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Ultrafast photocarrier dynamics in nanocrystalline ZnOxNy thin films Taeho Shin,1,* Eunha Lee,1 Soohwan Sul,1 Hyungik Lee,1 Dong-Su Ko,1 Anass Benayad,1 Hyun-Suk Kim,2,3 and Gyeong-Su Park1 1

Analytical Science Group, Samsung Advanced Institute of Technology, Suwon, Gyeonggi-do 443-803, South Korea 2

3

Display Laboratory, Samsung Advanced Institute of Technology, Suwon, Gyeonggi-do 443-803, South Korea Department of Materials Science and Engineering, Chungnam National University, Daejeon 305-764, South Korea *Corresponding author: [email protected] Received May 29, 2014; revised July 11, 2014; accepted July 25, 2014; posted July 25, 2014 (Doc. ID 213059); published August 20, 2014 We examined the ultrafast dynamics of photocarriers in nanocrystalline ZnOx Ny thin films as a function of compositional variation using femtosecond differential transmittance spectroscopy. The relaxation dynamics of photogenerated carriers and electronic structures are strongly dependent on nitrogen concentration. Photocarriers of ZnOx Ny films relax on two different time scales. Ultrafast relaxation over several picoseconds is observed for all chemical compositions. However, ZnO and oxygen-rich phases show slow relaxation (longer than several nanoseconds), whereas photocarriers of films with high nitrogen concentrations relax completely on subnanosecond time scales. These relaxation features may provide a persistent photocurrent-free and prompt photoresponsivity for ZnOx Ny with high nitrogen concentrations, as opposed to ZnO for display applications. © 2014 Optical Society of America OCIS codes: (310.6860) Thin films, optical properties; (160.5335) Photosensitive materials; (300.6530) Spectroscopy, ultrafast. http://dx.doi.org/10.1364/OL.39.005062

Oxide semiconductors have attracted considerable attention owing to their diverse applications, such as photocatalysts, memory devices, and transparent electronics [1,2]. Among these, nanocrystalline zinc oxynitride (ZnOx Ny ) thin films have been recently reported as a promising material for optoelectronic applications such as photo imaging sensors and displays [3,4]. While Zn3 N2 and ZnO have their own crystal structures, bulk ZnOx Ny is not supported by unique crystal structures. Previous structural analysis showed that the microstructure of ZnOx Ny films contains a few nanometer-sized crystallites together with an amorphous phase. Nonetheless, the band gap of ZnOx Ny varies from 1.1 (Zn3 N2 ) to 3.4 eV (ZnO), depending on the nitrogen concentration. Such band tunability offers a photosensitivity over a broad range of wavelengths [4]. Moreover, the material exhibits a high intrinsic mobility of ∼100 cm2 V−1 s−1 , extending its application to high-motion-speed displays. Despite its technological importance, however, the dynamics of photocarriers have not been examined in detail. In this Letter, we examine the ultrafast dynamics of photocarriers in ZnOx Ny thin films as a function of compositional variation using femtosecond differential transmittance spectroscopy. The nitrogen concentration influences the dynamics of photocarriers and electronic structures. In ZnO films, photocarriers relax over two different time scales. They relax as fast as 10–20 ps and then further relax on a time scale longer than several nanoseconds. The longer time-scale relaxation may originate from oxygen vacancies, which cause substantial persistent photocurrent (PPC) in ZnO-based photosensors under gate voltage control. On the contrary, as the nitrogen concentration increases, most photocarriers relax faster than 10 ps, via Auger recombination, and nanosecond timescale relaxation does not occur. These relaxation features account for prompt photoresponsivity and negligible PPC for ZnOx Ny with high nitrogen concentrations. 0146-9592/14/175062-04$15.00/0

The preparation of ZnOx Ny thin films was described in detail elsewhere [3,4]. In brief, 50 nm thick ZnOx Ny thin films were deposited on glass substrates at 323 K by reactive radio frequency magnetron sputtering, using a zinc target (99.995%) with mixed gas: Ar (99.99%), O2 (99.999%), and N2 (99.999%). The chemical composition was controlled by the gas ratio and determined by Rutherford backscattering spectrometry (RBS). Thin films with five different chemical compositions were prepared for the present study: ZnO, ZnO0.73 N0.27 , ZnO0.51 N0.49 , ZnO0.14 N0.86 , and Zn3 N2 . Femtosecond differential transmittance spectroscopy was carried out using a Tisapphire regenerative amplifier, which produced 1 kHz pulses with 60 fs duration at 800 nm. Pump wavelengths of 360 or 400 nm (3.44 or 3.10 eV) were used with an optical parametric amplifier and a subsequent nonlinear doubling crystal. Two separate white-light supercontinuum sources were generated as probe beams for UVvisible (330–850 nm) and near-infrared (NIR) (750– 1600 nm) transmittance measurements. After being transmitted through samples, the intensity of the probe beam was measured by either UV-visible or NIR spectrometers as a function of pump-probe delay times (0–8 ns). The sample stage was scanned over an area of 10 mm × 10 mm at a rate of 10 mm/s to avoid photoinduced damage. Figure 1 shows the normalized differential transmittance spectra of the thin films. They were measured at 2 ps after photoexcitation and obtained by combining two separate UV-visible and NIR measurements. Two-dimensional plots of differential transmittance spectra for ZnOx Ny films, except Zn3 N2 , and a detailed description of their spectral features can be found in our recent work [4]. Here, we briefly describe the static spectral features. The differential transmittance spectra of the ZnOx Ny films clearly demonstrate their electronic band gaps. The maximum positive transmittance change due to © 2014 Optical Society of America

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Fig. 1. Differential transmittance spectra of ZnOx Ny thin films obtained at 2 ps after photoexcitation. Each spectrum was normalized by its maximum intensity. Wavelength and energy of an excitation laser pulse for all samples (except for Zn3 N2 ) were 360 nm and 700 nJ, respectively. For Zn3 N2 , they were 400 nm and 100 nJ, respectively.

ground state bleaching is assigned as the band gap. The band gap is 3.38 eV (366 nm) for ZnO and 1.10 eV (1130 nm) for Zn3 N2 , which is in good agreement with previous optical measurements [5]. The smaller band gap of the Zn3 N2 film is attributed to the nitrogen 2p band formation (valence band) above the oxygen 2p band. The band gap difference between the films suggests that the width of the nitrogen 2p band is larger than 2.28 eV. Between these two extremes, the band gap of the ZnOx Ny decreases with increasing nitrogen concentration because the valence band maximum is increased due to the filling of the nitrogen 2p band. From the local maxima in the transmittance change, they are identified as 2.50 eV for ZnO0.73 N0.27 , 1.77 eV for ZnO0.51 N0.49 , and 1.35 eV for ZnO0.14 N0.86 . In ZnO0.73 N0.27 , two absorption peaks are observed. The peak of 3.46 eV originates from an oxygen-rich ZnOx Ny phase, while that of 2.50 eV is attributed to a nitrogen-rich ZnOx Ny phase. Interestingly, the band gap of the oxygen-rich ZnOx Ny phase is larger than that of the ZnO film. This may be explained as follows: the nitrogen 2p band overlaps with the oxygen 2p band slightly, and the density of states (DOS) is increased. Accordingly, the resulting highest occupied level—or valence band maximum—is lower than that of ZnO at low nitrogen concentrations, yielding the larger band gap. In addition to the band gap change, the absorption spectral width also varies as a function of chemical composition. While both ZnO and Zn3 N2 exhibit narrow spectral widths, other ZnOx Ny films show broad widths. This is likely due to inhomogeneous broadening that originates from diverse crystalline size and locally different stoichiometry of the ZnOx Ny sample because there is no bulk crystal structure supporting it [4]. Figure 2(a) shows the time-dependent dynamics of the photocarriers of the films at their corresponding peak wavelengths. Excited photocarriers undergo an ultrafast intraband relaxation on a subpicosecond time scale (∼500 fs), although the wavelength of the pump laser

Fig. 2. (a) Time-dependent transmittance change at peak wavelengths. They were normalized by their maximum values at t
0, and the y axis was truncated at 0.4. Inset shows the fast decay of the time-dependent signal up to 100 ps. (b) Fitting with multiple exponential functions for ZnO and Zn3 N2 films.

is significantly shorter compared to the band gaps (except for ZnO). Following intraband relaxation, they continue to relax further via an interband transition. A single exponential decay curve does not fit the transmittance signal, even over the first 20 ps period, so we tentatively define the decay time as the time until it decays to 1∕e of the initial maximum value. It is 14.1 ps for ZnO and 5.0 ps for Zn3 N2 . Other ZnOx Ny films also exhibit short decay times of ∼5–10 ps. Correlations between the decay time and nitrogen concentration are not observed for this ultrafast time scale. Longer time-scale (>100 ps) relaxation, however, is influenced by the nitrogen concentration. For the ZnOx Ny films with high nitrogen concentrations (y
0.49, 0.86, and 1.00), most of the photocarriers relax within 300 ps. The negligible nonzero transmittance change is still observed after 300 ps, but this is probably due to the elevated lattice temperature resulting from carrier– phonon coupling and not due to the residual photocarriers. However, for ZnO and the oxygen-rich phase of the ZnO0.73 N0.27 film, characterized by the peak energy of 3.46 eV, the transmittance change deviates substantially from the zero level, even at 7 ns, implying that relaxation of photocarriers is still taking place. The oxygen-rich phase of ZnO0.73 N0.27 shows the negative transmittance change at longer delay times, indicating a photoinduced absorption process of the excited photocarriers. The absorption occurs for at least 10 ns, suggesting that a significant amount of photoexcited carriers survive longer than 10 ns. Similar long time-scale relaxation is also observed in the ZnO film.

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For more quantitative analysis, the time-resolved data for ZnO and Zn3 N2 are fitted by a sum of multiple exponential decay functions, as shown in Fig. 2(b). At least three or four exponential functions are needed for Zn3 N2 and ZnO; otherwise, significant deviations occur. This implies that multiple independent relaxation processes are involved. For ZnO, the time constants with relative amplitudes from the fit are: 2.5 ps (0.45), 20 ps (0.34), 220 ps (0.15), and 8.6 ns (0.06). For Zn3 N2 , they are: 1.9 ps (0.36), 9.7 ps (0.31), and 56 ps (0.33). Direct comparison of the time constants between ZnO and Zn3 N2 is not possible because the excitation energy (or density) for Zn3 N2 (100 nJ at 400 nm) is lower than that for ZnO (700 nJ at 360 nm). However, it is expected that the time constants of Zn3 N2 would become shorter under the same excitation as ZnO due to Auger recombination. The major dynamical difference between ZnO and Zn3 N2 is that the slow relaxations of 220 ps and 8.6 ns occur only in ZnO. The time constant of 220 ps agrees with the photoluminescence (PL) lifetimes of free excitons in a bulk ZnO crystal [6] and thin ZnO films [7]. However, the slower relaxation of 8.6 ns has not been observed previously, including in time-resolved photoluminescence (TRPL) experiments. A previous TRPL experiment reported a long PL lifetime of 1 ns for the free exciton band, which results from a slow radiative recombination (>1 ns) and a fast nonradiative recombination (