I. A. Andreev

InAsSbP/InAs heterostructures for thermophotovoltaic converters: Growth technology and properties

A liquid phase epitaxy (LPE) technique for growing indium arsenide (InAs)based narrow-bandgap semiconductor compounds for thermophotovoltaic (TPV) applications has been developed. InAs-based multicomponent solid solutions and InAs/InAsSbP heterostructures with Eg = 0.35–0.6 eV are promising materials for TPV converters that operate at an emitter temperature of 1000–2000°C. The sensitivity of new TPV elements is extended toward longer wavelengths (up to 3.8 μm), which ensures effective conversion of low-energy photons. The epitaxial films of quaternary InAsSbP solid solutions were obtained by LPE from a supercooled solution melt and by the liquid phase electroepitaxy technique with controlled doping of a growth solution from a single liquid source of components. The films have homogeneous compositions and highly perfect crystal structures. The values of reverse saturation currents in n-InAs/p-InAsSbP heterostructures are close to theoretical predictions.

Thermophotovoltaic Converters on Indium Arsenide-Based Compounds

Thermophotovoltaic converters based on multicomponent solid solutions of III–V compounds, specifically, InAsSbP/InAs heterostructures (Eg = 0.35–0.60 eV), that are intended for fabricating IR emitters heated to 1000–2000°C are studied. The use of such narrow-gap heterostructures makes it possible to advance the sensitivity of the elements into the long-wave range and utilize the thermal energy of low-temperature sources more efficiently. Fresh physical approaches to fabricating epitaxial quaternary InAs-based InAsSbP solid solutions with a low carrier concentration and heterostructures with sharp interfaces are presented. Quaternary InAsSbP solid solutions and other related heterostructures offer a number of advantages, such as the possibility of growing perfect structures lattice-matched with the substrate, stress-free interfaces, good electrical and photoelectrical properties (low dark currents and a high external quantum efficiency), and the possibility of flexibly controlling the energy gap by varying the composition of the solid solution. It is shown that InAsSbP films grown on an InAs substrate by liquid-phase epitaxy from supercooled liquid solution and liquid-phase electro-epitaxy with replenishment of liquid solution by growing layer components are uniform in composition and have a perfect crystal structure. Thermophotovolatic p-InAsSbP/n-InAs diode-type heterostructures obtained by the above methods are found to have saturation dark currents close to theoretically predicted values and a wide range of spectral sensitivity, which makes them candidates for thermophotovoltaic elements.

Photodiodes for a 1.5–4.8 µm spectral range based on type-II GaSb/InGaAsSb heterostructures

Long-wavelength photodiodes based on LPE-grown type-II heterostructures in lattice-matched GaSb/InGaAsSb/GaInAsSb and GaSb/InGaAsSb/AlGaAsSb systems were fabricated and studied. Band energy diagram engineering for heterostructures with wide-and narrow-gap layers allows the photodiode parameters to be controlled by varying the conditions at heterointerfaces. Electrical and photoelectric characteristics and the dark current mechanisms in the heterostructures were investigated. The optimal photodiode structure was selected that consists of two type-II broken-gap heterojunctions and one p-n-junction in the narrow-gap active layer. Room-temperature detectivity Dλ* Hz1/2/W at λ=4.7 µm was obtained. Type-II heterostructures may help develop high-efficiency uncooled photodiodes for the 1.6–4.8 µm range for gas analysis, environmental monitoring, and also the diagnostics of combustion and explosion products.

Passivation of infrared photodiodes with alcoholic sulfide solution

The effect of passivation with the solution of sodium sulfide (Na2S) in isopropyl alcohol on the room-temperature performance of the GaInAsSb/GaAlAsSb and InAs/InAsSbP photodiodes is investigated. After such a treatment the dark current density of the GaInAsSb/GaAlAsSb photodiodes at a reverse bias of 0.1 V is reduced from 5.5 × 10−2 to 2.1 × 10−3 A/cm2 and a zero-bias resistance-area product (R0A) is improved from 1.0 to 25.6 Ω cm2. For the InAs/InAsSbP photodiodes, the dark current density at U = −0.1 V is decreased from 1.34 to 8.1 × 10−1 A/cm2, while the R0A value increases from 4.4 × 10−2 to 7.3 × 10−2 Ω cm2. The method offers long-term stability of the photodiode performance.

High-efficiency GaInAsSb/GaAlAsSb photodiodes for 0.9-to 2.55-µm spectral range with a large-diameter active area

The results of a study aimed at the fabrication of high-sensitivity photodiodes for the 0.9-to 2.55-µm spectral range with a photosensitive area diameter as large as 1–3 mm are presented. A large range of photodiodes based on GaSb/GaInAsSb/GaAlAsSb heterostructures with a long-wavelength edge of spectral sensitivity λ=2.4 and λ=2.55 µm have been developed. The special features of the photodiodes are their high monochromatic current sensitivity in the spectral peak, high operating speed, and low reverse dark current density. The detectivity of photodiodes estimated from the measured noise level and monochromatic current sensitivity in the spectral peak reaches D*(λmax, 1000, 1)=(0.8–1.0)×1011 Hz1/2 cm W−1.

Photodiodes based on InAs/InAs0.88Sb0.12/InAsSbP heterostructures for 2.5–4.9 μm spectral range

Photodiodes with a photosensitive area diameter of 0.3 mm operating at room temperature in a middle-IR (2.5–4.9 μm) wavelength range have been created based on InAs/InAs0.94Sb0.06/InAsSbP/InAs0.88Sb0.12/InAsSbP/InAs heterostructures grown by liquid phase epitaxy. Distinguishing features of the proposed photodiodes are a high monochromatic responsivity, which reaches a maximum of 0.6–0.8 A/W at λmax = 4.0–4.6 μm, and a low dark current density of (1.3–7.5) × 10−2 A/cm2 at a reverse bias of 0.2 V. The differential resistance at zero bias reaches up to 700–800 Ω. The detection ability of photodiodes in the spectral interval of maximum sensitivity reaches (5–8) × 108 cm Hz1/2 W−1.

Molecular beam epitaxy of thermodynamically metastable GaInAsSb alloys for medium IR-range photodetectors

The features of growth of GaInAsSb alloy with In content as high as 25 mol % lattice-matched to GaSb by molecular beam epitaxy are studied. These alloys are promising for use as the active region of photodetector structures of the mid-infrared range. The results of the study of these alloys by double-crystal X-ray diffractometry, scanning electron microscopy, and photoluminescence are presented. It is shown that the GaxIn1 − xAsySb1 − y alloys with x < 0.8 grown at 500°C show the degradation of structural and optical properties as the layer thickness is increased. In layers with thicknesses above the critical one, spinodal decomposition is observed in complete agreement with thermodynamic calculations of the location of the boundaries of immiscibility regions. The possibilities to optimize the molecular beam growth of the GaxIn1 − xAsySb1 − y alloys (x < 0.75) with high optical and structural quality, as well as the characteristics of photodetectors based on the GaInAsSb/AlGaAsSb heterostructures, are discussed.

Photovoltaic Detector Based on Type II Heterostructure with Deep AlSb/InAsSb/AlSb Quantum Well in the Active Region for the Midinfrared Spectral Range

Photodetectors for the spectral range 2–4 μm, based on an asymmetric type-II heterostructure p-InAs/AlSb/InAsSb/AlSb/(p, n)GaSb with a single deep quantum well (QW) or three deep QWs at the heterointerface, have been grown by metal-organic vapor phase epitaxy and analyzed. The transport, luminescent, photoelectric, current-voltage, and capacitance-voltage characteristics of these structures have been examined. A high-intensity positive and negative luminescence was observed in the spectral range 3–4 μm at high temperatures (300–400 K). The photosensitivity spectra were in the range 1.2–3.6 μm (T = 77 K). Large values of the quantum yield (η = 0.6−0.7), responsivity (Sλ = 0.9−1.4 A W–1), and detectivity (D*λ = 3.5 × 1011 to 1010 cm Hz1/2 W−1) were obtained at T = 77–200 K. The small capacitance of the structures (C = 7.5 pF at V = −1 V and T = 300 K) enabled an estimate of the response time of the photodetector at τ = 75 ps, which corresponds to a bandwidth of about 6 GHz. Photodetectors of this kind are promising for heterodyne detection of the emission of quantum-cascade lasers and IR spectroscopy.

Electrical properties of isotype N+-GaSb/n0-GaInAsSb/N+-GaAlAsSb type-II heterojunctions

Band diagrams and current-voltage and capacitance-voltage characteristics of isotype N+-GaSb/n0-GaInAsSb/N+-GaAlAsSb heterostructures have been studied. Dark-current flow mechanisms have been analyzed. It is shown that a staggered type-II heterojunction can behave as a Schottky diode and its current-voltage characteristics exhibit rectifying properties over the entire temperature range 90–300 K. The thermionic-emission current predominates at high temperatures and low voltages. This current is due to thermal excitation of electrons from GaInAsSb to GaSb over the barrier at the heterointerface. A comparison of the relevant theoretical and experimental data confirmed that the tunneling charge transport mechanism plays the key role at low temperatures under both forward and reverse biases.

Fast-response p-i-n photodiodes for 0.9–2.4 μm wavelength range

Fast-response, uncooled p-i-n photodiodes with a long-wavelength spectral sensitivity boundary at λ = 2.4 μm have been created on the basis of GaSb/GaInAsSb/GaAlAsSb heterostructures. A low doping level (1014–1015 cm−3) in the active layer ensured a low capacitance of the photodiode structure (below 1 pF at a sensitive area diameter of 100 μm) and a record small response time (on a level of 100–150 ps). The photodiode pass band reaches up to 2 GHz. The proposed devices are characterized by a small dark current level (500–1000 nA at a reverse bias voltage of 1–3 V) and a detection ability reaching 9 × 1010 cm Hz1/2 W−1 in a spectral interval of maximum sensitivity within 1.9–2.2 μm.