Heteroepitaxial Growth of GaSb Nanotrees with an Ultra-Low

Mar 20, 2012 - College of Physics and Microelectronics Science, Key Laboratory for Micro-Nano Physics and Technology of Hunan Province, Hunan Universi...
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Heteroepitaxial Growth of GaSb Nanotrees with an Ultra-Low Reflectivity in a Broad Spectral Range Chenglin Yan,†,* Xiaopeng Li,†,∥ Keya Zhou,‡ Anlian Pan,§ Peter Werner,† Samuel L. Mensah,† Alexander T. Vogel,† and Volker Schmidt† †

Max Planck Institute of Microstructure Physics, Weinberg 2, 06120 Halle, Germany Department of Materials and Chemical Engineering, Hanyang University, Ansan, Gyeonggi-do 426-791, South Korea § College of Physics and Microelectronics Science, Key Laboratory for Micro-Nano Physics and Technology of Hunan Province, Hunan University, Changsha 410082, People’s Republic of China ∥ Martin-Luther-University, Halle-Wittenberg 06120 Halle, Germany ‡

S Supporting Information *

ABSTRACT: We report on the growth of GaSb nanotrees on InAs {1̅ 1̅ 1̅}B substrates by chemical beam epitaxy. GaSb nanotrees form by the nucleation of Ga droplets on the surface of B oriented GaSb nanowires followed by the epitaxial growth of branches catalyzed by these Ga droplets. In the tip region, the trunks of the GaSb nanotrees are periodically twinned, which is attributed to a change of the effective V/III ratio at the later stage of growth as a consequence of the change in surface structure. The reflectivity of a forest of nanotrees was measured for a broad spectral range and compared to the reflectivity of a GaSb (1̅ 1̅ 1)̅ B wafer and of GaSb nanowires. At wavelengths from 500 to 1700 nm, the presence of GaSb nanotrees decreased the reflection by three orders of magnitude compared to a blank GaSb substrate.

KEYWORDS: Gallium antimonide, III−V semiconductors, low-band gap semiconductors, nanostructured materials, semiconductor growth, chemical beam epitaxial growth

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view. Decreasing the reflectance by epitaxial nanostructures seems to be a promising means to increase the absorption of light.12−15 Engineered three-dimensional arrays of nanostructures enable implementation of optoelectronic devices with significantly improved performances.1,16−21 Boden et al. demonstrated that the reflectivity could be optimized through tuning the geometry of the nanostructures in order to achieve a low reflectance in a specific wavelength range.22 Cylindrical nanocones and nanowires23,24 as well as dual-diameter nanopillar arrays21 have been also presented to decrease the reflectivity for maximizing optical absorption. To enhance the broad band optical absorption efficiency, a GaSb nanotree structure is fabricated with tapered nanowire trunks and a high density of side branches for maximizing the effective absorption coefficient. The thereby created absorber material exhibits an impressive absorbance beyond 99% over wavelengths from 500 to 1700 nm. This approach toward nanotree-based highperformance near-infrared photonic devices essentially mimics the strategy of biological structures in their ambition to capture more sunlight.

he ongoing miniaturization of electronic devices has triggered worldwide interest in the epitaxial growth of complex bottom-up designs and derived device concepts.1−3 However, fundamental issues, such as lattice and thermal expansion mismatch, are an obstacle for the epitaxial integration of III−V nanowires. Chemical beam epitaxy (CBE) is an attractive growth technique especially for the epitaxial growth of III−V nanowires on lattice-mismatched substrates with precise control of interfacial structure and material composition.4−6 As an important direct low-band gap semiconductor with Eg = 0.73 eV, GaSb is of interest for near- and mid-infrared optoelectronic devices.7−9 Despite great advances in the epitaxial growth of III−V nanowires for many emerging optoelectronic applications, there is no report on the epitaxial growth of GaSb nanotrees. In this letter, we demonstrate the heteroepitaxial growth of the bottom-up grown GaSb nanotrees on InAs (111)B substrates. The GaSb nanotrees are composed of GaSb nanowire trunks and epitaxial GaSb branches, both terminated with metal Ga droplets. Advances in nanostructuring techniques have resulted in an increased interest in enhancing absorption in photonic nanodevices by means of nanostructuring.10 Increasing absorption is of great importance to improve performance, as it helps to reduce the amount of active material while maintaining efficiency,11 at least from the optical point of © 2012 American Chemical Society

Received: November 2, 2011 Revised: February 22, 2012 Published: March 20, 2012 1799

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Figure 1. SEM images of GaSb nanotrees. (A,B) oblique views of GaSb nanotrees and (C,D) top view images of GaSb nanotrees. Please note in (D) the staggered surface in the region near the tip.

(1̅ 1̅ 1̅)B substrates were immediately transferred into the vacuum system. The growth of the GaSb nanotrees was achieved at a substrate temperature of 465 °C with a TMGa flux of 0.4 sccm and a constant TESb-to-TMGa flux ratio of 2.5. After 30−60 min of growth, precursors were turned off, and the samples were cooled down under vacuum. To characterize the GaSb nanotrees, we first employed scanning electron microscopy (SEM). Field-emission SEM micrographs of the as-grown nanotrees are shown in Figure 1. Figure 1a gives an oblique view on a forest of pine-like GaSb nanotrees. One can clearly see the multitude of downwardfacing branches, each having a droplet at its tip, extending from the GaSb nanowires being also terminated by a droplet. Since the amount of Au originating from the Au-colloid particles used

A CBE system was utilized for the epitaxial growth of vertically aligned GaSb nanotrees on InAs (1̅ 1̅ 1)̅ B substrates using colloidal gold particles to initiate the growth. All our experiments were performed using trimethylgallium (TMGa) and triethylantimony (TESb) as metal organic precursors. Precursor fluxes to be introduced into the growth chamber were controlled by commercial mass flow controllers. The TESb precursor was injected into the cold wall growth chamber through a thermal cracker cell held at temperatures around 625 °C, whereas the TMIn precursor was introduced into the chamber through an injector held at about 70 °C. InAs (1̅ 1̅ 1)̅ B wafers, cleaned in a solution of 2HCl:2H2O:HNO3 for 10 min in an ultrasonic bath, were used as substrates in our experiments. After the deposition of the Au colloids, the InAs 1800

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Figure 2. Schematic representation and SEM images of GaSb nanotrees showing their development in time. Growth time is (A) 30, (B) 40, and (C) 60 min.

[111̅]B). Figure 1d shows a close-up of the treetop in top view. Please note the staggered surface structure in the tapered region below the droplet, which is potentially indicative of the presence of planar defects inside of this part of the crystal; we will return to this point later on. In order to elucidate the bottom-up growth of GaSb nanotrees, a series of time-dependent growth experiments was conducted. In Figure 2, SEM images of the GaSb nanotrees for different process duration of 30, 40, and 60 min are displayed. One can see that the growth of the nanotrees can be approximately described as a two-step process. In the first step, as shown in Figure 1a, tapered and [1̅ 1̅ 1]̅ -oriented GaSb nanowires grew perpendicular to the InAs (111)B surface. In this phase, the surface of the nanowires is still almost void of any branches. However, one can see that in some places Ga droplets nucleated on the side faces of the GaSb tree trunks and that, yet rarely, branches started to grow. In the second phase, the branching commences. After 40 min of GaSb growth by CBE (see Figure 2b), the density of Ga droplets as well as the density of branches increased considerably as compared to Figure 1a. In addition, also the stem of the nanotree increased in diameter. This process continues until after 60 min (see

to initiate the growth is substantially less than the volume of the droplets, one can surmise that the droplets are mostly consisting of Ga, which is liquid at the growth temperature. One can furthermore see that the GaSb nanotrees are vertically aligned with respect to the substrate surface, which indicates that the nanotrees preferentially grew in the [1̅ 1̅ 1]̅ B direction as defined by the InAs (1̅ 1̅ 1̅)B substrate. Figure 1b shows a magnified view (tilt angle 45°) of three nanotrees exhibiting a 120° rotational symmetry with regard to the dominant type of branches. This 120° symmetry with respect to the preferred branches is also clearly visible in Figure 1c, displaying a top view SEM of three nanotrees. However, one can also see that concerning the direction of the branches, the left most nanotree seems rotated by 180° with respect to the other two, which indicates a planar twin defect in the initial phase of nanotree growth. Due to this randomly occurring 180° rotation as well as to the short and tapered shape of the branches, it is hardly possibe to unambiguously determine the growth direction of the branches. However, given the known general preference for III−V crystal growth in B direction, it seems not unlikely that the preferred type of branches did grow in the direction of the downward facing ⟨111⟩B directions (i.e., [1̅11]B, [11̅1]B, 1801

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Figure 3. (A) EDX elemental analysis of a droplet at the tip of a branch. (B) EDX of the EDX elemental analysis of the nanotree trunk. The nonlabeled minor peaks at 0.28 and 0.53 eV are the carbon and oxygen Kα lines. (C−E) Bright-field TEM images of GaSb nanotree; zone axis is [110]. (C) Magnified view to the tip region showing regular twinning. (D,E) Magnified view to the tip region; note that the twinning in (E) extents into the branches.

Ga nature of the catalyst particles suggests that the self-catalytic growth mechanism is responsible for the GaSb nanotree growth. Self-catalyzed growth of Ga-V nanowires is a frequently observed and reported phenomenon.7,25 Typically, the Ga droplets at the tip of the nanotrees have diameters of a few hundred nanometers. It seems likely that the gold, originating from the 2 nm Au colloid particles used to initiate the GaSb nanowire growth, is completely dissolved in the Ga droplet. At temperatures greater than 400 °C, more than 10 atomic percent of Au can maximally be dissolved in liquid Ga; and a rough estimate considering the size of the Au-colloid particles and the volume of the droplets shows that Au concentration should be less than ∼1%. Thus in the later stage of growth, Au has to be considered a minor impurity to a catalyst particle that essentially consists of pure Ga. This assumption is consistent with the EDX measurement of Figure 3a that does not reveal any significant Au contribution. An EDX analysis of the droplets terminating the side branches (not shown) leads to equivalent results. Also the droplets at the branch tips essentially consist of Ga, so that also the branches can be

Figure 2c) the nanotrees developed a substantial density of side branches, augmenting both volume and impressiveness of the nanotrees. In addition to branching, also lateral growth on the sides apparently contributes to the increase in volume. One can summarize that the GaSb nanotrees show a tendency of first growing mostly in height; subsequent to this period of vertical growth, the nanotrees tend to expand increasingly in width and volume both by radial growth and by developing a higher density of protruding side branches. Thus, in some sense, the development of the GaSb nanotrees parallels the development of most common trees in nature. Before discussing the nanotree growth process in more detail, let us take a look at the composition of the droplets, catalyzing the growth of the nanotree stem and the growth of the branches as well as the composition of the nanotree itself. Figure 3a shows an energy dispersive X-ray (EDX) elemental analysis of the composition of the droplet located at the tip of the nanotree performed on a Philips CM20 transmission electron microscope. As can be seen from EDX analysis of Figure 3a, the droplet essentially consists of pure Ga. The pure 1802

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the effective V/III ratio, i.e., an increase of the overall Ga supply at constant precursor flux may indeed occur in CBE. TMGa, the Ga precursor we are using, is a rather stable molecule. DenBaars et al.28 found its decomposition temperature to be about 500 °C. Under their conditions28 only about half of the precursor decomposed at 465 °C. It therefore seems likely that at 465 °C, the substrate temperature we applied, surface decomposition is incomplete in the sense that a good percentage of the injected TMGa precursor molecules that initially hit the substrate surface will desorb undecomposed. At our pressures of 10−4−10−5 mbar, corresponding to mean free paths in the centimeter range, and given that a cold wall reactor is used, these molecules desorbing from the flat substrate surface are presumably gone for good. Yet, for a dense array of nanowires or nanotrees, the situation is different. Undecomposed TMGa molecules desorbing from the substrate surface will with a certain probability hit the surface of a nanowire/ nanotree, which will give these molecules a second chance for decomposition and Ga release. The increase in surface area due to the growth of the nanostructures will therefore increase the integral decomposition efficiency and thereby lead to an increased Ga supply. Since Sb supply is constant, this corresponds to an effective V/III ratio that continuously decreases with time. We believe that this is what causes the larger density of Ga droplets on the surface of the nanotrees. Such an effective V/III decrease could also be consistent with the more pronounced radial overgrowth at longer growth times, as visible in the micrographs of Figure 2, as it is known at least for InSb that the probability for this direct antimonide deposition is facilitated by lower V/III ratios.4 The morphology of the nanotrees provides another indication for a decrease of the effective V/III ratio. As can be seen in Figure 3c−e, the nanotrees exhibit a periodic sequence of twin defects in their tip region. Such twin-plane superlattices have also been observed for other III−V materials, like InAs29 and InP.30 One first guess might be that the creation of these twinned regions, as shown, e.g., in Figure 3e, is effected during cool down after switching off the precursors. However, the twinned region is quite long compared to the short necks that are often found and that are invariably related to sample cooling. The length of the twinned region in Figure 3d is about twice the diameter of the nanotree at its tip, which would require a substantial amount of Sb stored in the droplet. What furthermore speaks against a creation during cool down is that the twinning, as shown in Figure 3e, even extents into the branches, so that with the proviso that the branches did not grow during cool down as well, the twinned region must have been grown beforehand. That such a periodic twinning is indicative for a decrease of the V/III ratio is in accordance with our observation for InSb. At high V/III ratios, defect free zinc blend InSb nanowires can be synthesized. At lower V/III ratios, not only radial overgrowth becomes more pronounced but also the tendency to incorporate planar defects increases. In addition there is a V/III ratio window at which regular twinning can be observed. Thus, corollary to our InSb observations, the presence of the periodic twinning in GaSb might be considered as an indication for a decrease of the effective V/III ratio. Nowadays GaSb is mainly employed as a material for nearand mid-infrared optoelectronic devices. Self-catalyzed GaSb nanotrees, besides being interesting from a crystal growth point of view, might be an attractive means for increasing the absorbance of high-performance NIR photonic devices. To test their functionality in this regard, the specular reflectance of the

assumed to have been grown in a self-catalyzed, vapor−liquid− solid type manner. For completeness, in Figure 3b, an EDX measurement of the elemental composition of the nanotree trunk is shown. In addition to the presence of Ga, the trunk exhibits a substantial amount of Sb, as expected for GaSb. Figure 3c shows a lowmagnification bright-field TEM image of two GaSb nanotrees. Not considering the tip region, which exhibits a high density of planar defects, one can deduce from the bending contours and the absence of other contrastsbest visible in case of the nanotree on the right-hand sidethat the nanotree is indeed single crystalline. It was verified by diffraction (not shown) that the material is zinc blend GaSb, as expected, since the IIIantimonides generally show a stronger tendency than the other III−V materials to crystallize in the zinc blend phase. To understand the nature of the nanotree growth process, in particular the origin of the Ga droplets on the nanotree side (c.f., Figure 3c), one has to consider the characteristics of chemical beam epitaxy of III−V crystals. As said, the precursors of TMGa and TESb are employed for GaSb nanotree growth. After injection into the growth chamber, the antimony precursor is thermally precracked in a commercial cracker cell, creating a flux of mainly Sb2 molecules. The gallium precursor, TMGa, in contrast, decomposes predominantly on the surface of the substrate. Upon decomposition, Ga diffuses on the substrate surface, making surface diffusion an essential step for the CBE growth III−V nanostructures.26 Together with the Sb supplied from the gas phase, there are several alternative processes that can potentially occur. The first, desorption of elemental Ga, is rather unlikely, as the vapor pressure of Ga is extremely low at the growth temperatures applied. The second is that the diffusing Ga, together with the Sb from the gas phase, directly forms GaSb on the surface of the substrate or the surface of the nanowire/nanotree, leading to thin growth in the first and radial growth in the latter case. By comparing Figure 2a with Figure 2c, it becomes apparent that some radial overgrowth of the existing nanowires/nanotrees did occur. The third possibility is that Ga diffuses on the surface until it reaches a Ga droplet, where at a sufficient Sb supply, crystal growth via the vapor−liquid−solid mechanism can take place. From our point of view this is what can be safely assumed to be responsible for the growth of the initial GaSb nanowires, as shown in Figure 2a, as well as for the growth of the branches, as shown in Figure 2c. The fourth possibility, and this is the one of first importance for the nanotree growth, is that the diffusing Ga causes the formation of Ga droplets, which may then serve as catalysts for the growth of the branches. The growth mechanism by which the GaSb nanotrees were synthesized differs decisively from the results shown by the other groups2,27 They obtained their multibranched nanowires or nanotrees, respectively using a multistep nanoparticle-catalyzed process. The formation of Ga droplets is something that can be observed specifically if a too low V/III ratio is applied. At too low Sb supply, the excess Ga that due to the lack of Sb is not transformed into GaSb naturally tends to form droplets on the surface. Considering the electron micrographs in Figure 2, the question is why the tendency of droplet formation seems to be much more pronounced at the later stage of growth, whereas in the initial phase (c.f., Figure 2a) only very few small Ga droplets can be observed on the nanotree trunks’ surface. Considering that the V/III precursor flux ratio is kept constant by means of the mass flow controllers, the most obvious explanation seems to be a change of the effective V/III ratio. Such an increase of 1803

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fabricated GaSb nanotrees was determined as a function of wavelength. Figure 4 shows the reflectivity of GaSb nanotrees

Figure 5. (A) Schematic diagram of the simulated geometry of a GaSb nanotree on InAs substrate. (B,C) Simulated cross-sectional, electrical field distribution at a wavelength of 800 nm in GaSb nanotree and nanowire arrays. The calculated reflectivity of the GaSb nanotrees and nanowires is 0.05% and 6.38%, respectively.

Figure 4. Specular reflectance measurement of GaSb and InAs wafers, GaSb nanowires (on InAs), and GaSb nanotrees with few branches (on InAs) as well as GaSb nanotrees with fully developed branches (on InAs), respectively. The step at 1000 nm is due to a change of the detector.

related to the nucleation of Ga droplets on the side surface of the GaSb nanowires. The presence of the GaSb is explained by a decrease of the effective V−III ratio as a consequence of the higher integral precursor cracking probability induced by the change of the surface structure. It was furthermore found that the GaSb nanotrees exhibited a region of periodic twin defects at their tip, which can be interpreted as a further indication for a decreased V/III ratio. Measurements of the reflectivity of GaSb nanotrees indicate that the reflectivity of the nanotree covered substrate is about three orders of magnitude lower than the reflectivity of a GaSb wafer. Self-catalyzed GaSb nanotrees could serve as effective antireflection coatings to improve the absorption properties of GaSb infrared detectors.

and nanowires (both on InAs substrate) for wavelengths from 0.5 to 1.7 μm. As a reference the reflectance of GaSb and InAs wafers is also displayed. One can see that the growth of GaSb nanotrees considerably reduced the reflectivity in particular if compared to the reflectance of the blank GaSb and InAs substrates. The InAs substrate covered with fully branched GaSb nanotrees shows a reflectivity that is about three orders of magnitude smaller than the reflectivity of the GaSb wafer. Due to its unique structure combining tapered nanowire trunks with a high density of branches, effecting an irregular surface structure, the effective absorption of the GaSb nanotree covered substrate is close to 100%. With the aid of the commercial finite element method (FEM) software COMSOL Multiphysics, we calculated the reflectance of both GaSb nanotrees and nanowires at a wavelength of 800 nm. Details of the simulation method and parameters are provided in the Supporting Information. The geometry of the model is shown in Figure 5 A, and the corresponding electric field distribution in a cross section parallel to the light polarization is shown in Figure 5B,C, respectively. Our calculation showed that the GaSb nanotree structure exhibits an ultralow reflectivity of 0.05%, compared with a reflectance of 6.38% for GaSb nanowires without branches. It is also worth mentioning that the bulk GaSb substrate could reflect more than 40% of the incident light, which is determined by Fresnel equations. The calculated reduction of reflectance agrees very well with the experimentally measured results, highlighting the merits of the GaSb nanotrees in significantly reducing optical reflection. In an effective medium approximation, this extremely low reflectivity presumably could be explained as a consequence of the graded gradual change of the effective refractive index, as the average volume ratio of air to GaSb continuously increases from the nanotree tips down to the substrate surface. In summary, a CBE growth method was reported for the epitaxial growth of GaSb nanotrees on InAs {1̅ 1̅ 1̅}B substrates. It is found that the growth of the nanotree stems as well as the branches is essentially Ga catalyzed. Studying the time dependence of the GaSb nanotree growth revealed that the branches grow somehow belatedly, after the growth of the strongly tapered GaSb nanowires, and that their growth is

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details and detailed description of the simulation method. 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] Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the support from the Alexander von Humboldt foundation, the National Basic Research Program of China (no. 2012CB932703), and the National Natural Science Foundation of China (NSFC) (no. 90923014).

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