Donor compensation in the depletion layer of CdF<Subscript>2</Subscript> crystals with a Schottky barrier

RF response and capacitance-voltage and current-voltage characteristics of n-type semiconductor crystals CdF₂:In, CdF₂:Ga, and CdF₂:Y with a Schottky barrier were studied. Specific features of these characteristics are accounted for based on the assumption that the charge transport from the metal to the depletion layer is due to the formation of Cd⁰ excitations in the contact layer, which occurs because of the supply of electron pairs from the metal (Au). These excitations compensate donors in the space-charge region of ∼1 μm thickness, adjacent to the contact. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 38, No. 1, 2004, pp. 72–78. Original Russian Text Copyright ? 2004 by Shcheulin, Kupchikov, Angervaks, Ryskin.     

<Emphasis Type="Italic">p</Emphasis><Superscript>+</Superscript>-Si-<Emphasis Type="Italic">n</Emphasis>-CdF<Subscript>2</Subscript> heterojunctions

Boron diffusion and the vapor-phase deposition of silicon layers are used to prepare ultrashallow p⁺-n junctions and p⁺-Si-n-CdF₂ heterostructures on an n-CdF₂ crystal surface. Forward portions of the I–V characteristics of the p⁺-n junctions and p⁺-Si-n-CdF₂ heterojunctions reveal the CdF₂ band gap (7.8 eV), as well as allow the identification of the valence-band structure of cadmium fluoride crystals. Under conditions in which forward bias is applied to the p⁺-Si-n-CdF₂ heterojunctions, electroluminescence spectra are measured for the first time in the visible spectral region.     

An impurity band in Hg<Subscript>3</Subscript>In<Subscript>2</Subscript>Te<Subscript>6</Subscript> crystals doped with silicon

The influence of silicon impurity on the energy-band spectrum in the Hg₃In₂Te₆ semiconductor compound, which incorporated a high concentration of stoichiometric vacancies, was studied on the basis of the results of electrical and optical measurements. It is shown that silicon impurity forms an impurity band of donor states whose density can be approximated by a Gaussian distribution with a peak at E_c-0.29 eV. The emergence of the impurity band is accompanied with the formation of a quasi-continuous spectrum of localized states in the band gap (E_g=0.74 eV); the density of these states is shown to increase as the doping level increases. All states merge into a continuous band if the impurity concentration N_Si>4.5×10¹⁷ cm^?3. Experimental data are explained on the basis of the effect of impurity self-compensation, in which case donor impurity states arise simultaneously with acceptor states of defects.     

Optical and photoelectrical properties of AgCd<Subscript>2</Subscript>GaS<Subscript>4</Subscript> single-crystal compounds

The poorly known AgCd₂GaS₄ single-crystal compounds that crystallize in a rhombic structure (space group Pmn2₁) are studied. Deviations from the stoichiometric composition of the samples and random occupation of the cation sublattice sites by Ag and Ga ions result in violation of long-range order in the atomic arrangement and make the AgCd₂GaS₄ compounds structurally closer to disordered systems. In this case, it is found that the fundamental optical-absorption edge is smeared and shifted to longer wavelengths, and is adequately described by the Urbach rule. In addition, a broadening of the spectral peaks of photoconductivity and luminescence is observed. The concentration of charged point defects responsible for the smearing of the absorption edge is calculated. It is found to be 1.2×10²⁰ cm^?3. The AgCd₂GaS₄ single crystals are photosensitive semiconductors. From the position of the absorption edge, the optical band gap of the compound is estimated (E _g0 = 2.28 eV at T = 297 K). The photoluminescence spectra of the AgCd₂GaS₄ single crystals are similar to the spectra of defect-containing CdS single crystals; for the AgCd₂GaS₄ crystals, the emission peaks are shifted to longer wavelengths with respect to the peaks for CdS crystals by Δλ = 0.06?0.1 μm. From the analysis of the experimental data, some conclusions on the nature of photoactive centers in AgCd₂GaS₄ compounds are drawn.     

Effect of gadolinium impurity on transmittance and reflectance of Hg<Subscript>3</Subscript>In<Subscript>2</Subscript>Te<Subscript>6</Subscript> crystals

The results of measuring the optical transmission and reflection spectra in the transparency region of Hg₃In₂Te₆:Gd semiconducting crystals are reported. It is shown that, in the entire wavelength range under investigation (2–25 µm), doping with Gd results in an increase in the continuous structureless absorption by impurities and defects, which introduce a quasi-continuous spectrum of localized states within the band gap. A decrease in transmittivity is accompanied by a decrease in the wavelength-independent reflectivity. Variation in the refractive index is attributed to changes in the bonding and electronic polarization of Hg₃In₂Te₆:Gd crystals. It is ascertained that polarization constants depend linearly on the strength of internal electric fields which exist in the vicinity of impurity defects.     

Design of the H<Subscript>10</Subscript><Superscript>□</Superscript> to H<Subscript>01</Subscript><Superscript>°</Superscript> Sector-Type Mode Converter at Ka-Band

A design procedure for the H₁₀^□ to H₀₁^° sector-type mode converter at Ka-band is described in detail. As an example, an H₁₀^□ to H₀₁^° sector-type mode converter with an input rectangular port size as 7.112×3.556 mm² and another output circular port diameter as 17.2 mm is designed and measured for insertion losses and wanted mode pattern. The mode converting efficiency from H₁₀^□ mode to H₀₁^° mode is about 91.2%. The insertion losses of the converter are lower than 0.1525 dB with 2GHz band.     

Effect of copper doping on kinetic coefficients and their anisotropy in PbSb<Subscript>2</Subscript>Te<Subscript>4</Subscript>

In anisotropic PbSb₂Te₄ and PbSb₂Te₄:Cu single crystals, nine main independent components of the Hall, electrical-conductivity, thermopower, and Nernst-Ettingshausen effects and their anisotropy in the range 77–450 K have been studied. PbSb₂Te₄ single crystals exhibit a high hole concentration (p ≈ 3 × 10²⁰ cm⁻³). Copper exhibits a donor effect and significantly (approximately by a factor of 2) reduces the hole concentration in PbSb₂Te₄. The temperature dependences of the kinetic coefficients, except for the Hall effect, have a form typical of the one-band model. The significant anisotropy of the Hall coefficient R ₁₂₃/R ₃₂₁ ≈ 2 at low temperatures corresponds to the multi-ellipsoid model of the energy spectrum of holes in PbSb₂Te₄. An important feature of the data on transport phenomena is the high thermopower anisotropy (ΔS ≈ 60–75 μV/K) in the mixed conductivity region caused by the mixed scattering mechanism. Data on the anisotropy of the transverse Nernst-Ettingshausen effect confirm the mixed mechanism of hole scattering; in the cleavage plane, scattering at acoustic phonons dominates, while in the trigonal axis direction, impurity scattering appears significant. Doping with copper enhances the role of impurity scattering in the direction of the trigonal axis c ₃; as a result, two components of the Nernst-Ettingshausen tensor Q ₃₂₁ and Q ₁₃₂ in the PbSb₂Te₄:Cu single crystal are positive at low temperatures, whereas, in the undoped crystal, only the Q ₃₂₁ component is positive.     

Superionic conductivity in TlGaTe<Subscript>2</Subscript> crystals

Temperature dependences of electrical conductivity σ(T) and permittivity ɛ(T) of one-dimensional (1D) TlGaTe₂ single crystals are investigated. At temperatures higher than 305 K, superionic conductivity of the TlGaTe₂ is observed and is related to diffusion of Tl⁺ ions via vacancies in the thallium sublattice between (Ga³⁺Te₂^{2− −} nanochains. A relaxation character of dielectric anomalies is established, which suggests the existence of electric charges weakly bound to the crystal lattice. Upon the transition to the superionic state, relaxors in the TlGaTe₂ crystals are Tl⁺ dipoles ((Ga³⁺Te₂²⁻)⁻ chains) that arise due to melting of the thallium sublattice and hops of Tl⁺ ions from one localized state to another. The effect of a field-induced transition of the TlGaTe₂ crystal to the superionic state is detected.     

Dielectric properties of layered FeGaInS<Subscript>4</Subscript> single crystals in an alternating electric field

The results of investigations of the frequency and temperature dependences of dielectric losses and the imaginary part of the dielectric permittivity in FeGaInS₄ single crystals are presented. Their experimental values are determined. It is established that the loss tangent and the imaginary part of the permittivity of FeGaInS₄ single crystals in a field with frequencies of 10⁴–10⁶ Hz decrease inversely proportional to the frequency (tanδ ~ 1/ω), and the conductivity is characterized by the band–hopping mechanism. For FeGaInS₄, the relaxation time is calculated, and it is established that there is a mechanism of electron polarization caused by thermal motion in this crystal.     

Photoreflection studies of the dopant activation in InP implanted with Be<Superscript>+</Superscript> ions

Photoreflection spectroscopy is used to study the activation of impurity in InP crystals implanted with 100-keV Be⁺ ions at a dose of 10¹³ cm^?2 and then subjected to thermal annealing for 10 s. After annealing at temperatures no higher than 400°C, lines characteristic of crystalline InP are not observed in the photoreflection spectrum, which indicates that the crystal lattice has become disordered as a result of the ion implantation. If the annealing temperature is in the range from 400 to 700°C, the lines related to the fundamental transition in InP (1.34 eV) and the transition between the conduction band and the subband, which has split off from the valence band owing to a spin-orbit interaction (1.44 eV), are observed in the spectrum, which indicates that the InP crystal structure is restored. The dopant is activated in samples annealed at 800°C, as indicated by the Franz-Keldysh oscillations observed in the corresponding photoreflection spectra. Free-carrier concentration is determined from the oscillation period and is found to be equal to 2.2 × 10¹⁶ cm^?3.     

Effect of microwave treatment on current flow mechanisms in Au-TiB<Subscript>x</Subscript>-Al-Ti-n<Superscript>+</Superscript>-n-n<Superscript>+</Superscript>-GaN-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> ohmic contacts

The temperature dependences of the contact resistivity ρ_c of Au-TiB_x Al-Ti-n⁺-n-n⁺-GaN-Al₂O₃ ohmic contacts have been studied before and after microwave treatment followed by nine-nonth room-temperature sample storage. The temperature dependences of ρ_c of initial samples were measured twice. The first measurement showed the temperature dependence typical of ohmic contacts; the repeated measurement in the temperature region above 270 K showed a ρ_c increase caused by metallic conductivity. After microwave treatment, the metallic conductivity in the ohmic contact is not observed. This is presumably associated with local heating of metal Ga inclusions under microwave irradiation and the formation, due to high chemical activity of liquid gallium, of compounds of it with other metallization components. In this case, the temperature dependence of ρ_c is controlled by ordinary charge transport mechanisms. After nine-nonth room-temperature storage, the temperature dependence of ?c is described by the tunneling mechanism of charge transport.     

Electrical properties of In<Subscript>2</Subscript>Se<Subscript>3</Subscript> single crystals and photosensitivity of Al/In<Subscript>2</Subscript>Se<Subscript>3</Subscript> Schottky barriers

In₂Se₃ single crystals ∼40 mm long and 14 mm in diameter were grown by the Bridgman method. The composition of grown single crystals and their crystal structure were determined. The conductivity (σ) and Hall constant (R) of grown single crystals were measured and the first Schottky barriers Al/n-In₂Se₃ were fabricated. Rectification and photovoltaic effect were detected in the new structures. Based on the study of the photosensitivity spectra of Al/n-In₂Se₃ structures, the nature of the interband transitions and band gap of In₂Se₃ crystals were determined. It was concluded that the new structures can be applied to develop broadband photoconverters of optical radiation.     

Exact self-compensation of conduction in Cd<Subscript>0.95</Subscript>Zn<Subscript>0.05</Subscript>Te:Cl crystals in a wide range of Cd vapor pressures

The process of self-compensation in Cd_0.95Zn_0.05Te:Cl solid-solution crystals has been studied by annealing single crystals under a controlled Cd vapor pressure, with subsequent measurements of the Hall effect, photoluminescence, carrier lifetime and mobility, and photocurrent memory in the annealed crystals. By means of this annealing, conditions of thermal treatment that make it possible to fabricate low-conductivity samples with a low carrier density, 10⁷–10¹¹ cm^?3, are defined. In these samples, a p → n conduction inversion is observed at a higher free-carrier density (n, p ≈ 10⁹ cm^?3) and the dependence of the electron density on the Cd vapor pressure exhibits a more gentle slope than in the case of CdTe:Cl crystals. The obtained data are discussed in terms of a self-compensation model in which intrinsic point defects act as acceptors with deep levels. This level is attributed to a Zn vacancy, which remains active at high Cd pressure.     

Magnetic susceptibility of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript> alloys

The magnetic susceptibility of Czochralski-grown single crystals of Bi₂Te₃-Sb₂Te₃ alloys containing 0, 10, 25, 40, 50, 60, 65, 70, 80, 90, 99.5, or 100 mol % Sb₂Te₃ has been investigated. The magnetic susceptibility of these crystals was determined at the temperature T = 291 K and the magnetic field H oriented parallel (χ_‖) and perpendicularly (χ_⊥) to the trigonal crystallographic axis C ₃. A complicated concentration dependence of the anisotropy of magnetic susceptibility χ_‖/χ_⊥ has been revealed. The crystals with the free carrier concentration p ≈ 5 × 10¹⁹ cm^?3 do not exhibit anisotropy of magnetic susceptibility. The transition to the isotropic magnetic state occurs for the compositions characterized by a significantly increased (from 200 to 300 meV) optical bandgap.     

Uniform Cr<Superscript>2+</Superscript> doping of physical vapor transport grown CdS<Subscript>x</Subscript>Se<Subscript>1−x</Subscript> crystals

The Cr²⁺ doped CdS_0.8Se_0.2 crystals were grown by the vertical, self-seeded, physical vapor transport (PVT) technique. Good quality, crack- and inclusion-free single crystals were grown with an average Cr²⁺ concentration of 5 × 10¹⁸ cm⁻³. Different source-to-tip distances were used to improve the segregation coefficient (Cr_crystal/Cr_source) of the grown crystals. It was observed that lowering the source-to-tip distance increases the segregation coefficient dramatically. With a 2-cm source-to-tip distance, good quality crystals were grown with uniform Cr²⁺ concentration throughout the ingot. The segregation coefficient was found to be ∼0.85. The composition of the crystals was also found to be fairly uniform along the length and across the diameter.     

Magnetic properties of the FeIn<Subscript>2</Subscript>S<Subscript>4</Subscript> ternary-compound crystals

Crystals of the ternary compound FeIn₂S₄ are grown by directional crystallization of a melt (the horizontal Bridgman method). Composition of the crystals and their crystal structure are determined. Magnetic properties of the FeIn₂S₄ crystals are studied in the temperature range 4–310 K in magnetic fields of 0–140 kOe. It is shown that the crystals under study are paramagnets up to ∼12 K and their specific magnetic moment monotonically increases with decreasing temperature. The antiferromagnetic character of indirect interactions between Fe²⁺ cations is established. The most probable causes and the mechanism of the formation of the magnetic state in the FeIn₂S₄ crystals are discussed.     

A Single-Composition Trichromatic White-Light-Emitting Phosphor Based on Sr<Subscript>8</Subscript>ZnY(PO<Subscript>4</Subscript>)<Subscript>7</Subscript> Coactivated with Ce<Superscript>3+</Superscript>, Tb<Superscript>3+</Superscript>, and Mn<Superscript>2+</Superscript>

Ce³⁺/Tb³⁺/Mn²⁺-codoped Sr₈ZnY(PO₄)₇ (SZYP) white-emitting phosphors have been synthesized via solid-state reaction technology. The overlapping spectra between the excitation bands of Mn²⁺ and Tb³⁺ ions and the emission band of Ce³⁺ suggest that Ce³⁺ → Mn²⁺ and Ce³⁺ → Tb³⁺ energy transfer occurs. The emission hues exhibited by Ce³⁺/Tb³⁺- and Ce³⁺/Mn²⁺-codoped phosphors could be modulated from bluish to greenish region and from bluish to reddish region by simply adjusting the relative content of Ce³⁺/Tb³⁺ and Ce³⁺/Mn²⁺, respectively. SZYP:Ce³⁺,Tb³⁺,Mn²⁺ samples exhibited three dominant bands at 410 nm, 545 nm, and 600 nm, attributable to electronic transitions of Ce³⁺, Tb³⁺, and Mn²⁺ ions, respectively. Thus, color-tunable emission was achieved by accurately modulating the concentrations of Ce³⁺, Tb³⁺, and Mn²⁺ ions. SZYP:0.05Ce³⁺,0.11Mn²⁺,0.11Tb³⁺ was found to be an ideal white-light-emitting phosphor with color coordinates of (0.34, 0.33) and correlated color temperature of about 5144.83 K. The results indicate that Ce³⁺/Tb³⁺/Mn²⁺ -tridoped SZYP phosphors are potential single-component white-emitting candidates for application in ultraviolet- and white-light-emitting diodes.     

<Superscript>63</Superscript>Cu and <Superscript>115</Superscript>In NMR study of CuInS<Subscript>2</Subscript> semiconductor

Two CuInS₂ semiconductor samples synthesized from chemical elements with the equiatomic cation ratio (N _Cu/N _In = 1) and either an excess or stoichiometric sulfur content have been investigated by ⁶³Cu and ¹¹⁵In nuclear magnetic resonance. The spectra were recorded on a Bruker Avance-400 spectrometer at a temperature of 290 K and frequencies of 106.14 MHz (⁶³Cu) and 87.67 MHz (¹¹⁵In). Numerical simulation made it possible to determine the quadrupole coupling constants: 0.34 MHz (⁶³Cu) and 1.1 MHz (¹¹⁵In). For the samples synthesized with excess sulfur (above stoichiometry), distortions are revealed in both the ⁶³Cu and ¹¹⁵In spectra.     

AC Electrical Conductivity of FeIn<Subscript>2</Subscript>Se<Subscript>4</Subscript> Single Crystals

The results obtained in a study of the frequency and temperature dependences of the ac electrical conductivity of FeIn₂Se₄ single crystals are presented. It is found that the law σ ~ f ^S (0.1 ≤ S ≤ 1.0) is obeyed for electrical conductivity in the 295–375 K temperature range at frequencies of 2 × 10⁴–10⁶ Hz. It shown that the frequency dependence of the conductivity in an FeIn₂Se₄ single crystal can be accounted for in terms of the multiplet model, and, consequently, the conductivity in these single crystals is characterized by the band-hopping mechanism.     

Low-temperature instabilities of the electrical properties of Cd<Subscript>0.96</Subscript>Zn<Subscript>0.04</Subscript>Te:Cl semi-insulating crystals

The electrical properties in the temperature range 295–430 K and low-temperature (4.2 K) photoluminescence of Cd_1?xZn_xTe:Cl semi-insulating crystals grown from melts with a variable impurity content (C _Cl ⁰ = 5 × 10¹⁷–1 × 10¹⁹ cm^?3) are investigated. Nonequilibrium processes leading to a decrease in carrier concentration are observed in all the samples at low temperatures (T = 330–385 K). These changes are reversible. The activation energy of these processes E _a is found to be 0.88 eV. As with semi-insulating CdTe:Cl, the observed phenomena can be explained by a change in the charge state of background copper atoms: Cu_Cd ? Cu_i. The introduction of Zn changes the ratio of the concentrations of shallow-level donors Cu_i and Cl_Te from their levels in the initial material.