Radiation Dosimeter Based on Metal-Oxide-Semiconductor Structures Containing Silicon Nanocrystals
Transcript of Radiation Dosimeter Based on Metal-Oxide-Semiconductor Structures Containing Silicon Nanocrystals
Radiation dosimeter based on Metal-Oxide-Semiconductor structures
containing silicon nanocrystals
Nicola Nedev1,a, Emil Manolov2,b, Diana Nesheva2,c , Kiril Krezhov3,d, Roumen Nedev1,4,e, Mario Curiel5,f, Benjamin Valdez1,g, Alexander Mladenov3,h
and Zelma Levi2,i 1Institute of Engineering, Autonomous University of Baja California, Benito Juarez Blvd. esc. Calle
de la Normal, s/n, C. P. 21280 Mexicali, B. C., Mexico
2Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee Blvd,
1784 Sofia, Bulgaria
3Institute for Nuclear Research and Nuclear Energy, Bulgarian Academy of Sciences, 72
Tzarigradsko Chaussee Blvd, 1784 Sofia, Bulgaria
4Technical University of Sofia, FKSU, 8 Kliment Ohridski Blvd., 1000 Sofia, Bulgaria
5Centro de Nanociencias y Nanotecnología, Universidad Nacional Autónoma de México, P.O. Box,
356, 22800 Ensenada, B.C. México
[email protected],email,
Keywords: MOS Dosimeter, Nanocrystals, Metal-Oxide-Semiconductor structures, γ-radiation
Abstract. MOS structures containing silicon nanocrystals in the gate dielectric have been tested as
dosimeters for ionizing radiation. Before irradiation the nanocrystals have been charged with
electrons by applying a pulse to the gate electrode. The γ-irradiation with doses in the range 0-100
Gy causes approximately linear variation of the flatband voltage, resulting in sensitivities of
~ 2.5 mV/Gy. At higher doses the sensitivity decreases because of decrease of the oxide electric
field.
Introduction
Since the first publication which proposes the usage of a Floating Gate Metal-Oxide-
Semiconductor Field Effect Transistor (FG MOSFET) as solid state dosimeter [1] important work
has been carried out in order to clarify the advantages of such dosimeters [2,3] and to optimize their
operation [4-8].
In this work we develop further our idea for application of a FG MOSFET as a solid state
dosimeter substituting the continuous floating gate by a distributed of silicon nanocrystals (Si NCs)
as discrete charge storing nodes embedded in a SiO2 matrix [9]. An important advantage of such a
device, especially when operated in an environment of ionizing radiation, is that a single leakage
path in the SiO2 will not lead to a complete discharge of the dosimeter. A schematic cross-section of
a MOSFET with Si nanocrystals is presented in Fig. 1. The operation is based on generation of
electron-hole pairs in the SiO2 when the transistor is exposed to ionizing radiation and separation of
the generated carriers by the local internal electric field created around each NC by a preliminary
charging of Si NCs. For example if the NCs are negatively charged the holes generated in the SiO2
are swept towards the nanocrystals, where they recombine with a part of the trapped electrons and
reduce the net charge, while the generated electrons are swept towards the gate electrode.
Key Engineering Materials Vol. 495 (2012) pp 120-123Online available since 2011/Nov/15 at www.scientific.net© (2012) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/KEM.495.120
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Experimental Details
MOS capacitors with area of 2 × 10-3
cm2 were fabricated on p- and n-type (100) c-Si wafers
Fig. 1. Schematic cross-section of a MOS
transistor with silicon nanocrystals.
with resistivity of 1 and 4-6 Ω.cm, respectively. On both types of substrates a 3.9 nm thick SiO2
layer was thermally grown in dry O2 atmosphere followed by an ~ 15 nm thick SiOx film (x = 1.15)
prepared by thermal evaporation of SiO in vacuum. Prior to oxidation the wafers were cleaned
chemically by a standard for the microelectronics procedure. On the top of the SiOx film an
additional SiO2 layer having thickness of about 60 nm was deposited by radio frequency sputtering.
After the formation of the gate dielectric stack the samples were annealed at 1000o
C for 60 minutes
in nitrogen. Our previous results have shown that annealing of SiO1.1 under these conditions leads to
phase separation and formation of silicon nanocrystals with diameter of ~5-6 nm (Fig. 2) [10,11] in
a SiO2 matrix [12,13].The thickness of the oxide formed between the nanocrystals and the adjacent
films is ≥ 3 nm. After the annealing Al metallization was carried out through a mask and the top
electrodes (referred from now on as control gates) of the MOS capacitors were formed.
Fig. 2. Cross-section HRTEM micrograph of a
SiOx film with x = 1.1 and thickness of 15 nm
annealed at 1000 oC for 60 min.
In order to adjust the initial flatband voltage, ∆VFB0, the structures were charged by applying
voltage pulses to the control gate with positive or negative polarity and with various amplitudes and
durations. The positive pulses charge Si nanocrystals with electrons injected from the crystalline
silicon wafer, while the negative ones charge NCs with holes. Here we present results only for
structures charged negatively because they exhibited better retention of the trapped charge compared
to the positively charged structures.
Capacitance/Conductance – Voltage (C/G-V) measurements at 1 MHz were carried out using
Agilent E4980A Precision LCR Meter controlled by Agilent B1500A Semiconductor Device
Analyzer.
The samples were subjected to various integral gamma irradiation doses from 3 to 200 Gy which
were accumulated in steps at a dose rate of 37 Gy/h. The γ-radiation modification was carried out in
air of 75 to 80% humidity by means of the 38 000 Ci 60
Co source (average energy Eγ = 1.25 MeV)
of the Institute for Nuclear Research and Nuclear Energy.
Results and Discussion
Fig. 3 shows the shift of the high frequency C-V curve after charging a p-type MOS structure
with six consecutive pulses, each of them with amplitude of +10 V and duration of 5 s. The initial
curve is also presented. Parallel shifts to the positive voltages are seen with a change of the flatband
Key Engineering Materials Vol. 495 121
voltage value from ~0.14 V, after the first pulse, to VFB = 0.83 V after the last (sixth) one. The shifts
in this direction correspond to a gradual increase of the negative charge in the gate dielectric due to
charging of the nanocrystals [14] with electrons. The characteristic after each charging pulse was
measured in both directions in a narrow voltage range (0 - 2V), in order to avoid changes in the
charge state of the nanocrystals; no hysteresis has been obtained.
-3 -2 -1 0 13.0x10
-11
4.0x10-11
5.0x10-11
6.0x10-11
7.0x10-11
8.0x10-11
initial
6 consecutive pulses
+10V,5s
Cap
acita
nce (
F)
Gate voltage (V)
Fig. 3. C-V curves measured at
1 MHz of a MOS structure with Si
nanocrystals charged with 6
consecutive pulses, each of them with
amplitude of +10 V and duration of
5 s. The initial curve is also
presented.
Fig. 4 shows the time variation of the flatband voltage VFB of two p-type MOS structures, with
negatively charged NCs and different initial ∆VFB0. Both structures exhibit very good retention
characteristics, e.g. when the initial flatband voltage shift is 0.44 V all trapped electrons remain on
the nanocrystals after 68 h (100% retention), while when ∆VFB0 = 0.83 V the trapped charge
remaining on the nanocrystals is ~97 % after 44 h. Similar curves were measured for structures
fabricated on n-type silicon.
100
101
102
103
104
105
0.0
0.2
0.4
0.6
0.8∆V
FB0=0.83V
∆VFB0
=0.44V
∆V
FB (
V)
Time (s)
0 50 100 150 2000
100
200
300
400
∆VFB0
=0.80V
∆VFB0
=0.67V
∆VFB0
=0.74V
∆V
FB (
mV
)
Dose (Gy)
Fig. 4. Retention characteristics of MOS structure
on p-Si, in which the Si nanocrystals in the oxide
are charged with electrons. The structures were
kept short-circuited in the intervals between the
measurements.
Fig. 5. Flatband voltage changes of p-Si
MOS structures charged with electrons
versus absorbed γ dose.
122 Materials and Applications for Sensors and Transducers
The effect of the ionizing gamma radiation was studied by irradiating MOS structures preliminary
charged with electrons. Fig. 5 shows changes of the flatband voltage versus absorbed dose for three
capacitors having an initial flatband voltage shift of ∆VFB0 = 0.8, 0.74 and 0.67 V. The three curves
have similar shape within an initial interval, 0 - 100 Gy, in which approximately linear dependence
between ∆VFB and the dose is observed. The obtained sensitivities for the linear region are S~2.1,
2.8 and 2.3 mV/Gy, respectively, and no correlation between ∆VFB0 and S was found. The reduced
sensitivity at doses higher than 100 Gy can be explained by discharging of nanocrystals and
decrease of the oxide internal electric field.
Conclusion
It has been demonstrated that MOS structures containing silicon nanocrystals in the gate
dielectric are promising for application as dosimeters for ionizing radiation. Before irradiation the
structures have been charged with electrons by applying a voltage pulse or pulses with positive
polarity and appropriate amplitude and duration. By varying the pulse parameters the value of the
initial flatband voltage shift can be varied. The γ-irradiation with doses in the range 0-100 Gy
causes approximately linear variation of the flatband voltage, resulting in sensitivities of ~ 2.5
mV/Gy. At higher doses the sensitivity decreases because of decrease of the oxide electric field.
Acknowledgements
The authors gratefully acknowledge the financial support of Autonomous University of Baja
California, Mexico and the Bulgarian National Fund for Science (grant NFNI DO-224/2008).
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Materials and Applications for Sensors and Transducers 10.4028/www.scientific.net/KEM.495 Radiation Dosimeter Based on Metal-Oxide-Semiconductor Structures Containing Silicon Nanocrystals 10.4028/www.scientific.net/KEM.495.120
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