分子束磊晶成長技術對砷化鎵系應變層高電子移動率電晶體特性之改良

Abstract

本論文利用分子束磊晶(molecular beam epitaxy, MBE)成長技術製備InAs/GaAs應變層超晶格(strained layer superlattice, SLS)或InAs/InxGa1-xAs (x = 1→0)/AlyGa1-yAs (y = 0→0.3)成份漸次變化(composition-graded)結構，做為砷化鎵系應變層高電子移動率電晶體(GaAs-based pseudomorphic high electron mobility transistor, PHEMT)元件的非合金化歐姆電極接觸層(non-alloyed ohmic contact layer)，此結構不須經合金化退火處理，因此歐姆金屬層與半導體層之間無相互擴散反應，將這兩種非合金化歐姆電極接觸層結構分別應用於以AuGe/Ni金屬作為歐姆電極的PHEMT元件上時，以傳導線模式(transmission line model, TLM)量測分析，其比接觸電阻值(specific contact resistance, rc)可分別低至6.61x10-7及1.05x10-7 W-cm2。本研究亦利用MBE系統中的銦元素，作為元件再生長(regrowth)技術的鈍化層(passivation layer)原料，厚度雖僅有數10Å，卻已足夠阻絕大氣中的碳及氧污染，此銦鈍化層可經由熱去吸附(thermal desorption)方式去除，確保後續PHEMT元件再生長，實驗結果顯示元件依然保有良好的特性，且其再生長界面並無明顯載子殘留現象發生。上述調整結構PHEMT元件的電性測試結果，1.0 μm閘極元件之gme值可達330 mS/mm，而ft及fmax分別為26.5及48 GHz，最後並將測得之微波電性結果，運用微波設計系統(microwave design system, MDS)軟體模擬小訊號等效電路模式(small-signal equivalent circuit model)。

The InAs/GaAs strained layer superlattice (SLS) structures and InAs/InxGa1-xAs (x = 1→0)/AlyGa1-yAs (y = 0→0.3) composition-graded structures as non-alloyed ohmic contacts were applied, respectively, to GaAs-based pseudomorphic high electron mobility transistors (PHEMTs) grown by molecular beam epitaxy (MBE). The non-alloyed structures had an excellent surface morphology and sharp metal pad edges due to non-annealing process. While applying one of these two non-alloyed structures to PHEMTs with AuGe/Ni metals as the ohmic contact, the limitation of small-dimensional devices can be excluded. Transmission line model (TLM) measurements with a four-point configuration showed that the specific contact resistances rc were as low as 6.61x10-7 and 1.05x10-7 W-cm2 for the non-alloyed PHEMTs with the SLS structure and the composition-graded structure, respectively. Furthermore, a thin layer of indium, removed by thermal desorption prior to the regrowth of PHEMTs, was used as a surface passivation layer. X-ray photoelectron spectroscopy (XPS) measurements showed that an indium layer as thin as several tens of angstrom (Å) was adequate for the protection of underlying III-V epilayers from carbon and oxygen contamination while being exposed to the atmosphere. Capacitance-voltage (C-V) depth profiles revealed negligible residual carriers near the regrowth interface. The DC and microwave characteristics of the modulated PHEMTs with a gate of 1.0 μm in length indicated that the extrinsic transconductance gme was as high as 330 mS/mm, the current gain cut-off frequency ft 26.5 GHz, and the maximum oscillation frequency fmax up to 48 GHz. Finally, the small-signal equivalent circuit model of the modulated PHEMTs were also simulated and investigated by a microwave design system (MDS).