The Origin of the Yellow Luminescence Band in Be-Doped Bulk GaN

: Photoluminescence (PL) from Be-doped bulk GaN crystals grown by the High Nitrogen Pressure Solution method was studied and compared with PL from GaN:Be layers on sapphire grown by molecular beam epitaxy and metalorganic chemical vapor deposition techniques. The yellow luminescence band in the latter is caused by the isolated Be Ga acceptor (the YL Be band), while the broad yellow band in bulk GaN:Be crystals is a superposition of the YL Be band and another band, most likely the C N -related YL1 band. The attribution of the yellow band in bulk GaN:Be crystals to the Be Ga O N complex (a deep donor) is questioned.


Introduction
GaN with a bandgap of 3.50 eV attracts significant attention from researchers due to its applications in light-emitting and high-power devices [1][2][3][4].Point defects that may detrimentally affect the devices' efficiency and longevity are still poorly understood.Photoluminescence (PL) is a powerful research tool that reveals the properties of point defects, such as charge state, transition levels, and carrier capture coefficients [5][6][7].
The most-studied defect-related PL band in GaN is the yellow luminescence (YL) band [8][9][10].In undoped GaN grown by metalorganic chemical vapor deposition (MOCVD), the YL band with a maximum at 2.17 eV and zero-phonon line (ZPL) at 2.59 eV (labeled the YL1 band) is commonly observed due to contamination with carbon and attributed to the C N acceptor [10][11][12][13][14].The YL1 band can also be recognized in PL spectra from undoped GaN grown by other techniques such as the High Nitrogen Pressure Solution (HNPS) method [15] and the hydride vapor phase epitaxy (HVPE) method [12].Very low concentrations of carbon (10 15 -10 16 cm −3 ) in HVPE-grown undoped GaN lead to the quantum efficiency of the YL1 band close to unity thanks to a relatively high holecapture cross-section of the C N defect [16,17].This explains why the C N -related YL1 band is omnipresent in undoped GaN and GaN doped with various impurities.
A different (as will be shown below) yellow band is observed in GaN doped or implanted with beryllium (Be) [9,[17][18][19].A renewed interest in doping III-nitrides with Be is fueled by recent findings that shallow Be-related acceptors can potentially be used for conductive p-type GaN and AlN [20][21][22].We recently studied PL from more than 100 Be-doped GaN samples grown by MOCVD and molecular beam epitaxy (MBE), as well as GaN samples grown by HVPE and implanted with Be or co-implanted with Be and F [23,24].In almost all these samples, the YL band is the dominant defect-related PL band with absolute internal quantum efficiency approaching unity in some samples.This YL band, with a maximum at 2.15 eV and specific properties (labeled the YL Be band), is now reliably attributed to the isolated Be Ga acceptor [25].
In fair agreement with the theoretical predictions of Lany and Zunger [26], deep and shallow states of the Be Ga acceptor are found in PL experiments [25].Two deep polaronic configurations with a hole localized at a neighboring N atom in the c direction and one of three other directions have −/0 transition levels at 0.38 ± 0.03 eV (configuration Be2) and 0.33 ± 0.05 eV (Be1), respectively, above the valence band maximum (VBM) in the limit of low temperatures.Electron transitions from the conduction band to these levels are responsible for the YL Be2 and YL Be1 components of the YL Be band, respectively [25].The shallow state with a delocalized hole (Be3) is at 0.24 ± 0.02 eV above the VBM.Electron transitions from the conduction band to the shallow level cause the ultraviolet luminescence (UVL) band, with the main peak at 3.26 eV (labeled the UVL Be3 ).Although the UVL Be3 band resembles the Mg-related UVL band (the UVL Mg band with the main peak at 3.28 eV), these two bands can be reliably distinguished thanks to their specific temperature dependences [25].In particular, the ratio of intensities of the UVL Be3 and YL Be2 bands, I UVL Be3 /I YL Be2 , is the same for all GaN:Be samples, and it can be fitted with the following expression: where δ is approximately equal to the ratio of the electron-capture coefficients for the Be3 and Be2 states of the Be Ga acceptor (18 ± 8) and ∆E is the energy difference between these states (140 ± 10 meV).This property allows us to reliably recognize PL from the Be Ga acceptor in GaN:Be samples, even when other PL bands contribute to the PL spectrum.
Another unique feature of the YL Be band (observed in all studied GaN:Be layers on sapphire) is two-step quenching and complete disappearance at temperatures close to room temperature.The first step at about 100 K is attributed to a replacement of the YL Be1 component by the YL Be2 component after a captured hole transits from Be1 to Be2 over a potential barrier.The second quenching begins at about 200 K.The activation energy of this quenching is 0.30 eV in n-type samples, which led us to erroneously assume that the quenching is caused by the thermal emission of holes from the deep polaronic state to the valence band [23].However, a more detailed analysis indicated that the quenching is caused by the thermal emission of holes from the shallow Be Ga state (Be3, at ~0.20 eV at these temperatures) to the valence band.The larger nominal activation energy and relatively high temperatures at which the quenching of PL from such a shallow level occurs are explained by the feeding mechanism [25].Namely, holes from the deep polaronic state are excited to the shallow state before they escape to the valence band, so that the measured activation energy of the quenching is the sum of the shallow state energy (E 3 ) and the energy difference between the shallow and deep states (∆E).
A similar and very bright YL band with a maximum at 2.1-2.2eV is observed in bulk GaN:Be crystals produced by the HNPS growth method [27][28][29].However, the properties of the YL band in these crystals (labeled YL bulk , hereafter) appear to be different from those of the YL Be band in GaN:Be layers grown by MBE or MOCVD.In particular, the YL bulk band is strong at room temperature, and its quenching begins only at T > 500 K [28].It can be excited resonantly, with photon energies down to ~2.7 eV [28,29].These features of the YL bulk band indicate that the related transition level is at least 0.8 eV above the VBM, much deeper than the levels of the YL Be band.Teisseyre et al. [27] attributed the YL bulk band to the Be Ga O N complex (the YL BeO , hereafter).The assignment is based on theoretical predictions [27] and seems reasonable because these samples contain very high concentrations of Be and O (about 10 19 cm −3 ).However, the 0/+ transition level of the Be Ga O N complex is calculated at 0.26 eV [27].Moreover, first-principles calculations by another research group predict that the Be Ga O N complex is electrically neutral and does not have transition levels in the band gap [23].The controversy can be resolved if we assume that more than one PL band contributes to the broad YL bulk band.Lamprecht et al. [28] proposed that the YL bulk band consists of three components, with maxima at 1.88 eV (red band), 2.08, and 2.12 eV (yellow).The 2.08 eV component was attributed to the Be Ga O N complex, while the 2.12 eV component was thought to be caused by the C N acceptor.Time-resolved PL experiments using a shutter indicated that all three components decay very slowly, so the YL bulk signal could be observed for several seconds [28].
In this work, we propose a solution to the problem of the yellow band in Be-doped GaN.The Be Ga -related YL Be band with its specific properties is found in bulk GaN crystals grown by the HNPS technique.It is buried under a stronger yellow band in bulk GaN:Be crystals (temporarily labeled YL X ).The YL X band is caused by electron transitions from the conduction band (or from the shallow donors) to a deep level at 0.8-0.9eV above the VBM and is likely the C N -related YL1 band.

Materials and Methods
Be-doped GaN crystals were grown by the HNPS method at a growth temperature of 1450 • C and at 1 GPa of nitrogen pressure [30,31].Beryllium doping was at a level of 10 19 cm −3 .The typical lateral size of spontaneously crystallized GaN:Be hexagonal platelets did not exceed 5 mm and the thickness was 150 µm.According to secondary ion mass spectrometry (SIMS) measurements performed at the Institute of Physics of the Polish Academy of Sciences with CAMECA IMS6F microanalyzer (Gennevilliers, France), GaN:Be crystals grown under similar conditions contain 3 × 10 19 cm −3 of Be atoms, 2 × 10 19 cm −3 of O, and 5 × 10 17 cm −3 of C. The samples are semi-insulating.Point defects in similar crystals were previously studied by PL [27][28][29]31], positron annihilation [32], optically detected magnetic resonance (ODMR) [33], infrared absorption [34], and photo-induced electron paramagnetic resonance (photo-EPR) [35,36].
Steady-state PL was excited with a HeCd laser, dispersed by 1200 rules/mm grating in a 0.3 m monochromator, and detected by a cooled photomultiplier tube.Other details of PL experiments can be found elsewhere [7,13].The as-measured PL spectra were corrected for the measurement system's spectral response, and PL intensity was additionally multiplied by λ 3 , where λ is the light wavelength, to present the PL spectra in units proportional to the number of emitted photons as a function of photon energy [13].

PL Bands in Be-Doped GaN Grown by MOCVD and MBE Techniques
The shape and position of the YL Be band are well reproduced in many GaN layers grown by MOCVD and MBE. Figure 1 shows representative PL spectra at T = 18 K.The shape of the YL Be band can be fitted (the dashed line) with the following expression obtained in the one-dimensional configuration coordinate model [37]: Here, S e is the Huang-Rhys factor in the excited state of the defect; d FC g = E * 0 − hω max is the Frank-Condon shift in the ground state; E * 0 = E 0 + 0.5hΩ e , where E 0 is the ZPL energy and hΩ e is the energy of the effective phonon mode in the excited state; and hω and hω max are the photon energy and position of the PL band maximum, respectively.The ∆ is a minor shift of the PL band maximum due to sample-dependent reasons such as in-plane biaxial strain in thin GaN layers grown on sapphire substrates (blue shift) or local electric fields (red shift).
In addition to the YL Be band, PL spectra from Be-doped GaN grown by MOCVD and MBE contain two UVL bands: the Mg Ga -related UVL Mg band, with the first peak at 3.27 eV, and the Be-related UVL Be band, with the first peak at 3.38 eV.Both peaks are followed by a few LO phonon replicas.The UVL Mg band is caused by mild contamination with Mg during growth (especially significant in our MOCVD GaN:Be samples).The UVL Be band was found in the majority of GaN:Be samples.In MBE-grown samples, it required activation by annealing at T > 750 • C (Figure 1).Our earlier attribution of this band to the shallow state of the Be Ga acceptor [20] was later rejected, and the band is preliminary attributed to a complex defect involving Be [25].Normalized PL spectra at T = 18 K from MBE GaN:Be (sample 0408a, as grown and annealed at 800 °C) and two MOCVD GaN:Be (samples R68 and R87).The BeGa-related YLBe band is very strong in all the samples (the quantum efficiency is close to unity).The dashed line is calculated using Equation (2) with the following parameters: Se = 22, E0 * = 3.2 eV, and dFC g = 1.05 eV.The UVLBe band (presumably caused by a Be-containing complex) emerges in MBE GaN:Be samples only after thermal annealing.In MOCVD GaN:Be, the UVLBe band is strong in some samples (such as R87) but could not be resolved in other samples (R68).The MgGa-related UVLMg band is often observed in MOCVD GaN:Be samples because of contamination with Mg.NBE is the near-band-edge emission.
Here, Se is the Huang-Rhys factor in the excited state of the defect; dFC g = E0 * − ħωmax is the Frank-Condon shift in the ground state; E0 * = E0 + 0.5ħΩe, where E0 is the ZPL energy and ħΩe is the energy of the effective phonon mode in the excited state; and ħω and ħωmax are the photon energy and position of the PL band maximum, respectively.The Δ is a minor shift of the PL band maximum due to sample-dependent reasons such as in-plane biaxial strain in thin GaN layers grown on sapphire substrates (blue shift) or local electric fields (red shift).
In addition to the YLBe band, PL spectra from Be-doped GaN grown by MOCVD and MBE contain two UVL bands: the MgGa-related UVLMg band, with the first peak at 3.27 eV, and the Be-related UVLBe band, with the first peak at 3.38 eV.Both peaks are followed by a few LO phonon replicas.The UVLMg band is caused by mild contamination with Mg during growth (especially significant in our MOCVD GaN:Be samples).The UVLBe band was found in the majority of GaN:Be samples.In MBE-grown samples, it required activation by annealing at T > 750 °C (Figure 1).Our earlier attribution of this band to the shallow state of the BeGa acceptor [20] was later rejected, and the band is preliminary attributed to a complex defect involving Be [25].
The best way to recognize the BeGa-related YLBe band is to investigate the temperature evolution of the PL spectrum, the characteristic features of which were reproduced in all studied GaN:Be samples grown by MBE and MOCVD [25].The YLBe band is quenched in two steps.At T ≈ 100 K, the YLBe band intensity drops (usually by a factor of about 2) and the band abruptly redshifts by up to 50 meV.This behavior is explained by the relaxation of the bound hole from the polaronic state Be1 at 0.33 eV to the polaronic state Be2 at 0.38 eV.At T > 180 K, the second quenching is observed (Figure 2).In conductive n-type GaN:Be samples, the second quenching reveals an activation energy of 0.30 eV, while in semi-insulating samples, the quenching occurs by the abrupt and tunable quenching (ATQ) mechanism [38].Normalized PL spectra at T = 18 K from MBE GaN:Be (sample 0408a, as grown and annealed at 800 • C) and two MOCVD GaN:Be (samples R68 and R87).The Be Ga -related YL Be band is very strong in all the samples (the quantum efficiency is close to unity).The dashed line is calculated using Equation ( 2) with the following parameters: S e = 22, E 0 * = 3.2 eV, and d FC g = 1.05 eV.The UVL Be band (presumably caused by a Be-containing complex) emerges in MBE GaN:Be samples only after thermal annealing.In MOCVD GaN:Be, the UVL Be band is strong in some samples (such as R87) but could not be resolved in other samples (R68).The Mg Ga -related UVL Mg band is often observed in MOCVD GaN:Be samples because of contamination with Mg.NBE is the near-band-edge emission.
The best way to recognize the Be Ga -related YL Be band is to investigate the temperature evolution of the PL spectrum, the characteristic features of which were reproduced in all studied GaN:Be samples grown by MBE and MOCVD [25].The YL Be band is quenched in two steps.At T ≈ 100 K, the YL Be band intensity drops (usually by a factor of about 2) and the band abruptly redshifts by up to 50 meV.This behavior is explained by the relaxation of the bound hole from the polaronic state Be1 at 0.33 eV to the polaronic state Be2 at 0.38 eV.At T > 180 K, the second quenching is observed (Figure 2).In conductive n-type GaN:Be samples, the second quenching reveals an activation energy of 0.30 eV, while in semi-insulating samples, the quenching occurs by the abrupt and tunable quenching (ATQ) mechanism [38].
The most important feature of the temperature evolution of PL is the emergence of a UVL band (UVL Be3 ) at T > 140 K (Figure 2).The shift of the UVL maximum with temperature is shown in Figure 3.At T < 25 K, the UVL band is caused by donor-acceptorpair (DAP)-type transitions of electrons from shallow O N donors to shallow Mg Ga acceptors.At T > 50 K, the DAP peaks are replaced with similar peaks caused by electron transitions from the conduction band to the Mg Ga acceptors (eA transitions).At T > 130 K, the UVL Mg band is quenched by the ATQ mechanism.A similar in shape but slightly red-shifted UVL Be3 band emerges at higher temperatures.It can be seen from Figure 3 that the ZPL of the UVL Be3 is red-shifted by 13 ± 3 meV from the ZPL of the UVL Mg band.Since the transition level of the shallow Mg Ga acceptor is located at 223 ± 3 meV above the VBM at T = 18 K [12], we conclude that the shallow −/0 transition level of the Be Ga acceptor (the Be3 state) is located at 236 ± 5 meV above the VBM.With increasing temperature, the Be3 level moves towards the valence band (0.20 eV at T ≈ 200 K) [25].The most important feature of the temperature evolution of PL is the emergence of a UVL band (UVLBe3) at T > 140 K (Figure 2).The shift of the UVL maximum with temperature is shown in Figure 3.At T < 25 K, the UVL band is caused by donor-acceptor-pair (DAP)-type transitions of electrons from shallow ON donors to shallow MgGa acceptors.At T > 50 K, the DAP peaks are replaced with similar peaks caused by electron transitions from the conduction band to the MgGa acceptors (eA transitions).At T > 130 K, the UVLMg band is quenched by the ATQ mechanism.A similar in shape but slightly red-shifted UVLBe3 band emerges at higher temperatures.It can be seen from Figure 3 that the ZPL of the UVLBe3 is red-shifted by 13 ± 3 meV from the ZPL of the UVLMg band.Since the transition level of the shallow MgGa acceptor is located at 223 ± 3 meV above the VBM at T = 18 K [12], we conclude that the shallow −/0 transition level of the BeGa acceptor (the Be3 state) is located at 236 ± 5 meV above the VBM.With increasing temperature, the Be3 level moves towards the valence band (0.20 eV at T ≈ 200 K) [25].PL spectra from MOCVD GaN:Be sample R87 at selected temperatures and P exc = 0.005 W/cm 2 .The Be Ga -related YL Be band is quenched by the ATQ mechanism at T > 160 K.The Mg Ga -related UVL Mg band disappears at T > 130 K and is replaced with the Be Ga -related UVL Be3 band emerging at higher temperatures.The most important feature of the temperature evolution of PL is the emergence of a UVL band (UVLBe3) at T > 140 K (Figure 2).The shift of the UVL maximum with temperature is shown in Figure 3.At T < 25 K, the UVL band is caused by donor-acceptor-pair (DAP)-type transitions of electrons from shallow ON donors to shallow MgGa acceptors.At T > 50 K, the DAP peaks are replaced with similar peaks caused by electron transitions from the conduction band to the MgGa acceptors (eA transitions).At T > 130 K, the UVLMg band is quenched by the ATQ mechanism.A similar in shape but slightly red-shifted UVLBe3 band emerges at higher temperatures.It can be seen from Figure 3 that the ZPL of the UVLBe3 is red-shifted by 13 ± 3 meV from the ZPL of the UVLMg band.Since the transition level of the shallow MgGa acceptor is located at 223 ± 3 meV above the VBM at T = 18 K [12], we conclude that the shallow −/0 transition level of the BeGa acceptor (the Be3 state) is located at 236 ± 5 meV above the VBM.With increasing temperature, the Be3 level moves towards the valence band (0.20 eV at T ≈ 200 K) [25].In agreement with the model proposed in Ref. [25], the ratio UVL Be3 /YL Be of PL intensities is the same in all GaN:Be samples.This ratio is described in a wide range of temperatures with Equation (1), where ∆E is the energy distance between the Be2 and Be3 states (∆E = 140-145 meV).Figure 4 shows the dependence for two samples.In the MOCVD GaN:Be sample, the UVL Mg band is quenched and replaced by the UVL Be3 band at T > 140 K (compare Figures 3 and 4 for this sample).
In agreement with the model proposed in Ref. [25], the ratio UVLBe3/YLBe of PL intensities is the same in all GaN:Be samples.This ratio is described in a wide range of temperatures with Equation ( 1), where ΔE is the energy distance between the Be2 and Be3 states (ΔE = 140-145 meV).Figure 4 shows the dependence for two samples.In the MOCVD GaN:Be sample, the UVLMg band is quenched and replaced by the UVLBe3 band at T > 140 K (compare Figures 3 and 4 for this sample).A remarkable feature of this model is that the second step of the YLBe quenching is caused by the thermal emission of holes, not from a deep but from the shallow Be3 level, to the valence band.In particular, the activation energy of 0.30 eV obtained from this quenching in conductive n-type GaN:Be samples is the sum of the Be3 ionization energy and ΔE [25].From this, the deepest polaronic state of the BeGa acceptor is found at 0.38 ± 0.03 eV above the VBM in the limit of low temperatures (≈0.30eV at T = 200 K).The unique interplay of the UVLBe3 and YLBe bands will be used below to recognize the YLBe band in bulk GaN crystals.

The Yellow Band in Bulk GaN:Be Grown by High-Pressure Technique
Figure 5 shows PL spectra from bulk GaN:Be excited above-bandgap (with the 325 nm line of the HeCd laser at 3.81 eV) and below-bandgap (with the 442 nm line of the HeCd laser at 2.80 eV).The spectra are compared with the PL spectrum from MBE-grown GaN:Be.In the case of the usual above-bandgap excitation, a very weak near-band-edge (NBE) emission is detected at 3.47 eV.The UVLMg band appears as a structureless shoulder at about 3.2 eV.We assume that its ZPL and LO phonon replicas are unresolved because of strong local electric fields caused by short-range potential fluctuations in heavily doped and compensated GaN:Be [39].A remarkable feature of this model is that the second step of the YL Be quenching is caused by the thermal emission of holes, not from a deep but from the shallow Be3 level, to the valence band.In particular, the activation energy of 0.30 eV obtained from this quenching in conductive n-type GaN:Be samples is the sum of the Be3 ionization energy and ∆E [25].From this, the deepest polaronic state of the Be Ga acceptor is found at 0.38 ± 0.03 eV above the VBM in the limit of low temperatures (≈0.30eV at T = 200 K).The unique interplay of the UVL Be3 and YL Be bands will be used below to recognize the YL Be band in bulk GaN crystals.

The Yellow Band in Bulk GaN:Be Grown by High-Pressure Technique
Figure 5 shows PL spectra from bulk GaN:Be excited above-bandgap (with the 325 nm line of the HeCd laser at 3.81 eV) and below-bandgap (with the 442 nm line of the HeCd laser at 2.80 eV).The spectra are compared with the PL spectrum from MBE-grown GaN:Be.In the case of the usual above-bandgap excitation, a very weak near-band-edge (NBE) emission is detected at 3.47 eV.The UVL Mg band appears as a structureless shoulder at about 3.2 eV.We assume that its ZPL and LO phonon replicas are unresolved because of strong local electric fields caused by short-range potential fluctuations in heavily doped and compensated GaN:Be [39].
The yellow band (YL bulk ) at ~2.1 eV is broad and very strong, in agreement with earlier reports [28,29].With increasing excitation intensity, P exc , from 10 −5 to ~100 W/cm 2 , it blueshifts by about 0.1 eV.After a laser pulse at T = 18 K, the PL intensity decays nonexponentially, approximately as t −1.2 at time delays t between 1 and 1000 s, and the band redshifts by almost 0.1 eV.Such behavior agrees with the presence of potential fluctuations.
The yellow band in bulk GaN:Be crystals can also be excited below the bandgap.The spectrum obtained after excitation with the 442 nm line of the HeCd laser (2.8 eV) is shown in Figure 5 with filled circles.This band (temporarily labeled YL X ) is also broad and its maximum is at 2.07 eV.Interestingly, the sum of the YL Be band in MBE GaN:Be and the YL X band matches the shape of the YL bulk band observed after the above-bandgap excitation (Figure 5).This is the first indication that the YL bulk band may consist of two unresolved broad bands, one of which is the well-studied Be Ga -related YL Be band.A stronger argument follows from the analysis of the evolution of the PL spectrum with temperature, as discussed below.The yellow band (YLbulk) at ~2.1 eV is broad and very strong, in agreement with earlier reports [28,29].With increasing excitation intensity, Pexc, from 10 −5 to ~100 W/cm 2 , it blueshifts by about 0.1 eV.After a laser pulse at T = 18 K, the PL intensity decays nonexponentially, approximately as t −1.2 at time delays t between 1 and 1000 s, and the band redshifts by almost 0.1 eV.Such behavior agrees with the presence of potential fluctuations.
The yellow band in bulk GaN:Be crystals can also be excited below the bandgap.The spectrum obtained after excitation with the 442 nm line of the HeCd laser (2.8 eV) is shown in Figure 5 with filled circles.This band (temporarily labeled YLX) is also broad and its maximum is at 2.07 eV.Interestingly, the sum of the YLBe band in MBE GaN:Be and the YLX band matches the shape of the YLbulk band observed after the above-bandgap excitation (Figure 5).This is the first indication that the YLbulk band may consist of two unresolved broad bands, one of which is the well-studied BeGa-related YLBe band.A stronger argument follows from the analysis of the evolution of the PL spectrum with temperature, as discussed below.
Figure 6 shows the PL spectrum from bulk GaN:Be at selected temperatures.The intensity of the YLbulk band changes insignificantly with increasing temperature from 18 to 320 K.The intensity of the UVL band decreases between 18 and 140 K and then increases (between 140 and 200 K).While the fine structure of the UVL band cannot be resolved, the increase in the UVL intensity is very similar to that observed in the GaN:Be samples grown by MBE and MOCVD [25].Figure 6 shows the PL spectrum from bulk GaN:Be at selected temperatures.The intensity of the YL bulk band changes insignificantly with increasing temperature from 18 to 320 K.The intensity of the UVL band decreases between 18 and 140 K and then increases (between 140 and 200 K).While the fine structure of the UVL band cannot be resolved, the increase in the UVL intensity is very similar to that observed in the GaN:Be samples grown by MBE and MOCVD [25].Figure 7 compares PL spectra at T = 200 K for bulk GaN:Be (at two crystal locations) and GaN:Be layers grown by MBE and MOCVD (arbitrarily shifted vertically to match the UVL intensity).The ratio of intensities UVLBe3/YLBe at the selected temperature must be the same in all Be-doped GaN samples.Assuming that the UVLBe3 band emerges with the Figure 6.PL spectra from bulk GaN:Be at selected temperatures and P exc = 0.005 W/cm 2 .The YL bulk band consists of unresolved YL Be and YL X bands.The Mg Ga -related UVL Mg band at 18 K is replaced by a similar Be Ga -related UVL Be3 band at T > 120 K (both labeled UVL).
Figure 7 compares PL spectra at T = 200 K for bulk GaN:Be (at two crystal locations) and GaN:Be layers grown by MBE and MOCVD (arbitrarily shifted vertically to match the UVL intensity).The ratio of intensities UVL Be3 /YL Be at the selected temperature must be the same in all Be-doped GaN samples.Assuming that the UVL Be3 band emerges with the temperature [25], we conclude that the YL Be band in bulk GaN is hidden under a stronger yellow band of different origin (YL X ). Figure 6.PL spectra from bulk GaN:Be at selected temperatures and Pexc = 0.005 W/cm 2 .The YLbulk band consists of unresolved YLBe and YLX bands.The MgGa-related UVLMg band at 18 K is replaced by a similar BeGa-related UVLBe3 band at T > 120 K (both labeled UVL).
Figure 7 compares PL spectra at T = 200 K for bulk GaN:Be (at two crystal locations) and GaN:Be layers grown by MBE and MOCVD (arbitrarily shifted vertically to match the UVL intensity).The ratio of intensities UVLBe3/YLBe at the selected temperature must be the same in all Be-doped GaN samples.Assuming that the UVLBe3 band emerges with the temperature [25], we conclude that the YLBe band in bulk GaN is hidden under a stronger yellow band of different origin (YLX).The first step of the YL Be quenching is expected at T ≈ 100 K but cannot be resolved.The YL Be band hides under a stronger yellow band at T < 200 K and completely disappears at temperatures approaching room temperature, as expected [25].We estimate that the YL X band is stronger than the YL Be band by a factor of 2-5 at T < 200 K.
We also studied the temperature dependence of the YL bulk full width at half maximum (FWHM or W) and compared it with that for the YL Be band in the GaN:Be samples grown by MOCVD (Figure 9).For PL bands that are broad due to strong electron-phonon coupling, the W(T) dependence can be fitted with the following expression [6]: where hΩ e is the energy of the effective phonon mode.For MOCVD-grown GaN:Be samples, the fit with hΩ e = 38 meV is relatively good, and the value of the single fitting parameter is typical for defects in GaN (between 23 and 56 meV) [37,40,41].However, the W(T) dependence for the YL bulk band in bulk GaN:Mg shows abnormal narrowing with increasing temperature from ~170 to 250 K.This temperature region roughly corresponds to the region where the YL Be band is quenched.We conclude that at T > 250 K, the contribution of the YL Be band to the YL bulk band can be ignored and only the YL X band remains.The FWHM of the YL X band is smaller than that of the YL Be band, while the parameter hΩ e is larger (Figure 9).The first step of the YLBe quenching is expected at T ≈ 100 K but cannot be resolved.The YLBe band hides under a stronger yellow band at T < 200 K and completely disappears at temperatures approaching room temperature, as expected We estimate that the YLX band is stronger than the YLBe band by a factor of 2-5 at T < 200 K.
We also studied the temperature dependence of the YLbulk full width at half maximum (FWHM or W) and compared it with that for the YLBe band in the GaN:Be samples grown by MOCVD (Figure 9).For PL bands that are broad due to strong electron-phonon coupling, the W(T) dependence can be fitted with the following expression [6]: where ħΩe is the energy of the effective phonon mode.For MOCVD-grown GaN:Be samples, the fit with ħΩe = 38 meV is relatively good, and the value of the single fitting parameter is typical for defects in GaN (between 23 and 56 meV) [37,40,41].However, the W(T) dependence for the YLbulk band in bulk GaN:Mg shows abnormal narrowing with increasing temperature from ~170 to 250 K.This temperature region roughly corresponds to the region where the YLBe band is quenched.We conclude that at T > 250 K, the contribution of the YLBe band to the YLbulk band can be ignored and only the YLX band remains.The FWHM of the YLX band is smaller than that of the YLBe band, while the parameter ħΩe is larger (Figure 9). Figure 10 shows the temperature dependence of the YLbulk band maximum (shifted up by 23 meV for Pexc = 0.005 W/cm 2 ) in comparison with the dependence for the YLBe band in the MOCVD and MBE GaN:Be samples.The sudden shift at about 100 K is similar in all the samples, and it can be explained by replacing the YLBe1 with the YLBe2, PL bands associated with two deep polaronic states of the BeGa acceptor [25].At T > 230 K, the band maximum corresponds to the YLX band because the YLBe band is quenched.Note that the YLbulk band shifts significantly with excitation intensity (up to 0.1 eV with a variation of Pexc from 10 −5 to ~100 W/cm 2 ), whereas the shift of the YLBe band in the MOCVD and MBE GaN:Be samples is insignificant.Figure 10 shows the temperature dependence of the YL bulk band maximum (shifted up by 23 meV for P exc = 0.005 W/cm 2 ) in comparison with the dependence for the YL Be band in the MOCVD and MBE GaN:Be samples.The sudden shift at about 100 K is similar in all the samples, and it can be explained by replacing the YL Be1 with the YL Be2 , PL bands associated with two deep polaronic states of the Be Ga acceptor [25].At T > 230 K, the band maximum corresponds to the YL X band because the YL Be band is quenched.Note that the YL bulk band shifts significantly with excitation intensity (up to 0.1 eV with a variation of P exc from 10 −5 to ~100 W/cm 2 ), whereas the shift of the YL Be band in the MOCVD and MBE GaN:Be samples is insignificant.all the samples, and it can be explained by replacing the YLBe1 with the YLBe2, PL bands associated with two deep polaronic states of the BeGa acceptor [25].At T > 230 K, the band maximum corresponds to the YLX because the YLBe band is quenched.Note that the YLbulk band shifts significantly with excitation intensity (up to 0.1 eV with a variation of Pexc from 10 −5 to ~100 W/cm 2 ), whereas the shift of the YLBe band in the MOCVD and MBE GaN:Be samples is insignificant.Temperature dependence of the YL bulk band maximum in bulk GaN:Be for P exc = 0.005 W/cm 2 (blue-shifted by 23 meV for convenience of comparison) and 0.13 W/cm 2 (solid symbols) and the YL Be band in the MOCVD and MBE GaN:Be samples at P exc = 0.005 W/cm 2 (empty symbols).
To further explore the properties of the YL X band, we conducted PL experiments by using a high-temperature cryostat (Figure 11).The YL X band is quenched by the ATQ mechanism, which is typical for defects in semi-insulating GaN [38].Formally, it can be described with the commonly used expression: However, unlike the PL quenching in conductive n-type GaN, the parameter E A does not have a physical meaning [17].The quenching is very abrupt, yet the slope of the quenching could be smoothened by potential fluctuations [42], or other reasons [38].The quenching is tunable by excitation intensity.
To further explore the properties of the YLX band, we conducted PL experiments by using a high-temperature cryostat (Figure 11).The YLX band is quenched by the ATQ mechanism, which is typical for defects in semi-insulating GaN [38].Formally, it can be described with the commonly used expression: However, unlike the PL quenching in conductive n-type GaN, the parameter EA does not have a physical meaning [17].The quenching is very abrupt, yet the slope of the quenching could be smoothened by potential fluctuations [42], or other reasons [38].The quenching is tunable by excitation intensity.

Discussion
The results of this study indicate that at least two PL bands contribute to the YLbulk band, with a maximum at about 2.1 eV in bulk GaN:Be grown by the HNPS method.These are the YLBe band (with a maximum at 2.15 eV) associated with the isolated BeGa acceptor and the YLX band (at ~2.06 eV) caused by a different defect.Since the YLBe band is well

Discussion
The results of this study indicate that at least two PL bands contribute to the YL bulk band, with a maximum at about 2.1 eV in bulk GaN:Be grown by the HNPS method.These are the Be band (with a maximum at 2.15 eV) associated with the isolated Be Ga acceptor and the YL X band (at ~2.06 eV) caused by a different defect.Since the YL Be band is well studied [25], we will focus below on the analysis of the YL X band, which is the strongest PL band at all studied temperatures.

Type of Transition
The YL X band at T = 18 K can be excited below-bandgap, with photon energies of 2.8 eV (Figure 5).Earlier, the onset of the PL excitation (PLE) spectrum of the yellow band was found at 2.8 eV at 7 K, and the YL bulk band could be excited with a 2.78 eV laser [28].At room temperature (when we expect only the YL X band), the PLE spectrum also shows an onset at about 2.7-2.8eV [29].We assume that the YL X band is caused by electron transitions from the conduction band (or from shallow donors) to a deep defect level, as it occurs for almost all defects in GaN [7,17].Then, the results of the current work and those of previous studies indicate that the related defect level is located no closer than 0.8 eV to the VBM.Nonexponential and slow PL decay after a laser pulse agrees with the assumed type of transition.This PL cannot be caused by transitions from an excited state to the ground state of some defect because such transitions are characterized by exponential PL decay, even at very low temperatures, and positions of such PL bands are not affected by local electric fields [7,17].At this point, transitions from a deep donor to an acceptor (or a deeper donor) cannot be excluded since such transitions are possible in heavily doped GaN and are characterized by slow, nonexponential decay.If this was the case, the defect level responsible for the YL X band would be much closer to the VBM than 0.8 eV.However, the quenching of the YL X band at T > 450 K (Figure 11 and the data in Ref. [28]) favors the assumption that the related defect level is at 0.8-0.9eV above the VBM.It was shown in Ref. [17] that for defects with relatively strong PL in GaN, the quenching begins at T = 400-500 K (at P exc ≈ 0.1 W/cm 2 when the quenching occurs by the ATQ mechanism) if the defect level is located at 0.6-1.0eV above the VBM.Transitions from the conduction band (or from shallow donors) to a defect level at ~0.8 eV above the VBM may be characterized by a nonexponential, slow decay of PL and by relatively large PL band shifts with a variation of P exc if local electric fields (e.g., due to potential fluctuations) are present.ODMR and photo-EPR studies of the yellow band in similar bulk GaN:Be crystals support this type of transition [33,35].We exclude the possibility of electron transitions from a defect level located in the upper part of the gap to the valence band as the origin of the YL X band.Such transitions cannot be observed because photogenerated holes are captured by various acceptors with the multiphonon nonradiative mechanism many orders of magnitude faster than via radiative transitions [7].

On Identification of the YL X Band in Bulk GaN
The observed broad YL bulk band in bulk GaN:Be consists of at least two unresolved bands, one of which is a well-studied YL Be band associated with the isolated Be Ga acceptor.Another PL band (temporarily labeled YL X ) is caused by electron transitions from the conduction band (or shallow donors) to a deep defect (donor or acceptor), with a transition level at 0.8-0.9eV above the VBM.Since the main difference between GaN:Be layers grown by MBE or MOCVD methods and bulk GaN:Be crystals grown by the HNPS method is a high concentration of oxygen in the latter, it is tempting to attribute it to the Be Ga O N complex, which is expected to be a deep donor [27].However, the first-principles calculations using the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional [43] predict that the 0/+ level of the Be Ga O N defect is located at 0.26 eV above the VBM [27], or that this defect has no transition levels in the bandgap [23].Whereas fair agreement of theoretical predictions with experimental findings is observed for the Be Ga acceptor, the disagreement for the 0/+ level of the Be Ga O N defect is significant (Table 1).Another possibility is that the YL X band is the C N -related YL1 band slightly broadened and redshifted because of potential fluctuations (Figure 12).Both PL bands are quenched at T > 500 K in semi-insulating GaN (at P exc ≈ 0.1 W/cm 2 ).The concentration of carbon impurities in the studied bulk GaN:Be samples is about 5 × 10 17 cm −3 , which is enough to cause a strong yellow band.
Solids 2024, 5, FOR PEER REVIEW 13 carbon impurities in the studied bulk GaN:Be samples is about 5 × 10 17 cm −3 , which is enough to cause a strong yellow band.Below, we will discuss two candidates for the YLX band (CN acceptor and BeGaON donor).Parameters of defects that may contribute to the YLbulk band are given in Table 2.The hole-capture coefficient, Cp, for the CN acceptor is (3.7 ± 1.6) × 10 −7 cm 3 /s.The Cp for the BeGa acceptor was estimated from the YLBe quenching as 4 × 10 −7 cm 3 /s.However, careful analysis of PL quenching in numerous MOCVD GaN:Be samples (which contain C and Be, with concentrations of about 10 17 cm −3 and (1-30) × 10 17 cm −3 , respectively) shows that the Cp for the BeGa acceptor is larger, which is consistent with the capture of holes by a shallow acceptor level located at 0.2 eV above the VBM.It was shown in Ref. [25] that Cp = 10 −6 cm 3 /s agrees with the experimental data.Note that for another shallow acceptor (MgGa), Cp is also 10 −6 cm 3 /s.We will consider this value hereafter for the BeGa acceptor that captures holes at its shallow level at 0.2 eV above the VBM [25].The Cp for the BeGaON complex is expected to be significantly smaller because of the donor nature of this defect.The Cp for donors in GaN varies from 10 −10 to 4 × 10 −8 cm 3 /s [17].At low enough temperatures (before the PL quenching begins), the PL intensity from defects in GaN (both conductive n-type and semi-insulating) is proportional to the concentration of defects and their hole-capture coefficient, i.e., Ii PL = ACpiNi for defect i, where A is a constant.From the experiment, IX PL /IBe PL = 2−5.Below, we will discuss two candidates for the YL X band (C N acceptor and Be Ga O N donor).Parameters of defects that may contribute to the YL bulk band are given in Table 2.The hole-capture coefficient, C p , for the C N acceptor is (3.7 ± 1.6) × 10 −7 cm 3 /s.The C p for the Be Ga acceptor was estimated from the YL Be quenching as 4 × 10 −7 cm 3 /s.However, careful analysis of PL quenching in numerous MOCVD GaN:Be samples (which contain C and Be, with concentrations of about 10 17 cm −3 and (1-30) × 10 17 cm −3 , respectively) shows that the C p for the Be Ga acceptor is larger, which is consistent with the capture of holes by a shallow acceptor level located at 0.2 eV above the VBM.It was shown in Ref. [25] that C p = 10 −6 cm 3 /s agrees with the experimental data.Note that for another shallow acceptor (Mg Ga ), C p is also 10 −6 cm 3 /s.We will consider this value hereafter for the Be Ga acceptor that captures holes at its shallow level at 0.2 eV above the VBM [25].The C p for the Be Ga O N complex is expected to be significantly smaller because of the donor nature of this defect.The C p for donors in GaN varies from 10 −10 to 4 × 10 −8 cm 3 /s [17].At low enough temperatures (before the PL quenching begins), the PL intensity from defects in GaN (both conductive n-type and semi-insulating) is proportional to the concentration of defects and from the conduction band to the shallow level, and a broad band at about 3 eV, which is caused by electron transitions from the conduction band to the deep state of the Be Ga O N defect.The calculated barrier between the and deep states is 0.24 eV [27].The same calculations predict that the polaronic state of the isolated Be Ga defect is deeper (0.55 eV above the VBM), and the deep states of the Be Ga and Be Ga O N defects shift similarly with hydrostatic pressure [27].
We agree that component A of the yellow band can be the YL1 band caused by the C N acceptor but suggest that component B is caused by the isolated Be Ga defect (the −/0 transition level at 0.38 eV above the VBM and the PL band maximum at 2.15 eV).With increasing p, this level shifts towards the valence band and, at p ≈ 8 GPa, intersects the shallow level of the Be Ga (at 0.24 eV above the VBM at p = 0).At higher pressures, the UVL Be3 band of the Be Ga acceptor emerges even at T = 10 K because this configuration becomes the most stable.It may be interesting to test the above explanation by investigating MOCVD GaN:Be samples under hydrostatic pressure.As for the Be Ga O N complexes, we expect that they are either nonradiative, weakly radiative, or "inert" defects, so that their contribution to PL can be ignored even if their concentration is about 3 × 10 19 cm −3 .
Our interpretation of the PL results is consistent with the results of photo-EPR studies on similar bulk Be-doped GaN crystals [35,36].Willoughby et al. [35] initially attributed the photo-EPR signal from Be-doped GaN crystals to a Be-related acceptor, with the −/0 transition level at 0.7 eV above the VBM.However, these authors later revised their attribution and explained this photo-EPR signal by recombination of electrons with holes bound to the C N acceptor [36].Note that slow transients of the photo-EPR signal, explained by trapping electrons by a Be-related defect level close to the conduction band [36], could be caused by potential fluctuations and local electric fields.This is also consistent with the very slow PL decays for the YL bulk band [28].The attribution of the YL bulk band to a superposition of the YL Be and YL1 bands may also explain the results obtained by Glaser et al. [33].They observed ODMR signals from bulk GaN:Be crystals grown by the HNPS method (with a carbon concentration of 10 17 cm −3 ) and noted that the g-values for the yellow band in these samples were very close to those for the YL band in undoped GaN (i.e., for the C N -related YL1 band).

Conclusions
The broad yellow luminescence band in bulk GaN crystals grown by high-pressure technique and doped with Be consists of two unresolved bands: the Be Ga -related YL Be band and the YL X band, which is most likely the C N -related YL1 band.At T > 200 K, the YL Be band is quenched, while the quenching of the YL X band begins only at 400-500 K.We conclude that the yellow band, previously attributed to the Be Ga O N complex based on first-principles calculations and PL experiments with hydrostatic pressure [27], is in fact the Be Ga -related YL Be band, whereas the Be Ga O N defect is either optically inactive or its contribution to PL is negligible.At the same time, the concentration of the Be Ga O N complexes in these bulk GaN crystals significantly exceeds the concentration of the isolated Be Ga defects.

Figure 1 .
Figure 1.Normalized PL spectra at T = 18 K from MBE GaN:Be (sample 0408a, as grown and annealed at 800 °C) and two MOCVD GaN:Be (samples R68 and R87).The BeGa-related YLBe band is very strong in all the samples (the quantum efficiency is close to unity).The dashed line is calculated using Equation (2) with the following parameters: Se = 22, E0 * = 3.2 eV, and dFC g = 1.05 eV.The UVLBe band (presumably caused by a Be-containing complex) emerges in MBE GaN:Be samples only after thermal annealing.In MOCVD GaN:Be, the UVLBe band is strong in some samples (such as R87) but could not be resolved in other samples (R68).The MgGa-related UVLMg band is often observed in MOCVD GaN:Be samples because of contamination with Mg.NBE is the near-band-edge emission.

Figure 1 .
Figure 1.Normalized PL spectra at T = 18 K from MBE GaN:Be (sample 0408a, as grown and annealed at 800 • C) and two MOCVD GaN:Be (samples R68 and R87).The Be Ga -related YL Be band is very strong in all the samples (the quantum efficiency is close to unity).The dashed line is calculated using Equation (2) with the following parameters: S e = 22, E 0 * = 3.2 eV, and d FC g = 1.05 eV.The UVL Be

Figure 2 .
Figure 2. PL spectra from MOCVD GaN:Be sample R87 at selected temperatures and Pexc = 0.005 W/cm 2 .The BeGa-related YLBe band is quenched by the ATQ mechanism at T > 160 K.The MgGarelated UVLMg band disappears at T > 130 K and is replaced with the BeGa-related UVLBe3 band emerging at higher temperatures.

Figure 2 .
Figure 2.PL spectra from MOCVD GaN:Be sample R87 at selected temperatures and P exc = 0.005 W/cm 2 .The Be Ga -related YL Be band is quenched by the ATQ mechanism at T > 160 K.The Mg Ga -related UVL Mg band disappears at T > 130 K and is replaced with the Be Ga -related UVL Be3 band emerging at higher temperatures.

Figure 2 .
Figure 2. PL spectra from MOCVD GaN:Be sample R87 at selected temperatures and Pexc = 0.005 W/cm 2 .The BeGa-related YLBe band is quenched by the ATQ mechanism at T > 160 K.The MgGarelated UVLMg band disappears at T > 130 K and is replaced with the BeGa-related UVLBe3 band emerging at higher temperatures.

Figure 3 .
Figure 3. Dependence of UVL peak position on temperature for MOCVD GaN:Be (sample R134).The UVL Mg band is quenched at T ≈ 130 K (P exc = 0.0047 W/cm 2 ) and T ≈ 170 K (P exc = 0.13 W/cm 2 ) by the ATQ mechanism.At these temperatures, the UVL Mg band is replaced by the UVL Be3 band.The lengths of the arrows are 11 and 15 meV.

Figure 5 .
Figure 5. PL spectra at T = 18 K and P exc = 0.005 W/cm 2 from bulk GaN:Be (excited with HeCd laser at 325 nm and 442 nm) and MBE GaN (sample 0408a, not annealed).The PL spectrum excited at 442 nm is arbitrarily shifted vertically.The dashed and dotted lines are calculated using Equation (2) with the following parameters: S e = 7.4, E 0 * = 2.67 eV, and d FC g = 0.50 eV, ∆ = −0.1 eV (curve 1, the YL1 band); S e = 11, E 0 * = 2.8 eV, and d FC g = 0.74 eV (curve 2, the YL X band); and S e = 21, E 0 * = 3.2 eV, and d FC g = 1.05 eV (curve 3, the YL Be band).The empty circles are the sum of 2 and 3.The UVL Mg band is caused by contamination with Mg.

Solids 2024, 5 , 8 Figure 6 .
Figure 6.PL spectra from bulk GaN:Be at selected temperatures and Pexc = 0.005 W/cm 2 .The YLbulk band consists of unresolved YLBe and YLX bands.The MgGa-related UVLMg band at 18 K is replaced by a similar BeGa-related UVLBe3 band at T > 120 K (both labeled UVL).

Figure 7 .
Figure 7. PL spectra at T = 200 K and Pexc = 0.005 W/cm 2 from bulk GaN:Be (in two locations of the sample) compared with the PL spectra from MOCVD GaN:Be (sample R26) and MBE GaN:Be (sample 0020-1).The spectra for the MBE and MOCVD samples are shifted vertically to match the intensity of the UVLBe3 band in all three samples.

Figure 8
Figure 8 shows an Arrhenius plot for the studied PL bands in bulk GaN:Be.The expected temperature dependence of the MgGa-related UVLMg intensity is shown with a dotted line.The temperature dependences for the BeGa-related YLBe band (shown with dashed lines) are simulated to fit the UVLBe3 dependences by fixing the UVLBe3/YLBe ratio according to Equation (1) with earlier-found parameters.

Figure 7 .
Figure 7. PL spectra at T = 200 K and P exc = 0.005 W/cm 2 from bulk GaN:Be (in two locations of the sample) compared with the PL spectra from MOCVD GaN:Be (sample R26) and MBE GaN:Be (sample 0020-1).The spectra for the MBE and MOCVD samples are shifted vertically to match the intensity of the UVL Be3 band in all three samples.

Figure 8
Figure8shows an Arrhenius plot for the studied PL bands in bulk GaN:Be.The expected temperature dependence of the Mg Ga -related UVL Mg intensity is shown with a dotted line.The temperature dependences for the Be Ga -related YL Be band (shown with dashed lines) are simulated to fit the UVL Be3 dependences by fixing the UVL Be3 /YL Be ratio according to Equation (1) with earlier-found parameters.The first step of the YL Be quenching is expected at T ≈ 100 K but cannot be resolved.The YL Be band hides under a stronger yellow band at T < 200 K and completely disappears at temperatures approaching room temperature, as expected[25].We estimate that the YL X band is stronger than the YL Be band by a factor of 2-5 at T < 200 K.We also studied the temperature dependence of the YL bulk full width at half maximum (FWHM or W) and compared it with that for the YL Be band in the GaN:Be samples grown by MOCVD (Figure9).For PL bands that are broad due to strong electron-phonon coupling, the W(T) dependence can be fitted with the following expression[6]:

Figure 8 .
Figure 8. Temperature dependences of PL intensities at Pexc = 0.005 and 0.13 W/cm 2 from bulk GaN:Be (divided by Pexc for convenience of comparison).The dashed lines show the expected dependences of the YLBe component of the YL band.The solid lines are expected dependences of the UVLBe3 band.The UVLBe3/ YLBe ratios are described by Equation (1), in which EA = 140 meV and δ = 18 (Pexc = 0.005) and δ = 13.5 (Pexc = 0.13).The dotted line shows the expected dependence for the UVLMg band described by Equation (4), with EA = 170 meV and C = 3.5 × 10 6 .

Figure 8 .Figure 9 .
Figure 8. Temperature dependences of PL intensities at P exc = 0.005 and 0.13 W/cm 2 from bulk GaN:Be (divided by P exc for convenience of comparison).The dashed lines show the expected dependences of the YL Be component of the YL band.The solid lines are expected dependences of UVL Be3 band.The UVL Be3 / YL Be ratios are described by Equation (1), in which E A = 140 meV and δ = 18 (P exc = 0.005) and δ = 13.5 (P exc = 0.13).The dotted line shows the expected dependence for the UVL Mg band described by Equation (4), with E A = 170 meV and C = 3.5 × 10 6 .Solids 2024, 5, FOR PEER REVIEW 10

Figure 9 .
Figure 9. Temperature dependence of the FWHM of the YL bulk band in bulk GaN:Be for P exc = 0.005 and 0.13 W/cm 2 (solid symbols) and the YL Be band in three MOCVD GaN:Be samples (empty symbols).The dependences for the MOCVD samples are fitted using Equation (3), with hΩ e = 38 meV (solid lines).The dashed line, obtained using Equation (3) with W(0) = 0.475 eV and hΩ e = 60 meV, illustrates a possible dependence for the YL X component of the YL bulk band.

Figure 10 .
Figure 10.Temperature dependence of the YLbulk band maximum in bulk GaN:Be for Pexc = 0.005 W/cm 2 (blue-shifted by 23 meV for convenience of comparison) and 0.13 W/cm 2 (solid symbols) and the YLBe band in the MOCVD and MBE GaN:Be samples at Pexc = 0.005 W/cm 2 (empty symbols).

Figure 10 .
Figure 10.Temperature dependence of the YL bulk band maximum in bulk GaN:Be for P exc = 0.005 W/cm 2 (blue-shifted by 23 meV for convenience of comparison) and 0.13 W/cm 2 (solid symbols) and the YL Be band in the MOCVD and MBE GaN:Be samples at P exc = 0.005 W/cm 2 (empty symbols).

Figure 11 .
Figure 11.Abrupt and tunable quenching of the YLbulk band in bulk GaN:Be.The PL intensity is divided by Pexc for convenience of comparison.The solid curve is calculated using Equation (4), with C = 8 × 10 17 and EA = 1.6 eV.

Figure 11 .
Figure 11.Abrupt and tunable quenching of the YL bulk band in bulk GaN:Be.The PL intensity is divided by P exc for convenience of comparison.The solid curve is calculated using Equation (4), with C = 8 × 10 17 and E A = 1.6 eV.

Figure 12 .
Figure 12.Comparison of the YLX band in bulk GaN:Be with the YL1 band in MOCVD GaN:C at T = 300 K.

Figure 12 .
Figure 12.Comparison of the YL X band in bulk GaN:Be with the YL1 band in MOCVD GaN:C at T = 300 K.

Table 1 .
Calculated and experimentally found values (all in eV) of the charge transition levels relative to the VBM and PL band maxima (for E g = 3.50 eV) for defects that may contribute to the YL bulk band in GaN, assuming that the YL X band is caused by the Be Ga O N complex.

Table 2 .
Possible parameters of main defects in bulk GaN:Be.