Article pubs.acs.org/crystal
Directions and Breakup of Self-Running In Droplets on Low-Index InP Surfaces Songphol Kanjanachuchai*,† and Chanan Euaruksakul‡ †
Semiconductor Device Research Laboratory, Department of Electrical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 10330, Thailand ‡ Synchrotron Light Research Institute, 111 University Avenue, Muang District, Nakhon Ratchasima 30000, Thailand S Supporting Information *
ABSTRACT: The nucleation and dynamics of multiple generations of In droplets formed from Langmuir evaporation of InP (001), (111)A, and (111)B surfaces are reported. In situ mirror electron microscopy reveals that the majority of first-generation, or mother, droplets break up immediately before they run from the nucleation sites, leaving behind daughter droplets and etch trails where more droplets emerge. These subsequent droplets grow with time and run once a critical size is reached. The breakup and running characteristics are explained in terms of crystallography, viscosity, chemical potential, and temperature and will likely affect the growth processes and designs of various droplet-catalyzed nanostructures and devices.
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interface.13 Similarly, Au droplets as small as 20 nm have been found to move laterally along the Au/Si interface upon annealing.14 Since Au and group III (Ga and In) droplets are important catalysts for NW growth, the self-running droplet phenomenon implies a self-running catalyst mechanism and, thus, a possibility to grow NWs for function and to simultaneously move them laterally (together with the catalyst droplets) for integration. So far, running Ga droplets have been observed on GaP (111)B, GaAs (001), (111)A and (111)B surfaces, and the gross features of their dynamics are well-understood.12,13,15,16 Running In droplets, on the other hand, are relatively unknown as their only reports are on a single surface, InAs (111)B, obtained by fast temperature ramp,17 making it difficult to repeat in different settings. Since the first prediction,12 the existence of self-propelled or self-running In droplets remains to be proven beyond doubt. In this article, we report the first observation of self-running In droplets on three important low-index InP surfaces: the (001), (111)A, and (111)B planes. The nucleation of In droplets and their subsequent dynamics are observed in situ using a highly surface sensitive mirror electron microscopic (MEM) technique. We found the running In droplets share some commonalities with, but also exhibit striking differences from, their Ga counterparts. In particular, we found that the In droplets almost always break up before running, leaving behind etch trails with complex branches from where more droplets
INTRODUCTION For the past decade, metallic droplets have been extensively employed in the growth of semiconductor nanostructures, particularly quantum dots1 (QDs) and nanowires2,3 (NWs). III−V QDs can now be routinely grown with near crystallographic perfection and with varying degrees of geometrical complexity by droplet epitaxy, where group III droplets are first formed and subsequently crystallized by exposure to group V species.4,5 Various semiconducting NWs can also be grown with good crystallinity using Au or group III droplets via the vapor− liquid−solid (VLS) or similar mechanism.6 These surface metallic droplets are static during droplet epitaxy of QDs but are mobile during VLS growth of NWs, where droplets are transported away from the original sites. The direction of the catalyzing droplets depends mainly on surface energetics at the droplet/semiconductor interface.6 For (111)B-oriented substrates, for example, zincblende NWs grow vertically, resulting in vertical transport of the droplets.7 For other low-index orientations, NWs may grow normal and/or at an angle normal to the surface, pushing the droplets off the original surface correspondingly. The ability to engineer droplet transports or dynamics during growth has enabled process control from the individual nanostructure level8,9 to the integration level.10,11 The majority of catalyst-assisted NW growths, however, rely on droplets transport in the vertical direction. Planar NWs, in contrast, rely on droplets lateral dynamics under unconventional growth conditions.6 Recently, Tersoff et al. reported the first observation of self-running group III droplets where Ga droplets formed from noncongruent evaporation of GaAs move laterally.12 These droplets have to reach a critical diameter around 1.9 μm before they started running along the Ga/GaAs © 2013 American Chemical Society
Received: November 13, 2013 Revised: December 19, 2013 Published: December 20, 2013 830
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emerge. The droplet dynamics are qualitatively explained in terms of chemical potential, crystallography, viscosity, and temperature. Our in situ observation of the In droplets on three different crystallographic planes unequivocally prove the existence of the self-running In droplets. The insights gained from our studies are not only of fundamental importance to those working in reactive wetting,18,19 branched structures,20 and droplet dynamics,21,22 but also of technological importance since InP is a basic optoelectronic material.23 Ultimately, the branching and breaking-up characteristics of self-running In droplets may lead to additional crystal design parameters, with a potential to increase the functionality and to enable high-level integration of self-catalyzed III−V micro- and nanostructures.
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Figure 1. Nomarski DIC images showing self-running In droplets on InP (a) (001) and (b) (111)A. The insets show ensembles of running droplets at early stages. The scale bars are 10 μm and also apply to the insets.
EXPERIMENTAL METHODS
The InP (001), (111)A, and (111)B epitaxial ready substrates are scribed and loaded into Elmitech’s low-energy electron microscopy (LEEM III) system without any chemical processing. The substrates are outgassed overnight by backside radiative heating at temperatures well below the congruent temperature (TC). The substrate temperature T is measured by a thermocouple and a pyrometer. The temperature is raised by increasing the filament current IF while the surface is imaged in real time. The base pressure of the system is 2 × 10−10 Torr. The pressure rises during heating but is kept below 5 × 10−9 Torr throughout the experiments. We are able to reproduce the running droplets by noticing critical surface changes and adjusting IF correspondingly. First, the originally smooth, featureless surface roughens as a result of thermal deoxidation. IF is kept constant as soon as surface roughening occurs. After a few minutes, the surface regains its original smoothness. In fact, even smoother surface is obtained, judging from the disappearance of oxide scratches and pinholes. IF ramping then resumes, until the surface roughens again, but this time as a result of noncongruent sublimation which occurs when T > TC. The greater loss of P results in an In-rich surface upon which In droplets nucleate, run, and ultimately merge with one another in the stated order. At the first sign of In droplets nucleation, IF ramping is halted and slightly reversed to prevent rapid decomposition. If this critical step is bypassed, the surface group III droplets will quickly reach late-stage coalescence growth regime typically found for both Ga/GaAs and In/InP systems.24 By slightly decreasing IF at the onset of the second surface roughening, we were able to control the droplet density and to slow down the subsequent running droplets at will. The process can then be observed in “slowmotion.” Generally, the droplets are allowed to run for approximately 30 min during which MEM images are taken. The active temperature TA for the observation of droplets dynamics on the InP (001), (111)A, and (111)B surfaces are 369, 388, and 344 °C, respectively. The uncertainty in thermocouple’s readings is estimated to be less than 10 °C. These temperatures can be taken as the upper limits of TC for the corresponding surfaces, since to nucleate droplets, the substrate temperature must be raised beyond TC. Afterward, IF is ramped down to zero. The substrates are allowed to cool, removed from the LEEM system, and characterized by Nomarski differential interference contrast (DIC) and scanning electron microscopy (SEM). The droplets are confirmed to be In-rich by energy dispersive X-ray spectroscopy (EDS, see Supporting Information for detailed spectra).
substrate through diffusion, and In from the substrate is incorporated into the droplet.25 The In droplets on the InP (001) surface, hereafter referred to as the In (001) droplets, have approximately equal probabilities of running along the [110] and [1̅1̅0] directions, as seen in Figure 1a. But In droplets on the (111)A surface, hereafter the In (111)A droplets, run or try to run along the [011̅] direction only, as seen in Figure 1b. Due to material competition and the stochastic nature of droplets nucleation, ensembles at various stages of formation appear across both surfaces. The insets in Figure 1 (panels a and b) are representative of early stage ensembles on InP (001) and (111)A surfaces and clearly show the bidirectionality of the In (001) droplets and the unidirectionality of the In (111)A droplets, respectively. On the other hand, the In droplets on the InP (111)B surface, hereafter the In (111)B droplets, do not exhibit preferential direction. To understand the factors that shape the In droplet ensembles and the branching geometries, the self-running In (001), (111)A, and (111)B droplets will be discussed in sequence and compared with their Ga counterparts. The evolutions of typical In droplet ensembles on InP (001), (111)A, and (111)B surfaces are shown in series of MEM snapshots in Figure 2 (panels a, b, and c, respectively). Each series comprises six frames taken at various intervals (see the Supporting Information for animated versions, incorporating many more frames). In MEM imaging, metallic droplets appear as dark circles surrounded by bright caustic rings.26 The coexistence of stationary and mobile droplets in all cases shown in Figure 2 (panels a−c) indicates that the In droplets must reach a critical size before they start running, similar to Ga droplets on GaAs (001).13 In most cases, the departing In droplets break up, leaving behind a small droplet or two at the point of departure. Ga droplets, on the other hand, move in whole, leaving behind only etch pits.12,16,27 The criterion that determines whether or not a droplet breaks up when dislodged from its nucleation site is difficult to single out due to the many inter-related parameters involved in reactive wetting (e.g., liquid−solid reaction, mass transport, and nanoscale surface ordering).15,18 Droplet breakup is a longstanding research problem that is not fully understood,18 but viscous forces play a critical role in several droplet systems.28,29 Generally, the forces driving liquid spread on a solid substrate are counteracted by a viscous force (Fv) proportional to the liquid droplet’s viscosity (μ), according to Fv = μUR/h, where U is the contact line velocity and R and h are the droplet radius and height, respectively.18 Since the sizes of In and Ga droplets are similar, this leaves viscosity μ as a most-likely factor
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RESULTS AND DISCUSSION Figure 1 (panels a and b) are DIC images showing overall morphology of the postannealed InP (001) and (111)A surfaces, respectively. The droplets on both surfaces have wide size distributions, are almost circular, and are clearly separated into ensembles. In each ensemble, the droplets are interconnected by shallow, tapered etch trails. The trails result from thermal etching, or reactive wetting, of the InP surface by the liquid In droplets. Previous studies have established that as a result of thermal decomposition, P is driven out of the 831
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Figure 2. Series of MEM snapshots of self-running In droplets on InP (a) (001), (b) (111)A, and (c) (111)B surfaces during thermal decomposition. The time at the bottom of each frame is in min:sec, relative to the left-most frame of each figure.
This new droplet grows and may run toward or away from the mother droplet as will be explained later. The In (001) droplets thus run in both directions with roughly the same probabilities, avoiding the trails of other droplets. This characteristic is similar to Ga (001) droplets, indicating that the forces driving the In and Ga (001) droplets toward a more roughened area arise from the same origin (i.e., the chemical potential gradients between the roughened surroundings and the smooth etch pits).12 The In (111)A droplets in Figure 2b start moving toward the [011̅] direction at approximately the same critical diameter (0.7 μm) as the (001) droplets, then leave behind daughter droplets that later branch out at approximately 45° from the [011]̅ direction as better seen in the SEM image in Figure 3b. In Figure 2b, the upper and lower mother droplets (marked 4 and 6, respectively) each leaves behind a daughter droplet (5 and 7, respectively) which moves sideways to one side while another daughter droplet is about to move sideways to the other side. Some daughter droplets move in the opposite direction to the mother droplets, indicating that a small Fd component exists in the [01̅1] direction. But after a short distance (a few μm), they all make a 135° turn (see the Supporting Information for more images). The overall morphology of thermally decomposed InP and GaAs (111)A surfaces differ in two main aspects. First, the triangular etch pits which dominate the GaAs (111)A surface are absent on the InP (111)A surface. Second, the subsurface dislocation networks are present in the GaAs (111)A but are absent in the InP (111)A substrate. This is deduced from the absence of sharp droplet turns as expected if droplet paths cross with dislocations as reported earlier.16 The temperature needed to observe the running In droplets on InP (111)A surface reported here is 388 °C, much lower than 660 °C needed to observe the running Ga droplets on GaAs (111)A surface reported earlier.16 The etch pits and, to a certain extent, dislocation networks are thermally generated; their absence in low-temperature annealed InP (111)A substrates are thus not surprising. The In (111)B droplets movement is nearly directionless. They seem to move unpredictably under similar surface conditions. Between frames 1 and 3 of Figure 2c, droplet 8 is seen to move in a near straight line on a flat surface for more than 5 μm before it changes direction upon collision with a stationary droplet on its path. Figure 3c, however, shows a similar-sized droplet whirling for more than 10 μm on a similarly flat surface. The whirling action is implied from the arc-looking etch trail decorated by daughter droplets along the edges. The nonstraight line motion of In (111)B droplets possibly stems from the (111)B surface being more chemically active than the (001) and (111)A surface:31 the free surface energies γ for the (001) and (111) surfaces of InP are 1.8 and
influencing the observed droplet breakup behavior. Viscosity is temperature-dependent. It is thus only meaningful to compare the viscosities of liquid Ga and In at active temperatures (TA), where self-running Ga and In droplets are observed. For In (001) droplets at TA = 369 °C, μIn = 1.15 mPa s, and for Ga (001) droplets at TA = 630 °C, μGa = 0.60 mPa s.30 In droplets thus suffer from greater viscous force, making them more sticky and break up more readily than Ga droplets. While breakup is common among the In droplets on the three surfaces, the directionality of the subsequent movement differs considerably and depends critically on surface orientation. The first In (001) droplet to move in Figure 2a, marked 1, runs toward the [110] direction, leaving the second droplet, marked 2, to run toward the [1̅1̅0] direction, and the third droplet, marked 3, to initially run sideway and finally toward the [1̅1̅0] direction. Daughter droplets (2 and 3) originate from the departure point of the mother droplet (1) as is also evident in the inset of Figure 1a, which shows that mother droplets, regardless of running direction, can leave behind small droplets. Daughter droplets do not immediately run after the breakup because they are too small; the droplet driving force Fd varies linearly with droplet diameter.12,13 As thermal decomposition proceeds, daughter droplets gain additional materials and when a critical diameter is reached are driven out of the etch pits, running away from the mother droplet. While moving, the mother and daughter In droplets also etch the InP substrates, gaining more materials and increasing in size. This causes the etch trail to widen as droplets run. Fd acts against Fv, resulting in a stick−slip motion evidenced in the terraced trails better seen in the SEM image in Figure 3a, in line with previous observation on running Ga droplets on GaAs (001) surfaces.27 When sticking dominates during movement, a running droplet may further break up; an example can be seen between frames 4 and 6 of Figure 2a, where the circular perimeter of the mother droplet (1) is distorted and from where a new droplet emerges.
Figure 3. False-colored SEM images showing self-running In droplets on InP (a) (001), (b) (111)A, and (c) (111)B surfaces after thermal decomposition. The scale bars are 2 μm. The labels “m” and “d” mark the mother and daughter droplets, respectively. 832
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1.0 J/m2, respectively.32 This makes the (111)B droplets more sensitive to the immediate surrounding. The nondirectionality of In (111)B droplets is in stark contrast to Ga (111)B droplets, which were found to move along the [011̅] direction.15,16 The reasons are 2-fold. First, the (111) surface of InP is more chemically active than those of GaAs: γ values for the (111) surfaces of GaAs and InP are 1.4 and 1.0 J/m2, respectively.32 Second, In droplets are more chemically active than Ga droplets as γ values of In are lower than Ga for equivalent, low-index orientations.33 All of the In droplets above move at similar speeds (∼200 nm/min), suggesting that the underlying forces in the three systems are in the same order of magnitude and differ only in direction. The directionality of the self-running In droplets on all the investigated surfaces can be explained by considering the relative importance of two types of forces: a diffusion-related force Fdiff along low diffusion barrier direction(s) and a driftrelated force Fdrift originated from spatial variation in chemical potentials Δμ, as schematically shown in Figure 4a. The former
equipotential contour, and possibly the Fm direction. The latter determines whether a droplet moves in a straight line or not. In the case of Figure 4b, droplet motion does not change the Fm direction; the droplet’s path is thus straight. In the case of Figure 4c, in contrast, droplet motion does change the Fm direction; the droplet’s path is thus nonstraight. The vector sum of Fdiff and Fm guides the running droplets. The seemingly complex droplet motion patterns evidenced by the etched pits of the three surfaces can be explained simply by considering the relative strength between Fdiff and Fm and the effects that the trails have on the direction of Fm. Figure 4 (panels d and e) summarize the key characteristics of the force vectors acting on the In (001) and (111)A droplets, respectively. For In (001) droplets, the running mother droplet (m in Figure 4d) experiences a net force in the [110] direction, while the daughter droplet at the other end of the trail (not shown) experiences a net force in the opposite direction. The motion of the mother and daughter droplets does not affect the Fm direction, both thus run away from each other. The newly nucleated droplet (n in Figure 4d) meanwhile experiences a zero Fdiff along the [110] direction, and a nonzero Fm to the left of the figure, away from the smooth etch trail to the right. The n droplet’s initial motion is thus approximately orthogonal to the m droplet’s running trail. The latter is slightly tapered, and the lower half of the n droplet thus collects slightly more material than the upper half. Fm thus has a small downward component. Though the n droplet’s initial movement is sideways toward the bottom left of the figure, its trail rotates Fm, tips the balance of Fdiff, and drives the droplet toward the more energetically favorable [1̅1̅0] direction. For In (111)A droplets, the running mother droplet (m in Figure 4e) experiences a net force in the [011]̅ direction, and its motion does not affect the Fm direction, similar to the In (001) mother droplet. The newly nucleated droplet (n in Figure 4e) however is driven by the vector sum of Fdiff along the [011]̅ direction and Fm to the left. The n droplet’s running trail thus makes an angle θ to the m droplet’s. The angle θ indicates the relative amplitude of the two forces; the observed θ ≈ 45° implies |Fdiff| ≈ |Fm|. For In (111)B droplets, the absence of preferred droplet direction indicates that drift dominates over diffusion, or |Fdiff| ≪ |Fm|. This means that the size of the solid blue arrow in Figure 4c is small compared with those of the dashed red arrow. In such a case, if a droplet moves along a straight line or, in other words, the angle θm between Fm and the droplet trail is zero, the droplet will keep moving along the same straight line. This explains the observation in Figure 2c. But if the droplet deviates from the line, for example, by a local thermal or chemical inhomogeneity, θm will be nonzero, and the resulting etch trail and modified equipotential contour will sustain such condition. This explains the observation in Figure 3c, where the mother (m) droplet moves clockwise while the daughter (d) droplet moves counter-clockwise (see the Supporting Information for MEM images with larger fields of view).
Figure 4. Schematic drawings of diffusion-related force Fdiff (solid blue arrows) and drift-related force Fdrift (dashed red arrows) acting on In droplets while (a) stationary, (b) running straight, (c) running nonstraight, (d) running on InP (001), and (e) running on InP (111)A. Equipotential contours are shown as dashed curves/circle. The labels “m” and “n” mark the mother and newly nucleated droplets, respectively.
is global in scope and results from the group III atoms’ preference to hop along group V dimers.34 The latter is local in scope and is affected by the droplet’s immediate surroundings. For stationary droplets surrounded by homogeneously roughened surfaces, Δμ is isotropic and Fdrift sums to zero. Instantaneous inhomogeneities (thermal, chemical, or otherwise), however, give rise to a nonzero Fdrift sufficient to release the droplets,12 setting them off in motion and changing the Δμ landscape. Figure 4b shows a schematic drawing of a running droplet and a possible profile of an equipotential contour. The direction of the arrows reflect the direction of the net forces. Fdrift is thus nonzero and points away from the trail. When Fdrift dominates, the droplet would move along the direction of maximum force Fm. As the droplet moves, the newly exposed etch trail inevitably affects the force distribution, the
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CONCLUSION InP (001), (111)A, and (111)B surfaces are thermally decomposed under ultrahigh vacuum conditions. Surface events during the decomposition are observed in real-time via mirror electron microscopy. As a result of noncongruent evaporation, In droplets form on the three surfaces, propel themselves from the nucleation sites, and run away. The majority of droplets 833
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(16) Kanjanachuchai, S.; Euaruksakul, C. ACS Appl. Mater. Interfaces 2013, 5, 7709−7713. (17) Mandl, B.; Stangl, J.; Hilner, E.; Zakharov, A. A.; Hillerich, K.; Dey, A. W.; Samuelson, L.; Bauer, G.; Deppert, K.; Mikkelsen, A. Nano Lett. 2010, 10, 4443−4449. (18) Kumar, G.; Prabhu, K. N. Adv. Colloid Interface Sci. 2007, 133, 61−89. (19) Zheng, C. X.; Tang, W. X.; Jesson, D. E. App. Phys. Lett. 2012, 100, 071903. (20) Glotzer, S. C.; Solomon, M. J. Nat. Mater. 2007, 6, 557−562. (21) Chaudhury, M. K.; Whitesides, G. M. Science 1992, 256, 1539− 1541. (22) Xu, X.; Qian, T. Phys. Rev. E 2012, 85, 061603. (23) Duan, X.; Huang, Y.; Cui, Y.; Wang, J.; Lieber, C. M. Nature 2001, 409, 66−69. (24) Shorlin, K.; Zinke-Allmang, M. Surf. Sci. 2007, 601, 2438−2444. (25) Lowes, T. D.; Zinke-Allmang, M. Phys. Rev. B 1994, 49, 16678− 16683. (26) Kennedy, S. M.; Zheng, C. X.; Tang, W. X.; Paganin, D. M.; Jesson, D. E. Ultramicroscopy 2011, 111, 356−363. (27) Wu, J.; Wang, Zh. M.; Li, A. Z.; Benamara, M.; Li, S.; Salamo, G. J. PLoS ONE 2011, 6, e20765. (28) Castrejón-Pita, A. A.; Castrejón-Pita, J. R.; Hutchings, I. M. Phys. Rev. Lett. 2012, 108, 074506. (29) Javadi, A.; Eggers, J.; Bonn, D.; Habibi, M.; Ribe, N. M. Phys. Rev. Lett. 2013, 110, 144501. (30) Assael, M. J.; Armyra, I. J.; Brillo, J.; Stankus, S. V.; Wu, J.; Wakeham, W. A. J. Phys. Chem. Ref. Data 2012, 41, 033101. (31) Gatos, H. C.; Lavine, M. C. J. Electrochem. Soc. 1960, 107, 427− 433. (32) Ayers, J. E. Heteroepitaxy of Semiconductors: Theory, Growth, and Characterization; CRC Press: Boca Raton, 2007; p 36. (33) Vitos, L.; Ruban, A. V.; Skriver, H. L.; Kollár, J. Surf. Sci. 1998, 411, 186−202. (34) Kley, A.; Ruggerone, P.; Scheffler, M. Phys. Rev. Lett. 1997, 79, 5278−5281.
breakup before running, leaving a few daughter droplets and running trails that further nucleate droplets along the edges. These subsequent generations of droplets also grow and run. The self-running directions are dictated mainly by crystallography and history, following the direction of maximum force, which may be maintained or continually varied during reactive running. The microscopic behaviors of In droplets reported here, together with those of Ga droplets reported earlier,12,13,16 not only serve as model test systems for fundamental reactive wetting and running studies, but may also serve as a novel design avenue that can be exploited for the growth of various droplet-based micro- and nanostructures.
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ASSOCIATED CONTENT
S Supporting Information *
EDS spectra, animated videos of self-running In droplets, and additional SEM and MEM images of In (111)A and (111)B droplets. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel: +66-81-646-3496. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge W. Busayaporn, T. Chokamnuai, N. Jearanaikoon, P. Photongkam, and P. Rattanadon for assistance during MEM imaging. S.K. acknowledges Prof. S. Panyakeow for continuous supports and grants from the Ratchadaphiseksomphot Endowment Fund of Chulalongkorn University (RES560530147-EN).
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