|An Organic Solid State Injection
We report on electrically driven amplified spontaneous emission and lasing in tetracene single crystals using field-effect electrodes for efficient electron and hole injection. For laser action, feedback is provided by reflections at the cleaved edges of the crystal resulting in a Fabry-Perot resonator. Increasing the injected current density above a certain threshold value results in the decreasing of the spectral width of the emission from 120 millielectron volts to less than 1 millielectron volt because of gain narrowing and eventually laser action. High electron and hole mobilities as well as balanced charge carrier injection lead to improved exciton generation in these gate-controlled devices. Moreover, the effect of charge-induced absorption is substantially reduced in high-quality single crystals compared with amorphous organic materials.
Bell Laboratories, Lucent Technologies,
600 Mountain Avenue, Murray Hill, NJ 07974-0636, USA.
Semiconductor lasers are widely used in modern science and technology. Compared with conventional inorganic semiconductors,organic semiconductors offer potential advantages with respectto easy processing, lower cost, and flexibility. Hence, electricallydriven lasers based on organic semiconductors might find a widerange of applications. Optically excited lasing and amplifiedspontaneous emission have been observed in a wide range of semiconductingpolymers, small molecules, and organic single crystals (1-9).Moreover, amorphous or nearly amorphous organic and polymericsemiconductors have been very successfully used in thin-film organiclight-emitting devices (OLEDs). These devices typically requireinjection current densities of 1 to 10 mA/cm2 to achieve brightnesses of order 100 cd/m2 (4). For laser applications, substantially higher currentdensities will be required. Reduced luminescence efficienciesat high injection current densities and charge-induced absorptionhave been identified as major problems for electrically pumpedOLEDs (4, 10, 11). Another limitingfactor is the low charge carrier mobility. As an electricallydriven device is strongly influenced by the transport propertiesof the semiconductor, we focus on organic materials that exhibithigh mobilities for electrons as well as holes.
Materials properties. Mobilities on the order of 2 cm2 V1 s1 can be achieved in tetracene at room temperature. This is more than four orders of magnitude higher than mobilities in materials used in conventional OLEDs (4, 8, 10).Tetracene also offers a reasonably high photoluminescence quantumyield, and electroluminescence has been reported for single-crystaldiodes (12). Optically pumped amplified spontaneousemission has been observed in tetracene-doped (13,14) and undoped anthracene single crystals (15).To minimize extrinsic influences such as grain boundaries, defects,or disorder, we investigated high-quality single crystalline samplesto unveil the intrinsic opto-electronic properties of this material.
Device design. To ensure facile electrical contacts and balanced injection of electrons and holes, we used two field-effect electrodes. Tetracene single crystals were grown from the vapor phase in a stream of inert gas (16). Typical samples exhibit smooth faces of some mm2 and thicknesses in the range of about 1 to 10 µm. Field-effect device structures were prepared on freshly cleaved crystal surfaces. Source and drain electrodes (Au for holes and Al for electrons, respectively) were thermally evaporated through a shadow mask defining a 25-µm channel length and several hundred µm channel width. An amorphous Al2O3 layer was sputtered onto the crystal,resulting in a capacitance of Ci = 50 nF/cm2 between the gate electrode and the crystal. Finally, transparent Al-doped ZnO gate electrodes were deposited. A schematic of the structure is shown in Fig. 1A.
Fig. 1. (A) Structure of the tetracene single-crystal device that uses two field-effect electrodes for charge injection. Source, drain electrodes, a gate dielectric layer, and gate electrodes were deposited on the top and bottom of a single crystal. Electrons and holes are injected by applying a positive (bottom) or negative (top) source-gate voltage, respectively. In addition, a voltage (about 5 V) was applied across the crystal between both field-effect electrodes (between drain 1 and source 2 as well as between source 1 and drain 2). Electroluminescence and amplified stimulated emission are detected from the edge of the crystal. (B) Schematic energy band diagram illustrating the directions of electron and hole current flow in the device.
It has been recently demonstrated that ambipolar charge transport, i.e., n- as well as p-channel activity, can be obtained in high-quality single-crystal polyacene field-effect transistors (FETs) (17). Hence, gate-controlled electrodes can realize efficient electron as well as hole injection, with the field-induced charge acting as a heavily doped "contact" to the crystal. Furthermore, charge carrier mobilities as high as 103 to 105 cm2 V1 s1 are achieved at low temperatures, and Fractional Quantum Hall transport has been observed in similar devices at temperatures below 4 K (18). Typical threshold voltages Vt of the FETs are -0.6 V for p-channel and +1 V for n-channel operation. The carrier concentration in the channel region can be adjusted by the applied gate voltage. Organic semiconductor crystals can act as a waveguide, as has been shown for optically pumped oligothiophene crystals (19, 20). However, in the present structure, the Al2O3 layer (~150 nm) is too thin to serve as a cladding layer. Thus, a planar multimode waveguide is formed by the entire structure with air as the cladding material on both sides. This means that at low injection current densities, a substantial fraction of the guided modes are in the lossy ZnO layers. The geometry of the device is such that there are many modes, and an analysis of the optics of this structure will be published elsewhere. An overall internal loss on the order of 100 cm1 is estimated for this multilayer structure. Feedback is provided by cleaving the crystal perpendicular to the waveguide structure, resulting in a Fabry-Perot-type resonator. Assuming an ideal reflectivity of 8% for these cleaved edges, mirror losses on the order of 50 to 100 cm1 are estimated for a resonator length of several hundred µm. However,we expect the actual reflectivity at the facets of the crystalto be lower than 8% because of imperfections in the cleaving.In such structures, it is expected that gain guiding becomes importantat high injection current densities.
Device operation. Applying a negative gate voltage to the top gate and a positive voltage to the bottom gate, holes and electrons are induced at the top and bottom tetracene/Al2O3interface, respectively. The electron (ne) and hole (nh) concentration can be estimated with the gate voltage Vg and capacitance Ci (nCiVg/e). Densities on the order of 1013 cm2 are achieved for Vg of 50 V. If a potential difference is applied between the electron gas and hole gas induced by the two gates, the electrons and holes will flow to the bottom and top, respectively, resulting in a net current across the device (Fig. 1B). This voltage, which represents the voltage drop in the sample, was maintained at 5 V in the experiments reported here. However, the current density is varied by the applied gate voltages. This changes the conductivity of the electron and hole gases that form the "ohmic contacts" to the bulk of the crystal. Thus, current injection into the device is essentially contact limited. As a result of the injection of electrons and holes, excitons are formed and light is emitted. The emission intensity at 5 K as a function of the pulsed current density (10 µs, 100 Hz) up to 1700 A/cm2 (Fig. 2) shows that at relatively low current densities j (<1 A/cm2), three peaks can be observed (see Fig. 3). They can be identified as radiative recombination from the first excited state to the ground state (0-0) and its vibronic progressions (0-1, 0-2, ...) (21). The spacing between the bands of 170 meV corresponds to intramolecular vibrations. When the current density is increased by increasing the source-gate voltages, the 0-1 band becomes dominant and its width narrows from about 120 meV to 10 meV for 30 A/cm2. Finally, at even higher j (>500 A/cm2), the linewidth collapses to less than 1 meV (see Fig. 4) and is limited by the resolution of the measurement setup. The abrupt spectral narrowing in combination with the distinct threshold (30 A/cm2) is typical of amplified spontaneous emission along with gain guiding. The second decrease of the linewidth at higher current densities (>500 A/cm2) indicates the onset of laser action. The estimated gain becomes comparable to the overall resonator losses, resulting in laser action at these current densities. The large difference between thresholds for observing gain narrowing and laser action is because of the low reflectivities of the end mirrors, which lead to a high mirror loss.
Fig. 2. Optical power as a function of current density for pulsed excitation (10 µs, 100 Hz) at 5 K. The insets show the change of the emission spectrum as a function of excitation.
Fig. 3. Normalized electroluminescence (EL) intensity at 5 K as a function of energy for various current densities. The spectral narrowing of the 0-1 peak (at 2.15 eV) is visible. The right-hand picture shows the magnified spectral dependence for pulsed current densities exceeding 200 A/cm2 on an expanded energy scale.
Fig. 4. (left). Full width at half maximum (FWHM) of the emission spectrum as a function of excitation current density (5 K, pulsed). The linewidth collapses from about 120 meV to less than 1 meV. At the highest current densities, the linewidth is limited by the resolution of the setup.
Fig. 5. (right). Optical power as a function of current density for pulsed excitation (10 µs, 100 Hz) at room temperature. The insets show the change of the emission spectrum as a function of excitation. RT, room temperature.
Above the threshold for the amplification of spontaneous emission, Fabry-Perot fringes characteristic of a resonator with gain are observed. The fringe spacings are consistent with axial mode spacings of a cavity of appropriate length (22,23) (see Fig. 3 and insets in Fig. 2).The reduction in the number of peaks with increasing pump poweris also a clear indication of laser action at current densitiesexceeding 500 A/cm2. At the highest pump current densities, a single resolution limitedpeak is observed at 575.7 nm. The transition from amplified spontaneousemission to laser action occurs near 500 A/cm2; this can also be discerned from the slopes in Fig. 2. As pointed out above, the threshold for observing effects of opticalgain is much lower (~30 A/cm2). Gain narrowing is also observed at low temperatures (<200 K) for continuous wave (CW) excitation. Emission spectra taken at 300 K are shown in the inset of Fig. 5 for pulsed excitationat two injection densities. Here again we observe signatures ofamplified spontaneous emission and lasing, with a single peakabove current densities of 1600 A/cm2.
The use of two-dimensional gases of electrons and holes is an effective way to inject electrons and holes into a nominally undoped semiconductor. Doping of such semiconductors is possible and has been demonstrated. However, the diffusion of such dopants is likely to adversely affect the optical properties. Thus, the formation of electron and hole gases with field-effect structures is a promising way to realize efficient ohmic contacts to inject large carrier densities required for laser action in organic single crystals. We reiterate that while the source-gate voltage is being modulated, the potential difference across the organic crystal is held fixed at 5 V. In our arrangement, it is the source-gate voltage modulation that is responsible for the changes in intensity as well as gain narrowing of the emission spectra. Moreover, the injection of charge carriers can be controlled and balanced by individually adjusting the source-gate voltages of both metal-insulator-semiconductorstructures. As the threshold voltage Vt was similar for the n-and p-channel FETs, the source-gate voltages were chosen to bethe same for electron and hole injection.
Future prospects. Organic semiconductors can be approximated by a four-level system (24), unlike semiconductorssuch as GaAs, which are three-level systems. The natural stokesshift that exists between the absorption and emission spectrameans that the threshold pump density required to achieve transparencyis much less than in materials such as GaAs. Indeed, all thatis required is to overcome the losses of the system. Optimizationof the materials used in the structure shown in Fig. 1 will help lower the optical losses of the system. In amorphousOLEDs, carrier-induced transparency is overwhelmed by the inducedabsorption (25). In sharp distinction, in single crystalsof tetracene, our data imply that the gain is higher than thecombined losses--residual and carrier-induced. The above factor,as well as the much higher mobilities and transparencies nearthe absorption edge, are advantages of single crystals over amorphousand polycrystalline organic semiconductors for injection laserapplications. Thus, although disordered organic/polymer semiconductorshave proven to be very successful for light-emitting device applications,our results indicate that high-quality single crystals hold promisefor injection lasers based on organic semiconductors.
The above inferences suggest that in an optimized device, the threshold
current density (for laser action) could be more than two orders
of magnitude less than the maximum injection density used in
these experiments. This in turn indicates that room temperature CW-laser
action is a strong possibility in materials such as tetracene. The
structure used for this study can be improved upon in many ways
to lower the threshold for stimulated emission. Use of a low-loss
waveguide is an obvious way. The incorporation of a high-quality cavity
or resonator will facilitate true room temperature CW-laser action
by reducing the threshold.
14 June 2000; accepted 4 July 2000
Volume 289, Number 5479, Issue of 28 Jul 2000, pp. 599-601.
Copyright © 2000 by The American Association for the Advancement of Science.
Fig-4 & Fig-5