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New emission line in highly excited GaN
Journal of Luminescence 12/13 (1976) 611 615 © North-Holland Publishing Company
NEW EMISSION LINE IN HIGHLY EXCITED GaN J.M. HVAM Institute of Physics, Odense University, DK 5000 Odense, Denmark
and E. EJDER Lund Institute of Technology, Department of Solid State Physics, S 22007 Lund, Sweden
A new emission band (P-band) with peaks at 3.453 eV and 3.448 eV is observed in the photoluminescence from GaN at 1.8 K and high excitation intensities. The intensity of the P-band luminescence varies quadratically with the free exciton density. The kinetics of the P-band emission and its spectral position is well explained by an exciton— exciton interaction process.
I. Introduction The luminescence from GaN has recently been studied by several authors [1]. Detailed investigations of the photoluminescence at low temperatures were performed by Dingle et al. [2] showing a strong luminescence line near the band gap (Eg = 3.50 eV), ascribed to the decay of excitons bound to neutral donors (12 -line), and a pair recombination band in the energy range 3.00—3.30 eV. Stimulated emission and laser action was also reported in GaN at high pumping intensities [3]. In the present work we investigate the near band gap luminescence at low temperatures and high excitation powers. With increasing excitation a structure appears in the spectra and a new emission line is growing up at the low energy side of the bound exciton line. The new luminescence is discussed in terms of an exciton—exciton interaction process, and it is suggested that the observed transitions are responsible for the stimulated emission, previously reported [3].
2. Material and experiment The GaN crystals used were grown by vapour-phase epitaxy on sapphire employing the method of Maruska and Tietjen [4]. They were undoped, and the detailed growth conditions were as in ref. [5]. Undoped GaN is generally of low resistivity. 611
612
J.M. Hvam, E. Ejder/New emission line in highly excited GaN
However, the particular layers used in these experiments turned out unusually highohmic. The layers were relatively thick (150 200 pm) reducing the residual stress at the surface, and the samples with minimum broadening of the 12-line at low excitation were selected. From this and electrical measurements carrier concentrations n 1017 cm are estimated. The measurements were performed at 1.8 K with the crystals immersed in liquid helium. The source of excitation was a pulsed N2-gas laser (Avco C950) with a peak power of 100 kW and a pulse length of 10 ns. The luminescence emitted from the excited surface of the sample was analyzed with a Spex 1302 double spectrometer followed by a photomultiplier and a fast box-car integrator system.
3. Results and discussion The decay time of the near band gap luminescence was fast and limited by the time constant of the apparatus (15 ns), whereas the decay time in the pair recombination band was several microseconds [21.Using a gate width of the detection WAVELENGTH (A) 3500
3550
3600
3650
3700
GaN
—1H
j
~
~1F’~
1.8K
~1o
~Hooi
00001 Fig. 1. Luminescence spectra for different 350 PHOTON excitation 345 ENERGY(eV) intensities 340 1~.Io 335 — 1 corresponds to 2. 2.5 MW/~m
J.M. Hvam, E. Elder/New emission line in highly excited GaN
613
system of 20 ns, the spectral intensity of the longer wavelength radiation was weaker than the near band gap luminescence by more than two orders of magnitude. Fig. 1 shows spectra of the near band gap luminescence for different excitation powers over three orders of magnitude, the maximum excitation intensity being 2. At the lowest excitation level the spectrum consist of a single about 2.5 MW/cm line, with a half width of 6 meV, centered at 3.470 eV. This line is identified as the 1 2-line, previously reported [21.With increasing excitation the emission line broadens, particularly to the high energy side, and further more a new emission band, denoted the P-band in fig. 1, appears on the low energy side of the 12-line. The P-band grows more than linearly with excitation power, and it contains some structure at the highest excitation level, with peaks at 3.453 and 3.448 eV. The peak intensities of the 12-line and the P-band are shown in fig. 2 as a function of excitation power. At low excitation levels the excitation dependences approach a linear and a quadratic dependence for the 12-line and the P-band, respectively (dashed lines). With increasing excitation power the peak intensities tend to saturate. This is to be expected considering the strong broadening of the spectra observed in fig. 1. On the other hand, from fig. 2 it is clear that the intensity of the P-band increases faster than that of the 12-line in the entire excitation range. This is illustrated in the insert of fig. 2, where the peak intensity of the P-band is plotted versus that of the 12-line, revealing a quadratic relationship. It is reasonable to assume the amplitude of the 12line to be proportional to the density nex of free excitons. Hence, the transitions responsible for the P band luminescence are proportional to ~ indicating an exciton exciton interaction process. Indeed, the spectral position of the P-band, about an
I2~LINE INTENSITY
iiooi
0.01
0.1
EXCITATION INTENSITY 10 (RELATIVE UNITS) Fig. 2. Kinetics of the luminescence lines as a function of excitation intensity. The insert shows the peak intensity of the P(2)-line versus peak intensity of the I2-line.
614
f.M. Hvam, E. Elder/New emission line in highly excited GaN
exciton binding energy below the free exciton energy, agrees with transitions involving the inelastic scattering between two free excitons. Such a transition was first proposed by Benoit a Ia Guillaume et a!. to occur in CdS [6], and later reported in ZnO, CdS and CdSe by Magde and Mahr [71.We consider here an interaction between two free excitons in their ground state (n = 1) resulting in the scattering of one exciton into a photon-like state, leaving the crystal as luminescence, while the remaining cxciton is scattered into an excited state, n = 2, 3, ~o. This process implies possible emission lines with peak energies at [8] 2), ii = 2,3 (1) F~(0) ~ G(1 1/n where Ee\ and G are exciton ground state energy and binding energy, respectively. Applying the numerical values Eex = 3.475 eV and & = 2~meV [51we find from eq. (1): E~( 2) = 3.454 eV and E~() = 3.447 eV. These energies are indicated with arrows in fig. 1 and the agreement with experiment is convincing. It appears that with increasing exciton density the process involving scattering to an n = 2 state (P(2)-line) is the first to become active, whereas at the highest pumping level scattering illto higher excited states also takes place. The strong broadening of the spectra at high pumping is ascribed to a rise in effective temperature of the free excitons (hot excitons). Exciton temperatures of 30 40 K are to be expected undes our conditions and with a calculated half width of 2.5 kT of the P(2)-line [8], this corresponds to a width of about 10 meV. The broadening to the high energy side of the 12-line is ascribed to the admixture of free exciton luminescence which has a low energy cut-off atE cx and an exponential high energy tail as observed (fig. 1). At the highest pumping level the emission in the region between the 12-line and the P-band seems to increase rapidly. Whether this is caused by a general broadening, or new recombination mechanisms are involved cannot be decided from the present experiments. At 77 K the free exciton luminescence was increased compared to the 12-line owing to the thermal dissociation of bound excitons. Likewise the P-band luminescence was observed even more intensely than at helium temperatures, supporting the interpretation in terms of free exciton interactions. Preliminary investigations at 77 K showed that stimulated emission in the P-band was easily obtained. From theory and from experiments on other materials [8] it is well known that the exciton exciton collision process is favourable for achieving population inversion and thereby optical gain. Hence, we believe the exciton exciton interaction to be responsible for not only the P-band luminescence observed in the present work, but also the stimulated emission and laser action, previously reported [3]. ...,
...,
References [1) J.I. Pankove, J. Luminescence 7 (1973) 114, and references therein. 121 R. Dingle, D.D. Sell, S.F. Stokowski and M. Ilegems, Phys. Rev. B4 (1971) 1211; R. Dingle and M. ilegems, Solid State Commun. 9 (1971) 175.
f.M. Hvam, E. Elder/New emission line in highly excited GaN [3] [4] [5] [6] [7] [81
615
R. Dingle, K.L. Shaklee, R.F. Leheny and R.B. Zetterstrom, Appl. Phys. Letters 19 (1971) 5. H.P. Maruska and J.J. Tietjen, AppI. Phys. Letters 15 (1969) 327. B. Monemar, Phys. Rev. BlO (1974) 676. C. Benoit ala Guillaume, J.M. Debever and F. Salvan, Phys. Rev. 177 (1969) 567. D. Magde and H. Mahr, Phys. Rev. Letters 24 (1970) 890. J.M. Hvam, Phys. Stat. Sol. (b) 63 (1974) 511.
Discussion S. Shionoya: You don’t see a luminescence line due to excitonic molecules. Would you comment on this? J.M. Hvam: I do see a strong increase of the luminescence in the region between the l 2-line and the P-band at the highest excitation levels. However, I would hesitate to interprete this from the present experiments. T. Goto: Do you think any possible emission process where the P-emission is associated with the 12-bound eXcitofl? J.M. Hvam: I have not considered this possibility. If you refer to the model by Honerlage and Riissler, however, this model predicts a shift of the line with increasing excitation power. No such shift is observed in my experiments. I. Broser: Do you have any explanation for your observation that the ratio of the probability for exciton collision processes with scattering into the n 2 states to that of scattering into higher states depends highly on the excitation intensity? J.M. Hvam: No I have no explanation for this. There exist some calculations for scattering probabilities to different excited states, but, being no theoretician, I do not know how reliable they are.
NEW EMISSION LINE IN HIGHLY EXCITED GaN J.M. HVAM Institute of Physics, Odense University, DK 5000 Odense, Denmark
and E. EJDER Lund Institute of Technology, Department of Solid State Physics, S 22007 Lund, Sweden
A new emission band (P-band) with peaks at 3.453 eV and 3.448 eV is observed in the photoluminescence from GaN at 1.8 K and high excitation intensities. The intensity of the P-band luminescence varies quadratically with the free exciton density. The kinetics of the P-band emission and its spectral position is well explained by an exciton— exciton interaction process.
I. Introduction The luminescence from GaN has recently been studied by several authors [1]. Detailed investigations of the photoluminescence at low temperatures were performed by Dingle et al. [2] showing a strong luminescence line near the band gap (Eg = 3.50 eV), ascribed to the decay of excitons bound to neutral donors (12 -line), and a pair recombination band in the energy range 3.00—3.30 eV. Stimulated emission and laser action was also reported in GaN at high pumping intensities [3]. In the present work we investigate the near band gap luminescence at low temperatures and high excitation powers. With increasing excitation a structure appears in the spectra and a new emission line is growing up at the low energy side of the bound exciton line. The new luminescence is discussed in terms of an exciton—exciton interaction process, and it is suggested that the observed transitions are responsible for the stimulated emission, previously reported [3].
2. Material and experiment The GaN crystals used were grown by vapour-phase epitaxy on sapphire employing the method of Maruska and Tietjen [4]. They were undoped, and the detailed growth conditions were as in ref. [5]. Undoped GaN is generally of low resistivity. 611
612
J.M. Hvam, E. Ejder/New emission line in highly excited GaN
However, the particular layers used in these experiments turned out unusually highohmic. The layers were relatively thick (150 200 pm) reducing the residual stress at the surface, and the samples with minimum broadening of the 12-line at low excitation were selected. From this and electrical measurements carrier concentrations n 1017 cm are estimated. The measurements were performed at 1.8 K with the crystals immersed in liquid helium. The source of excitation was a pulsed N2-gas laser (Avco C950) with a peak power of 100 kW and a pulse length of 10 ns. The luminescence emitted from the excited surface of the sample was analyzed with a Spex 1302 double spectrometer followed by a photomultiplier and a fast box-car integrator system.
3. Results and discussion The decay time of the near band gap luminescence was fast and limited by the time constant of the apparatus (15 ns), whereas the decay time in the pair recombination band was several microseconds [21.Using a gate width of the detection WAVELENGTH (A) 3500
3550
3600
3650
3700
GaN
—1H
j
~
~1F’~
1.8K
~1o
~Hooi
00001 Fig. 1. Luminescence spectra for different 350 PHOTON excitation 345 ENERGY(eV) intensities 340 1~.Io 335 — 1 corresponds to 2. 2.5 MW/~m
J.M. Hvam, E. Elder/New emission line in highly excited GaN
613
system of 20 ns, the spectral intensity of the longer wavelength radiation was weaker than the near band gap luminescence by more than two orders of magnitude. Fig. 1 shows spectra of the near band gap luminescence for different excitation powers over three orders of magnitude, the maximum excitation intensity being 2. At the lowest excitation level the spectrum consist of a single about 2.5 MW/cm line, with a half width of 6 meV, centered at 3.470 eV. This line is identified as the 1 2-line, previously reported [21.With increasing excitation the emission line broadens, particularly to the high energy side, and further more a new emission band, denoted the P-band in fig. 1, appears on the low energy side of the 12-line. The P-band grows more than linearly with excitation power, and it contains some structure at the highest excitation level, with peaks at 3.453 and 3.448 eV. The peak intensities of the 12-line and the P-band are shown in fig. 2 as a function of excitation power. At low excitation levels the excitation dependences approach a linear and a quadratic dependence for the 12-line and the P-band, respectively (dashed lines). With increasing excitation power the peak intensities tend to saturate. This is to be expected considering the strong broadening of the spectra observed in fig. 1. On the other hand, from fig. 2 it is clear that the intensity of the P-band increases faster than that of the 12-line in the entire excitation range. This is illustrated in the insert of fig. 2, where the peak intensity of the P-band is plotted versus that of the 12-line, revealing a quadratic relationship. It is reasonable to assume the amplitude of the 12line to be proportional to the density nex of free excitons. Hence, the transitions responsible for the P band luminescence are proportional to ~ indicating an exciton exciton interaction process. Indeed, the spectral position of the P-band, about an
I2~LINE INTENSITY
iiooi
0.01
0.1
EXCITATION INTENSITY 10 (RELATIVE UNITS) Fig. 2. Kinetics of the luminescence lines as a function of excitation intensity. The insert shows the peak intensity of the P(2)-line versus peak intensity of the I2-line.
614
f.M. Hvam, E. Elder/New emission line in highly excited GaN
exciton binding energy below the free exciton energy, agrees with transitions involving the inelastic scattering between two free excitons. Such a transition was first proposed by Benoit a Ia Guillaume et a!. to occur in CdS [6], and later reported in ZnO, CdS and CdSe by Magde and Mahr [71.We consider here an interaction between two free excitons in their ground state (n = 1) resulting in the scattering of one exciton into a photon-like state, leaving the crystal as luminescence, while the remaining cxciton is scattered into an excited state, n = 2, 3, ~o. This process implies possible emission lines with peak energies at [8] 2), ii = 2,3 (1) F~(0) ~ G(1 1/n where Ee\ and G are exciton ground state energy and binding energy, respectively. Applying the numerical values Eex = 3.475 eV and & = 2~meV [51we find from eq. (1): E~( 2) = 3.454 eV and E~() = 3.447 eV. These energies are indicated with arrows in fig. 1 and the agreement with experiment is convincing. It appears that with increasing exciton density the process involving scattering to an n = 2 state (P(2)-line) is the first to become active, whereas at the highest pumping level scattering illto higher excited states also takes place. The strong broadening of the spectra at high pumping is ascribed to a rise in effective temperature of the free excitons (hot excitons). Exciton temperatures of 30 40 K are to be expected undes our conditions and with a calculated half width of 2.5 kT of the P(2)-line [8], this corresponds to a width of about 10 meV. The broadening to the high energy side of the 12-line is ascribed to the admixture of free exciton luminescence which has a low energy cut-off atE cx and an exponential high energy tail as observed (fig. 1). At the highest pumping level the emission in the region between the 12-line and the P-band seems to increase rapidly. Whether this is caused by a general broadening, or new recombination mechanisms are involved cannot be decided from the present experiments. At 77 K the free exciton luminescence was increased compared to the 12-line owing to the thermal dissociation of bound excitons. Likewise the P-band luminescence was observed even more intensely than at helium temperatures, supporting the interpretation in terms of free exciton interactions. Preliminary investigations at 77 K showed that stimulated emission in the P-band was easily obtained. From theory and from experiments on other materials [8] it is well known that the exciton exciton collision process is favourable for achieving population inversion and thereby optical gain. Hence, we believe the exciton exciton interaction to be responsible for not only the P-band luminescence observed in the present work, but also the stimulated emission and laser action, previously reported [3]. ...,
...,
References [1) J.I. Pankove, J. Luminescence 7 (1973) 114, and references therein. 121 R. Dingle, D.D. Sell, S.F. Stokowski and M. Ilegems, Phys. Rev. B4 (1971) 1211; R. Dingle and M. ilegems, Solid State Commun. 9 (1971) 175.
f.M. Hvam, E. Elder/New emission line in highly excited GaN [3] [4] [5] [6] [7] [81
615
R. Dingle, K.L. Shaklee, R.F. Leheny and R.B. Zetterstrom, Appl. Phys. Letters 19 (1971) 5. H.P. Maruska and J.J. Tietjen, AppI. Phys. Letters 15 (1969) 327. B. Monemar, Phys. Rev. BlO (1974) 676. C. Benoit ala Guillaume, J.M. Debever and F. Salvan, Phys. Rev. 177 (1969) 567. D. Magde and H. Mahr, Phys. Rev. Letters 24 (1970) 890. J.M. Hvam, Phys. Stat. Sol. (b) 63 (1974) 511.
Discussion S. Shionoya: You don’t see a luminescence line due to excitonic molecules. Would you comment on this? J.M. Hvam: I do see a strong increase of the luminescence in the region between the l 2-line and the P-band at the highest excitation levels. However, I would hesitate to interprete this from the present experiments. T. Goto: Do you think any possible emission process where the P-emission is associated with the 12-bound eXcitofl? J.M. Hvam: I have not considered this possibility. If you refer to the model by Honerlage and Riissler, however, this model predicts a shift of the line with increasing excitation power. No such shift is observed in my experiments. I. Broser: Do you have any explanation for your observation that the ratio of the probability for exciton collision processes with scattering into the n 2 states to that of scattering into higher states depends highly on the excitation intensity? J.M. Hvam: No I have no explanation for this. There exist some calculations for scattering probabilities to different excited states, but, being no theoretician, I do not know how reliable they are.