Max-Planck-Institut für extraterrestrische Physik

- Infrared/Submillimeter Astronomy -

GaAs Photoconductors

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