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Epitaxial Growth Of GaN-based HEMT On Large Diameter Silicon Substrates

Posted on:2017-06-18Degree:DoctorType:Dissertation
Country:ChinaCandidate:S M LiFull Text:PDF
GTID:1318330482999485Subject:Optical Engineering
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Since the first AlGaN/GaN-based high electron mobility transistor (HEMT) was developed in 1993, great efforts have been devoted to improve the performance of this polarization-based electrical device. However, compared to the widely used LED, GaN-based HEMT is still suffering from the high cost. In order to commercialize the HEMT device, GaN-on-Si technology is one of the effective methods due to the advantages of low cost, large size, good thermal conductivity, and silicon-compatible device manufacturing. Nevertheless, the large lattice and thermal mismatches between GaN and Si(111) lead to a high dislocation density and even cracks. As a result, it's challenging to grow thick crack-free high-quality HEMT on Si(111).To improve the performance of GaN-based HEMT, this dissertation mainly focuses on the research of HEMT grown on Si(111) including the epitaxial designs of AIN buffer, AlGaN transition layers, wafer warpage, highly resistive (HR) layer, and AlGaN/GaN/Ga(Al)N heterostructure. Achievements are as follows:In the aspect of AlN-on-Si(111), the thermal cleaning time, TMA1 preflow time, one-step and two-step growths, and AIN thickness were studied. It was found thatl?5 min for thermal cleaning,12-15 s for TMA1 preflow time, one-step growth, and 250 nm were the optimum growth conditions for AIN buffer. Moreover, adopting SiH4 during the thermal cleaning process was found to help recover the Si(111) surface and improve the AIN morpglogy. The linewidth of AIN XRD (0002) rocking curve was 1014", indicating a high crystalline quality for AIN grown on Si(111).For the design of stress control layer, we proposed a two-step graded AlGaN structure to introduce the compressive stress in advance and successfully achieved a mirror-like crack-free high-quality GaN film. TEM and Williamson-Hall measurements demonstrated that not only many dislocations terminated at the GaN/AlGaN2/AlGaN 1 interfaces, but also a large number of residual dislocations were bent and annihilated. The root mean square (RMS) of 5 ?m×5 ?m GaN surface was 0.31 nm and (0002) and (1012) rocking curve line widths were 305" and 336" respectively, which were among the best results in the world.For the control of wafer warpage, based on the analysis of strain evolution during the growth of GaN/AlGaN2/AlGaNl/AlN structure, we observed that A1N grown on Si(111) was under a slightly tensile stress (0.66 GPa), while AlGaNl, AlGaN2, and GaN suffered from the compressive stresses (-3.57 GPa,-2.41 GPa, and -0.75 GPa). In order to decrease the curvature of HEMT wafer at room temperature, we developed two methods to reduce the bow. One was adopting a thicker silicon substrate and the other was decreasing the thickness of stress control layers (AlGaN/AIN).For the design of HR layer in the HEMT structure, firstly we established a quantitative relationship between growth conditions and carbon concentrations ([C]), which could help to grow Ga(Al)N buffer layers with a given [C]. Then, electrical breakdown characteristics of AlxGa1-xN buffer layers grown on Si(111) were investigated by varying the carbon concentration ([C]:from ?1016 to 1019 cm-3), Al-composition (x=0 and 7%), buffer thickness (from 1.6 to 3.1 ?m), and substrate type (n type and p type). It was found that the improvement of off-state breakdown voltage (VBD) for GaN-based HEMT by increasing [C] was observed to be limited when [C] exceeding 1×10 cm-3. By increasing the HR Al0.07Ga0.93N buffer thickness from 1.6 to 3.1 ?m, the leakage current was greatly reduced down to 1 ?A/mm at a bias voltage of 1000 V.For the design of AlGaN/GaN/Ga(Al)N heterostructure, we studied the impact of Ga(Al)N back barrier, GaN channel, AlN interlayer, AlGaN barrier, and cap layer on the HEMT performance through the integrated analysis of theoretical calculations and experiments. With the reduced [C]<1017 cm-3 in GaN, improved AlGaN/GaN interface, decreased alloy scattering, and high-quality AlGaN barrier, a Hall result of ?=2094 cm2/Vs, ns=1.23×1013 cm-2, and R?=243 ? was achieved.Based on the above studies, we successfully grown the 4-?m-thick mirror-like crack-free GaN-based HEMT structure on large diameter silicon substrates (4 inch and 6 inch). After the 6" HEMT wafer was processed into devices with 1.7×2.8 mm2 in area, the measured output current reached 19 A and the specific on-resistance was 11.9 m?·cm.
Keywords/Search Tags:GaN Growth, Large Diameter Silicon Substrate, High Electron Mobility, Transistor, AlGaN/AIN Stress Control Layers, Wafer Warpage, Highly, Resistive Layer, Back Barrier
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