Dependence of photoluminescence spectra of epitaxial Pb1 ? xEuxTe (0 ≤ x ≤ 0.1) alloy layers on conditions of growth

The photoluminescence spectra of the Pb_{1 − x} Eu _x Te (0 ≤ x ≤ 0.1) alloy are studied at low temperatures. Epitaxial layers were grown by molecular-beam epitaxy at different temperatures. Along with the basic line corresponding to interband radiative recombination, additional emission lines are observed in the low-energy region of the spectra. Some nonuniformities are observed at the sample surface (within an area smaller than 1% of the total surface area). The concentration and size (1–10 μm) of the nonuniformities decrease with increasing temperature of growth. The additional emission lines are attributed to local nonuniformities in the layer. The dependence of the band gap of the Pb_1−x Eu _x Te (0 ≤ x ≤ 0.1) alloy on the composition parameter x is determined at 77.4 K. The dependence is nonlinear and adequately described by the relation E _g [eV] = 0.213 + 4.8x − 18.4x ².     

Passivation of infrared photodiodes with alcoholic sulfide solution

The effect of passivation with the solution of sodium sulfide (Na₂S) in isopropyl alcohol on the room-temperature performance of the GaInAsSb/GaAlAsSb and InAs/InAsSbP photodiodes is investigated. After such a treatment the dark current density of the GaInAsSb/GaAlAsSb photodiodes at a reverse bias of 0.1 V is reduced from 5.5 × 10⁻² to 2.1 × 10⁻³ A/cm² and a zero-bias resistance-area product (R ₀ A) is improved from 1.0 to 25.6 Ω cm². For the InAs/InAsSbP photodiodes, the dark current density at U = −0.1 V is decreased from 1.34 to 8.1 × 10⁻¹ A/cm², while the R ₀ A value increases from 4.4 × 10⁻² to 7.3 × 10⁻² Ω cm². The method offers long-term stability of the photodiode performance.     

Effect of Homogenization on the Indentation Creep of Cast Lead-Free Sn-5%Sb Solder Alloy

Creep behavior of cast lead-free Sn-5%Sb solder in unhomogenized and homogenized conditions was investigated by long time Vickers indentation testing under a constant load of 15 N and at temperatures in the range 321–405 K. Based on the steady-state power law creep relationship, the stress exponents were found for both conditions of the material. The creep behavior in the unhomogenized condition can be divided into two stress regimes, with a change from the low-stress regime to the high-stress regime occurring around 11.7 × 10⁻⁴ < (H _V /E) < 18 × 10⁻⁴. The low stress regime activation energy of 54.2 kJ mol⁻¹, which is close to 61.2 kJ mol⁻¹ for dislocation pipe diffusion in the Sn, and stress exponents in the range 5.0–3.5 suggest that the operative creep mechanism is dislocation viscous glide. This behavior is in contrast to the high stress regime in which the average values of n = 11.5 and Q = 112.1 kJ mol⁻¹ imply that dislocation creep is the dominant deformation mechanism. Homogenization of the cast material resulted in a rather coarse recrystallized microstructure with stress exponents in the range 12.5–5.7 and activation energy of 64.0 kJ mol⁻¹ over the whole ranges of temperature and stress studied, which are indicative of a dislocation creep mechanism.     

Electrophysical properties of solid solutions of silver in PbTe

The thermopower coefficient α₀ and the electrical conductivity σ of Pb_{1 − x} Ag _x Te solid solutions, where x = (0–0.007), are measured at T = 300 K. The hole concentration p is calculated. All samples are of the p type. With increasing silver content, α₀ decreases, while p and σ increase. For undoped crystals, α₀ = 251.0 μV/K, p = 1.1 × 10¹⁸ cm⁻³, and σ = 165 Ω⁻¹ cm⁻¹. At the silver-solubility limit for x = 0.007, α₀ = 193.8 μV/K, p = 2.3 × 10¹⁸ cm⁻³, and σ = 216 Ω⁻¹ cm⁻¹. The hole concentration in all samples is much lower than the concentration of introduced silver atoms. The hole gas in Pb_{1 − x} Ag _x Te solid solutions is weakly degenerate in the entire silver-concentration range.     

Epitaxial Growth and Device Design Optimization of Full-Vertical GaN <Emphasis Type="Italic">p</Emphasis>-<Emphasis Type="Italic">i</Emphasis>-<Emphasis Type="Italic">n</Emphasis> Rectifiers

The optimization of growth parameters, epitaxial structure, and device design for full-vertical gallium nitride (GaN) p-i-n rectifiers grown on n-type 6H-SiC substrates employing AlGaN:Si conducting buffer layers have been studied. The Al _x Ga_1−x N:Si (x = ~0.1) nucleation layer is calibrated to be capable of acting as a good buffer layer for subsequent GaN growth as well as to provide excellent electrical properties. Two types of full-vertical devices were fabricated and compared: one without any current guiding and the other with the current guiding in the p-layer. The reverse breakdown voltage for rectifiers with a relatively thin 2.5-μm-thick i-region without p-current guiding was found to be over −330 V, while one with p-current guiding was measured to be over −400 V. Devices with p-current guiding structures exhibit reduced reverse leakage current by an order of magnitude >4 at −100 V.     

Current flow mechanism in ohmic contact to <Emphasis Type="Italic">n</Emphasis>-4<Emphasis Type="Italic">H</Emphasis>-SiC

Current flow in an In-n-4H-SiC ohmic contact (n ≈ 3 × 10¹⁷ cm⁻³) has been studied by analyzing the temperature dependence of the per-unit-area contact resistance. It was found that the thermionic emission across an ∼0.1-eV barrier is the main current flow mechanism and the effective Richardson constant is ∼2 × 10⁻² A cm⁻² K⁻¹.     

Electronic properties and deep traps in electron-irradiated <Emphasis Type="Italic">n</Emphasis>-GaN

The study is concerned with the effect of electron irradiation (with the energies E = 7 and 10 MeV and doses D = 10¹⁶−10¹⁸ cm⁻²) and subsequent heat treatments in the temperature range 100–1000°C on the electrical properties and the spectrum of deep traps of undoped (concentration of electrons n = 1 × 10¹⁴−1 × 10¹⁶ cm⁻³), moderately Si-doped (n = (1.2−2) × 10¹⁷ cm⁻³), and heavily Si-doped (n = (2−3.5) × 10¹⁸ cm⁻³) epitaxial n-GaN layers grown on Al₂O₃ substrates by metal-organic chemical vapor deposition. It is found that, on electron irradiation, the resistivity of n-GaN increases, this is due to a shift of the Fermi level to the limiting position close to E _c −0.91 eV. The spectrum of deep traps is studied for the initial and electron-irradiated n-GaN. It is shown that the initial properties of the irradiated material are restored in the temperature range 100–1000°C, with the main stage of the annealing of radiation defects at about 400°C.     

Component Overpressure Growth and Characterization of High-Resistivity CdTe Crystals for Radiation Detectors

Spectrometer-grade CdTe single crystals with resistivities higher than 10⁹ Ω cm have been grown by the modified Bridgman method using zone-refined precursor materials (Cd and Te) under a Cd overpressure. The grown CdTe crystals had good charge-transport properties (μτ _e = 2 × 10⁻³ cm² V⁻¹, μτ _h = 8 × 10⁻⁵ cm² V⁻¹) and significantly reduced Te precipitates compared with crystals grown without Cd overpressure. The crystal growth conditions for the Bridgman system were optimized by computer modeling and simulation, using modified MASTRAPP program, and applied to crystal diameters of 14 mm (0.55′′), 38 mm (1.5′′), and 76 mm (3′′). Details of the CdTe crystal growth operation, structural, electrical, and optical characterization measurements, detector fabrication, and testing using ²⁴¹Am (60 keV) and ¹³⁷Cs (662 keV) sources are presented.     

The <Emphasis Type="Italic">α</Emphasis> − λ − <Emphasis Type="Italic">μ</Emphasis> and <Emphasis Type="Italic">α − η − μ</Emphasis> Small-Scale General Fading Distributions: A Unified Approach

In this paper, a general small-scale fading model for wireless communications, that explores the nonlinearity and at the same time the inhomogeneous nature of the propagation medium, is presented, studied in terms of its first-order statistics of the envelope, and validated by means of field measurements and the Monte Carlo simulation. It is indeed a novel distribution with many advantages such as its generality, its physical interpretation that is directly associated with the propagation channel, and its mathematical tractability due to its simple and closed-form expression. By fitting to measurement data, it has been shown that the proposed distribution outperforms the widely known fading distributions. Namely, the α − λ − μ model, which can be in fact called α − η − μ format 2 model, can also be obtained from the α − η − μ format 1 model by a rotation of the axes. Both formats are combined, in order to result to a unified model in a closed form that may describe the propagation environment in a variety of different fading conditions. Its physical background is hidden behind the names of its parameters. The unified model includes the already known general distributions α − μ′, η − μ, λ − μ (η − μ format 2), and their inclusive ones as special cases.     

Photovoltaic detector based on type II heterostructure with deep AlSb/InAsSb/AlSb quantum well in the active region for the midinfrared spectral range

Photodetectors for the spectral range 2–4 μm, based on an asymmetric type-II heterostructure p-InAs/AlSb/InAsSb/AlSb/(p, n)GaSb with a single deep quantum well (QW) or three deep QWs at the heterointerface, have been grown by metal-organic vapor phase epitaxy and analyzed. The transport, luminescent, photoelectric, current-voltage, and capacitance-voltage characteristics of these structures have been examined. A high-intensity positive and negative luminescence was observed in the spectral range 3–4 μm at high temperatures (300–400 K). The photosensitivity spectra were in the range 1.2–3.6 μm (T = 77 K). Large values of the quantum yield (η = 0.6−0.7), responsivity (S _λ = 0.9−1.4 A W^–1), and detectivity (D* _λ = 3.5 × 10¹¹ to 10¹⁰ cm Hz^1/2 W⁻¹) were obtained at T = 77–200 K. The small capacitance of the structures (C = 7.5 pF at V = −1 V and T = 300 K) enabled an estimate of the response time of the photodetector at τ = 75 ps, which corresponds to a bandwidth of about 6 GHz. Photodetectors of this kind are promising for heterodyne detection of the emission of quantum-cascade lasers and IR spectroscopy.     

Multifunctional ZnO-Based Thin-Film Bulk Acoustic Resonator for Biosensors

Zinc oxide (ZnO) and its ternary alloy magnesium zinc oxide (Mg _x Zn_1−x O) are piezoelectric materials that can be used for high-quality-factor bulk acoustic wave (BAW) resonators operating at GHz frequencies. Thin-film bulk acoustic resonators (TFBARs) are attractive for applications in advanced communication and in various sensors as they offer the capability of monolithic integration of BAW resonators with radio-frequency integrated circuits (RF ICs). In this paper we report Mg _x Zn_1−x O-based TFBAR biosensors. The devices are built on Si substrates with an acoustic mirror consisting of alternating quarter-wavelength silicon dioxide (SiO₂) and tungsten (W) layers to isolate the TFBAR from the Si substrate. High-quality ZnO and Mg _x Zn_1−x O thin films are achieved through a radio-frequency (RF) sputtering technique. Tuning of the device operating frequency is realized by varying the Mg composition in the piezoelectric Mg _x Zn_1−x O layer. Simulation results based on a transmission-line model of the TFBAR show close agreement with the experimental results. ZnO nanostructures are grown on the TFBAR’s top surface using metal- organic chemical vapor deposition (MOCVD) to form the nano-TFBAR sensor, which offers giant sensing area, faster response, and higher sensitivity over the planar sensor configuration. Mass sensitivity higher than 10³ Hz cm²/ng is achieved. In order to study the feasibility of the nano-TFBAR for biosensing, the nanostructured ZnO surfaces were functionalized to selectively immobilize␣DNA, as verified by hybridization with its fluorescence-tagged DNA complement.     

Thermoelectric Oxides: Effect of Doping in Delafossites and Zinc Oxide

The electrical resistivity (ρ) and Seebeck coefficient (S) of the three delafossites CuFe_0.9Cr_0.1O₂, CuCr_0.98Mg_0.02O₂, and CuRh_0.9Mg_0.1O₂ have been measured and their power factor (PF) calculated. These p-type oxides show PF values at 800 K from 1.4 × 10⁻⁴ W K⁻² m⁻¹ to 6.9 × 10⁻⁴ W K⁻² m⁻¹. In contrast to delafossites containing Fe or Cr, for which ρ exhibits a regime, the Rh-based delafossite shows a metallic regime from 5 K to 1000 K. This points toward the role of the transition-metal electronic configuration in the transport properties. Otherwise, similar PF values are obtained in the case of the n-type Al-doped ZnO. For these oxides, the ρ and PF values are minimum and maximum, respectively, for x = 0.01. However, the presence of spinel impurities even for x = 0.01 in Zn_1−x Al _x O or for x = 0.02 in CuCr_1−x Mg _x O₂ calls into question the role of the doping element in the physical properties. This should motivate a deeper insight into the physics of thermoelectric oxides.     

Dispersion of the refractive index of epitaxial Pb1 ? xEuxTe (0 ≤ x ≤ 1) alloy layers below the absorption edge

The transmittance spectra of epitaxial Pb_{1 − x} Eu _x Te (0 ≤x ≤ 0.1) alloy layers are exploited to study the dispersion of their refractive index in the spectral range from 650 to 8000 cm⁻¹ (below the absorption edge). The refractive index and the position of the absorption edge as functions of the composition parameter of the alloys are determined at two temperatures, 80 and 295 K. The refractive index is calculated in the context of the classic wave concepts of propagation of electromagnetic radiation. The experimentally determined dispersion dependences are described by the empiric Sellmeier expression of the second order. From analysis of the transmittance of the layers, it follows that the band gap of the epitaxial Pb_{1 − x} Eu _x Te alloys increases with increasing temperature at x < 0.5 and decreases at x > 0.5.     

Ultraviolet Photodetection Properties of a Pt Contact on a Mg<Subscript>0.1</Subscript>Zn<Subscript>0.9</Subscript>O/ZnO Composite Film

We report on the ultraviolet (UV) photodetection properties of a Pt contact on a sol-gel Mg_0.1Zn_0.9O/ZnO composite structure on a glass substrate. In the dark, the current–voltage (I–V) characteristics between the Pt and Ag contacts on the top of the ZnO film were linear while that on the Mg_0.1Zn_0.9O/ZnO composite film were rectifying, suggesting the formation of a Schottky diode on the latter. The ideality factor, n, and the reverse leakage current density, J _R , of the Schottky diode were greater than 2 and 2.36 × 10⁻² A cm⁻² at −5 V, respectively. Under ultraviolet light, the I–V characteristics become linear. The maximum photo-to-dark current ratio observed was about 63. The composite film showed good sensitivity to UV light with wavelengths of less than 400 nm, though the photoresponse process was found to be slow.     

AUTOCORRELATIONS OF l-SEQUENCES WITH CERTAIN SHIFTS

In this paper, the autocorrelations of maximal period Feedback with Carry Shift Register sequences （l-sequences） are discussed. For an l-sequence a with connection integer q = p^e（e ≥ 2） and period T = p^t-1（p- 1）, and for any integer i, 1 ≤ i ≤ e/2, by calculating the number of certain sets, it is shown that the autocorrelation of a with shift τ= kT/2p^i is Ca（τ） =（-1）^k-1 T/p^2i-1, where 1 ≤ k ≤ 2p^i - 1, and gcd（k,2p^i） = 1. This result shows there do exist some shifts such that the autocorrelations of l-sequences are high although most autocorrelations are low. Such result also holds for the decimations of l-sequences.     

Observation of indium-vacancy and indium-hydrogen interactions in Hg1−xCdxTe

We have used a nuclear hyperfine technique, perturbed γγ angular correlation (PAC), to study the interactions between¹¹¹In and native defects and impurities in Hg_1−xCd_xTe. The PAC technique uses the quadrupole interaction of¹¹¹In with local electric field gradients to characterize the local environment of this donor dopant. We observed that when In was diffused into a bulk or thin film sample of Hg_1−xCd_xTe (x=0.21 and x=0.3) at 350°C and the sample was slow cooled, the In occupied sites with near-cubic symmetry, presumably the substitutional metal site. However, when the sample was quenched, a fraction of the In was incorporated into defects characterized by quadrupole interaction strengthsv _Q1 andv _Q2 and asymmetries of ν₁=ν₂=0.08. These defects are attributed to the trapping of a metal vacancy at a next-nearest neighbor site to the In atom. The introduction of hydrogen by boiling the samples in distilled water for >4h eliminated the previously observed PAC signals and created defects characterized byv _Q3=35 MHz, ν₃ <0.1 andv _Q4=MHz, ν₄ <0.1. These defects are attributed to the decoration of the In-V_Hg complex by a hydrogen atom. Hall effect measurements showed that hydrogenation increased the hole concentration in p-type quenched samples and even converted n-type indium-doped samples to p-type. A possible model for hydrogen incorporation which includes self-compensation by vacancy creation is suggested.     

Charge carrier transport in Ge<Subscript>20</Subscript>As<Subscript>20</Subscript>S<Subscript>60</Subscript> chalcogenide semiconductor films

Charge-carrier transport in Ge₂₀As₂₀S₆₀ films has been studied using the transit time method under low-injection conditions at room temperature. It was found that drift mobilities of electrons and holes in Ge₂₀As₂₀S₆₀ films are close to each other, i.e., μ _e ≈ μ _h ≈ 2 × 10⁻³ cm² V⁻¹ s⁻¹ at T = 295 K and F = 5 × 10⁴ V/cm. It was shown that the time dependence of the photocurrent during carrier drift and the voltage dependence of the drift mobility allowed the use of the concept of anomalous dispersive transport. Experimental data were explained using the model of transport controlled by carrier trapping by localized states with energy distribution near conduction and valence band edges described by the exponential law with a characteristic energy of ∼0.05 eV.     

Absorptivity of semiconductors used in the production of solar cell panels

The dependence of the absorptivity of semiconductors on the thickness of the absorbing layer is studied for crystalline silicon (c-Si), amorphous silicon (a-Si), cadmium telluride (CdTe), copper indium diselenide (CuInSe₂, CIS), and copper gallium diselenide (CuGaSe₂, CGS). The calculations are performed with consideration for the spectral distribution of AM1.5 standard solar radiation and the absorption coefficients of the materials. It is shown that, in the region of wavelengths λ = λ _g = hc/E _g , almost total absorption of the photons in AM1.5 solar radiation is attained in c-Si at the thickness d = 7−8 mm, in a-Si at d = 30–60 μm, in CdTe at d = 20−30 μm, and in CIS and CGS at d = 3−4 μm. The results differ from previously reported data for these materials (especially for c-Si). In previous publications, the thickness needed for the semiconductor to absorb solar radiation completely was identified with the effective light penetration depth at a certain wavelength in the region of fundamental absorption for the semiconductor.     

Optical absorption in PbGa2Se4 single crystals

Optical measurements are performed in a PbGa2Se4 single crystal. The nature of the optical transitions is determined in the interval of photon energies 2.24–2.46 eV in the temperature range 77–300 K. It is shown that indirect and direct optical transitions take place in the energy intervals 2.28–2.35 eV and 2.35–2.46 eV, corresponding to E _gi =2.228 eV and E _gd =2.35 eV, respectively, at 300 K. The temperature coefficients of E _gi and E _gd are equal to −0.6×10⁻⁴ eV/K and −4.75×10⁻⁴ eV/K, respectively. Fiz. Tekh. Poluprovodn. 33, 39–41 (January 1999)     

Conductivity of layers of a chalcogenide glassy semiconductor Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript> 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 ∝ U ⁿ , 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/F ₀) (where F ₀ = 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.