D. Burckel

Nanoheteroepitaxy for the integration of highly mismatched semiconductor materials

We describe an ongoing study of nanoheteroepitaxy (NHE), the use of nanoscale growth-initiation areas for the integration of highly mismatched semiconductor materials. The concept and theory of NHE is briefly described and is followed by a discussion of the design and fabrication by interferometric lithography of practical sample structures that satisfy the requirements of NHE. Results of NHE growth of GaAs-on-Si and GaN-on-Si are described, following the NHE process from nucleation through to coalescence. Micro-Raman measurements indicate that the strain in partially coalesced NHE GaN-on-Si films is <0.1 GPa.

Defect reduction mechanisms in the nanoheteroepitaxy of GaN on SiC

This article describes defect reduction mechanisms that are active during the growth of GaN by nanoheteroepitaxy on (0001) 6H SiC. Nanoheteroepitaxial (NHE) and planar GaN epitaxial films were grown and compared using transmission electron microscopy, photoluminescence, x-ray diffraction, and time resolved photoluminescence. It was found that in addition to the previously reported defect reduction mechanism that results from the high compliance of nanoscale nuclei, other independent defect reduction mechanisms are also active during NHE including: (i) filtering of substrate defects, (ii) improved coalescence at the nanoscale, and (iii) defect termination at local free surfaces. Also, it was found that the biaxial strain in the GaN film could be significantly reduced by using a “grouped” NHE pattern geometry. Time resolved photoluminescence measurements on NHE GaN samples with this geometry showed a more than tenfold increase in carrier lifetime compared to GaN grown on planar SiC.

Subfeature speckle interferometry

A novel speckle technique, subfeature speckle interferometry, is introduced that relies on the amplitude interference of two independent speckle patterns, originating from coherent illumination, by use of an optical system that produces interferometric quality interference fringes on a scale comparable with the speckle correlation length. Examples are given for in-plane translation, sample tilt, and temperature measurement (strain). A temperature measurement accuracy sigma = 0.92 degrees C is realized. In contrast to traditional full-field speckle crosscorrelation techniques, this technique requires only a small number of detector elements with minimal signal processing and is compatible with many real-time sensor applications.

Record breaking solar cells : ZnxCd(1-x)Te graded bandgap nanoarrays.

CdTe is the leading material for thin-film solar cells due to ease of processing and reduced cost. However, low conversion efficiencies due to defects in the material are still a problem in these devices. We propose implementing micro and nano-enabled pseudomorphic growth of ZnCdTe to dramatically increase the efficiency of CdTe solar cells. Simulations predict that defect-free films are possible in graded ZnCdTe nanoislands below 90 nm as well as 24 % efficient solar cells with the use of ZnCdTe grading. Selective growth of CdTe showed single grains when the island sizes decreased below 300 nm. The simulation and experimental results demonstrate for the first time the ability to use nanopatterned substrates to enhance uniformity and efficiency in CdTe thin film solar cells. More than 20 reports, presentations, and papers were produced as part of this work and the results from this project enabled UTEP and Sandia to secure to grants.

Fabrication of nanostructures with interferometric lithography

Summary form only given. Nanostructures, features with characteristic dimensions between 1- and 100-nm, are the focus of much current attention as a result of the unique physical properties offered in this mesoscopic regime. As semiconductor-processing technology has matured, the ability to create small structures has improved. Traditional nanostructure fabrication involves serial processing, e.g. e-beam lithography. For many applications, such as heterostructure crystal growth, a large-area parallel processing capability is vital. One optical lithography technique, interferometric lithography (IL), is a powerful technique for making nanostructures, providing an inexpensive method to generate nano-scale features over large areas.