Patterned Growth and Transfer of ZnO Micro and Nanocrystals with Size and Location Control

ZnO microand nanostructures have been produced using a large number of different synthetic routes, and the applications that utilize their unique properties keep increasing. The 3.3 eV direct bandgap and 60 meV exciton binding energy is exploited in ultraviolet optoelectronics, room-temperature lasing, and solar cells applications; extremely long photocarrier lifetimes have been observed yielding UV photodetectors with 10 internal gain; the optical properties in combination with n-type conduction support transparent transistor and display applications, while the piezoelectricity is utilized in power generation and force sensing applications. The integration of these device prototypes on a wafer scale will require access to ZnO microand nanostructures with a variety of dimensions at known locations. A wet-chemical approach is desirable for reasons of processing cost when compared with gas-phase methods, and a number of patterned and seeded growth methods have been reported. Patterned self-assembled monolayers with hydrophobic and hydrophilic endgroups have been used on silver or silicon substrates yielding densely packed 400 nm diameter and 2mm long ZnO nanorods in regions that were 2mm wide with empty areas in between. Out-of-plane orientation varied but has been improved by seeding ZnO nanocrystals through thermal oxidation of zinc acetate. Perfect vertical orientation, however, requires substrates such as GaN, MgAl2O4, [20] or sapphire, which can be partially masked with photoresist to achieve patterned growth. Qualitatively all of these methods produce nanorods in the seeded or unmasked areas with limited control over the location and density. Continued growth leads to coalescence into a polycrystalline film as the diameter increases with grain boundaries and defects in between. Continued growth in combination with photoresist has also been reported to lead to a lateral overgrowth; a previously reported concept to produce high-quality GaN thin films. For ZnO on MgAl2O4, lateral growth over patterned photoresist improved the dislocation density by a factor of 100 compared to the window region containing coalesced nanorods. Subsequent growth using a second window yielded continuous ZnO thin films with reduced dislocations.