Faceted and Vertically Aligned GaN Nanorod Arrays Fabricated ...

NANO LETTERS

Faceted and Vertically Aligned GaN Nanorod Arrays Fabricated without Catalysts or Lithography

2005 Vol. 5, No. 9 1847-1851

Parijat Deb,*,† Hogyoung Kim,‡ Vijay Rawat,† Mark Oliver,† Sangho Kim,§ Mike Marshall,| Eric Stach,† and Timothy Sands†,§ School of Materials Engineering, Department of Physics, and School of Electrical and Computer Engineering, Purdue UniVersity, West Lafayette, Indiana 47907, and The Frederick Seitz Materials Research Laboratory, UniVersity of Illinois at Urbana-Champaign, Urbana, Illinois 61801 Received June 8, 2005; Revised Manuscript Received August 1, 2005

ABSTRACT Monocrystalline, vertically aligned and faceted GaN nanorods with controlled diameter have been synthesized by selective organometallic vapor phase epitaxy (OMVPE) onto GaN exposed at the bottom of pores in silicon dioxide templates patterned by reactive ion etching through self-organized porous anodic alumina films. This process is free of foreign catalysts, and the nanorod diameter control is achieved without the need for low-throughput nanolithographic techniques. The use of conventional OMVPE growth conditions allows for the straightforward adaptation of conventional doping and heterostructure growth as will be necessary for the fabrication of nanorod-based strain-relaxed electrically pumped lasers and light-emitting diodes.

Gallium nitride nanowires, nanorods, and nanopillars are intriguing structures for applications in nanophotonics.1-12 The high-aspect-ratio form factor combined with the possibility of quantum confinement suggest the feasibility of fabricating electrically pumped nanolasers that could be used in information storage, as optical interconnects, or as nearfield probes in studies of biological systems. Beyond carrier confinement and the potential advantages of the high-aspectratio form factor, nanowires, -rods, and -pillars offer the possibility of expanding the range of band engineering of coherent heterostructures through the additional elastic strain relief afforded by the presence of the additional free surface. The large lattice mismatch between GaN and its alloys with InN and AlN can be accommodated in longitudinal heterostructures if the radius of the nanowire, -rod, or -pillar is below a critical value, which is typically about an order of magnitude larger than the comparable critical thickness of a planar thin film heterostructure.13-17 This strain relief mechanism should enable greater InN mole fractions and/or larger InN-rich quantum dots in the active region of GaN-based light emitters, thereby expanding the portion of the visible spectrum that is accessible to efficient GaN-based emitters * Corresponding author. E-mail: [email protected]. † School of Materials Engineering, Purdue University. ‡ Department of Physics, Purdue University. § School of Electrical and Computer Engineering, Purdue University. | Materials Research Laboratory. 10.1021/nl0510762 CCC: $30.25 Published on Web 08/12/2005

© 2005 American Chemical Society

from today’s blue and green, toward yellow, orange, and red. The possibility of using strain engineering to synthesize GaN nanostructures, thus enabling fabrication of a phosphorfree monolithic white LED for applications in solid-state lighting motivates the present work. To make such a heterogeneous device practical, a process for fabricating dislocation-free controlled-diameter nanorod arrays without foreign catalysts, postgrowth fluidic assembly, or nanolithography is arguably necessary. In this paper, a novel process is reported, yielding nanorod arrays with controllable dimensions defined by self-organized templates over large areas. The elimination of the catalyst obviates the associated concerns of contamination by the catalyst metal, dimensional control (both in diameter and length), and control of alloying and doping profiles as mediated by the catalyst particle during vapor-liquid-solid (VLS) growth. The process described here also benefits from a conventional GaN deposition method, organometallic vapor phase epitaxy (OMVPE), that is easily adapted to alloying with AlN and InN as well as doping with Si and Mg, employing conventional growth conditions similar to those used today for the commercial production of GaN-based LEDs. There have been several reports describing the growth of GaN nanowires and nanorods using catalysts such as Fe, Au, and Ni and employing templates such as carbon nanotubes.18-23 Low-temperature growth using hydride vapor phase epitaxy (HVPE) on unpatterned sapphire substrates has also been

Figure 1. (a) Cross-sectional FESEM image after two-step anodization and pore widening. (b) FESEM image of top surface of SiO2 pattern after removal of porous anodic alumina (PAA) template.

shown to yield GaN and (InGa)N nanorod structures with various morphologies.11,24,25 GaN and (InGa)N one-dimensional structures have also been grown by molecular beam epitaxy.26,27 However, these previous synthesis methods do not enable controlled variation of nanorod diameters on the same wafer and either use catalysts or do not employ conventional LED growth methods or conditions. Porous anodic alumina (PAA), on the other hand, has a hexagonally ordered pore structure with diameters that can be controlled by varying the anodization potential. Recent work by Wang et al.28 has demonstrated the applicability of these templates toward the creation of GaN nanostructures by a subtractive process using inductively coupled plasma etching of GaN films. In this paper, we describe a process that uses reactive ion etching to transfer a pattern into a silica template to enable selective growth of faceted GaN nanorods of controlled diameter. Slight variations of this process can be used to incorporate (In,Ga)N heterostructures, p-n junctions, and controlled nanorod diameter variation on the same substrate. The starting substrates for the present work were undoped 5 µm GaN films grown by HVPE on c-plane sapphire. The roughness of the GaN film was less than 5 nm root mean square (rms) (measured by atomic force microscopy over an area of 5 µm by 5 µm). The GaN surface was prepared by ultrasonication in acetone and methanol for 5 min each, followed by immersion in 50 vol % HCl for 10 min. A 60 nm thick SiOx film was then deposited on the HVPE GaN film using an electron beam evaporator at a deposition rate of 0.3 nm/s. Without breaking vacuum, a 1 µm thick Al film was deposited at 1 nm/s, yielding an Al surface with 10 nm rms roughness. The chamber pressure before e-beam evaporation of SiOx and Al was below 6 × 10-7 Torr. The sample edges were coated with nail polish to prevent exposure of GaN during the subsequent anodization process. A two-step anodization was then carried out in a 0.4 M oxalic acid solution at 4 °C and a potential of 40 V. The two-step anodization helps in improving the pore order in the PAA film.29 The first anodization proceeded until the current began to decrease, an observation that was correlated by crosssectional scanning electron microscopy to a reduction in the Al thickness to approximately 200 nm. The alumina was then dissolved by immersing the sample in a phosphoric acidchromic oxide solution (3.5 mL of H3PO4 + 4.5 g of CrO3 1848

+ 100 mL of deionized H2O) at 60 °C. The second anodization was then carried out until the current in the circuit dropped significantly (to ∼20 mA for a 2 in. wafer). To ensure complete conversion of the entire Al into its oxide, the voltage was maintained for 60 s after the current had stabilized at this low value. The PAA template serves as a mask to transfer the pore pattern into the SiOx layer. Such a process was found to be necessary, as GaN nucleates on PAA under conventional growth conditions. In contrast, GaN does not readily nucleate on silica, allowing for selective growth only in regions of exposed substrate.30 The pore diameter in the as-anodized alumina structure was about 10-15 nm. We have experimentally observed that this initial pore diameter is insufficient for transferring the desired pattern into the SiOx film using reactive ion etching. We think this limitation is due to the divergence of the ion beam which makes it difficult for the ions to reach the bottom of the pores when the aspect ratio of the PAA template is high. Hence, the porous anodic alumina was subjected to a pore widening step by immersion into a 4 vol % H3PO4 solution at room temperature for 30 min. The resulting pore diameter was approximately 60 nm, as can be seen in Figure 1a. Next, the sample with the porewidened alumina pattern on the SiOx film was subjected to reactive ion etching. The gases used were SF6 and O2 with flow rates of 10 sccm each. The power was set at 100 W and the chamber pressure was maintained between 55 and 60 mTorr, resulting in a dc bias of 400-425 V. These reactive ion etching (RIE) parameters and the etching time must be precisely controlled to ensure etching through the barrier oxide and the SiOx film, with minimal attack of the underlying GaN film. For the present case, the etching was carried out for 2 min, which was sufficient to form a throughhole pattern in the SiOx film without significant etching of the GaN film. The remaining PAA template was removed by immersing the sample in a 5 vol % H3PO4 solution at 50 °C for 60 min. Figure 1b shows a top view of the SiOx template on GaN. The pore diameters in the present study were found to be controllable in the range from 50 to 200 nm by adjusting the anodization potential, the pore widening time, or the RIE process conditions. For example, anodization at 55 V combined with pore widening for 45 min yielded pores with 100 nm diameter. Nano Lett., Vol. 5, No. 9, 2005

Figure 2. (a) FESEM image of GaN nanorods showing faceting in the nanostructures. (b) FESEM top view images of the GaN nanorods indicating very narrow size distribution and even coverage over the underlying GaN film surface. (c) and (d) FESEM images of GaN nanoods having mean nanorod diameters of 50 nm (image c) and 100 nm (image d). The insets in parts c and d are images of 50 and 100 nm diameter nanorods at the same magnification.

The growth of the GaN nanorods on the GaN substrate exposed within the pores in the SiOx template was performed in an Aixtron 200HT OMVPE reactor. The growth parameters were chosen to ensure a vertical growth regime even at high temperatures to prevent the nucleation of stacking faults in these nanostructures. A vertical growth regime is achieved when the growth rate perpendicular to the substrate plane is higher than the rate parallel to the plane, resulting in high-aspect ratio structures. Previous studies of lateral epitaxial growth of GaN on SiOx patterned GaN substrates indicated that a hydrogen carrier gas along with a relatively low V/III ratio will result in vertical growth of GaN.31 For the present work, the V/III ratio was 1500 with ammonia as the nitrogen source and trimethylgallium as the Ga source. The reactor pressure was maintained at 13 kPa with hydrogen as the carrier gas. The total flow rate of all gases taken together was kept at 10 slm. The growth temperature was 1020 °C, a conventional substrate temperature for GaN layer growth by OMVPE. During the temperature ramp up, a high overpressure of NH3 (the partial pressure of NH3 was calculated to be 6.76 kPa at room temperature, the total reactor pressure being 13 kPa and the only other gas species being hydrogen) was maintained over the sample to prevent decomposition of the exposed GaN. Growth was carried out for 5 min, which for conventional film growth under the same growth conditions gave a film thickness of 100 nm. Finally, Nano Lett., Vol. 5, No. 9, 2005

the SiOx film was removed by immersing the sample in buffered oxide etch for 20 s. Figure 2a shows the final GaN nanorod structures after the SiOx pattern removal. Parts b and c of Figure 2 show the nanorod structures in plan view at different magnifications. To vary the GaN nanorod diameter controllably, the anodization potential was changed to 55 V and the pore widening time to 45 min with all other process parameters unchanged. The resulting mean nanorod diameter was increased to 100 nm, as shown in Figure 2d. Field-emission scanning electron microscopy (FESEM) revealed prismatic faceting along with a pyramidal cap that formed during the self-terminated growth as the nanorod grew beyond the silica template. On increase of the growth time, the nanorod tip came out of the silica template and started growing laterally, still maintaining the pointed tip morphology, a growth mode similar to that of epitaxial lateral overgrowth using low V/III ratios.31 The GaN nanorod diameter distribution measured from the sample imaged in Figure 2c was found to be 50 ( 10 nm and in Figure 2d to be 100 ( 10 corresponding to PAA templates anodized at 40 and 55 V, respectively. The pointed tip morphology of these nanostructures could find use in field emission devices as is suggested by the very bright nature of these nanorods in the FESEM micrographs. Note that not all of the pores in the silica template were found to host nanorods. The factors that control the pore filling are under investigation. 1849

Figure 3. (a) [112h0] zone axis microdiffraction pattern from the underlying GaN film. (b) [112h0] zone axis microdiffraction pattern from a single GaN nanorod. (c) Selected area diffraction aperture position. (d) The selected area diffraction spot pattern from one GaN nanorod and the underlying GaN.

Samples for transmission electron microscopy (TEM) were prepared using a dual-beam focused ion beam instrument. The procedure involved making a thin section using the Ga ion beam, picking up the sample using a tungsten Omniprobe, and welding the specimen to a 3 mm TEM grid using platinum.32 The faceting as seen from the FESEM images could be observed even in the TEM images to some extent (not shown). Also, the absence of any threading dislocations extending from the GaN film into the nanorods, when seen at different sample tilts, suggests that the nanorods are free of extended defects. Parts a and b of Figure 3 show the [112h0] zone axis microdiffraction patterns obtained from the GaN film and the GaN nanorod, respectively. The symmetry along the zone axis confirmed that the nanorod is growing in the same orientation as the underlying GaN film. To further confirm this observation, a selected area diffraction (SAD) pattern was obtained from a sample region containing both the GaN film and a nanorod. The SAD aperture position is shown in Figure 3c, and the corresponding [112h0] zone axis diffraction pattern is shown in Figure 3d. The SAD pattern confirmed that the nanorods were growing epitaxially on the underlying GaN film. From the FESEM and TEM studies, we conclude that the cap of the nanorods comprises six {11h01} facets and the vertical walls are prismatic planes of the {11h00} type. The diffuse circular rings in the selected area diffraction patterns are due to the presence of Pt and 1850

Au/Pd that were used to protect the sample during the TEM sample preparation. In summary, we have demonstrated a new nonlithographic and catalyst-free technique for controllably growing GaN nanorods in place, without the need for postgrowth assembly. FESEM and TEM analyses show that the nanorods grow epitaxially on the underlying GaN film. This process provides control of the nanorod diameter by varying the PAA pore diameter, which in turn can be controlled by the applied voltage during anodization or by a subsequent pore widening process. The synthesized nanostructures are faceted and are expected to be free of any extended defects. Further work on growing graded (AlGa)N and (InGa)N nanorods is ongoing and is expected to enable a broader range of latticemismatched heterostructures than is currently possible using planar heteroepitaxy. Acknowledgment. This work was supported by the National Science Foundation (ECS-0424161). The focused ion beam work for TEM sample preparation was carried out in the Center for Microanalysis of Materials, University of Illinois, which is partially supported by the U.S. Department of Energy under Grant DEFG02-91-ER45439. We thank Brian Johnson from the School of Electrical and Computer Engineering, Purdue University, for discussions on the Nano Lett., Vol. 5, No. 9, 2005

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