научная статья по теме ODMR EVIDENCE OF ELECTRONIC CASCADE IN MULTIPLE ASYMMETRICAL (CDMN)TE QUANTUM WELLS Физика

Текст научной статьи на тему «ODMR EVIDENCE OF ELECTRONIC CASCADE IN MULTIPLE ASYMMETRICAL (CDMN)TE QUANTUM WELLS»

Pis'ma v ZhETF, vol. 102, iss. 4, pp. 257-261 © 2015 August 25

ODMR evidence of electronic cascade in multiple asymmetrical

(CdMn)Te quantum wells

A. S. Gurin*, D. O. Tolmachev*, N. G. Romanov*1^, B. R. Namozov*, P. G. Baranov*, Yu. G. Kusrayev*,

G. Karczewskix * loffe Institute, 194021 St. Petersburg, Russia x Institute of Physics, Polish Academy of Sciences, PL-02608 Warsaw, Poland

Submitted 29 May 2015 Resubmitted 7 July 2015

Exchange-coupled complexes consisting of Mn ions and holes were revealed by optically detected magnetic resonance in the narrowest quantum wells of asymmetrical multiple-quantum-well structures (CdMn)Te/(CdMg)Te. Calculations were performed to estimate the parameters of the complexes (exchange interactions and hole (/-factors) and simulate the spectra. The formation of such complexes implies the directional electron tunneling from narrow to larger wells, i.e. an electronic cascade, which results in the creation of the excess hole concentration in the narrowest and intermediate-width quantum wells.

DOI: 10.7868/S0370274X15160080

In diluted magnetic semiconductors (DMS), strong interaction between the two spin subsystems - free electrons and localized spins of impurity transition ions is known to give rise to striking effects, e.g., the giant Zeeman splitting of both the conduction and valence bands [1]. Since the magneto-optical properties of DMS are determined by magnetic impurity ions (Mn in most cases) it is important to clarify the structure of Mn-related centers. Electron paramagnetic resonance (EPR) is a method of choice for the study of transition metal ions [2]. However, conventional radiospectroscopy techniques are hardly applicable for low-dimensional systems because of a small active volume, not high enough sensitivity and the absence of spatial selectivity. High sensitivity, extreme resolution and spatial selectivity of optically detected magnetic resonance (ODMR) allow to overcome these difficulties and make this technique very suitable for a study of defects, carriers and excitons in quantum wells (QWs), superlattices (SLs), quantum dots (QDs) and nanocrystals [3].

Using ODMR the giant exchange interaction of free carriers with the localized spins of magnetic ions was discovered [4]. Later, ODMR was used for investigation of bulk DMS (ZnMn)S [5], (ZnMn)Te [6], (CdMn)Te [7, 8] and DMS-based nanostructures (see [9, 10] and references therein) but no structural information on Mn-related centers was reported. Anisotropy of ODMR spectra found in (CdMn)Se QDs [11] and sub-monolayer QWs [12] allowed to reveal a strain-induced axial fine

-^e-mail: nikolai.romanov@mail.ioffe.ru

structure splitting of the Mn2+ energy levels that appears because of low dimensionality.

It was shown [13] that manipulating the density of the two-dimensional hole liquid affects the ferromagnetic properties of magnetic quantum wells and drive the system between the ferromagnetic and paramagnetic phases, in a direction which can be selected by an appropriate design of the structure. This offers new tools for patterning magnetic nanostructures as well as for information writing and processing. Excess hole concentration can be created in not specifically doped QWs owing to the surface states [14]. In such (CdMn)Te quantum wells with 2D hole gas, unusual behavior of Mn spin system was observed in spin-flip Raman-scattering experiments [15] where a reduction of the effective Mn ^-factor was found. The effect was ascribed to magnetic soft mode [16] of collective excitations in strongly coupled spin system. New anisotropic ODMR spectra in (CdMn)Te quantum wells containing 2D hole gas [17] were ascribed to the complexes consisting of manganese ions exchange-coupled to localized holes.

In this work, we report on peculiarities of ODMR spectra in a different type of systems, i.e. asymmetrical QW structures containing (CdMn)Te QWs with different width separated by large (CdMg)Te barriers. The evidence of the excess hole concentration that appears because of the cascade charge transfer between QWs is presented.

A schematic of the (CdMn)Te/(CdMg)Te het-erostructure under study is shown in the inset in Fig. 1. The structure was grown by molecular-beam epitaxy on a (OOl)-oriented GaAs substrate with a thick (100 nm)

258

A. S. Gurin, D. O. Tolmachev, N. G. Romanov et a 1.

I

§

i-J Ph

10 mil

720 740 760 780

X (nm)

Fig. 1. The scheme of the triple (CdMn)Te/(CdMg)Te QW structure under study with QWs of 4, 6, and 10 nm (inset) and high resolution photoluminescence (PL) spectra recorded at 1.5 K at different magnetic field values. Dashed lines show the PL spectra measured at 2 K with lower spectral resolution, which was used in ODMR experiments

(CdMg)Te buffer layer and contained three (CdMn)Te quantum wells with the widths of 4, 6, and 10 nm. The Mn content was ca. 1 %. The QWs were not doped. They were separated by rather large (30 nm) (CdMg)Te barriers and covered with a lOOnm (CdMg)Te cap layer. Our ODMR study of QWs covered with such a thick cover layer have shown that no excess hole concentration is created in such QWs.

Photoluminescence (PL) was excited above the band gap using a 650 nm semiconductor laser (ca. 1 W/cnr), and detected with a triple monochromator and a CCD camera. In ODMR measurements, a singlegrating monochromator and a photomultiplier tube were used. The 35 and 94 GHz ODMR spectra were recorded at a temperature of 1.8 to 2Iv via PL intensity at a fixed wavelength which was chosen in such a way that the PL intensity were proportional to the PL line shift in the absence of the microwave field. For ODMR measurements at 94 GHz a quasi-optical microwave circuit shown in the inset in Fig. 2 was used. Its specific features were a high power microwave generator (100 mW) and a field concentrator described in Ref. [18]. In the 35 GHz ODMR spectrometer the sample was placed

3.1 3.2 3.3 3.4 3.5 3.6 3.7 B (T)

Fig. 2. Block diagram of the 94 GHz ODMR spectrometer (inset) and ODMR spectra recorded at 2 K in the (CdMn)Te QW structure by monitoring the PL intensity at the wavelengths marked by arrows. Solid and dashed lines show PL spectra in magnetic fields of 3 and 4T, respectively

iii the center of a cylindrical microwave cavity with holes for excitation and detection of PL. The sample was mounted on a rotating sample holder, which enables the ODMR measurements at different orientations of the sample relative to the magnetic field.

High-resolution PL spectra measured in different magnetic fields at a temperature of 1.5 Iv in the (CdMn)Te/(CdMg)Te QWs structure under study are shown in Fig. 1. The magnetic field was directed along the growth axis [001]. For each QW the spectrum consists of two emission lines - excitons (X) and charged excitons (trions, T). They manifest similar shifts to lower energy (larger wavelength) with increasing magnetic field. In ODMR experiments, we used a lower spectral resolution. The corresponding PL spectra measured at the magnetic field values of 3 and 4 T are shown in Fig. 1 by dashed lines.

Fig. 2 shows 94 GHz ODMR spectra of three QWs recorded via PL intensity at the wavelengths marked by

arrows with microwaves applied in cw mode. The angle between the growth axis [001] and the magnetic field was 9 = 55°. In the absence of microwaves the field dependencies of the PL intensity were close to linear and the shifts of the PL lines produced by increasing magnetic field and resonant microwaves were inside these linear dependencies. Thus, the PL intensity was proportional to the shift of the PL line. The ODMR spectra measured at both slopes of the PL line were found to be identical, which confirms reasonableness of such an approach.

The ODMR spectra are anisotropic. The baseline corrected 94 and 35 GHz ODMR spectra in three QWs are shown in Figs. 3a and b for the sample orientation

t

ä

Q O

(CdMn)Te/(CdMg)Te QWs

10 nm

3.20

3.40

3.60

Mn-holes 4nm

6 nm

10 mil

1.00

1.20

1.40

1.60

B (T)

Fig. 3. 94 GHz (a) and 35.1 GHz (b) ODMR spectra for 4, 6, and 10 nm QWs measured at 2K. The baseline is subtracted. The angle 6 between the c-axis and magnetic field

is 55°

0 = 55° in which two ODMR lines are better resolved. They belong to two different spin systems, i.e. the nar-

row lines can be attributed to isolated Mn ions [11, 12] and the wide lines can be ascribed to exchanged-coupled complexes [17] formed by a localized hole and Mn ions. The narrow ODMR line shifts slightly to lower magnetic fields while the wide ODMR line shifts from lower (effective ^-factor > 2) to higher fields with increasing angle 9 between the magnetic field and [001] growth axis of the structure.

The 94 and 35 GHz spectra are similar but they are very different for 4, 6, and 10 nm QWs. It is clearly seen that the narrow ODMR line of isolated Mn ions is observed in the widest QW and is present in the intermediate 6 nm QW while the wide ODMR line of the complexes (Mn-hole) dominates the spectrum of the narrowest 4 nm QWs, is still observed in 6 nm QW and is nearly absent in 10 nm QW. The formation of such complexes implies that an excess hole concentration is created in the narrowest (4 nm) and intermediate (6 nm) QWs. It is the largest in the narrowest well.

The excess hole concentration seems to appear as a result of the directional electron tunneling from narrower to wider QWs. Accumulation of excess holes happens in spite of very low tunneling probability caused by a large (30 nm) barrier thickness. Surface states have no effect on the hole concentration in our sample because of a thick cup layer [14]. Although ODMR spectra can not provide direct information on the hole concentration it can be roughly estimated from comparison of the ODMR spectra in the structure under study and the (CdMn)Te/(CdMg)Te QW with 2D hole gas that was studied in Refs. [15] and [17] as being of the order of fO11. It is the largest in the narrowest QW.

The (/-factor of individual Mn2+ ions is know

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