NANO LETTERS
GaN Nanorod Schottky and p−n Junction Diodes
2006 Vol. 6, No. 12 2893-2898
Parijat Deb,*,†,| Hogyoung Kim,‡,| Yexian Qin,‡,| Roya Lahiji,‡,| Mark Oliver,† Ronald Reifenberger,‡,| and Timothy Sands†,§,| School of Materials Engineering, Purdue UniVersity, West Lafayette, Indiana 47907, Department of Physics, Purdue UniVersity, West Lafayette, Indiana 47907, School of Electrical and Computer Engineering, Purdue UniVersity, West Lafayette, Indiana 47907, and Birck Nanotechnology Center, Purdue UniVersity, West Lafayette, Indiana 47907 Received September 12, 2006; Revised Manuscript Received October 19, 2006
ABSTRACT Conductive atomic force microscopy has been used to characterize single GaN nanorod Schottky and p−n junction diodes. The ideality factor, reverse breakdown voltage, and the Schottky barrier height of individual nanorod diodes were compared to those from conventional thin-film diodes. Large-area contacts, enabling diodes with arrays of GaN nanorods in parallel, were also fabricated and their electrical characteristics investigated. The defect-free nature of the GaN nanorods and enhanced tunneling effects due to nanoscale contacts have been invoked to explain the electrical behavior of the nanorod diodes.
III-V compound semiconductors in one-dimensional forms such as nanowires and nanorods are intriguing structures for optoelectronics and nanophotonics.1-12 The possibility of quantum confinement, coaxial heterostructures, correlated photon emission, and photonic crystal effects make GaN nanorods potential contenders for nanolasers and nanoLEDs that may prove useful for information storage, optical interconnects, and general illumination.13,14 The onedimensional form factor also enables fabrication of coherent heterostructures with larger lattice mismatches than can be grown coherently as planar heterostructures. Coherent heteroepitaxy with the composition varying along the nanorod or nanowire axis requires the radius of the nanorod or nanowire to be below a critical value that is about an order of magnitude larger than the equilibrium critical thickness of a comparable thin-film heterostructure.15-19 In the context of GaN, the larger surface area-to-volume ratio in nanorods and nanowires offers lateral elastic strain relaxation that can be utilized to accommodate either larger mole fractions of InN in coherent (In,Ga)N quantum dots, or larger quantum dots, thereby broadening the range of electroluminescence wavelengths toward the red portion of the spectrum. This possibility of fabricating a monolithic, phosphor-free LED that emits white light by utilizing nanoscale strain engineering is the motivation for the present work. * Corresponding author. E-mail:
[email protected]. † School of Materials Engineering. ‡ Department of Physics. § School of Electrical and Computer Engineering. | Birck Nanotechnology Center. 10.1021/nl062152j CCC: $33.50 Published on Web 11/02/2006
© 2006 American Chemical Society
Device prototypes using semiconductor nanowires and nanorods have already been demonstrated.32 However, a thorough understanding of the electrical behavior of nanodevices employing nanorods is critical to designing and fabricating a high performance electronic or optoelectronic device. Current transport through interfaces has been seen to differ in nature (e.g., tunneling versus thermionic) as the lateral size of the contact is varied.24-25 Also, the reduction in the density of extended defects in the case of nanostructures offers unique advantages in terms of lower leakage and recombination currents as compared to the thin-film counterparts, provided that surface recombination is suppressed. Previous studies of GaN nanowire p-n junction diodes have involved synthesizing the nanowires, dispersing them in a solvent, and making lithographically defined contacts.30-31 However, for fabricating a device using one-dimensional GaN nanostructures, it is essential to develop a process to make contacts to individual nanowires or nanorods as well as to arrays of nanowires or nanorods. In this paper, we report Schottky and ohmic contacts to n- and p-type GaN nanorods and thus the fabrication of Schottky and p-n junction diodes using both individual GaN nanorods and nanorod arrays. Contacts to individual GaN nanorods were made using conductive atomic force microscopy (CAFM) tips. The values of the ideality factors and reverse breakdown voltages of Schottky and p-n junction diodes employing single GaN nanorods have been compared to the values from conventional GaN thin-film diodes. The effect of large-area contacts to arrays of ∼105 nanorods in parallel on the electrical
Figure 2. Schematic diagram of the experimental setup for single nanorod Schottky and p-n junction diode characterization using the conductive atomic force microscopy (CAFM) technique.
Figure 1. Field emission scanning electron microscopy (FESEM) image of GaN nanorods. The inset shows a TEM bright field image of a single nanorod.
characteristics has also been investigated in this study. The implications of the significant differences between individual nanorod diodes, nanorod array diodes, and planar diodes for the design and performance of electronic and optoelectronic devices are discussed. The GaN nanorods were synthesized using a template approach employing porous anodic alumina films as an etch mask to define nanoscale growth windows in a silica mask.20 Subsequent selective area growth by organometallic vaporphase epitaxy (OMVPE) yields arrays of faceted epitaxial GaN nanorods. The details of this catalyst-free synthesis method can be found in ref 20. The nanorods consist of six prismatic planes of the {11h00} type and a pyramidal cap made up of six {11h01} type planes. A self-limiting growth condition of low V-III ratio (1350) and hydrogen as the carrier gas (detailed growth conditions in reference 20) was employed to yield nanorods with only the pyramidal cap of the nanorod exposed above the silica template (Figure 1). The nanorods have a diameter of 50 ( 5 nm and a total length of around 100 nm, determined by the silica template thickness of 60 nm and the exposed pyramidal cap height of about 30-40 nm. Thin GaN films grown under the same conditions as the unintentionally doped nanorods are n-type with a carrier concentration of 8 × 1016 cm-3 as determined by capacitance-voltage (C-V) measurements. Nanorods were also p-type doped with Mg using Cp2Mg as the precursor, followed by rapid thermal annealing at 950 °C for 30 s in nitrogen ambient. Thin films grown under the same conditions were found to be p-type with a hole concentration of 2 × 1017 cm-3 as determined by C-V measurements. For the nanorod p-n junction diode, the growth was controlled to achieve an undoped length of about 15 nm and a p-type doped length of about 85 nm. Using the carrier concentrations for the n- and p-type regions as determined from planar films, the width of the depletion region on the p-doped side of the junction would be about 70 nm in the thin-film case, and hence the nanorods are not expected to be completely depleted of carriers in the unbiased condition.21 Note that this simple analysis neglects the effects of band bending at the pyramidal and prismatic GaN nanorod 2894
Figure 3. Three-dimensional AFM image of GaN nanorod caps exposed above the SiO2 template.
surfaces and the effect of the Pt nanoscale contact (expected to be ohmic) in close proximity to the junction. Schottky diodes employing individual GaN nanorods were realized using a CAFM tip. The metal coating on the silicon AFM tip was chosen to be Ti (15 nm)/Pt (35 nm) so as to ensure a Schottky contact to an n-GaN nanorod and an ohmic or low-barrier contact to a p-GaN nanorod. A flat region on the same wafer (containing no nanorods) was used to make the ohmic contact to the n-GaN underlayer with indium metal. The schematic diagram of the experimental setup is shown in Figure 2 and an AFM scan of the surface containing exposed GaN nanorod caps can be seen in Figure 3. The shape of the nanorods is better represented in the transmission electron microsope (TEM) image displayed as the inset to Figure 1. The large-area contacts to arrays of nanorods were made using electron-beam evaporated Pt contact pads with 100 nm thickness and a size of 100 µm × 100 µm. The electrical characteristics of individual nanorod Schottky diodes were compared to those of the nanorod arrays as well as to the characteristics obtained from planar Pt/n-GaN diodes. A similar study was carried out for the GaN nanorod p-n junction diodes. For both the Schottky and p-n junction diodes, I-V characteristics were collected from ten individual nanorods. The average values of barrier height, ideality factor, and reverse leakage current over these ten measurements for both types of diodes are presented here. Figure 4a shows the I-V characteristics from a single n-GaN nanorod Schottky diode obtained using the CAFM technique. Figure 4b shows the CAFM I-V characteristics with the AFM tip on the SiO2 surface, exhibiting negligible current as expected for this SiO2 thickness and voltage range. The forward bias characteristics were analyzed using the thermionic emission model as follows.21 Nano Lett., Vol. 6, No. 12, 2006
1.0 ((0.18) V and the reverse leakage current was ∼1 × 10-3 nA. Considering the measured CAFM tip radius of about 20 nm, the Schottky contact area was assumed for the purposes of modeling to be a disk with a diameter of ∼15 nm. The Schottky barrier height was calculated from the I-V data to be 0.62 ((0.02) eV. A variation of (5 nm in the assumed contact diameter would result in a change in the calculated Schottky barrier height value by (0.02 eV. Motayed et al.27 fabricated GaN nanowire Schottky diodes utilizing focused ion beam (FIB) induced Pt deposition and found the ideality factor and the Schottky barrier height (SBH) to be 18 and 0.2 eV, respectively. They attributed such nonideal behavior to the ion damage occurring during Pt deposition using the FIB technique at the surface of the nanowires. The lower values of the ideality factor obtained in the present study thus suggest the absence of any significant surface damage induced by the CAFM contacting technique. Thin-film Schottky diodes with Pt as a Schottky metal were also fabricated using the same growth condition as for the unintentionally doped GaN nanorods. Figure 4c presents the I-V characteristics for the thin-film Schottky diodes. The ideality factor and the SBH for the thin-film Schottky diode were determined to be 1.28 ((0.06) and 0.94 ((0.02) eV, respectively. Compared to the thin-film Schottky diode, the ideality factor in the CAFM Schottky diode is high; however, similar results have been reported in studies on ZnO nanorod Schottky diodes (η ∼ 7-9).23 It has been shown that, in the case of a nanoscale Schottky contact where the diode size is smaller than the depletion width, the tunneling current becomes significant.24,25 The potential profile that develops in a Schottky diode, where the contact size is smaller than the depletion width, is described by25 a V(r) ) ‚Vs r Figure 4. (a) I-V characteristic for a single GaN nanorod Schottky diode using a CAFM tip as a Schottky contact (Schottky barrier height ∼0.62 ( 0.02 eV, ideality factor ∼6.5 ( 0.6, estimated contact area ∼175 nm2), the inset shows the forward bias I-V characteristics on a logarithmic scale and (b) I-V characteristic when the tip is placed on SiO2 surface. (c) I-V characteristics for a thin-film Schottky diode (Schottky barrier height ) 0.94 ( 0.02 eV, ideality factor ) 1.28 ( 0.06), the inset shows the forward bias I-V characteristics on a logarithmic scale. (d) I-V characteristics of a GaN nanorod array Schottky diode comprising ∼105 nanorods in parallel.
I ) I0[exp(qV/ηkBT) - 1]
(1)
I0 ) AA*T2 exp(-qφB/kBT)
(2)
where I0 is the reverse saturation current, η is the ideality factor, A is the Schottky contact area, A* is the effective Richardson constant, taken to be 26.4 Å/cm2K2 for n-GaN,22 and φB is the Schottky barrier height. An ideality factor of 6.5 ((0.60) was measured in the voltage range of 1-1.5 V. The turn-on voltage, measured at a current of 0.005 nA per nanorod for the single GaN nanorod Schottky diode, was Nano Lett., Vol. 6, No. 12, 2006
for r g a
(3)
where Vs is the potential at the metal-semiconductor interface, a is the diode size, r is the distance from the metal-semiconductor interface into the semiconductor, and V(r) is the potential in the semiconductor as a function of the distance from the interface (r). This reduction in the barrier width results in an increased probability of tunneling along with thermionic transport through the Schottky barrier (thermionic field emission) and is one possible reason for the increase in the effective ideality factor and the reduction in the effective barrier height in the single GaN nanorod Schottky diodes as compared to their planar thin-film counterparts.24-25 The I-V characteristics of a large-area contact to a n-GaN nanorod array is shown in Figure 4d. A nearly ohmic I-V characteristic suggests a dramatic lowering of the Schottky barrier height for the array of nanorods (∼105) contacted in parallel, relative to the result obtained from individual nanorods by CAFM. This unusual ohmic behavior can be explained by the presence of metal-semiconductor contact regions with very small convex radii of curvature at the perimeter of the pyramidal cap (depicted in the schematic diagram of Figure 5), leading to enhanced electric fields and, 2895
Figure 5. Schematic diagram of large-area GaN nanorod array Schottky diode employing arrays (∼105) of nanorods. The circles in the figure depict the expected regions with electric field enhancement
hence, increased tunneling currents. Another potential factor contributing to the nearly ohmic behavior of the array contacts is the contact interface crystallography; the evaporated metallization on the GaN nanorod array makes contact primarily to the nonpolar pyramidal {11h01} plane, whereas the CAFM Schottky contact to the apex of an individual nanorod is made primarily to the polar (0001) contact plane. The anisotropic electronic structure of GaN and its interface with Pt would therefore be expected to yield different barrier heights and ideality factors for the two types of contacts.26 The nearly ohmic behavior of the evaporated contact to the GaN nanorod array suggests the possibility of using the small radius of curvature features at the perimeter of nanorods to substantially lower the contact resistance of ohmic contacts to p-GaN. Prior investigations of ohmic behavior of nanosized Pt islands on planar GaN28 and of nanoscale protrusions of metals into GaN29 support the argument that fieldenhancement effects can have a marked effect on current transport in contacts to GaN. It should be noted here that the above-mentioned experiment was repeated several times with different GaN nanorod samples to minimize the possibility that an isolated macroscopic defect was responsible for the nearly ohmic I-V characteristics. Furthermore, as shown later in the paper, the p-GaN nanorods when contacted exhibit rectifying characteristics, which suggests the absence of any significant surface-related carrier transport. The single nanorod p-n junction characteristics and the large-area nanorod array contact characteristics are shown in parts a and b of Figure 6, respectively. The large-area contact involves approximately 1.1 × 105 nanorods. The turnon voltage, defined at 0.005 nA per nanorod, for the single nanorod p-n junction diode as measured using the CAFM technique was 3.2 ((0.22) V and, using the diode equation of I ) I0 exp[(qV/ηkBT) - 1], the ideality factor was calculated to be 10.4 ((0.74). Cheng et al.28 fabricated GaN nanowire p-n junction diodes with lateral contacts and reported an ideality factor of 5.5-6.5, which is consistent with the observations in the present study. The reverse leakage current for the single GaN nanorod p-n junction diode was ∼0.05 nA and the breakdown voltage was ∼4 V. By comparison, the reverse breakdown voltage of the nanorod array p-n junction was found to be substantially higher (∼20 V) and the turn-on voltage was found to be 2896
Figure 6. (a) I-V characteristics from a single GaN nanorod p-n junction diode (ideality factor 10.4 ( 0.74, estimated junction area ∼2000 nm2). (b) I-V characteristics from an array of GaN p-n junction nanorod diodes (ideality factor 11.2 ( 0.56, estimated junction area ∼2 × 108 nm2). The insets show the forward bias I-V characteristics on a logarithmic scale. (c) I-V characteristics for thin-film p-n junction diode (ideality factor ∼20, estimated junction area ∼104 µm2). The inset shows the forward bias I-V characteristics on a logarithmic scale.
lower (∼2.0 V). The ideality factor for the array, however, was slightly higher (11.2 ( 0.56). A possible reason for the negligible variation of the ideality factor from the individual nanorod to the array is that, by changing the p-type contact area, there is no expected change in the depletion width and the built-in potential at the p-n junction, which is near the base of the GaN nanorods. In the forward-biased condition, the high contact resistance due to the CAFM tip contact to the GaN nanorod results in a higher turn-on voltage for the single nanorod case as compared to the nanorod array contact. However, in the reverse biased condition, the metal/ p-GaN interface acts as a Schottky contact with a finite barrier height that the carriers have to overcome to be injected into the semiconductor. In the CAFM tip-nanorod contact case, because of field enhancement, tunneling transport of carriers across the barrier significantly increases the probability of carrier injection into the semiconductor, which eventually leads to breakdown at lower voltages as compared to the large-area contact case. Thus, the application of a large-area contact to an array of GaN nanorod p-n junction diodes using a thin evaporated metallization results in a dramatic increase in the reverse breakdown voltage and Nano Lett., Vol. 6, No. 12, 2006
Table 1. Measured Parameters Describing the I-V Characteristics of Individual GaN Nanorods (CAFM), Nanorod Arrays, Planar Thin-Film Schottky, and Planar p-n Junction Diodesa Schottky diode φB (barrier height) (eV)
η (ideality factor)
turn-on voltage (V)
reverse breakdown voltage (V)
6.5 ( 0.6
0.62 ( 0.02
1.28 ( 0.06
0.94 ( 0.02
10.4 ( 0.74 11.2 ( 0.56 20
3.2 ( 0.22 2 10
4 20 20
η (ideality factor) CAFM array planar thin film
p-n diode
a CAFM: conductive atomic force microscopic contacting technique to single GaN nanorods. Array: large-area contact to ∼105 GaN nanorods in parallel. Thin film: conventional thin-film planar diode.
a reduction in the forward turn-on voltage, enhancing the electrical characteristics of the p-n junction nanorod diode array. Thin-film p-n junction diodes were also fabricated using similar growth conditions as used for the GaN nanorods with a p-type layer thickness of 400 nm, and p-n junction diodes were fabricated using the same metals as in the nanorod p-n junction case. The macroscopic contact size was also kept the same (100 µm × 100 µm for the p-type contact) and the I-V characteristics are shown in Figure 6c. The ideality factor was calculated to be ∼20, much higher than the value calculated for the nanorod p-n junction (∼11). The GaN nanorods fabricated using the templated approach are expected to be free of any extended defects, and preliminary analysis by TEM supports this conjecture.20 The lack of extended defects in nanorods is expected to reduce both the leakage and the generation-recombination currents. The nanorod surface is expected to be depleted of carriers due to the surface pinning of the Fermi level, yet recombination at such a surface is expected to increase the recombination current in the absence of any surface passivation. However, the results of this study suggest that the improvement in the ideality factor attributed to the elimination of extended defects more than overcomes the increase in the ideality factor due to surface recombination effects. Table 1 summarizes the results of GaN nanorod Schottky and p-n junction diodes, employing CAFM and nanorod array contacts. The thin-film diode characterization results are also included in the table. In conclusion, we have fabricated single GaN nanorod Schottky and p-n junction diodes and characterized them using the CAFM technique. The Schottky diode nanorods showed significant nonideality (η ) 6.5 ((0.60)) and lower Schottky barrier heights, as expected from enhanced tunneling current through a barrier of decreased width due to the small radius of curvature of the nanoscale contact. Individual nanorod p-n junction diodes exhibited a turn-on voltage of ∼3.2 V and ideality factor of 10.4 ((0.74). The reverse breakdown voltage was increased from ∼4 to 20 V by application of large-area contacts to nanorod arrays with the reverse current densities remaining approximately constant. Also, by eliminating extended defects in the p-type GaN region, a reduction in the ideality factor, and thus an improvement in the device characteristics, were achieved relative to the thin-film planar diode. These results are expected to provide the experimental demonstration and insight necessary for the design of practical, high-perforNano Lett., Vol. 6, No. 12, 2006
mance electronic and light-emitting devices based on GaN nanorod arrays. Acknowledgment. We thank Applied Materials Inc. for their support of Parijat Deb as an Applied Materials Graduate Fellow. This material is based upon work supported by the Department of Energy under award no. DE-FC26-06NT42862 and by the National Science Foundation (ECS-0424161). References (1) Lieber, C. M. MRS Bull. 2003, 28, 486. (2) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353. (3) Han, W.; Redlich, P.; Ernst, F.; Ruhle, M. Appl. Phys. Lett. 2000, 76, 652. (4) Chen, C. C.; Yeh, C. C. AdV. Mater. 2000, 12, 738. (5) Li, J. Y.; Chen, X. L.; Qiao, Z. Y.; Cao, Y. G.; Lan, Y. C. J. Crystal Growth 2000, 213, 408. (6) Cheng, G. S.; Zhang, L. D.; Zhu, Y.; Fei, G. T.; Li, L. Appl. Phys. Lett. 1999, 75, 2455. (7) Yoshizawa, M.; Kikuchi, A.; Mori, M.; Fujita, M.; Kishino, K. Jpn. J. Appl. Phys., Part 2, 1997, 36, L459. (8) Logan, R. A.; Thurmond, C. D. J. Electrochem. Soc. 1972, 119, 1727. (9) Park, Y. J.; Son, M. O.; Kim, E. K.; Min, S. K. J. Korean Phys. Soc. 1998, 32, 621. (10) Elwell, D.; Feigelson, R. S.; Simkins, M. M.; Tiller, W. A. J. Crystal Growth 1984, 66, 45. (11) Kim, H. M.; Kim, D. S.; Park, Y. S.; Kim, D. Y.; Kang, T. W.; Chung, K. S. AdV. Mater. 2002, 14, 991. (12) Tang, C. C.; Fan, S. S.; Dang, H. Y.; Li, P.; Liu, Y. M. Appl. Phys. Lett. 2000, 77, 1961. (13) Duan, X.; Huang, Y.; Agrawal, R.; Lieber, C. M. Nature 2003, 421, 241. (14) Huang, Y.; Duan, X.; Lieber, C. M. Small 2005, 1, 142. (15) Luryi, S.; Suhir, E. Appl. Phys. Lett. 1986, 49, 140. (16) Zubia, D.; Hersee, S. D.; J. Appl. Phys. 1999, 85, 6492. (17) Hersee, S. D.; Zubia, D.; Sun, X.; Bemmena, R.; Fairchild, M.; Zhang, S.; Burckel, D.; Fraunglass, A.; Breuck, S. IEEE J. Quantum Electron. 2002, 38, 1017. (18) Ertekin, E.; Greaney, P. A.; Sands, T. D.; Chrzan, D. C. Mater. Res. Soc. Symp. Proc. 2003, 737, F10.4.1. (19) Ertekin, E.; Greaney, P. A.; Chrzan, D. C.; Sands, T. D. J. Appl. Phys. 2005, 97, 114325. (20) Deb, P.; Kim, H.; Rawat, V.; Oliver, M.; Kim, S.; Marshall, M.; Stach, E.; Sands, T. Nano Lett. 2005, 5, 1847. (21) Sze, S. M. Physics of Semiconductor DeVices; Wiley: New York, 1981. (22) Hacke, P.; Detchprohm, T.; Hiramatsu, K.; Sawaki, N. Appl. Phys. Lett. 1993, 63, 2676. (23) Park, W. I.; Yi, G. C.; Kim, J. W.; Park, S. M. Appl. Phys. Lett. 2003, 82, 4358. (24) Smit, G. D. J.; Rogge, S.; Klapwijk, T. M. Appl. Phys. Lett. 2002, 80, 2568. (25) Smit, G. D. J.; Rogge, S.; Klapwijk, T. M. Appl. Phys. Lett. 2002, 81, 3852. 2897
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NL062152J
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