GaN基太赫兹器件的新结构及材料生长研究
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摘要
相比于其他传统半导体材料，GaN太赫兹器件具有更大的电子有效质量，更高的纵向声子能量，更快的子带间电子散射，更大的负阻区电流峰谷比和更高的二维电子气密度等，使其在太赫兹领域中具有一定的优势。然而GaN材料的生长技术和传统的半导体材料相比依然不够成熟，GaN材料中的缺陷密度依然很高，这会直接导致GaN太赫兹器件的性能衰减，更甚至于击穿和烧毁。为了推动GaN太赫兹器件的应用，必须首先解决相关器件材料中的结晶质量问题。传统的结晶质量改善可以通过横向外延过生长技术(ELOG)或者插入层技术来达到。然而ELOG技术包括了多步生长以及掩膜技术，实现起来不仅繁琐而且代价昂贵；插入层技术是可以过滤位错缺陷，但是由于插入层技术通常采用异质材料，那么必然存在晶格失配情况，这会在插入层界面处引入失配位错，从而不能有效直接地降低位错密度。针对这些问题，本文结合GaN太赫兹器件的特殊结构，对生长过程进行了调整以及优化，从而有效过滤了位错密度，明显改善了器件渡越区中结晶质量。并且通过一系列的实验测试，获得了一些令人振奋的结果，主要研究结论如下：
1，论文报道了采用MOCVD法生长实现了底层Notch掺杂加速区(BNL)结构和顶层Notch掺杂加速区(TNL)结构的GaN基太赫兹耿氏器件外延结构，并进行了相关的材料测试分析，并通过自主建立的外延缺陷生长动力学模型解释了器件有源区生长位错湮灭的机理。
2，论文报道了一种改进的四元件小信号电容-电压（C-V）测试等效模型，该模型被用来消除测试中的频率离散现象。接着我们提出一种新颖的数学模型来计算GaN缓冲层中的体缺陷密度。在GaN太赫兹耿氏二极管中，体缺陷的存在（尤其是深能级点缺陷）会影响器件的高场输运特性。为了分析这类缺陷，我们采用了PL谱，改进的C-V，SIMS，HRXRD，以及TEM等测试对结构中的点缺陷进行了深入的研究。研究提出热退火处理可以使点缺陷密度分布降低30%左右，有效改善了GaN中深能级缺陷引入的电荷俘获效应会导致射频功率和转换效率的明显衰减。
3，论文报道了设计和实验获得了亚微米渡越区长度的带有Al组分三级递进AlGaN层的AlGaN/GaN异质结太赫兹耿氏二极管外延结构，详细研究AlGaN热电子注射(HEI)层对GaN渡越区位错的阻断和结晶质量的影响，进一步通过分析验证揭示了位错从缓冲层穿透并进入AlGaN HEI层时，螺位错和刃位错在AlGaN/GaN异质界面发生弯曲和随之而来的湮灭现象，指出采用底层AlGaN-HEI层结构可以作为AlGaN/GaN异质结THz耿氏器件的最佳选择。同时研究了通过赝晶生长的Al组分线性渐变AlGaN插入层来湮灭N极性GaN中位错，提出了一种有效改善N极性GaN结晶质量的方法以而改善N极性太赫兹器件的特性。
4，论文报道了针对InAlN/GaN异质界面的结晶质量优化过程，采用PMOCVD生长InAlN层，然后生长一层低温（LT）GaN保护层，这样可以保证在紧着的高温GaN生长时InAlN中不会发生In簇的形成以及In组分再分布。从TEM测试中可以看出，高质量的InAlN/GaN异质界面不仅可以有效过滤位错，而且可以防止失配位错的产生。因此，PMOCVD生长InAlN加上LT-GaN层可以获得高质量的InAlN/GaN异质界面，从而改善材料的整体结晶质量，最终为相关太赫兹器件应用提供一种有价值的参考。
Compared to other conventional semiconductor material, GaN terahertz device hasmore electron effective mass, higher longitudinal phonon energy, faster electronscattering between sub-band, greater negative resistance current peaks valley, highertwo dimension electron gas density and so on, showing that GaN has evident advantagesin the field of terahertz application. However, the growth of GaN material technologycompared with traditional semiconductor materials is still not mature enough to obtainhigh crystal quality, the high defect density in GaN materials is still high, therefore, itcan directly result in GaN terahertz device performance attenuation, and more evenbring in breakdown and burning. In order to promote the application of terahertz GaNdevice, the related devices material quality problem must be first taken into account.The traditional improvement of crystal quality can be achieved byEpitaxial-lateral-overgrowth (ELOG) or interlayer technologies. However, ELOGtechnology requires multi-step regrowth and mask technology, which is cumbersomeand costly to be achieved. Interlayer technology can filter dislocation defect, but it canintroduce large lattice-mismatch with GaN layer therefore lead to generation of misfitdislocations at the interface, which could not effectively reduce the dislocation density.To solve these problems, this paper takes the special structure of GaN terahertz devicesinto consideration, and then carry out the adjustment and optimization for growthprocess, thus effectively filter the dislocation density, significantly improved the crystalquality in the transit region of devices. Some encouraging results have been obtainedthrough a series of experimental tests, the main research conclusions are as follows:
1, We report the GaN based THz Gunn diode epitaxial growth through MOCVD,including the bottom notch-doping accelerating region structure (BNL) and the topnotch-doping accelerating region structure (TNL). Subsequently, the relatedmeasurement and analysis are carried out, and independent establishment of the growthdynamics model of the epitaxial defect is proposed to explain the annihilation ofdislocation in the active region.
2, We propose an improved small-signal equivalent model to eliminate frequencydispersion phenomenon in capacitance-voltage (C-V) measurement, and introduce anew mathematic method to calculate the amount of bulk defect existing in the GaNbuffer layer. As well known, the existence of bulk defect (especially deep-level pointdefect) can degrade high-field transport characteristic during further device operation.To have a further investigation, we make use of PL, improved C-V, SIMS, HRXRD, and TEM measurements to take a deep insight into the bulk defect in the device. In addition,the experiment indicates that the annealing treatment reduces the point defect density ofsamples by around30%, which can weaken the charge-trapping effect introduced bydeep-level crystal defects in GaN and encourage the improvement of radio frequencypower and conversion efficiency in terahertz device operation.
3, We report the design and experiment of the sub-micron transit regionAlGaN/GaN heterostructure THz Gunn diode with a tri-graded AlxGa1-xN layer. Wecarry out a detailed investigation about the effects of AlGaN hot electron injecting layer(HEI) on blocking the dislocation in GaN transit region and improving the crystalquality. In the following, the related experiment demonstrates that the bending andannihilation of the screw and edge dislocations occur at the AlGaN/GaN heterointerface.We propose that the bottom AlGaN-HEI structure is a promising candidate for thefabrication of GaN Gunn diodes in terahertz regime. At the same time, we report on thedislocation annihilation in N-polar GaN with a pseudomorphicly growngraded-Al-fraction AlGaN interlayer. The improvement for the crystal quality ofN-polar GaN can be introduced in optimizing the N-polar related THz deviceperformance.
4, We report the crystal quality optimization of the InAlN/GaN heterointerface.PMOCVD was employed to grow InAlN interlayer. Subsequently, a low temperature(LT)-GaN protective layer was deposited on the InAlN interlayer, to prevent the risk ofindium cluster formation and indium redistribution during the following hightemperature top GaN growth. According to the TEM measurement, it demonstrates thatthe InAlN/GaN heterointerface can not only filter the dislocations with great efficiency,but also avoid the generation of misfit dislocation. Therefore, the InAlN layer grown byPMOCVD and the LT-GaN layer can significantly bring in high quality InAlN/GaNheterointerface, thus improve the crystal quality of the whole material. In brief, ourwork provides a valuable reference to the related terahertz devices application.
1，论文报道了采用MOCVD法生长实现了底层Notch掺杂加速区(BNL)结构和顶层Notch掺杂加速区(TNL)结构的GaN基太赫兹耿氏器件外延结构，并进行了相关的材料测试分析，并通过自主建立的外延缺陷生长动力学模型解释了器件有源区生长位错湮灭的机理。
2，论文报道了一种改进的四元件小信号电容-电压（C-V）测试等效模型，该模型被用来消除测试中的频率离散现象。接着我们提出一种新颖的数学模型来计算GaN缓冲层中的体缺陷密度。在GaN太赫兹耿氏二极管中，体缺陷的存在（尤其是深能级点缺陷）会影响器件的高场输运特性。为了分析这类缺陷，我们采用了PL谱，改进的C-V，SIMS，HRXRD，以及TEM等测试对结构中的点缺陷进行了深入的研究。研究提出热退火处理可以使点缺陷密度分布降低30%左右，有效改善了GaN中深能级缺陷引入的电荷俘获效应会导致射频功率和转换效率的明显衰减。
3，论文报道了设计和实验获得了亚微米渡越区长度的带有Al组分三级递进AlGaN层的AlGaN/GaN异质结太赫兹耿氏二极管外延结构，详细研究AlGaN热电子注射(HEI)层对GaN渡越区位错的阻断和结晶质量的影响，进一步通过分析验证揭示了位错从缓冲层穿透并进入AlGaN HEI层时，螺位错和刃位错在AlGaN/GaN异质界面发生弯曲和随之而来的湮灭现象，指出采用底层AlGaN-HEI层结构可以作为AlGaN/GaN异质结THz耿氏器件的最佳选择。同时研究了通过赝晶生长的Al组分线性渐变AlGaN插入层来湮灭N极性GaN中位错，提出了一种有效改善N极性GaN结晶质量的方法以而改善N极性太赫兹器件的特性。
4，论文报道了针对InAlN/GaN异质界面的结晶质量优化过程，采用PMOCVD生长InAlN层，然后生长一层低温（LT）GaN保护层，这样可以保证在紧着的高温GaN生长时InAlN中不会发生In簇的形成以及In组分再分布。从TEM测试中可以看出，高质量的InAlN/GaN异质界面不仅可以有效过滤位错，而且可以防止失配位错的产生。因此，PMOCVD生长InAlN加上LT-GaN层可以获得高质量的InAlN/GaN异质界面，从而改善材料的整体结晶质量，最终为相关太赫兹器件应用提供一种有价值的参考。
Compared to other conventional semiconductor material, GaN terahertz device hasmore electron effective mass, higher longitudinal phonon energy, faster electronscattering between sub-band, greater negative resistance current peaks valley, highertwo dimension electron gas density and so on, showing that GaN has evident advantagesin the field of terahertz application. However, the growth of GaN material technologycompared with traditional semiconductor materials is still not mature enough to obtainhigh crystal quality, the high defect density in GaN materials is still high, therefore, itcan directly result in GaN terahertz device performance attenuation, and more evenbring in breakdown and burning. In order to promote the application of terahertz GaNdevice, the related devices material quality problem must be first taken into account.The traditional improvement of crystal quality can be achieved byEpitaxial-lateral-overgrowth (ELOG) or interlayer technologies. However, ELOGtechnology requires multi-step regrowth and mask technology, which is cumbersomeand costly to be achieved. Interlayer technology can filter dislocation defect, but it canintroduce large lattice-mismatch with GaN layer therefore lead to generation of misfitdislocations at the interface, which could not effectively reduce the dislocation density.To solve these problems, this paper takes the special structure of GaN terahertz devicesinto consideration, and then carry out the adjustment and optimization for growthprocess, thus effectively filter the dislocation density, significantly improved the crystalquality in the transit region of devices. Some encouraging results have been obtainedthrough a series of experimental tests, the main research conclusions are as follows:
1, We report the GaN based THz Gunn diode epitaxial growth through MOCVD,including the bottom notch-doping accelerating region structure (BNL) and the topnotch-doping accelerating region structure (TNL). Subsequently, the relatedmeasurement and analysis are carried out, and independent establishment of the growthdynamics model of the epitaxial defect is proposed to explain the annihilation ofdislocation in the active region.
2, We propose an improved small-signal equivalent model to eliminate frequencydispersion phenomenon in capacitance-voltage (C-V) measurement, and introduce anew mathematic method to calculate the amount of bulk defect existing in the GaNbuffer layer. As well known, the existence of bulk defect (especially deep-level pointdefect) can degrade high-field transport characteristic during further device operation.To have a further investigation, we make use of PL, improved C-V, SIMS, HRXRD, and TEM measurements to take a deep insight into the bulk defect in the device. In addition,the experiment indicates that the annealing treatment reduces the point defect density ofsamples by around30%, which can weaken the charge-trapping effect introduced bydeep-level crystal defects in GaN and encourage the improvement of radio frequencypower and conversion efficiency in terahertz device operation.
3, We report the design and experiment of the sub-micron transit regionAlGaN/GaN heterostructure THz Gunn diode with a tri-graded AlxGa1-xN layer. Wecarry out a detailed investigation about the effects of AlGaN hot electron injecting layer(HEI) on blocking the dislocation in GaN transit region and improving the crystalquality. In the following, the related experiment demonstrates that the bending andannihilation of the screw and edge dislocations occur at the AlGaN/GaN heterointerface.We propose that the bottom AlGaN-HEI structure is a promising candidate for thefabrication of GaN Gunn diodes in terahertz regime. At the same time, we report on thedislocation annihilation in N-polar GaN with a pseudomorphicly growngraded-Al-fraction AlGaN interlayer. The improvement for the crystal quality ofN-polar GaN can be introduced in optimizing the N-polar related THz deviceperformance.
4, We report the crystal quality optimization of the InAlN/GaN heterointerface.PMOCVD was employed to grow InAlN interlayer. Subsequently, a low temperature(LT)-GaN protective layer was deposited on the InAlN interlayer, to prevent the risk ofindium cluster formation and indium redistribution during the following hightemperature top GaN growth. According to the TEM measurement, it demonstrates thatthe InAlN/GaN heterointerface can not only filter the dislocations with great efficiency,but also avoid the generation of misfit dislocation. Therefore, the InAlN layer grown byPMOCVD and the LT-GaN layer can significantly bring in high quality InAlN/GaNheterointerface, thus improve the crystal quality of the whole material. In brief, ourwork provides a valuable reference to the related terahertz devices application.
引文
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