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prescribed /0- In the present circuit, however, reference current
/ t provides an additional degree of freedom. Thus, with the
area and resistance ratios from eqn. 3 as nominal design
values, a user can accurately obtain a desired 70 simply by
trimming Iy. Expensive on-chip resistance trimming is not
required.
Conclusion: A submicroampere controlled current source has
been described. The basic CCCS is integrable using standard
bipolar processing and provides linear current control over
four orders of magnitude with good temperature stability. The
circuit is self starting, eliminating the need of a separate startup circuit. Addition of an OA results in VCCS capability.
These significant improvements over a recently reported
technique1 makes the proposed design well suited to precision
applications such as low current references, large bit DA con-
vertors, voltage/current controlled low frequency oscillators,
long duration linear sweep generators and timers.
A. NEDUNGADI
1st April 1981
Department of Electrical Engineering
Indian Institute of Technology
Kanpur, 208016, India
References
1
BARKER, R. w. j . , and HART, B. L.: 'Novel submicroampere current
source design technique for monolithic circuits', Electron. Lett.,
1980, 16, pp. 609-611
2 HART, B. L.: Translinear circuit principle: a reformulation', ibid.,
1979, 15, pp. 801-803
3
VAN KESSEL, TH. j . , and VAN DE PLASSCHE, R. J.: integrated linear
basic circuits', Philips Tech. Rev., 1971, 32, pp. 1-12
0013-5194/81/090320-03$!.50/0
ACOUSTIC PROPERTIES OF EVAPORATED
CHALCOGENIDE GLASS FILMS
Indexing terms: Acousto-optics, Glass
Acoustic properties of characteristic acoustic impedance,
longitudinal velocity and density for evaporated chalcogenide
glass films of As-S, As 2 Se 3 and As-S-Se systems are determined by reflection loss measurements in an acoustic transmission line composed of sapphire/chalcogenide-glassfilm/water system in the U H F range. It is found that, by
appropriately selecting the As/S/Se composition ratio of the
evaporation source, the characteristic acoustic impedance of
the evaporated chalcogenide glass film can be controlled
within a range of 5-53-9-30 x 106 k g m " 2 s ~ 1 .
In chalcogenide glassfilms,especially in compositions of As2S3
and As2Se3, electrical, optical and structural properties have
been extensively studied because of their very important materials as photoconductof, semiconductor and optical memory
media,1 7 but no literature investigating their acoustic properties has been published, except one example of using the As2S3
film as an acoustic antireflection coating layer for matching a
large acoustic discontinuity at the sapphire/water interface in
an acoustic microscope lens.8 Generally, the acoustic properties of films prepared by evaporation or sputtering are considered to be more or less different from the bulk acoustic
properties as well as structural and composition properties.
In this letter, acoustic properties, namely characteristic
acoustic impedance, longitudinal velocity and density for evaporated chalcogenide glass films of As-S, As2Se3 and As-S-Se
systems are investigated. Determination of the acoustic
properties is made by the technique of reflection loss measurements in the UHF range for an acoustic transmission line,
which is composed of sapphire/chalcogenide-glass-film/water
system including the film to be measured.
very high
impedance
material
Z
0
very low
impedance
material
Z,
fJ727H
Fig. 1 Acoustic transmission line for determining acoustic properties of
chalcogenide glass films
322
To describe the measurement principle of acoustic properties of films, we consider an acoustic transmission line as
shown in Fig. 1. Thin films (characteristic acoustic impedance:
Zx) to be measured are formed between very high acoustic
impedance material (Zo) and very low acoustic impedance
material (Z2). Reflection loss RL at the a-a' boundary is given
as follows when the acoustic loss offilmcan be neglected:
RL = - 20 In | (Zin - Z0)/(Zin + Zo) |
(1)
where Zin = Zi(Z2 +jZx tan pl)/(Zi +jZ2 tan j?/), and
P = 2n/k = Inf/v.fxs the acoustic frequency and v, X and / are
the sound velocity, the wavelength and the film thickness, respectively. Reflection loss in eqn. 1 has a frequency dependence
and is maximum at pi = (In — l)n/2, i.e. / = (In — l)A/4, where
n is a positive integer. Eqn. 1 is then written as
RL = - 2 0 In \(Z\ - Z0Z2)/(Z\ + Z0Z2)
(2)
at / = A/4 (n = 1). The magnitude of reflection loss is strongly
dependent on the acoustic impedance offilm.Therefore, from
the measurement of the frequency dependence of reflection loss
in the acoustic transmission line as shown in Fig. 1, the maximum value of reflection loss and the acoustic frequency at a
quarter wavelength are easily found out, so that we can determine the acoustic properties of the film: Z t from eqn. 2 when
Z o and Z 2 are known, v from the relation oft; =fX = 4/7 when
/ is given, and p from the relation of Z t = pv. Here, the
acoustic transmission line of Z-cut sapphire/chalcogenideglass-film/water system is used for measurements on acoustic
properties of the films. Characteristic acoustic impedances of
Z-cut sapphire and water (at 20°C) are 44-56 x 106 and
1-48 x 106 kgm~ 2 s~ 1 , respectively. In this transmission line,
measurements of the film impedances with a range of 4 x 106
to 15 x 106 kg m~2 s~l can be performed with high sensitivity.
A series of evaporation sources of the 9 different compositions of As-S-Se system with a purity of 9999% were prepared
as listed in Table 1. The chalcogenide glass films were evaporated on one surface of the Z-cut sapphire rods of 6 mm
length at a substrate temperature of 25°C and a pressure of
1-3 x 10" 5 torr by heating Mo-wire-wound fused quartz crucible with weighted evaporation sources. The crucible temperature and the melting points of the evaporation sources were
checked by using a chromel-alumel thermocouple and by monitoring evaporation pressures. Deposition rates offilmswere
1-1-5 ^m/min. All the films were proved to be amorphous by
the X-ray analysis. Film thicknesses were measured by an
interference microscope. ZnO piezoelectric transducers with
an electrode of 1 mm in diameter to generate and detect longitudinal acoustic waves of a centre frequency around 450 MHz
were fabricated with a film thickness of about 5.7 fim by DC
sputtering.
ELECTRONIC LETTERS 30th April 1981
Vol.17
No. 9
To determine the acoustic properties of evaporated chalcogenide glass films, the frequency response of reflection loss
for each sample was measured by using the pulse mode measurement system developed by the present authors. 9 1 x Fig. 2
shows the experimental results automatically measured in a
frequency range of 200 to 700 MHz for an amorphous As-S
film of 1-25 pirn thickness whose evaporation source is the
As 2 S 3 system (sample 4 in Table 1). The maximum reflection
loss of 19-1 dB is measured at a frequency of 483 MHz. From
eqn. 2 the value of 191 dB is calculated to correspond to the
film with a characteristic acoustic impedance of 7-27 x 106
k g m ~ 2 s ~ 1 . Then the longitudinal velocity and the density of
the film are determined to be vL = 2-42 x 103 m s " 1 and
p = 300 x 103 kgm" 3 , respectively. For the bulk vitreous
As 2 S 3 , the acoustic properties are given as Z = 832 x 106
Y = 2 - 6 0 x l 0 3 m s " 1 and p = 3-20 x 103
kgm
kgm 3, respectively.12 Apparently, the values of film acoustic
properties are lower by about 6-13° o than those of bulk
properties. One of the reasons for the diiTerences may be due to
the variation of composition and the decrease of film density
by evaporation.
In conclusion, acoustic properties of characteristic acoustic
impedance, longitudinal velocity and density for evaporated
chalcogenide glass films of As-S, As 2 Se 3 and As-S-Se systems
have been investigated by reflection loss measurements in
acoustic transmission line composed of Z-cut-sapphire chalcogenide-glass-film water system in UHF range. As a result, it has
been demonstrated that the characteristic acoustic impedance
of chalcogenide glass film can be easily controlled in the
As-S-Se system with a range of 5-53-9-3O x 106 kg m~ 2 s" J
by taking the appropriate As S Se composition ratio for the
evaporation source.
Acknowledgment: The authors are very grateful to Y. Ohmachi
for his helpful suggestions on As 2 S 3 and As 2 Se 3 films and to
M. Aihara for the X-ray analysis.
24th March mi
J. KUSHIBIKI
H. MAEHARA
N. CHUBACHI
Department of Electrical Engineering, Faculty of Engineering
Tohoku University, Sendai, Japan
Table 1 ACOUSTIC PROPERTIES O F EVAPORATED AMORPHOUS As-S-Se FILMS
Sample
number
Composition
(As:S:Se)
atomic ° o
1
2
3
4
5
6
7
8
9
40:0:60*
40:30:30
40:40:20
40:60:0**
34:66:0
29:71:0
24:76:0
20:80:0
16:84:0
Film
thickness
Centre
frequency
Maximum
reflection
loss
/*m
MHz
dB
1-29
116
1-25
1-25
1-42
1 52
1-25
1-35
1-32
419
487
460
483
427
397
473
430
432
17-4
28-2
35-9
191
171
15 9
13 5
110
8-7
Similar experiments were performed for other evaporated
amorphous As-S, As 2 Se 3 and As-S-Se films to determine the
acoustic properties. The experimental results are tabulated in
Table 1. For the As 2 Se 3 film, the values of acoustic properties
are lower than those of bulk properties (Z = 10-44 x 106
k g m " 2 s - 1 , vL = 2-25 x 103 m s " 1 and p = 4-64 x 103
kgm~ 3 ), 1 3 and similarly in the case of As 2 S 3 film. For the As-S
films, all the measured parameters decrease monotonically with
increasing sulphur content, i.e. Z = 5-53-7-27 x 10 6 kgm~ 2 s~ 1 ,
vL = 2-28-2-42 x 103 m s" 1 and p = 2-42-300 x 103 kg m " \
respectively. For the As-S-Se films, the acoustic properties
were observed to vary between As 2 S 3 and As 2 Se 3 films, and Z
and p increased with increasing selenium content while v,
decreased.
Characteristic
acoustic
impedance
10 6 kgm" 2 s
l
9-30
8-44
7-99
7-27
706
6 91
6-55
608
5-53
Longitudinal
velocity
103ms-'
216
2-25
2-31
2-42
2-42
2-41
2-37
2-33
2-28
Density
103 kgm
3
4 31
3-75
3-46
300
291
2-87
2-76
2-61
2-42
References
1 MYERS, M. B., and FELTY, E. J.: 'Structural characterizations of
vitreous inorganic polymers by thermal studies', Mat. Res. Bull.,
1967, 2, pp. 535-546
2 BRANDES, R. c , LAMING, F. p., and PEARSON, A. D. : 'Optically formed
dielectric gratings in thick films of arsenic-sulfur glass', Appl. Opt.,
1970, 9, pp. 1712-1714
3 LAKATOS, A. I., and ABKOWITZ, M.: 'Electrical properties of amorphous Se, As 2 Se 3 , and As 2 S 3 \ Phys. Rev., 1971, B3, pp. 1791-1800
4 OHMACHI, Y., and IGO, T.: 'Laser-induced refractive-index change in
As-S-Ge glasses', Appl. Phys. Lett., 1972, 20, pp. 506 508
5 AST, D. G.: 'Structural and electrical properties of evaporatedamorphous and vitreous-amorphous V-VI compounds', J. Vac. Sci.
Technol., 1973, 10, pp. 748-752
6 DENEUFVILLE, J. p., MOSS, s. c , and OVSHINSKY, s. R.: 'Photo-
structural transformation in amorphous As 2 Se 3 and As 2 S 3 films',
J. Non-Crystalline Solids, 1973/74, 13, pp. 191 223
7 SOLIN, s. A., and PAPATHEODOROU, G. N.: 'Irreversible thermo-
structural transformations in amorphous As 2 S 3 films: a lightscattering study', Phys. Rev., 1977, B15, pp. 2084 2090
8 BRIDOUX, E., NONGAILLARD, B., ROUVAEN, J. M., BRUNEEL, C ,
THOMIN, G., and TORGUET, R.: 'Optimization of a transmission
acoustic microscope', J. Appl. Phys., 1978, 49, pp. 574 579
9 KUSHIBIKI, j . , SANNOMIYA, T., and CHUBACHI, N . : 'Performance of
sputtered SiO 2 film as acoustic antireflection coating at
sapphire/water interface', Electron. Lett., 1980, 16, pp. 737 738
10 KUSHIBIKI,
J., and
CHUBACHI,
N . : 'Double-layer
acoustic
antireflection coating for sapphire/water interface', ibid., 1981, 17,
pp. 59 61
11 KUSHIBIKI, j . , SANNOMIYA, T., and CHUBACHI, N.: 'A novel acoustic
200
300
measurement system for pulse mode in VHF and UHF ranges' (to
be submitted)
12 DIXON, R. w.: 'Photoelastic properties of selected materials and
their relevance for application to acoustic light modulators and
scanners', J. Appl. Phys., 1967, 38, pp. 5149 5153
13 OHMACHI, Y., and UCHIDA, N.: 'Vitreous As 2 Se 3 : investigation of
acousto-optical properties and application to infrared modulator'.
ibid., 1972,43, pp. 1709 1712
400
500
600
700
frequency,MHz
|i72/2|
Fig. 2 Experimental results of frequency response oj reflection loss for
evaporated amorphous As-S film (1-25 \im thick) whose composition oj
evaporation source is the As2S3 system (sample 4)
ELECTRONIC LETTERS
30th April 1981
Vol.17
No. 9
0013-5194/81/090322-0251.50/0
323
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