We report on the experimental determination of the spontaneous polarization of wurtzite- (Mg,Zn)O by examination of the recombination dynamics of polar ZnO/(Mg,Zn)O quantum wells (QWs). The thickness-dependent decay time of the unscreened single-exciton states inside the QWs was modeled by a self-consistent solution of Schrödinger- and Poisson-equation to deduce the total polarization across the QW for different Mg-contents inside the barriers. By the separation of the piezoelectric components of the polarization, a linear increase in spontaneous polarization with increasing Mg-content x of P/x = (0.151 ± 0.015) C/m2 was determined for Mgx Zn1−x O.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第18281号 / 工博第3873号 / 新制||工||1594(附属図書館) / 31139 / 京都大学大学院工学研究科電子工学専攻 / (主査)教授 川上 養一, 教授 藤田 静雄, 准教授 須田 淳 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

InGaN based light emitting devices operating in the blue and near UV spectral regions are commercialized and used in many applications. InGaN heterostructures experience compositional inhomogeneity and thus potential fluctuations, such that regions of higher indium composition are formed and correspond to lower potentials. The indium rich regions form localization centers that save carriers from non-radiative recombination at dislocations, thus despite the large defect density, their quantum efficiency are surprisingly large. However, the conventional c-plane InGaN QWs suffer from high internal piezoelectric and spontaneous fields. These fields are detrimental for the performance of such structures as they lead to the quantum confined stark effect causing red-shift of the emission as well as reducing the electrons and holes wavefunctions overlap, thereby reducing the radiative recombination rate. However, growth of InGaN QWs on semipolar and nonpolar planes greatly reduced the polarization fields. Semipolar and nonpolar QWs experience an outstanding property which is polarized luminescence, opening a new frontier for applications for InGaN emitting devices. While nonpolar QWs have larger degree of polarized emission than semipolar QWs, semipolar QWs can emit in longer wavelengths due to their higher indium uptake. In this thesis, semipolar 20¯21 and nonpolar m-plane InGaN/GaN QWs were investigated. Photoluminescence, spectral and polarization dynamics were all studied in order to form a whole picture of the carrier dynamics in the QWs. Time resolved photoluminescence measurements were conducted for following carriers distribution between extended and localized states. Both the semipolar and nonpolar samples showed efficient luminescence through short radiative recombination times, as well as carrier localization in lower potential sites after thermal activation of excitons. Carrier localization was found to be benign as it didn’t degrade the performance of the samples or decrease the polarization ratio of their emission. However, the structures showed modest potential variations with the absence of deep localization centers or quantum dots. High polarization ratios were measured for both samples, which is well-known for nonpolar QWs. The high polarization ratio for the semipolar sample is of great importance, thus semipolar 20¯21 QWs should be considered for longer wavelength emitters with highly polarized spontaneous emission.