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Zhu, Ying

University of British Columbia

Porous germanium (PGe) possesses unique physical and chemical properties, such as large surface specific area, tunable porosity, and tunable refractive index, making it attractive for applications in templates, waveguides, and lithium battery anodes. Electrochemical etching is a cost-effective method for fabricating PGe, offering high nanocrystal density. However, several challenges remain: (1) Current studies on PGe formation focused on bulk Ge (hundreds of micrometers thick), unsuitable for the nm- or μm-scale semiconductor devices; (2) Achieving uniform pores in n-type Ge is challenging due to inadequate hole production, and the effects of doping have not been well studied; (3) Epitaxial Ge-on-Si (epi-Ge) has Ge thickness in the nm to μm range and contains a high density of threading dislocations (TDs), beneficial for pore formation. There are limited studies on PGe formed by electrochemical etching epi-Ge, and the influence of TDs on the electrochemical etching behaviors is unexplored; and (4) the optical properties of PGe-on-Si are not characterized previously, impeding its optical applications. This thesis presents a comprehensive investigation of PGe prepared by bipolar electrochemical etching of epi-Ge. Uniform PGe structures were successfully achieved in both p- and n-type epi-Ge. The space charge region model was used to interpret the pore formation mechanisms. Three steps were observed in the pore formation process, and there was an optimal cathodic time range for stable pore growth. Penetrating pores were formed in the epi-Ge layers during the porosification process through preferential etching along TDs, which was independent of doping type and concentration. These pores followed TD lines, could penetrate the entire Ge layer of 1.3 μm and further porosificate the Si substrate. The diameter of the penetrating pores at TD sites increased with the total etching time, and an optimal anodic current density should be selected to obtain stable pore structures. Four optical models based on effective medium approximation were developed to elucidate optical properties with different levels of complexity. The optical constants, anisotropy, and porosity gradient of PGe were extracted by fitting ellipsometer measurement data and could be tuned by adjusting the etching time, cathodic pulse time, and doping concentration.

Text

2024-08-08

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/88908

Materials Engineering

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/