Thin crystalline gallium arsenide optoelectronic devices
Abstract
The numerous existing and developing applications for two closely related devices, III-V light emitting diode (LEDs) and solar cells, demand improved device efficiencies. Removing the substrate should increase the efficiency of both LEDs and solar cells by eliminating the absorption losses in the substrate. We have used the phenomenon of photon recycling in thin-crystalline device geometries to enhance efficiencies of LEDs and solar cells. GaAs LEDs were fabricated and removed from the substrate by the epitaxial lift-off process. Devices with and without an underlying GaAs substrate were then characterized by optical and electrical measurements. Efficiency enhancements of up to a factor of six were achieved. By carefully analyzing the electrical and optical measurements, we demonstrate that the device operation can be explained in terms of accepted theories for radiative recombination and photon recycling which supports our hypothesis that the efficiency enhancement is due to photon recycling in the thin-crystalline device structure. Electrical and optical characterization of ELO LEDs is also shown to be a convenient diagnostic tool for examining recombination losses in thin-crystalline solar cells. Thin crystalline solar cells were fabricated and characterized by I-V and QE measurements. Alloyed ohmic contacts are used extensively for GaAs devices. However, alloyed contacts produce rough interfaces that do not make good reflectors needed for many optoelectronic devices. Non-alloyed ohmic contacts to optoelectronic devices could make good reflectors, if one uses highly reflective metal like Au to make an ohmic contact. Ex-situ non-alloyed contacts to n-GaAs were made by using low temperature molecular beam epitaxy. Ag and Ti/Au contacts to this structure exhibited specific contact resistivities of mid 10$\sp{-7}\ \Omega$-cm$\sp2$. Low temperature molecular beam epitaxy of GaAs with high concentrations of Be followed by an anneal under As over pressure was used to minimize the fast diffusing interstitial Be concentration in p$\sp{++}$-GaAs. Non-alloyed Ti/Au ohmic contacts to such p-type GaAs exhibited specific contact resistivities of about 10$\sp{-7}\ \Omega$-cm$\sp2$. A new amalgamation technique was developed for mounting thin crystalline devices on a substrate different from a host substrate.
Degree
Ph.D.
Advisors
Woodall, Purdue University.
Subject Area
Condensation|Electrical engineering
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