New process approaches to metalorganic vapor phase epitaxy of III-nitrides for high power HEMTs
As a promising candidate for future microwave power devices, GaN-based high-electron mobility transistors (HEMTs) have attracted much research interest. However, group III-nitride growth related issues – predominantly the high defect densities due to lacking lattice matched substrates – limit the quality and life time of the HEMTs. In the AlGaN/GaN heterostructure the GaN buffer layer is particularly important for that matter. Several requirements need to be satisfied, such as high buffer resistivity, low dislocation densities and smooth and defect free interfaces. Poor reproducibility and uniformity of the heterostructure is an additional challenge. Research aim of this work is to develop a MOVPE based growth process that allows the reliable and efficient deposition of high quality material for HEMT devices based on AlGaN/GaN heterojunctions on sapphire substrates. Usage and development of adequate growth control and monitoring instruments is essential to meet these premises. Direct wafer surface temperature determination is a must during growth, though it is challenging for transparent substrates. A new calibration method is demonstrated that uses the in-situ measured, temperature dependent band-gap shift of SiC. This technique allows adjusting any MOVPE system to identical run-by-run wafer temperatures. Using this new temperature determination approach the cause of irreproducibility in an horizontal reactor is investigated. The two major origins of irreproducibility are parasitic deposits that form on the cold reactor walls which allow for insufficient temperature control. Thus, a new deposition approach is introduced that reduces the formation of parasitic depositions, by injecting the metalorganics near the substrate unlike the conventional process. Experiment and modelling verify the suitability of the new growth approach for enhanced reproducibility. The new approach leads to a fourfold reactor uptime. The structural, morphological and electrical characteristics of the grown layers are comparable for both processes. It has been found that inversion of the precursor supply has a drawback when considering the thickness uniformity. In order to achieve a homogeneous distribution of growth species over the substrate, flow mechanical aspects in our reactor system are investigated. The gas velocity and the gas "nature" are therefore important optimization parameters. For that purpose the influence of different carrier gases – nitrogen and hydrogen – on growth is examined. Using the inverted inlet configuration, uniformity can only be improved by changing the nature of the carrier gas, thus, modifying the diffusive transport and decomposition of growth species. This way, in N2 atmosphere a standard deviation in thickness of less than 3% is achieved. Nitrogen is crucial for improved thickness homogeneity, but apparently nitrogen not only deteriorates the surface morphology but it affects the growth mode in the early stages of deposition. It is found that nitrogen enhances untimely start of 2D growth, which leads to GaN layers with a larger number of edge type dislocations. Dislocations, in turn, influence the strain in the layer, leading to more relaxed GaN layers when grown in nitrogen ambient. Dislocation scattering is the very probable source of the decline in electrical data when nitrogen is used as carrier gas. As a result a larger "coalescence thickness" is preferred, since it reduces the density of dislocations. Therefore a strategy was developed with which good quality material can be obtained despite the nitrogen ambient needed for uniform layers. Uniform layers with smooth surfaces and state of the art electrical and structural quality material were achieved by using only small amounts of hydrogen during the coalescence phase in otherwise nitrogen ambient. We managed to increase the Hall mobility at room temperature up to 671 cm^2/Vs with a carrier concentration of 9x10^16cm^-3 and a FWHM for the (002) and (102) reflections of 238 and 349 arcsec respectively. The inverted inlet combines the reproducibility of the growth process with a good growth control for AlGaN. Due to the high incorporation efficiency a minimal amount of adduct formation occurred. Tight alloy composition and thickness control was achieved with less than one percent deviation over the wafer for device relevant aluminum contents in either hydrogen or nitrogen as carrier gas. AlGaN/GaN heterostructures grown on the GaN template. Despite state of the art sheet conductivity values the heterojunctions exhibit additional conductivity due to residual donors in the layer. A strategy for growing semi-insulating GaN was introduced using nitrogen as the carrier gas. Thus, acceptor like edge type dislocations are systematically generated that compensate the background carrier concentration. One key factor for varying the density of these dislocations is the morphology development in coalescence. Decreasing the coalescence time by using nitrogen in combination with shorter annealing times is beneficial for obtaining high resistive layers with smooth and specular surfaces. Electron mobilities at room temperature of 1490 cm^2/Vs at a sheet carrier concentrations of 9.88 <10^12cm^-2 demonstrate the suitability of HEMT structures grown on "nitrogen GaN". The DC and RF characteristics of the produced HEMT structures are comparable to state of the art HEMT results as published in literature. These findings finally prove the functionality and feasibility of our MOVPE growth process. Thus, new holistic approaches to III-nitride monitoring and growth are presented. High reliability, reproducibility and minimum downtimes of the equipment is proven, optimized AlGaN/GaN film deposition shown and transferability of process temperatures to any reactor demonstrated.
info:eu-repo/classification/ddc/620, Ingenieurwissenschaften, MOCVD-Verfahren, Wide-gap-Halbleiter, Heterostruktur, Nitride, Pyrometrie, HEMT, GaN, MOVPE, III-nitrides, in-stu reflectometry
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