Investigation of temperature dependent electrical

Thin Solid Films 564 (2014) 367–374

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Thin Solid Films journal homepage: www.elsevier.com/locate/tsf

Investigation of temperature dependent electrical properties of Ni/Al0.26Ga0.74N Schottky barrier diodes A. Akkaya a, T. Karaaslan a, M. Dede b, H. Çetin c, E. Ayyıldız a,⁎ a b c

Department of Physics, Faculty of Science, Erciyes University, 38039 Kayseri, Turkey Nanomagnetic Instruments Ltd., 06800 Ankara, Turkey Department of Physics, Faculty of Arts and Sciences, Bozok University, 66100 Yozgat, Turkey

a r t i c l e

i n f o

Article history: Received 15 May 2013 Received in revised form 8 May 2014 Accepted 9 May 2014 Available online 15 May 2014 Keywords: Metal–semiconductor devices Electrical properties Aluminum gallium nitride Schottky barrier diodes Current–voltage–temperature characteristics Gaussian distribution Thermionic emission theory Barrier inhomogeneity

a b s t r a c t The current–voltage (I–V) characteristics of the Ni/Al0.26Ga0.74N Schottky barrier diodes (SBDs) were measured in the temperature range of 100–310 K by the step of 10 K. The forward I–V characteristics were analyzed on the basis of the thermionic emission theory. The characteristics of diode parameters such as the Schottky barrier height (SBH) and the ideality factor were investigated as a function of temperature. An experimental SBH value of about 1.021 eV was obtained for the Ni/Al0.26Ga0.74N SBD at 300 K. The experimental results show that the values of the ideality factor decrease while the values of the SBH increase with increasing temperature. The temperature dependence of the SBH was explained on the basis of a thermionic emission mechanism with the Gaussian distribution of the SBHs due to the SBH inhomogeneities at the metal–semiconductor interface. The values of the mean barrier height Φb0 and the standard deviation σs0 were 1.362 eV and 133 meV in the temperature range of 210–300 K, 1.204 eV and 111 meV in the temperature range of 100–210 K, respectively. The modified Richardson plots according to inhomogeneity of the SBHs have a good linearity in the corresponding temperature range. The values of Richardson constant A* were found to be 31.46 Acm−2 K−2 and 33.36 Acm−2 K−2 in the temperature ranges of 210–310 K and 100–210 K, respectively. The obtained Richardson constant values are in good agreement with the theoretical value of 34.56 Acm−2 K−2 known for n-type Al0.26Ga0.74N. © 2014 Elsevier B.V. All rights reserved.

1. Introduction In the last decades, gallium nitride (GaN) and its ternary alloys, especially aluminum gallium nitride (AlxGa1 − xN), have been widely used in the fabrication of high power electronics and optoelectronics devices due to the properties of direct wide-band gap, high electron saturation velocity and large breakdown field [1–4]. GaN-based electronic and optoelectronic devices, including visible light emitting diodes [5,6], metal–semiconductor field effect transistors [7], high electron mobility transistors (HEMTs) [8,9], and ultraviolet detectors [10,11], have already been present. The nitride material growth technology which supports the optical device efforts has also proven to be compatible with the development of electronic devices. It is well-known that the performance and stability of these devices not only count on the quality of the semiconductor grown but also rely critically on the performance of metal contacts. However, Schottky contacts formed on GaN and its alloys are not well behaved. Especially, they suffer from large leakage current under reverse bias [12–14]. These large leakage currents degrade the current–voltage (I–V) characteristics of the gate and increase its power consumption. A large barrier height leads to small leakage currents and ⁎ Corresponding author. E-mail address: [email protected] (E. Ayyıldız).

http://dx.doi.org/10.1016/j.tsf.2014.05.007 0040-6090/© 2014 Elsevier B.V. All rights reserved.

higher breakdown voltage which results in the improved noise and power performance of HEMTs [15]. The barrier height is an important parameter of the metal–semiconductor (MS) Schottky barrier diodes (SBDs) which control both the width of the depletion region in the semiconductor and the carrier current transport through the MS interface. Although the SBDs have been investigated for more than four decades, the fundamental mechanism which determined the nature and formation of the SBH are not still completely understood [16–28]. Most of the methods used for Schottky contact formation resulted in complex interface morphologies, adatom-induced semiconductor surface disruption, atomic inter diffusion, metal clustering, alloy or compound formation and structural chemical changes of the semiconductor surface [29,30]. Therefore, there is a large spread of metal/nitride Schottky barrier heights (SBHs) reported in the literature [30–40]. On the other hand, defects introduced unevenly in epitaxial layers grown on the lattice mismatched substrates can also be major causes for lateral variations in the SBH. The Schottky contacts to AlGaN for a variety of elemental metals have been studied extensively. Kumar et al. [15] have studied the current–voltage and the capacitance–voltage (C–V) measurements at room temperature of iridium Schottky contacts formed on Al0.25Ga0.75N layer grown by metal organic chemical vapor deposition (MOCVD) on the sapphire substrate. They reported that there is an excellent

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A. Akkaya et al. / Thin Solid Films 564 (2014) 367–374

agreement between the values of barrier height obtained from both I–V and C–V measurements. Sawada et al. [38] have investigated the interface properties of Au/Al0.20Ga0.80N structures formed on oxide-etched and intentionally oxidized surfaces by using I–V–T and capacitance–voltage– temperature (C–V–T) measurements. The analysis of the obtained characteristics has supported the previously proposed “surface patch” model, where the patches with low SBHs cause a leakage current [41,42]. Arehart et al. [39] have explored the carrier trapping properties and current transport behavior of Ni/n-Al0.30Ga0.70N SBDs by a combination of deep level optical spectroscopy (DLOS), thermally based deep level transient spectroscopy (DLTS), I–V–T and internal photoemission measurements. The DLTS and DLOS results indicated a high trap concentration in excess of the doping density, which likely contributes to substantial carrier compensation. They have reported that these high trap concentrations in the bulk were consistent with the I–V–T results, which also suggested trap-related modification of the Schottky barrier profile near the surface. Lim et al. [37], investigating the electrical characteristics of Ni/n-AlGaN SBDs, have shown that there is the strong influence of barrier height inhomogeneity on the temperature dependence of apparent barrier heights obtained through I–V measurements. The SBHs of the gate electrode are an important parameter for device performance. Analysis of the I–V characteristics of the SBDs obtained only at room temperature does not give detailed information about the charge transport process and about the nature of the barrier formed at the MS interface. In fact, it neglects many possible effects that cause non-ideality in the I–V characteristics of the diode and reduce the SBH. The temperature dependence of the I–V characteristics gives a better picture of the conduction mechanism and allows one to understand different aspects that shed light on the validity of various processes involved [31–48]. In this work, the I–V characteristics of SBDs fabricated with a standard photolithography and lift-off techniques on n-Al0.26Ga0.74N epitaxial layer grown by MOCVD on a silicon substrate were investigated over the temperature range of 100–310 K. The values of the apparent barrier height and the ideality factor were obtained from the forwardbiased I–V curves by using the thermionic emission theory (TET). The temperature dependence of the SBH of the Ni/n-Al0.26Ga0.74N SBDs was interpreted on the basis of the existence of the double Gaussian distribution of the SBHs around mean values due to SBH inhomogeneities at the MS interface. Simulation results, based on the double Gaussian distribution parameters, are in good agreement with the experimental forward bias I–V curves.

evaporation for Au and a standard lift-off process was used to pattern to form Schottky contacts with a diameter of 0.5 mm. The I–V–T measurements of the Ni/n-Al0.26Ga0.74N SBDs were accomplished by employing a computer-controlled HP 4140B picoamperemeter and liquid nitrogen cooled cryostat in the temperature range of 100–310 K by steps of 10 K in the dark. The temperature accuracy is better than ±1 K in the temperature range of 100–310 K. 3. Results and discussion The temperature dependent I–V characterization of the SBDs under forward bias is one of the most common experimental techniques and is used to find the significant parameters such as the ideality factor n and the SBH Φb ruling the current transport across the MS contact. 3.1. Temperature dependent current–voltage characteristics of the Ni/ Al0.26Ga0.74N Schottky barrier diodes The semi-logarithmic forward and reverse I–V characteristics of the Ni/Al0.26Ga0.74N SBD in the temperature range of 100–310 K by the step of 10 K are shown in Fig. 1. It can be seen from Fig. 1 that the Ni/ Al0.26Ga0.74N SBD has a good rectifying property at the temperature range studied and it is obviously temperature-dependent. However, the reverse bias current density of Ni/Al 0.26 Ga 0.74 N SBD at − 1 V was between 7.881 × 10−6 A cm−2 and 8.416 × 10−7 A cm−2 at 300 K and 100 K, respectively. In similar works, Zhang et al. [14] reported that the reverse bias current density at −1 V was about 9 × 10−4 A cm−2 and 5 × 10−5 A cm−2 at 300 K and 110 K respectively. Hsu et al. [50] reported that the reverse bias current density at −1 V for Pt/GaN/ AlGaN SBDs was about 5 × 10−7 A cm−2 at room temperature. These values are high, which is a common result for GaN and its ternary alloys because of its own nature. The experimental I–V curves were analyzed using the thermionic emission theory (TET); it is assumed that the SBH is homogeneous in a SBD. According to the TET, the current I across a SBD under forward bias V is given by (for V ≥ 3kT/q) [51,52] qV I ¼ I o exp n kT

10-3

2. Experimental procedure

10-4 10-5

Current(A)

In this study, unintentionally doped (uid) n-type Al0.26Ga0.74N epitaxial layers grown by MOCVD on a Si (111) substrate were used. The epistructure of the wafer consists of 20 Å thin layer of GaN cap layer for protection purposes, 180 Å of AlGaN layer (x = 0.26), 1 μm thick layer of undoped GaN which forms a two-dimensional electron gas (2DEG) at the AlGaN interface, 1.1 μm buffer layer and high resistivity Si (111) substrate with a resistivity of 10 kΩ-cm. The room temperature sheet carrier concentration and electron mobility of the 2DEG induced at the hetero interface were 2 × 1012cm−2 and 1500 cm2/Vs, respectively [49]. The substrates were cleaned consecutively with acetone, methanol, trichloroethylene, deionized water (18 MΩ) for 5 min using ultrasonic agitation in each step. The substrates were then dried with high-purity nitrogen. After cleaning organic residuals, the substrates were dipped in aqua regia to remove the native oxide from the front surface of the substrate and then, in boiling a 0.5 M KOH solution to reduce the surface roughness. The Ti (25 nm)/Al (105 nm) metallization was deposited using magnetron dc sputtering for Ti and thermal evaporation for Al and a standard lift-off process was used to pattern the contacts. The contacts were annealed at 850 °C for 1 min in flowing high purity (6 N) argon gas in a quartz tube furnace. The Ni/Au (30 nm/50 nm) metallization was then deposited using magnetron dc sputtering for Ni and thermal

10-6

ð1Þ

100K 110K

210K 220K

120K 130K 140K 150K 160K 170K 180K 190K 200K

230K 240K 250K 260K 270K 280K 290K 300K 310K

10-7 10-8 10-9 -1.0

-0.5

0.0

0.5

1.0

Voltage(V) Fig. 1. The semi-logarithmic reverse and forward bias I–V characteristics for the Ni/ Al0.26Ga0.74N SBD in the temperature range of 100–310 K.


where

369

ideality factor

1.00

barrier height

ð2Þ

2.50

here Io, V, n, T, A, A*, q, k, and Φbo are the saturation current at zero bias, the applied bias voltage, the ideality factor, the temperature in Kelvin, the effective diode area, the effective Richardson constant, the electron charge, the Boltzmann constant, and the apparent barrier height at zero bias determined from the I–V data, respectively. In Eq. (2) it is assumed that the effective Richardson constant A* keeps constant over the temperature range of our experimental measurement. The theoretical value of the effective Richardson constant is given by A* = 4πqm*k2/h3, where h is the Planck constant and m* is the electron effective mass for AlGaN. A theoretical Richardson constant value of 34.56 Acm−2 K−2 was used for Al0.26Ga0.74N. This value is based on linearly interpolated effective electron masses using m* = 0.48 m0 for AlN and m* = 0.22 m0 for GaN [2]. In Eq. (1), n is a dimensionless parameter introduced to account for the departures from the TET (ideally n = 1). The experimental values of the apparent SBH and the ideality factor were determined from intercepts and slopes of the linear regions of the forward-bias lnI versus voltage plots according to the TET at each temperature, respectively. Least square fittings and other calculations are performed via computer program SeCLaS [53]. This program reads I–V measurement results of each temperature from data file and allows the user to select a linear region and then automatically add least square fit to the selected region. Ideality and SBHs were calculated using a fit equation and TET. As shown in Table 1, the values of the SBH and the ideality factor of the Ni/Al0.26Ga0.74N SBD varied from 0.550 eV and 2.643 at 100 K to 1.033 eV and 1.319 at 310 K, respectively. In our case, the high values of the ideality factor may be attributed to the presence of a thin native oxide layer between the metal and semiconductor [22]. It is well known that the semiconductor surface depending on the surface preparation method is inevitably covered with a thin insulating film of about 10–30 Å thickness [22]. Fig. 2 shows the variation of the SBH and the ideality factor as a function of temperature. As can be seen, the values of ideality factor decrease while the values of SBH increase with increasing temperature. Such a temperature dependent behavior of the SBH and the ideality factor is commonly observed in real SBD and attributed to the lateral inhomogeneity of the SBH at the MS interface [25–27]. Similar trends have been reported for diodes on AlGaN [33–37] as well as any other

2.25

Table 1 The experimental parameters obtained from the forward bias I–V characteristics in the temperature range of 100–310 K for the Ni/Al0.26Ga0.74N SBD. T

n

Φb (eV)

I0 (A)

100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300

2.443 2.333 2.209 2.059 1.965 1.900 1.844 1.793 1.746 1.677 1.632 1.589 1.558 1.521 1.494 1.445 1.407 1.384 1.366 1.353 1.346

0.556 0.580 0.611 0.655 0.690 0.719 0.746 0.771 0.796 0.828 0.853 0.878 0.897 0.918 0.936 0.957 0.971 0.989 1.001 1.007 1.021

1.160 6.515 9.178 1.918 7.835 4.541 2.346 1.121 4.775 1.088 3.731 1.074 3.781 1.123 3.638 8.878 2.809 7.136 2.109 7.434 1.766

× × × × × × × × × × × × × × × × × × × × ×

10−22 10−22 10−23 10−22 10−22 10−21 10−20 10−19 10−19 10−18 10−18 10−17 10−17 10−16 10−16 10−16 10−15 10−15 10−14 10−14 10−13

Ideality factor (n)

0.90

0.80 2.00 0.70

1.75

Barrier height (eV)

qΦbo 2 Io ¼ A A T exp − k T

0.60

1.50

120

160

200

240

280

320

Temperature (K) Fig. 2. The SBH and the ideality factor of the Ni/Al0.26Ga0.74N SBD as a function of temperature. The continuous curves represent the calculated values of the SBH and the ideality factor using Eqs. (7) and (8) for the double Gaussian distribution of SBH havingΦb0 = 1.204 eV and σS0 = 111 meV in the temperature range of 100–210 K and Φb0 = 1.362 eV and σs0 = 133 meV in the 210–300 K range, ρ2 = −0.039, ρ3 = −0.015 V in the temperature range of 100–210 K and ρ2 = −0.152, ρ3 = −0.008 V in the 210–300 K range.

semiconductors [25–27,43–48]. As explained in [54,55], since the current transport across the MS interface is a temperature-activated process, electrons at low temperatures are able to surmount the lower barriers. Therefore, the current transport will be dominated by the current flowing through the patches of lower SBH, leading to a larger ideality factor. As the temperature increases, more and more electrons have sufficient energy to surmount the higher barriers. As a result, both SBH and n are strongly dependent on temperature. The nature and origin of these anomalies in some studies have been explained on the basis of the TET which takes into account the lateral barrier distribution of the SBHs due to inhomogeneities prevailing at the MS interface. Eq. (1) shows that any variations of the SBH result in strong variations of the current I. In fact, it is reasonable to assume that the SBH varies laterally. Ideally, the reverse current should saturate according to the TET [51, 52]. However, in practice and as shown in Fig. 1 this is rarely the case. More usually, the barrier lowering necessary to explain the lack of saturation is greater than that due to the image force. One of the the commonest causes of a field-dependent barrier height is the presence of an interfacial layer. [51]. In addition, the soft reverse characteristics are another sign of presence of the barrier inhomogeneity [27]. For the evaluation of the apparent SBH, one may also use the Richardson plot of the saturation current. Then, Eq. (2) can be rewritten as ln

Io T2

qΦbo : ¼ ln AA − kT

ð3Þ

Fig. 3 shows the Richardson plot of (ln (Io/T2) vs. 1000/T) for the Ni/ Al0.26Ga0.74N SBD. The Io values were derived from the straight line intercept of forward-bias lnI versus bias voltage (V) plots at each temperature and were given in Table 1. According to Eq. (3), the plot ln (Io/T2) versus 1000/T yields a straight line with a slope determining the zero-bias barrier height and the intercept at the ordinate gives the Richardson constant for a known diode area A. The experimental data were seen to fit asymptotically to a straight line only at higher temperature. The value of the Richardson constant A* obtained from the intercept of the straight portion at the ordinate was found to be 1.196 × 10−4 A cm−2 K−2, which is much smaller than the theoretical value of 34.56 A cm−2 K−2 for electrons in undoped Al0.26Ga0.74N. A SBH value of

370


value of 1.2 eV given for the Ni/Al0.26Ga0.74N SBD at 300 K by Sugawara et al. [59].

for Φ b1 =1.362eV and σ1 =0.133eV

-35

for Φ b2 =1.204eV and σ2 =0.111eV Richardson plot

-36

3.2. Analysis of inhomogeneous barrier height

-42

-105

-48

ln(I0/T2)

ln (I0/T2)- σi2/(2k2T2))

-70

-54 -140 -60 -175 4

6

8

10

1000/T (K-1) Fig. 3. The Richardson plot [ln (Io/T2) vs. 1000/T] and the modified Richardson plot [ln(Io/ T2) −q2σo2/2k2T2 vs. 1000/T] of the Ni/Al0.26Ga0.74N SBD. The solid lines show the best fitting of the data in the temperature ranges of 100–210 and 210–300 K, respectively.

The increase in the values of the ideality factor and the decrease in the values of the apparent SBH, the deviation from linearity in the Richardson plot with a decrease in the sample temperature and the presence of the linear relationship between the temperature-dependent SBH and ideality factors obtained from the experimental forward bias I–V data can be explained by the lateral inhomogeneities of the SBHs that consist of low and high barrier regions [27,54]. As reported in the literature [25, 26], the interface between the metal and semiconductor is not atomically flat but rough with the result of lateral fluctuations of the SBH. There are two main methods used to explain the Schottky barrier inhomogeneity. One of them is Tung's pinch-off model [27,55] which suggested the interaction between neighboring patches with different SBH. The other approach is the parallel conduction model based on different patches having different barrier heights which do not affect each other. According to the latter model, the total current through the diode is the sum of the currents flowing on all these patches present in the whole contact area and given by [24,26]

Io ¼ A A T 0.695 eV from the slope of this straight line was obtained for the device. The bowing of the experimental ln(Io/T2) versus 1000/T plot is caused by the temperature dependence of the SBH. As will be discussed below, the deviation in the Richardson plots may be due to the laterally inhomogeneous SBHs and potential fluctuations at the MS interface that consist of low and high barrier regions, that is, the current through the SBD will flow preferentially through the lower barrier in the potential distribution [25–27,33–37,43–48]. The homogeneous SBH rather than the mean values of the apparent SBHs of the SBDs should be used to discuss theories on the physical mechanisms that determine the SBHs of the MS contacts [54–58]. The homogeneous barrier height may be obtained by linear extrapolation of the experimental SBHs versus ideality factors plot to n = 1.0 [58]. As can be seen from Fig. 4, a linear relationship for the two different temperature regimes between the temperature-dependent barrier heights and ideality factors obtained from the experimental forward bias I–V data was determined. The values of the homogeneous SBH for the Ni/Al0.26Ga0.74N SBD were obtained to be 1.206 eV and 1.084 eV for the temperature ranges of 210–310 K and 100–210 K, respectively. The laterally homogeneous barrier height of the Ni/Al0.26Ga0.74N SBD in the temperature range of 210–310 K is in close agreement with the

Φ b= -0.555n + 1.761

Barrier height (eV)

1.20

Φ b= -0.387n + 1.471

þ∞ qV qΦb −1 ∫ P ðΦb Þ exp dΦb exp − nkT kT −∞

ð4Þ

where P(Φb) is the normalized distribution function used to describe the inhomogeneity of the SBH. In the case of a Gaussian distribution of SBHs with a mean value of Φb and a standard deviation of σs, P(Φb) is given by 2 ! Φb −Φb 1 p ffiffiffiffiffiffi exp − P ðΦb Þ ¼ 2σ 2s σ s 2π

ð5Þ

where σ p1 ffiffiffiffi is the normalization constant. According to the model of S 2π Werner and Güttler [26] at thermodynamic equilibrium (V = 0), the temperature dependence barrier height of the lateral SBD can be described by the following equation [26] Φap ¼ Φb0 −

qσ 2S0 2kT

ð6Þ

where Φap is the apparent SBH measured experimentally at 0 V, and 0 V, Φb0and σs0are the Gaussian parameters of the barrier height distribution. The temperature dependence of the standard deviation is usually small and can be neglected. The observed variation of the ideality factor with temperature in this model is given by [26] ! 1 qρ −1 ¼ −ρ2 þ 3 nap 2kT

ð7Þ

where nap is the apparent ideality factor obtained experimentally, ρ2 is the voltage deformation coefficient of the mean barrier height change Δ Φb ðV Þ under the bias, Δ Φb ðV Þ ¼ Φb ðV Þ−Φb0 ¼ ρ2 V; and ρ3 is the voltage deformation coefficient of the standard deviation change Δσs(V) 2 under the bias, Δσs2(V) = σs2(V) − σs0 = ρ3 V. The apparent SBH increases with increasing temperature according to Eq. (6). At a given temperature, the change of the SBH under the bias can be written as [26,60]

1.05 0.90 0.75 0.60 0.45

2

T>210K

1.5

qρ

1 ΔΦb ðV Þ ¼ −ρ2 þ 3 ¼ −1 V: n 2kT

T