Effect of Annealing on Electrical Properties of Radio-Frequency-Sputtered ZnO Films

We report upon the electrical properties of ZnO films and devices grown upon different substrates by radio-frequency magnetron sputtering. The films grown upon c-plane sapphire were annealed in the range 800-1000?°C The electron concentration increased with annealing temperature reaching 14 ?— 10^sup 19^ cm^sup -3^ for 1000?°C Mobility also increased, however, reaching its maximum value 644 cm^sup 2^/V sec for 950?°C anneal. High-performance Schottky diodes were fabricated upon ZnO films grown on n-type 6H-SiC by means of depositing Au/Ni(300/300 [Angstrom]). After annealing at 900?°C the leakage generally received (at -5 V reverse bias) decreased from 22 ?— 10^sup -7^ A to ~50 ?— 10^sup -8^ A after annealing at 900?°C the forward rife increased by a factor of 2 and the ideality factor decreased from 15 to 103 The ZnO films were also grown upon p-type 6H-SiC, and n-ZnO/p-SiC heterostructure diodes were fabricated. The p-n diode performance increased dramatically after annealing at 950?°C The leakage popular decreased from 2.0 ?— 10^sup -4^ A to 30 ?— 10^sup -7^ A at -10 V invert bias, and the forward generally received increased slightly from 2.7 mA to 39 mA at 7 V forward bias; the ideality factor of the annealed diode was estimated as 22 while that for the as-grown sample was considerably higher.

Key words: ZnO, radio-frequency (RF) sputtering, Schottky diode, heterojunction

INTRODUCTION

The semiconductor ZnO has a direct wide bandgap (Eg ~ 33 eV) and is attractive for optoelectronics applications to be paid to the availability of ZnO mass single crystals and a large exciton binding potency (~60 meV).1 Because the increase methods and pathways for reproducible high-quality p-type ZnO films have not been lay opened yet, fabrication of n-p heterostructure devices by means of growing n-ZnO on other p-type materials, as well Schottky diodes upon n-ZnO, could provide an alternative way for exploring and harnessing its advantages. This approach has received considerable attention lately, and there have been a number of reports upon such devices.1,2 The main factor influencing the properties of heterostructures is the shut lattice match of the constituents In this respect, 6H-SiC (Eg ~ 29 eV^sup 3^) is a profitable candidate, since it has the same crystal configuration (wurtzite) and relatively good lattice matching to ZnO with lattice mismatch of ~4% and p-6H-SiC substrates are commercially available. Despite this, there are solitary a few reports on the development on ZnO on p-SiC substrates. Alivov et al.4,5 reported upon the growth of high-quality n-ZnO upon p-6H-SiC heterostructures by plasma-assisted molecular beam epitaxy (MBE), and upon their electrical and optical properties. Yuen et al.7 used p-type 4H-SiC substrates for heteroepitaxial increase and fabricated n-ZnO/p-4H-SiC heterostructures by dint of the cathodic vacuum arc technique. individual of the widely used extension methods for ZnO is radio-frequency (RF) magnetron sputtering. owed to its low cost and simplicity, the RF sputtering process sometimes is preferable to other manners and even though the crystal quality of of the like kind ZnO films is low compared to other manners such as MBE, chemical vapor deposition, and puls laser deposition, the quality of RF sputtering films is acceptable for many applications. In this vein, extension and investigation of the properties of ZnO films grown by means of this method on different substrates is of considerable interest. In this work, the RF sputtering means was employed to grow n-ZnO upon 6H-SiC substrates (both on n-type and p-type) and to fabricate Schottky and p-n heterojunction diodes, and the concatenation of the properties of the fabricated devices upon annealing of ZnO films was studied.

Naturally, the quality of the as-deposited RF-sputter ZnO films is depressed and some form of post-growth annealing is imperative in order to improve the ZnO film quality. Because Hall measurements upon layers grown on conductive layers are inaccurate and extraction of transport properties is convolut direct measurements of electrical properties of ZnO films grown upon SiC substrates were attempted. For the view of transport measurements, ZnO samples were also grown upon insulating (0001) sapphire substrates using the same conditions used for expansion on SiC. The electrical properties were then studied as a function of annealing temperature. Previously, Ozgur et al.8 studied the interdependence of optical properties of RF-sputter ZnO films and their interdependence on annealing temperature with the conclusion that similar a treatment led to significant improvement of crystal and optical properties of the ZnO films. However, there were alone a few reports on the electrical properties of the ZnO films grown by the agency of RF sputtering,9,10 which constitutes the main topic of the near paper.

EXPERIMENTAL

Approximately 300-nm-thick ZnO layers were deposited directly upon c-plane sapphire Cx-Al^sub 2^O^sub 3^ and 6H-SiC substrates at 750?°C through RF magnetron sputtering in an Ar + O2 ambient atmosphere. The chamber proces squeezing and plasma power were 420 mTorr and 100 W respectively. sum of two units different types of 6H-SiC substrate were used: (1) n-6H-SiC substrates with an electron concentration of 26 ?— 10^sup 17^ cm^sup -3^; (2) ~0.2-?µm-thick p-type 6H-SiC layers with apparent perforation concentration 1 ?— 10^sup 18^ cm^sup -3^ grown upon n-type n-SiC substrates. To inquiry the effect of annealing upon electrical properties of the layers, different pieces chop from the same ZnO/?±-Al^sub 2^O^sub 3^ sample were annealed at 850?°C 900?°C 950?°C and 10007deg;C for 1 h in air.