=Paper=
{{Paper
|id=Vol-2507/230-235-paper-40
|storemode=property
|title=Current-Voltage Characteristics of Aluminium and Zinc Implanted Silicon for Radiation Detection Applications
|pdfUrl=https://ceur-ws.org/Vol-2507/230-235-paper-40.pdf
|volume=Vol-2507
|authors=Duke Oeba,Joseph Bodunrin,Sabata Moloi
}}
==Current-Voltage Characteristics of Aluminium and Zinc Implanted Silicon for Radiation Detection Applications==
https://ceur-ws.org/Vol-2507/230-235-paper-40.pdf
Proceedings of the 27th International Symposium Nuclear Electronics and Computing (NEC’2019)
Budva, Becici, Montenegro, September 30 – October 4, 2019
CURRENT-VOLTAGE CHARACTERISTICS OF
ALUMINIUM AND ZINC IMPLANTED SILICON FOR
RADIATION DETECTION APPLICATIONS
D.A. Oeba1, J.O. Bodunrin1, S.J. Moloi1,a
1
Department of Physics, CSET, University of South Africa, Private Bag X6, Florida, 1710, Science
Campus, Christiaan de Wet and Pioneer Avenue, Florida Park, Johannesburg, South Africa
E-mail: a moloisj@unisa.ac.za
Crystalline p-silicon was implanted with Aluminium and Zinc at the fluence of 1.0 1017 ions cm-2.
A change in silicon conductivity due to the implantation was investigated by the use of current-voltage
(I-V ) technique at room temperature. The qualitative analysis of the I-V characteristics showed that
the implantation reduces the measured current of the material indicating that the conductivity of the
material has decreased after ion implantation. The I-V trends of the metal-implanted silicon show
ohmic I-V behaviour. A change in parameters such as saturation current, ideality factor, and
Schottky barrier height due the implantation was also investigated in this study. The results, in
general, show that in silicon, Aluminium and Zinc are responsible for conversion of silicon from
lifetime to relaxation material. A material exhibiting relaxation behaviour has been found promising to
be radiation-hard. This conversion of material to relaxation behaviour indicates that both metals are
suitable dopants to improve radiation-hardness of silicon.
Keywords: Silicon, Aluminium, Zinc, Schottky diodes, Current-voltage
Duke Oeba, Joseph Bodunrin, Sabata Moloi
Copyright © 2019 for this paper by its authors.
Use permitted under Creative Commons License Attribution 4.0 International (CC BY 4.0).
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Proceedings of the 27th International Symposium Nuclear Electronics and Computing (NEC’2019)
Budva, Becici, Montenegro, September 30 – October 4, 2019
1. Introduction
A study on interaction of radiation with silicon has given rise to a wide range of applications
such as switches to electronic devices and radiation detectors for high radiation physics. It has been
experimentally [1 -4] and theoretically found [5] that metals embedded in silicon play an important
role in proving radiation hardness of material for detection applications. The metals in the materials
generate defects that are responsible for the suppression of radiation damage hence improve radiation
hardness of the material. This formidable effect of metals in production is due to the mid-gap defect
introduced by metals in the material. The mid gap defect is a defect level that is situated at the center
of silicon energy gap where it interacts equally with both bands to reduce the conductivity of the
material [6]. In this case, carriers are generated and recombined at the same rate to maintain the
intrinsic behavior of the material [6]. Material rich of this defects exhibits ohmic I-V behavior; a
feature that has not been fully understood nor explained. It is therefore, important that more studies on
I-V properties of the metal-doped silicon based devices are conducted in order to justify the suitability
of the relaxation theory which was established longtime ago [7 -9] but still not welcome in the field of
semiconductors. Relaxation theory has been found promising to explain this ohmic behavior and it
could be alternative to explain defects in silicon. Additionally, understanding of this ohmic behavior
would lead to acceptable recommendations for fabrication of efficient silicon radiation detector to be
used in high energy physics experiments.
Studies available on literature are based on metal dopants that are expensive like gold and
platinum; hence, there is a need for alternative dopants. Aluminum (Al) and Zinc (Zn)-doped silicon
based diodes were fabricated and characterized by I-V technique at room temperature in order to
investigate a change in electrical properties of the devices due to doping. The material behavior was
then inferred from the devices to establish the suitability of the material for detector fabrication of
radiation-hard detector after doping.
2. Experimental details
2.1 Sample preparation
A p-silicon of resistivity 1-20 Ω.cm and the thickness of 275 25.0 µm was used in this work.
The wafer was diced into 0.6 cm x 0.6 cm pieces using a laser cutter. The standard procedure of
cleaning silicon samples was followed to remove any grease and to reduce the oxide layer [10-11].
After the cleaning process, all pieces were mounted in the chamber for aluminum and zinc
implantation. The implantation was carried out using an ion implanter set up at iThemba LABS,
Gauteng, South Africa. Aluminum and Zinc were implanted onto the polished side of silicon pieces
at the energy of 160 keV and fluence of 1.0 1017 ions cm-2.
2.2 Diode fabrication
Schottky diodes were fabricated on unimplanted, aluminium-implanted and zinc- implanted
crystalline p-silicon. Prior to diode fabrication, silicon pieces were cleaned again using the standard
procedure [10-11]. The pieces were then loaded into an evaporation chamber for formation of
Schottky contacts. The contacts were achieved by evaporation and deposition of 100 nm Aluminium
through a mask of 0.6 mm diameter holes. The deposition was carried at 10-6 mbar at the rate of 1Å/s.
The Ohmic contact was then realized by evaporation and deposition of gold onto the back
(unpolished) surface of the pieces. The finished devices each consists of 6 diodes on a piece and with
one common Ohmic contact.
2.3 Diode characterization
The I-V measurements were carried at room temperature using Keithley 6487 Picoammeter
with a built-in voltage source. The data were taken over the range of -2 to 2V within a voltage step of
0.01V. Throughout the experiments, the current limit was set at 2.5mA while the time between
measurements was maintained for 1s to allow the device to stabilize.
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Proceedings of the 27th International Symposium Nuclear Electronics and Computing (NEC’2019)
Budva, Becici, Montenegro, September 30 – October 4, 2019
3. Results and discussion
The current through the diodes is related to the applied voltage (V) [12] as:
𝑒𝑉
𝐼 = 𝐼𝑠 [𝑒𝑥𝑝 (𝜂𝑘𝑇 − 1)] (1)
where η and IS is the ideality factor and saturation current, respectively.
The saturation current IS is derived from the straight linear fitting at zero bias and is expressed as:
−𝑒𝛷
𝐼𝑠 = 𝐴𝐴∗ 𝑇 2 ( 𝑘𝑇 𝐵 ) (2)
where A is the diode area, 𝛷𝐵 is barrier height and 𝐴∗ is the Richardson constant and is 32 A-cm-2-K-2
for p-type silicon. The ideality factor was calculated from the linear region of the slope of forward bias
ln I versus V plot as
𝑒 𝑑𝑉
𝜂 = 𝑘𝑇 𝑑(𝑙𝑛𝐼) (3)
𝑑𝑉
Where, 𝑑(𝑙𝑛𝐼) is the reciprocal of the slope of the linear region of 𝑙𝑛𝐼 versus V plot.
η is a measure from the deviation of ideal diode behaviour. For the ideal diode, η is equal to 1 but in
practice, it is always greater than 1for real Schottky diode.
The evaluated IS from equation 2 is used to calculate the Schottky barrier height as:
𝑘𝑇 𝐴𝐴∗ 𝑇 2
𝛷𝐵 = 𝑒 𝑙𝑛 ( 𝐼𝑠
) (4)
3.1 Undoped p-silicon
Figure 1shows current-voltage characteristics of the diodes fabricated on undoped p-silicon
both in linear (a) and logarithmic (b) scales. It is observed from figure 1 (a) that the trends exhibit
typical characteristics of semiconductor diode, indicating that the diodes were well fabricated. In
figure 1(b), it can be seen that the forward current increases slightly with voltage up to 0.05V
thereafter, it then increases drastically up to 1.2V. At the higher voltage, there is a tendency of the
trend to flatten due to the effect of series resistance that comes into play when the applied voltage is
large [11-12]. The reverse current on the other hand, increases gradually indicating that it depends
slightly on the applied voltage. These results are expected since the device was fabricated on a
relatively ‘defect-free’ material.
8 1
10
(b)
(a)
Undoped P-Silicon
Undoped P-Silicon
Foward bias
6 Foward bias
Reverse bias
Reverse bias
Temperature = 298 K
Temperature = 298 K -1
10
Current (mA)
Current (A)
4
2
-3
10
0
-5
10
-2 -1 0
-2 -1 0 1 2 10 10 10
Voltage (V) Voltage (V)
Figure 1. I-V characteristics of diodes fabricated on Undoped p-silicon in a linear (a) and a logarithmic
(b) scales
3.2 Aluminium-doped p-silicon
The I-V behavior of diodes fabricated on aluminium-doped p-silicon is shown in
figure 2. It can be seen from figure 2 (a) that the forward current is linear at the lower voltage
and increases sharply after 0.7V. The trends in figure 2 (a) are similar to those in figure 1 (a)
apart from that in this case the current starts to increase exponentially at 0.7V. In figure 2 (b),
the I-V plots exhibits a greater difference from that in figure 1 (b). For example, the reverse
and forward current trends have come close to each other indicating that they are equal,
particularly at lower voltages. Unlike in the case of undoped p-Si, the reverse current of
Aluminium-doped silicon diodes increases linearly with voltage for the whole voltage range.
It has to be noted that the forward current at 2V is 8.0 10-2 mA lower than 101 mA of
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Proceedings of the 27th International Symposium Nuclear Electronics and Computing (NEC’2019)
Budva, Becici, Montenegro, September 30 – October 4, 2019
undoped silicon indicating a decrease in charge carriers generation rate. This decrease in the
rate can be noticed by a change in the forward current trend from exponential to linear
increase after doping with aluminium. This change in I-V behaviour indicates that properties
of the material have been changed due to aluminium doping.
-1
0.4 10
(a) (b)
Al-doped -P-Silicon Al-doped- P-Silicon
0.3 Foward bias Foward bias
Reverse bias Reverse bias
Temperature = 298 K -2 Temperature = 298 K
10
Current (mA)
Current (A)
0.2
0.1
-3
10
0.0
-4
10
-1 0
-2 -1 0 1 2 10 10
Voltage (V) Voltage (V)
Figure 2. I-V characteristics of diodes fabricated on aluminium-doped p-silicon in a linear (a) and a
logarithmic (b) scales
3.3 Zinc-doped p-silicon
To study the effects of zinc on properties of p-type silicon, I-V characteristics of diodes
fabricated on Zn-doped p-silicon were investigated. The trend is different from those of figure 1 (a)
and figure 2 (a). It can be seen from figure 3 (a) that the forward current increases linearly with
voltages. Two different regions are observed, with the one at voltages higher than 1.5V being higher.
The reverse current is found to have increased by a factor of two after doping with zinc. In logarithmic
scale both trends, forward and reverse currents, shows are linear with a considerable gap between
them, unlike in the case of Al-doped p-silicon-based diodes
1
10
2
(a) (b)
Zn-doped- P-Silicon Zn-doped- P-Silicon
Foward bias 10
0 Foward bias
Reverse bias Reverse bias
1
Temperature = 298 K Temperature = 298 K
Current (mA)
Current (A)
-1
10
0
-2
10
-1
-3
10
-1 0
-2 -1 0 1 2 10 10
Voltage (V)
Voltage (V)
Figure 3. I-V characteristics of diodes fabricated on zinc-doped p-silicon in a linear (a) and
logarithmic (b) scales
From the linear part of the I-V plots, diode parameters were calculated and represented in table 1.
Table 1. A summary of device parameters evaluated from I-V plots for diodes fabricated on p-silicon
Parameter Undoped Aluminum- doped Zinc-doped
IS (μA) 2.718 1.452 2.565
η 1.245 1.539 2.650
𝛷(V) 0.1814 0.1962 0.1892
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Proceedings of the 27th International Symposium Nuclear Electronics and Computing (NEC’2019)
Budva, Becici, Montenegro, September 30 – October 4, 2019
The parameters calculated from the diodes fabricated from undoped, aluminium and zinc-
doped p-silicon diodes are presented in table 1. The saturation currents of the diodes fabricated on
metal doped silicon are found to be lower than that of undoped p-Si based diode. This decrease in
saturation current indicates that the resistivity of the material has increased due to compensation
resulting in a low density of charge carriers contributing to the measured current. A high resistivity is
the most important parameter for the metal to be used for fabrication of a radiation –hard detector [13].
Based on the saturation currents evaluated from these devices, it can be concluded that aluminium is
the most suitable dopant to improve radiation- hardness of the material.
The ideality factor of zinc-doped diode (2.65) and zaluminium-doped diode (1.539) is
large than the undoped diode (1.245). This high value of ideality factor in metal-doped p-
silicon is attributed to other effects, such as the organic layer effect, inhomogeneity of the
film thickness and the existence of series resistance [6, 10].
It can be seen from table 1 that aluminium-doped silicon diode has high value of Schottky
barrier height (0.1962) compared to diode fabricated from undoped silicon (0.1814). From literature
[9] it has been established that the high value of Schottky barrier height is due to donor levels present
in the energy gap of p-type silicon. The donor levels are responsible for compensation of majority
carriers that causes increase in resistivity.
3.4 Overview and discussion
The effect of metals dopants on the electrical properties of the diodes can be compared in
terms of the data presented in figure (4). It can be seen that in silicon both metals are responsible for a
change in device behavior from lifetime, exponential, behavior to ohmic behavior. This ohmic
behavior indicates that the diodes have been fabricated on relaxation material [14]. The relaxation
behavior is due to a defect level that is situated at the center of energy gap ( ̴ 0.56 eV) in the case of
silicon [2]. At this position, this defect interacts with conduction and valence bands where it generates
and /or recombine charge carriers at the same rate. Since the density of charge carriers generated
would be equal to the one of charge carriers recombined, the magnitudes of both currents, forward and
reverse, would be the same. In this case, the material has attained its intrinsic likeness since the charge
carrier distribution is due to a defect at intrinsic fermi-level position. Thus, the fermi-level is pinned at
the intrinsic level making the device behaviour ohmic. The position of fermi energy pinned at the
intrinsic level is independent of incident radiation [15]. Since the electrical properties of the material
depend on the position of the fermi energy, it can therefore be concluded that electrical properties of
the metal-doped p-Si based diodes would be independent of the incident radiation.
In comparing the effects of metals on the properties of the material, it can be seen that a diode
fabricated on aluminium-doped p-silicon shows ohmic behaviour for almost the whole voltage region.
A high region of ohmic behaviour is due to the high density of ‘midgap’ defects, defect centres that
are responsible for relaxation behaviour of silicon [14]. Based on these results the diodes fabricated on
aluminium-doped silicon could be more resistant to radiation-damage than the one fabricated on zinc-
doped p-silicon.
1 1 1
10 10 10
(a) (b)
(b)
Undoped P-Silicon Al-doped- P-Silicon
Zn-doped- P-Silicon
10
0 Foward bias 0 Foward bias 10
0
10 Foward bias
Reverse bias Reverse bias
Temperature = 298 K Temperature = 298 K Reverse bias
Temperature = 298 K
Current (mA)
-1 -1
Current (mA)
-1
10 10 10
Current (mA)
-2 -2
10 -2
10 10
-3 -3
10 -3
10 10
-4 -4
10 -4
10 10
-2 -1 0
10 10 10 10
-1 0
10 10
-1 0
10
Voltage (V) Voltage (V)
Voltage (V)
Figure 4. I-V characteristics of diodes fabricated on undoped, Al and Zn-doped p-silicon in a linear (a)
and a logarithmic (b) scales
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Proceedings of the 27th International Symposium Nuclear Electronics and Computing (NEC’2019)
Budva, Becici, Montenegro, September 30 – October 4, 2019
4. Conclusion
In this work the diode were well fabricated on undoped, Aluminium and Zinc-doped p-Silicon.
A change in electrical properties of the diodes due to metal dopants was investigated by the current-
voltage technique. The diodes fabricated on metal doped silicon show ohmic I-V behavior indicating
that they were fabricated on relaxation material. An aluminium-doped p-Si based diode shows an
ohmic region higher than that of Zinc-doped p-Si based diode. The diode fabricated on relaxation
material has high resistivity and is resistant to radiation damage. This high resistivity of the diodes was
confirmed by low leakage current evaluated from the metal-doped p-Silicon based diodes.
In general, both metals are promising dopants for radiation hardness of silicon with aluminium being
the most promising one. In addition, further investigations are needed to confirm the diode property
independency of radiation. It is also important to carry out charge collection efficiency measurements
on the diodes.
5. Acknowledgement
This work is based on the research supported wholly by the National Research Foundation of
South Africa (Grant numbers 105292 and 114800).
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