以脈衝雷射沉積和連續光雷射退火封裝高品質的鍺薄膜製作紅外光偵檢器

Fabrication of high quality germanium thin film for infrared ray photodetector by pulsed laser deposition and scanning continuous wave laser annealing

陳亮宇(Liang-Yu Chen)

指導教授 ： 林明緯

國立清華大學/原子科學院/工程與系統科學系/碩士(2020年)

摘要

紅外光在熱影像學和高頻寬光纖通訊上佔有很重要的地位。然而常用之基於銦鎵砷合金和磷化銦合金所製成之中紅外光元件卻相對較昂貴。隨著近年 高頻寬 通訊 技術的普及，相應的紅外光元件的需求也持續提高因此越來越多研究試著找出新製程與材料以製造出低成本且高頻寬的紅外光偵測器。
在所有可行的材料中，鍺有相當大的潛力。相對於銦、鎵之類的三族或五族半導
體鍺基元件可提供更高的頻寬與相對低廉的價格，讓它成為高頻通訊的首選材料。
但因鍺為一種非直接能隙材料使得 鍺光電原件的效率較為低落，而且因鍺的晶格常數較矽大使得在矽上成長鍺薄膜時會在鍺薄膜內造成大量的缺陷和收縮應力。近
年有研究指出可以藉由分段成長鍺薄膜並階段輔以高溫熱退火操作可顯著降低線差排(threading dislocation)並將壓應變(compressive strain)轉換為張應變 (tensile strain)同時還能減少能隙。
在此工作中，我們嘗試使用脈衝雷射沉積和掃描式連續光熱退火在矽基板上成長
鍺薄膜並封裝成中紅外光偵測器。 在使用脈衝雷射沉積以製作薄膜的操作中結果顯
示鍺靶材緻密度可顯著影響製出的鍺薄膜品質。此外在使用連續光雷射執行退火操作時， 可藉由改變光束功率 、 腔體內之環境氣壓、光束掃描樣品表面之速度、 掃描次數以及基板初始溫度來最佳化退火後樣品之張應變量和減少線差排密度 。 檢測工具選用X光繞射 (X-Ray diffraction)量測張應變並用 掃描式電子顯微鏡 (Scanning Electron Microscope)量測線差排。結果顯示將有一微米厚鍺薄膜之試片加熱至 450oC後並用20 W雷射功率與400 torr氬氣的環境氣壓執行退火時， 線插排密度可由 107 cm-2降至2×106 cm-2。然而改變雷射掃描次數不會明顯改變薄膜張應變量。
我們也發展後續的元件製程包含使用藥品參雜製作PIN二極體的P層，蝕刻鍺
薄膜將多餘的部分去除並蒸鍍電極將樣品封裝成一個光偵測器以量測鍺光偵測器的 暗電流和對 900至 1600 nm紅外光之響應率。相比於未退火之樣品，退火操作可使元件對1550 nm紅外光的響應率提升約50倍至0.4 A/W。然而此元件在-2 V外加偏壓的暗電流密度也增高至 5×10-2 A/cm-2，推測應為脈衝雷射沉積製程中，薄膜表面所產生的鍺液滴 (particulate)造成的影響 。

關鍵字

脈衝雷射沉積 ； 連續光雷射退火 ； 鍺薄膜

並列摘要

Infrared (IR) detection is important in the thermography and in the development of next-generation broadband fiber communication. However, commercial IR products made by InGaAs or InP are typically very expensive, primary because of the cost of these materials. In these years, the demand of IR photodetector rises along with the rapidly maturing technologies of high-bandwidth communication. Efforts are being made to invent new manufacturing processes and/or novel materials that can ultimately facilitate the fabrication of low-cost, high-bandwidth IR detectors in the near future.
Germanium (Ge) represents a promising material for fabricating novel IR detector because it has a higher bandwidth and a lower price than that of III or V semiconductors like indium, gallium, and arsenic. However, the indirect bandgap structure in Ge can possibility result in a low efficiency in photoelectric conversion. Furthermore, since Ge has 4% larger lattice constant than that of silicon (Si), the threading dislocation and the compressive strain will appear when depositing a Ge thin film upon a Si substrate. Under this condition, a multi-step deposition method including a related thermal annealing operation, can be a valid approach to decrease the density of threading dislocation (TD) and to convert the compressive strain into a tensile train, as well as the realization of a lowered bandgap that can extend the cur-off wavelength of IR detection [1].
My research focuses on developing a combined method of the pulsed laser deposition (PLD) and the scanning continuous wave laser annealing (SCWLA) that can be applied to fabricate a Ge-based IR detector. For the study about PLD, the results show that the consistency of used Ge target is the major factor that determine the quality of produced Ge thin film. Regarding the SCWLA, parameter search is conducted to optimize the laser power, the pressure of ambient gas inside the chamber, the scan speed of laser beam, and the number of scans for reducing the TD density and increasing the tensile strain in the annealed Ge thin film. Here, the appeared TD density is measured by the Scanning Electron Microscope and the tensile strain inside the file is identified by the X-ray diffraction. Results show that when the sample with the 1-µm thick Ge film is heated to 450oC, the TD density can be reduced from 107 cm-2 to 2×106 cm-2 with the use of the 400-torr argon pressure and the 20-W laser power for annealing. However, no prominent change to the tensile strain of the Ge film is observed by vary the scan number of the annealing laser beam.
A fabrication process of an IR detector, including the doping of Ge film into a p-type layer of a p-i-n junction, the etching of Ge film, and the coating of electrodes on the sample is also implemented. For the fabricated IR detector, the responsivity increases about 50 times higher to 0.4 A/W at 1550 nm when compared to the one without annealing. However, the dark current density rises to 5×10-2 A/cm-2 at -2 V biased voltage, which might be resulted from the particulate generated in the Ge film during the PLD process.

並列關鍵字

Pulsed laser deposition ； continuous wave laser annealing ； Ge thin film

參考文獻

[1] J. Michel, J. Liu, and L. C. Kimerling, "High-performance Ge-on-Si photodetectors," Nature Photonics, vol. 4, no. 8, pp. 527-534, 2010/08/01 2010.

[2] Z. Huang, Germanium photodetector integrated with silicon-based optical receivers. 2019.

[3] L. M. Giovane, H.-C. Luan, A. M. Agarwal, and L. C. Kimerling, "Correlation between leakage current density and threading dislocation density in SiGe p-i-n diodes grown on relaxed graded buffer layers," Applied Physics Letters, vol. 78, no. 4, pp. 541-543, 2001/01/22 2001.

[4] Y. Ishikawa and S. Saito, "Ge-on-Si photonic devices for photonic-electronic integration on a Si platform," IEICE Electronics Express, vol. 11, no. 24, pp. 20142008-20142008, 2014.

[5] S. B. Samavedam, M. T. Currie, T. A. Langdo, and E. A. Fitzgerald, "High-quality germanium photodiodes integrated on silicon substrates using optimized relaxed graded buffers," Applied Physics Letters, vol. 73, no. 15, pp. 2125-2127, 1998/10/12 1998.