Research On Mbe Growth And Transport Characteristics Of HgTe/CdTe Thin Films And Quantum Wells

Strained HgTe has inverted band structure as a typical topological insulator material.Most researches have focused on uniaxially strained HgTe?100?.In this thesis,molecular beam epitaxy?MBE?was used to achieve the growth of high-quality HgTe?111?thin films and quantum wells on Cd Te/Ga As composite substrates.In addition,the strain characteristics of HgTe?111?thin films were studied theoretically and experimentally.The low-temperature transport properties of HgTe?111?quantum wells were systematically studied.In addition,we also tried to grow medium-wave HgCdTe thin film materials and made preliminary device verification.The main research contents are as follows:?1?Realized the growth of high-quality HgTe thin film.Firstly,the structure and principle of the MBE system are introduced,as well as the particularity of the Hg-based MBE system.Using the MBE system,Cd Te single crystal buffer layers with?111?crystal directions were grown on Ga As?100?substrates.The root mean square?RMS?roughness of the 1?m thick Cd Te?111?buffer layers is 0.8 nm,the atomic level surface flatness is achieved,so we chose Cd Te?111?as the buffer layer to grow the HgTe?111?film.Compared with the Cd Te buffer layer,the growth of the HgTe thin film is more difficult,its growth temperature is very low,and the temperature range is small?<3??.Through real-time monitoring of reflection high-energy electron diffraction?RHEED?,the growth conditions of HgTe thin films were optimized.High-quality HgTe thin films were finally obtained.In addition,the XRD,Raman and EDS of HgTe thin films were preliminary investigated.?2?The strain and interface characteristics of HgTe?111?film were systematically studied.Based on the force balance theory,the strain release critical thickness of HgTe?111?thin films on Cd Te substrate is 284nm calculated by Matthews equations.Through HRXRD combined with reciprocal space pattern?RSM?,the strain release characteristics of HgTe?111?samples with different thicknesses were studied.And the experimental results consistent with the theoretical calculations.In addition,through high-resolution transmission electron microscopy?HRTEM?testing,it was found that the HgTe thin film grown at 157? had twin-like defects at the HgTe-Cd Te interface.In the early stage of HgTe growth,RHEED real-time monitoring showed twin-like pattern;but for the HgTe thin film grown at 160?,the HgTe-Cd Te interface in the HRTEM and RHEED results exhibit high crystal quality.For HgTe?111?thin films,the orientation of defects such as misfit dislocations are[011],parallel to the HgTe?111?surface.RHEED can effectively monitor the?111?surface defects based on the principle of slightly incident high energy electrons diffraction.?3?The transport characteristics of the HgTe?111?quantum well were studied.For HgTe?111?quantum wells with different thicknesses and different growth conditions at 157? and 160?,the magnetic transport characteristics were systematically studied.It was found that the thin HgTe?111?quantum well grown at 157? has positive magnetoresistance at low-temperature and negative magnetoresistance at high-temperature;however,the HgTe?111?quantum well grown at 157? with a thickness of more than 67nm and grown at 160? show positive magnetoresistance at low and high temperature.The negative magnetoresistance characteristic of HgTe?111?quantum well may be origin from the grain boundary scattering of HgTe-Cd Te interface defects.After the Cd Te cap layer is selectively removed by etching solution,the positive to negative magnetoresistance transition temperature of?13nm and?31nm samples are about 120K,which is very close to the reported inverted to normal energy band transition temperature?113.8K?of the 8nm HgTe quantum well.Thus,we assume that the electron transport is protected by defects insensitive topological states at low temperatures,and exhibit positive magnetoresistance characteristics;when the temperature exceeds 120K,the HgTe quantum wells change to conventional normal energy bands,and a large number of interface defects scattering leads to negative magnetoresistance.In addition,the conductive channel properties of the two HgTe quantum wells were studied through the weak anti-localization?WAL?effect.Through the HLN model fitting,it was found that the edge conductivity of the?13nm HgTe quantum well is obvious,while the coupling effect of the edge and bulk conduction channel of the?31nm HgTe quantum well is robust.As the thickness increases,the bulk conduction mode dominates,and the influence of interface defects becomes weaker.The high-temperature negative magnetoresistance characteristic does not appear in the?67nm HgTe quantum well.?4?The HgTe quantum well Hall device was fabricated,and the weak anti-localization characteristics were studied.Using?13nm high-quality HgTe?111?quantum well samples grown at 160?,Hall devices were fabricated through UV lithography.Through gate voltage modulation,the WAL effect of HgTe was systematically studied.Combined with the HLN and Golub model fitting,the characteristics of the HgTe quantum well decoherence length and spin orbit length at different temperature and carrier concentration are studied.The decoherence length l?is up to 1400nm.There are strong spin-orbit coupling and conductive channel coupling effects in the sample.In addition,we obtained a weak Sd H oscillation signal,and made a preliminary analysis.?5?MBE growth of HgCdTe thin film material is achieved.The AFM surface roughness of the 2.9?m Hg_0.713Cd_0.287Te thin film is less than 5nm.The surface defect density is about 600 cm^-2,and?422?peak FWHM is about 115 arcsec.Through the fitting analysis of Fourier infrared transmission spectrum,the thickness uniformity and composition uniformity of the 3inch HgCdTe sample are less than 1%.In addition,photoconductive devices was prepared,and the photoelectric response characteristics of 3.5—5.5?m wavelength range at 77K were higher than 0.8?normalized?,and the response curves of different devices are almost the same.Finally,we tried to grow a two-dimensional electron gas?2DEG?structure of HgTe/Hg_0.3Cd_0.7Te quantum well,and explored the relationship between carrier concentration and In doping temperature.It provides a basis for the growth of HgCdTe infrared detectors and high mobility two-dimensional electron gas materials.