Research Activities

Semiconductor wafer
A high-mobility two-dimensional electron system (2DES) can be realized in AlGaAs/GaAs modulation-doped heterostructures. Typical wafers consist of a GaAs cap layer, an AlGaAs doping layer, an AlGaAs spacer layer, a GaAs channel layer, and buffer layers grown on a GaAs substrate. Typical high-quality atomically-smooth layers are grown by Molecular Beam Epitaxy (MBE) technique. Various nanodevices, such as quantum dots, quantum wires, quantum point contacts and edge channels, can be fabricated from an MBE wafer. [photo: 2" GaAs wafer]

Photolithography system
Photolithography is used to fabricated patterns greater than a few microns. We have a MIKASA spin coater [MS-A100] and a photo mask aligner [M1-S] system to transfer photomask patterns to the resist layer on a semiconductor wafer. After spreading a resist layer on a substrate, ultraviolet light is exposed through a glass photomask. The irradiated regist will be removed after development. The resist patten can be used for lift-off process and etching process.

Electron beam lithography / Proximity effect correction
FuPECElectron beam lithography is used to fabricate fine patterns less than 100 nm. We can utilize a JEOL EB system at TokyoTech through NanoNet program.
We have home-made proximity effect correction program [FuPEC], which adjusts the dose of electron beam for meshed regions. This is essential for writing complicated patterns with dense and sparse regions. The FuPEC program converts DXF data into J01 format with ShotRank modulation.

Vacuum evaporator with electron beam and resistance heater
Metal electrodes (gate electrodes or Ohmic contacts) can be fabricated by depositing Au/Ti or AuGeNi alloy on a wafer. Our vacuum evaporator [K-Science] is equipped with an eletcron beam gun (3 melting pots) and a resistance-heating tungsten boat. An sample holder can be tilted and rotated during the evaporation. The vacuum chamber can be evacuated below 1E-6 Pa by using a TMP(Turbo molecular pump, 550L/s) and a RP (Rotary pump, 250L/min).

Draft chamber for chemical processes
Two draft chambers for acid and organic materials are used for chemical processes, such as wet chemical etching, surface cleaning, developments in photo and EB lithography processes. Ventilation, scribber and gas absorption units are equipped for clean and safety opertion.

Manual Probe Station

ManualProberThis laboratory-made low-cost manual probe station MPS1000 is designed to measure current-voltage characteristics and microwave characteristics of nano devices at room temperature. Fabricated nanodevices are screened by testing at room temperature.
Details can be found here.

Manual wire bonding (West Bond)
Nanodevices are usually mounted on a specially-designed microwave sample holder with external circuits (if required), and electrical connections are made by the wire bonding machine. Typically 25um gold wire is used for the purpose. Ultrasonic power is introduced to the capillary to make a wire bonding to the bonding pad of the sample. For better microwave transmission, the sample is mounted next to the end of coaxial cable.

Variable Temperature Insert (VTI) with 9T magnet
The VTI Insert (Cryo Inductories of America) provides low-temperature electrical measurement at variable tamperature ranging from 1.5K to above 300K. A superconducting magnet (Cryo Magnetics) can generates magnetic field up to 9T. We use this cryostat to investigate fundamental transport characteristics as well as microwave transport.

Helium 3 Cryostat with 12/14T magnet
He3 cryostatOxford Helium3 cryostat Heliox is used to measure DC and RF characteristics of nano devices at low temperature below 300 mK. The cryostat is easy to handle, and the hold time exceeds 90 hrs (by specification). Two coaxial cables are installed to feed high-frequency signal to the sample. The sample can be placed in a shielding box with RF filters. The cryostat is equipped with 12T superconducting magnet.

Dilution refrigerator with 10/12T magnet
Oxford Kelvinox HA400 [HA: HighAccess option, Nominal cooling power: 400uW] provides precise electrical measurement at extreley low tempertature (7 mK min.). It has a large space at 4K, 1K, 50mK, and MC (mixing chamber) plates for cryogenic curcuits like cold amplifiers (4K) and passive components. We have 6 coaxial cables (made of Niobium) for high frequency measurements, and 24 twisted pair cables (48 wires) for standard dc measurement with a common grounding at the MC. Electromagnetic noise is filtered by RC filters and copper powder filters. The cryostat is equipped with 10/12T superconducting magnet.

Cryogen-free (dry) dilution refrigerator with 12T magnet
Oxford TRITON [Nominal cooling power: 400uW@100mK] is useful for long-term electrical measurement without supplying liquid Helium. Various cold electronics can be placed in the large-space plates (400mm) at 50K, 4K, and base temperature.

Microwave and pulse generators
We have strong specialty on high-speed high-frequency measurements for studying quantum dynamics and coherent dynamics. Typical equipments are
- Anritsu high-speed pulse pattern generator (12.5 GHz, 2ch, photo)
- Microwave generator (20GHz)
- Vector network analyzer (20 GHz)
- Sampling oscilloscope (50GHz) with TDR (Time domain reflectometry) module (20 GHz)
- Tektronics arbitrary wave generator (10GS/s, 2ch).
- Spectrum analyzer (32 GHz)

The product of the minimum time resolution (< 0.1 ns) and the thermal energy (<10ueV below 100mK) can be smaller than the Planck's constant (4.14 ueVns), where quantum mechanical characteristics of electrons may appear.

Cryogenic electronics
Some electronic circuits (usually, passive components) are need to be operated at low temperature (<100mK). We designed and fabricated impedance matched copper powder filter, thin coaxial cable for filtering higher-order mode, DC block, LC resonators for impedance transformer, and so on.

Low-noise DC measurement system
Low-noise measurement setup is crucial to measure quantum mecahnical behavior in semiconductor nanostructures.
The 16 channel 16bitx2 DAC unit (left) provides stable DC voltages (10V, 5V, 2V full scale) with 22.5-bit resolution. The current amplifier with optical isolation amplifier (right) is designed to measure small currenr (approx. 20 fA/sqrtHz).

LabVIEW automated program FMA
FMA (Flexible Measurement and Analysis) software library is used to control various instruments through GPIB interface, accumulate various measurement data in a file format fma, analyze the stored data in various ways. Each data file involves three-dimensional array of measured value with scanning two variables (Var1 and Var2). The measurement can be repeated by scanning another three variables (Var3, Var4, and Var5).

Helium recovery monitor
Liquid Helium is major refrigerant for low-temperature experiments. We have to recycle Helium in order to run experiments at low cost. Our home-made Helium recovery monitor watches the Helium recovery rate and purity continuously, and alart the users via e-mail, if the recovery rate becomes low. Daily, weekly, monthly charts of Helium recovery data are availabe from the web. Typical recovery rate in our lab. is 95-96%. The Helium will be liquified and deliverd from the Research Center for Low Temperature Physics at TokyoTech.

Sketch book
The first thing you need to start with is to make a rough sketch of your idea. Interests on nanoscience, theoretical considerations, sample fabrication, measurement schemes, experimental details and so on... These idea can be brushed up by rewriting the sketch many times and by discussions with colleagues, students, and professors. For the purpose, FujisawaLab original functional pen is useful.