Hot wall epitaxy of II–VI compounds: CdS and CdTe

CdS and CdTe films were grown by hot wall epitaxy on single-crystal CdTe, BaF₂ and SrF₂ substrates. The films grow epitaxially and do not show any misorientations under usual X-ray investigations. N-type films were grown by coevaporation of indium. Typical electron concentrations of up to 2 × 10¹⁷ cm^-3 in CdTe and 3 × 10¹⁸ cm^-3 in CdS were obtained. P-type CdTe layers were also obtained using antimony as a dopant, with hole concentrations up to 10¹⁸–10¹⁹ cm^-3. Room temperature values of the electron mobility up to 600 cm² V^-1 s^-1 for CdTe and 230 cm² V^-1 s^-1 for CdS were obtained. Minority carrier diffusion lengths larger than 1 μm were measured in n-CdTe layers grown on p-CdTe substrates. Deep level transient spectroscopy was used to characterize carrier traps in the CdTe films. The concentration of the traps was a function of the growth conditions and could be drastically reduced with annealing of the samples.     

Structure-property relationships in heteroepitaxial InAs and InSb thin films

Heteroepitaxial thin films of InSb and InAs were evaporated onto sapphire and onto semi-insulating GaAs substrates. An in-line high vacuum evaporation set-up was used to deposit the semiconductor films by the three-temperature method at an ambient pressure of about 10^-5 mbar. The orientation and structure of the films were investigated as functions of substrate temperature and substrate orientation.The highest carrier mobilities were observed in (100)-oriented InAs and InSb films deposited on GaAs (100) substrates; these films had room temperature mobilities of 20 000 and 40 000 cm² V^-1 s^-1 respectively. InAs films on (0001) sapphire planes always showed (111) orientation with a maximum carrier mobility of 13 000 cm² V^-1 s^-1 at room temperature.     

The influence of the substrate temperature on the preparation of thin film In2Te3

An optimum substrate temperature T_o of 453 K was found for the preparation of thin film In₂Te₃ by electron beam evaporation. Films prepared at this temperature are stoichiometric and exhibit a maximum hole mobility of 48 cm² V^-1s^-1 and a maximum hole concentration of 4.2 × 10¹⁸ cm^-3. These observations can be explained on the basis of the Vincett-Barlow-Roberts theory which predicts a value for T_o/T_b, where T_b is the boiling point, of 0.3. The electrical activation energies of the films were also determined and are discussed in terms of the possible defects present.     

Effect of stacking type in precursors on composition, morphology and electrical properties of the CIGS films

The copper-indium-gallium (CIG) metallic precursors with different stacking type (A: CuGa/CuIn/CuGa/glass and B: CuInGa/CuIn/CuInGa/glass) were prepared onto glass substrates by magnetron sputtering method. In order to prepare Cu(In_1?xGa_x)Se₂ (CIGS) thin films, the CIG precursors were then selenized with solid Se powder using a three-step reaction temperature profile. The influence of stacking type in precursors on structure, composition, morphology and electrical properties of the CIGS films is investigated by X-ray diffraction, energy dispersive spectrometer, scanning electron microscope and Hall effect measurement. The results reveal that the stacking type of the precursor has a strong influence on composition, morphology and properties of the CIGS thin films. The atomic ratios of Cu/(In+Ga)/Se of the CIGS films A and B are 1.61:1:2.11 and 1.39:1:2.04, respectively. The better quality CIGS thin films can be obtained through selenization of metallic precursor of CuInGa/CuIn/CuInGa/glass. The CIGS films are p-type semiconductor material. The hole concentration, resistivity and hole mobility of the CIGS thin films is 2.51 × 10¹⁷ cm^?3, 3.11 × 10⁴ Ω cm and 19.8 cm² V^?1 s^?1, respectively.     

Properties of epitaxial ZnO:P films

Epitaxial ZnO:P films have been produced by annealing ZnP₂ substrates in atomic oxygen and characterized by X-ray diffraction, atomic force microscopy, Hall effect measurements, X-ray photoelectron spectroscopy, and photoluminescence measurements. The X-ray diffraction patterns of the films showed the 002 peak, indicating that their c axis was normal to the substrate surface. According to the Hall effect data, the layers were p-type, with a resistivity of ～20 Ω cm, hole mobility of ～9 cm²/(V s), and hole concentration of ～7.8 × 10¹⁷ cm^?3. The photoluminescence spectra of the ZnO:P films showed a peak at 3.356 eV (neutral acceptor bound exciton). Our results indicate that the ZnO:P films contain the P_Zn-2V_Zn defect complex as a shallow acceptor responsible for their p-type conductivity.     

Nanocrystalline copper selenide thin films by chemical spray pyrolysis

Thin films of copper selenide were deposited onto amorphous glass substrates at various substrate temperatures by computerized spray pyrolysis technique. The as deposited copper selenide thin films were used to study a wide range of characteristics including structural, surface morphological, optical and electrical, Hall Effect and thermo-electrical properties. X-ray diffraction study reveals that the films are polycrystalline in nature with hexagonal (mineral klockmannite) crystal structure irrespective of the substrate temperature. The crystalline size is found to be in the range of 23–28 nm. The SEM study reveals that the grains are uniform with uneven spherically shaped and spread over the entire surface of the substrates. EDAX analysis confirmed the nearly stoichiometric deposition of the film at 350 °C. The direct band gap values are found to be in the range 2.29–2.36 eV depending on the substrate temperature. The Hall Effect study reveals that the films exhibit p-type conductivity. The values of carrier concentration and mobility for the film are found to be 5.02 × 10¹⁷ cm^?3 and 5.19 × 10^?3 cm² V^?1 s^?1; respectively for film deposited at 350 °C.     

Electronic transport near the surface of indium antimonide films

Surface layer carrier concentrations and mobilities of n-type InSb films were separated from those of the bulk of the film through the magnetic field dependence of the Hall coefficient coupled with conventional galvanomagnetic measurements. This technique yields a surface electron density for the top plus bottom of the film of (3?5) × 10¹³ cm^?2 and a surface layer mobility of 1500–2000 cm² V^?1 s^?1, both largely independent of temperature. The surface density is found to decrease when the free surface is anodized.     

Electrical transport properties of thallium-doped p-type PbTe films

Hall measurements were made in the temperature range 77–700 K on thallium- doped p-type epitaxial films of PbTe having free carrier concentrations in the range 5×10¹⁷?6×10¹⁹cm^?3 at 300 K. The various band parameters, i.e. mobility, effective mass and population ratios for light and heavy holes, were estimated as functions of temperature and carrier concentration. These results were compared with reported data on bulk samples.     

Photo-electronic properties of InAs0.07Sb0.93 films

The temperature dependent photoconductive and surface barrier photovoltaic spectral responses of InAs_0.07Sb_0.93 films with an impurity density ～5.5×10¹⁵ cm^-3, grown by liquid phase microzone crystallization, are shown to be a function of their fundamental energy bandgaps shifted to values lower than those of InSb. From charge carrier transport measurements their absolute zero bandgap is calculated as ε_g0=0.197 eV, in good agreement with theory. In contrast with previously reported data on InAs_xSb_1-x solid solution bulk and single crystal layers, the electron mobility of InAs_0.07Sb_0.93 films increases with decreasing temperature reaching a value of ～1.32×10⁵ cm²/Vs at 77 K.     

The synthesis and characterization of Cu<Subscript>2</Subscript>ZnSnS<Subscript>4</Subscript> thin films from melt reactions using xanthate precursors

Kesterite, Cu₂ZnSnS₄ (CZTS), is a promising absorber layer for use in photovoltaic cells. We report the use of copper, zinc and tin xanthates in melt reactions to produce Cu₂ZnSnS₄ (CZTS) thin films. The phase of the as-produced CZTS is dependent on decomposition temperature. X-ray diffraction patterns and Raman spectra show that films annealed between 375 and 475 °C are tetragonal, while at temperatures <375 °C hexagonal material was obtained. The electrical parameters of the CZTS films have also been determined. The conduction of all films was p-type, while the other parameters differ for the hexagonal and tetragonal materials: resistivity (27.1 vs 1.23 Ω cm), carrier concentration (2.65 × 10⁺¹⁵ vs 4.55 × 10⁺¹⁷ cm^?3) and mobility (87.1 vs 11.1 cm² V^?1 s^?1). The Hall coefficients were 2.36 × 10³ versus 13.7 cm³ C^?1.     

Structural,optical and electrical properties of PbS and PbSe quantum dot thin films

PbS and PbSe were prepared by hot injection method. The powders were used for preparing the corresponding films by using thermal evaporation technique. The structural, optical and electrical properties of PbS and PbSe thin films were investigated. The structural properties of PbS and PbSe were investigated by X-ray diffraction, transmission electron microscopy and energy dispersive X-ray techniques (EDX). PbS and PbSe films were found to have cubic rock salt structure. The particles size ranged from 1.32 to 2.26 nm for PbS and 1.28–2.48 nm for PbSe. EDX results showed that PbS films have rich sulphur content, while PbSe films have rich lead content. The optical constants (absorption coefficient and the refractive index) of the films were determined in the wavelength range 200–2500 nm. The optical energy band gap of PbS and PbSe films was determined as 3.25 and 2.20 eV, respectively. The refractive index, the optical dielectric constant and the ratio of charge carriers concentration to its effective mass were determined. The electrical resistivity, charge carriers concentration and carriers mobility of PbS at room temperature were determined as 0.55 Ω cm, 1.7 × 10¹⁶ cm^?3 and 656 cm² V^?1 s^?1, respectively, and for PbSe films they were determined as 0.4 Ω cm, 9 × 10¹⁵ cm^?3 and 1735 cm² V^?1 s^?1, respectively. These electrical parameters were investigated as a function of temperature.     

Peaked structure in a single-crystal film

A peaked structure, similar to that previously observed in the field effect mobility versus gate voltage experiments on silicon inversion layers, has been observed on single-crystal films. These films and contacts to them are the same continuous crystals; only the thicknesses are different. A double modulation circuit, which measures the field effect mobility and the conductivity simultaneously, is sensitive enough to measure field effect mobilities of less than 1 × 10^-5 m² V^-1 s^-1. The fine structure is reproducible in great detail, and some regularity seems to appear at very low temperatures.     

Electrical properties of silver selenide thin films prepared by reactive evaporation

The electrical properties of silver selenide thin films prepared by reactive evaporation have been studied. Samples show a polymorphic phase transition at a temperature of 403 ± 2 K. Hall effect study shows that it has a mobility of 2000 cm²V^?1s^?1 and carrier concentration of 10¹⁸ cm^?3 at room temperature. The carriers are ofn-type. X-ray diffraction study indicates that the as-prepared films are polycrystalline in nature. The lattice parameters were found to bea= 4.353 Å,b= 6.929 Å andc = 7.805 Å.     

Activation of p-type conduction in ZnO:N films by annealing in atomic oxygen

ZnO:N epitaxial films have been grown by reactive magnetron sputtering. The effect of annealing in atomic oxygen on the structural and electrical properties of the ZnO:N films has been studied by X-ray diffraction, atomic force microscopy, Hall effect measurements, and X-ray photoelectron spectroscopy. By annealing at temperatures from 500 to 700°C, we have obtained p-type ZnO:N films with a resistivity of ～57 Ω cm, hole mobility of ～2.7 cm²/(V s), and hole concentration of ～6.8 × 10¹⁷ cm^?3. X-ray photoelectron spectroscopy results suggest that the p-type conductivity of the films is due to a decrease in the concentration of (N₂)_O and V _O donors.     

Carrier properties of B atomic-layer-doped Si films grown by ECR Ar plasma-enhanced CVD without substrate heating

Abstract

The atomic-layer (AL) doping technique in epitaxy has attracted attention as a low-resistive ultrathin semiconductor film as well as a two-dimensional (2-D) carrier transport system. In this paper, we report carrier properties for B AL-doped Si films with suppressed thermal diffusion. B AL-doped Si films were formed on Si(100) by B AL formation followed by Si cap layer deposition in low-energy Ar plasma-enhanced chemical-vapor deposition without substrate heating. After fabrication of Hall-effect devices with the B AL-doped Si films on unstrained and 0.8%-tensile-strained Si(100)-on-insulator substrates (maximum process temperature 350°C), carrier properties were electrically measured at room temperature. Typically for the initial B amount of 2?×?10¹⁴ cm^?2 and 7?×?10¹⁴ cm^?2, B concentration depth profiles showed a clear decay slope as steep as 1.3 nm/decade. Dominant carrier was a hole and the maximum sheet carrier densities as high as 4?×?10¹³ cm^?2 and 2?×?10¹³ cm^?2 (electrical activity ratio of about 7% and 3.5%) were measured respectively for the unstrained and 0.8%-tensile-strained Si with Hall mobility around 10–13 cm² V^?1 s^?1. Moreover, mobility degradation was not observed even when sheet carrier density was increased by heat treatment at 500–700 °C. There is a possibility that the local carrier (ionized B atom) concentration around the B AL in Si reaches around 10²¹ cm^?3 and 2-D impurity-band formation with strong Coulomb interaction is expected. The behavior of carrier properties for heat treatment at 500–700 °C implies that thermal diffusion causes broadening of the B AL in Si and decrease of local B concentration.     

Large-sized-mismatched group-V element doped ZnO films fabricated on silicon substrates by pulsed laser deposition

Pulsed laser deposition has been utilized to synthesize impurity-doped ZnO thin films on silicon substrate. Large-sized-mismatched group-V elements (A^V) including P, As, Sb and Bi were used as dopants. Hall effect measurements show that hole concentration in the order of 10¹⁶-10¹⁸ cm⁻³, resistivity in the range of 10-100 Ω cm, Hall mobility in the range of 10-100 cm²/Vs were obtained only for ZnO:As and ZnO:Bi thin films. X-ray diffraction measurements reveal that the films possess polycrystallinity or nanocrystallinity with ZnO (002) preferred orientation. Guided by X-ray photoemission spectroscopy analyses and theoretical calculations for large-sized-mismatched group-V dopant in ZnO, the A_Zn^V-2V_Zn complexes are believed to be the most possible acceptors in the p-type A^V-doped ZnO thin films.     

Electrical properties of post-oxidized In2O3:Sn films

In₂O₃:Sn (ITO) films were prepared on soda lime glass by evaporating suboxides and oxidizing these in air. The specific resistivity of the films showed a broad minimum of about 2×10^-3ω cm for a doping level range of 10–30 wt.% SnO₂ and they had decreased mobility (10 cm² V^-1 s^-1) and carrier density (2×10²⁰ cm^-3) compared with high performance ITO films obtained by reactive evaporation onto a high temperature substrate. Unlike the behaviour of films treated in vacuum or in forming gases, the change in specific resistivity with oxidation for our films exhibited three characteristic stages: a sharp decrease accompanied by increasing transparency in the first stage, a steep increase in the second and a very slow increase in the third. The lowest resistivity was attained at the end of the first stage with a slight absorption. This change in resistivity was explained qualitatively on the basis of the diffusion model. It was also found that the resistivity could be further reduced if the residual gas pressure during the deposition was maintained as low as possible.     

Electrical effects of thallium on the Hall mobility in p-PbTe thin films

The Hall coefficient and Hall mobility were measured for epitaxial p-type PbTe films doped with thallium. It was observed that doping of the films with thallium leads to a decrease in the mobility and an increase in the concentration of charge carriers. The mobility μ_D limited by defect scattering was calculated and was found to be independent of temperature and to decrease with increases in the carrier concentration p. For films with carrier concentrations exceeding 7 × 10¹⁸ cm^-3 the value of μ_D decreases as p^{-$\text{4}\text{3}$}, whereas for films with lower carrier concentrations the decrease in μ_D with p is at a slower rate.     

Plasma exposure inducing crystallization of indium oxide film with improved electrical and mechanical properties at room temperature

A novel plasma exposure technique has been introduced into conventional magnetron sputtering process to enhance the crystallization of indium oxide (In₂O₃) films at room temperature. The effect of plasma exposure technique with different pulsed DC voltages on the electrical and mechanical properties of In₂O₃ films was investigated. It is observed that film crystallization can be significantly enhanced when the pulsed DC voltage (|V _p|) is higher than |?500 V| (|V _p| > |?500 V|). By applying the plasma exposure process, In₂O₃ films prepared at room temperature with thickness of 135 nm shows low resistivity of 4.11 × 10^?4 Ω cm, mobility of 42.1 cm²/Vs, and transmittance over 80 % in the visible range. Compared with the In₂O₃ films without plasma exposure process, the In₂O₃ films with plasma exposure show better crystallization and remarkably higher nanohardness. The plasma exposure technique is a useful candidate technique for enhancing film crystallization at low temperature.     

Growth and characterizations of dual ion beam sputtered CIGS thin films for photovoltaic applications

The growth of CIGS thin films on soda-lime glass substrates at different substrate temperatures by dual ion beam sputtering system in a single-step route from a single quaternary sputtering target with the composition of Cu (In_0.70 Ga_0.30) Se₂ was reported. The effects of the substrate temperature on structural, optical, morphological and electrical properties of CIGS films were investigated. Stoichiometry of one such film was investigated by X-ray photoelectron spectroscopy. All CIGS films had demonstrated a strong (112) orientation located at 2θ ~26.70^o, which indicated the chalcopyrite structure of films. The value of full-width at half-maximum of (112) peak was reduced from 0.58° to 0.19° and crystallite size was enlarged from 14.98 to 43.05 nm as growth temperature was increased from 100 to 400 °C. However, atomic force microscope results showed a smooth and uniform surface at lower growth temperature and the surface roughness was observed to increase with increasing growth temperature. Hall measurements exhibited the minimum film resistivity of 0.09 Ω cm with a hole concentration of 2.42 × 10¹⁸ cm^?3 and mobility of 28.60 cm² V^?1 s^?1 for CIGS film grown at 100 °C. Film absorption coefficient was found to enhance nominally from 1 × 10⁵ to 2.3 × 10⁵ cm^?1 with increasing growth temperature from 100 to 400 °C.