Optical Observation of Quantum-Size Effect in the Amorphous Silicon/Amorphous Silicon Carbide Multilayer
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OPTICAL OBSERVATION OF QUANTUM-SIZE EFFECT IN THE AMORPHOUS SILICON/AMORPHOUS SILICON CARBIDE MULTILAYER
Kiminori HATTORI, Takeshi MORI, Hiroaki OKAMOTO, and Yoshihiro HAMAKAWA Faculty of Engineering Science, Osaka University, Toyonaka, Osaka, 560 Japan
ABSTRACT A staircase structure reflecting quantum-size effect has been observed in differential optical absorption spectra in a-Si:H/a-SiC:H multilayer and a-Si:H ultrathin single layer. The threshold energies for the identified subband transition are found to be consistent with those expected from one-dimensional quantum-well model involving a conservation rule for the subband index. The experimental approaches introduced here will open up new possibilities for investigating the quantized band structure as well as for establishing the design concept of functional elements based on quantum size effects.
1. INTRODUCTION Quantum-size effects in semiconductor multilayers give rise to a density of states that increases in discrete steps. This structure is expected to be reflected in the optical absorption spectrum even in the case of amorphous semiconductor multilayers. The observed absorption spectrum, however, does not exhibit clear quantum features 1l].This seems to imply that the optical structures associated with quantized density of states are largely reduced due to an non-direct nature of optical transition process in amorphous To identify such structures in unresolved absorption spectrum materials. Optical requires experimental approaches with an extreme sensitivity. (differential) modulation requirement.
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In this paper, we report on the use of photothermal modulation (PTM) spectroscopy to study interband optical transitions in a-Si:H/a-SiC:H The PTM spectroscopy is a conventional thermal multilayered structures. modulation spectroscopy £23, in which the temperature of the sample is modulated by heat produced by light absorption. The PTM spectra measured on the a-Si:H/a-SiC:H multilayered structures drastically changes from a linear to a staircase form with decreasing the a-Si:H well layer thickness below 50A. The observation indicates that the three-dimensional (3D) parabolic band transition is turned into transitions between quantized band states We will also present (subband states) in the a-Si:H quantum well (QW). subband absorption spectra in a-Si:H ultrathin single layers (single QW structures), which are manifested by the wavelength-differential technique. 2. EXPERIMENT The multilayered structures were prepared on glass substrate by rf plasma chemical vapor deposition using a 1:9 SiH /H gas mixture for the a-Si:H layers and 1:13:126 SiH /CH4 /H, gas mixture 5o a-SiC:H layers. Each layer was formed in separate c;Namber4 with interruption on the plasma. The thicknesses of the sublayers were determined by the deposition time and the deposition rates for the thick films. The a-Si:H well layer thickness was vanied from 20A to 1O00A while keeping the total well layer thjckness at 3000A, and the a-SiC:H barrier la
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