利用拉曼散射分析聚焦式鎵離子束成像對砷化銦表面造成之影響

Abstract

本論文利用 focus ion beam (FIB) 之離子束對砷化銦表面成像，並使用Raman 量測觀察其破壞的程度。由於從變功率之 Raman 量測原生砷化銦試片，觀察到此砷化銦試片易受到雷射產生之載子影響產生 LO-plasmon couple mode (L-)，並觀察到當雷射功率降低至 2 kW/cm2 可排除此信號。接著量測受到 30 keV 之離子束成像之區域，出現受到晶格破壞之影響，產生被破壞的LO信號。使用spatial correlation model 擬合此信號，無論在高注量或低注量下，其correlation length呈現較短且對注量較無變化之趨勢。由於FIB 具有較高電流密度且較短停留時間，同一區域受到離子束破壞較嚴重，除此之外，FIB之加速電壓操作在較低電壓，因此傾向於在表面造成破壞，correlation length 較小。而correlation length 與離子注量較無關係，我們認為在進行離子成像時，因砷化銦之熱導係數較小，局部產生溫度較高，因此有修復晶格之現象。降低離子加速電壓後成像，隨注量變化較明顯，是因晶格受到離子碰撞產生之熱能較少，因此加熱修復晶格效應較小，correlation 變化較劇烈。由破壞區域產生的TO信號隨注量提升，且在高加速電壓下 TO 信號較明顯，原因是在大注量下發生 collision cascade 次數較多，破壞厚度隨注量增加，而較高之加速電壓可破壞之深度較大，因此 TO 信號較大，但是未破壞的 LO 反而在較高加速電壓信號較強。由於表面破壞較少，而在表面釋放熱能較大，因此表層也有未破壞的 LO 信號，整體 LO 信號較強。

In this thesis, the effect of focused ion beam (FIB) imaging on the crystallinity of InAs was investigated by Raman scattering. The as-grown InAs sample shown a LO-plasmon couple mode (L-) at high laser power due to low carrier concentration. We lower the laser power (2 kW/cm2) to avoid this effect. The FIB ion imaging region (beam energy at 30 keV) shown a damaged LO mode, and the spatial correlation model was applied to fit this mode. The fitting result gives a short correlation length characteristic and invariant with ion fluence. We attribute this large damage in InAs surface to the high beam current density and the low dwell time of the FIB imaging process. In addition, low ion energy will prefer to nuclear stopping which will lead to damage the sample. In the other hand, due to the low InAs thermal conductivity, the imaging process also lead to a high local temperature in the imaging region and annihilated the defects, which reduce the relation between fluence and correlation length. Lower the beam energy, the correlation length shows a relation to ion fluence because of the low local temperature which suppresses the effect of annihilated the defects. Moreover, the TO mode intensity from damaged layer increase with fluence due to the collision cascade effect increase the damaged layer thickness. At beam energy of 30 keV , the damaged layer thickness is thicker than that of beam energy of 5 keV/16 keV. However, the undamaged LO at beam energy of 30 keV shows a higher intensity. We attributed to large annihilated defects at surface region which revived the damaged crystallinity.