By depositing Si-rich SiOx nano-rod in nano-porous anodic aluminum oxide (AAO) membrane using PECVD, the spatially confined synthesis of Si quantum-dots (Si-QDs) with ultra-bright photoluminescence spectra are demonstrated after low-temperature annealing. Spatially confined SiOx nano-rod in nano-porous AAO membrane greatly increases the density of nucleated positions for Si-QD precursors, which essentially impedes the route of thermally diffused Si atoms and confines the degree of atomic self-aggregation. The diffusion controlled growth mechanism is employed to determine the activation energy of 6.284 kJ mole−1 and diffusion length of 2.84 nm for SiO1.5 nano-rod in nano-porous AAO membrane. HRTEM results verify that the reduced geometric dimension of the SiOx host matrix effectively constrain the buried Si-QD size at even lower annealing temperature. The spatially confined synthesis of Si-QD essentially contributes the intense PL with its spectral linewidth shrinking from 210 to 140 nm and its peak intensity enhancing by two orders of magnitude, corresponding to the reduction on both the average Si-QD size and its standard deviation from 2.6 to 2.0 nm and from 25% to 12.5%, respectively. The red-shifted PL wavelength of the Si-QD reveals an inverse exponential trend with increasing temperature of annealing, which is in good agree with the Si-QD size simulation via the atomic diffusion theory.
© 2011 OSA
Self-assembled Si quantum dots (Si-QDs) buried in Si-rich SiOx film are attractive light-emitting materials of photonics on chip technology. The dominant mechanisms for photoluminescence (PL) emissions of Si-QD are considered as direct band to band radiative re-combinations, which are attributed to either the breakdown of the k-conservation rule in Si-QD, or the enhanced indirect re-combinations, assisted by Si phonons . However, an inherent limitation of its widespread application is the relatively low quantum efficiency of light emissions, due to the unfavorable density of excess Si atoms and wide range of size distribution of the Si-QDs. Therefore, the PL enhancement by synthesizing the Si-QD buried in the Si-rich SiOx host matrix with uniform size distribution and high density still remains bottlenecked at current stage [2,3]. The synthesis of a functional organosilicon micro-network, with ultra-low size variations, has attracted a great deal of attention, as previously demonstrated, by a well-defined size distribution concept within spatially confined reaction sites . Not long ago, Cheng et al. have preliminarily demonstrated a simple and effective approach to rapidly synthesize high-quality nanocrystalline Si buried in columnar structure by using inductively coupled plasma-enhanced chemical vapor deposition (PECVD) [5,6]. The formation mechanism of such specific nanocrystalline Si thin films with controllable microstructure and high crystallinity by adjusting the hydrogen flux density and the energy of ion species impinging on the surface under non-equilibrium plasma conditions were proposed . A review of this work suggests it is a feasible method for confining size distribution, as the tunable scale of the reaction sites were changed, either by spatial dimension from two-dimension (2D) to one-dimension (1D), or by the enhancement of the surface area of the host matrix. However, the influence of surface traps on the optical and transport properties of such low-dimension (rod, wire, or pillar) structures could be solved by either passivating the surface dangling band or by using a high-bandgap shell around the nano-rod.
Nano-porous anodic aluminum oxide (AAO) membranes, formed by the anodic oxidation of aluminum, have been extensively used as the templates for growing arrays of nano-rods, due to their self-aligned nano-channels of extremely high aspect ratios and high area density, up to 1011 pores cm−2 [7–10]. Recent reports suggested that the use of regular surfaces of roughened structures, such as AAO with uniform pore size distribution, could benefit from the synthesis of nano-grain materials. For example, the AAO host matrix can effectively enhance the formation of diamond nuclei in patterned nano-diamond films, which also facilitates electron transport from Si to the top surface of the nano-diamond film . Furthermore, the use of an anodization technique to produce 2D porous AAO, in order to improve the output power of light emitting diodes was recently reported. They indicated that the porous AAO, with a refractive index (n) of 1.6~1.7, could be treated as an intermediary for reducing Fresnel and total-internal-reflection losses without destroying the structure of the LEDs . On the other hand, several reports have observed that field-emission characteristics are strongly influenced by surface morphology, due to the Fowler–Nordheim (FN) tunneling effect, and indicated that the turn-on electric field, as required for electron emissions, is effectively decreased with the increased surface roughness [13,14]. In our previous work, we demonstrated that the high-aspect-ratio Si nano-pillar array helped to enhance FN tunneling-based carrier injections, facilitated complete relaxation on the total internal reflection, and confined the size of Si atom self-aggregation from the Si-rich SiOx host matrix, which increased quantum efficiency by at least one order of magnitude, thus, improving light extraction from a nano-roughened device surface [15,16]. Furthermore, it is desirable to have an understanding of the growth kinetics in order to predict the relationship between atomic diffusion and Si-QD nucleation. If the growth kinetics of the Si-QD are know, it would be possible to predict any corresponding changes in the physical properties of the SiOx host matrix. Therefore, one conceptually simple and elegant method for achieving this goal is to confine the spatial synthesis of Si-QDs in the SiOx nano-rod, as deposited in nano-porous AAO membrane.
In this work, we investigate the PL characteristics of size controlled Si-QDs by employing the nano-porous AAO membrane as the template for growing Si-rich SiOx nano-rods, which achieve spatial confinement for the synthesis of Si-QD. An ultra-bright PL can be emitted from the SiOx nano-rod in a nano-porous AAO membrane. The synthesizing mechanism for such a spatially confined synthesized Si-QD is buried between the 1D SiOx nano-rods, and 2D thin-film is elucidated. High-resolution transmission electron microscopy (HRTEM) is performed to determine the spatially confined size distribution of the Si-QDs. To realize the concept of spatial confinement synthesis between the Si atomic diffusion theory and the Si-QD nucleation mechanism, the effective diffusion characteristics of Si atoms in sub-stoichiometric SiOx must also be determined. Study on precisely controlled, uniformly distributed Si-QD size for improving the standard deviation of Si-QD size dependent spectral line width is emphasized. The relationship between Si-QD size and annealing temperature for controlling emission wavelengths, and temperature’s effect on spatially confined synthesis of Si-QD are proposed to improve light emission power.
2. Experimental setup
The nano-porous AAO membrane was used as a template to deposit 1D Si-rich SiOx nano-rods, through PECVD at RF power, working pressure, working temperature, and N2O/SiH4 flow ratio of 50 W, 0.5 Torr, 350°C and 5.5, respectively. All SiOx-on-AAO samples were deposited within 120 seconds. To precipitate Si-QD, the 1D Si-rich SiOx nano-rods, embedded in nano-porous AAO specimens, were annealed in a quartz furnace with a nitrogen gas flow at a temperature varying between 350 and 800°C. To confirm the results of spatially confined synthesis of Si-QDs, a SiOx film, grown on (100)-oriented p-type Si substrate, is prepared for comparison. In particular, the Si-rich SiOx films were annealed at 1100°C , for precipitating the Si-QD in the host matrix, without spatial confinement.
Subsequently, the room-temperature PL of the samples pumped by He-Cd laser at a wavelength and average power of 325 nm and 6.3 W cm−2, respectively, was analyzed using a fluorescence spectrophotometer (CVI, DK240), with a single-grating of 0.06 nm resolution, and converted into an electrical signal by a photomultiplier (Hamamatsu, model R928), with an operational voltage of 1.0 volts. In addition, the working distance between the focusing lens and the sample was fine-tuned to maximize the PL intensity. The HRTEM (JEOL JEM-2010) and TEM X-ray energy-dispersive spectrometer (TEM-XEDS, Link ISIS 300) were employed to characterize the Si-QD size and O/Si composition ratio within the 1D SiOx nano-rods and 2D SiOx thin-film. In addition, Si-QD distributions were obtained by processing HRTEM images, and the lattice spacing of Si-QD was confirmed using related software (Digital Micrograph 3.3.1 and Origin R7.0 SRO).
3. Results and Discussion
The electron micrographs of the Si-rich SiOx nano-rod in the nano-porous AAO membrane are shown in Fig. 1 . It is worthy noting that the low-plasma and low-temperature PECVD based technology is able to fill the pores of the nano-porous AAO membrane. Previously, Ostrikov et al. also proposed a similar nano-array fabrication technique to improve the precise throughput of energetic ion condensation by introducing the dielectric nano-porous templates to reduce nano-pore clogging with tuning pulsed bias . After PECVD growth, the nano-porous AAO membrane is filled with Si-rich SiOx material, with an average rod diameter of 97 ± 3 nm (see A region in upper part of Fig. 1). Our process show a very similar result with those reported by Wu et al.  that the low-temperature and low-plasma-power deposition can be advantageous to fill dense nano-pores or nano-tube array with Si-rich SiOx. The charged Si/O particles can reach deeply into the high-aspect-ratio AAO trench membrane when the reactant gas is discharged to create low-power plasma under a relatively weak microwave electromagnetic field. Furthermore, the lower-left and lower-right insets of Fig. 1 are the magnified HRTEM images, which clearly show the SiOx nano-rods in the nano-porous AAO membrane after spatially confined synthesis (see lower-left inset), and the abundant Si-QDs are buried in the localized SiOx nano-rod (see lower-right inset) with their diameter confined at 2.0 ± 0.1 nm. To study the assembled Si-QD, room-temperature PL of two post-annealed Si-rich SiOx samples on AAO membrane and Si wafer are measured, as shown in Figs. 2(a) and 2(b). Each PL spectrum consists of only one PL band corresponding to the emission from the Si-QD, which indicates a conspicuous difference between PL intensity and emission-wavelength. As growing the Si-rich SiOx on a nano-porous AAO membrane, it exhibits a strong PL intensity, with the relatively narrow line width of 140 nm at 492 nm, whereas, the PL intensity red-shifts to 596 nm, with greater attenuated intensity, up to two orders of magnitude, and broadened line width (210 nm) when growing the SiOx on the Si substrate. The related PL wavelength correlates well with Delerue’s empirical equation and associates [19,20]. Apparently, the Si-QD size shrinks from 2.6 ± 0.2 to 2.0 ± 0.1 nm and causes a PL blue-shift wavelength Δλ = 96 nm, which is attributed to the spatially confined synthesis of the small-size Si-QDs within the Si-rich SiOx rod. Furthermore, the TEM-XEDS analysis of the O/Si composition ratio of Si-rich SiOx samples is illustrated in the Fig. 2(b) inset. The measured O/Si composition ratios of the spatially confined SiOx nano-rod and planar SiOx thin-film are both 1.5, indicating that the excessive Si atom density in the aforementioned two samples is identical.
To realize the correlation between Si atom diffusion and nucleation within the spatially confined synthesized SiOx nano-rods, the effective diffusion characteristics of interstitial Si atom in sub-stoichiometric SiOx is simulated. The interstitial diffusion mechanism occurs at high impurity concentrations, with its characteristic parameters (such as diffusion length) varying according to the annealing temperature/duration, and by the stoichiometry of the host matrix . Under a driving force F, the Si-QDs can eventually be found by moving the Si atom directly from one interstitial site to other interstitial sites following the diffusion equation of21], the diffusion equation can be rewritten as a function of Gibbs free energy by J = −D0,Si[∂CSi/∂r] + [CSiD0,SiΔGm/akT] under a driving force of ΔGm/a, where T and k are the temperature and Boltzmann constant, respectively, ΔGm is the change of Gibbs free energy, and the a is the distance between neighboring lattice planes. An increased ΔGm or decreased lattice constant is necessary to inverse the atomic diffusion. According to the thermodynamic theory, the change of Gibbs free energy could be related to ΔGm = ΔΗ−ΤΔS, where ΔΗ and ΔS are the enthalpy and the entropy, respectively. In our case, the negative diffusion flux caused by Si atom nucleation from low-concentration to a high-concentration region, occurs by ΔH<0 and ΔS>0 at a given annealing temperature, which can produce the ΔGm<0 condition and favor the interstitial Si atom diffusion and nucleation.
On the other hand, the atomic diffusion theory has directly correlated the proportionality of Si-QD radius to the SiOx composition ratio, the environmental temperature, and the duration of heat treatment. According to Fick's first and second Laws, the diffusion length ofwith D* = D0exp(−Q/kT) is proportional to the square root of the time and diffusion coefficient , which provides a confinement on the maximum movement of excessive Si atoms, within the SiOx matrix, at a given annealing temperature and time. When the diffusion length is larger than the bounded size, finite Si atoms can be aggregated to form the Si-QD of precisely controlled size. For example, it takes nearly 624 Si atoms to form a Si-QD with a diameter of 2 nm in the SiOx host matrix an O/Si composition of 1.5, which corresponds to the excess Si atom density of 2.82 × 1020 cm−3. By using the excess density of a Si atom to estimate the spheral radius of the formed Si-QD of 2 nm, we can obtain the spheral radius of 1.74 nm aggregate of Si-QD, and the spheral radius is called the diffusion length. Theoretically, the diffusion length of 2.84 nm, as determined using the above model , is larger than the above value. However, the diffusion length determined by using either excess Si atom density or theoretical method do not larger than the bounded size (nano-porous AAO membrane sideward), it is interesting to understand other possible causes. According to the nucleation mechanism, Gibbs free energy is reduced during Si-QD nucleation, while the grain boundary and defective position exhibit the lowest nucleation energy to achieve the lowest Gibbs free energy. If the atoms are unable to diffuse to the grain boundary, which will nucleate in the randomly distributed defective position within the SiOx host matrix, then the Si-QD are enlarged, and eventually saturate at a size set by the diffusion length of the excessive Si atoms within the SiOx matrix.
In comparison with an as-grown SiOx film, the SiOx nano-rod with buried Si-QD as-deposited in a nano-porous AAO membrane shows defect related PL emissions at 415–455 nm of an intensity increasing over two orders of magnitude (see Fig. 2(a) inset). Such a blue-green emission results mainly from the activation of dense radiative defects, such as weak oxygen bonding or neutral oxygen vacancy (NOV) defects in the SiOx host matrix. According to the fundamental theory of impurities in crystals , the interstitial atoms are vacancies formed in pairs (called Frenkel defects). These vacancies induce interstitial atoms to randomly form distributed defective positions within the SiOx host matrix. In other words, spatially confined synthesis enhances defect density to promote more nucleated positions, which can impede the route of diffusion and confine the degree of atomic aggregation. A schematic illustration is presented in Fig. 3 to explain such a spatially confined synthesis of Si-QD in a 1-D SiOx nano-rod surrounded by a nano-porous AAO membrane. Annealing induced Si atomic diffusion within a 1-D SiOx nano-rod is restricted by its defect density. However, when annealing the SiOx film grown on Si substrate, the Si-QD can be aggregated by the excessive Si atoms diffusing within the diffusion length. Therefore, the size of Si-QD buried in a 1D SiOx nano-rod structure is smaller than that buried in 2D SiOx film on a smooth Si wafer. Furthermore, the diffusion controlled growth mechanism is employed to determine the activation energy of 6.284 kJ mole−1 for Si atom diffusion, which also indicates that the decreasing O/Si composition ratio of SiOx host matrix reduce activation energy, and promote Si atomic aggregation.
To further support the existence of Si-QD within the SiOx nano-rod deposited among nano-porous AAO membrane with an O/Si composition ratio of 1.5, the high-resolution TEM images are shown in Fig. 4 . The average diameter and maximum volume density of Si-QD, with a distinct lattice spacing of 0.31 nm, buried in SiOx thin-film are about 2.6 ± 0.2 nm and 5.71 × 1017 cm−3, respectively. In comparison, the Si-QD within SiOx nano-rod buried in a nano-porous AAO membrane shows tiny Si-QD with a smaller diameter of only 2.0 ± 0.1 nm, but a higher volume density of 5.51 × 1018 cm−3 (see Figs. 4(a) and 4(b)). The shrunken size and enlarged volume density strongly correlates with the geometrically confined aggregation of excessive Si atoms. Such a spatially confined effect also leads to a smaller standard deviation of size, varied from 25% to 12.5% (Figs. 4(c) and 4(d)). By changing the host matrix of the deposited SiOx from a smooth Si wafer to a nano-porous AAO membrane, the color of the PL blue-shifts its central wavelength, decreasing from 588 nm to 492 nm to manifest the shrinking size of Si-QD . The spatial distribution of the Si-QDs is quantified by statistically analyzing the distances between the nearest neighbors with a previously reported method . Note that the variance coefficient of Si-QDs should be as large as possible to reach uniform spatial distribution in the host matrix. The distance variance of the distributed Si-QDs shown in Figs. 4(a) and 4(b) are 76% and 47%, respectively, indicating that the more uniform distribution of Si-QDs inside AAO membrane is observed due to the numerous nucleation defects with extremely low free-energy. A bright PL with white-light emitting pattern is contributed by the Si-QDs precipitated within the spatially confined SiOx nano-rods deposited in nano-porous AAO membrane. In contrast, the PL of Si-QDs self-aggregated in SiOx grown on Si wafer show a dark-red-light (see Fig. 5 ). The smaller size of Si-QD buried in nano-rod structure eventually leads to a higher volume density than that buried in the SiOx film. The emitting centers were enhanced by increasing the volume density of Si-QD, which combines the enlarging bandgap effect of Si-QD with decreasing size (d) to promote the PL intensity yield.
The PL of the SiOx nano-rod varied with annealing temperatures from 350 to 800°C, and reveals a red-shifted and slightly broadened significant peak at the central wavelength as varied from 450 to 600 nm, and shown in Fig. 6 . When the annealing temperature is below 500°C, the PL reveals a blue emission pattern as the Si-QDs within the SiOx are extremely small. The PL intensity reached its maximum after annealing at 500°C as the precipitation of large-scale Si-QD is initiated. The Si-QD related PL of SiOx nano-rods grown in porous AAO membrane reveals a super-luminescent blue spectrum with an intensity (normalized to thickness) of 0.17 count/nm at around 485-500 nm. In contrast, the same SiOx film grown on a smooth Si wafer only exhibits a PL intensity of 0.08 count/nm. The spectral linewidth of normalized PL clearly indicates a less confined size distribution for Si-QDs within the SiOx when annealing at higher temperature. Note that the PL can further broaden and red-shift to 600 nm for the 800°C annealed sample, however, the PL intensity greatly attenuates due to both the enlargement of Si-QD size and the reduction of the emission photon energy.
On the other hand, the nano-porous AAO membranes become easily cracked due to the extreme bending strain occurring at such high temperatures. According to the diffusion theory, the diffusion length of the excessive Si atoms is proportional to the square root of the diffusion constant with a Boltzmann distribution function of exp(−1/kT), thus providing a lengthening diffusion range for the excessive Si atoms and a enlarging size for the self-aggregated Si-QD with increasing temperature. To elucidate the relationship on diffusion-constant dependent size of Si-QD embedded in 1D SiOx nano-rod with varying annealing temperature, we employ Fick's first, second laws, and delerue’s formula for theoretically fitting the obtained spectra, the combined formula is described as Eq. (2) for annealing temperature ranging from 350 to 800°C shown in Fig. 6 also indicates a good agreement with the experimental data. The simulation clarifies that the diffusing constant eventually saturates at relatively high annealing temperature, such that the Si-QD size can no longer expand to cause a red-shifted PL by increasing temperature.
The observation of a PL signal from SiOx nano-rods in nano-porous AAO membrane, at extremely low annealing temperatures, also corroborates that the Si-QD precipitation can be initiated due to surface effects of a nano-scale structure, as compared to the bulk case . Note that many unusual thermodynamic behaviors are associated with spatial confinement synthesis. Fitting the melting temperature of the semiconductor QDs by a phenomenological model reveals a significant deviation with increasing size. Goldstein et al. indicated that the CdS melting data agrees well with the phase equilibrium model , and the same model predicts a rapid rise in Tm with the increasing particle size of the GaAs . Such a low annealing temperature is less than those reported in any previous work . This phenomenon elucidates that the monotonically decreasing dimension of the SiOx host matrix can concurrently contribute to the spatially confined precipitation of Si-QD and low temperature phase-separation between Si-QD and SiO2 matrices, thus facilitating super-luminescent PL with enhanced power, blue-shifted wavelength, and narrowing spectral line width.
We have demonstrated the spatially confined synthesize of a SiOx nano-rod in nano-porous AAO membrane, with ultra-bright PL after lower annealing temperature of 500°C. A higher concentration, with a volume density of 5.51 × 1018 cm−3 and a tiny size diameter of only 2.0 ± 0.1 nm, of the excessive Si-QD can be easily obtained. HRTEM results indicate that the geometric form variations between 2D to 1D could also effectively reduce the standard deviation of Si-QD size from 25% to 12.5%. The spatially confined synthesis greatly increases the density of nucleated positions for Si-QD precursors, which essentially impedes the route of thermally diffused Si atoms and confines the degree of atomic self-aggregation during annealing. By using a diffusion controlled growth mechanism to estimate the diffusion characteristics, we obtain the activation energy of 6.284 kJ mole−1 with a diffusion length of 2.84 nm for a SiO1.5 nano-rod in nano-porous AAO membrane. These results also indicate that the varied composition ratio of SiOx host matrix will affect the value of activation energy. In contrast, the same SiOx film grown on smooth Si wafer only exhibits a PL intensity of 0.08 count/nm. The PL could further broaden and red-shift to 600 nm for the 800°C annealed sample, however, the PL intensity greatly attenuates, due to both the enlarged Si-QD size and the reduction of the emission photon energy. The impact of this work elucidates that the monotonically decreasing dimension of SiOx host matrix can concurrently contribute to the spatially confined precipitation of Si-QD and low temperature phase-separation between Si-QD and SiO2 matrices, thus facilitating super-luminescent PL of enhanced power, blue-shifted wavelength, and narrowing spectral line width.
This work was supported by the National Science Council of the Republic of China, Taiwan, under grants NSC-98-2218-E-002-022, and NSC-98-2623-E-002-002-ET.
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