Low resistance metal semiconductor contacts : low power nano-electronics and sensing

PhD ThesisMetal semiconductor (MS) contacts are essential in nearly every electronic device. High electrical contact resistance degrades device performance, especially at smaller device geometries. The contact resistance normally scales inversely with the cross-sectional area of the MS contact, and this results in poor electrical conduction in small geometries. Additionally, experiments confirm that surface effects dominate over bulk properties, especially at nanoscale geometries. These conditions impose several restrictions in implementing various device technologies. The electronic properties of metal-semiconductor contacts in some important semiconductors such as Si, Ge, GaAs, among others are found to be largely insensitive to the metal workfunction and semiconductor doping level, due to a phenomenon called Fermi level pinning (FLP). FLP can severely degrade device performance, and creates several fabrication challenges. Many semiconductors lose their applicability in mainstream electronics due to restrictions imposed by this effect. FLP effects are practically observed in many semiconductors doped below 1019 cm−3 and are most pronounced in lightly doped and (~intrinsic) pure crystals. This thesis explores material engineering methods to improve contact to semiconductors, without resorting to heavy doping. Large area metal contacts (length/ diameter (d)~ 50-300 μm) are fabricated on Si and Ge. Three key approaches are investigated: (1) Modifying interface dipoles and blocking Metal Induced Gap States (MIGS) using ~ nm thick charged oxide interlayers, implementing planar metal interlayer semiconductor (MIS) contacts (Chapter 4). (2) Exploiting geometric field enhancement in nanostructured hybrid contacts (Chapter 5) and (3) Exploiting voltage controlled non-equilibrium electron heating in island metal films. The contacts produced by these methods (2) and (3) are the first experimental demonstrations to show that limitations imposed by FLP can be overcome by modifying the contact material geometry alone, without using heavy doping. Applying mV range bias to these metallizations causes hot carrier emission from these contact’s nanostructured surfaces. Hot carriers are non-equilibrium, energetic carriers that easily overcome the FLP effect in the semiconductor. High conductivity is observed due to the hot carrier effect over a broad range of temperatures –from 4.2 K, tested up to 500 K- despite using low doping in the semiconductor (ND ~ 6.4 × 1014 cm−3). Novel transport processes are revealed by hot carrier tunnelling and emission mechanisms, which improve conductivity in semiconductors, and will potentially be applicable to other low dimensional materials as well. The results in Chapter 5 show an interesting demonstration of hot carrier edge scaling current injection used to achieve Ohmic contact to low doped n-Ge. This contact scheme presents a ii promising alternative to improving conductivity extrinsically, without using heavy doping, and in a scalable manner. Chapter 6 also contains a proof of concept demonstration. It is shown that closely spaced networks of metal nano-islands of critical dimensions are susceptible to non-equilibrium electron heating, when they receive power in the form of voltage controlled tunnel current. This leads to elevated electron temperatures (~103 K) relative to a cold lattice (at ambient temperature). Hot carriers easily overcome small (few eV) electrostatic barriers e.g. Schottky barrier. Consequently, Ohmic conduction is observed at room temperature, and near ballistic hot carrier conduction is observed at 4.2 K through the entire low doped wafer (thickness 0.5 mm, ND ~ 6.4 × 1014 cm−3). The wide scope of these findings may find promising applications in nanoelectronic engineering and applied science. There is considerable incentive to continue the research, and obtain a wider range of materials capable of similar effects, described further in the thesis outlook (Chapter 7). Advancing this research further will translate to applications in high speed switching, sensing, optoelectronics and energy harvesting. It is anticipated that these technologies will be applicable to many semiconductors and can be adapted into heterostructures, using advanced fabrication methods

Voltage Controlled Hot Carrier Injection Enables Ohmic Contacts Using Au Island Metal Films on Ge

We introduce a new approach to creating low-resistance metalsemiconductor ohmic contacts, illustrated using high conductivity Au island metal films (IMFs) on Ge, with hot carrier injection initiated at low applied voltage. The same metallization process simultaneously allows ohmic contact to n-Ge and p-Ge, because hot carriers circumvent the Schottky barrier formed at metal/n-Ge interfaces. A 2.5x improvement in contact resistivity is reported over previous techniques to achieve ohmic contact to both n- and p- semiconductor. Ohmic contacts at 4.2 K confirm nonequilibrium current transport. Self-assembled Au IMFs are strongly orientated to Ge by annealing near the Au/Ge eutectic temperature. Au IMF nanostructures form, provided the Au layer is below a critical thickness. We anticipate that optimized IMF contacts may have applicability to many material systems. Optimizing this new paradigm for metalsemiconductor contacts offers the prospect of improved nanoelectronic systems and the study of voltage controlled hot holes and electrons.Peer reviewe

Electroless Nickel Deposition:An Alternative for Graphene Contacting

We report the first investigation into the potential of electroless nickel deposition to form ohmic contacts on single layer graphene. To minimize the contact resistance on graphene, a statistical model was used to improve metal purity, surface roughness, and coverage of the deposited film by controlling the nickel bath parameters (pH and temperature). The metalized graphene layers were patterned using photolithography and contacts deposited at temperatures as low as 60 °C. The contact resistance was 215 ± 23 ω over a contact area of 200 μm × 200 μm, which improved upon rapid annealing to 107 ± 9 ω. This method shows promise toward low-cost and large-scale graphene integration into functional devices such as flexible sensors and printed electronics

The Busboy Problem: Efficient Tableware Decluttering Using Consolidation and Multi-Object Grasps

We present the “Busboy Problem”: automating an efficient decluttering of cups, bowls, and silverware from a planar surface. As grasping and transporting individual items is highly inefficient, we propose policies to generate grasps for multiple items. We introduce the metric of Objects per Trip (OpT) carried by the robot to the collection bin to analyze the improvement seen as a result of our policies. In physical experiments with singulated items, we find that consolidation and multi-object grasps resulted in an 1.8x improvement in OpT, compared to methods without multi-object grasps. See https://sites.google.com/berkeley.edu/busboyproblem for code and supplemental materials

The Busboy Problem: Efficient Tableware Decluttering Using Consolidation and Multi-Object Grasps

We present the "Busboy Problem": automating an efficient decluttering of cups, bowls, and silverware from a planar surface. As grasping and transporting individual items is highly inefficient, we propose policies to generate grasps for multiple items. We introduce the metric of Objects per Trip (OpT) carried by the robot to the collection bin to analyze the improvement seen as a result of our policies. In physical experiments with singulated items, we find that consolidation and multi-object grasps resulted in an 1.8x improvement in OpT, compared to methods without multi-object grasps. See https://sites.google.com/berkeley.edu/busboyproblem for code and supplemental materials

Assessment of commercially available bone graft material in the implant placed socket to enhance the osteointegration

Background: Alveolar bone resorption occurs in the majority of patients following teeth extraction. The present study was conducted to assess commercially available bone graft material in the implant placed socket to enhance the osteointegration. Materials & Methods: The present study comprised of 64 patients of both genders. Concentrated growth factor (CGF) was prepared according to Sacco’s protocol, using the patients’ own venous blood. Extraction of mandibular first molars was carried out and implants were immediately placed with CGF grafting. A Cone Beam Computed Tomography (CBCT) was taken immediately after implant placement and after six months of undisturbed healing to assess the quantity and quality of new bone formed around implants. Results: Out of 64 patients, males were 40 and females were 24. The mean bone height was on buccal side immediately was 9.04 and after 6 months was 11.3, on lingual side immediately was 10.6 and after 6 months was 11.8, on distal side immediately was 8.3 and after 6 months was 11.0, on mesial side immediately was 7.5 and after 6 months was 11.2. The difference was significant (P< 0.05).&nbsp