Pierson Hugh O. Handbook of carbon, graphite, diamond and fullerene. New Jersey / carbon

266

Carbon,

Graphite,

Diamond,

and

Fuiierenes

Effect

of

impurities

and

Structural

Defects.

As

seen

above,

diamond

would

be

the

ideal

transparent

material

if it were

totally

free

of

impurities,

particularly

nitrogen,

and had a perfect

structure. Howeverthese

conditions

are never completely achieved,

and impuritiesand

crystal-lattice

defects

and other

obstacles

to

the

free movement

of

photons

affect

its

transmittance.

These

obstacles

add

a number

of absorption

bands

to the

two

mentioned

above,

particularly

in the

IR region

as shown

in Table

11.4.

in spite

of this,

diamond

remains

the

best

optical

material.

Lattice vacancies

(missing

atoms)

may considerably

alter

the valence

bonds

and cause electrons to be exited by a much smaller

amount

of energy

(such

as produced

by a photon

of red light)

that would

normally

be required

in a perfect

lattice.

A diamond

containing

such

lattice

vacancies

appears

blue since the red components

of light

(the one with

less photon

energy)

are

absorbed.

A minimum

of one

vacancy

per 1Or’ atoms

is necessary

for the

blue

color

to be noticeable.tll]

Type

Optical

absorption

bands

la

IR: 6-13 microns

UV: ~225

nm

lb

IR: 6-l 3 microns

UV: <225

nm

Ila

Closer

to

ideal

crystal.

No absorption

in the range

cl 332 cm-‘. Continuous

absorption

below

5.4eV

Ilb

IR: no significant

absorption

from 2.5-25 microns

UV: absorption

at 237 nm

X-ray transmission

of diamond

is excellent

by virtue

of its low atomic

number

and, in thin sections,

it even allows the

transmission

of character-

istic x-rays generated

by low-energy

elements such as boron,

carbon,

and

oxygen.

In this

respect, it

compares

favorably

with

the

standard

x-ray

window

material:

beryllium.

The

x-ray transmission

of

a 0.5 mm-thick

diamond

of the characteristic

radiation

of a series of elements

is shown in

-

hka

Figure 11.14.

X-ray

transmission

of

a 0.5 mm-thick diamond window of the

characteristic

radiation

of a series

of elements.[“]

Sound

waves are carried by vibrations

in the

low-frequency

range (a

few hundred

Hertz),

unlike thermal conductivity

and optical

absorption

which are associated

with high-frequency

vibrations.

Structure

and Properties

of Diamond

269

The

structure

of diamond

favors

low-frequency

transmission

and

the

material

has high sound

velocity.

Measurements

of

up to 20

km/s

are

reported.

By comparison,

the speed

of sound in beryllium

is 12.89

km/s and

in silicon

slightly

less than

10

km/s.tlll

Resistivity,

Q-m

Type

I and most Type Ila

10’8

Type

Ilb

103-

105

Dielectric

Constant

at 300 K

5.70

+ 0.05

Dielectric

Strength,

V/cm

106

Saturated electron velocity, 1O7cm/s

2.7

Carrier mobility, cm*/Vs

Electron

2200

Hole

1600

Pure

single-crystal diamond, with a bandgap

of 5.48

eV, is one

of the

best solid

electrical insulators (see Sec. 6.2).trg]

The high

strength

of the

electron bond makes it unlikely

that

an electron

would

be exited

out

of the

valence

band.

In pure

diamond,

resistivity

greater

than

10’s

S&m has been

measured.

However,

as with

the optical

properties,

the

presence

of impurities

can

drastically

alter

its electronic

state

and

the inclusion

of s$

(graphite)

bonds

will considerably

decrease the

resistivity

and render

the

material

useless

for

electronic

applications.

The

dielectric

constant

of

diamond

(5.7)

is low

compared

to

that

of

other

semiconductors

such

as silicon

or germanium

but not as low as most

organic

polymers

(in the

2 to 4 range)

or glasses

(approximately

4).

9.3

Semiconductor

Diamond

The

semiconductor

properties

of

diamond

are

excellent

and

it

has

good

potential

as

a

semiconductor

material.t4]

It is an indirect bandgap

semiconductor

and has the widest

bandgap

of any semiconductor

(see Sec.

6.2).

When

a semiconductor

material

is heated,

the probability

of electron

transfer

from thevalence

band to the conduction

band becomes

greater

due

to the thermal

excitation

and,

above

a certain

limiting

temperature,

the

material

no

longer

functions

as a semiconductor.

Obviously

the larger

the

bandgap,

the smaller

the

possibility

of electron

transfer

and

large-bandgap

semiconductors

can

be

used

at higher

temperature.

This

is the

case

for

diamond

which

has an upper limit semiconductor

temperature

of 500°C or

higher.

In comparison,

the

upper

limit

of silicon

is 150°C and that

of GaAS

is 250°C.

Diamond

can be changed

from

an intrinsic

to an extrinsic

semiconduc-

tor at room

temperature

by doping

with

other elements

such as boron

and

phosphorus.t4)t2*)

This

doping

can

be accomplished

during

the synthesis

of

diamond

either

by high pressure

or especially

by CVD

(see Ch. 13, Sec.

4.4).

Doped

natural

diamond

is also

found

(Type

Ilb)

but

is rare.

Diamond

has an excellent

electron-carrier

mobility

exceeded

only

by

germanium

in

the p-type and by

gallium

arsenide

in the

n-type.

The

saturated

carrier velocity,

that

is, the

velocity

at which

electrons

move

in

high

electric fields, is higherthan

silicon,

gallium

arsenide,

or silicon carbide

and,

unlike

other

semiconductors,

this

velocity

maintains

its

high

rate

in

high-intensity

fields

as shown

in Fig.

11 .15.

I

I

I

104

105

105

Electric

Field

(V/cm)

10.0 MECHANICAL

PROPERTIES

OF DIAMOND

10.1 Summary of Mechanical

Properties

It should

be emphasized

that

the

strength

properties

are

difficult

and

expensive

to

measure

due to the

lack

of diamond of the required

test

size

and configuration.

The

data

presently

available

show

a

material

of

considerable

strength

and

rigidity.

The

mechanical

properties

of diamond

are

summarized

in

Table

11 .7 .t23] The

properties

were

measured

on single-crystal

diamond

either

natural or produced by high pressure.

For comparison purposes,

the table

includes

the

properties

of a high-strength

ceramic,

namely

alumina.

272

Carbon,

Graphite,

Diamond,

and Fullerenes

Table

11.7. Mechanical

Properties

of Diamond and

Alumina at 23°C

Property

Diamond

Alumina

Density,

g/cm3

3.52

3.98

Young’s

modulus,

GPa

910-

1250

380 - 400

Compression strength, GPa

8.68

- 16.53

2.75

Knoop

hardness,

kg/mm*

overall

5700

- 10400

2000 - 2100

111 plane

7500

- 10400

100 plane

6900

- 9600

Poisson’s

ratio

0.10

- 0.16

0.22

Coefficient

of friction

in

air

0.05

- 0.1

in vacuum

near 1

10.2 Hardness

The very fact that diamond

is the

hardest

known

material

makes

it

difficult

to

measure

its hardness

since

only another

diamond

can be used

as an

indenter.

This

may explain

the

wide

variations

in

reported

values

which

range from 5,700

to over

10,400

kg/mm*.

The hardness

of diamond is compared

with that of other

hard materials

in Fig.1 1.16.

The

test

method

is the

Knoop

hardness

test

which

is

considered

the most accurateforcrystalline

materials.

The

hardness

is also

a function

of the crystal

orientation

as shown

in Table

11.7.

The

hardness

of

diamond

can also

be

determined

form the

elastic

coefficients

as

there

is

a linear

relation

between

hardness

and

these

Diamond

10000

“E

f

Reported

g

8000

Range

2

f

6

6000

I

t

CBN

x

8

g

4000

PSiC

Y

AI,O,

w,c

20000

lLd!-

Diamond

behaves

as

a brittle

solid

and fractures readily

along

its

cleavage planes. Cleavage

occurs

mostly

along

the

(11 I}

planes

but

can

also occur along the other

planes since the energy differences

between

planes

is small

as shown in Table 11 .8.t23] Cleavage

velocity in all these

planes

is considerable

and

has been measured

at several

thousands

of

meters

per second.

Structure and Properties of Diamond

275

The

by-product

of the

reaction

is carbon

dioxide

which,

being

a gas,

provides

no

surface

passivation.

A

fresh

diamond

surface

is

always

exposed

and oxidation

proceeds

parabolically

with

temperature.

Oxidation

in air is less rapid with the onset

of the

reaction

at approximately

500°C.

Under normal

conditions,

oxygen

is

adsorbed

on

the

surface

of

diamond

after

exposure

to air (or oxygen)

for a period

of time.

However,

no

adsorption

occurs

if the temperature

is below

-78°C. From

0 - 144°C

oxygen

is chemisorbed.

CO,

is formed

from

244

to 370°C

by the

interaction

of 0,

with the

diamond

surface.[24] Adsorbed

oxygen

and carbon

oxides

account

for the

hydrophillic

characteristic

of diamond.

As mentioned

in Sec.

10.4,

the formation

of surface

oxides

is also

an important

factor

in the

control

of

frictional

properties.

At 600°C and low pressure,

the

presence

of residual

oxygen

results

in

the formation

of a dense

film of

graphite.

11.2 Reaction with Hydrogen

Like oxygen,

hydrogen

is chemisorbed

on the surface

of diamond

but

not until

a temperature

of 400°C is reached.

This

chemisorption

is probably

the result

of

the

formation

of

surface

hydrides.

Diamond

is

generally

considered

inert to molecular

hydrogen

(as opposed

to graphite).

However

attack

by atomic

hydrogen

occurs

above

1000°C.

Yet

diamond

is far

less

reactive

than

graphite,

a characteristic

which

is used

to good

advantage

in

the deposition

of diamond

films

and

the

selective

elimination

of

the

co-

deposited

graphite

(see

Ch.

13).

11.3 General Chemical Reactions

Diamond

is resistant

to all

liquid

organic

and

inorganic

acids

at room

temperature.

However

it can

be etched

by several

compounds

including

strong

oxidizers

such

as sodium

and

potassium

nitrates

above

500°C

by

fluxes

of sodium

and

potassium

chlorates,

and by molten

hydroxides

such

as NaOH.

It is resistant

to alkalis and

solvents.

At approximately

1000°C

it reacts

readily

with

carbide-forming

metals

such

as Fe, Co,

Ni, Al, Ta,

and

B. This

characteristic

provides

the

mechanism

of high-pressure

synthesis

(see Ch. 12, Sec.

4.5). Generally

speaking,

diamond

can

be considered

as

one of the

most-chemically

resistant

material.