二六族化合物半導體及奈米結構的光學性質

Abstract

本論文的第一部份使用鑽石高壓及半導體光頻譜技術，探討二六族化合物硒化鎘鋅(鎘含量佔0至32莫耳百分比)和碲化錳鋅(錳含量佔0至26莫耳百分比)半導體薄膜的光學聲子模態和晶體結構特性。藉由觀察拉曼散射光譜可以確定硒化鎘鋅以及碲化錳鋅的光學聲子模都屬於中間態。我們以二次多項式去擬合縱向光學聲子頻率隨壓力變化的關係，求得格留乃森 (Grüneisen) 參數。在鎘濃度較高的硒化鎘鋅樣品中發現由外加壓力引起的共振拉曼散射增強效應，這個共振效應也被用來研究在常溫常壓時拉曼訊號相當微弱之碲化錳鋅半導體。高壓拉曼實驗結果顯示，半導體轉變為金屬相的開始壓力會隨著鎘和錳元素的參雜濃度增加而下降。當鎘(錳)濃度從0增加到32莫耳百分比(0增加到26莫耳百分比)時，相轉變壓力從13.6降至9.4 (15.7至10.3) GPa。此外，實驗結果顯示在常壓時，錳離子的參雜會增加碲化鋅半導體的離子性質，然而外加壓力卻會降低碲化錳鋅的離子性。 利用拉曼光頻譜來觀察以氯摻雜之n型硒化鋅薄膜半導體 (電子濃度範圍從8.2 × 1015 cm-3 到1.8 × 1018 cm-3) 的聲子振動、電子濃度以及晶體結構的特性。實驗發現，氯摻雜之n型硒化鋅半導體的拉曼光譜會受到縱向光學聲子以及自由電子耦合效應的影響。因此藉由拉曼散射效率方程式以及介電方程式來擬合譜線，可以求得樣品的電子濃度和載子遷移率。實驗發現半導體轉變為金屬相的開始壓力會隨著電子濃度的增加而下降，這個現象指出樣品參雜電子也會造成晶體的不穩定。此外，我們發現外加壓力會使得縱向光學聲子和自由電子的耦合效率變低，可能是因為加壓造成另一個深層施子能態的產生，使得樣品的電子濃度降低所造成。 本論文的第二部份我們發現由Kohlrausch提出的指數延伸定律與等電性碲硒化鋅半導體螢光的衰減頻譜有極高的關聯性。當碲原子的濃度增加時，延伸參數 (β) 會先降低而後增加，這個結果可以用跳躍遷移 (hopping-transport) 以及能量轉移的模型來解釋。當等電性碲原子捕捉態 (trap states) 的數量增加時，碲硒化鋅半導體光激發光的衰減速率會變慢、光頻譜的半寬會變寬；然而，當碲原子局域性能態與共價帶的邊緣能態開始混成之後，碲硒化鋅半導體光激發光的生命期和光頻譜的半寬都會產生下降的趨勢。 最後，我們用時間解析光頻譜技術來討論碲化鎘膠質量子點之間能量轉移的現象。量子點間的能量轉移的效率不但會與小尺寸量子點的放射譜和大尺寸量子點的吸收譜的重疊程度有關，還會與量子點之間的距離有關。在混合的量子點溶液和固態薄膜中，發現小尺寸的量子點的光激螢光強度和生命期會變低，然而大尺寸的量子點的光激螢光強度和生命期會變高，這些實驗結果是鄰近量子點極性耦合產生的共振能量轉移的直接證據。在混合的量子點固態薄膜中，隨著量測能量的降低，延伸參數 (β) 會隨之增加，並且趨向一，這個現象直接反映了有效率的能量轉移從小的量子點至大的量子點。

The first part of this thesis explores the optical phonon modes and crystal characteristics of Zn1–xCdxSe (0 ≦ x ≦ 0.32) and Zn1-xMnxTe (0 ≦ x ≦ 0.26) thin films using high-pressure techniques and optical spectroscopy. The phonon Raman spectra of ZnCdSe and ZnMnTe all exhibit intermediate phonon modes. The pressure-dependent longitudinal optical phonon frequencies and the Grüneisen parameter were obtained by quadratic polynomial fitting. The pressure-driven resonant Raman scattering effect was observed in Zn1–xCdxSe with a high Cd concentration, and it was exploited to analyze the crystal characteristics of ZnMnTe, which has weak Raman signals under ambient conditions. The pressure at the onset of metallic phase transition (Pt) declines as the Cd and Mn contents increase. As the Cd (Mn) concentration increases from 0 to 0.32 (0 to 0.26), the Pt falls from 13.6 to 9.4 (15.7 to 10.3) GPa. The vibrational, electronic, and crystalline characteristics of n-type chlorine-doped ZnSe (ZnSe:Cl) layers with a carrier concentration from 8.2 × 1015 to 1.8 × 1018 cm-3 were also studied by Raman spectroscopy. The spectral lineshapes of the longitudinal-optical-phonon and plasmon coupling (LOPC) modes are analyzed using the Raman scattering efficiency and the dielectric function to determine the electron densities and mobility. The metallic phase transition pressure of ZnSe:Cl layers decreases as the carrier concentration increases, indicating that n-type doping reduces crystal stability. Additionally, the pressure-induced weakening of the LOPC efficiency suggests that pressure tends to degrade the n-type characteristic of ZnSe:Cl because of the emergence of a new deep donor-like state. In the second part of this thesis we found that the Kohlrausch’s stretched exponential law is correlated well with the PL decay profiles of ZnSe1−xTex. As the Te concentration increases, the stretching exponent β initially declines and then monotonically increases. This result can be understood using the hopping-transport and energy transfer model. An increase in the number of isoelectronic Te localized traps reduces the PL decay rate and increases the linewidth, whereas the hybridization of the Te localized states with the valence-band edge states reduces both the lifetime and the linewidth. Finally, the transfer of electronic energy between CdTe colloidal quantum dots (QDs) was studied using time-resolved photoluminescence (PL) spectroscopy. The efficiency of energy transfer in QDs depends not only on the spectral overlap of small dots emission and large dots absorption, but also on the inter-dot distances. The quenching of the PL intensity (lifetime) of small dots, as well as an enhancement of large dots in mixed solution and a solid film are evidence of a resonant transfer of energy due to dipolar coupling between proximal QDs. In a solid with mixed QDs, the stretching exponent β increases as the probe-energy declines, and approaches one, implying efficient energy transfer from smaller to larger QDs.