Due to its wide band gap and high carrier mobility, ZnO is an attractive material for light-harvesting and optoelectronic applications. Its functional efficiency, however, is strongly affected by defect-related in-gap states that open up extrinsic decay channels and modify relaxation timescales. As a consequence, almost every ZnO sample behaves differently, leading to irreproducible or even contradicting observations. Here, a complementary set of time-resolved spectroscopies is applied to two ZnO samples of different defect density to disentangle the competing contributions of charge carriers, excitons, and defects to the non-equilibrium dynamics after photoexcitation: Time-resolved photoluminescence, excited state transmission, and electronic sum frequency generation. Remarkably, defects affect the transient optical properties of ZnO across more than eight orders of magnitude in time, starting with photodepletion of normally occupied defect states on femtosecond timescales, followed by the competition of free exciton emission and exciton trapping at defect sites within picoseconds, photoluminescence of defect-bound and free excitons on nanosecond timescales, and deeply trapped holes with microsecond lifetimes. These findings do not only provide the first comprehensive picture of charge and exciton relaxation pathways in ZnO, but also uncover the microscopic origin of previous conflicting observations in this challenging material and thereby offer means of overcoming its difficulties.
Effects of strain on the valence band structure and exciton-polariton energies in ZnO
采用基于HSE+ G 0 W 0方法的现代第一性原理计算、基于形变势框架的k-p模型和偏振光致发光测量，对纤锌矿ZnO的能带结构和激子-极化激元跃迁随单轴应力的变化进行了深入研究.发现价带的有序性[A（G7），B（G9），C（G7）]即使对于高的单轴和双轴应变也是稳健的。考虑自旋轨道相互作用的影响，得到了A、B、C价带的单轴压力系数、分裂率及其光学跃迁的理论结果。激子变形势的推导和静水压力的应力率确定的基础上的单轴和双轴应力的结果。此外，利用准立方近似下的基本激子函数，考虑激子态间的相互作用，建立了激子-极化激元的交换相互作用和纵向-横向劈裂的应力依赖性理论。结果表明，这些影响的考虑是至关重要的，在ZnO中的光学光谱的应力依赖性的准确描述。理论结果进行比较，不同的ZnO衬底的单轴压力和实验值的函数的偏振光致发光测量在文献中报道的表现出良好的协议与计算的压力系数。
Experimental realization of purely excitonic lasing in ZnO microcrystals at room temperature: transition from exciton-exciton to exciton-electron scattering
Ultrafast screening and carrier dynamics in ZnO: Theory and experiment
当载流子密度高于莫特密度时，库仑屏蔽破坏了激子共振。这与带隙重整化和带填充一起严重影响了光谱。我们已经实验研究了这些影响的超快泵浦-探测反射率测量ZnO单晶在激子共振周围的各种波长，并在一个广泛的载流子密度范围。理论上我们确定了ZnO在300 K时的Mott密度为$1.5\times10^{24}$ m $^{-3}$。采用场论方法，我们推导并求解了Bethe-Salpeter阶梯方程，计算了密度相关的反射率和吸收光谱。建立了载流子动力学模型，包括三光子吸收、载流子冷却和表面附近的载流子俘获。基于我们模型的理论反射率与实验数据之间的一致性非常好。
Clustered vacancies in ZnO: Chemical aspects and consequences on physical properties
ZnO中点缺陷的化学性质，它们的偏析，簇或复合物的形成是一个重要的研究领域。在这份报告中，1.2 MeV的Ar离子束被用来将颗粒ZnO中的缺陷。用XPS、PL和Raman光谱研究了辐照能量密度为1 x 10^14和1 x 10^16 ions/cm 2时缺陷态的演化。XPS研究表明，在Ar离子辐照的ZnO中存在氧空位（VO）。Zn（LMM）俄歇谱清楚地识别出辐照样品中涉及金属锌的跃迁。所有样品的10 K光谱中都可见IZn相关浅施主束缚激子（DBX）的强PL发射。虽然整体PL大大减少与辐照无序，DBX强度增加的最高通量辐照样品。拉曼光谱研究表明，高能离子束对锌和氧亚晶格都有损伤。从涉及VO，IZn和IO的缺陷复合物的代表性拉曼模式是可见的照射后，与中间通量。注量的进一步增加在某种程度上显示了无序的均匀化。对于该样品还注意到电阻的巨大降低。当然，高辐照能量密度诱导的常规（和高电阻）的颗粒状ZnO的晶界（GB）结构的定性修改。低电阻路径，涉及IZn相关的浅施主，跨越GB可以被假定为解释电阻降低。随着辐照能量密度的增加，开孔体积（VZn和VO）越来越多地聚集，最终转化为空洞。结果作为一个整体已经阐明了一个模型，强调可能的演变新的缺陷的微观结构，是明显不同的GB相关的障碍。在此基础上，对无序ZnO中常见的辐射硬度、着色和铁磁性等现象给出了定性解释。
Effective masses, Burstein-Moss shift, and bandgap renormalization in degenerate Al-doped ZnO from broadband ellipsometry and Hall measurements
Pontificia Universidad Católica del Perú（秘鲁天主教宗座大学）；Helmholtz-Zentrum Berlin für Materialien und Energie GmbH（柏林亥姆霍兹材料与能源中心有限公司）；Eindhoven University of Technology（埃因霍温理工大学）
Many linked processes occur concurrently in strongly excited semiconductors, such as interband and intraband absorption, scattering of electrons and holes by the heated lattice, Pauli blocking, bandgap renormalization and the formation of Mahan excitons. In this work, we disentangle their dynamics and contributions to the optical response of a ZnO thin film. Using broadband pump-probe ellipsometry, we can directly and unambiguously obtain the real and imaginary part of the transient dielectric function which we compare with first-principles simulations. We find interband and excitonic absorpti…
We unravel the influence of quasiparticle screening in the non-equilibrium exciton dynamics of monolayer WS$_2$. We report pump photon energy-dependent exciton blue and red-shifts from time-resolved-reflectance contrast measurements. Based on a phenomenological model, we isolate the effective impact of excitons and free carriers on the renormalization of the quasi-free particle band gap, exciton binding energy and linewidth broadening. By this, our work does not only provide a comprehensive picture of the competing phenomena governing the exciton dynamics in WS$_2$ upon photoexcitation, but al…
The microscopic origin of slow carrier cooling in lead-halide perovskites remains debated, and has direct implications for applications. Slow carrier cooling has been attributed to either polaron formation or a hot-phonon bottleneck effect at high excited carrier densities (> 10$^{18}$ cm$^{-3}$). These effects cannot be unambiguously disentangled from optical experiments alone. However, they can be distinguished by direct observations of ultrafast lattice dynamics, as these effects are expected to create qualitatively distinct fingerprints. To this end, we employ femtosecond electron diffract…
In organic semiconductors working under ambient circumstance, there are remarkable quantum effects lack of comprehensive understanding, and an exotic composite particle named charge transfer (CT) exciton is normally regarded as the key ingredient. Another essential substance is the phonons stemming from intra- and inter-molecular vibrations, and the relevant diagonal electron-phonon couplings give rise to spatial localization and the off-diagonal couplings refer to the dispersion of electron wavefunctions. In this work, we first propose a toy model based upon the fracton physics to phenomenolo…
