| About this Abstract |
| Meeting |
2011 Electronic Materials Conference
|
| Symposium
|
2011 Electronic Materials Conference
|
| Presentation Title |
C4, Combined XSTM and High Resolution XRD Study for Quantitative Structural Descriptions of Type-II Superlattice IR Detectors |
| Author(s) |
Michael Yakes, Syed Qadri, Kevin Matney, Changyun Yi, Ed Aifer, Nadeemullah A Mahadik |
| On-Site Speaker (Planned) |
Nadeemullah A Mahadik |
| Abstract Scope |
The type-II superlattice (T2SL) material system has made great progress towards infrared devices with state of the art performance using band structure engineering to develop new designs. In order to fully understand the material effects on device quality we believe it is essential to determine the structural properties in as much detail as possible. We have developed a powerful approach to accomplishing this using atomic scale resolution cross sectional scanning tunneling microscopy (XSTM) to identify the microscopic structure in small regions of a given sample, and then incorporating this information into detailed simulations of the XRD spectra. This technique yields a detailed quantitative description of the structure on a macroscopic scale. XRD and dynamical simulation of the x-ray rocking curves are well-established and powerful tools for characterizing such structural properties as composition, thickness, and strain in complex heterostructures composed of multiple superlattice regions. Typically, the SL period is estimated from the satellite peak separation, and the average SL lattice constants are estimated from the position of the 0th order satellite peak. More detailed structural information may also be obtained by simulating the XRD spectra using the intended structure and composition using an automated fitting procedure with a set of free parameters and arbitrary tolerances. However, the success of such a fit depends critically on appropriate choice of the independent variables and their ranges of uncertainty, therefore without a reliable knowledge of the microstructure this approach is unlikely to provide fitting results that can be used with confidence. Instead of using idealized parameters from the growth design, we use inputs derived from XSTM images which yield local structural and compositional parameters with atomic scale precision. Our XSTM images give local information on the lattice thickness, interface roughness, interface bonding, and group V cross incorporation. High resolution x-ray measurements of multiple reflections from the same sample were performed using a Rigaku four-circle diffractometer and an 18 kW generator equipped with Cu-anode. The incident x-ray beam was monochromated to CuKα 1 radiation by using two Ge(220) channel-cut crystals. This arrangement provided an angular resolution of 3.6 arcseconds in 2θ. A full dynamical XRD fitting was then performed using the commercially available Bede RADS simulator, in conjunction with initial parameters and tolerances provided by the XSTM measurements. When the structural and compositional values from the XRD fit were incorporated into the band structure algorithm, the recalculated energy gaps were in better agreement with the measured photoluminescence than the gaps derived from the intended structure. We believe that this combined approach of the microscopic XSTM measurements with the statistical XRD probe and simulation will allow a more reliable picture of the superlattice structure on the wafer scale. |
| Proceedings Inclusion? |
Undecided |