Abstract:
A method and system for preparing a light transmitting and electrically conductive oxide film. The method and system includes providing an atomic layer deposition system, providing a first precursor selected from the group of cyclopentadienyl indium, tetrakis (dimethylamino) tin and mixtures thereof, inputting to the deposition system the first precursor for reaction for a first selected time, providing a purge gas for a selected time, providing a second precursor comprised of an oxidizer, and optionally inputting a second precursor into the deposition system for reaction and alternating for a predetermined number of cycles each of the first precursor, the purge gas and the second precursor to produce the oxide film.

Description:
The United States Government has certain rights in this invention pursuant to Contract No. DE-AC02-06CHI1357 between the United States Department of Energy and UChicago Argonne, LLC as operator of Argonne National Laboratories. 
    
    
     The present invention is directed to a method and system for synthesis of transparent conductive oxide coatings and is also related to an article of manufacture. More particularly, the invention is directed to a method and system for preparation of indium oxides, tin oxides and indium-tin oxides which are highly transparent and exhibit low resistivity. 
     BACKGROUND OF THE INVENTION 
     Indium oxide (In 2 O 3 ), tin oxide (SnO 2 ) and indium tin oxide (ITO), have substantial advantages for use as transparent conducting oxides which are employed, for example, in optoelectonic devices, flat panel displays and photovoltaic devices and which are also useful in gas sensors and as catalysts. In these types of applications, it can be helpful for device performance to have precise control over film thickness and composition, and some applications require the ability to coat high aspect ratio geometries or porous materials. In 2 O 3  thin films can be deposited using a variety of methods including sputtering, chemical vapor deposition, and atomic layer deposition (ALD). Of these techniques, ALD shows the most significant promise as this method affords excellent control over both the thickness and the composition of the deposited film. Most importantly, ALD offers excellent deposition conformality that enables the coating of porous materials with aspect ratios in excess of 1000. 
     Previously, In 2 O 3  deposition by ALD has been accomplished using InCl 3  with either H 2 O or H 2 O 2  as the oxygen source. Although useful for coating planar surfaces, this method suffers from several limitations. First, the InCl 3  chemistry requires high growth temperatures of ˜300-500° C. and yields a low growth rate of only 0.25-0.40 Å/cycle. In addition, the InCl 3  has a very low vapor pressure and must be heated to 285° C. just to saturate a planar surface. Furthermore, the corrosive HCl byproduct can damage the deposition equipment. But the greatest limitation of the InCl 3 /H 2 O method, especially for coating nanoporous materials, is that InCl 3  can etch the deposited In 2 O 3 . Consequently, nanoporous materials require very long precursor exposures that are likely to completely remove the In 2 O 3  from the outer portions of the nanoporous substrate. 
     An improved ALD process for In 2 O 3  has also been sought for many years and a number of alternate precursors have been investigated including β-diketonates (In(hfac) 3  (hfac=hexafluoropentadionate), In(thd) 3  (thd=2,2,6,6-tetramethyl-3,5-heptanedioneate), and In(acac) 3  (acac=2,4-pentanedionate)) and trimethyl indium, (In(CH 3 ) 3 ). Unfortunately, these efforts were unsuccessful. No growth was observed using β-diketonates with water or hydrogen peroxide, while trimethyl indium did not yield self-limiting growth. 
     SUMMARY OF THE INVENTION 
     A method and system are described for producing light transmitting (including light transparent) as well as highly conductive oxides which includes without limitations indium oxide, tin oxide, indium-tin oxide and doped variations of these oxides. Atomic layer deposition is preferably used to reactively form these articles with a high degree of control of the chemistry, as well as forming the desired layers more rapidly and also depositing the oxide layers onto substrates of high aspect ratio and porous materials. These oxides are prepared by use of a precursor of cyclopentadienyl indium for the indium oxide films and tetrakis (dimethylamino) tin for preparation of tin oxide films. Reactive preparation of these films was accomplished by introduction of ozone and/or hydrogen peroxide as part of an alternating exposure with the indium and tin precursors. The number of cycles of each component and time of each cycle can be adjusted to achieve a desired deposition result. Various dopants can also be added as part of the preparation process to produce a wide variety of optical and electrical characteristics for the product film. These and other features of the invention will be described in more detail hereinafter with reference to the figures described below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a schematic of a system for producing a conducting oxide thin film; 
         FIG. 2A  illustrates In 2 O 3  thickness growth versus InCp exposure time; and  FIG. 2B  illustrates In 2 O 3  thickness growth versus ozone exposure time; 
         FIG. 3A  illustrates In 2 O 3  thickness as a function of ALD cycles; and  FIG. 3B  illustrates In 2 O 3  growth rate versus deposition temperature. 
         FIG. 4A  illustrates a plan view SEM image of a 100 nm ALD In 2 O 3  film deposited on Si(100); and  FIG. 4B  illustrates a cross-sectional image of a 100 nm ALD In 2 O 3  film deposited on Si(100) 
         FIG. 5A  illustrates a back scattered electron image of an AAO membrane 70 microns thick, a 200 nm pore diameter and coated at 275° C. with In 2 O 3  13 nm thickness;  FIG. 5B  illustrates an EDAX spectrum from the center of the specimen of  FIG. 5A ; 
         FIG. 6A  illustrates SnO 2  growth rate as a function of TDMASn exposure time; and inset  FIG. 6B  illustrates SnO 2  growth rate as a function of bubbler temperature; and  FIG. 6C  illustrates SnO 2  growth rate as a function of H 2 O 2  exposure time; and  FIG. 6D  illustrates SnO 2  thickness as a function of oxygen source. 
         FIG. 7A  illustrates SnO 2  thickness as a function of ALD cycles; and  FIG. 7B  illustrates SnO 2  growth rate versus deposition temperature; 
         FIG. 8A  illustrates a plan view SEM image of a 92 nm ALD SnO 2  film deposited on Si(100); and  FIG. 8B  illustrates a cross-sectional view SEM image of a 92 nm ALD SnO 2  film deposited on Si(100); 
         FIG. 9A  illustrates growth rate versus percentage of SnO 2  cycles for ITO films; and  FIG. 9B  illustrates SnO 2  content versus percentage of SnO 2  cycles for ITO films. 
         FIG. 10A  illustrates resistivity of ITO deposited on glass at 275° C. as a function of SnO 2  cycles; and  FIG. 10B  illustrates optical transmission as a function of percent of Sn cycles; 
         FIG. 11A  illustrates ITO growth rate as a function of deposition temperature; and  FIG. 11B  illustrates ITO resistivity as a function of deposition temperature. 
         FIG. 12A  illustrates QMS signal for m=66 (cyclopentadiene, solid line) versus time during alternating exposures to InCp (dotted line) and O 3  (dashed line) at 250° C. using the timing sequence: 2-5-2-15;  FIG. 12B  illustrates QMS signal for m=44 (CO 2 ) recorded using the same conditions as  FIG. 12A ; and 
         FIG. 13A  illustrates QCM signal versus time during alternating exposures to InCp and O 3  at 250° C. using the timing sequence: 2-5-2-15;  FIG. 13B  Illustrates expanded view of  FIG. 13A  showing correlation between QCM signal (solid line) and exposures to InCp (dotted line) and O 3  (dashed line). 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     A system for producing a conducting oxide film in accordance with the invention is indicated generally at  10  in  FIG. 1 . In the preferred embodiment shown in  FIG. 1  the system  10  includes device  15  comprised of a conventional atomic layer depositing (“ALD”) apparatus and method, such as described in U.S. Pat. No. 4,058,430, which is incorporated by reference herein. Other known deposition systems are useable. The system  10  can include various sources for precursor vapors. The sources can be in gaseous, solid, or liquid form, but the partial pressure of the precursor is typically adjusted by heating, cooling, or pressure regulation to be preferably in the range of approximately 0.01 to 10 Torr which is appropriate for reaction and deposition. The system  10  can therefore include precursor sources  16  and  17  as well as additional precursor sources controlled by valves  18  and a valve control  19 . The system  10  can further include a purge gas source  21 , such as a N 2  gas source which flows in the direction indicated by the arrows in  FIG. 1 . The ALD device  15  can also include a heater  22  operated using a control  24 . Various monitoring and measurement devices can be employed to ensure deposition of the desired product oxide onto a selected substrate or collection of substrates  20  and deposition monitored by various devices, such as a mass spectrometer  26  and quartz crystal microbalance  28 . A thin film is deposited onto the selected substrate  20  by means of alternating saturated surface chemical reactions. These reactions are carried out by inputting gaseous or vaporized source materials alternately into the device  15 , and rinsing the reactor with an inert gas, such as from the purge gas source  21 , between the source material inputs. In the device  15 , the film growth occurs through saturated surface reactions with film growth being self-controlled. Consequently, in such a well-controlled form of the device  15  film thickness and film elemental composition can be precisely controlled by the number of reaction cycles. 
     In, the case of producing In 2 O 3 , the ALD process was performed using alternating exposures to cyclopentadienyl indium (InCp, Strem, electronic grade 99.999+% In) and ozone. The InCp is held in a stainless steel bubbler maintained at 40° C., and the tubing connecting the bubbler to the ALD reactor is maintained at 200° C. to prevent the deposition of InCp on the reactor walls. Ultrahigh purity nitrogen (99.999%) at a mass flow rate of 60 sccm was sent through the bubbler during the InCp exposures and was diverted to bypass the bubbler following the InCp exposures. The ozone was produced using a commercial ozone generator (Ozone Engineering L 11 ) using a feed of ultrahigh purity oxygen at a flow rate of 400 sccm to produce ˜10% ozone in oxygen. 
     The ALD timing sequences can be expressed as t 1 -t 2 -t 3 -t 4 , where t 1  is the exposure time for the first precursor, t 2  is the purge time following the first exposure, t 3  is the exposure time for the second precursor, t 4  is the purge time following the exposure to the second precursor, and all units are given in seconds (s). The timing sequence for In 2 O 3  ALD was typically 2-4-2-2 s; but this is subject to typical changes of the device  15 , gas flow reaction and other experimental variables, all of which can be readily adjusted to achieve the advantageous results set forth herein. 
     A QCM was installed in the ALD reactor of the device  15  in place of the substrate  20  enabling in situ measurements during the In 2 O 3  growth. These measurements utilized a Maxtek BSH-150 bakeable sensor and AT-cut quartz sensor crystals with a polished front surface obtained from the Colorado Crystal Corp., part no. CCAT1BK-1007-000. The QCM measurements were made using a Maxtek TM400 film thickness monitor interfaced to a personal computer. In addition, the ALD reactor was equipped with a QMS (Stanford Research Systems RGA300) located downstream of the QCM in a differentially pumped chamber separated from the reactor tube by a 35 μm orifice and evacuated using a 50 L/s turbomolecular pump. 
     In 2 O 3  ALD films were deposited on 1 cm×2 cm Si(100) and glass substrates. Prior to loading, the substrate  20  was ultrasonically cleaned in acetone and then 2-propanol and blown dry using nitrogen. After loading, the substrate  20  was allowed to outgas in the ALD reactor for 10 min at the deposition temperature (typically 250° C.) in 1 Torr of flowing ultrahigh purity nitrogen. Next, the substrate  20  was cleaned in situ using a 60 s exposure to 10% ozone in oxygen at a pressure of 2 Torr and a mass flow rate of 400 sccm. We observed a reactor conditioning effect in which the thicknesses of the In 2 O 3  films deposited immediately following Al 2 O 3  growth were thinner than expected. To compensate for this effect, we always deposited an In 2 O 3  buffer layer on the inside of the reactor using ˜100 InCp/O 3  cycles following deposition of a different material. 
     SEM images were acquired using a Hitachi S4700 scanning electron microscope with a field emission gun electron beam source, secondary electron and backscattered electron detectors, and an energy dispersive analysis of X-rays (EDAX) detector for elemental analysis. AFM measurements were performed on a Digital Instruments Dimension 3000 with a NanoScope IIIa controller operated in tapping mode. XRD measurements were taken on a Rigaku Miniflex Plus diffractometer. Ellipsometric measurements of the In 2 O 3  films deposited on Si(100) surfaces were performed using a J. A. Woolam Co. M2000 variable angle spectroscopic ellipsometer using a table of refractive indexes for In 2 O 3  supplied with the instrument. Optical absorption spectra were acquired from ALD In 2 O 3  films deposited on glass using the M2000 operated in transmission mode and were fit to a model using the same In 2 O 3  optical constants. AAO membranes (Whatman Anodisc 13) with pore diameters of 200 nm and a membrane thickness of 70 μm were also coated by In 2 O 3  and will be described hereinafter. Prior to SEM analysis, cleaved cross sections of the membranes were embedded in conducting epoxy and polished with progressively finer diamond polishing compound ending with 0.25 μm. 
     Various detailed in situ measurements and evaluations were performed as part of the characterization of the process of producing transparent conducting oxides. These measurements and evaluations are set forth in detail in the Example hereinafter. 
     In 2 O 3  was deposited on various substrates, including (100) silicon and glass substrates.  FIG. 2A  shows the results of uptake measurements made while varying the exposure time to InCp using the timing sequence x-2-2-2 at 250° C. substrate temperature. This figure demonstrates self-limiting behavior for InCp for exposure times of ˜2 s.  FIG. 2B  shows a similar graph exploring the effect of increasing ozone exposures using the timing sequence 2-2-x-2 and demonstrates self-limiting In 2 O 3  growth for ozone exposure times beyond ˜2 s. Increasing the InCp and O 3  purge times did not affect the In 2 O 3  growth rates, indicating that purge times ≧2 s are sufficient to avoid mixing the precursors. For the remainder of the measurements, a timing sequence of 2-4-2-2 was used unless otherwise noted. It should be noted that these timing sequences are subject to modification given the parameters, such as, geometry and operational details of the device  15 , the precursor, the temperature of the substrate  20  and gas flows. 
       FIG. 3A  reveals nearly linear In 2 O 3  growth over the range of 50-1000 InCp/O 3  cycles at an average growth rate of 2.0 Å/cycle. Detailed inspection of the plot reveals that the In 2 O 3  growth rate actually increases somewhat with the number of cycles from 1.3 to 2.0 Å/cycle over the range of 50-2000 ALD cycles. Without limiting the scope of the invention, this increase probably results from an increase in surface area with the evolution and growth of the In 2 O 3  nanocrystals. Gradual changes in ALD growth rates have been observed previously for nanocrystalline materials in which the morphology or crystal size evolves with film thickness. The effect of substrate deposition temperature on the In 2 O 3  growth is shown in  FIG. 3B . The In 2 O 3  growth rate drops precipitously below 200° C.; and without limiting the scope of the invention, this may occur because 200° C. is the threshold temperature below which ozone no longer decomposes to form oxygen radicals necessary for In 2 O 3  growth. Between 200 and 450° C., the In 2 O 3  growth rate is nearly constant at 1.2-1.4 Å/cycle. At 500° C., it was difficult to control the film thickness because most of the InCp decomposed at the leading edge of the sample holder in the reactor. 
     We also examined the variation in In 2 O 3  film thickness along the flow direction of the reactor. Using the 2-4-2-2 timing sequence, the film thickness was constant for the first ˜15 cm of the reactor, after which the film thickness dropped off by 53% over 22 cm. The film uniformity improved using 15 s of O 3  exposure times so that the thickness decreased by only 33% over 22 cm, but the thickness variation was unaffected using longer InCp exposure times. We attribute this behavior to the depletion of ozone along the flow direction of the reactor. The results of the film growth studies are summarized in the Table where we compare the InCp/O 3  process with the existing In 2 O 3  ALD process using InCl 3 /H 2 O 2 . 
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 
               
             
             
               
                   
               
               
                 Comparison of InCl 3  and InCp Precursors for In 2 O 3 ALD 
               
             
          
           
               
                   
                 indium precursor InCl 3   
                 InCp 
               
               
                   
                   
               
             
          
           
               
                 oxygen source 
                 H 2 O 2   
                 O 3   
               
               
                 ALD temperature window (° C.) 
                 300-500 
                 200-450 
               
               
                 precursor temperature (° C.) 
                 285 
                 40 
               
               
                 growth rate (Å/cycle) 
                 0.40 
                 2.0 
               
               
                 precursor etching? 
                 yes 
                 no 
               
               
                   
                 Ritala 
               
               
                 reference 
                 1998 
                 this work 
               
               
                   
               
             
          
         
       
     
     XRD measurements for a 176 nm film deposited on glass at 275° C. match closely cubic, polycrystalline In 2 O 3 . AFM studies reveals a relatively rough, nanocrystalline topography for a 100 nm In 2 O 3  film deposited on Si(100) and yield a root-mean-squared (RMS) roughness of R=3.96 nm for a 1×1 μm scan. The RMS roughness increases somewhat to R=4.9 and 5.8 nm for scan sizes of 2×2 and 10×10 μm, respectively. Nanocrystals with a lateral dimension of 50-100 nm are evident in the plan view SEM image for the 100 nm In 2 O 3  sample on Si(100) shown in  FIG. 4A , and the cross-sectional SEM images ( FIG. 4B ) demonstrate that the In 2 O 3  films are dense and free of voids, pinholes, or cracks. 
     The optical transmission spectrum of a 173 nm thick In 2 O 3  film deposited on glass yields an average transmission of the In 2 O 3  film over the wavelength range 400-1000 nm of T=90.0% and is comparable to ALD In 2 O 3  films deposited previously using InCl 3 /H 2 O. This film had a resistivity of 16×10 −3  Ωcm which is somewhat higher than the value of (3-6)×10 −3  Ωcm obtained using InCl 3 /H 2 O, suggesting that the O 3  used in our process produces a more perfect In 2 O 3  stoichiometry with fewer oxygen vacancies resulting in increased resistivity. 
     Anodic aluminum oxide (“AAO” hereinafter) membranes were also coated with In 2 O 3  using 80 InCp/O 3  cycles at 250° C. The AAO had an initial pore diameter d=200 nm and thickness L=70 μm such that the aspect ratio is L/d=350. To allow gaseous diffusion of the precursors into the high aspect ratio pores, relatively long ALD cycle times of 60-15-60-15 were used.  FIG. 5A  shows a backscattered electron image recorded from the middle of a cleaved cross section of the AAO membrane. The white lines visible on the edges of the AAO nanopores are the In 2 O 3  coating and appear brighter than the surrounding Al 2 O 3  because the higher-Z indium backscatters electrons more efficiently. Because this sample was polished prior to imaging, the white lines are not likely caused by edge contrast. In fact, the pore structure was practically invisible in secondary electron images (not shown).  FIG. 5B  shows an EDAX spectrum taken from the same location at the center of the AAO membrane, and the prominent In L α  peak at 3.29 keV demonstrates that the ALD In 2 O 3  completely infiltrates the high aspect ratio AAO membrane. The intensity of the In Lα peak decreases by only 20% between the edge and the middle of the membrane, indicating very high conformality of the ALD In 2 O 3  coating. We also coated much higher aspect ratio AAO membranes (L/d=2300) using an identical treatment, but the coating was less conformal and the In Lα signal decreased by ˜90% between the edge and the middle of the membrane. This is most likely due to a decrease in concentration of the O 3  or oxygen radicals along the very high aspect ratio pores. 
     In the case of producing SnO 2 , ALD was performed using alternating exposures to tetrakis(dimethylamino) tin (TDMASn, Gelest, &gt;95% purity) and hydrogen peroxide (H 2 O 2 , Aldrich, 30 wt % in water). The TDMASn is held in a stainless steel bubbler maintained at 40° C., and the tubing connecting the bubbler to the ALD reactor is maintained at 150° C. to prevent condensation of the TDMASn on the reactor walls. All of the measurements in  FIGS. 6A-6D  were performed by depositing SnO 2  films on Si(100) substrates that were first coated with ˜1 nm ALD Al 2 O 3  using 10 cycles of trimethyl aluminum and H 2 O 2 . The ALD Al 2 O 3  was used to prepare a chemically reactive surface with a high density of surface hydroxyl groups to facilitate the subsequent SnO 2  ALD. The thicknesses of the SnO 2  film was then determined using spectroscopic ellipsometry for the data in  FIGS. 6A-6D .  FIG. 6A  shows the effect of varying the TDMASn exposure time on the SnO 2  growth rate and demonstrates self-limiting growth for exposure times exceeding ˜2 s.  FIG. 6B  shows the effect of varying the TDMASn bubbler temperature on the SnO 2  growth rate and exhibits saturation for bubbler temperatures exceeding 30° C.  FIG. 6C  shows the effect of varying the H 2 O 2  exposure time on the SnO 2  growth rate and shows saturated growth for H 2 O 2  exposure times greater than ˜0.5 s.  FIG. 6D  presents the SnO 2  growth rates observed using different oxygen containing precursors and shows that the H 2 O 2  yields the highest SnO 2  growth rate. 
       FIG. 7A  shows the effect of varying the number of TDMASn/H 2 O 2  cycles between 10 and 750 cycles. These films were deposited on Si(100) substrates at 150° C. using the timing sequence 1-5-1-5 after first depositing 1 nm ALD Al 2 O 3 . This figure yields an average growth rate of 1.2 Å/cycle.  FIG. 7B  shows the effect of varying the substrate temperature on the resulting SnO 2  growth rate. The SnO 2  growth rate decreases steadily between from 1.58 Å/cycle at 50° C. to 0.83 Å/cycle at 325° C., and this gradual decrease is consistent with a decrease in the number of surface hydroxyl groups as has been observed previously in Al 2 O 3  ALD. The SnO 2  growth rate increases abruptly to 2.56 Å/cycle at 325° C. In addition, the SnO 2  films become less uniform in thickness at this higher temperature as indicated by the large error bars for these thickness measurements in  FIG. 7B . These findings are consistent with the onset of thermal decomposition of the TDMASn precursor leading to non-self limited growth. 
       FIG. 8A  shows a plan-view SEM image of the SnO 2  film with a thickness of 916 Å deposited on Si(100) at a temperature of 150° C. This SEM image is nearly featureless, and this is consistent with the amorphous structure observed in the XRD data measured for the film of similar thickness deposited on glass. The cross-sectional SEM image of this film in  FIG. 8B  shows a smooth, conformal and flat film with no evidence of granularity as would be seen for a crystalline film. AFM analysis of this film shows a very smooth surface and yields an RMS roughness of 0.85 nm. Optical transmission measurements for an SnO 2  film with a thickness of 140 nm deposited on glass yields an average transmission in the range of 400-1000 nm of 94%. 
     The refractive indices for the SnO 2  films deposited by ALD using 100 cycles of TDMASn/H 2 O 2  were determined by spectroscopic ellipsometry versus deposition temperature. At deposition temperatures above 200° C., the refractive index for the SnO 2  films was relatively constant in the range of 1.83-1.91. Below 200° C., the refractive index decreased steadily with deposition temperature to a value of 1.62 at a deposition temperature of 50° C. The elemental composition (Sn, O, C, and N content) of these films was determined using XPS measurements. The C content remains relatively constant at 5-6% and the N content is undetectable above 200° C. Below 200° C., the C and N contents increase with decreasing deposition temperature reaching 10 and 2%, respectively. In addition, the resistivity of the SnO 2  films deposited by ALD decreased with increasing deposition temperature from 2.8×10 −1  cm at 150° C. to 1.9×10 −3  Ωcm at 200° C. 
     The refractive index and elemental composition measurements for the films deposited above 200° C. are consistent with pure SnO 2 . SnO 2  has a refractive index of 1.9. The constant value of 5-6% C probably results from contamination at the surface of the film from the air transfer between the ALD reactor in the device  15  and the XPS system. In contrast, the lower refractive index, lower conductivity and lower purity observed at the lower deposition temperatures are consistent with residual dimethylamino ligands remaining in the films. If we assume ˜5% surface carbon contamination, then the C:N ratio in the films at the lower temperatures is ˜2:1 as would be expected from dimenthylamine. While not limiting the scope of the invention, the lower refractive index is consistent with a lower density film as would be expected from these contaminants. It is plausible that the surface reactions do not proceed to completion at the lower deposition temperatures. Longer H 2 O 2  exposures, or possibly using O 3  or oxygen radicals may lower the concentrations of C and N impurities in these films deposited at lower temperatures. 
     The film thickness determined from this SEM image  FIG. 8B  is 93 nm, in excellent agreement with the ellipsometric thickness measurement of 91.6 nm. This good agreement strengthens the finding that the refractive index from the ellipsometric measurements is 1.86, which agrees well with previous determinations of the refractive index for SnO 2 , and suggests that the stoichiometry of our films is Sn:O=1:2. The XPS measurements on the SnO 2  films deposited by ALD yielded a stoichiometry of Sn:O=1:1. However, it is likely that the SnO 2  films deposited by ALD have an oxygen deficient surface layer, and only this surface layer is probed by the XPS measurements. Similar XPS measurements have been made on SnO 2  films deposited by ALD using SnCl 4 . 
     As in the case of In 2 O 3  described hereinbefore, conformal coating ability of this new ALD SnO 2  process was completed by coating an AAO membrane with a pore diameter of d=200 nm and a thickness of L=60 microns yielding an aspect ratio of L/d=300. After coating with an SnO 2  film with a thickness of 38 nm, the AAO membrane annealed in air at 400 C for 4 hours to induce crystallization thereby improving the contrast in SEM. SEM examination showed that the SnO 2  crystals are disposed on the inner surfaces of the two AAO pores, and the ALD SnO 2  film with a thickness of ˜40 nm is also clearly evident as lining the inner walls of the two pores in a manner, such as was shown for In 2 O 3  in  FIG. 5A . The conformal SnO 2  films are more evident in the backscattered electron image of the same region of the AAO membrane that was collected simultaneously. The high contrast in this image results because the higher average atomic number of the SnO 2  (27) as compared to the surrounding Al 2 O 3  (15) yields a much higher intensity of backscattered electrons. 
     In a further aspect of the invention, the separate process for the ALD In 2 O 3  and the ALD SnO 2  can be combined in different, controlled ratios to produce indium-tin oxide (ITO) and thereby to modify or improve the optical transmission and electrical conductivity.  FIG. 9A  shows the results of varying the percentage of SnO 2  cycles on the resulting ALD growth rate as determined by both ellipsometric measurements and XRF measurements.  FIG. 9B  shows the variation in SnO 2  content for the ITO films as a function of the percentage of SnO 2  cycles. The SnO 2  content is higher than predicted by a simple rule of mixtures formula because the SnO 2  impedes the In 2 O 3  ALD resulting in lower than expected In 2 O 3  content for these films. Nevertheless, the SnO 2  content can be precisely controlled in these ITO films over the range 0-40%. 
       FIGS. 10A and 10B  show the electrical resistivity and optical transmission of 40 nm ITO films on a glass substrate  20  at a deposition temperature of 275° C. for the glass substrate  20 . There is a forty times increase in conductivity with Sn doping and an increase from 80% to 92% in transmission. 
       FIG. 11A  shows the effect of varying the substrate temperature on the ITO growth rate for ITO films prepared using 5% SnO 2  cycles. The temperature dependence for the pure In 2 O 3  and SnO 2  films are also shown for comparison. The ITO growth rate is nearly constant at 1.2-1.5 Å/cycle in the temperature range 200-325° C. The ITO growth rate increases dramatically at 350 C and this probably results from the thermal decomposition of the TDMASn precursor as this change is also observed for the pure SnO2 films. The ITO growth rate decreases dramatically below 200° C. and this probably results because the O 3  precursor for the In 2 O 3  ALD is no longer activated at this low temperature as indicated by the drop-off in the In 2 O 3  growth rate below 200° C.  FIG. 11B  shows the result of varying the substrate temperature on the resistivity of the ITO films prepared using 5% SnO 2  cycles. The ITO resistivity is nearly constant at 3-4×10 −4  Ohm cm over the same temperature range of 200-325° C. for which constant growth rate is seen in  FIG. 11A . 
     In 2 O 3  forms the basis for an important class of transparent conducting oxides (TCOs) that see wide use in optoelectronic devices, flat-panel displays and photovoltaics. In addition, In 2 O 3 , has applications in the fields of gas sensors and catalysis. SnO 2  is also a widely used TCO material, especially when doped with fluorine or antimony. In 2 O 3  doped with SnO 2  (typically 10 weight percent) is one of the most widely used TCO materials. ITO is used in a wide variety of optoelectonic devices such as liquid crystal displays, touch panels, flat panel displays, plasma displays, organic light-emitting diodes, and solar cells. In addition, ITO is used in optics to make infrared reflecting coatings (hot mirrors) for architectural applications as well as for antistatic coatings. 
     The following non-limiting example illustrates various aspects of the invention. 
     EXAMPLE 
     In situ QCM and QMS measurements were used to investigate the mechanism for In 2 O 3  ALD using InCp and O 3 . These measurements were performed at 250° C. using the timing sequence 2-5-2-15. The In 2 O 3  ALD process can be described by a generalized reaction scheme:
 
*+InCp→InCp x *+(1 −x )products  (1)
 
InCp x *+oxidant→InO 1.5 *+( x )products  (2)
 
     In these reactions, the surface species are designated with an asterisk and x is the fraction of Cp ligands remaining on the surface following each InCp exposure. The gas-phase products, the initial reactive sites, and the oxidant are all left unspecified but will be determined from the in situ measurements. 
     QMS measurements were performed to determine the gas-phase products of the InCp and ozone half-reactions. Representative QMS data recorded during the InCp and O 3  half-reactions are shown in  FIG. 1   a,b , respectively. As shown in  FIG. 12A , a peak at m=66 appears during the InCp half-reaction, but not during the O 3  half-reaction. When the InCp exposure follows an O 3  exposure, the m=66 peak has a sharp spike at the leading edge followed by a smaller plateau that persists as long as the InCp dosing valve is held open as illustrated by the first two ALD cycles in the figure. However, if no O 3  exposure precedes the InCp exposure, the sharp spike in the m=66 peak is absent as shown by the final two ALD cycles in  FIG. 12A . From these observations we conclude that the sharp feature represents a gas-phase product of reaction 1, while the smaller plateau is simply decomposition of the InCp precursor in the QMS. 
     Similarly, we see a sharp spike in the m=44 signal coincident with the O 3  exposures that are preceded by InCp exposures ( FIG. 12B ); however, this spike is not seen when the InCp exposures are absent. Consequently, we conclude that m=44 is a gas-phase product of reaction 2. The small peak in the m=44 data coincident with the InCp exposures is present whether or not the InCp exposure is preceded by an O 3  exposure, and therefore this is a crack of InCp rather than a reaction product. The large, slow transient feature in the m=44 signal that appears during the purge cycle of each O 3  exposure results from a CO 2  impurity in the ultrahigh purity oxygen which is pumped slowly by our system. 
     By collecting QMS data over the mass range 12-115 amu, we discovered that m=44 (CO 2 ) is the only product of the O 3  reaction, while the InCp reaction yields the following products (and relative abundances): m=66 (100), 65 (67), 39 (53), and 40 (33). This mass pattern matches closely the fragmentation pattern for cyclopentadiene (C 5 H 6 ). We also looked for the cyclopentadienyl dimer at m=132, but we found none. It is surprising that we do not observe water during the O 3  half-reaction. One explanation is that the hydrogen from the Cp ligands in reaction 2 remains on the surface as hydroxyl (OH) groups that subsequently react with InCp to form HCp (cyclopentadiene, m=66). This would explain why no m=18 is observed in reaction 2, while the main product from reaction 1 is m=66 rather than m=65. 
     The ratio of gas-phase products measured during the InCp and O 3  half-reactions can be used to calculate x in eqs (1) and (2). By integrating the product mass peaks observed during the InCp exposures and correcting for variations in electron multiplier gain, quadrupole transmission, and ionization efficiency, we calculate that the amount of Cp released during eq (1) is (in arbitrary units) 1−x=15. Similarly, after correcting for the relative effusion rate of CO 2  versus cyclopentadiene, the amount of CO 2  released during eq 2 is 5x=13.8, where the quantity 5x accounts for the fact that five CO 2  molecules are released from each Cp ligand. Combining these expressions, (1−x)/(5x)=1.09 so that x=0.15. 
       FIG. 13A  also shows the QCM data recorded simultaneously with the QMS measurements demonstrating that alternating InCp/O 3  exposures results in a linear mass increase versus time. The slope of the data in  FIG. 13A  yields a net mass change of 55 ng/cm 2 /cycle. Assuming a bulk density for In 2 O 3  of 7.19 g/cm 2 , this corresponds to a growth rate of 0.76 Å/cycle. As described in the next section, this growth rate is lower than the 1.3-2.0 Å/cycle measured on Si(100) substrates. This difference arises because the QCM is located 33 cm downstream from the substrates where the O 3  concentration is lower. 
       FIG. 13B  shows an expanded view of the QCM data for two ALD cycles. There is an abrupt mass increase during each InCp exposure and a transient mass decrease coincident with each O 3  exposure followed by a slow increase such that the net mass change produced by the O 3  exposures is almost zero. We attribute the apparent mass decrease during the O 3  exposures to a transient heating of the QCM produced by the thermal decomposition of O 3  or the oxidation of the Cp ligands. The mass changes produced by the individual half-reactions can predict the fraction of Cp ligands remaining on the surface after the InCp exposures. Using the relationship R=Δm/Δm 2 , where Δm is the mass change from one complete cycle and Δm 2  is the mass change during reaction 1, we calculate from eqs 1 and 2 and the atomic masses that Δm(InO 1.5 )=139 and Δm 2 [In+(x)Cp]=115+65x so that R=139/(115+65x). From  FIG. 13B , R=1.0 so that x=0.37 which implies that, on average, 37% of the Cp ligands remain on the surface after reaction 1. This value is somewhat higher than the value x=0.15 calculated from the QMS data. This difference probably arises from uncertainties in the QMS parameters used to calculate the cyclopentadiene and CO 2  concentrations or from inaccuracies in the QCM data caused by the temperature-induced transient feature during the O 3  exposures. Nevertheless, the primary conclusion from the QCM data is the same as from the QMS data: a majority of the Cp ligands are lost during the InCp exposures, and the small fraction remaining is subsequently removed during the following O 3  exposure. 
     One additional finding from the in situ measurements is that the reactive oxygen species during the In 2 O 3  ALD is most probably oxygen radicals formed by the thermal decomposition of ozone (O 3 →O 2 +O). This process occurs primarily on the In 2 O 3  surface but also to a lesser degree on other surfaces (e.g., Al 2 O 3 ) or possibly in the gas phase. The In 2 O 3  growth rate drops off abruptly at 200° C. to nearly zero (see  FIG. 3B ) suggesting that we must exceed a threshold temperature for ozone decomposition to enable In 2 O 3  growth. This interpretation is supported by the observation that the m=48 QMS signal from ozone is ˜10× larger at temperatures below 200° C. Furthermore, In 2 O 3  appears to catalyze O 3  decomposition more efficiently than Al 2 O 3  or gas-phase decomposition. When we first executed InCp/O 3  cycles following Al 2 O 3  growth, we determined the m=48 signal is initially high but decreases nearly to zero following 40-50 In 2 O 3  cycles. Moreover, in situ QCM measurements reveal that the In 2 O 3  growth is initially inhibited following Al 2 O 3  growth. Taken together, these results suggest that ozone decomposes on the growing surface to form a more active oxidizing species. This species is most likely a surface-bound oxygen radical or possibly a surface peroxide. 
     To summarize the in situ measurements, we can rewrite eqs (1) and (2) with the unknown surface species, gaseous products, and oxidant filled in:
 
5OH*+O*+6InCp→5O—In*+O—In(Cp)*+5C 5 H 6   (3)
 
5O—In*+O—In(Cp)*+19O*→5O—In(O) 0.5 —OH*+O—In(O) 1.5 *+5CO 2   (4)
 
     In reaction 3, the initial reactive sites are five OH groups and one surface oxygen species. Six InCp molecules react with the surface liberating five cyclopentadiene molecules and leaving one Cp ligand on the surface. In reaction 4, surface-bound oxygen species formed by the decomposition of O 3  release the carbon from the remaining Cp ligand as five CO 2 , but the hydrogen atoms remain to reform five new hydroxyl groups. Consequently, reaction 4 regenerates the initial surface and forms In 2 O 3  with the proper stoichiometry, In/O=1:1.5. Reactions 3 and 4 yield x=⅙=0.17, which is in the range of x=0.15-0.37 determined from the in situ measurements. Although somewhat speculative, this mechanism has the appeal that the single remaining Cp ligand will exactly balance the five OH groups so that no hydrogen-containing products are released during the O 3  reaction. Although the indium oxidation state is not explicit in reactions 3 and 4, the conversion from In 1 + to In 3 + probably occurs mostly during the ozone step. In situ measurements using infrared absorption spectroscopy could verify the existence of OH groups following the O 3  exposures. If oxygen radicals are indeed the active oxidizing species in this process, then substituting an oxygen plasma in place of the O 3  may allow In 2 O 3  growth below 200° C. 
     The foregoing description of embodiments of the present invention have been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the present invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the present invention. The embodiments were chosen and described in order to explain the principles of the present invention and its practical application to enable one skilled in the art to utilize the present invention in various embodiments, and with various modifications, as are suited to the particular use contemplated.