AVS 64th International Symposium & Exhibition
    Applied Surface Science Division Tuesday Sessions
       Session AS+TF-TuA

Paper AS+TF-TuA12
Band Energy Alignment Studies at Heterojunction by X-ray Photoelectron Spectroscopy (XPS)

Tuesday, October 31, 2017, 6:00 pm, Room 13

Session: Problem Solving Using Surface Analysis in the Industrial Laboratory
Presenter: Jisheng Pan, Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), Singapore
Correspondent: Click to Email
The performance of any type of hetero-junction device is determined by two kinds of interface parameters: the band discontinuities and the built-in potential. Therefore, determining heterojunction band offsets and tuning them to a desired application would have an obvious impact on the optimization of the devices. Many techniques have been developed to determine the interfaces and to understand the microscopic origin of the interface properties. XPS is more widely used technique to study band alignment of heterojunction, probably due to its capability to simultaneously detect interface chemical and electronic properties which can be exploited for fully understanding of distinct correlations between the thin film material characteristics and device performance. There are two ways to obtain energy band offsets using XPS. One is direct measurement of valence band spectrum from interface, and this spectrum is simply considered as a superposition of substrate-related and overlayer-related valence band spectra. A nonlinear least squares fit is performed to separate it to substrate-related and overlayer-related valence band spectra to determine two valence band maxima and, valence band offset (VBO) of interface. The conduction-band offset CBO is deduced from VBO and suitable reference gap values of two materials at interface. It can be seen that the accuracy of the band offsets determined through this way depends on the fitting procedure. In order to overcome the above problem and obtain reliable band offset data, a method was proposed to determinate band offsets by combination of core level and valence band spectra. No fitting procedure is involved in this method. However, accurate XPS determination of band alignment in this way requires careful consideration of many other possible effects. In this paper, we have studied the effects of chemical shift, differential charging, band bending and photoemission final state on the determination of heterojunction band offsets using Kraut’s method. The method has also been applied to determinate energy-band alignments of molybdenum disulphide (MoS2) monolayer on high-k dielectric oxides such as Al2O3 and ZrO2. The VBO at monolayer MoS2/Al2O3 (ZrO2) interface was measured to be 3.31 eV (2.76 eV), while the CBO was 3.56 eV (1.22 eV). For bulk MoS2/Al2O3 interface, both VBO and CBO increase by 0.3 eV, due to the upwards shift of Mo 4dz2 band. The symmetric change of VBO and CBO implies Fermi level pinning by interfacial states. Our finding ensures the practical application of both p-type and n-type MoS2 based complementary metal-oxide semiconductor and other transistor devices using Al2O3 and ZrO2 as gate materials.