Lu Xie

For the formation of nano-scale Ge channels in vertical Gate-all-around field-effect transistors (vGAAFETs), the selective isotropic etching of Ge selective to Ge_0.8Si_0.2 was considered. In this work, a dual-selective atomic layer etching (ALE), including Ge_0.8Si_0.2-selective etching of Ge and crystal-orientation selectivity of Ge oxidation, has been developed to control the etch rate and the size of the Ge nanowires. The ALE of Ge in p⁺-Ge_0.8Si_0.2/Ge stacks with 70% HNO₃ as oxidizer and deionized (DI) water as oxide-removal was investigated in detail. The saturated relative etched amount per cycle (REPC) and selectivity at different HNO₃ temperatures between Ge and p⁺-Ge_0.8Si_0.2 were obtained. In p⁺-Ge_0.8Si_0.2/Ge stacks with (110) sidewalls, the REPC of Ge was 3.1 nm and the saturated etching selectivity was 6.5 at HNO₃ temperature of 20 °C. The etch rate and the selectivity were affected by HNO₃ temperatures. As the HNO₃ temperature decreased to 10 °C, the REPC of Ge was decreased to 2 nm and the selectivity remained at about 7.4. Finally, the application of ALE in the formation of Ge nanowires in vGAAFETs was demonstrated where the preliminary I_d–V_ds output characteristic curves of Ge vGAAFET were provided.

Gate-all-around (GAA) field-effect transistors have been proposed as one of the most important developments for CMOS logic devices at the 3 nm technology node and beyond. Isotropic etching of silicon–germanium (SiGe) for the definition of nano-scale channels in vertical GAA CMOS and tunneling FETs has attracted more and more attention. In this work, the effect of doping on the digital etching of Si-selective SiGe with alternative nitric acids (HNO₃) and buffered oxide etching (BOE) was investigated in detail. It was found that the HNO₃ digital etching of SiGe was selective to n⁺-Si, p⁺-Si, and intrinsic Si. Extensive studies were performed. It turned out that the selectivity of SiGe/Si was dependent on the doped types of silicon and the HNO₃ concentration. As a result, at 31.5% HNO₃ concentration, the relative etched amount per cycle (REPC) and the etching selectivity of Si_0.72Ge_0.28 for n⁺-Si was identical to that for p⁺-Si. This is particularly important for applications of vertical GAA CMOS and tunneling FETs, which have to expose both the n⁺ and p⁺ sources/drains at the same time. In addition, the values of the REPC and selectivity were obtained. A controllable etching rate and atomically smooth surface could be achieved, which enhanced carrier mobility.

With the development of new designs and materials for nano-scale transistors, vertical Gate-All-Around Field Effect Transistors (vGAAFETs) with germanium as channel materials have emerged as excellent choices. The driving forces for this choice are the full control of the short channel effect and the high carrier mobility in the channel region. In this work, a novel process to form the structure for a VGAA transistor with a Ge channel is presented. The structure consists of multilayers of Si_0.2Ge_0.8/Ge grown on a Ge buffer layer grown by the reduced pressure chemical vapor deposition technique. The Ge buffer layer growth consists of low-temperature growth at 400 °C and high-temperature growth at 650 °C. The impact of the epitaxial quality of the Ge buffer on the defect density in the Si_0.2Ge_0.8/Ge stack has been studied. In this part, different thicknesses (0.6, 1.2 and 2.0 µm) of the Ge buffer on the quality of the Si_0.2Ge_0.8/Ge stack structure have been investigated. The thicker Ge buffer layer can improve surface roughness. A high-quality and atomically smooth surface with RMS 0.73 nm of the Si_0.2Ge_0.8/Ge stack structure can be successfully realized on the 1.2 µm Ge buffer layer. After the epitaxy step, the multilayer is vertically dry-etched to form a fin where the Ge channel is selectively released to SiGe by using wet-etching in HNO₃ and H₂O₂ solution at room temperature. It has been found that the solution concentration has a great effect on the etch rate. The relative etching depth of Ge is linearly dependent on the etching time in H₂O₂ solution. The results of this study emphasize the selective etching of germanium and provide the experimental basis for the release of germanium channels in the future.