The Effect of Cr³⁺ and Mg²⁺ Impurities on Thermoluminescence and Deep Traps in β-Ga₂O₃ Crystals

Abstract

Thermally stimulated luminescence (TSL) of β-Ga2O3 single crystals doped with Cr3+ and Mg2+ impurities was investigated. Based on the correlation between the Cr3+ concentration and light sum accumulated in the thermoluminescence (TL) glow peak at 285 K, it was concluded that doping of β-Ga2O3 with Cr3+ ions leads to the formation of electron traps manifested in this peak. The activation energy of peak at 285 K is equal to Ec-0.55 eV and close to E1. Thus the Cr3+e− centers can be a candidate for E1. The high-temperature TL glow peak at 385 K (Ec-0.94 eV) is related to oxygen vacancies which are created in gallium oxide doped by Mg2+ ions to compensate for the negative charge formed by the substitution of gallium sites by magnesium ions.The co-doping of β-Ga2O3 crystals with Cr3+ and Mg2+ impurities leads to the appearance of a new TL glow peak at 320 K with an energy close to E*2 (Ec-0.7). It is suggested that this peak is formed by defect complex, in particular, oxygen vacancies with Cr3+ or Fe3+ ions.

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

“…While the unambiguous assignment of these traps is notoriously difficult, some of these levels have been attributed to intentional or unintentional dopants, such as Fe, Cr and Mg or to intrinsic defects, such as O vacancies, or complex defects involving them. In particular, a defect complex involving Fe and intrinsic defects, with an energy level lying 0.7 eV below the conduction band minimum, was reported, and a correlation between its concentration and the Cr 3+ luminescence was observed 22 . Additionally, the charge transfer level Fe 2+ /Fe 3+ (energy level lying 0.78 eV below the conduction band minimum 29 ) was proposed to act as a charge transfer channel to Cr ions in the 4+ charge state 35 .…”

“…Regarding the optical activation of the Cr 3+ ions in β -Ga 2 O 3 , the excitation processes are not fully understood yet, due to the large number of possible defect states in this material that can be responsible for the sensitization. Such processes include energy or charge transfer between traps in β -Ga 2 O 3 and Cr 3+ ions 22 – 25 . Recent IBIL measurements monitored at the Cr 3+ emission wavelengths show a strong enhancement of the emission intensity as a function of the irradiation fluence in a conductive sample, thus suggesting that irradiation-induced defects play a major role in the activation of this luminescence 23 .…”

“…In view of this, there is a plethora of defect levels located within the wide bandgap of β -Ga 2 O 3, as reported in several theoretical and/or experimental works employing techniques such as deep level transient/optical spectroscopy (DLTS/DLOS) or thermoluminescence (TL) 22 , 24 , 29 – 35 . While the unambiguous assignment of these traps is notoriously difficult, some of these levels have been attributed to intentional or unintentional dopants, such as Fe, Cr and Mg or to intrinsic defects, such as O vacancies, or complex defects involving them.…”

“…Therefore, while the Cr 3+ emission has been shown to depend on the electrical conductivity 23 , 26 , 27 and on the presence of certain defect levels within the band gap 22 , the full excitation processes are still unclear. In this context, this paper presents a detailed IBIL study performed on Cr-doped β -Ga 2 O 3 using two different ions (H + and He + ) in order to assess the evolution of the IBIL intensity associated with the Cr 3+ ions with the irradiation fluence.…”

Magneto-optical properties of Cr3+ in β-Ga2O3

“…It was suggested that the internal transitions between the 2 E and 4 A 2 states of Cr 3þ give rise to two sharp emission (or absorption) lines at around 1.78 and 1.80 eV at room temperature (RT), which were labeled as R1 and R2 lines. 18,19,[22][23][24][25][26][27][28][29][30] This conclusion was based 19,22,23,25 on a similarity of the transition energies with the internal transitions of Cr 3þ in Al 2 O 3 . [31][32][33] They were further supported by the analysis of higher-lying excited states detected in absorption 19,28 and emission 18,22,[24][25][26][27][28] spectra of Cr-doped b-Ga 2 O 3 , as well as photoluminescence excitation spectra of the R lines 24,26,28 using the Tanabe-Sugano diagrams.…”