Photoluminescence and absorption spectra of undoped and Si-doped GaN films have been studied. It was shown that moderate Si doping improves the optical properties of GaN epitaxial layers. The further increase of doping concentration results in a red shift and broadening of bound exciton line. The broadening seems to be due to potential fluctuations caused by randomly distributed donor impurities. The analysis of temperature dependence of GaN photoluminescence spectra allowed us to reveal the E 1 2 optical-phonon replica of free exciton. The observed flattening of absorption edge with doping can be described in terms of Urbach rule.
PACS 71. 78.40.Fy, 78.66.Hf ZnS x Se 1-x alloy films with and without near-surface quantum wells were deposited on the GaAs substrates in the photo-induced MOCVD growth process. The reflection spectra of the structures have been studied experimentally and theoretically. The spectra from the film containing a photo-induced near-surface quantum well show additional reflectance peaks as compared to the quantum well free film. The number and position of the additional peaks are governed by the lateral local composition x . A theoretical model explaining the principal exciton-mediated features of the experimental reflectivity spectra is developed. The resonant spectral structure of the reflectivity is shown to be due to the center-of mass quantization of both light-and heavy-hole excitons inside the quantum well and due to free excitons in the thick (barrier) part of the ZnS x Se 1-x film. 1 Introduction During the last two decades, the optical properties of semiconductors with nearsurface localized excitons have been intensively investigated (see, for example, [1][2][3][4][5][6][7][8][9][10][11] and references therein). Extrinsic near-surface potential wells, where the exciton center-of-mass motion is quantized, can be created by means of surface treatments such as electron and ion bombardment, heating, doping, illumination, and the application of an electric field. Also, sufficiently wide and deep extrinsic potential wells, having a large number of excitonic bound states, can be formed in semiconductor solid solutions by varying their composition in the near-surface region [1,6,7]. Near-surface localized excitons manifest themselves in optical spectra (specular [1-9,11] and diffuse [10] reflection, transmission [4]) as resonances at frequencies very close to their corresponding eigenvalues. For this reason, the excitonic optical spectra turn out to be very useful in characterizing both optical and structural properties of the nearsurface region of solids.In the present paper we investigate experimentally and theoretically the exciton quantization in nearsurface potential wells of the ZnS x Se 1-x based planar structures. It is noteworthy that the ZnS x Se 1-x alloys are of much interest now because of their potential applications in optoelectronic devices in blue-nearultraviolet spectral range. The behaviour of excitons in ZnS x Se 1-x quantum wells is especially important for understanding fundamental optical properties of the materials suitable for the design of light emitters, heterostructure lasers as well as other waveguiding devices utilizing these materials.
The influence on the excitonic luminescence and reflectance spectra of an electric field applied to a Schottky-barrier GaN structure was investigated in dependence on the uncompensated donor concentration. It was discovered that quenching of the luminescence takes place under a reverse bias, while an increase in the intensity occurs under forward bias. Rotation of the exitonic reflectance spectrum induced by an electric field was observed. These effects were attributed to the variation of the thickness of the exciton-free layer under the influence of the applied voltage. As a result, it was shown that the excitonic luminescence and reflectance spectra are well modulated by the space charge and that the character of the modulation is dependent on the uncompensated donor concentration in the sample.
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