Optimization and characterization of vanadium-based contacts to nitrogen-type aluminum gallium nitride

With improvements in growth and processing of nitride-based devices, there has been an increasing need for low resistance ohmic contacts to wide band gap nitrides. The semiconductor, AlxGa1-xN, provides a material system suitable for a number of applications, provided for by adjustment of the AlN mole fraction. Thus, there is a need for a universal metallization that can consistently produce low specific contact resistances for a range of AlxGa1-xN compositions. Although conventional ohmic contacts to n-type AlxGa1-xN are based on four-layer Ti-based metallizations, recently our research group has shown that V-based contacts outperform Ti-based contacts as the x in AlxGa1-x N increases. When the identity of the third layer in a V/Al/X/Au layer structure was studied, where X is a transition metal, the V/Al/V/Au contact on AlN-rich AlxGa1-xN showed promise over the other V-based metallizations by providing low specific contact resistances at annealing temperatures as low as 700°C. Up until this work, research based on V-based contacts was limited and further optimization of this metallization was needed. This thesis is split into three sections based on AlxGa1-x N composition. The first is on ohmic contacts to n-Al0.58Ga 0.42N for bottom-emitting ultraviolet light emitting diodes (UV LEDs). The V- and Ti-based metallizations are optimized on both as-received and plasma-etched material, and differences between the metallization/semiconductor interactions are explored. The second section reports on contacts to Al0.27Ga 0.73N/GaN heterostructures for high electron mobility transistors. The last section details a newly developed V/Al/V/Ag contact to n-GaN and n-Al xGa1-xN, which explores the effect replacing the traditional Au cap with a different metal. Throughout the thesis, the focus remains on V-based metallizations and their adaptation to different compositions and conditions of AlxGa1-xN.;Contacts to n-Al0.58Ga0.42N are the first metallizations to be described in this thesis. The aluminum nitride-rich AlxGa 1-xN is important for bottom-emitting ultraviolet light emitting diodes due to its wide, direct band gap. As x increases in AlxGa 1-xN to facilitate shorter emission wavelengths, it may become more difficult to form low resistance ohmic contacts to the semiconductor. Since the nitride material often requires a sapphire substrate, both n- and p-type contacts are made to the same side of the LED device and plasma etching is required to expose the buried n-type layer. A blanket layer of n-Al0.58 Ga0.42N is plasma-etched with a BCl3/Cl2 /Ar etch chemistry to mimic conditions under which bottom-emitting UV LEDs may be processed. Several V/Al/V/Au and Ti/Al/Ti/Au contacts are tested to the as-received n-Al0.58Ga0.42N and compared to metallizations contacted to the plasma-etched material.;When compared on the as-received n-Al0.58Ga0.42N, the V-based contacts provided lower specific contact resistances at lower temperatures than the Ti-based contacts. Transmission electron microscopy (TEM) showed that for the annealed V-based contacts, limited reaction with the Al0.58Ga0.42N occurs. With the Ti-based contact, the reaction region is deep, penetrating 50 nm into the semiconductor. Interestingly, the V-based metallization does not require deep reaction depths in order to provide a low resistance contact.;On the plasma-etched n-Al0.58Ga0.42N, the specific contact resistances of V- and Ti-based contacts were more similar. Cross-sectional TEM uncovered similarities in reaction depth and phase formation. Both metallizations exhibited limited reaction depths, unlike Ti-based contacts as-received Al 0.58Ga0.42N. Both metallizations formed thin layers of aluminum nitride at the interface. The crystalline aluminum nitride was penetrated by metal channels, which connect the top part of the annealed metallization to the plasma-etched n-Al0.58Ga0.42N. The metal channels differ in composition, size and density between the V- and Ti-based contacts. The metal channels were comprised of only the Al-Au phase for the Ti-based contacts, while they were made of V-Al-Au-N and Al-Au phases for the V-based contacts. The importance of the aluminum nitride layer is discussed.;Low resistance ohmic contacts are also important for efficient high electron mobility transistors. Here, ohmic contacts are also required to exhibit smooth morphology and contact edges. A V/Al/V/Ag metallization has been developed for ohmic contacts to Al0.27Ga0.73N/GaN heterostructures with a thin GaN cap. (Abstract shortened by UMI.)...