Growth of GaN nanowall network on Si (111) substrate by molecular beam epitaxy
© Zhong and Hane; licensee Springer. 2012
Received: 6 November 2012
Accepted: 12 December 2012
Published: 27 December 2012
GaN nanowall network was epitaxially grown on Si (111) substrate by molecular beam epitaxy. GaN nanowalls overlap and interlace with one another, together with large numbers of holes, forming a continuous porous GaN nanowall network. The width of the GaN nanowall can be controlled, ranging from 30 to 200 nm by adjusting the N/Ga ratio. Characterization results of a transmission electron microscope and X-ray diffraction show that the GaN nanowall is well oriented along the C axis. Strong band edge emission centered at 363 nm is observed in the spectrum of room temperature photoluminescence, indicating that the GaN nanowall network is of high quality. The sheet resistance of the Si-doped GaN nanowall network along the lateral direction was 58 Ω/ . The conductive porous nanowall network can be useful for integrated gas sensors due to the large surface area-to-volume ratio and electrical conductivity along the lateral direction by combining with Si micromachining.
KeywordsGaN nanowall network GaN growth on Si substrate Porous GaN Hall measurement N/Ga ratio TEM
GaN semiconductors exhibit excellent properties in optical devices and high-power/high-frequency electronics, such as light-emitting diodes , laser diodes , and AlGaN/GaN high-electron mobility transistors . Much attention also has been paid to GaN nanostructures because nanoscale materials, such as nanowires , nanotubes , and nanorods , are dislocation-free and strain-free with a large surface area-to-volume ratio [7, 8]. Due to these characteristics, GaN nanostructures exhibit superior performance to conventional planar GaN. An optoelectronic device using GaN nanowires was demonstrated in .
Though these GaN nanostructures (nanotube, nanowire, and nanocolumn) are exhibiting promising properties, fabrication of an electronic device based on them is complicated because the separation of nanostructures inhibits electric current from flowing among these nanostructures. In the case of a photo detector based on GaN nanowires, the detector was fabricated on an individual nanowire . Fabrication of an electronic device on an individual nanowire is highly difficult.
Nanowalls are attractive due to their porous surface and material continuity along the lateral direction. Carbon [11, 12], ZnO [13, 14], and NiO  nanowalls have been investigated. Kesaria et al. reported the growth of a GaN nanowall network on a sapphire substrate [16–18]. In these papers, transformation among the GaN nanowall network, GaN nanocolumn, and GaN film is observed by changing the growth condition. On one hand, the width of the GaN nanowall is in nanoscale and, in terms of property, is as good as a separated nanostructure . On the other hand, unlike nanotubes and nanowires, the GaN nanowall network is continuous along the lateral direction. Because of this characteristic, the GaN nanowall network is expected to be fabricated to nanodevices as easily as the GaN film. A gas sensor was fabricated on a ZnO nanowall network using the same technology as film device . Especially, using Si substrate, Si-based micromachining as well as integrated circuit can be applied to an integrated sensor .
In this paper, GaN nanowall networks were grown on Si (111) substrate by molecular beam epitaxy (MBE). Growth of GaN on silicon makes it compatible with the most mature silicon-based semiconductor technology. Characterization of the GaN nanowall was carried out. Adjustment of the nanowall width ranging from 30 to 200 nm is achieved by adjusting the N/Ga ratio. Hall mobility and carrier concentration of the Si-doped GaN nanowall network were measured using Hall measurement system.
The GaN nanowall network was deposited on Si (111) substrate using a Riber 32 MBE system equipped with a N2 RF plasma source (RFS-N/TH, Veeco Instruments Inc., Plainview, NY, USA). The base pressure of the growth chamber is 3.0 × 10−10 Torr. The purity of N2, Ga, and Al is 99.9999%. A 380-μm-thick Si (111) substrate with a resistivity larger than 5,000 Ω·cm was cleaned in alcohol, followed by standard RCA process. Then, it was dipped in HF:H2O = 1:50 for a few seconds to remove the silicon oxide layer on the surface of the Si substrate as well as to form a hydrogen-terminated surface. After cleaning of the substrate, it was immediately blown dry by N2 and transferred to a loading chamber for pre-heating at a pressure of 10−7 Torr to remove residual hydrocarbons .
In our previous work , we intentionally nitrided the Si substrate before the growth of GaN, and we observed GaN nanocolumns on this nitrided Si substrate. For the samples shown in this paper, we pre-deposited several monolayers of Al before igniting the N2 plasma source to avoid nitridation of the substrate, followed by growth of an about 40-nm-thick AlN buffer layer. Then, ten pairs of AlN (5 nm)/GaN (15 nm) multilayer were grown on the AlN buffer layer. Finally, six GaN samples were grown on the multilayer with various N/Ga ratios from 980 to 180 at 700°C. Adjusting of N/Ga ratio was achieved by changing the temperature of the Ga cell while N2 flow was kept constant. The N/Ga ratio is determined by the N flux/Ga flux. For convenience, Ga and N fluxes are given in terms of corresponding beam equivalent pressures measured by a Bayard-Alpert gauge. A Si-doped GaN nanowall network was also grown with a N/Ga ratio of 400 under the same growth procedure. Solid Si effusion cell heated at 1,200°C was used for Si doping.
Field emission scanning electron microscopy (FESEM; S-4500, Hitachi Ltd., Tokyo, Japan), transmission electron microscope (TEM; Hitachi HF 2000, Hitachi Ltd.) and X-ray diffraction (XRD; PW3040/60 X'pert PRO, PANalytical B.V., Almelo, The Netherlands) were used for characterization. A photoluminescence (PL) spectrum analyzer with He-Cd laser (325 nm, 200 mW) as excitation source was also used to investigate the optical property of the GaN nanowall network. Hall parameters of the Si-doped GaN nanowall network were carried out using the Hall measurement system.
Results and discussion
Therefore, as indicated by Figure 2a,b,c,d,e, the width of the nanowall can be controlled from 30 to 200 nm by adjusting the N/Ga ratio. In a highly nitrogen-rich condition, the Ga adatoms diffuse over a short distance before getting nitrided, promoting three-dimensional nucleation to form the hexagonal GaN nanowall network . With the decrease of the N/Ga ratio, the Ga diffusion distance increases, leading to the change of the nanowall width as shown in Figure 2a,b,c,d,e. When the N/Ga ratio is further decreased to below 180, the nitrogen sticking probability is reduced. Thus, the Ga diffusion distance is increased, forming the GaN film.
With the decrease of the N/Ga ratio, the intensity of the emission peak centered at 363 nm increases fast and becomes dominant for the samples grown with N/Ga ratios smaller than 800. Meanwhile, the violet emission at 418 nm decreases gradually with the N/Ga ratio and disappears for the samples grown with N/Ga ratios less than 400. Only the band edge emission at 363 nm with a FWHM of about 12.8 nm is observed in the spectra corresponding to N/Ga ratios of 400 and 300, indicating that GaN networks grown under these conditions are of high quality.
Therefore, this nanowall network structure is promising in fields where a large surface/volume ratio is needed, for instance, gas sensors based on surface change after exposing to a particular gas. Compared with separated nanostructures, such as nanowires and nanoparticles, its continuous characteristic along the lateral direction makes it much easier to fabricate to various electronic devices. Moreover, Si substrate is helpful for integrated sensors through the combination with silicon micromachining as well as conventional Si electronics.
Continuous GaN nanowall network was grown on Si (111) substrate by MBE under N2-rich condition. GaN nanowalls overlap and interlace with one another, together with large numbers of holes, forming a continuous GaN nanonetwork. XRD and PL results show that the GaN nanowall network is of high quality. By adjusting the N/Ga ratio, the nanowall width can be varied from 30 to 200 nm. This kind of nanostructure can be fabricated to electronic nanodevices as easily as GaN film. In addition, growth of GaN on silicon makes it compatible with the most mature silicon-based semiconductor technology.
The authors are grateful to F. R. Hu for his great help in operating the MBE system and F. Iguchi as well as T. Miyazaki for their help in the XRD and TEM measurements. The authors would also like to thank Y. Kanamori, T. Wu, and T. Sasaki for the discussion. This work was supported by the research projects, Grant-in-Aid for Scientific Research (A 24246019) and μSIC. A. Zhong appreciates the China Scholarship Council (CSC) for the financial support.
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