GaAs(100) quantum wells and the sample surface

Superlattices

and Microstructures,

Vol. 72, No. 2, 1992

COUPLING BETWEEN NEAR-SURFACE AND TBE SAMPLE SURFACE

267

In, Gal-x

As

/ GaAs(l00)

QUANTUM

WELLS

2. Sobiesierski’ and D-1. Westwood Department of Physics and Astronomy University of Wales College of Cardiff P.O.Box 913. Cardiff CFl 3TB. Wales, U.K.

(Received 4 August

1992)

Low temperature PL measurements are reported for a series of In6~26Gag.74As/GaAs(160) quantum well (QW) samples, with surface-barrier thicknesses ranging from 8 to 566A GaAs. Each sample consists, in growth order. of l&, 3Si and 58h wells, separated by 106@A GaAs barriers. The intensity and peak energy of the PL arising from the 58;h well provide a measure of the coupling between this near-surface QW and the sample surface. At the same time. the PL lineshapes obtained from the other two wells allow us to correct any minor variations in growth parameters which might exist within the range of samples studied. The decrease in PL intensity from the near-surface QW only becomes significant for surface-barriers below 106;b in thickness. A 58%L GaAs ? whilst a top barrier results in a PL red-shift of 6t2 ?eV, 25A GaAs surface barrier leads to a much broader PL band, red-shifted by 86t2 meV. with a peak energy corresponding to band edge recombination within the strained In6.26GaQ.74As layer. No luminescence has been observed from the 50A Ing.26Ga6.74As/GaAs surface QW.

Introduction

the

The use of non-invasive structures

is

particular,

optical probe

spectroscopy as of semiconductor

now well be

quantum high

the

strength

PL signal

Interface Hokkaido

address: Quantum University,

Japan.

0749-6036/92/060267+05$08.00/0

(PL)

used to examine environment

semiconductor with relatively

Present

In

photoluminescence

measurements can electronic local

*

established.

well (QW) sensitivity, is

the in

samples, since

proportional

Research

a

Centre

number of within Whilst

investigations wells fabricated there which wells lattice

carriers the the

which

potential majority

have involved within bulk

have been several have addressed

recent surface

become well of

quantum material, studies quantum

[l-41. PL measurements for matched In0.53Ga0.47As/InP

both [2]

and strained InAs/InP [4] revealed efficient carrier

epilayers capture

for

confinement within such However, similar studies,

surface QW’s. performed on

060

strained In,Gal_,As/GaAs ML InAs/GaAs structures

to

Electronics, N13 W8 Sapporo,

total

captured region.

detect

any

significant

0 1992

have and

(~~0.20) and have failed

1-2 to

luminescence

Academic

Press Limited

268

Superlattices

arising

from

[5].

In

lattice

to

coupling and

Hence,

this

of have

GaAs

surface

followed

the in-situ

performed effect

at of

150K,

3Ga0.

surface.

In

exhibited intensity,

that

thickness

of

reduced

QW

and

case, a

the

1501.

carried

out

main

under

reasons.

Al,Gal_,As/GaAs

system,

confining

potential

adjusted

independently,

alloy the

composition

strained.

thermal

activation

strained to

of So,

low

measurements. surface

QW’s

to

it

oxidation

QW's

[71

result

being

altered,

gases

of in

but

InAs/InP

the

not

too

layer

PL

unlike

the

both

the can

The

PL

sensitive

grown

to

to

establish in

both

composition.

the

lib

variations

in

fluctuations on

well

possible

alloy

whilst,

of

of

exist,

for

one

well in

the

is

thickness

other

alloy hand,

Potential

VARIABLE GaAs SURFACE-BARRIER

of for the will

to

limit of

the

advisable been

IOOOAGaAs

PL

--IOA Infia,_./+s ---

shown near-

[6],

and

Substrate

surface efficiency

drastically.

at

might

energy

choice

Surface

Fig.1 Schematic

were

energy

for

and

than

composition,

potential samples

peak

within

be

Experimental The

which

more

lie

need

limits

the

ensures

the

The

the

time,

structure

at

[6].

thickness

PL

(T<77K) has

used

re-cycling

same

wells

length reflects

have

out is

three

the

barrier

GaAs/Al,Gal_,As

different

prolonged

the

which

temperature it

all

rather

as

determines order

QW’s

of

in

chosen

carriers

Secondly,

exposure

emission

value

be

well in

of

InxGal_,As

perform

that

system

thickness

remain

can

that

absorption

the

the

restricts

carrier

of

This affords

between

also the

solely

it

coupling

At

3011 The

data.

since

inter-well

thickness

with

5011 wells.

PL

and

occur.

variations,

sample

width

the

which

In,Ga1_,As/GaAs

maximum

well

of can

total

conditions

where

and

electronic

501

the

determined

chosen

themselves

confidence

which

Firstly,

was

order),

and

been

in

3Odi and

separating 1lA

of

nm (1.97

depicted

111,

temperature

structure

carried

632.8

(growth

the has

low

widths

top

wells

low

power

is

of

barriers

composition

amount

of

500°C,

by

GaAs(100)

of

the for

samples

variation the used

a

The

were

structure

both

inside

excitation

consists

In0,26Ga0.74As 1OOOA GaAs

minimal

an

semi-

on

reactor.

a wavelength It

from

MBE

measurements

sample

1.

the

Preliminary ex-situ

The

(MBE)

substrates,

using

at

the

significant

reported,

1 W/cm2

study

meV,

PL

4.2K,

between

PL a

40

at

excitation

reduction

A10,3Ga0.7As

are

of

the

with

below

and

near-surface

the

to

We

series

to

drastic

up

measurements two

a

together of

used

out

which

measurements,

within 7As

both

red-shift

for

was

near-

Moison

a

temperature

Vol. 12, No. 2. 1992

epitaxy

V60H

Semi con

wells

varied.

coupling

states

GaAs/Alo.

a

of

PL

electronic

confined

been

approach

the

thickness

is

where

on

of

VG

from

radiative

the

barrier

[3],

elegant

as

QW

focusses

the

GaAs(100)

well

states.

within

QW,

from

confined

proximity

on

occurring

co-workers

was

the

has

InxGal_,As

the

the

beam

insulating

figure

reflect

surface

molecular

eV).

and

PL

must

study

which ace

recombination surface

QW’s

particular

surf

since exhibit

of

InxGal_xAs

influence

sample

the

capture

between

the

for

absence

In,Gal_,As/GaAs

strong

high

However,

carrier

be

even

very

invariably

the

states

for a

QW.

efficient

surface

may

velocity

QW’s

confinement,

large

there

produce

surface

InxGal_xAs/GaAs

the

that

recombination

both

the

dislocations,

InAs,

InAs/GaAs

material

case,

means

misfit

ML

surface

a

surface-well

latter

mismatch

sufficient 1-2

the

the

and Microstructures,

in

confining

1n0.26Ga0.74AS/ in this study.

Superlattices the

PL

30A

and

on

and Microstructures,

peak

the

energies

501

wells

exact

Results

Vol. 12, No. 2, 1992

associated are

far

with

more

the

dependent

composition.

and

269 1.493

2

obtained

displays

from

samples

barrier

thicknesses

and

The

in

25x. the

with

a the

with

GaAs

main

200h,

material.

The

changes

in

thickness,

PL

peaks

from

GaAs

surface

to

radiative

first

electron

with

surf

ace

observed

in

50X

assumed

that

energies

for

observed

the

sample barrier

heavy

1lK

hole

struct;re!elT~~l’;d~~fi~~~:

“;i’

bizd

“z:

in

well,

I

3oA

the

asymmetry

this

well,

to

lower

PL

surface

GaAs 66i2

meV.

PL

band

has

the

peak

to

direct

within from

Figure

3

5011

L

1.40

1.50

Energy/&

a

degree

4.2K GaAs

PL

spectra

quantum

surface (b)lOOA,

we1 1

barrier (c)

for samples

1n0.26Ga0.74As/ with GaAs

thicknesses 50A

and

(d)

of 251.

(a)

2OOA,

as

observed

barrier For for

, to

which QW

and

surface

reflects length

of

PL

sample

evidence

surface

for 100X

intensity

near-surf

overlap

the

the

between

below

effective

the

use

of

occurs

barriers

This

even

can

the

7As

indicated

thickness.

that

exists

the

.3Ga0

PL curve

we

coupling

comparison,

the

well

latter

thicknesses

GaAs/Alo for

for

loft

indication

of

the from

IpL(50:30),

plot The

an

onset

already

of

indicating

coupling

surface

tunneling

obtained

measurements

as

The

in

layer.

was

obtained

wells

near-surface

distance

the and

ratio

similar

flat,

surface.

coupling

of

corresponds

QW,

30h well

ex-situ of

QW’S

25A

recombination

IpL(30:10).

(50:30)

data

a

by

width

eV

the

3Oh

the

these

GaAs . Fig.2

meV.

QW with

intensities

essentially

the

slight

considerably 1.236

plots

and

of

IpL

6f2

In0.26Ga0.74As

with

intensities, is

a

some

the

surface

PL

together

for

a

[51.

integrated

ratio

is

1 ineshape

with

whatsoever

50h

barrier

PL

band-to-band

the

in

red-shifted

increased

emission

shift

ace

50h

is

strained

previously

the

of

the

of

certainty

near-surface

the

case,

energy

the

PL

NO

this

of

5015. there

together

barrier

In

a

to

50h

to

from

30.X

samples

any

to

energy

arising

the

the

surf

reduced

from

in variation

all

thus

GaAs

pronounced

be

composition

the

is

The

I

for

peak

can

for

for

determine

thickness

shift

QW’s

fluctuations

meV

is

can

been

PL

corresponds

This

When

has

in 30A

in

energy

shifts

it

energy

i2

which

peak

QW.

ZOOA GaAs Surface barrier

peak

to

we

associated

The

reproducibility which

pronounced

variation and

of

slight

Hence,

to

PL

amounts

PL

(a)

the

composition.

with

soA

than

1lA

GaAs

between

more energy

the

x=0.26+.0025.

r

a

fig.2.

carbon

the

occurrence

peak

solely

measured,

states

well

attributed alloy

have PL

the

observed,

transitions and

the

the

to

within

well

should on

spectra lOOA,

acquired

2OOft

correspond between

PL

of

three

spectrum

4.2K

electron

barrier samples,

Discussion

from

recombination

effect Figure

arises

eV

acceptor

around

ace of

200h

coup1 ing between

the

state

and

270

Superlattlces

IO -

z5

lQmm

(b)

I

??

~-~~~~~_~_~~_~-~ / (aI

d _$l.-0

4’

2

I

x

,

Vol. 72, No. 2. 1992

and Microstructures,

c

100-

IO I

0

200

100

300

400

Kx,

0

I

50

100

GaAs Thickness/A

1000/T (K-II Fig.4

Fig.3 Ratio

of

for:

integrated

(a)

4.2K

50A-to-30A

30A-to-10A

intensities and

emission,

GaAs

of

PL

emission

well

thickness

surf

Integrated surf

(b)

versus

ace

the

penetration

The

into

penetration

wavefunct

(b)

ions

should

26Ga0.74As/GaAs

and

Hence,

the

coup1 ing

distance

difference

in

surf

of

Further of

for

1001

emphasise

presented

GaAs

Arrhenius

has that plot

obtained

from

surface

barrier.

activation 1001

sample

the

been is

similar there for

thickness.

well

to

the

that

is

surface

5OOa,

(e)

25A.

portion

to

of

curve

comes

coup1 ing

The

barriers Decreasing

shown

non-

longer

be

carriers

for

the

the

GaAs PL sample

assumptions variations the

be the

would

77-100K

thorough

to in

measurements it

of PL

is of

ex-situ

with

From

in-situ

it

near-surface and

dependence, a more

by

that

increase

the

simple

possible

temperatures

suffice

have

even

parameters.

undertaken

of

providing

temperature

that

ion

a

to

of

means

to no

QW

allows

growth

surface

will

between

structure

that

in the

of

step

74As/GaAs

made

GaAs

change

we

measurements,

5001

sample

However,

sample

follow

surface,

as

the

proximity

act ivat

to

sample

to

the

oxidised

becomes

which layer.

a

each

to

QW barriers.

InO, 2SGa0,

the

top

in

knee

for

rate-limiting

conclusion,

possible

result

the

curve,

thermal the

In

of

recombination

simple

of

no

the

the

over

barrier

a

(a)

and

barrier

case,

radiative

PL

surface

256: GaAs

not

vertically

with

a

near-

4.

part

of:

vertically

pronounced

latter

dependent

shifted

sample

with

increased

figure

linear

intensity

the

surface.

the

shifted

rounding

the

added

surface

linear

energy in

a

the

So,

the

surface

in

to

particularly

tunneling

an

temperature

corresponding

to

in

the

of

have

GaAs

between

series

of

above

oxidised

coup1 ing

measurements curve

an

of

of

in

PL

either

evidence QW and

a

we for

or energy, we have

since

complication

present, values

state

length,

from

At

been the

50A

(d)

near-

GaAs

(a).

values

clear

length

75A,

has with

pronounced

change a

overlap

the

A10.3Ga0,7As/GaAs

distinct

(c)

(b)

for

thicknesses

lOOA,

501

from

well,

QW

altered

be

tunneling

extract

ace

surface

the

indicates

the

state. to

degree

of not

QW

barrier.

between

systems.

tried

the

surface

depth

significantly

surface

of

the

intensity

quantum

surface-barrier

barrier.

depth

wavefunctions

PL

ace

Curve

In0

I

I

study

spectroscopy.

seem would to

be

Superlattices

and Microstructures,

Acknowledgement - This work was funded under the Low Dimensional Structures and Devices (LDSD) initiative of the United Kingdom Science and Engineering Research Council.

References 1.

2.

3.

271

Vol. 72. No. 2, 1992

R.M.Cohen,M.Kitamura and Z.M.Fang, Appl. Phys. Lett.a, 1675 (1987). E.Yablonovitch, H.M.Cox and T.J.Gmitter, Appl. Phys. Lett.52, 1002 (1988). J.M.Moison, K.Elcess, F.Houzay, J.Y.Marzin, J.M.Gerard, F.Barthe and

4.

5. 6.

7.

8.

M.Bensoussan, Phys. Rev. Ba, 12 945 (1990). Z.Sobiesierski, S.A.Clark, R.H.Williams,A.Tabata, T.Benyattou, G.Guillot, M.Gendry, G.Hollinger and P.Viktorovitch, Appl. Phys. Lett.B, 1863 (1991). Z.Sobiesierski "unpublished". F.Houzay, J.M.Moison, K.Elcess and F.Barthe, Superlatt. and Microstruct. 9, 507 (1991). and Z.Sobiesierski, S.A.Clark R.H.Williams, Appl. Surf. Sci. a58, 703 (1992). See, for example, Handbook of Optical Constants of Solids, edited by Edward Palik (Academic, London, 1985).