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The emission and waveguiding properties of individual GaN microwires as well as devices based on an n-GaN microwire / p-Si (100) junction were studied for relevance in optoelectronics and optical circuits. Pulsed photoluminescence of the GaN microwire excited in the transverse or longitudinal direction demonstrated gain. These n-type GaN microwires were positioned mechanically or by dielectrophoretic force onto pre-patterned electrodes on a p-type Si (100) substrate. Electroluminescence from this p-n point junction was characteristic of a heterostructure light-emitting diode. Additionally, waveguiding was observed along the length of the microwire for light originating from photoluminescence as well as from electroluminescence generated at the p-n junction.
©2011 Optical Society of America
1. Introduction
The III-nitride material system is the basis of the entire UV and blue semiconductor laser diode and light-emitting diode (LED) market [1, 2]. Recently, nano/micro structures have emerged for use as the building blocks of the next generation micro- and nano- photonic devices [3–5]. Integrating semiconductor based optical waveguides, light detectors, and light emitters or lasers within Si microelectronics would allow hybrid optical/electronic circuitry.
Top-down processing of III-nitride thin films has been used to produce micron-scale interconnects and light-emitters. Bottom-up processing of nano/microstructures has a number of advantages including the strain-free growth of a single crystal with large surface area and natural facets, and superior mechanical and electrical properties [3–5]. Breakthroughs in growth and synthesis techniques of nano/micro wires has allowed for the rapid fabrication of high quality nano/micro wires. One approach that amalgamates top-down and bottom-up processing is a nanowire or microwire / thin film homojunction or heterojunction [6–10]. Huang et al. reported multi-color electroluminescence (EL) from p-Si/n-type nanostructures by fluidic assembly [3]. Chen et al. demonstrated EL from n-ZnO nanowire / p-GaN thin film heterostructures by MOCVD [8]. Previously, the authors of this article reported EL from n-ZnO nanoflowers / p-GaN thin film heterostructures and from n-GaN nanostructures / p-GaN thin film homojunction [9, 10]. Another interesting property of microwires is the intrinsic wave-guiding by total internal reflection [11,12]. Wave-guiding occurs when the light is confined in a core layer due to the difference in refractive indices between the inner layer and outer layer. The condition that the refractive index of the inner layer, e.g., 2.4, in GaN, is larger than that of the surrounding layers, e.g, 1 in air, is required for total internal reflection. For photonics, microwires alleviate the need for a potentially deleterious etch that can introduce optical scattering centers [11,12]. This work demonstrates an approach to directly place GaN microwires within a processed Si (100) structure and shows that this microwire acts as a light waveguide over the several micron length of the wire. On-chip optical interconnects, emitters, and detectors are possible components to convert electrical/optical signals and route optical signals within Si micro-electronics – and potentially address the interconnect bottleneck [13–15].
2. Experimental details
The near atmospheric pressure growth technique employs thermal gradients within the solution to grow GaN crystals. By changing the composition of the solution, the growth direction of the crystals can be controlled. The GaN microwires were grown from a solution held at 0.2 Mbar and 800 °C for 100–120 h. After growth, the microwires were cleaned with acid and collected. The resulting wires have diameters ranging between 1 and 10 μm and lengths up to 1 mm, but averaging between 100 and 300 μm.
A 300nm thick SiO2 layer was thermally grown on a heavily doped p-type Si substrate (Boron doped, 1.0~10.0 ohm•cm, Silicon Technology Corporation) to isolate the metal contact to the n-GaN microwire. To form the bottom electrode, Ti/Au (20nm/80nm) was deposited by an e-beam evaporator on the back side of a Si substrate. The top electrode (Ti/Au 20nm/80nm) was defined by photolithography, followed by electron-beam evaporation. After the lift-off process, reactive ion etching (RIE, O2 10 sccm, SF6 10 sccm, 100 watt) was performed to etch the SiO2 layer for 30 seconds. To remove any residual SiO2, a buffered oxide etchant (BOE) was used. The GaN microwire was mechanically transferred by probe-tip or positioned by dielectrophoretic force as described in [9, 10]. A schematic and SEM images of the p-Si thin film / n-GaN microwire heterojunction LED are shown in Fig. 1 , where the undercut from the BOE wet etch is observable. The undercut is preferred to form the contact to the GaN microwire. The CW photoluminescence (PL) spectrum of an as-grown GaN microwire was taken with the 325nm wavelength excitation source from a He-Cd laser (Kimmon Corporation). Stimulated emission (SE) was excited with a 266nm pulsed (4nsec, 20Hz) Nd:YAG laser. Current-Voltage characteristics from n-GaN/p-Si heterojunction were obtained by an Agilent 4155C Parameter Analyzer.
Fig. 1 (a) Schematic image and (b) optical microscopy image of the n-GaN microwire / p-Si thin film heterojunction LED. (c) An SEM image that shows the undercut formed by wet etching.
3. Results and discussion
Figure 1(b) shows an optical image of the p-Si thin film / n-GaN microwire heterojunction LED. Discoloration points along the wire are from growth-related defects. The SiO2 layer is approximately 300nm in thickness and confirms that the wet etch undercut yielded a step edge (Fig. 1(c)).
Photoluminescence spectra of individual GaN microwires were obtained with light collected from the cylindrical wall (transverse orientation, Fig. 2(a,c) and from the rod end (longitudinal orientation, Fig. 2(b,d) as a function of excitation level. The sharpening of the peak and the non-linear increase in the peak intensity is characteristic of a gain mechanism, exhibiting a threshold near 1MW/cm2 in both orientations. The green curve, Fig. 2(c) and the red curve, Fig. 2(d), correspond to a part of the spectrum (with the wavelength indicated in the figure) that increase approximately linearly with power, representing spontaneous emission. The blue curve Fig. 2(c) and the purple curve Fig. 2(d) correspond to the difference between the intensity of the nonlinearly growing peak near 375nm and the spontaneous background (green and red curves), and represents the enhanced emission component.
Fig. 2 Photoluminescence spectra of a GaN microwire in the (a) transverse and (b) longitudinal orientations. The super-linear increase in emission is evident when the peak intensity is plotted as a function of input power density in the (c) transverse and (d) longitudinal orientation in a (c) log and (d) linear intensity scale.
In the transition from spontaneous emission to stimulated emission, nanowires are often observed to exhibit an amplified spontaneous emission region where propagation losses begin to be compensated by gain creating longitudinal cavity modes that resonate between the reflective end facets [16]. Work is underway to further characterize this transition from an amplified spontaneous emission regime that is super-linear with pump-power to a stimulated emission regime that is linear with pump-power [17].
Figure 3 displays the CW PL spectrum of a GaN microwire positioned on a Si (100) substrate patterned with a contact metallization. The PL spectrum of the GaN microwire shows an intense near-UV band edge emission with a peak at approximately 376 nm, which indicates the high quality of GaN microwire. The transition near 500nm has been attributed to Ga-vacancy defects in thin films [18]. Similarly, Li and Wang directly observed blue and red luminescence in the bulk of GaN nanowires and attributed these transitions to Ga-vacancy complexes [19].
Fig. 3 PL spectrum of GaN microwire displays strong near bandedge emission. Inset: Optical image of the n-GaN microwire on a p-Si substrate with the metal electrode visible as a circle. Optical stimulation with a 325nm wavelength CW laser source produced photoluminescence at a point near the contact edge (black arrow in the inset). Light generated at the PL point traveled the length of the wire without apparent scattering and escaped into free space at the microwire tip.
The inset microscope image of Fig. 3 shows that light is emitted only at the photoluminescence point and the end of the waveguide. This confirms that the generated light traveled through the GaN waveguide without significant scattering. Light generated at the photoluminescence point with a propagation direction within approximately 24° of the surface normal can escape as emitted light as described by Snell’s law [2]. The remaining light suffers total internal reflection and can only escape at the end of the wire.
Figure 4 shows qualitatively that the EL from the p-n heterojunction increases with the increasing bias voltage. The location of carrier injection, recombination, and light emission is at the physically formed p-n point junction located at the right end of the microwire in each sub-picture in Fig. 4. Similar to the photoluminescence result in Fig. 3, light generated at the p-n junction can escape through the light cone, suffer from total internal reflection and escape from the other (i.e., left) end of the microwire, or scatter or be absorbed at defects along the nanowire. As expected, the location where the p-n junction exists displays the brightest emission. The majority of the remaining emitted light appeared at the other end of the nanowire. Light emission was also observed as scattered spots along the microwire, which are attributed to the photons that escaped through local nonuniformities such as growth-induced step-edges [16].
Fig. 4 Optical microscopy images of the LED devices biased under increasing forward bias. The primary light source at the right tip of the microwire coincides with the location of the n-GaN microwire / p-Si junction. Waveguiding produced light emission at the opposite (left) tip of the microwire.
The electron affinity and band gap are 4.05 and 1.1eV, respectively, for Si and the electron affinity and band gap of GaN are 4.1 and 3.44eV, respectively (Fig. 5(b) ) [20,21]. The I-V curve in Fig. 5(a) of the p-Si thin film / n-GaN microwire heterojuction LED displayed a turn-on voltage near 1V, which is approximately the energy necessary to align and inject electrons from the conduction band of the GaN to the conduction band of Si. The gradual increase in current with increasing voltage is attributable to the increasing number of holes with sufficient energy to surmount the valence band discontinuity. Efficient injection of holes from the p-Si into the n-GaN is necessary for EL and, as expected, experimentally EL appeared starting near 5V. In Fig. 4, it was observed that the color of the EL changed when biased due to the voltage dependent injection mechanism. Additionally, defect states within the bandgap can contribute mid-gap transitions that generate red and green emission (Fig. 5(c)).
Fig. 5 (a) I-V characteistic of the n-GaN microwire / p-Si thin film heterojunction LED. (b) Band diagram of the n-GaN microwire / p-Si substrate heterojunction LED (c) Band diagram under a forward bias. Major forward conduction began at approximately 5V.
GaN and Si are known to form a native oxide and these oxides likely behave as a tunnel barrier at the p-n junction. Although an oxide etchant was used to strip the surface, the native oxides likely hampered the contact formation, which caused the I-V characteristic to deviate from an ideal p-n junction. Nevertheless, the EL spectra and I-V curve suggested the emission mechanism is distinctive to a p-n heterojunction.
4. Conclusion
This study demonstrated electroluminescence from an n-GaN microwire / p-Si (100) thin film heterojunction LED. Optical and electrical excitation of light propagated along the microwire as a wave-guide owing to the high index of refraction contrast of GaN and air (via Snell’s law) as well as the intrinsically smooth sidewalls of the microwires. The n-GaN microwire / p-Si heterojunction LED displayed an I-V curve characteristic of a p-n junction. The major forward conduction began at about 5V, which slightly deviated from the ideal p-n junction equation, which is attributed to the some residual native oxide at the junction interface. Scattered light emission spots along the microwire resulted from a small number of local nonuniformities in the GaN microwire.
Acknowledgements
The research at Korea University was supported by a Human Resources Development grant from the Korea Institute of Energy Technology Evaluation and Planning (KETEP) funded by the Ministry of Knowledge Economy (No. 20104010100640) and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0028021 and 2011-0004270). The research at NRL was partially supported by ONR.
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