Low-temperature growth of highly crystalline β-Ga2O3 nanowires by solid-source chemical vapor deposition
© Han et al.; licensee Springer. 2014
Received: 30 May 2014
Accepted: 29 June 2014
Published: 10 July 2014
Growing Ga2O3 dielectric materials at a moderately low temperature is important for the further development of high-mobility III-V semiconductor-based nanoelectronics. Here, β-Ga2O3 nanowires are successfully synthesized at a relatively low temperature of 610°C by solid-source chemical vapor deposition employing GaAs powders as the source material, which is in a distinct contrast to the typical synthesis temperature of above 1,000°C as reported by other methods. In this work, the prepared β-Ga2O3 nanowires are mainly composed of Ga and O elements with an atomic ratio of approximately 2:3. Importantly, they are highly crystalline in the monoclinic structure with varied growth orientations in low-index planes. The bandgap of the β-Ga2O3 nanowires is determined to be 251 nm (approximately 4.94 eV), in good accordance with the literature. Also, electrical characterization reveals that the individual nanowire has a resistivity of up to 8.5 × 107 Ω cm, when fabricated in the configuration of parallel arrays, further indicating the promise of growing these highly insulating Ga2O3 materials in this III-V nanowire-compatible growth condition.
77.55.D; 61.46.Km; 78.40.Fy
In the past decade, gallium oxide (Ga2O3), as a large-bandgap (approximately 4.9 eV) semiconductor, has attracted extensive attention in the area of insulating oxides for the metal-oxide-semiconductor (MOS) technology as well as the active materials for the solar-blind deep ultraviolet detectors [1–6]. In particular, when high-mobility III-V compound semiconductor nanomaterials, such as GaAs, InAs, GaSb, and InSb nanowires (NWs), have been successfully illustrated with their great technological potentials in next-generation electronics [7–9], Ga2O3-based gate dielectrics are of significant importance to be achieved and to outperform the conventional silicon technology, due to their excellent stability and relatively high dielectric constant (approximately 14.2) as compared to that of SiO2 (approximately 3.9) or even the typically used high-κ Al2O3 (approximately 8) [1, 10].
Till now, there are several effective integrations of Ga2O3-based gate dielectrics demonstrated in thin-film III-V field-effect transistors (FETs). For instance, Ga2O3 and Ga2O3/Gd2O3 composite materials have been shown to be excellent gate dielectrics for GaAs, In x Ga1 − xAs, and GaN thin-film transistors with the low interface state density and high breakdown field strength [2, 3, 7, 11]. However, there are still very few studies focused on Ga2O3 dielectrics prepared directly on III-V NWs since the typical thermal oxidizing method is challenging to be executed on the small-diameter NWs, while the atomic-layer-deposited (ALD) high-κ HfO2 and Al2O3 dielectrics often have significant interfacial defects when performed on NW materials . In this case, it is necessary to explore other alternative dielectrics such as Ga2O3 achieved by other advanced techniques in order to tackle this issue for the versatile high-mobility III-V NW devices.
Among many Ga2O3 phases, the monoclinic β-Ga2O3 is the most stable phase, being a promising gate dielectric alternative; nevertheless, it often requires synthesis at high temperatures to maintain its excellent crystallinity. For example, β-Ga2O3 NWs are usually prepared at above 1,000°C, employing Ga metal as the source in the chemical vapor deposition (CVD) , and sometimes even high-energy arc plasma is utilized when using GaN as the starting material . As most III-V NWs are synthesized at a moderate temperature in the range 400°C to 600°C via vapor-liquid-solid (VLS) and/or vapor-solid-solid (VSS) mechanisms [15–18], a compatible low-temperature β-Ga2O3 growth technique is therefore essential to grow dielectrics laterally on III-V NWs while not degrading the III-V NW materials with high vapor pressures.
Recently, we have adopted various III-V material powders as precursor sources for the NW growth by CVD, such as obtaining GaAs, InP, GaSb, etc. at a temperature of 500°C to 600°C [19–21]. Here, in this report, we perform detailed studies on the synthesis behaviors and fundamental physical properties of β-Ga2O3 NWs at this moderate growth temperature in a similar CVD growth system. It is revealed that highly crystalline and insulating β-Ga2O3 NWs are successfully grown on the amorphous SiO2 substrate, which provides a preliminary understanding of the β-Ga2O3 NWs attained by the solid-source CVD method, and further enables us to manipulate the process parameters to achieve high-quality gate dielectrics laterally grown on III-V semiconductor NWs for the coaxially gated NW device structures .
Synthesis of Ga2O3 NWs
The Ga2O3 NWs were synthesized in a dual-zone horizontal tube furnace, where the upstream zone was used for evaporating the solid source and the downstream zone for the NW growth, as reported previously . At first, 50-nm Au colloids (standard deviation of approximately 5 nm, NanoSeedz, Hong Kong) were drop-casted on SiO2/Si substrates (50-nm thermally grown oxide) to serve as the catalyst, which were then placed in the middle of the downstream zone with a tilt angle of approximately 20°. The solid source, GaAs powders (approximately 1.0 g), was contained in a boron nitride crucible, which was then positioned in the upstream zone with a distance of approximately 10 cm away from the substrate with catalysts. During the NW growth, the substrate was initially heated to the preset growth temperature (580°C to 620°C) and the source was then heated to the required source temperature (900°C). Mixture of argon (Ar, 99.9995% purity, 100 sccm) and oxygen (O2, 99.9995% purity) in different flow ratios (100:1 to 100:100) was used as the carrier gas to transport the thermally vaporized precursors to the downstream. After the growth of 1 h, the source and substrate heater were stopped together and cooled down to room temperature under the Ar and O2 flow.
Characterization of Ga2O3 NWs
Surface morphologies of the grown Ga2O3 NWs were examined with a scanning electron microscope (SEM; FEI/Philips XL30, Hillsboro, OR, USA) and transmission electron microscope (TEM; Philips CM-20, Amsterdam, The Netherlands). Crystal structures were determined by collecting X-ray diffraction (XRD) patterns on a Philips powder diffractometer using Cu Kα radiation (λ = 1.5406 Å) and by selected area electron diffraction (SAED; Philips CM-20). Elemental analysis was performed using an energy-dispersive X-ray (EDS) detector attached to JEOL CM-20 (Akishima-shi, Japan) to measure the chemical composition of the grown NWs. For the TEM and EDS analyses, the Ga2O3 NWs were first suspended in an ethanol solution by ultrasonication and drop-casted onto a copper grid for the corresponding characterization. The reflectance spectrum was measured with a LAMBDA 750 spectrophotometer (PerkinElmer, Waltham, MA, USA) at room temperature.
The Ga2O3 NW arrays were fabricated by contact printing on SiO2/Si substrates (50-nm thermally grown oxide) as reported previously . Typically, a pre-patterned SiO2/Si substrate coated with a photoresist was used as the receiver, while the donor NW chip was flipped onto the receiver and slid at a rate of 10 mm/min with a pressure of 50 g/cm2. After photoresist removal, the Ga2O3 NW arrays were left on the patterned region. Then, photolithography was utilized to define the electrode regions, and a 100-nm-thick Ni film was thermally deposited as the contact electrode followed by a lift-off process. The electrical performance of the fabricated NW arrays was characterized with a standard electrical probe station and Agilent 4155C semiconductor analyzer (Santa Clara, CA, USA).
Results and discussion
Highly crystalline β-Ga2O3 NWs are synthesized by a solid-source chemical vapor deposition method employing GaAs powders as the source material and mixture of Ar and O2 as the carrier gas. The NWs grown at the Ar:O2 flow ratio of 100:2 are long (>10 μm) with a uniform diameter of approximately 100 nm and smooth surfaces. X-ray diffraction and selected area electron diffraction results confirm the monoclinic structure of the obtained NWs with varied growth orientations along the low-index planes. Furthermore, the reflectance spectrum demonstrates the bandgap of β-Ga2O3 NWs being 4.94 eV, while the electrical measurement deduces the corresponding resistivity of 8.5 × 107 Ω cm. All these results indicate the successful synthesis of a large-bandgap Ga2O3 material in III-V-compatible growth conditions, illustrating the promising potential for dielectric materials used for III-V nanowire-based metal-oxide-semiconductor technology.
This research was financially supported by the Early Career Scheme of the Research Grants Council of Hong Kong SAR, China (Grant Number CityU139413), the National Natural Science Foundation of China (Grant Number 51202205), the Guangdong National Science Foundation (Grant Number S2012010010725), and the Science Technology and Innovation Committee of Shenzhen Municipality (Grant Number JCYJ20120618140624228) and was supported by a grant from the Shenzhen Research Institute, City University of Hong Kong.
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