Structural Evolution and Growth Mechanism of Self-Assembled

Mar 4, 2013 - ... supersaturation accounts for this diameter-independent growth rate. ...... D. H.; Huang , Y.; Rogers , J. A. Semiconductor Wires and...
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Structural Evolution and Growth Mechanism of Self-Assembled Wurtzite Gallium Nitride (GaN) Nanostructures by Chemical Vapor Deposition V. Purushothaman and K. Jeganathan* Centre for Nanoscience and Nanotechnology, School of Physics, Bharathidasan University, Tiruchirappalli, India 620024 ABSTRACT: We present the fabrication of self-assembled wurtzite gallium nitride (GaN) nanostructures with manifold morphologies through chemical vapor deposition under controlled deposition conditions. The different nanostructures including vertical standing whiskered nanowires (NWs), entangled NWs, nanorods, micro/nanotowers, highly transparent ultrathin nanosheets, and hexagonal microcrystals were evolved by the direct reaction of metal Ga with NH3 using a self-catalytic process by varying the important growth parameters such as temperature, source to substrate distance, and the reactor pressure. The growth mechanism of GaN nanostructures with manifold morphologies was interpreted with a surface diffusion model by accounting the direct impingement and surface migration of adatoms. Electron microscopy studies combined with a selected area electron diffraction pattern recorded on the NWs show wurtzite structure with preferential growth direction of (0001). X-ray diffraction studies on different nanostructures show that the hexagonal GaN contains neither cubic GaN nor Ga2O3 phases. Room temperature photoluminescence spectra reveal high optical quality of the nanostructures grown under either equimolar ratio or slightly nitrogen-rich regime, and, interestingly, GaN microcrystals grown under Ga-rich conditions were dominated by the defect induced green and yellow luminescence.

1. INTRODUCTION In the past decade semiconductor nanostructures, such as nanowires, nanorods, nanosheets, and nanotubes have received a great deal of attention for understanding new structures and utilizing them as a building block for future nanoscale devices.1−5 Semiconductor nanostructures have the potential to be used as both interconnect and functional units for the fabrication of electronic, optoelectronic, electrochemical and electromechanical devices because of their outstanding electrical, optical, thermal, and mechanical properties caused by their size effects.1−3 Among the various semiconductor nanostructures, gallium nitride (GaN), which has a direct and wide band gap of 3.4 eV at room temperature is a promising candidate for short wavelength optoelectronic devices, such as light emitting diodes (LEDs) and laser diodes (LDs) and high power and high temperature operation devices because of its high thermal and chemical stability, high breakdown field, and high saturation drift velocity.4,5 Wurtzite GaN nanostructures with different morphologies such as nanowires,6 nanosheets,7 and nanoribbons8 have been synthesized successfully through high temperature vapor routes for nanodevice applications. Today most of the one-dimensional nanostructures have been fabricated via vapor−liquid−solid (VLS) mechanism.9 The VLS mechanism utilizes a noble metal catalyst10 while the other mechanism termed as the vapor−solid−solid (VSS) mechanism,11 relies on the direct crystallization from the vapor. Generally VSS growth takes place well below the eutectic point temperature of catalyst and the growing material. For catalyst© 2013 American Chemical Society

assisted growth mode (for both the VLS and VSS) the dominant morphology is generally nanowires because nucleation and growth are defined by a catalyst particle, and the growing NW is bound with the restriction of the droplet size. However, undesired contamination may occur in NWs from the catalyst.12−14 To overcome such contamination of unwanted materials, a self-catalytic approach has been engaged.15 For the growth of compound semiconductors the excess of one of the materials can act as the liquid forming catalyst, which is the preferred site for arriving precursor atoms. From this perspective the synthesis of different morphologies of GaN using the self-catalytic approach have been reported;16−19 however, there are no reports available to understand the morphological evolution of different nanostructures and crystallographic growth orientation among these GaN structures as a function of variable growth parameters. Such information would be critically important for building up devices, by tuning the characteristics of highly anisotropic physical properties of GaN. Designing, controlling, and rational growth of one-dimensional nanoarchitectures with desired configuration plays a central role in this field. In this article, we demonstrate that by tuning the important parameters such as temperature, source to substrate distance, and reactor pressure, GaN nanostructures with different morphologies could be Received: December 7, 2012 Revised: February 28, 2013 Published: March 4, 2013 7348

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Figure 1. Scanning electron microscopy images of GaN nanostructures evolved from the series of experiments with the constant gas flow and sourceto-substrate distance and by varying reactor pressure from 760 to 200 Torr and temperature from 850 to 950 °C.

transmission electron microscope (HRTEM) (JEOL, 2010− 200 kV) equipped with selected area electron diffraction (SAED) was carried out to examine the crystalline nature of a single GaN NW. TEM samples were prepared by scratching GaN NWs from the substrate, dispersing them in ethanol, and casting a drop of the suspension on a Cu-grid coated with porous carbon thin film. The crystal structure of GaN nanostructures were characterized by X-ray diffraction (XRD) pattern using XPERT-PRO PAN analytical diffractometer with Cu−Kα radiation at λ = 1.5406 A°. The diffraction patterns were recorded using θ−2θ geometry between 10° and 80° with a step size of 0.05u for the structural identification. Optical characteristics of the ensemble of NWs were recorded using room temperature photoluminescence. PL spectra for ensembles of NWs were dispersed using a monochromator (Horiba Jobin Yvon −0.55M), with 325 nm He−Cd laser focused through an objective lens, and the resulting laser spot incident on the sample had a diameter of ∼10 μm. The photoluminescence signal from the sample is analyzed through a charge coupled device.

selectively prepared using CVD by the direct reaction of Ga vapor with NH3. The growth mechanism of transposed nanostructures with reference to VLS and VS is explained based on the direct impingement and surface diffusion of radicals.

2. EXPERIMENTAL DETAILS Metal Ga (Alfa Aesar 99.999%) in a quartz crucible is placed in the upstream of the Si (111) substrate with the desired sourceto-substrate distance ranging from 2 to 20 mm. Si substrates (1 × 1 cm2) were cleaned by the standard RCA (Radio Corporation of America) procedure prior to loading into the quartz reactor. Then the reactor is pumped to 1 × 10−3 mbar and flushed with the nitrogen (N2) gas in order to overcome the residual oxygen. The temperature of source and substrate is always maintained to be almost equal with a fluctuation of ±2 °C using the two zone CVD reactor. The growth temperature is varied between 850 and 970 °C with constant nitrogen (N2) and ammonia (NH3) gas flow rates of 500 SCCM (Standard cubic centimeter) and 200 SCCM, respectively. Ammonia is carried inside the reactor with a separate quartz tube and cracked just in front of the substrate in order to reduce the parasitic deposition of GaN and source nitridition. The growth duration is fixed for 1 h and then the reactor is turned off for natural cooling. The reactor pressure is varied at 760, 200, and 300 Torr through the throttle valve and capacitance monometer, while the other growth parameters remain constant. The morphology and compositional variation of GaN NWs were studied using field emission scanning electron microscopy (FESEM) (Carl Zeiss, Σigma) equipped with energy dispersive X-ray analysis (EDX) (Oxford instruments). High resolution

3. RESULTS AND DISCUSSION Figure 1 show FESEM images of GaN nanostructures (NSs) evolved from the series of experiments with different reactor pressure and temperature. When the temperature is ramped to the desired value, the carrier gas is flown inside the reactor, which transports the metal Ga vapors onto the substrate and forms a thin metal droplet layer of Ga. With a delay of 30 s NH3 is exposed into the reactor, the III/N ratio suddenly changes depending on the Ga vapor pressure and NH3 flow rate. At 850 °C, the vapor pressure of Ga is very low which results in the formation of GaN nanoparticles (NPs) on the 7349

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substrate even after the extended growth duration of 1 h. When the pressure of the reactor is reduced to 300 Torr, the NPs distribution is reduced with the increase of particle size. Further reduction in reactor pressure to 200 Torr leads to the formation of networks of NPs. 3.1. Transit in Growth Mode from VLS to VS. As the temperature is ramped to 900 °C, at constant source-tosubstrate distance and reactor pressure of 760 Torr, the entangled nanowires (NWs) having an average diameter of 60 nm and length of 500 nm were formed. Interestingly small droplets are clearly visible on the top of the GaN NWs, which is the characteristic of the VLS growth mechanism. The quasialigned nanorods (NRs) with the average diameter of 40 nm and the length of 200 nm were obtained as the reactor pressure reduced to 300 Torr. It is worth noting that there is no droplet on the tip of the rod, and further the length of the rod is significantly reduced as the reactor pressure is lowered, which can be related to the direct transformation of vapor to solid under the VS growth mechanism. Further reducing the reactor pressure to 200 Torr, the average distribution and size of the NRs are slightly varied but the feature appears to be similar to 300 Torr. Despite the growth duration of 1 h, the size features (diameter and length) remain in the nanometer regime irrespective of growth pressure at 760, 300, and 200 Torr. This is because of the lower Ga pressure at 900 °C which could be insufficient for the higher growth rate of NWs. To elucidate the scenario of pressure-dependent growth mechanism, the vapor pressure of Ga is increased by increasing the temperature of the Ga cell (950 °C) and the growth is carried out with different pressure ranges as shown in the morphological phase diagram (Figure 1). Figure 2a shows the FESEM image of entangled GaN NWs grown at 950 °C under 760 Torr. Figure 2b shows the Ga metal droplet on the tip of the NWs and the inset of Figure 2b shows the TEM image of the droplet on the apex of the NW. The average diameter of the NWs is 80 nm and length varies up to

several micrometers. The enhancement of the growth rate and containing the diameter of NWs by the presence of liquid alloy droplet on the tip is associated with the VLS mechanism.11 In the case of self-catalytic GaN NWs, the Ga metal droplet is normally not observed on the apex as the growth proceeds at higher temperature that results in consumption or desorption of Ga. In contrast to the earlier reports, we have noticed Ga metal droplets on the apex as well as on the surface of the NWs in which the former is responsible for the enhancement of axial growth and the later activates branched growth. Here the reason for such metal particles on the NWs can intimately be related to the initial growth that takes place at Ga-rich conditions, and subsequent growth is continued by the supply of sufficient metal Ga under the pressure of 760 Torr. Initially the Ga metal beam was transported to the substrate at 950 °C, which causes the formation of a liquid droplet layer. The supply of NH3 and carrier gas contributes to the Ga-rich GaN nucleation. The adatom diffusion of nitrogen radicals in a Ga liquid droplet is expected to be high under 760 Torr,20 favoring crystallization at the substrate−liquid interface, and then growth proceeds by a continuous supply of Ga and NH3 that keeps the liquid droplets alive on the tip of the wire. Elemental analysis carried out using the EDX (Figure 2c) reveals higher Ga content on the droplet over the body of the NWs from which we can conclude that the droplet contains excess Ga which acts as the sink for arriving adatoms. The surface diffusion and direct impingement of radicals are responsible for the enhancement of the growth rate, and the catalyst accumulates on the surface of the wire also promoting branched growth under the VLS approach. It is well understood that the control of metal Ga vapor pressure is a very difficult process as the vapor pressure of metal Ga steeply increases above 950 °C under 760 Torr pressure.21 Hence, careful selection of the initial nucleation and control of the III-N ratio is mandatory for the self-catalytic approach with the catalyst displayed on the tip under 760 Torr. The HRTEM image (Figure 2d) recorded on the body of the NW reveals the highly crystalline nature of the NWs.22 The interplanar spacing is about 0.269 nm which shows that the NW is grown along the cdirection.6 On the other hand, when a similar growth process takes place at the reduced pressure of ∼300 Torr, the structure completely changes from entangled to quasi-aligned vertical NWs22,23 with an average diameter of 150 nm, and length of 3 μm as shown in Figure 3a. Figure 3b shows the EDX recorded on the NWs which has the elemental composition of 51.3 and 48.7% for Ga and N, respectively. Figure 3c shows a TEM image recorded on the top part of a single GaN NW clearly indicating that the NW is straight and smooth with a uniform diameter and there is no catalyst particle present on the tip of the NW. The inset of Figure 3c shows the SAED pattern recorded on the body of NW which can be indexed with the wurtzite GaN. The HRTEM image of vertical standing GaN NW is shown in Figure 3d. The visible lattice fringes on the HRTEM image of the NW illustrate its high crystalline quality without any sheathed amorphous layer. The interplanar spacing is about 0.268 nm and confirms that individual NWs grow consistently along c-direction.6 When the reactor is kept at the reduced pressure, the Ga metal beam will be flown partly away by the carrier gas and maintains thermodynamical equilibrium. Once the NH3 is exposed the entire metal Ga droplet is expected to be converted into GaN nanodots which forbids accumulation of Ga liquid droplets on GaN. The continuous supply of Ga

Figure 2. (a) Scanning electron microscopy images of entangled GaN NWs grown under the pressure of 760 Torr. (b) Metal droplet on the tip of the nanowire. The inset of panel b shows the bright field transmission electron microscopy image of Ga-droplet on the tip of the NW. (c) Elemental analysis of single GaN nanowire on the droplet and body of the wire. (d) High resolution transmission electron microscopy image recorded on the body of the NW shows visible fringes. 7350

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Figure 4, top. Temperature profile measured at 950 °C in both the zones clearly shows (Figure 4, top) the flat temperature

Figure 3. (a) Scanning electron microscopy images of vertical aligned GaN NWs grown under reduced (200 Torr) pressure. (b) Energy dispersive X-ray spectrum recorded on the GaN NWs. The elemental composition for the ensemble of NWs is shown in the inset. (c) Transmission electron microscopy image of single GaN NW where there is no trace of any metal catalyst. Inset of (c) shows selected area electron diffraction pattern recorded on the single GaN NW. (d) High resolution transmission electron microscopy image of a single GaN nanowire grown by the VS route.

through the carrier gas and NH3 at constant flow rate facilitates the further growth of GaN nucleation to vertical standing NWs through a vapor−solid (VS) reaction. The growth rate of vertical GaN NWs is greatly reduced as compared to the entangled NWs in the VLS approach, due to absence of liquid catalyst on the tip. This slower growth rate is presumably a result of weaker surface reactivity of vapor−solid interaction over vapor−liquid interaction in VLS growth. The NWs grown under reduced pressure leads to a columnar structure with neither liquid nor solid catalyst on the tip. Hence it is very difficult to reiterate the nature of the catalyst involved in the VS mechanism. However, the microscopic and compositional analysis envisages that no catalyst is involved in the growth of NWs under direct VS transformation. The presence or absence of catalyst depends on temperature and precursor pressure which does not hinder the NWs growth but transit the growth structure between entangled and vertical standing wires governed by the VLS and VS mechanism, respectively. It clearly reiterates that, regardless of state of the catalyst, NWs continues to grow along the c-axis of GaN but the growth mode strongly transits under vapor pressure.24 A deep insight in the growth mode of GaN NWs shows that , when the reactor pressure is 760 Torr, the growth is governed by the VLS mechanism and while the reactor is kept at reduced pressure (300 and 200 Torr) the growth is likely driven by the VS mechanism. However in situ analysis on the growth is required to exactly reiterate the absence of catalytic adlayer on the NWs during the growth. Further, it is worth mentioning that the density of NWs grown at 950 °C under 200 Torr is considerably lower as compared to that of the sample grown under 300 Torr (Figure 1) due to the formation of a thin GaN layer at the interface between the nanowires and the Si substrate. 3.2. Substrate Position-Dependent Morphology of Nanowires. To probe the change in morphology with the substrate positioning, two small pieces of Si (111) substrates were placed in the position (a) upright on the top of the source metal Ga and (b) 10 mm away from the source as shown in the

Figure 4. (Top) Schematic diagram of the chemical vapor deposition (CVD) reactor and its corresponding temperature profile in the bottom part. The temperature profile clearly indicates that the substrate and the source temperature are equal in all the cases. (Bottom) Plot for the vapor pressure of Ga with temperature and distance. From the plot it is evident that the vapor pressure increases abruptly at above 950 °C.

region of about 20 cm on the CVD reactor. Figure 4, bottom panel represents the plot for temperature versus gallium vapor pressure and source distance versus hallium vapor pressure.21 The temperature for the growth is fixed at 950 °C with the pressure of 200 Torr. After the growth duration of 60 min the substrate that is placed on upright on the source is fully covered with a gray colored appearance. Figure 5 panels a and b show the FESEM investigation on the sample that reveals the formation of high dense nanowires with a very wide spectrum of diameters from 200 nm to 1 μm with an average length of 3.5 μm. Here the reason for such high dense nanowires could be because of the higher Ga content on the substrate. As the substrate is placed just upright on the source, during the ramping of the furnace to the desired temperature all the evaporated Ga sticks to the substrate until the temperature is reached. Once the NH3 is supplied, the Ga converts into GaN, resulting in dense nanowires. Figure 5c shows the TEM images recorded on the GaN NWs. Figure 5d shows the HRTEM image recorded on the body of the NW, which contains a high density of defects. The inset shows the SAED pattern recorded on a single NW. From the HRTEM and SAED it is clear that the NWs contain high defect levels including cubic inclusions and the amorphous shell. The diameter versus length plot for the sample positioned in Figure 5a does not give any specific relation in which the 7351

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Figure 5. (a,b) Scanning electron microscopy images of quasi-aligned GaN NWs grown on the substrate placed directly top of the Ga metal source. The wires have a wide diameter range from 50 nm to 1 μm with the length of few micrometers. (c) Bright-field transmission electron microscopy image of GaN NWs. (d) High resolution transmission electron microscopy image recorded on the body of the NW. Inset shows the selected area electron diffraction pattern.

Figure 6. (a) Scanning electron microscopy image of quasi-aligned GaN NWs grown by self-catalytic approach using chemical vapor deposition. (b) Energy dispersive X-ray spectrum recorded on the body of the GaN NW with the elemental composition is shown in the inset. (c) Bright-field transmission electron microscopy image of ultralong GaN NW. (d) High resolution transmission electron microscopy image of GaN NW and the inset shows the selected area electron diffraction pattern of a single NW.

micro/nanowires show a diameter-independent growth rate as reported earlier.25,26 The higher supersaturation accounts for this diameter-independent growth rate. Considering the fact that all the NWs are quasi-aligned with response to substrate positioning geometry, here we suggest that the evaporated species virtually incorporate in the top part of the growing wires. For instance the NWs were governed by the main contributions from the direct impingement of the vapor species and the adatom diffusion; however, the surface diffusion of adatoms makes a little sense on the growth rate for this specific growth. The supersaturation of the vapor on the liquid phases may increase with increasing precursor flow, and high supersaturation will result in bigger size nucleation than the critical radius (rc) driven by Gibbs free energy. In general, for NWs with thinner radius, the growth rate is higher and for NWs with larger radius, the growth rate is almost independent of its diameter.27,28 In this case, the NWs with larger radius (r > rc) were grown under this condition, and hence the growth rate dependence of diameters can be neglected. 3.3. Whiskered Ultralong GaN Nanowires. Figure 6a shows a FESEM image of high yield straight GaN NWs on the Si substrate. When the substrate is placed at 10 mm away (position b) from the source under the reactor pressure of 200 Torr, it exhibits ultralong whiskered GaN NWs. These NWs covered the whole substrate and were free from tapering. Detailed SEM examination shows that the NWs have an average diameter of 150 nm and the length was up to a few tens of micrometers, indicating the high aspect ratio (length/ diameter) of 100. For thinner wires an aspect ratio of about 200 was achieved. The inset of Figure 6a shows the diameter distribution of GaN NWs. Figure 6c shows the bright field TEM image of long GaN NWs. The NW looks homogeneous and free of domain boundaries, indicating the single-crystalline nature. The NW diameter is apparently very uniform from top to bottom without any tapering effect. HRTEM recorded on the body of the NW is shown in Figure 6d. The inset of Figure 6d shows the SAED pattern recorded on the body of single GaN NWs. Detailed analysis of the diffraction pattern shows

that our NW took the wurtzite structure and grew it along the [0001] direction. TEM and SAED observations over many NWs and also at different locations of each NW showed a similar diffraction pattern and lattice fringes oriented along the [100] crystallographic direction. The self-catalytic VLS approach is expected to be the reason for such lengthier NWs and the growth mechanism is explained below. Initially metal Ga is transported to the substrate for 2 min using N2 carrier gas to form a thin metal droplet layer. Then 200 SCCM of NH3 is introduced without interrupting Ga vapor. Once the metal droplet is formed the nitrogen radicals readily react with the Ga droplet to form GaN nanodots to act as the seed (or) nucleation center for the subsequent growth of GaN NWs. The position of the NW is dictated by the Ga metal droplet, and the size of the nuclei restrict the diameter of the NW. Our assumption is quite reasonable because of various factors that the growth rate of the GaN NWs is much higher which is around 0.25 μm/min. Some ultralong wires are present on the substrate with lengths exceeding 100 μm with a growth rate of about 1.67 μm/min, suggesting that the growth is via self-catalytic VLS and not through a vapor−solid process as the direct reactivity between Ga and N is poor and requires high binding energy. However, a metal droplet at the end of the growth is not observed. A few monolayers of metal Ga on the top of the growing NW may be present, which is good enough to act as a catalyst for the one-dimensional growth. 3.4. Manifold Morphologies with Higher Vapor Pressure. For the next set of the samples, the temperature and pressure were kept fixed at 970 °C (for higher Ga vapor pressure) and 200 Torr, while the substrate-to-source distance was varied. Small pieces of Si (111) substrate were placed from 2 to 20 mm distance from the source, where the vapor pressure of the metal Ga varied approximately from 1 to 0.2 Pa as shown in Figure 4b. The manifold morphologies of GaN nanostructures for different vapor pressure with distance are shown in Figures 7−11. With a smaller source to substrate distance of about 2−5 mm, a thick layer of metal Ga is formed on the 7352

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Figure 7. (a,b) Hexagonal shaped GaN microcrystals formed with the source-to-substrate distance of 2−5 mm with the temperature of 950 °C. (c) Energy dispersive X-ray spectrum recorded on the single hexagonal crystal showing higher Ga content than N content.

Figure 8. (a,b) Scanning electron microscopy images of a hierarchical micro/nanotower like GaN NSs with the source-to-substrate distance of 6−10 mm. (c) Ultrasmall nanoneedles with a diameter of 50 nm grown on the top of the NW. (d) Energy dispersive X-ray spectrum recorded on the nanotower with the inset showing the elemental composition.

surface of the substrate. The Ga is converted into GaN hexagonal microcrystals under the exposure of NH3 as shown in Figure 7. The diameter of the crystal is around 3 μm. EDX recorded on the microcrystals reveals (Figure 7b) the Ga-rich condition still after the crystallization process of 60 min. In addition the formation of polycrystalline GaN crust at the edge of the substrate is clearly evident as shown in Figure 7 b. As the substrate placed very near to the Ga source, accumulation of Ga during early stage of the growth favors microcrystals. Also, it is worth noting that even after the reaction of 1 h, the microcrystals exhibits Ga-rich condition as evidenced by the EDX which shows that all the Ga is not converted into GaN. Further increasing the distance to 6−10 mm we have obtained micro/nanotowers, with the basal diameter of 2 μm and the length higher than 50 μm, and it appears to be that the NWs are decorated with the microcrystals, which finally appear to be micro/nanotowers. Further increase in the distance leads to the formation of microstructures such as sword like edges, micro/ nanosheets. The evolution of different structures of GaN at 970 °C under the reactor pressure of 200 Torr can be explained by the variation of Ga vapor pressure with a difference in sourceto-substrate distance, for instance, Ga pressure is very high near to the source and decays with distance. The growth kinetics with the surface diffusion model is employed to explain the evolution of some interesting NSs. 3.5. Micro/Nanotowers. Figure 8 panels a and b show FESEM images of GaN micro/nanotower like structures. The structural analysis illustrates that the micro/nanotower is composed of two types of structures with distinct morphology. The first one is the wire-like structures and the second is the microcrystals around the nanowires with the base crystals having wider diameter than the crystals stacked on the top of the wires. The growth mechanism for such a structure can be explained as follows. During the initial stage of growth, the metal Ga may act as catalyst for the GaN NWs governed by VLS process. The lateral growth of stacked microcrystal layer below the critical length of the NWs is due to the secondary nucleation on the side surface of the NW.29 Initially the nucleation is formed for the growth of NWs, and the NWs growth takes place with high growth rate from both the contribution of direct impingement and surface diffusion of

adatoms. With time, the adatoms could not reach the top of the growing NW as the migration length becomes shorter than the length of the wire. This leads to the accumulation of crowded adatoms on the base of the NWs that forms the first layer of microcrystals on the side facets of the NWs.30 With time the consumption of radicals leads to an increase in the size of the first layer microcrystals, and the new layer of microcrystals forms on the top of the existing layer as due to limited adatom mobility along the radial direction. By this way hexagonalshaped microcrystals stack on the growing NW. The GaN crystals on the base of the NWs have a diameter of around 1−2 μm, while the crystals at the top layer have an average diameter of 300 nm. These hexagonal patterned microcrystals formed on the side faces of the NWs looks like the NWs decorated with the crystals. In the mean time, the NWs growth rate is also reduced as the adatoms diffusion length is reduced but does not stop, hence the NWs have the opportunity to grow by means of utilizing the adatoms from the direct impingement in the growth regime, and the contribution of diffusion kinetics is negligible for the growth of NWs on the top part of the microcrystals.31,32 We also found another type of micro/ nanostructure as shown in Figure 8c, in which ultraslim NWs extend from the existing NWs, indicating the tapering effect of NWs. The growth can be accounted as purely direct impingement. EDX analysis on the microrod shows higher Ga content over NW on the top of the microcrystals as shown in Figure 8c. The nucleation and subsequent growth of NWs followed by the micro/nanotowers is the result of the reduced III/N ratio of 51.7:48.3, as the growth proceeds. Figure 9 shows the schematic representation of the growth mechanism for the micro/nanotowers. It is worth noting that NWs initially form followed by the decoration of microcrystals on the surroundings of the NW, which appears to be a tower-like structure. 3.6. Nanosheets. Figure 10 panels a−c show the nanosheets (NSHs) with an average thickness of about 25 nm and length vary up to a few tens of micrometers. The NSHs are transparent to electrons (structures below the sheets are clearly visible). NSHs of compound semiconductors including GaN have already been reported,33−37 and these structures 7353

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interaction between the VS and VLS approach in which the former is weaker and the later is stronger in their respective growth process. Thus the adatom diffusion at different temperature and pressure could be the possible reason for the evolution of various micro/nanostructures. Adatom diffusion length is higher at high temperature and low pressure.41 Further, high supersaturation of Ga increases the adatom mobility on the polar surfaces of GaN (0001) which is always metal terminated.42 At Ga-rich condition, weak Ga−Ga bonds formed, which serve as the lubricant for the adatom diffusion. Here as the temperature of the source and substrate are held high, the excess Ga content along the high thermal energy induces the increased adatom mobility which subsequently forms smooth surfaces.20 Here, we picture that the NSH growth takes place from the NW nucleus due to desired Gibbs free energy, following similar growth mechanisms of NWs but extended along the a-plane, and the axial growth at c-direction dominates. In general, GaN NWs grow in the c-axis having {0001} facets along the sidewalls. These facets are expected to have high surface energy and consequently enhance the VS growth rate along low-index planes of GaN. The proposed growth mechanism implies that the nanosheets can only be observed under growth conditions allowing for sufficient VS growth. That is, such an extension of NWs to sheets is the coexistence of two simultaneous growth behaviors, one for the faster growth normal to the wire and the other with slower growth mostly perpendicular to the NSHs. The adatom surface diffusion phenomenon is anticipated to be higher on the side surfaces of wires and specifically on the polar axis. The origin of this direction-dependent surface diffusion process can likely be traced to the “wettability” of polar versus nonpolar surfaces during growth.43 Nevertheless, the tapered nanosheets appear to be catalyst-guided NWs as another segment grew epitaxially on one side of its lateral surface plane of the wire having nonpolar a-plane nanosheets. Though a metal droplet could not be seen on the HRSEM results, the extension of epitaxial NWs from the edge of the NSHs could be interpreted as either catalyst-assisted growth44 or VS route. Further an extensive in situ and ex situ analysis is rather important to elucidate the growth mechanism and to understand the nonpolar nature of nanosheets. The X-ray diffraction pattern for the various GaN nanostructures including quasi-aligned, entangled, nanotowers, and microcrystals is shown in Figure 12. It is obvious that quasialigned GaN NWs have comparatively sharp and dominant diffraction peaks indexed for (100), (002), and (101) planes, exhibiting the wurtzite nature of GaN. Entangled GaN NWs have the peak positions very similar to quasi-aligned NWs in

Figure 9. Schematic representation of the proposed growth mechanism for the nanotower-like structures. From the figure it is clear that that the initial formation of nanowires transforms into the nanotower-like structure by adding the crystals to the side facets of the wire.

Figure 10. (a−c) Scanning electron microscopy images of manosheets with an average thickness of about 25 nm formed with the source-tosubstrate distance of 15−20 mm. (d) Energy dispersive X-ray spectrum recorded on the GaN nanosheets.

were always obtained under the growth conditions of reduced pressure around 200−300 Torr and high source vapor pressure38 which may follow the VLS or VS growth mechanism. Generally an external metal catalyst has been used for such sheet-like structures;39 however, in many reports,34−37 there are no catalysts found on the edges of the NSHs even when an external catalyst is used. Two possibilities for the growth of NSHs from the nucleation process can be enumerated. One is where the nucleation formed for the growth of NWs is expanded along the a-plane of GaN by the secondary nucleation on the surface of the NW into NSH, and the second one is the formation of a two-dimensional sheet-like nucleation which continues further as a sheet. The Gibbs free energy for the nucleation of two-dimensional nanostructures such as NW and sheet is calculated,40 and it is found that the nucleation energy of the NW nuclei is lower than that of the two-dimensional nuclei for sheet. The thermodynamical condition for the formation of NW nuclei is more desirable than that of a sheet like nuclei. In a typical VS process, the incorporation of adatoms into the solid phase involves two steps. One in which there is direct impingement of atoms and the second where there is adatom diffusion followed by the subsequent incorporation at specific site/plane where the Gibbs free energy is minimum. Direct impingement of adatoms on the top of the NW is always similar in both VLS and VS processes, and the only variation is the

Figure 11. Schematic illustration for the growth mechanism of GaN nanosheets. It is worth noting that there is no catalyst present at the apex of the NW in any stage of the growth; however, a few layer of Ga could be present at the top the growing NW and not be resolved using the ex situ analysis. 7354

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the GaN microcrystals which exhibits the broad emission at 3.35 eV that can be assigned to high defects at the boundaries of microcrystals. In addition, all the nanostructures show blue luminescence (BL), green luminescence (GL), and yellow luminescence at 3.22−3.26 eV and 2.96−3.09 eV and 1.9−2.26 eV, respectively. GaN micro crystals shows high intense of defect related YL and BL emission which is dominant than its characteristics band edge emission, which can be attributed to a high level of VN and VNIGa related defects,46 and the results are supported by the nitrogen deficiency in the elemental analysis. The dominant band edge emission demonstrates the high quality of the nanostructures, particularly for quasi-aligned and entangled NWs. The broad band edge is attributed to the coalescence of structures as the spectra recorded on the ensembles. However, we are in the process of doing PL spectra on isolated single wires and sheets. They are expected to reveal a wealth of information about optical transitions. A detailed PL analysis with respect to nanostructures along the structural identification by HRTEM will be published in our forthcoming article.

Figure 12. X-ray diffraction spectra recorded for different nanostructures including entangled NWs, quasi aligned NWs, GaN microcrystals, GaN micro/nanotowers.

4. CONCLUSION In summary, we have reviewed the fabrication of GaN nanostructures with manifold morphologies using Ga/NH3 CVD. By varying the important growth parameters such as reactor pressure, temperature, and vapor pressure (source-tosubstrate distance), we have accomplished different structures including nanowires, nanorods, nanosheets, micro/nanotowers, highly transparent ultrathin nanosheets, microcrystals, etc. The evolution of different nanostructures has been explained using a surface diffusion model. The interplay of growth mechanism and the transition of nanostructures under various growth conditions have an important impact on understanding the evolution of III−N NSs. This interplay can be used to achieve a nanostructure with controlled morphologies by means of reacting Ga vapor with reactive ammonia at elevated temperatures under variable reactor pressures. The GaN nanosheets are transparent to electrons, and the fabrication of such large area sheets is expected to open a potential opportunity for electronics and optoelectronics devices.

addition to the other diffraction peaks of hexagonal GaN. However, in spite of the broad peak for entangled and other nanostructures, the GaN contains neither cubic inclusions nor Ga2O3 phases in manifold nanostructures. 3.7. Optical Properties. Optical properties of selfassembled GaN nanostructures were analyzed using the room temperature photoluminescence (PL) studies. All the nanostructures and their corresponding emission energies are given in Table 1. Figure 13 shows the PL spectra recorded for various Table 1. Photoluminescence Data for Various Nanostructures and Their Corresponding Emission Energies quasi-aligned NWs (Figure 6) entangled NWs (Figure 2) nanosheets (Figure 10) nanotowers (Figure 8) microcrystals (Figure 7)

NBE (eV)

BL (eV)

GL (eV)

YL (eV)

3.41 3.39 3.40 3.42 3.35

3.26 3.22 3.26 3.22

2.96 3.01 3.05 3.00 3.09

1.9 2.26 2.23 2.14 1.92

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +91-431-2407057. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS K.J. thanks the Department of Science and Technology (DST), Govt. of India, for the financial assistances under Project No. SR/FTP/PS-64/2007 and SR/NM/NS-77/2008. V.P. acknowledges CSIR, Govt. of India, for the award of senior research fellowship (SRF). The authors acknowledge Dr. P.V. Satyam, Institute of Physics, Bhubaneswar, India, for TEM measurements. V.P. acknowledges P. Sundara Venkatesh for the technical assistance in photoluminescence measurement and fruitful discussions.

Figure 13. Room temperature photoluminescence spectra recorded for various nanostructures.

nanostructures (entangled NWs, Figure 2; quasi aligned NWs, Figure 6; microcrystals, Figure 7; micro/nanotowers, Figure 8; and nanosheets, Figure 10). All the nanostructures show a near band edge emission of GaN at 3.39−3.42 eV that matches well with that of the MOCVD grown GaN film,45 except for that of

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REFERENCES

(1) Huang, Y.; Duan, X. F.; Cui, Y.; Lieber, C. M. Gallium Nitride Nanowire Nanodevices. Nano Lett. 2002, 2, 101−104.

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(2) Li, Y.; Xiang, J.; Qian, F.; Gradecak, S.; Wu, Y.; Yan, H.; Blom, D. A.; Lieber, C. M. Dopant Free GaN/AlN/AlGaN Radial Nanowire Heterostructures as High Electron Mobility Transistors. Nano Lett. 2006, 6, 1468−1473. (3) Ponce, F. A.; Bour, D. P. Nitride Based Semiconductors for Blue and Green Light Emitting Devices. Nature (London) 1997, 386, 351− 359. (4) Orton, J. W.; Foxon, C. Group III-Nitride Semiconductors for Short Wavelength Light Emitting Devices. Rep. Prog. Phys. 1998, 61, 1−75. (5) Kim, H. M.; Cho, Y. H.; Lee, H.; Kim, S. I.; Ryu, S. R.; Kim, D. Y.; Kang, T. W.; Chung, K. S. High-Brightness Light Emitting Diodes Using Dislocation-Free Indium Gallium Nitride/Gallium Nitride Multiquantum Well Nanorod Arrays. Nano Lett. 2004, 4, 1059−1064. (6) Duan, X.; Lieber, C. M. Laser Assisted Catalytic Growth of Single Crystal GaN Nanowires. J. Am. Chem. Soc. 2000, 122, 188−189. (7) Bae, S. Y.; Seo, H. W.; Park, J.; Yang, H.; Park, J. C.; Lee, S. Y. Single Crystalline Gallium Nitride Nanobelts. Appl. Phys. Lett. 2002, 81, 126. (8) Li, Z. J.; Chen, X. L.; Li, H. J.; Tu1, Q. Y.; Yang, Z.; Xu, Y. P.; Hu, B. Q. Synthesis and Raman Scattering of GaN Nanorings, Nanoribbons and Nanowires. Appl. Phys. A: Mater. Sci. Process. 2001, 72, 629− 632. (9) Kuykendall, T.; Pauzauskie, P.; Lee, S.; Zhang, Y.; Goldberger, J.; Yang, P. Metalorganic Chemical Vapor Deposition Route to GaN Nanowires with Triangular Cross Sections. Nano Lett. 2003, 3, 1063. (10) Wagner, R. S.; Ellis, W. C. Vapor Liquid Solid Mechanism of Single Crystal Growth. Appl. Phys. Lett. 1964, 4, 89. (11) Givargizov, E. I. Fundamental Aspects of VLS Growth. J. Cryst. Growth 1975, 31, 20−30. (12) Allen, J. E.; Hemesath, E. R.; Perea, D. E.; Lensch-Falk, J. L.; Li, Z. Y.; Yin, F.; Gass, M. H.; Wang, P.; Bleloch, A. L.; Palmer, R. E.; et al.et al. High-Resolution Detection of Au Catalyst Atoms in Si Nanowires. Nat. Nanotechnol. 2008, 3, 168−173. (13) Perea, D. E.; Allen, J. E.; May, S. J.; Wessels, B. W.; Seidman, D. N.; Lauhon, L. J. Three-Dimensional Nanoscale Composition Mapping of Semiconductor Nanowires. Nano Lett. 2006, 6, 181−185. (14) Sadan, M. B.; Barthel, J.; Shtrikman, H.; Houben, L. Direct Imaging of Single Au Atoms Within GaAs Nanowires. Nano Lett. 2012, 12, 2352−2356. (15) Chen, Y.; Cui, X.; Zhang, K.; Pan, D.; Zhang, S.; Wang, B.; Hou, J. G. Bulk Quantity Synthesis and Self-Catalytic VLS Growth of SnO2 Nanowires by Lower Temperature Evaporation. Chem. Phys. Lett. 2003, 369 (1−2), 16−20. (16) He, M.; Minus, I.; Zhou, P.; Mohammed, S. N.; Halpern, J. B.; Jacobs, R.; Sarney, W. L.; Riba, L. S.; Vispute, R. D. Growth of Large Scale GaN Nanowires and Tubes by Direct Reaction of Ga with NH3. Appl. Phys. Lett. 2000, 77, 3731. (17) Jacobs, B. W.; Crimp, M. A.; McElroy, K.; Ayres, V. M. Nanopipes in Gallium Nitride Nanowires and Rods. Nano Lett. 2008, 8, 4353−4358. (18) Jacobs, B. W.; Ayres, V.; Crimp, M. A.; McElroy, K. Internal Structure of Multiphase Zinc-Blende Wurtzite Gallium Nitride Nanowires. Nanotechnology 2008, 19, 405706. (19) ElAh, A. M. S.; He, M.; Zhou, P.; Harris, G. L.; Riba, L. S; Felt, F.; Shaw, H. C.; Sharma, A.; Jah, M.; Lakins, D.; et al.et al. Systematic Study of Effects of Growth Conditions on the (Nano-, Meso-, Micro) Size and (One-, Two-, Three) Dimensional Shape of GaN Single Crystals Grown by a Direct Reaction of Ga with Ammonia. J. Appl. Phys. 2003, 94, 7749. (20) Takeuchi, N.; Selloni, A.; Myers, T. H.; Doolittle, A. Adsorption and Diffusion of Ga and N Adatoms on GaN Surfaces: Comparing the Effects of Ga Coverage and Electronic Excitation. Phys. Rev. B 2005, 72, 115307. (21) Yu, L.; Liu, N.; Wang, X.; Hu, Z. A General Low-Temperature CVD Synthetic Route to Fabricate Low Melting Point Metal Compounds 1D Nanostructures. J. Alloys Compd. 2009, 1−2 (478), L21−L24.

(22) Purushothaman, V.; Ramakrishnan, V.; Jeganathan, K. Interplay of VLS and VS Growth Mechanism for GaN Nanowires by a Self Catalytic Approach. RSC Adv. 2012, 2, 4802−4806. (23) Purushothaman, V.; Ramakrishnan, V.; Jeganathan, K. Whiskered GaN Nanowires by Self Induced VLS Approach Using Chemical Vapor Deposition. CrystEnggComm 2012, 14, 8390−8395. (24) Kodambaka, S.; Tersoff, J.; Reuter, M. C.; Ross, F. M. Germanium Nanowire Growth Below the Eutectic Temperature. Science 2007, 316 (5825), 729−732. (25) Kallesøe, C.; Wen, C. Y.; Mølhave, K.; Bøggild, P.; Ross, F. M. Measurement of Local Si-Nanowire Growth Kinetics Using in Situ Transmission Electron Microscopy of Heated Cantilevers. Small 2010, 6 (18), 2058−2064. (26) Kodambaka, S.; Tersoff, J.; Reuter, M. C.; Ross, F. M. DiameterIndependent Kinetics in the Vapor-Liquid-Solid Growth of Si Nanowires. Phys. Rev. Lett. 2006, 96, 096105. (27) Dubrovskii, V. G.; Sibirev, N. V. General Form of the Dependences of Nanowire Growth Rate on the Nanowire Radius. J. Cryst. Growth 2007, 304, 504−513. (28) Borgstro, M. T.; Immink, G.; Ketelaars, B.; Algra, R.; Bakkers, E. P. A. M. Synergetic Nanowire Growth. Nat. Nanotechnol. 2007, 2, 541−544. (29) Yan, Y. G.; Zhang, Y.; Zeng, H. B.; Zhang, L. D. In2O3 Nanotowers: Controlled Synthesis and Mechanism Analysis. Cryst. Growth Des. 2007, 7, 940−943. (30) Katkar, A. S.; Chu, Y. C.; Chu, L. W.; Chen, L. J. ChromiumDoped Germanium Nanotowers: Growth Mechanism and Room Temperature Ferromagnetism. Cryst. Growth Des. 2011, 11, 2957− 2963. (31) Song, T.; Park, W. I.; Paik, U. Epitaxial Growth of One Dimensional GaN Nanostructures with Enhanced Near Band Edge Emission by Chemical Vapor Deposition. Appl. Phys. Lett. 2010, 96, 011105. (32) Yang, Y. H.; Wang, B.; Yang, G. W. Growth Mechanisms of One Dimensional Zinc Oxide Hierarchical Structures. Nanotechnology 2006, 17, 5556. (33) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Nanobelts of Semiconducting Oxides. Science 2001, 291, 1947−1949. (34) Li, L.; Wu, P.; Fang, X.; Zhai, T.; Dai, L.; Liao, M.; Koide, Y.; Wang, H.; Bando, Y.; Golberg, D. Single-Crystalline CdS Nanobelts for Excellent Field-Emitters and Ultrahigh Quantum Efficiency Photodetectors. Adv. Mater. 2010, 22, 3161−3165. (35) Fang, X. S.; Ye, C. H.; Zhang, L. D.; Wang, Y. H.; Wu, Y. C. Temperature Controlled Catalytic Growth of ZnS Nanostructures by the Evaporation of ZnS Nanopowders. Adv. Funct. Mater. 2005, 15, 63−68. (36) Baca, A. J.; Ahn, J. H.; Sun, Y.; Meitl, M. A.; Menard, E.; Kim, H. S.; Choi, W. M.; Kim, D. H.; Huang, Y.; Rogers, J. A. Semiconductor Wires and Ribbons for High Performance Flexible Electronics. Angew. Chem., Int. Ed. 2008, 47, 5524−5542. (37) Dai, Z. R.; Pan, Z. W.; Wang, Z. L. Novel Nanostructures of Functional Oxides Synthesized by Thermal Evaporation. Adv. Funct. Mater. 2003, 13, 9−24. (38) Hao, Y.; Meng, G.; Wang, Z. L.; Ye, C.; Zhang, L. Periodically Twinned Nanowires and Polytypic Nanobelts of ZnS: The Role of Mass Diffusion in Vapor−Liquid−Solid Growth. Nano Lett. 2006, 6, 1650−1655. (39) Weigand, C. C.; Bergren, M. R.; Ladam, C.; Tveit, J.; Holmestad, R.; Vullum, P. E.; Walmsley, J. C.; Dahl, O.; Furtak, T. E.; Collins, R. T.; et al.et al. Formation of ZnO Nanosheets Grown by Catalyst-Assisted Pulsed Laser Deposition. Cryst. Growth Des. 2011, 11, 5298. (40) Zheng, J.; Yang, Y.; Yu, B.; Song, X.; Li, X. [0001] Oriented Aluminum Nitride One Dimensional Nanostructures: Synthesis, Structure Evolution, and Electrical Properties. ACS Nano 2008, 2, 134−142. (41) Mula, G.; Adelmann, C.; Moehl, S.; Oullier, J.; Daudin, B. Surfactant Effect of Gallium During Molecular Beam Epitaxy of GaN on AlN (0001). Phys. Rev. B 2001, 64, 195406. 7356

dx.doi.org/10.1021/jp3120572 | J. Phys. Chem. C 2013, 117, 7348−7357

The Journal of Physical Chemistry C

Article

(42) Zywietz, T.; Neugebauer, J.; Scheffler, M. Adatom diffusion at GaN (0001) and (0001) Surfaces. Appl. Phys. Lett. 1998, 73, 487. (43) Li, H.; Chin, A. H.; Sunkara, M. K. Direction Dependent Homoepitaxial Growth of GaN Nanowires. Adv. Mater. 2006, 18, 216−220. (44) Wang, B.; Yang, Y.; Xu, N.; Yang, G. Mechanisms of Size Dependent Shape Evolution of One Dimensional Nanostructure Growth. Phys. Rev. B 2006, 74, 235305. (45) Shan, W.; Schmidt, T.; Yang, X. H.; Song, J. J.; Goldenberg, B. Optical Properties of Wurtzite GaN Grown by Low Pressure Metalorganic Chemical Vapor Deposition. J. Appl. Phys. 1996, 79, 3691. (46) Reshchikov, M. C.; Morkoç, H. Luminescence Properties of Defects in GaN. J. Appl. Phys. 1997, 97, 061301.

7357

dx.doi.org/10.1021/jp3120572 | J. Phys. Chem. C 2013, 117, 7348−7357