Study of InN epitaxial films and nanorods grown on GaN template by RF-MOMBE
© Chen et al.; licensee Springer. 2012
Received: 5 July 2012
Accepted: 9 August 2012
Published: 21 August 2012
This paper reports on high-quality InN materials prepared on a GaN template using radio-frequency metalorganic molecular beam epitaxy. We also discuss the structural and electro-optical properties of InN nanorods/films. The X-ray diffraction peaks of InN(0002) and InN(0004) were identified from their spectra, indicating that the (0001)-oriented hexagonal InN was epitaxially grown on the GaN template. Scanning electron microscopic images of the surface morphology revealed a two-dimensional growth at a rate of approximately 0.85 μm/h. Cross-sectional transmission electron microscopy images identified a sharp InN/GaN interface and a clear epitaxial orientation relationship of InN // GaN and ()InN // ()GaN. The optical properties of wurtzite InN nanorods were determined according to the photoluminescence, revealing a band gap of 0.77 eV.
The narrow band gap InN has been attracting considerable attention for optoelectronic and high-speed electronic devices, thanks largely to its narrow direct band gap energy of 0.7 eV, high electron mobility, and electron saturation velocity[1–4]. Recent reports have revealed promising results in the application of InN epilayers for chemical sensors. Improvements in growth techniques over the past few years have enabled the fabrication of high-quality InN epilayers through molecular beam epitaxy (MBE), radio-frequency metalorganic molecular beam epitaxy (RF-MOMBE), sputtering, and metalorganic chemical vapor deposition (MOCVD).
Nonetheless, the epitaxial growth of InN remains a considerable challenge because a suitable lattice-matched substrate has yet to be identified, and growth conditions are greatly restricted by the low-dissociation temperature of InN. InN is usually grown on a sapphire substrate using various buffer layers, such as ZnO, GaN, AlN, and 6H-SiC[9–12]. The lattice mismatch of InN and sapphire is approximately 25%, but when GaN is used as a substrate it is only 10%, which has led to the widespread adoption of GaN as a buffer layer.
Previous studies on the deposition of InN on a substrate of GaN have demonstrated a substantial improvement in the nucleation of InN. Ajagunna et al. reported that the morphology of InN depends on the N/In flux ratio used in nitrogen RF-plasma source molecular beam epitaxy (RF-MBE). Kryliouk et al. indicated that the diameter, density, and orientation of nanorods could be controlled by temperature; the selection of substrate, and HCl/TMIn and N/In inlet molar ratios, during growth. Shubina et al. claimed that the narrow band gap of InN may be due to the presence of In nanoclusters, indicating that pure InN would have a large band gap. The development of growth techniques, particularly in MBE, has significantly improved the quality of InN films. Nonetheless, the growth rate remains lower than that of MOCVD. Thus, the RF-MOMBE system-combining characteristics of both MBE and MOCVD techniques are suitable for the mass production of InN growth. The growth rate of RF-MOMBE is higher than the conventional MBE and is capable of producing high-quality epitaxial InN films/nanorods. Although significant progress has been made, more research is required to improve our understanding and optimization of the heteroepitaxy of InN on GaN.
However, few studies were reported about InN films growth using RF-MOMBE. MOCVD, plasma-assisted molecular beam epitaxy (PA-MBE), and metalorganic vapor phase epitaxy are the widely used techniques in InN epitaxial growth. Compared with the PA-MBE growth method, the RF-MOMBE technique generally has the advantage of a high growth rate for obtaining epitaxial nitride films[10, 17]. Also, our previous study indicated that using RF-MOMBE-growth-InN-related alloys were higher than the growth rate of PA-MBE.
In this paper, the InN materials were grown by RF-MOMBE on the sapphire substrate using a GaN template. This paper studies how V/III flow rate and RF power influence the growth of InN films and nanorods on a GaN template using RF-MOMBE.
We prepared InN films/nanorods on c-plane sapphire substrate with GaN template using MOMBE system with an RF source to activate the nitrogen. The 4-μm-thick GaN template grown by MOCVD was commercially available. A turbo molecular pump evacuated the growth chamber, reaching a base pressure of 1 × 10−9 Torr. The group III precursors, i.e., trimethylindium (TMIn) and trimethylaluminum, were delivered to the growth chamber by heating the metalorganic sources without carrier gases. The active nitrogen radicals were supplied by a radio-frequency plasma source. Also, the V/III flow rate was controlled by mass flow controller. During the InN growth, N2 flow rate were fixed at 0.6 sccm (sccm denotes cubic centimeter per minute at STP) for film and 1 sccm for rods, respectively. In our experiments, the V/III ratio was changed by adjusting the TMIn flow rate from 0.8 sccm (sample 1) to 0.4 sccm (sample 2).
Experimental parameters for the deposition of InN films/nanorods
RF-plasma power (W)
The surface and cross-sectional morphologies were examined using a Hitachi S-4300 field-emission scanning electron microscope (FE-SEM) (Hitachi China Ltd., Beijing, China). Structural properties were characterized by X-ray diffraction (XRD, Siemens D5000, Siemens, Cary, NC, USA) and transmission electron microscopy (TEM, Philips Tecnai 20, North Billerica, MA, USA). The Φ scan and full width at half maximum (FWHM) of the ω-scan rocking curves of InN film was measured by high-resolution X-ray diffraction (Bede D1; Bede Scientific Instruments Limited, Durham, UK). The electrical properties were measured by Hall Effect Measurement System using the van der Pauw configuration with 0.32 T magnetic field at room temperature. For Hall tests on InN, In pellets are used as ohmic metal contacts. Below, we describe metal contact fabrication using indium metal balls. The In metal pellets are 99.99% pure with a metal ball diameter of about 1 mm. The optical properties were assessed by photoluminescence at 13 K using a diode-pumped solid state laser emitting at a wavelength of 532 nm as the excitation source. The collected luminescence was directly projected into a grating spectrometer and detected with extended InGaAs detector.
Results and discussion
Table1 shows the growth parameters of two InN samples fabricated using RF-MOMBE. Each InN sample was grown on a 4-μm-thick GaN buffer layer. The InN nanorods were grown under N-rich condition, and the InN films were close to stoichiometry confirmed by TEM-EDX analysis.
In summary, this study investigated the characteristics of InN films/nanorods in epitaxial growth on the GaN template using RF-MOMBE. The structural properties shown in XRD pattern reveal the good crystallinity of InN materials grown on GaN without any metallic In phase. The TEM images indicate the sharp interface of the epitaxially grown InN/GaN. In addition, the PL spectra illustrate a band gap of 0.77 eV in the InN nanorods. These results indicate that an improvement in the quality of InN material can be achieved using heteroepitaxy on GaN template, and further investigation on the morphological revolution of InN is underway.
This work was supported by the National Science Council (NSC) of Taiwan under contract number NSC- 101-2112-M-182-003-MY3.
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