J Radioanal Nucl Chem (2014) 302:425–431 DOI 10.1007/s10967-014-3295-7

Influences of Co-60 gamma-ray irradiation on electrical characteristics of Al2O3 MOS capacitors Senol Kaya • Ercan Yilmaz

Received: 1 March 2014 / Published online: 22 June 2014 Ó Akade´miai Kiado´, Budapest, Hungary 2014

Abstract Effects of gamma-ray irradiation on the electrical characteristics of Al2O3 MOS capacitors such as barrier height, acceptor concentration, series resistance and interface state parameters have been studied by analyzing capacitance–voltage (C–V) and conductance–voltage (G/ x–V) measurements. The fabricated MOS capacitors were irradiated with gamma-rays at doses up to five grays. C– V and G/x–V measurements were recorded prior to and after irradiation at high frequency. The results show that the measured capacitance and conductance values decreased with increasing in irradiation dose and C–V and G/x curves has been shifted toward the negative voltages. Moreover, the series resistance (Rs) and density of interface states increased with increasing in irradiation dose and density of interface states (Dit) were calculated as order of 1012 eV-1cm-2 prior to and after irradiation. Due to presence and variations in the Rs values, the corrected and the measured C–V and G/x–V exhibited different behaviors. Therefore other electrical characteristics were assessed from corrected Cc characteristics. It was observed that acceptor concentration decreased with increasing in barrier height of device due to changes in interface states and diffusion potential. Keywords Radiation effects  Al2O3 MOS capacitor  Interface states  Series resistance  Barrier heights

S. Kaya  E. Yilmaz Physics Department, Abant Izzet Baysal University, 14280 Bolu, Turkey S. Kaya (&)  E. Yilmaz Nuclear Radiation Detectors Research and Development Center, 14280 Bolu, Turkey e-mail: [email protected]

Introduction It is well-known that the gate dielectric material and interface between dielectric layer and semiconductor have crucial effects on the usability and suitability of metal– oxide–semiconductor (MOS) devices [1–3]. The studies related to design and characterization of the MOS devices have been performed to its better performance since the last few decades [4–6]. Due to several possible sources of errors, the electrical characteristics of MOS capacitors deviate from expected ideal behavior. These errors could be related to the existence of charges inside in the dielectric and/or at the dielectric-semiconductor interface and series resistance [7–10]. Therefore, these sources of errors should be taken into account in the calculations. It is well known that MOS devices are extremely sensitive to ionizing radiation. Influences of radiation on the electrical characteristics of MOS devices are complicated in nature and may cause deviation during decades of device operation [11]. Radiation can generate traps and trapped charges in devices due to interactions with the atoms of the dielectric and semiconductor [12, 13]. The generated traps and trapped charges may be lead to performance degradation in electronic devices [14]. Electronic devices used in various environments such as space, nuclear industry and radiotherapy can contain numerous types of oxides and insulators. However, little knowledge exists in the literature about the radiation hardness of different high-k systems. The radiation hardened MOS devices, require for wide band gap, high effective dielectric constant and thermally stable material. Therefore, Al2O3 could be one of the suitable dielectrics due to its large band gap of Eg = 8.6 eV, the dielectric constant 8.4, the band off set to Si and its thermal stability. We focused on this work for Al2O3 dielectrics to be used in personal monitoring at low

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doses in medical applications. Hence, irradiated gamma doses were limited to maximum five grays in this study. The influences of gamma irradiation on Al2O3 MOS capacitors have been investigated in the literature [15–18] since last decades and briefly, the results demonstrate that the characteristics of devices can be affected by irradiation due to generated and trapped charge in the structure. However, the responses of barrier height, doping concentration, Rs and Dit to irradiation have not been holistically investigated. Studies show [19–22] Rs parameter of devices depends on irradiation doses and can be masking real behavior of device. Therefore, effects of irradiation on MOS devices must be investigated by considering Rs parameter. In this work, in order to investigate influences of irradiation on these electrical characteristics of Al2O3 MOS capacitors, the samples were irradiated by using the Co-60 gamma ray source from 1.5 to 5 grays. The electrical characteristics of the device were investigated from high frequency (1 MHz) C–V and G/x–V measurements and discussed for different exposure doses.

Experimental details The Al2O3-based MOS devices were fabricated using p-type (100) silicon wafer with a nominal resistivity 10 Xcm. Following a standard radio corporation of america cleaning process, oxide layers were deposited by magnetron sputtering. The base pressure was 1.5 9 10-6 Torr and the sputtering gas pressure was 1.0 9 10-3 Torr. A quasireactive gas mixture of Ar (97 %) and O2 (3 %) were used in order to obtain the desired stoichiometry. An in situ annealing temperature of 450 °C was used during sputtering. MOS capacitors were fabricated by forming ohmic contacts onto back side, and circular electrodes onto front side by Al evaporating. To form the front contacts, metal circular shadow mask with 1 mm diameter was used. In order to reduce defect concentrations of devices, the samples were annealed at 400 °C for 30 min in H2 environment under the atmospheric pressure. The thickness of Al2O3 film was calculated to be about 140 nm from the oxide capacitance measurement in the strong accumulation region for Al2O3 MOS devices. In order to investigate the influence of gamma ray irradiation on the MOS capacitors, fabricated MOS devices were irradiated using the GAMMACELL 220 Co-60 radioactive source from 1.5 to 5 grays at a dose rate of 0.018 Gy/s. The C–V and G/x–V measurements of the devices were performed by a HIOKI 3532-50 LCR before and after each irradiation doses at high frequency (1 MHz). All of the C–V and G/x–V measurements were carried out at room temperatures in the dark condition with 0.2 V increment steps.

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J Radioanal Nucl Chem (2014) 302:425–431

Capacitance (Farad)

426

4,0x10

-10

3,5x10

-10

3,0x10

-10

2,5x10

-10

2,0x10

-10

1,5x10

-10

-10

No Rad 1.5 Gy 2.5 Gy 3.5 Gy 5.0 Gy

-5

0

5

10

Voltage (V)

Fig. 1 The C–V curves of an Al2O3 MOS capacitor before, after gamma irradiation for 1 MHz

Results and discussion The influences of irradiation on the measured capacitance are illustrated in Fig. 1 for prior to and after irradiation. The three distinct regimes of capacitance curves named as accumulation–depletion–inversion before and after gamma ray irradiation are given in Fig. 1. The maximum reduction in the capacitance in accumulation regime was observed as 3.2 % on the irradiated devices compared to virgin samples. The rise in leakage through the oxide is responsible for the slight decreases in the capacitance in accumulation region [19]. Similarly, the capacitance in depletion regime decreased by 3.5 % after five Gray. These changes in the depletion regions may be attributed to the contribution of interface states capacitance generated by irradiation to the measured capacitance [22]. It is also observed from Fig. 1, the important modifications for curves are translations of flatband voltages (Vfb) toward negative voltage axis. The type of generated trap states in the MOS devices such as interface states and oxide traps are responsible for these voltage shifts [23, 24]. In other words, the negative voltage shifts are due to trapping of generated holes. It is wellknown that electrons are much more mobile than holes. Therefore, during ionization they can easily sweep out into the devices leaving behind a hole in the oxide [25]. The generated holes can be trapped at oxide and/or interface defects. Moreover, the variation in the DVfb respect to irradiated dose was fitted to the function in order to investigate DVfb and dose relations. The obtained results have been plotted in Fig. 2. Compatibility of the regression line with measured DVfb is 98 % which indicates the perfect match. In addition, linear matched regression line for irradiated Al2O3 MOS capacitor is well-agreement for thicker Al2O3 dielectric layers in the literature [18]. This linearity is one of the important factors for devices to be used in radiation sensors [12, 26]. Therefore, we advise that Al2O3 may be used as dielectric in MOS-based radiation sensors.

J Radioanal Nucl Chem (2014) 302:425–431

427

4.0 3.5

ΔV (V) fb

3.0 2.5 2.0 Measured ΔV fb

1.5

Fitted curve y=0.7022x+ 0.2129 Rsqr =0.98

1.0 1

2

3

4

5

Dose (Gy)

Fig. 2 The flatband voltage shifts on C–V curves of an Al2O3 MOS capacitor after gamma irradiation

1.05x10 -10

No Rad. 1.5 Gy 2.5 Gy 3.5 Gy 5.0 Gy

G/ω (Siemens)

9.00x10 -11 7.50x10 -11 6.00x10 -11 4.50x10 -11

for before and after maximum. Conductance increases very slightly before the maxima while decreasing in after the maxima. Before the maxima especially in accumulation regions the dominant effects on the conductance measurements is series resistance, however the interface states are more effective in the depletion and inversion regions [28]. Therefore, changes in distribution of series resistance and interface state with irradiation may be the reason of the variations in conductance characteristics. It is also important to note that the peak values of conductance located in depletion regions slightly decrease with increasing in radiation doses and the peak position of measured conductance shifts toward negative voltages. These behavior may be due to the generation of the lattice defects in the form of vacancies, defect clusters, and dislocation loops near the dielectric/semiconductor interface owing to the increases in radiation dose [20]. Studies showed that series resistance is a substantial factor to specify noise ratio of devices in terms of radiation doses which lead to deviations in electrical properties of MOS structure [21, 29]. Using the C–V and G/x–V characteristics in accumulation, the Rs can be calculated for before and after irradiation by using Eq. 1. [1, 30]:

3.00x10 -11

Rs ¼

1.50x10 -11 -10

-5

0

5

10

Voltage (V)

Fig. 3 G/x–V characteristics before and after irradiation of the Al2O3-based MOS capacitor

The conductance method [1, 27] is based on the conductance losses resulting from the exchange of the majority carriers between the interface states when a small voltage signal is applied to the MOS devices. The behavior of G/x as a function of voltage at various radiation doses (from 1.5 to 5 Gy) for Al2O3 MOS capacitor is shown in Fig. 3. It is seen that the distributions of the conductance are different

Gma

ð1Þ

2

ðGma Þ þ ðxCma Þ2

where x is the angular frequency, Cma and Gma are expressed as the measured capacitance and conductance in accumulation region, respectively. The calculated Rs are given in Table 1 and the calculated Rs values are also used to stabilize the measured C–V and G/x–V curves due to calculation of the real Dit. As seen in Table 1, the series resistance increase with increasing in radiation dose. This is attributed to the reordering and restructuring of the defects under irradiation. In order to remove the effects of Rs on the measured capacitance (Cm) and conductance (Gm) characteristics and evaluate the real interface trap density Dit of Al2O3 MOS

Table 1 The various parameters for Al2O3 MOS capacitor before and after irradiation doses Irradiation dose (Gy) 0

Rs (Ohm) 618

Gc,max 9 10-11 (Siemens)

Cc 910-10 (Farad)

6.78

Dit 91012 (eV-1 cm-2)

VD (eV)

Na 9 1016 (cm-3)

EF (eV)

DuB (meV)

u (eV)

1.16

-1.304

3.63

0.1618

38.3

1.18

1.26

0.376

3.60

0.1620

27.9

0.50

1.37

0.986

3.32

0.1641

34.8

1.12

1.70

1.896

3.21

0.1654

40.6

2.02

1.78

2.990

3.15

0.1657

45.4

3.11

3.10 1.5

620

6.59 3.17

2.5

646

6.64 3.23

3.5

662

5.77 3.40

5

681

5.30 3.44

123

428 4,5x10 -10 4,0x10

Cm (No Rad.) Cm (5 Gy) Cc (No Rad) Cc (5 Gy)

-10

3,5x10 -10

-2

D (eV cm )

-1

3,0x10 -10 2,5x10 -10

2x10

12

2x10

12

2x10

12

2x10

12

1x10

12

1x10

12

1x10

12

1x10

12

it

Capacitance (Farad)

(a)

J Radioanal Nucl Chem (2014) 302:425–431

2,0x10 -10 1,5x10 -10 -10

-5

0

5

10

15

G/ω (Siemens)

1

2

3

4

5

Dose (Gy)

Voltage (V)

(b)

0

Fig. 5 The interface state density versus irradiation dose of Al2O3 MOS capacitor

Gc (No Rad.) Gm (No Rad) Gc (5 Gy) Gm (5 Gy)

1.05x10-10 9.00x10-11 7.50x10-11 6.00x10-11 4.50x10-11 3.00x10-11 1.50x10-11 0.00 -10

-5

0

5

10

15

Voltage (V)

Fig. 4 The voltage dependent graphs of the corrected a capacitance and b conductance curves before and after irradiation

device before and after irradiation, C–V and G/x–V curves were corrected by the obtained Rs values. The corrected capacitance Cc and conductance Gc are calculated from following equations [1, 30, 31] Cc ¼

½ðGm Þ2 þ ðxCm Þ2 Cm a2 þ ðxCm Þ2

ð2Þ

and Gc ¼

½ðGm Þ2 þ ðxCm Þ2 a a2 þ ðxCm Þ2

ð3Þ

where a = (Gm) - [(Gm)2 ? (xCm)2]Rs, Cm and Gm are measured capacitance and conductance, respectively. The distributions of Cc and Gc/x as a function applied voltages are given in Fig. 4a, b prior to and after irradiation, respectively. The figures demonstrate that values of the corrected capacitance are greater than the measured values of capacitance. On the contrary, the values of the corrected conductance are smaller than the measured conductance due to the removing effect of the Rs. However, there still exists the peak in the corrected G/x–V measurements and these peaks are still located in corresponding depletion edge of devices which are -2.65 and 2.70 V for virgin and five Gy irradiated samples, respectively. The existence of the peak in the corrected conductance curves demonstrates that the charge transfer can take place through the interface. In addition, variations in the capacitance and

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conductance after corrections prove that the presence of the series resistance could lead to change the electrical characteristics of the devices. C–V and G/x–V characteristics in accumulation can be used to calculate the oxide capacitance Cox for the device through relation [1, 21] "   # Gc;ac 2 Cox ¼ Cc;ac 1 þ ð4Þ xCc;ac where Cc,ac is the corrected capacitance and Gc,ac is conductance in accumulation region, respectively. Cox is calculated to be approximately 4.17 9 10-10 F by using Eq. 4. The oxide thickness t can be evaluated from the corrected C–V curve in accumulation using the equation Cox = eie0A/t, where A is the front contact area MOS capacitor, ei = 8.4e0 and e0 are the permittivity of the oxide layer and free space, respectively. The oxide thickness t was calculated as about 140 nm. Interface state density (Dit) at dielectric/semiconductor interface is a useful parameter to specify quality of MOS devices. In order to calculate Dit, Hill–Coleman technique [32] confirmed by Konofaos [33] and Dakhel [34] can be used. Therefore, the densities of interface state are evaluated from Dit ¼

2 " Aq 

Gc;max =x # 2 2 Gc;max =xCox þð1  Cc =Cox Þ

ð5Þ

where, q is the electrical charge, A is the area of the MOS capacitor, Cox is the capacitance of oxide layer in accumulation region of Cc–V curve which is equal to 4.17 9 10-10 F, Gc,max/x is peak values of corrected G/x–V curve, Cc is corrected capacitance of the MOS capacitor corresponding to Gc,max/x. These required values to calculate interface state density are given in Table 1. The calculated Dit values of Al2O3 MOS capacitor before and after irradiation are displayed in Fig. 5. Table 1 and Fig. 5 depict that the Dit values

C

-2

3,0x10

19

2,5x10

19

2,0x10

19

1,5x10

19

1,0x10

19

5,0x10

18

429

No Rad. 1.5 Gy 2.5 Gy 3.5 Gy 5.0 Gy

C

-2

(Farad )

J Radioanal Nucl Chem (2014) 302:425–431

-4

-2

0

2

4

6

8

10

Voltage (V)

Fig. 6 The C-2 c –V characteristics for the Al2O3MOS capacitor at 1 MHz before and after gamma irradiation

increase with increasing in irradiation dose. This is due to the increasing in defects concentrations on MOS devices by irradiation. However, the calculated Dit values of MOS devices are about order of 1012 eV-1 cm-2. This order of Dit values is not high enough to pin Fermi level of Si substrate corrupting device operation [19, 33, 34] over given dose range. It can be concluded that calculated Dit values cannot hinder the construction of MOS device. Nonlinear rise in the trend of Dit with respect to dose have also been observed from Fig. 5. The passivation of dielectric layer from semiconductor may show variation for different exposed dose [26]. Therefore, this behavior of Dit is attributed the variations on the passivation of dielectric layer. Due to assess some other main electrical characteristics of Al2O3 MOS capacitors such as diffusion potential VD, acceptor concentration Na, and barrier height uB values, the C-2 c –V plots given in Fig. 6 were obtained from the corrected C–V characteristics prior to and after irradiation. Figure 6 demonstrates that the C-2 c –V variation is linear in the wide voltage range (DV & 2.0 V) at high frequency. This linearity of the curve is attributed to the uniformity of the Na in the depletion region [35]. The relation between Cc and V can be expressed as [1, 8, 36]; Cc2 ¼

2ð V0 þ V Þ es e0 qA2 Na

states in valance band and DuB is the image force barrier lowering and can be calculated from [36] rffiffiffiffiffiffiffiffiffiffiffiffiffi qEm ð8Þ DuB ¼ 4pes e0 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi where Em ¼ 2qVD Na =e0 es is the maximum electric field detailed in literature [37]. These calculated electrical characteristics are also given in Table 1. The intercept of C-2 versus V plot in Fig. 6 is positive voltage value for c non-irradiated sample which indicates that a fairly large number of negative charges are trapped in MOS structure due to fabrication process. However increasing in radiation doses, the Vo shifts towards the negative voltages owning to trapped positive charges generated by irradiation in the structure. As can be seen in Table 1, the uB almost enhance with increasing in irradiation dose. The increase in barrier height is due to an increase in VD [38]. Devices had widebarrier height decrease charge injection from the substrate into the dielectric and thus the tunneling effects into the structure may decrease resulting from higher value of barrier height. In addition, acceptor concentrations given in Table 1 decrease with increasing in irradiation doses. The decline in acceptor doping density with rise in radiation dose is attributed to generation and recombination through the interface states on dielectric/semiconductor interface. Usually, there exist various kinds of states with different lifetimes at the semiconductor interface [35]. Interface states capacitance may contribute the measure capacitance at low frequencies. Hence, total measured capacitance is higher than bare capacitance [28, 39]. If the measurements are performed at sufficiently high frequencies, the interface state charges cannot follow the voltage signal, and do not contribute to the measured capacitance. Therefore, the generated interface states effects on acceptor concentration may be reduced.

ð6Þ Conclusion

where V is the applied voltage and V0 (= VD - kT/q) is the intercept of the C-2 c versus V plot with the voltage axis at 1 MHz prior to and after irradiation. The Na is the acceptor concentration of p-type Si and value was obtained from the slope of C-2 c versus V plot. The value of the barrier height (uB) can be obtained from the reverse voltage C-2 c –V characteristics by following the relation [1, 8, 36];   kT kT NV uB ¼ V0 þ þ EF  DuB ¼ VD þ ln  DuB q q Na ð7Þ where EF is the energy difference between the bulk Fermi level and valance band edge, NV is the effective density of

The effects of gamma-ray irradiation on Al2O3 MOS capacitor have been analyzed using high frequency C– V and G/x–V characteristics of devices at various irradiation doses. The obtained results demonstrate that the capacitance and conductance curves shifts toward negative voltages and decrease slightly owning to the generated defects and trapped positive charges at the Al2O3/Si interface. Only the changes in measured capacitance is 3.2 % in accumulation regime, therefore it is acceptable for materials used in electronic component of devices at given dose ranges, however the changes in flatband voltage is higher than radiation tolerant materials [40, 41]. Hence, to use Al2O3 as a dielectric layer in MOS based

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microelectronic technology the voltage shift must be calibrated to given doses. Moreover, linear relation between DVfb and irradiated doses has been observed. Therefore it can be concluded that Al2O3 layer is more appropriative to use in MOS based radiation sensors. In addition, it is seen that the series resistance parameter and density of interface states increase with increasing in irradiation dose. However, the calculated Dit values are not high enough to pin the Fermi level of the Si substrate disrupting the device operation over given dose ranges. It is also observed that the measured and corrected curves exhibit different electrical behavior. Therefore, Rs distribution is important factor for device characteristic which lead to deviate of real characteristics of device. The decline in the acceptor concentration may be attributed the variations on the interface states and the barrier height of device almost increase due to changes in diffusion potential. In summary, trapped charges and generated defects densities by irradiation cause important changes in the device characteristics and Rs effects should be taken into account during the calculation to get accurate results. Acknowledgments This work is supported by Ministry of Development of Turkey under Contract Number: 2012K120360.

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