Crystalline phase-change materials [Elektronische Ressource] : disorder, medium-range order and electrical switching / Philipp Johannes Merkelbach

Sujets

• Physik

Informations

• Publié par: rheinisch-westfalischen_technischen_hochschule_-rwth-_aachen
• Publié le: 01 janvier 2012
• Nombre de lectures: 15
• Langue: English
• Poids de l'ouvrage: 11 Mo

Extrait

Crystalline Phase-Change Materials:
Disorder, Medium-Range Order and Electrical Switching
Von der Fakultät für Mathematik, Informatik und Naturwissenschaften
der RWTH Aachen University zur Erlangung des akademischen Grades
eines Doktors der Naturwissenschaften genehmigte Dissertation
vorgelegt von
Diplom-Physiker
Philipp Johannes Merkelbach
aus Köln
Berichter: Universitätsprofessor Dr. Matthias Wuttig,
Prof. Dr. Theo Siegrist
Tag der mündlichen Prüfung: 25.11.2011
Diese Dissertation ist auf den Internetseiten der Hochschulbibliothek online verfügbar.Contents
1. Introduction 1
1.1. Phase-Change Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2. Resonant bonding in crystalline Phase-Change materials . . . . . . . . . . . . 5
1.3. Treasure Map of Phase-Change materials . . . . . . . . . . . . . . . . . . . . . . 8
1.4. Crystal Structure and Disorder in Phase-Change Materials . . . . . . . . . . . . 12
2. Disorder Induced Localization 17
2.1. Resistivity of solids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.2. Metal-Insulator Transition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.2.1. Ioffe-Regel condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.2.2. Types of Metal-Insulator Transition . . . . . . . . . . . . . . . . . . . . . 20
2.2.3. Energy scales of MIT transitions . . . . . . . . . . . . . . . . . . . . . . . 21
2.3. Temperature dependance of the Resistivity in GeTe and GeSb Te . . . . . . . 242 4
2.4. Low temperature measurements of GeSb Te . . . . . . . . . . . . . . . . . . . 282 4
2.5. X-ray diffraction analysis of the MIT . . . . . . . . . . . . . . . . . . . . . . . . . 31
2.6. Transport parameter of GeTe and GeSb Te . . . . . . . . . . . . . . . . . . . . 382 4
2.7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3. Crystallographic Analysis of Disorder 49
3.1. Neutron Pair Distribution Function technique . . . . . . . . . . . . . . . . . . . 49
3.1.1. Diffraction by disordered systems . . . . . . . . . . . . . . . . . . . . . . 50
3.1.2. Pair distribution function . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
3.2. Beamline D4 at Institut Laue-Langevin, Grenoble . . . . . . . . . . . . . . . . . 57
3.2.1. Sample geometry, data treatment, and data analysis . . . . . . . . . . . 58
3.3. Modeling of the short to medium range order in phase-change materials . . . 63
iiiContents
3.4. Structure of GeTe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
3.5. Comparison of the structure of GeTe and GeSb Te . . . . . . . . . . . . . . . . 682 4
3.6. Structure of GeSb Te . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 712 4
3.6.1. Rhombohedral model for GeSb Te . . . . . . . . . . . . . . . . . . . . . 712 4
3.6.2. Displacement model from DFT calculations . . . . . . . . . . . . . . . . 72
3.7. Annealing effects on structure of GeSb Te . . . . . . . . . . . . . . . . . . . . . 782 4
3.8. Summary of Crystallographic Analysis of Disorder . . . . . . . . . . . . . . . . 81
4. Electrical Switching in Phase-Change Materials 83
4.1. Working principle of phase-change memory . . . . . . . . . . . . . . . . . . . . 84
4.2. Disorder encoded multi-level storage in phase-change memory . . . . . . . . 87
4.3. Phase-change memory cell preparation . . . . . . . . . . . . . . . . . . . . . . . 88
4.4. Pulsed Electrical Tester . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
4.5. Electrical switching in GeTe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
4.6. Summary of electrical switching in phase-change materials . . . . . . . . . . . 98
5. Summary and Outlook 99
Appendix 102
A. New stoichiometric frontiers 103
B. Additional Plots 109
ivList of Figures
1.1. Property contrast between amorphous and crystalline phase of a covalent
bonded material and a phase-change material. . . . . . . . . . . . . . . . . . . 4
1.2. Infrared reflectance spectra of Ge SbTe film. . . . . . . . . . . . . . . . . . . . 62 4
1.3. Resonant bonding in GeTe. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.4. Resonant bonding vs. distortion. . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.5. Treasure map for phase-change materials. . . . . . . . . . . . . . . . . . . . . . 11
1.6. EXAFS data of GeSb Te measured at different temperatures. . . . . . . . . . . 152 4
1.7. EXAFS data of GeTe: amorphous vs. crystalline. . . . . . . . . . . . . . . . . . . 15
1.8. EXAFS data of GeSb Te : amorphous vs. crystalline. . . . . . . . . . . . . . . . 162 4
2.1. Classification of metal and insulator. . . . . . . . . . . . . . . . . . . . . . . . . 18
2.2. Comparison of two metal-insulator transition concepts: Mott and Anderson. 23
2.3. Temperature dependent sheet resistance of GeTe and GeSb Te . . . . . . . . . 252 4
2.4. Effect of annealing on the resistivity and its temperature dependance for
different GeSbTe alloys. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
2.5. Low temperature resistivity of different annealed crystalline GeSb Te films. . 292 4
2.6. Variable range hopping model fitted to GeSb Te . . . . . . . . . . . . . . . . . . 302 4
2.7. Transport parameters of VO as a function of temperature. . . . . . . . . . . . 322
2.8. X-ray diffraction pattern of GeTe and GeSb Te . . . . . . . . . . . . . . . . . . . 332 4
2.9. Ge segregation in the grain boundaries of GeTe. . . . . . . . . . . . . . . . . . . 35
2.10.Changes of the FWHM of GeTe and GeSb Te on annealing. . . . . . . . . . . . 372 4
2.11.Summary of the transport parameters of GeTe and GeSb Te . . . . . . . . . . . 402 4
2.12.The Mott criterion applied to GeSb Te . . . . . . . . . . . . . . . . . . . . . . . 432 4
2.13.Disorder induced localization: Energy scales and Density of states. . . . . . . 44
vList of Figures
2.14.Schematic presentation of the unique crystal structure and bonding situation
in GeSb Te . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 472 4
3.1. Differential scattering cross section per atom for a monoatomic system. . . . 55
3.2. Examples of real-space functions. . . . . . . . . . . . . . . . . . . . . . . . . . . 56
3.3. Instrument D4 at Institut Laue-Langevin. . . . . . . . . . . . . . . . . . . . . . . 59
3.4. Total structure factor and pair-distribution function of crystalline GeTe and
GeSb Te . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 612 4
3.5. Evolution of the pair distribution function G(r) with increasing q . . . . . . 62max
3.6. Effects of rhombohedral distortion and atomic displacement on the pair dis-
tribution function G(r). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
3.7. Structure of GeTe. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
3.8. Fit of a rhombohedral model to the total pair-distribution function of GeTe
(range 2.5 - 20.5 Å). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
3.9. GeTe: Short to medium range order PDF . . . . . . . . . . . . . . . . . . . . . . 68
3.10.Total pair disotributiuon function G(r) of GeTe and GeSb Te . . . . . . . . . . 692 4
3.11.Comparison of the short to medium range order of GeTe and GeSb Te . . . . 702 4
3.12.Rhombohedral fit of the pair-distribution function G(r) for r = 2.5 - 20.7 Å for
crystalline GeSb Te annealed at 150 °C. . . . . . . . . . . . . . . . . . . . . . . 722 4
3.13.Comparison between the short to medium range order and the long range
order in crystalline GeSb Te annealed at 150°C. . . . . . . . . . . . . . . . . . 732 4
3.14.Comparison of DFT data with the experimental data. . . . . . . . . . . . . . . . 76
3.15.Displacement pattern in GeSb Te calculated by DFT. . . . . . . . . . . . . . . 772 4
3.16.Annealing effect on short to medium range order Pair Distribution Function
of GeSb Te . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 792 4
3.17.Neutron PDF fit parameters of GeSb Te on annealing. . . . . . . . . . . . . . 802 4
4.1. Working principle of a PCRAM cell. . . . . . . . . . . . . . . . . . . . . . . . . . 85
4.2. Threshold switch in Ge Sb Te . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 862 2 5
4.3. Enabling multi-level PCRAM cells with a new approach. . . . . . . . . . . . . . 88
4.4. PCRAM base wafer and single cell. . . . . . . . . . . . . . . . . . . . . . . . . . . 89
4.5. AFM image of the bottom electrode of a PCRAM cell. . . . . . . . . . . . . . . . 91
4.6. SET window for switching GeTe. . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
viList of Figures
4.7. Cell resistance after application of SET pulses with different amplitude and
length. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
4.8. Change of SET speed with initial RESET resistance. . . . . . . . . . . . . . . . . 97
A.1. Temperature dependent sheet resistance of different co-sputtered GeTe-Sb Te2 3
samples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
A.2. X-ray diffractogramm of a co-sputterd GeTe-Sb Te sample. . . . . . . . . . . 1052 3
A.3. FT-IR spectra of a co-sputterd GeTe-Sb Te sample . . . . . . . . . . . . . . . . 1062 3
B.1. Res