Excitons in Wide-Gap Semiconductors: Coherence, Dynamics, and Lasing

Abstract

The role of excitonic excitations in wide-gap II-VI and nitride semiconductors is reviewed with focus on the recent research at Bremen University. More-particle excitations such as trions and biexcitons are studied including their contributions to nonlinearities in the coherent regime and to lasing. Specific questions concerning laser processes in wide-gap materials are addressed. IntroductionWide-gap semiconductors such as most of the Zn and Cd based II-VIs or the nitride based blue to UV emitters are model materials for exciton and Coulomb correlation effects on the whole time and a large intensity scale since excitons in these compounds are much more stable as in conventional III-V semiconductors widely used for optoelectronic applications so far. This in particular holds for lower-dimensional structures exhibiting exciton binding energies exceeding those of the optical phonons. Thus, even at high electron-hole densities and rather elevated temperatures, lasing is still strongly governed by Coulomb correlation effects [1,2], what makes wide-gap materials rather different from III-Vs concerning this important aspect of application. Also with respect to very basic phenomena such as higher-order contributions to coherent multi-wave mixing, or propagation effects, the role of exciton correlation is extremely significant [3][4][5][6][7]. This paper is a short review of the recent activities of the Bremen group in the above-mentioned fields.

Vertically aligned and hexagonal crystal ZnSe nanoribbon arrays on Zn substrates

“…As a Zn-based II-VI compound, ZnSe is a direct band gap semiconductor with roomtemperature band gap energy of 2.8 eV. Therefore, ZnSe has attracted great interest in the field of thin films, quantum wells, and bulk crystals material researches [1][2][3][4][5][6]. ZnSebased optoelectronic devices have also been the subject of intensive study [7].…”

Zinc Selenide Nanoribbons and Nanowires

“…Wide-gap II−VI semiconductors are efficient emitters in the blue to ultraviolet spectral region, and excitons in these compounds are much more stable than those in the conventional III−V semiconductors that are widely used for optoelectronic applications. , Recent studies showed that the low-dimensional nanostructures of Zn-based II−VI wide-gap semiconductors exhibit exciton binding energies exceeding their optical phonon energies. As a consequence, the quantum yield of the II−VI-based devices is expected to be comparable to or greater than that of the common III−V devices.…”

“…This suggests that ZnSe is potentially a good material for short-wavelength lasers and other photoelectronic devices. Therefore, ZnSe is of great interest as a model material in such forms as thin films, quantum wells, and bulk crystals, ,,− and ZnSe-based optoelectronic devices have been the subject of intensive research,…”

“…This suggests that ZnSe is potentially a good material for short-wavelength lasers and other photoelectronic devices. Therefore, ZnSe is of great interest as a model material in such forms as thin films, quantum wells, and bulk crystals, 1,2,[6][7][8][9] and ZnSe-based optoelectronic devices have been the subject of intensive research, 5 In the pursuit of nanoelectronics, one-dimensional semiconductor nanostructures have attracted intense interest with regard to the fundamental issues concerning low dimensionality, spaceconfined transport, and device applications. 10 In recent years active research has focused on semiconductor wirelike nanostructures, including those of compound semiconductors (GaN, 11 GaAs, 12 InAs 13 ), elemental semiconductors (Si, 14 Ge 15 ), and oxide semiconductors (Ga 2 O 3 ).…”

Gas Phase Growth of Wurtzite ZnS Nanobelts on a Large Scale

“…Semiconductor materials have been extensively researched due to their potential applications in optical, photocatalytic, and optoelectronic �elds [1][2][3][4][5][6][7]. Among them, wide band gap II-VI semiconductors are efficient emitters from blue to UV spectra range and are likely candidates to replace materials like GaN in light emitting laser diodes [8]. In particular, Zinc sul�de, a direct wide band gap semiconductor (3.7 eV) with exciton binding energy of 40 meV [9], has a high refractive index [10] and a high transmittance in the visible range [11].…”