| 摘要: | 單光子雪崩崩潰光二極體 (Single-photon Avalanche Diode, SPAD) 是利用一光子吸收產生載子觸發衝擊游離的機制來偵測低強度的光，它在自動駕駛車的光達 (Light Detection And Ranging, LiDAR) 系統、三維影像 (3D imaging) 、光纖通訊 (Fiber-optic communication) 等近紅外光 (Near-Infrared) 偵測的領域皆有應用。砷化銦鎵/砷化銦鋁 (In0.47Ga0.53As/In0.48Al0.52As) 單光子崩潰二極體分別以砷化銦鎵作為吸收層偵測波長 1550 奈米的光，而砷化銦鋁作為累増層使電子加速並達到衝擊游離 (impact ionization) ，與較早開始研究的磷化銦 (InP) 來比較，砷化銦鋁有著以下潛力，以崩潰電壓隨溫度變化來說，擁有相同厚度的砷化銦鋁累増層的元件對溫度比較不靈敏，所以可操作溫度比較彈性;還有因為砷化銦鋁有較高雪崩崩潰機率 (avalanche breakdown probability) ，所以在相同暗計數率 (dark count rate) 下，光偵測效率 (photo detection efficiency) 會比較高，以上兩點使砷化銦鋁成為砷化銦鎵單光子崩潰二極體中累増層材料的另一選擇。
本研究中設計與製作正面收光且分離吸收、電荷、累增層 (separate absorption, charge and multiplication, SACM) 的平台式 (mesa type) 雪崩崩潰光二極體 (avalanche photodiode, APD) ，嘗試優化電荷層中的摻雜濃度來達到想要的電場分布、適當的崩潰電壓和擊穿電壓，使累増層的電場大於雪崩閥值且同時吸收層的電場小於雪崩閥值，製程上有在沉積保護層前使用硫化銨 ((NH4)2S) 對裸露的側壁硫化處理來減少表面漏電流。選用適當的保護層與金屬層厚度，製作出SPAD元件並量測其電流-電壓特性，在室溫下崩潰電壓約為 47 伏，隨著溫度下降，崩潰電壓的溫度係數在200K以下為 52 mV/K 和 200K以上為 16 mV/K，另一顆為 60 和 21 mV/K，由活化能大小分析其漏電流來源；為降低二次崩潰的影響，我們以閘控模式電路 (gated mode circuit) 操作，先紀錄崩潰訊號之波形，再進一步降溫至 77 K量測暗計數率與光計數率，並探討造成暗計數隨溫度變化趨勢的主要因素，以及討論元件光偵測效率低的原因。;Single-photon Avalanche Diode (SPAD) is used to detect low power light via absorbing one photon and generating carriers to induce impact ionization process. SPAD attracts great interest in the field of near-infrared detection such as light detection and ranging (LiDAR), 3D imaging, fiber-optic communication, etc. In0.47Ga0.53As/In0.48Al0.52As SPAD, consisting of InGaAs absorption layer for the detection of near infrared light and InAlAs multiplication layer for achieving impact ionization process, have several advantages in comparison with the SPAD using a InP multiplication layer. The breakdown voltage of SPAD with InAlAs multiplication layer is more stable to the temperature than that of SPAD with InP multiplication layer. InAlAs also has higher avalanche breakdown probability than InP, hence higher photon detection efficiency is expected. Therefore, InAlAs becomes an alternative candidate for next generation of InGaAs SPAD.
In this work, we design and fabricate top-illuminated and mesa type SACM avalanche photodiodes. With the aid of technology computer-aided design (TCAD) methods, we optimize the doping concentration of charge layers for gaining high enough electric field in the multiplication layer and low enough electric field in the absorption layer. We apply sulfur treatment on the exposed sidewall by using (NH4)2S before depositing passivation layer to reduce surface leakage current. With appropriate thickness of passivation and bonding pad, we have successfully carried out SPAD devices. The basic current-voltage characteristics of SPAD have been measured and analyzed. The breakdown voltage is around 47 V at room temperature and the temperature coefficient of breakdown voltage is 52 mV/K below 200K and 16 mV/K above 200K, 60 and 21 mV/K for another one. We analyzed the origin of dark current via different activation energy. For reducing the afterpulsing effect, we use gated mode operation. The avalanche signal is acquired under dark condition. Then the temperature dependences of dark count rate and photon count rate are measured for temperatures down to 77K. Finally, we discuss the main factors affecting the dark count rate and the photon detection efficiency of our devices. |
