Enhanced field-effect mobility (>250 cm2/V·s) in GaN MOSFETs with deposited gate oxides via mist CVD

We report an enhanced field-effect mobility (>250 cm2·V−1·s−1) in GaN MOSFETs. High mobility was achieved by reducing the oxidation of the GaN surface, which was a major factor affecting channel mobility in GaN MOSFETs. Among various gate oxide deposition methods, mist CVD using O3 suppressed GaN surface oxidation. The best field-effect mobility was observed using mist CVD-deposited gate oxides, achieving a peak mobility of 266 cm2·V−1·s−1 with a high threshold voltage of 4.8 V.

We report an enhanced field-effect mobility (>250 cm 2 •V −1 •s −1 ) in GaN MOSFETs.High mobility was achieved by reducing the oxidation of the GaN surface, which was a major factor affecting channel mobility in GaN MOSFETs.Among various gate oxide deposition methods, mist CVD using O 3 suppressed GaN surface oxidation.The best field-effect mobility was observed using mist CVD-deposited gate oxides, achieving a peak mobility of 266 cm 2 •V −1 •s −1 with a high threshold voltage of 4.8 V. © 2024 The Author(s).][3] Vertical MOSFETs are suitable for invehicle inverters that control the main motors owing to their high threshold voltage and large current operation.][6] One major factor determining the losses in MOSFETs is channel mobility.Whereas SiO 2 /SiC MOSFETs have a channel mobility of approximately 100 cm 2 [10][11][12] Al 2 O 3 / GaN, 13) and AlSiO/GaN 14) MOSFETs have reported values of approximately 200 cm 2 •V −1 •s −1 ; this is an advantage of GaN over SiC.However, further improvement in channel mobility is required for achieving an on-resistance of 1 mΩ•cm 2 or lower in GaN MOSFETs.5) However, achieving threshold voltages exceeding 4 V has been challenging because of the positive polarization charges at the AlN/GaN interface, resulting in low threshold voltages below 2 V. Therefore, we investigated the high mobility of SiO 2 /GaN that has a high band offset and reliability. 16)Previous reports have indicated that GaO x at the SiO 2 /GaN interface affects the field-effect mobility (μ FE ) in GaN MOSFETs.8][19][20][21] Previous reports have discussed the channel mobility of GaN MOSFETs with SiO 2 film deposition using plasma-enhanced CVD (PE-CVD), 10,22) remote PE-CVD, 12,23) atomic layer deposition (ALD), 24) and low pressure CVD (LPCVD).25) However, reports on enhancing the mobility through oxidation control are still limited. I this study, we used PE-CVD, ALD, and mist CVD [26][27][28] to reduce GaN surface oxidation and investigated GaN surface oxidation, channel mobility, and interface state density (D it ) at SiO 2 /GaN.
We fabricated three types of samples to evaluate GaN surface oxidation, channel mobility, and interface state density.
The oxidation evaluation samples were Mg-and Si-doped GaN epitaxial layers grown via metal organic CVD (MOCVD) on n + -GaN (0001) substrates, as shown in Fig. 1(a).The designed Mg concentration was 5 × 10 17 cm −3 .The oxidation evaluation process was performed as follows: the samples were placed in the deposition chamber, and the deposition process was performed without Si sources, only by supplying oxidant gases under the conditions shown in Table I.
For the channel mobility evaluation samples, we fabricated lateral MOSFETs, as shown in Fig. 1(b).The epitaxial layer structure was the same as that of the GaN surface oxidation evaluation samples.The n + source regions of the MOSFETs were formed by Si ion implantation with a dosage of 3 × 10 15 cm −2 and an acceleration energy of 30 keV.Using a 100 nm SiO 2 capping layer, activation annealing was performed at 1050 °C in an N 2 ambient atmosphere for 5 min that simultaneously activated the p-GaN.After removing the SiO 2 capping layer using buffered HF (BHF), a surface wet treatment, H 2 SO 4 + H 2 O 2 and dilute HF, was applied for native oxide removal.An SiO 2 film was deposited as the gate oxide layer via ALD using O 2 plasma, PE-CVD using O 2 plasma, and mist CVD using O 3 .Post-deposition annealing (PDA) was performed at 800 °C in an N 2 ambient atmosphere for 5 min.The equivalent SiO 2 thicknesses after PDA were 106, 97, and 103 nm for the ALD, PE-CVD, and mist-CVD samples, respectively.Finally, a 160 nm Ni film was formed for the gate, source, body, and drain electrodes through sputtering deposition and wet etching using a resist patterning mask.
The n-type MOS capacitors, as shown in Fig. 1, were fabricated for D it evaluation.The Si doping concentration in n-GaN was 5 × 10 16 cm −3 for the mist CVD sample and 3 × 10 16 cm −3 for the ALD and PE-CVD samples.The fabrication process, including gate oxide layer deposition, PDA, and front-side electrode formation, was the same as that of the lateral MOSFETs.A 500 nm Al film was formed for the backside electrode.
Figure 2 shows the relationship between the oxidation time and the oxidation ratio of the samples under various oxidation conditions.The oxidation ratio of the oxidation samples was calculated by analyzing X-ray photoelectron spectroscopy (XPS).Peak decomposition analysis of Ga-O and Ga-N components at Ga 3d core-level spectra was performed and the oxidation ratio on the GaN surface was calculated using the following formula: , where [Ga-O] and [Ga-N] represent the intensity at each component area. 21)mong the ALD samples, the oxidation ratio decreased in the order of O 2 plasma, O 3 , and H 2 O. Additionally, the longer the oxidation time, the higher the oxidation ratio.In addition, when comparing ALD and mist CVD samples with the same oxidation time and the same oxidant gas O 3 , the mist CVD sample showed a lower oxidation ratio.We assume that this difference was due to variations in the O 3 concentration, differences in the heater configuration in the deposition equipment, and variations in the gas flow through the chamber.
Figure 3 shows the (a) I d -V g and (b) μ FE -V g curves of the lateral MOSFETs.The channel length and width were 100 μm.The I d -V g characteristics of the MOSFETs were measured under the following conditions: V d = 0.1 V, V s = 0 V, V b = 0 V, and V g = −10 to 30 V at 300 K.The channel mobility was calculated as the field-effect mobility as expressed in Eq. ( 1).
where L denotes the channel length, W denotes the channel width, C ox denotes the gate oxide capacitance per unit area, and V d denotes the drain voltage.The threshold voltages, defined as V g , when I d = 1 × 10 −8 A were 4.4 V, −9.0 V, and 4.8 V for the PE-CVD, ALD, and mist CVD samples, respectively.Figure 4 shows the relationship between the estimated oxidation ratio and the peak field-effect mobility.The oxidation ratio was determined from the total oxidant gas supply time for each sample and the oxidation evaluation results, as shown in Fig. 2. A clear relationship was observed, and the highest field-effect mobility of 266 cm 2 •V −1 •s −1 was achieved in the MOSFET fabricated by mist CVD using O 3 .The obtained field-effect mobility was more than 2 times higher compared with the previous report of a GaN-MOSFET with a similar threshold voltage of 4.2 V and field-effect mobility of 119 cm 2 •V −1 •s −1 . 12)o directly observe GaO x at the SiO 2 /GaN interface, we performed cross-sectional scanning transmission electron microscopy with electron energy loss spectroscopy (STEM-EELS) evaluation of these MOSFETs.Figure 5 shows the composition distributions of Ga, N, Si, and O at the SiO 2 /GaN interface, obtained using a linear scanning analysis method.The ideal composition ratio of the atomic densities of Ga and N in the GaN unit cell is one-to-one; however, in the ALD samples, the ratio of N to Ga decreased at the SiO 2 /GaN interface, indicating the presence of thick GaO x .We noted that mist CVD using O 3 was an effective SiO 2 deposition method for suppressing GaO x .
Figure 6(a) shows the quasi-static and 100-kHz capacitancevoltage (C-V ) characteristics.The quasi-static C-V curves were almost coincident with the 100-kHz C-V curves on each   sample.The flat-band voltage shifts were −2.4 V, 1.5 V, and −3.6 V for the ALD, PE-CVD, and mist CVD samples, respectively.Figure 6(b) shows the energy distributions of D it obtained using the C-Ψ s method. 29)The D it at E c −E t = 0.2 eV were 8.0 × 10 10 cm −2 •eV −1 , 1.2× 10 12 cm −2 •eV −1 , and 2.1 × 10 12 cm −2 •eV −1 for the mist CVD, PE-CVD, and ALD samples, respectively.Thus, a clear relationship between the peak field-effect mobility, D it , and oxidation ratio was obtained.
For the threshold voltage, the difference between the experimental and theoretical voltages was larger than the flat-band voltage shifts of the n-type capacitor, indicating that the difference was primarily caused by the defects at the SiO 2 /p-GaN interface and/or p-GaN surface rather than the defects in the SiO 2 film and SiO 2 /n-GaN interface.In this study, an Mg doping concentration of 5 × 10 17 cm −3 was applied to obtain the high threshold voltage.However, this high concentration could lead to an increase in Coulomb scattering. 12,22,23)Furthermore, to enhance the channel mobility with a high threshold voltage, the impact of the factors that contribute to the decrease in the threshold voltage must be minimized, and then, the Mg doping concentration must be reduced.
In summary, the oxidation ratios under various oxidant gases were evaluated for p-GaN epitaxial layers.For ALD, the oxidation ratio on the GaN surface decreased in the following order: O 2 plasma, O 3 , and H 2 O. Mist CVD using O 3 exhibited the lowest oxidation ratio.GaN MOSFETs with mist CVD-SiO 2 exhibited a normally off operation with a high threshold voltage of 4.8 V and a peak field-effect mobility of 266 cm 2 •V −1 •s −1 .The GaO x layer at the mist CVD-SiO 2 /GaN   064002-3 © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd interface was thinner than that of the ALD and PE-CVD samples, and D it was as low as 8.0 × 10 10 cm −2 •eV −1 at E c −E t = 0.2 eV, which was consistent with the high fieldeffect mobility.We believe that the high channel mobility obtained will play an important role in GaN power devices.

Fig. 1 .
Fig. 1.Cross-section of samples; (a) epitaxial layer for oxidation evaluation, (b) lateral MOSFET for channel mobility evaluation, (c) n-type MOS capacitor for D it evaluation.

Fig. 2 .
Fig. 2. Oxidation ratio on the p-GaN surface under various conditions.The oxidation ratio of the oxidation samples is calculated by analyzing XPS.

Fig. 3 .
Fig. 3. (a) I d -V g and (b) μ FE -V g curves of lateral MOSFETs.The channel length and width were 100 μm.The I d -V g characteristics of the MOSFETs were measured under V d = 0.1 V, V s = 0 V, V b = 0 V, and V g = −10 to 30 V at 300 K.

Fig. 4 .
Fig. 4. Relationship between the estimated oxidation ratio and the peak field-effect mobility.Based on the oxidation evaluation results shown in Fig.2, the oxidation ratio was calculated by the total oxidant gas supply time.

Fig. 6 .
Fig. 6.(a) Quasi-static and 100-kHz C-V of the n-type MOS capacitor.The gate voltage sweep direction is from −20 to 20 V for both quasi-static and 100-kHz C-V measurement.(b) Distribution of D it as a function of energy extracted using the C-Ψ s method.

Table I .
Experimental setup of various oxidation conditions.