B. E. Zhurtanov

Light-emitting diodes based on GaSb alloys for the 1.6–4.4 μm mid-infrared spectral range

The available publications concerned with fabrication and study of light-emitting diodes (LEDs) intended for operation in the 1.6–4.4 μm spectral range; based on GaSb substrates; and grown by liquid-phase epitaxy, which makes it possible to form fairly thick layers lattice-matched to GaSb, are reviewed. In these studies, the active region consists of the GaInAsSb compound in LEDs for the spectral ranges 1.8–2.4 and 3.4–4.4 μm and the AlGaAsSb compound for the spectral region 1.6–1.8 μm. The wide-gap AlGaAsSb confining layers contain up to 64% of Al, which is an unprecedentedly high content for liquid-phase epitaxy. Asymmetric (GaSb/GaInAsSb/AlGaAsSb) and symmetric (AlGaAsSb/GaInAsSb/AlGaAsSb) heterostructures have been fabricated and studied. Various types of designs that make it possible to improve the yield of radiation generated in the active region have been developed. The measured external quantum yield of emission is as high as 6.0% at 300 K for the LEDs operating at the wavelengths 1.9–2.2 μm. A pulsed optical-radiation power of 7 mW at a current of 300 mA with a duty factor of 0.5 and 190 mW at a current of 1.4 A with a duty factor of 0.005 have been obtained. The external quantum emission yield of ∼1% has been obtained for LEDs that emit in the spectral range 3.4–4.4 μm; this yield exceeds that obtained for the known InAsSb/InAsSbP heterostructure grown on an InAs substrate by a factor of 3. The measured lifetime of minority charge carriers (5–0 ns) is close to the theoretical lifetime if only the radiative recombination and impact CHCC bulk recombination are taken into account. The impact recombination is prevalent at temperatures higher than 200 K for LEDs operating in the spectral range 3.4–4.4 μm and at temperatures higher than 300 K for LEDs operating in the spectral range 1.6–2.4 μm.

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.

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.

Properties of GaSb-based LEDs with grid ohmic contacts

We studied light-emitting diodes (LEDs) based on GaSb-GaInAsSb-GaAlAsSb heterostructures with the emission due to electron transitions from the conduction band to levels of intrinsic double-charged acceptors. Parallelepiped-shaped LEDs with a round or gridlike contact on the surface of the epitaxial layer were studied experimentally and theoretically. The relations describing the current spreading from round and grid contacts are presented. It is shown that grid contacts, in contrast to round ones, provide a uniform distribution of current over the emitting layer. The current density under a grid contact is smaller than under a round one by a factor of 20, which is especially important for long-wavelength (λ≳2 µm) LEDs, in which the contribution of the nonradiative Auger recombination is significant. An emission power of 3.5 mW at 300 mA current is obtained from grid-contact LEDs.

Impact-ionization-stimulated electroluminescence in isotype n-GaSb/n-AlGaAsSb/n-GaInAsSb heterostructures

We have studied electroluminescence in n-GaSb/n-AlGaAsSb/n-GaInAsSb heterostructures with isotype heterojunctions, in which the quantum efficiency of emission is increased due to the additional production of electron-hole pairs as a result of the impact ionization that takes place near the heterointerface. The impact ionization in such heterostructures is possible due to the presence of deep wells in the energy band structure.

Photodiodes based on n-GaSb/n-GaInAsSb/p-AlGaAsSb heterostructures grown using rare-earth elements for the 1.1–2.4 μm spectral range

Photodiodes sensitive in the wavelength range of 1.1–2.4 μm have been created based on n-GaSb/n-GaInAsSb/p-AlGaAsSb heterostructures with a narrow-gap n-GaInAsSb layer (Eg ≅ 0.5 eV) grown in the presence of a rare-earth element (holmium). The electron concentration in the narrow-gap layer is n = 1 × 1016 cm−3, which is about one-fourth of that in an analogous structure grown without the rare-earth element. The proposed structure is characterized by increased quantum efficiency and response speed.

Improving parameters of GaSb/GaInAsSb/AlGaAsSb photodiode structures with thin active regions for the 1.0–2.5 μm wavelength range

We have studied and optimized the properties of photodiodes with a red photosensitivity threshold at 2.5 μm, which have been created on the basis of GaSb/GaInAsSb/AlGaAsSb heterostructures with Ga0.76In0.24As0.22Sb0.78 active regions of variable thickness and a reflecting contact on the rear side. It is established that a decrease in the thickness of the active region and the reflection of radiation from the rear side lead to an improvement in the parameters of photodiodes and related thermophotovoltaic converters in the 1.0–2.5 μm wavelength range. The optimized structures showed a detection ability of D*λ = (3–6) × 1010 cm Hz1/2/W and an open-circuit photo emf up to Voc = 0.2 V.

High-efficiency LEDs based on n-GaSb/p-GaSb/n-GaInAsSb/P-AlGaAsSb type-II thyristor heterostructures

A new type of light-emitting diodes (LEDs), a high-efficiency device based on an n-GaSb/p-GaSb/n-GaInAsSb/P-AlGaAsSb thyristor heterostructure, with the maximum emission intensity at wavelength λ = 1.95 μm, has been suggested and its electrical and luminescent characteristics have been studied. It is shown that the effective radiative recombination in the thyristor structure in the n-type GaInAsSb active region is provided by double-sided injection of holes from the neighboring p-type regions. The maximum internal quantum efficiency of 77% was achieved in the structure under study in the pulsed mode. The average optical power was as high as 2.5 mW, and the peak power in the pulsed mode was 71 mW, which exceeded by a factor of 2.9 the power obtained with a standard n-GaSb/n-GaInAsSb/P-AlGaAsSb LED operating in the same spectral range. The approach suggested will make it possible to improve LED parameters in the entire mid-IR spectral range (2–5 μm).

High-efficiency 3.4–4.4 μm light-emitting diodes based on a p-AlGaAsSb/n-InGaAsSb/n-AlGaAsSb heterostructure operating at room temperature

Light-emitting diode structures operating at room temperature were obtained based on a p-AlGaAsSb/n-InGaAsSb/n-AlGaAsSb heterostructure with high Al content in the boundary layers formed on a p-GaSb(100) substrate. This structure ensures a threefold increase in the output radiant power and the external quantum yield (∼1%) as compared to the known InAsSb/InAsSbP heterostructure grown on an InAs substrate. A considerable increase in the pulsed output radiant power is explained by a more effective confinement of nonequilibrium charge carriers in the active region and by a decrease in the nonradiative recombination level, which is achieved by creating an isoperiodic structure.