Doping behavior of iodine in Hg0.8Cd0.2Te+

Iodine doped single crystal samples of Hg_0.8Cd_0.2Te were annealed at temperatures varying from 450 to 600°C in Hg vapor and quenched to room temperature. Hall effect measurements at 77 K on the crystals cooled to room temperature indicate the samples to be n-type after anneals at high Hg pressures whereas they turn p-type after anneals at low Hg pressures; the electron concentration increases with increase in Hg pressure. The results are explained on the basis that the crystals are saturated with (Hg,Cd)I₂, with the iodine being present as donors occupying tellurium lattice sites (I_Te) and a fraction being present as (I_TeV_Hg)’ species formed from the iodine on tellurium lattice sites (I_Te) pairing with the doubly ionized native acceptor defects (V^Hg/11). The solubility of the I_Te species increases with increase in Hg pressure, whereas that of the (I_TeV_Hg)’ species increases with decrease in Hg This work was supported by NASA under contract NAS8-33245 pressure. Equilibrium constants for the incorporation of the iodine species as well as the pairing reaction have been established     

Bulk growth of Hg0.8Cd0.2Te single crystals by a combined CVT-seeding technique

The growth of high quality Hg_0.8Cd_0.2Te bulk single crystals by CVT, combined with an in-situ seeding technique, is reported here for the first time. For this purpose, a temperature difference of 590° → 540° C with a gradient of 40°-50° C/cm at the solid-vapor interface, and about 0.1 atm of HgI₂ as a transport agent, were employed. The bulk crystals have the expected stoichiometry and compositional homogeneity. Etch pit densities of 10⁴-10⁵ cm⁻² on the (111) face and hitherto unreported etch pits on the (100) face were observed in this work. Possible origins of the sub-grain structure are discussed.     

Interface formation between deposited Sn and Hg0.8Cd0.2Te

The structure of the interface formed by the reaction of deposited Sn on Hg_0.78Cd_0.2Te(lll)B was investigated by hemispherically scanned x-ray photo-electron spectroscopy including x-ray photoelectron diffraction (XPD). The interface formation was found to proceed as follows: At the onset of Sn deposition, Hgis expelled and substituted by Sn in the topmost monolayer of the Hg0 78Cd0 22Te lattice while the zinc-blende structure of the original surface is maintained. With further Sn deposition (and further loss of Hg), an epitaxial layer of cubic SnTe (with inclusions of CdTe) was found to grow. At room temperature, the SnTe growth stopped after a few monolayers, and the epitaxial growth of cubic a-Sn was observed to start on top of it. At elevated deposition temperatures, the SnTe intermediate layer continued to grow up to several 100dgA.     

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.     

Hg0.8Cd0.2Te native defects: Densities and dopant properties

We examine the native defect equilibrium in HgCdTe, including cation and anion vacancies, interstitials, and antisites in the analysis. A gradient correction to the local density functional has been added to the defect formation enthalpies calculated within the local density approximation, and preliminary predictions of the dominant ionization states are made. Temperature-dependent defect formation entropies and the temperature dependence of the pre-exponentials are incorporated into the calculation of the defect densities. Degenerate Fermi-Dirac statistics are used for the electronic equilibration, and the intrinsic reaction constant as a function of composition and temperature is calculated. We theoretically substantiate the doubly ionized mercury vacancy as the dominant defect in HgCdTe, and expect the doubly ionized mercury vacancy densities to be comparable in HgZnTe. We predict that tellurium antisites are donors and will be present for some annealing conditions in sufficient quantities to be measured and possibly to affect device performance.     

The existence region of the Hg0.8Cd0.2Te phase field

The existence region of the Hg0.8Cd_0.2Te phase field has been re-examined using three techniques: Hall measurements, p-n junction depth measurements on material which had been annealed to produce an n-type skin following equilibration at high temperature, and quantitative modelling of experimental observations of gettering of silver in (Hg,Cd)Te. The results we obtain are generally in good agreement with previously reported measurements, although some systematic disagreement is found, particularly under tellurium saturated conditions for temperatures less than 450‡C.     

非晶态碲镉汞的暗电导率和吸收边

研究了非晶态碲镉汞(x=0.2)薄膜的暗电导率随温度变化关系,发现非晶态结构的碲镉汞材料具有明显的半导体特性,其室温禁带宽度在0.88～0.91 eV之间,与通过光学方法获得的结果相符.在80～240K的温度区间非晶态碲镉汞(x=0.2)的暗电导率从1×10-8Ω-1·cm-1缓慢增大到5×10- 8Ω-1-cm-1,温度大于240K时,其电导率剧烈增大到1×10-5Ω-1·cm-1,说明在240K附近非晶态碲镉汞材料的导电机制发生了变化,这对非晶态碲镉汞材料的应用研究具有重要意义.还研究了退火过程对非晶态碲镉汞薄膜电导率的影响,结果表明140℃退火后非晶态碲镉汞薄膜发生了部分晶化.     

非晶态碲镉汞薄膜晶化过程的椭圆偏振光谱研究

采用椭圆偏振光谱技术研究了非晶态碲镉汞薄膜在不同退火条件下的结构性能.结果表明非晶态碲镉汞薄膜在退火过程中的成核晶化是在薄膜内部均匀发生的,对于不同晶化程度的薄膜,其光学常数谱具有明显的特征,通过对光学常数谱的分析研究可以对非晶态碲镉汞薄膜的晶化程度进行量化表征,从而控制退火条件,优化材料质量.     

A comparison of the diffusion of iodine into CdTe,Hg0.8Cd0.2Te and Zn0.05Cd0.95Te

Studies on the diffusion of iodine into CdTe, mercury cadmium telluride (Hg_0.8Cd_0.2Te, referred to as MCT) and zinc cadmium telluride (Zn_0.5Cd_0.95Te, referred to as ZCT) in the temperature range of 20 to 600°C are compared and discussed. The concentration profiles were measured using a radiotracer sectioning technique. As with the diffusion studies using the halogens into CdTe, the profiles were composed of four parts to which a computer package consisting of the sum of four complementary error functions (erfc) gave satisfactory fits. The diffusivity for the diffusion of iodine into MCT was faster than for the diffusion into CdTe, which was faster than for the diffusion into ZCT. The high diffusivity for the fastest profile part at 20°C indicates that when iodine is diffused from the vapor into these materials, it is not a suitable long term stable dopant in devices where sharp junctions are required.     

The direct determination of the vacancy concentration andP-T phase diagram of Hg0.8Cd0.2Te and Hg0.6Cd0.4Te by dynamic mass-loss measurements

A modified mass-loss measurement technique is employed, for the direct,in- situ determination of the metal vacancy formation in (Hg_1x2212;itxCd _x )_1−y Te _y (s) withx = 0.2 and 0.4. Forx = 0.2, the metal vacancy concentrations are determined between 336 and 660° C for three different compositions(y) within the homogeneity region and range from 2.4 to6.8 x 10¹⁹cm₋₃. The enthalpy of formation of a singly-ionized metal vacancy is derived to be between 0.17 and 0.45 eV depending upon the deviation from stoichiometry (compositiony). Forx= 0.4, three samples of different y-values give the metal vacancy concentrations from 1.9 to 5.5 x 10¹⁹cm⁻³ between 316 and 649° C, and the enthalpy of vacancy formation between 0.25 and 0.40 eV. Compared to the recent data on HgTe(s), these experimental results show a slight but significant decrease in the enthalpy of vacancy formation from HgTe to Hg_0.8Cd_0.2Te, which supports theoretical predictions of the bond weakening effect of Cd for the latter alloy system. Based on the simultaneously determined equilibrium Hg partial pressures within the homogeneity range, the vacancy concentration-partial pressure isotherms are constructed. The Hg partial pressures are also measured along the three-phase boundaries of the solid solutions of bothx = 0.2 and 0.4, and these are in close agreement with published data obtained by optical absorption measurements.     

The mechanisms of hole scattering in p-Hg0.8Cd0.2Te crystals at low temperatures

The resistivity and Hall effect were investigated in p-Hg_0.8Cd_0.2Te crystals that contained from 1.5×10¹⁵ to 1.7×10¹⁸ cm^?3 Cu atoms. The measurements were carried out in the temperature range of 4.2–100 K. It is demonstrated that, in order to correctly determine the Hall mobility of holes at low temperatures, one should exclude the contribution of hopping charge transfer. It was found that heavy holes are scattered at 77 K by each other, by impurity ions, by composition fluctuations, and by lattice vibrations. In compensated crystals, holes are scattered only by lattice vibrations at low temperatures. For uncompensated crystals, when calculating the mobility, it is necessary to make allowance for the hole scattering by positively charged centers formed due to trapping of excess holes by acceptors.     

Cd0.8Mn0.2Te晶体的生长及其性能

通过适当的工艺措施,采用传统布里奇曼法生长了尺寸为φ30mm×120mm的Cd0.8Mn0.2Te晶体.对晶体进行了X射线粉末衍射、X射线双晶摇摆曲线、紫外-可见光光谱、红外透过率及电阻率测试.测试结果表明,晶体结构为立方型,半峰宽较低,吸收边为720nm,对应禁带宽度为1.722eV,晶体的红外透过率和电阻率都较高.并讨论了晶体中的缺陷对红外透过率和电阻率的影响.     

Cd0.8 Mn0.2Te晶体的生长及其性能

通过适当的工艺措施,采用传统布里奇曼法生长了尺寸为φ30mm×120mm的Cd0.8Mn0.2Te晶体.对晶体进行了X射线粉末衍射、X射线双晶摇摆曲线、紫外-可见光光谱、红外透过率及电阻率测试.测试结果表明,晶体结构为立方型,半峰宽较低,吸收边为720nm,对应禁带宽度为1.722eV,晶体的红外透过率和电阻率都较高.并讨论了晶体中的缺陷对红外透过率和电阻率的影响.     

Special features of hopping conduction in p-Hg0.78Cd0.22Te crystals under conditions of dual doping

At T=4.2–125 K, the electrical conductivity and Hall effect were studied in p-Hg_0.78Cd_0.22Te crystals that contained 3×10¹⁶ cm^?3 Cu atoms and 1.83×10¹⁶ cm^?3 of Hg vacancies (either simultaneously or independently of each other). In such crystals, the ?₁ conductivity over the valence band is dominant at temperatures above 10–12 K, whereas the hopping conduction is prevalent at temperatures below 8–10 K. In the samples containing copper atoms and mercury vacancies simultaneously, conductivity with variable-range hopping is observed. It is found that the ?₁ conductivity of the copper-doped crystals is independent of the presence of mercury vacancies, whereas the hopping conductivity increases appreciably if these vacancies are introduced into the undoped crystal. This phenomenon is attributed to attachment of holes to the neutral mercury vacancies. The energy of this attachment is calculated, and it is found that this energy is equal to 3.7 meV for the ground state. The fluctuation-related broadening of the impurity band in the solid solutions gives rise to the overlap of the impurity bands formed by the copper acceptor levels and by the levels of holes attached to vacancies.     

Donor-like localization and activated conduction in compensatedn-Hg0.8Cd0.2Te

We report first results on the activated resistivity inn- type Hg_0.8Cd_0.2Te crystals heavily compensated through native acceptor defects (double acceptor mercury vacancies). Three activation energies are reported, all of which increase with increasing compensation. In conjunction with the variation of Hall coefficient and Hall mobility with temperature we conclude, i) formation of (donor⁺-acceptor⁻) pairs during stoichiometric annealing, ii) distortion of second acceptor states of the paired mercury vacancies to raise them near, or in the conduction band, and iii) the pair states, when occupied with electrons in the presence of excess unpaired donors, behave like localized donors, and when unoccupied, like electron traps and/or recombination centres.     

Effect of an electric field on the relaxation of photoconductivity in n-Hg0.8Cd0.2Te crystals

The effect of a pulling electric field on the relaxation curves of the photoconductivity of n-Hg_0.8Cd_0.2Te crystals has been investigated. It is shown that as the field intensity increases, the relaxation time of the slow component increases and that of the fast component decreases. The contribution of the slow component also decreases. This behavior of photoconductivity relaxation is due to the change in the energy-band bending near macrodefects in the presence of an electric field and to a change in the feeding of nonequilibrium charge carriers to the macrodefects. Fiz. Tekh. Poluprovodn. 31, 936–938 (August 1997)     

在TEA-CO2强激光脉冲作用下Hg0.8 Cd0.2 Te晶片表面的组分变化

利用场发射扫描电子显微镜对TEA-CO2强激光脉冲辐照的Hg0.8Cd0.2Te晶片表面进行了观察,并利用电镜自带的能谱分析仪对其表面进行了成分分析.在单脉冲能量为1.91 J,峰值功率密度为2.63×107W/cm2的脉冲CO2激光辐照下,晶片表面呈现出熔融迹象,且晶片表面的化学组分比发生明显的变化.理论与实验研究结果表明:激光急速加热使晶体表面的Hg-Te键破坏,从而导致Hg损失,而Hg损失程度与热作用过程的时间有关.随着脉冲作用次数的增加,多脉冲的连续作用使Hg损失加剧,晶片表面成分变化更加突出.     

高阻碲锌镉单晶体的生长及其性能观测

报道了采用富Cd原料的无籽晶垂直布里奇曼法生长高阻碲锌镉Cd0.8Zn0.2Te(CZT)单晶体的新工艺,对所生长的晶体作了X射线衍射分析、红外透过率测试、光吸收截止波长测量及电学性能测试.晶体在4400～450cm-1范围内的红外透过率达到50%,截止吸收波长为787.6nm,带隙为1.574eV,室温电阻率达到2×1010Ω·cm,已接近本征Cd0.8Zn0.2Te半导体的理论值.用该晶体制作的核探测器在室温下对241Am和109Cd放射源均有响应,并获得了比较好的241Am-59.5keV吸收谱.结果表明改进的方法是一种生长室温核辐射探测器应用的高阻CZT单晶体的简便有效的新方法.     

高阻碲锌镉单晶体的生长及其性能观测

报道了采用富Cd原料的无籽晶垂直布里奇曼法生长高阻碲锌镉Cd0.8Zn0.2Te(CZT)单晶体的新工艺,对所生长的晶体作了X射线衍射分析、红外透过率测试、光吸收截止波长测量及电学性能测试.晶体在4400～450cm-1范围内的红外透过率达到50%,截止吸收波长为787.6nm,带隙为1.574eV,室温电阻率达到2×1010Ω·cm,已接近本征Cd0.8Zn0.2Te半导体的理论值.用该晶体制作的核探测器在室温下对241Am和109Cd放射源均有响应,并获得了比较好的241Am-59.5keV吸收谱.结果表明改进的方法是一种生长室温核辐射探测器应用的高阻CZT单晶体的简便有效的新方法.     

Electron-probe microanalysis of doped PbTe and Pb0.8Sn0.2Te single crystals

The distribution of the elements in naturally faceted PbTe and Pb_0.8Sn_0.2Te single crystals doped with zinc and cadmium is analyzed. The results show the formation of a multilayer structure with an outer layer of ZnTe (CdTe) on top of a metallic sublayer. Fiz. Tekh. Poluprovodn. 31, 980–982 (August 1997)