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Abstract
In this work, photoluminescence (PL), quantum efficiency and carrier dynamics are investigated in indium arsenide (InAs) nanowires (NWs) with various surface treatments, including a molecular beam epitaxy (MBE)-grown semiconductor shell passivation, sulfur-passivation, alumina (Al2O3) coating by atomic layer deposition (ALD) and polydimethylsiloxane (PDMS) spin-coating. The ALD-dielectric layer-coated InAs core-shell NWs show a maximum 13-fold increase in PL intensity. In contrast to the previous reports, this enhancement is found to be due to increased radiative rate from an enhanced Purcell factor, better thermal conductance and higher carrier injection within the NWs instead of improved surface quality. Numeric simulations confirm the experimentally observed increased radiative rate. Further improvements are suggested with even thicker capped InAs NWs. Carrier lifetime in surface-treated NWs is extended and shows long-term stability, critical for practical devices.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Narrow bandgap [1], high electron mobility [2] and integration with silicon (Si) [3] make indium arsenide (InAs) nanowires (NWs) competitive candidates for next generation infrared optoelectronic and photonic devices [4,5]. However, abundant surface states [6,7] facilitate non-radiative recombination [8] and largely reduce electron mobility [9,10] in NW structures. NWs are also more prone to oxidation due to the large surface-to-volume ratio [11]. An effective surface treatment for InAs NWs is therefore crucial. A widely used treatment is sulfur passivation by ammonium sulfide solution, but unfortunately this approach lacks long-term stability [12,13]. Wider bandgap semiconductor shells can result in photoluminescence (PL) [14,15] and carrier lifetime enhancement [16]; however, they must be dislocation-free, whereas the critical thickness for a modest In0.8Al0.2As shell on an InAs NW is only 2 nm [17] which barely improves the NW cross section and thermal conductance. For a thick dielectric shell, Alexander et al. [18] demonstrated an order of magnitude enhancement in mobility of InAs NWs with a 90-nm-thick aluminum oxide (Al2O3) shell, while Holloway et al. [19] reported a decrease in mobility with atomic layer deposition (ALD) dielectric shell that directly passivated the NWs. Apart from these contradictory results, the effect of the dielectric passivation on optical properties of InAs NWs, for instance light emission intensity, remains an open question. The PL enhancement post thick dielectric coating for other types of NWs was simply claimed to be due to a reduction of surface states [20,21].
We agree that the contribution from reduced surface state density may be significant, yet a dielectric shell increases the effective refractive index and the fractional mode confinement. This leads to an enhanced local electric field and consequently improved light emission through the Purcell effect [22]. However, the increased effective refractive index will cause reduction in light extraction efficiency [23]. Moreover, factors like thermal conductance are not trivial. Heating in NWs caused by the high pump fluence is severe due to the low thermal conductance of the interwire medium, which is vacuum. The presence of a thick dielectric coating can increase the NW cross-section and improve thermal conductivity. This will cause lower local temperature and thus stronger light emission. Therefore, a full understanding of the observed photoluminescence (PL) enhancements from dielectric shells requires disentangling the interplay of these various effects: surface properties, mode confinement, thermal conductivity, and light extraction. Such understanding also paves the way to further improvements.
In this work, we perform a variety of surface treatments, including a molecular beam epitaxy (MBE)-grown wide-band indium aluminum arsenide (In0.8Al0.2As) semiconductor shell, sulfur-passivation, an ALD-dielectric, and a polydimethylsiloxane (PDMS) spin-coating on self-catalyzed InAs NWs. Through ultrafast spectroscopy, reflection/absorption spectroscopy and quantum efficiency measurements, as well as scanning electron microscopy (SEM) and transmission electron microscopy (TEM), we analyze contributions to the observed large increase in PL. Finite-difference time-domain (FDTD) simulations confirm our analysis. Additionally, stability of the optoelectronic performance of the NWs using different surface treatments is investigated with aging tests.
2. Methods
All the NWs were grown on undoped Si(111) substrates prepatterned by selective area epitaxy (SAE). SAE uses a mask to prevent NW nucleation except where desired. This allows control over NW growth kinetics and dimensions by varying the pitch of the nanohole. A pitch of 300 nm nanoholes in a square lattice was chosen to target NW dimensions of 2 µm in length and 100 nm in diameter. A 50-nm SiNx film was sputtered on a Si(111) substrate for the SAE mask. A 200 nm thick electron beam lithography (EBL) resist was spin-coated onto each piece followed by a 10 nm film of aluminum (Al) to prevent charging during exposure. Nanohole array exposure was performed on a Raith Voyager EBL system using a dose of 300 µC-cm-2 over an area of 2.5 mm by 2.5 mm. After EBL exposure, the Al layer was removed with a wet etch, the resist was developed, and the nanohole pattern was transferred into the SiNx with a fluorine-based reactive ion etch (RIE), leaving a few nanometers of SiNx at the bottom of each hole so as to not damage the growth surface with ion bombardment. Each substrate was dipped in 2% hydrofluoric acid (HF) to clear the holes down to the epi-ready Si(111) before growth. More fabrication details can be found in Ref. [24]. A Veeco Gen20 solid-source MBE was utilized to grow self-catalyzed InAs NWs with a V-to-III true flux ratio of 40 for 3.5 hours at 475°C for the InAs core same as previously reported [25]. Twenty percent of the indium flux was replaced with Al for 25 minutes, resulting in a shell thickness of ∼5 nm. InAs/In0.8Al0.2As core-shell NWs were grown on multi-array substrates.
Sulfur-passivation was employed to remove the native oxide for InAs core-only (CO) NWs before dielectric deposition. NWs were immersed in hydrated ammonium sulfide ((NH4)2Sx: H2O = 1:10) solution for 5 min to remove the native oxide with minimum etching of the InAs [26]. The Al2O3 layer was conformally coated on NWs by ALD at 110℃. Layer thickness variation was realized by tuning the number of ALD purge cycles. PDMS was spin-coated on NWs at 4500 rpm for 35 s and then heat-cured at 130℃ for 20 min, resulting in a 3 µm layer. Both ALD-Al2O3 and PDMS were transparent to both the pump and emission light. Nine samples were studied, as summarized in Table 1:
Table 1. A summary of all the nine NW samples investigated in this study.a
3. Results and discussion
3.1. Electron microscopy and elemental mapping analysis
Figure 1 shows the scanning electron microscope (SEM) images for some representative NWs. The CO NWs (Fig. 1 (a)) have diameter (102 ± 6) nm and length (2.1 ± 0.4) µm, and the CS NWs (Fig. 1 (b)) are (126 ± 12) nm thick and (2.1 ± 0.3) µm long. Note that the In0.8Al0.2As shell is around 5 nm. Two dielectric-coated CS NWs are also shown in Fig. 1(c) (Sample B) and (d) (Sample F). Scanning transmission electron microscopy (STEM) energy dispersive spectroscopy (EDS) mapping of the cross section of Sample B is also presented in Fig. 1(e)-(h) for As, In, Al and O elements, respectively. A Zeiss Nvision 40 was employed to slice the NWs to prepare the cross-sectional TEM samples. The sample was transferred to a Si wafer from MBE prepared samples. A protective Pt layer was deposited on the area of interest by electron beam evaporation before the sample was exposed to a Ga ion beam. The prepared sample was loaded into a FEI Talos (S)TEM with a X-FEG (field-emission gun), specialized in high-resolution STEM imaging. It is equipped with a SuperX energy-dispersive spectrometer (EDS), allowing fast and precise EDS mapping. The STEM images show that the capping is moderately uniform on the NWs, and there is a sharp interface between the NWs and the Al2O3 layer.
Fig. 1. SEM images for (a) Sample I: InAs core-only NWs prior to dielectric deposition, (b) Sample A: InAs-InAlAs CS NWs, (c) Sample B: CS NWs with 43 nm ALD-Al2O3 and (d) Sample F: CS NWs with 96 nm Al2O3. (e), (f), (g), (h) respectively illustrate As, In, Al and O distribution in cross sectional Sample B by the STEM-EDS elemental mapping. The bottom left scale bars in (a), (b), (c) and (d) are 500 nm and the bottom right scale bars in (e), (f), (g) and (h) are 50 nm.
3.2. Photoluminescence enhancement measurements
The PL measurement was conducted by pumping the NWs with a 25 kHz sinewave-modulated laser diode with a wavelength of 825 nm and a spot radius of ∼ 500 µm. Light emission at 77 K for the CS NWs pre- and post-ALD are illustrated in Fig. 2. A 2.5-times PL increase was observed in Sample B (Fig. 2(a)) with 43 nm Al2O3; this factor increased to ∼8.5x with 50 nm Al2O3 (Fig. 2(b)) and to 12.7x with 68 nm Al2O3 (Fig. 2(c)) and then decreased to ∼3x with 96 nm Al2O3 (Fig. 2(d)). The enhancement is maximum when the thickness is around 68 nm as the trend in Fig. 2(f) illustrates. We encapsulated another CS NW with a 65-nm Al2O3 layer and then spin-coated a 3-µm transparent PDMS layer to fill the interwire gap (Sample G). A 13.2-fold improvement was obtained (Fig. 2(e)). Note that the PL signal for CO NWs is below our detection limit and the enhancement factor of CO NWs will be discussed later in this report. (The PL spectrum of Sample D and H can be found in Supplement 1 Fig. S1)
Fig. 2. Normalized PL intensity of InAs-InAlAs CS NWs pre and post ALD-Al2O3 with thickness of (a) 43 nm, (b) 50 nm, (c) 68 nm, (d) 96 nm and (e) 65 nm with a following 3 µm spin-coated PDMS layer. (f) PL enhancement factor versus Al2O3 thickness. The black data (0 nm) in (a)-(d) are for Sample A.
The PL enhancement was previously claimed to be solely caused by reduced surface states post coating [20,21]. However, minor surface quality tuning was expected by the subsequent encapsulations with the existence of the InAlAs shell, and the PL increase cannot be attributed to better surface quality alone, contradicting those claims [20,21]. Moreover, a blue-shift was observed in the spectra post encapsulation (Fig. 2(a)-(d)). One explanation is that encapsulation can lower lattice temperature in NWs under high pump fluence. The electron temperature for one representative sample (Sample E) was extracted from the Fermi-tail fits [27,28] to be a remarkably high 344 K before encapsulation and 147 K afterwards (Supplement 1 Fig. S2). Because the electrons thermalize with the lattice on a time scale of picoseconds [29,30], the electron temperature is the same as the lattice temperature under continuous-wave excitation. Therefore, the lattice is cooler with the dielectric encapsulation. The expected blue-shift from the temperature change (344 K to 147 K) can be estimated from the better studied zinc-blende (rather than wurtzite) InAs, where a 0.3 µm blue-shift is predicted in Ref. [31], consistent with the blue-shift we observe. The blueshift can also be induced by compressive strain [15,31]. But there is no sign of significant strain when Al2O3 is below 65 nm while compressive strain contributes to the slightly increased blue-shift in Sample F which has a 96 nm-thick Al2O3 coating (Fig. 2(d)), consistent with the two-times decrease in lifetime post encapsulation in that sample (Fig. S4) which will be discussed later in Section 3.3. The lack of a blueshift in Sample G may be from a redshift caused by tensile strain induced by the PDMS offsetting the blueshift caused by reduced NW temperature. The shrinkage of the PDMS layer as it is cooled would lead to a tensile radial strain.
A broadband light absorption spectrum was measured for the NWs (Supplement 1 Fig. S3(a, c, e); see Refs. [16] and [32] for details on the measurement technique). Light absorption is crucial to determine the carrier injection level in ultrafast measurements, which will be discussed in the next section. Higher light absorption was observed in CS NWs post ALD-dielectric encapsulation in the short wavelength range (<1.6 µm), which matches numeric simulations (Supplement 1 Fig. S3 (b, d, f)). This increase is due to the improved light coupling into the absorptive NW core from the non-absorptive coating [33,34]. The carrier injection level in the coated CS NWs is thus comparatively higher under the same incident light fluence, and more light emission is expected. Yet, the light absorption at the PL pump wavelength (830 nm) increased less than two times, so is only one component of the PL enhancement.
3.3. Lifetime and aging measurements
Ultrafast pump-probe spectroscopy [16,35] was performed to obtain carrier recombination rates and lifetimes with pump and probe wavelength of 1.65 µm and 7.8 µm, respectively. The normalized recombination rate R(ΔN) can be expressed as the ABC model [36]:
in which ${\textrm{A}_{\textrm{SRH}}}{\; },{\; }$B and $\textrm{C}$ are respectively the SRH, radiative and Auger coefficient which are temperature dependent. $\Delta \textrm{N}$ is the optically generated excess carrier density which is linearly dependent on light absorption (Fig. S3) and is determined as described in our previous reports [16,32,35]. From the Fermi-tail fits (Supplement 1 Fig. S2), ${n_0}$, the background carrier density, was extrapolated to be (1.35 ± 0.79)× 1017 cm-3 for Sample A and (1.95 ± 0.51)× 1017 cm-3 for Sample G from the same growth.Figure 3 shows the recombination rates for several representative samples. Figure 3(a) shows the minority carrier (MC) recombination rate R(0) in CO NWs (Sample I) was reduced by two-fold after sulfur-passivation followed by thin ALD-Al2O3. Since R(0) is dominated by SRH, the reduced rate is a result of the diminished surface defects [37]. Comparing R(0) of Sample I (black squares in Fig. 3(a)) with Sample A (black squares in Fig. 3(b)), we observe an approximately 20x reduction in Sample A’s R(0) due to the presence of the 5 nm wide-band semiconductor shell. This 20x reduction will lead to a 20-fold increase in quantum efficiency due to predominant SRH recombination (Eq. (2)) with $\textrm{R}({\Delta N} )\approx {\; }{A_{SRH}}$ when the excess carrier density is less than 1016 cm-3 and consequently gives rise to at least one order of magnitude PL increase, which is consistent with previous literature [14,15].
Fig. 3. Recombination rate R versus carrier density ΔN for (a) Sample I (CO) pre and post surface passivation, (b) Sample A (CS), (c) Sample G (CS + 65 nm ALD-Al2O3 +3 µm PDMS) before and after 2 hours in a humidity chamber. (d) Minority carrier lifetime for Samples A and G versus time in the humidity chamber. The solid lines in (a), (b), (c) are the fits for the corresponding sample using Eq. (1). The dashed lines in (d) are the exponential decay with offset fits for the carrier lifetimes.
R(0) for Samples A and G (0 h in Fig. 3(b, c)) are comparable, indicating minor surface quality tuning from the Al2O3 and PDMS coating and no dislocation formation. Because the semiconductor shell already provides a diffusion barrier and surface passivation, the ∼13x enhancement (Fig. 2(e)) is not from better surface quality (analyzed further in the next section). Thicker Al2O3 (96 nm) introduces dislocations to NWs through strain relaxation and causes a two-fold reduction in minority carrier lifetime (τMC) (Supplement 1 Fig. S4).
Aging tests were also conducted by leaving both Sample A and G in a heated humidity chamber (∼45˚C, relative humidity: 35%) for hours (Fig. 3(d)). Without the dielectric encapsulant, τMC for Sample A was observed to degrade three-times after 2 hours. Yet τMC for Sample G only dropped by 7.1%. The long term carrier lifetime for the former is 224 ps and for the latter is 679 ps. This comparative low aging rate and high long-term carrier lifetime shows that the encapsulation effectively stabilizes the NW optical properties, consistent with previous reports [38].
3.4. Quantifying enhancements: Purcell factor, light extraction, and thermal management
The radiative coefficient is expected to be strongly influenced by the surrounding optical medium due to the Purcell effect [22]. The Purcell effect states that the radiation rate is proportional to the local density of states (LDOS), which is related to photonic density of states (DOS) around the emitting frequency and the local electric field of each mode. Correspondingly, coating the NW has two positive effects here. First, the DOS increases approximately as n3, where n is the refractive index. Since the NWs are much smaller than a single mode volume, the emitter mostly sees air, giving a lower Purcell factor compared to a bulk material. Replacing the interwire vacuum (n=1) by Al2O3 (n∼1.7) and PDMS (n∼1.4) increases the DOS and Purcell factor. Second, the electromagnetic fields of each mode distribute mainly outside the thin NWs. The coating serves as an anti-reflection layer between air and InAs, which improves the coupling between the NW and air and enhances the local electric fields in the NWs, also improving the LDOS and Purcell factor.
The Purcell factor and the light extraction coefficient are calculated with Meep, an open-source electromagnetics simulation software package via the FDTD method [39]. It can be proven that the spontaneous emission rate at a point in an optical structure is proportional to the power emitted by a Hertzian dipole at the same location. To get the Purcell factor, a dipole oscillating perpendicular to the NW axis [40] is placed at the center of a NW array with infinite length (inset of Fig. 4(a)). Absorbing boundary conditions (yellow region) are used around the simulation area to mimic an open space. The electric and magnetic fields were calculated after the simulation. Then the power outflow through a surface is calculated by integrating the normal component of the Poynting vector along the surfaces. The total power generated is the summation of the power outflow through six surfaces enclosing the dipole. The Purcell factor is the ratio between the power generated by a dipole in a NW array and the power by a dipole in a homogeneous InAs material. (More information about Purcell simulation can be found in Supplement 1 S5)
Fig. 4. (a) the Purcell factor, (b) the light extraction efficiency versus coating thickness for NW array from numeric simulations with or without PDMS filled in the interwire gap; IQE results for (c) uncoated Sample A (CS) and (d) Sample G with 65 nm ALD-Al2O3 + 3 µm spin-coated PDMS layer with modulation duty cycles from 1% to 40% at 77 K, (e) the extrapolated B coefficient for Sample A (gray dots) and Sample G (black dots) with no heating. The solid lines in (e) are the fits using Eq. (2). (f) Purcell factor versus InAs core thickness for a single infinite long NW with (60 nm) or without (0 nm) ALD- Al2O3. Note that the interwire medium is either air or PDMS in (f). The insets in (a) and (b) show the simulation geometry; the red arrows are the dipole and blue lines are the integration surfaces.
The numeric simulation predicted a sixfold Purcell factor increase with a 60-nm Al2O3 coating and PDMS in the interwire space (Fig. 4(a)). To compare this prediction of Purcell factor enhancement to experiment, we measure the internal quantum efficiency of the NWs without heating, which is determined by the external quantum efficiency and the light extraction efficiency. Prior to discussing measurement of the Purcell factor, we must first determine NW light extraction efficiency, and discuss thermal management.
To get the light extraction efficiency, an air/NW interface is introduced (see the inset in Fig. 4(b)). The extracted power is the power outflow through a surface close to the interface. The light extraction coefficient is the ratio between the extracted power and the total power generated by the dipole. To avoid unnecessary complexity, we do not include the substrate in the simulations since the reflected power from the substrate is smaller than 10% of the total power.
Numerical simulations give the extraction efficiency ${\eta _{ext}}$ of 0.31 and 0.19 for Samples A and G (Fig. 4(b)), respectively, much higher than light extraction efficiency in multilayer structures (∼1/4n2 ∼ 0.02). A simple explanation is the electromagnetic field in a multilayer structure can be trapped in lateral directions forever, which is impossible in a NW. The lower ${\eta _{ext}}$ in Sample G is caused by the higher effective refractive index due to the additional Al2O3 (n∼1.7) and PDMS (n∼1.4) coating compared with Sample A which has no coating (n = 1), and leads to a ∼1.6 reduction of PL.
The external quantum efficiency (EQE) of NWs can be reduced by heating. To find conditions under which NW heating is eliminated, the NWs were measured under variable pump duty cycle. The EQE was measured by counting the photons emitted by the sample per optically injected electron-hole pair using a method described previously [16,32]. The internal quantum efficiency (IQE) can then be determined as: IQE = EQE/ηext and is demonstrated in Fig. 4(c, d) as a function of carrier density. The pump laser was modulated by a square-wave with a repetition rate of 100 Hz and various duty cycles to gauge heating in NWs during the pulses. The IQE dropped by 66.1% for Sample A (Fig. 4(c), CS) while by only 27.9% for Sample G (Fig. 4(d), CS + Al2O3+PDMS) at $\Delta \textrm{N}$ of ∼ $9 \times {10^{15}}\; c{m^{ - 3}}$ from 1% to 40% duty cycles. This indicates that heating is less severe with the coatings, leading to lower local temperature and hence more emission. Since the pump laser used in the PL measurement was modulated by a sinewave (effectively 50% duty cycle) giving the same maximum injection, the heat management has at least a two-fold contribution to the 13.2-times PL improvement.
We are now able to quantify the experimentally observed Purcell factor enhancement. Having separately measured the IQE without heating and the total recombination rate $\textrm{R}({\Delta N} )$ (Figs. 3 and 4), we can calculate the radiative rate by the definition of the IQE:
By applying Eq. (2) below 1015 cm-3 with low duty cycles where heating is removed (Fig. 4(e)), the radiative coefficients were determined to be (1.57 ± 0.01) ×10−10 and (5.07 ± 0.04) ×10−10 cm3s-1 for Samples A and G, respectively. Thus, we observed a 3.2-fold Purcell factor increase in radiative rate post coating, close to the sixfold enhancement predicted by the simulations.
Moreover, from Fig. 4(f), the 60 nm dielectric coating + PDMS can provide an even larger Purcell factor enhancement in thin NWs. When the radius is 20 nm, this enhancement is as high as 11. The Purcell factor is much larger for thick bare NWs. NWs with 350 nm radius have a Purcell factor peak of ∼1.6 without additional coating, even higher than bulk material, and ∼190 times that of the uncoated NW investigated in this paper (blue dot in Fig. 4(e) with 0 nm coating, Purcell factor = 0.0083). Surface recombination and thermal management are also improved with thicker NWs. Considering the predominant surface recombination is inversely related to the NW diameter, a 5.6-fold reduction in SRH is expected compared with ∼63 nm NW radius in this work. The ${\eta _{ext}}$ can remain unchanged by increasing the NW pitch; along with the one order of magnitude enhancement from the semiconductor shell, a very large PL increase can potentially be obtained, enabling high performance InAs NWs based optoelectronics.
4. Conclusions
In conclusion, an effective surface encapsulation is demonstrated for InAs NWs that may enable high optoelectronic performance, stable, light-emitting devices. We demonstrated a 13-fold improvement in the PL of CS NWs encapsulated by 65-nm ALD-Al2O3 and 3-µm spin-coated PDMS layer compared to CS NWs with no encapsulation. In contrast to previous literature, the comparable carrier lifetimes in CS NWs before and post dielectric deposition demonstrate no further reduction in surface recombination. Numerical simulations confirm an increased Purcell factor which successfully explains the encapsulation-induced radiative improvement. Moreover, the thinner the NW is, the more Purcell factor enhancement can be increased by the coating. The lattice is also found to be much cooler post encapsulation under high excitation due to increased thermal conductivity. Additionally, enhanced pump absorption is observed in the coated CS NWs. Consequently, rather than simply diminished surface defects, the 13-fold enhancement is a result of an enhanced radiative rate, higher thermal conductivity, and higher excitation level. Simulations also indicate that more orders of magnitude increase can be obtained in thicker CS NWs. Finally, the carrier lifetime in the encapsulated NWs remains high even after fifteen days of exposure to hot humid air, demonstrating long-term stability of the NWs, and making stable optoelectronic devices possible.
Funding
Basic Energy Sciences (DE-AC02-06CH11357); Office of Science (DE-AC02-06CH11357); U.S. Department of Energy (DE-AC02-06CH11357); National Science Foundation (EPMD-1608714).
Acknowledgements
The authors gratefully acknowledge the financial support by National Science Foundation through grant EPMD-1608714. Use of the Center for Nanoscale Materials, an Office of Science user facility, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Supplemental document
See Supplement 1 for supporting content.
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