a review on thermal donors in cz-silicon and their … · in this article, an ... fz – si is very...

A REVIEW ON THERMAL DONORS IN CZ-SILICON AND THEIR

IMPACT ON ELECTRONIC INDUSTRY

Rajeev Singh

Asst. Professor, Department of Physics, Arignar Anna Government Arts & Science College, Karaikal (UT of Puducherry) -609605, India

P. B. Nagabalasubramanian

Asst. Professor, Department of Physics, Arignar Anna Government Arts & Science College, Karaikal (UT of Puducherry) -609605, India

Abstract

CZ-silicon is the primary material for integration of many

electronic components since 1960 after the replacement of

Germanium. Many impurities are inbuilt in the crystal.

Among these, presence of Oxygen & Nitrogen prominently

affects the electrical and optical properties. In this article, an

attempt has been made to discuss about oxygen related

donors, their identification and kinetics of formation. We

will also try to project the positives and negatives related to

the problem in particular relating to electronic industry and

its future prospects.

Keywords: CZ-silicon, Thermal donors, Oxygen

Need of hour

Silicon is mostly used material in Integrated Chips and Solar

panels manufacturing. In the present era, it is essentially

required to increase the number of circuits in a single chip

and thereafter the number of operations in them. For

reducing the packaging cost and failure rate of chips, the

heat generation must be lower. In power devices, the

excessive leakage current can be produced due to defects

below the surface. The interstitial type dislocation loops in

the active regions of the devices may cause degradation of

minority carrier lifetime, alter diffusion profiles and may

shift p-n junction characteristics [1-3]. In very lightly doped

silicon, these oxygen related thermal donors (TDs) can

modify the resistivity of material and hence they can affects

numerous resistivity-dependent device parameters. The

oxide precipitates present in the active region may increase

the leakage currents in the p-n junctions; reduce refresh

times in DRAM memories etc [4]. The proper

characterization and understanding regarding the role of

oxygen related impurities is essential to optimize the yield.

Comparison and importance of silicon over other

semiconductors

Many other semiconductors and their compounds have been

examined as a replacement of silicon e.g. GaAs, InP etc.

GaAshas certain technical advantages over silicon. Due to

higher bandgap energies and motilities,GaAs power

amplifiers have higher gain even at lower operating voltages.

Its band gap can be varied also with inserting one type of

atoms, i.e. GaAlAs, GaInP etc. Since GaAs has direct band

gap so it is also convenient for Laser &optoelectronics [5].

The device components of GaAs generate less noise than

silicon and most other types of semiconductor components

and hence they are useful in weak-signal amplification

applications. There are some drawbacks also in GaAs e.g.

the out diffusion of As at high temperatures, adjustment of

the threshold voltage etc [6]. The thermal conductivity of

GaAs is about three times lower than that of silicon. Silicon

is roughly a thousand times cheaper than germanium, i.e. the

cost of 8 inch diameter silicon wafer is around $ 5 but for

GaAs is around $ 5000/. Therefore, GaAs devices are only

used in specific applications where their special capabilities

justify their higher cost. However, some researchers in

Stanford University are working to prepare the lower cost

chips of GaAs [7].

Production of silicon single crystal

Single crystals of silicon are produced primarily by

Czochralski (CZ) method [Fig.1]. However, for power

devices, Salnick et al developed a new growth techniqueto

grow untraditionally doped CZ-grown silicon [8]. The other

method is Float Zone (FZ) method. FZ – Si is very soft for

most commercial applications and crumbles very easily

because it contains over two to three orders of magnitude

less oxygen than Cz-Si. The oxygen precipitates helps to

suppress stacking faults. Presence of oxygen makes the Si

more resistant to thermal stress during processing [9].

Therefore, CZ-Si is used for integrated circuit designs

having many thermal processing steps.

In Czochralski method, the polycrystalline silicon is melted

in a silica crucible heated by a resistor or by RF currents

[Fig 1]. Under steady conditions and after stabilizing the

melt flow, the seed is lowered towards the melt. Now, the

crystal and the crucible rotate in opposite directions. After

dipping the seed into the melt, a stable interfere between

melt and seed crystal is adjusted [2] and a large amount of

International Journal of Applied Engineering Research ISSN 0973-4562 Volume 14, Number 2, 2019 (Special Issue) © Research India Publications. http://www.ripublication.com

Page 43 of 47

mailto:[email protected]

mailto:[email protected]

oxygen from crucible is incorporated in to the melt. Oxygen

is acting as a gettering agent so it promotes the internal

gettering process. However, a certain level of oxygen

concentration is quite essential for mechanical strength of

the wafer. Many types of impurities are found in crystal melt.

The impurity concentration in the solid part CS differs from

the concentration CL in the liquid part just at the interface.

The ratio of Cs to CL is called segregation coefficient ‘k’

and it has different values for different materials in which

oxygen has the value 1.3 greater than all other types of

impurities [6]. Now, the seed is crystallized after pulling

from the melt upward at the solid-liquid interface [2]. In this

process, the seed must be not touched to the melt surface to

avoid the generation of dislocations due to thermal shock.

Therefore, the process of necking is performed for

dislocations free growth.

Fig.1: CZ-Si growth (1) Silica crucible, (2) carbon suscepter,

(3) Graphite heater, (4) Single crystal Silicon.

Review of research and development in the subject

Oxygen, the most common impurity, is being introduced

from the crucible during the early stages of growth of the

crystal. Its behavior is different under lower and higher

temperatures annealing. The annealing at round 450o C leads

to a change of its material’s electrical resistivity. Upon heat

treatments, the oxygen impurities in silicon interact with

other defects already present in the crystal. These oxygen

atoms in interstitial sites are electrically inactive but they

create, upon low temperature annealing, high concentrations

of electrically active centers [10]. These centers were

discovered by Fuller et al in 1954. They were called as

Thermal Double Donors or Thermal Donors. The types of

oxygen donors are given in the table [10-12].

Kaiser et al reported the oxygen related thermal donor

formation kinetics with dependence of interstitial

concentrations and a model known as KFR model was given

by them in which only three complexes of Si with 2-4

oxygen atoms are electrically active [13]. Wada and Inoue

[14] modified this model by considering electron emission.

Table: Oxygen related Donors with their generation temperature

S.

No.

Type of

Oxygen

donors

Annealing

Temperature

Annealing

Time Reference

1 Thermal

donor (TD) 300-500°C <105 min

Fuller et al.

(1954) [10]

2 New donor

(ND) 550-800°C <105 min

Kanamori and

Kanamori

(1979) [11]

3

New thermal

donor

(NTD)

450-550°C >105 min

Kamiura et

al. (1990)

[12]

In OSB model [15] Si atom at the center and surrounded by

five or more oxygen atoms give birth to donor activity. In

another model by Mathiot [16], three oxygen atoms with

one or more atoms in interstitial position form thermal

donors. McQuaid et al [17] proposed that oxygen loss

during low temperature anneals followed second order

kinetics. At higher temperature (T > 450 oC), the rate of the

loss of oxygen concentration were less than expected rate of

(Oi-Oi) interaction and tended to vary with increasingly high

powers of oxygen concentration [18]. Murin et al [18] had

also investigated the Radiation induced complexes of

oxygen, carbon, vacancy and interstitials in silicon by

absorption spectroscopy. According to Newman et al, TD (n)

centers have only a few oxygen atoms clustered together

[19-21]. They proposed that oxygen dimers O-O may

diffuse more rapidly than isolated oxygen atoms [21].

Aforesaid workers and many others have tried to look into

the problem of oxygen related donors (ODs) from different

angles and contributed reasonably in order to evolve a

widely accepted common consensus for the proper

understanding of the exact mechanism [22].

Ono et al. [23] have studied anomalous ring-shaped

distributions of oxygen precipitates in wafers using infrared

absorption spectroscopy and X-ray topography. FTIR

measurements showed homogenous distributions of oxygen

precipitates, interstitial oxygen and thermal donors. The

results also showed that the thermal donors and interstitial

silicon atoms in the wafers do not affect oxygen

precipitation. Therefore, Ono concludes that native point

defects, impurities or their clusters in as grown wafers are

not responsible for inhomogeneous precipitation after

thermal annealing. The anomalous distributions should be

strongly affected by already existing precipitates except

OSF nuclei. The density of OSF nuclei, which were also

heterogeneously nucleated at high temperatures during

crystal growth, is too low to directly affect the

inhomogeneous precipitation.

Capaz et al [24] identified the process of enhanced oxygen

diffusion due to interaction between interstitial oxygen and

hydrogen atoms. This interaction is also supported by IR

absorption following an annealing after implantation of

hydrogen atoms into Silicon [25-26]. Newman [27] has very

beautifully presented an overall scenario of past and present

state of art clearly indicating that the processes involved

therein are not yet fully understood.


Page 44 of 47

The behavior of Oi in the presence of N is not yet fully

understood. It exhibits electrically inactive NNO defect.

Many N/O experiments have been suggested for our

understanding of how the nitrogen atoms diffuse, and form

the complexes with either interstitials or with dimers [28-30].

The hydrogen passivation and carbon suppression of thermal

donors have also been studied [31]. Yang et al [32-33] have

beautifully tried to look into the behavior of STDs in silicon

doped with isotopic oxygen and also grown-in defects in

nitrogen-doped CZ-Silicon. Some results on electrical

properties of H implanted Si annealed under high

hydrostatic pressure have been reported by Kaniewska and

Misiuk [34]. We have been consistently pursing the research

work [35].

Till now, sixteen neutral (TDD1-TDD16) and nine

positively charged species (TDD1-TDD9) of thermal double

donors have been detected and confirmed through infra-red

absorption spectra [36]. M. Pesola et al gave the

confirmation about the structures of TDD0, TDD1 to TDD7

on the basis of accurate total energy calculations and made a

good agreement with Chadi [37-38]. The other possible

structures for all the TDDs and the most probable average

number of oxygen atoms involved in a single TDD has been

given by us [39].

Measurement & Techniques

There are many techniques for detection and measurement

of donors such as Hall Effect, Four probes, FTIR, DLTS,

XRD, PL etc. Here we are giving only a brief introduction

about the resistivity measurement and FTIR.

(a) Resistivity

The resistivity of unannealed/annealed Si samples will

be measured with the help of most commonly used

collinear four-probe method the number of carriers are

determined with the help of Irvin’s curve [40]. To

ascertain the carrier concentration, the results are also

be supplemented by Hall measurements.

(b) Donor Generation

After receiving the desired heat treatment in air or

nitrogen ambient, the samples are cleaned in

hydrofluoric acid in order to eliminate the surface oxide.

Now by measuring its new resistivity, the

corresponding number of carriers is again obtained

from Irvin’s curves [40]. We can deduce the number of

donors generated or eliminated during the given heat

treatment by calculating the difference of number of

carriers with the approximation that the mobility

remains constant.

(c) FTIR Spectroscopy

FTIR measurements will be used for determination of

the oxygen and carbon absorption coefficients at room

temperature from 3500 cm-1 to 600 cm-1 band

respectively enabling us, to know about the

reduction/precipitation of oxygen/carbon contents in the

annealed/unannealed samples. TD (n), where ‘n’ ranges

from 0 to 16, may also be identified through FTIR

spectra.

(d) Precipitated oxygen and carbon concentration

These concentrations are measured initially in the wafers

received as such. Then again the concentrations of Oi

and Cs were determined after the scheduled heat

treatment. The difference between these concentrations

of annealed and unannealed samples helps to find the

precipitated oxygen and carbon during heat treatment of

samples.

(e) Activation energy of thermal donors

In the low temperature annealing (< 500 oC), the rate of

loss of interstitial oxygen atoms (Oi) may be related by

trapping with other such atoms [41]. Assuming that

backward reaction rate is negligible in comparison to the

forward reaction rate, the kinetics of Oi loss would be

controlled solely by the interaction rate of two randomly

diffusing Oi atoms [42].

2[ ]8 [ ]i

id O DR O

dt

Where R = radius of capture associated with dimer

formation, D = Diffusion coefficient for oxygen

Now, integrating the above expression,

1 18

[ ] [ ]i i i o

DRtO O

or1 1( ) ( ) 8C t C O DRt

where1 1( ) and ( )C t C O

are reciprocal values of

oxygen concentration prior to the anneal and after

anneals for time ‘t’. Plot of [1 1( ) ( )C t C O ] versus

time is expected to be linear having the gradient equal

to8 DR . Then, we can get the value of diffusivity of

oxygen for a particular temperature. Now, the value of D

so obtained is substituted in the expression

0.17exp /D E kT cm-2s-1. Hence, the value of

activation energy for the diffusion of oxygen can be

calculated [42]. We can find the formation of donors

species at different temperature for different annealing

time.

Conclusion

In spite of more than sixty years of research about thermal

donors, many fundamental questions are unanswered

regarding their structures and their method of formation.

Some of them are as follows:

What are the exact structures and formation processes

for different types of thermal double donors?

How many types of donors can exist?


Page 45 of 47

What is the role of silicon interstitials in the formation

of TDDs? They are part of defect structures or they are

working as a catalyst for defect formation.

What is the role of oxygen dimers?

The high temperature (T > 700 oC) anneals are well

understood now a days. But, the lower temperature

processing is still a matter of debate. It is strongly needed to

improve the understanding about this because the device

processing temperatures and the size of future devices are

decreasing continuously.

References

[1] H. Yamagishi, I. Fusegawa, N. Fujimaki& M.

Katayama, “Recognition of D defects in silicon single

crystal by preferential etching and effect on gate oxide

integrity”, Semiconductor Science and Technology,

vol. 7 (1), pp. A135-A140, 1992

[2] Lukas Valek, Jan Sik, “Modern aspects of bulk crystal

and thin film preparation (Chapter-3)”, Book published

by Intechopen, Online Jan 13, 2012.

[3] T. Abe, H. Harada & J. Chikawa, “Swirl defects in

Float-zoned silicon crystals”, Physics B+C, vol. 116

(1-3), pp. 139-147, 1983.

[4] D. K. Schroder, “Lifetime in silicon, In: gettering and

defect engineering in the semiconductor technology”,

Kittler, M. (Ed.), Vaduz, FL, pp. 383-394, 1989

[5] Garrett, J. Hayes, Bruce M. Clemens, “Laser liftoff of

Gallium arsenide Thin Films”, MRS Communications,

vol. 5 (1) pp. 1, 2015.

[6] B. Leroy, “Silicon wafers for integrated circuit

Process”, Revue Phys Appl, vol. 21, pp. 467-488, 1986.

[7] Tom Abate, Stanford University, Stanford News

Service, March 27, 2015.

[8] Z. A. Salnick, Y. A. Miklyaev, O. S. Salnick, A. V.

Naumov, A. V. Phomichev, “Untraditionally doped

CZ-grown silicon for power devices”, J. of Crystal

Growth, vol. 172, pp. 120, 1997.

[9] G. A. Rozgonyi, R. P. Deysher and C. W. Pearce, “The

identification, annihilation and suppression of

nucleation sites responsible for silicon epitaxial

stacking faults”, J. Electrochem. Soc., vol. 123, pp.

1910, 1976

[10] C.S. Fuller, J.A. Ditzenberger, N.B.Hanney and E.

Buehler, “Resistivity changes in silicon induced by

heat treatment”, Phys Rev, vol. 96, pp. 833, 1954; Acta

Metall. vol. 3, pp. 97, 1955.

[11] A. Kanamori and M. Kanamori, “Comparison of two

kinds of oxygen donors in silicon by resistivity

measurements”, J. Appl. Phys., vol. 50, pp. 8095, 1979.

[12] Y. Kamiura, M. Suezawa, K. Sumino and F.

Hashimoto, “Optical properties of new kinds of

thermal donors in silicon”, J. Appl. Phys., vol. 29, pp.

1937, 1990.

[13] W. Kaiser, H. L. Frisch and H. Reiss, “Mechanism of

the formation of donor states in heat treated silicon”,

Phys Rev, vol. 112, pp. 1546, 1958.

[14] K. Wada and N. Inoue, “Suppression of thermal donor

formation in heavily doped n-type silicon”, J. Appl.

Phys, vol. 57, pp. 5145, 1985.

[15] A. Ourmazd, W. Schroter and A. Bourret, “Oxygen

related thermal donors in silicon”, J. Appl Phys vol. 56,

pp. 1670, 1984.

[16] D. Mathiot, “Thermal donor formation in silicon: A

kinetic model based on self-interstitial aggregation”,

Appl. PhysLett, vol. 52, pp. 904, 1987.

[17] S. A. McQuaid, M. J. Binns, C. A. Landos, J. H.

Tucker, A. R. Brown and R. C. Newman, “Oxygen

loss during thermal donor formation in cz-silicon: New

insight into oxygen diffusion mechanism”, J. Appl

Phys, vol. 77, pp. 1427, 1995.

[18] L. I. Murin, J. L. Lindstroem, G. Davies and V. P.

Markevich, “Evolution of radiation-induced carbon-

oxygen related defects in silicon upon annealing: LVM

studies”, Nucl. Instrum Methods Phys Res., Sect. B,

vol. 253, pp. 210, 2006.

[19] W. Gotz, G. Pensl and W. Zulehner, “Observations of

five additional donor species TD12 to TD16 and of

regrowth of thermal donors at initial stages of the new

oxygen donor formation in Czochralski-grown silicon”,

Phys Rev B, vol. 46, pp. 4312, 1992.

[20] R. C. Newman, J. H. Tucker, N. G. Semaltianos, E.C.

Lighowlers, T. Gregorkiewicz, I. C. Zevenbergen and

C. A. J. Ammerlaan, “Infrared absorption in silicon

from shallow thermal donors incorporating hydrogen

and a link to the NL10 paramagnetic resonance

spectrum”, Phys Rev B, vol. 54, pp. 6803, 1996.

[21] R. C. Newman, “Oxygen precipitation and thermal

donors in silicon”, Mater Res Soc Symp Proc, vol. 59,

pp. 205, 1986.

[22] Early stages of Oxygen Precipitation in Silicon, edited

by R. Jones (KulwerAcedemic Publishers, Dordrecht,

1996) and references therein.

[23] H. Ono, T. Ikarashi, S. Kimura, A. Tanikawa and K.

Terashima, “Early stages of oxygen precipitation in

silicon”, pp. 509-516, 1996.

[24] R. B. Capaz, L. V. C. Assali, L. C. Kimerling, K. Cho

and J. D. Joannopoulos, “Mechanism for hydrogen-

enhanced oxygen diffusion in silicon”, Phys Rev B, vol.

59, pp. 4898, 1999.

[25] B. Hourahine, R. Jones, S. Oberg, P. R. Briddon, V. P.

Markevich, R. C. Newman, J. Hermansson, M.

Kleverman, J. L. Lindstrom, L. I. Murin, N. Fukata and

M. Suezawa, “Stable hydrogen pair trapped at carbon

impurities in silicon”, Phys B: Condensed Matt, vol.

308-310, pp. 197, 2001.

[26] R. E. Pritchard, M. J. Ashwin, J. H. Tuker, R. C.

Newman, E. C. Lightowlers, M. J. Binns, S. A.

McQuaid and R. Faslter, “Interaction of hydrogen

molecules with bond-centered interstitial oxygen and

another defect center in silicon”, Phys Rev B, vol. 56,

pp. 13118, 1997.

[27] R. C. Newman, “Oxygen diffusion and pericipitation in

Czochralski silicon”, J. of Phy: Conden Mater, vol. 12,

pp. 335, 2000.

[28] Naohisa Inoue, HidenoriOyama, Kaori Watanbe,

Hirofumi Seki & Yuichi Kawamura, “Behaviour of


6 of 47

nitrogen in si crystal during irradiation and post-

annealing”, AIP Conf. Proceed., vol. 1583, pp. 19,

2014.

[29] C. P. Ewels, Ph. D. Thesis, University of Exeter, 1997.

[30] S. Singh, B. C. Yadav, R. Singh, “Role of nitrogen on

formation of oxygen related donors in step annealed

cz-silicon”, J. Optoelectronics & Adv Mater, vol. 10

(6), pp. 1522, 2008.

[31] Rajeev Singh, Shyam Singh & B. C. Yadav, “Effect of

nitrogen and carbon, in the formation of shallow

thermal donors in cz-silicon”, I. Journal of Mater

Science, vol. 12 (1), pp. 81, 2017.

[32] D. Yang, M. Klevermann and L. I. Murin, “Shallow

thermal donors in silicon doped with isotopic oxygen”,

Physica B, vol. 302, pp. 193, 2001.

[33] X. Yu, D. Yang, X. Ma, J. Yang, M. Liben Ii and D.

Que, “Grown-in defects in nitrogen-doped czochralski

silicon”, J. Appl Phys, vol. 92, pp. 188, 2002.

[34] M. Kaniewska and A. Misiuk, “Electrical properties of

hydrogen-implanted Si annealed under high

hydrostatic pressure”, Crys Res Tech, vol. 38, pp. 336,

2003.

[35] Rajeev Singh, Shyam Singh, Bal Chandra Yadav,

“Kinetics of new thermal donors (NTDs) in cz-silicon

based on FTIR analysis”, AIP Conference Proceed.,

vol. 1953, pp. 050072, 2018.

[36] B. Pajot, H. Compain, J. Leroueille and B. Clerjaud,

“Spectroscopic studies of 450 oC thermal donors in

silicon”, Physica, vol. 117B & 118B, pp. 110, 1983.

[37] M. Pesola, Young Joo Lee, J. Von Boehm, M.

Kaukonen& R. M. Nieminen, “Structures of thermal

double donors in silicon”, Phy Rev Lett, vol. 84 (23),

pp. 5343, 2000.

[38] D. J. Chadi, “Core structure of thermal donors in

silicon”, Phys. Rev. Lett, vol. 77, pp. 861, 1996.

[39] Shyam Singh, Rajeev Singh, B. C. Yadav, “Thermal

donor formation in cz-silicon with reference to

dimmers, trimers and V-O interaction”, Physica B, vol.

404, pp. 1070, 2009.

[40] S. M. Sze and J. C. Irvin, “Resistivity, mobility and

impurity levels in GaAs, Ge and Si at 300 oC”, Solid

State Electronics, vol. 11, pp. 599, 1968.

[41] V. Dubey and S. Singh, “Role of oxygen and carbon

on donor formation in step-annealed cz-silicon”, J.

Phys Chem Solids, vol. 65, pp. 1265, 2004.

[42] Om Prakash and Shyam Singh, “Effect of oxygen on

donor formation in cz-silicon”, Indian J. Phys, vol. 69

A (5), pp. 509, 1995.


Page 47 of 47

a review on thermal donors in cz-silicon and their … · in this article, an ... fz – si is very...

Documents