Journal of Non-Crystalline Solids 137&138 (1991) 543-546 North-Holland

JOURNAL OF

NON-CRYSTALLINE SOLIDS

THE MECHANISM OF SUBNANOSECOND CARRIER RECOMBINATION IN a-Sl:H

R. VANDERHAGHENol, A. MOURCHID(II(z) ,D. HULIN{Z),D. A. YOUNGer, W. L. NIGHAN Jro), P.M. FAUCHET(~)

1 Laboratoire de Physique des Interfaces et Couches Minces (UPRA 258 du CNRS) Ecole Polytechnique, 91128 PALAISEAU Cedex FRANCE

2 Laboratoire d'Optique Appliqu~e (URA 1046 du CNRS) ENSTA-Ecole Polytechnique, 91120 PALAISEAU FRANCE

3 Laboratory for Laser Energetics, University of Rochester, ROCHESTER N.Y.14623 USA 4 Department of Electrical Engineering Princeton University, PRINCETON N.J.08544 USA

We report on the non-radiative picosecond recombination of photogenerated ( free or trapped) carriers in a- Si:H and alloys. The mechanism is bimolecular multiphonon, with a limited number of recombining states. The recombination efficiency of tail states is discussed.

1. INTRODUCTION

This paper reports our investigation of ultrafast

trapping and recombination of carriers in a-Si:H

and alloys. We use the techniques of femtosecond

pump-probe spectroscopy 1. An ultrashort laser

pump pulse, at 2 eV (or 1.4 eV), photoexcites a

density of free (or trapped) carriers N. A probe pulse

tests the resulting photoinduced change in the

optical properties at various time delays.

2. EXPERIMENTAL ARRANGEMENT

The ultrashort optical pulses are generated from

a colliding pulse mode-locked dye laser and

amplifier system, producing pulses of tunable

wavelength, with less than 100 fs in duration and at

a repetition rate of 20 Hz. The pump beam, at 0.62 #m (2 eV), is focused to a diameter of 1 mm. Free

carriers are injected above the mobility edge of the

sample (the measured gap for a-Si:H is 1.7 eV on a

Tauc plot), with a carrier density ranging from more than 1019 tO less than 1021 cm -3. The technique of

self phase modulation is used to generate a white

light continuum. A fraction of this beam is selected in the infrared spectrum at 0.88 #m, amplified in a

three stages IR dye amplifier, and can be also used

as a pump 2 to inject trapped carriers. The other

fraction of the continuum is selected with

interference filters and used as a probe beam.

The undoped a-Si:H and the alloys with C or Ge

are deposited on silica and their thickness is less

than one absorption length at 2 eV. They have been

prepared by rf glow discharge, with a deposition

rate of 1 ,&,/s and a substrate temperature of 230 °C.

We measure the absolute change in reflection

and transmission of the probe pulse as a function of

time delay with respect to the pump pulse. For each

delay point, the complex index of refraction, n + ik,

is calculated using the "thin absorbing film"

equations. This directly provides the induced

change in real refractive index (An) and the photoinduced absorption (Acz = 4~Ak/X). These

quantities are measured at least at two probe wavelengths, 1 lam where Ak is only sensitive to the

carriers, and 0.52 #m where there is a

superposition of carrier effect and lattice heating 3.

3. CARRIER DYNAMICS

We have shown previously that both electronic

and thermal effects induce transient changes in the

real and imaginary parts of the index. The

electronic contribution results from intraband absorption with an effective cross-section o. The

short wavelength probes are sensitive also to

thermal effects through the increase of interband

absorption which follows thermal band gap

0022-3093/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved.

546 R. Vanderhagen et aL /Mechanism of subnanosecond carrier recombination ih a-Si:H

density, where this mechanism is the most efficient,

the mean excess energy of the remaining carriers

should be around 1 eV. Taking into account the

measured 5 1 eV/ps thermalization time, there

should be a 1 ps delay between recombination and

lattice heating, which is incompatible with the 100 fs

upper limit 2 for the delay between recombination

and complete lattice heating. We thus attribute this

bimolecular recombination to a mutiphonon

mechanism. The recombination efficiency is the

same for free carriers and for trapped carriers at

least 100 meV below the band edge, but it is

reduced for carriers in deep traps : at very low carrier density, we observe a residual Ak which

depends on the sample decreases very slowly and

cannot be attributed to a thermal effect. The saturation value of '~eff measured at very

high N excludes a recombination process between

hot carriers. At least one carrier needs to be cold (in

a limited number of states near the band edge) or

trapped at the top of the band tail.

When the carriers are directly generated in the

band tail within 100 meV of the extended states

(with the IR pump in a-Si:H or the 2 eV pump in a-

Si:C:H), the recombination can be fitted with the

same parameters as when the carriers are in the

extended states in a-Si:H. Therefore, carriers in

these traps, are as efficient for recombination as

when they are in the extended states

The nearly constant 'teff(Ak) at low N could be

attributed either to ~ or to recombination. However,it

is observed as well at t = 0 + and at longer time.; moreover, Ak decreases roughly exponentially over

one order of magnitude. We then attribute it to

bigger recombination efficiency of a limited number

of tail states.

7. CONCLUSIONS

We have observed, for a-Si:H and its alloys, a

bimolecular multiphonon recombination of carriers

in the extended states or in the band tail states. This

recombination involves one cold or trapped carrier.

Carriers trapped at the top of the band tai l , at least

down to 100 meV below the edge, exhibit the same

recombination efficiency. Some from these tail traps

are more efficient for recombination, and deeper

traps are less efficient. Finally, the observed

recombination cannot be an Auger type.

ACKNOWLEDGEMENTS

The work at Rochester is supported by ONR

(N00014-91-J-1139) and NSF (ECS-9196000).

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