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Spontaneous and stimulated emission from CdSe at high excitation levels
J. Phys. Chem. Solids
Pergamon Press 1971. Vol. 32, pp. 2193-2199.
Printed in Great Britain.
S P O N T A N E O U S A N D S T I M U L A T E D EMISSION FROM CdSe AT H I G H E X C I T A T I O N LEVELS I. FILINSKI* Chemical Laboratory B, Technical University of Denmark, Lyngby, Denmark and
B. WOJTOWICZ-NATANSON I nstitute of Experimental Physics, University of Warsaw, Warsaw. Poland and J. M. HVAM
Physics Laboratory 1I1, Technical University of Denmark, Lyngby. Denmark (Received 6 June 1969; in revised form 23 November 1970) Abstract-Luminescence of CdSe single crystals excited by high intensity light pulses has been
investigated in the temperature region 4-2-77"K. The rate of increase of the intensity of several free and bound exciton lines as a function of excitation intensity has been determined, The line with the highest rate of increase has been interpreted as being due to interaction of two free excitons. Electron-beam pumped laser emission from this line has been obtained at low temperatures and its contribution to the laser emission at higher temperatures is discussed. 1. INTRODUCTION
ALTHOUGH lasing transitions have been observed in several I I - V I compounds the mechanism giving rise to the lasing lines is still not wholly understood. In a recent paper C. Benoit a la Guillaume e t al.[1] have shown that at least three different processes can lead to laser effects in CdS. Some significant changes occurring in the luminescence of CdSe when the intensity of exciting light is greatly increased haved been reported in the last few years[2-5]. From measurements of the dependence of the intensity of luminescence on the intensity of excitation interesting information may be obtained concerning the nature of recombination centers and the mechanism of luminescence and laser action. This paper reports more detailed studies of these effects in CdSe in the region of exciton lines. 2. EXPERIMENTAL
Thin CdSe platelets were used in the measurements. The luminescence was excited *On leave from the University of Warsaw.
by high intensity pulses from a Xenon flash tube (20 p.p.s., 0.2 Joule pei" flash). Calibrated neutral filters were used for attenuation of the excitation intensity. The recombination radiation was analyzed by a SPM-2 grating monochromator (with 2 0 A / m m dispersion) or with a PGS-2 spectrograph (with 7.8 A/mm dispersion in the investigated spectral region) and was recorded by a photo-multiplier. Spectra were taken with a P A R boxcar integrator followed by a logarithmic converter. The lowest temperature was obtained by immersing crystals in liquid Helium. Carbon and Platilaum resistors were used for temperature measurements. The laser emission was obtained by pulsed electron-beam excitation (200 nsec., 50 Hz). The beam energy was 30 keV and currents up to 40 mA were used. Platelets that were cleaved to form a cavity perpendicular to the electron-beam (laser geometry) were mounted on a copper cold finger in a Helium dewar. In spontaneous emission measurements the light emitted directly from the bombarded face was detected. The output from the photomultiplier was led to a sampling oscilloscope,
2193
2194
I. F I L I N S K I et al.
sampling on the leading edge of the pulse in order to avoid the influence of heating during the pulse. 3. RESULTS AND;DISCUSSION
The dependence of th~ intensity of the most prominent lines in the exciton spectrum on the intensity of exciting light was measured at 4.2~ This dependence can be expressed by the formula llum ~ 1 '~ exc
(1)
is the intensity of luminescence and le.~c the intensity of exciting light. The values of the power a are given in Table 1. As the temperature was increased from 4.2 to 77~ the spectrum shifted towards longer wavelengths due to the variation of the band gap. The positions of the maxima in this temperature range are shown in Fig. 1. The shifts of the phonon lines A1-LO and A r 2 L O are seen to be weaker corresponding to shifts of 1-5 and 0.5 kT respectively towards higher energies relative to the band gap in agreement with the results from Ref. [6]. With the change of temperature a significant change of spectral w h e r e llum
distribution of emitted intensity is observed (Fig. 2). The intensity of the recombination radiation of the free exciton A~ is nearly a linear function of the excitation intensity, as should be expected for high excitation intensities at 4.2~ (at very high excitation intensity, however, this function can become sublinear [4,7]), at which the number of free excitons should be proportional to the number of created pairs of carriers. The Is line is ascribed to the radiative recombination of an exciton bound to a neutral donor[8]. The binding energy of this complex measured from the spectrum as the distance on energy scale between this line and the A1 line is 4 m e V . With increasing temperature the intensity of the 12 line diminishes rapidly, because of the small binding energy of the complex. The dependence on the excitation intensity is linear, as could be expected if the number of these shallow neutral donors is independent of the excitation intensity. ( T h e number of free excitons increases linearly with the number of incident photons). The dependence of the intensity of the
Table 1. Positions o f some o f the exciton lines in CdSe at 4-2~ and the respective values o f a obtained by photoexcitation Symbol
A~ /z 11 P A~-LO I~-LO
A ~-2LO
Exciton complex
free exciton Fg-F7 in state n = I exciton bound to neutral donor exciton bound to neutral acceptor ? free exciton with emission of I phonon exciton bound to neutral acceptor with emission of I phonon free exciton with emission of 2 phonons
Wavelength
Energy
a":
A, 6789
eV 1"826
1" 1
6803
1"822
1.0
6823
1'817
1"6
6847 6889
1.811 1.799
1-9 > 1
6924
1.790
> 1
6993
1.773
*Because of some overlapping of the peaks and of a strong background the accuracy of a is about 20 per cent.
CdSe A T H I G H E X C I T A T I O N L E V E L S
2195
1.830 CdSe 1.82C
1.810
A,;Lo "~
+ x
1,800
B
x
+
+ i
T
2, l
r
r
li-LO
o
+
~. 1.790
+
++ 1380
A1-2LO ~
1.770
v& p o n XOA S " Laser emission
o ~
Loser emission from Ref.4 1.760
' 10
• 20
' 30'
4tO Temperot ure
' 50~
' 60
=n 7_
80'
(~
Fig. 1, Position of spontaneousand stimulated emission lines vs. temperature. The spontaneous emission was excited by a flash lamp. The laser emission was electron-beam excited with a beam energy of 30 keV and beam currents of I-5 mA.
I1 line on excitation is superlinear with a = 1-6. One may expect this value of power since this line is due to recombination of excitons bound to neutral acceptors[8]. Its intensity should therefore be proportional to both the concentration of free excitons and of neutral acceptors. As CdSe is always
n-type, acceptors will tend to be ionized in unexcited samples. Hence, far from saturation, the concentration of neutral acceptors should be proportional to the number of free holes generated by the light. The concentration of free holes, on the other hand, will increase as the square root of the excitation
2196
I. F I L I N S K I et al.
I2
Odk
CdSe
II
P
LAr'~
20
-~
LAr'~
40
60
5~'K
I
I
I
I
1.83
1.82
1.81
1.80
I
1.79
1
1.78
|
t77
1.76
1.75
Photon energy (eV)
Fig. 2. Photoexcited luminescence spectra at different temperatures.
intensity, as can be seen from the following increases as the square of the intensity of argument. The number of free excitons which exciting light and the position of which is is proportional to the product n 9p of electron shifted from the free exciton line by the and hole concentrations is observed to in- exciton binding energy. This line was excrease almost linearly with light intensity. plained by Haynes as arising from the decay Assuming n = p (low temperature) one gets of excitonic molecules. The binding energy a square root dependence for the hole con- of the excitonic molecule is expected to be centration. With values o~= 1 for free exci- very small[9] (in case of CdSe it could be tons and ot----0.5 for neutral acceptors one estimated as Eb < 3 meV), and one should should expect ot = 1.5 for the 11 line. not expect to observe this line at elevated For the P line we have found o~= 1-9, temperatures. As the P line persists even to i.e. a value higher than that for the 11 line. 50~ (see Fig. 2) we think that in our case a In the previous paper [2] the P line was collision of two excitons resulting in distentatively ascribed to the recombination of sociation of one of them into a free electron free excitons in the excited state n----2 with and a free hole and a radiative recombination simultaneous emission of a longitudinal of the other is a more probable process than optical phonon. It should be noted, however, the mechanism proposed by Haynes. The that the P line is separated from the free ex- hv of emitted photons would then be about citonA1 line by 15 meV, i.e. by the value equal Eo--2Ex, where Ex is the binding energy of a to the free exciton binding energy. Haynes [9] free exciton and Eo the energy gap. Assuming has observed in Si a line the intensity of which such an exciton-exciton collision process one
CdSe A T H I G H E X C I T A T I O N L E V E L S
can expect that the intensity of the P line should increase as the square of that of the exciting light. This interpretation of the origin of the P line is the same as was first proposed by C. Benoit a ia Guillaume et al. for one of the three processes leading to laser transitions in C d S [ I , 10, 11]. A new luminescence line observed in ZnO, CdS and CdSe at low temperatures and high excitation was ascribed the same recombination mechanism[12]. In all the three materials this line is separated by the exciton binding energy from the A1 line. We have obtained electron-beam pumped laser emission in CdSe in the temperature range from 25 up to above 100~ The laser lines were relatively broad containing several unresolved modes. The number of modes was increasing drastically with increasing excitation. Hence the spectra were recorded as
2197
close to the threshold as possible. Fig. 3 shows spontaneous and stimulated emission spectra corresponding to a sample temperature of 35~ In the spontaneous spectrum the 1~ line is dominant which was typical for the samples used in the laser measurements (Cd-rich compared to the optically excited samples). In the laser geometry we observed stimulated emission at wavelengths close to the P line as well as the A1-LO line (but not in between). Whether laser emission was obtained in one band or the other or in both bands simultaneously was dependent on sample, position on sample, and excitation in a nonsystematic way. In the actual recordings of Fig. 3 the laser emission in the A1-LO band had a somewhat higher threshold than in the P band. With increasing temperature the two laser lines approached each other and above 60~ we only observed laser emission in one band
12
ql- (o, II
C d Se
35~
30 k V
(o) Spontaneous emission (b) ( c )
Stimutoted
emission.
>-
tti-.1
-LO A1
A1-2LO 6800
68 50 6 900 WaveLength ( ~ )
69 50
7000
Fig. 3. Electron-beamexcited emissionspectra at 35~ The intensityof the laser lines have been reduced by a factor of about 50. Beamcurrents I mA in (a) and (b) and 4 mA in (c).
2198
I. F I L I N S K I et al.
which on the other hand exhibits a shift that is considerably larger than the shift of the band gap, here represented by the A1 line (see Fig. 1). At 80~ the shift relative to the band gap is about 2 0 ~ e V . The threshold for laser action did not increase appreciably from Helium temperature up to above 100~ the threshold current being 1-2 mA in this temperature range. Our results at the lower temperatures are consistent with the laser emission that has earlier been observed in CdSe[4,5]. At 77~ however, Ref. [4] reports laser emission in the A 1 - L O band which we have not been able to obtain. The threshold for laser action from the A1-LO process has been calculated[13] to rise exponentially with temperature at higher temperatures due to the growth in phonon population. The peak position of neither the A~-LO line[13] nor the exciton-exciton line[ll] is expected to be shifted drastically with temperature increase. However, the large shift of the position of the laser line only requires that the maximum net gain shifts accordingly, which may as well be caused by the temperature and wavelength dependent absorption tail that has been observed below the lowest exciton energy in several I I - V I compounds [14, 15]. This absorption (described by the empirical Urbach's rule) has recently been shown[16] to be able to explain a similar temperature shift of the laser line in CdS assuming a relatively broad gain curve following the band gap, and with an extended low energy tail. One would expect the exciton-exciton line to be broad and with a low energy tail in contrast to the A~-LO line[13]. We therefore suggest that the laser emission at elevated temperatures is due to the same process as the low temperature laser action on the P-line, namely the excitonexciton collision process. This is also supported by the continuous and smooth shift of the laser line. We have observed a similar shift of the laser line at elevated temperatures in CdS and ZnO[17] and the contribution of the absorp-
tion tail to this shift is under investigation to appear in a later publication. 4. CONCLUSION
The measured excitation dependences of the different exciton lines are in agreement with the identifications of the lines. The near quadratic dependence of the P line together with its position an exciton binding energy below the A1 line strongly suggest this line to be due to an exciton-exciton collision process. At low temperatures we have obtained laser action from the P line as well as the A1-LO line. The continuous shift of the laser line with increasing temperature in connection with the slow rise in threshold current points towards a recombination process with a low energy tail in photon energy and with no inverse absorption process connected to it. These conditions are apparently met by the exciton-exciton collision process although the exact lineshape has not been evaluated for this process at finite temperatures. The position of the laser line is probably largely determined by the onset of the background absorption. Acknowledgements-We would like to thank Professor L. Sosnowski and Dr. T. Skettrup for stimulating discussions and for their interest in this work. We are also indebted to Professor N. I. Meyer, Professor R. W. Asmussen, Dr. J. Mycielski, Dr. W. Wardzynski and Professor I. Balslev for critical discussions of the results. One of us (B.W-N.) wishes to express her gratitude to Professor R. W. Asmussen for his kind permission to execute a part of this work in his laboratory. REFERENCES 1. B E N O I T A LA G U I L L A U M E C., D E B E V E R J. M. and S A L V A N F., Phys. Rev. 177,567 (1969). 2. R A Z B I R I N B. S., F I L I N S K I I. and W O J T O W I C Z N A T A N S O N B., Phys. Status Solidi 18, K37 (1966). 3. G R U N J. B., M Y S Y R O W I C Z A., R A G A F., B I V A S A., L E V Y R. and N I K I T I N E S., Phys. Status Solidi22, K155 (1967). 4. P A C K A R D J. R., C A M P B E L L D. A. and T A I T W. C.,J. appl. Phys. 38, 5255 (1967). 5. H U R W I T Z C. E., I I - V I Semiconducting Compounds-1967 Int. Conf., p. 682. W. A. Benjamin, New York (1967). 6. S E G A L L B. and M A H A N G. D., Phys. Rev. 171, 935 (1968).
CdSe AT H I G H EXCITATION LEVELS 7. MYSYROWlCZ A., G R U N J. B., BIVAS A., LEVY R. and N I K I T I N E S., Phys. Left. 25A, 286 (1967). 8. GROSS E. F., RAZBIRIN B. S., FEDOROV V. P. and N A U M O V Yu.P., Phys. Status Solidi 30, 485 (1968). 9. HAYNES J. R.. Phys. Rev. Lett. 17,860 (1966). 10. BENOIT A LA G U I L L A U M E C., DEBEVER J. M. and SALVAN F., I I - V I Semiconducting Compounds-1967 Int. Conf. p. 669. W. A. Benjamin, New York (1967). 11. BENOIT A LA G U I L L A U M E C., DEBEVER J. M. and SALVAN F., IX Int. Conf. Semiconductor Physics, p. 581. Moscow, 'Nauka', Leningrad (I 968).
2199
12. M A G D E D., MAHR H., Phys. Rev. Lett. 2,4, 890 (1970). 13. HAUG H.,J. appl. Phys. 39, 4687 (1968). 14. SEGALL B., Phys. Rev. 163,769 (1967). 15. G N A T E N K O Yu.P. and K U RI K M. V., Soviet Phys-Solid State 12, 892 (1970). 16. LEHENY, R. F., SHAKLEE K. L., IPPEN E. P., N A H O R Y R. E. and SHAY J. L., Appl. Phys. Lett. 17, 494 (1970). 17. HVAM J. M., Proceedings of the Tenth Interna-
tional Conference on the Physics of Semiconductors, U.S.A.E.C. 1970, p. 71, Cambridge, Mass. (1970).
Pergamon Press 1971. Vol. 32, pp. 2193-2199.
Printed in Great Britain.
S P O N T A N E O U S A N D S T I M U L A T E D EMISSION FROM CdSe AT H I G H E X C I T A T I O N LEVELS I. FILINSKI* Chemical Laboratory B, Technical University of Denmark, Lyngby, Denmark and
B. WOJTOWICZ-NATANSON I nstitute of Experimental Physics, University of Warsaw, Warsaw. Poland and J. M. HVAM
Physics Laboratory 1I1, Technical University of Denmark, Lyngby. Denmark (Received 6 June 1969; in revised form 23 November 1970) Abstract-Luminescence of CdSe single crystals excited by high intensity light pulses has been
investigated in the temperature region 4-2-77"K. The rate of increase of the intensity of several free and bound exciton lines as a function of excitation intensity has been determined, The line with the highest rate of increase has been interpreted as being due to interaction of two free excitons. Electron-beam pumped laser emission from this line has been obtained at low temperatures and its contribution to the laser emission at higher temperatures is discussed. 1. INTRODUCTION
ALTHOUGH lasing transitions have been observed in several I I - V I compounds the mechanism giving rise to the lasing lines is still not wholly understood. In a recent paper C. Benoit a la Guillaume e t al.[1] have shown that at least three different processes can lead to laser effects in CdS. Some significant changes occurring in the luminescence of CdSe when the intensity of exciting light is greatly increased haved been reported in the last few years[2-5]. From measurements of the dependence of the intensity of luminescence on the intensity of excitation interesting information may be obtained concerning the nature of recombination centers and the mechanism of luminescence and laser action. This paper reports more detailed studies of these effects in CdSe in the region of exciton lines. 2. EXPERIMENTAL
Thin CdSe platelets were used in the measurements. The luminescence was excited *On leave from the University of Warsaw.
by high intensity pulses from a Xenon flash tube (20 p.p.s., 0.2 Joule pei" flash). Calibrated neutral filters were used for attenuation of the excitation intensity. The recombination radiation was analyzed by a SPM-2 grating monochromator (with 2 0 A / m m dispersion) or with a PGS-2 spectrograph (with 7.8 A/mm dispersion in the investigated spectral region) and was recorded by a photo-multiplier. Spectra were taken with a P A R boxcar integrator followed by a logarithmic converter. The lowest temperature was obtained by immersing crystals in liquid Helium. Carbon and Platilaum resistors were used for temperature measurements. The laser emission was obtained by pulsed electron-beam excitation (200 nsec., 50 Hz). The beam energy was 30 keV and currents up to 40 mA were used. Platelets that were cleaved to form a cavity perpendicular to the electron-beam (laser geometry) were mounted on a copper cold finger in a Helium dewar. In spontaneous emission measurements the light emitted directly from the bombarded face was detected. The output from the photomultiplier was led to a sampling oscilloscope,
2193
2194
I. F I L I N S K I et al.
sampling on the leading edge of the pulse in order to avoid the influence of heating during the pulse. 3. RESULTS AND;DISCUSSION
The dependence of th~ intensity of the most prominent lines in the exciton spectrum on the intensity of exciting light was measured at 4.2~ This dependence can be expressed by the formula llum ~ 1 '~ exc
(1)
is the intensity of luminescence and le.~c the intensity of exciting light. The values of the power a are given in Table 1. As the temperature was increased from 4.2 to 77~ the spectrum shifted towards longer wavelengths due to the variation of the band gap. The positions of the maxima in this temperature range are shown in Fig. 1. The shifts of the phonon lines A1-LO and A r 2 L O are seen to be weaker corresponding to shifts of 1-5 and 0.5 kT respectively towards higher energies relative to the band gap in agreement with the results from Ref. [6]. With the change of temperature a significant change of spectral w h e r e llum
distribution of emitted intensity is observed (Fig. 2). The intensity of the recombination radiation of the free exciton A~ is nearly a linear function of the excitation intensity, as should be expected for high excitation intensities at 4.2~ (at very high excitation intensity, however, this function can become sublinear [4,7]), at which the number of free excitons should be proportional to the number of created pairs of carriers. The Is line is ascribed to the radiative recombination of an exciton bound to a neutral donor[8]. The binding energy of this complex measured from the spectrum as the distance on energy scale between this line and the A1 line is 4 m e V . With increasing temperature the intensity of the 12 line diminishes rapidly, because of the small binding energy of the complex. The dependence on the excitation intensity is linear, as could be expected if the number of these shallow neutral donors is independent of the excitation intensity. ( T h e number of free excitons increases linearly with the number of incident photons). The dependence of the intensity of the
Table 1. Positions o f some o f the exciton lines in CdSe at 4-2~ and the respective values o f a obtained by photoexcitation Symbol
A~ /z 11 P A~-LO I~-LO
A ~-2LO
Exciton complex
free exciton Fg-F7 in state n = I exciton bound to neutral donor exciton bound to neutral acceptor ? free exciton with emission of I phonon exciton bound to neutral acceptor with emission of I phonon free exciton with emission of 2 phonons
Wavelength
Energy
a":
A, 6789
eV 1"826
1" 1
6803
1"822
1.0
6823
1'817
1"6
6847 6889
1.811 1.799
1-9 > 1
6924
1.790
> 1
6993
1.773
*Because of some overlapping of the peaks and of a strong background the accuracy of a is about 20 per cent.
CdSe A T H I G H E X C I T A T I O N L E V E L S
2195
1.830 CdSe 1.82C
1.810
A,;Lo "~
+ x
1,800
B
x
+
+ i
T
2, l
r
r
li-LO
o
+
~. 1.790
+
++ 1380
A1-2LO ~
1.770
v& p o n XOA S " Laser emission
o ~
Loser emission from Ref.4 1.760
' 10
• 20
' 30'
4tO Temperot ure
' 50~
' 60
=n 7_
80'
(~
Fig. 1, Position of spontaneousand stimulated emission lines vs. temperature. The spontaneous emission was excited by a flash lamp. The laser emission was electron-beam excited with a beam energy of 30 keV and beam currents of I-5 mA.
I1 line on excitation is superlinear with a = 1-6. One may expect this value of power since this line is due to recombination of excitons bound to neutral acceptors[8]. Its intensity should therefore be proportional to both the concentration of free excitons and of neutral acceptors. As CdSe is always
n-type, acceptors will tend to be ionized in unexcited samples. Hence, far from saturation, the concentration of neutral acceptors should be proportional to the number of free holes generated by the light. The concentration of free holes, on the other hand, will increase as the square root of the excitation
2196
I. F I L I N S K I et al.
I2
Odk
CdSe
II
P
LAr'~
20
-~
LAr'~
40
60
5~'K
I
I
I
I
1.83
1.82
1.81
1.80
I
1.79
1
1.78
|
t77
1.76
1.75
Photon energy (eV)
Fig. 2. Photoexcited luminescence spectra at different temperatures.
intensity, as can be seen from the following increases as the square of the intensity of argument. The number of free excitons which exciting light and the position of which is is proportional to the product n 9p of electron shifted from the free exciton line by the and hole concentrations is observed to in- exciton binding energy. This line was excrease almost linearly with light intensity. plained by Haynes as arising from the decay Assuming n = p (low temperature) one gets of excitonic molecules. The binding energy a square root dependence for the hole con- of the excitonic molecule is expected to be centration. With values o~= 1 for free exci- very small[9] (in case of CdSe it could be tons and ot----0.5 for neutral acceptors one estimated as Eb < 3 meV), and one should should expect ot = 1.5 for the 11 line. not expect to observe this line at elevated For the P line we have found o~= 1-9, temperatures. As the P line persists even to i.e. a value higher than that for the 11 line. 50~ (see Fig. 2) we think that in our case a In the previous paper [2] the P line was collision of two excitons resulting in distentatively ascribed to the recombination of sociation of one of them into a free electron free excitons in the excited state n----2 with and a free hole and a radiative recombination simultaneous emission of a longitudinal of the other is a more probable process than optical phonon. It should be noted, however, the mechanism proposed by Haynes. The that the P line is separated from the free ex- hv of emitted photons would then be about citonA1 line by 15 meV, i.e. by the value equal Eo--2Ex, where Ex is the binding energy of a to the free exciton binding energy. Haynes [9] free exciton and Eo the energy gap. Assuming has observed in Si a line the intensity of which such an exciton-exciton collision process one
CdSe A T H I G H E X C I T A T I O N L E V E L S
can expect that the intensity of the P line should increase as the square of that of the exciting light. This interpretation of the origin of the P line is the same as was first proposed by C. Benoit a ia Guillaume et al. for one of the three processes leading to laser transitions in C d S [ I , 10, 11]. A new luminescence line observed in ZnO, CdS and CdSe at low temperatures and high excitation was ascribed the same recombination mechanism[12]. In all the three materials this line is separated by the exciton binding energy from the A1 line. We have obtained electron-beam pumped laser emission in CdSe in the temperature range from 25 up to above 100~ The laser lines were relatively broad containing several unresolved modes. The number of modes was increasing drastically with increasing excitation. Hence the spectra were recorded as
2197
close to the threshold as possible. Fig. 3 shows spontaneous and stimulated emission spectra corresponding to a sample temperature of 35~ In the spontaneous spectrum the 1~ line is dominant which was typical for the samples used in the laser measurements (Cd-rich compared to the optically excited samples). In the laser geometry we observed stimulated emission at wavelengths close to the P line as well as the A1-LO line (but not in between). Whether laser emission was obtained in one band or the other or in both bands simultaneously was dependent on sample, position on sample, and excitation in a nonsystematic way. In the actual recordings of Fig. 3 the laser emission in the A1-LO band had a somewhat higher threshold than in the P band. With increasing temperature the two laser lines approached each other and above 60~ we only observed laser emission in one band
12
ql- (o, II
C d Se
35~
30 k V
(o) Spontaneous emission (b) ( c )
Stimutoted
emission.
>-
tti-.1
-LO A1
A1-2LO 6800
68 50 6 900 WaveLength ( ~ )
69 50
7000
Fig. 3. Electron-beamexcited emissionspectra at 35~ The intensityof the laser lines have been reduced by a factor of about 50. Beamcurrents I mA in (a) and (b) and 4 mA in (c).
2198
I. F I L I N S K I et al.
which on the other hand exhibits a shift that is considerably larger than the shift of the band gap, here represented by the A1 line (see Fig. 1). At 80~ the shift relative to the band gap is about 2 0 ~ e V . The threshold for laser action did not increase appreciably from Helium temperature up to above 100~ the threshold current being 1-2 mA in this temperature range. Our results at the lower temperatures are consistent with the laser emission that has earlier been observed in CdSe[4,5]. At 77~ however, Ref. [4] reports laser emission in the A 1 - L O band which we have not been able to obtain. The threshold for laser action from the A1-LO process has been calculated[13] to rise exponentially with temperature at higher temperatures due to the growth in phonon population. The peak position of neither the A~-LO line[13] nor the exciton-exciton line[ll] is expected to be shifted drastically with temperature increase. However, the large shift of the position of the laser line only requires that the maximum net gain shifts accordingly, which may as well be caused by the temperature and wavelength dependent absorption tail that has been observed below the lowest exciton energy in several I I - V I compounds [14, 15]. This absorption (described by the empirical Urbach's rule) has recently been shown[16] to be able to explain a similar temperature shift of the laser line in CdS assuming a relatively broad gain curve following the band gap, and with an extended low energy tail. One would expect the exciton-exciton line to be broad and with a low energy tail in contrast to the A~-LO line[13]. We therefore suggest that the laser emission at elevated temperatures is due to the same process as the low temperature laser action on the P-line, namely the excitonexciton collision process. This is also supported by the continuous and smooth shift of the laser line. We have observed a similar shift of the laser line at elevated temperatures in CdS and ZnO[17] and the contribution of the absorp-
tion tail to this shift is under investigation to appear in a later publication. 4. CONCLUSION
The measured excitation dependences of the different exciton lines are in agreement with the identifications of the lines. The near quadratic dependence of the P line together with its position an exciton binding energy below the A1 line strongly suggest this line to be due to an exciton-exciton collision process. At low temperatures we have obtained laser action from the P line as well as the A1-LO line. The continuous shift of the laser line with increasing temperature in connection with the slow rise in threshold current points towards a recombination process with a low energy tail in photon energy and with no inverse absorption process connected to it. These conditions are apparently met by the exciton-exciton collision process although the exact lineshape has not been evaluated for this process at finite temperatures. The position of the laser line is probably largely determined by the onset of the background absorption. Acknowledgements-We would like to thank Professor L. Sosnowski and Dr. T. Skettrup for stimulating discussions and for their interest in this work. We are also indebted to Professor N. I. Meyer, Professor R. W. Asmussen, Dr. J. Mycielski, Dr. W. Wardzynski and Professor I. Balslev for critical discussions of the results. One of us (B.W-N.) wishes to express her gratitude to Professor R. W. Asmussen for his kind permission to execute a part of this work in his laboratory. REFERENCES 1. B E N O I T A LA G U I L L A U M E C., D E B E V E R J. M. and S A L V A N F., Phys. Rev. 177,567 (1969). 2. R A Z B I R I N B. S., F I L I N S K I I. and W O J T O W I C Z N A T A N S O N B., Phys. Status Solidi 18, K37 (1966). 3. G R U N J. B., M Y S Y R O W I C Z A., R A G A F., B I V A S A., L E V Y R. and N I K I T I N E S., Phys. Status Solidi22, K155 (1967). 4. P A C K A R D J. R., C A M P B E L L D. A. and T A I T W. C.,J. appl. Phys. 38, 5255 (1967). 5. H U R W I T Z C. E., I I - V I Semiconducting Compounds-1967 Int. Conf., p. 682. W. A. Benjamin, New York (1967). 6. S E G A L L B. and M A H A N G. D., Phys. Rev. 171, 935 (1968).
CdSe AT H I G H EXCITATION LEVELS 7. MYSYROWlCZ A., G R U N J. B., BIVAS A., LEVY R. and N I K I T I N E S., Phys. Left. 25A, 286 (1967). 8. GROSS E. F., RAZBIRIN B. S., FEDOROV V. P. and N A U M O V Yu.P., Phys. Status Solidi 30, 485 (1968). 9. HAYNES J. R.. Phys. Rev. Lett. 17,860 (1966). 10. BENOIT A LA G U I L L A U M E C., DEBEVER J. M. and SALVAN F., I I - V I Semiconducting Compounds-1967 Int. Conf. p. 669. W. A. Benjamin, New York (1967). 11. BENOIT A LA G U I L L A U M E C., DEBEVER J. M. and SALVAN F., IX Int. Conf. Semiconductor Physics, p. 581. Moscow, 'Nauka', Leningrad (I 968).
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12. M A G D E D., MAHR H., Phys. Rev. Lett. 2,4, 890 (1970). 13. HAUG H.,J. appl. Phys. 39, 4687 (1968). 14. SEGALL B., Phys. Rev. 163,769 (1967). 15. G N A T E N K O Yu.P. and K U RI K M. V., Soviet Phys-Solid State 12, 892 (1970). 16. LEHENY, R. F., SHAKLEE K. L., IPPEN E. P., N A H O R Y R. E. and SHAY J. L., Appl. Phys. Lett. 17, 494 (1970). 17. HVAM J. M., Proceedings of the Tenth Interna-
tional Conference on the Physics of Semiconductors, U.S.A.E.C. 1970, p. 71, Cambridge, Mass. (1970).