Morphology Control in As-Grown GaN Nanoporous Particles Joan J. Carvajal*,†,‡ and J. Carlos Rojo†,§ Department of Materials Science and Engineering, State UniVersity of New York, Stony Brook, New York 11794, and Fı´sica i Cristal · lografia de Materials (FiCMA), UniVersitat RoVira i Virgili (URV), Campus Sescelades, c/ Marcel · lı´ Domingo s/n, 43007, Tarragona, Spain
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 1 320–326
ReceiVed May 14, 2008; ReVised Manuscript ReceiVed September 18, 2008
ABSTRACT: The influence of different parameters, such as temperature, flow rate of NH3, and pressure, in the crystal growth process of GaN micron-size nanoporous particles through the direct reaction of Ga and NH3 has been studied. Temperature influences porosity and the coalescence of the individual pores. Flow rate of NH3 influences the degree of nanoporosity of the particles. Pressure is the main parameter controlling the external shape of the particles by modulation of the crystal growth rate along or perpendicular to the c crystallographic direction. It also seems to play a role in controlling the size of the pores that can be obtained. Programmed and controlled changes in pressure during the growth experiment resulted in interesting nanoporous structures with benefits for extraction of light and fabrication of electrical contacts on these particles.
1. Introduction Recent advances in the technologies to produce porous semiconductor materials has propelled the use of these materials in the fabrication of enhanced devices for advanced optoelectronics,1,2 sensors,3,4 interfacial structures5 and catalysis,6 as porous semiconductors exhibit unique properties when compared to their bulk counterparts.7 Among these materials, wide bandgap semiconductors (SiC, GaN, AlN, ZnO, BN, etc.) are expected to enable novel technologies in optoelectronics, magnetism, catalysis, and biotechnology due to the band gap shift, efficient luminescence, high surface area, and size selective adsorption that porous semiconductors show.8 The actual application of these materials does, however, critically hinge on the development of processing methods able to precisely control the optical and electrical properties of the resulting porous materials. Furthermore, the use of these porous semiconductors is partially restrained by an incomplete understanding of pore-forming mechanisms. Particular interest in the production of porous GaN arises from the potential to shift the absorption band edge of this material further into the ultraviolet due to quantum confinement effects. Porous GaN thin films have also found application as buffer layers or templates for heteroepitaxial growth of lattice-mismatched materials with low density of defects.9,10 These porous templates could be potentially important for rich-Al AlGaN and rich-In InGaN epitaxial growth for deep UV and green applications, respectively, where the lattice and thermal expansion mismatch between a foreign substrate or even GaN and the active layer results in defects generation that adversely affect the lifetime, reliability, and performance of opto-electronic devices. Another area of technological interest can arise from the strong photoresponse that porous GaN exhibit when illuminated by a broad spectra light source with photon energies from 2.5 to 4.0 eV.11 This particular effect makes porous GaN structures even more attractive than bulk GaN for the development of sensors and detectors operating in the visible and the UV wavelengths of the electromagnetic * Corresponding author. E-mail:
[email protected]. † State University of New York. ‡ Universitat Rovira i Virgili. § Present address: GE Global Research.
spectrum due to the increase in surface area. It has been shown that GaN and some of its alloys exhibit a photocatalytic response with application in the production of hydrogen by water splitting.12 Here, the use of porous GaN structures is clearly attractive due to the enhanced effect resulting from the higher surface-volume ratio when compared with its bulk counterparts. Finally, nanoporous GaN particles represent optically homogeneous media through which the electromagnetic radiation can propagate without internal scattering but where the presence of pores induces an optical anisotropy that may be of particular importance for nonlinear optical and photonic applications when a certain order exists in the position of the pores.13 Recently, we reported a novel and simple technique for the synthesis of GaN porous structures by the direct reaction of Ga and NH3 in a chemical vapor deposition (CVD) system.14 This novel technique presents several advantages over other techniques used to produce porous GaN films that are based in corrosion and etching methods, as it allow us to produce porous GaN without any postgrowth treatment. Corrosion techniques may induce surface damage;15 pores tend to coalesce with etching time16 and the induced porosity is highly dependent on the uniformity of the substrate used.17 This makes control of pore morphology and arrangement in the GaN film difficult. As these parameters are generally regarded as fundamental during the fabrication of nanoporous materials, exploiting the properties of porous GaN has been hampered, because it requires good control over its morphology, surface chemistry, optoelectronic properties, and in situ in-plane patterning on substrate. Our novel method for the production of as-grown GaN micronsized nanoporous particles represents a new opportunity to overcome these difficulties. In this paper, we report the influence of different reaction parameters, such as temperature, flow rate of ammonia, and pressure, on the external morphology and nanopore density of the GaN micron-sized nanoporous particles synthesized by the direct reaction of Ga and NH3. A careful control of these parameters will allow us to establish different strategies to produce GaN micron-sized nanoporous particles with the desired morphology and density of nanopores, as we discuss in the last section of the paper.
10.1021/cg800498y CCC: $40.75 2009 American Chemical Society Published on Web 11/20/2008
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Figure 1. SEM micrographs of the GaN particles obtained at (a) 1073 K, (b) 1123 K, (c) 1173 K, and (d) 1223 K. Flow rate of NH3 and total pressure of the system were kept at 75 sccm and 15 Torr, respectively, during the experiments.
2. Experimental Details 2.1. Crystal Growth. GaN micron-sized nanoporous particles were grown on the surface of a hot-pressed boron nitride (BN) plate AXO5 (Saint Gobain Advanced Ceramics) by the direct reaction of metal Ga (99.99999%) and NH3 (99.999%, Specgas Inc.).14 The reaction was carried out in a quartz tube inserted in a horizontal Khantal-resistance furnace. An excess amount of pure Ga was placed in a quartz boat at the bottom of the quartz tube. The BN plate, used as substrate for the nucleation of the nanoporous GaN particles, was placed in the quartz tube standing at a vertical distance of ∼2 cm from the Ga source. Previous to the start of the chemical reaction, the quartz tube was degassed to a pressure down to 10 mTorr. After that, pure ammonia gas was introduced into the quartz tube through a mass-flow controller. No additional carrier gas was used. The temperature of the furnace was increased at a rate of 100 K/min from room temperature to a preset reaction temperature, at which the system was maintained for a period of 60 min under a constant flow of NH3. Different series of experiments were carried out using different reaction parameters inside the reactor tube. In a first series, the temperature of the reaction was changed in 50 K steps between 1073 and 1223 K, while the flow rate of ammonia was maintained at 75 sccm and the pressure of the system was kept at 15 Torr. In a second series the temperature was maintained at 1173 K and the pressure at 15 Torr, while the flow rate of ammonia was changed at 25 sccm steps between 50 and 100 sccm. Finally, while keeping constant the temperature at 1173 K and the flow rate of NH3 at 75 sccm, the total pressure in the quartz tube was set at three different values: 1.5, 15, and 150 Torr. After the influence of temperature was evaluated, flow rate of NH3 and pressure on the production of nanoporous GaN particles, a new set of experiments was undertaken, where the pressure was changed abruptly from 15 to 1.5 Torr and from 1.5 to 15 Torr after 30 min of the beginning of the reaction. In these experiments, the reaction was maintained between 60 and 180 min at the latter pressure.
To stop the reaction, the furnace was cooled down quickly to room temperature, while the NH3 was stopped and the system was kept under vacuum at a pressure around 10 mTorr. The resulting GaN nanoporous structures were collected on the surface of the BN substrate facing to the Ga source, which was covered with a light yellow layer. 2.2. Morphological and Structural Characterization. The morphology, size, and composition of the resulting GaN micron-sized nanoporous particles were analyzed by scanning electron microscopy (SEM) in a LEO 1550 SFEG-SEM equipped with a Robinson BackScatter detector and an energy-dispersive X-ray (EDX) spectrometer. The particles were also analyzed morphologically and structurally under a JEOL FEM 3000F TEM high-resolution transmission electron microscope (HRTEM) available at the Center of Functional Nanomaterials in the Brookhaven National Laboratory, operating at 300 kV. The as-grown GaN nanoporous particles were scraped from the surface of the BN substrate and mixed with ethanol. Several drops of this ethanolic suspension were deposited on a copper TEM grid covered by a holey carbon film and the ethanol was evaporated off.
3. Results and Discussion 3.1. Effect of Temperature. Figure 1 shows SEM pictures of the GaN particles with different morphologies obtained at different temperatures, ranging from 1073 to 1223 K, while keeping the flow rate of NH3 at 75 sccm and the pressure of the system at 15 Torr in all the experiments. At 1073 K (see Figure 1a), small nonporous GaN particles, 150 nm in size, were obtained on the top of the BN plate. The particles show a prismatic shape with sharp edges. At 1123 K (see Figure 1b), nonporous GaN particles with a prismatic shape were obtained with larger sizes, around 350 and 500 nm.
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Figure 2. Morphology of the GaN nanoporous particles obtained at 1173 K, a flow rate of NH3 of 75 sccm, and a total pressure of the system of 15 Torr: (a) SEM micrograph of a single particle; (b) simulation of the morphology of the particle shown on (a) with the Miller indexes corresponding to the different faces corresponding to the wurtzite structure; (c) TEM micrograph of a small single particle; (d) simulation of the morphology of the particle shown on (c) with the Miller indexes corresponding to the different faces that can be identified on the wurtzite structure.
At 1173 K, GaN nanoporous micron-size particles were obtained, as can be seen in Figure 1c. The particles were found to be very homogeneous in size (∼1.5 µm) and morphology. A close analysis of the particles revealed a pyramidal shape with a prismatic shape similar to a hexagonal pyramid with smooth lateral faces, and most important, a very clearly defined porous structure. All the particles present pores with diameters ranging from 40 to 100 nm, and only observed on the basal plane of the micron-size particles. The walls among pores have a thickness less than 50 nm, as can be seen in the inset included in Figure 1c. The SEM micrographs taken from these particles revealed a quasi-alignment of the pores, parallel to each other, with their axis perpendicular to the (0001) basal plane of the GaN particles. Figure 1d shows the particles obtained at 1223 K. Although the external shape of these particles is almost the same than that of the particles obtained at 1173 K, they are larger, with dimensions ranging between 1.5 and 5 µm. Also, the well-defined nanopore structure obtained at 1173 K has been substituted by a network of ridges running along the basal plane of the particles. This network of ridges is thought to be formed by the coalescence of neighbor pores. However, in the most external part of the basal plane, some nanopores with a circular cross section can still be seen. A simulation of the external morphology of the particles shown in Figure 1c using a commercial software18 is shown in Figure 2, together with some additional SEM and TEM pictures of the particles. This simulation indicates that the growth orientation is parallel to the 〈0001〉 direction. Wurtzite GaN crystallizes in the hexagonal system, with the P63mc as the spatial group of symmetry. This means that the symmetry of the structure does not include an inversion center; the structure is polar. As a consequence of this lack of inversion center, the {hkıjl} and the {hkıjjl} forms are not required to be equivalent in the morphology of the particle. They can show different growth rates. Taking into account these considerations, the morphology of the particles can be described as a hexagonal pyramid
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confined by the six {101j1} planes, the basal (0001j) plane, and the incipient six {101j1j} planes. Note that we are not able to discriminate theoretically between the c+ and the c- crystallographic directions. This means that we could also describe the morphology of the particles as pyramids confined by the six {101j1j} planes, the basal (0001) plane, and the incipient {101j1} planes. However, we have chosen the former nomenclature as it has been reported that the N-terminated face in GaN (usually named as the (0001j) face, tends to show a roughness higher than the Ga-terminated face (usually the (0001) face)).19 Independently of the nomenclature adopted, this analysis indicates that the pores, which seem to be perpendicular to the basal plane, are running parallel to the c crystallographic direction of the wurtzite structure of GaN. This hypothesis was confirmed by the characterization of these particles by HRTEM. Figure 3a is a low-magnification image taken from a typical porous GaN particle after the particle was tilted to obtain the image from the low index 〈112j0〉 zone axis. The (0001j) basal plane, that is the one that contains the pores, could not be observed due to the thickness of the particle; this is why no features of the pores are observed in these micrographs as this makes the lateral {101j1} forms perpendicular to the electron beam, while the (0001j) basal plane is tilted by an angle of 60° with respect to the plane of observation of the sample. Figure 3b is a selected area electron diffraction (SAED) pattern taken from the corner of the particle that is shown in the selected area in Figure 3a, opposite to the basal plane. From the measurements and calculation of the resulting d-spacings observed in the high-resolution TEM images, the zone axis was proven to be 〈112j0〉. All the diffraction peaks in this pattern could be indexed according to the wurtzite structure of GaN, which indicates that the nanoporous GaN particle is single crystalline in the regions measured. However, the diffraction streaks observed in some of the peaks along g ) 0001 suggest the existence of thin lamellar defects lying parallel to the (0001) basal plane. These streaks are evident in the (11j00) diffraction peaks of the pattern, and show a splitting of the intensity of the diffraction peak in the main spot and a smaller one located to the north of the main spot in the case of the (11j00) diffraction peak and to the south of the main spot in the case of the (1j100) peak. More highly magnified observations reveal the existence of stacking faults along the c-crystallographic direction, confirming the observation extracted from the diffraction streaks in the SAED pattern. These stacking faults are marked with arrows in Figure 3c. Finally, Figure 3d shows a higher magnification image of the corner of the GaN nanoporous particle shown in Figure 3a, and opposite to the basal plane. The two-dimensional atomic structure can be clearly observed in the inset of this picture with resolved d-spacings of 0.52 and 0.28 nm for the (0001) and (101j0) planes, respectively, which are consistent with wurtzite GaN.20,21 Some of the particles obtained at 1223 K and shown in Figure 1d crystallized with a peculiar shape similar to an hourglass shape. This hourglass shape morphology is due to an asymmetric reflection twinning, composed by a plane perpendicular to the c crystallographic direction, with one part of the twin longer than the other. This morphology was previously observed in ZnO crystals formed by a hydrothermal process,22 although in that case, the basal planes of the crystals (in this case two basal planes, one in each part of the twin) were completely flat. Figure 4 shows the simulation of the external morphology of these particles together with additional SEM pictures. This reflection twinning along the c crystallographic direction has important implications for the domains in the wurtzite GaN structure.
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Figure 3. Transmission electron micrographs of a GaN nanoporous particle: (a) low-magnification image of a single particle; (b) SAED [112j0] zone axis pattern; (c) magnified image of the selected area depicted in (a) showing the presence of a few stacking faults (marked with arrows in the picture); (d) high-magnification image of the selected area depicted in (c). The inset shows the two-dimensional atomic structure with resolved d-spacings for the (0001) and (101j0) planes.
Figure 4. (a) SEM micrograph, and (b) simulation of the morphology of a GaN particle grown at 1223 K, a flow rate of NH3 of 75 sccm, and a total pressure of the system of 15 Torr showing an hourglass shape morphology due to an asymmetric reflection twinning.
Controlling the position and the orientation of these twinning structures may open up a new route to create as-grown periodically poled GaN with potential to generate wavelengths from the far-infrared to the near UV by sum- or differencefrequency generation.23 3.2. Effect of the Flow Rate of NH3. We performed experiments at flow rates of 50, 75, and 100 sccm while fixing the temperature of the reaction at 1173 K and the total pressure of the system at 15 Torr during the reaction time. At a flow rate of ammonia of 50 sccm, a large quantity of small GaN nanoporous particles with diameters ranging between 500 and 700 nm were obtained, as it can be seen in Figure 5a. Polyhedra with a hexagonal symmetry are linked together forming large aggregates, with particles growing along different directions. These particles form an irregular film on the surface of the BN flakes. The hexagonal polyhedra tend to show pores on one of their surfaces, although they are not as well defined as they were in the previous case when the experiment was performed at a flow rate of 75 sccm of NH3 (see Figure 1c). These incipient pores are 40-100 nm in diameter and are clearly visible on the surfaces of the GaN particles perpendicular to the main hexagonal morphologies. The individual particles tend to show a central core, more prominent and free of pores, surrounded by a external part with pores. The number of pores seems to be reduced when compared with the number of nanopores obtained in the GaN particles grown at a flow rate of 75 sccm of NH3, also due to the reduced size of these particles when compared to the previously obtained When we increased the flow rate of NH3 up to 100 sccm, prismatic particles with diameters ranging between 700 nm and 1.1 µm are obtained (Figure 5b). The particles show different external morphologies, with some of them showing morphol-
ogies similar to truncated hexagonal bipyramidal prisms, where the pores, in most of the cases a single pore with a diameter of around 150 nm (Figure 5b), are found on one of the truncated bases of the bipyramids. Other particles show hexagonal prismatic shapes, with one or multiple pores on the basal planes perpendicular to the c crystallographic direction (Figure 5c). Finally, some particles show an hourglass shape (Figure 5d). In these particles, multiple nanopores can be observed in the two basal planes of the hourglass morphology. 3.3. Effect of the Total Pressure of the System. Pressure seems to be critical to control the morphology, and subsequently, the rate of growth of GaN particles. Changes in the reactor pressure leads to dramatic changes in the morphology of the GaN particles, from small prismatic particles to large and thin platelets. However, the porosity is not lost in any of the morphologies obtained. Different experiments were undertaken at total pressures of the reactor (always measured downstream of the direction of flow of NH3) of 1.5 and 150 Torr, while the temperature of the reaction and the flow rate of ammonia were kept at 1173 K and 75 sccm, respectively. These experiments where performed additionally to the ones where we kept the total pressure of the system at 15 Torr, and whose results have been described in detail in the previous sections. At 1.5 Torr, hexagonal GaN platelets, ∼250 nm in thickness and up to 6 µm in diameter, were obtained (Figure 6a). These GaN platelets show an almost perfect hexagonal morphology, which indicates that the larger faces of the particles are formed by the {0001} planes. Low pressure causes a dramatic increase in the growth rate perpendicular to the c crystallographic direction, making the particles more developed along the (0001) plane, while their dimension in c direction is up to 24 times smaller. However, the faces of these platelets exhibit both smooth and rough surfaces. It has been reported that the N-terminated (0001j) face is less stable and tends to be rougher than the Ga-terminated (0001) face.19 Then, while still speculative, these differences in the termination of the faces of GaN could explain the presence of flat and rough surfaces in Figure 6a. However, differently from previous reports that can be found in the literature, the roughness of the N-terminated face in these platelets is organized in the form of pores distributed along one of the surfaces of the plates. The larger pores that can be observed in the rough surfaces of these particles have diameters of ∼100 nm, and show an almost elliptical shape when observed under the high-resolution transmission electron microscope. Figure 6b,c shows HRTEM pictures of one of those pores. While the lower magnification image in Figure 6b shows the almost
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Figure 5. SEM micrographs of the GaN particles obtained at a flow rate of NH3 of (a) 50 sccm and (b,c, and d) 100 sccm. The crystal growth experiments were performed at 1173 K keeping the total pressure of the system at 15 Torr. Panel (b) shows several particles showing a single pore in one of their faces. Panel (c) shows a hexagonal prism-shaped GaN nanoporous particle presenting a single pore in one of their basal planes, while the other basal plane is attached to the substrate. Panel (d) shows several GaN nanoporous particles with an hourglass shape morphology. The presence of pores in the two parts of the particles show the asymmetric reflection twinning responsible for this shape.
Figure 6. (a) SEM micrographs of the GaN nanoporous plates obtained for a total pressure of the system of 1.5 Torr. (b) Low-magnification TEM micrograph of one of the pores showing an almost oval shape. (c) High-resolution TEM micrograph of the same pore shown in panel (b) showing the facets that form the pore. The two-dimensional atomic structure is also evident in this picture, showing that both the deeper (inside the pore) and the tallest part of the particle (the surface of the plate) have the same atomic structure, confirming the single crystal nature of these particles.
elliptical shape of these pores, the high-resolution image in Figure 6c shows the two-dimensional atomic structure of the particle. These images were taken with the rough face of the particles, in which the pore is located, perpendicular to the electron beam. This indicates that these faces are growing perpendicular to the c crystallographic direction, as can be deduced from the hexagonal symmetry observed in the disposition of the atoms in the structure. It can also be seen in the atomic resolution picture that the atomic structure is continuous between the top level of the pore (the plane of the face where
the pore is lying) and its deeper part. This is a further indication of the single crystalline nature of these particles. When we performed the experiment at a total pressure of 150 Torr, small hexagonal prismatic GaN particles were obtained (Figure 7a). The diameters of these particles are ∼600 nm. The simulation of their external morphology is shown in Figure 7b. Their morphology is confined by the six {101j1} planes, the six {101j1j} planes and the (0001j) and the (0001) basal planes. When we compare this morphology with the one observed in particles obtained at 15 Torr (Figure 1c), the six
Morphology Control in GaN Nanoporous Particles
Figure 7. (a) SEM micrograph of the GaN nanoporous plates obtained for a total pressure of the system of 150 Torr. (b) Simulation of the morphology of one of the particles shown in (a). All the experiments were performed at 1173 K and a flow rate of NH3 of 75 sccm.
{101j1} planes are more developed in this new morphology, but an asymmetry still exists in the development of these faces compared with the {101j1} planes, revealing again the absence of an inversion center in the symmetry of the structure. The most interesting feature in these particles is, however, that they still show nanopores with diameters below 15 nm, on their basal planes. In particles that are lying on the substrate out of the c crystallographic direction, we can observe an incipient roughness on both basal planes, running parallel to the c crystallographic direction. While at low pressures (1.5 Torr) we have a fast growth perpendicular to the c crystallographic direction, and at intermediate pressures (15 Torr) we have a fast grow along the c crystallographic direction; at high pressures (150 Torr) we have a more balanced growth both perpendicular and parallel to the c crystallographic direction, generating more isometric particles. After we analyzed these results, we explored the possibilities to establish different strategies to grow GaN micron-size particles controlling their external morphology and, if possible, also controlling the porosity to some extent. Figure 8a shows the results of a experiment in which we started the growth process at a pressure of 15 Torr and then we decreased it quickly down to 1.5 Torr 30 min after the beginning of the experiment, and maintained the system at this latter pressure for 1 h. During the whole experiment, the temperature was maintained at 1173 K and the flow rate of NH3 at 75 sccm. The GaN particles obtained show an almost perfect hexagonal pyramidal shape, with smooth lateral faces and a basal plane formed by a core area where the pores are formed and an external area with a smooth surface free of pores. After
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analyzing this morphology in detail, we believe that the core porous area of the basal plane was grown at the beginning of the experiment, when keeping the pressure at 15 Torr. The external area seems to be grown at a faster growth rate perpendicular to c direction than the central core porous area, coinciding with the growth conditions found in our previous experiments performed at 1.5 Torr. Then, by changing quickly the pressure from a medium value to a low value, we are modifying the conditions of growth from a situation with a fast growth along the c crystallographic direction to a situation with a fast growth perpendicular to this direction. This procedure allow us to control better the external morphology of the particles, adjusting it to an almost perfect hexagonal pyramid, in which the particles do not show anymore the incipient {101j1j} planes. Also the porosity is restricted only to the central part of the basal plane of the particles, which provide a flat and smooth area around this porous section in which electrical contacts may be fabricated, if needed, without covering the pores of the particles. Figure 8b shows the GaN particles obtained when we started the growth process at a pressure of 1.5 Torr and increased it quickly up to 15 Torr 30 min after the beginning of the experiment, and maintained this latter pressure for an additional 60 min. During the whole experiment, the temperature was maintained at 1173 K and the flow rate of NH3 at 75 sccm. From the morphology obtained in these particles, they seem to start the growth process as plates, as it was expected from a growth process at low pressure, and then, when we abruptly changed the growth conditions to provide a faster growth rate along the c axis, these plates started to grow in thickness. However, it seems that the edges of these plates started to grow faster than their central part. This probably may be due to the large number of available kinks at the edges of the particles compared with the flat central area. This may also be an indication that we are not only changing the growth rate along the c axis during this growth process, but also that we are favoring a 3D crystal growth mechanism at medium pressure values, while we have a 2D crystal growth mechanism at low pressures. Then, as the edges of the plates are growing faster along the c axis than the central part of the plates, the particles tend to grow leaving only one pore in the central part of most of the particles. Note that using this procedure we are getting something similar to what can be considered the opposite structure to that obtained starting the growth process at a medium pressure level and changing it faster to a low level. Here we are getting particles with a larger flat basal plane and a small porous region on top of it. This morphology also presents
Figure 8. SEM micrographs of the GaN nanoporous particles obtained in experiments where we changed the pressure of the system after 30 min of the beginning of the experiment (a) from 1.5 to 15 Torr, and (b) from 15 to 1.5 Torr, keeping the temperature and the flow rate of NH3 constant at 1173 K and 75 sccm, respectively.
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benefits to fabricate electrical contacts on them, as they have a larger useful flat area to do it. Also, although reduced in extension, the particles maintain a porous structure, with benefits for the extraction of light from these porous structures, one of the main problems of solid state lighting.
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in the characterization of the GaN particles by HRTEM. J.J.C. also thanks the Commission for Cultural, Educational and Scientific Exchange between the United States of America and Spain, and the Spanish Government for a Fulbright-MEC Fellowship.
4. Conclusions
References
We studied how the different parameters of the reaction, including temperature, flow rate of NH3 and pressure affect the crystal growth process of GaN micron-size nanoporous particles by the direct reaction of metal Ga and NH3. Temperature is the parameter that produces the largest effect in terms of producing porous or nonporous particles. At high temperatures, however, it has also been observed that pores tend to coalesce forming ridges on the basal planes of the particles. The flow rate of ammonia affects to the degree of porosity of the particles, from incipient porosity at low flow rates to the production of particles with a single pore at the central part of the particle at high flow rates of NH3. Finally, pressure is the parameter that most affected the growth rate along or perpendicular to the c crystallographic direction. This has a huge influence on the external morphology of the particles, ranging from hexagonal thin plates to hexagonal prisms. After a careful analysis of these results, we were able to establish different strategies to obtain particles with different morphologies combining growth segments in one experiment. This allowed us to control not only the external morphology of the particles, but also the area of nanoporosity of the particles and at some extent, the number of pores. Compared to other reported approaches, this simple, direct, and inexpensive process is unique in that it results in the formation of nanoporous micron-size particles during the growth process without requiring any additional postgrowth treatment. The morphologies obtained are expected to have benefits when fabricating electrical contacts on these particles as well as in the extraction of light that can be achieved from the nanoporous areas.11
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Acknowledgment. This work has been partially supported by the College of Engineering Dean’s Office of the SUNY Stony Brook and the Center for Advanced Technology in Diagnostic Tools and Sensor Systems. TEM characterization was performed at the Center for Functional Nanomaterials of the Brookhaven National Laboratory (BNL). The authors thank E. Sutter from the Center for Functional Nanomaterials at BNL for her help
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