University thesis:
Dissertation, Universität Freiburg, 2020
Footnote:
Description:
Abstract: Autonomous driving, increased data volumes, and the pursuit of world-wide networking call for new material solutions in order to satisfy this unabated demand for innovative technology. Efficient devices that work with high output power and frequencies in the millimetre wave range are highly sought after. Due to their advantageous polarisation properties, Gallium nitride based high electron mobility transistors (HEMT) are the su- perior technology to use at high output power while covering a wide range of frequencies. The conventional AlGaN/GaN HEMT can only be operated up to a limited drain voltage. However, permanent connectivity demands simultaneously high operating voltages and high transistor current density, i.e. sheet carrier density.<br><br>For GaN-based HEMTs with AlScN as novel barrier layer material, analytical calcu- lations of predicted sheet carrier densities ranging from 4 × 10 13cm-2 to 6 × 1013 cm-2 in dependence on composition and Sc concentration. Furthermore, when deposited lattice matched Al0.82Sc0.18N/GaN, no critical thickness limits the barrier thickness and an addi- tional degree for barrier design is given, making this material a very appealing alternative to conventional nitride based HEMTs. Successful deposition has been demonstrated by molecular beam epitaxy (MBE). As opposed to metalorganic chemical vapour phase de- position (MOCVD) which is commonly used by commercial companies, MBE is mainly applied for material research due to its considerably lower wafer throughput.<br><br>Developing a growth process using the technique MOCVD for GaN-based HEMTs with AlScN barrier material will allow for this material to be available for a wide field of commercial applications, among them high frequency and high power transistors. In this thesis, the successful development of a growth process for AlScN/GaN heterostructures by MOCVD is presented. When this work was started, however, no deposition of conven- tional nitrides alloyed with a sufficient amount of Sc had been achieved using MOCVD as growth technique. This fact is traceable to the absence of precursor material with a reasonable vapour pressure.<br><br>50 - 150 nm AlScN/GaN heterostructures targeting a sufficient Sc incorporation, phase purity and low surface roughness were successfully deposited after a technical upgrade of the MOCVD reactor. Growth parameters, including temperature, reactor pressure and V/III ratio were determined. Sc concentrations of up to 30 % in the Al1− Sc N layer could be exposed.<br><br>Based on this, 5 - 20 nm AlScN/GaN heterostructures for HEMTs high crystal quality and surface roughness as low as 0.2 nm could be realised. The presence of a 2DEG in the channel for Al0.84Sc0.16N/AlN/GaN heterostructures deposited at elevated temperatures and high V/III ratios was confirmed by capacitance-voltage measurements.<br><br>Optimised samples showed very promising electron transport properties with sheet carrier densities and electron mobilities in the range of 2.8 × 1013 cm-2 and 800 cm2/Vs, resulting in a sheet resistance of 320 Ω/ . The deviation in sheet carrier concentration from analytical calculation is mainly attributed to the strong interdiffusion. This results in a grading of the barrier composition and subsequent flattening of the quantum well.<br><br>In comparison to MBE, a process in which the insertion of an AlN interlayer between the AlScN barrier and GaN buffer has been recommended, no conclusive results for the enhancement neither in sheet carrier density nor in electron mobility could be observed. The reason for this phenomenon is assumed to be heavy interdiffusion of Al, Sc, and Ga combined with high oxygen incorporation leading to enhanced impurity and interface roughness scattering. Thus, when electron transport properties of samples deposited by MBE are compared to MOCVD, a similar quality is found.<br><br>Ohmic contact metallisation resulted in contact resistances of 0.6 Ωmm after rapid thermal annealing. Output and transfer characteristics revealed a saturation current of up to 1450 - 1700 mA/mm with a transconductance above 650 mS/mm. This correlates to the results obtained by AlScN-HEMTs grown by MBE. The newly developed structures and MOCVD growth process will contribute significantly to the commercialisation of high performance AlScN barrier material HEMTs for high frequency and power electronic applications