III族氮化物奈米粒成長與光學特性研究

Abstract

本論文初期我們透過微螢光激發光譜(μ-PL)分析氮化鋁鎵薄膜表面直徑約6 □m的hillock微結構光學特性。實驗結果顯示相關於hillock的發光譜峰為351 nm (IH)，而在其周圍之氮化鋁鎵薄膜發光譜峰約為341 nm (Imatrix)；IH的強度隨著量測位置由邊緣往hillock中心移動時其強度明顯增強，半高寬由76 meV降低至53 meV，此量測結果顯示hillock為一種高發光強度之微結構。從變溫μ-PL的量測顯示IH譜峰與溫度關係呈現S型之變化，其轉換溫度約為120 K，此外紅位移亦較Imatrix來的小，這些結果均顯示出hillock相較於周圍之氮化鋁鎵薄膜而言，具有較低之鋁組成含量；另一方面，我們亦發現hillock之尺寸變大時(從6μm增加到11μm)，其鋁組成約降低~2%。 本論文提出一種特別的「流量調制磊晶法」製程技術用以成長III族氮化物奈米粒結構。首先，我們利用此技術成功地成長GaN奈米粒在低晶格不匹配度之Al0.15Ga0.85N緩衝層上，由奈米粒密度與成長溫度關係圖中，我們發現使用該方法成長之奈米粒在較低與較高成長溫度區間主要分別由反應物於薄膜表面之擴散機制與吸附機制主導；因為該方法在通入TMGa反應氣體時，是先形成Ga金屬，而Ga金屬與Al0.15Ga0.85N緩衝層晶格長數分別為4.51與3.18Å，晶格不匹配度高達41.8%，故此方法成長GaN奈米粒應為Volmer-Weber成長模式。另一方面，我們進一步深入研究GaN奈米粒之光學特性，針對平均高度分別為6.5, 7, 8.5 nm之奈米粒進行變溫螢光光譜研究，在PL峰點能量與溫度倒數關係圖中，使用Varshini 方程式模擬實驗資料，我們得到低溫區之侷限能量隨著奈米粒尺寸縮小而減小；此結果與使用Arrhenius方程式模擬PL積分強度與溫度倒數關係圖所獲得之低溫區活化能結果一致；而高溫區之活化能亦隨著奈米粒尺寸縮小而減小，此高溫區活化能代表著奈米粒內之載子因溫度升高躍遷至氮化鋁鎵能障層氮空缺能階所需之能量。 另一方面，我們亦成功地在GaN緩衝層上成長InN奈米粒，在覆蓋GaN披覆層後進行PL光學分析，當InN奈米粒高度降低至6.5 nm時，其PL峰點能量藍位移至1.07 eV；在溫度相依之PL光譜量測中，我們發現到PL譜峰位置與量測溫度之關係似乎不依循Varshini方程式之預測曲線，相較於InN塊材而言，其發光波長具有較高的溫度穩定性。而在PL積分強度與溫度倒數關係圖得知，InN奈米粒之高溫區活化能為73 meV明顯高於InN塊材之43 meV，顯示奈米粒之thermal quench程度相較於塊材而言較弱，意即InN奈米粒具有更佳之光學特性。最後，我們分別比較使用流率調制磊晶(FME)與MOCVD兩種成長方法成長之InN奈米粒結構與光學特性，實驗結果顯示在FME成長方法中，In adatom之擴散活化能明顯低於MOCVD成長方法；另外我們發現到使用FME成長InN奈米粒時，在In-rich的條件下成長之InN奈米粒擁有較佳的光學特性，當NH3流率調低至500 sccm時，在未覆蓋GaN披覆層下，已可以獲得PL光譜半高寬達63 meV之InN奈米粒。

In this thesis, the spatial variation of the optical properties of hillocks in Al0.11Ga0.89N films has been studied by using microphotoluminescences (μ-PL) microscopy. Theμ-PL spectrum revealed a strong emission peak (IH) at 351 nm from the hillock, besides the near-band-edge peak emission (Imatrix) at 341 nm. Moreover, the IH intensity increases significantly and its full width at half maximum (FWHM) decreases from 76 to 53 meV by probing across the hillock center. These indicated that the hillock is a strong emission structure. The temperature-dependent μ-PL measurements showed that the IH also has the S-shape behavior with a transition temperature of ~120 K which is lower than that of Imatrix. The redshift of IH is also smaller than Imatrix. Both indicated that the Al composition in hillocks is lower than the surrounding area. Moreover, we also observed that the Al composition decreased ~ 2% as the diameter of hillock increased from 6μm to 11μm. Otherwise, we proposed a new technique for fabrication III-nitride nanoparticles in flow rate modulation epitaxy (FME). Firstly, the self-organized GaN dot structure is successfully grown on a slightly lattice-mismatched Al0.11Ga0.89N epilayer using FME growth technique. From the variation of dot density with growth temperature, we can observe that the GaN dot growth is controlled predominately by the surface diffusion of Ga adatoms at substrate temperatures below 915℃ and by re-evaporation at higher temperatures. Because of the special alternating gas supply feature in FME, during the Ga source step, it is the Ga metal that is deposited on the underlying Al0.11Ga0.89N layer. This is because of the large lattice mismatch of 41.8% between the Ga metal (4.51 Å) and Al0.11Ga0.89N (3.18 Å). We consider that the GaN dot growth in this study is mainly through the Volmer-Weber growth mode. Moreover, the temperature dependent PL studies showed that at low temperature the localization energy, which accounts for de-trapping of excitons, decreases with the reducing dot size. The decrease in emission efficiency at high temperature is attributed to the activation of carriers from the GaN dot to the nitrogen vacancy (VN) state of the Al0.11Ga0.89N barrier layer. The activation energy decreases with reducing dot size. Secondly, the self-organized InN dots successfully grown on GaN epilayer using pulsed-growth mode growth technique. The PL properties of InN dots embedded in GaN were also investigated. We observed a systematic blueshift in the emission energy as the average dot height was reduced. The widely size-tunable emission energy can be ascribed to the size quantization effect. Temperature-dependent PL measurements show that the emission peak energies of the dots are insensitive to temperature, as compared with that of bulk film, indicating the localization of carriers in the dots. A reduced quenching of the PL from the InN dots was also observed, implying superior emission properties for the embedded InN dot structures. Finally, FME was also employed to synthesize self-assembly InN dots on GaN/sapphire substrate. Experimental results clearly indicate the adatom diffusion activation energy in FME is greatly reduced as compared to that in conventional growth method. We also demonstrate that InN dots prepared by FME under In-rich growth conditions possess much better optical quality than under N-rich growth conditions. Consequently, relatively high PL intensity with linewidth as narrow as 63 meV was realized for InN dots grown by FME at a NH3 background flow rate of 500 sccm, without any encapsulating layer.