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HAL Id: jpa-00249491

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Submitted on 1 Jan 1996

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Investigation of the Effect of Discharge Plasma Stabilization by a Semiconductor

N. Lebedeva, V. Orbukh, B. Salamov

To cite this version:

N. Lebedeva, V. Orbukh, B. Salamov. Investigation of the Effect of Discharge Plasma Stabi- lization by a Semiconductor. Journal de Physique III, EDP Sciences, 1996, 6 (6), pp.797-805.

�10.1051/jp3:1996155�. �jpa-00249491�

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Investigation of the Elllect of Discharge Plasma Stabilization by a

Sendconductor

N.N. Lebedeva (*), V.I. Orbukh and B-G- Salamov Baku State University, Baku 370148, Azerbaijan

(Received 19 June 1995, accepted 14 March 1996)

PACS.52.40.Hf Solid-plasma interactions; wall effects; probes; sheaths

Abstract. The present paper deals with the urgent problem of the gas discharge plasma

contact with a semiconducting electrode. The phenomenon of gas discharge stabilization by

a semiconducting electrode has been investigated in metal-thin (10 -100 ~m) gas gap-high ohmic semiconductor structure. Gallium arsenide (10~ -10~ ohm cm) has been used as the semiconductor. It has been found that the deposition of a metallic film on particular regions of

a semiconductor surface leads to current filamentation on the corresponding regions. This fact confirms the conclusion that the stabilization effect is determined by boundary conditions on a plasma-semiconductor contact.

1. Introduction

Recently, a plane gas discharge cell with a high-ohmic and photosensitive semiconductor plate,

as the electrode, has found practical application. The so-called "photo-ionization systems" [1-4]

used for high-speed and non-silver photography [5,6] as the IR-image converters [7], and the devices for visualization of electrical and structural defects in high-ohmic semiconductors [8-10]

have been developed on its basis.

The effect of such systems is based on the formation of the gas discharge in the gap between

a metallic anode and a semiconductor photosensitive high-ohmic cathode. The discharge in such a cell has the following peculiarities: I) a narrow discharge gap (a few tens of microns);

2) a distributed resistance of the semiconductor electrode; 3) a uniform distribution of the gas

glow over the whole area of the electrode; 4) a local control of the current and the discharge glow due to a change in the resistance of the semiconductor electrode.

At present, it is well known that the electric current is realized as a thin filament in a gas-

discharge gap between two metallic electrodes. If one of the electrodes is a dielectric, then the

discharge stabilization takes place, I-e- the filaments are absent and hence a uniform plasma glows along the section of the structure. Generally, such a physical phenomenon is explained by the essential role of the electrode resistance of the dielectric distributed over the whole

volume ill].

In the present paper we study the discharge stabilization effect in a plane gas-discharge cell with a semiconductor electrode and consider the possibilities of an increase in the glowing of the gas.

(*) Author for correspondence

@ Les Editions de Physique 1996

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798 JOURNAL DE PHYSIQUE III N°6

2. Experimental

The behaviour of the current-voltage (I U) characteristics is determined by the type of the discharge. Therefore the experimental part of our work involved the measurements of the I U characteristics of a gas-discharge cell with either total or partial localization of

a semiconductor electrode resistance, and with a distributed resistance of a semiconductor electrode at a simultaneous discharge photographing. For this purpose, the I- U characteristics and the discharge glow have been studied for the following systems (Fig. I): a) the discharge

gap between two metallic electrodes connected in series with an external load resistance jr);

b) the discharge gap between two metallic electrodes connected in series with a localized resistance of a semiconductor electrode; c) the discharge gap between a metallic and a partially localized resistance of a semiconductor electrode; d) the discharge gap between a metallic and

a distributed resistance of a semiconductor electrode.

The chromium doped high-ohmic, 10~ -10~ fl cm, GaAs plate with a photosensitivity of 0.89 -1.6 ~lm was used as a semiconductor electrode (cathode). The diameter of the GaAs

plate was 3 cm and the thickness was 1.2 mm. The surface of the plate on which the voltage

was applied was illuminated through a thin Ni-layer. The semiconductor resistance changed by 2-3 orders in this case.

The discharge was photographed through a thin metallic net used as the anode. The dis- charge gap between the anode and the semiconductor plate was determined by calibrated mica spacers with thickness 10 100 ~lm. The plane gas discharge cell was placed into a chamber with a pressure of15 600 Torr and a constant voltage up to I kV.

To realize the systems "b" and "c" (Fig. I), the surface of a semiconductor plate from the side of a gas-discharge gap was coated with a metallic film. In the case of the system "b" this

layer was solid over the whole area of the semiconductor, so that the semiconductor resistance

was completely localized between the metallic layers on its surface. In the case of system "c"

a partial localization of the semiconductor resistance was realized. For this purpose, a solid

layer area was separated into particular contacts. Such contacts (current concentrators) were deposited on the GaAs plate by vacuum evaporation of the metal through the mask with the

+~r

a

~R,

b

~ d

Fig. 1. The discharge gap between: a) two metallic electrodes connected in series with an external load jr); b) two metallic electrodes connected in series with

a localized resistance of a semiconductor lL; c) metallic electrode and partially localized semiconductor electrode resistance; d) metallic and

distributed semiconductor electrode resistance.

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I(a) xio'~

5Q

~

25

33o 34o

Fig. 2. The I U characteristics and a photograph of filament in the systems "a" and "b" in

Figure1.

holes. The smallest structure of concentrators was obtained with the help of a mask with 3 holes per I mm~, the concentrator area was 5 x 10~~cm~.

The I U characteristics were recorded with gradually varying voltage at a speed of 0.6

1.0 V s~~ The maximum sensitivity along the current axis was 10~~ A cm~~, while the

maximum sensitivity along the voltage axis was I V cm~~ The radiation intensity of the gas

discharge was fixed both by a photomultiplier and on the photographic film.

3. Results and Discussion

3.I. THE DISCHARGE BETWEEN TWO METALLIC ELECTRODES WITH A SERIES EXTERNAL

RESISTANCE OR WITH A LOCALIZED RESISTANCE OF A SEMICONDUCTOR ELECTRODE.

When coinciding the external resistance (r) with a semiconductor electrode resistance in "a"

and "b" systems (Fig. I), the I U characteristics of the both systems were found to be

completely equivalent. The typical I U characteristic is shown in Figure 2. The current

jump in the hysteresis loop corresponds to the appearance of the filament in the gap (insertion

in Fig. 2). As a rule, one filament is observed and it can move over the electrode area.

Theoretically the I U characteristic of such system can be obtained from the I U char- acteristic of the discharge in the gap and the I U characteristic of the ohmic resistance connected in series with the gap. Due to a superlinear dependence of the Town-send ioniza- tion coefficient, the gas discharge has a non-linear I U characteristic with the region of a

Negative Differential Resistance (NDR) Rd;f

"

() < 0 (the region AB, curve I, Fig. 3a), U is a voltage on the gas gap, I is the current through the system, e is the external voltage in

the circuit. The equation U

= e Ir is graphically shown as a straight line called "the load line" cutting off the length equal to e on the U-axis, and making an angle with the U-axis (the

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800 JOURNAL DE PHYSIQUE III N°6

1

~ ~ ~dif

al

J

~ ~dif

hi (b)

J

1

r > RJif

i

Cl

(hi

Fig. 3. The plot of the I U characteristic for the system "a" in Figure 1for different comparative values of r and Rd>f in the NDR region. a) r < Rd>f, b) r

= Rd,f, c) r > Rd>f.

cotangent of the angle is equal to r). With the given e and r only the states simultaneously satisfying both "the load lines" and the I U characteristics of the discharge are possible in the system considered. With increasing external voltage on the system "the load line" shifts to the right in the plot. With the known current value corresponding to different e, one can

graphically construct the I U characteristics of the system considered (Fig. 3, curve 2). The shape of the I U characteristic of the system depends on the relationship between Rd;f in the NDR region and the resistance r. Figure 3a shows the case when r < Rd;f. In this case the I U characteristic of the system is characterized by the hysteresis loop which increases

as r decreases. Figure 3b depicts the I U characteristic for i~

= Rd;fi in this case the jump

of the current is observed on the I U characteristic of the system. The I U characteristic of the system when r > Rd>f is shown in Figure 3c. In this case, the I U characteristic of the

system is characterized by the bending point.

Using the graphical method, on can build the I U characteristic of the gas discharge

gap based on the measured I U characteristic of the system and the I U characteristic

of ohmic resistances connected in series with the gas discharge gap. The measured I U

characteristic of the system ~i>ith a localized semiconductor resistance (curve I) and the I U characteristic of a semiconductor (curve 2) are shown in Figure 4. The curve 3 (Fig. 4) is

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Ijaj xlo~~

>

lo

e iBi

5a « » >5a

Fig. 4. The I U characteristics of the system "b" in Figure 1 (curve 1), of a semiconductor

(curve 2) and a gas discharge gap (curve 3).

a graphically constructed I U characteristic of the gas discharge gap. According to the above-mentioned, the hysteresis loop on the curve 3 (Fig. 4) indicates that the semiconductor resistance r < Rd;f is in the NDR region. The measurement of the I U characteristic of the system at different resistances and a parallel photographing of the filament in the gap have shown that with increasing resistance of the semiconductor the filament area decreases. In this

case, the hysteresis loop area and the NDR region also decrease in the I U characteristics of

the system and of the discharge, respectively.

The intensity of filament glow depends on the semiconductor resistance value, I-e- with

increasing the resistance the intensity decreases. The dependence ofthe discharge glow intensity of the filament on the external voltage is similar to the I U characteristic of the system.

3.2. DISCHARGE IN A SYSTEM WITH PARTIALLY LOCALIZED RESISTANCE OF THE SEMICON-

DucToR ELECTRODE (THE SYSTEM "c", FIG. I). The hysteresis loop and the current

jumps followed by striking of particular filaments (Fig. 5) are typical of the I U charac- teristics of a system with current concentrators. As seen in Figure 6, the number of filaments is determined by the number of concentrators, the filaments occur at the edge of the concen-

trators, the increase in the number of concentrators and the decrease of their area lead to an increase in filament number and to their narrowing. The striking voltage of filaments depends nonmonotonically on the gas pressure, reaching its minimum at p = 150 Torr and decreases with decreasing the semiconductor resistance.

3.3. DISCHARGE IN A SYSTEM WITH A DISTRIBUTED RESISTANCE OF THE SEMICONDUCTOR

ELECTRODE (THE SYSTEM "d", FIG. I). In such a system, the discharge is observed in the gap between the metallic anode and the free surface of the semiconductor cathode. The above system differs from the previous cases by the fact that the semiconductor resistance is

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802 JOURNAL DE PHYSIQUE III N°6

I(al x10~~

~

410 41z 414

Fig. 5. The I U characteristic of the system "c" in Figure 1.

Fig. 6. The photographs of a discharge in the system "c" in Figure with different current con- centrator number.

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4&Js %Km %©m

1(a) x10~~

1

z

/ .'

~

e(B)

2o 4o 34o J6o 1e1

Fig. 7. The photographs of a stabilized gas-discharge in the system "d" in Figure 1 and the I U characteristics for the system "d" in Figure 1 (curve 1), a semiconductor (curve 2) and

a gas-discharge

gap between a metallic electrode and a distributed resistance of a semiconductor electrode (curve 3).

not localized but it is distributed over the entire volume and area of the semiconductor. A uniform discharge glow is observed over the whole electrode area on discharge photographs (insertion in Fig. 7). Neither the current jump, nor the hysteresis loop but only the bending point characterizing a rapid increase of the current are observed in the I U characteristic of such a system (Fig. 7). Using a graphical method, the I U characteristic of a stabilized

discharge (Fig. 7, curve 3) has been constructed from the measured I U characteristics of the system and a semiconductor (Fig. 7, curves I and 2). As seen in Figure 7, the NDR region is not observed in this characteristic. We refer to the transition from the glowing filament to a uniform glow over the whole electrode area as discharge plasma stabilization. Our experiments lead us to attribute the occurrence of the above stabilization to the distributed resistance of the semiconductor electrode.

In all three systems ("b", "c" and "d", Fig. I) the resistance of a semiconductor can be

kept constant by means of a photoactive illumination. The comparison of the light intensity

in one filament, in the entire filament system and in the stabilized discharge showed that for the same current, the intensity of the gas glow was the same for all three cases. This results

indicates that due to the distributed semiconductor electrode resistance, the total energy of the filament is distributed uniformly over the electrode area. At the transition to concentrators, the filament is divided into separate filaments. With an increase in the number of concentrators, the filament section starts to overlap with the concentrator area.

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804 JOURNAL DE PHYSIQUE III N°6

The area ofplasma glow in the system with concentrators is lower than that of the stabilized plasma glow. Therefore the density of glowing in the system with concentrators exceeds the

density of the stabilized plasma. The use of concentrators with an area of S

= 5 x 10~~ cm~

and a density of 400 cm~~ leads to

a fivefold increase in the gas radiation density. In this case, the resolution of the system decreases.

Thus, the analysis of the I U characteristics of the systems considered and the comparison of the discharge photographs allow us to make the following conclusions:

1) The mechanism of the current passage in a gas discharge gap limited by two low-ohmic electrodes remains unchanged at the transition to the narrow (10 100 ~lm) gaps: discharge

filamentation takes place and an NDR region is observed in the I U characteristic of the

discharge.

2) The localized resistance connected in series with the discharge gap affects the filament pa- rameters restricting it in area and glow intensity.

3) The partial delocalization of a semiconductor electrode resistance due to the current con-

centrators leads to the separation of one filament into a filament system.

4) The distributed resistance of a semiconductor electrode of the order of 10~ -10~ fl stabilizes the discharge over the whole electrode area, I.e. filamentation is impossible.

5) The I U characteristics of stabilized discharge, unlike that of filamentary discharge, have

no NDR region.

6) The glow intensity of one filament, the filament system and the stabilized discharge is the

same for constant current.

It follows from our experiment that the transition from conditions of current filamentation to uniform glow conditions (at the prescribed semiconductor resistance) is unambiguously

determined by boundary conditions on the surface.

Particularly, even a local (S

= 5 x 10~~ cm~) metallization of a semiconductor surface leads to filamentation. This phenomenon can be explained as follows: the filamentation of the

electric current presupposes a sharp decrease of electric field in the filament volume without any considerable change in the electric field in the remaining part. This is possible when the

magnitude of the field screening on electrodes is considerably lower than the gas gap thickness.

Therefore, in the case when one of the electrodes is a high-ohmic semiconductor, a uniform discharge glow (no filaments), takes place.

The metallization of a semiconductor surface sharply decreases the magnitude of the field

screening and electric current filamentation is observed.

References

[lj Paritskii L-G- and Kasymov Sh.S., USSR Patents 19460/18-10 and 197820/18-10 (1973).

[2j Agaronov B-S-, Zeinally A.Kh., Lebedeva N-N- and Paritskki L-G-, USSR Patent 535451

(1978).

[3j Astrov Yu.A., Egorov V-V-, Kasymov Sh.S., Murugov V-M-, Paritskii L-G- and Ryvkin S-M-, Novoye fotograficheskoe ustroistvo dlya issledovaniya kharakteristik lazernogo is-

lucheniya, Kuantouaya Etectronika 4 (1977) 1681.

[4j Paritskii L-G- and Ryvkin S-M-, Ispolzovanie poluprovodnikov dlya osushestvleniya fo-

tograficheskogo protsessa v dlinnovoInovoi oblasti spektra, Sou. Phys. Semicond. 4 (1970)

764.

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[5j Zeinally A.Kh., Lebedeva N-N-, Paritski L-G- and Salamov B-G-, Image Recording on

Bismuth Film in an Ionization-type Photographic System, J. Photogr. Sci. 39 (1991) l14.

[6j Salamov B-G-, Akinoglu B-G-, Elliatlioglu S., Allakhverdiev K-R- and Lebedeva N-N-, Enhancement of the Resolution of a Semiconductor Photographic System in a Magnetic Field, J. Photogr. Sci. 42 (1994).

[7j Lebedeva N.N., Salamov B-G- and Zeinally A.Kh., Spektralnye kharakteristiki fotografich-

eskoi sistemy ionizatsionnogo tipa, Zhurn. Techn. Fiz. 57 (1987) 1985.

[8] Lebedeva N-N-, Orbukh V-I- and Zeinally A.Kh., Visualization of Electrical Inhomo- geneities in Semi-insulating Gallium Arsenide, Fiz. Tekn. Potuprouodn. 27 (1993) l134.

[9] Lebedeva N-N-, Salmov B-G-, Orbukh V-I- and Nagiev V-M-, Gas-discharge Vizualizer of Inhomogeneities in High-Resistance Semiconductors, Instrum. and Ezper. Techn. 37

(1994) Part 2.

[10] Lebedeva N-N-, Salamov B-G-, Akinoglu B-G- and Allakhverdiev K-R-, Visualization of Electrical Inhomogeneities in High-ohmic Semiconductor Plates by an Ionization-type Photographic System, J. Phys. D: Appt. Phys. 27 (1994)1229.

[llj Blaszuk P-R-, US Patent 3-743-881 (1973).

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