Rapid melt growth of crystalline germanium for solar energy harvesting applications
Recent development of energy conversion devices namely photovoltaic (PV) cells or solar cells and thermophotovoltaic (TPV) cells require the use of bulk germanium as substrate material for efficient device performance. Germanium is employed in solid-state terrestrial or space energy conversion devic...
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| Format: | Thesis |
| Published: |
2014
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| Subjects: | |
| Online Access: | http://eprints.uthm.edu.my/7899/ http://eprints.uthm.edu.my/7899/1/nurfarina_zainal.pdf |
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| Summary: | Recent development of energy conversion devices namely photovoltaic (PV) cells or
solar cells and thermophotovoltaic (TPV) cells require the use of bulk germanium as
substrate material for efficient device performance. Germanium is employed in
solid-state terrestrial or space energy conversion devices due to its excellent
electrical properties. With a bandgap of 0.66 eV, energy from infrared region of solar
or thermal spectrum can be absorbed and converted into electrical energy. At
present, multi-junction solar cells with bulk germanium substrates show the highest
performance but have complex and expensive manufacture processes. The major
contributor to the high cost is the germanium substrate, which is an expensive and
scarce material. One of the possibilities to reduce cost is by using thin film instead of
thick bulk germanium. To date, development of thin film germanium for energy
conversion devices has not been established. By providing germanium on insulator
structures a good quality thin film germanium can be attained and thus, offer a low
cost route. The rapid melt growth (RMG) technique has been proposed, where it
potentially produces thin film germanium with quality similar to that of bulk
material. In the existing RMG technology germanium thickness has been limited to
100 nm. For photovoltaic applications thicker germanium films are required to
increase energy absorption for eficient performance. For these reasons, alternative
RMG routes to producing thin films for energy conversion devices have been
investigated. The feasibility and efficacy of the RMG technique has been evaluated
for different germanium deposition methods namely physical vapour deposition
(PVD) and chemical vapour deposition (CVD). In this study, a novel RMG
technology has been established in order to produce high quality thick germanium
films (above 100 nm). Experiments have been conducted with various anneal
temperature, germanium patterns, substrate and crucible materials to achieve
optimum crystalline quality. Micro-Raman spectroscopy has shown that the
technology has the capability to produce thick germanium films that are free from
major defects such as cracks and delamination, with crystalline quality similar to that
of bulk material. This thesis highlights the use of PVD germanium compared to
CVD germanium in minimising silicon diffusion through the crystal growth process,
which has resulted in longer length of crystal germanium. The mechanism of
germanium lateral crystallisation has been investigated in this thesis by depositing
germanium onto non-silicon substrates. A prototype germanium photovoltaic has
been fabricated to demonstrate the potential of the RMG technique in production of
future solar cells. |
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