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26 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 57, NO. 1, JANUARY 2010

Recent Advances in ZnO-BasedLight-Emitting Diodes

Yong-Seok Choi, Jang-Won Kang, Dae-Kue Hwang, and Seong-Ju Park, Member, IEEE

(Invited/Review Paper)

Abstract—ZnO has attracted considerable attention for opti-cal device applications because of several potential advantagesover GaN, such as commercial availability of bulk single crystalsand a larger exciton binding energy (∼60 meV compared with∼25 meV for GaN). Recent improvements in the control ofbackground conductivity of ZnO and demonstrations of p-typedoping have intensified interest in this material for applications inlight-emitting diodes (LEDs). In this paper, we summarize recentprogress in ZnO-based LEDs. Physical and electrical properties,bandgap engineering, and growth of n- and p-type ZnO thin filmsare also reviewed.

Index Terms—p-type ZnO, zinc oxide, ZnO-based light-emittingdiodes (LEDs).

I. INTRODUCTION

IN THE upcoming information, digital, and multimedia age,light-emitting diodes (LEDs) based on wide-bandgap semi-

conductors have drawn much attention. The high efficiency,fast switching time, high color gamut, low power consump-tion, semipermanence, and low heat output of the LED haveled to many new applications. The backlight units in liquid-crystal displays have been replaced by high-efficiency LEDs.As the efficiency of LEDs was further improved, many prod-ucts equipped with LEDs have been reported. To meet therequirement of high-brightness LEDs for illumination, mobileappliances, automotive, and displays, it is necessary to de-velop new wide-bandgap semiconductors such as ZnO, whichhas excellent structural and physical properties compared toGaN. ZnO, furthermore, is inexpensive, relatively abundant,chemically stable, easy to prepare, and nontoxic. We alreadyreviewed ZnO growth and doping, metal ohmic contacts, etch-

Manuscript received June 16, 2009; revised September 23, 2009. Firstpublished November 10, 2009; current version published December 23, 2009.This work was supported in part by the Ministry of Knowledge Economy andthe Korea Science and Engineering Foundation under Grant R17-2007-078-01000-0 funded by the Korean government and in part by the World ClassUniversity program at the Gwangju Institute of Science and Technology (GIST)under Project R31-2008-000-10026-0 funded by the Ministry of Education,Science and Technology of Korea. The review of this paper was arranged byEditor S. Pearton.

Y.-S. Choi, J.-W. Kang, and S.-J. Park are with the Department of NanobioMaterials and Electronics, GIST, Gwangju 500-712, Korea, and also with theDepartment of Materials Science and Engineering, GIST, Gwangju 500-712,Korea (e-mail: [email protected]).

D.-K. Hwang is with Materials Research Institute, Northwestern University,Evanston, IL 60208 USA (e-mail: [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TED.2009.2033769

TABLE IBAND PARAMETERS OF ZnO AT 6 K

ing of ZnO, and ZnO-based LEDs in a previous report [1]. Inthis paper, we summarize recent results of ZnO-based LEDs,which were mostly published after the previous report [1].The organization of this review is given as follows. First, ZnOmaterials properties such as physical and electrical propertiesand bandgap engineering are described in Section II. This isfollowed by the growth of n- and p-type ZnO thin films inSection III. Section IV is devoted to recent reports of ZnO-based LEDs.

II. ZnO MATERIAL PROPERTIES

AND BANDGAP ENGINEERING

Recently, ZnO has attracted much attention for its applicationto LEDs, varistors, scintillators, solar cells, and transparentelectronics [2]–[5]. In this section, we focus on the materialsproperties of ZnO such as physical properties, electrical prop-erties, and bandgap engineering.

A. Physical Properties

ZnO is a II–VI compound semiconductor with a hexagonalwurtzite structure whose lattice constant is 3.25 Å. ZnO is adirect bandgap semiconductor. The band edge at the Γ-point istriply degenerated and split due to the effect of the crystallineelectric field and the spin–orbit coupling. Table I shows theatmospheric pressure bandgap, the pressure dependence of thebandgap, the exciton binding energy, and exciton energy levelsat 6 K [6]. Further, the splitting energies due to the crystalline

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CHOI et al.: RECENT ADVANCES IN ZnO-BASED LIGHT-EMITTING DIODES 27

electric field Δcf and the spin–orbit coupling Δsc are 39.4 and−3.5 meV, respectively [6]. The negative value of Δsc is dueto coupling with the Zn 3d orbital [7], [8]. ZnO has a numberof advantages over GaN, the wide-bandgap semiconductor cur-rently utilized in the short-wavelength optoelectronics industry.Some of these advantages include a large exciton binding en-ergy (∼60 meV), a higher radiation hardness, simple processingdue to amenability to conventional chemical wet etching, andthe availability of large area substrates at relatively low materialcosts [9], [10].

B. Electrical Properties

The electrical properties of undoped ZnO film have beenfound to be n-type due to the formation of native defect. Thebackground carrier concentration is typically 1016−1017 cm−3

[11]. The electron mobility of undoped ZnO film varies, de-pending on the growth method but is usually 120–440 cm2V−1 ·s−1 at room temperature [11]. The high carrier concentrationachieved by n- and p-type doping is ∼1020 electrons · cm−3

and ∼1019 holes · cm−3, respectively [12]. The carrier mobilityof doped ZnO film is relatively lower compared to that ofundoped ZnO films due to carrier scattering mechanism suchas ionized-impurity scattering, polar optical-phonon scattering,acoustic-phonon scattering through deformation potentials, andpiezoelectric interactions [11]. The typical value of mobilityat room temperature for low n- and p-type doping is 200 and5–50 cm2V−1 · s−1, respectively [13]. The hole mobility ismuch lower than electron mobility due to the difference ofeffective mass and carrier scattering mechanism.

C. Bandgap Engineering

An important step in order to design ZnO-based LEDs isthe realization of bandgap engineering to create barrier lay-ers and quantum wells (QWs) in device structures. Recently,CdxMgyBezZn1−x−y−zO alloys have attracted much attentionbecause their optical devices operate in the UV and visible re-gion. CdO is an n-type semiconductor with a direct bandgap of2.2–2.6 eV and an indirect one at 0.8–1.3 eV [14]–[16]. Whencompared to ZnO, the CdO has a different crystal structure: acubic rocksalt structure with a lattice constant of 4.69 Å. CdOhas been used to grow CdxZn1−xO alloys that are expected toachieve low energy bandgaps. Because the ionic radius of Cd2+

(0.74 Å) is close to that of Zn2+ (0.60 Å), the wurtzite phase,which is a stable phase of ZnO, is expected to be conservedwhen a rocksalt-structured CdO is alloyed. Because of the largelattice mismatch and the difference in crystal structure betweenCdO and ZnO, however, the thermodynamic solubility limit ofCd is 2 mol% [17]. Makino et al. [17] reported that the bandgapcould be tuned to the lower energy side by 0.3 eV at x = 0.07in comparison with that of ZnO. Shigemori et al. [18] reportedthat plasma-assisted metalorganic chemical vapor deposition(MOCVD) enabled the growth of CdZnO with a Cd composi-tion of 69.7% and a bandgap of 1.85 eV. On the other hand, thelarger bandgap of MgZnO alloys compared to ZnO will enableits use for the growth of multiple QWs optical devices operatingin the UV region. MgO has a stable rocksalt structure with

a lattice constant of 4.22 Å. According to the phase diagramof the ZnO–MgO binary system, the thermodynamic solubilitylimit of MgO in ZnO is less than 4 mol% due to large latticemismatch and the difference in crystal structure [19]. In spiteof a large structural dissimilarity between ZnO and MgO, thegrowth of MgZnO with a Mg composition of 51% (bandgap of4.44 eV) has been reported and used for fabrication of solar-blind optical detectors [20], 2-D electron gas MgZnO/ZnOheterostructures [21], and transistors [22]. Ohtomo et al. [23]reported that the solid solubility of MgO in ZnO is 33 mol%for the thin-film alloys grown under metastable conditions.Above 33%, MgO was reported to segregate from the wurtziteMgZnO lattice limiting its maximum bandgap to 3.9 eV. Toovercome the problem with phase segregation of Mg from theMgZnO alloys, Ryu et al. [24] proposed the BeZnO alloy,which has much larger energy bandgap. BeO has a wurtzitestructure with a bandgap of 10.6 eV. Since BeO and ZnOshare the same hexagonal symmetry, phase segregation is notdetected in BeZnO alloys. Ryu et al. reported that the Be con-centration is varied over the entire range from 0 to 100 mol%without any phase segregation.

III. DOPING OF ZnO

A. n-Type ZnO

The as-grown ZnO material is unintentionally n-type, andit is widely believed that intrinsic defects such as the zincinterstitial (Zni) and oxygen vacancy (VO) are source of donorsin ZnO [25]. Vanheusden et al. [26] reported that the freecarrier concentration was much larger than VO in their samples,and Look et al. [27] suggested that Zni rather than VO isthe dominant native shallow donor in ZnO materials. It wasalso suggested that hydrogen atoms were incorporated to formhydrogen-related donors, such as a substitutional hydrogen(HO) and an interstitial hydrogen (Hi), which resulted in an in-crease in the donor concentration of ZnO [28], [29]. Intentionaln-type doping has been accomplished using group III elementssuch as Al, Ga, and In, which can easily substitute Zn ions.Many groups have attempted doping with Al, Ga, and In forapplications to n-type layers in light-emitting devices as wellas transparent ohmic contacts. Doping method with Al [30], Ga[31], and In [32] can produce highly conducting n-type ZnOfilms with a carrier concentration of > 1020 cm−3 range. Thegroup VII elements F and Cl, which substitute oxygen ion, havealso been suggested for n-type dopants. Gordon [33] reportedF-doped ZnO grown by chemical vapor deposition (CVD) witha resistivity of 4 × 10−4 Ω · cm. Chikoidze et al. [34] grewCl-doped ZnO films by the MOCVD method.

B. p-Type ZnO

In order to develop ZnO-based LEDs, the most importantissue is the fabrication of low-resistivity p-type ZnO. Becauseunintentionally undoped ZnO shows typically n-type proper-ties, acceptors may be compensated by native defects such asZni, VO, or background impurities such as hydrogen. VO inZnO makes deep donor level located at ∼1.0 eV below the



bottom of the conduction band and can compensate acceptorsin p-type ZnO [35]. Zni also makes shallow donor level locatedat ∼30 meV below the bottom of the conduction band [27].Atomic hydrogen is well known to behave as an exclusive shal-low donor in bulk ZnO [28], [29]. Hydrogen is ubiquitous andcan easily be incorporated in ZnO during the crystal growth. Togrow p-type ZnO, the acceptor concentration should be higherthan the unintentional donor concentration. The acceptors inZnO include group I elements such as Li [36]–[40], Na [41],Cu [42], and Ag [43], [44], Zn vacancies, O interstitial, andgroup V elements such as N [39], [45]–[75], P [71], [76]–[89],As [90]–[94], and Sb [95]–[99]. p-type doping in ZnO may bepossible by substituting either group I elements (Li, Na, and K)for Zn sites or group V elements (N, P, As, and Sb) for O sites.It has been believed that the most promising dopants for p-typeZnO are the group V elements, although theory suggests somedifficulty in achieving a shallow acceptor level [100]. Recently,another p-type doping mechanism was proposed for group Velements (P, As, and Sb). P, As, and Sb substitute Zn sites,forming a donor, then it induces two Zn vacancy acceptors asa complex form (PZn−2V Zn, AsZn−2V Zn, and SbZn−2V Zn)[101], [102]. However, the choice of p-type dopant and growthtechnique remains controversial and the reliability of p-typeZnO and the doping mechanism are still subjects of debate.Nitrogen has been used to p-type dopant due to similar ionicradius compared to oxygen and availability of gas source suchas N2, N2O, NO, NO2, and NH3. Wang et al. [75] reported onthe growth of p-type ZnO thin films prepared by oxidation ofZn3N2 thin films deposited by dc-magnetron sputtering. Unfor-tunately, reliability of N-doped p-type ZnO is still problematicdue to low solubility of nitrogen and the formation of N–N-related complexes in ZnO [13], [103]. Aoki et al. [76] reportedP-doped p-type ZnO film by expose Zn3P2 film to excimer laserradiation in high pressure nitrogen or oxygen ambient. Similarresults were obtained by Lee et al. [77], who also transformed aZn3P2 layer on ZnO/sapphire to p-type ZnO by laser annealing.Kim et al. [78] prepared p-type ZnO thin films showing holeconcentrations of 1017−1019 cm−3 by sputtering a ZnO targetdoped with P2O5 at high temperatures followed by a thermalannealing process. The possibility of p-type doping with largegroup V elements, such as As and Sb, has also been exploredby Limpijumnong et al. [102] using the first-principles calcula-tion. Ryu et al. [90], [91] reported the formation of As-dopedp-type ZnO films grown by hybrid beam deposition on GaAsand sapphire substrates. The Sb-doped p-type ZnO film wasfirst achieved by exposing an Sb film deposited on ZnO filmto KrF excimer laser radiation [95]. However, the residual Sbmetal film on top of the ZnO layer and the nonuniformity of Sbdoping might be a potential problem for device performance.Xiu et al. [96] fabricated Sb-doped p-type ZnO films by amolecular beam epitaxy (MBE) and reported p-type conduc-tivity with a very low resistivity of 0.2 Ω · cm, a high carrierconcentration of 1.7 × 1018 cm−3, and a high Hall mobility of20.0 cm2V−1 · s−1. In contrast to p-type doping of ZnO film,p-type doping of ZnO nanostructures has been difficult andrarely achieved. Recently, p-type conduction in ZnO nanowireswas reported by using N or P as the dopant by a simpleCVD method [104], [105]. Xiang et al. [104] confirmed p-type

TABLE IISURVEY OF THE ELECTRICAL PROPERTIES OF p-TYPE DOPED ZnO

TOGETHER WITH THE DOPANT, PL PEAK, AND SUBSTRATE

conductivity of P-doped p-type ZnO nanowires by temperaturedependent photoluminescence (PL) and measurement of ZnOnanowire back-gate field-effect transistors at room temperature.The calculated field-effect mobility and hole concentrationfor a quasi-1-D system are ∼1.7 cm2V−1 · s−1 and ∼2.2 ×107 cm−1, respectively. Table II is a survey of dopants, growthtechnique, substrates, and electrical properties of p-type ZnO.

IV. ZnO-BASED LEDs

A. Heterojunction LEDs

ZnO is very attractive for blue/UV LEDs and high-temperature/transparent electronics. However, ZnO-basedLEDs have suffered from the lack of reproducible and high-quality p-type material. A number of studies have reported onthe heterojunction LEDs using n-type ZnO deposited on p-typematerials of GaN, Si, and conducting oxides.



TABLE II(Continued.)

Rogers et al. [106] fabricated an n-ZnO/p-GaN:Mg het-erostructure on a c-Al2O3 substrate. A ZnO layer was grownon top of the p-type GaN layer using pulsed laser deposition(PLD) in oxygen ambient with a KrF excimer laser (248 nm).I–V characteristics exhibited nonlinear rectifying behaviorwith a forward-bias voltage of 3.3 V, which corresponds tothe bandgap of ZnO. The electroluminescence (EL) emissionsfrom the n-ZnO/p-GaN heterostructure under forward bias atroom temperature show a single peak with a λmax at 375 nm.Yang et al. [107] also fabricated heterojunction LEDs with an-ZnO/p-GaN structure on a c-Al2O3 substrate. For the growthof the heterojunction, a 620-nm-thick Mg-doped p-type GaNlayer was deposited on the c-Al2O3 substrate using MOCVD

TABLE II(Continued.)



Fig. 1. EL spectra of an n-ZnO/p-Al0.12Ga0.88N heterostructure LED at 300and 500 K (Ic = 20 mA) [108].

and a deposition of 750-nm-thick undoped n-type ZnO layerwas followed using the same MOCVD system. Based on theresults of X-ray diffraction and atomic force microscopy, itwas concluded that the obtained n-ZnO/p-GaN heterojunctionstructure was structurally in high quality. The EL spectra ex-hibited two peaks centered at 410 and 470 nm. A comparisonof EL and PL spectra of the n-type ZnO and p-type GaN layersshowed that two EL emission peaks were attributed to a radia-tive recombination in both n-type ZnO and p-type GaN layers.

Alivov et al. [108] fabricated n-ZnO/p-AlGaN heterojunc-tion LEDs, taking advantage of the fact that AlGaN is alsowell matched to ZnO and can be doped p-type. A layer ofn-type ZnO with a thickness of 1 μm was grown on thep-type Al0.12Ga0.88N using CVD stimulated by RF-dischargeplasma. The devices exhibited a rectifying diode-like behaviorwith a threshold voltage of ∼3.2 V, a high reverse breakdownvoltage of almost 30 V, and a small reverse leakage current of∼10−7 A at room temperature. As shown in Fig. 1, under for-ward bias, the device showed intense UV EL with a peak emis-sion near 389 nm (3.19 eV) and a full-width at half-maximum(FWHM) of 26 nm with no other emission bands. This emis-sion was found to be stable at temperatures up to 500 Kand was shown to originate from recombination within ZnOdue to large conduction band offset.

To improve optical characteristics with ZnO-based hetero-junction LEDs, double and triple heterostructure LEDs weredemonstrated [109], [110]. Ohashi et al. reported a ZnO-based double heterostructure system that consisted of ann-type ZnO cap layer, an n-type Mg0.12Zn0.88O cladding layer,an n-type Zn1−xCdxO emission layer, and an N-doped p-typeMg0.12Zn0.88O barrier layer on p-type 4H-SiC substrates [109].The PL emission energies of red-region Zn1−xCdxO filmsindicated almost a constant value of 1.9 eV. As shown in Fig. 2,EL spectra under current injection showed that the emissionsof blue and red are coming from different places. When theinjection current level is as low as 40 mA, blue emissionscoming from the p-type 4H-SiC substrate were observed. Onthe other hand, when a higher current of around 120 mA wasinjected, the red emission appears. Osinsky et al. [110] fabri-cated a p-n junction LED that has a MgZnO/ZnO/AlGaN/GaN

Fig. 2. EL spectra of ZnO-based double heterojunctions under current injec-tion [109].

triple heterostructure. Energy band diagrams of the LED struc-ture incorporating piezoelectric and spontaneous polarizationfields were simulated, revealing a strong hole confinement nearthe n-ZnO/p-AlGaN. Electron-beam-induced current measure-ments confirmed the presence of a p-n junction located at then-ZnO/p-AlGaN interface. The triple heterostructure p-n junc-tions showed rectifying characteristics with a threshold voltageof ∼3.2 V and a UV light emission at ∼390 nm. Experimentalspectral dependence of the photocurrent confirmed the exci-tonic origin of the optical transition at 390 nm. Light emissionsmeasured up to 650 K provided additional confirmation of theexcitonic nature of the optical transitions in the devices.

Mares et al. reported visible EL from a hybridn-MgZnO/CdZnO/p-GaN LED heterostructure with a singleCd0.12Zn0.88O QW [111]. PL measurements showed strongluminescence centered at 430 nm with an FWHM of 90 nmwithout any low-energy defect luminescence. The measuredEL spectra also showed no significant defect luminescencebands in the EL emission, which is indicative of the high-quality growth. EL spectra showed, however, higher energyemission than PL. Mares et al. demonstrated that PL spectradiffer from the EL emission due to spatially indirect opticaltransitions between the electrons confined in the QW and holesaccumulated at the GaN/CdZnO interface.

Heterojunction LEDs have been constructed mostly by de-positing n-type ZnO or n-type MgZnO on various p-type semi-conductor layers. However, there have been several attemptsto fabricate the heterojunction LEDs constructed by depositingp-type ZnO on various n-type semiconductor layers.

Mandalapu et al. [112] fabricated heterojunction LEDs bygrowing Sb-doped p-type ZnO layer on an n-type Si substrate.A thin undoped ZnO film was grown at low temperature onn-type Si (100) substrate as a buffer layer, followed by Sb-doped ZnO layer at a higher temperature by MBE. The re-sultant thicknesses of undoped and Sb-doped ZnO layers are50 and 370 nm, respectively. Finally, thermal activation of theSb dopant was performed in situ in vacuum at 800 ◦C for30 min. The I–V characteristics showed a rectifying behavior



Fig. 3. (a) Temperature-dependent EL spectra obtained at an injection currentof 110 mA. EL from LED is obtained at 9, 50, 100, 200, and 300 K.(b) Temperature-dependent EL spectra in the UV region only [112].

with a higher leakage current at both higher temperatures andhigher biases, which is possibly attributed to the band alignmentof wide-bandgap p-type ZnO and narrow-bandgap n-type Si.Fig. 3(a) shows the EL spectra obtained from LEDs at aninjection current of 110 mA at different temperatures. The near-band edge (NBE) emission at ∼381 nm was observed. Theother peaks around 485, 612, and 671 nm related to intrinsicdefects were also observed. Fig. 3(b) shows the spectra focusingon the UV region only. With increasing temperature, both thesmall UV peak and the NBE emission redshift and appear asa single peak at higher temperatures due to the temperature-induced bandgap variation. The intensity of emissions dropsthroughout the spectra with increasing temperature, which istypical due to the increase in nonradiative recombination athigher temperatures. The intensity of UV peaks is low com-pared to deep-level emissions probably due to low radiativeefficiency and self-absorption effect induced by deep levels.

Hwang et al. [113] reported on the growth and device prop-erties of p-ZnO/n-GaN heterojunction LEDs. A GaN bufferlayer was deposited on a (0001) sapphire substrate, followedby deposition of an n-type GaN layer by MOCVD. After thegrowth of the n-type GaN layer, a P-doped ZnO layer wasdeposited by an RF-magnetron sputtering system. To activatethe p-type dopant, a rapid thermal annealing (RTA) process

Fig. 4. (a) I–V characteristics of a p-ZnO/n-GaN heterostructure illustratingthe rectifying behavior. Inset: I–V characteristics of Ti/Al and NiO/Au metalcontacts on n-GaN and p-ZnO films. (b) EL spectrum of a p-ZnO/n-GaNheterostructure at an injection current of 60 mA [113].

was adopted at a temperature of 800 ◦C under a N2 ambient.Fig. 4(a) shows rectifying characteristics of a p-ZnO/n-GaNheterojunction with a threshold voltage of 5.4 V, and the insetof Fig. 4(a) shows the I–V characteristics of the Ti/Al andNiO/Au metal contacts on n-type GaN and p-type ZnO films,respectively, indicating that good ohmic contacts are formedon both electrodes. The PL spectra showed emission peaks ofn-type GaN and p-type ZnO at 365 and 385 nm, respectively.Fig. 4(b) shows the EL spectrum of a heterostructure at aninjection current of 60 mA. The EL emission peak at 409 nmwas attributed to the recombination of holes and electrons inthe p-type ZnO region. A comparison of EL and PL spectra ofthe p-type ZnO layer shows that the EL emission peak appearsat a lower energy region around 409 nm rather than at 385 nm.This lower energy EL emission peak is attributed to the bandoffset of 0.13 eV at the p-type ZnO and n-type GaN interface.Table III is a survey of structure, method, and emission color ofZnO-based heterojunction LEDs [106]–[121].

B. Heterojunction LEDs With ZnO Nanostructures

The optical devices using ZnO nanostructures have attractedmuch attention due to their optical properties arising fromquantum confinement such as enhanced radiative recombina-tion of carriers. The research for application to LEDs usingthe ZnO nanostructures has focused on 1-D nanostructure



TABLE IIISURVEY OF STRUCTURE, METHOD, AND EMISSION

COLOR OF ZnO BASED HETEROJUNCTION LEDs

due to the crystal orientation of ZnO. Various approaches forgrowing ZnO nanostructures have been developed by manymeans, such as MOCVD [122]–[124], CVD [125], the solutionmethod [126]–[128], electrodeposition [129], PLD [130], andthe vapor-phase transport method [131]. However, the difficul-ties also exist here due to the absence of a stable and reliablep-doping method in the fabrication of heterojunction LEDsusing ZnO nanostructures. Therefore, most of heterojunctiondevices using ZnO nanostructure have been realized fromdifferent p-type materials such as p-type GaN [122]–[124],[130], Si [126], NiO [127], and p-type polymers [125], [128],[129]. The properties of heterojunction LEDs with ZnO nanos-tructures are summarized in Table IV together with the LEDstructure, the growth method of ZnO nanostructures, and theemission peak position [122]–[131].

As a promising approach for LEDs using ZnO nanostruc-tures, 1-D ZnO nanostructure/p-GaN heterojunction structureswere suggested by several groups because these materials

TABLE IVSURVEY OF STRUCTURE, METHOD, AND EMISSION COLOR OF

HETEROJUNCTION LEDs WITH ZnO NANOSTRUCTURES

have the same wurtzite crystal structure, relatively similarbandgap energy, and a low lattice parameter misfit of 1.9%. TheEL emission of 1-D ZnO nanostructure/p-GaN heterojunctionstructures is observed at UV, blue, green, and yellow, whichis generally considered from excitonic recombination, radiativerecombination related to Mg acceptor level, and ZnO defectstates such as Zni and VO [122]–[124], [130].

Park and Yi [122] reported on a p-n heterojunction LED com-posed of n-type ZnO nanorod on Mg doped p-type GaN filmgrown by using MOCVD. ZnO nanorod arrays were alignedvertically to the substrate surface due to the growth directionalong the c-axis of ZnO. Room temperature EL spectra ofn-ZnO/p-GaN nanorod heterojunction LEDs were measuredat the various reverse-bias voltages because EL emission wasnot observed at a forward-bias voltage in the range of up to10 V. The main emission peak of EL spectra was a wide yellowemission band centered at 2.2 eV, and the intensity increasedwith the reverse bias increased from 3 to 7 V. The blue emissionpeak at 2.8 eV and UV emission peak at 3.35 eV were revealedabove reverse-bias voltage of 4 V.

Jeong et al. [123] proposed p-GaN film/ZnO nanowire/n-ZnO film heterojunction LEDs to improve the carrier in-jection efficiency of nanostructured junctions. ZnO nanowirearrays of this junction device were formed on Mg-dopedp-type GaN films by MOCVD. Al-doped ZnO films for in-jection of electrons into ZnO nanowire arrays were grownby RF-magnetron sputtering. Fig. 5 shows scanning electron



Fig. 5. (a) SEM image tilted by 45◦. (b) Schematic of the Mg-doped GaNfilm/ZnO nanowire array/Al-doped ZnO film structures for nanometer-sizedGaN/ZnO heterojunction diode applications. Inset: Photograph of blue-lightemission from the heterojunction diodes observed through a microscope at theforward current of 10 mA [123].

Fig. 6. EL spectra of (a) film-based GaN/ZnO heterojunction diodes and(b) ZnO-nanowire-inserted GaN/ZnO heterojunction diodes [123].

microscope (SEM) images and a schematic diagram of ZnO-nanowire-inserted GaN/ZnO heterojunction LEDs. Film-basedZnO/GaN heterojunction structures were also fabricated forthe optical and electrical characterization of ZnO-nanowire-inserted GaN/ZnO heterojunction LEDs. The EL spectra of thefilm-based and ZnO-nanowire-inserted GaN/ZnO heterojunc-tion diodes are shown in Fig. 6. The EL spectra of film-basedheterojunction show broad blue emission peaks. The blue

Fig. 7. Typical I–V curve of n-ZnONR/p-CuAlO2/p+-Si LEDs. Inset: I–Vcurve of n-ZnO NR/p+-Si LEDs [131].

emission peak centered at 440 nm was enhanced and slightlyshifted to 425 nm with the forward current increased to 20 mA.The enhancement and blueshift of the blue emission peakswith an increased forward current were due to the electroninjection from the ZnO nanowires to the GaN films for radiativeelectron–hole recombination in the GaN films. The I–V charac-teristics of two heterojunction devices showed different leakagecurrent at a reverse-bias voltage of 10 V and turn-on voltagedue to the differences in the interface between ZnO nanowireor ZnO films and p-type GaN.

Park et al. [124] fabricated a heterojunction LED by growingZnO nanorod on p-type GaN. The ZnO films as electron injec-tion layers were homoepitaxially grown by growing ZnO filmson ZnO nanorod arrays using MOCVD. This heterojunctionLEDs showed asymmetry rectifying I–V characteristics witha small breakdown voltage, which is due to a leakage currentat the junction. An EL spectrum showed a strong peak centeredat 440 nm and a broad peak around 560 nm. The blue emissionfrom the ZnO nanorods/GaN heterojunction indicated that theemission was caused by the p-type GaN [120]–[122]. A visiblebroad peak centered at around 560 nm was related to transitionsby ZnO defect states.

Sun et al. [126] fabricated a p-n heterojunction LED com-posed of n-type ZnO nanorods and p-type Si. ZnO nanorodswere grown on a p-type Si substrate by using an aqueoussolution method. High-molecular-weight polymers were usedto fill the space between the individual ZnO nanorods. The I–Vcharacteristics of n-ZnO nanorods/p-Si heterojunction showeda low threshold voltage under reverse bias and a high resistanceunder forward bias because of an inconsistent band structureat the interface and the nonideal contact. In addition, thelarge valence band offset (∼2 eV) at the interface of n-ZnOnanorods/p-Si caused easy tunneling of electrons between theelectrode and ZnO nanorods. The EL spectra of LED werecomposed of narrow UV emission peak centered at 387 nmand broad greenish emission peak centered at 535 nm, whichwere related to excitonic transition and transition from a shal-low donor level of VO to the valence band, respectively. Theintensity of UV emission increased more quickly than that ofthe green emission by increasing the applied bias.

Ling et al. [131] fabricated a n-ZnO nanorod/p-CuAlO2 filmheterojunction LED on p-type Si substrates. Vertically alignedZnO nanorods grown by the vapor-phase transport method



Fig. 8. (a) Room temperature PL spectrum of ZnO nanorods andthe schematic of the light transmission path in our device (inset).(b) Room temperature EL spectra and the EL photograph (inset) ofn-ZnO nanorods/p-CuAlO2/p+-Si LED. (c) Room temperature EL spectraand the EL photograph (inset) of n-ZnO NR/p+-Si LED [131].

served as channels for injection of electron and optical confine-ment. A p-type CuAlO2 film grown by dc-magnetron sputteringwas used as the hole injection layer. In addition, the freespace between ZnO nanorod arrays was filled with hydropho-bic poly(methyl-methacrylate) (PMMA) as an insulating andsupporting layer. Fig. 7 shows rectifying I–V characteristics ofn-ZnO nanorods/p-CuAlO0/p+-Si and n-ZnO nanorod/p+-SiLEDs. The LED with the p-type CuAlO2 film indicated acomparatively low turn-on voltage of 4 V and a high break-down voltage above 40 V compared to the LED withoutp-type CuAlO2. As shown in Fig. 8, UV (band edge) emissionand greenish (defect-related) emission were observed on theroom temperature PL and EL spectra of LEDs. While ELspectra of LED with a p-CuAlO2 layer showed an rising UVemission intensity with increasing injection current, as shown inFig. 8(b), relatively intense green emission was obtained fromthe LED without CuAlO2. It indicated that the p-type CuAlO2

layer can be used to improve the LED performance of ZnO/SiUV LEDs.

Fig. 9. Zinc oxide homostructural p-i-n junction shows EL in forward bias atroom temperature. EL spectrum from the p-i-n junction (blue) and PL spectrumof a p-type ZnO film measured at 300 K. The p-i-n junction was operated byfeeding in a direct current of 20 mA [132].

Nadarajah et al. [129] realized flexible LEDs using n-typeZnO nanowire and p-type polymer. ZnO nanowires were grownon an ITO-coated polymeric substrate by electrodeposition. Thetop anode contact for hole injection consisted of a thin poly(3,4-ethylene-dioxythiophene) poly(styrenesulfonate) layer and aAu film. The measured I–V characteristics showed the diodecharacteristics and the rectification. The EL spectra of flexibleLED using ZnO nanowires showed a broad emission bandcovering the wavelength range of 500–1100 nm. In addition,ZnO nanowires in this flexible LED remained attached even asthe substrate was bent over a curvature radius less than 10 μm.

C. Homojunction LEDs

Tsukazaki et al. [132], [133] proposed a repeated tempera-ture modulation technique as a reliable and reproducible way tofabricate p-type ZnO. Thin films of ZnO and junction deviceswere grown by laser MBE using nitrogen as a p-type dopant.The p-i-n homojunction structure was grown in a layer-by-layermode, keeping an atomically flat interface. The I–V character-istics show fairly good rectification with a threshold voltage ofabout 7 V. The threshold voltage is higher than the bandgapof ZnO (3.3 eV), which is mainly due to the high resistivityof the p-type ZnO layer. The EL spectrum shows luminescencefrom violet to green regions with multireflection interferencefringes. The EL spectrum apparently shows a redshift comparedto exciton emission of undoped ZnO layer at 3.2 eV. This ispartly due to the low hole concentration in p-type ZnO; electroninjection from undoped ZnO to p-type ZnO overcomes holeinjection from p-type ZnO to undoped ZnO. The PL spectrum(black) of a p-type ZnO film is also shown in Fig. 9. The higherenergy side peak around 430 nm in the EL spectrum matcheswell with the PL spectrum.

Lim et al. [134] realized a UV emitting diode using a ZnOp-n homojunction. Ga-doped n-type ZnO layers with a



Fig. 10. EL spectra of a p-n homojunction ZnO LED operated at forwardcurrents of 20 and 40 mA; PL spectrum of p-type ZnO obtained at roomtemperature [134].

thickness of 1.5 μm were grown on a c-Al2O3 substrate by RF-sputtering deposition at 900 ◦C, using a ZnO target mixed with1 wt.% Ga2O3. For the p-n homojunction ZnO LED, a layerof p-type ZnO with a thickness of 0.4 μm was grown on then-type ZnO layer at 900 ◦C using a ZnO target mixed with1 wt.% P2O5. For the ZnO LED with carrier confinementlayers, two Mg0.1Zn0.9O epitaxial layers, each of 40 nm thick-ness, were deposited by cosputtering MgO and ZnO targets onn-type ZnO. The samples were subjected to an RTA process for5 min at 800 ◦C in a nitrogen atmosphere in order to activatethe p-type ZnO layers. The LED shows clear rectification with athreshold voltage of 3.2 V, which is in good agreement with theZnO bandgap energy of 3.37 eV. Fig. 10 shows an EL spectraof the homojunction ZnO LED, with an NBE emission at380 nm (3.26 eV) and broad deep-level emissions at approx-imately 640 nm. A comparison of EL and PL spectra of thep-type ZnO film suggests that the recombination of electronsand holes occurs mostly in p-type ZnO. The carrier diffusionlengths of the electrons in p-type ZnO and the holes in n-typeZnO were estimated to be 290.2 and 97.8 nm, respectively.As shown in Fig. 11(a), a ZnO LED with two 40-nm-thickMg0.1Zn0.9O films as the carrier confinement layers and a40-nm-thick n-type ZnO layer as the active region was grownon c-Al2O3. Fig. 11(b) shows EL spectra of the ZnO LEDwith Mg0.1Zn0.9O layers operated at forward currents of 20 and40 mA. With increasing injection current, a redshift of the bandedge emission was observed because of bandgap narrowingcaused by heat generation. Unlike the case of the band edgeemission, the deep-level emission was not blueshifted. Thisresult indicates that the deep-level emission is from the p-type ZnO excited by UV light emitted from the confinedn-type ZnO layer that does not show any deep-level emissionin the PL spectrum.

Ryu et al. presented results for ZnO-based UV LEDs thatemploy a Be0.3Zn0.7O layer and a BeZnO/ZnO active layerbetween n-type and p-type ZnO [135]. The active layer is com-

Fig. 11. (a) Schematic of a ZnO LED containing Mg0.1Zn0.9O barrier layersfor carrier confinement in n-type ZnO. (b) EL spectra of the ZnO LED withMg0.1Zn0.9O layers operated at forward currents of 20 and 40 mA [134].

Fig. 12. EL spectrum measured at room temperature in continuous currentmode of a p-n junction ZnO-based LED having a BeZnO active layer. The pri-mary spectral emission peak is located near 363 nm and arises from localized-exciton emissions in the QWs. The secondary peak centered near 388 nm isfrom impurity-bound exciton emissions in ZnO [135].

posed of seven QWs for which undoped Be0.2Zn0.8O and ZnOform barrier and well layers, respectively. The thickness of eachBeZnO barrier layer is about 7 nm, and each ZnO well layer isabout 4 nm. The p-type ZnO and BeZnO layers are formed withAs as the acceptor dopant while the n-type ZnO and BeZnOlayers were formed with Ga as the dopant. I–V measurementsdemonstrate p-n junction characteristics for the ZnO-basedstructures, featuring a high turn-on voltage and a low reverse-bias current as shown in the insert. Fig. 12 shows the intensityof the EL spectral output of one of the ZnO-based LEDs withincreasing injection current. The peaks located near 388 nm(bound exciton) and 550 nm (green band) are the dominantfeatures at low forward currents under 20 mA, at which the in-tensities of EL peaks at 388 and 550 nm have become saturated.



Fig. 13. EL spectra of the ZnO LED measured at room temperature undervarious injection currents. Inset: Normalized EL spectra in which the blueshiftof the emission peak is clearly seen [137].

The peak at 388 nm is the impurity (donor or acceptor)-boundexciton emission. The peak at 550 nm is shifted to shorterwavelengths as current injection is increased; the peak locationis around 520 nm at an injection current of 50 mA. This behav-ior indicates impurity-involved emission such as from donor–acceptor pair recombination for the 550-nm peak. The peaklocated near 363 nm (localized exciton) becomes the prominentspectral feature at current injection levels above 20 mA. Thepeak observed at 363 nm has not been previously reported forZnO. Its emission could be from band-to-band recombination,such as from localized-exciton peaks in the active layer of theQWs, as has been observed in GaN-based LEDs.

Xu et al. [136] reported on the growth of p-type ZnO thinfilms by MOCVD using NO plasma as both the oxygen andnitrogen sources. N-doped ZnO thin films were grown by theplasma-assisted low-pressure MOCVD method. ZnO homo-junction has a rectifying behavior with a turn-on voltage of2.3 V, which is a little lower than ZnO bandgap. The reductionof the turn-on voltage might be attributed both to the highdefect concentration in the interface and to the low carrierconcentration in the n-type side of the junction. At a forwardcurrent of 20 mA, weak luminescence was detected in theregion between yellow and blue (430–600 nm). As the current isincreased to 40 mA, besides the emissions at the visible region,a UV emission at around 375 nm was also detected.

Wei et al. [137] fabricated ZnO p-n junction LEDs onc-Al2O3 substrates by plasma-assisted MBE. A gas mixture ofN2 and O2 was used as the p-type dopant, by which the double-donor doping of N2(O) was avoided significantly. At a lowtemperature of 100 K, the I–V curve of the ZnO LED showeda good rectification characteristic with a low leak current atreverse-bias voltages and a threshold voltage 4.0 V at forwardbias. The p-n junction LED kept a good rectifying characteristiceven at temperatures up to 300 K. Fig. 13 shows the EL spectrameasured at room temperature under forward injection currentsof 2.42, 2.65, 2.86, 3.09, and 3.31 mA. Under the injection

Fig. 14. EL spectra of a p-n ZnO diode at room temperature, with increasinginjection current from 60 to 100 mA. Inset: PL spectrum of the same device atroom temperature [138].

current of 2.42 mA, the emission band is located at 2.83 eVwith an FWHM of 500 meV. With increasing injection current,the emission band shifted to the higher energy side. The insetshows the normalized EL spectra, which shows the peak energyof the emission band shifting from 2.83 to 2.95 eV when theinjection current changed from 2.42 to 3.31 mA. The blueshiftof the emission band with the injection current suggests thatthe EL mechanism originated from the donor–acceptor pairrecombination in the p-type ZnO layer.

Kong et al. [138] and Chu et al. [139]showed a ZnOp-n junction grown on an n-type Si (100) substrate using MBE.A thin MgO buffer layer was deposited at 350 ◦C to reducethe lattice mismatch between Si and ZnO, followed by 410 nmGa-doped n-type ZnO and 410 nm Sb-doped p-type ZnO. Thediode showed fairly good rectification behavior with a thresholdvoltage of 2.5 V. Fig. 14 shows the EL spectra obtained atdifferent injection currents. An NBE emission at 3.2 eV startsto appear when the current is 60 mA. Afterward, the intensity ofthis emission increases as the injection current increases from60 to 100 mA. Moreover, the intensity of this emission peakincreases evidently at a current of 60–80 mA, while it changesless significantly at 80–100 mA, which is due to the heateffect as a result of the increasing current through the diode.When injection current increases from 60 to 100 mA, the UVemission peak also slightly shifts from 385 to 393 nm. This istypical in a radiative recombination process for a direct bandgapmaterial because heat induced by increased injection currentwill decrease its effective bandgap. The output power of thisLED is estimated to be only 1 nW at a drive current of 100 mA.

Zhao et al. [140] fabricated a ZnO homojunction on thec-plane sapphire substrate by MOCVD. Diethylzinc, trimethylantimony, and high-purity O2 were used as Zn, Sb precursor,and oxidizer, respectively. The unintentionally doped ZnO witha thickness of 500 nm was used as an n-type layer. The Sb-doped p-type ZnO layer with a thickness of 300 nm was grownon top of the n-type ZnO layer. From the X-ray photoelectronspectroscopy spectrum, the Sb 3d3/2 peak and 3d5/2 at thebinding energy of 539.3 and 530.2 eV indicated that Sb shouldexist as SbZn rather than SbO in the Sb-doped ZnO film. Aclear rectifying behavior of the ZnO p-n homojunction wasdemonstrated by the I–V curve. The forward turn-on voltageand reverse breakdown voltage were 3.3 and 5.0 V, respectively.No noticeable changes in the spectrum shape were observed



TABLE VSURVEY OF STRUCTURE, METHOD, AND EMISSION COLOR OF

ZnO-BASED HOMOJUNCTION LEDs

under injection currents of 40, 60, and 80 mA. The EL spectrumcould be fitted to three independent peaks centered at 3.00, 2.31,and 1.74 eV. On comparing EL spectra with the PL spectra,the emission peak at 3.0 eV on the EL spectrum is associatedwith NBE emission. A dominated emission peak at 1.74 eVwith a high energy shoulder of 2.31 eV is observed in the ELspectra. The emission of 2.31 eV should be assigned to theintrinsic defects related emission. Considering the difference inthe emission position of the EL and PL, the emission of 1.74 eVin the EL spectrum should be Sb-related deep-level emission.

There have been several attempts to reduce hydrogenincorporation at or near the surface and mesa sidewall thatwas introduced when the LED structure was fabricated bywet etching and photolithography [141], [142]. Kim et al.[141] reported that annealing at 350 ◦C for 5 min in O2

improved I–V characteristics and EL intensity. ZnO-basedLEDs were fabricated on c-plane sapphire using P-dopedZnO/Zn0.9Mg0.1O/ZnO/Zn0.9Mg0.1O/Ga-doped ZnO p-i-nstructures using PLD. The LED fabrication started withdevice isolation, followed by p-mesa definition using dilutephosphoric acid solution. The resistivity of P-doped ZnO wasdecreased significantly with exposure to the etchant due toincorporation of hydrogen as a donor. Although hydrogenatoms can easily diffuse into ZnO films, they also can bedriven out of the films by thermal annealing. The annealedLEDs showed rectifying I–V characteristics with a turn-onvoltage of 2.2 V and an EL peak at 385 nm. Wang et al. [142]have investigated the passivation effects of dielectric materials(SiO2 and SiNx) on ZnO LEDs. The 300-nm SiO2 films weredeposited after mesa formation. After deposition of SiO2,there were no diode rectifying characteristics and no lightemission for the as-fabricated LEDs. After annealing at 350 ◦C,under an O2 ambient, however, diodes showed rectifying I–Vcharacteristics and EL emission at 385 nm and a broad defectband ranging from around 500 to 1000 nm at room temperature.The 350 ◦C annealing would have significant impact on thediode characteristics through removing the acceptors in thep-ZnO by removal of hydrogen. The annealed SiNx-passivatedZnO LEDs showed leaky diode characteristics and no lightemission. The difference in I–V characteristics and ELproperty between SiO2- and SiNx-passivated LEDs may resultfrom residual hydrogen in the dielectric. The SiNx-passivatedLED has a significant concentration of hydrogen left suchas N–H and Si–H bonds, whereas no hydrogen bonding wasdetected either for unpassivated or SiO2-passivated LEDs. Theproperties of ZnO-based homojunction LEDs are summarizedin Table V, together with the structure, growth method, andemission peak position [76], [132]–[156].

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Yong-Seok Choi was born in Korea in 1983. Hereceived the B.S. degree in material science and engi-neering in 2005 from Chonnam National University,Gwangju, Korea, and the M.S. degree in materialscience and engineering in 2007 from the GwangjuInstitute of Science and Technology (GIST), wherehe is currently working toward the Ph.D. degree inmaterial science and engineering in the Departmentof Materials Science and Engineering under the su-pervision of Prof. Park.

He is also with the Department of Nanobio Ma-terials and Electronics, GIST. His current research interests include growth ofZnO by MOCVD and ZnO-based optoelectronic devices.

Jang-Won Kang received the B.S. degree in physicsin 2007 from Kyunghee University, Seoul, Korea,and the M.S. degree in material science and en-gineering in 2009 from the Gwangju Institute ofScience and Technology (GIST), Gwangju, Korea,where he is currently working toward the Ph.D.degree in material science and engineering in theDepartment of Materials Science and Engineeringunder the supervision of Prof. Park.

He is also with the Department of Nanobio Ma-terials and Electronics, GIST. His current research

interests include ZnO-based optoelectronic devices.

Dae-Kue Hwang received the B.S. degree in materi-als science and engineering from Chonbuk NationalUniversity, Jeonju, Korea, in 2002, the M.S. andPh.D. degrees in materials science and engineeringfrom the Gwangju Institute of Science and Technol-ogy, Gwangju, Korea in 2004 and 2008, respectively,with a focus on ZnO light-emitting diodes (LEDs),ZnO and GaN heterojunction LEDs, transparent con-ducting oxide for optoelectronic devices, ZnO fieldeffect transistors (FETs) and ZnO nanowire FETs.

He is currently a Postdoctoral Fellow with the Ma-terials Research Institute, Northwestern University, Evaston, IL. His researchinterests include photonic crystal, surface plasmons for biosensor, atomic layerdeposition of oxide materials, and dye-sensitized solar cells.

Seong-Ju Park (M’00) was born in Korea in 1952.He received the B.S. degree in chemistry and theM.S. degree in physical chemistry from Seoul Na-tional University, Seoul, Korea, in 1976 and 1979,respectively, and the Ph.D. degree in physical chem-istry from Cornell University, Ithaca, NY, in 1985.

From 1985 to 1987, he was a PostdoctoralFellow with the IBM T.J. Watson Research Center,Yorktown Heights, NY. From 1987 to 1995, he waswith the Electronics Telecommunications ResearchInstitute, Taejeon, Korea, as a Principal Researcher.

In 1995, he joined the faculty of the Department of Materials Science andEngineering, Gwangju Institute of Science and Technology (GIST), Gwangju,Korea, where he is currently a Professor. He is also with the Department ofNanobio Materials and Electronics, GIST. He is the Director of the SamsungLED Research Center, GIST. He is also the Director of GIST Technology In-stitute, GIST. He is a member of Korean Academy of Science and Technology.He is the author or a coauthor of more than 230 refereed papers. His currentresearch interests include growth and fabrication of GaN LEDs, Si quantumdot LEDs/Flash memory, and ZnO LEDs/TFT displays.


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