STRUCTURE AND ELECTRICAL PROPERTIES OF ELECTRON IRRADIATED CdSe THIN FILMS

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1 Journal of Optoeletronis an Avane Materials ol. 6, o. 1, Marh 24, p STRUCTURE AD ELECTRICAL PROPERTIES OF ELECTRO IRRADIATED C THI FILMS L. Ion a*, S. Antohe a, M. Popesu b, F. Sarlat, F. Sava b, F. Ionesu b a University of uharest, Faulty of Physis, M gurele-ilfov, PO MG-11, Romania b ICDFM uharest, M gurele-ilfov, Romania alahia University, Târgovi te, Romania Strutural an eletrial properties of polyrystalline C thin films irraiate with highenergy eletrons are analyze. The samples were prepare by vauum eposition onto optial glass substrate. Their struture an the temperature epenene of the eletrial resistane were etermine, both before an after irraiation with 6 Me eletrons an fluenies up to 1 16 eletrons/m 2. There were no measurable hanges in the rystalline struture of the films after irraiation. Eletrial properties are ontrolle by a efet level of onor type, possibly a selenium vaany, with two harge states having ionization energies of about.4 e, respetively.22 e. Irraiation inreases signifiantly the onentration of those efets. (Reeive February 28, 23; aepte after revision January 28, 24) Keywors: Polyrystalline thin films, C, Eletron irraiation 1. Introution The optial an eletrial properties, as well as its goo hemial an mehanial stability, reommens Camium lenie (C) as a semionutor well suite for optoeletroni appliations, suh as photo-etetion or solar energy onversion. It is a well known fat that the quality of the evies base on C thin films strongly epens on the strutural an eletroni properties of the films obtaine uner various experimental onitions. Many reports have been publishe on the preparation an eletrial properties of the vauum-eposite films of A II - I ompouns [1-6]. ut, espite this onsierable amount of work, only a few stuies on the influene of the ionizing raiations on their eletrial properties have been performe [7-11]. In this paper we present the results of a strutural an eletrial investigation on polyrystalline thin films of C subjete to high-energy eletron irraiation. The films were prepare by thermal vauumevaporation onto glass substrates. 2. Experimental proeures C thin films, 4 µm thik, were prepare by thermal vauum evaporation from a single soure onto an optial glass substrate. The pressure in the evaporation ell was kept below Torr uring the eposition of the films. The evaporator onsiste of a quartz ontainer heate to 75 C, the substrate being maintaine at 22 C uring the eposition. To improve the strutural an hemial homogeneity of the films, they were subsequently thermally treate in vauum at 3 C, for 1 min. Then four Al ontats were evaporate on C. The ontats, 1 mm 1 mm in surfae,.3 µm thik an separate by 1 mm, were plae in line. The obtaine strutures were subjete to irraiation with eletrons supplie by a betatron. The samples were irraiate at room temperature with 6 Me eletrons to a fluene of * Corresponing author: luian@soli.fizia.unibu.ro

2 114 L. Ion, S. Antohe, M. Popesu, F. Sarlat, F. Sava, Feliia Ionesu 1 15 eletrons/m 2, irraiation iretion being perpeniular to the surfae of the samples an the thermal effet uring irraiation was negligible. The struture of the samples was investigate, before an after irraiation, with a θ-2θ X-ray iffratometer, using Cu-K α (λ= Å) line. Line profiles were reore in a step-sanning regime with (2θ)=.5. The temperature epenene of the eletrial resistane an I- harateristis were reore by introuing the samples in a He lose yle ryostat an ontating the probes with soft Ag wire. During the measurements the pressure in ryostat was below 1-4 Torr. Eletrial properties were measure with a Keithley 24 soure-meter, in the temperature range allowe by our experimental setup. 3. Experimental results A. Struture Fig. 1 shows the experimental XRD pattern of two samples. It iniates the presene of wurtzite-type (hp) struture, whih is known to be the stable one for this material. The films are oriente with (1) rystalline iretion perpeniular to their surfae. The position of the peaks yiels hp lattie onstants of 4.296Å (a h ) an 7.3Å ( h ). While a h is onsistent with the reporte bulk value, h is somewhat larger. 2 I (a.u.) Sample hp (1); 2 - hp (2); 3 - hp (11); 4 - hp (12); 5 - hp (11); 6 - hp (13); 7 - hp (2); 8 - hp (112); 9 - hp (21); 1- hp (4); θ (eg) Fig. 1. Experimental X-ray iffration pattern of a C thin film sample. A simple inspetion of the iffration pattern shows that both broaene lines an sharp refletions are present. An explanation for this [12] has been given as ue to faulting. In the ase of rystals mae up of lose-pake layers of atoms, staking faults (mistakes in the normal hexagonal or ubi paking orer) may easily our. The energy of prouing suh efets is very low. For refletions of the type (hk), (l) an (hkl) with h=k=3n, there is no hange in struture fator on rossing the fault, therefore no broaening is observe. In ontrast, for (hkl) refletions with h- k=3n±1, the struture fator hanges signifiantly at eah fault, whih results in broaening the line. The amount of broaening epens on the number of staking faults. A measure of the oherene length of the perioi strutural pattern orresponing to a partiular peak an be obtaine from its with through the well-known Sherrer formula:.9λ D eff =, (1) δ osθ where D eff is the oherene length, λ is the X-ray wavelength, θ is the angle where the peak ours an δ is its full with at half-maximum. D eff values, alulate for three samples from (2) peak, are iniate in Table 1.

3 Struture an eletrial properties of eletron irraiate C thin films 115 Table 1. Coherene length, as etermine from (2) peak. Sample D eff (Å) Within the limits of experimental resolution, no hange in iffration pattern aspet or in line broaening was observe following eletron irraiation at fluenes up to 1 16 eletrons/m 2 (Fig. 2). Therefore there are no major hanges in the rystalline struture of the films, as a result of eletron irraiation at energy an fluenes iniate above. It may be onlue that at most point-like efets (vaanies, interstitials or their assoiation in some more omplex efets) o our in the struture of the films sample 1, non-irraiate proba 1, irraiate (E=6 Me, F=1 15 e - /m -2 ) irraiate, E=6 Me, F=1x1 15 e - /m 2. Sample 1, hp (2) peak 2 2 I (a.u.) 15 I (a.u.) 15 non-irraiate Fig. 2. X-ray iffration pattern an hp (2) peak for sample 1, before an after irraiation.. Eletrial properties The temperature epenene of the eletrial resistane, reore for one sample, before an after irraiation, is shown in Fig. 3. All the stuie samples are of n-type, as onfirme by Hall effet ata (Fig. 4). Irraiation parameters are also iniate in legen. 1 1 before irraiation after irraiation, E=6 Me, F=5x1 13 e - /m R (Ω) 1 5 θ (eg) θ (eg) /T (K -1 ) Fig. 3. Temperature epenene of eletrial resistane, before an after irraiation.

4 116 L. Ion, S. Antohe, M. Popesu, F. Sarlat, F. Sava, Feliia Ionesu This epenene is of ativate type: R(T ) = E R exp a (2) but has ifferent ativation energies in the upper an lower region of the investigate temperature range. The values of the parameters entering eq. (2), as obtaine by numerial fit, are iniate in Table 2. Table 2. Parameters esribing the temperature epenene of the eletrial resistane of C thin films (eq.2). R 1 an E a1 are the values orresponing to higher temperature range, while R 2 an E a2 orrespon to lower temperatures. Sample R 1 (Ω) E a1 (e) R 2 (Ω) E a2 (e) Comments non-irraiate sample; irraiate, 6 Me eletrons, e - /m 2 ; Experimental ata moel - isrete levels moel - gaussian istributions 1 2 R H (m 3 /C) /T (K -1 ) Fig. 4. Temperature epenene of the Hall onstant (sample 3). Points represent experimental ata, while lines represent moel results as esribe in text. At room temperature an own to 25 K, the experimental results suggest a ban onution mehanism, ontrolle essentially by a eep onor level (E 1 ) loate at about.4 e below the bottom level of the onution ban. This is also supporte by the Hall effet ata (Fig. 4). In this region the temperature epenene of the eletrial resistane is entirely ue to the rapi erease in the onentration of the free arriers (eletrons), that are graually reapture by the onor levels they originate from. After irraiation a higher value of R 1 is reore, so the mobility of free arriers iminishes, probably as a result of an inrease onentration of ionize onors. It is worth pointing out that in the low temperature region the pre-exponential oeffiient R 2 rops by almost an orer of magnitue after irraiation (see Table 2). Also a signifiant erease of the ativation energy of the Hall onstant R H is observe. This suggests a hange in the onution mehanism at temperatures below 23 K. The experimental results may be explaine by onneting the eletrial onution mehanism in this temperature range with the motion of eletrons over some other (onor) states (E 2 ), loate at about.18 e above the E 1 level where the Fermi level is pinne (Fig. 5).

5 Struture an eletrial properties of eletron irraiate C thin films 117 Fig. 5. Energy levels orresponing to a onor efet with two ionization states. It was suggeste [2,11] that E 1 level oul be relate to a efet implying selenium vaany. ut suh a efet an exist in two harge states, simply an oubly ionize. We believe that E 1 an E 2 levels orrespon to two harge states of a efet involving a selenium vaany, an respetively: E2 E1 e e ;. (3) Due to ompensation, only the seon of the above proesses generates free eletrons, the Fermi level is pinne at E 1 below onution ban an the ban onutivity epens on temperature aoring to E1 kt σ b e. That is the situation observe from room temperature own to 25 K, the ativation energy of the eletrial resistane being E a1 =E 1. At lower temperatures the number of free eletrons is exponentially small, an a hopping mehanism omes into first plae. Loalize eletrons an hop from a state to a state or from a to, et. The former situation orrespons to hopping from an oupie E 1 state to an empty one, requires partial ompensation, results in small ativation energy of the onutivity an shoul be the most important proess at very low temperature. The latter situation orrespons to hopping between E 2 (neutral) states; the number of E1 E2 kt hopping eletrons is exponentially small e, but their mobility is higher ue to a larger loalization raius of the E 2 state. For that reason the above mentione mehanism oul beome ominant in some finite temperature range. It results in an ativation energy of the onutivity E a2 =E 1 -E 2, an also an explain the experimentally observe erease of the pre-exponential fator R 2. Eletron irraiation generates new suh efets, the overlap of the loalize wave funtions beomes more pronoune an hene an exponentially smaller R 2 value results. For testing the above assumption (onor efets with two ionization states), a moel for the temperature epenene of the Hall onstant for a C rystal with a onor efet having two ionization states an a partially ompensating aeptor efet was also evelope, the results being presente in Fig. 4. For the sake of simpliity, the bottom level of the onution ban was hosen as energeti referene (E =, Fig. 5). At some given temperature onor efets an exist in equilibrium in neutral (onentration ), singly ( ) or oubly ionize ( ) states. Aeptor levels are fully oupie, ue to ompensation. The equation expressing eletrial neutrality of the rystal reas: where a is aeptor onentration, an: a = n 2, (4) =. (5)

6 118 L. Ion, S. Antohe, M. Popesu, F. Sarlat, F. Sava, Feliia Ionesu Equations relating the quantities appearing in eq. (5), are given by stanar Fermi statistis for a ope semionutor: g = g F E exp, = F E exp 2 1 g g, (6) where g s are the egeneray fators of the orresponing states (g = g =1, g =2 aounting for the two spin states), E 1 an E 2 are ionization energies as introue in eq. (3) (see also fig. 5). Using eqs. F (5)-(6) an taking into aount that n = exp, eq. (4) beomes: k T E2 1 exp n k T n a = n E 1 E2 1 exp exp 2 2n. (7) The above equation was numerially solve, with =2.23 T 3/ m -3, typial for C, E 1 =.4 e, E 2 =.22 e an a ompensation fator k= a / =.1. The results are iniate as soli line in Fig. 4. The ashe line represents the results obtaine as iniate above, but for a gaussian-like ontinuous istribution of onor levels, entere at E 1 an E 2 an.1 e wie. It is obvious that the isrete levels solution is loser to experimental ata. 4. Conlusions Struture an eletrial properties of C thin films irraiate with high-energy eletrons were investigate. The films ontain wurtzite-type C, (1) preferentially oriente in the growth iretion, with some amount of staking faults. The oherene length as etermine from (2) iffration line broaening, was about 1Å for all analyze samples. o hanges in the general aspet of the iffration pattern, or in line broaening were observe after irraiation, within the limits of experimental resolution. It may be onlue that this type of irraiation, in analyze onitions, proues only some type of point-like efets in the rystalline struture of the films. Eletrial properties of the films an be explaine by assuming that they are ontrolle by a eep onor efet, possibly selenium vaany, having two ionization states, with ionization energies of.4 e, respetively.22 e (eq. 3). In the temperature range extening from room temperature own to about 25 K, the ban onution mehanism is ominant, while below 23 K eletrial onution is ue to eletrons hopping over singly fille onors. After irraiation the onentration of these onors inreases; this an explain the observe behavior of resistane pre-exponential fators R 1 an R 2. A moel was evelope for alulating the temperature epenene of the Hall onstant for a semionutor rystal with a efet existing in two ionization states. Its results agree reasonably well with experimental ata. A shift of ativation energies of the eletrial resistane was also observe after irraiation with high energy eletrons. Suh an irraiation generates new efets of vaany an/or interstitials type [13]. Due to a better overlapping of the loalize wave funtions an also ue to Coulomb interations, the efet levels are sattere in energy, resulting a wier impurity ban. The texture of the films an also play a role: eletron irraiation an affet the energeti potentials at grain bounaries, etermining a shift of the mobility ege in the onution ban. These two effets oul be responsible for the observe shifts of ativations energies.

7 Struture an eletrial properties of eletron irraiate C thin films 119 Referenes [1] L. L. Kazmerski, W.. erry, C. W. Allen, J. Appl. Phys. 43(8), , (1972). [2] Kazuo-Shimizu, Japanese Journal of Applie Physis 4(9), 627, (1965). [3].. Ismail, R. D. Goul, Phys. Stat. Sol. (a), 115, 237, (1989). [4] R. D. Goul,.. Ismail, Int. J. Eletron. 69, 19-24, (199). [5] A. M. Anriesh,. I. erlan, L. A. Malahova, J. Optoeletron. Av. Mater 5(4), 817 (23). [6] M. Lisa, E. Pentia, G. Sarau, L. Pintilie, I. Pintilie, T. otila, J. Optoeletron. Av. Mater. 5(4), 849 (23). [7]. A. Kulp, Phys. Rev. 125, 1865, (1962). [8] H. Ohyama, K. Hayama, Phys. Stat. Sol. (a) 142, K.117, (1994). [9] I. Spânulesu, I. areanu,. åltåteanu, I. Z. Abi, T. Khalass, Thin Soli Films 143, 1-6, (1986). [1] S. Antohe, L. Ion,. Ruxanra, J. Appl. Phys. 9(12), 5928 (21). [11] S. Antohe, L. Ion,. A. Antohe, J. Optoeletron. Av. Mater. 5(4), 81 (23). [12] E. F. Kaelble (e.), Hanbook of X-Rays, hapt. 17, MGraw-Hill, ew York, USA, [13] J. W. Corbett, Soli State Physis, vol. 7 suppl. ( Eletron Raiation Damage in mionutors an Metals ), Aaemi Press, ew York, 1966.

Chapter 2: One-dimensional Steady State Conduction

Chapter 2: One-dimensional Steady State Conduction 1 Chapter : One-imensional Steay State Conution.1 Eamples of One-imensional Conution Eample.1: Plate with Energy Generation an Variable Conutivity Sine k is variable it must remain insie the ifferentiation