1. Introduction

Group III-nitride compound semiconductors have attracted considerable interest for development of optoelectronic devices such as light emitting diodes (LEDs) and laser diodes [1,2]. However, the performance of GaN-based LEDs grown on sapphire substrate was limited by the relatively poor crystalline quality of the GaN epitaxial layers because the lack of lattice-matched substrates makes the growth of GaN complicated [3]. As a result GaN epitaxial layers usually contain high threading dislocation (TD) densities around 10⁸–10¹⁰ cm⁻². For the growth of low dislocation density GaN films, epitaxial lateral overgrowth (ELO) method has been studied in the past years due to the possibility of the ~4 orders of magnitude reduction of the TD density in the ELO regions compared to the regions of conventional vertical growth [4]. Furthermore, the performance of GaN-based LEDs is limited by the total internal reflection at the interface of the semiconductor and outer medium. The critical angle of total internal reflection is low and 23 degrees for air/GaN. Therefore, only a small fraction of light generated in the active region of LEDs can escape into the surrounding environment [5].

Recently, technologies of embedded air void structures in GaN-based LEDs have been applied to improve light extraction efficiency (LEE) and internal quantum efficiency (IQE) using a patterned dielectric layer, a tungsten ELO mask, a honeycomb cone structure, etc [6–8]. Particularly, some research groups have studied the output power enhancement of GaN-based LED occurring when the LED structure is overgrown on the nanopillar templates [9,10]. During this process, coalescence overgrowth of nanopillar template not only improves crystal quality, but also produces additional scattering of the emitted photons, leading to higher LEE [9,10]. However, there are still problems in applying this structure. The density of pores in the overgrown GaN layer is the key parameter when trying to improve the LED performance. So far, the overgrowth of GaN LEDs was done directly on the nanopillar templates, without trying to optimize the selective overgrowth process by suppressing vertical growth at the bottom of the nanopillar structure by masking these areas. As a result the density of pores turned out to be lower than expected and the area of the overgrown layer comprised by vertically grown material with a high dislocation density was large compared to the area of laterally overgrown material with a low dislocation density [11,12].

In what follows we demonstrate that, with the SiO₂ nanoparticles introduced in between the nanopillars, the GaN films could be overgrown in predominantly lateral growth fashion thus decreasing the dislocation density in the overgrown film and leaving behind a layer of high density of pores contributing to the light scattering. It is shown that such a structure is beneficial to the LED performances.

2. Experimental methods

Figure 1 shows a schematic illustration of the InGaN/GaN LED embedded with SiO₂ nanoparticles. 3-μm-thick undoped GaN (u-GaN) films were grown in the conventional way on c-plane sapphire substrates by metal organic chemical vapor deposition (MOCVD). For the fabrication of nanopillar GaN template, a 100-nm-thick SiO₂ layer was deposited as an interlayer on top of the u-GaN surface by plasma-enhanced chemical vapor deposition. After the SiO₂ deposition, a 10-nm-thick Ni layer was deposited on the SiO₂ interlayer by e-beam evaporation. The Ni/SiO₂/GaN samples were then subjected to rapid thermal annealing (RTA) under N₂ flow at 850 °C for 1 min to form the self-assembled Ni metal clusters. Inductively coupled plasma (ICP) etching was conducted to etch the exposed SiO₂ film using the Ni nanodots as etch mask under a gas mixture of O₂ and CF₄. Then, the u-GaN was further etched down to 1.5 µm by the ICP etching with Cl₂ gas [13]. Finally, the Ni nanodots/SiO₂ remaining on the top of nanopillars were removed by a buffered oxide etchant to obtain the vertically arranged nanopillar templates. The SiO₂ nanoparticles were embedded on nanopillar GaN template by spin coating (5000 rpm, 30 s). We used commercial colloidal SiO₂ nanoparticles with a diameter of 100 nm. The u-GaN template (reference) and the SiO₂-embedded nanopillar GaN template were then loaded into MOCVD chamber for the epitaxial lateral overgrowth of GaN. After well-coalesced GaN seed layers were formed on top of the GaN nanopillars, a Si-doped 3-μm-thick n-GaN layer was grown, followed by the multi-quantum well (MQW) region consisting of five undoped InGaN(2 nm)/GaN(10 nm) quantum wells and, finally, by a 200-nm-thick Mg-doped p-GaN top contact layer.

 

Fig. 1 A schematic illustration of the SiO₂-embedded LED structure.

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The cross section images of the SiO₂-embedded LED were obtained using a field emission scanning electron microscopy (FE-SEM). The crystalline quality of the films was assessed by high-resolution triple-axis rocking curves of X-ray diffraction (XRD) for the symmetrical (0002) and asymmetrical (10-12) reflection. Room temperature photoluminescence (PL) mapping measurements were carried out with a 325 nm line of a 25 mW He-Cd laser as an excitation source. The current-voltage (I-V) and optical output (L-I) characteristics were measured by means of an on-wafer probing with indium contacts. The results were compared to those obtained on the reference structure without SiO₂ nanoparticles (otherwise the structure and growth conditions were the same as for LEDs with the nanopillar template).

3. Results and discussion

The Ni nanodot arrays were formed on SiO₂/u-GaN after the RTA process. The SiO₂ layer between thin Ni layer and u-GaN surface helps to form highly dense and spherical Ni nanodots by offering dewetting conditions for Ni film during the annealing process. With the Ni nanodots/SiO₂ on u-GaN template, the ICP etching was done to fabricate the vertical and highly dense nanopillar GaN arrays with a diameter of 100~150 nm, a length of ~700 nm and a density of ~1 × 10⁹ cm⁻². The nanopillar size and shape was controllable by the Ni layer thickness, the RTA time and temperature, and the ICP etching conditions [13].

Figure 2 shows the SEM images of the nanopillar GaN structure after the coating of SiO₂ nanoparticles. Figure 2(a) and 2(b) are plane view images before the overgrowth, and Fig. 2(c) is a cross sectional image after the overgrowth of LED structure. The densely embedded SiO₂ nanoparticles are clearly seen between GaN nanopillars before and after the overgrowth. We believe that the embedded SiO₂ nanoparticles serve as a ELO mask facilitating selective lateral overgrowth [14], resulting in the formation of the air voids between nanopillars. In this step, it is important that the seed GaN of nanopillar top region should be clearly exposed with the remaining regions covered with SiO₂ nanoparticles. The density of embedded SiO₂ nanoparticles can be controlled by the spin coating conditions and the concentration in SiO₂ suspension in deionized water. As shown in the Fig. 2(d), the optical microscope image displays a smooth morphology of the SiO₂-embedded LED surface after the overgrowth.

 

Fig. 2 (a,b) Plane and tilted view FE-SEM images of the embedded SiO₂ nanoparticles between nanopillar GaN templates. (c) Cross-sectional FE-SEM images of the SiO₂-embedded LED structure regrown by MOCVD and (d) Normalski image of surface morphology after regrowth.

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The laterally overgrown GaN regions would consist of a continuous well coalesced layer with lower dislocation density than in the template because some of the threading dislocations are bent out of the growth direction. Indeed, XRD ω-scans (not shown here) exhibited much decreased full width at half maximum (FWHM) values for the SiO₂ embedded LED structure compared to the reference sample. The FWHM values of (0002) GaN were 362 and 302 arcsec and those of (10-12) GaN were 422 and 352 arcsec for the reference and the SiO₂ embedded LED, respectively. Note that the (0002) reflection is mostly sensitive to screw dislocations and the (10-12) reflection is sensitive to all dislocations [15]. In Fig. 2(c) at the interface with the nanopillar template, we see the presence of a large density of SiO₂ particles and voids or pores formed during the lateral overgrowth. These features should lead to additional light scattering in the structure and, as a consequence, to improved LEE. In fact, the SiO₂-embedded LED displayed a strong increase (about 300%) in PL intensity compared with the reference. We attribute this PL enhancement to the increased LEE due to a large density of the SiO₂ particles and the air voids between the GaN nanopillars.

We note that additional factors for this increased PL intensity could be a lower strain in the MQW region due to a partial strain relaxation by the nanopillar portion of the film and could be a higher indium concentration in the quantum wells due to the anisotropy of indium incorporation for the vertical and lateral growth (see e.g [16,17].). The former should decrease the magnitude of the piezoelectric field and related quantum confined Stark effect, which should result in the blue shift of the PL peak and in increased PL intensity because of the stronger spatial overlap of the electron and hole wavefunctions in the MQW (see e.g. Ref [18].). The latter can result in a higher indium concentration in the MQW and a red shift of the PL peak. The PL peak positions were 441 nm and 447 nm for the reference and the SiO₂ embedded LED, respectively. This indicates a higher indium concentration in the MQW. We also examined XRD patterns of both samples in (0002) ω-2θ mode which exhibited clear satellite peaks of InGaN/GaN MQWs. After careful curve fittings the average indium compositions in the MQWs were calculated to be 14.7% for the SiO₂-embedded LED and 13.5% for reference LED. This is coincident with the red shift of PL peak [19].

Electrical measurements also showed considerable performance enhancement. The L-I characteristic curves in Fig. 3 undoubtedly demonstrate the superiority in the light output for the SiO₂ embedded LED compared with the reference sample (~300% emission enhancement at 20 mA). Note that the PL and EL data in this study were obtained from on-wafer test scheme, and exact optical powers can only be measured by using integrating spheres. Both samples had a similar forward voltage of ~3.5 V at 20 mA (not shown). Again, the significant improvement in the light output is mostly attributable to the light scattering by the embedded SiO₂ nanoparticles together with the air voids. In addition, enhancements in crystalline quality of the regrown GaN may also have contributed to this improved emission efficiency.

 

Fig. 3 Schematic L-I characteristics of the reference LED and SiO₂-embedded LEDs, respectively.

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The electroluminescence (EL) peak position in the SiO₂ embedded LED was virtually independent of the driving current, which was not the case for the reference sample where the peak shifted from ~444 nm considerably to the shorter wavelength (see Fig. 4 and the inset of the figure comparing the peak wavelength dependences in the two cases in question). The strong blue shift of EL peak in GaN LEDs is commonly attributed to the strong impact of QCSE and related space separation of electron and hole wave functions in the QW. Increasing the injection level shield the polarization field and increases the electron and holes wave functions overlap, thus increasing the EL efficiency and causing the blue shift of the EL peak. A much weaker blue shift for the SiO₂ embedded LED is most likely to be a direct consequence of a lower strain in the MQWs and hence lower magnitude of polarization field and QCSE [18].

 

Fig. 4 EL peak wavelength as a function of injection current (5~350 mA) of the SiO₂-embedded LED. The inset image is the graph of peak shift as a function of injection current of the reference LED and the SiO₂-embedded LED, respectively.

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4. Conclusion

InGaN/GaN MQW LEDs with SiO₂ nanoparticles embedded in nanopillar GaN template was shown to create a layer with a high density of SiO₂ nanoparticles and air voids at the interface of the nanopillar template and the overgrown GaN film. The light scattering at the interfacial layer of the SiO₂ nanoparticles and voids improved the light extraction efficiency of the structure. The LEDs embedded with SiO₂ nanoparticles also showed an improved crystalline quality, a decreased strain and an increased indium concentration in the MQWs. These results were attributed to enhanced lateral overgrowth over the SiO₂ nanoparticles. These features resulted in decreased amount of non-radiative recombination due to dislocations in the MQW region and decreased magnitude of polarization field and related magnitude of the QCSE, which in turn led to an increase of IQE and a much weaker dependence of the EL peak wavelength on the driving current as compared to reference structure with a higher amount of strain and higher dislocation density. The combination of these effects led to a remarkable improvement of PL and EL efficiency of the structures.