Paper Session I-B - Characterizing Space-Grown Degenerate Narrow Gap Semiconductors by Scanning Tunneling Optical Spectroscopy

We consider the II-VI narrow gap semiconducting alloys Hg(1-x)Cd(x)Te, Hg(1-x)Zn(x)Te, Hg(1-x)Zn(x)Se, for which empirical equations exist that give each alloy’s forbidden energy band gap Eg(x) as a function of its stoichiometry as characterized by the value x . These materials are important to NASA for two reasons. They are useful for making infrared detectors, and they are best grown in microgravity to optimize their uniformity. The equations can be inverted to yield the stoichiometry parameter x provided that the value of Eg can be determined experimentally, for example, by optical absorption measurements. We have investigated an alternative method, which should yield appreciably better spatial resolution, in which scanning tunneling optical spectroscopy (STOS) is used to measure the enhancement of the current that is due to photoexcitation of carriers at the tunneling junction in an STM. We present a simplified working model for low temperature calculations of STOS. Our major conclusions are: (a) for the degenerate case, knowledge of ND - NA (donor density minus the acceptor density) can be used to deduce the true band gap from the apparent band gap, (b) the low temperature tunneling current may have a sharper onset, depending on the diffusion length, at the band gap than does the optical absorption, and (c) our simplified formulation allows for quick, straightforward evaluation of many different cases and is in essential agreement with more detailed analysis

Silicon Carbide Solar Cells Investigated

The semiconductor silicon carbide (SiC) has long been known for its outstanding resistance to harsh environments (e.g., thermal stability, radiation resistance, and dielectric strength). However, the ability to produce device-quality material is severely limited by the inherent crystalline defects associated with this material and their associated electronic effects. Much progress has been made recently in the understanding and control of these defects and in the improved processing of this material. Because of this work, it may be possible to produce SiC-based solar cells for environments with high temperatures, light intensities, and radiation, such as those experienced by solar probes. Electronics and sensors based on SiC can operate in hostile environments where conventional silicon-based electronics (limited to 350 C) cannot function. Development of this material will enable large performance enhancements and size reductions for a wide variety of systems--such as high-frequency devices, high-power devices, microwave switching devices, and high-temperature electronics. These applications would supply more energy-efficient public electric power distribution and electric vehicles, more powerful microwave electronics for radar and communications, and better sensors and controls for cleaner-burning, more fuel-efficient jet aircraft and automobile engines. The 6H-SiC polytype is a promising wide-bandgap (Eg = 3.0 eV) semiconductor for photovoltaic applications in harsh solar environments that involve high-temperature and high-radiation conditions. The advantages of this material for this application lie in its extremely large breakdown field strength, high thermal conductivity, good electron saturation drift velocity, and stable electrical performance at temperatures as high as 600 C. This behavior makes it an attractive photovoltaic solar cell material for devices that can operate within three solar radii of the Sun

Single Wall Carbon Nanotube-polymer Solar Cells

Investigation of single wall carbon nanotube (SWNT)-polymer solar cells has been conducted towards developing alternative lightweight, flexible devices for space power applications. Photovoltaic devices were constructed with regioregular poly(3-octylthiophene)-(P3OT) and purified, >95% w/w, laser-generated SWNTs. The P3OT composites were deposited on ITO-coated polyethylene terapthalate (PET) and I-V characterization was performed under simulated AM0 illumination. Fabricated devices for the 1.0% w/w SWNT-P3OT composites showed a photoresponse with an open-circuit voltage (V(sub oc)) of 0.98 V and a short-circuit current density (I(sub sc)) of 0.12 mA/sq cm. Optimization of carrier transport within these novel photovoltaic systems is proposed, specifically development of nanostructure-SWNT complexes to enhance exciton dissociation

Electrodeposited CuInSe2 Thin Film Junctions

We have investigated thin films and junctions based on copper indium diselenide (CIS) which have been grown by electrochemical deposition. CIS is a leading candidate for use in polycrystalline thin film photovoltaic solar cells. Electrodeposition is a cost-effective method for producing thin-film CIS. We have produced both p and n type CIS thin films from the same aqueous solution by simply varying the deposition potential. A CIS pn junction was deposited using a step-function potential. Stoichiometry of the single layer films was determined by energy dispersive spectroscopy. Carrier densities of these films increased with deviation from stoichiometry, as determined by the capacitance versus voltage dependence of Schottky contacts. Optical bandgaps for the single layer films as determined by transmission spectroscopy were also found to increase with deviation from stoichiometry. Rectifying current versus voltage characteristics were demonstrated for the Schottky barriers and for the pn junction

Progress Towards III-V Photovoltaics on Flexible Substrates

Presented here is the recent progress of the NASA Glenn Research Center OMVPE group's efforts in the development of high efficiency thin-film polycrystalline III-V photovoltaics on optimum substrates. By using bulk polycrystalline germanium (Ge) films, devices of high efficiency and low mass will be developed and incorporated onto low-cost flexible substrates. Our progress towards the integration of high efficiency polycrystalline III-V devices and recrystallized Ge films on thin metal foils is discussed

Quantum Dots Investigated for Solar Cells

The NASA Glenn Research Center has been investigating the synthesis of quantum dots of CdSe and CuInS2 for use in intermediate-bandgap solar cells. Using quantum dots in a solar cell to create an intermediate band will allow the harvesting of a much larger portion of the available solar spectrum. Theoretical studies predict a potential efficiency of 63.2 percent, which is approximately a factor of 2 better than any state-of-the-art devices available today. This technology is also applicable to thin-film devices--where it offers a potential four-fold increase in power-to-weight ratio over the state of the art. Intermediate-bandgap solar cells require that quantum dots be sandwiched in an intrinsic region between the photovoltaic solar cell's ordinary p- and n-type regions (see the preceding figure). The quantum dots form the intermediate band of discrete states that allow sub-bandgap energies to be absorbed. However, when the current is extracted, it is limited by the bandgap, not the individual photon energies. The energy states of the quantum dot can be controlled by controlling the size of the dot. Ironically, the ground-state energy levels are inversely proportional to the size of the quantum dots. We have prepared a variety of quantum dots using the typical organometallic synthesis routes pioneered by Ba Wendi et al., in the early 1990's. The most studied quantum dots prepared by this method have been of CdSe. To produce these dots, researchers inject a syringe of the desired organometallic precursors into heated triocytlphosphine oxide (TOPO) that has been vigorously stirred under an inert atmosphere (see the following figure). The solution immediately begins to change from colorless to yellow, then orange and red/brown, as the quantum dots increase in size. When the desired size is reached, the heat is removed from the flask. Quantum dots of different sizes can be identified by placing them under a "black light" and observing the various color differences in their fluorescence (see the photograph)

Chemically Deposited Thin-Film Solar Cell Materials

We have been working on the development of thin film photovoltaic solar cell materials that can be produced entirely by wet chemical methods on low-cost flexible substrates. P-type copper indium diselenide (CIS) absorber layers have been deposited via electrochemical deposition. Similar techniques have also allowed us to incorporate both Ga and S into the CIS structure, in order to increase its optical bandgap. The ability to deposit similar absorber layers with a variety of bandgaps is essential to our efforts to develop a multi-junction thin-film solar cell. Chemical bath deposition methods were used to deposit a cadmium sulfide (CdS) buffer layers on our CIS-based absorber layers. Window contacts were made to these CdS/CIS junctions by the electrodeposition of zinc oxide (ZnO). Structural and elemental determinations of the individual ZnO, CdS and CIS-based films via transmission spectroscopy, x-ray diffraction, x-ray photoelectron spectroscopy and energy dispersive spectroscopy will be presented. The electrical characterization of the resulting devices will be discussed

Small-polaron hopping conductivity in bilayer manganite La1.2_{1.2}Sr1.8_{1.8}Mn2_{2}O7_{7}

We report anisotropic resistivity measurements on a La$_{1.2}$Sr$_{1.8}$Mn$_{2}$O$_{7}$ single crystal over a temperature $T$ range from 2 to 400 K and in magnetic fields $H$ up to 14 T. For $T\geq 218$ K, the temperature dependence of the zero-field in-plane $\rho_{ab}(T)$ resistivity obeys the adiabatic small polaron hopping mechanism, while the out-of-plane $\rho_{c}(T)$ resistivity can be ascribed by an Arrhenius law with the same activation energy. Considering the magnetic character of the polarons and the close correlation between the resistivity and magnetization, we developed a model which allows the determination of $\rho_{ab,c}(H,T)$. The excellent agreement of the calculations with the measurements indicates that small polarons play an essential role in the electrical transport properties in the paramagnetic phase of bilayer manganites.Comment: 4 pages, 3 figures, to appear in Physical Review

Strain effect on electronic transport and ferromagnetic transition temperature in La0.9_{0.9}Sr0.1_{0.1}MnO3_{3} thin films

We report on a systematic study of strain effects on the transport properties and the ferromagnetic transition temperature $T_{c}$ of high-quality La$_{0.9}$Sr$_{0.1}$MnO$_{3}$ thin films epitaxially grown on (100) SrTiO$_{3}$ substrates. Both the magnetization and the resistivity are critically dependent on the film thickness. $T_{c}$ is enhanced with decreasing the film thickness due to the compressive stain produced by lattice mismatch. The resistivity above 165 K of the films with various thicknesses is consistent with small polaronic hopping conductivity. The polaronic formation energy $E_{P}$ is reduced with the decrease of film thickness. We found that the strain dependence of $T_{c}$ mainly results from the strain-induced electron-phonon coupling. The strain effect on $E_{P}$ is in good agreement with the theoretical predictions.Comment: 6 pages and 5 figures, accepted for publication in Phys. Rev.

Electrical Characterization of Defects in SiC Schottky Barriers

We have been investigating the effect of screw dislocation and other structural defects on the electrical properties of SiC. SiC is a wide-bandgap semiconductor that is currently received much attention due to its favorable high temperature behavior and high electric field breakdown strength. Unfortunately, the current state-of-the-art crystal growth and device processing methods produce material with high defect densities, resulting in a limited commercial viabilit