C (P_Hg≈2 atm), in addition to p-type activation, we observe a significant improvement of p/n junction characteristics independent of the As source; i.e. As doping either in situ, by diffusion, or ion implantation. A study of this As activation of As-doped MBE HgCdTe as a function of anneal temperature reveals a striking similarity to results observed for As diffusion into HgCdTe and implanted As activation as a function of temperature. The observed dependence of As activation on partial pressure of Hg at various temperatures in the range of 250 to 450°C suggests that As acts as an acceptor at high Hg pressure (>1 atm) and as a donor at low Hg pressure (<1 atm) even under Hg-rich conditions.     

Growth method,composition, and defect structure dependence of mercury diffusion in CdxHg1–xTe

Mercury radiotracer diffusion results are presented, in the range 254 to 452°C, for bulk and epitaxial Cd_xHg_1–xTe, and we believe this to be the first report for metalorganic vapor phase epitaxy (MOVPE) grown Cd_xHg_1–xTe. For all growth types studied, with compositions of x_Cd=0.2±0.04, the variation of the lattice diffusion coefficient, D_Hg, with temperature, under saturated mercury partial pressure, obeyed the equation: D_Hg=3×10⁻³ exp(−1.2 eV/kT) cm² s⁻¹. It was found to have a strong composition dependence but was insensitive to changes of substrate material or crystal orientation. Autoradiography was used to show that mercury also exploited defect structure to diffuse rapidly from the surface. Dislocation diffusion analysis is used to model defect tails in MOVPE Cd_xHg_1–xTe profiles.     

液相外延碲镉汞贯穿型缺陷研究

液相外延碲镉汞材料的贯穿型缺陷会在后续器件制备中导致多个盲元的形成。采用共聚焦显微镜对该类缺陷的深度进行了表征,并对缺陷底部的成分进行了测试。使用聚焦离子束(Focused Ion Beam, FIB)将缺陷挖开后对其进行观察。对于贯穿型缺陷较多的碲锌镉衬底外延生长碲镉汞薄膜,统计后发现碲镉汞表面的贯穿型缺陷与衬底缺陷存在一定的对应关系,因此推测液相外延贯穿型缺陷起源于碲锌镉衬底缺陷。     

Comparison of characterization techniques in p-on-n HgCdTe LWIR photodiodes technology

In this paper standard techniques for characterization of HgCdTe liquid phase epitaxial layers (LPE) were presented. The performance of long wavelength p-on-n HgCdTe photodiodes fabricated by arsenic diffusion was described. The correlation between LPE HgCdTe material parameters and properties of the infrared photodiodes was demonstrated.     

Variation of arsenic diffusion coefficients in HgCdTe alloys with temperature and hg pressure: Tuning of p on n double layer heterojunction diode properties

Arsenic diffusion coefficients were measured in HgCdTe alloy films at 375,400, and 425°C. The diffusion coefficients displayed a strong dependence on Hg pressures at all three temperatures, increasing by 1000 x with decreasing Hg pressure. The Hg pressures employed were selected to span a portion of the single phase field at each temperature. These investigations added to the diffusion measurements at 350°C reported earlier. The behavior reported at all three temperatures generally resembled the behavior observed at 350°C. For example, at 375°C, the diffusion coefficient increased from 1.6 x 10^-14 cm²/s for a Hg overpressure of ~1.25 atm, corresponding to the Hg saturated end of the phase field, to 3.2 x 10^-12 cm²/s, when the Hg overpressure was decreased to 0.2 atm, at a point within the phase field, but still considerably removed from the Tesaturated end. For mercury pressure ranges generally within an order of magnitude from the Hg saturated end point, the transport behavior remained simple classical and the diffusion coefficients varied approximately as ∝ P _-He ^-3 at all three temperatures. With continued decrease in the mercury pressure, the transport behavior still remained simple classical, but the dependence of the diffusion coefficients on mercury pressure became more complex, progressing from a P _-Hg ^-3 dependence to a P _-He ^-1 dependence. This transition was most apparent at 425°C. With additional decrease in the Hg pressure, the transport behavior became more complex and could no longer be described by a simple classical or a monocomponent concentration independent Fickian model. This range was not studied during the present investigations. Additional progress in tuning the positioning of the p/n junction for double layer heterojunction films was possible from the results obtained. This was demonstrated by comparing diode performances obtained on heterojunctions films subjected to seleccted anneal sequences.     

不同钝化层结构对HgCdTe热退火Hg空位调控影响

对不同钝化层结构的分子束外延（MBE）生长的HgCdTe外延材料的Hg空位浓度控制进行研究。获得了更高Hg空位浓度调控范围的外延材料,为后续新型焦平面器件的研发提供基础。研究发现,在热退火过程中,HgCdTe外延材料的Hg空位浓度的变化随着钝化层结构的不同而发生改变。且这种改变是因为HgCdTe表层的钝化层的存在改变了原始热退火的平衡态过程。同时,通过二次离子质谱（SIMS）测试以及相应的理论拟合进行了验证。     

As离子注入碲镉汞的p+n结红外探测器

讨论了制作pn结的另一种途径,即在n型HgCdTe材料上离子注入As+,然后通过辐射快速退火形成反型p+层来获得p+n结.通过对制作工艺及实验结果的讨论,阐述了两种制作pn结工艺的特点.实验表明,采用辐射快速退火工艺可以在短时间内完成对HgCdTe注入表面的缺陷退火和杂质激活,而不会改变HgCdTe 组份.已用此方法在注入As+的n型HgCdTe样片上制作出性能较好的p+n结光伏红外探测器.     

碲镉汞As掺杂技术研究

对于MBE原位掺杂,HgCdTe的N型掺杂比较容易,而P型掺杂相对来说难度比较大。作为掺杂杂质的As表现出两性掺杂行为,在富Te的条件下生长,As有很大的几率进入到阳离子位置处。而As必须进入Te位才能参与导电,表现为P型。因此,采取了多种方法,现已获得10^16-10^18坤cm^-3掺杂水平的P型材料。在成功实现As的掺杂后,研究人员对激活退火做了一些研究。研究发现,需要在汞压下经过高温退火,As原子才能占据Te位成为受主杂质。对As在碲镉汞中的扩散系数也进行了研究。     

采用(211)碲锌镉衬底制备碲镉汞液相外延薄膜

文中对(211)晶向的CdZnTe衬底进行液相外延生长HgCdTe。获得的碲镉汞液相外延材料的组分为0. 30 ～0. 33,薄膜厚度为10 ～15μm,表面缺陷密度为500cm- 2 ,材料的FWHM达到24弧秒,位错腐蚀坑密度约为2 ×105 cm- 2 ,该材料的表面形貌与采用(111)晶向衬底的HgCdTe外延材料有较大区别。     

Comparison of the diffusion of Hg into CdTe and Hg0.8Cd0.2Te

In this paper, results published recently on Hg diffusion in the important infrared detector material, Hg_0.8Cd_0.2Te, and its common substrate material, CdTe, are compared and discussed. As is customary with diffusion studies in II-VI semiconductors, two component profiles were obtained in the majority of cases, each profile giving two values of the diffusivity. The values of D for the mercury diffusion in CdTe were much less than corresponding values in Hg_0.8Cd_0.2Te diffusion proceeds by volume diffusion followed by short circuit diffusion, whereas in CdTe diffusion is rate limiting volume diffusion involving a slow stream and a fast stream. From pressure dependency measurements, it is proposed that the slow component occurs by an interstitial mechanism at low P_Hg and a vacancy mechanism at high P_Hg, whereas in Hg_0.8Cd_0.2Te the reverse occurs for the fast diffusion component. In CdTe, the slow component increases systematically with etch pit density, whereas in Hg_0.8Cd_0.2Te, the diffusivity is independent of the quality of the material. From the comparisons, it is seen that there are some common features in the diffusion of Hg in the two materials but there are also some clear and distinct differences.