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higher temperatures, but it still exists at lower temperatures. In
order to check this more carefully, we used a linear amplifier
which terminates the Ge detector in zero impedance, but the
same saturation behaviour below the threshold was observed
not only at c.w. but also for pulsed operation (repetition
rate = 40 pps, pulse width = 85 us). In each light output pulse
we could not see any evidence of heating below the threshold.
The common observed behaviour is that the light output increases sublinearly up to Ilh/2 and linearly thereafter until
reaching threshold. The physical explanation for this has not
yet been given.
In conclusion, we have demonstrated the usefulness of a
chemical etch in the monolithic fabrication of laser diodes, but
there remain problems to be solved, such as the reproducibility
in each process, electrical contacts to etched lasers and the
uniformity of facet passivation.
We would like to thank A. A. Ballman and W. B. Bonner for
supplying us with InP substrates, P. K. Tien and M. A. Pollack
for their useful advice and encouragement, R. J. Martin for his
helpful advice in fabrication, R. E. Nahory for his comments
on laser characteristics, L. F. Mollenauer for his suggestions on
light detection and J. H. McGee for his assistance in the
measurements.
K. IGA*
B. I. MILLER
Bell Laboratories
Holmdel, NJ 07733, USA
T.,
WAKAO,
K.,
a n d SAKAMOTO,
Y.:
'GalnAsP/InP facet lasers with chemically-etched end mirrors',
Jpn. J. Appl. Phys., 1979, 18, pp. 2035-2036
3
IGA, K., POLLACK,
M. A., MILLER,
B. I., a n d MARTIN,
R. J.:
GalnAsP/InP DH lasers with a chemically-etched facet', IEEE J.
Quantum Electron., 1980, QE-16, to be published
4
diffusion source(In-Zn)
Fig. 1 Experimental set-up for Zn diffusion in InP
GaAs-GaAlAs double heterostructure lasers with polycrystalline
GaAsP lasers—II. Lasers with etched mirrors', IEEE J. Quantum
Electron., 1977, QE-13, pp. 628-631
KAMBAYASHI,
exhaust
1549/11
ITOH, K., ASAHI, K., INOUE, M., and TERAMOTO, i.: 'Embedded-stripe
IGA, K.,
V/////////////////A
InP substrate
References
2
furnace
9th September 1980
* On leave from Tokyo Institute of Technology
1
The ordinary way to diffuse Zn or Cd in III-V compounds is
the sealed-ampoule technique.12 This method often results in
a veiled surface due to metal condensation during ampoule
cooling. It is also now well known that even a short heat
treatment induces out-diffusion of the group V volatile
element. Guha and Hasegawa3 have shown that there is a
considerable amount of phosphorus evaporated from InP
when heating at temperatures above 400°C. Davies et a/.4 have
shown that thermal erosion could be avoided by annealing in a
mixture of PH 3 and H2.
Starting from these statements, we have developed an open
ampoule diffusion process in which InP surface erosion is
avoided by a PH 3 /H 2 atmosphere.
InP <100> single-crystal substrates (2 x 1016 electrons
cm" 3 ) were purchased from MR or Sumitomo. They were
etched in a bromine-methanol solution prior to diffusion.
Ohmic contacts were evaporated on before electrical
measurements: Au-Ge on the n-type side and Au-Be on the
p-type side.
WRIGHT, P. D., NELSON, R. J., and CELLA, T.: 'lnGaAsP hetero-
structure lasers (A = 1-3 n) with etched reflectors', Appl. Phys. Lett.,
1980, 36, pp. 518-520
5 MILLER, B. I., and IGA, K.: 'GalnAsP/InP stripe laser with etched
mirrors fabricated by a wet chemical etch', ibid., 1980, 37, p. 339
6 IGA, K., and MILLER, B. I.: 'GalnAsP/InP laser with monolithically
integrated monitoring detectors', Electron. Lett., 1980, 16, pp.
342-343
7 WRIGHT, P. D., and NELSON, R. J.: unpublished work
8 KAMBAYASHI, T., KITAHARA, c , and IGA, K. : 'Chemical etching of InP
and GalnAsP for fabricating laser diodes and integrated circuits',
Jpn. J. Appl. Phys., 1980, 19, pp. 79-85
The experimental set-up is shown in Fig. 1. In a quartz tube
furnace flushed by nitrogen, the diffusion source (In + Zn)and
the sample are put in an open ampoule, aperture towards the
gasflow.Atfirst,the ampoule is in the cold zone of the furnace,
and the N 2 flow is replaced by a H2 flow, then by a PH 3 /H 2
flow. Then the ampoule is pushed into the hot zone. After
diffusion time t, the ampoule is pulled into the cold zone and
the PH3 is switched off. In our set of experiments, we have used
3 1/min H2 with 1 % PH 3 and a diffusion source of 10 g on In in
which 0-7 g of Zn was dissolved.
We performed Zn diffusion for various times t, the temperature being held at 550°C. The mirror-like wafer surface was
always conserved. Then junction depth was measured by staining a cleaved face into a solution of K3Fe(CN)6 (10 g) in
KOH(N) (100 cm3). The results are plotted versus t1/2 in Fig.
2. From the slope of this line, the Zn diffusion coefficient is
about 1-3 x 1O10 cm2 s~ * at 55O°C. Furthermore, the junctions
are found to be quite planar.
On another sample diffused at 615°C for 20 min, several
time.min
0013-5194/80/230830-03
25
SI.50/0
100
15
OPEN AMPOULE DIFFUSION IN InP
Indexing terms: Diffusion, Semiconductors
We describe a new diffusion process to obtain Zn-doped
p + -InP layers in an open ampoule in a PH 3 /H 2 atmosphere.
Using this process, we are able to control the diffusion and to
realise planar and abrupt junctions while conserving the
mirror-like InP surface.
For many devices, it is desirable to realise p-n junctions in InP.
Such junctions can be obtained by ion implantation or by
thermal diffusion. In this letter, we shall describe a simple Zn
diffusion process which allows us to get highly planar shallow
junctions while keeping a mirror-like wafer surface.
832
c
15
10
t
1/2
.(min)
1/2
1549/? |
Fig. 2 Junction depth versus til2 at 550°C
ELECTRONICS LETTERS 23rd October 1980
Vol. 16 No. 22
characterisation experiments were carried out:
64
—Zn profile by s.i.m.s. (secondary ion mass spectrometry)
using a Cs ion beam;
—junction staining as previously described;
—hole concentration in the diffused layers by differential
Hall measurements combined with chemical sectioning in
Br 2 :HBr:H 2 O (1:17:90);
—electron concentration in the substrate by C/V
measurements.
19
_•
10
••. zinc atoms
o
0
0
18
holes
o o
o b
S 17
- 10
'
initial concentration
electrons
A A * lifi *+*&£-
16
10
n-type
p-type
;
depth, um
Fig. 3 Zinc, hole and electron profiles in diffused lnP
t = 615°C, 20 min in hot zone of furnace
Zn 6 4 distribution measured by s.i.m.s.
Hole distribution determined from differential Hall measurements
Electron distribution measured by C/V measurements
The results are reported in Fig. 3. It should be noted that the
maximum [Zn] concentration is an estimated 3 x 1019 cm" 3
(Ref. 5) and that there is a steep decrease of [Zn] near the stained
depth. On the other hand, the hole concentration is rather flat
at 2 x 1018 cm" 3 (i.e. only a small fraction of Zn acts as an
acceptor), and decreases very steeply near the junction as also
supported because electron concentration is not changed in the
substrate. The hole mobility is about 70 cm2 V" 1 s" 1 .
We have described a quite simple Zn diffusion process into
InP using an open ampoule in a PH 3 /H 2 atmosphere. We
have thus obtained an excellent surface quality, abrupt shallow
junction in a reproducible way. We plan now to investigate the
influence of Zn and P partial pressure on the Zn surface concentration before explaining the large difference between [Zn]
and the hole concentration.
P. N. FAVENNEC
L. HENRY
M. GAUNEAU
H. L'HARIDON
G. PELOUS
1CM/T0H, CNET
22300 Lannion, France
26th August 1980
References
1
2
TUCK, B., and HOOPER, A.: J. Phys. D, 1975, 8, pp. 1806-1813
HOOPER, A., and TUCK, B.: Solid-State Electron., 1976, 19, pp.
3
GUHA, s., and HASEGAWA, F.: Solid-State
513-517
Electron., 1977, 20, pp.
27-28
4
DA VIES, D., EIRUG, POTTER, w. T., and LORENZO, i. p.: J. Electrochem.
Soc, 1978, 125, pp. 1845-1848
5
GAUNEAU, M.: to be published
0013-5194/80/230832-02 $1.50/0
ELECTRONICS LETTERS 23rd October 1980
ELECTROFLUORESCENT COLLOID LIGHT
SWITCH
Indexing terms: Electrqfluorescence, Display devices
A low field electro-optic light switch mechanism is proposed,
based on changes in the fluorescence from dye molecules absorbed to sepiolite clay particles. When tagged with acridine
orange and suitably oriented in an electric field, sepiolite
suspensions give fluorescence effects orders of magnitude
greater than those for dye-tagged liquid crystals. Uses of the
system are discussed.
There is enormous commercial interest in electro-optic display
systems. Often, the active principle of these devices is the
change in one or other optical property of a suitable fluid when
it is subjected to an electric field. The optical transmission
characteristics of certain liquid crystals are perhaps the best
known phenomena in current commercial use. It has been suggested that dichroic1-2 or fluorescent34 dyes may be incorporated into the liquid crystal matrix and electrically induced
dichroism or fluorescence detected respectively as the active
display principle.
In this letter we report experiments which indicate that dyetagged sepiolite mineral suspensions give rise to extremely
large electrofluorescence effects which are much greater than
can be obtained with liquid crystals under comparable conditions. The potential of such suspensions as high contrast
electro-optical switches is indicated thereby.
When light is incident on afluorescentmolecule or chemical
group (fluorophor) it is absorbed and re-emitted at increased
optical wavelength. This is a twofold process involving absorption and emission transition moments which occur along
specific molecular directions. The absorption and hence the
fluorescence is maximised when the incident light is polarised
parallel to the absorption moment and zero when perpendicular to it. Alignment of a collection of fluorophors relative to
each other and the linearly polarised incident light affords a
means of switching from zero to optimal absorption and hence
fluorescence. In principle, a solution of fluorescent dye
molecules can be aligned in an electric field. In practice, the
molecules are of such small size and their electrical anisotropy
(permanent dipole moments and polarisabilities) so small that
excessive electric fields would be needed. This can be obviated
by binding the dye molecules to sepiolite mineral particles
which have high geometric anisotropy and high electrical
anisotropy. It is imperative that the dyes attach to the material
with very high directional order.
Sepiolite disperses in water and other media as rigid charged
needle-like particles about one micrometre in length. These
particles can be fully aligned in relatively low amplitude electric fields (~ 1 kV cm" 1 ). The charge on the particle surface
readily binds ionic dyes. In particular, the structure is such that
there are channels or gutters in the surface which run parallel
to the needle axis. Dyes such as acridine orange which are
cationic tabular molecules can be absorbed into the gutters
with their long axis (the direction of the optical transition
moments) parallel to the gutter and hence to the long axis of
the mineral.5 Such dye-tagged particles therefore meet the
criterion that in relatively low amplitude electric fields their
transition moments are strongly aligned.
Preliminary experiments have been made on dilute aqueous
sepiolite suspensions (8 x 10"5 gml" 1 concentration) to
which acridine orange (2 x 10"7 gml" 1 concentration) had
been added. Light of 488 nm wavelength was polarised in a
vertical V or horizontal H plane and was incident upon the
suspension held in a 1 cm square glass cell. The vertically and
horizontally polarised emitted fluorescent light at wavelengths
exceeding 515 nm was recorded at 90° observation angle in the
horizontal plane. Electricfieldsof up to 4 kV cm" * amplitude
were applied in the form of short duration pulses in the vertical
direction. By recording the transient changes in the polarised
components of thefluorescence,both the magnitudes and response times of the phenomenon could be recorded.
A representative transient response is shown in Fig. 1. The
rate processes are indicative of the rotary relaxation of the
sepiolite particles. From the components of fluorescence
measured at 90°, the totalfluorescenceintensities / integrated
Vol. 16 No. 22
833
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