Excitonic and Defect-Related Photoluminescence in Mg3N2 - The

Article pubs.acs.org/JPCC

Excitonic and Defect-Related Photoluminescence in Mg3N2 Yuki Uenaka and Takashi Uchino* Department of Chemistry, Graduate School of Science, Kobe University, Kobe, Nada 657-8501, Japan S Supporting Information *

ABSTRACT: Exciton luminescence in semiconductors has in general a larger oscillator strength than that of band-to-band transitions and is hence desirable to utilize it as practical phosphors. However, binding energies of excitons are usually not large enough to be stable at room temperature except for the case of ZnO. We show that magnesium nitride (Mg3N2) exhibits excitonic photoluminescence (PL) in the violet-blue (∼2.9 eV) region at room temperature. The violet-blue PL spectra are characterized at least by two PL components peaking at ∼2.85 and ∼2.82 eV, which are assigned to the free exciton transition and its first longitudinal-optic phonon replica, respectively. In addition, Mg3N2 exhibits yellow-orange (∼2 eV) PL emission bands, which most likely result from a lattice defect in the form of a nitrogen vacancy. The excitonic PL features at room temperature indicate that Mg3N2 is a direct band gap semiconductor with a relatively larger exciton binding energy and is hence a promising material that can be used for visible light emitters and detectors.

1. INTRODUCTION Structure and properties of Mg3N2, which is the only reported Mg−N compound with an antibixbyite structure,1,2 have attracted renewed interest since it emerges as a parasitic phase during Mg-doping of group III nitrides XN (X = Al, Ga, In).3−5 The presence of the Mg3N2 phase in group III nitrides is not desirable because it decreases the free hole concentration,6 hence limiting the doping efficiency. However, Mg3N2 and related nitride compounds have recently found applications in their own right as high-thermal conductivity ceramics7 and reversible hydrogen storage sources.8−10 It has also been reported that Mg3N2 is a suitable precursor for the preparation of a metastable high-pressure bcc-Mg phase at very moderate pressures.11 These recent experimental findings have elucidated that Mg3N2 is a potentially interesting material in various fields of science and technologies. In contrast to the structural, chemical, and physical characteristics, however, the electronic and optical properties of Mg3N2 have not been fully investigated and understood. Previous ab initio band structure calculations of Mg3N2 have predicted that that there exist strong interactions between N 2p and Mg 3s, 3p states in the top of the valence band, resulting in a very flat dispersion curve with little anisotropy.12−14 Based on these calculations, Mg3N2 could be regarded as a direct (or a nearly direct) band gap material, where the indirect band gap is predicted to be only slightly (∼0.01 eV) smaller than the direct band gap.13 However, optical diffuse reflectance measurements on Mg3N2 powders have demonstrated that there is a substantial, although not too large, energy difference (∼0.3 eV) between the indirect (2.85 eV) and direct (3.15 eV) band gaps.15 To obtain reliable information on the band-edge features, high quality single crystals of Mg3N2 need to be examined. Unfortunately, however, bulk single-crystalline © 2014 American Chemical Society

Mg3N2 cannot easily be synthesized, which prevents us from performing precise optical absorption measurements of Mg3N2. In this work, we hence employ photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy in order to overcome the above problem. This method has less stringent requirements on sample size or shape as compared with optical absorption and reflection. Furthermore, luminescence excitation/emission spectroscopy enables us to obtain complementary and consistent information on the excitation processes under band gap and below band gap excitations, as has been successfully applied to a range of wide band gap semiconductors.16 To our knowledge, detailed PL measurements have not been performed on Mg3N2 previously.

2. EXPERIMENTAL SECTION The sample used in this work is commercial Mg3N2 powder (Sigma-Aldrich, ≥99.5% purity, trace metal basis, particle size less than 45 μm). We confirmed from powder X-ray diffraction measurements that the sample powder is a single-phase polycrystalline Mg3N2, as shown in Figure S1 of the Supporting Information. Steady state PL spectra of the sample were recorded on a spectrofluorometer (JASCO, FP 6600) by using a monochromated xenon lamp (150 W). Time-resolved PL measurements in the nano- to microsecond time range were carried out with a 150 lines/mm grating and a gated imageintensified charge coupled device (Princeton Instruments, PIMAX:1024RB) by using the third harmonic (355 nm) of a pulsed Nd:yttrium−aluminum−garnet (YAG) laser (SpectraPhysics, INDI 40, pulse width 8 ns, a repetition rate 10 Hz) as Received: March 27, 2014 Revised: May 15, 2014 Published: May 19, 2014 11895

dx.doi.org/10.1021/jp503023t | J. Phys. Chem. C 2014, 118, 11895−11901

The Journal of Physical Chemistry C

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Figure 1. (a) PL spectra in the violet-blue region obtained from Mg3N2 in the temperature range of 77−350 K. The excitation energy is 3.54 eV. The broken lines drawn on some peaks are for guidance. (b) Temperature-dependent peak positions of the first (filled squares) and second (filled circles) highest energy peaks shown in (a). The data for the first highest energy peak were fitted with eq 6, and those for the second highest peak energy with eq 1. The inset shows the energy separation between the first and second highest energy components, and the solid line is the result of a linear fit.

dependence of the energy separation has been usually observed for the free exciton transition and its longitudinal optic (LO) phonon replicas in direct-gap semiconductors such as ZnO.16,17 A general relationship for the peak position of the exciton transition (EX) and its first LO phonon replica (EX‑1LO) can be written as follows:18

an excitation source. All the PL measurements were carried out in a closed-cycle N2 cryostat in the temperature region from 77 to 400 K under vacuum (∼10−2 Pa) to avoid a possible reaction of Mg3N2 with atmospheric water.

3. RESULTS 3.1. Photoluminescence Characteristics in the VioletBlue Spectral Region. First, we analyze the PL spectra of Mg3N2 in the violet-blue (∼2.7−3.0 eV) region. We found that the violet-blue PL signals are observed under excitation by photons with energies higher than the reported band gap energy of ∼3 eV. The spectral shape depends hardly on the excitation energy, suggesting that the interband and/or excitonic excitations are responsible for this PL emission. Figure 1a shows typical violet-blue PL spectra measured under 3.54 eV excitation in the temperature region from 77 to 350 K. In the temperature region from 77 up to ∼200 K, the PL spectra consist at least of three components, as shown by the guided broken lines in Figure 1a. With further increase in temperature, these PL components broaden and merge, accompanied by an appreciable red-shift of the respective PL peak positions. Figure 1b plots the peak energies of the two highest energy PL components as a function of temperature. We should note that the peak energy of the first highest peak can be rather accurately determined in the temperature region from 77 to 150 K, as can be seen in Figure 1a. In the temperature region from 175 to 275 K, however, there exists an inaccuracy in its peak energy (∼±0.007 eV) because of the line broadening and merging effects mentioned above, and for temperature higher than 300 K its peak energy is not well discerned and determined. On the other hand, the position of the second highest energy peak can be well recognized in the whole temperature range from 77 to 350 K. Using the data shown in Figure 1b, we then calculate the energy separation between the two highest energy components, ΔE, as shown in the inset of Figure 1b. We found that ΔE decreases linearly with temperature in the temperature region up to 275 K, including the lower (T < 150 K) temperature region where the ΔE value can be rather accurately estimated. Such a linear temperature

E X‐1LO = E X − ℏω +

⎛1 ⎞ ⎜ + l⎟kBT ⎝2 ⎠

(1)

where ℏω represents the LO phonon energy, kB is Boltzmann’s constant, and l is a constant related to the transition probability of the phonon assisted annihilation for the exciton. According to eq 1, a linear fit to the data yields the values of ℏω from the intercept (see the inset of Figure 1b), predicting the LO phonon energy of ℏω = 95.9 meV. According to the Raman spectra of Mg3N2 reported by Heyns et al.,19 Mg3N2 has a principal Raman band at 379 cm−1 (47 meV) along with several weaker components in the wavenumber region from ∼200 cm−1 (∼25 meV) to ∼620 cm−1 (∼77 meV). These observed Raman bands are lower in energy than the predicted LO phonon mode, which may be indicative of the inaccuracy of our estimation of the LO phonon energy. Another possible cause for the discrepancy is the effect of atmospheric decomposition of Mg3N2. It has been demonstrated that Mg3N2 is sensitive with respect to moisture and can be eventually decomposed into Mg(OH)2 and NH3.19,20 Although the present PL measurements are carried out under vacuum (∼10−2 Pa), we cannot completely exclude the possibility that our samples are subject to (partial) decomposition during measurements. It has been demonstrated that the Raman spectrum of the decomposition product of Mg3N2 is quite different from that of Mg3N2, yielding an additional Raman band at 1080 cm−1 (134 meV).19 Thus, it could be possible that a partial decomposition of Mg3N2 is responsible for the predicted high LO phonon energy. The value of l is also worth mentioning. As for the first LO phonon replica of the free exciton with the kinetic energy ε, the transition probability W can be often expressed by a power law with the exponent of l:18 11896


The Journal of Physical Chemistry C W(ε) ∝ εl

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spectra measured at temperatures of 77, 225, and 300 K, and Figure 3b shows the temperature dependence of the PLE spectra as a contour plot. The PL peak intensity shows a gradual decrease with increasing temperature under the interband excitation (Eex > ∼3.1 eV). Under the near-bandedge excitation (∼2.8 < Eex < ∼3.0 eV), however, the PL intensity shows an irregular temperature dependence as manifested by an increase in the PLE peak intensity with increasing temperature. To highlight more clearly the effect of Eex and temperature on the PL intensity, we replot the data shown in Figure 3 in a different form. That is, the PL intensities measured under a certain value of Eex are plotted as a function of temperature (see Figure 4). As shown in Figure 4a, the PL intensity shows a normal thermal quenching behavior under excitation by photons with energies Eex > ∼3.1 eV. We found that the temperature dependence of the PL intensity I(T) shown in Figure 4a is well described by the following Mott-type equation:22

(2)

Note, however, that this relationship is true only for direct band gap semiconductors in which the phonon wave vector qphonon is comparable to the exciton wave vector Kexc, i.e., qphonon ≈ Kexc.21 In indirect band gap semiconductors, in which |qphonon| ≫ |Kexc|, W(ε) is constant, or l = 0 because the phonons no longer “feel” the spatial extent of the excition.21 Thus, the value of l can be used to evaluate whether the relevant excitonic emission occurs in a direct band gap semiconductor or in an indirect one. We obtained the value of l = 2.6 from the slope of the fitted line. This strongly suggests that the excitonic process in a direct bandgap semiconductor is responsible for the present violet-blue PL emission. Thus, Mg3N2 can be viewed as a direct band gap semiconductor in terms of the exciton−LO phonon coupling. It should be noted, however, that the estimated value of l = 2.6 is substantially higher than the often reported value of l ∼ 1.16,18 It has been theoretically predicted that W is proportional to ε, i.e., l = 1, for the parabolic dispersion of the exciton band.18 The estimated value of l = 2.6 might result from the inaccuracy of our estimation, as in the case of the LO phonon energy mentioned earlier. Otherwise, the large estimated value suggests that Mg3N2 yields a highly complex exciton−polariton dispersion curve at K ≈ 0. The wave vector of the (photon-like) exciton polariton in the final states for the LO phonons may not be completely negligible in Mg3N2, thus resulting in a marked change in the kinetic energy dependence of W(ε). 3.2. Photoluminescence Characteristics in the YellowOrange Spectral Region. Figure 2 shows a yellow-orange PL

I (T ) =

I0 1 + A exp( −E NR /kBT )

(3)

where I0 is the emission intensity at T = 0 K, ENR is an effective activation energy for nonradiative recombination, and A is a constant. The fitted values of ENR are typically ∼0.30 eV for Eex > ∼3.1 eV (see Figure 4c). Under excitation by photons with energies Eex < ∼3.0 eV, however, the PL intensity does not follow eq 3 but tends to show a thermally assisted luminescence behavior, or a negative thermal quenching behavior, as shown in Figure 4b. To further confirm this, we measured the PL spectra at different temperatures under 2.95 eV photon excitation (see Figure 5). Figure 5a shows the PL spectra measured at temperatures of 150, 225, and 300 K, and the temperature dependence of the PL spectra is shown in Figure 5b as a contour plot. It is clear from Figure 5b that the PL peak intensity reaches its maximum at ∼225 K, in harmony with the results shown in Figure 4b. A possible explanation for this thermally activated PL process under the near-band-edge excitation will be given in section 4.2. 3.3. Time-Resolved PL Measurements. In addition to the steady state PL mentioned above, we also carried out timeresolved PL measurements under the interband excitation using a 355 nm (3.49 eV) pulsed Nd:YAG laser. Time-resolved PL transients under the near-band-edge excitation were not obtained because of the limitations of the excitation light sources. As for the violet-blue PL signals, we were unable to obtain the decay profiles because the luminescence decays faster than our time resolution of 10 ns. Note, however, that the expected (sub)nanosecond PL lifetime is consistent with our proposal that the violet-blue PL results from the excitonic process in a direct bandgap semiconductor. On the other hand, the intensity of the orange-yellow PL as a function of time elapsed after excitation was successfully obtained by using the present time-resolved detection system. Figure 6 shows the PL decay curves of the 2.2 eV PL peak measured at different temperatures. We see that the decay line shape is strongly nonexponential, exhibiting decay components on the tens-of-nanoseconds to submillisecond time scale. We found that the all the decay data are well fitted to the following stretched exponential function23

Figure 2. Room temperature PL and PLE spectra of Mg3N2. The PL spectrum was recorded with excitation at 2.9 eV and the PLE spectrum with emission at 2.2 eV.

spectrum along with a corresponding PL excitation (PLE) spectrum measured at room temperature. One sees that the PL spectrum is characterized by a peak at 2.2 eV and its low-energy shoulder at ∼2.0 eV. In the PLE spectrum, a sharp peak is observed at 2.9 eV. These results demonstrate that the yelloworange PL band is most effectively induced through near-bandedge excitation. The PLE spectrum also exhibits a flat profile in the energy region from ∼3.2 to ∼5.0 eV, indicating that the yellow-orange PL occurs under the band-to-band excitation as well. We also found that the PL peak intensity is influenced not only by temperature but also by excitation energy Eex. Figure 3 illustrates the temperature dependence of the PLE spectra monitored at the 2.2 eV PL peak. Figure 3a shows the PLE

I(t ) = Ii exp[−(t /τ )β ] 11897

(4)



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Figure 3. (a) PLE spectra in the yellow-orange region measured at temperatures of 77, 225, and 300 K. The monitored energy is 2.2 eV. (b) Temperature dependence of the PLE spectra are shown as a contour plot. The solid black line indicates the temperature dependence of the exciton transition energy predicted by eq 6.

the temperature variation of τ is well described by the equation26 τ0 τ (T ) = 1 + A exp( −E NR /kBT ) (5) where τ0 is the radiative lifetime at T = 0 K. The fitted values of τ0 and ENR are 38.8 μs and 0.19 eV, respectively. Although the latter value is a bit smaller than the value (∼0.3 eV) obtained from eq 3, it can safely be said the values of ENR obtained independently from eqs 3 and 5 are in reasonable agreement with each other.

4. DISCUSSION 4.1. Violet-Blue PL Band. We have shown in section 3.1 that the violet-blue PL signals in Mg3N2 result from the excitonic emission and its phonon replicas. We suggest that the observed excitonic emission is not from bound excitons but from those of free excitons. This is because the interaction of LO phonons with bound excitons is usually very weak, and the bound excitons and their phonon replicas are not, in general, detectable at temperatures higher than ∼100 K.27,28 If this assumption is valid, the temperature dependence of the higher energy peak, or the free-exciton transition energy, should obey analytical equations to describe the temperature dependence of the band gap since the binding energy of free exciton is expected to be nearly independent of temperature. Among such equations, a three-parameter function proposed by O’Donnell and Chen29 is shown to be compatible with reasonable assumptions about the influence of phonons on the band gap energy. They write

Figure 4. PL peak intensities in the yellow-orange region as a function of temperature measured under excitation (a) with photons of 4.13, 3.26, and 3.10 eV and (b) with photons of 3.02, 2.95, 2.88, and 2.82 eV. Solid lines in (a) and (b) indicate the best fit of the data to eqs 3 and 7, respectively. (c) Fitted values of ENR in eq 3. (d) Fitted values of ENR and EA in eq 7.

where I(t) is the time-dependent luminescence intensity, τ is a characteristic decay time, β is a dispersion exponent, and Ii is the emission intensity at t = 0. In general, values of β < 1 represent the existence of a broad distribution of decay times. A trap-controlled dispersive transport of photoexcited carriers and the subsequent radiative recombination process often result in a decay profile with a stretched exponential function.24,25 As shown in Figure 7, the fitted values of τ and β remain almost constant at temperatures up to ∼200 K, followed by a substantial decrease for higher temperatures. This temperature dependence is basically similar to that of the PL peak intensity obtained under excitation with photons of energy higher than ∼3.1 eV (see Figure 4a). As in the case of I(T) given in eq 3,

Eg (T ) = Eg (0) − S⟨ℏω⟩[coth(⟨ℏω⟩/2kBT ) − 1]

(6)

where Eg(0) is the band gap energy at zero temperature, S is a dimensionless coupling constant, and ⟨ℏω⟩ is an average phonon energy. Thus, we use the form of eq 6 and replace Eg with the exciton transition energy EFX. As shown in Figure 1b, the temperature dependence of the higher energy peak is well represented by eq 6, supporting our assumption that the luminescence of free exciton is responsible for the present violet-blue PL emission. The fitted values of EFX(0), S, and ⟨ℏω⟩ are 2.961 ± 0.003 eV, 4.65 ± 0.45, and 32.9 ± 4.1 meV, respectively. Using these fitted parameters, we can estimate the value of EFX at 300 K to be 2.84 eV, which is in good agreement 11898



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Figure 5. (a) PL spectra in the yellow-orange region measured at temperatures of 150, 225, and 300 K. The excitation energy is 2.95 eV. (b) Temperature dependence of the PL spectra is shown as a contour plot.

conclude that the optical absorption in the energy region below ∼3 eV will not result from indirect band-to-band transitions. It should also be noted that the predicted average phonon energy ⟨ℏω⟩ ∼ 33 meV lies well within the range of the observed Raman spectra of Mg3N2 mentioned in section 3.1,18 also supporting the discussion mentioned above. 4.2. Orange-Yellow PL Band. As mentioned in section 3.2, PLE spectral features of the orange-yellow emissions depend strongly on temperature (see Figure 3). The most noticeable feature is that the PLE peak around 3 eV develops and shifts to lower energies as temperature increases. Accordingly, the PL intensity obtained under the near-bandedge excitation (∼2.8 < Eex < ∼3.0 eV) exhibits the apparently anomalous temperature dependence, as demonstrated in Figure 4b. In the contour plot of the temperature-dependent PLE spectra shown in Figure 3b, we imposed the temperature -dependent exciton transition energy predicted by eq 6 (see the solid black line in Figure 3b). It is clear that the PLE peak position decreases in energy with temperature just in a similar way as the exciton transition energy (or band gap energy) does. It is hence most likely that the temperature induced band gap shrinking underlies the observed temperature dependence of the PLE peak at ∼3 eV. It is interesting to point out that a very similar thermally assisted emission process has been reported from the “green”

Figure 6. PL decay curves for the yellow-orange PL measured at different temperatures. The excitation and observation energies were 3.49 and 2.2 eV, respectively. The solid lines are the fit of the data with eq 4.

with the reported “indirect” band gap energy (2.85 eV) of Mg3N2 observed at room temperature.15 This close correspondence allows us to suggest that the expected excitonrelated absorption has been erroneously considered as the absorption due to indirect optical transitions. In addition to the exciton-related absorption, defect-related absorption is also likely to occur in a similar energy region, as will be shown in section 4.2. On the basis of the above considerations, we can

Figure 7. Fitted values of (a) τ and (b) β obtained from the fitting of the PL decay curves with eq 4. The solid line shown in (a) indicates the fit of the data with eq 5. 11899



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PL band in ZnO under the near-band gap excitation.30−32 As for the green PL band in ZnO, shallow donor electrons are likely to contribute to the thermally assisted PL process by being thermally excited from the donor levels to the conduction band.32 We have recently derived an analytical expression to represent the complex temperature dependence of the green PL emission in ZnO on the basis of the thermally assisted energy transfer from the excited state to the emission state via extended conduction states.32 If we assume that the rate constant of the thermally assisted emission process is described by the Arrhenius-type equation with an activation energy EA, the emission intensity is described by I (T ) = I 0

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ASSOCIATED CONTENT

* Supporting Information S

XRD pattern of the as-received Mg3N2 sample and the detailed derivation of eq 7 on the basis of a three level kinetic model. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Ph +81 78 803 5681; Fax +81 78 803 5681 (T.U.).

1 + A 2 exp( −EA /kBT ) A1 + A 2 exp( −EA /kBT ) + A3 exp(−E NR /kBT )

Notes

The authors declare no competing financial interest.

■

(7)

The detailed derivation of eq 7 is given in the Supporting Information. If the above scheme can be applied to the yelloworange PL bands in Mg3N2, the PL intensity shown in Figure 4b can be fitted with eq 7. As demonstrated in Figure 4b, the excellent fits were obtained for all the experimental data measured under excitation by photons with energies ∼2.8 eV < Eex < ∼3.0 eV. Figure 4d shows the fitted values of EA and ENR as a function of Eex; EA shows a steady increase with decreasing Eex below ∼3 eV, whereas ENR yields an almost constant value of ∼0.35 eV. A similar anticorrelation between EA and Eex was also observed in the green PL signals in ZnO,32 which has been interpreted in terms of the phonon-assisted excited process from the photoexcited electrons in the sub-band gap states to the emission level. Thus, we consider that the temperaturedependent behaviors for the yellow-orange emission in Mg3N2 can be interpreted in a similar manner as those for the green emission in ZnO. Finally, we will give a comment on the structural origin of the yellow-orange emission center in Mg3N2. As for ZnO, an oxygen vacancy (VO) is believed to be responsible for the green PL emission although its true microscopic origin is still a matter of debate.16,32,33 From the analogy of the green PL band in ZnO, a nitrogen vacancy (VN) is an assumed candidate for the emission center of the yellow-orange PL band in Mg3N2. In accordance with this assumption, previous density functional theory calculations14 have predicted that the most abundant point defect in Mg3N2 is a nitrogen vacancy.

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5. CONCLUSIONS We have shown that Mg3N2 exhibits two different PL emission bands in the violet-blue (∼3 eV) and yellow-orange (∼2 eV) regions of the visible spectrum. The violet-blue PL signals are attributed to free excition emission and its phonon replicas. Mg3N2 is hence considered to be a direct band gap semiconductor with a relatively large exciton binding energy. On the other hand, a lattice defect (possibly a nitrogen vacancy) is likely to be responsible for the yellow-orange PL signals. As for the yellow-orange PL, thermally enhanced emissions are observed when the excitation energy lies around and below the direct band gap energy. These emission characteristics are very similar to those observed in ZnO, except the fact that the PL emissions in Mg3N2 occur at lower energy as compared with those in ZnO. Thus, Mg3N2 may potentially find an application in the field of visible optoelectronic devices in the spectral region where ZnO does not emit light. 11900



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