Photoluminescence and infrared spectroscopy for the study of defectsin silicon for photovoltaic applications

Simona Binetti n, Alessia Le Donne, Adele SassellaMilano-Bicocca Solar Energy Research Center (MIB-SOLAR) and Department of Materials Science, University of Milano-Bicocca, via Cozzi 55,I-20125 Milano, Italy

a r t i c l e i n f o

Article history:Received 30 October 2013Received in revised form3 February 2014Accepted 4 February 2014

Keywords:mc-SiSoG-SiImpuritiesDefectsIR spectroscopyPhotoluminescence

a b s t r a c t

It is widely known that photoluminescence (PL) and infrared (IR) spectroscopies are among theexperimental tools extensively used in the last decades for the study of impurities and defects in siliconfor both microelectronic and photovoltaic applications. This review paper reports the main historicalachievements and recent developments obtained in this field by PL and IR, paying particular attention tothe most useful data for the study of defects in silicon for photovoltaic applications.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

The spectroscopic study of impurities and defects in silicon iswidely recognized as one of the keystone of the outstandingdevelopment of silicon based devices in the last 60 years. Photo-luminescence (PL) and infrared (IR) spectroscopies are among theexperimental tools who contributed to the main achievements in thisfield. Oxygen and carbon (i.e. the main non-dopant impurities inelectronic grade and solar grade silicon, respectively), along withtheir aggregates, have been studied for more than 50 years both byPL and IR. Infrared spectroscopy is sensitive both to isolated inter-stitial or substitutional impurities and to more complex impurity-related structures or defects, through the vibrational response ofspecific bonds. This permits in some cases to quantify the content ofdopants and non-dopant impurities, in some others to get worthyinformation on the defect type and characteristics. As well, dopants,point and extended defects in monocrystalline silicon (mono-Si) havebeen extensively inspected by PL at low temperature. In particular,the emission of light from dislocations, identified for the first time in1976 [1], was the subject of hundreds of papers concerning mono-Siand, more recently, multicrystalline silicon (mc-Si) for photovoltaics(PV). Apart from this specific application, PL and PL mapping [2,3] ofsilicon wafers and solar cells strongly emerged as valuable character-ization tools since it was observed that luminescence from mc-Si isspatially inhomogeneous and correlated to the diffusion length of the

minority carriers [4–8]. Due to the fast capability of detecting areascontaining defects, such as grain boundaries and dislocation/defectclusters both in mc-Si wafer and solar cells, PL mapping has been formany years the workhorse of the characterization methods for Sibased PV. PL studies on the presence and role of impurities anddefects were recently back in the limelight as solar grade silicon(SoG-Si) became a source for low cost but less pure Si feedstock[9,10]. As a matter of fact, besides a variety of metal impurities,SoG-Si often contains a large amount of both acceptor and donorimpurities which gives rise to compensation. Careful analyses of bothmetal and dopant impurities are therefore mandatory in order to set-up suitable strategies to reduce their impact on solar cell perfor-mance [11–13].

This review paper provides first of all an overview of the mainhistorical results and recent developments obtained in the study ofimpurities and defects in silicon by PL and IR. Last but not least, sincemany papers on this topic are relatively dated and often knownmainly to an audience which dealt with silicon for microelectronics,the second ambitious aim of this work is to bring out the most usefuldata for the study of defects in silicon for PV applications.

2. Dopant impurities in silicon

2.1. Determination of dopant content by low temperature IR

Low temperature IR is a powerful tool to determine theconcentration of dopants in silicon materials, since below 15 K

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n Corresponding author. Tel.: þ39 0264485177; fax: þ39 0264485400.E-mail address: simona.binetti@unimib.it (S. Binetti).

Please cite this article as: S. Binetti, et al., Photoluminescence and infrared spectroscopy for the study of defects in silicon forphotovoltaic applications, Solar Energy Materials and Solar Cells (2014), http://dx.doi.org/10.1016/j.solmat.2014.02.004i

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the concentration of free carriers is negligible. The early approachto such an analysis was reported by White [14], while laterdevelopments allowed the determination of residual impuritiesin high resistivity silicon [15].

At low temperatures (To15 K), the silicon IR spectrum consistsof a series of intense absorption bands (superimposed to theintrinsic Si response) due to electron or hole transitions from theground state of neutral impurities to a series of hydrogenic-likelevels lying close to the respective band edge [16]. As an exampleof the characteristic absorption bands typical of a shallow impur-ity, those related to boron are reported in Table 1 [17].

The strongest band related to each impurity is generally usedfor the quantitative analysis required to determine the concentra-tion of the impurity itself. A standard procedure for the determi-nation of electrically active boron, phosphorus, arsenic, aluminum,antimony, indium and gallium content in crystalline silicon isreported in the ASTM standard F1630-95 [18]. Such a procedure,based on the application of the Beer's law with proper calibrationfactors (based on resistivity measurements [19]), is designed forsilicon samples thicker than 1 mm.

As far as compensated materials like SoG-Si are concerned, thesample should be illuminated with an incident white light to allowneutralization of compensated impurities. In this condition, the excesselectrons and holes rapidly neutralizes the ionized centers, allowing

both acceptor and donor impurities (namely, B and P) to be detected ina simultaneous illumination–absorption spectrum (see Fig. 1).

Actually, such an analysis in highly compensated silicon ishampered by a band broadening phenomenon proportional tothe compensation ratio RC [15,20], which makes very difficult toidentify the B and P peaks. Independently of the compensationratio, a similar band broadening phenomenon associated to theinternal stress, which often occurs in low thickness samples,hampers as well the identification of the B and P peaks.

2.2. Determination of dopant content by low temperature PL

When silicon is excited at low temperatures by photons withenergy higher than the bandgap, electron–hole pairs are produced.They can recombine in a number of ways, some of which give riseto luminescence peaks in the range between 0.7 and 1.17 eV,associated to defects or impurities.

When the donor and acceptor concentrations are higher than1015 at/cm3, at 4.2 K the free excitons (FE) are all virtually localizedat the dopants giving rise to impurity-specific bound exciton (BE)luminescence. BE lines are sharp with photon energy given by:

hν¼ Eg�Ex�EB ð1Þwhere Eg is the energy gap, Ex is the binding energy of the FE andEB is the binding energy of the exciton bound to the neutral donoror acceptor, which is about one tenth of the donor or acceptorionization energy.

In general, the luminescence intensity associate to a particular peakdepends not only on the relative and absolute concentration of variousimpurities or defects, but also on the excitation power density, on theoperating temperature and on non-radiative recombination processes.As a consequence, PL is usually considered as a qualitative techniqueable to evaluate relative contents of impurities and defects. Never-theless, Tajima demonstrated that quantitative information on donorand acceptor concentrations could be inferred from the ratio between

Table 1Absorption bands of boron and related electronictransitions.

Line (cm�1) Transition

245.2 1S(3/2)-2P3/2278.5 1S(3/2)-2P5/2309.3 1S(3/2)-3P3/2320.4 1S(3/2)-3P5/2

Fig. 1. Infrared peaks of B and P in compensated p-type Si (ρZ7000 Ω cm, thickness 10 mm) collected at 15 K with different spectral widths. (a) and (c) Fundamental bandtransition excited; (b) and (d) without fundamental band excitation. Reprinted with permission from [15]. Copyright [1975], AIP Publishing LLC.

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BE and FE lines [21], provided that the FE response is detectable (i.e.for dopant content lower than 1015 at/cm3). As the concentration ofresidual donor and acceptor in SoG-Si ranges between 1015 and1017 at/cm3, specific variations of this method were developed byTajima and coworkers [22,23]. In [22] the authors exploited theincrease of the FE line intensity with the excitation power, whichoccurs at a higher rate with respect to the BE one. A proper excitationpower was then selected to detect both BE and FE line, whoseintensity ratio was used to obtain a calibration curve. In [23] theauthors exploited a higher operating temperature to enhance the FEemission, then evaluating the B and P concentrations on the basis ofthe BE to FE ratio at that temperature. Both approaches providereliable dopant contents in the concentration range between 1�1014

and 2�1016 at/cm3 in good agreement with secondary ion massspectrometry (SIMS) and inductively coupled plasma mass spectro-scopy (ICP-MS). However, it should be remarked that both approachesare designed for compensated mono-Si, so that their application tocompensatedmc-Si can be hampered by the presence of further defectrelated PL emission lines [24,25].

As far as a qualitative analysis of compensated silicon isconcerned, a fingerprint of the simultaneous presence of B and Pin the Si matrix is an emission related to a transition between therespective donor and acceptor levels. The recombination energy ofsuch donor–acceptor pair (DAP) luminescence is given by:

hν¼ Eg�ðEDþEAÞþe2

4πε0εrr� ar6

ð2Þ

where Eg is the energy gap, ED and EA are the donor and acceptorionization energy, e is the electron charge, ε0 the low-frequencyrelative dielectric constant of vacuum, εr is the low-frequencyrelative dielectric constant of Si, and r is the DA pair separation.The last term in Eq. (2) accounts for Van der Waals interactions,where a is a constant for a given DAP.

DAP luminescence is a well known phenomenon, mainlyobserved and investigated in compound semiconductor like GaAs,GaP, CdTe and SiC. Conversely, after the first works published inthe 1960s [26,27], the scientific community paid little attention toDAP in Si [28], due to the very low compensation ratio typical ofelectronic grade (EG) silicon.

PL spectra collected at 12 K on compensated mono-Si sampleswith different compensation ratios revealed a broad band with anasymmetric shape peaked at around 1.05 eV (see Fig. 2a), whichshowed a blue shift for increasing excitation power typical of DAP[29]. The same DAP transition between B and P levels has beenobserved as well in PL spectra collected at 12 K on SoG mc-Sisamples (see Fig. 2b), confirming that PL provides an easy andreliable detection tool of compensation.

The fine structure at 4.2 K of the higher energy side of DAPluminescence in compensated Si has been studied for the first timeby Tajima et al. in [30,31] where a comparison between the experi-mental and the theoretical spectrum, deduced from the number ofpossible DA pairing as a function of transition energy, is reported.These works suggest the possibility of a quantitative determination ofdonor and acceptor impurities from the spectral shape analysis,considering that the pair distribution depends on the donor andacceptor concentrations. Such an approach could lead to an accurateidentification and quantification of donor and acceptor impurities inthe 1015–1017 at/cm3 range in any type of compensated sample.Furthermore, ED and EA can be obtained by a comparison betweenthe theoretical spectrum of DAP luminescence (Eq. (2)) and theobserved fine spectral structure. If a close agreement is obtained usingthe generally accepted P and B ionization energies then P and Bimpurities are unlikely to form complexes like B–P pairs.

Tajima and coworkers focused their attention also on the finestructure of the DAP luminescence involving Ga-acceptors inhighly compensated Si co-doped with Ga for PV applications [32].

3. Non-dopant impurities in silicon

As previously mentioned, oxygen and carbon are the main non-dopant impurities in EG-Si and mc-Si for PV, respectively. Oxygenatoms in silicon are electrically inactive and predominantly enterinterstitial sites, while carbon is an electrically inactive substitu-tional impurity. Oxygen is present at levels above the solidsolubility limit both in EG-Si, mc-Si and SoG-Si, while the carboncontent is relatively low in EG-Si and higher than the solidsolubility limit in mc-Si and SoG-Si [33]. Other unintentionalimpurities, detected mainly in SoG-Si, are nitrogen and 3d transi-tion metals (Fe, Ti, V, Mn, Cu). The latter are present in general asinterstitials at levels above the solid solubility limit [33,34] and areusually investigated by characterization techniques other than IRand PL [35].

3.1. Analysis of oxygen, carbon and nitrogen in silicon by IR

The presence of interstitial oxygen (Oi) in silicon is manifested bythe IR absorption peaks due to the vibrational modes of the quasi-molecule Si–Oi–Si: the bending vibration at 515 cm�1 and the sym-metric and antisymmetric stretching at 1107 and 1205 cm�1, respec-tively. A typical example is given in Fig. 3 for a Czochralski siliconsample with 5�1017 at/cm3 interstitial oxygen. The symmetricstretching band at 1107 cm�1 is involved in the quantitative determi-nation of the Oi content, which requires the comparison with FloatZone O-free silicon spectra at low temperatures due to the presence ofthe silicon crystal phonon response in the same spectral region[36,37]. Due to the strong enhancement of the interstitial oxygen

Fig. 2. (a) PL spectra at 12 K of SoG mono-Si with different compensation ratios(Rc¼1, 1.24, 1.68, 18.34), grown according to the Czochralski method from 10%phosphorus containing SoG-Si feedstock blended with 90% highly purity poly-Sielectronic grade feedstock; (b) PL spectrum at 12 K of SoG mc-Si, grown frommetallurgic Si feedstock (sample details are reported in [25]).

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absorption band below 77 K, low temperature IR measurements areparticularly effective for the quantitative determination of the Oi

content in samples with low Oi concentration (r3�1014 at/cm3) orvery low thickness. The conversion factors for the determination of theOi content (in at/cm3) from IRmeasurements at different temperaturesare reported in [37].

Oxygen related defects, mainly oxide precipitates [37] andthermal donors, were extensively studied in the literature due totheir influence on the electrical and mechanical properties ofsilicon and on the performance of the devices based on it. TypicalIR spectra of silicon show some absorption peaks related to oxideprecipitates in the region between 1200 and 1250 cm�1, whichhave been subjected to detailed studies for getting a uniqueinterpretation. Indeed, the spectral position and the line shape ofthese IR bands depend not only of the precipitate content, butmostly on their shape and composition. This is why a quantitativeestimation of the precipitate content is difficult, requiring anaccurate knowledge of their characteristics [38,39]. When a pre-cise identification of the precipitate related IR response is possible,a more accurate determination of the Oi content can also beobtained, as demonstrated by low temperature measurements[40,41]. In silicon for solar cells [42] oxygen preferentially segre-gates close to structural defects, giving rise to IR absorption bandsin the range between 980 and 1100 cm�1. These signals being inthe same region of the Oi related absorption, low temperaturemeasurements are needed to distinguish them from each other.

As far as other oxygen related defects are concerned, e.g. thermaldonors, no direct IR response can be found. However, IR absorptionlines associated to electronic transitions from the ground state intoexcited states of thermal donors can be observed at low temperaturein the region between 300 and 900 cm�1 [43–45].

Substitutional carbon (Cs) in silicon gives rise to an IR peak atabout 605 cm�1 (at 77 K), which can be used for the quantitativeevaluation of Cs content at different temperatures [46]. Such anevaluation however needs some care, since the Cs peak is super-imposed to the intrinsic Si phonon line at 610 cm�1. Carbonimpurities in silicon also originate defects and complexes invol-ving the host atoms, mainly studied in electron irradiated siliconcrystals [47–50]. Complexes involving carbon and oxygen arepresent as well, leading to different vibrational modes in themedium IR spectral region, which are detectable preferentially atlow temperature [51–53]. The main ones at 1104 and 1052 cm�1

(at 77 K) are associated to precise atomic configurations, whichgive rise to perturbed Oi modes or new vibrations from specificcenters, as calculated by Kaneta [54]. The presence of C relateddefects [42] and precipitates (namely, SiC) [55] leading to theformation of traps and to metal impurities gettering, should beconsidered with care in mc-Si and SoG-Si.

Nitrogen in silicon is an impurity with high diffusion coeffi-cient, which has been investigated in N-rich Si monocrystalsgrown by adding Si3N4 into the melt [56]. The main N relateddefects are Ni–Ni pairs, where the two nitrogen atoms occupyinterstitial sites in a locally distorted silicon lattice, and severalN–O complexes, some of which are also electrically active. Ni–Ni

pairs give two main vibrational bands at about 770 and 965 cm�1,the latter being used for the quantitative evaluation of thenitrogen content both in mono-Si [57,58] and in mc-Si or SoG-Si[59,60]. Isolated interstitial nitrogen also exists, with three mainpeaks related to vibrations of the Si–N bond, at 550, 773, and885 cm�1. Furthermore, interactions of nitrogen with interstitialoxygen leads to different complexes which show the followingvibrational modes: three bands at 802, 996, and 1027 cm�1 areassociated to the electrically inactive complex of the Ni–Ni pairwith Oi, while two peaks at 810 and 1018 cm�1 are related to thecomplex of the Ni–Ni pair with two Oi [61]. While in O-rich mono-Si proper thermal treatments are required to form N–O complexes,as-grown mc-Si naturally contains N–O complexes in concentra-tions higher than 1015 at/cm3 [59]. Finally, interactions of nitrogenwith other defects, in particular dislocations, and with carbon [42]are known. The latter, which play a role in the formation of SiCprecipitates, are particularly relevant in SoG-Si where both nitro-gen and carbon are present at high levels [33].

3.2. Analysis of oxygen, carbon and nitrogen in silicon by PL

The electrically inactive interstitial oxygen and substitutionalcarbon, as expected, do not introduce any luminescence line, butoxygen and carbon agglomerates and complexes are involved indifferent photoluminescence centers that will be summarized inthe following.

3.2.1. Luminescence related to old thermal donorsIt is well known that a long dwelling at temperatures around

450 1C during the cooling cycle of oxygen rich silicon ingots, or anythermal treatment in the same temperature range, leads to thegeneration of oxygen related donors. They are known as oldthermal donors (OTD), to distinguish them from other thermaldonors formed in the temperature range 600–700 1C and labeledas new thermal donors (NTD). In spite of thousands of paperspublished on the topic, which make OTD one of the most studieddefect center in semiconductors, a model which consistentlyexplains all their features, including their optical properties, isstill lacking. Anyway it has been generally accepted that a thermaldonor consists of a family of similar shallow double donor defectsrather than a unique defect.

After the first works by Tajima [62] and Heijmink [63] onexcitons bound to thermal donors, it was observed that oxygenrich silicon annealed at 450 1C for several hours exhibits an intenseand narrow zero-phonon emission at 0.767 eV, generally labeledas P line [64–68] (see Fig. 4a). A carbon isotopic effect, experi-mentally observed in the P line, demonstrated the involvement ofcarbon in the OTD center [69].

Since the P line is still detectable at room temperature [65] (seeFig. 4b), sometimes it has been used as diagnostic tool. As anexample, on the basis of the PL intensity of the P line observed atroom temperature from a crystal pulled at low growth rate, it wasfound [70] that the effect of the thermal history of a crystal pulledat 0.8 mm/min was equivalent to an annealing at 500 1C for 3 h.

3.2.2. Luminescence related to oxide precipitatesAs previously mentioned, oxygen is present at levels above the

solid solubility limit in EG-Si, mc-Si and SoG-Si [33]. Any post-growthannealing can therefore induce the formation of SiOx precipitates.

Fig. 3. IR absorbance spectrum of a Czochralski silicon sample with 5.1�1017 at/cm3

interstitial oxygen content, normalized to a Float Zone silicon sample of the samethickness to better evidence the oxygen related response.

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Furthermore, it is well known that the growth of precipitates fromsilicon oxide embryos at temperatures as high as 800 1C, or more,occurs with the simultaneous emission of self-interstitials, which, afterclustering, are the precursors of dislocations. Heterogeneous precipita-tion of oxygen at dislocations is a favorite process as well, sodislocation in silicon are very often decorated with oxygen or oxideprecipitates. Therefore, distinguishing whether luminescence phe-nomena are associated to dislocations or to oxide precipitates isanything but a trivial problem.

Tajima and coworkers [71] assigned for the first time in 1991 aluminescence line to oxide precipitates. This band, labeled as Db

and peaked at about 0.820 eV from 77 to 150 K and at 0.768 eV at300 K, was identified as being due to oxide precipitates on thebasis of its correlation with the concentration of precipitatedoxygen. In a series of papers [72–76], Pizzini and coworkerssupported with a wide set of experimental results the original

hypothesis by Tajima [71,77] that oxide precipitates in siliconpresent light emission in the same energy range of D1 and D2lines, typical of dislocations (see below). The results of these works[72–76] demonstrated not only that luminescence associated todislocations and to oxide precipitates can be discriminated, butalso that the emission at 0.817 eV can be assigned to SiOx

precipitates. These findings demonstrated that photoluminescenceis much more sensitive than IR spectroscopy for the detection ofoxygen complexes and even more convenient than TEM, when thedensity of oxide nuclei and precipitates is of the order of 1010–1011 at/cm3 [78].

As far as mc-Si is concerned, several examples of the PLeffectiveness in detecting oxide precipitates were reported. PLmeasurements on both mc-Si and Ribbon edge film growth (EFG)Si submitted to the different solar cell processing steps (texturiza-tion, junction formation and antireflection coating deposition)demonstrated [79] that the junction formation has not only abeneficial effect related to the gettering of metal impurities, but italso induces the segregation of oxide precipitates at dislocations.Furthermore, Tajima and coworkers [80] recently succeeded indistinguishing between oxide precipitates and dislocations relatedemissions in PL intensity maps collected at room temperaturearound small angle grain boundaries (SA-GBs). Their resultsshowed that patterns corresponding to secondary defects/impu-rities trapped by the strain field around dislocation clustersforming SA-GBs and to preferential oxygen precipitation on dis-locations are not observed for SA-GBs with an angle of o11,suggesting that oxygen precipitation occurs on SA-GBs with anangle of 1–21.

3.2.3. Luminescence related to carbon and nitrogenCarbon related luminescence in silicon is associated to different

defect complexes formed by carbon and the host matrix or otherimpurities. The three-atom defect Ci–Si–Cs is responsible for the so-called zero-phonon G line at 0.969 eV [81, 82], which usually arises insilicon after irradiation and subsequent annealing up to 200 1C.Thonke et al. [83] demonstrated however that the G line center canbe formed without radiation damage by an annealing at 1200 1C for7 h, followed by a rapid quenching in silicone oil at room temperature.

The defect complexes involving carbon and oxygen are insteadbelieved to be responsible for the C line at 0.789 eV [84–88] (seeFig. 5), since this band is affected by changing either the carbon orthe oxygen isotope [89, 90]. Upon annealing at temperature higherthan 350 1C for about 20 min or prolonged irradiation at roomtemperature, the C line disappears indicating that the relateddefect center is deactivated.

Both G and C lines have been used to determine the carbon contentin silicon [91]: in the case of Si samples with known oxygen contentthe former allowed carbon concentrations of 1016 to 1017 at/cm3 to bedetermined within an uncertainty of 730%, while the latter enabledthe determination of the lowest levels of carbon (E1014 to 1015

at/cm3), almost independently of oxygen concentration.A further defect complex involving carbon and oxygen has been

associated to the so called H line at 925.5 meV (see Fig. 5)observed both in irradiated mono-Si and in mono and mc-Sisubmitted to any thermal treatment or dwelling in the tempera-tures range around 450 1C [64, 66, 67].

As in the case of carbon, nitrogen related luminescence insilicon is associated to different defect complexes formed bynitrogen and other impurities. Some sharp PL lines detected at4.2 K in the region between 0.745 and 0.773 eV have beenassociated to nitrogen–carbon and nitrogen–carbon–oxygen com-plexes, labeled as N1 to N5 centers [92, 93]. These lines have beenobserved mainly in mono-Si subjected to nitrogen implantationor to sequential nitrogen and carbon implantation followed by

Fig. 4. (a) PL spectrum collected at 12 K on EG mono-Si annealed at 470 1C for 24 h;(b) PL spectrum collected at 300 K on EG mono-Si annealed at 470 1C for 24 h.

Fig. 5. PL spectrum at 12 K of SoG mc-Si, grown from metallurgic Si feedstock(sample details are reported in [25]).

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thermal annealing. Not only Dornen et al. demonstrated that N1(745.6 meV) and N5 (772.4 meV) centers have monoclinic sym-metry [94], but they showed as well that the N centers incorporateboth nitrogen and carbon and that N3 (761.5 meV), N4 (767.4 meV)and N5 need additional interactions with other impurities (prob-ably oxygen) to form [93, 95].

Finally, a further PL line associated to nitrogen at 1.1223 eV hasbeen observed at 4.2 K by Tajima et al. [96] in mono-Si intention-ally doped with nitrogen.

4. Analysis of extended defects in silicon by PL

The most studied extended defects in mono-Si are dislocations,which are present at high levels in mc-Si, along with grainboundaries. Dislocations are responsible for several PL bands,while grain boundaries do not introduce radiative recombinationcenters. Nevertheless, they can be effectively monitored by PLmapping, as the presence of non-radiative recombination centersreduces the intensity of the band-to-band luminescence, givingrise to dark lines in the PL patterns [4].

As previously mentioned, light emission from dislocations wasreported for the first time by Drozdov et al. [1] and consists of four PLbands, labeled as D1, D2, D3 and D4 with photon energies at lowtemperature of 0.807 eV, 0.870 eV, 0.935 eV and 1.0 eV, respectively(see Fig. 6). Since 1976, dislocations and their interactions withimpurities in silicon have been extensively studied not only by PL,but also by DLTS and EPR measurements [97–100]. For the sake ofclarity, it should be remarked that, in spite of so many years ofinvestigations, very little has been definitively established about theinfluence of impurities on the electrical activity of dislocations andtheir related effects on D bands. Nevertheless, any new finding on thistopic gets us closer to limit the deleterious effects of dislocations onthe efficiency of mc-Si solar cells.

A generally accepted idea is that D3 and D4 lines are associated tothe intrinsic nature of dislocations and in particular to the carrierrecombination at straight segments of splitted 601 dislocations[99, 101–105]. Furthermore, the D3 line has been often consideredas a phonon replica of the D4 [106,107], despite in many cases theintensity of D4 exceeds the intensity of D3 (see Fig. 6). As far as D1 andD2 emission lines are concerned, they are detected even in theabsence of D3 and D4, supporting the theory that the D1/D2 andthe D3/D4 pairs have different origins. However, the real origin of D1and D2 is still unclear. It has been shown that their features (energypositions and intensity) are affected by the presence of impurities likeoxygen [72–74, 80], nitrogen [108, 109] andmetals [110, 111]. Seibt andcoworkers [112] reported that electron-hole pair recombination atdislocation sites mainly runs via its shallow levels and is stronglyincreased by impurities bound to the dislocation core. Arguirov et al.[113] suggested that removal or deactivation of recombinating impu-rities from dislocations (e.g. by gettering) eliminates deep level

recombination routes, but does not influence the shallow levels. Kittleret al. [114] demonstrated as well that in mc-Si a relatively lowcontamination level of dislocations in the order of 10 impurityatoms/μm of the dislocation length induces D1 luminescence at roomtemperature. From a general point of view, the high temperaturestability of D1 luminescence has been modeled in [107]. In the samework [107], D2, D3 and D4 lines are attributed to the excitation ofelectrons and holes from the D2, D3, and D4 centers to the mobilityedge of the dislocation related energy band. The activation energy ofthis excitation is rather small and the D2, D3, and D4 PL linesdisappear therefore rather quickly with increasing temperature sinceall carriers are trapped at the D1 centers which are the deepest. D1being therefore still visible at room temperature, dislocation clusters,either decorated with oxide precipitates/metal impurities or not,appear as dark lines in the band-to-band luminescence mappingand as bright lines in the 0.8 eV mapping [80]. The relative intensity ofD1 to D4 depends on the dislocation density and can vary along thesame sample. Furthermore, the relative intensity of D bands cangreatly vary from sample to sample and is affected by thermaltreatments at relatively low temperature [115], as shown in Fig. 7.

5. Concluding remarks

Thanks to the huge amount of studies reported in the literaturesince the sixties of the last century, many impurities, defects andcomplexes in silicon for both microelectronic and photovoltaic appli-cations can be monitored by IR and PL spectroscopies. Not onlyimpurities and defects can be detected even at low concentrations,but both techniques are sensitive to the chemical species, are non-destructive and do not need particular sample preparation andhandling. These powerful detection tools however should be appliedwith care onmc-Si, SoG-Si or upgradedmetallurgical silicon due to thecomplexity of these materials (e.g. presence of several defects, mutualinteractions among impurities or defects and inhomogeneity along theingot). Despite both the quantitative determination of impurities anddefects by IR and defect study by PL often require low temperaturemeasurements and small samples, any study on silicon for PV candefinitely benefit from the synergy of room temperature PL mappingand low temperature IR and PL.

References

[1] N.A. Drozdov, A.A. Patrin, V.D. Tkachev, Recombination radiation on disloca-tions in silicon, Pisma Zh. Eksp. Teor. Fiz. 23 (11) (1976) 651–653 (Sov. Phys.JETP Lett. Vol. 23, pp. 597-599).

[2] H.J. Hovel, Scanned photoluminescence of semiconductors, Semicond. Sci.Technol. 7 (1992) A1.

[3] V. Higgs, F. Chin, Xue Cheng Wang, Scanning photoluminescence for wafercharacterization, Solid State Phenomena 63–64 (1998) 421.Fig. 6. PL spectrum at 12 K of a dislocated mono-Si sample.

Fig. 7. PL spectra collected at 12 K both on as grown and annealed EFG Si samples.

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