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We investigate the structural properties of molecular-beam-epitaxy coalescence overgrowth of GaN columns at the nanoscale with transmission electron microscopy and other characterization techniques. Two samples grown over nanocolumns of different widths and spatial densities (columns/area) are compared. It is found that columns with a larger cross section (~500 nm) and correspondingly lower spatial density normally lead to un-coalesced overgrown domains ranging 5-8 μm in size. On the other hand, the overgrowth on the columns of a smaller cross section (~100 nm) and correspondingly higher density results in coalesced domains ranging from 1 to 5 μm in size. It is believed that among the smaller, more closely spaced columns the strain distribution resulting from overgrowth is more effective in leading to the uniformity of crystalline orientation, and hence successful coalescence. The optical characterization leads to the conclusion that the defect density in the sample grown on smaller columns is lower when compared with that grown on larger columns.
©2013 Optical Society of America
1. Introduction
Because of lateral strain relaxation, GaN nano-columns have been widely investigated for reducing the threading dislocation (TD) density present in typical GaN heteroepitaxy [1–14]. Unlike other nanostructures, such as carbon nano-tubes, GaN nano-columns grown with molecular beam epitaxy (MBE) can be quite straight, parallel, and vertical with respect to the substrate. A decrease of TD density enhances the quality of optoelectronics or electronics devices based on such a material. However, the fabrication of optoelectronic or electronic devices based on the pristine column structures is normally difficult and not scalable. Therefore, the coalescence overgrowth of such GaN columns is important to provide a high-quality GaN template for practical device fabrication [15,16]. Attempts of coalescence overgrowth of GaN nano-columns, including those containing InGaN/GaN quantum wells, with MBE have been reported [17–19]. However, the detailed studies, such as the nanostructures and qualities of overgrowth layers and the dependence of overgrowth result on the column condition, have not been discussed yet. Such a study is useful for optimizing the growth conditions of the columns and the coalescence layer. In addition to the GaN nano-columns-based approach, significant advances have been reported for addressing high performance mid / deep UV sources by using AlGaN-based [20–23] and AlInN-based [24,25] material systems.
In this paper, we report the structural properties of coalescence overgrowth of GaN columns by plasma-assisted molecular beam epitaxy (PA-MBE). The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images are shown for understanding the sample nanostructures. The results of X-ray diffraction (XRD) and temperature-dependent photoluminescence (PL) measurements provide us further information about the crystalline and optical properties of the coalescence overgrowth samples. Two samples of different column sizes and densities are compared to see the different qualities of the overgrown layers. In section 2 of this paper, the PA-MBE growth conditions for the GaN columns and the overgrowth layers are reported. Also, the experimental conditions are discussed. Then, the SEM and TEM images of the two samples are presented in section 3. The related XRD and PL results are shown in section 4. Discussions on the dependence of the coalescence overgrowth on column condition are given in section 5. Finally, the conclusions are drawn in section 6.
2. Sample growth conditions and experimental procedures
The GaN columnar structures were grown with PA-MBE on c-plane sapphire substrates. A 500-nm thick layer of a refractory metal (titanium) was deposited on the backside of the sapphire substrates for thermal absorption. An in situ cleaning method, where Ga metal was deposited onto the substrate and subsequently evaporated, was used to remove organics from the sapphire surface. This technique has been shown to improve both the crystal and optical qualities more effectively when compared with other cleaning methods. To start the growth, sapphire substrates were heated to approximately 710 °C under an active nitrogen flux provided by an EPI Unibulb plasma source. Temperature measurements were made using an infrared optical pyrometer. A thin AlN buffer layer (< 20 nm) was deposited to assist in the formation of the nanostructures. The substrate temperature was then increased for the column growth. The sample with a lower column density and larger column diameters (sample A) was grown with a substrate temperature of 750 - 760 °C, whereas the substrate temperature for another sample of a higher column density and smaller column diameters (sample B) was above 800 °C. From previous experiments with no overgrowth, the average column diameter for the growth conditions of sample A was approximately 500 nm, whereas average column diameter for the growth conditions of sample B was ~100 nm, as measured by scanning electron microscopy. For the columnar growth, the Ga-flux and N2 flow rate was ~3.5 × 10−7 Torr and 0.7 sccm, respectively. For both samples, the GaN columns were grown under nitrogen-rich conditions. The substrate temperature for the coalescence overgrowth was 700 – 710 °C, with the Ga-flux and N2 flow rate at ~5.5 × 10−7 Torr and 0.35 sccm, respectively. Compared to the column growth, the conditions of overgrowth had a lower growth temperature and a lower nitrogen flow rate, giving the effect of a metal-rich surface known to produce smooth films in GaN [26].
The TEM investigations were performed using a Philips Tecnai F30 field emission electron microscope with an accelerating voltage of 300 kV and a probe forming lens of Cs = 1.2 mm. The XRD measurement was performed using a Bede D1 system. The PL measurements were carried out with the 325-nm line of a 35 mW He-Cd laser as the excitation source. The samples were placed in a cryostat for temperature-dependent measurements ranging from 10 to 300 K.
3. Scanning electron microscopy and transmission electron microscopy results
Figure 1(a) shows a plan-view SEM image of sample A. In this figure, hexagonal domains of 5 μm in typical size (between two parallel sides) can be seen. Some hexagonal domains are incomplete and some others seem coalesced into larger domains of irregular shapes. Those domains of different heights are clearly un-coalesced although they are closely arranged. Certain smaller domains of irregular shapes and smaller heights are squeezed between the larger domains. A site-to-site competition of available gallium flux appears to be the origin of the different domain heights. When the larger domains are formed first, the smaller domains have no space for further growth. Figure 1(b) shows a cross-section SEM image of the same sample. Here, above the substrate and the thin AlN nucleation layer, three layers can be identified, including the overgrown layer of individual up-side-down bell-shaped structures (forming the domains in Fig. 1(a)) at the top, the clearly separated columnar structure in the middle, and a tightly filled GaN layer with residual columnar morphology at the bottom. The overgrown layer is estimated to be 6-7 μm in thickness. The sizes and heights of those bell-shaped structures vary from domain to domain. Their tops show hexagonal or other irregular shapes, as already shown in Fig. 1(a). Some of them seem to be coalesced; however, most of them are separated from each other. The cross-section dimension of the columns in the middle layer are around 1 μm or larger. The estimated column density in this layer is 2.85 x 107 cm−2. The GaN structure in the bottom layer of the epitaxial growth only appears when the growth parameters are changed into the Ga-rich conditions to promote thin film overgrowth. It appears to be the residual GaN that has nucleated in between the columns.
Figure 2(a) shows a TEM image of sample A demonstrating the junction of a column and the overgrown bell-shaped structure. The GaN column has a cross-section dimension of 1.7 μm. It is interesting to see the existence of a neck structure at the junction between the column and the overgrown layer. In other words, the cross section dimension at the point of starting overgrowth is a minimum. The crack line, indicated with an arrow, was produced during the preparation of the sample for TEM observation. During the early stage of overgrowth, the column expands its cross-section dimension rapidly. The expansion slows down after 2-3 μm overgrowth. The stage of fast cross-section expansion leads to a slant angle of about 30 degrees. This bell-shaped overgrown structure is isolated from others to form a hexagonal top feature of 6-7 μm in size. The neck structure formed at the starting point of overgrowth can be due to a process that the column is thickened during the overgrowth stage. In this stage, certain Ga atoms migrate to the column sidewalls and form GaN, creating an additional shell layer around the original column. As the overgrowth continues, a shell layer makes the column thicker and the formation of the neck structure. Note that the formation of the shell layer is not necessarily symmetric.
Fig. 2 (a) TEM image of an overgrown column in sample A and (b) a high-resolution TEM image of the junction between the column and its overgrown bell-shaped structure, as circled in part (a).
In other TEM images, we have observed differences in the structure with the overgrowth from the borders of the thickened columns in this sample. The original column cross section dimension before overgrowth is assumed to be smaller than the observed value of ~1 μm (Fig. 1(b)). As mentioned early, SEM measurements of other samples with similar growth conditions for the columns and no overgrowth layer showed column diameters to be approximately 500 nm for sample A. To understand the crystalline structure at the junction between the overgrowth layer and a column, the circled portion in Fig. 2(a) is magnified to give the atomic-scale image in Fig. 2(b). Here, three regions can be identified, including the column at the bottom, the overgrowth layer at the top and the transition region in between. The transition region shows a slightly poorer atomic arrangement. Certain strain distributions can be seen from the variation of contrast.
We have extensively examined the nanostructures of those columns with TEM. In more than 30 high-quality TEM images of those columns, we could not find any evidence of TD. Careful examination of the overgrown layers on those columns in both samples also leads to the conclusion that the overgrown domains are TD free.
Figure 3(a) shows a plan-view SEM image of sample B. Here, one can see that the domains become smaller (1-3 μm) and more irregular in shape. The irregular domain shapes imply better coalescence among overgrown columns. Figure 3(b) shows a cross-section SEM image also demonstrating three layers on the AlN nucleation layer. The top layer of about 3 μm in thickness consists of highly packed long bell-shaped structures, which represent the major results of the overgrowth. Some of the bell-shaped structures are clearly coalesced. In the middle layer, some columns are coalesced during the overgrowth stage. The estimated column density in this layer is 1.24 x 108 cm−2. In Fig. 4(a), we show a TEM image demonstrating the coalescence of two neighboring bell-shaped structures, which individually have a cross-section dimension of about 1.25 μm. To better observe the transition between a column and the overgrowth portion, we magnified the circled portion in Fig. 4(a) to give Fig. 4(b). Here, one can see that the transition is quite smooth. The neck structure, as shown in Fig. 2(a) for sample A, is unclear in sample B. The cross-section dimension does not change dramatically in this sample. Such a difference can be attributed to the denser column distribution (the smaller space between the columns) and the smaller column dimension (the smaller side-wall surface) in this sample. These two factors make the GaN attachment to the column side wall for forming the shell-like layer difficult. Therefore, the observed column cross-section dimension is comparatively closer to that before overgrowth than for sample A. However, during the overgrowth stage, GaN can still fill in the gaps between columns to form the tightly filled bottom layer right above the AlN nucleation layer.
Fig. 4 (a) TEM image of several overgrown bell-shaped structures in sample B with two of them being coalesced; (b) Magnified TEM image of the circled portion in part (a).
4. X-ray diffraction and photoluminescence measurements
Figure 5 shows the XRD rocking curves in the (0002) and (10-12) planes of the two samples. The full-widths at half-maximum (FWHM) of those curves are 885 and 1051 arcsec in the (0002) and (10-12) planes, respectively, for sample A. XRD FWHM values for sample B are 703 and 2556 arcsec in the (0002) and (10-12) planes, respectively. The large rocking curve widths, which are larger than those of a typical high-quality GaN thin film of around 200 arcsec in the (0002) plane and 300 arcsec in the (10-12) plane, originate from the superposition of the results from many un-coalesced domains of different crystalline orientations. In an individual domain, the crystal quality can be quite high. It is difficult to precisely judge the superiority of the crystal quality between the two samples based on the above XRD data. However, sample B gives the smaller rocking curve width in the (0002) plane, implying that the crystal orientations among un-coalesced domains in this sample can be more uniform. In particular, the crystalline tilts of different domains are more uniform. Nevertheless, its larger rocking curve width in the (10-12) plane implies a larger variation of the twist angle among different domains in sample B.
Figures 6(a) and 6(b) show the PL spectra at 10 and 300 K, respectively, of the two samples. Because the PL excitation laser can penetrate GaN for only a few μm, the PL results mainly come from the overgrown layers of the samples. At 10 K, both the PL spectral peaks are located at 357 nm (3.473 eV). Several secondary peaks, either strong or weak, can be observed on the long-wavelength side. The major peak at 357 nm is due to the emission of the free exciton A [6]. The secondary peak at 363 nm (3.416 eV) is attributed to the defects near the interface between a column and the AlN nucleation layer [6] or the surface states on the columns [27]. The peak at 373 nm (3.33 eV) originates from the emission of excitons localized in the extended defects [28]. Then, the peak at 379 nm (3.272 eV) is due to the existence of cubic GaN structure in the nano-columns [29]. The cubic structures were indeed observed in our SEM and TEM measurements. Finally, the peak at 393 nm (3.155 eV) comes from the recombination of donor-acceptor pairs [30,31]. The spectral positions of those secondary peaks are quite consistent between the two samples, implying that the same kinds of energy state exist in the two samples. The relatively weak secondary peaks in sample B imply that coalescence overgrowth is more successful in this sample. This comparison is also true in the PL spectra at 300 K. At 300 K, the major PL peak red shifts to 363 nm (3.416 eV).
In Fig. 7, we show the temperature-dependent variation of the integrated PL intensity of the two samples. The ratio of the integrated PL intensity at 300 K over that at 10 K is used as an estimate of the internal quantum efficiency (IQE), which reflects the defect density in the sample [32]. The reduction of PL intensity as temperature increases is mainly due to the flow of carriers into defects. From Fig. 7, one can read that the IQEs of samples A and B are 0.86 and 5.75%, respectively, implying that the defect density in sample B is significantly smaller than that in sample A. Because many defects come from the domain boundaries, the higher IQE in sample B confirms that the coalescence conditions in this sample are better. The growths of GaN on nanopatterned sapphire substrates had been reported resulting in improved internal quantum efficiency in GaN-based LEDs [33–36].
5. Discussions
Based on the SEM and TEM images shown above, we have an observation that a high density of narrow columns is preferred for coalescence overgrowth. The condition for growing narrow columns leads to a high column density. Column diameters are controlled through the substrate temperature and nitrogen plasma overpressure to create a large V:III ratio. A reduction in the column diameter can be achieved by reducing the surface diffusion coefficient. Additional nucleation sites are then formed, leading to a higher column density. In sample B, although the domain structure still exists and its domain size is not necessarily larger than that of sample A, the coalescence between different columns looks better. In the case of larger columns, although a column can be overgrown to become a domain of several μm in size at the top, the coalescence between columns seems more difficult. It does not seem possible for a single column to be overgrown up to a domain of several hundred μm in size, which is large enough for device fabrication indicating that coalescence is critical for device applications. The difficulty of coalescence between columns can be due to the different heights and crystal orientations among columns and their overgrown domains. In the overgrowth of narrower columns of a larger density, the individual domains can touch each other before they become quite large in cross section dimension. In this situation, it is easier to apply a strain to a thinner column (by a thicker column) and force it to follow the crystal orientation of a nearby thicker column in the subsequent overgrowth process. In other words, the thinner columns merge into the thicker columns to become coalesced. On the other hand, in the case of larger columns of a lower density, individual columns are overgrown to become quite large in cross section before they become close. In this situation, the strain conditions between columns appears less conducive for coalescence.
6. Conclusion
We have demonstrated the nanostructures of PA-MBE coalescence overgrowth of GaN columns with TEM, SEM, XRD and PL measurements. Two samples of different column sizes and densities were compared. It was found that columns of a larger cross section (~500 nm) and a corresponding lower density normally led to un-coalesced overgrown domains of 5-8 μm in size. On the other hand, the overgrowth on the columns of a smaller cross section (~100 nm) and a corresponding higher density resulted in the coalesced domains of 1-5 μm in size. Generally speaking, an overgrown layer from narrower columns of a higher density is more tightly packed. In this situation, the individual columns are better coalesced. It is supposed that among the columns of smaller cross sections and higher densities, the strain induced by overgrowth among the columns is more effective in leading to the uniformity of crystalline orientation and hence successful coalescence. The optical characterization led to the conclusion that the defect density in the sample of smaller columns is lower when compared with that of larger columns.
Acknowledgment
This research was supported by the National Science Council, The Republic of China, under the Grants NSC 100-2221-E-194-043, 101-2221-E-194-049, 102-2221-E-194-045, and 102-2622-E-194-004-CC3.
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