Optical properties of aluminum oxynitrides deposited by laser-assisted CVD

Optical properties of aluminum oxynitrides deposited by laser-assisted CVD Hulya Demiryont, L. R. Thompson, and G. J. Collins

Composition-dependent

optical properties of (Al2 0 3 ) 1-,-(AlN), films deposited by the laser-assisted chemi-

cal vapor deposition (L-CVD)technique are investigated. Three distinct substrate temperature regions give rise to three distinct film compositions from A12 0 3, through oxynitride composites, and ultimately to AIN.

Optical parameters of the films deposited on fused quartz substrates are deduced from spectrophotometric transmittance

characteristics in the spectral region ranging from 0.18 to 3.5,um. The optical energy gap of the

films is determined from optical absorption spectra. Fourier transfer infrared spectra (4400-400cm-') are used to characterize the chemical composition of the films.

1.

sisted CVD are examined in the composition region

Introduction

Aluminum nitride (AlN) is a suitable thin film material for use in electrooptic devices at high power in the

blue-ultraviolet spectral region because of its wide band gap (Eg

6 eV)lA along with its favorable ther-

mal and electrical properties. AlN is a thermally and chemically stable dielectric material with a refractive index of ,2.1.5,6 It exhibits strong piezoelectric constants which allows its use for acoustoelectronic devices.7' 8 AN also shows weak dichroism leading to

birefringence9 of the order of 10-2. The growth of aluminum nitride coatings in crystalline or amorphous form has been carried out previously by ion-beam sputtering, 10 "'1reactive rf magnetron sputtering,' 2 reactive evaporation,'3 reactive ion-beam deposition,'4 as well as by thermal and plasma CVD15,1 6 techniques.

The technological importance of aluminum oxide (AI2 03 ) stems from its electrical insulator properties

and from the efficacy of the films as a protective barri-

er on aluminum against both chemical corrosion and mechanical wear. A1203 films can be made by various

methods including anodic oxidation,' 7,' 8 reactive evaporation,' 9 reactive and rf sputtering20 and pyrolytic CVD,2 ' and laser-enhanced CVD.2 2

In this study, the optical properties of (Al2 -

0 3)1-,-(AlN), composite films deposited by laser-as-

The authors are with Colorado State University, Engineering Research Center, Fort Collins, Colorado 80523. Received 11 October 1985. 0003-6935/86/081311-08$02.00/0. © 1986 Optical Society of America.

ranging from c = 0 to c = 1.0. A spectrophotometric

transmissivity investigation has been made in the 0.18-3.5-,tm spectral region. These measurements are complemented by Fourier transfer IR spectroscopy of the AlN films in the 4400-400-cm'1 region. II.

Experimental Procedure

Aluminum nitride, aluminum oxide, and aluminum oxide-aluminum nitride composite films were deposited on fused quartz and (100) silicon wafers by laserenhanced chemical vapor deposition, the L-CVD technique. 2 2 Figure 1 is a schematic diagram of the L-CVD

deposition system. The details of this new deposition technique are given in our earlier study.2 2 This technique provides both the AlN, A12 0 3 , and (Al 203)1-(AlN)c composite films with various composition parameters c (0 < c < 1.0), as well as the Al-rich films of

all three compositions. The presence of oxygen in our films occurs via vacuum leaks in our deposition apparatus. We do not control the 02 leak rate other than to maintain fixed background vacuum during pump down, P _ 10-6 Torr. Evidence of residual water vapor was also observed in our infrared spectra as described in Sec. III.B. In this study, an ArF excimer laser operating at 193 nm was used to dissociate the feedstock reactant gases trimethylaluminum (TMAl) and ammonia (NH3 ). Both TMAI (Ref. 23) and NH3 (Ref. 24) absorb 193-nm radiation strongly and dissociate to form free radicals. Residual 02 does not strongly absorb 193-nm radiation. Photodissociated free radicals condense on the substrate underneath the beam dissociation region to form a film. The other deposition parameters affecting AlN film composition and associated optical properties are given in Table I. 15 April 1986 / Vol. 25, No. 8 / APPLIEDOPTICS

1311

I0°r

-T)

BEAM NH,

H, CARRIER

T-

T,'r

H, PURGE

IA

50F

(aO)

A1203

I-

w I-_I0f

z

_---------------------

To

I.-

I Fig. 1. Schematic diagram of the L-CVD deposition apparatus.

The substrate temperature T will also help determine the resulting film stoichiometry because the heterogeneous surface reactions and sticking coefficients for radical species are determined by T, values. In our study the chemical composition parameter c of (Al203) ,-(AlN), composite films was controlled primarily by varying the substrate temperature T, while keeping the other L-CVD parameters the same (see Table I). This is in contrast to conventional thermal CVD where T, serves both to create free radicals via thermal dissociation and to drive the heterogeneous surface reactions between free radicals. In our case the free radicals are created via photodissociation. Each free radical will have a varying sticking coefficient to the substrate which varies with T. The samples deposited on quartz substrates were examined by a Beckman transmission spectrophotometer while those deposited on silicon substrates were examined by a Fourier transfer infrared (FTIR) spectrophotometer. Ill.

Results and Discussion

A.

Spectrophotometric Measurements

Table 1. Typical DepositionParameters for AluminumOxyniltrideL-CVD

Trimethyl aluminum (TMAI) Ammonia (NH 3 )

Laser

transition (193 nm) Avg absorbed power, 0.5 W

Background Working:

-10-6

Torr

3.0 Torr

NH 3 /TMAI flow rate ratio

50/0.01

Substrates

Fused quartz, Corning 7059 glass Si wafers

Substrate temperature 1312

Fig. 2.

AN I

( b)

-

0

0

0.2

0.4 0.6 0.8 1.0 ' 1.8 WAVELENGTH (m)

2.0

Transmission vs wavelength spectra of (a) A120 3 and (b)

AIN films on fused quartz substrates. T+ and T- are the envelope curves of the transmission spectra; Tois the transmittance of bare substrate.

2(a) and (b) illustrate spectrophotometric transmissivity curves of A120 3 and AlN films on quartz substrates, respectively. The refractive index of substrate n is obtained from transmittance Toof the bare substrate given by n(X) =

1+ 1-T -TO(,\)

0

2

G\)

(1)

Optical parameters of a weakly absorbing film-substrate system are obtained from the following equa2 n(\) = 2 (8n,(X)C(X) + [n,(X) + 1]211/ + 8n,(X)C(X) 2

+ [n,(X) -

]21112)

k(X) =-L(A), 47rd

(2) (3)

1m-1

dii+,

d = M- 1

(4)

i=i

Excimer laser operating at ArFX

Pressure

To

i

(

tions 2 5 :

Optical parameters including refractive index n, extinction coefficient k, and film thickness d of L-CVD deposited film samples were deduced from spectrophotometric transmittance characteristics. Figures

Feedstock gases

5C

A

-30-500 0 C

APPLIEDOPTICS / Vol. 25, No. 8 / 15 April 1986

where Xiand Xi+,correspond to the ith and i + 1th extrema of the T-\ curve. The corresponding film thickness is given by di = {4 [n(Xi) _n(Xi+l)

(5)

The quantities related to Eqs. (2)-(4) are L(A) = In u(X) + [u2(X) - C2 (A) + (X)]1/2 2a

(6)

2.1r

4

2.0-

o-0 n -A _-- -A. *-----*K 11-A

0o

U

0 C z

ore-

*snow

dT X= Xm T'(X) =T(X)and dT = -for

t-X1 L- CVD

_--0

T-X plots (see Fig. 2), providing the following conditions at the localized minima and maxima:

kxI0

ION-BEAM

dX

20

1.9

T-(X) =T(X) and

dX

mx

ddo = -for dT X= Xmn. dX

(11)

dX

I.8

0 U. UJ

Figure 3 illustrates the spectral dependence of calculated n and k values of A12 03 films deposited by two different experimental techniques obtained by theoretical analysis of the respective T-X experimental curves: ion-beam sputter deposition (IBSD) and L-

I6

I.?

Ad~

i

~

~

In ',

0C

I.6*

C

I

1.6-

I

t-

0.5

04

0.2

1.2

1.0

0.8

I.4

CVD, respectively. As can be seen in Fig. 3 both IBSD and L-CVD deposited A120 3 films exhibit the same

- 10 1.6

(pm) WAVELENGTH Fig. 3. Spectral dependence of the refractive index and extinction coefficient of A12 03 films deposited by ion-beam sputtering and

L-CVD techniques. The optical characteristics of the films are identical.

[

n(X)

8n (X)

J

(7)

optical behavior, being nonabsorbing and nondispensive with a refractive-index value of 1.666 in the spectral region of X > 0.2 ,um. The high refractive index 2 6

indicates that both L-CVD and IBSD A12 03 samples are pinhole free, having a high packing density. The dramatic increases in n and k values in the shorter wavelength region (X < 0.2 Atm)are due to the onset of electronic transitions through the energy gap of the oxide. 2 7 28

Figures 4(a) and (b) show n vs X and k vs Xcurves of a(X) =

[n(X) + 1]'[n(X) + n.(X)] 2 16n,(X)n (X)

C(A) = T()

-

7'(X)

(9)

(10)

T(X) + (X) 2T+(X)Ti(X)

where T+(X) and T-(X) are the envelope curves of the

three typical L-CVD film samples: A12 03 , the (Al2 03)0 .6 6-(AlN)0 .3 3 composite film, and pure AlN films. The measured refractive index of these films increases from 1.666 to 2.08 with increasing nitride concentration (from 0 to 100%). Dispersion and absorption edges also shift slightly to higher wavelengths with an increasing nitride component in the composite material. Table II summarizes the n and c values vs substrate temperature T,. c values were determined from the

2.7 -

kXIo

za$

AIN

;

, eO

it

2.5.-

X

W a 2.3- A120 Z 2.1

Nl

An_

.2.07

T * 475 IC

w z 1,.18I,Ts U.1 .9F

300 'C

20

U.

0 0 z 0 C.)

z

10

I-

WAVELENGTH (m)

x

1.0

WAVELENGTH (m)

Fig. 4.

Spectral dependence of the deposited film optical parameters: (a) n vs Xplot and (b) k vs Xplot where Xd and Xa are dispersion and absorption edges, respectively. T, is the substrate temperature; c is the volume concentration of AIN in composite film. 15 April 1986 / Vol. 25, No. 8 / APPLIEDOPTICS

1313

Table II.

L-CVD Samples Examined In This Study

Label

T (C)

n (0.4 Am < X)

128 127 125 119 126 146 114 106 107 108 116 137 133 129 113

100 150 200 250 300 300 350 400 400 400 420 450 450 475 490

1.666 1.700 1.774 1.756 1.808 1.725 1.842 1.968 1.916 1.848 1.950 1.998 2.08 2.08 2.07

IC)0

c 0 0.07 0.24 0.20 0.32 0.13 0.40 0.67 0.55 0.40 0.66 0.785 1.0 1.0 1.0

I `2 CI 01

:--I sI?

.O xI

X

I

9

300 Ts (C)

500

Fig. 5. c vs T plot of L-CVD samples deposited under reactor vacuum conditions given in Table I. and ®points correspond to the films deposited at high and low residual oxygen levels, respec-

tively.

(12)

e

is the dielectric

constant of the (Al2 03)lic-(AlN)c composite film with volume concentration c. Figure 5 illustrates more clearly that the substrate temperature dependence of the deposited film composition has three distinct substrate temperature regions where A12 03 , A12 03 -AlN composites and, AlN films are

obtained, respectively. This dependence of film composition with substrate temperature shows that in LCVD the thermal heterogeneous surface reactions still play a dominant role. Oxygen affinity of aluminum is

much stronger than nitrogen affinity in the 100-200°C region, thus aluminum oxide films are obtained. The presence of oxygen arises due to residual air leaks in our vacuum chamber during the deposition process. Unfortunately, we were unaware of the critical need for higher vacuum (p < 10-7 Torr) prior to our L-CVD experiments. We did, however, keep our background reactor pressure at the 10- 6-Torr level. An increase in c values with increasing T is consistent with the known decrease in the sticking coefficient of oxygen with increasing temperature.3 0 It can only be surmised that the sticking coefficient of the nitrogen radicals crosses that of 02 at ,200°C. Alternatively, the total rate for nitride reactions exceeds that for oxide reactions at ,200C. Chemical reactions leading to the condensation of aluminum products on the substrate surface are judged to be (CH3 )3A + residual 02 + NH 3 hl'A1 2 03 + volatile products for the low substrate temperature region (TS 100°C, see Fig. 5).

The nitrogen reaction pathways on the surface are minimized either due to low sticking coefficient values or due to the higher reactivity of oxygen. Oxygen arises either from vacuum leaks in our reactor or residual water vapor. In the intermediate T region AlN-AI2 0 3 composites are formed because of the increased nitride formation rate. Again we surmise that, either because of the decrease in the sticking coefficient of oxygen compared to nitrogen-free radi1314

I

100

where el and C2 are the individual dielectric constants of A120 3 and AlN, respectively, and

/

w I

high frequency dielectric constant e (= n 2 ) by using the Drude equation,2 9 c = e1 + C(e2 -el),

z

APPLIEDOPTICS / Vol. 25, No. 8 / 15 April 1986

cals with increasing substrate temperature or because of the difference in reactivities of 02 and N free radicals on the surface, (Al2O3 )1 _,-(AlN), composites with 0 < c < 1 are obtained for the T value ranging from -100 to -450°C. For the high substrate temperature region (TS > 450°C) L-CVD deposition leads to primarily nitride products: (CH3) 3A + NH 3 h AlN + volatile products. Above 450°C the surface reaction is dominated by nitrogen-free radical heterogeneous reactions and 02 pathways are judged negligible. In Sec. B we also see the disappearance of water vapor incorporated in our films. Lower NH 3/TMAl flow rate ratios lead to Al-rich films in all substrate temperature regions investigated. An increase in the residual oxygen level as measured by decreases in the ultimate reactor vacuum level prior to deposition was observed to lead to a decrease in the c value at a constant substrate temperature (see Fig. 5). Figures 6(a) and (b), respectively, illustrate the effect of varying beam-substrate distance I on observed deposition rate and refractive index of the film formed at T =4750C. The depositionrate decreases approximately linearly with increasing [Fig. 6(a)], with 5 cm being the extreme edge of the deposition region. The linear decrease in the film thickness with increasing beam-substrate separation 1 arises because of the decreased gradient of photodissociated free radicals with increasing 1. This reduces the diffusion flux term J = DVN of free radicals to the substrate surface. Figure 6(b) illustrates the changing value of the index n vs 1for the corresponding samples of Fig. 6(a). Note that n increases slightly with increasing 1 in Fig. 6(b) (e.g., n = 1.98 for 1

0 and n = 2.08 for 1

2.1 cm).

The higher deposition rate in Fig. 6(a) corresponds to the lower refractive-index film of AlN in Fig. 6(b), implying that thermally driven atom migration on the surface is not able to deal with the density of the films deposited at high deposition rate. The packing density p is defined as the ratio of the density of the film material pf to the density of the bulk

3000

_

(a) # 133 T. = 450

04;

C

2500l-

z C.

I

2000_

IO

u

IE

-

( b) 0

2.001_ 0

0

_U

z

I0

w 1=

1 95

E.

4r

2

4

6

DISTANCE, F(cm) BEAM-SUBSTRATE

Fig. 6. (a) Beam-substrate distance-dependent deposition rate and (b) refractive index of laser-deposited

AIN films.

8

E (eV)

T, = 450'C

Fig. 7.

Absorption spectra of L-CVD A12 03 and AIN films

(constant).

material Pb. Packing density and refractive index2 9 are related by n + 2

pf

nf-1

Pb

nf + 2 nb-1

(13)

.

10000

I

20000

Assuming that the bulk value of the refractive index of 0~~~~~~~~~~~Ia

0

single crystal AlN is 2.16 (Ref. 4) and the correspond-

ing density Pb = 0.55, the packing density of L-CVD AIN is p = 0.956 for I = 2.1 cm and p = 0.897 for I 0. Optical absorption spectra of (A12O3 )jc-(AlN)c composite films were examined in the spectral region of 0.1&< X (am) 6.0 eV (AI203 ) orE > 6.7 eV (AlN) the values of (aE) 4/ 7 increase at a faster rate with photon energy (see Fig. 8), suggesting the existence of a second absorption process at the transi-

from (aE)-m vs E plots, fitting for m values until experimental data show a linear dependence. L-CVD

tion energies of approximately Eg = 6.0 eV for AlN and Eg = 6.7 eV for A12 03. Since the apparent absorption

where E is the photon energy corresponding to the

A12 03 , AlN, and their composites fit Eq. (14) best for m

1.75. Figure 8 illustrates (aE) 4 /7 vs E plots of A12 03 and AlN films when E is the energy of the probe photons. Optical energy gap Eg is defined by extrapolating the =

linear portion of the absorption spectrum to aE = 0. As can be seen in this figure, there are two linear

edge is sensitive to defects and impurities, we choose the highest energy limit observed for the energy gap. The origin of the absorption starting from 4.6 eV to an energy of 6.0 eV for AlN and from 5.1 eV to 6.7 eV for

is not clear at this time. Indirect transitions or transition via impurity states3 2 are usually responsible for the multislopes observed in absorption spectra A12 03

15 April 1986 / Vol. 25, No. 8 / APPLIEDOPTICS

1315

such as those plotted in Fig. 8. The high energy gap, high refractive index, high density, and low absorption level indicate that excess metallic impurities do not exist in our L-CVD samples. Since the band structure calculations of AlN predict a direct band gap,33 -35 transitions via low level impurities such as carbon may be responsible for this lower energy absorption region. Figure 9 illustrates composition-dependent energy

Eg

gap plots of (AI2 0 3)1-c-(AlN)c samples.

0f 6.0

(9 5.0

\

dence, i.e., energy difference of Eg

I--

ICcr

20

40 60 AIN (%)

80

100 AIN

0.4 for Eg > 6.1 eV (or c (or c 0.30).

Fig. 9. Eg vs c and Eim vs c plots of L-CVD (Al 2 O3)j.c-(AlN)c composite films. c 30% is a critical concentration. For c < 0.3 A12 0 3-like optical properties dominate films, while for c > 0.30 AIN-

9

d

1.8

0 z

0 Cr LI.

1 7

wi I. I o

6.0

0.301 and m = 4 for Eg

6.5 Eg. (eV)

Fig. 10. n vs Eg plot of L-CVD (Al 2 0 3) 1-c-(AlN)c composite films. An n = A _ relation is obtained: m, is a power factor; A, is a constant; m = 0.4 for Eg 6.1 eV; and m = 4 for Eg 6.1 eV.

Infrared Spectroscopy

Fourier transfer infrared (FIR) absorbance spectra of A12 0 3 -AlN composite samples were examined in the spectral range from 4400 to 400 cm-' using a Bio-Rad model QS100 spectrometer. The main absorption peaks observed in the FTIR spectrum are given in Table IV. The intensity of all the peaks listed in Table IV exhibited substrate temperature and hence film composition dependence. Substrate temperature dependence of peak intensity suggested that the peaks centered around 3450 cm-' (strong, s) and 1640 cm-' (weak, w) are related with impurity water bands.3 6 These bands were completely eliminated for samples deposited above 3000C substrate temperature. The absorption band observed at around 460 cm-' was strong for the films deposited at low substrate tem-

TableIll. OpticalProperties of A1203 andAINFilmSamplesa Energy gap

Refractive index Material A1 203

and impurity

= 0.3jum 1.666

gap states Eg = 6.7 eV Eim =5.1 eV

1.67 1.67 1.55

AlN

2.08 2.16

Eg = 6.0 eV Eim = 4.6 eV Eg = 5.9-4.8 eV

1.99

Eim = 2.8 eV 5.9 eV

Remarks L-CVD T

=

1000C

References This study

e beam T= 400 C e beam T = 300'C Anodic oxidation

26 26 18

L-CVD T = 4500 C

This study 5

Sc; Eg decreases with

increasing nitrogen

a sc, single crystal; 1316

pc, polycrystalline;

6.1

In Table III, optical properties of L-CVD AlN and

B.

A

a X w

1.5 eV is

those given in the literature using other film deposition methods.

n cc Egm m 4 for E .E 6.1 eV m = 04 for Eg 6.1 eV

o 2.0

Eim

A1203 films obtained in this study are compared with

like optical properties dominate. 2 1

-

independent of the AlN concentration within the A12 03 . Optical properties of L-CVD aluminum oxide-nitride composites are A12 03 -like for c 30% and AlN-like above this critical concentration (see Fig. 9). Figure 10 shows the energy gap-refractive-index dependence observed for A12 0 3 -AlN composite films. This dependence fits the naE-m relation, where m =

ZII

4.50 A1203

As can be seen

in this figure, both the high energy absorption edge Eg and the low energy absorption threshold due to the impurities Eimexhibit the same concentration depen-

a.

im, impurity state.

APPLIEDOPTICS / Vol. 25, No. 8 / 15 April 1986

pc, pyrolysis T = 800-1100 0 C

5

Table IV.

Infrared AbsorptionBandsObserved on L-CVD (Al2 03) 1j 0-(AIN)¢ CompositeFilms

Observed

Substrate temperature

peak locations

dependence of

values

peak intensities

Identification

(in cm-')

References

2.9 6.1

Decreases with increasing T,

(OH);'1

-3450 -1640

39 39

460

21.78

Peak intensity decreases with increasing T,

A1203 molecules

459

37, 38

-1180 680

8.45 14.7

Becomes dominant with increasing Ts

AIN molecules

1183 1057 714 700 675

39 39 39 37 40

in (cm-') -3450 -1640

in (gim)

peratures but absorption decreases in intensity for the films deposited at increasing substrate temperatures. This is consistent with the identification of the absorption band at around 460 cm-' to a vibrational mode of the A12 03 molecule.3 7 ' 38 AlN shows a strong character-

istic IR band at -680 cm-' and a weak band at around 1180 cm-', both mentioned in previous literature.3 9 IV.

Literature

Conclusions

Optical properties of (AI2O3 )1_c-(AlN)c composite films formed by the L-CVD technique were examined vs the film composition. Nonabsorbing and nondispersive films in the spectral region of 0.25 < X (,gm) < 2.5 of A120 3, AIN, and their composites can be obtained

by the L-CVD technique. In this nondispersive region the refractive indices of the samples were 1.67 for A120 3 and 2.08 for AIN. Oxynitride films exhibited

composition-dependent optical characteristics, i.e., AlN-like optical properties are dominant for c > 0.3 and AI20 3 -like for c 0.3. Photon energy dependence of absorption spectra of L-CVD samples exhibits an apparent optical absorption edge Eg as well as an ab-

sorption threshold Eim,probably related to impurities. Impurity absorption onset energy is located at 5.1 eV

Index and Birefringence of Aluminum Nitride," Phys. Status Solidi 20, K29 (1967). 7. M. T. Wouk and D. K. Winslow, "Vacuum Deposition of AIN Acoustic Transducers," Appl. Phys. Lett. 13, 286 (1968). 8. J. H. Collins, P. J. Hagan, and G. R. Pulliam, Ultrasonics 218 (Oct. 1970). 9. H. Yamashita, K. Fukui, S. Misawa, and S. Yoshida, "Optical

Properties of AIN Epitaxial Thin Films in Vacuum Ultraviolet Region," J. Appl. Phys. 50, 896 (1979).

10. H. Birey-Demiryont, S. Pak, J. F. Wagner, and J. R. Sites, "IonBeam Sputtered AlO1-,N, Encapsulating Films," J. Vac. Sci. Technol. 16, 2086 (1979).

11. H. Birey-Demiryont, S. Pak, and J. R. Sites, "Photoluminescence of Gallium Arsenide Encapsulated with Aluminum Nitride and Silicon Nitride," Appl. Phys. Lett. 35, 623 (1979). 12. T. Shiosaki, T. Yamamoto, T. Oda, and A. Kawabata, "LowTemperature Growth Piezoelectric AIN Films by Reactive Pla-

nar Magnetron Sputtering," Appl. Phys. Lett. 36, 643 (1980). 13. S. Yoshiada, S. Misawa, and A. Itoh, "Epitaxial Growth of AIN on Sapphire by Reactive Evaporation," Appl. Phys. Lett. 26,461 (1975). 14. S. Bhat, S. Ashok, S. J. Fonash, and L. Tongson, J. Electron. Mater. 14,405 (1985). 15. H. Arnold, L. Biste, D. Bolze, and G. Eichhorn, Krist. Tech. 11, 17 (1976).

16. J. Bauer, L. Biste, and D. Bolze, "The Symetrized Combination

for A12 03 and 4.6 eV for AlN samples. Energy gaps of A1 2 03 and AlN films were 6.7 and 6.0 eV, respectively.

of Plane Waves of Wurtzide," Phys. Status Solidi A24, 659 (1967). 17. L. Young, Anodic Oxide Films (Academic, New York, 1961).

The authors would like to thank the Office of Naval Research and the National Science Foundation for their support of this study.

18. H. Birey-Demiryont, "Anodization Rate and Augmentation

References 1. J. Pastranak and L. Roskovcova, "Optical Absorption Edge of AIN Single Crystals," Phys. Status Solidi 26, 591 (1968). 2. J. Bauer, L. Biste, and D. Bolze, "Optical Properties of Aluminum Nitride Prepared by Chemical and Plasmachemical Vapor Deposition," Phys. Status Solidi 39, 173 (1977). 3. W. M. Yim, E. J. Stofko, P. J. Zanzucchi, J. I. Pankove, M. Ettemberg, and S. L. Gilbert, "Epitaxially Grown AIN and Its Optical Band Gap," J. Appl. Phys. 44, 292 (1973). 4. G. A. Cox, D. D. Cummins, K. Kawabe, and R. H. Tredgold, "On

Sn-SnOt Cermet Films Deposited by Reactive Evaporation,"

the Preparation, Optical Properties and Electrical Behavior of Aluminum Nitride," J. Phys. Chem. Solids 28, 543 (1967). 5. T. L. Chu and R. W. Kelm, J. Electrochem. Soc. 122,991 (1976). 6. J. Pastranak and L. Roskovcova, "The Dispersion of Refractive

Factor of Anodic Aluminum Oxide Films," J. Appl. Phys. 50, 2906 (1979). 19. H. Demiryont and N. Tezey, "Characterization of Al-AlOx and Thin Solid Films 101, 345 (1983). 20. K. Ando and K. Matsumura, "Current Efficiency in the Plasma Anodization of Aluminum," Thin Solid Films 52, 153 (1978). 21. E. Ferrieu and B. Pruniaux, J. Electrochem. Soc. 116, 1008 (1969). 22. R. Solanki, W. H. Ritchie, and G. J. Collins, "Photodeposition of

Aluminum Oxideand AluminumThin Films," Appl. Phys. Lett. 43, 454 (1983). 23. D. J. Ehrlich, R. M. Osgood, Jr., and T. F. Deutsch, IEEE J.

Quantum Electron. QE-16, 1233(1980). 24. V. M. Donnelly, A. P. Baronavski, and J. R. McDonald, "Excited

State Dynamics and Bimolecular Quenching Processes for NH 2 (A2A,)," Chem. Phys. 43, 289 (1979).

15 April 1986 / Vol. 25, No8 / APPLIEDOPTICS

1317

25. E. Aktulger, "Optical Parameters of a Weakly Absorbing Film

by Spectrophotometric Transmissivity," Ph.D. Thesis, Istanbul U., Turkey (1983).

26. E. Ritter, "Dielectric Film Materials for Optical Applications," Phys. Thin Films 8, 1 (1975). 27. E. E. Khawaja and S. G. Tomlin, "AIN/GaAs Structures Grown by Molecular Beam Epitaxy," Thin Solid Films 98, 75 (1982). 28. F. Rubio, J. M. Albella, J. Denis, and J. M. Martinez-Duart, "Optical Properties of Reactively Sputtered Ta 2O5 Films," J. Vac. Sci. Technol. 21, 1043 (1982).

29. R. Jacobson, "Inhomogenous and Coevaporated Homogenous Films for Optical Applications," Phys. Thin Films 8, 51 (1975). 30. T. H. Allen, "Properties of Ion Assisted Deposited Silica and Titania Films," Proc. Soc. Photo-Opt. Instrum. Eng. 325, 93 (1982).

31. J. Tauc, "Optical Properties of Non-Crystalline Solids," in Optical Properties of Solids, F. Abeles, Ed. (North-Holland, Amsterdam, 1971), p. 277.

32. A.T. Collins, E. C. Lightowlers,and P. J. Dean, "Lattice Vibration Spectra of Aluminum Nitride," Phys Rev. 158,833 (1967). 33. B. Hejda, "Energy Band Structure of AIN," Phys. Status Solidi 32, 407 (1969). 34. S. Bloom, "Band Structure of GaN and AIN," J. Phys. Chem. Solids 32, 2027 (1971).

35. P. Jones and A. H. Lettington, "Electronic Band Structure of Wide Band-Gap Semiconductors GaN and AIN," Solid State Commun. 11, 701 (1972). 36. H. Burger, W. Vogel, and V. Kozhukharov, Infrared Phys. 25, 395 (1985).

37. A. Takase et al., Jpn. J. Appl. Phys. 21, 1447(1982). 38. A. Takase et al., J. Mater. Sci. Lett. 1, 529 (1982). 39. E. G. Gerova, N. A. Ivanov, and K. I. Korov, "Deposition of AIN

Thin Films by Magnetron Reactive Sputtering," Thin Solid Films 81, 201 (1981). 40. E. G. Brame, J. L. Margrave, and V. W. Melcoche, J. Inorg. Nucl. Chem. 5, 48 (1957).

OSA Board Changes

William B. Bridges of the California Institute of Technology has been elected to serve as vice president of the Optical Society of America in 1986. Three new directors at large were also elected:

Jay M. Eastman of Optel Systems, Inc., in Pittsford, NY; William T. Silfvast of AT&T Bell Laboratories in Holmdel, NJ; and George

I. Stegeman of the Optical Sciences Center at the University of Arizona. Bridges, an OSA fellow, has been a professor of electrical engi-

neering and applied physics at California Institute of Technology in Pasadena, CA, since 1977. In 1983, he became the Carl F. Braun professor of electrical engineering there. He is the discoverer of laser oscillation in noble gas ions, and he spent several years developing high-power visible and UV ion lasers for military uses. He has

published 30 papers, holds seven patents, and is coauthor, with C. K. Birdsall, of ELECTRONDYNAMICS OFDIODE REGIONS (Academic Press, 1966). Bridges has served OSA in various capacities,

most recently on the OSA board of directors 1982-85. Jay M. Eastman became president of Optel Systems, Inc., in 1983

and is an adjunct associate professor at the University of Rochester's Institute of Optics. He was formerly director of the University's Laboratory for Laser Energetics. Prior to that, he was a member of the technical staff at Spectra-Physics, Inc. Now completing a twoyear term as chair of the Technical Council, Eastman has also served as chair for numerous OSA meetings and on various program com-

mittees. He is also a past president of the Rochester local section of OSA.

William T. Silfvast joined the technical staff at AT&T Bell Laboratories in 1967and was named a distinguished member of the technical staff in 1983. He is a pioneer in the field of metal vapor lasers and has discovered more than 200 laser transitions in over 30

elements. His current research interests are autoionizing states and the use of short-wavelength lasers to study new physics. Silfvast served on the executive committee of the Technical Council in 1983 and has also been active in planning the Conference on Lasers and

Electro-Optics. He is on the 1986 CLEO steering committee and was cochair of CLEO '84. George I. Stegeman has been a professor of optical sciences at the University of Arizona since 1980 and was formerly on the faculty 1318

APPLIEDOPTICS / Vol. 25, No. 8 / 15 April 1986

at the University of Toronto. He is coauthor, with Fred Hopf, of APPLIED CLASSICAL ELECTRODYNAMICS, VOL. I, LINEAR OPTICS, published in 1985 by Wiley. The second volume, on nonlinear optics, will be published in 1986. Stegeman's research is

currently focused on nonlinear optical interactions at surfaces, specifically for nonlinear guided wave all-optical signal processing and surface spectroscopy. He has also been involved in research on wave interactions at surfaces. A fellow of the Society, he has served on

numerous committees and was cochair of the 1977and 1982 annual meetings. A board of editors is now in place (1 Jan.) with Anthony

J.

DeMaria as chair for the first two-year term. DeMaria is with United Technologies Research Center in East Hartford, CT. The board will consist of the editors for all the primary journals published or copublished by the Optical Society of America. DeMaria is also chair, for 1985-86, of the coordinating committee for the Journal of Lightwave Technology, copublished by the Society and the Institute of Electrical and Electronics Engineers. An OSA fellow, he was

president of the board of directors in 1981and served on the Society Objectives and Policy Committee from 1982to 1984. In addition, DeMaria chaired the Technical Council during 1976-77,and he was chair of the technical group on lasers during 1972-74. He was editor of the IEEE Journal of Quantum Electronics from 1977to 1982. Alexander A. Sawchuk is beginning a two-year term as chair of the Technical Council, after serving as vice chair during 1984-85. During 1980-81 he was a member of the Technical Council, repre-

senting the technical group on information processing and holography. Sawchuk, an OSA fellow,is with the Department of Electrical Engineering at the University of Southern California. He was cochair of the Society's 1983topical meeting on signal recovery, chair of the 1984OSAannual meeting, and a program committee member for the 1985 topical meeting on machine vision. Jean M. Bennett took office as president of the Society on 1 Jan.

She is on sabbatical at the University of Alabama, Huntsville, on leave from the Naval Weapons Center in China Lake, CA. Serving

with her as president-elect is Robert G. Greenler of the University of Wisconsin, Milwaukee. F. Dow Smith, the treasurer, is with the New England Collegeof Optometry in Boston, MA. All three are OSA fellows.