Atomic Layer Deposition of Gallium Oxide Films as Gate Dielectrics in AlGaN/GaN Metal–Oxide–Semiconductor High-Electron-Mobility Transistors
© Shih et al. 2016
Received: 29 January 2016
Accepted: 20 April 2016
Published: 30 April 2016
In this study, films of gallium oxide (Ga2O3) were prepared through remote plasma atomic layer deposition (RP-ALD) using triethylgallium and oxygen plasma. The chemical composition and optical properties of the Ga2O3 thin films were investigated; the saturation growth displayed a linear dependence with respect to the number of ALD cycles. These uniform ALD films exhibited excellent uniformity and smooth Ga2O3–GaN interfaces. An ALD Ga2O3 film was then used as the gate dielectric and surface passivation layer in a metal–oxide–semiconductor high-electron-mobility transistor (MOS-HEMT), which exhibited device performance superior to that of a corresponding conventional Schottky gate HEMT. Under similar bias conditions, the gate leakage currents of the MOS-HEMT were two orders of magnitude lower than those of the conventional HEMT, with the power-added efficiency enhanced by up to 9 %. The subthreshold swing and effective interfacial state density of the MOS-HEMT were 78 mV decade–1 and 3.62 × 1011 eV–1 cm–2, respectively. The direct-current and radio-frequency performances of the MOS-HEMT device were greater than those of the conventional HEMT. In addition, the flicker noise of the MOS-HEMT was lower than that of the conventional HEMT.
Gallium nitride (GaN)-based semiconductor materials are useful not only in optoelectronic devices but also in millimeter-wave power devices, especially for the fabrication of high-electron-mobility transistors (HEMTs) [1, 2]. For microwave power applications, an AlGaN/GaN HEMT must exhibit high speed, high radio-frequency (RF) power performance, and a high breakdown voltage . Nevertheless, a high gate leakage current is the factor most responsible for limiting the direct-current (DC) and RF power performances of conventional Schottky gate HEMTs . Metal–oxide–semiconductor HEMTs (MOS-HEMTs) can decrease the gate leakage current when incorporating a variety of gate oxide/insulators, including electron beam (EB)-evaporated Pr2O3 and Er2O3 [5, 6], thermally oxidized TiO2/NiO , sputtered Al2O3 , and atomic layer-deposited HfO2 and Al2O3 [9, 10].
Among the established dielectric deposition methods, atomic layer deposition (ALD)—a low-temperature chemical vapor deposition technique in which layer-by-layer deposition occurs based on surface-limited reactions—is attractive because of its accurate control over thickness, excellent step coverage, conformity, high uniformity over large areas, low-defect density, good reproducibility, and low deposition temperatures arising from the self-limiting reactions . These features make ALD a strong candidate for manufacturing nanoscale dielectric layers for electronic devices. Indeed, ALD has been exploited to prepare a variety of high-dielectric-constant (high-k) materials (e.g., Al2O3 , HfO2 , ZrO2 ) that are used widely in Si-based devices. ALD-deposited high-k materials, including HfO2, Sc2O3, and Al2O3, have been employed as gate dielectric and surface passivation layers to improve the properties of HEMTs . In addition, such binary oxides are thermodynamically stable when they are contacted with III–V semiconductors. Among the high-k materials, trivalent Ga2O3 is a promising material for application as a gate dielectric and passivation layer in III–V semiconductor-based devices because its large band gap (4.9 eV) and moderate dielectric constant (10.6) can help to decrease the leakage current . It was also reported that Ga2O3 could be a good candidate as a gate dielectric of AlGaN/GaN HEMTs due to the good interface characteristics .
Several groups have reported the ALD growth of Ga2O3. Shan et al. performed thermal ALD of GaN using [(CH3)2GaNH2]3 and O2 plasma as precursors . In 2012, Comstock and Elam described the ALD of Ga2O3 films from trimethylgallium and ozone . In 2013, Donmez et al. applied low-temperature ALD to grow Ga2O3 thin films from trimethylgallium and O2 plasma . A temperature window of 100–400 °C has been reported for this process.
In this present study, we prepared high-quality Ga2O3 thin films through remote plasma atomic layer deposition (RP-ALD) using triethylgallium (TEG) and O2 plasma. The remote plasma configuration avoided plasma-induced damage because the wafer was not exposed directly to the plasma, and low-temperature growth mode could realize selective growth by the lift-off method, it made the process much easier and convenient. After investigating the ALD window and characteristics of the Ga2O3 films, we examined their deposition on AlGaN Schottky layers. Comparing the DC and RF characteristics with those of conventional systems, our proposed ALD Ga2O3 dielectrics on AlGaN/GaN HEMTs appear to be very promising devices.
The thickness and optical characteristics of the Ga2O3 thin films were measured through spectroscopic ellipsometry (SE, Elli-SE, Ellipso Technology) in the wavelength range 280–980 nm at an incident angle of 70°. The film thickness was confirmed using high-resolution transmission electron microscopy (HRTEM). The chemical compositions and bonding states in the films were characterized using X-ray photoelectron spectrometry (XPS) with Al Kα (1486.6 eV) radiation; pre-sputtering was performed for 10 s to remove any contamination from the surface. The crystal structure of the Ga2O3 films were characterized by high-power grazing incidence the X-ray diffractometer (GI-XRD; Rigaku TTRAX 3, 18 kW) in θ−2θ mode with Cu Kα radiation. Atomic force microscopy (AFM; Bruker, Edge) was used to evaluate the roughness of the Ga2O3 surface and interface.
The epitaxial structure was grown on a 2-in silicon carbide substrate using a Nippon Sanso SR-2000 metal-organic chemical vapor deposition system (MOCVD). The epilayer consisted of a 26-nm Al0.275Ga0.725N barrier layer, a 1-nm AlN inter layer, a 2-μm GaN layer, a 0.7-μm Al0.07Ga0.93N transition layer, and a 300-nm AlN buffer layer. All epitaxial layers were unintentionally doped. The HEMT structure exhibited a sheet charge density of 1.02 × 1013 cm–2 and a Hall electron mobility of 1880 cm2 V–1 s–1 at 300 K.
Results and Discussion
Characteristics of ALD Ga2O3
Characteristics of Ga2O3 MOS-HEMT
Figure 12b presents the values of αH plotted with respect to (V GS–V th), measured at a value of f of 100 Hz. The average values of αH for the conventional HEMT and Ga2O3 MOS-HEMT were 1.4 × 10–2 and 1 × 10–3, respectively. The flicker noise spectral density of the Ga2O3 MOS-HEMT was lower than that of the conventional HEMT because of its lower number of interfacial states.
We have used remote plasma ALD to deposit Ga2O3 films that we then applied in AlGaN/AlN/GaN HEMTs on silicon carbide substrates. The thin Ga2O3 films prepared through RP-ALD exhibited saturation of the growth rate upon increasing the TEG pulse time and plasma time. The film thickness varied linearly with respect to the number of ALD cycles. This behavior is consistent with the growth of Ga2O3 following the ALD mode. The ALD Ga2O3 films possessed excellent uniformity and the Ga2O3–GaN interfaces were smooth. The fabricated Ga2O3 MOS-HEMT exhibited enhanced gate insulating and surface passivation effects, resulting in superior DC and RF performance relative to those of the conventional HEMT. Moreover, the low leakage current and low interfacial state density of the Ga2O3 MOS-HEMT provided a measured SS of 78 mV decade–1 and an I ON/I OFF ratio that was greater than 107 times that of the conventional HEMT. These attractive features of the HEMT incorporating the ALD-prepared Ga2O3 gate dielectric suggest that ALD-prepared Ga2O3 might find further applicability in other high-power devices in the near future.
We thank Nano Device Labs (NDL), Hsinchu, Taiwan, for the low-frequency noise and load-pull measurements. This study was supported financially by the National Science Council (NSC), Taiwan, under contract no. NSC-102-2221-E-182-060 and Chang Gung Memorial Hospital BMRP 591.
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