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Home , Boule (crystal), Crystal growth, Czochralski method, Epitaxy, Single crystal

Physics and Material Science of Nanostructures

PHYS 570P

Prof. Oana Malis Email: [email protected]

Course website: http://www.physics.purdue.edu/academic_programs/courses/phys570P/ Today

Bulk semiconductor growth • Single‐ techniques Nanostructure fabrication • Epitaxial growth  MBE

Ref. Ihn Chapter 2 Types of Crystalline :

• Different levels of may exist ranging from to totally non-crystalline – Single crystal silicon – Multi-crystal silicon – Polycrystalline – Ribbon silicon – Amorphous silicon • The main difference between each is the crystal grain size and their growth technique Different Forms of Silicon

Crystal Type Symbol Crystal Grain Common Growth Size Techniques Single-crystal sc-Si > 10 cm Czochralski (Cz), Float-Zone (FZ) Multicrystalline mc-Si 10cm Cast, Spheral, Sheet, ribbon Polycrystalline pc-Si 1m – 1mm Evaporation , CVD, sputtering

Process flow from starting material to polished . Semiconductor-Grade Silicon

Steps to Obtaining Semiconductor Grade Silicon (SGS) Step Description of Process Reaction Reduction of silica: produce metallurgical grade silicon 1 SiC (s) + SiO (s)  Si (l) + SiO(g) + CO (g) (MGS) by heating silica with 2 (97% pure) Purify MG silicon through a chemical reaction to produce a silicon-bearing gas of 2 Si (s) + 3HCl (g)  SiHCl3 (g) + H2 (g) + heat (SiHCl3) Additional purification via distillation CVD: SiHCl3 and react in a process called 3 Siemens to obtain pure 2SiHCl3 (g) + 2H2 (g)  2Si (s) + 6HCl (g) semiconductor- grade silicon (SGS)

Single Crystal Growth Techniques

• Czochralski Growth (Cz) – Most single crystal silicon made this way – Lower quality silicon than FZ with Carbon and present – Cheaper production than FZ – Produces cylinders and circular wafers • Float Zone (FZ) – Better Quality than Cz – More Expensive than Cz – Produces cylinders and circular wafers • Pure Silicon is melted in a crucible under or inert gas and a is dipped into the melt • The seed crystal is slowly withdrawn and slowly rotated so that the molten silicon crystallizes to the seed (Rock Candy) • The melt temperature, rotation rate and pull rate are controlled to create a of a certain diameter Modern CZ Crystal Growth

• The raw Si used for crystal growth is purified from SiO2 (sand) through refining, fractional distillation and CVD.

• The raw material contains < 0.01 ppb impurities except for O ( 1018 cm-3) and C ( 1016 cm-3)

• Essentially all Si wafers used for ICs today come from Czochralski grown .

• Polysilicon material is melted, held at close to 1415 °C, and a single crystal seed is used to start the crystal growth.

• Pull rate, melt temperature and rotation rate are all important control parameters. Czochralski Technique

Spinning rod with “Seed” Crystal lowered into the molten silicon

Slowly pulled up to allow silicon to crystallize on the seed layer

Molten Silicon Once to the size desired, the crystal is pulled faster to maintain the needed diameter CZ crystal growth (cont.)

• Sequence of photographs and drawings illustrating CZ crystal growth. The charge is melted, • the seed is inserted, the neck region is grown at a high rate to remove and finally the growth is • slowed down to produce a uniform crystal. Czochralski Growth

• Entire of silicon produced as one big crystal • Very high quality material with few defects • No boundaries between crystals because it is one crystal in one orientation • Si crystal inevitably contains oxygen impurities dissolved from the quartz crucible holding the molten silicon

13 12” (30 cm) Drawback of the CZ method

• The only significant drawback to the CZ method is that the silicon is contained in form in a crucible during growth and as a result, impurities from the crucible are incorporated

• in the growing crystal. Oxygen and carbon are the two most significant contaminants.

• These impurities are not always a drawback, however. Oxygen in particular can be very useful in mechanically strengthening the silicon crystal and in providing a means for gettering other unwanted impurities during device fabrication. Lacture # 3 15 Float Zone Method • Produced by cylindrical polysilicon rod that already has a seed crystal in its lower end • An encircling inductive heating coil melts the silicon material • The coil heater starts from the bottom and is raised pulling up the molten zone • A solidified single crystal ingot forms below • Impurities prefer to remain in the molten silicon so very few defects and impurities remain in the forming crystal Dopant Concentration Nomenclature

Concentration (/cm3) Material 14 14 16 16 19 19 Dopant < 10 10 to 10 10 to 10 >10 Type (Very Lightly Doped) (Lightly Doped) (Doped) (Heavily Doped) -- - + Pentavalent n n n n n -- - + Trivalent p p p p p

Basic Process Steps for Wafer Preparation

Wafer Lapping Crystal Growth and Edge Grind Cleaning

Shaping Etching Inspection

Wafer Slicing Polishing Packaging Slicing into Wafers • Ingots are cut into thin wafers • Two Techniques – Wire sawing – blade sawing • Both results in loss of silicon from “kerf losses”  silicon saw dust • Time consuming • Water Cooled, Dirty Ingot Diameter Grind

Preparing crystal ingot for grinding Internal diameter wafer saw

Diameter grind

Flat grind Wafer Polishing: single or double side

Upper polishing pad

Wafer Slurry Lower polishing pad Wafer Notch and Scribe

Notch Scribed identification number

Chemical Etch of Wafer Surface to Remove Sawing Damage Silicon Wafer Fabrication Review • Raw materials (SiO2) are refined to produce electronic grade silicon with a purity unmatched by any other available material on earth. • CZ crystal growth produces structurally perfect Si single crystals which are cut into wafers and polished. • Starting wafers contain only dopants, and trace amounts of contaminants O and C in measurable quantities. • Dopants can be incorporated during crystal growth • Point, line, and volume (1D, 2D, and 3D) defects can be present in crystals, particularly after high temperature processing. • Point defects are "fundamental" and their concentration depends on temperature (exponentially), on level and on other processes like implantation which can create non-equilibrium transient concentrations of these defects. Nanostructure fabrication Top-down versus bottom up: An analogy If we want to make a very small tree we can either…

Get a very big piece of wood Plant a seed and then control and carve it into a much its growth to form a fully- smaller model tree functioning bonsai tree (TOP DOWN APPROACH) (BOTTOM UP APPROACH)

The wafers grown through the described bulk techniques are rarely used in direct device manufacture, but are used as substrates instead.

Solution: grow one or more layers (of some <m thickness) over them.

The epitaxial growth techniques have low growth rate (as low as one single layer per second in some techniques) which allows a high precision size control in the growth direction, which is essential for the heterostructure variety that is nowadays used in devices. Epitaxy

The extended growth of single-crystal films on single-crystal substrates

Characterised by a well defined crystal orientation relationship between the film (A) and the substrate (B), e.g. (for cubic materials):

(110)A//(110)B [001]A//[001]B

Homoepitaxy: Heteroepitaxy:

- Same film and substrate - Different film and substrate - Essentially no lattice materials mismatch - Significant lattice mismatch - Material properties - Material properties affected by unaffected biaxial strain Molecular Beam Epitaxy MBE

Molecular beam epitaxy • growth in high vacuum or ultra high vacuum (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. and ) 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. Effusion cells

Basic schematic Real example Temperature control is very important since small temperature controls equilibrium vapour pressure, aperture and hence deposition rate

close to equilibrium with gas

Shutter over aperture – must operate very quickly to ensure fast switching between beams of different Heat elements (e.g. In, Ga) and achieve sharp interfaces. 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 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) 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. RHEED: 3D nanostructures

Flat sample Sample with small 3D islands

Eletrons scattered at surface Electrons transmitted through island Streaky RHEED patterns 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. 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 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. As4). • 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 mobility and dopant ) Temperature measurement

• Thermocouples (± 1 oC) • Pyrometers (± 50 oC)

A junction is created between two Intensity of emission from heated black different metals, which produces a body is correlated to its temperature voltage related to a temperature Hence measure intensity of emission at difference between the junction a particular wavelength to determine T and a voltmeter held at RT But emission of real body (wafer) is (Seebeck effect) different from black body, so emissivity ε has to be determined Apparent temperature varies with the Attached to the back of the sample emissivity of the film or the sample mounting plate  For transparent films, emission from doesn’t measure the surface T substrate effects measured T! Temperature measurement

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

You don’t know what the real temperature is! Quantum Dots Grown Using MBE

“Electronic Structure of InAs Pyramidal Quantum Dots”: http://www.sst.nrel.gov/research/InAs.html

Three-dimensional AFM image of CdSe QDs deposited on ZnCdMgSe barriers. The inset shows a histogram of the AFM QD height [ Courtesy: Prof. Tamargo’S group- CCNY ].