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single-moded at / = 0-633 pm and the overall fibre diameter
was 110 |im. The fibre was spun at various rates in the range
of 250-50 turn/m.
To analyse the fibre an He-He laser was used to illuminate
a 2 m length of fibre. The output was found to be planepolarised for a plane-polarised input and was circularly polarised for a circularly polarised input.
The beat length was measured under plane-polarised illumination by two methods. First, the rotation of the polarisation
at the fibre output was monitored as the fibre was cut back,
the length removed for a 180° rotation corresponding to a
beat length. Secondly, the variation of Rayleigh scatter transverse to thefibrewas observed giving a direct measurement of
beat length. The two measurements were in excellent agreement. Table 1 shows the beat lengths obtained for various
twist rates.
Table 1 MEASURED TWIST RATES AND BEAT
LENGTHS ON OUR SAMPLE FIBRE
Twist rate
Optical rotation rate
T/m
T/m
mm
50
83
100
116
160
250
50
83
100
116
14
10
10
6
5
4-3
35
50
Beat length
Table 1 clearly shows that the optical rotation for this fibre
closely follows the twist rate for rates less than or equal to
116 turn/m. For faster twist rates the optical rotation rate
tends to slip well behind. In fact, the resulting optical rotation
rate never exceeds the maximum of 116 turn/m irrespective of
the twist rate.
Conclusion: The technique of producing a strongly circularly
birefringent fibre by twisting an azimuthally inhomogeneous
core has been demonstrated with the simplest structure possible. A beat length of 4-3 mm at X — 0-633 /zm has been
achieved in a single-lobe fibre with a core radius of 10 \m\ and
an overall fibre diameter of 110/xm. This fibre should find
new and exciting applications in the areas of sensors, devices
and optical communications. We have only reported preliminary results here. We are confident that with improved
designs,f i.e. increasing the An value and reducing the lobe
core radius, we will shortly achieve substantially shorter beat
lengths with this technique.
MCINTYRE, P., and SNYDER, A. w.: 'Light propagation in twisted
anisotropic media: Application to photoreceptors', J. Opt. Soc.
Am., 1978, 68, pp. 149-157
HOT-ELECTRON INJECTION BY GRADED
AI I Ga 1 _ x As
Indexing terms: Semiconductor devices and materials, Hot
electrons
Triangular barriers of Al^Gaj^As are used as hot-electron
injectors. The Al^Ga^^As is linearly graded from x = 0 to
0-2 or 0-3 over distances of order 500 A to provide barriers
in the range 200 to 300 meV. In the present application the
barrier forms the injector of a hot-electron spectrometer.
The development of MBE and MOCVD growth techniques
allows precise control over doping and composition on a
length scale of a few Angstroms. This has led to a renewed
interest in GaAs hot-electron transistors in which hot electrons are injected across thin (<1000A) heavily doped
(1018 cm"3) base regions. The emitter and collector barriers
used so far have been provided by planar doped barriers1 or
square Al^Ga^^As barriers2 (x ~ 0-3). We propose an alternative to the thin (~ 100 A) square AlGaAs tunnelling emitter
barriers. Instead Al^Ga^^s is linearly graded from x = 0 to
0-2, 0-3 over a distance of order 500 A.3 The aluminium concentration is then dropped sharply to zero. Electrons are thermally excited over the triangular barrier in forward bias. The
lower capacitance of these graded-gap diodes over equivalent
tunnel diodes will result in improved high-frequency performance.
The graded gap forms the hot-electron injector of a hotelectron spectrometer4 designed to investigate relaxation of
hot carriers in heavily doped GaAs. The graded gap injects
electrons into one side of a base, after traversing which the
electron energy spectrum is determined by a planar doped
barrier whose height is a linear function of the reverse basecollector bias. An example of such a structure is shown in the
inset of Fig. 1. The material as grown by MBE on a (100) n+
substrate at a substrate temperature of 630°C. The planar
doped barrier was grown first followed by a 1000 A base
Si-doped to 1018 cm" 3 . On approaching the growth of the
AlGaAs barrier the substrate temperature was raised to
Acknowledgments: This work was supported by the UK
Science & Engineering Research Council.
C. D. HUSSEY
R. D. BIRCH
12th December 1985
Department of Electronics & Information Engineering
The University
Southampton SO9 5NH, United Kingdom
Y. FUJII
PDB
< 2
E
G-G
0-5
12th December 1985
Institute of Industrial Science
University of Tokyo
Tokyo, Japan
n-GaAs 1018cnrf3S i5000A
AlxGQi-xAs x«g.2
n-GaAs 1018 cm"3 Si
GaAs
References
P + GaAs10 18 cm" 3
1
GaAs
DYOTT, R. B., COZENS, j . R., and MORRIS, D. G.: 'Preservation of
polarisation in optical fibre waveguides with elliptical cores', Electron. Lett., 1979, 15, pp. 380-382
2
n-GaAs 10 18 cm" 3 Si
500A
1000 A
E
B
200A
120A
C
2 U 00 A
5000A
n + substrate
BIRCH, R. D., PAYNE, D. N., and VARNHAM, M. p.: 'Fabrication of
polarisation-maintaining fibres using gas phase etching', ibid.
1982, 18, pp. 1036-1038
3 SMITH, A. M. : 'Birefringence induced by bends and twists in singlemode optical fibres', Appl. Opt., 1978, 17, pp. 52-56
4 ROSS, J. N.: 'The rotation of polarisation in low birefringent monomode optical fibres due to geometric effects', Opt. & Quantum Electron., 1984, 16, pp. 455-461
5 PAPP, A., and HARMS, H.: 'Polarisation optics of liquid core optical
fibres', Appl. Opt., 1977, 5, pp. 1315-1319
6 ULRICH, R., and SIMON, A.: 'Polarisation optics of twisted singlemode fibres', ibid., 1979, 18, pp. 2241-2251
130
Fig. 1 Current/voltage characteristics for graded gap {G-G) and planar
doped barrier {PDB) diodes of area 8 x 10~9 m2 and 7 x 10~8 m2,
respectively
Al^Ga,_ x As graded from x = 0 to 0-3. Zero bias planar doped
barrier height is 0-316V
ELECTRONICS LETTERS 30th January 1986 Vol. 22 No. 3
680°C. The triangular barrier was formed by opening the
shutter to the aluminium Knudsen cell and subsequently
reducing gradually the cell temperature from 1150 to 950°C
and hence the aluminium flux to grade the Al^Ga^^As to
GaAs over 500 A. During the grading the substrate temperature was reduced back to 630°C. In processing the wafers
anodic etch techniques were used to reveal the buried base
layer followed by a shallow ohmic AuGe/Ni/Ti/Au contact.
In Fig. 1 we plot typical current/voltage characteristics for
graded-gap and planar-doped diodes. In strong forward bias
the characteristics are limited by series resistance from the
thin base. The hot-electron spectrum is given by the differential of the collector current with base-collector voltage. Examples of hot-electron spectra are shown in Figs. 2 and 3. These
Monte-Carlo simulation
In Fig. 2 we include a Monte-Carlo simulation of scattering in
this structure. The role of Landau damping is found to be
important in this model. This suggests that much of the low
energy part of the spectra is the result of electrons excited
from the base. The scattering of hot electrons in heavily doped
GaAs and the correlation of Al^Gaj.^As growth with transmission electron microscopy studies will be discussed in detail
in two further papers elsewhere.
In conclusion, we have described a technique for generating
or injecting hot electrons by graded Al^Gaj^As structures.
Such injectors are suitable as emitters for hot-electron transistors. The hot-electron spectra demonstrate the severe scattering that results in heavily doped GaAs and indicate the
need for thin (< 500 A) base widths in the design of efficient
hot-electron transistors.
This work was funded by the Procurement Executive of the
UK Ministry of Defence, sponsored by the Division of Components, Valves & Devices.
A. P. LONG
P. H. BETON
M. J. KELLY
T. M. KERR
GEC Research Limited
Hirst Research Centre
Wembley, Middx. HA9 7PP, United Kingdom
.-C 1
LU
28th November 1985
References
1
0
50
100
E.meV
HOLLIS, M. A., PALMATEER, S. C , EASTMAN, L. F., DANDEKAR, N. V., a n d
SMITH, P. M.: 'Importance of electron scattering with coupled
plasmon-optical phonon modes in GaAs planar doped barrier
transistors', IEEE Electron Device Lett., 1983, EDL-4, pp. 440-443
150
Fig. 2 Hot-electron spectrum for a base width 1000 A, doping level
1018 cm~2 and injection energy ~200 meV
2
HASE, I., KAWAI, H., IMANAGA, S., KANEKO, K., a n d WATANABE, N . :
'MOCVD-grown AlGaAs/GaAs hot-electron transistor with a
base width of 30 nm\ Electron. Lett., 1985, 21, pp. 757-758
Al x Gai_ x As graded from x = 0 to 0-2. Energy is referred to the
Fermi level. Also shown is a Monte Carlo simulation of scattering
in this structure
3
GOSSARD, A. C , BROWN, W., ALLYN, C. L., a n d WIEGMANN, W.:
'Molecular beam epitaxial growth and electrical transport of
graded barriers for nonlinear current conduction', J. Vac. Sci.
Technoi, 1982, 20, pp. 694-700
4
HAYES, J. R., LEVI, A. F. j . , and WIEGMANN, w.: 'Hot-electron spec-
troscopy', ibid., 1984, 20, pp. 851-852
,W b =1000A
ROOM-TEMPERATURE OSCILLATIONS IN A
SUPERLATTICE STRUCTURE
Indexing terms: Semiconductor devices and materials, Superlattices
We report on the observation of negative differential resistance driven oscillations at room temperature in which the
active element is a novel superlattice structure.
Fig. 3 Hot-electron spectra for base widths 1000 A and 2000 A, doping
level 5 x 1011 cm'3 and injection energy ~300 m
spectra were obtained at 4-2 K. The spectrum in Fig. 2 is from
the structure shown in Fig. 1, where injection energy is
~ 200 meV and the base width and doping are 1000 A and
1018 cm"3, respectively. Fig. 3 shows spectra where the
AlGaAs barrier is graded to 0-3 and the base doping reduced
to 5 x 1017 cm"3. The base widths are 1000 and 2000 A.
We note the following points from the hot-electron spectra:
(i) In all cases the hot electrons have suffered severe scattering
in the base regions. Only the 1000 A/5 x 1017 cm" 3 base
structure shows any evidence for ballistic electron transport
(ballistic transport is evidenced by a contribution to the spectrum at the injection energy); in all cases the peak is much
closer to the Fermi energy.
(ii) The results imply mean free paths of order 300-500 A,
which is much less than that due to optic-phonon scattering in
GaAs.
(iii) The results may be understood in terms of the coupled
optic phonon-plasmon scattering1 mode which will occur for
such high electron concentrations.
Transport through semiconductor superlattices was first
investigated by Esaki and Chang.1 They found that, as the
voltage drop across each period exceeded the miniband width,
structure appeared in the current/voltage (I/V) characteristics
owing to the formation of high field domains in which interminiband tunnelling occurred. We have recently reported the
observation of negative differential resistance (NDR)2 in a
novel superlattice structure which contains a high field region,
greatly enhancing this effect. The structure contains two sections of superlattice coupled by a tunnel barrier at which most
of the applied bias is dropped. This tunnel barrier is in fact
one superlattice layer made anomalously thick. The superlattice sections, which are doped to place the Fermi level in
the first miniband, act as energy filters, defining the allowed
initial and final states between which tunnelling can occur.
The relative positions of the minibands on each side of the
tunnel barrier at several biases are shown in Fig. 1; allowed
states are shaded with occupied states heavily shaded. The
magnitude of the current passed is determined by the overlap
in energy between occupied initial and unoccupied final states,
which in turn depends on the bias applied. At low bias values
the final states are in the first miniband while at high biases
they are in the second; in both these cases current can flow.
Between these regimes there is a range of biases where the
ELECTRONICS LETTERS 30th January 1986 Vol. 22 No. 3
131
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