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line narrowing’ (FLN). At low dopant concentrations (i
lob),
in the absence of significant cross-relaxation of excitation
between dissimilar ion sites, fluorescence from the fibre will be
characteristic of the excited subset of ions. The narrow inhomogeneous linewidth of neodymium in phosphate glasses
compared with silica-based glasses indicates that less site-tosite variations are expected in such materials and hence less
pump wavelength dependency is observed.
Conclusions: A number of Nd : silica fibre types with a range
of additives have been investigated for spectral stability with
reference to the pump wavelength. The results show a worstcase correlation between pump wavelength and fluorescence
wavelength of approximately O.lsM.32nm/nm at room temperature. Single-mode fibres fabricated from Nd : phosphate
glass do not show such a large correlation and the worst case
sensitivity is approximately 0.05nm/nm. This fibre type also
shows a stability over the pump wavelength range 80&840nm
of % 200 ppm. Coarse wavelength stabilisation of & 15nm at
an appropriate pump wavelength will be sufficient to produce
wavelength stability of the order of lOppm, which is suitable
for high accuracy FOG applications, with the phosphate fibre
type.
Acknowledgments: Acknowledgments are due to M. P.
Varnham of British Aerospace, Stevenage, UK for useful discussions relating to this work. The work was funded under a
contract from British Aerospace, Stevenage, UK.
P. R. MORKEL
22nd March 1990
E. M. TAYLOR
J. E. TOWNSEND
D. N. PAYNE
Optical Fibre Group
Dept. of Electronics and Computer Science
University of Southampton
Highfield, Southampton SO9 5”.
United Kingdom
to control the transport of ballistic hot electrons.’ To realise
such devices, it is necessary to develop a technique to form
ultrafine structures of one type of semiconductor buried in
another type of semiconductor. One promising method for
this purpose is the formation of ultrafine structures using
lithography followed by regrowth. We have developed the
technique to form such structures by using electron beam
lithography and wet chemical etching and realised 70nmpitch InP rectangular corrugations? It is important to
develop the regrowth technique to allow us to embed ultrafine
structures. Until now, such regrowth was reported only for
fine structures with periods over ~ O O N ~ , whereas
~ . ~
this
process must be sensitive with respect to the size and pitch of
structures to be embedded.
In embedding fine structures by regrowth, one of the serious
problems is their thermal deformation3 and the deformation is
more drastic with decreasing size. Therefore, it is important to
optimise the regrowth conditions to suppress the thermal
effect, especially in the case of quantum-sized structures.
Here we report the formation of 7Onm-pitch InP corrugations buried with GaInAs by organometallic vapour phase
epitaxy (OMVPE) to preserve their rectangular shapes.
We carried out GaInAs regrowth on InP rectangular corrugations of 70nm pitch, with their tops covered with thin
GaInAs. First, the rectangular patterns on OMWE-grown
wafer were obtained as follows. On the wafer, the resist
pattern was formed by electron beam lithography and transferred to InP by two-step wet chemical etching, in which the
thin GaInAs layer plays the rBle of the etching mask in InP
etching. The details of this process have been reported previously.2 The etched structure is shown in Fig. la. After conventional cleaning using organic solvents, the wafer was
loaded into the OMVPE reactor and GaInAs was grown on
it, as shown in Fig. Ib.
220.70nrn
I
I
-
GalnAs
InP
LO-1 00 nrnI
ReferenceS
GalnAs
Po, H.,HAKUMI, F., MANSPIELU, R. I., MCWLLUM, B. c., NMINELLI,
P., and S M T ~ E.:
, ‘Neodymium fibre laser at 0.905, 1.06
R.
and
1.4pm’. Proc. OSA Meeting, Seattle, 1986
I
InP
substrate
and CRAIG, s. P.:
‘IOmW superfluorescent single-mode fibre source at 1060nm’,
Electron. Lett., 1987,23, (24), pp. 132&1321
LIU, K., DIGONNET, M., SHAW, H. J., AMSLW B. I.,
I
BURNS, W. K., UULING, 1. N., GOLDBWG, L., MOELLW, R. P., VILLARRUEL, c. A.,SNITZER, E, and Po, H.: ‘Fiber superfluorescent sources
for fiber gyro applications’. Roc.Cod. Optical Fibre Sensors
OFS’89, Paris, 1989, pp. 137-142
TAVWR, E M., et al.: ‘Application-specificoptical fibres manufactured from multicomponent glasses’. Proc. MRS Conf., Boston,
1989
s. A., and WEBER, M. I.: ‘Observation of fluorescence line
narrowing, hole burning, and ion-ion energy transfer in neodymium laser glass’, Appl. Phys. Lett., 1979,35, (1)
BRA-
b
Fs 1 Regrowth process
a Etched structure
BURIED RECTANGULAR GalnAs/lnP
CORRUGATIONS OF 70nm PITCH
FABRICATED BV OMVPE
Indexing terms: Semiconductor deuices and materials, Indium
compounds, Gallium compounds, Vapour deposition
Rectangular InP corrugations of 70nm pitch and 40nm
depth were buried with GaInAs by OMVPE so as to preserve the rectangular shape. A low regrowth temperature and
short heating up time in an atmosphere of high PH, partial
pressure are effective in the suppression of thermal defonnation during regrowth.
The wave properties of electrons in ultrafine artificial semiconductor structures are very attractive for the creation of new
devices. We have proposed the use of electron wave diffraction
ELECTRONICS LElTERS 21st June 1990
Vol. 26
No. 13
6 Regrown structure
The OMVPE system’ used for the regrowth has a horizontal reactor with RF heating and operates at a low pressure of
1.0 x 104Nm-’. Triethylindium (TEI), triethylgallium (TEG),
phosphine (PH,) and arsine (ASH,) were used as sources. The
typical V/III ratio and growth rate for GaInAs were 29 and
l%jan/h, respectively. The growth temperature was 520°C
close to the lowest temperature at which good morphology
can be obtained under these conditions. With TEI good morphology can be obtained at lower temperatures than with
trimethylindium, the conventional indium source.
First, GaInAs regrowth on InP corrugations of 70nm pitch
and lOOnm depth was attempted. The heating up time, from
the start of heating to the start of regrowth, was 4.5 min in an
atmosphere of PH,. The partial pressure of PH, was
94Nm-*. In this condition, the periodicity of the corrugation
was conserved after the regrowth, as shown in Fig. 2.
However, the rectangular shapes became round. Moreover,
the periodicity of the corrugation was frequently broken by
the connection of a few InP mesas.
We inferred that the main reason for this deformation was
the thermal effect during the heating up period, which was
a75
determined by the stabilisation of the temperature of a thermocouple behind the susceptor. However, we confirmed that
100nm
Fig. 2 InP corrugationswith GaInAs buried under original conditions
the actual temperature rose more quickly through measurement with an IR pyrometer. Hence, we shortened the heating
up time to suppress the deformation. We began t o grow
GaInAs as soon as the temperature reached 500°C and the
heating up time was shortened to 1.5min. When 30s had
passed since the start of growth, the stabilised temperature of
520°C was reached. The partial pressure of PH, during the
heating up period was raised to 2.2 x lo3Nm-’.
Collapse of the InP rectangular mesa before regrowth was
another possibility for the deformation, especially in the case
of the connection formed between a few InP mesas. This collapse was frequently observed after the etching and cleaning
processes, before regrowth, due to the high aspect ratio. This
problem was overcome simply by decreasing the depth of corrugation to 40nm. Necessary modifications in the etching and
cleaning processes are to be carried out to obtain a deep mesa.
We attempted GalnAs regrowth on the 70nm pitch, 40nm
depth InP corrugations with the improvement in the conditions mentioned above. A 50nm thick GaInAs layer and an
InP layer were grown on the InP ultrafine corrugation. Fig. 3
shows the cross-sectional view of the buried InP corrugations.
Acknowledgments: We wish to acknowledge Prof. Y. Suematsu, President of Tokyo Institute of Technology, for his
continuous support, Assoc. Prof. S. Arai for fruitful discussions and Mssrs. S. Tamura and M. Yabuki for experimental support. This work was supported by the Scientific
Research Grant-in-Aid and by University-Industry Joint
Research in ‘Mesoscopic Electronics’, both from the Ministry
of Education, Science and Culture, Japan.
T. YAMAMOTO
Y. MIYAMOTO
M. OGAWA
E. INAMURA
K. FURUYA
Department of Electricaland Electronics Engineering
Tokyo Institute of Technology
2-12-1,O-okayarna,Meguro-Ku, Tokyo, 152,Japan
17th April 1990
Referenees
FURUYA, K.: ‘Novel high-speed transistor using electron-wave diffraction’, J. Appl. Phys., 1987,62, pp. 1492-1494
INAMURA, E., MIVAMOTO,Y., TAMVRA, s., TAKASUGI, T., and FURWA,
K.: ‘Wet chemical etching for ultrafine periodic structure: rectangular InP corrugations of 70nm pitch and lOOnm depth’, Jpn.
J. Appl. Phys., 1989.28, pp. 2193-2196
NAGAI, H.,NOOUCH, Y., and MATSUOKA, T.: ‘Thermal deformation
of surface corrugations on InGaAsP crystals’, J. Cryst. Growth,
1985,71, pp. 225-231
RAZEGHI, M., BLONDUU, R., KAZMIFXSKI, K., KR*KOWSKIM, M., DE
CREMOUX, E., OUCHEMIN,1. P., and muLEY, I. c.: ‘CW operation of
t.57pm GaJn, _,As,P,_,InP distributed feedback lasers grown
by low-pressure metalorganic chemical vapor deposition’, Appl.
Phys. Lett., 1984,45, pp. 784-786
MIYAMOTO,Y., UESAKA, K., TAKADOU, M., NRWA,
K.,and SUEMATSU,
Y.: ‘OMVPE conditions for GalnAs/lnP heterointerfaces and
superlattices’,J. Cryst. Growth, 1988,93, pp. 35>358
POLARISATION-MAINTAINING FIBRE USING
A NOVEL GLASS SYSTEM
.,
%U*,.”
(1
,
1
.*
.
H
100nm
Fig. 3 InP corrugations buried with GalnAs
in
an atmosphere of high
PH, partial pressure
It can be seen that InP corrugations with 70nm pitch and
40nm depth have been buried with GaInAs preserving their
rectangular shapes over a wide region. It can also be seen that
the flatness is rapidly recovered in this regrowth, because the
heterointerface between the regrown GaInAs and InP is
almost flat, although the distance between this interface and
the corrugation is only 30nm.
In our experiment, we fixed the flow rate of the regrowth
sources. However, with a higher VPII ratio and lower growth
rate, it is possible to decrease the growth temperature further.
Thus, much finer structures can be obtained, without deformation, by modifying the growth conditions.
To enable us to apply this technique to new devices, we will
carry out necessary studies of the electrical properties of the
regrown interface as the next step.
In summary, we buried 70nm pitch InP corrugations with
GaInAs by OMVPE such that those rectangular shapes were
preserved. Lowering the growth temperature and heating up
in an atmosphere of high PH, partial pressure in a short
period are effective in suppressing thermal deformation during
regrowth.
876
Indexing terms. Optical fibres, Birefringence, Refractive index
profile, Optical properties of substances
A new glass dopant system, AIPO, is used to form the stressproducing regions of a polarisationmaintainingoptical fibre.
The advantages of this glass system are that it can match
match the refractive index of silica and does not have a fundamental absorption peak near the optical frequencies of
interest, allowing it to be placed in close proximity to the
fibre core to optimise birefringence. This material was used
for the cladding of a preform, which was then flattened to
yield an elliptical fibre upon drawing. This fibre demonstrated an extinction ratio at 1.3/rm of 42.5dB for a 10m
length and 22dB for 500111 for h = 1.3 x 10-’m-’. The beat
length at 1 = 0.488/nn was about 9mm, corresponding to a
modal birefringence of 5.3 x IO-’.
Introduction: Stress-induced birefringence prevents mixing of
orthogonal modes caused by perturbations along the length of
a single mode fibre.’ The stress producing material must have
a thermal expansion coefficient substantially different from
that of silica for the birefringence to be created by thermal
contraction between the core and the anisotropically crosssectioned stress-inducing regions.’ Typically, borosilicate
compositions are used to create the required thermal expansion mismatch. However, this material, as a result of the E O
bond, has a long wavelength absorption edge starting around
1.lpm. In spite of this constraint, low loss birefringent fibre
for operation at wavelengths above 1.1p n has been fabricated
For example, a polarisation
using a barrier layer
holding parameter h of 1.7 x 10-6m-1 for lOOm lengths and
losses of less than O.3dB/km at 1.3 and 1.5pm have been
reported for rectangular PM fibres. Another successful fibre
design is the so-called PANDA fibre, which has two highly
ELECTRONICS LETTERS
21st June 1990
Vol. 26
No. 13
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