n-Si(111) Schottky diodes

Solid-State Electronics Vol. 34, No. 7, pp. 727-729, 1991 Printed in Great Britain. All rights reserved

0038-1101/91 $3.00 + 0.00 Copyright © 1991 Pergamon Press plc

EFFECTS OF HYDROGENATION ON THE ELECTRICAL CHARACTERISTICS OF Ni/n-Si(lll) SCHOTTKY DIODES P. P. SAHAY, 1 M. SHAMSUDDIN 2 and R. S. SRIVASTAVA 1 ~Department of Physics and 2Department of Metallurgical Engineering, Institute of Technology, Banaras Hindu University, Varanasi 221 005, India (Received 17July 1990; in revised form 11 December 1990) Abstract--Hydrogenation effects on the electrical characteristics of Ni/n-Si(111) Schottky diodes have been reported from (l-V) and (C-V) studies. It has been concluded that hydrogen lowers the work function of nickel and also generates interfacial traps at the Si-SiO2 interface. Slight passivation of deep donor states responding to the lower frequency test signal has also been observed after hydrogenating the diode.

1. INTRODUCTION

teristics. However, due to its low diffusivity in nickel[9,10] hydrogen takes a long time to diffuse into the nickel as compared to palladium.

Recently, hydrogenation effects on the electrical characteristics of metal-semiconductor Schottky diodes have become an active area of interest for semiconductor researchers because of their possible applications in the fabrication of hydrogen-sensing devices. The hydrogenation effects on Pd/Si Schottky diodes have been extensively studied by many researchers[ 1-7]. The exact role of hydrogen in affecting Pd/Si Schottky diodes is subject to some controversy, but it has generally been concluded that hydrogen lowers the effective work function of Pd, and hence modifies the diode characteristics. In this paper we report an extensive study of the effects of hydrogenation on the electrical characteristics of Ni/n-Si(111) Schottky diodes.

(a) (I-I/) studies Forward ( I - V ) characteristics of three stages of the diode are shown in Fig. 1. The zero-bias barrier height, ~bBo, and the ideality factor, r/, have been calculated from ( I - V ) characteristics[l 1], using an effective Richardson constant of 110 A cm -2 K-2[12]. The results are summarized in Table 1. The ideality factor in the linear region has been found at a forward bias voltage of 0.13 V. The high value (1.4) of r/for the as-deposited (i.e. before hydrogenation) diode indicates the presence of a thin interfacial SiO2 (~-20 ~ or less) layer between nickel and silicon suggesting thereby the diode to be a Ni-thin SiO2-Si structure. The formation of a thin SiO2 layer is unavoidable during the fabrication of the device by the conventional techniques discussed in Section 2. After hydrogenation r/ has increased to 1.93, which may be due to the generation of interfacial traps by hydrogen at the Si-SiO2 interface[4,5]. By hydrogenation treatment q~Bohas been found to decrease. This is attributed to a change in work function of nickel due to formation of a hydrogenrelated dipole layer at the interface[13,14], and also to the appearance of interfacial traps at the Si-SiO2 interface. The observed increase in the forward current through the diode can also be understood as quantum mechanical tunneling of electrons between the generated interfacial traps and the nickel, so as to provide additional current paths.

2. EXPERIMENTAL DETAILS

Ni/n-Si(111) Schottky diodes were fabricated by the thermal vacuum deposition of Ni on n/n ÷ Si(111) epitaxial wafers at ,,, 10 -5 torr pressure. Details of the fabrication technique has already been reported in our previous paper[8]. For hydrogenation the diodes were kept in an evacuated chamber (,,, 10 -2 torr) and then hydrogen gas was slowly passed into the chamber to achieve a pressure of 1 atm. Dark ( I - V ) and ( C - V ) measurements were carried out by means of HP 4140 B picoammeter and a computer controlled HP 4277A LCZ meter, respectively. The transient capacitance response of the diode at 1 MHz and zero-bias was also monitored by the same LCZ meter. All measurements including the hydrogenation and dehydrogenation treatments were conducted at room temperature (300 K). 3.

RESULTS AND DISCUSSION

The hydrogen adsorbed in the nickel acts as a source of hydrogen for modifying the diode charac-

(b) Transient capacitance response The transient capacitance response of the diode at 1 MHz and zero-bias during hydrogen adsorption and desorption cycle is shown in Fig. 2. During the hydrogen adsorption cycle the original capacitance A (270 pF) at zero-bias first increases rapidly and then

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P. P. SAHAY e t al.

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Table 1. Effect of hydrogenation on 4)Boand q at room temperature

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Fig. 1. Forward (I-V) characteristics of three different stages of the diode. (©) Before hydrogenation (point A of Fig. 2); ( 0 ) after hydrogenation (point B of Fig. 2); ( × ) after dehydrogenation (point C of Fig. 2). slowly achieves a practically constant value (1330 pF) after about 12 h. This constancy has been observed up to 24 h (region B). The increase in the diode capacitance has been explained in terms of the change in work function of nickel due to adsorption of hydrogen[13-15] and the generation of interfacial traps at the Si-SiO2 interface. The adsorbed atomic hydrogen reduces the work function of the internal

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Ni layer due to formation of a hydrogen-related dipole layer at the interface. This consequently reduces the barrier height of the diode, resulting in rapid increase in the diode capacitance. Generation of interfacial traps at the Si-SiO2 interface due to subsequent diffusion of adsorbed atomic hydrogen in nickel through the thin SiO2 layer[4] also contributes in reduction of the barrier height. This produces a slight increase in the diode capacitance. The constant value of the diode capacitance in the region B gives an idea of the dynamic equilibrium of the diode at ambient pressure of hydrogen. During the hydrogen desorption cycle, a reverse phenomenon has been observed. The diode capacitance first decreases sharply and then slowly achieves a practically constant value of 305 pF (region C in Fig. 2). The decrease in the diode capacitance is due to the increase in the barrier height which happens because of the increase in the work function of the internal Ni layer.

(e) (C-V) studies The measured ( C - V ) and (G-V) characteristics of the diode have been found to be frequency dependent which gives an idea of the interface states model of the diode structure. The (C-V) characteristics do not show saturation under sufficient forward bias voltage, which suggests that the series interfacial layer capacitance is not able to hold charge. The increase of the capacitance after hydrogenation and their frequency dispersion suggest the generation of inter-

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Fig. 2. Transient capacitance response of the diode during hydrogen adsorption and desorption cycle at 300 K.

Effects of hydrogenation on Schottky diodes

35f

ation seems to passivate slightly those deep donor states[17-20] which respond to the low frequency (10 kHz) test signal. Further studies are in progress to evaluate the density and distribution of the interface states from ( C - V ) characteristics.

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Acknowledgement--Authors are thankful to the Ministry of Human Resources Development, Government of India, New Delhi for financial assistance.

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REFERENCES

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Fig. 3. ( C - 2 - V ) characteristics of three different stages of the diode. ( - - ) Before hydrogenation (point A of Fig. 2); ( - - ) after hydrogenation (point B of Fig. 2); (. . . . ) after dehydrogenation (point C of Fig. 2). facial traps at the Si-SiO2 interface[4]. The ( C - 2 - V ) characteristics of three different stages of the diode are shown in Fig. 3. The barrier height calculated from ( C - 2 - V ) characteristics[11] has also been found to decrease after hydrogenation, which supports our results based on ( l - V ) studies. The estimated values of effective carrier concentration from the deep depletion region of ( C - 2 - V ) characteristics[l l] are given in Table 2. F r o m the table it is observed that in all cases the effective carrier concentration is higher at lower frequencies. This can be explained in terms of the involvement of ionized deep states[16] which contribute to the effective carrier concentration at lower frequencies. It is also observed that hydrogenTable 2. Effect of hydrogenation on effectivecarriec concentration at room temperature Effectivecarrier concentration Before After After hydrogenation hydrogenation dehydrogenation (point A of Fig. 2) (point B of Fig. 2) (point C of Fig. 2) Frequency (x 1016cm -3) (× 1016cm-3) (x 1016cm-3) 1 MHz 1.12 1.12 1.12 100 kHz 1.27 1.27 1.27 10 kHz 1.46 1.40 1.40

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