Faceting of local droplet-etched nanoholes in AlGaAs

Nanoholes, drilled in the (001) surface of AlGaAs by local Al droplet etching, are shown to consist of faceted inner walls. The most prominent facets of the inverted pyramidlike nano-sized etch pits are the {111}A and {$1\overline{1}1$}B surfaces, which differ in their atomic surface terminations. In the [110] direction, the {111} facets change to {112} and/or {113}, which are both stepped surfaces with (111)A terraces. Etching-temperature-dependent data indicate that this facet transition seems kinetically hindered up to etch temperatures above 660 °C, at which point the walls along $\left[1\overline{1}0\right]$ have already evolved completely towards {$1\overline{1}1$}B facets. The redeposited ring material outside the nanohole develops facets with indices of (115) and higher, thereby forming relatively flat structures. The facets and their indices are unraveled by a combination of atomic force microscopy, scanning electron microscopy, and x-ray diffraction, which is performed on an ensemble as well as on a single hole using nanodiffraction. These results imply that this nanoconfined etching process can be largely understood in a vapor-liquid-solid scheme, which includes the bulk thermodynamics in the Al-Ga-As system, the surface energies of low index facets, and their etch rates and surface terminations.

Figure 1

Overview of the sample etched with Al at a temperature of $T=625{}^{\circ }\mathrm{C}$. (a) Cross-sectional schematic of the multilayer structure with Al droplet etched nanoholes in the topmost layer. The Al content in the AlGaAs layers is 0.35. (b) AFM image of the sample surface with nanoholes of low density ($1\times {10}^7{\mathrm{cm}}^2$). The inset shows a magnified hole illustrating the anisotropic shape of the wall around the nanohole. (c) AFM line scans through the hole center along [110] (blue) and $\left[1\overline{1}0\right]$ (red) directions with equal $x$- and $z$-axis scales. The vertical dotted lines indicate the extension of the nanohole. (d) Derivatives of the AFM line scans shown in (c). The vertical dotted lines indicate the extension of the nanohole and the horizontal dotted lines indicate the angular values of {111} facets (55°) and {112} (35°). The arrows indicate the positions where the slope abruptly changes along the [110] direction. This plot further shows that inside the hole, where material was etched, the steepest walls occur. Most of the surface of deposited material outside the nanohole has much flatter orientations.

Figure 2

Microscopy results. (a) AFM of a single nanohole as in Fig. 1. The solid contour lines highlight the presence of inner facets. The dashed lines indicate the edges between the different facets. Due to the fact that the hole is not exactly square, these edges are also not at exactly 90° to each other. This is explained by the inner facet wall changing from (111) to (112) along the [110] direction. (b) High-resolution SEM of a single nanohole and ring. The inset shows contour lines, which highlights the nearly square shape and the presence of inner facets.

Figure 3

Ensemble reciprocal space maps around the 111 GaAs Bragg peak. (a) Map in the $(H+K=2,L)$ plane, which is perpedicular to the $(H,H,L)$ plane. The inset shows schematically the planes mapped out in reciprocal space around the Bragg peak (black dot). The weak, nonuniform butterfly-like intensity distribution around the central Bragg peak arises due to scattering from facets. The directions (white lines) and indices of the steepest observed facets, as discussed in the text, are indicated. (b) Map in the $(H,H,L)$ plane. The directions (white lines) and indices of the steepest facets are indicated. (c) Atomic-scale surface model showing Ga (blue) and As (green) of the ($1\overline{1}1$)B surface. (d) The (112) surface is stepped with close-packed, Ga-terminated (111)A terraces and As-terminated (001) steps. (e) The (111)A Ga-terminated surface is shown in the top view.

Figure 4

Single hole reciprocal space maps around the 113 GaAs Bragg peak using a 300-nm x-ray beam. Shown are the $(H,H,L)$ [(a)–(c)] and $(H+K=2,L)$ planes [(d)–(f)]. Data were taken with the x-ray beam aligned on the nanohole [(a) and (d)] and in between the holes [(b) and (e)]. The difference maps [(c) and (f)] highlight the weak nonuniform intensity distribution around the central Bragg peak which arises due to scattering from facets, the directions (black lines) and indices of which are also indicated. The other lines [vertical in (a)–(c) and diagonal in (d)–(f)] originate from the x-ray insensitive junctions between neighboring chips on the detector.

Figure 5

AFM results of the average nanohole shape at varied process temperature $T$. Shown are the average values of different shape parameters, which are obtained from multiple measurements on different holes. The error bars represent the scatter in the obtained values. (a) Width of the nanohole opening averaged over the [110] and $\left[1\overline{1}0\right]$ directions. (b) Nanohole depth $d$ giving the distance between the hole bottom and the planar surface. The inset shows a cross-sectional sketch of a hole with the characteristic sizes. (c) Average hole side-facet angle $\alpha$ calculated from the the hole width and depth. The values for various $\left(11n\right)$ facets are also indicated (dotted lines). (d) AFM profiles along [110] and $\left[1\overline{1}0\right]$ directions from a nanohole fabricated at $T=600{}^{\circ }\mathrm{C}$, (e) a nanohole fabricated at $T=630{}^{\circ }\mathrm{C}$, and (f) a nanohole fabricated at $T=665{}^{\circ }\mathrm{C}$. The (111)-, (112)-, and (113)-type facets are indicated for comparison (dotted lines).