Patterned growth of InGaN/GaN quantum wells on freestanding GaN grating by molecular beam epitaxy
© Wang et al; licensee Springer. 2011
Received: 7 September 2010
Accepted: 4 February 2011
Published: 4 February 2011
We report here the epitaxial growth of InGaN/GaN quantum wells on freestanding GaN gratings by molecular beam epitaxy (MBE). Various GaN gratings are defined by electron beam lithography and realized on GaN-on-silicon substrate by fast atom beam etching. Silicon substrate beneath GaN grating region is removed from the backside to form freestanding GaN gratings, and the patterned growth is subsequently performed on the prepared GaN template by MBE. The selective growth takes place with the assistance of nanoscale GaN gratings and depends on the grating period P and the grating width W. Importantly, coalescences between two side facets are realized to generate epitaxial gratings with triangular section. Thin epitaxial gratings produce the promising photoluminescence performance. This work provides a feasible way for further GaN-based integrated optics devices by a combination of GaN micromachining and epitaxial growth on a GaN-on-silicon substrate.
81.05.Ea; 81.65.Cf; 81.15.Hi.
It's of significant interest to conduct the fundamental research as well as the applied study on the epitaxial growth on patterned GaN-on-silicon substrate [1–9]. Commercial GaN-on-silicon substrates make this research feasible , and novel epitaxial structures can be generated with smooth facets and are free of etching damage. It can also provide a great potential for further integrated GaN optics devices by a combination of the epitaxial growth, etching of GaN and silicon micromachining.
Compared to other growth techniques, the selective growth of GaN by molecular beam epitaxy (MBE) is relative difficult [11, 12]. The substrate also impacts on the epitaxial growth. As the epitaxial growth of GaN on patterned Si or SiO2 substrates, GaN nanocolumns are easily formed due to random nucleation [13, 14]. Selective area growth of GaN can produce periodic GaN nanocolumns with the assistance of nanostructured Ti-mask [15, 16]. Recently, the selective growth of GaN by MBE is realized on patterned GaN-on-silicon substrate without introducing additional dielectric mask . The shape and the growth area have the dominant influence on the realization of the selective growth by MBE. This approach enables easy fabrication and scaling, opening the great potential for a large variety of novel GaN-based devices.
In this study, we extend our research on the patterned growth of InGaN/GaN quantum wells (QWs) on freestanding nanoscale GaN gratings by MBE. Various freestanding GaN gratings are processed on a GaN-on-silicon substrate by a combination of electron beam (EB) lithography, fast atom beam (FAB) etching of GaN, and deep reactive ion etching (DRIE) of silicon. The patterned growth by MBE is performed on the prepared GaN template. Through the introduction of nanoscale grating structures, the selective growth occurs and depends on the grating period and the grating width. The optical performances of the resultant epitaxial gratings are characterized in photoluminescence measurements.
The patterned template is put into a high vacuum chamber and cleaned at the temperature of 280°C for 12 h. Then the template is transferred into the growth chamber and cleaned at the temperature of 800°C for 60 min. A 140-nm-thick buffer layer is deposited at the temperature of 700°C, and a 200-nm high-temperature GaN layer is then grown at the temperature of 780°C. The six-pair 3 nm InGaN/9 nm GaN MQWs is subsequently deposited at the temperature of 620 to approximately 640°C. Finally, a 10-nm GaN layer is grown at the temperature of 620°C.
Experimental results and discussion
Figure 3b,c,d show the epitaxial structures on the 700-nm-period GaN gratings with the grating width W of approximately 500, approximately 350, and approximately 250-nm, respectively. Compared with unpatterned GaN substrate, grating structures locally change the diffusion conditions of adatoms from neighboring areas. Coherent growth is suppressed, and the selective growth takes place on the grating ridge with a preferential growth process on the low-energy side facets. As the grating width W decreases, the area of the grating ridge is reduced. Thus, the surface diffusion can be sufficiently enhanced, resulting in complete coalescence between two side facets. Epitaxial gratings with smooth facets are observed in Figure 3c,d. Especially, Figure 3d demonstrates that the selective growth can also occur in the grating openings. Compared with Figure 3b, it can be concluded that a critical growth area is needed for the selective growth. When the growth area is too small, the selective growth is suppressed. On the other hand, it's difficult to complete the selective growth if the growth area is too large. The critical growth area might be dependent on the surface diffusion, which could be improved by adjusting the grating parameters.
In summary, various freestanding GaN gratings are fabricated on a GaN-on-silicon substrate by a combination of EB lithography, FAB etching of GaN and DRIE of silicon. The patterned growth of InGaN/GaN QWs is performed on the processed GaN template by MBE. Nanoscale grating structures locally change the diffusion conditions of adatoms from neighboring areas, and the selective growth takes place with a preferential growth process on the low-energy side facets. Coalescences between two side facets are achieved to generate epitaxial gratings with triangular section, and the patterned growth depends on the grating period P and the grating width W. Thin epitaxial gratings produce the promising photoluminescence performance. This work provides a feasible way for further GaN-based integrated optics devices by a combination of GaN micromachining and MBE growth on a GaN-on-silicon substrate.
This work was supported by the Research Project, Grant-In-Aid for Scientific Research (19106007). Yongjin Wang gratefully acknowledges the Japan Society for the Promotion of Science (JSPS) for financial support.
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