Probing the Cr3+ luminescence sensitization in β-Ga2O3 with ion-beam-induced luminescence and thermoluminescence

Abstract

Ion-beam-induced luminescence (IBIL) measurements were performed in Cr-doped β-Ga2O3 using both protons and helium ions, showing a strong enhancement of the Cr3+ luminescence upon ion irradiation. Theoretical modelling of the IBIL intensity curves as a function of the fluence allowed estimating the effective cross-sections associated with the defect-induced IBIL enhancement and quenching processes. The results suggest that sensitizing the Cr3+ luminescence is more efficient for H+ than for He+ irradiation. Thermoluminescence (TL) studies were performed in the pristine sample, with no TL signal being observed in the spectral region corresponding to the Cr3+ emission. In agreement with the IBIL study, upon ion irradiation (with either protons or helium ions), this TL emission is activated. Moreover, it can be quenched by annealing at 923 K for 10 s, thus revealing the role played by the defects induced by the irradiation. These results show that the irradiation-induced defects play a major role in the activation of the Cr3+ luminescence, a fact that can be exploited for radiation sensing and dosimetry.

Thermoluminescence Studies of Proton-Irradiated Cr-, Mg-Codoped β-Ga2O3

“…Under these conditions, SRIM simulations predict that the first ∼25 μm of the sample were damaged, with projected ion ranges of 8.74, 15.6, and 24.1 μm for each of the three ion energies, respectively, as shown in the Supporting Information. These irradiation conditions have been previously employed in similar samples . Considering that the penetration depth of the electrons employed in this study is ∼500 μm, both the irradiated and the pristine regions are probed.…”

“…Moreover, these works also show an enhancement in the Cr 3+ luminescence yield with ion irradiation in the case of an electrically conductive sample . It is, however, clear that the optical activation is intrinsically associated with the presence of defect levels that can sensitize the Cr 3+ emission. ,,, …”

“…For the previously mentioned reasons, there are several recent studies regarding the optical properties of Cr-doped β-Ga 2 O 3 . In particular, at RT, the characteristic luminescence associated with Cr 3+ extends at least from 650 to 850 nm, comprising two sharp R -lines (namely, R 2 centered at ∼690 nm and R 1 at ∼697 nm) superimposed on a broad band with a maximum at ∼720 nm. , This emission has been previously explored in different applications, ranging from tunable optical microcavities to luminescent thermometers or radiation detectors. , …”

“…In fact, recent works suggest that the conditions under which the Cr 3+ ions become optically active may be related with the sensitization of the emission by processes such as energy or charge transfer between defect levels in β-Ga 2 O 3 and Cr 3+ ions. − In this context, previous ion-beam-induced-luminescence (IBIL) measurements show a strong enhancement of the Cr 3+ emission intensity as a function of the irradiation fluence as well as a reduction in electrical conductivity, thus suggesting that irradiation-induced defects play a major role in the activation of this luminescence. , These results are in line with several works showing a clear positive correlation between the Cr 3+ luminescence yield and the electrical conductivity of the sample. Notably, semi-insulating samples have been found to exhibit higher luminescence yields compared to conductive ones. ,,, It has been previously suggested that this dependence is related with different Cr 2+ /Cr 3+ concentrations due to different Fermi-level positions inside the bandgap. , While the Fermi-level position can be pinned lower in the bandgap by deep acceptors, such as Mg, recent works comparing the Cr charge state in Mg-doped semi-insulating samples and conductive samples show that the charge state is 3+ in both cases . Moreover, these works also show an enhancement in the Cr 3+ luminescence yield with ion irradiation in the case of an electrically conductive sample .…”

“…Given its wide bandgap and its monoclinic lattice with several different inequivalent positions, the number of different possible defect states in β-Ga 2 O 3 is quite high, as reported in several theoretical and/or experimental works resorting to techniques such as deep-level transient/optical spectroscopy (DLTS/DLOS) or thermoluminescence (TL). − ,− Although it is notoriously difficult to unambiguously assign the observed levels with the corresponding defects, some of the experimentally measured trap levels have been attributed to intentional or unintentional dopants, such as Fe, Cr, and Mg, to intrinsic defects, such as O vacancies, or complex defects involving them (see Supporting Information). An energy level lying 0.7 eV below the conduction band has been reported and attributed to a complex involving Fe and intrinsic defects in multiple works. − Notably, a correlation between the density of these defects and the Cr 3+ luminescence was observed .…”