Da escala micro para a escala nano

1. Da escala micro para a escala nano Como são produzidos os semicondutores ? As técnicas de crescimento epitaxial permitiram a miniaturização MBE – Molecular Beam Epitaxy CBE – Chemical Beam Epitaxy MOVPE – Metalorganic Vapor Phase Epitaxy

2. MBE Alto vácuo Pressão 10-10 Torr

3. MOVPE • MOCVD - Metalorganic Chemical Vapor Deposition • OMCVD - Organometallic Chemical Vapor Deposition • OMVPE - Organometallic Vapor Phase Epitaxy

5. Princípio de deposição • (CH3)3Ga + AsH3→ GaAs + 3 CH4 • (1-x) (CH3)3Ga + x(CH3)3Al + AsH3→ AlxGa1-xAs + 3 CH4

7. Epitaxial Growth TMGa AsH3 GaAs Substrate

8. AlAs GaP InP InAs GaxAl1-xAs InxGa1-xP InxGa1-xAs InxAl1-xAs GaAs

9. a’ > a InAs a a Strained layers Lattice matched AlGaAs Strained InAs GaAs GaAs

10. 3D a 0D

11. De 3D a 0D • 3D E = Eg + h2k2/8pm Density of states r(E) = 21/28mc3/2 (E-Eg)1/2/h3 • 2D • E = Eg + Eqz +h2k//2/8p2m • Eqz= qz2h2/8md2 • Density of states • r(E) = 4pm/h2 • 0D • E = Eg+Eqz+Eqy+Eqx • Eq(z,y.x)= qz,y,x2h2/8md2 • Density of states • r(E) = # of dots g /Vol • 1D • E = Eg+Eqz+Eqy+h2kx2/8p2m • Eqz,y= qz,y2h2/8md2 • Density of states • r(E) = 8Lm1/2/h2½(E-Eq)1/2

12. Pontos quânticos O que são estas estruturas 0D? • Estruturas com confinamento 3D numa escala menor que o raio de Bohr levando a uma quantização 3D. • Comportamento atômico. • 1980 foram fabricados os primeiros pontos quânticos de ZnS em vidro. • Existem várias maneiras de produzí-los.

13. Estrutura de banda

14. Sintonia de estruturas de PQs Fafard 2003

15. Top-down vs bottom-up Top-down: Photolithography Electron beam lithography X-rays Extreme ultraviolet light Scanning probe methods Bottom-up: Self-assembled quantum dots Scanning probe methods

16. Comparando os métodos • Lithography Advantage: The electronics industry is already familiar with this technology. Disadvantage: The necessary modifications will be expensive. UV-light and x-rays can damage the equipment. • Scanning Probe Advantage: STM and AFM are very versatile, they can move particles in a patterned fashion. Disadvantage: Too slow for mass production. • Bottom-up Methods Advantage: Controlled chemical reactions can cheaply and “easily” produce nanostructures. Disadvantage: Cannot produce designed, interconnected patterns.

17. Pontos quânticos auto-organizados

18. Métodos diferentes de crescimento

19. Stranski-Krastanow

20. Princípio de formação de pontos quânticos por MOVPE • Uma diferença importante no parâmetro de rede numa heteroestrutura, leva a um aumento na energia elástica que será aliviada com a formação de ilhas de dimensões que podem ser inferiores ao raio de Bohr. • Para materiais descasados um aumento na tensão elástica com o aumento na espessura torna a superfície rugosa. O crescimento 2D camada a camada é interrompido e num segundo passo, a nucleação 3D se inicia. Numa terceira etapa as ilhas 3D se desenvolvem em tamanho consumindo o material que está móvel na superfície. Seifert 2000

22. Espessura da wetting layer

23. Dots’ parameters • Dot density 108 to 1011 cm2 • Dot size 4 – 20 nm height, 20 – 50 nm base width • Dot shape Pyramidal, truncated pyramidal, lens- and cone-shaped How to determine these parameters?

24. Scanning Tunneling Microscopy(Nobel Prize to Rohrer and Binnig in 1986)

25. Atomic Force Microscopy Determination of size distribution and density of quantum dots

26. Example of AFM Results

27. Transmission Electron Microscopy Two geometries: Plain view Cross section InAs/GaAs Cross section gives information about shape, size and composition. Samples are thinned down to a thickness of the order of 1mm. 104 – 106 atoms per dot

28. TEM image of an InAs/InGaAs/InP dot HREM images Landi et al 2005

29. Photoluminescence • The laser beam usually probes an ensemble of quantum dots. The FWHM gives information on the uniformity of the dot size distribution. • For a density of 1010 cm-2, one probes about 106 dots for a 100 mm laser spot. • Single dot spectroscopy requires low dot density and processing to isolate one dot.

30. Examples of Photoluminescence of Dots Luminescence of an ensemble of dots with resolved excited states. Linewidths of the order of 20-30 meV. Single dot spectroscopy. Linewidths of the order of meV. Signal is time averaged. s p d f Fafard et al 2000 Finley et al 2001

31. Electroluminescence for two injection levels reveals the Pauli principle. Photocurrent measurements show absorption to the ground state (s) and to three excited states (p, d, f). Mowbray et al 2005

32. Growth parameters • Temperature • Higher temperature, lower density, larger size. • Deposition time • Longer times, more material, larger dots. • Fluxes of gases/ Growth rate • Higher growth rates, smaller dots, higher density. • Annealing time For the same amount of material the dot density and the dot size show inverse behavior

33. Tgrowth = 500°C • Height increases • FWHM decreases Effect of temperature on InAs/InGaAs/InP Tgrowth = 520°C

34. Effect of temperature on InAs/InGaAs/InP Reduction of the PL FWHM in agreement with AFM results PL intensity for higher energies decreases → larger dots

35. Deposition time increases→ Dot density increases InAs/InGaAs/InP

36. In flux / growth rate dependence In flux: 30 sccm 60 sccm 66 sccm 76 sccm Tgrowth: 520 oC tgrowth: 4.2 s InAs / InP

37. InAs / InGaAs / InP Attempting to reach higher densities InAs /InP

38. Same scale: from 2.0 108 to 2.0 1010 dots cm-2 InAs/InGaAs Tg =490oC H  9 nm InAs/InP Tg = 490oC H  12 nm

39. Stacks of quantum dots • For device applications it is important to have several layers of dots. • Nature has helped. In general dots spontaneously grow on top of each other.

40. Multi-layers of quantum dots Surface QDs 200 nm 20 nm TEM Images of Stacked Quantum Dots AFM image Landi et al 2005

41. Red-shift with increasing number of stacks Effect of number of stacks on dots’ properties Vertical coupling increases the average dot height Landi et al 2004

42. Controlled site deposition of quantum dots on a patterned surface Dots grown away from the patterned region Patterned substrate using AFM Dots’ formation on designated sites Fonseca Filho et al 2005