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con substrate.10,11 For example, we fabricated ZnO p-n ho- series of two-layer-structured ZnO p-n homojunctions,

APPLIED PHYSICS LETTERS 89, 053501 2006mojunctions composed of a NAl codoped p-type ZnO layerand an Al-doped n-type ZnO layer.11 Recently, Tsukazaki etal.12 claimed the room-temperature electroluminescencefrom a ZnO p-i-n homojunction grown on a ScAlMgO4 sub-strate. All of these progresses have focused attention on thedevelopment of light emitting diodes in the ZnO system.

For the improved device performance, an acceptable ho-mostructural junction is demanded. The characteristics of ho-mojunctions are mainly controlled by the layer material, con-tact electrode, and device structure. At this early stage ofZnO light emitting diode development, the p-type layer iscertainly crucial considering its instability. But it should benoted that the other facets also play important roles in fabri-cating ZnO homojunctions. However, there have been fewreports on this related field, except the study on contact elec-trodes for forming Ohmic behavior.13 In this letter, we dem-

p-ZnO: N,Al /n-ZnO:Al and n-ZnO:Al/ p-ZnO: N,Al,on sapphire substrates, as shown in Fig. 1. They were madein a layer-by-layer growth mode. In-Sn alloy contact spotswere deposited on both layers, followed by annealing for30 s at 400 C in a pure Ar ambient. This rapid thermalannealing treatment is necessary to decrease contact resis-tance and to increase adhesive force. The inset in Fig. 1shows the surface current-voltage I-V characteristics of the

TABLE I. Electrical properties of p-ZnO: N,Al, n-ZnO:Al, andi-Zn,CdO films derived from Hall-effect measurements at room tempera-ture.

Sample

GrowthtimeC

Resistivity cm

Hallmobilitycm2/V s

Carrierconcentration

cm3

p-ZnO: N,Al 500 2.64 1.62 1.451018n-ZnO:Al 400 0.0085 3.7 1.971020Electrical characterization of ZnO-basJ. G. Lu,a Z. Z. Ye,b G. D. Yuan, Y. J. Zeng, F.State Key Laboratory of Silicon Materials, Zhejiang UnivPeoples Republic of ChinaS. B. ZhangNational Renewable Energy Laboratory, Golden, Colorad

Received 10 December 2005; accepted 13 June 20

Electrical characteristics have been studied for ZnO pof device structures for improved performance. Cformation of abrupt junctions. The current-voltagerectification behavior. The p-ZnO: N,Al /n-ZnOsubstrates combining with the intrinsic ZnO buffer lwith the forward turn-on voltage of 1.4 V and tintroduction of an intrinsic Zn,CdO layer, the reshomojunction exhibits a high reverse breakdown voPhysics. DOI: 10.1063/1.2245221

Recent success in p-type doping in ZnO has opened adoor for its practical applications to short-wavelength opto-electronic devices, such as light emitting diodes and lasers,which can be an alternative to those based on GaN.1 Ascompared with GaN, ZnO has a larger exciton binding en-ergy 60 meV, cf. 25 meV for GaN, which can ensure ahighly efficient emission at room temperature.2 Followingthe growth of p-type ZnO films, considerable efforts havebeen made to fabricate ZnO-based homostructural p-n junc-tion diodes, with positive results obtained in recentliteratures.312 Guo et al.3 fabricated a ZnO homostructuraldiode with the p-ZnO:N/n-ZnO junction grown on a ZnOwafer. Aoki et al.6 produced a ZnO homojunction diode bydirectly forming a P-doped p-type ZnO layer on an intrinsicn-type ZnO wafer. Ryu et al.8 adopted As-doped p-type ZnOto fabricate the p-ZnO:As/n-ZnO homojunction on a SiCsubstrate. Xiong et al.9 reported the properties of p-n homo-junctions prepared by oxygen control in the sputteringplasma. Further light on ZnO-based diodes was added bycombining a N-III III=Al, Ga, and In codoped p-type ZnOlayer with a n-type ZnO layer on a quartz, sapphire, or sili-aElectronic mail: lujianguo@zju.edu.cnbElectronic mail: yezz@cmsce.zju.edu.cn

0003-6951/2006/895/053501/3/$23.00 89, 05350Downloaded 19 Dec 2006 to 130.158.130.96. Redistribution subject tohomojunctionsge, L. P. Zhu, and B. H. Zhao

y, Hangzhou 310027,

401

ublished online 31 July 2006

nd p-i-n homojunctions, with optimizationitance-voltage measurements confirm theacteristics exhibit their inherent electrical

homojunctions fabricated on sapphirehave acceptable p-n diode characteristics,everse breakdown voltage of 5.3 V. Bynt p-ZnO: N,Al / i-Zn,CdO/n-ZnO:Al

of 18 V. 2006 American Institute of

onstrate the electrical characterization of ZnO-based homo-junctions, describing the design and optimization of devicestructures with improved performance.

Four kinds of ZnO thin films were involved in this work.They are undoped ZnO, Cd-doped ZnO Zn,CdO,Al-doped ZnO ZnO:Al, and NAl codoped ZnOZnO:N,Al. In our previous reports, we have provided aNAl codoping method as an effective approach to achievep-type ZnO, and the resultant p-type conductivity was dem-onstrated to be stable and controllable.11,14,15 Thus, we useZnO:N,Al films as the p-type layer. All the four kinds ofZnO films were prepared by using a magnetron sputteringsystem. Details of the growth process could be foundelsewhere.14,16 The electrical properties of p-ZnO: N,Al,n-ZnO:Al, and i-Zn,CdO films, with their respectivegrowth processes exactly the same as that adopted in fabri-cating homojunctions except that the growth time was differ-ent, are listed in Table I, measured by the van der Pauwconfiguration on sapphire substrates, with the film thicknessabout 350 nm.

By using these ZnO films, we fabricated twoi-Zn,CdO 350 751 0.84 9.851015

2006 American Institute of Physics1-1 AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

p-ZnO: N,Al and n-ZnO:Al films in contact with the al-loys. The Ohmic behaviors are confirmed by the fairly linearI-V dependencies.

The I-V characteristics of the p-ZnO: N,Al /n-ZnO:Al homojunction are shown in Fig. 1, which displayan apparently electrical rectifying behavior that is consistentwith the formation of a typical p-n junction at the interface.The forward turn-on voltage appears at 2 V, and the re-verse breakdown voltage is 4 V. A low leakage current isalso observed under reverse bias. For comparison, we fabri-cated a n-ZnO:Al/ p-ZnO: N,Al homojunction by usingp-type ZnO as the bottom layer and n-type ZnO as the toplayer, whose I-V characteristics are also shown in Fig. 1. Itstill exhibits an evident asymmetric feature, but comparedwith that of the former homojunction, the degradation in per-formance can be readily identified, with increased forwardturn-on voltage and decreased reverse breakdown voltage.Investigations on p-type ZnO films indicated that a long-timepostannealing process in a conventional ambient such as O2is usually harmful to their p-type conductivity.17 As weknow, the following n-type layer growth process after depo-sition of p-type layer actually plays an annealing effect tothis p-type film already grown on the substrate. It possiblyanswers for this degradation. Accordingly, to improve diodeperformance, the n-type ZnO layer should be deposited priorto the p-type layer in fabricating ZnO homojunctions.

Capacitance-voltage C-V measurements were per-formed on these junctions. Based on Andersons abrupt junc-tion diffusion model, the unit area capacitance for a homo-junction can be expressed as

C = 0rqNAND2NA + NDV0 V1/2

. 1

Here, q is the electronic charge, 0 is the vacuum permittiv-ity, r is the relative permittivity r=8.75 for ZnO, V0 is thebuilt-in voltage, and NA and ND are the carrier concentrationsin the p-type and n-type layers, respectively. Figure 2 showsa plot of 1 /C2 as a function of V for the p-ZnO: N,Al /

FIG. 1. I-V characteristics of two-layer-structured ZnO p-n homojunctions p-ZnO: N,Al /n-ZnO:Al and n-ZnO:Al/ p-ZnO: N,Al. The insetsshow their schematic diagrams and the I-V dependencies of InSn alloycontacts on p-ZnO: N,Al and n-ZnO:Al films.

053501-2 Lu et al.n-ZnO:Al diode. It is linear in the 03.5 V reverse biasrange, which confirms that the junction is electrically abrupt.From Fig. 2, the V0 value of 2.1 V can be obtained.If we assume ND=1.971020 cm3, the average carrier con-Downloaded 19 Dec 2006 to 130.158.130.96. Redistribution subject tocentration NA in the p-type layer is found to be8.721017 cm3, which is in agreement with Hall mea-surements for similar ZnO:N,Al films.

To further improve the diode behavior, we fabricated ap-ZnO: N,Al /n-ZnO:Al junction by introduction of a100-nm-thick intrinsic ZnO buffer layer on a sapphire sub-strate at 350 C. Figure 3 shows the I-V characteristics ofthis ZnO p-n homostructural diode. They exhibit an inherentand acceptable rectification behavior, and the results are re-producible. The forward turn-on voltage occurs at 1.4 V,and the breakdown voltage is 5.3 V in reverse bias with aquite low leakage current. A maximum forward-to-reversecurrent ratio of 90 occurs at 2.0 V. As can be seen, thediode behavior improved significantly compared with thatwithout a buffer layer, demonstrating the importance of ZnObuffer layer in enhancing the device performance. In thiscontext it is worthy to campare the I-V characteristics mea-sured by other workers for ZnO p-n homojunctions. Theturn-on voltage commonly appears in the range from1 to 3 V, such as 0.5 V for p-ZnO:N/n-ZnO,3 1 V forp-ZnO:P/n-ZnO,6 1.5 V for n-ZnO/ p-Zn,MgO:P,7 and3 V for n-ZnO/ p-ZnO: N,In homojunctions.10 Thesevalues are smaller than the band gap energy of ZnO3.37 eV. The low turn-on voltage remains under consider-

FIG. 2. Inverse of square of capacitance as a function of applied voltage fora p-ZnO: N,Al /n-ZnO:Al diode.

Appl. Phys. Lett. 89, 053501 2006FIG. 3. I-V characteristics of a ZnO p-n homojunction p-ZnO: N,Al /n-ZnO:Al by introduction of an intrinsic ZnO buffer layer on a sapphiresubstrate. The insets are the junction schematic and the I-V plot in semilogform. AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

ation, but it seems to be acceptable as qualified by electrolu-minescence in oxide p-n junctions.3,6,18 These data are re-markably low compared with other wide-gap materials suchas GaN and ZnSe. For this reason, ZnO-based devices arevery favorable for long-time operation without seriousOhmic heating at the contact.

The inset in Fig. 3 shows the semilog plot of I-V curves.The measured forward current has two distinct regions. Forlow bias V0.7 V the current increases exponentially withthe applied voltage, while for high bias V0.7 V the cur-rent increases in proportion to the power of voltage. Thediode ideality factor can be determined by using theusual junction rectification model in the low bias region:

I = I0expqV/kT 1 , 2

where I0 is the saturation current, k is Boltzmanns constant,and T is the absolute temperature. The ideality factor de-rived from the equation is 4.3. This value is somewhat highas compared with that of the ideal p-n junctions =12.Nevertheless, the ideality factor obtained here is similar to,but often less than, those observed for ZnO p-n junctionsreported elsewhere, such as 325 for p-ZnO:As/n-ZnO,81020 for n-ZnO/ p-Zn,MgO:P,7 and 4.710.6 forp-ZnO:N/n-ZnO homojunctions.5 The high ideality factorsuggests that there are multiple current transport mechanismsin the junctions, such as conventional carrier recombinationin the space-charge region, as well as deep-level-assistedtunneling and/or parasitic rectifying junctions within the de-vice. Further investigations should be done for improvedjunction quality.

It is well known that the band gap of ZnO can be tunedby alloying with Cd, and the resulting Zn, CdO alloys al-low luminescence covering a wide range from2.3 to 3.37 eV theoretically. The p-ZnO/ i-Zn,CdO/n-ZnO homojunction composed of a narrow-band-gap Zn,CdO film as the active layer can be used for lasers withpotentially high quantum efficiency. Figure 4 illustrates thep-i-n junction structure combining with an intrinsic ZnO

FIG. 4. I-V characteristics of a ZnO p-i-n homojunctionp-ZnO: N,Al / i-Zn,CdO/n-ZnO:Al by adopting an intrinsic Zn,CdOfilm as the active layer. The insets illustrate the schematic of the p-i-njunction, as well as the band gap energies of the p-ZnO: N,Al,i-Zn,CdO, and n-ZnO:Al films derived from their respective opticalabsorption spectra.

053501-3 Lu et al.buffer layer and the I-V characteristics of this diode. It dis-plays a fairly good rectification. The p-ZnO: N,Al /i-Zn,CdO/n-ZnO:Al homojunction diode, on the oneDownloaded 19 Dec 2006 to 130.158.130.96. Redistribution subject tohand, has a reverse breakdown voltage as high as 18 V,with a relatively low reverse leakage current, but on the otherhand, the forward turn-on voltage also appears at a highvalue of about 4.2 V, which is presumably owing to the ad-ditional film and interface resistivities by introduction of anintrinsic Zn,CdO layer as compared with two-layer-structured homojunctions. The rectifying behavior of p-i-njunction has also been observed in the p-ZnO:N/ i-ZnO/n-ZnO:Ga homojunction,12 with a forward turn-on voltageappearing at 7 V and a reverse breakdown voltage 10 V.By the way, attempts to observe the room-temperature bandedge emission from the p-i-n homojunction provided herewere unsuccessful probably due to the polycrystal nature ofZnO films grown by a sputtering system. Alternately, wehave observed the electroluminescence in suchlike ZnO ho-mojunction diodes fabricated by pulsed laser deposition withimproved device quality, which will be reported separately.

In summary, we have fabricated ZnO-based p-n andp-i-n homojunctions on sapphire substrates. The homojunc-tions are electrically abrupt, exhibiting apparent rectifyingcharacteristics, and they are reproducible. The device struc-ture was designed and optimized to obtain improved perfor-mance. This study is expected to provide further insight onthe emergence of homostructural diodes in the ZnO system.The challenges to be resolved in the near future will be 1 tostabilize the p-type behavior in ZnO, 2 to further optimizethe device structure, and 3 to control the surface and inter-face states in ZnO homojunction diodes.

This work was supported by the Key Project of NationalNatural Science Foundation of China under Grant No.50532060.

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