L. P. Kazakova

Optical and electrical properties of porous gallium arsenide

Photoluminescence (PL), Raman scattering, and carrier transport have been studied for the first time in porous GaAs prepared on (111) oriented wafers of n-type crystalline GaAs (faces A and B). Peaks of the main PL band from faces A and B were observed at 1.82 and 1.88 eV, respectively. The electron drift mobility was found to be ∼4×10−4 cm2 V−1 s−1. The nanocrystallite size in porous GaAs was determined both from PL spectra and from the Raman shift. The obtained values are close or equal to 6–8 nm.

Carrier transport in porous silicon

Carrier transport in porous silicon layers has been studied by the time-of-flight method in the strong injection mode at temperatures T=290–350 K and electric field strengths F=(1.5–7)×104 V cm−1. The electron and hole drift mobilities μe≈2×10−3 cm2 V−1 s−1 and μh≈6×10−4 cm2 V−1 s−1 were obtained at T=292 K and F=4×104 V cm−1. An exponential temperature dependence of drift mobility with activation energy of ∼0.38 and ∼0.41 eV for, respectively, electrons and holes was established. It is shown that the type of time dependences of the photocurrent associated with carrier drift and the superlinear dependence of the transit time on the reciprocal of the voltage applied to a sample allow use of the concept of space-charge-limited currents under the conditions of anomalous dispersive transport. The experimental data are accounted for in terms of the model of transport controlled by carrier trapping into localized states with energy distribution near the conduction and valence band edges described by an exponential function with a characteristic energy of ∼0.03 eV.

Conductivity of layers of a chalcogenide glassy semiconductor Ge2Sb2Te5 in high electric fields

Effect of high electric fields on the conductivity of 0.5-1-µm-thick layers of a chalcogenide glassy semiconductor with a composition Ge2Sb2Te5, used in phase memory cells, has been studied. It was found that two dependences are observed in high fields: dependence of the current I on the voltage U, of the type I ∝ Un, with the exponent (n ≈ 2) related to space-charge-limited currents, and a dependence of the conductivity σ on the field strength F of the type σ = σ0exp(F/F0) (where F0 = 6 × 104 V cm−1), caused by ionization of localized states. A mobility of 10−3–10−2 cm2 V−1 s−1 was determined from the space-charge-limited currents.

Carrier drift mobility in porous silicon carbide

The time-of-flight technique was used to determine the drift mobilities of electrons and holes in porous silicon carbide produced by surface anodization of n-type 4H-SiC wafers. The electron and hole mobilities at 300 K in an electric field of 104 V/cm were μe=6×10−3 and μh=3×10−3 cm2 V−1 s−1, respectively. The low values of the mobilities are accounted for by carrier capture in localized states.

Transient photocurrent and photoluminescence in porous silicon

Transient photocurrent in porous silicon samples subjected to prolonged storage in air has been studied using the time-of-flight technique at temperatures in the range 300–350 K and electric field strengths of 104–105 V/cm. Photoluminescence has been studied in the same samples at T=300 K using time-resolved spectroscopy. A conclusion is made on the basis of comparison of the data obtained that localized states play important part in both the processes at characteristic times of these processes falling within the microsecond range.

Controlling the U−-center density in Se-As chalcogenide-glass semiconductors by doping with metals and halogens

Temperature dependences of the drift mobilities of electrons and holes are investigated in chalcogenide-glass semiconductors with composition Se95As5, both without impurities and with the impurities Ag and Br. The data obtained indicate that the localized states that control the transport of charge carriers are U−-centers, and that the change in the magnitude of the drift mobility after doping is caused by a change in the concentration of these centers. Estimates of the concentrations of positive and negatively charged intrinsic defects show that their values are similar, equalling ∼1016 cm−3 in impurity-free glasses with the composition Se95As5 and lying in the range 1013–1017 cm−3 when these glasses are doped with Ag, Br, and Cl. It is established that halogen impurities change the concentration of U−-centers most strongly (by two to three orders of magnitude). Analysis of the data obtained shows that the percentage of electrically active Br and Cl impurity atoms is 1%, while for Ag atoms it is 10−2%.

Transient current in amorphous, porous semiconductor-crystalline semiconductor structures

The time-of-flight technique in the weak signal mode (i.e., under conditions of small charge drift in the sample) is used to study the transient photocurrent in amorphous (porous) semiconductor-crystalline semiconductor structures. Amorphous Se-As materials, porous Si, and crystalline Si and CdSe were incorporated in the structures. The carrier drift mobilities in the amorphous and porous layers of the structures were determined. The appearance of a cusp on the curves of the transient current is shown to be caused by acceleration of carriers passing through the interface between the amorphous (porous) layer and the crystal. It is established that the carrier acceleration influences the drift mobility and the dispersion parameters.

Drift mobility of carriers in porous silicon

The drift mobility of carriers in porous silicon has been studied in a wide temperature range (190–360 K) at electric field strengths of 2×103–3×104 V/cm. An exponential temperature dependence of the hole drift mobility with an activation energy of d ∼ 0.14 eV was established. The density of localized states controlling the transport is evaluated.

Voltage oscillations in the case of the switching effect in thin layers of Ge-Sb-Te chalcogenides in the current mode

The I–V characteristics obtained for layers of chalcogenide vitreous semiconductor of the Ge–Sb–Te system in the current-generator mode are investigated. The instability region, i.e., the region of conductivity oscillations, observed in the case of the switching effect under conditions of a set current is revealed. The key parameters describing these oscillations and the conditions of their occurrence are investigated in detail. Analysis of the obtained data shows that, to explain the oscillations in the instability region, it is necessary to take into account an increase in the current density and the process of heat exchange between the current filament that arises in the film upon switching and the environment.

Electrical conductivity and optical properties of tellurium-rich Ge-Sb-Te films

The Ge-Sb-Te based (GST) films are intensively studied because they are suitable for reversible phase-change storage media. Ge2Sb2Te5 films are most frequently used for memory applications while Te enriched GST films are also promising but not well studied. In this work Te enriched GST films with two different compositions and various thicknesses (120 - 750 nm) were prepared by thermal evaporation of previously synthesized Ge15Sb5Te80 and Ge15Sb15Te70 glasses. Optical transmission and electrical conductivity measurements in sandwich and planar contact configuration were carried out. Temperature measurements of the planar electrical conductivity were performed in the range 77 - 300 K. Values of around 0.7 eV have been determined for the optical band gap of both compositions; the film refractive index is in the range 3.8 - 4.5. The sandwich conductivity is in the interval (2.5 - 5)×10−4 (Ω.cm)−1 while the planar conductivity is around 104 times greater. The obtained results are discussed in terms of existence of an amorphous volume part as well as a crystalline surface layer in the films.