Low-frequency noise in gallium nitride epitaxial layers with different degrees of order of mosaic structure

The correlation between the noise level 1/f and the degree of mosaic-structure order in gallium nitride epitaxial layers was studied for the first time. Samples with a doping level of N _d ?N _a ≈8×10¹⁶ cm^?3 and a relatively high degree of order were characterized by the Hooge parameter α≈1.5×10^?3. This value is unprecedently low for thin GaN epitaxial films. The Hooge parameter was significantly higher for samples with N _d ?N _a ≈1.1×10¹⁸ cm^?3 and a low degree of order despite the fact that α generally decreases with increasing doping level at the same degree of order. Thus, the degree of mosaic-structure order affects not only the optical and electrical characteristics but also the fluctuation parameters of GaN epitaxial layers.     

Electrical properties and luminescence spectra of light-emitting diodes based on InGaN/GaN heterostructures with modulation-doped quantum wells

The distribution of charged centers N(w), quantum efficiency, and electroluminescence spectra of blue and green light-emitting diodes (LED) based on InGaN/AlGaN/GaN p-n heterostructures were investigated. Multiple InGaN/GaN quantum wells (QW) were modulation-doped with Si donors in GaN barriers. Acceptor and donor concentrations near the p-n junction were determined by the heterodyne method of dynamic capacitance to be about N _A ≥ 1 × 10¹⁹ cm^?3 ? N _D ≥ 1 × 10¹⁸ cm^?3. The N(w) functions exhibited maxima and minima with a period of 11–18 (±2–3 nm) nm. The energy diagram of the structures has been constructed. The shifts of spectral peaks with variation of current (J=10^?6–3×10^?2 A) are smaller (13–12 meV for blue and 20–50 meV for green LEDs) than the corresponding values for the diodes with undoped barriers (up to 150 meV). This effect is due to the screening of piezoelectric fields in QWs by electrons. The dependence of quantum efficiency on current correlates with the charge distribution and specific features in the current-voltage characteristics.     

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.     

A 4<Emphasis Type="Italic">H</Emphasis>-SiC<Emphasis Type="Italic">p-i-n</Emphasis> diode fabricated by a combination of sublimation epitaxy and CVD

The possibility of fabricating heavily doped (N _a ?N _d ≥ 1 × 10¹⁹ cm_?3) p⁺-4H-SiC layers on CVD-grown lightly doped n-4H-SiC layers by sublimation epitaxy has been demonstrated. It is shown that a Au/Pd/Ti/Pd contact, which combines a low specific contact resistance (～2 × 10^?5 Ω cm²) with high thermal stability (up to 700°C), is the optimal contact to p-4H-SiC. The p-n structures obtained are used to fabricate packaged diodes with a breakdown voltage of up to 1400 V.     

High-temperature nuclear-detector arrays based on 4 <Emphasis Type="Italic">H</Emphasis>-SiC ion-implantation-doped <Emphasis Type="Italic">p</Emphasis><Superscript>+</Superscript>-<Emphasis Type="Italic">n</Emphasis> junctions

Results obtained in a study of spectrometric characteristics of arrays of four detectors based on 4H-SiC ion-implantation-doped p ⁺-n junctions in the temperature range 25–140 °C are reported for the first time. The junctions were fabricated by ion implantation of aluminum into epitaxial 4H-SiC layers of thickness ≤45 μm, grown by chemical vapor deposition with uncompensated donor concentration N _d ? N _a = (4–6) × 10¹⁴ cm^?3. The structural features of the ion-implantation-doped p ⁺-layers were studied by secondary-ion mass spectrometry, transmission electron microscopy, and Rutherford backscattering spectroscopy in the channeling mode. Parameters of the diode arrays were determined by testing in air with natural-decay alpha particles with an energy of 3.76 MeV. The previously obtained data for similar single detectors were experimentally confirmed: the basic characteristics of the detector arrays, the charge collection efficiency and energy resolution, are improved as the working temperature increases.     

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.     

Electron mobility in the inversion layers of fully depleted SOI films

The dependences of the electron mobility μ_eff in the inversion layers of fully depleted double–gate silicon-on-insulator (SOI) metal–oxide–semiconductor (MOS) transistors on the density N _e of induced charge carriers and temperature T are investigated at different states of the SOI film (inversion–accumulation) from the side of one of the gates. It is shown that at a high density of induced charge carriers of N _e > 6 × 10¹² cm^–2 the μeff(T) dependences allow the components of mobility μ_eff that are related to scattering at surface phonons and from the film/insulator surface roughness to be distinguished. The μ_eff(N _e ) dependences can be approximated by the power functions μ_eff(N _e) ∝ N _e ^?n . The exponents n in the dependences and the dominant mechanisms of scattering of electrons induced near the interface between the SOI film and buried oxide are determined for different N _e ranges and film states from the surface side.     

Interaction of infrared radiation with free carriers in mesoporous silicon

Features of absorption and reflection of infrared radiation in the range 500–6000 cm^?1 are investigated; these features are associated with free carriers in the layers of mesoporous Si (porosity, 60–70%) formed in single-crystal p-Si(100) wafers with a hole concentration of N _p ≈10²⁰ cm^?3. It is found that the contribution of free holes to the optical parameters of the samples decreases as the porosity of the material increases and further falls when the samples are naturally oxidized in air. The experimental results are explained in the context of a model based on the Bruggeman effective medium approximation and the Drude classical theory with a correction for additional carrier scattering in silicon residues (nanocrystals). A comparison between the calculated and experimental dependences yields a hole concentration in nanocrystals of N _p ≈10¹⁹ cm^?3 for as-prepared layers and shows a reduction of N _p when they are naturally oxidized.     

Silicon light-emitting diodes with strong near-band-edge luminescence

Electroluminescence (EL) in the region of interband transitions from silicon light-emitting diodes (LEDs) fabricated by cutting a solar cell with an area of 21 cm² and external quantum efficiency η_ext of EL up to 0.85% has been studied at room temperature. Despite the considerable decrease in η_ext because of the cutting and Auger recombination, record-breaking values of the total power emitted by a diode (up to W = 8 mW) and emitted power per unit area (up to P ₀ = 65 mW/cm²) were achieved at pulse currents of up to 10 A and structure areas in the range S = 0.1–0.9 cm². The EL decay kinetics was measured for LEDs with different areas. The emission pattern of a Si LED with a textured surface and the emission intensity distribution along different directions in the plane of the emitting area of the LED were measured.     

Electronic Processes in CdIn<Subscript>2</Subscript>Te<Subscript>4</Subscript> Crystals

The results of investigations of electrical, optical, and photoelectric properties of CdIn₂Te⁴ crystals, which were grown by the Bridgman method are presented. It is shown that electrical conductivity is determined mainly by electrons with the effective mass m_n = 0.44m₀ and the mobility 120–140 cm²/(V s), which weakly depends on temperature. CdIn₂Te₄ behaves as a partially compensated semiconductor with the donor-center ionization energy E_d = 0.38 eV and the compensation level K = N_a/N_d = 0.36. The absorption-coefficient spectra at the energy hν < E_g = 1.27 eV are subject to the Urbach rule with a typical energy of 18–25 meV. The photoconductivity depends on the sample thickness. The diffusion length, the charge-carrier lifetime, and the surface-recombination rate are determined from the photoconductivity spectra.     

Photoluminescence of electron-hole plasma in semi-insulating undoped GaAs

The dependence of the photoluminescence spectrum of electron-hole plasma in semi-insulating undoped GaAs on the concentration of the background carbon impurity N_C(3×10¹⁵ cm^?3≤N_C≤4×10¹⁶ cm^?3) is studied at 77 K. It is established that the density of the electron-hole plasma, which is equal to n _e?h ≈1.1×10¹⁶ cm^?3 in crystals with the lowest impurity concentration at an excitation intensity of 6×10²² photons/(cm² s), decreases considerably as the value of N_C increases in the range mentioned above. A decrease in the density of the electron-hole plasma with increasing N_C is attributed to the effect of fluctuations in the carbon concentration N_C, which give rise to a nonuniform distribution of interacting charge carriers and to localization of holes in the tails of the density of states of the valence band.     

Anomalous electron spin splitting in InAs pumped by injection

The electroluminescence and photoluminescence of bulk n-InAs with a high concentration of donors (N _d ≈ 5 × 10¹⁶ cm^?3) is studied experimentally in magnetic field in the Faraday layout of the experiment. Under the conditions of electrical injection, the photon energy corresponding to the electroluminescence peak exceeds the energy band gap E _g. When a magnetic field is applied, the energy of the peak becomes lower than E _g, and the peak splits into two circular-polarized components. The splitting depends on the injection current. In moderate magnetic fields (about 2 T), the splitting can be much more profound than the calculated splitting corresponding to the well-known g factor of electrons in InAs. The effect is attributed to different degrees of magnetic freezing-out of electrons with different spin orientation. The maximum in the dependence of the degree of polarization of photoluminescence on the magnetic field and the behavior of the photoluminescence line width support the model suggested.     

Photosensitive properties of metal-containing polydisalicylidene azomethines

Photosensitive properties of new metal-containing polydisalicylidene azomethines were studied. It was shown that polymer properties are controlled by the nature of the metal atom (its electron affinity energy A _a and ionic radius r _i ) included in “nonclassical” polyconjugation. The photosensitivity S _0.1 of studied polymers is ～10⁵cm²/J, and the quantum yield of free-carrier photogeneration is η ≈ 0.10–0.15, which corresponds to the level of organometallic complexes that have found application in optoelectronics.     

Carrier Lifetimes in Lightly-Doped <Emphasis Type="Italic">p</Emphasis>-Type 4H-SiC Epitaxial Layers Enhanced by Post-growth Processes and Surface Passivation

We investigated limiting factors of carrier lifetimes and their enhancement by post-growth processes in lightly-doped p-type 4H-SiC epitaxial layers (N _A ～ 2 × 10¹⁴ cm^?3). We focused on bulk recombination, surface recombination, and interface recombination at the epilayer/substrate, respectively. The carrier lifetime of 2.8 μs in an as-grown epilayer was improved to 10 μs by the combination of V_C-elimination processes and hydrogen annealing. By employing surface passivation with deposited SiO₂ followed by POCl₃ annealing, a long carrier lifetime of 16 μs was obtained in an oxidized epilayer. By investigating carrier lifetimes in a self-standing p-type epilayer, it was revealed that the interface recombination at the epilayer/substrate was smaller than the surface recombination on a bare surface. We found that the V_C-elimination process, hydrogen annealing, and surface passivation are all important for improving carrier lifetimes in lightly-doped p-type epilayers.     

Electrical properties of Si:H/<Emphasis Type="Italic">p</Emphasis>-Si structures fabricated by hydrogen implantation

Hydrogenated silicon (Si:H) layers and Si:H/p-Si heterostructures were produced by multiple-energy (3–24 keV) high-dose (5×10¹⁶–3×10¹⁷ cm^?2) hydrogen implantation into p-Si wafers. After implantation, current transport across the structures is controlled by the Poole-Frenkel mechanism, with the energy of the dominating emission center equal to E _c ?0.89 eV. The maximum photosensitivity is observed for structures implanted with 3.2×10¹⁷ cm^?2 of hydrogen and annealed in the temperature range of 250–300°C. The band gap of the Si:H layer E _g ≈2.4 eV, and the dielectric constant ?≈3.2. The density of states near the Fermi level is (1–2)×10¹⁷ cm^?3 eV^?1.     

Effective electron mass in a Mn<Subscript>x</Subscript>Hg<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Te system

The reflection spectra of n-Mn_xHg_1?xTe single crystals and epitaxial layers were measured at 300 K. The effective electron mass was determined for the samples with x=0.06–0.10 and an electron concentration N>6×10¹⁶ cm^?3. The calculated values of effective electron mass are close to experimental values.     

Specific features of recombination processes in CdTe films produced in different temperature conditions of growth and subsequent annealing

The steady-state and kinetic characteristics of photoconductivity and photoluminescence and the thermally stimulated conductivity spectra of the GdTe layers deposited by vacuum evaporation onto heated substrates are studied in relation to the substrate temperature. The measurements are carried out at temperatures, illuminations, and wavelengths ranging from 4.2 to 400 K, from 10¹⁰ to 10²³ photon/cm², and from0.4 to 2.5 μm, respectively. A certain optimal range of substrate temperatures T _s ≈ 450–550°C, at which the as-prepared layers exhibit a high resistivity, a high photosensitivity, and the best structural quality, is established. In the spectra of these layers, a new luminescence band at hv _m = 1.09 eV is observed along with the known photoluminescence band at hv _m = 1.42 eV. It is established that this new band is due to intracenter transitions rather than recombination transitions. The nature of radiative recombination centers in the layers is discussed. It is suggested that the d electrons of cations can be involved in the formation of chemical bonds of local centers in CdTe.     

Internal quantum efficiency of stimulated emission of (λ=1.55 µm) InGaAsP/InP laser diodes

The stimulated emission (η _i ^st ) of InGaAsP/InP separate-confinement double heterostructure lasers operating at λ=1.5–1.6 μm has been studied experimentally and theoretically. Laser heterostructures with a varied design of the waveguide layer were grown by MOCVD. The maximum internal quantum efficiency η _i ^st ≈97% was obtained in a structure with a double-step waveguide characterized by minimum leakage into the p-emitter above the generation threshold. The high value of η _i ^st is provided by low threshold and nonequilibrium carrier concentrations at the interface between the waveguide and p-emitter. The calculation yields η _i ^st values correlating well with the experimental data. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 37, No. 2, 2003, pp. 243–248. Original Russian Text Copyright ? 2003 by Skrynnikov, Zegrya, Pikhtin, Slipchenko, Shamakhov, Tarasov.     

Thermoelectric figure of merit in solid solutions with phonon scattering by off-center impurities

The Seebeck coefficient and the electrical and thermal conductivities (S, σ, and κ) of ternary PbTe_1?xSe_x (x=0.1 and 0.3) and quaternary PbTe_1?2xSe_xS_x (x=0.025, 0.05, 0.1, and 0.15) solid solutions have been studied. Polycrystalline samples with an electron density of (0.5–5.0)×10¹⁸ cm^?3 were used; their quality was monitored by comparing the measured and calculated mobility values at 85 K. A considerable decrease in mobility and an anomalous trend in the σ(T) curve near 77 K were revealed in quaternary alloys with x?0.1; for x=0.15, unusual behavior of κ(T) was also found. According to estimates, the lattice thermal conductivity of this material is temperature-independent in the 80-to 300-K temperature range. This means that a reduction in phonon-phonon scattering with an increase in temperature is completely compensated by an increase in the scattering on impurities. The observed anomalies of σ(T) and κ(T) are considered assuming the possible of off-center location of sulfur atoms at the lattice sites. The thermoelectric figure of merit Z of the studied alloys has been determined in the range 80–300 K. In spite of decreasing mobility, the maximum Z was obtained in a quaternary compound with x=0.1: at 300 K, Z_max=2×10^?3 K^?1 with a carrier density of ～3×10¹⁸ cm^?3; at 175 K, Z_max=1.5×10^?3 K^?1 with the density decreasing to 5×10¹⁷ cm^?3. The obtained data indicate that the introduction of off-center impurities rises Z at T?300 K.     

High-voltage (900 V) 4 <Emphasis Type="Italic">H</Emphasis>-SiC Schottky diodes with a boron-implanted guard <Emphasis Type="Italic">p-n</Emphasis> junction

High-voltage (900 V) 4H-SiC Schottky diodes terminated with a guard p-n junction were fabricated and studied. The guard p-n junction was formed by room-temperature boron implantation with subsequent high-temperature annealing. Due to transient enhanced boron diffusion during annealing, the depth of the guard p-n junction was equal to about 1.7 μm, which is larger by approximately 1 μm than the projected range of 11 B ions in 4H-SiC. The maximum reverse voltage of fabricated 4H-SiC Schottky diodes is found to be limited by avalanche breakdown of the planar p-n junction; the value of the breakdown voltage (910 V) is close to theoretical estimate in the case of the impurity concentration N = 2.5 × 10¹⁵ cm^?3 in the n-type layer, thickness of the n-type layer d = 12.5 μm, and depth of the p-n junction r _j = 1.7 μm. The on-state diode resistance (3.7 mΩ cm²) is controlled by the resistance of the epitaxial n-type layer. The recovery charge of about 1.3 nC is equal to the charge of majority charge carriers that are swept out of an epitaxial n-type layer under the effect of a reverse voltage.