Phillip P. Jenkins

Measurement of the settling rate of atmospheric dust on Mars by the MAE instrument on Mars Pathfinder

The Materials Adherence Experiment (MAE) on Pathfinder was designed to measure the rate of dust settling from the Martian atmosphere onto the solar array of the Mars Pathfinder Sojourner Rover. The MAE measurements indicate steady dust accumulation at a rate of about 0.28% per day. This value is consistent with the performance of the lander solar arrays, which decreased in power at an estimated rate of 0.29% per day over the same period.

Gallium arsenide transistors: realization through a molecularly designed insulator.

A GaAs-based transistor, analogous to commercial silicon devices, has been fabricated with vapor-deposited cubic GaS as the insulator material. The n-channel, depletion mode, GaAs field-effect transistor shows, in addition to classical transistor characteristics, a channel mobility of 4665.6 square centimeters per volt per second, an interfacial trap density of 1011 per electron volt per square centimeter, and a transconductance of 7 millisiemens for a 5-micrometer gate length at a gate voltage of 8 volts. Furthermore, the GaAs transistor shows an on-to-off resistance ratio comparable to that of commercial devices.

Enhancement of photoluminescence intensity of GaAs with cubic GaS chemical vapor deposited using a structurally designed single-source precursor

A two order‐of‐magnitude enhancement of photoluminescence intensity relative to untreated GaAs has been observed for GaAs surfaces coated with chemical vapor‐deposited GaS. The increase in photoluminescence intensity can be viewed as an effective reduction in surface recombination velocity and/or band bending. The gallium cluster [(t‐Bu)GaS]4 was used as a single‐source precursor for the deposition of GaS thin films. The cubane core of the structurally characterized precursor is retained in the deposited film producing a cubic phase. Furthermore, a near‐epitaxial growth is observed for the GaS passivating layer. Films were characterized by transmission electron microscopy, x‐ray powder diffraction, and x‐ray photoelectron and Rutherford backscattering spectroscopies.

Rare Earth Doped High Temperature Ceramic Selective Emitters

As a result of their electron structure, rare earth ions in crystals at high temperature emit radiation in several narrow bands rather than in a continuous blackbody manner. This study develops a spectral emittance model for films of rare earth containing materials. Although there are several possible rare earth doped high temperature materials, this study was confined to rare earth aluminum garnets. Good agreement between experimental and theoretical spectral emittances was found for erbium, thulium and erbium-holmium aluminum garnets. Spectral emittances of these films are sensitive to temperature differences across the film. Emitter efficiency is also a sensitive function of temperature. For thulium aluminum garnet the efficiency is 0.38 at 1700 K but only 0.19 at 1262 K.

A Single-Event-Hardened Phase-Locked Loop Fabricated in 130 nm CMOS

A radiation-hardened-by-design phase-locked loop (PLL)-designed and fabricated in 130 nm CMOS-is shown to mitigate single-event transients (SETs). Two-photon-absorption (TPA) laser tests were used to characterize the error signatures of the PLL and to perform single-event upset (SEU) mapping of the PLL sub-components. Results show that a custom, voltage-based charge pump reduces the error response of the PLL over conventional designs by more than two orders of magnitude as measured by the number of erroneous PLL clock pulses following a single-event. Additionally, SEU mapping indicates a 99% reduction in the vulnerable area of the radiation-hardened-by-design (RHBD) charge pump over a conventional design. Furthermore, the TPA experiments highlight the importance of the voltage-controlled oscillator in the overall SET response of the PLL implementing the RHBD charge pump.

Electronic passivation of n‐ and p‐type GaAs using chemical vapor deposited GaS

We report on the electronic passivation of n‐ and p‐type GaAs using chemical vapor deposited cubic GaS. Au/GaS/GaAs fabricated metal‐insulator‐semiconductor (MIS) structures exhibit classical high‐frequency capacitor versus voltage (C‐V) behavior with well‐defined accumulation and inversion regions. Using high‐ and low‐frequency C‐V, the interface trap densities of ∼1011 eV−1 cm−2 on both n‐ and p‐type GaAs are determined. The electronic condition of GaS/GaAs interface did not show any deterioration after a six week time period.

High efficiency indium gallium arsenide photovoltaic devices for thermophotovoltaic power systems

The development of indium gallium arsenide (Eg=0.75 eV) photovoltaic devices for thermophotovoltaic power generation is described. A device designed for broadband response had an air mass zero efficiency of 11.2 % and an internal quantum yield of over 90% in the range of 800 to 1500 nm. Devices designed for narrow‐band response have also been developed. Both structures are based on a n/p junction which also makes them applicable for integration into indium phosphide based, monolithic, tandem solar cells for solar photovoltaic applications.

Design of an achievable, all lattice-matched multijunction solar cell using InGaAlAsSb

A design for a realistically achievable, multijunction solar cell based on all lattice-matched materials with >50% projected efficiencies under concentration is presented. Using quaternary materials such as InAlAsSb and InGaAlAs at stochiometries lattice-matched to InP substrates, direct bandgaps ranging from 0.74eV up to ∼1.8eV, ideal for solar energy conversion, can be achieved. In addition, multi-quantum well structures are used to reduce the band-gap further to <0.7 eV. A triple-junction (3J) solar cell using these materials is described, and in-depth modeling results are presented showing realistically achievable efficiencies of AM1.5D 500X of η ∼ 53% and AM0 1 Sun of η∼ 37%.

Modeling and analysis of multijunction solar cells

The modeling of high efficiency, multijunction (MJ) solar cells away from the radiative limit is presented. In the model, we quantify the effect of non-radiative recombination by using radiative efficiency as a figure of merit to extract realistic values of performance under different spectral conditions. This approach represents a deviation from the traditional detailed balance approximation, where losses in the device are assumed to occur purely through radiative recombination. For lattice matched multijunction solar cells, the model predicts efficiency values of 37.1% for AM0 conditions and 52.8% under AM1.5D at 1 sun and 500X, respectively. In addition to the theoretical study, we present an experimental approach to achieving these high efficiencies by implementing a lattice matched triple junction (TJ) solar cell grown on InP substrates. The projected efficiencies of this approach are compared to results for the state of the art inverted-metamorphic (IMM) technology. We account for the effect of metamorphic junctions, essential in IMM technology, by employing reduced radiative efficiencies as derived from recent data. We show that high efficiencies, comparable to current GaAs-based MJ technology, can be accomplished without any relaxed layers for growth on InP, and derive the optimum energy gaps, material alloys, and quantum-well structures necessary to realize them.

InGaAs PV device development for TPV power systems

Indium gallium arsenide (InGaAs) photovoltaic devices have been fabricated with bandgaps ranging from 0.75 eV to 0.60 eV on Indium Phosphide (InP) substrates. Reported efficiencies have been as high as 11.2 percent (AMO) for the lattice matched 0.75 eV devices. The 0.75 eV cell demonstrated 14.8 percent efficiency under a 1500 K blackbody with a projected efficiency of 29.3 percent. The lattice mismatched devices (0.66 and 0.60 eV) demonstrated measured efficiencies of 8 percent and 6 percent respectively under similar conditions. Low long wavelength response and high dark currents are responsible for the poor performance of the mismatched devices. Temperature coefficients have been measured and are presented for all of the bandgaps tested.