Superlattices and Microstructures 64 (2013) 213–219

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Superlattices and Microstructures

j o u r n 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 ca t e / s u p e r l a t t i c es

Room temperature deposition of graded ZnOhomojunction from a single sputter target

0749-6036/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.spmi.2013.08.018

⇑ Tel.: +886 972506900; fax: +886 73645589.E-mail address: ianbu@hotmail.com

Ian Y.Y. Bu ⇑Department of Microelectronics Engineering, National Kaohsiung Marine University, 81157 Nanzih District, Kaohsiung City, Taiwan,ROC

a r t i c l e i n f o

Article history:Received 5 May 2013Received in revised form 14 July 2013Accepted 22 August 2013Available online 14 September 2013

Keywords:Zinc oxideGraded junctionSolar cellsCo-doping

a b s t r a c t

The study demonstrates fabrication of a novel graded ZnO homon-junction by sputtering process using just a single aluminum dopedzinc metallic target. Specifically, the ZnO homonjunction consistsof n-type ZnO:Al/p-type, graded ZnO:Al:N layers deposited underdifferent N2 flow rate. The deposited layers were extensively char-acterized by scanning electron microscopy, X-ray diffraction, andenergy dispersive X-ray spectroscopy and photoluminescencemeasurements. It was found that ZnO:Al:N thin film propertieswere strongly affected by the amount of incorporated nitrogen.From the current voltage (I–V) characteristics measured from thefabricated devices shows that the proposed graded junction out-performs non-graded ZnO homonjunction by exhibiting reducedturn on voltage and better ON/OFF ratio than non-graded junction.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Over the past decade, there have been considerable research interests in zinc oxide (ZnO) due to itspotential application in optoelectronics [1–3], cosmetics [4,5] and electronics devices [6–8]. ZnO is awurtzite-structured semiconductor with a wide-bandgap (3.37 eV) and large exciton binding energy(60 meV). As-deposited ZnO films are usually n-type due to its intrinsic defects [9–12]. Although, suchdoping-asymmetry property has been exploited in fabrication of transparent conductive electrodes forsolar cell applications [13,14], it has also presented difficulty in obtaining device-quality p-type zincoxide [10,15]. The difficulty arises due to the combination of self-compensation effects and low solu-bility of acceptor dopants [16]. Theoretically, it has been shown that p-type ZnO doping can beachieved through co-doping method using the combination N and Al [17,18]. Co-doping of ZnO

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through the combination of N and Al is attractive due to its cost-effectiveness and general acceptanceby the semiconductor industry [12,19,20].

In the past, p-type ZnO thin films have been successfully synthesized through sputtering [21], sol–gel [22], chemical vapor deposition (CVD) [23], laser ablation [24] and hydrothermal method [25].Amongst the different deposition methods, sputtering is preferred due to its extensive usage in thesemiconductor industry.

Due to the aforementioned doping asymmetry problem in ZnO, most of previous studies on ZnO-based junctions focused on adapting ZnO with an well established p-type metal oxides such as CuO2

[26–28] and NiO [29] to form heterojunctional devices. Although such studies on ZnO heterojunctiondevices are of research interests, their device performances are often inadequate due to the latticemismatch between ZnO and the selected p-type metal oxide. Past studies have revealed a clear trendof transition from n-type ZnO:Al to ZnO:Al:N as nitrogen incorporation in ZnO increases [30,31]. Sub-sequently, ZnO based pn junction devices have been fabricated through the combination of sputteredn-type ZnO:Al and p-type ZnO:Al:N (NAZO) thin film [30,31]. To our best knowledge, there is no studyon graded ZnO:Al:N homojunction. It is expected that by adapting a graded ZnO homojunction resultsin smoother band alignment and superior electrical performances than the heterojunctioncounterpart.

In this study, ZnO homojunctional devices were fabricated through sequential sputtering of n-typeZnO:Al and graded ZnO:Al:N thin films. N2 was selected as p-type dopant due to its general acceptanceby the semiconductor industry. The optoelectronic properties of the layers were thoroughly character-ized through X-ray diffraction (XRD), photoluminescence (PL), and current–voltage (I–V) measure-ments. The proposed process utilizes conventional sputtering method using just a single metallicZn/Al target, which are suitable for large-area deposition and can be scaled up for industrialintegration.

2. Experimental

Corning glasses (Eagle 2000) were used as the substrates. Prior to deposition, the glass substrateswere cleaned using ultrasonic agitation within acetone, methanol and water, for duration of 30 min ineach solvent. Metallic Zn:Al and Zinc oxide derivative depositions were performed using a 2-in. Directcurrent magnetron-sputtering coater with a Zn:Al target (99.99% pure, doped with 1 wt% Al). First thesubstrates were introduced to the sputter chamber (with target-to-substrate distance at 9 cm) andpumped down to base pressure of around 3 � 10�6 mTorr using the turbo pump. Then high puritynitrogen, oxygen and argon gases were introduced into the chamber using mass flow controllers.For all the depositions, the working pressure, power, temperature and duration were set to�5 � 10�4 mTorr, 33 �C, 120 W and 15 min, respectively.

A FEI Quanta 400 F Environmental Scanning Electron Microscope (SEM) was used to evaluate thestructure of the NAZO samples. Deposition rates of the thin film were extracted by tilted SEM images.Chemical composition was determined via energy dispersive X-ray spectroscopy (EDS) within thesame chamber. The crystal orientation of the NAZO samples was determined using a SiemensD5000 X-Ray diffractometer with Cu Ka radiation. Photoluminescence (PL) of the NAZO films weremeasured by excitation from a 325 nm He–Cd laser at room temperature. Fig. 1 shows the schematicdiagram of the fabricated device. Sequentially, metallic Zn:Al (deposited under Ar set at 5 sccm), n-type ZnO:Al (AZO) (deposited under Ar and O2 set at 5 sccm) and graded, p-type NAZO (deposited un-der Ar, O2 set at 5 sccm and N2 set between 1–25 sccm) were synthesized on glass and plastic acetatesubstrate. For comparative purposes, a non-graded NZO (10 sccm)/AZO junction was also fabricated.Before electrical measurements, Ohmic contacts were deposited onto the fabricated p–n junctionvia pre-fabricated mask, using sputter condition identical to the first Zn:Al metallic layer.

3. Results and discussion

The structure properties of the ZnO-based films were evaluated using a SEM. Fig. 2a shows the rep-resentative SEM image of the ZnO films deposited under N2 flow 25 sccm. For the AZO sample, Fig. 2a,

Fig. 1. Schematic diagram of the proposed graded device.

Fig. 2. Representative scanning electron microscope images of the deposited (a) ZnO:Al:N and (b) tilted SEM image of thegraded junction (Both scale bar equals 2 lm).

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it can be observed from the SEM image that the film consists of granular structure with grain size of50 nm. Fig. 2b shows the tilted SEM image of the graded junction. It can be observed from the SEMimage that the graded ZnO consists of columnar structure normal to the substrate, which is typicalfeature of sputtered ZnO [32]. Fig. 2b also shows that the variation in nitrogen flow during sputteringprocess has not resulted in noticeable changes between film interfaces. Furthermore, the surface of thegraded device is highly texturized. Normally an antireflective layer is deliberately etched onto the sur-face of photovoltaic devices to enhance light absorption and enhance power conversion efficiency.Such antireflective layers are fabricated by dry or wet etch that requires extra process step. By adapt-ing graded ZnO homojunction, this additional process can be avoided.

Fig. 3 shows the XRD peaks of the NAZO samples deposited under N2 flow of 1, 5, 10 and 25 sccm,respectively. With the exception of NAZO film deposited under N2 flow of 25 sccm, all other films ex-hibit a braod peak centered at 34.4�. This peak corresponds to (002) orientation and indicates forma-tion of hexagonal wurtzite ZnO. It is important to note that the XRD peak intensity decreases withincreasing N2 flow, which suggest degradation of crystallinity. The data correlates well with previousstudies on ZnO:N thin films that indicated decrease in XRD peak as N increases [33]. The degradation isdue to differences in the radius of N2 and Zn, which results in the formation of substitution (NO–ZO),(NO–Zi), vacancy defect and interstitial NO–VO[31].

EDS is a useful tool for the determining the chemical composition deposited thin films. Fig. 4 showsthe representative EDS analysis of the deposited NAZO films. It is reassuring that Zn, O, Al and N arepresent within the deposited thin films. The peak centered at around 2 keV from Au coating was elim-inated from analysis. The EDS data indicated that the NAZO films is slightly Zn rich at Zn:O ratio at1.06. It is expected that the N2 are dissociated into monatomic nitrogen by generated plasma.

Fig. 3. XRD Pattern of the sputtered ZnO:Al:N at flow rate between 1 and 25 sccm flow of nitrogen.

Fig. 4. Typical EDS composition analysis of the deposited ZnO:Al:N thin film at N2 set at 10 sccm.

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PL spectroscopy is a common method used to evaluate the defects present in the ZnO films. In gen-eral, two peaks centered at around 3.14 eV and 2.6 eV have been observed for ZnO thin films. Theultraviolet (UV) peak, 3.14 eV, is often associated with the recombination of free excitons betweenconductive band and valence band of the ZnO and near band edge emission, whereas the peakcentered at around 2.6 eV has been attributed to the transition between the vacancy of oxygen andinterstitial oxygen. Fig. 5 presents room temperature PL spectra of NAZO as a function of incrementalnitrogen flow. It can be observed that as the N2 flow increased from 1 to 25 sccm the PL peak shiftsfrom 2.74 eV to 3 eV, accompanied with significant decrease in PL intensity. The reduction in PL

Fig. 5. Photoluminescence emission spectra of the deposited ZnO:Al:N thin film at N flow rate between 1 and 25 sccm.

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intensity suggests film degradation and correlates well with XRD data presented in Fig. 3.Twotheoretical models have been proposed to explain the origin of PL peak at 3.04 eV. The first model[34] attributed this peak to zinc vacancy under excessive zinc films. In the second proposed model,He and co-workers [35] predicted zinc vacancy at 0.3 eV and assigned the peak (at around 3.04 eV)to transition between the conduction band and zinc vacancy. The EDS data suggested that this peakoriginated from Zn-rich films. Further increase in N2 flow results in appearance of additional peakat around 2.4 eV. The origin of 2.4 eV is still under debate and has been attributed to oxygen vacancy,interstitial Zn, and Zinc vacancies [34,36,37]. Our EDS data suggest that the 2.4 eV peak could berelated to nitrogen incorporation as an acceptor, as observed by other researchers [38–40].

The electrical properties of ZnO-based p–n junction were evaluated through current voltage (I–V)measurements. Fig. 6 shows the I–V characteristics of the graded and non-graded ZnO homojunctionaldevice. Both of the fabricated devices shows rectifying behavior and confirms that the depositedgraded NAZO is p-type. From the plotted I–V curve for non-graded junction, the reverse current,ON/OFF ratio and turn on voltage were extracted to be 4.8 � 10�5 A, 8.52 and 4 V, respectively,whereas for the graded ZnO junction the corresponding parameters, the reverse current, ON/OFF ratioand turn on voltage were extracted to be 1.6 � 10�6 A, 170 and 3 V, respectively. Clearly, the gradeddevice outperforms the non-graded devices due to better interface formed between the AZO and NZO.It is believed that the reduction in leakage current is due to suppression of recombination current from

Fig. 6. Dark I–V characteristics of the fabricated graded and non-graded ZnO PN junction.

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defect induced recombination center. By adapting a graded doping structure the graded ZnO junctionbecame less resistive than non-graded device due reduced junction barrier abruption and leads to lar-ger current at same applied voltage.

4. Conclusion

In summary, layers of ZnO:Al:N films have been grown on glass and plastics acetate sheets by themagnetron sputtering process. SEM images reveal the films consist of small grain islands, that de-creases as N2 flow is increased. XRD data suggest that the films are polycrystalline with some prefer-ence to the c-axis. From the measured IV characteristic of the p–n junction devices, it have been shownthat the graded devices outperforms non-graded device due to the reduction in band discontinuity andhomogenous interface.

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