半導體/絕緣層/半導體異質接面結構於太陽能電池及光檢測器之應用

Abstract

本論文研究利用脈衝式雷射沉積（Pulsed Laser Deposition，PLD）技術在n-型Si基板上沉積非晶態銦鋅氧化物（a-IZO）薄膜，並以氧化矽（SiOx）做為IZO與Si基板間的絕緣層而製成SISHJS（Semiconductor-Insulator-Semiconductor Het-ero-Junction Structure，SISHJS）結構。以濕式製程成長之SiOx層、在基板溫度150°C鍍製IZO層的SISHJS元件呈現之光電轉換效率為2.2%、開路電壓（Open-circuit Voltage，Voc）為0.24 V、短路電流密度（Short-circuit Current Density，Jsc）為28.4 mA/cm^2、填充因子（Fill Factor，FF）為33.6%；研究結果並顯示SiOx絕緣層厚度及品質在SISHJS太陽能電池性能中扮演著決定性角色，利用乾式熱氧化製程方式，改變基板溫度調控SiOx絕緣層厚度製備SISHJS太陽能電池的研究發現基板溫度於200至300°C之間可得到厚度約為1.8至2.0 nm的SiOx層，其可改善元件性能，所得之Voc、Jsc、FF及光電轉換效率值分別為0.35 V、28.6 mA/cm^2、34.3%及3.4%；元件性能的改善應源自乾式熱氧化法能形成緻密的SiOx絕緣層並伴隨低晶体缺陷所致。為了能更進一步提升SISHJS太陽能電池元件的性能，本研究藉著改變a-IZO層的In/(Zn+In)比例調變a-IZO的載子密度，從而降低a-IZO/SiOx界面的界面捕捉密度（Interface Trapping Density，Dit），以探討其對SISHJS元件光伏特性的影響。實驗結果顯示高In含量的a-IZO薄膜可抑制a-IZO/SiOx界面的Dit值，推測原因為a-IZO/SiOx界面得到額外電荷載子補償，而能有效地提升SISHJS太陽能電池特性，在AM1.5模擬日光照明條件下量測所得之Voc為0.38 V、Jsc為45.1 mA/cm^2、FF為49.7%以及光電轉換效率值為8.4%。 在SISHJS於光檢測器的應用研究部分，當元件被能量高於Si能隙的入射光子照射時，SISHJS具備近紅外光至紫外光波長範圍的光檢測能力。此獨特光檢測特性的基礎機制起因為SISHJS界面的蕭特基能障（Schottky Barrier）對主要載子穿隧行為的抑制作用。SISHJS光檢測器在波長為650 nm可見光與波長為365 nm紫外光的照射下，可獲得的光響度（Photo-response）分別為35 AW^–1及6.15 AW^–1，且擁有低於80 msec的光響應時間。

The semiconductor-insulator-semiconductor hetero-junction structure (SISHJS) comprising of a transparent conductive oxide (TCO) film directly deposited on the n-type Si substrates clad with a SiOx insulating layer were fabricated by pulsed laser deposi-tion (PLD). First, the SISHJS solar cells containing amorphous indium zinc oxide (a-IZO) and the thin SiOx layer grown by wet process were prepared. Such a device fabricated at low substrate temperature of 150°C exhibits a conversion efficiency of 2.2%, open-circuit voltage (Voc) of 0.24 V, short-circuit current density (Jsc) of 28.4 mA/cm^2 and fill factor (FF) of 33.6%. Analytical results indicated that the thickness and quality of the SiOx layer plays a decisive role in the performance of SIS solar cells. Accordingly, the dry thermal oxidation was employed to grow the SiOx layer and its thickness was further adjusted by varying the substrate temperature during PLD. It was found that, with the SiOx layer thickness about 1.8 to 2.0 nm obtained at sub-strate temperatures of 200 to 300°C, the device performance was improved to have Voc, Jsc, FF and conversion efficiency of 0.35 V, 28.6 mA/cm^2, 34.3% and 3.4%, respectively. The dry thermal method resulted in dense SiOx layer with less crystalline defects and, hence, the improved device performance. In order to further improve the conversion efficiency value of device, the interface trap density (Dit) at the a-IZO/SiOx interface has to be reduced. We ex-amined the influences of carrier density in the a-IZO layer on the photovoltaic (PV) characteristics of SISHJS devices by varying the In/(Zn+In) ratio while keeping the SiOx/Si part identical. The increase of In/(Zn+In) ratio in a-IZO layer apparently low-ered the Dit at the a-IZO/SiOx interface, presumably due to the charge compensation at the interface. The device exhibited the excellent SIS solar cells performance with Voc of 0.38 V, Jsc of 45.1 mA/cm^2, FF of 49.7% and conversion efficiency of 8.4% under the AM1.5 il-lumination condition. This thesis study also found the TCO-based SISHJS is potentially sensitive to the lights with wavelengths ranging from near-infrared (NIR) to ultraviolet (UV) when the devices are irradiated by incident photons with energies above the optical energy band gap (Eg) of Si. The underlying mechanism for such a unique characteristic was attributed to the suppression of majority carrier tunneling resulted from the Schottky barrier established at interfaces of the SISHJS. The SIS photo-detectors made of the SISHJS exhibited the excellent photo-response of 35 AW^–1 and 6.15 AW^–1 under the illumination of 650-nm visible light and 365-nm UV light, respectively, with a quick photo-response time less than 80 msec.