Physics and Material Science of Semiconductor Nanostructures

Transcription

1 Physics and Material Science of Semiconductor Nanostructures PHYS 570P Prof. Oana Malis Course website:

2 Lecture 5 Bulk semiconductor growth Single crystal techniques Nanostructure fabrication Epitaxial growth MBE, MOCVD Fabrication techniques Bottom up approaches: self assembly Ref. Ihn Chapter 6

3 Top down approach Mesa-etched dot 1µm~100nm Get a very big piece of wood and carve it into a much smaller model tree (TOP DOWN APPROACH)

4 Top down versus bottom up If we want to make a very small tree we can either Get a very big piece of wood and carve it into a much smaller model tree (TOP DOWN APPROACH) Plant a seed and then control its growth to form a fullyfunctioning bonsai tree (BOTTOM UP APPROACH)

5 Kondo corral STM image Bottom-up method Interference pattern of twodimensional electron gas on Co/Cu(111) D.M.Eigler et al. PRL 86(2001)2392

6 Cleaved-edge overgrowth (CEO) Nanowire growth by MBE

7 V-groove nanowires Pseudowires V-shape due to different etching directions Growth of barrier material Growth of wire material Growth of 2nd barrier material to sharpen groove again wire

8 Short Period-AlGaAs/GaAs quantum wires (QWR) Array Laser Diode with SiO 2 Current Blocking Layer AlGaAs/GaAs QWRs (5x20 nm 2 ) GaAs substrate OUTPUT POWER [mw] SP-QWAL Uncoated facets (200 x 500 m) I th = 143 ma J th = 0.14 ka/cm 2 P max = 9 mw diff = 17 % c = 825 nm Appl. Phys. Lett. 69(7), 955 (1996) IEEE Photon. Tech. Lett. 9(1), 2 (1997) CURRENT [ma]

9 Self-assembly of nanostructures

10 Simple example: Quantum wire growth on periodically-facetted surfaces

11 Quantum wire growth on periodically facetted surfaces The first self-assembly route for semiconductor nanostructure we will consider is the growth of quantum wires on periodically facetted surfaces. To motivate our discussion, we begin with an example: AFM image of self-assembled InGaAs quantum wires grown on a facetted GaAs surface. Quantum wires of this type were incorporated into a semiconductor laser which exhibited linear polarisation of its light output due to the anisotropy of the active region. Ohno et al. J. Vac. Sci. Technol. B, 22, (2004)

12 Reminder: Crystal habit planes The shapes of natural minerals often reflect their underlying crystal symmetry to some extent This is often because the crystals grow with low energy facets forming large areas What happens if we deliberately produce a crystal surface which is at an angle to these low energy facets?

13 Formation of periodically faceted surfaces When the crystal is sliced through at a random angle, many of the atoms are in positions where they cannot fulfill their bonding By reorganising the atoms to form low energy facets, many atoms are now in more energetically favorable positions

14 Example of a periodically facetted surface These STM images shows a TaC(110) surface which has broken up into an array of facets following annealing. The period of the structure along [1-10] is 6a (Image from Zuo et al. Surf. Sci. 301, 233 (1994))

15 Vicinal surfaces and macroscopic step bunching A vicinal surface occurs when a crystal surface is at a small angle to a low Miller index plane. It ideally consists of flat terraces with low Miller indices, with neighbouring terraces separated by equallyspaced steps of monolayer height As with the faceted surfaces we have already discussed, there is an energy associated with each step edge. Hence, it is possible to reduce the overall energy of the system by forming step bunches. However, the contribution of elastic stress to the total energy will again increase as the step bunches get further apart, so again an optimum step spacing exists which minimises the total energy.

16 Vicinal surfaces and macroscopic step bunching: possible configurations Here, the step bunches form 2 facets: the original low index facet and another facet at 90 to that Here, the steps form an array of alternating singular facets (terraces) and vicinal facets. The vicinal facets may reconstruct (i.e. rearrange the atoms) to reduce the local surface energy further.

17 Vicinal surfaces and macroscopic step bunching: Example 200 nm 200 nm STM image of a vicinal Si(111) surface. The net surface orientation is 4 off (111) towards (-211). (7 7) reconstructed terraces and unreconstructed step bunches 10 steps high are observed. From: Williams et al. Surf. Sci. 294, 219 (1993).

18 Heteroepitaxial growth on facetted surfaces The formation of surfaces with nanoscale separation is all very good but to form quantum wires, we need to have regions of a lower bandgap material surrounded by a higher bandgap material. So what happens when we deposit a second material (heteroepitaxy) on our faceted surface? It could cover the surface homogeneously It could form isolated large islands It could form lots of small islands

19 Heteroepitaxial growth on facetted surfaces: quantum wires Often, the periodic facets provide a template for the shape of the islands which are deposited. Hence, quantum wires are formed. Isolated large wire Smaller wires, with a distribution of lengths

20 Influence of interface energy on heteroepitaxial growth mode So, what controls whether we get a homogenous surface layer or the growth of islands? A key factor is the energy of the interface(s) between the periodically faceted substrate surface and the epitaxial layer. If the interface energy is low, the deposited material will tend to wet the substrate i.e. we will get a homogenous coverage. If the interface energy is high, the deposited material will tend to form islands. To derive the equilibrium island shape, we would also need to consider the lattice mismatch between the cluster and the island and the resulting strain energy. In a real system, kinetic effects will also play a role. For instance if diffusion over the surface is slow, this will tend to favour the formation of lots of small clusters, rather than isolated large clusters.

21 Example of quantum wire formation on a periodically facetted surface Efremov et al. (Physica E 23 (2004) ) grew a GaAs quantum wire superlattice on a periodically facetted AlAs(311) surface. Following the growth of an initial layer of GaAs wires, more AlAs is deposited and reforms the originally facetted surface, allowing more layers of wires to be deposited. The left hand image is a high resolution TEM image of the quantum wire array. The right hand image is a simulation of an HRTEM image for an ideal array.

22 Self-assembled quantum dots

23 Formation of InAs quantum dots (S-K mode) InAs (a 0 =6.06 Å) GaAs (a 0 =5.65 Å) (i) (ii) 2D layer growth 2D-3D transition Ripening phase Elastic strain energy stable 2D (i) metastable 2D (ii) S-K morphology 2D + 3D E a (i) Strain exists but lattice matched (ii) Coherently strained defect-free 3D island Growth time W. Seifert et al., J. Crystal. Growth (1997)

24 Typical characteristics of self-assembled In x Ga 1-x As quantum dots formed on GaAs (100) substrate (100) 240K PL Intensity (a. u.) 200K 150K QD 100K ev QW 17K ev Photon Energy (ev) 1.3 Monolayers of InAs, T sub : 500 O C N QD : 3.4 X /cm 2, QD Size : ~ 40 nm => Resulting in irregular positioning and distribution

25 Typical structures of multi-stacked InAs QDs/GaAs S 15 periods (Volcano-like defect) Vertically aligned QDs [001] [110] 6 periods Well aligned

26 Multi-stacked InAs QDs grown by MBE GaAs AlGaAs GaAs InAs Q.D. GaAs Well aligned QD array Large strain high In mole fraction ~ 30 nm Small strain Large strain ~ 55 nm 1.7 ML InAs at 470 C GaAs/AlGaAs at 510 C ~ 6 nm

27 The Structure of QD VCSEL 1.3 m Intermixed QD oxide layer oxide layer top DBR bottom DBR GaAs/Al(Ga)As DBR GaAs/Al(Ga)As DBR GaAs

28 Infra-red Photodetector Using Quantum Dots A maximum responsivity of 4.7 A/W has been recorded at 10 K and bias-voltage 9 V. High responsivity has been seen up to 190 K. Tokyo University, K. Hirakawa lab., Appl. Phys. Lett. 75(10), 1428 (1999)

29 Schematic illustration for selective formation of QDs Sputtering or CVD GaAs (100) substrate Oxide layer PMMA Oxide layer Patterning of PMMA by E-beam Lithography Oxide Mask Electron-beam or laser holography Etching and Removing of PMMA Oxide layer Molecular beam epitaxy Selective growth of QDs (InGaAs) Etching Removal of oxide layer Wetting layer

30 InGaAs QDs Selectively Formed on the Patterned Oxide Layer QDs 0.1 um 0.3 um Ga 2 O 3 QDs GaAs substrate QDs Ga 2 O 3 mask layer

31 Sample structures for selective QD growth using strained SL GaAs 180 ML InAs QDs (2ML) GaAs 20ML GaAs b ML In x Ga 1-x As a ML GaAs buffer 0.5 μm GaAs (001) sub N periods n periods Strained layer - a ML InGaAs/ b ML GaAs SL *Averaged composition is fixed at x=0.2-20ml GaAs (act as barrier) x a 4MLs 2MLs 1ML b 1ML 3MLs 4MLs n N

32 Surface morphologies of the InAs QDs, QWRs (2 o -off (100) GaAs substrates, 1x1 m 2 AFM images ) Changing the thickness of GaAs buffer layer transformation of the terrace width Growing optimal thickness of InAs layer for wirelike QDs Control of interval between wirelike QDs InAs wirelike QDs GaAs buffer GaAs buffer GaAs Buffer 43 ML / 55 nm / 2.0 ML 70 ML / 75 nm / 2.3 ML 120 ML /91 nm / 2.5 ML The thickness of GaAs buffer layer / The terrace width / The optimal thickness of InAs layer

33 Single electron device using self-assembled QDs InAs quantum dots Al lever-arm T = 300 K 200 GaAs cap layer 4 nm InAs QD layer GaAs buffer layer 20 nm S.I. GaAs Substrate Al lever-arm Current (pa) di/dv (ns) InAs quantum dots nm Bias Voltage (V)

34 SET structures using In(Ga)As/GaAs wire-like SAQDs Source Drain Source Gate Drain Control Back Gate Upper Gates Back Gate

35 Synthesis of Nanowires Methods of Nanowire synthesis VLS (Vapour Liquid Solid) method Modification of VLS CVD (Chemical Vapour Deposition) LCG (Laser Ablation catalytic Growth) Low temperature VLS method FLS (Fluid Liquid Solid) mechanism SLS (Solution Liquid Solid) mechanism OAG (Oxide Assisted Growth)

36 Semiconductor nanowires Catalytic (VLS) crystal growth Key features: gold particle liquid Au-InP eutect vapor time nanowire nanoscale diameter (few to 100 nm) High aspect ratio (1-100 micron long) Versatility in composition

37 Possible nanowire structures heterojunctions hollow p-n junctions coaxial

38 Before growth InP wire on SiO 2 Zinc Blende [111] direction 10 nm

39 InP Tubes 200 nm Bakkers et al., JACS 2003, 125, nm Zinc Blende crystal structure

40 50 nm Coaxial wires InP GaP Counts Ga P 200 In Position (nm) Group III modulation

41 Heterojunctions Au GaAs GaAs GaAs GaP GaP GaP 100 nm GaP GaP: 1.8 nm/sec GaAs: 5.0 nm/sec Section length (nm) GaAs GaP Group V modulation Growth time (sec)

42 Heterostructures nanowires (Chemical beam epitaxy, MOVPE) InAs InP InAs Björk et al., NanoLetters 2, 87 (2002) (Samuelson s group Lund) Almost atomically sharp interfaces No strain-induced dislocations (stress can relax at the surface)