AuSb structure

ARTICLE IN PRESS Materials Science in Semiconductor Processing 11 (2008) 53–58

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Fabrication and electrical properties of Al/aniline green/n-Si/AuSb structure ¨ . Gu¨llu¨, A. Tu¨ru¨t S- . Aydog˘an , O ¨ rk University, Faculty of Sciences and Arts, Department of Physics, 25240 Erzurum, Turkey Atatu

a r t i c l e in fo

abstract

Available online 13 January 2009

The current–voltage (I–V), capacitance–voltage (C–V) and capacitance–frequency (C–f) characteristics of Al/aniline green(AG)/n-Si/AuSb structure were investigated at room temperature. A modified Norde’s function combined with conventional forward I–V method was used to extract the parameters including barrier height (BH) and the series resistance. The barrier height and series resistance obtained from Norde’s function was compared with those from Cheung functions, and it was seen that there was a good agreement between the BH values and series resistances from both methods. The C–V characteristics were performed at 10 and 500 kHz frequencies, and C–f characteristics were performed 0.0, +0.4 and 0.4 V. & 2008 Elsevier Ltd. All rights reserved.

Keywords: Schottky contact Schottky barrier height Series resistance Electrical characteristics Aniline green

1. Introduction In the last few decades the organic semiconductors and organic–inorganic semiconductor interfaces have been a subject of intensive research. The potential of these materials for photonic and electronic device applications has already been demonstrated. A successful fabrication of organic and organic-on-inorganic heterostructures useful for electroluminescent devices and photodetectors has been realized [1–7]. Recently, non-polymeric and/or polymeric semiconducting organic compounds have been employed particularly in electronic devices due to their stability and barrier height (BH) enhancement properties. Semiconducting organic materials can be used at different in condensed matter physics applications, such as organic lightemitting diodes, organic field effect transistors, photovoltaic (PV) and solar cells, organic spintronics and so on. Furthermore, more recently, electronic systems are moving to the ultimate scale of molecular entities, as demonstrated by the growing interest in understanding

 Corresponding author. Tel.: +90 442 231 4073; fax: +90 442 236 0948.

E-mail address: E-mail address: [email protected] (S- . Aydog˘an). 1369-8001/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2008.11.004

transport through organic molecules bridging two metal contacts [8,9] and metal/organic material/semiconductor structure [10–13]. Schottky diodes made by introducing a thin methyl red and methyl violet organic layer on inorganic semiconductors have been shown to exhibit promising characteristics for diode applications [12,13]. Among the semiconducting organic materials, AG is considered a good candidate for organic semiconductor device fabrication such as Schottky device and solar cell. Because, it offers a possibility of low-cost and large-area devices. Aniline green with molecular formula C23H25ClN2 (4-[(4-dimethylaminophenyl)-phenyl-methyl]-N,N-dimethyl-aniline) used in this study is a typical aromatic azo compound. The molecular structure of the aniline green is given in Fig. 1. The structure of azo dyes has attracted considerable attentions recently due to their wide applicability in the light-induced photo isomerization process, and their potential usage for the reversible optical data storage [12]. Organic/inorganic semiconductor structures or metal/ semiconductor (MS) contacts [14] are of great importance since they are present in most semiconductor device. It is well known that the interfacial properties of these contacts have a dominant influence on device performance, reliability and stability. There is a native thin

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54

Al top contact

AG (organic compound) n-Si semiconductor AuSb alloy (ohmic contact) Fig. 2. A schematic cross-section of the Al/AG/n-Si/AuSb structure.

Fig. 1. Chemical structure of aniline green (AG).

insulating layer of oxide on the surface of the semiconductor in most practical MS contacts. This layer converts the MS structure into a metal/insulator/semiconductor device [15]. Besides, it can be constructed as an organic thin film between metal and inorganic semiconductor intentionally. This film modifies some electrical parameters of the devices. For example, Schottky barrier heights of MS contacts can be manipulated by insertion of a dipole layer between the semiconductor and the organic film. So far many attempts have been made to realize a modification and the continuous control of the barrier height using an organic semiconducting layer, an insulating layer and/or a chemical passivation procedure at certain metal/inorganic semiconductor interfaces, or to determine characteristic parameters of organic film [16–28]. Campbell et al. [16] have used organic thin film to introduce a controlled dipole layer at the semiconductor/organic interface and thus change the effective Schottky barrier height. They reported that the effective Schottky barrier could be either increased or decreased by using organic thin layer on inorganic semiconductor. Also, thicker organic interlayers of the conjugated molecules have also been successfully used to modify effective barriers [23–27]. The aim of this study is to fabricate a Al/AG/n-Si/AuSb and to study the suitability and possibility of organic-oninorganic semiconductor contacts for use in barrier modification of Si MS diode. For this purpose, we investigate some junction parameters of the structure by the electrical measurements such as; current–voltage, capacitance–voltage and capacitance–frequency.

AG organic layer was directly formed by adding 4 mL of the aniline green organic compound solution (0.2 wt% in methanol) on the front surface of the n-Si wafer, and evaporated by itself for drying of solvent in N2 atmosphere for 1 h. The thickness of the AG film was calculated as 60.1 nm from the high-frequency C–V characteristics. Then, Al metal was evaporated on the AG layer at 105 Torr (diode area ¼ 7.85  103 cm2). In this way the Al/AG/n-Si/ AuSb structure was obtained. A schematic cross-section of the Al/AG/n-Si/AuSb structure is shown in Fig. 2. The I–V and C–V–f measurements of this structure were performed by KEITLEY 487 picoammeter/voltage source and HP 4192A (50 Hz–13 MHz) LF impedence analyzer, respectively. 3. Results and discussion According to the thermionic emission (TE) theory, the current in Schottky barrier diodes (SBDs) can be expressed as [15]      qFb qV 1 , (1) I ¼ AA T 2 exp  exp nkT kT where   qFb I0 ¼ AA T 2 exp  kT

is the saturation current, Fb is the effective barrier height at zero bias, A* is the Richardson constant and equals to 112 A cm2 K2 for n-type Si , where q is the electron charge, V is the bias voltage, A is the effective diode area, k is the Boltzmann’s constant, T is the temperature in Kelvin, n is the ideality factor, and it is determined from the slope of the linear region of the forward bias ln I–V characteristic through the relation:

2. Experimental procedure n¼ In this study, n-Si wafer with (1 0 0) orientation, 400 mm thickness and 1–10 O-cm resistivity was used and then, the n-Si wafer was chemically cleaned using the RCA cleaning procedure (i.e. 10 min boil in NH3+H2O2+ 6H2O followed by a 10 min HCl+H2O2+6H2O at 60 1C) before making contacts. The ohmic contact was made by evaporating AuSb alloy on the back of the substrate, then was annealed at 420 1C for 3 min in N2 atmosphere. The native oxide on the front surface of the n-Si substrate was removed in HF+10H2O solution. Finally, it was rinsed in de-ionized water for 30 s and was dried in N2 atmosphere before forming an organic layer on the n-type Si substrate.

(2)

q dV . kT dðln IÞ

(3)

n equals to one for an ideal diode. However, n has usually a value greater than unity. High values of n can be attributed to the presence of the interfacial thin native oxide layer and a wide distribution of low-SBH patches, and therefore, to the bias voltage dependence of the SBH [15,28,29]. Fb is the zero-bias barrier height, which can be obtained from the following equation: ! kT AA T 2 ln Fb ¼ . (4) q I0

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the energy-level alignment of the lowest unoccupied molecular orbital (LUMO) with respect to the conduction band minimum (CBM) of the inorganic semiconductor at the organic–inorganic semiconductor interface. It is well known that the downward concave curvature of the forward bias current–voltage plots at sufficiently large voltages is caused by the presence of the effect of Rs, apart from the interface states, which are in equilibrium with the semiconductor [30]. If the series resistance effect is low, the non-linear region will be narrow [31]. The values of the Rs were calculated using a method developed by Cheung and Cheung [32]. According to Cheung and Cheung [32], the forward bias I–V characteristics due to the TE of the Schottky diode with the series resistance can be expressed as   qðV  IRs Þ I ¼ I0 exp , (5) nkT

1.0E-4

1.0E-6

1.0E-7

where the IRS term is the voltage drop across series resistance of device. The values of the series resistance can determined from following functions by using Eq. (5):

Al/AG/n-Si/AuSb

dV nkT ¼ þ IRs , dðln IÞ q 1.0E-9

-1.0

-0.5

0.0 0.5 Voltage (V)

1.0

1.5

2.0

Fig. 3. The current-voltage characteristics of the Al/AG/n-Si/AuSb structure.

Fig. 3 represents the forward bias current–voltage (I–V) characteristics of the Al/AG/n-Si/AuSb structure. The values of the n and Fb obtained from I–V characteristics using Eqs. (3) and (4) are 3.4870.05 and 0.7670.02 eV, respectively. The high values in the ideality factor may be caused by other effects such as inhomogeneities of film thickness, non-uniformity of the interfacial charges and the effect of the series resistance. As can be seen, the Fb value of 0.76 eV obtained for the Al/AG/n-Si/AuSb structure is remarkably higher than that achieved with conventional Al/n-Si metal/semiconductor contacts, where Fb is 0.50 eV [15]. This may be ascribed to AG interlayer modifying the effective barrier height by influencing the space-charge region of the inorganic silicon substrate [27]. Thereby, it is known that the AG film forms a physical barrier between the metal and Si inorganic substrate, preventing the metal from directly contacting the Si surface. The AG organic layer appears to cause to a significant modification of interface states even though the organic–inorganic interface becomes abrupt and unreactive [26,27]. Thus, the change in barrier height can qualitatively be explained by an interface dipole induced by the organic layer passivation [4,26]. Kampen et al. [4] have observed by photoemission spectroscopy investigations that the sulphur passivation reduces the surface band bending on n-type doped GaAs, and on the other hand, the band bending on the surfaces of p-typedoped GaAs increases. Similarly, Zahn et al. [3] have indicated that the initial increase or decrease in effective barrier height for the organic interlayer is correlated with

HðIÞ ¼ V 

    nkT I ln , q AA T 2

(7)

and H(I) is given as follows: HðIÞ ¼ nFb þ IRs .

(8)

A plot of ðdV=dðln IÞÞ vs. I will be linear and gives Rs as the slope and ðnkT=qÞ as the y-axis intercept from Eq. (6). Fig. 4 shows the plot of ðdV=dðln IÞÞ vs. I at room temperature. The values of n and RS have been calculated as n ¼ 3.8470.05 and Rs ¼ 4.3870.15 kO, respectively. It is observed that there is a relatively difference between the n values obtained from the forward bias ln I–V plot and from the dV/d(ln I)–I curves. This may be attributed to the existence of the series resistance and interface states, and to the voltage drop across the interfacial layer [33].

0.60

3.30 Al/AG/n-Si/AuSb 3.20

0.50

3.10 dV/dln(I)

-1.5

(6)

0.40 3.00 0.30

2.90 2.80

0.20

0.10 0.0E+0

H(I)

Current (A)

1.0E-5

1.0E-8

55

2.70 2.0E-5

4.0E-5 6.0E-5 Current (A)

8.0E-5

Fig. 4. A plot of ðdV=dðln IÞÞ vs. I and H(I) vs. I obtained from forward bias current-voltage characteristics of the Al/AG/n-Si/AuSb structure.

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H(I) vs. I will be linear, and the slope of this plot provides a different determination of RS. Using the value of the n obtained from Eq. (6) the value of Fb is obtained from the y-axis intercept. The H(I) vs. I plot is shown in Fig. 4. From H(I) vs. I plot, Fb and Rs were calculated as 0.7770.03 eV and 4.1570.15 kO respectively. Furthermore, Norde proposed an alternative method to determine the value of the series resistance. The following function has been defined by the modified Norde’s method [34,35]:   V kT IðVÞ ln FðVÞ ¼  , (9) g q AA T 2 where g is an arbitrary integer (dimensionless) greater than n. I(V) is the current obtained from the I–V curve. Once the minimum of the F vs. V plot is determined, the value of barrier height can be obtained from Eq. (10), where F(V0) is the minimum point of F(V), and V0 is the corresponding voltage.

Fb ¼ FðV 0 Þ þ

V0

g



kT . q

(10)

Fig. 5 shows the F(V)–V plots of the junction. The value of the series resistance was obtained from Norde’s method for Al/AG/n-Si/AuSb junction. From Norde’s functions RS value is determined as Rs ¼

kTðg  nÞ . qI

(11)

From the F–V plot, the some parameters of the structure have been determined as Fb ¼ 0.8370.03 eV, Rs ¼ 4.1470.15 kO by using F(V0) ¼ 0.76 V, V0 ¼ 0.41 V values. There is a good agreement the values of Rs obtained from the forward bias ln I–V, Cheung functions and Norde functions. The effect of the series resistance is usually modeled with series combination of a diode and a resistance Rs.

The voltage across the diode can be expressed in terms of the total voltage drop across the diode and the resistance Rs. The very high series resistance behavior may be ascribed to decrease of the exponentially increasing rate in current due to space-charge injection into the AG thin film at higher forward bias voltage. But, Norde’s model may not be a suitable method especially for the high ideality factor of the rectifying junctions, which are non-agree with pure thermionic emission theory. This non-ideal behavior was attributed to effects of the bias voltage drop across the interfacial organic layer and series resistance and inhomogeneity of the interface. Capacitance measurement is one of the most important non-destructive methods for obtaining information on rectifying contacts interfaces. In some contacts the capacitance under forward bias is larger than the spacecharge capacitance predicted by basic theory. The difference between the measured and the space-charge capacitance is called the excess capacitance and is attributed to interface states. The interface states can be created by crystal lattice discontinuities (dangling bonds), interdiffusion of atoms or a large density of crystal lattice defects close to the metal/semiconductor interface [15] or organic materials/semiconductor interface. The C–V measurements of the structure were carried out at 10 and 500 kHz frequencies. Differential capacitance measurements on a Schottky barrier measure the response of the barrier to an alternating current (a.c.) voltage superposed on a direct current (d.c.) voltage. When the d.c. voltage corresponds to a reverse bias, the differential capacitance represents the response of the depletion layer to the a.c. signal. Fig. 6 shows the forward and reverse bias C–V characteristics of the structure measured at 10 and f ¼ 500 kHz frequency, at room temperature. The value of the capacitance is increasing with forward bias till a point where it reaches a maximum value, and saturates after that. Measurement of the depletion region capacitance

0.88

5.0E+3 10 kHz

Al/AG/n-Si/AuSb

0.86

Al/AG/n-Si/AuSb

4.0E+3

Capacitance (pF)

0.82 0.80

3.0E+3

500 kHz

-7

1.6x10

2.0E+3

-7

1.2x10 -2

C-2 (pF )

F (V)

0.84

0.78

-8

1.0E+3

4.0x10

0

0.76 0.74 0.00

-8

8.0x10

0.20

0.40

0.60 V (V)

0.80

1.00

Fig. 5. F(V) vs. V plot of the Al/AG/n-Si/AuSb structure.

1.20

0.0E+0 -2.00 -1.50 -1.00 -0.50

0.00

0.0x10 -0.4

0.0

0.50

1.00

0.4 0.8 Voltage(V)

1.50

1.2

2.00

Voltage (V) Fig. 6. The forward and reverse bias C–V characteristics of the Al/AG/ n-Si/AuSb structure at 10 and f ¼ 500 kHz frequencies.

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under forward bias is difficult because the diode is conducting and the capacitance is shunted by a large conductance. However, the capacitance can be easily measured as a function of the reverse bias [28]. The inset in Fig. 6 depicts C2–V plot from C–V data of the Al/AG/nSi/AuSb device at 500 kHz. The non-linearity seen in the inset in Fig. 6 that indicates a non-uniform dopant density profile is ascribed to the interface states introduced by the native oxide layer plus AG organic layer and the surface irregularities/defects that cause the variation of the effective area. Assuming dielectric constant es ¼ 11.9e0, the carrier concentration of n-Si was obtained as 1.25  1017 cm3. The voltage axis intercept of the C2–V plot gives a value of 0.7670.03 V for diffusion voltage. Also, Schottky barrier has been calculated as 0.787 0.03 eV. The junction capacitance is measured as a function of frequency and voltage. The capacitance–frequency (C–f) measurements of this structure were carried out at the various biases (0.00, +0.4 and 0.4 V). These plots are seen at Fig. 7. As shown in this figure, the values of the measured capacitance are become almost constant up to the certain frequency value. Besides, the higher values of capacitance at low-frequency originate from the excess capacitance resulting from the interface states in equilibrium with the n-Si that can follow the a.c. signal. That is, the interface states at lower frequencies follow the a.c. signal, whereas at higher frequencies they cannot follow the a.c. signal. The values of the capacitance at the highfrequency region are only space-charge capacitance. The voltage and frequency dependences of the junction capacitance are due to the particular features of a Schottky barrier, impurity level, high series resistance, etc. At low frequency, the capacitance measured is dominated by the depletion capacitance of the Schottky diode, which is bias-dependent and frequency-independent. As the frequency is increased, the total diode capacitance is affected not only by the depletion capacitance, but also by the bulk resistance and the dispersion capacitance, which is frequency-dependent and associated with hole or electron emission from slowly responding deep impurity levels [15].

6.0E+3

Capacitance (pF)

5.0E+3

Al/AG/n-Si/AuSb V=+0.4 V

4.0E+3

V=0.0 V

3.0E+3 2.0E+3

V=-0.4 V

1.0E+3 0.0E+0 1.0E+3

1.0E+4

1.0E+6 1.0E+5 Frequency (Hz)

1.0E+7

Fig. 7. The forward bias C–f characteristics of the Al/AG/n-Si/AuSb structure at various voltages.

57

In organic/inorganic semiconductor contact applications, in order to keep the technological difficulties and unknowns to a minimum, silicon is generally chosen as the substrate semiconducting material. In this structure, deposition of organic materials on the inorganic semiconductor can generate large number of interface states at the semiconductor surface that strongly influence the properties of the Al/AG/n-Si/AuSb structure. When these structures are considered as Schottky diodes, the devices comprise a high-resistivity layer (the depletion layer) in series with a low-resistivity layer, which has its own capacitance and resistance. In addition, the native oxide layer, which is occurred at the right time the cleaning procedure, between AG and n-Si can affect the capacitance [29].

4. Conclusions In summary, we have investigated the I–V, C–V and C–f characteristics of the Al/AG/n-Si/AuSb structure. The values of the ideality factor, series resistance and barrier height obtained from two methods were compared, and it was seen that there was an agreement with each other. The downward concave curvature of the forward bias current–voltage characteristics at sufficiently large voltages is caused by the presence of the effect of series resistance. Thus, the concavity of the forward bias current–voltage characteristics increases with the increasing series resistance value. The high resistance values have given the high ideality factors. Besides, the higher values of capacitance at low frequencies were attributed to the excess capacitance resulting from the interface states in equilibrium with the n-Si that can follow the a.c. signal. References [1] Forrest SR, Kaplan ML, Schmidt PH. J. Appl. Phys. 1984;55:1492. [2] Urbach P, Felbier F, So¨rensen A, Kowalsky W. Japan. J. Appl. Phys. 1998;37:1660. [3] Zahn DRT, Park S, Kampen TU. Vacuum 2002;67:101; Zahn DRT, Kampen TU, Mendez H. Appl. Surf. Sci. 2003; 212–213:423. [4] Kampen TU, Schuller A, Zahn DRT, Biel B, Ortega J, Perez R, Flores F. Appl. Surf. Sci. 2004;234(1–4):341. [5] Kampen TU, Park S, Zahn DRT. J. Vac. Sci. Technol. B 2003;21:879. [6] Ginev G, Riedl T, Parashkov R, Johannes H-H, Kowalsky W. Applied Surface Science 2004;234:22–7. [7] Ginev G, Riedl T, Parashkov R, Johannes H-H, Kowalsky W. J. Phys. Condens. Matter 2003;15:2611. [8] Petty MC, Bryce M. An Introduction to Molecular Electronics. New York: Oxford University Press; 1995. [9] Jortner J, Ratner MA. Molecular Electronics. Washington, DC: American Chemical Society; 1997. [10] Aydin ME, Yakuphanoglu F, Eom JH, Hwang DH. Physica B 2007;387:239. [11] Gupta RK, Singh RA. Materials Science in Semiconductor Processing 2004;7:83–7. [12] Gullu O, Turut A, Asubay S. J. Phys.: Cond. Mat. 2008;20:045215. [13] Gullu O, Baris O, Biber M, Turut A. Appl. Surf. Sci. 2008;254:3039. [14] Maeda T, Takagi S, Ohnishi T, Lippmaa M. Materials Science in Semiconductor Processing 2006;9:706–10. [15] Rhoderick EH, Williams RH. Metal-Semiconductor Contacts. second ed. Oxford: Clarendon; 1988. [16] Campbell IH, Rubin S, Zawodzinski TA, Kress JD, Martin RL, Smith DL, et al. Phys. Rev. B 1996;54(20):14321. [17] Migahed MD, Fahmy T, Ishra M, Barakat A. Polymer Testing 2004;23:361.

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