The influence of GaN column diameter DGaN on structural properties was systematically investigated for InGaN nanocolumns (NCs) grown on top of GaN NCs. We demonstrated a large critical layer thickness of above 400 nm for In0.3Ga0.7N/GaN NCs. The structural properties were changed at the boundary of DGaN=D0 (∼120 nm). Homogeneous InGaN NCs grew axially on the GaN NCs with DGaN≤D0, while InGaN-InGaN core-shell structures were spontaneously formed on the GaN NCs with DGaN>D0. These results can be explained by a growth system that minimizes the total strain energy of the NCs.

A new method of two-step selective area growth (SAG) by RF-plasmaassisted molecular beam epitaxy is developed, enabling the growth of uniform arrays of thin GaN nanocolumns (NCs) with diameters <50 nm. In the SAG, the migration-enhanced epitaxy mode with an alternating supply of Ga and active nitrogen was employed during the initial growth of NCs on small-nanohole-patterned substrates to complete the crystal nucleation in the nanoholes. Once the nucleation occurred, the growth mode to the simultaneous supply of Ga and nitrogen is immediately switched. In the second step, the growth temperature is increased and the nitrogen flow rate to suppress the lateral growth rate is decreased. A high-density uniform array of very thin NCs in a triangular lattice with a diameter of 26 nm and a lattice constant of 60 nm is demonstrated; the NC density is 3.2 × 10 10 cm −2 .Introduction: GaN nanocolumns (NCs), which are independent onedimensional nanocrystals, were first fabricated on (0001) sapphire substrates [1, 2] and then on (111) Si substrates [3] through self-assembly by RF-plasma-assisted molecular beam epitaxy (RF-MBE). The selfassembled GaN NCs have been utilised in the fabrication of InGaN-based light-emitting diodes (LEDs) on Si [4][5][6][7]. However, randomness of the size and position of the NCs was inevitably introduced by the self-assembly of NCs, which is initiated by random and spontaneous nucleation, frequently resulting in the multicolour emission of LEDs in microscale areas [5]. At the same time, precise control of the NC size and position was achieved by the development of selective area growth (SAG) [8][9][10][11]. However, the uniform arrays of GaN NCs fabricated by SAG had NC diameters (D) larger than ∼100 nm [8][9][10][11][12]. It was therefore considered a challenge to grow well-ordered thin GaN NCs with D < 100 nm, even though the diameter of self-assembled NCs typically varies from 50 to 100 nm [1,5]. In axial InGaN/GaN heterojunction NCs, the in-plane spatial separation of electrons and holes occurs, reducing the internal quantum efficiency (IQE) [13]. According to a theoretical prediction, however, reducing the NC diameter to 40 nm significantly increases the electron-hole ground state overlap, thus providing a promising approach for achieving a higher IQE [14]. In this Letter, we report having developed a two-step SAG method for improving the growth control of thin NCs and having demonstrated a high-density array of very thin NCs in a triangular lattice with D = 26 nm and a NC density of 3.2 × 10 10 cm −2 . The lattice constant (L) was 60 nm.

The effect of the structural properties on the optical characteristics was investigated for In0.3Ga0.7N nanocolumns (NCs) grown on GaN NCs as a function of GaN column diameter, DGaN. With increasing DGaN, the photoluminescence spectra changed from single-peak to double-peak emissions at the diameter D0 where InGaN axial NCs change to InGaN–InGaN core–shell NCs. For the core–shell NCs, the volume recombination probabilities of the InGaN cores did not change with DGaN. Whereas the surface recombination probability of the InGaN cores exponentially decreased because of the spontaneous formation of InGaN shells for DGaN > D0, it drastically increased for DGaN ≤ D0.

We experimentally demonstrated the strain relaxation effect in uniform GaN/Al0.19Ga0.81N quantum wells on GaN nanocolumn (NC) arrays with various column diameters and periods created using the Ti-mask selective area growth technique. The photoluminescence (PL) emission from the GaN well layer was not affected by the period of the NC arrays. As the column diameter decreased, the PL peak energy of the GaN well layer blueshifted, whereas that of the GaN NC underlayer remained almost unchanged. This blueshift was reproduced with the calculated strain relaxation effect, indicating that the strain in the GaN well layer decreased as the column diameter decreased.

Emission mechanisms in regularly arrayed InGaN/GaN quantum structures on GaN nanocolumns were investigated, focusing on the spatial emission distribution at the nanocolumn tops and the carrier recombination dynamics. The double-peak emission originated from the dot- and well-like InGaN areas with different In compositions was observed. From the results regarding the spatial emission distribution, we proposed a simple analytical approach to evaluating the carrier recombination dynamics using the rate equations based on the two energy states. The considerable six lifetimes can be uniquely determined from the experimental results. Carrier transfer from the high- to the low-energy state is dominant at high temperatures, producing the increased total emission efficiency of the inner low-energy area. In addition, the internal quantum efficiency should not be simply discussed using only the integrated intensity ratio between low and room temperatures because of the carrier transfer from high- to low-energy states.