以電漿輔助化學氣相沉積外延高品質鍺薄膜

Abstract

這項實驗中使用自製的電漿輔助化學氣相沉積系統，透過四氯化鍺蒸氣與氫原子之間的反應，讓單晶鍺薄膜生長在矽基板表面；為了達到高結晶品質、低雜質含量、高薄膜鍍率以及低表面粗糙度等目標，我們對生長溫度、氣體流量、射頻功率和處理步驟進行了優化。 對結晶品質而言，生長溫度是非常重要的因素，室溫生長的鍺薄膜呈現非晶態，而生長溫度在 100~600℃ 之間呈現單晶態；450℃ 時結晶品質顯著提升，600℃ 時結晶品質顯著降低，這種結晶行為推測受到四氯化鍺蒸氣與氫原子之間的反應機制所影響，和氫的脫附現象也有關聯。 而生長溫度也會影響雜質脫附，室溫生長的鍺薄膜有氯殘留，但生長溫度高於 100℃ 的情況下幾乎完全脫附，其含量低於 EDX 的檢測極限；而生長溫度低於 300℃ 的鍺薄膜有氫殘留，並在薄膜生長後或是後續退火中造成薄膜剝落，但生長溫度高於 450℃ 的情況下幾乎完全脫附，在薄膜生長後或是後續退火中皆不會發生剝落。 氣體流量、射頻功率和處理步驟皆會改變四氯化鍺蒸氣和氫原子在反應腔體中的濃度，對薄膜鍍率和表面粗糙度而言是非常重要的因素；低氣體流量或低射頻功率的情況下，鍺薄膜容易受到來自起泡器和氣體管線中水氣或氧氣的影響，導致鍺薄膜被蝕刻並造成表面粗糙化；適當生長參數下可以降低其影響，並有效降低表面粗糙度。 電漿輔助化學氣相沉積所生長的單晶鍺薄膜在維持低雜質含量(低於 EDX 的檢測極限)、高薄膜鍍率(78 nm/min)和低均方根粗糙度(0.786 nm)的情況下，線性缺陷密度約為 10^4 1/cm^2；所製造的 700 nm 鍺薄膜光偵測器之暗電流密度以及響應率為 4.2∙10^(-4) A/cm^2 和 0.08 A/W (＠ 1550 nm)。

In this study, germanium thin films epitaxially deposited on silicon substrates by using a home-made plasma-enhanced chemical vapor deposition system (PECVD) with GeCl_4/H_2 as precursors. We optimized the growth temperature, gas flow rate, RF power, and operating procedures to achieve a impurity-free and high-quality monocrystalline germanium thin film with a low surface roughness and high deposition rate. The growth temperature is important for the crystalline quality. For the growth temperature in range of 100~600 ℃, the germanium thin film was monocrystalline. The crystalline quality significantly improved at 450 ℃, but was reduced at 600 ℃. This behavior may be attributable to the desorption of hydrogen and the reaction mechanism between germanium tetrachloride and hydrogen atoms. The growth temperature is also important for the the desorption of impurity. Under a growth temperature of approximately 25 ℃, chlorine was present in the germanium thin film, but no chlorine was detected for a growth temperature above 100 ℃. With a growth temperature below 300 ℃, a high hydrogen content remained in the germanium thin film, which can lead to blistering, but no blistering was detected after growth or during post growth annealing for a growth temperature above 450 ℃. The Gas flow rate, RF power, and operating procedures can change the concentration of germanium tetrachloride vapor and hydrogen atoms in the reaction chamber, which are important for the deposition rate and surface roughness. For a low gas flow rate or low RF power, the germanium thin film can be easily etched by oxygen and water vapor which come from the bubbler and gas pipelines. This etching can cause a rough surface, but for appropriate growth parameters, a low root mean square (RMS) of surface roughness can be achieved. For the monocrystalline germanium thin film grown by PECVD with a low impurity content (below the detection limit of energy-dispersive X-ray spectroscopy), low RMS surface roughness (0.786 nm) and high deposition rate (78 nm/min), the threading dislocation density (TDD) was approximately 10^4 1/cm^2. The Dark current density and responsivity of a 700 nm germanium thin film PIN photodetector were 4.2∙10^(-4) A/cm^2 and 0.08 A/W (＠ 1550 nm).