光電所所訊

High two-dimensional electron mobility in a SiGe/strained Si/SiGe quantum well

Professor Chee Wee Liu

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

臺灣大學光電所劉致為教授

The two-dimensional electron gas (2DEG) mobility has reached 2 × 10⁶ cm²/V s at 0.3 K for 15 nm strained Si well with a 526 nm Si_0.86Ge_0.14 barrier layer. The SiGe/strained Si/SiGe heterostructure was grown on p-type (001)-oriented Si at 550℃ by hot-walled ultrahigh vacuum chemical vapor deposition. The factors that limit the mobility are the background charges, the threading dislocation density and the roughness at the Si/SiGe heterojunction. Reduction of Ge concentration in the SiGe barrier layer and the relaxed buffer layer that reduces the threading dislocation density and the roughness, effectively enhances the mobility of the 2DEG in the Si channel. The background doping was greatly reduced by high purity gases and the long-time baking of quartz tube.


Figure 1. Electron density dependence of the 2DEG mobility for the sample with Si_0.86Ge_0.14 relaxed buffer layer measured at T = 0.3 K. The maximum value of 2DEG mobility of the Si_0.86Ge_0.14 sample reaches 2 × 10⁶ cm²/V s at the electron density of 1.4 × 10¹¹ cm^-2.	Figure 2. The 2DEG peak mobility as a function of Ge concentration in the relaxed buffer layer. The inset demonstrates the SiGe/Si/SiGe heterostructure schematically.

Threshold voltage controlled by gate area and gate recess in inverted trapezoidal trigate AlGaN/GaN MOS-HEMTs with PEC and PEALD oxides

Professor Lung-Han Peng

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

臺灣大學光電所彭隆瀚教授

An enhancement-mode, AlGaN/GaN inverted trapezoidal trigate MOS-HEMT wrapped with double-layered oxides, i.e., PEC-grown Ga₂O₃/(Al,Ga)₂O₃ and PE-ALD Al₂O₃, was demonstrated to exhibit positive V_th = 1.2V with current on/off ratio of 10⁸ and sub-threshold swing of 160mV/dec. HRTEM analysis revealed trapezoid-shape channel sidewalls due to crystallographic wet chemical etching by using molten KOH, and a reaction rate-limited process at which the outer surfaces of the nonpolar/semipolar GaN facets are converted into crystalline α-Ga₂O₃ passivation layers with preferential growth of (003) α-Ga₂O₃ upon the c-plane of GaN. From DC characteristics analysis of diverse gate recess depths and different gate areas of fabricated devices, we suspected that there is negative polarization in the interface of double-layer gate oxide and barrier to compensate the strong polarization charge induced by AlGaN/GaN heterostructure, and hence, to lead to higher threshold voltage than other HEMTs without gate oxide. These observations can be ascribed to combined effects of (a) interfacial negative space charge of 3.2 μC/cm² in the gate-recessed device to partially compensate the spontaneous charge, and (b) side-wall passivation to preserve the high mobility channel. It relaxes the constraint of using nanometer scale channel width to achieve positive value of V_th and results in a cut-off frequency of unity current gain/power gain at f_T/f_max = 9/36GHz at gate length/width of L_G /L_w = 250 /360nm.

Fig. 1 (a) Top-view SEM micrograph of inverted trapezoidal trigate AlGaN/GaN MOS-HEMT with gate recess, (b) 3D structural diagram of this device, (c) the f_T and f_MAX plot corresponding to L_G=250nm E-mode AlGaN/GaN inverted trapezoidal trigate MOS-HEMT, and DC measurements for (d) I_DS-V_DS family curves, (e) transfer characteristics and transconductance curve in linear-scale, (f) transfer characteristics of I_DS-V_G in log-scale and I_G-V_G gate leakage current measured in a three-terminal configuration.

Ref: Po-Chun Yeh, Yun-Wei Lin, Yue-Lin Huang, Jui-Hung Hung, Bo-Ren Lin, Lucas Yang, Cheng-Han Wu, Tzu-Kuan Wu, Chao-Hsin Wu, and Lung-Han Peng," Threshold voltage controlled by gate area and gate recess in inverted trapezoidal trigate AlGaN/GaN MOS high-electron-mobility transistors with photoenhanced chemical and plasma-enhanced atomic layer deposition oxides", Applied Physics Express, 8, 084101, 2015.