Field emission properties of Ge-doped GaN nanowires
Introduction
As a well-known III–V semiconductor material, Gallium nitride (GaN) has been widely studied because of its intrinsic structure with a direct large band gap (∼3.4 eV) allowing it to be used in optoelectronic devices, such as light emitting diodes, laser diodes in the blue to ultraviolet region [1], [2], [3], [4], [5], and so on. Ge and Ga atoms are neighbor in the periodic table of chemical elements, and have diameter similarities, so Ge can be expected to be incorporated into the GaN lattice without significant lattice distortion [6], [7], [8], [9]. Ge-dopted GaN and Ge-dopted GaN nanowires (NWs) have been studied in theoretical and experimental [10], [11], [12]. The field emission properties of Ge-dopted GaN NWs will be studied in our work.
GaN has a low work function (4.1 eV) and low electron affinity (2.7–3.3 eV), which is attracting a lot of attention as a cathode material for field emission devices [13]. As a kind of one-dimensional (1D) nano-material with high aspect ratio, GaN NW possess excellent performance of field emission properties. The field emission properties of NWs could be enhanced by improving the field enhancement factor β which depends on the geometry and surface state of the emitter [14]. For example, Layer-structure, nanopencils, helical, ropy GaN NWs, and other different morphology GaN NWs are responsible for superior field emission properties [15], [16], [17], [18], [19], [20] because of their special morphology, which has enhanced the field emission factor. On the other hand, The field emission properties of NWs could be enhanced by decreasing the work function which could be improved by doping. Many research groups have studied the field emission properties of doped GaN NWs [21], [22], [23], [24]. Up to now, how Ge impurity to influence field emission properties of GaN NWs has not been reported. In this paper, we will study the influence of Ge impurity to field emission properties of GaN NWs both in theoretically and experimentally.
Section snippets
Calculation methods
In this work, the DFT calculations was performed using the VASP package within the plane-wave-pseudo-potential approach [25]. Exchange-correlation effects were treated with the generalized gradient approximation (GGA), the interaction between the valence electrons and the ionic core was described by ultrasoft pseudo-potentials and the Perdew and Wang (PW91) gradient-corrected functional [26], [27]. The cutoff energy for the plane-wave basis set was 350 eV. Geometry optimization was done with
Experiment details
In a horizontal tube atmosphere furnace, Ga2O3 powder and Pt-coated Si (111) substrate were placed in a quartz boat with the distance of 1 cm between them (GeO2 was placed 1 cm at the front of Ga2O3 when synthesizing Ge-doped GaN NWs), and the quartz boat was pushed to the flat-temperature zone of the horizontal tube furnace. N2 gas was introduced for a given duration to remove the remaining gas, then the furnace was heated to 1100 °C at a rate of 10 °C/min, in this process, NH3 gas was
Conclusions
The pure and the three Ge-doped GaN NWs have been synthesized via CVD in single-crystalline form with the hexagonal wurtzite structure, and the NWs show a uniform density and smooth surface. Furthermore, Ge-doped GaN NWs possess low turn-on fields and high current density. The DFT calculated results show that Ge doping provides local electron states near the Fermi level which will supply more electrons that can tunnel through the barrier to vacuum at a given operating voltage, and the Fermi
Acknowledgements
This work was supported by the National Natural Science Foundation of China (No. 51042010) and the Natural Science Key Project Foundation of Shaanxi Province, China (No. 2013JZ018).
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