The preexponential factor in Mott’s law for variable-range-hopping conduction in lightly compensated p-Hg0.8Cd0.2Te crystals

Hopping conduction with variable range was studied in undoped p-Hg_0.8Cd_0.2Te crystals with the Hg-vacancy concentration varying from 10¹⁶ to 4×10¹⁷ cm^?3 in the temperature range of 4.2–125 K. The temperature and concentration dependences of the preexponential factor ρ_0M in Mott’s law was determined. It is found that the temperature dependence of ρ_0M is in good agreement with the theoretical calculations performed in the one-particle approximation. At the same time, ρ_0M depends heavily on the Hg-vacancy concentration and parameter T ₀ in Mott’s law, which is in contradiction with this theory. The experimental dependence of ρ_0M on both parameters has a power-law form; the exponent is equal to 2.3–2.4. It is assumed that the concentration dependence of ρ_0M is typical of double-charged acceptors. This fact can be explained by the capture of a second hole by the Hg vacancy.     

The concentration dependence of acceptor-state radii in p-Hg0.78Cd0.22Te crystals

Hopping conduction in undoped p-Hg_0.78Cd_0.22Te crystals containing native double-charged acceptors (Hg vacancies) with concentrations of 10¹⁶–10¹⁸cm^?3 was studied. Electrical conduction with a variable hopping range is dominant in the entire concentration range at temperatures below 6–16 K. The measured parameters of this conduction were used to calculate the acceptor-state radius as a function of vacancy concentration N _A . It is shown that, for N _A <4×10¹⁷ cm^?3, the low-temperature conduction occurs via the vacancy states whose radius is independent of N _A . For N _A >5×10¹⁷ cm^?3, the hopping conduction is governed by the states of uncontrolled shallow-level impurity acceptors. The radius of the state for these defects increases with increasing N _A owing to an increase in the effective permittivity of the medium.     

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.     

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.     

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.     

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.     

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     

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)     

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.     

Ionization energy of copper in Hg0.8Cd0.2Te crystals under conditions of light and intermediate doping

Resistivity and the Hall effect were studied in copper-doped p-Hg _0.8 Cd _0.2 Te crystals in the temperature range of 4.2–125 K and the range of Cu concentrations from 2.6×10¹⁵ to 2×10¹⁸ cm^?3. It is shown that the conventional method for determining the ionization energy of impurities from the slope of the dependence R _H (T) is inapplicable in this case. In order to obtain the correct results, it is necessary to take into account the structure of the impurity band and the screening of the impurity charge with free charge carriers. A simplified model of the impurity band is suggested; this model makes it possible to calculate the ionization energy of acceptors under conditions of light doping and a small degree of compensation. This approach is used to find that ionization energy of copper depends only slightly on the copper concentration at T=0 and is equal to E _A =7.6 meV for an isolated acceptor, which coincides with the theoretical value. At finite temperatures, the ionization energy of acceptors decreases appreciably as a result of screening.     

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.     

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.     

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.     

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.     

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.     

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.     

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)     

Metallic conductivity over an acceptor band of lightly compensated copper-doped p-Hg0.78Cd0.22Te crystals

The conductivity and Hall effect of heavily doped p-Hg_0.78Cd_0.22Te:Cu crystals were studied in the temperature range of 4.2–125 K. The conductivity over the impurity band is of a metallic type for the acceptor concentration N _A>3.8×10¹⁷ cm^?3. The conductivity and the Hall coefficient governed by the delocalized charge carriers in the impurity band are independent of temperature. The sign of the Hall effect is positive in the metallic conductivity range. Near the metal-insulator transition point, the Hall mobility increases linearly with the acceptor concentration and is independent of the acceptor concentration at N _A>1.6×10¹⁸ cm^?3. The metallic conductivity is proportional to N _A in the concentration range under study at N _A<3.1×10¹⁸ cm^?3. The Anderson transition occurs at the Cu concentration N _A=1.4×10¹⁷ cm^?3 in the A ⁺ impurity band, which is formed by positively charged acceptors. Minimum metallic conductivity corresponding to this transition equals 5.1 Ω^?1 cm^?1. It is shown that ?₂ conductivity in the subthreshold region is defined by delocalized carriers in the upper Hubbard band only for fairly heavy doping (N _A>1.4×10¹⁷ cm^?3). For N _A<1.4×10¹⁷ cm^?3, the hopping conductivity is observed.     

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.