Transient Photoconductivity of Ternary CdSSe Nanobelts As

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Transient Photoconductivity of Ternary CdSSe Nanobelts As Measured by Time-Resolved Terahertz Spectroscopy Junpeng Lu,†,§ Hongwei Liu,†,‡,§ Sharon Xiaodai Lim,† Sing Hai Tang,† Chorng Haur Sow,*,† and Xinhai Zhang*,‡ †

Department of Physics, National University of Singapore, 2 Science Drive 3, 117542 Singapore Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link, 117602 Singapore

‡

ABSTRACT: The frequency- and fluence-dependent transient photoconductivity in ternary CdSSe nanobelts is investigated using timeresolved terahertz spectroscopy. The carrier density and mobility are extracted by modeling the measured complex photoconductivity using the Drude−Smith model. Within the first few picoseconds of excitation, both the carrier density and mobility reach their maximum values and then decay gradually over tens to hundreds of picoseconds. The decay of free carriers is mainly attributed to fast surface trapping and structuraldefect-mediated recombination. The surface trapping saturates rapidly with increasing excitation fluence attributable to the low trapping density on the nanobelt surface caused by self-passivation of surface defects during the growth process.

method to identify electrical properties in semiconductors because it is noncontact, its frequency range is consistent with the typical carrier scattering rate, and the dynamics can be investigated with subpicosecond to nanosecond time scales. The THz wave is dispersed and attenuated by intraband transitions in proportion to the conductivity. Instantaneous carrier mobility and density can be extracted by appropriately modeling the measured complex photoconductivity. Recent studies of bulk or nanomaterial semicoductors using TRTS have examined transient photoconductivity in GaAs wafers,12 thin films,13 and nanowires,14 nanoporous InP,15 and ZnO wafers and nanowires.16−18 A few reports also employed TRTS to investigate carrier dynamics based on two-photon absorption in bulk ZnTe and CdTe.19 In this work, we present a study on the frequency-dependent, complex photoconductivity in photoexcited ternary CdSSe nanobelts using TRTS. The corresponding plasma frequency and scattering time are extracted by modeling the experimental data with the Drude−Smith model. Both the carrier density and mobility are calculated from the model fitting parameters, reaching their maximum values within the first ∼4 ps after photoexcitation and decaying gradually over tens to hundreds of picoseconds. The initial electron mobility is up to 81.6 cm2 V−1 s−1, and it decreases to about 41.5 cm2 V−1 s−1 within 500 ps at a laser fluence of 40 μJ/cm2. Systematic investigations are carried out to study the dynamics of the carrier density as a function of the excitation fluence. The decay of free carriers is attributed to fast surface trapping and structural-defect-

CdSSe is a wide band gap semiconductor with a tunable band gap from 1.73 to 2.44 eV, facilitating its use in diverse applications as a waveguide,1 light emitter,2,3 field-effect transistor,4,5 and light absorption material in nanostructured photoelectrodes.6 Free carrier and electron−hole dynamics based photoconductivity and charge transport are central to each of these applications. Therefore, developing a deep understanding of exciton creation, cooling, free carrier formation, trapping, and recombination is critical to optimize their performance. Carrier dynamics in semiconductors can be investigated by introducing electrons into the conduction band via doping, direct photoexcitation, interfacial electron transfer from an adsorbed photon sensitizer, thermal excitation, or an electron beam. The free carrier density and mobility as well as sample temperature, morphology, and defect distribution all affect the dynamics of exciton formation, carrier transport, and recombination. The photoexcited carrier population is depleted by exciton formation, radiative recombination, and nonradiative recombination at defect trap states. Recently, the carrier dynamics in CdSSe nanocrystals and nanobelts have been studied using time-resolved photoluminescence spectroscopy.7−11 However, this ultrafast technique mainly relies on band edge recombination; as a result, the measurements of carrier lifetimes in CdSSe nanostructures are hindered by the large defect densities, which reduce the photoluminescence efficiency. Both carrier population and mobility can be monitored by time-resolved terahertz (THz) spectroscopy (TRTS), which makes use of an optical pump to photoexcite conduction band electrons and valence band holes in semiconductors followed by a THz probe to measure the conductivity. TRTS is an ideal © XXXX American Chemical Society

Received: May 2, 2013 Revised: May 15, 2013

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Figure 1. (a) SEM, (b) TEM, and (c) HRTEM images of CdSSe nanobelts. The (d) XRD pattern and (e) EDS spectrum of CdSSe nanobelts show an atom ratio of S/Cd = 0.75. (f) PL spectrum of CdSSe nanobelts.

monitored by measuring the differential transmission, ΔT/T0, of THz waveforms at the peak amplitude as a function of the delay time between the terahertz probe and optical pump pulse. Here, ΔT is the time-dependent transmission change of the THz wave due to optical excitation, and T0 is the transmitted intensity of THz pulses without optical excitation. All the measurements were carried out in a dry nitrogen purge environment at room temperature. Figure 2a displays the pump-induced differential transmission signal, ΔT/T0, of CdSSe nanobelts as a function of the time t after excitation at an excitation-pulse fluence of 40 μJ/cm2. ΔT/T0 drops within 3.5 ps as soon as the pump pulse arrives, after which the nanobelts exhibit a biexponential decay in ΔT/T0. The fast relaxation time of CdSSe nanobelts is fitted to be 158 ps, while the slow relaxation time is 1020 ps. As demonstrated in our previous report, these two processes are attributed to surface trap and nonradiation structural-defect-related recombination, respectively.7,20 The shape of the decay in ΔT/T0 changes with the excitation fluence, tending to a monoexponential conductivity decay at lower excitation fluence, as illustrated in Figure 2b, which presents ΔT/T0 at a different excitation fluence range of 5−50 μJ/cm2. The disappearance of the fast process at 5 μJ/cm2 results from the lower density of surface states and the dominance of structural-defect recombination. By fixing the pump−probe delay time and recording the corresponding differential transmission, ΔT/T0, we determined the frequency dependence of the complex photoconductivity, Δσ(ω,t), of the CdSSe nanobelts, which provides a highresolution time-resolved monitor of quasi-particle dynamics in the material. To obtain Δσ(ω,t) from ΔT/T0, we employed the following expression:

mediated recombination by the biexponential fitting theory analysis.20 These results shed light on the optimization of optoelectronic devices based on CdSSe nanobelts. In this study, we investigate CdSSe nanobelts grown from a facile one-step chemical vapor deposition method on a C-plane sapphire substrate.5 The side-view scanning electron microscopy (SEM) image of the sample is shown in Figure 1a. These nanobelts have a thickness of about 30 nm, a width of 100−200 nm, and an average length extended to several tens of micrometers. The low-magnification transmission electron microscopy (TEM) image (Figure 1b) reveals that the nanobelts have a uniform morphology and smooth surface. Figure 1c displays the high-resolution TEM (HRTEM) image revealing that all the nanobelts are high-quality single crystals with growth along the [010] direction. The XRD pattern (Figure 1d) of the sample exhibits a pure wurzite structure without impurity phases. By applying Vegard’s law from the (100) or (002) peak, we calculated the sample with an “S” composition of S/Cd = 0.75, which is in good agreement with the energy-dispersive X-ray spectrometry (EDS) result, as shown in Figure 1e. The CdSSe nanobelts exhibit a narrow near-band-edge emission peak located at 580 nm without the appearance of a defect-emission peak, as shown in the photoluminescence (PL) spectrum in Figure 1f. The dynamics of photoexcited carriers are measured by TRTS. The sample is excited by an ultrafast 400-nm light generated with a Ti:sapphire regenerative amplifier laser system, which provides ∼35-fs pulses at a repetition rate of 1000 Hz. The THz probe beam with a spectrum range from 0.3 to 5 THz was generated with the air-plasma technique21 and detected by a THz air-biased-coherent-detection (THz-ABCD) method.22 The samples were excited at 45° incidence while the THz wave passed through the samples at normal incidence. The transient behavior of the photoexcited carriers was

Δσ(ω , t ) = B

̃ (ω , t ) ε0c(n1 + n2) ΔTsam d T0̃ (ω)

(1)

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where ε0 is vacuum permittivity, c is the speed of light, d is the thickness of the CdSSe nanobelts (measured from the SEM image), n1 and n2 are the refractive indices of nitrogen and sapphire, respectively, and ΔT̃ sam(ω,t)/T̃ 0(ω) is the transient differential transmission at decay time t. The extracted complex photoconductivity of the CdSSe nanobelts at several pump delay times after the photogeneration of carriers is shown in Figure 3. Here, the real part of conductivity (solid circles) can be regarded as the resistive response of the nanobelts, while the imaginary part (open circles) can be associated with an additional inductive or capacitive response.14 During the first 4.9 ps after excitation, the real part of the photoconductivity presents a sharp increase with the delay time (Figure 3a). This indicates a large amount of free carriers are generated by photon excitation. However, over the next 500 ps, the real part of the photoconductivity gradually decreases associated with the free carrier relaxation. Despite of the general increasing and decreasing process, a peak exists in the real part of the photoconductivity, accompanied by a corresponding zero crossing in the imaginary part (Figure 3b) over the whole 500 ps process. This feature is initially located at a lowfrequency position during the photocarrier generation process (2.8 ps) and shifts to the higher frequency range over the free carrier relaxation process (after 3.5 ps, at which point ΔT/T0 presents the largest amplitude). Furthermore, the frequency of this feature does not change with the delay time over the whole relaxation process. Therefore, we attribute it to a carrierdensity-independent mode related to excitonic transition23,24 rather than a carrier-density-dependent mode such as a localized surface plasmon.14,25 Such a surface plasmon is expected to occur for nanoparticles and is also observed in GaAs nanowires,14 in which the carrier dynamics are dominated

Figure 2. (a) Time-dependent differential THz transmission, ΔT/T0, pumped by 400 nm excitation at 40 μJ/cm2. (b) Excitation fluence dependence of time-dependent differential THz transmission. The solid lines are the results of best exponential fitting.

Figure 3. Time-resolved complex conductivity of photogenerated electrons and the carrier density and electron mobility extracted from the fitting model. The (a) real and (b) imaginary aspects of the photoconductivity are given at a series of pump−probe delay times at an incident excitation fluence of 40 μJ/cm2. Contour mapping of (c) experimental and (d) fitting results of frequency-dependent photoconductivity as a function of the delay time. (e) The carrier density decays with an exponential fit with the delay time as a guide to the eye,14 and (f) the electron mobility decreases linearly with the delay time. C

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Figure 4. (a) Excitation fluence dependence of the real part of the photoconductivity at a pump−probe delay t = 4.1 ps. Excitation fluence from top to bottom panel: 50, 40, 31.5, 20, 10, and 5 μJ/cm2. Circles are experimental data, and solid lines are from fitting. (b) Extracted plasma frequency as a function of the square root of the excitation fluence. (c) Extracted lifetimes of photoconductivity as a function of the incident excitation fluence.

density during the decay process can be best fitted by an exponential function14 (solid line in Figure 3e). The maximum electron mobility, 81.6 cm2 V−1 s−1, is presented at the earliest delay time measured just after full excitation, 4.9 ps, and the electron mobility decreases monotonically with increasing delay time after the arrival of the pump pulse. Electrons are excited well beyond the conduction band minimum, and we might expect the electron mobility to increase as electrons cool to the conduction-band minimum where the band curvature is highest. The electron mobility decreases linearly to 41.5 cm2 V−1 s−1 with the delay time over 500 ps. This value is higher than those of CdS0.25Se0.75 (14.8 cm2 V−1 s−1), CdSe (9.6 cm2 V−1 s−1), and CdS (1.7 cm2 V−1 s−1) measured from field effect transistor (FET) devices,4,33,34 indicating the longer persistence of the free carriers and highlighting the prospect of implementation of CdSSe nanobelts in fast nanoscale electronics. The observed drop in the photocarrier density with delay is indicative of the ultrafast trapping or recombination of the free carrier population. An experiment employing a fixed pump− probe delay (4.1 ps) and a series of incident excitation fluences, P, was carried out to facilitate the comparison of plasma frequencies, ωp, extracted from experimental data with those anticipated assuming linearity between the photogenerated carrier density and the excitation fluence. The plasma frequencies were deduced from fits to the photoconductivity spectra using the Drude−Smith model as shown in Figure 4a. Evidently, at the lower excitation fluence, the real part of the photoconductivity is positive and has no significant frequency dependence, which indicates the presence of photoexcited free carriers with a large scattering rate.14 However, as discussed earlier, the photocurrent gradually increases with increasing pump fluence and exhibits more obvious frequency dependence with increased photocarrier density. The extracted plasma frequencies are plotted in Figure 4b as a function of the excitation fluence. While the plasma frequency increases with

by surface effects.26,27 Here, the features reveal the transformation of photogenerated unbound electron−hole pairs into excitons, which requires a transient phonon bath into which the momentum and binding energy can be permanently released.23 We employ the Drude−Smith model to extract the photoconductivity of the carriers in the nanobelts. It introduces an extra parameter, C, related to incomplete randomization of free carrier momentum upon scattering and is described by32 σ(ω) =

⎞ ε0τ0ωp2 ⎛ C ⎜1 + ⎟ 1 − iωτ0 ⎝ 1 − iωτ0 ⎠

(2)

where ωp = Ne /(m*ε0) is the plasma frequency, in which N is the carrier density, m* is the electron effective mass, and e is the charge of a single electron, and τ0 is the momentum scattering time. Excellent fits to the experimental data are obtained over the whole range of pump−probe delay time, as presented with the solid lines in Figure 3a,b. More intuitive observation is revealed by the contour map shown in Figure 3c, which displays the real and imaginary parts of the photoconductivity as a function of the pump−probe time delay. Figure 3d is the theoretical fitting results extracted from the model as described below and in eq 2. The contours highlight increments of 77 Ω−1 cm−1, and the feature peak frequency is clearly revealed in the real parts of Figure 3c,d. The real part photoconductivity value can be measured to be higher than 710 Ω−1 cm−1. This value is much higher than those measured for nanostructures under the same excitation fluence.18,28−31 The extracted carrier density and calculated electron mobility (μ = (eτ(1 + C)/m*)) are presented as a function of the pump−probe delay in parts e and f, respectively, of Figure 3. As expected, the photocarrier density increases sharply just after the arrival of the excitation pulse and reaches a maximum value of about 2.2 × 1019 cm−3 within 5 ps, after which it gradually decays to less than 50% of its maximum level over 500 ps as carriers are lost through recombination, trapping, or exciton formation. The carrier 2

2

D

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the excitation fluence, it almost scales according to ωp ∝ P1/2 ∝ N1/2. This suggests that the photogenerated carrier density in the first 4.1 ps is linear with respect to fluence. As the defectrelated recombination or trapping strongly affects the carrier lifetime, it is essential to obtain an estimate of the trapping influence for photoconductivity. To investigate this behavior, biexponential curves are fitted to a range of pump−probe scans (shown as solid lines in Figure 2) to extract both the fast process decay time (τ1) and slow process decay time (τ2), with the results plotted as a function of the excitation fluence as shown in Figure 4c. Evidently, a clear dependence of τ1 on the excitation fluence can be seen, while τ2 does not show an obvious change with increasing excitation fluence. Therefore, as described previously,7,20 the biexponential relaxation process is attributed to surface trapping and structural-defect recombination, respectively. The obvious dependence of τ1 on the excitation fluence is primarily due to the saturation of surface traps by the large photogenerated carrier population, which leads to a reduction of available trap states with increasing excitation fluence. However, the photogenerated carrier density at all measured excitation fluences is lower than the available structural-defect density states so that τ2 does not show a clear dependence on the excitation fluence. In conclusion, we have investigated the transient photoconductivity of CdSSe nanobelts on picosecond to subnanosecond time scales using a noncontact pump−probe based on TRTS. The observed photoconductivity dynamics differs strongly from those for binary CdS and CdSe nanobelts, which display more surface trap density.7 In CdSSe nanobelts, the photocarriers experience an ultrashort lifetime by surface defect trapping and a longer lifetime in structural-defect recombination. This behavior is critically due to the low trap density on the nanobelt surface caused by the self-passivation of surface defects during the growth process.7 These results facilitate the possibility for implementation of ternary nanobelts in optoelectric devices, whose performance can be optimized through passivation of surface defects. The photocarrier density and electron mobility at highly excited states for the nanobelts at room temperature were extracted from the experimental data by modeling the photoconductivity response that forms 4.9 ps after excitation. The resulting values can be calculated to be as high as 2.2 × 1019 cm−3 and 81.6 cm2 V−1 s−1, respectively, demonstrating that the 1D ternary CdSSe nanobelts possess excellent photoelectronic properties.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.H.S.); [email protected] (X.Z.). Author Contributions §

Junpeng Lu and Hongwei Liu contributed equally.

Notes

The authors declare no competing financial interest.

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