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Abstract
Catalyst-free, position-controlled indium arsenide (InAs) nanowires (NWs) of variable diameters were grown on Si (111) by selective-area epitaxy (SAE). Ultrafast pump-probe spectroscopy was conducted, from which carrier recombination mechanisms on the NW surface and interior were resolved and characterized. NWs grown using SAE demonstrated high optical quality, showing minority carrier lifetimes more than two-fold longer than that of the randomly-positioned (RP) NWs. The extracted SAE-InAs NW interior recombination lifetime was found to be as long as 7.2 ns, 13X longer than previous measurements on RP-NWs; and the surface recombination velocity 4154 cm · s- 1. Transmission electron microscopy revealed a high density of stacking defects within the NWs, suggesting that interior recombination lifetime can be further increased by improving NW interior crystalline quality.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Semiconductor nanowires (NWs) have garnered considerable attention due to their applications in the next-generation nanoscale electronics and optoelectronics [1]. Future integrated NW devices require more specific, precise and intricate NW arrangements, for which selective area epitaxy (SAE) is the technique of choice [2,3]. SAE utilizes a thin dielectric mask with lithographically defined openings on top of a substrate to control the NW placement, directionality, aspect ratio and filling factor, which are elements that can be adjusted to tune a device’s performance [2]. By allowing predefined patterning, SAE could improve the flexibility and complexity of structure designs in NW quantum networks for Majorana braiding investigations [4]. GaN nanowire two-dimensional photonic crystal (2DPC) based laser array has been reported. A near continuum of lasing wavelengths is realized by suitably adjusting the lattice constant and diameter of the NWs [5]. In addition, the periodic structure of SAE NW arrays could help enhance optical absorption and reduce reflection in photovoltaics [6]. NW-based devices such as light-emitting diodes (LEDs) [7], vertical surrounding-gate transistors [8], plasmonics and photonic crystals [9] have been successfully fabricated using SAE geometry.
Among III-V materials, indium arsenide (InAs) possesses unique physical properties such as direct, narrow bandgap and high electron mobility [10]. In NW form, InAs is an excellent platform for investigation of quantum confinement effects due to the large InAs bulk exciton Bohr radius of 35 nm [11,12]. The reduced carrier scattering in NW quasi one-dimensional structures, combined with the low InAs electron effective mass has enabled fabrication of InAs NW ballistic transistors [13]. Moreover, InAs NWs have a lower effective index of refraction than their bulk counterpart, which facilitates efficient light extraction, and is particularly ideal for applications such as LEDs [14]. InAs NWs monolithically integrated onto silicon (Si) exploit Si’s robustness, low optical loss in the mid-infrared region, and the maturity of CMOS technology, which are crucial factors that promote NW device manufacturability.
The development of novel, high-efficiency NW devices requires high quality material with long carrier lifetimes, and low surface recombination velocities [15,16]. InAs NWs, commonly grown in a randomly-positioned (RP) manner where NWs nucleate arbitrarily on unmasked substrates, have shown short lifetimes. Boland et al. reported a room temperature photoconductivity lifetime of 130 ps for RP InAs NWs grown on a gallium arsenide (GaAs) substrate [17]. Joyce et al. [18] studied carrier lifetime as well as carrier transport in Au catalyzed InAs NWs grown on InAs substrate by metal organic chemical vapor deposition (MOCVD) and measured the room temperature surface recombination velocity to be 3000 cm/s, carrier mobility to be 6,000 cm2/(V·s). Zhang et al. [19] disentangled carrier recombination on the sidewall, ends and interior of RP InAs/InAlAs core-shell NWs on Si, finding interior carrier lifetimes of 559 ps, and 89 ps at 77K and room temperature, respectively, and surface recombination velocities varying from 5,000 to 8,000 cm/s at 77K to room temperature.
SAE InAs NWs have shown longer lifetimes than RP InAs NWs, and promising Auger rates compared to InAs planar materials. Li et al. [14] resolved Shockley-Read-Hall (SRH), radiative, Auger recombination rates in InAs/InAlAs NWs grown on Si, and found the NW Auger recombination rate is about one order of magnitude smaller than that of the InAs planar materials. Minority carrier lifetime was found to be about 700 ps at 77K, reducing to 150 ps at 300K. Although SAE NW devices have been widely reported, a study that separates the different competitive recombination pathways in various parts of the InAs NWs grown by the SAE approach, especially on highly lattice-mismatched Si has not yet been reported.
In this work, we demonstrate the high optical quality of SAE core-only InAs NWs grown on electron-beam lithographically patterned, silicon nitride (SiNx) masked Si (111) substrate. A series of NWs with variable diameters were grown, and NW minority carrier lifetimes were experimentally determined by non-contact ultrafast pump-probe spectroscopy at 77 K. 77K, which is readily achievable by liquid nitrogen cooling, is the cryogenic operation temperature for many NW-based optoelectronic devices [20,21]. Using the geometrical dependence of a diameter series, the surface and interior recombination rates in SAE-InAs NWs were resolved. Record long InAs NW interior recombination lifetime was achieved.
2. Methods
NW samples were grown on Czochralski, epi-ready (111) Si substrates. For SAE mask fabrication, a 50 nm SiNx dielectric layer was first magnetron sputtered onto the Si (111), followed by spin-coating a 200 nm ZEP520A electron beam lithography (EBL) resist, and a 10 nm evaporated anti-charging aluminum (Al) film. Periodic nanohole arrays with an area of 2.5 mm × 2.5 mm, 50 nm nanohole diameter and 300 nm pitch and were written by Raith Voyager EBL system with a dose of 300 µC·cm-2. After exposure, the Al layer was removed by a wet etch. The nanohole pattern was subsequently developed, and transferred into the SiNx layer with a fluorine-based reactive ion etch (RIE), with a few nanometers of SiNx left at the bottom to prevent damages of epi-ready surface caused by ion bombardment. More fabrication details can be found in Ref. [22]. The patterned templates were subsequently RCA cleaned, followed by a 2% hydrofluoric acid (HF) wet etch to clear the holes, before entering the molecular beam epitaxy (MBE) reactor for NW growth.
Growths were conducted in a GEN20 MBE system utilizing dual filament SUMO III cells and valved cracker V cells. Growth temperatures used were ∼ 475 – 490°C, and indium (In) impingement rate was fixed at 0.06ml/s. The SAE mechanism obviates the use of foreign metal catalysts. Six InAs NW samples were synthesized, with variable diameters of 51.3 nm, 67.6 nm, 77.9 nm, 83.7 nm, 94.8 nm and 130.0 nm. Two methods were mainly adopted for the width control. The first one was changing the true V/III flux ratio. When As flux (V) was decreased, In adatoms (III) built up on the NW sidewalls due to a slower In incorporation by arsenic at the NW tips, which led to enhanced NW radial growth; conversely an increased As (V) flux favored NW axial growth. The NW growth front is at the tip of NW, and III adatoms will arrive at the NW growth front through three routes: (1) direct impingement on growth front; (2) adsorption onto the substrate, then diffusion to growth front via NW sidewalls; (3) adsorption of onto NW sidewalls, then diffusion to growth front. When the supplied V flux increases, chances of group III adatoms getting incorporated by V adatoms at NW tips increase, therefore axial growth becomes more favorable. When the supplied V flux is lowered, the III adatoms can’t be incorporated by V adatoms at the NW tips fast enough. Instead, they build up on NW sidewall, resulting in a higher III adatoms density on the sidewall which consequently promotes NW radial growth [23,24]. The second method employed was changing the NW growth time, as increased time led to NW radial growth promotion. After MBE growth, a Hitachi S-4800 field emission scanning electron microscope (FE-SEM) was implemented to examine the NW morphology. A JEOL 2100F transmission electron microscope (TEM) was employed for the NW dark field imaging and selective area electron diffraction (SAED).
An ultrafast pump-probe differential transmission measurement was performed on the as-grown, vertically-positioned NWs on Si at 77 K. The technique is described in detail elsewhere [25]. Briefly, NWs were photo-excited by a pump pulse (150 fs duration, 1kHz repetition) with photon energy of 751 meV (1.65 μm), generating excess carriers across the NW band gap. A sub-bandgap (149 meV, 8.3 μm) probe beam (150 fs duration, 1 kHz repetition) detected the presence and density of carriers in the NWs in (differential) transmission through free carrier absorption. The pump laser spot radius is 650 µm. The average incident pump power was varied from ∼200 to ∼800 µW with repetition rate of 1 kHz, giving average power density of 50–200 mW·cm-2, or pulse fluence of 50–200 µJ·cm-2. The probe spot radius is ∼ 300 µm and the fluence of the probe is very weak (<<50 µJ·cm-2).
3. Results and discussion
Representative SEM images of SAE InAs NWs with different diameters ($d$) are presented in Fig. 1(a)-(b). As seen from the SEMs, high-yield, well-aligned, uniform NW arrays with excellent selectivity and periodicity were fabricated. NWs grew in the [111]B crystallographic direction perpendicular to the Si substrate surface with good verticality illustrated in [26], and in cross-section show hexagonal (110) facets. An image of non-selective, RP-NWs grown on unmasked Si is show in Fig. 1(c) for comparison. NWs grow preferentially along [111]B crystallographic direction, irrespective of the underlying substrate orientation [27,28]. NWs grown on (111) substrate suffer from twin stacking faults which will be discussed later in the work. Stacking faults have been shown to be mitigated by Sb incorporation in InAs NWs [29], by growth on (100) substrates in related material systems, and by other novel approaches [28]. The NW diameter control from 51.3-130 nm was successfully achieved by SAE. Four of the NW arrays in the series have lengths ($l$) ranging from 1.25-1.69 µm, one has the shortest length ($d$ = 51.3 nm, $l$ = 1.08 µm, Fig. 1(a)), and one has the longest length ($d$ = 95 nm, $l$ = 2.1 µm). However, according to our previous study [14], differences in NW length have no effect on the NW carrier lifetime when the length is above 1µm.
Fig. 1. Representative SEM images of SAE-NWs with variable diameters (a) $d$ = 51.3 nm, (b)$\; d$ = 78 nm; and (c) RP InAs-InAlAs NWs of $d$ = 95 nm. Images are tilted 30°. Inset of (b) shows a top down view of the SAE-NWs. The SEMs demonstrate the non-tapered geometry and excellent size homogeneity of SAE-NWs.
The 77 K differential transmission ($\Delta T/T$) temporal decay of two representative InAs NW samples with diameters in the extremities of the investigated range (51.3 nm and 130 nm) are presented in Fig. 2(a), along with multi-exponential fits to the data. It can be seen from Fig. 2(a) that the smaller NW diameter is associated with the faster decay. The multi-exponential decay is attributed to different excess carrier density $({\Delta N} )$ dependent recombination mechanisms within the NWs: Shockley-Read-Hall (SRH), radiative, and Auger. Using a procedure previously described [14], the normalized recombination rate $R\; $as a function of excess carrier density of all NW samples is obtained from the measured carrier lifetime data. Figure 2(b) depicts the $R({\Delta N} )$ data of the two aforementioned NW samples fitted with equation:
where ${n_0}$ is the NW background carrier density, and A, B, C are SRH, radiative, and Auger coefficients, respectively. From $R({\Delta N} )$, the minority carrier lifetime ${\tau _{MC}}$, which is the NW lifetime in the limit of zero excess carrier density ($\Delta N$=0), was extracted. Minority carrier recombination rate ${R_{MC}}( = 1/{\tau _{MC}}$) is plotted for all investigated NWs at 77 K, as presented in Fig. 3.Fig. 2. (a) $\Delta T/T$ decays and (b) recombination rates plotted against excess carrier densities of two NW samples at 77 K, one with the smallest diameter of 51.3 nm (grey square), and one with the biggest diameter of 130.0 nm (black circle) in the investigated diameter series. NW with smaller diameter features faster decay. In (a), multiexponential decays given by $y = a \times {e^{ - bt}} + m \times {e^{ - nt}} + p \times {e^{ - qt}}$ were applied to fit the data with least squares to obtain derivatives of the experimental curves. In (b), the red and blue lines are the fit to the corresponding data by Eq. (1).
Fig. 3. 77 K recombination rate of the InAs NW diameter series, plotted against inverse NW diameter, fitted with equation ${R_{MC}} = \frac{{4S}}{d} + {R_{int.}}.$ The adjusted R2 is 0.90.
As can be seen from Fig. 3, ${\tau _{MC}}$ displays a pronounced dependence on the NW diameter. This observation correlates to the typical NW surface-mediated recombination mechanism, where recombination rate is proportional to the (high) NW surface-to-volume ratio. The rich surface states on NWs are key for sensitive biological detection [30,31]. However, they essentially act as Shockley-Read-Hall recombination centers, and reduce carrier lifetime as the NW diameter gets smaller while the NW surface-to-volume ratio gets higher. A photo-excited carrier will travel less distance before it encounters a surface defect state and recombines with it, which leads to a decrease in NW lifetime. For the NW with the largest diameter of 130 nm, the 77 K ${\tau _{MC}}$ was measured to be 787.4 ps, while ${\tau _{MC}}$ of the NW with the smallest diameter of 51.3 nm reduced to 320.5 ps. Note that because of the variations in the NW diameters within a single sample and the small signal-to-noise ratio in the pump-probe measurement, the data have relatively big error bars.
We compare the ${\tau _{MC}}$ of SAE InAs NWs to that of the RP InAs/InAlAs core-shell NWs grown on un-patterned, H2O2-oxidized Si substrates previously reported by our group [19]. The growths of RP NWs were conducted using similar MBE parameters as SAE. Remarkably, SAE core only InAs NWs display 77K minority carrier lifetimes over two times longer than that of the RP core shell InAs/InAlAs NWs. For example, the RP InAs/InAlAs NWs of 95 nm diameter were measured to have ${\tau _{MC}}$ of 252 ps, while SAE-InAs of 98.5 nm diameter have ${\tau _{MC}}$ of 552.5 ps, which is 2.2 times longer; the ${\tau _{MC}}$ of 130 nm diameter RP InAs/InAlAs NWs was determined to be 290 ps, while ${\tau _{MC}}$ of 130 nm SAE InAs NWs was 787.4 ps, which demonstrates a 2.7 times increase. Note that the SAE NWs were unpassivated; with passivation the ${\tau _{MC}}$ of SAE InAs NWs are predicted to be even longer.
The minority carrier lifetime ${\tau _{MC}}$ of a NW can be represented by [32]:
where S is the surface recombination velocity,$\; {R_{int}}$ is the interior minority carrier recombination rate and d is the NW diameter. A fit of Eq. (2) was applied to 1/$\; {\tau _{MC}}$ plotted against 1/$\; d$ in Fig. 3, and S, ${R_{int}}$ were obtained from the slope, and intercept of the fit, respectively. The 77 K S of the SAE InAs NW was extracted to be $4154({ \pm 40.5} )\; cm \cdot {s^{ - 1}}$, comparable with that of the RP InAs/InAlAs core-shell NWs, which is $5388\; ({ \pm 1596} )\; cm \cdot {s^{ - 1}}$ [19]. However, as reported previously, the presence of a wider bandgap InAlAs shell can enhance the minority carrier lifetime by a factor of two due to effective surface passivation by the shell [14]. Thus the surface velocity of core shell SAE NWs is expected to be reduced below that of RP core-shell NWs, indicative of better surface quality. The 77 K ${R_{int}}$ was measured to be $0.139({ \pm 0.101} )\; n{s^{ - 1}}$. As previously mentioned, the data from the ultrafast measurement (teal triangles in Fig. 3) have big error bars, which result in the considerable error bar on the extracted ${R_{int.}}$ Nevertheless the measured data points demonstrate an excellent linearity and nicely fits Eq. (2), validating our measurement and analysis approach. Compared to the reported minority carrier recombination rate of planar InAs materials [33,34], which are less than $0.005\; n{s^{ - 1}}$ at 77 K, this value is about two orders of magnitude larger. However, it is an order of magnitude smaller than the ${R_{int}}$ measured in RP InAs/InAlAs NWs (${\sim} \; 1.79\; n{s^{ - 1}}$) [19]. The upper limit of measured ${R_{int}}$ (${\sim} \; 0.24\; n{s^{ - 1}}$) is still seven times smaller than $1.79\; n{s^{ - 1}}$. To the best of our knowledge, this is the longest InAs NW interior minority carrier lifetime reported in literature so far, further proving the much improved material and optical property achieved by SAE compared to the RP approach.The surface recombination rate, calculated by $4S/d$, is 3.24 $n{s^{ - 1}}$ in NWs with the smallest diameter $51.3\; nm$, and 1.28 $n{s^{ - 1}}$ in NWs with the largest diameter $130\; nm$, which are both one order of magnitude higher than the NW interior recombination rate (0.139 $n{s^{ - 1}}$). Note that the InAs NWs investigated in this work have dimensions (diameter = 51.3-130 nm) larger than InAs Bohr radius (∼ 35 nm for bulk InAs) [11,35], thus quantum confinement of electronic carriers is insignificant in the NWs presented here. This shows the surface recombination dominates the interior recombination in our selective-area InAs NWs at all investigated diameters at 77 K. Passivation methods include the addition of a higher bandgap shell, such as InAlAs [36] or InP [37], chemical passivation [38], and encapsulation with dielectrics or polymers [39].
To gain an insight into the interior crystalline quality, SAE InAs NWs were characterized by TEM. III–V bulk materials typically stabilize in the zincblende (ZB) crystallographic phase. In NWs, an additional wurtzite (WZ) phase is commonly observed [40], which is not naturally occurring in bulk due to its thermodynamic instability in bulk form [41]. Phase perfection is important to achieve long carrier lifetime in III-V semiconductors. For instance, Furthmeier et al. [42] reported a long carrier lifetime of 11.2 ns in stacking-fault-free WZ GaAs NWs, exceeding the lifetime of polytypic GaAs NWs. Twin boundaries could reduce the acoustic phonon velocity in InAs NWs [43]. Researchers have performed time-resolved photoluminescence measurements on both WZ and ZB indium phosphide (InP) films and reported shorter lifetime for films of wurtzite structure, from which enhanced light emission was suggested [44]. As another member of the III-V family, InAs is expected to have similar properties. The higher band gap reported in WZ InAs in comparison to their zincblende counterparts [45,46] will suppress Auger recombination since Auger rate decreases as band gap increases [47,48]. Therefore, we expect an enhanced radiative rate and a reduced Auger rate for WZ structure compared with ZB ones. Phase-pure InAs NWs of either ZB or WZ structure have been achieved [21,49]. However, these efforts usually involve the incorporation of metallic catalysts, such as Au, which forms deep level traps in Si, and is incompatible with Si CMOS technology.
TEM inspection revealed the SAE InAs NWs in this study have predominantly WZ structure, with a high density of WZ/ZB stacking faults and small angle rotation domains in the NW interior, as seen from the low magnification TEM in Fig. 4(a). The diffraction spot pattern shown in Fig. 4(b) is characteristic of predominantly wurtzite material, while elongation of the spots are indicative of WZ/ZB stacking faults [31,49]. These observations agree with other results for catalyst- free NWs [50,51]. These results also suggest the planar defects and disorder could contribute to the faster carrier decay, and account for the two orders of magnitude larger interior minority recombination in SAE InAs NWs than in planar InAs [33,34]. We propose that using methods which enhance NW phase purity, such as Sb incorporation [29], or growth temperature elevation [51], the NW interior recombination rate can be further reduced, and NW minority carrier lifetimes prolonged. Limited by the resolution, we are not able to directly quantify defects in RP NWs and SAE NWs from the SAEDs, however the study conducted by Hertenberger et al. [52] showed that X-ray diffraction rocking curve peak width of the SAE and RP InAs NWs, and found that the full width half maximum (FWHM) of the SAE-NWs peak was 0.62°, a factor of 2 smaller than that of the RP-NWs, which is 1.24°, which indicates an improved suppression of crystal tilt and better material quality. This conclusion is consistent with our finding of shorter minority carrier lifetimes of RP NWs compared to SAE NWs.
Fig. 4. The (a) dark field TEM and (b) SAED of InAs NWs grown on Si (111). White lines in (a) evidence the stacking faults of the NW. The large number of stacking defects results in a distortion in the NW crystal structure, causing the elongation of reflection spots in the SAED pattern.
4. Conclusions
In conclusion, ultrafast pump-probe spectroscopy was performed on selective-area InAs NWs grown on lithographically patterned, highly lattice-mismatched SiNx/Si (111) substrates at 77 K. The minority carrier lifetimes ${\tau _{MC}}$ demonstrated a strong geometrical dependence, from which carrier recombination on the NW sidewalls, and interior was resolved. SAE NWs show ${\tau _{MC}}$ over two-fold longer than that of the RP InAs/InAlAs NWs grown under similar MBE conditions. The surface quality of SAE NWs was found be to better than that of the RP NWs. Moreover, a very low interior recombination rate was extracted to be $0.139\; n{s^{ - 1}}$, which corresponds to an exceptionally long NW interior minority carrier lifetime of $7.2\; ns$. This value is an order of magnitude longer than that of the RP InAl/InAlAs NWs, and to the best of our knowledge, is the longest reported in literature for InAs NWs. These findings show that NWs fabricated using SAE approach have superior optical quality, critical for InAs NW on-chip devices. The surface recombination was found to dominate the interior recombination for the entire investigated diameter range measured at 77 K. TEM inspection revealed that the SAE NWs are predominantly wurtzite, and display a considerable density of stacking fault defects within the interior, suggesting with better phase purity, interior recombination can be further reduced.
Funding
National Science Foundation (EPM-1608714); U.S. Department of Energy (DE-AC02-06CH11357); Office of Science (DE-AC02-06CH11357); Basic Energy Sciences (DE-AC02-06CH11357).
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.
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