Structural changes in the surface layer of technologically treated silicon by ion implantation, chemical etching, and their combined action have been investigated by the X-ray diffractometry methods. The functional and quantitative differences in the thickness dependences of strains, values of maximum strain, level of lattice disturbance and extension of elastic strains nave been revealed after different steps of treatment. The essential modification of photoluminescence spectra was observed in the porous layer after implantation by phosphorus ions in the process of natural aging

Fodchuk, I.

Lytvynchuk, I.

Swiatek, Z.

Semiconductor Physics Quantum Electronics & Optoelectronics

Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)

231' 2004, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of UkraineSemiconductor Physics, Quantum Electronics & Optoelectronics. 2004. V. 7, N 3. P. 231-235.PACS: 61.43.GtThe influence of ion implantation by phosphorouson structural changes in porous siliconZ. Swiatek1, I. Lytvynchuk2, I. Fodchuk21Institute of Metallurgy and Materials Science, Krakow, Poland2Chernivtsi National University, Chernivtsi, UkraineAbstract. Structural changes in the surface layer of technologically treated silicon by ionimplantation, chemical etching, and their combined action have been investigated by the X-ray diffractometry methods. The functional and quantitative differences in the thicknessdependences of strains, values of maximum strain, level of lattice disturbance and extensionof elastic strains nave been revealed after different steps of treatment. The essential modifica-tion of photoluminescence spectra was observed in the porous layer after implantation byphosphorus ions in the process of natural aging.Keywords: ion implantation, porous silicon, photoluminescence, X-ray diffraction.Paper received 13.08.04; accepted for publication 21.10.04.1. IntroductionPorous silicon represents the c-Si single crystal, in whichthe extensive set of small pores is created by the chemicaletching. The density of pores in some samples can be solarge, that there is their overlapping, and the unetchedparts of silicon acquire the form of the coral-like systemof filaments of variable cross-sections [16].In view of the perspective of porous silicon for practi-cal using at the creation of silicon light-emitting devicesit is necessary to intensive study of porous silicon proper-ties. This is due to the fact that on the way of realizationof similar devices there are serious problems related toinstability of structural, optical, luminescent and othercharacteristics of porous silicon. The one of the supposedsolutions of this problem is using the ion implantationand certain modes of heat treatment [710]. However,post-implantation defects formed in the result of ion im-plantation and annealing do not give the possibility touse in full measure the electric and structural proportiesof amorphous surface layer of material. The presence oflocal strains in the transitional gap of Siam/Sicr hetero-boundary often results in the displacement of absorptionbands [8].The purpose of this work is to research structuralchanges in surface layers of the silicon exposed to differ-ent types of technological treatments (ion implantation,chemical etching and their combined action) by the meth-ods of X-ray diffractometry and atomic force microscopy.2. Object of researchesTo determine the structural changes arising due to differ-ent technological treatments of (111) surface, a sample ofthe high-perfect Czochralsky grown silicon was used. Thissample was exposed to the chemical etching and polish-ing. Thereafter the chemical cleaning of surface was car-ried out. Four regions were created on the sample surface(corresponding designations are shown in Fig. 1): the firstwas initial material, second and third were exposed toion implantation, third and fourth were exposed to thechemical etching in the HF, HNO3 solution for 40 se-conds. Due to chemical etching a porous structure wasformed in third and fourth regions. Moreover, the thirdregion was additionally exposed to ion implantation byphosphorus before etching. The energy of implanted ionswas E = 180 keV, dose was about Q = 8⋅1014 ion/cm2.Implantation was carried out in such a way that the ionchanneling was impossible. At these doses and energiesof implanted ions a surface layer was destroyed with for-mation of numerous disordered regions with the consid-erable concentrations of point defects, which caused thechanges of material optical properties [810].3. Experimental partThe symmetric, asymmetric and skew-asymmetric Bragggeometries of X-ray diffraction with (111), (333), (311)232SQO, 7(3), 2004Z. Swiatek et al.: The influence of ion implantation by phosphorous on ...reflections of CuKα  radiation were used for X-ray diffrac-tometry and topography researches.X-ray rocking curves were measured using a three-axisdiffractometer. Two germanium monochromators wereplaced on the first two axes of diffractometer and the in-vestigated silicon crystal was located on the third axis.The monochromators in mutually dispersion arrangementwith the symmetric (333) reflection practically suppressedpi-polarization of the characteristic CuKα1 radiation.Highly collimated incident beam had the following spec-tral and spatial charachteristics: 7105 −⋅≈∆λλ and 2 ′′=δθ . 20 nm1 12 23 34 4µm112 23 34 420 nmµm  1 3 4 2 Fig. 1. Atomic-force microscopy (AFM): part „1 is initial material; part „2 implanted by P ions; part „3 implanted by P ions andchemically etched (porous part); part „4 chemically etched (porous part).Z. Swiatek et al.: The influence of ion implantation by phosphorous on ...233SQO, 7(3), 20044. Theoretical partSimulation of X-ray diffraction patterns for crystal withthe damaged surface layer is carried out using kinematicand dynamic theories of X-ray scattering. In the dynamictheory contrary to kinematic, the effects of multiple scat-tering are taken into account, and therefore this theorydevoid of limitations on the layer thickness. Thus it iseffectively used for the analysis of rocking curves obtainedfrom crystals exposed to different external influences [11,1415].It should be noted that using skew-asymmetric dif-fraction for certain reflective planes and rotating crystalaround the normal to the reflective plane we can gradu-ally decrease extinction depth by more than two orders[16]. Its magnitude can be smaller or comparable withthe effective thickness of the damaged surface layer.The rocking curves were calculated within the frame-work of the kinematic theory for X-ray reflection. By thefunctional definition of strain profile dzdz /)()( ∆=ε  anddistortions of the surface layer W(z) (W = 1  eL, where Lis the Debye-Waller factor), it was achieved a satisfac-tory agreement with the experiment. The functionaldependences ε(z) and W(z) were specified in the form of aset of exponential and sinusoidal maxima with variousheights and widths located at the determined depths. Atthe same time, the ordinate of each point of rockingcurves is the function of the whole strain profile ε(z). Afirst approximation of this strain profile is determinedby a solution of the inverse problem of X-ray diffractionusing experimental rocking curves [14, 17].Within the framework of the kinematic theory theamplitude of X-rays reflected from N layers is the sumof amplitudes reflected from each j-th layer and cor-rected for the phase shift caused by (∆d/d)j and Wj. It isgiven by [17]:( )∑=−⋅Φ+−⋅=NjjjLjNnnieqA1,2sin2sin21expδδδ(1)where ohklBFmceVdiq ⋅⋅=22sinθλ is the amplitude of wavereflected from the single crystallographic plane of per-fect crystal in the case when the amplitude of the incidentwave is equal to 1; V is the unit cell volume; δj is the phasediference between reflected beams from neighbouringcrystallographic planes in the j-th layer:( )jBj ddd /2cos4 ∆⋅+∆⋅⋅= piθθλpiδ ; (2)Φj is the total phase shift after transmission through thej  1 layers:∑−==Φ11jijij n δ , Φ1 = 0. (3)The Debye-Waller factor is given by the expression:2222sin8jBj UL ⋅⋅= θλpi, (4)where λ is the radiation wavelength, 2jU  is the root-meansquare displacement of lattice sites in the j-th layer causedby the defects of structure. Distortions jLj eW−−=1  areequal to 0 in the j-th layer for a perfect crystal and to 1 inthe case of the fully disordered lattice.To obtain a satisfactory agreement between theoreti-cal and experimental rocking curves, the method of mini-mization of non-negative discrepancy was used in thecase of direct problem solution [13]:( )( ) ( )( ) ,  2,10,,11,10,,2,1∆∆−∆==∆∑=kåıðkkåıðkikIxxIIxxxFθθθρθKK(5)where [ ] 120 )(11)( −−∆+−=∆ Wkkk θθθρ  is the weightfunction, θk is the abscissa of the k-th point in the rockingcurve, θ0 is the abscissa of the main maximum correspon-ding to X-ray scattering from the crystal volume, W isreverse to the halfwidth of the rocking curve for an idealcrystal.The whole profile of strains was divided into thin lay-ers as well as the profile of distortions. In the first stage ofoptimization, the values of (∆d/d)j and Wj were deter-mined in each thin layer. Then variation of the obtainedvalues had been carried out so long as the greatest possi-ble agreement (<3%) between theoretical and experimen-tal rocking curves was achieved (Figs 24). The experi-mental data processing was accepted as correct for thechosen strain model, if the value of the fitting criterion Fwas within the range of fn/212 ±=χ  (here pf nnn −=indicates a number of freedom degrees at the fitting, n isa number of experimental points, np is a number of vari-able parameters) [17].It should be noted that computer processing theseX-ray rocking curves with the fitting procedure and mini-mization of discrepancy function does not guarantee theuniqueness of obtained results without the detailed analy-sis of influence degree of different parameters on the formof rocking curves and precision of determination of theseparameters.5. Results5.1. X-ray researchesQuantitative estimations of structural changes in themarked parts of the silicon sample were defined from theanalysis of rocking curves for (111) and (333) reflectionsof Cu˚α1-radiation (Figs 24).The analysis of diffraction curves of X-ray reflectionin Figs 24 shows that the external influences (mentionedabove) on the surface of the porous Si crystal produce thechanges of the rocking curve form especially its tails234SQO, 7(3), 2004Z. Swiatek et al.: The influence of ion implantation by phosphorous on ...in comparison with the initial part of the sample. Thehalfwidth of X-ray rocking curves and maximum valuesof intensity slightly increase. The general view of rock-ing curves indicates that there are near-surface layerswith various levels of distortions, extensions and straindistributions in the treated parts of sample. These partsare characterized by expanded alternating-sign strains(ε) in the direction perpendicular to the surface. The pa-rameters of strains in different parts are:„2: depth extension of strains is L ~ 0.44 µm, the great-est positive strain is +maxε ~ 1.5⋅104 at +maxz == 0.15 µm, the greatest negative strain is −maxε ~~ 0.5⋅104  at −maxz  = 0.42 µm;„3: L ~ 0.6 µm, +maxε ~ 1⋅104 at +maxz = 0.17 µm, −maxε ~~ 0.2⋅104  at −maxz  = 0,4 µm.„4: L~ 0.58 µm, −maxε ~0.2⋅104 at −maxz  = 0.12 µm withthe following strain decay according to the expo-nential law to a depth of 0,34 µm, and next−maxε ~ 0.2⋅104  at −maxz  = 0.43 µm.It should be noted that another structure defects caninfluence on real values of strains, for example, tetrahe-dral packing defects of the vacancy type, dislocation loopsof the interstitial type [14] as well as compression of acrystal lattice owing to formation of a various sortprecipitations such as SiÑ.According to data of atomic force microscopy (Fig. 1)the cross-sections of silicon filaments and isolated parts(clusters) after chemical etching (part „4) had minimalsizes of the order of several nanometres. Therefore, sur-face distortions are insignificant as judged from bothchanges of interplanar spacing and strain extension. It isconfirmed by the behaviour of tails on the experimen-tal rocking curves (Figs 34). The main contribution tothe increase of the diffusion component of X-ray scatter-ing on rocking curve tails is made, apparently, by thechange of the surface layer density (degree of porosity is~40 %) and by the surface relief.As would be expected, the greatest value of averagedeviation of heights from the mean plane of the surfacerelief Ra = 0.398 nm is inherent to the sample part „3. Inthe parts „2 and „4, values of Ra are 0.288 nm and0.303 nm, respectively. The initial part of the crystal ischaracterized by the lowest value Ra = 0.17 nm.5.2. PhotoluminescenceThe analysis of photoluminescence spectra obtained overthe interval of 24 months was carried out to amplify theinvestigations of structural features of all the parts of thisporous silicon sample. Under action of light generatedluminescence the spectral bands of luminescence shiftedinto the red area for sample parts „3 and „4 (Fig. 5).This shift occurred due to intensity redistribution betweendifferent bands. According to [7], two components in thespectrum correspond to two groups of silicon filaments,which differ one from another by the filament diameter.In other words, there is a dependence of the energy (wave-length) of recombination radiation on the structural fea-tures of the porous part. Taking into account the fact thatlongwave component of spectrum reduces more slowlyfor porous parts in comparison with the initial part therelative intensity of this component in the spectrum grows,as confirmed by experimental data.As a result of ageing of porous silicon, the broade-ning (by ~30 %) of the main band and insignificant in-60 40 20 0 20 40 601E30.010.1q, arc.  sec . ca lc u lat ed ex pe r im en ta lln (I/I  ) 0.2 0.41012D ×d d/ 10  –4z , mµFig. 2. Experimental and calculated rocking curves for the part„2. (333) reflection of Cu˚α-radiation. The profile of strainsshown on the right60 40 20 0 20 40 60q, ar c. se c . c a lc u la te d e x p e r im e n ta lln ( /  )I I D ×d d/ 10  –4z , mµ1E30.010.1 0.2 0.4 0.601Fig. 3. Part „3. Notations are similar to those in Fig.2.60 40 20 0 20 40 60 c a lcu late d e x p e r im en ta lln ( /  )I ID ×d d/ 10  –4z , mµ1E41E30.010.1 0.2 0.4 0.610Fig. 4. Part „4. Notations are similar to those in Fig. 2.Z. Swiatek et al.: The influence of ion implantation by phosphorous on ...235SQO, 7(3), 2004crease (by 23 %) of intensity maximum were observed.The displacement of intensity to the longwave area andappearance of peak peculiar to porous silicon were re-vealed for the porous part implanted by phosphorus.The effect of luminescence fatigue  autocratic re-duction of the photoluminescence intensity at the sameconditions of excitation  was observed in the obtainedspectra (Fig. 5b). For 60 min, the intensity was reducedby half, and spectral band of luminescence was shiftedby 2535 µm. According to the majority of modern mo-dels, the spectral position of the visible luminescence bandfrom porous silicon depends on the cross-section of sili-con filaments with quantum sizes [17]. The reason ofphotoluminescence fatigue consists in the change ofconcentration of the centres of non-radiating recombina-tion. The same concentrations of non-radiating centrescause a different probability of recombination for differ-ent band components. The time and spectral positions ofband components are determined by structural featuresof porous layer, namely, by adsorbed molecules and vari-ous surface defects that cause randomized potential. Theinfluence of this potential on the carriers drift depends oncross-sections of quantum silicon filaments.6. Conclusions1. The quantitative differences of thickness depen-dences of strains, values of the maximal strain, distortionlevel and extension of elastic strains were revealed forthe parts of silicon sample subjected to implantation byphosphorus ions and chemical etching.2. The significant alternating-sign strains in direc-tion perpendicular to the surface are peculiar to treatedparts of silicon sample: ε ~ 1.5 ⋅ 104  for part „2, ε ~ 104for „3, ε ~ 1.2 ⋅ 104 for „4. The extension of strains incrystal depth is 300 nm and maximum of negative strainslocalised at the depth of ~400 nm.The average heights of roughnesses (Rà) of character-istic surface for the parts „14 are 0.17 nm, 0.288 nm,0.398 nm, and 0.303 nm, respectively.3. The porous layer obtained on silicon surface andmodified with previous implantation by phosphorus ionsis characterized by an essentially different photolumi-nescence spectrum in comparison with the spectrum fromthe usual porous layer. The changes of the spectrum in-tensity as well as the displacement of the spectral band tolongwave area are observed for porous parts of siliconsample under conditions of natural ageing for 24 months.References1. D. Buttard, G. Dolino, D. Bellet // Solid State Communica-tions, „109. pp. 1-5 (1999).2. E.V. Astrova, V.V. Ratnikov, R.F. Vitman // FTP, 31(5).pp. 1261-1268 (1997).3. D.N. Gorechev, L.V. Belekov, O.M. Sreseli // FTP, 34(6).pp. 1130-1134 (2000).4. E.V. Astrova, T.N. Vasyunkina // FTP, 36(5). pp. 593-596(2002).5. S.P. Zimin // FTP, 34(3). pp. 359-363 (2000).6. M.E. Kompan, V.E. Hartsiev, I.Yu. Shabanov // FTT, 39(12),pp. 2137-2140 (1997).7. U. Rissel, Õ. Ruge, Ionnaya implantatsiya. M.: Science, 1983.234 p.8. Z.T. Kuznitskiy // Neorganicheskiye materialy. 33(2). pp. 142-146 (1997).9. A.G. Kazanskiy, S.M. Petrushko, N.V. Rigov // FTP, 33(3)pp. 332-335 (1999).10. V. Chamard, C. Pichat, G. Dolino // Solid State Communica-tions, 118, pp. 135-139 (2001).11. M. Servidori, F. Gembali // J. Appl. Cryst., 21, pp.176-181(1988).12. M. Nemiroff, V.S. Speriosu // J. Appl. Phys., 58(10) (1985).13. A.V. Goncharskiy, A.V. Kolpakov, A.A. Stepanov // Metro-logiya, 11, pp. 19-23 (1986).14. S.A. Stepanov, E.A. Kondrashkina, A.N. Chuzo // Poverh-nost, 9. pp. 112-118 (1988).15. R. Fabbri, G. Lulli, R. Nipoti, M. Servidori // Nuclear Instru-ments and Methods in Physics Research B80/81, pp. 624-627(1993).16. I.M. Fodchuk, O.S. Kshevetskiy // Metallofizika, 14(5),pp. 57-62 (1992).17. S.I. Geludeva, B.G. Zaharov, M.V. Kovalchuk, V.G. Kon,E.A. Sozontov, A.I. Sosfenov // Crystallographiya, 33(6).pp. 1352-1356 (1988).l, µmb300 400 500 600 700 8000.20.40.60.81.0*10 porous partI, a.u. porous part*10porous part+ P implantation300 400 500 600 7000.00.20.40.60.81.0l, µmI, a.u.porous partporous part+ P implantation  = 60 mint    = 30 mint   = 3 mintaFig. 5. Photoluminescence spectra from different parts of siliconsample: a) just treated sample, b) after 24 months.

The influence of ion implantation by phosphorous on structural changes in porous silicon

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The influence of ion implantation by phosphorous on structural changes in porous silicon

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Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)