Novel of Normally-off GaN HEMT Device Structure by ... - CS ManTech

Novel of Normally-off GaN HEMT Device Structure by Using Nano-rods Technology Chwan-Ying Lee, Young-Shying Chen , Lurng-Shehng Lee, Chien-Chung Hung, Cheng-Tyng Yen, SuhFang Lin, Rong Xuan, Wei-Hung Kuo, Tzu-Kun Ku, and Ming-Jinn Tsai Electronics and OptoElectronics Research Laboratories (EOL), ITRI No.195, Sec.4, Chung Hsing Rd., Chutung, Hsinchu, 31040, Taiwan, R.O.C. *Email: [email protected]; Phone: +886-3-591-3490 Keywords: GaN on Si, Normally-off, nano-rod Abstract This paper reports a novel process by introducing nano-rods technique into AlGaN/GaN high-electronmobility-transistor (HEMT) device. This process adapts nickel to form the nano-rod hard mask pattern and then transfers to the electron supply AlGaN layer of the twodimensional electron gas (2DEG) device, followed by SF6 irradiation and p-GaN layer encapsulation. This device with the novel gate structure exhibits normally-off characteristic. The threshold voltage of this nano-rod device is higher than 0.5V and the breakdown voltage is higher than 1500V.

INTRODUCTION The High-Electron-Mobility Transistor (HEMT) device based on AlGaN/GaN hetero structure has low resistance characteristic by taking advantage of two-dimensional electron gas (2DEG) induced by piezoelectric polarization mechanism, so this device has been attracting considerable attention and intensively studied as for the next-generation power electronic devices. However, the conventional HEMT device typically has a negative threshold voltage because of the inherent existence of the 2DEG channel, and thus it becomes a normally-on device. This is rather inconvenient of use for some safety-concerned applications. Several techniques to achieve the normally-off property of the AlGaN/GaN devices have been reported, such as the fluoride-based plasma treatment method[1][2], the use of ptype layers underneath the gate region[3]-[5] to lift up the conduction band, or thin down the electron supply AlGaN layer to form a recessed gate structure[6][7]. This recess would weaken the electric field contributed by a lower piezoelectric polarization, and generate a lower carrier concentraion in the 2DEG layer for Vt improvement. However, this approach is difficult to control the remaining AlGaN thickness and therefore exhibits poor device uniformity. Although many papers adopted various methods to achieve normally-off property, it sacrificed device turn-on performance in some cases. The purpose of this study is to

develop a normally-off device with minimum impact on the drain current by proposing a novel imprint method to form the nano-rod or nano-strip gate structures. PROCESS PROCEDURE A nano-rod hard mask was formed by thermally annealed Ni particles around the gate region, followed by partially AlGaN etching and then implanting F-ion by SF6 plasma irradiation. And then the P-type GaN is selectively grown on the AlGaN to help to raise the potential of the 2DEG channel region. This device combines all the benifits of Fion implantation, p-GaN gate and recessed gate devices to exhibit the normally-off property. Owing to the AlGaN layer under the gate is not fully removed, the piezoelectric polarization can still be high enough so that the drain current can be almost comparable to the conventional HEMT device. Figure 1 illustrates the schematic structure of the proposed device structure.

S

G

D

p-AlGaN u-AlGaN u-GaN

F-ion

Buffer Substrate Figure 1. Schematic representation of device cross-sectional structre in this investigation.

Some key process steps of this device are shown in Fig. 2. Figure 2a shows the typical HEMT device structure (uAlGaN/u-GaN/buffer/substrate) as the starting material and

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then deposits the oxide material as for the etching hard mask layer. Figure 2b shows the hard mask opening and the

oxide u-AlGaN u-GaN Buffer Substrate (a)

Figure 3 shows the schematic representation of process steps of the imprint method. The Nickel material was deposited on SiO2 layer and then thermally annealed to form the nano-rod pattern around the gate region and then the oxide hard mark was formed by the ICP reactive ion etching.

Substrate (b)

Substrate (c)

Nano-rod structure formed in the oxide layer by using imprint method. The nano-rod pattern was transferred into AlGaN layer and then the SiO2 layer was removed as shown in Fig. 2c. Then, the F-ion was implanted by SF6 plasma irradiation as shown in Fig. 2d, the thick oxide layer was used as hard mask. Last, the p-GaN layer was selectively grown on the gate region as shown in Fig. 2e. After that, the standard process of ohmic contact on the source and drain region (Fig. 2f) and Schottky contact on the gate region (Fig. 2g) was performed.

SF6

oxide

SiO2

Ni SiO2

1.6 mmGaN thick GaN

1.6 mmGaN thick GaN

Sapphire substrate substrate (a) GaN grown followed by SiO2 deposition

Substrate

Sapphire substrate substrate (b) Ni deposition.

GaN

GaN substrate (c) Ni particle formed by thermal annealing

substrate (d)SiO2 (d) nano rods by SiO Nanorodsformed formed by ICP RIE.

Figure 3. Schematic representation of Nano-rod formation.

(d) EXPERIMENTAL RESULTS

p-AlGaN

Figure 4 shows the tilt SEM view of one Ni nano-rod structure around gate region. The area ratio occupied by these nano rods is about 38% . The threshold voltage and the output characteristic of this device can be adjusted by designing different area ratio of the nano-rod structure. We believe the partially etched AlGaN layer, i.e. nano-rod structure, can has better tradeoff characteristic as compared to the fully etched AlGaN layer, i.e. gate-recessed structure. And this result proves the feasibility of the proposed nanorod process.

Substrate (e) S

D

Substrate (f)

G D

S

Substrate (g)

Figure 2. Key process steps for the novel device.

Figure 4. SEM view of Ni nano-rod structure.


Figure 5 shows a cross-sectional SEM image of the nano-rod structure after oxide hard mask opening. This nano-rod process is unlike the recessed-gate process, in which the whole area of AlGaN layer underneath the gate electrode is removed. The piezoelectric polarization may decrease too much from fully etched the AlGaN layer of the recessed-gate structure. So, the drain current is not easy to maintain a higher level. The etch process of the recessedgate process usually not easy to control and sometimes damages the 2DEG layer, and therefore contributes to nonuniformity of threshold voltage and decrease of drain current. As to the nano-rod structure, certain ratio of the 2DEG layer remains underneath the gate region, the device can have higher drain current as compared to the typical recessed-gate structure.

The threshold voltage Vth can be extracted from the linear extrapolation of the Id-Vgs plot in Fig. 6. A positive Vth value of 1V was achieved from the nano-rod HEMT device, while a negative value of -2V was presented from the standard AlGaN/GaN HEMT device. Figure 7 shows the reverse blocking characteristic of the proposed nano-rod HEMT device. The breakdown voltage of this device is higher than 1500V. This is a good result that exhibits high current, normally off, and large breakdown voltage at the same time by the novel nano-rod structure.

Ni

5000Å SiO2 rods

GaN Template Figure 5. Crossectional SEM view of SiO2 nano-rod structure.

Figure 6 shows the comparison of the DC transfer characteristics on different HEMT devices. The nano-rod device exhibits a peak drain current Id of 170 mA/mm at gate to source voltage Vgs of 5V. This value does not degrade too much as compared to peak Id of 220 mA/mm at Vgs of 4V from the standard HEMT device.

Figure 7. Reverse blocking characteristic of the novel device.

This is a preliminary result that shows good device properties on the on-state current, off-state voltage, and gate control capability. However, we still need to optimize device performance and also simplify the nano-rod process. Figure 8. shows alternative approach by nano-imprint process, which can easily achieve different pattern density of the nano-rods. The tradeoff characteristics of on-state current and threshold voltage can therefore be optimized according to experiment by changing the nano-rod area ratio.

Figure 8. Proposed a simplified nano-imprint process flow for nano-rod GaN device

Figure 6. Comparison of DC transfer characteristics on different GaN-on-Si HEMT devices. The nano-rod device shows normally-off property.

From the nano-rod device, the on-state drain current can be improved and modulated by increasing the area ratio of the nano-rods underneath the gate region. In addition, the threshold voltage can also be increased by adding the F-ion implantation and the P-GaN expitaxy layer to exhibit a normally-off characteristic. The breakdown voltage can still


maintain high enough value due to the discontinued 2DEG layers formation by the nano-rod hard mask patterning. We will continue to develop and optimize the device property and target to achieve a nano-rod device with threshold voltage higher than 1.5V and drain current larger than 300 mA/mm. CONCLUSIONS In conclusion, we have demonstrated a nano-rod AlGaN/GaN HEMT device with normally-off, high on-state current and high breakdown voltage performances. The positive threshold voltage and high drain current performance is attributed to the nano-rod structure underneath the gate region because of the discontinued 2DEG layers formed so that the high current maintained and the threshold voltage can be modulated by F- irradiation and adopt P-GaN layer. This device also exhibits high breakdown voltage and low leakage current. The nano-rod AlGaN/GaN device is very promising for the high current and high voltage motor drive applications. REFERENCES [1] H. Mizuno, et al., Phys. Stat. Sol. (c ), vol.4, no. 7, p.2732 , July, 2007. [2] Y.Cai et al., IEEE Electron Device vol. 53, no. 9, p. 2207, 2006. [3] N. Tsuyukuchi, et al., Jpn. J. Appl. Phys., vol.45, no.11, p.L-319, 2006. [4] Y.Uemoto, et al., IEEE Electron Device vol. 54, no.12, p.3393, 2007. [5] X.Hu, et al., Electron. Lett., vol.36, no.8, p.753, 2000. [6] C. Chen, et al., IEEE Electron Device Letters, vol. 32, no. 10, p. 373, 2011. [7] R. Chu, et al., IEEE Electron Device Letters, vol. 32, no. 5, p. 632, 2011.

ACRONYMS HEMT: High Electron Mobility Transistor
