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Title: Investigation of intrinsic defects and versatility of Zn-doping in SnO2 thin films using pulsed laser deposition
Author: Porte, Yoann Bernard Henri
ISNI:       0000 0004 5918 641X
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2016
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n-type transparent conducting oxides (TCOs) have been studied extensively and are now commercially available. The development of p-type counterparts lags someway behind, despite the opportunity preparing such materials offers. Interest in tin oxide (SnO2) thin films and their applications has grown significantly over recent years, particularly for the preparation of p-type TCOs. However for this to be realised, limitations such as the ionisation energy of acceptor defects and the existence of compensating mechanisms must be overcome. Even though the effectiveness of acceptor doping is essential, the presence of compensating defects as the film growth environment changes is critical for stable p-type TCOs. Here a growth study of SnO2 deposited using pulsed laser deposition (PLD) reveals intrinsic acceptor defects have limited contribution to the conductivity as theoretically predicted. The conductivity remains n-type despite significant variation in the oxygen content of the growth environment and temperature - which is attributed to the large concentration of tin interstitial defects. To limit donor defect concentration, elevated temperature and oxygen pressure were set as the growth conditions with the introduction of Zn as an acceptor dopant. Here, the introduction of Zn acceptors changes the behaviour of intrinsic defects such that measured charge carrier concentrations increase by some 3-orders of magnitude compared with undoped films, suggesting the presence of compensating mechanisms. n-type behaviour persists, even after oxygen annealing, and film resistivity also increases. This is attributed to the formation more acceptor defects that act as electron compensation centres. Zn is reported to act not only as an acceptor in SnO2 but also to enhance n-type conductivity: here this behaviour is also confirmed. By studying the electrical, optical and structural properties of Zn doped SnO2, films of surprisingly low resistivity (2.9 x 10-3 Ω.cm) were formed, which exhibited preferential (200) on Zn addition. An observed decrease in bandgap was found when Zn doping was increased, from 3.6 eV in undoped films down to 3.4 eV in crystalline films and 3.2 eV in amorphous films. The work is concluded by investigating low deposition temperature where low resistivity highly transparent films are formed at temperatures as low as 100 degree celcius.
Supervisor: McLachlan, Martyn ; Anthopoulos, Thomas Sponsor: King Abdullah University of Science and Technology
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available