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In the blue frame below is the content of the MPE web page
"GaAs Photoconductors"
in the archived version
June 24, 2010.

GaAs Photoconductors

The GaAs BIB detector development program for far infrared astronomy is continued now as a common effort of four institutions: MPE (Germany), UC Berkeley, LBNL and NPS Monterey (USA). The actual  research program is supported by a three year NASA grant. Current experimental work is assisted by theoretical modeling.

Rationale for a GaAs BIB (Blocked Impurity Conduction Band) Detector Array Development:

  • GaAs has the shallowest stable dopant (e.g., Te: 5.7 meV) of any technically well-explored semiconductor.
  • Extension of the photoconductive cut-off wavelength to 330 µm (30 cm-1) with GaAs BIB devices is expected,  whereas the present cut-off wavelength limit of stressed Ge:Ga photoconductors is around 210 µm, cf. Fig. 1.
  • GaAs BIB detector  should get rid of g-r noise characteristic of bulk GaAs photoconductors.
  • Manufacture of planar structured two-dimensional detector arrays at an affordable price seems feasible.
  • Complexity advantage of photoconductors over bolometers and their more demanding cooling techniques

Fig. 1 FIR absorption spectra of n-doped GaAs
Fig. 2: sketch of a GaAs BIB pilot sample

Experiment status:

  • GaAs layers are grown using the Liquid Phase Epitaxy process. A centrifuge system with magnetic bearings was set up at UCB, Fig. 3. Growth parameters are under permanent optimization, but manufacture of high purity blocking layers is a challenge (unwanted donor concentration now below 1013 cm-3 with a small sapphire crucible).
  • With a full size sapphire crucible, growth of multi-layer structured devices will be possible.
  • Controlled doping of n-GaAs (e.g. with Te) is proven. Verification of  the reproducibility of the growth process and  manufacture of a BIB pilot sample, Fig. 2, are the next goals.

GaAs centrifuge growth system at UCB

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