R.D. Underwood

Anisotropic epitaxial lateral growth in GaN selective area epitaxy

Epitaxial lateral mask overgrowth which occurs during GaN selective epitaxy has been studied using linear mask features. The lateral growth varies between its maximum and minimum over a 30° angular span and exhibits hexagonal symmetry. Vertical growth follows an opposite trend, with lateral growth maxima, and vertical growth minima occurring for lines parallel to the GaN 〈10•0〉. Large variations in the lateral growth are also obtained through variations in the growth temperature and NH3 flow. Under proper growth conditions, lateral to vertical growth rate ratios of up to 4.1 can be achieved, resulting in significant lateral mask overgrowth and coalescence of features without excessive growth times.

Selective area epitaxy of GaN for electron field emission devices

Selective area epitaxy of GaN by MOCVD has been used to fabricate arrays of hexagonal pyramid structures for electron field emission devices. The reactor temperature and pressure have been found to strongly affect the resulting pyramid morphology. Growth at 76 Torr results in improved pyramid shape and uniformity compared to growth at atmospheric pressure. Optimized arrays of pyramids produced emission currents of 80 μA at 1100 V, when biased across 0.5 mm in UHV.

Selective-area regrowth of GaN field emission tips

Abstract Field emission current has been observed for the first time from GaN. Single-crystal GaN pyramids were grown in arrays by selective-area metalorganic chemical vapor deposition (MOCVD) on GaN thin films using a dielectric mask. GaN does not deposit on the dielectric mask and growth of hexagonal pyramid structures occurs only in mask openings. The pyramids were biased negatively with respect to a metal anode and an emission current of 0.8 μA at 2000 V was observed.

GaN field emitter array diode with integrated anode

GaN field emission pyramids are grown by self-limiting, selective-area metalorganic chemical vapor deposition. The self-limitation provides the potential of high uniformity of the pyramids and the selective-area growth allows one to define regular arrays of GaN pyramids for field emitter arrays (FEAs). Fabrication of an integrated anode lowered the operating voltage of the FEAs by narrowing the anode-cathode distance compared to devices with an external anode. A maximum emission current of 0.15 μA/tip has been observed for voltages of 570 V with an emitter-anode separation of 2 μm.

Piezoelectric surface barrier lowering applied to InGaN/GaN field emitter arrays

A method of lowering the surface barrier for field emission by using the piezoelectric effect is presented. The piezoelectric effect produces a surface dipole that decreases the surface barrier, which in turn decreases the turn-on voltage of the field emitter. Calculations show that significant reduction of the tunneling barrier can be effected with relatively thin layers of strained InGaN on GaN field emitter arrays. Dramatic reduction of the turn-on voltage from 450 V (GaN field emitter array) to 70 V (InGaN/GaN field emitter array) was observed and can be attributed partly to surface barrier lowering.

TELLURIUM-DOPED AL0.43GA0.57AS/(IN0.2)GAAS MODULATION DOPED HETEROSTRUCTURES BY MOLECULAR-BEAM-EPITAXY

Te has previously been demonstrated to have a shallower deep donor (DX‐center) level than Si in AlGaAs. In this work, Te‐doped Al0.43Ga0.57As/GaAs and pseudomorphic Al0.43Ga0.57As/ In0.2Ga0.8As modulation‐doped heterostructures (MDHs) grown by MBE have been studied. The conduction band offset ΔEc in the pseudomorphic AlGaAs/InGaAs material system has a maximum at 43% Al mole fraction. This allows maximum carrier confinement in the quantum well. Two‐dimensional electron densities and mobilities 2.36×1012 cm−2 and 7794 cm2/V s at 300 K and 2.17×1012 cm−2 and 24 379 cm2/V s at 77 K (in the dark) have been obtained in Te‐doped pseudomorphic MDHs.

Field emission from selectively regrown GaN pyramids

Summary form only given. Field emission is the phenomena by which electrons quantum mechanically tunnel into vacuum from a solid or liquid by the application of an electric field. Once in vacuum, the electron transport is not restricted by collisions with the lattice. The higher velocity of electrons in vacuum than in a crystal allows the operation of high efficiency, high speed devices such as flat-panel displays and microwave power sources. The theory of field emission was first explained correctly by Fowler and Nordheim in 1928. The ideal field emitter will have a high concentration of electrons, a low work function (or electron affinity), and an atomic-sized tip. A sharp tip is necessary to provide high field near the surface yet avoid arcing. Tips in molybdenum have been deposited, and tips in silicon have been etched. Work in GaN has focused on selective-area epitaxial regrowth of the tips on a patterned GaN layer. Recently, the first observation of field emission from GaN was reported. The sharpness of the tips, the hardness of the material, and the large uniform arrays that can be produced by regrowth make GaN an excellent candidate for a viable field emission cathode.

GaN FEA diode with integrated anode

GaN field emission pyramids are grown by a self-limiting, selective-area metalorganic chemical vapor deposition (MOCVD). Thc self limitation ensures high uniformity of the resulting pyramids and the selective-area growth allows one to define regular arrays of GaN pyramids ,for FEAs. Fabrication of an integrated anode lowered the operating voltage of the FEAs by narrowing the anode-cathode distance compared to devices with an external anode. A maximum emission current qf 0.15 @/tip has been observed for voltages of 570 V with an emitter-anode separation of 2 pm.

InGaN/GaN field emitters with a piezoelectrically-lowered surface barrier

The operating characteristics of field emitters are exponentially dependent on two device parameters: the field enhancement factor and the surface energy barrier height. In addition to these device parameters, the practical use of field emitters as electron sources is dependent on the stability and uniformity of field emission. We have investigated field emission from GaN field emitters because GaN has a high resistance to sputtering and is chemically inert. One drawback of GaN field emitters is that they are not easily sharpened. Thus, to lower the operating voltage of GaN-based field emitters we have investigated the lowering of the surface energy barrier using a strained-layer of InGaN on GaN field. The piezoelectric polarization produced in the strained InGaN forms a dipole much like the dipole formed by coating a surface with an electropositive adsorbate.