GeSi Quantum Dot Superlattices

Transcription

1 GeSi Quantum Dot Superlattices ECE440 Nanoelectronics Zheng Yang Department of Electrical & Computer Engineering University of Illinois at Chicago

2 Nanostructures & Dimensionality Bulk Quantum Walls Quantum Wires Quantum Dots ρ 3D (E) ρ 2D (E) ρ 1D (E) ρ 0D (E) Energy Energy Energy Energy 2 ρ 3D (E) ~ E 1/2 ρ 2D (E) ~ E 0 ρ 1D (E) ~ E -1/2 ρ 0D (E) ~δ(e)

3 Contents Growth of GeSi QDSLs Molecular-beam epitaxy; S-K growth mode; Raman Spectra of QDSLs Background on Raman scattering; Optical phonons; Acoustic phonons. PL Spectra of QDSLs 3 QDSL: Quantum Dot Superlattice

4 Applications of Ge/Si Quantum Dot (Superlattices) Electronic Applications such as resonant tunneling diodes with improved quality. Optoelectronic Applications such as photo-detectors, photo-emitters, and lasers in mid-infrared range. Thermoelectric Applications such as novel thermoelectric materials with large ZT values. 4 Quantum Information Applications

5 Contents Growth of GeSi QDSLs Molecular-beam epitaxy; S-K growth mode; Raman Spectra of QDSLs Background on Raman scattering; Optical phonons; Acoustic phonons. PL Spectra of QDSLs 5 QDSL: Quantum Dot Superlattice

6 Molecular-Beam Epitaxy (MBE) F-vdM: Frank-van der Merwe V-W: Volmer-Weber 6 S-K: Stranski-Krastanow

7 Self-Assembled Growth of GeSi Quantum Dot Superlattices Schematics of GeSi QDSLs 7 Si spacer layer Self-assembled Ge quantum dots Ge wetting layer Si buffer layer Si substrate

8 TEM Image of Ge/Si Quantum Dot Superlattices Vertical correlation of the quantum dots is evident!! 10-period, 540 C Ge 1.2 nm/si 20 nm Si spacer layer Self-assembled Ge quantum dot

9 AFM Images of Ge/Si Quantum Dot Superlattices µm 9 Growth temperature: 600 o C; Ge nominal thickness: 1.5 nm. Base size of the dots ~ 70 nm; Height of the dots ~ 15 nm.

10 The Samples Used in the Following Experiments 10

11 More AFM Images of Ge/Si Quantum Dot Superlattices (1) μm 0.5μm 0.5μm μm

12 More AFM Images of Ge/Si Quantum Dot Superlattices (2) nm 12

13 More TEM Images of Ge/Si Quantum Dot Superlattices 20 Periods

14 Contents Growth of GeSi QDSLs Molecular-beam epitaxy; S-K growth mode; Raman Spectra of QDSLs Background on Raman scattering; Optical phonons; Acoustic phonons. PL Spectra of QDSLs 14 QDSL: Quantum Dot Superlattice

15 Discovery of Raman Spectroscopy In 1928, Raman discovered that the spectrum of scattered lines of CCl 4 liquid not only consisted of the Rayleigh lines but a pattern of lines of shifted frequency the Raman spectrum. Mr. Raman won the Nobel Prize of Physics in 1930, for his work on the scattering of light and for the discovery of the effect named after him. 15 C.V. Raman ( )

16 Stokes S Anti-Stokes S S L S L q q 16 S L q S L q

17 Raman Spectra of Ge/Si Quantum Dot Superlattices Acoustic Mode From Sample 226 Si-Si Raman Intensity (arb.unit) Ge-Ge Optical Mode Si-Ge Raman Shift (cm -1 )

18 Raman Spectra of Optical Phonon Modes (1) From Samples 137 and 136 Si-Si Raman Intensity (a.u.) Ge-Ge Ge QDs Si-Ge Si-Si LOC WLs Raman Shift (cm -1 ) WL: wetting layer

19 Raman Spectra of Optical Phonon Modes (2) 298 From Samples 210, 226, and 187 Raman Intensity (a.u.) Sample 226 Sample 210 Sample 187 Bulk Ge: 300 cm Raman Shift (cm -1 )

20 Frequency Shift of Optical Phonon Modes Redshift Ge-Ge optical phonon frequency Blueshift Phonon Confinement ~1cm 1 Strain Linear Chain Model 1 or RWL Model 2,3 Biaxial Strain Model 1, Liu J L et al, JAP 92, 6804 (2002) 2 Yang Z et al, Chin. Phys. Lett. 20, 2001 (2003) 3 Richter H et al, Solid State Commun. 39, 625 (1981)

21 Red Shift from Phonon Confinement Phonon Confinement phononconfinement A a d Ge Richard, Wang and Ley Model: Solid State Communication, 39, 625 (1981); APL, 69, 200 (1996); PRB, 55, 9263 (1997); JAP, 86, 1921 (1999) a Ge 0.566nm Lattice constant of germanium 21 A cm Red shift ~ 1 cm -1 Constant parameters d the height of the Ge dot nearly same as the result of Linear Chain Model!

22 Blue Shift from Compressive Strain Effect (1) Strain induced blue shift strain p zz q( xx yy ) s 1 (~ 300cm 1 ) Frequency of the bulk Ge zone center LO phonon p q s 2 27 s 2 Ge deformation potentials 22 zz yy 2C C xx xx C C kbar Biaxial strain model PRB, 59, 4980 (1999) 482.5kbar

23 Blue Shift from Compressive Strain Effect (2) Strain induced blue shift strain 1 C12 1 q p xx xx 414.5(cm 0 C11 ) 23 Fully Strained a a Si Ge 0.543nm 0.566nm Partially Strained (due to the strain relaxation from the atomic intermixing at the Si/Ge interface) xx a Si a a Si Ge (cm ) xx xge 1- x 1- x x strain asi age xx x? a Si Ge

24 Composition of the Ge/Si Quantum Dots I I GeGe SiGe a Vegard s Law Si Ge xage (1 x) a 1- x x x B x 2(1 x) 1 B 3.2 I Si I GeGe SiGe Raman Intensity (a.u.) Sample 226 Sample 210 Sample Raman Shift (cm -1 ) 24 I GeGe : Integrated Intensity of Ge-Ge peak; I SiGe : Integrated Intensity of Si-Ge peak; x ~ 0.5

25 Composition of the Ge/Si Quantum Dots Composition of the quantum dots APL, 62, 2069 (1993) I I GeGe SiGe x B 2(1 x) B 3.2 x 1 I I GeGe SiGe Vegard s Law a Si Ge xage (1 x) a 1- x x Si xx a Si 1- x a Ge Si x 1- x a Ge x Ge 1 x 1 (1 x) a a Si Ge x a ( a Si Ge 0.96) 25 xx 0.02 when x 0.5 strain 8(cm 1 )

26 Summary (Raman optical phonon part) Optical phonon modes in MBE-grown self-assembled Ge/Si QDSLs were studied by Raman spectroscopy. Si-Si, Ge-Ge, and Si-Ge peaks were observed in the Raman spectra, arising from Si substrate, optical phonon modes of Ge in the QDs, and the SiGe alloys, respectively. The red-shift and blue-shift of the optical phonon modes in the Raman spectra induced by phonon confinement and strain within the Ge QDs were investigated. 26

27 Contents Growth of GeSi QDSLs Molecular-beam epitaxy; S-K growth mode; Raman Spectra of QDSLs Background on Raman scattering; Optical phonons; Acoustic phonons. PL Spectra of QDSLs 27 QDSL: Quantum Dot Superlattice

28 (a) 10 periods 100 nm Raman Intensity (a. u.) Folded acoustic phonons Optical phonons 1 µm Ge-Ge Si-Ge Si-Si (b) 20 periods 100 nm Raman Shift (cm -1 ) R am an Shift (cm -1 ) Intensity d cos( qd) cos( V d )cos( V k 1 d ) sin( 2k V (c) periods 100 nm Z. Yang, J. Liu, Y. Shi, Y. Zheng, and K. Wang, J. Nanoelectron. and Optoelectron. 1, 86 (2006). 1 1 q =0.10nm Wave Vector (/d) Phonon Frequency (cm -1 ) Normalized Raman Intensity d )sin( V 2 2 ) 1st order 2nd order 3rd order Superlattice Periods

29 Rytov s Elastic Continuum Theory 29 In the model, the acoustic phonon dispersion can be approximately written as 2 d1 d2 k 1 d1 d cos( qd) cos( )cos( ) sin( )sin( v v 2k v v where k v v v1 and v2 d 2 and d 1 1 and and d d 1 d 2 sound velocity thickness in Ge and Si layers density )

30 Raman Spectra of Acoustic Phonon Modes (1) Raman Intensity (a.u.) Sample 137 Sample 136 Sample 138 From Samples 136, 138, 137 (nm) 0.6, 1.2, 1.5 Folded Acoustic Phonons in Ge/Si superlattices. d cos( qd) cos( V Rytov s Model 1 1 d )cos( V k 1 d ) sin( 2k V 1 1 d )sin( V 2 2 ) Si sub Raman Shift (cm -1 )

31 Raman Spectra of Acoustic Phonon Modes (2) Raman Intensity (a.u.) Si Sub Sample 226 Sample 210 From Samples 210 and 226 (nm) 1.2, Raman Shift (cm -1 ) 31 Intensity increase with the thickness of Ge layer.

32 Raman Spectra of Acoustic Phonon Modes (3) 32 Raman Intensity (a.u.) Sample 269 (2) Sample 264 (10) Sample 265 (20) Sample 266 (35) Sample 183 (50) Raman Shift (cm -1 ) Intensity increases with increased periods of QDSL layers. The amplitudes of the lowfrequency Raman scattering peaks associate with each QDs layer when the Ge QDs in different layers are vertical correlated!

33 Summary (Raman acoustic phonon part) Low-frequency Raman scattering spectra of selfassembled Ge/Si QDSLs were first time observed in experiments on non-resonant Raman scattering mode. These Raman peaks were arisen from the folded acoustic phonons in the Ge/Si QDSLs. It has been observed that the intensity of the lowfrequency Raman peaks is closely related to the Ge and Si layer thickness and the number of the periods of the Ge QDSLs. The smaller periods, the lower intensity of the Raman peaks. The results were discussed with Retov model. 33

34 Contents Growth of GeSi QDSLs Molecular-beam epitaxy; S-K growth mode; Raman Spectra of QDSLs Background on Raman scattering; Optical phonons; Acoustic phonons. PL Spectra of QDSLs 34 QDSL: Quantum Dot Superlattice

35 Photoluminescence Spectroscopy Continuum states Electronic bound state Laser energy E CB k h excitation h emission VB PL Intensity 35 A laser excites electrons from the valence band into the conduction band, creating electron-hole pairs These electrons and holes recombine (annihilate) and emit a photon. The number of emitted photons (intensity) as a function of energy, which is photoluminescence (PL).

36 Photoluminescence E (Normal semiconductor vs. Quantum Well) E C E G laser emitted light emitted light hν=2.4 ev hν=1.5 ev (GaAs) hν 1.2 ev (QDs) E V 36

37 Photoluminescence in Ge/Si Quantum Dot Superlattices PL Intensity (arb. unit) T=10K Ge QDs From Sample 265 TO+NP Si-TO Ge WLs TO NP Si-TA or Si-NP Photon Energy (ev)

38 Temperature Dependent Photoluminescence Spectra From Sample 265 PL Intensity (arb. unit) 10K 30K 50K 80K 120K 160K 200K Photon Energy (ev)

39 Temperature Dependence of PL Intensity in Nano-Crystallines r Early Theory Arrhenius Type R exp( ) r r T T Recent Theory Combination of Arrhenius and Berthelot Type T T Berthelot Type Rhop B exp( ) B 39 I I T ) T 1 exp( T ( 0 B Tr T ) T B a m * e 2 k B

40 Fitting of the PL Intensity 10K PL Intensity (arb. unit) 30K 50K 80K 120K 160K PL Intensity (arb. unit) 1 200K Photon Energy (ev) Photon Energy (ev)

41 Fitting of the PL Intensity 10K PL Intensity (arb. unit) 30K 50K 80K 120K 160K PL Intensity (arb. unit) K Photon Energy (ev) Photon Energy (ev)

42 Fitting of the PL Intensity 10K PL Intensity (arb. unit) 30K 50K 80K 120K 160K PL Intensity (arb. unit) K Photon Energy (ev) Photon Energy (ev)

43 Fitting of the PL Intensity 10K PL Intensity (arb. unit) 30K 50K 80K 120K 160K PL Intensity (arb. unit) K Photon Energy (ev) Photon Energy (ev)

44 Fitting of the PL Intensity 10K PL Intensity (arb. unit) 30K 50K 80K 120K 160K PL Intensity (arb. unit) K Photon Energy (ev) Photon Energy (ev)

45 Fitting of the PL Intensity PL Intensity (arb. unit) 10K 30K 50K 80K 120K 160K PL Intensity (arb. unit) 200K Photon Energy (ev) Photon Energy (ev)

46 Fitting of the PL Intensity PL Intensity (arb. unit) 10K 30K 50K 80K 120K 160K PL Intensity (arb. unit) K Photon Energy (ev) Photon Energy (ev)

47 Fitting of the PL Intensity 47 I T / I 10K I I T ) T 1 exp( T ( 0 B Tr ) T Experimental Data Fitted Curves Temperature (K) Fitting Parameters T B = 31.6 K; T r = 0 K; ν = 0.025

48 Estimation of the Electron Effective Mass in the QDs T B a m * e 2 k B a : the dimension of the quantum dots ; m e* : the electron effective mass in the quantum dots ; m 0 : the rest electron mass; T B : fitting parameters. 48 m e* 1.4. m a 0 2 a (nm)

49 Summary (Photoluminescence part) Photoluminescence experiments were performed on MBE-grown Ge/Si QDSLs. The temperature-dependence of the PL intensity has been reported and fitted. 49 Based on the analyses of the fitting parameters, the electron effective mass of the QDs was estimated.

50 References Z. Yang, Y. Shi, J. Liu, B. Yan, Z. Huang, L. Pu, Y. Zheng, and K. Wang, Chinese Physics Letters 20, 2001 (2003). Z. Yang, Y. Shi, J. Liu, B. Yan, R. Zhang, Y. Zheng, and K. Wang, Materials Letters 58, 3765 (2004). Z. Yang, J. Liu, Y. Shi, Y. Zheng, and K. Wang, Journal of Nanoelectronics and Optoelectronics 1, 86 (2006). 50