Characterization of 6.1 Å III-V materials grown on GaAs and Si: a comparison of GaSb/GaAs epitaxy and GaSb/AlSb/Si epitaxy

GaSb p–i–n photodiodes were grown on GaAs and Si, using interfacial misfit arrays, and on native GaSb. For the samples grown on GaAs and Si, high-resolution transmission electron microscopy images revealed interface atomic periodicities in agreement with atomistic modeling. Surface defect densities of ~View the MathML source were measured for both samples. Atomic force microscopy scans revealed surface roughnesses of around 1.6 nm, compared with 0.5 nm for the sample grown on native GaSb. Dark current and spectral response measurements were used to study the electrical and optoelectronic properties of all three samples

Characterisation of Ga(1-x)In(x)Sb quantum wells (x~0.3) grown on GaAs using AlGaSb interface misfit buffer

GaInSb multiple quantum wells (MQW) grown on GaAs using an AlGaSb interface misfit (IMF) metamorphic buffer layer technique exhibit superior infrared photoluminescence (PL) at room temperature compared with MQW grown directly on GaSb. PL emission was obtained in the range from 1.7 μm (4 K) to 1.9 μm (300 K) from Ga1-xInxSb samples containing five compressively strained QW with In content x~0.3. Structural and optical characterisation confirms that the AlGaSb IMF growth technique is promising for the development of photonic devices operating at extended wavelengths based on GaAs substrates

Transport of modulation-doped Al0.2Ga0.8Sb/GaSb heterojunctions

Mobilities and carrier densities of modulation doped Al0.2Ga0.8Sb/GaSb heterostructures are presented for the first time. The structures studied were grown by molecular beam epitaxy and consisted of a single heterojunction with Te compensation doping to reduce the intrinsic p-type background. Hall measurements were performed from 30–300 K, giving p-type mobilities peaking at 3240 cm2/Vs, a considerable improvement over previous reported bulk mobilities for samples with compensation doping. Growth trials on bulk material have also been carried out to investigate the optimum growth conditions for future structures, with the aim of minimising the occurrence of natural growth defects in GaSb, which act as acceptors. Together these measurements lay the ground work for (magneto)transport studies of two-dimensional charge-carriers in AlxGa1-xSb/GaSb heterostructures, which has not been previously reported

Impact ionisation in Al0.9Ga0.1As0.08Sb0.92 for Sb-based avalanche photodiodes

We report the impact ionisation coefficients of the quaternary alloy Al0.9Ga0.1As0.08Sb0.92 lattice matched to GaSb substrates within the field range of 150 to 550 kV cm-1 using p-i-n and n-i-p diodes of various intrinsic thicknesses. The coefficients were found with an evolutionary fitting algorithm using a non-local recurrence based multiplication model and a variable electric field profile. These coefficients not only indicate that an avalanche photodiode can be designed to be function in the mid-wave infrared, but also can be operated at lower voltages. This is due to the high magnitude of the impact ionisation coefficients at relatively low fields compared to other III-V materials typically used in avalanche multiplication regions

InSb-based quantum dot nanostructures for mid-infrared photonic devices

Novel InSb quantum dot (QD) nanostructures grown by molecular beam epitaxy (MBE) are investigated in order to improve the performance of light sources and detectors for the technologically important mid-infrared (2-5 μm) spectral range. Unlike the InAs/GaAs system which has a similar lattice mismatch, the growth of InSb/InAs QDs by MBE is a challenging task due to Sb segregation and surfactant effects. These problems can be overcome by using an Sb-As exchange growth technique to realize uniform, dense arrays (dot density ~1012 cm-2) of extremely small (mean diameter ~2.5 nm) InSb submonolayer QDs in InAs. Light emitting diodes (LEDs) containing ten layers of InSb QDs exhibit bright electroluminescence peaking at 3.8 μm at room temperature. These devices show superior temperature quenching compared with bulk and quantum well (QW) LEDs due to a reduction in Auger recombination. We also report the growth of InSb QDs in InAs/AlAsSb ‘W’ QWs grown on GaSb substrates which are designed to increase the electron-hole (e-h) wavefunction overlap to ~75%. These samples exhibit very good structural quality and photoluminescence peaking near 3.0 μm at low temperatures

Delta doping and positioning effects of type II GaSb quantum dots in GaAs solar cell

GaSb quantum dot (QD) solar cell structures were grown by molecular beam epitaxy on GaAs substrates. We investigate the reduction in open-circuit voltage and study the influence of the location of QD layers and their delta doping within the solar cell. Devices with 5 layers of delta-doped QDs placed in the intrinsic, n- and p-regions of a GaAs solar cell are experimentally investigated, and the deduced values of Jsc, Voc, fill factor, efficiency (η) are compared. A trade-off is needed to minimize the Voc degradation while maximizing the short circuit current density (Jsc) enhancement due to sub-bandgap absorption. The voltage recovery is attributed to the removal of the QDs from the high-field region which reduces SRH recombination. The devices with p- or n-doped QDs placed in the flat band potential (p- or n-region) show a recovery in Jsc and Voc compared to devices with delta-doped QDs placed in the depletion region. However, there is less photocurrent arising from the absorption of sub-band gap photons. Furthermore, the long wavelength photoresponse of the n-doped QDs placed in the n-region shows a slight improvement compared to the control cell. The approach of placing QDs in the n-region of the solar cell instead of the depletion region is a possible route towards increasing the conversion efficiency of QD solar cells

Type II GaSb/GaAs quantum rings with extended photoresponse for efficient solar cells

The introduction of GaSb quantum dots (QDs) within a GaAs single junction solar cell is attracting increasing interest as a means of absorbing long wavelength photons to extend the photoresponse and increase the short-circuit current. The band alignment in this system is type-II, such that holes are localized within the GaSb QDs but there is no electron confinement. Compared to InAs QDs this produces a red-shift of the photoresponse which could increase the short-circuit current and improve carrier extraction. GaSb nanostructures grown by molecular beam epitaxy (MBE) tend to preferentially form quantum rings (QRs) which are less strained and contain fewer defects than the GaSb QDs, which means that they are more suitable for dense stacking in the active region of a solar cell to reduce the accumulation of internal strain and enhance light absorption. Here, we report the growth and fabrication of GaAs based p-i-n solar cells containing ten layers of GaSb QRs. They show extended long wavelength photoresponse into the near-IR up to 1400 nm and enhanced short-circuit current compared to the GaAs control cell due to absorption of low energy photons. Although enhancement of the short-circuit current was observed, the thermionic emission of holes was found to be insufficient for ideal operation at room temperature

Low bandgap GaInAsSb thermophotovoltaic cells on GaAs substrate with advanced metamorphic buffer layer

Thermophotovoltaic (TPV) devices based on GaInAsSb lattice matched to GaSb (100) substrates have demonstrated high external quantum efficiencies (EQEs) in the mid-infrared spectral range, making them promising candidates for waste heat recovery from high temperature “blackbody” sources. In this work, the GaInAsSb alloy has been integrated onto more cost-effective GaAs (100) substrates by using advanced metamorphic buffer layer techniques in molecular beam epitaxy (MBE), which included an interfacial misfit (IMF) array at the GaSb/GaAs interface followed by GaInSb/GaSb dislocation filtering layers. The threading dislocations in the GaInAsSb region can be efficiently supressed, resulting in high quality materials for TPV applications. To determine the performance of the GaInAsSb TPV on GaAs it was compared with a cell grown lattice matched onto GaSb substrate having the same structure. The resulting TPV on GaAs exhibited similar dark current-voltage characteristics with that on GaSb. Under illumination from an 800 °C silicon nitride emitter, the short circuit current density (Jsc) from the GaInAsSb TPVs on GaAs reached more than 90% of the control cell on GaSb, and the open circuit voltage (Voc) exceeded 80% of the cell on GaSb. The EQE from the TPV on GaAs reached around 62%, the highest value reported from this type of TPV on GaAs. With improved TPV structure design, large area GaInAsSb TPV panels on GaAs substrates can be realized in the future for waste heat energy recovery applications

Room temperature mid-infrared emission from faceted InAsSb multi quantum wells embedded in InAs nanowires

There is considerable interest in the development of InAsSb-based nanowires for infrared photonics due to their high tunability across the infrared spectral range, high mobility, and integration with silicon electronics. However, optical emission is currently limited to low temperatures due to strong nonradiative Auger and surface recombination. Here, we present a new structure based on conical type II InAsSb/InAs multiquantum wells within InAs nanowires which exhibit bright mid-infrared photoluminescence up to room temperature. The nanowires are grown by catalyst-free selective area epitaxy on silicon. This unique geometry confines the electron–hole recombination to within the quantum wells which alleviates the problems associated with recombination via surface states, while the quantum confinement of carriers increases the radiative recombination rate and suppresses Auger recombination. This demonstration will pave the way for the development of new integrated quantum light sources operating in the technologically important mid-infrared spectral range

Land cover change and carbon emissions over 100 years in an African biodiversity hotspot

Agricultural expansion has resulted in both land use and land cover change (LULCC) across the tropics. However, the spatial and temporal patterns of such change and their resulting impacts are poorly understood, particularly for the pre-satellite era. Here we quantify the LULCC history across the 33.9 million ha watershed of Tanzania's Eastern Arc Mountains, using geo-referenced and digitised historical land cover maps (dated 1908, 1923, 1949 and 2000). Our time series from this biodiversity hotspot shows that forest and savanna area both declined, by 74% (2.8 million ha) and 10% (2.9 million ha), respectively, between 1908 and 2000. This vegetation was replaced by a five-fold increase in cropland, from 1.2 million ha to 6.7 million ha. This LULCC implies a committed release of 0.9 Pg C (95% CI: 0.4-1.5) across the watershed for the same period, equivalent to 0.3 Mg C ha(-1) yr(-1) . This is at least three-fold higher than previous estimates from global models for the same study area. We then used the LULCC data from before and after protected area creation, as well as from areas where no protection was established, to analyse the effectiveness of legal protection on land cover change despite the underlying spatial variation in protected areas. We found that, between 1949 and 2000, forest expanded within legally protected areas, resulting in carbon uptake of 4.8 (3.8-5.7) Mg C ha(-1) , compared to a committed loss of 11.9 (7.2-16.6) Mg C ha(-1) within areas lacking such protection. Furthermore, for nine protected areas where LULCC data is available prior to and following establishment, we show that protection reduces deforestation rates by 150% relative to unprotected portions of the watershed. Our results highlight that considerable LULCC occurred prior to the satellite era, thus other data sources are required to better understand long-term land cover trends in the tropics. This article is protected by copyright. All rights reserved