ABSTRACT
Dub M. M. Investigation of heat-resistant ohmic contacts to semiconductor diamond. – Manuscript.
The Ph. D thesis for a scientific degree of the doctor of philosophy, field 172 – telecommunications and radio engineering. V. E. Lashkaryov Institute of Semiconductor Physics NAS of Ukraine, Kyiv, 2020.
The dissertation is devoted to the research of multilayer heat-resistant ohmic contacts to diamond, as semiconductor material perspective for high-frequency transistors, powerful microelectronic devices and extreme electronic devices (X-ray and nuclear radiation detectors, temperature- and radiation-resistant microelectronic devices). Сreation and future commercialization of these microelectronic devices is possible only under the condition of formation of the reliable ohmic contacts.
From the literature data analysis, it was established that the most common is boron doping. This is due to the relatively low activation energy of boron ≈ 0.37 eV as compared to nitrogen ≈ 1.7 eV, high values of mobility and solubility in diamond. After comparing the contact resistances for the same metallization type for poly- and monocrystalline films, it was found that at low doping levels > 1018 cm–3, monocrystalline films have two orders of magnitude lower contact resistance. With increasing the concentration, this difference is leveled. Taking into account the contact resistance value from the doping level and annealing temperature, it is established that the most promising contact-forming layer is titanium, which in the process of heat treatment allows forming the TiC phases with low resistivity.
We investigated the morphological properties of the formed contacts. Auger profilometry and X-ray difflactometry were performed for contact structures before and after thermal treatment. It was found that the Au/Ti/C contact system has a significant oxygen content before and after thermal annealing, which may indicate vacuum problems during the contact deposition. After annealing, the oxygen content in the contact was increased significantly due to the diffusion of titanium through Au layer. As a result, the degradation of the subcontact region and as a consequence of ohmic contact is observed, when the annealing temperature increased to 800 °С. Aforementioned can be explained by the significant oxygen content in the contact area. As a result of X-ray diffractometry, the peaks of the TiOx and TiC phases were not observed before and after annealing. It is possible to assume that these composites are in an amorphous or nanocrystalline state.
The received method for optimizing titanium contact layer using an additional Mo anti-diffusion barrier. Optimization of the contact layer Ti allows to form a thin layer with a ratio of Ti:C equal to 1:0.96 after rapid thermal annealing at 800 °C. It is ascertained that in the cases of two-layer metallization of Ti/Au, Au contact layer with a thickness of 100 nm is not enough to avoid the diffusion of carbon to the surface, and oxygen - deep into the contact. As a result, when the RTA is above 600 °C, the contact degrades. An additional diffusion barrier based on a 60 nm thick prevents contact degradation up to 800 °C.
We investigated the electrophysical properties and current transfer mechanisms of ohmic contact. It was established that the ohmic contact in the Au/Ti/C and Au/Mo/Ti/C structures is formed by magnetron sputtering on a diamond wafer heated to 350 °C. For Au/Ti/C contact metallization the optimal annealing temperature is 600 °C. The contact resistance in this case is ρc = 6.3·10–3 Ohm∙cm2 at the doping concentration range Na = 2.5·1019 cm–3. There is the degradation of the contact and as a consequence of the increase of the contact resistance after increasing of the annealing temperature. We demonstrated possibility of utilizing a radial ohmic contact as integrated thermal sensor. The conductivity in the temperature range 20–400 °C varies by two orders of magnitude at low applied voltages, which is sufficient for the operation of protection circuits.
Fabricated contact structure of Au/Mo/Ti/C was characterized by the increased thermal stability of contacts up to 800 °C. The contact resistance in this case is ρc = (6.12 ± 3.78)·10–5 Ohm∙cm2 at a doping concentration of Na = 1.3·1019 to 6.6·1019 cm–3. We analyzed the obtained results in comparison with the already existing metallizations contacts and confirmed the prospects of the created metallization.
The heat-resistant ohmic contact system Au/Mo/Ti to diamond was used in the formation of microstrip transmission lines of hybrid microwave integrated circuits on diamond substrates of high thermal conductivity, which operated in the temperature range 20–400 °C. The loss-angle tangent is not more than 10–4. Operation of transposition line at temperatures of 25–400 °C showed the stability of the impedance device characteristics.
Key words: ohmic contact, diamond, rapid thermal annealing, hybrid integrated circuit.
