ОПТИКА И СПЕКТРОСКОПИЯ, 2014, том 117, № 3, с. 438-442
СПЕКТРОСКОПИЯ КОНДЕНСИРОВАННОГО СОСТОЯНИЯ
y%K 535.51:538.9
EFFECTS OF PROCESS PARAMETERS ON THE OPTICAL CONSTANTS OF HIGHLY TEXTURED V2O5 THIN FILMS
© 2014 n V. V. Atuchin*, **, V. A. Kochubey*, **, L. D. Pokrovsky*, **, V. N. Kruchinin***, and C. V. Ramana****
*Laboratory of Optical Materials and Structures, Institute of Semiconductor Physics, SB RAS, 630090 Novosibirsk, Russia **FunctionalElectronics Laboratory, Tomsk State University, Tomsk 634050, Russia; Laboratory of Semiconductor and Dielectric Materials, Novosibirsk State University, 630090 Novosibirsk, Russia ***Laboratory for Ellipsometry of Semiconductor Materials and Structures, Institute of Semiconductor Physics,
630090 Novosibirsk, Russia ****Department of Mechanical Engineering, University of Texas at El Paso, El Paso, 79968 Texas, USA
E-mail: atuchin@thermo.isp.nsc.ru Received January 16, 2014
The optical properties of the highly-textured V2O5 thin films grown on Si(100) by sputter-deposition at various oxygen reactive pressures were investigated in detail. The profiles of the optical constants, namely the refractive index and extinction coefficient, of V2O5 films were evaluated in the photon-energy range of 1— 5 eV. At photon-energy above 2.5 eV, the dispersion behavior in optical constants is explained based on Lorentz-Drude model. The refractive index dispersion fits to a Cauchy's relation at photon-energy below 2.5 eV, where the V2O5-film is mostly transparent. The optical transitions across the bandgap occur at energy ~2.5—3.2 eV depending on the V2O5 growth conditions and film-microstructure. The highly-textured and c-axis oriented V2O5-films, fabricated under optimum conditions of temperature and oxygen partial pressure, exhibit excellent optical characteristics similar to V2O5 single crystals.
DOI: 10.7868/S0030403414090037
INTRODUCTION
Vanadium pentoxide (V2O5) is an important material for many technological applications [1]. The most stable oxide in the V—O system exhibits a semiconductor-metal transition at ~250°C. V2O5 thin films exhibit multicolored electrochromism and have high potential for use in electrochromic display devices, color filters, and other optical and optoelectronic devices [1—8]. V2O5 thin films can also be used in variable reflectance mirrors, smart windows, and surfaces with tunable emittance for temperature control of space vehicles [1, 3, 5, 6]. The layered-structure and lithium-ion intercalation ability makes V2O5 an interesting electrode material for application in lithium-microbatteries [9—12]. It was reported that the heat treatment of bulk and thin-film V2O5 results in nano-structurization of material with a final chemical composition and geometry dependent on temperature and atmosphere composition [10, 13]. These nano-phase V oxides exhibit enhanced functionality and are promising for industrial applications. Furthermore, most recently, it was demonstrated that a thin V2O5 layer can significantly enhance the hole-injection from a p-SI anode which finds application in organic light-emitting diodes (OLED) [14]. The decrease in 'turn-on voltage' and enhanced maximum 'current ef-
ficiency' of the ^-Si/V2O5 device, when compared to other configurations or any other oxides, could potentially benefit the future OLED technology.
Despite the wide range of technological applications, it is well known that the optical and electrical properties of V2O5 films are highly dependent on the microstructure, which in turn are controlled by the film-fabrication technique, growth conditions, and post-deposition processes [15]. Evaluation of the optical properties and profiles of optical constants, the refractive index (n) and extinction coefficient (k), is even more important to optimize the conditions to produce high-quality V2O5 films for optical applications. A comprehensive understanding of the optical constants will further enhance our ability to meet the requirements of a given device-application involving V2O5 optical films. In this work, spectroscopic ellipsometry (SE), which is known to be a sensitive and nondestructive method for structure and optical characterization of thin-film materials, has been employed in combination with reflective high energy electron diffraction (RHEED) to study the optical properties of the textured and oriented V2O5/Si(100) system in a photon-energy (E) range of1—5 eV [16, 17]. The most significant feature of the work is that the highly-textured and c-axis oriented V2O5 films grown by magnetron-sputter deposition exhibit optical characteristics
similar to high-quality optical V2O5 films. Our results demonstrate that the fine tuning and/or optimization of thermodynamic variables namely temperature and oxygen partial pressure results in the formation of high-quality optical V2O5 films. The qualitative and quantitative information obtained on the optical properties of highly-textured V2O5 films is reported and discussed in this article.
EXPERIMENTAL
V2O5 thin films were deposited using reactive DC magnetron-sputtering of a V-metal target in the O2 + Ar atmosphere. The deposition was made onto chemically well-cleaned Si(100) substrates. The deposition chamber was evacuated down to a pressure of 5 x 10-4 Pa prior to film-fabrication. The (O2 + Ar) gas mixture was introduced into the chamber for reactive deposition and the pressure maintained during deposition was ~0.1 Pa. The DC power during deposition was 255 W. The distance between V-target and substrate was maintained at 23 cm. The substrate temperature (Ts) during deposition was 290°C, an optimum temperature to produce crystalline V2O5-films. Optimization of Ts was performed by detecting the V2O5 crystallinity as a function of Ts. At Ts < 290°C, V2O5 films were amorphous and adhesion to the Si substrate was poor. Therefore, Ts was maintained at 290 ± 2°C to produce V2O5 films as a function of varying oxygen reactive-atmosphere (pO2) in the range of 30—50%. The grown V2O5 films were characterized for phase composition and surface-texture using reflection high energy electron diffraction (RHEED) studies made using the EFZ4 device (Carl Zeiss). An electron-beam of 65 keV is directed at the sample and the images are recorded by the electrons diffracted from surface. To probe the optical properties, SE measurements were made on the V2O5 films using a Spectroscan ellipsom-eter in the spectral range of 250 < X < 900 nm at an incidence angle of 70°.
DISCUSSION
The most important observation derived from RHEED patterns is the V2O5-film texture and morphology, which is sensitive to the reactive gas-mixture and highly dependent on the pO2. The formation of V2O5 textured film with a preferred [001] orientation has been found in RHEED patterns for deposition with a pO2 in the range of 32.7-48.9%. The RHEED patterns obtained as a function of pO2 are shown in Fig. 1. The RHEED patterns shown in Figs. 1a, b, and c represent V2O5 films grown at a pO2 of 32.7, 40.8, and 48.9% (referred to samples 1, 2, and 3, respectively, from hereafter), respectively. The surface phase in all these samples has been identified as V2O5 but the texture development is different as seen in
4
(b)
(c)
Fig. 1. RHEED patterns of V2O5 thin films grown at pO2 = 32.7% (a), 40.8% (b), and 48.9% (c).
RHEED patterns. As it appears, higher pO2 results in the faster oxidation and formation of V-oxide phase. However, the crystal grains are small and less ordered. This is, perhaps, due to higher pO2, which decreases the inter-mass-transport on the Si-substrate surface and introduces morphological disorder. The size of V2O5 grains increases with decreasing pO2 leading to a semi-spot structure in RHEED pattern (Fig. 1a).
Optical constants and bandgap of V2O5 thin films have been primarily probed by SE, which measures the relative changes in the amplitude and phase of the linearly polarized monochromatic incident light upon oblique reflection from the film surface [16, 17]. The experimental parameters obtained by SE are the angles T (azimuth) and A (phase change), which are related to the microstructure and optical properties, defined by
p = Rp/Rs = tanTexp(ZA),
(1)
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ATUCHIN et al.
where Rp and Rs are the complex reflection coefficients of the light polarized parallel and perpendicular to the plane of incidence, respectively. The spectral dependencies of ellipsometric parameters W and A can be fitted with appropriate models to extract film thickness and the optical constants (n and k) based on the best fit between experimental and simulated spectra [17—20]. The spectral dependencies of the parameters, W and A, determined for sputter-deposited V2O5 thin films on Si are shown in Fig. 2. The general trend of the curves is more or less similar for all the V2O5 films. The dispersion relations of n(X) and k(X) are derived from the single-layer optical model which consists of [air]/[isotropic homogeneous film (V2O5)]/[isotropic substrate (Si)]. In the spectral range X > 500 nm, where V2O5-film can be considered as transparent (k = 0), the refractive index dispersion was simulated with three parameter Cauchy polynomial:
n(X) = a + b/X2 + c/X4. (2)
In the spectral range X < 500 nm the Lorentz-Drude dispersion model was used. In this model,
8(E) = 8 w--
J\D
E — iE2dE
m
+1
n=1
A„E„
En E + ir nEnE
(3)
20
e
-d
15
10
if \ • /-it ]\
\\ /♦'/ • \ /.'/
/v
\\ /•'/ />
120
f 80
40
where 8^ is a high frequency dielectric constant (8 at E ^ ro) and is a non-dispersive term, second term is a Drude term representing the free carrier contribution, E is the photon energy (eV), and E1D & E2D are the constants. Third term is related the interband transitions described by Lorenz damped harmonic oscillators, where An, En, and rn are strengths, energies and broadenings, respectively, ofoscillator with number n [2].
The n and k profiles derived using W(E) and A(E) data are shown in Fig. 3 for V2O5 films gro
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