Research Article www.acsami.org
Interface Energy Alignment of Atomic-Layer-Deposited VOx on Pentacene: an in Situ Photoelectron Spectroscopy Investigation Ran Zhao, Yuanhong Gao, Zheng Guo, Yantao Su, and Xinwei Wang* School of Advanced Materials, Shenzhen Graduate School, Peking University, Shenzhen 518055, China S Supporting Information *
ABSTRACT: Ultrathin atomic-layer-deposited (ALD) vanadium oxide (VOx) interlayer has recently been demonstrated for remarkably reducing the contact resistance in organic electronic devices (Adv. Funct. Mater. 2016, 26, 4456). Herein, we present an in situ photoelectron spectroscopy investigation (including X-ray and ultraviolet photoelectron spectroscopies) of ALD VOx grown on pentacene to understand the role of the ALD VOx interlayer for the improved contact resistance. The in situ photoelectron spectroscopy characterizations allow us to monitor the ALD growth process of VOx and trace the evolutions of the work function, pentacene HOMO level, and VOx defect states during the growth. The initial VOx growth is found to be partially delayed on pentacene in the first ∼20 ALD cycles. The underneath pentacene layer is largely intact after ALD. The ALD VOx is found to contain a high density of defect states starting from 0.67 eV below the Fermi level, and the energy level of these defect states is in excellent alignment with the HOMO level of pentacene, which therefore allows these VOx defect states to provide an efficient holeinjection pathway at the contact interface. KEYWORDS: interface energy alignment, atomic layer deposition, vanadium oxide, photoelectron spectroscopy, defect states
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INTRODUCTION Organic electronic devices are promising for numerous important applications,1 such as active-matrix displays,2 radiofrequency identification tags,3 flexible sensors,4 and flexible bioelectronics.5 However, the device performance is often limited by the contact resistance, because of the often poor charge injection at the organic/metal interfaces.6,7 Large contact resistant can severely limit the effective carrier mobility, switching speed, and saturated output current for organic fieldeffect transistors (OFETs),8,9 and also hinder the downscaling of transistor channel length for high-frequency applications.6,8 Recently, a great number of efforts have been devoted to lower the contact resistance by carefully engineering the organic/ metal interfaces.7,10 As a simple and straightforward approach, inserting a thin metal oxide interlayer (e.g., V2O5,11,12 MoO3,13,14 Al2O3,15 etc.) between the organic and contact metal has been found very effective for lowering the contact resistance. But the origin of this improvement is still not well understood.7 The metal oxide interlayers could be prepared by solutionbased processes,11,13 thermal evaporation,12,15 or atomic layer deposition (ALD), which was an emerging fabrication method recently demonstrated by our group.16 ALD is a highly controllable thin-film deposition technique which employs alternating self-limiting surface reactions and provides for highly reproducible, large-scale uniform thin film growth with atomic precision control of film thickness.17,18 These features make ALD highly suitable for preparing well-controlled, highquality interlayers in organic devices, although the process © XXXX American Chemical Society
compatibility with organic materials should be cautioned (e.g., low process temperature using nonoxidizing reagents16). As we previously demonstrated,16 with a gentle ALD process using nonoxidizing coreactant of H2O at a process temperature as low as 50 °C, an ultrathin vanadium oxide (VOx) layer could be safely deposited directly on organic materials, and serve as an efficient hole-injection interlayer to greatly reduce the contact resistance in OFET devices. Meanwhile, the excellence in film controllability by ALD can also enable one to study the mechanism and the role of the inserted interlayer for the improved contact resistance. To demonstrate this concept, we, in this work, fabricated an organic/oxide interface by the above ALD process of VOx on pentacene, and carefully investigated the X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) in situ during the ALD process. With the in situ spectroscopic results herein, we were able to understand the chemistry and energy alignment at the pentacene/VO x interface, and this understanding would be highly beneficial for future contact engineering by ALD in general.
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EXPERIMENTAL SECTION
In situ experiments were performed on a modified XPS system (Thermo Scientific, Escalab 250Xi), as schematically illustrated in Figure 1a. The system consisted of an ALD chamber, an analysis Received: October 9, 2016 Accepted: December 26, 2016 Published: December 26, 2016 A
DOI: 10.1021/acsami.6b12832 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 1. Schematic illustrations of (a) in situ experimental setup and (b) sample structure.
Figure 2. High-resolution in situ (a) V 2p, O 1s, and (b) C 1s XP spectra taken from 0 cycles (i.e., initial surface) through 80 cycles of ALD VOx on pentacene. The cycle numbers are denoted on each of the spectrum curves. (c) Extracted VOx film thickness with respect to the number of ALD cycles. Square (blue) and diamond (red) symbols correspond to the films deposited on pentacene and Si substrates, respectively. The method of extracting film thickness is described in the text.
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chamber, and a connecting intermediate chamber for transferring samples without vacuum break. ALD of VOx was performed in the ALD chamber at 50 °C, using tetrakis(dimethylamino)vanadium (V(dma)4) and H2O vapor as the vanadium precursor and oxygen source, respectively. V(dma)4 and H2O were kept at room temperature in two separate containers, respectively, and were alternately dosed into the ALD chamber during the ALD process. Each ALD cycle consisted of 5 s V(dma)4 dosing, 120 s N2 purging, 1 s H2O dosing, 120 s N2 purging, and an additional pumping step to ∼1 × 10−5 Torr to bring down the water residue to a sufficiently low level. The effective exposures of V(dma)4 and H2O for each ALD cycle were roughly 0.015 and 0.040 Torr s, respectively. For (quasi) in situ photoelectron spectroscopic characterizations, the samples were transferred from the ALD chamber under vacuum via the intermediate chamber (base pressure ∼3 × 10−7 Torr) to the analysis chamber. The analysis chamber was equipped with a monochromatic Al Kα X-ray source (energy resolution of 0.45 eV) and a He I (21.22 eV) discharge lamp (energy resolution of 0.1 eV), which were used for XPS and UPS experiments, respectively. In XPS, the binding energy was referenced to Au 4f7/2 (83.96 eV). To avoid damage from X-ray, the XP spectrum was always taken on a new spot for each measurement. For UPS, the samples were all biased by −10 V for better measurement of the secondary-electron cutoffs. For in situ ALD of VOx, pentacene substrate samples were prepared by thermally evaporating 80 nm of pentacene on Si(100) wafers (Figure 1b). The base pressure for the thermal evaporation was 3 × 10−4 Torr, and the evaporation rate was ∼0.4 Å s−1. For comparison, in situ ALD of VOx was also performed on Si(100) reference wafers. The Si substrates were treated with UV/ozone for 5 min prior to ALD, and the native surface oxide was not intentionally removed. Spectroscopic ellipsometry (JA Woollam, M-2000) was used to measure the thickness of the ALD VOx films grown on the Si substrates.
RESULTS AND DISCUSSION
In situ XPS and UPS were employed to monitor the deposition process of VOx on pentacene. Figure 2ab presents the highresolution XP spectra of V 2p, O 1s, and C 1s core levels taken from 0 cycles (i.e., initial surface) through 80 cycles of VOx deposited on a pentacene substrate. The associated survey spectra were provided in Figure S1. Prior to ALD, the initial pentacene surface (0 cycles) showed an expected dominating carbon peak at binding energy (BE) of 284.29 eV,19,20 along with a tiny oxygen peak at 532.25 eV, which was probably due to the surface oxidation of pentacene in air.19,21 These features remained generally the same after 1 cycle of VOx deposition, except for very small shifts of the peak positions as will be discussed later. But, in contrast to the ALD VOx film grown on a Si substrate (Figure S2), the one-cycle film grown on pentacene did not show clearly observable peaks for vanadium, which suggested a delay of growth nucleation on pentacene surface (vide infra). After 10 ALD cycles, the XP spectrum (Figure 2a) started to show vanadium peaks (V 2p). These vanadium peaks became more prominent when 20 or more cycles of VOx were deposited, and, at the same time, the intensity of the oxygen peak (O 1s) was also much increased. The BE of the V 2p3/2 peaks were all roughly 516.4 eV, which indicated that the valence state for vanadium in the deposited VOx film was mainly +4 (reported values for V5+ and V4+ were 517.4 and 516.2 eV, respectively22). This valence state was also consistent with the nominal +4 vanadium state in the V(dma)4 precursor, suggesting that the role of H2O was to mainly substitute the V−N bonds with V−O bonds rather than to induce redox reactions directly on the vanadium atoms.23 As for the concurrently increasing O 1s peak, the BE was roughly B
DOI: 10.1021/acsami.6b12832 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 3. In situ UP spectra showing the evolution of (a) the secondary-electron cutoff and (b) HOMO regions for 0−80 cycles of VOx deposited on pentacene. (c) Plot of work function versus the thickness of the ALD VOx layer.
was lower than that on Si. ALD growth normally starts from the reaction of the metal precursor with the reactive surface groups on substrate. Our UV/ozone-treated Si substrate carried a fair amount of hydroxyl groups, which could serve as the active nucleation sites for reacting with the metal amide precursors, V(dma)4, to afford metal−oxygen bonds.29−31 On the other hand, hydrocarbons are normally inert for this type of chemical reactions,17 so the ALD nucleation is often found delayed on a hydrocarbon surface (e.g., polyethylene32). Therefore, the suppression of the initial growth on pentacene was probably due to the lack of reactive sites on the aromatic pentacene molecules, which could lead to nucleation delay and island formation at the early stage of ALD.33 After 20 cycles, the VOx growth behavior returned back to normal, and the growth rate (slope) was the same as for the Si case. Therefore, at least 20 ALD cycles were needed for the VOx film to coalesce and form a continuous, full layer on pentacene. In addition, a close examination of the peak positions revealed that all the major core-level peaks (O 1s, V 2p3/2, and C 1s) showed small but noticeable shifts of a few hundredths of eV toward lower BE as the VOx layer grew thicker, and these shifts could be ascribed to the shift of the Fermi level as will be discussed later. Nevertheless, no change of the line shape was observed for the pentacene C 1s peak, which suggested that the underneath pentacene was largely intact by ALD VOx. In situ UP spectra were taken in parallel with the XPS measurements. Figure 3ab displays the secondary-electron cutoff and HOMO regions of the in situ UP spectra for 0− 80 cycles of VOx deposited on pentacene. Since the secondaryelectron cutoff energy was related to the surface work function by subtracting it from the incident photon energy, the work function was accordingly calculated and plotted in Figure 3c as a function of VOx thickness. The work function increased from 4.40 to 4.60 eV for the first 40 cycles, and then remained constant as the deposition continued to 80 cycles. The initial increase was likely due to the introduction of interfacial dipole by the ALD VOx, and once the covering ALD layer was enough thick (≥40 cycles), the work function became a constant reflecting the Fermi level of VOx. As for the HOMO region of the UP spectra (Figure 3b), a clear peak at ∼1.1 eV below the Fermi level was observed for 0−20-cycle samples, and this peak was due to the emission from the HOMO level of pentacene.15,34 The intensity of this HOMO level emission gradually diminished as the ALD of VOx proceeded from 0 through 20 cycles, along with which a small but noticeable BE shift from 1.12 to 1.05 eV was also observed for the HOMO peak (HOMOpk) (see also Figure S4). Note that the onset of the HOMO emission (HOMOon) was always 0.42 eV above
530.5 eV, which was also in consistence with the reported value for VOx.24 We also noticed that the line shape of the O 1s peak had an asymmetry toward higher BE (∼531.8 eV), which could be ascribed to the contribution from hydroxyl oxygen,24 because the hydroxyl groups were the terminating surface groups after each ALD cycle and they could also be partially incorporated in the oxide, given such a low-temperature process using water as the coreactant.25 Careful peak fitting was performed for O 1s (Figure S3), and the results showed that the percentage for hydroxyl oxygen was roughly 15−20% (Table S1). Also, based on the peak areas of V and O, the x in VOx was found to be fairly close to 2.0 (Table S1). As for the C 1s core level (Figure 2b), the intensity continuously diminished as the deposition proceeded. This phenomenon was expected due to the attenuation effect from the growing ALD VOx layer on top. The thickness of the VOx layer (d) could be extracted from the areal intensity ratio of the V 2p and C 1s peaks (IV2p/IC1s) via the following equation26,27 ⎞ ⎛ IV 2p d = λ sin θ ln⎜ + 1⎟ ⎠ ⎝ βIC1s
(1)
where λ is the effective attenuation length of the V 2p photoelectrons in the ALD VOx layer, θ (= 90°) is the ∞ photoelectron takeoff angle, and β (= I∞ V2p/IC1s) is the ratio of V 2p and C 1s intensities for “infinitely thick” VOx and pentacene samples, respectively. A ∼40 nm thick VOx film deposited by ∞ the same ALD process was used to measure I∞ V2p; whereas IC1s was directly taken from the spectrum for the initial pentacene surface prior to in situ ALD (i.e., 0 cycles). Since the value of λ was unknown for our nonstandard ALD material, we also performed the same in situ XPS/ALD experiments on a flat Si reference substrate, where the ALD film thickness could be accurately measured by ellipsometry and used to extract λ for our ALD VOx material. As the details provided in Figure S2, the obtained λ was 2.80 nm, which was a reasonable value for an oxide28 and, therefore, was used to calculate the thickness for other VOx layers. The evolution of the VOx thickness on pentacene was plotted in Figure 2c, with respect to the ALD cycle number. In the same figure, the evolution for the VOx deposited on a Si reference substrate was also plotted for comparison. On Si, the film thickness followed a good linear relation with the cycle number, and the growth rate, extracted from the linear slope, was about 0.32 Å/cycle, which was well consistent with our previous ex situ results.16 On the other hand, for the film deposited on pentacene, the initial film growth (0−20 cycles) seemed to be partially suppressed, as the effective film thickness C
DOI: 10.1021/acsami.6b12832 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Based on the above XPS and UPS results, the energy diagram for the pentacene/VOx interface could be constructed. Here, we present the diagram for the 40-cycle VOx case (Figure 5),
HOMOpk (toward Fermi level). Eventually, after 60 ALD cycles, the pentacene HOMO emission was completely screened, and therefore, the corresponding UP spectra (60 and 80 cycles) reflected only the features for the coated VOx layer. The valence band maximum (VBM) for VOx was primarily composed of O 2p states,35,36 which was found at ∼3.2 eV below the Fermi level in the spectra (see also Figure S5). On the other hand, the partially filled V 3d states, contributed from V4+(3d1), could hybridize with O 2p37 and form defect states (DS) above the valence band.38 As denoted in Figure 3b, the onset (DSon) and peak (DSpk) of the defect states were found at 0.67 and 1.64 eV below the Fermi level, respectively. Since the electronic structures for transition metal oxides are generally very sensitive to fabrication/ambient conditions (e.g., cation oxidation state and oxygen vacancy level39), it was therefore particularly important to perform the aforementioned in situ measurements for accurate energy alignment analysis. The traces for the HOMOpk and DSon positions from the UPS results were plotted with respect to the VOx thickness (Figure 4), which could allow us to derive the energy alignment
Figure 5. Energy diagram for the pentacene/VOx interface. Values are shown in eV.
because 40 cycles were beyond the nucleation stage (Figure 2c) and produced stable values for the work function (Figure 3c) and the HOMO/DS positions (Figure 4), and, more importantly, 40 cycles of ALD VOx were found to give out the minimum contact resistance in practical OFET devices.16 The LUMO of pentacene was drawn at 2.68 eV above its HOMO,42 and the conduction band minimum (CBM) of VOx was estimated from the optical absorption of a thick ALD VOx sample (Figure S6). As shown in Figure 5, the HOMO band of pentacene was found to be in great overlap with the defect band of ALD VOx, indicating that the HOMO and DS were well aligned at the pentacene/VOx interface. Also, noticing that the energy offset between the HOMOpk and DSon was of a constant value (0.31 eV) regardless of the VOx thickness (Figure 4), their relative positions should not alter with the VOx thickness as well. Therefore, the matched alignment between the HOMO and DS should be the case for all the thicknesses in this study. The energy-aligned VOx defect states were highly important for efficient hole injection in actual devices where the oxide layer was inserted at the organic/metal contact interface. The energy-matched defect states could greatly assist the charge tunneling/hopping process from the contact metal to pentacene HOMO, and therefore effectively reduce the contact resistance in device.7 Also, since the ALD VO x was amorphous,29 the density of the defect states should be fairly high, which would be beneficial for this defect-assisted injection process. On the other hand, common high work function metals (e.g., gold) usually suffer from so-called “push back” effect6,43 by adjacent organic molecules, resulting in much lowered effective work functions that no longer match up with the organic HOMO levels, and the essence of the “push back” effect is the tails of the metal electron wave functions that extended out into vacuum being pushed back by the adjacent organic molecules.6 But in the case that ALD VOx was present, the defect states in VOx could still accommodate the metal electron wave functions to leak out, and therefore the “push back” effect would presumably be much alleviated. The above reasonings implied that, to maximize the overall hole-injection efficiency, a full coverage of the VOx layer should be needed,
Figure 4. HOMO peak energy (HOMOpk) and defect states onset energy (DSon) in dependence on the thickness of the ALD VOx layer.
at the pentacene/VOx interface.40 Meanwhile, the energy alignment could also be derived from the XPS core-level shifts.40,41 Following the standard procedure described in the literature,41 the traces for the core levels were obtained from their BE by subtracting their energy differences to the HOMOpk (for C 1s) or DSon (for V 2p3/2 and O 1s) position, which are all material constants (see Figure S7 for details).40 As shown in Figure 4, the traces obtained from the valence UPS and core-level XPS agreed very well, and also the BE for HOMOpk and DSon showed parallel shifts, which ensured that the interface energy offset obtained from these experiments was reliable.40 The shift in HOMOpk was toward the Fermi level and in consistence with the shift of the work function (Figure 3c), which corroborated the presence of dipole at the pentacene/VOx interface, but nevertheless the shift was fairly small (0.1−0.2 eV) and occurred mostly at the early nucleation stage of ALD (first 20 cycles). The energy offset between HOMOpk and DSon was obtained as a constant of 0.31 eV for all the thicknesses, which was a critical value for the following analysis. D
DOI: 10.1021/acsami.6b12832 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 6. (a) High-resolution XP spectra showing the evolution of V 2p and O 1s peaks as exposing the ALD VOx to air. XPS were taken for the ALD VOx as deposited (VOx), after 24 h air exposure (VOx-24h), and after 120 h air exposure (VOx-120h). XPS for thermally evaporated V2O5 (PVD V2O5) was also included for comparison. (b) UP spectra showing the corresponding evolution of the defect states.
results presented herein not only enriched our understanding of the role of the oxide interlayer, but they also brought in new insights for the engineering of contact interfaces by ALD.
which corresponded to >20 cycles for passing beyond the early nucleation stage in ALD (Figure 2c). On the other hand, the ALD VOx had a poor conductivity, presumably because the defect states were localized in amorphous VOx and the charge mobility was low, and thus a thin VOx layer was favored for the injection. Therefore, among the cycle numbers investigated in this study, one would naturally predict that 40 ALD cycles of VOx (0.94 nm) would give out the highest efficiency for hole injection in actual devices, and this prediction was indeed in excellent agreement with our previous device characterization results (see Figure S8 for details).16 In addition, to extend the applicability of our findings under vacuum to the actual device fabrication environment where air break was often involved after ALD and before contact metal evaporation,16 we also examined the evolution of the XP and UP spectra when exposing the ALD VOx to air. As the XP spectra shown in Figure 6a, a clear shift toward higher BE was found for the V 2p3/2 peak, which suggested that the surface VOx was indeed oxidized in air. But the oxidation rate seemed quite slow, and also the oxidation of vanadium did not reach complete +5 state, as, even after 120 h of air exposure, the XP spectrum still showed a prominent shoulder associated with V4+ on the lower BE side of the V 2p3/2 peak. Since the lower-state V4+(3d1) was the main contributor to the defect states,37 the defect states were still observable in the UP spectrum after 120 h of air exposure (Figure 6b). Therefore, short air breaks during device fabrication should not alter our conclusions regarding the role of the defect states in ALD VOx.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b12832. Additional XP and UP spectra, Tauc plot from UV−vis spectrometry, schematic illustration of the relationship between XPS core-levels with HOMOpk/DSon, and contact resistance results from device characterizations (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Ran Zhao: 0000-0002-4689-1836 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by NSFC (Grant Nos. 51302007, 11404011, and 51672011), Guangdong Natural Science Funds for Distinguished Young Scholar (Grant 2015A030306036), Guangdong Innovation Team Project (2013N080) and Shenzhen Science and Technology Innovation Committee (Grant KQCX20150327093155293 and JCYJ20150629144835001).
CONCLUSIONS
In this work, we carefully investigated the interface of the ALD VOx grown on pentacene by in situ photoelectron spectroscopy. Benefited from the in situ XPS and UPS characterizations, we were able to monitor the ALD growth process of VOx and trace the evolutions of the work function, pentacene HOMO level, and VOx defect states during the growth. The initial VOx growth was found to be partially delayed on pentacene in the first ∼20 ALD cycles, and thereafter the growth behavior returned to the normal linear behavior as on reference Si substrate. The underneath pentacene layer remained largely intact after the ALD. The deposited VOx was found to contain a high density of defect states starting from 0.67 eV below the Fermi level, and the energy level of these defect states was in excellent alignment with the HOMO level of pentacene, which therefore could allow these VOx defect states to provide an efficient hole injection pathway at the contact interface. The
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DOI: 10.1021/acsami.6b12832 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
