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Coherent and directional emission at 1.55 μm from a PbSe colloidal quantum dot electroluminescent device on silicon is demonstrated. The quantum dots are sandwiched between a metallic mirror and a distributed Bragg reflector and are chemically treated in order to increase the electronic coupling. Electrons and holes are injected through ZnO nanocrystals and indium tin oxide, respectively. The measured electroluminescence exhibits a minimum linewidth of ~3.1 nm corresponding to a cavity quality factor of ~500 at a low injection current density of 3 A/cm2, and highly directional emission characteristics.
©2011 Optical Society of America
1. Introduction
Several technologies are currently being investigated to realize a silicon complementary metal oxide semiconductor (CMOS)-compatible lightwave network for inter- and intra-chip optical interconnects. A key requirement is an on-chip integrated light source that can preferably emit at wavelengths ~1.3-1.6 μm. A promising approach is to employ chemically synthesized nanocrystals such as Pb(S, Se) and CdSe colloidal quantum dots (QDs) as the gain media and couple their emission to the resonances of suitable high quality-factor (high-Q) cavities.
In this context, these nanocrystals have been coupled to various types of resonant cavities formed in photonic crystals [1, 2] and glass capillaries [3] to demonstrate optically pumped amplified spontaneous emission and lasing. For a more practical device on a silicon platform, an electrically injected device was realized by integrating the QDs immersed in a conjugated polymer matrix with suitable charge transport layers and a photonic crystal (PC) microcavity [4]. However, the long-chain oleate ligand coating surfaces of PbSe QDs functions as a barrier to impede the carrier injection from electron and hole transporting layers into QDs, which requires a high turn-on voltage. Moreover, the small overlap between the cavity field and the QDs in Ref. 4 limits the Purcell enhancement and makes lasing less possible.
Here, we demonstrate electrically injected coherent and directional emission at ~1.55 μm from PbSe QDs by embedding the light emitting region between highly reflective distributed Bragg reflector (DBR) and a metal mirror, fabricated on (001) silicon substrate.
2. Device heterostructure
The device heterostructure is schematically shown in Fig. 1(a) . The light emitting region consists of PbSe QD layers, an indium tin oxide (ITO) with poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) as a hole transporting layer (HTL), and ZnO nanocrystals as an electron transporting layer (ETL), similar to an excitonic solar cell structure [5]. PbSe QDs are synthesized by a non-coordinating solvent technique [6] and 6 nm size of PbSe QDs are chosen for 1.55 μm emission. Ethanedithiol (EDT) treatment on PbSe QDs was employed to increase the electronic coupling between PbSe QDs by displacing long chain oleate ligand [5].
Fig. 1 (a) Schematic of the device heterostructure fabricated on silicon substrate. The light emitting region consisting of ITO, PEDOT:PSS, and ZnO, is clad by Ag and the bottom DBR which is made of SiO2 and amorphous-Si; (b) numerically calculated electric field intensity (blue line) in the device with refractive index profile (green line). The maximum electric field spatially coincides with a layer of PbSe QDs, which is highlighted.
PEDOT:PSS provides a smoother surface for ITO, which prevents morphologically induced electrical shorts and allows holes to be efficiently injected into the PbSe QDs. ZnO nanocrystals are also chemically synthesized by hydrolysis and condensation of zinc acetate dihydrate by potassium hydroxide in ethanol [7] and treated by ultraviolet (UV) light to enhance the conductivity, which is known to passivate electron traps on the ZnO surfaces [7, 8]. The measured absorbance spectrum shows a shoulder peak around 320 nm, corresponding to the bandgap energy of 3.9 eV. This in turn gives an average particle size of 3.5 nm [9]. ZnO nanocrystals are inherently n-type and, therefore, serves as an efficient ETL [10]. The injected carriers form excitons in the QD layer, leading to recombination and light emission.
In order to tune a resonance wavelength, 100-500 nm thick SiO2 is additionally inserted between ITO and DBR, which tailors the cavity length from λ/2 to λ. The 5-period highly reflective bottom DBR deposited on silicon substrate is composed of amorphous silicon (a-Si) as the high refractive index layer (nr ~3.48) and SiO2 as the low refractive index layer (nr ~1.52). The top mirror is formed by a layer of Ag instead of Al to attain a higher reflectivity for near infrared light. Measured reflectivity spectrum of DBR and Ag mirror exhibits a very high reflectivity of ~98.7% at λ ~1.55 μm. The bandwidth of the photonic stopband spans the wavelength range of 1.3-1.6 μm due to a large index difference between a-Si and SiO2. This avoids the Bragg mode overlap with the QD emission and ensures single mode operation.
The Q factor of the λ-cavity device calculated with the measured reflectivity of DBR and Ag mirror is about 500. Figure 1(b) shows the electric field calculated by finite difference time domain (FDTD) method with the refractive index profile shown. In order to account for the dispersive property of the metal, the dielectric function of Ag is modeled as a sum of Lorentzian functions [11]. The maximum electric field spatially coincides with a layer of PbSe QDs, which is highlighted in this figure, and the calculated overlap of the electric field in a cavity with the PbSe QD layer is 20%.
3. Fabrication
The fabrication starts with the deposition of alternating 255 nm thick SiO2 and 111 nm thick a-Si on (001) silicon substrate by plasma enhanced chemical vapor deposition (PECVD) at 300°C. Deposition rate and conditions were carefully calibrated to achieve the smooth surfaces needed for a high quality mirror. 250 nm thick ITO was sputtered on the DBR at room temperature and annealed at 250°C for 30min in nitrogen atmosphere to increase the film conductivity and improve the surface morphology.
After cleaning samples with sonication in acetone and isopropyl alcohol (IPA), the ITO surface was further treated with oxygen plasma to remove organic residue and increase the work function. PEDOT:PSS (Clevios, P VP Al 4083) was filtered through 0.45 μm polyvinylidene difluoride (PVDF) syringe filter and then spin-coated at 2,000 rpm and annealed at 150°C for 10min in nitrogen environment. After the sample was cooled down to room temperature, it was immediately transferred into an inert ambient with both oxygen and water concentration lower than 5ppm for subsequent processing.
PbSe QDs suspended in chlorobenzene was spin-cast on top of the PEDOT:PSS layer and 0.1M of EDT in acetonitrile was dropped on the surface. After the lapse of 2 mins. to allow EDT to react with the oleate ligand of PbSe QDs, the excess EDT in acetonitrile was spun away. This procedure was repeated three times to form a 100 nm thick and uniform QD layer [5], and the QD film was baked at 80°C for 30min to remove solvent residue. The areal density of the PbSe QD film is estimated to be ~1012 /cm2 with a QD size of 6 nm. After deposition of a PbSe QD layer, ZnO nanocrystal in ethanol was spin-coated and baked at 80°C for 30min. Spin-cast films were visually inspected under the microscope and showed no cracks or pinholes. Finally, the Ag cathode with an area of 1mm2 was thermally evaporated on top of the ZnO nanocrystal layer with a shadow mask.
4. Results and discussions
The current density-voltage (J-V) characteristics of the fabricated device were measured at room temperature. Figure 2 shows the measured J-V characteristic in a log-log plot. The slope of the plot increases around 2.3 V, which is believed to be the onset of electron and hole injection into the QDs. The current below the onset is limited by space-charge-limited conduction and therefore does not contribute to charge injection into the quantum dots [12]. A decrease of the J-V slope is observed as the bias is increased above 5 V, which indicates that the carrier injection is limited by series resistance of either the ETL or the HTL. The measured J-V characteristics plotted on a linear scale, including the reverse bias region, shows good rectifying behavior (inset of Fig. 2).
Fig. 2 Measured current density-voltage characteristics of the fabricated device in log-log plot. The increase of the slope at 2.3 V indicates the onset of carrier injection into the QDs. The inset shows J-V characteristics in linear scale, which exhibiting a good rectifying behavior.
For electroluminescence (EL) measurements, a forward bias was applied between the ITO anode and the Ag cathode. In order to avoid heating, a pulsed current source was used with a repetition rate of 50 Hz and a duty cycle of 50%. The light output from the device was analyzed with a high-resolution monochromator and detected with a liquid nitrogen cooled Ge detector using phase lock-in amplification.
Figure 3(a) shows the normalized EL spectra of a control device without DBR and devices with DBR. The output spectrum of the control device exhibits a broad emission between 1.4 and 1.65 µm, with a full-width-at-half-maximum (FWHM) of ~120 nm. On the other hand, the devices with DBR and two different thickness of SiO2 spacer show cavity resonances at λ = 1563.1 (Device 1) and 1603.7 nm (Device 2), respectively. The narrowest linewidth observed is ~3.1 nm at an injection current density of 3 A/cm2 from Device 1, which is ~38 times less than that of the control device and corresponds to a cavity Q factor of 500. The additional peak at λ = 1619.3 nm was also observed on the shoulder of the dominant peak in Device 2. This abnormal peak is due to the non-uniform cavity thickness. The light emission from the device with DBR was also measured at angles ranging from 0 to 45 degrees with respect to the cavity axis, as shown in the angular emission characteristics (Fig. 3(b)). Also shown is the Lambertian emission pattern for comparison. The device with DBR exhibits a highly directional light output parallel to the cavity axis, showing a smaller divergence compared to the Lambertian emission pattern.
Fig. 3 (a) Room temperature electroluminescence spectra from the control sample without a DBR and two devices with different cavity sizes. Resonant modes are tuned by varying the cavity size. The narrowest linewidth is ~3.1 nm at λ = 1563.1 nm; (b) angular emission characteristics recorded from the device with DBR at angles ranging from 0 to 45 degrees with respect to the cavity axis. The Lambertian pattern is also shown for comparison.
The measured light-current density characteristics shown in Fig. 4 displays the linear increase of light output as the current density increases. The onset of light output inferred from the extrapolation of the linear fit to the measured points is around 2.4 V, which coincides with the turn-on estimated from the J-V characteristics. As the current density increases above 4 A/cm2, the integrated light output becomes saturated due to heating.
Fig. 4 Measured voltage-current density and light-current density characteristics of the device. The integrated light output increases linearly with current. The extrapolation of the linear fit indicates a turn-on voltage of ~2.4 V.
5. Summary
To summarize, we demonstrate coherent emission at room temperature from electrically injected PbSe QDs placed between a metallic mirror and DBR on silicon. A minimum linewidth of ~3.1 nm was measured, corresponding to a cavity Q factor of 500, for a low injection current density of 3 A/cm2. The device also exhibits a highly directional emission, which is favorable for efficient fiber coupling. This directional and coherent silicon based light source using colloidal QDs is of interest as a viable technology for optical interconnects as the device is miniaturized.
Acknowledgments
The work at the University of Michigan is supported by the Air Force Office of Scientific Research under Grant No. FA9550-09-1-0634 and that at Pennsylvania State University (PSU) is supported by the Army Research Office under Grant No. DAAD19-02-D-0001. The work at the PSU is also partially supported by the National Science Foundation under Grants EECS-0846818 and EECS-0824816. The authors would like to thank Prof. Lingjie (Jay) Guo for his support during fabrication. One of us (JH) would like to thank the Samsung Scholarship Program.
References and links
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