金屬有機氣相沉積成長技術在氮化鎵基高電子遷移電晶體之應用

Abstract

近年來三族氮化物之高電子遷移率電晶體 (High Electron Mobility Transistors, HEMTs)，其高頻、高功率之應用潛力已被成功地驗證。 當三族氮化物 HEMTs 被應用於商業市場時，雖然已經過多年，但是仍然有許多的研究投注在效率與可靠度之改善上。本研究之目標即是開發氮化鎵 (GaN) HEMT所需之金屬有機氣相沉積(Metal Organic Chemical Vapor Deposition, MOCVD)成長技術，並藉由調變能障層之應力及消除於GaN緩衝層內之非刻意摻雜，以提升GaN元件之實用效能。 本研究首先在兩英吋的藍寶石單晶基板上成長AlGaN/GaN HEMTs 之結構，這是因為此基板成本較低廉，並且可獲得高品質磊晶結構。在此基板成長之AlGaN/GaN HEMT，因殘餘氧之故導致有高漏電流之特性。為消除高漏電流只有抑制此氧雜質問題才能解決。因此，為開發出高崩潰電壓之AlGaN/GaN HEMTs，必須能同時抑制自藍寶石基板之氧擴散，或者是去除由前驅物殘存之氧雜質，才能得到。 為提升HEMT元件之性能，首先必須要成長出高結晶品質之AlGaN 能障層，並且進行應力調變，方可達成可靠的性能改善。這些優化之成長參數，特別是氣壓值與三/五比，在成長高品質AlGaN時，對於相分離的抑制以及Al組成的非均質現象而言，非常重要。在AlGaN/GaN異質結構中之殘留應力，則來自於晶格常數之不匹配度，是可以藉由改變高溫成長於AlGaN與GaN之AlN中間層 (interlayer) 厚度，而獲得改善。另外，Al摻入AlGaN的量以及結晶品質，亦會受AlN中間層引發之應力影響。因此，使用優化之條件成長AlGaN/GaN HEMTs於2吋藍寶石基板上，將可獲得具有優良電性與均勻性之磊晶片。 此外，本研究展示以MOCVD 進行其結構、能障層應力以及非刻意之碳摻雜等工程調變方式，提升AlGaN/GaN HEMTs 其電器性能之成果。所提出之HEMT前瞻結構，即以高溫-低溫-高溫交錯方式成長AlN緩衝層，再配合高溫AlN中間層，則可將HEMT的崩潰電壓推升至200V以上。此結構設計主要是藉由插入高溫AlN中間層位於傳統HEMT結構中，可降低AlGaN能障層之拉應力而明顯改善表面平整度。進而大幅提升46%二維電子氣(2 Dimensional Electron Gas, 2DEG)之電子遷移率，達到 1900 cm2/Vs 。同時，此以高溫-低溫-高溫交錯方式成長之AlN，取代了傳統結構中之高溫緩衝層，則可增進氮化鎵之結晶品質，明顯提升HEMTs之性能。此前瞻之HEMTs結構，在製作出元件後，其直流 (DC) 特性大幅提升， 與傳統以AlN緩衝層結構相較，其洩極 (Drain) 最大電流增加率達35.5% (~ 680A/mm)；電導 (transconductance) 則有 15% (114 mS/mm)增加率。此插入高溫AlN中間層之方式，確實可降低AlGaN能障層之拉應力，因此對於AlGaN/GaN HEMTs 可靠度之提升而言，深具潛力。 除了在藍寶石基板上成長氮化鎵HEMT之研究外，高頻應用之AlGaN/GaN HEMT 結構，亦於兩吋(及三吋)半絕緣 (Semi-insulating, SI) 碳化矽基板以及高阻值(High resistivity, HR) 4吋矽晶片上成功開發。於碳化矽基板成長之AlGaN/GaN HEMT， 其特性已符合高頻、高功率之X波段(8-12 GHz) 軍用雷達應用需求。為降低成本，亦成功於高阻值的四吋矽晶片上開發高性能AlGaN/GaN HEMT，其通道電子密度與電子遷率移分別高達 0.8 x1013/cm2 及1560 cm2/Vs。同時此結構之微波損耗特性在40GHz頻率下低於-1.1 dB/mm適合於高頻應用。

In recent years High Electron Mobility Transistors (HEMTs) based on the nitride material system have successfully proven their potential as high-power and high frequency devices. While nitride-based HEMTs have been available in the commercial market for a few years, more study for improvements in efficiency and reliability is still object of present research. The objective of this work is to develop MOCVD growth technologies for GaN-based HEMTs and to improve performances of AlGaN/GaN HEMTs by modifying strain in barrier and by unintentionally doping in GaN buffer. AlGaN/GaN HEMTs structure are firstly grown on 2-inch c-plane sapphire substrate, which is relatively cheap and available with high quality. AlGaN/GaN HEMT grown on sapphire substrate shows high leakage current, which causes by residual oxygen. The low leakage current of AlGaN/GaN HEMTs grown on sapphire only can be achieved by eliminating both the diffusion of oxygen from sapphire substrate and n-type buffer layer due to oxygen impurities originating from MOCVD sources. To improve performances of HEMT devices, a good crystallinity AlGaN barrier is essentially required and strain-modified barrier may result in an improvement in reliability. The optimization of growth parameters, especially pressure and III/V ration, is very important to suppress phase separation including an inhomogeneous Al-composition and to achieve high crystal quality of AlGaN film. The residual strain in AlGaN layer grown on GaN induced by lattice mismatch can be modulated by varying thickness of a high temperature (HT) AlN interlayer (IL) inserted between AlGaN and GaN layers. Moreover, the AlGaN crystal quality and Al incorporation are influenced by strain induced by the HT AlN interlayer. Using optimum growth conditions, an AlGaN/GaN HEMTs structure grown on 2-inch sapphire substrate introduces good electrical characteristics and uniformity. Additionally, improvements in electrical characteristics of AlGaN/GaN HEMTs grown using MOCVD by engineering structure, barrier strain, and unintentional carbon incorporation are demonstrated. An advanced HEMT structure with a high-low-high (HLH) temperature AlN buffer and a HT AlN interlayer (IL) presents a breakdown voltage higher than 200 V. The HT AlN IL inserted in the middle of the conventional HEMTs structure introduces a reduction in the tension of AlGaN barrier, which results in an improvement on surface morphology. As a consequence, the 2DEG mobility increases remarkably by 46% (1900 cm2/Vs). The HLH AlN buffer substituting for the HT AlN buffer leads to the enhancement of GaN crystalline quality, which contributes to the performance improvement for HEMTs. The advanced HEMT using both an AlN IL and a HLH AlN buffer produces increases in the DC maximum drain current by 35.5% (~ 680A/mm) and in the transconductance by 15% (114 mS/mm) in comparison with the normal HEMT with an AlN buffer. Indeed, the reduction of AlGaN barrier tensile strain by inserting the HT AlN IL is promising for an improvement in AlGaN/GaN HEMTs reliability. Following the study of growth of GaN-based HEMTs on sapphire, AlGaN/GaN HEMT structures for high-frequencies application are successfully developed on 2-inch ( and 3-inch) semi-insulating (SI) 4H-SiC and HR 4-inch silicon substrates. The AlGaN/GaN HEMT structure on SI SiC results meet requirements for high-frequency and high-power applications in use of military radars which operate in particular frequency X-band (8-12 GHz). Looking to lower costs, AlGaN/GaN device on high-resistivity 4-inch (111) Si substrate has also been developed, exhibiting channel electron sheet densities of 0.8 x1013/cm2 and mobility of 1560 cm2/Vs. The designed structure is suitable for high-frequency application, which have been documented by RF loss less than -1.1 dB/mm at 40GHz.