Picosecond transient absorption and excitonic luminescence at low temperature in undoped ZnO

• Published in: Physica Status Solidi (b) 2008-12, Vol.245 (12), p.2680-2683
• Main Authors: Ucer, K. Burak, Wall, R. Andrew, Lipke, Kyle C., Williams, R. T.
• Format: Article
• Language: English
• Subjects: 71.35.Cc; 78.47.jc; 78.55.Et; 82.53.Hn; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Exact sciences and technology; Ii-vi semiconductors; Optical properties and condensed-matter spectroscopy and other interactions of matter with particles and radiation; Photoluminescence; Physics
• ISSN: 0370-1972; 1521-3951
• DOI: 10.1002/pssb.200879895

Transient optical absorption is observed upon band‐gap excitation of undoped single‐crystal ZnO. Using 2‐photon excitation by 300 fs pulses of 410 nm light to create electron–hole pairs and an infrared continuum pulse to probe absorption, the evolution of induced absorption has been measured in the time range 2–100 ps at 17 K. Approximately 38% of the induced absorption decays with a time constant of 50 ps. Based on measurements of 50 ps lifetime for neutral donor‐bound exciton luminescence (D0X) in the same samples in our laboratory and the known dominance of D0X luminescence at low temperature in many studies of undoped ZnO, we assign the 50 ps infrared absorption to photoionization of the bound excitons. The observed spectral shape of the absorption band rises monotonically with increasing wavelength from 950 nm to 1600 nm, very similar to expectation of the shape of free‐carrier absorption. Since both the internal electron–hole binding energy and the binding of center‐of‐mass motion to the neutral donor are much less than the transition energies of 0.8–1.3 eV observed in absorption, it can be argued that the broad infrared absorption signal is a measure of the combined population of free carriers, shallow trapped carriers, and bound or free excitons. Based on this hypothesis, we deduce from the time‐dependent infrared absorption that the quantum efficiency of D0X luminescence in this sample is approximately 38%. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)