A. Khoshakhlagh

Strain relief by periodic misfit arrays for low defect density GaSb on GaAs

We demonstrate the growth of a low dislocation density, relaxed GaSb bulk layer on a (001) GaAs substrate. The strain energy generated by the 7.78% lattice mismatch is relieved by a periodic array of 90° misfit dislocations. The misfit array is localized at the GaSb∕GaAs interface and has a period of 5.6nm which is determined by transmission electron microscope images. No threading dislocations are visible. The misfits are identified as 90°, rather than 60°, using Burger’s circuit analysis, and are therefore not associated with generation of threading dislocations. A low dislocation density and planar growth mode is established after only 3 monolayers of GaSb deposition as revealed by reflection high-energy electron diffraction patterns. Calculations corroborate the materials characterization and indicate the strain energy generated by the 7.78% lattice mismatch is almost fully dissipated by the misfit array. The low dislocation density bulk GaSb material on GaAs enabled by this growth mode will lead to new devices, especially in the infrared regime, along with novel integration schemes.We demonstrate the growth of a low dislocation density, relaxed GaSb bulk layer on a (001) GaAs substrate. The strain energy generated by the 7.78% lattice mismatch is relieved by a periodic array of 90° misfit dislocations. The misfit array is localized at the GaSb∕GaAs interface and has a period of 5.6nm which is determined by transmission electron microscope images. No threading dislocations are visible. The misfits are identified as 90°, rather than 60°, using Burger’s circuit analysis, and are therefore not associated with generation of threading dislocations. A low dislocation density and planar growth mode is established after only 3 monolayers of GaSb deposition as revealed by reflection high-energy electron diffraction patterns. Calculations corroborate the materials characterization and indicate the strain energy generated by the 7.78% lattice mismatch is almost fully dissipated by the misfit array. The low dislocation density bulk GaSb material on GaAs enabled by this growth mode will lead to n...

GaSb∕GaAs type II quantum dot solar cells for enhanced infrared spectral response

The authors report an enhanced infrared spectral response of GaAs-based solar cells that incorporate type II GaSb quantum dots (QDs) formed using interfacial misfit array growth mode. The material and devices, grown by molecular beam epitaxy, are characterized by current-voltage and spectral response characteristics. From 0.9to1.36μm, these solar cells show significantly more infrared response compared to reference GaAs cells and previously reported InAs QD solar cells. The short circuit current density and open circuit voltages of solar cells with and without dots measured under identical conditions are 1.29mA∕cm2, 0.37V and 1.17mA∕cm2, 0.6V, respectively.

III/V ratio based selectivity between strained Stranski-Krastanov and strain-free GaSb quantum dots on GaAs

The authors demonstrate and characterize type-II GaSb quantum dot (QD) formation on GaAs by either Stranski-Krastanov (SK) or interfacial misfit (IMF) growth mode. The growth mode selection is controlled by the gallium to antimony (III/V) ratio where a high III/V ratio produces IMF and a low ratio establishes the SK growth mode. The IMF growth mode produces strain-relaxed QDs, where the SK QDs remain highly strained. Both ensembles demonstrate strong room temperature photoluminescence (PL) with the SK QDs emitting at 1180nm and the IMF QDs emitting at 1375nm. Quantized energy levels along with a spectral blueshift are observed in 77K PL. Transmission electron microscope images identify the IMF array and crystallographic shape for both types of QD formation. Atomic force microscope images characterize QD geometry and density.

Bias dependent dual band response from InAs∕Ga(In)Sb type II strain layer superlattice detectors

We report on the multispectral properties of infrared photodetectors based on type II InAs∕Ga(In)Sb strain layer superlattices using an nBn heterostructure design. The optical and electrical properties of the midwave and long wave infrared (MWIR-LWIR) absorbing layers are characterized using spectral response and current-voltage measurements, respectively. The dual band response is achieved by changing the polarity of applied bias. The spectral response shows a significant change in the LWIR to MWIR ratio within a very small bias range (∼100mV), making it compatible with commercially available readout integrated circuits.

Formation and optical characteristics of strain-relieved and densely stacked GaSb∕GaAs quantum dots

The authors report the formation and optical characteristics of type-II, strain-relieved, and densely stacked GaSb∕GaAs quantum dots (QDs) using an interfacial misfit (IMF) growth mode. A moderate V/III ratio during the growth of GaSb QDs produces strain-relieved QDs facilitated by the IMF array without Sb segregation associated with defects and threading dislocations. In contrast, a low V/III ratio establishes conventional Stranski-Krastanov QDs. The strain-free nature of the IMF QDs allows densely packed, multistacked ensembles which retain very high crystalline quality demonstrated by x-ray diffraction, room-temperature photoluminescence, and electroluminescence. The possibility for dense stacking enabled by the strain-relieved growth mode may prove beneficial for QD sensors, emitters, and solar cells.

Simultaneous interfacial misfit array formation and antiphase domain suppression on miscut silicon substrate

The authors describe simultaneous interfacial misfit (IMF) array formation along with antiphase domain (APD) suppression in highly mismatched (Δa0/a0=13%) AlSb grown on a 5° miscut Si (001) substrate. Strain energy from the AlSb/Si heterojunction is accommodated by a self-assembled two-dimensional array of pure 90° dislocations confined to the interface. The 13% lattice mismatch establishes the AlSb/Si IMF period of ∼3.46 nm. This IMF spacing is well matched to the step length of the 5° miscut Si (001) substrate. Furthermore, the miscut substrate geometry suppresses APD formation due to the double step height. The resulting bulk material has both very low defect density (∼7×105/cm2) and very low APD density (∼103/cm2) confirmed by transmission electron microscope images. This material is expected to be desirable for electronic III-V devices on Si substrates.

Lasing characteristics of GaSb/GaAs self-assembled quantum dots embedded in an InGaAs quantum well

The authors report the optical characteristics of GaSb∕GaAs self-assembled quantum dots (QDs) embedded in an InGaAs quantum well (QW). Variations in the In composition of the QW can significantly alter the emission wavelength up to 1.3μm and emission efficiency. Lasing operation at room temperature is obtained from a 2-mm-long device containing five stacked GaSb QDs in In0.13Ga0.87As QWs at 1.026μm with a threshold current density of 860A∕cm2. The probable lasing transition involves electrons and holes confined in the QW and QDs, respectively, resulting in a large peak modal gain of 45cm−1. A significant blueshift of the electroluminescence peak is observed with increased injection current and suggests a type-II band structure.

Performance improvement of InAs/GaSb strained layer superlattice detectors by reducing surface leakage currents with SU-8 passivation

We report on SU-8 passivation for performance improvement of type-II InAs/GaSb strained layer superlattice detectors (λcut-off∼4.6 μm). Optical and electrical behavior of SU-8 passivated and unpassivated devices was compared. The dark current density was improved by four orders of magnitude for passivated single diodes at 77 K. The zero bias responsivity and detectivity at 77 K was equal to 0.9 A/W and 3.5×1012 Jones for SU-8 passivated single pixel diodes. FPA size diodes (24×24 μm2) were also fabricated and they showed responsivity and detectivity of 1.3 A/W and 3.5×1012 Jones, respectively at 77 K.

Epitaxial growth and formation of interfacial misfit array for tensile GaAs on GaSb

The authors report the formation of an interfacial misfit (IMF) array in the growth of relaxed GaAs bulk layers on a (001) GaSb surface. Under specific conditions, the high quality IMF array has a period of 5.6nm and can accommodate the 7.78% tensile GaAs∕GaSb lattice mismatch. The misfit site is identified as a 90° edge dislocation using Burger’s circuit theory and confirmed by high-resolution cross-section transmission electron microscopy (TEM) images. The resulting GaAs bulk material is both strain-free and highly crystalline. Plan-view TEM images show threading dislocation density of ∼3×106∕cm2. This material demonstration will enable novel device structures including an embedded GaSb active region in GaAs device matrix.

GaSb quantum-well-based “buffer-free” vertical light emitting diode monolithically embedded within a GaAs cavity incorporating interfacial misfit arrays

The authors demonstrate a monolithic, electrically injected, vertically emitting GaSb∕AlGaSb light emitting diode (LED) emitting at 1.6μm comprised of a hybrid GaAs∕GaSb-based structure. The LED is comprised of a GaSb∕AlGaSb quantum well/barrier active region embedded within high index contrast GaAs∕AlGaAs distributed Bragg reflectors (DBRs) using two interfacial misfit (IMF) arrays to relieve the strain induced from the high 8% lattice mismatch between the material systems. The first IMF is formed under compressive strain conditions to enable strain-free, defect-free deposition of GaSb active region directly on the lower GaAs∕AlAs DBRs without need for thick buffer. The second IMF is formed under tensile conditions to enable the upper GaAs∕AlAs DBRs on the GaSb active region. The device demonstrates a maximum output power of 3.5μW. Initial diode optical and electrical characteristics along with IMF band structure are discussed.