Paper NM-WeM1
Nanoscopic Analysis of Semiconductor Heterointerfaces by Kelvin Probe Force Microscopy

Wednesday, December 14, 2016, 8:00 am, Room Hau

The electronic structure of heterointerfaces is known to determine the performance of semiconductor devices. This led to the series rigorous studies on interfaces, i.e., Fermi level pinning or Schottky barrier formation, for decades. However, such studies were not easy and hence the results not conclusive, due to factors such as the reproducibility in preparing identical interfaces or limitation in the availability of tools that could characterize the interfaces on an atomic scale. Kelvin probe force microscopy (KFM) is an atomic force microscopy-based technique that provides us with the opportunity to tackle such problems by mapping the surface potential and topograph on nanometer scale simultaneously. Here, we report the application of KFM for the study of 1) InAs quantum dots (QDs) grown on GaAs(001) and 2) Si heterojunction (SHJ) solar cell structure.

We previously reported that the I-V characteristics of the InAs QD on GaAs(001) vary as a function of QD size, and that the interface may be ohmic-like when the QD diameter is about 100 nm while it exhibited Schottky diode-like behavior when the diameter is about 20 nm [1]. The phenomenon was naively interpreted in terms of a balance between Fermi level pinning in the conduction band of the InAs eventually overcoming that at mid gap for GaAs surrounding the InAs QD as its size is enlarged. Here, we performed KFM on QDs whose diameters are around 20-30 nm. We observe that a ring-like dip of surface potential is formed at the peripheral of the QDs (Fig.1) in agreement with previous work [2]. We converted the surface potential map into band alignment by using the known electron affinity of the bulk materials, as a function of depth from the surface. We find that the Fermi level of the dip region lies in the conduction band suggesting the presence of a source of electric contact, while the Fermi level in the wetting layer and the majority of the QD lies in the band gap (Fig.2).

We also performed KFM measurements on the cleaved interfaces of SHJ solar cell and related structures. The SHJ structure we employed is ITO/p-aSi/i-aSi/n-cSi(001). We observe that while the workfunction, and thus the band alignment, of i-aSi/n-cSi(001) interface is abrupt, p-aSi/i-aSi is not, and also that ITO/p-aSi interface may be blurred as a result of interdiffusion (Fig.3). This is in strong contrast from the reference sample Ag/Si(001) from which we observe a nearly ideal Schottky-like band alignment.

We will discuss the possible mechanisms and implication of these results.

[1] I. Tanaka, I. Kamiya, H. Sakaki, et al., Appl. Phys. Lett. 74, 844 (1999).

[2] S. Ono and T. Takahashi, Jpn. J. Appl. Phys. 44, 6213 (2005).