V. Kasiyan

Development of AIIBVI Semiconductors Doped with Cr for IR Laser Application

Electrical and optical measurements obtained with CdSe single crystals, doped with chromium from gas-source CrSe over a wide temperature range (500-1050 °C), are compared with ZnSe annealed in liquid metal (Zn). These annealing processes are intended to control the concentrations of the impurity and the intrinsic defects. Low temperature annealing of CdSe crystals in CrSe atmosphere allows to obtain a high electron mobility up to 9000 cm 2 /Vs at 80 K and demonstrates the low native defect concentration. A high temperature annealing gives rise to increased electron concentration with decreased mobility. Optical absorption measurements show that at the high annealing temperature effective doping with Cr takes place. The impurity absorption beyond the absorption edge is interpreted by the excitation of Cr ++ and Cr + deep levels.

Observation of local electron states linked to the quasi-Fermi level

We report on the observation of semiconductor local electron states linked to the quasi-Fermi level and, consequently, not characterized by the defined position in the energy spectrum which is familiar for shallow and deep impurities. This type of local electron states have been found in the doped narrow-gap semiconductor Pb1−xSnxTe(In). The binding energy of these states is less than 10 meV providing photoresponse at the wavelengths exceeding 100 μm.

Porosity-induced blueshift of photoluminescence in CdSe

Porous CdSe layers have been fabricated by anodic etching of n-type single crystalline substrates with different values of conductivity. The morphology and porosity of the layers thus produced were found to be controlled by the conductivity of the material, anodization voltage, and conditions of in situ UV illumination. The porosity-induced changes in the photoluminescence spectra were studied. The decrease of the skeleton size down to 10–20nm was found to result in a blueshift of the excitonic emission lines by 10meV which was attributed to quantum-size effects in the nanocrystalline CdSe porous skeleton. An increase of the exciton–LO-phonon interaction by a factor of 1.5 in a weak-to-intermediate confinement regime was deduced from the analysis of temperature dependence of the free exciton luminescence line.

Infrared detectors based on semiconductor p-n junction of PbSe

P-n junctions based on physical vapor deposition of thin PbSe films and conductivity type inversion from n- to p-type are developed and characterized over a wide range of temperatures and bias voltages. Photosensitivity and diode characteristics in the thin film PbSe diode structures were found at temperatures up to 300 K. The values of the measured and estimated parameters of these structures demonstrate their high photodetector performance and the potential for development of IR detectors with optimal sensitivity at the highest possible operating temperature.

Photoconductivity of oxidized nanostructured PbTe(In) films

Photoconductivity of as-grown and oxidized nanocrystalline PbTe(In) films has been studied in the dc and ac modes at temperatures 4.2–300 K. The electric transport in the films is defined by two mechanisms: conductivity through barriers at grain boundaries and transport along inversion channels at the grain surface. Modification of the transport mechanisms induced by oxidation is considered. Relatively weak oxidation results in an increase in the contribution of grain barriers to conductivity followed by an enhancement of the photoconductivity amplitude. Instead, this contribution drops in the case of deep oxidation resulting in a photoresponse reduction. It is shown that the main mechanism of charge transport in deeply oxidized films at low temperatures is hopping along inversion channels at the grain surface. It is demonstrated that the photoconductive response of nanocrystalline materials may be optimized by variation of the oxidation level, measurement frequency and temperature.

Characterization of high-temperature PbTe p-n junctions prepared by thermal diffusion and by ion implantation

Two types of high-quality PbTe p-n junctions, prepared by thermal diffusion of In4Te3 gas [thermally diffused junction (TDJ)] and by ion implantation [implanted junction (IJ)] of indium (In-IJ) and zinc (Zn-IJ), have been characterized. Capacitance-voltage and current-voltage characteristics have been measured over a temperature range from ∼10to∼180K. The saturation current density J0 in both diode types was ∼10−5A∕cm2 at 80K, while at 180K, J0∼10−1A∕cm2 in TDJ and ∼1A∕cm2 in both IJ diodes. At 80K, the reverse current started to increase markedly at a bias of ∼400mV for TDJ and at ∼550mV for IJ. The ideality factor was about 1.5–2 for both diode types at 80K. Both diode types were linearly graded. The height of the junction barrier, the concentration and the concentration gradient of the impurities, and the temperature dependence of the static dielectric constant have been determined. The zero-bias resistance times area product (R0Ae) at 80K is 850Ωcm2 for TDJ, 250Ωcm2 for In-IJ, and ∼80Ωcm2 for Zn-IJ, w...

Photothermal effect in narrow band gap PbTe semiconductor

In this paper we report the observation of photothermal effect in PbTe p-n junction. The effect is expressed in photosignal generation due to illumination by 100 ns pulse CO2 laser with photon energy less than PbTe forbidden gap.

Mid-infrared photoluminescence of PbSe film structures up to room temperature

Lead salt materials are of high interest for midinfrared optical emitters and detectors for molecular spectroscopy. The IV-VI narrow gap semiconductors have a multivalley band structure with band extrema at the L point of the Brillioun zone. Due to the favorable mirrorlike band structure, the nonradiative Auger recombination is reduced by one or two orders of magnitude below that of narrow gap III-V and II-VI semiconductor compounds1. The photoluminescence in the midinfrared range for PbSe film structures, excited by a semiconductor laser diode, is investigated. The PbSe films were prepared by Physical Vapor Deposition (PVD) using an electron gun. A PbSe crystal doped with 0.1 at% Bi was used as a source for the fabrication of thin layers. Starting from the assumption that the rate of nucleation is a predominate factor in determining grain size, thin films were fabricated on substrates that had been maintained at various temperatures of deposition process2. Amorphous glass and Kapton polyimide film was used as substrate. The growth rate was 0.2 nm/s. Films were thermally treated at high oxygen pressure in a heated encapsulated system. Microstructure has been studied using XRD, AFM and HRSEM. For PbSe structures photoluminescence at temperature as high as 300 K is demonstrated.

PbTe(In) films with variable microstructure forphotodetection in IR and terahertz range

The work deals with studies of the grain size and surface state effect on photoelectric and transport properties of PbTe(In) films in the temperature range from 4.2 K up to 200 K under irradiation of a blackbody source and terahertz laser pulses. The PbTe(In) films were deposited on insulating substrates kept at the temperatures TS equal to -120 (see manuscript) 250C. AFM, SEM, Auger spectroscopy and X-ray diffraction were used to study the film microstructure. Increase of the TS value led to mean grain size growth from 60 up to 300 nm. All films had a column-like structure with the columns nearly perpendicular to the substrate plane. It is shown that microstructure of the films strongly affects the photoconductivity character in the terahertz region of the spectrum. Positive persistent photoresponse is observed at low temperatures in the polycrystalline films. For these films transport and photoelectric properties are determined by the grain volume and impurity state specifics. Nanocrystalline films have all features of non-homogeneous systems with band modulation. For these films only negative photoconductivity is observed in the whole temperature range. Possible mechanisms of the photoresponse formation are discussed.

Photoconductivity of PbTe:In films with variable microstructure

It is shown that the microstructure and features of formation of surface states in nanocrystalline and polycrystalline PbTe:In films most significantly affect the character of photoconductivity in the spectral range of 1–2.5 THz. We present the results of a study and comparative analysis of the character of conductivity of PbTe:In films in the temperature range from 4.2 to 300 K in a static mode and in variable electric fields with a frequency of up to 1 MHz with illumination with white light and under the effect of high-power terahertz laser pulses with a wavelength of up to 280 μm.