with Sub-100-nm Silica Nanoparticles by Molecular Beam Epitaxy

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Lithography-free Nanoscale Patterned Growth of GaAs on Si(001) with Sub-100-nm Silica Nanoparticles by Molecular Beam Epitaxy S. C. Lee,*,† L. R. Dawson,† S. H. Huang,‡ and S. R. J. Brueck† †

Center for High Technology Materials and Department of Electrical and Computer Engineering, University of New Mexico, 1313 Goddard SE, Albuquerque, New Mexico 87106, United States ‡ Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, United States ABSTRACT: Lithography-free, nanoscale patterned, molecular beam epitaxial growth of GaAs on Si is demonstrated on a Si(001) substrate coated with a dense, single-particlehigh, stack of ∼80-nm-diameter silica nanoparticles (NPs). The NP stack plays the role of a high aspect ratio, deep sub-100 nm opening area SiO2 pattern which is small enough for nanoscale patterned growth of large lattice-mismatched heterostructures. The GaAs, selectively grown through the interparticle spaces, proceeds over the NP stack initially with island formation and ultimately buries the NP stack by epitaxial lateral overgrowth and coalescence. X-ray diffraction confirms that the GaAs grown over the NP stack has highly improved crystallinity as compared with a reference growth on unpatterned Si. This is explained by necking of the defects propagating along {111} by the small opening, high aspect ratio characteristic of the NP stack.

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train-relieved, defect-free, epitaxial growth of heterostructures with large lattice-mismatch is an important direction for next generation electrical/optoelectronic devices and multifunctional integrated circuits. The growth of device-quality GaAs on Si of misfit, f ∼ 4.2%, has been investigated for the last several decades. Various strategies have been reported: low-high twostep growth with thermal cycling;1 postgrowth annealing;2 strained superlattice buffers;3 growth on 2°-off Si(001) and patterned substrates;4,5 and nanoscale patterned growth (NPG) supported by the Luryi Suhir model.6,7 Strain-relieved, dislocation-free In0.06Ga0.94As/GaAs heteroepitaxy (f ∼ 0.4%) has been demonstrated with NPG for a pattern period of 355 nm, fabricated by interferometric lithography and dry etching.8 While there is some controversy on the applicability of the model,9 calculations universally suggest that a larger misfit requires a smaller pattern period for strain relief without formation of misfit dislocations and that a pattern period less than 100 nm is required for GaAs on Si.6,7,10 12 This scale is beyond the reach of many easily accessible large-area lithography techniques. This patterning scale is, however, achievable by using a selfassembled structure that provides an array of holes at a sub100 nm regime over an entire substrate without sophisticated lithography technologies. In this work, ∼80 nm-diameter silica nanoparticles (NPs) which provide cost-effective, large scale, sub-100 nm coating are employed for the surface patterning in NPG of GaAs on Si. NPs have been used previously for maskless nanopatterning.13 A simple spin coating is applied to cover large areas providing a nonlithographic substrate patterning. Commercial NPs of non-negligible size dispersion (>10%) were used for this initial experiment. Nonetheless, the NP stack is uniform on a macroscopic scale, can cover full wafers, and provides a high aspect ratio that is adequate to examine large lattice-mismatched heteroepitaxy. r 2011 American Chemical Society

The basic procedure of the NPG of GaAs on Si by silica NPs is schematically illustrated in Figure 1. First, a 100-nm-thick GaAs buffer layer was deposited on an oxide-free, nominally (001)oriented Si substrate at 450 °C by molecular beam epitaxy (MBE) (Figure 1a). This relatively thick GaAs buffer layer was grown on a clean Si surface to avoid any issues of contamination/ oxidation of the Si surface during the ex-situ NP coating process and to induce complete strain relaxation. The sample was removed from the growth chamber after the buffer deposition and cut into two pieces: one for control and the other for coating with a single-particle-thick stack of silica NPs of diameter d ∼ 80 nm by spin coating (Figure 1b). In Figure 1, a hexagonal array of identical NPs is assumed, but the actual configuration resulting from the spin-coating is a mixture of square and hexagonal arrays due to the particle size and shape fluctuations. Colloidal concentration and spin speed were controlled to form a dense single-particle-thick stack of NPs on the GaAs buffer. In this work, an ∼8 wt % concentration, colloidal silica NP suspension was employed, and spin speed was set to 4000 rpm. These conditions are highly effective over a wafer up to ∼50 mm in diameter. After a bake at 110 °C for 10 min to drive off residual water remaining from the spin coating, the sample covered with the NP stack was reloaded into the growth chamber for a second GaAs deposition. For selective epitaxy of the second GaAs layer, the growth temperature was set to 600 °C. This is higher than the temperature for the thermal desorption of native oxide from the buffer surface (∼580 °C). Before deposition of the second layer, however, the temperature of the reloaded sample was kept at Received: October 14, 2010 Revised: June 26, 2011 Published: July 26, 2011 3673

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Figure 1. (a d) Schematic growth procedure of the NP stack sample. (e) Schematic illustration of a part of an NP hexagonal array in top and side view. The parameters used in the text (opening width, s, NP diameter, d) are indicated. (f) Top view SEM image of the NP stack on the GaAs buffer.

620 °C for ∼20 min under As-rich conditions (As beam equivalent pressure ∼3  10 6 Torr) for complete removal of the native oxide that might be formed on the GaAs buffer during the ex situ process. This procedure was confirmed with a control sample will be discussed below. Optical pyrometry was employed to monitor temperatures. After cooling the sample to 600 °C, the second GaAs layer was selectively grown from the buffer layer through the interparticle spaces in the NP stack with the growth rate of 0.03 monolayer (ML)/s (Figure 1c and e). Here, the interparticle space means the volume enclosed by NPs in a singleparticle-thick stack. In the case of the hexagonal array shown in Figure 1e, it corresponds to the volume enclosed by three closepacked spherical NPs on the GaAs surface that form a triangular shape with the lateral dimension (or opening width), s, in top view. Then, as defined in Figure 1e, s of individual interparticle √ spaces formed by the d = 80 nm NPs, is 3d/4∼35 nm in a hexagonal array. As seen in Figure 1f, however, the actual NP stack employed in this work varies somewhat as a result of the particle size fluctuations and ranges from 10- to 20-nm, larger than, but on the same order as, the average spacing of misfit dislocations in completely relaxed GaAs on Si(001) ∼ 4.7 nm.14 Thus, the effective pattern was a ∼80 nm-period SiO2 hexagonal symmetry mask with a ∼10- to 20-nm opening in each period. At the specified growth conditions, selective NPG is available over this NP stack.15 The second growth layer seeded on the GaAs buffer layer grew through the interparticle spaces (Figure 1c) and ultimately covered the NP stack by lateral growth and coalescence (Figure 1d). The thickness of the second GaAs deposition was ∼1 μm.

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Figure 2. (a) Low magnification TEM image revealing a whole cross section of the NP stack sample. Inset: an SEM image of an NP-covered sample with ∼150 nm deposition. (b and c) Detailed cross sections around the NP stack. An arrow in part b indicates unfilled interparticle space (vacancy). Other black arrowheads and a white arrowhead in part c point to stacking faults along and a defect off from{111}, respectively.

The inset of Figure 2a shows scanning electron microscopy (SEM) images taken from the sample at a stage between Figure 1c and d. The deposition thickness corresponds to ∼150 nm or roughly 2d. In Figure 1f, a noticeable NP size/ shape fluctuation is observed from the top view of the NP stack coated on the GaAs buffer. The actual interparticle space thus can vary and is often considerably smaller than that estimated from Figure 1e. Because of this variation, the growth does not proceed evenly; as seen below, the island-type growth initiated from the wider interparticle spaces prevails at the beginning of selective NPG over the NP stack. As growth proceeds, the GaAs islands coalesce into a single GaAs layer over the NP stack. Figure 2 presents cross-sectional transmission electron microscopy (TEM) images taken from the sample at the stage of Figure 1d. As mentioned earlier, the top surface of the GaAs in Figure 2a is not very flat as a result of the uneven filling process and associated coalescence. This is partly because NP nonuniformity randomizes the nanoscale faceting that dynamically proceeds on the surface of the island-type epilayer grown along individual interparticle spaces in the filling process and ultimately affects the top surface morphology with the irregular facet evolution during the lateral growth and coalescence over the NP stack. Up to 1 μm deposition, the top surface has a roughness comparable to the NP size fluctuation of ∼80 nm. Figure 2b reveals the details of the filling process. In this 3674

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Figure 4. XRD curve from the NP stack sample. The peaks around ∼32° are from (002) reflection. The inset is a plot of normalized intensity vs angle around the (004) reflection from the NP stack and the control sample. The green arrow indicates the weak peak from the NP stack sample.

Figure 3. (a) A two-beam bright field TEM image taken near the NP stack. A vacancy similar to that in Figure 2b is indicated by an arrow. Like the case of Figure 2c, black arrowheads indicate some of the defects blocked by the NP stack. (b) A plan-view TEM image taken from the top surface of the NP stack sample.

figure, most interparticle spaces are filled up by GaAs. However, as indicated by an arrow in Figure 2b, an unfilled space (vacancy) that is relatively narrower than other interparticle spaces in the given cross section is also observed. The apparent ratio of unfilling to filling measured from TEM is ∼0.1. The presence of such vacant interparticle spaces means that the vertical overgrowth is followed by lateral growth leading to the coalescence of the GaAs islands over the NP stack. As seen in Figure 2c, the NP stack effectively blocks most of the defects in the dark area under the individual NPs and the stacking faults indicated by black arrowheads which evolve from the GaAs buffer. This is feasible because the NP stack has a small open area ratio (= the area of interparticle space in the top view/ unit cell area ∼ 0.04 in Figure 1f)√and high aspect ratio {[= NP height/opening width = d/s (=4/ 3 in a hexagonal array)] > 1; cf. Figure 1e} which provides an additional necking effect to block misfit dislocations as well as stacking faults propagating from the GaAs buffer along {111}.16,17 Also, the stacking faults extending from the buffer but terminated in the middle of the NP stack in Figure 2c prove the continuity of the crystal structure from the buffer to the second epitaxial layer in spite of the presence of the NP stack. On the other hand, other defects such as the line on the cleaved (110) plane (white arrowhead) that is off from {111} appear in Figure 2c. While in-depth study of the defects observed in Figure 2 is beyond the scope of this work, such a plane defect could result from uneven coalescence between the GaAs islands observed in Figure 2 and will be reduced in density with a more uniform starting NP distribution. Figure 3a is a two-beam bright field cross-sectional TEM image that confirms the filling process and defect formation/ distribution observed in Figure 2. Like the case of Figure 2b, all

interparticle spaces wider than the vacancy indicated with an arrow are filled with GaAs. Furthermore, most of the defects generated in the buffer are confined in the same region by the NP stack and physically blocked by it although some of them pass it through interparticle spaces. The contrast between the buffer and the second GaAs layer (below and above the NP stack) in Figure 3a implies a difference of the crystalline quality and stress distribution. This contrast is similar to and consistent with that observed in Figure 2. Figure 3b is a plan-view TEM taken from the top surface which enables the measurement of misfit dislocation density. As seen in Figure 3b, dislocation is extremely rare at the top surface, and its density calculated from TEM does not exceed 2  105/cm2.18 Some of the noticeable contrast change in the plan-view image could be due to the rough top surface with varying transmission associated with the thickness fluctuation. Figures 2 and 3, therefore, provide clear evidence of the role of the NP stack in NPG—blocking of the defects generated in the strain relief process during growth of the GaAs buffer. Figure 4 is an X-ray diffraction (XRD) curve obtained from the as-grown sample. In this figure, the GaAs(004) and Si(004) reflection peaks dominate. The inset to Figure 4 shows the (004) reflections from the samples with and without (control) a NP stack but under the same growth procedure and conditions are shown. In the inset, as indicated by an arrow, the NP stack sample has a weak peak to higher angles which is absent in the control sample. Several possible explanations are available for this shoulder peak but local orientation deviation in the coalescence process is the most probable. Peak splitting in XRD for epitaxial lateral overgrowth (ELO) of GaAs on patterned substrates has been reported.19 As seen in the inset of Figure 2a and Figures 2b and 3a, the growth proceeds with island formation at an initial stage, and all the interparticle spaces were not filled up but the NP stack was ultimately covered with GaAs in the second layer growth. These observations imply that ELO is another major growth modality as the growth proceeds beyond the NP stack. The vertical lattice constants of the NP stack sample and the control sample from the inset are 0.5635 and 0.5639 nm, respectively, slightly less than the lattice constant of bulk GaAs = 0.5653 nm.20 It is not clear whether this results from tensile stress due to the difference of thermal expansion coefficients between GaAs and Si especially in case of the (001) 3675

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Crystal Growth & Design substrate employed in this work, without the 2° tilt often used for GaAs on Si growth.21 Further experiments are required to analyze the role of the NP stack on strain relief for GaAs on Si. As shown in the inset of Figure 4, the (004) reflection peak has a significantly reduced (∼1/4) full width at half-maximum (fwhm) compared with the control sample. Also, as indicated in the inset, the peak intensity is much higher than that from the control sample, ∼30. These results provide clear evidence that the crystallinity is highly improved through the necking of the misfit dislocations and stacking faults by the NP stack. The open area ratio depends on the NP arrangement in the coating process. For example, it is 0.21 in a square array but decreases to 0.09 in the hexagonal array shown in Figure 1. The actual open area ratio in Figure 2a is reduced further and only ∼0.04, owing to the accommodation to the NP size nonuniformity. This means only ∼4% of propagating defects can pass through each unit cell. For perfect necking of the defects along {111}, the aspect ratio of the NP stack must be greater than tan 54.7°. With θ ∼ 60° in Figure 1e, trigonometry roughly shows that the finite aspect ratio of the NP stack accompanies at least additional ∼40% reduction of defect propagation through each interparticle space in a hexagonal array. From the geometry and the experimental results, it is conjectured that the defect density is reduced at least 100 with the NP stack in Figure 2. This rough estimate based on the NP stack shown in Figure 1f supports the reduced dislocation density near the top surface measured from plan-view TEM in Figure 3b. Sub-100-nm NPG will have an impact on future large-latticemismatch heteroepitaxy. Silica NPs are available with a range of sizes, down to ∼6 nm diameter or less, and with varying degrees of monodispersion. A hexagonal close-packed, double or triple NP stack formed by such small particles should have sufficiently high aspect ratio and extremely small open area ratio for the perfect blocking/necking effect discussed above. Also, smaller NPs can relieve the peak splitting by ELO observed in XRD with earlier coalescence when the film is thinner. Once the NP stack is fully covered by uniform ELO and early coalescence, the growth rate, lowered for the selective growth mode, can be increased for any additional growth. Further study examining the trends with different size silica NPs is presently underway. By resolving some critical issues (size uniformity and particle arrangement in coating processes), NPG by sub-10 nm NP stacks can be a promising solution not only to III V on Si but also to large latticemismatched III V on GaAs, such as GaSb on GaAs, where the Luryi Suhir model can be directly examined. In summary, lithography-free NPG of GaAs on Si(001) has been examined with ∼80-nm-diameter single stack silica NPs. The NP stack, playing the role of a sub-100-nm masking film for selective epitaxy, is characterized by high aspect ratio and ∼10 20-nm-wide interparticle spaces, small enough for GaAs on Si. Epitaxial growth is performed with the growth of a 100-nmthick GaAs buffer on a Si(001) substrate at 450 °C, ex situ NP spin-coating over the GaAs buffer, and a second selective epitaxy of 1-μm-thick GaAs over the NP stack by ELO and coalescence at 600 °C. The NPs effectively block most of the defects propagating along {111} such as misfit dislocations and stacking faults as a result of high aspect and small open area ratios, and lead to highly improved crystallinity. The process is quite simple and involves only a few, conventional processing steps; the absence of any nanoscale lithographic steps make it cost-effective and easily accessible to deep sub-100-nm NPG for large lattice-mismatched heteroepitaxy.

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was partially supported by Sandia National Laboratories. ’ REFERENCES (1) Shimizu, M.; Sugawara, K.; Sakurai, T. J. Jpn. Assoc. Cryst. Growth 1986, 13, 253. (2) Lee, J. W.; Shichijo, H.; Tsai, H. L.; Matyi, R. J. Appl. Phys. Lett. 1987, 50, 31–33. (3) Yamaguchi, M.; Nishioka, T.; Suge, M. Appl. Phys. Lett. 1989, 54, 24–26. (4) Hashimoto, A.; Fukunaga, T.; Watanabe, N. Appl. Phys. Lett. 1989, 54, 998–1000. (5) Lingunis, E. H.; Haegel, N. M.; Karam, N. H. Appl. Phys. Lett. 1991, 59, 3428–3430. (6) Luyri, S.; Suhir, E. Appl. Phys. Lett. 1986, 49, 140–142. (7) Zubia, D.; Hersee, S. D. J. Appl. Phys. 1999, 85, 6492–6496. (8) Lee, S. C.; Dawson, L. R.; Pattada, B.; Brueck, S. R. J.; Jiang, Y.-B.; Xu, H. F. Appl. Phys. Lett. 2004, 85, 4181–4183. (9) Xie, Y. H.; Bean, J. C. J. Vac. Sci. Technol. 1990, B8, 227–231. (10) Fischer, A.; Richter, H. Appl. Phys. Lett. 1992, 61, 2656–2658. (11) Huang, F. Y. Phys. Rev. Lett. 2000, 85, 784–787. (12) Ye, H.; Lu, P.; Yu, Z.; Han, L. Semicond. Sci. Technol. 2009, 24, 025029. (13) See, for example: Haynes, C. L.; van Duyne, R. P. J. Phys. Chem. 2001, B105, 5599–5611. (14) Asai, K.; Kamei, K.; Katahama, H. Appl. Phys. Lett. 1997, 71, 701–703. (15) Lee, S. C.; Brueck, S. R. J. Appl. Phys. Lett. 2009, 94, 153110. (16) Typical NPG using SiO2 films has an aspect ratio significantly lower than 1. See refs 8 and 15 for details. (17) Langdo, T. A.; Leitz, C. W.; Currie, M. T.; Fitzgerald, E. A.; Lochtefeld, A.; Antoniadis, D. A. Appl. Phys. Lett. 2000, 76, 3700–3702. (18) The dislocation density was based on the multiple observation spots (>20) and their spatial distribution on the specimen for plan-view TEM. For this reason, only the upper limit on it was proposed as an experimental result. (19) Zytkiewicz, Z. R.; Domagala, J. Appl. Phys. Lett. 1999, 75, 2749–2751. (20) Semiconductors, Group IV Elements and III-V compounds. In Data in Science and Technology; Springer-Verlag: 1991. (21) Pearton, S. R.; Abermathy, C. R.; Caruso, R.; Vernon, S. M.; Short, K. T.; Brown, J. M.; Chu, S. N. G.; Stavola, M. J. Appl. Phys. 1988, 63, 775–783.

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