Cathodoluminescence spectra of gallium nitride nanorods
© Tsai et al; licensee Springer. 2011
Received: 13 September 2011
Accepted: 14 December 2011
Published: 14 December 2011
Gallium nitride [GaN] nanorods grown on a Si(111) substrate at 720°C via plasma-assisted molecular beam epitaxy were studied by field-emission electron microscopy and cathodoluminescence [CL]. The surface topography and optical properties of the GaN nanorod cluster and single GaN nanorod were measured and discussed. The defect-related CL spectra of GaN nanorods and their dependence on temperature were investigated. The CL spectra along the length of the individual GaN nanorod were also studied. The results reveal that the 3.2-eV peak comes from the structural defect at the interface between the GaN nanorod and Si substrate. The surface state emission of the single GaN nanorod is stronger as the diameter of the GaN nanorod becomes smaller due to an increased surface-to-volume ratio.
Recently, the applications of semiconductor materials in optoelectronic devices grow rapidly. Among them, due to the high thermal conductivity, wide direct bandgap, and chemical stability, III-V family nitride-based semiconductors, including aluminum nitride [AlN], gallium nitride [GaN], and indium nitride [InN], and their alloys have attracted lots of studies in the applications on light-emitting diodes and laser diodes. The bandgaps for AlN, GaN, and InN are 6.2 eV, 3.4 eV, and 0.65 eV, respectively. By varying the composition of these three nitride-based materials, the emission light energy will range from 0.65 eV to 6.2 eV . Through the studies of the fundamental properties of the nitride-based materials, one can get more insight into the applications of these materials.
Semiconductor nanowires have attracted a lot of attention due to the large surface-to-volume ratio in the nanoscale dimension and their applications on nanodevices [2, 3]. Since the first investigations of GaN nanorods (also called nanowires or nanocolumns) in 1997 [4, 5], these one-dimensional [1D] GaN nanorods have attracted a lot of studies on the growth methods [6–9], physical properties [10–13], and their applications [14–16]. The geometric structures will greatly affect the optical and electrical properties of these nitride-based materials. It has been reported that 1D GaN nanorods have higher photoluminescence intensity than two-dimensional GaN due to the large surface-to-volume ratio . Furthermore, in the applications of GaN materials, the defects of GaN will affect the electrical and optical properties of the GaN-based devices greatly and thus affect the performance and reliability of the devices .
In this work, we studied the surface topography and optical properties of the GaN nanorod cluster and single GaN nanorod via field-emission scanning electron microscopy [FE-SEM] and cathodoluminescence [CL]. The vertically aligned GaN nanorods were grown on a Si(111) substrate at 720°C without a buffer layer via plasma-assisted molecular beam epitaxy [PAMBE] [18, 19]. Temperature-dependent CL spectra of the GaN nanorods were carried out to study the defect states of GaN nanorods. CL spectra at different positions along the length of the nanorod were also measured to investigate the size-dependent properties of GaN nanorods.
GaN nanorod growth
The PAMBE system used in the GaN nanorod growth was Veeco EPI 930 (Veeco Instruments Inc., Plainview, NY, USA). Ultra-high pure nitrogen gas (99.9999% purity) was supplied for the radio-frequency plasma source via a mass flow controller. The Ga source (99.999995% purity) was loaded in a Knudsen effusion cell. The base pressure of the PAMBE chamber was pumped down below 3 × 10-11 Torr by a cryogenic pump. Before starting the growth process of the GaN nanorods, the Si substrate was cleaned by acetone, isopropanol, and deionized water, respectively, with ultrasonication to remove residual surface contamination. Then, the native oxide of the Si substrate was removed by a diluted hydrofluoric acid [HF] solution (HF:H2O = 1:5) for 5 min. The hydrogen-terminated Si(111) substrate was then transferred to the growth chamber. Prior to the growth of the GaN nanorods, the Si substrate was further annealed at 900°C for 30 min to remove atomic hydrogen  and residual native oxide  with an orderly 7 × 7 reflection high-energy electron diffraction pattern. Thereafter, the substrate was cooled down to the growth temperature of 720°C to adjust the beam equivalent pressure [BEP]. When Ga BEP was well-controlled to about 2.5 × 10-7 Torr by changing the temperature of the Knudsen effusion cell, N BEP was then adjusted to about 2.5 × 10-5 Torr. By preserving the growth temperature and the BEPs (Ga and N) for 3 h, the GaN nanorod cluster was successfully grown on the Si substrate.
Separation and position of a single GaN nanorod
In order to separate and position a single GaN nanorod for CL measurement, the nanorods were scratched from the as-grown GaN nanorod substrate by a small tweezer and then knocked down on a Si substrate with a little hammer. GaN nanorods were dissolved in ethanol with an ultrasonicator for 10 min. Thereafter, the GaN nanorods were dropped on a gold-coated Si substrate covered with a copper-based network for identifying the position of the GaN nanorods.
FESEM and CL measurement systems
The measurement system used in this work is FE-SEM (JEOL JSM-7000F, JEOL Ltd., Akishima, Tokyo, Japan). The best resolution can be approximately 1.5 nm at an acceleration voltage of 35 kV. The CL measurement was performed by the JSM-7000F FE-SEM (JEOL Ltd., Akishima, Tokyo, Japan) equipped with a Gatan MonoCL system (Gatan, Inc., Pleasanton, CA, USA). The spectrum range of the CL measurement was 200 nm to 2,300 nm.
Results and discussion
where y0 is baseline offset, A i is the area under each Gaussian curve from the baseline, x i is the center of each Gaussian peak, n = 3 is the number of peak, and w i is approximately 0.849 of the full width at half maximum for each peak. There exist three peaks of the photon energy at 3.21 eV, 3.35 eV, and 3.45 eV which correspond to Y7, Y4, and NBE, respectively. The 3.35 eV or Y4 peak observed in GaN has been assigned to the excitons bound to the stacking faults in the as-grown GaN samples . Additionally, the Y4 and Y7 lines are reported to simultaneously appear among the GaN epilayers. For low-temperature CL measurements (T = 20 K), we can compare the CL spectra performed at different locations: the top of the GaN nanorod cluster and the side of the GaN nanorod cluster. The results show that the intensity ratio of the Y7/NBE as shown in Figure 2c became larger than that measured in the GaN nanorod cluster (shown in Figure 2a). Accordingly, we could suggest that the Y7 line arises from the junction between the GaN nanorods and Si substrate because the junction contains more defects, owing to the broken GaN nanorods or randomly aligned short GaN nanorods. Furthermore, the CL spectra carried out on top of the GaN nanorod cluster show a strong surface-state peak but without the Y4 line. In contrast, the Y4 line appears in the CL spectra measured on the side of the GaN nanorod cluster. The result further reveals that the surface state is due to the tip of the GaN nanorod.
where m e, m h, h, and D are the effective electron mass, effective hole mass, Plank constant, and diameter of nanorod, respectively. For GaN, m e and m h are 0.22 m 0 and 1.1 m 0 (m 0 = 9.11 × 10-31kg is the electron mass) , respectively. The estimated ΔE at the bottom of the GaN nanorod (D ≈ 35 nm) is larger than the ΔE at the top of GaN nanorod (D ≈ 85 nm) by 5.3 meV which is smaller than the experimental value of 15 meV. Based on Equation 3, we can estimate that if the effective diameters of the GaN nanorod at R1 and R5 are 21 nm and 51 nm, respectively, which are about 60% of the measured values, the shifted energy will approach the experimental result of 15 meV. The reduction on the effective diameter of the GaN nanorod could be related to the band-bending effect caused by the Fermi level pinning of the GaN nanorod [28, 29]. Furthermore, the peak intensity is strong as the CL spectra are carried out at the bottom of the GaN nanorod, which is mainly due to the size effects. The size effects come from the increase of the surface state density of GaN due to the large surface-to-volume ratio and the variation of electronic states because of the diameter difference. However, in CL spectra measurements, the results cannot conclude which effect dominates the increased intensity. That can be further confirmed by other measurements or experimental setups in the future.
In summary, the as-growth GaN nanorod cluster and the single GaN nanorod via PAMBE growth were studied by FE-SEM and CL spectroscopy. The emissions from the NBE, surface state, Y4 and Y7 defect states of the GaN nanorod cluster, and the single GaN nanorod were investigated and analyzed. The results show that the CL spectra of the GaN nanorod cluster and the single GaN nanorod are sensitive to the change in temperature and structure of GaN. For the GaN nanorod cluster and the single GaN nanorod, the NBE line position will blueshift with the decreasing temperature, and the intensity of the CL spectra for the surface state of 3.4 eV will increase with the decreasing temperature. However, the Y7 defect line did not appear in the single GaN nanorod; therefore, we can deduce that the source of the Y7 line came from the structural defect existing between the GaN nanorods and the Si substrate. Furthermore, the position-dependent CL spectra of the single GaN nanorod revealed that the surface state of the single GaN nanorod is strongly influenced by the diameter of the GaN nanorod. These studies give us more insight in the fundamental properties of GaN nanomaterials and provide useful information in the applications of GaN nanorod-based devices.
This work is supported by the National Science Council of Taiwan under the contract numbers NSC 99-2112-M-110-012-MY2 and NSC 98-2923-M-110-001-MY3. Additional funding support from the potential program project of National Sun Yat-sen University is also acknowledged.
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