新型氮化物InAlN半导体异质结构与HEMT器件研究
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摘要
GaN基高电子迁移率晶体管（HEMT）具有大的禁带宽度、高的临界击穿场强、高的电子饱和漂移速度、以及强的自发和压电极化效应产生的具有优越输运特性的二维电子气（2DEG）等出色的材料性能，非常适合应用于高温、高压、高频大功率电子器件，在过去二十年中得到了广泛而深入的研究并取得了重大进展。为了满足对器件性能日益增长的需求，人们在材料外延技术、新材料应用、器件结构设计和器件制备工艺等方面作了大量努力，以期使器件性能接近理论预测极限。其中，晶格匹配无应变InAlN/GaN异质结材料与HEMT器件成为目前宽禁带氮化物半导体和微电子领域的研究热点和前沿。和常规AlGaN/GaN异质结相比，InAlN/GaN异质结势垒层中不存在应变弛豫和逆压电效应，这减轻了势垒层内在应变产生的缺陷对2DEG迁移率和面密度的影响，提高了HEMT器件在高温和高压下长时间工作时的可靠性。同时，InAlN势垒层有更强的自发极化效应，即使没有压电极化效应，晶格匹配InAlN/GaN异质结也能以较薄的势垒层产生高密度的2DEG，可以提高HEMT器件的电流驱动能力和输出功率密度。薄的势垒层能有效抑制HEMT器件尺寸缩小引起的短沟道效应，避免槽栅干法刻蚀工艺对2DEG沟道造成的损伤，降低HEMT器件制备工艺的复杂性和难度，进而提高工艺的一致性和重复性。此外，晶格匹配InAlN/GaN HEMT有极高的化学和热稳定性，可以在1000oC的高温环境下工作而没有明显的性能退化。然而，高质量InAlN及其异质结构材料的外延生长极其困难，常出现晶相分离和组分分布不均等现象，一直阻碍其优势的体现和广泛应用。国内针对InAlN/GaN异质结材料和HEMT器件的研究还很不成熟，尚处于起步阶段。本文即在此背景下，重点围绕InAlN半导体异质构材料的外延生长和结构设计、HEMT器件制备和性能分析等方面展开工作，取得的主要研究成果如下：
1.提出了一种先高后低的阶变V/III比技术，实现了高质量GaN基板材料的生长。研究发现，高温AlN阻挡层提高GaN基板结晶质量的能力有限，但会在顶层GaN中引入额外的压应变，该压应变有助于提高GaN基异质结的输运特性。而先高后低的阶变V/III比技术能明显提高GaN基板材料的质量，用此技术生长了表面光滑、低位错密度和低背景载流子浓度的高质量GaN基板，其（002）面和（102）面高分辨率X射线衍射（HRXRD）摇摆曲线半高宽（FWHM）分别降低到70arcsec和348arcsec。这种技术从GaN基板自身生长角度出发来提高结晶质量，而没有引入其他外来结构，简化了GaN基板材料的生长程序，为高质量InAlN基异质结构材料生长打下了坚实基础。
2.提出了一种脉冲式金属有机物化学气相淀积（PMOCVD）材料生长技术，成功地在c面蓝宝石衬底上生长了高质量近晶格匹配InAlN/AlN/GaN异质结材料。分别对外延压强、TMIn脉冲宽度和外延温度等参数进行了生长优化，研究了其对InAlN/AlN/GaN异质结性能的影响，获得了近晶格匹配In0.17Al0.83N材料的最佳生长参数和条件，并实现了对InAlN合金材料组分和厚度的有效控制和设计。生长的InAlN/AlN/GaN异质结表面光滑，表现出明显的原子台阶流，无铟滴析出和晶相分离，表面均方根粗糙为0.3nm，室温下2DEG迁移率和面密度分别为1402cm2/V s和2.01×1013cm-2，在低温77K时迁移率高达5348cm2/Vs，2寸晶圆片上2DEG方块电阻均值为234/□，不均匀性为1.22%。和常规连续式MOCVD技术相比，PMOCVD技术表现出了明显的优势和应用前景，从生长方法上给氮化物材料外延提供了借鉴。同时解释了PMOCVD技术提高InAlN及其异质结构材料质量的生长机制。
3.研究了AlN界面插入层厚度对InAlN/AlN/GaN异质结性能的影响，获得了1.2nm的最优厚度。研究发现，AlN界面插入层厚度影响InAlN/AlN/GaN异质结输运特性和表面形貌。超薄AlN界面插入层的引入，能有效增加InAlN/AlN/GaN异质结的导带断续并改善界面质量，降低界面粗糙度和合金无序散射，提高2DEG迁移率和面密度。无AlN插入层的InAlN/GaN异质结，2DEG迁移率在室温和低温77K下分别为949cm2/Vs和2032cm2/Vs，而引入1.2nm的AlN界面插入层后，2DEG迁移率分别提高到1425cm2/Vs和5308cm2/Vs。同时，AlN界面插入层厚度为1.2nm时，InAlN/AlN/GaN表现出最好的表面形貌。从散射机制和能带结构方面，分析和讨论了AlN插入层对InAlN/AlN/GaN异质结2DEG输运特性的影响机制。
4.成功把InAlN/AlN/GaN异质结的PMOCVD生长技术从蓝宝石衬底移植到半绝缘SiC衬底上，自主研制出国内首个基于SiC衬底的高性能InAlN/AlN/GaNHEMT器件。制备在2DEG迁移率和面密度分别为1032cm2/Vs和1.59×1013cm-2的InAlN/AlN/GaN异质结材料上，栅长（LG）和栅宽（WG）分别为0.8μm和2×50μm的HEMT器件，其漏极最大输出电流密度和最大跨导分别为1A/mm和310mS/mm，特征频率（fT）和最大振荡频率（fMAX）分别为18GHz和39GHz，其fT×LG达到14.4GHz·μm。研究发现，蓝宝石衬底上InAlN/AlN/GaN HEMT器件的栅极反向漏电大于SiC衬底上的，为了进一步降低蓝宝石衬底上InAlN/AlN/GaN HEMT器件的栅极反向泄漏电流，采用原子层淀积技术生长了3nm Al2O3绝缘栅介质，自主研制了InAlN/AlN/GaN MOS-HEMT器件。
5.提出了一种低温氮气和高温氢气相结合的GaN沟道生长技术，用PMOCVD技术成功生长了首个高质量近晶格匹配InAlN/GaN/InAlN/GaN双沟道异质结材料。该技术避免了后续高温GaN沟道生长时底层InAlN势垒层的晶相分离和表面形貌退化，并给顶层2DEG提供了高质量沟道。研究了顶层GaN沟道厚度对双沟道异质结材料输运特性的影响，获得了顶层GaN沟道的最佳厚度为20nm。在此厚度下，生长的InAlN/GaN/InAlN/GaN异质结未发生应变弛豫和相分离，表面rms粗糙为0.2nm，InAlN势垒层中不存在寄生导电沟道。室温下2DEG迁移率和面密度分别为1414cm2/Vs和2.55×1013cm-2，解决了单沟道异质结中常以牺牲2DEG面密度来提高迁移率的矛盾，并明显提高了2DEG的高温迁移率。低达172/□的方块电阻在InAlN基氮化物异质结材料中创立了最高指标，此项成果被《应用物理快报》选为研究亮点。
6.自主研制出InAlN/GaN/InAlN/GaN双沟道HEMT器件，并对其性能进行了深入测试和分析。制备的LG为0.8μm、WG为2×50μm的双沟道HEMT器件，漏极最大输出电流密度和最大跨导分别为1059mA/mm和223mS/mm，fT和fMAX分别为10GHz和21GHz，其fT×LG达到8GHz·μm。该器件在直流输出和交流小信号特性方面均表现出明显的双沟道特性，在器件级水平上验证了双沟道异质结外延技术的成功实践。同时，该HEMT器件栅漏肖特基二极管表现出极低的反向漏电，栅漏电压（VGD）为-10和-20V时的反向泄漏电流密度分别为1和40μA/mm。器件三端击穿电压为16V，实际破坏性击穿电压为26V。
7.在高温气氛中铟元素表面活化剂作用下，用PMOCVD技术成功生长了高质量超薄势垒AlN/GaN异质结材料，并实现了AlN/GaN HEMT器件。研究了AlN势垒层生长温度和厚度对AlN/GaN异质结输运特性的影响，在830oC生长的势垒层厚度为4nm的AlN/GaN异质结，室温下2DEG迁移率和面密度分别为1398cm2/Vs和1.3×1013cm-2。和已报道的用常规MOCVD技术外延的结果相比，我们在较低的生长温度下获得了低的2DEG方块电阻（344/□）。制备的LG和WG分别为0.6μm和2×50μm的AlN/GaN HEMT器件，漏极最大输出电流密度和最大跨导分别为305mA/mm和95mS/mm。
8.用PMOCVD技术生长了近晶格匹配高Al组分AlGaN沟道InAlN/AlGaN异质结材料。通过AlGaN/AlN超晶格结构来过滤位错和释放应力，显著提高了高Al组分AlGaN沟道材料的结晶质量。通过调节TMIn脉冲的流量和宽度，生长了不同组分的近晶格匹配InxAl1-xN/AlyGa1-yN异质结，其2DEG方块电阻低于已报道的常规AlxGa1-xN/AlyGa1-yN异质结材料的。此结构把晶格匹配的概念推向更高Al组分，为InAlN及其异质结构材料在高压电力电子器件中的潜在应用做了初步探索。
综上所述，本文充分利用InAlN合金材料的优越性能，对新型氮化物InAlN半导体异质结构和HEMT器件做了研究。通过材料生长技术的创新和新型器件结构的设计，解决了从材料外延到器件制备等领域内的基本技术问题，填补了国内研究的空白，拓展了宽禁带氮化物材料与器件的研究领域，为后续InAlN基材料和器件的发展铺平了道路和奠定了重要基础。
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.
1.提出了一种先高后低的阶变V/III比技术，实现了高质量GaN基板材料的生长。研究发现，高温AlN阻挡层提高GaN基板结晶质量的能力有限，但会在顶层GaN中引入额外的压应变，该压应变有助于提高GaN基异质结的输运特性。而先高后低的阶变V/III比技术能明显提高GaN基板材料的质量，用此技术生长了表面光滑、低位错密度和低背景载流子浓度的高质量GaN基板，其（002）面和（102）面高分辨率X射线衍射（HRXRD）摇摆曲线半高宽（FWHM）分别降低到70arcsec和348arcsec。这种技术从GaN基板自身生长角度出发来提高结晶质量，而没有引入其他外来结构，简化了GaN基板材料的生长程序，为高质量InAlN基异质结构材料生长打下了坚实基础。
2.提出了一种脉冲式金属有机物化学气相淀积（PMOCVD）材料生长技术，成功地在c面蓝宝石衬底上生长了高质量近晶格匹配InAlN/AlN/GaN异质结材料。分别对外延压强、TMIn脉冲宽度和外延温度等参数进行了生长优化，研究了其对InAlN/AlN/GaN异质结性能的影响，获得了近晶格匹配In0.17Al0.83N材料的最佳生长参数和条件，并实现了对InAlN合金材料组分和厚度的有效控制和设计。生长的InAlN/AlN/GaN异质结表面光滑，表现出明显的原子台阶流，无铟滴析出和晶相分离，表面均方根粗糙为0.3nm，室温下2DEG迁移率和面密度分别为1402cm2/V s和2.01×1013cm-2，在低温77K时迁移率高达5348cm2/Vs，2寸晶圆片上2DEG方块电阻均值为234/□，不均匀性为1.22%。和常规连续式MOCVD技术相比，PMOCVD技术表现出了明显的优势和应用前景，从生长方法上给氮化物材料外延提供了借鉴。同时解释了PMOCVD技术提高InAlN及其异质结构材料质量的生长机制。
3.研究了AlN界面插入层厚度对InAlN/AlN/GaN异质结性能的影响，获得了1.2nm的最优厚度。研究发现，AlN界面插入层厚度影响InAlN/AlN/GaN异质结输运特性和表面形貌。超薄AlN界面插入层的引入，能有效增加InAlN/AlN/GaN异质结的导带断续并改善界面质量，降低界面粗糙度和合金无序散射，提高2DEG迁移率和面密度。无AlN插入层的InAlN/GaN异质结，2DEG迁移率在室温和低温77K下分别为949cm2/Vs和2032cm2/Vs，而引入1.2nm的AlN界面插入层后，2DEG迁移率分别提高到1425cm2/Vs和5308cm2/Vs。同时，AlN界面插入层厚度为1.2nm时，InAlN/AlN/GaN表现出最好的表面形貌。从散射机制和能带结构方面，分析和讨论了AlN插入层对InAlN/AlN/GaN异质结2DEG输运特性的影响机制。
4.成功把InAlN/AlN/GaN异质结的PMOCVD生长技术从蓝宝石衬底移植到半绝缘SiC衬底上，自主研制出国内首个基于SiC衬底的高性能InAlN/AlN/GaNHEMT器件。制备在2DEG迁移率和面密度分别为1032cm2/Vs和1.59×1013cm-2的InAlN/AlN/GaN异质结材料上，栅长（LG）和栅宽（WG）分别为0.8μm和2×50μm的HEMT器件，其漏极最大输出电流密度和最大跨导分别为1A/mm和310mS/mm，特征频率（fT）和最大振荡频率（fMAX）分别为18GHz和39GHz，其fT×LG达到14.4GHz·μm。研究发现，蓝宝石衬底上InAlN/AlN/GaN HEMT器件的栅极反向漏电大于SiC衬底上的，为了进一步降低蓝宝石衬底上InAlN/AlN/GaN HEMT器件的栅极反向泄漏电流，采用原子层淀积技术生长了3nm Al2O3绝缘栅介质，自主研制了InAlN/AlN/GaN MOS-HEMT器件。
5.提出了一种低温氮气和高温氢气相结合的GaN沟道生长技术，用PMOCVD技术成功生长了首个高质量近晶格匹配InAlN/GaN/InAlN/GaN双沟道异质结材料。该技术避免了后续高温GaN沟道生长时底层InAlN势垒层的晶相分离和表面形貌退化，并给顶层2DEG提供了高质量沟道。研究了顶层GaN沟道厚度对双沟道异质结材料输运特性的影响，获得了顶层GaN沟道的最佳厚度为20nm。在此厚度下，生长的InAlN/GaN/InAlN/GaN异质结未发生应变弛豫和相分离，表面rms粗糙为0.2nm，InAlN势垒层中不存在寄生导电沟道。室温下2DEG迁移率和面密度分别为1414cm2/Vs和2.55×1013cm-2，解决了单沟道异质结中常以牺牲2DEG面密度来提高迁移率的矛盾，并明显提高了2DEG的高温迁移率。低达172/□的方块电阻在InAlN基氮化物异质结材料中创立了最高指标，此项成果被《应用物理快报》选为研究亮点。
6.自主研制出InAlN/GaN/InAlN/GaN双沟道HEMT器件，并对其性能进行了深入测试和分析。制备的LG为0.8μm、WG为2×50μm的双沟道HEMT器件，漏极最大输出电流密度和最大跨导分别为1059mA/mm和223mS/mm，fT和fMAX分别为10GHz和21GHz，其fT×LG达到8GHz·μm。该器件在直流输出和交流小信号特性方面均表现出明显的双沟道特性，在器件级水平上验证了双沟道异质结外延技术的成功实践。同时，该HEMT器件栅漏肖特基二极管表现出极低的反向漏电，栅漏电压（VGD）为-10和-20V时的反向泄漏电流密度分别为1和40μA/mm。器件三端击穿电压为16V，实际破坏性击穿电压为26V。
7.在高温气氛中铟元素表面活化剂作用下，用PMOCVD技术成功生长了高质量超薄势垒AlN/GaN异质结材料，并实现了AlN/GaN HEMT器件。研究了AlN势垒层生长温度和厚度对AlN/GaN异质结输运特性的影响，在830oC生长的势垒层厚度为4nm的AlN/GaN异质结，室温下2DEG迁移率和面密度分别为1398cm2/Vs和1.3×1013cm-2。和已报道的用常规MOCVD技术外延的结果相比，我们在较低的生长温度下获得了低的2DEG方块电阻（344/□）。制备的LG和WG分别为0.6μm和2×50μm的AlN/GaN HEMT器件，漏极最大输出电流密度和最大跨导分别为305mA/mm和95mS/mm。
8.用PMOCVD技术生长了近晶格匹配高Al组分AlGaN沟道InAlN/AlGaN异质结材料。通过AlGaN/AlN超晶格结构来过滤位错和释放应力，显著提高了高Al组分AlGaN沟道材料的结晶质量。通过调节TMIn脉冲的流量和宽度，生长了不同组分的近晶格匹配InxAl1-xN/AlyGa1-yN异质结，其2DEG方块电阻低于已报道的常规AlxGa1-xN/AlyGa1-yN异质结材料的。此结构把晶格匹配的概念推向更高Al组分，为InAlN及其异质结构材料在高压电力电子器件中的潜在应用做了初步探索。
综上所述，本文充分利用InAlN合金材料的优越性能，对新型氮化物InAlN半导体异质结构和HEMT器件做了研究。通过材料生长技术的创新和新型器件结构的设计，解决了从材料外延到器件制备等领域内的基本技术问题，填补了国内研究的空白，拓展了宽禁带氮化物材料与器件的研究领域，为后续InAlN基材料和器件的发展铺平了道路和奠定了重要基础。
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.
引文
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