On temperature dependence of the optically active behavior of an infrared active defectin siliconYi Shi, Fengmei Wu, Youdou Zheng, Masashi Suezawa, Masato Imai, and Koji Sumino Citation: Applied Physics Letters 66, 1945 (1995); doi: 10.1063/1.113285 View online: http://dx.doi.org/10.1063/1.113285 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/66/15?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Temperature dependence of optical anisotropy of birefringent porous silicon Appl. Phys. Lett. 96, 243102 (2010); 10.1063/1.3453449 Characterization of optically active defects created by noble gas ion bombardment of silicon J. Appl. Phys. 83, 4075 (1998); 10.1063/1.367227 Temperature dependence of optical constants for amorphous silicon Appl. Phys. Lett. 60, 2186 (1992); 10.1063/1.107074 Temperature dependence of the infrared active translational modes of solid carbon disulphide J. Chem. Phys. 82, 1476 (1985); 10.1063/1.448422 Temperature dependence of the optical properties of silicon J. Appl. Phys. 50, 1491 (1979); 10.1063/1.326135

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On temperature dependence of the optically active behavior of an infraredactive defect in silicon

Yi Shi, Fengmei Wu, and Youdou ZhengDepartment of Physics and Institute of Solid State Physics, Nanjing University, Nanjing 210008, People’sRepublic of China

Masashi Suezawa, Masato Imai,a) and Koji SuminoInstitute for Materials Research, Tohoku University, Sendai 980, Japan

~Received 19 September 1994; accepted for publication 26 January 1995!

Optically active behaviors of the infrared active defect, so-called higher order bands, have beeninvestigated at different temperatures in fast neutron irradiated silicon. It is found that the opticallyactive decay follows logarithmic time dependence with a decay time of about 105 s, which is nearlytemperature independent below 80 K. The residual absorption remains up to heating temperatures of180 K. The experimental findings are discussed in terms of the relaxation characteristic ofphotoexcited carriers governed by neutron irradiation induced defect clusters. ©1995 AmericanInstitute of Physics.

Many efforts have been made to understand defects andtheir effects on the electrical and optical properties of semi-conductors, which are very important topics for both physicsand device applications. In many cases, however, the charac-teristics observed are very complex, involving both proper-ties of the defect itself and the material, which must be stud-ied carefully.1,2 In this letter, we will present the firsttemperature-dependent investigation with infrared absorptionon the optically active behavior of an important infrared ac-tive defect, so-called higher order bands~HOB!, and the re-laxation characteristic of photoexcited carriers at low tem-peratures in neutron irradiated silicon. The HOB consists ofmore than 40 absorption bands in the range of 600–1400cm21, appearing mainly in fast neutron irradiated silicon fol-lowed by annealing at 400–600 °C.3–7 There are many simi-lar points between the HOB and the thermal donors~TD!.8

An interesting optically active process of the HOB was firstobserved by Corelliet al. using the dual beam method.4 Re-cently, the property of such an optically active process hasbeen proposed to be related to slow relaxation of photoex-cited carriers, which results from the presence of fast neutronirradiation-induced defect clusters.7,8 Phenomena for slowrelaxation of photoexcited carriers have been observed withpersistent photoconductivity measurements at above liquid-nitrogen temperatures in neutron irradiated silicon.9,10 Nev-ertheless, the temperature dependence and the microscopicmechanism for the optically active behavior of the HOB andrelaxation characteristic of photoexcited carriers governed bythe defect clusters have been unclear. These involve not onlythe defect state giving rise to the HOB but also a variety ofother kinds of defect states in an irradiated silicon. Althougha number of investigations with various experimental tech-niques, such as Hall effect, deep-level transient spectroscopy~DLTS!, infrared absorption~IR!, electron paramagneticresonance~EPR!, and so on~see Ref. 11!, have been made todetermine defect states in neutron irradiated silicon, no clear

picture has been established yet. The present investigationprovides a chance to get a deeper insight into the propertiesof the HOB as well as of the defect clusters.

Samples studied in this work were prepared from an ini-tial high resistivity ~.2000 V cm! float-zone~FZ! siliconcrystal. The irradiation was performed with a high fluence offast neutrons~831018 cm22, neutron energyEn.1.0MeV! in a light-water reactor, where the Cd ratio was about20. After neutron irradiation, the ingot was cut to approxi-mate 1.5 mm thickness, and then polished to obtain opticalsurfaces. The samples were sealed within evacuated quartzcapsules and annealed at 450 °C for different annealing timefrom 1 to 100 h. Infrared absorption measurements weredone using a JEOL JIR-100 Fourier-transform infrared~FTIR! spectrometer. The temperature was controlled by anAir Products liquid-helium cryostat in the range from 6 to300 K. Except for where specially noted, the measurementsat different temperatures were taken in such a way that thesample was always heated up to a room temperature andallowed to relax to equilibrium after each measurement, thencooled down in darkness to the desired temperature. This isto ensure that the measurements obtained for each tempera-ture have the same initial condition. The light source of thespectrometer was filtered with a germanium wafer at roomtemperature for all measurements. An additional illuminatinglight, obtained from a tungsten lamp through a monochro-mator, was used for optical excitation.

The absorption bands of the HOB cannot be detectedbefore illumination. More than 40 sharp absorption bands ofthe HOB are clearly observated under a band-edge lightillumination.7 These absorption bands broaden slightly withincreasing temperature, and disappear above 90 K. No shiftfor the position of the prominent absorption bands is ob-served. It is found experimentally that the absorption coeffi-cients depend weakly on temperature in the temperature re-gion of 6 to 70 K under illumination. These behaviors aresimilar to those of the single ionized state of thermal donorTD1in neutron irradiated Czochralski silicon.8 Since the ab-sorption of the HOB is identified as electronic transition, the

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

1945Appl. Phys. Lett. 66 (15), 10 April 1995 0003-6951/95/66(15)/1945/3/$6.00 © 1995 American Institute of Physics

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change of an absorption coefficient reflects the transforma-tion of the corresponding defect state. The defect state givingrise to the HOB has been proposed to be located slightlybelow the TD~1/11! level.8 Due to heavy radiation dam-age, the Fermi level is pinned near the middle of the bandgap in the present samples even after annealing for 100 h.7

Before illumination, therefore, the occupancy of these defectstates is too low to be observed. Under illumination, thegeneration of photoexcited carriers leads to a shift of thequasi-Fermi level towards the conduction band. As a result,this occupancy increases, and then the absorption becomesdetectable.

The optically active process of the HOB has been mea-sured at different temperatures in detail. After illuminatinglight is turned on, the absorption begins to increase and ap-proaches a saturation value. As the illuminating light isswitched off, the absorption decays slowly to a residual valuerather than the initial value. In Fig. 1, we show the timedependence of the absorption coefficient at 1102 cm21 of thesample annealed for 40 h during the photoexcitation~left-hand side! and decay~right-hand side!. The temperature is 7K, and the photon energy of the illuminating light is 1.23 eV.The ratio of the illuminating-light intensities for curves 1 to3 is 1:0.2:0.06; for curve 4 the intensity is the same as that ofcurve 2, but the illuminating time is only 400 s. Here, theband at 1102 cm21 with the highest absorption peak is usedas a representative result. In the processes of the photoexci-tation and the decay, the variations of the absorption coeffi-cients can be quite well described with exponential and loga-rithmic functions of time, respectively.7 The decay timeconstanttd estimated from the present data is about 105 s.Thus the residual absorption remains, practically unchangedduring our measurement. The absorption decay curves forseveral representative temperatures are shown in Fig. 2. Mostimportantly, it can be clearly observed that the decay behav-iors are almost the same at the temperatures below 80 K, thatis, thetd is nearly temperature independent in this tempera-ture region.

Owing to the traps in a defect cluster, a large number ofmajority carriers are trapped, and the defect cluster is elec-

trically charged, then a spherical space charge of oppositepolarity is formed, creating a potential barrier. This potentialbarrier spatially dissociates photoexcited carrier pairs. At lowtemperatures, only a small fraction of the majority carrierswill have sufficient energy to overcome the barrier and re-combine. This, consequently, results in slow relaxation ofphotoexcited carriers.9,10 Introducing defect clusters is one ofthe main features of fast neutron irradiation.12,13 In general,the annealing-out temperature for these defect clusters ishigher than that for point defects.11 After the annealing at450 °C, most point defects have been annealed out. In thepresent samples, cluster defects have been detected.14 Thesedefect clusters are able to result in slow relaxation of photo-excited carriers at low temperatures. Within consideredrange, the occupancy of the defect state giving rise to theHOB is proportional to the excess electron concentrationgenerated by the illuminating in bulk. Consequently, the ab-sorption reflects the variation of the excess electron concen-tration. On the basis of the macroscopic potential barriermodel, we have successfully given a good description for thephotoexcitation and the decay observed in Fig. 1, where thedecay time constanttd is assumed to be proportional toexp~Vd0 /KT!.7 Here,Vd0 is the potential barrier height atequilibrium. It seems that thetd is thermally activated, i.e.,is very sensitive to temperature. In fact, thetd here is nearlyconstant in the temperature region below 80 K. It is conceiv-able that the relaxation of photoexcited carriers is also tem-perature independent in this temperature region. One pos-sible interpretation is the potential barrier heightVd0

increases with the temperature.Figure 3 shows the isochronal annealing of the residual

absorption coefficient at 1102 cm21 of the samples annealedfor 40 and 100 h, respectively. When the absorption decaysto the residual value, the sample is heated up in darkness andkept for 10 min at each temperature, and then followed bymeasuring at 7 K. We note that there are three annealingstages. The residual absorption under consideration is foundto remain almost unchanged up to;80 K. In the temperatureregion of 90,T,120 K, an annealing flat is observed. Whenheated up to higher temperatures, furthermore, the residualabsorption decreases and disappears rapidly at about 180 K.

FIG. 1. Time dependence of the absorption coefficient at 7 K from thesample annealed for 40 h during photoexcitation~left-hand side! and decay~right-hand side!.

FIG. 2. Absorption decay curves for five selective temperatures of thesample annealed for 40 h.

1946 Appl. Phys. Lett., Vol. 66, No. 15, 10 April 1995 Shi et al.

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These indicate that the effects of defect clusters on the relax-ation of photoexcited carriers are complicated. Similar be-haviors of the residual absorption annealing imply that thedefect states are almost the same in the two samples annealedfor 40 and 100 h. Indeed, the evolution of defects in anirradiated silicon is dominantly controlled by the annealingtemperature.11

Some groups reported temperature dependencies of therelaxation of photoexcited carriers affected by defect clustersin neutron irradiated silicon at above liquid-nitrogentemperatures.15–17 However, the results were not coincidentwith each other, due to complications of defect states at adefect cluster. In principle, temperature dependencies of therelaxation of photoexcited carriers may be due to two rea-sons. First, it results from variations of the carrier recombi-nation at the core of a defect cluster, and second, it resultsfrom variations of the potential barrier height. At low tem-peratures, generally, the carrier recombination through quan-tum mechanical tunneling is important, which is temperatureindependent.18 However, the variations of the potential bar-rier height with the temperature is more complex, associatedwith the detailed defect states at a defect cluster. Invokingsimple theoretical analysis, it was assumed that the potentialbarrier height is approximately proportional to the tempera-ture only for some distributions of defect states.17,19From thepresent observations, the origins of such variation with tem-perature are not exactly known.

There are similar points in effects of the potential barrieron the carrier recombination process between defect clusters

and dislocations. Recently, the investigations on dislocationsin silicon with the temperature-dependent electron beam in-duced current~EBIC! technique20,21 showed different tem-perature dependencies of carrier recombination process, de-pending on their defect states at dislocations. Kusanagiet al.suggested that the potential barrier height increases withtemperature in plastic deformed silicon.20

In conclusion, the temperature-dependent investigationon the optically active behavior of the HOB in a fast neutronirradiated silicon has been presented. Experimental resultsshow that the decay behavior with a long decay time is al-most temperature independent below 80 K, and the residualabsorption remains up to 180 K. These behaviors are pro-posed to be related to the relaxation characteristics of photo-excited carriers at different temperatures, which may be gov-erned by the variation of the potential barrier of the defectclusters. Detailed experimental investigation and physicalmodeling remain to be done.

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FIG. 3. Isochronal annealing of the residual absorption. The absorptionswere measured at 7 K.

1947Appl. Phys. Lett., Vol. 66, No. 15, 10 April 1995 Shi et al.

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