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ENGINEERING OHMIC CONTACTS TO III-V, III-N AND 2D DICHALCOGENIDES: THE IMPACT OF ANNEALING AND SURFACE PREPARATION ON CONTACT RESISTANCE
Restricted (Penn State Only)
Materials Science and Engineering
Doctor of Philosophy
Date of Defense:
February 09, 2017
Suzanne E. Mohney, Dissertation Advisor
Suzanne E. Mohney, Committee Chair
S. Ashok, Committee Member
Joan Redwing, Committee Member
Roman Engel-Herbert, Outside Member
A new era of smart interconnected systems of electronic devices has begun, with cloud computing (big data) and artificial intelligence amplifying the capacity and impact of these systems on human society. Behind this technological revolution lie Si-based complementary metal–oxide–semiconductor (CMOS) logic devices as well as high-power and high-frequency radio-frequency (RF) electronics, which are key enablers of the current progress. However, as we look into the future, the trend set by Moore’s law for ever smaller, higher-performance, and more power-efficient devices is reaching its limit, and soon Si may no longer serve as the material of choice in some aggressively scaled transistors. For example, InGaAs is being considered as channel materials to replace Si. Similar efforts are also underway to replace Si with GaN in RF applications. Moreover, transition metal dichalcogenides such as MoS2 are interesting for flexible electronics applications. As these new materials take hold, key processes such as contact metallization must be investigated and optimized in order to improve the performance of next-generation devices. This dissertation discusses studies of Ohmic contacts to GaN, InGaAs, and MoS2, focusing on how surface preparation and annealing affect contact resistance. During the first phase of this dissertation, we investigated Ti/Al contacts to N-polar GaN/AlGaN heterostructures. The resistance of Ti/Al-based contacts was found to depend sensitively on their interfacial composition. Limiting the thickness of the first layer deposited (either Al or Ti) to a few nanometers produced low contact resistances after annealing for 60 s at 500 °C. The lowest contact resistance of 0.10 Ω·mm (specific contact resistance, ρ_c, of 3 × 10−7 Ω·cm2) was achieved with 3 nm of Al as the first deposited layer. Cross-sectional transmission electron microscopy (TEM) studies revealed a thin Ti–Al-Ga–N layer adjacent to the GaN in this annealed Al/Ti/Al contact, while the contact resistance was higher when the interfacial layer contained only Ti, Ga, and N. The simultaneous presence of Al and Ti next to GaN at the onset of reaction was found to be critical for achieving the lowest contact resistance. Drawing from lessons learned about surface preparation and alloyed contacts to GaN, in the second phase of this dissertation, we investigated Ni-based alloyed contacts to InP-capped and uncapped n+¬-In0.53Ga0.47As (ND = 3×1019 cm−3). Contacts with specific contact resistances of 4.0 × 10−8 ± 7 × 10−9 Ω·cm2 and 4.6 × 10−8 ± 9 × 10−9 Ω·cm2 were achieved for the capped and uncapped samples, respectively, after annealing at 350 °C for 60 s. By using a pre-metallization surface treatment of ammonium sulfide, ρ_c decreased further to 2.1 × 10−8 ± 2 × 10−9 Ω·cm2 and 1.8 × 10−8 ± 1 × 10−9 Ω·cm2 on epilayers with and without 10-nm InP caps, respectively. Cross-sectional TEM micrographs revealed that the ammonium sulfide surface treatment more completely eliminated the semiconductor’s native oxide at the contact interface, which we believe caused the reduced contact resistance both before and after annealing. In the third and final phase of this dissertation, alloyed Ag contacts to few-layer (FL)-MoS2 were investigated. Similar to the two contacts investigated earlier in this dissertation, annealing was critical for achieving low contact resistance. The contact resistances of the as-deposited samples were 0.8–3.5 Ω·mm, while the annealed contacts exhibited lower contact resistances of 0.2–0.5 Ω·mm for MoS2 from 5 to 14 layers or ~3 to 9 nm thick. TEM micrographs of the annealed contacts revealed that the Ag was epitaxial on MoS2. Electron energy-loss spectroscopy (EELS) spectra collected near the Ag/MoS2 contacts showed limited interdiffusion at the metal/semiconductor interface as well as the presence of Ag in the underlying MoS2. Furthermore, thanks to the effective pre- and post-metallization surface preparation, no gross interfacial contaminants (oxygen and resist residue) appeared, indicating the formation of an intimate contact between the Ag and MoS2. Overall, this dissertation shows the importance of surface preparation and annealing in producing low-resistance Ohmic contacts to GaN, InGaAs, and MoS2. Therefore, the contact metallization processes developed here may eventually aid in the development of next-generation electronics based on these semiconductors.
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