Prominent c-axis oriented Si-doped ZnO thin film prepared at low substrate temperature in RF magnetron sputtering and its UV sensing in p-Si/n-SZO heterojunction structures

doi:10.1016/j.jpcs.2020.109907

Journal of Physics and Chemistry of Solids

Volume 151, April 2021, 109907

https://doi.org/10.1016/j.jpcs.2020.109907 Get rights and content

Highlights

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Si doping in ZnO controlled by growth temperature, RF power and dopant source coverage on the sputtering target.
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Substitutional doping at Zn²⁺ lattice site by Si in Si⁴⁺ and/or Si³⁺ configuration.
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Lattice constant ‘c’ reduces when Si substitute Zn, following VEGARD's law.
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At elevated doping Si resides at the interstitial position, increases the lattice constant, stress and strain.
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p-Si/n-SZO heterojunction for potential device application in UV detection.

Abstract

Considering the effectiveness of silicon (Si) doping in zinc oxide (ZnO) to produce Si-doped zinc oxide (SZO) for good quality transparent and conducting oxide (TCO) films, different varieties of ZnO and SZO films have been grown by changing the substrate temperature (T_S), radiofrequency (RF) power and coverage (Ψ%) on the erosion area of the ZnO target using the c-Si wafer as the source of Si dopant in RF magnetron sputtering. Prominent c-axis orientation in SZO thin films and its gradual evolution on parametric changes have been systematically studied. For undoped ZnO films grown via a gradual increase in the T_S, the lattice constant (c), lattice strain and stress do not change substantially. However, the grain size significantly increases, as shown by the changes in the position and full width at half maxima (FWHM) of the X-ray diffraction (XRD) peak corresponding to the (002) plane. Increasing the RF power increases the grain size and moderately decreases the c. On the contrary, upon the introduction of Si doping and its systematic escalation at increased Ψ%, the grain size linearly decreases. Upon increasing the RF power, the grain size in the SZO network increases and the magnitude of c decreases enormously, which signify the increased substitutional incorporation of the Si dopant in its Si⁴⁺ ionized state at the Zn²⁺ lattice site. In the SZO samples the magnitude of c, strain and stress decrease up to a certain level and then increase at elevated substrate temperature, indicating the occurrence of increased doping via Si incorporation, in contrast, at the interstitial position in the ZnO network. The RMS roughness, average roughness and surface area increase linearly with the RF power for both ZnO and SZO films. Preliminary achievements using the SZO films toward ultraviolet (UV) sensing utilizing its p-Si/n-SZO heterojunction structures has been presented.

Graphical abstract

Introduction

Transparent and conducting oxide thin films, popularly known as TCO coatings, are regularly used in different optoelectronic devices such as solar cells, flat panel displays, thin film transistors and many others [[1], [2], [3], [4], [5], [6], [7], [8]]. The commonly used TCOs include indium tin oxide (ITO; In₂O₃:Sn) [9,10] and fluorine-doped tin oxide (FTO; SnO₂:F) [11]. However, indium can diffuse into the layer sequentially stacked on it in devices fabricated using conventional plasma processing [12,13]. On the other hand, for FTO a higher deposition temperature is an essential requirement [14]. To get rid of these intrinsic constraints in device fabrication, zinc oxide (ZnO) has been used as an alternate TCO material. ZnO films possess very good visible range transparency, electrical conductivity and excellent chemical stability, which cause its promising use as a suitable optical material [15,16]. The energetically favoured orientation of ZnO along the <001> direction, known as the c-axis of the wurtzite structure, results from the lowest surface energy with the (002) plane [17]. The c-axis growth is oriented perpendicular to the substrate surface and the piezoelectric properties of the film along this direction can be used in acoustic wave devices (SAW) [18]. In addition, the (002) plane can facilitate charge carrier transport along the direction perpendicular to the substrate surface and reduce the carrier path length in the case of thin film solar cells, by decreasing the series resistance and thereby, improve the conversion efficiency of the device [2]. The TCO properties of ZnO can be improved by doping aluminium (Al), gallium (Ga), In, antimony (Sb) and F, each of which possess a single extra charge carrier per dopant [[19], [20 ], [21], [22], [23], [24], [25], [26], [27]]. However, Si or Sn can accommodate two extra charge carriers per atom due to Zn²⁺_1–x(Si/Sn)⁴⁺_xO^2– formation and thereby, can further reduce the resistivity [28,29]. The atomic size of Sn is 2.25 Å, while that of Zn is only 1.39 Å. Accordingly, a major lattice mismatch in the ZnO:Sn matrix is quite reasonable [30]. On the contrary, the atomic size of Si is ~1.17 Å, which is similar to Zn, and hence, the use of Si as the dopant in the ZnO matrix can result in a reduced lattice mismatch [31]. Additionally, for Si-based devices processed using plasma-CVD [32,33], Si-doped zinc oxide (ZnO:Si or SZO) does not create any dopant diffusion related inconvenience in device performance. Among the various deposition methods reported, sputtering is extremely suitable for manufacturing large area devices with uniform film properties. The SZO film was first investigated by Minami et al. using RF magnetron sputtering [34]. Subsequently, many other research groups have tried to develop SZO films using different methods, however, most people use high growth temperatures and/or involve high-temperature post-deposition heat treatment that introduce an obvious restriction in the device fabrication [[35], [36], [37], [38], [39], [40], [41], [42], [43]].

In the present work, a systematic study of highly c-axis oriented crystalline ZnO:Si (SZO) thin films has been carried out using radiofrequency (RF) magnetron sputtering. Effective doping at an optimum growth temperature of ~150 °C and the dependence of the crystallite size of both ZnO and SZO films on the applied RF power, along with the corresponding dopant induced changes in the lattice stress and strain have been investigated in detail. In addition, a preliminary study has been performed to explore the feasibility of applying p-Si/n-SZO heterojunction structures as ultraviolet (UV) detectors.

Section snippets

Experimental

ZnO and silicon doped ZnO (ZnO:Si or SZO) thin films were grown from a 7.62 cm diameter circular ZnO target (~99.999% purity) in a RF magnetron sputtering system using Corning® Eagle 2000™ and p-type Si (100) wafer substrates. In the sputtering process, the ejection of atoms from the target material mainly occurs from a localized area, called the erosion area, which covers a circular band residing in between the centre and periphery. Rectangular (5 mm × 2 mm) strips of lightly dopes n-type Si

Results and discussion

Initially, two sets of samples were prepared by varying the substrate temperature (T_S) at 50, 100, 150, 250 and 350 °C; (i) ZnO (Set-I) and (ii) SZO with Ψ = 1.5% (Set-II). The XRD spectra (2θ = 10–80°) obtained for the ZnO films grown at different T_S are plotted in Fig. 1a. It is notable that the prominent peak corresponding to the (002) crystallographic plane is observed along with a relatively negligible peak attributed to the (004) plane. Upon increasing the T_S, the peak intensity of the

Conclusions

Si-doping in the ZnO network has been pursued to produce SZO films. Various ZnO and SZO films have been grown at different substrate temperatures (T_S) and RF power as well as varying the coverage (Ψ%) on the erosion area of the ZnO target using c-Si wafer as the Si dopant source in the RF magnetron sputtering process. Both ZnO and SZO thin films have been found to possess intense c-axis orientation, the parametric variations of which have been systematically studied by monitoring the changes in

Declaration on authors’ contribution

The authors declare that they have equal contributions in this publication.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The work has been done under projects funded by Department of Science and Technology (Nano-Mission Program) and Council of Scientific and Industrial Research, Government of India. One of the authors (LK) acknowledges CSIR, GoI, for providing a Senior Research Fellowship.

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Prominent c-axis oriented Si-doped ZnO thin film prepared at low substrate temperature in RF magnetron sputtering and its UV sensing in p-Si/n-SZO heterojunction structures

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Declaration on authors’ contribution

Declaration of competing interest

Acknowledgements

Appl. Surf. Sci.

Sol. Energy Mater. Sol. Cell.

J. Non-Cryst. Solids

Phys. E Low-dimens. Syst. Nanostruct.

Thin Solid Films

Appl. Surf. Sci.

Thin Solid Films

Sol. Energy Mater.

Appl. Surf. Sci.

Mater. Res. Bull.

Appl. Surf. Sci.

Surf. Coating. Technol.

Vacuum

Supper. Micro.

J. Mol. Struct.

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Sol. Energy Mater. Sol. Cells

Sol. Energy Mater. Sol. Cells

Physica E

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Physica B

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