氮化鎵奈米結構與鈦酸鍶/釕酸鍶異質結構之載子動力學

Abstract

本論文利用飛秒雷射建構時間解析光譜系統，分別研究兩種不同材料的載子超快現象。第一部分為分析氮化鎵(Gallium nitride)薄膜與奈米柱之差異。第二部分為氧化物鈦酸鍶(SrTiO3, STO)及釕酸鍶(SrRuO3, SRO)所組成之STO/SRO/STO(001)異質結構。氮化鎵奈米柱比起薄膜擁有許多優點，像是沒有晶格缺陷、提高發光效率以及與基板間沒有張力。氮化鎵薄膜的光致螢光光譜存在能隙(3.4 eV)以外的波長(3.36 eV及3.30 eV)但在奈米柱光譜中並沒有發現。我們發現利用此激發 探測光反射系統同樣檢驗出此頻帶；同時也發現相對薄膜，奈米柱結構限制的光激載子的擴散通道。 我們利用兩種不同架構激發 探測光反射系統研究STO/SRO異質結構，我們展示SRO在材料內部時同樣可以做為聲子產生器。根據STO中布里淵散射結果，我們準確的得到STO近紫外光附近的的折射率。在變溫實驗(80 K~300 K)中，我們發現STO與SRO相變溫度分別為105 K，160 K，在相變過程中，我們發現聲子的生命週期減短。

In this thesis, we study the carrier dynamics of (i) GaN nanorod/film, and (ii) strontium titanate (SrTiO3)/strontium ruthenate (SrRuO3) heterostructures by utilizing time-resolved pump-probe spectroscopy. In the first part, the photoluminescence spectrum of GaN thin film exhibits side peaks at 3.36 eV and 3.30 eV besides bandgap 3.40 eV, while the spectrum of GaN nanorods shows only one peak at 3.40 eV. These phenomena were also observed in wavelength-dependent pump probe traces, and the absorption peaks were resolved for GaN thin film and nanorods. The carrier diffusion and surface trapping effects have also been investigated. The second part, two different pump-probe spectroscopy setups were employed to study STO/SRO heterostructures. We demonstrate that SRO beneath STO thin film can be served as phonon transducer. According to the Brillouin scattering, the refractive index of STO around near UV region was accurately obtained. In the temperature-dependent experiment (80K~ 300 K), we discovered dramatic changes of time-resolved reflectivity during the phase transition of STO (105 K) and SRO (160 K), respectively. The lifetimes of phonon were also shorter during the phase transitions of both materials.