PhD dissertation defense : Hyun-Yong Yu

Selective Heteroepitaxial Growth of Ge for Monolithic Integration of MOSFETs and Optical Devices

Hyun-Yong Yu

Department of Electrical Engineering, Stanford University

Advisor: Prof. Krishna C. Saraswat

 

Date: Thursday August 20th, 2009

Time: 9:30 AM (Refreshments at 9:15 AM)

Location: Paul Allen Building(Formerly CIS-X) CISX auditorium

 

Abstract

As Si bulk CMOS devices approach their fundamental scaling limit, diverse research is being done to introduce novel structures and materials. Its high carrier mobility and possible monolithic integration with Si based devices have prompted renewed interest in Ge based devices. For optical applications, it was challenging to make conventional Si photodetectors operate in 1.3-1.55μm wavelength range, due to its relatively large indirect (1.1eV) and direct (3.4eV) bandgaps. However, Ge’s smaller direct band gap energy (0.8eV) corresponding to ~1.55μm in wavelength and possible monolithic integration with Si CMOS technology make Ge a strong candidate for photodetectors. I demonstrate high performance Ge MOSFETs and optical devices which can be monolithically integrated to Si technology, by employing novel Ge heteroepitaxial growth and in-situ dopoing technique.

In the first part of this talk, we will talk about selective Ge heteroepitaxial growth on Si and in-situ doping technique for n+/p junction. Surface roughness of heteroepitaxially gorwn Ge on Si is considerably reduced by high temperature hydrogen annealing. Ge growth and hydrogen annealing steps are repeated until desired epi layer thickness is reached. High quality Ge film (minimal dislocation (1x10⁷cm^-2) and very smooth surface (0.65nm (RMS)) is achieved selectively on Si using SiO₂ window. For abrupt and box shaped n+/p junction in Ge, in-situ phosphorus doping using PH₃ is employed during the epitaxial growth. Temperature dependency of the dopant activation was investigated associated with the shallower and abrupt junction formation. This diode shows better characteristics (on/off ratio and on current density) compared with conventional ion-implanted junction.

In the second part of the talk, we will talk about high performance Ge MOSFETs and optic devices fabricated using selective Ge heteroepitaxial growth on Si. For n-MOSFETs, in-situ doping technique is used to form source and drain with very low series resistance and shallow junctions. p-MOSFET is fabricated with high-k/metal gate stack. Results show the highest electron mobility ever reported on (100) Ge n-MOSFETs and ~80% enhancement of hole mobility over Si universal mobility for p-MOSFETs. I also demonstrate normal incidence p-i-n photodiodes on selectively grown Ge. Enhanced efficiency in the near infrared regime and the absorption edge shifting to longer wavelength is achieved due to residual tensile strain. Measured responsivities are promising towards monolithically integrated on-chip optical links and in telecommunications.