MOCVD growth of high reflective GaN/AlGaN distributed Bragg reflectors
Introduction
GaN, AlN, InN and their alloys have been highly popular as optical devices in the blue-ultraviolet region and have created much interest for high-temperature, high-power electric devices at microwave frequencies. GaN-based high brightness light emitting diodes (LEDs) and edge emitting laser diodes (LDs) have demonstrated continuous-wave operation at room temperature [1]. Recently, GaN-based vertical cavity surface emitting lasers (VCSELs) with a distributed Bragg reflector (DBR) have been studied in many research groups because of their many advantages for various applications such as full color display, photolithography, super high-density optical memory and bright white light source [2]. While the fabrication of smooth edge mirror is difficult for edge emitting lasers, VCSELs can get the smooth mirror without a cleavage technique. However, it is necessary to fabricate high reflective mirrors on both sides of the active region to form their short cavity length. In this material system, GaN/AlGaN multilayers are usually used as high reflective mirrors on the bottom side [3].
To realize a high reflective DBR, the contrast of the refractive index between two materials of the multilayers and the flat surface of each layer are important [4]. Because the difference of refractive index between GaAs and AlAs, so many periods of GaN/AlGaN layered structures are required to obtain a high reflectivity. When making such numbers of periods, many cracks are grown and the surface flatness is spoiled. More recently, optical pumping VCSEL, which performed the cavity by GaN/AlGaN DBR for the bottom side mirror and ZrO2/SiO2 dielectric mirror for the top, was reported at room temperature [5], and many authors reported the GaN/AlGaN based reflectors [4], [6], [7], [8], [9], [10], [11]. We have obtained improved characteristics of InGaN MQW LED on sapphire by use of the 15 pairs of GaN/AlGaN DBR [12].
In this letter, we reported the high reflective GaN/Al0.60Ga0.40N DBRs with crack-free surface fabricated under the low-pressure condition.
Section snippets
Growth of GaN/AlGaN DBRs
A metal organic chemical vapor deposition (MOCVD) equipment with horizontal quartz reactor (Nippon Sanso, SR-2000) was employed for the growth of GaN/AlGaN DBRs. (0 0 0 1)-oriented, 2-in diameter sapphire substrates were used for the growth of samples. Trimethylgallium (TMGa) and trimethylaluminum (TMAl) were used as group III and ammonia (NH3) as group V source materials, respectively. The surface morphology of the DBRs was studied with an optical microscope and the atomic force microscopy (AFM)
Results and discussion
We prepared four samples and changed the thickness of GaN layer grown under the atmospheric pressure condition (AP-GaN) to 50, 500, 1500 and 6000 nm, respectively. Fig. 1 shows the maximum reflectivity of multilayers as a function of thickness of the AP-GaN layer. The measurement was carried out on the nine areas with a diameter of 7 mm for each sample, taken across the 2-in substrate to study the distribution. In the case of 50 nm, the reflectivity was low and had a large distribution. It was
Conclusions
We fabricated the low-pressure grown GaN/Al0.60Ga0.40N DBR on AP-GaN by MOCVD. Optimizing the thickness of AP-GaN, the surface morphology has been improved. We also fabricated 45.5 pairs of GaN/Al0.60Ga0.40N DBR with a reflectivity of over 98% at 421 nm. This DBR had a flat surface free of cracks, even though the total thickness of DBR was about 4.5 μm. We believe that this DBR should play an important role in the realization of GaN-based VCSELs.
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