S. A. Ringel

Deep level defects throughout the bandgap of (010) β-Ga2O3 detected by optically and thermally stimulated defect spectroscopy

Deep level optical spectroscopy (DLOS) and deep level transient spectroscopy (DLTS) measurements performed on Ni/β-Ga2O3 Schottky diodes fabricated on unintentionally doped (010) substrates prepared by edge-defined film-fed growth revealed a rich spectrum of defect states throughout the 4.84 eV bandgap of β-Ga2O3. Five distinct defect states were detected at EC − 0.62 eV, 0.82 eV, 1.00 eV, 2.16 eV, and 4.40 eV. The EC − 0.82 eV and 4.40 eV levels are dominant, with concentrations on the order of 1016 cm−3. The three DLTS-detected traps at EC − 0.62 eV, 0.82 eV, and 1.00 eV are similar to traps reported in Czochralski-grown β-Ga2O3, [K. Irmscher et al., J. Appl. Phys. 110, 063720 (2011)], suggesting possibly common sources. The DLOS-detected states at EC − 2.16 eV and 4.40 eV exhibit significant lattice relaxation effects in their optical transitions associated with strongly bound defects. As a consequence of this study, the Ni/β-Ga2O3 (010) Schottky barrier height was determined to be 1.55 eV, with good c...

Multi-junction III-V photovoltaics on lattice-engineered Si substrates

Dual junction (DJ) In/sub 0.49/Ga/sub 0.51/P/GaAs solar cells were grown on compositionally graded Ge/Se/sub 1-x/Ge/sub x//Si (SiGe), fabricated and characterized. The DJ solar cells exhibited open-circuit voltage (V/sub OC/) values in excess of 2 V for both AM0 and AM1.5 illumination. The high V/sub OC/ values result from maintaining very low defect densities in these highly lattice-mismatched structures by using SiGe graded layers and monolayer-scale control over the III-V/Ge interface formation. Comparisons made with identical cells grown on GaAs substrates reveal that the DJ solar cell on SiGe retained 91% of the V/sub OC/ and 99% of the short circuit current density achieved by the homoepitaxial DJ cell, demonstrating the potential for high efficiency multi-junction solar cells grown on SiGe. In addition, modeling shows that In/sub 0.49/Ga/sub 0.51/P/GaAs DJ cells should be more tolerant of the low residual dislocation densities characteristic of lattice-engineered SiGe substrates than single junction GaAs cells, indicating great promise for achieving a high efficiency III-V multijunction cell technology on Si.

Monolithically integrated thin film III-V/Si solar panel on wafer for active power management

We have demonstrated a monolithically integrated solar panel on Si that allows scaling of cell output voltage on the wafer level. Our design also incorporates integrated bypass diodes and the possible incorporation of CMOS for active power management at the materials integration level. In addition, we have demonstrated the first GaAsP/SiGe dual junction solar cell on Si that provides the ideal bandgaps for the highest efficiency for solar spectra between AM0 and AM1.5. Combined with future, CMOS-based active power management, the solar panel on wafer (SPOW) design enables maximum power output and ideally managed power profiles under non-ideal, and time-varying illumination conditions. The result is improved reliability, lower system cost, and higher specific power over conventional III-V PV array technology.

Impact of annealing and V:III ratio on properties of MBE grown wide-bandgap AlGaInP materials and solar cells

The growth and properties of wide bandgap (Al/sub x/Ga/sub 1-x/)/sub 0.51/In/sub 0.49/P layers and solar cells grown by solid source molecular beam epitaxy were examined to correlate the impact of growth conditions and in-situ annealing on photovoltaic performance. A P/sub 2/:III flux ratio of 12 was found to optimize the optical qualities of Ga/sub 0.51/In/sub 0.49/P epilayers. Ga/sub 0.51/In/sub 0.49/P solar cells were grown and subjected to different in-situ annealing conditions. The effect of annealing on material quality and device performance was characterized through deep level transient spectroscopy (DLTS) and photoluminescence (PL), which revealed a trend in non-radiative recombination. The results suggest that removal of a deep level near E/sub C/ - 0.78 eV in the n-type base is responsible for the observed improvement in current collection seen after anneal. After refinement of the Ga/sub 0.51/In/sub 0.49/P growth, wider bandgap material was investigated for future use in a high temperature/high intensity solar cell. A range of (Al/sub x/Ga/sub 1-x/)/sub 0.51/In/sub 0.49/P materials, with direct bandgaps from 2.09 eV to 2.26 eV have been successfully demonstrated using digital alloying and conventional bulk growth.

1EV GAN x AS 1-x-y SB y material for lattice-matched III–V solar cell implementation on GaAs and Ge

The effect of different arsenic species (As<inf>2</inf> or As<inf>4</inf>) on the quality of molecular beam epitaxy (MBE) grown GaNAsSb materials (samples A and B) and GaAs/ GaNAsSb/GaAs p⁺ n⁻ n⁺ devices (samples C and D) were investigated. The improvement in material quality in sample B, as well as the improvement in diode and solar cell characteristics in sample C, may suggest a successful defect density manipulation using As<inf>2</inf> overpressure for GaNAsSb growth.

Identifying the source of reduced performance in 1-stage-grown Cu(In, Ga)Se 2 solar cells

Traps in Cu(In,Ga)Se2 (CIGS) deposited by listage and 3-stage co-evaporation processes were characterized using conventional deep level transient and optical spectroscopies (DLTS/DLOS) and scanning-DLTS to quantify and map the deep levels associated with the different growth approaches. Two defects were observed at EV + 0.44/0.50 eV and EV + 0.97 eV. The total deep level trap concentration was found to vary primarily due to the EV + 0.97 eV trap, which was ∼4X larger in the 1-stage sample. This large increase correlates with the reduced open circuit voltage and conversion efficiency typically observed in 1-stage co-evaporation grown CIGS, suggesting that controlling the presence and concentration of this level may be important to enable high-efficiency CIGS solar cells using simpler processes.Traps in Cu(In, Ga)Se2 (CIGS) deposited by listage and 3-stage co-evaporation processes were characterized using conventional deep level transient and optical spectroscopies (DLTS/DLOS) and scanning-DLTS to quantity and map the deep levels associated with the different growth approaches. Two defects were observed at EV + 0.44/0.50 eV and EV + 0.97 eV. The total deep level trap concentration was found to vary primarily due to the EV + 0.97 eV trap, which was ~4X larger in the 1-stage sample. This large increase correlates with the reduced open circuit voltage and conversion efficiency typically observed in 1-stage co-evaporation grown CIGS, suggesting that controlling the presence and concentration of this level may be important to enable high-efficiency CIGS solar cells using simpler processes.

Impact of the Ga/In ratio on defects in Cu(In, Ga)Se 2

The impact of the composition of a 3-stage-process deposited CIGS on the deep level spectrum was investigated using deep level transient and optical spectroscopies to probe defects throughout the bandgap to have a complete picture of the defect spectrum. It is shown that the defect spectra depend strongly on the Ga/In ratio with both higher numbers of trap levels and higher trap concentrations (by >65%) on average in the higher Ga content films. These facts can likely explain the reduced CIGS solar cell efficiency at higher Ga content.

80nm In 0.52 Al 0.48 As/In 0.53 Ga 0.47 As/InAs 0.3 P 0.7 Composite channel HEMT with an f T of 280GHz

We have demonstrated a high performance 80nm gate In_0.52Al_0.48As/In_0.53Ga_0.47As/InAs_0.3P_0.7 composite channel HEMT. The device has a g_m as high as 1 S/mm and an f_T 280 GHz. To complete this study, the noise and power performance of such device will be further studied and compared with conventional InGaAs channel devices in near future.