Time‐resolved spectroscopy of photoluminescence has been studied in single‐crystals of transition‐metal doped β‐Ga2O3, particularly focusing on Cu‐ and Cr‐doping effects on donor‐acceptor recombination. Cr‐doped β‐Ga2O3 still exhibits the photoluminescence due to the radiative recombination of donor‐acceptor pairs but its temporal evolution is greatly suppressed, resulted from non‐radiative decay channels through the d‐d transitions in Cr3+ ions. The most of 3d transition metals give similar effects. On the other hand, in Cu‐doped β‐Ga2O3 the luminescence spectrum and its temporal evolution are less sensitive to doping, reflecting no Cu‐induced state inside of the band gap. The optical absorption spectrum also indicates that the most of 3d transition metals, except for Cu and Cr, greatly reduce the number of conducting carriers. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Cr‐doping effects on optical properties of β‐Ga2O3 have been studied in single‐crystalline samples, grown by floating‐zone method. The photoluminescence, which is observed as an intense and broad peak in nominally pure β‐Ga2O3, is found be abruptly suppressed by Cr‐doping. This suppression demonstrates that a small amount of Cr3+ ions acts as non‐radiative centers for photo‐excited carriers. In Cr‐doped β‐Ga2O3 the residual photoluminescence shows UV and green emissions, peaking at about 3.3 eV and 2.4 eV, respectively. The photoluminescence excitation spectrum of the UV peak has a polarization‐dependent maximum at the fundamental absorption edge that is arising from the band‐to‐band excitations. Therefore, the radiative decay channels for photo‐excited carriers partly survive in Cr‐doped β‐Ga2O3. We have also found a Cr‐induced absorption band located just below the fundamental absorption edge (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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Optical properties of Si‐doped β‐Ga2O3 have been investigated by the time‐resolved spectroscopy, as well as by the conventional cw‐spectroscopy. The green photoluminescence is observed for photo‐excitations below the optical band gap Eg. The excitation spectrum of the green photoluminescence indicates a weak absorption band, presumably induced by Si‐doping, located at 0.2–0.3 eV below the conduction band minimum. The time‐resolved spectroscopy has revealed that the green photoluminescence manifests itself as the slower decay component. On the other hand, the fastest decay component, which is distributed from the UV to green regions, can be characterized by a time constant of ∼100 ns, comparable to that for un‐doped β‐Ga2O3.

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