Novel Ohmic Contact Schemes for Iii-v Semiconductors

Open Access
Author:
Yearsley, Joshua David
Graduate Program:
Materials Science and Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
July 19, 2013
Committee Members:
  • Suzanne E Mohney, Thesis Advisor
  • Roman Engel Herbert, Thesis Advisor
  • Joan Marie Redwing, Thesis Advisor
Keywords:
  • Ohmic contacts
  • metal/semiconductor contacts
  • doping
  • materials science
Abstract:
This thesis addresses a number of techniques to reduce the specific contact resistance of Ohmic contacts to lightly doped n-In0.53Ga0.47As epilayers and nanostructures. It also explores metal reactions to and electrical measurements of nanostructured n-In0.53Ga0.47As. Solid phase regrowth (SPR) contacts were used to introduce excess Si dopant to heavily dope the near-surface region of InGaAs. By optimizing the Pd/Si atomic ratio of a Pd/Si/Pd/Ti/Au contact, specific contact resistances of 9×10^−8 ± 2×10^−8 Ω-cm^2 and 1.8×10^−8 Ω-cm^2 were achieved for lightly (ND = 1×10^17 cm^−3) and heavily doped (ND = 3×10^19 cm−3) epilayer samples, respectively. These values are record lows compared to SPR contacts previously reported in literature. Cross-sectional transmission electron microscopy (XTEM) and current-voltage temperature (I-V-T) measurements supported the SPR mechanism as the predominant cause of the reduced specific contact resistance in these contacts. To further lower the specific contact resistance to lightly doped n-In0.53Ga0.47As epilayers, an ammonium sulfide surface treatment was used in conjunction with the SPR contact. This surface treatment significantly reduced the specific contact resistance to a minimum of ρc = 2.6×10^−8 ± 1.7×10^−8 Ω-cm^2. This value is only slightly higher than the lowest reported values on n-In0.53Ga0.47As reported in the literature, but is achieved on an epilayer with much lower dopant concentration (ND = 5×10^17 cm^−3) compared to the epilayers used in the literature (n > 3.5×10^19 cm^−3). The ammonium sulfide surface treatment did not reduce the specific contact resistance of a Pd/Ti/Au contact, suggesting that SPR is essential for enabling the surface treatment. XTEM showed that the morphology of this sulfur-treated SPR (S-SPR) contact did not differ from that of the SPR contact. Secondary ion mass spectrometry (SIMS) suggested but did not confirm that S from the surface treatment diffused into the InGaAs layer as a co-dopant. A Fin-TLM (transmission line method) structure was designed and fabricated to investigate the electrical properties of lightly (ND = 5×10^17 cm^−3) and heavily doped (ND = 3×10^19 cm^−3) etched n-In0.53Ga0.47As nanofins with diameters as low as 50 nm. The results of these measurements showed that the SPR contact could yield a low specific contact resistance on the heavily doped nanofins (ρc = 1.07×10−7 ± 5×10−8 Ω-cm^2) fairly close to the measured value of that contact on the respective planar layer. However, the measured specific contact resistances of SPR contacts to the lightly doped nanofins were between 1×10^−3 Ω-cm^2 and 1×10^−6 Ω-cm^2, much higher than those achieved on planar contacts to lightly doped (ND = 1.0×10^17 cm^−3) InGaAs. The extracted sheet resistances of these lightly doped samples were also much higher than those of the planar samples, suggesting that surface depletion of the nanofins affected the electrical measurements. Although depositing a thin layer of Al2O3 reduced the extracted sheet resistance and its deviation, it did not significantly reduce the measured specific contact resistance. Beyond the possibility of surface depletion, the higher contact resistance may have been caused by different reactions of the deposited metals with the different faces of the nanofin geometry. Etched n-InGaAs nanofins of different orientations with deposited Pd and Ni were annealed at 200◦C and 400◦C for 2 min to study the reaction of these metals with the nanofins. At both 200 and 400◦C, nanofins partially covered with Pd decomposed significantly, while nanofins partially covered with Ni did not. Energy-dispersive spectroscopy (EDS) maps showed no Pd diffusion down the length of the fins, suggesting a vapor- phase reaction mechanism. Auger electron spectroscopy (AES) maps of the Pd contact surface supported this hypothesis. Nanofins completely covered with Pd annealed at 200◦C did not show this decomposition behavior, but did show orientation-dependent reaction behavior. This thesis provides valuable information for hardware engineers and designers as aggressively scaled transistors continue to transition towards non-planar structures and require even lower specific contact resistances to perform at ultra-high frequencies.