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Photoelectrical Properties of Semiconductor in Contact with Gas Discharge Plasma
N. Lebedeva, V. Orbukh, B. Salamov, M. Özer, K. Çolakoǧlu, Ş. Altındal
To cite this version:
N. Lebedeva, V. Orbukh, B. Salamov, M. Özer, K. Çolakoǧlu, et al.. Photoelectrical Properties of Semiconductor in Contact with Gas Discharge Plasma. Journal de Physique III, EDP Sciences, 1997, 7 (5), pp.1039-1044. �10.1051/jp3:1997173�. �jpa-00249627�
Photoelectrical Properties of Semiconductor in Contact with Gas
Discharge Plasma
N.N. Lebedeva (~), V.I. Orbukh (~), B-G- Salamov (~>*), M. 0zer (~),
K. ~olako(Iu (~) and $. Altindal (~)
(~) Physics Department, Baku State University, 370145 Baku, Azerbaijan
(~) Physics Department, Faculty of Arts and Sciences, Gazi University, 06500 Teknikokullar, Ankara, Turkey
(Received 30 September 1§96, revised 27 Januar; 19§7, accepted ii February 1997)
PACS.52.80 -s Electric discharges
PACS 73.40.Sx Metal-semiconductor-metal structures
Abstract. The plasma contact effect on spectral characteristics of the longitudinal photo- conductivity of GaAs has been investigated in the present paper The sample studied was a GaAs:Cr high-resistivity (p
= 10~ Q cm) plate of thickness d
= 1 mm and diameter +~ 20 mm- One electrode (thin semi-transparent Ni layer) was deposited on the plate surface, and another electrode (Sn02 film) was separated from the plate surface by the gas discharge air gap The
constant voltage applied to the electrodes was higher than the breakdown voltage. The semicon-
ductor was illuminated both from the side of the Ni-contact and through plasma contact. The measured spectral characteristics of photocurrent were different in the strong absorption region.
When the semiconductor was illuminated through plasma contact the photocurrent was 5-2 times higher than for the Ni-contact illumination The observed phenomenon can be explained by the change of surface recombination velocity of non-equilibrium carriers in the semiconductor
due to the bombardment of the semiconductor surface by plasma
1. Introduction
A plane gas discharge cell with a high-ohmic photosensitive semiconductive plate has been used in ionization photographic systems [1,2], IR-converters [3, 4], non-silver photography is,6]
and devices for visualization of electrical and structural defects in semiconductors [7].
Using such cells with a photosensitive semiconducting GaAs:Cr, Si:Zn, Si:Pt cathode [8-10],
devices for transformation and photography of the IR image, on semiconductor photographic
ionization systems as they are called, have been developed. The semi-insulating GaAs doped
with Zn or Cr, and Si doped with Zn or Au was used as a photosensitive semiconducting
cathode in the IR region. In the case of GaAs:Zn it has a sensitivity of up to I
= 1.7 ~lm
at room temperature ill>. A high sensitivity of up to 1
= 3 ~lm for Si:Zn and 1
= 10.6 ~lm
for Si:Cu is obtained at 95 K- The impurity of a semiconductor defines the sensitivity of the device in_the IR region. The gas discharge is realized by free charged particles-electrons and (°) Author for correspondence (e-marl: balatlquark.fef.gazi.edu.tr). On leave from Physics Depart- ment, Baku State University, Azerbaijan
@ Les #ditions de Physique 1997
1040 JOURNAL DE PHYSIQUE III N°5
3
L L
~ ~
- ~
case A case B
5
Fig 1. Scheme of the gas discharge cell with the semiconductive electrode: 1) GaAs semiconductor plate; 2) transparent conductive Sn02 layer on a glass substrate; 3) semi-transparent Ni layer; 4) gas discharge gap; 5) calibrated packings
ions with the energy considerably exceeding the thermal energy. Moreover, the discharge is
accompanied by a short-wave glowing.
Hence, the two-layer cell of photoconductor-plasma considered is a converter transforming
and amplifying a relatively low-powered photon flux incident on the receiving surface of the semiconductor into the flux of high-energy particles, i-e- electrons, ions, photons. The possibil-
ities of registration and visualization of the image formed by a semiconductor in the discharge plasma are rather widespread.
Studies of the characteristics of such devices showed that they are strongly affected by the
quality of the semiconductor electrode. This can be observed in part when uniform illumination of a semiconductor gives rise to a nonuniform pattern of the discharge radiation.
One can expect that the photoelectric phenomena will be characterized by the following
features: 1) the recombination velocities on the surfaces in contact with plasma and metal should differ from each other; 2) the enriching in nonequilibrium carriers can take place on
the surface in contact with the plasma. Therefore the field effect on recombination velocities
on both surfaces will be different; 3) the discharge effect on a semiconductor surface can be considered as a new mechanism of current amplification in a semiconductor; 4) the sensitivity
of the system in the fundamental absorption region may be highly increased in the case of
decreasing surface recombination velocity of non-equilibrium carriers in the semiconductor.
The above problems are studied in the present paper for the first time, and the effect of the
plasma on the shape of spectral characteristics of GaAs.Cr used as an electrode in a plane gas discharge cell are studied.
2. Experimental
To solve the above problem, a plane gas discharge cell shown in Figure I was used. The n-type high-ohmic (p = 10~ Q cm) GaAs:Cr plate of thickness d
= I mm and diameter
+~ 20 mm was
used as a semiconductive cathode (I)- The photosensitivity region is 0.8-1-8 ~lm- One of the
plate surfaces was coated with a thin Ni layer (3) to which a constant potential was applied.
The second electrode (2) was a transparent Sn02 layer on a glass substrate and by means of dielectrical calibrated packings is) it formed a narrow 10-80 /Jm gap (4) with a free surface of the semiconductor. The semiconductor was illuminated either through a thin semi-transparent
Ni layer (case A, Fig. I), or through a transparent conductivity Sn02 film on the glass and a
gas discharge gap (caseB, Fig. I). To equalize the light fluxes L absorbed by a semiconductor
in the above two cases, the light absorption on Ni and Sn02 layers was taken into account.
The absorption in the Ni and Sn02 film was known from the measurements carried out. The cell was placed into the chamber from which the air was pumped out up to the pressure of p = 200 Pa. In this case the breakdown voltage UB, was about 300 V. At the applied constant
voltage U, a direct current was observed between 300-800 V. The density of the current was constant due to the distributed resistance of the semiconducting electrode [12>.
When a semiconductor is illuminated a longitudinal photoconductivity occurs, and a gas
discharge current can be distributed over the whole area of the semiconducting cathode, causing
a glow of gas discharge. Thus, a semiconductive plate in a gas discharge cell has two types of
contact: the first contact is a standard contact (a metal layer on its surface) and the second
contact is a plasma contact (a gas discharge plasma with a semiconductor surface). This
allowed the problem of the effect of contact type on a longitudinal photoconductivity of the semiconductor to be solved. The photocurrent was recorded in the range of10~~-10~~ A- To determine the sign of majority nonequilibrium carriers in studied samples, we investigated
the longitudinal photoconductivity of the similar GaAs:Cr plate with the semi-transparent Ni
layer deposited on both sides.
3. Results and Discussion
In the case of longitudinal photoconductivity, a spectral distribution of the photocurrent [13]
depends on direction of the applied field (E). This dependence is characterized by the value
r =
~P~~ ii)
Iph-
where I~h+ and I~h- are the currents in the field. Where (E ii L) denotes that the direction of the electric field is parallel to the direction of light propagation, and (E it L) denotes that
the direction of the electric field is opposite to that of light propagation. A large difference of r from equation (I) should be observed for highly absorbed light when Kd > l~ where K is a
light absorption coefficient~ and d is the length of crystal in the direction of light propagation.
If r < I, then the majority carriers are electrons. In this case the electric field (E it L)
promotes the passage of electrons generated on the illuminated surface through the volume of
a semiconductor: generation of a photocurrent I~~- occurs in this case. By contrast, when
(E ff L) the electric field r < I (I~~+ < I~~- prevents the passage of electrons through the semiconductor.
The spectral characteristics of the photocurrent in a GaAs plate with two metallic Ni con-
tacts for par. (E ii L) and oppos. (E it L) field to the direction of light are shown in Fig-
ure 2 (curves I and 2 respectively). A considerable difference of photocurrents is observed for
< 1.0 ~lm. It can be seen from Figure 2 that in the (E it L) field the ratio I~~- /I~h+ is higher than in the (E ff L) field. This fact indicates that the majority of nonequilibrium
carriers of the photocurrent in the considered semi-insulating GaAs are electrons. In the range of 0-9-1.8 ~lm the photoconductivity in GaAs:Cr is attributed to chrome impurities.
Now~ consider a gas discharge cell which allows the study of a longitudinal photoconductivity
and the same GaAs sample illuminated from the side coated with a thin Ni layer (case A), or
from the side of plasma contact (case B)- In both cases the light absorption intensity was the same, while the field was opposite to the direction of the light propagation (E it L).
Figure 3 shows the spectral characteristics of photocurrent when a semiconductive GaAs plate in a gas discharge cell is illuminated from the side coated with Ni layer (curve I) and of a gas discharge plasma (curve 2)- In the range of 0.74-0.83 ~lm the photocurrent, when
1042 JOURNAL DE PHYSIQUE III N°5
ph orb. units
~ (2)
zo
s
o? o.9 1,i 1.3 1.5 1? i.'
I I/Lm)
Fig. 2 Spectral characteristic of GaAs longitudinal photoconductivity, when the light direction coincides (curve 1) and is opposite (curve 2) to field direction.
a GaAs plate is illuminated from the side of a gas discharge plasma, is 1.5-2 times higher as compared to photocurrent at the metallic contact illumination This region (0 74-0.83 ~lm) for GaAs has a large absorption coefficient (k = 10~~-10~~ cm~~). In this case the light is absorbed in a narrow (10~~-10~~ cm) semiconductor layer and therefore the observed effect
can be explained by a different velocity of surface recombination
Using the method proposed in [14], the surface recombination velocity S, with metallic and
plasma contacts was determined from the above measurements. For highly absorbed light,
when
Kl » I Kd » 1 (2)
where is a diffusion length, the following expression is valid for S-
~ ~ Iii °fir/k2)l~~
~~~
where
= /bT
= (1÷ 2) 10~~ cm, D is an ambipolar coefficient D
= 250 cm~ s~~,
T is the life time of majority carriers T = 10~~ s, d is a distance between contacts, d
= I mm;
a = I~hi /I~h~i I~hi, I~h~, Ki, K2 are the photocurrents for two wavelength and absorption
coefficients for the strong absorption region, when the conditions ill are fulfilled.
As seen from a spectral characteristic of the absorption coefficient in GaAs, 1 < 0.87 ~lm
corresponds to the conditions (2)- For a standard contact the surface recombination velocity Si
1 (arb. units)
0.72 0.74 0.76 0.78 0.80 0.82 0.84 0.86
(/Lm)
Fig- 3. Spectral characteristic of GaAs photocurrent at the illumination through Ni-contact
(curve 1) and gas discharge plasma (curve 2)
was determined from the equation (3), where Iphi and I~h~ are defined from Figure 3, curve 1, for 11
" 0.78 ~lm and 12 " 0.82 ~lm. For a plasma contact the surface recombination velocity
52 was determined similarly from Figure 3, curve 2. We obtained the following values of the surface recombination velocity Si " IA x 10~ cm s~~ and 52 = 0-9 x 10~ cm s~~. It is seen that the Si/52 = 1-S is similar to the photocurrent ratio measured for plasma and metallic
contact at 0.73 ~lm < 1 < 0.83 ~lm-
Thus the increase in longitudinal photoconductivity of a semiconductor on plasma contact,
can be explained by decrease of the surface recombination velocity of nonequilibrium carriers in the semiconductor, on being in contact with the gas discharge plasma. The observed phe-
nomenon can be attributed to the change of recombination processes due to bombardment of
the semiconductor surface by the charged plasma particles. It should be noted that during the
illumination of the sample through the plasma contact the observed increase in photo-current
cannot be attributed to additional illumination by the gas discharge itself.
The direct measurements of a spectral composition of the gas discharge radiation in the range of 0-3-1-8 ~lm were carried out in iii. The above radiation was observed only in the UV/ blue spectrum (0.336-0 45 ~lm), while in the IR region the radiation was not revealed. Therefore
we consider that the plasma radiation does not affect the photocurrent of the semiconductor.
It should be noted that the investigations carried out have shown that the reproducibility of the obtained results is independent of the details of the GaAs surface preparation.
Thus, the effect of the gas discharge plasma on the photoconductivity of a GaAs:Cr semi- conductive plate has been investigated in the present paper. We have shown that due to the interaction of plasma with the semiconductor, the photoconductivity increases in the range of strong absorption. The results obtained can be used to improve the output characteristics of the semiconductor photographic systems of an ionization type.
1044 JOURNAL DE PHYSIQUE III N°5
Acknowledgments
This work is supported by Scientific and Technical Research Council of Turkey (research project 1309) and Gazi University (AFP) FEF (research projects 05 /96-4, 05/96-7 and 05/96-12)
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