Epitaxy of novel AlScN/GaN and AlYN/GaN heterostructures by metal-organic chemical vapour deposition

Sustainable transmission of large data flows is key in an increasingly digitalized modern world. Monolithic microwave integrated circuits (MMIC) based on gallium nitride (GaN) are used for large volume data transmission. This technology is currently the only available solution for output powers exceeding 1 W at Ka-band frequencies of 27 to 40 GHz. AlGaN/GaN high-electron mobility transistors (HEMT) are responsible for signal amplification in these circuits. The core of these devices is the heterojunction of a high bandgap barrier layer with a high polarization (e.g. AlGaN) and a low bandgap channel layer with lower polarization (e.g. GaN) leading to the formation of a potential well in the lower bandgap material and a two-dimensional electron gas (2DEG). The maximum drain current density I(D) of the transistor scales with the sheet carrier density n(s) in the 2DEG. Barriers of Al₁₋ₓScₓN provide up to 4.5 x higher n(s) tha9n AlGaN and an a lattice parameter matching to GaN that allows for the growth of strain-free structures, potentially increasing the lifetime of devices. Growth of Al₁₋ₓScₓN/GaN heterostructures by metal-organic chemical vapour deposition (MOCVD) is required for mass production of semiconducting epitaxial layers. However, the growth of AlScN by MOCVD is challenging due to the low vapour pressure of the conventional Sc precursor Cp₃Sc which allows only for very low growth rates of 0.006 nm/s even if the precursor is kept at 155°C and a heated gas mixing system is installed. This work shows that high thermal budget, generated by low growth rates and high growth temperatures, promotes not only atom diffusion across the heterojunctions but also supports strain-related effects that lead to the formation of linearly graded, quarternary AlGaScN interlayers between the Al₁₋ₓScₓN barriers and the GaN channels, as well as Sc concentration gradients in the barrier. A linearly graded interlayer broadens the potential well at the heterojunction and causes a loss of 2DEG confinement. The novel Sc precursor (MCp)₂ScCl is used to double the growth rate at unchanged source temperature, while (EtCp)₂Sc(bdma) and (EtCp)₂Sc(dtbt) allow for the reduction of the source temperature to 100°C and a simultaneous increase of growth rate by a factor of six and ten, respectively. Up to a growth rate of 0.015 nm/s, graded interlayers with type I stacking faults, edge and screw dislocations are observed. Sc grading in the barrier occurs up to growth rates of 0.034 nm/s. Abrupt interfaces and homogeneous barriers are obtained at a growth rate of 0.067 nm/s, even at high growth temperatures of 1100°C. Alternatively to growth rate increase, AlN interlayers can be employed to suppress compositional grading. The electrical performance of Al₁₋ₓScₓN/GaN heterostructures is very sensitive not only to interfacial grading but also to the precursor purity grade and impurity incorporation at low growth temperatures, as Sc oxidizes easily. Carbon impurities trap electrons and reduce the n(s) of the 2DEG. Al(Ga)ScN/GaN HEMTs grown with the precursor (MCp)₂ScCl achieve a output power P(out) and a power added efficiency PAE of 8.4 W/mm and 42.0 %, respectively, at 30 GHz. This is the highest reported combination of P(out) and PAE achieved for class-AB continuous wave operation at Ka-band frequencies on metal-polar GaN-based HEMTs so far. The criticality of Sc as raw material and the difficulty in chemical handling of it brings the attention to Al₁₋ₓYₓN. Y has similar properties to Sc but is more abundant and easier to extract and purify. In this thesis, the growth of Al₁₋ₓYₓN and Al₁₋ₓYₓN/GaN heterostructures by MOCVD is demonstrated for the first time. With the precursor (EtCp)₂(iPr-amd)Y, several hundred nanometer thick wurtzite Al₁₋ₓYₓN layers with Y concentrations up to 40 % were grown, however the purity level was not sufficiently high to allow for 2DEG formation. In contrast, 2DEGs with n(s) of 1-2 x 10¹³ cm⁻², electron mobilities μ of 1000 cm²/(Vs) and excellent C-V characteristics were grown with the precursor (MCp)₃Y. At 7 K, record high μ of 3234 cm²/(Vs) was achieved.