hydrothermal growth of bismuth silicate (bso)

Journal of Crystal Growth 128 (1993) 871-875 j . . . . . . . . C R Y S T A L North-Holland G R O W T H

Hydrothermal growth of bismuth silicate (BSO)

John Larkin, Meckie Harris , J. E m e r y Cormier

Electronic Materials, Rome Laboratory, Hanscom AFB, Massachusetts 01731, USA

and

Al ton Arming ton

Parke Mathematical Laboratory, Carlisle, Massachusetts 01741, USA

Bismuth silicate (Bii2SiO20) is a promising material for use in optical signal processing. It is a photorefractive material with good response at argon laser wavelengths (488 nm) and is well suited to holography and four wave mixing applications. It exhibits a high response speed and good sensitivity, although the gain is far less than that of the ferroelectric materials (barium titanate, etc.). Most of the currently available material is obtained by the Czochralski growth process or the directional gradient freeze process. Nominally undoped crystals from these processes yield material of acceptable research quality but crystal uniformity and reproducibility have been a problem. Improved growth techniques are needed for advanced applications. Hydrothermal growth of this material dramatically changes the "intrinsic" optical properties. There are indications that lower temperature aqueous growth prevents the formation of defects created in the melt processes. This yields a baseline material ideally suited for the study of these defects. In addition, by modifying the optical properties with dopants, material tailored to specific wavelengths and applications may possibly be produced. Growth procedures and preliminary optical characterization results are reported.

1. Introduction

Bismuth silicate, Bil2SiO20 is a photorefractive material having I23 symmetry and the sillenite structure. It is characterized by oxygen tetrahedra surrounding silicon atoms which occur at the center and the corners of the unit cell. For this reason it is often referred to as "body centered cubic". In addition, a variety of divalent and trivalent ions can be incorporated into the sillenite structure (e.g. Ga, Fe, Zn) at different Bi /metal stoichiometries. This has led to a model which would permit Bi 5+ to occupy some of the tedrahedral sites to provide charge compensation [1]. Bismuth silicate is conventionally grown by the Czochralski technique from stoichiometric melts, although microcrystals have been produced near room temperature by precipitation in basic solutions [1].

Commercial crystals of bismuth silicate of suf- ficient quality for most current research applications are available. These crystals may not be suitable for advanced applications since optical homogeneity and crystal-to-crystal reproducibility have been a problem [2]. The nature of the pho- toelectric "absorption tail" necessary to the photorefractive process is not well understood and a variety of mechanisms for this optical feature have been proposed [3]. This dominant optical feature may be sensitive to variations in the typical melt growth environment (at a temperature of 895°C). In addition to the possibility of purely thermal defects, these crystals are grown in an ambient air environment, leading to the possibility that at least some of the bismuth in the stoichiometric melt is present as Bi 5+. Growth in inert or reducing atmospheres results in decom- position of the melt, producing bismuth metal

0022-0248/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

872 J. Larkin et al. / Hydrothermal growth of bismuth silicate

which attacks the crucible. The hydrothermal environment, by contrast, is normally neutral to oxidation and reduction. In addition, the much lower temperature (about 400°C) should result in a lower number of purely thermal defects. This technique is capable of producing crystals of large size and identical consistency while requiring only stable temperature control once the internal growth conditions have been established. The process will generally run under automatic temperature controls for periods in excess of 30 days [4]. Under ideal growth conditions crystal number and size should only be limited by the amount of starting nutrient, vessel size and number of seeds.

Several crystals grown in experimental size autoclaves at the Rome Laboratory Hydrothermal Research Facility at Hanscom AFB in Mas- sachusetts have been free of the typical structural defects encountered in commercial material, i.e. veils, bubbles, precipitates, and other strain producing phenomena [5]. Remaining problems in the hydrothermal technique yet to be solved in- clude orientation of preferred growth, optimum growth gradients, and seed/interface irregulari- ties. Additionally, the need for reusable liners that preclude the use of expensive noble metal materials that require refinement and refabrica- tion has to be investigated.

2. Growth

The hydrothermal growth of BSO requires the use of a high pressure vessel or autoclave (fig. 1) capable of withstanding pressure of at least 1000 atm at an internal operating temperature of 400°C. The autoclaves used in our experiments were National Forge pressure vessels with modi- fied Bridgman seals and inside dimensions of 1.125 inch by 12 inches or 3 inches by 36 inches. The vessels were heated by upper and lower sets of strap heaters powered by L&N zero-crossing power supplies and controlled by programmable LFE PID controllers. A Hewlett Packard HP1000 computer was used for data acquisition and some alarm functions. Sensors for data acquisition and alarm functions consisted of top and bottom imbedded type K thermocouples used for control,

_\

(A)

(B) (G)

(D)

(E) (F)

Fig. l. Autoclave used for bismuth silicate hydrothermal growth: (A) sealed platinum liner; (B) bismuth silicate seeds; (C) baffle; (D) bismuth silicate untrient; (E) liner support stand; (F) 3 foot pressure vessel; (G) vessel sealing assembly.

two additional type K thermocouples to monitor strap temperatures, and an electronic pressure transducer.

Seed material and nutrient (source material) for the hydrothermal process were prepared by Czochralski growth from stoichiometric melts. Reagents used to prepare melts were either Johnson Matthey 99.8% or Grade 1 bismuth silicate and Johnson Matthey Puratronic silicon oxide. Crystals weighing approximately 300 g were grown in a resistance furnace from 140 cm 3 platinum crucibles containing 450 g of melt. Crystals were pulled at 4 mm/h with a rotation rate of 70 rev/min over an 8 h period. The resulting crystals were then withdrawn to a point just above the melt and annealed in-situ at 700°C for 48 h. They were subsequently cooled to room temperature over a 36 h period. The lower purity melts were used as nutrient in experiments to establish basic hydrothermal growth conditions whereas high purity reagents were used to produce optical quality Czochralski and, ultimately, hydrothermal crystals. Seed crystals of approximate dimensions 30 m m × 10 mm × 1.5 mm were also cut from the Czochralski material in the (100), (110), or ( l i d

J. Larkin et al. / Hydrothermal growth of bismuth silicate 873

orientation. Some nutrient material was also produced by decanting bismuth silicate melts into room temperature graphite crucibles. This material was porous and friable but its use was discon- tinued because of the possibility that some of the bismuth was being reduced to the metal.

Due to the corrosive nature of solutions containing bismuth compounds, (metals in the autoclave walls can reduce bismuth oxide to bismuth metal) the entire growth experiment must be isolated from the autoclave. For this reason, the experiment is enclosed in a sealed platinum liner. Cylindrical platinum tubes of dimensions 1 inch × 11 inches or 2 inches x 12 inches (OD), and having a wall thickness of 0.05 inch, were used in the smaller or larger autoclaves respectively. Nu- trient material consisting of 1.5 to 2.0 cm pieces of BSO was placed in the lower (sealed) end of the liner. Additional SiO2 in the form of high purity quartz (5 to 15 g, based in part on the volume of the liner) is often added to promote solubility and to control the composition of the solution. A silver seed rack and perforated baffle assembly is lowered into the liner. The baffle rests on an indentation at the midpoint of the liner, and the seeds, which are attached by silver wires, are in the upper zone. A fitted disk with a small, open platinum vent tube (1 mm ID) is now heliarc welded to the top of the liner.

Sodium hydroxide solutions of normality between 1.5N and 5.0N have been used as the solvent (mineralizer). The solutions are prepared using Strem Chemical 99.8% or Apache Chemi- cal 99.999% NaOH and deionized water and are introduced through the vent tube. The amount of solution is calculated to produce an internal pressure within the liner of between 4500 and 10,000 psi. The vent tube is then crimped and heliarc welded. The 1 inch liners are inserted in the

1 smaller autoclaves above a 5 inch spacer to pre- vent the liner from contacting the relatively cold spot at the bottom of the vessel. The 2 inch liners are placed in a steel rack designed to suspend them at the midpoint of the 3 inch autoclaves. Either 1.0N NaOH or 1.5M NazSiO 3 solution is now added to the annular space between the liner and the autoclave in an amount calculated to provide a pressure balance across the liner

wall at operating temperatures. The autoclave is then sealed.

After closing the autoclave, the system was ramped to growth temperatures by the LFE Con- trollers over 72 h and then held for 25 days for the 1 inch experiments or 35 days for the 2 inch experiments. The bottom, nutrient zone, of the autoclave was held at a constant temperature (ranging from 360 to 400°C) in various experiments, while the top zone, seed area, of the autoclave was held at a lower constant temperature to establish a 5 to 25°C gradient (temperatures based on imbedded thermocouples). Tem- perature stability for the course of the experiment was _+ 0.1°C. Upon completion of the growth period the system was ramped to room temperature over 72 h.

3. Experimental results and discussion

3.1. Hydrothermal growth

Little or no growth occurred at NaOH normalities less than 4N or nutrient zone temperatures less than 390°C. Excessive, unoriented growth on the seeds and spontaneous nucleation on the walls of the liner occurred with normalities above 4N, nutrient temperatures above 390-C, or gradients much above 5°C. Experiments using the lower purity NaOH showed no visible differences in crystal quality. Experiments using Czochralski grown nutrient produced better crystals. Limited experimental data indicates that the preferred growth orientation is (100). Most of the crystals were colorless in contrast to the straw color of the Czochralski crystals (fig. 2). Some were pale green, perhaps as a result of minor leaks in the platinum liner. Growth was generally somewhat better in the two inch liners, perhaps because the gradient varies more smoothly in this configura- tion.

3.2. Uniformity of Czochralski crystals

An undoped Czochralski grown crystal from a typical growth run was sliced perpendicular to the growth axis and polished for optical absorption

874 J. Larkin et al. / Hydrothermal growth of bismuth silicate

Fig. 2. Hydrothermally grown bismuth silicate crystals.

measurements. The optical absorption results are shown in fig. 3. Unfortunately, the ordering of the slices within the boule was lost in the transfer from the performing vendor. Thus the slices are presented in random order. The absorption coefficient was calculated at three wavelengths: 5000,

o

6000 and 7500 A. These results should be re- garded as approximate since the index of refrac- tion was estimated at 8500 A and not adjusted as a function of the wavelength. As might be ex- pected, since 5000 A is in the absorption "tail", the coefficients are much higher in this curve. The values vary about 20% in all these curves. One could speculate that this variation is due to fluctuations in the number of defect centers and could have a direct influence on the photorefractive properties. We are preparing a second set of

10

I-. z i l l - 8 o u . I t . I1/ 0 6 o z 0 i - 4

t: 0

m 2

I I i i ~ I t i , i

• 7 5 0 0 A

• 6 0 0 0 A

• 5 0 0 0 A

0 I I I I I I I I I 110 111 0 1 2 3 4 5 6 7 8 9 12

S A M P L E N U M B E R

Fig. 3. Absorption coefficient of slices from a Czochralski BSO boule.

sequentially ordered optical samples to further determine the cause of this variation. We also intend to perform refractive index measurements and will report these results in the future.

3.3. Optical absorption and purity

A comparison of the absorption coefficients of Czochralski crystals with hydrothermal crystals is shown in fig. 4. An undoped hydrothermally grown crystal (BS42) has a much lower absorption and shorter wavelength cutoff than a typical undoped Czochralski sample (BSOll0F). This has also been reported by Martin et al. [6]. Sample BS122A is a Czochralski aluminum doped crystal.

Y w

O t l .

O O

Z O

I--

O

Fig.

i i

8 '

7

6 i

i l

I

\

• B S 4 2 • B S O 1 2 2 A A B S 1 1 • B S O 1 1 0 F

s ~ \

4 0 0 5 0 0 5 0 0 7 0 0 8 0 0

W A V E L E N G T H ( N M )

4. Absorption coefficient of hydrothermal and Czochral- ski BSO.

J. Larkin et al. / Hydrothermal growth of bismuth silicate 875

This dopant reduces the absorption tail of melt grown materials. It is also essentially transparent throughout the visible, but its cut off is at somewhat longer wavelengths in the ultraviolet. If the photoconductivity of the Czochralski material is due to deep donors, the effect of aluminum may be to introduce even deeper levels which empty the levels responsible for the visible absorption. BSll is a vanadium doped hydrothermal crystal which had a greenish tint. This was the only photorefractive hydrothermal specimen of the samples shown. The spark source mass spec- troscopy analysis of this material showed vanadium at about 200 parts per million atomic and also some iron and sodium. We are presently studying vanadium doped material both by the Czochralski and the hydrothermal technique.

[6], indicate that traps in hydrothermal material are reduced by at least two orders of magnitude below those found in Czochralski material. Fu- ture studies will involve the addition of dopants and of oxidizing materials to the hydrothermal environment.

Acknowledgements

Professor Wallace Leigh, Alfred University, is acknowledged for optical and TSC measurements, and Tack Chi Fu, Tufts University, for photorefractive measurements.

References

4. Conclusions

Hydrothermal bismuth silicate appears to be truly intrinsic in its properties. If colorless hydrothermally transported material is used to produce a melt for the growth of a Czochralski crystal, the resulting crystal again exhibits the typical straw color of melt grown material. This strongly indicates that thermal or oxidation driven traps are being formed. Thermally stimulated current measurements reported by Martin et al.

[1] H.S. Horowitz, A.J. Jacobson, J.M. Newsam, J.T. Lewandowski and M.E. Leonowicz, Solid State Ionics 32/33 (1989) 678.

[2] A.R. Tanguay, Jr., S. Mroczowski and R.C. Parker, J. Crystal Growth 42 (1977) 431.

[3] B.C. Grabmaier and R. Oberschmid, Phys. Status Solidi (a) 96 (1986) 199.

[4] A.F. Armington, Progr. Crystal Growth Characterization 21 (1990) 97.

[5] M.T. Harris, J.J. Larkin, E. Cormier and A.F. Armington, presented at 9th AACG West Meeting, Fallen Leaf Lake, CA, June 1991.

[6] J.J. Martin, M. Harris and J. Larkin, J. Crystal Growth, to be published.

hydrothermal growth of bismuth silicate (bso)

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