The High Photo-Sensitivity of Low-Temperature Poly-Silicon Lateral p-I-n Diode Utilizing Reflective Layers

In past research, most of Semiconductor photo-detectors were fabricated in thick single-crystal silicon wafer in order to obtain high photo sensitivity. They are widely used in the electronics industry in a variety of areas from ambient light sensors, image scanner to artificial retina systems. However, most of them suffered from high-temperature and complexity to the fabrication process. Therefore, in this work a polycrystalline silicon lateral PIN diode utilizing reflective layers (RL) by LTPS technology was proposed to achieve high photo sensitivity for the first time. When combined with photo-detectors, LTPS technology will become a promising one-panel solution in wearable chips or systems application [1].

    The lateral thin-film diodes were fabricated on a glass substrate using the conventional LTPS process [2]. In which, the intrinsic region was definition by overlapping boron and phosphorous implant regions. In the past research, the space charge region of devices was confined to intrinsic region mostly [3], so that RL was designed upon the intrinsic region by using photolithography that was formed of tri-layer film stacks consisted of silicon nitride, silicon dioxide and molybdenum. This device was demoted as RL-PIN described in Fig1. In order to examine our RL-PIN device, a very low intensity planar light source (~10mw/m² at peak) with 0^o incident angle was supplied as backside illumination through the substrate BK-7 glass. In general, the multiple dielectric layers named buffer layer were used to isolate the device RL-PIN and substrate BK-7 glass. The buffer layer included oxide and nitride in this work. After that, it was found that the incident light from backside illumination obtained the high transmittance over 85% to polycrystalline silicon region based on Fabry-Perot theory calculation.For further investigation, photo-sensitivity was defined per watt incident light illumination generated the photo current to estimate the photo-sensitivity performance. In our research, the RL-PIN achieved a very high photo-sensitivity above 10⁶mA/W with reverse bias which was higher three orders than without RL ones. In our implementation of RL-PIN, silicon nitride, silicon dioxide and molybdenum were used as a RL having high reflectivity above 98% to reflect the incident light through the intrinsic region. Furthermore, a bi-layer of silicon nitride and silicon dioxide as the buffer layer also provided the reflectivity above 27% to the other interface of the intrinsic region. The reflectivity could be calculated according to (1). Both of interfaces of the intrinsic region were associated RL and buffer layer with reflectivity above 98% and 27%, respectively. Furthermore, the polycrystalline silicon was not only placed in between RL and buffer layer but also had higher reflectivity around 5.0 higher than the others. Therefore, it was found that the optical-cavity phenomenon in the intrinsic region was occurred to improve light absorption and enhance photo-sensitivity.    

    In conclusion, backside illumination had been first adopted in RL-PIN was fabricated by LTPS technologies to block incident light for high-performance photo sensors which can achieve very high photo-sensitivity due to the optical-cavity phenomenon for scientific applications. The RL-PIN was of increasing interest due to their application to mobile display with embedded sensors.

Reference

[1] Yoshitaka Nishizaki, Takehiko Yamashita,Takehiro and Satoshi Inoue IDW ’07 107.

[2] Se Hwan Kim, Seung Hoon Lee, and Jin Jang IEEE electric device letters, vol. 31, no.5, MAY 2010

[3] IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 56, NO. 1, JANUARY 2009