UV photo-detectors with high responsivities for wavelengths shorter than 300nm are very important for many applications such as flame detection for fire safety, UV astronomy, and in particular, a detector with a response curve closely matching the erythemal action spectrum that describes human skin sensitivity to UV radiation is strongly desired. This study focuses on the development of a novel solar-blind UV photodetectors in four main areas: (i) Growth of ZnMgS-based II-VI thin films by the molecular beam epitaxy (MBE) technique; (ii) study of the structural transition of ZnMgS thin film occurring at a critical thickness during the MBE growth; (iii) photoluminescence (PL) measurements on ZnMgS thin films and (iv) fabrication and characterization of ZnMgS Schottky barrier photodiodes....[ Read more ]

UV photo-detectors with high responsivities for wavelengths shorter than 300nm are very important for many applications such as flame detection for fire safety, UV astronomy, and in particular, a detector with a response curve closely matching the erythemal action spectrum that describes human skin sensitivity to UV radiation is strongly desired. This study focuses on the development of a novel solar-blind UV photodetectors in four main areas: (i) Growth of ZnMgS-based II-VI thin films by the molecular beam epitaxy (MBE) technique; (ii) study of the structural transition of ZnMgS thin film occurring at a critical thickness during the MBE growth; (iii) photoluminescence (PL) measurements on ZnMgS thin films and (iv) fabrication and characterization of ZnMgS Schottky barrier photodiodes.

Various ZnMgS-based II-VI thin films have been successfully grown on GaP substrates by MBE. Ichino et al. [Ref 1] have studied the MBE growth of ZnMgS alloy thin films on GaP (100) substrates up to Mg composition about 0.45 using excess S beam pressure provided by an elemental S source together with ZnS and Mg source cells. In this work, we used a similar growth scheme to study the structural transition of the ZnMgS alloys covering the entire Mg composition range and revealed that a structural transition will occur at a critical thickness whose value is sensitively dependent on the Mg composition.

Room-temperature PL measurements on the as-grown ZnMgS alloy layers were preformed using an excitation wavelength at 275nm. Their luminescence peaks are quite close to the expected band edge; the origin of these emissions could either result from a band-to-band transition or be of excitionic type. Different excitation peak power density have been used; however, no features of any deep-level emission were detected.

The ZnMgS-based Schottky-barrier-type photodetector structures were fabricated using an approach similar to what my supervisior, I.K. Sou, has developed for the ZnSSe structures². The bottom electrode consists of a few thousand Å thick n⁺ - Zn_0.95Mg_0.05S:Al. An undoped Zn_1-xMg_xS layer as the active layer was grown on top of the bottom electrode layer. A 100-Å-thick Au layer with area of 3 x 3 mm² was deposited on top of the undoped layer using a thermal evaporation technique in a separated vacuum chamber to serve as the top transparent electrode.

Room-temperature photoresponse measurements were performed on these detectors and abrupt long-wavelengths cutoffs covering 325, 305, 295 and 270 nm were achieved for devices with Mg composition of 16%, 44%, 57% and 75%, respectively. The response curve of the Zn_0.43Mg_0.57S device offers a close match to erythemal action spectrum that describes human skin sensitivity to UV radiation.

ZnMgS is proved to be a good candidate as the active material for UV photodetection applications that require excellent solar rejection and tunable turn-on wavelength capabilities.

This experimental study has revealed some of the interesting issues about using ZnMgS-based II-VI thin films as new UV detection materials. The results show that the developed Schottky-barrier diode structures are suitable for UV detector applications that require fast and solar-blind response.