D.A. Shiau

Monolithically series-interconnected GaInAsSb/AlGaAsSb/GaSb thermophotovoltaic devices with an internal backsurface reflector formed by wafer bonding

GaInAsSb/AlGaAsSb/GaSb thermophotovoltaic (TPV) cells were monolithically interconnected in series to build open-circuit voltage Voc. GaInAsSb epitaxial layers were transferred to GaAs by wafer bonding with SiOx/Ti/Au, which provides electrical isolation of individual cells and forms an internal backsurface reflector. This configuration is compatible with monolithic series interconnection of TPV cells; can mitigate the requirements of filters used for front-surface spectral control; and has the potential to improve TPV device performance. Wafer-bonded GaInAsSb TPV cells exhibit nearly linear voltage building. At a short-circuit current density of 0.4 A/cm2, Voc of a single TPV cell is 0.2 V, compared to 0.37 and 1.8 V for 2- and 10-junction series-interconnected TPV cells, respectively.

Analysis of Recombination Processes in 0.5–0.6 eV Epitaxial GaInAsSb Lattice‐matched to GaSb

This work summarizes recent data on minority carrier lifetime in n‐ and p‐type double heterostructures (DHs) of 0.5–0.6 eV GaInAsSb confined with GaSb and AlGaAsSb cap layers. Recombination times were measured by time‐resolved photoluminescence (TRPL) and by optical frequency response (OFR) to sinusoidal excitation. It was shown that one of the mechanisms responsible for interface recombination in GaSb/GaInAsSb/GaSb DHs is thermionic emission of carriers over the heterobarrier. Considerable improvement of carrier confinement was obtained with 1 eV AlGaAsSb cap layers. Optimization of the epitaxial growth resulted in a recombination velocity at GaInAsSb/AlGaAsSb interface as low as 30 cm/s.

Extremely low surface recombination velocity in GaInAsSb∕AlGaAsSb heterostructures

Low surface recombination velocity is critical to the performance of minority carrier devices. Minority carrier lifetime in double heterostructures (DHs) of 0.53-eV p-GaInAsSb confined with 1.0-eV p-AlGaAsSb, and grown lattice-matched to GaSb, was measured by time-resolved photoluminescence. The structures were designed to be dominated by the heterointerface while minimizing the contribution of photon recycling to minority carrier lifetime. Surface recombination velocity as low as 30cm∕s for DHs was achieved. This value is over an order of magnitude lower than that reported in previous studies.

Ohmic contacts to n-type GaSb and n-type GaInAsSb

An investigation with the objective of improving n-type ohmic contacts to GaSb-based devices is described. This study involves a series of n-GaInAsSb and n-GaSb samples with varying doping, grown on semi-insulating (SI) GaAs substrates. These samples were fabricated into mesa-etched, transfer-length method (TLM) structures, and the specific-contact resistivity and sheet resistance of these layers as a function of majority electron concentration were measured. Extremely low specific-contact resistivities of about 2 × 10−6 Ω-cm2 and sheet resistances of about 4 Ω/▭ are found for n-type GaInAsSb doped at about 3 × 1018 cm−3.

Wafer‐Bonded Internal Back‐Surface Reflectors for Enhanced TPV Performance

This paper discusses recent efforts to realize GaInAsSb/GaSb TPV cells with an internal back‐surface reflector (BSR). The cells are fabricated by wafer‐bonding GaInAsSb/GaSb device layers to GaAs substrates with a dielectric/Au reflector, removing the GaSb substrate, and subsequently processing the layers using standard photolithographic techniques. The internal BSR enhances optical absorption within the device, while the dielectric layer provides electrical isolation. This approach is compatible with monolithic integration of series‐connected TPV cells and can mitigate the requirements of filters used for front‐surface spectral control.

Monolithic Series‐Interconnected GaInAsSb/AlGaAsSb Thermophotovoltaic Devices Wafer Bonded to GaAs

GaInAsSb/AlGaAsSb/GaSb epitaxial layers were wafer bonded to semi‐insulating GaAs wafers for monolithic series interconnection of thermophotovoltaic (TPV) devices. SiOx/Ti/Au was used as a bonding layer to provide electrical isolation and to serve as an internal back‐surface reflector (BSR). The minority‐carrier lifetime in WB BSR structures is more than two times longer than that of control structures without a BSR. WB GaInAsSb/AlGaAsSb TPV cells were fabricated and monolithically interconnected in series. These cells exhibit nearly linear voltage building. At a short‐circuit current density of 0.4 A/cm2, Voc of a single TPV cell is 0.2 V, compared to 0.37 and 1.8 V for 2‐ and 10‐junction series‐interconnected TPV cells, respectively.

Effect of growth interruption on surface recombination velocity in GaInAsSb/AlGaAsSb heterostructures grown by organometallic vapor-phase epitaxy

The effects of growth interruption on the quality of GaInAsSb/AlGaAsSb heterostructures grown by organometallic vapor phase epitaxy are reported. In-situ reflectance monitoring and ex-situ characterization by high-resolution x-ray diffraction, 4K photoluminescence (PL), and time-resolved PL indicate that GaInAsSb is extremely sensitive to growth interruption time as well as the ambient atmosphere during interruption. By optimizing the interruption sequence, surface recombination velocity as low as 20 cm/s was achieved for GaInAsSb/AlGaAsSb double heterostructures.

Non-Contact Determination of Free Carrier Concentration in n-GaInAsSb

GaSb-based semiconductors are of interest for mid-infrared optoelectronic and high-speed electronic devices. Accurate determination of electrical properties is essential for optimizing the performance of these devices. However, electrical characterization of these semiconductors is not straightforward since semi-insulating (SI) GaSb substrates for Hall measurements are not available. In this work, the capability of Raman spectroscopy for determination of the majority carrier concentration in n-GaInAsSb epilayers was investigated. Raman spectroscopy offers the advantage of being non-contact and spatially resolved. Furthermore, the type of substrate used for the epilayer does not affect the measurement. However, for antimonide-based materials, traditionally employed Raman laser sources and detectors are not optimized for the analysis wavelength range dictated by the narrow band gap of these materials. Therefore, a near-infrared Raman spectroscopic system, optimized for antimonide-based materials, was developed. Ga{sub 0.85}In{sub 0.15}As{sub 0.13}Sb{sub 0.87} epilayers were grown by organometallic vapor phase epitaxy with doping levels in the range 2 to 80 x 10{sup 17} cm{sup -3}, as measured by secondary ion mass spectrometry. For a particular nominal doping level, epilayers were grown both lattice matched to n-GaSb substrates and lattice-mismatched to SI GaAs substrates under nominally identical conditions. Single magnetic field Hall measurements were performed on the epilayers grown on SI GaAs substrates, while Raman spectroscopy was used to measure the carrier concentration of epilayers grown on GaSb and the corresponding SI GaAs substrates. Compared to Hall measurements, Raman spectra indicated that the GaInAs/Sb epilayers grown on GaSb substrates have higher free carrier concentrations than the corresponding epilayers grown on SI GaAs substrates under nominally identical conditions. This is contrary to the assumption that for nominally identical growth conditions, the resulting carrier concentration is independent of substrate, and possible mechanisms will be discussed.

Wafer bonding and epitaxial transfer of GaSb-based epitaxy to GaAs for monolithic interconnection of thermophotovoltaic devices

GaInAsSb/AlGaAsSb/InAsSb/GaSb epitaxial layers were bonded to semi-insulating GaAs handle wafers with SiO{sub x}/Ti/Au as the adhesion layer for monolithic interconnection of thermophotovoltaic (TPV) devices. Epitaxial transfer was completed by removal of the GaSb substrate, GaSb buffer, and InAsSb etch-stop layer by selective chemical etching. The SiO{sub x}/TiAu provides not only electrical isolation, but also high reflectivity and is used as an internal back-surface reflector. Characterization of wafer-bonded epitaxy by high-resolution x-ray diffraction and time-decay photoluminescence indicates minimal residual stress and enhancement in optical quality. 0.54-eV GaInAsSb cells were fabricated and monolithically interconnected in series. A 10-junction device exhibited linear voltage building with an open-circuit voltage of 1.8 V.

Ohmic Contacts to n-type GaSb and n-type GaInAsSb

An investigation with the objective of improving n-type ohmic contacts to GaSb-based devices is described. This study involves a series of n-GaInAsSb and n-GaSb samples with varying doping, grown on both n-GaSb and semi-insulating GaAs substrates. These samples were fabricated into mesa-etched TLM structures, and the specific contact resistivity and sheet resistance of these layers as a function of majority electron concentration were measured. Extremely low specific contact resistivities of about 2 x 10{sup -6} {Omega}-cm{sup 2} and sheet resistances of about 4 {Omega}/{open_square} are found for n-type GaInAsSb doped at about 3 x 10{sup 18} cm{sup -3}.