标题: 掺镁氧化锌材料之激子–声子交互作用研究
Influence of phonons on excitonic characteristics in Mg-incorporated ZnO
作者: 潘晴如
谢文峰
光电工程学系
关键字: 氧化锌;氧化镁;ZnO;MgO
公开日期: 2007
摘要: 我们利用热扩散与溶胶-凝胶法制备氧化镁锌合金半导体。 利用拉曼散射与光激发光实验研究氧化镁锌合金的晶格动力学、组成扰动以及激子相关特性(即激子与声子、自由载子或其他激子的交互作用)在掺杂不同镁成分之下的变化情形。 我们藉由x-光绕射与光激发光的光谱概略地估计氧化镁锌合金中镁所取代锌的比例。 从室温光激发光与吸收光谱谱峰的蓝位移以及晶格常数的改变,可以证明镁离子掺入氧化锌晶体中而且成功地占据锌离子的位置。
一般来说,由于合金引起的组成成分扰动会使得拉曼光谱中各种声子模态发生变化,如局部晶格改变引发的振动模态、因扰动而活化的振动模态、声子频率的位移、谱形变宽与不对称等特性。 因此,我们利用拉曼光谱分析当镁掺入氧化锌奈米结构后,氧化镁锌晶格振动模态的变化情形。 除了氧化锌原有的声子模态之外,我们亦观察到383与510 cm-1两个异常的声子模态,据推测可能来自于镁相关的振动模。 由于镁离子与锌离子在半径上的差异,镁的掺入会产生某些新的晶格缺陷抑或是原有的晶格缺陷因扰动而活化进而出现一些额外的振动模。
我们用spatial correlation model探讨组成成分的扰动造成拉曼光谱谱形变宽与不对称的行为;而这个理论模型的概念是考虑声子的传播受到合金位能异动(alloy potential fluctuations)的局限与晶粒分布(grain size distribution)的影响。 因此可藉由分析氧化镁锌奈米粉末掺镁的含量多寡与E2(high)声子的频率变化之关联性得知。 我们发现随着镁含量的增加,合金位能异动造成晶粒相对变小,那是由于受到氧化镁对氧化锌溶解度的限制。 多余的镁离子无法取代氧化锌晶体中锌离子的位置因而形成氧化镁群环绕氧化镁锌晶体。
在辐射放光过程中双激子(biexciton)的产生是束缚两个激子形成,因此激发能量与辐射放光的强度之间会有接近于2的一个理想指数关系。 我们在氧化锌粉末中观察到声学声子(acoustic phonon)与光学声子(optical phonon)在双激子形成的过程中扮演着不可或缺的角色。 研究发现当温度低于某个程度,也就是激子动能低于光学声子动能时,激子或载子(carriers)冷却(relaxation or cooling)过程仅有声学声子的放射,此时的指数值是小于1的;然而当温度提高到使得激子动能接近于光学声子最低动能的时候,激子或载子冷却过程多了光学声子的参与,此时的指数值则是接近于理论上的理想值。
最后,我们使用变温光激发光光谱研究掺杂不同镁成分的氧化镁锌粉末其激子与纵光学声子(longitudinal optical phonon)耦合强度的变化。 激子与纵光学声子的耦合强度可经由分析激子的能量随着温度变化所产生的位移来决定。 除此之外,观察到随着镁成分的提高激子的束缚能相对增加,其原因可能是激子位能受到合金引发的组成扰动而产生变化。 激子束缚能的增加是由于激子波尔半径(Bohr radius)的减小造成激子极性减弱所致,因而降低了激子与纵光学声子的耦合强度。
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.
URI: http://140.113.39.130/cdrfb3/record/nctu/#GT009024811
http://hdl.handle.net/11536/37914
显示于类别:Thesis


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