Nearband gap optical behavior of sputter deposited α and α+βZrO2 filmsCheeKin Kwok and Carolyn Rubin Aita Citation: Journal of Applied Physics 66, 2756 (1989); doi: 10.1063/1.344484 View online: http://dx.doi.org/10.1063/1.344484 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/66/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Pressure dependence of the band gaps in Si quantum wires Appl. Phys. Lett. 64, 3545 (1994); 10.1063/1.111219 Anisotropic optical properties of YBa2Cu3O7 J. Appl. Phys. 71, 6002 (1992); 10.1063/1.350454 Indirect band gap in αZrO2 J. Vac. Sci. Technol. A 8, 3345 (1990); 10.1116/1.576548 Resonance effects in Raman scattering in YBa2Cu3O7 J. Appl. Phys. 66, 977 (1989); 10.1063/1.344451 The transition from αZr to αZrO2 growth in sputterdeposited films as a function of gas O2 content, raregas type,and cathode voltage J. Vac. Sci. Technol. A 7, 1235 (1989); 10.1116/1.576261

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TABLE n. Probabilities oftunneHng and radiative deexcitation of Fccnters at low temperatures.

P R Luminescence Host r (s I) (8 I) J7"

Nar 3.6 9.5 X 1010 _10" IX 10 Nob

NaBI' 4.83 3X 1OJ!) _106 3.3x 10 5 No Liel 6.59 5.7X 10" _106 1.8X 10- 4 No LiBr 6.34 7.7X 109 -10' 1.3 X 10 4 No LiP 8.42 2X 109 -10" 5X 10' No NaCl 18.21 5.4 X ]()4 8X 10' 0.93 Yes NaF 23.47 5.9X 102 -10" 1 Yes KF 32.83 2AX 10- 2 4.8X 10" 1 Yes RbI 34.93 1.5X 10-' ~ \05 I Yes RbF 36.74 4.3X 10 4 2.4X 10" 1 Yes KI 36.98 I.7X 10 4 4.5 X 10' 1 Yes KBf 38.58 3.8XlO 5 9X 10' 1 Yes RbBr 40.56 7X 10 " 1.2X 10" 1 Yes KCI 40.62 6.7X 10" 1.8X 10" Yes RbCI 46.57 1.1><10 8 1.7 X 10" Yes CsF 63.43 <10

, 2X 10' Yes

CsI 72.98 < 10- g Yes CsBr 14l.88 < 10 -14 6.5X 10' Yes CsCI \99.6\ <10 14 Yes

a rl~ predicted efficiency. \> No and yes refer to the observation of luminescence.

model for several alkali halides. The values shown in Table I were extracted from Ref. 6 where the authors studied mainly the temperature dependence of the linewidth in absorption of F centers. These values are the same used by Bartram and Stoneham in the crossover model. 2 The parameter A given by the relation S / A is exactly the one used in Ref. 2.

By using these values in Eq. (3) and setting n = 0, we have the effective tunneling probability from the minimum of the potential curve of the excited state. The results of this calculation are presented in Table II.

As one can see, for five alkali halides, Nal, NaBr, Liel, LiBr, and LiF, the tunneling rates (P) are bigger than the radiative rates for luminescence (R), making the deexcita­tion extremely nonradiactive. For the remaining alkali ha­lides (Table II), the tunneling rates are smaller than the radiative rates making the deexcitation via luminescence with full efficiency in most of the cases. The efficiency of luminescence, r" was calculated by the relation 77 = R / (P + R).

With this model we could explain all the experimental observations regarding the F-center luminescence. The tun­neling rates derived from this model, for NaBr and NaI, are in the same order of magnitude as the ones obtained by Bal­dacchini et al. in Ref. 3, for the F center in the same host materials. Also, the predictions of 71 below unity for NaCl derived from this model, is in agreement with the experimen­tal observations. R The F excited electron always reaches the minimum ofthe potential curve (relaxed excited state) from where the system can either be relaxed to the ground state or ionized to the conduction band producing an F' center de­pending on the temperature of the system.

ID. L Dexter, C. C. Klick, and G. A. Russel, Phys. Rev. 100,603 (1955). 2R. H. Bartram and A. M. Stoneham, Solid State Commun. 17, 1593 (1975); A. M. Stoneham and R. H. Bartram, Solid-State Electron. 21, 1325 (1978).

'G. Baldacchini, D. S. Pan, and F. Luty, Phys. Rev. B 24, 2174 (1981). 'G. Spinolo, Phys. Rev. 137, 1495 (1965). 'G. Baldacchini, G. P. Gallerano, U. M. Grassano, and F. LUly, Phys. Rev. n 27, 5039 (1983).

hR. K. Dawson and D. Pooley, Phys. Status Solidi 35, 95 (1969). 7c. C. Klick, D. A. Patterson and R. S. Knox, Phys. Rev. A 133, 1717 (1964 ).

RS.Honda and M.Tomura, J. Phys. Soc. lpn. 33,1003 (1972).

Nearaband gap optical behavior of sputter deposited u" and a + f3~Zr02 films Chee-Kin Kwok and Carolyn Rubin Aita Materials Department and the Laboratory for Surface Studies, University of WL~consin-Milwaukee, p. 0. Box 784, Milwaukee, Wisconsin 53201

(Received 27 February 1989; accepted for publication 24 May 1989)

The functional dependence of the optical absorption coefficient on photon energy in the 4.9-6.5 eV range was determined for a- and a + /3-Zr02 films grown by reactive sputter deposition on fused silica. Two allowed direct interband transitions in a-ZrOz were identified, with energies equal to 5.20 and 5.79 eV. Modification of these transitions in a + /3-zr02 is reported.

Monoclinic a-Zr02 is the only stable form for an infi­nite-size Zr02 crystal at standard temperature and pressure. However, it is well known2

-5 that when the crystallite size is

small, a-ZrOz and one of its high-temperature polymorphs, tetragonal (3-Zr02' coexist. /3-Zr02 in the composite is not a metastable phase nor the result of stabilization by impurities. It is a consequence of the dominance of the surface energy contribution to the Gibbs free energy of formation when the

crystallite surface area-to-volume ratio exceeds a critical val­ue. 4

We previously reported6 that phase transition from a­Zr--->a-Zr02 in the sputter-deposited Zr-O system involves the formation of a + /3-Zr02 structures. The shape of the fundamental optical absorption edge was found to be differ­ent for films that contain a /3-Zr02 component compared to mms that are single-phase a-Zr02• The functional depen-

2756 J. Appl. Phys. 66 (S), 15 September 1989 0021-8979/89/182756-03$02.40 @ 1989 American Institute of Physics 2756

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dence of the absorption coefficient a on the incident photon energy hv is reported here, Two optical transitions across the band gap of a-ZrOz are identified. Modification of these transitions in a + /3-Zr02 films is reported.

Films were grown on Supersil II fused silica in a hot-oil pumped rf diode sputter deposition system using a 13-cm­diam, 99.8% pure Zr target and discharges containing 2% O2 in a rare gas, Ne (film A), Ar (fUm B), or Kr (film C). The discharge was operated at a cathode voltage of - 1.3 k V (p-p) and a total pressure of 1 X 10- 2 Torr. Further details of the deposition process are given in Ref. 6, as well as a discussion of the changes in growth environment and target oxidation state that accompany a change in rare gas type.

Film thickness was measured post-deposition from a masked step using a profilometer. Growth rate was calculat­ed from this measurement. Crystallography was determined by double-angle x-ray diffraction (XRD) using unresolved 1.5418-A CuKa radiation. Calibration was done using the {1O T 1} peak of an a-quartz standard at 28 = 2.66 ± 0.02°, with a full width at half maximum (FWHM) intensity of 0.18°.

A Perkin-Elmer Model 330 UV-Visible-NIR doubJe­beam spectrophotometer with a specular reflection attach­ment was used to measure the transmittance T and reflec­tance R of 0.19-0.25 pm (6.5-4.9 eV) near-normal incidence radiation at room temperature in laboratory air. For a film of thickness x, a is related to T and R,7

T = [(1 -- R) 2 exp ( - ax) JI [1 - R 2 ex p ( -- 2ax)]. (1)

Film thickness and growth rate are recorded in Table 1. The major XRD peaks are shown in Fig, 1. No adjustment in intensity was made for thickness difference between sam­ples. The XRD pattern of Film A consists of a single peak at 28 = 28.10 attributed to {I In a-Zr02 planes. M The pattern of Film B consists of a peak at 26 = 28.2° attributed to {II n a-Zr01 planes, and a broad asymmetric peak attributed to {Ill} /3-Zr02 planes at 28 = 30.3° and {Ill} a-Zr02 planes at 2(J = 31.6°, The pattern of Film C has a third peak, attrib­uted to five sets of closely spaced a- and (3-Zr02 planes whose indices are given in Fig, 1. The sloping baseline on all patterns was caused by diffraction from the microcrystallites of the fused silica substrate.

High-resolution patterns were taken of the {II n a­Zr02 peak of all films and the FWHM was used to calculate the minimum dimension d of a {II 1}-oriented a-Zr02 crys­tallite in a direction perpendicular to the substrate, after sep­arating out the instrument contribution to peak broaden­ingY Values of d are recorded in Table I. Ca1culations4 based

TABLE I. Film thickness (x), growth rate (r), minimum crystallite diame­ter Cd), and the energy of two optical transitions (E, and E,) in a- and ex + f3-zr0 2 films sputter deposited on fused silica.

x r d HI E2 Film Gas (kA)a (A/min)" (A) (eV) (eV)

A Ne-2%02 1.0::t: 0.2 15.6± 3 126 5,20 5.79 B Ar-2%02 2.4 ± 0.4 37 ± 6 88 5.14 C Kr-2%02 3.3 ± 0,3 51 ±5 70 5.12

a:t: 10%.

2757 J. AppL Phys., Vol. 66, No.6, 15 September 1989

FILM B

>-..... ~ w.l

§ E2 x

FILM A

29 (nEG)

FIG. 1. X-ray diffraction pattern, for a-Zr02 (film A) and a-+- ,B-ZrO, (films Rand C) sputter deposited on fused silica,

on a balance between the surface and volume contributions to the Gibbs free energy of formation of the a and (3 phases predict a crystallite diameter of 306 A at which the two phases coexist. The value obtained from our experimental data is -0.3 of the predicted value.

Figure 2 shows a as a function of hv. For an films, the

FIG. 2, The absorption coefficient as a function of incident photon energy for a-ZrOc and a -/. j3-Zr02 films sputter deposited on fused silica. The sol­id curves represent Eqs. (2) and (3) applied to film A.

c. Kwok and C. R. Aita 2757

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[a] CbJ

5.0 x 109' x 1010 rx lD9

q.O

~ 3.0

N

~ 2.0

1.0

0 5.0 t 5.5 6.0 6.5 5.0 5.5 6.0

£1 E2 PHOTOII EI!EIIGY (EVl

dependence of a on hv is logarithmic at the onset of absorp­tion. The inverse slope of the exponential increase of a with hv is on the order of kT -·1 for many crystalline semiconduc­tors and insulators.7

.!O Here, dOn a)ld(hv) is equal to (2.5 kT) - I for film A, (4.6 kT)-1 for film B, and (3.6 kT) -I for film C.

For film A, the dependence of a on hv takes the form

a 2 =C1(hv-E1 ) (2)

between hv = 5.28-5.64 eV, and

(3)

between hv = 5.90-6.37 eV. For films Band C, the depen­dence of a on hv initially follows Eq. (2). However, a satu­rates with increasing hv. The higher energy optical transi­tion that occurs in Film A given by Eq. (3) does not occur in films Band C. Figure 3 shows a 2 as a function of hv. El and E2 were obtained from the value of hv at which a = 0 and are recorded in Table I.

From the functional dependence of a on hv for film A, we identify here, for the first time, two allowed direct inter­band transitions 7 that occur in a-ZrOz. The energies of the transitions, E\ and Ez, are 5.20 and 5.79 e V, respectively. Band calculations for a-Zr02 have yet to be carried out. Therefore, we cannot yet associate these optical transitions with specific electronic transitions. The coexistence of a­and /3-Zr02 phases results in the following modifications:

2758 J. Appl. Phys., Vol. 66, No.6, i 5 September 1989

ee]

~ 138

I FIG. 3. The square of the absorption coefficient as a function of incident photon energy for (a) film A showing two iinear regions, (b) film B, and (c) film C. The first linear region for film A is reproduced in the appropriate scale for comparison in (b) and (c).

5.0

(1) elimination of the high-energy transition; (2) a shift of E 1 to lower energy (5.20 e V for film A to 5.14 e V for film B to 5.12 e V for film C); and (3) ultimately, a decrease in the value of a over the entire absorption edge. The reasons for these modifications in terms of changes in short-range Zr-O order will be the subject of future research.

This work was supported under U. S. Army Research Office Grant No. DAAG03-89-K-0022 and by a gift from Johnson Controls, Inc. to the Wisconsin Distinguished Professorship (CRA).

'I. P. Abriata and J. Garces, Bull. Alloy Phase Diag. 7,116 (1986). 2R. C. Garvie, 1. Phys. Chern. 69.1238 (1965). .lE. C. Subbarao, H. S. Maiti, and K. K. Srivastava, Phys. Status Solidi A 221.9(1974).

'R. C. Garvie, J. Phys. Chern. 82, 218 (1978). 'M. Ruhle, J. Vac. Sci. Technol. A 3, 749 (1985). 6C.oK. Kwok and C. R. Aita, J. Vac. Sci. Techno!. A 7.1235 (1989). 7See, for example, J. I. Pankove, Optical Processes in Semiconductors (Prentice-Hal!, Englewood Cliffs, NJ, 1971).

x ASTM Joint Committee on Powder Diffraction Standards, 1974, Powder Diffraction File Nos. 13 0 307 (aoZrO,) and 14-534 ({3-ZrOol.

"L, V. Azarotf, Elements of X-Ray 6 ystallography (McGr~w-Hi\I, New York, 1(68), pp. 549-558. The term "minimum dimension" is used here to acknowledge that we arc assuming the limiting case of peak broadening due to finite crystalline size with no contribution from random lattice strain.

"'P. Urbach, Phys. Rev. 92,1324 (1953).

C. Kwok and C. R. Aita 2758

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