Electrical Properties of Silver Impurities and their Annealing Behaviour in p-Type Fz Silicon

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Electrical Properties of Silver Impurities and their Annealing Behaviour in p-Type Fz Silicon

G. Adegboyega, L. Passari, M. Butturi, A. Poggi, E. Suzi

To cite this version:

G. Adegboyega, L. Passari, M. Butturi, A. Poggi, E. Suzi. Electrical Properties of Silver Impurities

and their Annealing Behaviour in p-Type Fz Silicon. Journal de Physique III, EDP Sciences, 1996, 6

(12), pp.1691-1696. �10.1051/jp3:1996207�. �jpa-00249551�

Electrical Properties of Silver Impurities and their Annealing

Behaviour in p-Type Fz Silicon

G-A- Adegboyega (~,*), ^{L. Passari} (~), ^{M.A. Butturi} (~), A. Poggi (~) and E. Suzi (~)

(~) Department of Electronic and Electrical Engineering, Obafemi Awolowo University, Ile-Ife, Nigeria

(~) Dipartimento di Fisica-Universita, Via Paradiso 12, 44100 Ferrara, Italy (~) CNR-Istituto LAMEL, Via P. Gobetti 101, 40129 Bologna, Italy

(Received 6 February 1996, revised 26 July 1996, accepted 13 September 1996)

PACS.71.55.-I Impurity and defect levels PACS.72.80.-r Conductivity of specific materials

PACS.78.30.-j Infrared and Raman spectra

Abstract. The electrical activity of silver

well

its annealing properties in 10 Q

p-type Fz silicon substrate

studied by

of the four-point probe and minority carrier lifetime measurements. Silver atom concentration in the range 10~~ to 10~~ cm~~ consistently

showed

donor type behaviour in the material and its presence led _to

a

reduction of up ^to two orders of magnitude in the lifetime of minority carriers by the formation of deep-level traps. ^Isochronal annealing of silver contaminated specimens showed

gettering of the Ag impurities with resulting temperature dependent changes in the resistivity

well

the

minority carrier lifetime values. Analysis of

results shows that

a

large fraction of the silver impurity atoms present forms the deep level defects and both the deep- and donor-levels appear to originate from the

1. Introduction

Of all the group IB elements (Cu, Ag and Au), the physical properties of Ag in silicon is the least well understood. Copper, _{as a} major contaminant in silicon device manufactur- ing _process ^and gold, _{as a} minority carrier lifetime controlling center in silicon, have both

been subject of detailed study and review [1-6]. Copper, with _a diffusion coefficient of about 0.76 _x 10~5 cm~ s~~ _at 500

C, is known to be the fastest diffusing species in Si, with relevant diffusion mechanism being fully interstitial. However, due to its high mobility, _copper related complexes and deep-level _traps have been identified. Gold also has _a high ^diffusion constant in Si. It diffuses mainly by the interstitial-substitutional kick-out mechanism with about 90%

remaining substitutional and 10% interstitial. Gold is amphoteric in Si with deep donor and acceptor ^levels at about Ev ₊ 0.35 eV and Ev + 0.63 eV, respectively.

The electrical and optical properties of silver in silicon have been studied in _a number of investigations [7-27]. The picture that emerges from these studies is that Ag diffuses relatively

(*) Author for correspondence

@ Les #ditions _{de Physique 1996}

1692 JOURNAL DE PHYSIQUE III N°12

easily in silicon and its presence results in two deep levels: _an acceptor level at about Ec

0.54 eV and _a donor level at about Ev + 0.34 eV. Rollert _et al. [21] ^{have shown} by _means

of neutron activation analysis _on silver diffused specimens that interstitial silver, Ag; is the most frequently occurring configuration by which Ag is incorporated in dislocation-free silicon and that Ag solubility of about 7.4 _x 10~~ atoms cm~~

can be achieved at a temperature ^of 1300

C. The experimental tool most frequently employed to probe silver related deep levels in Si is the DLTS (deep level transient spectroscopy). However, of the large amount of silver

expected to be incorporated in Si from diffusion experiments similar to Rollert et al. [21), only a

concentration level of the order of10~~ cm~~ _{silver atoms} (about I%), not specifically connected with Ag;, has been observed in the DLTS experiment [17,18, 20, 22]. ^{There is} still, therefore,

a puzzle _on the whereabout of the majority of the Ag impurity atoms in Si. It is noteworthy that recent Electron Paramaglietic Resonance (EPR) data [25, 26) ^{have indicated at least six}

new prominent spectra ^related to silver and its complexes in _n- and p-type ^{silicon substrates} in addition to the hitherto known donor and acceptor ^levels. One of these _new spectra, ^with

an estimated impurity concentration of about 10~3 _atoms cm~3, has been identified with _an isolated interstitial silver. No defect has _so far been identified with _an isolated substitutional silver. The newly observed spectra _may account for _some of the silver impurity atoms when their microscopic structures are identified.

In this report, we have used four-point probe and minority carrier lifetime measurements to

investigate the effects of varying silver impurity concentration levels _on the electrical charac- teristics of p-type Fz silicon. In addition, the behaviour of this impurity in the substrate under isochronal annealing at different temperatures was also investigated.

2. Experimental

A 7.5 _cm diameter, 0.038 _cm thick B-doped p-type (100) oriented Fz silicon wafer _was used in this investigation. The wafer, with _a resistivity of about 10 Q _{cm, was} divided into eight parts and each part was subjected to the following treatment:

A control, _{as grown}

B specimen A, annealed (1000 °C, 6 h)

C specimen ^{A with} Ag diffusion (1000 °C, 6 h)

D specimen C, annealed (400 °C, 2 h)

E specimen C, annealed (600 °C, 2 h)

F specimen A with Ag diffusion (1150 °C, 6 h)

G specimen F, annealed (400

C, 2 h)

H specimen F, annealed (600 °C, 2 h)

For the diffused specimens, 5N _pure silver _was evaporated under _{a vacuum} pressure of about 2 _x 10~~ _Torr,

on to the back (unpolished) surface of the wafer. All diffusions and annealing

were carried _out in _a tube furnace under flowing high-purity nitrogen _gas and specimens _were quenched to _room temperature by moving ^them to one end of the furnace tube. After dif- fusion, the remaining silver film _was etched off before annealing commenced. The specimens

were characterized by _{a computer} controlled four-point probe system ^for resistivity and carrier

concentration values. Minority carrier lifetime measurement _was performed _on the specimens

by the PEM-PC (photoelectromagnetic-photoconductive) method [28, 29].

3. Results and Discussion

Very few data exist in the literature _on the solubility and the diffusion constant of Ag in Si [7,21) and there exists considerable disagreement in -the published values. For example,

at 1573 K, Boltaks and Shih-yin _[7] found _a solubility value for Ag of1 _x 10~~ _atoms cm~3,

whereas Rollert et al. [21] ^found a solubility value of 7A _x 10~5 _atoms cm~3 Although Boltaks and Shih-yin _[7] indicated that the diffusion rate of Ag is comparable to that of Au in Si,

their determined value8 of 3 _x 10~~ _{to 2.4}

x 10~~ cm~ s~~ _{for the} diffusion coefficient of Ag in Si between l100 and 1350 °C differ significantly from that of Au which has _a value of about

2.3 x10~5 cm~s~~ _{at 1200} °C [2]. However, Rollert et al. [21] gave a diffusion coefficient value of 3.6 _x 10~5 cm~ s~~ _for Ag in Si at 1100 °C. This value is within the usual range for fast dilfusants. The diffusion and solubility data of Rollert et al. [21] ^{have been assumed in}

our analysis. Our diffusion conditions _are such that would give rise to complementary _error

function profiles. The density of Ag impurity atoms _{as a} function of distance and time at a

particular diffusion temperature _can, therefore, be written _as:

C(x, t)

=

Coerfc[~/2/Dt)] (I)

where Co is the impurity concentration at the unpolished surface which in this _{case can} be assumed _to be equal to the solubility value of Ag in Si _at the corresponding temperature, ~

is the distance measured from this surface, t is the time of diffusion and D is the diffusion coefficient _at the diffusion temperature.

Using the diffusion and solubility data of Rollert et al. [21] at the corresponding temperatures the diffusion time employed and the thickness of _our specimen, the Ag impurity concentration, 1AlAg, ^at the polished surface would be NAg (l150

C)

=

1.45 x10~5 cm~3 and NAg (1000 °C)

=

7.27 _x 10~~ cm~3 _These figures _compare favorably well with the solubility values, Co of about 1.48 _x 10~5 cm~3 and 7.53 _x 10~~ cm~~ _at temperature ^{of1150 and} 1000 °C respectively. We will therefore _assume the solubility values for _our analysis. The four point probe system ^used in this work gives the resistivity and the carrier concentration values of the sample under test.

Table I shows the results obtained from the probe system. Lifetime measurement results _as well

as the diffusion, annealing and relevant solubility data for _our Ag-diffused silicon specimens

are also shown in Table I.

Comparison of specimens A and B of Table I shows that the 1000

C anneal for six hours under nitrogen _gas ambient has little _{or no} effect _on the resistivity, carrier concentration and the minority carrier lifetime of the substrate. Therefore, contamination by the difsusion system is negligible. Comparing specimens A, C and F, however, shows that the introduction of silver into specimens C and F results in partia1compensation of the acceptors and higher _NAg value gave rise to a higher ^{level of} compensation. This donor type ^behaviour is not surprising as

Ag ^is known to be amphoteric in Si [18, 20, 22]. However, the low donor concentration, Nd values of 4 x 10~~ and 9 x 10~~ cm~~ respectively for specimens C and F _are surprising. ^These

values correspond to about 5.3% and 6.1% of the NAg introduced into specimens C and F

respectively. The presence of Ag in Si is known to give rise to a donor level at Ev ₊ 0.34 eV.

With _an ionization energy of about 0.3 eV, however, _one would expect ^little _(~~J 5%) _{or no}

ionization of this donor level at room temperature. ^The low level of acceptor compensation observed here could be due to _a low level of ionization of this donor level. In the absence of

ionization of the donor level, two possibilities could give rise _to the observed donor behaviour.

The presence of Ag impurity atoms could lead to the formation of _a shallow donor level with

low concentration _or Ag _may be involved in the formation of boron containing complex. The

deactivation of B acceptor atoms resulting from such _a complex formation _can also give rise to

the observed donor behaviour. More work will be required to resolve this issue.

1694 JOURNAL DE PHYSIQUE III N°12 Table I. Relevant diffusion data, resistivity and lifetime measurement results _on silver-

diffused p-type ^Fz silicon

Specimen A B C D E F G H

Ag diffusion temperature 1000 1000 1000 1150 1150 1150

(°C) for 6 h

NAg(10~~cm~~) 0.75 0.75 0.75 1.48 1.48 1.48

from Ref.

Temperature (°C) 1000, 6 h 400 600 400 600

of anneal for 2 h

Resistivity in cm) 10.32 10.33 10.51 10.43 10.39 10.73 10.63 10.55

Acceptor concentration 1.30 1.30 1.26 1.27 1.28 1.21 1.23 1.25

Na (10~~cm~~)

Donor concentration 4.0 3.0 2.0 9.0 7.0 5.0

Nd (10~~cm~~) from NAg

Carrier Lifetime, T 18.40 17.29 0.23 1.19 1.38 0.13 0.82 1.07

Table I shows that the incorporation ^of Ag impurities in the Si substrate has _a devastating

effect _on the minority carrier lifetime. Comparison of specimens A, C and F shows that _a silver atom impurity concentration of about 7.5 x10~~ cm~~ reduced the lifetime value, _Tn from 18.40 to 0.23 ~ls. Almost doubling the Ag impurity content (specimen F) reduced _Tn from 18.40 to 0.13 ~ls. Such _a significant reduction in _Tn is due to the presence of Ag-related _traps.

For _a p-type semiconductor, the electron lifetime, _T, is given by:

T

(anuthNt)~~ (2)

where _an is the electron capture cross section, _uth

(3kT/m)~/~ the carrier thermal velocity

and Nt the trap density. If _an and _{uth are} assumed to be constant for measurement taken at a particular temperature, then equation (2) _can be written _as:

Nt

cT~~ (3)

where _c is _a constant.

If the experimental values for

in Table I _are substituted into equation (3) for specimens C and F respectively, _we have

Ntc

4.35c (4)

and

NtF

7.69c. (5)

However, if the density of traps is proportiona1to the density of the silver impurity atoms present, then if Ntc

"

4.35c, NtF should _{assume a} value NtF,

8.5c. Therefore,

NtF

0.9NtF; (6)

Equation (6) ^{show that} a large percentage ^{of the silver} impurity atoms present ^forms the deep- level traps, while the remaining small fraction probably exists in other forms such _as complexes,

precipitates _or shallow levels.

To date, _no clear picture exists about the microscopic structure of the defects with which silver deep levels _are associated in silicon. For example, Rollert et al. [21] ^{indicated that} the overwhelming majority of Ag atom impurity exists _on the interstitial sites, Ag;, and that substitutional silver, Ags, either does not occur or occurs only in very low concentrations.

Using EPR investigations, Son et al. [26] ^{have identified that iwlated} Ag; represents only

a small fraction of the total amount of silver impurity atoms present. ^{One of the} recently observed silver related spectra [25,26] has been attributed _{to a} Ag-Fe complex formed by silver with the inadvertent iron impurity almost always present ^{in silicon wafers. When} present ^{in B-} doped Si substrates, Fe is known to from FeB donor pairs [30, 31]. ^{The formation of} a Ag-Fe-B complex with _a resulting boron acceptor atom deactivation can, therefore, not be ruled out in silver contaminated B-doped silicon wafers. As _more experimental data become available, the

microscopic nature of the silver impurity in Si would become clearer.

A comparison of the lifetime values of specimens C, ^D and E having the _same silver impurity

concentration of 0.75 _x 10~~ _atoms cm~3 _{and also} of specimens F, G and H having the _same silver impurity concentration of1.48 _x 10~5 _atoms cm~~ shows that annealing of Ag contami- nated specimens decreases the density of the silver-induced deep-level traps. This is reflected in the improved lifetime values of annealed specimens D and E and also of specimens ^{G and H.}

This reduction in the trap densities shows that _some Ag impurity atoms either diffuse out of the Si substrate _or precipitate in small clusters during annealing. The precipitated Ag is elec-

trically inactive and _{can no} longer perform the function of aiding carrier recombination. Our results show that the quantity of outdiffused _or precipited Ag impurity is higher at annealing temperature ^{of 600 °C than 400 °C. In addition} to decreasing the trap density, annealing also has beneficial effects _on the resistivity and carrier concentration values of silver contaminated

specimens _as both values tend to that of the original _or control specimen. As the annealing temperature increases from 400 °C to 600

C, Nd decreases. This decrease in the Nd value is accompanied by _a decrease in the Nt _as observed by the increase in the value _T in Table I.

This observation suggests a close connection between Nd and Nt. In other words, the _same

impurity _energy level is _most likely _to be responsible for the donor behaviour _as well _as the

deep-level trap.

4. Conclusion

Silver proved to be _a donor impurity in p-type ^{Si. The} deep level traps ^{formed due} to the presence of silver led _{to a} decrease of _{to two} orders of magnitude in the minority carrier lifetime. Annealing ^of silver contaminated specimens decreased the silver related defects. The donor and deep levels _{appear to} originate from the _same defect _source.

Acknowledgments

One of the authors (GAA) acknowledges the support of the ICTP Programme ^for Training

and Research in Italian Laboratory, Trieste, Italy, for his stay at the Istituto CNR-LAMEL, Bologna, Italy, where most of the measurements were carried out.

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