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Dual-Color Emission in Hybrid IIINitride/ZnO Light Emitting Diodes

Gon Namkoong, Elaissa Trybus1, Maurice C. Cheung2, W. Alan Doolittle1,

Alexander N. Cartwright2, Ian Ferguson1, Tae-Yeon Seong3, and Jeff Nause4

Electrical and Computer Engineering Department, Old Dominion University, Norfolk, VA 23529, U.S.A.1School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA 30332, U.S.A.2

Department of Electrical Engineering, University at Buffalo, The State University of New York, Buffalo, NY 14260, U.S.A.3Department of Materials Science and Engineering, Korea University, Seoul 136-712, Korea4Cermet Inc., Atlanta, GA 30318, U.S.A.

Received November 29, 2009; accepted January 22, 2010; published online February 12, 2010

We report dual-color production of the blue and green regions using hybrid nitride/ZnO light emitting diode (LED) structures grown on ZnO

substrates. The blue emission is ascribed to the near-band edge transition in InGaN while green emission is related to Zn-related defect levels

formed by the unintentional interdiffusion of Zn into the InGaN active layer from the ZnO substrates.

# 2010 The Japan Society of Applied Physics

DOI: 10.1143/APEX.3.022101

Z

nO/IIInitride heterojunction semiconductors are of

technological interest for the development of bright

light emitting diodes (LEDs). However, the propertiesof ZnO/IIInitride heterojunction devices1,2) have a great

potential for scientific impact. The uniqueness of ZnO has

attracted interests in developing high-efficiency optoelec-

tronic devices, such as the low-threshold UV lasers3) and

short wavelength LEDs.4) In particular, ZnO materials have

excellent luminous efficiency because of the large exciton

binding energy of 60 meV ($26meV for GaN).5) The various

wavelengths of ZnO materials can be easily obtained with

different dopants, such as ZnO:W for blue, ZnO:V for yellow

and ZnO:(Y,Eu) for red emission.6) The strong luminescence

and easy tuning of wavelength in ZnO will be beneficial to

the design of multi-wavelength LEDs. However, the lack of

reliable p-type ZnO hinders the development of ZnO-based

optoelectronic devices.7) Therefore, direct integration of III-

nitride emitters onto ZnO via p-type GaN has the potential to

produce advanced LEDs by combining strong luminescence

of ZnO and lattice-matched InGaN. The use of ZnO

substrates has already demonstrated the improved structural

quality of IIInitride materials.810) An additional advantage

is that high quality and low defect density (105 cm2) of ZnO

substrates are available at low costs.

Until now, most of the ZnO/IIInitride heterojunction

devices are epitaxially grown on p-GaN1,11) or p-AlGaN

templates.12) None of the studies address heterojunction

device performance on ZnO substrates because p-type GaNhas not been successfully grown on ZnO substrates. The lack

of progress is due to the volatility of the ZnO substrates13)

and the tendency for n-type compensation from oxygen.

Herein, we present, for the first time, the achievement

of p-type GaN layer on ZnO and the characteristics of

heterojunction ZnO/nitride LEDs. Furthermore, it is found

that the careful control of impurities from ZnO into InGaN

active layer can produce dual wavelengths of blue and green

emissions not observed from IIInitride/ZnO heterojunction

grown on p-GaN1) or p-type AlGaN templates.12)

IIInitride epilayers were grown on Zn-face ZnO

substrates from Cermet Inc. using molecular beam epitaxy

(MBE).10) 50-nm-thick InGaN and 0.4-m-thick Mg-

doped GaN were grown under metal-rich conditions. Hole

concentration was measured to be 3{5 1017 cm3 for

Mg-doped GaN. The indium composition of InGaN on Zn-

face ZnO was confirmed by X-ray diffraction measurementusing a Phillips Xpert Pro MRD. Optical measurement was

performed at room temperature (RT) using a 325 nm HeCd

laser, a 405 nm pulsed diode laser (Picoquant), and the

400 nm second harmonic of a Ti:sapphire femtosecond

pulsed laser (Coherent RegA-Mira). Using the femtosecond

($200 fs) pulses for excitation, the backscattered time-

resolved photoluminescence (PL) spectra and decays were

measured using a Hammatsu C4334 Streak Camera attached

to a Chromex 250 IS spectrograph. Electroluminescence

(EL) spectra were measured from a device of 350 350

m2. Ohmic contacts of Ni/Au and Ti/Al/Ti/Au were

formed on p- and n-type ZnO, respectively.

The present work uses low growth temperatures of 500

550 C with a Mg to achieve p-type GaN layer on Zn-polar

ZnO substrates. As-received ZnO substrate showed n-type

conductivity with electron concentration of 3 1016 cm3.

To compare the device performance, a GaN pn diode was

grown with similar structure on a sapphire substrate,

which has 0.15-m-thick Mg-doped p-GaN layer and 1.0-

m-thick Si-doped n-type GaN. Hall carrier concentrations

of n-type and p-type layers were estimated $1 1018 and

$3 1017 cm3 for electrons and holes, respectively. The

currentvoltage (IV) characteristics of hybrid GaN/ZnO

pn diodes and GaN pn diodes on sapphire substrates

are shown in Fig. 1. The devices show reasonable IVcharacteristics, with the less mature GaN/ZnO based diode

having higher reverse leakage currents. However, the IV

characteristic indicates that Mg-doped GaN on ZnO is

indeed a p-type conductive layer. Moreover, it is found that

current density of GaN/ZnO pn diodes is $4 times larger

than that of GaN pn diodes on sapphire substrates. The high

current density of hybrid pn diodes can be attributed to the

conductive ZnO substrates while a poor thermal conductivity

(35 W/mK) of sapphires restricts the operating currents.

Therefore, highly conductive ZnO substrates are promising

in terms of device performance (1) providing higher current

injections into the active layers and (2) more emission

intensity in LEDs.

For the hybrid p-GaN/n-ZnO heterojunction diodes, the

forward turn-on voltage was about 2.8 V which is close to the

values reported by Chuang et al.14) The ideality factor of theE-mail address: gnamkoon@odu.edu

Applied Physics Express 3 (2010) 022101

022101-1 # 2010 The Japan Society of Applied Physics

http://dx.doi.org/10.1143/APEX.3.022101http://dx.doi.org/10.1143/APEX.3.022101http://dx.doi.org/10.1143/APEX.3.022101http://dx.doi.org/10.1143/APEX.3.022101http://dx.doi.org/10.1143/APEX.3.022101http://dx.doi.org/10.1143/APEX.3.022101

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hybrid diodes was $5, indicating that there exist multiple

transport mechanisms such as defect-assisted tunneling and

carrier recombination in the space charge region. Eventhough we used the low growth temperature, real-time

reflection high-energy electron diffraction (RHEED) indi-

cates the formation of interfacial layer at the initial growth

stage.10) Therefore, such an interfacial layer contains defects

which may allow for tunneling path or carrier recombination

during the operation of p-GaN/n-ZnO diodes.

The achievement of p-type GaN on ZnO has lead us to

grow and fabricate a heterojunction of p-GaN/In0:07Ga0:93N/

ZnO LEDs. The IV characteristics of heterostructure

devices are presented in Fig. 2(a). EL at an applied current

of 40 mA shows near-UV peak at 396 nm. As the applied

current increases from 40 to 60 mA, the yellow emission at

560 nm shows a drastic increase in intensity. Moreover, weak

blue emission at 483 nm is also observed.

Near-UV emission at 396nm can be attributed to band

edge emission of the active In0:07Ga0:93N layer. At a higher

injection current of 60 mA, the band edge emission of

GaN at 360 nm appears with yellow emission at 560 nm,

indicating that the radiative recombination occurs in GaN

layer. The PL spectra of p-GaN/InGaN/ZnO measured at

RT are shown in inset of Fig. 2(a). As seen from the figure,

the PL spectrum consists of intense yellow luminescence

emission with a wavelength of$550 nm. The broad yellow

band is commonly observed and is attributed to Mg-related

defects in Mg-doped GaN layers.15) Therefore, highercurrents inject electrons from the n-ZnO to the InGaN and

into the p-GaN.

The EL peak at 483 nm is not clear since PL emissions of

the p-GaN/InGaN/ZnO are not correlated to EL peaks. To

investigate the possible origin of blue peak, a 50-nm-thick

In0:07Ga0:93N was grown on ZnO at 515C and was examined

with transmission electron microscopy (TEM) and time-

resolved PL analysis. A TEM image of the sample is shown

in the inset of Fig. 3(a). The image shows that the interface

between the In0:07Ga0:93N and the ZnO is fairly planar. The

corresponding energy dispersive spectroscopy (EDS) of

InGaN grown on ZnO substrates indicates that the inter-

diffusion occurred at the interface of InGaN/ZnO substrates.

Zn and O atoms indeed diffused into InGaN layers and Ga

and N also diffused into ZnO. These impurities from ZnO can

significantly affect optical properties of the InGaN layer, and

they can be a possible cause of the broad PL spectrum of the

In0:07Ga0:93N/ZnO around the PL peak at 483 nm, as shown

in Fig. 3(b). To deepen our understanding of this broad

emission, transient measurement by time-resolved PL was

performed. The intensity decay for the defects related PL is

usually non-exponential and can be fitted into the stretched

exponential decay:16,17)

Fig. 1. IV characteristic of GaN pn junction (black) on sapphire and

p-GaN/n-ZnO (red). Device size was 350 350m2.

(a) (b)

Fig. 2. (a) IV characteristic and (b) EL spectra of p-GaN/In

GaN/n-ZnO with different forward currents. Inset of (a) shows

photoluminescence of p-type GaN/InGaN/ZnO structures measured

at RT.

(a)

(b)

(c)

Fig. 3. (a) TEM image of InGaN on ZnO and corresponding EDS

profile and (b) photoluminescence and (c) time resolved PL (TRPL)

of In0:07Ga0:93N grown on ZnO, indicating the power law decay of

It I0t1:4.

G. Namkoong et al.Appl. Phys. Express 3 (2010) 022101

022101-2 # 2010 The Japan Society of Applied Physics

8/3/2019 Dual-Color Emission in Hybrid IIINitride/ZnO Light Emitting Diodes

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It I0 exp t

eff

" #; 1

or the power-law decay18)

of:

It I0tm; 2

where eff is the effective lifetime and is the stretched

component parameter. The potential fluctuation of band-edge

transition, typically present in InGaN as a result of the

randomly localized indium clusters in potential wells, can be

fitted into the stretched exponential decay.16,17) However, it is

seen from a Fig. 3(c), the PL decay near the 483 nm transition

measured at RT exhibits a power-law decay that fits well with

the exponent of m 1:4. The decay kinetics following the

power-law decay usually represent possible tunneling driven

radiative recombination via various trap centers or defect

levels.1921) Since interdiffusion of Zn and O into the InGaN

may creates various defect levels, such as deep acceptor22)

and donor defect states,23) a power-law decay in InGaN layer

can be related to the radiative recombination though such

defect centers. Moreover, the broad PL peaks possibly caused

by Zn interdiffusion profiles into InGaN indicate the

broadened acceptor levels.24) It should be noted that Zn is

an acceptor in nitride materials and occupies deep energy

state of$0:5 eV above the valence band in ternary InGaN

materials, as Nakamura et al.22) indicated. Therefore, PL

spectrum at 483 nm (2.57 eV) should be related to Zn-related

deep acceptor states.

Since interdiffusion of Zn and O into InGaN activelayer creates multiple defect levels, the control of indium

composition can produce dual wavelengths in nitride/ZnO

LEDs. For this purpose, hybrid LEDs were grown on n-type

ZnO with higher indium composition of 14% in active

InGaN layer. EL spectra of p-GaN/In0:14Ga0:86N/n-ZnO

heterostructure were measured under different forward

currents and are shown in Figs. 4(a) and 4(b). The EL

spectra of heterojunction LEDs consist of two different

wavelength spectra of blue and green emissions. At the

forward current of 40 mA, green emission at 516 nm is

dominant and can be attributed to Zn related band emission

in InGaN. Further increase in the forward current to 60 and

100 mA increases the EL intensity. Moreover, the blue

emission spectra also show the peak position shift toward

the shorter wavelength from $432 nm (2.87 eV) to 411nm

(3.01 eV) as injection current increases from 60 to 100 mA,

respectively. The photographs of the hybrid LEDs results in

bluish white color because of the dual emissions of green

and blue, as shown in Fig. 4(b). Non-uniform EL spectra

observed in Fig. 4(b) are possibly related to the non-optimal

growth and fabrication conditions, in conjunction with very

rough surface of nitride/ZnO, formation of interfacial layers,

and non-uniform p-type spreading layers of Au metals.

In conclusion, hybrid IIInitride/ZnO LEDs were demon-strated by achieving p-type GaN on ZnO substrates.

Unintentional interdiffusion of the Zn and O from ZnO into

InGaN layer creates multiple defect energy levels which are

responsible for green emission in hybrid LEDs. Furthermore,

multi-quantum well (MQW) structures in the active layer

may produce bright dual wavelengths if impurities from the

ZnO diffusing into the InGaN active layer are carefully

controlled.

Acknowledgments Distribution Statement A (Approved for Pub-

lic Release, Distribution Unlimited). The publication of this article was

partially supported by National Science Foundation.

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(a) (b)

Fig. 4. (a) Electroluminescence spectra of p-GaN/In0:14Ga0:86N/ZnO

LEDs and (b) photographs of the LEDs at different forward currents.

G. Namkoong et al.Appl. Phys. Express 3 (2010) 022101

022101-3 # 2010 The Japan Society of Applied Physics

http://dx.doi.org/10.1002/adma.200305729http://dx.doi.org/10.1002/adma.200305729http://dx.doi.org/10.1002/adma.200305729http://dx.doi.org/10.1002/adma.200305729http://dx.doi.org/10.1063/1.1940736http://dx.doi.org/10.1063/1.1940736http://dx.doi.org/10.1063/1.1940736http://dx.doi.org/10.1063/1.121620http://dx.doi.org/10.1063/1.121620http://dx.doi.org/10.1063/1.121620http://dx.doi.org/10.1063/1.121620http://dx.doi.org/10.1116/1.2180255http://dx.doi.org/10.1116/1.2180255http://dx.doi.org/10.1116/1.2180255http://dx.doi.org/10.1116/1.2180255http://dx.doi.org/10.1063/1.122077http://dx.doi.org/10.1063/1.122077http://dx.doi.org/10.1063/1.122077http://dx.doi.org/10.1002/adfm.200304335http://dx.doi.org/10.1002/adfm.200304335http://dx.doi.org/10.1002/adfm.200304335http://dx.doi.org/10.1038/nmat1284http://dx.doi.org/10.1038/nmat1284http://dx.doi.org/10.1038/nmat1284http://dx.doi.org/10.1143/JJAP.45.L611http://dx.doi.org/10.1143/JJAP.45.L611http://dx.doi.org/10.1143/JJAP.45.L611http://dx.doi.org/10.1143/JJAP.45.L611http://dx.doi.org/10.1143/JJAP.45.L611http://dx.doi.org/10.1063/1.2206883http://dx.doi.org/10.1063/1.2206883http://dx.doi.org/10.1063/1.2206883http://dx.doi.org/10.1063/1.2206883http://dx.doi.org/10.1063/1.2206883http://dx.doi.org/10.1063/1.2120912http://dx.doi.org/10.1063/1.2120912http://dx.doi.org/10.1063/1.2120912http://dx.doi.org/10.1063/1.2120912http://dx.doi.org/10.1088/0957-4484/18/5/055608http://dx.doi.org/10.1088/0957-4484/18/5/055608http://dx.doi.org/10.1088/0957-4484/18/5/055608http://dx.doi.org/10.1088/0957-4484/18/5/055608http://dx.doi.org/10.1088/0957-4484/18/5/055608http://dx.doi.org/10.1063/1.1632537http://dx.doi.org/10.1063/1.1632537http://dx.doi.org/10.1063/1.1632537http://dx.doi.org/10.1016/j.cap.2004.01.039http://dx.doi.org/10.1016/j.cap.2004.01.039http://dx.doi.org/10.1016/j.cap.2004.01.039http://dx.doi.org/10.1016/j.cap.2004.01.039http://dx.doi.org/10.1063/1.2822817http://dx.doi.org/10.1063/1.2822817http://dx.doi.org/10.1063/1.2822817http://dx.doi.org/10.1063/1.2822817http://dx.doi.org/10.1088/0953-8984/10/48/029http://dx.doi.org/10.1088/0953-8984/10/48/029http://dx.doi.org/10.1088/0953-8984/10/48/029http://dx.doi.org/10.1088/0953-8984/10/48/029http://dx.doi.org/10.1063/1.122843http://dx.doi.org/10.1063/1.122843http://dx.doi.org/10.1063/1.122843http://dx.doi.org/10.1002/pssc.200461498http://dx.doi.org/10.1002/pssc.200461498http://dx.doi.org/10.1002/pssc.200461498http://dx.doi.org/10.1002/pssc.200461498http://dx.doi.org/10.1088/0953-8984/18/4/020http://dx.doi.org/10.1088/0953-8984/18/4/020http://dx.doi.org/10.1088/0953-8984/18/4/020http://dx.doi.org/10.1063/1.441677http://dx.doi.org/10.1063/1.441677http://dx.doi.org/10.1063/1.441677http://dx.doi.org/10.1063/1.441677http://dx.doi.org/10.1134/1.1485662http://dx.doi.org/10.1134/1.1485662http://dx.doi.org/10.1134/1.1485662http://dx.doi.org/10.1016/0022-0248(94)91163-0http://dx.doi.org/10.1016/0022-0248(94)91163-0http://dx.doi.org/10.1016/0022-0248(94)91163-0http://dx.doi.org/10.1016/0022-0248(94)91163-0http://dx.doi.org/10.1007/s11664-997-0007-xhttp://dx.doi.org/10.1007/s11664-997-0007-xhttp://dx.doi.org/10.1007/s11664-997-0007-xhttp://dx.doi.org/10.1007/s11664-997-0007-xhttp://dx.doi.org/10.1063/1.116351http://dx.doi.org/10.1063/1.116351http://dx.doi.org/10.1063/1.116351http://dx.doi.org/10.1063/1.116351http://dx.doi.org/10.1063/1.116351http://dx.doi.org/10.1063/1.116351http://dx.doi.org/10.1063/1.116351http://dx.doi.org/10.1007/s11664-997-0007-xhttp://dx.doi.org/10.1016/0022-0248(94)91163-0http://dx.doi.org/10.1134/1.1485662http://dx.doi.org/10.1134/1.1485662http://dx.doi.org/10.1063/1.441677http://dx.doi.org/10.1088/0953-8984/18/4/020http://dx.doi.org/10.1002/pssc.200461498http://dx.doi.org/10.1063/1.122843http://dx.doi.org/10.1088/0953-8984/10/48/029http://dx.doi.org/10.1088/0953-8984/10/48/029http://dx.doi.org/10.1063/1.2822817http://dx.doi.org/10.1063/1.2822817http://dx.doi.org/10.1016/j.cap.2004.01.039http://dx.doi.org/10.1063/1.1632537http://dx.doi.org/10.1063/1.1632537http://dx.doi.org/10.1088/0957-4484/18/5/055608http://dx.doi.org/10.1088/0957-4484/18/5/055608http://dx.doi.org/10.1063/1.2120912http://dx.doi.org/10.1063/1.2206883http://dx.doi.org/10.1063/1.2206883http://dx.doi.org/10.1143/JJAP.45.L611http://dx.doi.org/10.1143/JJAP.45.L611http://dx.doi.org/10.1038/nmat1284http://dx.doi.org/10.1002/adfm.200304335http://dx.doi.org/10.1063/1.122077http://dx.doi.org/10.1063/1.122077http://dx.doi.org/10.1116/1.2180255http://dx.doi.org/10.1063/1.121620http://dx.doi.org/10.1063/1.1940736http://dx.doi.org/10.1002/adma.200305729