| 摘要: | 由於積體電路製程的快速發展，元件尺寸愈發的小型化，傳統的封裝方式已經無法負荷微型製程的需求，將矽鍺光電元件直接成長於矽基板上的「矽光電子學技術」因而應運再生，而「矽基磊晶鍺薄膜技術」更是矽光電子學中的關鍵技術。本論文中，我們以 ”電子迴旋共振化學氣相沈積法” 研發矽基磊晶鍺薄膜，並以矽基磊晶鍺薄膜技術為基底，進行薄膜磊晶成長與分析，並將其應用於矽基鍺光偵測器與矽基III-V族太陽能電池等光電元件。由於電子迴旋共振化學氣相沈積法具有低表面損傷、高解離率、低溫成長等優點，在沉積矽基磊晶鍺薄膜時，能夠較有效率的提升薄膜的結晶率，並降低缺陷密度與表面粗糙度。此外，本研究的低溫薄膜成長技術除了可進一步減少熱應力在薄膜中造成的缺陷外，還可減少高溫所造成元件整合限制，有效地促進低溫製程的開發。
本論文研究主軸可分為三部分：
一、薄膜磊晶成長與分析：此項研究使用電子迴旋共振化學氣相沈積法超低溫(180°C) 磊晶成長非摻雜與重摻雜矽基鍺薄膜，並進行磊晶薄膜的特性、應力與介面分析。由於矽和鍺的晶格不匹配(4.2%)及熱膨脹係數差異(55%)，容易導致矽基磊晶鍺的品質不佳。而本項研究成功以超低溫(180°C)、高氫稀釋比(H2/GeH4= 16) 磊晶成長高品質的非摻雜矽基鍺薄膜，其X光繞射之半高寬可達495 arcsec。此項成果為本研究的主要成果之一，不但突破了傳統化學氣相沉積所需的高溫製程，同時也解決了矽鍺間熱膨脹係數差異的影響，更使磊晶鍺薄膜之品質達到不錯的水準。除此之外，本研究藉由調變ECR-CVD製程參數，可控制鍺薄膜之應變，其控制範圍從0.58%的壓縮應變到0.15%的拉伸應變。研究結果發現，腔體壓力由2.6pa提升至4.7pa時，鍺薄膜將會由0.58%的壓縮應變轉變為0.09%的拉伸應變；將磁場電流由40A提升至55A時，鍺薄膜將會由0.15%的拉伸應變轉變為0.15%的壓縮應變。而藉由調整鍺薄膜內的拉伸應變，將可使鍺材料的截止波長產生紅位移，使其光吸收範圍擴大，甚至使鍺材料由間接能隙轉變為直接能隙，實現矽鍺光二極體、矽鍺雷射之目標。而傳統的鍺薄膜大都是以高溫退火或調整結構等方式來進行應變調製，本研究則創新以調變成膜參數之方式，控制鍺薄膜內之應變，成功使鍺薄膜之截止波長由1550 nm移動到1650 nm。此項突破成功地使鍺薄膜光吸收範圍增加，對於提高光偵測器或太陽能電池之效率皆有極大之幫助。
另一方面，對光偵測器來說，為了減少響應時間的延遲，一般接合面均要求儘量接近光偵測器的表面，本研究則藉由調整氫氣流量(0到60sccm)成功成長厚度為30nm、摻雜濃度可達8×1020 cm-3的重摻雜P型超薄磊晶鍺薄膜，藉由薄型摻雜層的開發，將可大幅減少響應時間的延遲。此外，雖然重摻雜薄膜擁有極好的電性，但是高摻雜相對也容易造成較多的缺陷，也較不易形成單晶結構。而本項研究則兼具重摻雜、超薄、單晶幾項特點，其良好的電性(Resistivity可低於5x10-4 Ω-cm) 與薄型厚度(30nm)可減少光偵測器響應時間的延遲；若和多晶結構相比，單晶結構缺陷也較少，能有效地降低光偵測器之暗電流。
二、低溫矽基鍺光偵測器研發。本項研究是將上述研究之矽基磊晶鍺薄膜製作成垂直型矽基鍺光偵測器，其光響應可達0.17(A/W)，暗電流密度可達0.542 (mA/cm2)，而目前文獻中垂直型矽基鍺光偵測器之響應大多在0.1~0.3(A/W)之間；暗電流密度則介於50~0.5 (mA/cm2)之間，故本研究所開發之低溫矽基鍺光偵測器之特性有一定的水準，其並不會輸給高溫矽基鍺光偵測器。此外本研究顯示，藉由調整光偵測器中的重摻雜P型超薄磊晶鍺薄膜之氫氣流量，可減少光偵測器之暗電流。實驗結果顯示將氫氣流量由60 sccm降於0 sccm，光偵測器之暗電流可由10-6 A降至10-7A。
矽基鍺光偵測器其優點是易於微小化與積體化的整合於矽晶片上，若結合波導光路、雷射光源、光調變器等元件，將可發展以矽光電子學為主的光通訊技術。一般傳統磊晶製程，如分子束磊晶法或是超高真空化學氣相沉積法，往往需要超過600 ˚C的高溫，而過高的溫度常會形成熱應力、元件整合溫度受限及高熱成本等重重困難。而本研究之光偵測器從薄膜磊晶到元件製作，溫度都不超過200°C，對於改善上述之困難有極大之幫助。
三、III-V族薄膜與元件堆疊於矽基鍺薄膜特性分析：本項研究是將III-V族薄膜與元件磊晶堆疊於矽基鍺薄膜上，並進行III-V族薄膜與元件特性分析，其中鍺薄膜是由前述ECR-CVD所成長，III-V族薄膜則是由MOCVD所成長，此項研究目的是藉由鍺薄膜來解決III-V族與矽材料間的晶格不匹配問題。目前III-V族光電元件整合於矽晶片是以封裝整合於矽基板上，但隨著元件持續縮小，傳統封裝方式將逐漸不適用。如能將III-V族元件直接成長於矽基板上，除了可降低成本外，更有利於大規模量產製造。此外，對於太陽能電池來說，則可結合了III-V族太陽能電池高效率與矽基板低成本的兩項優點，將可利於開發出高效率、低成本的新形態太陽能電池。而本研究則藉由調變非摻雜本質鍺薄膜之氫氣流量(60~90sccm)，接著進行退火來研發適合當作緩衝層的磊晶鍺薄膜。研究結果顯示當氫氣流量在80sccm時，磊晶鍺薄膜品質最好，其結晶率可達94%； X光繞射之半高寬可達507arcsec。而將此薄膜進行700°C、5分鐘之高溫退火後，其X光繞射之半高寬可由507降至407 arcsec。接著我們將單晶GaAs薄膜成長於上述矽基鍺薄膜上，其X光繞射之半高寬可達220 arcsec、缺陷密度為106 cm-3。此成果已實現III-V族光電元件直接成長於矽基板上之可行性，並符合未來微小化，低成本的趨勢。
;In this information explosion era, the topic of energy and communication are payed attention to many people. The Ge/Si technology provide many advantage to develop those topic, such as: quasi-direct bandgap, lattice match between GaAs and Si materials, easy monolithic integrated on Si substrate. Therefore, the development of Ge/Si device is significantly payed attention for scientists. In order to obtain high performance Ge/Si device, the Ge epilayers quality is very important. In this study, we investigated the Ge epilayers by high plasma density electron cyclotron resonance vapor deposition method and apply these films for solar cells and photodetectors development.
A suitable Ge film growth method can significantly enhance the Ge epilayer quality. The electron cyclotron resonance chemical vapor deposition provide a number of advance to grow Ge epilayers, for example: high crystallinity, low growth temperature and less ion damage to the surface. In this dissertation, the study can divide into three parts:
In the first part, we use electron cyclotron resonance chemical vapor deposition to investigate the strain behavior in Ge epilayers grown on silicon at a low temperature of 220°C. The strain in the Ge epilayers is transformed from compressive (-0.567%) to tensile (0.15%) as the process pressure decreases and main coil current increases. This tensile strain is due to intrinsic stress in the Ge epilayers at high process pressure and low main coil current. Besides, the Ge atoms have higher kinetic energy and shorter mean free path at low process pressure and high main coil current, which causes atomic bombardment effect on the Ge surfaces frequently. Thus, the intrinsic stress in Ge epilayers become compressive. The absorption coefficient of tensile and compressive strain in Ge films are measured using a UV–VIS-NIR spectrophotometer. The results show the absorption coefficients of the tensile strain Ge epilayer has a redshift condition on the absorption edge compare with compressive strain Ge epilayers. Finally, the structure information of the Ge epilayers is identified by atomic force microscopy and transmission electron microscopy. This strain control technology is modulated by film growth parameter, which can adjust the Ge bandgap for the device requirement.
In second part, we study the heavily boron-doped hydrogenated Ge epilayers are grown on Si substrates at a low growth temperature (220°C). The quality of the boron-doped epilayers is dependent on the hydrogen flow rate. The optical emission spectroscopic, X-ray diffraction and Hall measurement results demonstrate that better quality boron-doped Ge epilayers can be obtained at low hydrogen flow rates (0 sccm). This reduction in quality is due to an excess of hydrogen in the source gas, which breaks one of the Ge-Ge bonds on the Ge surface, leading to the formation of unnecessary dangling bonds. The structure of the boron doped Ge epilayers is analyzed by transmission electron microscopy and atomic force microscopy. In addition, the performance, based on the I-V characteristics, of Ge/Si photodetectors fabricated with boron doped Ge epilayers produced under different hydrogen flow rates was examined. The photodetectors with boron doped Ge epilayers produced with a low hydrogen flow rate (0 sccm) exhibited a higher responsivity of 0.144 A/W and a lower dark current of 5.33×10-7 A at a reverse bias of 1 V.
In third part, we present high quality GaAs epilayers grow on virtual substrate with 100nm Ge buffer layers. The thin Ge buffer layers were modulated by hydrogen flow rate from 60 to 90 sccm to improve crystal quality by electron cyclotron resonance chemical vapor deposition (ECR-CVD) at low growth temperature (180°C). The GaAs and Ge epilayers quality was verified by X-ray diffraction (XRD), and spectroscopy ellipsometry (SE).The full width at half maximum (FWHM) of the Ge and GaAs epilayers in XRD is 406 arcsec and 220 arcsec, respectively. In addition, the GaAs/Ge/Si interface is observed by transmission electron microscopy (TEM) to demonstrate the epitaxial growth. The defects at GaAs/Ge interface are localized within a few nanometers. It is clearly showed that the dislocation is well suppressed. The quality of the Ge buffer layer is the key of III–V /Si tandem cell, Therefore, the high quality GaAs epilayers grow on virtual substrate with 100nm Ge buffer layers is suitable to develop the low cost and high efficiency III-V/Si tandem solar cells. |
