超寬波段熱載子矽基紅外光偵測器之研究

Abstract

本論文主要探討熱載子式金/矽基之光偵測器操作於光子能量小於矽之能隙吸收波段，也就是通訊波段紅外光及中紅外光波段，研究由金屬端產生熱載子之偵測機制。搭配降溫系統，我們排除了由熱產生之熱電流（thermal current），進而以短路電流模式（ISC）與開路電壓模式（VOC）說明熱載子之收集與溫度間的關係。本實驗室為第一個利用降溫系統探討熱載子式矽基紅外光偵測器之熱載子收集機制之團隊。並且在中紅外光波段，突破Fowler thereoy之預測，光子能量小於蕭特基能障之波段仍可被偵測，本研究認為其所產生之電訊號，主要來自於量子之穿隧效應（quantumn tunneling），同時，本研究亦藉由表面鈍化（surface passivation）之製程，排除由金屬與半導體表面缺陷所貢獻之缺陷能階（quantumn assisted tunneling）之可能性; 其所製作出元件有以下優點1.矽基光偵測器由於匹配於矽基互補式金氧半導體製程技術 (Silicon-Complementary Metal-Oxide-Semiconductor, Si-CMOS)，因此在低成本大量製造上占有相當大的優勢2.熱載子式元件之光響應速度快，較不會產生延遲3.元件不需要搭配降溫系統，可於室溫下操作。 論文第一部分主要探討近紅外光波段之熱載子有效收集與溫度間之關係，探討熱載子經過蕭特基接面後如何被背電極接收。搭配降溫量測系統，得以排除由熱產生之熱電流，進而確定產生之光電流與光電壓為光激發熱載子所貢獻，而於低溫之操作環境下，發現蕭特基能障會隨溫度逐漸降低，此部分於電壓模式之降溫量測實驗中可以看到電壓光響應隨溫度降低而增加得到證實，由文獻及實驗發現在本研究操作的溫度區間（e.g.室溫至攝氏-100度間）。將元件分成三部分，分別對溫度效應與熱載子在為金屬中平均自由徑、半導體阻及蕭特基能障作一系列之探討。電壓模式與電流模式最大之不同為，當熱載子躍過蕭特基能障後，其半導體端原本維持電中性之環境被破壞，因而產生靜電荷，而此靜電荷會進一步在金屬半導體之兩端產生壓降，此即開路電壓（Voc）。然而電流模式之產生需再多一步驟，即當熱載子跨過蕭特基能障後，需利用擴散之方式經過整個半導體，才能經由背電極所接收。當溫度下降後，金屬阻值降低，n型矽半導體的阻值略為下降且等效蕭特基能障亦下降，然而光電流並未提高，電流模式並未隨溫度降低而增加電流響應之原因，本研究認為由熱載子產生之電流隨溫度降低之後無法有效擴散，因此擴散電流隨溫度下降而降低，所得之光電流響應於低溫時並未獲得提升，然而光電壓在室溫下且波段1310奈米約為1.24 V/W 降至攝氏-100度後提升提升超過100倍達142.2 V/W，其為等效蕭特基能障隨溫度下降之有力證據。 論文第二部分，首先關注下一世代通訊波段2微米之偵測能力，元件亦為金與n型矽半導體之蕭特基元件。理論上，從Fowler theory預測，當入射光光子能量小於蕭特基能障，將無法貢獻光電流響應，然而本元件於中紅外光波段亦能達到偵測效果，此部分探討其不受Fowler theory能障限制之主要原因。首先光電流之產生可初略分為以下幾種產生之方式1.由金屬端入射，照光後熱載子利用跳躍越過蕭特基能障。2. 由金屬端入射，照光後熱載子利用穿隧效應穿過蕭特基能障。3.照光後，矽因本質吸收進而產生電子電洞對。4.缺陷能階所貢獻。本研究利用實驗及理論排除了1、3、4項之可能性。並進一步以量子穿隧現象解釋光電流之產生。根據金之能帶圖，越低光子能量之狀態密度（ density of state）越高，將其與穿隧機率作理論計算，於較短波段之中紅外光（e.g.2微米及3.25微米）與光電流光電壓貢獻吻合，然而較長波段（e.g.6微米及10.6微米）實驗值與理論趨勢亦相符。因此推測於中紅外光波段相較於近紅外光，其偵測機制主要以自於金屬本身所產生熱載子之狀態密度及穿隧機率之量子穿隧效應所產生。綜合上述，本論文主要探討近紅外光波段至中紅外光波段熱載子之有效收集機制，利用降溫系統，證實等效蕭特基能障隨溫度下降而降低，並大幅增加光電壓響應。

In this thesis, we mainly discuss the gold (Au)/silicon (Si) of hot-carrier-based photodetectors operate when photon energy is less than the band gap of silicon(Si), that is, the optical communication of near mid-infrared (NIR) and mid-infrared (MIR) spectral regimes. With the cooling system, we excluded the thermal current generated by heat. We measured the short circuit current (ISC) and open circuit voltage (VOC) generated from hot carriers to discuss the relationship between the collection efficiency and temperature of hot carrier based device. Our team is the first one to study that the collection mechanism of hot carriers based Si photodetectors by using a cooling system. In the MIR region, we break through the prediction of the Fowler theroy that the light with photon energy less than Schottky barrier can still be detected. We suggest the electrical signals would be generated by quantum tunneling effect. We also used the surface passivation processes to reduce the defected-energy levels contributed by surface defects between metal and semiconductor. The devices have the following advantages: 1. Device is compatible with Si based semiconductor processes. 2. Hot carrier based device has rapid response time. 3. Device detecting MIR signals can operate at room temperature. In the first part of this thesis, we discussed the temperature effect on collection Efficiency of hot carriers in NIR region. The hot carrier would be received by the back electrode after passing through the Schottky junction. The thermal current generated by the heat would be greatly reduced by cooling system. Therefore, the photocurrent and the photovoltage are mainly contributed from the hot carriers generated by photon excitation. In low operating temperature, the equivalent Schottky barrier is gradually decreased with temperature. A series of discussions of temperature effects on mean free path of hot carrier in the metal, resistance of semiconductor, and Schottky barrier height are investigated. The tremendous difference between voltage mode and current mode is that when the hot carriers cross over the Schottky barrier, the electric neutrality of semiconductor is changed, thereby generating an electrostatic charge. The electrostatic charge would have voltage drop across the semiconductor and metal that is the open circuit voltage (Voc). However, the generation of the photocurrent requires an additional step, that is, after the hot carrier surmount the Schottky barrier, it needs to pass through the entire semiconductor by diffusion to be received by the back electrode. When temperature decreases, the resistance of metal is decreased, the resistance of n-type Si is slightly decreased and the equivalent Schottky barrier is decreased as well. However, why photocurrent was not increased? We suppose that the current generated by hot carriers cannot diffuse efficiently in low temperature. The diffusion current would be decreased with decreasing temperature. However, the response of photovoltage was greatly increased with decreasing temperature that is an evidence that the equivalent Schottky barrier is decreased with decreasing temperature. In the second part of this thesis, we firstly focus on detecting light having a wavelength of 2 μm for a future Si photonics based communication system. In this study, we also utilized hot carriers and Si based photodetector to detect light having a wavelength in MIR regime. However, Fowler theory predict only light having photon energy larger than the Schottky barrier can be detected. This study explores the main reasons why MIR light still could be detected. For a hot carrier based device, after generating hot carriers, there are several paths to transport the carriers through Schottky barrier. We exclude some possibility such as free carrier absorption and trap assisted tunneling. Then we suggest the quantum tunneling effect may be the main mechanism for the detection of MIR light by hot carrier based device having a barrier height larger than photon energy. The experimental result is shown in accordance with the theoretical prediction in the shorter-wavelengths of 2 μm and 3.25 μm. For the MIR light having a wavelengths of 6 μm and 10.6 μm, the experimental results are also shown in line with theoretical trend. This thesis mainly discusses the effective collection mechanism of hot carriers based photodetectors working in both NIR and MIR regimes. The effective Schottky barrier could be decreased as decreasing the temperature and further drastically increase the response of photovoltage of hot carrier and Si based devices.