The Growth of InAlN/AlN/GaN Heterostructures by MOCVD for High Electron Mobility Transistor Applications
Chang, Edward Yi
本實驗將利用有機金屬化學氣相沉積方式成長氮化銦鋁/氮化鋁/氮化鎵異質結構於藍寶石基板上，首先將著重於磊晶成長的參數如:成長壓力、成長溫度對於氮化銦鋁材料成長之影嚮，發現成長的壓力和溫度影響了氮化銦鋁材料的成長速率與鋁的化合效率。在成長參數固定後也對氮化銦鋁/氮化鋁/氮化鎵異質結構的參數做最佳化以改善電性，在元件結構中影響電性的因素如:氮化銦鋁厚度、氮化鋁厚度、氮化鋁/氮化鎵的界面。為了在氮化銦鋁/氮化鋁/氮化鎵結構中有好的界面，上述這些因素是相當關鍵的。當成長氮化鋁之前的壓力穩定時間是15秒時與氮化鋁厚度1.5奈米時，高電子遷移率電晶體(HEMT)有著片電阻519 Ω/☐、電子遷移率890 cm2/V•s、載子濃度1.350×1013 cm−2。
此外，為了提高HEMT元件的崩潰電壓，也進行了高溫氮化鋁緩衝層的厚度對崩潰電壓影響的研究，藉由增加氮化鋁緩衝層的厚度，可以有效地減少因從基板向外擴散的氧原子而造成的漏電。並且從X光繞射、表面蝕刻凹洞密度(EPD)分析以及霍爾量測發現，當氮化鋁緩衝層的厚度較厚時擁有較佳的氮化銦鋁/氮化鋁/氮化鎵晶體品質與電性，當氮化鋁緩衝層235奈米時片電阻、電子遷移率和載子濃度分別進一步提高為334 Ω/☐、1190 cm2/V•s、1.53×1013 cm−2。當使用雙接點量測時，此結構在200伏特之下的漏電只有0.1mA/mm。最後HEMT元件採用閘極長度1μm、源極到汲極7μm，從DC測量得到，最大汲極電流為890 mA/mm，轉移電導202 mS/mm、off-state崩潰電壓超過200V。|
Due to the characteristics of wide bandgap, high breakdown field, high electron mobility and excellent thermal conductivity, GaN electronic devices have become one of the most promising candidates for next generation high-power applications. Among the GaN based transistors, InAlN/GaN high electron mobility transistors (HEMT) have attracted a lot of attentions in recent years. For the InAlN/GaN HEMT, the InAlN material is lattice matched to GaN when the indium composition is 18%. Besides, the InAlN/GaN structure also has larger conduction band offset and higher spontaneous polarization field. Therefore, it can provide superior electrical performance as compared to conventional InGaN/GaN HEMT with good reliability if the material growth issues can be solved. In this study, InAlN/AlN/GaN heterostructures were grown on sapphire substrates by MOCVD. In the first part, the study was focused on the influence of epitaxial growth parameters, such as growth pressure and temperature, on the InAlN layer. It is found that the growth pressure and temperature affect the growth rate and Al incorporation efficiency in the InAlN material. The optimization of growth parameters were carried out to improve the electrical properties of the InAlN/AlN/GaN heterosturcture after the growth parameters were fixed, the device structure that affect the electrical properties, such as the InAlN barrier thickness, AlN spacer thickness and the interface between AlN spacer and GaN were discussed. These factors are key to achieve a good interface for the InAlN/AlN/GaN structure. By using pressure state time of 15 sec prior to the AlN spacer layer growth and a 1.5-nm thick spacer layer, HEMT structure with sheet resistance of 519 Ω/☐, carrier mobility of 890 cm2/V•s and sheet carrier density of 1.350×1013 cm−2 has been achieved. Further, in order to improve the HEMT device breakdown voltage, the effect of the HT-AlN buffer layer thickness was also investigated. By increasing the thickness of AlN buffer layer, the leakage current caused by the out-diffusion of O atoms from the substrate can be effectively reduced. Furthermore, from the X-ray diffraction, surface etching pit density (EPD) method and Hall effect measurements, InAlN/AlN/GaN with better crystal quality and improved electrical properties was obtained when thicker AlN buffer layers were used. By using a 235-nm thick AlN buffer, the sheet resistance, carrier mobility, and sheet carrier density were further improved to 334 Ω/☐, 1190 cm2/V∙s and 1.53×1013 cm−2, respectively. The leakage current of the buffer was only 0.1mA/mm when measured at 200 V by using two-terminal method. Finally, HEMT devices with gate length of 1 μm and source-drain spacing of 7 μm were fabricated. The DC measurement showed that a maximum drain current of 890 mA/mm, a tranconductance of 202 mS/mm and an off-state breakdown voltage of more than 200V have been achieved.
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