Ultrafast dynamics of pure many-body effect and its competition with bandgap widening via electron-phonon coupling in PbTe films
Ultrafast dynamics of pure many-body effect and its competition with bandgap widening via electron-phonon coupling in PbTe films
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纯多体效应的超快动力学及其与 PbTe 薄膜中电子声子耦合带隙展宽的竞争
DOI：
10.1088/1361-6641/ab3ba4
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发表时间：
2019
期刊：
影响因子：
--
通讯作者：
Tianshu Lai
中科院分区：
文献类型：
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作者：
Zhaohui Luo;Tianyu Shu;Zhifeng Chen;Tianran Jiang;Huizhen Wu;Tianshu Lai
带隙重整化(BGR)，或称带隙变窄效应，是半导体物理中多体理论预言的多体效应之一。然而，在实验上从未观察到纯的BGR效应，尽管在GaAs和CdTe半导体中报道了BGR效应与晶格加热引起的带隙变窄的混合效应。PbTe具有正的带隙温度系数。它可以消除晶格加热引起的带隙变窄，从而有可能观察到纯粹的BGR效应。然而，由于PbTe具有巨大的介电屏蔽性，其BGR效应很弱，因此到目前为止还没有报道。利用近红外飞秒分辨瞬变差分(TDT)光谱研究了PbTe薄膜中光激发载流子的超快动力学。观察到了一种复杂的类振荡衰减动力学。负凹陷是BGR效应的指纹特征，在光激发载流子的热化过程之后立即出现。因此，我们首次在PbTe中观察到了纯BGR效应，有力地支持了多体理论。同时，我们还观察到了从瞬时吸收增强到吸收饱和的动态转变，揭示了通过载流子-声子耦合引起的缓慢晶格加热引起的带隙展宽。基于双温模型进行了模拟计算，支持晶格加热引起的带隙展宽导致动态相变。建立了一个唯象动力学模型，该模型包括三个动力学部分：光激载流子的热化、热化载流子的复合和晶格加热引起的禁带展宽，用于最好地拟合与激发功率相关的TDT动力学。在不同的激励功率下，得到了三个动态分量的时间常数和强度。时间常数随激发功率的变化揭示了载流子-载流子和载流子-声子散射在热化过程中的不同作用，以及自由载流子和激子复合在热载流子衰变过程中的作用。
Bandgap renormalization (BGR), or bandgap narrowing effect, is one of the many-body effects predicted by many-body theory in semiconductor physics. However, pure BGR effect has never been observed experimentally although the mixed effect of BGR with lattice-heating induced bandgap narrowing was reported in GaAs and CdTe semiconductors. PbTe has a positive temperature coefficient of bandgap. Its lattice-heating induced bandgap narrowing can be eliminated so that the pure BGR effect is possibly observed. However, that has never been reported so far because the BGR effect is weak in PbTe due to its huge dielectric screening. We study the ultrafast dynamics of photo-excited carriers in PbTe films using near-infrared femtosecond-resolved transient differential transmission (TDT) spectroscopy. A complex oscillation-like damped decay dynamics is observed. A negative dip, a fingerprint signature of BGR effect, occurs immediately after the thermalization process of the photo-excited carriers. As such, we have observed a pure BGR effect in PbTe for the first time, strongly supporting the many-body theory. Meanwhile, we have also observed a dynamic transition from a transient absorption enhancement to absorption saturation, revealing the emergence of a slow lattice-heating induced bandgap widening via carrier-phonon couplings. Based on a two-temperature model, a simulation computation is carried out, supporting the lattice-heating induced bandgap widening leads to the dynamic transition. A phenomenological dynamic model is developed, including three dynamic components: the thermalization of the photo-excited carriers, recombination of the thermalized carriers and lattice-heating induced bandgap widening, and used to best fit the excitation-power dependent TDT dynamics. Time constants and strengths of the three dynamic components are obtained for different excitation powers. Time constant variation with the excitation power reveals different roles of the carrier–carrier and carrier-phonon scattering in the thermalizing process, as well as of free carrier and exciton recombination in the decaying process of hot carriers.