The large number of coronal ejections produced by solar storms may bring serious attacks on the Earth, which may lead to the short circuit and malfunction of satellites in space and the destruction of the surface power transmission system as well as the communication system ^[1,2]. For example, the Carrington incident in 1859 witnessed the devastation of human’s first power system caused by a solar storm. In 1989, another small-scale solar storm led to a blackout in Quebec, Canada; in 2003, a huge solar storm paralyzed two satellites, disrupting communications throughout the world ^[3]. Therefore, in order to make a timely response to solar storms, it is necessary to monitor their generation and evolution.

In fact, since vacuum ultraviolet (VUV) (10–200 nm) light is the most suitable information carrier with its lowest sun background noise ^[2,4], as shown in Fig. 1, solar- and astrophysicists have been working hard over the past 20 years to explore the new-generation semiconductor-based VUV detectors to study the formation of solar storms and the corona ^[5–11]. Researchers have already made encouraging progress and have even integrated, tested, and applied the new-generation VUV sensors on satellites ^[2]. However, to achieve real-time monitoring of the dynamic evolution of solar storms, the VUV detector is required to have two of the most critical capabilities: 1) a response speed as fast as possible (close to or even faster than the evolution speed of the monitored solar storm and corona), and 2) the capability of VUV imaging.

To satisfy the first requirement of improving the response speed of the VUV detector, we have made a series of efforts over the past five years. For example, in 2015, we applied micro-/nanostructures to VUV photoconductive detectors ^[5]. With the help of low-dimensional conductive channels and their inner defect-free advantages, a response speed in milliseconds was achieved. Recently, through the combination of graphene and AlN films, we have produced the first photovoltaic detector that responds to VUV selection, taking a huge leap forward in response rise time to 80 ns, which is 104 to 106 times faster than the previous photoconductive devices ^[6,7].

To fulfill the second requirement of enabling VUV imaging, here, we further extended the transfer, doping, and cutting-edge technologies of graphene to VUV light detection. We fabricated integrated hybrid heterostructure (HH) arrays of p-Gr/AlN/p-Si on a Si wafer for a total of 20 units, each with a size of 200μm×5μm. With the line arrays, we have successfully demonstrated VUV photovoltaic imaging. As a result, both the response speed and the imaging capability of the detector meet the requirements as a VUV imaging device. This result attests to the successful progress of a semiconductor VUV device from being a light-intensity detection unit to being an imaging array.

As a matter of fact, AlN is actually one of the most desirable materials for VUV detection ^[5,6,12], not only because it has exactly the right bandgap of 6.2 eV (corresponding to 200 nm) to realize a perfect VUV selective response, but also because it has strong radiation resistance, which enables long-term stable work in space ^[9,13]. In order to create a Si-integrated VUV photovoltaic line array detector, we grew a 100-nm epitaxial film of AlN on the p-Si substrate with a (111) surface. The X-ray diffraction (XRD) pattern [Fig. 2(a)] shows a very symmetrical (002), (111) diffraction peak of AlN and Si, demonstrating that AlN film grows along the c axis and has good crystal orientation and high crystalline quality, which is also reflected in a high-resolution transmission electron microscope (TEM) image [Fig. 2(c)]. The photo of the scanning electron microscope (SEM) [Fig. 2(b)] and the elemental analysis prove the uniformity of the chemical compositions of the film. The X-ray photoelectron spectroscopy (XPS) pattern further exhibits clearly the Gaussian symmetric emission peaks of Al2p and N1s, whose peak positions are close to the ideal values ^[13]. All the proofs have demonstrated that AlN epitaxial film has relatively pure chemical components and is without other obvious nonideal chemical valences. The characterization mentioned above indicates the high epitaxial quality of AlN film, which results from the related small-lattice mismatch between the (111) plane of Si and the (002) plane of AlN ^[14].

Based on the synthesized high-quality epitaxial film, we used the two-dimensional processing technology of graphene to further fabricate a line array detector consisting of 20 VUV photovoltaic units of 200μm×5μm [Fig. 3(a)], which has a hybrid heterostructure of p-Gr/AlN/p-Si. This kind of structure has three advantages. First, graphene has a high transmittance of up to 96% for VUV light, which provides the most important VUV transparent conductive window for VUV photovoltaic devices ^[6,15]. Second, the back-to-back structure of p-i-p can hinder the diffusion of hot carriers and help the device suppress noise. Third, using p-type Si as a substrate is beneficial to the on-chip integration of the device.

After obtaining the VUV arrays, we used the focus of the quartz object lens to acquire circular VUV spots of a definite diameter [as shown in Fig. 3(b)] and conducted an imaging test. The results show that only the photovoltaic units irradiated by the VUV spots can produce photocurrent output, and the output current value is directly proportional to the intensity of light exposure. When the y axis is fixed, we can obtain the current output of the whole line array and perform line imaging, as shown in Fig. 3(c). Further, the device was moved in steps of 200 μm in the direction of the y axis, and the line array was subjected to line-to-line current detection every time the device was moved. Ultimately, the picture of the VUV pattern imaging was obtained.

In order to further understand the electrical parameters of various aspects of the device, we selected one photovoltaic unit and conducted comprehensive VUV photoresponse measurements. First of all, the I-V curves of the unit device [Fig. 4(a)] in a dark state and under the illumination of 185 nm VUV light reflect a typical photovoltaic response, which shows an open circuit voltage of 1.4 V. Encouragingly, at an external bias of 0 V, the light/dark current ratio of the device is up to 7 orders of magnitude [Figs. 4(a) and 4(b)], showing an ultrahigh signal-to-noise ratio under the current detection mode ^[17]. This point is again proved by the extremely low noise voltage density [Fig. 4(c)] of the device.

The mechanism of generating the photovoltaic effect under VUV illumination is shown in Fig. 4(d). When p-type graphene contacts with AlN, the holes of graphene will diffuse to AlN, and the energy band of AlN at the interface will bend up to form a potential energy barrier and produce an internal electric field in the depletion region of the interface. When VUV light passes through graphene into AlN, most of it will be absorbed in the depletion region formed in AlN, resulting in photogenerated electron–hole pairs. Under the influence of the internal electric field, the electrons and holes are separated and move in opposite directions (electrons drift towards AlN, and holes move to graphene). After overcoming the internal resistance of the device, a 1.4 V open circuit voltage is formed between both ends of the device, and VUV photovoltaic response is demonstrated.

We have further evaluated the spectral selectivity of the device. As the spectral-dependent photoresponsivity [Fig. 4(e)] shows, the device has obvious photovoltaic sensitivity to λ<200nm, and its response cutoff wavelength is near 206 nm, which is consistent with the characteristic of spectral absorption of AlN. The photoresponsivity (Rλ) is an indicator of the photodetector’s ability to detect monochromatic light ^[18–21], which is defined by the formula Rλ=ΔI/PS, where ΔI refers to photocurrent (Ip) minus dark current (Id), P refers to the incident optical power density, and S means the light-absorbing area of the device. Typically, the photoresponsivity at 185 nm is 5.13 mA/W. The corresponding external quantum efficiency (EQE=hcR/eλ ^[22–25], where h stands for the Planck constant, c refers to light speed, e signifies electronic charge, and λ means the incident light wavelength) is 3.4%.

Since VUV detection is mainly applied to the real-time monitoring of the dynamic process of solar storms, the detector should be endowed with a rapid rise-and-recover response speed ^[26]. Here, we use a 10 ns pulsed VUV laser of 193 nm to perform a switching temporal-response test on the device. Figure 4(f) shows the result of the detector’s photovoltaic response to a single pulse, and the inset manifests the enlargement of the rising edge. It is encouraging that the rise response time of the device is 120 ns, and the decay time is 2 ms. This relatively long decay time is usually due to the junction capacitance existing inside the device and to the trap states. The response speed of the reported device is 10,000–100,000 times that of the traditional nonimaging photoconductive unit device (see Table 1).

In summary, we have reported, to the best of our knowledge, the first imaging-capable integrated VUV photovoltaic arrays with ultrafast response speed, using p-type graphene with unexpected ultrahigh VUV transmittance (>96%) as the transparent electrode and high-quality AlN film as the VUV-sensitive layer. In addition, the reported device has displayed nanosecond-level ultrafast temporal response, which can be attributed to the high-quality structure of the device and the high mobility and hole-collection-efficiency of the graphene window. This device combines the traditional metal organic chemical vapor deposition (MOCVD) epitaxy technology with the latest two-dimensional material fabricating technology, which provides a new path for the development of the late-model UV imaging devices.