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Ammonia speeds MBE growth of GaN-on-silicon HEMTs

Gallium nitride transistors with good structural and electrical characteristics have been produced by ammonia MBE on a 100 mm silicon substrate
Researchers from Nanyang Technological University, Singapore, claim that they have produced the first crack-free, GaN-based HEMTs on 100 mm silicon substrates by ammonia MBE.

The substrate that they have used, silicon, is an excellent choice for producing GaN HEMTs, because it is cheap, available in large diameter formats and can produce transistors that can be processed through depreciated silicon lines.

By far the most common method for depositing the epilayers of the HEMT on silicon is MOCVD. However, according to the researchers from Singapore, this has several weaknesses compared with MBE.

In their opinion, MBE is a more flexible growth technique that delivers sharper interfaces and enables growth at lower temperatures, which aids the management of thermal mismatch between substrate and epilayers. What’s more, they claim that MBE allows in-situ monitoring of the growth surface, leading to real time growth process control at the monolayer scale.

The Singapore team favours ammonia MBE over its plasma-assisted (PA) variant, because it is difficult to control the III-V ratio with the latter technique. In addition, due to the low nitrogen molecular cracking efficiency – typically 1 percent to 10 percent – growth rates are typically below 0.4 µm/hr for PA MBE.

Using ammonia MBE, the team from Singapore have produced crack-free epilayers at growth rates of up to 0.75 µm/hr, nearly double that typically used in PA MBE. Faster growth doesn’t just save time – it also improves morphology, by accelerating the transition from three-dimensional to two-dimensional growth and suppressing defect formation.

This trimming of the defect density is revealed in cross-sectional images of the lower part of the epitaxial stack: The 50 nm-thick AlN nucleation layer, 200 nm-thick GaN and AlN stress mitigation layers, and the GaN buffer. The nucleation layer and lower GaN layer are riddled with defects, but many dislocations terminate at the interface between the second AlN layer and the GaN buffer, and a substantial proportion of those that propagate into this second buffer bend and interact within the first few hundred nanometres.

Estimations based on X-ray diffraction analysis suggest that the density of screw-type dislocations in the GaN buffer falls from 7.7 x 109 cm-2 to 2.1 x 109 cm-2 when the buffer thickness is increased from 0.9 µm to 1.7 µm. Resistance mapping of the HEMT epiwafers, which have a 28 nm-thick Al0.25 Ga0.75N barrier and a 2 nm-thick GaN cap deposited on the GaN buffer, show an average sheet resistance of 368 Ω /square.

Meanwhile, room-temperature Hall measurements reveal that the carrier density and mobility of the two-dimensional electron gas are 1.2 x 1013 cm-2 and 1350 cm2/Vs. Cool the sample to 90K, and mobility rises to 4290 cm2/Vs.

To determine the electrical characteristics of the buffer layer, engineers formed test structures with two ohmic contacts with a gap of 5 µm. A structure with a 1.7 µm-thick buffer produced a buffer leakage current of 2.6 x 10-4 mA/mm at 20 V and had a ratio between on-current and off-current of 7.3 x 106.

HEMTs with 0.3 µm T-shaped gates were formed on high-resistivity silicon. These transistors delivered a peak drain current of 768 mA/mm, produced a maximum transconductance of 190 mS/mm and exhibited a threshold voltage of -4.53 V.

Further details of this work have been published in the paper, " Demonstration of AlGaN/GaN High-Electron-Mobility Transistors on 100-mm-Diameter Si(111) by Ammonia Molecular Beam Epitaxy", by N. Dharmarasu et al in Applied Physics Express 5 091003 (2012). DOI:10.1143/APEX.5.091003

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