Deterministic Fabrication of Uniform and Bright InGaN Quantum Dot Emitters toward Exciton Bohr Radius Regime by a Combination of Dry and Anisotropic, Controlled Wet Etching Techniques

Photoelectrochemical and boiling tetramethylammonium hydroxide (TMAH)-assisted etching techniques are demonstrated as controlled, anisotropic methods to reliably and deterministically fabricate uniform InGaN quantum dots (QDs) in GaN nanowires. Photoelectrochemical etching in a dilute H2SO4 solution is facilitated in two possible ways. First, it ensures no chemical reaction with GaN in the absence of light. Second, under photoexcitation, H2SO4 inhibits oxide accumulation on the semiconductor surface, which confirms uninterrupted QD etching. Perfect modulation of etch bias and duration, along with a gradual shrinkage in QD diameter, opens up the possibility of extremely controlled etching toward the exciton Bohr radius (∼3 nm) regime. This is being investigated for the first time to the best of my knowledge. Strong quantum confinement is observed through a significant blue shift in the photoluminescence peak with longer photoelectrochemical etching. But direct photoelectrochemical etching of large-diameter dry-etched nanowires to fabricate QDs is detrimental to QD stability. Large mechanical strain at a QD/GaN interface causes nanowires to break. Alternatively, TMAH-assisted etching causes radial decay of the entire nanowire while ensuring better uniformity of QD diameter and nanowire height across the array. It also corrects the nanowire sidewall to near-vertical. Advantages of both techniques are simultaneously availed for the first time by combining them to fabricate selectively etched, uniform and mechanically stable QDs in near-vertical nanowires. This process shows the pathway to fabricate QDs from optically written large nanodisks at low cost. This combined etching technique leads to the formation of nanowires with a high surface-to-volume ratio and enhanced emission power density from QDs. The size uniformity of these bright QDs will make them promising sources of indistinguishable single photons with a reasonable count rate for room-temperature quantum optics applications. These site-controlled QDs will also pave the way for next-generation on-chip photonic integration.