Effect of Layer Relaxation on the Internal Photoemission in Pt/Si1−xGex Schottky Barrier Type Infrared Detectors

A Schottky type infrared detector fabricated on a p-type Si1−xGex substrate has a higher cut-off wavelength than one on a pure Si substrate because the barrier height of the Schottky junction on p-type Si1−xGex decreases with the Ge content and the induced strain in the Si1−xGex layer. We have studied the effect of the strain relaxation on the internal photoemission and I-V characteristics of a Pt/Si1−xGex Schottky junction with x=0.14. It is shown that the cut-off wavelength of the diode made on a strained Si0.86Ge0.14 layer is higher than that on a Si substrate as expected. This shows the possibility of tuning the range of these detectors in the mid-infrared region. However, the thermal relaxation in the Si0.86Ge0.14 layer is found to reduce the cut-off wavelength to lower values, showing that the difference between the Fermi level of the metal and the valence band edge increases with the layer relaxation. This effect should be taken into account when a Schottky type infrared detector is manufactured on a strained Si1−xGex film. I-V characteristics of the junctions also indicate an increase of the barrier height with the relaxation of Si1−xGex. These results demonstrate the band edge movements in a Si ixGex layer experimentally agree with the expected changes in the band structure of the Si1−xGex layer with strain relaxation.