Research On The Growth And Physical Property Modulation Of Gallium Oxide Epitaxial Thin Film

In the last decade,β-Ga₂O₃ materials and devices have become the focus of academic.Due to its extremely large breakdown electric field and large-scale,high-quality and low-cost wafers,β-Ga₂O₃ has broad prospects in power electronics and solar-blind detectors.The rapid development of device applications has made the demand for epitaxial thin films with controllable doping and high crystalline quality more urgent.However,there are still many difficulties to be overcome in the preparation and related properties ofβ-Ga₂O₃ materials,especially the low crystal quality and difficult doping of epitaxial films,which limit its research and application in devices application.In addition,the lack of p-type Ga₂O₃ and the low doping activation rate under high doping concentration remain to be solved.Although the research on n-type doping is relatively mature,there is still difficult to continuously control the carrier concentration at low doping concentration.Therefore,the development of epitaxial film growth methods with high crystal quality and high doping activation rate and the control of material properties by different means are the focus ofβ-Ga₂O₃ materials and device research.Elastic strain can transform ultra-wide bandgap semiconductors into normal bandgap semiconductors or even metals,which provides a possible solution to the problems of p-type doping deficiency and poor ohmic contact inβ-Ga₂O₃ materials.However,under experimental conditions,β-Ga₂O₃ possesses highly anisotropic monoclinic crystal structure and poor mechanical flexibility,resulting in a complicated stress distribution under the elastic strains.Consequently,elastic strain-induced modulation of the bandgap ofβ-Ga₂O₃mainly focuses on the theoretical calculation,and is difficult to achieve in practical experiments.In this thesis,the epitaxial growth method combining two-step treatment process and pulsed laser deposition was used to prepare theβ-Ga₂O₃ epitaxial films with atomically flat and high doping activation rate.The effects of surface treatment process and growth oxygen pressure on the crystalline quality of epitaxial films were systematically explored.A simple and effective method is proposed to control the band gap of the material by elastic strain,and then the carrier concentration of the epitaxial film can be continuously controlled byαparticle irradiation.It provides reliable theoretical and experimental support for solving the problems of low crystal quality,low doping activation rate,and lack of p-type doping inβ-Ga₂O₃ epitaxial films.The specific research contents of this thesis are arranged as follows:（1）The growth ofβ-Ga₂O₃ epitaxial films with high crystalline quality and high doping activation efficiency on homogeneous substrates was successfully achieved by using a pulsed laser deposition growth method combined with a two-step surface treatment process by adjusting the annealing temperature,growth oxygen pressure and substrate temperature.The preparation of semi-insulating Fe-dopedβ-Ga₂O₃ epitaxial films with atomic-level step morphology was reported for the first time,and the growth of multilayer atomic-level flat epitaxial films was completed.The measurement results confirmed the high crystalline quality of the epitaxial films.The Fe dopedβ-Ga₂O₃ semi-insulating film,with a carrier concentration down to 4.39 x 10⁷ cm^-3 and a square resistance of up to 2.26 x 10¹¹Ω/□,is comparable to commercial Fe doped single crystal substrates;the Sn dopedβ-Ga₂O₃epitaxial layer,with a carrier concentration of 2.01 x 10²⁰ cm^-3 and a doping activation rate of 41.1%,breaks through theβ-Ga₂O₃ doping activation rate of 41.1%,breaking the bottleneck of low doping activation rate in the epitaxial growth process.（2）The ability of high-energy particle irradiation to continuously control the carrier concentration of semi-insulating Fe-dopedβ-Ga₂O₃ epitaxial films was explored,and the effects on the crystal structure,crystal quality and basic physical properties of the films were investigated.Firstly,the experimental results of irradiated films showed that the lattice of the epitaxial films was slightly distorted due to the bombardment of ions during the irradiation process,but the crystal structure and crystal quality did not change significantly.Then,the irradiated Fe-dopedβ-Ga₂O₃ epitaxial films were used to fabricate Schottky barrier diodes,which were systematically tested.The test results show that the carrier concentration in semi-insulating Fe-dopedβ-Ga₂O₃ epitaxial films is continuously controllable in the range of 6.52×10¹³ to 2.25×10¹⁵ cm^-3 with the increase of irradiation dose.Measurements of the Fe 2p peak suggest that high-energy particle irradiation may invalidate the compensatory effect of Fe doping,releasing trapped carriers.Irradiation effectively controls the carrier concentration of the epitaxial layer in the low concentration range,which makes up for the blank of epitaxial growth.With the increase of irradiation dose,the current characteristics of the device are gradually enhanced,and the breakdown voltage is decreased.The breakdown field strength of the diode device without p-type termination and field plate structure is as high as 2.4 MV/cm.The experimental results show that the irradiation introduces defects in the epitaxial layer and increases the carrier concentration.By changing the irradiation dose,the purpose of regulating the carrier concentration of the epitaxial layer can be achieved.（3）A simple and effective method is proposed to apply continuous elastic strain toβ-Ga₂O₃ materials and to reversibly modulate their optical bandgap.The modified two-point bending method ensures that the sample can withstand greater elastic strain without cracks and defects in the absence of additional stresses on the substrate.The highly anisotropic elastic modulus in theβ-Ga₂O₃ material has been calculated using the first-principle based on density generalisation theory.The results of the tests on bent samples showed that the optical bandgap of theβ-Ga₂O₃ sheets decreases continuously with increasing elastic strain.Free-standingβ-Ga₂O₃ sheets exhibit a sizeable reduction in the bandgap up to 30%（from4.9 e V to 3.4 e V）.The bending experiments confirm that the modulation of the elastic stress optical bandgap is continuous and reversible and that no cracks or defects are produced during bending.In agreement with first principles calculations,mechanistic research experiments confirm the presence of elastic strain and strain gradients in the bent samples.The respective experimental results show that in the bentβ-Ga₂O₃ samples the variation in the optical bandgap is esulted from combined effects of the elastic strain and strain gradients.