Op2 Devices - Semiconductors

Front 1

Examples of some Si uses

Back 1

Integrated circuitry: photo diodes, laser crystals, Strain gauges,

Front 2

What are the requirements of grown silicon?

Back 2

Uniformity in:

Composition, dopant distribution, impurities, electrical properties, luminescent properties, defects and flaws.

Front 3

What levels of purity are required? Whaat is a normal level of doping

Back 3

purity of parts per 1000 billion, doping 10^22, about 1%

Front 4

First stage of Si production, what is the product?

Back 4

Vacuum arc remelting. SiO in, Si out by C reduction. Still very impure, produces Metal grade silicon (MGS)

Front 5

Describe the Siemens process and resulting Si

Back 5

Stage 1 - react Si with HCL - forms SiHCl3.

Stage 2 - Fractional distillation - this compoun is exceptionally volatile and can be fairly easily removed

Stage 3- reduction back into pure electronic grade Si

Conc of impurities reduced to few ppb max, other than C and O

Front 6

What process for Si crystal growth? Important factors

Back 6

Czochralski growth

temperature gradient - too shallow and you get constitutional supercooling too step you get convection and non uniform dopant distribution

S/L interface speed - must be controlled

Front 7

What is the impurity segragation coefficient and what is it for

Back 7

gives effective segragation when equilibrium cannot be established

Front 8

Derive the conc of impurities in a solid product

Back 8

Front 9

What is the result of poorly controlled/ non uniform T gradient?

Back 9

Breaks down into cells - impurities segregate to cell boundaries. Non uniform leads to fluctuations in impurity concentration

Front 10

Show how velocity is related to crystal radius. What does this mean?

Back 10

main point: v+B=A/r, so if you increase the velocity you must decrease the radius of the crystal, or melt back will occur

Front 11

Sketch the Czochralski process

Back 11

Front 12

Conc of O in Si. Advantages of O in Slicon

Back 12

Most abundant impurity - Keff=1.25

Improves mechanical properties - O hardens the material

Gettering - impurities segregate to SiO2 and improve purity

Front 13

disadvantages of O

Back 13

Can nucleate dislocations

Require surface treatment for removal at top layer where electrical props are otherwise hindered

uncontrolled doping effect as a donar

Front 14

What are BMDs and COPS

Back 14

Bulk micro defects - provide gettering, removal of harmful transition metals, from the surface

COPS crystal originated particles - vacancies diffuse to eachother and cause microvoids, which fill with supersaturated oxygen. Very harmful to electronic gating properties

Front 15

How do TM impurities affect props

Back 15

Most are fine - low level transition metals 10^10 in 10^22 Si atoms provide very small minority carrier currents. Only not okay for radiation detectors

B is problematic - 10^13 atoms due to Keff 0.8 resulting in max resistivity of 300Ohmm^-1. Not good for high power uses

Front 16

How can we get rid of B in Si growth. Pros and cons

Back 16

Float zone growth - melt a portion of the Si and move it up to the top of the Si wafer. Impurities follow the melt up due to their low Keffs. High purity and lifetime, but reduces O so poorer mech props and quite expensive as radius is limited

Front 17

Why is GaAs harder than Si in Czorch

Back 17

Volatile - loss of compounds results in non stoichiometry

Extended phases fields - always some variation in composition

Low yield strenght - always some dislocations

Sensitive to stoichiometry - over 48% As is n doped, which is required for high resistivity

under is p doped each leading to low resistivities

Front 18

How is good stoichiometry achieved for GaAs Czorch

Back 18

Use encapsulation - high pressure argon or boron at the VP of the liquid is used to encapsulate the liquid GaAs, preventing vapourisation. Allows close control of properties

Front 19

Different types of epitaxial growth and the stages? Which stage is arguably most important

Back 19

hetero and homo epitaxy, same or different phase.

1)strong bonding to surface bond strength (Ea)

2) diffusion - most important, low activation (Ed) for diffusion gives good surface coverage

3) formation fo clusters (Ec)

Front 20

main type of growth for epitaxial

Back 20

Frank Van der Mewre - Ea>Ec, like to bond to surface so diffuse till find a surface atom. Grows in a plane.

Front 21

What is epiitaxial growth called for small lattice difference? Expression for energy and sketch of energies we must consider

Back 21

Psuedomorphically. lattice shrinks to fit

Front 22

what kind of structure forms at interface of epitaxia

Back 22

dislocations of 60 degree type form on the surface

Front 23

Effects of dislocations on interface properties

Back 23

recombination centres - can cause heating, activate more sources and generate more dilocations.

Increase resistivity, also normally bad

Front 24

What is CVD? What controls composition and growth rate? normal limiting factors?

Back 24

Chemical vapour deposition - vapourize and spray onto the surface. Adsorbed, then thermally decomposes and diffuses away to farm even layer. Comp is controlled by flow rate of the gases, partial pressures, and the rate limiting factors. growth rate on T, and everything above. Limited by supply of substrate or reaction rate on the surface

Front 25

Sketch growth rate of a CVD substrate as a function of flow explaining features

Back 25

A: fully controlled by flow

B: some supersaturation

C: complete supersaturation, growth limited by reaction rate

Front 26

T vs growth rate for CVD. explain regions

Back 26

A:Surface reaction kinetics

B:surface diffusion

C: Transport path limited

Front 27

Where do we use it

Back 27

Si - different dopings of Si give very good electrical properties and clean junctions, high resistivities and complex devices

Halides

Metal oxides

Front 28

Halide VPE fundamentals and main prooblem

Back 28

uses PH3 or AsH3 and pure In or Ga which forms GaCl or InCl.

Near eq, so comp affect phases produced

huge amounts of HCl produced, which disposal

N

Front 29

pros and cons of HVPE

Back 29

simple and can control phases by flow

but can't do Al, or anything that doesn't have a vapour phase, and high Ts lead to impurity inclusions. limited to nm thick

Front 30

Metallo organic CVD differences and pros and cons

Back 30

just uses metal alkyls of group 3 which are more readily accessible that put metal vapours, can be done at low T due to instability which mean purer and thinner layers can be grown. But even worse gases produced

Front 31

MBE

Back 31

molecular beam deposition - each element is vapourised in separate chambers and flowed in. Huge supersaturations. Low T gives one layer at a time, perfect epitaxy, and limited interdiffusion. Ultra pure