A. A. Dobrovolsky

Photoconductivity of nanocrystalline SnO2 sensitized with colloidal CdSe quantum dots

A highly reproducible photoresponse is observed in nanocrystalline SnO2 thick films sensitized with CdSe quantum dots. The effect of the SnO2 matrix microstructure on the photoconductivity kinetics and photoresponse amplitude is demonstrated. The photoresponse of the sensitized SnO2 thick films reaches more than two orders of magnitude under illumination with the wavelength of the excitonic transition of the quantum dots. Long-term photoconductivity kinetics and photoresponse dependence on illumination intensity reveal power-law behavior inherent to the disordered nature of SnO2. The photoconductivity of the samples rises with the coarsening of the granular structure of the SnO2 matrix. At the saturation region, the photoresponse amplitude remains stable under 104 pulses of illumination switching, demonstrating a remarkably high stability.

Features of Vanadium Impurity States in Lead Telluride

Temperature dependences of resistivity, magnetic susceptibility, and charge-carrier concentration and mobility in single-crystalline PbTe:V samples with a varied impurity content are investigated. It is shown that vanadium forms a donor level located ∼20 meV below the conduction-band bottom. The electron mobility is as high as 105 cm2 V−1 s−1 in the samples with NV ≤ 0.21 at % and proves to be more than an order of magnitude higher in the samples with the highest vanadium content NV = 0.26 at %. In the same samples, the real part of the conductivity is characterized by a pronounced frequency dependence. An increase in vanadium concentration is accompanied by a decrease in the effective magnetic moment of impurity atoms. The features of electron transport in PbTe:V may be caused by a variable vanadium valence and by the effect of interimpurity correlations.

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.

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.

Deep impurity levels in vanadium-doped Pb1?xMnxTe solid solutions

Measurements of electric and magnetic properties of Pb1−xMnxTe(V) solid solutions have been performed in the temperature range 4.2–300 K. It is demonstrated that the Fermi level position in this semiconductor is defined by formation of an impurity level within the bandgap of the material. Realization of a semi-insulating state at low temperatures is accompanied by absence of the persistent photoconductivity effect and by high homogeneity of electrical parameters in Pb1−xMnxTe(V) which is a unique combination. Measurements of electric properties in the ac field reveal that the main mechanism of the low temperature transport is the hopping conductivity. Pb1−xMnxTe(V) demonstrates a paramagnetic behavior at all temperatures, at least above 10 K.

Photoconductivity of structures based on the SnO2 porous matrix coupled with core-shell CdSe/CdS quantum dots

Embedding of quantum dots into porous oxide matrixes is a perspective technique for photosensitization of a structure. We show that the sensitization efficiency may be increased by the use of core-shell quantum dots. It is demonstrated that the photoresponse amplitude in a SnO2 porous matrix with CdSe/CdS quantum dots depends non-monotonously on the number of atomic layers in a shell. The best results are obtained for SnO2 matrixes coupled with the quantum dots with three atomic layers of a shell. Mechanisms responsible for the structure sensitization are discussed.

Photoconductivity of composite structures based on porous SnO2 sensitized with CdSe nanocrystals

The introduction of CdSe nanocrystals (colloidal quantum dots) into a porous SnO2 matrix brings about the appearance of photoconductivity in the structures. Sensitization is a consequence of charge exchange between the quantum dots and the matrix. Photoconductivity spectral measurements show that the nanocrystals embedded into the matrix are responsible for the optical activity of the structure. The photoconductivity of the structures sensitized with different-sized quantum dots is studied in the temperature range from 77 to 300 K. It is shown that the maximum photoconductivity is attained by introducing nanocrystals of the minimum size (2.7 nm). The mechanisms of charge-carrier transport in the matrix and the charge-exchange kinetics are discussed.

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.

Effect of oxidation on the conductivity of nanocrystalline PbTe:In films in an alternating electric field

Temperature and frequency dependences of components of complex impedance for nanocrystalline PbTe:In films in the temperature range of 4.2–300 K and the frequency range from 20 Hz to 1 MHz have been studied. The films were deposited onto a cooled glass substrate and then annealed in an oxygen atmosphere at temperatures of 300 and 350°C. The charge-carrier transport in the studied films is controlled by charge transport over inversion channels at the surface of grains and by transitions through barriers at the grain boundaries. Parameters (resistance and capacitance) corresponding to each of above-mentioned mechanisms were determined. Dominant contribution to conductance of the film annealed at 350°C is made by inversion channels. It is shown that the transport of charge carriers over inversion channels in the region of low temperatures is realized by hopping conductivity.