Ultra-low-frequency photocurrent (PC) self-oscillation has been investigated in an In0.15Ga0.85As/Al0.15Ga0.85As quantum-well (QW) diode in details as a function of temperature, excitation power and wavelength. The PC oscillation frequency increases with increasing temperature and illumination power at excitation wavelengths below the leading n = 1 heavy-hole exciton resonance line under reverse bias conditions. The illumination wavelength dependence shows a clear evidence for beating due to two oscillators when photoexcitation by shorter wavelength below 1050 nm is used. These results suggest that the low-frequency PC self-oscillation with a characteristic frequency of about 0.01-0.1 Hz is caused by oscillating electric fields due to two types of photogenerated charge carriers trapped at deep localized centers within the QW regions. (c) 2006 Elsevier B.V. All rights reserved.