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.
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 ~10<sup>12</sup> cm<sup>-2</sup>) 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.
In this work we report on the characterization of InAsNSb dilute nitride alloys and mutli-quantum well structures. InAsN
epilayers with room-temperature photoluminescence emission have been successfully grown by MBE on InAs and GaAs
substrates. By careful attention to growth conditions, device quality material can be obtained for N contents up to ~3%
with band gap reduction which follows the band anti-crossing model. Mid-infrared light-emitting diodes containing ten period
InAsNSb/InAs multi-quantum wells within the active region were fabricated. These devices exhibited
electroluminescence up to room temperature consistent with e-hh<sub>1</sub> and e-lh<sub>1</sub> transitions within type I quantum wells in good agreement with calculations. Comparison of the temperature dependence of the EL with that of type II InAsSb/InAs reveals more intense emission at low temperature and an improved temperature quenching up to T~200 K where thermally activated carrier leakage becomes important and further increase in the QW band offsets is needed. This
material system shows promise for use in mid-infrared diode lasers and other optoelectronic devices.
We report the molecular beam epitaxial growth of narrow gap dilute nitride InAsN alloys onto GaAs substrates using a
nitrogen plasma source. The photoluminescence (PL) of InAsN alloys with N-content in the range 0 to 1% which
exhibit emission in the mid-infrared spectral range is described. The sample containing 1% N reveals evidence of
recombination from extended and localized states within the degenerate conduction band of InAsN. A comparison of
GaAs and InAs based material shows little change in PL linewidth such that the change in substrate does not cause
significant reduction in quality of the epilayers. The band gap dependence on N content in our material is consistent
with predictions from the band anti-crossing model. We also report the growth of InAsSbN/InAs multi-quantum wells
which exhibit bright PL up to a temperature of 250 K without any post growth annealing. Consideration of the power
dependent PL behaviour is consistent with Type I band alignment arising from strong lowering of the conduction band
edge due to N-induced band anti-crossing effects.
We report the successful growth of InAsN onto GaAs substrates using nitrogen plasma source molecular beam epitaxy. We describe the spectral properties of InAsN alloys with N-content in the range 0 to 1% and photoluminescence emission in the mid-infrared spectral range. The photoluminescence emission of the sample containing 1% N reveals evidence of recombination from extended and localized states within the degenerate conduction band of InAsN. A comparison of GaAs and InAs based material shows little change in FWHM suggesting the change in substrate does not cause significant reduction in quality of the epilayers. Material grown is consistent with predictions from the band anti-crossing model (BAC model).
We report the molecular beam epitaxial growth of InSb quantum dots (QD) inserted as sub-monolayers in an InAs matrix
and grown using Sb<sub>2</sub> and As<sub>2</sub> fluxes. These InSb QD nanostructures exhibit intense mid-infrared photoluminescence up
to room temperature. The nominal thickness of the sub-monolayer insertions can be controlled by the growth
temperature (T<sub>Gr</sub> = 450-320 °C) which gives rise to the variation of the emission wavelength within the 3.6-4.0 μm range
at room temperature. Light emitting diodes where fabricated using ten InSb QD sheets and were found to exhibit bright
electroluminescence with a single peak at 3.8 μm at room temperature. A comparative analysis of the optical properties
of the structures grown using (Sb<sub>2</sub>,As<sub>2</sub>) and (Sb<sub>4</sub>,As<sub>4</sub>) is also presented.
This paper describes the characteristics of a separate confinement heterostructure laser design based on type-I
InAsSbP/InAsSb multiple quantum wells (MQW). An 8×8 band k.p method was used to calculate the band structure.
The optical gain of the active region containing InAsSb QW was calculated using a free carrier gain model. Other
properties such as behavior of the fundamental optical TE mode and refractive index profile were also determined. These
were used for simulation of the resulting device properties and to estimate the threshold modal gain and threshold current
density for the InAsSb MQW laser. Suitable InAsSbP cladding layer and waveguide/barrier materials have been
determined. The strain, critical thickness, band offset, optical gain, Auger coefficient and threshold current density have
been calculated at various Sb contents (x). The lowest current density is found for the composition range between
0.12<x<0.16, where the estimated laser threshold current density is 1.85-1.86 kA cm<sup>-2</sup>.
GaInAsPSb is a new narrow gap semiconductor material, which is suitable for the fabrication of semiconductor light sources for the mid-infrared spectral range. Unique physical properties of the alloy are discussed and its advantages for mid-infrared optoelectronic devices are considered. Liquid phase epitaxy (LPE) growth conditions for GaInAsPSb homogeneous high crystal quality layers lattice-matched onto GaSb substrates were determined. Spectra of photoluminescence (PL) emission were investigated. Homojunction p-i-n light-emitting diodes (LEDs) based on this pentenary alloy were fabricated and electroluminescence (EL) peaking near 4.0 μm at room temperature was observed.
In this work we report on a specially optimized type-I InAsSb/InAsSbP double heterostructure (DH) ridge laser grown by liquid phase epitaxy (LPE). To remove residual impurities and reduce Shockley-Read recombination, the active region was purified using a Gd gettering technique. In addition free carrier absorption loss was minimized by the introduction of two undoped quaternary layers with the same composition of the cladding layers either side of the active region. The inserted layers also helped alleviate inter-diffusion of unwanted dopants towards the active region during or after growth and reduced current leakage of the device. The diode lasers operate readily in pulsed mode at elevated temperatures and emit near 3.45 μm at 170 K with a threshold current density as low as 118 A/cm<sup>2</sup> at 85 K. Compared to the conventional 3-layer DH laser, the optimized 5-layer structure with reduced optical loss can raise the maximum lasing temperature by 95 K to ~210 K.
We report on the liquid phase epitaxy (LPE) growth of an optimized double heterostructure (DH) 3-4 μm laser and the use of linear rapid slider boat technology for the production of quantum well (QW) structures based on InAsSb/InAsSbP. Typical characteristics of some of these prototype sources are presented and analyzed, including the results of SEM, X-ray diffraction, photo- and electro-luminescence characteristics of prototype DH & QW devices. The optimized 5 epi-layer diode lasers operate readily in pulsed mode at elevated temperatures and emit near 3.45 μm at 170 K with a threshold current density as low as 118 A/cm<sup>2</sup> at 85 K. Coherent emission was obtained up to 210 K. LPE growth of InAsSb QW has been successfully obtained experimentally. The QW structure has been confirmed by SEM and electroluminescence measurements at different temperatures.
The fabrication and characterization of heterojunction phtodiodes for room temperature operation in the mid-infrared (2-5 μm) spectral range is described. Liquid phase epitaxy was employed to fabricate two different devices containing In<sub>0.97</sub>Ga<sub>0.03</sub>As and InAs<sub>0.89</sub>Sb<sub>0.11</sub> active regions appropriate for phtodetection at 3.3 μm and 4.6 μm, corresponding to the absorption bands of methane and carbon monoxide. Basic detector characteristics have been measured and were found to compare favourbly with other available detectors in this wavelength range. A simple analystical model was developed to help design and study the corresponding device physics governing the performance of the detectors and was found to give good agreement with the experimentally measured values.
We have proposed a new physical approach for the design of mid-IR lasers operating at λ = 3.2 - 3.26 μm based on type II heterojunctions with a large asymmetric band-offset at the interface (Δ<i>E</i><sub>C</sub> > 0.6 eV and Δ<i>E</i><sub>V</sub> > 0.35 eV). These high potential barriers produce effective electron-hole confinement at the interface and results in a tunnel-injection radiative recombination mechanism within the device due to reduce leakage current from the active region. The creation of high barriers for carriers leads to their strong accumulation in the active region and increases quantum emission efficiency of the spatially separated electrons and holes across the heteroboundary. Our approach also leads to the suppression of non-radiative Auger-recombination and a corresponding increase in the operation temperature of the laser. The active region of the laser structure consists of the type II heterojunction formed by narrow-gap In<sub>0.83</sub>Ga<sub>0.17</sub>As<sub>0.82</sub>Sb<sub>0.18</sub> (E<sub>g</sub> = 0.393 eV at 77 K) and wide-gap Ga<sub>0.84</sub>In<sub>0.16</sub>As<sub>0.22</sub>Sb<sub>0.78</sub> (E<sub>g</sub> = 0.635 eV at 77 K) layers lattice-matched to InAs substrate.
Electroluminescence is reported for the first time from InAs<SUB>0.75</SUB>Sb<SUB>0.25</SUB> quantum dot light emitting diodes. The quantum dots were grown from the liquid phase at 590 degree(s)C on an InAs (100) substrate and embedded within the undoped active region of an InAs homojunction pin diode. At 4K and 250 mA injection current, three transitions were identified, centered at 4.01, 3.80 and 3.63micrometers , associated with the sp, p and d states of the confined holes inside the quantum dot. Each of the transitions exhibits a blue shift with increasing injection current, but the electroluminescence results indicate the presence of a phonon bottleneck in these devices. The quantum dot electroluminescence was observed to persist up to room temperature.
There is considerable interest in the realization of room temperature mid-infrared diode lasers for a variety of applications, including remote gas sensing, infrared countermeasures and molecular spectroscopy. However the maximum temperature of operation in narrow gap III-V component alloys is limited by strong non-radiative Auger recombination and various band structure engineering techniques are being investigated to provide Auger suppression. In our work we are investigating the possibility of obtaining a practical 3.3micrometers laser by making use of radiative recombination across single type II hetero-interfaces. Because transitions occur between confined electron and hole states localized on either side of the heterojunction where the potential wells are triangular, there exists the possibility of tailoring the wave-function overlap to give good Auger suppression while still maintaining high radiative output. At the same time growth form the liquid phase offers potentially lower SRH recombination. We compared two such heterojunctions (InAs<SUB>0.94</SUB>Sb<SUB>0.06</SUB>/InAs and Ga<SUB>0.96</SUB>In<SUB>0.04</SUB>As<SUB>0.11</SUB>Sb<SUB>0.89</SUB>/ InAs) grown by rapid slider LPE and report on the photoluminescence and electroluminescence from the interfaces. The dependence of these interface transitions on temperature, excitation intensity, band offset and polarization is reported, with a view towards incorporating these in the active region of a practical laser.