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Journal of Metallurgy

Volumeย 2012ย (2012), Article IDย 531915, 9 pages

http://dx.doi.org/10.1155/2012/531915

## Electrical Properties of Rapidly Annealed Ir and Ir/Au Schottky Contacts on n-Type InGaN

Department of Physics, Sri Venkateswara University, Tirupati 517 502, India

Received 23 August 2011; Accepted 24 October 2011

Academic Editor: Stefanoย Gialanella

Copyright ยฉ 2012 V. Rajagopal Reddy et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

The effect of annealing temperature on electrical characteristics of iridium (Ir) and iridium/gold (Ir/Au) Schottky contacts to n-type InGaN have been investigated by means of current-voltage (*I-V*) and capacitance-voltage (*C-V*) techniques. It is observed that the barrier height of Ir/n-InGaN and Au/Ir/n-InGaN Schottky diodes increases after annealing at for 1โmin in ambient compared to the as-deposited. However, the barrier heights are found to be decreased somewhat after annealing at for the both Ir and Ir/Au Schottky contacts. From the above observations, it is clear that the optimum annealing temperature for both Ir and Ir/Au Schottky contacts is . Moreover, the barrier height (), ideality factor (*n*) and series resistance () are determined using Cheungโs and Norde methods. Besides, the energy distribution of interface state densities are determined from the forward bias *I-V* characteristics by taking into account the bias dependence of the effective barrier height. Based on the above results, it is clear that both Ir and Ir/Au Schottky contacts exhibit a kind of thermal stability during annealing.

#### 1. Introduction

III-V nitride semiconductors, particularly gallium nitride (GaN) and related alloys, have received much attention because of the (or their) wide range of practical applications. Among the ternary compounds of group III nitride materials, InGaN has drawn a great deal of attention. GaN and InGaN have been considered as the most important and crucial materials used for the fabrication of light emitters which were active in entire visible and part of the near UV spectral regions. The development of high-quality Schottky contacts to InGaN is difficult because in obtaining high-quality InGaN layers with high composition of In, their low carrier mobility (compared to GaN), and a large amount of surface defects in InGaN layers [1]. Hence, the fabrication of Schottky contacts to InGaN with a high barrier height and good thermal stability is still a challenge.

Many research groups have extensively investigated the electrical properties of n-GaN Schottky diodes using various metal schemes such as W, WSi_{x} [2], Ru, Ru/Au [3], Rh, Rh/Au [4], Ni/Au [5], and Re [6]. Kim et al. [2] investigated the thermal stability of W and WSi_{x} Schottky contacts on n-type GaN and reported that the WSi_{x} contacts showed improved thermal stability compared to pure W contacts. They also reported that the WSi_{x} contact was stable up to temperatures >600ยฐC with the Schottky barrier height (SBH) of ~0.5โeV. Rajagopal Reddy et al. [3] studied the influence of thermal annealing temperature on the electrical properties of Ru and Ru/Au Schottky contacts to n-type GaN. They found that the Ru Schottky contacts exhibit a kind of thermal stability during annealing compared to the Ru/Au contacts. Rajagopal Reddy et al. [4] investigated the effect of annealing temperature on electrical properties of Rh and Rh/Au Schottky contacts to n-type GaN. They reported that there was a significant improvement in the electrical characteristics of Rh/Au Schottky contacts upon annealing compared to the Rh Schottky contacts. Miura et al. [5] investigated the thermal annealing effects on Ni/Au-based Schottky contacts on n-GaN and AlGaN/GaN after inserting high work function metals such as Pt, Ir, Pd, or Mo. Venugopalan and Mohney [6] showed that the Re Schottky contact was thermally stable on n-GaN and reported a barrier height of 0.82โeV and 1.06โeV after annealing at 500ยฐC by *I-V* and *C-V* methods. Jang et al. [7] investigated the Schottky barrier characteristics of Pt contacts on n-InGaN by the *I-V* and *C-V* methods. They found that there was a large difference in SBHs obtained by the TE and TFE modes using the *I-V* data. Xue et al. [8] fabricated large-area Au/Pt/In_{0.2} G_{0.8โ}N Schottky contacts for photovoltaic devices and investigated the current transport mechanisms of the Schottky diodes with different background carrier concentrations.

There are, however, only limited works on metal-InGaN () Schottky barriers. Hence, the main aim of the present work is to fabricate and characterize the Ir-based Schottky contacts on InGaN. To the best of our knowledge, Ir metal scheme has not been explored as Schottky contacts on InGaN. In this work, Ir is selected because it has a high work function of 5.46โeV. From a device point of view, it is important to know what happens to metal contacts when they are annealed. Therefore, our main focus in the present work is to investigate the electrical characteristics of Ir and Ir/Au Schottky contacts on n-InGaN by means of current-voltage (*I-V*) and capacitance-voltage (*C-V*) measurements as a function of annealing temperature.

#### 2. Experimental Details

2โ*ฮผ*m-thick unintentionally doped GaN layer on a 40โnm-thick nucleation layer/(0001) sapphire substrate was grown by metal organic chemical vapor deposition (MOCVD). This was followed by the growth of ~0.25โ*ฮผ*m-thick n-InGaN: Si () with 10% of indium (In) composition. Prior to the metal deposition, the n-type InGaN layer was first ultrasonically degreased with warm trichloroethylene followed by acetone, methanol, and ethanol for 5โmin in each step and then rinsed with deionized (DI) water. This degreased layer was then dipped into boiling aquarezia [] for 10โmin to remove the native oxide. For ohmic contacts, bilayer of Ti (25โnm)/Al (100โnm) was deposited on InGaN and then annealed at 650ยฐC for 1โmin in flowing N_{2} atmosphere using rapid thermal annealing (RTA) system. Ir (50โnm) and Ir/Au (20โnm/30โnm) Schottky contacts were fabricated on n-InGaN with a circular diameter of 0.7โmm using electron beam evaporation system with a vacuum pressure of . In order to investigate the thermal annealing effect on Schottky diodes, the Ir and Ir/Au contacts were annealed at 300ยฐC, 400ยฐC, and 500ยฐC for 1โmin in N_{2} ambient. The electrical characteristics of Schottky contacts were determined by means of current-voltage (*I-V*) and capacitance-voltage (*C-V*) characteristics using Keithley source measure unit (model no. 2400) and deep level transient spectrometer (DLS-83 D), respectively.

#### 3. Results and Discussion

##### 3.1. Current-Voltage (*I-V*) Characteristics

Figures 1(a) and 1(b) show the semilogarithmic plots of forward and reverse *I-V* characteristics of Ir and Ir/Au Schottky contacts measured as a function of annealing temperature. It is noted that the properties of Ir and Ir/Au Schottky diodes are uniform over different temperatures. It is observed that the leakage current of as-deposited Ir Schottky diode is at โ1โV. For the contact annealed at 300ยฐC, the leakage current decreases to , whereas for the contacts annealed at 400ยฐC and 500ยฐC, the leakage current values are and at , respectively. In the case of as-deposited Ir/Au Schottky diode, the leakage current at โ1โV is . For the contacts annealed at 300ยฐC, 400ยฐC, and 500ยฐC, the obtained leakage current values are , , and at โ1โV, respectively. The smallest leakage current values are obtained for both Ir and Ir/Au Schottky contacts after annealing at 300ยฐC, accompanied by the corresponding increase in the barrier height. This indicates that the electrical characteristics of both the Ir and Ir/Au contacts are improved upon annealing at 300ยฐC compared to the as-deposited contacts. It also is noted that the reverse leakage current decreased for Ir/n-InGaN Schottky diode compared to the Au/Ir/n-InGaN Schottky diode upon annealing temperature.

The barrier height and the ideality factor of a metal-semiconductor contact are determined using thermionic emission theory and are given by [9]
with
Here, is the saturation current, is the electron charge, is the applied voltage, is the series resistance, is the absolute temperature, is the ideality factor, is the Boltzmannโs constant, is the contact area, and is the effective Richardson constant. The theoretical value of was 23โAโcm^{โ2}โK^{โ2} based on the effective mass () of the n-InGaN [7] and was used to deduce . In general by using a linear curve fit to the forward characteristics of ln(*I*) against data, barrier height () and ideality factor () can be determined from the -intercept and the slope of the fitted curve, respectively. Calculations yield that the Schottky barrier height (SBH) of as-deposited Ir/n-InGaN Schottky diode is 0.79โeV. For the contacts annealed at 300ยฐC, 400ยฐC, and 500ยฐC, the calculated barrier heights are 0.85โeV, 0.84โeV, and 0.80โeV. The Schottky barrier height estimated in the case of Ir/Au contact is 0.70โeV (as-deposited), 0.74โeV after annealing (at 300ยฐC), 0.71โeV (400ยฐC), and 0.67โeV (500ยฐC).

The ideality factor is determined from *I-V* plot, specifically at small forward currents, where the effect of series resistance is considered to be small. In case of Ir and Ir/Au Schottky diodes, the typical values of ideality factor are found to be 1.60 and 1.51 (as-deposited), respectively. However, it is observed that the ideality factors are improved to 1.10 for Ir and 1.23 for Ir/Au Schottky contacts upon annealing at 300ยฐC. The ideality factor calculated from the forward bias *I-V* plot is found to be greater than unity. This may be due to a potential drop in the interface layer and the presence of excess current and the recombination current through the interfacial state between the semiconductor/insulator layers [10]. Also, when the bias is applied, part of the voltage is dropped across the interface layers. Thus, the barrier height becomes dependent on the bias voltage, leading to an ideality factor greater than one. Moreover, the larger values of ideality factors are attributed to secondary mechanisms that include interface dipoles due to interface doping or specific interface structure as well as fabricate-induced defects at the interface [11, 12]. The high value of ideality factor can also be attributed to the presence of a wide distribution of low-SBH patches caused by laterally inhomogeneous according to Tung [13]. Another possibility may be the image-force effect, recombination generation, and tunneling mechanisms that could be lead to the ideality factor value greater than unity [14]. The barrier heights and the ideality factors of Ir and Ir/Au Schottky diodes as a function of annealing temperature are given in Table 1.

Generally, at low voltages, the linear forward bias current-voltage (*I-V*) characteristics in the semilogarithmic scale deviate considerably from linearity, which is due to the effect of parameters such as and when the applied voltage is sufficiently large. The series resistance values are evaluated using the method developed by S. cheung and N. cheung [15]. Cheungโs functions given as
should give a straight line for the data of downward curvature region in the forward bias *I-V *characteristics. The term is the current drop across the series resistance of Schottky diodes. Figures 2(a) and 3(a) show the experimental versus plot for the Ir and Ir/Au Schottky diodes, respectively. The values of and are calculated from the slope and -axis intercepts of the versus plot, respectively. From Figures 2(b) and 3(b), it is evident that the plot of versus gives a straight line with the -axis intercept equal to , and the slope parameter can be used to check the consistency of Cheungโs method. From (3), the values of ideality factor and series resistance for Ir/InGaN and Au/Ir/InGaN Schottky diodes are calculated. The values of barrier height and series resistance for Ir/InGaN and Au/Ir/InGaN Schottky diodes are calculated from (5) which is given in Table 1. Thus, it can be clearly seen that there is relatively a difference between the values of ideality factor calculated from the downward curvature regions of forward bias *I*-*V* plots and from the linear regions of the same characteristics. This can be attributed to the existence of effects such as series resistance and the bias dependence of Schottky barrier height according to the voltage drop across the interfacial layer and change of the interface states with the bias in the low-voltage region of the current-voltage plot.

The accurate evaluation of the barrier height from the standard ln *(I)-V* plot can be effected by the high series resistance. To include this effect, Norde method is also used to compare the SBH of Ir and Ir/Au Schottky contacts. The Norde function , being plotted against , is given by [16, 17]
The Schottky barrier height is given by
where is the minimum value of , and is the corresponding voltage. Measurements showed that the SBHs are 0.81โeV for as-deposited, 0.88โeV for 300ยฐC, 0.86โeV for 400ยฐC, and 0.82โeV for 500ยฐC for Ir Schottky contacts. In the case of Ir/Au Schottky contacts, the calculated barrier heights are 0.73โeV for as-deposited, 0.76โeV for 300ยฐC, 0.74โeV for 400ยฐC, and 0.71โeV for 500ยฐC, respectively. It is observed that these values are in good agreement with those calculated by *I-V* method.

##### 3.2. Capacitance-Voltage (*C-V*) Characteristics

Capacitance-voltage (*C-V*) characteristics of Ir and Ir/Au Schottky contacts are measured at a frequency of 1โMHz with an ac modulation of 100โmV in the dark. Figures 4(a) and 4(b) show plots of as a function of bias voltage for as-deposited and annealed contacts of Ir and Ir/Au Schottky contacts, respectively. The *C-V* relationship for Schottky diode is given by [9]
where is the flat band voltage, is the donor concentration, is the area of the Schottky contact, and is the permittivity of the semiconductor (). The x-intercept of the plot of versus , is related to the built in potential by equation , where is the absolute temperature. The barrier height () is given by the equation where . The density of states in the conduction band edge is given by , , and its value was for InGaN at room temperature. Measurements showed that the barrier heights of Ir Schottky contacts are 1.17โeV for as-deposited, 1.21โeV for 300ยฐC, 1.11โeV for 400ยฐC, and 1.07โeV for 500ยฐC annealed contacts. For Ir/Au Schottky contacts, the estimated barrier heights are 1.00โeV (as-deposited), 1.03โeV after annealing the contacts (at 300ยฐC), 0.96โeV (400ยฐC), and 0.93โeV (500ยฐC) by *C-V* method.

Figure 5(a) shows a plot of barrier height of Ir/n-InGaN as a function of annealing temperature. It can be seen clearly from Figure 5(a) that the barrier height of Ir/n-InGaN Schottky diode increases upon annealing at 300ยฐC for 1โmin in the N_{2} ambient compared to the as-deposited contact. However, there is a slight decrease in the Schottky barrier height for the contacts annealed at 400ยฐC and 500ยฐC. The interface states and chemical reactions between metals and semiconductors at the interface may be the reason for the variation in the barrier heights [18]. From Figure 5(a), it is also noted that the ideality factor is improved and reaches near unity value (1.10) after annealing at 300ยฐC. However, the values of ideality factor slightly increase upon annealing at 400ยฐC and 500ยฐC compared to the contact annealed at 300ยฐC.

The plot of barrier heights and ideality factors of Ir/Au Schottky contacts as a function of annealing temperature is shown in Figure 5(b). It is noted that the barrier height calculated for the as-deposited Ir/Au contacts is less than annealed contacts (300ยฐC and 400ยฐC). When the contact is annealed at 300ยฐC, it is found that the Schottky barrier height increases for Ir/Au contact. Rajagopal Reddy et al. [3] had observed similar behavior in Au/Ru/n-GaN Schottky diode. Also, Miura et al. [5] had observed similar behavior in Ni/Pt/Au, Ni/Au, and Ni/Mo/Au Schottky contacts on n-GaN. Upon annealing temperature, the increase in the barrier height may be due to the reduction in the nonstoichiometric defects in the metallurgical interface [19, 20]. Also, it is observed that the increase in the barrier height is accompanied by the corresponding decrease in the reverse leakage current. Duboz et al. [21] had reported that the metal/n-GaN interface remains unaffected even after annealing. They also found that the nature of the metal does not play a major role in the improvement of barrier height on annealing temperature. Many researchers had observed the pinning of Fermi level at metal/n-GaN interface because of the interface defects. Further, a modification in the defect density on annealing could change the pinning, which leads to the change of barrier height. Significant improvement in the electrical characteristics in the case of Ir/n-InGaN Schottky diode is observed upon annealing temperature compared to the Au/Ir/n-InGaN Schottky diode. It is also observed that the electrical characteristics of Ir Schottky contacts are fairly more thermally stable than that of Ir/Au Schottky contacts. Also, Figure 5(b) shows that the ideality factor value decreases upon annealing at 300ยฐC. With a further increase in the annealing temperatures up to 500ยฐC, the ideality factor is slightly increased.

It can also be seen from Figures 5(a) and 5(b) that the barrier heights obtained from *I-V* measurements are lower than those obtained from *C-V* measurements. This difference in the values of and may be due to the presence of native oxide layer at the metal/InGaN interface [22]. Lowering the barrier height by an image force due to current flow across the barrier can also have some effect [9]. However, with high-temperature annealing, the metal perhaps comes into intimate contact with the semiconductor. Furthermore, the differences in the barrier height calculated from *I-V* and *C-V* methods may also cause the Schottky barrier height inhomogeneities that prevail at the metal semiconductor interface [23]. Another possibility is that the surface damage at the metal-semiconductor interface affects *I-V* measurements because defects may act as recombination centers for trap-assisted tunneling currents. *C-V* measurements are normally less prone to interface states. Hence, the calculated barrier heights are considered more reliable, though the depletion width can be altered by interface defects if they are deeper into the space charge region [24]. It may also be the transport mechanism in these diodes which is not purely due to thermionic emission. To these diodes, obtained from *I-V* method is voltage or electric field sensitive unlike the obtained from *C-V. *

##### 3.3. Interface State Density (*N*_{SS}) from *I-V* and *C-V* Measurements

It is noted from Figures 6(a) and 6(b) that the interface state density has an exponential increase with the bias from the midgap towards the top of the conduction band. According to Card and Rhoderick, for a real Schottky diode having interface states in equilibrium with the semiconductor, the ideality factor becomes greater than unity and is given by [9, 25] where is the density of interface states, is the width of the semiconductor, and and are the permittivity of the semiconductor and the interfacial layer, respectively. The voltage-dependent ideality factor can be expressed as [26]. The expression for the interface state density can be given as In the n-type semiconductor, the energy of the interface states with respect to the bottom of the conduction band at the surface of the semiconductor is given by [25, 27, 28] where is the effective barrier height. The voltage dependence of the effective barrier height is contained in the ideality factor through the relation where is the voltage coefficient of . The effective barrier height is given by [29] From Figures 6(a) and 6(b), it can be seen that the value decreases with the increase in the value. The increase in values forms from midgap towards the bottom of the conduction band which is probably due to the increase in the series resistance (). The change in the charge of the interface states and thus the interface state energy distribution due to the potential drop across the interfacial layer varies with the bias. It alters the diffusion potential and therefore the depletion capacitance [9, 30โ33].

#### 4. Conclusions

We have investigated the electrical properties of Ir and Ir/Au Schottky contacts on n-type InGaN () by *I-V* and *C-V* measurements as a function of annealing temperature. Measurements showed that the barrier height of as-deposited Ir Schottky contact is 0.79โeV (*I-V*) and 1.17โeV (*C-V*). Upon annealing at 300ยฐC for 1โmin in nitrogen ambient, the Schottky barrier height increases to 0.85โeV (*I-V*) and 1.12โeV (*C-V*). After annealing at 400ยฐC and 500ยฐC, the Schottky barrier heights decrease to 0.84โeV (*I-V*), 1.11โeV (*C-V*) and 0.80โeV (*I-V*), 1.07 (*C-V*). In the case of as-deposited Ir/Au Schottky contacts, the estimated barrier height is 0.70โeV (*I-V*) and 1.00โeV (*C-V*). However, the Schottky barrier height increases upon annealing temperature at 300ยฐC, and the corresponding value is 0.74โeV (*I-V*) and 1.03โeV (*C-V*). Further, with an increase in annealing temperature up to 500ยฐC, the barrier height decreases to 0.67โeV (*I-V*) and 0.93โeV (*C-V*), respectively. Based on the above results, the optimum barrier heights are obtained after annealing at 300ยฐC for both Ir and Ir/Au Schottky contacts. Therefore, the optimum annealing temperature for both Ir and Ir/Au Schottky contacts is 300ยฐC. The values of diode parameters such as ideality factor (), series resistance (), and barrier height () are also calculated by Cheungโs and Nordeโs methods. The interface state density has an exponential raise with bias from the midgap towards the top of the conduction band. From the above observations, it is clear that the electrical properties of Ir and Ir/Au Schottky contacts are improved with an increase in annealing temperature compared to the as-deposited one. Moreover, both Ir and Ir/Au Schottky contacts showed relatively thermal stability during annealing temperature.

#### Acknowledgment

The authors thank the University Grant Commission (UGC), New Delhi, for providing the financial assistance (Grant no. 39-530/2010 (SR)).

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