DOI:10.30919/es8d1003

Received: 30 Jan 2020
Revised: 07 Mar 2020
Accepted: 08 Mar 2020
Published online: 10 Mar 2020

Temperature dependent studies on radio frequency sputtered Al doped ZnO thin films

Pankaj K. Bhujbal, Habib M. Pathan and Nandu B. Chaure*

 

Department of Physics, Savitribai Phule Pune University, (formerly University of Pune) Pune 411007, India

* E-mail: n.chaure@physics.unipune.ac.in

Abstract

In this work, highly conductive and transparent Al: ZnO (Al doped ZnO, i. e., AZO) thin films were grown by radio frequency (RF) magnetron sputtering technique at a typical deposition temperature. The effect of deposition temperature on the structural, morphological, optical and electrical properties was studied. The x-ray diffraction (XRD) studies revealed a hexagonal wurtzite crystal structure for all AZO layers with a (002) preferred orientation along the c-axis. Columnar, compact, uniform grain growth of the layer was observed from atomic force microscopy (AFM) images. The deposition temperature had an influence on the surface roughness and average grain size of deposited films, which could be confirmed by means of AFM images. Optical studies confirmed that both optical band gap energy and urbach energy were influenced by the substrate temperature. Highly transparent films with an energy band gap ranging from 3.48 to 3.65 eV were obtained upon changing the deposition temperature from 22 to 400℃. The presence of defects was confirmed by photoluminescence (PL) spectra. A systematic measurement of the electrical parameters like barrier height, and ideality factor of the devices (Ag/Al:ZnO) was carried out with the help of I-V characteristic. This study may be useful for the design and fabrication of AZO based electrodes for solar cell applications.

Keywords: Al:ZnO; RF magnetron sputtering; Urbach energy; Ideality factor; Deposition temperature


1. Introduction

Transparent conducting oxides (TCOs) have numerous applications in optoelectronics and electronic devices like light emitting diodes (LEDs), flexible electronic devices, flat panel displays, solar cells,[1-3] sensors,[4-6] etc. Indium tin oxide (ITO) is one of the most commonly used TCOs, however, the toxicity, limited availability and high cost of the indium are major issues. Therefore, the prime concern is to find an alternative to the ITO material.

Al doped zinc oxide (Al:ZnO, i.e., AZO) is the most promoting alternative because of its environmental stability and transparency. ZnO is a n-type semiconductor with a wide band gap. Pristine ZnO shows low and unstable electrical properties, like conductivity and mobility. Doping with some transition metals can modify the properties of ZnO thin films. For example, group III elements like Ba,[7] Al,[8] Ga[7, 9] and In[7] are deposited into ZnO, which allows to achieve both high optical transparency and high conductivity. Among the III group elements, Al is commonly used to improve the properties of pristine ZnO, i. e., transparency, stability, and conductivity. AZO thin films have gained much interest due to their large exciton binding energy (60 meV) at 22, wide optical energy band gap (3.37 eV) and high chemical stability.[10] Along with a high optical transparency in the visible range, AZO films have superior electrical conductivity, good thermal and chemical stability[11]

Numerous methods and techniques have been reported to deposit AZO films like chemical vapor deposition (CVD),[12] RF magnetron sputtering, [11,13,14,15] sol-gel method,[16] spray pyrolysis,[17-19] and pulsed laser deposition.[20-23] RF magnetron sputtering is one of the promising methods for the deposition of AZO films because it offers film deposition at lower temperatures as well as a better adhesion than other methods. It is popularly used in industry because its parameters can be easily controlled, the films can be obtained with a high packing density and strong adhesions at a relatively high deposition rate.[24] The physical such as electrical properties of AZO films are influenced by various deposition parameters like Al2O3 content in the target, deposition time, RF power, Ar gas pressure, and deposition temperature.[25] Thus, it is important to study the effects of the deposition parameter on the deposition of the AZO films. Deposition temperature is one of the most significant parameters as it can tune electrical, optical and structural properties.

In order to study various electrical parameters like barrier height, ideality factor, saturation current, etc, Ohmic and Schottky contacts are normally made. For AZO, it is more difficult to fabricate rectifying or Schottky contacts than Ohmic contacts. The reasons can be the diffusion of metal into the semiconductor, chemical reactions between metal and semiconductor and the defects in the surface region.[26-27] Schottky contacts are mostly used in many applications like the stand-alone photovoltaic systems, power supply, etc. Many researchers found that n-ZnO along with Au, Ag, and Pd (low-reactive metals) can form comparatively high Schottky barriers.[28-30] For example, Ozgur et al. reported that to make the Schottky barrier, a high work function metal has to be applied to the surface of a ZnO.[27,31] The thermal stability of Au/n-ZnO Schottky junction has not been studied. It has reported that at high temperatures (>330K), Au based contacts have some serious issues like the degradation of samples with the thermal cycling and their poor I-V characteristics.[29,30,32,33] Simpson et al.[34] found that the thermal stability of the Au/ n- ZnO based Schottky contact was lower than that of Ag based Schottky contact. In 2002, Sheng et al.[35] developed the Ag/ZnO Schottky diode, studied its electrical characteristics and reported the electrical parameters like ideality factor and barrier height as 1.33 and 0.89 eV. Keskenler et al.[27] fabricated an Ag/n-ZnO/p-Si/Al heterojunction diode by the sol-gel spin coating technique and found the barrier height and ideality factor of 0.71 and 2.03 eV, respectively. In 2013, 

Dondapati et al.[36] investigated the optical as well as plasmonic properties of AZO thin films for various substrate temperatures to understand the fundamentals of carrier generation and transport characteristics. Aliasghar et al.[37] prepared AZO film by DC-magnetron sputtering and found that the band-gap energy and urbach energy of AZO film was 3.2 and 0.4 eV, respectively. They also calculated the barrier height and ideality factor to be about 10 and 0.3 eV, respectively for Au/Al: ZnO devices. Pathirane et al.[38] reported the AZO/Ag nanowire (NW) electrodes for solar cells with Ag NW coated on the AZO. However, the barrier height and ideality factor for this particular device were not reported.

Herein, the AZO films were grown by RF magnetron sputtering and the effects of substrate temperature on the microstructural, morphological, optical and electrical properties were investigated and the relations between them were established. The carrier generation and current transport mechanisms of AZO films were also studied. The optical behavior of AZO films along with urbach energy and band gap energy was described as well. In order to study the electrical measurement, the Ag contacts on the AZO film were made by using the thermal evaporation technique.

2. Experimental

2.1 AZO thin film preparation

An AZO film was grown by the RF magnetron sputtering onto a microscopic glass substrate. The substrates were cleaned in acetone and ethanol by using an ultrasonic cleaning. Fig. 1A shows a schematic diagram of the RF magnetron sputtering system, which was used to grow AZO thin films.

Fig. 1 Schematic diagram of (A) RF magnetron sputtering, (B) thermal evaporation system, and (C) proposed device, used for electrical measurements.

 

The AZO target with a Cu bonding (99.99% purity, Al: ZnO = 2:98 wt% ) was procured from Testbourne Ltd. UK. The turbo molecular pump was used to evacuate the sputtering chamber at a base pressure lower than 10-4 torr. The flow rate of Ar (80 sccm) was controlled by a mass flow controller. The RF power was set at a constant 200 W. The AZO film depositions were carried out in the temperature range from 22 to 400 for 15 minutes for all samples. Table 1 shows the parameters used for the deposition of AZO film.

Table 1. The parameters used for the deposition of AZO film.

Parameter

Number

Target

AZO

Thickness of the target (mm)

3

Diameter of the target (mm)

76.2

Weight ratio of Al to ZnO

2 : 98

Chamber base pressure (torr)

< 10-4

Ar flow rate (cm³/min)

80

RF power (W)

200

Deposition time (min)

15

Temperature ()

22-400

 

2.2 Fabrication of Device (Ag / Al:ZnO)

Fig. 1C shows the fabricated Al /Al:ZnO device for the present study. For the fabrication of proposed device, the Ag metal contacts were made on the top surface of AZO films by thermal evaporation technique, (Fig. 1B). In the present systematic study, the effects of deposition temperature on the microstructural, morphological, optical andelectrical properties of Ag /Al: ZnO junction were investigatedby the thermionic emission theory.

2.3. Characterizations

The physical properties, microstructures, morphologies, optical properties and electrical properties of the AZO thin films were studied systematically under the influence of deposition temperatures. The optical band gap, specular transmittance, urbach energy and absorption were carried out by JASCOV-670 UV-Vis spectrophotometer. The electronic structures and defect related transitions of AZO films were studied using a Perkin Elmer LS-55 photoluminescence spectrometer. The film thicknesses were obtained from the fringes observed in the transmission spectra using Swanepole’s calculations. The structural properties of the AZO films were performed to investigate the crystallinity, crystallite size and crystal orientation of the deposited films using X-ray diffractometer, model Bruker D8 with Cu Kα radiation having a wavelength of 0.154nm. The atomic force microscopy (AFM) was used to study the surface morphology. The DC electrical measurements, i. e. ideality factor, saturation current, barrier height, electrical conductivity and resistivity of the deposited AZO films, were carried out by MODEL 

4200-SCS Semiconductor Characterization System and four-probe measurement unit.

 

3. Results and discussion

3.1 X-ray diffraction (XRD)

Fig. 2 shows the typical XRD pattern of AZO films deposited at different deposition temperatures. Fig. 3 shows the magnified part of the XRD patterns for the detailed study of (004) diffraction peaks. The AZO films were grown at 22 to 400℃. All films show the peak at about 34°, corresponding to the (002) reflection of hexagonal wurtzite crystal structure (JCPDS card No 36-1451).[11,39,40] It has been observed that the (002) and (004) diffraction peak intensity increases with increasing the deposition temperature up to 300℃ due to the enhancement in the crystallinity of AZO films. This enhancement may be related to the increases of atomic mobility and surface diffusion of the adsorbed species, the reduction of structural defects, and the increased size of the grains. With the additional increase in the deposition temperature, the intensity of (002) and (004) peak was found to be decreased and an additional peak (101) appeared around 72.5°. This could be associated with the decreased crystallinity at a higher temperature (such as 400 ℃), which is attributed to the stress and lattice mismatch[13,41] between the surface of microscopic glass substrate and AZO layer.

Fig. 2 XRD patterns of AZO films deposited at (a) 22, (b) 100, (c) 200, (d) 300 and (e) 400℃.

Debye formula was used to determine the average crystallite size (d) of AZO films by using Equation (1):

                                    (1)