Magnetic soft mode behaviour investigated via multi-spin flip Raman spectroscopy on near surface Cd_1tn1_1tn-_1tnxMn_1tnxTe-Cd_1tn1_1tn-_1tnyMg_1tnyTe quantum wells [Elektronische Ressource] / vorgelegt von Christian Kehl

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• Physik

Informations

• Publié par: julius-maximilians-universitat_wurzburg
• Publié le: 01 janvier 2010
• Nombre de lectures: 51
• Poids de l'ouvrage: 3 Mo

Extrait

Magnetic soft mode behaviour investigated
via Multi-Spin Flip Raman Spectroscopy on
near surface Cd Mn Te/Cd Mg Te1−x x 1−y y
Quantum wells
Dissertation zur Erlangung des
naturwissenschaftlichen Doktorgrades
der Julius-Maximilians-Universität Würzburg
vorgelegt von
Christian Kehl
aus Werneck
Würzburg, 2010Eingereicht am: 02. Dezember 2010
bei der Fakultät für Physik und Astronomie
1. Gutachter: Prof. Dr. J. Geurts
2. Gutachter: Prof. Dr. W. Ossau
der Dissertation
1. Prüfer: Prof. Dr. J. Geurts
2. Prüfer: Prof. Dr. W. Ossau
3. Prüfer: Prof. Dr. W. Kinzel
im Promotionskolloquium
Tag des Promotionskolloquiums: 28. März 2011
Doktorurkunde ausgehändigt am: ....................für meine Eltern und GroßelternContents
1 Introduction 1
2 Theory and Basics 5
2.1 Cd(Mn)Te Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1.1 CdTe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1.2 CdMnTe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.2 Semimag. Semiconductors . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.2.1 Mn states . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.2.2 Magnetization . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.2.3 Exchange interaction among Mnons . . . . . . . . . . . . . . . 11
2.2.4 s/p exchange interaction . . . . . . . . . . . . . . . . . . . . . 13
2.3 Quantum Well . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.3.1 CdMnTe/CdMgTe Quantum Well . . . . . . . . . . . . . . . . . 18
2.3.2 Magnetic anisotropy of the valence band . . . . . . . . . . . . . 25
2.3.3 The limit: Total Anisotropy . . . . . . . . . . . . . . . . . . . . 30
2.4 Theory of MultiFRS . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
2.4.1 Raman spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . 32
2.4.2 Macroscopic view . . . . . . . . . . . . . . . . . . . . . . . . . . 33
2.4.3 Microscopic view . . . . . . . . . . . . . . . . . . . . . . . . . . 34
2.4.4 PR Raman scattering . . . . . . . . . . . . . . . . . . . . . . . . 36
2.4.5 MultiR Raman scattering . . . . . . . . . . . . . . . . . . . . 39
2.5 Kavokin’s theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
3 Setup and samples 51
3.1 Twoolour experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.2 QW samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
4 Results: 030807A-I 55
4.1 PL - characteristics - . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
4.1.1 Identification of PLignals . . . . . . . . . . . . . . . . . . . . . 55
4.1.2 Belowarrier excitation . . . . . . . . . . . . . . . . . . . . . . 61
4.1.3 Abovearrier . . . . . . . . . . . . . . . . . . . . . . 66
4.1.4 Twoolour excitation . . . . . . . . . . . . . . . . . . . . . . . . 68
4.1.5 Conclusion of the PL behaviour . . . . . . . . . . . . . . . . . . 69
4.2 Influencing the Mnactor . . . . . . . . . . . . . . . . . . . . . . . . 70
4.2.1 Mnactor determination . . . . . . . . . . . . . . . . . . . . . 70
4.2.2 Varying abovearrier excitation . . . . . . . . . . . . . . . . . . 71
ICONTENTS CONTENTS
4.2.3 Dependence on Beld . . . . . . . . . . . . . . . . . . . . . . . 74
4.2.4 Temperature dependence aspect . . . . . . . . . . . . . . . . . . 75
5 Magnetic soft mode 77
6 Cap impact 83
6.1 13nm cap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
6.2 15nm cap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
6.3 17nm cap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
6.4 19nm cap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
6.5 Conclusion of 031709BIV . . . . . . . . . . . . . . . . . . . . . . . . . 100
7 Discussion PL 105
8 Summary 115
9 Zusammenfassung 119
10 Indices 123
Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
List of pictures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
List of tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
A Phys. & math. add-ons 141
A.1 k p theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
A.2 BRILLOUIN function and band splitting . . . . . . . . . . . . . . . . . 143
A.3 Magnetic polaron in 2D in its exchange field . . . . . . . . . . . . . . . 145
A.4 Bloch equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148
A.5 Auxiliary calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
A.5.1 Differential equation . . . . . . . . . . . . . . . . . . . . . . . . 149
A.5.2 Stationary State . . . . . . . . . . . . . . . . . . . . . . . . . . 151
A.6 QW Exciton spins in an external Beld . . . . . . . . . . . . . . . . . 152
B Formality 153
IIChapter 1
Introduction
"Smaller, faster, cheaper" are the keywords for the development of data storage media
in the chiproducing industries for more than twenty years. Latest fashioned and
promising for discovering a new way of data storage is spintronics. This is a branch
of science and technology which employs both the charge and the spin of the carrier
(electron). This requires materials which exhibit both ferromagnetic and semiconduc
tor properties in order to combine permanent (i.e. stable and nonanishing) magnetic
storage and the conventional electronics of semiconductors in one device [ZLR10].
Basic research was required on lowimensional semiconductor heterostructures with
the most recent focus on nonmagnetic host materials that can be par
2+ 2+tially substituted by a small amount of transitionetal ions like Mn and Co that
promote magnetism.
These studies triggered the ongoing discussion about a carriernduced ferromagnetic
phase transition in dilutedagnetic III semiconductors (DMS) through RKKY
+interaction [HWA 97]. In this context theoretical studies on the coherent dynamics
of localized spins (e.g.: Mnons) coupled with a twoimensional hole gas (2DHG) in
DMS QWs was done by K.V. KAVOKIN [Kav99]. His main result is the time evo
2+lution of the (Mn ) spin system in an inlane magnetic field affected by a 2DHG,
resulting in the reduction of its Larmor frequency and thus of the Mnactor under
the influence of an oscillating effective field of holes. This is called magnetic soft mode
+behaviour [KAK 09].
The experimental access to the Mnactor is Multipinlip Raman scattering, also
called Multiaramagneticesonance (PR) Raman scattering. A lot of research was
2+done on this method by resonantly exciting Mn ons in Cd Mn Te QW structures1−x x
+embedded in Cd Mg Te in the 1990th by STÜHLER et al [SHSW94], [SSD 95],1−y y
+[SSD 96], [Stü95].
This research status is the motivation for this thesis, using MultiR Raman spec
troscopyinthisthesisunderthenewaspectofembeddingaCd Mn Te/Cd Mg Te1−x x 1−y y
QWclosetothesurfaceresultinginanearurfacenduced popingoftheQW.More
over, the hole concentration in the QW is tuned by photoenerated carriers through
abovearrier laser illumination. Simultaneously, MultiR Raman scattering is in
duced by resonant QWxcitation. In these twoolour experiments the carrier influ
ence on the Mnactor is probed. The result is the influence of the MultiR signals
1CHAPTER 1. INTRODUCTION
by photoenerated carrier (holes) using a twoolour experimen t i.e. one light source
exciting above and one resonantly below the QW barrier.
Figure 1.1 gives an overview of the new aspects done in this thesis (red) and the
already existing research procedure by STÜHLER et al (black).
For the experiment a resonant monochromatic light source is needed to serve as
experiment material result physical
meaning(semimagnetic)
excitation semiconductor
in QWresonant
nano structure(SF-probe)mono-
&
surface-inducedchromatic variation of
-1p-doping Raman-shift / [cm ]light source of resonant
(cap-thickness d)
light source
2+ B / [T]intensity Mn Multi-
2+Spin-Flips Mn -Voigt-
Quantum Wellmagnetic field geometry g-factor
QWB|| QW-layerB +{
influence of
1.7 - 4.2K magneticthe
variation of photoluminescence soft-mode
variable of excitoncap thickness d behaviour
excitation and triontemperature
2.0above E CdMgTeCdMgTe
influence ofCdMnTeQW-barrier
(photo-generated)&
carrieradditional variation of
concentrationlight source light source
(2DHG)
intensity 1.9ond
2+(charge carrier Mn Multi-
influence) Spin-Flips Hole concentration
z
Figure 1.1: Chart of the main research blocks used for achieving the interesting results in
this thesis. The first blocks present the requirements to both the experiment and the material
for observing and investigating the Multipinlip signals in general (black). The additional
requirements for observing the soft mode behaviour via Multipin Flip Raman spectroscopy
are marked red.
a probe for the Multipin Flips which appear at small temperatures (here: 1.7 -
4.2K) and in an applied magnetic field perpendicular to the growth direction (z) of the
semimagnetic QW. The twoimensional hole gas (2DHG) in the QW is induced by
embedding the QW close to the surface (cap layer thickness: 139nm) resulting in a
nearurface induced poping of the QW. This 2D