Specific features of photoconductivity in thin n-PbTe:Ga epilayers

Photoconductivity and transient processes in thin (0.2–0.3 µm) n-PbTe:Ga epilayers were studied. The films were grown by the hot-wall technique on BaF₂ 〈111〉 substrates. Photoelectric properties of the samples were investigated in the temperature range from 4.2 to 300 K. A GaAs light-emitting diode and miniature incandescent lamp were used as sources of pulsed and continuous infrared radiation, respectively. The most important characteristic of the films is the very wide temperature range of photosensitivity. At an illumination power density of 10^?5 to 10^?4 W/cm², the temperature at which the films become photosensitive T _C is as high as 150 K, which exceeds T _C for thicker films (2–3 µm) and high-resistivity single crystals of n-PbTe:Ga by 40 and 70 K, respectively. An analysis of the transient behavior of photoconductivity shows that photoexcited carriers are uniformly distributed over the volume of the thin films. A barrier for recombination of nonequilibrium charge carriers was estimated for slow relaxation processes.     

Mobility of charge carriers in double-layer PbTe/PbS structures

The method of hot-wall epitaxy is used to grow epitaxial heterostructures of p-PbTe/n-PbS on BaF₂ substrates. Hall-effect measurements are analyzed in order to obtain the dependence of the effective carrier mobility on thickness and temperature in the ranges 0.1–2 μm and 100–300 K, respectively. It is found that this mobility depends on the thickness of the sample and on the individual thicknesses of its constituent layers. The effective mobility is calculated under the assumption that charge carriers are scattered by the surface of the structure and by dislocations that form at the heterojunction boundary. Fiz. Tekh. Poluprovodn. 32, 1064–1068 (September 1998)     

Electrical characterization of current conduction in Au/TiO2/n-Si at wide temperature range

In this study, we have examined Au/TiO₂/n-Si Schottky barrier diodes (SBDs), in order to interpret in detail the experimental observed non-ideal current–voltage–temperature (I–V–T) characteristics. I–V characteristics were measured in the wide temperature range of 80–400 K. TiO₂ was deposited on n-Si substrate by reactive magnetron sputtering. The zero-bias barrier height (ϕ_B0) and ideality factor (n) show strong temperature dependence. While n decreases, ϕ_B0 increases with increasing temperature. Experimental results show that the current across the SBDs may be greatly influenced by the existence of Schottky barrier height (SBH) inhomogeneity. These temperature behaviors have been explained on the basis of the thermionic emission (TE) theory with Gaussian distribution (GD) of the barrier heights (BHs) due to BH inhomogeneities at metal–semiconductor (M/S) interface. From this assumptions, obtaining Richardson constant value of the A^* 121.01 A/cm² K² is perfect agreement with the theoretical value of 120 A/cm² K² for n-type Si. Hence, behaviors of the forward-bias I–V characteristics of the Au/TiO₂/n-Si (SBDs) can be successfully explained on the basis of a TE mechanism with a double Gaussian distribution of the BHs.     

Analysis of characteristics of photodetectors based on InGaAs heteroepitaxial structures for 3D imaging

The 320 × 256 focal plane arrays based on р ⁺ -B–n-N ⁺ tetralayer heterostructures with a wide-gap barrier layer have been investigated. The heterostructures with a narrow-gap n-InGaAs absorbing layer were grown by means of metalorganic vapor phase epitaxy on InP substrates. The band discontinuity between the In_0.53Ga_0.47As absorbing layer and the In_0.52Al_0.48As barrier layer is removed by growing a thin four-component n-AlInGaAs layer with the bandgap gradient variation. Delta-doped layers included into the heterostructures make it possible to lower the barrier in the valence band and eliminate the nonmonotonicity of energy levels. The experimental study of the dark current has been performed. It has been revealed that the average value of the dark current does not exceed 10 fA for the photodiode arrays with a pitch of 30 μm.     

Properties of barrier contacts with nanosize TiB x layers to InP

Structural and electrical properties of Au-TiB _x -nn ⁺ n ⁺⁺-InP and TiB _x -nn ⁺ n ⁺⁺-InP multilayer barrier structures on standard (“rigid”) and soft (“porous”)n ⁺⁺-InP substrates have been studied, with the semiconductor layers deposited by vapor-phase epitaxy, metallic layers formed by magnetron sputtering, and porous substrates fabricated by electrochemical etching of the standard InP. Samples on porous substrates have the following advantages: leakage currents in their reverse current-voltage characteristics are ten times lower; the range of the exponential rise in current in the forward characteristics is an order of magnitude wider; the changes in the ideality factor and the Schottky barrier height, observed as the contact area varies by a factor of 100, are three and ～10 times smaller, respectively; and the structure of the layers is more stable in annealing at up to 800°C.     

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.     

Influence of Annealing on Electrical Properties of an Organic Thin Layer-Based n-Type InP Schottky Barrier Diode

The electrical properties of a fabricated Au/polymethylmethacrylate (PMMA)/n-InP Schottky barrier diode have been analyzed for different annealing temperatures using current–voltage (I–V) and capacitance–voltage (C–V) techniques. It is observed that the Au/PMMA/n-InP structure shows excellent rectifying behavior. The extracted barrier height and ideality factor of the as-deposited Au/PMMA/n-InP Schottky contact are 0.68 eV (J–V)/0.82 eV (C–V) and 1.57, respectively. However, the barrier height (BH) of the Au/PMMA/n-InP Schottky contact increases to 0.78 eV (J–V)/0.99 eV (C–V) when the contact is annealed at 150°C for 1 min in nitrogen atmosphere. Upon annealing at 200°C, the BH value decreases to 0.72 eV (J–V)/0.90 eV (C–V) and the ideality factor increases to 1.48. The PMMA layer increases the effective barrier height of the structure by creating a physical barrier between the Au metal and the n-InP. Cheung’s functions are also used to calculate the series resistance of the Au/PMMA/n-InP structure. The interface state density (N _ss) is found to be 6.380 × 10¹² cm^?2 eV^?1 and 1.916 × 10¹² cm^?2 eV^?1 for the as-deposited and 150°C-annealed Au/PMMA/n-InP Schottky contacts, respectively. These results indicate that the interface state density and series resistance have a significant effect on the electrical characteristics of Au/PMMA/n-InP Schottky barrier devices. Finally, it is noted that the diode parameters change with increasing annealing temperature.     

Characterization of n-Type and p-Type ZnS Thin Layers Grown by an Electrochemical Method

Electrodeposition of n-type and p-type thin-film layers of ZnS was carried out using a simple two-electrode system and aqueous solutions of ZnCl₂ and (NH₄)₂S₂O₃ with different Zn²⁺ concentrations. X-ray diffraction measurements show that the ZnS layers deposited from both solutions are amorphous. Optical absorption measurements show low absorbance of the layers with energy bandgap in the range of 3.68 eV to 3.78 eV after postdeposition annealing. Photoelectrochemical cell measurements show that both n-type and p-type ZnS thin layers can be electrodeposited by simply changing the concentrations of the deposition solutions. With higher Zn²⁺ concentration in the bath, n-type ZnS films were deposited, while p-type ZnS films were deposited with lower Zn²⁺ concentration. The estimated resistivity of layers from both solutions using I–V measurements were 3.0 × 10⁴ Ω cm and 2.0 × 10⁴ Ω cm, respectively, for n-ZnS and p-ZnS. Scanning electron microscopy shows that the deposited films consist of particles with good surface coverage of the glass/fluorine-doped tin oxide substrate.     

Hafnium-ntype silicon Schottky barriers

This paper describes the electrical properties of hafnium-/n-type/silicon contacts. These contacts were found to be Schottky barriers with a low barrier height. Polished and chemically cleaned 〈111〉 silicon wafers with a donor concentration N_d = 7 × 10²² m^?3 were used to fabricate experimental Schottky barrier structures. For the Schottky barrier height φ_bn and the ideality factor n values were found of 0.47 V and 1.07–1.11, respectively. It is concluded that due to their low forward voltage drop and good rectifying properties, Hf-nSi contacts can be applied in microwave Schottky barrier diodes.     

Growth characteristics and physical properties of PbTe/BaF2 prepared under nonequilibrium conditions

PbTe/BaF₂ films were grown under nonequilibrium conditions by laser-modulated epitaxy. The structural properties of the layers were investigated by x-ray crystallographic methods and scanning tunneling microscopy. It was established that the films obtained under nonequilibrium conditions on (III)BaF₂ substrates are granular (d?250 Å) with (001) orientation. The electrophysical and photoelectric properties of the films depend on the technological conditions of growth and are determined by states at intergrain boundaries.     

Radiation effects and interphase interactions in ohmic and barrier contacts to indium phosphide as induced by rapid thermal annealing and irradiation with γ-ray 60Co photons

The radiation resistance of Au-Pd-Ti-Pd-n ⁺⁺-InP ohmic contacts and Au-TiB _x -n-n ⁺-n ⁺⁺-InP barrier contacts—both initial and subjected to a rapid thermal annealing and irradiated with ⁶⁰Co γ-ray photons with doses as high as 10⁹ R—has been studied. Before and after external effects, the electrical characteristics of the barrier and ohmic contacts, distribution profiles for components, and phase composition in the metallization layers have been measured. In ohmic Pd-Ti-Pd-Au contacts subjected to rapid thermal annealing and irradiation, a significant distortion of the layered structure of metallization occurs; this distortion is caused by the thermal and irradiation-stimulated transport of Pd over the grain boundaries in polycrystalline Ti and Au films. However, the specific contact resistance ρ _c does not change appreciably, which is related to a comparatively unvaried composition of the contact-forming layer at the Pd-n ⁺-InP interface. In the initial sample and the sample subjected to the rapid thermal annealing at T = 400°C with the Au-TiB _x -n-n ⁺-n ⁺⁺-InP barrier contacts and irradiated with the dose as high as 2 × 10⁸ R, a layered structure of metallization is retained. After irradiation with the dose as high as 10⁹ R, in the samples subjected to a rapid thermal annealing at T = 400°C, the layered structure of metallization becomes completely distorted; however, this structure is retained in the initial sample. The electrical properties of the contact structure appreciably degrade only after irradiation of the sample preliminarily subjected to a rapid thermal annealing at T = 400°C.     

Specifics of MOCVD formation of InxGa1−x N inclusions in a GaN matrix

MOCVD-grown heterostructures with one or several In_xGa_1?x N layers in a GaN matrix have been studied by transmission electron microscopy. In heterostructures with thick InGaN layers, a noncoherent system of domains with lateral dimensions (～50 nm) on the order of the layer thickness (～40 nm) is formed. In the case of ultrathin InGaN inclusions, nanodomains coherent with the GaN matrix are formed. The content of indium in nanodomains, determined by the DALI method, is as high as x≈0.6 or more, substantially exceeding the average In concentration. The density of the nanodomains formed in the structures studied is n≈(2–5)×10¹¹ cm^?2. In the structures with ultrathin InGaN inclusions, two characteristic nanodomain sizes are observed (3–6 and 8–15 nm).     

Enhanced barrier height of AuIn1−xGaxAsyP1−y Schottky diodes

Barrier heights of Au Schottky diodes made on In_1?xGa_xAs_yP_1?y epitaxial layers with interfacial films have been measured. The interfacial layer was either a thin (?25 Å) natural oxide film or an oxide film grown by HNO₃ oxidation for which the thickness was much greater (300–500 Å). Room temperature forward-biased current-voltage and capacitance-voltage measurements show that enhancement of the barrier heights ranging from 0.15 to 0.32 eV, depending on composition, is obtained for the natural oxide films. The enhancement is much less for the HNO₃ oxide layers. The best value of the ideality factor n = 1.4 and saturation current densities J_s ranging from 10^?4 to 10^?8 A cm^?2 are obtained. For the thin natural oxide interfacial films, the results can be explained reasonably with a thermionic emission model taking into account the interfacial oxide layer.     

CdHgTe heterostructures for new-generation IR photodetectors operating at elevated temperatures

The parameters of multilayer Cd_xHg_1–xTe heterostructures for photodetectors operating at wavelengths of up to 5 μm, grown by molecular-beam epitaxy (MBE) on silicon substrates, are studied. The passivating properties of thin CdTe layers on the surface of these structures are analyzed by measuring the C–V characteristics. The temperature dependences of the minority carrier lifetime in the photoabsorption layer after growth and thermal annealing are investigated. Samples of p ⁺–n-type photodiodes are fabricated by the implantation of arsenic ions into n-type layers, doped with In to a concentration of (1–5) × 10¹⁵ cm^–3. The temperature dependences of the reverse currents are measured at several bias voltages; these currents turn out to be almost two orders of magnitude lower than those for n ⁺–p-type diodes.     

The effects of surface states and series resistance on the performance of Au/SnO2/n-Si and Al/SnO2/p-Si (MIS) Schottky barrier diodes

We have fabricated two types of Schottky barrier(SBDs),Au/SnO₂/n-Si (MIS1) and Al/SnO₂/p-Si (MIS2), to investigate the surface (N_ss) and series resistance (R_s) effect on main electrical parameters such as zero-bias barrier height (Φ_Bo) and ideality factor (n) for these SBDs. The forward and reverse bias current–voltage (I–V) characteristics of them were measured at 200 and 295 K, and experimental results were compared with each other. At temperatures of 200 and 295 K, Φ_Bo, n, N_ss and R_s for MIS1 Schottky diodes (SDs) ranged from 0.393 to 0.585 eV, 5.70 to 4.75, 5.42×10¹³ to 4.27×10¹³ eV^?1 cm^?2 and 514 to 388 Ω, respectively, whereas for MIS2 they ranged from 0.377 to 0.556 eV, 3.58 to 2.1, 1.25×10¹⁴ to 3.30×10¹⁴ eV^?1 cm^?2 and 312 to 290 Ω, respectively. The values of n for two types of SBDs are rather than unity and this behavior has been attributed to the particular distribution of N_ss and interfacial insulator layer at the metal/semiconductor interface. In addition, the temperature dependence energy density distribution profiles of N_ss for both MIS1 and MIS2 SBDs were obtained from the forward bias I–V characteristics by taking into account the bias dependence of effective barrier height (Φ_e) and R_s. Experimental results show that both N_ss and R_s values should be taken into account in the forward bias I–V characteristics. It has been concluded that the p-type SBD (MIS2) shows a lower barrier height (BH), lower R_s, n and N_ss compared to n-type SBD (MIS1), which results in higher current at both 200 and 295 K.     

Technological Fabrication Features of Microwave Device with Schottky Barriers

At present, research and development of heterojunctions are conducted in the directions of searching for new compositions and technological regimes for the creation of ohmic and barrier transitions for gallium arsenide. The transition to silver-based metallization, which has large thermal and electrical conductivity comparing with gold and a relatively low diffusion coefficient to gallium arsenide, should improve the technical characteristics of the devices. One of the most important technological operations in the formation of Schottky ohmic contacts and barriers is thermal annealing. Silver to gallium arsenide contacts are made in vacuum by the method of thermal evaporation. The deposition and thermal treatment regimes for creating ohmic contacts of Ag–Ge–In/n–n⁺ GaAs with specific contact resistance ρ_c = (5...7)+10^–5 Ω.cm² are developed. The influence of the substrate temperature during the silver deposition and the annealing temperature on the height of the Schottky barrier Ag/n–n⁺ GaAs, the injection coefficient γ and the nonideality factor η is established.     

Effect of bismuth impurity on carrier density in PbSe:Bi:Se epitaxial layers

The dependence of the Hall carrier density on bismuth concentration, n, p=f(N _Bi), in PbSe:Bi:Se/BaF₂ films has been studied. The films were grown by vacuum condensation from two independent molecular beams (PbSe:Bi and Se₂) mixed directly at the surface of a (111)BaF₂ substrate heated to 350°C. The bismuth concentration in the stock was 0–0.3 at. %. Two specific portions can be distinguished in the experimental n, p=f(N _Bi) dependence. At N _Bi>0.0375 at. %, the electron density is close to N _Bi; at low bismuth concentrations, N _Bi<0.0375 at. %, the linear run of the n=f(N _Bi) dependence is violated, and the conduction changes to p-type. All the doped films under study are saturated with selenium. This is a necessary condition for obtaining the highest electron densities in the films at N _Bi corresponding to the linear portion of the n=f(N _Bi) dependence. The results are discussed in terms of a thermodynamic model of the impurity interaction with intrinsic defects in PbSe, taking into account the amphoteric behavior of bismuth atoms in lead selenide.     

A study of thick 3C-SiC epitaxial layers grown on 6H-SiC substrates by sublimation epitaxy in vacuum

3C-SiC epitaxial layers with a thickness of up to 100 μm were grown on 6H-SiC hexagonal substrates by sublimation epitaxy in vacuum. The n-type epitaxial layers with the area in the range 0.3–0.5 cm² and uncompensated donor concentration N _d − N _a ∼ (10¹⁷–10¹⁸) cm⁻³ were produced at maximum growth rates of up to 200 μm/h. An X-ray analysis demonstrated that the epitaxial layers are composed of the 3C-SiC polytype, without inclusions of other polytypes. The photoluminescence (PL) spectrum of the layers was found to be dominated by the donor-acceptor (Al-N) recombination band peaked at hv ≈ 2.12 eV. The PL spectrum measured at 6 K was analyzed in detail. It is concluded that the epitaxial layers obtained can serve as substrates for 3C-SiC-based electronic devices. Original Russian Text ? A.A. Lebedev, V.V. Zelenin, P.L. Abramov, E.V. Bogdanova, S.P. Lebedev, D.K. Nel’son, B.S. Razbirin, M.P. Shcheglov, A.S. Tregubova, M. Suvajarvi, R. Yakimova, 2007, published in Fizika i Tekhnika Poluprovodnikov, 2007, Vol. 41, No. 3, pp. 273–275.     

Auger electron spectroscopic analysis of barium fluoride surfaces exposed to selenium vapor

(111)-oriented BaF₂ substrates were exposed to selenium vapor in an atmospheric pressure hydrogen ambient at three temperatures: 592, 629 and 696° C. Auger electron spectroscopy analysis of exposed surfaces showed formation of BaSe patches, BaSe layers, and BaSe₃/BaSe composite layers at these three temperatures, respectively. We speculate that BaSe reaction layers form epitaxially on BaF₂ since cations in both materials have the same sublattice structure and the materials have a relatively small, 6 percent, lattice parameter mismatch. Such structure-preserving substrate surface reactions can occur during IV-VI semiconductor epitaxy on BaF₂ and can play an important role in early stages of semiconductor growth.     

Dielectrical,conduction mechanism and thermal properties of rhodanine azodyes

In this paper, we report on the differential scanning calorimetry analysis (DSC) and thermogravimetric analysis (TGA) performed for 5-(4'-derivatives phenylazo)-2-thioxothiazolidin-4-one (HL_n) (n=1, R=OCH₃; n=2, R=CH₃; n=3, R=H; and n=4, R=NO₂) in the temperature range 46–800 °C. The values of the thermal activation energies of decomposition of HL₁, HL₃ and HL₄ are found in the range 59.10–299.72 kJ/mol. The molecular and electronic structures of the investigated compounds (HL_n) were also studied using quantum chemical calculations. The alternating current conductivity (σ_ac) and dielectrical properties of HL_n were investigated in the frequency range 0.1–100 kHz and temperature range 303–500 K. The temperature and frequency dependence of the real and the imaginary dielectrical constants are studied. The values of the thermal activation energy for derivatives under investigation were calculated at different frequencies. The values of thermal activation energies of electrical conductivity ΔE₁ and ΔE₂ for all ligands decrease with increasing the test frequency. The activation energies, ΔE₁ and ΔE₂, increase according to the following order p-(NO₂>H>CH₃>OCH₃). This is in accordance with that expected from Hammett's substituent coefficients (σ^R). The conductivities are found to be dependent on the structure of the compounds. The values of σ_ac are related to the frequency as σ_ac α ω^S where the behavior of the exponent S determines the operating conduction mechanism. The correlated barrier hopping (CBH) is the dominant conduction mechanism for HL_n. The values of maximum barrier height (W_m) were calculated.