Carrier Recombination Processes in Gallium Indium Phosphide

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Carrier Recombination Processes in Gallium Indium Phosphide Nanowires Wei Zhang, Xulu Zeng, Xiaojun Su, Xianshao Zou, PierreAdrien Mante, Magnus T Borgström, and Arkady Yartsev Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b01159 • Publication Date (Web): 27 Jun 2017 Downloaded from http://pubs.acs.org on June 28, 2017

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Carrier Recombination Processes in Gallium Indium Phosphide Nanowires Wei Zhang,†,# Xulu Zeng,‡,# Xiaojun Su,† Xianshao Zou,† Pierre-Adrien Mante,† Magnus T. Borgström,‡ Arkady Yartsev†,* †

NanoLund and Division of Chemical Physics, Lund University, Box 124, 221 00 Lund, Sweden ‡

NanoLund and Division of Solid State Physics, Lund University, Box 118, 221 00 Lund, Sweden

Table of Contents Figure

ABSTRACT: Understanding of recombination and photoconductivity dynamics of photo-generated charge carriers in GaxIn1−xP NWs is essential for their optoelectronic applications. In this paper, we have studied a series of GaxIn1−xP NWs with varied Ga composition. Time resolved photo-induced

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luminescence, femtosecond transient absorption and time resolved THz transmission measurements were performed to assess radiative and non-radiative recombination and photoconductivity dynamics of photo-generated charges in the NWs. We conclude that radiative recombination dynamics is limited by hole trapping whereas electrons are highly mobile until they recombine non-radiatively. We also resolve gradual decrease of mobility of photo-generated electrons assigned to electron trapping and de-trapping in a distribution of trap states. We identify that the non-radiative recombination of charges is much slower than the decay of the photoluminescence signal. Further, we conclude that trapping of both electrons and holes as well as non-radiative recombination become faster with increasing Ga composition in GaxIn1−xP NWs. We have estimated early time electron mobility in GaxIn1−xP NWs and found it to be strongly dependent on Ga composition due to the contribution of electrons in the X-valley. KEYWORDS: Gallium Indium Phosphide, nanowire, mobility, photoconductivity, carrier recombination Semiconductor nanowires (NWs) are future building blocks for optoelectronic devices such as solar cells,1−3 light-emitting diodes,4 and photodetectors5. Among NW materials, Gallium Indium Phosphide (GaxIn1−xP) NWs with bandgap tunable from 1.34 to 2.26 eV, have potential applications in single/multiple junction solar cells, light emitting diodes and tunable NW lasers.6−8 Because of the small dimensions, NWs can exhibit different electronic and optical properties as compared to planar structures of the same materials. First, NWs have high surface-to-volume ratio, and surface states may trap charges and act as non-radiative recombination centers.9 Therefore, surface trapping is an important issue for the application of NWs based devices. Second, the crystalline structure of NWs may differ from that of planar structures, and thus influence the opto-electronic properties of devices.10 Further, altered Ga/In composition is expected to influence the NW surface and bulk features, in particular, due to a variation of the


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charge mobility and alloy disorder effect.11−17 Despite thorough studies of radiative recombination processes in planar structure of GaxIn1−xP,18,19 carrier recombination processes in GaxIn1−xP NWs are not clear yet. Understanding carrier recombination and photoconductivity dynamics of photogenerated charge carriers in GaxIn1−xP NWs is essential for their applications. In this paper, we have investigated the dynamics of photo-generated charge carriers in a series of GaxIn1−xP NWs with varying Ga compositions. We have combined time resolved photoinduced luminescence (TRPL), fs-transient absorption (TA) and time resolved THz spectroscopy (TRTS) measurements to examine the dynamics of the radiative and non-radiative charge recombinations and photoconductivity in these NWs. We attribute the observed decay of photoinduced luminescence (PL) lifetime and intensity to trapping of photo-generated holes considering that the PL decay time increases with increasing excitation density due to the trap filling effect. Electron trap filling in Ga0.54In0.46P and Ga0.35In0.65P NWs was also observed in the electron mobility dominated TRTS kinetics. We find that both electron and hole trapping rates increase with increased Ga content in GaxIn1−xP NWs. With the help of TA measurements, we conclude that the observed non-radiative recombination of charges is much slower than PL decay. This conclusion is consistent with the observation of long-lived photoconductivity. We also conclude that carrier mobility does not decay much on the timescale of 100−500 ps and that the electron mobility decreases with increase of Ga composition of GaxIn1−xP NWs. By comparing of TRPL, TRTS and TA we conclude that a small part of photo-generated charges recombine radiatively as most of the holes get trapped and recombine with highly mobile electrons in a nonradiative manner. We observe that the non-radiative recombination speeds up with increase of Ga composition as suggested by a comparison of the TA kinetics measured in NWs with different Ga composition. GaxIn1−xP NW arrays were grown on InP (111) B substrates by metal organic vapor phase epitaxy (MOVPE),20 see supporting information S1 for details of the NW growth. Four GaxIn1−xP


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NW arrays with different Ga composition were grown by adjusting the trimethylindium (TMI) molar fraction while keeping the trimethylgallium (TMG) molar fraction constant (χTMG = 1.36×10−3). The GaxIn1−xP NW arrays placed in a hexagonal pattern with a pitch of 500 nm. The NWs have the diameter D=185 nm and the length L≈2.0 µm, both independent of the TMI molar fraction (Supporting information S2). To conduct TRTS measurement, NWs were embedded in a colorless First Contact polymer film (Photonic Cleaning Technologies LLC, Supporting information S3), and then peeled off from the substrates,21 resulting in NWs inside the polymer matrix. TMI molar fractions of the studied NWs are listed in the table 1. The morphology of the asgrown NWs was inspected by LEO 1560 field-emission scanning electron microscopy (SEM). JEOL 3000F high resolution transmission electron microscopy (TEM) was used to analyze the crystal structure of different NWs. These NWs were examined by means of time-resolved techniques of TRPL, TRTS and TA. Further experimental details can be found in the Supporting Information (see supporting information S1). Table 1. TMI molar fractions (χTMI), PL peak energy, Ga composition, and PL decay time (τPL) in GaxIn1−xP NWs.

χTMI (×10−5)

2.4

3.0

3.6

4.2

PL Peak (eV)

1.92±0.01

1.77±0.01

1.66±0.01

1.56±0.01

GaxIn1−xP

Ga0.54In0.46P

Ga0.43In0.57P

Ga0.35In0.65P

Ga0.26In0.74P

τPL (ps)

14±2

23±2

30±3

130±9

First, we consider the PL measurements of the GaxIn1−xP NWs. In general, PL selectively reflects radiative recombination of mobile photo-generated charge carriers. Figure 1 (a) shows the PL emission spectrum from GaxIn1−xP NWs grown using different TMI molar fractions. We observe the PL emission redshift with increasing TMI molar fractions. A close to linear


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correlation between the maximum of the PL emission energy obtained by a Gaussian fit and the TMI molar fraction was observed (Figure 1 (b)).

Figure 1. (a) Normalized time-integrated PL spectra of as grown GaxIn1−xP NW arrays with varied TMI molar fractions after photoexcitation at 3.1 eV. (b) PL emission energy and intensity of as grown GaxIn1−xP NW arrays as a function of the TMI molar fraction measured under the same effective excitation.

Since the position of the PL maximum is correlated with Ga composition, x, in GaxIn1−xP NWs, x can be effectively tuned by controlling the molar fraction of TMI. By using the emission energy of the NWs we have calculated x in GaxIn1−xP NWs (Supporting information S4),22 which is also shown in Table 1. We conclude that x varies in the range of 0.26−0.54, which corresponds to a direct bandgap range in Zinc-Blende bulk and thus is suitable for applications in optoelectronic devices.6,18,22−24 Accounting for the accumulation time and excitation fluency in the PL measurements we have re-calculated intensity of the time- and spectrally- integrated PL spectra for all NW samples (Figure 1(b)). These values represent the number of emitted photons PL by the NWs for the same number of absorbed excitation photons and therefore can be considered as a relative PL quantum yield (Q). We observe that Q is clearly decreasing with increase of x. We have also measured the absolute PL quantum yield (QY) for the as-grown Ga0.43In0.57P NWs by an integration sphere


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method (Supporting information S5) and obtained the value of ~10−6 under ~300 suns CW excitation .

Figure 2. (a) TRPL decays of as grown Ga0.54In0.46P NW array under the indicated excitation photon fluxes. (b) TRPL kinetics of as grown GaxIn1−xP NW arrays with varied Ga composition after photoexcitation at 3.1 eV. The excitation photon flux is 4.0×1011 cm−2. TRPL kinetics were fit by two-exponential decay functions, the fitting parameters are given in the table S1. (c) Normalized photoconductivity kinetics of Ga0.35In0.65P and Ga0.54In0.46P NWs after photoexcitation at 3.1 eV at a fluence of 3.5×1011 photons cm−2 pulse−1, blue lines are mono-exponential ﬁts to the decays at early times after photoexcitation. (d) Normalized photoconductivity kinetics of Ga0.54In0.46P NWs at indicated photoexcitation fluencies after photoexcitation at 3.1 eV, the kinetics were fitted by two-exponential decay functions, the fitting parameters are given in the table S2.


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To understand the underlying physical mechanisms of the carrier dynamics, we have analyzed the TRPL kinetics of GaxIn1−xP NWs for varied x. In semiconductor NWs, the recombination dynamics of photogenerated charges can be described by the following general rate equation:25,26

dN N = − AN 3 − γN 2 − dt τ trap

(1),

where N is the concentration of photo-generated electron-hole pairs, A is the Auger non-radiative recombination constant, γ is the bi-molecular radiative recombination rate, and 1/τtrap is the charge trapping rate. As will be shown later, trapped charges undergo a much slower non-radiative recombination. If the trapping rate is low, for example due to low concentration of traps, charge dynamics should be dominated by bi-molecular and Auger recombination processes. In this case, PL kinetics should exhibit non-exponential decay, and the PL is expected to decay faster with increasing excitation flux. However, we observe slower PL decays at higher excitation power (Figure 2 (a) and supporting information S6), suggesting that TRPL kinetics of GaxIn1−xP NWs are not dominated by bi-molecular or Auger recombination. This behavior is in contrast with the PL dynamics in intrinsic InP NWs, where PL decay was reported to speed up with excitation fluence.25 Therefore, TRPL dynamics of GaxIn1−xP NWs is most likely controlled by charge trapping. Slowing down of the dominated by trapping PL dynamics can be understood as follows. At low fluence, charge carriers are efficiently captured by empty traps. This leads to a fast PL decay. With increasing fluence some traps remain filled by the photo-generated charge carriers at the arrival of the next excitation pulse. Consequently, the charge trapping rate decreases, and we observe a slower PL decay.25,27 The PL decay is x-dependent: the higher the x the faster the PL decays (Figure 2 (b)). Taking the time at which the emission decays to 1/e of its initial value,28 we can quantify the PL decay time of GaxIn1−xP NWs, as shown in table 1. In general, radiative recombination and thus the PL emission intensity depend on the product of the momentary hole and electron distribution.


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Therefore, both electron and hole trapping may influence PL emission, see the energy diagram in Figure 3. Faster PL decay with increasing x suggests that the carrier trapping rate (trapping of electrons or holes or both) increases with increasing x in GaxIn1−xP NWs. We note that TRPL decay is non-exponential in our case; this non-exponential PL decay may be related to an inhomogeneous distribution of traps and thus of trapping rate in the NWs.20,28,29 Inhomogeneous character of traps can be considered in terms of a conventional Density of States (DOS) concept.30 The traps then would be distributed in space and in energy so that mobile charges at specific positions of NW could encounter traps with different energy levels. This distribution of energy values of traps would result in a different trapping rate of mobile charges at every trap location leading to a non-exponential function of the trapping-dominated TRPL decay. In contrast to PL, TRTS is selectively sensitive to photoconductivity in the NWs and hence to the product of mobility and concentration of mobile photo-generated charges. Consequently, TRTS kinetics can be used to analyse carrier mobility dynamics as well as carrier recombination processes. In planar GaxIn1−xP, significantly higher electron mobility than that of holes has been reported,31−34 thus we presume that photoconductivity is mainly arising from photo-generated electrons. In intrinsic GaxIn1−xP NWs, the photoconductivity (∆σ) dynamics can be described by the following rate equation:

d (∆σ ) N (t ) = eµe × (− AN (t ) 3 − γN (t ) 2 − ) dt τ e − trap

(2),

where N is the concentration of electrons, µe is the electron mobility, and 1/τe-trap is the electron trapping rate.


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Figure 3. Schematic of charge recombination processes in GaxIn1−xP NWs with low and high Ga compositions used in our study. Double curved lines represent photoexcitation and emission processes; Blue curved lines represent carrier trapping-detrapping processes; Dashed-dotted curved lines represent the coupled electron scattering between X and Γ vallys; Dashed curved lines represent charge relaxation processes; “NR” represent non-radiative recombination process; straight lines represent trap states. In the NWs with high Ga compositions, the deep trap (doted lines) density is higher than that in low Ga composition NWs, resulting in faster decay of TRTS at early time.

Figure 2 (c) shows normalized ∆σ decay of the GaxIn1−xP NWs with x = 0.35 and 0.54 measured under the same excitation fluency. Apparently, ∆σ decays non-exponentially and the slope of the TRTS kinetic decreases with time. To assign the ∆σ decay process, we examined TRTS kinetics under varied excitation fluencies, as shown in Figure 2 (d) and Supporting information S7. We clearly see a slowing down of the ∆σ decay with increase of excitation fluency on few tens of ps for x = 0.54 and few hundreds of ps for x = 0.35. This result opposes the speeding up of charge recombination with excitations as expected for bi-molecular and Auger processes and agrees with the conclusion on lack of contributions of bi-molecular and Auger


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recombination in the TRPL kinetics based on their excitation dependences. Thus, similarly to the TRPL dynamics, we attribute this TRTS fluency dependent dynamics to trapping of electrons combined with trap filling. As ∆σ is a product of charge concentration and mobility, another explanation of the observed intensity dependency of TRTS kinetics might be an increase of the charge mobility at higher excitations. This explanation is not appealing though as increased charge concentration at high excitations should lead to more efficient charge-charge scattering and thus to lower mobility.9 Hence, the decay component of TRTS kinetics of GaxIn1−xP NWs has a contribution from electron trapping. Nevertheless, as the TRTS signal is still clearly pronounced at the delay time of ~1 ns, we have to conclude that photo-generated electrons are still highly mobile at this delay time. The observed at all excitations decay of ∆σ can be related to recombination of electrons and holes (the decrease of the photo-generated charge carrier density) and to a mobility retardation due to electron trapping. We have resolved this uncertainty via analysis of kinetics measured by TA, see Figure 4 and supporting information S8. Eventually, TA reports on the return of a system under study to the initial equilibrium state that the system occupied before photo-excitation. Therefore, TA kinetics can reflect the dynamics of electrons, holes or the sum of charge carriers depending on the probe photon energy. Thus in our experiments, the overall charge recombination via both radiative and non-radiative channels has been resolved in TA decay kinetics. As is obvious from the Figure 4, the overall recombination in GaxIn1−xP NWs speeds up with increase of x. We can fit the early part of the ∆σ decay by a mono-exponential decay function following the procedure suggested earlier.9,35,36 In this way, we can estimate the early ∆σ decay as 50 (±5) and 990 (±90) ps for x= 0.54 and 0.35, respectively. For the TA dynamics (Figure 4), the same fit approach results in the early decay time of ~230 (±10) ps for x= 0.54, which is clearly much faster than ~1.67 (±0.15) ns for x= 0.35 in agreement with the obviously different TA dynamics


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on the Figure. Both photoconductivity and the overall concentration of photogenerated charges decay faster at higher Ga composition.

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Ga0.54In0.46P Ga0.35In0.65P 0

100 200 300 400 Delay time (ps)

500

Figure 4. The absorption recovery dynamics (decay of the negative TA) of Ga0.54In0.46P and Ga0.35In0.65P NWs embedded in first contact polymer after photoexcitation at 2.3 eV at a fluence of ~4×1011 photons cm−2 pulse−1. The probe energies of Ga0.54In0.46P and Ga0.35In0.65P NWs are 1.91 eV and 1.66 eV respectively, both close to the bang gap of the materials. The dashed lines are fitting curves based on monoexponential decay functions at early timescales.

In general, TA is not sensitive to the charge carrier mobility. Yet, we can distinguish the decays of the charge mobility and density by comparing the recovery dynamics of the NW absorption (decay of the negative TA) and of the ∆σ decay. In the measured TA kinetics, the slow decay component is very similar to the TRTS decay (see Supporting information Figure S8-2, a and b) while the fast decay is less pronounced. Assuming that TA kinetics reflect only photogenerated charge density we can evaluate the relative electron mobility change via dividing normalized ∆σ by normalized TA. The above assumption is not valid for the first 5 ps as TA


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signal shows some rise there, most probably due to the state filling process, whereas the concentration of photogenerated charges should not rise further after the pump pulse. Apparently, for both x= 0.35 and 0.54 the mobility experienced a few percent decay on ~100 ps time scale (see Supporting information Figure S8-2, c and d). Additionally, for x= 0.54, we observe much more pronounced (~20%) and faster mobility decay on sub-20 ps time scale (see Supporting information Figure S8-2, d). To rationalize the observation of the trapping-related slow decay of electron mobility, we consider DOS of electron traps as associated with trapping and de-trapping processes. The detrapping is naturally thermally activated with the rate dependent on the de-trapping energy barrier height. If we assume that electron traps are shallow, then at high enough temperatures most of the charges will be able to eventually leave the traps and become essentially mobile. Further, the mobile charges will diffuse over the NW and face traps with different trapping and de-trapping rates, as shown in the energy diagram Figure 3. Thus, after some time, charges will be predominantly trapped at the deepest traps and the time intervals between charges being trapped and mobile will be shifted towards the former. Experimentally, this should be observed as the decay of ∆σ, which could be expressed in terms of a decay of the average charge mobility in the equation (2). The earlier mobility decay in the Ga0.54In0.46P NWs does not fit this scenario and could be associated with a nearly permanent electron trapping by some deep traps present at high x. To conclude on the dynamics of the photo-generated holes, we have compared TRPL and TRTS kinetics at similar excitation fluences, as shown in Figure 5. Again, we use a single exponential fit to approximate the initial dynamics. Early TRPL decays with 16 (±2) ps and 81 (±2) ps for x= 0.54 and 0.35, respectively, which is much faster (about 3 and 12 times, respectively) than the early TRTS decay for both x. As noted above, TRTS decay is associated with decrease of concentration and mobility of electrons, therefore the TRPL decay cannot be due to decay of the electron concentration or their trapping thus it has to be induced by hole trapping.


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Furthermore, as no PL has been observed at long times, trapped holes cannot escape from the, most probably, deep traps. Interestingly, we conclude that hole trapping is much faster than electron trapping in GaxIn1−xP NWs. The other conclusion that follows from the comparison of TRPL and TRTS is that only a small part of photo-generated charges recombines radiatively. If opposite would be correct, we have to see a substantial decay of the TRTS signal due to the decrease of the electron concentration contrary to our observations. This conclusion agrees well with the measured absolute PL quantum yield of 10−6.

Figure 5. Normalized TRTS and TRPL kinetics of (a) Ga0.54In0.46P and (b) Ga0.35In0.65P NWs embedded in first contact polymer. Blue lines are fitting curves based on mono-exponential functions.

Many factors, such as semiconductor doping, carrier diffusion coefficient and trap nature and density in NWs may influence carrier trapping. In planar GaxIn1−xP, the electron diffusion coefficient is much higher than that of the hole, which could lead to faster electron trapping. Such an explanation does not fit our observation. Alternatively, the trap density may be much higher for holes than for electrons in GaxIn1−xP NWs. Consequently, the hole trap density should increase with increasing Ga composition. Charge trapping of GaxIn1−xP NWs may be induced by crystallographic defects28 and surface states.37,38 We have examined crystal structures of GaxIn1−xP NWs by HRTEM and found that all


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NWs exhibit similar surface roughness and crystal structure (Supporting information S9). These observations suggest that charge trapping in GaxIn1−xP NWs is not dominated by the crystal polytypism in NWs. However, we cannot completely exclude the influence of bulk defects, in particular, point defects and Ga-composition inhomogeneity along the NWs. Furthermore, the increase of x in GaxIn1−xP may result in formation of deep charge trap states associated with Ga vacancies.39,40 Deep traps should have much higher driving force value for the charge trapping process compared to the relatively shallow InP-type of traps41,42 and thus should lead to faster trapping with increase of Ga composition x. Such deep traps in GaxIn1−xP NWs may be responsible for the hole and earlier electron trapping with increase of x observed in this study. Additionally, according to Shockley–Read–Hall model Ga-related deep traps should result in faster non-radiative recombination with increase of x, in agreement with our experimental results. Further studies will be necessary to prove and quantify the role of shallow and deep traps and the overall effect of the Ga composition in the dynamics of charge trapping and non-radiative recombination. From the photoconductivity measurements, we can evaluate the apparent electron mobility µ via dividing the measured ∆σ by the number of photo-generated charge carriers eNexc:43−45

ξµ =

∆σ 1 ∆E NW = −(n1 + n2 )ε 0 c eN exc eF E NW

(3)

where ξ is a quantum yield of mobile charge generation, µ is carrier mobility respectively, n1 and n2 are refractive indices of the media surrounding the photo-excited NWs, F is the absorbed photon fluence in photons per cm−2,

∆E NW is the transmitted THz electric field change, see ENW

supporting information S10. We assume that at early time scales (t < 3 ps), ξ = 1, as all absorbed photons are converted to mobile charges,46 which have not started to recombine yet according to the TA kinetics. Hence, we can estimate carrier mobility at the fluence of 3.5×1011 photons/cm2 by the equation (3), and obtain 2120 (±40), 1760 (±50) and 490 (±25) cm2/V×s for InP,


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Ga0.35In0.65P and Ga0.54In0.46P NWs, respectively. Further dynamics of electron mobility can be obtained via division of the photoconductivity signal with the TA. We note here that carrier mobility decreases severely with increasing Ga composition in GaxIn1−xP NWs. In a semiconductor, carrier-carrier and carrier-ionized impurity scatterings may limit carrier mobility. However, we would not expect much difference for these scattering mechanisms in the studied intrinsic GaxIn1−xP NWs with different x, as the effective electron/hole masses and the ionization impurities carrier concentrations are not expected to change much with increasing x.47−49 Although surface roughness and stacking faults may also play important roles in carrier scattering9 as they appear not to differ with x, supporting information S9, scattering induced by stacking faults and surface roughness should not be essential in our measurements. The Ga-dependent material constituent variation could cause decrease of electron mobility. In general, two possible scattering mechanisms are originating from the increase in x: alloy scattering and direct-to-indirect band gap transition. In the former model, the decrease in mobility originates from electron scattering by randomly positioned Ga atoms and by Ga clusters due to a segregation of Ga and In in GaxIn1−xP NWs. Both of these types of alloy scattering can significantly influence carrier transport properties in ternary alloys and are expected to increase with x in the range of 0-0.5.11−17 For the direct-to-indirect bandgap transition, electrons in the conduction band will be predominantly located in the Γ-valley for direct bandgap, while for the indirect bandgap, electrons will be in the X-valley. In the intermediate case, which is relevant to our study, electrons will be partially located in both X- and Γ-valleys, see the energy diagram in Figure 3. The difference of scattering rate in each valley will thus induce a change of mobility at the composition corresponding to the transition.22,31 We observe that the experimentally revealed trend of mobility seems to be better represented by a direct-to-indirect bandgap transition model than by alloy scattering (Supporting information S11). This observation is similar to the results reported for bulk GaxIn1−xP.31 However, within this explanation we also notice that the transition


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between direct-to-indirect bandgap in NWs seems to be shifted towards lower Ga composition compared to bulk GaxIn1−xP. The origin of this change is not clear, as multiple mechanisms could lead to this effect. For instance, the specifics of the crystallinity of the NWs could change the nature of its bandgap and thus charge mobility.50 Further studies are needed to conclude on this phenomenon. In conclusion, we have investigated recombination dynamics of photo-generated charges in GaxIn1−xP NWs as a function of Ga composition. The fast decay of the photoluminescence is due to efficient trapping of photo-generated holes with a distinct effect of trap filling at high excitation fluencies. We also recorded a pronounced photoconductivity signal linked to mobile photo-generated electrons with a much longer lifetime than that of photoluminescence. Further, we proposed an electron trapping observed as slowing down of the early photoconductivity decay with excitation fluency. We assessed the overall decay of photo-generated charge concentration dominated by non-radiative recombination by transient absorption spectroscopy. Comparing the decays of photoconductivity and transient absorption, we evaluate the electron mobility dynamics. We considered diffusion of photo-generated electron in the NWs over a spatial and energetic distribution of shallow trap states to rationalize the slower (~100 ps) part of the electron mobility decay. The fast hole- and electron- trapping rates and the non-radiative recombination rate are all strongly dependent on Ga composition in GaxIn1−xP NWs, in agreement with expected formation of deep traps under increased Ga composition. The strong dependence of the early time electron mobility on Ga composition in GaxIn1−xP NWs most probably originates from the direct-toindirect bandgap transition.

ASSOCIATED CONTENT Supporting Information. Experimental details, calculations, and additional figures. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

Author Contributions #

Wei Zhang and Xulu Zeng contributed equally to this work.

ACKNOWLEDGEMENT This work was performed within the NanoLund at Lund University, supported by the Swedish Research Council, Swedish Energy Agency, the People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme (FP7-People-2013-ITN) under REA grant agreement No 608153, PhD4Energy, Knut and Alice Wallenberg Foundation, China Scholarship Council and Crafoord Foundation.

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