Characterization of Molecular Modified Surface States by Wavelength- and Time-Dependent Two-Photon Photoemission Spectroscopy A. Kadyshevitch and R. Naaman* Department of Chemical Physics, Weizmann Institute of Science, RehoVot, 76100 Israel R. Cohen and D. Cahen* Department of Material and Interface, Weizmann Institute of Science, RehoVot, 76100 Israel J. Libman ² and A. Shanzer* Department of Organic Chemistry, Weizmann Institute of Science, RehoVot, 76100 Israel ReceiVed: January 6, 1997; In Final Form: March 13, 1997 X Wavelength-dependent two-photon photoemission (WD-TPPE) spectroscopy was used to investigate the surface state properties of CdTe crystals before and after the adsorption of specially designed organic molecules. One photon was used to modify the population of the surface states and a second photon to eject electrons from the substrate. We measured the dependence of the photoemission signal on the energy of the first photon and on the delay between the two light pulses. The energy of surface states, relative to the bands, was found to correlate with the relaxation time of the semiconductor surface after being photoexcited. This is explained in terms of a simple kinetic model for electron transfer. These findings demonstrate that the properties of surface states of semiconductors can be manipulated by adsorbing suitable organic molecules on the semiconductor surface and that the WD-TPPE method is a useful tool for optoelectronic characterization of semiconductor surfaces, with sensitivity exceeding that of most commonly used techniques. Introduction Surface states are known to have a large effect on the electronic properties of semiconductors. Therefore, understand- ing their nature is important in developing semiconductor-based electronic devices. Particularly, for developing fast devices one has to avoid slow trapping states. Hence the ability to control relaxation processes at semiconductor surfaces is of great importance. In this report we provide new insight in the parameters affecting surface state properties. We demonstrate the ability to manipulate these properties by chemisorption of organic molecules, using a newly developed noncontact measur- ing tool. The most important parameter in defining the properties of surface states is the rate at which charge can be transferred between the semiconductor bulk and the surface states. This rate is affected by two variables, the coupling and the energy difference between the surface states and the bands (either the valence or the conduction band). Experimentally it is difficult to measure each of the above variables separately. Several methods have been used to determine the lifetime of excited states in semiconductors. Among these we note the following non-contact ones: In the photoluminescence (PL) decay technique, 1 the decay time of the fluorescence of the semiconductor is measured following its photoexcitation or electron excitation. The method is restricted to relaxation times faster than the electron-hole recombination time in the bulk semiconductor and is applicable only to cases were emission occurs with relatively high yield. In addition, the technique does not provide direct information on the energy of the surface states relative to the bands’ energy. 2 Surface photovoltage measurements (contact potential differ- ence, CPD, under illumination) represent another method used for surface state characterization. 3 The time resolution of this method is relatively poor, and thus only long “averaged” relaxation times can be obtained with it. Another noncontact method for lifetime determination, de- veloped recently, is two-photon, time- and angle-resolved photoemission spectroscopy (TPPE). 4 The method was applied to semiconductors 5 and was found to be very useful also for the study of metals. 6,7 Recently the method was applied to the study of electron tunneling at metal-dielectric interfaces. 8 Here we present results of wavelength-dependent two-photon photoemission (WD-TPPE) spectroscopy which is a variation of the TPPE technique. The method enables one to measure electron transfer between surface states and the bulk semicon- ductor with high sensitivity, while simultaneously defining the position of these states relative to the bands’ energy. We show that it is possible to manipulate the surface state properties by adsorbing specially designed organic monolayers on the semi- conductor surface. The adsorbed molecules induce a shift of the surface state energies relative to the valence and conduction bands and thereby affect directly the rate of charge transfer between them and the semiconductor bulk. The WD-TPPE method is simple to apply, can be used for all types of semiconductor surfaces, is applicable over a broad time range, and has high sensitivity for the detection of surface states, especially surface traps, that exceeds by far that of other common methods. 9 It thus provides new insight into the properties of these states. In our measurements the first wavelength tunable light pulse (the pump) excites electrons either from the surface states or from the valence band, while the second pulse (the probe) has enough energy to eject photoelectrons from the valence band states. Since in this process photoelectrons are ejected only from the valence band, we are monitoring the effect of the excited state population on the work function of the crystal. ² This work is dedicated to the memory of Dr. J. Libman who passed away in March 1997. X Abstract published in AdVance ACS Abstracts, April 15, 1997. 4085 J. Phys. Chem. B 1997, 101, 4085-4089 S1089-5647(97)00080-1 CCC: $14.00 © 1997 American Chemical Society