Vidhya Chakrapani

Charge Transfer Equilibria Between Diamond and an Aqueous Oxygen Electrochemical Redox Couple

Undoped, high-quality diamond is, under almost all circumstances, one of the best insulators known. However, diamond covered with chemically bound hydrogen shows a pronounced conductivity when exposed to air. This conductivity arises from positive-charge carriers (holes) and is confined to a narrow near-surface region. Although several explanations have been proposed, none has received wide acceptance, and the mechanism remains controversial. Here, we report the interactions of hydrogen-terminated, macroscopic diamonds and diamond powders with aqueous solutions of controlled pH and oxygen concentration. We show that electrons transfer between the diamond and an electrochemical reduction/oxidation couple involving oxygen. This charge transfer is responsible for the surface conductivity and also influences contact angles and zeta potentials. The effect is not confined to diamond and may play a previously unrecognized role in other disparate systems.

Understanding the Role of the Sulfide Redox Couple (S2–/Sn2–) in Quantum Dot-Sensitized Solar Cells

The presence of sulfide/polysulfide redox couple is crucial in achieving stability of metal chalcogenide (e.g., CdS and CdSe)-based quantum dot-sensitized solar cells (QDSC). However, the interfacial charge transfer processes play a pivotal role in dictating the net photoconversion efficiency. We present here kinetics of hole transfer, characterization of the intermediates involved in the hole oxidation of sulfide ion, and the back electron transfer between sulfide radical and electrons injected into TiO(2) nanoparticles. The kinetic rate constant (10(7)-10(9) s(-1)) for the hole transfer obtained from the emission lifetime measurements suggests slow hole scavenging from CdSe by S(2-) is one of the limiting factors in attaining high overall efficiency. The presence of the oxidized couple, by addition of S or Se to the electrolyte, increases the photocurrent, but it also enhances the rate of back electron transfer.

Electrochemical pinning of the Fermi level: mediation of photoluminescence from gallium nitride and zinc oxide.

Charge transfer between diamond and an electrochemical redox couple in an adsorbed water film has recently been shown to pin the Fermi level in hydrogen-terminated diamond. Here we show that this effect is a more general phenomenon and influences the properties of other semiconductors when the band lineup between the ambient and electronic states in the semiconductor is appropriate. We find that the luminescent intensities from GaN and ZnO change in different, but predictable, ways when exposed to HCl and NH3 vapors in humid air. The effect is reversible and has been observed on single crystals, nanowires, flakes, and powders. These observations are explained by electron exchange between the oxygen electrochemical redox couple in an adsorbed water film and electronic states in the semiconductor. This effect can take place in parallel with other processes such as defect formation, chemisorption, and surface reconstruction and may play an important, but previously unrecognized, role when electronic and optical measurements are made in air.

Studies of Adsorbate-Induced Conductance of Diamond Surfaces

The surface conductance of hydrogen-terminated diamond was investigated under air, HCl, and NH 3 vapors using tungsten point contacts, gold/titanium ohmic contacts, and aluminum rectifying contacts. In all cases the conductance decreased with increasing pH. Upon imposition of a step temperature change, there is a transient increase in conductance followed by a gradual decrease to a steady state value. The transient conductance maxima increased with temperature. The steady state conductance decreased with increasing temperature, increased at higher humidity, and was quenched by organic liquids that wet the diamond surface. The results are in agreement with the electrochemical transfer-doping model.

Fabrication of 28nm pitch Si fins with DSA lithography

Directed Self-Assembly (DSA), as an extension of current state-of-the-art photolithography, has demonstrated the capability for patterning with resolution and cost effectiveness beyond the capability of other techniques. Previous studies of DSA have reported encouraging benchmarks in defect density and throughput capability for the patterning step, and such results provide a foundation for our ongoing efforts to integrate the DSA patterning step into a robust process for fabricating device layers. Here we provide a status report on the integration of two chemoepitaxy DSA patterning methods for the fabrication of 28nm pitch Si fin arrays. In addition to the requirements for a robust pattern transfer process, it is also important to understand the pattern design limitations that are associated with DSA. We discuss some of the challenges and opportunities associated with developing efficient device designs that take advantage of the capabilities of DSA.

Electrochemical Charging of CdSe Quantum Dots: Effects of Adsorption versus Intercalation

Effects of electrochemical charging of quantum dots (QDs) have been reported previously, wherein optical and electrical properties could be modulated through cation adsorption and electron injection into the quantum-confined 1Se states. In this work, we report two different modes of electrochemical double-layer charging in CdSe QDs and their effects on the electronic and optical properties. We show that the charging mechanism at the interface involves cation intercalation for smaller ions, such as Li+, Na+, or K+, and cation adsorption for larger bulky ions, such as tetrabutylammonium ions, where steric hindrance precludes intercalation. As a result, while cation adsorption leads to an increase in the absorbance in the mid-infrared spectral range, cation intercalation into the CdSe core results in an absorbance increase from the visible to infrared spectral range, an enhancement in radiative lifetime of e-, an increase of 158% in the intensity of band-edge photoluminescence, and strong emission in the near-infrared spectral range as a result of the formation of Se vacancies. The nature of charging mechanisms is discussed using the results of combined photoluminescence, radiative lifetime, and X-ray photoemission studies. The cation-coupled electronic and optical modulation reported here in CdSe QDs have important implications for electrochromic smart windows, photovoltaics, and other devices.

Optimization of low-diffusion EUV resist for linewidth roughness and pattern collapse on various substrates

This paper will report on our development of low diffusion EUV resists based on polymer-bound PAG technology. With our low diffusion resist, a wide process window for 30-nm hp of 280nm DOF over a 10% exposure range is achieved on a prototype ADT fullfield scanner. Linewidth roughness of 3.1nm is also achieved. Excellent resist profiles can be achieved on organic ULs or Si hardmask materials. This resist also shows only 1.1 nm carbon growth on witness plate mirrors for cleanables, and no reflectivity loss after mirror cleaning. These results clearly pass for use on all NXE exposure tools. We also have shown good pattern transfer for a Si HM stack using this resist. Finally, we report 17-nm hp resolution at a dose of 14.5mj for a higher absorption resist.

Co-adsorption of water and oxygen on GaN: Effects of charge transfer and formation of electron depletion layer

Species from ambient atmosphere such as water and oxygen are known to affect electronic and optical properties of GaN, but the underlying mechanism is not clearly known. In this work, we show through careful measurement of electrical resistivity and photoluminescence intensity under various adsorbates that the presence of oxygen or water vapor alone is not sufficient to induce electron transfer to these species. Rather, the presence of both water and oxygen is necessary to induce electron transfer from GaN that leads to the formation of an electron depletion region on the surface. Exposure to acidic gases decreases n-type conductivity due to increased electron transfer from GaN, while basic gases increase n-type conductivity and PL intensity due to reduced charge transfer from GaN. These changes in the electrical and optical properties, as explained using a new electrochemical framework based on the phenomenon of surface transfer doping, suggest that gases interact with the semiconductor surface through electrochemical reactions occurring in an adsorbed water layer present on the surface.