The authors observed the photoluminescence (PL) spectra of amorphous InGaZnO (a-IGZO) for the first time. At liquid nitrogen temperature, even weak near-band-edge emission was clearly observed at a wavelength of similar to 400 nm (3.1 eV) accompanied by a much stronger broad deep emission peaking at around 700 nm (1.77 eV) for 1-mu m-thick samples deposited by sputtering on sapphire substrates at room temperature. The PL intensity of each emission strongly depends on the electron concentration of a-IGZO ranging from 10(16) to 10(18) cm(-3). As the carrier concentration increased, the PL intensity of the broad deep emission decreased. The near-band-edge emission energy of 3.0 eV (413 nm) was in good agreement with the estimated absorption energy of 3.03-3.08 eV (403-409 nm) at 77 K. The depth profile of the carrier concentration of the a-IGZO layer was estimated using step-etching Hall measurements and was found to be uniform. The width of the depletion layer was determined by the film-thickness dependence of the sheet carrier concentration. If the substrate-side depletion layer is negligible, they estimated the upper limit of V-bi as similar to 1.9 eV (653 nm), in the middle of the bandgap, when assuming an 11.5 value for permittivity.