Table Of ContentSpringer Theses
Recognizing Outstanding Ph.D. Research
Tobias Nowozin
Self-Organized
Quantum Dots
for Memories
Electronic Properties
and Carrier Dynamics
Springer Theses
Recognizing Outstanding Ph.D. Research
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Tobias Nowozin
Self-Organized Quantum
Dots for Memories
Electronic Properties and Carrier Dynamics
Doctoral Thesis accepted by
the Technical University of Berlin, Germany
123
Author Supervisor
Dr. TobiasNowozin Prof.Dieter Bimberg
InstituteforSolid StatePhysics InstituteforSolid StatePhysics
Technical University ofBerlin Technical University ofBerlin
Berlin Berlin
Germany Germany
ISSN 2190-5053 ISSN 2190-5061 (electronic)
ISBN 978-3-319-01969-7 ISBN 978-3-319-01970-3 (eBook)
DOI 10.1007/978-3-319-01970-3
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Supervisor’s Foreword
The past several decades have seen incredible progress in mankind’s ability to
create, process, and store information. This progress was driven by the miniatur-
izationofsilicon-basedsemiconductorcomponentsandcircuits,suchaslogicand
memory nodes, contacts, etc., and relied both on breakthroughs and gradual
improvements of our lithographic tools. In particular semiconductor memories
havebenefittedgreatlyfromdownscaling.Reductiontohalfthefeaturesizeresults
in a fourfold increase in storage density. However, the simple downscaling
approach will soon reach its limits, when feature sizes, like the thickness of
insulating layers, become so small that interface roughness will lead to uncon-
trollable tunneling leakage. Hence, radically new roads must be explored for
continuing the improvement of semiconductor memories in the not so distant
future after 2020. It was the awareness of this threat that led the authors of the
International Technology Roadmap for Semiconductors (ITRS) to demand new
functionalities of memories.
Atpresent,thesemiconductormemorymarketissharedbytwomemorytypes,
each occupying distinct fields of application. The Dynamic Random Access
Memory(DRAM)dominatesasthemainworkingmemoryinpersonalcomputers
and mobile devices, owing to its short access time. Random access, however,
comes at the cost of a short data storage time in the ms-range, making energy-
expensive refresh cycles necessary. In contrast, the Flash memory is non-volatile
andcanstoreinformationformorethanadecadewithoutanyenergyconsumption,
buthas the drawbackof slow write times.Merging the advantagesofDRAMand
Flashwillyieldamemorywithcompletelynewfunctionality:fastaccessandnon-
volatility.
ThepresentPh.D.thesisinvestigatessuchamemoryconceptwhichindeedhas
the potential to merge DRAM and Flash. It uses self-organized III–V semicon-
ductorquantumdots(QDs)indiodestructuresasstoragenodes,andaModulation-
Doped Field-Effect Transistor (MODFET) to access the information. The wide
variety of III–V materials that can be combined to double heterostructures allows
tailoringthebarrierstoretainstoredinformationformanyyears,ifnothundredsof
years.Thusfastwritetimes(thephysicallimitsareinthepicosecondrange)canbe
combined with long storage times (non-volatility). Several of the currently
important challenges are in the center of this thesis: First, novel charge-retaining
quantum dot heterostructures, such as GaSb/GaAs and InGaAs/GaP, with
v
vi Supervisor’sForeword
particularly deep potentials for holes are presented and studied as candidates for
increasingthestoragetime.Combiningexperimentalobservationswiththeresults
ofnumericalcalculationstheauthordiscoversthatthecarriercapturecrosssection
of the quantum dot also plays a decisive role as control parameter for the storage
time.AnequallyimportantachievementisthecouplingofalayerofQDstoatwo-
dimensional hole gas in a p-MODFET structure, allowing the determination of
emissionandcaptureconstantsasafunctionoftemperature.Atlowtemperatures,
for the first time in such a system, many-particle effects are observed, leading to
detailed insight into the energetics of such coupled zero-dimensional/two-dimen-
sional electronic systems.
Berlin, July 2013 Prof. Dieter Bimberg
Abstract
This work investigates the electronic properties of and carrier dynamics in
self-assembledsemiconductorquantumdots(QDs)bymeansofstaticcapacitance–
voltage (C–V) spectroscopy, Deep-Level Transient Spectroscopy (DLTS), and
time-resolvedcurrentmeasurements.Thebackgroundandmotivationistofurther
increasethestoragetimeinamemorydevicebasedonself-assembledQDs,andto
studytheread-outprocessinsuchamemory.Hence,theworkconsistsoftwoparts.
In the first part, the electronic properties, such as the hole localization energy
and the apparent capture cross section, are studied for various material systems.
GaSb/GaAs QDswith andwithoutAl Ga As barriersare studied byDLTS.The
x 1-x
localization energies found for the GaSb/GaAs QDs are between 460(±20) meV
and 760(±20) meV. The maximum localization energy of 800(±50) meV is
reachedwithadditionalAl Ga Asbarriers.Theapparentcapturecrosssections
0.3 0.7
are between 1 (cid:2) 10-12 cm2 and 5 (cid:2) 10-11 cm2. Based on the theory of thermal
emission, a storage time at room temperature between 100 ns and 80 ms is
extrapolatedforthesevalues.Comparedtopreviouswork,thelocalizationenergy
in QDs was increased, but due to a smaller apparent capture cross section, the
storage time is still smaller than previously achieved values. For the first time,
GaSb/GaAsquantumrings(QRs)arestudiedbyDLTS.Asaresultoftheirsmaller
sizecomparedtoQDs,alocalizationenergyofjust380(±10)meVisfound,while
the apparent capture cross section is comparable to the one of QDs. Also for the
first time, a sample based on QDs in a GaP matrix is investigated. A mean acti-
vation energy of 450(±20) meV with an apparent capture cross section of
2 (cid:2) 10-13 cm2 is found for In Ga As/GaAs/GaP QDs, resulting in a room
0.25 0.75
temperature storage time of 3 ls.
In the second part, the coupling of a layer of InAs QDs to an adjacent two-
dimensional hole gas (2DHG) is studied for a series of samples with different
tunneling barriers. C–V measurements and time-resolved current measurements
reveal the level-splittings of the many-particle hole levels in the InAs QDs. The
values are found in good agreement withpredictionsfrom 8-bandk(cid:2)p theory.For
the first time, these measurements are also performed at temperatures above 4 K.
The emission and capture processes between the QDs and the 2DHG are studied
andgiveinsightintothetypeoftheemissionandcaptureprocesses,whichchange
from pure tunneling to thermal-assisted tunneling when the temperature is
increased.
vii
viii Abstract
Basedontheresultsofthiswork,recommendationsfornovelheterostructuresare
given: GaSb/Al Ga As with an Al-content beyond 30 %, GaSb P /Al Ga P
x 1-x x 1-X y 1-Y
QDs, and nitride-based QDs. These materials further increase the localization
energyandthestoragetimeinaQD-basedmemory,possiblytomorethan10years
at300K.Anenhanceddevicedesignoptimizesthedeviceforfastoperationatroom
temperature.
Acknowledgments
Firstandforemost,IwouldliketothankProf.Bimbergforgivingmethechanceto
workonsuchaninterestingsubjectasself-organizedquantumdots,andletmedo
my Ph.D. thesis in his group. Also, I thank the committee chairman Prof.
M. Lehmann, and the second referee Prof. A. Lorke.
There are two colleagues I am particularly indebted to: A. Marent and
M. P. Geller, which have fostered and guided me through the thesis.
In addition, I would like to thank many other people for their support:
A.Högner, L. Bonato,M. Narodovitch, A.Wiengarten, A.Beckel,B. Marquardt,
A. Schliwa, G. Hönig, A. Glacki, G. Stracke, A. Strittmatter, R. J. Young,
M. Hayne, Wei-Hsun Lin, Shih-Yen Lin, C. J. Reyner, Baolai L. Liang,
D. L. Huffaker, K. Schattke, M. Stubenrauch, K. Posilovic, D. Arsenijevic´,
P. Moser, G. Larisch, R. Schmidt, S. Bock, W. Hofmann, E. P. Smakman,
J. K. Garleff, P. M. Koenraad, U. Grupe, R. Koskinas, H. Farrell, I. Rudolph,
D. Nitzsche, S. Ludwig, T. Hoang, H. Schmeckebier, J.-H. Schulze, G. Fiol,
C. Meuer, and the rest of the Bimberg group.
I thank my parents and C. Stoll for their continuous support.
Tobias Nowozin
ix
