MBE-Grown MgO Thin Film Vacuum Ultraviolet Photodetector With Record High Responsivity of 3.2 A/W Operating at 400 oC

Lianjie Xin, Kewei Liu , Member, IEEE, Yongxue Zhu, Jialin Yang, Zhen Cheng, Xing Chen, Binghui Li, Lei Liu, and Dezhen Shen

Abstract — In this work, a high performance vacuum ultraviolet (VUV) photodetector (PD) based on MgO thin film has been fabricated and characterized from room tempera- ture to 400 ◦C for the first time. At 25 ◦C, the device exhibits a low dark current of 100 fA, a large VUV/UVC rejection

ratio of over 104, a high responsivity of 0.865 A/W under 185 nm illumination, and a short response time of 1.25 µs at the bias of 20 V. The excellent thermal stability has also been demonstrated even at high temperature up to 400 ◦C, exhibiting a record-high responsivity (3.2 A/W), a main- tained quick response speed (1.25 µs) and a large VUV/UVC

rejection ratio (>103), which is obviously better than any other reported VUV detectors based on ultra-wide bandgap semiconductors. Additionally, this MgO PD demonstrates exceptional repeatability and long-term operating stability at both room temperature and elevated temperature. These findings underscore the outstanding performance of the MgO VUV PD, rendering it highly suitable for demanding operational conditions.

Index Terms— MgO, MBE, vacuum ultraviolet photode- tector, high-temperature.

I.       INTRODUCTION

space science [1], [2], electronic industry [3], [4], basic science

and other related disciplines [5], [6], [7], [8], [9]. In general, the application of VUV detection often has to face extreme environments, such as ultra-high/-low temperatures, strong radiation and so on. To tackle these challenges, there has been considerable development and research focused on VUV PDs employing ultra-wide bandgap (UWBG) semiconductors [10], such as AlN [11], [12], [13], [14], [15], [16], [17], BN [18],

[19] and MgO [20], due to their strong radiation resistance and thermal stability. Till now, AlN-based VUV PDs are the most studied, and the devices with metal–semiconductor–metal (MSM) structures based on single-crystal and polycrystalline AlN thin films have been extensively reported with excellent performance even at high  temperature,  but  the responsivity of  most  devices  is  still  not  very  high,  generally  less  than

100 mA/W  [14],  [15],  [17].  Compared  with  AlN,  BN has a higher band edge absorption coefficient, and thus a large responsivity of 2.75 A/W at 160 nm has been demonstrated in a typical MSM-structured high-quality 2D few-layered h-BN photodetector [18]. However, the development of BN PDs is restricted by the material’s size and crystalline qual- ity,  and  the  responsivity  of  an  amorphous  BN  PD    under

VUV  light  is  only  4.8  µA/W  at  10  V  bias  [19].  In addi-

Manuscript  received  1  February  2024;  revised  11  March    2024;

accepted 20 March 2024. Date of publication 26 March 2024; date of current version 26  April  2024.  This  work  was  supported  in part by the National Natural Science Foundation of China under Grant 62074148, Grant 61875194, Grant 11727902, Grant 12304111,  Grant

12304112, and Grant 12204474; in part by the National Ten Thousand Talent Program for Young Top-Notch Talents, Youth Innovation Promo- tion Association, Chinese Academy of Science (CAS), under Grant 2020225; in part by Jilin Province Young and Middle-Aged Science and Technology Innovation Leaders and Team Project under Grant 20220508153RC; and in part by Jilin Province Science Fund under Grant 20220101053JC and Grant 20210101145JC. The review of this letter was arranged by Editor R.-H. Horng. (Corresponding author: Kewei Liu.)

Lianjie Xin, Kewei Liu, Xing Chen, Binghui Li, Lei Liu, and Dezhen Shen are with the State Key Laboratory of Luminescence and Appli- cations, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130033, China, and also with the Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China (

Yongxue Zhu, Jialin Yang, and Zhen Cheng 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.

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

Digital Object Identifier 10.1109/LED.2024.3381114

tion, it is worth noting that the  preparation  of high-quality AlN and BN films usually requires a higher temperature (>800  ◦C)  [19],  [21],  and  the  strict  preparation conditions

and high costs hinder the large-scale development of their VUV PDs.

In contrast, MgO, as a typical UWBG oxide semiconductor (band gap: 7.8 eV), is easier and less costly to prepare high-quality  films,  which  can  be  grown  at  relatively    low

temperatures (<500 ◦C). In addition, rocksalt structure   MgO

has a high melting point of 2850 ◦C and a strong radiation hardness [22], [23], [24], [25], [26], [27], [28]. Even more

remarkably, a high responsivity of 1.86 A/W has been reported in a two-dimensional MgO-based VUV detector under an illumination of 150 nm light at 4 V due to the strong VUV absorption ability and high charge-collection efficiency of pho- togenerated carriers in MgO [20]. Therefore, it is expected that MgO has great prospects for highly sensitive VUV detection applications in extreme environments. However, up to now, there are very few reports on the research of MgO VUV detectors, especially their working characteristics in extreme environments (such as high temperatures) are still   blank.

Fig. 1.  (a) Top view and cross-sectional view SEM images and (b) XRD

ω-2θ scans of MgO film on the sapphire substrate.

In this work, MgO thin film was prepared by molecular beam epitaxy (MBE) on c-Al2O3 substrate at 450 ◦C, followed by a high temperature post annealing at 1000 ◦C. And a VUV PD with a planar MSM structure was constructed on MgO film and characterized at different temperatures from 25 ◦C to

400 ◦C. At 25 ◦C, MgO PD has a low dark current (∼100   fA

at 20 V), a high responsivity (0.865 A/W at 185 nm) and a large VUV/UVC rejection ratio (more than 104). More inter- estingly, even at 400 ◦C, the device still maintains an excellent VUV  detection  performance  with  a  high  responsivity     of

3.2 A/W at 185 nm, a low dark current of ∼10 pA and a large VUV/UVC rejection ratio of >103  at 20 V,  which are much

better than that  of  any  other  reported  VUV photodetectors at high temperature. Furthermore, it showcases exceptional long-term stability and reliability during high-temperature operation.

II.       MATERIAL EPITAXY AND DEVICE  FABRICATION

MgO film was grown on c-Al2O3 substrate at 450 ◦C by MBE. Prior to  growing,  the  c-Al2O3  substrate  was treated at 650 ◦C for 1 hour to make its surface cleaner. During 3-hour growth, the temperature of Mg source and O flux were controlled at 280 ◦C and 1.1 sccm, respectively. Subsequently, the MgO film underwent  an  annealing  process  at  1000 ◦C in an O2 atmosphere for one hour.  After  that,  a  MSM PD was constructed by preparing  Pt  interdigital  electrodes  on the annealed MgO film by photolithography and magnetron sputtering.

The morphology and structural properties of thin films were studied using scanning electron microscope (SEM) (HITACHI S-4800), and a Bruker D8GADDS X-ray diffractometer (XRD). Agilent B1500A semiconductor device analyzer was used  to  characterize  the  time-dependent  photocurrent   (I-t)

curves and current-voltage (I-V) characteristics curves of the device. The vacuum (∼ 1 Pa) and high temperature environ- ments required for the test are provided by a vacuum heating platform.

III.       RESULTS AND DISCUSSION

The top-view and the cross-sectional SEM images of MgO film are shown in Fig. 1a. The  thickness  of  the  MgO film can be estimated to be about 170  nm.  The  surface  of the film was uniform and appeared as evenly distributed triangular particles, corresponding to the (111) crystal plane of MgO. Fig. 1b shows the XRD ω-2θ scans of MgO film prepared on sapphire template. In addition to the (0006) peak of the

Fig. 2. I–V characteristics (a) in the dark and (b) under 185 nm illumi- nation at different temperatures. (c) Time-dependent current measured at different temperatures under 185 nm light and 20 V applied bias.

(d) Responsivity of the device as a function of wavelength at 25 ◦C and

400 ◦C under 20 V bias.

sapphire at 2θ = 41.68◦, only one sharp peak can be observed at 2θ = 36.89◦, which is assigned to the (111) plane of cubic rocksalt structure MgO. The XRD result is in good  agreement

with SEM image.

To investigate the optoelectric properties of MgO film, the photodetector with MSM structure (Pt interdigital electrodes with finger length of 3 mm, finger width of 20 µm and finger spacing of 20 µm.) has been demonstrated in this work. The I-V characteristic curves of the device were measured in both the absence of light (dark state) and under 185 nm illumination at various test temperatures are shown in Fig. 2a and 2b, respectively. It is clear that the device has an ultralow dark current of 100 fA at 25 ◦C under 20 V bias. As the temperature increases, the dark current of the device gradually becomes larger, but it is still very low at 400 ◦C, only about 7 pA at 20 V. Similarly, the photocurrent of the device also  shows an increase with increasing the temperature. Under 185 nm illumination (35 µW/cm2) at 20 V, the photocurrents of the device at 25 ◦C and 400 ◦C are 9.8 nA and 42 nA, respec- tively. The I-t characteristics of the device were examined by intermittently switching on and off the 185 nm lamp at various temperatures under a constant voltage of 20 V. As shown in Fig. 2c, the device has good stability as well as repeatability at both room and high  temperatures.

Responsivity is another important parameter for a photode- tector, and the responsivity as a function of wavelength of MgO VUV PD is shown in Fig. 2d at 20 V bias. As shown in Fig. 2d, the responsivity of device at 185 nm is as high as 0.865 A/W at 25 ◦C. More interestingly, the responsivity could be increased to 3.2 A/W as a high operating temperature of 400 ◦C, corresponding an external quantum efficiency (EQE) of 2146%, which is much higher than that of any other previ- ously reported VUV PDs. The record high responsivity may be associated with the oxygen vacancies induced photoconductive

gain  in  oxide  materials  [29],  [30].  In  addition,  it  should be  noted  that  the  responsivity  of  the  device  is  below   the

instrumental detection limit (∼1 × 10−7  A/W) at wavelengths

TABLE I

COMPARISON TABLE  FOR PERFORMANCE PARAMETERS OF UWBG SEMICONDUCTOR VUV   PHOTODETECTORS

Fig. 3. (a) Relationship between time (µs) and normalized ∆I, the inset shows the variation of device fall time with load resistance (the red line represents a linear fit to the data). (b) Plots of decay time versus applied voltage and test temperature.

longer than 310 nm both at 25 ◦C and 400 ◦C. And the VUV/UVC rejection ratios (R185/R255) at 25 ◦C and 400 ◦C are larger than 104and 103, respectively. This indicates that the device has excellent VUV spectral selectivity. The increase in the responsivity with increasing the operating temperature may

be associated with the narrowing of the band-gap energy [31] and the increase of the density-of-state distribution caused by lattice expansion at high temperatures  [32].

To delve deeper into the device’s response speed, the tran- sient response characteristics of the MgO PD was examined by 193 nm ArF excimer laser. As shown in Fig. 3a, at 20 V bias voltage and load resistance of 10 kK, 90-10% decay time is only about 1.25 µs. The inset of Fig. 3a shows the variation of the device’s decay time as a function of the resistance of series resistor, and a linear relationship between decay time and load resistance shows that the decay time of the device is limited by the  resistance-capacitance  (RC)  time constant of the test system [15]. Fig. 3b shows the decay time versus applied voltage and test temperature. It can be clearly seen that whether the test temperature is increased or the bias is increased, the decay time remains almost  unchanged.

It is also important to study the stability of MgO PD. Fig. 4a

illustrates the I-t curves of the device operating  continuously at 400 ◦C for one hour. There is almost no fluctuation in the photocurrent and dark current of the device, which shows that MgO VUV PD can work very stably under high temperature environment. Moreover, we conducted a long follow-up  test on MgO PD and the results are shown in Fig. 4b. It should be mentioned here that our MgO PDs were stored in a drying cabinet at a temperature of around 24 ◦C and a humidity of about 3%. Clearly, MgO PD shows very good stability at both

Fig. 4. (a) I-t plot of the device continuously tested for one hour under 185 nm light at 400 ◦C. (b) Long-term operating stability of the device at 25 ◦C and 400 ◦C.

room temperature and high temperature during the 120 days tracking test.

The main performance parameters of VUV PDs based on UWBG semiconductors are summarized in the Table I. It is obvious that our MgO VUV detector has excellent overall performance, especially its responsivity (3.2 A/W at 185    nm

at  20  V)  at  400  ◦C  high  temperature  is  the  highest value

reported so far. The rejection ratio, dark current and response time of our device are also better than those of most other reported devices. The good performance of this MgO VUV PD may be associated with the suitable band  gap  of  the MgO material, large VUV absorption coefficient [33], and the photoconductivity gain induced by the oxygen vacancies in the oxide [29],  [30].

IV.       CONCLUSION

In summary, MgO thin  film  was  prepared  by  MBE  at 450 ◦C, and a high-performance VUV PD was demonstrated by preparing Pt interdigital electrodes on it. The MgO PD shows a high responsivity of 865 mA/W (185 nm), low dark

current of 1 × 10−13 A, high VUV/UVC rejection ratio of more than 104  at 25 ◦. Even at 400 ◦, the device still maintains

a highly sensitive, stable and fast response to the VUV light with a record high responsivity of 3.2 A/W at 185 nm. Overall, MgO thin film exhibits excellent VUV photoresponse at both room and high temperatures, and is expected to be used for cost-effective high-temperature VUV photodetection.