Extremely High- and Low-Density of Ga Droplets on GaAs{111}A,B

Dec 10, 2014 - Formation processes of Ga droplets on polar (111)A and (111)B surfaces of GaAs have been investigated. A single Ga atom forms a stable ...
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Extremely High- and Low-Density of Ga Droplets on GaAs{111}A,B: Surface-Polarity Dependence Akihiro Ohtake,*,† Neul Ha,†,‡ and Takaaki Mano† †

National Institute for Materials Science (NIMS), Tsukuba 305-0044, Japan Graduate School of Engineering, Kyushu University, NIMS, Tsukuba 305-0044, Japan

‡

ABSTRACT: Formation processes of Ga droplets on polar (111)A and (111)B surfaces of GaAs have been investigated. A single Ga atom forms a stable nucleus on the (111)A surface, so that the formation of extremely high-density of Ga droplets is achieved (2.8 × 1012 cm−2). On the (111)B surface, the initial Ga deposition on both As-rich (2 × 2) and Ga-rich (√19 × √19) reconstructions leads to the formation of a twodimensional GaAs layer having a more Ga-rich (3 × 2) reconstruction. The Ga droplets are formed on the (3 × 2) surface with their densities being 4 orders of magnitude lower than those for the (111)A orientation.

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INTRODUCTION Semiconductor quantum dots (QDs) are the subject of great interest for optoelectronic device applications. For selfassembly of QDs, droplet epitaxy1 has recently received considerable attention, because of its flexibility regarding the choice of the QD materials. In this method, liquid droplets of a group III element are first formed, which are crystallized by the subsequent irradiation of a group V flux. The size, shape, and density of QDs could be well-controlled by the growth conditions, such as temperature,2−5 the beam fluxes of group III and V elements,4,7 crystal orientation of substrates,3,6 and the atomic structure of the initial surfaces.8 While QDs are usually grown on the (001)-oriented substrate, the use of the (n11) orientations (n = 1−5) has recently attracted increasing interest because of the unique properties of (n11)-QDs:3,6,9,10 Highly symmetric excitons with significantly reduced fine-structure splitting are achieved in QDs fabricated on the (111)A6 and (111)B substrates,9 which have great potential for the generation of entangled photon pairs. For (311)A and (511)A, while the formation of highdensity Ga droplets was reported on GaAs(311)A, amounting to 7.3 × 1011 cm−2,3 the density on the (511)A is even lower than that on the (001) substrate.10 In this paper, we report on the formation processes of Ga droplets on the GaAs{111}A, B surfaces, especially paying attention to the effect of the surface polarity. It has been wellestablished that the (111)A and (111)B polar surfaces exhibit altogether different reconstructed structures.11 The Gastabilized (√19 × √19)R23.4° and As-stabilized (2 × 2) reconstructions are formed on the (111)B surface. On the other hand, on the (111)A surface, only a Ga-stabilized (2 × 2) reconstruction is observed under the conventional molecularbeam epitaxy (MBE) conditions. Here, we show the evidence © 2014 American Chemical Society

that the size and density of Ga droplets strongly depend on the polarity of the initial {111} surfaces. The extremely highdensity of Ga droplets exceeding 1012 cm−2 is formed on the (111)A surface. On the other hand, the density on the (111)B surface is lower by more than 4 orders of magnitude, irrespective of the initial surface reconstructions. We show that a single Ga adatom becomes a stable nucleus on the (111)A surface, which leads to the formation of high density of droplets.

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EXPERIMENTAL PROCEDURE

The experiments were performed in a system of interconnecting ultrahigh vacuum chambers for MBE growth and for online surface characterization by means of scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS).12 The n-type doped (Si, (1−4) × 1018 cm−3) GaAs(111)A and (111)B substrates were employed in this study. The native oxide was removed by heating the samples at 640 °C. Thin nondoped homoepitaxial layers were grown on the thermally cleaned (111)A [(111)B] substrate with an As4/Ga flux ratio of ∼100 [∼20] and at a growth temperature of 450 °C [650 °C]. After a subsequent thermal annealing at 600 °C under the As4 flux, the (111)A and (111)B surfaces show clean Ga-stabilized (2 × 2) and (√19 × √19) structures, respectively. The As-rich (2 × 2) reconstructions on (111)B were obtained by cooling the sample below 450 °C and by turning off the As flux. On the other hand, to maintain the Ga-rich (√19 × √19) structures at lower temperatures, the As flux was turned off at 560 °C, then the sample was kept at 460 °C until the residual As pressure was decreased to below 1 × 10−10 Torr. Ga was deposited at a rate of 0.025 ML/s at a substrate temperature of 200 °C. Here, 1 ML is defined as 6.26 × 1014 atoms/cm2, which is the site-number density of the unreconstructed GaAs(001) surface. During Received: October 16, 2014 Revised: November 27, 2014 Published: December 10, 2014 485

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the Ga deposition, residual As pressure, measured at the sample position, was kept below 5 × 10−11 Torr. All the STM images were acquired at room temperature using electrochemically etched tungsten tips. XPS measurements were carried out by using monochromatic Al Kα radiation (1486.6 eV). Photoelectrons were detected at an emission angle of 35° from the surface.

However, the present STM results clearly show that initially deposited Ga atoms are consumed to the formation of droplets without filling up the vacancy site. In addition, the (2 × 2) RHEED patterns were visible even after the 1 ML deposition, while the surface with the Ga-vacancy site being occupied by the Ga adatom has the (1 × 1) symmetry. This is in good agreement with the previous calculations: the vacancy site is not the most energetically stable one for Ga adatoms.14 As shown in Figure 1b,c, altogether different growth features were observed on the (111)B surface: two-dimensional islands are formed on the (111)B surfaces, irrespective of the initial reconstructions. Since (√19 × √19) and (2 × 2) reconstructions on the (111)B surface are enriched by ∼0.6 and 1.5 ML of As, respectively, as compared with the (111)A(2 × 2) reconstruction, it is most likely that the initially arrived Ga atoms on the (111)B surfaces are bound with excess As atoms to form a more Ga-rich two-dimensional layer. The initial (√19 × √19) and (2 × 2) reconstructions were almost fully covered by the two-dimensional islands at ∼1.0 and ∼1.5 ML, respectively [Figures 1(b)-C and 1(c)-C]. The formation of Ga droplets was confirmed by atomic force microscopy (AFM) observations, as shown in panels (b)-D and (c)-D in Figure 1. The averaged size of the droplets is significantly larger than that on the (111)A surface (170 nm for (2 × 2) and 190 nm for (√19 × √19)) and the density is more than 4 orders of magnitude lower than that on the (111)A-(2 × 2) surface [2.0 × 107 cm−2 for (2 × 2) and 1.5 × 107 cm−2 for (√19 × √19)].15 We note that the density of droplets are too low to be observed by STM. The critical thickness for the formation of Ga droplets on (111)B is confirmed by the XPS measurements. Figure 2 shows

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RESULTS AND DISCUSSION Figure 1a,b,c shows STM images of the Ga-rich (111)A-(2 × 2), Ga-rich (111)B-(√19 × √19), and As-rich (111)B-(2 × 2)

Figure 1. Typical filled-state STM images showing the nucleation of Ga droplets on the Ga-rich (111)A-(2 × 2) (a), Ga-rich (111)B-(√19 × √19) (b), and As-rich (111)B-(2 × 2) (c) surfaces. The image dimension is 200 nm × 200 nm. The insets show magnified images (8 nm × 8 nm). (b)-D and (c)-D show AFM images (5 μm × 5 μm).

surfaces, respectively, taken before (A) and after the deposition of Ga atoms (B−D). On the Ga-rich (111)A-(2 × 2) surface, the Ga atoms form droplets at the very initial stage of the growth (0.05 ML). The size of droplets increases as the growth proceeds averaged diameter = 3.1 nm at 0.05 ML and 7.2 nm at 1 ML), while the density of droplets is almost constant in the range of 0.05−1.0 ML (8 × 1011 to 1 × 1012 cm−2). Thus, it is likely that the arriving Ga atoms after the 0.05 ML deposition have a higher probability to be incorporated into existing droplets than to form a new nucleus. The GaAs(111)A-(2 × 2) surface has the Ga-vacancy buckling structure, in which one Ga atom per (2 × 2) unit cell is missing at the outermost Ga layer.13 Since the layer-bylayer epitaxial growth of GaAs on the (111)A surface requires the Ga-vacancy sites to be completely occupied by Ga atoms, it is naturally expected that the initial deposition of Ga atoms leads to the preferential adsorption at the Ga-vacancy sites.

Figure 2. Photoelectron intensity ratios of As 3d/Ga 3d plotted as a function of Ga thickness. The arrows indicate the critical thicknesses for the formation of Ga droplets on (111)B. Lines are guides for the eyes.

photoelectron intensity ratios of As 3d/Ga 3d as a function of Ga thickness. As indicated by triangles and squares, the As/Ga ratios for the (111)B-(√19 × √19) and -(2 × 2) surfaces linearly decrease with increasing Ga film thickness below 0.8 and 1.6 ML, respectively (arrows in Figure 2), beyond which the values decrease more gradually. Such an abrupt change in the slope indicates the onset of three-dimensional growth that covers the surface at a much reduced rate: since the average height of Ga droplets is 35−40 nm, which is much larger than the escape depth of As 3d and Ga 3d photoelectrons (1.3−1.6 nm), the Ga 3d photoelectrons generated at the inner part of 486

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large droplet are hardly detected, which results in a lower reduction rate in the As/Ga ratio. On the other hand, the results for (111)A-(2 × 2) (circles) show a monotonic decrease. Since the height of Ga droplets on (111)A is typically 0.4−1.0 nm, which is much smaller than those on (111)B, the Ga 3d photoelectrons generated at the inside of droplets are detected without significant attenuation, so that the Ga/As ratio shows a higher reduction rate than those for Ga droplets on (111)B. The surface of the two-dimensional island formed on the (111)B surface shows three-domain (3 × 2) reconstructions, as can be seen in the insets of panels (b)-C and (c)-C in Figure 1. The (3 × 2) reconstructions formed on the initial (2 × 2) surface are rather disordered as compared to the case on the (√19 × √19) surface. Although the (3 × 2) reconstruction on GaAs(111)B has not been previously reported, and the atomic structure is unknown, the surface composition could be estimated from the present XPS measurements. The (111)B(√19 × √19) and -(2 × 2) surfaces have a hexagonal-ring structure and As-trimer structure, respectively. The As coverages of these models are 0.84−0.89 ML16 for (√19 × √19) and 1.75 ML17 for (2 × 2),18 and the (3 × 2) reconstructions are completed after the deposition of 0.8 and 1.6 ML on (√19 × √19) and (2 × 2), respectively. Since the deposition of 1 ML Ga corresponds to the Ga coverage of 86.6% on (111) surfaces, the surface As coverage of the (3 × 2) structure is roughly estimated to be 0.14−0.35 ML, which is rather close to the value for the (111)A-(2 × 2) structure (0.25 ML). Here, it is interesting to note that the droplet densities on the A-(2 × 2) and B-(3 × 2) surfaces differ by more than 4 orders of magnitude, while surface compositions of A-(2 × 2) and B(3 × 2) are nearly equal. It is plausible to consider that the atomic structures of these surfaces play an important role in the formation of Ga droplets. However, no definitive answer is available without further studies on the atomic structure of the (3 × 2) surface. Figure 3a,d shows STM images of Ga droplets on GaAs(111)A formed at different substrate temperatures. The size and density of droplets increases and decreases, respectively, with increasing substrate temperature. The droplet density at 30 °C is 2.8 × 1012 cm−2, which is significantly higher than the values on the (311)A substrate (7.3 × 1011 cm−2).3 Figure 4a shows the density of Ga droplets plotted as a function of substrate temperature. The results for (001)-c(4 × 4)β are also shown for comparison. The droplet densities on (111)A-(2 × 2) are significantly higher than those for c(4 × 4). The measured droplet densities (n) below 200 °C are wellreproduced by a well-known scaling law predicted by classical nucleation theory19

⎛ E ⎞ n ∝ jF p exp⎜ ⎟ ⎝ kBT ⎠

Figure 3. STM images of Ga droplets on GaAs(111)A formed at different substrate temperatures: 300 (a), 200 (b), 100 (c), and 30 °C. The image dimension is 200 nm × 200 nm. −1 ⎡1 ⎛ E ⎞ ⎛ E + Er ⎞⎤ n = jF ⎢ exp⎜ − ⎟ + tr exp⎜ − ⎟⎥ ⎢⎣ ν kBT ⎠⎥⎦ ⎝ kBT ⎠ ⎝ p

(2)

where ν is the vibrational frequency and tr is the ripening time. Since the sample was quenched to room temperature just after the droplet formation, we assumed that the ripening time tr is equal to the growth time and that the ripening hardly takes place during the cooling. Under these assumptions, using the value of Er = 1.67 eV, the experimental behaviors above 200 °C could be explained by the extended scaling law for both (111)A and c(4 × 4)β, as shown by dashed curves. The activation energy for (111)A (0.13 eV) is significantly smaller than that estimated for (001)-c(4 × 4) (0.32 eV). At first sight, the present result is peculiar, because the droplet density is generally decreased as the activation energy is increased. According to classical nucleation theory,19 E = p(Es + Ei/i), and p = i/(i + 2.5) for three-dimensional nucleus, where i is the critical cluster size, Ei is the formation energy of the critical nucleus, and Es is the energy barrier for adatom diffusion. The values of p and i could be estimated by plotting the droplet density as a function of the deposition rate. As shown in Figure 4b, the dependence of the droplet density on the deposition rate shows a power low, and the critical nucleus size i on (111)A can be determined to be 1. This means that a single Ga atom becomes a stable nucleus on the (111)A surface. Since Ei is the binding energy that is gained when i single adatoms form a cluster of the size i, the value of Ei is 0 eV. Thus, we obtained Es = 0.45 eV, which is in good agreement with that obtained from the previous DFT calculations (0.4 eV).14 On the other hand, for c(4 × 4), the value i is estimated to be 5. Using the value of Ei/i = 0.15 in ref 4, Es is estimated to be 0.33 eV,20 which is quite close to that (0.30 eV) reported in ref 4. From these results, we conclude that the diffusion of Ga atoms on the (111)A-(2 × 2) surface is less enhanced, resulting in the higher droplet density.

(1)

Here, F is the deposition rate of Ga, kB is the Boltzmann’s constant, and j is a constant. The activation energy (E) of 0.13 eV yields a good agreement between the experiment and the scaling model below 200 °C (solid lines in Figure 4). On the other hand, above 200 °C, the measured density is lower than the value predicted by the scaling law. The reduction of the droplet density could be ascribed to the onset of Ostward ripening: Heyn et al.4 proposed the extended scaling law 487

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(2) Mano, T.; Kuroda, K.; Mitsuishi, K.; Noda, T.; Sakoda, K. J. Cryst. Growth 2009, 311, 1828−1831. (3) Jo, M.; Mano, T.; Sakuma, Y.; Sakoda, K. Appl. Phys. Lett. 2012, 100, 212113. (4) Heyn, Ch.; Stemmann, A.; Schramm, A.; Welsch, H.; Hansen, W.; Nemcsics, Á . Phys. Rev. B 2007, 76, 075317. (5) Jo, M.; Mano, T.; Sakoda, K. Cryst. Growth Des. 2011, 11, 4647− 4651. (6) Mano, T.; Abbarchi, M.; Kuroda, T.; McSkimming, B.; Ohtake, A.; Mitsuishi, K.; Sakoda, K. Appl. Phys. Express 2010, 3, 065203. (7) Mano, T.; Kuroda, T.; Sanguinetti, S.; Ochiai, T.; Tateno, T.; Kim, J.; Noda, T.; Kawabe, M.; Sakoda, K.; Kido, G.; Koguchi, N. Nano Lett. 2005, 5, 425−428. (8) Ohtake, A.; Mano, T.; Hagiwara, A.; Nakamura, J. Cryst. Growth Des. 2014, 14, 3110−3115. (9) Treu, J.; Schneider, C.; Huggenberger, A.; Braun, T.; Reitzenstein, S.; Höfling, S.; Kamp, M. Appl. Phys. Lett. 2012, 101, 022102. (10) AbuWaar, Z. Y.; Wang, Z. M.; Lee, J. H.; Salamo, G. J. Nanotechnology 2006, 17, 4037−4040. (11) Woolf, D. A.; Westwood, D. I.; Williams, R. H. Appl. Phys. Lett. 1993, 62, 1370−1372. (12) Ohtake, A. Surf. Sci. Rep. 2008, 63, 295−327. (13) Tong, S. Y.; Xu, G.; Mei, W. N. Phys. Rev. Lett. 1984, 52, 1693− 1696. (14) Taguchi, A.; Shiraishi, K.; Ito, T. Phys. Rev. B 1999, 60, 11509− 11513. (15) In this comparison, we assume that the total amounts of Ga atoms forming the droplets on the three types of surfaces are nearly identical to 1 ML, because ∼0.8 ML and ∼1.6 ML of Ga atoms are not consumed to the formation of Ga droplets on the (√19 × √19) and (2 × 2) surfaces, respectively. (16) Koga, H. Phys. Rev. B 2010, 82, 113301. (17) Biegelsen, D. K.; Bringans, R. D.; Northrup, J. E.; Swarts, L.-E. Phys. Rev. Lett. 1990, 65, 452−455. (18) The As coverage of 1 ML corresponds to the ideal Asterminated (111)B surface. (19) Venables, J. A.; Spiller, G. D. T.; Hanbücken, M. Rep. Prog. Phys. 1984, 47, 399−459. (20) We note that the value for c(4 × 4) is smaller than that estimated in our earlier paper (0.49 eV),8 because the analysis in ref 8 used the value of i = 2.5, as in the case in ref 4.

Figure 4. Densities of Ga droplets on GaAs(111)A plotted as a function of substrate temperature (a) and the deposition rate (b). The results in (b) were obtained at a substrate temperature of 200 °C. The results for GaAs(001)-c(4 × 4)β are also shown for comparison. The amount of Ga atoms consumed for the droplet formation is 1 ML for both surfaces.

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CONCLUSION We have studied the effect of the surface polarity on the formation processes of Ga droplets on GaAs{111}A,B. The size and density of droplets strongly depend on the polarity of the initial {111} surface. The density of Ga droplets on the (111)A surface exceeds 1012 cm−2, which is more than 4 orders of magnitude higher than that on the (111)B surface. It turns out that a single Ga atom becomes a stable nucleus, resulting in the formation of high density of droplets.

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NOTE ADDED AFTER ASAP PUBLICATION This paper was published ASAP on December 19, 2014, with an error to Figure 4. The corrected version was reposted on December 22, 2014.

AUTHOR INFORMATION

Corresponding Author

*Phone: +81-29-860-4198. Fax: +81-29-860-4753. E-mail: [email protected] (A.O.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Helpful discussions with Dr. Y. Sakuma and Dr. M. Jo are gratefully acknowledged. REFERENCES

(1) Koguchi, N.; Takahashi, S.; Chikyow, T. J. Cryst. Growth 1991, 111, 688−692. 488

dx.doi.org/10.1021/cg501545n | Cryst. Growth Des. 2015, 15, 485−488