Umesh K. Mishra

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.

GaN planar-doped-barrier electron emitter with piezoelectric surface barrier lowering

Electron emission from a GaN planar-doped-barrier hot-electron emitter with piezoelectric surface barrier lowering (i.e., no cesiation) was reported. The piezoelectric effect in the InGaN, which is produced by the strain from the pseudomorphic growth of InGaN on top of GaN, was utilized to lower the surface barrier, replacing the cesiation used before which is the major reason for the degradation. The potential drop produced by the piezoelectric field can lower the effective electron affinity. The structure of the planar electron emitter consisted of an In0.1Ga0.9N cap layer to lower the surface barrier, a i-p-i layer to form the planar-doped-barrier, and n+ GaN buffer to provide electrons. The sample was grown by molecular beam epitaxy and was fabricated to a diode-based planar emitter with an air-bridge anode. The emission current about 5×10−5u2009A/cm2 was measured with efficiency of 5.7×10−7. The low current and efficiency are due to the oxidation of the surface, dislocation-assisted tunneling current, an...

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.

Device-scaling constraints based upon delay-time arguments

Extracted delay times provide information useful for device scaling. In this work a novel parameter extraction technique that permits delay times associated with the physical operation of the transistor, along with element values for an equivalent circuit, to be determined from terminal S-parameter measurements. The technique is employed to investigate the operation of mm-wave AlInAs/GalnAs/InP heterojunction bipolar transistors. High current phenomena, such as the onset of the Kirk Effect, are clearly evident. The results indicate that the base region delay is dominant in determining the high frequency operation of these devices.