Engineering topics
Publications and source records attributed to Woodall, J. M..
Multimetallic layers produce stable, low-resistance contacts for p-type GaAs and p-type GaAlAs devices. Contacts present no leakage problems, and their series resistance is too small to measure at 1 Sun intensity. Ohmic contacts are stable and should meet 20-year-life requirement at 150 C for GaAs combined photothermal/photovoltaic concentrators.
The Pd contact to GaAs was studied using backscattering, Auger analysis, and sheet resistance measurements. Several metallurgical phases were present at low temperatures, but PdGa was the dominant phase in samples annealed at 500 C. Ti/Pd/Ag contacts appeared to have the lowest contact resistance. Etchback epitaxy (EBE) was compared to saturated melt epitaxy (SME) method of growing liquid phase epitaxial layers. The SME method resulted in a lower density of Ga microdroplets in the grown layer, although the best solar cells were made by the EBE method. Photoluminescence was developed as a tool for contactless analysis of GaAs cells. Efficiencies of over 8 percent were measured at 250 C.
High-efficiency p-Ga(1-x)Al(x)As, p-GaAs, n-GaAs solar cells are made by isothermally soaking n-GaAs substrates in an undersaturated Zn-doped Ga-Al-As melt. This one-step growth procedure produces a graded band gap p-Ga(1-x)Al(x)As layer 0.2-0.4 micron thick. Efficiencies of 18.5% AM0 and 21.9% AM1 have been measured.
Single step process produces gallium arsenide solar cells having uniform and continuous Ga(1-x)Al(x)As layer thinner than 0.5 micrometers, with an efficiency of 18.5 percent.
Anodization technique produces GaAs pn-junction solar cells exhibiting improved response to high energy photons and higher open circuit voltages through reduction of reflection loss.
Bimetallic contacts utilizing palladium and aluminum, gold, silver, or chromium, are used in reduction of series resistance in GaAlAs/GaAs solar cells, thereby improving cell reliability.
The etch-back epitaxy process is described for producing thin, graded composition GaAlAs layers. The palladium-aluminum contact system is discussed along with its associated problems. Recent solar cell results under simulated air mass zero light and at elevated temperatures are reported and the growth of thin polycrystalline GaAs films on foreign substrates is developed.
The procedures for the liquid phase epitaxial (LPE) growth of high efficiency p-Ga(1-x)Al(x)As,p-GaAs,n-GaAs solar cells have been developed. The methods are based on forming the structure by a one-step process in which the Zn-diffused p-n junction in the n-type GaAs substrate forms in conjunction with the LPE growth of the Zn-doped p-Ga(1-x)Al(x)As layer. For structures with 1-10 micron thick Ga(1-x)Al(x)As layers, an isothermal soak of the GaAs substrate in a saturated Ga-Al-As: Zn melt followed by ramp cooling produces good cells. For structures with less than one micron thick Ga(1-x)Al(x)As layers, it is necessary to isothermally soak the GaAs substrate in an undersaturated melt, and ramp cooling is not required.
Technique, suitable for monolithic device fabrication methods, permits growth of LED structures on GaP substrates by liquid-phase epitaxial method, thus obviating needs for growing thick layers and for removing substrates. High efficiency infrared LED's can be developed as pumping sources for Nd:YAG lasers.
Crystal growth procedures, fabrication techniques, and theoretical analysis were developed in order to make GaAlAs-GaAs solar cell structures which exhibit high performance at air mass 0 illumination and high temperature conditions.
The three types of solar cells investigated were: (1) one consisting of a nGaAs substrate, a Zn doped pGaAs region, and a Zn doped Ga(1-x)Al(x)As layer, (2) one consisting of an nGaAs substrate, a Ge doped pGaAs region, and a pGa(1-x)Al(x)As upper layer, and (3) one consisting of an n+GaAs substrate, an nGa(1-x)Al(X)As region, a pGa(1-x)Bl(X) As region, and a pGa(1-y)Al(y)As upper layer. In all three cases, the upper alloy layer is thin and of high Al composition in order to obtain high spectral response over the widest possible range of photon energies. Spectral response, capacitance-voltage, current-voltage, diffusion length, sunlight (or the equivalent)-efficiency, and efficiency-temperature measurements were made as a function of device parameters in order to analyze and optimize the solar cell behavior.
Violet cells with 500-1000 A junction depths have been made in GaAs by narrow junction diffusion followed by anodization. The best AM0 efficiencies obtained by this technique have been 10.5% (14% at AM1). GaAlAs-GaAs structures with very thin GaAlAs layers are much more promising, and efficiencies of over 18% at AM0 have been measured (21.9% at AM1).
Transparent conducting coating is applied to narrow pn junction surface to provide ohmic contact for majority carrier flow. Coating can be made thick to prevent series resistance problem.
Relatively good GaAs solar cells can be made from poor quality substrates by making the junction deep (more than 1 micron) instead of shallow and by leaching both the p and n GaAs regions during the growth process. Air-mass-zero efficiencies of 14.7% (19% AM1) have been obtained from substrates with starting substrate thickness of 0.6 micron.
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Ga(1-x)Al(x)As light-emitting diode structures have been grown on GaP substrates by the liquid-phase-epitaxial method. In spite of the large differences in lattice constants and thermal-expansion coefficients, room-temperature efficiencies up to 5.5% in air have been observed for a peak emission of 8500 A. Using undoped GaP substrates, which are transparent to the infrared and red portions of the spectrum, thin structures of Ga(1-x)Al(x)As with large external efficiencies can now be made.