Optoelectronic properties and Seebeck coefficient in SnSe thin films

SnSe thin films of thickness 180 nm have been deposited on glass substrates by reactive evaporation at an optimized substrate temperature of 523±5 K and pressure of 10^-5 mbar. The as-prepared SnSe thin films are characterized for their structural, optical and electrical properties by various experimental techniques. The p-type conductivity, near-optimum direct band gap, high absorption coefficient and good photosensitivity of the SnSe thin film indicate its suitability for photovoltaic applications. The optical constants, loss factor, quality factor and optical conductivity of the films are evaluated. The results of Hall and thermoelectric power measurements are correlated to determine the density of states, Fermi energy and effective mass of carriers and are obtained as 2.8×10¹⁷cm^-3, 0.03 eV and 0.05 m₀ respectively. The high Seebeck coefficient≈7863 μV/K, reasonably good power factor≈7.2×10^-4 W/(m·K²) and thermoelectric figure of merit≈1.2 observed at 42 K suggests that, on further work, the prepared SnSe thin films can also be considered as a possible candidate for cryogenic thermoelectric applications.     

Specific thermoelectric properties of lightly doped Bi<Subscript>2</Subscript>(TeSe)<Subscript>3</Subscript> solid solutions

Electrical and thermoelectric properties of a lightly doped n-Bi₂Te_2.7Se_0.3 solid solution have been studied in the temperature range 77–300 K. The results are compared with data for the compound PbTe_0.9Se_0.1 with a similar magnitude of the Seebeck coefficient S at 84 K. Along with lower thermal conductivity, Bi₂Te_2.7Se_0.3 has a higher electrical conductivity σ and a much weaker temperature dependence. As a result, the power coefficient S ²σ in optimal samples begins to decrease only when the density of minority carriers becomes significant. In this case, |S| considerably exceeds the standard value of 200 μV/K. The reduction of the electron density reduces the thermoelectric figure of merit Z at its maximum and slightly lowers the temperature of the maximum; therefore, the expected effect on the average value of Z in the range 77–300 K is absent. Similar behavior is observed in Bi₂Te_2.88Se_0.12, although the effect is less pronounced. The experimental results are discussed taking into account possible changes in the dominant scattering mechanisms, carrier density, and electron energy spectrum. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 38, No. 7, 2004, pp. 811–815. Original Russian Text Copyright ? 2004 by Konstantinov, Prokof’eva, Ravich, Fedorov, Kompaniets.     

Manifestation of percolation conductivity of short-channel field-effect transistors in the spectrum of shallow interface states

The effective density of shallow interface states N _ss is investigated in the temperature range T=77–300 K using the field-effect method in short-channel (0.5–5 μm) Si-MNOS and GaAs-based FET’s with high (greater than 10¹² cm⁻²) concentrations of built-in charge in the subgate insulator. A peculiarity of the density of electronic states N _ss was found having the form of a peak, which manifests itself more distinctly at lower temperatures, higher concentrations of built-in charge, and shorter gates. The peak was observed at the same values of the channel conductance G∼q ²/h, regardless of variations in the above-enumerated parameters, the thickness of the sub-gate insulator, and the channel-length-to-width ratio. This means that the energy depth of the peak (∼40–120 meV) varies in proportion to T, which contradicts the current understanding of the interface states caused by both the fluctuation potential (FP) and surface defects or traps. The results are interpreted within the framework of percolation theory applied to the conductivity of strongly disordered systems. The N _ss peculiarity is associated with a transition from the conductivity of a two-dimensional effective solid, which occurs when the fluctuation potential is strongly screened by surface electrons, to conductivity via a quasi-one-dimensional potential trough organized by local regions with reduced surface potential under conditions of a strong fluctuation potential. Fiz. Tekh. Poluprovodn. 31, 1460–1467 (December 1997)     

Radiation effect on the optical and electrical properties of CdSe(In)/p-Si heterojunction photovoltaic solar cells

The efficiency and radiation resistance of solar cells are graded. They are then fabricated in the form of n-CdeSe(In)/p-Si heterojunction cells by electron beam evaporation of a stoichiomteric mixture of CdSe and In to make a thin film on a p-Si single crystal wafer with a thickness of 100 μm and a resistivity of ～ 1.5 Ω·cm at a temperature of 473 K. The short-circuit current density (j_sc), open-circuit voltage (V_oc), fill factor (ff) and conversion efficiency (η) under 100 mW/cm² (AM1) intensity, are 20 mA/cm², 0.49 V, 0.71 and 6% respectively. The cells were exposed to different electron doses (electron beam accelerator of energy 1.5 MeV, and beam intensity 25 mA). The cell performance parameters are measured and discussed before and after gamma and electron beam irradiation.     

Photovoltaic effect in the impurity absorption region of Si-structures with blocked impurity conductivity

A study is made of the field dependence of the photoconductivity in two-layer Si:Sb-and Si:Bstructures with blocked impurity-band conductivity and different thicknesses of the undoped (blocking) layer. The impurity concentration in the doped (active) layer was ≈10¹⁸ cm⁻³. Measurements were made at temperatures T=4–15 K for high (Φ∼10¹⁶ photons/cm² · s) and low (Φ<10¹⁴ photons/cm² · s) incident photon fluxes. A photovoltaic effect is observed in the Si:B structures with a thin (3 μm) blocking layer. It is found that a photovoltage develops for photons with energies exceeding the ionization energy of boron and its magnitude is independent of the photoexcitation intensity (for Φ>10¹³ photons/cm² · s) and, in the limit of low temperatures, it is close to the activation energy ɛ ₃ for jump conductivity in the active layer. The photovoltaic effect is explained by ballistic transit of the blocking layer by holes emitted from the contact which are then cooled in the active layer, as well as by the presence of a potential barrier ≈ɛ ₃ between the active and blocking layers. These factors are taken into account in a model for describing the major features of the dependence of the photovoltage on temperature and on the photon intensity and energy. Fiz. Tekh. Poluprovodn. 33, 456–463 (April 1999)     

Properties of p +-n structures with a buried layer of radiation-induced defects

The p ⁺-n structures based on n-type Si with dopant density 1.7×10¹³–1.2×10¹⁴ cm⁻³ were irradiated with ²³⁸Pu α particles. A layer containing radiation-induced defects with a density of the order of 3×10¹³ cm⁻³ was produced at a depth of 20 μm. This defect density gave rise to intense draining of nonequilibrium carriers in the injection-extraction regime with stationary injection as well as with pulsed generation by single particles. This makes it possible to treat the damaged layer as a plane, introduced into the bulk, with an infinite surface recombination rate. The radiation-induced defects also participated in decreasing the conductivity. A characteristic space charge distribution and, correspondingly, a bias dependence of the capacitance are observed in the structure under reverse bias. Despite the presence of formally three charge regions, four sections appear on the capacitance curve. This latter effect is due to the “additional” charge step arising in the contact potential difference field and is characteristic of compensated deep levels in semiconductors. Fiz. Tekh. Poluprovodn. 32, 359–365 (March 1998)     

Effect of the charge state of defects on the light-induced kinetics of the photoconductivity of amorphous hydrated silicon

The photoconductivity and defect density in films of nondoped a-Si:H soaked with light (W=114 mW/cm², λ<0.9 μm) for 5 h were investigated. It is shown that σ_ph ~ t ^-γ and N _D ~ t ^β, where γ>β or γ⋍β, depending on the position of the Fermi level prior to light soaking, i.e., depending on the charge state of the defects: D ⁻ and D ⁰ or D ⁺ and D ⁰. It is also shown that the light-induced kinetics of σ _ph is affected by a transition of the defects into the D ⁰ state because of a corresponding shift of the Fermi level during light soaking. Fiz. Tekh. Poluprovodn. 32, 345–348 (March 1998)     

Staebler-Wronski effect as a function of the Fermi level position and structure of nondoped, amorphous, hydrated silicon

The photoconductivity degradation rates γ (σ _ph∼t ^−γ ) of nondoped, amorphous, hydrated silicon films deposited at T _s =300–400 °C and subjected to illumination for 5 h at 300 K (light source 100 mW/cm², λ<0.9 μm) were investigated. It was shown that the degradation rate γ depends on the preillumination position of the Fermi level ɛ _c −ɛ _F and often is not directly related to the hydrogen content in the film. It was found that there are correlations between the value of γ and the bonds in the silicon-hydrogen subsystem [isolated SiH and SiH₂ complexes, clusters (SiH)_n, and chains (SiH₂)_n]. Fiz. Tekh. Poluprovodn. 32, 484–489 (April 1998)     

Photoluminescence of cadmium telluride recrystallized by nanosecond pulsed laser irradiation

The influence of nanosecond pulsed laser irradiation on the morphology and low-temperature photoluminescence of n-type CdTe at energy densities sufficient to melt the material (0.2–0.5 J/cm²) is investigated. After recrystallization the material has an “orange-peel” appearance. The low-temperature photoluminescence spectrum corresponds to low-grade single-crystalline p-type CdTe containing a large number of dislocations and clusters of point defects. The laser treatment has a long-range effect, significantly altering the impurity-defect system in a way characteristic of n→p conductivity conversion at distances greater than 50 μm from the site where laser radiation is absorbed. Fiz. Tekh. Poluprovodn. 32, 32–35 (January 1998)     

Highly Transparent Low Resistance Ga doped ZnO/Cu grid double layers prepared at Room Temperature

Ga doped ZnO(GZO)/Cu grid double layer structures were prepared at room temperature(RT). We have studied the electrical and optical characteristics of the GZO/Cu grid double layer as a function of the Cu grid spacing distance. The optical transmittance and sheet resistance of the GZO/Cu grid double layer are higher than that of the GZO/Cu film double layer regardless of the Cu grid spacing distance and increase as the Cu grid spacing distance increases. The calculated values for the transmittance and sheet resistance of the GZO/Cu grid double layer well follow the trend of the experimentally observed transmittance and sheet resistance ones. For the GZO/Cu grid double layer with a Cu grid spacing distance of 1 mm, the highest figure of merit(Φ_TC=6.19×10^-3 Ω^-1) was obtained. In this case, the transmittance, resistivity and filling factor(FF) of the GZO/Cu grid double layer are 83.74%, 1.10×10^-4 Ω·cm and 0.173, respectively.     

Photosensitivity of In-SiO2-Cd0.28Hg0.72Te structures with a nontransparent field electrode

The photoeffect in a metal-insulator-semiconductor (MIS) structure that incorporated a Cd_0.28Hg_0.72Te compound, a low-temperature pyrolitic SiO₂, and an In layer with a thickness of 500 nm and an area of 0.5×0.5 mm² was studied. For a MIS structure with a nontransparent field electrode, the observed photoeffect consists in variation in the capacitance and high-frequency electrical conductivity of the MIS structure; this photoeffect is caused by photocarriers that are formed outside the MIS structure and reach this structure either due to diffusion or along the surface channel. This happens if the MIS structure is in the state of inversion; in this case, an eddy electric current formed crosses the induced p-n junction and closes on itself at the MIS structure periphery. It is assumed that this current and the extra voltage across the p-n junction are related by the Shockley formula. The following parameters were evaluated: the coefficient β in the Shockley formula (β characterizes the nonideality of the p-n junction); the product R ₀ A of the resistance for zero bias by the area of the p-n junction; and the surface area ΔS of collection of the charge carriers. It was found that β=1.52, R ₀ A=2.7×10⁵ Ω cm², and ΔS corresponds to a stripe that goes along the MIS-structure perimeter and has a width of 15 μm. The MIS structure studied is considered as a photodetector. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 34, No. 7, 2000, pp. 822–826. Original Russian Text Copyright ? 2000 by Ovsyuk, Vasil’ev, Mashukov.     

The use of SiC triode structures as detectors of nuclear particles

The possibility of the internal amplification of the signal by a factor of approximately 100 for SiC-based detectors of short-range ions was demonstrated. The detectors were fabricated from the growth of p-epilayers on 6H-SiC n ⁺-substrates. The film thickness was approximately 10 μm, and the doping level was 2.8×10¹⁵ cm⁻³. The Schottky barriers were formed on the films by magnetron sputtering of Ni. The structure of the detectors was n-p-n ⁺, and their parameters were investigated in the floating-base mode. The α particles from ²⁴⁴Cm with an energy of 5.8 MeV were used, and the increase in the signal (E) with a rise in applied voltage (U) was investigated. The structures irradiated are equivalent to the phototriodes. The superlinear rise of E was observed with a considerable (by a factor of tens) amplification of the nonequilibrium charge introduced by the α particle. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 36, No. 3, 2002, pp. 375–378. Original Russian Text Copyright ? 2002 by Strokan, Ivanov, Savkina, Davydov, Bogdanova, Lebedev.     

Effect of successive implantation of Ag+(Cu+) and Xe+ ions on the recombination properties of CdxHg1−x Te crystals

The effect of successive double implantation of Ag⁺(Cu⁺) and Xe⁺ ions on the recombination properties of Cd_xHg_1−x Te (0.2<x<0.3) crystals has been investigated. It is shown that after implantation of ions of one chemical element, followed by diffusion thermal annealing at temperatures below 150–200 K, recombination through local levels lying 30±5 meV below the conduction band bottom dominates. Successive double implantation of Ag⁺(Cu⁺) and Xe⁺ ions followed by diffusion thermal annealing changes the course of the temperature dependence of the lifetime of the nonequilibrium charge carriers. It was determined that for Cd_xHg_1−x Te crystals with x⋍0.20–0.25 in the temperature interval 700–200 K the lifetime of the nonequilibrium charge carriers is low (τ<0.15 μs) and does not depend on the temperature. For Cd_xHg_1−x Te crystals with x⋍0.3 recombination of nonequilibrium charge carriers occurs through two types of levels: in the temperature range 140–200 K — deep levels E _t1⋍E _c −51 meV and at lower temperatures (77–140 K) — through shallower levels E _t2⋍E _c −(16±2) meV. Fiz. Tekh. Poluprovodn. 31, 786–789 (July 1997)     

Luminescence and electrical properties of InGaN/AlGaN/GaN light emitting diodes with multiple quantum wells

Luminescence spectra of light-emitting diodes based on InGaN/AlGaN/GaN heterostructures with multiple quantum wells are studied for currents in the range J=0.15 μA-150 mA. The comparatively high quantum efficiency for low J(J _max=0.5–1 mA) is a consequence of a low probability for the nonradiative tunnel current. The current-voltage characteristics J(V) are studied for J=10⁻¹²–10⁻¹ A; they are approximated by the function V=ϕk+mkT· [1n(J/J ₀)+(J/J ₁)^0.5] + J · R _s. The portion of V∞(J/J ₁)^0.5 and measurements of the dynamic capacitance indicate that i-layers adjacent to the active layer play an important role. The spectra are described by a model with a two-dimensional density of states with exponential tails in multiple quantum wells. The rise in T with increasing J is determined from the short-wavelength decay of the spectrum of the blue diodes: T=360–370 K for J=80–100 mA. An emission band is observed at 2.7–2.8 eV from green diodes at high J; this band may be explained by phase separation with different amounts of In in the InGaN. Fiz. Tekh. Poluprovodn. 33, 445–450 (April 1999)     

High-pressure preparation and thermoelectric properties of Bi0.85Sb0.15

Nanocrystalline Bi_0.85Sb_0.15 powders were prepared by a novel mechanical alloying method. The bulk samples were formed by applying a pressure of 6 GPa at different pressing temperatures and times. Electrical conductivity, Seebeck coefficients, and thermal conductivity were measured in the temperature range 80–300 K. The Seebeck coefficient reaches a maximum value of −173 μV/K at 150 K. The largest figure of merit, 3.46 × 10⁻³ K⁻¹, achieved in this experiment is 50% higher than that of its single-crystal counterpart at 200 K.     

Resistivity variation of semi-insulating Cd1−xZnx Te in relationship to alloy composition

We investigated the resistivity variation of semi-insulating Cd_1−xZn_xTe used as room temperature nuclear radiation detectors, in relationship to the alloy composition. The resistivity and the zinc composition were determined using leakage current measurements and triple axis x-ray diffraction lattice parameter measurements, respectively. While the zinc content of the nominally x_Zn∼0.1 ingot varied monotonically according to the normal freezing behavior with an effective segregation coefficient of k_eff=1.15, the resistivity was found to vary non-systematically throughout the ingot. Furthermore, the “expected” relationship of higher zinc content with higher resistivity was not always observed. For example, wafer regions of x_Zn∼0.12 and x_Zn∼0.08 exhibited resistivity values of ∼10¹⁰ and ∼10¹¹ Ω·cm, respectively. In general, the experimental resistivity values can be explained by calculated values which take into account a compensating deep level defect and various electron and hole mobility values. The relative influence of the parameters that govern the resistivity (n,p, μ_e, and μ_h) are quantitatively investigated.     

Electrical conduction in polyimide between 20 and 350° C

Although conduction in polyimides at elevated temperatures has been widely reported, measurements at ordinary device temperatures have been less well documented. Quantitatively reproducible low field conduction measurements on two device-grade polyimides (PMDA-ODA, BTDA-ODA/MPDA) in the temperature range of 20–350° C and under dry conditions are reported. Aluminum—polyimide—aluminum capacitors are prepared by spin coating an aluminized silicon wafer with between two and four coats of polyimide (prebake at 135° C for 10 min between coats). Samples are cured in dry nitrogen at 400° C for 45 min. Final thickness ranged between 3.3 and 6.6 μm. To permit rapid equilibration of moisture between the film and ambient, the upper electrode is patterned into multiple 25 μm stripes with 5 μ spaces for a total area of 5.1 cm². After a bake-out at 120° C under dry air and subsequent equilibration in a dry ambient at the test temperature, a voltage step is applied to the sample and the current versus time is recorded for 16,000 sec (the charging current). The sample is then shorted, and the discharging current is recorded. Below 100° C, both charging and discharging currents are dominated by a reversible polarization that follows a power law (approximately t^−0.8). Isochronal plots of the polarization current reveal a linear dependence on the applied voltage for fields in the range 10⁴–10⁵ V/cm. The polarization current is nearly independent of temperature and is well modeled by the Lewis molecular dipole theory of polarization. Above 150° C, the current is increasingly dominated by a relatively constant transport current, defined as the difference between charging and discharging currents. This current is ohmic over the field range examined, and shows a complex, activated temperature dependence. For PMDA-ODA the transport current has an activation energy (E _a ) of 0.5 eV below 175° C and 1.5 eV above that temperature. For BTDA-ODA/MPDA the E_a is 0.6 up to 250° C and 2.1 eV above. This corresponds to a resistivity of 9 × 10¹⁸ Ω-@#@ cm at 23° C and 3.5 × 10¹⁴ Ω-cm at 200° C for PMDA-ODA and 5 × 10¹⁹ Ω-cm at 23° C and 5.6 × 10¹³ Ω-cm at 300° C for BTDA-ODA/MPDA. This work demonstrates that the low temperature behavior of polyimide cannot be extrapolated from high temperature measurements. Work sponsored in part by E. I. DuPont de Nemours & Co., Inc.     

Current-voltage characteristics of Si:B blocked impurity-band structures under conditions of hopping-transport-limited photoresponse

The photoconductivity of Si:B blocked-impurity-band (BIB) structures with boron concentration in the active layer ∼10¹⁸ cm⁻³ has been studied. Measurements were performed in the temperature range 4.2–10 K at different intensities of the exciting radiation 10¹⁰–10¹⁵ photons/cm²·s. Photoexcitation at 5.5 μm was realized using a semiconductor laser. At temperatures below 6 K and low bias voltages (<0.5 V) the current-voltage characteristics were found to have a threshold-like character. The threshold voltage rises as the temperature is lowered and the radiation intensity is increased. A model based on the Frenkel’-Poole effect in the impurity band has been developed. This model can be used to numerically describe the current-voltage characteristics with accuracy better than 5%. As a result, it is found that the photoconductivity rises and then reaches a plateau as the radiation intensity increases. Under these conditions, as under equilibrium conditions (in darkness), the hopping conductivity also depends exponentially on the electric field. This fact is explained in terms of the destruction by the electric field of (A ⁺-A ⁻) impurity complexes which appear under nonequilibrium conditions. Fiz. Tekh. Poluprovodn. 32, 192–199 (February 1998)     

Transparent flexible plastic substrates for organic light-emitting devices

In this paper, we describe the properties of flexible plastic substrates with a transparent conducting electrode (TCE), which are important for organic light-emitting devices (OLEDs). Specifically, we have evaluated the TCE electrical resistivity, surface roughness, electrode patterning, optical transmission, and the substrate water vapor/oxygen transmission. We have studied the effect of ultraviolet (UV)-ozone treatment on the TCE surface by using contact angle measurements and x-ray photoelectron spectroscopy (XPS). A decrease in the advancing contact angle by 30–40° and an increase of oxygen content on the TCE surface by 10 at.% were observed after the UV-ozone treatment. These changes facilitate the polymer adhesion to the TCE surface and increase the TCE surface work function, respectively. A sheet resistance of 12–13 Ω/□, an optical transmission greater than 80% over the visible range, and a surface roughness of 1.4–2.2-nm RMS over 50×50 μm² have been obtained for the plastic substrates. These properties are adequate for OLED applications based on United States Display Consortium specifications. Finally, we have found that a combination of hydrogenated amorphous silicon-nitride and silicon-oxide layers deposited on one side of the substrate at low-temperature reduces the water vapor and oxygen transmission rates (TRs) to less than 10⁻⁵ g/cm²-day-atm and about 10⁻⁷ cc/cm²-day-atm, respectively.     

A low noise interface circuit design of micro-machined gyroscope

The analyses of MEMS gyroscope interface circuit on thermal noise, 1/f noise and phase noise are made in this paper. A closed-loop differential driving circuit and a low-noise differential detecting circuit based on the high frequency modulation are designed to limit the noise. The interface chip is implemented in a standard 0.5 μ m CMOS process. The test results show that the resolution of sensitive capacity can reach to 6.47 × 10^-20 F at the bandwidth of 60 Hz. The measuring range is ± 200°/s and the nonlinearity is 310 ppm. The output noise density is 5.8°/(h·√Hz). The angular random walk (allen-variance) is 0.092°/√h and the bias instability is 2.63°/h.