Toby J. Garrod

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.

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.

50% continuous-wave wallplug efficiency from 1.53μm-emitting broad-area diode lasers

Long-wavelength InP-based diode lasers emitting at 1.53 μm have been optimized for maximum continuous-wave (CW) electrical-to-optical power conversion efficiency, so-called wallplug efficiency (WPE). Efficient electron and hole capture into a single-quantum-well (SQW) active region as well as suppression of electron and hole leakage out of the SQW result in high values for the internal differential efficiency: ∼97% for long-cavity (≥2 mm) uncoated-facet devices and ∼85%–89% for short-cavity (1.5 mm) optimized facet-coated devices. The characteristic temperature of the slope efficiency, T1, reaches a high value of 323 K. Doping-level optimization of the p-cladding layer and the use of the SQW result in low values for the internal loss coefficient: ∼1.1 cm−1 for long-cavity (≥2 mm) uncoated-facet devices and ∼1.5–2.0 cm−1 for short-cavity (1.5 mm) optimized facet-coated devices. In turn, a maximum CW WPE value of 50% is achieved at room temperature and ∼1 W output power from conductively-cooled 100 μm-wide-...

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.

High-power and high-efficiency distributed feedback (DFB) lasers operating in the 1.4-1.6 μm range for eye-safe applications

Incorporating monolithic distributed feedback (DFB) gratings into broad-area (BA) diode lasers results in ten times narrower spectral width and four-to-five times lower thermal shift in emission wavelength. We report on our progress to obtaining a high-power, high-efficiency DFB diode pump in the 1.4-1.6 μm wavelength range for use in industrial and military, eye-safe applications. Results for Fabry-Perot diode lasers emitting at 1530 nm are also discussed. We report on an index-guided, single-emitter design (0.15 x 0.01 cm2) capable of producing 2.5 W of continuous-wave output power at room-temperature with a peak power conversion efficiency of 50%.

High-power and high-efficiency broad-area diode laser emitting at 1.5 μm

Long-wavelength diode lasers, emitting at 1.5x μm, have been optimized for maximum continuous-wave (CW) electroto- optical power conversion efficiency (PCE) and output power. A maximum CW PCE value of 50% is achieved at room-temperature from a 0.10 x 1.5 mm2 diode laser with a CW output power of 2.5 W from a laser structure with a single-quantum-well (SQW) active region. Reliability tests show no degradation when run at 5A, 40°C for < 4000 hours of operation.

Properties of Dilute-Nitride-Antimonide Materials Grown by Metalorganic Vapor Phase Epitaxy (MOVPE) for Solar Cell Application

Bulk dilute-nitride-antimonide materials are grown by MOVPE nominally lattice-matched to the GaAs substrate with bandgap energies in the 1-1.25eV range. Single-junction solar cells demonstrate a peak efficiency of 7.22% with high open circuit voltages (Voc=0.72V).

High-power surface emitting distributed feedback (SE-DFB) lasers

High-power SE-DFB lasers have intrinsically narrow, wavelength stabilized spectra and high spatial brightness as a result of monolithic curved gratings. Performance results of high-power SE-DFB lasers in the 9xx - 15xx nm range are discussed.

Two-dimensional conduction-band engineering for performance optimization of quantum cascade lasers

This paper presents the deep-well quantum cascade laser (DW QCL) structures of novel active-region designs for virtually suppressing carrier leakage as well as for further increasing the CW η_wp,max value. Preliminary results are T₁ values as high as 797 K over the 20-60^oC range in heatsink temperature.

Long wavelength surface-emitting distributed feedback (SE-DFB) laser for range finding applications

We report on our progress to obtaining a 1540 nm SE-DFB laser with curved, second order grating capable of producing extremely high peak power for eye-safe range finding applications. Higher peak power can be extracted from SE-DFB lasers since the emission area is three orders of magnitude larger than for edge emitters. Ten-times narrower spectral width and four-times lower thermal shift of emission wavelength allows for the use of narrower filters on the range finder detector, providing a higher signal-to-noise ratio. Furthermore, a curved second-order diffraction grating in our SE-DFB laser allows for a higher brightness source, which makes transmitter optics geometry more compact and simpler.