phys. stat. sol. (c) 4, No. 7, 2411–2414 (2007) / DOI 10.1002/pssc.200674924

© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Nitride-organic semiconductor hybrid heterostructures

for optoelectronic devices

Hyunjin Kim1, Cuong Dang

1, Yoon-Kyu Song

1, Qiang Zhang

1, William Patterson

1,

A.V. Nurmikko*1

, K.-K. Kim2, S-Y. Song

2, and Jung Han

2

1 Division of Engineering, Brown University, Providence, RI 02912, USA 2 Department of Electrical Engineering, Yale University, New Haven, CT 06520, USA

Received 3 November 2006, revised 8 January 2007, accepted 11 January 2007

Published online 31 May 2007

PACS 73.40.-c, 73.50.-h, 85.60.-q

We explore hybrid gallium nitride-organic semiconductors as composite layered thin film materials and

report on initial results of fundamental studies of carrier transport across a junction composed of InGaN

and selected organic thin films, with the eventual application goal towards versatile optoelectronic devi-

ces.

© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction The subject of joining organic and inorganic semiconductors includes some early inves-tigations in the 1980s using Si or GaAs as the substrate for depositing thin organic films or assembled monolayers [1, 2]. More recently, II-VI colloidal nanocrystals have been incorporated within thin films of small organic molecules and polymers as structures for exploring light emitting [3] and photovoltaic [4] devices. One report has appeared on optical properties of polymer thin film/InGaN QW combination [5]. Here we explore hybrid material synergy, with III-nitrides as the high performance inorganic plat-form for integration with organics, ultimately for device applications include lighting and photovoltaics, as well as (bio)chemical sensors.

2 Charge transport across nitride/organic

semiconductor hetero-junction For the hybrid nitride/organic heterostructures, a core challenge is to understand the elec-tronic properties at/near the interface. We have performed initial experiments on pla-nar hybrid heterojunctions, employing In-GaN and GaN epitaxial thin films and QWs with organic thin films according to edu-cated guesses for matching the electronic states of the two media (i.e. the bandgap of the III-nitride approximately equals the HOMO-LUMO energy separation in the organic semiconductor partner, with elec-tron affinities in comparable range). Figure 1 shows a rough initial “roadmap” over the

* Corresponding author: e-mail: Arto_Nurmikko@brown.edu, Phone: +1 401 863 2869, Fax: +1 401 863 9120

F4-TCNQ

BCP

PTCDA

C60

PCBM

PTCBI

CBP

Alq3

a-N

PD

CuPc

P3HT

PEDOT

ITO Al

-8

-7

-6

-5

-4

-3

-2

Energ

y (

eV

)

Materials

InN

GaN

CB/LUMO

VB/HOMO

Fig. 1 Valence and conduction bands for InGaN and HOMO-

LUMO levels for selected organic material (plus Al and ITO).

2412 Hyunjin Kim et al.: Nitride-organic semiconductor hybrid heterostructures for optoelectronic devices

© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-c.com

compositional range of the InGaN ternary with and for selected small organic molecules and polymers, combining electron affinity data from surface science studies in the literature [6]. The organics are in established use in today’s LEDs, photovoltaics, and TFTs. The physics at the III-nitride/organic semiconductor interface can be divided into (a) electron-hole charge transport, and (b) excitation transfer, such as inter-mediated by excited electronic states (excitons) via Coulomb interaction across the interface. In this paper we focus on carrier transport experiments in planar devices, including their photoresponse. Figure 2 shows a schematic of a nitride-organic semiconductor test device struc-ture, based on n-type GaN single crystal epitaxial layer or InGaN SQW with a thin (few nm) GaN cap (Si doped, n ~ 1x1018 cm–3; grown by MOCVD) onto which a patterned Ti/Al ohmic contact structure (100 Å/1000 Å) was evaporated. After wet chemical etching to clean the n-GaN sur-face, organic thin films (~500 Å) were deposited under high vacuum (below 5x10–7 Torr) on the active area using an organic thermal evaporator and shadow mask technique. A metal contact was evaporated in-situ, immediately following the organic material deposition to ensure the integrity of the interface. With this metallic encapsulation to protect the organic layers from water and oxygen, we have to date investigated half a dozen common organic materials (typical in organic LEDs); The organic materials were chosen based on the energy line up of conduction/balance band and HOMO/LUMO (e.g. Fig. 1) and used as hole transporting layers in the heterostructure; For example, in Alq3 case, ‘valence-like’

band or highest occupied molecular orbital (HOMO) is offset from the GaN valence band maximum by an energy, ∆EV = 0.95 eV and the “conduction-like’ band or lowest unoccupied molecular orbital (LUMO), is then offset from the InGaN conduction band minimum by ∆EC = 0.15 eV, which shows relatively small energy barrier at the heterostructure interface. Examples of I-V data for n-GaN/α-NPD and n-GaN/Alq3 junctions are shown in Fig. 3 at room temperature, with both showing rectifying charac-teristics. The inset shows schematically the “hypothetical” band offsets that might be expected at zero bias without interface charge, dipoles, etc. The I-V trace for the n-GaN/Alq3/Al is relatively soft, sug-gesting that some mixture of electron and hole transport might be occurring through the Alq3 junction.

n-GaN

Sapphire substrate

Ti/Al Organic layer (~500 Å)

Metal contact (Al, Pd, or Au)

SiO2

(-) (+)

Fig. 2 Schematic of n-GaN/organic planar junction test “device”

structure (typical active area: 0.4 mm2).

-10 -8 -6 -4 -2 0 2 4-6

-4

-2

0

2

4

6

Cu

rre

nt

(mA

)

Voltage (V)

n-GaN

Alq3

3.4

6.9

5.95

3.25

4.28

Al

-15 -10 -5 0 5

0

5

10

Curr

ent

(mA

)

Voltage (V)

n-GaN

a-NPD

3.4

6.9

5.7

2.6

5.1

Au

Fig. 3 I-V characteristics of n-GaN/Alq3 and n-GaN/α-NPD: organic layers are used for hole transporting materials.

phys. stat. sol. (c) 4, No. 7 (2007) 2413

www.pss-c.com © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

By contrast, the I-V trace for the n-GaN/α-NPD/Au junction is reproducibly very crisp, and the forward

bias conditions show (modest) electroluminescence near the bulk bandgap of GaN as well. This suggests

that the hole transport layer, α-NPD, is dominant in injecting holes into GaN. This experiment has been

refined by imbedding a single 4 nm InGaN electron-hole pair “collector” quantum well within the n-GaN

host, very near (~2-6 nm) the adjacent organic thin film interface in an Al/n-GaN/InGaN/Alq3/Au device

structure. Again, weak and spatially inhomogeneous electroluminescence is obtained under forward bias,

at the bandgap energy of the InGaN (blue), but not from the organic (Alq3 or α-NPD) side of the junction

(even though in photoluminescence experiments under near UV excitation both the InGaN QW and the

organic partner show distinct and high degree of spontaneous emission).

These planar heterojunction structures have also been tested as “photodiodes”. Figure 4b displays a

typical photocurrent spectrum at room temperature, which shows a strong photoresponse (when com-

pared to an all-inorganic nitride QW pn junction reference LED). The spectrum is dominated by the

InGaN QW absorption (the short wavelength falloff of the photoresponse is due to the absorption by the

GaN buffer layer, as we illuminated the sample through the transparent sapphire substrate). Note that the

amplitude of the photovoltaic response is ‘nonlinear’, i.e. that an initial reverse bias is required to acquire

a measurable signal. Also, the thin film heterostructure is clearly able to sustain very large internal fields

as we have driven devices with organic layer thicknesses below 10 nm to |Vb | >20 V. The photoresponse

of the device is comparably fast when compared to the typical InGaN LED structure, which implies that

the response time is not significantly impeded by the low mobility of organic materials. From the charge

transport point of view, these results on the organic/n-nitride LEDs and photodiodes suggest that the

charge transport across the heterojunction is dominated by holes being able to cross the barrier in the

hybrid organic-inorganic interface. This follows as the electroluminescence or photocurrent response

were spectrally dominated by the InGaN part of the hybrid junction devices. Even in the case of an ener-

getically favorable hypothetical band lineup for electron transport based on bulk property of constituent

materials, e.g. n-GaN/Alq3 (or NPD) photodiode devices (e.g. Fig. 1), there was no clear evidence of

dominant electron transport in either the electroluminescence or photocurrent spectra, which are well

matched with other organic/inorganic heterojunctions reported [7]. The work is currently being ex-

panded, to include studies of the nitride doping (p-type vs. n-type), choice of a wider range of organic

molecules, GaN substrate orientation, and surface chemical functionalization.

Additionally, we have also employed conducting thin polymer films, deposited on the GaN-based

heterostructures, for vertical transport and optical evaluation. The polymer system offers a comple-

Fig. 4 (a) “Hypothetical” conduction/valence bands and HOMO/LUMO line up for the n-InGaN QW/Alq3 planar

heterojunction: Alq3 is used for hole transporting material (b) spectral photoresponse of the device operating in

reverse bias.

n -GaN

Alq3

3.4eV

6.5eV

5.95eV

3.25eV

5.1eV

AuInGaN MQW

Eg ~3.1eV

4.0eV

Al

280 300 320 340 360 380 400 420 440 460-500

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

Photo

curr

ent (n

A)

wavelength(nm)

0V-1V-2V

-3V-4V-5V-6V

-7V-8V-9V-10V-11V-12V-13V

InGaN QW 280 300 320 340 360 380 400 420 440 460-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Photocurrent(µA)

Wavelength(nm)

0V

-1V

-2V

-3V

-4V

-5V

-6V

-7V

-8V

-9V

-10V

-11V

-12V

-13V

(a) (b)

2414 Hyunjin Kim et al.: Nitride-organic semiconductor hybrid heterostructures for optoelectronic devices

© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-c.com

ment/alternative to the small organic molecules (as in e.g. the field of organic LEDs), which shows better

conformance to nanopatterned nitride device structures which we are also interested in exploring. .

Polymer selection was based on the anticipated HOMO/LUMO alignment with conduction/valence band

of InGaN. We have found that using chemical vapor deposition polymerization (CVD-P) and spin coat-

ing of PF (polyfluorene) based co-polymer (poly[9,9-dioctylfluorenyl-2,7-diyl)-co-1,4-benzo-{2,1-3}-

thiadiazole)]-end-capped with DMP renders the best performance in terms of consistence, morphology,

and optical efficiency. Electrical injection was carried out on a p-fluorene/n-InGaN p-n diode leading to

rectifying behavior similar to the small molecular case emphasized in this paper. These results will be

reported elsewhere in more detail.

3 Brief comment on excitation transport across the heterojunction While not included in this report,

we have also begun to employ spectroscopic and transient optical experiments on the hybrid In-

GaN/organic bilayer heterostructures to look for signatures of possible exciton intermediated coupling

across the heterointerfaces, as predicted theoretically by Agranovich [8, 9]. In this instance, one looks for

excitation transfer produced by resonantly coupled excitons across the interface, propelled by dipole-

dipole interaction and without the necessity of charge crossing the interface. Claim to such “Forster-

transfer” between the Wannier and Frenkel excitons has been recently made [8, 9]; our own work, to be

described elsewhere, suggests that the process maybe be effective at least at cryogenic temperatures.

4 Conclusion The experimental results summarized in this paper demonstrate that encouraging material

and electronic compatibility and connectivity can be readily achieved in a nitride/organic semiconductor

junction. The strongly rectifying and robust photocurrent respond suggest that even at this early material

development stage, the wide-gap III-V/organic semiconductor system is electronically quite active. This

early promise gives us a good platform and scientific rationale (as well as significant motivation) for the

continued research.

Acknowledgements This work was supported by the U.S. Department of Energy and the National Science Foun-

dation Biophotonics program BES- 0423566.

References

[1] F.F. So et al., IEEE Trans. Electron Devices 36, 66 (1989).

[2] P.M. Whitmore et al., Phys. Rev. Lett. 50, 1092 (1983).

[3] S. Coe et al., Nature 420, 800 (2002).

[4] W. Huynh et al., Science 295, 2425 (2002).

[5] G. Heliotis et al., Adv. Mater. 18, 334 (2006).

[6] C. Kendrick et al., Appl. Surf. Sci. 123, 405 (1998) and related work by the Kahn group at Princeton University.

[7] Chun-Yu Lee et al., Appl. Phys. Lett. 89, 23116 (2006).

[8] V. M. Agranovich et al., Solid State Commun. 92, 295 (1994).

[9] V. M. Agranovich et al., J. Phys.: Condens. Matter 10, 9369 (1998).