physics, chemistry and application of nanostructures. reviews and short notes to nanomeeting 2003, 2003, p.596

Talapin Non-linear optical properties of IV-VI semiconductor quantum dots 136 A... Arshinov NANOSTRUCTURE BASED DEVICES InGaN/GaN quantum wells: fabrication, optical properties and app

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Physics, Chemistry

a n d A p p l i c a t i o n

of Nanostructures

Reviews and Short Notes to Nanomeeting 2003

Minsk, Belarus 20 - 23 May 2003

Editors

V E Borisenko Belarusian State University of Informatics and Radioelectronics, Belarus

S V Gaponenko Institute of Molecular and Atomic Physics, Belarus

V S Gurin Belarusian State University, Belarus

Published by

World Scientific Publishing Co Pte Ltd

5 Toh Tuck Link, Singapore 596224

USA office: Suite 202, 1060 Main Street, River Edge, NJ 07661

UK office: 57 Shelton Street, Covent Garden, London WC2H 9HE

British Library Cataloguing-in-Publication Data

A catalogue record for this book is available from the British Library

PHYSICS, CHEMISTRY AND APPLICATION OF NANOSTRUCTURES

Reviews and Short Notes to Nanomeeting 2003

Copyright © 2003 by World Scientific Publishing Co Pte Ltd

All rights reserved This book, or parts thereof, may not be reproduced in any form or by any means, electronic or mechanical, including photocopying, recording or any information storage and retrieval system now known or to be invented, without written permission from the Publisher

For photocopying of material in this volume, please pay a copying fee through the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, USA In this case permission to photocopy is not required from the publisher

ISBN 981-238-381-6

(Marseille, France)

INTERNATIONAL ORGANIZING COMMITTEE

BELARUSIAN NATIONAL ORGANIZING COMMITTEE

FOREWORD

The first years of the XXI-st century have brought new fundamental knowledge and novel applications of nanostructures Nanoelectronics and nanophotonics, bioinformatics and molecular electronics are extensively progressing on the basis of recent achievements in nanotechnology The results obtained are discussed at

NMOMeemc;-2001 (20-23 May, 2003), which is the International Conference on

Physics, Chemistry and Application of Nanostructures traditionally organized each two years in Minsk (Belarus)

The book that you keep in your hands collects invited reviews and short notes

of contributions to NANOMEEWG-2001 The papers in the book are arranged in

traditional sections: Physics of Nanostructures, Chemistry of Nanostructures, Nanotechnology and Nanostructure Based Devices Both basic and applied researches are presented Among different results characterizing our knowledge about the nanoworld, one can note an increased interest to Ge/Si quantum dot systems, photonic crystals, carbon nanostructures, biological molecules, self-scrolled semiconductors, epitaxial GaAIN onto Si Their indeed astonishing properties promise a birth of novel approaches to information processing Scanning probe techniques and nanochemistry, self-organization and self-assembling have got new i mpetus to be applied in nanotechnology The examples can be found in the book The style of the presentations has been mainly preserved in its original form

We deeply acknowledge Sponsors provided the financial support for the Conference

Victor E Borisenko Minsk and Marseille Francois Arnaud dAvitaya January 2003

Co-chairmen of NANOMEE11NC;-2001

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CONTENTS

Foreword vii PHYSICS OF NANOSTRUCTURES

Si/SiGe nanostructures: challenges and future perspectives (invited) 3

D Grutzmacher

Spin resolved inverse photoemission from layered magnetic

nanostructures (invited) 11

R Bertacco, L Dud, M Marcon, M Portalupi, F Ciccacci

Nonlinear optical properties of one-dimensional photonic

crystals (invited) 19

C Sibilia, G D 'aguanno, M Centini, M C Larciprete,

M Bertolotti, M Scalora, M Bloemer

Tunable three-dimensional photonic crystals based on opal-V02

A V Gur'yanov, M I Samoilovich, M Yu Tsvetkov,

E B Intushin, Yu I Chigirinskii

Time-resolved luminescence of europium complexes in bulk and

nanostuctured dielectric media 43

E P Petrov, D A Ksenzov, T A Pavich, M I Samoilovich,

A V Gur'yanov

Synchrotron investigations of electron-energy spectra in silicon

nanostructures 47

E P Domashevskaya, V A Terekhov, V M Kashkarov,

E Yu Manukovskii, S Yu Turishchev, S L Molodtsov,

D V Vyalikh, A F Khokhlov, A I Mashin, V G Shengurov,

S P Svetlov, V Yu Chalkov

Strain-induced photoluminescence red shift of InGaAs /GaAs

microtubes 51

A V Vorob'ev, V Ya Prinz, A I Toropov, A A Lutich,

A A Gaiduk, S V Gaponenko, D Grutzmacher

Effects of doping and nonradiative defects in GaAs superlattices 55

V K Kononenko, D V Ushakov, H W Kunert

Scattering matrix approach to large-scale photonic crystal circuits 59

S Mingaleev, K Busch

Asymptotic analysis of radiation pattern of a classical dipole in a photonic

crystal: photon focusing caustics 64

D N Chigrin, C M Sotomayor Torres

Conservation laws for the integrated density of states in arbitrary

quarter-wave multilayer nanostructures 68

S V Zhukovsky

Peculiarities of light transformation in finite three-layered periodic

nanostructures 72

S N Kurilkina, M V Shuba

Laser pulse compression in Fibonacci multilayer nanostructures 76

L N Makarava, S V Zhukovsky, A V Lavrinenko

Synthesis of thin film titania photonic crystals through

an infiltrating sol-gel process 80

S Kuai, X Hu, G Bader, S Badilescu, V V Truong

FTIR study of vertically etched silicon as ID photonic crystal 84

V Tolmachev, E Astrova, T Perova

Large optical anisotropy of ID photonic crystal fabricated

by vertical etching of silicon 88

E V Astrova, V A Tolmachev, A D Remenyuk, T S Perova

Optical diagnostics of nanometer dielectric Alms by combining

ellipsometry and differential reflectance 96

P Adamson

Photonic and nonlinear-optical media based on nanostructured

semiconductors 100

P K Kashkarov

Optical properties of multilayer heterostructures based on ZnSe/ZnS 103

P I Kuznetsov, G G Yakushcheva, V I Kovalev, M V Ermolenko

Confined optical vibrations in ZnSe quantum dots 107

A I Belogorokhov, L I Belogorokhova, V Yu Timoshenko, P K Kashkarov

Intradot carrier relaxation in radiation-damaged InGaAs/GaAs

quantum dot heterostructures I l l

A Cavaco, M C Carmo, N A Sobolev, F Guffarth, H Born,

R Heitz, A Hoffmann, D Bimberg

Enhanced photoluminescence of Tb3+ and Eu3+ induced by energy

H Elhouichet, L Othman, A Moadhen, M Oueslati, M Ferid,

B Canut, J A Roger

Whispering gallery mode emission from a core-shell system of CdTe

nanocrystals on a spherical microcavity 120

Yu P Rakovich, J F Donegan, N Gaponik, A L Rogach

Photoluminescence up-conversion in CdTe nanocrystals 124

K I Rusakov, A A Gladyshchuk, D Talapin, A Eychmuller

Enhanced photoluminescence of semiconductor nanocrystals

near metal colloids 12$

O S Kulakovich, M V Artemyev, A Yaroshevich, S Maskevich

Evolution of optical phonons in CdSe/ZnS quantum dots:

Raman spectroscopy 132

A V Baranov, T S Perova, A Moore, Yu P Rakovich,

J F Donegan, D Talapin

Non-linear optical properties of IV-VI semiconductor quantum dots 136

A M Malyarevich, V G Savitsky, N N Posnov, K V Yumashev,

A A Lipovskii, E Raaben, A A Zhilin

Synchrotron investigations of electron-energy spectra in

III-V nanostructures 140

E P Domashevskaya, V A Terekhov, V M Kashkarov,

S Yu Turishchev, S L Molodtsov, D V Vyalikh, Zh I Alferov,

I N Arsentyev, I S Tarasov, D A Vinokurov, V P Ulin

Luminescence of Ge/Si quantum dots subjected to radiation

damage and hydrogen passivation 144

A Fonseca, J P Leitao, H Presting, H Kibbel

Raman scattering of zeolites under low-intense visible excitation:

role of reduced Cu cluster incorporated in zeolites pores 148

N Strekal, V Petranovskii

Surface plasmon resonances and light selection in metal-dielectric

nanostructures of various spatial arrangement 151

A D Zamkovets, S M Kachan, A N Ponyavina

Optical nonlinearity of copper nanoparticles synthesized by ion

implantation in silicate glass 155

A L Stepanov, R A Ganeev, A I Ryasnyansky, T Usmanov

The optical response of silver island films embedded in fluoride

and oxide optical materials 158

O Stenzel, P Heger, N Kaiser

Properties of nano-sized particles formed during double-pulse

laser ablation in liquids 163

V A Ageev, V S Burakov, A F Bokhonov, S N Isakov, M I Nedel'ko,

V A Rozantzev, N V Tarasenko

Mn photoluminescence kinetics in quantum dots 167

L I Gurinovich

Field enhancement near the annealed nanostructured gold

detected by optical spectroscopy with the probe biomolecules 171

N Strekal, V Askirka, S Maskevich, I Sveklo, I Nabiev

Planar Cu nanostructure: experimental and theoretical integral

light scattering characteristics 175

A Ya Khairullina, T I Ol'shanskaya, V A Babenko, V M Kozhevin,

D A Yavsin, S A Gurevich, S M Kachan

High-order harmonic generation by carbon nanotubes: density matrix

V V Uglov, Y Pauleau, F Thiery, J Pelletier, V M Anishchik,

A K Kuleshov, M P Samtsov, S N Dub

Electronic structure of metallic single-wall carbon nanotubes:

tight-binding versus free-electron approximation 186

N A Poklonski, E F Kislyakov, S L Podenok

Conductivity of metal - linear carbon chains with metal

inclusions - metal structures 190

D G Kolomiets, O M Ivanyuta, A D Gorchinskiy,

E V Buzaneva, P Scharff

Influence of Si(lll)-[(V3xV3)/30°]-Cr surface phase on growth and

conductivity of disordered iron 2D layers on Si(lll) 194

Af G Galkin, S A Dotsenko, S Ts Krivoshchapov, D L Goroshko

Modelling vertical tunneling in semiconductor multiple quantum well

structures: effect of the disorder in layer parameters 198

A V Dmitriev, O V Pupysheva

Electronic properties of nanocrystalline chromium disilicide 201

V L Shaposhnikov, A E Krivosheev, A B Filonov

Conductivity oscillations during formation of disordered 2D Yb layers

on Si(l 11) 206

N G Galkin, S A Dotsenko, D L Goroshko, S A Gouralnik,

A N Boulatov

Anisotropy of energy spectrum and transport properties of 2D

carriers in uniaxially strained GaAs/AIGaAs 210

E V Bogdanov, N Ya Minina, A V Polyanskiy, A M Savin,

O P Hansen, C B Sorensen

The photon-assisted transport in mesoscopic devices 214

A H Aly

Electron beam scattering from potential fluctuations in a

two-dimensional electron gas 219

E G Novik, H Buhmann, L W Molenkamp

Correlation of morphology and electrical conduction in

nanostructured perylene pigment films 223

A N Lappo, A V Misevich, A E Pochtenny, O M Stukalov,

G K Zhavnerko

Effect of doping concentration on the electron-phonon coupling

in degenerate silicon film 227

P Kivinen, A Savin, P Torma, J Pekola, M Prunnila, J Ahopelto

Conduction of nanowires formed between metallic electrodes 231

W Nawrocki, M Wawrzyniak

Oxidized silicon nanoclusters: a theoretical study 235

M Luppi, S Ossicini

D I Tetelbaum, O N Gorshkov, S A Trushin, A N Mikhaylov,

D G Revin, D M Gaponova, S V Morozov, G A Kachurin,

S G Yanovskaya

Composite nanostructures based on porous silicon host 244

V Bondarenko, G Troyanova, M Balucani, A Ferrari

Nanoporous anodic oxide on aluminum - titanium alloys 249

5 K Lazarouk, A A Leshok

Birefringence and photonic band gap in porous alumina films 253

V A Melnikov, G M Zaitsev, L A Golovan, V Yu Timoshenko,

P K Kashkarov, S A Gavrilov, D A Kravchenko

Anisotropic light scattering by porous anodic alumina 256

A A Lutich, I S Molchan

Photoluminescence excitation spectroscopy of erbium incorporated

with iron in oxidized porous silicon 260

V Bondarenko, N Kazuchits, M Balucani, A Ferrari

Impurity states in implanted porous anodic alumina 264

N N Cherenda, G V Litvinovich, A L Danilyuk

Evidence for energy transfer between Eu and Tb in

porous silicon matrix 268

A Moadhen, H Elhouichet, B Canut, C S Sandu, M Oueslati,

J A Roger

Electroluminescent xerogels fabricated in porous anodic alumina 273

/ S Molchan, N V Gaponenko, D A Tsyrkunov, J Misiewicz,

R Kudrawiec, V Lambertini, P Repetto

Periodic nanostructures fabricated by anodic oxidation

of valve metal films 277

V Sokol, A Vorobyova, E Outkina

Optical spectroscopy of porous composites with Si nanocrystals 281

A Gorchinskiy

Magnetic properties of nanoparticles formed in sol-gel films

by ion irradiation or thermal processing 285

J C Pivin, E Vincent

Deposition of nanoparticles on a cold substrate from a laminar gas flow 291

S P Fisenko, A I Shnip

Commensurate long-period nanostructures in alloys 294

S V Eremeev, O I Velikokhatnyi, I I Naumov, A I Potekaev,

V V Kulagina, V N Udodov

Chromatic polarization conversion of terahertz radiation by

a density-microstructured two-dimensional electron system 298

V V Popov, O V Polischuk

Exciton-phonon coupling of localized quasi-2D excitons

in semiconductor quantum well heterostructures 302

/ V Bondarev, S A Maksimenko, G Ya Slepyan,

I L Krestnikov, A Hoffmann

Lattice matching between bulkRu2Si3 and nanocrystalline RuSi2 306

L I Ivanenko, V L Shaposhnikov, E A Krushevski

CHEMISTRY OF NANOSTRUCTURES

Nanocluster superlattices grown at solution surfaces (invited) 313

Excitonics of I-VII semiconductors (invited) 320

Immunolabeling of membrane proteins and cells by highly

fluorescent cadmium selenide nanocrystals 331

M Artemyev, V Oleinikov, D Klinov, I Bronstein, W Offen,

A Sukhanova, J Devy, H Kaplan, I Nabiev

Luminescent coding by quantum dots: microcapsules loaded with

semiconductor nanocrystals 335

A Rogach, N Gaponik, I Radtchenko, H Weller

In vitro bioactivity testing of Z r 02 nanopowders prepared by

MW-assisted hydrothermal synthesis 338

F Bondioli, S Braccini, C Leonelli, G C Pellacani,

G Lusvardi, G Malavasi

Copper nanoparticles within amorphous and crystalline

dielectric matrices 342

V S Gurin, D L Kovalenko, V P Petranovskii

UV-visible characterization of gold clusters and nanoparticles

in beta zeolite 346

/ Tuzovskaya, N Bogdanchikova, M Avalos, A Simakov,

A Pestryakov

Manganese carbonate particles preparation by colloidal aggregation

for hollow polyelectrolyte capsules fabrication 349

Yu A Fedutik, A A Antipov, G B Sukhorukov

Impurity molecule trapping in growth of nanoparticles by

deposition from gas phase 353

V V Levdansky, J Smolik, P Moravec

Formation of nanopores and coagulation of nanograins

in cemented tin films 357

T N Vorobyova, A S Tselesh

Comparative DFT calculations of silver clusters

D K Ivanov, N P Osipovich, S K Poznyak, E A Streltsov

Investigation of monolayers by potentiodynamic electrochemical

impedance spectroscopy 373

G A Ragoisha, A S Bondarenko

Self-forming of silicon surface nanorelief near edges of chemical

masks during anisotropic etching 377

K A Soldatenko, A V Yukhnevich

Formation of silver nanoparticles from a

(2,3-dyhydroxy-4,6-di-tert-butylphenylthio-)acetic acid silver complex 381

M C Parfenova, V E Agabekov, A A Chernyavskaya,

N V Loginova, G I Polozov

S V Serezhkina, G P Shevchenko, S K Rakhmanov

Sol-gel synthesis of Fe-containing silica glasses 389

A A Boiko, E N Poddenezhny, V A Boiko, L V Sudnik

Structure and optical properties of CdSexTei.x in glass matrix 392

/ V Bodnar, V S Gurin, A P Molochko, N P Solovei

Formation and optical properties of ultrafine I-III-VI2 particles

in silicate glass matrices 396

/ V Bodnar, A P Molochko, N P Solovei

Structure evolution during laser sintering of fine powders 400

M K Arshinov, A N Tolochko

Peculiarities of electrochemical synthesis of nanosized Si02 films 403

/ L Baranov, L S Stanovaya, L V Tabulina

Inorganic particles formation in nanoengineered polymer capsules 407

D G Shchukin, G B Sukhorukov

Nanocrystalline perovskite-like Sr-Ba-Fe-Co oxides: stability

under reducing conditions 411

M I Ivanovskaya, L S Ivashkevich, A S Lyakhov, 1.1 Azarko,

V V Zyryanov, N F Uvarov

Synthesis and behavior of monomolecular films from

2,4-heneicosanedione and its metallocomplex 415

/ V Paribok, G K Zhavnerko, V E Agabekov, T Ondarcuhu

Cluster mechanisms of nanocrystal formation 419

Structure and nanohardness of PVD composite nanosized Ti-Zr-N films 429

V V Uglov, V V Khodasevich, S V Zlotski, Zh L Prikhodko, S N Dub

Synthetic approach for preparation of nanometer-sized non-linear

optical advanced materials 433

V V Lisnyak, N V Stus, R M Barabash, S A Alekseev,

M S Slobodyanik, P Popovich, D Stratiychuk

NANOTECHNOLOGY

Germanium quantum dots in Si02: fabrication and

characterization (invited) 439

A Nylandsted Larsen, A Kanjilal, J Lundsgaard Hansen, P Gaiduk,

N Cherkashin, A Claverie, P Normand, E Kapelanakis, D Tsoukalas,

K.-H Heinig

Mechanisms of island vertical alignment in Ge/Si(001) quantum-dot

multilayers (invited) 447

V Le Thanh

Enhanced luminescence of lanthanides from xerogels in

porous anodic alumina (invited) 460

N V Gaponenko

Advanced scanning probes as applied to self organized organic

systems (invited) 468

H Fuchs

New precise nanostructures: semiconductor shells and their

well ordered arrays 471

V Ya Prinz

Characterization of nanocrystalline silicon films by beam induced

current in the scanning tunneling microscope 475

E Nogales, B Mendez, J Piqueras, R Plugaru

Pulsed laser annealing of germanium nanoclusters in silicon 478

V A Volodin, A V Dvurechenskii, M D Efremov, A I Nikiforov,

A I Yakimov, E I Gatskevich, G D Ivlev, D A Orehov

Regular structures on silicon surface formed under compression

plasma flow 481

V M Astashynski, S I Ananin, V V Askerko, E A Kostyukevich,

A M Kuzmitski, S P Zhvavy, J Puric, M M Kuraica, I Dojcinovic,

JV A Sobolev, G D Ivlev, E I Gatskevich, D N Sharaev,

J P Leitdo, A Fonseca, M C Carmo, A B Lopes,

V V Shvartsman, A L Kholkin, H Kibbel, H Presting

AFM investigation of highly ordered nanorelief formation by anodic

treatment of aluminum surface 500

S A Gavrilov, V M Roschin, A V Zheleznyakova, S V Lemeshko,

B N Medvedev, R V Lapshin, E A Poltoratsky, G S Rychkov,

N N Dzbanovsky, N N Suetin

Quasi-ID channels in Si delta-doped GaAs grown on vicinal

(111)A GaAs substrates 503

V A Rogozin, V A Kulbachinskii, V G Kytin, R A Lunin,

A V Derkach, I S Vasil'evskii, G B Galiev, V G Mokerov

Nucleation of superconducting phase in multilayered nanostructures 507

S L Prischepa, V N Kushnir, M L Delia Rocca, C Attanasio

Ceramic filter materials with graded micro/nanoporous structure

fabricated by laser sintering 512

N K Tolochko, M K Arshinov

NANOSTRUCTURE BASED DEVICES

InGaN/GaN quantum wells: fabrication, optical properties and

application in light emitting devices (invited) 517

G P Yablonskii

Carbon nanotubes in microelectronic applications (invited) 525

F Kreupl, G S Duesberg, A P Graham, M Liebau, E Unger,

R Seidel, W Pander, W Honlein

Quantum-confined impurities as single-atom quantum dots:

application to terahertz emitters (invited) 533

P Harrison, M P Halsall, W -M Zheng, J -P R Wells,

I V Bradley, M J Steer

InGaN/GaN quantum well heterostructures grown on silicon for

UV-blue lasers and light emitting diodes 541

G P Yablonskii, E V Lutsenko, A L Gurskii, V N Pavlovski,

V Z Zubialevich, H Kalisch, A Szymakowski, Y Dikme,

R H Jansen, J F Woitok, B Schineller, M Heuken

Electrical properties of DNA-based switching diode 545

J A Berashevich, A B Filonov, V E Borisenko

Nano-size Sn0 2 films deposited by SILD method: structural and

gas response characterization 549

G Korotcenkov, V Macsanov, V Brinzari, V Tolstoy, J Schwank

Electrical conductivity and electroluminescence of planar nanocomposite

structures: gold island film - aluminum oxyquinoline 553

R D Fedorovich, T A Gavrilko, A A Marchenko, O V Mirzov,

V B Nechytaylo, G A Puchkovskaya, L V Viduta, A G Vitukhnovsky,

A G Naumovets

Textured porous silicon for efficient light detection in UV, VIS

and NIR spectrum ranges 557

N N Vorozov, V A Yakovtseva, S A Volchek, P S Smertenko,

T Ya Gorbach, V P Kostyhv

Relaxation processes in rare Earth doped crystals as studied by high

resolution fourier spectroscopy (invited) 560

M N Popova, B Z Malkin

Author index 569

PHYSICS OF NANOSTRUCTURES

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Embedding SiGe and Ge quantum structures into the Si host crystal opens up new paths for the integration of ultra fast electronic and opto-electronic devices into the mature Si micro- electronics In this paper some of these paths are discussed and the challenging problems of materials research are addressed Special emphasis is put on the Dot-FET concept and on the possibility of light emission from Si/SiGe quantum structures

1 Introduction

In 1952 H Welker discovered the semiconducting properties of III/V compounds [1], leading to a lot of enthusiasm about future applications Today, these materials dominate clearly the market for opto-electronic devices and are widely used for high speed microelectronics [2,3] In fact, the appearance of III/V semiconductors was an additional stimulation for the development of ever faster and smaller Si devices In particular, the concept of hetero- and quantum well structures opening the field of band gap engineering, was extremely fruitful for the progress of III/V opto-elec-tronic and high frequency devices [4] With the introduction of SiGe this path be-came also available for the Si technology The invention of the Si/SiGe hetero-bipolar transistor (HBT), allowed the design of Si based high frequency devices and HBTs exceeding transit frequencies of 200 GHz have been realized [5] These de-vices now enter the domain of wireless communication technology, even though for high end applications III/V devices are still superior [6] More recently, Si channels with tensile strain embedded in a relaxed SiGe lattice draw a lot of attention, due to the high electron mobility in the strained Si and the potential compatibility with Si CMOS technology [7] Thus, SiGe technology has entered the roadmap for the de-velopment of future generations of Si microprocessors However, several obstacles have to be circumvented before this technology may become available Typically, a heavily dislocated SiGe buffer layer is introduced to account for the lattice mis-match between the Si substrate and the relaxed SiGe film, which carries the strained

Si layer [8] The high amount of threading dislocations, the reduced thermal ductivity of SiGe compared to Si and the necessity to integrate p- and n-type devices on the same chip certainly are challenging problems for this technology

con-Si is an indirect semiconductor, thus not suitable for the fabrication of electronic devices Adding optical functionality to Si microelectronics is one of die most challenging problems but may revolutionize communication technology [9,10] The key device would be an efficient emitter, i.e a laser

opto-In this paper the potential to address some of this obstacles and challenges by using Ge quantum dots and Si/SiGe quantum well structures are discussed with respect to applications in micro- and opto-electronics

2 Perspectives of Ge quantum dot structures

Ge dots on Si (100) assemble via the Krastanov mode of growth Typically, two shapes can be distinguished, "hut" cluster with a square

Stranski-or elongated shape and (105) side facets [11,12] and multiple facetted "dome" cluster [13] The latter ones occur typically at higher temperatures and Ge coverages than the hut cluster High densities and small cluster sizes are achieved at deposition temperatures < 500°C Depending on the growth temperature, the diameter' of Ge islands can be controlled, in the range from 15-

300 nm If pre-deposition of C is used smaller sizes are possible and the temperature dependence is relaxed [14] Fig 1 shows a typical in-situ STM image [15] of a Ge hut cluster deposited by molecular beam epitaxy (MBE) at 520°C using 4.8 ML of Ge The size of this island

is about 60x60 nm These islands harbor a complex strain distribution [16] The top of the islands is largely relaxed exhibiting a larger :than

Si lattice constant Consequently, after embedding the islands in Si by overgrowth, the Si

in the vicinity of the dot is strained The strain field is easily visible in the cross sectional TEM micrograph presented in Fig 2 The image shows two islands stacked on top of each, other The strain field induces the nucleation of the second island on top.of the first [17]

Remarkably, the size of these Ge islands is in the range, of the gate length and width.of next generation CMOS transistors [7] Accordingly, it has been proposed

to use the strained Si on top of islands to create a channel for electrons and the Ge island itself to create a channel for holes [18] This concept of a dot based field ef-fect transistor (Dot-FET) would harbor fast p- and n-type devices on the same

Figure 1 STM image of a 60x60 nm Ge

hut cluster deposited by MBE at 520°C

^iS!

Figure 2 THM micrograph of stacked

Ge islands imaging the strain fields

structure In addition, since no relaxed SiGe buffer layer is needed, the related

problems due to the high threading dislocation density and the low thermal

conduc-tivity of relaxed SiGe buffers can be circumvented The use of Ge dots in FETs requires exact positioning of ordered dots at preset locations Recently, it has been demonstrated that by using shallow grooves such a positioning of individual dots is

possible [19]

Further constrains of this concepts may arise from the non-uniformity of the strain field above the dots So far only limited knowledge about the size, the uniformity and the strength of the strain fields induced by Ge dots is available Certainly the lateral dimensions of the Ge dots have to exceed the dimensions of the gate of the transistor It is well known, that Ge dots intermix during the overgrowth with Si, leading to a shape transformation by transporting material from the top to the pedestal of the dot [20] It can be expected that this shape transformation leads

to an increase in the uniformity of the strain-field on top of the dot, but at the same time the intermixing reduces Ge concentration of the dot and thus lowering the strain in the Si cap Future research has to focus on the relation between shape and strain fields of Ge dots and the impact of non-uniformity strain, distributions within the channel on the' performance of the device Certainly dots having a high Ge concentration and- thus inducing strong strain fields and providing rather uniform strain fields at the same time would be most beneficial

Currently detailed studies on the intermixing during the early states of growth of Ge dots by Si are performed The intermixing can be reduced by lowering the growth temperature or by using a surfactant In the latter case, growth was inter-rupted after the Ge dot deposition (720°C) and the sample was transferred' to a neighboring chamber equipped with a hydrogen plasma source The surface was covered with H and than growth was resumed in the MBE chamber at 500°C Pig 3 shows a cross sectional TEM of

over-this sample exhibiting a Ge

island of nearly 300 nm in

diameter and 30 nm in height

The island preserved the shape

of a dome cluster, as indicated F"g«re 3 Ge dome cluster embedded In SI using a H

by the still present facets, surfacta°t layer

suggesting that no intermixing occurred during the overgrowth It can be predicted that such an island induces a strong strain field in the Si cap layer

Fig 4 shows STM views of Ge islands during the early stages of Si growth, a) uncapped dots, b) after 1 monolayer (ML) and c) after the deposition of

over-5 ML of Si at 300 and 340°C respectively The uncapped sample exhibits dome and hut cluster The islands preserve their shape after a capping of 5 ML, in contrast to experiments performed with an overgrowth temperature of 620°C [20] However, after the deposition of 1 monolayer of Si a new type of small quadratic islands oc-curs, which is rotated by 45° in comparison to the normal Ge hut cluster It is as-sumed that these islands consist of Si and that at these low temperatures the Si does

not intermix with the Ge islands or Ge wetting layer Most likely Si does not wet Ge uniformly and 3-dimensional growth occurs The lack of intermixing for low tem-perature o¥ergrowth is also indicated by photoluminescence (PL) spectra [21] Dot structures emitting at energies < 650 meV indicate the presence of Ge islands under compressive strain with a Ge concentration close to 100% The strong confinement

of holes in these islands opens a new path to enhance the luminescence efficiency of

Ge quantum dots for opto-electronics

iiu: roiii/atiun of a Ge Oot-H;.T requires exact alignment ul'ihe i»,a'.e on n>p **••

? *> * i

tfj>uru 4 S'l M linage^ of Gc dome and hut cluster deposited al 620°C and owr-prowi wuli r.; • ML h)

3 ML ( 3 0 0 U C ) and c) 5 ML (340 0 C) of Si Black arrows point to cluster, which arc rotated by 45° with respect to Ge hut clusters

the buried Ge dot This might be achieved by a self alignment using the effect of vertical stacking of Ge islands as illustrated in Fig 2 The self alignment of gates may be obtained as follows On top of the structure a layer of uncapped Ge islands

is deposited, which align to the buried islands Next, a sacrificial layer is evaporated under shallow angles, leaving a side facet of the surface dots uncovered The Ge dots can be selectively etched opening up windows for the gate stack deposition The gate layers can be lifted from the field areas by etching the sacrificial layer [22]

3 Perspectives of Si/SiGe quantum well structures

In this chapter the focus is put on novel application for Si/SiGe quantum well structures aside from the applications in CMOS and HBT devices discussed in the infroduction and references therein

J 1 Self scrolling Si/SiGe micro- and nanotubes

The scaling down of sizes for high speed, large integration Si microelectronic vices not only puts constrains on transistors, but also on capacitors and coils, which are up to now rather spacious devices The self scrolling process of strained layer hetero- and quantum well structures [23,24] offer routes to considerably reduce the area consumed by these devices The self-scrolling of strained bilayers has been

de-Etch

Figure 5 Schematic view of the self-scrolling process of SiGe/Si bi-layer (Fig courtesy of

V Prinz)

demonstrated for III/V as well as for

Si/SIGe films The concept is schematically

illustrated in Fig, 5 The structure consists

of an undoped Si film and a p ^ SiGe/Si

bi-layer The thickness of Si and SiGe as well

as the Ge concentration determines the

diameter of the tube Typically film widths

of 1-100 nm at a Ge concentration of

10-40% have been used in our experiments

[25] For applications as capacitors or coils,

the bilayers have to be additionally coated with an insulating and/or a metal layer Here we introduce a simple process to fabricate Si/SiGe/Cr nanotubes

The pseudomorphic Si0.8Ge0ySi (12/50 nm) heterostructure heavily doped with boron (5 TO19 cm"3) was grown on a (001) n-type Si-wafer at 400°C by MBE Afterwards, a 20-nm thick chromium layer was evaporated onto the structure The choice of chromium as an upper-layer material was motivated by the fact that the use of this metal for preparing electron-beam lithography masks represents a well-established technology, and chromium is stable to alkaline etchants, harboring simultaneously high internal stress when deposited onto silicon Electron lithogra-phy was used to define a pattern on the surface of the initial planar structure Fig 1 illustrates the mask used After the resist has been developed the pattern was trans-ferred into the underlying SiGe/Si/Cr film using Cl2 and SF6 reactive ion etching (RIE) Next, the structure was dipped into KOH to form defined facets This dip is very crucial, since it determines from which side of the mesa the structure starts to scroll during the dip in the subsequent etch in the aqueous 3.7% NH4OH solution Without the dip the result would be a ring like structure.[25]

The correct orientation of the mesa and the dip in the KOH etch leads to the formation of a (111) facet along the short side of the mesa The (111> facet is not attacked by the selective etch and thus the scrolling is initiated from the long side The highly strained SiGe/Si/Cr structure is detached from the substrate The Cr film

deposited onto Si is under tension, while the SiGe layer is under compression As

a result, the free-standing SiGe/Si/Cr structure undergoes bending due to the internal elastic stress and forms the microtube depicted in Fig 6 The length

of the tube is >200 ftim and the diameter amounts to 4 |im About 80% of the length of the tube is completely detached from the substrate Future work will concentrate on transferring this technology to much smaller tubes The fabrication of capacitors will also Figure 6 I«'ive standing, >200 j.un lung Si/SiGc

microtobe, 0 4 |im

require an insulating film separating the p4"* bilayer and the Cr film The growth of multiple bilayers potentially permits the fabrication of dense arrays of tubes Finally

it has to be pointed out that this technology may also be used for the fabrication of biosensors as well as microfluidic and micromechanic devices

Figure 7, Illustration of the working frequencies of

semiconductor devices unravelling the THz gap SiGe

QC lasers would have the potential to fill this gap

(figine courtesy of D Paul)

3.2 Quantum cascade structures

Even though quite intense photoluminescence has been observed in Si based nanostructures and optical gain has been reported for

Si clusters embedded in an 5i02

matrix [26], the goal to achieve strong electroluminescence, i.e fabri-cating a Si based laser, has been out

of reach so far for concepts based on interband transistions Quantum cascade (QC) laser structures, which were first demonstrated using III/V quantum wells [27], appear to be a powerful alternative also for Si Si/SiGe quantum cascade lasers have the potential to fill the so called THz-gap for wavelengths ranging from 20

to 100 jun as illustrated in Fig 7 Big portions of the THz regime are not accessible for III/V devices, due to the strong interaction with phonons But, strong absorption lines of many molecules in the THz regime allow for interesting applications in medicine, biology and chemistry, besides the potential of' Si based opto-electronic devices for communication technology

First results on pseudomorphic Si/SiGe QC structures were promising and electroluminescence was observed in the mid-IR regime [28] To expand the design freedom, increase the number of cascades and to incorporate a waveguide the concept was transferred

to strain compensated Si/Sio.2Ge0.8 QC structures grown on relaxed Si0.5Ge03 buffer layers [29] Following the bound to continuum design [30] structures with up to 30 periods were deposited using MBE at 300°C and a rate of 0.25 A/s The design

Figure 8 Tl:M micrograph of 30 period QC structure

grown by MBE at 300°C, a) overview and b)

5 cascades at the bottom of the structure

using a chirped superlattice with increasing Si barrier width and a simultaneous decrease of the SiGe well width within the cascade, shifts the HH ground state from well to well to higher in energy At a bias voltage of 70 kV/cm these states will line

up and the minibands are formed Note that only ground states are involved in this design A cross sectional view taken by TEM of a structure containing 30 cascades, each containing 28 individual layers ranging in width from 0.4 to 2.8 nm is depicted

in Fig 8 These TEM micrographs indicate that there is no structural degradation through the whole stack of layers The interfaces in the top cascades are as perfect

as at the bottom of the structure Extended studies using ray diffractometry and ray reflectivity give evidence that the QC structures are strain compensated towards the Si0.5Ge0.5 relaxed buffer layer, having an excellent reproducibility within the structure and from sample to sample and have abrupt interfaces

x-Fig 9 depicts a electroluminescence spectrum of the sample containing

15 cascades A pronounced peak is observed at 176 meV, which agrees well to the 156 meV expected from the design of the sample The full width at half maximum amounts to

46 meV The linewidth can be explained by interface roughness, since the Si barriers are very thin in the active region, down to only 0.4 nm and the wave function extend

v n EI • i ~- * t K • J over several SiGe quantum wells In

Figure 9 Electroluminescence spectra of a 15 period ^

SiGe QC structures addition also the non-parabolicity of

the heavy hole states cannot be neglected and will c ontribute to the linewidth A more detailed discussion of the electroluminescence of these QC structures also in context with the VI characteristics can be found in ref [29]

4 Conclusion

SiGe nanotechnology offers several viable paths for industrial applications in evolving future markets It has the potential to cover the needs of mainstream mi-croelectronic as well as niche market applications Three subjects with interesting future perspectives have been discussed in detail The Dot-FET may provide the advantages of high mobility n- and p-type channels without the use of problematic relaxed SiGe buffer layers with their low heat conductivity and high defect density

It is envisioned that self-scrolling of 3-dimensional nanoshells may relax space constrains on microchips by a compact fabrication of capacitors and coils, but may also enter into other fields like micromechanics and biotechnology Finally, Si/SiGe quantum cascade structures might be suitable to fabricate a Si based laser

unpolarized, 4.7V, 550mA, 10%dc, 94kHz

TM 5.1V, 650 IUA, 20%dc, 94kHz

TE 5.1V 650mA, 20%dc, 94kHz

AtAt4

Acknowledgement

The author likes to acknowledge the colleagues at the PSI: L Diehl, 0 Leifeld,

O Kirfel, S Mentese, S Tsujino, H Sigg, E Muller, S Stutz, E Deckhardt and

T Neiger for their valuable scientific contributions and technical support Special thanks to S Golod and V Prinz (ISP-RAS) for fabricating nano- and micro tubes Support of P Waegli (ETHZ) in obtaining SEM pictures and of Y Campidelli,

O Kermarrec and D Bensahel (STMicroelectronics) for supply of relaxed SiGe buffer layers is acknowledged Different aspects of this work have been financed by the Swiss National Science Foundation and the European Community (SiGeNET)

References

1 H Welker., Z Naturforschung 70 744 (1952)

2 R V Steele, Laser Focus World 38 61 (2002), and 38 81 (2002)

3 B Boratynski, Optica Applicata 32 437 (2002)

4 F Capasso, J Electrochem Soc 135 C194 (1988)

5 S J Jeng, et al., IEEEElectr Device L 11 542 (2001)

6 A Scavennec, Microelectronics Reliability 41 1563(2001)

7 U KQnig, et.al., Solid-State Electronics 41 1541 (1997)

8 C Rosenblad, et.al., Mat Sci Eng B 74 113 (2000)

9 P Ball, Nature 409 974 (2001)

10 S S Iyer, Y H Xie, Science 260 40 (1993)

11 Y W Mo, et al., Phys Rev Lett 65 1020 (1990)

12 B Voigtlander, A Zinner, Appl Phys Lett 63 3055 (1993)

13 D J Eaglesham, M Cerullo, Phys Rev Lett 64 1943 (1990)

14 A Beyer, et al., Appl Phys Lett 11 3218 (2000)

15 O Leifeld, et al., Appl Phys A 66 S993 (1998)

16 O G Schmidt, et.al., Appl Phys Lett 81 2614 (2002)

17 C Teichert, et al., Phys Rev B 53 16334 (1996)

18 O G Schmidt, K Eberl, IEEE T Electron Dev 48, 1175 (2001)

19 O G Schmidt, et al., Appl Phys Lett 114139 (2000)

20 E Muller, et al., Inst Phys Conf Ser 169 163 (2001)

21 M W Dashiell, et.al., Appl Phys Lett 80 1279 (2002)

22 D Griitzmacher, European patent disclosure, EP 01122320.3

23 V Y Prinz, et al., Physica E 6 828 (2000)

24 V Y Prinz, et al., Inst Phys Conf Ser 166 203 (2000)

25 V Y Prinz, et al., Nanotechnology 12 399 (2001)

26 L Pavesi, et.al., Nature 408 440 (2000)

27 J Faist, et al., Science 264 553 (1994)

28 G Dehlinger, et al., Science 290 2277 (2000)

29 L Diehl, et.al., Appl Phys Lett 81 4700 (2002)

30 J Faist, M Beck, T Aellen, Appl Phys Lett 78 147 (2001)

INVITED

SPIN RESOLVED INVERSE PHOTOEMISSION FROM LAYERED

MAGNETIC NANOSTRUCTURES

R BERTACCO, L DUO, M MARCON, M PORTALUPI, F CICCACCI

INFM- Dipartimento di Fisica, Politecnico di Milano Piazza Leonardo da Vinci 32, 20133 Milano, Italy

We report on the use of spin polarized electron beams in the study of electronic states in solids, referring in particular to the Inverse Photoemission spectroscopy In this technique the empty electron states are investigated, and the spin resolution allows to study their spin character, yielding valuable information in magnetic systems Examples of application to layered magnetic nanostructures are given: in particular we present data on Fe/Cr/Fe(001) multilayers, ultrathin Fe films grown on ZnSe(OOl), and LaSrMnO/SnTiO junctions

1 Introduction

The ability of preparing thin metal films has recently generated a great deal of interest in their structural, electronic and magnetic properties, which may be radically different from their bulk counterparts From a technological point of view, the understanding of such phenomena is promising for applications, as in the case of high-density magneto-optical storage media or of devices based on spin dependent transport properties A crucial role in determining the magnetic properties of such 2-dimensional and related structures (multilayers) is played by the electronic states that are localized at the surface or at the interface between different layers In magnetic systems these states may be efficiently probed by means of spin-polarized electron spectroscopies, thanks to the possibility of a direct identification of their

spin character offered by spin resolution In classical 3d ferromagnets there are less unoccupied than occupied d bands, resulting in less overlapping states in the empty

region of the spectrum It is then experimentally simpler to picture out the dispersion relation of the exchange split minority and majority bands, making the study of empty states in ferromagnetic structures particularly appealing This task can be accomplished by means of spin resolved Inverse Photoemission (IPE): in this spectroscopy spin-polarized electrons are sent onto a solid surface while detecting the photons emitted in the radiative transitions towards the unoccupied states to be probed The first spin-resolved IPE studies on bulk ferromagnets date back to the early eighties [1]

In the last decade, our group has contributed to this research field with a number of experiments on empty states In our laboratory, in fact, a spin resolved IPE apparatus has been set-up by coupling standard ultra high vacuum (UHV)

techniques to an appositely designed spin resolved electron gun and high efficiency band pass photon detector [2] The systems allows clean surface preparation and ultrathin film deposition via Molecular Beam Epitaxy (MBE), surface characterization via Low Energy Electron Diffraction (LEED) and X-ray

photoemission Spectroscopy (XPS), and in situ IPE measurements As usual in

electron spectroscopies, measurements are performed in magnetic remanence (no applied magnetic filed) with the sample magnetized along a crystal easy axis (e.g., the [100] direction in case of Fe) The polarization of the electron beam, produced

by a negative affinity GaAs photocathode, can be switched from parallel to antiparallel with respect to the sample magnetization IPE spectra are collected in

the isochromat mode, i.e., by varying the beam energy and detecting only photons of

a fixed energy ( in our case hv—9A eV), among the ones emitted in the electron

decay towards the empty states (for details on the experimental apparatus, see Refs [2])

Different low dimensionality magnetic systems have been investigated, including surfaces, adsorbates, thin films, interfaces and multilayers In the following we p resent a pplication o f s pin r esolved IPE t echnique t o s tudy 1 ayered magnetic nanostructures, namely Fe/Cr/Fe(001) multilayers, Fe/ZnSe(001) ultrathin films, and LaSrMnO/SnTiO interfaces

structure above the Fermi level (E F) by means of spin resolved IPE In particular, we

focus on films with a relatively large thickness (>7 monolayer, ML;

1 ML =1.43 A), while results on the early stages of the Cr/Fe(001) interface formation and ultrathin films can be found elsewhere [6]

A set of data for different Fe/Cr/Fe(001) trilayers are presented in Fig 1 The left hand side of the Figure shows a sketch of the investigated samples, consisting of

a clean Fe(OOl) substrate on top of which Cr and Fe films were deposited at room temperature at typical rates of 0.5-1.5 A/min The data discussed here were taken on trilayers with different values of the Cr spacer thickness, whereas the top Fe overlayer was in any case 7 ML thick

Figure 1 Spin resolved IPE spectra taken from clean Fe(001) and Fe/Cr/Fe(001) trilayers with an 11 ML

and 12 ML thick Cr spacer In the IPE spectra the majority- (continuous lines) or minority-spin (dotted lines) character is referred to the Fe substrate The structure of the sample is also sketched

The right hand side presents spin resolved IPE spectra for the Fe/Crl 1ML/Fe(001) and the Fe/Crl2ML/Fe(001) systems, as well as those from the clean Fe(001) substrate, for a direct comparison The coincidence of both short and long periods of the FM-AF transition when increasing the Cr spacer thickness from

11 to 12 ML [5], makes these systems very well suited to observe the switching of the magnetic coupling between Fe layers In the spectra of Fig 1, continuous (dotted) lines refer to data obtained for parallel (antiparallel) spin alignment between the incoming electrons and the majority electrons inside the Fe substrate Thus, the structures Bl and B2 appearing in different spin channels of the Fe(001) spectrum have to be attributed to transitions towards majority- and minority-spin states, respectively, and constitute a clear evidence of the sample magnetic ordering [7] In the case of the Fe/Cr/Fe trilayer spectra, we note first that the Fe overlayer is thick enough (7 ML) to hinder any sizable contribution from the underlying Cr film The present measurements can then be interpreted in terms of pure Fe contribution, while the only role of the Cr spacer is to mediate the exchange interaction with the

substrate Moreover, the reduction of the Bl peak intensity in the IPE spectra from the trilayer samples with respect to the clean substrate indicates a progressive decrease of the surface order when increasing the overall film thickness [2,7] By looking a 11 he p olarization d ependence, a n AF c oupling i n t h e l l C r M L c ase i s clearly seen: the spin character of the features present in the IPE spectra is indeed interchanged with respect to the clean surface In the 12 Cr ML case, the spectra display the same spin character as the clean surface, indicating that now the magnetization of the Fe overlayer is in the same direction as the substrate below This is a direct spectroscopic evidence of the switching from AF to FM coupling between the topmost Fe film and the buried Fe substrate when adding a single layer

to the spacer, i.e increasing the Cr film thickness from 11 to 12 ML

3 Fe/ZnSe(001) ultrathin films

The control of the electron spin in semiconductors adds one degree of freedom, resulting in a very intriguing problem which holds potentials for the realisation of a new class of electronic devices with enhanced or completely new performances In these systems attempts are done for controlling the carrier spin rather than its

charge, adding the spin-up spin-down magnetic dualism to the conventional electron

hole dualism, ruling all semiconductor devices Spin electronics, nowadays

commonly dubbed spintronics, is a fascinating and emerging field whose scientific

and technological relevance is continuously increasing, that combines small scale (nanometric) magnetic elements with conventional semiconductor electronics [8] In principle a convenient way to inject a spin polarized current into a semiconductor is based on the use of ferromagnetic metals like Co or Fe, fabricating hybrid ferromagnetic/semiconductorheterostructures However a fundamental problem to

be faced is the reactivity of transition metals with semiconductors, which can lead to magnetically dead layers, and in turn suppress the spin polarization across the interface In this frame, interfaces fabricated on ZnSe substrates appear to be quite promising, offering more interesting properties than those based on more widely employed semiconductors, such as GaAs At variance with the Fe/GaAs heterojunctions [8], in fact, recent studies on Fe/ZnSe(001) have shown that such interface is magnetically sharp, with Fe magnetic moment similar or even larger than bulk [9,10] Furthermore the magnetic properties are stable up to 300 °C and the magnetism seem to be preserved in ultrathin films (coverages of the order of

1 ML), both results being important for device applications We have prepared a clean ZnSe(OOl) substrate by UHV annealing of a ZnSe(OOl) epilayer grown on GaAs and passivated with a Se overlayer Depending on the annealing temperature (300 to 420 °C), an (lxl) or a c(2x2) Zn rich surface reconstruction was obtained, as revealed by the LEED pattern, while XPS analysis indicated a clean surface with the correct Zn and Se stoichiometry Despite the different initial conditions, we do not find any influence of the surface reconstruction in our IPE data Ultrathin Fe layers

have been then deposited at a rate of about 0.5 ML/minute, with the sample kept at

170 °C, i.e the optimal temperature for good epitaxial growth [10]

Fig 2 presents a stack of IPE spectra taken at different Fe coverage, along with the reference spectra corresponding to the substrate (continuous line) and to a clean and well ordered Fe(001) surface (top spectra) The features A and D in the spectrum from clean ZnSe can be assigned to transitions between bulk states, as they display a sizable angular dispersion, typical of band-like states The semiconductor behaviour is clearly evident from the delayed onset of the spectrum with respect to

the Fermi level, E F The onset corresponds to the semiconductor conduction band

minimum (CB), which, as estimated from the graphic extrapolation shown in Fig 2,

is located around 1.3 eV above E F After deposition of 1ML of Fe new states appear

at E F , reflecting a metallic behaviour, while the features A and D shift towards

higher energies This shift ( S~ 0.6 eV) is related to the band bending induced by the

Schottky barrier formation upon metal deposition At 2 ML coverage the states at

E F , arising from Fe, grow up while A and D are attenuated The energy position of

these to features does not change (within 0.1 eV) with respect the situation of 1 ML This indicates that the Schottky barrier height is stable upon further Fe deposition The spectra referring to 1 and 2 Fe ML are spin integrated: no trace of spin polarization is seen up to 6-8 ML coverage, both at room temperature and at 100K

At 8 ML a difference between the spin-up and spin-down channels clearly appears at

~ 2 eV above E F, in the region of the peak B2 of the pure Fe surface As noted

above, the absence of Bl is a common feature of a poor quality Fe surface [2,7], and indicates a non perfect layer by layer growth The situation at 15 Fe ML is more or

less the same, even if there is a trace of Bl and we can observe the appearance of the peak C

The present results show that the magnetic properties of ultrathin Fe films on ZnSe are quite different from those of bulk, in a completely similar way as in GaAs based interfaces In particular, it is shown that spin injection into the semiconductor

is possible only by using Fe films thicker than 8 ML This will have strong impact

on devices applications

4 La0.7Sro.3Mn03 and La 0 7Sro.3Mn0 3 /SrTi03 interfaces

The search for 100% spin-polarized materials is a vital research area for spin electronics In this sense manganites seem very promising systems, and in particular

in the case of La0.7Sr0.3MnO3 (LSMO), a quasi half-metallic behavior at low temperature has been recently observed [11,12] Despite these encouraging results, the electronic structure of this oxide is still not well known Especially for the unoccupied density of states there is no experimental confirmation of theoretical calculations which predict a gap for the minority states We present here an analysis

of the electronic states of LSMO just above the Fermi level at different temperatures The films were grown by Pulsed Laser Deposition (PLD) on a SrTi03

(STO) substrate [13,14] In some cases they were covered by a STO layer As the sample had been grown in a separate PLD system and then transferred in the electron spectroscopies apparatus, X-ray Photoemission Spectroscopy revealed the presence of some carbon and oxygen at the surface Due to the difficulties inherent

to any method for cleaning the surface of a complex oxide without alteration of the surface stoichiometry, measurements have been performed onto the sample as received As a matter of fact this seems not so critical, since we succeeded in detecting the expected high spin polarization of LSMO through the contamination layer

In F ig 3a n ormal incidence IPE spectra from a free LSMO surface, taken at

100K and 300K are reported in the region near Ef LSMO is a ferromagnet with a Curie temperature (T c ) around 350 K: however, on the basis the small value of the

surface magnetization at 300K [12,13], we consider the room temperature spectra as

representative of the non-magnetic insulating behavior above T c In fact, at 300K,

there is no trace of spin polarization and the spin-integrated spectrum (bottom spectrum in Fig.3a) clearly displays the presence of a gap extending ~ 1 eV above

Ef The transition towards a low temperature half-metallic state is evident from the

spectra taken at 100K (top-spectra in Fig 3a), where two distinct line-shapes for the majority- (full dots) and minority-spin channels (empty dots) are visible The sample appears metallic for majority electrons and insulating for minority electrons, as it results from the delayed onset of the spin down channel Due to the very low

counting rate at E F and to the effect of the rescaling procedure to 100% polarization

of the incident electron beam, data present a sizeable scattering This prevents from

a precise determination of the spin polarization at E F , which is however definitely

above 90% On the other hand the delayed onset of the minority channel is clearly

visible, and can be related to the onset of the t 2g minority band The position of the

low energy edge of this band with respect to E F can be estimated from the energy

difference 5 between the minority and majority-channel onset: we find

5 = 4 0 0 ± 5 0 m e V

(a) (b)

Figure 3 (a): Spin integrated IPE spectrum from LMSO taken at 300K (squares in the bottom) and spin

resolved data (full and empty dots on top) taken at 100K (b): Spin resolved IPE spectra taken at 100K

from a LSMO film covered by two monolayers of STO A smoothing of experimental data at 100K is

plotted for the two spin channels: majority spin - continuous line, minority spin - dashed line

The results for the STO/LSMO interface at 100K, where the sample is

ferromagnetic, are reported in Fig 3b The sample consists of 2 ML of STO grown

on LSMO(OOl) in the typical conditions employed for tunneling junctions [11,13]

Also in this case we find a delayed onset of the minority spin electrons and a value

for 8 which is very close to that previously found for the free surface:

380 ± 50 meV Our findings then clearly indicate that the LSMO half-metallicity is

preserved also when a STO/LSMO interface is created, in agreement with the very

high value of the tunneling magnetic resistance observed in similar junctions

[11,13]

Acknowledgements

We thank V H Etgens (Paris) and J P Contour (Orsay) for providing ZnSe

substrates and LaSrMnO samples, respectively Thanks are also due to Yu Mamaev

(St Petersburg) for making available strained GaAs photocathodes used as polarized

electron sources in some experiments This work has been partly supported by the

European Community through project UE-INTAS 99-125

References

1 J Unguris, A Seller, R J Celotta, and D T Pierce, Phys Rev Lett 4 9 1 047

(1982); H Scneidt, M Globl, V Dose, and J Kirschner, Phys Rev Lett 51

1688(1983)

2 G Chiaia, S De Rossi, L Mazzolari, and F Ciccacci , Phys Rev B 48 11298

(1993); F Ciccacci and S De Rossi, Phys Rev B 51 11538 (1995); F Ciccacci,

Phys Scrip T66 190 (1996)

3 A M N Niklasson, B Johansson, L Nordstrom, Phys Rev Lett 8 4544

(1999)

4 M N Baibich, J M Broto, A Fert, F N Van Dau, F Petroff, P Etienne,

G Creuzet, A Friederich, J Chazelas, Phys Rev Lett 61 2472 (1988);

T.G.Walker, A W Pang, H Hopster; Phys Rev Lett 69 1121 (1992);

J Unguris, R J Celotta, D T Pierce, Phys Rev Lett 67 140 (1991); Ibid 69

1125(1992)

5 B Heinrich, J F Cochran, D Venus, K Totland, D Atlan, S Govorkov,

K Myrtle, J Appl Phys 79 45618 (1996); A Davies, J A Stroscio,

D T Pierce, J Unguris, R J Celotta; J Magn Magn Mater 165 82 (1997)

6 G Isella, R Bertacco, M Zani, L Duo, F Ciccacci, Sol State Commun 116

283 (2000)

7 J Kirschner, M Globl, V Dose, H Scheidt: Phys Rev Lett 53 612 (1984);

S De Rossi, F Ciccacci, J Electron Spectrosc Relat Phenom 76 172 (1995)

8 G A Prinz, Phys Today 48 58 (1995), and references therein

9 B T Jonker, G A Prinz, J Appl Phys 69 2938 (1991)

10 M Marangolo, F Gustavsson, M Eddrief, Ph Sainctavit, V H Etgens,

V Cros, F Petroff, J M George, P Bencok, N B Brookes, Phys Rev Lett 88

217202 (2002)

11 M Viret, M Drouet, J Nassar, J P Contour, C Fermon, A Fert, Europhys

Lett 39 545 (1997)

12 J -H Park , E Vescovo, H -J Kim, C Kwon, R Ramesh, and T Venkatesan,

Phys Rev Lett 81 1953 (1998)

13 J M De Teresa, A Barthelemy, J P Contour, A Fert, R Lyonnet,

F Montaigne, A Vaures, P Seneor; E-MRS Proceedings, San Francisco, April

(1999)

14 R Bertacco, M Portalupi, M Marcon L Duo, F Ciccacci, M Bowen,

J P Contour, A Barthelemy, J Magn Magn Mater 242/5 2710 (2002)