超薄 ALD Al2O3 緩衝層增強 AlN 於 Si(100)上的生長與探討 AlN/Al2O3/Si HEMT 的應用潛能

Abstract

氮化鋁（AlN）由於其超寬能帶和優異的極化特性而被廣泛應用於III族氮化物的高電子遷移率電晶體（HEMT）。 AlN/通道材料異質接面附近累積的高 2DEG密度使元件具有一些理想的電特性，例如低導通電阻Ron、低轉導gm和高驅動電流。然而，基於AlN/Si異質結構的HEMT元件，雖擁有極大的能帶不連續與相較於傳統Al(Ga)N/GaN異質結構更高的極化感應電荷，卻尚未被提出。此外，矽基板由於其高可得性、大晶圓尺寸、低成本和高導電性等優點而具有顯著的優勢。這些優勢引起了這項研究的動機。 由於極化效應的強度很大程度上取決於AlN的晶體品質，因此在製造基於AlN/Si異質結構的HEMT元件之前，確保Si基板上AlN薄膜的晶體品質並分析其電特性至關重要。 首先，AlN/Al2O3/Si HEMT的TCAD模擬顯示出極高的導通電流密度Ids,sat = 21 A/mm、較低的Ron = 2 Ωmm和較大的轉導峰值gm,peak = 700 mS/mm，這顯示出比傳統的Al(Ga)N/GaN HEMT更好的表現。 在這項研究中，AlN薄膜在高度晶格不匹配的Si(100)基板上的GIXRD結果觀察到FWHM優於以MOCVD生長AlN於Si(111)基板上 [20] 的c-plane晶體結構。此外，超薄ALD Al2O3緩衝層的加入進一步提高了AlN薄膜的晶體品質。從GIXRD結果中觀察到，加入ALD Al2O3緩衝層可以減少微應變，並促進AlN的生長。 拉曼光譜觀察到的AlN E2(high)和Al(TO)聲子模式的進一步證實了以ALD-ALA在Si(100)基板上實現了AlN的生長。此外，較厚的AlN薄膜與ALD Al2O3緩衝層皆有助於減少面內雙軸應變。 AFM結果表明，具有較厚ALD Al2O3緩衝層的AlN表面形貌得到改善，在具有ALD 20 cycle Al2O3緩衝層的AlN薄膜上實現了具有平行台階和平台結構的原子級光滑表面形貌。 霍爾測量顯示AlN在高電阻率Si(100)異質結構上的電特性得到改善，驗證了AlN/Si間的極化效應，並揭示了ALD 20 cycle Al2O3 緩衝層使片載子濃度和片電阻率分別提高了1000倍和10倍。此外，I-V和R-V量測進一步證實ALD Al2O3緩衝層對Si基板電導率的增強。 C-V量測得到有ALD-ALA AlN的樣品中出現反型層，這進一步證明了極化效應引發的通道，並且在有ALD 20 cycle Al2O3的樣品中實現了最大片載子密度~8.2×1012 cm-2。 透過元件模擬、材料量測與電性分析，這項研究的結果顯示了基於AlN/Al2O3/Si異質結構的HEMT的潛力。

Aluminum Nitride (AlN) is widely used in the III-nitride based high electron mobility transistors (HEMTs) due to its ultra-wide bandgap and superior polarization property. The high 2DEG density accumulating near the heterojunction of AlN/channel material enables some desirable electric characteristics of the device such as low on-resistance Ron, low transconductance gm and high drive current. However, a HEMT device based on the AlN/Si heterostructure, which possesses larger band offset and higher polarization-induced charge compared to the conventional Al(Ga)N/GaN heterostructure has not been proposed yet. Moreover, Si substrates offer significant benefits due to their advantages in high accessibility, large wafer size, low cost, and high conductivity. These advantages motivate the research of this work. Since the strength of the polarization effect strongly depends on the crystal quality of AlN, ensuring the crystal quality and analyzing the electric characteristics of the AlN thin film on Si substrates are critical prior to the fabrication of a HEMT device based on the AlN/Si heterostructure. First, TCAD simulation of the AlN/Al2O3/Si HEMT demonstrated a large on-current density Ids, sat of 21 A/mm, low Ron of 2 Ωmm, and large maximum transconductance gm, peak of 700 mS/mm, which shows higher performance than the conventional Al(Ga)N/GaN HEMT device. In this work, c-plane crystal structure with superior FWHM to the MOCVD AlN on Si(111) substrates [20] was observed from the GIXRD results of ALD-ALA AlN thin films on large lattice-mismatched Si(100) substrate. Moreover, the incorporation of ultra-thin ALD Al2O3 buffer layers further enhanced the crystal quality of AlN thin films. Micro strain reduction was observed with the incorporation of ALD Al2O3 buffer layers from the GIXRD results, enhancing the growth of AlN. Raman spectroscopy further confirms the achievement of ALD-ALA grown AlN on Si(100) substrates with the observation of the E2(high) and A1(TO) phonon modes of AlN, and indicates in-plane biaxial strain reduction in thicker AlN films and the incorporation of ALD Al2O3 buffer layer. AFM results indicates the improvement of the surface morphology of AlN with thicker ALD Al2O3 buffer layers, achieving an atomically-smooth surface morphology with parallel step-and-terrace structure on AlN thin films with ALD 20 cycle Al2O3 buffer layers. The Hall measurement indicates electric characteristics improvements of the AlN on high-resistivity Si(100) heterostructure, validating the polarization effect between AlN/Si, and reveals a 1000 times and 10 times improvements in sheet concentration and sheet resistivity with the ALD 20 cycle Al2O3 buffer layer. Additionally, the I-V and R-V measurements further confirms the enhancement of the conductivity of the Si substrates with the ALD Al2O3 buffer layers. The C-V measurements further proved the polarization induced channel with the inversion layer in the samples with ALD-ALA AlN, and the largest sheet carrier density ~8.2×1012 cm-2 is achieved in the sample with ALD 20 cycle Al2O3. Trough device simulation, material characterization and analyzation of the electric characteristics, the results of this work show potential in the application of a HEMT device based on AlN/Al2O3/Si heterostructure.