以拉曼散射光譜分析氮化鎵與碳化矽薄膜之應變分佈 Analysis of strain in GaN and SiC films by Raman-scattering spectroscopy

Date
2012
Authors
鍾佩君
Pei-Chun Chung
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Abstract
我們測量氮化鎵與碳化矽薄膜的拉曼散射光譜,研究薄膜的晶格結構,並以直線掃描方式測量薄膜截面的拉曼散射光譜,分析拉曼特徵峰參數隨距離薄膜表面不同深度的變化,進一步建立膜厚與應變、殘餘應力的相關性。 首先,7.6 micrometre厚度氮化鎵薄膜的拉曼散射光譜顯示兩個顯著特徵峰(E2(high)與A1(LO)對稱性振動模),其頻率位置約為569 cm-1與739 cm-1,截面光譜顯示五個一階拉曼活性振動模,E2(low)、A1(TO)、E1(TO)、E2(high)、及E1(LO)拉曼特徵峰的頻率位置約為143 cm-1、531 cm-1、558 cm-1、567 cm-1及740 cm-1,這意味著,氮化鎵薄膜屬於六方烏采晶格結構。而5.0 micrometre厚度碳化矽薄膜表面與截面的拉曼散射光譜皆顯示TO與LO對稱性振動模,其頻率位置約為796.1 cm-1與970.5 cm-1,代表碳化矽薄膜為立方晶系結構。 接著,我們以直線掃描方式測量薄膜截面的拉曼散射光譜,發現7.6 micrometre厚度的氮化鎵薄膜之E2(high)對稱性振動模與5.0 micrometre厚度的碳化矽薄膜之TO振動模愈接近基板界面,其頻率位置展示藍移現象;反之,4.0 m厚度、及摻雜矽之4.0 micrometre與2.0 micrometre厚度氮化鎵薄膜之E2(high)對稱性振動模顯示紅移現象。 氮化鎵薄膜的E2(high)振動模出現紅移現象,代表薄膜內部存在伸張應變,此時薄膜與基板的晶格常數不匹配對薄膜應變影響較明顯;若是薄膜的E2(high)振動模出現藍移現象,則表示薄膜內部存在壓縮應變,此結果表示熱膨脹係數對應變的影響較顯著。氮化鎵薄膜與藍寶石基板交界面的應變量值約為8.0 × 10-4 ~ 1.16 × 10-3 。而碳化矽薄膜的TO振動模出現藍移現象,此與碳化矽薄膜的沉積溫度與速率有關,其與矽基板交界面處的應變量值約為1.78 × 10-4。
We present Raman-scattering studies of structural phases in GaN and 3C-SiC films. Cross-sectional Raman spectra are also used to detect the depth dependence of residual strain in both films. First, the Raman spectrum of 7.6 micrometre-thick GaN film shows two phonon modes at about 569 cm-1 and 739 cm-1, corresponding to E2(high) and A1(LO) symmetries. While the cross-sectional spectrum exhibits five first-order Raman modes at about 143 cm-1, 531 cm-1, 558 cm-1, 567 cm-1 and 740 cm-1 having symmetries E2(low), A1(TO), E1(TO), E2(high) and E1(LO). These results reflect the characteristics of wurtzite phase of GaN film. The TO and LO phonon modes are observed at about 796.1 cm-1 and 970.5 cm-1 in different configurations of 5.0 micrometre-thick 3C-SiC film, indicating its cubic structure. Second, we found that the peak positions of E2(high) and TO phonon modes exhibit a blueshift as one probes deeper from the surface into the sample toward the substrate in 7.6 micrometre-thick GaN film and 5.0 micrometre-thick 3C-SiC film, respectively. In contrast, the E2(high) phonon mode shows a redshift in 4.0 micrometre-thick GaN film and other Si-doped GaN films. The above data indicate two implications: (i) the blueshift observed in E2(high) phonon mode of GaN film is mainly due to the effect of lattice mismatch between film and substrate, while the redshift arises from the effect of thermal coefficient; (ii) the blueshift observed in TO phonon mode of 3C-SiC film is likely associated with different deposition conditions of the films. Finally, the values of strain on the interface between GaN film and sapphire substrate are ranging from 8.0 × 10-4 to 1.16 × 10-3. In the case of 3C-SiC film, it is about 1.78 × 10-4.
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拉曼散射, 氮化鎵, 碳化矽, 應變, Raman scattering, GaN, SiC, Strain
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