Selective Annealing Effects of Asymmetric Schottky-Type AlGaN Metal-Semiconductor-Metal UV-B Sensor
Authors: Park Byeong-Jun , Seol Jeong-Hoon , Hahm Sung-Ho 
Published: February 2020

We proposed and fabricated AlGaN asymmetric Schottky-type metal-semiconductor-metal (MSM) ultraviolet (UV) photodiode with as-deposited Ti/Al/Ni/Au and Ni/Au as Schottky contact metal schemes. III-nitride UV sensors have been studied for solar UV monitoring, fire alarm, and missile detection [1, 2]. While GaN based MSM type Schottky UV photodiodes were widely reported showing high responsivity attributable to low dark current and noise [3, 4], AlGaN MSM type Schottky UV photodiode has not been studied well. Despite the higher bandgap energy, the AlGaN MSM UV photodiodes have been reported relatively poor UVRR (UV-to-visible rejection ratio), which is attributed to the Schottky barrier lowering caused by high interface traps density [5].
Fig. 1(a) shows a schematic cross-section of the proposed asymmetric MSM UV photodiode. An epitaxial AlGaN/GaN structure grown by metal organic chemical vapor deposition (MOCVD) on a (0001) sapphire substrate with a thick u-GaN layer (3.5 μm) and a thick Al0.24Ga0.76N layer (300 nm) were used to fabricate UV-B sensor. Fig. 1(b) shows a mask layout with asymmetric electrodes and pad for local annealing, and Ti/Al/Ni/Au electrode was annealed locally by using the electrical breakdown of a sacrificial insulator. The local annealing method will be useful to change the contact property on the probing stage even after finishing the whole fabrication processes. We investigated the electrical and optical characteristics of device before and after the local annealing of Ti/Al/Ni/Au electrode. We defined the Schottky mode when is reverse-biased Ni/Au electrode (the other electrode Ti/Al/Ni/Au is forward-biased) and pseudo-photoconductive mode when the Ti/Al/Ni/Au electrode is reverse-biased (the other electrode is forward-biased).
Fig. 2 showed I–V characteristics before and after the selective annealing. In Fig. 2(a), when the bias was applied to the Ni/Au Schottky contact positive and negative 10 V, the dark current densities were 1.61×10-4 A/cm2 and 4.67×10-6 A/cm2, respectively. The current difference means that the Schottky barrier height of the AlGaN–Ni/Au contact is higher than that of the AlGaN–Ti/Al/Ni/Au contact [6]. Under 365 nm irradiation, photo-to-dark current ratio at -3 V bias was relatively high as 176.7 with lower dark current in Schottky mode.
As shown in Fig. 2(b), the dark current density at 10 V and -10 V bias after the selective annealing were 2.07 A/cm2 and 8.83×10-6 A/cm2, respectively. The dark currents before and after the selective annealing were similar in the Schottky mode operation while dark currents in the pseudo-photoconductive mode operation were significantly changed, which shows the Schottky barrier on the Ti/Al/Ni/Au side becomes thinner effectively by the selective annealing. The dark current density after a selective annealing from 0 V to -2 V bias was lower than that before the annealing due to the passivation effect reported [7]. The interface traps of Ti/Al/Ni/Au–AlGaN contact were thought to be passivated by local annealing, and the traps and the defects were reduced, leading to reduction of the dark current density.
Fig. 3 shows the spectral-photo responsivity for Schottky mode operation before and after the selective annealing of Ti/Al/Ni/Au contact. The cut-off wavelength was 316 nm and the UVRR before and after the selective annealing were 86 and 527 at -5 V bias, respectively. As shown in Fig. 3(a), the high responsivities under visible region of longer wavelengths beyond 400 nm were attributed to the deep level traps corresponding to the excitation energy above 3.1 eV exist at the AlGaN–Ti/Al/Ni/Au interface [8, 9]. After the selective annealing, we obtained more reliable responsivity and higher UVRR. The level of dark current density as well as the traps related visible wavelength response was significantly reduced due to the passivation effect.
In conclusion, an asymmetric Schottky barrier MSM UV photodiode with Ni/Au-AlGaN-Ti/Al/Ni/Au structure was fabricated and investigated. Ti/Al/Ni/Au electrode was annealed locally by using the electrical breakdown of a sacrificial insulator. After the selective annealing of Ti/Al/Ni/Au, the dark current at -1.7 V bias and the UVRR at -5 V bias were 2.4×10-12 A/cm2 and 527, respectively. These results were remarkable improvement compared to dark current of 4.05×10-10 A/cm2 and UVRR of 86 before annealing, which mean more reliable responsivity and higher UVRR can be achieved if epitaxial AlGaN layer quality be improved.

Figure 1