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GaN films grown on PSS are investigated by XRD, CL, SEM and TEM. There are low threading dislocations (TDs) with larger fill factor, which results in better electrostatic discharge (ESD) yield of LEDs. The effect of growth rate on dislocations in GaN films grown on PSS is investigated by TEM. It is found that dislocations density decreases as the growth rates decrease. And the performance of InGaN-based LEDs on different PSS is analyzed. The performance of LEDs grown on different PSS is determined by slanted angle and fill factor simultaneously.
©2011 Optical Society of America
1. Introduction
With emission in blue and green spectral range [1], III-nitride high-brightness light emitting diodes (LEDs) have been used recently in large-area full-color outdoor displays, signal lights and high performance back lighting units in liquid crystal displays. Though commercially available, the external quantum efficiency (EQE) of GaN-based LEDs still falls short of what is desired. With the disparate refractive indices of GaN (n = 2.4) and air (n = 1), the internal light has difficulty in escaping into the air from the semiconductor. The light reaching the surface with incident angle larger than the critical angle (24.6°) will not emit to the air, and instead it will experience total internal reflection and continues to be reflected within the LED until being absorbed [2,3]. Improving light extraction efficiency (LEE) has thus become the focus of researches. The techniques of surface roughening [4], nanoimprinting [5], have been used to improve LEE but not effectively. Currently, GaN based-LEDs grown on patterned sapphire substrates (PSS) have been studied by more and more researchers [6,7]. The PSS, which is a mask-free and therefore contamination-free method, reduces the threading dislocation density in GaN epilayers, and moreover, it enhances LEE of LEDs due to increased light escape probability [8]. Many types of PSS had been studied and successfully applied to the fabrication of high-efficiency LEDs [9,10]. And there have been a lot of reports focusing on the comparisons of the LEDs EQE between un-patterned sapphire substrate (un-PSS) and PSS [11,12]. However, the effect of PSS shape on the performance of InGaN LEDs has been less reported.
In this work, periodic pyramidal array PSS with various slanted angles and fill factors (f) were used as substrates. GaN films grown on PSS were investigated by X-ray diffraction (XRD), cathodoluminescence (CL), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Epitaxial growth of GaN-based LEDs on these PSS was carried out by metal-organic chemical vapor deposition. The electrostatic discharge (ESD) and output power of InGaN LEDs grown different PSS was tested for comparison.
2. Epitaxial growth of GaN films on PSS
The huge lattice constant mismatch between sapphire substrates and GaN films results in high density of dislocations in GaN films. A two-step technique was developed to reduce the misfit dislocation. The essential role of a low temperature (LT) grown GaN as a nucleation layer is to supply a high density of nucleation centers with the same orientation, and then lateral growth originating from these nucleation centers occurs at the initial stage of high temperature growth [13]. The lateral growth plays a key role to reduce dislocations. Figure 1 shows the SEM images of epi-layers grown on PSS at 1040°C after the LT-GaN nucleation layer at 530°C for various growth times: (a) 0 min, (b) 3 min, (c) 10 min and (d) 30 min, respectively. The growth selectively proceeds with GaN islands nucleating at specific locations rendered by the pattern. In the initial growth stage, the GaN nucleation islands (NIs) grow predominantly on the flat trench around the cones, while GaN nucleation on the cone sidewall is suppressed. Then the GaN NIs on the trench region coalesce, which resemble the growth on planar sapphire substrate, as shown in Fig. 1(b) and (c). Then the GaN films gradually overgrow the protruding cones and coalesce near the summit as the growth proceeds [14].
Fig. 1 The plan-view SEM images of the GaN epi-layers grown on PSS with the shape of 3.0/2.0/1.5 μm × μm × μm: (a) 0 min, (b) 3 min, (c) 10 min and (d) 30 min after GaN nucleation layer, respectively.
As discussed above, at the initial growth stage of GaN on PSS, the GaN islands nucleate predominantly on the trench, while a few GaN islands nucleate on sidewall of the cone. It is believed that GaN islands on the cone have large misorientation with GaN on the trench since they nucleate at various crystalline planes rather than c-plane. Therefore, it is expected that high density of dislocations will appear when GaN islands on the trench merge with GaN islands on the cone.
The effects of growth rates on coalescence and dislocation density in GaN films grown on PSS are investigated. Samples were grown at 1040 °C with V/III ratio of 1100 and the TMGa flow of 52 sccm, 45 sccm and 38 sccm, respectively. The growth rates obtained by laser interferometer for the three samples were around 2.2 μm/h, 1.9μm/h and 1.6 μm/h respectively, which decreased linearly with the decrease of TMGa flow. In order to observed the growth evolution for samples with various growth rates, growth was interrupted after it started 30 minutes, 30 minutes, and 60 minutes for sample A, B, and C with growth rate of 2.2 μm/h, 1.9μm/h and 1.6 μm/h respectively. Figure 2(a)-(c) shows the SEM images of sample A, B and C. Two issues are noted from the SEM observation. First, coalescence is delayed as growth rates decrease. Sample A and B are about to coalesce at 30 minutes as shown in the Fig. 2(a) and (b). At 60 minutes, sample A and B coalesce fully (images not shown here), while full coalescence does not happen for sample C yet. Second, GaN islands size and density on the cone decreases as growth rates decrease. Adatoms on the cone tend to diffuse to the trench. However, adatoms have a tendency to nucleate on the cone when growth rates increase since there are more incoming species. The GaN islands on sidewall are so large that they merge with the GaN materials laterally grown from the trench under a high growth rate for sample A. Instead, the GaN islands on sidewall are suppressed as the growth rates decrease and can be hardly found in sample C.
Fig. 2 (a)-(c) SEM images of epilayers grown on the PSS for various growth rates: 2.2 μm/h, 1.9μm/h and 1.6 μm/h, respectively. (d)-(f) Cross-section TEM images of the GaN epilayers corresponding to sample D, E and F, respectively.
3 μm-thick and fully-coalesced GaN epilayers with various growth rate were observed by cross-section TEM, which were shown in Fig. 2(d)-(f) for sample D, E, and F. As can be seen, the dislocations can be generated from three positions, i.e. summit, sidewall and trench. Dislocation density in the trench region is similar for all the samples, which has the same mechanism as the growth on the planar sapphire substrates. However, a lot of dislocations generate from sidewall for sample D with the growth rate is 2.2 μm/h, which is attributed to the coalescence of GaN islands from the trench with GaN islands from the cone sidewall. As the growth rate decreases to 1.9 μm/h, the amount of dislocations generating from sidewall decreases evidently as shown in Fig. 2(e). As growth rate decreases further, the amount of dislocations decreases greatly. There is only one dislocation on summit and nearly no dislocation on sidewall as shown in Fig. 2(f). Therefore, the dislocation density decreases as the growth rates decrease. The TDs density is estimated to be about 6.6 × 108 cm−2, 2.0 × 108 cm−2 and 5.1 × 107 cm−2 for sample D, E and F, respectively.
Macroscopic crystal quality of GaN films with different growth rates was characterized by high resolution X-ray diffraction (HR-XRD). Full width at half maximum (FWHM) of (002) and (102) rocking-curve peaks for the three samples decreases monotonously as the growth rates decrease shown in Fig. 3 . FWHM of sample F is the lowest which indicates that crystal quality of sample F is the lowest. It is concluded that better crystal quality comes from a low growth rate. These results are consistent with the observation of TEM.
Fig. 3 FWHM of (002) and (102) rocking-curve peaks of epilayers grown on the PSS for various growth rates: 2.2 μm/h, 1.9μm/h and 1.6 μm/h, respectively.
3. The effect of PSS shape on quality of GaN films
Four types of PSSs were prepared with their diameter/spacing/height being 3.75/1.25/1.85, 3.0/2.0/1.5, 2.4/0.6/1.5 and 2.0/1.0/1.5 μm/μm/μm, named sample G, H, I and J, respectively. The slanted angle (θ) of PSS sidewalls is calculated from tanθ = 2height/diameter, and fill factor (f) is defined as the equation of (PSS area/total areas). The slanted angles are 44.6°, 45°, 51.3° and 56.3° for the four kinds of substrates. And, fill factors (f) are 0.51, 0.33, 0.58 and 0.4, respectively.
To investigate the crystal quality of GaN films grown on these substrates, CL and TEM are employed to observe the dislocations distribution in the four GaN epilayers with the thickness of 3μm. Figure 4[(a)-(d)] shows the plan-view CL intensity spatial mapping of GaN films on sample A, B, C and D. The dark region in the band-edge luminescence mapping of GaN films can be attributed to non-radiative recombination at threading dislocations [15]. The density of dark regions is estimated to be 1.2, 5, 1.0 and 4 × 108 cm−2 which are consistent with the tendency of flat area ratio (1-f) of these PSSs. As discussed above, dislocation generation at the cone sidewall is suppressed by reducing the growth rates, dislocations are generated mainly from the trench region. Therefore, dislocation density reduces as the fill factor increases. In addition to CL and TEM, XRD was used to evaluate the epilayers crystal quality. They are plotted with the combination of CL dark density as a function of f in Fig. 5 . FWHM of the rocking curve for both the symmetric (002) and asymmetric (102) reflections decrease monotonically as the f increases from 0.33 to 0.58.
Fig. 5 The CL dark density and FWHM of the rocking curve for both the symmetric (002) and asymmetric (102) is plotted as a function of fill factor.
4. Performance of InGaN-LEDs grown on PSS
InGaN-LEDs were fabricated on sample G, H, I and J, renamed as LED-G, -H, -I, -J. The LED structures comprised a 25nm nucleation layer on the PSS, a 2.5 μm undoped-GaN layer film, a 2.0 μm n-GaN layer, an five periods InGaN/GaN multiple quantum well (MQW) with emission wavelength in the blue region (460 nm), a 20nm AlGaN electron blocking layer and a 150nm p-GaN layer. Trimethylgallium (TMGa) and ammonia gas (NH3) were used as precursors for Ga and N source, respectively. The device mesa with a chip size of 350 × 350 μm2 was then defined by inductively coupled plasma. The indium tin oxide (ITO) layer was deposited to form a p-side contact layer and a current spreading layer. The Cr/Au layer was deposited onto the ITO layer to form the p-side and n-side electrodes. The optical and electrical properties of LEDs were tested by LED chip/wafer prober and tester (IPT-6000) with an integrate sphere from FitTech Co., Ltd.
The ability to withstand electrostatic discharge (ESD) is important for a GaN-LED. After human body model (HBM) ESD 2000V impulses were applied to the LED chips on wafer (COW), the reverse current was measured under reverse voltage of 5 V using a parameter analyzer. If the reverse current is over 1 μA, the chip is considered to be damaged by ESD impulse. The data were obtained with a sampling rate of 50:1. The ESD yield of GaN-LEDs on the four PSSs is plotted in Fig. 6 comparing with f. The ESD yield increases as f increases. There are more TDs with more planar area, which contributed to worse ESD yield. There are larger and more leakage pathways as the f decreases from 0.58 to 0.33. In order to maintain or enhance ESD characteristics, the pits-related TDs that inherently occur during the growth of InGaN/GaN MQW and eventually terminate at the top surface of the LED should be minimized. Also it is widely accepted that TDs in GaN provide non-radiative recombination channels and charge leakage pathways [16]. So it is believed that ESD can be improved by decreasing TDs density. In another word, the improvement of ESD is achieved by increasing f or by increasing PSS area.
The optical and electrical properties of these bare chips were also tested with a sample ratio of 50:1. The forward voltages for the five chips are 3.05V, 3.02V, 3.10V and 3.15V, respectively. The output power of four samples is plotted as a function of slanted angles as shown in Fig. 7 . The output power improvement percentage is 26% for the LED grown on PSS with slanted angle 44.6° in comparison with that grown on PSS with slanted angle of 56.3°. The output power increases as the slanted angles decrease from 56.3°to 44.6°. The output power enhancement as the decreasing of slanted angles can be attributed to the reflection of PSS array, resulting in a wider range of critical angle for internal reflection. The escape probability in LEDs grown on PSS is significantly higher than when the slanted angles decrease. And the report of light extraction analysis with Monte Carlo ray tracing presented that the photon escape probability in LEDs grown on PSS is highest as slanted angles decrease to around 30° [17]. From the data of ESD and output power, it can be concluded that the performance of the LEDs grown on PSS is determined by slanted angles and fill factor simultaneously.
Fig. 7 The plot of output power of LEDs at injection current of 20 mA as the function of slanted angles.
5. Conclusion
In summary, epitaxial growth of GaN on PSS is observed by SEM. TEM is used to discover the TDs of GaN films grown on PSS, there is a conclusion that TDs density decreases as f increases. It is also found that the quality of GaN films grown on PSS is affected by growth rates. And ESD yield of LEDs will be improved by increasing f. LEDs were grown on various PSS with different slanted angles and f. The tested output power of these LEDs grown different PSS is determined by slanted angle and f in combination.
Acknowledgements
The work was supported by National Nature Science Foundation of China (Grant No. 61076119) and supported by Technical Corporation Innovation Foundation of Suzhou Industrial Park (Grant No.SG0962).
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