Influence of high dielectric HfO2 thin films on the electrical properties of Al/HfO2/n-Si (MIS) structured Schottky barrier diodes
Introduction
Transition metal oxide nanomaterials have always been suitable materials of appropriate choice for applications such as bipolar transistors, field effect transistors (FET), metal-oxide-semiconductors (MOS), metal-insulator-semiconductors (MIS) and the rectifying current, nature of the current flow in device (admittance theory) gives as a better understanding of the basics of semiconductor devices. The surface conditions between the metal and semiconductor determine the stability and reliability of the device performances in semiconductor devices [[1], [2], [3]]. Comparing with traditional p-n junction diode, the MIS diode possesses zero diffusion capacitance and smaller transition capacitances which leads to it having a broad range of advantages such as high operating frequency, fast switching speed and low forward voltage drop [[1], [2], [3], [4], [5]]. The various applications include Variety RF, microwave applications such as varactors, detectors, mixture, multipliers and low voltage reference circuits [1,2,6]. SiO2 has been used as the main dielectric gate oxide material for fabrication of the device in the semiconductor industry because of its excellent compatibility with Si semiconductor but it has high leakage current and power consumption, which affects the overall devices performance [[7], [8], [9]]. Hence the interfacial layer is essential to control the thickness and leakage current in a MIS type SBD [10]. We infer that the interfacial layer should have high dielectric materials such as TiO2, ZrO2, TaO2, HfO2, etc., [[11], [12], [13]].
Among those, HfO2 plays an important role in binding with SiO2 in MIS SBD diodes [14]. Hafnium has excellent physical and chemical properties because of high temperature refractory, which makes the material suitable for vast technological and industrial applications including nonvolatile memories, negative capacitance field-effect-transistors and energy storage/harvesting. This is due to their high chemical stability. Also, they have high melting temperature (2774 °C), high crystallographic density (- 10 g/cm3), energy band gap of 5.68 eV and low phonon frequency. And it has good dielectric nature, better resistive switching properties, high dielectric constant (~21), low leakage current and high optical band gap (~5.8 eV) [[15], [16], [17], [18]]. In previous reports, the HfO2 was used as the interfacial layer in Pt/HfO2/n-GaN SBDs [19]. The hafnium has various polymorphs including one stable monoclinic phase and four metastable phases which are cubic, tetragonal, orthorhombic 1 and orthorhombic 2 as present in HfO2 [20,22]. The manipulation of structural and crystal growth determines the electro-optical properties which are turns controlled by the fabrication methods [20]. The HfO2 thin films are fabricated by various physical and chemical methods such as atomic layer deposition [21], sputtering [22], spray deposition [23], and reactive electron beam evaporations. The jet nebulizer spray pyrolysis (JNSP) technique is cost-effective, time-saving and more efficient compared to other (high-cost) techniques [24,25].
In the present study, we have deposited a nanostructured HfO2 thin films at various substrate temperatures (400, 450, 500, 550 and 600 °C) through the JNSP technique. The structural, optical and electrical properties of these films were studied by XRD, FESEM, EDX, UV–Vis and I–V characterizations. Also, the prepared wide band gap HfO2 thin films was utilized as an interfacial layer (in between Al/p-Si interface) in fabricating MIS type Schottky barrier diodes. Finally, significant diode parameters like I0, n and ΦB were studied using the J-V method and discussed.
Section snippets
Preparation of HfO2 thin films
Hafnium (IV) chloride (HfCl4 with 99% purity) was purchased from Sigma Aldrich. To prepare a high-quality HfO2 thin film, a 0.3 mol of HfCl2 was dissolved in deionized water and stirred for 20–30 min with a magnetic stirrer. Before coating, all the glass substrates have been cleaned using isopropyl alcohol, soap solution and acetone separately. After that, the prepared solutions were sprayed on pre-cleaned glass substrates at various temperatures such as 400, 450, 500, 550 and 600 °C using JNSP
X-ray diffraction
Fig. 1 denotes the XRD patterns of nebulizer-sprayed HfO2 thin films fabricated at 400, 450, 500, 550 and 600 °C substrate temperatures. No diffraction peak was recorded for the HfO2 films deposited up to 450 °C which can be seen in Fig. 1. Resulting data implies the amorphous nature of the films. This may be due to the insufficient substrate temperature. If the substrate temperature is low, the adhered atoms come out slowly and solidify to form an amorphous film. Comparatively a lower kinetic
Conclusions
High-quality wide-bandgap HfO2 thin films were effectively deposited on a glass substrate at various substrate temperatures by the JNSP technique. The structural, optical and electrical properties of the coated films were evaluated. From XRD, a monoclinic crystal structure of HfO2 thin films with improved crystallite was observed after 450 °C. Micro-porous and ball-like grains have been recorded by the FE-SEM micrograph. A large number of grains with small porous-like surface morphology was
Credit author statement
P. Harishsenthil: Processed the samples, compiled and analyzed the data, and wrote the manuscript. J. Chandrasekaran.: Conceptualization, Supervision, and Validation R. Marnadu.: collected the samples, data Formal analysis: P. Balraju.: participated in the sampling process. C. Mahendran.: Reviewing and Editing. All authors have read the final manuscript and confirmed its content.
Acknowledgments
The authors gratefully acknowledge the financial support from the Department of Science and Technology-Science and Engineering Research Board, Government of India, for the major research project (EMR/2016/007874).
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