ISSN 10637397, Russian Microelectronics, 2011, Vol. 40, No. 8, pp. 562–566. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © O.A. Buzanov, N.S. Kozlova, E.V. Zabelina, A.P. Kozlova, N.A. Siminel, 2010, published in Izvestiya Vysshikh Uchebnykh Zavedenii. Materialy Elektronnoi
Tekhniki, 2010, No. 1, pp. 14–19.
Piezoelectric pressure sensors play an important
part in car engines, enabling one to monitor residual
pressure in the cylinders and, ultimately, to control
and CO emission by ensuring completeness of
combustion . To do this, they must be compact,
reliable, and robust, being exposed to a temperature
that varies by about 300
C and reaches 3000
C at the
instants of ignition. More specifically, piezoelectric
crystals for the sensors concerned must meet the fol
lowing requirements :
(i) Their piezoelectric modulus must be sufficiently
large and insensitive to temperature variations.
(ii) They must be free from phase transitions over a
temperature range to about 1200
(iii) They must not exhibit pyroelectricity.
(iv) Their physical properties must be free from
(v) They must be close in linear expansion coeffi
cient to the material of the package.
(vi) They must lend themselves to machining (cut
ting, lapping, drilling, etc.).
(vii) They must offer an adequate amount of piezo
(viii) Their surface must be compatible with appro
(ix) Their resistivity must be high enough over the tem
perature range of operation, at least 10
cm at 300
These specifications are satisfied by lanthanum gal
lium tantalate, La
, also known as lan
gatate. The material is used in mobile phones as well as
hightemperature pressure sensors based on the direct
piezoelectric effect [2, 3].
As regards langasite, or lanthanum gallium silicate,
, it is employed in electrooptical laser
shutters and surface and bulkacousticwave devices.
Doped with Nd
, or Tm
, langasite crystals are
useful as laser active media [4–11].
At the same time, the commercial growth of high
quality, homogeneous, large crystals of langatate or
langasite with the given composition and properties is
still a demanding task. Defining specifications on
feedstock, growth conditions, and postprocessing to
ensure desired crystal quality requires an adequate
understanding of the nature and formation of lattice
defects, as these make a considerable contribution to
the electrical and optical behavior of the crystal. It is in
fact possible to estimate the structural quality of crys
tals from their optical and electrical properties.
Determining the appropriate partial pressure of
oxygen is crucial for growing langatate or langasite
crystals. On the one hand, oxygen is detrimental to the
material of a crucible . On the other hand, lack of
oxygen in an atmosphere of growth causes a dissocia
tion of the melt and evaporation of the gallium subox
ide, making the melt deficient in gallium and oxygen.
A number of compositions have been proposed for an
atmosphere in which to grow langatate or langasite: a
nitrogen–oxygen mixture [6, 11], air , argon [12,
14], and an argon–oxygen mixture [2, 10, 15]. The last
one, with about 2% oxygen, is probably the most pop
ular when growing langasite crystals.
Langatate measuring sensors designed to operate at
high temperatures often suffer from surface degrada
tion beneath the currentcarrying coating, which is
applied to a polar
cut. It is therefore important to
select a conducting material properly.
The electrical properties of langatate crystals as and
their dependence on the growth atmosphere have not
been discussed much in the literature. Accordingly, this
study is concerned with the influence of the growth
Optical Transmission Spectra and Electrical Properties of Langasite
and Langatate Crystals as Dependent on Growth Conditions
O. A. Buzanov
, N. S. Kozlova
, E. V. Zabelina
, A. P. Kozlova
, and N. A. Siminel
FomosMaterials, Moscow, Russia
MISIS National University of Science and Technology, Moscow, Russia
—The optical transmission spectra and electrical properties of langasite (La
) and lan
) single crystals are studied experimentally in relation to the oxygen concentration in
an argon–oxygen atmosphere of growth. The variation of langatate conductivity with temperature under a
constant electric field is examined over the range 20 to 500
C. Its dependence on currentcarrying material
is investigated for an Ir, Au/Ti, or Ag/Cr coating.
: langatate, electrical conductivity, optical spectroscopy, growth atmosphere.
AND TECHNOLOGY: DIELECTRICS