Chemical and structural characterisation of III–V epitaxial layers

Physica 129B (1985) 81-91 North-Holland, Amsterdam

81

CHEMICAL AND STRUCTURALCHARACTERISATIONOF III-V EPITAXIALLAYERS M.J. Cardwell Plessey Research (Caswell) Limited, Allen Clark Research Centre, Caswell, Towcester, Northants NN12 8EQ, England The accurate characterisation of epitaxial layers is a key component in the research and development of advanced I I I - V optoelectronic and high speed devices. A detailed understanding of chemical impurities and structural defects in the material and their effects on device performance and r e l i a b i l i t y must be achieved. Furthermore the modern generation of I I I - V devices demands the analysis of thinner and thinner layers of differing composition, which presents more problems to the analyst. A review of the techniques used to measure the chemical composition and impurities of epitaxial layers is made. This includes direct chemical measurement, such as secondary ion mass spectrometry and Auger electron spectroscopy, as well as indirect methods. The methods used to assess the structural perfection of layers is described and comparisons made between a variety of high resolution methods.

1. INTRODUCTION Apart from the obvious need to measure the carrier

concentration

and

thickness

(vi) Determination

is required to f u l l y characterise the epitaxial III-V

used in

the fabrication of

the

lattice

matching

of

epitaxial layers a great wealth of information layers

of

between layers.

advanced

optoelectronic and high speed devices.

A l l of this information has to be measured on epitaxial buried

layers of submicron dimensions,

beneath

layers

of

differing

composition.

The data required is: 2. IMPURITY IDENTIFICATION (i)

The

identification

of

chemical

impurities in layers.

The techniques used to identify impurities in epitaxial layers f a l l into two main groups, indirect

( i i ) The

ability

of

profile

impurities

in

layers.

and direct methods.

methods u t i l i s e

a physical

of

the

chemical impurity in the semiconductor lattice and give additional

( i i i ) T h e measurement of the matrix composition

The indirect

property

information such as the

location of the impurity within the crystal.

of different layers. (a) (iv) Measurement of interface sharpness between layers.

Indirect Methods The i d e n t i f i c a t i o n

ionisation (v)

spectroscopy.

The identification of structural defects

been described in detail

in layers.

and consists

0378-4363/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Pubfishing Division)

of

shallow

donor

impurities is carried out using photothermal

of

the

This

technique

has

by Stillman et al l measurement of

the

M.J. Cardwell / IH-V epitaxial layers

82

photoconductive response of a sample at l i q u i d

bands are broad and any broadening due to other

helium

impurities is

temperatures

magnetic

field.

in

the

presence

The technique

is

of

a

extremely

l i m i t e d , however, any changes in

the position of the Fermi level

in the sample

sensitive in high purity m a t e r i a l , because the

because of excess acceptors or donors may change

photo

the charge state of the centre and hence the

response

impurity

is

almost

concentration

trations.

at

The peaks in

have been i d e n t i f i e d using

independent of very

low

the

concen-

photoluminescence spectra.

the spectra for GaAs

with

individual

donors

back doping experiments on high purity

The

third

indirect

identification

is

spectroscopy

method

deep

(3LTS).

of

level

This

impurity transient

technique

is

material by Wolfe et al 2, Low et al 3 and Ozeki

described

et al 4.

The disadvantage of this technique is

measurement of capacitance transients in a p-n

that

may only

junction or a Schottky barrier while varying the

it

be applied to

high

purity

by

Lang 9

and

consists

of

the

samples with low compensation ratios and is only

temperature.

useful as a tool to i d e n t i f y residual impurities

part of the depletion region are forced to emit

in e p i t a x i a l material.

electrons

Acceptors normally

in

near-band-edge available alloys

III-V

identified on

is

semiconductors

using

low

photoluminescence. the

III-V

extensive.

binary

are

temperature The

data

compounds and

Most of

the

acceptor

or

The deep levels within the swept holes

measured from the transient. also

define

responsible for

in

bulk

to

identify

residual

impurities

samples.

The most complete tabulation of the

and e p i t a x i a l

acceptor photoluminescence energy in been given

by Ashen et

al 5.

GaAs has

T h e s e authors

demonstrated that when the spectrum is i.e.

three

centre

parameters

adequately

to

the deep level.

An extensive

the centres observed in GaAs is

given by Mittonneau et al i°. be made q u a n t i t a t i v e insulating

The technique may

and can

samples

by

be extended to

using

optical

stimulation.

sharp,

the sample is unstrained and high p u r i t y ,

the

determine the nature of the impurity or defect catalogue of

used r o u t i n e l y

is

The capture cross

be obtained and these

normally

the

is

emission rate

section and a c t i v a t i o n energy of the centre may

species have been i d e n t i f i e d and catalogued and technique

and the

In addition the technique can be applied to field

effect

tranmsistors

by pulsing the gate

the peak positions are extremely reproducible

and measuring

from sample to sample. However, l i k e the photo-

current.

thermal ionisation spectroscopy the technique is

in the channel of the FET may be i d e n t i f i e d and

really

measured and correlations with the performance

only

applicable

to

residual

impurity

i d e n t i f i c a t i o n and because of the other variable

the

transient

in

the

drain

In this way the deep centres present

of the FET made.

recombination mechanisms in the sample, is only semi-quantitative. Deeper centres transition

metal

(b) in the m a te r i a l , caused by impurities,

h a v e also

studied using photoluminescence.

been

Most of the

data has been obtained for Cr and Mn in GaAs6, 7 and Fe in

InP 8.

The deep photoluminescence

Direct Methods The

two

methods

normally

used

for

the

i d e n t i f i c a t i o n and measurement of impurities in e p i t a x i a l layers are atomic absorption spectroscopy and secondary ion mass spectrometry.

M.J. Cardwell / III-V epitaxial layers Atomic absorption spectroscopy measurements are made on a whole wafer by completely removing the epitaxial layer.

83

TABLE 1 SIMS d e t e c t i o n l i m i t s f o r some common impurities/dopants in GaAs. (After Clegg12)

Sensitivities for heavy

metals such as Cr, Zn and Cu are at the sub and S are extremely d i f f i c u l t to measure.

The

technique is t o t a l l y destructive and is only specific to one element at a time.

The arrival

of secondary ion mass spectrometry (SIMS) has made the use of atomic absorption obsolete in the analysis of epitaxial material, although i t is

still

used to analyse bulk samples with a

high degree of accuracy and is also used to provide calibration samples for SIMS. Early

attempts at

profiling

impurities in

epitaxial layers were restricted to radio tracer measurements.

Detection Limit (atoms cm- 3) 4x 1013 2 x 1 0 TM l x 1 0 is lx1016 4 x 1 0 ~3 4 x 1 0 TM 4 x 1 0 TM 3 x 1 0 TM 2x1013 2x1013 6x1013 l x 1 0 is 5x1015 lx1016 5x1012 l x 1 0 is 5x1012

Element Be B C 0 Mg AI Si S Cr Mn Fe Cu Zn Ge Se Sn Te

parts per million level but donors such as Si

These measurements had the

disadvantages of only being able to monitor the radio-active species, which had to be added in a special

experiment,

applicable to

and

also

relatively thick

only

and impurities in GaAs as given by Clegg12.

layers as the

being

These sensitivities, with the a b i l i t y to profile

tracer was not normally present in sufficient

in submicron layers, are unmatched by any other

concentration

competing technique.

to

allow the analysis of small

volumes of material.

Despite these problems the

Despite this advantage over any other method,

technique was very successful in the analysis of

there

outdiffusion and was used by Tuck et al 11 to

encountered with SIMS that make the accurate

demonstrate the outdiffusion of Cr from a semi-

interpretation of the data d i f f i c u l t .

insulating substrate into a growing layer.

are:

This

are

still

some problems which may be These

technique has now been replaced by SIMS except for

self diffusion experiments of the matrix

(i)

The large variation of ion sensitivity.

elements. SIMS

is

the

preferred method for

the

( i i ) Contamination in the ion beam.

identification and profiling of III-V epitaxial layers.

The technique consists of the removal

(iii)Contamination from the system.

of material using a rastered ion beam and the collection and mass analysis of the sputtered fragments.

(iv) Rougheningof the ion crater.

Commercial machines are available

which have sensitivities for some impurities in

(v)

Matrix effects.

the tenth of a part per b i l l i o n range. Table 1 shows the sensitivities of some common dopants

(vi) Ion mixing.

84

M.J. Cardwell / II1-V epitaxial layers The large v a r i a t i o n in the ion y i e l d is due

lon mixing is observed when p r o f i l i n g from a

to the i o n i s a t i o n potential of the species and

high

i t s environment, hence the y i e l d is both element

tration

and matrix

for

some of the atoms being measured and knock them

standards are required and

into the low concentration layer, where they are

dependent.

q u a n t i t a t i v e work, also that

This

means that

some elements are normally analysed

using oxygen (02+ ) and some using caesium (Cs+)

concentration region layer.

into

a low concen-

The primary ions c o l l i d e with

detected as impurities.

This leads to measured

interface broadening effects.

The f u l l analysis of a sample therefore involves an ion source change and a repeat of the experi10El7

ment, which is very time consuming. Contamination

of

the

ion

beam,

from

lOE6/~'-----

impurities in the plasma ion source, may lead to the

implantation

these species.

and subsequent

detection

,- . . . . . . . . . . . . . . . . . . . . . .

of IOE-

The problem is normally overcome

by mass f i l t r a t i o n

of

/~--J

the primary beam which

~ 10E t 4

reduces the contamination by a factor of i00 13 Other sources of contamination can arise

from 10E-3

the residual gases in the vacuum system and also from

sputtering of

sample.

the

These effects

surfaces close to the may

be

reduced

1 0 E -2

by

IOEI1

maintaining an u l t r a pure vacuum, energy f i l t e r ing the detected species to discriminate against molecules such as N2 and CO and making the metal surfaces

close

to

the

As

sample from Ta or

IOE ~ 0

Ti

10

20

3'0 40

5'0

6'0

instead of stainless steel which leads to high Cr and Fe background levels. The

crater

optimising the

roughening may be

reduced

sputtering conditions

for

by each

FIGURE 1 SIMS P r o f i l e of an annealed Mg implant Si3N4 coated GaAs.

into

matrix. Because the ion y i e l d d i f f e r s from matrix to matrix,

apparent

abrupt

changes in

is

shown in Figure i ,

In I I I - V devices which depend on the presence

impurity

levels at matrix interfaces is observed. effect

3. COMPOSITIONALMEASUREMENTS

This

which is a SIMS

of heterojunctions i t

is imperative to be able

to determine the composition of the a l l o y with a

coated GaAs. The Mg has obviously r e d i s t r i b u t e d

high degree of accuracy. I f the layer is thick

in the GaAs and into the n i t r i d e cap.

I-2

profile

of

an annealed Mg implant into Si3N 4 However

the abrupt change at the Si3N4/GaAs interface is

~m)

the

layer

may

enough (greater than be

analysed

using

wavelength dispersive or energy dispersive X-ray

not real and is an a r t e f a c t of the SIMS tech-

analysis.

nique moving from one matrix to another.

This

very reproducible, provided that the electrons

The technique is highly accurate and

is obvious in a sample with large differences in

generating the X-ray emission are contained in

the matrix, but can be confusing in samples with

the layer to be analysed.

only small matrix differences as in for example

are used as standards for the analysis.

GaAs/GaAIAs heterojunctions.

The binary compounds

~~

M.Z Cardwell / III-V epitaxial layers As the demand for thinner and thinner layers grows the use of X-ray analysers has diminished and other techniques have replaced i t .

10Et7

The band

10E

gap and hence the composition of a ternary alloy may be measured by photoluminescnce or optical absorption.

This

may not

be easy in

multilayer structure and also is

85

10E -5---

a

--- As

insufficient

0 Ga

~ 10EI'4

for quaternary alloys where similar band gaps

=¢

may be obtained with differing compositions. The two direct method of analysis u t i l i s e d

...............

are SIMS and Auger electron spectroscopy (AES).

lo612

SIMS has been discussed in the previous section the sub parts per million level is obviously to

measure layer

contamination from the

composition.

~AJ

lO?

and with the a b i l i t y to determine impurities at suited

............... t

The

\

10E~0 Se 0 1() 2'0 3'0 4() 5'0 6'0 7() 8'0

beam and system are

negligible, but the other effects, roughening, matrix effects and ion mixing are s t i l l present. This is demonstrated in Figure 2, which is a SIMS profile of

a GaAs/GaAIAs heterojunction

bipolar transistor containing interface

structure.

layers

are

spikes due

epitaxial

reactor

heterojunction

The aluminium

clearly

to

FIGURE 2 SIMS P r o f i l e of GaAs/GaAIAs bipolar t r a n s i s t o r structure.

shown and

transients

in

may be observed.

Auger electron spectroscopy can under certain

the

optimised conditions reduce the effect of ion

The

mixing.

AES has a detection l i m i t of ~ 0.1

impurities being monitored (C and O) show a

atomic per cent and hence is unsuitable for the

similar increase at the interface and follow the

measurement of

Al

conductors.

signal.

This

is

not

due to

additional

impurity

However i t

profiles

for

matrix effects in the SIMS, where the sputtering

layers.

efficiency of these impurities in the GaAIAs is

exactly the same ways as SIMS.

higher than that in GaAs. Hence to obtain a

semi-

is eminently suitable

contamination at the interface, but is due to

the measurement of

in

composition in

III-V

The AES technique requires standards in

Profiling is achieved using two methods:

quantitative SIMS p r o f i l e of impurities across heterojunctions

many SIMS standards

and

(i)

Linescans along a low angle bevel.

laborious measurements have to be made. Ion mixing, in the analysis of very abrupt

( i i ) Sputter p r o f i l i n g .

interfaces, is a problem with SIMS analysis of heterojunctions. The limitations of the primary ion beam, that i t normal

to

the

should be reactive and is

surface means that

in

some

matrices and the III-V compounds in particular ion

mixing

effects

will

dominate

measurement of interface abruptness.

in

the

The technique of linescanning a bevel is a very simple and quick technique. A low angle bevel is produced by lapping or chemical etching and then sputter cleaned in the UHV apparatus. A small diameter (4 0.5 ~m) electron beam is scanned down the bevel and the Auger electron

86

M.J. ('ardwell / I11-V epHaxial layers

peak height measured f o r the elements r e q u i r e d . If

r l,

=~

Ar, 7 0 0 e V 18 ° M W J 9

the angle of the bevel and the beam diameter

i s known the l a y e r widths and i n t e r f a c e abruptness may be measured.

-g

The accurate measurement

2oo~

o f the bevel angle (< 0 . i °) is obtained by laser reflection. bevel

is

In p r a c t i c e not

a

the production of the

trivial

exercise

and

some

l a b o r a t o r i e s p r e f e r to s p u t t e r a f l a t bottomed c r a t e r and measure the Auger peaks throughout the layer s t r u c t u r e . and

obviously

may

flexible

///\/

150-

,oo0 / \/ ., ,o

/

v

-

i

=

This technique is mixing effects.

o

s i m i l a r in concept to SIMS suffer

from

the

same ion

AES systems are normally more

<

29.4A o

Sputter time

40

FIG[IRE 3 Auger s p u t t e r p o f i l e of GaAs/GaAIAs s u p e r l a t t i c e s t r u c t u r e grown by MOCVD.

in the mounting of the ion gun r e l a t i v e

to the sample and do not have the r e s t r i c t i o n choice of

ion species,

of

reduced by s p u t t e r i n g the sample w i t h heavy ions of low energy at a high angle of incidence.

The

effect

The

of t h i s may be seen in Figures 3-5.

figures

show three

Xe

700eV

9uA

18 ° M W J 9

lon mixing is normally

AES p r o f i l e s

of

GaAs/GaAIAs s u p e r l a t t i c e s t r u c t u r e .

the

same

Figure 3 is

E ~. 200 ~ 15oi ~ lOOi

obtained using an Ar + ion beam at 18 ° incidence and an i n t e r f a c e is

obtained.

sharpness (10%-90%) of 29.4 A

~ 5o

By r e p l a c i n g the Ar + beam with a

23.8A °

reduced to 23.8 A (Figure 4 ) . 5 the e f f e c t

Finally

in Figure

of i n c r e a s i n g the angle to 50 ° is

to again reduce the i n t e r f a c e width to 18 A. The

accurage

characterisation

impurity of

layers

needs

both

respect

the

systems

than competative. the

advantages

very

and thin

FIC,URE 4 Auger s p u t t e r p r o f i l e of same s t r u c t u r e as Figure 3, showing the e f f e c t o f using a heavier ion species.

compositional III-V

epitaxial

SIMS and AES and are

45 Sputter time

heavier ion Xe+ the apparent i n t e r f a c e width is

in

complementary

this

"~

Xe'

A

~

700eV,

50 °

MWJ9

147A°

rather

A more d e t a i l e d comparison of and

disadvantages

of

both

o

techniques is given by Morgan lh 4. STRUCTURAL CHARACTERISATION A d e t a i l e d study of the s t r u c t u r e and defects of

III-V

requires

epitaxial

layers

many techniques

is

which

difficult

and

vary

the

from

simple and inexpensive to the s o p h i s t i c a t e d and very time consuming and expensive. structural

information

that

is

The type of

needed is

the

18A o 22A ° 26A ° 28A ° Sputter time

40

FIGURE 5 Auger s p u t t e r p r o f i l e o f the same s t r u c t u r e as Figure 3, showing the e f f e c t of increased angle of incidence.

M.Z Cardwell / III-V epitaxial layers identification

and

distribution

of

extended

87

in the epitaxial layers.

The defects in buried

defects such as stacking faults and disloca-

hetero-epitaxial layers are normally character-

tions, micro-

ised

strain

or

precipitates and the degree of

lattice

matching between hetero-

by removing the overlying layers

using

selective etches and then dislocation etching the wafer in an etch developed for the layer in

epitaxial layers. The techniques used to characterise

these

question.

layers f a l l into three broad areas: (i)

Defect etching and optical microscopy

( i i ) X-ray techniques (iii)Electron microscopy. 5. DEFECTETCHING AND OPTICAL MICROSCOPY Defect etches offer a simple and inexpensive method by which to layers.

characterise e p i t a x i a l

The conventional etches produce pits at

defect sites on the surface and these pits are counted using an optical microscope to produce an etch p i t or defect density.

These etches

have to be calibrated against another technique, such

as X-ray

topography

or

transmission

electron microscopy to determine what type of defect is being revealed by the etch.

A typical

example of this calibration is shown in Figure 6.

FIGURE 6 Transmission electron micrograph of InP layer etched in a defect revealing etch. Note the correlation between the etch pits and stacking faults in the layer.

This sample is an InP epitaxial layer which

has been etched in a defect etch developed by Huber and Linh 15.

The sample was then thinned

Other dislocation etches, for example the AB etch 16 as used for GaAs, reveal more detail

from the back and the thinned area examined

about the type and distribution of the detects.

using both optical

This etch,

and transmission electron

instead of producing pits,

has a

The transmission electron image of

"memory effect ''17 of the defects in the layer

an area containing etch pits is shown in Figure

removed. Hence by repeated successive etching a

6.

three-dimensional image of the defects in the

microscopy.

A one to

one correlation

between the

elongated etch pits and stacking faults revealed

layer may be produced.

by TEM is obtained.

of the application of this etch is

I t is interesting to note

that only the faults in one direction etch as

A detailed description given by

Stirland IB.

elongated pits and that only the partial dislocations of the faults in the other direction are revealed by the etch.

Once a calibration of

6. X-RAY TECHNIQUES Two X-ray methods have found use in

the

this type has been carried out the etch is used

structural characterisation of III-V epitaxial

as a rapid quality control check of the defects

layers.

The f i r s t method is X-ray topography.

M.J, Cardwell / Ill-V epitaxial layers

88

Transmission X-ray topography is used r o u t i n e l y

method.

to assess GaAs and InP substrates and has the

the

advantage of resolution of a few microns in a

technique

totally

The e p i t a x i a l

addition of X-Y movement and computer control

may be analysed using

allows the mapping of mismatch and strain over

layer

non-destructive mode. on

the

reflection

surface

X-ray

topography.

However the

The strain in the layers is given by

broadening of is

the

totally

diffraction

peak.

non-destructive

The

and the

the whole sample.

technique is not easily applicable to very thin multilayer penetrate

structures several

Additionally if lattice

since

microns

there

is

parameters of

the

X-rays

into

the

will

a difference in the

any of

the

layers

Bragg condition for the topography w i l l major

use

of

X-rays

in

Electron microscopy can be applied to

the

characterisation

of

epitaxial

wafers

in

the many

modes:

not be

satisfied. The

7. ELECTRONMICROSCOPY

sample.

(i)

Cathodoluminescence

(ii)

Electron beam induced currents

multilayer

structures is double crystal d i f f r a c t i o n .

This

method is used r o u t i n e l y to measure very small differences epitaxial

in

lattice

parameter

layers and also any strain

of

the

(iii)Transmission electron microscopy.

present. The f i r s t

Figure 7 shows the X-ray d i f f r a c t i o n peaks from

two methods u t i l i s e

the spatial

One of

resolution of scanning electron microscopy and

the ternary or quaternary layers is matched to

reveal defects by t h e i r non-radiative properties

and coincident with the InP substrate peak on

and changes in c a r r i e r

the l e f t and the other layer shows a s i g n i f i c a n t

techniques are s i m i l a r to dislocation etching in

mismatch leading to the peak on the r i g h t .

the information revealed.

a InP/GalnAs/GalnAsP

layer structure.

This

dif f us io n

length.

The

Transmission electron microscopy, however, is

mismatch could not be determined using any other

the most powerful and informative of the methods used to assess structural 10000

mat er ial.

Sample: N30/198 Material: Tern/Quat/InP

(~

8000 Pos: 244.8 secs

ii

Va,ue, oo :s

4000

to

360

460

560

Relative Bragg Angle (Arc. secs)

electrons

to

be

normally achieved by The defects can be

A typical example of a TEM image is shown This example is a stacking f a u l t

in GaAIA layers. 200

the

is

seen d i r e c t l y and f u l l y analysed in the instruin Figure 8.

100

allow

and this

chemical and ion thinning. ment.

2000 0 0

The sample must be made thin enough

microns)

transmitted

Peak Data

A

~ 6000

imperfections in the

as

Any chemical impurities such

micro-precipitates

may

be

identified

by

energy dispersive X-ray analysis using a solid state X-ray detector f i t t e d to the microscope. Additional

FIGURE 7 Double crystal rocking curve GalnAs m u l t i l a y e r structure.

of

InP/GalnAsP/

precipitate

information

of

the

phase of

any

can be obtained by analysing the

electron d i f f r a c t i o n pattern of the inclusion.

M.J. Cardwell / III-V epitaxial layers This

technique was used to

precipitates arsenide19.

on

identify arsenic

dislocations

in

gallium

89

seen in the TEM micrograph because of the change in

atomic

number contrast

caused by the

variation of Al concentration.

t

E~oxy resin

2 - 3 mm

Layers

I I

I I I

Mechanicely polish I / both sides down

SiC, 6U, llJ diamond paste Ion milling

FIGURE 8 Transmission electron micrograph of faults in GaAs/GaAIAs layers. The analysis structures

is

of

sectional sample. be examined is

end up with

thin multilayer epitaxial The technique used to obtain

stuck face to

Ion 205~va20

stacking

achieved by preparing a cross-

this sample is given in Figure 9.

~15.

FIGURE 9 Schematic of the preparation technique used to produce cross-sectional TEM samples.

The wafer to face using an

epoxy. Thin bars are then cut from this double structure and thinned using polishing and ion thinning to produce a thin area in the region of interest. out

in

Electron microscopy is then carried

the normal way exceDt that a cross-

sectional obtained.

v i e w instead

of

a

p l a n view is

T h i s allows the characterisation of

interfaces and the defect relationship between layers and substrate to be observed. A typical example of Figure

10.

multilayer

a TEM cross-section This structure

sample is grown by

is a

shown in

GaAIAs/GaAs

MOCVD.

The

switching of the gases has led to a variation in the Al concentration

in the AIGaAs layer (as

shown by SIMS in Figure 2).

T h i s may be easily

FIGURE 10 Cross-sectional TEM Micrograph of GaAs/GaAIAs multilayer structure. Note the variation of Al concentration in the GaAIAs layers.

M.J, Cardwell / III-V epitaxial layers

90

The

characterisation

and

s u p e r l a t t i c e and auantum well

measurement

not be achieved without the use of TEM. measurement

of

the

of

8. CONCLUSIONS A

structures could

layer

width

The

would

be

wide

range

of

techniques

developed to allow the f u l l III-V

epitaxial

layers.

have

been

characterisation of The

difficulties

impossible by any other techninue except photo-

presented by very thin m u l t i l a y e r structures of

luminescence 2°,

differing

quantum well

However i f

structure

the layer is not a

or of

a width qreater

than a few hundred angstroms this method is not

compositions have been overcome and

the accurate feedback reauired for a high y i e l d growth process has been achieved.

applicable and electron microscopy must be used. An example of a TEM micrograph of a superlattice structure

is

shown in Figure 11.

This

ACKNOktEDGEMENTS

layer

The author acknowledges the help and advice

structure of period 125 A was grown by MOCVD and

of many colleagues at the Allen Clark Research

shows no A1 compositional v a r i a t i o n s as shown in

Centre

Figure I0.

personnel of the a n a l y t i c a l services department

The layer thickness measured by TEM

was used to c a l i b r a t e the Auger sputter p r o f i l e s

and

is

particularly

grateful

to

the

for the use of t h e i r data.

shown in Figure 5. REFERENCES

FIGURE 11 Cross-sectional TEM Micrograph of GaAs/GaAIAs s u p e r l a t t i c e , with a period of 125 A.

i.

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2.

C.M. Wolfe, G . E . Stillman and P.M. Korn, GaAs and Related Compounds, St. Louis 1976, Confo Ser. No.33b ( I n s t i t u t e of Physics, London, 1977) 120.

3.

T.S. Low, G.E. Stillman, P.M. Collins,C.M. Wolfe, S. Tiwari and L.F. Eastman, Appl. Phys. Letts. 40 (1982) 1034.

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atomic

arrangement

of

atoms at

hetero-

e p i t a x i a l interfaces and to determine any long range order or disorder effects in I I I - V a l l o y layers.

M.J. Cardwell / III- V epitaxial layers

91

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12. J.B. Clegg, Semi-insulating III-V Materials, Evian 1982 (Shiva, England, 1982) 80. 13. J.B. Clegg, in Secondary Ion Mass Spectrometry SIMS I I I , ed. A. Benninghoven (Springer-Verlag, 1982) pp.308. 14. A.E. Morgan, Nuclear Inst. and Methods in Phys. Res. 218 (1983) 401. 15. D. Huber and N.T. Linh, J. Cryst. Growth 29 (1975) 80.

18. D.J.Stirland, GaAs and Related Compounds, Edinburgh 1976, Conf. Ser. No.33a (Institute of Physics, London, 1977) 150. 19. A.G. C u l l i s , P . D . Augustus and D.J. Stirland, J. Appl. Phys. 51 (1980) 2556. 20. P.M. F r i j l i n k and J. Malvenda, Journal de Physique 5, 12 (1982) C5-185. 21. R. Sinclair, Sixth American Conference on Crystal Growth, Atlantic City, 1984. To be published in J. Cryst. Growth.