Selective area growth of a- and c-plane GaN nanocolumns by molecular beam epitaxy using colloidal nanolithography

Abstract

Selective area growth of a-plane GaN nanocolumns by molecular beam epitaxy was performed for the first time on a-plane GaN templates. Ti masks with 150 nm diameter nanoholes were fabricated by colloidal lithography, an easy, fast and cheap process capable to handle large areas. Even though colloidal lithography does not provide a perfect geometrical arrangement like e-beam lithography, it produces a very homogeneous mask in terms of nanohole diameter and density, and is used here for the first time for the selective area growth of GaN. Selective area growth of a-plane GaN nanocolumns is compared, in terms of anisotropic lateral and vertical growth rates, with GaN nanocolumns grown selectively on the c-plane.The main advantages of one-dimensional structures such as nanocolumns, compared to thin films, is the dislocation-and strain-free growth on different substrates. This higher crystal quality yields a better efficiency in devices such as light emitting diodes (LEDs) based on InGaN/GaN quantum disks (QDisks) [1][2][3][4].Much effort has been dedicated to the growth of self-assembled nanocolumns (NCs) by Plasma-Assisted Molecular Beam Epitaxy (PAMBE), allowing a better knowledge of the material physical properties and growth mechanisms [5][6][7][8]. However, the strong morphology dispersion, typical of a self-assembled process, hinders both the processing of nanodevices arrays and their electrical behavior (NCs merging, defect generation, current injection inhomogeneities). Indeed, arrays of self-assembled nanoLEDs show in general much lower electroluminescence efficiency than the corresponding one measured by photoluminescence (PL). In addition to that InGaN active regions embedded in self-assembled NCs of different diameters and lengths always show fluctuations in the composition, leading to multicolor emission. Arrays of nanostructures grown by selective area growth (SAG) provide a much better homogeneity in terms of morphology, electrical and optical characteristics. SAG of Ill-nitride NCs [9][10][11][12] and other nanostructures [13,14], is generally performed on very thin metal (Ti, Mo) or dielectric (Si0 2 , SiN) masks with an array of nanoholes.A relevant issue concerning optoelectronic devices based on Ill-nitrides is the presence of strong polarization fields that may reduce efficiency. This is the case in layers grown along the c-axis and, a huge effort is nowadays dedicated to the growth of high quality non-polar and semi-polar material [15], with a particular emphasis on non-polar LEDs [16].In the case of SAG of GaN NCs on c-plane GaN templates, the growth front is generally formed by semi-polar (r-planes) that yield a "pencil-like" profile [12]. This profile is then transferred to InGaN QDisks embedded within the GaN NC. Though in this structure the effects of internal fields in the active region of the device (i.e. nanoLED) may be reduced compared with polar planes, the most effective solution is to grow along non-polar directions, either by growing core-shell heterostructures on the latera...

ORDEREDGaN/InGaNNANORODS ARRAYS GROWN BY MOLECULAR BEAM EPITAXY FOR PHOSPHOR-FREE WHITE LIGHT EMISSION

“…As already mentioned, colloidal lithography is an easy, fast and cheap technique compared with others. 36 ' 37 Though not having a perfect periodicity, these masks are easily fabricated over wide areas (2 inch or more) in a short period of time, as compared to EBL, and they allow the study of the SAG process and optimization that will be then transferred to a more standard patterning technique like Nanoimprint Lithography (NIL).…”

Ordered Gan/Ingan Nanorods Arrays Grown by Molecular Beam Epitaxy for Phosphor-Free White Light Emission

“…37 The same conditions and critical steps are applicable for the SAG of GaN NRs either on c-planes or a-planes. Perfect selectivity on colloidal lithography masks is achieved on a-plane GaN/sapphire templates, as shown in Figure 12.…”

Electron tomography of (In,Ga)N insertions in GaN nanocolumns grown on semi-polar (112̄2) GaN templates

“…The growth started with a GaN deposition for 3 h followed by the deposition of (In,Ga)N for 160 s. A final step of GaN growth for 5 min has been applied to cap the (In,Ga)N. The SAG process proceeded on a Ti nanomask deposited onto the substrate. 4 The preferential growth direction is found to be perpendicular to the c-plane, what forces an inclined growth of the NCs in respect to the substrate (see Fig. 1).…”

“…The growth of (In,Ga)N on non-or semi-polar facets is pursued to circumvent the consequences of strong internal electric fields along the polar ⟨0001⟩ direction of the GaN wurtzite structure. 3 The major approaches suggest using either a corresponding substrate orientation 4,5 or radial NC heterostructures with preferential incorporation of indium on the side-facet of the NCs grown on polar substrates. 6,7 The microstructure and local chemical composition of those perfectly aligned NCs can be analyzed by high-resolution and analytical transmission electron microscopy (TEM).…”