ABSTRACT
Abstract. We present n-type AlN/GaN:Si distributed Bragg reflectors grown on 6H-SiC(0001) by plasma-assisted molecular beam epitaxy. The structures are free of cracks and exhibit a stopband centered around 450 nm with a FWHM of 41 nm. The measured reflectance is >99%. A comparison between Si-doped and undoped structures reveals no degradation of the reflectance due to the Si-doping. Room temperature vertical conductance measurements show the structures having ohmic I-V behavior. The resistivity at 77 K is only 2 times larger than the room temperature resistivity. 1
1. Introduction Vertical conductivity is vital for devices such as vertical cavity surface emitting lasers (VCSELs) and resonant cavity light emitting diodes (RCLEDs) because it is critical that the maximum current density coincides with the maximum of the optical mode within the active region. For conventional, laterally contacted nitride based devices, this can be achieved by proton implantation, which defines a current aperture by creating insulating regions. Since distributed Bragg reflectors (DBRs) are essential components of VCSELs and RCLEDs, it would be a great advantage if the DBRs themselves were vertically conducting, as this would reduce the number of ex-situ processing steps. For instance, proton implantation would no be longer necessary since the current aperture would be inherently defined through the vertical injection geometry. The relatively large difference of the refractive indices (An = 0.36) between AIN and GaN makes these materials very well suited for nitride-based DBRs for the blue and green spectral range. Because AIN is commonly believed to be an insulator by nature, there have not been any reports concerning the doping of GaN/AlN-DBRs. Further, the large conduction band offset between GaN and AIN (2.2 eV) seems to inhibit carrier transport across the heterointerfaces. Tensile strain building up in these structures due to the large lattice mismatch between GaN and AIN (2.4%) often leads to the formation of cracks which reduces the reflectivity and makes electronic device processing extremely difficult.
