We propose to utilize confocal Raman spectroscopy combined with high resolution atomic force microscopy (AFM) for nondestructive characterisation of the sidewalls of etched and passivated small pixel (24 μm×24 μm) focal plane arrays (FPA) fabricated using LW/LWIR InAs/GaSb type-II strained layer superlattice (T2SL) detector material. Special high aspect ratio Si and GaAs AFM probes, with tip length of 13 μm and tip aperture less than 7°, allow characterisation of the sidewall morphology. Confocal microscopy enables imaging of the sidewall profile through optical sectioning. Raman spectra measured on etched T2SL FPA single pixels enable us to quantify the non-uniformity of the mesa delineation process.
GaSb thermophotovoltaic cells fabricated using Molecular Beam Epitaxy (MBE) and ion implantation techniques are studied. Challenges including different defect formation mechanisms using MBE and ion-induced defects using ion implantation were investigated by cross-sectional Transmission Electron Microscopy (XTEM), X-Ray Diffraction spectroscopy (XRD) and Scanning Electron Microscopy (SEM). For MBE grown TPVs, several approaches were used to suppress defects, including substrate preparation and using different MBE reactors. For ion-implanted TPVs, different implant doses and energies were tested to minimize the crystal damage and various Rapid Thermal Anneal (RTA) process recipes were studied to maximize the crystal recovery. Large area TPV cells with 1 × 1 cm dimensions were fabricated using these techniques, then electrically and optically characterized. Ideality factors and dark saturation currents were measured and compared for various TPVs.
Through the application of a bias voltage, metamaterials can dynamically change their response, opening up new
technological possibilities. Combining design elements from three common metamaterial patterns, we have created a
metamaterial polarizing filter that will transmit all polarization orientations equally when in the static mode. When a
bias voltage is applied, the filter will minimize the transmission of x-polarized light in the wavelength band of interest.
Progress has been made on creating a sufficiently conductive metamaterial to enable the dynamic mode, as well as on
incorporating several filters into a monolithic stack. Fabrication methods and transmission results for the required
substrates will be discussed.
Low resistance ohmic contacts have been successfully fabricated on n-GaSb layers grown by MBE on semi-insulating (SI) GaAs substrates using the Interfacial Misfit Dislocation (IMF) technique. Although intended for photovoltaic applications, the results are applicable to many antimonide-based devices. The IMF technique enables the growth of epitaxial GaSb layers on semi-insulating GaAs substrates resulting in vertical current confinement not possible on unintentionally doped ~ 1e17 cm-3 p-doped bulk GaSb. Results for low resistance ohmic contacts using NiGeAu, PdGeAu, GeAuNi and GeAuPd metallizations for various temperatures are reported. Specific transfer resistances down to 0.12 Ω-mm and specific contact resistances of < 2e-6 Ω-cm2 have been observed.
Lasers with emission wavelength around 2 _m have been traditionally based on InGaSb quantum wells grown on
GaSb. An alternative is to use self assembled InAs Quantum Dashes grown on InP by the Stranski-Krastanov
growth mode. More speci_cally, InAs quantum dashes embedded in strained GaInAs quantum wells, grown in
InAlGaAs waveguides lattice matched to InP substrates have been successfully used as active medium in edge
emitting lasers with wavelengths in the range from 1.45 _m to 2.1 _m. Advantages of this material system compared
to the GaSb based system include easier lattice matching; i.e. only one group V element is involved. Many
optoelectronic properties of the InAs/InP quantum dash material system are similar to those of InAs quantum
dots grown on GaAs substrates. The latter material system has been very successfully used for VECSELs in the
wavelength region around 1 _m, leading to the highest power VECSEL at this wavelength, mode locking, wide
range tunability as well as intra cavity SHG to generate red light. A challenge in the material system based on
InP substrates is to fabricate a DBR. A lattice-matched DBR can consist of GaAsSb/AlAsSb. Alternatively one
can grow a metamorphic DBR based on either GaAs/AlAs or GaSb/AlSb. The latter requires the DBR to be
grown after the active region. The resultant VECSEL is then a bottom emitter, where the substrate has to be
removed. This can be achieved by introducing an etch stop layer between substrate and active region. Lastly,
the DBR can be grown separately and subsequently wafer bonded to the active region. This paper will discuss
details of these technologies and present results.
The antimonide based vertical external cavity surface emitting lasers (VECSELs) operating in the 1.8 to 2.8 Tm wavelength range are typically based on InGaAsSb/AlGaAsSb quantum wells on AlAsSb/GaSb distributed Bragg reflectors (DBRs) grown lattice-matched on GaSb substrates. The ability to grow such antimonide VECSEL structures on GaAs substrates can take advantage of the superior AlAs based etch-stop layers and mature DBR technology based on GaAs substrates. The growth of such III-Sb VECSELs on GaAs substrates is non-trivial due to the 7.78% lattice mismatch between the antimonide based active region and the GaAs/AlGaAs DBR. The challenge is therefore to reduce the threading dislocation density in the active region without a very thick metamorphic buffer and this is achieved by inducing 90 ° interfacial mist dislocation arrays between the GaSb and GaAs layers. In this presentation we make use of cross section transmission electron microscopy to analyze a variety of approaches to designing and growing III-Sb VECSELs on GaAs substrates to achieve a low threading dislocation density. We shall demonstrate the failure mechanisms in such growths and we analyze the extent to which the threading dislocations are able to permeate a thick active region. Finally, we present growth strategies and supporting results showing low-defect density III-Sb VECSEL active regions on GaAs.
We investigate experimentally and theoretically the influence of non-radiative carrier losses on the performance of
VECSELs under pulsed and CW pumping conditions. These losses are detrimental to the VECSEL performance
not only because they reduce the pump-power to output-power conversion efficiency and lead to increased
thresholds, but also because they are strong sources of heat. This heating reduces the achievable output power
and eventually leads to shut-off due to thermal roll-over. We investigate the two main sources of non-radiative
losses, defect recombination and Auger losses in InGaAs-based VECSELs for the 1010nm-1040nm range as well
as for InGaSb-based devices for operation around 2μm. While defect related losses are found to be rather
insignificant in InGaAs-based devices, they can be severe enough to prevent CW operation for the InGaSb-based
structures. Auger losses are shown to be very significant for both wavelengths regimes and it is discussed how
structural modifications can suppress them. For pulsed operation record output powers are demonstrated and
the influence of the pulse duration and shape is studied.
Systematic characterization of various types of intersubband transitions in the quantum dots in a well
(DWELL) infrared photodetectors has been presented. By changing the thickness of the quantum well,
the excited state energy can be tuned with respect to the barrier, without altering the quantum dot
ground state. Bound to continuum transitions offer very high extraction probability for photoexcited
electrons but poor absorption coefficient, while the bound to bound transitions have higher absorption
but poorer extraction probability. Bound to quasibound transition is optimum for intermediate values
of electric fields with superior signal to noise ratio. The bound to quasibound device has the detectivity
of 4×1011 cm.Hz1/2 W-1 (+3V, f /2 optics) at 77 K and 7.4×108 cm.Hz1/2 W-1 at 200 K, which is highest
reported detectivity at 200 K for detector with long wave cutoff wavelength. High performance focal
plane arrays have been fabricated with noise equivalent temperature difference of 44 mK at 80 K for
6.1μm peak wavelength.
We demonstrate a novel epitaxial process for the growth of low-dislocation density GaSb on GaAs. The
growth mode involves the formation of large arrays of periodic 90° misfit dislocations at the interface
between the two binary alloys which results in a completely strain relieved III-Sb epi-layer without the
need for thick buffer layers. This epitaxial process is used for the growth of antimonide active regions
directly on GaAs/AlGaAs distributed Bragg Reflectors (DBRs) resulting in 2 μm VECSELs on GaAs
We present an overview of the quantum design, growth and lasing operation of both IR and mid-IR OPSL
structures aimed at extracting multi-Watt powers CW and multi-kW peak power pulsed. Issues related to
power scaling are identified and discussed. The IR OPSLs based on InGaAs QW bottom emitters targeted at
wavelengths between 1015nm and 1040nm are operated in CW mode (yielding a maximum power of 64W)
and pulsed (peak power of 245W). The mid-IR top emitter OPSLs designed to lase at 2μm are based on a
novel lattice mismatched growth using InGaSb QWs and yield a maximum peak power of 350W pulsed.
We compare an InAs quantum dot (QD) vertical external-cavity surface-emitting laser (VECSEL) design consisting of 4
groups of 3 closely spaced QD layers with a resonant periodic gain (RPG) structure, where each of the 12 QD layers is
placed at a separate field antinode. This increased the spacing between the QDs, reducing strain and greatly improving
device performance. For thermal management, the GaAs substrate was thinned and indium bonded to CVD diamond. A
fiber-coupled 808 nm diode laser was used as pump source, a 1% transmission output coupler completed the cavity. CW
output powers over 4.5 W at 1250 nm were achieved.
External quantum efficiency of semiconductor photonic devices is directly measured by
wavelength-dependent laser-induced temperature change (scanning laser calorimetry) with very high
accuracy. Maximum efficiency is attained at an optimum photo-excitation level that can be determined with
an independent measurement of power-dependent photoluminescence. Differential power-dependent
photoluminescence measurement is used to quickly screen the sample quality before processing.
This presentation will overview the growth of an IMF based VECSEL structure operating at 2 μm with an InGaSb QW
active region (a0 = 6.09 Å) on GaAs/AlGaAs distributed bragg reflectors (DBR) (a0 = 5.65 Å). The use of the GaAs
substrate instead of GaSb results in a significant reduction in the surface defect density while allowing the use of a
mature GaAs/AlGaAs DBR technology. We shall provide photoluminescence results from 2 μm IMF based active
regions grown on GaAs substrates and compare the results with the same active regions grown on GaSb substrates. We
shall also provide extensive transmission electron microscopy, surface morphology and high-resolution x-ray diffraction
analysis of the material grown.