GaN Schottky Contact on the Pattern Sapphire Substrate with Reducted Threading Dislocation
Authors: Lee Cheng-Che , Chien Cheng-Yen , Cheng Wen-I , Yen Cheng-Wei , Hu Po-Yuan , Huang Kiu-Tshiong , Kuan Chieh-Hsiung 
Published: February 2020

INTRODUCTION
Recently, the interest in high quality GaN films has in-creased, GaN and its alloys show good performance at high-power, high-frequency transistors. Because of large mismatches of lattice constant and thermal expansion between GaN epitaxial film and sapphire substrate, however, the high-density dislocation ranging from 108-1010cm-2 does degrade the diode performance profoundly. Thus, growth of GaN with low threading dislocation(TD) has been more and more important. The single-step growth on pattern sapphire substrate(PSS) has been widely used for fabrication of reliable high quality GaN films. To improve the Schottky diode’s performance, high TD density must be considered. Using the PSS technique, we can significantly reduce the TD density.
For an ideal Schottky diode the I-V characteristic[1] is given by

I=I0exp(qV/nkT)
．
[1-exp(-qV/kT)]

and

I0=SAT2exp(-Фb/kT),

In this work, the PSS method developed by ourselves[2] is used to improve the GaN epitaxial film quality, which is then applied to the fabrication of Schottky diode on the GaN film. At last, different pattern dimensions are compared to find an optimum GaN Schottky diode.

DEVICE FABRICATION

The PSS pattern was defined by Electron-Beam lithography system. The arrayed nanostructure was designed to be arranged on the commercial c-plane(001) sapphire substrate. The dimensions of the arrayed nanostructure are related to the quality of GaN and can be defined as dimension: 600nm, means spacing: 600nm, diameter: 600nm of the fabricated PSS hole array. Next, the reactive ion etching system(RIE) was used to transfer the pattern from photoresist to SiO2 mask, and then the mixed liquid of H2SO4(96%) and H3PO4(85%) was used to etch substrate at 230°C for 60 minutes to make a pyramid shape pattern.
Metal-organic-chemical-vapor-deposition(MOCVD) was used to grow GaN thin films. The epitaxial growth began with a 25nm-thick GaN nucleation layer and then an insulating GaN of 4μm thickness as a device buffer layer. The buffer layer has a resistivity greater than 200MΩcm2 to prevent leakage. A 30nm unintentionally doped AlGaN layer and a 2nm GaN cap layer were then grown to complete the modulation doped structure.
Schottky devices of both on the planar region and on the pattern region were fabricated on the same substrate for comparison as shown in Fig.1. The process began with mesa iso-selectively etch AlGaN/GaN to mesa etch for electrical isolation. After the dry-etch isolation, Ge/Ti/Al/Ti/Au(10/20/100/55/100nm) were evaporated and annealed at 860°C for 20sec to form source and drain ohmic contacts.

THIN FLIM QUALITY ANALYSIS

To analyze the quality of GaN thin films, Raman spectrum and EPD(Etching Pit Density) are jointly performed on AlGaN/GaN samples on the planar and pattern regions. The Raman shift and full-width-half-maximum(FWHM) of Raman spectrum were measured to compare the strain and quality of GaN on the planar region and the PSS region. As shown in the Fig.2-1, Raman shifts of the PSS region in different dimension is lower than the planar region and closer to freestanding GaN(567cm-1)[3], which means that the PSS region has less stress accumulation. Fig.2-2 shows that the FWHM trend is the same as Raman shift, which means the quality is improved as the PSS dimension goes down. The narrowest Raman FWHM implies the lowest-stress of GaN and thus the optimum crystal quality, while the widest Raman FWHM of the conventional planar region implies the worst crystal quality of GaN.
The EPD experiment used H3PO4 at 230°C for 5 minutes. The dislocation density on the planar region is 3.87x106(cm-2), higher than 1.37x106(cm-2) on the pattern region. This proves that the FWHM of the Raman spectrum corresponds to the reliability of the crystal quality, and that our PSS method does improve the crystal quality.

CONTACT RESISTANCE ANALYSIS

Schottky contact resistance was measured as shown in Fig.4. As the PSS dimension goes down, the contact resistivity goes down. That is, the proposed GaN thin film on the pattern region has a better quality than that on the planar region. Also, the Schottky contact on the proposed pattern region has a lower resistivity than that on the conventional planar region.

Conclusion

We have fabricated high quality GaN thin films by MOCVD on the Pattern Sapphire Substrate(PSS). Both Raman spectrum and EPD show that the PSS can magnificently improve the GaN film quality. The defect density has a tendency to correlate with the electrical properties of the component. To reduce defect density and improve electrical performance, different pattern dimensions are designed in the Schottky diodes. By using our PSS technology, the EPD can be reduced from 3.87x106(cm-2) to 1.37x106(cm-2) and the Raman spectrum FWHM can be reduced from 2.46(cm-1) to 2.33(cm-1). Also, the Schottky contact resistivity can be reduced from 0.0657(Ωcm2) to 0.00266(Ωcm2).

Figure 1