In this work we show successful metalorganic vapor phase epitaxy (MOVPE) of an AlN/AlGaN distributed Bragg reflector (DBR) that is wavelength matched to GaN quantum dots (QDs) in an AlGaN lambda cavity on top. Full insight into the growth of these structures enables the epitaxy of resonant cavity deep UV single photon emitters.
The DBR was grown on an AlN/sapphire template. In order to obtain a high reflectivity as well as a sufficiently large stopband width, the refractive index contrast needs to be maximized. Additionally, the absorption of QD emission in the high gallium containing layer needs to be minimized. A compromise was found for nominal Al-concentration of 70 % in the AlGaN layers. The resulting DBR splits up into self-organized AlN/Al(X)Ga(1-X)N/Al(Y)Ga(1-Y)N trilayers, which add up to desired lambda/2-periods. Therefore, the stopband at 272 nm with a width of 6 nm shows a maximum reflectivity of 99.7 %.
GaN QDs were obtained by growth of GaN on AlGaN for 10 s with a V/III-ratio of 30 followed by a growth interruption of 30 s. The QDs exhibit sharp emission lines with a FWHM down to 1 meV in µ-PL measurements. The main intensity of the QD ensemble emission is in the range of 250 nm to 275 nm.
Finally, spatially resolved low temperature CL measurements show resonant DBR-enhanced GaN QD emission at 271 nm showing successful wavelength match between a AlN/AlGaN deep UV DBR and GaN QDs in an AlGaN lambda-cavity on top.
We systematically studied the desorption induced GaN/AlN quantum dot formation using cathodoluminescence spectroscopy directly performed in a scanning transmission electron microscope (STEM). The GaN films were grown by metal organic vapor phase epitaxy (MOVPE) on top of an AlN/sapphire-template. After the deposition of a few monolayers GaN at 960°C a growth interruption (GRI) without ammonia supply was applied to allow for quantum dot formation. A sample series with GRI durations from 0 s to 60 s was prepared to analyze the temporal evolution systematically. Each quantum dot (QD) structure was capped with AlN grown at 1195°C.
Without GRI the cross-sectional STEM images of the reference sample reveal a continuous GaN layer with additional hexagonally-shaped truncated pyramids of 20 nm height and ~100 nm lateral diameter covering dislocation bundles. Spatially averaged spectra exhibit a broad emission band between 260 nm and 310 nm corresponding to the continuous GaN layer. The truncated pyramids exhibit only drastically reduced CL intensity in panchromatic images.
Growth interruption leads to desorption of GaN resulting in smaller islands without definite form located in close vicinity to threading dislocations. Now the emission band of the continuous GaN layer is shifted to shorter wavelengths indicating a reduction of GaN layer thickness. By applying 30 s GRI these islands exhibit quantum dot emission in the spectral range from 220 nm to 310 nm with ultra narrow line widths. For longer growth interruptions the QD ensemble luminescence is shifted to lower wavelengths accompanied by intensity reduction indicating a reduced QD density.
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