AVS1996 Session EM+NS-FrM: Electronic Transport across Interfaces

Friday, October 18, 1996 8:20 AM in Room 204A

Friday Morning

Start	Invited?	Item
8:20 AM	Invited	EM+NS-FrM-1 Atomic and Mesoscopic-scale Characterizations of Semiconductor Interfaces by Ballistic-Electron-Emission Microscopy E. Lee (University of California, Santa Barbara) Ballistic-electron-emission microscopy (BEEM) [1] is a dual probe microscopy for nondestructive imaging and spatially resolved spectroscopy of buried semiconductor interfaces. For epitaxial metal-semiconductor (M-S) interfaces such as the prototypical CoSi\sub 2\/Si interface prepared by molecular-beam epitaxy (MBE), BEEM allows direct mapping of interfacial atomic steps and interfacial atomic structures [2]. By combining BEEM spectroscopy with conventional scanning-tunneling spectroscopy, one can, without elaborate modeling, measure the interfacial transmission probabilities, as functions of energy, for electrons to cross the M-S interfaces [3]. One can also apply BEEM to study semiconductor-semiconductor interfaces buried below the M-S interfaces. For example, for MBE grown GaAs/In\sub x\Ga\sub 1-x\As/GaAs(001) heterostructures capped with thin Au overlayers, BEEM can image electrically active misifit dislocations at the In\sub x\Ga\sub 1-x\As/GaAs interface bur! ied below the Au/GaAs interface [ In this talk, these and other atomic and mesoscopic-scale characterizations of the semiconductor interfaces using BEEM will be discussed. It will be shown that BEEM is an important tool for electronic and structural characterization of buried semiconductor interfaces, especially in combination with other tools such as transmission-electron microscopy and atomic force microscopy. [1] W. J. Kaiser and L. D. Bell, Phys. Rev. Lett. 60, 1406(1988). [2] E. Y. Lee, H. Sirringhaus, U. Kafader, and H. von K\um a\nel, Phys. Rev. B 52, 1816(1995). [3] E. Y. Lee, H. Sirringhaus, H. von K\um a\nel, Surf. Sci. Lett. 314, 823(1994). [4] E. Y. Lee, S. Bhargava, K. Pond, K. Luo, and V. Narayanamurti (to be published).
9:20 AM		EM+NS-FrM-4 Direct Observation of Momentum Conservation at the Au/Si Interface using BEEM L. Bell (Jet Propulsion Laboratory, Caltech) The verification of parallel momentum conservation at a metal/semiconductor interface is a fundamental issue in interface transport. In epitaxial structures the question is less controversial, since atomically abrupt interfaces can be achieved between materials with matching lattice nets. Parallel momentum conservation in the case of a non-epitaxial evaporated metal film is conceptually less straightforward. In addition, elastic scattering in the metal film can provide the necessary momentum for electrons to enter the semiconductor conduction-band, making evaluation of interface transport difficult. Ballistic-electron-emission microscopy (BEEM) is a recently developed method for probing interfaces with nanometer resolution. One early prediction based on the idea of momentum conservation was the BEEM spectrum for Au/Si(111), which was expected to differ dramatically from that of Au/Si(100). The predicted behavior was not observed; instead, previously reported BEEM spectra are nearly identical to those for Au/Si(100). In this talk, BEEM spectroscopy on Au/Si(111) structures as a function of Au thickness and temperature will be described. At 77K a direct signature of parallel momentum conservation at the Au/Si interface is observed in the BEEM spectra. The variation in spectral shape with both Au thickness and temperature places restrictions on allowable values of inelastic and elastic mean free paths in the metal, and also requires the presence of multiple electron passes within the Au layer. An independent indication of multiple reflections is directly observed in the attenuation of BEEM current with Au thickness.
9:40 AM		EM+NS-FrM-5 Schottky Barrier Fluctuations at the CoSi\sub 2\ /Si(100) Interface on a nm Scale H. Sirringhaus, T. Meyer, H. von K\um a\nel (Laboratorium f\um u\r Festk\um o\rperphysik, ETH, Switzerland) The potential profile around individual interfacial dislocations = at the epitaxial CoSi\sub 2\ /Si(100) interface has been probed with nm = spatial resolution by in situ, low temperature ballistic- electron- = emission spectroscopy (BEES). Partial misfit dislocations with a Burgers = vector b=3D1/4 <111> and other types of linear defects give rise to a = lowering of the apparent Schottky barrier by up to 0.1eV. The potential = profile around the defects is approximately Lorentzian with a full-width = at half maximum (FWHM) of 4nm. This is the first time that the = observation by BEES of spatial variations of the Schottky barrier can be = correlated with the atomic structure of the interfacial defects, which = is known from transmission electron microscopy (TEM). In contrast, = dislocations at the CoSi\sub 2\ /Si(111) interface give rise to = scattering of the carriers, but not to a change of the band lineup. The = data are relevant for the theoretical understanding of the relationship = between structural interfacial defects and the local band lineup at the = interface, as well as for the interpretation of I-V measurements on = macroscopic diodes.
10:00 AM		EM+NS-FrM-6 Experimental Determination of Quantum Dipoles at Semiconductor Heterojunctions Prepared by van der Waals Epitaxy (vdWe): Linear Correction Term for the Electron Affinity Rule (EAR) R. Schlaf (Colorado State University); O. Lang, C. Pettenkofer (Hahn-Meitner-Institut, Germany); N. Armstrong (University of Arizona); W. Jaegermann (Hahn-Meitner-Institut, Germany) The investigation of the semiconductor heterostructure band offset has been the focus of much experimental and theoretical work for more than 30 years. One of the issues discussed most is the occurence of interface dipoles of electronic and structural origin which influence the band offset heights. The measurement of the so called quantum dipoles (QD) at the interfaces has proved to be very difficult in the commonly investigated IV,III-V and II-VI systems due to the superposition of structural dipoles (SD). SDs occur in these systems due to chemical reactions, defects and interface reconstruction. The recently developed method of van der Waals epitaxy (vdWe) makes it possible to overcome these problems by growing layered semiconductor heterostructures. Layered compounds consist of chemicially inert sandwich layers which are only weakly bound by vdW forces. Epitaxial heterocontacts are easily prepared which are atomically abrupt and free of structural dipoles. We determined the band offset of eight layered heterocontacts consisting of combinations between SnS2, SnSe2, WSe2, MoS2, MoTe2, InSe, and GaSe. The comparison to offsets predicted by the electron affinity rule (EAR) revealed a systematic deviation. Due to the absence of SDs this deviation corresponds to the magnitude of the QD at the interface which allows the development of a QD-correction term for the EAR. The corrected EAR is still a linear rule, allowing the assignment of "characteristic energies" to each material for the calculation of the band offset. The error margin of the corrected EAR lies well within the experimental error of photoemission spectroscopy (PES) experiments, thus proving the general applicability of linear rules for the determination of the band offset.