Ultrafast metal-semiconductor-metal photodetectors on low-temperature-grown GaN

Abstract

We have fabricated and characterized ultrafast metal-semiconductor-metal photodetectors based on low-temperature-grown ͑LT͒ GaN. The photodetector devices exhibit up to 200 kV/ cm electric breakdown fields and subpicosecond carrier lifetime. We recorded as short as 1.4-ps-wide electrical transients using 360-nm-wavelength and 100-fs-duration laser pulses, that is corresponding to the carrier lifetime of 720 fs in our LT GaN material. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.1938004͔ Gallium nitride is a wide, direct band-gap semiconductor that makes it a very promising candidate for the fabrication of ultraviolet ͑UV͒ photodetectors. Various types of GaN photodetectors have been demonstrated recently, such as the Schottky barrier detector, 1 p-n junction, 2 p-i-n structure, 3 and metal-semiconductor-metal ͑MSM͒ photodiode. 4 Among these, MSM photodetectors exhibit superior performance in terms of the response speed, device noise, and fabrication simplicity. However, the GaN photodetector performance varied from sample to sample depending on the material quality, 5-8 in which growth temperature plays a major role. High quality GaN photodetectors with full width at half maximum ͑FWHM͒ of the response in the picosecond range ͑ϳ3.5 ps͒, which are the best published data, were presented by Li et al. 9,10 In this letter, we present the fabrication and properties of MSM photodetectors based on lowtemperature-grown ͑LT͒ GaN. Such material exhibits lower electron mobility and higher concentration of defects, thus subpicosecond carrier lifetime can be assumed, which essentially shortens the response time of these LT GaN photodetectors.The photodetector structure was grown by plasma induced molecular beam epitaxy. An AlN / GaN / AlN / GaN heterostructure buffer with thickness of 100/ 1500/ 5 / 500 nm was deposited on 6H i-SiC substrate. Subsequently, the active GaN layer was grown at 650°C with growth rate of 300 nm/ h. Although the material is monocrystalline, the surface exhibits a roughness of about 20 nm and a high value of defects initialized by the low growth temperature instead of the usually used temperatures in the range of 750-900°C. 11,12 The dislocation density is about 5 ϫ 10 9 cm −2 that is 50 times higher than for conventional GaN. MSM structures with finger width and spacing of 1 and 1.5 m, respectively, and an active area of 20ϫ 20 m 2 , as presented in Ref. 13, were patterned on the GaN surface using conventional photolithography and liftoff technique. The MSM electrodes consist of Ti/ Al/ Ni/ Au ͑35/ 200/ 40/ 50 nm͒ for forming ohmic contacts. Ohmic contact metallization was annealed at 900°C for 30 s. Devices with Schottky contacts ͑25 nm Ni, 300 nm Au͒ were fabricated for the sake of comparison. The whole surface except the MSM structures was coated with a 400-nm-thick SiO 2 layer. Ti/ Au coplanar strip lines ͑CPS͒ with thickness of 50/ 600 nm were fabricated on top of this insulator layer. Figure 1 shows typical current-voltage ͑I -V͒ characteristics of the photodetectors fabricated on LT...

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