Preparation and optimization of low-temperature grown GaAs photomixers

Abstract

The aim of this work was to design and prepare photomixer devices based on LT GaAs and to optimize them with respect to the maximal output power. Essential part of the photomixer is the MSM photodetector structure. For this reason a major part of the optimization process was done on photodetector structures. One set of our photomixers was optimized for the 460 GHz output frequency employing single dipole antenna. The second set of photomixers used broadband bow-tie and spiral antennas designed for the frequency range up to 1.6 THz and 3.7 THz, respectively. The influence of the growth temperature of GaAs on its desired properties (high resistivity, high electric breakdown field, low carrier lifetime, etc.) is presented in Chapter 2. It is well know that with increasing the growth temperature the carrier lifetime in this material increases as well. This is a major parameter for limitation of the maximal output frequency of a photomixer device, that is not RC-constant limited. The second important factor for optimization of the maximal photomixer output power is the mobility of photogenerated carriers. It is a well established fact that with increasing the growth temperature the mobility also increases in GaAs material. The output power of a photomixer device depends on both above mentioned parameters which are in trade-off. Thus, the growth temperature is one of the most important parameters contributing to the output power increase of our photomixers. Photomixers designed for 460 GHz require partially higher growth temperature of LT GaAs, 275-300°C, as shown in Chapter 6. Ion implantation is another possibility how to decrease carrier lifetime in GaAs. In this work we present results showing that employing this technique fabrication of materials with subpicosecond carrier lifetime is possible. Materials implanted with various ion doses (1012-1016 ions/cm2) and implantation energies in the range 82 keV-880 keV were prepared and their properties and dynamics before and after annealing were studied. Photodetectors based on 880 keV nitrogen-implanted GaAs show 50% higher sensitivity than our best LT GaAs photodetectors. These results outline new possibilities for increasing of photomixer output power because with right choice of implantation energy and dose it is possible to prepare materials with subpicosecond carrier lifetime and with higher mobility of photogenerated carriers than in previously reported materials. Next improvement, necessary for photomixer devices, is that MSM contact could be also fabricated not only on the surface of photoconductive material, but also in the material. Recessed contacts exhibit higher breakdown voltage, which depends on the depth of recession. In this work we also observed that photodetectors with recessed contacts are more sensitive in comparison to MSM fabricated on the surface. Recessed contacts fabricated by wet etching and IBE are presented in Chapter 3. In both cases we observed sensitivities from 40% to 200% higher in dependence on the recession depth than for non-recessed structures. Very important fact is that recessed contacts could for optimal recession depth more efficiently collect photogenerated carriers from deeper regions and so not only increase total number of photogenerated carriers, but also in shorter time, because of decreasing of distance to the collecting electrodes. This was also observed in this work. Our first results from the photomixers with broadbanding antenna show in frequency range from 100 GHz to 1 THz approximately 2 times higher output power than photomixers with non-recessed contacts. In the future combination of here presented improvements will be used to reach next improvement of electrical properties of photodetector and photomixer devices