Honghyuk Kim

Strain-compensated GaAs1−yPy/GaAs1−zBiz/GaAs1−yPy quantum wells for laser applications

GaAs1−zBiz/GaAs1−yPy strained-compensated quantum well (QW) structures for laser applications were grown by metalorganic vapor phase epitaxy. The band offsets for the GaAs1−zBiz/GaAs1−yPy heterojunction were calculated by the density functional theory, and the design of strain-compensated structures was undertaken by the zero stress analysis. The post-growth thermal annealing of the structures dramatically increases the photoluminescence intensity compared to that from as-grown GaAs1−zBiz QW samples. Transmission electron microscopy studies verified layer thicknesses as well as the presence of abrupt interfaces in the annealed GaAs1−zBiz/GaAs1−yPy QW structure. Electroluminescence measurements from ridge-waveguide devices show broad spectral emission characteristics and lasing was not observed up to a current injection of 4 kA cm−2.

High internal differential efficiency mid-infrared quantum cascade lasers

Implementation of the step-taper active-region (STA) design to 8-9 μm-emitting quantum cascade lasers (QCLs) has resulted in both high T0 and T1 values: 220 K and 665 K, and short lower-level lifetimes: 0.12 ps. In turn, the internal differential efficiency ηid, which is the product of the injection efficiency and the differential laser-transition efficiency, reaches values as high as 86 % for both 8.4 μm- and 8.8 μm-emitting QCLs. Such ηid values are 30-50% higher than those obtained from conventional QCLs emitting in the 7-11 μm wavelength range. Achieving both carrier-leakage suppression and miniband-like carrier extraction in mid-infrared (IR) QCLs leads to ηid values close to the fundamental limit of ~ 90 %. In turn, the currently employed fundamental wallplug-efficiency limits over the mid-IR wavelength range have to be increased by ~ 34 % (e.g., the wallplug-efficiency limit at λ= 4.6 μm increases from 29 % to 39 %). Preliminary results from STA-type 4.8-5.0 μm-emitting QCLs include 1.5 W CW operation, and 77 % internal differential efficiency; that is, 30-50% higher than the ηid values obtained from conventional 4.0-6.5μm-emitting QCLs.

Impact of Sb Incorporation on MOVPE-Grown “Bulk” InGaAs(Sb)N Films for Solar Cell Application

We have investigated the impacts of Sb incorporation on the microstructural, optical, electrical, and carrier dynamics properties of bulk InGaAsSbN films in a comparative study of InGaAsN and InGaAsSbN materials grown by metal-organic vapor phase epitaxy (MOVPE). These films were grown at the relatively high temperature of 600 °C and annealed at 800 °C for 30 min. Transmission electron microscopy studies indicate compositional and structural homogeneity of the InGaAsN and InGaAsSbN films. Low-temperature time-resolved photoluminescence measurements of the MOVPE-grown InGaAsN film show a longer minority carrier lifetime (~40 ns) than observed for the InGaAsSbN film (~26 - 27 ns). In addition, single-junction solar cells with an InGaAsN (InGaAsSbN) base layer exhibit an open-circuit voltage of 0.64 (0.58) V, a short-circuit current of 17.13 (16.89) mA/cm2, a fill factor (FF) of 77.55 (74.29)%, and an efficiency of 8.57 (7.31)%. Sb incorporation in InGaAsN adversely affects solar cell performance due to a reduced minority carrier lifetime correlated with the formation of defects and narrow depletion region width resulting from a higher background carbon impurity level.

Characteristics of OMVPE grown GaAsBi QW lasers and impact of post-growth thermal annealing

Laser diodes employing a strain-compensated GaAs1−xBix/GaAs1−yPy single quantum well (SQW) active region were grown by organometallic vapor phase epitaxy (OMVPE). High resolution x-ray diffraction, room temperature photoluminescence, and real-time optical reflectance measurements during the OMVPE growth were used to find the optimum process window for the growth of the active region material. Systematic post-growth in situ thermal anneals of various lengths were carried out in order to investigate the impacts of thermal annealing on the laser device performance characteristics. While the lowest threshold current density was achieved after the thermal annealing for 30 min at 630 °C, a gradual decrease in the external differential quantum efficiency was observed as the annealing time increases. It was observed that the temperature sensitivities of the threshold current density increase while those of lasing wavelength and slope efficiency remain nearly constant with increasing annealing time. Z-contrast sca...

Carrier dynamics in QW and bulk bismide and epitaxial lift off GaAs-In(Al)GaP double heterostructures grown by MOVPE for multi-junction solar cells

III-V multi-junction solar cells are based on a triple-junction design that consists of an InGaP top junction, a GaAs middle junction, and a bottom junction that employs either a 1eV material grown on the GaAs substrate or InGaAs grown on the Ge substrate. The most promising 1 eV materials under extensive investigation are the bulk dilute nitride such as InGaAsN(Sb) lattice-matched to GaAs substrate and the dilute-bismide quantum well materials, such as GaAsBi, strain-compensated with GaAsP barriers. Both approaches have the potential to achieve high performance triple-junction solar cells. In addition, space satellite applications utilizing III-V triple-junction solar cells can have significantly reduced weight and high efficiency. An attractive approach to achieve these goals is to employ full-wafer epitaxial lift off (ELO) technology, which can eliminate the substrate weight and also enable multiple substrate re-usages. For the present study, we employed time-resolved photoluminescence (TR-PL) techniques to study carrier dynamics in MOVPE-grown bulk dilute bismide double heterostructures (DH). Carrier lifetime measurements are crucial to optimizing MOVPE materials growth. We have studied carrier dynamics in GaAsBi QW structures with GaAsP barriers. Carrier lifetimes were measured from GaAsBi DH samples at different stages of post-growth thermal annealing steps. Post-growth annealing yielded significant improvements in carrier lifetimes. Based on this study, single junction solar cells (SJSC) were grown and annealed under a variety of conditions and characterized. The SJSC annealed at 600 – 650 °C exhibited improved response in EQE spectra. In addition, we studied carrier dynamics in MOVPE-grown GaAs-In(Al)GaP DH samples grown on GaAs substrates. The structures were grown on top of a thin AlAs release layer, which allowed epitaxial layers grown on top of the AlAs layer to be removed from the substrate. The GaAs active layers had various doping densities and thicknesses. Our TR-PL results from both pre- and post-ELO processed GaAs-In(Al)GaP DH samples are reported.

Single junction solar cell employing strain compensated GaAs0.965Bi0.035/GaAs0.75P0.25 multiple quantum wells grown by metal organic vapor phase epitaxy

Single junction solar cells employing 30-period and 50-period GaAs0.965Bi0.035/GaAs0.75P0.25 (Eg ∼ 1.2 eV) multiple quantum wells (MQWs) as base regions were grown by metal organic vapor phase epitaxy. Room temperature photoluminescence measurements indicated a peak spectral emission at 1.18 eV, and the spectral dependence of the external quantum efficiency measured from the fabricated devices shows the extended absorption edge relative to that of GaAs. The fabricated devices with anti-reflection coating employing a 50-period MQW structure exhibit 23% improvement in the conversion efficiency, 4% in the open-circuit voltage, 9% in the short-circuit current density, and 9% in the fill factor, compared to those from the devices employing a 30-period MQW structure in the base region, under AM1.5 direct illumination.Single junction solar cells employing 30-period and 50-period GaAs0.965Bi0.035/GaAs0.75P0.25 (Eg ∼ 1.2 eV) multiple quantum wells (MQWs) as base regions were grown by metal organic vapor phase epitaxy. Room temperature photoluminescence measurements indicated a peak spectral emission at 1.18 eV, and the spectral dependence of the external quantum efficiency measured from the fabricated devices shows the extended absorption edge relative to that of GaAs. The fabricated devices with anti-reflection coating employing a 50-period MQW structure exhibit 23% improvement in the conversion efficiency, 4% in the open-circuit voltage, 9% in the short-circuit current density, and 9% in the fill factor, compared to those from the devices employing a 30-period MQW structure in the base region, under AM1.5 direct illumination.

Strain-compensated Ga(AsP)/Ga(AsBi)/Ga(AsP) quantum-well active-region lasers (Conference Presentation)

Ga(AsBi) quantum well (QW) active regions are an alternate to dilute-nitride QWs for achieving lasers in the telecom wavelength regions (λ~1.3-1.55μm) on GaAs substrates. Ludewig et al first reported the successful operation of Ga(AsBi) single quantum well laser in 2013 [1] with low threshold current densities, Jth=1.56kA/cm2 where (AlGa)As was used as a barrier material for low Bi-content QWs to improve the electron confinement in the conduction band and reduce thermally activated carrier leakage from the QW. We implement here the use of tensile-strained Ga(AsP) as a QW barrier material, providing carrier confinement as well as potential for strain-balancing. Laser structures employing a single GaAs0.976Bi0.024 quantum well (SQW) with either GaAs0.8P0.2, Al0.15Ga0.85As, or GaAs barrier materials were grown by MOVPE on a nominally singular (001) GaAs substrate Ridge waveguide lasers, 25μm-wide and 1mm-long ridge, were fabricated and characterized under pulsed current conditions. The threshold current densities for devices are 5.9kA/cm2 and 5.8kA/cm2 for GaAsP barriers and Al0.15Ga0.85As barriers respectively, with a lasing wavelength of 960nm. Devices with GaAs barriers only lased at higher currents for a short wavelength transition ~900nm. While threshold currents are relatively high, no post growth thermal annealing was performed on these laser materials. Thermal annealing studies will be presented indicating significant improvement in QW luminescence and reduction in Jth can be achieved after the post-growth in-situ annealing. [1] Ludewig, P., Knaub, N., Hossain, N., Reinhard, S., Nattermann, L., Marko, I. P., and Volz, K. 2013. Appl. Phys. Lett., 102(24), 242115.

Time-resolved PL and TEM studies of MOVPE-grown bulk dilute nitride and bismide quantum well heterostructure

Among several approaches proposed to achieve high-efficiency III-V multi-junction solar cells, the most promising approach is to incorporate a bottom junction consisting of a 1 – 1.25 eV material. In particular, several research groups have studied MBE- and MOVPE-grown 1 – 1.25 eV bulk (In)GaAsN(Sb) dilute nitride lattice matched to GaAs substrates, but it is a challenge to grow dilute nitrides without introducing a number of localized states or defects. Localized states originating from random distributions of nitrogen sites in dilute nitrides behave as highly efficient traps, leading to short minority carrier lifetimes. As our group previously reported, carrier dynamics studies are indispensable in the optimization of dilute nitride materials growth to achieve improved solar cell performance. Also, bismide QW heterostructures have recently received a great deal of attention for applications in solar cells and semiconductor lasers because theoretical studies have predicted reduction in nonradiative recombination in Bicontaining materials. For the present study, we employed time-resolved photoluminescence (TR-PL) techniques to study carrier dynamics in MOVPE-grown bulk (In)GaAsN(Sb) materials nominally lattice matched to GaAs substrates. Compared to our previous samples, our present samples grown using different metalorganic precursors at higher growth temperatures showed a significantly less background C doping density. Carrier lifetimes were measured from such dilute nitride samples with low C doping density at various temperatures between 10K and RT. We also performed preliminary TR-PL measurements on MOVPE-grown bismide QW heterostructures at low temperatures. Carrier lifetimes were measured from as-grown and annealed bismide QW structures consisting of GaAsBi(P) wells and GaAsP barriers. Lastly, TEM cross sections were prepared from both dilute nitride and bismide samples for defect and composition analysis using a high resolution TEM.