The Manfra group occupies several labs across the Purdue campus. Material growth and device fabrication is performed in the Birck Nanotechnology Center. III-Nitride materials are grown in a Riber MBE system while high-mobility GaAs crystals are grown in a highly customized Veeco GenII MBE. The Birck Center also has a wide variety of characterization and fabrication tools utilized by the group including facilities for photolithography, e-beam lithography, wet- and dry-etching, e-beam metal evaporation, dielectric deposition by ALD and PECVD, thermal annealing, surface profilometry, SEM, AFM, XRD, and TEM. Low temperature electrical characterizations are performed in the group's low temperature lab located in the Physics building.
Low Temperature Measurement Lab
In our low temperature measurement laboratory we perform electrical transport experiments related to the fractional quantum Hall effect and quantum dot-based spin qubits. Low temperature measurements are carried out in a variety of cryostats depending on the experimental requirements. Initial characterization of materials and devices is done in a variety of 4K dipper probes in helium storage dewars. Following this initial screening, devices can be cooled to lower temperatures in large magnetic fields in one of two systems. A Janis He3 system allows samples to be cooled to T=300mK in fields up to 9T, and the system's top-loading designs allows samples to be thermally cycled in as little as 3 hours. Lower temperature experiments can be performed at T=10mK in fields as high as 17T in the Oxford Kelvinox dilution fridge. The group's ultra-low temperature capabilities are currently being expanded with the addition of a cryogen-free, top-loading Oxford Triton dilution fridge which will be capable of cooling samples to 10 mK in fields up to 8T. A wide array of low-noise electronic equipment enables very precise experiments to be carried out at the extremely low excitation levels required at low temperature work. The group also has shared access to the department's helium liquefier which enables reliable, long-term operation of the wet fridges.
Gallium Nitride MBE Lab
The gallium nitride (GaN) MBE system is a Riber 32. The machine includes a load lock, a sample outgassing chamber, and a main growth chamber. In the main growth chamber the system is configured with 2 gallium sources, 2 aluminum sources, a nitrogen plasma source, an indium source as well as a source for silicon. It is equipped with in-situ diagnostics including a reflection high-energy electron diffraction (RHEED) gun for measuring sample surface evolution as well as a residual gas analyzer. Projects include the growth of AlGaN/GaN and AlInN/GaN heterostructures both on polar and non-polar GaN substrates for intersubband device applications. In addition this system has been used to produce the highest 2DEG mobility ever reported in an AlGaN/GaN heterostructure.
Gallium Arsenide MBE Lab
Our GaAs MBE is a Veeco Gen II system highly customized to improve overall vacuum quality and resulting crystal purity. Meticulous vacuum hygiene results in base pressure ~1x10-12 torr and all the effusion cells in our system are custom designed by our group in an effort to reduce the thermal load during growth. The lab also houses 2 ancillary vacuum chambers used for cleaning and characterization of components before loading into the MBE. One vacuum chamber has been specifically designed for gallium purification and mates with the MBE so that components can be transferred between chambers without exposure to air. This MBE system has recently produced 2DEGs with low temperature mobility in excess of 30x106cm2/Vs as well as 2DEGs exhibiting the highest excitation gap at ν=5/2 ever measured.