AC CONDUCTIVITY AND DIELECTRIC PROPERTIES OF Al 2 O 3 THIN FILMS

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1 Jurnal f Ovnic Research Vl. 8, N. 6, Nvember December 0, p AC CONDUCTIVITY AND DIELECTRIC PROPERTIES OF Al O 3 THIN FILMS D. DEĞER *, K. ULUTAŞ, Ş. YAKUT İstanbul University, Science Faculty, Physics Department, Vezneciler, Istanbul- TURKEY. Al O 3 thin films f different thicknesses were prepared nt clean glass substrates using hmic aluminum electrdes. Their dielectric prperties and ac cnductivity have been investigated in the frequency range 0.-00KHz and within the temperature range K. Oxide-layer thicknesses f the films range between Å. The dielectric cnstant was fund t decrease with increasing frequency and increase with temperature in the given intervals. Only ac lsses have been investigated due t the smallness f dc lsses. The ac cnductivity satisfies the pwer law s. Here the s parameter is in the vicinity f 0.8 and it decreases with increasing temperature. This behaviur f s can cmply with CBH mdel. The activatin energy values calculated frm ac cnductivity and dielectric lss factr measurements are in gd agreement with each ther. The btained values agree with the mdel f hpping f charge carriers by thermal activatin between tw sites having a culmbic ptential well. Film thickness dependence f Temperature Cefficient f Capacitance (TCC) and Temperature Cefficient f Permittivity (TCP) f the Al O 3 thin films were als determined. (Received Nvember 3, 0; Accepted December 4, 0) Keywrds: Al O 3; B. cnductivity; Dielectric cnstant; Dielectric lss; Activatin energy; Thin films. Intrductin There are many researches abut dielectric prperties f different thin films because f their electrnic and ptical usage [-5]. Al O 3 films are frequently used fr many purpses such as refractry catings, antireflectin catings, anticrrsive catings, micrelectrnic devices, capacitance humidity sensrs and passivatin f metal surfaces particularly in field effect transistrs [6-]. Therefre, much wrk has been reprted n the dielectric prperties f Al O 3 films, but especially studies f the dielectric prperties f these films are rather rare at lw frequencies. On the ther hand, determinatin f the ac cnductivity mechanism f materials is very imprtant frm the pint f their usage in technlgy. Thus varius mdels have been prpsed t make clear the cnductin mechanisms in amrphus semicnductrs. These mdels are Quantum-Mechanical Tunnelling (QMT) mdel, small plarn tunnelling mdel, large plarn tunnelling mdel, atmic hpping mdel, and Crrelated Barrier Hpping (CBH) mdel [-5]. In ur previus wrk n Al O 3 films, ur aim was t determine the pssible plarizatin mechanisms []. In this wrk, we are determining the dielectric prperties and the frequency and temperature dependence f ac cnductivity f Al O 3 films, prepared by andic xidatin methd and cnfrnt the results with ac cnductivity mdels in amrphus semicnductrs. * Crrespnding authr: deger@istanbul.edu.tr

2 80. Experimental techniques A. Preparatin f the samples: Aluminum base electrdes f thickness abut 3000 A 5 were evaprated nt micrscpe slides at a pressure f apprximately 0 Trr. The aluminum films were prepared by evaprating 99.99% pure Al. These evaprated aluminum films were partly andized at rm temperature in a slutin f 3% tartaric acid made up t ph5.5 with NH 4 OH. After andizatin, they were cleaned in distilled water and left fr drying in a desiccatr fr abut 4h. The xide film thickness was calculated frm the andizing rati (3.5 A V ) []. Accrding t the applied vltage xide-layer thickness in the range f A are btained. The Al cunter electrdes were evaprated nt these andized films at the same evapratin cnditin realized fr the base electrdes. Hence Al/Al O 3 /Al capacitrs are frmed. B. Experimental setup: A Gen-Rad 65-A Scheering Bridge, a Gen-Rad 3 Null Detectr, and a Gd Will GFG-806D Generatr were used fr the capacitance and dissipatinfactr measurements. These measurements were made in apprximately 0-4 Trr vacuum. Fr each sample capacitr, we reversed the cupling and repeated the measurements twice; we chse thse that gave the same results. The same experimental results were btained under the same cnditins. The areas f capacitrs ~36 mm were measured with a travelling micrscpe. Temperatures f the samples during the study were measured with a cpper-cnstantan thermcuple. 3. Results 3.. Ac cnductivity Ac cnductivity is related t the energy lss btained frm the plarizatin mechanism that is suitable fr the material and gives imprtant infrmatin abut the structure f the material. In all amrphus semicnductrs and in sme plymers, ac cnductivity, ac ( ), ttal cnductivity, ( ) and dc cnductivity, tt dc satisfy the fllwing frequency relatin [6,7]: s ( ) ( ) A, () ac tt dc where is the angular frequency, s is the frequency expnent and A is a cnstant independent f frequency. Variatin f ac cnductivity f the 550A film with frequency at varius temperatures is shwn in Fig.. In the wrked thickness interval we btained the same behaviur. Accrding t this figure ac ( ) increases linearly with frequency and frm the slpe f these lines the frequency expnent is determined. It is knwn that the value f this expnent yields the type f cnductivity mechanism. As an inset we present the temperature variatin f s. Fr all wrked thicknesses s decrease as temperature increases.

3 8-6 00K 30K 300K -7 lg ac ( ) S T(K) lg Fig.: Frequency dependence f ( ) ac at varius temperatures. The inset figure shws the temperature dependence f s fr 550Å thick Al O 3 film. Several cnductivity mechanisms have been prpsed fr amrphus semicnductrs. Accrding t the QMT mdel s is abut 0.8 and it either increases rather slwly r stays the same as temperature increases. Our experimental results disagree with such a behavir hence the mechanism respnsible frm the cnductivity f Al O 3 cannt be QMT. OLPT cnductivity mechanism requires bth a frequency and temperature dependence f s. Accrding t this dependence, s starting frm at rm temperature, must first decrease and after passing a minimum again increase. It is clear that ur results d nt fllw such a behaviur. On the ther hand, fr CBH mdel, the behaviur f s ught t be just the behaviur that we deduced frm ur experimental results [8,9]. In this mdel s is given by 6kT s () B, Eg where kb is the Bltzmann cnstant, T is the temperature in Kelvin and E g is the ptical band gap f the material. Frm this equatin ne can btain the value f s at rm temperature by using the energy gap value.6 ev determined by Shiki et al. [0]. The thus calculated value (0.98) is in gd agreement with ur experimental value (0.93) within 5%. The Austin-Mtt frmula [] derived within the CBH mdel, gives [ NE ( F)] ph 4 ac( ) kte B [ln ] 5 3 (3) where NE ( F) is the density f states at the Fermi level, is the expnential decay parameter f lcalized states wave functins and ph is the phnn frequency. Assumptins invlved in this frmula have been discussed by Pllak []; the main ne f which is that hpping is between pairs f centres i.e., multiple-hpping prcesses can be neglected [3]. By using Hz and 0 A [4], the density f states have been calculated: the frequency dependence f ph 0 5 N( EF) 0 ev cm 3. Fig. shws NE ( F) at different temperatures. N( E F) increases with frequency and temperature as is seen frm Fig.. This result agrees well with the behaviur described by eq. (3) btained frm CBH mdel.

4 8 x K 40K 300K N(E)( ev - cm -3 ) f (Hz) Fig.: Frequency dependence f density f states at different temperatures fr Al O 3 film 450 A thick. Fig. 3 shws the temperature dependence f ( ) ac at different frequencies fr the 550 A thickness film. Similar results were determined in the investigated film thickness regin. It can be seen frm Fig. 3 that ln ( ) decreases linearly with decreasing temperature. This behaviur ac shws that there is a thermally supprted prcess between the lcalized states at the band gap f ac cnductivity r valance and cnductivity band tails. E e kt B 0 The abve frmula defines the relatin between the ac cnductivity and activatin energy. 3 0 Thus activatin energy is calculated frm the slpe f ln ac( ) versus curve. The T calculated value at different frequencies is 0.0 ± 9% ev within the experimental errr. -7 lg ac ( ) -8-9,3kHz khz 4kHz 0kHz 60kHz 00kHz /T (K - ) Fig.3: Temperature dependence f ( ) ac fr 550 A thick Al O 3 films at different frequencies.

5 On the ther hand, Fig. 4 shws the frequency dependence f activatin energy fr the studied films. ( ) decreases with increasing frequency and is independent f film thickness. E In ne f ur wrks, we prpsed that electrnic hpping mechanism was dminating at high frequencies. Therefre by increasing the frequency f the applied electric field we increase the hpping number in ne secnd [5, 6]. This means that thermal activatin energy belnging t hpping between tw lcal sites decreases with increasing frequency. 83 0,030 0,05 (ev) 0,00 0,05 0, f (khz) Fig.4: Frequency dependence f ΔE σ (ω) fr Al O 3 thin films f thickness 450 A. 3.. Dielectric cnstant Our specimens are in the frm f parallel plate capacitrs. Therefre we calculated the dielectric cnstant frm the usual parallel plate capacitr equatin Cd, A 0 where C is the capacitance, d is the xide layer thickness, 0 is the free space permittivity and A is the area f the dielectric layer. Film thickness is an imprtant parameter affecting the dielectric prperties f the material under investigatin, particularly in thin films. The thickness dependence f the dielectric cnstant at different frequencies and different temperatures in the Al O 3 thin films in the thickness range f A are shwn in Figs. 5 and 6. is almst independent f thickness and has a cnstant value f abut 9 fr the films whse thicknesses excess 500 A. Twards lwer thicknesses, is directly prprtinal with thickness. The effective thickness f insulatrs decreases with increasing density f vid [7, 8]. The decrease in can be explained by saying that the effective medium decreases as thickness decreases [9]. The frequency dependence f at different temperatures fr the film f thickness 70 A is shwn as a representative example in Fig.7. Frm this figure, it is clear that decreases as frequency increases but increases with increasing temperature.

6 Hz khz 8kHz d (A) Fig.5: Thickness dependence f fr Al O 3 thin films at different frequencies at 300 K K 330K 370K d ( A ) Fig.6: Thickness dependence f fr Al O 3 thin films at different temperatures at 400 Hz K 340K 370K f(hz) Fig.7: Frequency dependence f the fr Al O 3 thin films f thickness 70 A at different temperatures.

7 In a material placed in an electric field, several different plarizatin mechanisms arise, such as electrnic, inic, diplar and interfacial plarizatins. The ttal plarizatin is due t the sum f these nes. Each mechanism may dminate in different frequency regins. In ne f ur wrks [], we prpsed that the inic mechanism is dminant at lw frequency regin. When the frequency is increased the relaxatin time decrease and diplar mechanism lst its effect since it necessitates lnger time with reference t electrnic and inic nes. Thus the ttal plarizatin ges dwn and this causes t decrease with increasing frequency. This decreases the value f dielectric cnstant with frequency appraching a cnstant value at higher frequency crrespnding nly t interfacial plarizatin [6, 30]. When the temperature is increased, the structure relaxes and the rientatin f the plarized units becme easier and hence the value f rientatinal plarizatin increases. Dielectric cnstant increases as expected [3]. We calculated bth the thickness dependence f the Temperature Cefficient f Capacitance (TCC) and Temperature Cefficient f Permittivity (TCP) f Al O 3 thin films using the fllwing expressins, respectively 85 dc TCC C dt s d TCP = dt The calcuated values are given in Table as a functin f the film thickness at KHz frequency and 300K temperature Dielectric lss As is seen frm Fig. 8, increases with temperature at all frequencies [30, 3]. This behaviur f can be reasned as fllws: The behaviur f charge carriers under the effect f cnstant and variable electric fields yields energy lst. Energy lst due t dc cnductivity under the effect f cnstant electric field increases with temperature. This energy lst culd be thught t dc 9 be calculated by. As an example, dc.7 0 m fr the 000 A Al O 3 thin 0 film, at rm temperature and at KHz frequency; the energy lst fr dc cnductivity has been calculated t be But this value is smaller frm the experimentally determined ne. Hence the reasn fr this energy lst cannt be the dc cnductivity [30]. Therefre the energy lst is related t the dielectric relaxatin appearing due t alternating electric field [3]. In rder t understand the reasn f the energy lst under alternating electric field ne has t study the dielectric relaxatin K 00K 300K 340K f (Hz) Fig.8: Frequency dependence f fr Al O 3 thin film f thickness 80 A at different temperatures.

riginated as the result f interactin between electric

This means that gets ac higher with temperature [3-33].

temperature t values is shwn fr 450 A thick Al O 3 film

It can be b seen frmm this figure that as frequency

8 lnf min /T (K - ) Fig.9: The plt f ln f min versus 000/T. Table : The thickness dependence f TCCC and TCP at khz and 300K. The lsses related t relaxatin are the heat energy riginated as the result f interactin between electric field inducedd plarizing charge carriers with lattice and is directly prprtinal with ( ). With ac the increase f temperature the structure relaxes and the plarizatin gets easier. Hence ( ) increases and ac cnductivity lsses increase. This means that gets ac higher with temperature [3-33]. The frequency dependence f dielectric lss at varius temperature t values is shwn fr 450 A thick Al O 3 film in Fig. 8 as a typical example. It can be b seen frmm this figure that as frequency increases decreases and reaches a minimum and thereafter increases. As temperature ges higher the frequency at which reaches a minimum shifts twards higher frequencies. The activatin energy fr the bserved relaxatin prcess was evaluatedd frm f E e kbt min f0,

9 where f min is the frequency at which is minimum and f versus min T 87 E is the activatin energy. The plt f is shwn in Fig. 9. Frm the slpe f the straight line the activatin energy evaluated as 0.09eV. The activatin energy values calculated frm ac cnductivity and dielectric lss factr measurements are in gd agreement with each ther. Furthermre, these values agree with the mdel f hpping f charge carriers by thermal activatin between tw sites having a culmbic ptential well [9, 34]. 4. Cnclusin Amrphus thin films f Al O 3 in the thickness range A were prepared by andic xidatin technique. The dielectric cnstant was fund t decrease with increasing frequency s but increase with temperature in the given intervals. The ac cnductivity beys the law with a temperature dependence s (s < ) which ges dwn as temperature increases. The temperature dependence f s is attributed t the crrelated barrier hpping mdel. The activatin energy values calculated frm the ac cnductivity and dielectric lss factr measurements are in gd agreement with each ther. The btained values agree with the hpping mdel f charge carriers by thermal activatin between tw sites having a culmbic ptential well. The film thickness dependence f TCC and TCP f the Al O 3 thin films have als been determined. Acknwledgment This wrk was supprted by the Research Fund f The University f Istanbul, Prject N: References [] J. Van der Geer, J.A.J. Hanraads, R.A. Luptn, J. Sci. Cmmun. 63, 5 (00). [] Deger D., Ulutas K., J. App. Phys 8, 553 (000) [3] Tepehan F. Z., Ghdsi F.E., Ozer N., Tepehan G.G., Slar Energy Materials & Slar Cells 68, 355 (00) [4] Lancaster M.C. J.Phys.D.: Apply. Phys. 5, 33 (97) [5] Seyam M.A.M., Appl. Surface Sci. 8, 8 (00). [6] Tmbs NC, Wegener HA, Newman R, Kenny BT, Cppla AJ. Prc. IEEE 55, 68 (967) [7] Hashimt S, Peng JL, Gibsn WM. Appl. Phys. Lett. 47, 07 (985). [8] Zhang X-H, Dmercq B, Wang X, Y S, Knd T, Wang ZL, Kippelen B. Organic Electrnics 8, 78 (007) [9] Yan D, He J, Li X, Jianxin L, Huili Ding Z. Surf. Cat. Technl. 4, (00). [0] Chang Y-S, Ry N. J. Vac. Sci. Technl. 7, 303 (989). [] Nahar RK, Khanna VK, Sens. Actuatrs Int. J. Electrn. 5, 557 (98) [] M.K. Fayek, M.F. Mstafa, F. Sayedahmed, S.S. Ata-Allah, M. Kaiser. J. Magn. Magn. Mater., 0, 89 (000) [3] Y.C. Chung, H.I. Y, J. Mater. Res.6, 774 (00) [4] A.M.A.E. Ata, S.M. Attia, T.M. Meaz, Slid State Sci. 6, 6 (004). [5] S.A. Mansur, I.S. Yahia, G.B. Sakr, Slid State Cmmun. 50, 386 (00). [6] Jnscher AK. Nature 67, 673 (977). [7] Durand B, Taillades G, Pradel A, Ribes M, Badt JC, Belhadj-Taher N. J Nn- Cryst Slids 7-74, 306 (994). [8] Ellitt SR. Phils Mag. 36, 9 (977). [9] Ellitt SR. Phils Mag. B 37, 35 (978). [0] Shiiki K, Igarashi M, Kaijy U. J.Appl.Phys 4, 585 (003).

10 88 [] Austin LG, Mtt NF, Adv. Phys, 8, 4 (969) [] Pllak M. Phils Mag 3, 59 (97). [3] Mtt NF and Davis EA Electrnic prcesses in nn-crystalline materials, Clarendn press, Oxfrd,97. [4] Agarwal SC, Guha S, Narasimhan KL. J Nn-Cryst Slids 8, 49 (975). [5] Angell C. Ann Rev Phys Chem 43, 693 (99). [6] M.A.Afifi, A.E.Bekheet, E.Abd Elwahhab, H.E.Atyia; Vacuum 6, 9 (00). [7] Saxena U, Srivastava UN, Thin Slid Films 33, 85 (976). [8 ] Deger D, Ulutas K. Vacuum 7, 307 (004) [9 ] Tareev B. Physics f dielectric materials. Mscw: Mir Publishers, 979, p.07 [30] Stevels JM. The electrical prperties f glasess. Hantbuch der Physik.957, p. 350 [3] S.Yıldırım, K. Ulutaş, D. Değer, E. O. Zayim ve I. Turhan; Vacuum, 77, 39 (005). [3] Hegab N.A., Bekheet A.E., Afifi M.A., Wahab L.A., Shehata H.A., J Ovnic Research; 3, 7 (007). [33] A.M. Farid, A.E.Bekheet; Vacuum 59, 93 (000). [34] Shimakawa K. Phils Mag. B 46, 3 (98).

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