Prakash, Om

Singh, Shyam

IACS Institutional Repository

Indian J. Phys. 71A (4), 453 ^6 0  (1997)U P A—  an international journalThermal d on or form ation  in C Z -silicon  annealed  at 450°C in  a ir am bientOm Prakash and Shyam SinghDepartment o f  Physics, G. B. Pant University o f Agriculture &. Technology, Panlnagar-263 145, IndiaR e c e iv e d  16  A p r i l  1 9 9 6 , i i c c e p l e d  3 0  J a n u a r y  1 9 9 7A bstract : Exact kinetics o f donor formation and possible number o f oxygen atoms in a single thermal donor (T D ) are tw o aspects out o f  many problems related with oxygen related donors in C Z -silicon. which carry a mark o f  interiegalion. Boron doped p-type silicon wafers were annealed in air ambient at 450°C  for different durations and subjected to resistivity and KT1R studies It w as observed that ambients do not affect the process o f  TD generation S u ccessive increase in annealing tim es results in the exponential ginwth o f donors with a maximum o f - I  79 x  10 * 7  t m ^  obtained in oui samples annealed for 55 hours Annealing also caused a gradual decrease in absorption coefficient Maximum observed value o f oxygen and carbon precipitations was 2.362 ppma and I l(K) ppma respectively The diffusion coefficient for oxygen was found to b e -4 .1 7  x  l(T , 9 cm 2  s ' 1 O xygen and carbon reduction followed second order kinetics The activation energy for diffusion o f oxygen was found to be 823 eV and the number o f  oxygen  atom s involved  in a single TD formation appears to be seven from our experimental investigationsK eyw ords : Thermal donor, annealing, air ambient PA C S N os. : 71 55 Cn, 6 6 .30Jt. 61.72 Cc1. Introductiont /.ochrulski (CZ)-grown silicon (Si) is commonly used in microelectronic devices. It is well known that dissolved oxygen in silicon changes from an electrically neutral slate into a shallow donor (the so called thermal donor, TD) hy heat treatment around 450°C and this TL) disappears by annealing at 500°C. These thermal donors are supposed to be oxygen complexes with an increasing number of oxygen atoms being attached to them during heat treatment. Formation rates of these TDs are strongly dependent upon the processing temperature and upon the concentration of oxygen, typically of the order of r=I019 cm-1 in CZ Si. TDs are generally undesirable because these TDs prevent determination of the quantity of intentionally added dopant. The study of low temperature (400°C < T < 500°C)© 1997 I ACS7IA(4)-|0454 Om Prakash and Shyam Singhannealing process of CZ-grown silicon is of increasing importance for prospective applications of low temperature cycles to device technologies as well as for fundamental researches on the behaviour of oxygen in silicon.Even more than four decades of intensive research following their discovery [\\} chemical nature and the atomic structure of oxygen related donors (TDs) are not well understood. TDjc° (jc = 1 -  11) and nine singly charged TDx* (jc = 1 -  9) species have been observed in IR spectra [2]. Five new effective-mass-like TDjc° (jc = 12 -  16) species have also been observed [3]. These defects are actually with mean activation energies of 0.07 anil 0.15 eV for ionisation of charge states [4]. It has recently been established that accelerated diffusion of oxygen impurity atoms occurs in the temperature range 300-500 C corresponding to the formation and thermal stability of oxygen related donors [5].Infrared (1R) absorption has been a primary integrating tool for oxygen in silicon Localised vibrational modes for oxygen at interstitial sites in Si introduce a prominent absorption band at 1106 cm-1 and 605 cm"1 at room temperature respectively. Oxygen aggregate modes for TD defects have been followed from observations that several oxygen atoms are lost from interstitial sites per electrically active donor formed during annealing near 400°C [6J. The intensities for Si-O bands related to aggregates will be weak because only a fraction of the dispersed oxygen precipitate in aggregate formation. Mpieovn annealing time or temperature to increase the TD concentration introduces a broad spectium of the TD family members which can cause band overlap and broadening ol Si—0 absorption hands for the aggregates.A critical review of the relevant literature reveals that the present state of knowledge on the two issues viz. exact kinetics of the TD formation and the number of oxygen atoms involved in a single TD, is still surrounded by an air of uncertainty and one has to go a long way in order to have a widely accepted solution. With this sole motivation, a few results are reported in this article.2. Material and methodsC/ochralski grown boron doped (B = 0.4 x 1016 cm-3) silicon wafers used in the study wcic 400-470 |im thick with 80 mm diameter, as cut etched, <100> orientation and resistivity ranging irom 80-100 ohm-cm. These wafers were cut down into small pieces of the si/.c I x 2 cm2 and then subjected to heat treatment in air ambient at a constant temperature ol 450°C.2.1. Resistivity measurement and donor generation :The resistivity of silicon wafer was measured with a col linear four probe array (van der Pauw method) at room temperature and the number of carriers derived from Irvin's curve [71. Afterwards, the wafer was given desired heat treatment in air ambient and subsequently cleaned in hydrofluoric acid in order to eliminate the surface oxide. The resistivity of the annealed samples was again measured and the corresponding number ol charge carriers derived from the Irvin's curve. Assuming that the mobility remains constant,Thermal donor formation in CZ- silicon annealed at 450°C etc 455ihc difference of the carrier concentration between un-annealed and annealed samples gives ihe donors generated or annihilated during the heat treatment.-> 2 Oxygen and precipitated oxygen :Absorption coefficient needed to compute the oxygen content in the samples has been derived from 1106 cm-1 band by FTIR measurements using the procedure laid down by li/uku el at (8| The value of the absorption coefficient so obtained helped to obtain the interstitial oxygen concentration using the expression O, = (3.03 ±0.02) x I0 '7 olq c n r \  The initial oxygen concentration in the test wafer was measured without any heal treatment mi it. Allerwards, the wafer was given a specific heat treatment and calculated interstitial oxygen concentration. The difference between the two is attributed to precipitated oxygen during heal treatment.In ihe low temperature range (< 500°C) the rale of loss of interstitial oxygen (O,) should be related to trapping of diffusing O, atoms by other such atoms. O, dimers arc It tuned in this process.()| + O, h  Oz.Assuming that the backward reaction rate is negligible in comparison to (he forward icaciion rate, the kinetics of O, loss would be controlled solely by the interaction rate of two undomly diffusing O, atoms.dlO, 1/dt-  HtcDR[Oi ]2,wheie. R is the capture radius associated with dimer formation and D is the diffusing loelliueni lor oxygen. Integrating the above expression, we getI[0,11-------  + KnDRt[ 0 >HI C ( i r , - C ( « ) - 1 = 8 nDRt,wheie, ('(i) 1 and C(o)“' are reciprocal values of oxygen concentration prior to the anneals and alter anneals for t time. Plot of C(iY' -  C(o)_l versus time are therefore expected to be linear with a gradient equal to 8kDR.3. Results and discussion1 t Donor generation as a function o f annealing time :Resistivity measurement on the samples annealed for different durations have been performed to ascertain carrier concentration and in turn, donor concentration. Annealing also helped in reducing the inhomogeneities present in the unannealed samples up to a large extent. The resistivity variation as a function of annealing time is shown in Figure 1. The resistivity of the sample annealed upto =il hour, increased; but thereafter, for longer annealing period, the resistivity fell off exponentially. This behaviour clearly showed that456 Om Prakash and Shyam Singhp -type sample was converted to n-type due to generation of thermal donors [91. The number of donors generated due to annealing at 450°C is plotted as a function of annealing time in Figure 2. Donor generation was very rapid in the early stages of annealing but decreased0 10 20 30 i*0 50 60ANNEALING TIME AT «*50°C (h o u r s )Figure 1. Resistivity v s  annealing tune at 450°C.10 20 30 i*0 50 60ANNEAL ING  T IM E AT <*50 °C ( h o u rs  )F igure 2. Donor g c i/’ralion v s  annealing time at 450°Clater on. Our results are in agreement with Capper et al [10k Cazcarra and Zunino [11]' Cleland [12], Kumar and Singh [13], and Mathiot [14]. The hole concentration in unannealed sample was found to be 1.2 x lO1* c n r \  Donor concentration was found to decrease in the early stage of annealing (upto 1 hour) but it increased exponentially as a function of annealing time later on. However, even tor annealing times of 55 hours, the maximum in donor generation was not attained. As per trend of the curve ^5 hour, itThermal donor formation in CZ-silicon annealed at 450°C etc 457could he concluded that longer annealing durations would bring a saturation in donorconcentrations.Precise composition of the donor has been and still is a matter of debate. In his first ever model on the subject, Kaiser et al [15] assumed that in the 450°C region S i04 complexes are donors while those with more oxygen atoms are neutral. Some complexes with less than four oxygen atoms are also donors. They explained the maximum donor concentration as being the equilibrium between S i04 donors and neutral S i05 and SiOfl complexes. The donor concentration falls after attaining a maximum of the annealing time is further increased. This result was explained by assuming that more SiOs and S i06 units tuc produced from S i04 units [15]. This model could satisfactorily explain the stale of know ledge. Since then many models, varying in nature, have been put-forth. Silicon inierslitialcy as nucleus and surrounded by O-aloms is more befitting.2 FTIR results :FT1R measurements, performed on two types of samples : one with high oxygen and carbon content and the other with moderate oxygen and carbon content, were used to determine absorption coefficients corresponding to 1106 cm-1 and 605 cm-1 respectively for oxygen and cuihon in the un-annealed and annealed samples. A smooth and gradual decrease in the Milues of absorption coefficients of oxygen and carbon was noticed.The fractional change of the IR absorption coefficient (Xq as a function of annealing hint* al 450°C is shown in Figure 3. Measurement of absorption coefficients at 1106 cm 1Figure 3. The fractional change o f IR absorption coefficient as a function o f annealing tune at 450°Cicflccted a decrease in isolated interstitial oxygen during annealing treatment. However, ihere is no direct way of measuring the fraction of oxygen precipitated, as a broad peak due u> ihc precipitation, is superimposed on the 1106 cm '1 peak [16]. We also observed a weak and broad IR absorption peak at -1012 cm"1 in the FTIR spectra of annealed samples that458 Om Prakash and Shyam Singhcontain high concentration of thermal donors. This absorption may orginate from Si-o vibration in cores of thermal donors.3.3. Effect of carbon and ambient on donor formation :Carbon and oxygen precipitation increased with increase in annealing time in our experiments [ 111. Increase in the carbon precipitation with annealing time indicated a decrease in substitutional carbon concentration in annealed samples with increasing annealing lime. In high carbon content samples, the donor formation was suppressed. This can be explained by the formation of stable CO complexes or oxygen precipitates, which resulted in the permanent lowering in the number of interstitial oxygen (O,). Interstitial oxygen atoms expand the lattice whereas substitutional carbon atoms produce contraction When both impurities are present, there is clearly a degree of strain compensation and there arc direct infrared (1R) observations of G-O pairing, where the carbon atom is a nearest neighbour of the two Si atoms bonded directly to the O, atom [17]. We also found that the ambient docs not have any effect on the results as also opined by Bean and Newman [ 18|3.4. Diffusivity o f oxygen atom : \Differences of the reciprocal values of the actual oxygen concentration [C(/) 1 ] and initial oxygen concentration [Cfu)-1] is plotted in Figure 4. A straight line is obtained suggestingFigure 4. Plot o f  the differences o f ihe reciprocal values o f the actual oxygen  concentration (C (0 _ l ) and initial oxygen concentration (C(r>)- 1 ) v e r m s  annealing time for T = 450°Cthat the oxygen reduction can be described by second order kinetics [19]. From the gradient of the straight line equal to 8kDR, the diffusion coefficient D has been determined to be =4.17 x I0“19 cm2 s_1. Here, R is the distance at which the oxygen atoms bind together to form a complex. R = 5 x 10~8 cm was used in the calculation [20]. The value of D so obtained was substituted in the expression D = 0.17 exp (-E  /  kT) cm-2 s~l, which yields theThermal donor formation in CZ-silicon annealed at 450°C etc 459wiluc of aclivation energy for the diffusion of oxygen to be 0.823 eV. Even lower activation energies for oxygen diffusion as compared to the above value, viz, E0 = (0.7 ± 0.2) eV was lound 1201 for annealing at temperatures between 525 and 600°C. This low aclivation energy can be explained by sinks of diffused oxygen, or more likely, by interactions of the dil I used oxygen with that of other species like self-interstitial having a sufficiently high diflusivity at these temperatures.? 5 Average number o f oxygen atoms involved in a single TD :F ,om our experimental results, we have tried to find out the average number of oxygen atoms involved in a single TD. For this purpose, we took seven different samples of low Llirbon content and annealed these samples at 450°C for 55 hrs. By comparison between the numbci of donors generated and the amount of oxygen reduced, we found that it lakes an i\eragc ol about seven oxygen atoms for a single TD formation. There may be some loss of O, due to the formation of carbon-oxygen complexes and also due to the formation of some oxygen related complexes (centres) that are no longer electrically active. However, we have med to minimise the chances of forming carbon and oxygen complexes by taking low wirhon content samples.The average number of 7 oxygen atoms for a single TD formation is more logical as also opined by Lindstrom et al [21] and recently by McQuaid et al [22].4. SummaryTo get a glimpse of the outcome of the problem under consideration, we itemise the main lindings ol the present study as follows :1 The resistivity of the sample increased initially upto 1 hour of annealing, then decreased exponentially suggesting that the sample changed from p -type to w-type lor longer annealing duration due to the donor generation. Inhomogenilies present in the sample got reduced after annealing.2 Carrier concentration increased exponentially with increasing annealing time and became almost saturated for longer annealing times.3 Oxygen precipitation can be described by second order kinetics. The activation energy for the diffusion of oxygen comes out to be -0.823 eV.4. Average number of oxygen atoms involved in a single TD comes out to be 7.AcknowledgmentWe express our sincere thanks to Dr. R K Purohit and Dr, D R Sagar, Senior Scientists at Solid State Physics Laboratory, New Delhi for their help in FTIR measurements.ReferencesMl C S Fuller, J A Duzcnberg, N B Hanny and E Buehler P h y s  R e v . 96 833 (1954)I2J B Pajot, H Com pain, J Leroucillc and B Cleijand P h y s ic a  ( U tr e c h t )  B 117/118 1 10 (1983)HI W Gotz and G Pensl P h y s , R e v . B47 4 3 12 (1992)14] Proceedings o f the 13th Ini. Conf. on Defects in Semiconductors cds. L C Kimeritng and J M Parsey (jrj(The Metallurgical Society of American Institute of Mining) p 129, p 639 (1985)[5) G F Cerofilini, S Bertony, P FumaGally, L Meda and C Spaggiari Phys. Rev. B47 10174 (1993)|6| P Wagner and J Hage Appl. Phys. A49 123 (1989)[71 S M Szc and J C Irvin Solid State Electronics 11 599 (1968)18] T lizuka. S Takasu, M Tajima, T Arai, T Nozaki, N Inoue and M Watanake J. Electrochem Sn< 132 1707 (1985)|9| A Bom ret, J Thibault-Dcsscaux and D N Seidman J Appl Phys. 55 825 (1984)110| P Capper. A W Jones, E J Wallhouse and J G Wilkes J. Appl. Phys. 48 1646 (1977)[II] V Cazcarra and P Zunino J. Appl Phys. 51 4206 (1980)1121 W J Cleland J Electrochem. Soc. ■ Solid state Sa. Tech 129 1227 (1982)|I31 T Kumar and S Singh Bull. Electrochem 10 518 (1994)(14] D Malhiot Mater. Res Soc Symp. Proc 104 189 (1988)1151 W Kaiser, H L Frisch and H Reiss Phys. Rev 112 1546 ( 1958)110) J R Patel and B W Batterman J Appl. Phys. 34 2716 (1983)[17] R C Newman Mater S(i Enf». B36 I (1996)[18] A R Bean and R C Newman J Phys Chem. Solids 33 255 (1972) \[19] M Rciclie. J Reichcl and Niizchc Phys. Stat. Sol. (a) 107 851 (1988)]2()| R C Newman J Phys C18 1967 (1985)[211 J L Lindstrom, H Weman and G S Oehrlem Phys. Stat. Sohdi (a) 99 581 (1987)122] S A McQuaid, M J Binns, C A Londos. J H Tucker, A K Biown and R C Newman J Appl I'hu 77(4) 1427 (1995)460 Om Prakash and Shyam Singh

Thermal Donor Formation in CZ-Silicon Annealed at 450°C in Air Ambient

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Thermal Donor Formation in CZ-Silicon Annealed at 450°C in Air Ambient

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