Origin of high carrier mobility and low residual stress in RF superimposed DC sputtered Al doped ZnO thin film for next generation flexible devices

Highlights

• Investigated origin of high carrier mobility and low residual stress in AZO thin films for next generation flexible devices.

• Superimposing RF power onto DC Power controlled energy and flux of incident ions during sputtering process.

• Mixed RF/(RF+DC) sputtering process results in better crystallinity and low residual stress.

• XPS study shows a variation in defect density in AZO thin film with different RF/(RF+DC) ratios.

• The defects finally migrate to grain boundaries and controls the carrier mobility.

Abstract

In this work, the energy and flux of high energetic ions were controlled by RF superimposed DC sputtering process to increase the grain size and suppress grain boundary potential with minimum residual stress in Al doped ZnO (AZO) thin film. AZO thin films were deposited at different RF/(RF + DC) ratios by keeping total power same and were investigated for their electrical, optical, structural and nanoscale grain boundaries potential. All AZO thin film showed high crystallinity and orientation along (002) with peak shift as RF/(RF + DC) ratio increased from 0.0, pure DC, to 1.0, pure RF. This peak shift was correlated with high residual stress in as-grown thin film. AZO thin film grown at mixed RF/(RF + DC) of 0.75 showed high electron mobility, low residual stress and large crystallite size in comparison to other AZO thin films. The nanoscale grain boundary potential was mapped using Kelvin Probe Force Microscopy in all AZO thin film and it was observed that carrier mobility is controlled not only by grains size but also by grain boundary potential. The XPS analysis confirms the variation in oxygen vacancies and zinc interstitials which explain the origin of low grain boundaries potential and high carrier mobility in AZO thin film deposited at 0.75 RF/(RF + DC) ratio. This study proposes a new way to control the grain size and grain boundary potential to further tune the optoelectronic-mechanical properties of AZO thin films for next generation flexible and optoelectronic devices.

Introduction

High cost and scarcity of indium have made aluminium doped zinc oxide (AZO) an important material which might be used as a replacement of tin doped indium oxide (ITO) in next generation optoelectronic devices [1], [2], [3]. Al doped ZnO is a direct band gap (E_g = 3.2–3.3 eV) material which exhibits high transmittance (>80% in the visible region), high electrical conductivity (∼10³ S/cm), non-toxicity, less expensive and earth abundance in nature [4], [5], [6]. However, for next generation flexible devices low residual stress is equally important along with good optoelectronic properties in AZO thin films. Low carrier mobility of AZO thin films, however, is still an impediment that needs to be overcome and understood in depth [7], [8], [9], [10], [11], [12], [13]. AZO thin films are being used in solar cells, organic light emitting diodes (OLEDs), thin film transistors (TFTs) and photodetectors but due to inadequate electrical property, their usage in optoelectronic devices is limited [13], [14], [15], [16], [17]. Formation of grain boundaries in crystalline or polycrystalline thin films is the major cause that impedes the carrier mobility and hence their electrical properties. Scattering of carriers at the grain boundaries play a predominant role in controlling the electrical properties of AZO thin films. Therefore, many fabrication techniques have been employed to synthesize good quality AZO thin films including chemical vapour deposition (CVD) [18], [19], atomic layer deposition (ALD) [20], [21], pulse laser deposition (PLD) [22], [23], RF sputtering [7], [8], DC sputtering [10], [11] and other diverse techniques to control and improve their optoelectronic properties [24], [25].

Radio frequency (RF) and Direct current (DC) sputtering are the most widely and industrially adopted thin film deposition techniques for high purity and better controlled processing on large area. During the sputtering process, the energy and flux of ions indenting on the growing film, critically affect the physical and chemical properties of the growing films. In DC sputtering, high discharge voltage is required to maintain the glow discharge by secondary electron emission at target surface. While, in RF sputtering, ionization is achieved by the oscillating electrons in plasma, which results lower discharge voltage but high ions/neutral atoms ratio. Therefore, high discharge voltage in DC sputtering provides high energy to plasma species which have the tendency to modulate the physical and chemical properties of thin films. High discharge voltage in DC sputtering also gives high deposition rate which may give better electrical properties but due to high energy of impinging ions high residual stress can also be present in the growing film [26]. On the contrary, low discharge voltage and high ionization ratio in RF sputtering gives low energy and high flux of indenting ions on the growing film which results in low deposition rate and poor electrical properties but low residual stress [27], [28], [29]. In order to achieve desired and controlled physical and chemical properties of growing thin films, the energy and flux of plasma species need to be controlled. Therefore, the superimposition of RF power on to DC power is an exemplar technique use to tailor the physical and chemical properties of thin films by reducing the discharge voltage which subsequently reduces the energy and control the flux of ions in plasma. Norihiro et al. and Kumar co-workers observed the dependency of electrical and mechanical properties on the superimposition of RF power onto DC power along with structural and optical properties in AZO and IZO thin films, respectively[29], [30]. Similarly, Park and co-workers showed the effect of RF superimposed DC sputtering on optical and structural properties along with the chemical change in the composition of In in ZIO thin films with increase in the ratio of RF power to DC power (RF/(RF + DC) ratio) [31]. Hence, superimposition of RF power onto DC power has the tendency to modulate the electrical, structural, mechanical and optical properties of thin films [26], [31], [32], [33]. Ga doped zinc oxide (GZO) [34], Sn doped indium oxide (ITO) [27], [35] and indium doped zinc oxide (IZO) [29], [36] thin films were deposited by RF superimposed DC sputtering to obtain the desirable properties. RF, DC and RF/(RF + DC) sputtering techniques have been utilized to deposit high quality AZO thin films with high carrier mobility [8], [37], [38]. Wang and co-workers deposited AZO films using RF magnetron sputtering and Hall mobility of 7.86 cm²/Vs was reported [7]. The dependency of Hall mobility on the film thickness was observed. Hall mobility was reported to increase up to 7.86 cm²/Vs and the resistivity was reported to decrease from 1.2 × 10⁻² Ω cm to 4.2 × 10⁻³ Ω cm with increase in the film thickness. An improvement in electrical properties with increase in film thickness was attributed to increase in the grain size. Chetan et al. used RF sputtering to deposited AZO film which also showed the dependency of electrical property on the film thickness. The defect chemistry was found to be dependent on the film thickness that controls the electrical properties of AZO thin films [8]. Norihiro et al. deposited AZO thin films using RF/(RF + DC) sputtering and lowest resistivity of 8.24 × 10⁻⁴ Ω cm was reported at 0.44 RF/(RF + DC) ratio. Deterioration in electrical property was attributed to the structural damage by high energy ions during the deposition. The change in residual stress from compressive to tensile with an increase in RF/(RF + DC) ratio was also reported [30]. Nomoto et al. used RF/(RF + DC) sputtering to analyse the contribution of grain boundaries, using an indirect optical method, in the reduction of Hall mobility in AZO thin films by comparing Hall mobility with optical mobility. It was found that the intrinsic electrical properties were almost constant and the dominating factor limiting the Hall mobility was grain boundaries scattering [38]. In all these reports, the focus was given to improve the electrical property by different approaches However, the issues of less carrier mobility and their correlation with processing parameters are still not well understood. Moreover, the contribution of grain size and grain boundaries on the improvement of carrier mobility have not been studied yet in thin films. Therefore, a detailed study to understand the dependency of grain and grain boundaries on the carrier mobility along with residual stress is required for next generation cost effective and flexible device applications.

Here, we present a controlled increase in grain size and supressed grain boundaries potential along with minimising the residual stress in AZO thin film by superimposing RF power onto DC power during the sputtering process. The crystallite sizes, nanoscale grain boundary potential and residual stress were measured in AZO thin films deposited at different RF/(RF + DC) ratios. High mobility of 6.31 cm²/Vs was obtained for AZO thin film deposited at 0.75 RF/(RF + DC) ratio (AZO75) along with low compressive residual stress of −1.42 ± 0.03 GPa. It was observed that not only grain size but the grain boundary potential barrier also plays a crucial role in controlling the charge carrier mobility. The XPS analysis confirms the low oxygen vacancies and zinc interstitials which contribute to low grain boundaries potential and high carrier mobility in AZO thin film. Large crystallite/grain size and low defects were observed for AZO thin film deposited at 0.75 RF/(RF + DC) ratio that also explains the origin of high carrier mobility.