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Film on thermally treated Si has a higher threshold field for
the onset of electron injection but shows a trapping ledge in
the I-V characteristics. For surface modified Si, both ramp
and DC I-V characteristics show a leakage current considerably smaller compared with that for an untreated surface. This
may he attributed to the stacked oxide structure resulting in
reduced defect density by misaligning the micropores and
other interconnecting defects.’ The lowest value of leakage
current is obtained for the plasma treated silicon sample. It
may be due to the rise of substrate temperature to about
140°C prior to TEOS oxide deposition resulting in a film with
higher density. This is supported by the etch rate data of the
deposited oxides in a P-etch solution where the oxide with
plasma treated surface etches at the rate of 6.3 A/s compared
to the value of 14.3 A/s when the oxide deposition starts at
room temperature. Deposition at an elevated temperature also
plays a key role in reducing the water related traps.
The moderately high resistivity value in the range of
5 x 10’3-10’5Rcm indicates the absence of possible carbon
incorporation in the oxide film from the metal-organics. This
has been further confirmed by undetectable carbon in the film
by Auger electron spectroscopy analysis. The minimum values
of interface state densities D i i , estimated from high frequency
C-V characteristics by Terman method, lie in the range of
5.2 x 10” to 8.5 x 10iOeV-’cm-* and are comparable to
Dii of any good quality oxide. The breakdown strength is
highest for the oxide deposited on thin thermal oxide. The
oxygen plasma treated Si sample tends to breakdown at a
lower field with an average breakdown strength in the range
of 4 5 MVicm.
DESIGN OF A BROADBAND
HIGH-FREQUENCY ANTENNA O N GALLIUM
ARSENIDE USING THE EFFECTIVE INDEX
METHOD
Indexina terms: Antennas. Gallium Arsenide
~~
A broadband (1618GHz) microwave antenna has been
designed on a semi-indulsting GaAs substrate using effective
index theory designed for optical waveguides. Test results on
a prototype fabricated on Duroid 6010 have validated the
design, which is relevant to monolithic integration of highfrequency antennas with other electronic and optoelectronic
devices.
Introduction: Broadband antenna arrays are essential for
applications such as radar, frequency-agile microwave communication and electronic counter-measures. Such arrays are
most efficiently fed by means of the corporate feed configuration, which is compatible with single-output high power
sources, and allows frequency-independent beam steering
when non-dispersive phase shifters are incorporated in the
feed lines to the individual elements. However, at high frequencies (above 1 2 G H z or so) the small spacing between elements makes individual waveguide feed lines difficult to
attach, especially in two-dimensional arrays. T o alleviate this
problem, we propose the use of optical fibre or integrated
waveguide feeding, possibly to an optoelectronic transmit/
receive module monolithically integrated on a substrate such
as GaAs or InP. Such a module would consist of a high-speed
photodiode, laser diode or LED, an amplifier and a waveguide element, as illustrated in Fig. 1. When functioning in the
Acknowledgments: Authors are grateful to Prof. S. K. Lahiri,
Coordinator, Microelectronics Centre for helpful suggestions.
S. K. RAY
C K MAlTl
N. B. CHAKRABARTI
28th March 1990
Microelectronics Centre, Department of Electronics & ECE
IIT Kharagpur, India
References
1
SUGANO. T.
(Ed.): ’Application of plasma processes to VLSI tech-
nology’ (John Wiley and Sons, New York, 1985)
2 AOAMS,
A.e., and CAPIO,
c. D.: ‘The deposition of silicon dioxide
films at reduced pressure’, J. Electrochem. Soc., 1979, 126, pp.
1042-1046
3 CHIN,B. L., and V A N OE VEN,E. :P ‘Plasma TEOS process for
interlayer dielectric applications’, Solid State Technol., 1988, 34,
pp. 119-122
e. K., and CHAKRABARTI, N. B.: ‘Oxidation of
4 RAY,s. K., MAITI,
silicon in microwave oxygen plasma’. Proc Int. Sym. Electronic
Devices, Circuits and Systems, Kharagpur 1987. pp. 291-294
K., MAITI, c. K., LAHIRI, s. K.,and CHAKRABARTI,
N. B.:‘Low
temperature deposition of silicon oxide films by microwave plasma
CVD of TEOS’, Semicond. Sci. Technol., 1990, in press
6 KIROV,
K. I., GMRGIEV,
s. s., GEROVA,
E. v., and ALEKSANDROVA,
s. P.:
‘Investigation of SiO, layers deposited by plasma decomposition
of tetra-ethoxy silane in a planar reactor’, Phys. Stat. Sol., 1978,
48, pp. 609-613
A. K.: ‘Synthesis of high quality ultra-thin
7 ROY, P. K., and SINHA,
gate oxides for VLSI applications’, A T & T Tech. J., 1988, pp.
s.
5 RAY,
155- 174
transmitting mode, the module accepts an optical signal from
an optical fibre. This signal is converted by the photodiode
into an electrical signal which is then amplified and radiated
into free space by the antenna. In receiving mode, a microwave signal is detected by the antenna, is amplified and converted by the light source (a laser diode or LED) into an
optical signal which is coupled into another optical fibre.
Apart from providing a compact corporate feed configuration,
this arrangement allows for remote positioning of the antenna
with respect to the signal source, because of the low losses in
optical fibre transmission. Phase shifting could be accomplished under optical control by including coherent detection
of the optical signal at the input to the transmit/receive
module. Apart from the availability of light sources with SUEciently large modulation bandwidths, the major problem in
implementing such a scheme is the antenna.
Broadband antenna design: In this letter, we concentrate on
designing a suitable broadband antenna on GaAs, InP or
other suitable high-permittivity substrate. We have chosen a
slot-line structure (see Fig. 2) for compatibility with other
x - 0x1s
A
~~
.
n2
.~
nl
n3
,
n2
*y-axis
r S
Fig. 2 Slot-line structure used for borad band antenna design
cI1
output
oprical
fibre
I
sem insulating GaAs substrute
Fig. 1 Monolithically integrahle optoelectronic transmit/receive module
ELECTRONICS LETTERS
5th July 1990 Vol. 26 No. 14
waveguide components and for its straightforward fabrication: simple metallisation of a planar substrate is sufficient to
define the antenna, and no etching is required. Design and
analysis of such a structure is usually accomplished’ by
writing Maxwell’s equations with appropriate boundary conditions, expanding the fields in a set of basis functions in the
time or frequency domain, then deducing the weight functions
in the expansion by solving a set of complicated coupled integral equations, using one of the methods of moments.’ When
a high-permittivity substrate is used, the fields in the slotline
1083
may be considered to he contained entirely within the substrate, as in an air-clad optical w a v e g ~ i d e Thus
. ~ we postulate
that it should be possible to use optical waveguide design
techniques such as the effective index
to determine
the impedance of the slot-line
To apply the effective index method, we divide the slot-line
in Fig. 2 laterally into three regions, each of which is initially
considered semi-infinite in the y-direction, according to the
scheme shown in Fig. 3a. An effective relative premittivity E.,,
-
~~~
n2
n3
~
n4
n4
n4
~
outer regon
centre region
outer region
" n1,
"I
n1
x-axis
f
L'
~
~-
6'1
'J
Fig. 3 Subdiuision ofslot line antenna
a
Actual divisions
b Resulting effective index profile
(or effective refractive index ne,,, where n:, = E,,,) is calculated in each region from the TE and TM mode propagation
constants obtained by satisfying the boundary conditions at
each interface. The two-dimensional index distribution in Fig.
2 is then approximated by the one-dimensional effective index
distribution shown in Fig. 36, assumed semi-infinite in the
x-direction. The overall effective index is then calculated and
used to determine the impedance of the slot-line section. By
repeating these calculations for various slot widths, the principal design curve for the antenna is obtained, by which the
antenna can match the input line (usually 50Q) to free space
(usually 377Q). For optimum performance at a specified
design frequency, a tapered impedance transformation might
be used, but for enhanced broad-hand capabilities, a foursection Chebyshev quarter-wave transformer design was
chosen by reference to standard fractional bandwidth tables5
The balanced-to-unbalanced transformer, or balun, was
implemented using a Marchand design,6 again for enhanced
broad-band performance.
Antennafabrication and evaluation: A prototype structure was
designed as described above, a Ruhylith mask was laid out
using a commercial CAD system and transferred using standard photolithography to a copper-clad substrate formed of a
high-permittivity composite (Duroid 6010)' chosen for its
similar dielectric properties to semi-insulating gallium
arsenide. A three-element array was defined in this way, but
only the centre element was fed actively, to simulate the effects
of lateral array loading on the structure. Two wire bonds were
attached to connect the balun.
The standing-wave ratio (SWR) of the prototype antenna
was measured as a function of input frequency using a sweep
oscillator and microwave frequency network analyser
(Hewlett Packard HP8510), showing a cutoff at 14.3 GHz, a
centre frequency of 16GHz and an upper limit extending
beyond I8GHz. A dedicated anechoic test range was then
used to test the antenna in receiving mode, and a typical
electric field polar pattern at 18GHz is given in Fig. 4,
showing a beam width of approximately 1 radian with four
side lobes lOdB helow the principal lobe. These lobes should
be suppressed and the beam width narrowed in a uniformly
Fig. 4 Measured electricfield polar pattern at I 8 G H z
1084
ELECTRONICS LETTERS
5th July 1990
Vol. 26 No. 14
fed antenna array with a larger number ( 2 5 ) of elements,
leading to a compact and rugged structure with high directional gain. By constructing such a structure on semiinsulating GaAs, we shall demonstrate that such an antenna
array will also be compatible with monolithic optoelectronic
integration.
Conclusions: We have designed, fabricated and tested a novel
broadband high-frequency slot-line antenna on a highpermittivity substrate. The antenna was designed using the
effective index method common to optical waveguide theory,
a method which is considerably simpler than conventional
antenna design techniques based on methods of moments. The
antenna can, in principle, be fabricated on a substrate of semiinsulating gallium arsenide, indium phosphide or similar
material, for compatibility with optoelectronic integration.
Acknowledgment: We are grateful to Mr. Denis E. Mathews,
formerly of Alcoa Defense Systems Inc., San Diego, California,
for helpful discussions.
L. J. BACA
J. G. McINERNEY
Optoelectronic Device Physis Group
Center for High Technology Materials
University of N e w Mexico
Albuquerque, New Mexico 87131, U S A
24th M a y 1990
References
F., AUDA, H. A., and MAUTZ, I. R.:
‘Characteristic modes for slots in a conducting plane: TE case’,
I E E E Trans., 1987, AP-35,pp. 162-168
m I 0 , A. I., and MILLER, E. K.: ‘Techniques for low-frequency
problems’, in ‘Antenna handbook: Theory, applications and
design’, Eds LO, Y. T., and LE& s. w., (Van Nostrand Reinhold, New
York, 1988)Ch. 3
ADAMS, M. I.: ‘An introduction to optical waveguides’ (Wiley, Chichester, 1981)
KNOX,
R. M., and ~ U L I O S P.
, P.: ‘Integrated circuits for the millimeter through optical frequency range’. Proc. MRI Symp Submillimeter Waves, Polytechnic Press, Brooklyn, pp. 497-516
JONES, E., MA-,
I., and YOUNG,
D.: ‘Microwave filters, impedance
matching networks, and coupling structures’ (McGraw-Hill, New
York, 1964)pp. 259-281
CLQETE, I. H.: ‘Graphs of circuit elements for the Marchand balun’,
Microwave J., 1981, pp. 12S128
Duroid 6010 is manufactured by Rogers Corporation, Microwave
Materials Division, Box 3000, Chandler, Arizona, USA
KABALAN, K., HARRINGTON, R.
experiments have been also reported at 1.54pm with 30 and
9 ps pulses before and after fibre compression, respectively.’
The DFB laser was modulated near 2 G H z and an opticallypumped erbium-doped fibre was used for pulse amplification.
We report on improvements at 1.53pm using a microwavemodulated DFB laser with an antireflection (AR) coating on
one facet and the Bragg wavelength downshifted from gain
maximum. Single-mode pulses of less than 20ps are generated
from the laser at multi-gigaHertz repetition rates. Fouriertransform limited pulses of 4 ps width and 0.3 W peak power
are obtained after fibre compression. For the first time, we
show that the phase-coherence can be kept from pulse to
pulse.
Experimental s e t u p : The laser used in the experiments is a
400pm long double channel planar buried heterostructure
(DCPBH) made by CGE. The Bragg wavelength is 5 n m
shorter than the gain peak wavelength. The 3% AR coating
deposited on the main output facet ensures single-mode operation a t any bias with a rejection ratio higher than 40dB for
side modes. The threshold current is 53mA at 20°C and the
output power reaches 15mW at 100mA. The -3dB cutoff
frequency of the device is about 6 G H z at a 80mA current
bias. Gain-switching is achieved by a strong microwave
modulation of the injection current. A microwave synthetiser
H P 8341B followed by a 1 W amplifier is used for this
purpose. For pulse compression, the laser beam is coupled
into a 1 km single-mode dispersion-shifted optical fibre. At
1,53pm, the dispersion of the fibre is approximately -25ps/
nm/km and the total attenuation including coupling losses is
3 dB.
Three types of measurements are performed. A 15 GHz
bandwidth InGaAs photodiode is used in conjunction with a
TEK sampling oscilloscope for a direct analysis of the laser
pulse shape. The laser pulsewidth is measured with a collinear
type-I autocorrelator using a newly developed P O M crystal
for second-harmonic generation at 1.3-1.5pm. The time averaged spectrum of the optical pulse is measured by either a
monochromator with 0.05 nm resolution or a Fabry-Perot
(FP) interferometer having a maximum free-spectral-range of
120 GHz and a finesse of 80 (is., a resolution of 0.02 nm).
R e s u l t s : Fig. 1 shows the single-mode laser pulse width (full
width at half maximum, FWHM) measured before compression as a function of the microwave current. Measurements
100-
80a
U
)
-
=-KOI
BANDWITH-LIMITED 0.3W PICOSECOND
PULSES (4ps) FROM A 1.53pm
MICROWAVE MODULATED DFB LASER
WITH FIBRE COMPRESSION
Indexing terms: Lasers and laser applications, Pulse generation
Picosecond pulse emissions at I.53pm using a microwavemodulated DFB laser with an antireflection (AR) coating on
one facet and the Bragg wavelength downshilted from gain
maximum are reported. Single-mode pulses of less than 20ps
were generated from the laser at multi-gigaHertz repetition
rates. Bandwidth-limited pulses of 4ps width and 0.3 W peak
power were obtained after fibre compression. The pulse to
pulse coherence is revealed by the modulation sidebands in
the time-averaged spectrum of the laser emission.
Introduction; Gain-switching of distributed feedback (DFB)
semiconductor lasers is an attractive technique for producing
single-mode picosecond pulses at optical telecommunication
wavelengths. Recently, 22 ps pulses have been obtained at
1.3pm by this technique, leading to 3ps pulses after fibre
compression.’ The repetition rate was about IO MHz. Other
ELECTRONICS LETTERS
5th July 1990 Vol. 26 No. 14
.
? I
$403
Q
-
20 -
0
40
80
120
microwave current, mA
160
200
Lq
Fig. 1 DFB laser pulse width against microwave current
are made from the uncoated laser facet. The modulation frequency was 2.1GHz. The D C bias was optimised for each
microwave power with values scaling from 70 to 90mA. Measurements with the fast photodiode are used to verify the
single-peak pulse emission. The type of evolution reported in
Fig. 1 agrees with recent theoretical calculations by Lam4 It
clearly evidences the advantage of strong current modulation.
The pulsewidth is reduced by a factor of 4.5 when the current
amplitude is increased from 30 to 200mA, i.e., when the
microwave power is increased from 13 to 30dBm. Further
improvements appear to be feasible by using still more powerful microwave amplifiers (> 3OdBm). but with an increased
risk of degradation for the device. The 18ps pulsewidth
1085
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