Physics and Material Science of Semiconductor Nanostructures

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

2 Today Bulk semiconductor growth Single crystal techniques Nanostructure fabrication Epitaxial growth MBE MOCVD Bottom up approaches: self assembly Top down approach: Device fabrication techniques Ref. Ihn Chapter 2, 6

3 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)

4 Molecular Beam Epitaxy

5 Molecular beam epitaxy MBE Thin film growth in high vacuum or ultra high vacuum (UHV) (10 8 Pa). Deposition rates are typically slow (less than 1000 nm per hour) so high vacuum is required to achieve high purity material. Ultra-pure elements (e.g. gallium and arsenic) are heated in separate crucibles until they begin to slowly sublimate. The gaseous elements then condense on the substrate, where they may react with each other (e.g. Ga + As GaAs). The term molecular beam" implies that the evaporated atoms do not interact with each other or vacuum chamber gases until they reach the substrate.

6 Effusion cells Basic schematic small aperture Real example Temperature control is very important since temperature controls equilibrium vapour pressure, and hence deposition rate solid close to equilibrium with gas Heat Shutter over aperture must operate very quickly to ensure fast switching between beams of different elements (e.g. In, Ga) and achieve sharp interfaces.

7 Monitoring MBE: RHEED Reflection high-energy electron diffraction Same high-vacuum requirements as for molecular beams. Electron beam interacts with surface layers at glancing angle Elastically scattered electrons form streaks on screen if surface is flat GaAs (110) surface

8 RHEED: growth modes and surface reconstructions Growth modes: 2D growth: step flow 2D growth: layer-by-layer (Frank-van der Merwe) 3D growth: layer-plus-island (Stranski-Krastanov) 3D growth: island (Volmer-Weber)

9 RHEED: layer-by-layer growth Intensity oscillations of RHEED streaks can be used to find the growth rate Slow growth rates enable monolayer-by-monolayer growth. Complete monolayer smooth surface peak in RHEED intensity Partial monolayer rougher surface reduced RHEED intensity One period of RHEED oscillation gives time for growth of one monolayer.

10 RHEED: 3D nanostructures Flat sample Sample with small 3D islands Eletrons scattered at surface Streaky RHEED patterns Electrons transmitted through island Spotty RHEED patterns If a transition occurs from 2D to 3D growth then the electron beam will pass through the island, and the diffraction pattern will be more similar to electron diffraction from the bulk of the material i.e. spotty. Hence a transition from 2D growth to the formation of small 3D islands (quantum dots) leads to a change in the RHEED pattern from streaky to spotty, which can be a useful signature.

11 Other monitoring methods for MBE The existence of ultra-high vacuum in the MBE chamber, enables the use of RHEED which cannot be used in MOVPE (since the electrons would be scattered by gas species). Other (not so common) techniques used for in situ monitoring of MBE include: x-ray diffraction scanning tunnelling microscopy low energy electron diffraction and microscopy optical monitoring techniques

12 Growth temperatures For both MBE and MOVPE it s very important to know what the growth temperature is... It should be high enough to dissociate precursors in MOVPE but not so high that the film decomposes. (May also need to dissociate molecules in MBE e.g. As 4 ). Temperature will affect the film growth in many ways... - Relative sticking coefficients (usually lower with higher T) composition (including impurity levels) - Surface adatom mobilities defects & roughness - Surface reconstructions may get abrupt changes in growth modes/composition etc. - Native point defect concentrations and mobilities (important for dislocation mobility and dopant diffusion)

13 Temperature measurement Thermocouples (± 1 o C) A junction is created between two different metals, which produces a voltage related to a temperature difference between the junction and a voltmeter held at RT (Seebeck effect) Attached to the back of the sample or the sample mounting plate doesn t measure the surface T Pyrometers (± 50 o C) Intensity of emission from heated black body is correlated to its temperature Hence measure intensity of emission at a particular wavelength to determine T But emission of real body (wafer) is different from black body, so emissivity ε has to be determined Apparent temperature varies with the emissivity of the film For transparent films, emission from substrate effects measured T!

14 Temperature measurement Band edge monitoring Detects shift of band edge (optical absorption vs T) versus temperature Very expensive Only works for material systems of known composition with band gaps in the easily-detectable region Surface reconstructions (MBE only) Single crystal wafer heated under observation with RHEED; surface reconstructions change at a specific temperature can use for calibration Only works for a few material systems Temperature of transition has to be known! You don t know what the real temperature is!

15 Quantum Dots Grown Using MBE Electronic Structure of InAs Pyramidal Quantum Dots : AFM Three-dimensional AFM image of CdSe QDs deposited on ZnCdMgSe barriers. The inset shows a histogram of the QD height [ Courtesy: Prof. Tamargo S group- CCNY ].

16 Comparison of epitaxial technques 16

17 Chemical Vapor Deposition Precursor Reactor Energy Solid Products (thin films and powders) Gas Phase products Microfabrication processes widely use CVD to deposit materials in various forms, including: monocrystalline, polycrystalline, amorphous, and epitaxial. Materials grown by CVD include: silicon, carbon fiber, carbon nanofibers, filaments, carbon nanotubes, SiO 2, silicon-germanium, tungsten, silicon carbide, silicon nitride, silicon oxynitride, titanium nitride, and various high-k dielectrics. The CVD process is also used to produce synthetic diamonds.

18 Precursor Considerations Volatility vapor pressure - simple molecules with high vapor pressure are rare determined by molecular weight and molecularity (degree polymerized) - result of structure and bonding control - temperature, valving Stability, Reactivity, and Safety bond strength, bond dissociation energy - affects process temperature and film composition (purity) thermal stability in storage and delivery into the reactor reactivity of the precursor and byproducts towards other substances (including biological objects like us) Single-Source Precursor providing more than one element into the film simpler delivery system uniform elemental distribution at atomic level possible limited composition range Common Precursors hydrides: MHx - SiH4, GeH4, AlH3(NMe3)2, NH3, PH3... halides: MXy - TiCl4, TaCl5, MoF6, WF6,... metal-organics - metal alkyls: AlMe3, AliBu3, Ti(CH2tBu)4... metal alkoxides: Ti(OiPr)4, [Cu(OtBu)]4... metal dialkylamides: Ti(NMe2)4, Cr(NEt2)4... metal diketonates: Cu(acac)2, Pt(hfac)2... metal carbonyls: Fe(CO)5, Ni(CO)4... others: complexes with alkene, allyl, cyclopentadienyl,... ligands many precursors have mixed ligands

19 Energy sources and Reactor types Thermal Energy resistive heating - tube furnace quartz tungsten halogen lamp (very good heat source) - radiant heating radio-frequency - inductive heating laser as thermal energy source Photo Energy UV-visible light laser as photo energy source

20 MOCVD (also called MOVPE) H 2 +AsH 3 RF Coils Gas Exhaust Substrate Mixing Chamber Al(CH 3 ) 3 Ga(CH 3 ) 3 H 2 Ga(CH 3 ) 3 + AsH 3 => 3CH 4 +GaAs Ref. Ihn Chapter 2

21 MOCVD (MOVPE) Metal-organic vapour phase epitaxy Thin crystalline films created at high temperatures by chemical reactions between metalorganic and anion precursors. Some typical precursors for III-V compound semiconductors: Group III Group V Dopants Ga(CH 3 ) 3 (TMG) NH 3 SiH 4 (in H 2 or He) Ga(C 2 H 5 ) 3 (TEG) AsH 3 CH 4 PH 3 Cp 2 Mg Cp 2 Fe Ga(CH 3 ) 3 + NH 3 GaN + 3 CH 4 2 Ga(C 2 H 5 ) AsH 3 2 GaAs + 6 C 2 H 6 Carrier gas usually H 2, Ar and/or N 2, used to transport metalorganic precursors to reaction chamber.

22 Carrier gas MOCVD (MOVPE) schematic Dopant 1 Filter & purifier MFC MFC MFC = mass flow controller controls the amount of gas going into the reactor Dopant 2 Group V precursor Carrier gas MFC MFC MFC MOVPE reactor Scrubber unit Gp. III precursor (in bubbler) Heater Power supply

23 Example of a real GaN reactor

24 MOCVD Precursor purity Typical atomic density in a semiconductor: 5 x atoms cm -3 Typical doping density in semiconductor devices: cm -3 To avoid unintentional doping of devices, this implies a desired purity of better than 1 (10 14 /10 22 ) = %. Best available precursors are about % pure (referred to as 7 Nines or 7N), and are very expensive. Carbon contamination from metalorganic precursors may also be a source of unintentional doping, as may contaminated system components.

25 Monitoring MOCVD growth Typical in situ monitoring uses optical techniques Example: optical reflectivity measurements on GaN: laser beam transparent GaN epilayer Sapphire substrate Beams reflected from GaN surface and GaN/sapphire interface interfere. As film grows, nature of interference oscillates between constructive and destructive, resulting in oscillating intensity of reflected light This can be used to measure growth rate.

26 Comparison MOVPE Moderate pressure (100 mbar) Higher temperatures High throughput Hydrogen normally present Greater process drift Higher growth rates Highly toxic/explosive precursors Higher impurity levels $$$ MBE Ultra-low pressure (10-11 mbar) Lower temperatures Lower throughput No hydrogen unless deliberately added Precise thickness control Lower growth rates Safer precursors (usually!) Ultra-low impurity levels $$$$

27 Other Epitaxial Techniques Liquid Phase Epitaxy - Dipping or Tipping Chemical Beam Epitaxy - Combination of MBE and MOCVD

28 Laser Assisted Vapor Deposition He H 2 SiH 4, SF 6, H 2

29 Laser Assisted Catalytic growth Fig. 2 Schematic of a nanowire growth set up using a combination of laser ablation and vapor-liquid- solid scheme Fig.1 Semiconductor (GaAs) nanowires grown using laser assisted catalytic growth [1ref]. The scale bar corresponds to 50 nm.

30 PECVD (Plasma-Enhanced CVD)

31 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)

32 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

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

34 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

35 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]