I. INTRODUCTION

Ultraviolet (UV) detectors are widely used in various fields including industry, military, space research, and medical application ⁽¹⁾. Wide energy bandgap semiconductors are suitable materials for UV detection due to their solar blind property. Gallium nitride (GaN) is direct bandgap materials with a wide energy bandgap (>~3.4 eV) suitable for UV detection below ~ 360 nm wavelength region and allows stable operation at high voltage and temperature than Silicon based UV detector ⁽²⁾. Bandgap engineering in aluminum gallium nitride (Al$_{\mathrm{x}}$Ga$_{\mathrm{1-x}}$N) mole fraction can adjust cut-off wavelength 200 nm to 360 nm wavelength ⁽³⁾.

An AlGaN/GaN heterojunction has the unique properties of high interface channel carrier density, resulting from spontaneous and piezoelectric polarizations, and high channel mobility ⁽⁴⁾. Since the channel carrier density depends strongly on the surface potential, the phototransistor output signal is highly sensitive to changes in the surface potential, and this feature can be utilized for sensor implementation. AlGaN/GaN heterojunction-based phototransistors with high photoresponsivity and fast response characteristics have been reported ⁽⁵⁾.

In this study, we fabricated AlGaN/GaN heterojunction phototransistors and investigated the effect of ZnO nanodot coating layer on the photoresponsivity characteristics. ZnO thin film and related nanostructures have been widely studied for UV detection applications because of wide energy bandgap and high UV absorption characteristics ⁽⁶⁾. It was also reported that ZnO nanostructures have improved photoresponsivity owing to the high surface-to-volume ratio of the sensing area ^(6,7). However, to the best of our knowledge, there has been no study on a ZnO nanodot layer on AlGaN/GaN phototransistors.

II. DEVICE STRUCTURE AND FABRICATION

The epitaxial structure of the phototransistors consisted of a 4 nm GaN cap layer, a 20 nm Al$_{\mathrm{0.23}}$Ga$_{\mathrm{0.77}}$N layer, a 1 nm AlN layer, a 2.1 ${μ}$m unintentionally doped GaN layer, and a 3900 nm GaN buffer layer on a Si (111) substrate. The fabrication started with a solvent-cleaning process, and subsequently, an ohmic metal stack (Ti/Al/Ni/Au) was deposited and annealed by rapid thermal annealing at 820 $^{\circ}$C for 30 s in N$_{2}$ ambient. Device isolation was achieved through Cl$_{2}$/BCl$_{3 }$based plasma etching with an etch depth of 400 nm. A Ti/Au metal stack was evaporated for forming the pad electrodes. ZnO nanodots were prepared by a simple solution process with 5 mmol of zinc acetate dihydrate and 30 mL of dimethyl sulfoxide. To this solution, a solution obtained by dissolving 5 mmol of tetramethylammonium hydroxide in 10 mL of ethanol was added. The ZnO nanodots were precipitated by the acetone and dispersed using ethanol ^(8,9). The ZnO nanodots are spherical shapes with an average diameter of 3.5 nm. Finally, the synthesized ZnO solution was spin coated on the surface of the phototransistor. Cross-sectional schematics of phototransistors with and without ZnO nanodot layer are shown in Fig. 1(a) and (b), respectively. The TEM image of synthesized ZnO nanodots are shown in the inset of Fig. 1(c). The distance between two ohmic electrodes was 6 ${μ}$m with a channel width of 50 ${μ}$m. (i.e., detection area = 6 ${\times}$ 50 ${μ}$m$^{2}$).

Fig. 1. Cross-sectional schematics of AlGaN/GaN hetero-junction phototransistors (a) without a ZnO nanodot layer, (b) with a ZnO nanodot layer, (c) TEM image of synthesized ZnO nanodots.

Fig. 2. Dark current and photocurrent at the wavelength of 300~nm versus the bias voltage.

III. RESULT AND DISCUSSION

Plots of the dark current and photocurrent versus the bias voltage for two phototransistors are presented in Fig. 2 for comparison; the wavelength of the incident light was 300 nm. Notably, the phototransistor with a ZnO nanodot layer exhibited significantly lower dark current, which was because of the absorption of free oxygen at the ZnO surface with negatively ionized states ⁽⁶⁾. The negatively charged oxygen ions partially depleted the channel charges, resulting in a lower current density. Under UV illumination, electron-hole pairs were generated. The electrons moved to the AlGaN/GaN interface channel, while the holes migrated to the surface. The holes recombined with electrons trapped in the negatively ionized oxygen ⁽¹⁰⁾, resulting in an increase in the surface potential, which in turn enhanced the photocurrent. The difference between the dark current and photocurrent was more significant in the phototransistor with a ZnO nanodot layer, and therefore, the photoresponsivity of the phototransistor was higher. For example, the photoresponsivity increased by about 75% from 1.0 ${\times}$10$^{6}$ A/W to 1.75 ${\times}$10$^{6}$ A/W at the wavelength of 300 nm.

As shown in Fig. 3, the photoresponsivity was enhanced in the entire wavelength range investigated in the experiments where the bias voltage was fixed to be 0.3~V. It should be noted that the cut-off photoresponsivity characteristics were not observed for the phototransistor with a ZnO nanodot layer due to the many defect sites in the ZnO nanodot layer ⁽¹¹⁾.

Fig. 3. Photoresponsivity at 0.3 V versus the wavelength for the AlGaN/GaN heterojunction phototransistors with and without a ZnO nanodot layer.

Fig. 4. TEM EDS analysis results for samples (a) without a ZnO nanodot layer, (b) with a ZnO nanodot layer.

Fig. 4(a) and (b) show the transmission electron microscopy-energy dispersive X-ray spectroscopy (TEM-EDS) analysis results for the phototransistor without and with a ZnO nanodot layer, respectively. The thickness of the ZnO nanodot coating layer was 25 nm. The phototransistor with a ZnO nanodot layer exhibited a relatively higher oxygen cps value than that without a ZnO layer. This can explain the dark current reduction through free oxygen absorption at the ZnO surface.

IV. CONCLUSIONS

We introduced a ZnO nanodot layer on the surface of AlGaN/GaN heterojunction phototransistors by using a spin-coating process. The layer not only reduced the dark current but also significantly increased the photocurrent, which enhanced the photoresponsivity. Therefore, it is concluded that a ZnO nanodot layer is very effective in enhancing the photoresponsivity of AlGaN/GaN heterojunction phototransistor.