Radiation-produced defects in germanium: Experimental data and models of defects

The problem of radiation-produced defects in n-Ge before and after n → p conversion is discussed in the light of electrical data obtained by means of Hall effect measurements as well as Deep Level Transient Spectroscopy. The picture of the dominant radiation defects in irradiated n-Ge before n → p conversion appears to be complicated, since they turn out to be neutral in n-type material and unobserved in the electrical measurements. It is argued that radiation-produced acceptors at ≈E _C – 0.2 eV previously ascribed to vacancy-donor pairs (E-centers) play a minor role in the defect formation processes under irradiation. Acceptor defects at ≈E _V + 0.1 eV are absolutely dominating in irradiated n-Ge after n → p conversion. All the radiation defects under consideration were found to be dependent on the chemical group-V impurities. Together with this, they are concluded to be vacancy-related, as evidenced positron annihilation experiments. A detailed consideration of experimental data on irradiated n-Ge shows that the present model of radiation-produced defects adopted in literature should be reconsidered.     

Growth,structure, and properties of GaAs-based (GaAs)1–x–y (Ge2) x (ZnSe) y epitaxial films

The possibility of growing the (GaAs)_1–x–y (Ge₂) _x (ZnSe) _y alloy on GaAs substrates by the method of liquid-phase epitaxy from a tin solution–melt is shown. X-ray diffraction shows that the grown film is single-crystal with the (100) orientation and has the sphalerite structure. The crystal-lattice parameter of the film is a _f = 0.56697 nm. The features of the spectral dependence of the photosensitivity are caused by the formation of various complexes of charged components. It is established that the I–V characteristic of such structures is described by the exponential dependence I = I ₀exp(qV/ckT) at low voltages (no higher than 0.4 V) and by the power dependence J ~ V ^α, where the exponent α varies with increasing voltage at high voltages (V > 0.5 V). The results are treated within the framework of the theory of the drift mechanism of current transfer taking into account the possibility of the exchange of free carriers within the recombination complex.     

Effect of Dislocation-related Deep Levels in Heteroepitaxial InGaAs/GaAs and GaAsSb/GaAs <Emphasis Type="Italic">p</Emphasis>–<Emphasis Type="Italic">i</Emphasis>–<Emphasis Type="Italic">n</Emphasis> Structures on the Relaxation time of Nonequilibrium Carriers

The results of an experimental study of the capacitance–voltage (C–V) characteristics and deep-level transient spectroscopy (DLTS) spectra of p⁺–p⁰–i–n⁰ homostructures based on undoped dislocationfree GaAs layers and InGaAs/GaAs and GaAsSb/GaAs heterostructures with homogeneous networks of misfit dislocations, all grown by liquid-phase epitaxy (LPE), are presented. Deep-level acceptor defects identified as HL2 and HL5 are found in the epitaxial p⁰ and n⁰ layers of the GaAs-based structure. The electron and hole dislocation-related deep levels, designated as, respectively, ED1 and HD3, are detected in InGaAs/GaAs and GaAsSb/GaAs heterostructures. The following hole trap parameters: thermal activation energies (E _t ), capture cross sections (σ _p ), and concentrations (N _t ) are calculated from the Arrhenius dependences to be E _t = 845 meV, σ _p = 1.33 × 10^–12 cm², N _t = 3.80 × 10¹⁴ cm^–3 for InGaAs/GaAs and E _t = 848 meV, σ _p = 2.73 × 10^–12 cm², N _t = 2.40 × 10¹⁴ cm^–3 for GaAsSb/GaAs heterostructures. The concentration relaxation times of nonequilibrium carriers are estimated for the case in which dislocation-related deep acceptor traps are involved in this process. These are 2 × 10^–10 s and 1.5 × 10^–10 s for, respectively, the InGaAs/GaAs and GaAsSb/GaAs heterostructures and 1.6 × 10^–6 s for the GaAs homostructures.     

Defects of the structure of semiconductor superlattices grown on the basis of II–VI alloys

The specific features of defects of crystal lattices in multilayer device structures containing small-period (T ? 20 nm) superlattices of type I ZnSe/Cd _x Zn_{1 ? x} Se/ZnSe/.../ZnSe/(001)GaAs and type II ZnS/ZnSe_{1 ? x} S _x /ZnS/.../ZnS/(001)GaAs are studied by the methods of X-ray diffractometry and diffraction rocking pseudocurves, and partially by the X-ray topography. According to the data of quantitative analysis of the X-ray diffraction spectra, the periods of superlattices are in the range T _I = 11.3–16.1 nm (for the compositions Cd _x Zn_{1 ? x} Se with x ₁ = 0.047 and x ₂ = 0.107) for type I superlattices and T _II = 15.6–17.2 nm for type II superlattices (for the compositions ZnSe_{1 ? x} S _x with x ₁ = 0.20 and x ₂ = 0.10). The widths of diffraction peaks from both the ZnSe layers and small-period superlattices in the diffraction rocking pseudocurves considerably exceed their widths in the X-ray diffraction spectra. This fact proves that a pronounced plastic strain with the formation of series of rectilinear dislocations in the crossing slip systems took place in the studied device structures. In order to exclude the generation of dislocations in the growth processes, it is necessary to decrease the concentration of the solid solution to the values x < 0.047 for the first type of superlattices and to the values x ? 0.062 for the second type of superlattices, and to decrease the thickness of the ZnSe and ZnS layers.     

Electron transport in unipolar heterostructure transistors with quantum dots in strong electric fields

A model for the explaining specific features of the electron transport in strong electric fields in the quantum-dot unipolar heterostructure transistor (AlGaAs/GaAs/InAs/GaAs/InAs) is presented. It is shown that the two-step shape of the output current-voltage characteristic I _D (V _D ) and the anomalous dependence of the drain current I _D on the gate voltage V _G are caused by the ionization of quantum dots in the strong electric field at the drain gate edge. The ionization of quantum dots sets in at the drain voltage V _D that exceeds the V_D1 value, at which the I _D (V _D ) dependence is saturated (the first step of the I-V characteristic). With the subsequent increase in V _D , i.e., for V _D >V_D1, the I _D (V _D ) dependence has a second abrupt rise due to the ionization of quantum dots, and then, for V _D =V_D2>V_D1, the current I _D is saturated for the second time (the second step in the current-voltage characteristic). It is suggested to use this phenomenon for the determining the population of quantum dots with electrons. The model presented also describes the twice-repeated variation in the sign of transconductance g _m =dI _D /dV _G as a function of V _G .     

Determination of the absolute value of the semiconductor surface potential by the quasi-static capacitance-voltage characteristics of an MIS structure

A simultaneous analysis of the derivatives C_V′V_g of the experimental and ideal quasi-static capacitance-voltage characteristics (plotted as a function of the normalized differential capacitance of a metal-insulator-semiconductor (MIS) structure) allows identification of regions within the semiconductor band gap E_g, in which interface states are virtually aTSent and the relation between the surface potential ψ_S of the real semiconductor and the voltage V_g applied to the MIS structure may be readily ascertained. This allows an accurate enough determination of the additive constants ψ_S0(V_g0) necessary to calculate the dependence ψ_S(V_g) in the entire range of V_g by numerical integration of the experimental quasi-static C-V characteristic. The comparison of this dependence with the ideal one characterizes in detail the integral electronic properties of the semiconductor-insulator heterojunction: the E_g-averaged density of interface states, the qualitative pattern of their distribution over the band gap, and the flat-band voltage V_FB and its components caused by a charge fixed in the undergate insulator and a charge localized at boundary states. A high accuracy of the V_FB measurements allows detection of even a weak physical response of MIS structures to external factors or to variations in the heterojunction technology. Results of such an analysis for a typical SiO₂/Si interface of an n-Si-MOS (metal-oxide-semiconductor) structure are considered. The application of C_V′-C_V diagrams for analyzing the high-frequency C-V characteristics is considered.     

X-ray conductivity of ZnSe single crystals

The experimental I–V and current–illuminance characteristics of the X-ray conductivity and X-ray luminescence of zinc-selenide single crystals feature a nonlinear shape. The performed theoretical analysis of the kinetics of the X-ray conductivity shows that even with the presence of shallow and deep traps for free charge carriers in a semiconductor sample, the integral characteristics of the X-ray conductivity (the current–illuminance and I–V dependences) should be linear. It is possible to assume that the nonlinearity experimentally obtained in the I–V and current–illuminance characteristics can be caused by features of the generation of free charge carriers upon X-ray irradiation, i.e., the generation of hundreds of thousands of free charge carriers of opposite sign in a local region with a diameter of <1 μm and Coulomb interaction between the free charge carriers of opposite signs.     

Study of RF transistor with submicron T-shaped gate fabricated by nanoimprint lithography

The results devoted to the development of a method for creating an RF transistor, in which a T-shaped gate is formed by nanoimprint lithography, are presented. The characteristics of GaAs p-HEMT transistors have been studied. The developed transistor has a gate “foot” length of the order of 250 nm and a maximum transconductance of more than 350 mS/mm. The maximum frequency of current amplification f _t is 40 GHz at the drain-source voltage V _DS = 1.4 V and the maximum frequency of the power gain f _max is 50 GHz at V _DS = 3 V.     

Photoluminescence Studies of Si-Doped Epitaxial GaAs Films Grown on (100)- and (111)A-Oriented GaAs Substrates at Lowered Temperatures

The electrical properties and photoluminescence features of uniformly Si-doped GaAs layers grown on GaAs substrates with the (100) and (111)A crystallographic orientations of the surface are studied. The samples are grown at the same As₄ pressure in the growth temperature range from 350 to 510°C. The samples grown on GaAs(100) substrates possess n-type conductivity in the entire growth temperature range, and the samples grown on GaAs(111)A substrates possess p-type conductivity in the growth temperature range from 430 to 510°C. The photoluminescence spectra of the samples exhibit an edge band and an impurity band. The edge photoluminescence band corresponds to the photoluminescence of degenerate GaAs with n- and p-type conductivity. The impurity photoluminescence band for samples on GaAs(100) substrates in the range 1.30–1.45 eV is attributed to V_As defects and Si_As–V_As defect complexes, whose concentration varies with sample growth temperature. Transformation of the impurity photoluminescence spectra of the samples on GaAs(111)A substrates is interpreted as being a result of changes in the V_As and V_Ga defect concentrations under variations in the growth temperature of the samples.     

Increase the threshold voltage of high voltage GaN transistors by low temperature atomic hydrogen treatment

High-electron-mobility transistors (HEMTs) based on AlGaN/GaN epitaxial heterostructures are a promising element base for the fabrication of high voltage electronic devices of the next generation. This is caused by both the high mobility of charge carriers in the transistor channel and the high electric strength of the material, which makes it possible to attain high breakdown voltages. For use in high-power switches, normally off-mode GaN transistors operating under enhancement conditions are required. To fabricate normally off GaN transistors, one most frequently uses a subgate region based on magnesium-doped p-GaN. However, optimization of the p-GaN epitaxial-layer thickness and the doping level makes it possible to attain a threshold voltage of GaN transistors close to V _th = +2 V. In this study, it is shown that the use of low temperature treatment in an atomic hydrogen flow for the p-GaN-based subgate region before the deposition of gate-metallization layers makes it possible to increase the transistor threshold voltage to V _th = +3.5 V. The effects under observation can be caused by the formation of a dipole layer on the p-GaN surface induced by the effect of atomic hydrogen. The heat treatment of hydrogen-treated GaN transistors in a nitrogen environment at a temperature of T = 250°C for 12 h reveals no degradation of the transistor’s electrical parameters, which can be caused by the formation of a thermally stable dipole layer at the metal/p-GaN interface as a result of hydrogenation.     

Structurally complex two-hole and two-electron slow traps with bikinetic properties in <Emphasis Type="Italic">p</Emphasis>-ZnTe and <Emphasis Type="Italic">n</Emphasis>-ZnS crystals

Thermal-activation and photoactivation methods were used to ascertain the existence of two-hole traps in p-ZnTe crystals and two-electron traps in n-ZnS. It was found that these traps have a large number of energy states that are grouped in two series of levels: E_V+(0.46–0.66) eV and E_V+(0.06–0.26) eV in p-ZnTe and E_C?(0.6–0.65) eV and E_C?(0.14–0.18) eV in n-ZnS. Both the hole and the electron traps belong to the class of slow traps with bikinetic properties. These traps feature normal kinetic properties in the state with a single trapped charge carrier and feature anomalous kinetic properties in the state with two charge carriers. Multiple-parameter models allowing for a relation of traps in p-ZnTe and n-ZnS to the vacancy-impurity pairs distributed according to their interatomic distances and localized in the region of microinhomogeneities with collective electric fields that repel the majority charge carriers are suggested. The main special features of behavior of electron and hole traps are explained consistently using the above models.     

Schottky-Barrier Model Nonlinear in Surface-State Concentration and Calculation of the <Emphasis Type="Italic">I–V</Emphasis> Characteristics of Diodes Based on SiC and Its Solid Solutions in the Composite Charge-Transport Model

A modified Schottky-barrier model, which is nonlinear in terms of the surface-state concentration and contains a local quasi-Fermi level at the interface induced by excess surface charge, is proposed. Such an approach makes it possible to explain the observed similarity of the I–V characteristics of diodes with the Schottky barrier M/(SiC)_1–x(AlN) _x and those of heterojunctions based on SiC and its solid solutions taking into account Φ_g ≈ Φ_B. The results of calculations of the Schottky-barrier heights are consistent with the experimental data obtained from measurements of the photocurrent for metals (M): Al, Ti, Cr, and Ni. The I–V characteristics in the composite–additive model of charge transport agree with the experimental data for the n-M/p-(SiC)_1–x(AlN) _x and n-6H-SiC/p-(SiC)_0.85(AlN)_0.15 systems.     

High-efficiency dual-junction GaInP/GaAs tandem solar cells obtained by the method of MOCVD

Monolithic dual-junction GaInP/GaAs solar cells grown by the MOCVD method were studied. The conditions of the growth of ternary Ga _x In_1?x P and Al _x In_1?x P alloys lattice-matched to GaAs are optimized. Technology for fabrication of a tunneling diode with a high peak current density of 207 A/cm² on the basis of heavily doped n ⁺⁺-GaAs:Si and p ⁺⁺-AlGaAs:C layers is developed. Cascade GaInP/GaAs solar cells obtained as a result of relevant studies featuring a good efficiency of the solar-energy conversion both for space and terrestrial applications. The maximum value of the GaInP/GaAs solar-cell efficiency was 30.03% (at AM1.5D, 40 suns).     

Germanium laser with a hybrid surface plasmon mode

The possibility of creating an n⁺⁺-Ge laser with a hybrid surface plasmon TM mode is theoretically studied. The distribution of electromagnetic fields and the absorbance in the mode under study, the optical confinement factor, the gain, and the threshold current density in the laser under consideration are calculated. It is shown that the threshold current density at optimal layer thicknesses of an n⁺⁺-Ge laser with a hybrid surface plasmon TM mode can be 2–3 times lower than the experimentally observed threshold current density in an n⁺⁺-Ge laser with a conventional dielectric waveguide.     

<Emphasis Type="Italic">C-V</Emphasis> and <Emphasis Type="Italic">I-V</Emphasis> characteristics of ultrathin-oxide MOS structures: Identification and analysis

For an NMOS structure with 3.7-nm-thick oxide, dynamic I-V characteristics are digitally measured by applying an upward and a downward gate-voltage ramp. An averaging procedure is employed to deduce the tunneling (active) current component and the quasi-static C-V characteristic (CVC). Analyzing the depletion segment of the CVC provides reliable values of the semiconductor doping level, the oxide capacitance and thickness, and the sign and density of oxide-fixed charge, as well as estimates of the dopant concentration in the poly-Si region. These data are used to identify the Ψ_s(V _g), V _i(V _g), and I _t(V _i) characteristics, where Ψ_s is the n-Si surface potential, V _i is the voltage drop across the oxide, V _g is the gate voltage, and I _t is the tunneling current; the gate-voltage range explored extends to prebreakdown fields (～13 MV cm^?1). The results are obtained without recourse to fitting parameters and without making any assumptions as to the energy spectrum of electrons tunneling from the n-Si deep-accumulation region through the oxide. It is believed that experimental I _t-V _i and Ψ_s-V _g characteristics will provide a basis for developing a theory of tunneling covering not only the degeneracy and size quantization of the electron gas in the semiconductor but also the nonclassical profile of the potential barrier to electron tunneling associated with the oxide-fixed charge.     

Conductivity of Ga<Subscript>2</Subscript>O<Subscript>3</Subscript>–GaAs Heterojunctions

The effect of annealing in argon at temperatures of T_an = 700–900°C on the I–V characteristics of metal–Ga₂O₃–GaAs structures is investigated. Samples are prepared by the thermal deposition of Ga₂O₃ powder onto GaAs wafers with a donor concentration of N _d = 2 × 10¹⁶ cm^–3. To measure theI–V characteristics, V/Ni metal electrodes are deposited: the upper electrode (gate) is formed on the Ga₂O₃ film through masks with an area of S _k = 1.04 × 10^–2 cm² and the lower electrode in the form of a continuous metallic film is deposited onto GaAs. After annealing in argon at T_an ≥ 700°C, the Ga₂O₃-n-GaAs structures acquire the properties of isotype n-heterojunctions. It is demonstrated that the conductivity of the structures at positive gate potentials is determined by the thermionic emission from GaAs to Ga₂O₃. Under negative biases, current growth with an increase in the voltage and temperature is caused by field-assisted thermal emission in gallium arsenide. In the range of high electric fields, electron phonon-assisted tunneling through the top of the potential barrier is dominant. High-temperature annealing does not change the electron density in the oxide film, but affects the energy density of surface states at the GaAs–Ga₂O₃ interface.     

Study of deep levels in GaAs <Emphasis Type="Italic">p–i–n</Emphasis> structures

An experimental study of the capacitance–voltage (C–V) characteristics and deep-level transient spectroscopy (DLTS) of p⁺–p⁰–i–n⁰ structures based on undoped GaAs, grown by liquid-phase epitaxy at two crystallization-onset temperatures T_o (950 and 850°C), with optical illumination switched off and on, are performed. It is shown that the p⁰, i, and n⁰ layers of epitaxial structures are characterized by the presence of defects with deep donor- and acceptor-type levels in concentrations comparable with those of shallow donors and acceptors. Interface states are found, which manifest themselves in the C–V characteristics at different measurement temperatures and optical illumination; these states form an additive constant. A distinct temperature dependence of the steady-state capacitance of the structures is revealed. It is found that the injection of minority carriers under an applied positive filling pulse and optical recharging lead to modification of the structure and, correspondingly, the DLTS spectra of the p⁺–p⁰–i–n⁰ structures. It is revealed that the p⁺–p⁰–i–n⁰ GaAs structures grown at T_o = 850°C are characterized by a lack of interface states and that the recharging of acceptor-type deep traps under illumination does not change the C–V characteristics. The conventionally measured DLTS spectra reveal the presence of two hole traps: HL5 and HL2, which are typical of GaAs layers.     

Structural and Electrical Properties of Ag/<Emphasis Type="Italic">n</Emphasis>-TiO<Subscript>2</Subscript>/<Emphasis Type="Italic">p</Emphasis>-Si/Al Heterostructure Fabricated by Pulsed Laser Deposition Technique

We have investigated the structural and electrical characteristics of the Ag/n-TiO₂/p-Si/Al heterostructure. Thin films of pure TiO₂ were deposited on p-type silicon (100) by optimized pulsed laser ablation with a KrF-excimer laser in an oxygen-controlled environment. X-ray diffraction analysis showed the formation of crystalline TiO₂ film having a tetragonal texture with a strong (210) plane as the preferred direction. High purity aluminium and silver metals were deposited to obtain ohmic contacts on p-Si and n-TiO₂, respectively. The current–voltage (I–V) characteristics of the fabricated heterostructure were studied by using thermionic emission diffusion mechanism over the temperature range of 80–300 K. Parameters such as barrier height and ideality factor were derived from the measured I–V data of the heterostructure. The detailed analysis of I–V measurements revealed good rectifying behavior in the inhomogeneous Ag/n-TiO₂/p-Si(100)/Al heterostructure. The variations of barrier height and ideality factor with temperature and the non-linearity of the activation energy plot confirmed that barrier heights at the interface follow Gaussian distributions. The value of Richardson’s constant was found to be 6.73 × 10⁵ Am^?2 K^?2, which is of the order of the theoretical value 3.2 × 10⁵ Am^?2 K^?2. The capacitance–voltage (C–V) measurements of the heterostructure were investigated as a function of temperature. The frequency dependence (Mott–Schottky plot) of the C–V characteristics was also studied. These measurements indicate the occurrence of a built-in barrier and impurity concentration in TiO₂ film. The optical studies were also performed using a UV–Vis spectrophotometer. The optical band gap energy of TiO₂ films was found to be 3.60 eV.     

Electrical and Photoelectric Properties of the TiN/<Emphasis Type="Italic">p</Emphasis>-InSe Heterojunction

The conditions for fabricating photosensitive TiN/p-InSe heterojunctions by the reactive-magnetron sputtering of thin titanium-nitride films onto freshly cleaved p-InSe single-crystal substrates is investigated. The presence of a tunnel-transparent high-resistivity In₂Se₃ layer at the heterojunction is revealed from analysis of the I–V characteristics, and the effect of this layer on the electrical properties and photosensitivity spectra of the heterostructures is analyzed. The dominant current transport mechanisms through the TiN/p-InSe energy barrier under forward and reverse bias are determined.     

Diamond Diode Structures Based on Homoepitaxial Films

(m–i–p)-Structures with high-resistance epitaxial i-layers are fabricated on heavily doped p⁺-type substrates with platinum contacts. The structures are studied using several methods: optical and electron microscopy and luminescence, and electrophysical (C–V and I–V characteristics) methods and tested as detectors of ionizing radiation. It is shown that the (m–i–p)-structures are promising for development of several electronic devices (high-voltage diodes, detectors of ionizing radiation, and photovoltaic devices).