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

The studies were performed on β-Ga₂O₃ crystals doped with Cr³⁺ impurity with a concentration that varied from a background concentration to 0.05% and β-Ga₂O₃ crystals a doped with 0.05% Cr³⁺ and 0.1% Mg²⁺ impurities. β-Ga₂O₃ single crystals for the study were grown by the floating zone technique with radiation heating. The raw material was gallium oxide with 4 N purity. According to the supplier´s specification, gallium oxide contains the following main impurities in ppm (Max): Si ≤ 10, Fe ≤ 5, Cr ≤ 1, Mn ≤ 1, Co ≤ 1. Impurities of chromium oxide Cr₂O₃ and magnesium oxide MgO with 4 N purity at the appropriate concentration were added to the mixture for the manufacture of ceramic rods, which were subsequently used for the crystal's growth. The pressed polycrystalline rods were sintered at a temperature of 1300 °C for 10–12 h. The single crystals were grown along a crystallographic direction of [100]. The growth and rotation rates of the sintered rods were 2–4 mm per hour and 6 rpm, respectively. The growth atmosphere was the air. The β-Ga₂O₃ crystals obtained had a length of up to 30 mm and a diameter of 5–7 mm. Samples for study 0.5 mm thick were cleaved from the grown crystals along the plane (100).

It was found that the luminescence yield of chromium impurity and TSL in gallium oxide crystals highly dependent on the conductivity of the crystals. As-grown samples were high-conductive and had very low luminescence yield of chromium impurity and TSL was absent. Also, it was experimentally established that annealing of crystals in an oxygen atmosphere leads to both a decrease in conductivity and an increase in the luminescence yield. After prolonged annealing in an oxygen atmosphere, all samples became high resistivity and the luminescence yield was maximal.

Prior to all studies, crystals with different concentrations of Cr³⁺ impurity were subjected to the same annealing in an oxygen atmosphere for 100 h at a temperature of 1300 °C. It should also be noted that all samples of β-Ga₂O₃:Cr crystals became high resistivity after annealing in an oxygen atmosphere. The activation energies of the conductivity of annealed crystals were ∼0.8 eV for β-Ga₂O₃:Cr crystals and ∼1 eV for β-Ga₂O₃:Cr, Mg crystals. All studies of luminescence and TSL were performed on such high-resistivity crystals of the same thickness.

Photoluminescence measurements were carried out on spectrofluorometer CM2203 in the 230–820 nm spectral range. Luminescence spectra were obtained with a spectral resolution of 0.5 nm. Hamamatsu R928 photomultiplier was used for luminescence detection. Photoluminescence spectra were automatically corrected by lamp intensity and photomultiplier tube sensitivity, respectively.

To measure the thermoluminescence (TL) glow curves, the β-Ga₂O₃ samples were exposed to X-ray irradiation for 20 min at 225 K. X-ray irradiation was provided by a microfocus X-ray tube URS-002 with copper anticathode (U = 45 kV, I = 0.3 mA) through the beryllium window of a cryostat. The TL glow curves of gallium oxide were recorded on a red luminescence band of Cr³⁺ impurity in the linear heating mode at the temperature increase rate of 0.1 K s⁻¹. The emission wavelengths were selected by an SF-4A monochromator. The irradiation of the crystals and the subsequent registration of thermoluminescence were performed through a calibrated diaphragm.

As-grown unintentionally doped (UID) β-Ga₂O₃ crystals had a blue color. After annealing in an oxygen atmosphere at 1300 °C during 100 h, the undoped β-Ga₂O₃ crystals became colorless. β-Ga₂O₃ crystals doped with Mg²⁺ impurity was colorless immediately after growing, and their annealing in an oxygen atmosphere did not lead to any noticeable color changes of the crystals.³⁰ As-grown β-Ga₂O₃ crystals doped with Cr³⁺ impurity had a green coloration. The color saturation of crystals changed from weakly to dark green with increasing concentration of chromium impurity. The color of β-Ga₂O₃: Cr³⁺ crystals are caused by two broad absorption bands of Cr³⁺ ions in the visible region of the spectrum. Such broad absorption bands are characteristic for Cr³⁺ ions in the octahedral sites of crystals.^23–29

The luminescence yield of Cr³⁺ impurity in as-grown β-Ga₂O₃:Cr crystals was low. It should be noted that it differs by several times for samples grown from the same raw material, but under other growth conditions in particular growth rate, overheating of the melt zone, etc. We found out that the additional annealing of the as-grown β-Ga₂O₃:Cr crystals in an oxygen atmosphere leads to a strong increase in the Cr³⁺ impurity luminescence. After annealing during 100 h at a temperature of 1300 °C, the luminescence yield of Cr³⁺ ions increased by more than 100-times in comparison with the luminescence yield of as-grown β-Ga₂O₃:Cr crystals. It should also be noted that after such annealing the crystals of β-Ga₂O₃ doped with Cr³⁺ impurity become light green. At the same time, β-Ga₂O₃ crystals co-doped with Cr³⁺ and Mg²⁺ impurity had light green color immediately after grown and a high yield of Cr³⁺ impurity luminescence. The additional annealing of these β-Ga₂O₃:Cr,Mg crystals in the oxygen atmosphere did not lead to a noticeable increase of luminescence yield of Cr³⁺ impurity.

Figure 1 shows the luminescence spectra of β-Ga₂O₃ crystals. The relatively low yield of Cr³⁺ impurity luminescence was observed in UID β-Ga₂O₃ crystals with a background concentration of chromium impurity. Broad emission band 300–550 nm with a maximum near 410 nm and a weak Cr³⁺ impurity luminescence band in the "red" region 650–820 of the spectrum were observed (Fig. 1a). This broad band is complex and related to the luminescence of host defects in β-Ga₂O₃ crystals.^31,32 The spectral composition of the host emission did not change at chromium dopping, but the relative luminescence intensity of the chromium ions increased significantly in the β-Ga₂O₃:0.05%Cr³⁺ crystals (Fig. 1b). The co-doping with magnesium impurity of β-Ga₂O₃: 0.05% Cr³⁺, 0.1% Mg²⁺ crystals leads to a further increase in luminescence intensity over the entire spectral range (Fig. 1c).

Zoom In Zoom Out Reset image size

It is known that irradiation of gallium oxide produces non-equilibrium electrons and holes that can be trapped by defects or impurities. The heating of the irradiated crystal is accompanied by the release of trapped carriers and subsequent radiative (or non-radiative) recombination. In order to determine, which of the traps occur as a result of doping, we investigated the TL glow curves of β-Ga₂O₃ crystals with various Cr³⁺ impurity concentrations. Figure 2a shows the TL glow curve of a β-Ga₂O₃ crystal with a background concentration of Cr³⁺ impurity. The intensity of TL was low as well as the TL glow curve was complex. The deconvolution of this curve into elementary peaks was carried out by the method of temperature cleaning. Obtained results showed that it arises from the overlapping of several simple TL glow peaks.

Zoom In Zoom Out Reset image size

As a result of deconvolution, five elementary TL glow peaks can be distinguished in the temperature range 225–500 K. The resulting elementary peaks are located at 285, 300, 354, 385 and 430 K. The activation energies of these peaks were 0.55, 0.62, 0.78, 0.94, and 1.1 eV, respectively. The most intense was the TL glow peak located at 300 K (Fig. 2a). The peaks at 285 K and 354 K were approximately 30% of the intensity of this one. A negligible increase in the concentration of chromium impurity to ∼0.005% (Fig. 2b) leads to a slight increase in the total intensity of TSL. The maximum of the TL glow curve shifts to lower temperatures 290–295 K. The redistribution of the TSL intensity was also observed in favor of the TL glow peak at 285 K for β-Ga₂O₃:0.005% Cr crystals. The amplitude of this peak increases and becomes equal to the intensity of the peak at 300 K. The TL glow peaks at 354, 385 and 430 K have changed little compared to UID β-Ga₂O₃ crystals. Further growth of the chromium concentration to 0.01 at% (Fig. 2c) leads to a significant increase in the total TSL intensity. Moreover, the TL glow peak at 285 K becomes dominant on the TSL curve at such concentration. Its amplitude is more than one order higher than the amplitude of other peaks. There is a further increase in both the total and the relative intensity of the peak at 285 K at a concentration of chromium impurities of about 0.05% (Fig. 2d). This TL glow peak becomes the main one and it stored almost all the light sum upon irradiation of the β-Ga₂O₃:0.05% Cr crystals.

It should be noted, that the intensity of the TL glow peak at 285 K increases from 0.3 to 15 arbitrary units (see Fig. 2) with an increase of chromium concentration from background to 0.05%, relative to the TL glow peak with a maximum at 300 K. The low-intensity TSL peaks at 354, 385 and 430 K in β-Ga₂O₃:Cr crystals remained practically unchanged at chromium concentrations increasing.

The dependence of the total light sum stored in elementary TL glow peaks (integral under separated peaks) mentioned above on the concentration of chromium impurity in β-Ga₂O₃:Cr crystals is presented in Table I.

The TL glow curve of the UID β-Ga₂O₃ crystals (background content of Cr³⁺) doped with a 0.1% Mg²⁺ impurity is shown in Fig. 3a. The complex TL glow curve with a maximum in the temperature range of 380–390 K was observed for these crystals. This glow curve also is the result of the superposition of elementary peaks located at 285, 300, 320, 354, 385 and 430 K. A new peak located at about 320 K appears with the entering of magnesium impurity. Other peaks were observed in Cr³⁺ doped β-Ga₂O₃:Cr crystals (see Fig. 2). The TSL peak at 385 K is dominant. The intensity of the new TL glow peak at 320 K was about 0.4 of the peak intensity at 385 K. The intensity of the other peaks is relatively low.

Zoom In Zoom Out Reset image size

The TL glow curve, when the concentration of Cr³⁺ impurity increased to 0.05% and the concentration of Mg impurity remained 0.1% are shown in Fig. 3b. Increasing the concentration of chromium impurity leads to significant changes in the overall shape of the TL glow curve. The maximum of TL glow curve is shifted to the 320–330 K for the β-Ga₂O₃:0.05%Cr³⁺, 0.1% Mg²⁺ crystal. This TL glow curve is also the result of the superposition of the simplest TSL peaks described above. As can be seen, the intensity of the 320 K peak increased at the chromium concentration grows in β-Ga₂O₃:Cr, Mg crystals. Namely, the TL glow peak at 320 K becomes dominant, while the intensity of the peak at 385 K is only 0.3 from its. The intensity of the low-temperature TL glow peak at 285 K also increases in the Cr³⁺ and Mg²⁺ co-doped crystals compared to the β-Ga₂O₃:0.1% Mg²⁺ crystals. The intensities of the TSL peaks with maxima at 300 K and 354 K remained actually unchanged in β-Ga₂O₃: 0.05%Cr³⁺, 0.1% Mg²⁺ crystal.

The TL glow curves of β-Ga₂O₃ crystals doped with chromium and co-doped with chromium and magnesium revealed deep traps that manifest in peaks at 285, 300, 320, 354, 385 and 430 K. The traps depths relative to the bottom of the conduction band are E_C-0.55, E_C-0.62, E_C-0.7, E_C-0.78, E_C-0.94 and E_C-1.1 eV, respectively. The Fermi level in the investigated crystals should be located far from the conduction band minimum (CBM) in particular below the depth of the all available electronic traps, which are manifested in the TSL. With such a Fermi-level arrangement, most of these traps are free and can capture the electrons when exposed. At the same time, heating of the irradiated crystals results in the release of electrons with subsequent recombination on Cr⁴⁺ ions and the formation of excited (Cr³⁺)^* states. As a result, the luminescence in the red spectral region is observed.

In particular, traps with an activation energy of E_c-0.55 eV that revealed in the TL glow peak at 285 K are the main traps in the Cr³⁺-doped β-Ga₂O₃ crystals. The activation energy of this trap is close to the energy of E₁.⁸ The almost linear increase in the light sum stored in this TSL peak is observed when the concentration of Cr³⁺ impurity increases (see Table I). It should also be noted that the light sum is proportional to the concentration of the traps. The light sum stored in this TSL peak increases about 50 times with a growth of the concentration of Cr³⁺ impurity from the background to 0.05 at%. Accordingly, such concentration dependence of the 285 K TL glow peak intensity indicates the impurity nature of this trap level. Trapped electrons are localized on oxygen ions in the vicinity of the first coordination sphere of the impurity ion and form the center of Cr³⁺-e⁻ or recharge the impurity ions from Cr³⁺ to Cr²⁺ state.

The TL glow peak near 300 K with an activation energy of ∼E_c-0,62 eV is most noticeable in the UID β-Ga₂O₃ crystals but it was detected in all Cr³⁺-doped β-Ga₂O₃ crystals. Its intensity is weak and practically does not depend on the Cr³⁺ and Mg²⁺ impurity concentrations. Therefore, it can be assumed that the traps responsible for the peak at 300 K are formed by one of the background impurities which are entered into the crystal from raw materials for crystal growing. The position and the activation energy of this TL glow peak are in good agreement with the location and energy of the main TSL peak in β-Ga₂O₃:Fe crystals.¹⁴ The negligible discrepancy between the peak maxima may be due to different crystal heating rates during the TSL study (0.1 K s⁻¹ in our case and 1 K s⁻¹ in work¹⁴). Therefore, the weak TL glow peak at 300 K can be analog of the peak 310 K registered by authors.¹⁴

A peak of TSL with a maximum near 354 K was also observed in all investigated samples. Its intensity did not change with the entering of chromium and magnesium impurities. Its depth of Eс-0.78 eV coincides very well with the trap E₂, which is associated with an iron impurity.^14,18–21 Therefore, we assume that this TL glow peak at 354 K can be also caused by a background impurity of iron.

The peak at 385 K with activation energies of 0.94 eV is main in Mg²⁺ doped crystals. Magnesium is known to create acceptor levels near the valence band^{7,13,17,33,34} and cannot be responsible for this peak. The entering of a divalent Mg²⁺ impurity in gallium oxide requires charge compensation to maintain the electroneutrality of crystals. The oxygen vacancies must be formed to compensate for the negative charge in the crystals doped with Mg²⁺ ions.^13,34 More precisely, to compensate for the charge of each two magnesium atoms, one oxygen vacancy will be formed. So the TSL peak at 385 K can be associated with the release of electrons from the oxygen vacancies.

The weak peak of the TL with a maximum near 430 K has an activation energy of Eс-1.1 eV close to the energy level of the E₃ traps.⁸ This weak peak was noticeable in all the studied crystals. It can be created by one of the uncontrolled impurities or by intrinsic defects. Further studies are needed to establish the nature of this energy level.

Unlike β-Ga₂O₃:Cr crystals, in β-Ga₂O₃ crystals co-doped with Cr³⁺ and Mg²⁺ impurities, the main one is the TL glow peak at 320 K with activation energy 0.7 eV. This peak did not occur in TSL of β-Ga₂O₃:Cr crystals with different concentrations of Cr³⁺ ions. This additional peak is new and appears only in the co-doped β-Ga₂O₃:Cr, Mg crystals. Moreover, the intensity of this peak increases when chromium content growth at a constant concentration of magnesium ions and an accompanying decrease in the 385 K peak intensity (see Fig. 3). This allows us to assume that the formation of these traps, occurs due to the same defects as for peaks at 285 and 385 K in the crystals β-Ga₂O₃: 0.05%Cr and β-Ga₂O₃: 0.05%Cr, 0.1%Mg, respectively. Since the deep traps E_C-0,55 eV (peak at 285 K) are formed by Cr³⁺ ions, and the traps E_C-0,94 eV (peak at 385 K) are likely to be related to oxygen vacancies, then new traps E_C-0.7 can be formed by the association of these simple defects. Located near to the Cr³⁺ ion, a positively charged oxygen vacancy V_O additionally attracts the trapped electron, thereby shifting the trap energy level towards the middle of the band gap forming a new level E_C-0.7 (peak at 320 K). Moreover, the Ec-0.7 eV level is close to the level of the E₂* state that has been credibly related to native defects produced by irradiation.¹⁰ And this natural defect of E₂*, which was noticed in the spectra of DLTS of irradiated samples, can be the associate of oxygen vacancy with Fe³⁺ ions. A similar level but with a slightly higher value of energy (0.75 eV) in the gallium oxide irradiated with protons was also observed by the authors.¹¹ They suggest that the origins of these levels are more likely due to a defect complex than an isolated point defect. Accordingly, the associates responsible for the peak at 320 K can include vacancies of oxygen and Cr³⁺ or Fe³⁺. However, further studies are needed to uniquely establish the nature of traps responsible for this TL glow peak.

Figure 4 shows the energy scheme of the main traps in β-Ga₂O₃ crystals doped with Cr³⁺ (a) and co-doped with Cr³⁺ and Mg²⁺ (b) impurities. Chromium creates deep traps for electrons in gallium oxide with an activation energy of E_C-0.55 eV manifested in TL glow peak at 285 K (Fig. 4a, red column). New traps with energy E_C-0.7 eV are created by defects associations Cr³⁺−V_O in co-doped with Cr³⁺ and Mg²⁺ impurities β-Ga₂O₃ crystals. Deep traps E_C-0.94 eV are related to oxygen vacancies.

Zoom In Zoom Out Reset image size

The results of the TSL studies of β-Ga₂O₃ single crystals doped with Cr³⁺ and co-doped with Cr³⁺ and Mg²⁺ impurities are presented. The growth of the chromium content from the background concentration to 0.05% is accompanied by an increase of the intensity of this peak by about 50 times. It was established that doping with the Cr³⁺ leads to the formation of electron traps with a depth of E_C-0.55 eV manifested in the TL glow peak with a maximum at 285 K. A TSL peaks at 300 and 354 K were observed in all investigated samples and its intensity did not change with the entering of Cr³⁺ and Mg²⁺ impurities. They can be caused by a background impurity of iron. The deeper traps E_C-0.94 eV (T_m = 385 K) is related to oxygen vacancies which are formed in the crystals when doped with divalent impurities, for example, Mg²⁺ ions. The co-doping of β-Ga₂O₃ crystals with Cr³⁺ and Mg²⁺ impurities leads to the appearance of new traps with a depth of E_C-0.7 eV and, accordingly, a new TL glow peak with a maximum near 320 K appears.

It has been established, that the TSL peak at 285 K is caused by the release of the electrons trapped by the Cr³⁺ ions (Cr³⁺e⁻-centers) and the TSL peak at 385 K is caused by the release of the electrons from the defects related to oxygen vacancy. It was also suggested that the TSL peak at 320 K is caused by the release of the electrons from the associate centers formed by impurity and native defects in particular oxygen vacancies.

The obtained results indicate that at sufficiently high concentrations of dopants or intrinsic defects it is necessary to consider the interaction of closely spaced charged defects because such interaction can lead to changes in traps activation energy. Moreover, since chromium creates trap levels at E_c-0,55 eV, in the manufacture of high-resistive substrates of gallium oxide for high-power devices, it is necessary to use raw materials with the lowest possible content of chromium impurity.

The work was partially supported by the Ministry of Education and Science of Ukraine (project no. 0118U003612).