Department of Materials Science and Engineering and Frederick Seitz Materials Research Laboratory,. University of Illinois at UrbanaâChampaign, 1304 West ...
APPLIED PHYSICS LETTERS
VOLUME 78, NUMBER 3
15 JANUARY 2001
Electrical properties of Ta-doped SnO2 thin films prepared by the metal–organic chemical-vapor deposition method Sang woo Lee, Young-Woon Kim, and Haydn Chena) Department of Materials Science and Engineering and Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana–Champaign, 1304 West Green Street, Urbana, Illinois 61801
共Received 28 August 2000; accepted for publication 6 November 2000兲 Undoped and Ta-doped SnO2 (Sn1⫺x Tax O2) thin films were prepared on Corning 7059 glass substrates by the metal–organic chemical-vapor deposition method. The relative amount of Ta, C Ta⫽X Ta /(X Ta⫹X Sn), varied from 0 to 7.13 at. %. For the five compositions studied, the lowest resistivity at room temperature was 2.01⫻10⫺4 ⍀ cm at C Ta⫽3.75% with charge carrier density and mobility of 1.27⫻1021 cm⫺3 and 24.5 cm2/V s, respectively. In microstructural investigation, 3.75% Ta-doped film maintains a growth pattern of initial stage growth while 7.13% Ta-doped film has a high population of small grains at the interface, which results in large grains through competitive growth. The resistivity of the undoped film was 0.17 ⍀ cm with charge carrier density and mobility of 1.31⫻1018 cm⫺3 and 28.1 cm2/V s obtained from Hall measurement. This study suggests that Ta is an excellent n-type dopant in SnO2. © 2001 American Institute of Physics. 关DOI: 10.1063/1.1337640兴
Tin oxide is an n-type wide-band-gap semiconductor where inherent oxygen vacancies act as an n-type dopant.1,2 Many studies have been extensively conducted on this material in the form of thin film due to its useful electrical and optical properties.3–7 The most notable applications include transparent conducting electrodes 共TCE兲 in flat-panel displays and solar cells. In such applications, SnO2 is usually doped with a small amount of Sb or F to increase the electrical conductivity. Besides Sb- or F-doped SnO2, there are a variety of other transparent conducting oxides 共TCOs兲 such as doped In2O3, ZnO, or CdO. Among these, Sn-doped In2O3 共ITO兲 is most widely used. However, ITO seems to have reached its maximum capability in high-density applications. It has been recommended that for TEC applications a sheet resistance of 1 ⍀/square is desired.8 Without sacrificing the optical transparency, this cannot be achieved using currently existing TCOs. Thus, there have been several studies to explore material systems, such as GaInO3, 8 AgInO2, 9 and ZnO–In2O3 – SnO2, 10 with limited success in property improvement. In this letter, we report the processing conditions and electrical properties of Ta-doped SnO2 thin films with a thickness of 0.2 m prepared by the metal–organic chemical-vapor deposition 共MOCVD兲 technique. Thin films were deposited using a cold-wall, horizontal, low-pressure MOCVD system. Commercially available tin-tetra-butoxide, Sn(OC4H9) 4 , and tantalum-ethoxide, Ta共OC2H5) 5 , were used as the organometallic 共OM兲 sources. Ultra-high-purity 共UHP兲 O2 was used as the oxidant. The OM source vapor was transported by UHP N2 as the carrier gas. Detailed deposition parameters are included in Table I. Phase identification was done by x-ray diffraction 共XRD兲 共Philips XRG-3000 with Cu K␣ radiation兲. The Ta a兲
Author to whom correspondence should be addressed; now at: Department of Physics and Materials Science, City University of Hong Kong, Kowloon, Hong Kong.
concentration was determined by a combination of Rutherford backscattering spectrometry and secondary ion mass spectrometry 共SIMS兲 共Cameca IMS 5f SIMS兲. The resistivity of the films was measured by the four-point-probe technique using van der Pauw geometry. The carrier concentration was determined by Hall measurement under 4.5 T with 0.1 mA 共Quantum Design PPMS兲. The temperature range of the measurements was between 10 and 300 K. Figure 1 shows –2 scans of SnO2 films with various Ta contents. All the films are in rutile phase without any secondary phases indicating that Ta is incorporated into SnO2 substitutionally. 2 positions of each reflection match very well with those in JCPDS File No. 41-1445. At room temperature, the lowest resistivity 共兲 of 2.01 ⫻10⫺4 ⍀ cm was observed for the 3.75% Ta-doped film with a carrier concentration of 1.27⫻1021 cm⫺3 and a mobility 共兲 of 24.5 cm2/V s. The resistivity is inversely related to the carrier concentration and the mobility through the wellknown relationship ⫽(nq ) ⫺1 . In Ta-doped SnO2 thin films, the carrier concentration is the main factor controlling the resistivity, which originates from an addition of Ta atoms. For films up to 4.27% Ta, it has been found that Ta atoms became 100% electrically active, implying that Ta is an excellent n-type dopant in SnO2. With further increase in the carrier concentration, the resistivity increases due to a decrease in the mobility. TABLE I. Deposition parameters of MOCVD-derived Ta-doped SnO2 thin films. Substrate temperature: Reactor pressure: OM precursors:
600 °C 10 Torr Sn共OC4H9) 4 : Ta共OC2H5) 5 :
O2 flow rate: N2 flow rate: Growth rate:
Temperature Flow rate: Temperature: Flow rate:
55 °C 0–20 sccm 90 °C 0–60 sccm
200 sccm 900 sccm 30–40 Å/min
0003-6951/2001/78(3)/350/3/$18.00 350 © 2001 American Institute of Physics Downloaded 09 Dec 2002 to 144.214.24.138. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp
Appl. Phys. Lett., Vol. 78, No. 3, 15 January 2001
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FIG. 1. XRD spectra of SnO2:Ta thin films grown at 600 °C. Relative amount of Ta is indicated on the right side of each spectrum.
Figure 2 is a plot of , n, and of three selected samples as a function of temperature. The characteristic semiconductor behavior is observed in the undoped sample. The resistivity exhibits a typical negative-temperature coefficient phenomenon with two distinct regions. In the high-temperature region, carrier freezing becomes significant upon cooling down to the complete thermal excitation of electrons from the donor level to the conduction band. In the lowtemperature region, below approximately 80 K, the carrier concentration tends to increase with temperature. This can be explained as hopping conduction through impurity states, presumably oxygen vacancy states, within the band. These two regions are shown clearly in the mobility plot in Fig. 2共c兲. In the high-temperature region, the mobility increases with decreasing temperature. Over this region, the mobility seems to be mainly limited by ionized impurity scattering. Based on the Brooks–Herring model,11 the temperature dependence of mobility, limited by the ionized impurity, is determined as T 3/2. Over this region, the charge carrier concentration is even a stronger function of temperature and dominates over T 3/2, yielding the negative power-law behavior. However, this effect cannot compete with the carrier freezing, such that the resistivity increases upon freezing. In the low-temperature region, the mobility starts to decrease with decreasing temperature and is limited by neutral impurity scattering. In the case of the doped samples, metallic behavior is predominant where the resistivity and carrier concentration are approximately invariant with temperature. For comparison, the degenerate density limit is drawn in Fig. 2共b兲 as a solid and dashed line by solving the Fermi–Dirac statistics using two different effective masses (0.3m e and m e ). It is evident in Fig. 2共b兲 that the doped samples are degenerate semiconductors. A small increase in the carrier concentration in the relatively high-temperature region can be explained as electron donation from oxygen vacancies that act as an n-type dopant, as in the case of the undoped sample. Thus, the carrier concentration is a combination of electrons from the oxygen vacancies and Ta dopants. The mobility of the doped samples is a weak function of temperature, implying that the predominant scattering mechanism is by ionized im-
FIG. 2. 共a兲 Resistivity 共兲, 共b兲 carrier concentration 共n兲, and 共c兲 mobility 共兲 as a function of temperature. The solid and dashed straight lines in 共b兲 are degenerate carrier density limits calculating using 0.3m e and m e .
purities. There is a relatively large decrease of the mobility in the 7.31% Ta-doped sample over the entire temperature range. Figures 3共a兲 and 3共b兲 show cross-sectional transmission electron microscope 共TEM兲 view of bright- and dark-field images of films with 3.75% Ta, respectively. Figures 3共c兲 and 3共d兲 are those of the 7.13% Ta-doped film. A portion of the 共110兲 ring patterns was used to obtain the dark-field images in both films. Both films show a clean interface between the films and substrates, and neither secondary phase nor preferential growth were observed in selected area diffraction patterns. Twins with a 兵101其 twin plane are observed in both small and large grains of the 3.75% and 7.13% Tadoped films. Facets at the top of the films do not have a preferential plane but have 40°–65° from the growth plane normal. The 3.75% Ta-doped film has a continuous growth pattern following the initial pattern formed in the initial stage of growth, while the 7.13% Ta-doped film has a high density of small grains near the interface, which results in large grains as a result of competitive growth. This implies that the nucleation rate of the 7.13% Ta-doped films is much higher than that of the 3.75% Ta-doped films, which might be induced by the high reactivity of Ta with oxygen atoms. An increase in resistivity as the Ta concentration changes from 3.75% to 7.13% may be partially explained by the attribution
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352
Appl. Phys. Lett., Vol. 78, No. 3, 15 January 2001
Lee, Kim, and Chen
FIG. 3. Cross-sectional TEM micrographs of bright-field 共a兲 and 共c兲 and dark-field 共b兲 and 共d兲 images of the 3.75% Ta-doped 共a兲 and 共b兲 and 7.13% Ta-doped 共c兲 and 共d兲 SnO2 thin films.
of structural disorder, mainly grain-boundary scattering of small grains near the interface, in addition to the temperature and impurity scattering effects. In summary, we have reported the growth and properties of Ta-doped SnO2 thin films with relative Ta concentration between 0% and 7.31%. The film doped with 3.75% exhibited of 2.01⫻10⫺4 ⍀•cm, n of 1.27⫻1021 cm⫺3, and of 24.5 cm2/V s at room temperature. The electrical property measurement showed that doped films are degenerate semiconductors. This work was supported by U.S. Department of Energy Grant No. DEFG02-96ER45439 through the Frederick Seitz Materials Research Laboratory at the University of Illinois at Urbana–Champaign.
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