научная статья по теме UNUSUAL NARROWING OF THE ESR LINE WIDTH IN ORDERED STRUCTURES WITH LINEAR CHAINS OF GE/SI QUANTUM DOTS Физика

Текст научной статьи на тему «UNUSUAL NARROWING OF THE ESR LINE WIDTH IN ORDERED STRUCTURES WITH LINEAR CHAINS OF GE/SI QUANTUM DOTS»

Pis'ma v ZhETF, vol. 102, iss. 2, pp. 120-124 © 2015 July 25

Unusual narrowing of the ESR line width in ordered structures with

linear chains of Ge/Si quantum dots

A.F. Zinoviev.a+1\ Zh. V. Smagina+, A. V. Nenashev+*, L. V.Kulik* x, A. V. Dvurechenskii+*

+ Rzhanov Institute of Semiconductor Physics SB of the RAS, 630090 Novosibirsk, Russia * Novosibirsk State University, 630090 Novosibirsk, Russia x Voevodsky Institute of Chemical Kinetics and Combustion SB of the RAS, 630090 Novosibirsk, Russia

Submitted 4 June 2015 Resubmitted 15 June 2015

Electron states in ordered Ge/Si heterostructures with linear chains ol quantum dots (QDs) were studied by the electron spin resonance (ESR) method. A new ESR signal with principal (/-factor values Qzz = 1.9993 ± 0.0001, gxx = gyy = 1.9990 ± 0.0001 was detected. Unlike non-ordered QD structures, where ESR line broadening is usually observed (evidence ol Dyakonov-Perel mechanism efficiency), the structures under study demonstrate the narrowing ol ESR line when the external magnetic field deviates from the growth direction. The ESR line width is AH = 1.2 Oe for perpendicular magnetic field (along the growth direction) and AH = 0.8 Oe for in-plane magnetic field. The narrowing of ESR line can be explained by combination of two mechanisms. The first one is suppression of Dyakonov-Perel spin relaxation due to a settled direction of electron motion and finiteness of QD chains. The second one is cancelation of the wave function shrinking effect with decreasing the perpendicular component of the magnetic field.

DOI: 10.7868/S0370274X1514009X

Progress in building of the structures with quantum dots [1-3] allows not only to use them as a working element of new electronic devices, but also to use these structures as a model for understanding the fundamental issues of matter. On the basis of artificial atoms one can create objects with different topology (e. g. 2D or 3D crystals, or molecules with different spatial packing), and investigate their electrical, optical and spin properties as functions of the structure of these objects. Recently it has been shown that the spatial configuration of quantum dots (QDs) in two-dimensional (2D) structures can affect the spin dynamics in the system [4]. Self-assembling of isolated groups of closely located QDs leads to a fourfold increase of the longitudinal spin relaxation time T\ as compared with 2D tunnel-coupled QD arrays with homogeneous in plane distribution of QDs [5]. The effect is related to limitation of electron movement within isolated groups of several QDs.

In the present work, a change of spin dynamics at transition from 2D QD system to a system with finite QD linear chains was demonstrated by electron spin resonance (ESR) measurements. The unusual behavior of ESR line width was obtained: the ESR line narrows when magnetic field deviates from the growth direction

-^e-mail: aigul@isp.nsc.ru

of the structure. To explain this effect, we proposed a model of spin relaxation in finite QD linear chains. A principal difference between spin dynamics in infinite QD arrays and that in finite linear QD chains was found.

Samples were grown by molecular-beam epitaxy on the pre-patterned Si(100) substrates with resistivity ^lOOQ-cm. The patterned stripes along the [100] direction with the period of 180 nm were fabricated by nano-imprint lithography and following irradiation by Ge+ ions through the imprinted resist. The details of the trenches formation are described elsewhere [2]. On such substrates the QD structure with 5 QD layers was grown. Each QD layer was formed by the deposition of 7Ge monolayers at the temperature of T = 600 °C. Atomic force microscopy (AFM) of the structure with a single uncovered QD layer shows that QDs formed the linear chains (Fig. 1). Each QD has do me-like shape with the base size of about 80 nm and the height of about 16 nm. It is clearly seen that the QD lines are not continuous and consist of finite QD linear chains. Quantum dots are located closely to each other within each linear chain. Si spacers between QD layers were grown at temperature of T = 500 °C. Transmission electron microscopy of covered samples shows that Si overgrowth results in twofold decrease of QD height indicating essential Ge/Si intermixing. On top of the struc-

I 0.10 h

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30c

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3344 3348

Fig. 1. (a) -3x3//.m2 AFM image of the uncovered sample with linear chains of Ge/Si QDs grown on pre-patterned substrate Si(001). (b) - 0.4 x 0.4 /im2 STM image of typical 2D array of self-assembled Ge/Si QDs (growth details are described elsewhere [6])

ture, a 0.2 /tm epitaxial n-Si layer with Sb concentration > 5 • 1016 cnr3 was grown. At low temperatures electrons should leave the impurities and fill the levels in QDs. The ESR measurements were performed with a Bruker Elexsys 580 X-band ESR spectrometer using a dielectric Bruker ER-4118 X-MD-5 cavity at a temperature of 4.5 Iv. The sample was glued on a quartz holder, then the entire cavity with the sample was maintained at a low temperature in a helium flow cryostat (Oxford CF935).

New ESR signal with principal ^-factor values gzz = = 1.9993 ± 0.0001, n,, = gyy = 1.999 ± 0.0001 was detected. Anisotropy of ^-factor confirmed that electrons were localized in strained Si regions near QD apexes. The principal values of ^-factor and its anisotropy are smaller than that of QD structures grown at lower temperature [6], which is testimony to the larger Ge/Si intermixing and smaller strain in the structures under study. A narrowing of the ESR line in the tilted magnetic fields was observed. The ESR line width is AH « 1.2 Oe for perpendicular magnetic field (along growth direction Z) and AH « 0.8 Oe for in-plane magnetic field, ii||[110] (Fig. 2, right panel). This behavior

3352 3474 H (Oe)

3476 3478

Fig. 2. ESR spectra at different orientations of magnetic field, microwave power P = 0.063 mW. Right panel: the results obtained for structures with QD lines (QD arrangement is shown in Fig. la). Left panel: the results obtained for typical 2D array of self-assembled Ge/Si QDs (QD arrangement is shown in Fig. lb). For 6 = 0" the magnetic field is parallel to the growth direction of the structure [001], 6 = 90" corresponds to the magnetic field applied along the crystallographic direction [110]

is unusual for tunnel-coupled QD structures. In existing works the opposite effect was observed: the ESR lines were broadened when the magnetic field deviates from Z. For example, in our previous study of a dense array of self-assembled QDs the ESR line width changed four times with sample rotation in the magnetic field [6]. In the magnetic field applied along the growth direction the ESR line width is AH « 0.75 Oe and, in the perpendicular magnetic field, it increases up to AH « 3 Oe (see Fig. 2, left panel). Such broadening is the first sign of Dyakonov-Perel spin relaxation mechanism [7] domination and occurs due to the special in-plane arrangement of spin-orbit fields (Bychkov-Rashba fields [8]), leading to the anisotropy of spin relaxation processes in the system. It is easy to show that the transverse spin relaxation time To should decrease in such type 2D systems at deviation of external magnetic field from the growth direction [9]. Since AH ~ 1 /To for homogeneously broadened ESR lines, then the special orientation dependence of ESR line width is observed. This mechanism can determine spin dynamics in the low-conduction QD system where electron transport occurs in the hopping regime [10], as well as in a 2D electron gas system with high conductivity [11].

The main factor providing the efficiency of Dyakonov-Perel mechanism in the QD system is the randomness of electron tunneling transitions between QDs due to the disorder of the self-assembled QD array. In the structures under study the ESR line width broadening is absent. This effect can be associated with a change of spin dynamics caused by the ordering of QDs in the linear chains. Such type of ordering defines the directions of electron tunneling to be only along

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A. F. Zinoviev.a, Zh. V. Smagina, A. V. Nenashev, L. V. Kulik, A. V. Dvurechenskii

QD lines. Indeed the recent results of conductance measurements of similar structures with QD lines demonstrate a strong conductance anisotropy [12]. The conductance along QD lines is two orders larger than in perpendicular direction. During tunneling along a linear QD chain the electron spin rotates around the effective magnetic field direction determined by vector product Heff ~ [n x ec], where n is the tunneling direction, ez is the growth direction of the structure [6]. For example the electron tunneling from the beginning to the end of the QD chain is accompanied by the clockwise spin precession, and the tunneling in the opposite direction - by the anticlockwise one. In the ideal case of the infinite chain of identical tunnel-coupled QDs this does not lead to a loss of spin orientation in zero magnetic field. The nonzero external magnetic field applied along ez provides an additional Larmor precession and the spin relaxation process depends on the relation between the frequency of Larmor precession ujl and the mean time of hopping between dots 17, . Recently we have demonstrated that for a ring-like group of closely spaced QDs at uJUh 1 the stabilization of ¿^-polarization occurs [4]. Now we simulate the spin relaxation process for the linear chain of QDs.

The model is based on the existing results of the experimental study of hopping transport in Ge/Si QD structures [12,13] and the theoretical study of spin relaxation during hopping through QD arrays [14]. The model includes a strong tunnel coupling between QDs in the line. Hopping between any neighboring QDs is permitted with an equal probability for the back and forth motion. Each tunneling transition is accompanied by a spin rotation by the small fixed angle a = 0.01. This value is taken based on results of t

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