Electronic and optical properties of doped Cu2O and ZnS thin films
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- Institutt for fysikk 
In this thesis, two earth-abundant, easily fabricated semiconductors, Cu2O and ZnS, were investigated to explore their electronic and optical properties after doping. Fe doped ZnS has been studied as a potential material for passive Q-switches in miniaturized mid-infrared lasers, while K and N doped Cu2O has been proposed as a suitable absorber material for photovoltaic (PV) and photo-electrochemical (PEC) applications. Cu2O samples were made by annealing and controlled oxidation of Cu metal foils. A robust p-type doping of Cu2O using low/medium energy ion implantation was demonstrated. Using this method, extreme levels of doping were achieved as evidenced by a 350meV shift in the Fermi level towards the valence band maximum (VBM). The robustness of the nitrogen implanted samples was tested by exposing them to atmospheric contaminants, and elevated temperatures. The samples were found to survive an increase in temperature of many hundreds of degrees. The effect of n-type surface doping of Cu2O by K was also investigated. K was in-situ deposited on a clean polycrystalline Cu2O surface. With increasing K concentration up to 8:7 at:%, the valence band maximum (VBM) shifted by 225meV to higher binding energies as determined by photoelectron spectroscopy. A successful K + N co-doping in Cu2O was finally demonstrated. A possibility to precisely control the dopant concentration and Fermi energy level position in Cu2O, thereby controlling the occupation of states within the band gap is an important requirement if Cu2O is to be useful in low price PV and PEC applications. 1–4 μm thick ZnS:Fe films of high optical quality with Fe content up to 9 at:% were made on sapphire and silicon using vapor deposition at room temperature. Well-isolated optical absorption peaks were observed with iron concentrations up to 4 at:%, despite a high density of twin defects in the cubic crystal structure. Raman shifts of the ZnS:Fe films were composition dependent, and demonstrated the onset of disorder for Fe content over 4 at:%. Preferential crystalline orientation and phase purity were increased by the addition of even small amounts of Fe. Optical Kerr effect measurements showed a dramatic increase in _(3) above the concentration where Raman peaks indicate structural changes. Fe doping decreased the bandgap of ZnS and raised the Fermi energy level, which motivated the investigation of a heterojunction between ZnS:Fe and Cu2O in details. Using photoelectron spectroscopy techniques (XPS/UPS) conduction and valence band offsets at (6 at:% Fe doped ZnS)/Cu2O heterojunction were calculated, giving _EC = 0:22 eV and _EV = 0:93 eV, respectively. An improvement over undoped-ZnS/Cu2O heterojunction was demonstrated.