GaN Ultraviolet Photodetectors with Transparent Titanium Tungsten and Tungsten Electrodes

Abstract

GaN metal-semiconductor-metal ͑MSM͒ ultraviolet photodetectors with titanium tungsten ͑TiW͒ and tungsten ͑W͒ transparent electrodes were fabricated and characterized. It was found that the 10-nm-thick TiW and W films could provide a reasonably high transmittance of 78 and 65.4% at 300 nm, a low resistivity of 1.7 and 1.5 ϫ 10 −3 ⍀ cm, and an effective Schottky barrier height of 0.773 and 0.777 eV on u-GaN, respectively. We also achieved a peak responsivity of 0.192 and 0.15 A/W, a quantum efficiency of 66.4 and 51.8% from the GaN UV MSM photodetector with TiW and W electrodes, respectively. With a 3 V applied bias, the noise equivalent power of photodetectors with TiW and W electrodes were estimated to be 1.98 ϫ 10 −10 W and 3.36 ϫ 10 −10 cm Hz 0.5 W −1 , corresponding to the normalized detectivity ͑D * ͒ of 6.36 and 3.82 ϫ 10 9 cm Hz 0.5 W −1 , respectively.Light detection in the ultraviolet ͑UV͒ spectral range has drawn much attention in recent years. UV photodetectors are useful in various fields such as biological, chemical, and cosmetic applications. Recently, the primary means of UV light detecting was the use of silicon photodiodes. However, the most sensitive wavelength of Si photodiodes is not located in the UV region because roomtemperature bandgap energy of Si is only 1.2 eV. Thus, the responsivity of Si photodiodes is low in the UV region. To maximize the detector responsivity in the UV spectral region, one should choose wide-bandgap materials, such as SiC, GaN, and ZnSe, for the fabrication of solid-state UV detectors. Among these wide-bandgap materials, GaN has a direct bandgap and a high saturation velocity. These properties make GaN a promising candidate for UV detector applications. In the past few years, various types of GaN-based photodetectors have been proposed, such as p-n junction diodes, 1,2 p-i-n diodes, 3,4 p--n diodes, 5 Schottky barrier detectors, 6,7 and metalsemiconductor-metal ͑MSM͒ photodetectors. 8-14 Among these structures, MSM photodetectors are easy to integrate with field effect transistors ͑FETs͒. However, the responsivity of normal MSM photodetectors is limited by the presence of the opaque metal contact electrodes in general. Although back-side illumination can be used to solve this problem, it also creates critical problems in device processing and chip packaging. Previously, it has been shown that one can use transparent indium tin oxide ͑ITO͒ and titanium nitride ͑TiN͒ electrodes to solve this problem. 13,14 However, the transparence of ITO and TiN is very low in UV-A and UV-B regions. For application in the UV region, investigation of highly reliable and highly transparent electrodes has become important. In this paper, we report the deposition of titanium tungsten ͑TiW͒ and tungsten ͑W͒ onto the surfaces of GaN layers by radio frequency ͑rf͒ magnetron sputtering. The physical, optical, and electrical properties of the deposited TiW and W films were then characterized. Also, GaNbased UV MSM photodetectors with transparent TiW and W Schottky contact electrodes were fabri...

Giant UV Photoresponse of GaN-Based Photodetectors by Surface Modification Using Phenol-Functionalized Porphyrin Organic Molecules

“…16 Such observations have been attributed to certain imperfections in GaN, such as dislocations and point defects. These limitations cause Schottky barrier inhomogeneity, 17,18 allow tunneling of carriers during reverse-bias conditions (in the dark), [13][14][15]17,18 and trap or recombine the photogenerated carriers. 12,16 In addition to this, lowering of Schottky barrier height, due to surface electronic states on GaN, 19−25 is also responsible for allowing high reverse-bias current to flow in dark conditions, 26,27 thereby degrading the performance of the UV detector.…”

“…Different high-work-function Schottky metals such as Ni , and Pt have been used in the form of interdigitated electrode (IDE) geometry to collect maximum photogenerated carriers. To increase the working area of the device, usage of transparent electrodes such as titanium tungsten (TiW), tungsten (W), and graphene has been demonstrated.…”

Performance enhancement of GaN metal–semiconductor–metal ultraviolet photodetectors by insertion of ultrathin interfacial HfO2 layer

“…Various insulating materials such as SiO 2 , ZrO 2 , and Al 2 O 3 are reported. [14][15][16] However, low dielectric constant (k) value restricts the maximum permissible electric field to the device. Therefore, high-k insulating material is helpful in reducing the field strength within the dielectric and thus allowing better performance of the devices.…”

Fabrication and device characteristics of Schottky-type bulk GaN-based “visible-blind” ultraviolet photodetectors

“…Recently, various types of UV photodetectors such as p-n junction photodiodes, 1 p-i-n photodiode, 2 p--n photodetectors, 3 metal-semiconductor-metal ͑MSM͒ photodetectors, [4][5][6] and Schottky-barrier-type photodiodes, [7][8][9] have been fabricated. Compared to p-n junction photodiodes, MSM and Schottky barrier photodetectors are attractive because of their fabrication simplicity and high response speed.…”