Crystal Growth And Characterization Of Aluminum Nitride And Hexagonal Boron Nitride

As the third generation semiconductor materials,AlN and hBN have great application prospects in optoelectronics and microelectronics.However,due to its unique physical and chemical properties,the growth of AlN and h-BN single crystals with large size and high quality is still a challenging work.As the most effective growth method of AlN single crystal,physical vapor transport method has many technical difficulties,including necking of raw materials,difficulty in expanding diameter,slow growth rate,secondary nucleation and so on.In view of the above growth difficulties,this dissertation carried out the research on AlN crystal growth by flux and vaporization of aluminum in nitrogen,focusing on the selection of flux and the growth habit of AlN crystal at low temperature and low pressure.In the aspect of hBN growth by flux method,Cu-Ni binary flux is proposed for the first time to grow hBN,and the growth of hBN single crystal under mild growth conditions is realized.Based on this,Schottky deep UV detector is prepared,and the device has good detection performance.The growth habit of spontaneous nucleation of AlN crystals by flux method was studied.The growth mechanism of AlN single crystal grown with Ca₃N₂ flux was proposed.The crystallization habit of AlN crystal is summarized.Under different supersaturation conditions,AlN crystal morphology can be divided into three types:one-dimensional extended type,which is needle like and long columnar;the other is two-dimensional extended type,which is flake like;the third is three-dimensional extended type,which is granular.At low supersaturation,the crystal extends along the c-axis[0001].With the increase of AlN solubility,{01-10}becomes the fast growth surface,and the base surface of the crystal is(10-10).Compared with the amber AlN obtained by traditional PVT method,the origin of crystal color is preliminarily explored.The spontaneous nucleation and growth habit of hBN crystals by flux were studied.The hBN single crystal with maximum size of 6 mm and thickness of 20?m was obtained by using Cu-Cr binary flux system.The effects of growth atmosphere and raw material source on the quality and size of crystal were studied.We observed that the domain shape of hBN is mainly triangular,which is supposed to be caused by the release of thermal stress in the crystal.Metal-semiconductor-metal(MSM)deep ultraviolet photodetectors were fabricated by mechanical exfoliation of hBN.The specific detection rate of the detector at 215 nm is more than 3.68×10⁸ Jones,and the cutoff wavelength of the photodetector is about 270 nm,which indicates that the device can be an ideal candidate for solar blind photodetector.There are two main reasons for the slow growth of AlN crystal by PVT.The nitrogen,as one of the raw materials,would inevitably hinder the transmission of Al vapor,especially under high pressure(>500 Torr),the transmission rate of Al vapor becomes the limiting step of the growth rate.The other reason is the kinetic limitation of nitrogen adsorption.If the adsorption coefficient of AlN is set to 1,the adsorption coefficient of N₂ is 10^-3-10^-5,and only the adsorbed atoms participate in the crystal growth.According to the classical Langmuir adsorption theory,the adsorption rate is proportional to the partial pressure.In order to solve this contradiction,we propose to separate the aluminum source from the nitrogen source,limit the saturated vapor pressure of pure aluminum by using aluminum alloy,and realize the growth of AlN crystal at a lower nitrogen partial pressure(5 k Pa).The diameter of AlN polycrystal can reach 30 mm and the thickness is 10?15 mm.The growth rate can reach 0.45 mm/h,but the crystal quality needs to be further improved.At the same time,because the pressure of growth atmosphere is low,we can use inert gas as the growth switch.The difficulty of this method is to accurately control the partial pressure of aluminum and nitrogen.What's more,due to the low growth temperature,the growth equilibrium constant of aluminum nitride changes slowly with temperature,and the growth window is small,so we need to accurately control the temperature gradient.Combined with VR-PVT software and infrared temperature measurement device,we regulated the temperature gradient and systematically studied the effects of different raw material ratio,different temperature gradient and different nitrogen partial pressure on the crystallization quality.