Surface sensing behavior and band edge properties of AgAlS2: Experimental observations in optical, chemical, and thermoreflectance spectroscopy

Abstract

Optical examination of a chaocogenide compound AgAlS2 which can spontaneously transfer to a AgAlO2 oxide has been investigated by thermoreflectance (TR) spectroscopy herein. The single crystals of AgAlS2 were grown by chemical vapor transport (CVT) method using ICl3 as a transport agent sealed in evacuated quartz tubes. The as-grown AgAlS2 crystals essentially possess a transparent and white color in vacuum. The crystal surface of AgAlS2 becomes darkened and brownish when putting AgAlS2 into atmosphere for reacting with water vapor or hydrogen gas. Undergoing the chemical reaction process, oxygen deficient AgAlO2-2x with brownish and reddish-like color on surface of AgAlS2 forms. The transition energy of deficient AgAlO2-2x was evaluated by TR experiment. The value was determined to be ∼2.452 eV at 300 K. If the sample is kept dry and moved away from moisture, AgAlS2 crystal can stop forming more deficient AgAlO2-2x surface oxides. The experimental TR spectra for the surface-reacted sample show clearly two transition features at EW=2.452 eV for deficient AgAlO2-2x and EU=3.186 eV for AgAlS2, respectively. The EU transition belongs to direct band-edge exciton of AgAlS2. Alternatively, for surface-oxidation process of AgAlS2 lasting for a long time, a AgAlO2 crystal with yellowish color will eventually form. The TR measurements show mainly a ground-state band edge exciton of E OX 1 detected for AgAlO2. The energy was determined to be E OX 1 =2.792 eV at 300 K. The valence-band electronic structure of AgAlS2 has been detailed characterized using polarized-thermoreflectance (PTR) measurements in the temperature range between 30 and 340 K. Physical chemistry behaviors of AgAlS2 and AgAlO2 have been comprehensively studied via detailed analyses of PTR and TR spectra. Based on the experimental analyses, optical and chemical behaviors of the AgAlS2 crystals under atmosphere are realized. A possible optical-detecting scheme for using AgAlS2 as a humidity sensor has also been proposed

Wide Band Gap Chalcogenide Semiconductors

“…AgAlS 2 has a comparable band gap (∼3.2 eV) to CuAlS 2 , though CBD synthesis yields a gap of 2.3 eV. However, it has been reported as unstable in air, which limits its practical applications . There are few studied of its electrical properties, but it has potential to be alloyed with other chalcopyrite semiconductors.…”

“…However, it has been reported as unstable in air, which limits its practical applications. 274 There are few studied of its electrical properties, but it has potential to be alloyed with other chalcopyrite semiconductors. We also note that mixed ionicelectronic conductivity in this system (and in AgGaS 2 ) has been reported.…”

Optical behavior and structural property of CuAlS2 and AgAlS2 wide-bandgap chalcopyrites

“…For the Ag-based I-III-VI 2 compounds, previous reports mainly focused on AgInS 2 [14,15] to claim its usage on photocatalyst and luminescence property [16,17]. For AgAlS 2 , researchers claimed it is unstable in the air [18], and may also make a blue to ultraviolet (UV) emission for application in full-color optoelectronics due to its wide bandgap. A possible optical-detecting scheme for using AgAlS 2 as a humidity sensor had been proposed [18].…”

“…For AgAlS 2 , researchers claimed it is unstable in the air [18], and may also make a blue to ultraviolet (UV) emission for application in full-color optoelectronics due to its wide bandgap. A possible optical-detecting scheme for using AgAlS 2 as a humidity sensor had been proposed [18]. Detections of the light intensity change by transmission or reflection measurement may be a potential way for the humidity sensing application.…”

Polarized optical sensing and band-edge transitions in Ag(In0.5Al0.5)S2

“…9) The solid form of AgAlS 2 was previously found to be moisturesensitive and unstable in air. 10,11) Furthermore, the Ag-based I-III-VI 2 compounds were reported to have a specific valence band structure, 12) which may present a polarizationdependent optical character with respect to their band-edge transitions. However, to date, very few studies on the dichroic optical behavior of the band-edge transitions of Ag-based I-III-VI 2 compounds have been carried out.…”