Band Lineup of Van Der Waals-Epitaxy Interfaces
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Mat. Res. Soc. Symp. Proc. Vol. 448 01997 Materials Research Society
Quasi-van der Waals Epitaxy
Van der Waals Epitaxy (2D)
(3D on 2D)
film subsrat
vander Waalsgap
van der Waals gap
(a)
(b)
Fig. 1: Van der Waals epitaxy (vdWe) of layered chalcogenide interfaces (a) and quasi-van der Waals epitaxy of three-dimensional semiconductors on layered chalcogenide substrates (b). EXPERIMENTAL PROCEDURE All presented experiments were performed in a photoelectron spectrometer (VG, ESCALAB, or ADES 500) connected to a MBE chamber. The experimental setup allows PES and electron diffraction (LEED) measurements in-situ whereas STM data and TEM data were obtained exsitu. For details refer to Refs [7-10]. The valence band offsets AEv were determined directly by PES measuring substrateoverlayer differences of core level binding energies, which were carried out after each step of the growth sequence. A detailed description of this method can be found in Ref. [11]. The reference spectra for the determination of the valence band maximum were obtained from single crystals. The electron affinity X is accessible from the experimentally determined work function (given by the secondary electron onset), the measured difference of the Fermi level to the valence band maximum (EF-EV) and the known band gaps (EG=EC-EV) of the semiconductors. The interface dipole potential D across the interface is given from the difference of the electron affinity offset AEc(EAR) between single crystal electron affinity of substrate and layer material assuming the validity of the electron affinity rule (EAR) and the experimentally deduced offset AEc(exp) determined from the experimentally determined value AEv(exp) and the bandgaps EG. RESULTS AND DISCUSSION The goal of the experiments was the experimental determination of valence and conduction band offset of several heterojunctions and their comparison to the predictions of the electron affinity rule (EAR). The deviation gives a value for the electronic quantum dipole (QD) for each investigated interface. As determined from PES and LEED measurements all interfaces are nonreactive, and atomically abrupt. Epitaxial or strongly textured overlayer films are obtained. The results of the evaluation of the valence band offsets of vdWe layered chalcogenide heterointerfaces versus the EAR predicted values are summarized in Figure 2 (additional AEv values obtained from earlier experiments are added to the graph [12]). The dashed line represents the EAR limit. It is evident that the experimentally determined valence band offsets AEv(exp) are close to the EAR rule as may be expected for vdWe heterointerfaces as the ideal vdW (0001) faces are known to be free of active surface states.[5] However, a close inspection of the experimental results suggest a small but systematic deviation from the predicted EAR values AEv(EAR). The line fit through the data points indicates that the magnitude of the quantum dipoles depends on the EAR band offsets suggesting a linear correction term for the EAR. The value of the int
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