GaN MSM photodetectors with photo-CVD annealed Ni/Au electrodes
Introduction
Recently, much attention has been focused on nitride-based materials due to their wide direct bandgap, high-saturation velocity and high thermal conductivity. These excellent properties make nitride-based materials potentially useful in optoelectronic, high speed and high power devices. For example, the large bandgap energy of GaN-based materials makes them useful for ultraviolet (UV) photodetectors with high UV/Visible rejection ratio. Such devices are important components in a variety of military and commercial applications, such as space communication and ozone detection [1]. To our knowledge, various types of GaN-based photodetectors have been reported [2], [3], [4], [5], [6], [7], [8]. Among them, Schottky contact metal–semiconductor–metal (MSM) photodetectors are of planar configuration and are suitable for integration with field effect transistors (FETs) [9]. The lateral, planar structure of MSM photodetectors also results in extremely low capacitances, which match well with the requirements of large bandwidth and low-noise performance.
For MSM photodetectors, a low Schottky barrier height will result in a large dark current. On the other hand, transparency of the contact material determines the number of photons entering the semiconductor. To achieve high performance MSM photodetectors, it is thus important to choose metal contacts with large Schottky barrier heights and high transparencies. In the literatures, Ni/Au is often used as the semi-transparent ohmic contact on p-GaN [10], [11], [12]. However, it has also been found that Ni/Au could form good Schottky contact on n-GaN and n-AlGaN [13], [14], [15], [16].
In this study, Ni/Au semi-transparent contact layers were deposited onto unintentionally doped nitride epitaxial layers for the fabrication of MSM UV photodetectors. In some cases, the as-deposited Ni/Au contacts were annealed in O2 by photo-chemical vapor deposition (photo-CVD) to improve the performance of the devices. The properties of the fabricated photodetectors and the effects of photo-CVD annealing will both be reported.
Section snippets
Experiment
Samples used in this study were all grown on c-face (0001) sapphire substrates by metalorganic chemical vapor deposition (MOCVD) [13], [14], [15]. The undoped GaN samples consist a 25-nm-thick low temperature GaN nucleation layer, and a 2-μm-thick undoped GaN layer. Ni/Au (3/6 nm) was subsequently deposited onto the undoped GaN samples to serve as the Schottky contacts by thermal evaporation. Contact pattern of the fabricated MSM UV photodetectors consists of two inter-digitated contact
Results and discussion
Fig. 2 shows normalized light transmission spectra of the as-deposited and photo-CVD annealed Ni/Au layers on glass substrates. In this figure, the transmittance of each film was normalized with respect to the transmittance of the glass substrate. It was found that transmittance of the as-deposited Ni/Au film was smaller than 67% in the region between 350 and 700 nm. It was also found that the transmittance of the Ni/Au layers become larger after annealing. With a 500 °C photo-CVD annealing in O2
Summary
In conclusion, thin Ni/Au (3 nm/6 nm) bi-layer metal films annealed by photo-CVD were investigated. With proper annealing in oxygen by the photo-CVD systems, it was found that the transmittance of the deposited Ni/Au increased from 67 to 85% in the region between 350 and 450 nm. GaN MSM UV photodetectors with photo-CVD annealed Ni/Au contact electrodes were also fabricated. It was found that dark current of the detector became significantly smaller after annealing. With a 1 V applied bias, it was
Acknowledgements
This work is supported by the National Science Council under contract numbers NSC 92-2215-E-230-002 and NSC 93-2215-E-230-001.
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