The Electronic Structure Of N-type Doped Ga₂O₃ Thin Films

Gallium oxide(Ga2O3)is an emerging oxide semiconductor with an ultra-wide bandgap of 4.9 eV.It holds great potential for application in next-generation solar-blind ultraviolet optoelectronic devices and ultra-high power electronic devices,because of its ultra-wide bandgap,high theoretical breakdown electrical field(8 MV/cm)and large-area affordable substrates.These advantages offer a competitive edge over current wide bandgap semiconductors such as SiC and GaN.Achieving high-quality epitaxial thin films as well as the ability to tune its optoelectronic properties are crucial for the realization of high performance Ga2O3-based devices.However,at present,the research on the epitaxial growth of thin-film,the control of optical and electrical property,as well as the basic physical and chemical properties,doping mechanism for Ga2O3 is still in its infancy.The lack of high-quality epitaxy thin film is still limiting the development of new generation of power electronics based on Ga2O3 and its application in other fields.In view of the above problems,this thesis employed pulse laser deposition to epitaxially grow high-quality doped Ga2O3 thin films.Synchrotron-based photoemission spectroscopy characterizations and theoretical calculations were combined to gain deep understanding of the the electronic structure,transport mechanism,defect and doping chemistry,and optical properties of Ga2O3.These characterizations and calculations provided theoretical and experimental guidances for the doping optimization of Ga2O3 and their applications in high-power electronics and deep ultraviolet photoelectronics.The main research results are as follows:(1)We achieved record-high conductive and deep-UV transparent oxide thin films based on Si doped Ga2O3(SGO)by pulsed laser deposition.The combined hard x-ray photoemission spectroscopy(HAXPES)and theoretical calculations clearly demonstrated their superior optoelectronic properties exhibit strong correlations with the chemistry of Ga 4s derived conduction band(CB)and doping effect of Si.The high conductivity of SGO was attributed to the high density of the free electrons induced by Si doping filled at the highly dispersive Ga 4s derived CB,while the low work-function of SGO was owing to high energy position of Ga 4s derived CB and thus small electron affinity.Remarkably,with the introduction of much lower In 5s orbital into CB,its work-function can be tuned by alloying with In2O3.The present results advance our fundamental understanding and provide important guidance for use of SGO and its alloying in ultraviolet and organic-based opto-electronics and high-power electronics.(2)Using HAXPES and hybrid DFT calculations,we carried out a combined photoemission spectroscopy and theoretical calculation study on the electron structure of Si doped Ga2O3 thin films with a wide range of carrier density from 4.6×1018 cm-3 to 2.6×1020 cm-3.A bandgap reduction of as much as 0.3 eV was observed in heavily doped Ga2O3.Our results showed that the bandgap renormalization mainly results from the decrease of CB driven by mutual electrostatic interaction between free electrons.This was attributed to the lack of orbital mixing between Ga 4s derived CB with Si 3s dopant state.Hybrid DFT calculation revealed that the Si 35 state sits resonant inside the CB,leaving the host CB edge mostly unperturbed,giving rise to a small electron effective mass.This explained the higher mobility achieved in Si doped thin films and suggests that Si is a superior dopant compared to Sn and Ge.Our work advances our fundamental understanding and provide significant guidance for doping optimization of Ga2O3 and its use in high-power electronics and deep-UV optoelectronics.(3)HAXPES has been used in conjunction with soft x-ray photoemission spectroscopy(Soft-PES)and DFT calculations to investigate the band gap,valence electronic structure,and surface electronic properties of Sn-doped Ga2O3 thin films with a wide range of carrier density from 2.8×1018 cm-3 to 1.3 ×1020 cm-3.From comparisons of theoretical calculations and experimental measurements of VB spectra of Sn-and Si-doped Ga2O3,profound differences between the Sn and Si doping were found.The larger band-gap renormalization found in Sn doped sample was attributed to the more significant hybridization for Sn 5s than Si 3s dopant state with the host CB edge(Ga 4s)of Ga2O3.Therefore,a strongly perturb to the host CB edge and a lessdispersed CB edge with a lower mobility were expected by the introduction of Sn than Si,which was further supported by transport measurement.Furthermore,an in-gap state observed in Sn doped sample,maybe resulted from self-compensating Sn2+ related defects,explained the lower carrier concentration achieved in Sn doped sample.Finally,a comparison of the VB and core-level spectra excited with different photon energy,with different probing depth,provided evidence for Sn doping level would influence the Ga2O3 surface electronic property and band bending is extreme for the highly Sndoped Ga2O3 sample.The present results would advance our fundamental understanding the doping optimization of Ga2O3 and the physical mechanism that influences the band bending on the doped Ga2O3 surface,and further aid in the issues of metal contact formation on different n-type doped Ga2O3 surfaces.