" On trying daring ideas with Herb". P.M.Petroff Professor Emeritus Materials Department, University of California, Santa Barbara

Some coauthored papers and shared students T.Y.Liu, P.M.Petroff, and H.Kroemer, "Luminescence of GaAs-GaAlAs Superlattices Grown on Silicon Substrates: Effects of Superlattice Interfaces”.J,.Applied Physics.64, 12, 6810 ( 1988) H. Kroemer, P. M.Petroff, T. L.Liu, "GaAs on Si: State of the Art and Future Prospects”.J.Crystal Growth 95, 96 (1989) J.M. Gaines, P.M. Petroff, H. Kroemer, R.J. Simes, R. S. Geels and J. English, "MBE Growth of Tilted GaAs/AlAs Superlattices by Deposition of Fractional Monolayers on Vicinal (100) Substrates”J. Vac.Scien.Tech. B6, 4,1378 (1988) M. Tsuchiya, J. M. Gaines, R. H.Yan, R. J. Simes, P. O. Holtz, L. A. Coldren, and P. M. Petroff "Optical Anisotropy in a Quantum Well Wire Array With Two Dimensional Quantum Confinement". Phys Rev. Lett. 6,466 (1989). M.S. Miller, C.E. Pryor, L.A. Samoska, H. Weman, H. Kroemer, and P.M. Petroff, "Serpentine Superlattice in GaAs; Concept and Results”. The Physics of Semiconductors (ed. E.M.Anastassakis and J.D.Joannopoulos, World Scientific Publ.). p.1717 (1990) M.S. Miller, C.E. Pryor, H. Weman, L.A. Samoska, H. Kroemer, and P.M. Petroff, "Serpentine Superlattice: Concept and First Results". J.Cryst Growth 111, 323 (1991) M.S. Miller, H. Weman, C.E. Pryor, M. Krishnamurty, P.M. Petroff, H. Kroemer, and J.L. Merz, "Serpentine Superlattices of AlGaAs Grown on GaAs Vicinal Surfaces” Phys. Rev. Lett. 68, 3464 (1992).

Tilted superlattices, Serpentine superlattices and Self assembled quantum wires with J.Gaines and M.Miller Conventional quantum wells and superlattices : growth direction is normal to the substrate surface Interfaces are parallel to substrate surface AlAs GaAs InAs

Atoms will diffuse to steps. Steps will move in phase. Atoms stick to the step edges and do not climb steps. We are able to control deposition to 0.1 ML! Vicinal {100} surface h=2.8Å Periodic steps 1 o ->80Å 2 o ->40Å (GaAs) 0.5 (AlAs) 0.5 THE TILTED SUPERLATTICE WITH INTERFACES PARALLEL TO THE GROWTH DIRECTION

TILTED SUPERLATTICE AND QUANTUM WIRE SUPERLATTICE tgß= |p-1|/tg   p=1.1  2 o p=m+n p=0.9  =2 o (GaAs)m(AlAs)n, with p=m+n ≂ 1 and m or n>0.5 or <0.5

Serpentine Superlattice Modeling TEM cross section p=0.9p=1p=1.1 ß=-30 o ß=0 o ß=60 o Flux non uniformity solution: Parabolic quantum well profile with linear variations of p(t)=m+n --> Quantum wires

It is the constant testing of the assumptions which makes for progress in Science. Daring! How well does it work? PH YSICAL REVIEW LETTERS 8 JUNE 1992

MOLECULAR BEAM EPITAXY ON SUBSTRATES WITH LARGE LATTICE MISMATCH e.g.: THE HOLY GRAIL III-V layers ON SI With T.Y. LIU

Various solutions to a very old problem Dislocations Thick buffer layer: Dislocation interactions  cm -2 to 10 7 cm -2 Micro pillars: Image forces  cm -2 to 0 cm -2 Multiple strained layers or strain graded layers: dislocation interactions  cm -2 to 10 7 cm -2 Lateral Epitaxy Overgrowth (LEO): Dislocation filtering  cm -2 to 10 4 cm -2 Wafer fusion: interface defects (dislocations) and interface traps. Quantum dots as active medium. Lattice mismatch and thermal expansion coefficient  misfit and threading dislocations Dislocations are deep levels. Electrons or hole traps are thermally or optically ionized Solutions: a1a1 a1a1 F3 F2 F1 a2a2 Si (A) (B) a3a3 anan

Hybrid MBE-LPE growth of hetero-structures with large Lattice mismatch and differential thermal expansion coefficients. Decouple the substrate from the epitaxial layers during growth GaAs GaAs substrate Liquid Phase Epitaxy (LPE) MBE Hybrid MBE-LPE asas a2a2 anan Liquid layer Ga(L) P.M.Petroff Materials department, University of California, Santa Barbara DISLOCATIONS REMOVAL IN HYBRID HETEROSTRUCTURES

Misfit dislocations sources and dislocation interactions in thick buffer layer or multi-layer samples  dislocation density : cm -2 to 10 7 cm -2 b1 b2 b1b2 b1b2 b3 b1+b2+b3=0 Solutions 1

Solutions 2-3 Image forces  Dislocations elimination Micro-pillars: Problems: Lithography and regrowth Small areas for devices Lateral epitaxial overgrowth: Dislocation filtering Dislocation density: cm -2 to 10 4 cm -2 Dislocation density: cm -2 to 0 cm -2

Bonded interface ≈10 5 dislocations /cm 2 and interface traps: Yet the laser is working. The active medium: several layers of quantum dots with large carrier capture cross section and fast and efficient carrier radiative carrier recombination. Problem: Passivation of defects at the fused interface. Solution 4: Fusion

(j)(k) Dislocations Si Melted thin film L1 L2 L3 (f)(g)(h)(i) (a)(b) ( c)(d)(e) L4 Remelt of L1 layer for liquid solid equilibrium. Dislocation climb and image forces eliminate dislocations in L3. Cooling to 300K may introduce MD confined to the layer with lowest shear modulus??? Proposed method: Use as a first layer a low melting point layer. (L1 layer: eg. InSb) Ideal case Real World Cooling Growth

Does it work ? Yes and then No: Liu made a mistake! In fact we do not know. Be daring and lets try it again seriously !

Why LPE does not work for sharp hetero- interfaces GaInSb ternary system e.g: at 550C, Ga.34 In.66 Sb (S) Ga.1 In.9 Sb (L) Liquid Solid equilibrium requires to be on the same tie line -> solid will readjust its composition and -> remelt and resolidification of the substrate or epilayer L2.

(j)(k) (f)(g)(h)(i) Dislocations Si (a)(b) ( c)(d)(e) Melted thin film L1 L2 L3 L4 Remelt of L2 layer for liquid solid equilibrium. Dislocation climb and image forces eliminate dislocations in L3. e.g. nanowires grown by VLS Cooling to 300K may introduce MD in layer with lowest shear modulus??? Proposed method: Use as a first layer a low melting point layer ( e.g. InSb) Ideal case Real World Cooling Growth

Dislocations dynamics at a liquid solid interface in Si (CZ growth) Dislocations climb and glide to the liquid solid interface

(j)(k) (f)(g)(h)(i) Dislocations Si (a)(b) ( c)(d)(e) Melted thin film L1 L2 L3 L4 Remelt of L2 layer for liquid solid equilibrium. Dislocation climb and image forces eliminate dislocations in L3. Cooling to 300K may introduce MD in layer with lowest shear modulus( InSb Layer)??? Proposed method: Use as a first layer a low melting point layer ( e.g. InSb) Ideal case Real World Cooling Growth

F3 F2 F1 Si (A) (B) M1 M2 M3 Si (C ) Misfit strain and thermal strain effects in the substrate decoupled epitaxial film M1 M2 M3 Si (D) Finite element calculation, linear elasticity M.Finot et al. J.Appl. Phys. 81, Liquid –Solid surface tension : Complete wetting case Grow lattice parameter matched layers Mismatched layers

(j)(k) (f)(g)(h)(i) Dislocations Si (a)(b) ( c)(d)(e) Melted thin film L1 L2 L3 L4 Remelt of L2 layer for liquid solid equilibrium. Dislocation climb and image forces eliminate dislocations in L3. Cooling to 300K may introduce MD in layer with lowest shear modulus??? Proposed method: Use as a first layer a low melting point layer ( e.g. InSb) Ideal case Real World Cooling Growth