掺鎂氧化鋅材料之激子–聲子交互作用研究

Abstract

我們利用熱擴散與溶膠-凝膠法製備氧化鎂鋅合金半導體。 利用拉曼散射與光激發光實驗研究氧化鎂鋅合金的晶格動力學、組成擾動以及激子相關特性(即激子與聲子、自由載子或其他激子的交互作用)在掺雜不同鎂成分之下的變化情形。 我們藉由x-光繞射與光激發光的光譜概略地估計氧化鎂鋅合金中鎂所取代鋅的比例。 從室溫光激發光與吸收光譜譜峰的藍位移以及晶格常數的改變，可以證明鎂離子摻入氧化鋅晶體中而且成功地佔據鋅離子的位置。 一般來說，由於合金引起的組成成分擾動會使得拉曼光譜中各種聲子模態發生變化，如局部晶格改變引發的振動模態、因擾動而活化的振動模態、聲子頻率的位移、譜形變寬與不對稱等特性。 因此，我們利用拉曼光譜分析當鎂掺入氧化鋅奈米結構後，氧化鎂鋅晶格振動模態的變化情形。 除了氧化鋅原有的聲子模態之外，我們亦觀察到383與510 cm-1兩個異常的聲子模態，據推測可能來自於鎂相關的振動模。 由於鎂離子與鋅離子在半徑上的差異，鎂的摻入會產生某些新的晶格缺陷抑或是原有的晶格缺陷因擾動而活化進而出現一些額外的振動模。 我們用spatial correlation model探討組成成分的擾動造成拉曼光譜譜形變寬與不對稱的行為;而這個理論模型的概念是考慮聲子的傳播受到合金位能異動(alloy potential fluctuations)的侷限與晶粒分布(grain size distribution)的影響。 因此可藉由分析氧化鎂鋅奈米粉末掺鎂的含量多寡與E2(high)聲子的頻率變化之關聯性得知。 我們發現隨著鎂含量的增加，合金位能異動造成晶粒相對變小，那是由於受到氧化鎂對氧化鋅溶解度的限制。 多餘的鎂離子無法取代氧化鋅晶體中鋅離子的位置因而形成氧化鎂群環繞氧化鎂鋅晶體。 在輻射放光過程中雙激子(biexciton)的產生是束縛兩個激子形成，因此激發能量與輻射放光的強度之間會有接近於2的一個理想指數關係。 我們在氧化鋅粉末中觀察到聲學聲子(acoustic phonon)與光學聲子(optical phonon)在雙激子形成的過程中扮演著不可或缺的角色。 研究發現當溫度低於某個程度，也就是激子動能低於光學聲子動能時，激子或載子(carriers)冷卻(relaxation or cooling)過程僅有聲學聲子的放射，此時的指數值是小於1的;然而當溫度提高到使得激子動能接近於光學聲子最低動能的時候，激子或載子冷卻過程多了光學聲子的參與，此時的指數值則是接近於理論上的理想值。 最後，我們使用變溫光激發光光譜研究掺雜不同鎂成分的氧化鎂鋅粉末其激子與縱光學聲子(longitudinal optical phonon)耦合強度的變化。 激子與縱光學聲子的耦合強度可經由分析激子的能量隨著溫度變化所產生的位移來決定。 除此之外，觀察到隨著鎂成分的提高激子的束縛能相對增加，其原因可能是激子位能受到合金引發的組成擾動而產生變化。 激子束縛能的增加是由於激子波爾半徑(Bohr radius)的減小造成激子極性減弱所致，因而降低了激子與縱光學聲子的耦合強度。

MgZnO alloys with various Mg contents were successfully synthesized by thermal diffusion and sol-gel methods. We investigated on the lattice dynamics, compositional disorder, and exciton-related characteristics (i.e., interactions between excitons and acoustic phonons, LO phonons, free carriers and other excitons) of the MgxZn1-xO alloy semiconductors which depend critically on the Mg substitution by Raman and photoluminescence spectroscopy. Alloy compositions can be estimated from X-ray diffraction patterns and photoluminescence spectra. The blueshift of near-band-edge emission and excitonic absorption along with the variation of lattice parameters reveal that Mg2+ is incorporated into the ZnO host lattice and substitutes for Zn2+. In ternary semiconductor alloys, the Raman spectra show changes of various phonon modes with compositional disorder, including emergence of local vibration modes and disorder-activated modes, a shift in phonon frequency and changes of the linewidth and asymmetry. In addition to the host phonons of ZnO, two anomalous modes around 383 and 510 cm-1 are presumably attributed to the Mg-related vibrational modes. Due to the differences in the ionic radii of Mg impurity and host ions, the Mg-induced extrinsic or intrinsic lattice defects in either substitutional sites or the interstitial sites would become activated. As a result, disorder-activated scattering or local lattice vibration occur in Raman scattering. The broadening in linewidth and asymmetry can be investigated in terms of the modified spatial correlation model based on the finite correlation length of a propagation phonon due to the alloy potential fluctuations and grain size distribution. The microscopic nature of the substitutional disorder is discussed by analyzing the compositional dependence of the E2(high) phonon mode in MgxZn1-xO nanopowders. With increasing Mg concentration, the alloy potential fluctuations lead to a decrease in the grain size, which is induced by the surplus Mg2+ that could form MgO clusters surrounding the MgZnO crystalline. Moreover, it is well known that efficient exciton relaxation is required for bounding two cooled excitons to form biexciton. Acoustic and optical phonon scattering playing key roles in exciton relaxation are responsible for formation of biexcitons at various temperatures. Using ZnO powders, we observed a sublinear dependence on excitation power at low temperature, in which the relaxation process involves only emission of acoustic phonons due to the excitons having kinetic energy lower than those of the optical phonons. However, the exponent comes near theoretical value of 2 for participation of optical phonons when the exciton kinetic energy approaches to the energy of the lowest optical phonon. Finally, the coupling between exciton and longitudinal optical (LO) phonon was investigated in use of the temperature-dependent photoluminescence from MgxZn1−xO powders (0 □ x □ 0.05). The exciton binding energy increases to 73 meV for 5 at. % Mg incorporated powders. It is suggested that the localization of excitons originating from the compositional fluctuation takes place in MgxZn1−xO alloys. The strength of exciton-LO-phonon coupling was deduced from the energy shift of exciton emission with temperature variation. The increase of the exciton binding energy results from decrease of the exciton Bohr radius that is responsible for reducing the coupling strength of exciton-LO-phonon as increasing Mg content.