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

anicola@iing.mxl.uabc.mx,email,

bemman@issp.bas.bg,

cnesheva@issp.bas.bg,

dkrezhov@inrne.bas.bg,

eroumennedev@yahoo.com,

fcuriel@cnyn.unam.mx,

gbenval@iing.mxl.uabc.mx,

hmladenova@inrne.bas.bg,

ilevi@issp.bas.bg

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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Key Engineering Materials Vol. 495 123

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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