Self-assembled single-crystal bimodal porous GaN exhibiting a petal effect: application as a sensing platform and substrate for optical devices

This paper investigates the petal effect (hydrophobicity and strong adhesion) observed on single-crystal bimodal porous GaN (porous GaN), which has almost the same electrical properties as bulk GaN. The water contact angles of porous GaN were 100°–135° despite the intrinsic hydrophilic nature of GaN. Moreover, it was demonstrated that the petal effect of porous GaN leads to the uniform attachment of water solutions, enabling highly uniform and aggregation-free attachment of chemicals and quantum dots. These results indicate that porous GaN can be applied in quantum dot light-emitting diodes and as an analytical substrate.


Growth of porous GaN by halogen-free vapor-phase epitaxy
Bimodal porous single-crystal GaN (porous GaN) were fabricated by halogen-free vapor phase epitaxy (HF-VPE) 1-6 using graphite crucibles with a pyrolytic boron nitride coating. A 2 μm-thick GaN template grown via metalorganic chemical vapor deposition on a sapphire substrate was used as the seed crystal (MO template). The temperature of the growth susceptor was 1353 K, and the crucible was heated to approximately 1410-1480 K by radio frequency irradiation. The growth pressure was 4 kPa and growth duration was 9 min.

SEM analysis
Field-emission scanning electron microscopy (FE-SEM) images were acquired using a Hitachi S-4800 instrument. The accelerating voltage was approximately 3 kV and the emission current was approximately 10 A.

X-ray rocking curves
S-4 Figure S1 (a, b) shows the normalized rocking curves for the (0002) and (11 2 2) reflections of porous GaN, respectively. The full width at half maximum (FWHM) values in the -scan XRC data for the porous GaN layer are 292 and 699 arcsec for the (0002) and (112 2) reflections, respectively. These FWHM values are slightly smaller than those obtained for the MO-template seed layer (approximately 320 and 760 arcsec, respectively). These results suggested that the crystallinity of porous GaN was slightly higher than that of the MO template (i.e., the seed crystal), and crystal quality degradation such as upon dislocation increase did not occur during porous GaN growth. The -scan data for the GaN (112 2) planes in porous GaN from −180 °to 180° are shown in Figure   S1

Secondary ion mass spectrometry
Secondary ion mass spectrometry (SIMS) analyses were performed to identify the concentrations of anti-surfactant impurity boron for preparing bimodal porous structures, the donor dopant impurity silicon and oxygen, and the compensated impurity of carbon in porous GaN ( Figure S2). The resulting data show that porous GaN had a boron concentration of 2 × 10 19 atoms/cm 3 , which is almost the same as the previously reported value. 7 Considerably high (10 20 -10 21 atoms/cm 3 ) carbon and oxygen concentrations were confirmed, and the depth profiles of carbon and oxygen were quite similar. Porous GaN had a significantly high specific surface area and was exposed to the atmosphere;

S-8
The pore coverage ratio p of porous GaN was obtained from the SEM images of each porous GaN surface, which were converted to binary via image processing. Figure S4. S-10

van der Pauw-Hall measurements and electrical properties of porous GaN
To verify the electrical properties of porous GaN, van der Pauw-Hall measurements were conducted at 300 K. A Ti/Al/Ni (20/200/40 nm) ohmic contact was deposited by e-beam evaporation. The measurement apparatus was a RESITEST 8300 (TOYO Corporation).
The free-electron concentration, N, was estimated using the sheet carrier concentration, Nsheet, obtained from the van der Pauw-Hall measurements.   S-14

Quantum dots photographs and image analysis of those photographs
The substrate contains both porous GaN and flat GaN region, and the edge of this substrate was not formed porous GaN (Figure S6(a)). It can be observed from Figure   S6(a) that the attachment of quantum dots are surprisingly different between two region.
The image analysis of photograph of Figure 5(e) of manuscript was carried out. Figure   S6