In
fluence of Mg-containing precursor flow rate on the structural, electrical
and mechanical properties of Mg-doped GaN thin
films
Wen-Cheng Ke
a, Sheng-Rui Jian
b,*, I-Chen Chen
c, Jason S.-C. Jang
c, Wei-Kuo Chen
d, Jenh-Yih Juang
d aDepartment of Mechanical Engineering, Yuan Ze University, Chung-Li 320, TaiwanbDepartment of Materials Science and Engineering, I-Shou University, Kaohsiung 840, Taiwan cInstitute of Materials Science and Engineering, National Central University, Chung-Li 320, Taiwan dDepartment of Electrophysics, National Chiao Tung University, Hsinchu 300, Taiwan
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
< The GaN:Mg thin films are grown by MOCVD.
< Electrical properties of GaN:Mg thin films are measured by Hall measurement.
< Hardness and Young’s modulus of GaN:Mg thinfilms are measured by nanoindentation.
a r t i c l e i n f o
Article history: Received 1 January 2012 Received in revised form 19 July 2012 Accepted 31 July 2012 Keywords: Thinfilms CVD Hall effect Nanoindentation
a b s t r a c t
The effects of Mg-containing precursorflow rate on the characteristics of the Mg-doped GaN (GaN:Mg) were systematically studied in this study. The GaN:Mgfilms were deposited on sapphire substrates by metal-organic chemical-vapor deposition (MOCVD) with variousflow rates of 25, 50, 75 and 100 sccm of bis-(cyclopentadienyl)-magnesium (Cp2Mg) precursor. The structural, electrical and nanomechanical properties of GaN:Mg thinfilms were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), Hall measurement and nanoindentation techniques, respectively. Results indicated that GaN:Mg films obtained with 25 sccm Cp2Mg possess the highest hole concentration of 3.1 1017cm3. Moreover, the hardness and Young’s modulus of GaN:Mg films measured by a Berkovich nanoindenter operated with the continuous contact stiffness measurements (CSM) option showed positive dependence with increasingflow rate of Cp2Mg precursor, presumably due to the solution hardening effect of Mg-doping. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
Successful preparation of p-type GaN:Mg layer has been one of the vital challenges for realizing blue and green light emitting diodes (LEDs). The primary function of this layer is to supply enough concentration of holes so that, when injected into the active layer,
they will recombine with electrons and emit light efficiently. There-fore, a higher hole concentration in the p-GaN layer is beneficial to the increase of the light intensity of LEDs. In addition, the dopant concentration in the p-GaN layer also must be optimized to obtain ohmic metal contact, which is essential to reduce the operating voltage of the devices. However, since Mg dopants are deep acceptors with relative large ionization energy (w160 meV from the valence band), the maximum hole carrier concentration of p-GaN thinfilms obtained by Mg doping has been severely limited[1]. To date, several growth techniques, such as delta-doping[2], MgeO[3]or MgeZn
co-* Corresponding author. Tel.: þ886 7 6577711x3130; fax: þ886 7 6578444. E-mail address:srjian@gmail.com(S.-R. Jian).
Contents lists available atSciVerse ScienceDirect
Materials Chemistry and Physics
j o u rn a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / m a t c h e m p h y s
0254-0584/$e see front matter Ó 2012 Elsevier B.V. All rights reserved.
doping[4], and multilayered buffer[5], have been demonstrated with some successes in increasing the hole concentration of p-GaN thin films. High hole carrier concentrations (i.e. >1 1018cm3) with very
low resistivities in the modulation Mg-doped p-type AlxGa1xN/GaN superlattices has been reported and was attributed to the significant reduction in activation energy[6]. However, the hole concentration of the doped p-type AlxGa1xN/GaN superlattices mainly depends upon
the Al content in the AlxGa1xN layer. In order to achieve a high Al
content in the AlGaN thinfilms, the growth pressure must be reduced as much as possible such that the serious parasitic reaction of precursors TMAl and NH3in the MOCVD reactor can be minimized
[7,8]. The typical growth pressure of p-type AlxGa1xN/GaN doped
superlattices has been kept at aroundw50 mbar. It implies that, in order to grow single layer of either p-AlGaN or p-GaNfilms with high doping concentration, it is essential to reduce the growth pressure. Unfortunately, up to now, most of the previous reports of growing p-GaN thinfilms have been focused on high growth pressure practiced in the commercial MOCVD systems[1e5].
In addition to monitoring the electric properties through careful control of the processing parameters, successful fabrication of devices based on Mg-doped GaN thin films also requires better understanding of the mechanical characteristics of thefilms, since the contact loading during processing or packaging can signi fi-cantly degrade the performance of these devices. The mechanical properties of materials are size-dependent. Thinfilms may have very different mechanical properties from their bulk materials. Consequently, a precise measurement of the mechanical properties of GaN:Mg thinfilms is required to use them as structural/func-tional elements in the devices. Nanoindentation is a versatile and non-destructive technique for measuring the mechanical charac-teristics of small structures[9,10]or thinfilms and coatings[11e15] at very small scales, namely in the micron and sub-micron range. The load-displacement responses obtained during nanoindentation also provide substantial insights into the mode and onset of plastic deformations or fracture of the materials[16].
In the present study, we attempted to grow p-GaNfilms under the very low pressure of 50 mbar. The experimental results show that the decrease in growth pressure effectively increases the Mg doping efficiency for the p-GaN films. The p-GaN thin films ob-tained under the low pressure MOCVD processes all display excellent structural quality with very smooth surface, as revealed by X-ray diffraction (XRD) and atomic force microscopy (AFM). The electrical and nanomechanical characteristics of the present MOCVD-derived p-type GaN:Mg thinfilms were characterized by Hall measurements and by using a Berkovich nanoindentation system operated in the continuous contact stiffness measurement (CSM) mode, respectively. Effects of the Mg-doping concentration on the electrical and mechanical properties of the obtained GaN:Mg thinfilms are discussed.
2. Experimental details
The p-GaN thinfilms were grown in a horizontal MOCVD system using trimethylgallium (TMGa), ammonia (NH3) and bis
(cyclopentadienyl) magnesium (Cp2Mg) as the metal-organic precursors for Ga, N and Mg, respectively. A series of w0.8-
m
m-thick Mg-doped GaNfilms with different acceptor concentrations were grown on templates consisting of a layer of 0.2-m
m-thick undoped-GaN and a layer of LT-GaN both coated sequentially on the sapphire substrates, hereafter denoted as (un-GaN)/LT-GaN buffer/ sapphire. The growth temperature of the p-GaN thinfilms was set at 1100C. The concentration of Mg doping was controlled by varying theflow rate of Cp2Mg from 25 sccm to 100 sccm. Afterwards, annealing was carried out in a nitrogen environment at a tempera-ture of 700C for 30 min to depassivate the MgeH complexes, which turns out is a very crucial step of achieving high hole concentration in the resultant p-GaN thinfilms. The Mg concentration and surface morphology of p-GaNfilms were measured by a secondary ion mass spectrometer (SIMS) and AFM, respectively. The crystalline struc-tures of the GaN:Mg thinfilms were mainly characterized by the high resolution X-ray diffraction (HRXRD, BRUKER D8 DISCOVER)u
-scan rocking curve. The hole concentration of the p-GaN thinfilms was measured using the Hall system with high purity indium balls pressed onto the four corners as electrical contacts following the van der Pauw geometry configuration.The mechanical properties of the GaN:Mg thin films were characterized using an MTS NanoXPÒ system (MTS Cooperation, Nano Instruments Innovation Center, TN, USA). The nano-indentation measurements, using a three-side pyramidal Berkovich diamond indenter of 50 nm radius (faces 65.3from vertical axis), were conducted under the continuous stiffness measurement (CSM) procedures[17], which was accomplished by superimposing small oscillations on the primary loading signal and analyzing the resultant responses of the system by using a lock-in amplifier. Prior to real measurement, the indenter was loaded and unloaded three times to ensure that the tip was properly in contact with the surface of GaN:Mg thin films and that any parasitical phenomenon was completely excluded from the measurement. On the fourth time, the indenter was loaded at a strain rate of 0.05 s1until reaching an indent depth (hc) of 80 nm and was held for 30 s. Then, it was
withdrawn with the same strain rate until 10% of the peak load was reached. At least 20 indents were performed on each sample. Each indentation was separated by 50
m
m to avoid possible interferences between neighboring indents. We also followed the analytic method proposed by Oliver and Pharr[18]to determine the hard-ness and Young’s modulus of GaN:Mg thin films from the load-displacement results. In this way, hardness and Young’s modulus were obtained as a continuous function of penetration depth. 3. Results and discussionThe results of hole concentration for all the GaN:Mgfilms ob-tained by the room-temperature Hall measurements are listed in Table 1. It is evident fromTable 1that thefilm with the highest hole concentration of 3.1 1017cm3is the one obtained with a Cp2Mg
flow rate of 25 sccm and it starts to decrease monotonically to 4.7 1016cm3forfilm obtained with 100 sccm Cp2Mg flow rate,
albeit the SIMS analyses (as shown in Fig. 3(a)) have otherwise
Table 1
Properties of GaN:Mg thinfilms grown at various Mg flow rates. Mg-doped GaNfilmsaMgflow rate R
s(nm) H (GPa) Efilm(GPa) smax(GPa) Screw/edge dislocation density (cm2) Hole concentration (cm3)
25 sccm 0.72 20.5 0.9 308.2 2.6 6.8 0.3 1.56 108/9.64 108 3.1 1017 50 sccm 2.29 22.5 0.6 335.6 4.9 7.5 0.2 2.19 108/1.19 109 1.3 1017 75 sccm 5.28 25.1 0.8 376.1 3.7 8.4 0.3 2.44 108/1.22 109 5.5 1016 100 sccm 23.45 28.5 1.6 409.8 6.6 9.5 0.5 2.83 108/1.35 109 4.7 1016 19 1[25] 286 25[25] 6.3[25] 1.7e 6.0 1017[19] aThis study.
indicated that the Mg concentration increases from 0.07% to 0.27% for the 25 sccm and 100 sccm films, respectively. The apparent discrepancy may be attributed to the effect originated from hydrogen involved in the GaN growth processes. Hydrogen behaves as a donor and forms neutral (MgeH) complexes with acceptors, which further suppresses the formation of self-compensating native point defects (e.g. nitrogen vacancy, VN) [19,20]. As
a result, the high concentration of hydrogen incorporated during thefilm growth processes became the main factor limiting the hole concentration in the obtained GaN:Mg thinfilms. It has been sug-gested that the MgeH complexes can be depassivated to render the acceptor electrically active by thermal annealing in nitrogen environment.
Alternatively, the quality of the crystalline structure of GaN:Mg films can play a key role in influencing the eventual hole concen-trations. Wright et al.[21]proposed that the edge dislocations may behave as hole traps in p-type GaN thinfilms. In order to delineate the effects of Mg-doping on thefilm crystalline structure and hence on the resulting hole concentration, high resolution XRD measurements, especially the XRD spectra of (002) and (102) diffraction peaks were carried out.Fig. 1(a) shows the full width at half maximum (FWHM) values of the (002) peak. It indicates that the FWHM value decreases from 0.14to 0.11as theflow rate of Cp2Mg precursor decreases from 100 sccm to 25 sccm. Similarly, as shown in Fig. 1(b), the FWHM of the (102) diffraction peak also decreases from 0.20 to 0.17 with decreasing Cp2Mgflow rate. These results suggest that an increasing Cp2Mgflow rate may have resulted in increased density of screw and edge dislocations in the doped p-GaN thinfilms. Based on the premise that the [002] and [102] diffraction peaks are respectively affected by screw and edge dislocations, the respective dislocation density (
r
) can be calculated by using the following expression[22]:r
¼b
24:35 b2 (1)
where b is the absolute value of the Burgers vector and
b
is the FWHM of the corresponding diffraction peak. The obtained densi-ties of the screw and edge dislocations in the GaN:Mg thinfilms arelisted inTable 1. Since the atomic radius of Mg (w0.136 nm) is larger than that of Ga atom (w0.126 nm), an increasing strain field induced by Mg doping can be expected. Typically, in a strained thin film, any further deposition would result in formation of disloca-tions such that the accumulated strain can be accommodated and the morphology of the layer remains smooth. In this scenario, the reduced hole concentration with increasing Mg doping may be explained by the resultant dislocation densities.
Fig. 2displays the surface morphologies of the 25 sccm (left panel) and 100 sccm (right panel) GaN:Mgfilms. It is evident that the surface roughness (Ra) within a scanning area of 10 10
m
m2 shows a dramatic change from 0.72 to 23.45 nm (as listed in Table 1), when the Mg doping is increased and the surface morphology drastically changes from rather smooth continuous film to densely distributed hexagonal hillocks seen inFig. 2. We note that, in fact, such morphology changes were actually appear-ing when the Cp2Mg flow rate was increased to 75 sccm (not shown here). It has been reported that Mg doping can result in pyramidal-shaped extensive defects in the resultant GaN films when the Mg concentration exceeds 1020cm3[23]. This is quite consistent with the Mg concentrations of the present GaN:Mgfilms obtained by SIMS.Fig. 3(a) shows the SIMS depth profile analyses revealing the actual Mg atomic concentration in GaN:Mg thinfilms grown by the present low pressure growth technique. The proce-dure of converting the sputtering into the SIMS depth profile is briefly described as following. Firstly, a testing experiment for calculating the sputtering rate was performed for each sample before doing the element concentration depth profile analysis. The testing experiment comprises: (i) measuring the depth of craters created on the polished surface of GaN:Mg samples by surface profiler after a certain sputtering time; and (ii) dividing the measured depths by the sputtering time to obtain the sputtering rate. Since in each case the sputtering rate was obtained under exactly identical oxygen or cesium ion bombardment conditions, it could be correctly converted to the SIMS depth profiles of the corresponding GaN:Mg samples. It is evident that the Mg concen-tration remains rather uniform throughout the entire thickness of GaN:Mg thinfilm and at a depth of 0.4m
m the Mg concentrations were 1.07 1020, 4.09 1019, 1.62 1019and 6.22 1018atom/cm3,for the Mgflow rate of 100, 75, 50 and 25 sccm, respectively. The SIMS analysis of H concentration in GaN:Mg thinfilms is also dis-played inFig. 3(b). It can be observed that the H concentration is slightly increased with increasing Mgflow rate. It is, thus, quite reasonable to attribute the increase in hole concentration to the decrease of H concentration in GaN:Mg thinfilms[24].
It should be noted that the sample with the highest Mg concentration appears to be over-doped with an estimated Mg/ Ga ratio of 0.49%, which is responsible for the severely degraded surface morphology consisting of densely packed hexagonal hillocks seen in the right panel ofFig. 2. In addition, the over-doped GaN:Mg could also lead to significant reduction in hole concentration, since Mg may incorporate into electrically inac-tive complexes and/or generate compensating donors. Conse-quently, it is suggestive from the present results that, for simultaneously achieving high hole concentration and smooth surface morphology, the optimal Mg doping concentration should be kept around 6.22 1018 atom/cm3 in GaN:Mg thin
films obtained by the low pressure CVD process practiced in the present study.
Typical results of nanoindentation measurements for the GaN:Mgfilms investigated in the present study are displayed in Fig. 4. The load-displacement curve for GaN:Mg thinfilms with the highest Mg doping is shown in Fig. 4(a). The load-displacement response obtained by nanoindentation contains information about the elastic and plastic deformation of the indented materials. Thus, it is often regarded as the“fingerprint” of the films under investigation, because mechanical properties, such as the hardness and Young’s modulus, can be readily extracted from the load-displacement data [18]. The hardness is estimated by the expression:
H ¼ Pmax=Ac (2)
where Pmax is the peak load; Ac is the contact projected area
determined by the geometry of the indenter and the contact depth, hc. In the present work, Acis assumed to be describable by an area
function FðhcÞ and can be expressed as,
Ac ¼ FðhcÞ ¼ 24:69h2cþ 122:80h1cþ 212:77h1=2c 191:25h1=4c
32:77h1=8c ð3Þ
The elastic modulus is then determined from the following relation: Eeff ¼ 1 2
b
S ffiffiffiffiffip
Ac r (4) with S andb
being respectively denoted as the measured stiffness and a shape constant of 1.034 for a Berkovich indenter. The Eeff is the effective elastic modulus defined by1 Eeff ¼ 1 v2 film Efilm þ 1 v2 i Ei (5) which takes into account the fact that elastic displacements occur in both the thinfilm and an indenter. The elastic modulus Ei, and
Pois-son’s ratio, vi, of the Berkovich indenter used in this work are 1141 GPa
and 0.07, respectively[18]. The Poisson’s ratio of thin film ðvfilmÞ is
assumed to be 0.25[25]. By combining Eqs.(4) and (5), one obtains the Young’s modulus of GaN:Mg thin films, Efilm, as following:
Efilm ¼ 1 v2 film SEi ffiffiffi
p
p 2b
Ei ffiffiffiffiffi Ac p 1 v2 i Spffiffiffip
(6)Through the continuous contact stiffness measurements, the displacement dependence of hardness and Young’s modulus are obtained instantaneously. Fig. 4(b) displays the hardness versus penetration depth curves for GaN:Mg thinfilms deposited at the various Cp2Mgflow rates. All of the plots can be divided into two stages, namely, initial increase to a maximum value and subsequent precipitous drop to a relatively constant value. The increase in hardness at the early stage of penetration is usually attributed to the transition between purely elastic to elastic/plastic contact. As a result, the hardness cannot be accurately measured by the mean contact pressure at this stage. Only under the condition of a fully developed plastic zone does the mean contact pressure represent the hardness. When there is no plastic zone, or only partially formed plastic zone, the mean contact pressure is less than the nominal hardness [18]. On the other hand, the constant charac-teristic of hardness reached after thefirst stage is consistent with that of a single material; therefore, the hardness values obtained at this stage could be regarded as the intrinsic properties of thefilms. The obtained hardness for GaN:Mg thinfilms grown under Cp2Mg
Fig. 2. AFM image of GaN:Mg thinfilms with various Mg flow rates of (a) 25 sccm and (b) 100 sccm.
flow rate of 25, 50, 75 and 100 sccm are 20.5 0.9, 22.5 0.6, 25.1 0.8 and 28.5 1.6 GPa, respectively. A clear trend can be observed from this figure that the hardness increases with increasing Mg doping concentration. Similar tendency is also clearly observed for the Young’s modulus of GaN:Mg thin films as displayed in Fig. 4(c). The corresponding values of the Young’s modulus for are 308.2 2.6, 335.6 4.9, 376.1 3.7 and 409.8 6.6 GPa, respectively.
Back to the load-displacement curve displayed inFig. 4(a). It can be clearly observed that the loading segment of the curve consists of a series of serrations all the way up the maximum load with a single prominent pop-in event (indicated by the arrow) occurring at a relatively low critical pop-in indentation load Pcr. The pop-in
event is a signature of catastrophic plastic deformation associated with some kind of structural changes induced by nanoindentation. Previous studies of other semiconductor materials have revealed a number of plastic deformation mechanisms occurring as a result of mechanical loading by using nanoindentation and
cross-Fig. 3. SIMS analysis for GaN:Mg thinfilms with various Mg flow rates: (a) Mg and (b) H concentration profiles.
Fig. 4. Nanoindentation results: (a) the typical load-displacement curve for GaN:Mg thinfilms deposited at the Mg flow rate of 100 sccm; (b) hardnessedisplacement curves and (c) Young’s modulusedisplacement curves for GaN:Mg thin films.
sectional transmission electron microscopy (XTEM) techniques, including densification by micro-cracking[16]and slip by punching out of dislocation arrays [11,25e27]. Furthermore, the reverse discontinuities during unloading curve, the so-called “pop-out” event commonly observed in silicon and has been attributed to pressure-induced phase transition [28,29], is not observed here. Therefore, it is clear that the pop-in event may reflect the transition from perfectly elastic to plastic deformation, that is, it is the onset of plasticity in GaN:Mg thin film. The corresponding shear stress under the Berkovich indenter at indentation load Pcr, where the
load-displacement discontinuity occurs, can be determined by using the following relation[30]:
s
max ¼ 0:316PcrE2film
p
3R2!1=3
(7)
where R is the tip radius of the indenter. Comparisons of the mechanical properties of the present GaN:Mg thinfilms with those reported by previous studies are also presented inTable 1. It can be seen that Mg-doping does have the enhancement effects on various mechanical properties of the GaN:Mgfilms, presumably due to the solute hardening effects arisen from the size difference between Ga and Mg atoms.
Finally, in order to check if there is any influence from the substrate in a more quantitative manner, especially when the studying objects are thin films, it is essential to monitor the mechanical properties as a function of depth. For the present case, it is noted that the events of multiple pop-ins are coinciding nicely with sudden decreases in the hardness of measured materials[31]. As can be seen in Fig. 4(b), the hardness of GaN:Mg thin film decreases abruptly at the penetration depth ofw30 nm corre-sponding to the pop-in event. The hardness after the pop-in for GaN:Mg thinfilm remains nearly constant with small fluctuations, possibly associated with dislocation activities. Similarly, as shown inFig. 4(c), the Young’s modulus of GaN:Mg thin films also displays a sudden drop occurring around the same penetration depth and then remains relatively constant. Consequently, it is plausible to state that indentation with contact depths being less than 30% of the total film thickness is needed to obtain intrinsic thin film properties and minimize the influence from the substrate[32]. 4. Conclusion
To summarize, the structural features, electrical and mechanical characteristics of GaN:Mg thin films obtained by using MOCVD process with the various Cp2Mgflow rates are investigated by XRD, AFM, Hall measurement and nanoindentation techniques. The XRD and AFM results suggest that overdopedfilms resulting from high Cp2Mgflow rates may have led to degradations in film crystalline structure by introducing excessive dislocation density, which in turn not only dramatically alters thefilm surface morphology and surface roughness but also reduces the effective hole concentra-tions. The Hall measurements evidently indicate that the highest hole concentration of 3.1 1017cm3is achieved forfilms obtained
with the Cp2Mg flow rate of 25 sccm. Nanoindentation results
indicated that both the Young’s modulus and film hardness are increased with increasing Mg doping concentration. The typical values obtained for the former ranges from 20.5 0.9 to 28.5 1.6 GPa and that for the latter ranges from 308.2 2.6 to 409.8 6.6 GPa for GaN:Mg thin films deposited with various Cp2Mgflow rates of 25e100 sccm.
Acknowledgements
This work was partially supported by the National Science Council of Taiwan, under Grant Nos.: NSC98-2112-M155-001-MY3, NSC100-2221-E-214-024 and NSC101-2221-E-214-017. JYJ is partially supported by the NSC of Taiwan and the MOE-ATU program operated at NCTU. Author likes to thank Dr. Y.-S. Lai and Dr. P.-F. Yang for their technical supports.
References
[1] P. Kozodoy, H. Xing, S.P. DenBaars, U.K. Mishra, A. Saxler, R. Perrin, S. Elhamri, W.C. Mitchel, J. Appl. Phys. 87 (2000) 1832.
[2] C. Bayram, J.L. Pau, R. McClintock, M. Razeghi, Appl. Phys. Lett. 92 (2008) 241103.
[3] G. Kipshidze, V. Kuryatkov, B. Borisov, Y. Kudryavtsev, R. Asomoza, S. Nikishin, H. Temkin, Appl. Phys. Lett. 80 (2002) 2910.
[4] K.S. Kim, M.S. Han, G.M. Yang, C.J. Youn, H.J. Lee, H.K. Cho, J.Y. Lee, Appl. Phys. Lett. 77 (2000) 1123.
[5] X. Zhang, S.J. Chua, P. Li, K.B. Chong, W. Wang, Appl. Phys. Lett. 73 (1998) 1772. [6] E.L. Waldron, J.W. Graff, E.F. Schubert, Appl. Phys. Lett. 79 (2001) 2737. [7] G. Coli, K.K. Bajaj, J. Li, J.Y. Lin, H.X. Jiang, Appl. Phys. Lett. 78 (2001) 1829. [8] J. Li, K.B. Nam, J.Y. Lin, H.X. Jiang, Appl. Phys. Lett. 79 (2001) 3245. [9] L. Liu, G. Cao, X. Chen, J. Nanomater. 2008 (2008) 271763.
[10] T.H. sung, J.C. Huang, J.H. Hsu, S.R. Jian, Appl. Phys. Lett. 97 (2010) 171904. [11] R. Navamathavan, Y.-T. Moon, G.S. Kim, T.G. Lee, J.H. Hahn, S.J. Park, Mater.
Chem. Phys. 99 (2006) 410.
[12] S.R. Jian, Jason S.C. Jang, Y.S. Lai, P.F. Yang, C.S. Yang, H.C. Wen, C.H. Tsai, Mater. Chem. Phys. 109 (2008) 360.
[13] S.R. Jian, G.J. Chen, T.C. Lin, Nanoscale Res. Lett. 5 (2010) 935. [14] S.R. Jian, W.C. Ke, J.Y. Juang, Nanosci. Nanotechnol. Lett. 4 (2012) 598. [15] S.R. Jian, H.G. Chen, G.J. Chen, Jason S.C. Jang, J.Y. Juang, Curr. Appl. Phys. 12
(2012) 849.
[16] S.J. Bull, J. Phys. D. Appl. Phys. 38 (2005) R393. [17] X.D. Li, B. Bhushan, Mater. Charact. 48 (2002) 11. [18] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564.
[19] R.R. Lieten, V. Motsnyi, L. Zhang, K. Cheng, M. Leys, S. Degroote, G. Buchowicz, O. Dubon, G. Borghs, J. Phys. D: Appl. Phys. 44 (2011) 135406.
[20] S. Hautakangas, J. Oila, M. Alatalo, K. Saarinen, L. Liszkay, D. Seghier, H.P. Gislason, Phys. Rev. Lett. 90 (2003) 137402.
[21] A.F. Wright, U. Grossner, Appl. Phys. Lett. 73 (1998) 2751.
[22] J.C. Zhang, D.S. Jiang, Q. Sun, J.F. Wang, Y.T. Wang, J.P. Liu, J. Chen, R.Q. Jin, J.J. Zhu, H. Yang, T. Dai, Q.J. Jia, Appl. Phys. Lett. 87 (2005) 071908. [23] P. Vennéguès, M. Benaissa, B. Beaumont, E. Feltin, P. De Mierry, S. Dalmasso,
M. Leroux, P. Gibart, Appl. Phys. Lett. 77 (2000) 880.
[24] I. Waki, F. Fujioka, M. Oshima, H. Miki, A. Fukizawa, Appl. Phys. Lett. 78 (2001) 2899.
[25] C.H. Tsai, S.R. Jian, J.Y. Juang, Appl. Surf. Sci. 254 (2008) 1997.
[26] J.E. Bradby, J.S. Williams, J. Wong-Leung, M.V. Swain, P. Munroe, Appl. Phys. Lett. 78 (2001) 3235.
[27] J.E. Bradby, S.O. Kucheyev, J.S. Williams, J. Wong-Leung, M.V. Swain, P. Munroe, G. Li, M.R. Phillips, Appl. Phys. Lett. 80 (2002) 383.
[28] S.R. Jian, Nanoscale Res. Lett. 3 (2008) 6.
[29] S.R. Jian, G.J. Chen, J.Y. Juang, Curr. Opin. Solid State Mater. Sci. 14 (2010) 69. [30] K.L. Johnson, Contact Mechanics, Cambridge University Press, Cambridge, UK,
1985.
[31] J.E. Bradby, J.S. Williams, M.V. Swain, J. Mater. Res. 19 (2004) 380. [32] X.D. Li, H.S. Gao, C.J. Murphy, L.F. Gou, Nano Lett. 10 (2004) 1903. W.-C. Ke et al. / Materials Chemistry and Physics 136 (2012) 796e801 801
