Luminescence by IR-stimulation of metastable carriers in a-Si:H : Experiment and Monte-Carlo simulation

JOURNA L OF

Journal of NonCrystalline Solids 137&138 (1991) 587-590 NorthHolland

NON-CRYS LLINESOLIDS

LUMINESCENCE BY IR-STIMULATION OF METASTABLE CARRIERS IN a-Si:H EXPERIMENT AND MONTE-CARLO SIMULATION Hans-Peter VOLLMAR and Roland BINDEMANN Institut ffir Angewandte Photophysik, Technische Universit~t Dresden, Mommsenstrage 13, 0-8027 Dresden, FRG The time behaviour of the IR-stimulated luminescence transient in a-Si:H has been investigated in detail. The relaxation and recombination processes induced by the IR-excitation of metastable carriers are successfully described by a Monte-Carlo simulation. By comparison of the calculated luminescence transients with experimental findings the values of the absorption cross sections for electrons and holes in tail states have been determined. section for e and h in tail states have been determined.

1. INTRODUCTION The relaxation and recombination processes of photoexcited carriers depend strongly on the properties of lo-

2. EXPERIMENTS AND RESULTS

in the mobility gap of a-Si:H. These

For the measurements undoped a-Si:H (deposited by

properties have been carefully studied by different meth-

glow discharge on roughened substrates) with low defect

ods including experiments on IR-stimulated photo-

density (ndb=7 - 1015 cm -3) was used. The basic excita-

calized states

conductivity and photoluminescence and IR-quenched

tion of the sample placed in a He-gas-flow-cryostat

LESR 1. These effects observable at low temperatures are

was realized by a Kr-laser line (E= 1.83 eV) and the IR-

based on the excitation of electrons (e) and holes (h) into

excitation by a tungsten halogen lamp (150 W) with a

extended states from deep tail states where they were metastably trapped. In the case of photoluminescence

Ge-Filter (EIR<0.65 eV) and a LiF-prism monochromator. The integral intensity of the PL-band at 1.4 eV

stimulated by IR-irradiation after the end of basic ex-

was detected by a LN2-cooled S l-photomultiplier. The

citation the excited carriers relax by a repeated tunnelling

on/off switching of the basic- and IR-excitation as well as

within the tail states. During this tunnelling relaxation

the time-resolved data logging were computer controlled

also a non-radiative (NRR) and a radiative (RR) recom-

with a sampling time of 0.56 ms. The density of the metastable population nmp at the

bination is possible resulting in the appearance of a characteristic luminescence transient (LT). Due to the lack of the basic excitation the time behaviour of this LT sensitively and directly reflects the kinetics of the NRR- and RR- processes. The later process is exclusively caused by distant-pair recombination because

the metastable car-

tiers are trapped spatially at random. In the present paper we study in detail the time behaviour of the LT in dependence on the IR-power PIR,

the

delay time t d (time between the end of basic excitation and the beginning of IR-stimulation) and the defect density ndb. For the calculation of the LT the relaxation and

1.0

•

"%*%

i0 K

Z2 r(b

"-i % ~..

.'"'''2

E ~I0 . 5

~

<. \,., ~"-.~

EL

5 s

~dl = Zd2 =

10 s

~d3 =

30

Ed5 =

s

3600

s

.. .........

(b rr

o o o ° ° ~ 0.0 ....

I

I0 0

. . . . . . . .

i

Time

recombination processes connected with the IR-excita-

i

i

]01

i

rlllll

r

~

I

10 2 [ms]

tion are successfully described by a Monte-Carlo simulation (MCS)

for zero-temperature. By comparison with

FIGURE 1 The LT in dependence on t d at fixed PIR = 85 mWcm -2.

experimental findings the values of the absorption cross 0022-3093/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved.

588

H.-P. Vollmar, R. Bindeman /Luminescence by 1R-stimulation of metastable carriers in a-Si:H

beginning of the IR-excitation strongly depends on the

requieres more time and the LT shifts to longer times (cf.

delay time td. The resulting shift of the LT at rising td

fig.2).

and the increasing half-width are shown in figure 1. The

The time behaviour of the LT is also strongly influ-

time integral of the LT is a measure for the density of

enced by the defect density ndb (cf. fig. 3). With in-

radiatively recombining metastable carriers and can be

creasing ndb the probability for NRR increases what re-

exactly reproduced by a power law - td-m. The exponent

sults in a shift of the LT to shorter times and a nar-

m shows a clear temperature-dependence

rowing of its half-width.

(m(10 K) = 0.40; m(40 K) = 0.29; m(70 K) = 0.27).

1.0

the IR-photon energy EIR at fixed IR-photon flux ~ I R and on the temperature T will be published in a forth-

"".-',4X 2.."

78 K

2),

C ~b

@

E ~o.5

."

• °• •

PIR [rowcm-2] Pl = 105

"k~

"" "

"

I

"~

: 3

~

4 .~

• °°°°

The results concerning the dependence of the LT on

coming paper.

P2 = 75 P3 = 44

3. THE MONTE-CARLO SIMULATION

P4 =

23

the following processes induced by the IR-excitation : (i)

P5 =

10

The calculation of the LT requieres the simulation of tunnelling relaxation process (TRP), (ii) NRR- and RRprocesses, (iii) repeated excitation of relaxing carriers by

EE

0.0

....

I

. . . . . . . .

I00'

I

. . . . . . . .

101

I

102

Time

IR. Furthermore, in the simulation all changes of the

I

. . . . . . . .

density of the metastable carriers during the stimulation

193

process by NRR and RR have to be taken into account

[ms]

because they strongly influence the recombination rates. FIGURE 2 The LT in dependence on PIR at fixed td=30 s.

A detailed description of the simulation will be given in ref.2. In the present paper we give only the basic ideas of the simulation. The DOS model used and the as-

1.0

I;..-'~X

sumptions for the TRP are in accordance with that of the

f

78 K

.o*~°

C

MCS in ref.3. The initial situation of the simulation is characterized

•

by the randomly distributed metastable carriers trapped in I

:

deep tail states. The IR-photons excite the e (h) with the

CZ

absorption cross section O e (Oh) into extended states. After their subsequent fast thermalization5 (t< 10-12 s)

m cY

0.[3

iIill

i0 0

I

I

IIItl

'1

. . . . . . . .

I0 ]

.I

and capture by localized states near the mobility edge the . . . . . . . .

102 Time

I

,

,

, ,

103

rims]

FIGURE 3 The LT in dependence on the defect density. (2) is the LT of a sample with high defect density (realized by electron bombardment , 90 keV) and (1) the LT of the same sample annealed (low defect density).The LTs are normalized.

carriers start the TRP to unoccupied states with lower energy. Because of both the low density of metastable eartiers (1016...1017 cm -3) and their low absorption cross sections the IR-excited carriers can be treated as isolated, i.e. a mutual influence of the carriers during the TRP is negligible. Therefore, each carrier executes its seperate TRP

With decreasing PIR less carriers can be excited per time unit. Therefore, the whole IR-excitation process

by realization of

that tunnelling step with the

highest tunnelling transition rate p(R) =woex p{-2R/R o(E)}.

(1)

H.-P. Vollmar, R. Bindeman / Luminescence by IR-stimulation of metastable carriers in a-SkH

589

If this rate to the nearest unoccupied localized state is lo-

strongly depends on the used DOS-model, the capture

wer then that to the nearest defect state or to the nearest

mechanism of the defects, the density of metastable car-

carrier with opposite sign the concerned carder recom-

riers at the beginning of IR-irradiation, the absorption

bines non-radiatively or radiatively, respectively.

cross sections (I e and O-h and on the IR-power PIR.

For the capture of an e by a neutral dangling bond (D °) we assumed an immediately capture4 of a h due to the high capture cross section of the charged defect, i.e. e + D o - - - > D-

and D- + h - - - > D ° .

(2)

4. DISCUSSION For the DOS-model we used typical parameters of samples with low defect densities (cf. table 1 in ref.3) and for the prefactors of the tunnelling rates the values

Analogously we assumed (3)

wonr=1013 s-1 and wor=108 s-1 for the non-radiative

With increasing number of hopping steps of the con-

and radiative transitions5, respectively. For the localiza-

sidered carrier the distance to the next unoccupied lo-

tion lenghts of the relaxing carriers a E -1/2 dependence6

calized state and consequently its lifetime rises on the

on the energy separation from the nearest mobility edge

average. Due to the low carrier density the average dis-

was taken

tance to the nearest possible partner for recombination is

Roh(Evo)=7/~. The values of the absorption cross sec-

also large. For this reason the instantaneous lifetime of

tions IJ e and O h are unknown and have to be deter-

the carrier may increase to such a value that it has to be

mined by comparison of the calculations with the experi-

h + D o - - - > D + and D + + e - - - >

D° .

into account with Roe(Eco)=10 /~ and

high

mental results. The density of the metastable population

probability of renewed IR-excitation. In order to describe

is approximately known from LESR measurements1. For

this situation we defined a so called maximal stay-time

excluding a possible dependence of the absorption cross

considered

as

repeatedly

metastable with

a

tv(PiR , O) which is calculated in parallel for each tun-

sections

nelling step. If the actual tunnelling time tm of a transi-

monochromatic 1R-irradiation (EIR=0.62 eV). All meas-

on

the

IR-photon energy EIR

we

used

tion is smaller than tv the calculated hop and the radiative

urements used for the comparison with MCS were per-

or the non-radiative tunnelling transition takes place,

formed at 10 K in order to minimize the influence of

respectively. Otherwise the relaxing carrier will be re-

temperature.

peatedly excited and its renewed TRP starts. The actual

At first we determined the absorption cross sections

tunnelling time tm defines the moment for the transition

O e and O h by an appropriate variation of the values for

which

Oe,

is

calculated

by

assuming

an

exponential

Oh, and nmp(td) at fixed PIR and by comparison

dependence of the transition probability on time, i.e.

of the therewith calculated LTs with experimental ones

tm=-T(R), in(l-Z) where Z is a random number with

measured at different td. After that we calculated with

0 < Z < 1. The lifetime T(R) is the reciprocal value of

the determined values for (I e and O h the LTs as a

the tunnelling rate (1).

function of PIR at fixed

nmp(td) and compared them

The TRP are calculated in parallel both for all e and h

with the corresponding measured LTs. A very good

excited by IR per time interval. With increasing time the

agreement between the experimental findings and the

density of metastable carriers decreases because of the

calculations could be obtained under the assumption that

RR- and NRR- processes. The time of the possible RR of

the IR-irradiation results mainly in an excitation of meta-

a carrier results from the time of its IR-excitation, the

stable

holes.

The

determined

value

amounts

to

sum of the tunnelling times t m of all hops between the

Oh=(3.0...6.0)d0-16 cm2. Small values of O h within

tail states and the tunnelling time of the final radiative

this interval require a larger density nmp for the same td

transition. As result we get the density of radiatively re-

in the experiment. For the value (Jh=4.6 • 10-16 cm2

combining carriers per time unit in dependence on time.

we found for the delay times tdl = 5 s and td2 = 100 s

It is evident that the time behaviour of the calculated LT

nmp(tdl)=5.0 1017 cm-3 and nmp(td2)=7.5 • 1016 cm 3,

590

H.-P. Vollmar, R. Bindeman /Luminescence by IR-stimulation of metastable carriers in a-Si:H

respectively (cf. fig.4). It is noteworthy that as well for

mechanism for carrier capturing by defects the time be-

increasing td as for decreasing PIR both the retarded rise

haviour of the experimental LT could not be described by

of the LT and its shift on the time scale can be correctly

the MCS. The influence of the defect density on the time

reproduced by the MCS. For the metastable electrons we

behaviour of the LT can also be calculated by the MCS.

found O-e=(0.1...0.5 ) - O h. A further reduction of O e

Detailed investigations are in preparation.

causes only a negligible change of the time behaviour of the LT. However, a slight increase of

O e (i.e.

O-e=(0.5...1.0 ) • Oh) results in a faster rise of the LT

5. SUMMARY It could be demonstrated that the time behaviour of

because of the larger localization length and therefore the

the

shorter lifetimes of relaxing electrons.

tively the kinetics of the recombination processes. The

IR-stimulated luminescence transient reflects sensi-

luminescence transients calculated by Monte-Carlo simulations are in good agreement with the experimental

1.0 m~.

Z~

Tdl =

5

S

findings confirming the models for the DOS, the tunnelling relaxation and the radiative and non-radiative recombination processes used. The absorption cross section

E ~J

~0.5

of the metastably trapped h and e have been determined.

I _J

Owing to the unequivocal correlation between the time

~J

behaviour of the LT and the defect density it should be possible to elaborate a sensitive method for determining

EE

0.0

the dangling bond density basing of the mentioned meas1o 0

1o 1 Time [ms]

1o2

urements. ACKNOWLEDGEMENTS

FIGURE 4 Measured (circles,triangles) and calculated (solid lines) LTs for two differend delay times at fixed PIR, measured at 10 K. The LTs are normalized. The assumption that mainly holes are excited by IR-irradiation (EIR < 0.65 eV) is consistent with experimen-

We are indebted to Dr. H. Mell for providing samples. REFERENCES 1. R. Carius, W. Fuhs, AIP Conf. Proc. 120 (1984) 125

tal findings on photoinduced absorption7. From the simulation it is evident that also the time be-

2. H.-P. Vollmar, R. Bindemann, to be published

haviour of the LT is determined by the IR-excitated holes because of their larger absorption cross section. The rise of the LT is caused by the metastable holes firstly excited

3. O. Gutschker, R. Bindemann, Monte-Carlo simulations of Carrier Relaxation and Recombination in a-Si:H, this volume.

and their subsequent radiative recombination, whereas its long-time decay is mainly influenced by holes being repeatedly excited. Because of the decreasing density of carriers during the IR-irradiation the probability both for NRR and for repeated excitation increases. Therefore, the density of carriers excited per time unit and the probability of repeated excitation during the TRP are determined by the absorption cross sections ( O e , Oh) and PiR,, With other values for 0 e and O h or another

4. R.A. Street, D.K. Biegelsen, R.L. Weisfleld, Phys. Rev. 30B (1984) 5861. 5. R.A. Street, in Semiconductors and Semimetals 21B ed. by J. Pankove, Academic Press, (1984) 197. 6. M. Stutzmann, LNon-Cryst.Solids 97/98 (1987) 105. 7. H.A. Stoddart, Z. Vardeny, J. Tauc, Phys. Rev. 38B (1988) 1362.