Study On Epitaxial Growth Of GaN Based ?-? Group Nitrides And Related Devices

Since 2000,the group III nitride semiconductor heterostructures represented by indium nitride(InN),gallium nitride(GaN)and aluminum nitride(AlN)and their electronic devices have been developed rapidly.These semiconductors are widely used in the field of optoelectronics,power electronics and acoustic devices for their excellent optical and electrical properties.Among the III-N semiconductor optoelectronics,the extensive application of GaN-based light-emitting diode(LED),opened the era of semiconductor lighting.In the field of power electronic devices,strong critical breakdown of the electric field,high electron mobility and high saturation drift velocity of III-N group make the power electronic devices excellent electrical characteristics.In the field of radio frequency devices,high electron mobility transistors(HEMTs)based on GaN have been used in the radar and communication.In addition,AIN piezoelectric single crystal has the advantages of small dispersion,good thermal stability,high thermal conductivity,low thermal expansion coefficient,high corrosion patience,low electrical conductivity and low acoustic loss at the high frequency range.Especially,AIN has the highest surface acoustic waves(SAWS)propagation velocity and good piezoelectricity in all inorganic non-ferrous piezoelectric materials.All above properties make AIN an ideal material for the fabrication of high-frequency surface acoustic wave devices and bulk acoustic wave devices in recent years.Since III nitride intrinsic substrates are not available,GaN films have to be grown heteroepitaxially on other substrates such as SiC,sapphire(?-Al2O3)and Si substrates.The lattice mismatch and thermal mismatch between silicon carbide(SiC)substrate and GaN are very small.Therefore,the GaN and AIN thin films grown epitaxially on SiC substrates have low defect density and low internal stress.In addition,due to high thermal conductivity,high breakdown strength and otherexcellent electrical properties,SiC substrates are suitable for the fabrication of high power LED and HEMTs etc.The disadvantage for the use of SiC substrate is the expensive substrate price which leads to the high cost of related devices.Silicon(Si)substrate has the advantages of easy growing and processing,cheap,high electrical conductivity,relatively high thermal conductivity.However,there are greater lattice mismatch and thermal mismatch between Si and GaN.Therefore,it is more difficult to epitaxially grow high-quality GaN and other group III nitrides on Si substrate.In addition,Si has a large optical absorption coefficient,which is disadvantagous for LEDs.Compared with SiC,sapphire has larger lattice mismatch and thermal mismatch with GaN,which will lead to high defect density of GaN film and accumulation of internal stress.These shortcomings make the poor epitaxial film quality and the deteriotation of LED device performance.However,it is possible to grow a GaN film having a high crystal quality by controlling the growth pressure,the temperature and the selection of the carrier gas during the growth process.Low price and high transparency make sapphire the common commercial substrate for the fabrication of GaN based LED.In this disertation,group ? nitride films were epitaxial grown on different substrates by metal organic chemical vapor deposition(MOCVD)system manufactured by Vecco and Aixtron.The epitaxial growth mechanisms of group ?nitride materials were studied.The optical and electrical properties of corresponding LED and HEMT devices were characterized.By designing and optimizing the multilayer structure of epitaxial wafer,high-quality,high-performance group ?nitride epitaxial films were grown.We also study the growth of GaN-based LED epitaxial structures and the physical properties of the corresponding chip.The interaction between the two-dimensional electron gas(2DEG)and SAWs in the HEMT structures are theoretically explored.At the same time,we have further analyzed the characteristics of the grown AIN piezoelectric film in high frequency SAWs devices by using COMSOL finite element analysis method.A band gap adjustable acoustic filter based on the combination between SAWs and phononic crystals in the piezoelectric material system was simulated.This dissertation includes the following contents:1.The epitaxial growth of GaN-based LED structures and the measurement of the ?-? characteristic curve of LED chips.The GaN-based LED structures were epitaxially grown on a c-axis oriented Al2O3 substrates by MOCVD method.The effects of the thickness of the underlying undoped GaN layer and the growth rate of the n-type GaN layer on the surface internal stress and wavelength uniformity of whole epitaxial film were studied.The stress distribution on the surface of the epitaxial layer was deteced by Raman scattering spectoscopy.It was found that thinning of the underlying undoped GaN layer and the lowering of the growth rate of n-type GaN layer caused the blue shifting of high level E2 acostic mode.It proves that the compress stress on the epitaxial film is partially relaxed.The wavelength uniformity of LED epitaxial wafers was characterized by photoluminescence(PL)spectroscopy.Test results show that the uniformity of main wavelength,peak wavelength and color are improved by thinning the underlying undoped GaN layer and the lowering the growth rate of n-type GaN layer.At the same time,the FWHM of PL spectrum of LED becomes narrow.Thus,by optimization epitaxial growth condition,the GaN-based LED structures with low stress and good wavelength uniformity can be obtained.The traditional MOCVD epitaxial GaN-based LED process takes a long time,which leads to high cost of LED device.Here,we used the GaN films grown on Al2O3 substrates fabricated by hydride vapor phase epitaxy(HVPE)as new type substrates(HVPE-GaN substrate)of epitaxial GaN-based LED structures.The structure of the GaN-based LED was characterized by microscopy,PL spectroscopy and electroluminescence(EL)spectroscopy.The optical and electrical properties of the corresponding chip were characterized.After the use of new HVPE-GaN substrates for LED epitaxy,non-doped GaN buffer layer was no longer grown.It effectively reduces the MOCVD working time and lowers the cost of LED devices.The LED chips were fabricated on GaN based epitaxial wafer by micro-nano processing technology.The relationship between growth conditions(growth temperature,In mass flow,Mg mass flow)of the p-type GaN layer at low temperature and the performance of the chips were analyzed from the measured ?-? characteristic curve of the chips.The static voltage patience capacity of the chips was tested.It proved that static voltage patience capacity of the LED chips is higher than 4000V.Furthermore,the failure mechanism of GaN-based LED chips is analyzed.2.The effect of growth condition on the performance of HEMT epitaxial structures.The interaction between SAWs and 2DEG was studied theoretically.AlGaN/GaN HEMTs Structures were epitaxially grown by MOCVD method.The effects of growth pressure and carrier gas composition(H2 and N2)on the AlGaN barrier layer and its channel 2DEG mobility were studied.Atomic force microscopy(AFM)and scanning electron microscopy-Energy Dispersive Spectrometer(SEM-EDS)were used to characterize the quality of the epitaxial layer.The results show that the surface morphology of the AlGaN barrier layer becomes more regular,i.e.the widths of steps increase and the heights of steps decrease as the reaction chamber pressure increases from 50torr to 200torr.However,when the chamber pressure increases to 200 torr,the surface morphology of AlGaN barrier layer begins deteriorate.It is also found that the proper introduction of H2 will improve the structural quality of the AlGaN layer.When the AlGaN barrier layer was grown under pure N2 carrier gas,some growth islands appered on the surface.After H2 was introduce into carrier gas,growth islands disappeared.The 2DEG mobility in the tunnel of AlGaN/GaN HEMTs was measured by Hall effect test system.When the pressure of the reaction chamber is 100torr and the flow mass of H2 occupys 59%of the total carrier gas(H2+N2),2DEG mobility reaches 1545cm2/V·s.The interaction between SAWs and 2DEG is studied by constructing model and theoretical calculation.By adjusting concentration of 2DEG in AlGaN/GaN heterojunctions,the energy transmission of SAWs in this structure can be monitored.3.GaN and AIN epitaxial growth on SiC substrates by MOCVDThe compressive stresses of GaN layers with different thicknesses on 4H-SiC substrates were studied and corresponding structural qualities were assessed.The critical thickness of two-dimensional growth of GaN film by MOCVD technique on 4H-SiC substrate was systematically studied.By controlling the growth time,GaN layers with high structural quality and low surface roughness were obtained.After the growth condition optimization,a crack-free GaN thin film with the thickness of 600 nm was grown.The full width at half maximum(FWHM)of the(002)reflection rocking curve was measured to be 279.6 arcsec by high resolution X-ray diffractometry.Above high quality GaN film can be used as the buffer layer of the heteroepitaxial growth of GaN.The AIN epitaxial films were grown on 6H-SiC substrates by MOCVD technique.The qualities of the corresponding AIN films were compared with those of other AIN films grown by magnetron sputtering on the same substrates by means of HRXRD,AFM.HRXRD test results indicated that the reflection intensity of(006)of AIN grown by MOCVD is stronger than that of AIN grown by magnetron sputtering.It implies that the structural quality of AlN grown by MOCVD is better than that of AIN grown by magnetron sputtering.When the AIN film was grown on SiC substrate,it was subjected to a compressive stress due to lattice mismatch and a tensile stress due to thermal mismatch.For above two AlN films,the reflection peak position distances between substrate and AIN film are larger than the theroretical value.It indirectly indicated that the stress caused by lattice mismatch was dominant.AFM observation indicated that the root mean square roughnesses for two AIN film grown by MOCVD and magnetron sputtering are 2.42nm and 0.244nm,respectively.Some growth hillocks were observed on the surface of AIN film grown by MOCVD.In comparison,some growth islands begin to merge and a few growth steps can be seen on the surface of AIN film grown by magnetron sputtering.Thus,the surface of AIN film grown by magnetron sputtering is much smoother than that of AIN film grown by MOCVD.4.Simulation of SAWs filter based on AlN/SiC and SAWs-PnCs filterInter-digital transducers(IDTs),which can excite SAWs,was fabricated by means of micro-nano processing technology on the LiNbO3 single crystal substrates.The transmission performances of SAWs filter were measured by network analyzer and simulated by COMSOL finite element analysis method,respectively.The transmission characteristics of SAWs on AlN/6H-SiC piezoelectric film were simulated and the transmission modes of SAWs were analyzed.The transmission characteristics of SAWs in the phonon crystals(PnCs)with or without enhanced coupling thin slabs on piezoelectric substrates(LiNbO3?GaN and AlN)were studied by three-dimensional finite element method(3D-FEM).The results indicated that more energy bands of SAWs could be produced when the enhanced coupling thin slabs were introduced into between two adjacent cylinders of PnCs.In addition,as the thickness of the coupling enhancement slabs increases,the prohibited frequency band of SAWs corresponding to band-stop filters in electro-acoustic devices were widened.