SCANNING OPTICAL PATTERNS WITH ACOUSTIC SURFACE WAVES

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SCANNING OPTICAL PATTERNS WITH ACOUSTIC SURFACE WAVES

N. Moll, O. Otto, C. Quate

To cite this version:

N. Moll, O. Otto, C. Quate. SCANNING OPTICAL PATTERNS WITH ACOUS- TIC SURFACE WAVES. Journal de Physique Colloques, 1972, 33 (C6), pp.C6-231-C6-234.

�10.1051/jphyscol:1972650�. �jpa-00215168�

JOURNAL DE PHYSIQUE

Colloque C6, suppliment au no 1 1- 12, Tome 33, Novembre-Ddcembre 1972, page 23 1

SCANNING OPTICAL PATTERNS WITH ACOUSTIC SURFACE WAVES (*)

N. J. MOLL

(**),

0. W. OTTO and C . F. QUATE Microwave Laboratory Stanford University, Stanford, California, USA

RksurnB.

- On dkcrit un nouveau dispositif qui utilise un solide pour la production klectronique d'images. Son principe est I'emploi d'ondes acoustiques sur une surface piQoklectrique associke

a

une couche semi-conductrice adjacente. On propose un aperqu physique simple du fonctionne- ment de ce dispositif et prksente des rksultats expkrimentaux sous forme d'inscription

a

l'oscillo- scope d'images restitukes en l'utilisant.

Abstract.

- A

new solid state device for electronic resolution of images is described. The basis of the device is the use of piezoelectric acoustic surface waves coupled to an adjacent semiconductor

film.

Simple physical insights into the operation of the device are offered, and experimental results in the form of oscilloscope displays of images resolved with the device are presented.

1 . Introduction.

-

We have found a novel techni- que for rapidly scanning conductivity perturbations in semiconductor films, by using the piezoelectric fields of acoustic surface waves. Clearly such a technique is of particular interest for the resolution of optical patterns, but is equally applicable to the detection of any pattern which will modulate the conductivity of a semiconductor film. Infrared and acoustic images are examples of fields which can be used t o modulate the film conductivity.

Scanning of the conductivity is accomplished by means of a non-linear interaction between piezoelec- tric acoustic surface waves caused by coupling of the piezoelectric field of the surface waves through a semiconductor film situated about 1 000 A away

from the surface of the piezoelectric. FIG. la.

^-

Transient reduction of the attenuation of a signal wave (corresponding

to

increased output amplitude) due

to

2.

-

It is possible to use surface acoustic the presence of

a

large amplitude shorter duration acoustic

wave propagating along

with the

signal wave (coflow).

waves t o scan semiconductor conductivity because the attenuation experienced by a surface wave pro- pagating beneath a semiconducting film can be perturbed by the presence of a second acoustic wave. In figure la, the temporaly reduction in atte- nuation of the signal wave is due t o a second, fairly powerful, acoustic pulse one microsecond long pro- pagating along with the signal wave. The size of the relative perturbation changes with optical illumina- tion and, hence, with the conductivity of the semi- conductor. If the second acoustic wave is launched so that it propagates oppositely t o the signal wave it will perturb the signal wave in the region where the two waves overlap according to the conductivity

(*)

Supported in part

by

the

US

Office of Naval Research

under Contract Number N 00014-67-A-0112-0039

;

and

in

FIG. lb.

-

Top trace

:

duration

of the

perturbing scan pulse part by the US Army Research Office (Durham) under Contract

which

propagates oppositely to the signal wave (contraflow).

Number DAHC 04-71-C-0005. Lower trace

:

signal wave with

and

without perturbation pro-

(**)

On leave from Hewlett-Packard Laboratories. duced by scan pulse.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1972650

C6-232 N.

J.

MOLL, 0. W. OTTO AND C . F. QUATE

of the semiconductor at the overlap region. As the

waves propagate through one another they sample the conductivity of a different region of semiconductor, thus effecting a one-dimensional scan. The pertur- bation produced on a signal wave by an oppositely propagating

cc

scan

))

pulse is shown in figure lb.

The scanning interaction is illustrated schematically in figure 2.

LIGHT SOIJRCE

S I G N A L MODULATED

PULSE S I G N A L

P U L S E [Yz L i N $ 0 3 SURFACE WAVE

D E L A Y L I N E

FIG. 2. - Schematic of scanning device.

2.1 THE

^SCANNING INTERACTION. -

It is well established that the propagation constant, P, of a piezoelectric surface wave is perturbed by the presence of a conducting medium near the surface, thus resulting in a change in phase velocity and in atte- nuation of the wave [I], [2]. The perturbation for a given semiconductor-piezoelectric system is deter- mined by the conductivity, and the spacing from the piezoelectric, of the semiconductor. The thickness of the semiconductor is also important when it is smaller than 1/P, about 2.5 pm for our device. When a large amplitude surface wave propagates beneath the semiconductor, the rf piezoelectric field associated with the wave interacts with the charges through the non-linearity inherent in the current density equation, J

= pv

where J is current density,

p

is charge density, and

u

is charge drift velocity. This produces a static perturbation in charge density in the direction of the surface normal. Associated with this perturbation in charge density is a transverse acoustoelectric field, calculated and observed by Gulyaev

et

al. [3]. The static perturbation of the semiconductor will appear to any other surface wave as a change in the effective conductivity and spacing.

The behavior of a small amplitude signal wave can be expressed mathematically as

where 1 is the length of the semiconductor and S1 is the amplitude of the signal wave. The attenuation coefficient along the acoustic path is a(x) in the absence of any large amplitude acoustic waves,

ADl2 is the change in propagation constant of the signal wave produced by the presence of the large amplitude scan pulse. With moderate amplitudes AP,, is proportional to the square of the ampli- tude

S,

of the scan pulse

;

thus

where K(x) is a non-linear interaction strength related to the conductivity. If S2 is a short pulse, of width a, propagating oppositely to the signal wave S,, then the perturbation in time of the detected signal wave becomes

=

Slo(z) exp(- 51) exp a ( I S20 I K ( U2r) )

x = -

^{. (3)}

Thus the amplitude of the detected signal wave at time z is determined by the effective conductivity at the point

vz/2

along the semiconductor.

2.2 SURFACE STATES. - Up to this point, we have emphasized a method of resolving spatial variations in the semiconductor conductivity, pro- duced for example by optical carrier-pair generation.

The same method lends itself to resolving depletion width variations

;

the interaction strength K between acoustic waves depends not only on the bulk conduc- tivity of the semiconductor, but on the spacing of the carriers from the acoustic delay line. By employing surface states, we can optically modulate this spacing, by varying the depletion width at the semiconductor surface. This variation is more sensitive to the optical intensity than the modulation of the bulk conductivity.

To illustrate this effect we will discuss the specific case of an n-type semiconductor with a large number of acceptor type surface states. When the semiconduc- tor is in thermal equilibrium and there is no external field, the Fermi level is pinned at the surface state energy. Now, consider the following series of expe- riments.

1. We apply a short pulse of electric field, with the field pointing out of the semiconductor. The depletion width increases by an amount depending on the applied field. It then relaxes toward its thermal equilibrium value as the filled surface states are discharged by other thermally generated holes. When the field is removed, the depletion width returns nearly to its thermal equilibrium value.

2. We apply a short pulse of electric field pointing

into the semiconductor. The depletion width instan-

taneously decreases. Empty surface states are rapidly

filled by majority carriers which are diffusing across

the depletion region. Thus, the depletion region

rapidly relaxes to its equilibrium width. When the

field is removed, the depletion width increases to

its magnitude in 1, and slowly decreases towards

its equilibrium value.

SCANNING OPTICAL PATTERNS WITH ACOUSTIC SURFACE WAVES C6-233

3. When we apply an rf field, combining the results in 1, and 2, it is clear that after a few cycles the average depletion width is increased by charging of the surface states. At the same time there is an rf modulation of the depletion width. Figure 3a shows the effect of a short rf pulse on the propagation loss of a device with many surface states. The increased amplitude corresponds to moving carriers away from the delay line.

4. We shine light on the semiconductor, keeping in mind that the surface states are filled by some appropriate means. The optically generated holes are well in excess of those generated thermally and these holes are drawn to the surface as described in 1. They rapidly discharge the filled states. Figure 3b

the same spatial variation, and this variation can be read as before.

We can define a sensitivity parameter,

where

cx

is the attenuation/cm of the acoustic wave and

0

is the number of photons/cm2/s absorbed by .the semiconductor. Comparing the two imaging techniques, photoconductive (pc) and surface state discharge (ss) we see that

where W is the depletion region width.

Now,

a a a / d a

and

W dald

W are roughly of the same order of magnitude

;

the same relative changes in

^a

and W produce roughly the same change in attenuation. Then,

(a)

where

z,

is the majority carrier lifetime, ND is the

shows the same experiment as figure 3a, but wit

donor concentration, L is the film thickness,

zs

is the time between the charging event and the reading event, which must be shorter than the dark decay time, and W is the depletion width.

Since L

%

W,

zs % z,,

the surface states provide a far more sensitive device.

3. Experimental results. - The physics of the semiconductor-piezoelectric-light system having now been presented, some comments and results on its practical use as an imaging device are in order.

Present devices scan an optical

⁽⁽

line

⁾⁾

10 mm long

FIG. 3. - Surface state charging and discharging : a) in the dark,

b) illuminated.

room level illumina6on falling on the semiconductor.

Fm.

-

An oscilloscope Uace modulated with a

If the light intensity has some variation with the

signal wave produced by three light spots focussed on the

spatial coordinate, the depletion width will have

semiconductor.

C6-234 N. J. MOLL, 0. W. OTTO AND C. F. QUATE

by

.5

mm wide, at the focal plane. The signal output amplitude is analogous to the output of a single horizontal sweep from a vidicon tube. If we intensity modulate an oscilloscope spot as it is swept horizon- tally at an appropriate velocity, a magnified replica of the optical line is produced on the oscilloscope.

Such an image, of three light spots focussed on the semiconductor above the acoustic beam, is shown in figure 4. The device was operated here using the photoconductive effect. If the image is moved perpen- dicular to the

.5

mm x 10 mm line, at a velocity slow compared to the

.35

c m / p horizontal scan, successive lines of the image are resolved, as with a vidicon, and a two-dimensional image will be displayed if the oscilloscope trace is swept vertically

FIG. 5a.

- Picture of

the

Stanford University

emblem

pro- in syncronism with the perpendicular sweep of the

duced by intensity modulation of a

vertically

and

horizontally

swept

oscilloscope

trace.

The sky behind the tree has

been image. Such an image formed with surface state shaded

in to demonstrate gray-scale response.

charging is shown in figure 5a. This image shows horizontal resolution limits. at the focal lane. of

,. 56.

-Reproduction

of a

photograph

of C . F. Quate med

as

in part a, the acoustic

scan

direction being along

the vertical

axis.

around 7 lineslmm, which corresponds to the limit set by the 15 % bandwidth of the 200 MHz signal wave

;

a wider bandwidth would increase this reso- lution. The vertical resolution is determined by the beam width, 2 lineslmm in this photo. One future goal is, of course, the realization of an acoustically controlled vertical scan.

4. Conclusions. - Although the numbers mentioned above are not comparable to typical vidicon specifi- cations, neither were they achieved in a device intended to yield spectacular comparison, and we believe that this device does have certain special features unmatched by other imaging devices. It is entirely solid state. The semiconductor requires little or n o processing, and no photolithography.

Finally, the choice of semiconductor is not severely limited by technological considerations. Thus if there is a semiconductor which exhibits an internal photoelectric effect in a given portion of the electro- magnetic spectrum, an image composed of radiation from that region can be resolved.

Acknowledgments.

- 0.

W. Otto gratefully acknowledges the Fannie and John K. Hertz Foun- dation for its generous support.

References

[I] LAKIN

K. M.,

((~erturbation Theory for Electroma- sonics Symposium, San Francisco, California, gnetic Coupling to Elastic Surface Waves

on

Paper A-1 (1970).

Piezoelectric Substrates >>.

J. Appl, phys. 42 [3]

GULYAEV

Yu. V. et al.,

Theory of Electronic Absorp-

(1971) 899. tion and Amplification of Surface Acoustic

Waves

in

Piezoelectric Crystals

>>. Sov. Phys.

[2]

AULD

B.

A., ((Surface Wave Theory

>>. IEEE

Ultra- Solid

State 12

(1971) 2085.

DISCUSSION

P. DAS.

-

I am interested in your results about of varying the air-gap. The semiconductor wafer is convolution. Have you studied the effects of air-gap now much larger than the acoustic beam and in the and semiconductor thickness on the convolution future we plan to adjust the width to match that of

output

?

the acoustic wave. This should serve to increase

C . F. QUATE.

-

We have not studied the effect the Output.