Results from WP1 - Material and Epitaxy

Our goal is the development of mature epitaxial GaN and AlN substrates. GaN-on-Si will obtain improved reliability by different epitaxial buffer structures. AlN will be re-grown on templates with high epitaxial quality. Finally, the correlations between material properties and device performance will be investigated.

From the lab: 

 GaN on Si with very low sheet resistance obtained


One of the aims of the project is to boost the performance of GaN on Si. Two strategies are employed: on the one hand by increasing the voltage handling capability, and on the other hand by reducing the on-state resistance. On a material level, the latter aspect is achieved by reducing the sheet resistance of the 2DEG by re-engineering the classical heterostructure that uses AlGaN barrier layers grown on a GaN channel layer. Two different approaches were investigated: ultrathin AlN barriers and lattice-matched InAlN barriers. The wafers with an InAlN barrier showed an exceptionally low sheet resistance with a median value of 208 Ohm/sq (distribution displayed on left figure). This value is roughly half the value of what is typically achieved with the standard AlGaN barriers.

 

 Very high crystal quality AlN on sapphire characterized


In order to explore the next generation of ultra-wide bandgap materials, the InRel-NPower project is exploring the AlN material system. A first step is to obtain very high quality AlN starting material onto which modified HEMT structures will be grown using MOCVD. Several types of AlN on sapphire templates have been provided by our Japanese partners in the project, Prof. Miyake and Prof. Kangawa from Mie University and Kyushu University respectively. These wafers were investigated in depth by project partners at Fraunhofer IISB in Erlangen and the University of Gent which indicated very high crystal quality (FWHM values below 60 arsecs for the 0002 reflection) and a very homogenously smooth surface with an RMS roughness value of only 0.35nm.

Tapping-mode AFM measurements on AlN template substrate is displayed on the topography image on the left. The highly ordered morphology exhibits step edges in characteristic directions.

Fraunhofer IISB furthermore investigated the dislocation density of AlN templates by etching the surface with KOH/NaOH. The used etching parameters are optimized in a way that dislocation density can be determined in a reliable way, as illustrated in the picture on the right.