Electrical, Photoelectrochemical, and Photoelectron Spectroscopic

Subscriber access provided by UNIV OF NEBRASKA - LINCOLN

Article

Electrical, Photoelectrochemical and Photoelectron Spectroscopic Investigation of the Interfacial Transport and Energetics of Amorphous TiO/Si Heterojunctions 2

Shu Hu, Matthias Richter, Michael F Lichterman, Joseph A. Beardslee, Thomas Mayer, Bruce S. Brunschwig, and Nathan S. Lewis J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b09121 • Publication Date (Web): 15 Dec 2015 Downloaded from http://pubs.acs.org on December 19, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Electrical, Photoelectrochemical and Photoelectron Spectroscopic Investigation of the Interfacial Transport and Energetics of Amorphous TiO2/Si Heterojunctions Shu Hu1,2†, Matthias H. Richter2†, Michael F. Lichterman1,2, Joseph Beardslee2,4, Thomas Mayer5, Bruce S. Brunschwig1 and Nathan S. Lewis*,1,2,3

1

Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena,

CA 91125, USA. 2

Joint Center for Artificial Photosynthesis, California Institute of Technology, Pasadena, CA

91125, USA. 3

Kavli Nanoscience Institute, California Institute of Technology, Pasadena, CA 91125, USA.

4

Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.

5

Darmstadt University of Technology, Materials Science Department, Petersenstrasse 32, 64287

Darmstadt, Germany. *Corresponding author: [email protected] †

These authors contributed equally to this work.

1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Solid-state electrical, photoelectrochemical, and photoelectron spectroscopic techniques have been used to characterize the behavior and electronic structure of interfaces between n-Si, n+-Si, or p+-Si surfaces and amorphous coatings of TiO2 formed using atomic-layer deposition. Photoelectrochemical measurements of n-Si/TiO2/Ni interfaces in contact with a series of oneelectron, electrochemically reversible redox systems indicated that the n-Si/TiO2/Ni structure acted as a buried junction whose photovoltage was independent of the formal potential of the contacting electrolyte. Solid-state current-voltage analysis indicated that the built-in voltage of the n-Si/TiO2 heterojunction was ~ 0.7 V, with an effective Richardson constant ~ 1/100th of the value of typical Si/metal Schottky barriers. X-ray photoelectron spectroscopic data allowed formulation of energy band diagrams for the n-Si/TiO2, n+-Si/TiO2, and the p+-Si/TiO2 interfaces. The XPS data were consistent with the rectifying behavior observed for amorphous TiO2 interfaces with n-Si and n+-Si surfaces and with an ohmic contact at the interface between amorphous TiO2 and p+-Si.


Page 2 of 47

Page 3 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Introduction Efficient

operation

of

a

photoelectrosynthetic

or

photovoltaic-(PV-)

biased

electrosynthetic water-splitting cell 1 depends on the effective separation and collection of photogenerated charge carriers from small-band-gap light absorbers. In a fully integrated, intrinsically safe water-splitting system, explosive mixtures of H2(g) and O2(g) are not ever produced in the reactor. In this type of system, the photoanode is immersed in a locally alkaline or acidic aqueous electrolyte and transfers photogenerated holes to an oxygen-evolution reaction (OER) electrocatalyst.2–6 Semiconductor heterojunctions can provide the asymmetry required for the effective separation and collection of photogenerated charge carriers

7–11

and offer advantages

over metallurgical homojunctions, such as ease of processing,12 applicability to a wider set of semiconducting materials including polycrystalline semiconductors,13,14 opportunities for interfacial engineering,15,16 and perhaps most importantly, protection of the underlying smallband-gap semiconductor against corrosion.8,17–21 Recently, heterojunctions between TiO2 and small-band-gap semiconductors (including Si, CdTe, and III-V materials such as GaAs, GaP and GaInP) have been shown to be stable against photocorrosion while in contact with 1.0 M KOH(aq) and under simulated solar illumination for up to 2200 h of continuous water oxidation.17,20,22,23 Among these protected semiconductors, n-type Si (n-Si) forms a 0.4 V photovoltage, 0.77 V barrier-height junction with amorphous TiO2 deposited using atomic-layer deposition (ALD), in which the “leaky” amorphous TiO2 layer supported large anodic current densities despite a 2.2 eV energetic difference between the valence-band maxima of Si and TiO2.17 When used as gate dielectrics, such ALD-grown TiO2 films are electrically conductive in both positive and negative biases, so the electrical characteristics of this overlayer are concisely described as “leaky.” A 15 nm thick 3 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 47

MoO3 layer deposited on n-Si has also been shown to be able to support large hole-based current densities in solid-state devices despite a similar difference in the energies of the valence-band maxima of the heterojunction partners.24 The hole-transport behavior observed for amorphous TiO2 coatings is neither limited to ALD films nor limited to the amorphous phase of the TiO2 coatings.25

As-sputtered and

annealed ALD TiO2 films do not fully share the same characteristics of the as-grown ALD TiO2, in that the photovoltages produced by n-Si/annealed TiO2 films are generally lower and fewer defect states are detectable, relative to heterojunctions between n-Si and ”leaky” TiO2. When used as heterojunction partners with n-Si and contacted by a Ni overlayer, “leaky” ALD TiO2 films, annealed ALD TiO2 films, and sputter-deposited polycrystalline TiO2 films display similar rectifying behavior and electronic transport characteristics. Ambient-pressure synchrotron X-ray photoelectron spectroscopy has indicated that the Ni overlayer forms an ohmic contact to the underlying TiO2.26 An ohmic contact has recently been reported between a surface-diffused p+Si layer and the TiO2 in an np+-Si/Ti/TiO2/Pt buried-junction layered interface, with the predominant current flow ascribed to intrinsic charge-carrier conductivity in the conduction band of the TiO2 .27 The behavior of the np+-Si/Ti/TiO2/Pt system addresses the interfacial energetics of a system having an np+-Si homojunction coated with a 5 nm Ti metallic interfacial contact layer, but not the mechanism of anodic conduction across n-Si/TiO2 heterojunctions. Such holetransport behavior can be compared to the properties of p-type semiconductor window layers, e.g. Cu(I) based delaffosites,28 NiCo2O4,29 and ZnRh2O4.30 Solid-state electrical and photoelectrochemical techniques are useful for measurement of barrier heights and charge-transfer rates, and when combined with photoelectron spectroscopic techniques, provide a powerful means for determination of the energy-band relationships of 4 ACS Paragon Plus Environment

Page 5 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


semiconductor heterojunctions. The energy-band relationships can be obtained from X-ray and ultraviolet spectroscopies (XPS and UPS) of core-levels (CL) and the valence-band (VB), in conjunction with UPS work function measurements.31,32

As shown in Figure S1, XPS

measurements allow determination of the energy levels relative to the Fermi level of the material. The sampling depth of XPS is typically a few nm, and when thin overlayers are employed, shifts in the core-level binding energy (CLBE) and the valence-band maxima (VBM) can be monitored for both the substrate and the overlayer. This characteristic allows band level shifts and surface dipoles to be mapped as a heterojunction is built by stepwise increases in the overlayer thickness. Thus, photoelectron spectroscopy allows measurement of the positions of the band edges as a function of overlayer thickness. In a semiconductor, a change in band bending produces an equal change in the observed CL and VBM binding energies; however, the differences between the CLBEs and the VBM are independent of the band bending. Energy shifts for the VBM due to band bending of the substrate or the overlayer are reflected in the shifts of their respective CLBEs that are observed as the overlayer thickness is increased. Furthermore, concomitant monitoring of the shifts in the secondary electron-emission cut-off (work function) can reveal the presence of interfacial dipoles. Collectively, measurements of these energy shifts can yield a detailed picture of the interfacial energetics.33 We describe herein solid-state, photoelectrochemical, and photoelectron spectroscopic measurements to develop a detailed picture of the energetics of the heterojunction between n-Si and “leaky” TiO2 (n-Si/TiO2/Ni). We also present a detailed picture of the band energetics for interfaces between “leaky” TiO2 and p+- and n+-Si.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Experimental Atomic-Layer Deposition of TiO2 onto Si samples TiO2 was coated on Si substrates by ALD, as described previously.17 The substrates were either phosphorous-doped n-type Si (n-Si, 1.2 – 1.6 Ω·cm resistivity), degenerately n-type arsenic-doped Si (n+-Si, 0.003 Ω·cm resistivity) or degenerately p-type boron-doped Si (p+-Si, 0.003 Ω·cm resistivity). The Si substrates were cleaned by an RCA SC-1 etch [soaking the Si for 10 min in a 3:1 (by volume) solution of ~ 18.4 M H2SO4 and ~ 11 M H2O2] followed by immersion in 5 M hydrofluoric (HF) acid. The samples were then subjected to an RCA SC-2 procedure, which involved soaking the samples for 10 min at 75 °C in a 5:1:1 (by volume) solution of H2O, 11.6 M hydrochloric acid, and ~11 M H2O2(aq). An ALD cycle consisted of a 0.015 s pulse of H2O, a 15 s purge of N2(g) at 20 sccm, a 0.1 s pulse of tetrakis(dimethylamido)titanium (TDMAT) precursor, and another 15 s N2(g) purge. This ALD deposition cycle was repeated the desired number times to produce amorphous TiO2 overlayers of known thicknesses. The growth rate for the TiO2 ALD films is ~ 0.045 nm/cycle after the initial formation of an interfacial layer so that a 250 cycle layer of TiO2 is ~ 3 nm thick while a 1500 cycle layer is ~ 68 nm thick. Electrical Contacts to Si/TiO2 samples All TiO2-coated n-Si photoelectrodes were either metallized by 3 nm of Ni, with the resulting electrodes described as “n-Si/TiO2/Ni,” or by metal impurities introduced during Ar plasma etching to give “n-Si/TiO2 etched/metal” electrodes. Ni intermixes at the surface of TiO2 during Ni deposition17 and makes an ohmic electrical top contact with amorphous TiO2.26,34


Page 6 of 47

Page 7 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


As-grown TiO2 surfaces, as well as HF-etched and reactive-ion etched (RIE) TiO2 surfaces, did not facilitate the transfer of charge to liquid electrolytes under anodic potentials (Figure S2), whereas metallized TiO2 surfaces readily passed anodic current and exhibited ohmic behavior to all of the redox couples investigated below. Deposition of Ni Pads and Busbars (n-Si/TiO2/Ni samples) RF sputter deposition (AJA International, Inc.) of Ni at ~ 1 nm per min was performed using a sputtering power of 150 W and a pressure of 8.5 mTorr, with an Ar carrier-gas flow of 17 sccm. Ni pad and Ni busbar patterns were formed by optical lithography and lift-off processes. For the n-Si/TiO2/Ni pad samples, 500 µm diameter Ni pads (100 nm in thickness) were fabricated by RF sputter deposition of 100 nm thick Ni films on as-grown TiO2 surfaces that had been patterned with 500 µm diameter hole patterns in a Shipley S1813 (MicroChem) positive photoresist, followed by lift-off. The n-Si/TiO2/Ni busbar samples, in which each metal bar had contact dimensions of 25 µm in width and 4200 µm in length, and were separated by a 475 µm spacing and connected by a bus line, were fabricated by deposition of 100 nm Ni films on patterns that had been formed on TiO2 surfaces using nLOF2020 negative photoresist. The S1813 and nLOF2020 photoresists were removed using PG remover (MicroChem) and acetone (EMD Chemicals), respectively, followed by rinsing the sample with copious amounts of deionized H2O having a resistivity of 18 MΩ·cm as obtained from a Barnsted Nanopure system. Ar Plasma Etching (n-Si/TiO2-etched/metal samples) Samples denoted as “n-Si/TiO2-etched/metal” were made by RF plasma etching of TiO2 surfaces in the same AJA sputtering system that was used for the deposition of Ni pads and busbars. RF plasma etching was performed by Ar-ion sputtering for 33 s at 300 W and at a pressure of 30 mTorr with an Ar flow of 20 sccm. This etching process intermixed metal 7 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

impurities on the film surface and reduced the film thickness uniformly by ~ 2 nm, as indicated by spectroscopic ellipsometry (J.A. Woollam V-VASE) of TiO2 surfaces after etching (e.g. 1500 ALD cycles produced a TiO2 film that was 68 nm thick, and the thickness of that film was reduced to 66 nm by etching). The thicknesses were fitted using a Cauchy model, which included a silicon oxide layer at the n-Si and TiO2 interface and avoided interference from metalimpurity absorption on TiO2 surfaces. The TiO2 thicknesses obtained by the fits to the ellipsometric data were 3 nm for 250 ALD cycles and 68 nm for 1500 ALD cycles, respectively. After RF plasma etching, metal impurities including Ni, W, Ir, and Fe were observed by XPS (from Figure S11 in Hu et al.17) on the top of the etched TiO2 films and presumably facilitated electrical conduction in such interfaces. Photoelectrochemical, Solid-State, and Photoelectron Spectroscopic Measurements n-Si/TiO2/Ni and n-Si/TiO2-etched/metal samples were made into photoelectrodes for non-aqueous photoelectrochemistry, solid-state impedance and current density vs. voltage (J-V) measurements, and X-ray photoelectron spectroscopy. Non-Aqueous Electrochemical Measurements Non-aqueous photoelectrochemical current-density vs. potential (J-E) measurements were performed in a N2(g)-purged glove box. A Bio-Logic model SP-200 potentiostat was used in a 3-electrode setup, where the working electrode was an n-Si/TiO2/Ni or an n-Si/TiO2-etched/metal photoelectrode, the reference electrode was a Pt wire (0.5 mm diameter, 99.99% trace metals basis, Alfa-Aesar), and the counter electrode was a Pt gauze (100 mesh, 99.9% trace metal basis, Alfa-Aesar). Ohmic contact was made to the back of the samples using an In-Ga eutectic, and the sample was then attached to a coiled, tin-plated Cu wire using Ag paste. The working electrodes were bottom-facing, made by encapsulating the n-Si/TiO2/Ni or 8 ACS Paragon Plus Environment

Page 8 of 47

Page 9 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


n-Si/TiO2-etched/metal samples with Hysol 9462 epoxy at the end of glass tubing through which the Cu wire had been threaded. Three redox reagents, with mutually different formal reduction potentials, E0’, were used: an Fe(Cp*)2+PF6-/Fe(Cp*)2 redox solution that contained 5 mM of decamethylferrocenium hexafluorophosphate and 50 mM of decamethylferrocene (slightly above its solubility limit in CH3CN); an Fe(Cp)2+BF4-/Fe(Cp)2 redox solution that contained 0.5 mM of ferrocenium and 90 mM of ferrocene; and a Co(Cp)2+PF6-/ Co(Cp)2 redox solution that contained 50 mM of cobaltocene and 10 mM of cobaltocenium. Co(Cp)2, Fe(Cp*)2 and FeCp20 were purchased from Sigma Aldrich and were purified by sublimation in vacuo. Co(Cp)2+ · PF6-, Fe(Cp*)2+ · PF4−, and FeCp2+ • BF4− were purchased from Sigma Aldrich, recrystallized in a mixture of diethyl ether (ACS grade, EMD) and acetonitrile (ACS grade, EMD), and dried in vacuo before use. In all three solutions, 1.0 M LiClO4 (fused and dried in vacuo) was dissolved in dry CH3CN, to provide a supporting electrolyte. The Nernstian electrochemical potentials for the non-aqueous solutions were -0.13 V, -0.58 V and -1.39 V versus the formal potential of Fe(Cp)2+/0 for the Fe(Cp)2+/0, Fe(Cp*)2+/0, and Co(Cp)2+/0 solutions, respectively. The opencircuit potentials were measured using a 5 ½ digit Keithley Model 2000 Multimeter. Solid-State Electrical Measurements Solid-state J-V measurements of n-Si/TiO2/Ni pad samples in the dark were performed at temperatures between ~200 K and 300 K in a cryogenic probe system. The top electrical contacts for the n-Si/TiO2/Ni pad samples were formed via bonding of an Al wire between the Ni pads and the chip carrier that held the sample. The Ni pads established ohmic contacts to the TiO2 films.17 The bottom ohmic contacts were formed by scratching an In-Ga eutectic (SigmaAldrich) to the back sides of the n-Si. The substrates were then glued to the chip carriers using flash-dry Ag paste (SPI supplies). The temperatures of the n-Si/TiO2/Ni pad samples were 9 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 47

measured using a Si diode temperature sensor and were controlled by a LakeShore Model 321 Autotuning Temperature Controller. The J-V data were acquired by a Bio-Logic USA, SP-200 potentiostat using a two-electrode setup in which the working electrode was connected to top contacts (Ni pads) and the counter and reference electrodes were shorted and connected to the bottom contacts (n-Si back side). Solid-state light and dark J-V measurements were performed using the same TiO2-coated n-Si wafers. For these measurements, Ni busbars were patterned on the TiO2 surfaces and were used as top contacts instead of Ni pads. J-V measurements in the presence of illumination were performed under simulated AM 1.5G solar illumination (AAA grade), calibrated to 1-Sun by a reference Si photodiode. The sample temperatures were regulated at 293 K by a water chiller. Solid-state dark J-V and impedance measurements were performed in the dark using the same electrical connection. The built-in voltages, Vbi, of n-Si/TiO2 junctions were first determined from the impedance data using Mott-Schottky analysis. A 10 mV (peak-to-peak) AC signal with a frequency ranging from 2 MHz to 1 Hz was superimposed onto a DC bias, V, that was swept from -1.00 V to 0.40 V in 28 equal-voltage steps.

The differential capacitance, Cd, (area

normalized) of the n-Si space-charge region was calculated by fitting the data with a Randles circuit, in which a capacitor and resistor in parallel were connected in series with another resistor.35 The flat-band voltage, Vfb, which is equal to Vbi, was determined by linear fitting and extrapolation of the 1/Cd2 vs V plot, followed by application of the correction term kT/q (25.6 mV at 298 K), where k is Boltzmann’s constant. The barrier height can be extracted from the Mott-Schottky analysis by using the equation: ΦB/q = Vbi+(kT/q) × exp(NC/ND), where NC is the effective density of states in the conduction band of Si and ND is the dopant density of the Si.


Page 11 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Four-point probe measurements were performed using a Jandel RM3000 system. A cylindrical probe head that integrated a line of 4 tungsten carbide pins with spacings of 1 mm was used to make ohmic electrical contacts to surfaces of the n-Si, p+-Si and n+-Si wafers that were chosen in this study. A constant current, I, was set to flow through the outer two pins while the voltage, V, was measured between the inner two pins.

The sheet resistance was then

calculated using Ohm’s Law. The resistivity, , was calculated using the following relationship:

= 4.5324 , where d is the thickness of the wafer. Each four-point probe measurement was

performed 5 times with currents ranging from 0.1 to 1 mA to obtain a statistical average of the resistivity value. Photoelectron Spectroscopic Measurements X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) were performed using a Kratos Axis Ultra system with a base pressure of 1×10-9 Torr in the analysis chamber. Samples with TiO2 grown using 0, 10, 20, 40, 100 and 1000 ALD cycles on n+-Si (3 × 1019 cm-3) and p+-Si (4 × 1019 cm-3), as well as 0, 5, 10, 20, 40, 100 and 150 ALD cycles on n-Si (3.6 × 1015 cm-3), were transferred to the load-lock of the XPS system within 5 min after ALD growth. Photoelectron spectra for the Ti 2p, O 1s and Si 2p core levels were collected in the binding-energy ranges of 457 – 468 eV, 529 – 536 eV, and 97 – 106 eV, respectively. XPS core-level spectra were collected on thin ALD (< 150 cycles) TiO2 films that had been deposited on Si. In this thickness regime, X-ray core-level photoelectrons emitted below thin TiO2 overlayers can be detected and analyzed because such photoelectrons can escape through the surface overlayers. The inelastic mean-free path (IMFP) of photoelectrons at a kinetic energy of ~ 1386 eV (Si 2p photoelectron from X-rays at 1486.6 eV) is 2.68 nm through 11 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 47

TiO2 and is 3.09 nm through Si, whereas the IMFP for photoelectrons with a kinetic energy of ~ 1026 eV (Ti 2p photoelectron) is 2.12 nm through TiO2.36–38 Calculations of Band-Energy Diagrams The band-energy diagrams (see Figures 6, 7, and S6) were calculated from XPS and UPS measurements, and were developed in a step-wise fashion as schematically illustrated in Figure S3. The derivation of the energy diagrams for bare Si and its space charge regions (Figure S3a and S3b) is presented in the SI. As shown in Figure S3c, the potential drop across the SixTiyO2 oxide layer can be calculated from the experimentally obtained averaged core-level binding energies of the Siox and Si0 core level peaks by use of Equation (1). The following assumptions were made: 1) a linear energy drop, EPD, is present in the interfacial oxide; 2) the Siox core-level binding energy represents the averaged binding energy across the complete the oxide layer; 3) the experimental value for the core level binding energy of Siox for zero potential drop in the oxide layer (i.e. EPD=0) is given by core-level binding energy of Si0 plus the chemical shift for SiO2 of 3.8 eV* with respect to Si0 i.e. EB,Siox(EPD=0) = EB,Si + 3.8 eV* (note an “*” is used to denote literature values while a “**” denotes a calculated value); 4) if an additional potential drop, EPD, is present in the oxide layer, the oxide core-level binding energy is changed by the averaged value of ~ EPD/2. Thus, the energy drop EPD,Siox/2 in the interfacial oxide can be obtained by subtracting the measured Si0 binding energy and the chemical shift for Siox (3.8 eV* for SiO2) from the experimentally determined binding energy of the Siox, 2p core level i.e. E , = 2E, − E, − 3.8 eV


(1).

Page 13 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The value for the potential drop in Siox is indicated in the respective figures at the vacuum level of SiOx (and also later for SixTiyO2). At the interface between two semiconductors with different work functions φ1 and φ2, an interface dipole δ can be formed in addition to band bending (EBB) or a potential drop (EPD) in the respective semiconductors (shown in Figure S8). The thermodynamic equilibration of the Fermi levels requires ∆φ = φ2 - φ1 = EBB,1 + EBB,2 + δ, i.e. the interface dipole between Siox and Si can be calculated by: δ = ϕ − (ϕ + E , + E, )

(2).

Then, for the TiO2 covered Si, its interfacial band-energy diagram was obtained by tracking the shift of the Si0 and Siox photoemission lines (the shift of the Siox photoemission line maximum is indicated in Figure 5). The energy relations of the TiO2 overlayer with Si were derived by tracking the photoemission line shift of the Ti 2p CLBE for stepwise increases in the TiO2 layer obtained by XPS in conjunction with the position of the VBM and the work function of the underlying substrate, as obtained by UPS (Figure S3d). For n-Si, the VBM position was obtained from XPS instead of UPS. The CBM position of TiO2 was calculated using the optical band gap of 3.34 eV that was measured separately (see Figure S14 in Hu et al.17). The shift of the Ti 2p CLBE has two components: 1) band-bending in the TiO2 overlayer, and 2) the potential drop across the interfacial layer, as indicated by the shift of the SiOx photoemission line. When the shift of Ti 2p follows that of SiOx, band bending is negligible in the TiO2 overlayer. The Si/TiO2 interfacial dipole is a chemical property of the Si/TiO2 interface and was assumed to be the same magnitude and direction for all the Si/TiO2 interfaces (n-Si, n+-Si, and



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 47

p+-Si) investigated in this study. The dipole is indicated by ∆, in addition to the vacuum energylevel changes caused by interfacial electrical fields in the SixTixO2 interlayer. Results Electrical properties of n-Si/TiO2 interfaces Rectification behavior in the dark and photovoltaic behavior under illumination were observed in air for n-Si/TiO2/Ni solid-state devices that contained patterned Ni busbars to contact the TiO2 surface. Figure 1 shows the measured J-V performance of n-Si/TiO2, 68 nm (1500 ALD cycles)/Ni busbar devices, in the dark (black curve) and under simulated AM 1.5G illumination at 1 Sun (red curve).

The n-Si/ TiO2,

68 nm/Ni

busbar devices exhibited an open-circuit

photovoltage, Voc, of 0.39 ± 0.02 V, a short-circuit photocurrent density, Jsc, of 9.3 ± 1.4 mA·cm-2, a fill factor of 0.42 ± 0.01, and a photovoltaic system efficiency, ηPV, of 1.5 ± 0.2%, with 5 devices measured. Similarly, n-Si/TiO2, 3 nm (250 ALD cycles)/Ni busbar devices exhibited Voc = 0.31 ± 0.01 V and Jsc of 7.2 ± 0.7 mA·cm-2, with 4 devices measured. Figure 2 shows the open-circuit voltage of n-Si/TiO2/metal photoelectrodes in contact with 1.0 M LiClO4-CH3CN solutions that contained a series of one-electron redox couples: Fe(Cp)2+/0, Fe(Cp*)2+/0, and Co(Cp)2+/0. The Nernstian potentials of the solutions were -0.13 V, -0.58 V and -1.39 V versus E0’(Fe(Cp)2+/0), respectively. The n-Si/TiO2/metal electrodes contained either etched or unetched TiO2 on Si, with ALD TiO2 thicknesses ranging from 1 to 68 nm. The measured photovoltages were constant for a given type of photoelectrode in contact with any of the redox couples, and at least 6 electrodes of each type were measured. The TiO2 overlayers in order of thickness were 250 ALD cycles etched (1 nm) < 250 ALD cycles (3 nm) < 1500 ALD cycles etched (66 nm) < 1500 ALD cycles (68 nm), and devices made from


Page 15 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


corresponding TiO2 overlayers with n-Si produced photovoltages that increased as the TiO2 thickness increased in the order 1 nm < 3 nm ≈ 66 nm ≈ 68 nm. For example, the measured photovoltages of n-Si/TiO2, 68 nm/Ni photoelectrodes were 0.380 ± 0.009 V. The photovoltage for n-Si/TiO2, 1 nm/metal electrodes (the thinnest TiO2 layer in the comparison) was 0.151 ± 0.030 V, while the photovoltage of n-Si/TiO2, 3 nm/Ni electrodes (the second thinnest TiO2 layer in the comparison) was 0.388 ± 0.015 V, and was the same, within error, as the photovoltages exhibited by the remaining two sets of samples that had thicker TiO2 coatings (n-Si/TiO2, 66 nm/ metal and n-Si/TiO2, 68 nm/Ni electrodes). Variable-temperature electrical transport characteristics of n-Si/TiO2 interfaces The barrier height of n-Si/TiO2/Ni structures was determined from analysis of the solidstate electrical transport behavior in the dark. Figure 3a shows the rectifying solid-state J-V data for the n-Si/TiO2, 3 nm/Ni pad samples measured in the dark at 205 – 294 K. At temperatures below 200 K, the forward bias J-V data exhibited a negligible exponential region, because the resistance in the n-Si/TiO2/Ni pad samples dominated the forward-bias J-V behavior. From the linear region of the log-linear plot in Figure 3b, the exchange-current density, J0, and the diodequality factor, n, were determined by use of the Richardson-Schottky equation:39 ! = "∗ $ % exp (−

)∅+ ,-

)

)

. /exp (0,-. − 12 = !3 /exp (0,-. − 12

(3),

where "∗ is a temperature-independent pre-factor indicative of the charge-transfer rate between the n-Si and the TiO2, and is mathematically equivalent to the effective Richardson constant for thermionic emission, T is the absolute temperature, q is the absolute value of the unit charge of electrons, and ∅4 is the barrier height of the rectifying n-Si/TiO2 heterojunction. The diodequality factor, n, was ~ 1.1 for all of the J-V data, and was independent of temperature. Figure



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 47

! 3c shows a Schottky-type plot of 567 8 39$ % : ; 3 nm, the Mott-Schottky analysis showed a

constant barrier height, which agreed with the constant trend of photovoltages in contact with nonaqueous redox couples (Figure 2). Four-point probe measurements The resistivity measured using the 4-point probe method were 0.003 Ω·cm, 1.4 ± 0.2 Ω·cm, and 0.003 Ω·cm for n+-Si, n-Si, and p+-Si, respectively, indicating doping densities of 3 × 1019 cm-3, 3.6 × 1015 cm-3, 4 × 1019 cm-3. As discussed earlier, the values obtained for the n-Si samples were in agreement with those obtained from the Mott-Schottky analysis. Photoelectron spectroscopy of Si/TiO2 interfaces


Page 17 of 47

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5a shows the XPS data for the O 1s, Ti 2p and Si 2p core-level photoemission, stacked from bottom to top, for stepwise increases in ALD cycle numbers i.e. incremental increases of TiO2 thickness on an n-Si substrate. For thin TiO2 (

Electrical, Photoelectrochemical, and Photoelectron Spectroscopic

Recommend Documents