Uniform SiGe/Si quantum well nanorod and nanodot arrays fabricated using nanosphere lithography
© Chang et al.; licensee Springer. 2013
Received: 17 June 2013
Accepted: 5 August 2013
Published: 8 August 2013
This study fabricates the optically active uniform SiGe/Si multiple quantum well (MQW) nanorod and nanodot arrays from the Si0.4Ge0.6/Si MQWs using nanosphere lithography (NSL) combined with the reactive ion etching (RIE) process. Compared to the as-grown sample, we observe an obvious blueshift in photoluminescence (PL) spectra for the SiGe/Si MQW nanorod and nanodot arrays, which can be attributed to the transition of PL emission from the upper multiple quantum dot-like SiGe layers to the lower MQWs. A possible mechanism associated with carrier localization is also proposed for the PL enhancement. In addition, the SiGe/Si MQW nanorod arrays are shown to exhibit excellent antireflective characteristics over a wide wavelength range. These results indicate that SiGe/Si MQW nanorod arrays fabricated using NSL combined with RIE would be potentially useful as an optoelectronic material operating in the telecommunication range.
Over the past decades, there has been enormous interest in fabricating periodic semiconductor nanostructures, in which the semiconductor nanodot or nanorod array has shown its great potential for future applications in photonic crystals , nanoscale transistors , field electron emitters , biomaterials , and light-emitting devices . The well-known top-down techniques providing accurate size and geometric control in periodic semiconductor nanostructure patterning include laser interference lithography , nanoimprint lithography , ion beam lithography , and electron beam lithography . However, the cost and complexity of these techniques increase dramatically with the demand for reduced feature sizes over large areas. Nanosphere lithography (NSL) has emerged as an alternative nanofabrication technique, where a monodisperse or multidisperse nanosphere template acts as an etching or deposition mask to transfer its pattern to the underlying substrate [10–12]. The sizes of nanospheres can be tuned from 20 to 1,000 nm [13, 14], offering a simple and inexpensive solution to scale nanostructure feature sizes. More importantly, the location, density, and coverage of nanostructures can be well controlled. With improvements in the domain sizes of the self-assembled nanosphere arrays , NSL has great potential in fabricating nanoscale electronics, optoelectronics, thermoelectrics, and biosensors. Over the past decade, NSL has been used to nanopattern Si , GaAs , and glass  substrates. Recently, we also demonstrated the realization of SiGe nanorod arrays on SiGe virtual substrates using NSL combined with catalytic etching .
On the other hand, the idea of integrating optoelectronic and electronic devices into Si chips has always been highly attractive due to the benefits in cost, reliability, and functionality . However, Si is an indirect bandgap semiconductor and thus of limited use for optoelectronic applications. Many efforts have been made to resolve the low quantum efficiency of Si associated with its indirect bandgap. One important approach is the combination of Si with other semiconductor materials, such as Ge or Si1 − xGe x alloys for heterostructures. For this purpose, Si/Ge superlattices (SLs) , multiple quantum wells (MQWs) , and multiple quantum dots (MQDs)  have been demonstrated to adjust the bandgap and reduce nonradiative recombination. Choi et al. further reported that the formation of microdisks from the Si/Ge/Si single QW using electron beam lithography significantly enhanced the intrinsic photoluminescence (PL) transitions . Chen also fabricated pyramidal nanodots that possess Si/Ge SLs by chemical selective etching through a self-assembled Ge QD nanomask and found an obvious enhancement in PL emission . In addition, an improvement of light extraction from SiGe/Si MQWs with nanowall structures fabricated by electron cyclotron resonance plasma etching through a random Al-masked pattern was also reported . However, few studies reported the fabrication of periodic nanostructure arrays composed of SiGe/Si MQWs using NSL. In this study, we demonstrate the fabrication of optically active uniform SiGe/Si MQW nanorod and nanodot arrays from the Si0.4Ge0.6/Si MQWs using NSL combined with the reactive ion etching (RIE) process. Compared to the as-grown sample, we observe an obvious blueshift in PL spectra for the SiGe/Si MQW nanorod and nanodot arrays, which can be attributed to the transition of PL emission from the upper multiple quantum dot-like (MQD-like) SiGe layers to the lower MQWs. In addition, the SiGe/Si MQW nanorod arrays are also shown to exhibit excellent antireflective characteristics over a wide wavelength range.
It is crucial to obtain a hydrophilic surface to allow the self-assembly of PS nanosphere monolayer arrays. In the first step, the as-grown SiGe/Si MQW samples were ultrasonically cleaned in acetone and in a solution of 4:1 H2SO4/H2O2 at 80°C for 30 min to prepare a hydrophilic surface. The SiGe/Si MQW samples were then coated with 800-nm-diameter PS nanospheres to form highly ordered and close-packed nanosphere arrays. Subsequently, a mixture of SF6 and O2 was used to etch the samples at a working pressure of 25 mTorr for various durations to form the SiGe/Si MQW nanorod arrays. During the RIE etching, the inductively coupled plasma (ICP) power and bias of the etcher were kept at 50 W and 25 V, respectively. Finally, the PS nanosphere template was removed by ultrasonically cleaning in acetone solution. In addition, for the nanosphere resizing, O2 plasma RIE was used to shrink the PS nanospheres, allowing postspin feature size control.
The surface morphologies of the etched samples were examined by scanning electron microscopy (SEM; FEI Quanta 200F, Hillsboro, OR, USA). Transmission electron microscopy (TEM) was carried out with a JEOL 2100 TEM (Akishima, Tokyo, Japan) operating at 200 kV to reveal detailed information about the microstructures of the etched nanostructures. PL measurements were performed at 10 K to study the optical properties of the SiGe/Si MQW nanorod and nanodot arrays using a 514.5-nm line of an Ar+ laser. The PL spectra were recorded by a liquid nitrogen-cooled Ge photodetector with the standard lock-in technique. We also measured total hemispherical reflectance spectra in air on a spectrophotometer with an integrating sphere (300 to 2,000 nm, Hitachi U-4100, Chiyoda, Tokyo, Japan) for the etched SiGe/Si MQW nanostructures.
Results and discussion
In Figure 4b, we also find that in spite of the large material loss in the RIE process, the SiGe/Si MQW nanorod arrays exhibit a strong PL intensity comparable to that of the as-grown counterpart. We suggest that there exists a possible mechanism for PL enhancement. As mentioned above, this PL enhancement is difficult to be attributed to quantum confinement or indirect–direct bandgap transition since the mean diameter of the MQW nanorods is much larger than the exciton Bohr radius of Si and Ge. Some groups have reported the enhancement of PL intensity by laterally patterning the III-V or IV-IV heterostructures with the sizes similar to or larger than that in this study. A significant enhancement of the quantum efficiency in the PL spectra has been observed by forming GaN/AlGaN MQW microdisks of about 9-μm diameter and interpreted as a suppression of impurity-related transitions . Choi et al. also associated the PL enhancement with carrier localization in the 500- and 1,000-nm-diameter Si/Ge/Si microdisks fabricated by electron beam lithography, the existence of which suppresses impurity-related nonradiative combination . The similar mechanism may also contribute to the enhancement of PL intensity in our SiGe/Si MQW nanorod arrays. In addition, in this study, the high-density plasma generated during RIE process may severely damage the surface of SiGe/Si MQW nanorods and therefore form a 10- to 20-nm-thick amorphized layer on the surface. This may result in the formation of an effective ‘dead layer’ (indicated by DL in Figure 5a, b, c), in which nonradiative recombination processes dominate. This dead layer will further reduce the effective lateral size of the nanorods because carriers able to participate in optical process are confined to the undamaged region of the MQW nanorods. This factor may also act in the PL emission process and further enhance the PL intensity. For practice applications, this dead layer can be minimized by adjusting the RIE process parameters, such as reducing ICP power, decreasing reactive gas fluxes, and lowering the operating temperature. In addition, this damaged layer can be removed by an etchant .
In conclusion, this study demonstrates the fabrication of optically active uniform SiGe/Si MQW nanorod and nanodot arrays from the Si0.4Ge0.6/Si MQWs using NSL combined with reactive RIE. Compared to the as-grown sample, we observe an apparent blueshift in PL spectra for the SiGe/Si MQW nanorod and nanodot arrays, which can be attributed to the transition of PL emission from the upper MQD-like SiGe layers to the lower MQWs. A possible mechanism associated with carrier localization is proposed for the PL enhancement. Moreover, the SiGe/Si MQW nanorod arrays are shown to exhibit excellent antireflective characteristics over a wide wavelength range from the ultraviolet to infrared. This work offers a low cost and feasible alternative for designing and fabricating SiGe/Si nanostructured arrays as a potential material of multifunctionality.
H-TC is currently a Ph.D. candidate of National Central University (Taiwan). B-LW is a Master's degree student of National Central University (Taiwan). S-LC and TL are professors of the Department of Chemical and Materials Engineering at National Central University (Taiwan). S-WL is an associate professor of the Institute of Materials Science and Engineering at National Central University (Taiwan).
The research is supported by the National Science Council of Taiwan under contract no. NSC-100-2221-E-008-016-MY3. The authors also thank the Center for Nano Science and Technology at National Central University.
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