雷射熱退火與光電元件之模擬

Abstract

矽與鍺為符合摩爾定律重要的材料。鍺的載子遷移率較矽高，未來可應用在通道材料。鍺之吸收波段包含1310奈米及1550奈米為光通訊重要頻段，為波導型光偵測器的重要材料。矽之高介電質材料鈍化對於太陽能裝置是不可或缺的製程，聚乙烯醋酸乙烯酯薄膜下方的二氧化鈦/三氧化二鋁的薄膜可作為鈍化層，且為良好抗反射層。非晶矽太陽能電池增加表面結構可提高短路電流，然而高深寬比的表面結構會使得靜電場分布不均勻而有讓更多的載子複合，造成能量轉換效率下降。 論文的第一部分是利用三維數值方法模擬脈衝雷射熱退火於成長於絕緣層上矽基板和矽基板之磊晶鍺，磊晶鍺由化學氣相沉積且原地摻雜磷，隨著雷射之能量密度增加，磊晶鍺薄膜的模擬融化深度以及量測到的片電子密度也跟著增加，測得之片電子密度與模擬的融化深度呈正相關。雷射熱退火所造成之溫度變化與磷在鍺中的固態及液態溶解度可擬合量測之片電子密度與雷射能量密度之關係。若能量密度再增加使得矽與磊晶鍺薄膜皆融化，由穿隧式電子顯微鏡之截面圖與能量色散X-射線光譜可觀察到矽和鍺之間則會有互混現象。 透過降低n型與p型單晶矽之表面複合速率，矽基板之光致發光強度會因由電漿增強式原子層沉積之三氧化二鋁與二氧化鈦/三氧化二鋁堆疊層而增強。由於降低介電質中的介面陷阱密度與負電的氧化層電荷降低表面復合速率。二氧化鈦/三氧化二鋁堆疊層可當作抗反射層。透過合成氣體熱退火造成額外增加帶負電的氧化層電荷，可使二氧化鈦/三氧化二鋁的場效鈍化再被增強。二氧化鈦/三氧化二鋁亦可當作抗反射層。 可透過有限差異時域光學模擬結合電性模擬來研究具有表面紋理結構之非晶矽太陽能電池的損失機制。復合電流密度會受到表面紋理之身寬比的影響，深的表面紋理會造成非均勻的靜電場與增強的電子電洞對復合，使得短路電流密度下降。因此，即使較深的表面紋理結構可以降低反射，增加吸收，但仍有最佳化之深寬比。模擬結果顯示深寬比在0.2到0.4，寬度~400 nm，可得到最佳的能量轉換效率。 波導光偵測器模擬同樣採用時域有限差分計算光響應，再將光響應考慮進電性模擬。光偵測器的頻寬是利用脈衝響應與傅立葉轉換計算出。模擬考慮摻雜濃度對鍺的載子遷移率之影響。在1310 nm的波長下，鍺的寬度較波導寬時，模擬光偵測器響應率大於0.95A/W。然而因為電容值增加，頻寬因光偵測器的寬度增加而降低。而p與n摻雜的中間未摻雜區域之寬度對於頻寬有明顯的影響，當未摻雜區域寬度較小時，會產生較大的電容，而降低頻寬。未摻雜區域之寬度增加時，會造成電場降低，頻寬降低。可得到最佳的未摻雜區域之寬度200到300nm。

Si and Ge are important materials to meet Moore’s law and more-than-Moore. Ge has higher mobility than Si. Besides, heavily doped Ge can be source/drain stressor for p-type metal-oxide-semiconductor field-effect transistor with Si or SiGe channel. Ge is the important material for waveguide photodetector due to the absorption among 1310 and 1550 nm. Passivation of Si with high-k materials is important for solar devices. The TiO2/Al2O3 stack films show antireflection capability under ethylene-vinyl acetate layers. The texture structures of a-Si solar cells increases the short circuit current. However, deep textures degrades the power conversion efficiency due to the strong recombination caused by non-uniform distribution of static electric fields. In the first part of this dissertation, temperatures of pulsed laser annealed epi-Ge layers on SOI and bulk Si substrates are simulated using a 3D numerical method. Epi-Ge is phosphorus-doped by in-situ chemical vapor deposition doping. As the laser fluence increases, both the simulated melt depth of epi-Ge and the measured sheet electron density increase. The measured sheet electron density is positively correlated to the simulated melt depth of epi-Ge. To fit the sheet electron density, the solid and liquid solubility of P in Ge is used according to the simulated temperatures during the laser annealing. As both Si and epi-Ge are melted in the simulation, an intermixing between Ge and Si is observed by cross-sectional transmission electron microscopy and energy dispersive x-ray analysis. By reducing effective surface recombination velocities on both n-type and p-type monocrystalline Si, the photoluminescence intensity was improved by the Al2O3 and Al2O3/TiO2 stack layers deposited by the plasma-enhanced atomic layer deposition enhance. The low effective surface recombination velocity is due to the reduced interface trap density and negative oxide charges in the dielectrics. By the 2D numerical simulation, the correlation of the effective surface recombination velocity between the photoluminescence intensity is investigated. The AM1.5-weighted front side reflectance to 11.8% was reduced by the bilayer stacks without texture. The field-effect passivation of Al2O3/TiO2 films is further enhanced by a forming gas annealing due to the additional increase of the negative oxide charge density. The loss mechanisms of amorphous Si solar cells with realistic conformal layer structures was investigated by the finite-difference time-domain optical simulation combined with the electrical simulation. The recombination current densities are strongly influenced by the aspect ratio of the textures. The deep texture results in the non-uniformity of the static electric field and the significant recombination to decrease the short circuit current density. As a consequence, there is an optimal textured structures to enhance the power conversion efficiency, although the deeper texture can trap more light. Our calculations show for the studied design that an optimal aspect ratio between 0.2 and 0.4 for the depth to width texture ratio with widths around 400 nm lead to highest power conversion efficiency. The lateral waveguide photodetectors were simulated by the same method as the solar cell simulation. The impulse response and the fourier transform were used to obtain the bandwidth of photodetectors. The doping-dependent mobility model of Ge is included in our simulation. The bandwidth decreases with increasing width of photodetector, because the capacitance increases. There is an optimal gap width to obtain the optimal 3dB bandwidth. As the gap is close to zero, Si p-n junction forms, and the capacitance increases. As the gap is larger than ~400 nm, the strength of the electric field decreases.