Evaluation of InP:Fe parameters by measurement of two wave mixing photorefractive and absorptive gain

In this paper, we present two-wave mixing absorption gain measurements in InP:Fe in the 960–1035 nm wavelength range. The measured absorption gain is shown to be positive for long wavelength but changes sign for shorter wavelength. By simultaneously measuring the photorefractive gain and the absorption gain, we deduce the values of the photo-ionization cross sections related to the iron deep level trap. Finally, the study of the temperature dependence of the absorption gain allows us to evaluate a temperature shift of the iron level with respect to the conduction band of −4×10⁻⁴ eV/K.     

Optical properties of crystals of the solid solutions (InSb)1−x (CdTe)x

The reflection and absorption spectra of crystals of the solid solutions (InSb)_1−x (CdTe)_x in the wavelength interval 2.5–25 μm were measured within the limits of solubility of CdTe in InSb (x⩽0.05) at room temperature. Analysis of the experimental results confirmed the applicability of the Kane theory for all compositions investigated. The variation of the optical band gap ɛ _g ^opt and the effective mass m _c at the Fermi level as a function of composition was determined. It is shown that the minimum values m _c=0.8×10⁻² m ₀ and ɛ _g ^opt =0.07 eV are reached for x=0.02–0.03. Information about the predominant mechanism of scattering for each alloy is obtained from the absorption curves in the region of absorption by free charge carriers. X-Ray crystallographic investigations were performed and the change Δa(x) in the lattice constant of the solid solutions relative to pure InSb was determined. It is shown that the behavior of m _c(x) and ɛ _g ^opt is uniquely determined by Δa(x). In turn, Δa(x) is determined by the complicated character of the interaction of the dopants with one another and with the InSb lattice. Fiz. Tekh. Poluprovodn. 32, 303–306 (March 1998)     

Optical properties of photochromatic sulfur-doped chlorosodalite

Synthetic.photochromic sulfo-chlorosodalite, 6(NaAlSiO₄) ·2 NaCl(S), has been thoroughly investigated by measurements of optical absorption, photo-luminescence and cathodoluminescence. Depending on the sulfur ion form and concentration, the doped sodalite exhibits either sensitive tene-brescence or photoluminescence with long wavelength UV excitation. The photo-induced color absorption peaks at 5260A at 300°K with absorption coefficient, Δα_max >200 cm⁻¹ . This is by far the highest photo-induced absorption observed for synthetic chlorosodalite. At 80°K, the peak position of the absorption does not show significant shift within instrumental accuracy. In photoluminescence, the emission spectra as well as the excitation spectra are studied at both 300 and 78°K. Four characteristic spectral bands (IR, blue, red, and a band with oscillation in wavelength) are observed. The oscillatory S₂ ^- ion emission band starting about 2.35 eV and extending to lower energy and the IR band peaked at 1.4 eV are most efficiently excited by 3660A (3.4 eV), whereas the blue luminescence peaked at 2.7 eV has an excitation threshold of 3.9 eV. The red band is often masked by the oscillatory band and can be observed by higher energy excitation. The red and blue bands are also observable in the cathodoluminescence measurements of the sulfur-doped samples but not the undoped samples. Correlating the absorption, luminescence, and excitation spectral results, a quantitative model is derived to interpret the nature and the role of sulfur ions in the photochromic chlorosodalite material.     

Band Offsets in the Mg0.5Ca0.5O/GaN Heterostructure System

MgCaO offers promise as a gate dielectric for GaN-based metal-oxide-semiconductor (MOS) transistors, particularly in combination with environmentally stable capping layers such as Sc₂O₃. X-ray photoelectron spectroscopy (XPS) was used to measure the energy discontinuity in the valence band (ΔE _v ) of Mg_0.5Ca_0.5O/GaN heterostructures in which the MgCaO was grown by rf plasma-assisted molecular beam epitaxy on top of thick GaN templates on sapphire substrates. A value of ΔE _v = 0.65 eV ± 0.10 eV was obtained by using the Ga 3d energy level as a reference. Given the bandgap of 7.45 eV for the MgCaO, we infer a conduction band offset ΔE _C of 3.36 eV in the Mg_0.5Ca_0.5O system.     

Molecular-beam epitaxial growth of HgCdTe infrared focal-plane arrays on silicon substrates for midwave infrared applications

Molecular beam epitaxy has been employed to deposit HgCdTe infrared detector structures on Si(112) substrates with performance at 125K that is equivalent to detectors grown on conventional CdZnTe substrates. The detector structures are grown on Si via CdTe(112)B buffer layers, whose structural properties include x-ray rocking curve full width at half maximum of 63 arc-sec and near-surface etch pit density of 3–5 × 10⁵ cm⁻² for 9 μm thick CdTe films. HgCdTe p⁺-on-n device structures were grown by molecular beam epitaxy (MBE) on both bulk CdZnTe and Si with 125K cutoff wavelengths ranging from 3.5 to 5 μm. External quantum efficiencies of 70%, limited only by reflection loss at the uncoated Si-vacuum interface, were achieved for detectors on Si. The current-voltage (I-V) characteristics of MBE-grown detectors on CdZnTe and Si were found to be equivalent, with reverse breakdown voltages well in excess of 700 mV. The temperature dependences of the I-V characteristics of MBE-grown diodes on CdZnTe and Si were found to be essentially identical and in agreement with a diffusion-limited current model for temperatures down to 110K. The performance of MBE-grown diodes on Si is also equivalent to that of typical liquid phase epitaxy-grown devices on CdZnTe with R₀A products in the 10⁶–10⁷ Θ-cm² range for 3.6 μm cutoff at 125K and R₀A products in the 10⁴–10⁵ Θ-cm² range for 4.7 μm cutoff at 125K.     

Gain and internal losses in InGaAsSb/InAsSbP double-heterostructure lasers

We report on a study characterizing internal losses and the gain in InGaAsSb/InAsSbP diode-heterostructure lasers emitting in the mid-infrared (3–4 μm). Numerical simulations of the current dependence of the intensity of spontaneous emission above the laser threshold and of the differential quantum efficiency allowed us to determine the intraband absorption k ₀ ≈5.6×10⁻¹⁶ cm². The cavity-length dependence of the threshold current is used to estimate the internal losses at zero injection current α ₀≈5 cm⁻¹. Calculations of the internal losses at laser threshold showed that they increase more than fourfold when the cavity length is decreased from 500 μm to 100 μm. The temperature dependence of the differential quantum efficiency is explained on the assumption that intraband absorption with hole transitions into a split-off band occurs. It is shown that the maximum operating temperature of “short-cavity” lasers is determined by the intraband absorption rather than by Auger recombination. The internal losses are shown to have a linear current dependence. The separation of the quasi-Fermi levels as a function of current demonstrates an absence of voltage saturation of the p-n junction above threshold. Fiz. Tekh. Poluprovodn. 33, 759–763 (June 1999)     

Hopping conduction in heavily doped bulk n-type SiC

The electronic properties of heavily doped n-type 4H, 6H, and 15R SiC have been studied with temperature dependent Hall effect, resistivity measurements, and thermal admittance spectroscopy experiments. Hopping conduction was observed in the resistivity experiments for samples with electron concentrations of 10¹⁷ cm⁻³ or higher. Both band and hopping conduction were observed in all three polytypes in resistivity and Hall effect experiments. The hopping conduction activation energy ε₃ obtained from the resistivity measurements varied from 0.003 to 0.013 eV. The ε₃ value obtained from thermal admittance spectroscopy measurements were slightly lower. The nitrogen ionization levels were observed by thermal admittance spectroscopy only in those samples where hopping conduction was not detected by this experiment. Free carrier activation energy E_a for nitrogen was difficult to determine from temperature dependent Hall effect measurements because of the effects of hopping conduction. A new feature in the apparent carrier concentration vs inverse temperature data in the hopping regime was observed.     

Optical properties of CdSxTe1−x polycrystalline thin films

Thin films of CdS_xTe_1−x (0≤x≤ 1) have been prepared by vacuum evaporation from solid solutions. Rutherford backscattering spectrometry has been used to determine the thickness of the films, which is in the range 8–50 nm, and x-ray diffraction analysis has been used to determine the phase. The refractive index and extinction coefficient of the films has been calculated from reflectance and transmittance measurements for the wavelength region 250–3200 nm. Polynomial functions are given for each sample, which describe the variation in refractive index and extinction coefficient over the entire wavelength range. Least squares fitting to the absorption spectra revealed that the films all have a direct band gap, although photon energies required for indirect transitions have also been found. CdS_0.8Te_0.2 is found to have the lowest absorption coefficient at energies greater than 2.1 eV.     

MBE P-on-n Hg1−xCdxTe heterostructure detectors on silicon substrates

The capability of growing state-of-the-art middle wavelength infrared (MWIR)-HgCdTe layers by molecular beam epitaxy (MBE) on large area silicon substrates has been demonstrated. We have obtained excellent compositional uniformity with standard deviation of 0.001 with mean composition of 0.321 across 1.5″ radii. R₀A as high as 5 × 10⁷ ω-cm² with a mean value of 7 × 10⁶ Θ-cm² was measured for cut-off wavelength of 4.8 μm at 77K. Devices exhibit diffusion limited performance for temperatures above 95K. Quantum efficiencies up to 63% were observed (with no anti-reflection coating) for cut-off wavelength (4.8–5.4) μm @ 77K. Excellent performance of the fabricated photodiodes on MBE HgCdTe/CdTe/Si reflects on the overall quality of the grown material in the MWIR region.     

Capacitance-voltage profiling and thermal evolution of the conduction band-offset of unstrained Ga0.47In0.53As/InP single quantum well

We report the results of capacitance-voltage (C-V) and Deep Level Transient Spectroscopy (DLTS) measurements performed upon a Ga_0.47In_0.53As/InP quantum well structure. At room temperature, a conduction-band offset ΔE_c=(200±10)meV and charge densities σ_I=±(3±1)*10¹¹ times the electronic charge per cm² have been measured from C-V experiments. At lower temperature (T≤150K) we have observed an important decrease of the band-offset, considerably larger than a pure thermal effect. We have shown that the explanation lies in the presence of a high concentration of deep traps located at the well-barrier interfaces. Two species A and B have been detected through DLTS experiments with activation energies E_tA=90 meV and E_tB=195 meV, respectively. The filling of these trap levels at low temperature lowers the band offset from 200 to 120 meV, owing to band repulsion effects.     

Influence of native defects on the infrared transmission of undoped Ga<Subscript>1−x</Subscript>In<Subscript>x</Subscript>Sb bulk crystals

The below bandgap infrared transmission (up to 25 μm) in undoped Ga_1−xIn_xSb bulk crystals has been studied for the first time and found to be limited by native defects such as antisites and vacancies found in antimonide-based III–V compounds. For the gallium-rich alloy compositions (x<0.5 in Ga_1−xIn_xSb), the crystals exhibit p-type conductivity with an increase in net acceptor concentration and an increase in gallium content in the crystals. For x>0.5 (the indium-rich alloy compositions), the crystals exhibit n-type conductivity when the net donor concentration and indium content in the crystals increase. A correlation between the optical transmission and the residual carrier concentration arising from the native acceptors and donors has been observed. Due to donor-acceptor compensation, crystals with alloy compositions in the range of x=0.5−0.7 exhibit high optical transmission for a wide wavelength range (up to 22 μm). The light hole to heavy hole interband transition in the valence band and the free electron absorption in the conduction band have been found to be the two dominant absorption processes.     

Photoelectrical memory in GaAs/AlGaAs multilayer quantum-well structures

An increase in the dark current (by 2–3 orders of magnitude) in GaAs/Al_xGa_1−x As multilayer quantum-well structures with x⋍0.4 is observed after illumination of the structures with optical light (λ<1.3 μm). This increase is sustained for an extended time (more than 10³ s) at low temperatures. It then decreases to its initial value upon heating of the sample. A model of the barrier with local sag of the conduction band facilitating tunneling is proposed. The conduction band sag and the magnitude of the current grow due to optical ionization of uncontrolled deep level clusters present in the barrier and decrease due to subsequent capture of electrons from the conduction band by the deep levels upon heating. Fiz. Tekh. Poluprovodn. 32, 209–214 (February 1998)     

Effect of composition on deep levels in heteroepitaxial GexSi1−x layers and evidence for dominant intrinsic recombination-Generation in relaxed ge layers on Si

Electron traps, hole traps, and the dominant recombination-generation (R-G) centers have been investigated with deep level transient spectroscopy and current-voltage/temperature measurements in heteroepitaxial Ge_xSi_1-x alloys with x ranging from 0.15 to 1, grown on graded Ge_y.Si_1−y/Si substrates. For all samples with compositions x < 0.85, which retain the Si-like conduction band structure, we detect a dominant electron trap and R-G center whose activation energy is ΔE = 0.5 eV, independent of composition. This energy agrees with that of electron traps previously reported for plastically deformed (PD) Si, suggesting a connection to the Si-like band structure. This 0.5 eV level dominates the reverse leakage current over a wide range of growth and annealing conditions for the 30% Ge samples, indicating that the electronic state at ΔE = 0.5 eV is a very efficient R-G center, as would be expected from its midgap position. Alternatively, for strain relaxed, pure Ge (< 1), we detect electron traps at E_c − 0.42 eV and E_c − 0.28 eV, in agreement with the literature on PD Ge and Ge bicrystals. These energies are significantly different from those observed for x < 0.85, and we conclude that these changes in activation energy are due to changes in the conduction band structure for high Ge content. Moreover, in contrast with the Si-like samples (x < 0.85), the reverse leakage current in the relaxed Ge cap layer is not controlled by deep levels, but is rather dictated by intrinsic, band-to-band generation due to the reduced bandgap of Ge as compared to Si-like alloys. Only for reverse bias magnitudes which incorporate a significant portion of the graded buffer within the depletion region do R-G centers dominate the reverse leakage current. These results confirm the high quality of the strain-relaxed, pure Ge cap region which was grown on a Ge_ySi_1−y/Si step graded heterostructure (where y was increased from 0 to 1) by ultra high vacuum chemical vapor deposition. Finally, we report for the first time, what is apparently the dislocation kink site state at E_c − 0.37 eV, in a Ge_xSi_1−x alloy.     

Photoluminescence of erbium-doped aluminum oxide films with embedded silicon nanoparticles

Erbium-doped aluminum oxide films with embedded Si nanoparticles have been obtained by magnetron puttering of a composite (Al + Er₂O₃ + Si) target and subsequent electrochemical anodization at room temperature. The photoluminescence (PL) spectra of these films are measured in the temperature range 4.2–300 K. Efficient PL is observed at a wavelength of 1.54 μm without preliminary annealing of the samples, which indicates the possibility of activating Er³⁺ ions without any high-temperature treatment. The aluminum oxide films with embedded Si nanoparticles were observed to show stronger PL at a wavelength of 1.54 μm than similar films without Si nanoparticles. This effect can be explained by additional pumping of Er-based luminescence centers and energy transfer from the Si nanoparticles.     

An optical alternative to the hall test

Effective mass ratios, m*, of electrons in n-type InSb, GaAs, and near intrinsic and n-type Hg_1−xCd_xTe for 0.20 < × < 0.30 over the temperature range 77K < T < 296K were measured using Faraday rotation spectroscopy. m* ranged from 0.0186 to 0.0357 for InSb with carrier concentrations, N, in the range 1.76 < N < 110 × 10¹⁶ cm⁻³ at 296K, in good agreement with available values in the literature. Effective masses of HgCdTe were found to be about twice as large at room temperature as band edge effective mass, m*_be calculations. These calculations can be corrected for thermal excitation by adding a factor, m**, to the band edge calculation: m* = m** m*_be, where m** was found empirically to be m** = 4.52 × 10₋₃T + 0.78. The electron’s mobility is proportional to the ratio of the electron’s Faraday rotation to its absorption; that is, the absorption due to the intraband transitions of the electron itself, not the sample’s total absorption, which may include holes, interband transitions, and the like. The constant of proportionality, or the “mobility constant”, was measured in n-type GaAs and InSb doped above 18 × 10¹⁶ cm⁻³ using absorption directly. Both HgCdTe and InSb have large intrinsic carrier concentrations, on the order of 10¹⁶ cm⁻³. Hole absorption is the majority component of the sample’s absorption at lower n-type dopant concentrations. In these cases, the mobility constant was determined using an absorption cross section.     

Large VLWIR Hg1−xCdxTe photovoltaic detectors

Very long wavelength infrared (VLWIR; 15 to 17 μm) detectors are required for remote sensing sounding applications. Infrared sounders provide temperature, pressure and moisture profiles of the atmosphere used in weather prediction models that track storms, predict levels of precipitation etc. Traditionally, photoconductive VLWIR (λ_c >15 μm) detectors have been used for sounding applications. However, photoconductive detectors suffer from performance issues, such as non-linearity that is 10X – 100X that of photovoltaic detectors. Radiometric calibration for remote sensing interferometry requires detectors with low non-linearity. Photoconductive detectors also suffer from non-uniform spatial optical response. Advances in molecular beam epitaxy (MBE) growth of mercury cadmium telluride (HgCdTe) and detector architectures have resulted in high performance detectors fabricated in the 15 μm to 17 μmm spectral range. Recently, VLWIR (λ_c ∼ 17 μm at 78 K) photovoltaic large (1000 μm diameter) detectors have been fabricated and measured at flux values targeting remote sensing interferometry applications. The operating temperature is near 78 K, permitting the use of passive radiators in spacecraft to cool the detectors. Detector non-AR coated quantum efficiency >60% was measured in these large detectors. A linear response was measured, while varying the spot size incident on the 1000 μm detectors. This excellent response uniformity, measured as a function of spot size, implies that low frequency spatial response variations are absent. The 1000 μm diameter, λ_c ∼ 17 μm at 78 K detectors have dark currents ∼160 μA at a −100 mV bias and at 78 K. Interfacing with the low (comparable to the contact and series resistance) junction impedance detectors is not feasible. Therefore a custom pre-amplifier was designed to interface with the large VLWIR detectors operating in reverse bias. A breadboard was fabricated incorporating the custom designed preamplifier interfacing with the 1000 μm diameter VLWIR detectors. Response versus flux measurements were made on the large VLWIR detectors and non-linearity <0.15% was measured at high flux values in the 2.5×10¹⁷ to 3.5×10¹⁷ ph-cm⁻²sec⁻¹ range. This non-linearity is an order of magnitude better than for photoconductive detectors.     

Comparison of MOS capacitors on n- and p-type GaN

The electrical characteristics of both n- and p-type GaN metal-oxide semiconductor (MOS) capacitors utilizing plasma-enhanced CVD-SiO₂ as the gate dielectric were measured. Both capacitance and conductance techniques were used to obtain the MOS properties (such as interface state density). Devices annealed at 1000°C/30 min. in N₂ yielded an interface state density of 3.8×10¹⁰ cm⁻² eV⁻¹ at 0.19 eV from the conduction band edge, and it decreased to 1.1×10¹⁰ cm⁻² eV⁻¹ deeper into the band gap. A total fixed oxide charge density of 8×10¹² q cm⁻² near the valence band was estimated. Unlike the symmetric interface state density distribution in Si, an asymmetric interface state density distribution with lower density near the conduction band and higher density near the valence band was determined.     

The Role of oxygen diffusion in photoinduced changes of the electronic and optical properties in amorphous indium oxide

A stable increase by as much as 10⁸ in the conductivity of amorphous indium oxide to σ≥ 10³Ω^-1 cm₋₁ can be achieved by ultraviolet photoreduction. This treatment also increases the absorption coefficient, α(hυ), by up to a factor of 10₃ for hυ <1.5 eV due to free carrier absorption and causes a 0.1 eV shift of the absorption edge to the blue. These changes are controlled by the Fermi level, EF, which is presumably determined by doping due to oxygen vacancies. A diffusion constant D >3 x 10⁻¹² cm²/s for oxygen at 300K is determined from a constant flow experiment. Oxygen diffusion is verified by secondary ion mass spectrometry with ¹⁸O. The functions α(hυ) and σ(T) are simulated as E_F is varied using a simple density of states model appropriate for amorphous semiconductors. These simulations qualitatively agree with the experimental data if transitions from the conduction band tail to the conduction band are assumed to be forbidden.     

Absorption and photoluminescence spectroscopy of diffusion-doped ZnSe:Cr2+

ZnSe:Cr²⁺ is an attractive candidate as a room-temperature tunable solid-state laser with output in the 2–3 μm range. Passive absorption losses in this emission range currently limit laser performance. In this study, we use absorption and photoluminescence spectroscopies at 5 and 296K to address the origin of these optical losses. A series of diffusion-doped ZnSe:Cr single-crystal samples with Cr²⁺ concentrations in the range from 2×10¹⁷ cm⁻³ to 9×10¹⁹ cm⁻³ were obtained using CrSe powder as the dopant source. We find that trace amounts of Fe²⁺ produce absorption in the 2–3 μm range. Also, we have obtained data on a 680 nm absorption band observed in ZnSe:Cr which has been assigned to an internal transition of Cr²⁺. In our series of samples, the relative intensities of the 680 nm absorption band do not track the relative intensities of the 1.8 μm band (known to be due to Cr²⁺), although excitation near 680 nm does produce weak Cr²⁺ luminescence. Our absorption data do not support the current assignment of the 680 nm absorption as being an internal transition of the Cr²⁺ ion.     

Accurate determination of the band-offset of a quantum semiconductor structure from its capacitance-voltage profile: Application to an InP/Ga0.47In0.53As/InP single quantum well

We discuss the possible analysis of an electron distribution obtained by capacitance-voltage profiling for the determination of the conduction band offset of a single quantum well. We show that, for this method which requires only relatively light experimental equipment, a nonconsuming computational time interpretation can be set up within a quite satisfactory degree of accuracy. As an application, we report the study of a lattice matched InP/Ga_0.47In_0.53As/InP quantum well for which we get ΔE_c = (200±10) meV, in good agreement with other measurements upon this system.