Exciton-Phonon Stimulated Emission in ZnO Crystalline Film at Room Temperature
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Exciton-Phonon Stimulated Emission in ZnO Crystalline Film at Room Temperature N. N. Vasilyeva, *, E. N. Borisova, and B. V. Novikova a St.
Petersburg State University, St. Petersburg, Russia *e-mail: [email protected]
Received April 23, 2020; revised April 23, 2020; accepted April 28, 2020
Abstract—The near-edge luminescence of zinc oxide epitaxial film grown via molecular-beam epitaxy on a sapphire substrate is studied. Upon an increase in optical excitation at room temperature, the luminescence spectrum changes radically and a new band appears with a maximum of ~3.17 eV. It has features of stimulated emission, i.e., a threshold of nonlinear growth and a narrowing of the half-width. A model of a one-dimensional amplifier and experimental data were used to calculate the gain spectrum, with maximal value being about 170 cm–1. The theoretical approaches to calculate the Mott concentration were analyzed. It is shown for the first time that the observed stimulated emission near the threshold intensity originates from the second phonon replica of the exciton. Keywords: ZnO film, gain, excitonic origin DOI: 10.1134/S1063783420100352
1. INTRODUCTION Interest in the zinc oxide binary semiconductor (ZnO) has revived due to new possibilities in nanotechnology and its great potential for use in optoelectronics. The width of its band gap is ~3.4 eV, and the exciton binding energy is 60 meV. This makes it a competitor of another wide band gap semiconductor, GaN, for application in the amplified stimulated emission (ASE) of ultraviolet (UV) light sources in the form of light or laser diodes [1, 2]. The high binding energy and relative simplicity of growth technology favorably distinguishes ZnO from GaN and makes its use promising at elevated temperatures. A necessary step in the correct description and calculation of ASE is the identification of its nature. The main UV ASE mechanisms in ZnO are based on nearedge luminescence and are associated with excitons and degenerate electron–hole plasma (EHP). The excitonic mechanism is considered more preferable, because it basically gives a lower ASE threshold for optical excitation intensity Iexc (pump) and a higher gain index g. At low temperatures and small Iexc, ASE of an excitonic nature prevails [3]. At room temperature (RT), excitonic and plasma mechanisms are often difficult to distinguish, because the spectra of nonlinear bands of different natures are merged into one band. EHP can be visually identified if the considered sample does not degrade at sufficiently large Iexc. In this case, the plasma band appears on the long-wave side with respect to the primary nonlinear band. As Iexc increases further, it continues to separate or shift in the
same direction at a slower rate and becomes dominant (e.g., [4]). At lower pumping rates, excitonic and plasma mechanisms can be manifested in processes of the same physical nature, because ASE can be associated with both excitons and correlated electron-hole (e–h) pairs. One of these mechanisms is electron scattering on an
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