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Scientific Highlight
Achievement/Results
The term defects refers to atomic-scale imperfections in an otherwise periodic crystal lattice. Although small, point defects can have large effects on electronic and optical devices due through charge trapping and structural distortion. At the size scale of less than 1nm, characterizing point defects, their intensity, their spatial distribution, and effect on the local optoelectronic structure is a formidable task. IGERT Fellow Drew Steigerwald (Tolk’s group) discovered and developed a method to detect the presence of such defects using femtosecond laser pulses.
In these experiments ultrafast laser pulses are absorbed sample of interest and give rise to picosecond acoustic pulses roughly 10nm in size which travel into the bulk material. We then monitor the travelling acoustic wave using a time-delayed probe pulse. By studying exactly how the probe light interacts with the acoustic wave we can study the optical and electronic properties of our samples with ten nanometer resolution, without ever damaging sample.
Using this technique we have shown that point defects can be detected with high precision. Further, we have developed this technique and are able to measure the spatial distribution and predict with high confidence the exact number of defects present at any point below the surface. Our experiments can detect defect concentrations to almost 1/100,000 atoms. No other technique can boast the same combination of sensitivity, depth resolution, depth limits, ease of use, and sample preservation.
Currently we are studying how to extend this technique to the study of internal strain and interface properties, a common problem in thin film growth, with application in a variety of fields from photovoltaics to magnetic or spin-based information processing. Additionally, we are studying the exact interaction between defects and the optical response in out experiments. By understanding the dependence between the change in optical response and number of defects, we can estimate the extent of nearest neighbor interactions that propagate outward through the electronic structure surrounding a single point defect. This can have great impact on the use of nanoscale devices, since as the size scale decreases single defects have ever increasing influence.
Steigerwald has first authored 3 papers and 4 coauthored papers, have been published/submitted describing this work.
Address Goals
Discovery: The activity described above represents a significant advance in the contact-less detection of semiconductor defects. Defects and their control are crucial to all applications of semiconductors, from advanced complementary metal-oxide semiconductor (CMOS) technology to photovoltaics and electronic sensors. Creation of defects is often a natural by-product of device processing, ion implantation and its associated lattice damage creation are prime examples. In the case of ion implantation it is usually desirable to anneal (heal) the material thus eliminating the created defects. In other cases, defect creation is desirable as in applications where minority carrier lifetime is important. In all such work the control of the defects in terms of their intensity and their depth profile are critical to a successful application.
The new optical probe described above permits such detection using modern, ultrafast optical techniques. Competitive probes either create further damage (i.e. ion beam channeling) or require device-like processing (i.e. Deep Level Transient Spectroscopy). The development of this new probe will represent an advantage to a broad range of semiconductor science and technology. In particular, the sensitivity to the probe wavelength (relative to the nominal band-gap) reveals a new method of detecting and modifying optical materials in terms of band-edge broadening. The precise data represents a new challenge to solid state theory in terms of a quantitative analytical description of the band-edge broadening associated with defects. The practical aspect of the probe will undoubtedly be useful in the actual development of advanced semiconductor processing particularly for optical materials and devices.
Research Infrastructure: The research activity described above represents a major potential transformation in the monitoring and quantifying of structural defects in semiconductor materials. Up to now defect analysis involved energetic electron or ion probes (I. e STEM, ion channeling), which represent significant investments in relatively large and expensive facilities. Alternately, structural probes could be investigated indirectly through electronic measurements requiring device-like processing and the associated facilities as typically found in a clean-room. The optical probe represents a contact-less investigation of the semiconductor solid not requiring either device processing or energetic particle bombardment. As a result it represents a simplification in infrastructure and a precise and convenient way of monitoring device processing. In addition, because the technique is based on optical principles, it has intrinsic advantages for optical materials such as used in light emitting diodes, solid-state lasers and photovoltaics.
The research infrastructure advance represented here, is not the development of a new piece of hardware, but rather the invention of a new application of existing instrumentation which might replace the rather large, expensive and complex apparatus in current use. The early adoption of such techniques and set-ups can clearly lead to a manufacturing advantage via the employment of new experimental tools.
