P-type GaSb Substrate Research Articles

Device Characterization of a Sulfur-Implanted p$^{++}$/p GaSb Photovoltaic Camel Diode

The implantation and rapid thermal annealing of sulfur (S <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> ) ions has previously been shown to be an effective method in non-epitaxially attaining hole carrier concentrations as high as 1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> in gallium antimonide (GaSb). This technique was used to fabricate a photovoltaic diode by delta-doping the front surface of a p-type GaSb substrate and forming a p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">++</sup> /p junction. The steep potential created using this process is increased by strong Fermi level pinning at the metal/p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">++</sup> interface, resulting in a camel diode with a barrier height of 0.51 eV. A post-fabrication etch process succeeded in improving the short circuit current density to 41.8 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and the internal quantum efficiency to 90% by enhancing the carrier lifetime away from the front metal contact grid. Likewise, the open circuit voltage improved to 0.21 V, with an intrinsic fill factor above 40%. These results offer the potential of a significantly higher power output than similar non-epitaxial devices made on n-type GaSb substrates.

Type-II Superlattice Infrared Photodetectors With Graphene Transparent Electrodes

In this letter, we have demonstrated that graphene can act as a transparent electrode on the p-type GaSb substrate. Wavelength-insensitive and over 95% high transmittance are observed for the bi-layer graphene in the infrared light range. By using graphene as the transparent electrode, over ten-time responsivity enhancement is observed for the InAs/GaSb type-II superlattice infrared photodetector. The phenomenon is attributed to the shortened carrier transport paths resulted from the vertical current flow to the graphene electrode. The results have revealed the potential of graphene for the application of infrared transparent electrodes.

Development of n-type Te-doped GaSb substrates with low carrier concentration for FPA applications

Undoped GaSb is p-type with the residual acceptor concentration of about 1e17cm−3 due to the gallium vacancies and gallium in antimony site. Counter-doping of GaSb with low level of Te can reduce the net carrier concentration resulting in higher optical transparency in a broad IR spectral range. In this work, the carrier concentration, mobility and sheet resistance of n-type and p-type Te-doped GaSb substrates were measured using Hall method at 300K and 77K. The Hall carrier concentration data at 300K were correlated with the absorption coefficients of GaSb in the IR spectral range. An empirical relationship between these values was established. Based on this correlation, we discuss application of FTIR spectroscopy for non-destructive optical screening of the substrates that allows construction of the carrier concentration distribution map across GaSb wafers. Investigations of the electronic properties of the low-doped p-type and n-type GaSb substrates upon cooling down to 77K indicate the reduction of the hole carrier concentration background for both GaSb types. This is evident from the decrease in the Hall-measured carrier concentration for p-type GaSb. For n-type GaSb, an increase in the carrier concentration is observed due to the reduction of the hole carrier concentration background.

Increased p-type conductivity in GaNxSb1−x, experimental and theoretical aspects

The large increase in the p-type conductivity observed when nitrogen is added to GaSb has been studied using positron annihilation spectroscopy and ab initio calculations. Doppler broadening measurements have been conducted on samples of GaNxSb1−x layers grown by molecular beam epitaxy, and the results have been compared with calculated first-principle results corresponding to different defect structures. From the calculated data, binding energies for nitrogen-related defects have also been estimated. Based on the results, the increase in residual hole concentration is explained by an increase in the fraction of negative acceptor-type defects in the material. As the band gap decreases with increasing N concentration, the ionization levels of the defects move closer to the valence band. Ga vacancy-type defects are found to act as positron trapping defects in the material, and the ratio of Ga vacancy-type defects to Ga antisites is found to be higher than that of the p-type bulk GaSb substrate. Beside Ga vacancies, the calculated results imply that complexes of a Ga vacancy and nitrogen could be present in the material.

MBE growth of high absorption mid-IR type-II InAs/GaSb superlattices

Two kinds of short-period type II superlattices (SLs) InAs (6 monolayers (MLs))/GaSb (3 MLs) and InAs (8 MLs)/GaSb (8 MLs) which can serve for mid-infrared (MIR) detection have been grown by molecular beam epitaxy (MBE) on p-type GaSb (100) substrates. The cutoff wavelength for the two superlattices (SLs) was found to be around 4.8 µm at 300 K. The high resolution X-ray diffraction (HRXRD) measurements indicated that the InAs (8 MLs)/GaSb (8 MLs) SLs have better crystalline quality than that of the InAs (6 MLs)/GaSb (3 MLs) SLs. However, compared with infrared absorption in the 2.5–4.3 µm range, the optical absorption of InAs (6 MLs)/GaSb (3 MLs) SLs was more excellent. This can be attributed to increase probability of the electron and hole wave function overlap in the thinner period thickness.

Influence of the period thickness and composition on the electro-optical properties of type-II InAs/GaSb midwave infrared superlattice photodetectors

In this paper, the electro-optical properties of InAs/GaSb superlattice (SL) midwave infrared photodiodes with different periods were investigated. Three devices with different SL periods, but the same cut-off wavelength at 5 µm at 77 K, were grown by molecular beam epitaxy on p-type GaSb substrates. The optical and electrical behaviours were characterized and analysed. Our investigations show strong influence of the SL composition on both the material properties and photodetector performances, such as the background doping concentration, shape of the response spectra and the dark current behaviours.

Deposition and Characterization of Atomic Layer Deposited ZnS Thin Films on p-type GaSb(100) Using Diethylzinc Precursor and Hydrogen Sulfide

Diethylzinc precursor and hydrogen sulfide were used for the atomic layer deposition (ALD) of ZnS films on thioacetamide (TAM)-treated p-type GaSb substrates for the passivation of GaSb-based devices. The film growth rate was found to be 0.13±0.01 nm/cycle with an ALD window of 175 - 275 °C. X-ray photoelectron spectroscopy (XPS) analyses of as-deposited ZnS films showed that the resulting films were stoichiometric ZnS with no evidence of other sulfur species in the film. XPS indicated the surface of GaSb treated with TAM was a mixture of Ga-S and Sb-S without detectable oxides. After the ALD of 1 nm-thick ZnS on TAM-treated GaSb, the major component at the interface of ZnS/TAM-treated GaSb system was found to be gallium sulfide. Surface morphology and structure of as-deposited ZnS were studied with phase-shifting interferometry and grazing incidence X-ray diffraction.

Temperature dependence performances of InAs/GaSb superlattice photodiode

In this communication we report on temperature dependence performances of short period InAs/GaSb pin superlattice (SL) photodiode grown by Molecular Beam Epitaxy on p-type GaSb substrate. SL symmetrical structure with 3 μm thick active region, adapted for detection in the MWIR domain with cutoff wavelength varying from 4.6 μm to 5.5 μm in the temperature range 80–300 K, was processed in mesa photodiode in order to perform dark current measurements as a function of temperature. Extracted from current–voltage characteristics, R 0 A products above 1 × 10 6 Ω cm 2 at 80 K and around 0.15 Ω cm 2 at 200 K were measured, and the quantitative analysis of the J– V curves allowed us to identify the dominant dark current mechanism in each operating temperature range. As a result, SL photodiode is dominated by generation–recombination (GR) processes at low temperature and becomes diffusion limited above 140 K. Such results are discussed and minority carrier lifetimes were extracted from J– V curve fitting.

Intense Terahertz Radiation from InAs Thin Films

Terahertz (THz) radiation from InAs thin films grown by molecular-beam epitaxy on closely lattice-matched p-type GaSb (100) substrates and lattice-mismatched semi-insulating GaAs (100) substrates was investigated. The THz radiation intensity was measured from InAs films with thicknesses between 100 nm and 1.5 μm excited by a femtosecond laser pulse with a wavelength of approximately 780 nm. The radiation intensity increased as the InAs film thickness increased and it exceeded that from a bulk n-type InAs substrate with an electron concentration of 2.3 × 1016 cm−3 when the InAs film thickness was greater than about 500 nm. In addition, the THz intensity from a 1-μm-thick InAs film was greater than that from a bulk p-type InAs substrate. We ascribe this enhanced THz intensity to the wave reflected from the lower interface between the InAs film and the layer grown beneath it. We confirmed this by observing an increased pulse width due to constructive overlap of the reflected wave. The results demonstrate that InAs thin films are promising materials for THz emitting devices.

Metal-organic chemical vapour deposition growth of InAs/GaSb type-II superlattice photodiodes

Presented for the first time is the realisation of InAs/GaSb type-II superlattice photodiodes grown by metal-organic chemical vapour deposition. The photovoltaic detectors have a cutoff wavelength (50% power responsivity) of ~8 μm at 78 K. The active region was grown on a p-type GaSb substrate and consists of an InAs(5.1 nm)/ GaSb(2.1 nm) superlattice. The R 0 4 product is typically around 0.03 Ω-cm 2 at 78 K. The responsivity is typically around 0.6 A/W and the detectivity is 1.6 × 109 at 78 K.

Uncooled InAs/GaSb superlattice photovoltaic detector operating in the mid-wavelength infrared range

A p–i–n superlattice photovoltaic detector is presented, operating uncooled in the 3–5 µm mid-wavelength infrared region. The active zone of the detector device, grown by molecular beam epitaxy on p-type GaSb substrate, is made of 150 periods of strain compensated InAs/InSb/GaSb (10/1/10 monolayers) superlattice (SL). The SL detector exhibited at 293K a cutoff wavelength of 5.9 µm, an absorption coefficient of 5×103 cm−1 and a responsivity of 0.7 mA/W at 3.5 µm.

InAs/GaSb type-II superlattices for high performance mid-infrared detectors

The superlattice (SL) design parameters of a 50 period InAs/GaSb SL structure with InSb-like interfaces (IFs) were systematically varied around the 26 Å InAs/ 27 Å GaSb design in order to explore the parameter space for maximum photoresponse in the 3–5 μ m mid-infrared atmospheric window. Using previously optimized growth conditions, the SL structures were grown on p-type GaSb substrates by molecular beam epitaxy with precisely calibrated growth rates. The electrical properties of the SLs were characterized by magnetic field-dependent Hall effect measurements below the carrier freeze-out temperature of the p-type substrate. Multi-carrier analysis at 4.2 K determined an electron sheet carrier concentration of 8.5 × 10 10 cm - 2 with a mobility of 8200 cm 2 /Vs. Two sets of SLs were used in the optimization process: the first set with a fixed InAs width of 26 Å, and the second with a fixed GaSb width of 27 Å . As the GaSb layer width varied from 15 to 27 Å , the photoresponse cut-off wavelength shifted from 6.47 to 5.24 μ m. Similarly, as the InAs width varied from 26 to 13 Å, the cut-off wavelength shifted from 5.08 to 3.05 μ m. The strongest photoresponse in the 3–5 μ m mid-IR window was achieved with the InAs (20 Å)/GaSb (27 Å) SL design.

Efficient 4.2 µm light emitting diodes fordetecting CO 2 at room temperature

InAs0.91Sb0.09 light emitting diodes (LEDs) were grown on p-type GaSb substrates using liquid phase epitaxy (LPE). These devices exhibit efficient infrared emission at 4.2 µm and can be used to fabricate infrared carbon dioxide (CO2) gas sensors for the cost effective detection and monitoring of CO2 gas in various applications.

Injection InGaSbAs lasers emitting radiation of wavelengths 1.9–2.3μ at room temperature

Double GaAlSbAs/InGaSbAs/GaAlSbAs heterostructures were grown by the method of liquid phase epitaxy on (100) faces of p-type GaSb substrates. Pulsed lasing was observed in the wavelength range 1.9–2.3 μ at room temperature. The threshold current density at the wavelength of 2.29 μ was 20 kA/cm2. This was the longest wavelength emitted by an uncooled injection laser.

PInxGa1-xSb–nGa1-yAlySb Heterojunction Photodiodes

Highly efficient, lattice-matched heterojunction photodiodes were fabricated by liquid-phase-epitaxial growth of a pIn0.07Ga0.93Sb layer, followed by an nGa0.3Al0.7Sb layer on the p-type (111) GaSb substrate. The spectral photocurrent response of the diodes was nearly flat in the wavelength range of 1.0–1.8 µm. The external guantum efficiency of photodiodes was 32% at zero voltage bias in the vicinity of 1.5 µm where a minimum loss of optical fibre was reported.

Highly efficient pGaSb-nGa1−xAlxSb photodiodes

Highly efficient pGaSb-nGa1−xAlxSb photodiodes were fabricated by liquid-phase-epitaxial growth of an undoped p-type GaSb layer, followed by that of an n-type Ga1−xAlxSb layer with a composition at x=0.7 on a p-type (100) GaSb substrate. Spectral photocurrent response of the diodes measured at various bias voltages and room temperature was fairly flat between 1.0 and 1.7 μm. The external quantum efficiency of the photoresponse was 38% at zero bias and 54% at 1 V reverse bias in the vicinity of 1.2 μm.