No abstract

Ta/NiO x /Ni 81 Fe 19 /Ta multilayers were prepared by rf reactive and dc magnetron sputtering. The exchange-coupling field (H ex ) and the coercivity (H c ) of NiO x /Ni 81 Fe 19 as a function of the ratio of Ar to O 2 during the deposition process were studied. The composition and chemical states at the interface region of NiO x /NiFe were also investigated using the x-ray photoelectron spectroscopy ͑XPS͒ and peak decomposition technique. The results show that the ratio of Ar to O 2 has a great effect on the nickel chemical states in NiO x film. When the ratio of Ar to O 2 is equal to 7 and the argon sputtering pressure is 0.57 Pa, the x value is approximately 1 and the valence of nickel is ϩ2. At this point, NiO x is antiferromagnetic NiO and the corresponding H ex is the largest. As the ratio of Ar/O 2 deviates from 7, the exchange-coupling field (H ex ) will decrease due to the presence of magnetic defects such as Ni ϩ3 or metallic Ni at the interface region of NiO x /NiFe, while the coercivity (H c ) will increase due to the metallic Ni. XPS studies also show that there are two thermodynamically favorable reactions at the NiO/NiFe interface: NiOϩFeϭNiϩFeO and 3NiOϩ2Feϭ3NiϩFe 2 O 3 . These interface reaction products are magnetic defects at the interface region of NiO/NiFe, it is believed that these magnetic defects would have an effect on the exchange-coupling field (H ex ) and the coercivity (H c ) of NiO/NiFe.

Kesterite Cu2ZnSn­(S,Se)4 (CZTSSe) thin film is a promising material for optoelectronic devices. In this work, we fabricate Mo/CZTSSe/CdS/ZnO/ITO (ITO, indium tin oxide) heterojunction photodetectors with favorable self-powered characteristics. The photodetector exhibits exceptional high-frequency photoresponse performance whose −3 dB bandwidth and rise/decay time have reached 1 MHz and 240/340 ns, respectively. For further improvement, ultrathin Al2O3 layer prepared via atomic layer deposition (ALD) process is introduced at the Mo/CZTSSe interface. The influence of ALD-Al2O3 layer thickness and its role on the photoresponse performance are investigated in detail. The interfacial layer proved to serve as a protective layer preventing selenization of Mo electrode, resulting in the reduction of MoSe2 transition layer and the decrease of series resistance of the device. Accordingly, the −3 dB bandwidth is remarkably extended to 3.5 MHz while the rise/decay time is dramatically improved to 60/77 ns with 16 cycles of ALD-Al2O3 layer, which is 4–5 orders of magnitude faster than the other reported CZTSSe photodetectors. Simultaneously, it is revealed that the ALD-Al2O3 interfacial layer acts as an electron blocking layer which leads to the effective suppression of carrier recombination at the rear surface. Consequently, the responsivity and detectivity are enhanced in the entire range while the maximum values are up to 0.39 AW–1 and 2.04 × 1011 Jones with 8 cycles of ALD-Al2O3, respectively. Finally, the CZTSSe photodetector is successfully integrated into a visible light communication system and obtains a satisfying transfer rate of 2 Mbps. These results indicate the satisfying performance of CZTSSe-based thin film photodetectors with great potential applications for communication.

The Cu(In,Ga)Se 2 (CIGS) thin film has been commercialized as solar cells with great success, but its application for photodetectors still faces some practical challenges, including low detectivity and long response time. In this paper, the structure of the Mo/CIGS/CdS/ZnO/ITO heterojunction has been fabricated, and satisfactory performances of high detectivity and fast response time have been achieved by suppressing the dark current and enhancing the carrier mobility. The controllable growth of CIGS grains is accomplished through optimizing the selenization process, demonstrating that bigger grain sizes resulted in higher carrier mobility and better response characteristics. Particularly, the high rise/decay speed of 3.40/6.46 μs is achieved. Furthermore, the interface of the CIGS/CdS heterojunction has been modified by the Al 2 O 3 layer via the atomic-layer deposition (ALD) process. The dark current of the device is effectively suppressed by the ALD-Al 2 O 3 layer, which remarkably drops from ∼10 −7 to ∼10 −9 A. As a consequence, the detectivity rises from 3.08 × 10 11 to 1.84 × 10 12 Jones. In addition, the ALD-Al 2 O 3 layer shows a protective effect as well, which is positive for photoelectrical conversion. Besides, the wide linear dynamic range of 102.1 dB and large −3 dB bandwidth of 78 kHz are acquired. This work suggests that the CIGS-based heterojunction has great potential for high-performance thin-film photodetectors.