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MOCVD Growth And Characterization Of High Quality AlGaN/GaN Heterostructures

Posted on:2010-10-29Degree:DoctorType:Dissertation
Country:ChinaCandidate:J Y NiFull Text:PDF
GTID:1228330395462566Subject:Microelectronics and Solid State Electronics
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Due to a large conduction band offset of2.78eV between AlN and GaN and a strong polarization effect in the III-V nitrides, a large amount of sheet carriers called the two-dimensional electron gas (2DEG) is formed at the interface between the AlGaN and GaN heterostructure.The2DEG has high mobility because it is far from the ionized charge, making the AlGaN/GaN heterostructures to be the ideal material system for the fabrication of high electron mobility transistors (HEMTs). In order to improve the performance of the AlGaN/GaN HEMTs, one should enhance the product of the carrier concentration n and the mobility μ of the2DEG, which could be achieved by increasing Al composition of the AlGaN layer. However, the alloy disorder scattering and the interface roughness scattering increase greatly when the Al composition is above a certain value, resulting in the reduction of the mobility μ of the2DEG. Fortunately, the insertion of a AlN layer into the GaN buffer broke through the bottleneck of the enhancement of the conductivity of the AlGaN/GaN heterostructures. The subject of this work started from the design of the reactor of the metalorganic chemical vapor deposition (MOCVD) equipment. By optimizing the growth conditions of the GaN buffer layer and the AlN interlayer, we obtained the high quality of AlGaN/GaN heterostructures, therefore the high performance of AlGaN/GaN HEMTs.First of all, the finite element analysis of the temperature field and flow field in the reactor of the MOCVD system was carried out by using ANSYS simulations. The results of the simulations allowed us to improve the structure of the showerhead, showing three advantages as (1) the low consumption of raw materials required for the GaN growth;(2) the excellent surface morphology of epitaxial, and (3) the good uniformity of the GaN epilayers.The effect of reactor pressure which is one of the most important parameters in the MOCVD process on the growth rate, surface morphology, and crystalline quality of GaN films grown by MOCVD was investigated. It was found that the formation and the growth of initial high temperature GaN islands were associated significantly with the employed reactor pressure. Although the GaN surface became rough and the growth rate of the GaN films decreased with increasing of the reactor pressure, the crystalline quality of the GaN films improved by reducing the threading dislocations during the GaN islands growth and coalescence. Meanwhile, the measurement of electrical performance for the GaN films also proved that the utilization of high reactor pressure was valuable for the improvement of the mobility in GaN.Hall measurement with Van der Pauw method and the capacitance-voltage (C-V) characteristics were performed on AlGaN/GaN heterostructures. It was found that the value of sheet carrier density obtained from Hall measurement was larger than that deduced from C-V carrier density profile, which was ascribed to two reasons. On the one hand, Schottky contact deposited on AlGaN/GaN heterostructure changed the surface states of the AlGaN barrier layer, causing some electrons in2DEG to be extracted to the void surface states of the AlGaN layer. On the other hand, the accuracy of C-V measurement itself was influenced by the series resistance effect, which caused underestimation of the magnitude of the background carrier concentration.The influence of the AlN interlayer on the structural and stress properties of the GaN films were studied. Compared with the GaN films without an AIN interlayer, the one using an AlN interlayer reduced the dark pit density on the surface of the GaN in spite of the crystalline quality of the GaN was not markedly affected. The surface roughness of GaN decreased with the increasing growth temperature of the AIN interlayer. It was found from both the photoluminescence measurement and Raman measurement that the AlN interlayer introduced a compressive stress into the GaN film. The compressive stress in GaN increased with the increase of the growth temperature of the AlN interlayer. Furthermore, it was found that the stress in GaN has a linear relationship with the shift of E2Raman mode by using direct X-ray diffraction measurement.GaN epilayers and AlGaN/GaN heterostructures inserted by the AlN interlayers grown under different temperatures were also investigated. It was found that the surface of the AlGaN/GaN heterostructures with the AIN interlayer had less dark pits compared with the one without the AlN interlayer. The use of the AlN interlayer did not change the crystalline quality of both the GaN epilayers and the AlGaN/GaN heterostructures. However, both the carrier concentration n and the mobility μ of the2DEG in AlGaN/GaN heterostructures improved due to that the AlN interlayer enhanced the compressive strain of the GaN film. It was found that the increased compressive stress enhanced the piezoelectric polarization field in GaN, which consequently caused more accumulation of electrons at the AlGaN/GaN interface. On the other hand, the employment of the AlN interlayer reduced the lattice mismatch between the GaN and AlGaN and smoothed the AlGaN/GaN interface, and thus increased the2DEG mobility by weakening the interface roughness scattering. The1-μm gate-length AlGaN/GaN HEMTs based on the GaN buffer layer with the AIN interlayer were fabricated in this work. The measurements showed that the maximum drain current and transconductance were increased by42%and20%compared with the one without the AIN interlayer.
Keywords/Search Tags:metalorganic chemical vapor deposition, AlGaN/GaN heterostructures, highelectron mobility transistors, two-dimensional electron gas, AlN interlayer, stress
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