Electronic properties of electrodeposited semiconductor junctions

Chatman, Shawn (2009) Electronic properties of electrodeposited semiconductor junctions. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

This thesis describes the synthesis, structural properties, optical properties, and electronic properties of semiconductor junctions based on electrochemically deposited ZnO and CU₂O thin films. The first focus is characterizing the effect of deposition conditions (including applied potential and electrolyte composition) on the fundamental properties of these materials (including carrier concentration, band gap, and microstructure). Subsequent discussion addresses electrical conduction to and through ZnO/substrate junctions as a function of these deposition conditions. Finally, three device applications for these ZnO-based junction are explored: Schottky rectifiers, humidity sensors, and photovoltaic cells. Since electrical conduction to and through heterojunction interfaces is very important for maximizing the functionality of semiconductor devices, this thesis work is an essential step towards increasing the functionality of multi-layer ZnO-based heterojunction devices prepared by electrodeposition. -- Capacitive Mott-Schottky analyses showed that the carrier concentrations of our ZnO electrodeposits are dependent upon deposition potential, with higher net carrier concentration at more positive potentials. UV/Visible diffuse reflectance data indicates that band gap increases with more positive deposition potentials. Together, these results suggest that hydrogen is the dominant, yet unintentional, Moss-Burstein like dopant in our n -type ZnO. Furthermore, the range of carrier concentrations we can achieve (10¹⁸ - 10²¹ cm⁻³) is comparable to that obtained with intentional doping. This is significant because using deposition potential to change growth rate or morphology will simultaneously change electronic properties. -- The deposition potential studies evolved into a procedure for selective, one-step production of either ohmic or rectifying (Schottky) ZnO/metal junctions (Chatman et al., Appl. Phys. Lett., 2008, 92, 012103/1-3). Rectifying ratio and soft-breakdown characteristics can be influenced by tuning either deposition potential and p H (Chatman et al., ACS Appl. Mater. Interfaces, 2009, 1, 552-558). (In contrast, our CU₂O/metal junctions were always ohmic.) Rectifying ZnO/metal junctions are very susceptible to relative humidity compared with ohmic samples because of protonic conduction at the ZnO/air interface. Hydrophobic coatings applied to ZnO deposits greatly reduce the erratic sensitivity of ZnO at high humidities while retaining humidity sensitivity (Chatman et al., ACS Appl. Mater. Interfaces, 2009, 1, 552-558). -- ZnO/CU₂O pn junctions were prepared with different permutations of a two-step electrodeposition process. ZnO/CU₂O/metal junctions were determined to be light sensitive when the CU₂O layer was deposited using lactate-based electrolytes. Unfortunately, solar conversion efficiencies was too low to determine device efficiencies.

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