Temperature Dependent Current-Transport Mechanisms (CTMs) in the Al/Al2O3/p-Si (MIS) Diodes (SDs) Using Current-Voltage-Temperature (I-V-T) Characteristics in the Temperature Range of 200-320 K

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Optoelectronics, Applied Optics and Microelectronics

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Department of Advanced Technologies, University of Mohaghegh Ardabili

Namin, Ardabil, Iran Aug. 17-19, 2019

Temperature Dependent Current-Transport Mechanisms Si (MIS) Diodes (SDs) Using -

/p O 3

(CTMs) in the Al/Al 2

Current-Voltage-Temperature (I-V-T) Characteristics in the Temperature Range of 200-320 K

1Mehmet Kosal, ²Elif Marıl, ³Şemsettin Altındal

1Department of Physic, Faculty of Arts and Sciences, Harran University, Şanlıurfa-, Turkey

2Department of Property Protection and Security, Yenice Vocational School, Karabük University, Karabük,Turkey

3Department of Physics, Faculty of Sciences, Gazi University, Ankara, Turkey

Abstract- The reverse bias saturation-current (Io), ideality factor (n), zero-bias barrier height (BH:ΦBo), rectifying ratio (RR), series (Rs) and shunt (Rsh) resistances of the fabricated Al/Al2O3/p-Si (MIS) diodes have been investigated function of temperature. These values were extracted from the forward-reverse bias current-voltage (I-V) characteristics in wide range of temperature (200-320 K) and voltage ( 3V). The values of Rs and Rsh were obtained from the Ohm’s Law as a simple method. On the other hand, the Io, n, ΦBo, n.ΦBo, Rs, and Rsh values were found as 2.34 pA, 2.366, 0.41eV, 0.969 eV, 54.64 k , 4.69 G at 200 K and 1.166 nA, 1.786, 0.510eV, 0.911 eV, 4.78 k , 0.11 G at 320 K, respectively. It is clear that all these parameters are strong function temperature. While the value of ΦBo

increase with increasing temperature, the modified value of (n.ΦBo) by introduced the value of n in the reverse bias expression (Io=AA*T²exp(-q ΦBo/nkT)) decrease as like forbidden band gap with -4.927x10^-4 eV/K temperature coefficient. This negative temperature coefficient of BH is very close to the temperature dependent band gap of Si (-4.73x10^-4 eV/K). The higher value of n at low temperature was attributed barrier inhomogeneity and interfacial sapphire layer.

Keywords: Metal-insulator-semiconductor (MIS) type Schottky diodes; Current-transport- mechanisms; Temperature dependent

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Namin, Ardabil, Iran Aug. 17-19, 2019 1. Introduction

Current-transport/conduction mechanisms (CTMs/CCMs) in the metal-semiconductor (MS) diodes/structures with and without an interfacial insulator or polymer are dependent on various factors such as the thickness and homogeneity of interlayer, surface states (Nss) located at M/S interface, inhomogeneity of barrier height (BH), series resistance (Rs) and shunt resistance (Rsh) of diode, sample temperature and voltage [1-5]. Among them, temperature, barrier inhomogeneity, Rs, Rsh, and native or deposited an interlayer are more influenced optic, electric and dielectric properties of these structures. It is believed that the used a high-dielectric material as interfacial layer may be reduced the leakage current, Nss and increase of RR, Rsh and BH, respectively.

In the MS and MIS type diodes and solar cells, various CTMs may dominate the others at a certain temperature and at voltage regions, such as thermionic emission (TE), generation-recombination (GR), thermionic-field emission (TFE) and field emission (FE).

Additionally, a simultaneous contribution from two or more mechanisms could also be possible [6-11]. Among them TFE and FE are important at low temperatures and high doping concentration atoms. Therefore, the investigation of CTMs in the wide range of temperature and voltage is more important to get more information on the possible conduction mechanisms and nature of barrier height between metal and semiconductor.

The main aim of this study is to fabricate Al/Al2O3/p-Si (MIS) structures and investigate I-V-T characteristics in wide range of temperature (200-320 K) for better interpretation of the possible CTMs in them. Experimental results confirmed that the value of ΦBo increase with increasing temperature, whereas n decreases. Such behavior of them was explained with barrier inhomogeneity rather than tunneling mechanisms (TFE and FE).

2. Experimental Procedure

In this study, Al/Al2O2/p-Si (MIS) diodes were grown on the Boron-doped (p-Si) wafer with (100) surface orientation 300 µm thickness, 5.08 cm radius, and 1.39x10¹⁶ cm^-3 doping concentration (NA). Firstly, p-Si wafer was etched in a sequence of H2SO4 and H2O2, 20%

HF, a solution of 6 HNO3: 1 HF: 35 H2O, 20% HF and then it was also rinsed in deionized water with 18 M cm for 5min to remove organic impurities surface and then it was dried by high-pure dry nitrogen (N2) gas. Secondly, high purity Al (99.999%) layer with 120 nm was thermally evaporated on the whole back of the p-Si wafer at 10^-6 Torr and then it was annealed at 450 ^oC in nitrogen ambient to perform a good ohmic contact. Thirdly, a layer of Al2O3 was grown on the p-Si wafer by ALD technique. Finally, high purity Al (99.999%) dots with 120 nm thicknesses and 1 mm diameter was thermally evaporated on the Al2O3 layer. The forward and reverse bias current-voltage (I-V) measurements were

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performed by using a Keithley 2400 current voltage source-meter in the temperature range of 200-320 K.

3. Results and Discussions

The forward and reverse bias semi-logarithmic I-V plots of the Al/Al2O3/p-Si (MIS) structures were given in Fig. 1 for various temperatures (200-320 K). As can be seen in Fig. 1, LnI-V plots show a good rectifier behavior for each temperature, but RR decreases with increasing temperature. In addition, the value of current in the forward bias region increases with increasing voltage almost as linearly, but it deviated from the linearity because of Rs and interfacial Al2O3 layer effect. According to TE theory, the relation between I and V (≥ 3kT/q) can be expressed as given by [1-3].

exp 1

o

I I qV

nkT

   

         

⁽¹⁾

In Eq.1; Io, q, V, k and T quantities are well-known diode parameters given elsewhere [1-3]. The values of reverse-saturation current (Io) and n can be extracted from the intercept point and slope of the linear part LnI-V plot for each temperature as follow.

2

exp

^Bo

o

I AA T q

kT



  

     

⁽²⁾

(ln )

q dV n kT d I

 

  

 

⁽³⁾

Here; A is the diode area and A^* is the effective Richardson constant, which is 32 A cm^-

2 K^-2 for p-Si. Thus, the value ΦBo can be calculated from Eq (2) by using obtained value of Io and diode area [2-5]:

2 Bo

ln

o

kT AA T

q I







   

 

⁽⁴⁾

The voltage dependent profile of resistance (Ri) can be extracted from the I-V data by using Ohm’s Law as a simple method. At enough high forward biases, the value of Ri is corresponding to the real value of Rs, but at enough revers biases it is corresponding to the real value of Rsh for each temperature. Therefore, the values of Rs and Rsh were calculated at 3 V by using Ohm’s Law. The value of RR (=IF/IR at  3V) were also calculated and it decreases with increasing temperature. The decrease of RR with increasing temperature is the result of decreasing of bandgap and gained thermal energy by electrons. The obtained experimental values of Io, n, Bo, Rs, Rsh and RR are tabulated in Table 1, respectively.

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Fig 1. The forward and reverse bias semi-logarithmic I-V plots of the Al/Al2O3/p-Si (MIS) structure for various temperatures.

Table 1: The obtained experimental values of Io, n, Bo, Rs, Rsh and RR for the Al/Al2O3/p-Si (MIS) structure at various temperatures

T (K)

Io

(A)

n Bo

(eV)

n. Bo

(eV)

Rs

(k )

Rsh

(G )

RR

200 2.34x10^-12 2.366 0.410 0.969 54.64 4.69 8.59x10⁴ 220 7.40 x10^-12 2.208 0.432 0.955 45.63 3.78 8.28x10⁴ 240 1.85 x10^-11 2.065 0.456 0.942 34.98 2.61 7.45x10⁴ 260 5.04 x10^-11 1.965 0.475 0.934 23.08 1.63 7.06x10⁴ 280 1.40 x10^-10 1.880 0.491 0.923 16.30 0.91 5.55x10⁴ 300 4.02 x10^-10 1.820 0.502 0.914 8.67 0.41 4.74x10⁴ 320 1.17 x10^-09 1.786 0.510 0.911 4.78 0.11 2.33 x10⁴

-25 -23 -21 -19 -17 -15 -13 -11 -9 -7

-3 -2 -1 0 1 2 3

V (V)

Ln (I)

200 K 220 K 240 K 260 K 280 K 300 K 320 K

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As seen in the table 1 and Fig 2, the value ΦBo increases with increasing temperature almost as exponentially, but n decreases almost an exponentially. Such behavior of ΦBo

with temperature is disagreement with the negative temperature coefficient of forbidden bandgap (Eg) of the Si and BH for ideal diodes. In addition, the observed higher values of n especially at low temperatures is an evident to deviation from standard TE theory.

Higher values of n especially at low temperatures were attributed to the existence of interlayer, surface states and barrier in-homogeneities [12-14]. Similarly, as shown in Fig. 3, the values of Rs and Rsh decrease with increasing.

Fig 2. The changes in the ΦBo and n with temperature for the Al/Al2O3/p-Si (MIS) structure.

Fig 3. The changes in the Rs and Rsh with temperature for the Al/Al2O3/p-Si (MIS) structure.

0,40 0,42 0,44 0,46 0,48 0,50 0,52

190 210 230 250 270 290 310 330

T (K)

Bo (eV)

1,75 1,88 2,00 2,13 2,25 2,38 FBo

(eV) n

Polino m (FBo

(eV)) Ideality factor, n

Rs and Rsh

0 1 2 3 4 5

190 210 230 250 270 290 310 330

T (K) Rsh (GW)

0 10 20 30 40 50 60

Rsh (GW)

Rs (kW) at 3V Rs (k)W

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It can be seen in Table 1, the value of n changes with inverse temperature (1/T) as linearly [2].

n(T) =no+To/T (5)

In Eq.5, no and To are constants which were found to be 0.767 and 316.2 K by fitting.

In this case, the temperature dependence of the BH and n of the Al/Al2O3/p-Si (MIS) structure can be called as “To effect’’ or “To anomaly”. To effect can be also connected with the lateral inhomogenity of the BH or the role of the recombination and tunneling CTMs [15]. In this case, the value of n must be placed in Eq.2 as exp(-qFBo/nkT).

Therefore, the temperature dependent value of effective BH (FBef.) was calculated as (FBef =n.FBo) [15]. Thus, as can be seen in Table 1, the values of FBef. Change as linearly with temperature as (FBef.= FB(0K)+aT)=-4.93x10-4 eVK-1+1.063). It is clear that the obtained temperature coefficient (a) of modified BH is close to its theoretical value (=-4.73x10-4 eVK-1) for Si [1].

4. Conclusion

In this study, Al/Al2O3/p-Si (MIS) structures were fabricated and their CTMs have been investigated in wide temperature range of 200-320 K by using I-V-T characteristics to get more reliable and accuracy results on the nature of BH at M/S interface and possible conduction mechanism. Therefore, the main electrical parameters of them such as Io, n, Bo, Rs, Rsh and RR were obtained as function of temperature. All these parameters were found a strong function of temperature. While the value of ΦBo increase with increasing temperature, n decreases and such behavior of them was attributed the existence of many lower values of barriers or patches at around mean BH, surface states, and Al2O3 interfacial layer. It was found that the value of n changes with inverse of temperature (1/T) as linearly which is called as “To effect’’ or “To anomaly”. The increase of ΦBo with increasing temperature is also in-agreement with the reported negative temperature coefficient of forbidden bandgap of semiconductor. Therefore, the temperature dependent value BH ( Bef.) was modified as ( Bef =n. Bo) and it becomes decrease with increasing temperature as (= B(0K)+ T)=-4.93x10^-4 eVK^-1+1.063). It is clear that this negative temperature coefficient of BH is very close to the temperature dependent band gap of Si (-4.73x10^-4 eV/K). In conclusion, the changes in ΦBo and n with temperature were explained with barrier inhomogeneity rather than TE theory and tunneling mechanisms (TFE and FE).

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