Measuring the Surface Photovoltage of a Schottky Barrier under

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Measuring the Surface Photovoltage of a Schottky Barrier Under Intense Light Conditions: Zn/p-Si(100) by Laser TimeResolved Extreme Ultraviolet Photoelectron Spectroscopy Brett M. Marsh, Mihai Emilian Vaida, Scott Kevin Cushing, Bethany Rose Lamoureux, and Stephen R Leone J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06406 • Publication Date (Web): 06 Sep 2017 Downloaded from http://pubs.acs.org on September 12, 2017

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The Journal of Physical Chemistry

Measuring the Surface Photovoltage of a Schottky Barrier Under Intense Light Conditions: Zn/p-Si(100) by Laser Time-Resolved Extreme Ultraviolet Photoelectron Spectroscopy Brett M. Marsh, Mihai E. Vaida,* Scott K. Cushing, Bethany R. Lamoureux, and Stephen R. Leone

‡§

Department of Chemistry , and *Department of Chemistry, University of Central Florida, Orlando, Florida 32816, ‡ § and Department of Physics, University of California, and Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720

Abstract: A metal-semiconductor heterojunction is investigated by Auger and photoelectron spectroscopy to characterize the structural and electronic properties of the metallic film and to obtain the time-resolved electronic response induced by femtosecond laser excitation of the semiconductor material. The 3.5 ML Zn films deposited on p-type Si(100) at liquid nitrogen temperature grows in a layer-by-layer fashion. Electronic structure measurements by extreme ultraviolet (XUV) photoelectron spectroscopy indicate that the films are metallic in nature, creating a Schottky barrier at the 3.5 ML Zn/p-Si(100) interface. Utilizing a 35 fs, 800 nm pump pulse at a pump intensity of 2.5-6 x 109 W/cm2 to excite the Si and a timedelayed extreme ultraviolet pulse to probe the Zn, large transient surface photovoltage shifts of 0.3-2.2 eV are observed at carrier densities of 1.5-4.5 x 1020 cm-3. Three shifts are determined the Zn 3d core level, the photoemission onset, and the metallic Fermi level. The photovoltages increase with laser excitation intensity, and the Zn 3d core level exhibits the largest binding energy shifts due to pronounced screening of the core level. The large observed shifts are rationalized based on the energetics of band flattening and carrier accumulation in the metallic layer of the Zn/p-Si(100) heterojunction at high carrier densities. The observed carrier recombination dynamics are biexponential in character, with similar time constants for both the Zn 3d and photoemission onset binding energy shifts. The Zn 3d core level shifts are also found to be sensitive to the electron temperature. These results show that core-level photoemission can be used to monitor valence electron dynamics, allowing separation of charge dynamics in heterojunctions and solids composed of multiple-elements.

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Introduction The development of photocatalytic materials based on metal-decorated semiconductors has considerable promise for performing chemical transformations.1-3 The enhancement of photocatalytic activity by metal deposition can occur by trapping of electrons photoexcited in the semiconductor by the metal due to the Schottky barrier and possibly by excitation of electrons in the metal due to surface plasmon resonances.4-5 Trapping of electrons by the Schottky barrier leads to a shift in the metal Fermi level, which makes the catalyst more reductive.6 In this work the Zn/p-Si(100) heterojunction is investigated as a model Schottky barrier system using time-resolved extreme ultraviolet (XUV) photoelectron spectroscopy (XUV-PES) along with variable intensity pulsed laser excitation of the semiconductor band gap. The XUV-PES technique was recently used to observe electron injection in the n-TiO2/p-Si(100) heterojunction system to assess the electronic structure of defect-rich and defect-poor TiO2 films.7 Using a similar technique of measuring surface photovoltages, large surface charging due to electron transfer from the silicon semiconductor into a Zn metal film is observed with intense laser excitation, also providing measurements of the surface charging lifetimes. Due to the short mean free path of electrons generated by XUV pulses the experiment is intrinsically sensitive to the surface of the sample.8 While time resolved photoemission, in the form of two photon photoemission (2ppe) experiments,9-14 has been previously employed to study metal surfaces and heterojunctions, the present technique allows for interogation of core atomic energy levels, allowing for elemental specificity in measurements of the surface electron dynamics. It should be noted that high harmonic based XUV-PES experiments have been used to look at materials, typically with angular resolution,15-17 but have not been used to study the core level dynamics after excitation. The present work first explores the morphology of Zn films grown on p-Si(100) at liquid nitrogen temperature through the use of Auger electron spectroscopy. The morphology data is complemented by electronic structure measurements performed using static XUV-PES. Surface photovoltage changes and

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carrier recombination processes in the Zn/p-Si(100) heterojunction are observed through the use of femtosecond time-resolved XUV-PES pump-probe spectroscopy.

Experimental The apparatus, described previously,7 consists of (i) a laser-produced high harmonic generation (HHG) XUV source, pumped by a Ti:sapphire ultrafast laser system, and (ii) an ultrahigh vacuum (UHV) surface science chamber, which is used to prepare the samples for measurement. Femtosecond laser pulses are supplied by a Ti:Sapphire oscillator (KM Labs Griffin) pumped by a continuous 5 W frequencydoubled Nd:YVO4 laser. The resulting pulses are stretched and multipass-amplified in a Nd:YLF pumped Ti:sapphire amplifier (KM Labs Dragon), and the compessed amplified pulses have a temporal width of 24 fs, a typical average power at 1 kHz repetition rate of 2.2 W, and a central wavelength of 800 nm. The beam is then split, with one arm serving as a pump pulse and the other arm used to generate the XUV probe pulse. The probe arm is focused to 1014-15 W/cm2 at the exit of a semi-infinite gas cell filled with Ar gas at around 24 Torr. Under these conditions odd harmonics of the 800 nm fundamental are produced from the 7th to 29th order. The XUV beam is then dispersed by a plane grating in first order diffraction to select a single harmonic. The selected harmonic is focused onto the sample inside the UHV chamber by a combination of a cylindrical mirror and a toroidal mirror to a spot size of 0.2 mm. Additional harmonics are blocked by a slit at the entrance of the UHV chamber. The 800 nm pump beam (variable from 2.5 x 109 W/cm2 to 6 x 109 W/cm2) is brought onto the sample by an aluminum mirror in vacuum, which is located slightly above the XUV beam path. The pump beam, en route to the vacuum chamber, is passed through a computer controlled optical delay stage which allows for control of the temporal overlap between the pump and probe beams. The zero time delay between the two beams is determined by observing the photoelectron intensity of the 800 nm pump and the 800 nm infrared beam that is used to generate the HHG radiation.

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When the two beams are overlapped in space and time, an enhancement of the photoelectron signal is observed due to constructive interference between the beams. This measurement provides an instrumental response function of approximately 40 fs. Finally, the time response function of about 80 fs is measured between the 800 nm pump and the XUV probe beams using laser assisted photoemission spectroscopy (LAPE) technique.18 The zero-time delay obtained via transient multiphoton photoemission spectroscopy and LAPE are within 10 fs. The UHV end chamber is equipped with tools for preparation and characterization of surfaces. Among these are an Ar+ ion gun, used to clean the silicon (100) sample, a Zn oven, used to deposit the Zn film onto the silicon, and an Auger spectrometer, used to analyze surface composition and coverage. Photoelectrons are collected and analyzed by a time-of-flight photoelectron spectrometer (TOF-PES) with a 1 m long double-walled µ-metal inner tube and a microchannel plate (MCP) detector at the end of the drift region. The signal is then acquired via a 5 GHz multichannel scaler unit. The silicon sample is mounted onto a liquid nitrogen cooled cryostat, which is coupled to an xyzθ stage to allow manipulation of the sample position. The temperature of the sample holder is nominally 90° K when cooled with liquid nitrogen as measured by a thermocouple. The Zn films were grown by evaporation of zinc metal from a homebuilt evaporator. Briefly, the evaporator consists of a 4 cm long, 0.5 mm in diameter Ta filament wrapped around a piece of Zn metal. A current of approximately 4 A is applied to the Ta filament, which results in resistive heating of the filament and heating of the zinc. The sample is typically positioned 5 mm away from the entrance of the TOF-PES with the sample surface normal parallel to the spectrometer axis. This results in a laser beam incidence angle of 45° with respect to the surface normal. The sample position can be reproduced with an accuracy of 0.02 mm and 0.5°. The base pressure of the chamber is typically 2 x 10-10 Torr and rises to 7 x 10-10 Torr when the sample chamber is opened to the beamline, due to residual Ar gas from the HHG cell. Typical static

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photoemission spectra consist of an average of the collected photoemission spectra of 250,000 laser pulses. A typical transient spectrum has a similar number of collected pulses per time delay, with the 800 nm pump beam impinging on the sample at a time before or after the XUV pulse. In a transient PES experiment, time points between -5 to 5 ps are collected with 1 ps steps, while points outside this region are collected with approximately 20 ps steps. These time steps were chosen based on previously observed timescales of surface photovoltage decay,7 and shorter time behavior was not sought.

Results/Discussion Preparation of Zn films on Si(100) The p-doped Si(100) substrates (100 Ω−cm resistivity, boron doping) were used as the semiconductor substrate. The surface was first prepared by Ar+ sputtering for 1 hour (500 eV ion energy, 12 µA Ar+ current). The cleanliness of the sample was checked with Auger electron spectroscopy (AES) to ensure no carbon or oxygen peaks are observed within the noise of the Auger instrument, putting a limit of

Measuring the Surface Photovoltage of a Schottky Barrier under

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