(Color online) Intensity-vs-time evolution of the 3D features appearing on the RHEED pattern during the QD formation of sample T170. Insets show RHEED patterns at different formation times: (a) t = 25 s, (b) t = 70 s, (c) t = 90 s, (d) t = 110 s, (e) t = 142 s. A higher quality close-up of the 3D RHEED feature is inserted in inset (b) in order to highlight the presence of chevrons.
(Color online) AFM images of surface QDs: samples (a) T25, (b) T70, (c) T90, (d) T110. The vertical range Δz is indicated on each image.
Statistical analysis of surface QD heights (deducting the WL thickness) Δhsurf and QD diameters derived from AFM images represented in Fig. 2: samples (a) T25, (b) T70, and (c) T90. Black lines represent Gaussian fit to the height and diameter dispersion. The QD densities as well as the mean value and the standard deviation of the Gaussian fits are also reported.
(Color online) (a) Low-temperature PL spectra of GaN/AlGaN QDs for various formation times. The sample is excited at 244 nm (5.08 eV) with a 2.5 × 102 W cm−2 excitation density. For clarity, PL spectra are shifted by one decade along the y axis. (b) Influence of the density of excitation on the QD emission energy. Black circles, red square, green stars, and blue triangles correspond, respectively, to samples T25, T70, T90, and T110.
Emission energy as a function of the GaN/Al0.55Ga0.45N heterostructure height. Black spheres associate TEM and PL data collected from the WL of samples T25 and T110 and from the QDs of sample T25. Black solid lines are calculated PL energies of GaN/Al0.55Ga0.45N quantum wells for various electric fields F. Vertical and horizontal error bars correspond respectively to the TEM measurement dispersion and the PL full width at half maximum. Inset shows a TEM image of a buried QD. Dashed lines emphases QD and WL contours.
(Color online) (a) Variation of the GaN volume per unit area as a function of t: red stars and black circles correspond respectively to the volume of QDs emerging from the WL as measured by AFM and to the total GaN volume remaining on the sample surface Vtot . Vertical bars correspond to the error on Vtot due to the error on the built-in electric field. The black solid line is a linear fit to the data. The black arrow indicates the time of disappearance of the 3D RHEED features. The red dotted line is a guide for the eyes. (b) Variation of the QD mean height and the WL thickness before and after capping as a function of formation time. Black triangles and blue spheres correspond respectively to buried and surface QDs. Black squares and blue stars correspond respectively to buried and surface WLs. Vertical bars correspond to the error on the built-in electric field. Lines are guides for the eyes.
Values of the mean height of buried QDs and of the mean thickness of buried WLs as a function of the formation time t. Values are derived from the low-temperature and low-excitation PL emission energy assuming a F = 2.5 MV/cm built-in electric field.
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