LETTER pubs.acs.org/NanoLett
Controlled Growth of Ordered Nanopore Arrays in GaN Isaac H. Wildeson,†,§ David A. Ewoldt,‡,§ Robert Colby,‡,§ Eric A. Stach,‡,§ and Timothy D. Sands*,†,‡,§ †
School of Electrical and Computer Engineering, ‡School of Materials Engineering, and §Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907, United States
bS Supporting Information ABSTRACT: High-quality, ordered nanopores in semiconductors are attractive for numerous biological, electrical, and optical applications. Here, GaN nanorods with continuous pores running axially through their centers were grown by organometallic vapor phase epitaxy. The porous nanorods nucleate on an underlying (0001)oriented GaN film through openings in a SiN x template that are milled by a focused ion beam, allowing direct placement of porous nanorods. Nanopores with diameters ranging from 20-155 nm were synthesized with crystalline sidewalls. KEYWORDS: Nanopore, nanorod, GaN, organometallic vapor phase epitaxy, selective area growth, transmission electron microscopy
P
orous semiconductors have attracted considerable attention for their potential applications in sensing, catalysis, and as templates for lattice-mismatched heteroepitaxy.1-4 With controlled positioning of pores with diameters in the range of nanometers comes the promise of devices that manipulate ionic and molecular transport, 5,6 “nanocontainers” that can be employed in “smart” extraction and gene delivery systems,7,8 and the assembly of “quantum fortresses” within semiconductors.9,10 The III-nitrides are interesting materials for nanopore applications as they are mechanically robust and biologically compatible, as well as electrically and optically active.11 Nanopore structures have been previously fabricated within GaN by reactive ion etching (RIE)12 and inductively coupled plasma (ICP) etching,13-15 anodization and wet chemical etching,16,17 and growth of porous particles and nanowires.18,19 Direct RIE and ICP etching of nanopores results in damaged surfaces, thus degrading the quality of electrically active interfaces. Chemical etching and anodization methods result in nanopores with variations in uniformity and little control over position. For optimal functionality of GaN nanopores, controlled pore synthesis by direct epitaxy is desired.20-24 Goldberger et al.22,23 have demonstrated the epitaxial growth of GaN nanopores by the use of a core-sheath heterostructure in which GaN growth nucleates around a ZnO nanowire, which is subsequently etched to produce the final nanopore. Mei et al. fabricated GaN nanopores defined by freestanding AlN/GaN nanomembranes.24 While these techniques provide a high-quality interface at the nanopore walls, controlled variation of both the nanopore diameter and spacing on a single wafer is not easily achieved. In this paper, we report a fabrication method for porous GaN nanorods with a high degree of control over nanopore diameter and spacing. The growth templates for the porous nanorods are r 2010 American Chemical Society
created by milling openings through a dielectric mask that is deposited on top of a GaN thin film, and a small distance into the GaN as well, using a focused ion beam (FIB). Subsequent growth of GaN by organometallic vapor phase epitaxy (OMVPE) yields nanopore walls that are lined with crystalline GaN. The nanopore diameters were tuned by varying the size of the opening in the SiNx as well as by altering the growth time. The mechanism for nanopore formation is examined, and the geometry and crystal quality of the porous nanorods are investigated. The starting substrates for this work were 7 μm thick undoped GaN films grown on c-plane sapphire by hydride vapor phase epitaxy (HVPE). The GaN surface was cleaned by a 5 min soak in 50 vol % HCl followed by a 5 min rinse in deionized water. An 85 nm film of SiNx was then deposited on the GaN surface by plasma enhanced chemical vapor deposition at a rate of 0.7 nm/sec. Roughly 1 nm of Au/Pd was then sputtered onto the SiNx surface to minimize electron and ion beam drift during imaging and subsequent milling. An FEI Nova 200 FIB with a Ga ion source, operating at 30 kV and 10 pA, was used to pattern circular openings through the SiNx template (Figure 1a). The FIB milling penetrated through the SiNx film and 300-700 nm into the GaN, depending on the diameter of the opening in the SiNx template. In this study, the diameter of FIB-milled openings ranged from 165-375 nm. Following the FIB milling, samples were submerged in aqua regia (3:1, hydrochloric acid to nitric acid) for 30 s to remove the 1 nm Au/Pd and any residual Ga or Received: September 28, 2010 Revised: November 22, 2010 Published: December 20, 2010 535
dx.doi.org/10.1021/nl103418q | Nano Lett. 2011, 11, 535–540
Nano Letters
LETTER
diameters evaluated here, allowing tuned control of the nanopore dimensions (Figure 2b). The nanopore diameter, as measured from plan-view FESEM, is the narrowest diameter of the pore as it runs through the nanorod. SiNx openings with diameters of ∼165 nm or less possess no visible nanopores in FESEM after a 30 s growth. It should be noted that when the average nanopore diameter is less than 40 nm, a few of the nanorods within the array do not contain pores. Furthermore, the percentage of nanorods lacking pores within an array increases with decreasing average nanopore diameter and is observed to be the highest at ∼15% for the smallest average nanopore diameter observed in this study (∼20 nm). The absence of a pore in these few nanorods is likely influenced by the roughness of the FIB-milled openings (Figure 1a), which causes slight variation in opening diameters, resulting in some porous nanorods filling in more quickly than others. TEM analysis of a single porous nanorod reveals a continuous, 35 nm diameter pore extending through the nanorod and into the underlying FIB-milled cavity in the GaN (Figure 2c). The vertical walls that outline the nanopore are comprised of nonpolar facets (e.g., the {1100} and the {1120} planes). GaN epitaxy is observed to occur only through the openings within the SiNx, and the point of nucleation occurs below the SiNx/GaN interface and spans 80-200 nm into the FIB-milled cavity, depending on the diameter of openings in the SiNx. TEM further confirmed that the lining of the nanopore walls was crystalline, which may provide high-quality electrical interfaces for possible sensing applications. The nanopore diameter can also be altered by varying the GaN growth time. An increase in GaN growth time results in additional lateral growth and a corresponding decrease in nanopore diameter. After a growth time of 60 s, only nanorods grown through ∼350 nm or larger diameter openings still possessed visible nanopores. When investigating porous nanorods grown through 340 nm diameter openings for 30 s and porous nanorods grown through 355 nm diameter openings for 60 s, we find nanopore diameters of ∼155 and ∼30 nm, respectively (Figure 2b; Figure 3a,b). Alternatively, a ∼25 nm diameter nanopore can be achieved by employing a ∼180 nm diameter SiNx opening and a 30 s growth time (Figure 2b). This illustrates the ability to produce nanopores of the same diameter within hexagonal GaN nanorods of varying dimensions. As is evident from the linear relationship in Figure 2b (solid diamonds), the extent of lateral growth inward from the perimeter of the openings was found to be independent of the opening's diameter and averaged ∼80 nm for a 30 s growth in this study. The extent of lateral growth after 60 s of epitaxy averaged ∼160 nm (Figure 2b, open squares), illustrating that the lateral growth rate is uniform with time and averages ∼2.7 nm/sec in our study. The standard deviations of the nanopore diameters do not correlate with the size of the opening diameters, averaging (7 nm after 30 s of GaN growth and (11 nm after 60 s of GaN growth. The main source of this variation in nanopore diameter is believed to derive from the roughness of the FIB-milled openings. In addition to the decrease in nanopore diameter, the morphology of the nanopore also changed with additional GaN growth time. After a 30 s GaN growth through ∼350 nm diameter SiNx openings, the outline of the nanopores was comprised of semipolar {1101} planes for a substantial portion of the nanopore length (Figure 3a). However, after a 60 s GaN growth through ∼350 nm diameter SiNx openings, the structure of the porous nanorods changed such that the nanopore was primarily
Figure 1. Cross-section schematics and field emission scanning electron microscope plan-view images of a typical (a) array of overetched openings through SiNx on GaN prepared by focused ion beam milling and (b) an array of porous GaN nanorods subsequently grown by OMVPE.
amorphous GaN damaged by the ion beam. A 5 min soak in deionized water followed. Selective area epitaxy of GaN porous nanorods was performed in an Aixtron 200HT OMVPE reactor. Prior to growth, samples were heated to 1030 C in a mixture of 2:3 NH3/H2 and were held at this temperature for 3 min in an effort to recrystallize any remaining damaged GaN. The growth temperature and total pressure were 1030 C and 100 Torr, respectively. Trimethylgallium (TMG) and ammonia (NH3) were used as the Ga and N sources, respectively. Hydrogen was used as the carrier gas and the total flow of all gases was kept at 10 slm. A low V/III molar ratio of 1430 was used with a TMG flow rate of 88 μmol/min. The growth time for the porous nanorods under this investigation ranged from 30-90 s. Field emission scanning electron microscopy (FESEM; Hitachi S-4800, operating at 10 kV) was used to investigate the morphology of the porous nanorods as well as the relationship between nanopore diameter and size of opening in SiNx. Transmission and scanning transmission electron microscopy (TEM/STEM; FEI Titan 80-300, operating at 300 kV) were used to investigate the nanopore structure and the crystal quality of the nanorods. Faceted GaN nanorods with pores running axially through their centers were fabricated (Figure 1b). The hexagonal pyramidal structure of the cap of each GaN nanorod is primarily comprised of six semipolar {1101} planes. This faceted structure has been previously observed for GaN nanorods selectively grown through circular openings in a dielectric template,25,26 and promotes efficient dislocation filtering in solid GaN nanorods.27,28 The resulting nanopore diameters in this study ranged from 20-155 nm, depending on the diameter of the SiNx openings and the growth times. Figure 2a shows porous GaN nanorods grown for 30 s through variously sized openings patterned on the same substrate. The nanopore diameter decreases linearly with decreasing SiNx opening diameter in the range of the opening 536
dx.doi.org/10.1021/nl103418q |Nano Lett. 2011, 11, 535–540
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Figure 2. (a) Plan-view FESEM of GaN porous nanorods with varying pore diameters all grown for 30 s through different sized openings in SiNx on the same wafer. (b) Relationship between diameter of openings in SiNx and corresponding average pore diameter in GaN nanorods after 30 s (solid diamonds) and 60 s (open squares) of growth. The average nanopore diameters and standard deviations were calculated from plan-view FESEM of ∼20 randomly chosen porous nanorods from each array. For elliptically shaped nanopores, the nanopore diameter was determined by calculating the diameter of a circular nanopore of same area. (c) Cross-sectional TEM image of an example GaN nanorod with a single 35 nm diameter pore.
outlined by nonpolar facets (e.g., {1100} and {1120}), similar to porous nanorods grown through smaller SiNx openings (e.g., SiNx opening diameter
