Development of 3D III-Nitride Resonant Tunneling Diodes

• Applicants: Professor Dr. Thomas Hannappel; Dr.-Ing. Lisa Liborius
• Subject Area: Electronic Semiconductors, Components and Circuits, Integrated Systems, Sensor Technology, Theoretical Electrical Engineering
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 566428840

The electromagnetic frequency range from 0.3 to 3 THz has gained much attention for various applications such as contactless material identification, high resolution radar imaging, or wireless communication with high bandwidth. A very promising approach is the use of ultrafast electronic resonant tunneling processes to achieve amplification at THz frequencies. In the most basic setup, a resonant tunneling diode (RTD) consists of two highly n-doped layers (collector and emitter) separated by two quantum barriers (QBs) and a quantum well (QW). Their current-voltage characteristic exhibits a region, where the current drops even though the voltage is increased. This negative differential resistance, NDR, acts as a gain in an inductance-capacitance resonant circuit, compensating losses and leading to net signal generation. First attempts have been made to use higher bandgap semiconductor materials in RTD structures, i.e. GaN/AlGaN, to increase operating voltage and thus RF output power. Nonetheless, there are two major challenges that are currently limiting the development of GaN RTDs: heterostructure crystal quality and polarization field. A possible solution for both challenges may be achieved using 3D structures. The goal of this project is the development of m-plane GaN-based RTDs on sapphire substrates. A 3D structure grown by bottom-up selective area epitaxy will be used in order to gain access to the non-polar m-plane and reduce defect densities within the active layers. It consists of an n-doped GaN core and shell as electron reservoir, a GaN QW between two AlGaN QBs and two undoped GaN spacers and, lastly, an outer n-doped shell as second electron reservoir. For our approach, quantum based physical simulations of the GaN RTD structures will be performed in order to determine the optimum geometrical dimensions of the spacers, QBs and QWs. These findings are applied in the epitaxial growth development of the core-shell RTDs and vice versa. The crystal defect density greatly impacts the performance of the device. It is therefore necessary to analyze the influence of the epitaxial parameters and chosen geometry on the crystal quality by TEM, multi-tip STM (MT-STM), and EDX. To advance device technology development, single NW analytics is necessary, which will provide feedback for improving growth processes. Electrical characterization on individual freestanding nanowires will be performed with the help of the MT-STM, which should give direct insight into individual components. The results, together with the simulations, will be used to improve our understanding of the charge carrier transport as well as charge carrier separation and optimize the epitaxy of the RTDs. For oscillator applications, we consider the resistance and capacitance values of the structure, determining the R-C-time constant. The latter is a good indicator for the maximum attainable frequency. Eventually, on-chip oscillators in the >100 GHz frequency range will be developed.

• DFG Programme: Research Grants
• International Connection: USA
• Cooperation Partner: Professor Dr. Gerhard Klimeck