Magnetic field-induced exchange effects between Mn ions and free carriers in ZnSe quantum well through the intermediate nonmagnetic barrier studied by photoluminescence

0.57 MB·English

by  D. M. Zayachuk

Magnetic field-induced exchange effects between Mn ions and free carriers in ZnSe
quantum well through the intermediate nonmagnetic barrier studied by
photoluminescence
D. M. Zayachuk,1 T. Slobodskyy,2,3,∗ G. V. Astakhov,2,4 A. Slobodskyy,5,6
C. Gould,2 G. Schmidt,2,7 W. Ossau,2 and L. W. Molenkamp2
1Lviv Polytechnic National University - 12 Bandera St, 79013 Lviv, Ukraine
2Physikalisches Institut der Universita¨t Wu¨rzburg - 97074 Wu¨rzburg, Germany
3Present address: Institute for Synchrotron Radiation,
1
Karlsruhe Institute of Technology - 76344 Eggenstein-Leopoldshafen, Germany
1
4A. F. Physico-Technical Institute, Russian Academy of Sciences - 194021 St. Petersburg, Russia
0
5Light Technology Institute, Karlsruhe Institute of Technology (KIT), Kaiserstr. 12, 76131 Karlsruhe, Germany
2
6Zentrum fu¨r Sonnenenergie- und Wasserstoff-Forschung Baden-Wu¨rttemberg, Industriestr. 6, 70565 Stuttgart, Germany

n 7Present and permanent address: Institut fu¨r Physik, Universita¨t Halle - 06099 Halle, Germany
a (Dated: January 10, 2011)
J
7 Photoluminescence (PL) of the 50 nm Zn0.9Be0.05Mn0.05Se/ d nm Zn0.943Be0.057Se/
2.5 nm ZnSe/ 30 nm Zn0.943Be0.057Se structures is investigated as a function of magnetic
] field (B) and thickness (d) of intermediate Zn0.943Be0.057Se nonmagnetic barrier between the
ll Zn0.9Be0.05Mn0.05Se semimagnetic barrier and ZnSe quantum well at the temperature 1.2 K.
a The rate of the shift of different PL bands of the structures under study is estimated in low and
h
high magnetic fields. The causes of the shift rate increase under pass from low to high magnetic
-
s fieldsareinterpreted. Thepeculiarities oftheeffectoftheintermediatebarrierontheluminescence
e propertiesofthestructuresarepresented. Itisshownthatdeformationofadjacentlayersbythebar-
m rierplaysacrucialroleintheformationoftheseproperties,especiallyinformingtheMncomplexes
. in the Zn0.9Be0.05Mn0.05Se layer. The change of the band gap as well as of the donor and accep-
at tor levels energies undertheeffect of biaxial compression of the Zn0.9Be0.05Mn0.05Se layer by the
m Zn0.943Be0.057Seareestimated. ItisconcludedthattheZn0.943Be0.057Seintermediatebarrieralso
appreciably changestheeffect ofgiant Zeeman splittingof thesemimagnetic Zn0.9Be0.05Mn0.05Se
- barrierenergylevelsonthemovementoftheenergylevelsofZnSequantumwellinamagneticfield
d
n and on polarization of thequantumwell exciton emission.
o
c
[ I. INTRODUCTION are located in the same layer of a QW. This leads to a

strong interaction between the free carriers and the lo-
2
calized 3d-electrons of ions which intensifies the effects
v Quantumheterostructurescontaininglayersofdiluted
caused by the magnetic field. However, the presence of
1 magneticsemiconductors(DMS),usuallya3dMnorFe
5 based transition metal, have been extensively studied in magneticimpuritiesinaquantumwellstimulatesspinre-
2 laxationprocesses9–11. Hence,itispreferabletoseparate
the literature. The main focus has been made on both
0 the carriers from the magnetic media12–14. In this case,
fundamental and practical applications, especially these
1. designedfordifferentspin-electronicdevices. Presenceof the exchange interaction between the free 2D carriers
0 of nonmagnetic quantum well and the ions of magnetic
transition element ions provides the conditions for mag-
1 impurities in the barrier is driven by penetration of the
netic tuning of the heterojunction band alignment due
1 carrier wave function tails into the barrier.
to extraordinarily large spin-splitting of the DMS bands
:
v duetotheexchangeinteractionbetweenthesandpband Theeffectivedepthofawell,representedbythediffer-
Xi electrons and 3d5 electrons associated with the 3d ele- ence betweenthe positions ofthe bandedgesin adjacent
ment ions1–3. The s, p−d exchangeinteractionbetween layers, is strongly dependent on the magnetic field. At
r
a the local moments and the band electrons gives rise to a thesametime,theenergyofquantizationlevelsinashal-
rich spectrum of collective magnetic behavior. When an low well strongly depends on the well depth. Therefore,
external magnetic field is applied to DMS, they exhibit the electron states in a nonmagnetic quantum well ac-
twodistinctbandgaps,oneforeachspindirection1. This quire some characteristics of the DMS materials, specifi-
splitting ofconductionandvalence bands(giantZeeman cally sensitivity to the magnetic field1.
splitting) can lead to a sizable spin polarization of the It seems interesting to investigate the effect of mag-
carriersintheDMS.Thispropertyisusedtoinjectspin- netic field on the electron states in a nonmagnetic quan-
polarized carriers into nonmagnetic semiconductors4,5. tum well effected by a semimagnetic barrier. The bar-
Twotypicalapproachesareusuallyappliedtothe fab- rier changes its height and exchange interaction with
ricationofDMSheterostructures: aheterostructurewith the states. Separation of the two effects can provide
a DMS layer as a quantum well(QW)6 or a heterostruc- an additional degree of freedom in fabricating spintron-
ture with DMS layers as quantum barriers7,8. In the ics devices. In particular, an additional nonmagnetic
first case, both the free carriers and the 3d element ions layer introduced between a semimagnetic barrier and a
2
30 nm Zn Be Se Ec Ev
0.943 0.057
2.5 nm ZnSe nits)1600
u
d nm Zn0.943Be0.057Se arb. u1200 B = 0 Ta 1
150 nm Zn Be Mn Se y ( b 1
0.9 0.05 0.05 sit c 1
n
e 800 d 1
nt
GaAs Substrate E L I
P
400
FIG. 1. Schematic view of the sample layout (left) with the
2.84 2.85 2.86 2.87 2.88 2.89
correspondingenergybandprofile(right). Thespin-splitsub-
Energy (eV)
bands of the conduction band (EC) and the valence band
(EV) in the magnetic field are depicted by arrows. The
Zn0.943Be0.057Se barrier thickness d was varied as 0 (named FIG.2. ThePLspectraofthe150nmZn0.9Be0.05Mn0.05Se/
a1), 2.5 (b1), 7.5 (c1), and 12.5 nm (d1). d nm Zn0.943Be0.057Se / 2.5 nm ZnSe /
30 nm Zn0.943Be0.057Se structures in zero magnetic
fields: T = 1.2 K. ¯hνex = 2.94 eV.
nonmagnetic quantum well may be used for this pur-
pose. Since, in the rangeof our experimentalconditions, III. THE EXPERIMENTAL PL SPECTRA
the nonmagnetic barrier height only negligibly depends
on a magnetic field, the changes of an exchange inter- Fig. 2 shows the PL spectra for the structures under
action contribution will dominate in the field behavior study without the application of magnetic field. One
of the energy characteristics of the QW. In this paper canseethattheshapeofPLspectrumcriticallydepends
we show such a possibility using the example of quan- on the presence of an intermediate Zn Be Se
0.943 0.057
tum structures based on the ZnSe quantum well with barrier between the ZnSe QW and the semimagnetic
theZn0.9Be0.05Mn0.05SeandZn0.943Be0.057Sebarriers. Zn0.9Be0.05Mn0.05Se layer. This concerns the two ”evi-
Thebarriercompositionswereselectedbasedonthecon- dent”spectracharacteristics,i. e.,the numberofthe PL
dition of an approximate parity of the band gap of the bands and their intensity. The most striking feature of
barriers at zero magnetic field. In order to study the this dependence is adecreaseofthe number ofPLbands
field-induced exchange effects through the intermediate by one for the structures with an intermediate barrier.
nonmagnetic barrier between Mn ions and free carriers
The composition of the structure also affects the be-
in ZnSe quantum well, Zn Be Se barrier of dif-
0.943 0.057 havior of the PL bands in the magnetic field where
ferent thickness was introduced between the ZnSe and the bands split into two components with σ+ and σ−-
Zn Be Mn Se layers. Photoluminescence (PL)
0.9 0.05 0.05 polarization. Fig. 3 shows this effect using the example
propertiesofthestructureinmagneticfieldsupto5.25T
of different polarization PL spectra in a magnetic field
were investigated.
up to 1 T for a1 and d1 structures.
In order to quantitatively analyze the structure com-
position effect on the behavior of PL spectra in a mag-
netic field, it is necessary to separate the spectra into
elementary components. This turns out to be very com-
II. EXPERIMENTAL DETAILS plicatedproblem,especiallyintherangeofhighmagnetic
fields where different PL bands superimpose one above
another. As a result, the experimental PL spectrum can
The samples used in the PL experiments were grown
be reproduced in different ways by the same number of
by molecular beam epitaxy on a GaAs substrate. The
elementary bands. As an example, the data in Fig. 4
sample layout is depicted in Fig. 1. The nonmagnetic
illustrate this.
space layerd is used for the purpose of varyingthe mag-
In view of the mentioned ambiguity, decomposition
netic interaction in the structure.
of the experimentalspectra into elementary constituents
PL spectra at 1.2 K in magnetic fields up to 5.25 T was carried out using the method of averaging. Details
at Faraday geometry were measured to study the pe- of the method of decomposition will be discussed more
culiarities of the exchange interaction between the Mn in detail elsewhere. Here we emphasize that it is well
ions and free carriers. For optical excitation, we used a known12 that for the zinc-blende structure, the transi-
stilbene-3dyelaserpumpedbyultravioletlinesofanAr- tion matrix element of the transitions involving heavy
ionlaser. Fornon-resonantexcitation,thelaserenergyis holes (hh) is three times larger than that involving light
tuned to E = 2.94 eV exceeding the band gap of the holes (lh). Therefore, only those decompositions were
exc
Zn Be Mn Se barrier. taken into account where the PL intensity of the heavy
0.9 0.05 0.05
3
FIG. 3. The PL spectra of σ+ and σ−-polarization of thea1(top) and d1(bottom) structuresin magnetic fields.
FIG. 4. Two examples of a multitude of decompositions of the experimental σ+-polarized PL spectrum of the d1 structure in
5T magnetic fieldson theLorentzcomponentssatisfying arequirement ofa domination of theintensityof theheavyexcitons
(the left band) over the intensity of the light excitons (the second band from the left) in the ZnSe QW. Classification of the
L - L bandssee below in Sec. IV, their nature- in Sec. VIII.
1 4
excitons waslargerthan the PL intensity ofthe lightex- intensity of the Lorentz components within the error of
citons. Itwillbeshownbelowthatexactlytheseexcitons no more than 10 % of the true value.
form the longest wave PL bands of the structures under
study. Not less than 20 different decompositions cover-
ing the maximum range of parameter variations for each
IV. ENERGY POSITION OF THE PL BANDS
spectrum were used to determine the averaged values of
theparameters. Ananalysisshowedthatthedeviationof
experimentalvaluesoftheparametersofdifferentdecom- Fig. 5 shows the magnetic field dependences on the
energy positions of different PL bands for the structures
positionsfromtheiraveragedvalues inanystructuresdo
notexceed: (i)±0.0005eVfortheenergyofashortwave under study. Let us examine these dependences from
viewpoint of general and distinctive features in different
component of the spectra and ± 0.0015 eV for the long
wave components; (ii) ± 20 % of the averaged value for structures.
the full width at half maximum (FWHM) of the bands; The L1 band: It is the highestenergyPL bandinzero
± 60 % of the averaged value for the intensity of the magnetic fields. Its behavior in a magnetic field is the
bands. The total intensity of PL is reproduced by the sameforallstructures. Theσ+-polarizedL+ bandshifts
1
to the long-waverange,ifB increases. The σ−-polarized
4
FIG. 5. Magnetic field dependence of the energy positions of the Lorentz components of both σ+ (L+ - L+ ) and σ−-
1 4(2)
polarizations (L− - L− ).
1 4(2)
−
L bandtendstoshiftintheoppositedirectioninamag- The L band: This is the smallest energy PL band
1 4
netic field and practically disappears when B > 0.5 T. in zero magnetic fields. This band is a superposition of
twocomponentswithcloseenergy. We markthemL+ ,
The L2 band: It is the next PL band in zero magnetic 4(1)
fields. It behaves differently in different structures. σ+- L+ and L− , L− for σ+ and σ−-polarization, re-
4(2) 4(1) 4(2)
polarization: (i) the a1 structure: the L+ band slightly spectively. Their behavior is absolutely different for the
2
changes its position in low (< 1 T) magnetic fields and structures with and without an intermediate nonmag-
shifts appreciably to the long-wave range in high mag- neticZn Be Sebarrier. The a1 structure: (i)the
0.943 0.057
netic fields, when B increases; (ii) the b1 and c1 struc- energy position of the L bands changes if B changes
4
tures: the L+ band energy practically does not depend withinthewholerangeofmagneticfieldunderstudy;(ii)
2
on the magnetic field for B < 1.25 T, then starts to de- the L+ and L+ bands shift to the long-waverange if
4(1) 4(2)
crease somewhat. The band disappears in the magnetic −
B increases;(iii)ifthemagneticfieldisapplied,theL
field where its energy becomes equal to the L+ band en- 4(1)
ergy; (iii) the d1 structure: the L+ band po1sition does and L−4(2) bands shift to the short-wave range but they
2
not depend on the magnetic field. σ−-polarization: the change the direction of their shift if B > 0.5 T. In this
L− band energy does not depend on the magnetic field. rangetheyfollowthepositionsoftherespectivePLbands
H2owever, the range of magnetic fields where the band of σ+-polarization. The b1, c1, and d1 structures: (i) in
is observed is different for different structures. It is the low magnetic field, the energy of the bands practically
narrowest (B ≤0.5 T) for the a1 structure and includes doesnotdependonthemagneticfieldandthustheL+4(1)
all magnetic fields under investigation for the c1 and d1 and L− as well as L+ and L− coincide with each
4(1) 4(2) 4(2)
structures. other; (ii) in high magnetic field, all bands shift to the
The L band: This band displays the strongest de- long-wave range and the bands with different polariza-
3
pendence on the structure composition. It is observed tions gradually diverge. The thicker is the intermediate
only for the a1 structure without the intermediate barrier the larger is the divergence of both L+ and L−
4 4
Zn Be Sebarrier. TheL+ bandenergydecreases bands.
0.943 0.057 3
if B increases. On the contrary,in low magnetic fields of
−
approximately B < 1.5 T, the L band energy increases
3 −
if B increases. If B > 1.5 T, the L band shifts in the
3
opposite direction. In the fields above 2 T its energy po-
sitioncoincideswiththe energypositionoftheL+ band.
1
5
V. FULL WIDTH AT HALF MAXIMUM OF barrier Zn0.943Be0.057Se between the ZnSe and
THE PL BANDS Zn Be Mn Selayersofthestructurehasnoeffect
0.9 0.05 0.05
on the L band intensity in zero magnetic fields but ap-
1
Fig. 6 and Fig. 7 show the effect of both the inter- preciablyaugmentsthe intensityoftheotherPLbands-
mediate barrier and the magnetic field on the PL bands L2,QW(I(L4(1))+I(L4(2)))aswellasthetotalPLinten-
FWHM w for the structures under study. sity. All these intensities increase if d(Zn0.943Be0.057Se)
increases (Fig. 8).
The main features of this effect are as follows.
The L band: FWHM of the L+ band is the widest in Application of a magnetic field increases the intensity
1 1 of the short wave L+ PL band in any structures. At the
the a1 structure. In zero magnetic fields, ω decreases if 1
sametime,thefieldeffectonthebehavioroftheotherPL
the intermediate nonmagnetic barrier is applied and its
bandsisnotthatsimple. Forexample,inalowmagnetic
thickness increases. In low magnetic field B < 1 T, the
field B < 1 T, an intensity of the long wave L+ and
band behavior is different for different structures: from 4(1)
continuous decrease in the a1 structure to some increase L+ bandsincreasewhenB increasesinthea1structure
4(2)
in the d1 structure if B increases. For B > 1 T, w but decreases in another three structures. An intensity
monotonously decreases if B increases. Herein the band of the L+ band increases under transition from low to
2
ispracticallyofthesamewidthinallstructureswiththe high magnetic fields in the a1 structure but decreases
intermediate Zn0.943Be0.057Se nonmagnetic barrier. and comes off plateau in the d1 structure (Fig. 9).
The L band: FWHM of the L+ band increases ap- Applicationof a magnetic field alsoincreases the total
2 2
proximately three times under transition from low to intensity I of the σ+-polarized emission of the struc-
σ+
high magnetic fields for the a1 structure and somewhat tures but decreases the total intensity Iσ− of the σ−-
−
decreases for the d1 structure. FWHM of the L band polarized emission (Fig. 10). In high magnetic fields,
2
practicallydoesnotdependonB forthec1andd1struc- Iσ− first stabilizes and then displays a weak tendency to
tures where it is observed for any investigated magnetic increase. Herein Iσ− becomes much smaller than Iσ+.
fields. For the top magnetic field, B = 5.25 T ratio Iσ+/Iσ−
The L band: If a magnetic field is applied, FWHM changes from 0.04 in the a1 structure to 0.14 for the d1
3
of the L+ band somewhat decreases and that of the L− structure (see inset in Fig. 10, (b)).
3 3
band increases. After changing the shift direction in a In low magnetic fields, the characterof I (B) appre-
σ+
−
magnetic field, the L band FWHM decreases if B in- ciablydependsontheintermediatebarrierthickness: the
3
creases. In this range of the magnetic field it is approxi- thicker is the barrier the weaker is the dependence. In
mately four times larger than the L+ band FWHM. high magnetic fields, an effect of the intermediate bar-
3
The L4 band: FWHM of the L+4(1) and L+4(2) bands rierthickness onthe totalintensity Iσ+ ofthe structures
practically does not depend on B in low magnetic fields has got a pronounced non-monotonous character. The
B < 1 T and decreases if the intermediate nonmagnetic thinnest barrier (the b1 structure) does not increase Iσ+
barrierisappliedanditsthicknessincreases. Intheinter- with respect to the a1 structure. Similar increase of the
mediate magnetic fields, w(L+ ) and w(L+ ) sharply intermediatebarrierthicknessfrom7.5to12.5nm(thec1
4(2) 4(1)
andd1structures)practicallydoesnotchangetheparam-
rise in the structures with an intermediate barrier. The
eterI aswell. Thusitispossibletoreachtheprincipal
band widening depends on the barrier thickness d: it is σ+
increase of the total PL intensity I in the high mag-
the largest for the b1 structure where d is the smallest σ+
netic fields by changing the intermediate barrier thick-
and vice versa - it is the smallest for the d1 structure
where d is the largest. If B increases further, w(L+ ) ness withinthe limits of2.5- 7.5nm. The applicationof
4(2) the intermediate barrier also appreciably affects the rel-
and w(L+ ) starts to decrease. The FWHM decrease
4(1) ativeincreaseofIσ+(B)enfeebling it. Hereinthe barrier
slackens if the barrier thickness increases. For the d1 thickness is of minor importance (see inset in Fig. 10,
structure, w(L+ ) and w(L+ ) practically do not de- (a)).
4(2) 4(1)
pendonB inthehighmagneticfieldrange. Inthisrange
of the field for the a1 structure, w(L+ ) and w(L+ )
4(2) 4(1)
somewhat increase if B increases. VII. POLARIZATION OF THE PL BANDS
− −
FWHM of both L and L bands behaves like to
4(1) 4(2)
L+ and L+ bands FWHM. Polarization of the PL bands is traditionally deter-
4(1) 4(2) mined as ρ = (Iσ+ − Iσ−)/(Iσ+ + Iσ−). The shortest
−
L PL band disappears as early as B > 0.5 T in any
1
structure under study. Thus, the intermediate barrier
VI. INTENSITY OF THE PL BANDS
does not effect polarization of the L band, which arises
1
as soon as a magnetic field is applied and reaches 100 %
Fig. 8 and 9 show an effect of both the intermediate if B > 0.5 T. The state of matter is absolutely different
barrierandmagneticfieldonthe PLbandintensityI for for the longest L PL band. Fig. 11 demonstrates it by
4
the structures under study. the example of the L bands.
4(2)
The application of the intermediate nonmagnetic As one can see, the L band is not polarized up to
4(2)
6
FIG. 6. Magnetic field dependenceof theFWHM of the σ+-polarized bands of thea1 and d1 structures.
FIG. 7. Magnetic field dependenceof the FWHMof theL+ and L+ bandsfor all structures.
1 4(2)
VIII. DISCUSSION
)
s
1unit 2 LL12 In our previous work13 we analyzed the PL spectra
b. QW of the a1 structure and proposed the following interpre-
(ar 8 Total tation of the origin of different bands. Low magnetic
tensity 4 cfiaonenldadscu:cce(tpii)otnoartbrcaaonnmsdiptliEeoxnZCncbBoenetMwtaenienSneintgahneMdZnnth0E.e9MBenne0e.r0g5yMlne0vf.oe0lr5mSoesf
n ZnBeMnSe
I 0 the short wave L1 band; (ii) a donor-acceptor transition
0.0 2.5 5.0 7.5 10.0 12.5 ED -EA in Zn Be Mn Se forms
d (nm) ZnBeMnSe ZnBeMnSe 0.9 0.05 0.05
the L band; (iii) an indirect transition in real space be-
2
tween the 2D conduction band of the ZnSe QW EC
FIG.8. Barrier thicknessd(Zn0.943Be0.057Se)dependenceof and EMn forms the L band. High magnZentSice
PL signal intensity at zero magnetic field. ZnBeMnSe 3
fields: (i) a transition between EC and the bar-
ZnBeMnSe
rier valence band EV forms the L band; (ii) a
ZnBeMnSe 1
transition ED - EV forms the L band;
ZnBeMnSe ZnBeMnSe 2
(iii) an indirect transition EC - EV forms the
ZnSe ZnBeMnSe
L band. The transitions within the ZnSe QW EC
3 ZnSe
B ≈ 1 T for the c1 and d1 structures. At the same - EZVnSe forms the L4 band in any magnetic field. The
time, for the other two structures with the thinnest in- short wave component L4(1) of the L4 band is formed
termediate barrier and without a barrier the L band by emission of the light exciton, while the long wave
4(2)
starts to polarize as soon as a magnetic field is applied. component L is formed by emission of the heavy
4(2)
In the high magnetic fields, the L band polarization exciton. There are no reasons to expect that the in-
4(2)
saturates. Thepolarizationofsaturationisthelargestin termediate Zn0.943Be0.057Se barrier changes the nature
thea1structurewithoutanyintermediatebarrier(91%), of the PL bands within both the ZnSe QW and the
somewhat smaller for the b1 and c1 structures (89 - 88 Zn0.9Be0.05Mn0.05Se semimagnetic barrier. Therefore,
%), and noticeably smaller for the d1 structure with the the analysisthat followsthe originofthe L1, L2,andL4
thickestintermediatebarrier(71%). The polarizationof bands is the same as in the other three structures.
the L band is the same. In order to separate the contribution of the barrier
4(1)
7
FIG. 9. Magnetic field dependenceof the PL intensity of the σ+- polarized bands for both a1 (left) and d1 (right) structures.
Inset: PL intensity of both L and L bandsof σ+ and σ− polarization.
4(1) 4(2)
FIG.10. Magneticfield dependenceofatotalintensityofσ+ (left) andσ−-polarized (right)PL ofthestructuresunderstudy.
Insets: (left) - a total intensity normalized by its value in zero magnetic fields; (right) - a relation between a total intensity of
σ+ and σ−-polarized PL.
) 0.9
χN =αN −βN (2)
+ I 0.8 0 0 0
)/(I 00..67 where x˜ is the effective Mn concentration, αN0 and
- I 0.5 βN0 aretheexchangeintegralsbetweentheMnionsand
(I carriersforupperandlowerstatesresponsibleforthisPL
0.4 a1
band, respectively, <S > is the thermal averageof the
0.3 b1 Z
0.2 c1 Mn spin given by:
0.1 d1
0.0
5
0 1 2 3 4B (T)5 <SZ >= 2B5/2[5µBB/kB(T +Teff)] (3)
FIG. 11. Magnetic field dependence of the L4(2) band polar- B5/2 is Brillouin function of the argument in square
ization. brackets,µ isthe Bohrmagneton,k isthe Boltzmann
B B
constant, T is the temperature, and T is an empirical
eff
parameter representing antiferomagnetic interaction be-
height and exchange interaction in the field behavior of tween the Mn ions. In our calculations, the parameter
different PL bands, a quantitative analysis is needed. T was takento be equal to 1.75 K in accordancewith
eff
The L1 band shift in a magnetic field is caused by the the empirical ratio obtained in15 for Zn1−XMnXSe.
giant Zeeman splitting and may be written as1,15,16: A comparison of the experimental and the calculated
dataforallstructuresunderstudyispresentedinFig.12.
It clearly shows that there are two ranges of magnetic
E(B)=E(0)∓(χN )x˜<S > (1) fields for any structure where the rates of the L+ band
0 Z 1
8
shift (χN ) in the Zn Be Mn Se layer are differ- Zn Be Mn Se on the strains if the strained con-
0 0.9 0.05 0.05 0.9 0.05 0.05
ent: they are smaller for low and larger for high mag- tactlayersprovidethemaincontributiontotheemission
netic fields. We marked them by (χN ) and (χN ) , transitions. Note that the importance of the strained
0 1L 0 1H
respectively. TheparameterE(0)isalsodifferentforthe heterointerface for localization of excitons was already
rangesoflowandhighmagneticfields. Wemarkedthem emphasizedininitialinvestigationsofthe II-VIstrained-
E (0) and E (0), respectively. layer heterostructures20.
1L 1H
One can see in Fig. 12 that many of the parameters The value E (0) − E (0) has a sense of an
1H 1L
of the L band in the Zn Be Mn Se layer are acceptor level depth of the Mn complex in the
1 0.9 0.05 0.05
differentnotonly forlowandhighmagnetic fields. They Zn Be Mn Se strained surface layers. It is ap-
0.9 0.05 0.05
arealsodifferentforthe structures withandwithoutthe proximately one and a half times smaller in the com-
intermediate Zn Be Se barrier as well as for the pressedlayersincomparisonwiththetensedone: 10meV
0.943 0.057
structures of different barrier thickness. Let us examine for the Zn Be Mn Se barrier in the a1 structure
0.9 0.05 0.05
these parameters. against (15.5 ± 1.2) meV for the other three structures.
E1H(0) has a sense ofa band gapof the semimagnetic The values of (χN0)1H and (χN0)1L are determined
Zn0.9Be0.05Mn0.05Se barrier in zero magnetic fields. by the exchangeinteractionsplitting the electronic band
This parameter increases monotonously from 2.8932 eV and impurity levels. It is obvious from Fig. 12 that the
for the barrier in the a1 structure to 2.9019 eV for the (χN ) values arethe same for allstructures. It means
0 1H
same barrier in the d1 structure (Fig. 12). We explain that the αN and βN exchange integrals for C and V
0 0
the observed changes of the parameters E1H(0) as well bands of Zn0.9Be0.05Mn0.05Se layer develop with no ef-
as of the energy of L1 bands by both: (i) the effect fect of the 2D free carriers of the ZnSe QW. On the
of the strains of the Zn0.9Be0.05Mn0.05Se barrier sur- contrary, the (χN0)1L value is the same only for the
face layerscaused by the actionof the adjacent ZnSe or structures with the intermediate Zn Be Se bar-
0.943 0.057
Zn0.943Be0.057Se layers on the band gap value; (ii) the rier. For the a1 structure without any intermediate bar-
dominant contribution of the strained surface layers in rieritislargerbyapproximately22%. Itmeansthatthe
forming the emission bands of the semimagnetic barrier. exchangeintegralforelectronicstatesoftheMncomplex
Itiswellknownthatintheabsenceofstrain,themaxima in the structures under study develops appreciably with
of the heavy- and light-hole valence bands are degener- participation of the 2D free carriers of the ZnSe QW.
ateinthezinc-blendesemiconductors. Thebiaxialstrain In accordance with our previous analysis13 the band ex-
shifts and splits the heavy- and light-hole bands. If the change integrals for the Zn Be Mn Se layer are
0.9 0.05 0.05
strain is compressive, the band gap increases and coin- equal to αN = 0.104 eV, βN = - 0.264 eV. The value
0 0
cides with the heavy-hole-derived band gap. In the case of the exchange integral for electronic states of the Mn
of biaxial tension, the band gap shrinks and is associ- complex in the a1 structure is equal to 0.156 eV. Us-
ated with the light-hole transition17. The layers of the ing the obtained (χN ) value for the b1, c1, and d1
0 1L
structures under study have different lattice constants: structures we see that the application of intermediate
a(ZnSe) = 5.6684 ˚A, a(Zn0.943Be0.057Se) = 5.6382 ˚A, Zn0.943Be0.057Sebarrierdecreasesthisvalueto0.112eV,
a(Zn0.9Be0.05Mn0.05Se) = 5.6592 ˚A18,19. Therefore, i.e., by approximately 40 %.
theydeformeachother: ZnSetensesthesurfacelayersof
We explain these results in the following way. There
Zn Be Mn Se,andZn Be Secompresses
0.9 0.05 0.05 0.943 0.057 are two types of free carriers contributing to the ex-
them. Thus, the Zn Be Mn Se surface layers
0.9 0.05 0.05 change interaction in the strained surface layers of
that contact with the ZnSe layer, have a smaller band
the Zn Be Mn Se barrier: 3D carriers of the
0.9 0.05 0.05
gap,andcontactingwiththeZn Be Selayerhave
0.943 0.057 Zn Be Mn Se barrier and 2D carriers of the
0.9 0.05 0.05
a larger band gap than the strainless one. It enables us
ZnSe QW. The wave functions of 3D carriers are quite
to construct a dependence of the band gap of the strain
extended and span a large number of lattice sites of the
Zn Be Mn Selayersonthethicknessofboththe
0.9 0.05 0.05 Zn Be Mn Se barrier. On the contrary,only the
0.9 0.05 0.05
contact ZnSe and Zn Be Se layers which cause
0.943 0.057 tails of the wave functions of 2D carriers penetrate from
these strains. This dependence is presented in Fig. 13.
QW into the barrier. This penetration decreases or is
Fig. 13 shows that the band gap of the absent when the intermediate Zn Be Se barrier
0.943 0.057
Zn0.9Be0.05Mn0.05Se strain layers asymptotically appears between the ZnSe and Zn0.9Be0.05Mn0.05Se
approaches the value 2.9043 eV under compression by layers. Thus, we can unambiguously conclude that 2D
the Zn0.943Be0.057Se layer when d(Zn0.943Be0.057Se) carriers of the ZnSe QW give negligible contribution to
approaches 30 nm. Extrapolation of the data in the formation of exchange integrals for C and V bands
Fig. 13 also shows that the band gap of the strainless of the Zn Be Mn Se strained contact layers but
0.9 0.05 0.05
Zn0.9Be0.05Mn0.05Se is equal to 2.894 eV. Thus, the their contribution to the exchange interaction between
biaxial strain of the Zn0.9Be0.05Mn0.05Se layer by the the Mn complexes in these layers is can be described by
Zn0.943Be0.057Se layer can increase the band gap of the mixing of the 3d5 levels of Manganese with the states of
former by approximately 10 meV. 2D free carriers of the ZnSe QW. This is possible only
The obtained dependences of E(L+) from B may iftheMncomplexconcentrationinthestrainedlayersis
1
be explained by the dependence of the band gap of largerthantheconcentrationofMninthesitesofcrystal
9
FIG. 12. Magnetic field dependenceof theE(L ) for the different structuresunderstudy.
1
E(0) (eV)221H..990004 22E (eV) ..885650 22..885691 eeVV
L
Experiment 4(1)
2.896 Extrapolation 2.850 L
E1H(0)=2.9043 eV 4(2)
2.845
2.892
0 10 20 30 40 0 5 10 15
d (nm)
d(Zn Be Se) (nm)
0.943 0.057
FZInG0..9B13e.0.05EM1Hn0(.00)5Sves sdemoifmtahgeneltaiycerbsarcroinetracintintghewiitnhvestthie- FIG. 14. Energy of the L±4(1) and L±4(2) bands vs. the inter-
gatedstructures. dispositivefortheZn0.943Be0.057Selayers mediate Zn0.943Be0.057Se barrier thickness in zero magnetic
(compressionofZn0.9Be0.05Mn0.05Selayer)andnegativefor fieldsand theirapproximations.
ZnSe layer (tension of Zn0.9Be0.05Mn0.05Se layer).
±
nent L of the L band.
4(1) 4
lattice. Inotherwords,mostprobablytheMncomplexes A splitting between the energy of heavy and light ex-
in Zn Be Mn Se developinto the strainedlayers citonsinzeromagneticfieldsis equalto(2.0±0.3)meV
0.9 0.05 0.05
of a heterocontact. forallstructures. Atthe sametime, the energiesofboth
Let us now consider the long wave L PL band. The heavy and light excitons are different for different struc-
4
layer of the ZnSe QW experiences compression on the tures and increase with an increased thickness of the
partofbothZn0.9Be0.05Mn0.05SeandZn0.943Be0.057Se intermediate nonmagnetic barrier Zn0.943Be0.057Se be-
layers. As a result, its heavy- and light-hole bands split tween the Zn0.9Be0.05Mn0.05Se and ZnSe layers. The
± ±
and the heavy-hole band defines the band gap in any dependences of E(L ) and E(L ) on the thickness
4(1) 4(2)
structures under study. Therefore, the heavy excitons oftheintermediatebarrierincaseoftheabsenceofmag-
|±1i=|∓1/2,±3/2iformthelongwavecomponentL± netic field are shown in Fig. 14.
ofσ+ andσ−-polarization,respectively,andthelight4e(x2)- One can see that both E(L± ) and E(L± ) increase
4(1) 4(2)
citons |±1i= |±1/2,±1/2i form the short wave compo- if d(Zn Be Se) increases and asymptotically ap-
0.943 0.057
10
proach the energy 2.861 and 2.859 eV, respectively. The
cause of these changes is the same as in the case of the
L bands: the deformation effects. There is the follow-
1
ing ratio between the lattice constants of the layers of
the structures: a(ZnSe) > a(Zn Be Mn Se) >
0.9 0.05 0.05
a(Zn Be Se). As a result, the Zn Be Se
0.943 0.057 0.943 0.057
layermorestronglycompressestheZnSeQWlayerthan
the Zn Be Mn Se layer and its deformation ef-
0.9 0.05 0.05
fect increases if d(Zn Be Se) increases. There-
0.943 0.057
fore, the ZnSe layer band gap increases and the exciton FIG. 15. The experimental PL spectra of the a1 (a) and d1
energy increases too. For the a1 structure in zero mag- structures (b) of σ+ (black lines) and σ−-polarization (red
neticfieldsE(L± )=2.8478,andE(L± )=2.8455eV. lines) for B = 5.25 T in the energy range of the L bands.
4(1) 4(2) 4
It is seen that under the effect of the intermediate bar- Right - the shift between the energy positions of L+ and L−
4 4
rier, the heavy-hole-derived band gap of the ZnSe QW bandsvs. d(Zn0.943Be0.057Se) at 5.25 T.
increases by 13 meV extra.
Applicationofamagneticfieldsplitsthebandedgesin
the EV+ edge before the L bands change their po-
thewell. However,theZeemansplittingfornonmagnetic ZnSe 4
sition. In accordance with the energy diagram of the
ZnSe should be negligible taking into account the g-
factor value for electrons and holes21. Actually E(L± ) a1 structure13, the offset between the C and V bands
4(1) of the Zn Be Mn Se and 2D ZnSe layers dis-
± 0.9 0.05 0.05
andE(L )negligiblydependsonB onlyfortheb1,c1,
4(2) tributed as ∆EC/∆EV = 60/40which is typicalof these
andd1structuresandonlyinmagneticfieldsB <(2.0÷ materials7. Theintermediatebarriersomewhatdecreases
2.5)T (Fig. 5). For higher magnetic fields, the energyof thevalue(E (0)−E (0)),whichdeterminesthemu-
1H L4(2)
L4 exciton bands appreciably decreases. There ar±e more tual location of the Zn0.9Be0.05Mn0.05Se and 2D ZnSe
essentialchangesintheenergypositionsofbothL and layer band edges on the energy scale. Its magnitude for
4(1)
L± bandsinamagneticfieldforthea1structure,aswe the a1 structure is equal to 47.7 meV, and the aver-
4(2)
aged magnitude for the other three structures is equal
have emphasized above. It is obvious that the exchange
to (42.3± 1.5)meV. Ifwe sharethis value in proportion
interactionbetweenfreecarriersoftheQWandMnions
∆E /∆E =60:40 we find that EV -EV in
of the semimagnetic barrier, which is further modified C V ZnSe ZnBeMnSe
zero magnetic field for the b1, c1, and d1 structures is
by the intermediate nonmagnetic barrier, causes the ob-
± ±
equal to 16.9 meV. The energy of L and L bands
served changes. 4(1) 4(2)
The a1 structure is the structure with a shallow non- start to decrease only if the EV+ goes down to
ZnBeMnSe
magnetic QW having the adjacent layer of semimag- approximately 24.5 meV (B ≈2 T). However,one needs
netic semiconductor. Therefore, the effective depth of to remember that the effective barrier for both electrons
the well, given by the difference between the positions and holes forming the L+ bands goes down when B in-
4
of the band edges in the adjacent layers, is strongly de- creases. On the contrary, the effective barrier for elec-
−
pendent on the magnetic field, and in a shallow well the trons and holes forming the L bands goes up. At the
± 4 ±
energy of the size quantization levels is strongly depen- same time, the energy of L and L pair decreases
4(1) 4(2)
dent on the well depth. For the other structures, the in the range of high magnetic fields. It entirely bears
situation is radically different. Herein the positions of a resemblance to the behavior of both L± and L±
4(1) 4(2)
the band edges of the Zn0.943Be0.057Se layers adjacent bands in the a1 structure in this magnetic range. In our
to the QW negligibly depend on B and the height of previouswork13weexplainedsuchabehaviorofdifferent
the intermediate barrier also negligibly depends on B. polarizedexcitonbandsintheQWbytheinfluenceofthe
At the same time, an effective barrier height for elec- spin-flip processes caused by the degeneration of energy
trons and holes appreciably depends on B because the levels of the ZnSe QW and the Zn Be Mn Se
0.9 0.05 0.05
position of the band edges of the Zn0.9Be0.05Mn0.05Se layer. We believe that the same processes also occur in
layer depends on B. It changes a range of pene- thestructureswiththeintermediatenonmagneticbarrier
tration of the wave functions of the ZnSe QW well butthebarrierpresencesomewhatchangesthesituation.
free carriers in the barrier (Zn0.9Be0.05Mn0.05Se + The barrier weakens the interaction between the ZnSe
Zn0.943Be0.057Se) because the boundary conditions on QWandtheZn0.9Be0.05Mn0.05Selayer. Asaresult,the
the QW edge (Zn0.943Be0.057Se / ZnSe) change. How- spin-flip processes also weaken and the differently polar-
ever, a change of the height of the intermediate and ef- ized L bands gradually drift apart. The data in Fig. 15
4
fective barriers has got a different effect on the shift of clearly show this.
± ±
both L4(1) and L4(2) bands. As soon as an intermedi- Let us examine the L2 band. It is formed by
ate barrier height changes, the L4 band position also the emission transition inside the semimagnetic layer
changes. In case of an effective barrier, the situation Zn Be Mn Se but the presence of the interme-
0.9 0.05 0.05
is different. As it follows from the obtained data, the diate barrier in the structure and its thickness ap-
EV+ edgeshouldgodownappreciablylowerthan preciably changes this band behavior. First of all,
ZnBeMnSe