Electrical and Optical Transport Characterizations of Electron Beam Evaporated V Doped In₂O₃ Thin Films

Abstract

Vanadium (5 at. %) doped Indium Oxide (V: In₂O₃) thin films with different thicknesses (50 nm, 100 nm and 150 nm) were prepared onto glass substrate by electron beam evaporation technique in a vacuum of about 4×10^-3 Pa. X-ray diffraction (XRD) pattern revealed that the prepared films of thickness 50 nm are amorphous in nature. Temperature dependence of electrical resistivity was studied in the 300 < T < 475 K temperature range. The films exhibit a metallic behavior in the 300 < T < 380 K range with a positive temperature coefficient of the resistivity (TCR), whereas at T > 380 K, the conduction behavior turns into a semiconductor with a negative TCR. Optical studies revealed that the films of thickness 50 nm possess high transmittance of about 86 % in the near-infrared spectral region. The direct optical band gap lies between 3.26 and 3.00 eV depending on the film thickness.

Keywords:
Transparent conducting oxide; Activation energy; Indium oxide; V: In₂O₃ Thin films; Optical band gap

1. Introduction

Transparent conducting oxide (TCO) has become the essential part of the optoelectronic applications due to its excellent transparent and conductive properties. TCO has unique characteristics such as low electrical resistivity (< 10^-3 Ω-cm) and high optical transmittance in the visible region (> 80 %) with a wide energy band gap (3.70 eV)^1-3. Due to these combined unique properties, TCO has been used in a wide range of applications such as flat panel displays, photovoltaic cells, light emitting diodes, solar cells, barrier layers in tunnel junctions, and thin film transistor (TFT)^4-8.

Literature reports on TCO films revealed that some TCO thin films have interesting physical phenomena such as metal-semiconductor transitions (MSTs). These transitions occur at a wide temperature range and are observed in doped TCO thin films^9-15. Among the various TCO thin films like ITO, CdO, ZnO, SnO_2, Indium oxide (In₂O₃) has been widely used in flat panel displays, opto-electronic modulators, liquid crystal displays, solar cells, architectural glasses and photovoltaic devices. A variety of electrical properties such as metallic, semiconducting, or insulating behavior can be obtained depending on its stoichiometric form (In₂O₃)¹⁶. The structural and opto-electrical properties of oxide materials like In₂O₃ thin films are highly influenced by doping impurities. The impurities like tin (Sn), zinc (Zn), gallium (Ga), copper (Cu), zirconium (Zr), erbium (Er), molybdenum (Mo) and titanium (Ti) have been doped to In₂O₃ to produce technologically useful materials and studied extensively by many researchers^17-23. Compared to these dopants, vanadium is one of the interesting and suitable external dopant in our study because indium has a valence of three, and vanadium has a valence of five. In V: In₂O₃ samples, vanadium acts as a cationic dopant in the indium oxide lattice, which results in n-doping of the lattice by providing one or two electrons to the conduction band. If vanadium substitutes with indium, it provides free electrons into the lattice and the electrical conduction will increase but it also acts as a neutral impurity scattering center and decreases the electrical conduction when combined with interstitial oxygen atoms. Its smaller ionic radius (78 pm) compared with (94 pm) for In³⁺ causes local strains and enhance the formation of grain boundaries. A variety of deposition techniques have been used for the preparation of undoped and doped In₂O₃ films such as reactive thermal evaporation method, electro-deposition, radio frequency magnetron sputtering^24-27. Li et al. reported a transparent conducting V: In₂O₃ thin films for hole injection in organic light-emitting devices (OLEDs) prepared by a modification-specific reactive thermal co-evaporation method, which shows a minimum electrical resistivity of 7.95×10^-4 Ω-cm and good optical transmittance in the visible spectra range²⁴. Recently, Seki et al. fabricated transparent V: In₂O₃ thin films for OLED by spray chemical vapor deposition²⁸ and found the minimum resistivity of 1.08 × 10^-3 Ω-cm and average transmittance in the visible range of 84 %. V: In₂O₃ nanofibers can be used as a gas sensor at low temperature²⁹. Although there have been a number of investigations on the structural, electrical and optical properties of the above-mentioned doped In₂O₃ films, no systematic study has been paid on the electrical and optical properties using vanadium as a dopant.

In this article, vanadium (5 at. %) doped indium oxide (V: In₂O₃) thin films were prepared onto glass substrate by the electron beam evaporation technique. The influence of thickness on the electrical and optical properties of the V: In₂O₃ thin films were investigated.

2. Experimental details

Vanadium doped (5 at. %) indium oxide thin films were prepared onto glass substrate by electron beam evaporation technique in vacuum at ~ 3×10^-4 pa from In₂O₃ powder (99.999 % pure) and vanadium powder (99.999 % pure) obtained from Aldrich Chemical Company, USA. Each material was weighted by an electronic balance (Mettler TOLEDO, AB 204) having a resolution of ± 0.0001 g. The percentage of composition was determined as³⁰:

(1) $\mathit{Weight}\%\mathit{V}=\frac{{\mathit{w}}_{\mathit{V}}}{{\mathit{w}}_{\mathit{V}}+{\mathit{w}}_{\mathit{InO}}}\times 100\%$

Where W_v and W_InO are the weight of V and In₂O₃, respectively. When the chamber pressure was reduced to 3×10^-4 pa, deposition was started with beam currents 60 mA by turning on the low tension (LT) switch. The deposition rates of VIO thin films are about 12.5 nms^-1.

The thicknesses of the films were determined using interference Fizeau fringes method³¹. The electrical contacts required for resistivity measurements were made with silver paste (leading silver D-200) above the films. Structural property of the V: In₂O₃ film was carried out in a PHYLIPS PW3040 X'Pert PRO XRD System. X-ray diffractogram of the sample was recorded using monochromatic CuK_α radiation (λ=1.54187 Å), scanning speed 2 degree/min, starting from 8⁰ and ending at 80⁰. The surface morphology of the film was studied using in a HITACHI S-3400N Scanning Electron Microscopy (SEM) system.

Temperature dependent electrical resistivity (ρ) measurements were performed by Van-der-Pauw technique using a standard four-probe setup³². The electrical contacts were made in the four corners using silver paste (leading silver D-200) above the films. Resistance was measured by applying a current through the sample, and measuring the voltage by digital multi-meters (KENWOOD DL-711), respectively. The optical transmittance spectra of films were recorded from 300 < T < 1100 nm wavelength using a SHIMADZU UV- double beam spectrophotometer at room temperature. To determine the band gap of the V: In₂O₃ thin films optically, the plot of (αhν)² vs. (hν) was drawn for direct allowed transition. The value of the band gap was determined for the tangent of these curves which intersect the energy axis.

3. Results and Discussion

3.1. Structural Properties

XRD pattern of the as-deposited V: In₂O₃ thin film with thickness of 50 nm is shown in Figure 1. In the absence of sharp diffraction peaks in the XRD pattern indicates that the film is amorphous in nature. Similar result was observed in e-beam evaporated indium tin oxide films¹⁰.

Figure 1
XRD pattern of the as-deposited V: In₂O₃ film with thickness of 50 nm.

Scanning Electron Microscopy (SEM) was used to study the surface morphology of V: In₂O₃ thin films. Figure 2 shows the SEM micrograph of the as-deposited V: In₂O₃ thin film with different thicknesses. The films display uniform morphology and possess nearly distributed grains over the surface.

Figure 2
SEM micrograph of the as-deposited V: In₂O₃ film with different thicknesses.

3.2. Electrical properties

Figure 3 shows the resistivity versus temperature for a few typical V: In₂O₃ films with different thicknesses of 50 nm, 100 nm and 150 nm in the 300 <T < 475 K range. From Table 1 it is observed that the resistivity of V: In₂O₃ films strongly depended on the films thickness. At room temperature, the resistivity of the films decreases from 1.80×10^-1 to 1.10×10^-2 Ω-cm with increasing the films thickness. The decrease in resistivity with the films thickness may be attributed to the lesser relative contribution of the carrier scattering at the films surface³³. Vanadium doping affects the resistivity of the V: In₂O₃ thin films. The minimum resistivity of 1.10 ×10^-2 Ω-cm is observed in V: In₂O₃ which is larger than that for pure indium oxide (~2.00 × 10^-4 Ω-cm)³⁴ and indium tin oxide (~1.9 × 10^-4 Ω-cm) thin films³⁵. The reason for the increase in resistivity is that some V atoms may occupy interstitial positions and may also form defects such as V₂O and V₂O₅, which acts as a carrier traps rather than electron donors. As observed from Figure 3, the resistivity for all films increases with increasing the temperature up to 380 K, showing metallic behavior. At approximately 380 K, the resistivity begins to decrease rapidly, and above 415 K, ρ decreases slowly with further increasing the temperature. The films display a semiconductor behavior at T > 380 K, and a minimum resistivity of 9.00 × 10^-4 Ω-cm is found at 475 K. Metallic and semiconductor behavior are evidenced in the resistivity measurements as a function of temperature: a metal-semiconductor transition (MST).

Figure 3
Temperature dependence of the resistivity (ρ) as a function of temperature (T) of V: In₂O₃ thin films with different thicknesses.

Table 1
Dependence of resistivity on the films thickness at various temperatures.

No.	Thickness (nm)	Resistivity (Ω-cm)
		At 300 K	At 380 K	At 475K
1	50	1.80 ×10-1	3.30 ×10-1	9.00 ×10-4
2	100	3.60 ×10-2	6.60 ×10-2	1.40 ×10-3
3	150	1.10 ×10-2	2.10 ×10-2	1.70 ×10-3

The increase in resistivity with temperature is resulted mainly from a decrease in carrier concentration and Hall mobility. Some vanadium atoms may form point defect clusters which act as a carrier trap as explained above. The decrease in carrier concentration and mobility with increasing the temperature may be due to an increase in disorder of the crystal lattice, which causes phonon scattering and ionized impurity scattering and results in an increase in resistivity. At approximately 380 K, this point defect clusters are begun to rearrange and remove by small scale diffusion of Vanadium, thereby reducing in carrier scattering, consequently reducing the resistivity. Many theories have been developed to explain defect removal during annealing^36-38. Thus, rearrangement and elimination of point defect clusters leads to an increase in carrier concentration, thereby reducing resistivity.

Figure 4 shows that the films exhibit a positive TCR in the 300 < T < 380 K temperature range, whereas the films display a semiconductor behavior with a negative TCR in the 380 < T < 475 K range. The activation energy calculated from temperature dependent electrical conductivity measurements of V: In₂O₃ films. The plot of ln (σ) vs. 1000/T in the 380 < T< 475 K temperature range is shown in Figure 5. The plot is found to have two regions with two different slops with the point of inflection around 425 K. The presence of two regions with two different slopes in the plot may be due to the presence of two types of conduction in the V: In₂O₃ films, such as 380 < T < 425 K and above 425 K in the investigated temperature range. In the 380-425 K range, V may be provided electrons in the lattice which are activated to the localized state and this state moves toward the conduction band and thus increased conductivity. And above 425 K, impurity-impurity scattering may be increased which leads to decrease in the electrical conduction. Fitting the data using the linear equation:

(2) $\mathit{\sigma }={\mathit{\sigma }}_0\exp \left(-\Delta E/2K_{B}T\right)$

Figure 4
Variation of T.C.R. with temperature for the V: In₂O₃ thin films with different thicknesses.

Figure 5
Plot of ln (σ) versus 1000/T of V: In₂O₃ thin films with different thicknesses in the temperature range 385-475 K.

where ΔE is the activation energy, σ₀ is the pre-exponential factor and K_B is the Boltzmann constant and T is the absolute temperature. In the 380 < T < 425 K temperature range, almost average ΔE of 8.25 eV is found, and at T > 425 K, ΔE is found to decrease from 0.40 to 0.20 eV with increasing the films thickness.

3.3. Optical properties

UV-VIS spectra of the films were studied using the optical transmittance measurements which were taken in the spectral region from 300 < T < 1100 nm. The variation of optical transmittance as a function of wavelength for V: In₂O₃ thin films with various thicknesses, as shown in Figure 6. It is seen from the figure that transmittance is greatly affected by films thickness, the thinner films of thickness 50 nm show relatively higher transmittance of about 86 % in the near infrared region. The films of thickness 100 nm are transparent with transmission of about 80 % and the thicker films of thickness 150 nm are relatively less transparent with transmission of about 75 % in the near infrared region. The reduction of transmittance with thickness may be attributed to the optical scattering arising from longer optical paths and also to changes in the carrier concentration^39,40. In the visible region, transmittance of all films decreases sharply with decreasing the wavelength up to 620 nm, and after which, it remains almost constant with further increasing the wavelength up to 400 nm. This decrease of transmittance with the increase in photon energy is due to the absorption of free carrier.

Figure 6
Spectral transmittance of V: In₂O₃ thin films as a function of wavelength with different thicknesses.

The absorption coefficient (α) was calculated from the transmittance (t) using the relation⁴¹:

(3) $\mathit{\alpha }=1/d\left[\ln \left(1/\mathit{T}\right)\right]$

where d is the film thickness and T is the transmittance. Figure 7 shows the variation of absorption coefficient with the photon energy (hν) of V: In₂O₃ thin films of different thicknesses. The absorption coefficient for all films is found to be the order of 10⁴ cm^-1. The optical band gap (Eg) was determined from the absorption coefficient (α) and photon energy (hγ) using the following relation:

(4) $\mathit{\alpha }\mathit{hv}=\mathit{A}{\left(\mathit{hv}-\mathit{Eg}\right)}^{\mathit{m}}$

Figure 7
Absorption coefficient (α) as a function of photon energy (hυ) of V: In₂O₃ films with different thicknesses.

where E_g is the optical band gap, hγ is the energy of the incident photon, and A is a constant and m takes the value of ½ and 2 for direct and indirect allowed transition, respectively. The direct allowed bang gap is obtained by plotting (αhυ)² against photon energy and extrapolating the linear part of the plots to zero absorption (α=0). Figures 8 (a)-(c) show the (αhυ)² versus (hυ) plots for various thicknesses. It is observed that E_g decreases from 3.26 eV to 3.00 eV with increasing thickness. This is possibly due to the increase in particle size and decrease in strain and dislocation density. The doping of vanadium into the indium oxide reduces the band gap (band gap of pure indium oxide is 3.75 eV⁴²). This band gap reduction may be attributed to the decrease in carrier concentration because some V causes disorder in the indium oxide lattice and V atoms acts as a carrier trap instead of electron donors. This decrease in carrier density shifted the absorption edge towards lower energies. The band gaps of V: In₂O₃ films at various films thicknesses are inserted in Table 2. The resistivity, optical transmittance and direct band gap of the V: In₂O₃ thin films of the pervious works are compared in Table 3 with the present work.

Figure 8
(αhυ)² versus photon energy ( hυ) plots for V: In₂O₃ films with different thicknesses of (a) 50 nm, (b) 100 nm and (c) 150 nm.

Table 2
Optical band gap of V: In2O3 thin films with different thicknesses

No.	Thickness (nm)	AVT (%) (700-1000 nm)	Band gap (eV)
1	50	86	3.26
2	100	80	3.16
3	150	75	3.00

• AVT average transmittance

Table 3
Comparison of electrical and optical properties of V: In2O3 thin films with previous reported works of different TCO films by various techniques.

Technique	TCOs	(wt. %)	Thickness (nm)	ρ (Ω-cm)	AVT (%) (700-1100 nm)	Eg (eV)	Refs.
E-beam	V:In2O3	5.0 % V	50	1.80 ×10-1 (RT)	86	3.26	Present work
			100	3.60 ×10-2 (RT)	80	3.16
			150	1.10 ×10-2 (RT)	75	3.00
MSRTCE	V:In2O3	1.80 % V	-	7.95×10-4	84	-	[ 24 ]
		3.05 % V		8.05×10-4	84
SCVD	V:In2O3	1.50 % V	-	1.10×10−3	84	-	[ 28 ]
Thermal	Mo:In2O3	5.0 % Mo	-	5.20×10-4	90	3.68	[ 17 ]
PLD	Sn:In2O3	5.0 % SnO2	150	4.00×10-4	85	3.89	[ 35 ]
			170	2.00×10-4	92
MOCVD	Ga:In2O3	0.1 % Ga	-	2.20×10−3	78	3.89	[ 20 ]
		0.9 % Ga	-	1.90		4.46
RF Sputtering	Ti:In2O3	2.5 % TiO2	-	1.20×10-4	75	-	[ 43 ]
ALD	Zr: In2O3		-	3.70×10−4	-	-	[ 22 ]

• MSRTCE modification-specific reactive thermal co-evaporation, SCVD spray chemical vapor deposition, PLD pulsed laser deposition, MOCVD Metal-Organic Chemical Vapour Deposition, ALD atomic layer deposition

4. Conclusions

Vanadium (5 at. %) doped indium oxide thin film was prepared onto glass substrate by electron-beam evaporation method. XRD analysis indicated that the V: In₂O₃ film is an amorphous one. Temperature dependent electrical resistivity were carried out in the 300 < T < 475 K temperature range. The films exhibit a metallic behavior with a positive TCR in the 300 < T < 380 K range. Semiconductor conductivity with metal-semiconductor transition is observed at T > 380 K in V: In₂O₃. For the film with 50 nm thickness, the resistivity value is close to 9.00 × 10^-4 Ω-cm at 415 K and the transmittance is ~86 % in the near-infrared region. The optical band gap depends on the film thickness, which decreases from 3.26 to 3.00 eV. These structural, electrical and optical properties of V: In₂O₃ films are encouraging for application as a transparent conducting oxide and are suitable for many opto-electronic applications.

5. Acknowledgments

A. Islam is grateful to the Institute of Mining, Mineralogy & Metallurgy, BCSIR, Joypurhat, Bangladesh, for measuring the X- ray data. The authors also wish to thank Dr. Nuruzzaman and A. T. K. Karim for valuable discussions.

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• Erratum

  In the article "Electrical and Optical Transport Characterizations of Electron Beam Evaporated V Doped In2O3 Thin Films", DOI number: ;http://dx.doi.org/10.1590/1980-5373-mr-2015-0753, published in Mat. Res., 20(1): 102-108, in the first page where it read:
  Md. Ariful Islama *
  Ratan Chandra Royb
  Jakir Hossainb
  Md. Julkarnainb
  Khairul Alam Khanb
  ^aDepartment of Physics, Rajshahi University of Engineering & Technology - RUET, Rajshahi, Bangladesh
  ^bDepartment of Applied Physics & Electronic Engineering, University of Rajshahi, Rajshahi, Bangladesh
  It should be written:
  Md. Ariful Islama *
  Ratan Chandra Royb
  Jaker Hossainb
  Md. Julkarnainb
  Khairul Alam Khanb
  ^aDepartment of Physics, Rajshahi University of Engineering & Technology - RUET, Rajshahi, Bangladesh
  ^bDepartment of Applied Physics & Electronic Engineering, University of Rajshahi, Rajshahi, Bangladesh

Publication Dates

• Publication in this collection
  08 Dec 2016
• Date of issue
  Jan-Feb 2017

History

• Received
  08 Dec 2015
• Reviewed
  24 Aug 2016
• Accepted
  07 Sept 2016