State of the art InGaP2/GaAs/In0.28Ga0.72As inverted metamorphic (IMM) solar cells have achieved impressive results, however, the thick metamorphic buffer needed between the lattice matched GaAs and lattice mismatched InGaAs requires significant effort and time to grow and retains a fairly high defect density. One approach to this problem is to replace the bottom InGaAs junction with an Sb-based material such as 0.73 eV GaSb or ~1.0 eV Al0.2Ga0.8Sb. By using interfacial misfit (IMF) arrays, the high degree of strain (7.8%) between GaAs and GaSb can be relaxed solely by laterally propagating 90° misfit dislocations that are confined to the GaAs-GaSb interface layer.
We have used molecular beam epitaxy to grow GaSb single junction solar cells homoepitaxially on GaSb and heteroepitaxially on GaAs using IMF. Under 15-sun AM1.5 illumination, the control cell achieved 5% efficiency with a WOC of 366 mV, while the IMF cell was able to reach 2.1% with WOC of 546 mV. Shunting and high non-radiative dark current were main cause of FF and efficiency loss in the IMF devices. Threading dislocations or point defects were the expected source behind the losses, leading to minority carrier lifetimes less than 1ns. Deep level transient spectroscopy (DLTS) was used to search for defects electrically and two traps were found in IMF material that were not detected in the homoepitaxial GaSb device. One of these traps had a trap density of 7 × 1015 cm-3, about one order of magnitude higher than the control cell defect at 4 × 1016 cm-3.
Understanding and quantifying nonradiative recombination is a critical factor for the successful laser cooling of semiconductors. The usual approach to measuring the nonradiative lifetime employs pulsed photoexcitation and monitors the luminescence decay via time-resolved photon counting. We present an alternative approach that
employs phase fluorometry with a lock-in amplifier. A sinusoidally modulated diode laser is used for excitation. Lifetime data are extracted from the frequency dependent phase shift and amplitude response of the photolumi-nescence signal, detected by a photomultiplier tube. Samples studied include high quality AlGaAs/GaAs/AlGaAs and GaInP/GaAs/GaInP double heterostructures, grown by MBE and MOCVD. Data over a temperature range from 10 to 300 K is compared with results obtained in time-domain measurements.
We present the growth and characterization of high quality semiconductor laser cooling material. The structure consists of GaAs passivated by InGaP which has been reported to have a longest surface recombination lifetime. GaAs was grown on 10 degree miscut GaAs substrate and sandwiched between lattice matched In<sub>0.49</sub>Ga<sub>0.51</sub>P. This structure was grown by a low-pressure metal organic chemical vapor deposition (MOCVD) system. The material was grown in the temperature range of 550 to 700 °C at 60.8 Torr. The morphology of InGaP was improved by the growth on 10 degree miscut substrate along <110> direction, which is confirmed by X-ray diffraction (XRD). The uninterrupted growth technique and GaP separation layer are employed to prevent the indium segregation and P/As intermixing at the interface between InGaP and GaAs. The effects of V/III ratio, growth temperature and material precursors on material impurities were also studied. The carrier lifetimes were measured using the time resolved photoluminescence (TRPL) technique at cryogenic temperatures. The experimental results show that the carrier lifetime was increased by 5 times with the use of TBA as arsenic source in place of AsH<sub>3</sub>. Recent results show a highest room temperature carrier lifetime of 2 &mgr;sec.