Density control of GaN nanowires at the wafer scale using self-assembled SiN$_x$ patches on sputtered TiN(111)

TL;DR

This study demonstrates a method to precisely control the density of GaN nanowires across wafer scales by using self-assembled SiN$_x$ patches on sputtered TiN(111), enabling tunable nanowire ensembles with uniformity.

Contribution

It introduces a novel density tuning technique for GaN nanowires via SiN$_x$ patch deposition on TiN substrates, bridging the gap between existing growth regimes.

Findings

Nanowire density can be tuned over three orders of magnitude.

GaN nanowires nucleate on SiN$_x$ patches, not the TiN surface.

Photoluminescence shows excitonic transitions with blue shift and broadening.

Abstract

The self-assembly of heteroepitaxial GaN nanowires using either molecular beam epitaxy (MBE) or metal-organic vapor phase epitaxy (MOVPE) mostly results in wafer-scale ensembles with ultrahigh ($>10$ $\mu$m$^{-2}$) or ultralow ($<1$ $\mu$m$^{-2}$) densities, respectively. A simple means to tune the density of well-developed nanowire ensembles between these two extremes is generally lacking. Here, we examine the self-assembly of SiN$_x$ patches on TiN(111) substrates which are eventually acting as seeds for the growth of GaN nanowires. We first found that if prepared by reactive sputtering, the TiN surface is characterized by \{100\} facets for which the GaN incubation time is extremely long. Fast GaN nucleation is only obtained after deposition of a sub-monolayer of SiN$_x$ atoms prior to the GaN growth. By varying the amount of pre-deposited SiN$_x$, the GaN nanowire density could be tuned by three orders of magnitude with excellent uniformity over the entire wafer, bridging the density regimes conventionally attainable by direct self-assembly with MBE or MOVPE. The analysis of the nanowire morphology agrees with a nucleation of the GaN nanowires on nanometric SiN$_x$ patches. The photoluminescence analysis of single freestanding GaN nanowires reveals a band edge luminescence dominated by excitonic transitions that are broad and blue shifted compared to bulk GaN, an effect that is related to the small nanowire diameter and to the presence of a thick native oxide. The approach developed here can be principally used for tuning the density of most III-V semiconductors nucleus grown on inert surfaces like 2D materials.