GaN/Al0.5Ga0.5N (11-22) semipolar nanostructures: A way to get high luminescence efficiency in the near ultraviolet range

Abstract

The epitaxial growth of GaN/Al 0.5 Ga 0.5 N (11-22) semipolar nanostructures and their structural and optical properties are reported. The nanostructure formation results from a strain induced growth process (Stransky-Krastanov-like growth mode). Atomic force microscopy measurements show that depending on the amount of deposited GaN, the nanostructure shape evolves from an island shape to a string shape aligned along the [1-100] direction. Transmission electron microscopy experiments reveal that (11-20) and (11-23) lateral facets are formed, making with the (11-22) growth plane an angle of 32 and 12 , respectively, and giving a very asymmetric nanostructure shape. Photoluminescence (PL) experiments as a function of the excitation power and temperature show that the internal electric field is very low compared to the case of GaN/Al 0.5 Ga 0.5 N (0001) polar quantum dots (QDs). As a consequence, the PL emission is strongly shifted towards the UV range compared to polar QDs and the full width at half maximum of the PL peak is drastically reduced. Furthermore, a room temperature PL nanostructure emission 300 times more intense than that of the semi-polar GaN template is obtained despite a large density of defects in the surrounding matrix, as expected for efficient carrier localization inside the nanostructures.

Growth of GaN nanostructures with polar and semipolar orientations for the fabrication of UV LEDs

“…In fact, for a GaN deposited amount of 4 MLs (sp-GaN-4), isolated GaN QDs are observed, whereas for larger amounts of GaN (sp-GaN-6 to sp-GaN-16), the nanostructure morphology evolves towards elongated nanostructures aligned along <1 1 00>. 15 A similar alignment along [1-100] has already been reported in the case of GaN QDs on AlN (11 2 0). 22 In the case of a GaN deposited amount of 16 MLs (sp-GaN-16), the QDs are coalesced along <1 1 00> forming QWRs.…”

“…22 In the case of a GaN deposited amount of 16 MLs (sp-GaN-16), the QDs are coalesced along <1 1 00> forming QWRs. 15,16 From both series of samples, the nanostructure density and surface coverage have been found to increase when the GaN deposited amount is increased (from densities in the 10 10 cm -2 range up to a few 10 11 cm -2 ). To investigate the morphology of GaN nanostructures buried in Al 0.5 Ga 0.5 N, TEM measurements have also been performed.…”

“…4(b)), the nanostructures have an asymmetrical shape with the facet on the right side of the image corresponding to a (11 2 0) plane, the top facet to (11 2 2), and the facet on the left side to a (11 2 3) plane. 15 In both cases, the typical nanostructure width is found between 15 and 25 nm. As the GaN deposited amount is increased, the nanostructure height varies between 3 to 5 nm and 1 to 4 nm in the polar 21 and semipolar case, 16 respectively.…”

“…20 However, on a scale in the 100-nm range, no specific surface corrugation can be distinguished. 15 At this scale, the morphology consists of small islands ≤ 1 nm high and ~ 20 nm wide. The root mean square (rms) roughness measured from (2 x 2) µm 2 AFM images is 0.5 nm for Al 0.5 Ga 0.5 N (0001) and increases to 2.2 nm for Al 0.5 Ga 0.5 N (11 2 2).…”

“…QDs, QDashes, quantum wires (QWRs) and QWs) can be grown on Al 0.5 Ga 0.5 N surfaces by modifying different growth parameters: the GaN deposited thickness amount, the NH 3 pressure or the surface orientation. 13,14,15 Furthermore, we have demonstrated that the nanostructure design leads to particular optical properties (emitted wavelength range, temperature dependence, system dimensionality etc.). 14,16 In this paper, the formation, morphological and optical properties of (0001) "polar" and (11 2 2) "semipolar" GaN nanostructures will be presented and compared, using different experimental techniques.…”