N. D. Stoyanov

Superlinear electroluminescence due to impact ionization in GaSb-based heterostructures with deep Al(As)Sb/InAsSb/Al(As)Sb quantum wells

We report on the observation of superlinear electroluminescence (EL) in nanoheterostructures based on GaSb with a deep narrow Al(As)Sb/InAsSb/Al(As)Sb quantum well (QW) in the active region, grown by metal organic vapor phase epitaxy. Electroluminescence spectra for different driving currents were measured at temperatures of 77 and 300 K. It is shown that such structure exhibits superlinear dependence of optical power on the drive current and its increase of 2–3 times in the current range 50–200 mA. This occurs due to impact ionization in the Al(As)Sb/InAsSb quantum well in which a large band offset at the interface ΔEC = 1.27 eV exceeds ionization threshold energy for electrons in the narrow-gap well. Calculation of the size quantization energy levels is presented, and possible cases of impact ionization, depending on the band offset ΔEC at the interface and on the quantum well width, are considered. This effect can be used to increase quantum efficiency and optical power of light emitting devices (laser...

Ultimate InAsSbP solid solutions for 2.6–2.8-μm LEDs

Epitaxial layers of phosphorus-rich InAs1−y−xSbyPx solid solutions were obtained by liquid phase epitaxy (LPE). The films with x=0.32 were grown at 575 °C on isoperiodic (100)InAs substrates. It is shown that the growth of InAsSbP layers from a phosphorus-rich liquid phase is accompanied by saturation of the phosphorus content in the solid state. InAsSbP-based diode heterostructures emitting in the 2.6–2.8 μ m wavelength range were obtained, the output emission power of which is sufficient for detecting both natural and industrial gases in the atmosphere.

High-efficiency LEDs of 1.6–2.4 µm spectral range for medical diagnostics and environment monitoring

High efficient LED structures covering the spectral range of 1.6–2.4 µm have been developed on the basis of GaSb and its solid solutions. The electroluminescent characteristics and their temperature and current dependences have been studied. The radiative and nonradiative recombination mechanisms and their effect on the quantum efficiency have been investigated. A quantum efficiency of 40–60% has been obtained in the quasi-steady mode at room temperature. A short-pulse optical power of 170 mW was reached.

Superlinear electroluminescence in GaSb-based heterostructures with high potential barriers

The electroluminescence in isotype and anisotype light-emitting diode heterostructures grown by the method of liquid-phase epitaxy with large conduction-band offset ΔEc at the heterointerface between a narrow-band active region and a wide-band layer is studied. Two types of electroluminescence peaks are observed in the range of photon energies 0.28–0.74 eV at temperatures T = 300 and 77 K; in this case, a super-linear increase in the intensity and optical power of emission by a factor of 1.5–2 is observed in the range of pump currents 20–220 mA. This effect is attributed to the formation of additional electron-hole pairs as a result of impact ionization by hot electrons heated as a result of the band offset ΔEc in the conduction band at the n-AlGaAsSb/n-InGaAsSb and n-GaSb/n-InGaAsSb heteroboundaries. This effect can be used to increase the quantum efficiency of semiconductor emitters (light-emitting diodes, lasers) in the mid-infrared region.

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.

Photovoltaic detector based on type II p-InAs/AlSb/InAsSb/AlSb/p-GaSb heterostructures with a single quantum well for mid-infrared spectral range

Mid-infrared photovoltaic detector (PD) designed on the base of a type II p-InAs/p-GaSb asymmetric heterostructure with a deep AlSb/InAsSb/AlSb quantum well (QW) at the interface is reported. The heterostructures containing the single QW were grown by LP-MOVPE. Transport, electroluminescent and photoelectrical properties of these structures were investigated. Intense both positive and negative electroluminescence was observed in the spectral range 3-4 µm above room temperature (300-400 K). Spectral response in the mid-infrared range 1.2-3.6 μm was obtained at temperatures T=77-300 K. High quantum efficiency η=0.6-0.7 responsivity Sλ=1.4-1.7 A/W and detectivity Dλ* =3.5×1011 cm Hz1/2w-1 were achieved at 77 K. Such QW PDs are suitable for heterodyne spectroscopy and free space communication using quantum cascade lasers as well as for gas analysis and ecological monitoring applications.

Electroluminescence and photoelectric properties of type II broken-gap n-In(Ga)As(Sb)/N-GaSb heterostructures

Layers of n-InAs and n-InGaAsSb were grown by metalorganic vapor phase epitaxy and liquid phase epitaxy on N-GaSb substrates. The electroluminescence, current-voltage characteristics and photocurrent spectra of these heterostructures were studied at low temperatures. It was shown that GaSb/In(Ga)As(Sb) with InAs-rich narrow-gap solid solutions are broken-gap heterojunctions of type II at 77 and 300 K. Intense electroluminescence of the N-GaSb/n-In(Ga)As(Sb) heterostructures was found in the spectral range of 3–4 μm at 77 K. The origin of radiative recombination at the N-n type II broken-gap heterointerface is proposed and is in agreement with the experimental results for both systems.

Electroluminescent characteristics of InGaAsSb/GaAlAsSb heterostructure Mid-IR LEDs at high temperatures

The electroluminescent characteristics of an InGaAsSb/GaAlAsSb heterostructure LED emitting at 1.85 μm are studied in the temperature range 20–200°C. It is shown that the emission power exponentially drops as P ≅ 0.4exp(2.05 × 103/T) with a rise in temperature primarily because of an increase in the Auger recombination rate. It is found that band-to-band radiative recombination goes in parallel with recombination through acceptor levels, the latter causing the emission spectrum to broaden. With a rise in temperature, the activation energy of the acceptor levels decreases by the law ΔE≅ 32.9 − 0.075T and the maximum of the LED’s emission spectrum shifts toward the long-wavelength range (hνmax = 0.693 − 4.497 × 10−4T). Based on the dependence Eg = hνmax − 0.5kT and experimental data, an expression is derived for the temperature variation of the bandgap in the In0.055Ga0.945AsSb active area, Eg ≅ 0.817 − 4.951 × 10−4T, in the range 290 K < T < 495 K. The resistance of the heterostructure decreases exponentially with rising temperature as R0 ≅ 5.52 × 10−2exp(0.672/2kT), while cutoff voltage Ucut characterizing the barrier height of a p−n junction decreases linearly with increasing temperature (Ucut = −1.59T + 534). It is found that the current through the heterostructure is due to the generation-recombination mechanism throughout the temperature interval.

Portable optical water-and-oil analyzer based on a mid-IR (1.6–2.4 μm) optron consisting of an LED array and a wideband photodiode

An optical method for measuring the water and oil content using mid-IR (1.6–2.4 μm) LEDs and a wideband photodiode is suggested for the first time. This method is developed based on the absorption spectra of pure water, dewatered oil, and water—oil emulsions (cut oil) with different content of water and uses 10 types of LEDs in the spectral range 1.6–2.4 μm. It is shown that pure water heavily absorbs the LED radiation in the spectral range 1.85–2.05 μm, oil absorbs in the range 1.67–1.87 μm, and the LED radiation with a maximum at 2.20 μm is equally weakly absorbed by water and oil. An optical cell of the water-and-oil analyzer is designed on the basis of a three-element diode array with radiation maxima at 1.65 (detection of oil), 1.94 (detection of water), and 2.2 μm (reference signal) wideband photodiode covering the spectral range 1.3–2.4 μm. A calibration curve is derived that represents the dependence of the water concentration in oil on the amplitude of the reduced signal obtained by processing three signals from the LEDs. This optical method of measuring the water content in oil underlies a portable analyzer making possible online measurements directly in an oil well.

High-power InAs/InAsSbP heterostructure leds for methane spectroscopy (λ ≈ 3.3 μm)

Two designs of light-emitting diodes (LEDs) based on InAsSbP/InAs/InAsSbP double hetero-structures grown by metal-organic vapor phase epitaxy on p− and n-InAs substrates have been studied. The current-voltage and electroluminescence characteristics of the LEDs are analyzed. It is shown that the LED design with a light-emitting crystal (chip) mounted with the epitaxial layer down on the LED case and emission extracted through the n-InAs substrate provides better heat removal. As a result, the spectral characteristics remain stable at increased injection currents and the quantum efficiency of radiative recombination is higher. The internal quantum efficiency of light-em itting structures with an emission wavelength λ = 3.3–3.4 μm is as high as 22.3%. The optical emission power of the LEDs is 140 μW at a current of 1 A in the quasi-continuous mode and reaches a value of 5.5 mW at a current of 9 A in the pulsed mode.