Molecular beam epitaxial growth of normal and inverted two-dimensional electron gases in AlGaN/GaN based heterostructures

Abstract

Two-dimensional electron gases in AlGaN/GaN heterostructures grown by plasma induced molecular beam epitaxy have been formed by polarization induced interface charge effects. Growth of the “normal” structure (AlGaN on GaN) has formed a two-dimensional electron gas confined in the GaN when grown on sapphire with an AlN nucleation layer, SiC (Si face), or on GaN nucleated by organometallic vapor phase epitaxy on sapphire. Hall mobilities in GaN for normal interfaces are as high as 1238 cm2/V s at 300 K and 3182 cm2/V s at 77 K. Direct current results from field effect transistors fabricated from this material yield a maximum transconductance of 210 mS/mm and a current density of 710 mA/mm. Microwave measurements gave a ft of 27 GHz and a fmax of 37 GHz. When nucleating GaN directly on sapphire, an “inverted” structure (GaN on AlGaN) was used to create a two-dimensional electron gas. The difference in the normal and inverted structures is the polarity of the (Al)GaN layers. The flipping of the (Al)GaN polarity on sapphire is achieved through the use of the AlN nucleation layer.

High-Electron-Mobility AlGaN/GaN Transistors (HEMTs) for Fluid Monitoring Applications

“…The deposition was started growing a 10 nm AlN nucleation layer followed by a 2 mm thick GaN buffer layer. With this thin AlN nucleation layer the polarity of the thick GaN layer was changed from normally N-face to Ga-face [12]. Next a 30 nm thick Al x Ga 1--x N barrier layer with an aluminum content of 30% was grown to form a two-dimensional electron gas (2DEG) at the GaN/AlGaN interface (Fig.…”

Role of barrier and buffer layer defect states in AlGaN/GaN HEMT structures

“…A 7 nm thick AlN buffer layer was then grown at 800°C to control the GaN polarity. 2,3 The heterostructures are composed of a 25 nm AlGaN barrier layer of nominal 30% Al concentration grown on a GaN buffer layer of thickness 1 µm and then capped with 2 nm of GaN (see Fig. 1).…”

Characterization of AlGaN/GaN structures on various substrates grown by radio frequency-plasma assisted molecular beam epitaxy

“…It was recently reported that high temperature annealing of AlN nucleation buffer on sapphire resulted in Ga-polarity material by MBE. 13 However, since sapphire is not a polar substrate, this way of obtaining a Ga-polar film is achieved by weeding out the undesired N-polar nucleation layer. 14 This requires optimization of several steps including buffer deposition and anneal.…”