Micro-photoluminescence from CdSe quantum dots

Micro-photoluminescence from CdSe quantum dots

Journal of Crystal Growth 214/215 (2000) 778}781 Micro-photoluminescence from CdSe quantum dots Takeshi Ota *, Yasuhiro Murase , Tsuguki Noma , Kenz...

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Journal of Crystal Growth 214/215 (2000) 778}781

Micro-photoluminescence from CdSe quantum dots Takeshi Ota *, Yasuhiro Murase , Tsuguki Noma , Kenzo Maehashi , Hisao Nakashima , Kenichi Oto, Kazuo Murase The Institute of Scientixc and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan Department of Physics, Faculty of Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan

Abstract We have investigated CdSe quantum dots (QDs) by micro photoluminescence (PL). To reduce the number of QDs to be observed, Al masks with sub-micron size apertures are deposited on the sample surfaces, which make it possible to repeatedly trace the same excitation position in taking temperature dependent micro-PL spectra. Resolved sharp lines having linewidth &500 leV are observed. The results of temperature dependence reveal that the linewidths of QDs broaden with increasing temperature, which is considered to come from lifetime broadening of ground state by absorption of phonons. The linewidth broadening depends on the peak energy position of the QDs. The linewidths of lower-energy lines corresponding to larger size QDs, are more temperature-dependent than those of higher-energy lines corresponding to smaller ones. The di!erent linewidth broadening can be qualitatively explained by taking into consideration of di!erent energy level spacing between discrete states of the QDs.  2000 Elsevier Science B.V. All rights reserved. PACS: 63.20.Kr; 63.22.#m; 68.65.#g; 78.66.Hf Keywords: Single CdSe quantum dot; Micro-photoluminescence; Electron-beam lithography; Full width at half maximum; Temperature dependence

1. Introduction Recently self-organized semiconductor quantum dots (QDs) are of great interest as they provide zero-dimensional structures with delta-function density of states. In order to realize these quantum e!ects, many e!orts have been made for development of fabrication techniques. We have investi-

* Corresponding author. Fax: #81-6-6879-8414. E-mail address: [email protected] (T. Ota).

gated the formation of self-organized CdSe QDs on ZnSe (1 0 0) surfaces using molecular-beam epitaxy (MBE) [1]. In macroscopic photoluminescence (PL) spectrum, intense peak from the QDs is observed and its linewidth is &60 meV, which is due to the size #uctuations of QDs. To investigate the optical properties of single QD is very important for understanding the basic features of the QDs and, moreover, their application to optical devices. The density of QDs was estimated to be &100 lm\, using plan-view transmission electron microscopy (TEM). Previously, we have demonstrated to put Al masks with sub-micron size

0022-0248/00/$ - see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 2 2 8 - 1

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apertures on AlGaAs quantum wire (QWR) sample surfaces for reducing the number of the AlGaAs QWRs [2}4]. In this paper, we present the results of micro-photoluminescence (micro-PL) measurements of single CdSe QDs to clarify the optical properties of the QDs.

2. Experimental procedure CdSe QDs have been formed on ZnSe(1 0 0) surfaces using MBE [1]. First, the 200 nm thick GaAs bu!er layers were grown on GaAs (1 0 0) substrates. Second, 130 nm thick ZnSe bu!er layers were grown on the substrates at 2503C. Then, a few monolayers (ML) CdSe was deposited. Finally, 10 nm ZnSe cap layer covered these structures. The PL intensity of QDs was maximum at a CdSe coverage of 2.2 ML [1]. The QDs were clearly observed for 2.2 ML samples from a plan-view TEM image. This 2.2 ML sample was used for the micro-PL measurements. The plan-view TEM image revealed that the density and diameter of QDs were about 10 cm\ and 30 nm, respectively. In order to reduce the number of observed QDs, we put Al masks with sub-micron size apertures on the sample surfaces. The 100 nm thick Al masks were made by electron-beam lithography and lifto! technique. The diameters of the apertures were varied from 0.6 to 1 lm. Micro-PL measurements were performed as follows. The sample "xed on a cold "nger in a cryostat was cooled down using helium gas. The 458 nm line of argon-ion laser was used to excite the QDs. The excitation laser was focused to about 0.8 lm spot size through a microscope objective. The charge-coupled device (CCD) camera cooled by liquid nitrogen detected the luminescence through a monochromator. The spectral resolution is 380 leV. The micro-PL experiments were performed in the temperature range of 4 to 60 K.

3. Results and discussions Macroscopic PL spectrum at 4 K using 325 nm line of He}Cd laser revealed an intense peak of CdSe QDs at 2.37 eV, which is shifted towards

Fig. 1. Micro-PL spectra of CdSe QDs at 4 K: (A) without Al mask, (B) 1 lm and (C) 0.6 lm size aperture.

higher energies as compared to bulk CdSe (&1.84 eV), and its spectralwidth is &60 meV. This sample exhibited very strong green emission from CdSe QDs even at room temperature. Fig. 1 shows the micro-PL spectra of CdSe QDs, which are taken through several di!erent size apertures at 4 K. The excitation power of each spectrum was 100 lW. Fig. 1(A) shows a spectrum of microPL with focusing the excitation laser on the sample surface without Al masks. The wide full-width at half-maximum (FWHM) of the broad peak is due to size distribution of the QDs. With decreasing the size of the aperture from 1 to 0.6 lm, as shown in Fig. 1(B) and (C) respectively, the broad peaks observed through the apertures split into a number of anomalously sharp lines having linewidths of &500 leV. These sharp lines in Fig. 1(C) reveal zero-dimensional density of states of the QDs. The characteristic features of the spectrum in Fig. 1(C) are as follows. In lower-energy region ranged from 2.28 to 2.34 eV, the sharp lines are clustered close together in comparison with those in the higherenergy region ranged from 2.34 to 2.4 eV. The integrated intensities of the lower-energy lines are larger than those of the higher energy lines, indicating the di!erence of the e$ciency of each QD in gathering excitons. The di!erent e$ciencies may be attributed to the di!erent dot sizes. The linewidths of the lower-energy lines are narrower than those of the higher-energy lines. Gaussian-shaped broad background ranged from 2.28 to 2.4 eV is considered to exist under sharp peaks. These


T. Ota et al. / Journal of Crystal Growth 214/215 (2000) 778}781

Fig. 3. FWHMs of CdSe QDs at di!erent peak energy positions as a function of temperature.

Fig. 2. Temperature dependence of micro-PL spectra of CdSe QDs taken through the 0.6 lm aperture.

phenomena are also observed in other spectra through other apertures. Fig. 2 shows the micro-PL spectra taken through the 0.6 lm aperture, which is the same aperture in Fig. 1(C). The temperature ranged from 4 to 60 K. The excitation power is 100 lW. With increasing temperature, the linewidths of the sharp lines broaden, however, they are much smaller than k¹ (k: Boltzmann's constant) for temperature up to 60 K. The relative intensities of the sharp lines do not vary with increase of temperature, indicating that the thermal escape of the excitons from QDs and subsequent capture in another QD do not occur. Fig. 3 shows FWHMs of QDs at di!erent peak energy positions. With increasing temperature, the linewidth gradually broadens. The degree of the linewidth broadening is di!erent from line to line. The curves 1}4 represent the FWHMs of the lines at 2.25, 2.39, 2.36, and 2.42 eV, respectively, plotted

as a function of temperature. The results indicate that the degree of the linewidth broadening depends on the peak energy position, corresponding to the QD size, and that the linewidth of the smaller QD is less temperature dependent than that of the larger QD. In order to explain these phenomena qualitatively, we have performed simple theoretical analysis. The assumptions are that the exciton is "xed in the QD and the momentum conservation of both the exciton and the phonon is negligible. With increasing the temperature, excitons in the ground state are gradually scattered into higher states by absorption of acoustic phonons. The scattering rate of the ground state C(T ) is given as follows, C(¹)"C #C n(E , ¹),  J J where n(E , ¹) is Bose distribution that gives the J number of the phonons with energy E which J represents the transition energy between lth state and ground state. C is the scattering rate of the J excitons between lth state and ground state. The "rst term C represents scattering rate of the  ground state at zero temperature. The second term represents the contribution of scattering from the ground state to higher states by phonon absorption at temperature ¹. The values of E , C and the J J number of the levels in the QDs have not known,

T. Ota et al. / Journal of Crystal Growth 214/215 (2000) 778}781

Fig. 4. Calculated FWHM of the ground states as a function of temperature.


tures which varied from 1.0 to 0.6 lm. The masks with small apertures deposited on the sample surfaces enable us to repeatedly probe sub-micron size area and eliminate the thermal drift in taking temperature dependent spectra. The micro-PL spectra show that this technique is very useful for the investigation of single-quantum nanostructures. The results indicate that the linewidths of lower-energy lines with smaller energy level spacing are more temperature dependent than those of higher energy lines with larger one, which can be explained by taking into qualitative consideration of di!erent energy level spacing between discrete states of the QDs.

Acknowledgements therefore we present the qualitative temperature dependence of the FWHMs. Fig. 4 shows the calculated qualitative FWHMs of the ground states as a function of temperature. The FWHM increases with an increase of the temperature due to lifetime broadening, which corresponds to the increase of the scattering rate. The solid lines in Fig. 4 are model calculations which varied from smaller energy level spacing E to J larger one, corresponding to larger QD to smaller QD. C is assumed to be equal between the levels J of the QDs. These results reveal that with increasing the energy level spacing, the linewidth is less temperature dependent, which is qualitatively in good agreement with the experimental results, as shown in Fig. 3. From the excitation power measurements, it was con"rmed that the spectral broadening did not occur when the excitation power was less than 1 mW. This result strongly suggests that exciton}exciton interaction does not contribute to the temperature dependent linewidth.

4. Summary We have performed micro-PL measurements of CdSe QDs on ZnSe(1 0 0) surfaces through aper-

This research was partially supported by a Grant-in-Aid for Scienti"c Research (C) and by International Scienti"c Research Program: Joint Research from the Ministry of Education, Science, Sports and Culture, Japan.

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