Engineering And Study Of Doing And Alloying In Wide Bandgap Semiconductors

Wide-bandgap Semiconductors such as gallium oxide(Ga2O3),zinc oxide(ZnO),and zinc telluride(ZnTe)are promising in the versatile applications of short-wavelength optoelectronic devices and high-power electronic devices owing to their advantages of large bandgap,high breakdown field,and high radiation resistance,and.However,wide bandgap semiconductors generally have difficulty in simultaneously achieving n-type and p-type conduction.Futhermore,the electronic band structure and fundamental physical properties of their alloying materials are still lack of systematic research,which significantly limits their practical applications.For instance,investigation of the optoelectronic properties of the Ga2O3 alloy system is beneficial to expand its application to develop vacuum ultraviolet detection and power electronic devices.The ZnTeO alloy by isoelectronic oxygen doping is an alternative way to realize next-generation high-efficiency intermediate-band photovoltaic cells.Thus,the comprehensive understanding of wide-bandgap doping and alloying materials is important to enrich the knowledge of wide bandgap semiconductor materials and device and to promote the development of optoelectronic devices with novel functionality and improved performance.In this paper,high-quality(AlxGa1-x)2O3 alloys,ZnTeO highly mismatched alloys,and Cr-doped Ga2O3 material systems were prepared by in-situ alloying,isoelectronic ion implantation,and in-situ doping,respectively,targeting at the key problems in the field of Ga2O3 and ZnTe wide bandgap semiconductor materials.The bandgap engineering,defect behaviors and the relevant optical properties have been investigated compreshensively in the above-mentioned material systems,which provide a deep understanding on the fundamental properties and facilitate their potential applications in the field of high-power electronic devices,vacuum ultraviolet photodetectors and high-efficiency intermediate-band photovoltaic cells.Specific achievements are included as follows.1.Single-monoclinic ?-phase(AlxGai-x)2O3(0?x?0.54)alloying films with a highly preferred(201)orientation were grown by laser molecular beam epitaxy with a tunable bandgap ranging from 4.5 to 5.5 eV.The variation of lattice constant and the cell volume is consistent with Vegard's law as well as the DFT calculations.The electronic band structure of ?-(AlxGa1-x)2O3 alloys is revealed based on the combination of first-principles theoretical calculations and experiments.The results show that the(3-(AlxGa1-x)2O3 alloy has a typical indirect band gap characteristic,and the valence band maximu is located at the M point,and the energy difference with the f point of the valence band increases with the increase of the Al composition.The analysis on cathodoluminescence and XPS demonstrates that the surface upward band-bending of(AlxGa1-x)2O3 films is caused by Fermi level pinning effect,which results in surface depletion and high resistance of the films.These findings indicate that band modulation can extend the response wavelength into vacuum UV spectral region and also facilitates their applications in high power devices2.High-quality Cr3?doped Ga2O3 single crystalline films were prepared by in-situ doping on c-plane sapphire substrate through Mist-CVD technique.The unique luminescence properties of Cr3+ in Ga2O3 with different phases were investigated.It was found that Cr3? doped a-phase Ga2O3 epilayers exhibit sharp R-line zero phonon lines with a line width of less than 1.5 nm at a wavelength of 697 nm at room temperature,while no relevant spectral lines were observed in the fine spectra of Ga2O3 epilayers with ? and ? phases.The observed R-line zero phonon line of Cr3+doped ?-Ga2O3 shows distinguished polarization dependency.The power-dependent luminescence exhibits the sublinearity withI ? P0.85,which suggests its physical mechanism of radiative recombination through defect states induced by the activated transition metals.In terms of crystal field theory,the R-line zero phonon line in Cr3+:Ga2O3 is caused by the radiant transition of the activated state 2E to the ground state 4A2 of the Cr3+ ion located in an octahedral crystal field.The fluorescence lifetime of the electron at the 2E level is 2.0 ns.This kind of atomic luminescence property indicates that Cr3+:Ga2O3 is not only expected to be a new laser crystal,but also provides a room temperature single photon emission device for the application of quantum computing and quantum communication.3.Combining multi-step ion implantation and pulsed laser melt annealing technology,the ZnTeO highly mismatch alloy with optically active intermediate band energy level and effective band photo-response is realized.It was found that localized high temperature annealing induced by pulsed laser leads to the rapid melting and recrystallization of ZnTeO,which is similar to the process of liquid phase epitaxy.The process can effectively recover the lattice damage caused by ion implantation and prevent the diffusion of oxygen,but to some extent leads to the segregation of Te.Based on the analysis on absorption spectrum and photoluminescence spectroscopy,laser annealing can suppress the in-band luminescence of intrinsic defects such as zinc vacancies,and the isoelectronic doping of O will introduce the intermediate band at 0.45 eV under the minimum of the conduction band,while partially converted the localized state to extended state.The carrier dynamics were studied by time-resolved PL,indicating that the photogenerated electron lifetime in the intermediate banded electronic state reached approximately 1 ns.The photocurrent response results show that the absorption in the intermediate band is significantly enhanced,indicating that the carrier is rapidly separated in the intermediate energy level and then excited to the conduction band,thereby improving the absorption capacity of ZnTe for low-energy photons and the overall photoelectric conversion efficiency,which could benefit the development of high efficiency intermediate photovoltaic cells.4.The ZnTe quasi-two-dimensional nanobelt structure with low defect density and good luminescence properties was synthesized by optimizing the conditions of gold-catalyzed physical vapor deposition,and the growth mechanism of ZnTe nanowires and nanobelts was subsequently investigated in details.The results show that ZnTe nanowires,grown mainly along the[111]direction,have a higher density of stacking faults structures,and even form twin superlattices,while the upper and lower surfaces of the nanobelts are {111} planes and the sidewalls are {110},and the planar defect density is low.The first-principles calculations show that the {110} plane growth of ZnTe has lower formation energy,and thus the ZnTe nanowires are mainly obtained by gas-liquid-solid(VLS)growth mode under gold catalysis,while the nanosheets were obtained by continuously depositing ZnTe on the {110} plane of the nanowire grown along the[121]direction driven by a gas-solid growth mechanism.High-quality quasi-two-dimensional ZnTe nanosheets can provide a possible material system for the development of novel semiconductor micro/nanophotonic devices.