Dovbnya, A.N.

Dyomin, V.S.

Yefimov, V.P.

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

RADIATIVE PROCESSES OF AMORPHIZATION AND HYDROGENATION IN MONOCRYSTALLINE SILICONA.N. Dovbnya, V.P. Yefimov, V.S. DyominNational Science Center “Kharkiv Institute of Physics and Technology”,1, Academicheskaya Str., Kharkiv, 61108, Ukraine.PACS numbers: 61.82.-d, 61.82.Fk1  INTRODUCTIONThe creation of new composition materials for the production of the highly-efficient silicon photo-convert-ers with modified optical and electrophysical properties of Si-materials is achieved due to the amorphous-crys-talline phase transformation (a-Si/c-Si) and formation of a wide-zone cluster structure (a-Si:H). Strong and sharp electric fields gEBSF)(δ ∇−  are necessary for the con-trol of charge-carrier fluxes during their photo-genera-tion  in  the  semiconductor’s  bulk,  are  formed  by  a-Si:H(B,Al)/c-Si:H(B,Al)  interface  structures.  The method  of  amorphous-crystalline  transformation  and hydrogen-stimulated amorphization in  the bulk mono-crystalline  Si  structure  enables  the  development  of  a new technology for the creation of Si-photo-converters with a high conversion efficiency in a wide range of so-lar radiation wavelengths [1].  The main difficulties in the realization of this method are connected with a week migration of hydrogen at a great depth in the Si semi-conductor.  The  processes  described  will  form the  H-concentration at the required depth of Si semiconductor due spin splitting of Si:H compounds with the intensive electron beams and processes of the photo-stimulation of the volume diffusion. This will provide a continuous migration of hydrogen into the bulk material.2  FORMATION OF THE AMORPHOUS-CLUSTER STRUCTURESAmorphization of the c-Si structure is performed un-der irradiation with the intensive electron beam up to a doze of about 10-3 displacements per atom (d.p.a) in the dense xH4He ++ -plasma when the primary knocked-out atom acquires the energy ≥ε 5 keV. For the electron irradiation the primary energy of particles should be ε0 ≥ 10 MeV. Disordered domains are local defect clusters (50 to 1000 Å) surrounded by a space charge layer and blocking the flows of charge carriers (CC). At recoil en-ergy less than 5 keV (the threshold energy of forming the disordered structures) only the point defects are in-jected  into the  crystal.  At  point  disordering  of  lattice atoms the Frenkel pair components are being spaced for a distance minr , with 0aminr ≥  (a0 is the lattice con-stant). The threshold energy Ed of a primary knocked-out atom in silicon equals 140±5 keV. The concentra-tion of the amorphous phase is defined by correlation ( )00 εdrΦσndrN = , (1)where  =0n 55.19·1022 cm-3,  Φ  is the integral flux of the bombarding electrons with an energy of 0ε ,( ) ( )dεmaxεdrε dεεεdσεdrσ00 ∫= . (2)The differential cross sections of the electron scatter-ing on Si for different energies are calculated in [2]. Un-der one-pulse material irradiation with an intense elec-tron beam [3]  the temperature (T) of  the material  in-creases linearly and than falls down to the initial value( )xFSpCρτ2PT000⋅=  , (3)where 0P  is the electron beam power, τ  is the duration of the current  pulse,  0ρ  and  pC  are the density and heat  capacity  of  the  material  respectively,  0S  is  the electron beam area, ( ) ( )2/2l2xexpl21xF −=pi is the energy release profile, and l  is the depth of beam elec-tron penetration into the material. The re-crystallization of the structure is caused by the combinations of ther-mal effects which heat the material  up to the melting point 8.0∝  corresponding to the maximum crystalliza-tion  rate.  The  metastability  of  the  amorphous  state weakens when free charge carriers  are created due to splitting of covalent bonds. The annealing of point de-fects takes place at a temperature of  C300T ∝ . The partial crystallization of the amorphous Si takes place at a temperature of the order of C600T ∝ . To study the structure of a-Si, and a-Si:H materials, the methods of X-ray and electron diffraction as well as the combina-tion scattering of laser radiation are used. The IR spec-troscopy is used for the determination of the local con-figuration bonds of hydrogen. The abundance of chemi-cally bonded hydrogen atom influences on the degree of structure disordering, hence the energy value of the op-tical gap gE  in a-Si. On entering from 10 to 20 at. % of H into the a-Si structure, the value of gE  grows up to ( )eV1.81.7 ÷ .  The  c-Si(Al)-structure  forming  is  per-formed by the method of nuclear doping of Si<Al> us-ing  the  bremsstrahlung  gamma-quanta  with  energy 25MeVγE ≥   and irradiation doses 10-5 displacements per atom.13Al27/4.1s13Si28nβγ14Si28 →+→+ ,21413Al27pγ14Si28 +→+ .3  HYDROGENATION OF SILICONThe c-Si matrix hydrogenation needed for the amor-phous phase stabilization and recombination center neu-tralization is carried out in the course of the semicon-ductor irradiation with beam electrons in a hydrogen-he-lium plasma. The spatial hydrogen distribution over the semiconductor volume forms the profile of the a-Si/c-Si interface structure distribution in the matrix while the hydrogen concentration determines the value of electric field intensity. The diffusion is described in terms of hy-drogen  atom  capture  and  release  by  energy-bonded states defined as H traps. The capture of H leads to the creation of Si:H cluster structures.  The density of  the bond states changes depending on the H concentration, temperature and time. The average length of atom dis-placement is proportional to the distance between the re-gions of hydrogen capture. Point defects (vacancies, di-vacancies),  the  domains  of  the  disordered  structure, grain boundaries, and impurity atoms all they are traps. The diffusion of H takes place at the grain boundaries and between the lattice planes. At a low H concentra-tion,  the  cluster  dimensions are  restricted.  At  high H concentration the effective  diffusion is  determined by the capture of hydrogen atoms and their release from the cluster structures. The hydrogen migration in silicon and the profile of H concentration with the depth of material t)(x,nC  is described [4] by the formula:[ ]))erfc(x/S(tCt)(x,nC 0= , (4)where 0C  is the H concentration at the sample surface (x=0), x  is the running coordinate, 1/2t)eff(4DS(t) = , t is the diffusion time, and effD  is the effective diffu-sion coefficient. At high H concentration, the diffusion is independent on the degree of lattice disorder, and the atom arrangement in the lattice does not depend on the lattice structure. Weak Si-Si bonds will be splitted by H atoms, and only strong bonds will remain. As a result of the hydrogenation, the density of splitted spin couplings in silicon decreases. When splitting the weak bonds, the generation of spin-active defects takes place, and the de-fects by their passivation with H atoms form additional Si:H cluster structures. The Si(B) hydrogenation leads to the decrease of the electric conductivity of the struc-ture because of the neutralization of doped (B) atoms. The recovery of the electric conductivity occurs due to of  Al  atom production  in  the  process  of  the  nuclear transmutation of Si atoms. With increasing the sample thickness up to 1  μ m and at high H concentration, a good agreement is observed between the calculated and experimentally measured values of the cluster structure density.  However,  for  the  H  concentration  less  than 4·1018 cm-3 the diffusion concentration profile is already described by the damped exponent. The discrepancy be-tween  the  exponent  and  erfc  begins  in  the  region where the H concentration equals to the density of traps. The process of H migration into the solid-state Si is of activation  nature,  and  the  diffusion  coefficient  of  the process is determined by the expression)/kTaEexp(DeffD 0 −= . (5)The time (t) of the neutralization (passivation) of the splitted  spin  couplings  and  of  the  formation  of  Si:H cluster structures is determined by the expression)/kTaexp(Ett 0= , (6)where Ea is the activation energy, k and T are the Boltz-mann constant  and the temperature of the sample,  re-spectively, and =0t 10-11 s for Si: H compounds. At low H concentration, the energy level of deep traps is ap-prox. 1.7 eV and their concentration can reach approx. 1018  cm-3. As the H concentration increases, the activa-tion energy shifts to the neighboring peak of the density of states. The energy corresponds to the deep-occurring trap level of approximately 0.5 eV. The hydrogen mi-grates at the minimum of the concentration of the traps in the material determined by their energy state being between 0.5 and 1.7 eV. On capture of H by Si-Si bond centers, its diffusion into the depth of material ceases. In the material structure initially the deep-occurring traps and  then  the  shallow-occurring  ones  are  filled.  The equilibrium  H  distribution  in  the  shallow-occurring traps is established in the course of the diffusion. The higher times and temperatures are needed for a greater change  of  the  H  concentration.  The  H  concentration change  in  the  capturing  regions  is  equivalent  to  the change of hydrogen solubility and is determined by the chemical H potential between 1.2 and 1.3 eV. Accord-ing to the data of [4], the H concentration distribution drops from 1020 to 1018cm-3 in the thickness interval up to  0.3  μ m. But as is  shown in [2],  the c-Si  material thickness of approx. 500 μ m is needed for the full cap-ture of the solar spectrum radiation. Therefore, it is im-possible to solve by the existing means the problem of the H concentration diffusion for photocells. The forma-tion of the H concentration at the required depth in the Si material can be achieved only by the forced destruc-tion of the spin couplings of the compounds with the help of an intense electron beam thereby ensuring the continuous migration of hydrogen into the material. For this purpose the Si material is irradiated with the intense electron beam in the dense 4He + Hx+ plasma. The beam electrons shift the lattice and form additional radiation H traps while the ions  4He and Hx+ diffuse into the Si material. The  4He prevents from the recombination of the shifted lattice atoms with vacancies and intensify the c-Si structure amorphization process. The primary de-fects and radiation defects of the structure are neutral-ized by the hydrogen atoms with the formation of Si:H and H2 bounded states. The beam electrons destroy the spin  couplings  of  the  created  structures,  and  released hydrogen migrates into the material depth until it will be subsequently captured by H traps. The hydrogen-release process takes place only on the radiation treatment of the material by the electron beam. With the termination of the irradiation process, the H migration into the Si material stops. In this way, it is possible to form the de-sired profile of the H concentration distribution over the  ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2001. №5.Серия: Ядерно-физические исследования (39), с. 215-216.215Si material depth for the creation of the required distri-bution shape of the pulling ( δ -BSF) gΔE  electric field in the semiconductor [5].4  PHOTOSTIMULATION OF THE BULK DIFFUSIONOne  of  the  possible  methods  of  excitating  the phonons which stimulates the diffusion in solids is elec-tromagnetic  (laser)  irradiation.  The  photon-to-phonon energy  transfer  can  be  realized  by  IR-absorption  or stimulated combination scattering. Besides, the direct or indirect  absorption  at  defect  centres  is  possible.  The process of the photon-phonon interaction that leads to the stimulation of migration of atoms or ions consists of three stages [6]:• Photon conversion into the optical phonon;• Nonharmonic interaction that leads to the decay of long-wave longitudinal phonons created by relaxa-tion  of  hot  charge  carriers  on  longitudinal  and transversal acoustic phonons with energy-momen-tum conservation in  each event;• Transfer of the pseudo-momentum from acoustical phonons to atoms or  ions in the longitudinal  and transversal directions.The amplitude of the lattice atom vibrations caused by photon-phonon interaction and photon-to-phonon en-ergy transfer increases, and, therefore, the probability of doped  atom  migration  increases.  Acceleration  of  the atomic migration results  in increasing the temperature of solids from T to Teff, where Teff is the effective tem-perature that is created by photoexited and thermoexited phonons.  As is  known the temperature dependence of the coefficient of atom diffusion in solids is defined by Arrenius correlation (5). In the case of one-photon ab-sorption with subsequent decay of  the optical  phonon into acoustic ones, the exponent in eq. (5) can be repre-sented in the form [7]:( )[ ]∑ ++=s1ensn21effkTaE , (7)where ns and ne are the numbers of thermal and photoex-cited photon, respectively. One can see from formula (7) that excitation by photons ne increases Teff and, conse-quently,  increases  the  diffusion  coefficient  of  atoms. The formula of the diffusion coefficients with photoex-cited phonons (De) and without them (D) is defined by the formula:.effT1T1kaEexpDeD −= (8)The intensity of the required flux (Ne) of the laser ra-diation, that is necessary for observation of IR-absorp-tion influence on the diffusion of doping atoms in Si, is defined  by  the  value  == /VscceneN  =1027 photon/cm2,  where  cen  is the critical number of phonons excited by laser radiation, in the range of an el-ementary cell,  sc  is the velocity of sound. In the thin silicon  rod  the  velocity  of  sound  is  ==ρEsc1.1·106 cm/sec, where E is the Young’s modulus,  ρ  is the density of the material, V is the volume of the Si atom (for the Si atom radius =0r 1.34 Å, =V 1.03·10-23 cm3).  The  focused  laser  +3Nd:AIG  beam  of 108 W/cm2 produces  the  flux  of  the  order  of  1031 photon/cm2 and  the  flux  of  the  order  of 5·1026 photon/cm2 without  focusing.  Thus  using IR-lasers one can excite acoustic phonons and stimulate migration of doping atoms into the crystal in the bulk of Si. Developed processes of radiation disordening of lat-tice atoms and laser photo-stimulation of the hydrogen migration in the bulk of Si semiconductor allow to form amorphous-crystal  structures  of  high  effective  photo-cells for solar energetics.REFERENCES1. A.N.Dovbnya,  V.P.Yefimov,  S.V.Yefimov, A.N.Sleptsov.  Photoconverters  with  Heterointer-face Structure Powerful Electrical Systems // Prob-lems of Atomic Science and Technology. Issue: Nu-clear-Physics  Research (34).  1999,  v.  3, p. 113-114.2. A.N.Dovbnya, V.P.Yefimov, S.V.Yefimov. Radia-tion transformation of photovoltage material struc-ture // Problems of Atomic Science and Technology.  Issue: Physics of Radiation Damage and Science of  Radiation  Materials.  1999,  v.  1(73),  2(74), p. 143-147 (in Russian).3. V.V.Gann, A.I.Shukov, V.A.Kushnir, M.A.Krasno-golovets. Excitation of the Sound Waves in the Irra-diated  Electron  Beam  Materials  //  Problems  of  Atomic Science and Technology. Issue: Physics of  Radiation Damage and Science of Radiation Mate-rials. 1998, v. 3(69), 4(70), p. 51-52 (in Russian).4. N.H.Nickel, W.B.Jakson, J.Walker. Hydrogen Mi-gration in  Polycrystalline Silicon //  Phys.  Rev.  B. 1996, v. 53, N 12, p. 7750-7761.5. V.P.Yefimov,  V.Ye.Strelnitski,  S.V.Yefimov.  In-terface structures, Diamond Coatings and Their Ap-plication  //  Telecommunications  and  Radio  Engi-neering. 2001, v. 55 (4), p. 84-93.6. S.A.Ahmanov,  V.I.Emel’yanov  et  al.  Influence powerful laser radiation on the surface of the semi-conductors  and  metals  //  Uspekhi  Fizicheskikh Nauk. 1985, v. 179 (4), p. 675-745 (in Russian).7. T.D.Dzhafarov.  Photo-stimulation  of  the  atomic processes  in  the  semiconductors.  Moscow:  Ener-goatomizdat, 1984, 134 p. (in Russian).216

Radiative processes of amorphization and hydrogenation in monocrystalline silicon

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Radiative processes of amorphization and hydrogenation in monocrystalline silicon

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