An on-line Display
Method for Scanners Applied to Photo-detectors
During the development
of a solid-state device measurements are made to improve the method of
manufacture. If a certain parameter varies along the surface of the device,
a scanning system may be used for its examination. In the course of the
manufacture of infrared detectors of InSb in our laboratory a flying-spot
scanner and a display system were developed. These have been used to examine
the responsivity along the surface of the detectors.
A description is given of the scanner and of the display system.
obtained with the display system are shown on the enclosed photographs.
A modern photovoltaic infrared sensor (2x2
The number and type
of imperfections of the surface of a single crystal of semi-conducting
material will depend on treatments like sawing, lapping, etching and (chemical)
polishing. Besides these other sources of imperfections, like dust particles
and drying residuals, may contribute. In the following expedients will
be discussed, by which information can be obtained about the inhomogeneities
in the sensitivity of a photo-detector over its surface.
If a photo-detector
is scanned with a dot of light the output of the detector will correspond
to the photo-efficiency at the place of the spot on its surface. The characteristics
of the inhomogeneities in the photovoltaic response along the surface
should give indications about their causes and in some respect, of the
quality of the detector.
methods are in use for displaying the output of a scanning system on-line
on a X-Y oscilloscope. Principles are as follows
- The voltage corresponding
to the X and Y co-ordinate of the flying spot measure on the surface
of the device are connected to the horizontal and vertical inputs respectively
of the oscilloscope. From the output signal of the device a signal Z
is derived, which modulates the intensity of the spot on the screen.
From the three
signals, X, Y and Z ( Z being the output of the device) the following
two signals are derived: Z + A*X and Y+ B*X, where A and B are properly
These signals are
connected to the vertical and horizontal inputs of an oscilloscope. The
number of scans, which are displayed will be kept rather small in practice,
to restrict overlapping of parts of the successive scans.
A new design for the
on-line display, comparable with the second method, is described hereafter.
The method has the advantages, that in the pictures overlapping of areas
is avoided and that the pictures are well detailed. Experience has been
obtained with this method by application to the scanning of photovoltaic
Photovoltaic energy is the direct
transfer of (sun)light in electricity. Some materials have the property
to absorb light-photons and along this way to free tighten electrons.
This is called the photovoltaic effect if an electric voltage is generated.
of the applied method
The Cartesian co-ordinates
of a moving spot on the scanned surface will be denoted by X and Y, measured
on the surface. Y stands for the co-ordinate of the fast scan.
The output of the scanned device will be denoted by Z. X,Y and Z are represented
by voltages (Volts). The single-valued function Z(X,Y) can be represented
by the curved surface of an “object” in our visual three-dimensional space.
is depicted in fig.1, where the section with the Z-Y plane is marked
by dots. This section is sketched separately in fig. 2. Let (Y,Z) be a
point of the boundary of the section, and let Yp be a function
of Y and Z, defined by Yp = Y + Z * cotg(alpha), where Yp
can be considered as the point (Y,Z) projected on the Y-axis from a direction
of observation, given by the angle alpha.
Let the point (Y1,
Z1) be a point on the boundary, which is projected in Y1p.
Then from fig.2 we may conclude that (Y,Z) is visible to the observer,
when no point (Y1, Z1) can be found, for which Y1p
> Yp while Y1 < Y.
By placing the observer
far from the object we obtain the simple relation Yp=Y+Z.A
where A stands for cotg(alpha);, which is a constant now. The process of
making a section with the Y-Z plane will be called a scan. At every scan
during with Y increases we find that, for the visibility of the corresponding
point (Y,Z), Yp has to be larger than all previous values
arisen during the scan.
By the use of a peak-hold
circuit, of which a simple example is shown in Fig. 3, the visibility
of a point may be determined. Yp is used as the input signal
for the peak-hold circuit, the output will be denoted by Ym.
The diode in the circuit will be considered ideal. Obviously,
during the periods that Ym is increasing it will follow Yp,
at other moments it is constant and larger than Yp. The
value of the threshold Yp has to be chosen such that for the
initial points of all scans Yp is larger than Y0. A
complete image is built up by combining Ym-traces of successive
numbers of scans, at gradually increasing values of X, into one picture.
The X-scan will be called the slow scan. The capacitor has to be discharged
after every fast scan.
In Fig. 4 Y, Yp
and Ym are depicted for an arbitrary case. Characteristic of
the peak-hold circuit is that the visibility of a point is determined
by the diode, which is either conducting or non-conducting. The state
of the diode is also indicated by Ym which is either increasing
or constant. It will be obvious that points are visible only during the
conducting periods of the diode. By using Yp for the vertical
and X for the horizontal deflection of an oscilloscope and suppressing
the brightness of the spot on the screen during the non-conducting periods
of the diode we obtain pictures showing the curved surface without overlap,
provided there is also suppression during the back-sweep of the fast scan.
From Fig. 4 we may conclude that by using Ym instead of Yp
overlapping will not occur at all. Thus there is no necessity to
make a decision on the suppressing depending on the state of the diode.
The best results have been obtained by using Ym and suppression,
the latter only during relatively long periods of constant Ym .
This results in a contour intensification in the general pictures.
should be close together in order to obtain coherent pictures, a typical
number being 2000 scans per picture. With a frequency of 100 Hz for the
fast scan the generation of a picture will take 20 seconds. During that
period a photographic record is made of the display on the screen.
An outline of the flying-spot
scanner is shown in Fig. 5. In the experimental set-up a He-Ne laser Beam
(0.63 µm) was attenuated, expanded with a telescope, reflected by two mirrors
and focused on the surface of the device to be tested. A
mirror galvanometer has been used for the slow scanning. The fast scanning
was obtained from a circuit of which an outline is shown in Fig. 6.
All frequencies, except
that of the saw-tooth generator providing the X-signal, were derived from
a 100 kHz crystal oscillator. Two square waves of 100 Hz were obtained by
division, a manually adjustable phase shift between these square waves was
introduced with a univibrator which in Fig. 6 is denoted by “variable delay”.
From these waves a triangular and a sine-wave were derived by integrating
and low-pass filtering respectively.
Optics and mechanics of the flying-spot scanner
The circuit which generated the steering signals for the mirrors and oscilloscope
The sine-wave was
fed to the torsional scanner. Due to the small mismatch between the resonance-frequency
of the torsional scanner and the sine-wave frequency a phase-shift would
be introduced, which then was compensated by using the above mentioned
adjustable delay. The amplitude of the sine-wave was chosen such that
the dot of light was on the sensitive area of the device only during the
almost linear parts of the sine-wave. The output (Z) was amplified (frequency:
D.C. to 100 kHz) and added to the triangular wave, which represented the
Y co-ordinate. The result was Yp=Y+Z.A.. Signal Yp was
fed to the peak-hold circuit, which gave Ym as output. This
signal was connected to the vertical input of the oscilloscope. The saw-tooth
generator had an adjustable period up to 20 seconds. This signal was connected
to the horizontal input of the oscilloscope and to the X-mirror.
In Fig. 7 the layout
is given for the electronic-circuits and components.
The following three
photographs show the mechanical layout with the electronic circuits of
The pictures shown
in this paper are Ym-X displays. They have the appearance as
if a number of cross-sections was made through the “object” perpendicular
to the direction of observation. This is a result of using pulsed intensity
modulation in the display with a frequency of 100 kHz. By this presentation
a better display can be obtained of the steep parts.
In the experimental
set-up the intensity modulation was controlled such that it was suppressed
during the back-sweep of the X and Y scans and during the relatively long
periods of constant Ym. The intensity modulation could be
accentuated by using pulses of 3 µs width, obtained from the divider
circuit. A selection could be made from the repetition rates: 100, 50,
25 and 12,5 kHz.
When an extra pulse
is applied to the flip-flop which drives the Y-generator a phase shift
of 180 degrees will be introduced in the fast scan, and we
obtain due to the symmetrical scanning a similar picture, but seen from
the “opposite” side.
The detectors were
made of n-type single crystals of InSb. After sawing the original crystal
into cubes the damaged surface layers were removed by etching and chemical
polishing. A p-type surface layer was obtained by diffusion. A mesa-structure
was etched out of the cube by covering a part of the polished surface
and etching away the surrounding p-layer. Contacts
were made to both sides of the p-n junction, the n-side was soldered to
a covar strip. This strip was attached to the cold finger of a dewar for
liquid nitrogen. The mounting place was in front of a window with a low
absorption coefficient for both visual and infrared light.
Sawing and lapping
will cause cracks which may increase the recombination of electrons and
holes at the surface and, as in the case of a photovoltaic detector, within
the depletion layer. Also the reflection coefficient may be changed. The
cracks usually have the structure of tracks and therefore one may expect
to find tracks of decreased sensitivity on the surface (Fig. 7).
Dust particles and
drying residuals will change the sensitivity according to their optical
properties. They may either decrease the sensitivity by absorption or
increase it by anti-reflection effects and appear as spiks or holes (Fig.
If an anti-reflection
coating is deposited imperfections like holes will cause spots of altered
sensitivity just as dust particles may do. However, the effect depends
on the wavelength of the light (Fig. 9).
The resistance of
the surface layer of a photovoltaic detector causes a decrease of efficiency
with increasing distance from the contact-electrode. The effect of the
distance depends on the ratio of the surface impedance of the surface
layer and the junction resistance (Fig. 10).
At the boundaries of the p-n junction an increase of the sensitivity is
often found due to the decreased effect of bulk and surface recombination,
especially when the junction lies rather deep (Fig. 11). Of course there
is no sensitivity at the alloyed contact electrode and where the lead
covers the surface.
The causes of
imperfections mentioned above are due to the preparation of the diodes
but they will be mixed with the faults in the original crystal. The
pictures shown on Fig. 12a and 12b, and 13 are examples of diodes which
have no striking inhomogenities.
The causes for the
inhomogeneities of the responsitivity of this specimen are difficult to
determine. However, if the output is clipped at a level which is about
80% of the mean value we see that most of the inhomogeneities are not
The detector in Fig
13. shows a very homogeneous responsitivity along its surface. For a more
detailed result a reduced size of the spot and, according to this, a higher
cut-off frequency of the electronic circuits will be required.