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Synthesis and characterization of organic/inorganic heterostructure

films for hybrid light emitting diode

Toshihiko Toyama *, Tokuyuki Ichihara, Daisuke Yamaguchi, Hiroaki Okamoto

Department of Systems Innovation, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan

Available online 12 July 2007

bstract

Thin-film light emitting devices based on organic materials have been gathering attentions for applying a flat-panel display and a solid-state

ighting. Alternatively, inorganic technologies such as Si-based thin-film technology have been growing almost independently. It is then expected

hat combining the Si-based thin-film technology with the organic light emitting diode (OLED) technology will develop innovative devices. Here,

e report syntheses of the hybrid light emitting diode (LED) with a heterostructure consisting of p-type SiCx and tris-(8-hydroxyquinoline)

luminum films and characterization for the hybrid LEDs. We present the energy diagram of the heterostructure, and describe that the use of high

ark conductivities of the p-type SiCx as well as inserting wide-gap intrinsic a-SiCx at the p-type SiCx/Alq interface are effective for improving

evice performance.

2007 Elsevier B.V. All rights reserved.

ACS : 78.60.Fi; 73.21.Ac; 73.40.Lq; 72.80.Le; 79.60.�I; 79.60.Jv

www.elsevier.com/locate/apsusc

Applied Surface Science 254 (2007) 295–298

eywords: Light emitting diode; Thin film; Silicon carbide; Organic semiconductor; Electronic structure; Trapped-charge-limited current

1. Introduction

Thin-film light emitting devices have been gathering

attentions for applying a flat-panel display and a solid-state

lighting [1,2]. An organic light emitting diode (OLED) is one of

the most promising candidates because of high luminance,

excellent visibility, lightweight and mass-producibility [1,2].

Alternatively, Si-based thin-film technology has also excellent

mass-producibility as evidenced by productions of thin-film

transistor and thin-film solar cells [3]. Furthermore, we

investigated thin-film light emitting devices based on silicon

carbide (SiCx) alloys very previously [4]. So far, the organic and

inorganic technologies have been growing almost indepen-

dently, however, it is expected that combining the Si-based thin-

film technology with the organic light emitting diode

technology will develop innovative devices. For instance,

compared to the organic materials, Si-based thin-films usually

have excellent durability against humidity, thus improving the

* Corresponding author. Tel.: +81 66 850 6317; fax: +81 66 850 6316.

E-mail address: toyama@ee.es.osaka-u.ac.jp (T. Toyama).

169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

oi:10.1016/j.apsusc.2007.07.071

lifetime of OLED is to be possible due to the combined

technology.

Here, we describe syntheses of the hybrid light emitting diode

(LED) based on the organic/inorganic heterostructure consisting

of p-type SiCx and tris-(8-hydroxyquinoline) aluminum (Alq)

films, and we also mention the energy diagram of the

heterostructure as well as current transport properties of the

hybrid LED.

2. Experimental details

The structure of the hybrid LED was glass/transparent

conductive oxide (TCO)/p-type SiCx/Alq/Al. Regarding the

TCO layer, SnO2:F/indium tin oxide (ITO) double-coated film

was used [4]. The p-type SiCx layer was deposited on the TCO

layer by plasma enhanced chemical vapor deposition (PECVD)

from a mixture of H2, SiH4, C2H6, and B2H6 gases at 230 8C.

The excitation frequency was 40.68 MHz [5,6]. The other

deposition conditions are described elsewhere in detail [5–7].

Amorphous (a-) or microcrystalline (mc-) SiCx films were

prepared for the p-type SiCx. After short air breakage for

transferring the sample from the PECVD apparatus, Alq

(Kodak) and Al were deposited on the p-type SiCx sequentially

Fig. 2. Light emission spectra of hybrid LED (solid) and of OLED (dotted),

respectively.

T. Toyama et al. / Applied Surface Science 254 (2007) 295–298296

by thermal evaporation in high vacuum (<3 � 10�6 Torr). The

area of the Al electrode was 0.033 cm2. Typical thicknesses of

p-type SiCx and Alq layers were 15 and 70 nm, respectively. A

conventional OLED with a hole-transport material of 4,40-bis[N-(1-naphthyl)-N-(2-naphthyl)amino]biphenyl (TNB)

(Kodak) instead of p-type SiCx was also fabricated.

Ultraviolet-ray photoyield spectroscopy (UV-PYS) is often

used for identifying the band profile. An UV-PYS system

consisting of a vacuum monochromator and a channeltron was

used [8]. Electrons excited by the monochromatic light are

emitted from surfaces, and detected by the channeltron.

Device characterization was performed with Keithley 2400

current source/voltmeter (or voltage source/ammeter), Topcom

BM-7fast luminance meter, and Ocean Optics USB2000

spectrometer. The LEDs were set in vacuum in an optical

dewar, and the light emission was monitored through a glass

window. All measurements were carried out at room

temperature.

3. Results and discussion

Fig. 1 shows the UV photoyield spectrum of the a-SiCx film.

The photoemission from surfaces, Y, can be expressed as

Y � (E � Eth)5/2 where Eth denotes the emission threshold

energy correspond to the excitation energy from the Fermi level

to the vacuum level, and E the photon energy being in

accordance with an indirect optical excitation equation [9]. Eth

was estimated as around 5.3 eV, which is in good agreement

with the Eth of a-SiCx with low carbon contents reported by

Brown et al. [9]. On the other hand, the temperature dependence

of a-SiCx indicated the activation energy of 0.6 eV, and the

activation energy of a p-type amorphous silicon alloy

corresponds to the Fermi level above the valence band top

[3]. The optical bandgap of a-SiCx of 1.9 eV was estimated

from the transmittance spectrum in accordance with the Tauc

plot [3]. Based on the experimental results, the energy diagram

Fig. 1. UV photoyield spectrum of p-type a-SiCx. Inset shows energy diagram

of hybrid LED based on p-type a-SiCx/Alq heterojunction.

of the hybrid LED is depicted in the inset of Fig. 1. The energy

structure including the energy levels of the normal lowest

unoccupied molecular orbital (LUMO) and highest unoccupied

molecular orbital (HOMO) of Alq is referred to Ref. [10].

From the hybrid LED, yellowish-green light emission was

successfully observed. Fig. 2 shows a typical emission

spectrum of the hybrid LED compared to that of OLED.

The emission peaking at 540 nm of the hybrid LED arises from

the Alq layer. The difference in the wavelengths of 600–800 nm

would originate from the interference fringe due to the

refractive index of p-type SiCx being higher than that of TNB.

Fig. 3 displays a typical current–voltage (J–V) characteristic

of the hybrid LED. A clear rectification is confirmed. Inset

shows the double logarithmic plot of the J–V curve at forward

voltages. At low voltages below 8 V, the J–V curve show a

power law, J–Vm+1 with an exponent m of 1.7 close to m of

Fig. 3. J–V characteristic of hybrid LED with p-type a-SiCx. The hybrid LED

was driven by voltages. Inset shows log–log plot of the J–V characteristic for

forward voltages.

Fig. 4. L–J characteristics of hybrid LEDs with p-type mc-SiCx (solid) and a-

SiCx (dotted), respectively.

Fig. 5. J–V (a) and L–J (b) characteristics of hybrid LEDs with a structure of p-

type mc-SiCx/wide-gap a-SiCx/Alq. The hybrid LEDs were driven by current.

The thicknesses of wide-gap a-SiCx are varied [without wide-gap a-SiCx; (*),

2 nm; (&) 3 nm; (~) 5 nm (^)]. Discontinues in luminance occur in changing

measurement ranges.

T. Toyama et al. / Applied Surface Science 254 (2007) 295–298 297

trap-free space-charge-limited current (=1) [11]. Between 8 and

10 V, an apparent negative resistance behavior appears,

suggesting that a hole tunneling process occurs maybe at the

potential height valence band top between SiCx and HOMO

level of Alq as shown in Fig. 1 because light emission starts at

around 10 V. At high voltages over 15 V, another power law

with m = 8.9 appears, indicating that the trapped-charge-

limited current (TCLC) should dominate as the J–V character-

istics of OLED in the light emitting region [10].

Fig. 4 shows the luminance–current (L–J) characteristics of

the hybrid LEDs with p-type a- and mc-SiCx. Fig. 4 clearly

indicates that the use of mc-SiCx is effective for decreasing

operation current as well as for increasing maximum luminance.

The dark conductivity of a-SiCx was 5.3� 10�7 S cm�1, while

that of mc-SiCx was 1.1 � 10�2 S cm�1. High hole concentra-

tion and high hole mobility are usually responsible for the large

dark conductivity of mc-SiCx [3]. Alternatively, concerning

the optical absorption coefficient, a, at a wavelength of 540 nm,

a of a-SiCx was 3 � 104 cm�1, while a of mc-SiCx was

4 � 104 cm�1. Therefore, it is inferred that the dominant

mechanism for improving device characteristics is improving

the hole injection due to the high hole concentration and/or

improving hole transport due to the increased hole mobility

rather than the slight increase in the optical transmittance

through the p-type SiCx.

Besides, inserting highly resistive wide-gap a-SiCx with an

optical bandgap of 3.3 eV into the interface between the p-type

SiCx and Alq is effective for further improving L–J

characteristics, being similar to the a-SiCx-based p–i–n LEDs

[4]. In Fig. 5, the effects of wide-gap a-SiCx are demonstrated;

current–voltage (J–V) and L–J characteristics are depicted as a

function of thickness of the wide-gap a-SiCx layer. Due to

inserting the wide-gap a-SiCx layer, the operation current

decreased, and the exponent of the power law for TCLC also

decreased. The exponent, m, varied with the thickness of the

wide-gap a-SiCx, d; m = 13, 10, 7.2, and 7.6 were estimated for

d = 0 nm (or without the wide-gap a-SiCx), 2, 3, and 5 nm,

respectively. Therefore, as shown in Fig. 5(b), the maximum

luminance is obtained from the hybrid LED with the 3-nm thick

wide-gap a-SiCx of which exponent for TCLC is the minimum

among the samples, indicating a decrease in the trap density at

p-type SiCx/Alq interface. After further optimizations for the

thicknesses of p-type SiCx and Alq, the maximum luminance of

170 cd/m2 has been achieved.

Finally, a primitive lifetime test was carried out. The lifetime

of the hybrid LED until the half to the initial luminance was

measured with the following conditions: constant operating

current, in air atmosphere, and without passivation against

humidity. The lifetime of the hybrid LED was still as low as

220 min, however, it was almost equivalent or slightly longer

than the lifetime of OLED with a structure of ITO/SnO2/TNB/

Alq/Al.

T. Toyama et al. / Applied Surface Science 254 (2007) 295–298298

4. Conclusions

We have investigated on the hybrid LEDs for developing

stable flat-panel light emitting devices. The structure of the

hybrid LED was glass/ITO/SnO2/p-type SiCx/Alq/Al. The UV-

PYS and the activation energy of the dark conductivity revealed

the energy structure of p-type SiCx/Alq heterojunction. Highly

conductive mc-SiCx is effective for decreasing operation

current as well as for increasing maximum luminance. Besides,

inserting highly resistive wide-gap a-SiCx with an optical

bandgap of 3.3 eV into the interface between the p-type SiCx

and Alq is effective for further improving L–J characteristics,

indicating that the p-type SiCx/Alq interface plays a crucial role

in determining the device performance, and that the exponent of

the power law at the TCLC region is a good quality factor for

characterizing the p-type SiCx/Alq interface. Finally, the

maximum luminance of 170 cd/m2 has been achieved employ-

ing the p-type SiCx/wide-gap a-SiCx/Alq structure.

Acknowledgements

The authors would like to thank to Dr. Takahashi for her kind

advices on UV-PYS measurements. The author (TT) would like

to thank to Ms. Shinohara of Kodak Japan Ltd. for providing

organic materials. This work was partially supported by Grant-

in Aids of New Energy and Industrial Technology Development

Organization.

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