Research On Polycrystalline Diamond Semiconductor Material And MOSFET

As a typical representative of ultra-wide bandgap semiconductor materials,diamond has superior properties such as ultra-wide bandgap,ultra-high breakdown electric field strength,ultra-high thermal conductivity,high carrier mobility and high saturation speed.Diamond is a promising candidate for power,high frequency,and high temperature solid-state electronics.Therefore,it is also called"ultimate semiconductor".However,because of the difficulties in the preparation of high-quality diamond,n?p?-type doping and so on,the development of diamond semiconductor devices has been constrained.The growth mechanism of diamond is extremely complicated.For now,single crystal diamond can be only obtained by homoepitaxial or high-temperature and high-pressure methods,and the size enlargement is difficult.Therefore,polycrystalline diamond has attract much attention due to its large size.Through the hydrogen-plasma treatment,a two-dimensional hole gas?2DHG?can be formed on the polycrystalline diamond surface,which is similar to the heterojunction two-dimensional electron gas,and the device?commonly referred as H-terminated device?exhibits advantages in the power and microwave devices,etc..In addition,polycrystalline diamond,especially the nanocrystalline diamond films,which possess a high thermal conductivity and high surface flatness,has opens up the possibility for realizing the well-dissipated power device.Hence,in this work,we have studied the growth method and process of polycrystalline/nanocrystalline diamond to realize the preparation of nanocrystalline diamond films with high purity and high flatness.Furthermore,device structure and process of H-terminated MOSFET based on polycrystalline diamond has been also carried out.The devices has realized an extremely high output current density,which has reached the top level compared to the reported results.The main work of this article is shown as follows:1.Based on the Microwave Plasma Chemical Vapor Deposition?MWPCVD?method,we have studied the mechanism of how CH₄ concentration affect the key parameters such as the purity,morphology and grain size of the nanocrystalline diamond film,and the optimum process conditions for high purity and flatness has been obtained.Then,the purity and flatness of the nanocrystalline diamond film has been further improved by using the Ar/H₂forming carrier gas.The results show that with the decrease of the CH₄ concentration,the purity of the film increases,while it does not change significantly at the low CH4concentration,but the the ordered domain size and the formation of the surface of nanocrystalline diamond film will be deteriorated when the CH₄ concentration is over high or low.After Ar is added,the diamond film exhibits a high re-nucleation due to the strong etching effect of Ar,and the surface flatness and purity of the film exhibit a higher quality compared to the sample without Ar/H₂ forming carrier gas.2.Based on the X-ray Photoelectron Spectroscopy?XPS?technology,we've investigated?1?C 1s core level and valence band spectra of polycrystalline diamond;?2?Al 2p?Mo 3d?core level and valence band spectra of Al₂O₃?MoO₃?;?3?Al 2p?Mo 3d?and C 1s core level of Al₂O₃?MoO₃?/polycrystalline diamond interface sample,and the bandgap of Al₂O₃ and MoO₃ were obtained through O 1s core level and inelastic energy loss spectra.From the results mentioned above,the valence and conduction band alignment of Al₂O₃?MoO₃?/polycrystalline diamond were obtained.From the perspective of energy band structure,it was concluded that Al₂O₃ is more suitable to diamond hole devices as a gate dielectric due to its larger valence band offset and wider bandgap.3.Based on hydrogen-terminated technology and Atomic Layer Deposition?ALD?process,the characteristics of hydrogen-terminated polycrystalline diamond MOSFETs with gate dielectric grown by ALD under different temperature were studied.With the application of surface channel passivation,Au hard masking and the non-self-aligned gate technology,hydrogen-terminated device with an output current density of 329 mA/mm and a breakdown voltage of 143 V was obtained.Research shows that the hydrogen-terminated channel exhibit a highly sensitivity to temperature:Compared to the 2DHG density at lower temperature?200°C?,it has reduced about 38%at the high temperature?300°C?and the channel performance is significantly degraded.The stability of the device was greatly improved by covering Al₂O₃ on the surface channel as a gate dielectric as well as passivation layer.The non-self-aligned gate process significantly increases the breakdown voltage of the device.