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An investigation of the role of plasma conditions on the deposition rate of electrochromic vanadium oxide thin films
Journal of Non-Crystalline Solids 351 (2005) 1987–1994 www.elsevier.com/locate/jnoncrysol
An investigation of the role of plasma conditions on the deposition rate of electrochromic vanadium oxide thin films Michael Seman, Joey Marino, Wenli Yang, Colin A. Wolden
*
Chemical Engineering Department, Colorado School of Mines, 1500 Illinois Street, Golden, CO 80401, USA Received 28 August 2004; received in revised form 6 May 2005 Available online 20 June 2005
Abstract Plasma-enhanced chemical vapor deposition (PECVD) has been used to form amorphous vanadium oxide thin films from mixtures of VOCl3, O2, and H2. The deposition rate was examined as a function of PECVD operating conditions. Growth rates were found to be first order in VOCl3, and independent of both O2 and H2. High quality vanadium oxide films were also deposited without the use of hydrogen. Rates were observed to increase with rf power, and decrease with operating pressure. Maximum rates were an order of magnitude greater than typically observed with physical vapor deposition techniques. Optical transmission and electrochemical analysis were used to quantify the electrochromic response. After initial cycling films demonstrated high transparency across the visible, and the optical band gap increased with lithium intercalation. Lithium ion diffusion coefficients approached 1011 cm2/s, approximately an order of magnitude higher than literature values. 2005 Elsevier B.V. All rights reserved. PACS: 81.15.Gh; 78.20.J; 66.30.D; 78.66
1. Introduction Electrochromic materials are being pursued for numerous applications including sensors [1,2], smart windows [3], and display technology [4]. Solid-state electrochromic devices are composed of a stack of thin metal oxides whose optical transmission may be reversibly altered between clear and opaque in response to an applied voltage. The stack consists of an ion storage layer, an electrolyte, and an electrochromic layer sandwiched between two transparent conducting oxide (TCO) electrodes [4]. Vanadium oxide has been identified as a leading candidate to serve as the ion storage layer [5,6]. Physical vapor deposition (PVD) such as sputtering [7] and evaporation [8] are the dominating techniques employed for thin film synthesis. Plasma-
*
Corresponding author. Tel.: +1 303 273 3544; fax: +1 303 273 3730. E-mail address: [email protected] (C.A. Wolden).
0022-3093/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.05.016
enhanced chemical vapor deposition (PECVD) offers an attractive alternative due to its high deposition rates and low temperature capability. PECVD offers advantages for large area uniformity, and it is not susceptible to issues associated with target aging that can limit PVD techniques [2]. Our group is pursuing the complete synthesis of a solid-state electrochromic device by PECVD. To date we have produced high performance TCO electrodes [9,10] and electrochromic tungsten oxide [11,12] by PECVD. In this paper we take the next step, and describe PECVD synthesis of vanadium oxide for use as the ion storage layer. There has been limited use of PECVD for V2O5 synthesis. One group used oxygen and a complex organometallic precursor, VO(hfa)2 Æ H2O (hfa = 1,1,1,5,5,5hexafluoro-2,4 pentanedione), focusing on the dielectric properties of amorphous and nanocrystalline films [13,14]. Zhang et al. originated vanadium oxide PECVD using the three reactants VOCl3, O2, and H2, and examined the resulting films for application as cathodes in
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rechargeable lithium batteries [15]. They suggested that the reaction chemistry follows the following global pathway 4VOCl3 + 6H2 + 3O2 ! 2V2 O5 + 12HCl
ð1Þ
These authors reported that the addition of hydrogen accelerates the reaction by removing free chlorine. Optimized deposition rates were on the order of 60–70 nm/ min, which is a factor of five greater what is typically achieved using PVD techniques [16]. These films had a high storage capacity and demonstrated long-term stability. For electrochromic applications optical transparency is an important concern [6,17]. We are not aware of any reports of transmission characteristics of PECVDdeposited V2O5. Similarly, there are no reported investigations of ion transport in PECVD material, which can be important in determining device switching times. In this paper we present a comprehensive examination of PECVD deposition kinetics. We demonstrate rates that are an order of magnitude greater than conventional PVD processes. Fundamental optical and electrochemical characterization demonstrate that this material is highly suitable for application in electrochromic windows.
2. Experimental procedure and analysis Deposition of amorphous vanadium oxide was performed in a parallel plate, capacitively-coupled plasma chamber. Power was supplied by a 300 W rf power supply operating at 13.56 MHz and coupled using a match network. Further details of this custom built chamber are provided in the literature [18]. Vanadium oxide synthesis was accomplished using mixtures of vanadium oxytrichloride (VOCl3), oxygen, hydrogen, and argon. All flowrates were set using electronic mass flow controllers. Gases were introduced uniformly through a perforated showerhead that also served as the powered electrode. Films were deposited on the grounded electrode at ambient temperature. Silicon wafers and transparent conducting tin oxide coated glass with a sheet resistance of 11 X/sq purchased from a Libby–Owen–Ford (LOF) were used as substrates. Polished silicon served as an ideal substrate for film thickness and refractive index measurements using spectroscopic ellipsometry, while samples deposited simultaneously on LOF glass was used to evaluate electrochromic performance. Spectroscopic ellipsometry was used to measure the thickness of the films using a J.A. Woollam ellipsometer and the WVASE32 software. Scans were taken over the range of 400–1200 nm at 70 angle of incidence. The wavelength-dependent optical constants were fitted using a Cauchy model. A minimum of five measurements were made
on each sample, and the error bars in the deposition rate plots reflect the variation in film uniformity. In addition, the thickness values from ellipsometry were independently verified by profilometry measurements. Profilometry also confirmed that the deposition rates on silicon and LOF substrates were identical. The amorphous nature of the films was confirmed by X-ray diffraction using Cu Ka radiation. All film characterizations were performed at ambient conditions. Optical transmission was assessed using a Cary 5G UV–VIS–NIR spectrometer. The transmission spectra were taken over a wavelength range of 250–2500 nm. The absorption coefficient (a) as a function of wavelength was obtained from the transmission spectra. Optical band gaps were determined from linear fits to plots of (ae)1/2 vs. e. The linear relationship of (ae)1/2 vs. e is valid for indirect transitions, and has been successfully applied to vanadium oxide [6]. Lithium ions and electrons are simultaneously injected into film to form the vanadium bronze LixV2O5, a process termed intercalation, through the application of a series of potential steps. Transmission measurements were performed at several charge intercalation steps to yield the optical band gap as a function of ion insertion. Electrochemical performance was characterized using a Gamry model PC4 Potentiostat. Films deposited on LOF substrates were suspended in an electrolyte solution of 1 M lithium perchlorate in propylene carbonate. The LOF coated substrates served as the working electrode in a conventional three-electrode cell configuration. A graphite rod and saturated calomel electrode were suspended in close proximity to the working electrode and served as the counter and reference electrode, respectively. Both potentiostatic and galvanostatic intermittent titration techniques (PITT & GITT) were used to determine the diffusion coefficient as a function of the degree of intercalation. In general, each technique involves the displacement of lithium ions from a stable intercalation extent to a new equilibrium value through the imposition of a step change in either potential (PITT) or a short current pulse (GITT) [19]. In either case, measurement of the dependent variable as a function of time is used to obtain the diffusion coefficient. Derivation of both approaches from Ficks second law in conjunction with appropriate boundary conditions is given by Wen et al. [19]. The necessary relationships for PITT and GITT used in this work are given below. Measurement of diffusion coefficients by PITT is performed by applying a series of small voltage steps. After each step, the resulting current-time plot is integrated to determine the increase in charge insertion (Q). By evaluating the diffusion coefficient after each small change in potential one obtains DLi+ as a function of charge insertion, assuming of course that DLi+ is constant over each small step. In PITT, the transient current response
M. Seman et al. / Journal of Non-Crystalline Solids 351 (2005) 1987–1994
1989
the current response to a potential step is plotted versus time. Eq. (2) is valid at long times and this is confirmed by the linear fit to the data. Since film thickness is known, the diffusion coefficient is extracted directly from the slope of this line. GITT utilizes short constant current pulses to obtain the functionality of DLi+ with intercalation extent. In contrast to PITT, GITT measures the resulting transient changes in potential required to maintain constant current. At short times it is observed that the applied voltage, E, varies with time to the one half power as described by [19] dE 2IV M dE pffi ¼ pffiffiffiffiffiffiffi ð3Þ ðif t L2 =DÞ; d t zFS Dp dx
to an applied potential step at long times is given by [19]: 2 2QD p Dt IðtÞ ¼ 2 exp 2 ð2Þ ðfor t L2 =DÞ; L 4L where Q is the incremental intercalated charge as result of the step change in potential, D is the diffusion coefficient, and L is the film thickness. Eq. (2) represents the truncated solution to Ficks second law for the currenttime behavior in the PITT approach. To adequately model short time current-time behavior one would require the complete analytical series solution to Ficks second law [19]. However, at short times the current response includes a voltage drop across the electrolyte, a phenomenon often referred to as the IR drop. In addition, there is a small transient component of the current associated with the accumulation of charge at the electrolyte-electrode interface. These components fade on time scales much less than those of ion diffusion in the film, and thus the data is only analyzed at long times when the natural logarithm of the current vs. time behavior is found to be linear. Fig. 1(a) provides and example of the procedure used to evaluate diffusion coefficients using PITT. In this plot the natural log of
where I is the value of the current pulse, VM is the molar volume, z is the charge number, F is Faradays constant, S is the surface area, and x is stoichiometric molar ratio of lithium to vanadium oxide also referred to as the intercalation extent. Rearrangement for D yields the following expression used in the GITT approach [19]: 2 2 4 IV M dE dE pffi D¼ ðif t L2 =DÞ. ð4Þ p zFS dx d t
0.6
-7
b
E [V vs. SCE]
ln (I) [ln (Amps)]
a -7.5 -8 -8.5
0.5 New VOC
IR Drop
0.4 Constant Current Pulse
-9
Relaxation
0.3 -9.5
2
0 0
20
40
60
80
100
4
6
8
10
120
t1/2 [Seconds]1/2
Time [Seconds] 0.8
c
Eeq[V vs. SCE]
0.4 0
-0.4 -0.8 -1.2 0
0.5
1
1.5
2
x in LixV2O5 Fig. 1. (a) A typical plot of transient current response obtained from a PITT experiment; (b) a typical plot of transient voltage response obtained from a GITT experiment; and (c) the coulometric titration curve.
M. Seman et al. / Journal of Non-Crystalline Solids 351 (2005) 1987–1994
pffi In the above expression the value of dE=d t is directly obtained from measurement of voltage as a function of time during the constant current pulse, while dE/ dx is determined from the slope of the coulometric titration curve. The coulometric titration curve is obtained by plotting the steady-state equilibrium voltage of the working electrode vs. intercalation extent, x, after each galvanostatic titration step. An example of the voltage-time behavior from a typical GITT experiment is shown in Fig. 1(b). The slope of the line drawn through the data in the linear portion of the constant current pulse was used for the evaluation of the diffusion coefficient by Eq. (4). Eq. (4) remains valid as long as the duration of the current pulse is on a time scale less than the time scale of diffusion. Current pulses of our GITT experiments lasted 15 s. For typical diffusion coefficients obtained in this work the corresponding diffusion time scale was 45 s. At the end of the constant current pulse the potential relaxes to the open circuit voltage (VOC) value that is in equilibrium with the new degree of intercalation. As shown in Fig. 1(b), there is an almost instant jump in the voltage. This feature is termed the IR drop, and it is associated with potential drop across the electrolyte between the working and counter electrodes. After the IR drop, the potential slowly relaxes to the new VOC as the injected ions become evenly distributed through the film. The second term required to extract the diffusion coefficient using GITT is the value of dE/dx. Intercalation of ions into the vanadium oxide film results in a change of the open circuit voltage of the electrochemical cell. After the constant current pulse the film relaxes to a new steady-state open circuit voltage as shown in Fig. 1(b). The intercalation extent is determined by measuring the current during each pulse. The repetition of the galvanostatic steps and evaluation of the new open circuit voltage yields the coulometric titration curve, also referred to as electromotive force (emf). The coulometric titration curve derived from the repetition of current pulses is shown in Fig. 1(c). The value of dE/dx as a function of intercalation was obtained from the derivative of the curve shown in Fig. 1(c). Analysis of the data depicted in Fig. 1(b) and (c) was used with Eq. (4) to extract diffusion coefficient using GITT. There are advantages and limitations to both techniques. An advantage to PITT is that the voltage range is strictly controlled, thus any side reactions may be avoided if voltage ranges are set within proper limits. However, there exists a time dependent voltage drop across the electrolyte as well a small transient current related to the accumulation of charge at the electrolyteelectrode interface. An advantage to GITT is that the effects of this IR drop do not impact its analysis. A comparison between results obtained from both techniques is presented below.
3. Results 3.1. Deposition rate Figs. 2–5 describe the deposition rate dependence on plasma processing conditions. The parameters examined include the VOCl3 flowrate, H2:VOCl3 ratio, O2:VOCl3 ratio, rf power, and pressure. In all experiments these reactants were diluted with argon, which composed 50–70% of the total mixture. The argon fraction was varied within this range to maintain a constant total flowrate for each set of experiments. Additional experiments showed that the results were insensitive to the fraction of argon over this range. The base case parameters and ranges examined are summarized in Table 1. The ranges were selected to examine the plasma process to the broadest extent that was permitted by our equipment. Each parameter was varied individually while keeping all of the other values fixed at the base case condition unless otherwise noted. Fig. 2 shows the growth rate as a function of VOCl3 flowrate. The rate appears to be simply first order in VOCl3. Fig. 3 describes the dependence on both the H2:VOCl3 (squares) and O2:VOCl3 (circles) ratios. The rate is essentially independent of either parameter. Fig. 4 shows that the rate scales linearly with rf power. For a VOCl3 flowrate of 3.5 sccm a maximum deposition rate of >180 nm/min was obtained at 250 W. Fig. 5 shows the influence of operating pressure at two power levels. In both cases the rate decreases linearly with pressure, however the falloff was much steeper at higher powers.
90 80
Deposition Rate (nm/min)
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70 60 50 40 30
Power = 25 W Pressure = 200 mtorr
20
O2:VOCl 3 Ratio = 2 H2:VOCl3 Ratio = 0
10 0 0
0.5
1
1.5
2
2.5
3
3.5
4
VOCl3 Flowrate (sccm) Fig. 2. Deposition rate as a function of VOCl3 flowrate. Trendline used to guide the eye.
M. Seman et al. / Journal of Non-Crystalline Solids 351 (2005) 1987–1994
1991
70
60
Power = 25 W Power = 125 W
60
50
Deposition Rate (nm/min)
Deposition Rate (nm/min)
VOCl3 Flow = 1.25 sccm
40
30 O2:VOCl3 = 5 H2:VOCl3 = 5
20
Power = 25 W
O2:VOCl3 = 2
50
H2:VOCl3 = 0 Pressure = 200 mtorr
40
30
20
Pressure = 200mtorr 10
VOCl3 Flow = 2 sccm
10
0
0
0
2
4
6
8
10
100
200
H2:VOCl3 or O2:VOCl3 Ratios Fig. 3. Deposition rate as a function of the H2:VOCl3 ratio (squares, O2:VOCl3 = 5) and the O2:VOCl3 ratio (circles, H2:VOCl3 = 5). Trendline used to guide the eye.
200
400
500
600
Fig. 5. Deposition rate as a function of pressure for rf power = 25 W (squares) and rf power = 125 W (circles). Trendline used to guide the eye.
Table 1 Summary of plasma processing conditions and the parameter range explored
180 160
Deposition Rate (nm/min)
300
Pressure (mtorr)
140 120 100
Variable
Base case
Range
VOCl3 flowrate (sccm) O2:VOCl3 ratio H2:VOCl3 ratio rf Power (W) Pressure (mTorr)
2 2 0 25 200
1.5–3.5 1–7 0–10 25–250 125–600
80 VOCl3 Flow = 3.5 sccm
60
O2:VOCl3 Ratio = 2
40
100
H2:VOCl3 Ratio = 0
20
Pressure = 200 mtorr
0 50
100
150
200
75
250
rf Power (W) Fig. 4. Deposition rate as a function of rf power. Trendline used to guide the eye.
3.2. Optical and electrochemical performance The optical transparency spectra obtained from a typical vanadium oxide film of 150 nm in thickness is shown in Fig. 6. The data shown in Fig. 6 is transmission through only the vanadium oxide film, since contributions of the TCO coated substrate were subtracted out using a background scan. The oscillations in the curves are due to interference effects that depend on the film thickness and the refractive index. Optical band
%Transmission
0
50
25
as dep x=0.67 x=1.00 x=1.35
0 300
900
1500
Wavelength [nm] Fig. 6. Transmission spectra of a 150 nm thick V2O5Lix film at selected levels of intercalation (x).
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Fig. 8 summarizes the diffusion coefficients for Li+ in PECVD vanadium oxide obtained using both PITT and GITT techniques. A trend line has been drawn through the GITT data for clarity. Both techniques are in excellent agreement with each other. Typical of amorphous films, the diffusion coefficient is seen to be relatively insensitive of degree intercalation. The values decrease slightly with the intercalation extent. The maximum value of 1 · 1011 cm2/s in the as-deposited film is decreased by a factor of five to 2 · 1012 cm2/s at x = 1.76.
3.20 3.00
2.60 800
2.40 (αε αε)1/2 [cm eV]1/2
2.20
-1
Bandgap [ev]
2.80
2.00 1.80
0 2.5
3
ε [eV]
3.5
4. Discussion
4
1.60 0
0.5
1
1.5
2
2.5
4.1. Deposition rate
x in LixV2O5
Fig. 7. Optical band gap as a function of degree of intercalation. Inset: Example of a typical (ae)1/2 vs. e plot used to extract band gaps.
gaps were obtained from linear fits to plots of (ae)1/2 vs. e, the validation for indirect transitions [6]. Fig. 7 quantifies the change in optical band gap with increasing intercalation extent. An example of a typical linear fit to a plot of (ae)1/2 vs. e used to obtain the optical band gap is shown in the inset. The as-deposited film depicted in Fig. 6 displayed a slight yellow hue, and had an optical band gap of 2.13 eV. With increasing intercalation the observed yellow hue fades to a neutral grey, as the absorption tail progresses to lower wavelengths. The uptake in lithium was observed to increase the optical band gap by 1 eV, saturating at 3.13 eV at an intercalation extent of x = 1.75.
1.2 GITT
0.8
D x 10
-11
2
[cm /sec]
PITT
0.4
0 0
0.5
1
1.5
2
x in LixV2O5
Fig. 8. DLi+ as a function of intercalation obtained from both PITT (triangles) and GITT (circles) methods. Trendline used to guide the eye.
The maximum deposition rates achieved in this work are the highest that have been reported for vanadium oxide. They are approximately 3 times greater than a previous PECVD report [15], and an order of magnitude greater than conventional PVD processes [16,20]. Interestingly, the rate was observed to be independent of both the H2:VOCl3 and O2:VOCl3 ratios. As shown in Fig. 3 the rate remained constant at 50 nm/min in both cases. This is in contrast to the results of Zhang et al. [15] who observed that the rate increased with hydrogen addition up to H2:VOCl3 = 8.75. In our work it was observed that high quality films were deposited with no hydrogen at all, indicating that chlorine scavenging chemistry as described in Eq. (1) is not essential to this process. Films were also grown with O2:VOCl3 ratios as low as unity. We note that the plasma chemistry of this system much different than with tungsten oxide PECVD using WF6. In that system it has been clearly demonstrated by a number of groups [11,21,22] that hydrogen is required to scavenge atomic fluorine. The chemistry in these systems is quite different. Atomic chlorine is not as reactive as atomic fluorine. Moreover, while WF6 is a stable gas, the vanadium chlorides do not form volatile etch products. Although the plasma geometries are similar, there are a couple of significant differences in experimental conditions that may be responsible for the differences observed in this work compared to Zhang et al. [15]. First, their experiments employed a very large argon flowrate (500 sccm) to deliver VOCl3 from a bubbler. In our case the VOCl3 was delivered through a mass flow controller, and typical argon flowrates were just 20 sccm. It well known that argon can influence the plasma dynamics, and the higher total flowrates would also reduce the residence time in the reaction zone. Second, the Zhang et al. experiments were conducted at a pressure of 600 mTorr, while our base case was 200 mTorr. Plasma chemistry depends exponentially on the electron temperature, which decreases with pressure
M. Seman et al. / Journal of Non-Crystalline Solids 351 (2005) 1987–1994
due to the increased collision frequency. As shown in Fig. 5, the rate decreased substantially with pressure. The process may be more sensitive to the H2:VOCl3 ratio at these lower deposition rates. The results presented in Figs. 2–5 allow us to conjecture on the mechanism of vanadium oxide growth in a PECVD environment without hydrogen. The important plasma phase chemistry is illustrated in Eq. (5) þe
þO
VOCl3 ! liberates V and=or VO ! VO2ðgÞ
ð5Þ
As shown in Figs. 2 and 4, the rate is first order in both VOCl3 and rf power. It is hypothesized that the first step is electron impact dissociation of VOCl3, which liberates either elemental vanadium or the partial oxide VO. It is suggested that these two species would be the active precursors for deposition. The plasma also creates copious amounts of atomic oxygen. The atomic oxygen is expected to oxidize the precursors to form VO2. The free energy and reactivity of this higher oxide would be much lower, and as such they would not be expected to contribute significantly to film growth. The decrease in rate with pressure is attributed to a combination of factors. First, the electron temperature decreases with pressure, lowering the dissociation of VOCl3. Second, the oxidation process to form inactive oxides (VOx, x > 1) is controlled by neutral chemistry that scales as pressure squared.
1993
of magnitude. Likewise, there was little variation in the optical transparency of films as a function of plasma parameters. The GITT and PITT evaluation techniques compared quite favorably, agreeing within a factor of two over the range of intercalation. In the literature GITT is generally viewed to be the superior technique, and our results support that idea. The GITT results show much less scatter and noise than PITT. In PITT the value of the diffusion coefficient is highly sensitive to the slope obtained from semi-log plots such as that shown in Fig. 1(a). This accounts for the scatter that is shown in Fig. 8. In contrast, GITT is not as sensitive to the analysis procedure. GITT avoids issues associated with the IR drop and the formation of charging at the interface. The high transparency and efficient ion transport of this material shows promise for incorporation into electrochromic devices as the ion storage layer. Vanadium oxide also has a much higher ion storage capacity than tungsten oxide, where a fully intercalated film has a composition of LixWO3, with x 0.3. As such, relatively thin layers of vanadium oxide may be coupled with tungsten oxide films in electrochromic devices. Advantages of reducing the film thickness include enhanced transparency and shorter ion transport times. Currently we are fabricating solid state devices using PECVD tungsten oxide, PECVD vanadium oxide, and an organic electrolyte.
4.2. Optical and electrochemical performance To the best of our knowledge this is the first report of DLi+ for PECVD vanadium oxide. Overall, our reported values of DLi+ are among the highest reported for amorphous vanadium oxide in the literature. Julien et al. [23] reported values of 1012 cm2/s for flash evaporated films for Li concentrations of 0 < x < 3. While, McGraw et al. [24] reported diffusion coefficients of amorphous V2O5 grown by pulsed laser deposition that were on the order of 1013 cm2/s over the range of 0 < x < 1.5. It is evident from the literature that the film preparation technique strongly dictates the observed value of DLi+ in films of vanadium oxide. In addition, it is evident from this work that PECVD of vanadium oxide produces thin films with DLi+ values greater than those deposited by other forms of physical vapor deposition. In our previous work with tungsten oxide PECVD we found that electrochromic performance was highly sensitive to plasma conditions [11]. It was observed that small changes in plasma parameters, in particular rf power, could alter the diffusion coefficient and storage capacity by orders of magnitude. Relatively speaking, the electrochromic performance of vanadium oxide was much more robust with respect to plasma processing conditions. Although small variations in diffusion coefficients were observed, they were all the same order
5. Conclusions A comprehensive investigation of plasma processing on the deposition rate of electrochromic vanadium oxide was performed. The effects of precursor flowrate, gas composition, pressure, and power were examined. Rates in excess of 180 nm/min were achieved, an order of magnitude greater than conventional PVD technology. It was also observed that hydrogen had little impact on deposition, and that films could be deposited without H2. As-deposited films had an optical band gap of 2.13 eV, and it was observed to increase to as high as 3.13 eV with lithium intercalation. Lithium diffusion coefficients ranged between 1 · 1011 and 2 · 1012 cm2/s. These values are superior to those achieved by conventional PVD techniques in the literature. These films show great promise for application in electrochromic devices.
Acknowledgement The authors acknowledge financial support from the National Science Foundation through CAREER Award No. CTS-0093611.
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An investigation of the role of plasma conditions on the deposition rate of electrochromic vanadium oxide thin films Michael Seman, Joey Marino, Wenli Yang, Colin A. Wolden
*
Chemical Engineering Department, Colorado School of Mines, 1500 Illinois Street, Golden, CO 80401, USA Received 28 August 2004; received in revised form 6 May 2005 Available online 20 June 2005
Abstract Plasma-enhanced chemical vapor deposition (PECVD) has been used to form amorphous vanadium oxide thin films from mixtures of VOCl3, O2, and H2. The deposition rate was examined as a function of PECVD operating conditions. Growth rates were found to be first order in VOCl3, and independent of both O2 and H2. High quality vanadium oxide films were also deposited without the use of hydrogen. Rates were observed to increase with rf power, and decrease with operating pressure. Maximum rates were an order of magnitude greater than typically observed with physical vapor deposition techniques. Optical transmission and electrochemical analysis were used to quantify the electrochromic response. After initial cycling films demonstrated high transparency across the visible, and the optical band gap increased with lithium intercalation. Lithium ion diffusion coefficients approached 1011 cm2/s, approximately an order of magnitude higher than literature values. 2005 Elsevier B.V. All rights reserved. PACS: 81.15.Gh; 78.20.J; 66.30.D; 78.66
1. Introduction Electrochromic materials are being pursued for numerous applications including sensors [1,2], smart windows [3], and display technology [4]. Solid-state electrochromic devices are composed of a stack of thin metal oxides whose optical transmission may be reversibly altered between clear and opaque in response to an applied voltage. The stack consists of an ion storage layer, an electrolyte, and an electrochromic layer sandwiched between two transparent conducting oxide (TCO) electrodes [4]. Vanadium oxide has been identified as a leading candidate to serve as the ion storage layer [5,6]. Physical vapor deposition (PVD) such as sputtering [7] and evaporation [8] are the dominating techniques employed for thin film synthesis. Plasma-
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Corresponding author. Tel.: +1 303 273 3544; fax: +1 303 273 3730. E-mail address: [email protected] (C.A. Wolden).
0022-3093/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.05.016
enhanced chemical vapor deposition (PECVD) offers an attractive alternative due to its high deposition rates and low temperature capability. PECVD offers advantages for large area uniformity, and it is not susceptible to issues associated with target aging that can limit PVD techniques [2]. Our group is pursuing the complete synthesis of a solid-state electrochromic device by PECVD. To date we have produced high performance TCO electrodes [9,10] and electrochromic tungsten oxide [11,12] by PECVD. In this paper we take the next step, and describe PECVD synthesis of vanadium oxide for use as the ion storage layer. There has been limited use of PECVD for V2O5 synthesis. One group used oxygen and a complex organometallic precursor, VO(hfa)2 Æ H2O (hfa = 1,1,1,5,5,5hexafluoro-2,4 pentanedione), focusing on the dielectric properties of amorphous and nanocrystalline films [13,14]. Zhang et al. originated vanadium oxide PECVD using the three reactants VOCl3, O2, and H2, and examined the resulting films for application as cathodes in
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rechargeable lithium batteries [15]. They suggested that the reaction chemistry follows the following global pathway 4VOCl3 + 6H2 + 3O2 ! 2V2 O5 + 12HCl
ð1Þ
These authors reported that the addition of hydrogen accelerates the reaction by removing free chlorine. Optimized deposition rates were on the order of 60–70 nm/ min, which is a factor of five greater what is typically achieved using PVD techniques [16]. These films had a high storage capacity and demonstrated long-term stability. For electrochromic applications optical transparency is an important concern [6,17]. We are not aware of any reports of transmission characteristics of PECVDdeposited V2O5. Similarly, there are no reported investigations of ion transport in PECVD material, which can be important in determining device switching times. In this paper we present a comprehensive examination of PECVD deposition kinetics. We demonstrate rates that are an order of magnitude greater than conventional PVD processes. Fundamental optical and electrochemical characterization demonstrate that this material is highly suitable for application in electrochromic windows.
2. Experimental procedure and analysis Deposition of amorphous vanadium oxide was performed in a parallel plate, capacitively-coupled plasma chamber. Power was supplied by a 300 W rf power supply operating at 13.56 MHz and coupled using a match network. Further details of this custom built chamber are provided in the literature [18]. Vanadium oxide synthesis was accomplished using mixtures of vanadium oxytrichloride (VOCl3), oxygen, hydrogen, and argon. All flowrates were set using electronic mass flow controllers. Gases were introduced uniformly through a perforated showerhead that also served as the powered electrode. Films were deposited on the grounded electrode at ambient temperature. Silicon wafers and transparent conducting tin oxide coated glass with a sheet resistance of 11 X/sq purchased from a Libby–Owen–Ford (LOF) were used as substrates. Polished silicon served as an ideal substrate for film thickness and refractive index measurements using spectroscopic ellipsometry, while samples deposited simultaneously on LOF glass was used to evaluate electrochromic performance. Spectroscopic ellipsometry was used to measure the thickness of the films using a J.A. Woollam ellipsometer and the WVASE32 software. Scans were taken over the range of 400–1200 nm at 70 angle of incidence. The wavelength-dependent optical constants were fitted using a Cauchy model. A minimum of five measurements were made
on each sample, and the error bars in the deposition rate plots reflect the variation in film uniformity. In addition, the thickness values from ellipsometry were independently verified by profilometry measurements. Profilometry also confirmed that the deposition rates on silicon and LOF substrates were identical. The amorphous nature of the films was confirmed by X-ray diffraction using Cu Ka radiation. All film characterizations were performed at ambient conditions. Optical transmission was assessed using a Cary 5G UV–VIS–NIR spectrometer. The transmission spectra were taken over a wavelength range of 250–2500 nm. The absorption coefficient (a) as a function of wavelength was obtained from the transmission spectra. Optical band gaps were determined from linear fits to plots of (ae)1/2 vs. e. The linear relationship of (ae)1/2 vs. e is valid for indirect transitions, and has been successfully applied to vanadium oxide [6]. Lithium ions and electrons are simultaneously injected into film to form the vanadium bronze LixV2O5, a process termed intercalation, through the application of a series of potential steps. Transmission measurements were performed at several charge intercalation steps to yield the optical band gap as a function of ion insertion. Electrochemical performance was characterized using a Gamry model PC4 Potentiostat. Films deposited on LOF substrates were suspended in an electrolyte solution of 1 M lithium perchlorate in propylene carbonate. The LOF coated substrates served as the working electrode in a conventional three-electrode cell configuration. A graphite rod and saturated calomel electrode were suspended in close proximity to the working electrode and served as the counter and reference electrode, respectively. Both potentiostatic and galvanostatic intermittent titration techniques (PITT & GITT) were used to determine the diffusion coefficient as a function of the degree of intercalation. In general, each technique involves the displacement of lithium ions from a stable intercalation extent to a new equilibrium value through the imposition of a step change in either potential (PITT) or a short current pulse (GITT) [19]. In either case, measurement of the dependent variable as a function of time is used to obtain the diffusion coefficient. Derivation of both approaches from Ficks second law in conjunction with appropriate boundary conditions is given by Wen et al. [19]. The necessary relationships for PITT and GITT used in this work are given below. Measurement of diffusion coefficients by PITT is performed by applying a series of small voltage steps. After each step, the resulting current-time plot is integrated to determine the increase in charge insertion (Q). By evaluating the diffusion coefficient after each small change in potential one obtains DLi+ as a function of charge insertion, assuming of course that DLi+ is constant over each small step. In PITT, the transient current response
M. Seman et al. / Journal of Non-Crystalline Solids 351 (2005) 1987–1994
1989
the current response to a potential step is plotted versus time. Eq. (2) is valid at long times and this is confirmed by the linear fit to the data. Since film thickness is known, the diffusion coefficient is extracted directly from the slope of this line. GITT utilizes short constant current pulses to obtain the functionality of DLi+ with intercalation extent. In contrast to PITT, GITT measures the resulting transient changes in potential required to maintain constant current. At short times it is observed that the applied voltage, E, varies with time to the one half power as described by [19] dE 2IV M dE pffi ¼ pffiffiffiffiffiffiffi ð3Þ ðif t L2 =DÞ; d t zFS Dp dx
to an applied potential step at long times is given by [19]: 2 2QD p Dt IðtÞ ¼ 2 exp 2 ð2Þ ðfor t L2 =DÞ; L 4L where Q is the incremental intercalated charge as result of the step change in potential, D is the diffusion coefficient, and L is the film thickness. Eq. (2) represents the truncated solution to Ficks second law for the currenttime behavior in the PITT approach. To adequately model short time current-time behavior one would require the complete analytical series solution to Ficks second law [19]. However, at short times the current response includes a voltage drop across the electrolyte, a phenomenon often referred to as the IR drop. In addition, there is a small transient component of the current associated with the accumulation of charge at the electrolyte-electrode interface. These components fade on time scales much less than those of ion diffusion in the film, and thus the data is only analyzed at long times when the natural logarithm of the current vs. time behavior is found to be linear. Fig. 1(a) provides and example of the procedure used to evaluate diffusion coefficients using PITT. In this plot the natural log of
where I is the value of the current pulse, VM is the molar volume, z is the charge number, F is Faradays constant, S is the surface area, and x is stoichiometric molar ratio of lithium to vanadium oxide also referred to as the intercalation extent. Rearrangement for D yields the following expression used in the GITT approach [19]: 2 2 4 IV M dE dE pffi D¼ ðif t L2 =DÞ. ð4Þ p zFS dx d t
0.6
-7
b
E [V vs. SCE]
ln (I) [ln (Amps)]
a -7.5 -8 -8.5
0.5 New VOC
IR Drop
0.4 Constant Current Pulse
-9
Relaxation
0.3 -9.5
2
0 0
20
40
60
80
100
4
6
8
10
120
t1/2 [Seconds]1/2
Time [Seconds] 0.8
c
Eeq[V vs. SCE]
0.4 0
-0.4 -0.8 -1.2 0
0.5
1
1.5
2
x in LixV2O5 Fig. 1. (a) A typical plot of transient current response obtained from a PITT experiment; (b) a typical plot of transient voltage response obtained from a GITT experiment; and (c) the coulometric titration curve.
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pffi In the above expression the value of dE=d t is directly obtained from measurement of voltage as a function of time during the constant current pulse, while dE/ dx is determined from the slope of the coulometric titration curve. The coulometric titration curve is obtained by plotting the steady-state equilibrium voltage of the working electrode vs. intercalation extent, x, after each galvanostatic titration step. An example of the voltage-time behavior from a typical GITT experiment is shown in Fig. 1(b). The slope of the line drawn through the data in the linear portion of the constant current pulse was used for the evaluation of the diffusion coefficient by Eq. (4). Eq. (4) remains valid as long as the duration of the current pulse is on a time scale less than the time scale of diffusion. Current pulses of our GITT experiments lasted 15 s. For typical diffusion coefficients obtained in this work the corresponding diffusion time scale was 45 s. At the end of the constant current pulse the potential relaxes to the open circuit voltage (VOC) value that is in equilibrium with the new degree of intercalation. As shown in Fig. 1(b), there is an almost instant jump in the voltage. This feature is termed the IR drop, and it is associated with potential drop across the electrolyte between the working and counter electrodes. After the IR drop, the potential slowly relaxes to the new VOC as the injected ions become evenly distributed through the film. The second term required to extract the diffusion coefficient using GITT is the value of dE/dx. Intercalation of ions into the vanadium oxide film results in a change of the open circuit voltage of the electrochemical cell. After the constant current pulse the film relaxes to a new steady-state open circuit voltage as shown in Fig. 1(b). The intercalation extent is determined by measuring the current during each pulse. The repetition of the galvanostatic steps and evaluation of the new open circuit voltage yields the coulometric titration curve, also referred to as electromotive force (emf). The coulometric titration curve derived from the repetition of current pulses is shown in Fig. 1(c). The value of dE/dx as a function of intercalation was obtained from the derivative of the curve shown in Fig. 1(c). Analysis of the data depicted in Fig. 1(b) and (c) was used with Eq. (4) to extract diffusion coefficient using GITT. There are advantages and limitations to both techniques. An advantage to PITT is that the voltage range is strictly controlled, thus any side reactions may be avoided if voltage ranges are set within proper limits. However, there exists a time dependent voltage drop across the electrolyte as well a small transient current related to the accumulation of charge at the electrolyteelectrode interface. An advantage to GITT is that the effects of this IR drop do not impact its analysis. A comparison between results obtained from both techniques is presented below.
3. Results 3.1. Deposition rate Figs. 2–5 describe the deposition rate dependence on plasma processing conditions. The parameters examined include the VOCl3 flowrate, H2:VOCl3 ratio, O2:VOCl3 ratio, rf power, and pressure. In all experiments these reactants were diluted with argon, which composed 50–70% of the total mixture. The argon fraction was varied within this range to maintain a constant total flowrate for each set of experiments. Additional experiments showed that the results were insensitive to the fraction of argon over this range. The base case parameters and ranges examined are summarized in Table 1. The ranges were selected to examine the plasma process to the broadest extent that was permitted by our equipment. Each parameter was varied individually while keeping all of the other values fixed at the base case condition unless otherwise noted. Fig. 2 shows the growth rate as a function of VOCl3 flowrate. The rate appears to be simply first order in VOCl3. Fig. 3 describes the dependence on both the H2:VOCl3 (squares) and O2:VOCl3 (circles) ratios. The rate is essentially independent of either parameter. Fig. 4 shows that the rate scales linearly with rf power. For a VOCl3 flowrate of 3.5 sccm a maximum deposition rate of >180 nm/min was obtained at 250 W. Fig. 5 shows the influence of operating pressure at two power levels. In both cases the rate decreases linearly with pressure, however the falloff was much steeper at higher powers.
90 80
Deposition Rate (nm/min)
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70 60 50 40 30
Power = 25 W Pressure = 200 mtorr
20
O2:VOCl 3 Ratio = 2 H2:VOCl3 Ratio = 0
10 0 0
0.5
1
1.5
2
2.5
3
3.5
4
VOCl3 Flowrate (sccm) Fig. 2. Deposition rate as a function of VOCl3 flowrate. Trendline used to guide the eye.
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1991
70
60
Power = 25 W Power = 125 W
60
50
Deposition Rate (nm/min)
Deposition Rate (nm/min)
VOCl3 Flow = 1.25 sccm
40
30 O2:VOCl3 = 5 H2:VOCl3 = 5
20
Power = 25 W
O2:VOCl3 = 2
50
H2:VOCl3 = 0 Pressure = 200 mtorr
40
30
20
Pressure = 200mtorr 10
VOCl3 Flow = 2 sccm
10
0
0
0
2
4
6
8
10
100
200
H2:VOCl3 or O2:VOCl3 Ratios Fig. 3. Deposition rate as a function of the H2:VOCl3 ratio (squares, O2:VOCl3 = 5) and the O2:VOCl3 ratio (circles, H2:VOCl3 = 5). Trendline used to guide the eye.
200
400
500
600
Fig. 5. Deposition rate as a function of pressure for rf power = 25 W (squares) and rf power = 125 W (circles). Trendline used to guide the eye.
Table 1 Summary of plasma processing conditions and the parameter range explored
180 160
Deposition Rate (nm/min)
300
Pressure (mtorr)
140 120 100
Variable
Base case
Range
VOCl3 flowrate (sccm) O2:VOCl3 ratio H2:VOCl3 ratio rf Power (W) Pressure (mTorr)
2 2 0 25 200
1.5–3.5 1–7 0–10 25–250 125–600
80 VOCl3 Flow = 3.5 sccm
60
O2:VOCl3 Ratio = 2
40
100
H2:VOCl3 Ratio = 0
20
Pressure = 200 mtorr
0 50
100
150
200
75
250
rf Power (W) Fig. 4. Deposition rate as a function of rf power. Trendline used to guide the eye.
3.2. Optical and electrochemical performance The optical transparency spectra obtained from a typical vanadium oxide film of 150 nm in thickness is shown in Fig. 6. The data shown in Fig. 6 is transmission through only the vanadium oxide film, since contributions of the TCO coated substrate were subtracted out using a background scan. The oscillations in the curves are due to interference effects that depend on the film thickness and the refractive index. Optical band
%Transmission
0
50
25
as dep x=0.67 x=1.00 x=1.35
0 300
900
1500
Wavelength [nm] Fig. 6. Transmission spectra of a 150 nm thick V2O5Lix film at selected levels of intercalation (x).
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Fig. 8 summarizes the diffusion coefficients for Li+ in PECVD vanadium oxide obtained using both PITT and GITT techniques. A trend line has been drawn through the GITT data for clarity. Both techniques are in excellent agreement with each other. Typical of amorphous films, the diffusion coefficient is seen to be relatively insensitive of degree intercalation. The values decrease slightly with the intercalation extent. The maximum value of 1 · 1011 cm2/s in the as-deposited film is decreased by a factor of five to 2 · 1012 cm2/s at x = 1.76.
3.20 3.00
2.60 800
2.40 (αε αε)1/2 [cm eV]1/2
2.20
-1
Bandgap [ev]
2.80
2.00 1.80
0 2.5
3
ε [eV]
3.5
4. Discussion
4
1.60 0
0.5
1
1.5
2
2.5
4.1. Deposition rate
x in LixV2O5
Fig. 7. Optical band gap as a function of degree of intercalation. Inset: Example of a typical (ae)1/2 vs. e plot used to extract band gaps.
gaps were obtained from linear fits to plots of (ae)1/2 vs. e, the validation for indirect transitions [6]. Fig. 7 quantifies the change in optical band gap with increasing intercalation extent. An example of a typical linear fit to a plot of (ae)1/2 vs. e used to obtain the optical band gap is shown in the inset. The as-deposited film depicted in Fig. 6 displayed a slight yellow hue, and had an optical band gap of 2.13 eV. With increasing intercalation the observed yellow hue fades to a neutral grey, as the absorption tail progresses to lower wavelengths. The uptake in lithium was observed to increase the optical band gap by 1 eV, saturating at 3.13 eV at an intercalation extent of x = 1.75.
1.2 GITT
0.8
D x 10
-11
2
[cm /sec]
PITT
0.4
0 0
0.5
1
1.5
2
x in LixV2O5
Fig. 8. DLi+ as a function of intercalation obtained from both PITT (triangles) and GITT (circles) methods. Trendline used to guide the eye.
The maximum deposition rates achieved in this work are the highest that have been reported for vanadium oxide. They are approximately 3 times greater than a previous PECVD report [15], and an order of magnitude greater than conventional PVD processes [16,20]. Interestingly, the rate was observed to be independent of both the H2:VOCl3 and O2:VOCl3 ratios. As shown in Fig. 3 the rate remained constant at 50 nm/min in both cases. This is in contrast to the results of Zhang et al. [15] who observed that the rate increased with hydrogen addition up to H2:VOCl3 = 8.75. In our work it was observed that high quality films were deposited with no hydrogen at all, indicating that chlorine scavenging chemistry as described in Eq. (1) is not essential to this process. Films were also grown with O2:VOCl3 ratios as low as unity. We note that the plasma chemistry of this system much different than with tungsten oxide PECVD using WF6. In that system it has been clearly demonstrated by a number of groups [11,21,22] that hydrogen is required to scavenge atomic fluorine. The chemistry in these systems is quite different. Atomic chlorine is not as reactive as atomic fluorine. Moreover, while WF6 is a stable gas, the vanadium chlorides do not form volatile etch products. Although the plasma geometries are similar, there are a couple of significant differences in experimental conditions that may be responsible for the differences observed in this work compared to Zhang et al. [15]. First, their experiments employed a very large argon flowrate (500 sccm) to deliver VOCl3 from a bubbler. In our case the VOCl3 was delivered through a mass flow controller, and typical argon flowrates were just 20 sccm. It well known that argon can influence the plasma dynamics, and the higher total flowrates would also reduce the residence time in the reaction zone. Second, the Zhang et al. experiments were conducted at a pressure of 600 mTorr, while our base case was 200 mTorr. Plasma chemistry depends exponentially on the electron temperature, which decreases with pressure
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due to the increased collision frequency. As shown in Fig. 5, the rate decreased substantially with pressure. The process may be more sensitive to the H2:VOCl3 ratio at these lower deposition rates. The results presented in Figs. 2–5 allow us to conjecture on the mechanism of vanadium oxide growth in a PECVD environment without hydrogen. The important plasma phase chemistry is illustrated in Eq. (5) þe
þO
VOCl3 ! liberates V and=or VO ! VO2ðgÞ
ð5Þ
As shown in Figs. 2 and 4, the rate is first order in both VOCl3 and rf power. It is hypothesized that the first step is electron impact dissociation of VOCl3, which liberates either elemental vanadium or the partial oxide VO. It is suggested that these two species would be the active precursors for deposition. The plasma also creates copious amounts of atomic oxygen. The atomic oxygen is expected to oxidize the precursors to form VO2. The free energy and reactivity of this higher oxide would be much lower, and as such they would not be expected to contribute significantly to film growth. The decrease in rate with pressure is attributed to a combination of factors. First, the electron temperature decreases with pressure, lowering the dissociation of VOCl3. Second, the oxidation process to form inactive oxides (VOx, x > 1) is controlled by neutral chemistry that scales as pressure squared.
1993
of magnitude. Likewise, there was little variation in the optical transparency of films as a function of plasma parameters. The GITT and PITT evaluation techniques compared quite favorably, agreeing within a factor of two over the range of intercalation. In the literature GITT is generally viewed to be the superior technique, and our results support that idea. The GITT results show much less scatter and noise than PITT. In PITT the value of the diffusion coefficient is highly sensitive to the slope obtained from semi-log plots such as that shown in Fig. 1(a). This accounts for the scatter that is shown in Fig. 8. In contrast, GITT is not as sensitive to the analysis procedure. GITT avoids issues associated with the IR drop and the formation of charging at the interface. The high transparency and efficient ion transport of this material shows promise for incorporation into electrochromic devices as the ion storage layer. Vanadium oxide also has a much higher ion storage capacity than tungsten oxide, where a fully intercalated film has a composition of LixWO3, with x 0.3. As such, relatively thin layers of vanadium oxide may be coupled with tungsten oxide films in electrochromic devices. Advantages of reducing the film thickness include enhanced transparency and shorter ion transport times. Currently we are fabricating solid state devices using PECVD tungsten oxide, PECVD vanadium oxide, and an organic electrolyte.
4.2. Optical and electrochemical performance To the best of our knowledge this is the first report of DLi+ for PECVD vanadium oxide. Overall, our reported values of DLi+ are among the highest reported for amorphous vanadium oxide in the literature. Julien et al. [23] reported values of 1012 cm2/s for flash evaporated films for Li concentrations of 0 < x < 3. While, McGraw et al. [24] reported diffusion coefficients of amorphous V2O5 grown by pulsed laser deposition that were on the order of 1013 cm2/s over the range of 0 < x < 1.5. It is evident from the literature that the film preparation technique strongly dictates the observed value of DLi+ in films of vanadium oxide. In addition, it is evident from this work that PECVD of vanadium oxide produces thin films with DLi+ values greater than those deposited by other forms of physical vapor deposition. In our previous work with tungsten oxide PECVD we found that electrochromic performance was highly sensitive to plasma conditions [11]. It was observed that small changes in plasma parameters, in particular rf power, could alter the diffusion coefficient and storage capacity by orders of magnitude. Relatively speaking, the electrochromic performance of vanadium oxide was much more robust with respect to plasma processing conditions. Although small variations in diffusion coefficients were observed, they were all the same order
5. Conclusions A comprehensive investigation of plasma processing on the deposition rate of electrochromic vanadium oxide was performed. The effects of precursor flowrate, gas composition, pressure, and power were examined. Rates in excess of 180 nm/min were achieved, an order of magnitude greater than conventional PVD technology. It was also observed that hydrogen had little impact on deposition, and that films could be deposited without H2. As-deposited films had an optical band gap of 2.13 eV, and it was observed to increase to as high as 3.13 eV with lithium intercalation. Lithium diffusion coefficients ranged between 1 · 1011 and 2 · 1012 cm2/s. These values are superior to those achieved by conventional PVD techniques in the literature. These films show great promise for application in electrochromic devices.
Acknowledgement The authors acknowledge financial support from the National Science Foundation through CAREER Award No. CTS-0093611.
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