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n-InP Schottky contacts formed on chemically cleaned and air-exposed n-InP surface
ARTICLE IN PRESS
Physica B 394 (2007) 93–99 www.elsevier.com/locate/physb
Electrical characteristics of Au, Al, Cu/n-InP Schottky contacts formed on chemically cleaned and air-exposed n-InP surface H. C - etina,, E. Ayyıldızb a b
Department of Physics, Faculty of Sciences and Arts, Bozok University, 66100 Yozgat, Turkey Department of Physics, Faculty of Sciences and Arts, Erciyes University, 38039 Kayseri, Turkey
Received 1 February 2007; received in revised form 8 February 2007; accepted 8 February 2007
Abstract We fabricated Au, Al and Cu/n-InP (1 0 0) Schottky barrier diodes formed on chemically cleaned and air-exposed n-InP surfaces to investigate the influence of the air-grown oxide on electrical performance. The oxide layer was obtained by exposing to laboratory air for 1, 2, 4, 6 and 8 weeks before metal evaporation on a chemically cleaned InP surface. Oxide thicknesses were measured by elipsometry. The chemical composition of grown surface oxides was investigated using X-ray photoelectron spectroscopy (XPS). P2O5 and In(PO)4 were found on the air-exposed surface. Although In-oxides and In-P-oxides were found, no P-oxide was found on the chemically cleaned HF-etched surface. Both air-exposed and chemically cleaned samples have indium-rich surfaces. Furthermore, we have investigated the barrier height stabilities for 70 days. The results have shown that an increase in barrier height does not directly depend on crystal surface exposure time to air. Furthermore, Schottky metals deposited on the oxidized surface have determined an increase or decrease in barrier height with respect to reference samples. Metal–oxide, metal–crystal surface interactions are the main factors of determining Schottky barrier heights. r 2007 Elsevier B.V. All rights reserved. Keywords: Schottky contact; XPS; Barrier height
1. Introduction Metal film deposition on InP substrates has received much attention for many years because of an interest to develop fabrication technology for high-speed devices. In almost every case, the intimate Schottky barrier height was found to be rather lower than desirable. This increases the Schottky barrier leakage current, and the device performance is degraded [1–5]. Many attempts to passivate surface and increase the barrier heights of Schottky contacts have been performed by forming an interfacial layer between the metal and semiconductor [6–11]. Furthermore, the interfacial oxide layer has been used by various workers for passivation [12–17]. However, it is not clear whether a thin oxide is desirable or not for insulator/ III–V semiconductor MIS applications for long-term Corresponding author. Tel.: +90 354 242 10 21; fax: +90 354 242 10 22. E-mail address: [email protected] (H. C - etin).
0921-4526/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2007.02.013
device application. Although thin interfacial oxides could conceivably help to grade the interfacial mismatch between insulator and semiconductor, thereby minimizing the amount of interfacial strain and defect formation, oxides may cause poor MIS devices [1]. Besides, a thin native oxide may be the source of interfacial oxide traps that could cause Fermi-level pinning and electrical interface instabilities. For long-term device application, these instabilities give rise to suspect device reliability and performance. Therefore, further investigation is needed for the effects of oxide on barrier formation and long-term device application. In the laboratory environment, a chemically cleaned semiconductor crystal surface is usually covered with layers of native oxides even when it is exposed to clean room air for a very short time. Although native oxide is thin, this may affect device quality and barrier height. Furthermore, obtained high barrier height is sometimes related to involuntary surface oxidation during contact fabrication steps for metal-InP structures. It is important to know how
ARTICLE IN PRESS H. C - etin, E. Ayyıldız / Physica B 394 (2007) 93–99
much of air-grown oxide can affect the contact barrier height and electrical characteristics. Thus, we have investigated Au, Al and Cu/n-InP Schottky diodes to understand the influence of native oxide on the electrical performance and stability of diodes. Diodes with an interfacial layer have been formed by exposing a chemically cleaned InP surface to clean laboratory air before metal evaporation. X-ray photoelectron spectroscopy (XPS) has been used to investigate the chemical composition of surface oxides grown on the InP. The characteristic Schottky diode parameters such as ideality factors and barrier heights deduced from the current–voltage (I–V) characteristics have been compared with each other. 2. Experimental In this study, an undoped and one side polished n-type (1 0 0) InP substrate having a resistivity of 0.26 O cm and an electron mobility of 5280 cm2 V1 s1 was used. The wafer was chemically cleaned using trichloroethylene, acetone and methanol for 5 min and then rinsed in de-ionized water of 18.2 MO and dried in high-purity nitrogen. The substrate was dipped for 60 s in HF:H2O (1:10) solution to remove native oxide from the front surface of the substrate. After this treatment, the wafer was rinsed in deionized water for 10 s. The wafer was inserted into the deposition chamber immediately after the etching process. Ohmic contacts were produced by evaporation of In on the non-polished side of the wafer in a vacuum-coating unit and the wafer was thermally annealed at 350 1C for 60 s in flowing nitrogen in a quartz tube furnace. After that, the wafer was cut into six pieces of 5 mm 5 mm each. One of them was immediately inserted into the evaporation chamber to form the reference Schottky contacts. The metal gate, Au, was then deposited through a Mo mask by thermal evaporation in dots the shape of approximately 1 mm diameter, and their thickness (240 A˚) was monitored using a quartz oscillator. This sample has been called the reference diode. The second piece with Ohmic contact was exposed to clean laboratory air to obtain a native oxide layer on the polished side of n-InP for a week, before Schottky contacts were formed. The third piece for 2 weeks, the fourth piece for 4 weeks, the fifth piece for 6 weeks and the sixth piece for 8 weeks were exposed to clean laboratory air, before Schottky contacts were formed. The procedure mentioned above was repeated for Al and Cu/nInP Schottky contacts. All evaporation processes were carried out in a vacuum coating unit. The pressure during evaporation was about 1.5 106 mbar. Clean laboratory air in which samples were exposed had an average humidity of 40% at 25 1C, measured by a Lambrecht model hygrometer. The thicknesses of the oxide layer on the surfaces were measured using spectroscopic ellipsometry. XPS analysis was performed on the reference and oxidized InP wafer surfaces by using a SAGE 100 system
equipped with an MgKa source. Surface spectra were obtained for reference and 4 weeks air-exposed samples. The I–V characteristics were measured using an HP 4140B picoamperemeter at room temperature. Moreover, in order to determine the effect of the aging on the reference and oxidized contacts, I–V measurements were also repeated 7, 14, 28, 45, 56 and 70 days after fabrication of the Schottky diodes. 3. Experimental results and discussion 3.1. XPS measurements In order to obtain chemical composition of surface oxide, XPS was used. X-ray photoelectron spectrum obtained from reference and oxidized samples are shown in Fig. 1. An XPS P 2p spectra region is shown in Fig. 2 for the samples. It can be seen that the reference sample yields a P 2p peak at 129.25 eV. This energy corresponds to that of InP substrate [18]. The oxidized surface yields a P 2p peak at 130.8 eV. When this peak is compared to that of reference sample, the P 2p peak is shifted by 1.55 eV. Furthermore, the oxidized sample exhibits a peak at 135.6 eV. That peak corresponds to P2O5 compound [19]. The In 3d5/2 peak shapes are shown in Fig. 3. The reference sample has an XPS peak at 445 eV value. This energy corresponds to that of In(OH)3 [20]. The oxidized surface yields a peak at 445.6 eV. The peak arises from In2O3 compound [21]. These data indicate that oxide is present at the reference sample surface. The sample was immediately inserted the vacuum unit after chemically cleaned, but the sample might have been exposed to air for a short time. Oxide compounds may grow in the time interval. It is interesting that XPS results expose In-oxide and InP-oxide but not P-oxide on the reference sample surface. The reference sample has In-oxide-rich surface. The airexposed surface has both In-oxide and P-oxide. This
Intensity (a.u.)
94
In3d5/2 (b)
O1s
P2p C1s
(a)
0
200
400
600
800
1000
Binding energy (eV) Fig. 1. XPS spectra for n-InP: (a) reference sample and (b) 4 weeks airexposed sample.
ARTICLE IN PRESS H. C - etin, E. Ayyıldız / Physica B 394 (2007) 93–99 Table 1 Atomic concentrations obtained from XPS peak intensities
P
InP
P2O5
Intensity (a.u.)
(b)
(a)
126
128
130
132
134
136
138
140
Binding energy (eV) Fig. 2. XPS (P 2p) spectra for n-InP: (a) reference sample and (b) 4 weeks air exposed sample.
In(OH)3
In(PO4)
In2O3
Intensity (a.u.)
95
Element (at%)
Reference sample (%)
Four weeks air-exposed sample (%)
P In O C
18 29 22 31
13 19 32 36
water remove fluorine or chlorine from the substrate and barrier height change with the dipping is not observed. In this study, as the n-InP surface is not terminated by atomic fluorine, fluorine cannot affect the barrier height and oxidation can only cause barrier height change. In Table 1, atomic concentrations are shown. For 4 weeks air exposed sample, adsorbed carbon and oxygen increase from 31% to 36% and 22% to 32%, respectively. The origin of the carbon which exists on the reference sample surface may arise from chemical cleaning. This atomic ratio increase depends on the duration of the sample’s exposure to clean laboratory environment. The oxidized sample oxygen ratio is 1.45 times bigger than that of reference sample surface. As can be seen in the following section, this oxide can cause device electrical characteristics to change, depending on the used Schottky metal. 3.2. Current–voltage characteristics
(b) (a)
440
442
444
446
448
450
Binding energy (eV) Fig. 3. XPS (In3d) spectra for n-InP: (a) reference sample and (b) 4 weeks air exposed sample.
indicates that although reference sample surface has an oxide, reference and oxidized surface are different from each other. Kikuchi and Adachi studied [22] chemically cleaned InP (1 0 0) surfaces in aqueous HF solution using X-ray photoelectron emission spectroscopy. They found that the solution caused the removal of native oxide and left behind InP surface terminated by atomic fluorine. However, our XPS results show that there are no fluorine peaks on the HF-dipped, n-InP sample. In our study, water rinsing might cause removal of atomic fluorine after HF dipping. Vanalme et al. [23] prepared HF- and HCl-dipped p-InP substrate and investigated barrier height change with dipping by using BEEM. They show that although HF or HCl dipping left behind p-InP surface terminated by atomic fluorine or chlorine, short rinses in de-ionized
The forward bias current–voltage characteristics due to thermionic emission of a Schottky barrier diode for forward voltage in excess of a few kT/q can be expressed as [1,24] qV I ¼ I o exp , (1) nkT where n is ideality factor, k is Boltzmann constant, T is temperature, q is electron charge, V is applied voltage, and Io is saturation current given as qFbo 2 I o ¼ AA T exp , (2) kT where A is effective diode area, A is effective Richardson constant of 9.24 A cm2 K2 for n-type InP [1] and qFbo is barrier height at zero bias. The apparent or measured barrier height qFbo is given by A AT 2 qFbo ¼ kT ln . (3) Io Figs. 4–6 show the experimental semilog forward and reverse bias I–V characteristics of all samples immediately after fabrication at room temperature for Au, Cu and Al/nInP Schottky contacts, respectively. The apparent barrier heights of the devices were calculated from the y-axis intercepts of the semilog forward bias I–V characteristics according to Eq. (3). The values of n were calculated from the slope of the linear regions of the forward I–V
ARTICLE IN PRESS H. C - etin, E. Ayyıldız / Physica B 394 (2007) 93–99
Current (A)
Current (A)
96
1E-3
1E-3
1E-4
1E-5
-0.50
0.00
-0.25
Reference Sample 1 Week Air Exposed Sample 2 Weeks Air Exposed Sample 4 Weeks Air Exposed Sample 6 Weeks Air Exposed Sample 8 Weeks Air Exposed Sample
0.25
Reference Sample 1 Week Air Exposed Sample 2 Weeks Air Exposed Sample 4 Weeks Air Exposed Sample 6 Weeks Air Exposed Sample 8 Weeks Air Exposed Sample
1E-4
0.50
-0.50
-0.25
0.00
0.25
0.50
Voltage (V)
Voltage (V)
Fig. 4. Immediately after fabrication, experimental semi-log forward and reverse bias current vs. voltage characteristics of Au/n-InP Schottky barrier diodes at room temperature.
Fig. 5. Immediately after fabrication, experimental semi-log forward and reverse bias current vs. voltage characteristics of Cu/n-InP Schottky barrier diodes at room temperature.
characteristics according to
heights was frequently observed experimentally. The existence of Schottky barrier height inhomogeneity and interfacial traps may cause the soft reverse characteristics. Aluminum can react with anions of the compound semiconductors [25]. Furthermore, aluminum can react with surface oxide. These reactions may cause inhomogeneous Schottky barrier heights and interfacial traps. Table 2 shows surface oxide thicknesses that are measured by elipsometry and experimental barrier height values that are calculated with the help of Eq. (3) from yaxis intercepts of the semilog-forward bias I–V plots in the figure, and experimental ideality values that are calculated with the help of Eq. (4) from slopes of the semilog forward bias I–V plots at room temperature. From Table 2, one can see that oxide thicknesses are increasing with increasing air exposure time. In the time interval, we do not observe limiting oxide thickness. As previously mentioned, XPS results show that carbon atomic ratio increases depend on the duration of the sample’s exposure to clean laboratory environment. Carbon covers the sample surface and this thin carbon layer may contribute to measured oxide thicknesses. In Table 2, immediately after fabrication, barrier height value of the reference sample has been obtained as 0.50576 103 eV for Au/n-InP contacts. This value is in agreement with the literature [1,26]. It can be expected that air-grown oxide as an interfacial layer on
n¼
q dV . kT dðln IÞ
(4)
Fig. 4 shows the experimental semilog forward and reverse bias I–V characteristics that are obtained immediately after the Au/n-InP Schottky contacts formed. The reference sample has about 2.50 105 A saturation current value, which is lower than those of the others. Surface oxide has caused a bigger saturation current value than that of reference sample. It does not seem to be any soft or non-saturating behavior of the reverse bias region of the curves with increasing oxide thickness. Every I–V curve shape is identical to each other. Fig. 5 shows semilog forward and reverse bias I–V characteristics for Cu/n-InP contacts. The reference sample has about 1.17 103 A saturation current value. The other contacts have between 3.45 104 and 6.99 104 A saturation current values. Air-grown oxide causes lower saturation current value than that of reference sample. It does not seem that an orderly change of saturation current values depends on oxide thicknesses for the contacts. Fig. 6 shows semilog forward and reverse bias I–V characteristics for Al/n-InP contacts. No Schottky contacts except for reference and the samples exposed to air for 8 weeks show saturation in the reverse bias branch. This bias dependence of Schottky barrier
ARTICLE IN PRESS H. C - etin, E. Ayyıldız / Physica B 394 (2007) 93–99
Current (A)
the InP surface can result in improved device characteristics, because grown interface oxide could cause high barrier height and the reason could be discussed in terms of the Cowley–Sze model [27] with surface states at the oxidesemiconductor interface [28], or by charge trapped at the oxide–semiconductor interface [29]. As could be seen in Table 2, barrier heights are lower than that of reference sample for the contacts, which are formed on air-exposed surfaces. It could be shows that a positive fixed charge exists in the oxide, which causes the reduction of surface band bending and thus a reduction of Schottky barrier heights. Although positive fixed charge idea could be one
1E-3
1E-4 Reference Sample 1 Week Air Exposed Sample 2 Weeks Air Exposed Sample 4 Weeks Air Exposed Sample 6 Weeks Air Exposed Sample 8 Weeks Air Exposed Sample -0.50
-0.25
0.00
0.25
0.50
Voltage (V) Fig. 6. Immediately after fabrication, experimental semi-log forward and reverse bias current vs. voltage characteristics of Al/n-InP Schottky barrier diodes at room temperature.
97
of the factors to understand barrier height lowering with oxidation [30], it could not be the only one. Furthermore, defects could be effective on barrier height formation for III–V compound semiconductors [31]. As can be seen in Table 2, barrier height value was obtained as 0.4187 7 103 eV for the reference sample and 0.42771 103 eV for the 1 week air-exposed sample of Cu/n-InP contacts. It can be clearly observed that in contrast to Au/ n-InP contacts, barrier height decrease with surface oxidation has not been observed for Cu/n-InP contacts. The same condition can be seen in Table 2 for Al/n-InP contacts. These results have shown that an increase in barrier height does not directly depend on crystal surface exposure time to air. Furthermore, used Schottky metals, that are deposited on the oxidized surface have determined increase or decrease of barrier heights with respect to reference samples. Metal–oxide, metal–crystal surface interactions are the mainly factors of determining Schottky barrier heights. The other important feature of Table 2, sometimes obtained high barrier heights, was related to involuntary surface oxidation during fabrication steps of the structure, but this is not true. For the 8 weeks airexposed surface, oxide thickness and barrier height are 38 A˚ and 0.44172 102 eV, respectively. This barrier height is in good agreement with Schottky contacts formed on the chemically cleaned, non-oxidized sample surface [10]. The barrier height and its increase or decrease are determined by surface–Schottky metal interaction more than oxide thickness for metal/n-InP Schottky contacts. In Fig. 7, the barrier height values calculated from I–V characteristics as a function of time are shown for Au/nInP contacts. Reference contact barrier heights have different values from oxidized sample barrier heights with all measurements. It has been observed that surface oxide does not cause considerable aging of barrier heights for the Au/n-InP contacts. The barrier height values change up to 2 weeks for all samples and later they almost expose saturation with small change. The observed change may be related to the following explanation. In the Au/n-InP contacts, Au does not react with oxygen. Oxygen can diffuse through thin metal contact and be adsorbed by non-oxidized In and P atoms. Thus, electrical characteristics have changed. Furthermore, Au may diffuse into the
Table 2 For reference and air-exposed Au, Cu and Al/n-InP Schottky contacts, Schottky barrier heights measured immediately after metal contact fabrication, oxide thicknesses, ideality factors Samples
Oxide thickness (A˚)
Au/n-InP
Cu/n-InP n
qFbo (eV) Reference sample 1 week air exposed 2 weeks air exposed 4 weeks air exposed 6 weeks air exposed 8 weeks air exposed
23 26 28 30 33 38
3
0.50586 10 0.42287 103 0.44085 103 0.45183 103 0.44889 103 0.44182 102
Al/n-InP
qFbo (eV) 2
1.0583 10 1.1087 102 1.0484 102 1.0081 102 1.0284 102 1.1385 102
n 3
0.41887 10 0.42781 103 0.43483 103 0.42188 103 0.42882 103 0.43881 103
qFbo (eV) 2
1.1084 10 1.0982 102 1.1582 102 1.0981 101 1.1082 102 1.0981 101
n 3
0.43183 10 0.45181 103 0.44481 102 0.43182 103 0.41182 103 0.41187 103
1.0783 102 1.0782 102 1.1384 102 1.0481 102 1.1684 102 1.0581 101
ARTICLE IN PRESS H. C - etin, E. Ayyıldız / Physica B 394 (2007) 93–99
98 0.56
0.64 Reference Sample 1 Week Air Exposed Sample 2 Weeks Air Exposed Sample 4 Weeks Air Exposed Sample 6 Weeks Air Exposed Sample 8 Weeks Air Exposed Sample
0.52
Reference Sample 1 Week Air Exposed Sample 2 Weeks Air Exposed Sample 4 Weeks Air Exposed Sample 6 Weeks Air Exposed Sample 8 Weeks Air Exposed Sample
0.62 0.60 0.58 Barrier Height (eV)
Barrier Height (eV)
0.54
0.50 0.48 0.46
0.56 0.54 0.52 0.50 0.48 0.46 0.44
0.44
0.42 0.40
0.42
0.38 0
10
20
30
40
50
60
70
80
0
Fig. 7. Barrier height values from measured I–V characteristics as a function of time for Au/n-InP SBDs.
Reference Sample 1 Week Air Exposed Sample 2 Weeks Air Exposed Sample 4 Weeks Air Exposed Sample 6 Weeks Air Exposed Sample 8 Weeks Air Exposed Sample
Barrier Height (eV)
0.47 0.46
20
30
40
50
60
70
80
Fig. 9. Barrier height values from measured I–V characteristics as a function of time for Al/n-InP SBDs.
barrier height change is to be considerable for Al/n-InP contacts. Aluminum is known as a reactive metal and can react with III–V semiconductor surfaces or its oxides. The observed nature of variation of the barrier heights can be explained by aluminum-surface oxides chemical reactions. In the reaction duration, the metal–semiconductor interface has been modified and this modification can cause a change in electrical parameters.
0.49 0.48
10
Time (Day)
Time (Day)
0.45 0.44 0.43
4. Conclusion
0.42
In the present work, systematic changes of the electrical characteristics of Au, Al and Cu/n-InP Schottky contacts formed by evaporation on various time air exposure surfaces have been studied. We obtained air-grown oxide chemical composition and determined the atomic concentration of oxygen and other elements on the surface from XPS spectra. Furthermore, we measured grown oxide thicknesses by using ellipsometry. Obtained oxide thicknesses and atomic concentrations of oxygen showed that oxygen quantity on the surface increased with increasing exposure time to air. When the contact formed on the oxidized surface, air-grown oxide did not cause barrier height enhancement for Au, Al and Cu/n-InP contacts. These results have shown that an increase in barrier heights does not directly depend on crystal surface exposure time to air. Furthermore, the reactivity of Schottky metals deposited on oxidized surface has determined the increase or decrease in barrier heights with respect to reference samples. The aging of Au, Al and Cu/n-InP contacts formed on chemically cleaned and oxidized surfaces does not seem to be similar to each other. The aging effect of the Schottky contact has been observed depending on surface oxide thickness for Al/n-InP. Fabricated contact barrier height values and the contacts aging with time are determined by chemical reactivity of the used Schottky metal.
0.41 0.40 0
10
20
30
40
50
60
70
80
Time (Day)
Fig. 8. Barrier height values from measured I–V characteristics as a function of time for Cu/n-InP SBDs.
semiconductor. Anions and cations of the semiconductor may diffuse into the metal [32]. In the metal–semiconductor interface, complex interactions occur even at room temperature and interdiffusion can continue to change the detailed electrical nature of the contact days and months after the interface was formed. In Fig. 8, barrier heights of Cu/n-InP contacts are shown as a function of time. As can be seen in the figure, there is no considerable difference between the lines. Reference or oxidized sample barrier height values have not been distinguished. The effect of air-grown oxide on electrical characteristics has not been observed for Cu/n-InP contacts. In Fig. 9, the barrier heights of Al/n-InP contacts are shown as a function of time. As can be clearly seen, barrier height values change in 2 weeks as in the Au/n-InP contacts. However, when surface oxide is to be thicker,
ARTICLE IN PRESS H. C - etin, E. Ayyıldız / Physica B 394 (2007) 93–99
Acknowledgments This work has been supported by the Scientific Research Projects Unit of Erciyes University through Project No. 02012-11 EU+ BAP. We would like to thank Dr. Hosun Lee from Kyung Hee University and Dr. Seydi Dogan from Virginia Commonwealth University for ellipsometric measurements of InP samples and METU Central Laboratory for XPS analysis. The English text has been kindly checked by Lecturer Erkan Ozdog˘an from Erciyes University. References [1] C.W. Wilmsen, Physics and Chemistry of III–V Compound Semiconductor Interfaces, Plenum Press, New York and London, 1985. [2] K. Hattori, Y. Torii, Solid-State Electron. 34 (1991) 527. [3] N. Newman, M. Schilfgaarde, W.E. Spicer, Phys. Rev. B 35 (1987) 6298. [4] B. Singh, K.C. Reinhardt, W.A. Anderson, J. Appl. Phys. 68 (1990) 3475. [5] C. Barret, H. Maaref, Solid-State Electron. 36 (1993) 879. [6] H. C - etin, E. Ayyildiz, A. Tu¨ru¨t, J. Vac. Sci. Technol. B 23 (2005) 2436. [7] K. Vaccaro, H.M. Dauplaise, A. Davis, S.M. Spaziani, J.P. Lorenzo, Appl. Phys. Lett. 67 (1995) 527. [8] M. Schvartzman, V. Sidorov, D. Ritter, Y. Paz, Semicond. Sci. Technol. 16 (2001) L68. [9] R.R. Sumathi, N.V. Giridharan, R. Jayavel, J. Kumar, Mater. Lett. 51 (2001) 56. [10] S. Morikita, H. Ikoma, J. Vac. Sci. Technol. A 21 (2003) 226.
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Physica B 394 (2007) 93–99 www.elsevier.com/locate/physb
Electrical characteristics of Au, Al, Cu/n-InP Schottky contacts formed on chemically cleaned and air-exposed n-InP surface H. C - etina,, E. Ayyıldızb a b
Department of Physics, Faculty of Sciences and Arts, Bozok University, 66100 Yozgat, Turkey Department of Physics, Faculty of Sciences and Arts, Erciyes University, 38039 Kayseri, Turkey
Received 1 February 2007; received in revised form 8 February 2007; accepted 8 February 2007
Abstract We fabricated Au, Al and Cu/n-InP (1 0 0) Schottky barrier diodes formed on chemically cleaned and air-exposed n-InP surfaces to investigate the influence of the air-grown oxide on electrical performance. The oxide layer was obtained by exposing to laboratory air for 1, 2, 4, 6 and 8 weeks before metal evaporation on a chemically cleaned InP surface. Oxide thicknesses were measured by elipsometry. The chemical composition of grown surface oxides was investigated using X-ray photoelectron spectroscopy (XPS). P2O5 and In(PO)4 were found on the air-exposed surface. Although In-oxides and In-P-oxides were found, no P-oxide was found on the chemically cleaned HF-etched surface. Both air-exposed and chemically cleaned samples have indium-rich surfaces. Furthermore, we have investigated the barrier height stabilities for 70 days. The results have shown that an increase in barrier height does not directly depend on crystal surface exposure time to air. Furthermore, Schottky metals deposited on the oxidized surface have determined an increase or decrease in barrier height with respect to reference samples. Metal–oxide, metal–crystal surface interactions are the main factors of determining Schottky barrier heights. r 2007 Elsevier B.V. All rights reserved. Keywords: Schottky contact; XPS; Barrier height
1. Introduction Metal film deposition on InP substrates has received much attention for many years because of an interest to develop fabrication technology for high-speed devices. In almost every case, the intimate Schottky barrier height was found to be rather lower than desirable. This increases the Schottky barrier leakage current, and the device performance is degraded [1–5]. Many attempts to passivate surface and increase the barrier heights of Schottky contacts have been performed by forming an interfacial layer between the metal and semiconductor [6–11]. Furthermore, the interfacial oxide layer has been used by various workers for passivation [12–17]. However, it is not clear whether a thin oxide is desirable or not for insulator/ III–V semiconductor MIS applications for long-term Corresponding author. Tel.: +90 354 242 10 21; fax: +90 354 242 10 22. E-mail address: [email protected] (H. C - etin).
0921-4526/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2007.02.013
device application. Although thin interfacial oxides could conceivably help to grade the interfacial mismatch between insulator and semiconductor, thereby minimizing the amount of interfacial strain and defect formation, oxides may cause poor MIS devices [1]. Besides, a thin native oxide may be the source of interfacial oxide traps that could cause Fermi-level pinning and electrical interface instabilities. For long-term device application, these instabilities give rise to suspect device reliability and performance. Therefore, further investigation is needed for the effects of oxide on barrier formation and long-term device application. In the laboratory environment, a chemically cleaned semiconductor crystal surface is usually covered with layers of native oxides even when it is exposed to clean room air for a very short time. Although native oxide is thin, this may affect device quality and barrier height. Furthermore, obtained high barrier height is sometimes related to involuntary surface oxidation during contact fabrication steps for metal-InP structures. It is important to know how
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much of air-grown oxide can affect the contact barrier height and electrical characteristics. Thus, we have investigated Au, Al and Cu/n-InP Schottky diodes to understand the influence of native oxide on the electrical performance and stability of diodes. Diodes with an interfacial layer have been formed by exposing a chemically cleaned InP surface to clean laboratory air before metal evaporation. X-ray photoelectron spectroscopy (XPS) has been used to investigate the chemical composition of surface oxides grown on the InP. The characteristic Schottky diode parameters such as ideality factors and barrier heights deduced from the current–voltage (I–V) characteristics have been compared with each other. 2. Experimental In this study, an undoped and one side polished n-type (1 0 0) InP substrate having a resistivity of 0.26 O cm and an electron mobility of 5280 cm2 V1 s1 was used. The wafer was chemically cleaned using trichloroethylene, acetone and methanol for 5 min and then rinsed in de-ionized water of 18.2 MO and dried in high-purity nitrogen. The substrate was dipped for 60 s in HF:H2O (1:10) solution to remove native oxide from the front surface of the substrate. After this treatment, the wafer was rinsed in deionized water for 10 s. The wafer was inserted into the deposition chamber immediately after the etching process. Ohmic contacts were produced by evaporation of In on the non-polished side of the wafer in a vacuum-coating unit and the wafer was thermally annealed at 350 1C for 60 s in flowing nitrogen in a quartz tube furnace. After that, the wafer was cut into six pieces of 5 mm 5 mm each. One of them was immediately inserted into the evaporation chamber to form the reference Schottky contacts. The metal gate, Au, was then deposited through a Mo mask by thermal evaporation in dots the shape of approximately 1 mm diameter, and their thickness (240 A˚) was monitored using a quartz oscillator. This sample has been called the reference diode. The second piece with Ohmic contact was exposed to clean laboratory air to obtain a native oxide layer on the polished side of n-InP for a week, before Schottky contacts were formed. The third piece for 2 weeks, the fourth piece for 4 weeks, the fifth piece for 6 weeks and the sixth piece for 8 weeks were exposed to clean laboratory air, before Schottky contacts were formed. The procedure mentioned above was repeated for Al and Cu/nInP Schottky contacts. All evaporation processes were carried out in a vacuum coating unit. The pressure during evaporation was about 1.5 106 mbar. Clean laboratory air in which samples were exposed had an average humidity of 40% at 25 1C, measured by a Lambrecht model hygrometer. The thicknesses of the oxide layer on the surfaces were measured using spectroscopic ellipsometry. XPS analysis was performed on the reference and oxidized InP wafer surfaces by using a SAGE 100 system
equipped with an MgKa source. Surface spectra were obtained for reference and 4 weeks air-exposed samples. The I–V characteristics were measured using an HP 4140B picoamperemeter at room temperature. Moreover, in order to determine the effect of the aging on the reference and oxidized contacts, I–V measurements were also repeated 7, 14, 28, 45, 56 and 70 days after fabrication of the Schottky diodes. 3. Experimental results and discussion 3.1. XPS measurements In order to obtain chemical composition of surface oxide, XPS was used. X-ray photoelectron spectrum obtained from reference and oxidized samples are shown in Fig. 1. An XPS P 2p spectra region is shown in Fig. 2 for the samples. It can be seen that the reference sample yields a P 2p peak at 129.25 eV. This energy corresponds to that of InP substrate [18]. The oxidized surface yields a P 2p peak at 130.8 eV. When this peak is compared to that of reference sample, the P 2p peak is shifted by 1.55 eV. Furthermore, the oxidized sample exhibits a peak at 135.6 eV. That peak corresponds to P2O5 compound [19]. The In 3d5/2 peak shapes are shown in Fig. 3. The reference sample has an XPS peak at 445 eV value. This energy corresponds to that of In(OH)3 [20]. The oxidized surface yields a peak at 445.6 eV. The peak arises from In2O3 compound [21]. These data indicate that oxide is present at the reference sample surface. The sample was immediately inserted the vacuum unit after chemically cleaned, but the sample might have been exposed to air for a short time. Oxide compounds may grow in the time interval. It is interesting that XPS results expose In-oxide and InP-oxide but not P-oxide on the reference sample surface. The reference sample has In-oxide-rich surface. The airexposed surface has both In-oxide and P-oxide. This
Intensity (a.u.)
94
In3d5/2 (b)
O1s
P2p C1s
(a)
0
200
400
600
800
1000
Binding energy (eV) Fig. 1. XPS spectra for n-InP: (a) reference sample and (b) 4 weeks airexposed sample.
ARTICLE IN PRESS H. C - etin, E. Ayyıldız / Physica B 394 (2007) 93–99 Table 1 Atomic concentrations obtained from XPS peak intensities
P
InP
P2O5
Intensity (a.u.)
(b)
(a)
126
128
130
132
134
136
138
140
Binding energy (eV) Fig. 2. XPS (P 2p) spectra for n-InP: (a) reference sample and (b) 4 weeks air exposed sample.
In(OH)3
In(PO4)
In2O3
Intensity (a.u.)
95
Element (at%)
Reference sample (%)
Four weeks air-exposed sample (%)
P In O C
18 29 22 31
13 19 32 36
water remove fluorine or chlorine from the substrate and barrier height change with the dipping is not observed. In this study, as the n-InP surface is not terminated by atomic fluorine, fluorine cannot affect the barrier height and oxidation can only cause barrier height change. In Table 1, atomic concentrations are shown. For 4 weeks air exposed sample, adsorbed carbon and oxygen increase from 31% to 36% and 22% to 32%, respectively. The origin of the carbon which exists on the reference sample surface may arise from chemical cleaning. This atomic ratio increase depends on the duration of the sample’s exposure to clean laboratory environment. The oxidized sample oxygen ratio is 1.45 times bigger than that of reference sample surface. As can be seen in the following section, this oxide can cause device electrical characteristics to change, depending on the used Schottky metal. 3.2. Current–voltage characteristics
(b) (a)
440
442
444
446
448
450
Binding energy (eV) Fig. 3. XPS (In3d) spectra for n-InP: (a) reference sample and (b) 4 weeks air exposed sample.
indicates that although reference sample surface has an oxide, reference and oxidized surface are different from each other. Kikuchi and Adachi studied [22] chemically cleaned InP (1 0 0) surfaces in aqueous HF solution using X-ray photoelectron emission spectroscopy. They found that the solution caused the removal of native oxide and left behind InP surface terminated by atomic fluorine. However, our XPS results show that there are no fluorine peaks on the HF-dipped, n-InP sample. In our study, water rinsing might cause removal of atomic fluorine after HF dipping. Vanalme et al. [23] prepared HF- and HCl-dipped p-InP substrate and investigated barrier height change with dipping by using BEEM. They show that although HF or HCl dipping left behind p-InP surface terminated by atomic fluorine or chlorine, short rinses in de-ionized
The forward bias current–voltage characteristics due to thermionic emission of a Schottky barrier diode for forward voltage in excess of a few kT/q can be expressed as [1,24] qV I ¼ I o exp , (1) nkT where n is ideality factor, k is Boltzmann constant, T is temperature, q is electron charge, V is applied voltage, and Io is saturation current given as qFbo 2 I o ¼ AA T exp , (2) kT where A is effective diode area, A is effective Richardson constant of 9.24 A cm2 K2 for n-type InP [1] and qFbo is barrier height at zero bias. The apparent or measured barrier height qFbo is given by A AT 2 qFbo ¼ kT ln . (3) Io Figs. 4–6 show the experimental semilog forward and reverse bias I–V characteristics of all samples immediately after fabrication at room temperature for Au, Cu and Al/nInP Schottky contacts, respectively. The apparent barrier heights of the devices were calculated from the y-axis intercepts of the semilog forward bias I–V characteristics according to Eq. (3). The values of n were calculated from the slope of the linear regions of the forward I–V
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Current (A)
Current (A)
96
1E-3
1E-3
1E-4
1E-5
-0.50
0.00
-0.25
Reference Sample 1 Week Air Exposed Sample 2 Weeks Air Exposed Sample 4 Weeks Air Exposed Sample 6 Weeks Air Exposed Sample 8 Weeks Air Exposed Sample
0.25
Reference Sample 1 Week Air Exposed Sample 2 Weeks Air Exposed Sample 4 Weeks Air Exposed Sample 6 Weeks Air Exposed Sample 8 Weeks Air Exposed Sample
1E-4
0.50
-0.50
-0.25
0.00
0.25
0.50
Voltage (V)
Voltage (V)
Fig. 4. Immediately after fabrication, experimental semi-log forward and reverse bias current vs. voltage characteristics of Au/n-InP Schottky barrier diodes at room temperature.
Fig. 5. Immediately after fabrication, experimental semi-log forward and reverse bias current vs. voltage characteristics of Cu/n-InP Schottky barrier diodes at room temperature.
characteristics according to
heights was frequently observed experimentally. The existence of Schottky barrier height inhomogeneity and interfacial traps may cause the soft reverse characteristics. Aluminum can react with anions of the compound semiconductors [25]. Furthermore, aluminum can react with surface oxide. These reactions may cause inhomogeneous Schottky barrier heights and interfacial traps. Table 2 shows surface oxide thicknesses that are measured by elipsometry and experimental barrier height values that are calculated with the help of Eq. (3) from yaxis intercepts of the semilog-forward bias I–V plots in the figure, and experimental ideality values that are calculated with the help of Eq. (4) from slopes of the semilog forward bias I–V plots at room temperature. From Table 2, one can see that oxide thicknesses are increasing with increasing air exposure time. In the time interval, we do not observe limiting oxide thickness. As previously mentioned, XPS results show that carbon atomic ratio increases depend on the duration of the sample’s exposure to clean laboratory environment. Carbon covers the sample surface and this thin carbon layer may contribute to measured oxide thicknesses. In Table 2, immediately after fabrication, barrier height value of the reference sample has been obtained as 0.50576 103 eV for Au/n-InP contacts. This value is in agreement with the literature [1,26]. It can be expected that air-grown oxide as an interfacial layer on
n¼
q dV . kT dðln IÞ
(4)
Fig. 4 shows the experimental semilog forward and reverse bias I–V characteristics that are obtained immediately after the Au/n-InP Schottky contacts formed. The reference sample has about 2.50 105 A saturation current value, which is lower than those of the others. Surface oxide has caused a bigger saturation current value than that of reference sample. It does not seem to be any soft or non-saturating behavior of the reverse bias region of the curves with increasing oxide thickness. Every I–V curve shape is identical to each other. Fig. 5 shows semilog forward and reverse bias I–V characteristics for Cu/n-InP contacts. The reference sample has about 1.17 103 A saturation current value. The other contacts have between 3.45 104 and 6.99 104 A saturation current values. Air-grown oxide causes lower saturation current value than that of reference sample. It does not seem that an orderly change of saturation current values depends on oxide thicknesses for the contacts. Fig. 6 shows semilog forward and reverse bias I–V characteristics for Al/n-InP contacts. No Schottky contacts except for reference and the samples exposed to air for 8 weeks show saturation in the reverse bias branch. This bias dependence of Schottky barrier
ARTICLE IN PRESS H. C - etin, E. Ayyıldız / Physica B 394 (2007) 93–99
Current (A)
the InP surface can result in improved device characteristics, because grown interface oxide could cause high barrier height and the reason could be discussed in terms of the Cowley–Sze model [27] with surface states at the oxidesemiconductor interface [28], or by charge trapped at the oxide–semiconductor interface [29]. As could be seen in Table 2, barrier heights are lower than that of reference sample for the contacts, which are formed on air-exposed surfaces. It could be shows that a positive fixed charge exists in the oxide, which causes the reduction of surface band bending and thus a reduction of Schottky barrier heights. Although positive fixed charge idea could be one
1E-3
1E-4 Reference Sample 1 Week Air Exposed Sample 2 Weeks Air Exposed Sample 4 Weeks Air Exposed Sample 6 Weeks Air Exposed Sample 8 Weeks Air Exposed Sample -0.50
-0.25
0.00
0.25
0.50
Voltage (V) Fig. 6. Immediately after fabrication, experimental semi-log forward and reverse bias current vs. voltage characteristics of Al/n-InP Schottky barrier diodes at room temperature.
97
of the factors to understand barrier height lowering with oxidation [30], it could not be the only one. Furthermore, defects could be effective on barrier height formation for III–V compound semiconductors [31]. As can be seen in Table 2, barrier height value was obtained as 0.4187 7 103 eV for the reference sample and 0.42771 103 eV for the 1 week air-exposed sample of Cu/n-InP contacts. It can be clearly observed that in contrast to Au/ n-InP contacts, barrier height decrease with surface oxidation has not been observed for Cu/n-InP contacts. The same condition can be seen in Table 2 for Al/n-InP contacts. These results have shown that an increase in barrier height does not directly depend on crystal surface exposure time to air. Furthermore, used Schottky metals, that are deposited on the oxidized surface have determined increase or decrease of barrier heights with respect to reference samples. Metal–oxide, metal–crystal surface interactions are the mainly factors of determining Schottky barrier heights. The other important feature of Table 2, sometimes obtained high barrier heights, was related to involuntary surface oxidation during fabrication steps of the structure, but this is not true. For the 8 weeks airexposed surface, oxide thickness and barrier height are 38 A˚ and 0.44172 102 eV, respectively. This barrier height is in good agreement with Schottky contacts formed on the chemically cleaned, non-oxidized sample surface [10]. The barrier height and its increase or decrease are determined by surface–Schottky metal interaction more than oxide thickness for metal/n-InP Schottky contacts. In Fig. 7, the barrier height values calculated from I–V characteristics as a function of time are shown for Au/nInP contacts. Reference contact barrier heights have different values from oxidized sample barrier heights with all measurements. It has been observed that surface oxide does not cause considerable aging of barrier heights for the Au/n-InP contacts. The barrier height values change up to 2 weeks for all samples and later they almost expose saturation with small change. The observed change may be related to the following explanation. In the Au/n-InP contacts, Au does not react with oxygen. Oxygen can diffuse through thin metal contact and be adsorbed by non-oxidized In and P atoms. Thus, electrical characteristics have changed. Furthermore, Au may diffuse into the
Table 2 For reference and air-exposed Au, Cu and Al/n-InP Schottky contacts, Schottky barrier heights measured immediately after metal contact fabrication, oxide thicknesses, ideality factors Samples
Oxide thickness (A˚)
Au/n-InP
Cu/n-InP n
qFbo (eV) Reference sample 1 week air exposed 2 weeks air exposed 4 weeks air exposed 6 weeks air exposed 8 weeks air exposed
23 26 28 30 33 38
3
0.50586 10 0.42287 103 0.44085 103 0.45183 103 0.44889 103 0.44182 102
Al/n-InP
qFbo (eV) 2
1.0583 10 1.1087 102 1.0484 102 1.0081 102 1.0284 102 1.1385 102
n 3
0.41887 10 0.42781 103 0.43483 103 0.42188 103 0.42882 103 0.43881 103
qFbo (eV) 2
1.1084 10 1.0982 102 1.1582 102 1.0981 101 1.1082 102 1.0981 101
n 3
0.43183 10 0.45181 103 0.44481 102 0.43182 103 0.41182 103 0.41187 103
1.0783 102 1.0782 102 1.1384 102 1.0481 102 1.1684 102 1.0581 101
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98 0.56
0.64 Reference Sample 1 Week Air Exposed Sample 2 Weeks Air Exposed Sample 4 Weeks Air Exposed Sample 6 Weeks Air Exposed Sample 8 Weeks Air Exposed Sample
0.52
Reference Sample 1 Week Air Exposed Sample 2 Weeks Air Exposed Sample 4 Weeks Air Exposed Sample 6 Weeks Air Exposed Sample 8 Weeks Air Exposed Sample
0.62 0.60 0.58 Barrier Height (eV)
Barrier Height (eV)
0.54
0.50 0.48 0.46
0.56 0.54 0.52 0.50 0.48 0.46 0.44
0.44
0.42 0.40
0.42
0.38 0
10
20
30
40
50
60
70
80
0
Fig. 7. Barrier height values from measured I–V characteristics as a function of time for Au/n-InP SBDs.
Reference Sample 1 Week Air Exposed Sample 2 Weeks Air Exposed Sample 4 Weeks Air Exposed Sample 6 Weeks Air Exposed Sample 8 Weeks Air Exposed Sample
Barrier Height (eV)
0.47 0.46
20
30
40
50
60
70
80
Fig. 9. Barrier height values from measured I–V characteristics as a function of time for Al/n-InP SBDs.
barrier height change is to be considerable for Al/n-InP contacts. Aluminum is known as a reactive metal and can react with III–V semiconductor surfaces or its oxides. The observed nature of variation of the barrier heights can be explained by aluminum-surface oxides chemical reactions. In the reaction duration, the metal–semiconductor interface has been modified and this modification can cause a change in electrical parameters.
0.49 0.48
10
Time (Day)
Time (Day)
0.45 0.44 0.43
4. Conclusion
0.42
In the present work, systematic changes of the electrical characteristics of Au, Al and Cu/n-InP Schottky contacts formed by evaporation on various time air exposure surfaces have been studied. We obtained air-grown oxide chemical composition and determined the atomic concentration of oxygen and other elements on the surface from XPS spectra. Furthermore, we measured grown oxide thicknesses by using ellipsometry. Obtained oxide thicknesses and atomic concentrations of oxygen showed that oxygen quantity on the surface increased with increasing exposure time to air. When the contact formed on the oxidized surface, air-grown oxide did not cause barrier height enhancement for Au, Al and Cu/n-InP contacts. These results have shown that an increase in barrier heights does not directly depend on crystal surface exposure time to air. Furthermore, the reactivity of Schottky metals deposited on oxidized surface has determined the increase or decrease in barrier heights with respect to reference samples. The aging of Au, Al and Cu/n-InP contacts formed on chemically cleaned and oxidized surfaces does not seem to be similar to each other. The aging effect of the Schottky contact has been observed depending on surface oxide thickness for Al/n-InP. Fabricated contact barrier height values and the contacts aging with time are determined by chemical reactivity of the used Schottky metal.
0.41 0.40 0
10
20
30
40
50
60
70
80
Time (Day)
Fig. 8. Barrier height values from measured I–V characteristics as a function of time for Cu/n-InP SBDs.
semiconductor. Anions and cations of the semiconductor may diffuse into the metal [32]. In the metal–semiconductor interface, complex interactions occur even at room temperature and interdiffusion can continue to change the detailed electrical nature of the contact days and months after the interface was formed. In Fig. 8, barrier heights of Cu/n-InP contacts are shown as a function of time. As can be seen in the figure, there is no considerable difference between the lines. Reference or oxidized sample barrier height values have not been distinguished. The effect of air-grown oxide on electrical characteristics has not been observed for Cu/n-InP contacts. In Fig. 9, the barrier heights of Al/n-InP contacts are shown as a function of time. As can be clearly seen, barrier height values change in 2 weeks as in the Au/n-InP contacts. However, when surface oxide is to be thicker,
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Acknowledgments This work has been supported by the Scientific Research Projects Unit of Erciyes University through Project No. 02012-11 EU+ BAP. We would like to thank Dr. Hosun Lee from Kyung Hee University and Dr. Seydi Dogan from Virginia Commonwealth University for ellipsometric measurements of InP samples and METU Central Laboratory for XPS analysis. The English text has been kindly checked by Lecturer Erkan Ozdog˘an from Erciyes University. References [1] C.W. Wilmsen, Physics and Chemistry of III–V Compound Semiconductor Interfaces, Plenum Press, New York and London, 1985. [2] K. Hattori, Y. Torii, Solid-State Electron. 34 (1991) 527. [3] N. Newman, M. Schilfgaarde, W.E. Spicer, Phys. Rev. B 35 (1987) 6298. [4] B. Singh, K.C. Reinhardt, W.A. Anderson, J. Appl. Phys. 68 (1990) 3475. [5] C. Barret, H. Maaref, Solid-State Electron. 36 (1993) 879. [6] H. C - etin, E. Ayyildiz, A. Tu¨ru¨t, J. Vac. Sci. Technol. B 23 (2005) 2436. [7] K. Vaccaro, H.M. Dauplaise, A. Davis, S.M. Spaziani, J.P. Lorenzo, Appl. Phys. Lett. 67 (1995) 527. [8] M. Schvartzman, V. Sidorov, D. Ritter, Y. Paz, Semicond. Sci. Technol. 16 (2001) L68. [9] R.R. Sumathi, N.V. Giridharan, R. Jayavel, J. Kumar, Mater. Lett. 51 (2001) 56. [10] S. Morikita, H. Ikoma, J. Vac. Sci. Technol. A 21 (2003) 226.
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