Two-photon absorption of biexcitons in ZnS-based quantum wells

Two-photon absorption of biexcitons in ZnS-based quantum wells

ELSEMER Journal of Crystal Growth 184/185 (1998) 682-685 Two-photon absorption of biexcitons in ZnS-based quantum wells K. Yoshimura”, H. Watanab...

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ELSEMER

Journal

of Crystal

Growth

184/185 (1998) 682-685

Two-photon absorption of biexcitons in ZnS-based quantum wells K. Yoshimura”, H. Watanabea, Y. Yamadaav*, T. Taguchi”, F. Sasakib, S. Kobayashib, T. Tanib aDepartment of Electrical and Electronic Engineering, Yamaguchi University, 2557 Tokiwadai, Ube, Yamaguchi 755, Japan bElectrotechnical

Laboratory.

I-1-4 Umezono, Tsukuba, Ibaraki 305, Japan

Abstract Two-photon absorption processes of biexcitons in ZnS-based quantum-well structures have been studied by means of photoluminescence excitation spectroscopy. The peak position of two-photon absorption of biexcitons was dependent on the detected energy position in the photoluminescence excitation measurement. The energy difference between the one-photon exciton resonance and the two-photon biexciton resonance increased as the detected energy position was moved towards the lower-energy side of the biexciton luminescence. This result indicated the biexciton localization and the resultant increase in the biexciton binding energy. ‘0 1998 Elsevier Science B.V. All rights reserved. PACS:

78.66.Hf; 78.55.Et; 78.47. + p

Keywords:

Biexciton; Localization; ZnS-based quantum well; Two-photon

1. Introduction There has recently been an increasing interest in the formation of biexcitons (excitonic molecules) in semiconductor low-dimensional structures, especially in wide-band-gap II-VI quantum-well structures [l-13] because of their advantages of excitonic (biexcitonic) nature. It has also become clear that quasi-two-dimensional biexcitons play an important role in the formation of optical gain

*Corresponding author. Fax: [email protected].

+ 81 836 35 9449; e-mail:

0022-0248/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved. PII SOO22-0248(97)00757-4

absorption

[6,7,13]. Very recently, the binding energy of quasi-two-dimensional biexcitons has been derived analytically and the good agreement with the recent experimental observation has been reported [14,15] as compared with the earlier theory [16]. On the basis of the theoretical consideration, the effect of quantum confinement on biexcitons is expected to be larger than that on excitons. Therefore, the contribution of biexcitons to the formation of optical gain in II-VI quantum wells results from the large enhancement in the biexciton binding and its oscillator strength due to the effect of quantum confinement in a twodimensional case.

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Growth 1841185 (I 998) 682-685

Among II-VI materials, ZnS has a large biexciton binding energy of about 9 meV [17]. Therefore, it is expected that pronounced biexcitonic effects can be observed in ZnS-based quantum-well structures, and also that room-temperature biexcitons can be achieved in such structures. In the present paper, we study two-photon absorption processes of biexcitons in ZnS-based quantum-well structures, Cd,Zn, _,S-ZnS systems, in order to obtain the binding energy of biexcitons. We present experimental evidence for biexciton localization and the resultant increase in the biexciton binding energy.

2. Experimental

results and discussion I

3.55 The sample used in the present work was prepared by means of a low-pressure metalorganic chemical vapor deposition method using all gaseous sources. The MQW layer was grown on a (1 0 0)-oriented GaAs substrate, following the deposition of a 1.5 urn thick ZnS buffer layer. The MQW structure consisted of 50 periods of 3.5 nm thick CdO.zoZn,,soS well layers separated by 10.9 nm thick ZnS barrier layers. Fig. 1 shows the time-integrated luminescence spectra at 4.2 K taken from the MQW sample mentioned above under excitation power densities of(a) 0.2, (b) 0.8, (c) 4.7, (d) 15.8, and(e) 31.6 kW/ cm2. In this case, the luminescence spectra were observed under a condition of Xe-Cl excimer laser (308 nm) excitation. The repetition rate and the pulse width were 100 Hz and 2.5 ns, respectively. At the excitation power density as low as 0.2 kW/ cm2, the luminescence spectrum is dominated by the radiative recombination of n = 1 heavyhole excitons (denoted by X), which shows the peak at 3.661 eV. With increasing excitation power density, a shoulder (denoted by XX) on the lowenergy side of the n = 1 heavy-hole exciton line grows superlinearly (Zxx CT1;“) and becomes the dominant radiative-recombination process, which shows the peak at 3.638 eV. Then, the energy difference between the line X and the line XX is approximately 23 meV. On the basis of the spectral position as well as the superlinear dependence on the excitation power density, the line

3.60 3.65 3.70 3.75 PHOTON ENERGY (eV)

Fig. 1. Excitonic luminescence spectra at 4.2 K taken from a Cdo.lOZn O.,,S-ZnS MQW under excitation power densities of (a) 0.2, (b) 0.8, (c) 4.7, (d) 15.8, and (e) 31.6 kW/cm’.

XX is attributed to the radiative recombination of biexcitons. In order to confirm the above identification, the transient luminescence decays of both the exciton and the biexciton have been measured by means of a synchroscan streak camera. In this case, the excitation source was the fourth-harmonic light (300 nm) from an optical parametric generation/amplification (OPG/A) of amplified titanium sapphire laser pulses. The repetition rate and the pulse width were 1 kHz and 200 fs, respectively. The instrumental response of our system had an exponential decay-time constant of about 5 ps. Fig. 2 shows the time-resolved luminescence of the exciton (denoted by X) and the biexciton (denoted by XX) at 4.2 K under excitation energy density of 12.5 uJ/cm2. The luminescence intensities of both the exciton and the biexciton decay exponentially with decay-time constants of about 180 and 130 ps, respectively. This result clearly shows the expected temporal behavior of biexcitons: the shorter radiative lifetime of the biexciton as compared to that of the exciton. In order to obtain the binding energy of the biexciton precisely, we have performed two-photon

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TIME DELAY @s) Fig. 2. Time-resolved luminescence of the exciton (X) and the biexciton (XX) at 4.2 K taken from a CdO,,OZno.soS~ZnS MQW under excitation energy density of 12.5 pJ/cm2. The solid lines arc due to least-squares fits to the experimental data.

excitation spectroscopy of biexcitons. Fig. 3a shows the photoluminescence excitation (PLE) spectrum (denoted by the solid line) at 4.2 K taken from the MQW sample. The detected energy position was tuned at the peak of the n = 1 heavy-hole exciton luminescence. For reference, the photoluminescence (PL) spectrum is also shown by the dashed line. The IZ = 1 heavy-hole exciton resonance is clearly observed at 3.685 eV. Then, the difference in peak energy between the PLE and the PL spectra of the n = 1 heavy-hole exciton is about 24 meV. This energy difference, defined as a Stokes shift, results from the localization of excitons due to alloy composition and well width fluctuations. The two-photon excitation spectra of the biexciton are shown in Fig. 3b. In this case, the excitation source was the second-harmonic light from a dye laser pumped by a frequency-doubled Qswitched Nd3+ : YAG laser. The repetition rate and the pulse width were 100 Hz and 3 ns, respectively. Each two-photon excitation spectrum was obtained at the detected energy positions of (I) 3.638 eV, (II) 3.633 eV, and (III) 3.627 eV. Each detected energy position is shown in Fig. 3a. It can

1841185 (1998) 682-68.5

PHOTON ENERGY (eV) Fig. 3. (a) Photoluminescence (PL; dashed line) and photoluminescence excitation (PLE; solid line) spectra at 4.2 K taken excitation from a Cdc.zoZnc.sa S-ZnS MQW. (b) Two-photon spectra of biexcitons obtained at the detected photon energies of (I) 3.638, (II) 3.633, and (III) 3.627 eV.

clearly be seen from this figure that biexcitons are created directly from the ground state by a twophoton absorption process. When the detected energy position is tuned at the peak of the biexciton luminescence, the peak position of the two-photon absorption of biexcitons is observed at 3.668 eV as shown in the spectrum (I). Then, the energy difference between the one-photon exciton resonance and the two-photon biexciton resonance is 17 meV. This yields the biexciton binding energy of 34 meV, which is approximately 3.8 times larger than the binding energy of the biexciton in ZnS. It is noted here that two-photon biexciton resonance should appear at the center between one-photon exciton resonance and the peak of biexciton luminescence if biexcitons are free from localization. However, the two-photon biexciton resonance in our MQW sample appears at the higher-energy side relative to the center position. This observation clearly indicates the localization of biexcitons. It can also be seen from Fig. 3b that the twophoton biexciton resonance shifts towards the lower-energy side as the detected energy position is

K. Yoshimura et al. /Journal

of Crystal Growth 1841185 (1998) 682-685

moved toward the lower-energy side of the biexciton luminescence [(I) --f (III)]. Then, the energy difference between the one-photon exciton resonance and the two-photon biexciton resonance increases from 17 to 30 meV. As a result, the corresponding biexciton binding energy increases from 34 to 60 meV. We consider that this increase in the biexciton binding energy result from the biexciton localization which leads to an enhancement in three-dimensional confinement.

3. Conclusions Two-photon excitation spectroscopy of biexcitons in the Cdo,zoZno,saS-ZnS MQW has been performed in order to obtain the biexciton binding energy. The energy difference between the onephoton exciton resonance and the two-photon biexciton resonance increased as the detected energy position was moved towards the lower-energy side of the biexciton luminescence. We consider that the experimental data indicate the biexciton localization and the resultant increase in the biexciton binding energy.

Acknowledgements This work was partly supported Technology Research Foundation by a Grant-in-Aid for Scientific the Ministry of Education, Science Japan.

by the Electric of Chugoku and Research from and Culture of

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