Pergamon Solid State Communications, Vol. 91. No. 8, 631-634. pp. 1994

Elsevier Science Ltd Printed in Great Britain

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OPTICAL ACTIVE PROCESS OF HIGHER ORDER BANDS IN FAST NEUTRON IRRADIATED SILICON

Y. Shi, FM. Wu and Y.D. Zheng

Department of Physics, Nanjing University, Nanjing 210008, P.R. China

and

M. Suezawa, M. Imai* and K. Sumino

Institute for Materials Research, Tohoku University, Sendai 980, Japan

(Received 22 February 1994 by T. Tsuzuki )

Using Fourier transform infrared absorption measurement with excitation at low temperatures, we have investigated the optical active process involving higher order bands (HOB) in fast neutron irradiated float-zone silicon. It is found experimentally that the photoexcitation and decay process of the HOB as well as the saturation value of the absorption intensity are in good agreement with the predictions of the macroscopic barrier model for the slow relaxation of photoexcited carriers caused mainly by defect clusters.

Keywords: Radiation defects, infrared absorption, silicon.

IN RECENT YEARS, there has been a considerable amount of experimental and theoretical effort towards the understanding of defect properties under photo-excitation. These include EL2, DX centers in compound semiconductors [l, 21, and C,C; pairs [3, 41 and higher order bands (HOB) [4- 8] in silicon. Here, our study is emphasized on the HOB and its optical active process regarding to the photoexcitation and decay in fast neutron irradiated float-zone (Fz) silicon. The HOB consists of more than 40 sharp absorption bands identified as electronic transitions in the wavenumber range of 600-1400cm-‘, which appears only in the irradiated silicon with a high fluence of fast neutrons or other high energy particles followed by annealing at 400- 600°C. In a fast neutron irradiated silicon the Fermi level is pinned usually near the middle of the band- gap due to heavy radiation damage. To observe the HOB, therefore, a band-edge light illumination leading the shift of the quasi-Fermi level is required [5]. An interesting observation for the photo- excitation and decay of the absorption intensity of the HOB was reported by Corelli [5] based upon an

* Permanent address: Komatsu Electronic Metal Co. Ltd., Kanagawa 254, Japan.

examination of the residual absorption and the depletion in a neutron irradiated Fz-silicon. This led Corelli to suggest the defect associated with the HOB has three charge states within the band-gap, where the intermediate state is strong trap center and the final state gives rise to the HOB. Recently, the measurements have shown that the photoexcitation and decay of the absorption intensity of the HOB in a neutron irradiated Fz-silicon follow exponential and logarithmic time dependence, respectively, which was proposed to be related to a slow relaxation of photoexcited carriers [7, 81. This optical active process, apparently, is very important for the under- standing of the HOB. On the one side, the slow relaxation of photoexcited carriers has been investi- gated in various semiconductors. Several models have been proposed, as mentioned in the review of Queisser [9]. The phenomena for the slow relaxation of photoexcited carriers in neutron irradiated silicon have been observed with persistent photoconductivity measurement [lo, 111. However, no detailed investi- gation on this system at low temperatures has been performed so far. In the present work, we investigate this interesting optical active process of the HOB in fast neutron irradiated Fz-silicon at low temperatures under excitation-light illumination. The process of

631

632 HIGHER ORDER BANDS IN NEUTRON IRRADIATED SILICON Vol. 91, No. 8

the photoexcitation and decay is measured system- atically using a Fourier transform infrared (FTIR) Fz-Si I IOl3

spectrometer with a variety of the illuminating-light intensity. Analytic expressions on the basis of the

.T ‘E s

4

macroscopic barrier model give a satisfying inter- pretation for the observations. x3 .t:

An initial high resistivity (> 20000 cm) n-type Fz- 2

silicon single crystal was used in this experiment. 2 s 2

After the irradiation with a high fluence of fast _--------

neutrons (8 x 10’8cm-2, En > 1.0 MeV) at room 1 -----------

temperature in a light-water reactor, it was cut and 0 polished to approx. 1.5 mm thickness. The annealing 0

was done in vacuum condition at 450°C for a variety of annealing time from 10min to 100 h. Infrared Fig. 2. Time dependence of the absorption intensity absorption measurements were performed at 7 K at 1102 cm-’ during photoexcitation (left-hand side) using a JEOL JIR-100 FTIR spectrometer. The light and decay (right-hand side) at 7K in the sample

source of the spectrometer was filtered with a annealed for 40 h. Solid lines in the left are fitting

germanium wafer during all measurements. Samples curves with exponential function and that in the right

were cooled in darkness, and were measured without with logarithmic function.

or with an additional excitation-light obtained from a tungsten lamp through a monochrometer.

Figure 1 shows the infrared absorption spectra of the HOB in the sample annealed for 40 h as measured at 7 K. The absorption bands of the HOB cannot be detected before illumination (Fig. 1 (spectrum A)). The band 0; at 1136cm-’ means the absorption of interstitial oxygen atoms. More than 40 sharp absorption bands are clearly visible under a band- edge light illuminating (Fig. 1 (spectrum B)). Due to heavy radiation damage, the Fermi level is pinned near the middle of the band-gap in the present samples even after annealing at 450°C for 1OOh [8]. The defect level giving rise to the HOB has been proposed to be located slightly below EC-O.1 5 eV [7]. Since the absorption bands of the HOB are identified

as due to the electronic transitions, the change of the absorption intensity is caused from the transformation of the corresponding defect states. Before illumination, therefore, the occupation number of the defect state giving rise to the HOB is too low to be observed. Under a band-edge light illuminating, the generation of photoexcited carriers leads the shift of the quasi-Fermi level to the conduction band. As a result, the occupation number increases, and then the absorption becomes detectable.

__-...^I, .. J;L--_~ = A

lbSi x 2

1 1800 1600 1400 1200 1000 800 6ol

Wavenumber (cm-l)

Fig. 1. Infrared absorption spectra at 7 K in the sample annealed for 40 h. Spectrum A: measured without illumination, and spectrum B: measured with band-edge light illumination.

In Fig. 2, we present the time dependence of the photoexcitation (left hand side) and the decay (right hand side) of the HOB at 7 K in the sample annealed for 40 h. The ratio of the illuminating-light intensities for curves 1 to 3 is 1 : 0.2 : 0.06, for curve 4 the intensity is the same as that of curve 2, but the illuminating time is only 400s. It is found experi- mentally that the relative change rates of the absorption intensities of the prominent bands keep almost constants during the photoexcitation and decay. Thus, the band at 1102 cm-’ with the highest peak is used as a representation here. In the processes of the photoexcitation and the decay, the variations of the absorption intensity I can be quite well described with an exponential and a logarithmic function of time, respectively,

Z(t) = Z0[1 - exdtldl, (1) and

Z(t) = A0 ln[(~ + t0)/~dl, (2)

where 7, and rd are the time constants for photoexcitation and decay, respectively, ZO is the saturation value, to and A are parameters relating to the defect structures in a sample and the illuminating- light intensity [7, 81. The value rd estimated from the

Vol. 91, No. 8 HIGHER ORDER BANDS IN NEUTRON IRRADIATED SILICON 633

observed above, was proposed to be related to slow relaxation of photoexcited carriers caused by fast neutron-induced defect clusters at low temperatures. Investigations on the defect structures in neutron irradiated silicon have been carried out. The defects, such as dislocation loops, voids, rod-like defects, have been observed by electron microscopy in neutron irradiated silicon annealed at temperatures between 400 N 500°C [ 12, 131. In the present samples, the cluster-like defects have been detected with electron-dipole spin resonance method [ 141. These defects are able to play “macroscopic potential

Time (s) barrier” role and result in the slow relaxation of

Fig. 3. Plot of absorption intensity I decay vs photoexcited carriers at low temperatures [9]. - ln[(t + tO)/~d] in the samples annealed for 40 h On the basis of macroscopic barrier model [8, 11, (0) and 100 h (A). 1% 171, considering a quasi-equilibrium condition

where electron-hole pairs are homogeneously generated in the matrix at the rate G, the carrier recombination rate R, for electrons through defect clusters with the potential barrier height Vd is expressed by

present data is about lo5 s, thus, the residual absorption intensity remains at some level, practi- cally ,unchanging during our measurement. Further- more, if the illumination-induced absorption intensity is lower than the residual absorption, there is no obvious decay process (curve 4). In Fig. 3, we plot the absorption intensity decay according to equation (2), I vs - In[(t + r,-,)/rd], w h ere the experimental data are obtained from the samples annealed for 40 and 100 h. It also is interesting to note that the coefficient A and decay time constant 7d are dependent on the annealing condition. The dependence of the satura- tion value I0 on the illumination-light intensity is also measured. It is observed that the I,, increases with illumination proportionally to the logarithm of illuminating-light intensity in the range of low light intensity, as shown in Fig. 4. Moreover, similar observations are obtained in various annealed samples.

& = NNd[C,n(l -f)exp(-qVd/W - WI, (3) where N and ZVd are the density of defect clusters and the number of trap centers in a defect cluster, f is the occupation probability of the trap located at negatively charged state, C,, and E,, are the electron capture and emission rates, n is the electron concentration in the matrix, and q is the electron charge. Correspondingly, the parameters in equi- librium are denoted with the subscript symbol 0. At the steady state R, = G, the saturation value of the excess electron concentration An(oo) for /?V,n >> 1 is given by [8, 161

An(m)=~~[lnG+lnC~~~~“,,1, (4) ” 0 d 0 The optical active process of the HOB, as

10 :’ lo- = lo- 1 100 Light Intensity (au.)

Fig. 4. Dependence of the saturation value of the absorption intensity on the illuminating-light intensity at 7 K in the sample annealed for 40 h (0) and 100 h (A).

where p = q/kT and q = (fs -f)/fo. It should be noticed that the saturation value of the excess electron concentration An(oo) is dependent on the logarithm of the generation rate G as well as defect structure parameters in a sample.

Now let us assume that the illumination is switched off at the moment t = 0, and then the excess carriers decay in time from a steady state. Hence, the time-dependence of An can be described

by

An(t) =feln(I+“), Td

with

(5)

634 HIGHER ORDER BANDS IN NEUTRON IRRADIATED SILICON Vol. 91, No. 8

and

_i&Nd 1 -- lo = PI&, G’

where to is a parameter that is reversely proportional to the light intensity, the rd is the decay time constant. By introducing a new time variable 2’ = t + to into equation (5) it becomes a universal description of decay which is independent on the light intensity. The analytic expressions presented here indicate an essentially logarithmic time delay of photoexcited carriers. This is the most important feature for the macroscopic potential barrier model distinguishing from other models. Furthermore, there are the same factorfoNNd/PVdO in equations (4) and (5), which is the parameter of defect structures. For the photo- excitation process, not discussed here, the excess electron concentration An can be described by an essentially exponential type time dependence. It is important to note that the observations for the optical active process of the HOB are in good agreement with the predictions of the macroscropic barrier model.

The absorption bands of the HOB have been identified as due to electronic transitions, and the defect level giving rise to the HOB is proposed to be located slightly below E, - 0.15 eV in the band-gap. Within the considered range, namely, the quasi-Fermi level is located between the defect level and the Fermi level in the present samples, the occupation number of the defect state can be assumed to be proportional to the excess electron concentration An. Con- sequently, the variation of the absorption intensity I reflects that of excess electron concentration An. Indeed, our experimental situation satisfies these conditions. Another important evidence is obtained from the fast neutron irradiated Czochralski silicon, where the similarities for the photoexcitation and decay of the absorption intensities between the HOB and the thermal donor single ionized state TD+ have been observed [7]. This verifies further that the behavior of the optical active process of the HOB is related to the relaxation of photoexcited carriers, and the relaxation time of the defect state giving rise to the HOB is less than that of photoexcited carriers. The slow relaxation phenomena of defect states after photo-excitation have been investigated intensively in silicon and compound semiconductors involving usually in their metastable properties. So far, there has been no model concerning the metastable that could be used to interpret the present observations, in special the decay process. From our point of view, whether a theoretical model can thoroughly explain the observations greatly depends on whether or not it can successfully formulate the decay behavior.

From the present investigation on the optical active process of the higher order bands in the fast neutron irradiated Fz-silicon, it is found that the photoexcitation process follows an exponential type time dependence and the decay follows a logarithmic type, and the saturation value of the absorption intensity depends on the logarithm of the illumination-light intensity. Analytic expressions for the relaxation process of photoexcited carriers are derived with the macroscopic barrier model, which are in good agreement with the observations. The optical active process of the HOB is related to a slow relaxation of photoexcited carriers, which is caused mainly from the presence of defect clusters.

Acknowledgements - The authors would like to thank Drs T. Sekiguchi and I. Yonenaga for valuable discussions and help.

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