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 Table of Contents 
ARTICLE
Year : 2011  |  Volume : 34  |  Issue : 4  |  Page : 249-252  

A study of gamma radiation induced changes in electrical properties of Aℓ/TeO 2 /n-Si/Aℓ mos capacitor for dosimetric applications


1 MPTS, BARC, Mumbai, India
2 Department of Physics, Indian Institute of Technology, Kharagpur, India
3 CTS, BARC, Mumbai, India

Date of Web Publication17-Jan-2013

Correspondence Address:
G Chourasiya
MPTS, BARC, Mumbai
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0972-0464.106181

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  Abstract 

The study of the effects of ionizing radiation on MOS devices has been an active area of research due to their wide range applications. Some attempts have recently been made to investigate the influence of ionizing radiation on properties of the MOS capacitor prepared by replacing SiO 2 layer by any metal oxide layer of large band gap and then to understand its response. The effect of gamma radiation on electrical properties of the Aℓ/TeO 2 /n-Si/Aℓ MOS capacitor has been studied in detail for the first time in the present work in order to understand its applicability in the post-exposure gamma radiation dosimetry. The effect of gamma radiation on the real and imaginary parts of the permittivity, dielectric loss, series resistance, ac conductivity and surface state density has been determined. These properties have been obtained by analyzing C-V and G/ω-V characteristics, recorded at a frequency 1.0 MHz of the small ac signal, for the MOS structure exposed to different levels of the gamma radiation dose. The linear variation of the dielectric constant with the gamma radiation dose over a wide range of doses, observed corresponding to the accumulation region of the MOS capacitor, possesses high potential for its application as the post-exposure gamma radiation dosimeter.

Keywords: Dielectric properties, dosimeter, gamma radiation dose, tellurium dioxide


How to cite this article:
Chourasiya G, Maity T K, Sharma S L, Sarkar J, Vyas J C. A study of gamma radiation induced changes in electrical properties of Aℓ/TeO 2 /n-Si/Aℓ mos capacitor for dosimetric applications. Radiat Prot Environ 2011;34:249-52

How to cite this URL:
Chourasiya G, Maity T K, Sharma S L, Sarkar J, Vyas J C. A study of gamma radiation induced changes in electrical properties of Aℓ/TeO 2 /n-Si/Aℓ mos capacitor for dosimetric applications. Radiat Prot Environ [serial online] 2011 [cited 2020 Jun 6];34:249-52. Available from: http://www.rpe.org.in/text.asp?2011/34/4/249/106181


  1. Introduction Top


The study of the effects of ionizing radiation on MOS devices has been an active area of research due to their wide range applications over the past few decades. The main motivation behind such studies has been to develop novel post-exposure as well as real-time radiation dosimeters possessing high sensitivity. A fact is that MOSFETs are presently being used as radiation dosimeter in monitoring the radiation dose delivered to patients undergoing radiation therapy. [1] Any radiation dosimeter is required to possess linear response over the intended energy range with high sensitivity, real-time response, low noise and acceptable reliability. Several solid materials, with various geometric arrangements and physical detection techniques, have been used to meet these requirements. Amongst these, the radiation dosimeters based on MOS devices have attracted special attention of physicists as well as medical practitioners because of their superior sensitivity and excellent compatibility with the microelectronics technology.

The influence of the ionizing radiation on the MOS capacitor characteristics depends on both the dose and the parameters of the device structure including the oxide layer thickness. [2] The effect of the ionizing radiation on the C-V characteristics of the MOS capacitor is seen as a flat band voltage shift towards negative gate voltages on exposing it to ionizing radiation. [3],[4] This is due to the formation of the positively charged trap centers in the oxide layer as well as that at the interface of the oxide layer and semiconductor on exposure to ionizing radiation. [5] Recently, some attempts have been made to investigate the influence of ionizing radiation on different properties of the MOS capacitor prepared by replacing the oxide layer of silicon (SiO 2 ) by any metal oxide layer of large band gap and then to understand the response of the resulting MOS structure to different levels of the radiation dose of the ionizing radiation of different kinds. [6],[7],[8]

The effect of the gamma radiation exposure of different levels on different electrical properties of the Aℓ/TeO 2 /n-Si/Aℓ MOS capacitor has been studied in detail for the first time in the present work in order to understand its applicability in the post-exposure gamma radiation dosimetry. The effect of different levels of the gamma radiation dose on the real and imaginary parts of the permittivity, dielectric loss, series resistance, ac conductivity and surface state density has been determined quantitatively. These properties have been obtained by analyzing the capacitance-voltage (C-V) and conductance-voltage (G/ω-V) characteristics for the MOS structure, recorded at a constant frequency 1.0 MHz of the small ac signal, after exposing it to different levels of the gamma radiation dose. The dielectric properties of the structure have been found to be strongly influenced by the presence of radiation-induced defects, which are mainly associated with the oxygen vacancies. The linear variation of the dielectric constant with the gamma radiation dose over a wide range of doses, observed corresponding to the accumulation region of the MOS capacitor, possesses high potential for its application as post-exposure gamma radiation dosimeter. The details of these findings are presented and discussed below in the framework of developing post-exposure gamma radiation dosimeters.


  2. Experimental Details Top


Several samples of the Aℓ/TeO 2 /n-Si/Aℓ MOS capacitor, having tellurium oxide (TeO 2 ) layer of thickness of about 100 nm, were fabricated using phosphorous doped single crystal of silicon wafer of thickness 300 μm having (111) surface orientation. The doping level of the silicon wafer was about 4.16 × 10 15 cm−3 , giving resistivity of about 10 Ω cm. The wafer was chemically cleaned using the standard RCA cleaning procedure before preparing the MOS capacitor. The purpose of RCA cleaning procedure has been to remove organic contaminants (such as dust particles, grease, silica gel, etc.) from the wafer surface, then to remove oxide layer that might have been built up and finally to remove any ionic or heavy metal contaminants. After cleaning the surface, an ohmic contact of aluminium was grown on one side of the wafer by thermal evaporation in a vacuum ~10−5 mbar. The thickness of the ohmic contact was about 300nm. On the back side of the wafer, a thin layer of TeO 2 of thickness 100 nm was subsequently grown by thermal evaporation in a vacuum. On the top of the TeO 2 layer, another ohmic contact of aluminium was grown once again by thermal evaporation in a vacuum. In order to have good quality ohmic contacts, after preparation, each MOS capacitor was kept inside a furnace maintained at a constant temperature of about 450°C for about 10 minutes. In this manner, several samples of the Aℓ/TeO 2 /n-Si/Aℓ MOS capacitor were prepared. The capacitance versus voltage (C-V) characteristics and conductance versus voltage (G/ω-V) characteristics were subsequently recorded at a constant frequency 1.0 MHz of the small ac signal using a high precision impedance analyzer for several samples of the Aℓ/TeO 2 /n-Si/Aℓ MOS capacitor, after exposing them to different levels of the gamma radiation dose. As is well known that, at high frequencies (f ≥ 0.5 MHz), the interface state density maintains its equilibrium state as the charges at the interface cannot follow the ac signal. [9] The gamma radiation exposure of these samples of the MOS capacitor was carried out using a 60 Co source at room temperature. In all these measurements, a small test ac signal having V rms 500 mV was used.

Whenever a voltage is applied to gate of the MOS capacitor, keeping substrate at the ground potential, there occurs bending of the bands in ways that are dependent on both the polarity of the gate voltage and the substrate type. In the case of the n-type substrate, a positive gate voltage attracts mobile electrons to an ultra-thin layer close to the oxide-semiconductor interface making the layer effectively much more heavily doped n-type. This is termed as the accumulation condition. If now the gate voltage is changed to negative voltages, the mobile electrons are repelled from the interface forming a depletion layer (i.e., a layer of positive space charge) near the interface. This is termed as the depletion condition. As the gate voltage is made more and more negative, a point is reached when an ultra-thin layer close to the oxide-semiconductor interface becomes p-type extrinsic.

The dielectric properties of a material, placed between two parallel plate electrodes, can be expressed in many ways using different representations. The capacitance, dielectric constant and dielectric loss are important parameters in the selection of a material for its two-electrode device applications. The dielectric constant of the oxide layer of the MOS capacitor, when the gate voltage corresponds to the strong accumulation region, can be expressed as



Where ε′ is the real part of the relative permittivity (i.e., the dielectric constant), C c the corrected capacitance obtained from the measured capacitance after correcting it for the series resistance of the MOS capacitor, d ox the thickness of the oxide layer, A the overlapping area of the two parallel plate electrodes and εo (= 8.85 × 10−12 F/m) the permittivity of the vacuum. Also, the imaginary part of the relative permittivity ε′′ varies with the frequency of the ac signal in the manner given by:



Where, G c represents the corrected conductance obtained from the measured conductance after correcting it for the series resistance of the MOS capacitor and ω the frequency of the small ac signal.


  3. Results and Discussion Top


The capacitance and conductance of the Aℓ/TeO 2 /n-Si/Aℓ MOS capacitor, after exposing to different levels of the gamma radiation dose, have been measured for different gate voltages at an appreciably high frequency 1.0 MHz of the ac signal. [Figure 1] shows the typical measured capacitance versus gate voltage plots for the Aℓ/TeO 2 /n-Si/Aℓ MOS capacitor, exposed to different levels of the gamma radiation dose. Clearly, each of these C-V curves possesses three regions: Depletion-inversion-accumulation. It is further observed that the radiation induced dispersion is quite high in the strong accumulation region of these curves and that the capacitance increases with the gamma radiation dose up to a critical dose value in the strong accumulation region and decreases thereafter, a behavior attributed to a particular distribution of surface states at the TeO 2 -Si interface. Due to gamma irradiation, electron hole pairs are generated inside the metal oxide layer and at the interface of the metal oxide and semiconductor. These radiation generated electrons move away rapidly due to their much higher mobility in comparison to the holes. The holes, being relatively immobile, are trapped within the oxide layer and at the interface leading to net positive charge throughout the oxide layer and at the interface. [10] The radiation-induced defects may also serve as traps. [11] In the strong accumulation region, under the gamma irradiation, the radiation generated holes get accumulated near the interface leading to the reduction in the barrier potential causing capacitance enhancement up to a certain gamma radiation dose. The decrease of the measured capacitance in the strong accumulation region at higher gamma radiation doses can, however, be understood in terms of the leakage current through the series resistance of the Aℓ/TeO 2 /n-Si/Aℓ MOS capacitor.

[Figure 2] shows the typical measured conductance (G m /ω) versus gate voltage plots for the Aℓ/TeO 2 /n-Si/Aℓ MOS capacitor, exposed to different levels of the gamma radiation dose. Each of these G m /ω-V curves is once again observed to possess three regions: Depletion-inversion-accumulation. Further, the radiation induced dispersion is quite high in the strong accumulation region of these curves and that G m /ω is observed to increase with the gamma radiation dose up to a critical dose value in the strong accumulation region and decreases thereafter.
Figure 1: Measured capacitance versus gate voltage (C-V) plots for the Aℓ /TeO2/n - Si/Aℓ capacitor, exposed to different levels of the gamma radiation dose

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Figure 2: Measured conductance versus gate voltage (G/ w -V) plots for the Aℓ /TeO2/n - Si/Aℓ structure, exposed to different levels of the gamma radiation dose

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It is known that, at high frequency of the small ac signal, the electrical properties of the MOS structure in the strong accumulation region describe the basic dielectric behaviour of the oxide layer. Therefore, in the strong accumulation region of any MOS structure, the dielectric constant (the real part of the relative permittivity) of the metal oxide layer is of main concern. Furthermore, the dielectric constant is a measure of the extent to which the charge distribution in the material can be polarized by the application of an electric field. [Figure 3] shows the plots of the real part of the relative permittivity ε′ with the gate voltage for the MOS capacitor, exposed to different levels of the gamma radiation dose. Clearly, in the strong accumulation region of the MOS capacitor, the above Figure shows that the structural defects are quite homogeneous and that the dielectric constant increases with the gamma radiation dose up to a certain critical dose value and decreases thereafter.
Figure 3: Variations of the dielectric constant with gate voltage for the Aℓ /TeO2/n - Si/Aℓ capacitor, exposed to different levels of the gamma radiation dose

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[Figure 4] shows the variation of the dielectric constant with the gamma radiation dose at a forward bias of 4.0 V in the strong accumulation region. Obviously, the dielectric constant of the metal oxide layer increases with the gamma radiation dose up to 50 Gy and decreases beyond this dose value.

The linear variation of the dielectric constant with the gamma radiation dose over a wide range of the doses, in the strong accumulation region of the MOS capacitor, possesses high potential for its application as the post-exposure gamma radiation dosimeter in monitoring doses under a variety of practical situations from low levels of the doses such as involved in the teaching laboratories to high levels of the doses such as involved in medical applications, food irradiation, etc. For the dose determination involved in a teaching laboratory, the exposed structure is to be analyzed after two or three months and the same structure needs to be analyzed quite frequently in applications involving high doses.
Figure 4: Variation of the dielectric constant with gamma radiation dose at an applied voltage of 4.0 V in the strong accumulation region

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  4. Conclusions Top


The linear variation of the dielectric constant with the gamma radiation dose over a wide range of the doses, for the Aℓ/TeO 2 /n-Si/Aℓ MOS capacitor in the strong accumulation region, possesses high potential for its application as the post-exposure gamma radiation dosimeter.


  5. Acknowledgements Top


Authors acknowledge with thanks the financial assistance provided by the Department of Atomic Energy, Govt. of India in the form of a research project with sanction number 2011/36/23-BRNS.

 
  References Top

1.Hughes RC, Huffman D, Snelling JB, Zipperian TE, Ricco AJ, Kelsey CA. Miniature radiation dosimeter for in-vivo radiation measurements. Inter J Rad Onco Bio Phy 1988;14:963.  Back to cited text no. 1
    
2.Naruka K, Yodshida M, Maegushi J, Tango H. Radiation induced interface states of poly-silicon gate MOS capacitors using low temperature gate oxidation. IEEE Trans Nucl Sci 1983;30:4054.  Back to cited text no. 2
    
3.Arshak K, Korostynska O, Zleetni S. Review of various gamma radiation dosimeters based on thin and thick films of metal oxides and polymer materials. IEEE Trans Nucl Sci 2003;1:78.  Back to cited text no. 3
    
4.Yilmaz E, Kaleli B, Turan R. A systematic study on MOS type radiation sensors. Nucl Instr Meth B 2007;264:287.  Back to cited text no. 4
    
5.Brucker GJ, Gunten OV, Stassinopoulos EG, Sapiro P, August LS, Jordan TM. Recovery of damage in Rad-Hard MOS devices during and after irradiation by electrons, protons, alphas and gamma rays. IEEE Trans Nucl Sci 1983;30:4157.  Back to cited text no. 5
    
6.Karadeniz S, Tuðluoðlu N, Serin T. Substrate temperature dependence of series resistance in Al/SnO2 /p-Si (111) Schottky diodes prepared by spray deposition method. Appl Surf Sci 2004;233:5.  Back to cited text no. 6
    
7.Senthil Srinivasan VS, Patra MK, Choudhury VS, Pandya A. Gamma irradiation study of tin oxide thin films for dosimetric applications. J Optoelectronics Adv Mater 2007;9:3725.  Back to cited text no. 7
    
8.Senthil Srinivasan VS, Pandya A. Feasibility study of thin film Aℓ/SnO x /n-Si gate stack for gamma radiation dosimetry. Rad Meas 2009;44:325.  Back to cited text no. 8
    
9.Hill WA, Coleman CC. A single-frequency approximation for interface-state density determination. Solid-State Electronics 1980;23:987.  Back to cited text no. 9
    
10.Snow EH, Grove S, Fitzerald DJ. Effect of ionizing radiation on oxidized silicon surfaces and planer devices. Proc IEEE 1967;55:1168.  Back to cited text no. 10
    
11.Schwank JR, Winokur PS, Sexton FW, Fleetwood DM, Perry JH, Dresendorfer PV, et al. Radiation induced interface state generation in MOS devices. IEEE Trans Nucl Sci 1986;33:1177.  Back to cited text no. 11
    


    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4]


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1. Introduction
2. Experimental ...
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