Advanced HEMT Characteristics of Epitaxial Quality-improved GaN by Using Patterned Sapphire Substract
Authors: Chien Yen Cheng , Huang Chiu Chang , Yi Chen Kai , Yen Cheng-Wei , Kuan Chieh-Hsiung 
Published: February 2020

INTRODUCTION
Accomplishing with the booming market of personal communication services and the fifth generation (5G) mobile systems, the demands for high frequency and high power devices, such as radio frequency (RF) and microwave power amplifiers, becomes the most priority topic required to be dealt with. Considering the intrinsic electric properties, gallium nitride (GaN) is a suitable and important material for high-power device applications. However, there are many inevitable defects formed during epitaxial growth process due to the difference of the thermal expansion coefficient and the lattice constant mismatch between two materials. The reduction of the defect density is known to be the most fundamental way to fabricate high-power, high-efficient GaN electronic devices. In this paper, we proposed a critical technique to adjust the defects density of GaN, which was then introduced to fabricate HEMTs and yielded a well performed device.
DEVICE FABRICATION
The sapphire substrate used in this study was c-plane (0001), and was patterned with specific periodic structure with electron beam lithography in the first. The following wet-etching by using a mixture solution of sulfuric acid (H2SO4) and phosphoric acid (H3PO4) was processed to form the designed structure on the top of sapphire substrate as figure 1(a) and (b) shown. The substrate was then transferred to the epitaxial growth of GaN. The growth began with 3μm-thick insulating GaN as a device buffer layer, and then 30 nm unintentionally doped AlGaN layer doped 18% Aluminum. The as-prepared epi-wafer then processed with sequential procedures including, and the finial structure of HEMT were the same with the illustration of figure 1(c).

MATERIAL QUALITY ANALYSIS

In order to check the quality of epi-wafer, both Raman spectrum and EPD method were utilized here. We have compared two different regions of the sample, the PSS region and the planar region (without any surface structure). The shift of Raman peak and relative full width at half maximum (FWHM) were recorded and shown in the figure 2. From the figure 2(a), the peak-shift value from the PSS region is lower than planar region, and close to a free standing GaN (568.73 cm-1), which means less stress accumulated within the PSS region. Figure 2(b) shows that the FWHM in PSS region is 2.28, lower than 2.36 in planar region. Both the trends of Raman shift and FWHM are the same, which indicated a solid evidence of good crystal quality within PSS region.
The result of EPD method has been observed by SEM, which is shown in figure 3. The dislocation density within the planar region is about 1.06 x 107 cm-2, which is higher than the PSS region (~2.54 x 106 cm-2). This result proves the better crystal quality of PSS region as well as the former two measurements.

INFLUENCE OF SURFACE DEFECT DENSITY ON DIRECT CURRENT MEASUREMENT

A further HEMT application of the quality-improved sample had been executed. Figure 4 shows the electronic properties of the sample with and without PSS. When the device’s gate is biased from -4V to 1V, the device with PSS exhibits higher drain than the planar one. In figure 4(a), when the gate voltage equal to 1V, the saturation current of the planar sample is 272.2 mA/mm, while the PSS sample is 323.4 mA/mm, about 18.8% increasing. As to the on-resistance (Ron) value, it reduces to 7.7 Ω-mm under very small drain voltage, which is a 65.9% decreasing comparing to the planar sample. The transduction value, which represents the gain of the gate voltage to the current, increases from 78.3 (planar) to 90.5 (PSS) mS/mm, about 15.6% increases.

CONCLUSION

We demonstrated a method to acquire a better crystal quality of GaN with PSS. The sample has been verified by Raman analysis and the EPD observation, both shown obvious improvement of crystal quality with PSS. The edge defect density has dropped about 1 order. The advance of crystal quality improved the electrical performance of HEMT. We believe that the method will be an effective and efficient technique that is capable to be applied to more semiconductor materials.

Acknowledgments

This study was supported financially by the Ministry of Science and Technology (MOST), Taiwan, under contract no. MOST 105-2221-E-002-185-MY3.

References

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Yuen-Yee Wong ,et al.“The Roles of Threading Dislocations on Electrical Properties of AlGaN/GaN Heterostructure Grown by MBE” J ELECTROCERAM, 157 (7) H746-H749 (2010).

Fabio Alessio Marino , et al. ‘’ Effects of Threading Dislocations on AlGaN/GaN High-Electron Mobility Transistors’’ IEEE T POWER ELECTR, VOL. 57, NO. 1, JANUARY (2010).

Figure 1