Self-Powered p-GaN/i-ZnGa2O4/n-ITO Heterojunction Broadband Ultraviolet Photodetector With High

Working Temperature

Yongxue Zhu, Kewei Liu , Member, IEEE, Xiaoqian Huang, Peixuan Zhang, Qiu Ai, Zhen Cheng, Jialin Yang, Xing Chen, Binghui Li, Lei Liu, and Dezhen Shen

Abstract — A self-driven p-GaN/i-ZnGa2O4/n-ITO hetero- junction  broadband  ultraviolet  (BUV)  photodetector was

firstly demonstrated in this work with a high working temperature.  In  the  25-300  ?C  temperature  range,   the

device exhibits excellent and stable BUV    photodetection

performance. Even at 300 ?C,  a  large  peak responsiv- ity  of  ～132  mA/W,  a  broad  UV  response  band  ranging

from 250 to 400 nm, a high UV-to-visible rejection ratio of nearly 104, and a high ?3 dB cutoff frequency of 20 kHz

can be still observed at 0 V, which is obviously superior to the other reported high-temperature BUV heterojunc- tion photodetectors. The remarkable performance of our device at high temperature can be attributed to the excellent insulation and high crystalline quality of i-ZnGa2O4 layer, as well as the good electrical properties of p-GaN and n-ITO. Moreover, their wide and complementary band gaps make the device have a very broad UV detection band.

Index Terms— Broadband ultraviolet photodetector, het- erojunction, high-temperature, self-powered, ZnGa2O4.

I.       INTRODUCTION

Manuscript received 21 March 2023; accepted 26 March 2023.  Date

of publication 29 March 2023; date of current version 26 April 2023. This work was supported in part by the National Natural Science Foundation of China under Grant 62074148, Grant 11727902, and Grant 12204474; and in part by the National Ten Thousand Talent Program for Young Top-Notch Talents and Youth Innovation Promotion Association, Chinese Academy of Sciences (CAS), under Grant 2020225. The review of this letter was arranged by Editor T.-Y. Seong. (Corresponding author: Kewei Liu.)

Yongxue Zhu, Qiu Ai, Zhen Cheng, and Jialin Yang are with the State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130033, China.

Kewei Liu, Xiaoqian Huang, Peixuan Zhang, Xing Chen, Binghui Li, Lei Liu, and Dezhen Shen are with the State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130033, China, and also with the Center of Materials Science and Opto- electronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China

Color versions of one or more figures in this letter are available 

Digital Object Identifier 10.1109/LED.2023.3262755

require stable devices capable of working at high temperature, it is obviously necessary and important to realize thermally stable PDs [5], [6], [7], [8], [9], [10]. Benefiting from the complementary band gaps, the heterojunction PDs provide a huge potential for broadband detection [11], [12]. Moreover, the built-in electric  field  at  the  heterojunction interface could effectively separate the photo-generated carriers, allowing the device to work without external bias [13], [14], [15]. Because no additional bias voltage is required, the heterojunction detector in self-driven  operation  mode could be completely free from the restriction of dark current when operating at high temperature. Therefore, the self-powered heterojunction PDs formed from different wide-bandgap semiconductor materials, such as, r-GO/HR-GaN [2], β-Ga2O3/4H-SiC [16], Ga2O3/ZnO [17], diamond/β-Ga2O3 [18], Graphene/(AlGa)2O3/GaN [19], and so  on,  have become ideal candidates for  preparing  BUV  devices  that can meet the requirements of high-temperature applications. Although numerous excellent heterojunction devices have been demonstrated with broadband UV response and zero power consumption, the device performance often deteriorates rapidly as the operating temperature increases [2], [17], [19], [20]. The main reason for this phenomenon is that almost all the reported BUV heterojunction devices are based on simple p-n or n-n structures. With the increase of operating tem- perature, the ionization rates of the p- and n-layers increase, leading to the narrowing of the depletion region, which would reduce the quantity of photo generated carriers in   it.

Compared with p-n heterostructure, p-i-n  heterojunction has special advantages, such as higher responsivity, faster response speed and higher reliability, thus allowing higher operating temperature [21], [22], [23], [24], [25]. In this work, a self-driven BUV PD, which still has excellent optoelectronic

detection capability even at 300 ?C, has been demonstrated

for the first time on p-GaN/i-ZnGa2O4/n-ITO heterojunction. P-GaN, i-ZnGa2O4 and n-ITO with band gap  energies  of 3.40, 5.10 and 3.85 eV at room temperature were fabricated by molecular beam epitaxy (MBE), metal organic chemical vapor deposition (MOCVD) and radio-frequency magnetron sputtering, respectively.

Benefitting from the good crystalline quality and excellent insulation of i-ZnGa2O4  layer and high electrical conductivity

Fig. 1. (a) Side-view SEM image and (b) ω-2θ scanning pattern of the p-GaN/i-ZnGa2O4/n-ITO heterojunction.

of p-GaN and n-ITO, our p-i-n heterojunction device  can work in a wide temperature range with excellent performance. At 300 ?C, the device shows a broad UV response band ranging from 250 to 400 nm, a high peak responsivity  (Rpeak)

Fig. 2.  (a) Carrier density and mobility of p-GaN and n-ITO layers versus

2

of ～132 mA/W at 352 nm, UV-to-visible rejection ratio of

temperature. (αhν)

as a function of (hν) of (b) n-ITO, (c) p-GaN    and

nearly 104, and a high ?3 dB cutoff frequency of 20 kHz.

II.       MATERIAL EPITAXY AND DEVICE  FABRICATION

The Mg-doped p-GaN film was firstly grown on undoped GaN (u-GaN)/sapphire template at 750 ?C by MBE. During growth, the N2 flow rate was fixed at 1.8 sccm with radio frequency power of 350 W, and the temperatures of Mg and Ga cells were controlled at 343 and 1120 ?C, respectively. Subsequently, i-ZnGa2O4 film was deposited on the p-GaN layer by MOCVD. Diethylzinc and  trimethylgallium  with high purity nitrogen as carrier gas and 5N-purity O2 were employed as Zn, Ga and O sources, respectively. The chamber pressure was kept at 23 Torr and the substrate temperature was  maintained  at  650  ?C.  After  that,  n-type  indium    tin

oxide (ITO) was fabricated on i-ZnGa2O4 layer by radio- frequency magnetron sputtering. Ni/Au (～50/50 nm) and In (～100 nm) have been used as the ohmic contact electrodes to p-GaN and n-ITO, respectively.

The material characterization was performed by a scanning electron microscope (SEM) (HITACHI S-4800), a Bruker D8GADDS X-ray diffractometer (XRD), a Lakeshore Hall effect measurement system and an UV-3101PC scanning spec- trophotometer. Agilent B1500A semiconductor device ana- lyzer was used to measure the  time-dependent  current (I-t) and current-voltage (I-V) curves. 310 nm light was produced by light emitting diode (LED). A monochromator with a UV-enhanced Xe lamp (200 W) was used to measure the spectral response of the  device.

III.       RESULTS AND DISCUSSION

Fig. 1a presents the side-view SEM image of the p-GaN/ i-ZnGa2O4/n-ITO heterojunction. The thickness of n-ITO, i-ZnGa2O4  and  p-GaN  layers  can  be  estimated  to  be about

40 nm, 200 nm  and  600  nm,  respectively.  Fig. 1b  shows the ω-2θ scanning patterns of p-i-n heterojunction  prepared on  u-GaN/sapphire  template.  Besides  the  sapphire substrate

diffraction peak, the peaks at 2θ  = 31.1?  and 34.6?  can    be

respectively attributed to the (100) and (002) planes of    GaN.

And the diffraction peaks located at 18.85?, 37.56?, and 57.48? can be assigned to the (111), (222), and (333) crystal facets  of

ZnGa2O4, respectively [26]. The strong and narrow diffraction

(d) i-ZnGa2O4 layers at different temperatures.

Fig. 3. (a) C –V, and (b) I –V characteristics of the p-GaN/i-ZnGa2O4/ n-ITO   heterojunction   at   different   temperatures.   The   insets      of

(a) and (b) are the schematic device structure and the I –V characteris- tics of In/n-ITO and NiAu/p-GaN contacts, respectively.

peaks in XRD pattern indicate that both p-GaN and i-ZnGa2O4 have good crystalline quality.

To research the optical and electrical properties, optical absorption measurements and  Hall  effect  were  carried  out at various temperatures of 25, 100, 200,  and  300  ?C (see Fig. 2) The hall mobility and carrier density of p-GaN layer

were measured to  be  ～5.4  cm2/Vs  and  ～4.0  × 1018 /cm3 at  25  ?C.  With  the  increase  of  temperature,  the ionization

rate of acceptor impurity increases, leading to the increase in hole concentration. At the same time, Hall mobility decreases with increasing temperature. In contrast, the electron   concen-

tration and mobility of n-ITO at different temperatures were similar, which can be estimated to  be  ～1.4  ×  1020  /cm3 and ～25 cm2/Vs. Variation of (αhν)2 versus the photo energy (hν) of n-ITO, p-GaN and i-ZnGa2O4  under various  tempera-

tures are shown in Fig. 2b, 2c and 2d, respectively. The band gaps of ITO (3.85 eV at 25 ?C), GaN (3.40 eV at 25 ?C) and ZnGa2O4 (5.10 eV at 25 ?C) narrow with increasing temperature, and decrease by 0.19 eV, 0.15 eV and 0.20 eV from 25 to 300 ?C,  respectively.

To  further   investigate   the   photodetection  performance, a  cylindrical  p-GaN/i-ZnGa2O4/n-ITO  heterojunction device

with a diameter of ～0.66 mm was fabricated (inset of Fig. 3a).

The quasi-linear  I –V  curves (inset of Fig. 3b) of In/n-ITO/In

and NiAu/p-GaN/NiAu indicate that both p- and n-type con- tacts are Ohmic in nature. C-V characteristics of the device are characterized at different temperatures as shown in Fig. 3a.

TABLE I

COMPARISON TABLE  FOR SELF-POWERED HETEROJUNCTION HIGH-TEMPERATURE UV   PDs

Fig. 4. (a) Spectral response and (b) time-dependent current mea- sured from 25 to 300 ?C under 0 V applied bias. (c) Temporal photo response (at 300 ?C) under the modulated light of a 310 nm LED.

(d) (Imax ? Imin)/Imax versus illumination modulation frequency under various temperatures.

At 0 V, the capacitance of the detector is about 180 pF at 25  ?C, 184  pF  at  100  ?C, 200  pF  at  200  ?C, and  1.47 nF

at 300 ?C.

Below 200 ?C, the capacitance of the device almost does not change with the bias voltage, indicating that i-ZnGa2O4 layer has been completely depleted. When the temperature rises to more than 200 ?C, impurities in i-ZnGa2O4 layer would be ionized, leading to a slight narrowing of the space charge region, which further results in the increase of capacitance. Fig. 3b presents the dark I-V curves of the p-i-n PD  at different temperatures and clear rectification characteristics can be acquired.

Fig. 4a  shows  the  spectral  response  measured  from  25 to 300 ?C under 0 V bias. A broadband UV photoresponse (10% of Rpeak) ranging from 250 to 400 nm can be clearly observed at all temperatures. According to the experimental band gaps of three semiconductors in our device (ITO: 3.85 eV, ZnGa2O4: 5.10 eV, GaN: 3.40 eV), the response in the UVA/UVB region should be mainly attributed to p-GaN and n-ITO layers, while the response in UVC region is mainly associated with i-ZnGa2O4 layer. Additionally, with increasing the temperature, the long-wavelength cut-off edge determined by p-GaN layer shifted towards the longer wavelength region due to the decrease of the band gap. At 300 ?C, the peak responsivity at 352 nm is as high as 132 mA/W, and a high UV-to-visible  rejection  nearly  104  was  obtained.  To further

study the response speed characteristics, the I-t curve was measured by turning ON/OFF a 310 nm LED (0.768 mW/cm2) under various temperatures at 0 V (see Fig. 4b). Obviously, the device presents an excellent ON/OFF switching property with fast speed, good reproducibility and high stability at all temperatures. Due to the self-powered operating mode, the device maintains a high ON/OFF current ratio of more than

103 at 300 ?C. To further estimate the response speed, the normalized temporal response (at 300 ?C) of the device were measured at 0 V by oscilloscope under 310 nm modulated LED illumination with different modulation frequency (see Fig. 4c). And the rise and decay times can be estimated to be about 13 µs and 14 µs, respectively. Even at frequencies up to 50 kHz, the device still  maintains  good  optoelec- tronic detection capability. Fig. 4d plots the relative balance

(Imax?Imin)/Imax as a function of illumination modulation frequency at different temperatures, where  Imin  and  Imax    are

the minimum and maximum current obtained by switching ON/OFF 310 nm light at each frequency. As the modulation frequency increases, the relative balance gradually decreases, and the ?3 dB cutoff frequency of the detector exceeds 20 kHz at all temperatures (see Fig.  4d).

Some key parameters of the high-temperature self-powered heterojunction PDs were summarized in Table I. It  can  be seen that our device has the fastest response, the widest response band and the largest responsivity speed at high temperature. In fact, most of the reported devices have only observed the UV response at high temperature, while the detailed performance parameters are rarely studied. The high crystalline quality and excellent insulation of i-ZnGa2O4 layer, as well as the good electrical properties of p-GaN and n-ITO could account for the  superior  photodetection  performance of our device at high temperature. Moreover, their wide and complementary band gaps enable the broadband UV detection.

IV.       CONCLUSION

In summary, a high-temperature self-powered heterojunc- tion BUV PD was demonstrated based on p-GaN/i-ZnGa2O4/ n-ITO structure. Benefitting from the excellent insulation of i-ZnGa2O4 layer, good electrical properties of p-GaN and n-ITO, and their wide and complementary band gaps,  the p-i-n  heterojunction  PD  shows  a  high  peak  responsivity of

～132 mA/W, a broad UV response band ranging from 250 to 400 nm, a high UV-to-visible rejection ratio of nearly 104 and a high ?3 dB cutoff frequency of 20 kHz at 300 ?C. The result reported in this letter provides a feasible way for developing

BUV PDs with high operating  temperature.