InAlAs

Letter pubs.acs.org/NanoLett

Cite This: Nano Lett. 2019, 19, 4272−4278

Carrier Recombination in the Base, Interior, and Surface of InAs/ InAlAs Core−Shell Nanowires Grown on Silicon Kailing Zhang,†,‡ Xinxin Li,†,‡ Weitao Dai,†,‡ Fatima Toor,†,‡,§ and J. P. Prineas*,†,‡,§ †

Department of Physics and Astronomy, ‡Iowa CREATES, and §Department of Electrical and Computer Engineering, University of Iowa, Iowa City, Iowa 52242, United States

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S Supporting Information *

ABSTRACT: We report on carrier recombination within self-catalyzed InAs/ InAlAs core−shell nanowires (NWs), disentangling recombination rates at the ends, sidewalls, and interior of the NWs. Ultrafast optical pump−probe spectroscopy measurements were performed from 77293 K on the freestanding, variable-sized NWs grown on lattice-mismatched Si(111) substrates, independently varying NW length and diameter. We found NW carrier recombination in the interior is nontrivial compared to the surface recombination, especially at 293 K. Surface recombination is dominated by carrier recombination at the NW sidewall, while contributions from the highly strained, impure NW base are negligible.

KEYWORDS: minority carrier lifetime, surface recombination velocity, interior recombination rate, temperature dependence, III−V nanowire on silicon, pump−probe spectroscopy, molecular beam epitaxy

R

(MBE).21 In epitaxial growth, residual impurities remain at a substrate−epi interface after deoxidation, contaminating, and degrading initial epitaxial material. They are typically buried with a thick buffer layer and below a diffusion barrier.24 It is likely such contamination also occurs in the base layers of the NW. In the NW interior, irregularities such as short segment wurtzite/zincblende stacking faults, twin boundaries, and polytypism are commonly observed.25,26 These planar defects have been reported in vapor−liquid−solid (VLS) NWs,27,28 as well as in noncatalytic, self-induced NWs.29 NW surface states can originate from impurities such as oxygen adatoms adsorbed onto the surface of the NWs30 or native defects such as dangling bonds, vacancies, and antisites, which, in the case of InAs NWs, can cause surface Fermi level pinning.31,32 Most NW-based devices require long carrier lifetime and high carrier mobility, which are limited by these defect states. To enhance the device performance, it is crucial to study the origin, specifically, how defect states from NW sidewalls, interior, and end facets interact and compete as charge carrier nonradiative recombination drains. The III−V NW lifetime has been widely investigated previously. Boland et al. reported exceptionally long room temperature (RT) photoconductivity and photoluminescence lifetimes of 3.92 and 2.39 ns, respectively, in n-type modulation-doped GaAs/AlGaAs core−shell nanowires. 33 In ref 34, by increasing Sb

ecently, InAs nanowires (NWs) have sparked considerable interest1 in the research community due to their attractive properties such as narrow bandgap,2 high electron mobility,3 quantum confinement effects,4,5 and strong spin− orbit interaction.6,7 Such properties make them promising building blocks for photodetectors,8 transistors,9−12 single electron charge detectors,13 spintronics,6,14 and topological quantum devices.15 It is highly desirable to monolithically integrate InAs NWs onto silicon, as this combines the specified properties of InAs NWs with the mature, commercially dominant silicon CMOS technology. NW structures are generally plagued by a high density of defect states, which act as Shockley−Read−Hall (SRH) recombination centers and carrier traps, reducing NW photoluminescence,16 carrier lifetime,17 and carrier transport.18,19 Defect states can occur in any part of the NW base, sidewall, interiordue to different mechanisms. The base of NWs may contribute to nonradiative carrier decay and scattering through dislocations from lattice mismatch, or through residual impurities at the desorbed substrate interface. In the case of NW−substrate heteroepitaxy, the base of the NWs is strained due to lattice mismatch. Taraci et al.20 reported that for NWs with small diameters, the strain at the base of the NWs decreases along the NW axis on the order of a NW diameter. Beyond the critical diameters,21−23 strain relaxation takes place through the formation of misfit dislocations on the heterointerface. Consequently, NWs can be kinked, nonvertically aligned, or fail to form, as demonstrated in Au-catalyzed metal organic chemical vapor deposition (MOCVD) 22 and molecular beam epitaxy © 2019 American Chemical Society

Received: February 4, 2019 Revised: May 23, 2019 Published: June 17, 2019 4272

DOI: 10.1021/acs.nanolett.9b00517 Nano Lett. 2019, 19, 4272−4278

Letter

Nano Letters

between samples, all NWs were grown at 480 °C. InAs NW cores have an In0.8Al0.2As shell grown under identical conditions. Because of the high migration activation energy and short diffusion length of Al adatoms,44 the incorporation of Al in the NW epilayer leads to the natural formation of a uniform shell. Shell thickness is 5 ± 1 nm on the sidewall, and top segment length is 328 ± 26 nm, as measured by comparing scanning electron microscope (SEM) images of core−shell NWs to control core-only NWs. In the diameter series, four samples have similar length of 1.2−1.6 μm, and a fifth sample was grown with a slightly longer length of 2.2 μm. The detailed dimensions of each NW sample are presented in Table S1 (SI section 1). Representative SEM images of two NW samples with different diameters are shown in Figure 1. The figure reveals that the self-catalyzed NWs were nontapered, kink-free, and oriented vertically to Si(111) substrates, showing a good InAs/Si epitaxial relation.

concentration to 35%, the RT photoconductivity lifetime of InAs0.65Sb0.35 NWs was measured to be 486 ps, over a factor of 3 higher than that of the InAs NWs (130 ps). Li et al.35 disentangled SRH, radiative, Auger carrier recombination rates from 77293 K in InAs/InAlAs core−shell NWs grown on Si(111). This work found that carrier lifetime in the (wurtzite) InAs-based NWs was limited by Auger nonradiative recombination at higher carrier densities (>1018 cm−3), which were found to be about 1 order of magnitude lower than comparable (zincblende) planar materials. However, lifetime at lower carrier densities was found to be limited by SRH, with SRH lifetimes of 700 ps/150 ps at 77 K/293 K, respectively. The NW surface recombination velocity is typically extracted from NW carrier lifetime. While it has been investigated on a range of NW materials,36−38 it remains relatively underexplored for InAs NWs. Joyce et al.39 conducted ultrafast pump−probe measurements on a series of variable diameter MOCVD, Aucatalyzed InAs NWs grown on lattice-matched InAs(111) substrates, and reported a RT surface recombination velocity of 3000 cm/s, as well as a carrier mobility of 6000 cm2/V·s. However, these previous studies do not comprehensively separate out SRH (defect assisted) recombination rates from all the different regions of the NWsidewalls, interior, ends. In this Letter, we present the first study on the contributions from the sidewalls, interior, and end facets to the total carrier recombination in the self-catalyzed InAs/InAlAs NWs grown heteroepitaxially on Si(111) substrates by MBE. Foreign catalysts such as Au that can introduce deep level traps in the Si bandgap are eliminated, which is crucial for the direct integration of InAs NW devices into Si CMOS technology. Using temperature-dependent ultrafast pump−probe spectroscopy, minority carrier lifetimes were obtained in two series of NWs with independently varied length and diameter. From minority carrier lifetimes, which were measured to be primarily defect-assisted SRH lifetimes, parameters such as interior recombination rates, end, and sidewall recombination velocities were extracted from 77293 K. Competing nonradiative charge carrier recombination pathways in different parts of the NWs are resolved and compared. This resolution of pathways helps determine means for optimizing InAs NW optical quality, which is important for the potential realization of highefficiency InAs NW-based optoelectronics and photonics. InAs/InAlAs NWs were grown on undoped epi-ready Si(111) substrates by a GEN20 solid-source MBE, in a selfcatalyzed manner.40,41 This approach nucleates NWs at pinholes in a porous SiO2 layer on Si, the advantage being that no nanohole arrays need to be fabricated in a SiO2 layer as in selective area epitaxy; the nanohole array patterning can be laborious. Si substrates were simply etched in 2% hydrofluoric (HF) acid and pretreated with 30% hydrogen peroxide (H2O2) for oxide regrowth. Two separate series were grown: a length (l) series consisting of four NWs with ∼62 nm diameter, and lengths varying from 1.2 to 2.5 μm; and a diameter (d) series consisting of five samples with diameters varying from 40 to 129 nm. Length variations were realized by solely changing the InAs NW growth time, while the variable diameter NWs were realized two ways: (1) changing the thickness of the regrown SiO2 on Si(111)42,43 and (2) changing both the true V/III ratio and growth time. These methods were adopted instead of varying the growth temperature since temperature variation was found to have a pronounced influence over the NW zincblende/wurtzite crystal phase stacking sequence.29 To keep the NW overall crystal structures relatively uniform

Figure 1. Representative SEMs of InAs/InAlAs core−shell NWs from the variable diameter series (a) d = 40 nm, l = 1.6 μm. (b) d = 130 nm, l = 2.3 μm.

Utilizing time-resolved differential transmission optical pump−probe,45,46 the recombination rates and minority carrier lifetimes were measured in the InAs/InAlAs NW samples from 77 to 293 K. This contactless, nondestructive technique facilitates optical measurement on the as-grown, vertically aligned NWs. The temporally synchronized pump and probe beams are produced by two phase locked, tunable optical parametric amplifiers (OPAs), which are each pumped by the same kHz, amplified Ti:sapphire laser. Excess carriers in NWs were optically excited across the band gap by the pump beam (150 fs pulse, 1 kHz repetition rate) with photon energy of 953 meV (wavelength 1.3 μm), well above the conduction band edge of NWs, which is measured to be 461 meV (wavelength 2.7 μm) at 77 K. This population of excess carriers is probed by the sub-bandgap 149 meV (wavelength 8.3 μm) beam (150 fs pulse, 1 kHz repetition rate), which is absorbed by the NW free carriers. The change in free carrier absorption of the probe is dependent on the pump-induced excess carrier density. With a motorized optical delay stage in the pump beam path, the probe differential transmission ΔT/T through NWs with and without pump present was measured as a function of time delay. Pump and probe were spatially overlapped onto the samples that were mounted on a helium (He)-cooled cryostat, which enabled temperature-dependent measurements. Note that the pump beam spot radius (∼1000 μm) is three times larger than that of the probe beam (∼300 μm), so the spatial distribution of charge carriers in the probed area is relatively uniform. The pump and probe beam were incident on the as4273

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absorption measurement), repetition rate (1 kHz), photon energy, and spot radius (1000 μm), respectively. l and f are the length and fill factor (∼4.0%) of NWs, respectively. By varying the incident pump power, the dependence of ΔT/T at zero delay on the initial excess carrier density was detected and shown as the response curve in Figure 2b. The normalized recombination rate R(ΔN) was then obtained from the equation:

grown, free-standing NWs at an angle of approximately 15° to normal. Figure 2a shows a representative 77 K ΔT/T decay signal of a InAs/InAlAs core−shell NW sample (d = 130 nm, l = 2.3

( ΔT )

1 ∂ΔN 1 ∂ΔN ∂ T R(ΔN ) = − =− ΔN ∂t ΔN ∂ ΔT ∂t T

( )

(2)

where ∂ΔN/∂(ΔT/T) was extracted from the response curve (Figure 2b) and ∂(ΔT/T)/∂t from the decay curve (Figure 2a). Note the slight nonlinearity in the response curve is caused by free carrier absorption saturation at high carrier density. A hyperbolic fit was adopted to fit the data in Figure 2b. The excess carrier density-dependent recombination rate R(ΔN) of the NW sample is depicted in Figure 2c. The recombination of nonequilibrium carriers is primarily determined by three processes: SRH, radiative, and Auger recombination. For a bulk semiconductor structure, SRH recombination is mostly attributed to deep-level trap states in the bulk. However, for nanoscale devices, due to their high surface-to-volume ratio, the surface SRH recombination, facilitated by the large number of recombination centers on the surface becomes non-negligible. In an n-type NW sample, as is the case for unintentionally doped InAs, the recombination rate R(ΔN) can be represented by the equation R(ΔN ) = ASRH + B(ΔN + n0) + C(ΔN + n0)2

(3)

in which ASRH is the SRH coefficient, which includes contributions from the NW interior, surface, and ends, B is the radiative coefficient, C is the Auger coefficient, and n0 is the background carrier density. In the limit of ΔN = 0, R becomes the minority carrier recombination rate RMC RMC =

1 τMC

= ASRH + Bn0 + Cn0 2

(4)

where τMC is the minority carrier lifetime and ASRH, Bn0, and Cn02 are the SRH, radiative, and Auger recombination rates, respectively. First, we investigated the separate contributions from SRH, radiative, and Auger recombination to the total recombination rate in the NWs. The B coefficient of a representative NW sample with the largest diameter (d = 130 nm, l = 2.3 μm) in the series was independently resolved by an external quantum efficiency (EQE) measurement, and its background carrier density n0 was determined from a Fermi tail fit to the photoluminescence spectrum, as described in detail in SI sections 4 and 5. From the EQE measurement, the B coefficient was extracted as 2.18 × 10−10 cm3/s at 77 K. Substituting B and n0 back in to eq 3, ASRH and C coefficients were extracted by fitting the recombination rate data in Figure 2c with eq 4 (3.76 × 109/s and 1.22 × 10−27 cm6/s at 77 K, respectively). With ASRH, B, and C individually resolved, comparison of terms in eq 4 (or inspection of Figure 2c) shows that the NW minority carrier lifetime is dominated by the nonradiative SRH recombination, and 1/τMC may be interpreted primarily as ASRH. This interpretation turns out to be true for all the NW samples presented here at all investigated temperatures (see SI section 5). Auger recombi-

Figure 2. Representative graphs of (a) decay curve with initial excess carrier density of 5.8 × 1018 cm−3, (b) response curve, (c) excess carrier density-dependent recombination rate of the representative InAs/InAlAs NWs (d = 130 nm, l = 2.3 μm) at 77 K. The data points in panel (b) are extracted from the decay curves at zero delay with variable initial excess carrier densities. The red line in panel (b) is the best fit of the data, which goes through the origin (0, 0). The gray dots in panel (c) are measured recombination rates facilitated by eq 2, and the red line is the fit of the data by applying eq 3. The cyan dash, magenta dash-dot-dot, and purple dash lines in panel (c) represent SRH, radiative, and Auger recombination rates, respectively.

μm), with initial excess carrier density ΔN of 5.8 × 1018 cm−3. It can be seen that the ΔT/T decay curve is a multiexponential decay as opposed to a monoexponential decay, distinctly showing the carrier recombination rate dependency on the excess carrier density. ΔN can be calculated by PA ΔN = hc 2 R r λ πr lf (1) where P, A, Rr, hc/λ, and r are parameters associated with incident pump beam: average power, NW absorption at pump wavelength λ (1.3 μm) (see SI section 2 for details of the 4274

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the variations in the NW dimensions (SI section 1) and the small signal-to-noise ratio in the ultrafast pump−probe measurement as a result of the low NW filling factor (∼4%). We first look at the τMC of the NW length series, namely, four samples with a fixed NW diameter of ∼62 nm and lengths of 1.2−2.5 μm. The sole variation in the NW length allows for the investigation of carrier recombination at the NW tip and NWsubstrate heterointerface. Because of the relatively low NW absorption (∼40%) and high undoped-Si substrate reflection (∼50%) at 1.3 μm (measurements described in SI section 2), NWs are excited relatively uniformly along the length, so the entire length of the NW contributes to the optical response. To confirm, we simulated the electric field distribution of the pump within NW samples using COMSOL Multiphysics software (SI section 3) based on the finite-element method. The simulation and analysis showed that the initial carrier recombination in the NW is indeed relatively homogeneous along the axial direction at pump wavelength 1.3 μm. As illustrated in Figure 3a, τMC was observed to be relatively independent of the length of NWs, within uncertainty, at all temperatures. The average minority carrier lifetime of the length series is 184 ± 40, 150 ± 35, and 60 ± 15 ps at 77, 185, and 293 K, respectively. The length independence of τMC shows the material quality at the NW end facets does not make significant contribution to the minority carrier recombination. The NW−substrate interface could potentially contribute to reducing the carrier lifetime through misfit strain and impurities, as described earlier. However, despite a lattice mismatch as high as 11.6% between InAs and Si, and impurities in the initial segment, no degradation in material optical quality is evident. In contrast to the variable length NWs, the minority carrier lifetime, τMC, showed a substantial dependence on the NW diameter. This series consists of five NW samples with diameters ranging from 40 to 129 nm. Four of the samples have similar length 1.2−1.6 μm, while one has a length of 2.2 μm. However, based on the conclusions from the NW length series, the extra length of the fifth sample is not expected to affect the carrier lifetime. The dependence of 1/τMC on NW diameter is depicted in Figure 3b−d. 1/τMC depends linearly on the interior minority carrier recombination rate (Rinterior) and surface recombination velocity (S), and inversely with the NW diameter (d)47

nation dominates at high carrier densities, and the 2CnΔN term in eq 3 is negligible at the investigated carrier density ranges. Therefore, eq 3 can be written as R(ΔN ) ≈

1 τMC

+ BΔN + C ΔN 2

(5)

τMC can be extrapolated from a fit to the measured recombination rate versus carrier density data. This procedure was used to obtain τMC for the rest of the NW samples. The 77, 185, and 293 K minority carrier lifetimes τMC of the InAs/InAlAs NW length series are presented in Figure 3a, and the τMC of the diameter series, plotted as the reciprocal (RMC = 1/τMC) are shown in Figure 3b−d. Note that the error bars on the data are relatively big, especially at 293 K. This is caused by

1 τMC

=

4S + R interior d

(6)

1/ τMC versus 1/ d in Figure 3b−d was fitted with eq 6. S was obtained from the slope, and Rinterior from the intercept. Results are summarized in Table 1. The NW interior minority carrier lifetimes (R−1 interior), 559 ± 167, 426 ± 149, and 89 ± 19 ps at 77, 185, and 293 K, Table 1. Surface Recombination Velocities (S) at NW Interfaces and Interior Recombination Rates (Rinterior) Extracted at Three Different Temperatures from InAs/ InAlAs NW Diameter Series in Figure 3b−da Figure 3. (a) Minority carrier lifetime τMC at 77, 185, and 293 K for NWs with different lengths; reciprocal of τMC at (b) 77 K, (c) 185 K, and (d) 293 K for NWs with variable diameters. A pronounced dependence of 1/τMC on NW diameters was detected as shown in panels (b−d). The solid lines in (b−d) are the best fit of the data using eq 6.

S (cm·s−1) Rinterior (ns−1)

77 K

185 K

293 K

5388 ± 1596 1.79 ± 0.76

8615 ± 2500 2.35 ± 1.27

8250 ± 5145 11.2 ± 3.1

a

S shows a weak dependence on temperature, while Rinterior shows a more marked dependence on temperature. 4275

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diameter of 70.1 and 17.1 nm, for InAs/InAlAs NWs at 77 and 293 K, respectively. In conclusion, temperature-dependent ultrafast pump− probe measurements were performed on variable geometry InAs/InAlAs NWs grown self-catalyzed on silicon. Carrier recombination within the NW is individually resolved sidewalls, ends, and interiorand contributions separately investigated. Important parameters such as surface and interior recombination rates were extracted. Minority carrier lifetime, dominated by SRH recombination, was found to have a marked dependence on NW diameter, while varying little with NW length. Results show carrier recombination mainly originates from the NW sidewalls and interior, and not from NW ends, indicating optical material quality is not degraded in spite of high strain at the InAs NW−Si heterointerface and potential material contamination in the first few hundred nanometers. Finally, we found that the interior recombination is nontrivial compared to surface recombination. This is evidenced by a shift from primarily surface to interior recombination beyond a NW diameter of 130 nm at 77 K and the dominance of interior recombination at 293 K for NWs of all investigated diameters. Together these results show that achieving material quality in (self-catalyzed) InAs NWs comparable to that of equivalent planar materials will require more than passivation and encapsulation; and that NWs do not need a buffer layer in the way that planar structures do. This work provides insight into the improvement of self-catalyzed InAs NW optical quality, important for the development of mid-infrared InAs-based NW devices monolithically integrated onto silicon.

respectively, are at least 3 orders of magnitude shorter than the minority carrier lifetime of those reported in planar InAs materials,48,49 which are higher than 200 ns at 77 K. This could be attributed to the higher density of polytypic stacking defects, usually found in self-nucleated InAs NWs as a result of the foreign metal catalyst elimination.50,51 In the case of selfcatalyzed InAs NWs, Sb incorporation, even as low as 3.9%, has been shown to transform the InAs NW crystal phase from predominantly wurtzite to predominantly zincblende.51 Boland et al.34 observed an enhancement in the InAs NW photoconductivity lifetime with increasing Sb content, which is associated with the substantial reduction of NW structural defect densities due to Sb alloying. We propose that with higher crystal phase purity the NW interior minority carrier lifetime could be prolonged. The surface recombination values for the InAs NWs, summarized in Table 1, are comparable to findings of the surface recombination rate between epitaxial bulk InAs and air,52 which suggests a lower shell quality. The S at 293 K in Table 1 is also higher than that reported by Joyce et al. (3000 cm/s)39 on MOCVD-grown InAs NWs. One possibility for the high surface recombination velocity is that misfit dislocations may occur on the NW core−shell interface due to the 1.3% lattice mismatch between the InAs and In0.8Al0.2As. Another possibility is that the shell of the NW, which contains Al, oxidizes, so the NWs might benefit from a thicker shell or encapsulation. A thicker shell could reduce the probability of carriers from the NW core tunneling through the InAlAs barrier,53 and subsequently recombining with surface traps in the outer oxidized layers. Enhanced photoconductivity lifetime and electron mobility in GaAs/GaAlAs core−shell NWs have been observed due to increased shell thickness.54 An encapsulation of the InAs/InAlAs NWs could prevent the Al from oxidizing and stabilize the NWs. Rinterior may be compared to the surface recombination rate 4S Rs, obtained from R s = d , to see which dominates. At 77 K, the thinnest NWs (d = 41 nm) have an Rs of 5.31 ns−1, three times larger than Rinterior, while the thickest NWs (d = 130 nm) have an Rs of 1.66 ns−1, comparable to Rinterior. This suggests that the primary contribution to carrier recombination in these InAs-based NWs at 77 K shifts from surface to interior when NW diameter exceeds 130 nm. As temperature rises from 77 to 293 K, Rinterior increases by an order of magnitude, while S increases only slightly, by a factor of ∼1.5, suggesting increasing importance of interior recombination with temperature. At room temperature, surface recombination rates are 8.13 ns−1 and 2.54 ns−1 for NWs with d = 41 and 130 nm, respectively, both smaller than Rinterior, indicating that the NW minority carrier lifetime is essentially limited by recombination in the interior rather than surface recombination at 293 K. While at both temperatures, the NWs display a typical surfacemediated recombination behavior, i.e., Rs proportional to the NW surface-to-volume ratio, Rinterior dominates at 293 K. This finding highlights the challenge in fabrication of highly functional room temperature InAs NW-based devices. Our results show that contrary to previous studies,36,38,55 where only the effect of surface states are considered in analyzing NW carrier recombination lifetime (i.e., τ = 4S/d), Rinterior is nontrivial in our InAs/InAlAs core−shell NWs and could not be omitted in eq 6. We define the NW critical diameter dcrit to be the diameter at which total carrier lifetime is 1/e that of the interior minority carrier lifetime (R−1 interior) and estimate a critical

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.9b00517.

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NW geometrical, electrical, and optical data, including NW dimensions for all samples; measurement of NW background carrier density; measurement of NW light absorption; COMSOL simulation of NW absorption density distribution; and determination of SRH dominance in minority carrier lifetime in all samples (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

J. P. Prineas: 0000-0002-3552-1987 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support by National Science Foundation through grant EPM-1608714.

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REFERENCES

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Letter

Nano Letters

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