Magneto-optical properties of ZnMnTe/ZnSe quantum dots

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Magneto-optical properties of ZnMnTe/ZnSe quantum dots

W.C. Fan

a

, J.T. Ku

a

, W.C. Chou

a,n

, W.K. Chen

a

, W.H. Chang

a

, C.S. Yang

b

, C.H. Chia

c

a

Department of Electrophysics, National Chiao Tung University, HsinChu 30010, Taiwan, ROC

b

Graduate Institute of Electro-Optical Engineering, Tatung University, Taipei 104, Taiwan, ROC

c_{Department of Applied Physics, National University of Kaohsiung, Kaohsiung 81148, Taiwan, ROC}

a r t i c l e

i n f o

Available online 18 November 2010 Keywords:

A1. Nanostructures A3. Molecular beam epitaxy B1. Nanomaterials B1. Zinc compounds B2. Magneto-optic materials B2. Semiconducting II–VI materials

a b s t r a c t

The s+ and s circularly polarized time-integrated and time-resolved Photoluminescence (PL)

measurements were employed to investigate the carrier spin dynamics of ZnMnTe/ZnSe Quantum Dots (QDs) grown on GaAs substrates by molecular beam epitaxy. The Kohlrausch’s stretching exponential function well correlates both thes+ ands decay proﬁles. The measured spin relaxation time is

about 23 ns.

1. Introduction

Spin dynamics continues to receive attention in current semi-conductor research both for their basic materials science interest and for their possible future application in real spintronics devices

[1,2]. Semiconductor nanostructures, particularly self-assembled Quantum Dots (QDs), are of great interest in this regard because of the long spin times that could allow control and manipulation of the electron spin [3]. Recently, spin response time of ensemble CdSe/ZnMnSe QD system was studied by the time resolved Photo-luminescence (PL) measurements[4]. Nearly, complete spin polar-ization was observed at 5 K and magnetic ﬁeld above 1 Tesla (T). However, the CdSe/ZnMnSe QD system has type-I band alignment and fast exciton recombination time. In order to have a QD system of long recombination time and spatially separated electrons and holes for possible spin manipulation, the study of spin dynamics in type-II QD system is urgent. Recently, we have found that type-II Diluted Magnetic Semiconductor (DMS) ZnMnTe QDs can be grown on ZnSe buffer by Stranski–Krastanov mode with wetting layer thickness of about 2 Mono-Layers (MLs)[5]. In current report, the time integrated and time resolved PL measurements with

s

+and

s

circular polarization were carried out to investigate the spin

dynamics of ZnMnTe/ZnSe QDs. Long radiative recombination time and spin relaxation time were observed.

2. Experimental procedure

The sample studied in this paper was grown on GaAs (1 0 0) substrate by MBE system. Prior to the growth procedure, GaAs

(1 0 0) substrate was etched in a H2O2:NH4OH:H2O (1:5:50)

solution for 1 min at room temperature, rinsed in ﬂowing de-ionized water for about 2 min and dried with high purity N2.

De-sorption and growth procedures were monitored by the Reflection High Energy Electron Diffraction (RHEED). The effussion cell temperatures of Zn, Mn, Se, and Te were at 294, 695, 178, and 310 1C, respectively. The substrate temperature was fixed at 300 1C. The growth rates for ZnMnTe QDs and ZnSe buffer layer were 0.3 and 0.4 ˚A/s, respectively. The ZnSe buffer layer included several ML grown by Migration Enhanced Epitaxy (MEE) and a thickness of 50 nm grown by conventional MBE. Immediately after the deposi-tion of ZnSe buffer layer, the alternating supply method of ZnMnTe growth was performed. The alternating supply method for each ZnMnTe growth cycle described as follows: first the surface of ZnSe was exposed to Mn for 5 s, and then exposure to Zn and Te for 5 s alternately with 10 s interruption. The multiple (ZnMnTe/ZnSe) QDs sample has 5 periods with ZnMnTe coverage of 2.6 MLs and the ZnSe spacer thickness is 10 nm. Finally, the 50 nm ZnSe capping layer was grown on the QDs. The magneto-PL spectra were taken in the Faraday geometry. The sample was placed in an optical magnet cryostat and the emitted light was dispersed by a mono-chromator equipped with multichannel charged coupled device. The PL polarization was extracted by using a combination of quarter wave plate and linear polarizer. For the time-resolved PL, the GaN 405 nm pulsed laser diode was used.

3. Results and discussion

Fig. 1shows the time integrated PL spectra with

s

+ and

s

circular polarization at B ¼0 and 5 T for a 2.6 MLs ZnMnTe/ZnSe QDs sample. The large difference in PL intensity between both polarization results from the magnetic ﬁeld induced spin splitting Contents lists available atScienceDirect

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of holes in ZnMnTe QDs and electrons in ZnSe matrix. The circular polarization P ¼(I+I)/(I++ I)¼ 77% at 5 T, where I+ and Iare

the PL intensities of

s

+and

s

circular polarization, respectively.

The ﬁnite P is unexpected for B ¼0, when there is no expected preferential direction for Mn spin alignment. This non-zero circular polarization at B ¼0 is attributed to the formation of bound magnetic polaron caused by the exchange interaction between the localized holes and Mn 3d levels in the ZnMnTe QDs.

The circular polarization degree P as a function of B, obtained from low temperature, is shown inFig. 2. The P abruptly enhances at low B and gradually saturates at high B. The magnetic ﬁeld dependence follows the Brillouin function, which is a signature of Mn magnetism. Similar results were also observable for other DMS QDs[4,6]. The insert ofFig. 2shows the splitting of the heavy-hole band and conduction band of ZnMnTe QD at B 40 and the allowed transitions for

s

+exciton (heavy hole 32spin state and electron

1₂spin state) and

s

exciton (heavy hole +32spin state and electron

+1

2spin state). The light hole spin states are neglected due to the

large compressive strain induced heavy-light hole splitting in ZnMnTe QDs and light hole exciton energy is much higher than heavy hole exciton energy. The spin splitting of the conduction electron in ZnSe is also ignored due to very small g factor. Assuming that the exciton recombination time

t

Rfor both

s

+and

s

exciton

are the same. The circular polarization can be described by the two

following rate Eqs. (1) and (2) for the heavy-hole excitons of

s

+and

s

circular polarization: dn dt ¼G n

t

R þnþ

t

S ne DE=kT

t

S , ð1Þ dnþ dt ¼Gþ nþ

t

R nþ

t

S þne DE=kT

t

S ð2Þ where, nrepresents number density of

s

exciton and n+stands

for number density of

s

+ exciton.

t

Scorresponds to an effective

spin relaxation time between the two Zeeman levels. G+and Gare

generation rates for

s

+and

s

exciton, respectively. kT is thermal

energy and

D

E ¼ g

m

BB is the Zeeman splitting energy between

s

+

and

s

excitons. g is the effective g factor and

m

B is the Bohr

magneton. For steady state condition, the ratio

r

¼

t

S/

t

R can be

obtained by the experimental data using the following equation: P ¼ IþI IþþI ¼ nþn nþþn ¼ 1e DE=kT 1 þ eDE=kT _{1 þ}ts tR : ð3Þ

For a very slow spin relaxation (

t

S*

t

R), very large

r

¼

t

S/

t

R

results zero circular polarization. InFig. 2, P¼77% at 5 T, it results in approximate same order of magnitude for

t

Sand

t

R.

InFig. 2, the solid line is a fitting curve for the magnetic field dependence of circular polarization P by using Eq. (3) with fitting parameters, g factor and the ratio

r

¼

t

S/

t

R. The best ﬁt to the data

yieldsgmB

kT ¼0:9470:03 (T

1_{) and}

_t

S/

t

R¼0.3170.01. This indicates

that the spin relaxation time is about 3 times shorter than the exciton recombination time.

In order to determine

t

R, the decay proﬁles of time-resolved PL

were measured, as shown inFig. 3. The solid curve is a ﬁt using the Kohlrausch’s stretching exponential function

IðtÞ ¼ I0eðt=tÞ

b

, ð4Þ

where

t

is the exciton radiative recombination time (

t

R), and

b

is

the stretching exponential. The solid line ﬁts the experimental data reasonably.Fig. 3shows a very long decay time of about 76 ns due to the type-II band alignment induced slow exciton recombination. The spin relaxation time is then estimated to be about 23 ns. This spin relaxation time is much longer than other type-I QD systems

[4]and could be useful for spin manipulations.

The time-resolved PL spectra were also analyzed by

s

+and

s

circular polarization and correlated by the Kohlrausch’s stretching exponential function. The resultant data were shown inFig. 4. The lifetimes of

s

(

t

s) exciton decreases with the increasing

magnetic ﬁeld. On the other hand, the lifetimes of

s

+(

t

sþ) exciton

are almost independent of magnetic ﬁeld. The increasing magnetic ﬁeld results in the energy splitting of

s

+and

s

excitons, it further

Fig. 1. PL spectra withs+(solid line) ands(dashed line) circular polarization of a

2.6 ML multi-QD layers with 10 nm of spacer layer thickness at B¼ 0 T and 5 T.

Fig. 2. Plot of circular polarization as a function of magnetic ﬁeld at 10 K (circle). Insert: schematic conduction and heavy-hole band diagram of ZnMnTe/ZnSe quantum dots at B4 0. The spin splitting in ZnSe is ignored.

Fig. 3. Decay proﬁle of ZnMnTe/ZnSe QDs. The red solid curve is a ﬁt using the Kohlrausch’s stretching exponential function described in the text.

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enhances the spin relaxation from the higher energy

s

excitons to

the lower energy

s

+excitons. As a result,

t

sis shorter than

t

sþ.

The lifetime of

s

(

s

+) excitons is 55 (80) ns at 6 T. The difference

between

t

sþand

t

sis about 25 ns, which is very close to the value

obtained by Eq. (3).

4. Conclusion

We have investigated the spin dynamics of ZnMnTe/ZnSe QDs. The magnetic ﬁeld dependence of PL circular polarization degree

follows the Brillouin function and evidences the Mn magnetism in ZnMnTe QDs. The Kohlrausch’s stretching exponential function well correlates both the

s

+ and

s

decay proﬁles. The magnetic

ﬁeld dependence of PL circular polarization degree shows the long spin relaxation time of about 23 ns. The long spin relaxation time could be useful for spin manipulation.

Acknowledgements

This work was supported by the MOE-ATU and the National Science Council under the Grant no. of NSC 98-2119-M-009-015.

References

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Fig. 4. Lifetime as a function of magnetic ﬁeld. The squares represents+and circles

stand forscircular polarization. The curves are guide to eye.

W.C. Fan et al. / Journal of Crystal Growth 323 (2011) 380–382 382