7. Conclusions

In this thesis, Gunn diodes based on GaAs and GaN material systems were investigated with respect to microwave generation for automotive applications. Apart from the economical factor and the simple circuit requirement, the prerequisites for a microwave oscillator include: excellent frequency stability, adequate power output, low noise (in amplitude, frequency and phase modulation), variable tuning control and capability to be modulated in amplitude (AM), frequency (FM) or phase (PM). This dissertation describes the various efforts which were undertaken to improve Gunn diodes in order to achieve the targets described above.

The fabrication technology for planar GaAs and GaN Gunn diodes was developed and optimized. The GaAs Gunn diode structures were grown by MBE on 2-inch semi-insulating GaAs substrates in a Varian ModGen II machine and the GaN structures by MOVPE on 2-inch $Al_2 O_3$ substrates in an AIXTRON reactor at the Institute of Thin Films and Interfaces of Forschungszentrum Jülich. After the epitaxial growth, $2.2-3.0 \thickspace \mu m$ deep mesas have been processed. Dry etching has been chosen, because it allows an exact definition of the mesa, prevents unwanted under-etching and provides smooth and nearly vertical side-walls. The demand of deep mesas required high etch rates, which were achieved with a chlorine based ECR Reactive Ion Etching. In the case of GaN, standard Ti etching masks have been replaced by Ni ones, improving the etching selectivity five times. Self-aligned emitter and collector contacts have been deposited and alloyed. The devices have been electrically isolated with a wet chemical etching or Ar ion sputtering down to the semi-isolating substrate. The problem of the top contact (emitter) wiring was solved via low-parasitic air-bridges or with a top contact planarization, on which direct metal connections are deposited. By means of finite elemente temperature simulations, it has been demonstrated that the air-bridge choice improves dramatically the cooling of the diodes. The heat generated inside the active region of a Gunn diode corresponds to power densities higher than $140\thickspace kW/cm^2$ for GaAs and $1 \thickspace MW/cm^2$ for GaN. A heat-sink is required to cool the active region and to improve the performance and the reliability of the device. In typical Gunn diodes, the substrate is removed and a gold heat-sink is plated to the bottom contact. The airbridge technology showed up to be a valid alternative to the bottom heat-sink, which is not compatible with a planar process.

During this work, two different hot electron injectors for GaAs Gunn diode have been examined: a graded gap injector (GGI) and a novel resonant tunneling injector (RTI). The main task of a hot electron injector is to transfer as many electrons as possible from the $\Gamma$-valley to the L-valley at the very beginning of the device active region.
Within the framework of the cooperation project between Forschungzentrum Jülich and Robert Bosch GmbH [Pro04], GGI GaAs Gunn diodes have been studied and optimized. High quality planar GGI Gunn diodes have been fabricated using an air-bridge low-parasitic planar technology. RF evaluation of their performance up to 110 GHz shows the effectiveness of different graded gap injectors. The best results were found for graded AlGaAs barriers with maximum Al contents of 32% and 34%. An estimation of the possible operational modes is given for diodes used as microwave generators at 77 GHz, with application in automotive radar systems.
A second hot electron injector, the GaAs/AlAs double barrier resonant tunnelling injector has been proposed and designed. The RTI has been numerically simulated employing self-consistent real-time Green's functions [IM], in order to match the first transmission energy level for the given current density range ( $23-27 \thickspace kA/cm^2$) to the energy gap between the L- and the $\Gamma$-valley. GaAs Gunn diodes with RTI have been successfully fabricated and characterized. RTI Gunn diodes present clear evidence of the injector effectiveness both in DC and RF conditions. A comparison between the experimental results of traditional GGI Gunn diodes and novel RTI Gunn diodes is provided, pointing out the role of the two injectors in DC and at high frequencies.

The design, processing and characterization of a novel GaAs Gunn diode based VCO-MMIC^7.1 fulfilled the second objective of this work. A good integration of a planar GGI Gunn diode with a CPW resonator, a periodic slow-wave low-pass filter and interdigitated HF coupler resulted in a compact size. A maximal power of $3.77\thickspace mW$ at $46\thickspace GHz$ is reported. Further increase of the power output should be achieved by increasing the Gunn diode area and connecting the device top contacts with thick airbridges. A simple and straightforward processing technology makes our proposed microwave generator competitive with cavity oscillators and transistor based MMICs.

While the processing of the GaN Gunn diodes has been successfully completed, the measurement results of the fabricated diodes should be considered as preliminary. However, during the processing work, some new interesting results for nanoelectronic devices have been obtained. Our early GaN etching tests demonstrated the possibility to fabricate GaN nanocolumns with a simple and reproducible method. Among different types of nanostructures, the nanowires and nanotubes appeared to be extremely interesting as building blocks for nanoelectronics. In our novel top down approach, the nanowhiskers were processed via ECR-RIE etching GaN layers grown by MOVPE. With the advantage of a physical etching, nanocolumns with arbitrary doping concentration as well as thin AlN layer can be fabricated.