Steven Boeykens

Improvement of AlGaN∕GaN high electron mobility transistor structures by in situ deposition of a Si3N4 surface layer

We have made AlGaN∕GaN high electron mobility transistors with a Si3N4 passivation layer that was deposited in situ in our metal-organic chemical-vapor deposition reactor in the same growth sequence as the rest of the layer stack. The Si3N4 is shown to be of high quality and stoichiometric in composition. It reduces the relaxation, cracking, and surface roughness of the AlGaN layer. It also neutralizes the charges at the top AlGaN interface, which leads to a higher two-dimensional electron-gas density. Moreover, it protects the surface during processing and improves the Ohmic source and drain contacts. This leads to devices with greatly improved characteristics.

Deep defects in GaN/AlGaN/SiC heterostructures

Deep level transient spectroscopy (DLTS) measurements were carried out on GaN/AlGaN/SiC heterostructures prepared by low-pressure metalorganic vapor phase epitaxy. Si-doped n-GaN layers were grown using an n-AlGaN nucleation layer (8% and 30% of aluminum) on two kinds of p-type 4H-SiC substrates. The DLTS spectra of on-axis (0001) grown samples exhibit a dominant peak of a majority carrier trap with apparent activation energy close to 0.80 eV and capture cross section of about 5×10−14 cm2 regardless of the AlGaN composition. The energy of this deep level decreases with increasing electrical field due to Poole–Frenkel effect. Carrier capture kinetics indicates interacting point defects arranged along a line, probably a threading dislocation. Two additional traps (0.52 and 0.83 eV) were found in on-axis samples with 8% AlGaN composition. For 30% Al content, only a 0.83 eV level was detected. Majority carrier trap with activation energy of 0.66 eV was observed in the off-axis grown samples. This level is pro...

High electron mobility in AlGaN/GaN HEMT grown on sapphire: strain modification by means of AlN interlayers.

The performance of AlGaN/GaN High Electron Mobility (HEMT) transistors is directly related to the electrical characteristics of the two-dimensional electron gas formed at the interface thanks to the piezoelectric field. Modification of the Al content or thickness of the AlGaN layer can within a certain limit modify the carrier density and mobility in the 2DEG. However, further reduction of the sheet resistance requires strain engineering of the heterostructure. An effective way to reduce the sheet resistance, as well as to lower the threading dislocation (TD) density, is to perform strain engineering through the use of low temperature AlN interlayers inserted in the GaN buffer layer. From correlation of AFM, TEM and HRXRD mapping of the HEMT layers, the strain modification, as well as the mechanism reducing the TD density, can be explained by the highly defected nature of the AlN interlayer grown at low temperature, as well as its very small thickness. The LT AlN acts as a second nucleation layer for the GaN grown on top. Contrarily, when the AlN interlayer is grown at 1050°C, its high crystalline quality and the possibility to grow pseudomorphic and abrupt interfaces, allows its use at the AlGaN/GaN interface. Optimal combination of the AlGaN and AlN layer thickness leads to record values of the mobility at room temperature of 2050 cm2/Vs, for heterostructures grown on sapphire, which is approaching state-of-the-art for HEMT grown on SiC.

Growth, processing and characterization of GaN/AlGaN/SiC vertical n-p diodes

Application of SiC substrates instead of the most commonly used sapphire for the heteroepitaxial growth of III-Nitrides offers advantages as better lattice matching, higher thermal conductivity, and electrical conductivity. This namely offers interesting perspectives for the development of vertical III-Nitride devices for switching purposes. For example, an AlGaN/SiC heterojunction could improve the performance of SiC bipolar transistors. In this work, n-type GaN layers have been grown by MOVPE on p-type 4H-SiC substrates using Si doped Al 0.08 Ga 0.92 N or Al 0.3 Ga 0.7 N nucleation layers. They have been characterized with temperature dependent current-voltage ( I-V-T ), capacitance-voltage ( C-V ) techniques and transmission electron microscopy (TEM).