Lucile C. Teague

Contact-induced crystallinity for high-performance soluble acene-based transistors and circuits

The use of organic materials presents a tremendous opportunity to significantly impact the functionality and pervasiveness of large-area electronics. Commercialization of this technology requires reduction in manufacturing costs by exploiting inexpensive low-temperature deposition and patterning techniques, which typically lead to lower device performance. We report a low-cost approach to control the microstructure of solution-cast acene-based organic thin films through modification of interfacial chemistry. Chemically and selectively tailoring the source/drain contact interface is a novel route to initiating the crystallization of soluble organic semiconductors, leading to the growth on opposing contacts of crystalline films that extend into the transistor channel. This selective crystallization enables us to fabricate high-performance organic thin-film transistors and circuits, and to deterministically study the influence of the microstructure on the device characteristics. By connecting device fabrication to molecular design, we demonstrate that rapid film processing under ambient room conditions and high performance are not mutually exclusive.

Probing stress effects in single crystal organic transistors by scanning Kelvin probe microscopy

We report scanning Kelvin probe microscopy (SKPM) of single crystal difluoro bis(triethylsilylethynyl) anthradithiophene (diF-TESADT) organic transistors. SKPM provides a direct measurement of the intrinsic charge transport in the crystals independent of contact effects and reveals that degradation of device performance occurs over a time period of minutes as the diF-TESADT crystal becomes charged.

Wavelength dependence on the space charge collection in CdZnTe detectors

The distribution of the internal electric field in Cd1−xZnxTe (CZT) materials has significant effects on the charge collection ability. Light exposure at various wavelengths is a relatively unexplored process that alters charge collection at the anode contact. The use of multiple wavelengths can target charge carriers at various trap energies and positions throughout the crystal. The controlled illumination increases charge collection by releasing trapped electron and hole carriers in the crystal despite differences in light energy. Our study presents the results from our investigation of the effect of external illumination of CZT on the internal electric field via the Pockels effect. The space charge collection is further analyzed based on location and intensity relative to the specific wavelength of illumination.

Effect of sub-bandgap illumination on the internal electric field of CdZnTe

Post-growth manipulation of the internal electric field in CdZnTe crystals using sub-bandgap illumination is measured as a function of temperature through infrared (IR) transmission measurements. Using near sub-bandgap IR illumination, both the optical de-trapping of charge carriers and the reduction in carrier recombination increased the mobility lifetime in the crystal. The increased carrier transport is a direct result of decreased hole and electron trapping in addition to other underlying mechanisms. Concentration of the electric field near the cathode is also observed. We measured the electric field distribution with sub-bandgap illumination as a function of temperature via the Pockels effect.

Response of the Internal Electric Field in CdZnTe to Illumination at Multiple Optical Powers

Manipulation of CdZnTe (CZT) crystals using illumination is a useful tool for altering the internal electric field present under normal bias conditions. The interactions with carriers that are trapped at either terminal are visualized by the electric field distribution through polarization. In this report, we demonstrate an ability to selectively manipulate the internal electric field of CZT using multiple-wavelength light illumination at various optical powers. The internal electric field polarization can be controlled using changes in optical power. We also investigate the electric field distributions using multiple optical powers to examine the light response as a function of light penetration depth.

Use of Sub-bandgap Illumination to Improve RadiationDetector Resolution of CdZnTe

The performance of Cd1−xZnxTe (CZT) materials for room-temperature gamma/x-ray radiation detection continues to improve in terms of material quality and detector design. In our prior publications, we investigated the use of multiple wavelengths of light (in the visible and infrared) to target charge carriers at various trap energies and physical positions throughout crystals. Light exposure significantly alters the charge mobility and improves carrier collection at the anode contact. This study presents an investigation of material performance as a radiation detector during such illumination. The decrease in charge trapping and increase in charge collection due to a higher probability of free electron release from traps contributed to an increase in the resolution-based performance of the detector through controlled illumination. We investigated the performance improvement of CZT crystals with previously known levels of intrinsic defects and secondary phases, at various voltages, light-emitting diode (LED) light wavelengths, and shaping times. Although our setup was clearly not optimized for radiation detector performance, it demonstrated substantial resolution improvements (based on full-width at half-maximum using 662-keV gamma rays from 137Cs upon illumination with 950-nm light) of 16% to 38% in comparison with unilluminated CZT under similar conditions. This manuscript includes discussion of the electrooptic behavior and its effect on performance. Additional testing and fabrication of a detector that incorporates such LED light optimization could lead to improved performance with existing detector-grade materials.

Illumination response of CdZnTe

CdZnTe (CZT) semiconducting crystals are of interest for use as room temperature X- and γ-ray spectrometers. Several studies have focused on understanding the various electronic properties of these materials, such as the surface and bulk resistivities and the distribution of the electric field within the crystal. Specifically of interest is how these properties are influenced by a variety of factors including structural heterogeneities, such as secondary phases (SPs) and line defects as well as environmental effects. Herein, we report the bulk current, surface current, electric field distribution and performance of a spectrometer-grade CZT crystal exposed to above band-gap energy illumination.

AFM characterization of laser-induced damage on CdZnTe crystal surfaces

Semi-conducting CdZnTe (or CZT) crystals can be used in a variety of detector-type applications. CZT shows great promise for use as a gamma radiation spectrometer. However, its performance is adversely affected by point defects, structural and compositional heterogeneities within the crystals, such as twinning, pipes, grain boundaries (polycrystallinity), secondary phases and in some cases, damage caused by external forces. One example is damage that occurs during characterization of the surface by a laser during Raman spectroscopy. Even minimal laser power can cause Te enriched areas on the surface to appear. The Raman spectra resulting from measurements at moderate intensity laser power show large increases in peak intensity that is attributed to Te. Atomic Force Microscopy (AFM) was used to characterize the extent of damage to the CZT crystal surface following exposure to the Raman laser. AFM data reveal localized surface damage in the areas exposed to the Raman laser beam. The degree of surface damage to the crystal is dependent on the laser power, with the most observable damage occurring at high laser power. Moreover, intensity increases in the Te peaks of the Raman spectra are observed even at low laser power with little to no visible damage observed by AFM. AFM results also suggest that exposure to the same amount of laser power yields different amounts of surface damage depending on whether the exposed surface is the Te terminating face or the Cd terminating face of CZT.