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Nonpolar a-plane (11-20) GaN (a-GaN) layers with low overall defect density and high crystalline quality were grown on r-plane sapphire substrates using etched a-GaN. The a-GaN layer was etched by pulse NH3 interrupted etching. Subsequently, a 2-µm-thick Si-doped a-GaN layer was regrown on the etched a-GaN layer. A fully coalescent n-type a-GaN layer with a low threading dislocation density (~7.5 × 108 cm−2) and a low basal stacking fault density (~1.8 × 105 cm−1) was obtained. Compared with a planar sample, the full width at half maximum of the (11-20) X-ray rocking curve was significantly decreased to 518 arcsec along the c-axis direction and 562 arcsec along the m-axis direction.
© 2014 Optical Society of America
1. Introduction
Over the few decades, conventional c-plane III-V nitride light emitting diodes (LEDs) have achieved remarkable increases in performance owing to dramatic improvements in the quality of nitride materials. However, the device performance is strongly restricted by strong built-in electric fields originating from strain-induced piezoelectric and spontaneous polarizations in the [0001] direction. These polarization-induced fields along the c-axis cause the quantum-confined Stark effect, which leads to reduced radiative recombination rates in quantum wells (QWs) [1–4]. To overcome these problems, nonpolar and semipolar GaN have attracted considerable attention in the research field. However, nonpolar and semipolar GaN grown on sapphire substrates generally suffer from a high density of threading dislocations (TDs) and basal stacking faults (BSFs) owing to the high Ga-polar to N-polar wing growth rate ratio and the large lattice mismatch between GaN and the substrate [5–9].
Epitaxial lateral overgrowth (ELOG) is one of the most commonly used techniques to reduce the high defect density and improve crystalline quality. Although nonpolar a-plane (11-20) GaN (a-GaN) grown by ELOG has a low defect density in the wing area (on the mask), the window area (where these is no mask) still has a high defect density [6,10–12]. The high density of TDs in the window area can act as nonradiative recombination centers in multiple-quantum wells (MQWs) and decrease the LED device performance.
In this study, we report on nonpolar a-GaN layers with a low overall defect density, a high crystalline quality, and a good optical property grown on r-plane (1-102) sapphire substrates with −0.2°-off-axis tilted in the c-axis [0001] direction by metalorganic vapor phase epitaxy.
2. Experimental details
Nonpolar a-plane GaN layers were grown on r-plane sapphire substrates using pulse NH3 interrupted etching method. Trimethylgallium, trimethylindium, methylsilane, bis-magnesium, and ammonia (NH3) were used as the source materials of Ga, In, Si, Mg, and N, respectively.
Figure 1 depicts a schematic showing the growth of a-GaN using a pulse NH3 interrupted etching process. First, a 4-µm-thick undoped a-GaN layer, which was grown by modified two-step growth [13], was deposited on an r-plane sapphire substrate with a SiO2 nanopillar mask of 230 nm diameter by nanoimprint lithography (NIL) as shown in Fig. 1(a). Experimental details of the growth of the undoped a-GaN layer using the SiO2 nanopillar mask were previously reported [14]. A SiO2 nanopillar mask was again fabricated on the fully coalescent undoped a-GaN layer by NIL, as shown in Fig. 1(b), to enable the etching of a large area of GaN except for that below the SiO2 mask and obtain the rapid coalescence of a-GaN during the regrowth process. Figure 1(c) shows the a-GaN layer etching process below the fabricated SiO2 nanopillar mask. In general, GaN layers can be only etched along anisotropic direction by typical plasmas and wet etching. However, we tried the pulse NH3 interrupted etching method to obtain irregular and rough etched GaN surface, which can effectively bend the TDs.
Fig. 1 Schematic diagram of a-GaN layer growth: (a) a-GaN layer grown on r-plane sapphire substrate with SiO2 nanopillar mask, (b) fabrication of SiO2 nanopillar mask on a-GaN layer, (c) a-GaN layer etching process, (d) etched a-GaN layer, and (e) fully coalescent a-GaN layer.
It is well known that GaN can decompose during epitaxial growth. The decomposition of GaN can be described by several mechanisms, which are into gaseous Ga and N, liquid Ga and N, and sublimation of GaN as a diatomic or polymeric product [15,16]. The GaN decomposition rate is generally controlled by the temperature, pressure, and gas flow rate. To etch the a-GaN layer, we utilized the GaN decomposition phenomena. The reactor was set to 1050 °C and 75 Torr. A mixture of pure NH3 and H2 at a standard 5 L/min flow rate, which is the optimized flow rate in our reactor for obtaining uniformly etched samples, was injected, resulting in the rough and deep etching of the a-GaN layer. The mixture was injected for 10 sec for slow etching, then the NH3 flow was interrupted for 9 sec for fast etching. This was repeated 50 times in the initial etching of the a-GaN layer. Then, the a-GaN layer was sequentially etched for 30 min in the mixed NH3 and H2 flow to obtain a deeply etched a-GaN layer.
This pulse NH3 interrupted etching process can be used to obtain different GaN etching rates by changing the gas ambient [16]. Figures 2(a), 2(d), and 2(e) show images of etched a-GaN layers obtained under different gas flows. Figures 2(a) and 2(d) show etched a-GaN layers after 50 cycles of the pulse etching process using mixed NH3 and H2 and mixed NH3 and N2 flows, respectively. In the case of the mixed NH3 and N2 flow, the a-GaN layer was anistropically etched along the substrate. Even though the etched a-GaN layer was sequentially more etched for 30 min in the mixed NH3 and N2 flow, as shown in Fig. 2(e), it was only partially etched and had a small etching depth compared with that etched in the mixed NH3 and H2 flow as shown in Fig. 2(c). This was due to the GaN decomposition rate in the N2 flow being much lower than that in the H2 flow.
Fig. 2 Bird’s-eye views of etched a-GaN layers after pulse NH3 interrupted etching (50 cycles) in (a) mixed NH3 and H2 flow, and (d) mixed NH3 and N2 flow. (e) Etched a-GaN layer after pulse NH3 interrupted etching (50 cycles) and sequential etching (30 min) in mixed NH3 and N2 flow. (b) Plan-view and (c) cross-sectional SEM images along m-direction (inset: along c-direction) of a-GaN layer after pulse NH3 interrupted etching in mixed NH3 and H2 flow. (f) Cross-sectional and plan-view (inset) SEM images of a-GaN layer after sequential etching (45 min) in mixed NH3 and H2 flow.
Figure 1(d) shows the etched a-GaN layer with an irregular shape after pulse NH3 interrupted etching in the mixed NH3 and H2 flow. During the regrowth of a 2-µm-thick n-type a-GaN layer, a number of microscale voids formed under the SiO2 nanopillar mask during the coalescence of the a-GaN layer as shown in Fig. 1(e).
3. Results and discussion
Figure 2 shows a scanning electron microscopy (SEM) image of the etched a-GaN layer after pulse NH3 interrupted etching. In the plan-view SEM images, we observed the etching of the entire a-GaN layer excluding the SiO2 nanopillar mask as shown in Fig. 2(b). The irregular shape of the a-GaN layer along c- and m-directions etched to ~1.3 µm depth was observed in cross-sectional SEM images as shown in Fig. 2(c). The irregular and rough surface of the a-GaN layer can effectively bend the TDs and prevent their propagation during the lateral overgrowth process. Figure 2(f) shows the etched a-GaN layer after the sequential etching process (45 min) in the mixed NH3 and H2 flow. The sequential etching of GaN resulted in the poor uniformity of the etching depth and area owing to the constant etching rate. Triangular etching marks were also observed around the SiO2 mask as shown in the inset.
To investigate the density of TDs and BSFs, a fully coalescent n-type a-GaN layer was observed by transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). It is well known that TDs can be characterized when g = 11-20 via the |g·b| criterion in TEM imaging (g is the diffraction vector and b is the Burgers vector). Under a two-beam condition with g = 1-100, intrinsic I1-type BSFs are terminated by Frank–Shockley partial dislocations (PDs) with Burgers vector b = 1/6<20-23> [17,18]. Figures 3(a) and 3(b) show cross-sectional SEM and BF-STEM images of a fully coalescent n-type a-GaN layer, respectively. A number of nano- and microscale voids with an irregular shape were observed around the etched GaN and SiO2 nanopillar mask areas. The origin of these voids can be explained by the mass transport of GaN [19], which led to the bending and interruption of TDs. The TEM image in Fig. 3(d) shows clear evidence of mass transport. The upper area in which mass transport occurred clearly has a low TD density. Also, the light output power of LEDs can be increased via light scattering and reflection at the interface between voids and an a-GaN layer [20,21]. Figure 3(c) shows the nanopillar SiO2 mask area on an r-plane sapphire substrate. As illustrated in Fig. 1(a), a high density of TDs was observed in window areas owing to the use of the conventional ELOG process. The overall density of TDs was reduced from ~1.2 × 1010 cm−2 in a planar a-GaN layer (not shown here) to ~7.5 × 108 cm−2 in the a-GaN layer subjected to the pulse NH3 interrupted etching process. It is notable that although this TD density is higher than those for conventional ELOG samples in the wing area [6,10–12], we can fabricate an LED device using an epilayer with uniform low defect density.
Fig. 3 (a) Cross-sectional SEM image of fully coalescent n-type a-GaN layer. Cross-sectional STEM and TEM images viewed along m-direction with g = 11-20: (b) fully coalescent n-type a-GaN layer (STEM), (c) SiO2 nanopillar mask area on r-plane sapphire substrate (TEM), and (d) magnification of void area (TEM).
Figure 4 shows plan-view TEM images with g = 1-100 of a planar a-GaN layer and an a-GaN layer subjected to the etching process to observe BSFs, which are generated between the nucleation layer and the substrate and propagate through the a-GaN layer to the surface. The density of BSFs in the direction perpendicular to the c-axis [0001] was reduced from ~9.7 × 105 cm−1 in the planar a-GaN layer to ~1.8 × 105 cm−1 in the etched a-GaN layer. However, BSF terminations and short BSFs were observed more frequently in the planar a-GaN layer than in the etched a-GaN layer. This indicates that etching the a-GaN layer reduced the density of PDs, which terminate BSFs and prismatic stacking faults [22].
To evaluate the crystalline quality, the a-GaN layers were characterized by high-resolution X-ray diffraction measurements, employing Au Kα1 (1.27634 Å) as the X-ray source. Figure 5 shows the variation of the symmetric ω-scan full width at half maximum (FWHM) of the (11-20) X-ray rocking curve (XRC) with the in-plane rotation for various a-GaN layers. The etched a-GaN layer not only had the lowest FWHM but also exhibited the least anisotropic behavior of the (11–20) XRC FWHM in terms of the azimuth angle, which is related to anisotropic mosaicity [23]. The XRC FWHM values of the planar and etched a-GaN layers were 933 and 518 arcsec along the c-axis direction and 1033 and 562 arcsec along the m-axis direction, respectively. The high crystalline quality of the etched a-GaN layer was associated with the low density of extended defects, which is consistent with the TEM results, and the mosaic tilt/twist of the a-GaN layer [18].
To determine the optical properties of a-GaN LED devices, five-period InGaN (5 nm)/GaN (12 nm) MQW layers were grown on an n-type a-GaN layer with a doping concentration of 3 × 1018 cm−3 under N2 ambient. Subsequently, a 130-nm-thick Mg-doped p-type a-GaN layer was deposited. After the growth of the LED structure, the LED was activated at 725 °C in air ambient for 5 min by rapid thermal annealing. The hole concentration of the activated p-type a-GaN layer was 1.3 × 1018 cm−3 at room temperature. Conventional lateral LED devices with a size of 260 × 300 µm2 were fabricated. Figure 6 shows the dependences of the light output power of a-GaN LED devices as a function of injection current obtained by on-wafer measurements. The LED with the etched a-GaN layer had a much higher output power than the other LED devices. We attribute the high output power of the a-GaN LED to the etching process, which decreased the defect density and the density of nonradiative recombination centers in the MQWs, and increased light scattering by a number of nano- and microscale voids in the a-GaN layer. The inset in Fig. 6 shows the electroluminescence (EL) emission spectra of a-GaN LED devices at a current of 80 mA (100 A/cm2). The EL intensity of the etched a-GaN LED exhibits a high value with a narrow FWHM of 52 nm.
Fig. 6 Relative light output power and injection current of fabricated a-GaN LEDs in 10-80 mA range. The inset shows EL emission spectra and their FWHM at a current of 80 mA.
Figure 7 shows the EL emission spectra of LED with planar and etched a-GaN layers in 10-80 mA range. The planar a-GaN LED showed the broad FWHM and low EL intensity compared with the etched a-GaN LED as shown in Fig. 7. The peak emission wavelength shifted from 508.1 (planar a-GaN LED) and 503.2 (etched a-GaN LED) nm at 10 mA to 505.9 (planar a-GaN LED) and 501.4 (etched a-GaN LED) nm at 80 mA. These negligible peak shifts of 2.2 (planar a-GaN LED) and 1.8 (etched a-GaN LED) nm up to a current of 80 mA are consistent with the reduced polarization-related internal electric fields in a-GaN LED devices. The blue shift of a-GaN LED devices might be due to the band-filling effect of the localized energy states [24] and the screening of internal electric fields [25].
Fig. 7 EL emission spectra of LED with (a) planar a-GaN and (b) etched a-GaN layers and FWHM in 10-80 mA range.
4. Conclusion
Nonpolar a-GaN layers with the low overall defect density were grown on r-plane sapphire substrates using a pulse NH3 interrupted etching process. During the regrowth of the etched a-GaN layer, a number of microscale voids were formed by the mass transport of GaN. The TD density was significantly reduced from that in the etched GaN area. LED device based on an a-GaN layer subjected to pulse NH3 interrupted etching had higher performance than a planar LED device owing to light scattering and the reduced number of nonradiative recombination centers in the MQWs.
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