AlN Nucleation Layers for Selective-Area

Dec 18, 2015 - Polarity-Controlled GaN/AlN Nucleation Layers for Selective-Area Growth of GaN Nanowire Arrays on Si(111) Substrates by Molecular Beam ...
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Polarity-Controlled GaN/AlN Nucleation Layers for Selective-Area Growth of GaN Nanowire Arrays on Si(111) Substrates by Molecular Beam Epitaxy Matt D. Brubaker,* Shannon M. Duff, Todd E. Harvey, Paul T. Blanchard, Alexana Roshko, Aric W. Sanders, Norman A. Sanford, and Kris A. Bertness Physical Measurement Laboratory, National Institute of Standards and Technology, Boulder, Colorado 80305, United States S Supporting Information *

ABSTRACT: We have demonstrated dramatic improvement in the quality of selective-area GaN nanowire growth by controlling the polarity of the underlying nucleation layers. In particular, we find that N-polarity is beneficial for the growth of large ordered nanowire arrays with arbitrary spacing. Herein, we present techniques for obtaining and characterizing polarity-controlled nucleation layers on Si (111) substrates. An initial AlN layer, which is demonstrated to adopt Al- (N-)polarity for N- (Al-)rich growth conditions, is utilized to configure the polarity of subsequently grown GaN layers as determined by piezoresponse force microscopy (PFM), polarity-dependent surface reconstructions, and polarity-sensitive etching. Polarity-dependent surface reconstructions observed in reflection high-energy electron diffraction (RHEED) patterns were found to be particularly useful for in situ verification of the nucleation layer polarity, prior to mask deposition, patterning, and selective-area regrowth of the GaN NW arrays. N-polar templates produced fast-growing nanowires with vertical m-plane side walls and flat c-plane tips, while Gapolar templates produced slow-growing pyramidal structures bounded by (11̅02) r-planes. The selective-area nanowire growth process window, bounded by nonselective and no-growth conditions, was found to be substantially more relaxed for NW arrays grown on N-polar templates, allowing for long-range selectivity where the NW pitch far exceeds the Ga diffusion length.

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INTRODUCTION Gallium nitride nanowires (GaN NWs) have enabled the synthesis of nearly defect-free GaN and have produced a viable platform for fabrication of light-emitting devices on non- and semipolar planes. Increasingly, functional devices require nanowire arrays with site-specific nucleation and dimensional control, which can be obtained by use of selective-area growth techniques.1 Solid-state lighting has been one of the primary drivers in these efforts, where the NW diameter is used for tuning the emission wavelength,2 and densely packed, highaspect-ratio NWs increase the effective device area.3 Much of this work has been predicated on Ga-polar GaN templates, similar to conventional planar GaN LED devices, although nanowire arrays on N-polar GaN templates have also been demonstrated.4,5 Selectively grown NWs have also been obtained by catalyst-free molecular beam epitaxy (MBE) on silicon (111) substrates;6−15 however, the crystallographic polarity16 of the nucleation layers or the nanowire arrays have not been widely reported. Selective-area nanowire arrays grown on GaN-buffered Si(111) have been determined to exhibit Ga-polarity;15 however, the polarity of AlN-buffered Si(111) remains an open question. As the polarity influences many properties in III-nitride materials, including growth kinetics,17 incorporation of dopants18 and impurities,19 built-in polarization fields,20 and nanowire growth habit,4,21 detailed This article not subject to U.S. Copyright. Published XXXX by the American Chemical Society

knowledge of the nucleation layer polarity is essential. This is particularly important for the growth of GaN NW probes on Sibased AFM cantilevers,22 where the substrate choice is dictated by conventional micromachining technology. Often a thin AlN layer is used as the NW nucleation template for silicon (111) substrates, and this AlN layer can adopt either polarity configuration,23−25 suggesting that homogeneous Gaor N-polar selective-area nanowire arrays are possible on silicon (111) substrates. However, complications arise when the AlN layer exhibits mixed-polarity domains and a preferred nanowire growth polarity is active. These effects are increasingly recognized as key factors in spontaneous or random nanowire growth26−28 and have a significant effect on NW density, NW diameter, and coexistence of a film-like matrix layer.26,29 When used for selective-area growth templates, these mixed-polarity AlN nucleation layers typically produce low-quality and inhomogeneous NWs with multiple nucleation points per mask opening, as shown in Figure 1a. To address the issues described in the preceding text, we have developed a method for controlling and characterizing the crystallographic polarity of nucleation layers on silicon (111) Received: June 30, 2015 Revised: November 17, 2015

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conditions. The primary features used for interpreting the RHEED patterns are illustrated in Supporting Information Figure S1. Growth of the AlN nucleation layer on Si(111) was initiated by dosing the Si 7 × 7 reconstructed surface with a few monolayers of Al at a substrate pyrometer temperature of approximately 790 °C. After initiating the nitrogen plasma and stabilizing for ∼1 min, AlN growth was started (ΦN ≈ 220 nm/h; ΦAl ≈ 190−270 nm/h) and the substrate temperature was ramped to approximately 870 °C. These conditions correspond to the N-rich and Al-rich intermediate growth regimes,31 the latter of which was verified by the onset of Al droplet formation at lower substrate temperatures (see Supporting Information Figure S2). The next GaN nucleation layer was grown under stoichiometric fluxes (ΦGa = ΦN ≈ 300 nm/h) and a growth temperature of 670 °C (705 °C) for Ga- (N-)polar layers. These growth temperatures were partially optimized for smooth and dropletfree surfaces, as shown in Supporting Information Figure S3. For selective-area growth templates, a 70 nm thick, low-stress silicon nitride layer was deposited on the polarity-controlled GaN/AlN nucleation layers by low-pressure chemical vapor deposition (LPCVD). This layer was patterned by optical and/or electron beam lithography and etched using a CF4/O2 reactive ion etch (RIE) to produce mask openings with diameters ranging from ∼150 nm to several micrometers. Before reintroduction into the MBE system, the growth templates received a series of wet and dry cleaning processes to remove residual photoresist, organic contaminants, and native oxides. The templates were then degassed and exposed to active nitrogen flux, first at a substrate temperature of ∼600 °C to remove surface adsorbates and second at a substrate temperature of ∼850 °C and for a 5 min duration, which is intended to partially decompose the growth surface and to remove more strongly bound surface contaminants. Finally, selective-area and silicon-doped GaN regrowth was initiated using near-stoichiometric Ga and N fluxes (III/V = 0.8−1.1), nominal growth rates of 300−360 nm/h, and a substrate temperature of ∼850 °C. RHEED-Based Polarity Measurements. RHEED pattern analysis was utilized in determining the crystallographic polarity of GaN and AlN nucleation layers, specifically by in situ observation of polaritydependent surface reconstructions. These surface reconstructions are related to excess Ga on the GaN surface32 or excess Al on the AlN surface33 and produce RHEED patterns that depend on the overall surface stoichiometry and substrate temperature. With increasing Ga coverage, GaN produces a series of RHEED patterns: 1 × 1, 3 × 3, 6 × 6, and 6 × 12 patterns for N-polar surfaces; 2 × 2, 5 × 5, 6 × 4, and metal-rich “1 × 1” patterns for Ga-polar surfaces.32 In some cases, these reconstructions do not emerge at growth temperatures but are only observed postgrowth during cool down. We consider the N-polar 3 × 3 and the Ga-polar 2 × 2 patterns to be the most definitive and easiest to produce for GaN-polarity identification (summarized in Table 1). In development of nucleation layers, the surface stoichiometry was sometimes tailored to reveal these particular reconstructions: an N-rich surface was prepared by exposure to active nitrogen or through annealing to desorb excess group-III species; alternatively, a metal-rich surface was prepared by deposition of additional group-III fluxes. Similar tactics were employed for observation of polarity-dependent surface reconstructions in AlN layers, where 2 × 2 or 2 × 6 patterns are observed for Al-polar

Figure 1. FESEM images illustrating the influence of nucleation layer polarity on the structure of selectively grown GaN nanowires. The nanowires in panel a were grown on a presumably mixed-polarity nucleation layer, while the nanowires in panel b were grown on an Npolar nucleation layer. The mixed-growth mode illustrated in panel a persists even when identical hole size and pitch spacing are used (data not shown).

substrates and have demonstrated that high-quality, selectivearea NW growths are obtained for N-polar nucleation layers, as shown in Figure 1b. First, techniques for growing AlN and GaN/AlN nucleation layers with both polarity configurations were developed and evaluated by complementary characterization techniques, including ex situ piezoresponse force microscopy (PFM), in situ reflection high-energy electron diffraction (RHEED), and polarity-sensitive etching. To our knowledge, this is the first cross-correlation between these three types of polarity-sensitive measurements. Next, Ga- and N-polar GaN growth rates were investigated as a function of temperature by use of in situ reflectance spectroscopy. Here, polarity-dependent decomposition of GaN was identified as a limitation for temperatures relevant to selective-area NW growth. Lastly, selective-area NW arrays were grown on polarity-controlled nucleation layers on Si(111) and were characterized to clarify the effect of polarity on the NW growth habit. Comparisons were also drawn against similar polarity measurements and selective-area growths using commercially available Ga-polar GaN templates.

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EXPERIMENTAL SECTION

Molecular Beam Epitaxy Growth Conditions and Fabrication of Selective Growth Masks. Nucleation layers and nanowire arrays were grown by plasma-assisted MBE, using a commercially available system equipped with conventional effusion cells and a radio frequency (RF) nitrogen plasma source. Substrate temperatures were monitored with a backside pyrometer that was calibrated against blackbody radiation measurements of the Si(111) surface,30 where the reversible 7 × 7 to 1 × 1 reconstruction transition was observed at 820−830 °C. Group-III fluxes, calibrated against planar film growth rates under metal-limited conditions, are reported here in equivalent GaN (ΦGa) or AlN (ΦAl) film growth rates. Active nitrogen fluxes (ΦN) are also reported in equivalent growth rates and are determined by the spotty (metal-limited) to streaky (nitrogen-limited) transition in the RHEED patterns or by the maximum growth rate under nitrogen-limited

Table 1. Summary of Polarity Characterization Techniques and Associated Details Used in Characterizing Growth Templates.a characterization technique RHEED patterns PFM (positive tip bias cycle)

AlN GaN E-field/[0001] alignment deflection PFM phase

polarity-sensitive etch (hot H3PO4)

N-polar

Al-/Ga-polar

3×3 3×3 parallel positive in-phase pyramids

2 × 2, 2 × 6 2×2 antiparallel negative out-of-phase pits

a

RHEED patterns with polarity-dependent surface reconstructions were observed postgrowth after cooling the substrate, except in the case of Alpolar AlN, where surface reconstructions were observed during growth. B

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surfaces24,33 and a 3 × 3 pattern is observed for N-polar surfaces23,24 (also summarized in Table 1). The Al-polar AlN surface reconstructions were observed only during growth, unlike the Npolar AlN surface reconstructions, which were observed during cool down. While these RHEED-based polarity measurements provide a convenient method for in situ characterization of the dominant polarity, additional techniques (which are described next) are required to identify microscopic variations in the nucleation layer polarity. It should be noted that for selective-area NW growth templates, only a controlled exposure to active nitrogen was utilized to maintain a highquality surface for subsequent regrowth. Piezoresponse Force Microscopy. Surface maps of the crystallographic polarity and the topography of nucleation layers were obtained by PFM, a scanned-probe measurement based on the converse piezoelectric effect. In this technique, piezoelectric vibrations are induced in the film by a sinusoidal voltage bias on the probe tip and are detected through the cantilever deflection signal. The magnitude of the vibration is determined by the piezoelectric d33 coefficient, which linearly relates strain and electric field in the c-axis direction, neglecting the minor contributions of the shear piezoelectric components. Because the d33 coefficients in III-nitride materials are positive, an increase in the film thickness and deflection is expected when the induced E-field and [0001] direction vectors are parallel. For the Npolar configuration this results in in-phase behavior between the applied tip bias and the measured deflection signal at the bias frequency. Conversely, the bias and deflection signals are out-of-phase for the Al-/Ga-polar configuration. These phase-polarity assignments, which are identical to those published elsewhere for ZnO NWs,34 are summarized in Table 1 and were validated against polarity-sensitive etching in hot phosphoric acid. Details related to the PFM measurement conditions and application to mixed-polarity AlN films are published elsewhere.29 Planar GaN Growth Rates. In situ growth rate measurements were obtained by analyzing the maxima and minima of the normal incidence reflectance spectra. The instantaneous layer thickness (t) was calculated as t = (λimi)/4ni, where λi is the wavelength and mi is the integral interference order of the ith extremum. The wavelength dependence of the GaN index of refraction (ni) was accounted for by use of a dispersion relation published elsewhere.35 The temperature dependence of the index of refraction was neglected, leading to an overestimate of the film thickness at high growth temperatures. Using a high-temperature dispersion curve published elsewhere,36 we calculate this variation to be less than 7% over the range of growth temperatures used in this study. In any case, relative comparisons drawn between Ga- and N-polar GaN films at a given temperature are expected to be valid. Microscopy. NW arrays were characterized by field-emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), scanning transmission electron microscopy (STEM), and transmission electron microscopy (TEM) at 200 kV. TEM samples were prepared by focused ion beam (FIB), using a deposited platinum layer as a protective cap, followed by low-energy (850 eV) Ar milling.

Figure 2. N-polar AlN film grown under Al-rich conditions as characterized by (a) PFM topography, (b) PFM phase/polarity, and RHEED images from (c) ⟨112̅0⟩ and (d) ⟨101̅0⟩ azimuths. Plan-view FESEM images show the (e) as-grown and (f) polarity-sensitiveetched surfaces. The film has uniform N-polarity, as indicated by the in-phase PFM image in panel b, the 3 × 3 RHEED pattern in panels c and d, and the pyramidal etch structures in panel f. Al-rich and 2D growth conditions are indicated by the streaky RHEED patterns in panels c and d and the planar film surface in panel e.

confirmation of the N-polarity by revealing the expected hexagonal pyramids. The 2D growth mode that is characteristic of Al-rich growth conditions was evident in the streaky 1 × 1 RHEED pattern observed throughout growth and in the resulting planar surface morphology. In contrast, N-rich growth conditions produced Al-polarity layers, evidenced by the out-of-phase PFM signal, the 2 × 6 RHEED pattern observed during growth, and the hexagonal etch pits revealed during polarity-sensitive etching (see Figure 3). A low density of N-polar inversion domains (IDs) was observed in the Al-polar layers, often as a single ID in the 3 μm × 3 μm PFM scans. The columnar morphology and spotty RHEED patterns reflect the 3D growth mode typically observed under N-rich conditions. The 2 × 6 RHEED pattern was observed during growth and was typically visible only toward the end of the growth duration. For some growths we observed a 2 × 2 pattern, reported elsewhere as a more nitrogen-rich Al-polar reconstruction,33 instead of a 2 × 6 pattern. This may reflect an evolving surface stoichiometry during growth, possibly due to stabilization of the plasma source throughout the AlN layer. Conventionally, AlN films are grown near flux stoichiometry to promote smooth and planar surface morphology. However, polarity control at stoichiometric fluxes is difficult and often leads to mixed-polarity films, as shown in Supporting Information Figure S2. The tendency toward AlN-polarity reversal under excess Al has been reported elsewhere25 (albeit with an inverted polarity assignment from this work) and was attributed to an Al−Al bilayer introduced by the metal-rich surface. Nitrogen adatoms can bond to the Al−Al bilayer in

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DEVELOPMENT OF POLARITY-CONTROLLED NW NUCLEATION LAYERS ON SILICON (111) A series of AlN layers were grown to 100 nm thickness under N-rich or Al-rich growth conditions and with a substrate temperature sufficiently high to inhibit the formation of Al droplets. The film polarity was then determined by the RHEED, PFM, and polarity-sensitive-etch techniques described in Experimental Section and using the polarity assignments listed in Table 1. It was found that the AlN layer polarity could be controlled through the Al/N flux ratio, with the polarity transition occurring near stoichiometric fluxes. N-polar AlN layers were obtained for Al-rich growth conditions, as indicated by the inphase PFM signal and post-cool-down 3 × 3 RHEED pattern shown in Figure 2. Polarity-sensitive etching provided further C

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Figure 4. N-polar GaN film grown on N-polar AlN/Si as characterized by (a) PFM topography, (b) PFM phase/polarity, and RHEED images from (c) ⟨112̅0⟩ and (d) ⟨101̅0⟩ azimuths. The film has uniform Npolarity, as indicated by the in-phase PFM image in panel b and 3 × 3 reconstruction in panels c and d. The additional streaks interleaving those indicated by the arrows in panel d and offset toward the bottom of the image are from the next Laue zone in the diffraction pattern.

Figure 3. Al-polar AlN film grown under N-rich conditions as characterized by (a) PFM topography, (b) PFM phase/polarity, and RHEED images from (c) ⟨112̅0⟩ and (d) ⟨101̅0⟩ azimuths. Plan-view FESEM images show the (e) as-grown and (f) polarity-sensitiveetched surfaces. The film is almost uniformly Al-polar, as indicated by the out-of-phase PFM image in panel b and exhibits isolated and lowdensity N-polar inversion domains (IDs). The Al-polarity is confirmed by the 2 × 6 RHEED pattern in panels c and d and the hexagonal etch pits in panel f. N-rich and 3D growth conditions are indicated by the spotty RHEED patterns in panels c and d and by the columnar morphology observed in panels a and e. The spotty characteristic in panels c and d is partially suppressed due to image processing intended to enhance the surface reconstruction streaks. The spots at the righthand side of panel c are representative of the unprocessed pattern image.

produced 3 × 3 surface reconstructions (see Supporting Information Figure S4). Ga-polar GaN was obtained when grown on Al-polar AlN layers; however, a high density of N-polar inversion domains is introduced at the GaN/AlN interface. As shown in Figure 5, the GaN surface yields the Ga-polar 2 × 2 reconstruction during cool down, in agreement with the majority of the PFM phase image. The 2 × 2 reconstruction is observed on a nitrogen-rich surface,32 which was obtained by use of a procedure similar to that of the N-polar GaN layers, but with the plasma source left running during cool down. If the plasma source was turned off prematurely, a 3 × 1 pattern was sometimes observed, which could be confused with the 3 × 3 pattern. Verifying the absence or existence of the ×3 streaks in the ⟨101̅0⟩ RHEED azimuth was critical in assigning Ga- or Npolarity, respectively. To further clarify this ambiguity, the surface was exposed to additional active nitrogen which caused the 3 × 1 pattern to revert to the more definitive Ga-polar 2 × 2 pattern (shown in Supporting Information Figure S5 with supporting polarity-sensitive-etch tests). The Ga-polar GaN layers possess larger and more planar grains than the N-polar GaN layers. There are numerous N-polar IDs, visible as inphase domains in the PFM phase image shown in Figure 5b, with a density that far exceeds that observed in the Al-polar AlN layers alone (see Figure 3b), suggesting that the IDs are nucleated during the GaN layer. For comparison, a commercially available Ga-polar GaN template was also scanned by PFM and no IDs were observed in the 3 μm × 3 μm scan area (see Figures 5e,f) . IDs nucleated at the GaN/AlN interface in GaN/AlN/Si(111) layers have also been reported in a TEM study by Sanchez et al.37 and were correlated to pinholes in the AlN layer. It is possible that the 3D growth mode, inherent in the conditions used for obtaining Al-polar AlN, produced similar ID nucleation sites in this study. The GaN layer grown on top of the polarity-controlled AlN layers serves several purposes as a nucleation layer for selectivearea NW growths. First, the GaN layer can be grown under

either the N-polar configuration with an energetically stable 3fold coordination or in the singly coordinated Al-polar configuration. Here, we propose the former, that nitrogen adatoms assume the more energetically stable and 3-fold coordinated N-polar configuration with the Al bilayer. In this manner, N-polarity is induced under Al-rich growth conditions, while the native Al-polarity is preserved under N-rich conditions. In further development of polarity-controlled nucleation layers, GaN/AlN/Si structures were grown using the Al- and Npolar AlN layers described earlier. Both layers were grown to an approximate thickness of 40 nm. N-polar AlN layers were found to transfer their polarity to the subsequently grown GaN layers, as indicated by the uniform in-phase PFM signal and 3 × 3 RHEED pattern shown in Figure 4. The surface morphology of the GaN layer was somewhat rough and columnar with an interconnect network of grain boundaries. To expose the 3 × 3 reconstruction, the postgrowth GaN surface was exposed to a few monolayers of active nitrogen, after which the plasma source was turned off to allow sufficient surface Ga to remain. Continued active nitrogen exposure from flux leaking past the shutter would cause the surface to revert to a 1 × 1 pattern, in agreement with the nitrogen-rich surface reconstructions on Npolar GaN described by Smith et al.32 Polarity-sensitive-etch tests also confirmed the N-polarity of GaN surfaces that D

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GaN nucleation layer presents a homoepitaxial growth template for the subsequent selective-area NW growth.

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GAN GROWTH RATES ON N- AND GA-POLAR GAN SURFACES It is well-known that GaN NWs can be nucleated and grown spontaneously on a variety of surfaces, including the SiNx layer surface used in this study as a selective-area growth mask. In order to inhibit growth on the mask surface, the substrate temperature is increased beyond that used for spontaneous NW growths. While necessary for producing long-range selectivity, these high-growth temperatures can limit the GaN growth rate through thermal decomposition42 and Ga desorption processes. Moreover, growth kinetics can vary between Ga- and N-polar surfaces;43 therefore, we performed growth rate measurements using the polarity-controlled nucleation layers described previously at growth temperatures relevant to selective-area growth. After growth of the thin GaN/AlN nucleation layers and polarity confirmation by the 2 × 2 or 3 × 3 RHEED patterns, a thicker GaN layer was grown to a thickness of approximately 1 μm using stoichiometric Ga and N fluxes. In situ reflectance spectroscopy was then performed at various substrate temperatures, with growth rates measured during GaN growth (Ga and N shutters open) and during exposure to active nitrogen only (N shutter open), as shown in Figure 6. The growth rate Figure 5. Ga-polar GaN film grown on Al-polar AlN/Si as characterized by (a) PFM topography, (b) PFM phase/polarity, and RHEED images from (c) ⟨112̅0⟩ and (d) ⟨101̅0⟩ azimuths. The film is mostly Ga-polar, as indicated by the 2 × 2 reconstruction in panels c and d and out-of-phase regions in the PFM image in panel b; however, a high density of N-polar inversion domains (IDs) is observed. For comparison, commercially available Ga-polar GaN templates exhibited the expected out-of-phase behavior and did not possess IDs in (e) PFM topography or (f) PFM phase/polarity scans.

conditions that planarize the surface, i.e., close to flux stoichiometry, without inducing a flip in the layer polarity. The invariance of the GaN-polarity with metal-rich or N-rich growth conditions contrasts with that of AlN and has been reported by others.25 Second, GaN layers produced reconstructions for both Ga-polar (2 × 2) and N-polar (3 × 3) surfaces during a normal cool-down cycle, from which the nucleation layer polarity can be nondestructively verified, prior to removal from the MBE system for mask layer deposition and patterning. In our experiments, the Al-polar 2 × 6 pattern is only observed briefly during high-temperature growth and the post-cool-down N-polar 3 × 3 reconstruction often required additional Al dosing and re-evaporation steps to produce the necessary Al surface coverage. Characterization of the postgrowth reconstructions has proven especially useful when Al flux transients, similar to those reported elsewhere,38 create unexpected variations in the Al/N flux ratio and resulting polarity. Third, GaN may present cleaning-related benefits over AlN layers alone, as the GaN surface is less sensitive to oxidation than AlN and can be partially decomposed prior to growth during an in situ thermal preclean.39 Since oxygen plasma is utilized for photoresist removal and oxide interlayers can lead to polarity inversion,40,41 we speculate that this decomposition-based cleaning step is beneficial for maintaining the intended polarity across the regrowth interface. Lastly, the

Figure 6. Temperature-dependent growth rates for N- and Ga-polar GaN films measured by use of in situ reflectance spectroscopy. Interference peaks in the individual spectrum of panel a were analyzed to produce the growth rate curves in panel b, where the rate was first measured under Ga and N fluxes and then measured under N flux only. The growth rates for both film polarities are shown in panel c as a function of backside pyrometer substrate temperature (Tbsp), where a negative growth rate corresponds to decomposition. Closed and open circles correspond to growth rates obtained under Ga and N fluxes and under N fluxes only, respectively. The approximate temperature at which nonselective growth occurs in silicon nitride masked structures is indicated in panel c for reference.

was approximately 300 nm/h at low temperatures and declines at temperatures higher than 800 °C for both Ga- and N-polar surfaces. This sharp reduction in growth rate coincides with the onset of thermal decomposition, as indicated by the decomposition rates measured with only exposure to active nitrogen. The Ga-polar layers exhibit a more severe reduction in growth rate than N-polar layers, with the GaN growth rate becoming negative at a temperature of ∼850 °C. The temperature at which NWs nonselectively nucleate on a silicon nitride mask surface for these flux values is indicated in the E

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figure, suggesting a slow growth rate and narrow temperature window for conditions suitable for selective-area, Ga-polar growth. In contrast, the N-polar growth rate is observed to be higher for a given growth temperature, providing a wider process window between nonselective and no-growth conditions.

NW sidewalls or mask surfaces to the NW tip, and likely reflects the near-stoichiometic fluxes used for growth rate studies and selective-area growths. The NWs exhibit a slight increase in diameter near the base, which is also consistent with local N-limited (Ga-rich) growth conditions at the NW tip. As discussed elsewhere,44 a self-regulating increase in the NW diameter is observed under N-limited growth conditions. Larger NWs presented irregular facets and interior voids that have feature sizes similar to the grain boundaries in the nucleation layers (see Supporting Information Figure S3) and suggests that increased decomposition and suppressed growth rate occurs at grain boundary regions. Growth was not observed on SiNx mask surfaces at a growth temperature of 850 °C, either near hole pattern arrays or in field areas, suggesting long-range selectivity and Ga desorption from mask surfaces. We note the long-range selectivity in Figure 7f, where the pitch spacing of 10.4 μm far exceeds the Ga diffusion lengths of 100 and 400 nm, reported elsewhere for SiNx45 and Si12 surfaces, respectively. Growth was observed on mask surfaces for substrate temperatures less than ∼850 °C, except where Ga had diffused and incorporated at mask openings. Selective-area growth on Ga-polar templates produced pyramidal structures that were bounded by (11̅02) r-planes, as shown in Figure 8. The growth rate of Ga-polar GaN in the mask openings was slow in comparison to N-polar NWs described earlier. In fact, the overall growth height depended primarily on the mask opening size rather than the overall growth duration, suggesting that these features have obtained their equilibrium crystal shape. The proximity to equilibrium is also suggested by the nearly equal growth and decomposition rates observed at selective growth temperatures in the growth rate studies. Longer NWs were frequently observed to protrude from the Ga-polar pyramidal structures (see Figure 8c,d) with a growth rate and facet structure equivalent to the N-polar NWs described previously. Recalling the high density of N-polar IDs measured by PFM in the Ga-polar GaN nucleation layers and the similarity to N-polar NW growth rates and facet structure, we infer that the protruding NWs have N-polarity and originate in the GaN nucleation layer. These inferences are supported by the cross-section TEM image in Figure 8e, where the sidewalls of a fast-growing NW near the SiNx mask edge are observed to propagate through the GaN regrowth and nucleation layers. The contrast from the fast-growing NW sidewalls is not observed in the AlN layer, indicating that the polarity inversion creating the longer NWs does in fact originate in the GaN nucleation layer near the GaN/AlN interface. To further test this hypothesis, we also performed selective-area NW growths on Ga-polar GaN/sapphire templates using similar growth conditions. As shown in Figure 9, these growths produced pyramidal structures that were identical to growths on Ga-polar GaN/AlN/Si templates. However, no protruding N-polar NWs are observed in the GaN/sapphire growths, in agreement with the absence of inversion domains in PFM scans (see Figure 5e,f). The NW polarity, as determined by that of the underlying template layer, leads to two characteristic types of faceting that depend on the low-energy surfaces accessible for each growth template polarity. All relevant low-energy facets are observed when equilibrium crystal structures are grown on nonpolar aor m-plane surfaces, where growth occurs in both the + c- and −c-directions simultaneously for a single nanostructure. These types of selective-area growth studies have been performed by others for MBE46 and MOCVD47,48 techniques, producing

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SELECTIVE-AREA NW GROWTHS ON POLARITY-CONTROLLED NUCLEATION LAYERS For selective-area growth experiments, a 70 nm thick silicon nitride layer was deposited and patterned on the polaritycontrolled GaN/AlN nucleation layers. Silicon-doped GaN was then grown at a substrate temperature of ∼850 °C under conditions similar to those used in the growth rate studies described previously. As shown in Figure 7, N-polar templates

Figure 7. Selective-area NW growths on N-polar GaN/AlN/Si(111) templates. The nucleation layer polarity was verified by a 3 × 3 RHEED pattern after growth and prior to removal from the MBE system for mask fabrication. Nanowires with flat c-plane tips and mplane sidewall facets are produced in smaller mask openings (0.5 μm pitch, 100 nm diameter) patterned by e-beam lithography (a, b). Interior void regions are observed in larger mask openings with circular (c, d) and mesa-like (e) geometries that were patterned by optical lithography. The array dimensions are 5.6 μm pitch and 940 nm diameter in panel c and 10.5 μm pitch and 780 nm diameter in panel d. Long-range selectivity is shown in panel f, where the pattern dimensions (10.4 μm pitch) far exceed the cited Ga diffusion length values of 100−400 nm.12,45

produced relatively fast-growing NWs with flat c-plane tips and vertical m-plane sidewalls. High-quality NWs are produced when mask openings have dimensions similar to those of spontaneously nucleated NWs, as shown in Figure 7a,b. The NWs grow at approximately 200 nm/h and demonstrate that complete selectivity is readily obtained on N-polar nucleation layers at growth rates comparable to those of planar films. The correspondence between planar and NW growth rates indicates that the growth is likely N-limited, as an increase in axial growth rate would otherwise be expected due to Ga diffusion from the F

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Figure 9. Selective-area NW growths on Ga-polar GaN/sapphire templates. Slow-growing pyramidal features are produced (a, b), similar to Ga-polar GaN/AlN/Si (111) templates; however, no fastgrowing NWs are observed in circular or mesa-like (c) structures. The TEM cross-section image (d), obtained in bright field mode for the [11̅00] azimuth, shows a threading dislocation that originates in the underlying GaN layer, bends at the regrowth interface, and terminates at the surface of the selectively grown feature. The hole pattern dimensions are 5.5 μm pitch and 780 nm diameter in panel a and 10.6 μm pitch and 940 nm diameter in panel b.

Figure 10a, N-polarity is observed in GaN NWs that were grown on an N-polar GaN/AlN/Si template. Similarly, Ga-

Figure 8. Selective-area NW growths on Ga-polar GaN/AlN/Si (111) templates. The nucleation layer polarity was verified by a 2 × 2 RHEED pattern after growth and prior to removal from the MBE system for mask fabrication. The ideal growth habit is characterized by slow-growing pyramidal features (a) that are bounded by {11̅02} planes, as determined by sidewall angle measurements in AFM scans (b). Frequently, fast-growing NWs are also produced in these selectively grown features for both circular (c) and mesa-like mask openings (d). The TEM dark field cross-sectional image (e) obtained for the [11̅00] azimuth and g-vector parallel to the c-axis indicates that the fast-growing nanowire (labeled NW) originates at the GaN/AlN interface in the nucleation layer (indicated by white arrow). The hole pattern dimensions are 2.4 μm pitch and 620 nm diameter in panel a and 10.6 μm pitch and 940 nm diameter in panel c.

Figure 10. Annular bright field STEM lattice images of GaN regrowth sections for selective-area growth on an (a) N-polar GaN/AlN/Si template (corresponding images in Figure 7) and a (b) Ga-polar GaN/ sapphire template (corresponding images in Figure 9). The polarity is deduced from the nitrogen atom lattice position, indicated by the blue arrows, with respect to that of the Ga atom. In both panels a and b, the nucleation layer polarity is observed to propagate into the regrowth section. STEM images were obtained along the [11̅00] zone axis. Ga/ N atoms are indicated by green/blue spheres in the structural model.

equilibrium crystal shapes with arrow-like features that “point” in the +c-direction and are bounded by a (0001̅) N-face, (11̅00) m-planes, and (11̅0x) semipolar planes with x = 1, 2. For short growth durations or conditions far from equilibrium, a (0001) Ga-face or (1120̅ ) a-planes can sometimes be observed.47,49,50 Thus, for selective-area growth on the N-polar surface, where only the (0001̅) N-face and (101̅0) m-planes are accessible, flattipped NWs with vertical sidewalls are obtained.4,5 For selective-area growth on the Ga-polar surface at nearequilibrium conditions, where only the semipolar (101̅x) planes and (101̅0) m-planes are accessible, NWs with a pencil-shaped tip profile are obtained.49−54 We find that the polarity-dependent faceting and characteristic structures of our NW arrays, grown on polarity-controlled nucleation layers on Si (111) and on Ga-polar GaN/sapphire templates, are in good agreement with the preceding discussion and citations. We have further verified the correlation between polarity and characteristic morphology by obtaining STEM lattice images of the GaN regrowth sections, which indicate polarity through the relative lattice positions of the Ga and N atoms.55 As shown in

polarity is observed in the GaN pyramidal structures grown on Ga-polar GaN/sapphire templates (Figure 10b). In both cases the polarity in the NW regrowth section is observed to propagate that of the underlying nucleation layer, providing support for the polarity-dependent faceting discussed previously and further indicating that no polarity inversion has been induced by the masking process.

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CONCLUSION In summary, we have demonstrated that selective-area growth templates with N- or Ga-polarity can be reproducibly obtained on Si(111) substrates and that N-polar NW arrays grow faster, G

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over a broader range of conditions, and with a greater degree of homogeneity. The AlN layer polarity in nucleation layers is prescribed by use of Al- or N-rich growth conditions, while the subsequently grown GaN layer propagates the AlN layer polarity, allows for in situ polarity characterization via surface reconstructions, and potentially serves as a capping layer during mask fabrication. In general, Al-polar and Ga-polar buffer layers contained a significantly higher density of polarity inversion domains than N-polar buffer layers, which also degrades the quality of selective epitaxy arrays. RHEED-based polarity assignments, validated with polarity-sensitive-etch tests and PFM measurements, are a real-time check of nucleation layer polarity which can shift with small changes in growth conditions. NW arrays were grown at temperatures sufficiently high to inhibit growth on the mask surfaces, allowing longrange selectivity to prevail. Extremely low growth rates were observed for selective-area NW growth on Ga-polar templates and are likely related to surface decomposition processes that were activated by the high-temperature growth conditions. In contrast, N-polar NW arrays exhibited long-range selectivity and could be grown at near-planar growth rates. The long-range selectivity in these pitch-independent, N-polar NW arrays, where flux shadowing effects can be completely eliminated, opens new possibilities for MBE growth of core-sleeve structures and integration of single NW devices with Si (111) substrates.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b00910. Figures S1−S5 provide additional details relevant to growth and characterization of polarity-controlled GaN/ AlN/Si (111) nucleation layers, including the AlN growth diagram, cross-correlation of GaN nucleation layer RHEED, and polarity-sensitive-etch tests, GaN nucleation layer surface morphology, and calculated RHEED patterns (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Research performed in part at the NIST Center for Nanoscale Science and Technology. REFERENCES

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