B. A. Akimov

Dynamics of the semiconductor-metal transition induced by infrared illumination in Pb1-xSnx Te(In) alloys

Abstract The photoconductivity and quantum oscillation effects induced by infrared illumination have been investigated in Pb 1- x Sn x Te(In) samples. The impurity monopolar character of the photoeffect is established. It is shown that the time-dependence of the residual photoconductivity is described by a power law, the relaxation time being ≈ 10 4 s at temperatures T ⩽ 20 K.

Experimental study of negative photoconductivity in n-PbTe(Ga) epitaxial films

We report on low-temperature photoconductivity (PC) in n-PbTe(Ga) epitaxial films prepared by the hot-wall technique on -BaF_2 substrates. Variation of the substrate temperature allowed us to change the resistivity of the films from 10^8 down to 10_{-2} Ohm x cm at 4.2 K. The resistivity reduction is associated with a slight excess of Ga concentration, disturbing the Fermi level pinning within the energy gap of n-PbTe(Ga). PC has been measured under continuous and pulse illumination in the temperature range 4.2-300 K. For films of low resistivity, the photoresponse is composed of negative and positive parts. Recombination processes for both effects are characterized by nonexponential kinetics depending on the illumination pulse duration and intensity. Analysis of the PC transient proves that the negative photoconductivity cannot be explained in terms of nonequilibrium charge carriers spatial separation of due to band modulation. Experimental results are interpreted assuming the mixed valence of Ga in lead telluride and the formation of centers with a negative correlation energy. Specifics of the PC process is determined by the energy levels attributed to donor Ga III, acceptor Ga I, and neutral Ga II states with respect to the crystal surrounding. The energy level corresponding to the metastable state Ga II is supposed to occur above the conduction band bottom, providing fast recombination rates for the negative PC. The superposition of negative and positive PC is considered to be dependent on the ratio of the densities of states corresponding to the donor and acceptor impurity centers.

Band edge motion in quantizing magnetic field and nonequilibrium states in Pb1?x Sn x Te alloys doped with In

Galvanomagnetic and oscillation effects in Pb1−xSnxTe single crystals doped with 0.5 at % In have been studied in magnetic fields up to 60 kOe at temperatures from 4.2 to 30 K under hydrostatic pressure up to 18 kbar. Beyond the ultraquantum magnetic field limit (Huql) for the metallic state of Pb1−xSnxTe(In) alloys, Fermi level pinning by high-density quasilocal states takes place. In a strong fieldH>Huql the equationEF = const is valid instead of the equationn = const which is usual for degenerate semiconductors (EF is the electron or hole Fermi energy, andn is their concentration). This makes it possible to determine the direction of the band edge motion in the Pb1−xSnxTe energy spectrum in a quantizing magnetic field in the direct and inverse spectral regions. It is found that the charge carrier transitions between quasilocal and band states are of anomalously long duration (∼105 sec atT=4.2 K). By the application of a quantizing magnetic field we obtained a nonequilibrium metallic state of the system with a frozen or slowly diminishing Fermi surface. The characteristic time of the transition was found as a function of temperature and pressure. The relaxation kinetics of the nonequilibrium states induced by a quantizing magnetic field and infrared irradiation is discussed.

The electrical conductivity of polycrystalline SnO2(Cu) films and their sensitivity to hydrogen sulfide

The effect of doping with copper on the sensor properties and the electrical conductivity of polycrystalline SnO2(Cu) films has been investigated. It has been found that at room temperature the residual conductivity is observed after the films are exposed to H2S. This made it possible to determine the character of the low-temperature conductivity of the films for different degrees of saturation with hydrogen sulfide. A comparison of the obtained data with the results of layerwise elemental analysis suggested a model that explains the mechanism of the gas sensitivity of SnO2(Cu) to hydrogen sulfide. In contrast to the mechanisms, which are associated with the work done by the surface and which are standard for gas sensors, in the present case the change in the conductivity is due to the chemical reaction of the electrically active copper with sulfur in the entire volume of the film. This reaction determines the selectivity and high sensitivity of SnO2(Cu) to H2S.

Photoconductivity kinetics in high resistivity n-PbTe(Ga) epitaxial films

We present a study of photoelectric and kinetic properties of n-PbTe(Ga) epitaxial films prepared by the hot wall technique on BaF2 and SiO2-Si substrates. The measurements were performed under continuous and pulse illumination with use of a heat source and LED in the temperature range 4.2-300 K. The investigated samples were highly sensitive to infrared illumination at temperatures lower than 100-110 K. Two characteristic regions were clearly resolved in photoconductivity relaxation curves: a fast one at the beginning of relaxation (with lifetimes of about 10-3 s at 4.2 K) and a long duration relaxation tail. The obtained results are discussed in terms of the configuration diagram of a DX-like centre in PbTe(Ga). The experimental data obtained for the films are compared with properties of bulk material.

An operating regime based on switching effects for photodetectors of Pb1−xSnxTe(in) MBE films

Abstract Pb1−xSnxTe(In) (x = 0.20) MBE films have been prepared on BaF2 substrates. The characteristics of resistivity relaxation under the combined effect of a pulsed electric field and IR illumination have been investigated. We have shown that the films can be used as rather fast-response photodetectors operating in a regime of periodic accumulation and quenching of the signal.

Local unequilibrium states in Pb1−xSnxTe(In) (x=0.25)

Abstract It is shown that local long-life unequilibrium states are generated in Pb1−xSnxTe(In) alloys at the low temperatures under the action of the local light excitation. The character of the photoexcitation propagation in the darkened regions is discussed.

Conductivity of structures based on doped nanocrystalline SnO2 films with gold contacts

The conductivity of nanocrystalline Pt-, Pd-, and Ni-doped tin dioxide films on insulating SiO2 substrates is investigated in the temperature range 77–400 K. Doping allows variation of the resistance from 104 to 107Ω. It is established that, in contrast with a Au/single-crystal SnO2 contact, the gold contacts for the nanocrystalline material are ohmic in the entire temperature range and their contribution to the conductance of all the structures investigated does not exceed 5%.

Properties of p-PbTe (Ga) based diode structures

Single crystals of p-PbTe(Ga) with gallium density too low for complete compensation of uncontrollable impurities and intrinsic lattice defects and realization of Fermi level (FL) pinning were investigated for the purpose of producing diode structures In contact-[p-PbTe(Ga)]-Pt contact. It was found that the properties of the structures obtained have a number of features that distinguish them from In-[p-PbTe] Schottky barriers. The current-voltage characteristics (IVCs) of the experimental structures are not described by the standard relation of the Schottky theory not only in the region of reverse bias but also in the region of direct bias. Residual photoconductivity (PC) is observed under illumination with a thermal radiation source in the temperature range T<80 K; after the illumination is switched off, the IVCs are linear. Under constant illumination a photo-emf appears and the branches of the IVCs in the region of reverse biases rectify. The experimental results are discussed on the basis of the assumption that regions with n-type conductivity form and FL stabilizes near the nonohmic contact as a result of band bending. It has not been ruled out that n-PbTe(Ga) regions are initially present in the sample, but they are not manifested under ohmic contact conditions.

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.