Design Band Energy diagram of SnO 2 /CdS-CdTe Thin Film Heterojunction Using I-V and C-V Measurements

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1 The 4 th International scientific onference on Nanotechnology& dvanced Materials &Their pplications (INM 013)3-4 Nov, 013 dte Thin Film Heterojunction Using I- Rasha. bdullah Education ollege, University of Tikreet, Salahedden, +964, Iraq Mohammed.. Razooqi Education ollege, University of Tikreet, Salahedden, +964, Iraq Nada M. Saeed ollege of science, University of Baghdad / Baghdad. nadaalkhanchi@yahoo.com BSTRT SnO /ds-dte heterojunction has been fabricated by thermal evaporation technique, 0.05 µm thicknesses of SnO nanostructure was evaporated as thin layer to be used as an antireflection and as transparent conducting oxide. The prepared cell has been annealed at 573K for 180 minutes. The general morphology of SnO films was imaged by using tomic Force Microscope (FM), the image shows that the average grain size of the prepared film is constructed from nanostructure of dimensions in order of 7 nm. There are two wide peaks were presents at the x-ray pattern which were refers to SnO and is in agreement with the literature of merican Standard of Testing Materials (STM). The capacitance- voltage a measurement has studied at 10 Hz frequency, the capacitance- voltage measurements indicated that these cells are abrupt. The capacitance at zero bias, built in voltage, zero bias depletion region width and the carrier concentration have been calculated. The carrier transport mechanism for SnO /ds-dte heterojunction in dark is tunneling recombination. The value of ideality factor is 1.56 and the reverse saturation current is Band energy lineup for SnO / n-ds-p-dte heterojunction has been investigated by using I- and - measurements. Key words: n-ds-p-dte Heterojunction; - Measurements; I- Measurements; Energy Band iagram. رسم مخطط الطاقة للمفرق الهجين للغشاء SnO /ds-dte الرقيق باستخدام قياسات تيار جهد و سعة جهد الخالصة تم تضيم ا مفرقمال مفني م SnO /ds-dte بتق م مفتخي ما مفقام م مفضامواستمممت ر شاء من SnO ذت مفتاك ب مف اتا مفرضيا بسرك 0.05 µm كطخق شقاف ضم منع اامم س م تم تيم لن مفرقمال مفرضيما ب و م 375 مئولم تفرم 081 دق قم ت تم فضمب تاك مب شماء SnO بامتي مم مفرينا منفاتاتع (FM) س تق تخ ن بان مفغشاء ذت تاك ب عاعوا بر ل فيضي مفخيمووا 607

2 77 عاعومتاس تمن تضي ف تائج فضوصات ح ود منش مفس تخ ن بافشا عن ت ود قرت ن تممم تش ا فيراد مفرضيا طخقا فيرقال س منمالا فقضوصات مفرومد (STM) ت تم دوممم ق اممات مم نم ع م تمادد 011 هاتم,س ت قم مشماوت هملق مفق اممات مفمل من تيمك مفرقاول ه من مف وع مفضادت ت ق ت من ق ر مفس ع منعض از مفصمقاا ت نم مفخ ماء مفم م ي قمم ت اقصمما ب مم عري مم مفتيمم لنت ب رمما لمم,دمد عمماال م طقمم مف ممور فزعض مماز مفصممقاا ت تاك مم, مفضمامزت ممم عري م مفتيم لنت ت كاعمي م ااع ا م معتقمال مفضمامزت فيرقمال مفني م ع م مف مزم هم 01- أعتقال-معاد متضمادت ت كاعمي ق رم عامم مفرةاف م 31 ت 0 ت ت ماو منشمخاع هم 1 ت 01 6 أمخ مات ت تم مليا دومم ميطط ح,م مفطاق عن طالق ق امات ت او ن ت م ن ت INTROUTION ds/dt thin-film photovoltaic (P) technology plays a key role in today s fast growing photovoltaic industry [1]. etails about processing conditions like maximum processing temperatures are not known []. By the use of low growth temperatures not only production cost and energy payback time can be reduced, but also thermally sensitive substrates like polyimide film can be used []. dte, in the form of a compound, is stable in air and has no toxicity to the environment [3]. The material cost is relatively low compared to other solar cells, such as the Si based crystalline solar cell, which is currently dominating the solar cell market [3]. dte has been recognized as an attractive photovoltaic material because of its direct band gap (E g ) around 1.5 e, which is near the optimal value for p-n devices. The absorber thickness in commercial ds/dte Ps is in the range of 5-10 µm [4]. dte thickness could be as small as 1 µm to absorb about 90% of incident solar light [4]. admium sulfide (ds) belonging to the II-I group is one of the promising materials for optoelectronic devices [5]. apacitance vs. voltage is one of the most important techniques for the device characterization. This technique, together with the current voltage measurements permit to study the transport mechanism in the heterostructure and the basic physical parameters and junction features. The gained information is essential for understanding the photovoltaic loss mechanism, thus to improve the P-device performance to get higher efficiencies [6]. In this article, it is reported a study of capacitance voltage characteristics and study the current voltage characteristics of thermally deposited ds/dte heterojunctions, the band discontinuity and the interface charge density, which are obtained. EXPERIMENTL WORK SnO /ds-dte heterojunctions were produced via thermal evaporation technique. orning glass used as a substrate, coated by u of 0.05 µm thickness as back contact. p-dte layer with thickness of about 1. ± 0.05 µm evaporating on u back contact layer, and then n-ds layer with thickness of about 0. ± 0.05 µm evaporating on dte layer. Then nanostructure SnO of 0.05 µm thicknesses was evaporated as thin layer to be used as an antireflection coating and as transparent conducting oxide, finally l of 0.05 µm thicknesses as an electrode. The films have been deposited at substrate temperature 373 ± 5 K. ll materials have % purity. The thicknesses of the prepared films were measured using Fizue method. The contacts of ds on dte have been prepared in a sandwich configuration between l and u electrodes. The basic structure of the prepared heterojunction is shown in Figure (1). The Edward E306 coating system was used for all 608

3 deposition processes, under pressure of about 10-6 mbar, the prepared ds/dte heterojunctions annealed at 573 ± 5 K 180 minutes under vacuum of 30 mbar. Keithley 616 digital electrometer has been used to measure the resistance of dte films. apacitance-voltage (-) measurements under reverse bias voltage between zero to 1 volt with frequency of 10 Hz have been carried for ds/dte heterojunction by HP-R unit model 474 and 475 multifrequency LR meter. urrent-voltage (I-) measurements at the forward and reverse bias in dark, in addition to forward current measured at different ambient temperatures around room temperature of ds/dte heterojunction were done by Keithley digital electrometer 616 and.. power supply. RESULTS N ISUSSIONS - haracteristics for SnO /ds-dte heterojunction Mott-Shcottky plot ( - vs. ) for SnO /ds-dte heterojunction is shown in Figure (), the plots reveal straight line relationships which mean that the junction was an abrupt type [7]. The capacitance () strongly depends on the applied bias and the slope of - plot junction capacitance. In this case, it can be said that the u/dte contact is well formed. Similar results have been found by Jun et al [8]. The depletion layer width (W) is defined as [7]: 1 ( W qn N bi )( ( N 1 a N ) ) 1 (1) Where ε 1 and ε are the dielectric constants for n- and p-types semiconductor respectively, bi and a are the built-in and applied voltages respectively, q is the electron charge, N and N are the donor and acceptor concentration respectively. The doping concentration can be calculated from the following relation [7]: ( 1 ) q N N 1 1 ( bi ) a () Where is area of junction. The intercept of the line that results from plotting between - as a function of reverse bias represents the built-in potential and the carrier concentration can be calculated from the slope of this line from equation 3, the bi value is 0.9.The higher value of bi may be attributed to the midgap states that act as recombination centers that may arise either from states created as a result of junction fabrication or lattice mismatch between dte and ds [9]. The capacitance measurement relies on the fact that the width (W) of the space-charge - region (SR) of a semiconductor device junction changes with applied voltage. The value of depletion region width for SnO /ds-dte cells is 0.43 µm. The acceptor concentration of SnO /ds-dte heterojunction is (cm -3 ). The values of doping concentration suggests that the SnO /ds-dte interface is n + p type, elsewhere, we have shown that in the case of n + p type junction, recombination in the absorber space charge layer dominates over interface recombination [10]. The surface morphology of SnO The general morphology of SnO films was imaged by using tomic Force Microscope (FM), the image shows that the average grain size of the prepared film is constructed from nanostructure of dimensions in order of 7nm, as shown in Figure (3). 609

4 X-ray patterns of the nanostructure SnO The X-ray diffraction of SnO thin layer was obtained at two theta from 0 to 60 glancing angle; there are two wide peaks present at x-ray pattern, as shown in Figure (4). The result is in agreement with the literature of merican Standard of Testing Materials (STM), as listed in Table (1), the highest peak observed at Ɵ equal I- haracteristics for SnO /ds-dte heterojunction Figure (5) shows the I- haracteristic at dark of SnO /ds-dte heterojunction. In general, the I- characteristics for the SnO /ds-dte heterojunction at forward bias voltage show that the forward current consists of two regions; the first region represents recombination currents while the second represents the tunneling currents [11]. The carrier transport mechanism for this case is of a tunneling recombination type, in coherence with lnajjar et al [9]. The reverse saturation current was calculated from the intercept of the straight line with the current axis at zero voltage bias. The value of reverse saturation current is This result is in agreement with lnajjar et al [9]. The ideality factor (β) can be calculated from the following relationship [7]: q k T B F I ln I F S (3) Where F is the forward bias voltage, I F and I S are the forward bias and the saturation currents respectively, β can be calculated by applying equation 4 to the first region of the upper curve. The value of ideality factor t is 1.58, similar result have found by Huijin et al [1]. Band energy diagram of SnO /ds-dte solar cell The values of energy gaps of ds (E g1 ) is near.4 e [13] and of dte (E g ) is 1.44 e [14]. heterojunction has been investigated by using I- and - measurements [15]. I- measurements under forward bias in the temperatures range (91-301) K. The measurements were taken at low voltages to cancel the role of the tunneling effect. We can observe that the I S decreases with decreasing temperature due to resistance increase. Figure 6 shows the dependence of the I S on the 10 3 /T, from the slope of this figure the value of valence band offset ( E v ) can be deduced and by using the following equation [11]: I S q( E exp k BT ) (4) The value of E v which is calculated from the previous relation equals to 0.40 e. The total built-in voltage ( bi ) is 0.9 e, which is due to difference in work functions (ɸ) and it is equal to the sum of built-in voltages on both sides [11], therefore we can calculate the value of conduction band offset ( E c ) is 0.56 e, which calculated from the following equation [16]: 610

5 E E E E g1 g (5) The position of fermi level (E F ) in the two other side of heterojunction can be calculated by using the following equations [16]: E E F k B N T ln N (6) E F E k B N T ln N (7) Where N and N are the density of states concentrations of electrons (in conduction band) and holes (valance band) respectively. The values of (E -E F ) and (E F -E ) are found to be equal 0.49 and 0.53 e respectively. The band lineup model of SnO / n-ds/p-dte heterojunction was constructed depending on the yield of our data, as shown in Figure (7). Where subscripts 1 and in Figure (5) correspond to ds and dte respectively, and χ is the electron affinity. l Thin SnO film Thin n-ds film p-dte film u Electrode Glass substrate l Figure (1) the typical structure of typical dte/ds heterojunction. Figure () Mott- Shcottky plot and - curves SnO /ds-dte heterojunction. 611

6 Intensity (ount/second) Figure (3) The Surface Morphology of SnO film takes from tomic Force Microscope); the average grain size =7 nm. (110) (101) (11) Figure (4) the X-ray diffraction pattern of SnO thin film. Table (1) the value of d for all peaks of SnO thin film from X-Ray pattern. (hkl) ( Ɵ) ( Ɵ) d (XR) d egree STM o (STM) egree ( ) o ( ) (110) (101) (11) Figure (5) I- characteristics (forward and revere bias voltage) at dark for annealed SnO /ds-dte heterojunction. 61

7 Figure (6) Saturation current vs /T for a SnO /ds-dte heterojunction. Figure (7) Equilibrium energy band (band line up) diagram of SnO / n-ds/p-dte heterojunction. ONLUSIONS The prepared SnO /ds-dte heterojunctions are abrupt. The u/dte contact well formed. The depletion region widths, built in voltage, capacitance, and carrier concentration have been calculated. The values of doping concentration suggest that the SnO /ds-dte interface is n + p type. The carrier transport mechanism is tunneling recombination. We based on our results to modified band energy diagram which has already been proposed based on the analysis of electrical measurement. 613

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