以有機金屬化學氣相沉積於碳化矽基板上成長氮化鋁鎵/氮化鎵異質結構之高電子遷移率電晶體的應用

Abstract

本實驗中，是利用有機金屬化學氣相沉積成長氮化鋁鎵/氮化鎵高電子遷移率電晶體於半絕緣的碳化矽基板上。在第一部分，研究的重點是建立一個最佳化成長在碳化矽基板上成長氮化鋁鎵/氮化鎵高電子遷移率電晶體結構的製程，包括（i）氮化鋁緩衝層的厚度，（ⅱ）氮化鋁間隔層的厚度，以及（iii） 氮化鋁鎵阻礙層中鋁成分的調整。藉由X光繞射以及霍爾量測得知，使用較厚的氮化鋁緩衝層可有效改善氮化鋁鎵/氮化鎵異質結構的晶體品質和電子特性。緩衝層厚度在這項研究中的最佳值是120 nm，然而當氮化鋁緩衝層的厚度超過最佳化的厚度時，在材料表面上將產生裂紋。為了改善在氮化鋁鎵與氮化鎵之間的介面特性，實驗也插入一層薄的氮化鋁間隔層以防止散射效應對通道中電子的影響。實驗中使用不同成長時間 (0-15秒) 的氮化鋁間隔層，並且透過霍爾量測來觀察他們對氮化鋁鎵/氮化鎵異質結構電性的影響，發現當插入成長10秒的氮化鋁間隔層，可以得到最高的電子遷移率(1762 cm2/V•s)。在這個實驗中，將氮化鋁鎵厚度固定在23 nm，並針對不同的鋁成分(24 % - 29 %)做研究。從霍爾量測中，我們發現當氮化鋁鎵阻礙層中鋁成分為28 %時，會有最佳化的電性表現，其中片電阻、電子遷移率和片電子濃度分別為352 Ω/☐、1762 cm2/V•s、1.01×1013 cm−2。最後也進一步製作成高電子遷移率電晶體元件，採用的閘極長度為1.5 μm、源極到汲極為7 μm，從DC測量中得知，最大汲極電流為668 mA/mm，轉移電導160 mS/mm、off-state崩潰電壓約150V。 在完成基本的氮化鋁鎵/氮化鎵高電子遷移率電晶體結構後，實驗為了進一步提升電晶體的晶體品質與電子特性，也做了一些調整，包括使用在高壓成長的氮化鎵薄膜以及成長20 nm以下的氮化鋁鎵阻礙層。透過插入一層在500 torr高壓成長的氮化鎵薄膜於氮化鋁緩衝層上，會在氮化鎵成長時產生一段從三維到二維 (3D-2D)的過渡成長時間，並可以提高氮化鋁鎵/氮化鎵異質結構的晶體品質和電子特性，此電晶體的片電阻、電子遷移率和片電子濃度能夠被提升達到318 Ω/☐、1850 cm2/V•s、1.076×1013 cm−2。 成長很薄的氮化鋁鎵阻礙層這個方法是可以提高高電子遷移率電晶體元件在高頻率操作的性能。在這項研究中，將阻礙層厚度降低到10 nm，同時為了保持高的在二維電子氣通道中的電子濃度，我們也增加了在氮化鋁鎵阻礙層中的鋁成分達到40 %。在這個鋁成分為40 %的氮化鋁鎵/氮化鎵異質結構中，其片電阻、電子遷移率和片電子濃度分別為543 Ω/☐、887 cm2/V•s、1.39×1013 cm−2。結果顯示，當鋁成分上升，載子濃度有著顯著的增加，然而電子遷移率卻同時急遽的下降。材料特性研究中，發現這種快速變差的電性表現，來自於在氮化鋁鎵阻礙層中晶體品質與表面形貌的惡化。因此為了保持在這個結構中的高電子遷移率特性，如何調整成長參數來成長薄且含有高鋁成分的氮化鋁鎵阻礙層是未來重要且必須解決的議題。

In this study, AlGaN/GaN HEMT structures were grown on semi-insulating SiC substrates by using metal organic chemical vapor deposition (MOCVD) method. In the first part, the study is focused on growing the device structure for AlGaN/GaN HEMT on SiC substrate with optimum processing parameters. The growth parameters here includes (i) AlN buffer layer thickness; (ii) AlN spacer thickness; and (iii) the Al composition of AlGaN barrier layer. From the XRD and Hall measurement, it can be found that the crystalline quality and electrical performance of AlGaN/GaN heterostucture are improved by using thicker AlN buffer layer. However, cracks were generated on the material surface when the buffer layer thickness exceeds the optimum value of 120 nm. In order to improve the interface quality between AlGaN and GaN, a thin AlN spacer layer was inserted at the interface to reduce scatterings of channel electrons due to interface roughness. AlN spacer layers with different growth times (0 to 15s) were used and their influence on the HEMT structure was investigated by Hall effect measurement. In this study, the highest electron mobility obtained for HEMT structure was 1762 cm2/V•s when the growth time of AlN spacer was 10 sec. In this study, the thickness of AlGaN barrier was fixed at 23 nm and the Al composition was varied from 24 to 29%. From the Hall effect measurement, it was found that the barrier layer with 28% Al yielded the best electrical properties with sheet resistance of 352 Ω/☐, carrier mobility of 1762 cm2/V•s and sheet carrier density of 1.01×1013 cm−2. Finally, HEMT devices with gate length of 1.5 μm and source-drain spacing of 7 μm were fabricated. The DC measurement showed that the AlGaN/GaN HEMT on SiC has a maximum drain current of 668 mA/mm, a tranconductance of 160 mS/mm, and an off-state breakdown voltage of 150V.. After completing the basic device structure for AlGaN/GaN HEMT, some modifications were also introduced to further improve the HEMT performance. These modifications included the use of (i) a GaN layer grown at high pressure, and (ii) very thin AlGaN barrier layer (<20 nm). By inserting a GaN layer grown at high pressure (500 torr) right after the buffer layer, a 3-dimensional to 2-dimensional (3D-2D) transition growth was observed on the GaN layer growth. This transition could improve the crystal quality and electrical performance of the AlGaN/GaN heterostucture. By using this approach, AlGaN/GaN HEMT structure with improved sheet resistance of 318 Ω/☐, carrier mobility of 1850 cm2/V•s and sheet carrier density of 1.076×1013 cm−2was achieved. The very thin AlGaN barrier layer was used to improve the performance of HEMT device for high frequency operation in this study, the barrier thickness was reduced to 10 nm. In order to maintain high carrier density at the 2 dimensional electron gas (2DEG) channel, the Al composition in AlGaN was also increased up to 40%. For the AlGaN/GaN with 40% Al in the barrier layer, the sheet resistance was 543 Ω/☐, carrier mobility was 887 cm2/V•s and sheet carrier density was 1.39×1013 cm−2 .The results showed that, the electron mobility of the AlGaN/GaN HEMT decreases rapidly with the increased of Al composition. Further investigations showed that this degradation was attributed to the degradation of AlGaN barrier crystalline quality and surface morphology of AlGaN layer. Therefore, a modification on the growth parameter for AlGaN with high Al content should be carried out in the future in order to maintain the high electron mobility of the HEMT structure.