| Titre : | Caractérisation des défauts profonds dans le silicium amorphe hydrogéné et autres semi conducteurs photo-actifs de type V par la méthode de photo courant constant CPM |
| Auteurs : | Toufik Tibermacine ; Nouredine Sengouga, Directeur de thèse |
| Support: | Thése doctorat |
| Editeur : | Biskra [Algerie] : Université Mohamed Khider, 2011 |
| Langues: | Français |
| Mots-clés: | Spectre d‟absorption optique ; Densité des défauts profonds ; a-Si:H ; μc-Si :H ; SI-GaAs :Cr ; AC-CPM ; DC-CPM ; Illumination Intense. |
| Résumé : |
Le but de ce travail est de déterminer les propriétés optiques et électroniques en termes de coefficient d’absorption optique et densité d’états électronique d’un certain nombre de semi-conducteurs à savoir le silicium amorphe hydrogéné (a-Si:H), silicium microcristallin (μc-Si:H), arséniure de gallium dopé au chrome (SI-GaAs:Cr). Pour cette raison, on a mesuré le coefficient d'absorption optique des échantillons par la méthode de photo-courant constant en régime continu (DC-CPM) et en régime périodique (AC-CPM) pour plusieurs fréquences d’excitation optique. Ensuite on a convertit les spectres d’absorption mesurés en densité d'états électronique à l’intérieure du gap de mobilité. Une différence est observée concernant le coefficient d’absorption optique et la densité d’états profonds entre le régime périodique et le régime continu d’un côté et entre les différentes fréquences d’un autre côté. On a aussi, mesuré l’évolution du coefficient d’absorption et la distribution d'états électronique avec l’illumination. Cette mesure montre une augmentation de la densité de défauts profonds, une présence de défauts chargés plus importante que les défauts neutre et une saturation pour des temps d’illuminations très longues. En outre, on a développé un programme pour modéliser la technique CPM en mode DC et AC qui tient en compte toutes les transitions optiques possibles. Notre modélisation nous a démontré l’importance de considérer les deux coefficients d’absorption celles du aux électrons et celle due aux trous. Ceci est essentiel pour reconstituer la distribution des états occupés et non occupés en utilisant ces deux coefficients surtout pour des hautes fréquences. Au fur et à mesure que la fréquence augmente, les propriétés optiques en termes de spectre d’absorption optique sont plus en plus sous-estimées quant aux propriétés électroniques en termes de densité d’états électronique sont plus en plus bien déterminées.
We present in this thesis the optical and electronic properties of a number of semiconductor materials namely undoped and P-doped hydrogenated amorphous silicon a-Si:H prepared by Plasma Enhanced Chemical Vapour Deposition (PECVD), hydrogenated micro-crystalline silicon (c-Si:H) prepared by Very High Frequency Plasma-Enhanced Chemical Vapor Deposition (VHF-PECVD) and semi-insulating Cr-doped GaAs (SI-GaAs:Cr) prepared by the Liquid Encapsulated Czochralski (LEC) method. Sub-band gap optical absorption spectra of all samples have been measured by the constant photocurrent technique in dc and ac excitation (dc-CPM and ac-CPM). Then, these absorption coefficients are converted into electronic density of states (DOS) distribution within the mobility gap by applying the derivative method of Pierz et al. We present in this thesis the relationship between the optical excitation frequency and the optical and electronic properties of semiconductors materials in particular a-Si:H, c-Si:H and GaAs. We have developed a code program to simulate the dc and ac-CPM sub-band-gap optical absorption spectra. This numerical simulation includes all possible thermal and optical transitions between extended states and gap states. Our numerical results shows that (i) a discrepancy between dc mode and ac mode in absorption spectrum and gap state distribution particularly in defect region; (ii) extraction of DOS distribution using ac mode is better than using dc mode specially at high frequency (iii) DOS distribution can be reasonably reconstructed over a wide range of energy, especially at ultra high frequency, using both corresponding to optical transitions associated with free electrons and free holes creation, respectively. In addition and to validate our simulation results, we have measured for all samples at several frequencies. Our experimental results prove the simulation ones and showed that a significant difference between dc- and ac-absorption spectra is observed in defect region and that the determination of the density of the occupied states within the gap mobility of the material is better for high frequencies than for low frequencies. The evolution of the sub-band-gap absorption coefficient and the CPM-determined density of gap-states distribution within the gap versus the illumination time leads to: (i) an increase in the deep defect absorption without any significant changes in the Urbach tail (exponential part), (ii) a presence of more charged than neutral defects as predicted by the Vi defect pool model, and (iii) a saturation point of the degradation of both optical absorption coefficient and density of deep states of slightly P-doped sample measured by dc-CPM. The constant photocurrent technique in dc-mode as a spectroscopy method for the defect distribution determination is, therefore, most reliable to study the light soaking effect on the stability of hydrogenated amorphous silicon layers used in solar cells manufacturing. The constant photocurrent method in the ac-mode (ac-CPM) is also used in this work to determine the defect density of states (DOS) in microcrystalline silicon and to investigate the defect levels of semi-insulating Cr-doped GaAs from the optical absorption spectrum. The microcrystalline absorption coefficient spectrum is measured under ac-CPM conditions at 60Hz and then is converted by the CPM spectroscopy into a DOS distribution covering a portion in the lower energy range of occupied states. By deconvolution of the measured optical absorption spectrum of SI-GaAs: Cr, we have extracted the distribution of the deep defect states. Independently, computer simulations of the ac-CPM for both materials are developed. Using a DOS model for microcrystalline which consistent with the measured ac-CPM spectra and a previously measured transient photocurrent (TPC) for the same material, the total ac is computed and found to agree satisfactorily with the measured can Using a DOS model for gallium arsenide which consistent with the measured ac-CPM spectra and a previously measured modulated photocurrent (MPC) for the same material, the total ac is computed and found to agree satisfactorily with the measured ac The experimentally inaccessible components and, corresponding to optical transitions associated, respectively, with free electron and free hole creation, are also computed for both semiconductors. The reconstructed DOS distributions in the lower part of the energy-gap from and in the upper part of the energy-gap from fit reasonably well the DOS model suggested by the measurements. The results are consistent with a previous analysis, where the sub-gap ac-saturates to a minimum spectrum at sufficiently high frequency and the associated DOS distribution reflect reliably and exclusively the optical transitions from low energy occupied states. |
Exemplaires (2)
| Cote | Support | Localisation | Disponibilité | Emplacement |
|---|---|---|---|---|
| TH/1261 | Thèse doctorat | Bibliothèque centrale El Allia | Exclu du prêt | Salle de consultation |
| TH/1261 | Thèse doctorat | Bibliothèque centrale El Allia | Exclu du prêt | Salle de consultation |
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