Density Dependence of the Exciton Energy in Semiconductors

Abstract

We investigate both experimentally and theoretically the excitonic absorption in ZnSe in a temperature range between 2 and 60 K with increasing densities of carriers. For higher temperatures a weak redshift of the exciton resonance is found which turns into a blueshift for lower temperatures. While the widely used simplified treatment of the scattering processes within a static screening approximation fails completely to describe this thermally induced crossover, it can be explained by the interplay between Coulomb-Hartree-Fock renormalizations and carrier-carrier and carrier-polarization scattering including the dynamical screening. [S0031-9007 (98)06251-6] PACS numbers: 71.35.CcThe nonlinear behavior of the excitonic absorption is one of the most investigated manifestations of many-particle effects in a highly excited narrow-gap semiconductor. For instance the Mott transition observed for increasing excitation densities is well understood as a consequence of screening of the Coulomb interaction between charge carriers. While the absolute energy of the 1s exciton stays nearly unchanged in bulk samples over a wide range of carrier densities its oscillator strength disappears due to band gap shrinkage. This was proved in many experiments [1-3] and explained qualitatively by strong compensation of gap shrinkage and weakening of the Coulomb interaction due to screening inspecting the effective Wannier equation of an exciton embedded in a thermal plasma [4][5][6][7]. The situation is rather different in quantum wells, where a blueshift of the exciton with increasing density was observed [8,9]. Qualitatively, this behavior is caused by the reduced dimensionality, which leads to a reduction of the Coulomb interaction between carriers [10] as was confirmed by more elaborated calculations for a dense exciton gas model [11]. In GaAs͞Ga 12x Al x As quantum wells the transition from quasi-2D to 3D behavior was found to occur at well widths of 19 nm, i.e., already at less than two exciton Bohr radii [9,12].The present situation is characterized by two aspects. At first, recent experimental results shed new light on the commonly accepted view that the excitonic resonance stays nearly constant in a large density region. In GaAs͞Ga 12x Al x As quantum wells of 21 nm width the exciton shift was found to change from a weak blueshift under resonant excitation to a weak redshift under nonresonant excitation [13]. Also in bulk ZnSe, where the influence of many-particle effects on the exciton can be observed much more pronounced than in the III-V semiconductors (GaAs) due to the larger exciton binding energy, a weak blueshift of the exciton [14] at resonant excitation and low temperatures has been found recently. Second, substantial progress has been achieved during the last few years to find out which many-particle effects have to be included into the semiconductor-Bloch equations (SBE) in order to describe the nonlinear absorption features of laser pulses. In particular, inclusion of carrier-polarization scattering processes was sh...

Broadband terahertz study of excitonic resonances in the high-density regime inGaAs∕AlxGa1−xAsquantum wells

“…An important phenomenon in this context is the excitonic Mott transition: Driven by decreasing interparticle distance, excitonic bound states ultimately cease to exist such that a conducting plasma of unbound e-h pairs prevails [2,3,4,5]. Quasi twodimensional (2D) excitons in quantum wells, due to large binding energies and sharp optical resonances, are particularly suitable to study high-density many-particle effects of excitons [6,7].Numerous studies have investigated the near-infrared excitonic resonances just below the semiconductor band edge, and their broadening, bleaching, and energy shift due to photoexcited e-h pairs (see e. g. [8,9,10,11,12,13,14,15,16]). Broadening occurs via collisional interactions [8].…”

“…Numerous studies have investigated the near-infrared excitonic resonances just below the semiconductor band edge, and their broadening, bleaching, and energy shift due to photoexcited e-h pairs (see e. g. [8,9,10,11,12,13,14,15,16]). Broadening occurs via collisional interactions [8].…”

“…At elevated pump fluences, we observe broadening, bleaching, and a slight blue shift of the near-infrared 1s absorption line. These changes are explained by the joint effects of phase-space filling and screening [9,10,11,12,13,14,15]. To probe the internal response of the bound e-h pairs, we employ broadband THz probe pulses generated by optical rectification.…”

Interacting many-body systems in quantum wells: Evidence for exciton-trion-electron correlations

“…These differential spectra all evidence a clear blue-shift of the exciton line. This renormalization of the exciton resonance has been extensively studied in undoped-semiconductor quantum wells [13][14][15][16] and, is attributed to short-range exchange 14 having its origin in the Pauli exclusion-principle acting on the Fermi particles forming the excitons. This is a repulsive electron-electron and hole-hole interaction.…”

Many-Body Interaction Evidenced through Exciton-Trion-Electron Correlated Dynamics

“…3, we plot a set of differential reflectivity spectra, obtained when pumping selectively at the exciton resonance, for several delay times between pump and probe pulses and at temperatures of 5 K, 10 K, and 20 K. These differential spectra all show a clear blue-shift of the exciton line. This renormalization of the exciton resonance has been extensively studied in undoped-semiconductor quantum wells [21][22][23][24] and, is attributed to short-range fermion exchange [22], which is a repulsive electron-electron and hole-hole interaction. The exciton blue-shift appears in two-dimensional semiconductors [23] because the long-range Coulomb correlation effect is strongly reduced [22].…”