1. Introduction

Nano-optoelectronics have received a significant attention because of the potential applications in optical interconnections, light-emitting diodes (LEDs) and laser diodes (LDs) [1–5]. The integration of optical emitters such as LEDs and LDs into Silicon substrate is very important to the many applications including the opto-electronic integrated circuit [3–6]. The nature of the heteroepitaxy made the direct growth of LED/LD structures on Silicon substrate very difficult. Therefore, the optical emitters have been transferred to Silicon substrate after they were grown on other substrates or directly grown on Si substrate by inserting the complex strain relaxation layers [6, 7]. There are two kinds of approaches that have been commonly employed to obtain the nano-emitter structures, which are top-down and bottom-up methods [8–13]. In bottom-up method, the vapor-liquid-solid (VLS) technique using the nano-sized catalytic metals produced high quality nanowires and nanorods, but the poor controllability in length, diameter and carrier concentrations has been an issue Therefore, the development of a facile method for forming the nanostructures with uniform size, structure and quality attracted a lot of attention. Recently, an anodic-aluminum-oxide template was employed to improve the controllability [10]. However, it is still very complicated to demonstrate nano-LED structures with MQWs by using bottom-up approach [11]. Instead, top-down method that begins with (chemical or dry) etch of conventional thin-film LED wafer allows us a precise control in the optical and electrical properties (length, diameter, QWs and carrier concentrations) of the nano-LEDs [12, 13]. Kuo et al. reported that a nanorod LEDs with MQWs has ~79% extraction efficiency without a back reflector [12]. Furthermore, the top-down method is highly compatible with the advanced microfabrication processes. Therefore, we used top-down method to fabricate nano-sized LEDs by the combination of the nanospheres lithography and dry etch. Nanosphere lithography has been widely used to create the nanostructures because it is very simple and highly reproducible where the nanospheres can be used as etch mask or lift-offed after the metal mask was deposited. The diameter and length (height) of the nanostructures can be controlled by the diameter of the etch mask and the etching time, respectively. Kim et al. showed that GaN nanorods were successfully demonstrated by the similar technique [13].

In addition, the positioning of the nanostructures for the device applications is an issue because the electron-beam lithography and focused-ion beam techniques are time-consuming and costly. Several methods have been reported to position micro- and nano-structures precisely on wafer, including layer-by-layer, contact printing, fluidic flow-assisted, and electric field-assisted techniques [14–17]. Among them, the electric field-assisted technique (dielectrophoresis, DEP) has many advantages because it can position effectively and precisely the dielectric particles including nanostructures and bio-materials. DEP force can be generated when the particles are within a non-uniform electric field, where the force can be positive or negative, depending on the frequency and the nanostructure/medium materials. A positive DEP force positions particles to the target position by pulling the particles toward the highest gradient in the electric field. Kim et al. aligned GaN nanowires to the electrode at various frequencies and biases and measured the yield [18]. Motayed et al. fabricated a high efficiency homojunction LED that emitted a wavelength of 365 nm by aligning n-type GaN nanowires on a p-type GaN substrate using DEP force [19]. However, the electric-field-assisted positioning of the nanostructured LEDs with MQWs has been rarely reported.

In our experiments, uniform GaN nanorods with InGaN/GaN MQWs were obtained by using a simple top-down approach, followed by positioning the processed GaN nanorods under a positive DEP force onto the pre-patterned target electrode. Optical and electrical characterizations including SEM, cathodoluminescence (CL), current-voltage (I-V) characteristics and EL were used to characterize the processed nanorod-LEDs.

2. Experimental details

SiO₂ nanospheres were prepared by mixing ammonia with deionized water in an addition of ethanol through the hydrolysis of tetra-ethylorthosilicate, followed by sonication and centrifugation. SiO₂ nanospheres were spun-coated onto commercial blue LED structure with MQWs grown on sapphire substrate at 2000 rpm (Fig. 1(a) ). Inductively-coupled plasma (ICP) dry-etch method by using BCl₃ (5 sccm) and Cl₂ (30 sccm) at a pressure of 5 mTorr for 190 seconds were used to create uniform nanorods with MQWs (Fig. 1(b)). Nanorods had relatively uniform shapes, which are 1 µm of length and a 400 nm of diameter. They were controlled by the size of the SiO₂ nanospheres (mask material) and etch condition. Nanorods were tapered due to the sphere-shape of the mask material (nanospheres). ICP-etched sample was dipped in buffered-oxide-etchant solution (HF: H₂O = 1:6) to remove the residual SiO₂ nanospheres. Finally, this sample was chemically etched with KOH solution to minimize the plasma damage induced by ICP etch.

 

Fig. 1 Fabrication process (1) Deposition of SiO₂ nanospheres on LED structure by spin-coating (2) ICP etch (3) Dispersion of nanorods (4) Positioning the nanorods by DEP.

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SEM/CL (S-4700 Hitachi/ Gatan Mono CL3) was used to characterize the structural and optical properties of our nanorods with MQWs. Pre-patterned SiO₂/p⁺-Si substrate was prepared by depositing Ti/Au (20 nm /80 nm) on the backside of highly p-type doped Si wafer using an e-beam evaporator to make the backside electrode, followed by the conventional photolithography processes for the front-side patterning (Ti/Au (20 /80 nm)) on top of SiO₂. Then, rapid thermal annealing was performed at 300°C for 30 sec. To demonstrate the mushroom shape of Au/Ti/SiO₂ structures, the sample was etched with a combination of buffered-oxide-etchant solution and reactive ion etching (SF₆ (10 sccm) and O₂ (10 sccm)) at 100 W for 60 sec. Finally, Ti/Au (20 /80 nm) was deposited on the exposed p-Si layer by using an e-beam evaporator (Fig. 1(d)). The solution that contains the dispersed GaN-based nanorods LED with MQWs was dropped on the pre-patterned SiO₂/Si substrate, followed by applying an alternating current (AC) signal of 1 MHz and 6 Vpp (peak-to-peak voltage) to the circular Ti/Au pattern on top of the SiO₂ layer. An annealing process was performed at 450°C for 30 sec to improve the contact resistance between the nanorods and Ti/Au contacts.

EL images under various bias conditions were obtained by using a charge-coupled devices camera connected to a probe station. The nanorods after DEP processes was investigated by optical (microscope and SEM) and electrical method (Agilent 4155C Parameter Analyzer).

3. Results and discussion

Before the processed nanorods were transferred to the pre-patterned SiO₂/Si substrate, the uniformity and quality of the nanorods were characterized by using SEM/CL. Figures 2(a) and 2(b) show SEM and CL images of the processed nanorods after separated from the LED wafer, respectively. Note that Fig. 2(a) is an SEM image of the nanorods before the sonication process. The average length of the nanorods was approximately 1 µm, and the average diameter of the larger part was about 400 nm, and that of the narrower part was approximately 240 nm. The sphere-shape of the masking materials resulted in the tapering of the nanorods. Figure 2(b) is a CL image of Fig. 2(a). White stripe luminescences observed from the nanorods in Fig. 2(b) seems to be MQWs, where the white stripe was positioned at approximately 140 nm from the top of the nanorods. Figure 2(c) represents the CL spectrum that consists of the multiple peaks from InGaN/GaN MQWs, defects, and the GaN [20–23].

 

Fig. 2 (a) SEM image (b) CL image of the nanorods (c) CL spectrum from the nanorods.

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The DEP force on a spheroid can be expressed by the expression: F_DEP = A⋅ε_m⋅K(ω)⋅∇E², where A contains the information of the volume of the spheroid (in our case, nanorod LED), ε_m is the permittivity of the medium, K(ω) is the real part of the Clausius–Mossotti factor, and E is the magnitude of the electric field [17, 24]. It has been reported that a prolate spheroid can be approximated as a cylindrical particle [24]. K(ω) can be expressed as: K(ω)≡Re[(ε_p*- ε_m*)/ε_m*], where ε_p* and ε_m* are the complex permittivities of the particle and medium, respectively. This Clausius-Mosotti factor depends the signal frequency as well as the conductivity and permittivities of the medium and nanorods; therefore, the positive DEP force will attract the nanorods to the metal electrode where the electric field gradient is higher.

Figure 3(a) is the microscope image of the nanorods positioned by DEP force at a signal frequency of 1 MHz. GaN nanorods in the form of small black spots are shown around the circular metal electrode. SEM image shows that the nanorods were successfully positioned around the metal electrode (Fig. 3(b)). Figures 4(a) and 5(a) show the I-V characteristics at different bias conditions. Bright violet-blue ELs from the aligned nanorods were observed as shown in Figs. 4(b) and 5(b) under the negative and positive bias conditions, respectively. Insets of Figs. 4(a) and 5(a) depict the shape of the aligned nanorods and the light emission from the nanorods. Note that the nanorods that emit bright EL in Fig. 4(b) are different from those in Fig. 5(b). When the tapered nanorods were attracted and aligned, either the head (p-type GaN layer) or the tail (n-type layer) would randomly contact the metal electrode, which means that both structures (the nanorod on the left and on the right in Fig. 1(d)) are possible. Therefore, the nanorods on the left will turn on when the positive bias are applied to the circular electrode. Under the negative bias condition, the nanorods on the right in Fig. 1(d) will emit EL. Note that both nanorods in Fig. 1(d) emit EL only on the forward bias condition from the view point of the nanorods. Since the plasma etch such as ICP can induce the crystal damage, the red /yellow EL in Figs. 4(b) and 5(b) can be attributed to the existence of the defect sites in nanorods because the ICP etch can create the deep levels within the band-gap. These defect sites can also result in the high leakage currents in I-V characteristics. We believe that nano-emitters with MQWs produced by nanospheres lithography method have many advantages in the quality control of the nanorods and nanowires, compared with conventional VLS method. Also, the alignment of the nanorods by DEP force allows us to fabricate the nano-LED devices without the need of the complex electron-beam lithography and focused-ion beam methods.

 

Fig. 3 (a) Microscopic image and (b) SEM image around the metal electrode after DEP force was applied.

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Fig. 4 (a) I-V characteristics (Inset: schematic image after DEP) (b) EL image when the negative bias was applied to the top circular electrode. Note that the nanorods that emit EL are under forward bias condition.

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Fig. 5 (a) I-V characteristics (Inset: schematic image after DEP) (b) EL image when the positive bias was applied to the top circular electrode (Media 1). Note that the nanorods that emit EL are under forward bias condition.

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4. Conclusion

Uniform GaN-based nanorod LEDs with InGaN/GaN MQWs were fabricated by using an ICP plasma etch after SiO₂-based nanospheres lithography. Nanorods with InGaN/GaN MQWs were precisely aligned around the pre-patterned contacts on SiO₂/p-Si substrate assisted by DEP force. Bright blue-violet EL was observed from the aligned nanorods when the bias was applied to the circular metal electrodes, where the either of head or tail was attracted to the metal electrode. The nanorods that emit the EL when the positive bias was applied to the circular electrode were different from those under negative bias condition. Red-yellow EL can be attributed to the defect sites within the band-gap. Our simple method that combines the nanospheres lithography and DEP force is very effective in producing and positioning the nano-emitters with MQWs.