Study On The Novel Nitride InAlN Semiconductor Heterostructures And HEMT Devices

For the last two decades, GaN-based high electron mobility transistors (HEMT)have received widespread and extensive research, and significant progress has beenachieved for the potential applications in high temperature, high voltage, high frequencyand high power electronic devices because of the combination of unique fundamentalmaterial properties, such as large bandgap, large breakdown field, high electronsaturated drift velocity, and two-dimensional-electron-gas (2DEG) with excellenttransport property induced by strong spontaneous and piezoelectric polarization fields.To meet the increasing demands for device performance, tremendous efforts have beenmade in material epitaxial growth technology, new materials application, advanceddevice structure design as well as fabrication process, so as to push the deviceperformance even further and close to the limit of theoretical prediction. Among theproposed approaches, lattice-matched strain free InAlN/GaN heterostructure and HEMTdevice become the hotspot and frontier of research in the field of nitride wide bandgapsemiconductor and microelectronics. Compared to conventional AlGaN/GaNheterostructures, InAlN/GaN heterostructures eliminate the strain relaxation and inversepiezoelectric effect existing in barrier, and alleviate the deleterious effects of inherentstrain induced defects on2DEG mobility and sheet carrier density, thus improving theHEMT reliability operating under high temperature and high voltage for a long time.Also, high sheet carrier density can be formed in lattice-matched InAlN/GaNheterostructures with thin barrier relying on much stronger spontaneous polarizationeven though without contribution from piezoelectric polarization, providing highercurrent drive capability and output power density. The thin InAlN barrier can effectivelysuppress short channel effects in highly scaled HEMT, and reduces the technicalcomplexity and difficulty, improves the uniformity and repeatability of devicefabrication process across the wafer by avoiding the damage on the channel caused bygate recess dry etching. In addition, very high thermal and chemical stability has beendemonstrated for lattice-matched InAlN/GaN HEMT, which can operate under hightemperature of1000oC without showing distinct performance degradation. However,due to the phase separation and composition inhomogeneity, it is very difficult to growInAlN ternary alloy and related heterostructures with high quality, which impedes therealization of its advantages and vast application. The research on epitaxial fabricationand characteristics of InAlN/GaN heterostructures and HEMT devices remainsimmature and is under early stage in domestic. This dissertation mainly focuses on epitaxial growth and structure design of InAlN semiconductor heterostructures,fabrication and characterization of related HEMT. The major achievements are listed asfollowing:1. High quality GaN template is achieved by adopting graded ratio of V/III fromhigh to low. It is found that the capability of high-temperature AlN blocking layer toimprove the quality of GaN template is limited, but the additional compressive strain inthe upper GaN induced by high-temperature AlN blocking layer enables to improve thetransport property of heterostructures. However, aided by the technology of graded ratioof V/III, we obtain high quality GaN template with smooth surface morphology, lowdislocation density, and low background carrier concentration, whose full widths of halfmaximum of rocking curves of HRXRD (002) and (102) planes reduce to70and348arcsec, respectively. This technology improves the quality of GaN template by growthitself other than introduction of foreign structures, and simplifies epitaxy procedure andlays a solid foundation for growth of high quality InAlN-based heterostructurematerials.2. High quality nearly lattice-matched InAlN/AlN/GaN heterostructures aresuccessfully grown on c-plane sapphire substrate by proposed pulsed metal organicchemical vapor deposition (PMOCVD). Through the experimental optimization ofgrowth parameters, such as growth pressure, TMIn pulse duration, and growthtemperature, and studying their effects on the properties of InAlN/AlN/GaNheterostructures, respectively, the best growth parameters and conditions for epitaxy ofIn0.17Al0.83N are realized, and the composition and thickness of InAlN alloy can beeffectively controlled and designed. A smooth surface morphology with distinct atomicstep flows is observed for the as-grown InAlN/AlN/GaN heterostructures with a rootmean square (rms) roughness of0.3nm and without indium droplets and phaseseparation. A high electron mobility of1402cm2/Vs and a sheet carrier density of2.01×1013cm-2are obtained at room temperature, and the electron mobility dramaticallyreaches to5348cm2/Vs at77K. The heterostructures exhibit a low average sheetresistance of234/□with a nonuniformity of1.22%across the2in. wafer. Comparedto traditional continual MOCVD, PMOCVD is a promising approach and offers someadvantages, which provides reference to the nitride materials epitaxy from a growthmethod point of view. In addition, the growth mechanism of the improved materialquality of InAlN and related heterostructures by PMOCVD is interpreted.3. By studying on the effects of AlN interlayer thickness on the properties ofInAlN/AlN/GaN heterostructures, an optimal AlN interlayer thickness of1.2nm is obtained. It is showed that the AlN interlayer thickness influences the transportproperties and surface morphology of InAlN/AlN/GaN heterostructure. Introduction ofultrathin AlN interfacial interlayer could effectively increase the conduction band offsetof InAlN/AlN/GaN heterostructures and improve the interface quality, thus suppressingthe interface roughness and alloy disorder scattering and improving electron mobilityand sheet carrier density. For the InAlN/GaN heterostructures without AlN interlayers,the electron mobility is949cm2/Vs at300K and2032cm2/Vs at77K, while itincreases to1425cm2/Vs and5308cm2/Vs by the insertion of1.2nm AlN interlayer,respectively. Simultaneously, the InAlN/AlN/GaN heterostructures with the optimized1.2nm AlN interlayer exhibit a best surface morphology. The influencing mechanismsof AlN interlayer on the2DEG transport properties of InAlN/AlN/GaN heterostructuresare discussed and analyzed in detail form the standpoints of scattering mechanism andenergy band.4. The PMOCVD growth technology for InAlN/AlN/GaN heterostructuresoptimized on sapphire substrate is successfully transferred on semi-insulating SiCsubstrate, and a first domestic InAlN/AlN/GaN HEMT on SiC substrate is fabricatedindependently. The HEMT with a gate length (LG)of0.8μm and a gate width (WG)of2×50μm is made from an InAlN/AlN/GaN heterostructures with a electron mobility of1032cm2/Vs and a sheet carrier density of1.59×1013cm-2. The device exhibits amaximum drain current density of1A/mm, a maximum extrinsic transconductance of310mS/mm, and a current gain cutoff frequency (fT) and a maximum oscillationfrequency (fMAX) of18GHz and39GHz, respectively, and the product of fTand LGis14.4GHz·μm. It is found that the reverse gate leakage current of InAlN/AlN/GaNHEMT on sapphire substrate is larger than that of one on SiC substrate, and to reducethe reverse gate leakage current of InAlN/AlN/GaN HEMT made on sapphire substrate,an InAlN/AlN/GaN MOS-HEMT is processed by employing3nm Al2O3deposited byatomic layer deposition as gate insulation dielectric by ourselves.5. High quality nearly lattice-matched InAlN/GaN/InAlN/GaN double-channelheterostructures are successfully obtained for the first time by PMOCVD incombination of proposed innovative GaN channel growth technology, which is grownunder a low temperature in nitrogen ambient and a high temperature in hydrogen insequence. This approach prevents the lower InAlN barrier from degradation of surfacemorphology and avoids the risk of indium-cluster and redistribution during thesubsequent high temperature GaN channel growth, and provides a high quality channelfor the top2DEG. By investigating the effects of top GaN channel thickness on the transport properties of InAlN/GaN/InAlN/GaN heterostructures, an optimal top GaNchannel thickness of20nm is achieved. At this optimal thickness, the as-growndouble-channel heterostructures are free of strain relaxation and phase separation, with arms roughness of0.2nm. Also, no parasitic conduction channels are identified in theInAlN barriers. A high electron mobility of1414cm2/Vs is demonstrated along with asheet carrier density of2.55×1013cm-2at room temperature, which solves thecontradiction in the single-channel heterostructure that high electron mobility is alwaysgenerated by sacrificing2DEG density, and enhances the2DEG mobility at hightemperature. The low sheet resistance of172/□presented here sets a new benchmarkin InAlN-related nitride heterostructures so far. This achievement has been chosen as aresearch highlight by Applied Physics Letters.6. An InAlN/GaN/InAlN/GaN double-channel HEMT is demonstratedindependently, and a detailed measurements and analyses are performed. FabricatedHEMT with gate dimensions of0.8×100μm2exhibits a maximum drain current densityof1059mA/mm, a maximum transconductance of223mS/mm, a fTof10GHz, and afMAXof21GHz, respectively, and the product of fTand LGis8GHz·μm. The deviceexhibits a distinct double-channel behavior concerning with both static-output andsmall-signal performance, evidencing the successful practice of growth technology fordouble-channel heterostructures from the device level. Also, This device showsremarkably minimal gate Schottky diode reverse leakage current on the order of1and40μA/mm at VGD=–10and–20V, respectively. The three-terminal breakdown voltageis16V, and the actual destructive breakdown voltage is26V.7. High quality ultrathin AlN/GaN heterostructure is successfully realized byPMOCVD using indium as a surfactant under high temperature, and an AlN/GaNHEMT is obtained on these heterostructures. By optimization of growth temperatureand thickness of AlN barrier layer, a high electron mobility of1398cm2/Vs and a sheetcarrier density of1.3×1013cm-2are measured for the AlN/GaN heterostructures with abarrier layer thickness of4nm grown at830oC. Compared with the previously reportedresults achieved by conventional MOCVD, a low sheet resistance of344/□isachieved at a relatively lower growth temperature. A maximum drain current density of305mA/mm and a maximum transconductance of95mS/mm are demonstrated for theAlN/GaN HEMT with a gate geometry of (0.6×50)×2μm2.8. Nearly lattice-matched high Al-content AlGaN channel InAlN/AlGaNheterostructures are grown by PMOCVD. The quality of high Al-content AlGaNchannel is significantly improved by virtue of AlGaN/AlN superlattices to filter dislocation and to release stress. By adjusting the flow and duration of TMIn pulse,nearly lattice-matched InxAl1-xN/AlyGa1-yN heterostructures with different Al-contentAlGaN channel are realized, which have a low sheet resistance in comparison of that ofthe previously reported traditional AlxGa1-xN/AlyGa1-yN heterostructures in theliteratures. This structure pushes the lattice-matched concept to higher Al compositionand makes a preliminary probe into the potential application of InAlN and relatedheterostructures in high voltage power electronic device.Based above achievements, this dissertation presents a study on the novel nitrideInAlN semiconductor heterostructures and HEMT devices by making full use ofsuperior material properties of InAlN alloy. Relying on innovation of growth technologyand design of advanced device structure, some fundamental technological issues frommaterial epitaxy and device fabrication are resolved, which fills the research gap inChina and enlarges the research field of nitride material and device, and also paves theway and lays an important foundation for development of the InAlN-based material anddevice.