STOICHIOMETRY-DEPENDENT DEEP LEVELS IN N-TYPE GAAS

The photocapacitance (PHCAP) method in the constant capacitance condition has been applied to determine the energy and density of deep levels within the limits of a certain depletion layer thickness as a function of the photon energy and the applied voltage. Samples used were various n-type GaAs crystals annealed for a long time under excess As vapor pressure and rapidly cooled. The present PHCAP investigations have revealed several stoichiometry-dependent deep levels including some photoquenching levels. These are Ec-0.66 and -0.72 eV deep donors, the photoquenching level at 1.03, 1.25, and 1.41 eV. The As-vapor-pressure dependence of the density of these deep levels in quasithermal equilibrium were also clarified. The 1.25-eV photoquenching level density increased monotonically with increasing applied As vapor pressure and finally saturated at extremely high As vapor pressures. The density of Ec -0.72 eV deep donors showed a similar dependence on the As vapor pressure, but showed less saturation under higher As vapor pressures. The Ec -0.72 eV deep donor is quite different from the so-called EL2 level. But the temperature dependence of the PHCAP responses (ΔVph -T) revealed strong interaction effects among these defect structures. From the photoluminescence results and conventional deep-level transient spectroscopy measurements,it was clarified that both Ec -0.66 and -0.72 eV deep donors had larger Frank-Condon shifts, 0.28 and 0.24 eV, respectively, compared with that of the so-called EL2 level (0.12 eV). The PHCAP measurements after photoexcitation were carried out to detect de-ionization of the photoionized deep levels. Additional de-ionization of the photoionized state of the 1.41-eV photoquenching level was detected adjacent to those of the EL2 level. These de-ionization could be detected only in the course of the photoquenching phenomenon in n-type GaAs. The As-vapor-pressure dependence of these deep levels shows that interstitial As atoms play a vital role in forming deep levels in annealed GaAs crystals.