S. E. Mohney’s research while affiliated with Pennsylvania State University and other places

Incorporating two-dimensional transition metal dichalcogenides (TMDs) into electronic and optoelectronic applications requires a fundamental understanding of metal/TMD interactions. This work applies a fast and easy approach to observe reactivity between metal contacts and monolayer (1L) WS2 via Raman spectroscopy using both destructive and non-destructive methods. We compare findings from Raman spectra collected via a backside geometry and also from mechanically exfoliated metal/WS2 films after annealing with our previously published thermodynamic predictions for reactivity of bulk materials. The disappearance of the Raman-active phonon modes for WS2 suggests the consumption of WS2 through reactions with the continuous metal film, as observed completely for Ti upon deposition and nearly completely for Al after annealing at and above 100 °C. On the other hand, the persistence of multiple Raman-active phonon modes for WS2 confirms that Au, Cu, and Pd are unreactive with WS2 upon deposition and after cumulatively annealing for 1 h at 100, 200, and 300 °C, even though unreactive metal overlayers can shift some of the peaks in the spectrum. The metal/WS2 reactivity observed in this study is in excellent agreement with predictions from bulk thermodynamics, which can provide good guidance for studies of other metal/TMD systems. In addition, using a backside geometry for collecting Raman spectra can aid in fundamental studies of interfaces with TMDs.

Recent development of fabricating small core single-crystal silicon and germanium optical fibers using a visible laser crystallization technique is reviewed. These fibers have potential applications in fiber-based nonlinear optical devices and optoelectronic applications.

Molybdenum disulfide is a layered transition-metal dichalcogenide semiconductor that is attracting renewed attention for its potential use in future nanoscale electronics, optoelectronics, catalysis, tribology, and other fields. In all of these cases, the interaction between MoS2 and various transition metals is very important. In this work we survey the thermodynamics of the metal–Mo–S systems and the anticipated reaction products from transition metals and MoS2. We examined over 200 references on the reactions between transitions metals (M) and MoS2, compiled thermodynamic data, and used the thermodynamic data to predict M–Mo–S ternary phase diagrams for systems without experimentally determined diagrams. Where possible, experimental literature on the interactions between metals and MoS2 was used to corroborate our predicted diagrams and stable reaction products. Both the previously reported and newly predicted M–Mo–S phase diagrams fall into three categories. In the first category, the metal is in thermodynamic equilibrium with MoS2. In another, there is a driving force for the metal to reduce MoS2, with tie lines to sulfides of the contact metal dominating the phase diagram. In a final category, there is a very stable solid solution or ternary phase that dominates the phase diagram. Better understanding of the phase equilibria in the M–Mo–S systems will aid research on the use of MoS2 in a variety of fields.

Ohmic contacts to p- and n-type 4H-SiC using refractory alloyed W:Ni thin films were investigated. Transfer length measurement test structures to p-type 4H-SiC (N A = 3 × 1020 cm−3) revealed Ohmic contacts with specific contact resistances, ρ c , of ∼10−5 Ω cm2 after 0.5 h annealing in argon at temperatures of 1000 °C, 1100 °C, 1150 °C, and 1200 °C. Contacts fabricated on n-type 4H-SiC (N D = 2 × 1019 cm−3) by similar methods were shown to have similar specific contact resistance values after annealing, demonstrating simultaneous Ohmic contact formation for W:Ni alloys on 4H-SiC. The lowest ρ c values were (7.3 ± 0.9) × 10−6 Ω cm2 for p-SiC and (6.8 ± 3.1) × 10−6 Ω cm2 for n-SiC after annealing at 1150 °C. X-ray diffraction shows a cubic tungsten–nickel–carbide phase in the Ohmic contacts after annealing as well as WC after higher temperatures. Auger electron spectroscopy depth profiles support the presence of metal carbide regions above a nickel and silicon-rich region near the interface. X-ray energy dispersive spectroscopy mapping showed tungsten-rich and nickel-rich regions after annealing at 1100 °C and above. W:Ni alloys show promise as simultaneous Ohmic contacts to p- and n-SiC, offering low and comparable ρ c values along with the formation of Wx Niy C.

Ohmic contacts with extremely low specific contact resistances and carefully controlled morphologies are required for III-V digital logic and heterojunction bipolar transistors. Shallow, low-resistance Ohmic contacts to InGaAs with specific contact resistances near 10−8 Ω-cm2 have been achieved by heavily doping the semiconductor and subsequently depositing a relatively unreactive metal contact. However, the preparation of the InGaAs surface before deposition of the contact can greatly affect the contact resistance. In this work, palladium is shown to be particularly effective for making the contact less sensitive to the preparation of the semiconductor surface, as demonstrated using both electrical characterization of contacts to n- and p-type InGaAs and transmission electron microscopy. Furthermore, a properly chosen contact stack can induce the solid-phase regrowth (SPR) mechanism upon annealing, resulting in a shallow, heavily doped region beneath the contact and a low specific contact resistance, even on an originally lightly doped semiconductor. Hence, more flexibility is provided to the materials growth and device fabrication processes. We achieved a specific contact resistance of 9×10−8 Ω-cm2 using a Pd/Si-based contact to n-type InGaAs with ND =1×1017 cm−3. When an ammonium sulfide surface treatment was used in conjunction with SPR, we achieved a specific contact resistance of 2.6×10−8±1.7×10−8 Ω-cm2 for the same contact on lightly doped n-type InGaAs (ND =5×1017 cm−3). Finally, scaling of transistors has forced a transition to non-planar device architectures, such as fin field-effect transistors (FinFETs) and gate-all-around devices. Thus, it is critical that we investigate the formation of Ohmic contacts in these new geometries. In addition to the results for planar contacts, we also report effects of geometry on the reactions of contacts to InGaAs nanostructures.

Laser processing is a single step, attractive alternative to current multi-step formation of ohmic contacts between an aluminum metallization layer and a silicon substrate in solar cell devices. However, small changes in laser parameters such as pulse duration, power density and laser wavelength can result in significant differences in the contact geometry and electrical properties. Here, the effects of power density and pulse duration on the morphology, resistance and surface concentration of laser fired contacts (LFCs) are examined experimentally. The minimum fluence threshold for forming a contact with measureable resistance through the 100nm SiO2 layer is determined to be 8 J/cm2. In addition, when forming the contact, an outer rim region accumulates on the surface that is comprised of aluminum and silicon. As a result, the entire contact is actually governed by the size of an inner crater region plus this outer rim material, which is in contrast to results reported in the literature for nanosecond pulse durations. These results are in good agreement with independent results reported in the literature for LFCs processed on wafers with substantially different base resistivity and using significantly different processing parameters.

Multilayer ohmic contacts with differing first metal layers (M = Mo, Pd, Pt) beneath a Ti/Pt diffusion barrier and Au cap were fabricated on n+ and p+-InGaAs, and the relationship between their specific contact resistance and interfacial chemistry was examined. Palladium-based contacts offered the lowest specific contact resistances of ρc = 3.2×10−8 and 1.9×10−8 Ω-cm2 to n+- and p+-InGaAs, respectively. The low resistances of the Pd-based contact were correlated with the formation of a uniform PdxInGaAs phase in direct contact with InGaAs, as observed using transmission electron microscopy and energy dispersive spectroscopy. On the other hand, the Mo-based contact to n+ and p+-InGaAs had much higher specific contact resistances, even though its specific contact resistance on lightly doped n-InGaAs was nearly the same as that of the Pd-based contact. The cause of this discrepancy was identified to be the native oxide layer that remained between the contact and semiconductor in the Mo-based contacts, as revealed using transmission electron microscopy, energy dispersive spectroscopy, and electron energy loss spectroscopy. The effect of the native oxide in series with the Schottky barrier is significant only when the contact is doped heavily enough that tunneling of carriers through the Schottky barrier introduces a very low resistance.

Characterization of Zn1−xMnxSe nanowires grown by Au-catalyzed chemical vapor deposition (CVD) using transmission electron microscopy is reported. The nanostructures were found to grow in the form of nanowires, nanoribbons, and nanosaws. The fraction of manganese on the cation sublattice in the nanowires varied from x=0.1 to x=0.28. No correlation between Mn fraction and nanowire diameter was found for straight nanowires; however, in tapered nanowires the Mn fraction was found to increase as the diameter increased. The tapering, as well as the increase in Mn fraction, is likely caused by thin film deposition with a higher Mn fraction on the sides of the nanowires. Some of the nanoribbons were found to display extra diffraction spots at half-order positions in selected area diffraction. The half-order reflections were found to have originated from spontaneous atomic ordering of Mn and Zn on planes on the cation sublattice.