Correlation between electrical conductivity and luminescence properties in β-Ga2O3:Cr3+ and β-Ga2O3:Cr,Mg single crystals

Abstract

The photoluminescence, excitation, and absorption spectra as well as the electrical conductivity of β-Ga2O3:Cr and β-Ga2O3:Cr,Mg single crystals were studied. The as-grown β-Ga2O3:Cr crystals had a green color, the conductivity at about 10−2–10−3 Ω−1 cm−1, and a low yield of Cr3+ impurity luminescence. Annealing in oxygen atmosphere led to a strong increase in Cr3+ red luminescence yield, increase in the resistivity, and changes in the absorption and excitation spectra. Similarly, increases in the Cr3+ luminescence yield and resistivity were observed after codoping of β-Ga2O3:Cr crystals with magnesium (Mg2+). The registered changes in the Cr3+ luminescence yield, electrical conductivity, and in the absorption and excitation spectra are considered to be due to the shift in the Fermi level. In the as-grown β-Ga2O3:Cr crystals, the Fermi level is located near the bottom of the conduction band, and most chromium ions are in the Cr2+ charge state. Annealing in an oxygen atmosphere as well as codoping of the crystals with chromium and magnesium impurities moves the Fermi level toward the middle of the bandgap and recharges the chromium ions to the Cr3+ state.

Probing Local Structural Changes by Sharp Luminescent Infrared Nanophosphor for Application in Light-Emitting Diodes

“…The intensity of the anti-Stokes lines rises as the temperature increases. For the x = 0 sample, the broad band emission related to the spin-allowed 4 T 2 → 4 A 2 transition with a higher probability appears at a temperature higher than 400 K in the 650–850 nm range. − As expected for the sample with low x content, the energy difference of the 4 T 2 and 2 E states is smaller than the sample with higher x content, so the thermal occupancy of the 4 T 2 state is most noticeable.…”

Broadening Phosphor-Converted Light-Emitting Diode Emission: Controlling Disorder

“…The luminescence properties of Cr 3+ in Ga 2 O 3 are well known, therefore, we focus on mixed-ion samples x = 0.1, 0.4, and 0.8. ,,− Streak camera images at 10 K upon excitation at 440 nm and time-resolved emission spectra are shown in Figure , recorded at time ranges of 10 ms for x = 0.1, 5 ms for x = 0.4, and 1 ms for x = 0.8, along with emission spectra for two time ranges. Previous studies of Ga 2 O 3 :Cr 3+ show that its emission spectrum does not change with time, indicating a single luminescence center, unlike the time-dependent evolution of luminescence spectra for Al- and In-substituted materials .…”

“…For the sample of x = 0.1 at 100 K, a dominant R 1 line, a thermally occupied R 2 line, and phonon sidebands are observed. 30,31,33 Broad-band emission intensity increases with temperature as electrons activate from the 2 E to 4 T 2 state. At 250 K, the broad-band emission dominates, and for samples of x = 0.4 and 0.8, it extends from 650 to 1000 nm even at 100 K. With increased temperature for sample x = 0.1, the intensity of the R 2 emission increases relative to R 1 , until it dominates at a higher temperature (200 K), as a result of the higher radiative transition probability of R 2 than that of R 1 for strongly distorted tetrahedral Cr 3+ .…”

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.…”

“…Another argument for the assignment of the R-lines to Cr was based on the observed increase in the R-lines intensity in Cr doped Ga 2 O 3 . 19,22,24,28,29 Though reasonable, the later argument alone could not be considered as an unambiguous proof of the emission origin, since a change in the dopant concentration is well known to affect the Fermi level position in the sample and can also facilitate charge transfer between dopants and residual contaminants. For example, the same emission was also attributed to the 4 T 1 !…”