Effect of different types of surface treatment on the photoelectric and optical properties of CdTe

The photoconductivity and light absorption spectra of p-type CdTe single crystals subjected to different treatments have been investigated. It is shown that the photoconductivity and edge absorption in mechanically polished and plastically deformed crystals are determined by the damaged layer. The role of dislocations in the formation of the edge absorption and the photoconductivity spectrum is determined. Fiz. Tekh. Poluprovodn. 31, 1428–1430 (December 1997)     

Photoconductivity and luminescence of CuInSe2 single crystals at a high level of optical excitation

The luminance-current and spectral characteristics of photoluminescence of the CuInSe₂ single crystals are studied. The superlinear portion of the excitation-intensity dependence of photoconductivity at low excitation intensities in compensated p-CuInSe₂ crystals is explained on the basis of a recombination model. The emission band that peaked at 0.98 eV in the n-CuInSe₂ photoluminescence spectrum corresponds to radiative recombination of electrons at the donor level with a depth of 0.04 eV. The maximum in the band intensity corresponds to the energy gap between the trap level and the valence band.     

Photoconductivity of the germanium-doped solid solution p-GaAs0.94Sb0.06

A new mechanism of impurity photoconductivity in semiconductors has been discovered. The form of the long-wavelength photoconductivity spectra observed in p-GaAs_0.94Sb_0.06: Ge is satisfactorily explained in terms of resonance ionization of impurity levels by phonons excited during absorption of infrared radiation. Fiz. Tekh. Poluprovodn. 31, 475–477 (April 1997)     

Variable temperature hall effect on p-Hg1−xCdxTe grown on CdTe and sapphire substrates by liquid phase epitaxy

Variable temperature Hall effect measurements have been made down to 9–10K on p-type Hg_1−xCd_xTe grown by liquid phase epitaxy on both CdTe and sapphire substrates. Carrier freeze-out was usually observed throughout the measured temperature range. For most samples, the hole mobility was well-behaved and exhibited a maximum at ˜ 35K. Values of acceptor ionization energy E_A and donor concentration N_D were estimated from the data, using a model assuming significant compensation, which provided a good fit to the low temperature data. In addition, values of N_D were also estimated from an analysis of the low temperature mobility using the hole effective mass as a parameter to provide reasonable agreement between the N_D values calculated from the Hall coefficient and mobility data. The measured carrier concentration is a result of close compensation between stoichiometric acceptors and donors, with N_D usually in the low-10¹⁷ cm⁻³ range. Average values of E_A for as-grown, undoped x = 0.32 layers on CdTe and sapphire substrates are 7.4 and 6.6 meV, respectively. An activation energy of 0.84 meV was determined for a Cu-doped x = 0.32 layer that was annealed in Hg vapor to reduce the number of Hg vacancies. The average E_A for undoped Hg-annealed x = 0.22 layers on CdTe substrates is 2.35 meV. Layers with x = 0.32 grown on sapphire substrates have average carrier concentrations of 2.92 (σ = 0.54) × 10¹⁶ cm⁻³, compared with 4.64 (θ = 1.26) × 10¹⁶ cm⁻³ for the same composition layers grown on CdTe substrates.     

Electrical activity of iodine-diffused CdTe

The electrical activity of iodine in CdTe is discussed when iodine is introduced into the CdTe by diffusion from the vapour phase. It is compared both with the total concentration of iodine in the diffused CdTe slices and with the electrical activity in CdTe slices which have been annealed under Cd- and Te-saturated vapour pressures. Iodine-diffused slices of CdTe were profiled by secondary ion mass spectrometry (SIMS) to obtain the total iodine concentration and by capacitance-voltage (C-V) techniques to obtain the net concentration of electrically active iodine. After annealing with iodine in the form of Cdl₂, the slices were p-type, similar to those obtained when CdTe was annealed in either excess Te or Cd vapour, and they showed no significant increase in electrical activity. If Cd was added to the Cdl₂ diffusion source or the CdTe was given a subsequent anneal in cadmium vapour, the slices became n-type. The results indicated that in all cases a neutral layer composed of Cd _nI_m (m and n are integers) formed on the surface layers, whereas if Cd was involved in the diffusion, some of the iodine existed in an electrically active form deeper into the slice with a maximum concentration of active carriers given by N_D - N_A ≈ 10¹⁷ cm⁻³. © 1997 John Wiley & Sons Ltd.     

Dimethylarsine: Pyrolysis mechanisms and use for OMVPE growth

The pyrolysis mechanisms of dimethylarsine (DMAsH) have been studied mass spectrometrically in an atmospheric pressure flow tube reactor. In either the D₂ or He ambient, DMAsH will be converted to trimethylarsine (TMAs) at temperatures of 400–500° via a homogeneous CH₃ radical chain reaction. Supplemental CH₃ radicals, produced from azomethane ((CH₃)₂N₂) pyrolysis, have resulted in a significant increase in the pyrolysis rate for DMAsH. As temperature is increased beyond 500°, the product TMAs will decompose due to hydrogenolysis in D₂ and homolysis in He. At a GaAs surface, DMAsH pyrolyzes heterogeneously. The pyrolysis rate is further accelerated by the addition of trimethylgallium (TMGa). DMAsH has also been combined with TMGa to grow GaAs layers. The as-grown epilayers, at 1 atm and substrate temperatures of 570–720°, are allp-type with the net hole concentration dependent on the carrier gas. The use of N₂ leads to a higher concentration as compared to that in H₂. Photoluminescence spectra have indicated the acceptor to be carbon. A mechanism is developed to interpret these results based on the pyrolysis reactions determined from the kinetic studies.     

Influence of cation-vacancy imperfection on the electrical and photoelectric properties of the Cu1 − x Zn x InS2 alloy

The growth technology of single crystals of Cu_{1 ? x} Zn _x InS₂ alloys (x = 0–12) of n-type conductivity is developed. The formation mechanism of the alloy is investigated by X-ray structural analysis. It is shown that the single crystals have chalcopyrite structure, and the unit-cell parameters depend on the alloy composition. The temperature dependence of the electrical conductivity in the temperature range T = 27–300 K and the spectral distribution of the photoconductivity at T ≈ 30 K are investigated. Induced photoconductivity is found for CuInS₂-ZnIn₂S₄ with a content of ～8 and ～12 mol % ZnIn₂S₄ and thermally stimulated currents are investigated.     

Effect of bismuth impurity on carrier density in PbSe:Bi:Se epitaxial layers

The dependence of the Hall carrier density on bismuth concentration, n, p=f(N _Bi), in PbSe:Bi:Se/BaF₂ films has been studied. The films were grown by vacuum condensation from two independent molecular beams (PbSe:Bi and Se₂) mixed directly at the surface of a (111)BaF₂ substrate heated to 350°C. The bismuth concentration in the stock was 0–0.3 at. %. Two specific portions can be distinguished in the experimental n, p=f(N _Bi) dependence. At N _Bi>0.0375 at. %, the electron density is close to N _Bi; at low bismuth concentrations, N _Bi<0.0375 at. %, the linear run of the n=f(N _Bi) dependence is violated, and the conduction changes to p-type. All the doped films under study are saturated with selenium. This is a necessary condition for obtaining the highest electron densities in the films at N _Bi corresponding to the linear portion of the n=f(N _Bi) dependence. The results are discussed in terms of a thermodynamic model of the impurity interaction with intrinsic defects in PbSe, taking into account the amphoteric behavior of bismuth atoms in lead selenide.     

Mobility of minority charge carriers in <Emphasis Type="Italic">p</Emphasis>-HgCdTe films

Temperature dependences of electron mobility in p-Hg_1?xCd_xTe films (x=0.210–0.223) grown by molecular beam epitaxy are investigated. In the temperature range 125–300 K, mobility was found by the mobility-spectrum method, and for the range 77–125 K, it was found using a magnetophotoconductivity method suggested in this study. The method is based on the measurement of the magnetic-field dependence of photoconductivity. The magnetic field is parallel to the radiation and normal to the sample surface. The electron mobility is determined using the simple expression μ _n [m²/(V s)]=1/B _H [T]. Here, B _H is the induction of the magnetic field corresponding to a half-amplitude of the photoconductivity signal under zero magnetic field. In the temperature range 100–125 K, the results obtained by the magnetophotoconductivity and mobility-spectrum methods coincide. For the samples investigated, the electron mobility at 77 K is in the range 5–8 m²/(V s).     

Electrical properties of fine-grained polycrystalline CdTe

The temperature dependence of conductivity, the photoconductivity, and the hole mobility in fine-grained polycrystalline CdTe have been studied. The results obtained agree well with the concept of charged grain boundaries. The potential barriers at grain boundaries and the way in which these barriers affect the free carrier mobilities have been determined.     

Photoconductivity spectral characteristics of semiconductors with exponential fundamental absorption edge

An analysis has been performed of photoconductivity spectral characteristics of semiconductors with an exponential fundamental absorption edge as functions of the surface recombination rate and sample thickness. It is shown, in particular, that in crystals of Cd_xHg_1−x Te (x≈0.2) the spectral position of the photoconductivity maximum over a wide range of values of these parameters can be used, with an error not exceeding 1%, to determine the effective band gap and, consequently, the composition of the material. Fiz. Tekh. Poluprovodn. 33, 1295–1299 (November 1999)     

Absorptivity of semiconductors used in the production of solar cell panels

The dependence of the absorptivity of semiconductors on the thickness of the absorbing layer is studied for crystalline silicon (c-Si), amorphous silicon (a-Si), cadmium telluride (CdTe), copper indium diselenide (CuInSe₂, CIS), and copper gallium diselenide (CuGaSe₂, CGS). The calculations are performed with consideration for the spectral distribution of AM1.5 standard solar radiation and the absorption coefficients of the materials. It is shown that, in the region of wavelengths λ = λ _g = hc/E _g , almost total absorption of the photons in AM1.5 solar radiation is attained in c-Si at the thickness d = 7−8 mm, in a-Si at d = 30–60 μm, in CdTe at d = 20−30 μm, and in CIS and CGS at d = 3−4 μm. The results differ from previously reported data for these materials (especially for c-Si). In previous publications, the thickness needed for the semiconductor to absorb solar radiation completely was identified with the effective light penetration depth at a certain wavelength in the region of fundamental absorption for the semiconductor.     

Electrical properties of anisotype heterojunctions n-CdZnO/p-CdTe

Anisotype surface-barrier n-Cd_0.5Zn_0.5O/p-CdTe heterojunctions are fabricated by the high-frequency sputtering of a Cd_0.5Zn_0.5O alloy film onto a freshly cleaved single-crystal CdTe surface. The main electrical properties of the heterojunctions are studied and the dominant mechanisms of charge transport are established, namely, the multistage tunnel-recombination mechanism under forward bias, Frenkel-Pool emission, and tunneling under forward bias. The influence of the surface electrically active states at the heterojunction interface is analyzed and their surface concentration is evaluated: N _ss ?? 10¹⁴ cm^?2.     

Power broadening and nonlinear FIR magneto-photoconductivity in n-GaAs

The saturation of the photoconductivity due to 1s-2p₊ shallow donor transitions in n-GaAs has been investigated using a high power cw-FIR-laser. Magnetic field strengths were chosen in such a way that 2p₊ energy levels were either below or above the N=0 conduction band Landau level. In the former case 1s-p₊ transitions are found to be inhomogeneously broadened with a saturation intensity as low as 0.84 mW/cm², giving an effective lifetime of the 2p₊ state of 1.5 μs. Above the band edge the integrated photoconductivity does not saturate though the intensity-normalized peak photosignal decreases and the linewidth increases with raising intensity. This strange behaviour is tentatively attributed to optical excitations of 2p₊ electrons to higher lying electron Landau states.     

Effect of diffusion length and surface recombination on the photopleochroism of anisotropic crystals

Formulas were derived and numerical analysis was carried out for the dependences of photopleochroism coefficient P _i of the homogeneous anisotropic crystal on the diffusion length L and surface recombination rate. The polarization photoconductivity was considered in the region of both weak and strong optical absorption. The spectral contour of photopleochroism was shown to follow the optical dichroism curve at weak absorption and deviated from it or even reversed in sign in the case of appreciable recombination of charge carriers at the crystal surface at strong absorption. The limiting cases of zero and high surface recombination rates were considered for the dependence of the photopleochroism coefficient on the diffusion length. The dependences were analyzed by using the typical parameters of II-IV-V₂ ternary diamond-like semiconductors.     

Influence of holmium impurities on photoelectric properties of As2Se3 and (As2S3)0.3(As2Se3)0.7

The influence of a holmium impurity on the photoelectric properties of bulk and film As₂Se₃ and (As₂S₃)_0.3(As₂Se₃)_0.7 samples is studied. Measurements of the relative photoconductivity of bulk samples and the spectral distribution of the persistent photoconductivity in film samples showed an increase in the photoconductivity of materials doped with holmium to concentrations equivalent to 0.010–0.015 at %. The spectral distribution of the persistent photoconductivity and optical absorption showed that the band gap monotonically decreases from 1.88 to 1.85 eV for As₂Se₃ and from 2.05 to 2.00 eV for (As₂S₃)_0.3(As₂Se₃)_0.7 as Ho concentration increases to 0.015 at %, and then weakly increases to the values in initial pure materials.     

Photoelectric and photomagnetic properties of the gapless semiconductor CdxHg1−x Te in the infrared and millimeter spectral regions when an energy gap is opened

The magnetic-field and strain dependences of the photoelectric, photomagnetic, and photothermomagnetic characteristics are determined for the gapless semiconductor Cd_xHg_1−x Te (x=0.04–0.16), in which an energy gap is opened by external effects. In excitation by infrared radiation the photosignal is observed to increase sharply with an increase in the applied magnetic field or uniaxial elastic deformation. For radiation in the millimeter spectral range the photoresponse exhibits giant oscillations associated with the variation of the electron density. This phenomenon is confirmed by the field dependence of the photomagnetic Hall effect. It is shown that the photothermomagnetic effect is a differential signal relative to the photoconductivity signal. Fiz. Tekh. Poluprovodn. 31, 35–42 (January 1997)     

Optical properties of undoped and iodine-doped CdTe

A comprehensive study of the properties of undoped and iodine-doped CdTe structures by photoluminescence (PL) and photoreflectance (PR) is reported. Undoped bulk CdTe and iodine-doped CdTe layers grown by metalorganic molecular beam epitaxy on (lOO)-oriented CdTe and (211)B-oriented GaAs substrates with electron concentrations ranging from 10¹⁴ to mid-10¹⁸ cm^-3 were included in this study. Lineshape modeling of 80KPL and PR spectra indicated the presence of both free exciton and donor-hole transitions at the higher doping levels. Strong PL and PR signals were also observed at room temperature. If only a single transition is considered for the analysis of the 300K spectra, the PL emission peak and the PR transition energy both exhibit a strong dependence on electron concentration for doped layers. However, lineshape modeling of the room-temperature spectra indicated the presence of multiple transitions consisting of free exciton and direct band-to-band transitions. The use of two transitions resulted in a constant value of bandgap over the entire range of conductivities studied. A strong correlation remained between the broadening of the PR and PL spectra and excess carrier concentration N_D-N_A. In addition, the E₁ transition energy measured by PR was found to vary dramatically with growth conditions.     

Determination of donor and acceptor impurity concentrations in n-InP and n-GaAs

The Hall mobility of semiconducting materials is generally computed without accounting for the Hall factor,r _H. It is shown that for low magnetic fields this may lead to considerable errors in the calculation of the impurity concentrations,N _D andN _A, in both n-GaAs and n-InP of reasonable purity. Tables allowing the determination ofN _D andN _A under low magnetic field conditions are computed at room and liquid nitrogen temperatures. They account forr _H and for the inelastic nature of collisions with polar optical phonons without assuming Matthiessen's rule.     

Spectral sensitivity of p-Cu1.8S/n −-ZnS/n-(II-VI) heterostructures

Photosensitivity of multilayered p-Cu_1.8S/n ^?-(II-VI)/n-(II-VI) heterostructures beyond the fundamental-absorption edge of the wide-gap component is studied experimentally, and a simple model is suggested as an explanation of this photosensitivity. It is established that an effective method for reducing the photosensitivity of the structures beyond the ultraviolet spectral region consists in decreasing the probability of dominant tunneling processes, by increasing the thickness of the wide-gap layer, giving rise to a blocking barrier for photogenerated minority charge carriers. It is shown that the p-Cu_1.8S/n ^?-ZnS/n-CdSe heterostructures are promising for the development of efficient “solar-blind” detectors of ultraviolet radiation.