GaN MISIM Diode with High-k Dielectrics of ZrO2 and Al2O3 for UV Sensing
Authors: Lee Gil-Ho , Seol Jeong-Hoon , An Jong-Ki , Yun Ju-Young , Hahm Sung-Ho 
Published: February 2020

AlGaN ternary compound material has been actively studied for application of UV photodiode. Schottky type metal-insulator-semiconductor-insulator-metal (MISIM) diode structure has an advantage of low dark current that the diode could show highly UV selective property.[1] In this work, we fabricated a GaN MISIM diode structure two high-k dielectrics of ZrO2 and Al2O3 as insulation layer. The ZrO2 and Al2O3 were deposited by using atomic layer deposition (ALD),[2,3] we compared them for photo-electronic properties. As a result, the GaN MISIM UV photodiode with ZrO2 have better properties than that with Al2O3 because which has more defects related interface traps than that with ZrO2.
Fig. 1 shows a schematic cross-sectional structure of the fabricated MISIM type GaN UV photodiode. Following the deposition of ZrO2 and Al2O3 by atomic layer deposition (ALD) method, Ni and Au films were deposited with 20 and 15 nm thick for the inter-digitated Schottky metal electrodes by using an e-beam evaporator. The finger width, length, and spacing between two electrodes were 10, 200, and 10 um, respectively. After electrodes deposition, it was annealed at 300℃ for 30 sec rapid thermal processing (RTP) in an N2ambient for good interface quality.
Fig. 2 shows the static I-V characteristics of the fabricated GaN MISIM UV photodiodes under dark and under illumination of 365 nm UV. Firstly, under dark condition, Fowler-Nordheim (F-N) tunneling[4] started at 17.2 V in the MISIM photodiode with ZrO2, and started at 18.0 V in that with Al2O3 as shown in the insets of Fig. 2. Since the F-N tunneling through the oxide layer is closely related to the CBO(conduction band offset) between the dielectric and GaN layer, it has been expected to show different onset bias voltages for ZrO2 and Al2O3. The experimental results showed F-N tunneling at similar biases for the devices with ZrO2 and Al2O3, which was thought that the band gap of the thin Al2O3 film of amorphous phase (6.5 eV) was distinct from that of bulk α-Al2O3(8.8 eV).[5-7]
Secondly, under UV illumination of 365 nm, P-F emission and F-N tunneling appeared at 12.4 V and 14.4 V for MISIM photodiode with Al2O3, while neither P-F emission nor F-N tunneling were indicated for the device with ZrO2. P-F emission of indicated traps at Al2O3/GaN interface and the interface property of ZrO2/GaN was better than Al2O3­/GaN.
Fig. 3 shows the spectral photo-responsivities of the fabricated device. The cut-off wavelength was about 365 nm which reflected the absorption energy of epitaxial GaN layer. In Fig. 3(b), responsivity of the device with Al2O3 increases as applied bias voltage increases in visible region, while the dark current was not significantly fluctuated as shown in Fig. 2(b). This phenomenon was not shown in visible region for the device with ZrO2. It is assumed that the electrons were trapped by deep level response to wavelength between 400 nm and 550 nm, which were in Al2O3/GaN interface only.
Table 1 shows the electrical and optical characteristics of the devices with the two high-k dielectrics. The GaN MISIM UV photodiode with a ZrO2 layer showed slightly better photo-electronic properties than that with a Al2O3 layer instead of ZrO2, which is related with the reduced interface traps.

References

[1] C. J. Lee, Y. J. Kwon, C. H. Won, J. H. Lee, and S. H. Hahm, Appl. Phys. Lett, vol. 103, 111110 (2013).
[2] L. Aarik, H. Alles, A. Aidla, T. Kahro, K. Kukil, J. Niinist
ö
, H. Mändar, A. Tamm, R. Rammula, V. Sammelselg, and J. Aarik, Thin Solid Films 565(2014) pp. 37-44.
[3] J. Niinist
ö
, Atomic Layer Deposition of High-k Dielectrics from Novel Cyclopentadienyl-type Precursors, Dissertation for the Degree of Doctor of Science in Technology, Helsinki University of Technology (2006) pp. 19-30.
[4] R. H. Fowler and L. Nordheim, Proc. R. Soc. London, Ser. A. Vol. 119(1928), p. 173.
[5] J. Robertson, and B. Falabretti, J. Appl. Phys. 100, 014111 (2006).
[6] J. Yang, B. S. Eller, C. Zhu, C. England, and R. J. Nemanich, J. Appl. Phys. 112, 053710 (2012).
[7] V. V. Afanas’ev, and A. Stesmans, J. Appl. Phys. 102, 081301 (2007).

Figure 1