非晶碲镉汞薄膜的暗电导与光电导

This paper reports the dark conductivity and photoconductivity of amorphous Hg_(0.78)Cd_(0.22)Te thin films deposited on an Al_2O_3 substrate by RF magnetron sputtering.It is determined that dark conduction activation energy is 0.417 eV for the as-grown sample.Thermal quenching is absent for the as-grown sample during the testing temperature zone,but the reverse is true for the polycrystalline sample.Photosensitivity shows the maximum at 240 K for amorphous thin films,while it is higher for the as-grown ...     

Persistent photoconductivity of amorphous Hg0.78Cd0.22Te: In films

The persistent photoconductivity (PPC) of amorphous Hg_0.78Cd_0.22Te: In films has been studied under illumination by super-bandgap light (a He-Ne laser, hν=1.96 eV, 30 mW/cm²) and sub-bandgap light (1000 K Blackbody source, the largest photon energies hν_p=0.42 eV, 8.9 mW/cm²) in the range of 80-300 K. The persistent photoconductivity effect increases with increase in illumination intensity and illumination time. However, it decreases with increase in working temperature. The non-exponential decay of photoconductivity implies the presence of continuous distribution of defect states in amorphous Hg_0.78Cd_0.22Te: In films. These results indicate that the decay of photoconductivity is not governed by the carrier trapped in the intrinsic defects, but it may be due to light-induced defects under light illumination.     

Steady state and transient photoconductivity in amorphous thin films of Se0.8Te0.2

The present paper reports on steady state and transient photoconductivity measurements in amorphous thin films of Se_0.8Te_0.2. The paper also reports the effect of crystallization on the photoconductive behaviour. The photosensitivity(I _ph /I _d ) reduces by a factor of 5 and the decay of photocurrent becomes much slower in the films annealed above the crystallization temperature (state B) as compared to the films annealed below the crystallization temperature (state A). A detailed analysis of the photoconductive decay in the state B shows that the recombination within localized states may be the predominant recombination mechanism in this state.     

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.     

The minority carrier lifetime in doped and undoped p-type Hg0.78Cd0.22Te liquid phase epitaxy films

This paper will describe: (1) the first comparative study of recombination mechanisms between doped and undoped p-type Hg_1-xCd_xTe liquid phase epitaxy films with an x value of about 0.22, and (2) the first determination of τ_A7 ⁱ/τ_A1 ⁱ ratio by lifetime’s dependence on both carrier concentration and temperature. The doped films were either copper- or gold-doped with the carrier concentration ranging from 2 x 10¹⁵ to 1.5 x 10¹⁷ cm^-3, and the lifetime varied from 2 μs to 8 ns. The undoped (Hg-vacancy) films had a carrier concentration range between 3 x 10¹⁵ and 8 x 10¹⁶ cm^-3, and the lifetime changed from 150 to 3 ns. It was found that for the same carrier concentration, the doped films had lifetimes several times longer than those of the undoped films, limited mostly by Auger 7 and radiative recombination processes. The ineffectiveness of Shockley-Read-Hall (SRH) recombination process in the doped films was also demonstrated in lifetime vs temperature curves. The important ratio of intrinsic Auger 7 lifetime to intrinsic Auger 1 lifetime, τ_A7 ⁱ/τ_A1 ⁱ, was determined to be about 20 from fitting both concentration and temperature curves. The reduction of minority carrier lifetime in undoped films can be explained by an effective SRH recombination center associated with the Hg vacancy. Indeed, a donor-like SRH recombination center located at midgap (E_v+60 meV) with a capture cross section for minority carriers much larger than that for majority carriers was deduced from fitting lifetime vs temperature curves of undoped films.     

退火对非晶态碲镉汞薄膜稳定态光电导的影响

利用射频磁控溅射方法,在宝石衬底上制备了非晶态碲镉汞(a-HgCdTe)薄膜。对原生a-HgCdTe薄膜进行了不同退火时间和不同退火温度的热退火,在80～300K温度范围内,分别测量了原生和退火处理后的a-HgCdTe薄膜样品的稳定态光电导,研究了退火时间和退火温度对非晶态HgCdTe薄膜的稳定态光电导和激活能的影响。结果表明,原生和退火a-HgCdTe薄膜的稳定态光电导具有热激活特性;随着退火时间增加或退火温度升高,a-HgCdTe薄膜的晶化程度提高,导致光电导增大,光电导激活能降低。利用非晶-多晶转变机制讨论了实验结果。     

Electrical effects of dislocations and other crystallographic defects in Hg0.78Cd0.22Te n-on-p photodiodes

An understanding of the effects of dislocations in HgCdTe diodes is complicated by several issues such as the diode architecture, diode formation process, and the thermal history and location of the dislocations. To help decouple the effects of these factors, high stress films were used to lithographically introduce dislocations with different densities and locations during the fabrication process of ion implanted, n-on-p diodes. Both array and diode test structures were studied. After fabrication, the diodes were characterized with variable temperature I–V measurements and noise measurements. The diodes were then stripped and defect etched to quantify the density and distribution of the dislocations. The effects of these process-induced dislocations were analyzed and compared to the effects of as grown dislocations, subgrain boundaries and dislocations in other device architectures reported in the literature.^1,2 In general, high densities of either as grown or process-induced dislocations in n-on-p, ion implanted diodes severely degrade device performance by producing field dependent dark current At 77K, dislocation densities greater than the mid 10⁶ cm⁻² can produce dark current densities in excess of the diode diffusion current. Dislocations located near the outer periphery of the diode produce approximately ten times the dark current of interior dislocations. Grain boundaries, sub-grain boundaries, and twins also produce sufficient field dependent dark current to limit diode performance at 77K. The dark current produced by dislocations is nearly temperature in dependent, suggesting rather severe limitations on dislocation densities for low temperature diode operation.     

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.     

Hg0.6Cd0.4Te的激子光电导谱

报道了Ｈｇ０．６Ｃｄ０．４Ｔｅ红外光电导谱，在本征吸收边的低能侧观察到束缚激子引起的分立光电导响应     

The magnetic field dependence of R0A products in n-on-p homojunctions and p-on-n heterojunctions from Hg0.78Cd0.22Te liquid phase epitaxy films

The analysis of R₀A products as a function of magnetic field in n-on-p diodes using a simple diffusion current model has previously been shown to yield both J_ep/J_total ratio (the relative contribution of the p-side diffusion current) and μ_ep (the minority carrier, electron mobility). In this paper, we report the good agreement between the experimental and theoretical dependence of μ_ep on the hole concentration over a wide range between 1 x 10¹⁶ and 4 x 10¹⁷ cm⁻³ in n-on-p homojunction diodes fabricated on undoped p-type Hg_0.78Cd_0.22Te liquid phase epitaxial (LPE) films. The averaged J_ep/J_total ratio varied between 68 and 90% with the hole concentration. These J_ep/J_total ratios indicate that other leakage current mechanisms than the p-side diffusion current were not negligible. Also, for the first time, comparative measurements were made on p⁺/n heterojunction diodes consisting of As-doped Hg_0.07Cd_0.30Te and In-doped Hg_0.78Cd_0.22Te LPE layers. Unlike a typical change in R₀A products by a factor of 2–3 in n-on-p homojunction diodes, the R₀A products in p⁺/n heterojunction diodes at 7 kG were typically only 2–3% higher than that at the zero field. The typical J_ep/J_total ratio in p⁺/n heterojunction diodes was about 3–4 %, which confirms the general belief that the p⁺ cap layer, due to the high doping and a larger bandgap, contributes very little to the total leakage current.     

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.     

BF3-doped amorphous silicon thin films

BF₃ has been used as a p-type dopant in thin films of hydrogenated amorphous silicon (a-Si:H). The films were deposited in a capacitively coupled radio frequency (13.56 MHz) glow discharge system in which silane (SiH₄) was the principal gaseous component. The boron content in the films was measured by secondary ion mass spectroscopy (SIMS) using a calibrated boron-implanted silicon standard. The optical and transport properties of these films were measured and compared with films of similar boron film content deposited using B₂H₆ BF₃-doped films showed less sub-band gap absorption (as determined by photothermal deflection spectroscopy), no band gap narrowing, and higher conductivity prefactors σ_o for low doping levels compared to B₂H₆-doped films. The possible utility of lightly BF₃-doped a-Si:H as the i-layer in a p/i/n photovoltaic device is suggested.     

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.     

Facile solvothermal synthesis and photoconductivity of one-dimensional organic Cd(Ⅱ)-Schiff-base nanoribbons

One-dimensional (1D) organic nanoribbons built on N-p-nitrophenylsalicylaldimine cadmium complex were synthesized via a facile solvothermal route. Scanning electron microscope images revealed that the as-synthesized products were ribbon like with widths of 500 nm,thicknesses of about 50nm, and lengthes up to several hundred micrometers. Fourier transform infrared spectrum was employed to characterize the structure. The conductivity of a bundle of nanoribbons was also measured,which showed that Schiff base cadmium nanoribbons had good photoconductive property. This work might enrich organic photoconductive materials and be applicable in nano photoswitch devices in the future.     

Diffusion of Cd and Hg in liquid Te

The diffusion coefficients (D) of Cd and Hg in liquid Te were measured at 465-495° C. D was found to depend on concentration for concentrated solutions, and to have a minimum value for saturated solutions.     

As Doping in (Hg,Cd)Te: An Alternative Point of View

Acceptor doping of many II–VI compound semiconductors has proved problematic and doping of epitaxial mercury cadmium telluride (MCT, Hg_1−x Cd _x Te) with arsenic is no exception. High-temperature (>400°C) anneals followed by a lower temperature mercury-rich vacancy-filling anneal are frequently required to activate the dopant. The model frequently used to explain p-type doping with arsenic invokes an amphoteric nature of group V atoms in the II–VI lattice. This requires that group VI substitution with arsenic only occurs under mercury-rich conditions either during growth or the subsequent annealing and involves site switching of the As. However, there are inconsistencies in the amphoteric model and unexplained experimental observations, including arsenic which is 100% active as grown by metalorganic vapor-phase epitaxy (MOVPE). A new model, based on hydrogen passivation of the arsenic, is therefore proposed.     

Sensitive thermoelectric power and conductivity measurements on implanted polyparaphenylene thin films

A sensitive apparatus for determining the conductivity and thermoelectric power of conducting polymer films at temperatures between 150 and 450 K is described. Brittle polyparaphenylene films doped by ion implantation with either alkali (caesium, sodium) or halogen (iodine) ions are studied by this method. Analysis of the temperature dependence of both conductivity and thermopower shows that the doping process of this conjugated polymer is efficient only when low parameters of implantation are selected and appears around room temperature. At higher temperatures the thermopower exhibits an anomalous evolution that we attribute to oxygen migration. © 1997 John Wiley & Sons, Ltd.