Photon detection in tunnel-barrier structures

The use of tunneling in a structure consisting of a semiconductor, thin dielectric layer and metal offers promise as a practical photon detector free of many of the limitations ordinarily imposed by semiconductor materials. The basis for detection is the large increase in tunneling probability which can result when the kinetic energy of electrons is increased by photon absorption in the semiconductor. The barrier serves two functions: (1) it allows those photoelectrons at the barrier to tunnel through uninfluenced by trapping and recombination centers in the semi-conductor; and (2) because of its high impedance, the semiconductor resistance can be made as low as desired. Using thin-film tunnel barrier structures consisting of Te-Al2O3-Al, we have shown that a substantial increase is obtained in the speed of response as compared with that obtained with Te alone. In addition, although ordinary photoconductivity could not be detected in one sample in the Te layer alone, photocurrents were detected when the sandwich was operated in the tunneling mode. The spectral response is largely determined by that of the semiconductor, and the present device responds in the infrared. The similarity in the spectral response curves of the tunnel-barrier device and Te indicates that the generation of photoelectrons is a bulk property of the semiconductor and is not due to surface effects or excitation of electrons from traps in the insulator. Further, it suggests that the excited electrons are thermalized in the conduction band. The detectivity is similar to that of the Te alone.