GaSb-based materials can be used to produce high performance photonic devices operating in the technologically important mid-infrared spectral range. Direct epitaxial growth of GaSb on silicon (Si) is an attractive method to reduce manufacturing costs and opens the possibility of new applications, such as lab-on-a-chip MIR photonic integrated circuits and monolithic integration of focal plane arrays (FPAs) with Si readout integrated circuits (ROICs). However, fundamental material dissimilarities, such as the large lattice mismatch, polar-nonpolar character of the III-V/Si interface and differences in thermal expansion coefficients lead to the formation of threading dislocations and antiphase domains, which effect the device performance. This work reports on the molecular beam epitaxial growth of high quality GaSb-based materials and devices onto Si. This was achieved using a novel growth procedure consisting of an efficient AlSb interfacial misfit array, a two-step GaSb growth temperature procedure and a series of dislocation filter superlattices, resulting in a low defect density, anti-phase domain free GaSb buffer layer on Si. A nBn barrier photodetector based on a type-II InAs/InAsSb superlattice was grown on top of the buffer layer. The device exhibited an extended 50 % cut-off wavelength at 5.40 μm at 200 K which moved to 5.9 μm at 300 K. A specific detectivity of 1.5 x1010 Jones was measured, corresponding in an external quantum efficiency of 25.6 % at 200 K.
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 ~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.
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-hh1 and e-lh1 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 molecular beam epitaxial growth of InSb quantum dots (QD) inserted as sub-monolayers in an InAs matrix
and grown using Sb2 and As2 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 (TGr = 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 (Sb2,As2) and (Sb4,As4) is also presented.
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/cm2 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.