Low Dislocation Density and High Mobility GaN Layers for DHFET Channels Grown on High-Temperature AlN/AlGaN Buffer Layer by Ammonia MBE

Abstract

The growth of AlN buffer layer at extremely high temperature (1100-1150oC) in ammonia MBE STE3N2 system is shown to be the key step to obtain high quality GaN layers for DHFET channels. The buffer layer sequence from c-sapphire substrate involved AlN, AlGaN/AlN superlattice and AlGaN transition layers. TEM study showed gradual decrease of threading dislocation density from (2-4)×1010 cm-2 in AlN to (9-10)×108 cm-2 in the top GaN active layer. The improvement of structural quality resulted in substantial increase in electron mobility up to 600-650 сm2/Vs in a 1.5-μm-thick GaN top layer lightly doped with silicon up to n=(3-5)×1016 cm-3. These results correspond to a good quality MOCVD GaN grown on sapphire and several times better than in conventional MBE. Employing such a GaN layer in a double heterostructure (DH) with the cap AlxGa1-xN barrier layer (x=0.25-0.4) allows to change the electron sheet density, mobility and sheet resistance in a two dimensional electron gas in the range of 1300-1700 cm2/V.s, (1.0-1.8)×1013 cm-2 and 230-400 Ω/, respectively. Application of this technology and DH design for growing on SiC substrates enabled one to manufacture a DHFET with a gate length of 0.5 μm for 0.03-4.0 GHz extra-broadband power amplifiers having Pout=2.5 W, gain 17-25 dB and efficiency 30%.