Growth evolution and magneto-optical characteristics of self-assembled ZnTe/ZnMnSe quantum dots

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Growth evolution and magneto-optical characteristics of

self-assembled ZnTe/ZnMnSe quantum dots

Ling Lee

a,n

, Wen-Chung Fan

b

, Kun-Feng Chien

b

, An-Jye Tzou

b

, Wu-Ching Chou

b,n

a

Center of Nanoscience and Technology, Tunghai University, Taichung, Taiwan, Republic of China

b_{Department of Electrophysics, National Chiao Tung University, Hsinchu, Taiwan, Republic of China}

a r t i c l e

i n f o

Available online 10 November 2012 Keywords:

A1. Nanostructures A3. Molecular beam epitaxy B1. Zinc compounds B2. Magneto-optic materials

a b s t r a c t

The growth evolution and the magneto-optical characteristics of the self-assembled single-layer ZnTe/ ZnMnSe quantum dots grown by molecular beam epitaxy were investigated. As the ZnTe coverage is above 2.0 monolayers, a Stranski–Krastanov growth process is observed with a signiﬁcantly increased dot density on top of two-dimensional plateaus. As the coverage exceeds 2.7 monolayers, ripened dots dominate instead. Furthermore, a saturated degree of circular polarization of 76% at applied magnetic ﬁeld of 4 T and the formation of magnetic polarons with a formation time of 7.4 ns were demonstrated. &_{2012 Elsevier B.V. All rights reserved.}

1. Introduction

Recently dilute magnetic semiconductors (DMS) continue to receive attentions for spintronics [1–3]. In particular, magnetic quantum dots (QD) are of great interest because the control and manipulation of spin are superior to those of bulk and epilayer[4]. For example, we have reported a strong spin polarization at 10 K in the type-II ZnMnTe/ZnSe stacked QD [5,6]. The spatially separated electrons and holes in the type-II band structure provide an advantage of the long spin dephasing time [7]. On the other hand, non-magnetic QD embedded in a magnetic matrix also play as an important candidate for the development of spintronic devices. However, although a stronger polarization of the type-I aligned QD has been demonstrated[8], research on the type-II system is still rare. In this article, self-assembled single-layer ZnTe QD on magnetic ZnMnSe buffer single-layer were successfully achieved by molecular beam epitaxy (MBE). In addition to the studies on growth evolution, magneto-optical measurements were carried out to investigate the spin polarization and the formation of magnetic polarons (MP).

2. Experimental methods

The single-layer ZnTe/ZnMnSe QD were grown on GaAs(100) substrate by a Vecco Applied EPI 620 MBE system at 300 1C. The effusion cell temperatures of Zn, Mn, Se, and Te were ﬁxed at 294, 695, 178, and 310 1C, respectively. Prior to growth, the substrate was etched in a H2O2:NH4OH:H2O (1:5:50) solution, rinsed in ﬂowing

de-ionized water, dried with puriﬁed N2gas, and desorbed in the MBE chamber in sequence. The growth started from a ZnSe buffer layer and was followed by a 50-nm-thick ZnMnSe barrier layer. The Mn content in the barrier layer was extrapolated from its band gap energy. According to the dependence of band gap energy on Mn incorporation [9], the energy value of 2.790 eV indicates a Mn composition of about 5%. Immediately after the barrier layer, an alternating supply method was performed to deposit ZnTe with a growth rate of 0.03 nm/s. By a careful control of growth duration, ZnTe with coverage of 2.0, 2.2, 2.4, 2.7 and 3.0 monolayers (ML) were fabricated. The morphological properties were measured by tapping-mode atomic force microscopy (AFM).

For the magneto-optical investigations, capped specimens were fabricated with identical growth condition and followed by a 50-nm-thick ZnMnSe capping layer on top. The photolumi-nescence (PL) measurement was operated at 10 K, in which signals were excited by a Helium–Cadmium laser at 325 nm, analyzed by a Horiba Jobin-Yvon iHR550 0.5-m monochromator and detected by an LN2-cooled charge-coupled device with an energy resolution of 0.3 meV. The magneto-PL spectra were taken in the Faraday geometry in an optical magnet cryostat, in which the polarized signals were extracted by using a combination of a quarter wave plate and a linear polarizer in front of the monochromator. For the time-resolved PL, the signals were excited by a GaN pulsed laser diode at 405 nm, and detected by using a fast photomultiplier tube and a time-correlated single photon counting technique with an overall time resolution of 0.2 ns.

3. Results and discussion

Fig. 1(a) to (e) shows the AFM images of uncapped specimens with ZnTe coverage ranged between 2.0 and 3.0 ML. At both ends Contents lists available atSciVerse ScienceDirect

journal homepage:www.elsevier.com/locate/jcrysgro

Journal of Crystal Growth

n

Corresponding authors.

E-mail addresses: [email protected] (L. Lee), [email protected] (W.-C. Chou).

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of the scale bar the brightest and darkest indicate the relative height of 18 and 0 nm, respectively. The corresponding dot density as a function of coverage is plotted in Fig. 2.

Fig. 1(a) reveals two-dimensional-like (2D-like) plateaus on the 2.0-ML-thick specimen and almost no dots appear. When the coverage increases, three-dimensional (3D) dots appear on 2D-like plateaus with a sharp increase of density as shown in

Fig. 1(b) and (c). The rearrangement of ZnTe from 2D-like plateaus to 3D dots was attributed to the Stranski–Krastanov (S–K) process. According to the theoretical simulation[10], the mor-phology changes because of the minimization of total energy during the heterostructural growth under equilibrium. The total energy includes the surface energy of the deposited material and the strain energy arises from the lattice mismatch. When the lattice mismatch ranges from 6.7% to 7.9% the growth starts in 2D and changes to form 3D dots once the coverage exceeds a transition thickness in the range of 1.5–2.0 ML. In this study a lattice mismatch of 7.4% between ZnTe (lattice constant of 6.102 ˚A) and Zn0.95Mn0.05Se (lattice constant of 5.679 ˚A[11]) lies in this predicted regime; hence the S–K process is allowed. The solid line inFig. 2represents an experimental description of the coverage

y

dependent dot density

r

following the S–K process

[12]as

r

¼

r

Sð

y

CÞa, ð1Þ

in which

r

Sis the saturated density,

a

is the exponent, and

y

Cis the critical coverage of the transition. The best ﬁtted values in our results of

r

Sand

a

are 7.5 109cm2and 1.4, respectively. The transition coverage is 2.0 ML, which satisﬁes the theoretical prediction and the AFM image. Additionally, with further increase of coverage the AFM images exhibit a coexistence of large and small dots as shown in Fig. 1(d) and (e), respectively, for the 2.7-ML- and 3.0-ML-thick specimen. The corresponding dot densities are also lower than the predicted values. These phe-nomena imply the ripened process[10,13], which starts when the coverage exceeds 2.7 ML. Portions of the 3D dots enlarge at the expense of others and result in a coexistence of large and small dots as well as a reduced overall dot density.

In addition to the growth evolution, the optical properties of the capped specimens were also investigated for the basic studies on magneto-optical researches.Fig. 3shows the PL spectra at 10 K in which two emission bands were observed. The higher-energy band (marked by a dash line and labeled ‘H-band’) shows no clear

energy shift with increasing coverage. The emission could be explained by either the deep-level transition from the buffer layer

[14]or the transition from the plateau[15]. On the contrary, the peak positions of the lower-energy band (marked by dash-dot lines) are sensitive to the coverage. The decreasing emission energies together with the increasing coverage, as shown in the inset, were attributed to the reduced quantum conﬁnement of carriers in the ZnTe quantum dots. Therefore the lower-energy emissions were labeled ‘QD-band’. Moreover, all of the transition energies of the QD-band are lower than that of the ZnTe epilayer (2.37 eV at 10 K). A type-II band alignment was thus realized in which holes are conﬁned in ZnTe QD while electrons are dis-tributed in the ZnMnSe matrix in the vicinity of the QD and bound to holes via the Coulomb interaction.

In order to demonstrate the magneto-optical characteristics of ZnTe/ZnMnSe QD, the 2.7-ML-thick specimen was chosen because of its strongest emission intensity. The up-polarized (black solid line) and the down-polarized (gray dot line) components of PL spectra at 0 and 4 T are plotted inFig. 4(a). In the absence of the magnetic ﬁeld, the intensity of the up-polarized component Iþ and the down-polarized one I-_{are identical. Hence the degree of} circular polarization P, which is expressed by P¼(Iþ _I_)/ (Iþ_þI_{), is zero. On the contrary, a strong circular polarization}

Fig. 1. Plan-view atomic force microscopy images with ZnTe coverage of (a) 2.0, (b) 2.2, (c) 2.4, (d) 2.7, and (e) 3.0 ML.

Fig. 2. Dot density of ZnTe grown on ZnMnSe as a function of coverage. The solid curve represents the predicted densities by Eq.(1).

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of 76% at 4 T is observed. In the type-II band structure, the non-zero polarization is owing to the energy splitting of the conduc-tion band in ZnMnSe barrier layer. Transiconduc-tions between conﬁned holes and electrons which occupy the lower-energy (mS¼ 1/2) and the higher-energy (mS¼1/2) level allows the up-polarized and down-polarized components, respectively. At the low tem-perature of 10 K, the energy separation of about 5 meV between Iþ

and I

promotes electrons to relax to the spin-up level and inhibits the reverse process. Therefore the up-polarized transition dominates. Fig. 4(b) reveals that when the magnetic field increases the degree of circular polarization increases and satu-rates as the field strength exceeds 4 T. This dependence of the polarization on magnetic field strength is a typical characteristic of diluted magnetic semiconductors.

Furthermore,Fig. 4(a) shows that the splitting is signiﬁcantly smaller than that observed in the ZnMnSe epilayer with a value of 80 meV at 4 T[16]while the degree of circular polarization is comparable. Recently, such a paradox has been investigated for the ZnMnTe/ZnSe QD system[17]and been interpreted by the formation of magnetic polarons. During the formation process, the exchange interaction between the localized electrons in the vicinity of QD and Mn ions aligns the randomly oriented spin of Mn ions and reduces the QD-band transition energy. In order to realize the formation of the MP, the time evolution of PL peak position at B¼ 0 T was investigated as shown inFig. 5. The solid circles represent the evolution of the transition energy of QD-band for the 2.7-ML-thick specimen with a red-shift from 1.912 to 1.896 eV within 20 ns. In comparison, ZnTe QD embedded in the non-magnetic ZnSe matrix show no red-shift. Therefore other candidates such as carrier localization and band bending for the temporal evolution of the peak position were excluded. According to previous literatures[18], the formation dynamics could be described by a single exponential law as EðtÞ ¼ E t ¼ 0ð ÞEMP½1exp t=

t

MP

, ð2Þ

in which EMPof 16 meV is the MP binding energy and

t

MPof 7.4 ns is the formation time.

4. Conclusion

In this article, the type-II ZnTe/Zn0.95Mn0.05Se single-layer quantum dots were achieved by molecular beam epitaxy. The growth follows the Stranski–Krastanov process with transition coverage of 2.0 ML, whereas the ripened process dominates as

Fig. 3. PL spectra at 10 K of ZnTe with different coverage. The inset shows the dependence of corresponding peak position on ZnTe coverage.

Fig. 4. (a) 10 K up-polarized (Iþ

) and down-polarized (I

) PL spectra of QD at B¼0 and 4 T. (b) The degree of circular polarization as a function of the applied magnetic ﬁeld.

Fig. 5. Dependence of PL peak energies at 10 K of ZnTe QD embedded in a ZnMnSe (full circle) and a ZnSe (open circle) matrix.

L. Lee et al. / Journal of Crystal Growth 378 (2013) 222–225 224

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coverage exceeds 2.7 ML. The magneto-optical measurements revealed a saturated degree of circular polarization of quantum dots of 76% at 4 T and the formation of magnetic polarons with binding energy of 16 meV and formation time of 7.4 ns. These approaches provide the evidence of the exchange interaction and the alignment of the randomly oriented Mn spins nearby the QD.

Acknowledgment

This work was supported by the National Science Council under Grant no. NSC 100-2119-M-009-003.

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