Femtosecond Carrier Dynamics in In2O3Nanocrystals
© to the authors 2009
Received: 17 December 2008
Accepted: 9 February 2009
Published: 27 February 2009
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© to the authors 2009
Received: 17 December 2008
Accepted: 9 February 2009
Published: 27 February 2009
We have studied carrier dynamics in In2O3nanocrystals grown on a quartz substrate using chemical vapor deposition. Transient differential absorption measurements have been employed to investigate the relaxation dynamics of photo-generated carriers in In2O3nanocrystals. Intensity measurements reveal that Auger recombination plays a crucial role in the carrier dynamics for the carrier densities investigated in this study. A simple differential equation model has been utilized to simulate the photo-generated carrier dynamics in the nanocrystals and to fit the fluence-dependent differential absorption measurements. The average value of the Auger coefficient obtained from fitting to the measurements was γ = 5.9 ± 0.4 × 10−31 cm6 s−1. Similarly the average relaxation rate of the carriers was determined to be approximately τ = 110 ± 10 ps. Time-resolved measurements also revealed ~25 ps delay for the carriers to reach deep traps states which have a subsequent relaxation time of approximately 300 ps.
Indium oxide In2O3 is considered an important n-type wide-band gap semiconductor which has received a great deal of attention over the past few years due to its technological application in optoelectronic devices [1, 2] and sensors . Indium oxide is useful in these devices because of its high transparency in the visible part of the spectrum, high electric conductance, and its strong interaction with certain gas molecules. Furthermore, the growth of In2O3 nanocrystals (NCs) and nanowires (NWs) for sensor applications has also received attention in view of the large surface-area-to-volume ratio. These nanostructures have shown great promise for chemical and biological sensors [4–8] and as a result have attracted great interest by the nanostructure and sensing communities [9–18]. For example, In2O3 NWs configured as gas sensors have demonstrated greater room temperature sensitivity and selectivity than their commercial tin oxide thin-film counterparts. In addition, they have shown to be effective ultraviolet photo detectors .
Despite the extensive use of In2O3 as a transparent conducting material in optoelectronic devices, such as light emitting diodes, photovoltaic cells, liquid crystal displays, there has not been any detailed study on the photoinduced carrier dynamics. Therefore in this study we investigate the ultrafast carrier dynamics in In2O3 nanocrystals (average diameter ~500 nm) using two color pump-probe absorption spectroscopy [20–22], where the various important relaxation mechanisms have been identified. We find that Auger recombination appears to play a crucial role in the recovery of the photo-generated carriers in the In2O3 NCs within the first tens of ps and the Auger coefficient is 5.9 ± 0.4 × 10−31 cm6 s−1.
The average diameter of the NCs grown on the quartz substrate is approximately 500 nm whereas their estimated density is 8 × 107 NC/cm2. Further details on the growth and structure are given elsewhere . Steady-state transmission measurements provided an estimate of the NCs band gap from a plot of the square of the absorption versus photon energy to be approximately 3.5 eV . Furthermore the absorption curve shown as an inset in Fig. 1 depicts non-zero absorption below the band gap. Room temperature photoluminescence revealed a broad band luminescence covering a range between 350 and 460 nm with a peak at 390 nm. This is attributed to oxygen defects contained in the NCs, in agreement with previous reports [25–28] which suggest that oxygen vacancies are formed due to the incomplete oxidation during growth which act as donors resulting in the additional states below the band gap. In addition indium vacancies or interstitials in the NCs, may also be a contributing factor to the presence of the energy states below the band gap . Thus the conduction band tail extends over a large wavelength range resulting in PL due to the recombination from these defect states.
At this point we should mention that the In2O3NCs, given their relatively large size with the respect to the exciton Bohr radius (~2.4 nm), are not quantum confined; therefore the data from these structures can be analyzed within the framework of bulk-like material.
In this study, the dynamic behavior of carriers in In3O2 nanocrystals following femtosecond pulse excitation is investigated through the temporal behavior of induced absorption [20–22]. The experiments were carried using an ultrafast amplifier system running at 5 kHz. The source of short pulses was a self-mode-locked Ti:Sapphire oscillator generating 45 fs pulses at 800 nm. Part of the amplified energy was used in an Optical Parametric Amplifier system providing wavelength tunability in the UV range of the spectrum and thus a means of exciting the In3O2 nanocrystals. The rest of the energy was used to generate 400 nm from a BBO crystal and white light super continuum. The UV pulses from the OPA were used as the pump energy to excite the nanocrystals given that the expected band gap of this material is around 3.5 eV. The VIS–IR white light super continuum (500–1000 nm) which was use to probe the excited region was generated by focusing the 800 nm pulses on a 1-mm sapphire plate. Similarly a super continuum in the UV region of the spectrum was also generated with 400 nm pulses. The white light probe beam is used in a non-collinear geometry, pump-probe configuration, where the pump beam was generated from the OPA. Optical elements such as focusing mirrors were utilized to minimize dispersion effects and thus minimize the broadening of the laser pulse. The reflected and transmission beams were separately directed onto their respective detectors after passing through a band pass filter selecting the probe wavelength from the white light. The differential reflected and transmission signals were measured using lock-in amplifiers with reference to the optical chopper frequency of the pump beam. The temporal variation in the photo-induced absorption is extracted using the transient reflection and transmission measurements, which is a direct measure of the photoexcited carrier dynamics within the probing region [20–22]. Precision measurements of the spot size on the sample of the pump beam along with measurements of reflection and transmission at the pump wavelength provided accurate estimation of the absorbed fluence for the experiments in this study.
The first region corresponds to probing wavelengths below ~400 nm where the induced absorption change appears to be negative, and the second region corresponds to longer probing wavelengths where the change appears to be positive. For both probing regions there is an initial sharp change which is pulse-width limited reaching a maximum value, and then followed by a slow recovery toward equilibrium which persists over tens of picoseconds. The negative change in the induced absorption corresponds to what we refer to as “state filling”. This is associated with the occupation of states of the In2O3NCs by the photogenerated carriers following photoexcitation by the ultrafast laser pulse whose energy is above the band gap. Once the carriers occupy states that were normally unoccupied the absorption at the probing wavelength will appear reduced. Therefore, monitoring this negative change in absorption as a function of delay between the excitation and probing pulse is a direct measure of the temporal evolution of the photo-generated carriers at the probing wavelength state. On the other hand, if the probing energy is smaller than the band-gap energy, direct coupling from the valence band states to conduction band states will not be possible, therefore state filling will not be observed. However, under such probing conditions a positive change in the induced absorption maybe observable. This is due to secondary excitation of the photo-generated carriers to higher energy states due to the probing pulse. This positive photo-induced change depends on the number of photo-generated carriers present in the initial state and the coupling efficiency between the initial and final state. Therefore, the recovery signal is again a direct measure of the decay of the photo-generated carriers from the probing energy state. Here we should point out that in some cases state filling may be possible below the band gap when there are available energy states below the band edge which is the case for the In2O3NCs. The transient differential absorption measurements (Fig. 2) show state filling for probing wavelengths as long as 410 nm.
The recovery of state filling signal as seen for the shorter probing wavelengths (340 nm, 370 nm) in Fig. 2consists of two distinct temporal components, a fast and a much slower component. The fast component as we will show later on in this study is mainly due to Auger recombination, whereas the slower component which is of the order of 100 ps is associated with recombination or capture of the photo-generated carriers by various traps or surface-related states. It appears with increasing probing wavelength, the coupling from the valence bands to the available energy states below the band gap becomes weaker thus the state filling is reduced (this is in agreement with the broad photoluminescence spectra which drops to zero at ~460 nm). At the same time the contribution of secondary excitations increases, possibly, due to available higher energy states in the bands that the photo-generated carriers may couple by conserving energy and momentum. This is clearly evident from the observed increase in positive-induced absorption with increasing probing wavelength (Fig. 2). We should also point out that at some point both effects may be present as seen in Fig. 2at the probing wavelength of 410 nm. Furthermore, there appears to be a peak of positive-induced absorption at 600 nm which is attributed to a larger density of the coupled states at the particular probing wavelength.
The recovery seen in the positive photo-induced absorption, for the longer probing wavelengths, contains both temporal components seen for the shortest probing wavelengths; however, the fast Auger recombination component is much less pronounced. This is mainly because the number of carriers distributed among the probing states which are located below the band gap is less than that in the case of state filling which occurs near the band edge where most of the carriers relax before captured by traps or recombine. In addition near the band edge the probe couples to the electron and hole states, while at longer wavelengths it only interacts with electrons or holes separately. Since Auger recombination has a cubic carrier density dependence, this will cause a pronounced change on the temporal evolution of the photo-induced absorption.
It is also interesting to point out that the recovery of the induced absorption is much longer (~312 ps, see inset in Fig. 2) at the probing wavelength of 700 and 750 nm. Furthermore, the maximum signal appears to occur around 25 ps after the excitation pulse. The photo-generated carriers required a relatively long time to reach the probing states. This suggests that we are probing states that are much different than those we are probing with the shorter wavelengths where the maximum signal appears to be instantaneous (pulse-width limited). It is believed that we are probing deep traps states where the initial photo-generated carriers in the NCs have relaxed, at which point subsequent relaxation from these states is on the order of 300 ps.
Furthermore, we have investigated time-resolved dynamics at various excitation wavelengths with similar results. However, with increasing wavelength the signal becomes weaker. No measureable differential absorption signal was detected for pump wavelengths longer than 360 nm.
In conclusion we have investigated ultrafast carrier dynamics in In2O3NCs using pump-probe differential absorption white light measurements. State filling has been observed for probing wavelengths corresponding to energies above the band gap (3.5 eV) and just below the band edge due existence of shallow trap states. Positive-induced absorption (free carrier absorption) was the main contribution for wavelengths longer than 500 nm. Auger recombination appears to play a crucial role in the recovery of the photo-generated carriers in the first tens of ps. A simple differential equation model incorporating diffusion as well as carrier relaxation terms has provided a means to fit fluence dependence experimental data and obtain best-fit values for Auger recombination. The average Auger coefficient obtained from the fitted results was γ = 5.9 ± 0.4 × 10−31 cm6 s−1and carrier relaxation time constant τ = 110 ± 10 ps. Finally, differential absorption data clearly shows a long delay (approximately 25 ps) for the carriers to reach the probing states, which are believed to be deep traps states and ~300 ps for these carriers to move out of these states.
The study in this article was partially supported by the research programs: EPYNE/0504/06, ERYAN/0506/04, and ERYNE/0506/02 funded by the Cyprus Research Promotion Foundation in Cyprus.