The peculiarities of the photon assisted field emissions from GaN nanorods

GaN is a promising material for the high frequency, high power and high temperature electrical and optical ultraviolet devices due to its unique material parameters. The field and photofield emission may be developed as a method for determination of some of the band structure features. The peculiarities of electron field and photon-assisted field emission from GaN nanorods are presented here. The GaN field emitter rods were fabricated on a wafer with n-GaN active layer (5 mum) sandwiched between n⁺-GaN cap layer (100 nm) and n⁺-Si substrate. Then, active layer and top layer were grown with MOCVD using a standard method. A photoresistant mask had been used for the Argon plasma etching of the circular mesa with 400 nm height. Afterwards the samples were cleaned in Aceton and etched by the photoelectrochemical (PEC) etching of GaN. Some samples were GaN rods without PEC etching. Used field emission setup may achieve a vacuum ~2 times 10^-8 mbar. The cathode electrode was formed on the backside of the n⁺-GaN substrate. ITO coated quartz was used as an anode electrode. The distance between the GaN emitter and ITO anode was defined by a kapton spacer with either 7.5mum or 20 mum thickness and 1 mm diameter. The light was focused directly onto the field emitter in a high vacuum chamber through the quartz glass. Laser diodes with different wavelength (IR, red (R), green (G), UV) were used for the photo assisted field emission. As a rule, the curves with two slopes in FowlerNordheim coordinates were observed. The influence of light illumination on the field emission was discovered only on low voltage part of the curve. At higher voltages the experimental curves without and with illumination practically coincided. The slope of the low voltage part of F-N plots has decreased as a result of illumination. This fact points at the lowering of the energy barrier at field emission due to the electron excitation caused by light. The energy b- arrier heights at field and photofield emission were estimated. The experiment showed partial electron thermalization before the field emission into vacuum. As opposed to the inner conduction band the electron transitions at IR and G illumination at photoexcitation by UV, the interband (valence conduction bands) transitions are possible. The estimated photofield emission barrier would be higher in this case.