K. von Maydell

N. H. Nickel

Nickel N. H.

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Hydrogen equilibration in polycrystalline silicon

von Maydell, K.

Nickel, N.H.

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Hydrogen equilibration in polycrystalline siliconK. von Maydell and N. H. NickelaHahn-Meitner-Institut Berlin, Kekuléstr. 5, D-12489 Berlin, GermanyReceived 23 January 2007; accepted 19 February 2007; published online 29 March 2007Hydrogen equilibration in polycrystalline silicon was investigated as a function of annealing timeand temperature using electron spin resonance and hydrogen effusion measurements. During avacuum anneal at least 1.51021 cm−3 H atoms are mobile in the lattice, however, only about3.71018 cm−3 H atoms passivate Si dangling bonds. The results show that the annealing treatmentcan cause the vast majority of H atoms to accumulate in H stabilized platelets. Since defectpassivation preferentially occurs at grain boundaries and platelet nucleation and growth are confinedto the interior of single-crystal grains, H equilibration is governed by two spatially separatedprocesses. Moreover, the data demonstrate that the hydrogen density-of-states distribution isdynamic and changes in response to experimental parameters. © 2007 American Institute ofPhysics. DOI: 10.1063/1.2716348The properties of hydrogen in silicon have attracted agreat deal of interest.1 This manifests itself in the neutraliza-tion of dopant atoms and the passivation of localized defectssuch as Si dangling bonds. The latter attribute is commonlyutilized to improve the electrical and optical properties ofpolycrystalline silicon poly-Si, which contains a high con-centration of Si dangling bonds at grain boundaries.2 On theother hand, prolonged exposure of poly-Si to monatomic Hcan cause deleterious effects such as the formation of elec-trical and structural defects.3,4In poly-Si identical crystallites are separated by grainboundaries. These two-dimensional extended defects containdeep and shallow localized states. Posthydrogenation treat-ments at moderate temperatures e.g., 250–450 °C reducethe concentration of both classes of localized states and thekinetics of the grain-boundary passivation are consistent withdiffusion limited passivation.2 In addition to the passivationof localized states it has been shown that prolonged exposureof poly-Si causes deleterious effects such as the formation ofacceptor-like states that leads to electrical-type conversion3and the formation of hydrogen stabilized platelets.4 The lattereffect is controlled by the Fermi energy.5In this letter, we present experimental evidence thatmerely the migration of hydrogen in poly-Si is sufficient tosimultaneously cause beneficial and deleterious effects. Asimple vacuum anneal results in a decrease of the Sidangling-bond concentration and the formation of largeplatelet-like structures. While the passivation occurs prefer-entially at grain boundaries the formation of platelets is con-fined to the interior of the grains. Hydrogen equilibrationinvolves about 30% of all incorporated H atoms.Polycrystalline silicon thin films were prepared by thefollowing procedure. First, hydrogenated amorphous siliconthin films were grown by rf glow discharge decomposition ofsilane SiH4 at substrate temperatures ranging from160 to 335 °C. Then the specimens were crystallized atroom temperature and in high vacuum using the 308 nm lineof an excimer laser and employing the step-by-step crystal-lization technique.6 The resulting poly-Si films revealed anaverage grain size of 100–200 nm. Information on the defectdensities was obtained from electron spin resonance ESRmeasurements and the hydrogen density-of-states H DOSwas deduced from H effusion data as described in Ref. 7.Figure 1 shows ESR spectra of laser crystallized poly-Sibefore blue spectrum and after an additional vacuum annealat 400 °C for 30 min red spectrum. Both resonances ex-hibit a g value of 2.0055 that is commonly attributed to Sidangling-bond defects.8 From the ESR spectra a Si dangling-bond concentration of NS=6.21018 cm−3 was estimated foras-crystallized poly-Si. This is a typical value for the spindensity of completely crystallized poly-Si.9 A post-vacuum-anneal at TA=400 °C for 30 min resulted in a decrease of theintensity of the Si dangling-bond resonance red spectrum,and hence a decrease of the dangling-bond concentration toNS=3.81018 cm−3.The time and temperature dependence of NS is shown inFig. 2. For all poly-Si specimens prepared by laser crystalli-zation of hydrogenated amorphous silicon a-Si:H a de-crease of the Si dangling-bond density is observed after apost-vacuum-anneal triangles, squares, and circles in Fig.2. With increasing annealing temperature TA, the time tominimize NS decreases and at long annealing times tA, the SiaElectronic mail: nickel@hmi.deFIG. 1. Color online Electron spin resonance spectra of polycrystallinesilicon at 300 K. The blue spectrum shows the silicon dangling-bond reso-nance after laser crystallization. The spectrum shown by the red curve wasobtained after the sample was subjected to a post-annealing treatment for30 min at 400 °C and in vacuum.APPLIED PHYSICS LETTERS 90, 132117 20070003-6951/2007/9013/132117/3/$23.00 © 2007 American Institute of Physics90, 132117-1Downloaded 16 Jul 2007 to 134.30.4.55. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jspdangling-bond concentration saturates. Moreover, with in-creasing annealing temperature the number of additionallypassivated Si dangling bonds increases from 91017 cm−3 atTA=200 °C to 2.71018 cm−3 at TA=400 °C.The observed decrease of NS can be due to either struc-tural changes at the grain boundaries or the activation ofresidual hydrogen leading to the formation of additionalSi–H complexes. To clarify which of these two processes isresponsible for the defect passivation during post-vacuum-anneals, poly-Si specimens were prepared from sputter de-posited amorphous silicon. It is important to note that thesesamples contain a hydrogen concentration of less than0.1 at. %. After crystallization the samples exhibit adangling-bond concentration of NS=1.31019 cm−3. Thisvalue is about a factor of 3 larger than for samples where thestarting material was hydrogenated amorphous silicon. Asubsequent vacuum anneal for up to 16 200 s did not resultin the reduction of the dangling-bond concentration full dia-monds in Fig. 2. This clearly demonstrates that the observeddecrease of NS is due to the activation of residual H and thesubsequent passivation of Si dangling bonds.The inset of Fig. 2 shows the total change of the Sidangling-bond concentration, NS, as a function of the an-nealing temperature for poly-Si samples where the amor-phous starting material was deposited at the indicated sub-strate temperatures TS. Independent of the depositiontemperature of the starting material the number of passivatedSi dangling bonds increases with increasing TA. The largestvalue of NS=3.71018 cm−3 is obtained at an annealingtemperature of 400 °C for poly-Si where the a-Si:H startingmaterial was deposited at TS=230 °C. When the starting ma-terial is deposited at higher substrate temperatures circles inFig. 3 the decrease of the Si dangling-bond concentration isabout a factor of 4–6 smaller than in specimens deposited atTS230 °C. Most likely, this is due to the fact that the Hcontent of the amorphous starting material decreases withincreasing substrate temperature.10To elucidate the underlying microscopic mechanisms thehydrogen density-of-states distribution was derived from Heffusion measurements. The H DOS of “as-crystallized”poly-Si and after different post-crystallization treatments areshown in Fig. 3. The energy scale was adjusted to a relativescale where the zero of energy corresponds to a free H atom.Also shown are the energies per H atom of a number of Hcomplexes in crystalline silicon c-Si according to Refs. 11and 12. The influence of a post-vacuum-anneal of poly-Si atTA=350 °C for 2.5 h on the H DOS is shown in Fig. 3a.The as-crystallized poly-Si sample exhibits a broad hydrogendistribution between E=−2.5 and −3.7 eV red curve. How-ever, a simple post-vacuum-anneal causes a spectacularchange of the H DOS, namely, the formation of a pro-nounced peak located at E=−2.9 eV blue curve. By com-paring the position of the peak to the calculated H bindingenergies of hydrogen complexes in c-Si it can be assigned toH stabilized platelets. This is in accordance with a previousreport, which showed that platelets form preferentially in un-doped and n-type poly-Si.4Further insight into the passivation mechanism was ob-tained by comparing the influence of the vacuum anneals onas-crystallized to post-hydrogenated poly-Si. Therefore, a la-ser crystallized specimen was exposed to monatomic H in anoptically isolated remote plasma system for 1 h at 350 °C.Next, one-half of the sample was annealed in vacuum for2.5 h at TA=350 °C. Then the H DOS was derived from Heffusion measurements red curve in Fig. 3b. Post-hydrogenation did not alter the shape of the H DOS, how-ever, the total H concentration increased, which is consistentwith previous results.2,13,14 On the other hand, the additionalvacuum anneal also resulted in the formation of a pro-nounced peak located at E=−3.04 eV. The energetic positionof the peak suggests that it also originates from H stabilizedplatelets. Analyzing the area under the two peaks reveals thatabout 1.51021 cm−3 H atoms are accommodated in plate-lets after the vacuum anneal at TA=350 °C.The experimental data clearly show that hydrogenequilibration is governed by two distinctively differentmechanisms, namely, the passivation of Si dangling bondsand the formation of platelets. Both processes occur simul-taneously and are driven by H motion in the host lattice.According to the data shown in Fig. 3 the samples containabout 51021 cm−3 H atoms. After the vacuum anneals, atleast 30% of the H atoms have migrated in the specimens.However, only a small fraction of about 3.71018 cm−3 Hatoms have passivated additional Si dangling bonds. The ra-tio of H atoms passivating grain-boundary defects to H at-oms accommodated in platelets is a measure for the passiva-tion efficiency . For postannealing at TA=350 °C apassivation efficiency of 2.510−3 is obtained. This in-dicates that the migration of about 400 H atoms is required topassivate one Si dangling bond. For plasma passivation ofpoly-Si a much larger passivation efficiency of 0.055 isobserved.2The data show that most hydrogen atoms are accommo-dated in poly-Si without passivating preexisting defects.However, this requires that H be accommodated as diatomicFIG. 2. Density of silicon dangling bonds, NS, vs annealing time tA forvarious indicated annealing temperatures TA. The triangles, squares, andcircles represent data that were obtained from poly-Si where the initial ma-terial was hydrogenated amorphous silicon. The diamonds represent datawhere the initial amorphous Si film was grown by sputter deposition. Thesolid lines are fits to an exponential decay. The inset shows the total changeof the Si dangling-bond concentration, NS, as a function of the annealingtemperature TA. TS denotes the substrate temperature at which the a-Si:Hstarting material was deposited.132117-2 K. von Maydell and N. H. Nickel Appl. Phys. Lett. 90, 132117 2007Downloaded 16 Jul 2007 to 134.30.4.55. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jspcomplexes. Possible sites include strained Si–Si bonds neargrain boundaries and interstitial molecular H2. The passiva-tion of strained Si–Si bonds occurs by the formation of H2*complexes. These complexes consist of one H in a Si–Sibond center site forming a Si–H band and the second Hresiding in the antibonding interstitial site forming a secondSi–H bond.15 Since the main constituents of poly-Si are crys-talline grains and grain boundaries, the difference in passiva-tion efficiency must be related to the binding energy of H inthe reservoir. While  is significantly smaller after laser crys-tallization the binding energies of H in the reservoir must belarger than after plasma passivation. This is consistent withthe binding energies for H2* and H2 that amount to −1.65 and−1.92 eV, respectively. Hence, it is conceivable that lasercrystallization of a-Si:H causes the formation of molecularH2 while posthydrogenation preferably leads to the creationof H2* complexes.Although defect passivation and platelet formation hap-pen simultaneously, both effects occur spatially separated.ESR experiments performed on doped poly-Si showed thatdangling bonds are predominantly located at the grainboundaries.16 On the other hand, a detailed investigation onthe formation of platelets in poly-Si revealed that these Hstabilized two-dimensional structures form only in the inte-rior of the grains. Moreover, the presence of lattice strain inthe form of grain boundaries inhibits the formation ofplatelets.4 Thus, our data clearly show that H equilibration isgoverned by two spatially separated processes. Moreover, thedata in Fig. 3 clearly show that the H DOS is dynamic.Hydrogen creates its own traps preferably by forming multi-hydrogen clusters because of their low entropy. Since a con-siderable concentration of H atoms is required to account forthe peak in the H DOS, it can be concluded that the forma-tion of H stabilized platelets requires long range hydrogenmotion.In summary, we have presented direct experimental evi-dence that hydrogen equilibration in poly-Si is governed bytwo spatially separated processes, namely, defect passivationat grain boundaries and the formation of platelets within thegrains. H equilibration involves about 1.51021 cm−3 H at-oms of which only about 3.71018 cm−3 H atoms passivatedSi dangling bonds. The vast majority of H atoms are accom-modated in platelets. Moreover, the data clearly show thatthe H DOS is dynamic and changes in response to experi-mental parameters.1N. H. Nickel, Hydrogen in Semiconductors II Academic, San Diego,1999, Vol. 1.2N. H. Nickel, N. M. Johnson, and W. B. Jackson, Appl. Phys. Lett. 62,3285 1993.3N. H. Nickel, N. M. Johnson, and J. Walker, Phys. Rev. Lett. 75, 37201995.4N. H. Nickel, G. B. Anderson, and J. Walker, Solid State Commun. 99,427 1996.5N. H. Nickel, G. B. Anderson, N. M. Johnson, and J. Walker, Phys. Rev.B 62, 8012 2000.6P. Lengsfeld, N. H. Nickel, and W. Fuhs, Appl. Phys. Lett. 76, 16802000.7N. H. Nickel and K. Brendel, Phys. Rev. B 68, 193303 2003.8N. M. Johnson, D. K. Biegelsen, and M. D. Moyer, Appl. Phys. Lett. 40,882 1982.9P. Lengsfeld and N. H. Nickel, in Laser Crystallization of Silicon, editedby N. H. Nickel Elsevier Academic, Paris, 2003, Vol. 75, 119.10W. Beyer, in Hydrogen in Semiconductors II, edited by N. H. NickelAcademic, San Diego, 1999, Vol. 61, 165.11C. G. Van de Walle, Phys. Rev. B 49, 4579 1994.12W. B. Jackson, Curr. Opin. Solid State Mater. Sci. 1, 562 1996.13N. H. Nickel and I. E. Beckers, Phys. Rev. B 66, 075211 2002.14N. H. Nickel, W. B. Jackson, and J. Walker, Phys. Rev. B 53, 77501996.15K. J. Chang and D. J. Chadi, Phys. Rev. Lett. 62, 937 1989.16K. Brendel, N. H. Nickel, K. Lips, and W. Fuhs, J. Non-Cryst. Solids338-340, 262 2004.FIG. 3. Color online Hydrogen density-of-states distribution of laser crys-tallized poly-Si. The data plotted in a show the influence of a post-vacuum-anneal at TA=350 °C for 2.5 h on the H DOS while b shows theeffect of post-hydrogenation at 350 °C for 1 h followed by a vacuum annealat TA=350 °C for 2.5 h. The data are adjusted to a relative energy scalewhere zero energy corresponds to a free H atom. Calculated energies of Hcomplexes in single-crystal silicon are shown Refs. 11 and 12.132117-3 K. von Maydell and N. H. Nickel Appl. Phys. Lett. 90, 132117 2007Downloaded 16 Jul 2007 to 134.30.4.55. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

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Hydrogen equilibration in polycrystalline silicon

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