Charge Redistribution and Potential Barrier Reconstruction in SI GaAs Caused by EL2 State Change

(1)

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Charge Redistribution and Potential Barrier

Reconstruction in SI GaAs Caused by EL2 State Change

R. Kiliulis, V. Kažukauskas, J. Storasta, J.-V. Vaitkus

To cite this version:

R. Kiliulis, V. Kažukauskas, J. Storasta, J.-V. Vaitkus. Charge Redistribution and Potential Barrier

Reconstruction in SI GaAs Caused by EL2 State Change. Journal de Physique I, EDP Sciences, 1996,

6 (9), pp.1165-1187. �10.1051/jp1:1996122�. �jpa-00247239�

(2)

J. Pllys. I France 6

(1996)

1165-1187 SEPTEMBER1996, PAGE 1165

Charge Redistribution and Potential Barrier Reconstruction in

SI GaAs Caused by EL2 State Change

R.

Kiliulis,

V. Kaiukauskas

(*),

J. Storasta and J.-V. Vaitkus

Semiconductor Physics Department ^of ViInius University, ^Sauletekio al.9, bldg.3, 2054 ViInius, Lithuania

(Received ⁷ September1995, revised là January 1996, accepted 21

May1996)

~

PACS.71.55.Eq III-V semiconductors

PACS.72.20.Jv Charge _carriers: generation, recombination, lifetime, and trapping PACS.72.60.+g Mixed conductivity and conductivity transitions

Abstract. We report ^the êRects âssociated ^with ^the ^transition ôf ^trie ÊL2 ^defect ^to îts

metastable state EL2* and _mce _versa in semiinsulating

(SI)

GaAs. We investigated changes _in deep level spectra, ^the ^time ^evolution ^of ^the quenching _process, and the thermal _recovery of the normal state. It _was shown that the p-type state introduced by the transformation of EL2 and the associated charge transfer between diRerent defects exists in the dark _even above 150 K.

Therefore _a number of electron and hole traps can be observed separately by the thermally

stimulated measurements. A thermal quenching eRect of thermally stimulated currents and

thermally stimulated Hall mobility has been identified and numerically simulated. A model _is

proposed to explain charge transfer induced by the photoquenching of EL2, which is based _on the change of the compensation ratio of _some deep levels, not associated with EL2. Therefore it is not necessary to introduce the "EL2 family" concept. Furthermore, _we demonstrate that

the exhaustive analysis of the eRects associated with the EL2 transformation should include both charge redistribution between _numerous levels _in the band _gap and reconstruction of the

potential barrier network _as well. The evidence of _a prirnary lattice relaxation associated with

an intermediate excited EL2~ state _is demonstrated. Persistent _carrier eRects _are diRerent in

both EL2 states. This confirms that potential fluctuations _are modified during the thermal

quenching of the EL2 level. A cellular percolation ^model is presented.

1. Introduction

Among

all the

electrically

active defects present ⁱⁿ as-grown SI

GaAs,

the dominant donor EL2 is

particularly

_common ⁱⁿ a

bulk-grown

^material. ^EL2 ^is

responsible

for the SI

properties

of GaAs

[1-6]

as far _as it compensates the difference of _excess shallow carbon acceptors ^and shallow donors N~A N~D. Nevertheless it _was

pointed

out in [7] that

actually

there are

much _more ionized EL2+ defects in the

crystal

which amounts to some 10~6 cm~~ This

predominance

of EL2+ _to carbon acceptors requires the existence of other acceptors, e-g-, GaAs

(*) Author for correspondence je-mail:

Vaidotas.Kazukauskas©FF.VU.LT)

Q Les Éditions _de _Physique 1996

(3)

1166 JOURNAL DE PHYSIQUE I N°9

[7]. Concentration of native

defects, exceeding

or similar to that of EL2 _were

reported

in

[4,8-11],

which should be included in the compensation mechanism. Other compensation models _were

proposed

in

[6,12,13].

Also it _was

argued

that the

simple

3 level model lits quite well with the experimental data [14]. ^Efforts should be made _to elucidate the situation.

The

signature

of EL2 level is the existence of its metastable state EL2* below 120

K,

which

is

electrically, optically

and

magnetically

inactive

[15-17].

EL2*

fully

_recovers back to EL2 above _a temperature of

approximately

lso K. At the temperatures below the thermal _recovery of EL2

(120-lso K)

its normal

configuration

_can be restored

by

the illumination with either o.7 -1-o eV

photons

_or

photons

with energy

exceeding

1.4 eV [15]. ^The ^o.3 ^eV activation

energy of the thermal _recovery _of _EL2 _is lowered _to about o-1 eV

by

the _presence of free electrons [18]. ^Due to the

inactivity

of EL2* _no direct experimental

analysis

of its electronic

structure _was made. Some models _were proposed to

explain

EL2 ~ EL2* transition, which indude the excited neutral state EL2~ _as the intermediate _one

[16,19,20].

This state, associated with the _primary lattice

relaxation,

_is reached when the

initially

ionized EL2 becomes _neutral due to _an

optical

excitation. The

following

step includes the final lattice relaxation due to

Jahn-Teller distortion to _assure the metastable minimum of the

trapped

carriers _in coordinate-

configurational diagram

_[16]. This step is in competition with the _two other

possibilities, namely

the ionization of the _excited _state _and _the deexcitation _at smaller distortions [16].

The transformation of EL2 to its metastable _state modifies the compensation of SI GaAs

samples

_{[21, 22]} and thus the

photoquenching

effect properties should be

depeudent

_on EL2 and other defect concentrations. A

charge

transfer induced

by

the transformation has been observed in LVàI

absorption

_{[23, 24]} _and

photo-EPR [la,25]

experiments.

Nevertheless,

_except for

some other

particular

results

[là,17,

_26] _no detailed experimental and theoretical evidence _was presented _to show the influence of EL2 transformation _on other defects. On the other

hand,

the

diversity

of

photoquenching

_results _are _often _attributed _to _the _properties _{of the}

"EL2

family"

[8,

27-29].

We report the

investigation

of the

photoquenching

_and thermal _recovery effects of EL2 defects

by

optical spectroscopy,

thermally

stimulated and transient

photoconductivity

and Hall

mobility

measurements. A numerical simulation of

charge

transfer induced

by

the

photoquenching

effect is presented.

Charge

redistribution between EL2 and other defects

together

with _a

change

of the _associated potential barrier _structure

is demonstrated to be _an alternative _to the "EL2

family"

_concept.

Another

point

of interest is the influence of potential

inhomogeneities

_on _the _effects _associ-

ated with EL2. A random distribution of

chargea impurities

and defects

gives

rise to

spatially

distributed _band

bending [30,31]. Usually

EL2 and other defects _are accumulated around dislocations

[26,32-36],

that forms _a cellular structure

je-

_g.,

[14]) though

EL2 has been _seen _to be present in dislocation free SI LEC material [37]. Therefore, potential barriers _appear between the dislocations walls with

surrounding impurity

clouds and cells with lower defect

density

_[38].

Local band _gap variations induced

by microinhomogeneities

of the local compensation balance

cause a spatial separation of electrons _and

holes,

_a reduction of the carrier

mobility,

and the

scattering

of free _carrier thermal activation _energy values [39,

40].

It _was shown in

[41,42]

that potential fluctuations affect _carrier

transport

at low

light

intensities and temperatures below 330 K. In [43, 44] some basic ideas of _a

percolation

model have been

presented~

which will be

discussed in detail _in _connection _with _its influence

on the

photoquenching

effect. Thus, the influence of the

metastability

of EL2 _on other

deep

levels and _on the potential

fluctuations,

should be elucidated. Here _we present _a

complex

approach to the

charge

redistribution between

some

deep

levels, which is influenced

by

potential fluctuations

during

the EL2 transformation.

To _our

knowledge,

such _a

complex analysis

_is

being

_presented _for _the _first _time.

(4)

N°9 CHARGE REDISTRIBUTION, POTENTIAL BARRIER RECONSTRUCTION 1167

2.

Samples

and

Experiment

We

investigated

SI LEC and

Bridgman-grown

GaAs

samples, undoped, In-doped

and

Cr-doped.

Room temperature

resistivity typically ranged

from lo~ _to lo~ _Q _cm,

stationary

Hall

mobility

~IH from 200 to 5300 cm~

/(V si.

The

samples

_were

provided

with ohmic Au-Ge-Ni contacts.

The spectral distribution of

photoionization

_cross sections [45] was used to find the

optical

ionization

energies

of the traps

by fitting experimental

and theoretical _curves, which _were calculated

assuming

the

delta-function-type impurity

potent1al [46]. ^The

application

of this method for SI GaAs below lso K is burdened

by

a

possible

EL2 state

change

_upon infrared

illumination. EL2 _conversion to EL2" _can _occur in the 1.o 1.3 eV

spectral region and,

on the other hand, the _recovery of EL2* _to the normal state may take

place

in the

regions

o.7 -1.o eV and 1.3-1.5 eV. To prevent

this,

low

light

intensities _were

used,

_or, in _some _cases, _a

short-pulse

excitation _was

applied

for each

spectral point

with

pulse

duration

equal

to the stabilization time of the

experimental

system.

Thermally

stimulated current

(TSC)

spectroscopy was used to characterize

deep

and

moderately-deep

traps.

Marly

authors

[27,47-50] investigated

SI GaAs

by

this method.

However,

there _are considerable

discrepancies concerning

the trap parameters.

In _our _case the

heating

rates _was about 0.14

K/s.

Measurements of stationary and transient

photo-Hall

effect and photocurrent _[42] and TSC _were used to determine the contributions of different

sign

carriers and to evaluate the role of

microinhomogeneities, depending

_on the

excitation level and the temperature [43]. ^To êvaluate ^the înfluence ôf ^the

potential

fluctuation, associated with

inhomogeneities,

short circuit

photocurrent

and

photovoltage

_were measured.

Additionally,

TSC experiments at

higher

electric fields

(<

1 kV

/cm)

_[44] _were also doue.

In most _cases _we

paid

attention to the

changes

induced

by

the transition of EL2 to the

metastable state and not to the evaluation of the exact parameters of trie

deep

centers. The

samples

_were excited

by

_an He-Ne laser

(hv

= 1.08 eV and hv

= 1.97

eV),

with the

photon

flux of the order 3.5 _x 10~6 cm~~ s~~

3. Active Levels in the Band

Gap

Defect level spectra related both to normal and metastable EL2 states _were

investigated

in quasi-stationary conditions using

TSC, thermally

stimulated Hall

mobility (TSHM)

and spectral

photon

capture _cross section methods. In

Figure

la TSHM results _are

presented.

Curve 1 has been measured in _a normal EL2 state

In-type),

after _a short

(<

1

s)

illumination

by

1.08 eV

photons~

which _ensures the excitation of the

sample

but is not eilicient to

change

trie EL2 state. From trap densities

typically presented

in SI GaAs (10~5 -10~6

cm~~)

it is not

likely

that the

mobility

could be

notably

affected

by

ionized point defect

scattering.

Curve 2 represents an intermediate situation, obtained after 5 minutes of excitation

by

1.97 eV

light.

Curve 3 _was measured in EL2~ state, when

conductivity

became p-type âfter ¹⁰ ^min ôf êxcita- tion with

quenching light. According

to all

published data,

the metastable EL2* _state _cannot exist above 150 K temperature.

Kevertheless,

_curve 3 indicates that the p-type state remains above this temperature in the

dark, implying

that _recovery of the normal EL? _state and n-type

conductivity

_are caused

by

different mechanisms aiid do not _occur in _a _unique way in the dark and under

illumination,

_as will be discussed later.

The results of the TSC measurements using trie _same excitation conditions _are

presented

in

Figure

16.

According

to the

corresponding

Hall-effect

data, peaks Ei,

E2, Ex

(the origin

of this

peak

will be discussed

later)

and double

peak

E3 _are caused

by

electron traps. Sometimes

on the

rising

part of the El

Peak

_a shoulder is _seen, caused

by

holes released from _a

partially

filled Hi trap. ^Thermal activation energies of the traps, ^deduced from the initial

slope

of

thermally

cleaned TSC curves, were as follows: AEI

= 0.19 + 0.03 eV, AE2

= 0.29 + o.03 eV.

(5)

6000

_5000

ce ~

(4000

"fl

3000

/~

2000

~1000

2

0 ~ a

-1000

100 150 200 250 300 350

T

(K)

lÙ ^~~

Hi _H~

Ei

"t ~~

~

_~~ Î

t

,~2 ^~~

ÎÙ

Î~ ~l'~

j£

^j~

~

/ ~'

~ _Î

i "~~'"

Î ~~~ ^~ ,"

( fi

iÙ ^~~ ^Î ,' ~

%~X ~

~

90 140 190 240 290

T

(K)

Fig. 1. Thermally stimulated spectra of Hall mobility a) and _current b) after excitation _at ₁₀₀ _K temperature: 1) less than 1 _s with 1.08 eV light, 2) 5 _mm with 1.97 eV, 3) 10 _mm with 1.08 eV.

Curves 4 _were measured without excitation. Curve ₅ _in b) is _a result of the numerical simulation of trie thermal quenching of TSC. Here and henceforth, _except the Figure 8, positive mobility values

correspond to n-type ^and negative,- to p-type conductivity.

REX

=

AE2

within the _range of accuracy, and

AE3

= 0.34 + 0.03 eV for both _maxima of the

double peak E3. As it will be shown

later,

the minimum in the double

peak

E3 is caused

by

the decrease of

majority

carrier lifetime due _to the holes released from _a

partially

filled hole trap H2~

(thermal quenching

of

TSC).

The _excitation

by

1.97 eV

light (curve _{2) give} together

with the _same n-type peaks two hole trap

peaks,

labeled

H3

and H4. The _increase of

quenching light

illumination time led to the

gradual

decrease _of n-type peaks and the

development

of _p- type

peaks;

the

deepest

_traps reached _earlier saturation. The thermal activation energies of the traps deduced in p-type state _were the

following:

AHI

= 0.Il + 0.03 eV, AH2

= 0.40+ o.03

eV, AH~

= 0.45 + 0.03 eV, AH4

= 0.50 + 0.03 eV. Peak Ei is

usually

_seen _as

a shoulder _on the

decreasing

part ^of Hi

peak

in this _case. In this state TSHM in the temperature _range of

Hi

El

peaks

is close to zero,

indicating

_a mixed

conductivity

_regime. If the

initially

quenched sample

was heated in the dark _to temperatures below 260 K,

subsequently

cooled back and

shortly

(6)

-7.8

dark

= hv=1.17eV

~

~Î-8.8

é

= bo

-9.8

0 50 100 150

t

(s)

Fig. 2. An example of the infrared quenching of _a persistent photoconductivity. ^A persistent photoconductivity decay _was induced by the switching off of 0.72 eV light. The _arrow indicates trie

moment when 1.17 eV light _was applied. Here and henceforth, where _no special indications _are made, the experiment temperature _is (102 + 3) K.

excited

by

1.08 eV

light again,

the TSC _curves exhibited _an intermediate situation between _n- and p-type states, 1-e-, an

incomplete

_recovery of the initial state. The

complete

_recovery of n-type state has been observed

only

after the

heating

of the

sample.

in the dark to about 260 K

(thermal emptying

of the

deepest

hole trap

H4), confirming

that different traps take part in the

charge

redistribution. On the contrary, ^the illumination

by

_any

light

above 150 K led to the recovery of _an n-type conductivity

together

with EL2 restoration.

Spectral

distributions of

optical

_cross sections measured before and after the EL2

quenching

at 100 K revealed the

following photoionization

^threshold

^energies

ⁱⁿ ^n-type ^state: ^E~ ^0.40 ^eV, 0.76, 0.92 eV and 1.12 eV. In the

quenched

state the

photoionization energies

Ev ₊ 0.40

eV,

0.80, ^0.94

eV,

1.31 eV have been found.

Thus,

the

dominating deep levels,

are also different in

both EL2 states.

Most of the

investigated samples

have exhibited the infrared

quenching (IRQ)

^of

photocurrent

and of

persistent photoconductivity (PPC),

_as _was

reported also,

_e-g-, in [45,

49,51].

The mechanism of the

IRQ

is most

probably

the _same in both _cases. Indeed it _can be

supposed

that the initial

optical

excitation causes the _same trap

filling,

which remains constant in the first _case~ _or relaxes _very

slowly

to

equilibrium

if PPC is observed. We observed

quenching

ⁱⁿ

the

spectral region

0.39 1-Ù eV

(we

labeled it _as the A

band)

if the additiona1illumination

was in the

region

1-Ù 1.45 eV

(band B). And,

vice _versa, the

IRQ

of band B took

place

^if

the additiona1illumination _was with

light by

band A. The intensity ^of ^band ^B was chosen low

enough

to not initiate the effective

quenching

of EL2. As _an

example, IRQ

of PPC is

demonstrated in

Figure

2. After the illumination

by light

from band _A~ _a slow

photocurrent

relaxation is recorded. At the point ^labeled

by

_an _arrow the

light

^from ^band ^{B is} ^switched on.

After _a short

spike

of

photocurrent,

the

magnitude

^{of which}

depended

_on the intensities of

light

A and B and on the switch-on time of

light

B, _a

nearly exponential drop (IRQ

of

PPC)

of the

current was observed. If the intensity ^of ^band ^B was

high enough,

^the ^increase ^of

photocurrent

took

place

after the minimum _was reached. The traditional model of

IRQ

foresees _a scheme with at least two

deep

levels involved with different recombination and

generation

rates [52].

One of trie levels acts as a "Slow" center, which is activated

by

_an additional excitation and

supplies

hales to valence band. Later minonty carriers are

captured by

_a "fast" recombination

(7)

3

@$

5k

- ~

$Î~Î

_X

à ~cO°-g----

^~

/

~[

é çW j _~ ³

~

Î

₍ Î

$ ]

j

_~

tt ,

~ 0 39eV

(

₀ 68eV

(

05eV

x<

, o

O.ù 0.5 1.O 1.5

hv

(eV)

Fig. 3. Spectral dependence of the photo-ionization

cross section

a(

for minority carriers

obtained from infrared quenching of photocurrent by keeping its level constant. 1,3) experiment,as

2) obtained from 1 by subtracting calculated values; dashed _curves represent the calculated values.

The _arrows mark the positions of trie levels found.

center for

electrons,

thus

Àiminishing

investigated samples IRQ by

band A used to

disappear

around 200

K,

_i-e- well above tue temperature ai which tue metastable EL2* _can exist. Tue elfect of band B _was observed _even _ai

room temperature in _some samples,

especially

if

higher light

intensifies _were used.

In [51]

IRQ

of persistent

photocurrent

_or

spike conductivity

_was ascribed _to tue metastable behavior of defects

belonging

to tue "EL2

family". Indeed,

tue influence of EL2 is

obvious,

but tue above results show that

IRQ

elfects canner be

explained

in terms

only

of tue

metastability

of EL2, but orner defects should aise be considered. TO deduce tue nature of tue "slow" _centers tue spectral distribution of

photoionization

_cross section for

mmority

_carriers _was measured in

a normal EL? state

by keeping

tue

quenched

photocurrent constant

(Fig. 3).

Tue

following deep

level

positions

where obtained: Ev ₊ 0.39, 0.68 and 1.05 eV. Two spectral _as well _as tue temperature _regions, where

IRQ

elfects _were

observed,

mdicate tue influence of two dilferent electronic transitions.

Correspondingly,

bands A and B _can be attributed to tue influence of

tue _centers Ev ₊ 0.39 eV and Ev +1.05 eV. Tue first _one coincides within tue range of accuracy

with tue

H2

^level deduced from tue TSC measurements. This _is

supported by

tue

fact,

that H2 acts as a slow recombination _center in optical and thermal measurements

(tue

thermal

quenching

of TSC _as ii will be discussed

taler).

An electron trap E3 with _a thermal activation energy 0.34 eV is tue most

probable

candidate for tue center with

an

optical

activation energy of Ev + 1.05

eV, though

tue Frank-Condon _energy shift reaches _m 0.1 eV in ibis _case.

(8)

30

- 6000

~

5

'o

~~ _-

Q

4000

~

~ fi

1 _io

zooo

~ '~,,2 ^°

~'

/ ^'

~""~,

( Î

o ^~' ^"' 0

-10

0 50

t (s)

40 8000

~

30 5 6000

~

Î[ 1

~

₂₀ _,1 ₄₀₀₀

~

£

,/ b

fl

~z 10 ^" 2000

d

~ /

~ nz

o

2,,'

',--- o

~

-10 -2000

100 lZ0 140 160

T

(K)

Fig. 4. Quenching transients

a)

and thermal _recovery dependencies

b)

of: 1) photocurrent, 2) photovoltage and 3) ^Hall mobility _upon excitation by 1.08 eV light.

4. Transformation of EL2 Defect and trie Associated

Charge

Transfer

4.1. THE PROCESS OF EL2 QUENCHING AND RECOVERY.

Figure

4a shows trie

quenching

transients of

photocurrent, photo-Hall mobility (/tH)

and

photovoltage (Uph)

mobility

with _no

sign

inversion bas been observed. Tue _mea-

surements with masked and open

sample

contacts revealed that

photovoltage

was

generated

m each contact area and in tue bulk. Tue bulk value Uph is

analyzed

further. Tue important

pecuharity

of it is tue lime

delay

after tue excitation onset. Tue

recharge

of

scattering

centers

probably

occurs

during

this time. Further, tue rearrangement ^of

potential

fluctuations and

percolation patins

for carriers should

necessarily

be taken into account, because

photovoltage

(9)

l172 JOURNAL DE PHYSIQUE I N°9

io ^~~

-

io ^~~

Zl~~

_-io

10 ^~~~ ^~

~~ ^-iz

~~ ^-i~

~

0.4 0.8 1.Z 1.6

hv

(eV)

Fig. 5. Photocurrent spectra. ^Curves 1 and 2 correspond to normal EL2 state, ³ to _a metastable EL2* state. l _was measured just after cooling without trie initial preexcitation of the sample, 2 after

curve 1.

maxima

corresponding

to _an average electric field

equal

to 0.6

V/cm

_were _measured. In _some

samples puotovoltages uiguer

tuan I.à V bave been measured at hv

= 0.9 eV if _a

preceding

illumination witu I.o -1.45 eV

photons

_was used. A recovery process of tue _normal _EL2 _state witu temperature, under illumination

by

tue _same 1.08 eV

photons

is suown in

Figure

4b. Tue

puotovoltage

uad tue maximum

again

_m tue

typical

temperature

region

120 -140 K

(curve 2).

We evaluated tue _maximum dilference between tue quasi-Fermi

levels,

wuicu _can be reacued supposmg tuat tue nonequihbrium _carriers _are

elfectively

separated in space

by

_potential _bar-

riers: Fn

Fp

=

kTln(np/N~NV)

₊

_{EG Î53j.}

Here _n,p _are tue carrier

concentrations,

wuicu

were measured

experimentally during

tue EL2

quencuing,

N~ and Nv _are tue effective densities of states and

EG

is tue band _gap. Tue estimation gave tue _maximum

possible

dilference

m

tue

experimental

conditions _up _to _1.16 _eV. Tue above facts

show,

tuat

during

EL2 _m EL2*

mterconversions, asymmetrical barriers _appear in tue

bulk,

wuicu _are connected in series

yield-

ing

large puotovoltage

under

illumination,

and tueir

configuration

_is dilferent in botu _states.

Tue thermal activation energy of Uph was close to tue energy barrier of EL2 recovery

(about

0.3

eV).

Tue

puotocurrent

decreased _to tue _minimum and tuen mcreased to tue value of _an n-type state

(curve 1).'Tue

decrease at tue temperatures below 120 K

probably

takes

place

due to tue

change

of tue lifetime of tue

generated

_carriers. It is

supported by

tue measurements of tue

temporal decay

of tue

puotoconductivity

_as will be discussed taler. Tue recovery of EL2 ends

by

tue

conductivity

type

change

and ils increase to tue initial value of _an n-type state.

4.2. CHARGE REDISTRIBUTION _BETWEEN EL2 _AND _OTHER DEFECTS. On trie basis of

our results and _on tue literature data _a

qualitative

picture of tue trap

filling

_m botu EL2 states can be

presented,

wuicu will be taler detailed

by

tue results of numerical calculation. Earlier

single

_cases ^of tue

charge

transfer between EL2 and otuer intrinsic defects _were

analyzed, demonstrating

eituer tue neutralization of acceptors or tue ionization of donors

during

EL2 transformation. Tue

puotoconductivity

spectra presented in

Figure

5 demonstrate _tue _elfect of tuese

changes

well.

In

equilibrium,

tue Fermi level is controlled

by

_a dominant

midgap

donor EL2

il

_2]. Tue traps in tue upper ualf of tue band gap ^are empty ^and beneatu Fermi level (+~ 0.75

eV) tuey

_are filled

(10)

N°9 CHARGE REDISTRIBUTION, POTENTIAL BARRIER RECONSTRUCTION l173

witu electrons. This distribution is reflected

by

_curve 1 of

Figure

5, wuicu _was measured in

a normal EL2 state

starting

^from low

energies

wituout

preexcitation.

If tue

sample

is excited for _a short time witu

photons

^of energy ^> 0.8 eV, ^free ^electrons ^from

occupied

EL2 centers

are

generated,

wuicu _are later

captured by

otuer empty ^donors. ^IR ^and ^LVM

absorption

measurements

[23,

24,

54-56j

suowed

charge

redistribution between EL2 and _oxygen related

negative

U center OAS

(EL3) là?]

_or

Ga-O-Ga,

O VAS.

Simultaneously

voles from empty EL2 _are moved to tue valence band and

captured by

acceptors. ^Tuis ^is

supported by photo-

EPR and

puoto-ESR

measurements

je-

_g.,

[7,10,

25,

58,59]).

Tue appearance of neutral carbon acceptors

during

EL2 transformation _was detected

by

electronic Raman spectroscopy

il?]

and carbon related

absorption

spectra

[60j.

^Tue state

change

of

deep

accepter ^levels was

reported

in

[8,47,61-63j

and _are

supported by

_our experiments.

Gradually

neutral donors and acceptors

are created. At tuis stage ^tue

puotocurrent

is dominated

by

free

electrons,

because tue total donor amount in _an active EL2 state exceeds tuat of acceptors. ^Tuis distribution results in

curve 2 of

Figure

5, which _was recorded

just

after trie first _one.

If trie

sample

is further illuminated with

quenching photons

below 120

K,

inactive EL2* is created. It _can _no _more capture ^free ^hales ^and ^becomes ^filled

by

electrons. Thus bath donors and acceptors are

emptied

of electrons and filled witu voles. SO, ^tue

puotoconductivity

m curve

3 of tue

Figure

5 is dommated

by

voles. It _can be _seen tuat

puotoconductivity

values in tue

two EL2 states dilfer

significantly.

^Tue ^dilference ^reacues up to four orders of

magnitude

and

can be

explained only by assuming

^tuat

puotoconductivity

_near tue band

edge

^is ^dommated

net

by

intrinsic band-to-band transitions, ^but

by

extrinsic transitions _ma defect levels

(net only EL2),

tue

charge

state of wuicu is associated witu tuat of EL2. Tue

opposite

idea _was

proposed

in

[64j,

tuat tue spectrum at hv _>

EG

is associated witu tue

puotoionization

of EL2 atone. A

charge

transfer between tue _numerous donors and acceptors ^A~ + D+ _~ A° ₊ D° _was

proposed

to be _an alternative to tue

metastability

of tue EL2 model

[25j.

^We suppose tuat tuis

assumption

is net correct, ^because neutral donors exist witu

considerably

ionized acceptors ⁱⁿ

an n-type state. In _a p-type state, tue

picture

is tue _converse. Since EL2 is tue

only

defect wuicu accumulates electrons

during quencuing,

^tue

charge

redistribution

picture

should include tuis "reservoir" of electrons.

Heating

above120-150 K activates EL2

again.

^It ^suould ^be ^stressed, ^tuat a normal EL2

state does net

necessarily

_mean an n-type conductivity, because, _as it _was stated earlier, no

recovery of n-type occurs at 120-150 K in tue

dark, altuougu

normal EL2 is believed to recover

above tuis temperature

(EL2*

bas _never been observed above 150

K, tuougu

_a

uypotuesis

_was

proposed

in

[65j,

^tuat ^EL2* can exist at room temperature ^for a short

time).

Indeed, above 150 K tue level Hi is

tuermally

filled witu electrons and Ei, E2 ^become ^ionized. ^Otuer acceptors

H2, H3,

H4 remain neutralized in tue

dark,

_so

tuey

still _cause _a p-type conductivity. ^In [10, 25j

tue recovery of tue initial

charge

state of acceptors ^aise ^occurred

only

above 250 K in tue dark.

Tuis

implies

^tuat ^tue recovery of normal EL2 _occurs

turougu

^neutral

(filled) charge

state. If tue

ligut

is

applied during

EL2

quencuing

and _recovery

elfects, configurational changes

m tue

sample

_are

accoinpanied

and masked

by hgut-induced charge

redistribution between EL2 and otuer defects.

4.3. NUMERICAL MODEL _OF THE CHARGE TRANSFER PROCESS. Trie

charge

transfer

induced

by

EL2 _~ EL2* transformation _upon illumination witu

quencuing ligut

was simulated

numerically

_usmg _a

turee-deep-level

model. Tue _same scueme was used to model aise tue

tuermal

quencuing

elfect of TSC. EL2, _a donor

(D)

and _an acceptors

(A)

_are involved. Donor and acceptors ^levels can be assumed to be effective levels of _some donors and acceptors ^active ⁱⁿ

a limited temperature region. Tue scueme in

Figure

6 presents transitions taken mto account.

Here EL2 interacts witu botu bands. Donors and acceptors ^witu concentrations ND ^and

NA,

(11)

E~

~Du

~~*

CDn CDn

D

g*

r

~u Ca

jj~

CAn EL2

cap c~ a~

_~

a~~

c~

c~~

E~

^~

Fig. 6. The scheme of the electron transitions included in the model. For details _see the text.

mainly

interact witu

appropriate

bands. Tue

following

_set of equations describes tue

change

of tue

population

of electron in EL2

([EL2°j),

donors

IN[

_and _acceptors

_(Ni _),

_tue _free

carrier

concentration _n and p and tue transition of tue EL2 level _to its metastable _state EL2*:

dn

/dt

=

ion [EL2°j

+

çiaDnN£

_cn

[EL2+jn cDnN(n cAnN(n il +cDnN(Nc _exp[- (Ec

_ED

Tue last equation is tue

charge

balance _one. Tue EL2* is inactive and filled witu electrons. Tue last term in equations

(1)-(3)

describes tue tuermal generation of free carriers. It _was taken

into account

only

wuen

modeling

tue thermal

quencuing

of TSC, because in tuat _case optical

generation

_was absent. Tue thermal

generation

of _carriers from EL2 level _is _net effective due _to its low rate, wuicu dilfers _at least

by

_some orders of

magnitude

from tuat of _more suallow

levels,

active in tue temperature

region

beneatu 270 K.

[EL2*]

is tue metastable EL2 concentration.

an, ap are tue ionization _cross sections of

EL2,

_aDn of

donors,

_aAp of acceptors. _cn, _cp _are capture coefficients of EL2, _cDn, _cDp of

donors,

_cAn, _{cAp of} _acceptors. _{çi is}

photon

flux. a* is tue

cross section of tue

optical quencuing

of EL2. _r _is tue rate of tue _reverse transition from EL2* _to

EL2, describing

tue tuermal _as well _as tue

optical

_recovery _[26]. Tue

followmg

values _were used for calculations [9, 26,

66-70j

and references

tuerein):

a*

= x 10~17

cm2,

r = 2 x

10~3s~~,

an = ap = aDn " aAp = 1x10~1°

cm2,

_cn

= cp = cDn " cAp "

1x10~S cm3

s~~,

cDp " cAn " 1 x 10~~ cm3 s~~. Tue defect concentrations bave been cuosen _as given m

Table

I,

to fulfill tue condition

[EL2j

> NA >

ND,

wuicu is tue _case for SI GaAs. Maximal values of Hall

mobility

observed

experimentally

_m botu states

(/tn

= 12 _x 103

cm2/(V s),

~~ = -2 _x 103

cm/(V s))

bave been used.

Tue calculated

puotoquencuing

transients of

puotoconductivity,

Hall

mobility,

free _carrier concentrations

in, p)

and electron

population

_m _acceptors

(Ni

and donors

IN[

_are _presented

m

Figure

7. Curve 1 in ail

figures

demonstrates tue best comcidence _witu

our experimental

(12)

~~ ^-~

3

- _~

i 0

£

2

u 5

1 o ^~6

c©

b ~

l0 ^~~

Q ~

10 ^~° ^~

0 10 20 30 40

t

js)

12500

4 10000

~

Î~ 7500

1

7

~~

⁵⁰⁰⁰

~ 5

~ 2500 ₂

1

o ~ b

-2500

0 10 20 30 40

t

(s)

10 ^~~

_

~~ ⁹

~

io ^° 4

~

~ io ^~ ^~

~ io ^~

10 ^~ ^~

~ l

10 ^~

0 10 20 30 40

t

lS)

Fig. 7. Calculated photoquenching ^transients ^of photoconductivity a), Hall mobility b), _nonequi-

librium free electron c) and hole concentrations d), and electron population _m donors e) and acceptors f) at different defect concentrations (see Tab.

I).

The numbers of _curves correspond to that indicated

in Table I. Curve 4 from Figure 7c _is presented also _m Figure 7d

as a dashed _curve 8.

(13)

10 ^~~

, 3

, 2

10 ^" , ,

É~ ₅

'

1~9

, _' 8

'

O

°~

d ~

io 0 ¹⁰ ~

20

t lS)

6, 7

/5

~Î

~~ ~

ÎÙ

~

l ~

~

~$

e

~~ ^m

0 10 20 30 40

t

(s)

10 ^~~

10 ^~~ _ô

~7 iQ ^le

~~ ⁱⁱ ^~

10

~~

~

3

~~ ^s

0

t ^(s)

(14)

data. It _can be _seen, that trie

compensation

ratio determines tue

properties

^of ^tue

photo- quencuing

_process, ^and ^dilferent ^defect concentrations _can _cause trie

diversity

of

photoquench- ing

results observed in dilferent

samples.

A

temporal change

^of n to p ratio determines tue

behavior of PC and _/tH with _or without

conductivity

_type conversion.

Initially,

after trie

light

is tumed _on and EL2 is still _m trie normal state, trie

EL2,

donors and acceptons are

considerably

filled

by

electrons

(>

50% at t ₌

o).

Since

(EL2]

> NA _>

ND,

tue

photoconductivity

and trie

population

of traps are determined

mainly by

electrons.

Tuerefore tue values of _a, _n, _/tH,,

Ni, Ni

and

(EL2°]

_are

proportional

to tue concentrations of EL2 and otuer donors and

anti-proportional

to _an acceptor concentration. In tue final stage EL2* becomes

totally

filled witu electrons and inactive. Tuerefore tue

compensation

of otuer donors and acceptors is dominant. Tue concentration of acceptons

usually

exceeds tuat of donors, _so _a, _p, _/tH are

proportional

to NA and

anti-proportional

to ND- Donors

are

emptied

from electrons

(tueir occupation

is less tuan _a few _pet

cent)

and tue occupancy of acceptors ^is

equal

to tue total donor concentration. I.e., if NA »

ND,

acceptors

fully

compensate ^donors:

Ni

₌ ND- Tuis coincides witu _ouf

experiments

and

publisued

data _on LVM

(23, 24j, absorption

and

puoto-EPR (la,25].

On tue contrary, ^tue case of NA > ND

(curves

4, 6 and 7 in

Fig. 7)

bas revealed _an

interesting

situation: tue time transients of free uole concentration and electron

population

in acceptons

Ni

bave extremes. After tue _p maximum is

passed,

free carrier concentrations become

equal

_p m n

(curves

4 and 8

respectively, Fig. 7d)

and

N(

_m

N(.

In tuis _case /tH is

only

^reduced ^witu no

sign

inversion and PC does not exhibit enhancement. Note that

finally

donors and acceptons are not

necessarily totally filled,

tuis

being important

^for ^tue êvaluation ôf trap parameters ^from ^TSC ^results. În ^tue

case of enuancement of PC in _a p-type state, a

nonequilibrium

free uole concentration is mucu iower tuan tue defect concentration, tuus

explaining

tue

semiinsulating properties

^of ^GaAs in a

p-type state. Tue model also

explains

tue

possible

temperature

dependence

of

puotoquencuing

cuaracter.

Indeed,

at lower temperatures ^suallower traps ^take part in

charge

redistribution tuus

modifying quencuing

transients. Tuese issues _are consistent witu results _on

stoicuiometry

related TSC and

puotoquencuing

expenments. It was obtained

(8,47,60j,

tuat _an n-type beuavior _was

pronounced

in

As-ricu,

wuile _a p-type one in Ga-ricu

samples.

Tue autuors

assume, tuat most of TSC

peaks

_are defect and

probably

impunty

complexes involving

AsGa

and

tuey

_appear to be "EL2-like" _or "EL2*-like".

Trie

quencuing

transients follow _various laws:

non-exponential

_or

exponential

witu _more tuan

one time constant. Several decay constants bave been observed in _ouf PC

experiments

as well

as in

photocapacitance quenching experiments (28, 29j.

^In ^trie ^last ^works ^dilferent

exponential

decays were also ascribed to trie dilferent metastable defects

belonging

to trie "EL2

family".

There _are _more discussions in trie hterature _on trie "EL2

family" conception. Usually

ail defects which _are alfected

by

1.o 1.3 eV

quenching photons

_are ascribed to "EL2

family".

But it is

doubtful,

that _so many

deep

and shallow, intrinsic and extrinsic

le-

_g.

C, Zn,

Fe and OAS defects

can

belong

to "EL2

family".

Furthermore, as it _was mentioned earlier, trie p-type ^TSC

peaks

Table I. The values

of

concentrations

of EL2,

acceptons ^and ^donors ^used ⁱⁿ caicuiations

(m cm~3).

Concentration ^Curve ^N°

1 2 3 4 5 6 7

[E12j ^10~~ ⁵ ^x ^10~~ ^10~~ ^10~~ ^10~~ 10~° 10~°

NA 5 x lo~s ₅

x 10~5 lol~ lol~ ₅ _x 1015 5 _x 1015 ₅ _x 10~5

ND 10~~ 10~~ 10~~ 10~~ 10~5 5 _x 10~5 4.9 _x 10~5