Carrier Recombination Dynamics in Sulfur-Doped InP Nanowires

Letter pubs.acs.org/NanoLett

Carrier Recombination Dynamics in Sulfur-Doped InP Nanowires Wei Zhang,† Sebastian Lehmann,‡ Kilian Mergenthaler,‡ Jesper Wallentin,§ Magnus T. Borgström,‡ Mats-Erik Pistol,‡ and Arkady Yartsev*,† †

Division of Chemical Physics, Lund University, Box 124, 221 00 Lund, Sweden Department of Solid State Physics, Lund University, Box 118, 221 00 Lund, Sweden § Institute for X-ray Physics, University of Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany ‡

S Supporting Information *

ABSTRACT: Measuring lifetime of photogenerated charges in semiconductor nanowires (NW) is important for understanding light-induced processes in these materials and is relevant for their photovoltaic and photodetector applications. In this paper, we investigate the dynamics of photogenerated charge carriers in a series of as-grown InP NW with different levels of sulfur (S) doping. We observe that photoluminescence (PL) decay time as well as integrated PL intensity decreases with increasing S doping. We attribute these observations to hole trapping with the trap density increased due to S-doping level followed by nonradiative recombination of trapped charges. This assignment is proven by observation of the trap saturation in three independent experiments: via excitation power and repetition rate PL lifetime dependencies and by PL pump−probe experiment. KEYWORDS: Nanowire, doping, photoluminescence, hole trapping, carrier recombination

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NWs.20−26 To eliminate the otherwise dominant optical signal from InP substrate, NWs are typically broken off mechanically from the growth substrate and transferred to a substrate of different materials that does not contribute to optical signal of NWs.20,21 However, such transfer may affect the optical properties of NWs,19,27 and methods to characterize as-grown NWs are attractive. We have previously found that by choosing appropriate polarization and incident angle of excitation, we can strongly suppress the signal from the substrate.19 Also here we find that the time-resolved photoinduced luminescence (TRPL) signal of the substrate (nominally intrinsic InP) can be neglected as compared to that of S-doped InP NWs using this technique. In this work, we use TRPL to study charge recombination dynamics in as grown InP NWs with varied S-doping level. For intrinsic InP NWs, we find that PL decays faster with increasing excitation photon flux, which agrees with bimolecular recombination of photogenerated carriers. However, for the S-doped NW with low S concentration, the PL decay slows down and approaches a constant value with increasing excitation photon flux. This effect is explained by charge trapping combined with saturation of long-lived traps at high excitation power density. We confirm this hypothesis by PL pump−probe experiment. The effect of trap filling was less visible at higher S-doping level due to a (much) higher number

ver the past decades epitaxially grown semiconductor nanowires (NWs) have received intense interest as they have potential for applications in photovoltaic cells,1−3 photodetectors,4 light-emitting diodes,5 waveguides,6 biosensors,7 field-effect transistors,8,9 and so forth. Bulk InP occurs in cubic zincblende crystal structure, however, InP NWs commonly show a mix of zincblende and hexagonal wurtzite (WZ) crystal phase.10−15 In order to characterize the bulklike properties of WZ InP, great efforts have been spent to grow InP NWs with WZ crystal structure.9,14,16 In situ doping by use of hydrogen sulfide (H2S) as a precursor for n-type sulfur (S) doping was found as one of the methods to favor growth of WZ crystal structures and reduction of stacking faults in InP nanowires9,14 with pure WZ crystal structure occurring for sufficiently high H2S flows.17 Moreover, H2S has been shown to be a surface passivator for InP NWs18 which in combination with reduced stacking faults could result in a higher fluorescence quantum yield Q. However, in microluminescence measurements it was found that at low temperature (10 K) the overall PL intensity gradually decreased with increasing Sdoping level.14 Furthermore, ∼20 times decreased PL quantum yield of S-InP NWs compared to that of InP substrate was shown at room temperature.19 These studies indicate that nonradiative processes dominate recombination of photogenerated charges in S-doped NWs. In this study, we address the role of S doping in recombination of photogenerated charges and present a model that explains the decrease of integrated PL intensity with high S doping. Time-resolved spectroscopy has been intensively used to study charge recombination dynamics in epitaxially grown InP © XXXX American Chemical Society

Received: May 22, 2015 Revised: September 20, 2015

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DOI: 10.1021/acs.nanolett.5b02022 Nano Lett. XXXX, XXX, XXX−XXX

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I

II

III

IV

V

VI

VII

VIII

IX

χH2S ( × 10‑6) n ( × 1019 cm‑3)

0 0

0.4 0.31

2 0.80

3.9 1.28

7.9 1.47

12 2.25

16 2.37

31 2.47

63 4.58

Figure 1. The 30° tilted view SEM images of (a) as-grown nonintentionally doped and (b) S-doped InP NWs on InP substrate. (c) Length and diameter distribution of the NWs investigated depending on the H2S molar fraction supplied during growth. The scale bars are 500 nm in (a) and (b).

Figure 2. (a) Time-integrated PL spectra of S-doped InP NWs with different doping levels. (b) Spectral and temporal integrated PL intensity of Sdoped InP NWs as a function of χH2S after photoexcitation at 400 nm (3.1 eV). Inset in (b) shows log−log representation of the same graph and line corresponds to the slope −1. The excitation photon flux is 7.8 × 1012 cm−2. I−IX are the samples’ names as in Table 1.

absorption coefficient, and d is the film thickness. As the diameter and length of the NWs are varied slightly with Sdoping, the thickness of the representative films will also vary. At high absorption of excitation light by NWs (≥90%, see Supporting Information, S2), for the same I0 the maximum light in-coupling change with doping is less than 4%. More accurate description of light absorption requires thorough modeling28 as ray optics has been shown not applicable for regular NW arrays with absorptivity dependent on the NW sizes.1,29 Yet, for the set of irregular NW arrays we have studied the dispersion of the NW diameter is comparable to the variation of the mean diameters for different doping, see Figure 1a,b. Furthermore, light absorption in NWs is dominated by the top segment of NW1,29 thus the difference in the NW length is not critical. Altogether, we do not expect any significant variance in the NW light absorption for all studied doping levels. The crystal structure was investigated for selected samples by high-resolution transmission electron microscopy (HRTEM). We found a similar tendency as reported earlier for the use of H2S for Au-seeded InP nanowire growth14 namely an increase in wurtzite fraction for increasing amounts of H2S. Starting

of traps and faster nonradiative recombination. We consequently conclude that S doping introduces additional traps to InP NWs, which reduce the overall PL intensity. S-doped InP NWs were grown with molar fractions of H2S (χH2S) in the range of χH2S = 0−6.3 × 10−5. The NWs we have studied are listed in the Table 1; these NWs were investigated with scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), time-resolved photoluminescence (TRPL), and fs-transient absorption. More experimental details can be found in the Supporting Information (see S1). The grown NWs were characterized by scanning electron microscope (SEM) measurements, see Figure 1. Apparently, the average NW diameter has decreased and the average length has increased with increasing χH2S, thus the volume of an average NWs does not changed much with doping. Using a simplified assumption of linear light absorption and neglecting reflection (see Supporting Information, S2), we can approximate NWs by a film of InP of the same volume with the light absorptivity defined as 1 − T = ΔI/I0 = (I0 − I)/I0 = 1− exp(−μd), where T is transmission, I0 and I are the incident and transmitted light intensities respectively, μ is the linear B


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Nano Letters from heavily twinned pure zinc blende crystal structure with additional stacking defects (χH2S = 0) we observed an increase in WZ fraction up to pure wurtzite with almost no stacking defects present for the highest amount of H2S used (χH2S = 6.3 × 10−5). To examine the radiative recombination in as-grown InP NWs with different S doping levels, we conducted TRPL experiments. Figure 2a shows PL spectra of S doped NWs integrated over the time range of 0−1.7 ns. The PL spectra for the measured set of NW samples exhibit a shift of PL toward higher energies (blue shift) with increasing S-doping. Whereas the band gaps of ZB and WZ InP at room temperature are 1.338 and 1.408 eV,30 respectively, for S-doped NWs we observe a blue shift of the PL peak energy from ∼1.36 eV for intrinsic NWs to ∼1.75 eV at the highest doping level. The spectral width of the integrated PL spectra increases with S-concentration. Such a shift and broadening are expected for high n-doping as a result of the Fermi level shifting due to the Burstein−Moss effect at high densities of electrons in the conduction band.14,31,32 Because the energy at the half-maximum value on the high energy side of the PL spectra roughly indicates the position of the Fermi energy in a NW, the shift of the PL spectra can be used to measure carrier concentration.14 Using a simple parabolic band model, we have calculated the doped electron concentration (see Supporting Information, S3), as shown in Table 1. We found that the doping electron concentrations in most of the NWs are much higher than the photogenerated electron and hole concentrations in the excitation photon flux range used in our TRPL experiment. The Burstein−Moss shift is naturally coupled to the shift of absorption and will therefore affect the absorption of excitation light with energy close to the Fermi level. However, the excitation energy we use (3.1 eV) is well above the Fermi level, thus the excitation light absorptivity is very similar for all NW samples studied. Therefore, by accounting for the accumulation time in the PL measurements we can compare the intensity of the measured spectra for all NW samples and correlate it to the relative PL quantum yield (Q) defined as the time- and spectral-integrated PL intensity (Figure 2b). We observe that Q is strongly decreasing with S-doping level, for example, compared to sample II, Q is ∼6 times weaker in sample VI and ∼40 times weaker in sample IX. This phenomenon is consistent with the recent PL study of S-doped NWs at low (10 K) temperature.14 We also note that the PL Q of vertically standing S-doped (n ∼ 1019 cm−3) InP NWs is ∼20 times lower than that of the InP substrate at room temperature.19 These findings indicate that most of the absorbed excitation photons are not converted into emitted photons implying that radiative recombination is not the main recombination process in Sdoped InP NWs. To understand the underlying physical mechanisms of the carrier dynamics, we compared the TRPL kinetics in the differently doped InP NWs, see Figure 3. The PL decay time is doping-level-dependent: the stronger the doping, the faster the decay (shorter PL decay time). We have not observed any measurable long-lived PL signal on nanosecond time scale even with ∼100 times longer acquisition time. Such PL dynamics can be anticipated because the concentration of electrons that recombine with the photogenerated holes depends not only on the excitation photon flux but also on the doping.33 At low enough excitation fluencies, electrons originating from ionized dopants will dominate the recombination for all but intrinsic

Figure 3. TRPL decay of InP NWs with different sulfur contents as a function of time after photoexcitation at 400 nm (3.1 eV). The excitation photon flux is 7.8 × 1012 cm−2. I−IX are the samples’ names as in Table 1.

NW samples. The radiative recombination at higher doping level is expected to become faster as every hole can recombine with one out of a large number of excess electrons.33 It is important to note that within this explanation if all photogenerated charges would recombine radiatively the overall PL Q should not change with the doping level even though the rate of the radiative recombination increases. Apparently, if all holes eventually recombine with either photogenerated electrons or electrons originating from ionized donors the same number of PL photons is produced, which is equal to the number of absorbed excitation photons. If some of the photogenerated charges recombine nonradiatively, then the higher rate of radiative recombination at higher doping should increase the PL Q. Neither of these scenarios of doping electrons dominance in radiative recombination agrees with our experimental observations. Further, we consider the shape of the PL decay in all NW samples studied. For the intrinsic InP NWs, the PL decay cannot be fitted by a single exponential function and is strongly dependent upon incident photon flux, as shown in Figure 4a. It is expected that correlated electrons and holes (exciton) generated during absorption of a photon quickly dissociate into “free” electrons and holes at room temperature.34,35 Therefore, radiative recombination is dependent on the product of concentrations of electrons and holes and can be described as a bimolecular (second order reaction) type of process. Radiative recombination may compete with a nonradiative process, for example, when photogenerated charges become trapped in the NWs. At higher concentrations of free charges, another nonlinear recombination process, Auger recombination, may play an important role.34 Thus, recombination of free charge carriers in intrinsic InP NWs is governed by the following rate equation dN N = G(t ) − γN 2 − AN3 − dt τtrap

(1)

,where N is the concentration of photogenerated electron−hole pairs, G(t) is the generation rate of free charges determined by a pump pulse, γ is the bimolecular recombination rate, A is the Auger recombination constant, and 1/τtrap is the charge trapping rate. τtrap in the term − N/τtrap in eq 1 is a constant when the relative concentration of traps [tr] is much larger than N and the average distance between photogenerated charges and traps is determined by the trap−trap distance. When N is larger than [tr], the charge-to-trap distance is dependent on C


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Figure 4. (a) TRPL decays of intrinsic InP NWs under the indicated photon fluxes per single excitation pulse. (b) TRPL lifetime of intrinsic NWs as a function of incident photon flux. The lifetime is estimated by 1/e lifetime methods. The excitation wavelength is 400 nm (3.1 eV).

Figure 5. (a) TRPL decays of the sample II under the indicated excitation photon fluxes. Solid lines are fitting curves based on monoexponential functions. (b) TRPL lifetime of samples II, III, and VI as a function of incident photon flux. II, III, and VI are the samples’ names as in Table 1. The excitation wavelength is 400 nm (3.1 eV).

charge concentration N and the trapping time τtrap decreases with N. In our experiment, the PL of intrinsic InP NWs decays faster with increasing excitation photon flux and exhibits a nonexponential character, suggesting that bimolecular and Auger recombination may be essential in these NWs. If traps are crucial for PL decay in these NWs, then [tr] must be relatively low. We have conducted a femtosecond transient absorption (TA) study of intrinsic NWs and observed the longtime scale dynamics to be very similar in TRPL and TA at very similar excitation fluxes (see Supporting Information, S4). This suggests that radiative recombination is the dominant process in intrinsic NWs. For quantitative comparison of the measured kinetics, the time corresponding to e-times decrease of the PL amplitude was used. Such lifetimes are presented in Figure 4b for a range of incident photon flux. The decrease in the lifetime with excitation photon flux suggests faster recombination under higher excitation. In contrast to the intrinsic NWs, the measured PL decays of S-doped InP NWs can be fitted quite well by single exponential decay functions. At high S-doping, this procedure becomes less reliable as the decay time approaches the instrumental function. According to eq 1, exponential decay of PL will be observed when charge trapping is the dominant process and [tr] is large. Also, for (heavily) doped semiconductors at low enough excitation photon fluxes, radiative recombination could be dominated by the ionized electrons from the donors. Under these conditions, bimolecular radiative recombination turns into a pseudo-first order reaction with an exponential decay

function. Such behavior would agree with the exponential shape of the observed PL decays but this explanation has been shown above to be inconsistent with the Q measurements. In general, with increase of the excitation photon flux the concentration of the photogenerated charges may reach and even exceed the equilibrium carrier concentration. For such an excitation, the radiative recombination is expected to turn nonexponential and approach a bimolecular type of dynamics. At the excitations that we have used in TRPL experiments, the estimated concentration of the absorbed photons and therefore photogenerated charges of both signs is in the range of 5 × 1015 ∼ 1018 cm−3, which is lower than doping concentration even at the lowest S-doping level. Higher order recombination processes such as Auger recombination, could become prominent at higher excitation fluencies and would also lead to deviations of the PL kinetics from a single exponential function. Contrary to these considerations, at all excitation fluencies used we find that the PL decay is exponential in the Sdoped NWs. This observation further supports our earlier conclusion that radiative recombination due to majority charges induced by S-doping is not the main mechanism of recombination for these NWs and indicates that at the excitation photon fluxes used Auger recombination is not significant. With an increase of the excitation photon flux we have observed an unexpected behavior of the PL decay. Figure 5a presents the PL decays under different excitation photon fluxes in sample II. As the excitation photon flux increases, the PL decay rate decreases and then levels off. Figure 5b shows the PL D


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are in the NW volume and ∼7 nm if traps are located on the NW surface. The experimentally observed increase of the decay time in NWs with excitation photon flux could conceivably be explained by two types of trap filling processes. These processes might occur either separately or jointly. In the first process, traps are saturated during the leading part of the excitation pulse. This requires very fast trapping and a concentration of traps that is smaller than the concentration of photogenerated charges, [tr] < N. In this situation, we expect a bimolecular type of decay dynamics, which is in conflict with the single exponential decay of the PL that we observe. In the second trap filling process, the traps are filled at a constant rate by each excitation pulse. If the nonradiative recombination rate of the traps is sufficiently slow, we have a memory effect between the pulses because the traps are not completely empty when the next pulse arrives. In TRPL experiment, the excitation pulses arrive to the NW sample every ∼12 ns. After a sufficient number of excitation pulses, a balance between trap filling and emptying will result in some quasi-stationary level of the filled traps. This level of the trap filling depends on the excitation photon flux, on the number of available traps, and on their lifetime. Relatively high excitation photon fluxes are required to saturate the PL quenching for high trap concentrations. We observe a clear saturation effect for the sample II (Figure 5) in the sense that the decay time does not increase with photon flux anymore. For the higher doping levels, we are not able to reach saturation, supporting the hypothesis that the trap concentration increases with the S-doping level. To study the trap recovery process further, we decreased the excitation repetition rate in several steps from 80 down to 8, 4, 1.62, 0.8, and 0.4 MHz. For the sample II, we observe a clear effect of the PL decay rate increasing with decreasing excitation repetition rate, indicating that the long-lived trap model is valid for these NWs (Figure 7). Apparently, recombination of some trapped charges is very slow so that traps are still filled even at times longer than 2.5 μs.

lifetime as a function of the incident photon flux. For the NW samples II and III with the lowest doping level, the decay time increases with excitation. The maximum increase of the decay time in the excitation photon flux range used in these experiments anticorrelates with the doping level: the strongest effect was observed for the sample II, less pronounced effect for higher doping samples III and IV and almost no effect for sample V and others with even higher doping (see Supporting Information, S5). The excitation flux value when the PL decay time begins to rise increases as the doping level increases. We explain this effect by charge trapping, as shown in Figure 6. In our model, the electron−hole recombination via traps is

Figure 6. Schematic of charge recombination processes in S-doped InP NWs. PL is radiative recombination and NR represents nonradiative recombination.

slow and nonradiative, but the capture to traps is fast. At low excitation photon flux, charge carriers are quickly captured by the traps. This is seen as a fast PL decay. For increasing excitation photon flux, we start to fill some traps with charge carriers. Consequently, the charge trapping rate decreases and we observe a slower PL decay. Concerning the nature of the traps we note that electron traps should be saturated with electrons at high doping levels leading to a slow PL decay. However, this is in contradiction with experiments. On the contrary, hole trapping agrees with our observations. The trap concentration appears to increase with S-doping. Sulfur coverage of III−V NWs surfaces is usually considered as passivating surfaces mediator.18 Yet, chemical effect of H2S treatment of the NW surface can be very different from the effect of the InP electron doping induced due to replacing P atoms by S. Also, the S doping concentrations used in this study are very high and it is not inconceivable that sulfur can act as a trap at these concentrations. This is currently under active investigation (Wei et al. unpublished). A prolonged free carrier lifetime induced by trap filling under high excitation photon flux has been reported previously for GaAs36 and CdTe37 NWs, as well as in microcrystalline silicon.38 In our experiment, trap filling for sample II occurs at ∼4 × 1012 cm−2 of excitation photon flux. Assuming that most of the photogenerated holes are used for trap filling we can estimate the average distance between traps as ∼11 nm if traps

Figure 7. TRPL lifetime of samples II, III, IV, and V as a function of the laser repetition rate. The excitation wavelength is 400 nm (3.1 eV). II−V are the samples’ names as in Table 1.

To further cross-check the trap filling models, we have designed another type of PL experiment, which we call “timeresolved pump−probe PL”. In this experiment, we split the excitation beam into two pulses by a beam splitter, delay the arrival of one pulse to the sample, and focus both beams by a lens into the same spot on the sample, as shown in Figure 8a. The first, usually more intense pulse (1.4 × 1013 cm−2) is used E


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doped NWs, sulfur may induce some bulk defects, and these defects could act as nonradiative recombination centers.14 However, the reported trapping time induced by sulfur-doping related traps in S-doped bulk InP, which have similar doping concentration with sample II, is several orders longer than that of sample II when the effective lifetime is longest.41 Although charge dynamics in bulk could be more involved than in NWs, the dominating traps in S-doped NWs that we have studied may be associated with NW surface. In S-doped NWs, without photoexcitation electrons originating from donors can be trapped by surface states, forming acceptor-like centers, which may induce band bending and act as traps of photogenerated holes.40 Band bending due to doping has been reported in p-doped (Zn-doped) InP NWs42 and n-doped GaAs NWs40,43 and is used to describe the adsorption and desorption processes of oxygen ions processes in ZnO photodetectors.44 Here, the band bending model fits our observations. We suggest that at high excitation photon flux when a large number of carriers are photoinjected into a NW, the band bending decreases due to screening of the surface field, so that the hole trapping rate may decrease. For the sample II, the band bending due to doping is not very strong and can be influenced at high excitation. Thus, we observe the PL decay slowing down with increasing excitation photon flux. For high S-doped InP NWs (e.g., samples V−IX), the band bending according to this explanation is so severe that the excitation photon flux has much less influence on it. In summary, by using time-resolved photoluminescence, TRPL, we demonstrate that high S doping introduces traps in InP NWs and reduces the overall PL intensity. For intrinsic InP NWs, our measurements indicate that stacking faults, twins, ZB-WZ polytypism, as well as the surface of InP NWs have only a weak influence on the carrier lifetime. The main recombination mechanism of photogenerated charges in intrinsic InP NWs is radiative. In InP NWs with low S-doping level, we find that holes are captured by traps before they have a chance to recombine radiatively which is seen as a change in the PL decay time. These traps can be saturated at sufficiently high excitation power density. The trap saturation requires a higher excitation photon flux at higher S doping. From this we conclude that the concentration of traps increases with S concentration.

Figure 8. (a) Schematic of the pump−probe PL experiment. (b) TRPL decays of sample II at the probe photon flux of 1.3 × 1011 cm−2 (black), 1.3 × 1012 cm−2 (blue), and 5 × 1012 cm−2 (green) with (hollow) and without (solid) pump excitation (photon flux of 1.4 × 1013 cm−2). Solid lines are fitting curves based on monoexponential functions. The excitation wavelength is 400 nm (3.1 eV).

as a pump and the second pulse of varied photon flux (from 1.3 × 1011 cm−2 to 5 × 1012 cm−2) is a probe pulse. The delay between these pulses was set to ∼1.75 ns, which is much longer than the observed PL decay time given earlier. We set up the measurement such that only PL induced by the probe pulse is detected and resolved in the streak camera experiment. In the pump−probe PL experiment, when the pump is blocked, the PL decay slows down with increasing probe photon flux as has been presented above (Figure 8b). When the pump photon flux of 1.4 × 1013 cm−2 is applied, the probeinduced PL decay (∼35 ps, see Supporting Information, S6) hardly changes over the whole range of the probe photon flux variation. These probe photon fluxes fall into the range where we have observed change of the PL decay time earlier. In agreement with the second model of trap saturation, we assign the effect of the pump pulse to filling of the long-lived trap states. Charge trapping may be induced by surface traps,39,40 stacking defects, twins, ZB-WZ polytypism,21 and so forth. Trapping can separate electrons and holes spatially, preventing them from encountering each other and thus quenching PL emission. The similar decay kinetics of TRPL and TA that we observe in intrinsic InP NWs indicate that surface states (e.g., surface dangling bonds etc.) have weak influence on free carrier recombination at room temperature in such NWs. This is in contrast to GaAs NWs and consistent with the recent photoconductivity study of intrinsic InP NWs.23 High-resolution transmission electron microscopy (HRTEM) analysis shows that the densities of twins and ZBWZ polytypism are reduced with increasing S-doping level.14 The InP NWs with the highest S-doping concentration, which have a pure WZ structure, have the highest density of traps according to our TRPL and TA measurements. This observation suggests that the dominated traps of S-doped InP NWs are not induced by twins or ZB-WZ polytypism. For S-

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b02022. Experimental methods details, calculation of doped carrier concentration, fitting of pump−probe PL decays. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support of the Knut and Alice Wallenberg Foundation, Swedish Energy Agency, Swedish F


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Research Council, Crafoord Foundation and the Nanometer Structure Consortium at Lund University (nmC@lu).

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