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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

fir1
fir2
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.

fir3
GaAs centrifuge growth system at UCB

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