漸變蝕刻技術應用於多孔矽光感測器之研究

Graded-Etching Technology for Preparation of Porous-Silicon in Optical-Sensing Device Applications

傳統的多孔矽製程都是以固定陽極化蝕刻電流方式來形成多孔矽層，然而此法所製作出的多孔矽層通常厚度較薄，導致光響應無法提高；再者其光吸收響應頻譜比較平坦，所以也並不適合應用於需要偵測特定光波長之光感測元件的製作。本論文中我們研究以漸變蝕刻技術研製多孔矽層，藉著調變蝕刻電流大小與蝕刻時間來形成具不同結構的厚層多孔矽，並以其為基礎來製作具不同特定光波長響應的多孔矽光感測器。研究中發現，與固定蝕刻電流製程所形成的多孔矽元件比較，以漸變蝕刻電流製程所形成的多孔矽元件具有較高的光吸收響應。其中以線性遞增蝕刻電流方式所製成的元件，在入射光波長為660nm 下的光響應峰值為0.33A/W；而以線性遞減蝕刻電流方式所製成的元件，在入射光波長為540nm 下的光響應峰值為 0.25A/W。與固定蝕刻電流之元件比較，兩者分別約提高了43.5％與257％。同時我們也發現以蝕刻電流遞增方式製作的元件，其光響應峰值有往長波長偏移的趨勢；而以蝕刻電流遞減方式製作的元件則往短波長偏移。吾人認為這兩種不同製程所造成光響應偏移現象，與所形成的多孔矽層中之奈米結晶的大小尺寸分布(nanocrystal size distribution)有關。實驗中也發現，同時調變蝕刻時間與蝕刻電流的製程可以形成機械結構較強而且垂直孔洞分佈均勻之厚層多孔矽 (~35μm)結構。吾人認為此製程中的調變蝕刻參數可以補償固定蝕刻參數製程中所形成多孔矽層結構中孔隙率梯度(porosity gradient)，因此可形成孔隙率分佈較均勻且機械結構較強的厚層多孔矽。與固定蝕刻參數之元件比較，調變蝕刻參數所製作之多孔矽光感測器元件不僅具有較陡峭的光響應頻譜，其在入射光長為870nm 下的光響應峰值為0.16A/W，在10V 偏壓下之光/暗電流比(photo-to-darkcurrent ratio)為90，兩者也分別約提高了167％與36 倍。實驗結果證實漸變蝕刻參數製程可以形成孔隙分布均勻的厚層多孔矽，不僅可降低元件的暗電流以進一步提升元件之光/暗電流比，其中線性遞增蝕刻電流製程技術所形成之多孔矽層在近紅外線有突出的光吸收響應，因此這個製程技術應用於開發近紅外線多孔矽光感測器具有相當大的潛力。

Porous silicon layers (PSL’s) that produced from the constant anodization-current process were usually not thick enough to get high photo-resposivity. Furthermore, because those thin PSL’s usually led to quite flat light-absorption response spectra, they were not suitable for some specific optical-sensing applications. In this paper, we developed graded-etching technologies to form PSL’s, in which the anodization etching time and etching current were gradually changed to form different PSL’s structures. Basic photodetectors were fabricated based on the as-formed PSL’s in order to check the optoelectronic properties of the developed PSL’s. Compared with the constant etching current process, PSL’s devices fabricated from the graded-etching current process had higher photo-responsivity. Devices made from linearly increasing etching-current process(10mA?40mA) got a peak response of 0.33A/W at 660 nm, while devices made from linearly decreasing etching-current process (40mA?10mA) got a peak response of 0.25A/W at 540 nm. Both values are respectively about 43.5% and 257% higher than those of the devices fabricated from the constant etching-current process. In the mean time, we found that the peak photo-responsivity of devices based on an increasing etching-current process tended to shift toward long-wavelength, while that of devices based on a decreasing etching-current process was toward shore-wavelength. These phenomena were supposed to be related to the different nano-crystal size distribution in the PSL’s formed by the two different etching processes. It was also found that thick PSL’s (~35μm) with higher mechanical strength and more uniform pore distribution can be achieved by gradually changing simultaneously the etching time and the etching current. The preferable PSL structures produced was attributed to the modulated etching parameters in the anodization process, which might compensate the porosity gradient produced by the constant etching-current process. In addition, devices based on the modulated etching-parameter process showed a comparatively abrupt photo-response spectrum with a peak value of 0.16A/W at 870 nm. At a bias of 10V, the obtained photo-to-dark current ratio is about 90, which is 36 times larger than that got from the constant etching current process. Experimental results verified that the graded etching technologies can produce stronger thick PSL’s with uniform porosity distribution and the as-fabricated devices possessed lower dark-current, higher photo-to-dark current ratio. PSL’s made from linearly increasing etching-current process that had more abrupt responses in the near infrared (NIR) range. These indicated that such techniques have high potential in development of highly sensitive NIR photodetectors.