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Silicon tip arrays were fabricated by means of reactive ion etching followed by oxidation for final sharpening and molybdenum thin film coating. The field-emission (FE) properties of these Mo-coated p-Si tip arrays were systemically investigated by different measurement techniques. Integral measurements in diode configuration yielded a turn-on field (for 1 nA) of 22 V/μm and nearly stable FE currents up to 6.6 μA at 38 V/μm. The effective field enhancements factor extracted from the FN plots is about 180. Detailed investigations of these FE arrays were also performed by means of field emission scanning microscopy combined with electron microscopy. A rather limited efficiency of the tips (50% at 1500 V) and FE homogeneity (180 nA at 700 V) might be correlated with the varying morphology of the tips and the presence of oxides. Local I-V measurements of selected single tips revealed both activation and deactivation effects, which finally resulted in nearly reproducible I-V curves. Current stability measurements at a constant voltage showed rather large fluctuations (0.1-1 μA) of the FE current, which could be reduced up to 1.7% by using of a PID-regulated voltage source. SEM images showed unchanged tip shape after the current processing.
Aligned square arrays of black silicon (b-Si) on top of pillars were fabricated on p-type silicon substrate by a deep-etching step combined with a b-Si process. Two 10×10 arrays with pillar heights of 8 μm and 20 μm and one b-Si reference sample without pillars were investigated. Integral field emission (FE) measurements of the arrays yielded rather low onset-fields between 6.4 V/μm and 13.5 V/μm and field enhancement factors between 430 and 800. The I-V curves showed typical Fowler-Nordheim behavior for low fields, whereas a saturation region was observed at higher fields. The maximum integral current in the saturation region was 8 μA at a field of 20 V/μm. The stability of the emission current was investigated over 3 hours and revealed moderate fluctuations of ± 8% in the saturation region. Voltage scans showed well-aligned FE from nearly all pillars.
Precisely aligned arrays of sharp tip structures on top of elongated pillars were realized by using an improved fabrication process including an additional inductively-coupled-plasma reactive-ion etching step. Arrays of n-type and p-type silicon with 271 tips have been fabricated and investigated. Those structures have a total height of 5–6 µm and apex radii less than 20nm. Integral field emission measurements of the arrays yielded low onset-fields in the range of 8–12V=µm and field enhancement factors between 300 and 700. The I-E curves of n-type structures showed the usual Fowler-Nordheim behaviour, whereas p-type structures revealed a significant saturation region due to the limited number of electrons in the conduction band and a further carrier depletion effect caused by the pillar. The maximum integral current in the saturation region was 150 nA at fields above 30V=µm. An excellent stability of the emission current of less than ± 2% fluctuation was observed in the saturation region. For n-type Si a maximum integral current of 10 µA at 24V=µm and an average current stability with a fluctuation of ± 50% were measured.
In this contribution we report on field emission (FE) cathodes based on thin-film LED-technology and surface texturing. The FE cathode can be used both as an n-GaN cathode with an electrical contact at the top side metallization as well as a pn-GaN diode contacting only the p-GaN layer at the bottom side of the structure. The local and integral FE properties of the textured surface of the LED structure were investigated. For n-GaN an integral emission current up to 1.0 μA at an electric field of 19 V/μm was achieved. The pn-GaN diode measurements showed an integral current saturation behavior with two orders of magnitude lower FE currents. Regulated voltage scans obtained by FE scanning microscopy revealed a well-distributed emission over the whole cathode area. Measurements under pulsed tunable laser illumination and moderate electric fields indicated charge carrier generation in the buried double-heterostructures at photon energies below 3.5 eV besides normal photoemission above 4.1 eV.