p-InP Schottky barrier diodes

Vacuum 83 (2009) 1470–1474

Contents lists available at ScienceDirect

Vacuum journal homepage: www.elsevier.com/locate/vacuum

Determination of the laterally homogeneous barrier height of metal/p-InP Schottky barrier diodes ¨ . Gu¨llu¨ b, *, A. Tu¨ru¨t c S. Asubay a, O a

University of Dicle, Faculty of Science and Art, Department of Physics, Diyarbakir, Turkey Batman University, Faculty of Sciences and Arts, Department of Physics, Batman, Turkey c ¨ rk University, Faculty of Science, Department of Physics, Erzurum, Turkey Atatu b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 February 2009 Received in revised form 3 June 2009 Accepted 14 June 2009

We have reported a study of a number of metal/p-type InP (Cu, Au, Al, Sn, Pb, Ti, Zn) Schottky barrier diodes (SBDs). Each one diode has been identically prepared on p-InP under vacuum conditions with metal deposition. In Schottky diodes, the current transport occurs by thermionic emission over the Schottky barrier. The current–voltage characteristics of Schottky contacts are described by two fitting parameters such as effective barrier height and the ideality factor. Due to lateral inhomogeneities of the barrier height, both characteristic diode parameters differ from one diode to another. We have determined the lateral homogeneous barrier height of the SBDs from the linear relationship between experimental barrier heights and ideality factors that can be explained by lateral inhomogeneity of the barrier height. Furthermore, the barrier heights of metal–semiconductor contacts have been explained by the continuum of metal-induced gap states (MIGS). It has been seen that the laterally homogeneous barrier heights obtained from the experimental data of the metal/p-type InP Schottky contacts quantitatively confirm the predictions of the combination of the physical MIGS and the chemical electronegativity. Ó 2009 Elsevier Ltd. All rights reserved.

PACS: 73.30.þy 73.40.Ei 73.40.Ns 73.40.Sx Keywords: InP semiconductor Schottky diodes Metal–semiconductor–metal contacts Schottky barrier inhomogeneity

1. Introduction Metal–semiconductor contacts have been used in many applications such as gates for metal–semiconductor field-effect transistors, solar cells, and detectors [1–4]. The most characteristic parameter of a metal–semiconductor contact or Schottky contact is its barrier height (BH), that is, the energy separation between the Fermi level and the edge of the majority carrier band right at the interface [1–9]. The current transport across nearly ideal Schottky contacts fabricated the moderately doped semiconductors occurs by thermionic emission over the Schottky barrier [5–12]. The I–V characteristics of real Schottky contacts are described by two fitting parameters that are the effective barrier height Feff and the ideality factor, n [1–12]. The barrier height is likely to be a function of the interface atomic structure, and the atomic inhomogeneities at metal–semiconductor (MS) interface which are caused by grain boundaries,

* Corresponding author. Tel.: þ90 488 213 2782, fax: þ90 488 215 7201. ¨ . Gu¨llu¨). E-mail address: [email protected] (O 0042-207X/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2009.06.050

multiple phases, facets, defects, a mixture of different phases, etc.[13–22]. Song et al. [13] have also suggested that the barrier inhomogeneities can occur as a result of inhomogeneities in the interfacial oxide layer composition, non-uniformity of the interfacial charges and interfacial oxide layer thickness. In such cases, the current across the MS contact may be greatly influenced by the presence of the BH inhomogeneity [7–16]. Due to lateral inhomogeneity of the Feff, both parameters differ from one diode to another even if they were fabricated under experimentally identical conditions. However, their variations are correlated in that the Feff becomes smaller with increasing ideality factor. Extrapolation of such Feff versus ideality factor plot to the corresponding image force-controlled ideality factor nif gives the barrier heights of laterally homogeneous contacts [23–29]. In a metal/semiconductor system, the Fermi level within the semiconductor band gap determines the formation of the BH. In general, the exact position of Fermi level depends on the details of the interface [1–11]. The BH of metal/semiconductor contacts is almost independent on the metal work function. Such a behavior indicates that some mechanisms must be responsible for the Fermi level pinning, such as metal-induced gap states (MIGS) and defect

S. Asubay et al. / Vacuum 83 (2009) 1470–1474

1471

models [8–20]. In the ideal Schottky diodes (defect free interface), charge-neutrality level (CNL) of the MIGS may be considered to be the physical mechanism, which determines the barrier heights [1– 11]. These states originate from the tails of the metal wave functions into the semiconductor. No charge will be transferred across the interface when the Fermi level coincides with the CNL of the MIGS. In this case, the measured barrier height is called the zerocharge transfer BHs. This model does not account for any chemical reactions or imperfections at the interface. Fabrication-induced interface defects can exist in addition to the MIGS and alter the BH [1–3,8,9]. The defects give rise to additional discrete levels in the band gap and the Fermi level is pinned to one of these levels, possibly quite far away from the CNL. The BHs of laterally homogeneous contacts that determined by extrapolations of the effective BH versus ideality factor can be compared with the BHs predicted by the MIGS for ideal Schottky contacts [1–3,8,9]. The shift of the bind charge towards the more electronegative atom at MS interfaces corresponds with the net charge in the continuum of MIGS [1–3,8,9]. The MIGS and electronegativity concept combine the chemical and the physical concept and describe the chemical trend of the charge transfer at MS interfaces by the difference of the metal and the semiconductor electronegativities [1–3,8,9]. Our main goal is to determine laterally homogeneous BH of a number of metal/p-type InP (Cu, Au, Al, Sn, Pb, Ti, Zn) Schottky barrier diodes (SBDs) by help of the linear relationship between effective BHs and ideality factors which is experimentally [8,9,25– 32] and theoretically [20,21,27–29] confirmed. In addition, it is to show whether or not the experimental data of the laterally homogeneous barrier height for the metal/p-InP contacts verify the predictions of the MIGS theory. As is well-known, the homogeneous or uniform of the Schottky BH is an issue with important implications on the theory of Schottky BH formation and important ramifications for the operation of Schottky barrier diodes and contacts [20,21,27–29]. Moreover, Mo¨nch [8,9] mentioned that the homogeneous BHs rather than effective BHs of individual contacts or mean values should be used to discuss theories on the physical mechanism that determine the BHs of the MS contacts. Thus, provided the semiconductor substrate is well characterized then the homogeneous Schottky BH may be obtained even from the I–V characteristics of one contact [8,9,27–36].

the back side of the p-type InP is formed by sequentially evaporating Zn and Au layers on InP in a vacuum-coating unit of 106 Torr. Then, low resistance ohmic contact was formed by thermal annealing at 350  C for 3 min in flowing N2 in a quartz tube furnace. The Schottky contacts (Cu, Au, Al, Sn, Pb, Ti, Zn) have been formed by evaporating metals as dots with diameter of about 1 mm on the front surface of the p-InP. Cu metal is very diffusive metal. Therefore, we firstly deposited the other metals (Au, Al, Sn, Pb, Ti, Zn) to InP substrates. Cu metal was lastly deposited to InP wafer in the same chamber to get ride of contaminations during metallization. The numbers of dots tested for each metal in this study including Cu, Au, Al, Sn, Pb, Ti and Zn metals were 30,31,23,16,9,33 and 15, respectively. The current–voltage (I–V) characteristics were measured using a Keithley 487 Picoammeter/Voltage Source under dark conditions.

2. Experimental procedure

We have fabricated a large number of metal/p-type InP (Cu, Au, Al, Sn, Pb, Ti, Zn) Schottky contacts by thermal evaporation technique. The barrier height (BH) and ideality factor n values for the metals/p-InP/Zn–Au SBDs were obtained from their current– voltage characteristics (not given here) at 296 K (the room temperature) using Eqs. (2) and (3). The BH and ideality factor n values for the Cu/p-InP SBDs have varied from Flow ¼ 0.51 eV to Fhigh ¼ 0.70 eV, and ideality factor n from nlow ¼ 1.20 to nhigh ¼ 1.46. As can be seen, the experimental effective BHs and ideality factors obtained from the current–voltage (I–V) characteristics can differ from diode to diode even if they were identically prepared on the same sample. The statistical analysis yields the mean effective BH, CFbD ¼ 0.65  0.06 eV and the mean ideality factor, CnD ¼ 1.23  0.07 for the Cu/p-InP SBDs. The BH and n values for the Au/p-InP SBDs have varied from Flow ¼ 0.58 eV to Fhigh ¼ 0.72 eV, and ideality factor n from ¼ 1.14 to nhigh ¼ 1.47. CFbD ¼ 0.64  0.03 eV and CnD ¼ 1.25  0.07 values for the Au/p-InP SBDs were obtained from the statistical analysis. The parameters obtained for all of the metal/ p-type InP Schottky diodes fabricated by us are given in Table 1. It has been drawn the experimental effective BHs versus ideality factors plots for the metals/p-InP SBDs. These plots are given in Figs. 1–8. The effective BHs and ideality factors values were obtained from the experimental forward bias I–V characteristics. Figs. 1–8 reveal a pronounced correlation between the experimental

The samples have been prepared using cleaned and polished pInP (as received from the manufacturer) with (100) orientation and 4–8  1017 cm3 carrier concentration given by the manufacturer. We obtained a carrier concentration value of 6.0  1017 cm3 from the C2–V characteristics at room temperature [30]. Before making contacts, the p-InP wafer was dipped in 5H2SO4 þ H2O2 þ H2O solution for 1.0 min to remove surface damage layer and undesirable impurities and then in H2O þ HCl solution and then followed by a rinse in de-ionized water of 18 MU. Our cleaning procedure was frequently used in literature by many researchers [31–34]. For example, Shi et al.[31], Cetin et al. [32], Soylu et al.[33] and Contour et al. [34] have recently used this cleaning method. The effect of chemical etching by H2S04 þ H202 þ H20 mixture of InP substrates produced by molecular beam epitaxy has already been studied using X-ray photoelectron spectroscopy (XPS) by Contour et al.[34]. The final rinse in running de-ionized water does not produce any passivating oxide layer on the substrate surface. Oxidation observed on InP after these cleaning procedures occurs only during substrate handling in air [34]. H2O þ HCl solution was used to remove the oxide layer on InP surfaces. The wafer has been dried with high-purity nitrogen and inserted into the deposition chamber immediately after the etching process. Ohmic contacts on

3. Results and discussion When a nearly ideal SBDs is considered, it is assumed that the forward bias current of the device is due to thermionic emission current and considering the voltage dependence of the barrier height it can be expressed as [2,3]



I ¼ I0 exp

qV nkT



  qV 1  exp  ; kT

(1)

where

  qFeff ; I0 ¼ AA* T 2 exp  kT

(2)

is the saturation current density, Feff is the zero bias effective barrier height, A* is the effective Richardson constant and equals to 60 A/cm2K2 for p-type InP [2]; A is the diode area, n is an ideality factor and is a measure of conformity of the diode to pure thermionic emission. It is obtained from the slope of the straight line region of the forward bias ln I–V characteristics through the relation

n ¼

q dV : kT dðln IÞ

(3)

1472

S. Asubay et al. / Vacuum 83 (2009) 1470–1474

Table 1 The parameters obtained for all of the metal/p-type InP Schottky diodes fabricated by us.

Cu/p-InP Au/p-InP Al/p-InP Sn/p-InP Pb/p-InP Ti/p-InP Zn/p-InP

Flow

Fhigh

(eV)

(eV)

0.51 0.58 0.82 0.64 0.69 0.69 0.70

0.70 0.72 0.84 0.75 0.77 0.92 0.80

nlow 1.20 1.14 1.11 1.26 1.32 1.12 1.32

nhigh 1.46 1.47 1.18 1.62 1.53 1.96 1.56

CFbD (eV)

CnD

0.65  0.06 0.64  0.03 0.83  0.01 0.71  0.04 0.73  0.03 0.85  0.07 0.55  0.03

1.23  0.07 1.25  0.07 1.13  0.02 1.42  0.10 1.42  0.07 1.36  0.25 1.44  0.07

Fhom b0 (eV)

XM (eV) Ref. [3]

0.89 0.78 0.91 0.86 0.91 0.96 0.92

4.55 5.15 4.20 4.15 4.10 3.65 4.10

Barrier Height (eV)

Diodes

0.80 Au/p-InP

0.72

0.64

0.56 Φhom = 0.78 eV

0.48 effective BHs and ideality factors in that the effective BHs become smaller with increasing ideality factors. That is, there is a linear relationship between experimental effective BHs and ideality factors of Schottky contacts. This finding may be attributed to lateral barrier inhomogeneities of Schottky diodes [17–30,35–37]. It has been mentioned that higher ideality factors among identically prepared diodes were often found to accompany lower BHs. Already Ohdomari et al. [38], Chin et al. [39] Werner et al. [40] noted the correlation between effective BHs and ideality factors but only Schmitsdorf et al. [27] realized its significance by attributing to lateral barrier inhomogeneities of Schottky diodes. The ideality factors greater than unity are attributed to secondary mechanisms at the interface [1–9]. Ideality factors between 1.01 and 1.03 can be expected due to image force lowering of the Schottky barrier at the interface. By considering the image force lowering of the Schottky barrier height, we have obtained ideality factor value of about 1.03 for the metals/ p-InP SBDs that we have formed from p-InP semiconductor with free carrier concentration of 6.0  1017 cm3 at room temperature. The barrier inhomogeneities play an important role in determination of the Schottky diode parameters and therefore it has to be considered in the evaluation of experimental I–V characteristics. The high values of n can be attributed to the presence of a wide distribution of low-SBH patches caused by laterally barrier inhomogeneous. The laterally homogeneous BH values for the p-InP SBDs were obtained from the extrapolation of Feff versus ideality factor plot to the corresponding image force-controlled ideality factor niff, as can be seen in Figs. 1–7. Owing to the image force lowering, the experimental results show that there are a decrease in the effective I–V barrier height and an increase in the ideality factor for diodes formed on the more heavily doped samples [2,3].

1.10

1.20

1.30 Ideality Factor, n

The linear extrapolation of the experimentally observed effective BHs versus ideality factors curves to niff indeed gives the image hom  DFoif , of the laterally homogeforce lowered BHs,Fnif B ¼ FB o neous contacts, where DFiff is the image force lowering [27–30]. By considering the image force lowering of the Schottky barrier, we have obtained ideality factor values between niff ¼ 1.02–1.03 and barrier height difference values between DFoiff ¼ 0:05  0:06 eV for the metal/p-InP contacts (Nd ¼ 6.0  1017 cm3) using Eqs. (1.26a) and (3.14) in Ref.[1] or Eqs. (19.6) and (19.9) in Ref. [3]. The obtained experimental values for the metal/p-InP contacts are given in Table 1. Again, it has been attributed in Refs. [3,8,9] that the formation of the barrier heights to their chemical trends, i.e., to the differences in electronegativity between semiconductor and metal. In generalizing, the above concept, the transfer of charges at metal–semiconductor contacts will be characterized by the electronegativity difference of the metal and semiconductor in the contact. The combination of the physical MIGS and the chemical electronegativity concept yields the barrier height of ideal p-type Schottky contacts as [3,41]

Fhom ¼ Fbp  SX ðXm  Xs Þ; bp

(4)

where Xm and Xs are the electronegativities of the metal and semiconductor, respectively, and the value of Fbp ¼ 0.86 eV [41] is the zero-charge transfer barrier height or the CNL of the MIGS with

0.86 Al/p-InP/Zn-Au y = -0.20 x + 1.06

Cu/p-InP/Zn-Au y = -0.73 x + 1.58

0.75

Barrier height (eV)

Barrier Height (eV)

1.50

Fig. 2. Experimental barrier height versus ideality factor plot of the Au/p-InP Schottky barrier diodes at room temperature.

0.90

0.60

0.45

0.84

0.82 Φhom = 0.91 eV

Φhom = 0.89 eV

0.30 1.15

1.40

1.20

1.25

1.30 1.35 1.40 Ideality Factor

1.45

1.50

Fig. 1. Experimental barrier height versus ideality factor plot of the Cu/p-InP Schottky barrier diodes at room temperature.

0.80 1.100

1.125

1.150

1.175

1.200

Ideality factor Fig. 3. The experimental barrier height versus ideality factor plot of the Al/p-InP Schottky barrier diodes at room temperature.

S. Asubay et al. / Vacuum 83 (2009) 1470–1474

0.85

0.90 Zn/p-InP/Zn-Au y = -0.28 x + 1.15

Sn/p-InP

Barrier height (eV)

Barrier Height (eV)

0.80 0.75 0.70 0.65 0.60 0.55 1.20

Φhom = 0.86 eV

1.30

1.40

1.50

1.60

0.80

0.70

Φhom= 0.92 eV 0.60 1.30

1.70

Fig. 4. The experimental barrier height versus ideality factor plot of the Sn/p-InP Schottky barrier diodes at room temperature.

0.90

Barrier Height (eV)

Pb/p-InP/Zn-Au y = -0.30 x + 1.16 0.80

0.70

1.30

Φhom = 0.91 eV

1.35

1.40

1.45

1.35

1.40

1.45

1.50

1.55

1.60

Ideality Factor

Ideality Factor

0.60

1473

1.50

1.55

Ideality Factor Fig. 5. The experimental barrier height versus ideality factor plot of the Pb/p-InP Schottky barrier diodes at room temperature.

Fig. 7. The experimental barrier height versus ideality factor plot of the Zn/p-InP Schottky barrier diodes at room temperature.

respect to the valance-band at the surface of the semiconductor and SX ¼ 1.02 eV/(Miedema unit) [3,41] is the slope parameter of the MIGS line in the laterally homogeneous barrier heights versus electronegativity difference plot for n-InP. The electronegativity values for the metals are given in Table 1. The electronegativity value for InP is taken as 4.46 eV [41]. Fig. 8 displays the laterally homogeneous barrier heights of metals/p-InP Schottky contacts as a function of the difference of the metal and InP electronegativities. The parameters used in the figure are given in Table 1. The dashed lines in Fig. 8 are the linear least-squares fit to the experimental data. Thus, the experimental data of the laterally homogeneous barrier height for the metal/p-InP contacts are close to the MIGS line obtained for the metal/p-InP contacts using Eq. (4) and quantitatively verify the predictions of the MIGS theory. As can be seen from Fig. 8, the same close agreement between the linear leastsquares fit to the experimental data and theoretical line is also found. Deviations from the line predicted by Eq. (4) may be attributed to high fabrication-induced interface defects of donor type [3,8,9,41–43]. This result obtained for the metal/p-InP contacts exactly is agreement with that given by Mo¨nch [41]. In conclusion, from the experimental effective BHs versus ideality factors plots of the metals/p-InP SBDs given in Figs. 1–8, it has been seen that, even if Schottky contacts were identically

1.10

Barrier Height (eV)

1.00

Schottky Barrier Height (eV)

1.10

Ti/p-InP

0.90 0.80 0.70 0.60 0.50 1.00

Φhom= 0.96 eV

1.20

1.40 1.60 Ideality Factor

1.00

0.90

0.80

0.70

0.60 -1.00 1.80

MIGS theory

experimental data

0.00 0.50 -0.50 Electronegativity Difference (eV)

1.00

2.00

Fig. 6. The experimental barrier height versus ideality factor plot of the Ti/p-InP Schottky barrier diodes at room temperature.

Fig. 8. Laterally homogeneous barrier heights of metals/p-InP Schottky contacts as a function of the difference of the metal and InP electronegativities. The dashed lines are the linear least-squares fit to the experimental data. The solid MIGS line is drawn using Eq. (4) for the metal/p-InP contacts.

1474

S. Asubay et al. / Vacuum 83 (2009) 1470–1474

prepared on the same sample, the effective BHs decrease with increasing ideality factors which may be attributed to lateral barrier inhomogeneities of Schottky diodes, and that there is a linear relationship between effective BHs and ideality factors. Thereby, we obtained the laterally homogeneous BH values of the p-InP SBDs fabricated by us from this correlation between effective BHs and ideality factors using the method by Schmitsdorf et al. [27] It has been concluded that the MIGS line excellently reproduces the experimental results of the laterally homogeneous barrier height of the metal/p-type InP (Cu, Au, Al, Sn, Pb, Ti, Zn) Schottky contacts. References [1] Rhoderick EH, Williams RH. Metal–semiconductor contacts, Clarendon Press, Oxford. [2] Williams RH, Robinson GY. Physics and chemistry of III–V compound semiconductor interfaces. In: Wilmsen CW, editor. New York: Plenum Press; 1985. [3] Mo¨nch W. Semiconductor surfaces and interfaces. 3rd ed. Berlin: Springer; 2001. [4] Potje-Kamloth K. Chem Rev 2008;108:367. [5] Bayhan H, Ercelebi C. Semicond Sci Technol 1997;12:600–8. [6] Behnam A, Johnson J, Choi Y, Noriega L, Ertosun MG, Wu Z, et al. J Appl Phys 2008;103:114315. [7] Dimitruk NL, Borkovskaya OY, Dimitruk IN, Mamykin SV, Horvath Zs J, Mamontova IB. Appl Surf Sci 2002;190:455. [8] Mo¨nch W. J Vac Sci Technol B 1999;17:1867. [9] Kampen TU, Mo¨nch W. Surf Sci 1995;333:490. [10] Chand S, Bala S. Phys B 2007;390:179. [11] Dobrocka E, Osvald J. Appl Phys Lett 1994;65:575; Osvald J. Solid-State Electron 2006;50:228. [12] Okutan M, Basaran E, Yakuphanoglu F. Appl. Surf. Sci. 2005;252(5):1966. [13] Song YP, Van Meirhaeghe RL, Laflere WH, Cardon F. Solid-State Electron 1986;29:633.

[14] Werner JH, Gu¨ttler HH. J Appl Phys 1991;69:1522. [15] Hamida AF, Z Ouennoughi, Sellai A, Weiss R, Ryssel H. Semicond Sci Technol 2008;23:045005. [16] Mamor M, Sellai A, Bouziane K, Al Harthi SH, Al Busaidi M, Gard FS. J Phys D Appl Phys 2007;40:1351. [17] Kiziroglou ME, Zhukov AA, Lia X, Gonzalez DC, de Groot PAJ, Bartlett PN, et al. Solid State Commun 2006;140:508. [18] Osvald J, Horvath ZJ. Appl Surf Sci 2004;234(1–4):349–54. [19] Lucolano F, Roccaforte F, Giannazzo F, Raineri V. J Appl Phys 2007;102:113701. [20] Tung RT. Phys Rev B 1992;45:13509. [21] Sullivan JP, Tung RT, Pinto MR, Graham WR. J. Appl. Phys. 1991;70:7403. [22] Chand S, Kumar J. J Appl Phys 1997;82:5005. [23] Kim SW, Lee KM, Lee JH, Seo KS. IEEE Electron Device Lett 2005;26(11):787. [24] Cetin H, Ayyildiz E. Semicond Sci Technol 2005;20:625. [25] Im HJ, Ding Y, Pelz JP, Choyke WJ. Phys Rev B 2001;64(7):075310. [26] Ayyildiz E, Cetin H, Horvath Zs J. Appl Surf Sci 2005;252:1153. [27] Schmitsdorf RF, Kampen TU, Mo¨nch W. J Vac Sci Technol B 1997;15:1221. [28] Akkilic K, Aydin ME, Turut A. Phys Scrip 2004;70:364. [29] Leroy WP, Opsomer K, Forment S, Van Meirhaeghe RL. Solid-State Electron 2005;49:878. [30] Asubay S, Gullu O, Turut A. Appl Surf Sci 2008;254(11):3558. [31] Shi ZQ, Anderson WA. Indium phosphide and related materials. In: Third International Conference; 1991. p. 535–538. doi: 10.1109/ICIPRM.1991.147431. [32] Çetin H, Ayyildiz E. Appl Surf Sci 2007;253(14):5961. [33] Soylu M, Abay B. Microelectronic Eng 2009;86(1):88. [34] Contour JP, Massies J, Saletes A. Jpn J Appl Phys 1985;24(7):L563. [35] Duman S, Dogan S, Gurbulak B, Turut A. Appl Phys A 2008;91:337. [36] Ivaneo J, Horvath Zs J, van Tuyen V, Coluzza C, Almeida J, Terrasi A, et al. Solid State Electron 1998;42:229. [37] Sefaoglu A, Duman S, Dogan S, Gurbulak B, Tuzemen S, Turut A. Microelectronic Eng 2008;85(3):631. [38] Ohdomari I, Tu KN, d’Heurle FM, Kuan TS, Petersson S. Appl Phys Lett 1978;33:1028. [39] Chin VWL, Storey JWV, Gren MA. Solid-State Electron 1989;32:475. [40] Werner P, Jager W, Schuppen A. J Appl Phys 1993;74:3846. [41] Mo¨nch W. Appl Phys Lett 2008;93:172118. [42] Akkilic K, Turut A, Cankaya G, Kilicoglu T. Solid State Commun 2003;125:551. [43] Akkilic K, Kilicoglu T, Turut A. Phys B 2003;337:388.