Optimization and Characterization of Vanadium-Based Contacts to n-Type Aluminum Gallium Nitride

Open Access
Miller, Mary A.
Graduate Program:
Materials Science and Engineering
Doctor of Philosophy
Document Type:
Date of Defense:
November 30, 2007
Committee Members:
  • Suzanne E Mohney, Committee Chair
  • Joan Marie Redwing, Committee Member
  • Joseph Flemish, Committee Member
  • Jian Xu, Committee Member
  • James Patrick Runt, Committee Member
  • ohmic contacts
  • Vanadium
  • AlGaN
  • LEDs
  • HEMTs
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, Al<sub>x</sub>Ga<sub>1-x</sub>N, 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 Al<sub>x</sub>Ga<sub>1-x</sub>N compositions. Although conventional ohmic contacts to n-type Al<sub>x</sub>Ga<sub>1-x</sub>N 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 Al<sub>x</sub>Ga<sub>1-x</sub>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 Al<sub>x</sub>Ga<sub>1-x</sub>N 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 Al<sub>x</sub>Ga<sub>1-x</sub>N composition. The first is on ohmic contacts to n-Al<sub>0.58</sub>Ga<sub>0.42</sub>N 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 Al<sub>0.27</sub>Ga<sub>0.73</sub>N/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<sub>x</sub>Ga<sub>1-x</sub>N, 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 Al<sub>x</sub>Ga<sub>1-x</sub>N. Contacts to n-Al<sub>0.58</sub>Ga<sub>0.42</sub>N are the first metallizations to be described in this thesis. The aluminum nitride-rich Al<sub>x</sub>Ga<sub>1-x</sub>N is important for bottom-emitting ultraviolet light emitting diodes due to its wide, direct band gap. As x increases in Al<sub>x</sub>Ga<sub>1-x</sub>N 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-Al<sub>0.58</sub>Ga<sub>0.42</sub>N is plasma-etched with a BCl<sub>3</sub>/Cl<sub>2</sub>/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-Al<sub>0.58</sub>Ga<sub>0.42</sub>N and compared to metallizations contacted to the plasma-etched material. When compared on the as-received n-Al<sub>0.58</sub>Ga<sub>0.42</sub>N, 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 Al<sub>0.58</sub>Ga<sub>0.42</sub>N 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-Al<sub>0.58</sub>Ga<sub>0.42</sub>N, 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<sub>0.58</sub>Ga<sub>0.42</sub>N. 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-Al<sub>0.58</sub>Ga<sub>0.42</sub>N. 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 Al<sub>0.27</sub>Ga<sub>0.73</sub>N/GaN heterostructures with a thin GaN cap. Through a combinatorial approach to the optimization of metal layer thicknesses, a contact with a low specific contact resistance as well as a smooth morphology was developed. Some V/Al/V/Au and Ti/Al/Ti/Au contacts were fabricated for comparison; however, neither provided as good properties as those of the V/Al/V/Ag contacts. Cross-sectional TEM images of the V/Al/V/Ag contact to the Al<sub>0.27</sub>Ga<sub>0.73</sub>N/GaN heterostructure revealed some interesting results. Large grains of V-Al, Ag and V-Al-Ag are identified in the annealed metallization as well as a composite of several smaller grains of V-Al, Ag, V-Al-Ag, Al-O and Al-Ag. The composite contacts the Al<sub>0.27</sub>Ga<sub>0.73</sub>N layer. The majority of the composite is comprised of Ag-bearing phases. Since Ag has a lower work function than Au, it is likely that the work functions of the metal phases at the interface are lower than phases present in an analogous Au-bearing metallization. One of the most interesting results of the V/Al/V/Ag contact, however, is that the annealed metallization leaves the Al<sub>0.27</sub>Ga<sub>0.73</sub>N layer intact after annealing. In contrast, cross-sectional images of both of the optimized Au-bearing metallizations to the heterostructure show penetration of the 2DEG channel. Finally, the V/Al/V/Ag metallization is contacted to both n-GaN and n-Al<sub>0.58</sub>Ga<sub>0.42</sub>N, due to the superior properties observed on the Al<sub>0.27</sub>Ga<sub>0.73</sub>N/GaN heterostructures. Promising results were obtained for both semiconductor compositions. Cross-sectional TEM imaging of the V/Al/V/Ag metallization showed very similar phase formation, regardless of the composition of the semiconductor. Also, little change in the smooth morphology of the metallization occurred between V/Al/V/Ag contacts to n-GaN and n-Al<sub>0.58</sub>Ga<sub>0.42</sub>N. Specific contact resistance was also studied as a function of temperature. Curves that modeled thermionic emission, thermionic field emission and field emission dependence were matched against experimental data for all optimized contacts to n-GaN, Al<sub>0.27</sub>Ga<sub>0.73</sub>N/GaN heterostructures, and n-Al<sub>0.58</sub>Ga<sub>0.42</sub>N. Determination of doping density ranges and Schottky barrier heights is discussed. Ohmic contacts to n-GaN and n-Al<sub>0.58</sub>Ga<sub>0.42</sub>N were determined to have field emission dependence. Although experimental data for ohmic contacts to Al<sub>0.27</sub>Ga<sub>0.73</sub>N/GaN heterostructures appear to show thermionic field emission dependence on temperature, the data could not be modeled due to the presence of two barriers to current transport that may exhibit temperature dependence: the metal/semiconductor interface and the Al<sub>0.27</sub>Ga<sub>0.73</sub>N/GaN interface. The V/Al/V/Au, Ti/Al/Ti/Au, and V/Al/V/Ag metallizations were tested on three different Al<sub>x</sub>Ga<sub>1-x</sub>N compositions. Of the three metallizations, the V/Al/V/Ag metallization shows the most promising properties. Minimal changes in the metal layer thickness, annealing temperature and time were necessary to optimize the V/Al/V/Ag metallization to a range of compositions of Al<sub>x</sub>Ga<sub>1-x</sub>N. The V/Al/V/Ag metallization showed less of a dependence of metal layer thickness than the Au-bearing metallizations, and throughout the study, lateral diffusion of the metallization was not observed. A limited sensitivity to layer thickness makes the V/Al/V/Ag metallization more robust to processing variations. The V/Al/V/Ag metallization provides a reliable, low contact resistance and smooth morphology metallization for ohmic contacts to n-Al<sub>x</sub>Ga<sub>1-x</sub>N from x = 0 – 0.58.