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  1. (Daegu Kyungpook National University, Daegu, Korea.)

AlGaN sensors, UV-to-visible rejection ratio, ultraviolet, local breakdown, Al composition


Ultraviolet sensors have been studied for commercial and military applications such as solar UV monitoring, fire alarms, space-to-space communications, and missile detection (1,2). Nitride-based semiconductors are excellent candidates for UV sensors because of their chemical stability, high quantum efficiency, and excellent solar blindness, which is mainly due to their direct wide bandgaps. In order to improve the performance of nitride-based UV sensors, various device structures and experimental techniques have been applied over the last few decades (3-6). Low dark current densities and high UV-to-visible rejection ratios (UVRR), which are attributable to high-quality epitaxial layers and low defect densities are some of the most important parameters for nitride-based UV sensors (7). Heat treatment techniques using rapid thermal annealing (RTA) have been widely used not only to improve electrical characteristics such as transconductance and trap density, but also to change the contact behavior (8-10). Recently, the selective annealing method using local breakdown has been employed for heat treatment of metal-semiconductor-metal (MSM) AlGaN UV sensors (11). Unlike RTA, selective annealing can be done locally, preventing exposure to high temperature over larger regions. Notably, the Ti/Al/Ni/Au metal scheme can be used as a Schottky contact or ohmic contact depending on the heat treatment, and is the best scheme to prevent degradation of device performance during the selective annealing process.

In dual UV AlGaN sensors, which respond to either the UV-A and UV-B region depending upon their bias voltage, it is important to reduce the dark current density (12). High Al content and poor epitaxial quality have caused high leakage current in these sensors, raising the importance of heat treatment to improve device reliability. In this work, two types of AlGaN UV sensors with Al compositions of either 33% or 39% were fabricated using selective annealing, and their electrical and UV response characteristics were investigated using the Agilent HP4156C parameter analyzer, a 150 W xenon arc lamp with a monochromator (Oriel 74000), and a power meter (Newport 1930C). To investigate the effects of selective annealing on the interfaces inside the device, the change of the element content according to the depth direction of the selectively annealed regions was analyzed using transmission electron microscopy (TEM) energy dispersive X-ray spectroscopy (EDS).


1. AlGaN/GaN Substrates with Different Al Compositions

In order to illustrate selective annealing effects under different Al compositions, we fabricated AlGaN/GaN substrates with Al compositions of 33\% and 39\%. Epitaxial AlGaN/GaN layers and GaN buffer layers were grown by metal-organic chemical vapor deposition (MOCVD) on (0001) sapphire substrates. A 3.5 ${\mathrm{\mu}}$m GaN layer was grown using trimethylgallium (TMGa) and ammonia (NH$_{3}$) as sources, and a bulk 300 nm AlGaN layer was grown using trimethylaluminum (TMAl) as a source.

Table 1 summarizes the important parameters of two epitaxial samples used in this work. Fig. 1 shows PL spectra for each sample at room temperature; the PL peaks of the AlGaN samples were located at 288 nm and 298 nm. The bandgap energy for each Al sample composition was calculated using Eq. (1) (13).

$E_{g}=3.42+2.86x- x\left(1- x\right)$ [eV]

2. Fabrication process flow

Fig. 2 shows schematic cross-sectional and three-dimensional views of the proposed asymmetric AlGaN/ GaN MSM UV sensor. The first fabrication step is an initial cleaning using acetone, methyl alcohol, SPM (H$_{2}$SO$_{4}$:H$_{2}$O$_{2}$, 3:1), and SC2 (HCl:H$_{2}$O, 1:1) to remove contaminants from the wafer surface. After initial cleaning, electrodes were patterned using photolithography. Next, Ni (30 nm) and Ti/Al (30 nm/50 nm) were deposited using e-beam evaporation and lift-off processes. An 30 nm thick insulating layer of Al$_{2}$O$_{3}$ was

Fig. 1. Room temperature PL spectra of the materials used to fabricate the asymmetric MSM AlGaN UV sensors


Table 1. Important sample parameters


Sample 1

Sample 2

Al content [%]



PL peak [nm]



FWHM [nm]



E${_g}$ [eV]



layered using atomic layer deposition (ALD), and the insulator was selectively removed by dry etching, using plasma to create contact holes. Additionally, plasma ashing was carried out to remove the remaining organic materials, including the negative photoresist. Finally, Ni/Au (30 nm/50 nm) was deposited using e-beam evaporation and lift-off processes.

For selective local annealing, the oxide is layered between the metal electrodes 1 and 2, as shown in Fig. 2(a). When the bias between electrodes 1 and 2 is high enough, a high current may flow through the broken-down insulator and generate heat, which provides an annealing effect.

Table 2 shows the selective annealing conditions, which were achieved using a probe station with an Agilent HP4156C parameter analyzer. In order to optimize the selective annealing conditions, the bias voltage was incremented in steps from 5 V to 50 V to observe the occurrence of local breakdown. The breakdown of an insulator did not occur between 20 V and 30 V, but from 40 V to 50 V, a strong breakdown effect was found. Consequently, a bias voltage was applied 51 times from 39 V to 40 V at intervals of 0.02 V to achieve continuous annealing. The maximum current was set to 50 mA.

Fig. 2. Schematic cross-sectional view of asymmetric MSM AlGaN UV sensor (a) and three-dimensional view (b)


Table 2. Conditions for selective annealing by dielectric breakdown



Start [V]


Stop [V]




Step size [mV]


Compliance [mA]



1. I-V Characteristics

Fig. 3(a) shows the I-V characteristics of Sample 1, which had an Al composition of 39%. The dark current density at the forward bias before selective annealing was higher than that at the reverse bias, because work function of Ni is larger than that of Ti.

At a forward bias of 1 V, the dark current density was 8 × 10-4 A/cm2 before selective annealing and 0.2 A/cm2 after selective annealing, representing a 250-fold increase. As a result of selective annealing, the rectifying contact of the Ti/Al/Ni/Au electrode became a near-ohmic

Fig. 3. Sample 1, which had an Al composition of 39% (a) I-V characteristics before and after selective annealing, (b) Photoresponse characteristics after selective annealing


contact at forward bias, due to the formation of nitrogen vacancies. During annealing, nitrogen easily reacts with Ti to form TiN, and the remaining nitrogen vacancies act as donors on the AlGaN substrate, leading to high electron concentration. As a result of this high electron concentration, the Schottky barrier of the Ti/Al/Ni/Au electrode becomes thinner, changing the contact behavior.

In contrast, at reverse bias, differences in electrical characteristics before and after selective annealing were not clearly observed due to the high leakage current. The dark current density after selective annealing was 2.3 × 10-6 A/cm2 at -0.6 V, representing only a slight decrease as compared with the value of 6.4 × 10-6 A/cm2 before selective annealing.

The observed high leakage currents are not caused only by bulk defects and poor interface quality; they are also due to the surface leakage currents caused by GaO$_{2}$ on the AlGaN surface. On the AlGaN surface, Ga reacts with oxygen to form a thin layer of Ga$_{2}$O$_{3}$ and GaO$_{2}$, and the negatively charged chemical bonds of GaO$_{2}$ act as surface traps, leading to operational instability and performance degradation. After selective annealing, the dark current density was slightly lower than that before selective annealing. This decrease can be attributed to reduction of GaO$_{2}$, which is located on the AlGaN surface due to annealing.

In addition, the Ni/AlGaN interface as well as the Ti/AlGaN interface was expected to have annealing effects. Although the electrodes were isolated, the microscope image confirmed that the large breakdown voltage affected not only the Area 1 and the Area 2 but also a wide range of interdigitated fingers and surrounding insulators. It was expected that the annealing would affect the traps at the Ni/AlGaN interface. Under equilibrium conditions, most of the interface traps are filled with electrons below the Fermi level, and under the application of a reverse bias at the Ni/Au electrode, electrons captured by the interface traps may be emitted by a trap-assisted tunneling, which would contribute to leakage current. Interface traps at the Ni/AlGaN interface were thought to be passivated by the selective annealing, leading to a reduction in the dark current density.

Fig. 3(b) shows the photoresponse characteristics under 365 nm, 288 nm and dark conditions. Due to the high leakage current, there was no significant UV photoresponse at high voltages, which was attributed to poor epitaxial layer quality. In other words, the high leakage current of the proposed UV sensor indicates that the epitaxial layer quality was not optimal, due to incomplete epitaxial growth.

Fig. 4 shows the I-V and photoresponse characteristics of Sample 2, which had an Al composition of 33%, before and after selective annealing. The decrease in Al composition between Samples 1 and 2 substantially affected the epitaxial layer quality. Fig. 4(a) shows that a lower dark current density was measured in Sample 2 than in Sample 1. This occurs because lower Al compositions correspond to fewer defects and interface traps caused by lattice mismatch between Al and Ga atoms.

At a forward bias of 1 V, the dark current density was 9.1 × 10-6 A/cm2 before selective annealing, and significantly increased to 6.0 × 10-2 A/cm2 after selective

Fig. 4. Sample 2, which had an Al composition of 33% (a) I-V characteristics before and after selective annealing, (b) Photoresponse characteristics after selective annealing


annealing. In comparison, at a reverse bias of -0.2 V, the dark current density was 1.9 × 10-8 A/cm2 before selective annealing, and decreased to 7.7 × 10-10 A/cm2 after selective annealing. Consequently, Sample 2 shows better ohmic behavior than Sample 1. Fig. 4(b) shows the photoresponse characteristics of Sample 2 after selective annealing, which are improved as compared with Sample 1, due to the lower dark current density.

2. Spectral Responsivity

We measured the spectral responsivities of Samples 1 and 2 before and after selective annealing. As these samples had high dark currents, we reverse biased them between 0 and -1 V, where optical characteristics were clearly distinguishable. Fig. 5 shows the spectral responsivity of Sample 1. Under a forward bias and after

Fig. 5. The spectral responsivity of the AlGaN/GaN UV sensor of Sample 1, which had a 39% Al composition (a) before, (b) after selective annealing


selective annealing, photo-responsivity was not observed because the electrons supplied from the Ti/Al/Ni/Au electrode easily overcame the lower Schottky barrier height. Under a reverse bias, photoelectrons were the dominant charge carriers and the cut-off wavelength of 288 nm was attributed to the high Schottky barrier height of the Ni/Au electrode.

Before selective annealing, the measured UVRRs were low, with values of 44 and 11 at -0.3 V and -0.5 V, respectively. On the other hand, after selective annealing, the UVRRs increased to as high as 84 and 45 at -0.3 V and -0.5 V, respectively.

Fig. 6 shows the spectral responsivity of Sample 2 under various reverse bias conditions. The cut-off wavelength of Sample 2 was 298 nm. In Fig. 6(a), fluctuations in the curves were caused by interface traps, and high responsivity in the visible region was obtained.

Fig. 6. The spectral responsivity of the AlGaN/GaN UV sensor of Sample 2, which had a 33% Al composition (a) before, (b) after selective annealing


As shown in Fig. 6(b), after selective annealing, the responsivity showed reduced fluctuations and the UVRR improved. The UVRR was 19 at -1.0 V before selective annealing and increased 7-fold, to 135, after selective annealing.

Photoresponse characteristics of the samples improved overall after selective annealing. The improved UVRRs after selective annealing via the Ti/Al/Ni/Au electrode were attributed to reductions in the leakage current, which were due to surface passivation and reduction of the number of traps at the Ni/AlGaN interface. When annealing via the Ti/Al/Ni/Au electrode, the Ni/AlGaN interface is also affected by Joule heat, decreasing the number of interface traps and thereby reducing the dark current density.

Fig. 7. The atomic content along the depth direction in weakly annealed Area 1 (The top surface is Al${_2}$O${_3}$)


3. Transmission Electron Microscopy (TEM) Analysis

To characterize the effects of selective annealing on the interfaces and the surface of the device, we performed TEM EDS analysis. EDS is a device capable of qualitative analysis of samples. After the electron beam collides with the sample, internal electrons in the sample are emitted to the outside by incident electrons, and a vacancy is created and become unstable. The electrons in the upper orbit are shifted to the vacancy to be stabilized, and X-ray energy corresponding to the difference between the two energy orbits is emitted. Since X-ray energy has different values for each atom, the content of elements can be measured by counting X-rays.

Depicted in Fig. 2(a), Area 1 corresponds to an area where annealing effects were weak, and Area 2 corresponds to an area where annealing effects were strong, as dielectric breakdown occurred between Electrodes 1 and 2 of the MIM structure.

The atomic composition was characterized in the depth direction. Fig. 7 shows the atomic content as a function of position in the weakly annealed Area 1 of Sample 2, while Table 3 gives the average values of each element present in each sample region. Fig. 7 and Table 3 illustrate that the atomic content was in line with expectations for the respective layers, except by penetration of atoms such as N and O into the Ti layer.

Fig. 8 and Table 4 show the atomic composition as a function of position in the strongly annealed Area 2 of Sample 2 and the average values of each element present

Table 3. The average count of each element present in each layer at Area 1



Ti-Al interface

































Fig. 8. The atomic content along the depth direction in strongly annealed Area 2 (The top surface is Al${_2}$O${_3}$)


Table 4. The average count of each element present in each layer at Area 2



Ti-Al interface

































at each sample region, respectively. Compared to Area 1, Area 2 has unclear boundaries between each layer especially at the interfaces of the Ti layer. The insulating layer of Al$_{2}$O$_{3}$ was not clearly discernable. Also, Ti atoms were present within the Al layer, and N content in the Ti layer was significantly greater than in the weakly annealed Area 1. In addition, the strong annealing effectively formed a new layer, which was clearly observable as an Al hump and a Ga peak in Fig. 8 and Table 4.

The strongly annealed area showed interdiffusion between layers, and the N content of the Ti layer of Area 2 was higher than that of Area 1. It was thought that the local breakdown strongly contributed to modifying the rectifying contact into ohmic contact; the N content in the Ti layer was increased with the annealing strength. In other words, selective annealing provided sufficient energy to break the Ga-N bond, leading to N vacancies. The formation of a Ga layer demonstrates that selective annealing cause diffusion of atoms within the semiconductor, changing the electrical characteristics. But, the changes in contact behavior due to Ga diffusion warrant further study. As seen in Fig. 8, the O content increased in all layers, except the insulating layer of Al$_{2}$O$_{3}$ compared to Area 1. The increase indicates not only that O in the air penetrated into the device during selective annealing, but also that O in the device diffused into other layers. It is expected that dielectric breakdown in an N$_{2}$ atmosphere would reduce the O content.


To investigate the possibility of improving AlGaN UV sensor performance by using selective annealing, we fabricated asymmetric MSM AlGaN UV sensors with two different Al \% compositions. A Ti/Al/Ni/Au electrode was used to anneal selectively via the local breakdown of an insulator, modifying a back-to-back MSM UV sensor into a Schottky-type UV sensor. The sample with lower Al composition exhibited improvements in ohmic contact behavior, electrical, and photoresponse characteristics. The changes in contact behavior were attributed to the formation of a thin TiN layer and N vacancies. The change in the N content of the Ti layer, as measured by TEM, clearly showed that more interactions occurred in that layer during strong annealing than during weak annealing. After selective annealing of Sample 2, which had an Al composition of 33\%, the dark current density at a reverse bias of -0.2 V and the UVRR at a reverse bias of -1.0 V were 7.7 ${\times}$ 10$^{-10}$ A/cm$^{2}$ and 135, respectively. These results show significant improvements compared to dark current density of 1.9 ${\times}$ 10$^{-8}$ A/cm$^{2}$ and UVRR of 19 before the selective annealing, which can be attributed to the reduction in number of interface traps and surface passivation effects. Selective annealing using the dielectric breakdown of an insulator has been shown to be useful for altering contact behavior without additional processing steps and equipment, improving both dark current density and UVRR. Above all, it is important to optimize epitaxial AlGaN layer quality and annealing conditions to achieve more reliable responsivity and higher UVRR.


This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korea government (No. 2017R1D1A3B03028331). This work was also supported by the BK21 Four Project funded by the Ministry of Education, Korea.


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Byeong-Jun Park

Byeong-Jun Park was born in Daegu, Korea in 1992.

He received a B.S. degree from the School of Electronics Engineering, Daegu University, Korea in 2018 and an M.S. degree from the School of Electrical Engineering, Kyungpook National University in 2020.

Jeong-Hoon Seol

Jeong-Hoon Seol received a B.S. degree from the School of Semicon-ductor Electronics Engineering, Uiduk University in 2015 and an M.S. degree from the School of Electronics Engineering, Kyungpook National University in 2017.

Sung-Ho Hahm

Sung-Ho Hahm received a B.S. degree from the School of Electrical Engineering, Kyungpook National University (KPNU) in 1985, and received M.S. and Ph.D. degrees in electrical engineering from the Korea Advanced Institute of Science and Technology in 1987 and 1991, respectively.

From 1992 to 1996, he served as a deputy director in the Semiconductor Industry Division of the Korean Ministry of Trade and Industry.

From 2003 to 2004, he visited National University of Singapore as a Teaching Fellow.

From 2005 to 2006, he was a director of the National Education Center for Semiconductor Technology at KPNU.

Since 1996, he has worked at the School of Electronics Engineering, College of IT Engineering, Kyungpook National University.

His current research interests include GaN-based UV optoelectronic devices and simulations.