Electrical Conductivity of n-doped GaN-based Distributed Bragg Reflectors

Abstract

Vertical-cavity surface-emitting lasers (VCSELs) could find new applications in areas ranging from displays to bio-medicine if their wavelengths were extended into the blue spectrum. However, the realization of GaN-based VCSELs have proven to be challenging. One of the major hurdles have been to achieve epitaxially grown distributed Bragg reflectors (DBRs) with high reflectivity and vertical electrical conductivity. A vertically conductive DBR would allow the n-contact to be moved outside the cavity, reducing the cavity length and leading to significantly improved laser performance for blue-emitting GaN-based VCSELs. To electrically characterize a DBR, low-temperature annealed ohmic contacts to n-doped GaN are essential, since the DBRs often have built-in-strain and thus could crack when exposed to temperature rampings. Ohmic contact formation of Al, Ta/Al/Ta, and Ti/Al/Ti/Au contacts on n-GaN with different concentrations doping was studied for annealing temperatures up to 700°C by transmission line measurements. On highly doped n-GaN (n = 2.5 × 10^19 cm−3) the Al contacts were ohmic as-deposited and had a minimum specific contact resistivity of 3 × 10−7 cm^2. Al contacts had the lowest contact resistivity reported for ohmic contacts on n-GaN annealed under 500°C. Ta/Al/Ta contacts had lowest resistivity for annealing over 500°C and were thermally stable up to at least 700°C. No ohmic contact to low-doped n-GaN (n < 3 × 10^17 cm−3) was found. The effect of interlayers on the vertical electrical conductivity of AlN/GaN DBRs was investigated by IV-measurements on four different 10.5 DBR pair samples (Sample A-D). Sample A had no interlayers, samples B and C had one pair of AlN/GaN interlayers with thicknesses of 2 nm/2 nm and 0.5 nm/0.5 nm at each interface, respectively, and sample D had graded interlayers from GaN to AlN over 1.5 nm thickness. The electrical conductivity of 8 and 9 DBR pairs was investigated by etching mesas to different etch depths and depositing Al-contacts on top and on the etched surface next to the mesa. The series resistance scaled with mesa area and the lateral and contact resistance contribution was less than 10% of the total resistance. Sample A had a mean specific series resistance of 0.043 cm^2 for 8 pairs, which is comparable to state-of-the-art for n-doped AlN/GaN DBRs. The mean specific series resistances for Sample B and D were 4 times higher than that of sample A, and 10 times higher for Sample C, indicating that interlayers used for strain compensation can increase the electrical resistivity of n-doped AlN/GaN DBRs. An MBE-grown n-doped ZnO/GaN DBR was characterized in a similar manner and showed a specific series resistance of less than 10−3 cm2 for 3 pairs. The lateral and contact resistance were dominating over the vertical resistance, which made an accurate determination of the DBR conductivity difficult. Ga-droplets, which covered the DBR surface, were ruled out as the reason for the low vertical resistance by investigating small droplet-free mesas. This is the first report on electrically conducting ZnO/GaN DBRs.