p-Si metal–semiconductor (MS) structures

Microelectronics Reliability 51 (2011) 2205–2209

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Temperature dependent electrical and dielectric properties of Sn/p-Si metal–semiconductor (MS) structures Sß ükrü Karatasß ⇑, Zekeriya Kara _ Kahramanmarasß Sütçü Imam University, Faculty of Sciences and Arts, Department of Physics, 46100 Kahramanmarasß, Turkey

a r t i c l e

i n f o

Article history: Received 10 February 2011 Received in revised form 18 March 2011 Accepted 31 March 2011 Available online 21 April 2011

a b s t r a c t In this study, we investigated temperature dependent electrical and dielectric properties of the Sn/p-Si metal–semiconductor (MS) structures using capacitance–voltage (C–V) and conductance–voltage (G/x–V) characteristics in the temperature range 80–400 K. The dielectric constant (e0 ), dielectric loss (e0 0 ), dielectric loss tangent (tan d) and ac electrical conductivity (rac) were calculated from the C–V and G/x–V measurements and plotted as a function of temperature. The values of the e0 , e00 , tan d and rac at low temperature (=80 K) were found to be 0.57, 0.37, 0.56 and 1.04  107, where as the values of the e0 , e00 , tan d and rac at high temperature (=400 K) were found to be 0.75, 0.44, 0.59 and 1.21  106, respectively. An increase in the values of the e0 , e0 0 , tan d and rac where observed with increase in temperature. Furthermore, the effects of interface state density (NSS) and series resistance (RS) on C–V characteristics were investigated in the wide temperature range. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Measurement of dielectric properties (e0 , e00 and tan d) is an important part of the physical analysis of semiconductor, since it provides information valuable in all stages resource development ranging from formation evaluation to processing, process diagnostics, controls, and upgrading the product. In particular, the dielectric properties are known for their extreme sensitivity to change in MS. Therefore, the temperature dependence of these dielectric properties helps in identifying energy transport mechanisms in the material and is, important for modelling and monitoring conventional thermal processing, as well as in evaluating the potential of novel radio-frequency (RF) retorting techniques [1]. The dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric polarization. The dielectric behaviour of materials under external ac field has been the focus of numerous papers, in view of its high scientific and technological importance [2–4]. The temperature dependence electrical and dielectric properties (e0 , e00 and tan d) of Sn/p-Si (MS) structures are very important for different applications. Due to the technological importance of the metal–semiconductor structures in the electronics industry had been extensively studied both experimental and theoretically. In particularly, Si-contacts play one of the most important roles in Si-device or integrated circuit technology ⇑ Corresponding author. Tel.: +90 344 219 1310; fax: +90 344 219 1042. E-mail address: [email protected] (S ß. Karatasß). 0026-2714/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2011.03.041

represented by integrated circuit (IC), large-scale integration (LSI) and very large-scale integration (VLSI). The performance and reliability of metal–semiconductor structures are especially dependent on the formation of insulator layer between metal and semiconductor interface and series resistance of devices. Therefore, the temperature dependence electrical and dielectric changes are very important effect on determination of the MS structures [5– 7]. Also, the voltage and temperature dependence of series resistance (RS) at metal–semiconductor structures play an important role in the determination of the main parameters of the devices [4,8–13]. When voltage is applied across the M-S structure, the combination of the interfacial insulator layer, depletion layer and the series resistance of the device will share applied voltage. The values of the capacitance and conductance depend on various parameters, such as density of surface states, series resistance, insulator layer thickness and barrier height formation between metal and semiconductor. In recent studies, some researches [4,14–16] have reported an anomalous peak in the forward bias (C–V) characteristics. Among these very interesting study is presented by Chattopadhyay and Raychaudhuri, [15]. Chattopadhyay and Raychaudhuri have been shown that, in the presence of a series resistance, the C–V characteristics should exhibit a peak. Also, Kumar et al., Abo El Atta et al., and Zaki have studied the frequency and temperature dependence of the dielectric properties (e0 , e00 and tan d) [17–20]. They showed that the dielectric constant and loss tangent gives a peak at certain frequencies. Therefore, the temperature dependent electrical and dielectric characteristics are very important according to accuracy and reliability result. In general, there are several possible sources of error, which cause deviations of the ideal behaviour such as electrical

Sß. Karatasß, Z. Kara / Microelectronics Reliability 51 (2011) 2205–2209

and dielectric properties (e0 , e00 and tan d), which must be taken into account. These include the effects of interfacial insulator layer, interface state density, series resistance and formation of barrier height. Nevertheless satisfactory understanding of all details has still not been achieved. In previous study [4], we studied electrical and dielectric properties of Sn/p-Si (MS) structures in wide frequency range at room temperature. But, the aim of this study is to clarify temperature dependent electrical and dielectric properties of Sn/p-Si (MS) structures using capacitance–voltage (C–V) and conductance–voltage (G/x–V) measurements technique in wide temperature range at 500 kHz frequency and determine the variation of dielectric constant (e0 ), dielectric loss (e00 ), loss tangent (tan d) and ac electrical conductivity (rac) as temperature dependence.

4.0E-10

3.0E-10

2.5E-10

2.0E-10

2. Experimental procedure

1.5E-10 -0.3 0.0 0.3

0.6 0.9 1.2 1.5 1.8 2.1

2.4 2.7 3.0

Voltage (V) Fig. 1. The variation of the capacitance–voltage characteristics measured for various temperatures at 500 kHz of the Sn/p-Si (MS) structure.

3.2E-08 80K 100K

2.4E-08

Conductance ( S )

The Sn/p-type Si (MS) structures used in this study were prepared using mirror cleaned and polished as received from manufacturer p-type single crystal silicon wafers (boron doped) with (1 0 0) orientation and 6.248 X-cm resistivity. The wafer was chemically cleaned using the RCA cleaning procedure. The native oxide on the front surface of the substrate was removed in HF:H2O (1:10) solution and finally the wafer were rinsed in de-ionized water for 30 s. Then, low resistivity ohmic back contact to p-type Si (1 0 0) wafers was made by using Al, followed by a temperature treatment at 570 °C for 3 min in N2 atmosphere. The (MS) contacts were formed by evaporation of Sn dots with diameter of about 1.5 mm. All evaporation processes were carried out in a turbo molecular fitted vacuum coating unit at about 107 Torr. The capacitance–voltage (C–V) and conductance–voltage (G/x–V) measurements were performed in the temperature range of 80–400 K at 500 kHz by using a HP 4192A LF impedance analyzer and small sinusoidal test signal of 40 mVrms. The sample temperature was always monitored by using temperature-controlled Janes vpf-475 cryostat, which enables us to make measurements in the temperature range of 77–450 K and a lakeshore 321 autotuning temperature controller with sensitivity better than 70.1 K. All measurements were carried out with the help of a microcomputer through an IEEE-488 ac/dc converter card.

80K 100K 150K 200K 225K 250K 275K 300K 325K 350K 375K 400K

3.5E-10

Capacitance (F)

2206

150K 200K 225K

4.E-09

250K

1.6E-08

275K 300K

400 K

325K 350K

8.0E-09

375K 0.E+00

400K

0.0E+00 -4.0

-3.0

-1.0

-2.0

-1.0

0.0

0.0

1.0

2.0

Voltage (V) 3. Results and discussion The temperature dependence electrical and dielectric properties of Sn/p-Si (MS) structures are presented and discussed in this section. 3.1. Electrical properties Figs. 1 and 2 show the measured C–V and G/x–V characteristics of the Sn/p-type Si (MS) structure as a function of forward voltage at the temperature range of 80–400 K at high-frequency (500 kHz), respectively. As can be seen from Figs. 1 and 2, the capacitance (Cm) and conductance (G/x) plots are dependent on both the bias voltage and temperature. As can be seen from Fig. 1, the values of capacitance increased with increasing temperature. The increase in capacitance may be due to interfacial space charge formation [21–23]. The variation of conductance decreased with increasing temperature at forward bias, whereas the values of conductance increased with increasing temperature at reverse bias. Such behaviour attributed to the molecular restructuring and reordering of the interface states and series resistance or this situation of the conductance is attributed to particular distribution of surface states at between metal–semiconductor [7,21]. Therefore, the capaci-

Fig. 2. The variation of the conductance–voltage characteristics measured for various temperatures at 500 kHz of the Sn/p-Si (MS) structure.

tance and conductance are sensitive to temperature at high-and low-frequencies. The variations of C–V and G/x–V characteristics with temperature at high-frequency (500 kHz) at which only the free carriers within the majority bands were able to respond to the small excitation ac signal are shown in Figs. 1 and 2 for Sn/p-type Si (MS) structure, respectively. Series resistance (RS) is one of the important sources of small signal energy loss in MS Schottky structures. This causes a serious error in extraction of interfacial properties and doping profiles from the admittance measurements [4,24]. The real series resistance of SD can be obtained from the C–V and G/x–V measurements in strong accumulation region at high frequency (500 kHz). Then, the admittance Yma is given by [23]:

Y ma ¼ ½Gma þ jxC ma 

ð1Þ

where the series resistance is the real part of the impedance Zma = 1/Yma. Thus, the real series resistance of metal–semiconductor devices can be subtracted from the measured capacitance (Cma) and conductance (Gma) in strong accumulation region at high frequency

Sß. Karatasß, Z. Kara / Microelectronics Reliability 51 (2011) 2205–2209

density of interface states can be calculated by using the following equation [26];

500 450

Series Resistance (Ω )

400 350 300 250 200

80K 100K 150K 200K 225K 250K 275K 300K 325K 350K 375K 400K

NSS ¼

400 K

150

50

80 K

0 -2.0 -1.7 -1.4 -1.1 -0.8 -0.5 -0.2 0.1

0.4

0.7

1.0

Fig. 3. The variation of the series resistance as function of voltage for various temperatures at 500 kHz of the Sn/p-Si (MS) structure.

[23]. From C–V and G/w–V measurements in strong accumulation, the series resistance RS was calculated [5,25] through relation:

ðGma Þ

ð2Þ

G2ma þ ðxC ma Þ2

where x (=2pf) is the angular frequency, Gma and Cma are values of the conductance and capacitance obtained in strong accumulation region. Using Eq. (2), the values of RS were calculated as a function of bias at various temperatures as shown in Fig. 3. As can be seen from Fig. 3, the series resistance gives a peak as temperature dependence in the voltage range of 1.4 to 0.1 V. From Fig. 3, it is clearly seen that for each temperature the series resistance is almost independent of voltage at certain voltage region (voltage range 2 V between 1.4 V; and 0.1 > V). These behaviours considered that the trap charges have enough energy to escape from the traps located at metal–semiconductor interface in the Si band gap. The explanation of interface state distribution obtained from the C–V measurements as a function temperature of a MS Schottky structure with the series resistance can be expressed, and the

e ¼ e0  je00

ð4Þ

where j is the imaginary root of 1, the real part of the dielectric constant, e0 , is a measure of the energy stored from the applied electric field in the material and identifies the strength of alignment of dipoles in the dielectric. The imaginary part, e00 , or loss factor, is the energy dissipated in the dielectric associated with the frictional dampening that prevent displacements of bound charge from remaining in phase with the field changes [31]. In the e formalism, in the case of admittance measurements, the following relation holds

e ¼

Y Cm G ¼ j jxC 0 C 0 xC 0

ð5Þ

where Y  , C and G are the measured admittance, capacitance and conductance of the dielectric, and x the angular frequency (x = 2pf) of the applied electric field [7,32]. The real part of the dielectric constant (e0 ) at various temperatures are calculated using the measured capacitance values (Cm) at the strong accumulation region according to the relation [4,6,7,33,34];

2.0E+13

1.5E+13

-1

-2

ð3Þ

Dielectric properties (e0 , e00 and tan d) of various materials are finding increasing application, as fast and new technology is adapted for use in their respective industries and research laboratories. Earlier reports on dielectrics and modelling studies date back more than 80 years [28]. The dielectric constant is the ratio of the permittivity of a substance to the permittivity of free space. Thus, dielectric constant is a measurement of the quality and electrical properties of the material. Temperature dependencies of dielectric constant (e0 ), dielectric loss (e0 ), loss tangent (tan d) and ac electrical conductivity (rac) were evaluated from the data of capacitance and conductance measurements for Sn/p-Si Schottky structures in the temperature range of 80–400 K. It can be shown the real and imaginary parts of the relative permittivity of the MS can be defined in the following complex form [29,30],

2.5E+13

NSS (eV cm )

C2d q

3.2. Temperature dependence of dielectric properties (e0 , e00 and tan d)

Voltage (V)

1.0E+13

e0 ¼

5.0E+12

0.0E+00 120

ei ð1  C 2 Þ

where, NSS the density of acceptor-like interface states assuming over the energy range from barrier height to the Fermi level; C2 is a parameter inverse of the well-known ideality factor (n) which is a measure of conformity of the diode to pure thermionic emission and the values of n were determined by applying the slope of the C–V plots and d is the thickness of the interface layer and ei are the permittivities of the interfacial layer, respectively. The values of NSS for Sn/p-Si metal semiconductor structure calculated from Eq. (3) as a function of temperatures are shown in Fig. 4. As can be seen in Fig. 4, The NSS values decrease with increasing temperature due to the restructure and reordering of metal–semiconductor interface under the effect of temperature [25,27].

100

RS ¼

2207

170

220

270

320

370

420

Temperature (K) Fig. 4. Temperature dependence of interface states density obtained from the C–V measurements at various temperatures of the Sn/p-Si (MS) structure.

Cm Co

ð6Þ

where C 0 ¼ ðe0 A=dox Þ; A is the area of the sample, dox is the interfacial insulator layer thickness and e0 is the permittivity of free space charge (8.85  1014 F/cm), and Cm is measurement maximal capacitance of MS structure in the strong accumulation region, correspond to the oxide capacitance. The imaginary part, or loss factor (e00 ) is expressed as,

e00 ¼

Gm dox Ae0 x

ð7Þ

2208

Sß. Karatasß, Z. Kara / Microelectronics Reliability 51 (2011) 2205–2209

where Gm is the conductivity of device and x is the angular frequency. Also, the imaginary part can be expressed as follows,

7.0

e00 ¼ e0 tan d

6.0

ð8Þ

and thus loss tangent,

tan d ¼ e00 =e0

5.0

ð9Þ

rac ¼ xe0 e0 tan d

tan δ

The ac electrical conductivity (rac) of the dielectric material is given by the following equation [4,7,33,34]:

4.0

ð10Þ

3.0

The electrical conductivity provides a measure of the mobility of charge (e.g., electric current) when an electrical potential is placed across a device. Figs. 5–8 present the temperature depen-

2.0 1.0 0.0 60

0.80

100

140

180

220

260

300

340

380

420

Temperature (K) Fig. 7. Temperature dependence of loss tangent (tan d) at various temperatures of the Sn/p-Si (MS) structure.

0.75

0.70

ε'

2.2E-07 0.65 2.0E-07

σac (Ω cm)

-1

0.60

0.55

0.50 60

120

180

240

300

360

1.8E-07

420

Temperature (K)

1.6E-07

Fig. 5. Temperature dependence of dielectric constant (e0 ) at various temperatures of the Sn/p-Si (MS) structure.

1.4E-07 60

100

140

180

220

260

300

340

380

420

Temperature (K) 5.50 Fig. 8. Temperature dependence of ac electrical conductivity (rac) at various temperatures of the Sn/p-Si (MS) structure.

4.50

ε''

3.50

2.50

1.50

0.50

-0.50 60

120

180

240

300

360

420

Temperature (K) Fig. 6. Temperature dependence of dielectric loss (e0 0 ) at various temperatures of the Sn/p-Si (MS) structure.

dence of real part of the permittivity (e0 ),the imaginary part (e00 ), or loss factor, loss tangent (tan d) and electrical conductivity (rac) of the Sn/p-Si (MS) structures at different temperatures, respectively. As can be seen from these figures, the values of e0 , e00 , tan d and rac increases with an increase in temperature. It is seen that the values of the e0 , e00 , tan d and rac were found a strong temperature dependence. As the temperature rises, imperfections/disorders are occurred in the lattice and the mobility of the majority charge carriers (ions and electrons) increases. These results show that this MS structure possess better dielectric properties (e0 , e00 and tan d at temperatures higher than room temperature (according Ref. 4.), and the variation of the e0 , e00 , tan d with temperature is a general tendency in ionic solids. It may be due to space charge polarization caused by impurities or interstitials in the materials. The ac electrical conductivity (rac) curve of Sn/p-Si (MS) structure against temperature in the temperature range from 80 K to 400 K and at a constant frequency (500 kHz) is presented in Fig. 8. As can be seen from Fig. 8, the electrical conductivity

Sß. Karatasß, Z. Kara / Microelectronics Reliability 51 (2011) 2205–2209

increases with increasing temperature and especially shows a sharp increase in the electrical conductivity after about 100 K. Similar situations were observed in the literature [35–39]. The increase of the electrical conductivity leads to an increase of the eddy current which in turn increases the energy loss tan d and [4] This behaviour can be attributed to a gradual decrease in series resistance with increasing temperature [40]. It is clearly seen that the process of dielectric polarization in MS structure takes place through a mechanism similar to the conduction process.

4. Conclusion In this study, we investigated temperature dependent electrical and dielectric properties of Sn/p-Si (MS) structures using capacitance–voltage (C–V) and conductance–voltage (G/x–V) characteristics in the wide temperature range (80–400 K) and at a constant frequency (=500 kHz). It was shown that the electrical and dielectric properties of the C–V and G/x–V characteristics of Sn/p-Si (MS) structure were found to be strongly dependent to temperature. Also, the series resistance plot gives a peak, decreasing with increasing temperature. It has been shown that the RS strongly depends to temperature, and exponentially decreases with increasing temperature, and C–V and G/x–V characteristics confirm that the series resistance is important parameters that strongly influence the electrical and dielectric properties in Sn/p-Si (MS) structure. In addition, we can conclude that the values of dielectric constant (e0 ), dielectric loss (e00 ), loss tangent (tan d) and the ac electrical conductivity (rac) of Sn/p-Si (MS) structure strongly depends on temperature. The increase in ac electrical conductivity with increasing temperature may be attributed to charge centers created. The observed change in dielectric properties can be understood by considering the displacement damage introduced by temperature. Also, the increase in ac electrical conductivity (rac) can be attributed to a gradual decrease in series resistance with increasing temperature. Thus, in the future, it can be used contacts, integrated circuits, the other electronics devices an so on.

5-18 YLS). The Authors wish to thank to Kahramanmarasß Sütçü Imam University Scientific Research Project. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36]

Acknowledgements This work is supported by Kahramanmarasß Sütçü Imam University Scientific Research Project (BAB), FEF-Research Project (2010/

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[37] [38] [39] [40]

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