Marcie R. Black

Chapter 1 – Quantum Wells and Quantum Wires for Potential Thermoelectric Applications

In this chapter, the predictions of an enhancement in the thermoelectric figure of merit of low-dimensional material systems relative to their corresponding bulk counterparts, as well as the present state of experimental confirmation of these predictions, are reviewed. Progress with specific quantum well, quantum wire, and quantum dot materials systems, such as the lead salts, Si-Ge, and bismuth is discussed. To date most of the effort has gone into proof-of-principle studies, though actual demonstration of the highest thermoelectric figure of merit ( Z 3D T ) of any material to date has been seen in the low-dimensional system PbSe 0.98 Te 0.02 -PbTe, where the enhancement is attributed to quantum dot formation associated with the interface between the PbTe and PbSe 0.98 Te 0.02 . In this chapter, particular attention is given to a discussion of the structure and properties of bismuth quantum wires, which are still at an early state of research. Bismuth nanowires, however, offer significant promise for practical applications, because they can be self-assembled and are predicted to have desirable thermoelectric properties when they have wire diameters in the 5- to 10-nm range. Though temperature-dependent resistance measurements have been carried out for Bi nanowires in this diameter range, reliable thermoelectric measurements have not yet been reported.

Low‐loss polycrystalline silicon waveguides for silicon photonics

Photonic integrated circuits in silicon require waveguiding through a material compatible with silicon very large scale integrated circuit technology. Polycrystalline silicon (poly‐Si), with a high index of refraction compared to SiO2 and air, is an ideal candidate for use in silicon optical interconnect technology. In spite of its advantages, the biggest hurdle to overcome in this technology is that losses of 350 dB/cm have been measured in as‐deposited bulk poly‐Si structures, as against 1 dB/cm losses measured in waveguides fabricated in crystalline silicon. We report methods for reducing scattering and absorption, which are the main sources of losses in this system. To reduce surface scattering losses we fabricate waveguides in smooth recrystallized amorphous silicon and chemomechanically polished poly‐Si, both of which reduce losses by about 40 dB/cm. Atomic force microscopy and spectrophotometry studies are used to monitor surface roughness, which was reduced from an rms value of 19–20 nm down to ab...

Losses in polycrystalline silicon waveguides

The losses of polycrystalline silicon (polySi) waveguides clad by SiO2 are measured by the cutback technique. We report losses of 34 dB/cm at a wavelength of 1.55 μm in waveguides fabricated from chemical mechanical polished polySi deposited at 625 °C. These losses are two orders of magnitude lower than reported absorption measurements for polySi. Waveguides fabricated from unpolished polySi deposited at 625 °C exhibit losses of 77 dB/cm. We find good agreement between calculated and measured losses due to surface scattering.

Intersubband transitions in bismuth nanowires

Optical absorption associated with the one-dimensional joint density of states of an intersubband transition in bismuth nanowires is reported. The previously observed strong absorption in bismuth nanowires at ∼1000 cm−1 is here shown to depend on the wire diameter and on the polarization of the incident light. The absorption line shape, the decreasing frequency with increasing wire diameter, and the polarization dependence of the reflectivity, all indicate that this resonance is due to an intersubband absorption resulting from quantum-confinement effects.

Bismuth Nanowires for Potential Applications in Nanoscale Electronics Technology

Nanowires of bismuth with diameters ranging from 10 to 200 nm and lengths of 50 microm have been synthesized by a pressure injection method. Nanostructural and chemical compositional studies using environmental and high resolution transmission electron microscopy with electron stimulated energy dispersive X-ray spectroscopy have revealed essentially single crystal nanowires. The high resolution studies have shown that the nanowires contain amorphous Bi-oxide layers of a few nanometers on the surface. In situ environmental high resolution transmission electron microscopy (environmental-HRTEM) studies at the atomic level, in controlled hydrogen and other reducing gas environments at high temperatures demonstrate that gas reduction can be successfully applied to remove th oxide nanolayers and to maintain the dimensional and structural uniformity of the nanowires, which is key to attaining low electrical contact resistance.

New Directions for Low Dimensional Thermoelectricity

Low dimensionality provides opportunities to modify the properties of bulk materials dramatically and to control materials properties independently in a manner that is not possible for bulk materials. The special characteristics of low dimensional materials to enhance thermoelectric performance have already been demonstrated in quantum wells, quantum wires and quantum dots. The main focus of this review is a summary of advances made in the modeling of quantum dot superlattice nanowires. Several new research directions for low dimensional thermoelectricity or inspired by this research are briefly mentioned.

Thermoelectric transport properties of single bismuth nanowires

We present a novel technique for measuring the power factor (Seebeck coefficient and resistivity) of a single isolated Bi nanowire. Electron beam lithography is used to pattern electrodes on top of a 40 nm diameter Bi nanowire along with a microscopic heater and thermocouples. Running current through the heater generates a temperature gradient of 0.5 K over a distance of 10 /spl mu/m across the nanowire. Measurement of the Seebeck voltage of the nanowires was not possible due to the highly resistive and non-ohmic contacts. The non-linearity in the i(V) characteristics of the electrical contacts are understood by detailed modeling of the tunneling mechanism made through the electrical contacts. The prospect of using the poor contacts to perform tunneling spectroscopy of the electronic density of states of the nanowires is evaluated using this model.

Conventionally-processed silicon nanowire solar cells demonstrating efficiency improvement over standard cells

We fabricate silicon nanowire solar cells which, other than the texturing step, are processed by conventional silicon solar cell manufacturing equipment. The nanowires reduce reflection compared to standard multicrystalline cells, leading to higher short-circuit current. A less anticipated benefit is that the nanowires can also increase open-circuit voltage and fill factor. Compared to standard multicrystalline cells processed at the same time, our nanowire cells demonstrate a 0.4% efficiency improvement when only changing the texturing step and a 0.6% efficiency improvement when also adjusting the diffusion step. Our highest nanowire cell efficiency is 18.0% on a multicrystalline wafer, and 19.0% on a monocrystalline wafer.

4-Point Resistance Measurements of Individual Bi Nanowires

Abstract : We have synthesized single crystal bismuth nanowires by pressure injecting molten Bi into anodic alumina templates. By varying the template fabrication conditions nanowires with diameters ranging from 10 to 200nm and lengths of approx. 50 microns can be produced. We present a scheme for measuring the resistance of a single Bi nanowire using a 4-point measurement technique. The nanowires are found to have a 7nm thick oxide layer which causes very high contact resistance when electrodes are patterned on top of the nanowires. The oxide is found to be resilient to acid etching, but can be successfully reduced in high temperature hydrogen and ammonia environments. The reformation time of the oxide in air is found to be less than 1 minute. Focused ion beam milling is attempted as an alternate solution to oxide removal.

Overview of Bismuth Nanowires for Thermoelectric Applications

The goal of this workshop on thermoelectric materials “Beyond Bismuth Telluride” was to inspire researchers in the thermoelectrics field to think boldly about the future of Thermoelectrics Science and Technology and to identify what it would take to make a big step forward in this research area. The field of thermoelectrics advanced rapidly in the 1950s when the basic science of thermoelectric materials became well established, the important role of heavily doped semiconductors as good thermoelectric materials became accepted, the thermoelectric material bismuth telluride was discovered and developed for commercialization, and the thermoelectrics industry was launched. At that time it was established that the effectiveness of a thermoelectric material could in an approximate way be described in terms of the dimensionless thermoelectric figure of merit, ZT= S 2σT/κ where S, σ Tand κ are the Seebeck coefficient, the electrical conductivity, the temperature and the thermal conductivity. Over the following 3 decades 1960–1990, only incremental gains were made in increasing ZT, with Bi2Te3remaining the best commercial material at ZT≈ 1. During that 3 decade period, the thermoelectrics field received little attention from the worldwide scientific research community.lNevertheless the thermoelectrics industry grew slowly but steadily, by finding niche applications for space missions, laboratory equipment, and medical applications, where cost and efficiency were not as important as energy availability, reliability, and predictability.