Spectral Reflectance System for Low Substrate Temperature Substrate Measurement during Molecular Beam Epitaxy
Abstract
Far from equilibrium, small bandgap semiconductor alloys are in development for a range of infrared photonic applications. These materials, including Ge1-xSnx, Si1-x-yGeySnx, and the III-VBi alloys, may enable new infrared device applications. Epitaxial growth of these films by molecular beam epitaxy (MBE) requires very low growth temperatures, less than200 degreesC in some cases, far below the optimal conditions used for the host material. This is done in order to ensure significant incorporation of Sn or Bi solute atoms, and film composition and quality depend strongly on MBE growth temperature. However, existing non-contact temperature measurement methods are either not capable of measuring the temperatures of small bandgap semiconductor substrates or are not able to measure down to the low growth temperatures required. At Tufts University, the feasibility of high energy critical point reflectance thermometry (HCRT) has been demonstrated as a technique for measuring the temperature of substrates down to low temperatures. The HCRT technique makes use of spectral reflectance measurements to track the location of peaks from above band-gap, high energy critical points visible in the substrate materials reflectance spectrum. The locations of those critical point reflectance peaks vary with temperature, and so their locations can be used to determine substrate temperature across a broad range from cryogenic to high temperatures. HCRT overcomes many of the drawbacks of other non-contact measurement techniques, and will enable accurate process control for low temperature growth of epitaxial films. However, the repurposed optical equipment currently in use at Tufts University provides insufficient accuracy, temperature resolution, and spectral range to fully develop the HCRT technique for broad application and a wide range of common semiconductor substrate materials. Here, we propose acquisition of a spectral reflectance tool specifically optimized for the HCRT temperature measurement technique and capable of measuring a range of common semiconductor substrates. If awarded, the resources requested in this proposal will provide for acquisition of a dedicated, high sensitivity spectral reflectance system and support its use for development of the high energy critical point reflectance thermometry. This will be accomplished through in-situ molecular beam epitaxy (MBE) growth experiments and ex-situ temperature varying spectroscopic ellipsometry (SE) measurements using MBE and SE capabilities already present at Tufts. The spectral reflectance system acquired will improve the measurement accuracy and resolution of the HCRT technique as already applied, and extend it to a range of wafer substrate materials, including Ge, Si, GaSb, GaAs, InP, and InAs. Use of the proposed spectral reflectance system to develop the HCRT temperature measurement approach will benefit the scientific community and existing AFOSR research efforts to develop low temperature growth methods for SiGeSn and III-V-Bismide alloys. This will in turn enable the creation of new infrared device technologies based on these material systems that support critical DoD capabilities.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Feb 29, 2024
- Source ID
- FA95502310158
Entities
People
- Thomas E. Vandervelde
Organizations
- Air Force Office of Scientific Research
- Tufts University
- United States Air Force