Yongkun Sin

Narrow band gap (1 eV) InGaAsSbN solar cells grown by metalorganic vapor phase epitaxy

Heterojunction solar cell structures employing InGaAsSbN (Eg ∼ 1 eV) base regions are grown lattice-matched to GaAs substrates using metalorganic vapor phase epitaxy. Room temperature (RT) photoluminescence (PL) measurements indicate a peak spectral emission at 1.04 eV and carrier lifetimes of 471–576 ps are measured at RT from these structures using time-resolved PL techniques. Fabricated devices without anti-reflection coating demonstrate a peak efficiency of 4.58% under AM1.5 direct illumination. Solar cells with a 250 nm-thick InGaAsSbN base layer exhibit a 17% improvement in open circuit voltage (Voc), 14% improvement in fill factor, and 12% improvement in efficiency over the cells with a thicker (500 nm-thick) base layer.

Catastrophic Facet and Bulk Degradation in High Power Multi-Mode InGaAs Strained Quantum Well Single Emitters

Extensive investigations by a number of groups have identified catastrophic sudden degradation as the main failure mode in both single-mode and multi-mode InGaAs-AlGaAs strained quantum well (QW) lasers. Significant progress made in performance characteristics of broad-area InGaAs strained QW single emitters in recent years has led to an optical output power of over 20W and a power conversion efficiency of over 70% under CW operation. However, unlike 980nm single-mode lasers that have shown high reliability operation under a high optical power density of ~50MW/cm2, broad-area lasers have not achieved the same level of reliability even under a much lower optical power density of ~5MW/cm2. This paper investigates possible mechanisms that prevent broad-area lasers from achieving high reliability operation by performing accelerated lifetests of these devices and in-depth failure mode analyses of degraded devices with various destructive and non-destructive techniques including EBIC, FIB, and HR-TEM techniques. The diode lasers that we have investigated are commercial MOCVD-grown broad-area strained InGaAs single QW lasers at ~975nm. Both passivated and unpassivated broad-area lasers were studied that yielded catastrophic failures at the front facet and also in the bulk. To investigate the role that generation and propagation of defects plays in degradation processes via recombination enhanced defect reaction (REDR), EBIC was employed to study dark line defects in degraded lasers, failed under different stress conditions, and the correlation between DLDs and stress levels is reported. FIB was then employed to prepare TEM samples from the DLD areas for cross-sectional HR-TEM analysis.

Root cause investigation of catastrophic degradation in high power multi-mode InGaAs-AlGaAs strained quantum well lasers

Optimization of broad-area InGaAs-AlGaAs strained-quantum-well lasers has led to successful demonstration of high power and high efficient operation for industrial applications. State-of-the-art broad-area single emitters show an optical output power of over 20W and a power conversion efficiency of over 70% under CW operation. However, understanding of long-term reliability and degradation processes of these devices is still poor. This paper investigates the root causes of catastrophic degradation in broad-area lasers by performing accelerated lifetests of these devices and failure mode analyses of degraded devices using various techniques. We investigated MOCVDgrown broad-area strained InGaAs-AlGaAs single QW lasers at ~975nm. Our study included both passivated and unpassivated broad-area lasers that yielded catastrophic failures at the facet and also in the bulk. Our accelerated lifetests generated failures at different stages of degradation by forcing them to reach a preset drop in optical output power. Deep-level-transient-spectroscopy (DLTS) was employed to study deep traps in degraded devices. Trap densities and capture cross-sections were estimated from a series of degraded devices to understand the role that point defects and extended defects play in degradation processes via recombination enhanced defect reaction. Electron-beam-induced-current (EBIC) was employed to find correlation between dark line defects in degraded lasers and test stress conditions. Time-resolved electroluminescence (EL) was employed to study formation and progression of dark spots and dark lines in real time to understand mechanisms leading to catastrophic facet and bulk degradation. Lastly, we present our physics-of-failure-based model of catastrophic degradation processes in these broad-area lasers.

Impact of thermal annealing on bulk InGaAsSbN materials grown by metalorganic vapor phase epitaxy

Two different thermal annealing techniques (rapid thermal annealing (RTA) and in-situ post-growth annealing in the metalorganic vapor phase epitaxy (MOVPE) chamber) were employed to investigate their impact on the optical characteristics of double-heterostructures (DH) of InGaAsSbN/GaAs and on the performance of single-junction solar cell structures, all grown by MOVPE. We find that an optimized RTA procedure leads to a similar improvement in the photoluminescence (PL) intensity compared with material employing a multi-step optimized anneal within the MOVPE reactor. Time-resolved photoluminescence techniques at low temperature (LT) and room temperature (RT) were performed to characterize the carrier dynamics in bulk InGaAsSbN layers. Room temperature carrier lifetimes were found to be similar for both annealing methods, although the LT-PL (16 K) measurements of the MOVPE-annealed sample found longer lifetimes than the RTA-annealed sample (680 ps vs. 260 ps) for the PL measurement energy of 1.24 eV. InGaAsSbN-based single junction solar cells processed with the optimized RTA procedure exhibited an enhancement of the electrical performance, such as improvements in open circuit voltage, short circuit current, fill factor, and efficiency over solar cells subjected to the in-situ MOVPE annealing technique.

Carrier dynamics in MOVPE-grown bulk dilute nitride materials for multi-junction solar cells

Dilute nitride materials with a 1eV band-gap lattice matched to GaAs substrates are attractive for high-efficiency multi-junction solar cells. Carrier lifetime measurements are crucial in optimizing material growth and p-i-n field-aided carrier-extraction-device design. One research group has reported carrier lifetimes of MBE-grown bulk InGaNAsSb materials, but there has been no report of carrier lifetime measurements from bulk InGaNAsSb grown by MOVPE. In this study, we report the growth of bulk InGaNAsSb by MOVPE and the first carrier lifetime measurement from MOVPE-grown bulk InGaNAsSb materials with Eg= 1.0 - 1.2eV at 300K. We studied carrier dynamics in MOVPE-grown bulk dilute nitride materials nominally lattice matched to GaAs (100) substrates: 1μm thick In0.035GaN0.025As (Eg= 1.0eV at 300K) and ~0.2μm thick In(0.05-0.07)GaN(0.01-0.02)AsSb(0.02-0.06) layers (Eg= 1.2eV at 300K). Both structures are fully strained. The incorporation of N in InGaNAs leads to degradation in photoluminescence efficiency, but prior studies indicate the addition of Sb in MBE-grown InGaNAsSb improved the PL efficiency. Two-step post-growth thermal annealing processes were optimized to obtain maximum PL efficiencies that yielded a typical blue shift of 50 and 30meV for InGaNAs and InGaNAsSb, respectively. We employed a streak camera to measure carrier lifetimes from both as-grown and thermally annealed samples. Carrier lifetimes of <30psec were obtained from the InGaNAs samples, whereas carrier lifetimes of up to ~150psec were obtained from the InGaNAsSb samples. We discuss possible reasons for short carrier lifetimes measured from MOVPE-grown InGaNAs(Sb) materials.

Physics of failure investigation in high-power broad-area InGaAs-AlGaAs strained quantum well lasers

Continued improvements in broad-area InGaAs-AlGaAs strained quantum well (QW) lasers have led to unprecedented performance characteristics in these lasers including optical output powers of over 20 W and power conversion efficiencies of over 70% under CW operation. Catastrophic optical mirror damage (COMD) is responsible for failures in (Al)GaAs QW lasers, but InGaAs-AlGaAs strained QW lasers with optimized facet passivation predominantly fail by catastrophic optical bulk damage (COBD). Since COBD is relatively a new failure type, it requires physics of failure investigation to understand its root causes and then develop COBD-free lasers for high reliability applications including potential satellite systems. We recently proposed a model for degradation mechanism responsible the COBD process and this paper further investigates the root causes of COBD in the lasers using various failure mode analysis techniques. We investigated reliability and degradation mechanism in MOCVD-grown broad-area InGaAs-AlGaAs strained QW single emitters. During entire accelerated life-tests of the lasers we studied, time resolved electroluminescence (TR-EL) techniques were employed to observe formation of a hot spot and subsequent formation and progression of dark spots and dark lines through windowed n-contacts.

Catastrophic optical bulk damage (COBD) in high power multi-mode InGaAs-AlGaAs strained quantum well lasers

State-of-the-art broad-area InGaAs-AlGaAs strained quantum well (QW) lasers show an optical output power of over 20 W and a power conversion efficiency of over 70% under CW operation. Unlike broad-area (Al)GaAs QW lasers, broad-area InGaAs strained QW lasers show two failure types including facet catastrophic optical damage (COD) and bulk failure. Optimization of facet passivation processes has led to significant reduction in occurrence of facet COD (or COMD), but bulk failure (or COBD) has received little attention although it is crucial to understand degradation processes responsible for COBD and then develop COBD-free lasers for high reliability applications including potential satellite systems. Our group recently proposed a model for the COBD process and this paper further investigates the root causes of COBD in the broad-area lasers. We performed accelerated life-tests of MOCVD-grown broad-area strained InGaAs-AlGaAs single QW lasers at ~975 nm, which predominantly yielded catastrophic bulk failures. We employed various non-destructive techniques to study pre- and post-stressed lasers. Time resolved electroluminescence (TR-EL) was employed to observe formation and progression of dark spots and dark lines through windowed n-contacts during entire life-tests that eventually led to COBD. Deep level transient spectroscopy (DLTS) was employed to investigate trap characteristics in degraded devices at different stages of degradation to study the role that non-radiative recombination centers (NRCs) play in COBD processes. Time resolved photoluminescence (TR-PL) was employed to measure carrier lifetimes from both undamaged and damaged active areas to find correlation between dark line defects in degraded lasers and non-radiative recombination processes.

Characteristics of bulk InGaAsN and InGaAsSbN materials grown by metal organic vapor phase epitaxy (MOVPE) for solar cell application

Bulk, lattice-matched InGaAsSbN material has been grown by metal organic vapor phase epitaxy (MOVPE) for applications in concentrated multi-junction solar cells. By optimizing the growth conditions for high Sb and As partial pressures, we achieved background hole concentrations as low as 2 x 1018 cm-3. After thermal annealing, the background hole concentration increased from 2x1018 to 2 x 1019 cm-3, although PL intensity increased by a factor of 7. We recently grew single junction (1eV) solar cells incorporating dilute-nitride materials and devices were fabricated and characterized for solar cell application. Performance characteristics of these cells without anti-reflection coating included the efficiency of 4.25% under the AM1.5 (air mass) direct illumination, Voc of 0.7 V, and a spectral response extended to longer wavelength compared with GaAs cells.

Single Event Transients Induced by Picosecond Pulsed X-Ray Absorption in III–V Heterojunction Transistors

We perform measurements which show that focused, picosecond pulses of x-rays can be used to generate single event transients (SET) in a GaAs heterostructure field effect transistor (HFET) and a GaN high electron mobility transistor. X-ray pulses with photon energies of 8, 10 and 12 keV from the Advanced Photon Source at Argonne National Laboratory were used to excite transients. SETs are observed when x-ray pulses are incident upon metal layers above sensitive areas on the transistors. We use focused ion beam (FIB) cross-sectioning and scanning transmission electron microscopy energy dispersive x-ray spectroscopy (STEM-EDXS) to determine the compositional structure of the devices. We present a first order analysis of energy deposition in the devices and correlate it to the transient response to make preliminary interpretations of the results. We compare the x-ray transients from the GaAs HFET with transients generated by 750 nm and 870 nm femtosecond laser pulses. We also present results on the total dose susceptibility of the GaN HEMTs.

A Study of Degradation in High Power Multi-Mode InGaAs-AlGaAs Strained Quantum Well Lasers as Pump Lasers

Degradation processes in high power broad-area InGaAs-AlGaAs strained quantum well lasers were studied using electron beam-induced current (EBIC) techniques, time-resolved electroluminescence (TR-EL) techniques, and deep-level transient spectroscopy (DLTS). Accelerated lifetests of the broad-area lasers yielded catastrophic failures at the front facet and also in the bulk. EBIC was employed to study dark line defects generated in degraded lasers stressed under different test conditions. TR-EL was employed to study the intra-cavity intensity distribution in real time as devices were aged. DLTS was employed to study deep electron traps in both pristine and degraded laser diodes. Lastly, we present a possible scenario for the initiation of bulk degradation in the broad-area lasers.