- Email: [email protected]
Organic molecular beam deposition: technology and applications in electronics and photonics
Materials Science and Engineering B51 (1998) 58 – 65
Organic molecular beam deposition: technology and applications in electronics and photonics Achim Bo¨hler *, Peter Urbach, Dirk Ammermann, Wolfgang Kowalsky Institut fu¨r Hochfrequenztechnik, Technische Uni6ersita¨t Braunschweig, D-38092 Braunschweig, Germany
Abstract The organic molecular beam deposition technology allows the reproducible growth of complex layer sequences of various organic semiconductors in combination with dielectric films, different metallizations and indium-tin-oxide layers. The successful fabrication of devices for both electronic and photonic applications is discussed. Organic-on-inorganic heterostructure diodes based on crystalline thin PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) films on III– V-semiconductors are investigated with regard to microwave applications and secondly, organic light emitting diodes with bright emission in the blue, green and red spectral region and with low operation voltages are presented. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Organic molecular beam deposition; Organic semiconductor; Organic light emitting diode; Organic-on-inorganic heterostructure
1. Introduction
2. Organic materials and deposition technology
During the last decade, organic semiconductors have attracted increasing research interest from both academia and industry. Investigations of material properties, device structures and characteristics show that electronic and photonic devices can be successfully prepared from organic compounds. In contrast to spin or dipping techniques for the deposition of polymeric films, the organic molecular beam deposition (OMBD) technology allows the subsequent growth of thin solid films under ultrahigh vacuum conditions and leads to the fabrication of complex heterostructures with a layer thickness ranging from only molecular monolayers to several hundreds of nanometer. In this article two examples will be discussed: Crystalline thin films of the aromatic compound PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) deposited on n-type InP or GaAs substrates form quasi-Schottky contacts suitable for microwave mixer and detector applications. Multilayer structures composed of organic semiconductors with preferentially hole or electron transporting properties and highly fluorescent molecules allow to prepare organic light emitting diodes (OLEDs) for electroluminescence in the blue, green and red spectral region at low operation voltages.
Fig. 1 shows a schematic diagram of the organic molecular beam deposition (OMBD) system used for the growth of organic thin films. Following the concept of conventional molecular beam epitaxy (MBE) systems, this arrangement consists of two organic growth chambers, a metallization chamber, a sputter chamber and a preparation chamber for the substrates. Complex layer sequences of various organic semiconductors, dielectric or indium-tin-oxide (ITO) layers and various metallizations can be deposited without breaking the vacuum. Deposition of organic solids of different crystal structures and lattice constants on arbitrary inorganic semiconductor and amorphous substrates is possible because the molecules of organic crystals are only bonded by weak Van der Waals forces. For reproducible growth conditions the organic source materials are sublimated from effusion cells provided with mechanical shutters. The cell temperatures vary from 100 to 450°C depending on the organic material. Low deposition rates of 0.1–10 nm min − 1 and the possibility of substrate cooling with liquid nitrogen yield smooth and homogeneous thin films. This growth technique allows organic layers only a few nanometres in thickness to be achieved or even molecular monolayers with the high reproducibility necessary for organic on inorganic heterostructures and electroluminescent devices [1].
* Corresponding author. [email protected]
Fax:
+49
531
3915841;
e-mail:
0921-5107/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S 0 9 2 1 - 5 1 0 7 ( 9 7 ) 0 0 2 2 9 - 8
A. Bo¨hler et al. / Materials Science and Engineering B51 (1998) 58–65
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tracarboxylic dianhydride, Fig. 2(h) which is well suited for the organic on inorganic heterostructures is sublimed at 330°C and forms quasiepitaxial thin films at low growth rates of 0.1 nm min − 1. The molecules are then arranged parallel to the substrate surface with an intermolecular distance of 0.321 nm. Owing to this molecular arrangement the electrical and optical properties of PTCDA show distinct anisotropic behaviour. Therefore the conductivity perpendicular to the surface is several orders of magnitude higher than the conductivity in the molecular plane [2]. Molecular structures of the materials shown in Fig. 2(a)–(g) are used as compounds for the fabrication of organic light emitting diodes. The metal chelate complex Alq3 (tris–(8-hydroxychinoline) serves as emitter material for the green spectral region. It is also suitable as electron transport layer in double heterostructure devices. Blue emission is achieved from OXD-8 (1,3bis(N,N-dimethylamino-phenyl)-1,3,4-oxadiazole) an oxadiaziole derivative. Doping Alq3 with DCM (4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)
Fig. 1. Thin film processing technique: organic molecular beam deposition (OMBD).
The molecular structures of the organic materials used in the present investigations are shown in Fig. 2. The aromatic compound PTCDA (3,4,9,10-Perylenete-
-4H-pyr an), a well known laser dye, emission in the orange-red spectral region can be achieved. Preferentially hole transporting behaviour is observed for CuPc (copper phtalocyanine), TAD (N,N’-diphenyl-N,N’bis(3-methylphenyl)-1,1’-biphenyl-4,4’-di amine) [3] and the triphenylamine derivative starburst molecule. This novel class of organic compounds yields very homogeneous and stable thin films with a high glass transition temperature above 100°C which are well suited as hole injection layer due to the very low ionization potential of only about 5 eV [4]. PBD (2-(4-biphenylyl)-5-(4-tert-
Fig. 2. Molecular Structures of (a) Alq3; (b) OXD-8; (c) DCM; (d) CuPc; (e) TAD; (f) Starburst; (g) PBD; (h) PTCDA.
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Fig. 3. The structure of organic-on-inorganic (OI) diodes.
butylphenyl)-1,3,4-oxadiazole) serves as electron transport material in our devices.
3. Organic-on-inorganic (OI) heterostructures Crystalline thin films of the aromatic compound PTCDA deposited on n-type inorganic semiconductors form quasi-Schottky diodes with promising properties for nonlinear microwave circuits [5,6]. The planar structure of the devices is shown in Fig. 3. N-type InP or GaAs epitaxial layers of different doping concentrations on n + substrates are cleaned and etched to remove the oxide layer on the surface. The Ge/Ni/Au ohmic contacts are defined by conventional lift-off technique and alloyed by rapid thermal processing. To achieve small PTCDA contact areas and therefore, low junction capacitances, a second lithography and SiO2 evaporation or sputtering process is followed. The or-
ganic semiconductor layer and the Ag/Ti/Au top metallization are deposited in the OMBD-system under UHV and HV conditions, respectively. Since PTCDA is not completely resistant to organic solvents, the lateral definition of the contact can only be achieved by lift-off for thicknesses of at most 20 nm. For thicker organic layers, a lithography process with positive resist combined with etching of the metal layers was developed. Due to the anisotropy of the conductivity, the remaining PTCDA layer between different structures does not affect the device properties. In Fig. 4 the I–V characteristics of a PTCDA/InP sample with a doping concentration of n= 2.1017 cm − 3 and a conventional GaAs-Schottky-diode (HP 50822301) are compared. The OI-diode shows a remarkable forward characteristic with very low voltages and, in addition, satisfactory rectifying behavior with a reverse breakdown voltage of about 8 V. Therefore, a non-dcbiased operation in mixer circuits with an essentially reduced conversion loss at low local oscillator power levels can be expected. Furthermore, due to space charge limited current transport through the organic layer, the forward characteristic at higher currents can be tuned for special applications by the layer thickness, e.g. a nearly square relation is well suited for linear analog multipliers and power detectors. For dynamic characterization of the devices, DeLoach-measurements [7] in combination with C–Vmeasurements and an additional verification with network analyzers were carried out and allow to determine the lead inductance, the junction capacitance and the series resistance of the diodes. As expected, the results of these techniques were only dependent of the contact geometry and the properties of the inorganic semiconductor. Whereas samples with low doping concentrations show capacitances of several pF and series resistances of up to 5 V, comparable high capacitances up to 20 pF were found for higher doped samples with concentrations in the 1017 cm − 3 regime. However, they were compensated by series resistances below 1 V indicating negligible contributions of the organic layer and lead again to cut-off frequencies of several GHz.
Fig. 4. I –V-characteristic of a PTCDA/InP-diode in comparison with a conventional Schottky-diode (HP 5082-2301).
A. Bo¨hler et al. / Materials Science and Engineering B51 (1998) 58–65
Fig. 5. Microstrip layout of a single balanced mixer with OI-diodes.
To demonstrate the frequency conversion capabilities of OI-diodes, a single balanced mixer in microstrip technology was developed. With regard to the frequency regimes of mobile communication, the circuit was designed for a RF ranging between 1.8 and 1.9 GHz and an IF of 100 MHz. As shown in Fig. 5, the layout consists of a 90° hybrid coupler to symmetrically distribute the incoming signal on both diodes, two short and two open stubs to suppress the even and the odd harmonic frequencies of the LO, respectively and an IF filter network realized with three lumped SMD elements. In addition, due to the expected mismatching particularly of the OI-devices, special emphasis was taken on a quarter wave transformer in front of the diodes to avoid unnecessary losses. In Fig. 6, the measured conversion gain of one mixer circuit using an conventional Schottky-diode (HP 5082-2207) is compared with a simulated result of a second circuit based on the I –V- and C– V-characteristics of a PTCDA/ InP-diode. Whereas for high LO power levels the OIdiode could not reach the comparable low conversion loss of the conventional diode because of higher series resistances, the advantage of lower forward voltages becomes significant below 2 dBm and in this regime, the OI-diode is clearly superior.
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jected from opposite contacts into the organic layer sequence and transported to the emitter layer. Recombination leads to the formation of singlet excitons that decay radiatively. In more detail, electrons are injected from a low work function metal contact, e.g. Ca or Mg. The latter is chosen for reasons of stability. A wide-gap transparent indium-tin-oxide (ITO) or polyaniline (PANI) [8] thin film is used for hole injection. In addition, the efficiency of carrier injection can be improved by choosing organic hole and electron injection layers with a low HOMO (highest occupied molecular orbital) or high LUMO (lowest unoccupied molecular orbital) level, respectively. Charge carrier transport in organic thin films can only rarely be obtained by doping. Therefore, preferentially hole or electron transporting organic compounds with sufficient mobility have to be used to transport the charge carriers to the recombination site. The efficiency of electron-hole recombination leading to the creation of singlet excitons is mainly influenced by the overlap of electron and hole densities that originate from carrier injection into the emitter layer. Energy barriers for electrons and holes to both sides of the emitter layer allow to spatially confine and improve the recombination process. The generated singlet excitons will migrate with an average diffusion length of about 20 nm [9] followed by a radiative or non-radiative decay. Embedding the emitter layer into tranport layers with higher singlet excitation energies leads to a confinement of the singlet excitons and avoids non-radiative decay paths, e.g. contact quenching. Doping of the emitter layer with organic dye molecules allows to transfer energy from the host to the guest molecule in order to tune the emission wavelength or to increase the luminous efficiency [10].
4. Organic light emitting devices The principle of operation of organic light emitting diodes (OLEDs) is similar to that of inorganic light emitting diodes (LEDs). Holes and electrons are in-
Fig. 6. Conversion gain of a single balanced mixer with a PTCDA/ InP-diode and a conventional Schottky-diode (HP5082-2207).
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Fig. 7. Layer sequences and energy diagrams for OLEDs with (a) single layer; (b) single heterostructure; (c) double heterostructure and (d) multilayer structure with separate hole and electron injection and transport layers.
Efficient device operation not only depends on the choice of molecules with appropriate electronic and optical properties, but also on the design of the device structure. Fig. 7 shows the layer sequences and energy level diagrams of different structures used for our investigations. Electroluminescence is already achieved with a simple single layer device (Fig. 7(a)), however, the performance is poor since electrons and holes reach the opposite contact and excitons are quenched at the electrodes. The two-layer or single heterostructure device (Fig. 7(b)) introduces a separate hole transport layer. Holes are injected into the combined emitter and electron transport layer and recombine with electrons near the interface. An optimum thickness is found for the combined layer [11] as a result of sufficient distance of the interface to the metal contact and maximum thickness for a given operating voltage. The double heterostructure (Fig. 7( c)) well known from laser diodes allows to confine both charge carriers and exci-
tons. Unfortunately, energy barriers at the interfaces still impede the transport of electrons and holes from the contacts to the emitter layer. The complex multilayer structure shown in Fig. 7(d) has separate hole injection and transport layers to form a staircase-like path for holes. A similar layer sequence is used for electron injection. The hole blocking layer prevents holes from penetrating into the electron transport layers whereas the electron injection layer has an intermediate LUMO energy to enhance the electron injection from the Mg contact. For device characterization, current–voltage and luminance (optical output power)-current characteristics are investigated for cw-operation at room temperature and at normal ambient conditions. The luminance was determined with a Minolta LS-110 luminance meter. A large-area Si photodetector (Advantest) was used to measure the optical output power. The electroluminescence spectra were recorded with a 200 mm monochromator or an Anritsu optical spectrum analyzer. The internal quantum efficiency of the devices is estimated from the ratio of light generated within the structure to the light detected by a photodetector with limited aperture. Only about 11–15% of the total emission is measured due to Fresnel transmission losses, absorption and total reflection at the interfaces. A typical layer sequence of a green emitting device is depicted in Fig. 8(d). The single heterostructure device consists of a Starburst hole injection layer, a TAD hole transport layer and an Alq3 combined emitter and electron transport layer. The current–voltage and luminance-current characteristics of the green single heterostructure (20 nm Starburst, 50 nm TAD, 60 nm Alq3) are shown in Fig. 9. The turn-on voltage of about 4 V is mainly determined by the total layer thickness of the devices. The estimated internal quantum efficiency is about 4.7% and a luminous efficiency of 1.0 lm W − 1 is obtained. Fig. 10(a) shows the recorded electroluminescent spectrum of the green emitting diode with a peak wavelength of 535 nm and a half width of 105 nm.Blue emission with a peak wavelength of 480 nm and a half width of 95 nm was obtained for an OLED with OXD-8 as emitter material (Fig. 10(a)). There were three different blue emitting double heterostructure devices fabricated to investigate the effect of an additional hole injection layer (Fig. 8(a)–(c)). The current– voltage and luminance-current characteristics of OLEDs with 20 nm Starburst or TAD (15 nm Starburst and 5 nm TAD, respectively), 30 nm OXD-8 and 20 nm Alq are shown in Fig. 11. The turn-on voltage and the luminous efficiency are already improved for an OLED with the Starburst molecule as hole transport material compared to the standard TAD device. The combination of Starburst hole injection and TAD hole tranport layers allows both to achieve turn-on voltages below 7
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Fig. 8. Different realized heterostructures for the (a) – (c) blue, (d) green and (e) red spectral region.
V and to increase the quantum efficiency. An improvement with respect to turn-on voltage (10, 8 and 7 V, respectively), estimated internal quantum efficiency (2.2, 3.3 and 4.3%) and luminous efficiency (0.3, 0.5 and 0.8 lm W − 1) is deduced from the characteristics [12]. The energy level diagram allows to explain this result. Holes can efficiently be injected into the emitter layer for a combined Starburst/TAD layer sequence since no remarkable energy barrier occurs at the different layer interfaces. In contrast, holes injected into the TAD layer are efficiently transported to the emitter layer, however, the injection process from the ITO electrode into TAD is limited by the large energy barrier. On the other hand a single Starburst hole injection layer provides good injection from the ITO electrode but also a large energy barrier at the Starburst/OXD-8 interface. Thus a staircase-like HOMO level sequence provides a better hole injection and transport path and improves the overall device performance.
Doping the Alq3 emitter layer with DCM, a well known laser dye, also red emitting organic light emitting diodes can be realized. The device structure is depicted in Fig. 8(e). Depending on the doping concentration the emission spectrum could be tuned from green with a maximum at 530 for an undoped cell to 625 nm for an OLED with a 5% DCM doped emitter layer (Fig. 10(b)). Multilayer organic light emitting diodes have the ability to compete with other emissive technologies, e.g. plasma, vacuum fluorescence, or inorganic thin film electroluminescence displays. A luminance exceeding 100–1000 cd m − 2 and a luminous efficiency of 1–2 to 5 lm W − 1 required for indoor and outdoor flat panel applications [13], respectively, are possible. Low information content displays, e.g. alphanumeric displays, or sign boards, can be fabricated by photolithographic definition of contact patterns or OLED structures. Lightweight and flexible polyaniline (PANI) substrates are suitable for organic electroluminescent displays and offer a large potential for applications.
Fig. 9. Typical current–voltage and luminance – current characteriscs of a green emitting device.
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Fig. 10. Electroluminescence spectra of the (a) green and blue and (b) red emitting diodes.
spectral region with various multilayer structures were presented. Both applications have been fabricated by using organic molecular beam deposition technology, which offers ultra high vacuum growth conditions, precise layer thickness control, the possibility of substrate cooling and therefore, allows the deposition of homogeneous and smooth organic thin films. The organic-on-inorganic heterostructures have proved as devices for microwave applications with rectifying behavior, drastically reduced forward voltages and high cut-off frequencies in the GHz regime. For this reason, non-dc-biased mixers with improved frequency conversion at low power levels can be realized and were demonstrated with a single balanced mixer circuit in microstrip technology for 1.8 GHz commonly used in mobile communications. Furthermore, the PTCDA layer thickness as an additional fabrication parameter allows to tune the device characteristics for special applications, e.g. power detectors or analogue multipliers. Regarding the operation and performance of OLEDs as second application of the OMBD technique, improved heterostructure devices lead to drastically increased luminous and quantum efficiencies compared to single layer devices. Blue, green and red emitting OLEDs based on OXD-8, Alq3 and DCM doped Alq3, respectively, showed maximum values of 1.4 lm − 1W and 8.5%. Luminances exceeding 1000 cd m − 2 and sufficient lifetimes make organic light emitting diodes promising candidates for lightweight flat panel displays.
5. Summary In all, two different applications for organic semiconductors have been discussed. First organic-on-inorganic heterostructure diodes based on crystalline thin PTCDA films on III – V-semiconductors have been investigated with regard to microwave applications. Secondly organic light emitting diodes for the visible
Acknowledgements The research on organic materials is funded by the Bundesministerium fu¨r Bildung und Forschung (BMBF) and by the Volkswagen Stiftung. We gratefully acknowledge their generous financial support.
Fig. 11. Current – voltage and luminance current characteristics of the blue emitting device with different hole transport layers: Starburst/TAD (— ), Starburst ( – – – ) and TAD (–-–-–).
A. Bo¨hler et al. / Materials Science and Engineering B51 (1998) 58–65
References [1] C. Rompf, D. Ammermann, W. Kowalsky, J. Mater. Sci. 11 (1995) 845 – 848. [2] S. R. Forrest, M. L. Kaplan, P. H. Schmidt, J. Appl. Phys. 55 (6) (1984) 1492. [3] M. Stolka, J. F. Yanus, D. M. Pai, J. Phys. Chem. 88 (1984) 4707 – 4714. [4] Y. Shirota, Y. Kuwabara, H. Inada, et al., Appl. Phys. Lett. 56 (1994) 807 – 809. [5] P. Urbach, C. Rompf, D. Ammermann, W. Kowalsky, SSDM ’95, Osaka, Japan (1995), 401–403. [6] P. Urbach, D. Ammermann, W. Kowalsky, SSDM ’96, Yokohama, Japan (1996), 586–588.
.
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[7] B. DeLoach, IEEE MTT (1964) 15. [8] Y. Yang, E. Westerweele, C. Zhang, P. Smith, A. J. Heeger, J. Appl. Phys. 77 (1995) 694 – 698. [9] C. W. Tang, S. A. VanSlyke, J. Appl. Phys. 65 (1989) 3610– 3616. [10] Y. Hamada, T. Samo, K. Shibata, K. Kuroki, Jpn. J. Appl. Phys. 34 (1995) L824 – L826. [11] D. Ammermann, A. Bo¨hler, C. Rompf, W. Kowalsky, Proc. IEEE/LEOS Summer Topical Meeting on Flat Panel Display Technology, Keystone, CO, 1995, 31 – 32. [12] A. Bo¨hler, D. Ammermann, C. Rompf, W. Kowalsky, A. M. Richter, H.-H. Johannes, W.Grahn, Proc. SID, San Diego, CA, May 1996. [13] R.L. Moon, Proc. IS & T’s 48th Annual Conference, 1995, 386 – 391.
Organic molecular beam deposition: technology and applications in electronics and photonics Achim Bo¨hler *, Peter Urbach, Dirk Ammermann, Wolfgang Kowalsky Institut fu¨r Hochfrequenztechnik, Technische Uni6ersita¨t Braunschweig, D-38092 Braunschweig, Germany
Abstract The organic molecular beam deposition technology allows the reproducible growth of complex layer sequences of various organic semiconductors in combination with dielectric films, different metallizations and indium-tin-oxide layers. The successful fabrication of devices for both electronic and photonic applications is discussed. Organic-on-inorganic heterostructure diodes based on crystalline thin PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) films on III– V-semiconductors are investigated with regard to microwave applications and secondly, organic light emitting diodes with bright emission in the blue, green and red spectral region and with low operation voltages are presented. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Organic molecular beam deposition; Organic semiconductor; Organic light emitting diode; Organic-on-inorganic heterostructure
1. Introduction
2. Organic materials and deposition technology
During the last decade, organic semiconductors have attracted increasing research interest from both academia and industry. Investigations of material properties, device structures and characteristics show that electronic and photonic devices can be successfully prepared from organic compounds. In contrast to spin or dipping techniques for the deposition of polymeric films, the organic molecular beam deposition (OMBD) technology allows the subsequent growth of thin solid films under ultrahigh vacuum conditions and leads to the fabrication of complex heterostructures with a layer thickness ranging from only molecular monolayers to several hundreds of nanometer. In this article two examples will be discussed: Crystalline thin films of the aromatic compound PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) deposited on n-type InP or GaAs substrates form quasi-Schottky contacts suitable for microwave mixer and detector applications. Multilayer structures composed of organic semiconductors with preferentially hole or electron transporting properties and highly fluorescent molecules allow to prepare organic light emitting diodes (OLEDs) for electroluminescence in the blue, green and red spectral region at low operation voltages.
Fig. 1 shows a schematic diagram of the organic molecular beam deposition (OMBD) system used for the growth of organic thin films. Following the concept of conventional molecular beam epitaxy (MBE) systems, this arrangement consists of two organic growth chambers, a metallization chamber, a sputter chamber and a preparation chamber for the substrates. Complex layer sequences of various organic semiconductors, dielectric or indium-tin-oxide (ITO) layers and various metallizations can be deposited without breaking the vacuum. Deposition of organic solids of different crystal structures and lattice constants on arbitrary inorganic semiconductor and amorphous substrates is possible because the molecules of organic crystals are only bonded by weak Van der Waals forces. For reproducible growth conditions the organic source materials are sublimated from effusion cells provided with mechanical shutters. The cell temperatures vary from 100 to 450°C depending on the organic material. Low deposition rates of 0.1–10 nm min − 1 and the possibility of substrate cooling with liquid nitrogen yield smooth and homogeneous thin films. This growth technique allows organic layers only a few nanometres in thickness to be achieved or even molecular monolayers with the high reproducibility necessary for organic on inorganic heterostructures and electroluminescent devices [1].
* Corresponding author. [email protected]
Fax:
+49
531
3915841;
e-mail:
0921-5107/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S 0 9 2 1 - 5 1 0 7 ( 9 7 ) 0 0 2 2 9 - 8
A. Bo¨hler et al. / Materials Science and Engineering B51 (1998) 58–65
59
tracarboxylic dianhydride, Fig. 2(h) which is well suited for the organic on inorganic heterostructures is sublimed at 330°C and forms quasiepitaxial thin films at low growth rates of 0.1 nm min − 1. The molecules are then arranged parallel to the substrate surface with an intermolecular distance of 0.321 nm. Owing to this molecular arrangement the electrical and optical properties of PTCDA show distinct anisotropic behaviour. Therefore the conductivity perpendicular to the surface is several orders of magnitude higher than the conductivity in the molecular plane [2]. Molecular structures of the materials shown in Fig. 2(a)–(g) are used as compounds for the fabrication of organic light emitting diodes. The metal chelate complex Alq3 (tris–(8-hydroxychinoline) serves as emitter material for the green spectral region. It is also suitable as electron transport layer in double heterostructure devices. Blue emission is achieved from OXD-8 (1,3bis(N,N-dimethylamino-phenyl)-1,3,4-oxadiazole) an oxadiaziole derivative. Doping Alq3 with DCM (4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)
Fig. 1. Thin film processing technique: organic molecular beam deposition (OMBD).
The molecular structures of the organic materials used in the present investigations are shown in Fig. 2. The aromatic compound PTCDA (3,4,9,10-Perylenete-
-4H-pyr an), a well known laser dye, emission in the orange-red spectral region can be achieved. Preferentially hole transporting behaviour is observed for CuPc (copper phtalocyanine), TAD (N,N’-diphenyl-N,N’bis(3-methylphenyl)-1,1’-biphenyl-4,4’-di amine) [3] and the triphenylamine derivative starburst molecule. This novel class of organic compounds yields very homogeneous and stable thin films with a high glass transition temperature above 100°C which are well suited as hole injection layer due to the very low ionization potential of only about 5 eV [4]. PBD (2-(4-biphenylyl)-5-(4-tert-
Fig. 2. Molecular Structures of (a) Alq3; (b) OXD-8; (c) DCM; (d) CuPc; (e) TAD; (f) Starburst; (g) PBD; (h) PTCDA.
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Fig. 3. The structure of organic-on-inorganic (OI) diodes.
butylphenyl)-1,3,4-oxadiazole) serves as electron transport material in our devices.
3. Organic-on-inorganic (OI) heterostructures Crystalline thin films of the aromatic compound PTCDA deposited on n-type inorganic semiconductors form quasi-Schottky diodes with promising properties for nonlinear microwave circuits [5,6]. The planar structure of the devices is shown in Fig. 3. N-type InP or GaAs epitaxial layers of different doping concentrations on n + substrates are cleaned and etched to remove the oxide layer on the surface. The Ge/Ni/Au ohmic contacts are defined by conventional lift-off technique and alloyed by rapid thermal processing. To achieve small PTCDA contact areas and therefore, low junction capacitances, a second lithography and SiO2 evaporation or sputtering process is followed. The or-
ganic semiconductor layer and the Ag/Ti/Au top metallization are deposited in the OMBD-system under UHV and HV conditions, respectively. Since PTCDA is not completely resistant to organic solvents, the lateral definition of the contact can only be achieved by lift-off for thicknesses of at most 20 nm. For thicker organic layers, a lithography process with positive resist combined with etching of the metal layers was developed. Due to the anisotropy of the conductivity, the remaining PTCDA layer between different structures does not affect the device properties. In Fig. 4 the I–V characteristics of a PTCDA/InP sample with a doping concentration of n= 2.1017 cm − 3 and a conventional GaAs-Schottky-diode (HP 50822301) are compared. The OI-diode shows a remarkable forward characteristic with very low voltages and, in addition, satisfactory rectifying behavior with a reverse breakdown voltage of about 8 V. Therefore, a non-dcbiased operation in mixer circuits with an essentially reduced conversion loss at low local oscillator power levels can be expected. Furthermore, due to space charge limited current transport through the organic layer, the forward characteristic at higher currents can be tuned for special applications by the layer thickness, e.g. a nearly square relation is well suited for linear analog multipliers and power detectors. For dynamic characterization of the devices, DeLoach-measurements [7] in combination with C–Vmeasurements and an additional verification with network analyzers were carried out and allow to determine the lead inductance, the junction capacitance and the series resistance of the diodes. As expected, the results of these techniques were only dependent of the contact geometry and the properties of the inorganic semiconductor. Whereas samples with low doping concentrations show capacitances of several pF and series resistances of up to 5 V, comparable high capacitances up to 20 pF were found for higher doped samples with concentrations in the 1017 cm − 3 regime. However, they were compensated by series resistances below 1 V indicating negligible contributions of the organic layer and lead again to cut-off frequencies of several GHz.
Fig. 4. I –V-characteristic of a PTCDA/InP-diode in comparison with a conventional Schottky-diode (HP 5082-2301).
A. Bo¨hler et al. / Materials Science and Engineering B51 (1998) 58–65
Fig. 5. Microstrip layout of a single balanced mixer with OI-diodes.
To demonstrate the frequency conversion capabilities of OI-diodes, a single balanced mixer in microstrip technology was developed. With regard to the frequency regimes of mobile communication, the circuit was designed for a RF ranging between 1.8 and 1.9 GHz and an IF of 100 MHz. As shown in Fig. 5, the layout consists of a 90° hybrid coupler to symmetrically distribute the incoming signal on both diodes, two short and two open stubs to suppress the even and the odd harmonic frequencies of the LO, respectively and an IF filter network realized with three lumped SMD elements. In addition, due to the expected mismatching particularly of the OI-devices, special emphasis was taken on a quarter wave transformer in front of the diodes to avoid unnecessary losses. In Fig. 6, the measured conversion gain of one mixer circuit using an conventional Schottky-diode (HP 5082-2207) is compared with a simulated result of a second circuit based on the I –V- and C– V-characteristics of a PTCDA/ InP-diode. Whereas for high LO power levels the OIdiode could not reach the comparable low conversion loss of the conventional diode because of higher series resistances, the advantage of lower forward voltages becomes significant below 2 dBm and in this regime, the OI-diode is clearly superior.
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jected from opposite contacts into the organic layer sequence and transported to the emitter layer. Recombination leads to the formation of singlet excitons that decay radiatively. In more detail, electrons are injected from a low work function metal contact, e.g. Ca or Mg. The latter is chosen for reasons of stability. A wide-gap transparent indium-tin-oxide (ITO) or polyaniline (PANI) [8] thin film is used for hole injection. In addition, the efficiency of carrier injection can be improved by choosing organic hole and electron injection layers with a low HOMO (highest occupied molecular orbital) or high LUMO (lowest unoccupied molecular orbital) level, respectively. Charge carrier transport in organic thin films can only rarely be obtained by doping. Therefore, preferentially hole or electron transporting organic compounds with sufficient mobility have to be used to transport the charge carriers to the recombination site. The efficiency of electron-hole recombination leading to the creation of singlet excitons is mainly influenced by the overlap of electron and hole densities that originate from carrier injection into the emitter layer. Energy barriers for electrons and holes to both sides of the emitter layer allow to spatially confine and improve the recombination process. The generated singlet excitons will migrate with an average diffusion length of about 20 nm [9] followed by a radiative or non-radiative decay. Embedding the emitter layer into tranport layers with higher singlet excitation energies leads to a confinement of the singlet excitons and avoids non-radiative decay paths, e.g. contact quenching. Doping of the emitter layer with organic dye molecules allows to transfer energy from the host to the guest molecule in order to tune the emission wavelength or to increase the luminous efficiency [10].
4. Organic light emitting devices The principle of operation of organic light emitting diodes (OLEDs) is similar to that of inorganic light emitting diodes (LEDs). Holes and electrons are in-
Fig. 6. Conversion gain of a single balanced mixer with a PTCDA/ InP-diode and a conventional Schottky-diode (HP5082-2207).
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Fig. 7. Layer sequences and energy diagrams for OLEDs with (a) single layer; (b) single heterostructure; (c) double heterostructure and (d) multilayer structure with separate hole and electron injection and transport layers.
Efficient device operation not only depends on the choice of molecules with appropriate electronic and optical properties, but also on the design of the device structure. Fig. 7 shows the layer sequences and energy level diagrams of different structures used for our investigations. Electroluminescence is already achieved with a simple single layer device (Fig. 7(a)), however, the performance is poor since electrons and holes reach the opposite contact and excitons are quenched at the electrodes. The two-layer or single heterostructure device (Fig. 7(b)) introduces a separate hole transport layer. Holes are injected into the combined emitter and electron transport layer and recombine with electrons near the interface. An optimum thickness is found for the combined layer [11] as a result of sufficient distance of the interface to the metal contact and maximum thickness for a given operating voltage. The double heterostructure (Fig. 7( c)) well known from laser diodes allows to confine both charge carriers and exci-
tons. Unfortunately, energy barriers at the interfaces still impede the transport of electrons and holes from the contacts to the emitter layer. The complex multilayer structure shown in Fig. 7(d) has separate hole injection and transport layers to form a staircase-like path for holes. A similar layer sequence is used for electron injection. The hole blocking layer prevents holes from penetrating into the electron transport layers whereas the electron injection layer has an intermediate LUMO energy to enhance the electron injection from the Mg contact. For device characterization, current–voltage and luminance (optical output power)-current characteristics are investigated for cw-operation at room temperature and at normal ambient conditions. The luminance was determined with a Minolta LS-110 luminance meter. A large-area Si photodetector (Advantest) was used to measure the optical output power. The electroluminescence spectra were recorded with a 200 mm monochromator or an Anritsu optical spectrum analyzer. The internal quantum efficiency of the devices is estimated from the ratio of light generated within the structure to the light detected by a photodetector with limited aperture. Only about 11–15% of the total emission is measured due to Fresnel transmission losses, absorption and total reflection at the interfaces. A typical layer sequence of a green emitting device is depicted in Fig. 8(d). The single heterostructure device consists of a Starburst hole injection layer, a TAD hole transport layer and an Alq3 combined emitter and electron transport layer. The current–voltage and luminance-current characteristics of the green single heterostructure (20 nm Starburst, 50 nm TAD, 60 nm Alq3) are shown in Fig. 9. The turn-on voltage of about 4 V is mainly determined by the total layer thickness of the devices. The estimated internal quantum efficiency is about 4.7% and a luminous efficiency of 1.0 lm W − 1 is obtained. Fig. 10(a) shows the recorded electroluminescent spectrum of the green emitting diode with a peak wavelength of 535 nm and a half width of 105 nm.Blue emission with a peak wavelength of 480 nm and a half width of 95 nm was obtained for an OLED with OXD-8 as emitter material (Fig. 10(a)). There were three different blue emitting double heterostructure devices fabricated to investigate the effect of an additional hole injection layer (Fig. 8(a)–(c)). The current– voltage and luminance-current characteristics of OLEDs with 20 nm Starburst or TAD (15 nm Starburst and 5 nm TAD, respectively), 30 nm OXD-8 and 20 nm Alq are shown in Fig. 11. The turn-on voltage and the luminous efficiency are already improved for an OLED with the Starburst molecule as hole transport material compared to the standard TAD device. The combination of Starburst hole injection and TAD hole tranport layers allows both to achieve turn-on voltages below 7
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Fig. 8. Different realized heterostructures for the (a) – (c) blue, (d) green and (e) red spectral region.
V and to increase the quantum efficiency. An improvement with respect to turn-on voltage (10, 8 and 7 V, respectively), estimated internal quantum efficiency (2.2, 3.3 and 4.3%) and luminous efficiency (0.3, 0.5 and 0.8 lm W − 1) is deduced from the characteristics [12]. The energy level diagram allows to explain this result. Holes can efficiently be injected into the emitter layer for a combined Starburst/TAD layer sequence since no remarkable energy barrier occurs at the different layer interfaces. In contrast, holes injected into the TAD layer are efficiently transported to the emitter layer, however, the injection process from the ITO electrode into TAD is limited by the large energy barrier. On the other hand a single Starburst hole injection layer provides good injection from the ITO electrode but also a large energy barrier at the Starburst/OXD-8 interface. Thus a staircase-like HOMO level sequence provides a better hole injection and transport path and improves the overall device performance.
Doping the Alq3 emitter layer with DCM, a well known laser dye, also red emitting organic light emitting diodes can be realized. The device structure is depicted in Fig. 8(e). Depending on the doping concentration the emission spectrum could be tuned from green with a maximum at 530 for an undoped cell to 625 nm for an OLED with a 5% DCM doped emitter layer (Fig. 10(b)). Multilayer organic light emitting diodes have the ability to compete with other emissive technologies, e.g. plasma, vacuum fluorescence, or inorganic thin film electroluminescence displays. A luminance exceeding 100–1000 cd m − 2 and a luminous efficiency of 1–2 to 5 lm W − 1 required for indoor and outdoor flat panel applications [13], respectively, are possible. Low information content displays, e.g. alphanumeric displays, or sign boards, can be fabricated by photolithographic definition of contact patterns or OLED structures. Lightweight and flexible polyaniline (PANI) substrates are suitable for organic electroluminescent displays and offer a large potential for applications.
Fig. 9. Typical current–voltage and luminance – current characteriscs of a green emitting device.
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Fig. 10. Electroluminescence spectra of the (a) green and blue and (b) red emitting diodes.
spectral region with various multilayer structures were presented. Both applications have been fabricated by using organic molecular beam deposition technology, which offers ultra high vacuum growth conditions, precise layer thickness control, the possibility of substrate cooling and therefore, allows the deposition of homogeneous and smooth organic thin films. The organic-on-inorganic heterostructures have proved as devices for microwave applications with rectifying behavior, drastically reduced forward voltages and high cut-off frequencies in the GHz regime. For this reason, non-dc-biased mixers with improved frequency conversion at low power levels can be realized and were demonstrated with a single balanced mixer circuit in microstrip technology for 1.8 GHz commonly used in mobile communications. Furthermore, the PTCDA layer thickness as an additional fabrication parameter allows to tune the device characteristics for special applications, e.g. power detectors or analogue multipliers. Regarding the operation and performance of OLEDs as second application of the OMBD technique, improved heterostructure devices lead to drastically increased luminous and quantum efficiencies compared to single layer devices. Blue, green and red emitting OLEDs based on OXD-8, Alq3 and DCM doped Alq3, respectively, showed maximum values of 1.4 lm − 1W and 8.5%. Luminances exceeding 1000 cd m − 2 and sufficient lifetimes make organic light emitting diodes promising candidates for lightweight flat panel displays.
5. Summary In all, two different applications for organic semiconductors have been discussed. First organic-on-inorganic heterostructure diodes based on crystalline thin PTCDA films on III – V-semiconductors have been investigated with regard to microwave applications. Secondly organic light emitting diodes for the visible
Acknowledgements The research on organic materials is funded by the Bundesministerium fu¨r Bildung und Forschung (BMBF) and by the Volkswagen Stiftung. We gratefully acknowledge their generous financial support.
Fig. 11. Current – voltage and luminance current characteristics of the blue emitting device with different hole transport layers: Starburst/TAD (— ), Starburst ( – – – ) and TAD (–-–-–).
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