ApplPhysLett_91_263501

Realization of n-Zn1?x Mg x O/i-ZnO/SiO x/n+-Si heterostructured n-i-n light-emitting diodes by low-cost ultrasonic spray pyrolysis J.L.Zhao and X.W.Sun a?

School of Electrical&Electronic Engineering,Nanyang Technological University,Nanyang Avenue,

Singapore639798,Singapore

S.T.Tan,G.Q.Lo,and D.L.Kwong

Institute of Microelectronics,11Science Park Road,Science Park II,Singapore117685,Singapore

Z.H.Cen

School of Electrical&Electronic Engineering,Nanyang Technological University,Nanyang Avenue,

Singapore639798,Singapore

?Received12November2007;accepted26November2007;published online27December2007?

n-Zn1?x Mg x O/i-ZnO/SiO x/n+-Si heterostructured light-emitting diodes have been demonstrated by

low-cost ultrasonic spray pyrolysis.The current-voltage measurement shows typical characteristics

of a back-to-back diode due to the double Schottky barriers induced by the SiO x layer.Blue

electroluminescence peaking at460nm was observed at room temperature when a positive bias of

?4V was applied on the Si substrate.The electroluminescence is suggested to be dominated by the

donor-acceptor pair recombination in the i-ZnO layer,where the holes were injected from the

valence band of Si into the acceptor level of i-ZnO.?2007American Institute of Physics.

?DOI:10.1063/1.2824813?

ZnO has received overwhelming attention in blue/UV and white solid state lighting with potential advantages over the III-nitride system due to the higher exciton binding en-

ergy,lower growth temperature,and ease of wet etching.1,2

However,reliable and high quality p-type ZnO?lms are far

more dif?cult to achieve and which has stumbled many re-

searchers in the past decades.3,4As a result,a lot of efforts

have been devoted to hybrid heterojunction light-emitting

devices?LEDs?by growing n-type ZnO?lm on top of a

variety of p-type layers,such as GaN,AlGaN,Si,GaAs,SiC,

SrCu2O2,etc.2,5,6Among the attempts,ZnO/Si heterostuc-

tured LEDs have stirred up considerable interest for integrat-

ing ZnO onto Si.Although most of the works on ZnO/Si

heterostructured LEDs adopt a n-ZnO/p-Si heterostructure,

n-ZnO/n-Si isotype heterostructure has also been found promising for optoelectronic applications.7,8Very recently,

we developed a n-ZnO/SiO x/n+-Si heterostructured LEDs using metal-organic chemical-vapor deposition?MOCVD?

technique,which shows better electroluminescence?EL?per-

formance than n-ZnO/SiO x/p-Si heterostructured LEDs. The n-Si exhibits lower valence-band energy difference with

SiO x/ZnO than p-Si,favoring the hole injection from Si to ZnO.

As well known,high quality epitaxial ZnO layer is pre-

ferred for fabricating LEDs,which can be realized by

MOCVD,molecular beam epitaxy,and pulsed laser deposi-

tion.However,these deposition methods are generally ex-

pensive,resulting in high fabricating cost of the LEDs.

Therefore,development of polycrystalline ZnO-based LEDs

by low-cost techniques is of particular interest for some ap-

plications where high device performance is not required.

Ultrasonic spray pyrolysis?USP?has been widely used for

the growth of ZnO polycrystalline?lms,9and the

n-ZnO/p-GaAs LEDs has also been reported recently.10USP possesses the advantages of simplicity,low equipment cost,

low growth temperature,and high deposition rate.In this

letter,we demonstrated the n-Zn1?x Mg x O/i-ZnO/SiO x/n+-Si heterostructured LEDs by USP.A distinct blue emission cen-

tered at about460nm was observed at room temperature

when the Si substrate was positively biased.

The ZnO and Zn1?x Mg x O?lms used in this study were deposited by USP on n+-Si?100?wafers?carrier density of?1018cm?3?.The Si wafers were precleaned by sequen-tial ultrasonic baths of acetone and ethanol,resulting in a thin layer of SiO x??3nm determined by transmission elec-tron microscopy?on Si surface.The SiO x functions as a car-rier blocking layer for the light-emitting diodes.Aqueous solution of zinc acetate?Zn?CH3COO?2·2H2O,0.3mol/L?was used as the precursor to deposit undoped ZnO layer. The aerosol of the precursor was generated by a com-mercial ultrasonic nebulizer?frequency of1.65MHz?,and injected to the substrates heated at400°C.The thickness of ZnO layer was controlled at about200nm.After the depo-sition of ZnO?lm,a100-nm-thick In-doped Zn1?x Mg x O layer was deposited on ZnO by USP at the same temp-erature,using the mixed aqueous solution of zinc acetate?Zn?CH3COO?2·2H2O?,magnesium acetate ?Mg?CH3COO?2·2H2O?,and indium nitrate ?In?NO3?3·2H2O?as the precursor.The atomic ratio of Zn:Mg:In is controlled at16:4:1.The In-doped Zn1?x Mg x O layer is employed as the electron transport layer and trans-parent window for the LEDs.Au electrodes were deposited on the Zn1?x Mg x O layer and the back side of n+-Si wafers by direct-current magnetic sputtering,with thickness of about30and200nm,respectively.The Au electrodes on Zn1?x Mg x O were patterned into circular pads with diameter of1mm using a shadow mask.For the purpose of structural, optical,and electrical characterizations,single layers of un-doped ZnO and In-doped Zn1?x Mg x O were also deposited on Si and glass substrates with exactly the same condition.

X-ray diffraction analysis revealed that both ZnO and Zn1?x Mg x O?lms by USP exhibits polycrystalline wurtzite

a?Author to whom correspondence should be addressed.Also at Institute of

Microelectronics,11Science Park Road,Science Park II,Singapore

117685.Electronic mail:exwsun@https://www.sodocs.net/doc/f02512632.html,.sg.

APPLIED PHYSICS LETTERS91,263501?2007?

0003-6951/2007/91?26?/263501/3/$23.00?2007American Institute of Physics

91,263501-1

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structure ?not shown here ?.Figure 1shows the room-temperature photoluminescence ?PL ?spectra of both ?lms,which was excited by a 325nm He–Cd laser.The band-edge emissions are peaking at 3.31and 3.49eV,for ZnO and Zn 1?x Mg x O ?lms,respectively.The x is estimated to be ?0.2,assuming that the band gap of Zn 1?x Mg x O varies lin-early with x .9Other than the band-edge emission,undoped ZnO ?lm exhibits two defect emissions,i.e.,green-band and orange-band emissions centered at 2.42and 1.94eV,respec-tively.The green-band emission has always been attributed to the oxygen vacancies while the orange-band emission has been suggested to the oxygen interstitials.11,12No defect re-lated emission was observed in the PL of In-doped Zn 1?x Mg x O ?lm.The undoped ZnO ?lm exhibits weak n -type conduction,with resistivity in the range of 102–103?cm.The carrier concentration is estimated to be less than 1016cm ?3for such a high resistivity.On the other hand,the In-doped Zn 1?x Mg x O ?lm shows n -type conduc-tion,with the electron concentration of 1.2?1018cm ?3.

The typical current-voltage ?I -V ?characteristics of the n -Zn 1?x Mg x O /i -ZnO /SiO x /n +-Si heterostructured LEDs as well as the device structure are shown in Fig.2.Ohmic con-

tact was established for Au /n -Zn 1?x Mg x O,as shown in the inset of Fig.2.The I -V of the n -i -n diode shows nonlinear behavior at both forward and reverse biases.The native SiO x at the i -ZnO /Si interface acts as a double Schottky barrier for both n layers and the junction can be considered as a series of two back-to-back Schottky diodes.13EL can be ob-served from the diode at room temperature when a positive voltage is applied at the n +-Si substrate,while no EL is de-tected under reverse bias.The EL spectra of the emission with current injection from 80to 175mA are shown in Fig.3.The emission at lower injection current is centered at ?500nm,which shifts to higher energy with the increasing current.Blue emission centered at ?460nm can be clearly observed from naked eyes when the injection current is higher than 138mA.The inset of Fig.3shows the photo of the emitting LED biased with 4.5V and 175mA in a dark room.

The mechanism of current transport and electrolumines-cence can be understood by examining the energy-band structure of the heterostructures.Figure 4illustrates the energy-band diagrams of ZnO /SiO x /Si interface at equilib-rium and under bias that derived from Anderson model.Due to the considerable electron concentration in the i -ZnO ??1016cm ?3?,the Fermi level of the i -ZnO is close to the E C ?ZnO ?.When a positive voltage is applied on the Zn 1?x Mg x O,the SiO x /Si Schottky barrier is under forward bias and the electrons transport from the E C ?Si ?to the E C ?ZnO ?,giving rise to the conduction current.The turn-on voltage of negative-biased Si substrate is about 4.2V ?Fig.2?,which is in good agreement with the SiO x and Si conduc-tion band offset ??E e ?Si →ZnO ??3.92eV,as shown in Fig.4?a ??.Although excess electrons exist in i -ZnO ?injected from n +-Si ?,limited holes can be injected into i -ZnO from n -Zn 1?x Mg x O.Therefore,no EL signal was detected when a negative voltage was applied to Si substrate.On the other hand,understanding of the transport mechanism when Si substrate is positive biased is more challenging.Observation of EL from negatively biased Zn 1?x Mg x O indicates that holes have been effectively injected into i -ZnO from Si,as only radiative recombination of electron-hole pairs in i -ZnO layer will then give rise to light emission.At the interface of Si and SiO x ,the energy bands of Si bend upward at

positive

FIG.1.?Color online ?Room-temperature photoluminescence spectra of undoped ZnO and In-doped Zn 1?x Mg x O ?lms grown by ultrasonic spray

pyrolysis.

FIG. 2.?Color online ?Current-voltage characteristic of n -Zn 1?x Mg x O /i -ZnO /SiO x /n +-Si diode.The bottom right inset illustrates the diode structure and the top left inset shows the Ohmic contact between Au and n -Zn 1?x Mg x

O.

FIG.3.?Color online ?Room-temperature electroluminescence spectra with different injection currents.The inset shows the photo of an emitting LED at 175mA and 4.5V in a dark room.

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bias,which will induce an inversion layer under suf?cient applied electric ?eld.Besides,the E V ?Si ?is moving to lower-energy level under positive bias,resulting hole accumula-tions at the interface.When suf?cient bias is applied,the

E V ?Si ?is approaching the acceptor level in i -ZnO and those accumulated holes at the interface will then be swept into the acceptor level of i -ZnO through the SiO x barrier.The turn-on voltage of positive-biased Si substrate ?3.1eV ??see Fig.2?is

close to the barrier for hole inject from Si into i -ZnO ??E h ?Si →ZnO ??2.96eV,as shown in Fig.4?a ??.This suggests that the current of the LED under positive-biased Si substrate is governed by the hole injection from Si into i -ZnO.Mean-while,the electrons are injected from conduction band of n -Zn 1?x Mg x O into E C ?ZnO ?under bias,which accumulates near i -ZnO /SiO x interface due to the SiO x barrier.The re-combination of injected holes and electrons in conduction band or shallow donor level of i -ZnO will then give rise to EL.It is noted that the EL peak signi?cantly blueshifts with the increase of injection current,which is the typical charac-teristic of a donor-acceptor-pair emission.14Besides,the high-resistive characteristic of the i -ZnO ?lms and the exis-tence of the two defect related emissions in PL spectrum suggested that the i -ZnO ?lms might have both intrinsic do-nors and acceptors.Therefore,the EL emission is believed to be originated from the recombination of the electrons in shal-low donor level and the holes in deep acceptor level of i -ZnO.On the other hand,the near-band-edge UV emission has not been observed in this study,which is possibly due to the high defect density in USP-grown i -ZnO ?lm.

In summary,we have demonstrated

n -Zn 1?x Mg x O /i -ZnO /SiO x /n +

-Si heterostructured LEDs by a simple and low-cost spray pyrolysis method.It was found that the current-voltage characteristic of the diode shows back-to-back diode behavior due to the double Schottky barrier formed by the native SiO x insulating layer.Blue EL ??460nm ?was observed at room temperature when a positive voltage was applied on the Si substrate.The EL emission originates from the recombination of the elec-trons in the shallow donor level and the holes in the deep acceptor level of i -ZnO,where the holes were injected from the inversion layer formed in Si /SiO x interface.Such n -Zn 1?x Mg x O /i -ZnO /SiO x /n +-Si isotype diode is promising for the development of low-cost short-wavelength optoelec-tronic devices integrated on Si.

Financial supports from Singapore Agency for Science,Technology and Research ?A *STAR ?SERC Public Sector Fund ?No.0421010010?and Inter-RI project ?No.0521260095?are gratefully acknowledged.

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.FIG.4.?Color online ?Energy-band diagram at the i -ZnO /SiO x /n +-Si inter-face under thermal equilibrium ?a ?,positive bias at n -Zn 1?x Mg x O side ?b ?,and positive bias at Si substrate ?c ?.Downloaded 06 Jan 2011 to 202.114.78.123. Redistribution subject to AIP license or copyright; see https://www.sodocs.net/doc/f02512632.html,/about/rights_and_permissions