Chemical Vapor Deposition Synthesis of Novel Indium Oxide ...
InSitu Measurements of Work Function of Indium Tin Oxide ...
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In Situ Measurements of Work Function of Indium Tin Oxide after UV/Ozone Treatment Daisuke Yamashita 1,+ , Atsushi Ishizaki 1 and Tomoyuki Yamamoto 2,3 1 Research and Development Department, Riken Keiki Co., Ltd., Saitama 344-0057, Japan 2 Faculty of Science and Engineering, Waseda University, Tokyo 169-8555, Japan 3 Institute of Condensed-Matter Science, Waseda University, Tokyo 169-8555, Japan In situ measurements of work function of indium tin oxide after UV/ozone treatment were carried out using a photoemission yield spectrometer with an open counter. Although the work function increased just after UV/ozone treatment, it decreased as time passed and finally returned to the initial value. The continuous change in work function with exposure to air was observed under dry atmosphere and at various temperatures. The returning process at higher temperature proceeded faster than at lower temperature. By contrast, humidity has no influence on the work function recovery. The exponential decay of work function was consistent with the first-order reaction rate equation. The rate constant obeyed Arrhenius’ equation, and the activation energy was estimated to be 22 kJ/mol. [doi:10.2320/matertrans.MA201516] (Received January 26, 2015; Accepted March 6, 2015; Published April 10, 2015) Keywords: indium tin oxide, work function, ozone, photoemission yield spectroscopy, electrode material 1. Introduction Transparent conducting oxides (TCOs) have been widely studied because of the importance of their industrial applications. Among these TCOs, indium tin oxide (ITO) has been extensively investigated and already applied to the industrial use as electrodes for a variety of electronic devices, such as organic light emitting devices (OLEDs), 1) liquid crystal displays, 2) and solar cells, 3) because of its transparency in the visible light region and high electric conductivity. In OLEDs, ITO is used as an anode, or a hole- injecting electrode, based on its work function. 1) Since the work function of ITO affects the efficiency of charge injection into the organic layer, the work function plays an important role to improve the device performance. Surface treatment is one of the effective ways for the improvement in device performance. Thus, in situ measurement is expected to clarify the effect of a surface treatment or the stability of the surface. Such measurement can be carried out by using photoemission yield spectrometer in air (PYSA) which employed the open counter 4-7) as the detector. The open counter can detect and count a small number of low-energy photoelectron, and can operate in air under atmospheric pressure. Therefore, the observation of successive change in work function, such as Al exposed to air, 8) has been successfully performed using PYSA. In order to evaluate the effect of a surface treatment, PYSA is often employed for measurement of work function of ITO. 9-21) Several studies reported the change in work function caused by the UV/ozone treatment of the ITO 9-11) using conventional PYSA at an ambient condition. It is known that the work function of the ITO surface gradually decreased with exposure to air after UV/ozone treatment. In the current study, we carried out in situ measurement of work functions after UV/ozone treatment with exposure to air changing the humidity and temperature systematically, in order to evaluate dependences of humidity and temperature on recovery rate of work function. The time dependence of the change in work function was explained using chemical kinetics scheme. 2. Experimental Procedure The experimental setup of a PYSA is shown in Fig. 1. UV- rays emitted from a deuterium lamp were monochromatized by a grating monochrometer (Model CT-10, JASCO) and focused on a sample surface via an optical fiber. Here, increment of 0.10 eV for incident monochromatized UV-rays is chosen for the energy scanning up to 6.20eV, because UV- rays with energies greater than 6.20 eV are greatly absorbed by O 2 in air. The number of emitted photoelectrons from the sample surface was counted by the open counter in air. For the measurement under a dry air, dew point of atmosphere in the main chamber was kept below ¹35°C. The sample and the counting system were placed in a glovebox. Here, the counting system is constructed with an open counter and a control circuit. Details of this apparatus were reported in a previous paper. 22) For the measurement at various temper- atures, the sample was heated by a small rubber heater connected to temperature controller. 1 2 10 11 4 5 3 8 9 6 7 12 13 Fig. 1 Experimental setup of photoemission yield measurement. 1: deuterium lamp, 2: grating monochrometer, 3: optical fiber, 4: sample, 5: counting system, 6: measurement room 7: main chamber (glovebox), 8: antechamber, 9: gas piping, 10: gas purifier, 11: hygrometer, 12: heater, 13: temperature controller. Note that the measurement room can intercept external lights. + Corresponding author, E-mail: d-yamashita@rikenkeiki.co.jp Materials Transactions, Vol. 56, No. 9 (2015) pp. 1445 to 1447 Special Issue on Nanostructured Functional Materials and Their Applications © 2015 The Japan Institute of Metals and Materials
Transcript of InSitu Measurements of Work Function of Indium Tin Oxide ...
In Situ Measurements of Work Function of Indium Tin Oxideafter UV/Ozone Treatment
Daisuke Yamashita1,+, Atsushi Ishizaki1 and Tomoyuki Yamamoto2,3
1Research and Development Department, Riken Keiki Co., Ltd., Saitama 344-0057, Japan2Faculty of Science and Engineering, Waseda University, Tokyo 169-8555, Japan3Institute of Condensed-Matter Science, Waseda University, Tokyo 169-8555, Japan
In situ measurements of work function of indium tin oxide after UV/ozone treatment were carried out using a photoemission yieldspectrometer with an open counter. Although the work function increased just after UV/ozone treatment, it decreased as time passed and finallyreturned to the initial value. The continuous change in work function with exposure to air was observed under dry atmosphere and at varioustemperatures. The returning process at higher temperature proceeded faster than at lower temperature. By contrast, humidity has no influence onthe work function recovery. The exponential decay of work function was consistent with the first-order reaction rate equation. The rate constantobeyed Arrhenius’ equation, and the activation energy was estimated to be 22 kJ/mol. [doi:10.2320/matertrans.MA201516]
(Received January 26, 2015; Accepted March 6, 2015; Published April 10, 2015)
Keywords: indium tin oxide, work function, ozone, photoemission yield spectroscopy, electrode material
1. Introduction
Transparent conducting oxides (TCOs) have been widelystudied because of the importance of their industrialapplications. Among these TCOs, indium tin oxide (ITO)has been extensively investigated and already applied tothe industrial use as electrodes for a variety of electronicdevices, such as organic light emitting devices (OLEDs),1)
liquid crystal displays,2) and solar cells,3) because of itstransparency in the visible light region and high electricconductivity. In OLEDs, ITO is used as an anode, or a hole-injecting electrode, based on its work function.1) Sincethe work function of ITO affects the efficiency of chargeinjection into the organic layer, the work function plays animportant role to improve the device performance. Surfacetreatment is one of the effective ways for the improvement indevice performance. Thus, in situ measurement is expectedto clarify the effect of a surface treatment or the stability ofthe surface. Such measurement can be carried out by usingphotoemission yield spectrometer in air (PYSA) whichemployed the open counter47) as the detector. The opencounter can detect and count a small number of low-energyphotoelectron, and can operate in air under atmosphericpressure. Therefore, the observation of successive changein work function, such as Al exposed to air,8) has beensuccessfully performed using PYSA.
In order to evaluate the effect of a surface treatment,PYSA is often employed for measurement of work functionof ITO.921) Several studies reported the change in workfunction caused by the UV/ozone treatment of the ITO911)
using conventional PYSA at an ambient condition. It isknown that the work function of the ITO surface graduallydecreased with exposure to air after UV/ozone treatment. Inthe current study, we carried out in situ measurement of workfunctions after UV/ozone treatment with exposure to airchanging the humidity and temperature systematically, inorder to evaluate dependences of humidity and temperatureon recovery rate of work function. The time dependence of
the change in work function was explained using chemicalkinetics scheme.
2. Experimental Procedure
The experimental setup of a PYSA is shown in Fig. 1. UV-rays emitted from a deuterium lamp were monochromatizedby a grating monochrometer (Model CT-10, JASCO) andfocused on a sample surface via an optical fiber. Here,increment of 0.10 eV for incident monochromatized UV-raysis chosen for the energy scanning up to 6.20 eV, because UV-rays with energies greater than 6.20 eV are greatly absorbedby O2 in air. The number of emitted photoelectrons from thesample surface was counted by the open counter in air. Forthe measurement under a dry air, dew point of atmosphere inthe main chamber was kept below ¹35°C. The sample andthe counting system were placed in a glovebox. Here, thecounting system is constructed with an open counter and acontrol circuit. Details of this apparatus were reported in aprevious paper.22) For the measurement at various temper-atures, the sample was heated by a small rubber heaterconnected to temperature controller.
1
2
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8
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7
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Fig. 1 Experimental setup of photoemission yield measurement.1: deuterium lamp, 2: grating monochrometer, 3: optical fiber, 4: sample,5: counting system, 6: measurement room 7: main chamber (glovebox),8: antechamber, 9: gas piping, 10: gas purifier, 11: hygrometer, 12: heater,13: temperature controller. Note that the measurement room can interceptexternal lights.
+Corresponding author, E-mail: [email protected]
Materials Transactions, Vol. 56, No. 9 (2015) pp. 1445 to 1447Special Issue on Nanostructured Functional Materials and Their Applications©2015 The Japan Institute of Metals and Materials
In this study, ITO-coated glass (Furuuchi chemical, IN-100) was measured. For the UV/ozone treatment, photosurface processor (PL16-110, SENLIGHTS) was employed.ITO surface was treated by UV and ozone generated from amercury lamp for 30min. To estimate the work function ofITO surface, photoemission yield spectra were obtained usinga photoemission yield spectrometer (AC-5, Riken Keiki).Before UV/ozone treatment, photoemission yield measure-ment was made at room temperature (25°C, 50%RH).Immediately after the UV/ozone treatment, the sample wasplaced on the sample stage, and photoemission yield spectrawere taken as a function of time.
3. Results and Discussions
The square roots of photoemission yield against incidentphoton energies are shown in Fig. 2 for ITO before and justafter UV/ozone treatment. Threshold energy of photoem-ission, which corresponds to the work function, wasdetermined from the energy of an intersecting point betweena background line and the extended line of the squareroot of the photoemission yield. The square root of thephotoemission yield is often applied based on Fowler’sanalysis.23) The work function of ITO was determined here tobe 4.69 eV before UV/ozone treatment. Just after UV/ozonetreatment, work function increased up to 5.22 eV. This valueis very close to previously reported values, 5.25 « 0.1 eV,9)
5.15 eV,10) 5.2 eV.11) Sugiyama et al. reported that threefactors, i.e.,: (i) C-containing contaminants, (ii) the O/Inratio, and (iii) the In/Sn ratio on the ITO surface, affect thework function in the order: (ii) > (i) > (iii), and (i) is themajor reason of the increase in the work function due to theUV/ozone or O2 plasma treatments.24) As shown in Fig. 2,the slope of photoemission yield, Y, which characterizesthe escape probability of photoelectrons from the surface,slightly increased after UV/ozone treatment. When thesurface is covered with another layer, e.g. oxides on Si,2) n-alkylmercaptanes on Au,25) water on the TiO2,22) the numberof the photoelectrons that can reach the counter depends onthe layer thickness. If the layer of C-containing contaminantswere removed by UV/ozone treatment, the number ofphotoelectrons should increase. One possible explanationfor the low work function of contaminated ITO is a loweringof the vacuum level due to the adsorption of C-containingcontaminants.24) On the other hand, the oxygen defects in theITO surface are recovered with the exposure to UV/ozoneenvironment. This change led to an increase in the workfunction.9,24)
Figure 3 shows change in work function of specimen keptat ambient temperature (25°C). Each point represents theaverage of 3 replicates. The error bar denotes the range fromminimum to maximum. Work functions decrease graduallywith exposure to air and return to their initial values. Asshown in Fig. 3, work function recovery with exposure to airwas consistent with those observed under dry air. It wasindicated that humidity has no influence on the work functionrecovery.
Next, work functions of specimen kept at varioustemperatures were observed, which are shown in Fig. 4.Each point represents the average of 3 replicates. The error
bar denotes the range from minimum to maximum. Asshown in Fig. 4, the returning processes at higher temper-atures proceeded faster. Because change in work functionseems to be exponential decay, we try to apply a followingequation.
�ðtÞ ¼ ð�max ��minÞ expð�krtÞ þ�min ð1Þwhere ¯(t), ¯min and ¯max are the instantaneous, minimal andmaximal work functions, respectively. kr is the rate constant.The use of eq. (1) was merely a temporal dependence. Workfunction decreases at a rate which is proportional to itscurrent value. As shown in Fig. 4, the experimental curveswere well reproduced by eq. (1) after least-square fitting withthe rate constant as a single parameter. Equation (1) can beconverted to
ð�ðtÞ ��minÞ=ð�max ��minÞ ¼ expð�krtÞ ð2ÞConsequently, the decay of work function was consistentwith the first-order reaction rate equation. To confirm thetemperature dependence of rate constant, plots of ln(kr)against 1/T are shown in Fig. 5. The reaction has a rateconstant that obeys Arrhenius’ equation, because Fig. 5shows that there is good linearity between ln(kr) and 1/T.From this fittings, the activation energy was determined to be22 kJ/mol.
4.5 5.0 5.5 6.00
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Yie
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Fig. 3 Work function of ITO as a function of time at 25°C. © symboldenotes the result with exposure to air (dew point, 13 « 1°C). symboldenotes the result with exposure to a dry air (dew point, ¹40 to ¹35°C).
D. Yamashita, A. Ishizaki and T. Yamamoto1446
4. Conclusion
The time dependent change in work function of ITO afterUV/ozone treatment was elucidated using photoemission
yield spectroscopy changing the humidity and temperature.As a result of the UV/ozone treatment, work function ofsurface obviously increased, which decreased gradually astime passed and returned to their initial values after 150minat room temperature. The returning processes at highertemperatures proceed faster than at room temperature. It isalso confirmed that humidity has no influence on the workfunction recovery. The time dependence of the change inwork function was explained using chemical kinetics scheme,in which the exponential decay of work function withexposure to air was successfully explained. In addition theactivation energy is estimated to be 22 kJ/mol.
This kind of in situ measurement could be applied toevaluate the effect of a surface treatment or the stability of thesurface of other kinds of practical surfaces.
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Fig. 4 Work function of ITO after UV/ozone treatment at varioustemperatures as a function of time, (a) 95°C, (b) 75°C, (c) 60°C,(d) 40°C, (e) 25°C. Solid lines are fittings to eq. (1) in the main text.
0.0025 0.0030 0.0035-4
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Fig. 5 Plots of ln(kr) against 1/T. kr is the rate constant.
In Situ Measurements of Work Function of Indium Tin Oxide after UV/Ozone Treatment 1447
