Kalyan Nunna

GaSb/GaAs type-II quantum dots grown by droplet epitaxy

We demonstrate the formation of GaSb quantum dots (QDs) on a GaAs(001) substrate by droplet epitaxy using molecular beam epitaxy. The high crystal quality and bimodal size distribution of the QDs are confirmed using atomic force and transmission electron microscope images. A staggered type-II QD band structure is suggested by a photoluminescence peak that is blue shifted with increasing excitation intensity, a large emission polarization of 60%, and a long carrier decay time of 11.5 ns. Our research provides a different approach to fabricating high quality GaSb type-II QDs.

Structural Analysis of Highly Relaxed GaSb Grown on GaAs Substrates with Periodic Interfacial Array of 90° Misfit Dislocations

We report structural analysis of completely relaxed GaSb epitaxial layers deposited monolithically on GaAs substrates using interfacial misfit (IMF) array growth mode. Unlike the traditional tetragonal distortion approach, strain due to the lattice mismatch is spontaneously relieved at the heterointerface in this growth. The complete and instantaneous strain relief at the GaSb/GaAs interface is achieved by the formation of a two-dimensional Lomer dislocation network comprising of pure-edge (90°) dislocations along both [110] and [1-10]. In the present analysis, structural properties of GaSb deposited using both IMF and non-IMF growths are compared. Moiré fringe patterns along with X-ray diffraction measure the long-range uniformity and strain relaxation of the IMF samples. The proof for the existence of the IMF array and low threading dislocation density is provided with the help of transmission electron micrographs for the GaSb epitaxial layer. Our results indicate that the IMF-grown GaSb is completely (98.5%) relaxed with very low density of threading dislocations (105 cm−2), while GaSb deposited using non-IMF growth is compressively strained and has a higher average density of threading dislocations (>109 cm−2).

Characterization of GaSb/GaAs interfacial misfit arrays using x-ray diffraction

We report a nondestructive, large-area method to characterize dislocation formation at a highly lattice-mismatched interface. The analysis is based on x-ray diffraction and reciprocal space mapping using a standard, lab-based diffractometer. We use this technique to identify and analyze a two-dimensional array of 90° misfit dislocations at a GaSb/GaAs interface. The full width at half maximum of the GaSb 004 reciprocal lattice point is shown to decrease with increasing GaSb epilayer thickness, as expected from theoretical models. Based on these measurements, the variation in the spatial dislocation frequency is calculated to be 1%.

Short-Wave Infrared GaInAsSb Photodiodes Grown on GaAs Substrate by Interfacial Misfit Array Technique

We report GaInAsSb-based <i>p</i>-<i>i</i>-<i>n</i> photodiodes operating in the 2-2.4-μm wavelength range grown on GaAs (100) substrates using the interfacial misfit (IMF) array technique. A zero-bias dynamic-resistance-area product of 260 Ωcm² and a room temperature peak responsivity of 0.8 A/W (at 2 μm) with an estimated maximum detectivity (D*) of ~3.8×10¹⁰ cm Hz^1/2 W^-1 is obtained in the photodiodes at -0.2 V. These preliminary results of the IMF-based GaInAsSb detectors are comparable to similar detectors grown on native GaSb substrates demonstrating the potential of the IMF array growth mode to realize high-quality Sb-based infrared detectors on GaAs substrates.

Coexistence of type-I and type-II band alignments in antimony-incorporated InAsSb quantum dot nanostructures

Antimony-incorporated InAsSb quantum dots (QDs) are grown by molecular beam epitaxy on GaAs(001) substrates. The QD density increases ∼7 times while the QD height decreases ∼50% due to the increase of QD nucleation sites after Sb incorporation into the GaAs buffer layer and into the InAs QDs. These Sb-incorporated InAsSb QDs show red-shift in the photoluminescence (PL) spectrum and large energy separation between confined energy levels. More interestingly, besides the typical type-I QD transition, an additional peak from the recombination at wetting layer interface develops as the excitation laser intensity increases. This peak clearly exhibits type-II characteristics from the measurement of a large blue-shift of the PL peak and a long PL decay time. Finally, the mechanism of the coexistence of type-I and type-II band alignments is discussed.

Monolithically Integrated III-Sb-Based Laser Diodes Grown on Miscut Si Substrates

We report the formation and growth characteristics of an interfacial misfit (IMF) array between AlSb and Si and its application to III-Sb-based quantum-well broad-area edge-emitting laser diodes monolithically grown on an Si (001) substrate. A 13% lattice mismatch between AlSb and Si is accommodated by using the IMF array. A use of 5deg miscut Si substrates enables simultaneous IMF formation and suppression of an antiphase domain, resulting in a drastic suppression of dislocation density over the III-Sb epilayer and realization of electrically injected laser diodes operating at 77 K. The current-voltage characteristics indicate a diode turn-on of 0.7 V, which is consistent with a theoretical built-in potential of the laser diode. This device is characterized by a 9.1-Omega forward resistance and a leakage current density of 0.7 A/cm2 at -5 V and 46.9 A/cm2 at -15 V.

Band Alignment Tailoring of InAs1-xSbx/GaAs Quantum Dots: Control of Type I to Type II Transition

We report the growth of InAs(1-x)Sb(x) self-assembled quantum dots (QDs) on GaAs (100) by molecular beam epitaxy. The optical properties of the QDs are investigated by photoluminescence (PL) and time-resolved photoluminescence (TRPL). A type I to type II band alignment transition is demonstrated by both power-dependent PL and TRPL in InAs(1-x)Sb(x) QD samples with increased Sb beam flux. Results are compared to an eight-band strain-dependent k x p model incorporating detailed QD structure and alloy composition. The calculations show that the conduction band offset of InAs(1-x)Sb(x)/GaAs can be continuously tuned from 0 to 500 meV and a flat conduction band alignment exists when 60% Sb is incorporated into the QDs. Our study offers the possibility of tailoring the band structure of GaAs based InAsSb QDs and opens up new means for device applications.

Strain-balanced InAs/InAs1−xSbx type-II superlattices grown by molecular beam epitaxy on GaSb substrates

Strain-balanced InAs/InAs1−xSbx type-II superlattices (SLs) on GaSb substrates with 0.27 ≤ x ≤0.33 were grown by molecular beam epitaxy and demonstrated photoluminescence (PL) up to 11.1 μm. The calculated SL bandgap energies agree with the PL peaks to within 5 meV for long-wavelength infrared samples (9.5, 9.9, and 11.1 μm) and to within 9 meV for a mid-wavelength infrared sample (5.9 μm). X-ray diffraction measurements reveal average SL mismatches of less than 0.2%, and the PL full-width-at-half-maximums increase with the mismatch, confirming the importance of strain-balancing for material quality.

Compensation of interfacial states located inside the “buffer-free” GaSb/GaAs (001) heterojunction via δ-doping

We report the compensation of interfacial states formed by interfacial misfit dislocation (IMF) arrays via δ-doping. The IMF arrays are located inside the “buffer-free” heterojunction of GaSb/GaAs (001). The interfacial states are measured using surface photovoltage measurements and are positioned at 0.41, 0.49, and 0.61 eV. A higher reverse bias leakage current (IRB) was observed in the heterogeneous GaSb/GaAs IMF sample (73 μA at −5 V) compared to the homogeneous GaAs control sample (3.9 μA), which does not contain IMF. This increase in IRB is attributed to the interfacial states. Hence, the interfacial states are compensated by δ-doping the GaSb/GaAs interface using Te atoms. A low turn-on voltage of 0.85 V and a very low IRB of 0.1 nA were achieved for the δ-doped sample compared to the control and IMF samples. Hence, for optoelectronic applications, such as lasers, solar cells, and detectors, this compensated IMF technology is useful for integration of buffer-free III-Sb devices on an inexpensive GaA...

Structural properties of InAs/InAs1–xSbx type-II superlattices grown by molecular beam epitaxy

Strain-balanced InAs/InAs1−xSbx type-II superlattices (SLs) have been proposed for possible long-wavelength infrared applications. This paper reports a detailed structural characterization study of InAs/InAs1−xSbx SLs with varied Sb composition grown on GaSb (001) substrates by modulated and conventional molecular beam epitaxy (MBE). X-ray diffraction was used to determine the SL periods and the average composition of the InAs1−xSbx alloy layers. Cross-section transmission electron micrographs revealed the separate In(As)Sb/InAs(Sb) ordered-alloy layers within individual InAs1−xSbx layers for SLs grown by modulated MBE. For the SLs grown by conventional MBE, examination by high-resolution electron microscopy revealed that interfaces for InAs1−xSbx deposited on InAs were more abrupt, relative to InAs deposited on InAs1−xSbx: this feature was attributed to Sb surfactant segregation occurring during the SL growth. Overall, these results establish that strain-balanced SL structures with excellent crystallinit...