Observation of Hole Accumulation in Ge/Si Core/Shell Nanowires Using off-Axis Electron Holography

Abstract

Hole accumulation in Ge/Si core/shell nanowires (NWs) has been observed and quantified using off-axis electron holography and other electron microscopy techniques. The epitaxial [110]-oriented Ge/Si core/shell NWs were grown on Si (111) substrates by chemical vapor deposition through the vapor-liquid-solid growth mechanism. High-angle annular-dark-field scanning transmission electron microscopy images and off-axis electron holograms were obtained from specific NWs. The excess phase shifts measured by electron holography across the NWs indicated the presence of holes inside the Ge cores. Calculations based on a simplified coaxial cylindrical model gave hole densities of (0.4 ± 0.2) /nm(3) in the core regions.

Role of vacancies to p-type semiconducting properties of SiGe nanowires

“…This is possibly the main reason for the high-density of the hole gas detected in the Ge core /Si shell NW. [22][23][24][25] 2. Electronic structures (a) Vacancies in the RTP Si/Ge NW.…”

“…In experiments, a high density of hole carriers are found in the core of the Ge core /Si shell NW. [22][23][24][25] Park et al 28 studied the Si dangling bond on the surface and the Au impurity within the Si shell of the Ge core /Si shell NW to explain why a high density of holes is present in the Ge core /Si shell NW. However, the dangling bonds at the interfaces were not considered.…”

“…SiGe NW have aroused extensive attention because of their novel electronic properties. [12][13][14][15][16][17][18][19][20][21][22][23][24][25] In fact, composition-randomly-distributed SiGe NW (referred to as Si 1Àx Ge x ) have been successfully fabricated in the laboratory. [16][17][18][19][20][21] Importantly, the band gaps of Si 1Àx Ge x NW can be modulated over a wide range by tuning the composition of x and the area of the NW cross-sections.…”

“…The Ge core Si shell NW have attracted even more attention because of their highdensity free hole gas revealed in the experiments (without implementation of any doping strategy), as well as recently established high-performance FET based on Ge core Si shell NW. [22][23][24][25] Inspired by these previous experimental ndings on the SiGe core/shell NW, much of the recent research efforts have been devoted to the origin of the high-density free hole gas in the Ge core Si shell NW, and how to realize efficient electron and hole doping in the NW. The band offset at the radial heterojunction between the Ge core and the Si shell, the Si dangling bonds at the surface, as well as metal impurities (transition metal particles are usually used as catalyst for the growth of SiGe NW) were thought to be possible sources for contributing to the high-density hole gas in the Ge core Si shell NW.…”

Polarization‐Induced Charge Distribution at Homogeneous Zincblende/Wurtzite Heterostructural Junctions in ZnSe Nanobelts

“…The p‐n junction can be created in semiconductor single crystals by doping, thereby introducing impurity levels into the band gap, and forming a depletion region with built‐in potential difference across the junction. Heterojunctions consisting of materials with different band gaps exhibit considerable valence and/or conduction band offsets at the interfaces, which cause electrons or holes to reside in the same material2 (type I band alignment) or become separated in different materials3, 4 (type II band alignment). The strain due to lattice mismatch at the interface can also introduce band‐bending, facilitating higher density of charge localization at the interfaces 5.…”