利用原子層沉積技術成長氧化鋅薄膜於矽晶圓上之研究

Abstract

氧化鋅(Zinc Oxide, ZnO)為Ⅱ-Ⅵ族化合物半導體，具有直接能隙(Direct Bandgap)的能帶結構，能隙的大小為3.37eV，所對應的發光範圍在紫外光區段，並具有高達60meV的Exciton Binding Energy。此外，由於ZnO具有透光率高以及電阻率低的特性，亦可作為透明導電層的應用。因此ZnO被認為是未來非常重要的光電材料。本論文利用原子層沉積(Atomic Layer Deposition, ALD)技術在矽基板上成長高品質的ZnO薄膜。首先我們使用兩階段成長以及退火的方式在晶面為(111)的矽基板上成長具有(002)擇優取向的高品質ZnO薄膜，結果顯示緩衝層的晶體品質決定了整個ZnO晶體結構的優劣。此外，藉由ALD技術可以精準控制薄膜厚度與成分的特性，我們亦製作奈米混層結構，在ZnO中摻雜Al以增加電子濃度並降低電阻率，並製作出符合抗反射層所需折射率的薄膜。我們成功的在Al摻雜濃度為2%時得到最低電阻率為1.78×10-3 Ω-cm與折射率為1.96的ZnO:Al薄膜，並在波長範圍545nm至555nm間得到小於0.2%的反射率。最後我們將ZnO:Al薄膜成長在具有Al2O3表面鈍化層的矽基板上製作n-ZnO/Al2O3/p-Si穿隧二極體。當元件在逆向偏壓操作時，由於ZnO:Al薄膜具有良好的抗反射效果，元件吸收波長為532nm的入射光之光電轉換量子效率可達97.8%。另一方面，當元件操作在順向偏壓時，由於ZnO:Al薄膜內的缺陷提供了p-Si內的電洞利用穿隧效應進入ZnO:Al薄膜的途徑，這些電洞進而與ZnO:Al導電帶的電子結合而發光，因此我們量測到波長在590nm的可見光發光頻譜。

inc oxide is a II-VI semiconductor material with direct band-gap of 3.37eV corresponding to the light emission in the ultraviolet region. ZnO also has large excton binding energy (~60meV). In addition, ZnO can be used as transparent conducting oxide since it has low resistivity and high transparency in the visible region. As a result, ZnO is considered as a promising material for the application to the optoelectronics. In this thesis, we studied the growth of high-quality ZnO thin films on the silicon substrates by atomic layer deposition (ALD). We used two-step approach with high-temperature post-deposition annealing to grow ZnO with high (0002) orientation on the Si (111) substrate. It was shown that the crystal quality of the buffer layer significantly influenced the quality of the ZnO films. We also deposited the Al-doped ZnO (ZnO:Al) thin films to act as the transparent conducting oxide as well as the anti-reflection coating layer on the silicon substrates. The ZnO:Al thin film with the refractive index of 1.96 and the resistivity as low as 1.78×10-3 Ω-cm was achieved. The reflectivity of the ZnO:Al film on the silicon substrate was below 0.2% in the wavelength region between 545nm to 555nm. Finally, we deposited the n-type ZnO:Al thin film on the p-type silicon substrate with a thin Al2O3 surface-passivating layer to form an n-ZnO/Al2O3/p-Si tunneling diode. When the device was operated at reverse bias to act as a photodetector, the external quantum efficiency was up to 97.8% with the incident light at the wavelength of 532nm due to the anti-reflection coating effect of the ZnO:Al film. When the device was at forward bias, the holes in the p-type silicon tunneled into the defect states in ZnO:Al and then radiatively recombined with the electrons in ZnO:Al film, thus yielding an electroluminescence spectra at the wavelength at 590nm.