A model of photoinduced annealing of intrinsic defects in hexagonal CdS<Subscript>x</Subscript>Se<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> quantum dots

A possible mechanism of photoinduced annealing of intrinsic defects in quantum dots with a hexagonal crystal structure is justified on the basis of the studies of the kinetics of photoinduced decay of luminescence of CdS_xSe_1?x quantum dots synthesized in a glass matrix and ab initio calculations of chemical bond energies at the interface in the n(CdSe)-SiO_x-type cluster. The model proposed implies that photoinduced Se-O bond breaking at the anionic face results in an increase in electric field inside the quantum dot; this field stimulates cadmium vacancy diffusion to the surface. This model accounts for the degradation of luminescence and of the parameters of nonlinear optical devices observed during photoinduced annealing.     

Investigation of the photoresistance of granular CdS<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript> films obtained by screen printing

Samples of CdS _x Se_{1 ? x} films were obtained by screen printing. It is shown that samples represent a granular composite with a high oxygen content (up to 20%). It is proved that transitions in the forbidden region of the CdS _x Se_{1 ? x} solid solution are responsible for light absorption changing the material resistance. The dependence of CdS _x Se_{1 ? x} samples resistance on illumination is studied. It is discovered that this type of materials has a photocurrent/dark current ratio higher than ten to the fifth power. The dependence of the conductivity on the sample structure is established.     

Conditions for the formation of defectless quantum dots in the theoretical estimation of In<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>As/GaAs heterostructures

Critical thiknesses of formation mismatch dislocations and transition from two dimensional to three dimensional (2D–3D) mode during growth of In _x Ga_{1 − x} As/GaAs heterostructures by the Stranski-Krastanow mechanism were estimated using the energy balance model. The maximal magnitudes of those values and the intervals of the composition of solid solutions allowing the growth of defectless quantum dots were obtained.     

Optical properties of AgGa<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>In<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript>2</Subscript> alloys

Coarse-grained crystals of AgGaSe₂ and AgInSe₂ ternary compounds and their alloys are grown by planar crystallization of the melts. For the crystals produced in this way, the transmittance spectra near the fundamental absorption edge are studied. From the experimental spectra, the band gap (E _g) and its variation with composition are determined. It is established that E _g is a nonlinear function of the composition parameter x. The dependence E _g (x) is calculated theoretically in the context of the Van Vechten-Bergstresser model and Hill-Richardson pseudopotential model.     

The Power Factor of Bi<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript>Tl<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript>3</Subscript> Single Crystals

Single crystals of the ternary system Bi_2−x Tl _x Se₃ (nominally x = 0.0 to 0.1) were prepared using the Bridgman technique. Samples with varying content of Tl were characterized by measurement of lattice parameters, electrical conductivity σ _⊥c, Hall coefficient R _H(B║c), and Seebeck coefficient S(ΔT⊥c). The measurements indicate that incorporation of Tl into Bi₂Se₃ lowers the concentration of free electrons and enhances their mobility. This effect is explained within the framework of the point defects in the crystal lattice, with formation of substitutional defects of thallium in place of bismuth (Tl_Bi) and a decrease in the concentration of selenium vacancies (V_Se ^{+ 2} V_{\rm{Se}}^{ + 2} ). The temperature dependence of the power factor σS ² of the samples is also discussed. As a consequence of the thallium doping we observe a significant increase of the power factor compared with the parent Bi₂Se₃.     

Study of optical characteristics of structures with strongly strained In<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>As quantum wells

Results of photoluminescence (PL) studies of heterostructures with strongly strained InxGa_{1 ? x} As quantum wells (QWs) are presented. It is shown that the dependence of the PL intensity on the QW thickness has a maximum whose position depends on the composition of the In _x Ga_{1 ? x} As solid solution. The PL wavelength at the maximum intensity is 1.13 µm at a QW thickness of 60 µm at a QW thickness of 50 Å for x = 0.39 and 0.42, respectively.     

Electroluminescence and phototrigger effect in single crystals of GaS<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> alloys

The effects of switching and electroluminescence as well as the interrelation between these effects in single crystals of GaS _x Se_1?x alloys are detected and studied. It is established that the threshold voltage for switching depends on temperature, resistivity, and composition of alloys, and also on the intensity and spectrum of photoactive light. As a result, a phototrigger effect is observed; this effect arises under irradiation with light from the fundamental-absorption region. Electroluminescence is observed in the subthreshold region of the current-voltage characteristic; the electroluminescence intensity decreases drastically to zero as the sample is switched from a high-resistivity state to a low-resistivity state. Experimental data indicating that the electroluminescence and the switching effect are based on the injection mechanism (as it takes place in other layered crystals of the III-V type) are reported.     

Synthesis and Transport Properties of In<Subscript>4</Subscript>(Se<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Te<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>3</Subscript>

Polycrystalline samples of In₄(Se_1−x Te _x )₃ were synthesized by using a melting–quenching–annealing process. The thermoelectric performance of the samples was evaluated by measuring the transport properties from 290 K to 650 K after sintering using the spark plasma sintering (SPS) technique. The results indicate that Te substitution can effectively reduce the thermal conductivity while maintaining good electrical transport properties. In₄Te₃ shows the lowest thermal conductivity of all compositions tested.     

On the band gap of Cu<Subscript>2</Subscript>ZnSn(S<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>4</Subscript> alloys

The transmittance spectra of single-crystal Cu₂ZnSnS₄ and Cu₂ZnSnSe₄ compounds and Cu₂ZnSn(S _x Se_1–x )₄ alloys grown by chemical vapor-transport reactions are studied in the region of the fundamental absorption edge. From the experimental spectra, the band gap of the compounds and their alloys is determined. The dependences of the band gap on the composition parameter x of the alloy are constructed. It is established that the band gap nonlinearly varies with x and can be described as a quadratic dependence.     

Specific features of the temperature dependence of the conduction electron concentration in the narrow-gap and zero-gap states of Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Hg<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Te

Results of studies of the conductivity σ and the Hall coefficient R in the Cd _x Hg_{1 − x} Te crystals with x = 0.1, 0.12, 0.14, and 0.15 are analyzed in the temperature range T = 4.2–300 K and the magnetic field range B = 0.005–2.22 T. Using data on the R(B) in low and high magnetic fields and the data on σ(T), electron and hole concentrations and mobilities are determined. It is shown that the electron concentration n in the studied samples is almost independent of T in the range 4.2–15 K, while as T increases, it increases according to the law n ∝ T ^r (r > 3/2), where r = f(n, T, x). It is found that r varies from 1.7 at x = 0.1 to 3.1 at compositions with x = 0.14 and 0.15. The results for n(T) are compared with theory, taking into account nonparabolicity of the variance law for ⃛(T), and with the theory of impurity states in narrow-gap and zero-gap semiconductors. It is shown that the constancy of n(T) up to ∼15 K and the strong dependence n(T) (r > 3/2) at higher temperatures are caused by the intense ionization of electrons localized at acceptor states.     

Cathodoluminescence from dilute GaN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>As<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> solutions (<Emphasis Type="Italic">x</Emphasis> ≤ 0.03)

Cathodoluminescence from GaN _x As_1?x layers (0 ≤ x ≤ 0.03) was measured at photon energies ranging from the intrinsic absorption edge to 3 eV at room temperature. An additional emission band was visible in the visible range of the cathodoluminescence spectra. The intensity of this band is two orders of magnitude lower than the edge-emission intensity. The photon energy corresponding to the peak of this band and its FWHM are virtually independent of x and equal to ～2.1 and 0.6–0.7 eV, respectively. This emission is related to indirect optical transitions of electrons from the L _6c and Δ conduction-band minimums to the Γ₁₅ valence-band maximum.     

On the Dielectric Study of Se<Subscript>80−<Emphasis Type="Italic">x</Emphasis></Subscript>Te<Subscript>20</Subscript>Pb<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> (<Emphasis Type="Italic">x</Emphasis> = 0, 1 and 2) Glasses

In the present paper, the dielectric parameters such as the dielectric constant ε′(ω), dielectric loss ε″(ω) and alternating current (ac) conductivity have been investigated for bulk amorphous chalcogenide Se_80?x Te₂₀Pb _x (x = 0, 1 and 2) glasses in the frequency range 10 Hz to 500 kHz and within the temperature range from 300 K to 320 K. Dielectric constant ε′(ω) and dielectric loss ε″(ω) are found to be highly frequency (ω) and temperature dependent, and this behavior is interpreted on the basis of Guintini’s theory of dielectric dispersion. The ac conductivity (σ _ac) is found to be temperature independent and obey the power law ω ^s , where s < 1 and decreases as temperature rises. The obtained results are discussed in terms of the correlation barrier hopping model proposed by Elliot. The composition dependence of the dielectric constant, dielectric loss and ac conductivity are also discussed and reported here.     

Pb<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Sn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te epitaxial films for terahertz detectors

Liquid-phase epitaxy is used to fabricate Pb_0.8Sn_0.2Te films, undoped or doped with indium to different levels. The depth profiles of the carrier density and dopant concentration in the films are measured and examined. A uniform dopant concentration to a depth of 15 μm is obtained. Electrical-conduction inversion is observed at a temperature of 77.3 K as the doping level is varied. The liquid-phase epitaxial method is shown to be a more suitable technology for the reproducible manufacture of epitaxial films with a given carrier density, such as the ones used in terahertz detectors.     

Investigation of MBE-Growth of Mid-Wave Infrared Hg<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se

Undoped mid-wave infrared Hg_1?xCd_xSe epitaxial layers have been grown to a nominal thickness of 8–14 μm on GaSb (211)B substrates by molecular beam epitaxy (MBE) using constant beam equivalent pressure ratios. The effects of growth temperature from 70°C to 120°C on epilayer quality and its electronic parameters has been examined using x-ray diffraction (XRD) rocking curves, atomic force microscopy, Nomarski optical imaging, photoconductive decay measurements, and variable magnetic field Hall effect analysis. For samples grown at 70°C, the measured values of XRD rocking curve full width at half maximum (FWHM) (116 arcsec), root mean square (RMS) surface roughness (2.7 nm), electron mobility (6.6?×?10⁴ cm² V^?1 s^?1 at 130 K), minority carrier lifetime (～?2 μs at 130 K), and background n-type doping (～?3?×?10¹⁶ cm^?3 at 130 K), indicate device-grade material quality that is significantly superior to that previously published in the open literature. All of these parameters were found to degrade monotonically with increasing growth temperature, although a reasonably wide growth window exists from 70°C to 90°C, within which good quality HgCdSe can be grown via MBE.     

Type II broken-gap GaSb<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>As<Emphasis Type="Italic">x</Emphasis>/InAs heterojunction (<Emphasis Type="Italic">x</Emphasis> < 0.15): Evolution of the band diagram for the ternary solid solution

Layers of the GaSb_{1 ? x} As _x alloy with arsenic content in the range x = 0.06–0.15 have been grown for the first time on InAs (100) substrates by metal-organic vapor-phase epitaxy. A new approach to the calculation of the band diagram of the GaSbAs alloy is suggested. It is demonstrated on the basis of magnetotransport measurements in p-GaSbAs/p-InAs heterostructures and with the method suggested by the authors for the calculation of band diagrams for alloys in the GaSbAs system that, in the composition range under study, the GaSbAs/InAs heterojunction is a type II broken-gap heterojunction.     

Concentration dependence of the band gap of Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>In<Subscript>2</Subscript>S<Subscript>4</Subscript> alloys on their composition

The transmittance spectra of MnIn₂S₄ and FeIn₂S₄ ternary compounds and Mn _x Fe_{1 − x} In₂S₄ alloys in the fundamental absorption edge region are studied. The samples were grown by planar melt crystallization. From the experimental spectra, the band gaps of the compounds and their alloys are determined, and the concentration dependence of the alloy band gap on the component content is established. It is found that the band gap nonlinearly varies with the composition parameter x and can be described by a quadratic function.     

Pb<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te Alloys: Application Considerations

The search for alternative energy sources is at the forefront of applied research. In this context, thermoelectricity, i.e., direct conversion of thermal into electrical energy, plays an important role, particularly for exploitation of waste heat. Materials for such applications should exhibit thermoelectric potential and mechanical stability. PbTe-based compounds include well-known n-type and p-type compounds for thermoelectric applications in the 50°C to 600°C temperature range. This paper is concerned with the mechanical and transport properties of p-type Pb_0.5Sn_0.5Te:Te and PbTe<Na> samples, both of which have a hole concentration of ∼1 × 10²⁰ cm⁻³. The ZT values of PbTe<Na> were found to be higher than those of Pb_0.5Sn_0.5Te:Te, and they exhibited a maximal value of 0.8 compared with 0.5 for Pb_0.5Sn_0.5Te:Te at 450°C. However, the microhardness value of 49 H_V found for Pb_0.5Sn_0.5Te:Te was closer to that of the mechanically stable n-type PbTe (30 H_V) than to that of PbTe<Na> (71 H_V). Thus, although lower ZT values were obtained, from a mechanical point of view Pb_0.5Sn_0.5Te:Te is preferable over PbTe<Na> for practical applications.     

Low-temperature microwave magnetoresistance of lightly doped <Emphasis Type="Italic">p</Emphasis>-Ge and <Emphasis Type="Italic">p</Emphasis>-Ge<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Si<Subscript>x</Subscript>

The magnetoresistance of a lightly doped p-Ge_1?xSi_x alloy is studied in the range of compositions x = 1–2 at %. The results are compared with the available data for lightly doped p-Ge. The studies have been carried out using ESR measurements at a frequency of 10 GHz in the temperature range 10–120 K. It is established that micrononuniformity in the distribution of Si in the Ge lattice (Si clusters) suppresses the interference part of the anomalous magnetoresistance and, in addition, results in an averaging of the effects of light and heavy holes. This observation suggests a sharp decrease in the inelastic scattering time in the case of a Ge_1?xSi_x solid solution as compared to that of Ge.     

A Scanning Probe Microscopy Study of Cd<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Zn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te

The effects of several ex vacuo methods used in the surface preparation of Cd_1−x Zn _x Te (CZT) have been studied using noncontact atomic force microscopy, scanning tunneling microscopy, and scanning tunneling spectroscopy. Preparation techniques include mechanical lapping, hydroplane bromine-methanol polishing, and in vacuo annealing. The morphology, electrical homogeneity, and local density of states (LDOS) have been studied for each preparation method. Impurities and oxides quickly form on the surface after each preparation method. Annealing in ultrahigh vacuum causes the surface electronic structure to become inhomogeneous whilst the LDOS suggests a compositional change from an oxide surface to p-type CZT.     

Influence of Composition on the Thermoelectric Properties of Bi<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sb<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Thin Films

Bi_1?x Sb _x solid solutions have attracted much attention as promising thermoelectric (TE) materials for cooling devices at temperatures below ～200 K and as unique model materials for solid-state science because of a high sensitivity of their band structure to changes in composition, temperature, pressure, etc. Earlier, we revealed a non-monotonic behavior of the concentration dependences of TE properties for polycrystalline Bi_1?x Sb _x solid solutions and attributed these anomalies to percolation effects in the solid solution, transition to a gapless state, and to a semimetal–semiconductor transition. The goal of the present work is to find out whether the non-monotonic behavior of the concentration dependences of TE properties is observed in the thin film state as well. The objects of the study are Bi_1?x Sb _x thin films with thicknesses in the range d = 250–300 nm prepared by thermal evaporation of Bi_1?x Sb _x crystals (x = 0–0.09) onto mica substrates. It was shown that the anomalies in the dependence of the TE properties on Bi_1?x Sb _x crystal composition are reproduced in thin films.