Photopumped lasing properties in ZnCdSe—ZnSSe multiple quantum wells

JOURNAL OF

LUMINESCENCE ELSEVIER

Journal of Luminescence 59 (1994) 341—348

Photopumped lasing properties in ZnCdSe—ZnSSe multiple quantum wells S. Yamaguchi~,T. Shinzato, K. Ichino, Y. Kawakami, Sz. Fujita, Sg. Fujita Department of Electrical Engineering, Kvoto University, Kvoto 606, Japan (Received 25 April 1993; revised 15 December; accepted 11 January 1994)

Abstract Photopumped lasing properties in a temperature range between 4.2 K and room temperature in a Zn 083Cd017Se—ZnS010Se090 multiple quantum well structure prepared by metalorganic molecular beam epitaxy (MOMBE) have been investigated. Photoluminescence (PL) under low excitation, absorption and optical gain as well as the stimulated emission spectra have been systematically compared. It was estimated that the value of the Stokes shift between PL and absorption peak energies was as small as 5 meV at 4.2 K. However, the shift was hardly observed above 150 K. Low threshold excitation intensity for the lasing and lack of any difference in the values of full width at half maximum (FWHM) between the spectra of the optical gain and the heavy hole (HH) exciton absorption coefficient indicate that the exciton is related to the lasing mechanism at a temperature lower than —‘ 150 K.

1. Introduction

a blue—green LD has been reported in the latticematched (Zn, Mg) (S, Se) system [10,11].

Remarkable advance has been made in the past few years in the development of widegap Il—VI semiconductor quantum wells (QWs) such as ZnSe—Zn(S, Se), (Zn, Cd)Se—ZnSe, (Zn, Cd)Se— Zn(S, Se), (Zn, Cd)(S, Se)—Zn(S, Se), ZnSe— (Zn, Mg) (S, Se) and (Zn, Cd)S—ZnS for the realization of the optoelectronic devices operated in the green to the ultraviolet (UV) spectral region [1—23].Among these structures, (Zn, Cd)Se single QW (SQW) embedded in a ZnSe—Zn(S, Se) waveguide was allowed to demonstrate the first operation of injection laser diodes (LDs) by Haase et al. [14]. Most recently, the operation of a blue or

It has been recognized that the utilization of well designed waveguide structures and the introduction of stress to the wells [15] is effective for the realization of the LD exhibiting continuous wave (CW). oscillations at room temperature (RT). An understanding of the lasing processes is also vital to improve the performance of LDs. This is because the lasing mechanism in these structures is expected to be different from that of Ill—V semiconductors. Since the exciton screening density of Il—Vis is estimated to be much larger than that of III—Vs, it is expected that excitons can play an important role in the lasing process. This has been clearly shown at cryogenic temperatures in the

~

author,

study of photon or electron-beam pumped lasing mechanism for bulk crystals and single epilayers.

0022-2313/94/507.00 © 1994 — Elsevier Science By. All rights reserved SSDIOO22-2313(94)00008-Z

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/

Journal of Luminescence 59 (1994) 341—348

Ding et al. have pointed out that at low temperature excitons may relax to the low-energy states of the inhomogeneous spectral line due to the cornpositional fluctuations of the alloy, resulting in the formation of optical gain following exciton phasespace filling of those states [4], and more recently he has pointed out the possibility of the role of excitons even at RT in the QW structures in which the quasi-two-dimensional confinement are realized [21]. Most recently, Ando et al. [20] and Suemune et al. [2] have pointed out from photoluminescence (PL) under low excitation, photocurrent and stimulated emission spectra, that localized excitons may contribute to the formation of optical gain and the stimulated emission in a Il—VI QW system [16—22]. However, it is notable that these localization effects are due to the fluctuation of well widths and/or the compositional disorders of the alloy, which depend on the structural and crystal qualities of the sample and occasionally make it difficult to understand the detailed radiative recombination process for this kind of QW. Therefore, one has to be careful about the identification of the radiative recombination mechanism, particularly under high excitation. The localization effect due to the alloy disorder cannot be avoided even in perfect alloys because of the small Bohr radius in Il—VI widegap semiconductors. The broadening of the exciton line due to this effect for the sample has been analyzed using the model reported by Zimmermann [24], and the exciton linewidth of Zn1 _~Cd~Se was calculated to be about 9 meV ( ±2 meV), while the PL linewidth of the sample is about 12 meV. The interface of the ZnCdSe—ZnSSe studied here can be of high quality as indicated by the observation of RHEED specular beam oscillations during the growth [25]. In fact, in our previous paper we have shown that the flat and abrupt hetero-interfaces between (Zn, Cd)Se and Zn(S, Se) which are grown coherently on GaAs substrates could be formed by a metalorganic molecular beam epitaxy (MOMBE) technique [25]. In this paper we have investigated on the lasing properties of the (Zn, Cd)Se—Zn(S, Se) multiple QW (MQW) structures grown by MOMBE. Photoluminescence under low excitation, absorp-

tion and optical gain spectra as well as the stimulated emission have been compared. The results show that the difference in peak energy between the absorption and the optical gain is much larger than the Stokes shift of the photoluminescence.

2. Experimental procedures The sample assessed here is a Zn083Cd017 Se—ZnS0,10Se0,90 MQW structure which was grown by MOMBE on a (100) GaAs substrate, beginning with a 500A thick ZnSe buffer layer, followed by a 3000 A thick ZnS010Se090 clad layer. MQWs were formed by six 80-A-thick Zn0 83Cd0 17Se well layers, separated by 110-Athick ZnS010Se090 barrier layers, followed by 1000-A-thick ZnS010Se0,90 cap layer. Details of the growth procedure are described elsewhere [7]. The experimental alignment was as follows. For all optical measurements, the sample was set in a temperature variable cryostat. A CW 2 PL was under meaweak excitation of about 400 mW/cm sured using an excitation source of He—Cd laser (325 nm). For the absorption measurement, the GaAs substrate was selectively etched away, leaving a free-standing section of the epilayer. A N 2 laser (337 nm) with a pulse duration of about 10 ns at a 10 Hz repetition rate was used for the optical pumped gain and lasing measurement. For such measurements the sample surface was excited in an edge-to-edge geometry using a cylindrical lens, and the emission from the cleaved edge of the sample was focused on the entrance slit of a 1 m single monochromator followed by a photomultiplier or a gated photon counter. The optical gain was measured by the Schaklee’s technique [26] by which the optical gain is estimated by varying the length of the excited stripe of the sample. In the estimation of a linear amplification in log scale, the gain coefficient g is derived from the relationship 1 x l/g[exp(gl) 1], where I is the emitted intensity from the sample edge and I is the cavity length. For lasing experiments the sample was cleaved to approximately 1-mm-long resonators with uncoated facets. —

S. Yamaguchi et al.

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3. Results and discussion

(a)

Fig. 1 shows the temperature dependence of the absorption spectra of the sample investigated. The band gap difference between the ZnS010Se0,90 bar-

(1994) 341—348

343

Photon Energy (eV) 2.70 2.60 \.j.~n=1LH

1HH

4.2K

rier layer and the Zn0 83Cd0 175e QW layer is about 280 meV [9]. In this structure the conduc-

valence confinement tion band band offset for offset both (AE~)and (AEVhh) electron are theand estimated heavy heavy hole tohole be(HH) can 179 and 104 meV, respectively [9]. Therefore, good be expected. Indeed, n 1 heavy hole (HH) exciton transition can be identified up to room temperature. PL spectra as well as the absorption spectra at 4.2 K and RT are shown in Figs. 2(a) and (b), respectively. Since the PL emission at RT was too weak to be detected by a He—Cd laser excitation, the pulsed excitation by a N2 laser was employed. For this measurement the excitation was enset 2 sopower that the as lowshifts as about 10 W/cmexcitation can be ergy due toseveral high density =

I

I

450

460Wavelength 470

(b)

480 (nm)

490

Photon Energy (eV) 2.60 2.50

2.40

RT

3.0

Photon Energy 2.5(eV)

~b~brption

4.2K

_______

77K C 0

150K

0 U)

_____

200K

________

________________________ I I

460

480

500

520

250K Wavelength (nm) RT

400

Fig. 2. PL from the surface and absorption spectra taken from the same sample as shown in Fig. I, at 4.2 K (a) and at RT (b), respectively. In the absorption spectrum n = 1 HH and n = I LH exciton lines are depicted by arrows. ,

450 500 Wavelength (nm)

‘550

Fig. 1. Temperature dependence of absorption spectra of a Zn 0 83Cd0 17Se—ZnS0 10Se0 90 MQW sample whose well width is 80 A. The main peak is attributed to the n = I HH exciton line,

neglected, i.e., under several 10W/cm2 of excitation power, sheet carrier concentration in the active we adopt the region of the sample can 2, be where estimated to be less than around 4.8 x i0~cm

344

5. Yamaguchi et al. .1 Journal of Luminescence 59 (1994) 341—348

value of 100 ps as carrier lifetime, which was estimated by a time-resolved luminescence measurement. At 4.2 K the difference in peak energy between absorption (2.623 eV) and PL (2.628 eV) spectra is about 5 meV. This energy difference, defined as Stokes shift, arises from the localization of the

(a)

Energy (eV) 2.60

2.55

I

4.2K

nwell 1 width HH exciton. dependence Theoretical of the full analysis width at of half the maximum (FWHM) of the PL in this MQW system has shown that the dominant mechanism for the localization will be ascribed not to the fluctuation of the well width but to the alloy disorder [25]. At RT no Stokes shift can be observed as shown in Fig. 2(b). In order to investigate how the localized exciton becomes delocalized with increasing tern-

U) C

=

perature, peak energies of the absorption and the PL are plotted as a function of temperature as shown in Fig. 3. Since localized excitons are delocalized by thermal energies, the Stokes shift decreases with increasing temperature and becomes too small to be observed at temperatures higher than 150 K. Figs. 4(a) and (b) show the emission spectra from the cleaved facet as a function of pulsed excitation intensity measured at 4.2 K and RT, respectively. At 4.2 K the superlinear increase and the spectral narrowing of the emission which has a peak in the vicinity of 2.605 eV occurred at an excitation intensity (ith) of about 40 kW/cm2, indic-

Photon 2.65~

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x3~,~j095Ith 460

480 Wavelength (nm)

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Photon Energy (eV) 2.60

2.55

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ating the initiation of lasing. At RT the lasing peak :~‘ U) C

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0

100

200

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500

510

Wavelength (nm)

0

ctj

490

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300

Temperature (1<) Fig. 3. Peak energies of the absorption and the PL as a function of temperature.

Fig. 4. Emission spectra from the cleaved edge of the MQW sample at different excitation power densities measured at 4.2 K (a) and RT (b), respectively. In (a) the solid arrows indicate the peak energy position of PL emission from the sample surface while the dotted arrows show that of the absorption of the same sample. In (b) the solid arrows indicate the peak energy positions of both the absorption and PL emission from the same sample surface.

S. Yamaguchi et al.

/

Journal of Luminescence 59 (1994) 341—348

and ‘th were 2.5 16 eV and 200 kW/cm2, respectively. It is noted that the FWHMs of laser spectra are relatively large because the measurement resolution of 1 A is larger than the estimated interval of a longitudinal mode of about 0.25 A. As discussed later, the lasing line at each temperature is located at the lower photon energy side of both absorption and PL spectra under low power excitation. The net optical gain spectra with different excitation intensities together with absorption spectra at 4.2 K and RT are depicted in Figs. 5(a) and (b), respectively. The net optical gain (g) is given by the following equations:

345

(1)

citon—exciton collision, an exciton—LO phonon scattering process and localized excitonic recombination, have been proposed in bulk or epitaxially grown samples [28—33]as well as in MQW structures in Il—VI semiconductors [2—5,13, 20, 21, 23, 34] where the lasing mechanism has been discussed in terms of the energy difference between the stimulated and excitonic emission peaks. As shown in Figs. 5(a) and (b), although the spectral range of the high-energy tail of the gain overlaps with that of the low-energy tail of the absorption, no absorption was observed at the photon energy of the maximum optical gain at 4.2 K. These spectral features were observed up to about 77 K. This is in contrast to the samples of ZnSe—Zn(S, Se) MQWs by Suemune

(2)

et al. in [2]which and (Zn, by Ando emiset al. [20] the Cd)Se—ZnSe peak positionSQW of stimulated

where y is the gain coefficient in the active layer, Ta the fraction of the optical mode power within the active layer, cb, c~rand ; the loss constants of the unpumped ZnSSe clad layer, the reflection loss and the scattering loss at heterojunction interfacial imperfections, respectively. L is the cavity length and R the reflection coefficient between the sample and air. Since the refractive indices of Zn 0 83Cd0 17Se and Zn50 10Se090 at the lasing wavelength are calculated to be 3.06 and 2.71, respectively [9], Ta is estimated to be 0.11. The value of ~a is estimated to be 14.5 cm taking R 0.233 as due to Fresnel theory. By assuming the values of both Xb and ; of 10 cm 1, which was experimentally obtained in ZnSe—ZnSSe QWs [16], the gain coefficient in the active layer (y) was estimated as a function of wavelength. The values of y are also shown in the right axis in Figs. 5(a) and (b). Maximum gain coefficients at 4.2 K and RT are plotted as a function of excitation intensities in Figs. 6(a) and (b), respectively. As can be seen, the value at which gain g saturation occurs was estimated to be about 120 cm 1 (y 1000 cm 1) for 4.2 K, 100 cm (y ~ 2000 cm 1) for RT. Since it has been reported that the excitation intensity necessary to produce electron and hole plasma (EHP) in Il—VI widegap semiconductors is 2 [27], it is larger than of kW/cm expected thatsome the hundred carrier density just above the lasing threshold is well below the screening density of excitons at low temperature while it might be of comparable value at RT. So far, various excitonrelated lasing mechanisms, such as inelastic ex-

sion was located at the low-energy tail of photocurrent spectrum. It is noted that the FWHMs and peak energies of the gain spectra are almost constant within the excitation intensities assessed here. However, at RT the peak energies of the gain are located at the low-energy tail of the absorption spectra and shift toward high-energy side with in-

g

=

=

YEa —

—

c~b(i

—

Ta)

—;‘

—

1/L In R

-

=

-

-

creasing excitation intensity. The FWHMs of the absorption and gain spectra are plotted as a function of temperature in Fig. 7. At low temperatures (4.2 and 77 K), the FWHMs of the absorption spectra (about 16 meV) are close to those of the gain spectra (about 14 meV). Since the FWHM of absorption spectrum reflects the density of states of excitons, the good agreement of the FWHM in the absorption and gain spectra indicates that excitons play an important role in the optical gain at low temperature. At RT, however, the FWHM of the gain spectra (about 66 meV) is larger than that of the absorption spectra (about 48 meV) and the blue shift of the gain peak was observed with increasing excitation intensity. These features may indicate that excitons are at least partially screened and that EHP also contributes to the formation of optical gain. Since the optical path length parallel to the layers is longer than that in perpendicular direction, re-absorption effect has to be taken into account in order to discuss the emission mechanism from the PL data which is obtained from the cleaved facet. However, the situation is complicated because the absorption (or gain) coefficient changes with excitation intensities. Therefore, it is necessary to study the gain spectra, although the Shaklee’s

346

5. Yatnaguchi ef al.

(a)

Photon Energy (eV) 2.65 2.60 I

100-

—

O:l=lSlth 2Ig~ U: ll I

~‘E 50 0.9-

I

‘~

-1000

-~

:~ E CC _0

Ic~

Li:

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of Luminescence 59 (1994) 341—348

(a)

I

4.2K

C ID 0

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Lasing Threshold

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oC ‘E~ 2 ~.2K .2

500Power (kW/cm2) 1000 Excitation

0

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I

0

~ ~

0 465

I

I

470 475 Wavelength (nm)

(b)

480

Photon Energy (eV) 2.60 2.50 I 100

-

E 2000 .9-

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RT

U •

C

ID

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a)

0 :

0(3)

o0

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50 • : 1=4.01111 I

I

I

(3) I

0

(5 E

__

0’

Lasing Threshold • I

6 ~ xl ~

0

‘~

ID’-

AT

Fig.

~I.

6. Maximum

I

—.

500 1000 Excitation Power (kW/cm2) gain coefficients as a function of excitation

(~I~4

2

intensities taken at 4.2 K (a) and at RT (b).

2

0. ~

0

470

480 490 Wavelength (nm)

500

Fig. 5. Optical gain spectra with different excitation intensities measured at 4.2 K (a) and RT (b), respectively, together with absorption spectra in the region of n = I HH exciton transitions.

method is not valid for the assessment of the absorption coefficient. In order to discuss the lasing processes, the energy difference in peak energies between the ab-

sorption and gain spectra are plotted as a function of temperature as shown in Fig. 8. At 4.2 and 77 K the difference was about 18 meV. This value is substantially smaller than the energy difference of 28 meV which has been estimated in the Zn 085 Cd0 15Se—ZnS008Se092 MQW structure (well width: 50 A) by means of the photoluminescence excitation (PLE) spectroscopy [34]. If the lasing process takes place via exciton—exciton inelastic collision, the energy difference should correspond

S. Yamaguchi et al. / Journal of Luminescence 59 (1994) 341—348 I

tion. This is currently under investigation. It is noted that since exciton lines may be renormalized by many-body effects and also by heating effect due to high density photoexcitation, they have to be carefully determined by the PLE spec-

I

U

0 :absorption U :gain

60

5a)

0 40

troscopy or by the absorption measurements under high excitation condition; for detailed discussion of the mechanism see [34].

I 20’

347

a

a

4. Conclusions

Optical properties C 0

I 100

I

‘

200

I 300

Temperature (K) Fig. 7. FWHM of absorption and gain spectra of the sample as a function of temperature.

in

the

Zn0 83Cd017Se—

ZnS010Se090 MQW structure grown byIt MOMBE have been systematically investigated. has been shown that the Stokes shift was about 5 meV at 4.2 K and was hardly observed at temperatures higher than 150 K. The gain spectra have been systematically compared with PL and absorption spectra at each temperature. The results obtained in this work indicate that the localized excitons are not directly related to the lasing process in this sample at low temperatures.

3~3 a)

Acknowledgments

~ 20

in-Aid Researchin on Area, This for workScientific was supported partPriority by a GrantNew Functionality Materials Design, Preparation, and Control, from the Ministry of Education, Science and Culture, and also by The Murata Science Foundation.

a)

—

0 >‘

D) a) C

10

Ui

____________________________________ 0I 100 200 300 I

I

References

I

Temperature (1<) Fig. 8. Energy difference between absorption and lasing peaks as a function of temperature.

to the exciton binding energy [29]. Therefore, this collision process can be checked by using a set of the quantum well structures with various well widths and alloy compositions, particularly with absorption measurement under high density excita-

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