Organic Vapor Phase Deposition Research Articles

Thin organic heterostructures deposited via organic vapor phase deposition: spectroscopic ellipsometry characterization

Thin heterostructures and mixed layers of tris(8–hydroxyquinoline)-aluminum(III) (Alq 3) and N,N′-Di-[(1-naphthyl)-N,N′-diphenyl]-(1,1′-biphenyl)-4,4′-diamine ( α-NPD) were deposited on large-area silicon substrates by means of the recently developed organic vapor phase deposition (OVPD) method. Variable angle spectroscopic ellipsometry (VASE) was employed as a non-destructive technique to measure the thickness and optical constants of the single layers deposited by OVPD. Using the determined optical constants it is demonstrated that spectroscopic ellipsometry is capable of determining the thicknesses of individual layers in Alq 3/ α-NPD heterostructures. Furthermore, the percentage of mixing in uniformly mixed Alq 3− α–NPD layers can be determined from the analysis of the ellipsometric data. A simulation of ellipsometric parameters Ψ and Δ demonstrates that ellipsometry is a very suitable tool for in situ real-time thickness monitoring during the OVPD deposition process.

Controlled growth of a molecular bulk heterojunction photovoltaic cell

The power conversion efficiency of organic photovoltaic cells has increased with the introduction of the donor–acceptor heterojunction that serves to dissociate strongly bound photogenerated excitons1. Further efficiency increases have been achieved in both polymer2,3 and small-molecular-mass4 organic photovoltaic cells through the use of the bulk heterojunction (BHJ), where the distance an exciton must diffuse from its generation to its dissociation site is reduced in an interpenetrating network of the donor and acceptor materials. However, the random distribution of donor and acceptor materials in such structures can lead to charge trapping at bottlenecks and cul-de-sacs in the conducting pathways to the electrodes. Here, we present a method for growing crystalline organic films into a controlled bulk heterojunction; that is, the positions and orientations of donor and acceptor materials are determined during growth by organic vapour-phase deposition (OVPD5), eliminating contorted and resistive conducting pathways while maximizing the interface area. This results in a substantial increase in power conversion efficiency compared with the best values obtained by 'random' small-molecular-weight BHJ solar cells formed by high-temperature annealing, or planar double heterojunction photovoltaic cells using the same archetypal materials systems.

50.4: First Hybrid OLED by Organic Vapor Phase Deposition and Its Advantages in Deposition Rate Control for OLED Manufacturing

AbstractWe are discussing the latest results in the field of Organic Vapor Phase Deposition (OVPD) on basis of our experimental data. In particular the use of carrier gas and its controlled mass flow are adding an additional parameter to control deposition rates in OLED manufacturing by OVPD. Many advantages offered by this key parameter in this technology like stable deposition rates, wide range of adjustable deposition rates and doping concentrations are discussed.A small molecule hybrid OLED (SM‐HLED) consisting of a polymer layer of PEDT:PSS and two small molecule layers of α‐NPD and Alq3 was fabricated by OVPD. For this 3‐layer device we observed a turn on voltage of 2.5 V, a luminance brightness of 100 cd/m2 at 5.2 V and 300 cd/m2 at 6.5 V. Furthermore a passive matrix OLED‐display (PMOLED‐display) was demonstrated by OVPD. The display had a turn‐on voltage of about 3.0 V and showed a very homogeneous light emission.These device characteristics confirm that OVPD is close to be comparable to Vacuum Thermal Evaporation (VTE).

45.2: Self‐Organized Organic Thin‐Film Transistor for Flexible Active‐Matrix Display

AbstractWe developed a bottom contact, organic thin‐film transistor (OTFT) array on plastic using a self‐organized process. The pentacene thin‐film transistor, grown selectively between the source and drain using organic vapor phase deposition (OVPD), exhibited a field‐effect mobility of 1.1 cm2/Vs and on/off current ratio of >108. A 4 inch OTFT array with 50 ppi resolution was developed.

Self‐Organized Organic Thin‐Film Transistors on Plastic

The development of the self‐organized growth of pentacence thin films on the channel region of a thin‐film transistor (TFT) using surface modifications induced by organic vapor phase deposition is reported (see Figure). A bottom‐contact TFT on plastic using an organic gate insulator of cross‐linked poly‐(4‐vinylphenol) exhibited a field‐effect mobility of 1.2 cm2/Vs and an on/off current ratio of ∼ 107.

52.3: Invited Paper: OLED Manufacturing by Organic Vapor Phase Deposition

AbstractThe current status of the development of Organic Vapor Phase Deposition technology (OVPD) for the manufacturing of OLEDs will be reviewed. The basic OVPD principle is explained and typical performance data will be presented. Starting from a small scale R&D setup, the development of larger OVPD systems up to the first mass production unit is described. Special emphasis is put on the scalability of the process. The successful up‐scaling of the OVPD principle is supported by extensive numerical modeling that is also described.

Micropatterning of small molecular weight organic semiconductor thin films using organic vapor phase deposition

Using both analytical and experimental methods, we show that micron scale patterned growth of small molecular weight organic semiconductor thin films can be achieved by the recently demonstrated process of organic vapor phase deposition (OVPD). In contrast to the conventional process of vacuum thermal evaporation, the background gas pressure during OVPD is typically 0.1–10 Torr, resulting in a molecular mean free path (mfp) of from 100 to 1 μm, respectively. Monte Carlo simulations of film growth through apertures at these gas densities indicate that when the mfp is on the order of the mask-to-substrate separation, deposit edges can become diffuse. The simulations and deposition experiments discussed here indicate that the deposited feature shape is controlled by the mfp, the aperture geometry, and the mask-to-substrate separation. Carefully selected process conditions and mask geometries can result in features as small as 1 μm. Furthermore, based on continuum and stochastic models of molecular transport in confined geometries, we propose the in situ direct patterning growth technique of organic vapor jet printing. The high pattern definition obtained by OVPD makes this process attractive for the growth of a wide range of structures employed in modern organic electronic devices.

Effects of film morphology and gate dielectric surface preparation on the electrical characteristics of organic-vapor-phase-deposited pentacene thin-film transistors

Organic vapor phase deposition was used to grow polycrystalline pentacene channel thin-film transistors. Substrate temperature, chamber pressure during film deposition, and growth rate were used to vary the crystalline grain size of pentacene films on O2-plasma treated SiO2 from 0.2 to 5 μm, leading to room-temperature saturation regime field-effect hole mobilities (μeff) from 0.05±0.02 to 0.5±0.1 cm2/V s, respectively. Surface treatment of SiO2 with octadecyltrichlorosilane (OTS) prior to pentacene deposition resulted in μeff⩽1.6 cm2/V s, and drain current on/off ratios of ⩽108 at room temperature, while dramatically reducing the average grain size. X-ray diffraction studies indicate that the OTS treatment decreases the order of the molecular stacks. This suggests an increased density of flat-lying molecules, accompanying the improvement of the hole mobility at the pentacene/OTS interface.

27.4: Modeling and Fabrication of Organic Vapor Phase Deposition (OVPD) Equipment for OLED Display Manufacturing

AbstractThe Principle of Organic Vapor Phase Deposition (OVPD) using AIXTRON's proprietary Close Coupled Showerhead technology will be presented and discussed. This alternative deposition technology enables substrate scalability and multiple layer deposition with high growth rates and high material efficiency. CFD‐Simulations for the temperature distribution and the mass fraction of typically used Alq3 reveal excellent process control required for homogeneous deposition and reproducible mass production. The actual production equipment will be discussed.

Real time monitoring of layer growth on planetary reactor ® by reflectance-anisotropy-spectroscopy

Reflectance anisotropy spectroscopy (RAS/RDS) has been used for basic growth studies in metal–organic vapor-phase deposition epitaxy (MOVPE). Because of its sensitivity for the uppermost atomic monolayers RAS is a suitable tool for the investigation of a variety of physical properties, such as surface stiochiometry and reconstruction. The enhanced performance of the EpiRAS measurement tool, made by LayTec, is suitable for MOVPE device growth monitoring and control in an industrial AIX 2600 G3 MOVPE reactor in the 8×4 inch configuration with gas foil rotation ®. A brief introduction will be given to the basic surface physics and surface chemistry causing the optical signatures. Examples will be given concerning the optical, real time measurement of the growth of a GaAs/AlAs distributed Bragg reflector (DBR) and (Al x Ga 1− x ) 0.5In 0.5P multi quantum well (MQW) laser structures. The results prove that in-situ monitoring of multi layer structures can be investigated in detail by RAS. The DBR was chosen to optimize the growth rate and thickness which was checked via X-ray data.

Reduction of dislocation density in GaN films on sapphire using AIN interlayers

GaN (00.1) thin films, of thickness 1.25 to 2.25 μm grown on sapphire substrate (11.0) by metallo organic chemical vapor phase deposition (MOCVD) with different number of AlN interlayers, were characterized by triple crystal diffractometry and synchrotron white beam x-ray topography (SWBXT). The full width at half maximum (FWHM) of x-ray rocking curves from symmetric and asymmetric reflections was used to estimate the dislocation density in GaN films. It has been found that the edge dislocation density decreased from 1.63 × 1010 cm−2 to 1.23 × 1010 cm−2 and the screw dislocation density decreased from 2.0 × 108 cm−2 to 1.1 × 108 cm−2 when one AlN interlayer was inserted between the high temperature GaN layer. The dislocation density decreased further with the increase in number of interlayers. On the other hand the compressive stress in the GaN film increased from −0.29 GPa to −0.86 GPa. The compressive stress further increased as the number of interlayers increased but no cracking in the GaN film was observed. This could be due to better adhesion between the film and substrate due to interlayers. SWBXT in transmission from a GaN(00.1)/Al2O3(11.0) sample confirms the orientation of GaN and indicates that it is a single crystal with high dislocation density. SWBXT from the Al2O3 substrate shows cellular structure of dislocations.

Raman scattering studies of the ZnSe/GaAs interface

AbstractRaman spectroscopy was used to study the band bending at the interface of ZnSe/GaAs hetero‐structures. A series of samples, which contained a ZnSe buffer layer, 0–35 nm thick, grown at a lower temperature than the much thicker ZnSe epilayer, by metal–organic chemical vapor phase deposition, were investigated. Compared with that of the GaAs substrate, an enhancement of the intensity of the LOGaAs phonon was found in samples grown without and with a thick (≥28 nm) buffer layer, but not in a sample grown with a 4 nm thick buffer layer. The enhancement is attributed to the electric field induced Raman scattering, resulted from a strong band bending on the GaAs side of the hetero‐structure. The results suggest that the direction of the interfacial electric field on the GaAs side will reverse with increasing buffer layer thickness. Between this reversal, a near flat band condition can be achieved, as was found in a sample grown with a 4 nm buffer layer. This suggestion is consistent with the concomitant improvement of the structure of the epilayer and of the interfacial quality of the hetero‐junction, which unpins the Fermi level and affects the band bending. Copyright © 2001 John Wiley & Sons, Ltd.

State Filling in Type II Quantum Dots

Many-body states in self-organized type II GaSbAs/GaAs quantum dots grown by metal organic chemical vapor phase deposition are studied. With increasing exciton occupation the photoluminescence shows a blue shift and becomes significantly broader developing a saturated plateau structure. The results reveal the properties of many-hole states in GaSbAs quantum dots determined by Coulomb charging and state filling.

State Filling in Type II Quantum Dots

Many-body states in self-organized type II GaSbAs/GaAs quantum dots grown by metal organic chemical vapor phase deposition are studied. With increasing exciton occupation the photoluminescence shows a blue shift and becomes significantly broader developing a saturated plateau structure. The results reveal the properties of many-hole states in GaSbAs quantum dots determined by Coulomb charging and state filling.

Material transport regimes and mechanisms for growth of molecular organic thin films using low-pressure organic vapor phase deposition

We determine the physical mechanisms controlling the growth of amorphous organic thin films by the process of low-pressure organic vapor phase deposition (LP-OVPD). In LP-OVPD, multiple host and dopant molecular sources are introduced into a hot wall reactor via several injection barrels using an inert carrier gas, allowing for controlled film growth rates exceeding 10 Å/s. The temperature and carrier flow rate for each source can be independently regulated, allowing considerable control over dopant concentration, deposition rate, and thickness uniformity of the thin films. The rate of film deposition is limited either by the rate of condensation on the substrate or by the rate of supply from the source. The source-limited regime can be further classified into equilibrium or kinetically limited evaporation, coupled to convection- or diffusion-limited deposition. Models are developed to relate the rate of film growth to source and substrate temperature, and carrier gas flow rate. These models characterize and predict the performance of the LP-OVPD system used to grow high performance organic light emitting devices.

Structural properties of AlGaN/GaN heterostructures on Si(111) substrates suitable for high-electron mobility transistors

Transmission electron microscopy (TEM) investigations of metal organic vapor phase deposition grown AlxGa1−xN/GaN heterostructures on Si(111) containing an AlN high-temperature buffer layer have been carried out. The structural properties at the interface and in the epilayer as well as the electronic properties suitable for a high electron mobility transistor (HEMT) were analyzed and compared with systems grown on Al2O3(0001). High resolution TEM (HRTEM) at the AlN/Si(111) interface reveals a 1.5–2.7 nm thick amorphous SiNx layer due to the high growth temperature of TAlN=1040 °C. Therefore, a grain-like GaN/AlN region extending 40–60 nm appears and it is subsequently overgrown with (0001) orientated GaN material because of geometrical selection. The residual strain at the AlN/Si(111) interface is estimated to be εr=0.3±0.6% by Fourier filtering of HRTEM images and a moiré fringe analysis. This indicates almost complete relaxation of the large mismatch f(AlN/Si)=+23.4% which seems to be supported by the SiNx layer. Weak beam imaging and plan view TEM show typical threading dislocations in the epilayer with a density of 3×109 cm−2 extending along 〈0001〉 which sometimes form grain boundaries. An AlxGa1−xN/GaN interface roughness of 3 monolayers is estimated and a small AlxGa1−xN surface roughness of 1.5 nm is obtained by HRTEM and atomic force microscopy investigations which correspond to two-dimensional growth. C–V and Hall measurements reveal two-dimensional electron gas at the Al32Ga68N/GaN interface that has a sheet carrier concentration of 4×1012 cm−2. The electron mobility of 820 cm2/Vs measured at room temperature is applicable for a HEMT grown on Si(111).

The values of minority carrier diffusion lengths and lifetimes in GaN and their implications for bipolar devices

The wide bandgap semiconductors GaN and AlGaN show promise as the high voltage standoff layers in high power heterostructure bipolar transistors and thyristors due to their electric breakdown characteristics. Material properties which significantly influence the design and performance of these devices are electron and hole diffusion lengths and recombination lifetimes. We report direct measurements of minority carrier diffusion lengths for both holes and electrons by electron beam induced current. For planar Schottky diodes on unintentionally doped n-type and p-type GaN grown by metal organic vapor phase deposition (MOCVD), the diffusion lengths were found to be (0.28±0.03) μm for holes and (0.2±0.05) μm for electrons. Minority carrier lifetimes of approximately 7 ns for holes and 0.1 ns for electrons were estimated from these measured diffusion lengths and mobilities. In the case of GaN grown by halide vapor phase epitaxy (HVPE) diffusion lengths in the 1–2 μm range were found. We attempt to correlate the measured diffusion lengths and lifetimes with the structural properties of GaN and to explain why linear dislocations might act as a recombination centers. We calculate the performance of nitride based bipolar devices, in particular thyristor switches. The forward voltage drop across standoff layer of the nitride based thyristor switch is shown to significantly depend on the minority carrier (hole) lifetime.

On the formation and nature of nanometer size clusters on the surface of ZnSe epilayers

ZnSe epilayers were grown on GaAs (001) substrates by metal organic chemical vapor phase deposition, using different group VI–II precursor flow ratios. Atomic force microscopy (AFM) examinations of their surface show that epilayers grown with a high VI/II ratio are not as stable as those grown with a low ratio. When exposed to air, Se clusters would appear and grow on the surface of the unstable epilayers. The ripening process could take as long as 50 days at room temperature. Secondary electron and cathodoluminescence images indicate that the clusters are more likely to emerge from areas of high defect density. Moreover, AFM topographic images of epilayers at an intermediate stage of ripening suggest that clusters near surface depressions would grow in size at the expense of others. By using a low accelerating voltage and allowing the clusters to grow to a size larger than the electron interaction volume, we used energy dispersive x-ray analysis to show that the clusters are made up entirely of Se.

Macroscopically ordered thin films of an organic salt grown by low-pressure organic vapor-phase deposition

We demonstrate the process of low-pressure organic vapor-phase deposition (LP-OVPD) for the growth of crystalline thin films of the organic charge-transfer salt 4′-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) on amorphous TiO 2 substrates. LP-OVPD grown films exhibit polycrystallinity with long-range structural ordering limited only by substrate size. Film surface coverage and macroscopic ordering are strongly determined by the preparation of the TiO 2 surface prior to growth. The morphology and ordering of the films are also found to depend on the deposition temperature and the composition of the precursor molecules. These results suggest that LP-OVPD films are of sufficient quality for use in optical waveguide and related photonic devices.

Organic Vapor Phase Deposition

Organic vapor phase deposition (OVPD) opens new opportunities in the control of composition and structure of organic thin films. The use of this recently developed process is examined for the growth of four archetypal organic systems: a crystalline organic salt (the optically nonlinear charge transfer salt DAST, see Figure), amorphous and polymeric organic films, and an optically pumped organic laser (Alq3 doped with Rhodamine 6G).