Ultrathin CdSe quantum wells

ELSEVIER

Journal

of Crystal

Ultrathin KG.

Chinyama”y*,

Growth

184/185 (1998) 298-301

CdSe quantum wells

I.V. Bradley”, K.P. O’Donnell”, A.P. Chernushichb, V. Luzanovb

P.I. Kuznetsovb,

aDepartment qf Physics and Applied Physics, University qf Strathclyde, IV7 Rottenrow, Glasgow: G4 ONG, Scotland, UK b Institute ofRadioengineering and Electronics, Russian Academy of Sciences, Vvedensb 1, Fr?;azino, Moscow Region 14112V. Russian Federation

Abstract We report a survey of samples containing ultrathin layers of CdSe embedded in ZnSe or ZnSo.06Seo,94 matrices. CdSe single quantum wells (SQW) with well widths in the range 0.3-3 ML show several bands in the photon energy range from 1.7 to 2.4 eV. Similar bands have been attributed recently to self-organised quantum dots. Temperature-dependent photoluminescence (PL) shows that the bands in our samples vanish, without significant peak shift, at moderately high temperatures. Preliminary time-resolved PL shows that they also have relatively long lifetimes (ca. 3G250 ns). In all samples, the SQW exciton band lying at higher energies shows line narrowing with increasing temperature. Variations due to quantum-well thickness induce a correlation between the luminescence peak energy and the optical bandwidth which may be used to systematise our observations. 0 1998 Elsevier Science B.V. All rights reserved.

PACS: 78.55.C; 85.30.V Keywords:

Photoluminescence;

Deep bands; Exciton bands; Lifetime; Line narrowing; Correlation

1. Introduction In ultra-thin quantum-well structures, problems associated with growth disorder include interface roughness, interfacial alloy formation, and wellwidth fluctuations. Heterointerfaces between wells and barriers are rough within certain lateral re-

*Corresponding author. [email protected].

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0022-0248/98/$19.00 i(‘:t 1998 Elsevier Science B.V. All rights reserved. PII SOO22-0248(97)00674-X

gions that change in size at small terraces or islands with heights of monolayers: excitonic emission linewidths are closely related to the fluctuations of the heavy-hole excitons’ potential energy. Recently Xin et al. [l] demonstrated self-assembling of CdSe quantum dots in very thin CdSe-ZnSe QWs. A prominent feature of their PL spectra was a broad peak near 2.3 eV which they argue originates on CdSe quantum dots. We describe here luminescence characteristics of CdSe-ZnSe and ultra-thin QWs, presenting CdSe-ZnSo,06Seo,,, some preliminary results of a survey carried out,

K.G. Chinyama et al. I Journal

qf Crystal Growth 1841185 (1998) 298-301

299

using temperature-dependent and time-resolved PL measurements on five different SQW structures of various well thicknesses including one submonolayer (SML) well. 1e+7

2. Samples and measurement Samples were grown at atmospheric pressure in a metalorganic vapour phase epitaxy (MOVPE) system on (1 0 0)GaAs substrates. Sample CdSe-16 is composed of a 225 nm ZnS,.,,SeO.Ph buffer, a 3.3 monolayer (ML) CdSe well, and a 13 nm ZnSe cladding layer. Sample CdSe-20 consists of a 2.6 ML well sandwiched by a 150 nm buffer and 35 nm cladding layers of ZnSe. Samples CdSe-21 and 22 have 100 nm buffer and 100 nm cladding layers of ZnSo.obSeo,94, sandwiching 2.8 and 1.8 ML CdSe wells, respectively. Samples CdSe-9 consists of a 200 nm buffer and 35 nm cladding layer of ZnSe surrounding a 4 ML CdSe well. PL spectra were excited with a 50 mW Ar+ laser beam at a wavelength of 357 nm. The samples were attached to a cold head in a closed-cycle cryostat and the emission detected through a single-grating spectrometer by a GaAs photomultiplier tube with photon counting. Time-resolved PL spectra were obtained at higher excitation density using a tripled Nd : YAG laser at 355 nm with 7 ns pulses and integrating boxcar electronics.

3. Experimental

results

In four out of five samples, the PL spectra were found to exhibit, in addition to the single-well excitonic emission near 2.6-2.8 eV, deep bands between 1.7 and 2.4 eV. Fig. 1 shows the spectra for CdSe-16, -20, -21, and -22 measured at 13 K. The thicker wells CdSe-20 and -21 show single deep bands around 2.05 eV in addition to SQW emission at 2.58 and 2.55 eV, respectively. The thinner well CdSe-22 exhibits two deep bands centered at 2.27 and 1.96 eV, with SQW emission at 2.72 eV. For the much thinner CdSe-9 SQW (figure not given) the number of deep bands increases to three, centered at 2.32, 2.0, and 1.71 eV, dominating the SQW emission which appears at about 2.8 eV. In

1c?+6

le.a 1.6

1.6

2.0

2.2

Photon Energy

2.4

2.6

2.8

30

(eV)

Fig. 1. (a) PL spectra for CdSe-16, -20, -21, and -22 samples showing several deepbands in addition to the SQW emission. The sharp peaks around 2.8 and 1.7 eV are band-edge emissions from the cladding layer and the laser line, respectively.

the case of the thickest well, sample CdSe-16, no deep bands are observed. To gain some insight into the nature of the deep bands, temperature-dependent and time-resolved PL measurements were performed. In the case of samples with a single deep band, it was observed that the band does not exhibit significant alteration with rising temperature, except for the expected decrease in intensity. For the two-bands sample, there is again no band shift, but the low-energy band vanishes faster than the high-energy band with increasing temperature. The same observation applies to the three-bands case, but in this case the central band broadens and overlaps with the higher-energy band. The temperature-dependent measurements have also revealed PL line narrowing in the SQW emission of all samples, at temperatures up to over 100 K. Fig. 2 shows the PL

K.G. Chinyama et al. /Journal

150

100

Temperature,

200

250

of Crystal Growth 1841185 (1998) 298-301

300 Well Thickness

K

(ML)

60000

30 -

20 0 2.60

2.65

2.70

2.75

2.60

2.65

Photon Energy, eV

Fig. 2. (a) Temperature dependence of the FWHM of the PL from the four samples, showing the line narrowing. (b) PL spectra for CdSe-22 at different temperatures showing the gradual build-up of the barrier band-edge emission (small peak at high energy) as the temperature is increased.

full-width at half-maximum (FWHM) as a function of temperature. The last data point on each curve represents the temperatures at which the PL intensity drops to noise levels. Because of well-width fluctuations, PL mappings give peak energies corresponding to various well thicknesses. Fig. 3 clearly shows the correlation between the linewidth and the peak energy, hence, linewidth and well width. Time-resolved measurements show that the bands have a long lifetime, - 250 ns for the 2.27 eV band in CdSe-22 but only ca. 30 ns for the 2.32 eV band in CdSe-9. We have also calculated the transition energy of the heavy hole to conduction band ground-state energy as a function of well width based on the Rashba-Sheka-Pikus Hamiltonian for the top of the valence bands in wurzite structures [2,3]. The fit is shown in the inset of Fig. 3.

,.

_

n

- A _ . 2.3

Cd%-20 Cd%-21 Cd%?-22

2.4

2.5

2.6

PL peak position

2.7 (eV)

Fig. 3. PL linewidth plotted as a function of peak position. The data is derived from PL mappings of the samples. Inset shows the calculated transition energy of the heavy-hole exciton as a function of well width.

4. Discussion and conclusions The quick disappearance of the deep bands, their static behaviour with temperature, and their long lifetime indicate that these peaks probably do not originate from self-assembled quantum dots but rather from some defects in the buffer layer. This view is reinforced by the fact that these peaks are more prominent for thin QWs than for thick QWs. For even thicker ones, these bands do not appear. Further work is in progress to justify these observation and identify the exact nature of these bands. The reduction of the FWHM of SQW emission with temperature (by 30, 20, 15, and 8 meV for CdSe-16, -20, -21, and -22, respectively) and its increase again after reaching the minimum value, is

K.G. Chinvama et al. /Journal of Crystal Growth 184/185 (1998) 298-301

shown in Fig. 3. Within the temperature range in which this narrowing occurred, we observed that, while the PL peak slowly redshifted, the shape of the spectra also changed. The change in shape is due to the high-energy side of the emission peak decreasing faster than the low-energy side (see the representative spectra in Fig. 2b). As a result, the peak progressively narrows with temperature. Fig. 2b also shows the gradual build-up of the barrier layer band-edge emission at -2.83 eV with temperature signifying a gradual loss of excitons from the high-energy states of the QW into the barrier layer. The increase in linewidth at the higher temperatures is dominated by the increased exciton-phonon interaction and thermal inhomogeneous broadening processes. The PL mapping in Fig. 3 depicting the correlation between well thickness and linewidth shows a general decrease in linewidth with decreasing well width in three samples. This correlation can be explained in terms of the potential well fluctuations [4,5] which give rise to variations in the exciton potential energy within the plane of the well. We emphasise that these are preliminary results and work is still going on covering a large number of samples with varying well widths from the submonolayer regime to about 3 ML. In summary, the results show a variation of the exciton energy with sample position of order 5&100 meV, corresponding to well-thickness fluctuations, AW, of much less than a monolayer. In general, we have shown a strong correlation be-

301

tween the PL linewidth and the peak energy, and hence, of the linewidth with well thickness. Besides, our calculations show that the shape of the transition energy as a function of well width roughly fits this variation. The experimentally observed line narrowing is attributed to the loss of excitons from the high-energy states into the barrier. In addition, the temperature at which minimum linewidth occurs is found to be proportional to the well width. We have observed luminescence peaks that are superficially similar to the one which has been assigned to self-assembled quantum dots. However, the internal evidence does not concur with such an assignment in the case of our samples. It may be that the growth condition of our samples is incompatible with self-assembling of clusters. We deduce that the deep bands described above are due to some defects in the buffer layer although the exact nature of these bands is yet to be established.

References ct1 S.H. Xin, P.D. Wang, A. Yin, C. Kim, M. Dobrowolska,

J.L. Merz, J.K. Furdyna, Appl. Phys. Lett. 69 (1996) 3884. 121 CYi-P. Chao, S.L. Chuang, Phys. Rev. B 46 (1992) 4110. c31 Yu.M. Sirenko, J.-B. Jeon, K.W. Kim, M.A. Littlejohn, M.A. Stroscio, Phys. Rev. B 53 (1996) 1997. M F. Yang, M. Wilkinson, E.J. Austin, K.P. O’Donnell, Phys. Rev. Lett. 70 (1993) 323. F. Yang, E.J. Austin, K.P. O’Donnell, c51 M. Wilkinson, J. Phys.: Condens. Matter 4 (1992) 8863.