Semiconductor Materials Engineering - Nanoscale Structures

Very Clean Materials

• Impurities less than $10^{-9}$.
• Comparison:
  • Polycrystalline metals and dielectric crystals: Impurities ~0.001.
  • Amorphous polymers: Impurities ~0.01.
• Good mechanical parameters:
  • Hardness can exceed that of steels.
  • Allows for significant reduction in device dimensions.
  • Increases production yield.
• High thermal conductivity:
  • Devices can be placed more densely without concern for heating due to energy dissipation.

Unique Electrical Properties

• Electrical Conductivity:
  • Semiconductors fall between metals and dielectrics in terms of electrical conductivity.
  • Conductivity depends on the product of charge carrier concentration and their mobility.
  • Electron mobility in semiconductors is several orders of magnitude higher than in metals or dielectrics.
  • Even a small change in charge carrier density significantly alters the material's electrical conductivity.
  • This principle underlies the operation of many devices.

Optical Properties

• Dependence on Material Purity:
  • Optical properties are heavily influenced by the purity of the material.
  • Semiconductors are unique in their purity, enabling the observation of charge carrier recombination.
  • Recombination is due to direct electron transitions between energy levels of the crystal's constituent atoms, not impurity or defect centers.
  • This characteristic is widely used in semiconductor light sources, making them more efficient than other types of devices.

Desired Attributes of Semiconductor Devices

• High-Speed Electronics:
  • Large Frequency: Measured in GHz or even THz.
  • Applications: Mobile phones, satellite TV, computers.
• Optoelectronics:
  • Current ↔ Light conversion.
  • Applications: Lasers, light-emitting diodes (LEDs), fiber optic communication.
  • Solar cells, photodetectors.
• Miniaturization: Devices should be as small as possible.
• Speed: Devices should be as fast as possible.
• Cost-Effectiveness:
  • Low service costs.
  • Low energy consumption.
  • Inexpensive.

US Patent 2402662 A

• Light-sensitive electric device
• Publication date: June 25, 1946
• Official filing date: May 27, 1941
• Patent registration date: May 27, 1941
• Also published as US2443542
• Inventors: Russell S. Ohl
• Original assignee: Bell Telephone Laboratories Inc.
• Russel Ohl - First semiconductor device.

Liquid Phase Epitaxy (LPE)

• Growth occurs from a solution.
• Suitable for heterostructures and doping.
• Offers moderate uniformity and thickness control.
• "Workhorse" in optoelectronics (LEDs, PDs, etc.).

Gas Phase Epitaxy

• Metal-Organic Chemical Vapor Deposition (MOCVD) (typically III/V).
• Growth occurs from the gas phase.
• Dominates in optoelectronics and Si-based electronics (LPCVD).

Molecular Beam Epitaxy (MBE)

• Evaporation in a vacuum.
• Dominates in III/V semiconductor-based electronics and optoelectronics.
• Allows for abrupt junctions and versatile selection of chemical elements.
• Popular research tool.

Liquid Phase Epitaxy (LPE) Details

• First demonstrated in 1963.
• Thin film deposition from a solution onto a substrate with crystalline orientation.
• Solvent can be In, Ga, or other low-melting-point metal.
• Solution, e.g., As

Liquid Phase Epitaxy (LPE) Growth Methods

• Step-cooling: Thickness $\propto \sqrt{t}$
• Equilibrium cooling: Thickness $\propto t$
• Supercooling: Thickness $\propto t$
• $K$ is a constant dependent on the diffusion of each dissolved substance and the molar fraction of the soluble substance in the solution at the growth temperature.
• $t$ is the growth time.
• $R$ is the cooling rate.

Advantages of Liquid Phase Epitaxy (LPE)

1. Inexpensive and simple setup.
2. No high growth temperatures required ($350-900^{\circ}C$).
3. Relatively high growth rate.
4. Low concentration of point defects.

Disadvantages of Liquid Phase Epitaxy (LPE)

1. Compositional non-uniformity.
2. Insufficiently good morphology.

Gas Phase Epitaxy: Metal-Organic Chemical Vapor Deposition

• Group III: Metal-organic precursors.
• Group V: Commonly hydrides.
• Examples:
  • Arsenic, Phosphorus (As, P)
  • Hydrogen, Carbon (H, C)
  • Metal atom (Ga, In, Al)

Gas Phase Epitaxy System Components

1. Hydrides
2. Metal-organic sources
3. Ventilation manifold
4. H2 or N2 carrier gas
5. Quartz tube reactor with heated graphite susceptor
6. Throttle valve for total pressure control
7. System pump
8. Exhaust gas system

Advantages of Gas Phase Epitaxy

1. Most flexible.
2. Suitable for large-scale production.
3. Abrupt junctions.
4. Simple reactor, no ultra-high vacuum required.
5. Economically viable.

Disadvantages of Gas Phase Epitaxy

1. Low vapor pressure of many materials makes them difficult to transport via gas.
2. Potential for carbon contamination and unintentional hydrogen introduction.
3. Most parameters require precise control.
4. Harmful precursors.

Molecular Beam Epitaxy (MBE)

• Atomic layer by atomic layer growth.
• High-resolution transmission electron micrograph (TEM) showing a GaAs/AlAs superlattice.

Advantages of Molecular Beam Epitaxy (MBE)

1. Simple process.
2. High uniformity.
3. Abrupt junctions.
4. Extensive in-situ monitoring capabilities.

Disadvantages of Molecular Beam Epitaxy (MBE)

1. Problems with the P source.
2. Expensive operation.

Semiconductor Usage

• Si (excluding solar cells): 400 ha.
  • Applications: Microprocessors, memory, CCDs, etc.
• Other semiconductors (mainly GaAs and other III-V compounds): 10 ha.
  • Applications: Lasers, LEDs, microwave devices, etc.

Direct vs. Indirect Bandgap

• Direct Bandgap:
  • The conduction band minimum and valence band maximum are at the same point in the Brillouin zone (e.g., GaAs).
• Indirect Bandgap:
  • The conduction band minimum and valence band maximum are at different points in the Brillouin zone (e.g., Silicon).

Infrared (IR) Spectral Regions

• Near-Infrared (NIR): 800 nm to 1.5 μm (sometimes up to 3 μm).
• Mid-Infrared (MIR): 1.5 μm (or 3 μm) to ~6 μm (or 30 μm).
• Far-Infrared (FIR): Remaining portion up to 1 mm.
• Terahertz Frequencies: 0.1 THz to 10 THz (30 μm – 300 μm).

Methods for Modifying Band Structure Parameters

• Creating alloys of two or more semiconductors.
• Using heterostructures to create quantum-confined structures.
• Epitaxial growth on mismatched substrates.
• These methods, alone or in combination, are widely used in the design and fabrication of various electronic and optoelectronic devices.

Forbidden Energy Gaps

• Elemental Semiconductors:
  • Si: 1.12 eV (amorphous Si: 1.71 eV)
  • Ge: 0.661 eV
  • C (diamond): 5.46 eV
  • Te: 0.33 eV
• Binary Semiconductors:
  • GaAs: 1.41 eV (0.87 µm)
  • GaP: 2.26 eV
  • GaSb: 0.66 eV
  • GaN: 3.4 eV (0.36 µm)
  • InAs: 0.36 eV (3.4 µm)
  • InSb: 0.17 eV (7 µm)
  • InN: 0.65 eV (1.9 µm)
  • InP: 1.34 eV (0.9 µm)
  • SiGe: 1.12-0.66 eV
  • CdTe: 1.5 eV (0.8 µm)
  • ZnO: 3.3 eV (0.37 µm)
• The easiest way to change the properties of a material is to make an alloy of it with another material that differs in its characteristics.
• In the case of semiconductors, materials are alloyed to achieve two goals:
  • To obtain the required forbidden energy gap.
  • To obtain a material whose lattice constant matches the lattice constant of the substrate used in epitaxy.

Ternary Compounds

• Many parameters of ternary compounds depend on the parameters of the binary compounds that make them up, and vary linearly as the composition $x$ changes.
• Vegard's Law
• Example: $Al<em>xGa</em>{1-x}As$ lattice constant:
  • $a<em>{AlGaAs} = xa</em>{AlAs} + (1-x)a_{GaAs}$
• Example: $Al<em>xGa</em>{1-x}As$ bandgap:
  • $E<em>g^{AlGaAs} = xE</em>g^{AlAs} + (1-x)E_g^{GaAs} + cx^2$
• The compound becomes indirect.
• Virtual Crystal Approximation.

Semiconductor Technology Philosophy

• Make structures smaller and smaller.
• If needed materials don't exist, create them.
• Change electronic and optical properties using quantum confinement.
• At the nanometer scale, many new things happen.
• Requires searching for new fundamental physics, new applications, and new devices.
• Current advanced semiconductor technology relies on miniaturization and layer-by-layer growth.

Heterostructures

• Type I vs. Type II Heterostructures.
• In real junctions, charge carrier redistribution occurs, creating spatial charge regions and band edges distort.

AlGaAs/GaAs Heterostructure

• "Historical" AlGaAs/GaAs heterostructure.
• AlGaAs: depleted layer.
• GaAs: enriched layer.
• A 100-150 Å triangular potential well forms on the GaAs side, filled with two-dimensional electron gas.

Various Heterostructure Structures

• Quantum well (QW).
• Multiple quantum wells (MQW).
• Superlattice.
• Barrier.
• Double barrier with a quantum well.

Quantum Well (QW)

• Localized electron levels.
• Localized hole levels.
• New, larger forbidden energy gap.

Quantum Well Width and Energy

• From R. Dingle Festkoerperprobleme 15,21 (1975)
  • Well Width
    • 19.8 nm: 0.25 meV
    • 12.2 nm: 0.4 meV
    • 8.3 nm: 1.0 meV
    • 5.1 nm: ~ 5 meV
  • F. Pulizzi et al., Magnet Lab Nijmegen, 2001
    • 14 nm
    • 21 nm
    • 400 nm: 10 meV

Other Heterostructures

• Tunnel Barrier:
  • Used in hot electron transistors and varactors.
• Resonant Tunneling Diode

Resonant Tunneling Diode (RTD)

• The current-voltage characteristic (IVACH) is similar to that of a tunnel diode (Esaki diode).
• The parasitic capacitance of the diode is greatly reduced, which is why it is suitable for higher frequencies than the tunnel diode.
• Frequency band above 700 GHz.

Superlattices

• When quantum wells are close to each other, their levels overlap and the degeneracy is removed, splitting the levels into minibands.
• When the wells are far from each other, they act independently—MQW (multiple quantum wells).

Quantum Cascade Lasers

• Traditional semiconductor laser: electron transitions occur between conduction and valence band levels.
• Quantum cascade laser: electrons fall from a higher quantum well subband to a lower one.
• The same injected electron can emit multiple photons.
• The wavelength depends on the layer thickness, not the material.

Quantum Wires and Quantum Dots

• Quantum wells are already used in many devices; why not try confining electrons in the remaining two directions.

Quantum Wires: Top-Down Technology

• Selective chemical etching to obtain 1D structures.
• Confinement is only achieved when the wires are about 10 nm thick.
• It turns out that it is too difficult to produce nanometer-sized 1D (wires) and 0D (dots) using chemical etching.

Self-Organization for Quantum Wires

• Growth on grooved substrates.
• Thicker GaAs (wire) occurs in the AlGaAs trench bottom due to differences in growth rates on various planes and surface diffusion of adsorbed atoms.

Strained Layers

• Crystalline layers grown on a crystalline substrate.
• Substrate and layer lattice constants match - lattice-matched layers.
• The lattice constant in the layer is larger than in the substrate - compressed layers.
• The lattice constant in the layer is smaller than in the substrate - tensile strained layers.

Influence of Strain on Band Structure

• Unstrained
• Compressed
• Strained

Quantum Dots

• When mismatch is too large, layers turn into dots.

InAs Quantum Dots

• Pyramidal QD energy gap dependence on dot size.
• Dots form spontaneously by rearranging on GaAs (311)B substrates upon sequential growth of AlGaAs and strained epitaxial InGaAs layers.
• Typical quantum dot size is about 30-150 nm.

Quantum Dots in Solar Cells

• Theoretically achievable efficiency - 65%.

QDIP - Quantum Dot Infrared Photodetectors

• Quantum Dot Infrared Photodetectors (QDIP) are other rapidly progressing QD optoelectronic devices.
• They have good applications in the near and mid-wavelength IR ranges (1 - 20 mm).
• Interband and intraband electron jumps are used for IR detection.

Laser Diode History

• Heterolasers (3D).
• Quantum wells (2D).
• Quantum dots (0D).

Advantages of Lasers with Quantum Dots

• First laser diodes with QDs were created in 1994.
• They allow overlapping a wide spectral range (from 0.87 to 1.9 μm).
• The lowest threshold current density (6 A/cm2, 2005) has been achieved in a continuous operation laser, as well as the widest tunable wavelength range (210 nm, 2000).
• Parameters better than other laser diodes: maximum power emitted in continuous mode from a single channel (12 W, 2002), relaxation oscillation frequency (7.5 GHz, 2005), optical radiation resistance, etc.
• 2010. Fujitsu. Laser, 25 Gb/s stream.

Disadvantages of Quantum Dot Lasers

• QD inhomogeneity: Inhomogeneous line broadening due to different QD sizes.
• Spatial discreteness of QD distribution: Carriers recombine in bursts, there is no diffusion to replenish QDs with new ones.
• Internal optical losses: Free carrier absorption in cladding.

Nanowires

• Planar 1D technology proved not very fruitful.
• Around 2008, nanowires emerged - 1D structures grown perpendicular to the substrate.
• They are obtained in two main ways: using templates and VLS (vapor-liquid-solid) method. The latter is based on self-formation.

Vapor-Liquid-Solid (VLS) Growth

• Discovered when growing Si "whiskers" (Wagner and Ellis, 1964)
• Au/Si eutectic droplets formed at the growth temperature.
• Droplets were supersaturated with Si, entering them from the surrounding vapors.
• Si in the droplet condensed.
• This process started at the liquid and crystal interface and continued in the same direction.
• Highly anisotropic growth

GaAs Nanowire Growth on (111)B planes

• [111] is energetically the most favorable direction r Pout Pin.

GaAs Nanowire Temperature Dependence

• GaAs nanowires: Important to choose the correct temperature.

Other III-V Nanowires

• GaAs, InAs, and InP nanowires (wurtzite - hexagonal).
• AlGaAs (zinc blende - cubic).

Core-Shell Nanowires

• Low T → axial growth (GaAs core, 450 ºC)
• High T → radial growth (AlGaAs shell, 650 ºC)
• GaAs core is passivated by the AlGaAs shell.

Axial Heterostructure Nanowires

• Axial segments are obtained by alternating molecule flows, e.g., turning off TMG and turning on TMI
• Growth is interrupted for 1 to 5 minutes to allow the Au droplet to empty from the previous Group III material.
• Thin segments → axial quantum wells.
• Diffusion of additional atoms from the substrate is possible, so it is difficult to predict the exact composition.

Catalyst-Free VLS Growth

• (111) Si plane.
• Covered with oxide in which windows of the desired diameter are opened using microimprint technology.
• Nanowire growth begins in these windows.

Nanowires as Part of Electronic and Photonic Devices

• Overcome fundamental limitation of lattice mismatch.
• Future integrated optoelectronic circuits made of nanowires.

Single Nanowire Laser

• NW self-catalyzed VLS, 610oC -80 nm.
• Thickened to 340 nm (480oC and higher Ga and As flows).

Nanowire Solar Cell

• p-i-n junction, 2015. 13,8%. Islamabad

Nanocrystals

• A nanocrystal is a particle of material with a diameter of less than 100 nm, composed of atoms with or without a crystalline lattice (amorphous).
  • Properties differ greatly from the bulk crystal for diameters <5 nm.
  • Properties are closer to those of bulk crystals for diameters >20-50 nm.

Nanocrystals in History

• Nanoparticles were known and used in Roman times.
• Colloidal metals were used to dye glassware and fabrics and as therapeutic aid for arthritis treatment.

Nanocrystals and Diameter

• When the diameter of nanocrystals is much larger than the exciton diameter ab, the absorption spectrum is similar to that of the bulk crystal.
• When the diameter of the nanocrystal approaches or becomes smaller than ab, the absorption maximum shifts to the blue.

Nanoparticles Synthesis Methods

• Chemical synthesis by reduction/precipitation reactions (A).
• Physical nanoparticle synthesis (B).
• Biological synthesis, using microorganisms or plant extracts as reducing agents (C).

Chemical Methods for Nanocrystal Synthesis

• Colloid chemistry methods: Precipitation of insoluble salts from a mixture of soluble salt solutions.
  • Requires a stabilizer to ensure that the points do not get too large.
• Metastasis reaction: Molecule-precursors that initiate synthesis.
  • For example, CuInS quantum dots are produced this way.
• Metal-organic method: A precursor is injected with a syringe into a heated flask with TOPO (triocytlphospine oxide), which is vigorously stirred by blowing an inert gas.
  • CdSe quantum dots are produced this way.

Colloidal Materials

• Colloidal NCs are most often composed of elements of group II-VI
  • CdSe, CdTe, CdS
  • HgSe, HgTe, HgS
  • PbSe, PbS
  • ZnS
• The most common are CdSe and CdTe.

Nanocrystal Applications

• PbSe in glass matrix: Used for Ho laser mode synchronization.
• PbSe: Relaxation time decreases monotonically from 25 to 1 ps with decreasing radius from 2.9 to 1.4 nm. Dependence ~ $1/r^3$.

Fast Optical Switches

• CdTe crystals in a glass matrix.
• Excitation probing experiment.
• Excited by one or two beams with a delay of 1 and 2.5 ps.

Biological Marking

• Advantages compared to dye molecules:
  • Protein-coated QDs are very stable (>2 years).
  • Narrow spectral width (FWHM ~ 25nm).
  • Wide excitation spectrum.
  • High quantum yield (40-50%).
  • Compared to rhodamine 6G:
    • 20x brighter.
    • 100-200x more stable.

CdSe Quantum Dots

• By changing the size of nanocrystals, a wide spectrum of colors can be realized by excitation from a single source.

Quantum Dot Light Sources

• What makes quantum dot lasers superior to others?
  • Higher differential gain - wider modulation frequency band.
  • Electrons are trapped by QDs and cannot recombine in impurities and defects - lower threshold current.

Light-Emitting Diodes with QD

• By changing the size of the nanoparticles, it is possible to change the wavelength of the emitted wave.
• It is possible to produce white-light-emitting diodes.

Allotropes of Carbon

• a) diamond.
• b) graphite (composed of individual graphene layers).
• c) lonsdaleite.
• d) - f) fullerenes (C60, C540, C70).
• g) amorphous carbon.
• h) carbon nanotubes.

Graphene Band Structure

• Graphene is considered to be a semiconductor with a zero band gap because its conduction and valence bands meet at Dirac points.

Superconductivity in Graphene

• In 2018, a unique study found how to turn graphene into a superconductor under completely natural conditions.
• MIT, together with Harvard University scientists, found that two layers of graphene, stacked and rotated at a "magic angle" (1.1 degrees), allow electrons to pass with zero resistance.

Fullerenes

• Fullerenes are similar in structure to graphene, which consists of layers with hexagonal rings, but may also have pentagonal rings, which prevent the sheets from remaining flat.
• They can be hollow spheres, ellipsoids, tubes, or flat shapes.
• Spherical fullerenes are called "buckyballs", cylindrical ones - nanotubes.
• Graphene is an example of planar fullerene.
• In a C60 molecule, the distance from core to core is ~0.7 nm.

Unique Properties of Carbon Nanotubes

• Significantly stronger and more resistant to deformation compared to other materials.
• Extraordinary electrical properties.
• Stable up to 2800 oC (observed in vacuum).
• Charge-carrying capacity 1000 times better than copper wires.
• Has twice the thermal conductivity compared to diamond.
• By compressing or stretching, it is possible to change electrical properties by changing the quantum states of electrons.
• Depending on the structure (determined only by diameter and lattice angle), they are conductors or semiconductors.

Single-Walled vs. Multi-Walled Nanotubes

• Multi-Walled (MWNT):
  • Inner diameter 1.5 - 15 nm; Outer diameter 2.5 – 50 nm ~ 50 layers. Many structural defects.
• Single-Walled (SWNT):
  • Single atomic layer wall, diameter 1-1.5 nm. Unique mechanical properties.

Chirality

• The properties of nanotubes depend on the orientation of the hexagonal lattice with respect to the longitudinal axis of the nanotube.
• This property is called chirality.
• Chiral vector: $\vec{C<em>h} = n\vec{a</em>1} + m\vec{a_2} \equiv (n, m)$
  • Diameter: $d<em>t = \frac{C</em>h}{\pi}$
  • $C_h = a\sqrt{n^2 + m^2 + nm}$

Metallic vs Semiconducting Nanotubes

• If $3n + m$ is a whole number - metal.
• If not a whole number - semiconductor.
• Electrical conductivity is measured by the four-probe method.

Semiconducting Nanotubes

• As a rule, they are p-type.
• Basic hypotheses explaining p-type:
  • Influence of contact metal.
  • Impurities that entered during synthesis.
  • Influence of atmospheric oxygen.
• No one of these hypotheses has been confirmed or denied.

Empirical Guidelines

• The band gap is inversely proportional to the diameter of the tube.
• 1.6 nm diameter NV – 0.6 eV.
• The work function is F~4.5 eV, which is larger than most metals used for contacts (Au, Pt).
• Metallic NVs can transmit $10^9 A/cm^2$

Nanotube Production Technologies

• Electric discharge with carbon electrodes
• Laser evaporation, laser sputtering
• Chemical vapor deposition
• High-pressure technologies

Carbon Arc Method

• Historically the first method.
• The electric arc evaporates the carbon anode with catalysts (Ni and Co).
• He: 500 Torr, Current: 100 A and 35 V.
• The camera is water-cooled.
• Nanotubes settle in the internal walls of the tube.

Laser Vaporization

• Target: 1 at.% Ni and Co well-mixed in graphite powder.
• Ar gas flow.
• In an oven, temperature of 1200oC.
• Nd:YAG pulsed laser operating at 60Hz.
• Cleanest, but very low yield (~0.4 g/h).

Chemical Vapor Deposition (CVD)

• Idea: to create a certain catalyst distribution on the substrate in advance and grow nanotubes on it by CVD.
• The most important step: put the catalyst exactly where it needs to be.
• Advantage: nanotubes are grown in the places where they need to be.
• Source: hydrocarbons.

CVD Process Steps

• Apply photoresist.
• Expose the photoresist.
• Apply catalyst.
• Etch the resist.
• Grow carbon nanotubes.

High-Pressure (HiPCO) Method

• Single-walled nanotubes in the gas phase (1200C, 10 atm).
• CO+CO C+ CO2 catalyst (25 mTorr).
• CO flows through the catalyst particles at high pressure and high temperature.
• It is possible to produce single-walled nanotubes in kilograms.
• No cleaning is needed because CO is used instead of hydrocarbons.

Nanotube Applications

• Transistors:
  • Field-effect Transistor (FET).
  • Single-electron Transistor (SET).
  • Scanning microscope tips.
  • Field emission displays.

FET Transistor

• Similar to MOSFET
• p-type channel
• By annealing or doping with K, an n-type can be obtained
• Using both p- and n-type nanotubes, a CMOS analog can be obtained.

IBM Transistor

• At 4 K temperature, it acts as a single-electron device.

Nanotube Weaving

• Using AFM needle.
• Van der Waals forces fixed the changed shape.
• Band structure parameters change at bends.
• By weaving as needed, it is possible to create energy barriers.

N-Type Nanotubes

• Usually p-type, but can be made n-type.
• Doping with K in vacuum (IBM).
• Vacuum annealing (IBM).
• Electrostatic doping (TU Delft).
• Returns to p-type in contact with air.
• Needs to be coated with SiO2 or PMMA (Poly(methyl methacrylate)).

CNTFET Inverter

• p-i-n FET can be made from the same tube.

Nanotube Needles

• Most were made of metals and often broke.
• Carbon nanotubes have a higher length-to-thickness ratio, are more resistant, and have a sharper tip.
• GaAs surface scanned by AFM with a nanotube needle and with a traditional SiN needle.
• Resolution is 100 nm.