III-V compound Gallium Antimonide (GaSb), with a low bandgap of 0.72 eV at room temperature, is an attractive candidate for a variety of potential applications in optoelectronic devices. Ion implantation, among non-epitaxial methods, is a common and reliable doping technique to achieve local doping and obtain high-performance ohmic contacts in order to form a pn junction in such devices. An advantage of this technique over the diffusion method is the ability to perform a low-temperature process leading to accurate control of the dopant profile and avoiding Sb evaporation from GaSb surface occurring at 370 C.
In this work, the effect of protons and sulfur ions as two implant species on the electrical behavior of MBE-grown undoped GaSb on semi-insulating (SI) GaAs was investigated via the Hall Effect. Protons and sulfur ions were implanted at room temperature (27 C) and 200 C, respectively, and rapid thermal annealing (RTA) was implemented at various temperatures and durations upon encapsulated GaSb. The damage induced by protons enhanced the hole density of GaSb up to around 10 times, whereas mobilities showed both increase and decrease compared to the un-implanted one, depending on the dose. While the activation of sulfur donors at an elevated temperature was anticipated after annealing sulfur implanted GaSb, instead it led to increase in p-type concentration, as the residual damage originated from sulfur implantation dominated substitutional doping.
Furthermore, GaSb p/n photovoltaic devices were fabricated by applying sulfur implantation through silicon nitride layer at RT into an n-GaSb wafer (n-type base, p-type emitter). The device showed a rectifying current and photovoltaic characteristic. The J-V plot under AM1.5G illumination conditions, before and after an etch-back optimizing process, indicated lower short circuit current density J_sc, the same open circuit voltage V_oc, and higher fill factor FF, compared to the photovoltaic device with a p-type base. Also, both normalized series R_s and shunt R_p resistances in p/n diode indicated lower and higher values, respectively, as opposed to a GaSb p++/p diode, indicative of higher quality and lower manufacturing defects. / Master of Science / Generally, the photovoltaic effect is a process by which voltage or electric current is generated in a photovoltaic cell when exposed to light. A solar cell is a photovoltaic device, typically consisting a pn junction, that converts incident photon power into electrical power and delivered to a load to do electrical work for variety of applications. There are variety of methods to form a pn junction and fabricate such devices, among which ion implantation is a reliable doping technique. In this process, dopant ions are accelerated and smashed into a perfect semiconductor lattice, creating a cascade of damage that may displace a thousand atoms for each implanted ion and become activated after an annealing process. The ions themselves can act as either electron donors, make the semiconductor n-type, or electron acceptors, make it p-type.
In this work, sulfur ions and protons, as two implant species, were implanted into separate Gallium Antimonide (GaSb) substrates and the effect of each on the electrical behavior of GaSb was investigated by Hall effect experiment. Both species raised hole carrier concentration. This behavior was not expected for sulfur ions as they would be assumed to act as electron donors after activation and convert the GaSb surface to an n-type semiconductor. It was identified that this behavior is due to the domination of created defects during implantation over the number of activated sulfur donors. The same characteristics were predicted and verified for proton implantation as well, the effect of which is just leaving damage in the lattice.
Furthermore, to verify this method for converting n-type GaSb to p-type and fabricating a pn junction in GaSb for photovoltaic application, sulfur implantation into an n-type GaSb wafer was performed and optimized by removing the excess surface damage away from the device's metal contacts using wet etching. The device showed a diode-like rectifying current and photovoltaic characteristic. Some parameters such as short circuit current density J_sc, open circuit voltage V_oc, fill factor FF, and resistances (shunt and series) were measured and calculated using J-V plot under dark and illuminated conditions.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/102071 |
| Date | 25 January 2021 |
| Creators | Karimi, Ebrahim |
| Contributors | Electrical Engineering, Lester, Luke F., Hudait, Mantu K., Jia, Xiaoting |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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