Search

Amorphous hydrogenated silicon (a-Si:H) is a well-known semiconductor with metastable properties. Direct surface exposure, as it occurs e.g. in rf plasma equipments, introduces damage due to the charged particle bombardment. The paper deals with photoluminiscence properties of virgin, oxide layer covered and chemically treated (in KCN solutions) surfaces of a-Si:H and corresponding solar cell structures. The cyanide treatment improves the electrical characteristics of MOS structures as well as solar cells. X-ray diffraction at grazing incidence and reflectance spectroscopy complete the study. The photoluminescence measurements were performed at liquid helium temperatures at 6 K using an Ar laser and lock-in signal recording device containing the PbS and Ge photodetectors. Photoluminescence bands were observed as broad luminescent peaks between 1.05–1.7 eV. Two new peaks were detected at 1.38 and 1.42 eV. The evolution of the band at ∼1.2 eV related to microcrystalline silicon is investigated. The fitting and simulation of photoluminiscence spectra are presented. The surface luminescent properties of a-Si:H based structures (double layers, single thin film solar cells) before and after the passivation are compared with those of very thin oxide layers and chemically treated surfaces.

This paper deals with the formation of very thin insulating layers on crystalline (GaAs) and amorphous semiconductors (a-Si:H and a-SiGe:H) prepared by the impacts of particles of a very low energy. Plasma, ion beams and plasma immersion ion implantation (PIII) as the sources of impacting particles were used and compared. The last technique was applied successfully for the first time in the case of amorphous silicon-based semiconductors. More diagnostics techniques were used for the investigation of the transformation of the semiconductor surface properties. In the a-Si:H based MOS structures prepared by PIII technology, only two groups of defects 0.82 and 1.25 eV (D(z) and D(e), respectively) were found. We suppose that the PIII technology using the implantation at the sample voltage of ca. -1000V causes the formation of a-Si:H layers with missing group of D(h) states. The only decisive parameter determining the formation of two groups of states is the negative potential of the sample during the implantation. In aSiGe:H based MOS structures, three distributions could be prepared by a bias annealing procedure: 0.47, 0.58 and 0.95 eV corresponding to p-type (D(h)) intrinsic (D(z)) and n-type (D(e)) distributions, respectively.

The evolution of the programmed defect-state distributions in intrinsic hydrogenated amorphous silicon (a-Si:H) due to light soaking was qualitatively determined from charge deep-level transient spectroscopy. The defect-state distribution in a-Si:H was programmed by applying a particular bias voltage on the metal-oxide-semiconductor structure while annealing the structure above the equilibration temperature. The programmed distributions simulate defect-state distributions in different parts of an actual a-Si:H solar cell, particularly in the intrinsic regions close to the p/i and i/n interfaces.The defect-state distribution in the bulk of the intrinsic layer is characterized by comparable contributions from the positively charged defect states above midgap, Dh, neutral states, Dz, and negatively charged states below midgap, De. In the programmedp-type (n-type) defect-state distribution there is an excess of the Dh (De) states. Light exposure modifies the p-type distribution that evolves to a broad distribution of states with a maximum around midgap. This distribution is dominated by Dz states with substantial contributions from Dh and De states. In case of n-type distribution light soaking only slightly influences the distribution by removing a part of the Dh states and by a small increase of Dz and De states.

Computer simulations of single-junction hydrogenated amorphous silicon (a-Si:H) solar cells with different thickness of the intrinsic layer were carried out in order to study the role of charge gap states in their light-induced degradation. It is demonstrated that it is the decrease of positively charged states above midgap, Dh, and the increase of neutral states around midgap,Dz, and negatively charged states below midgap, De in the intrinsic layer that result in a drop of performance of the solar cells due to light soaking. These changes in the gap states are in accordance with our recent experimental results from the charge deep-level transient spectroscopy on undoped a-Si:H. The experimentally observed changes in the dark and illuminated J-V curves and spectral response could not be simulated with the same set of input parameters by only increasing the defect-state density in the intrinsic layer.

We used the charge deep level transient spectroscopy (DLTS) to determine the midgap density of states (DOS) distribution in undoped a-SiGe:H layers. By applying a bias voltage of -5 V and 0 V on the MIS structures and annealing them 10 minutes at equilibrium temperature of 490 K we moved the position of the Fermi level, E/sub F/, in the a-SiGe:H material, which has led to the different equilibrium distributions of the DOS. In case of applying -5 V, which moves E/sub F/ towards the valence band edge, the DOS distribution is dominated by a peak at 0.61 eV below the conduction band edge and in case of 0 V, when E/sub F/ is around midgap, a peak in the DOS distribution is formed at 0.80 eV. In both cases light soaking leads to an increase in the DOS and a formation of a subband at 0.90 eV.

The effect of a Fermi level shift in an intrinsic energy distribution of gap states of amorphous silicon prepared by glow discharge is investigated with the aim of simulating the energy distribution of gap states near the i–n (p–i) interface in a p–i–n (n–i–p) device. Therefore we carried out experiments in which the Fermi level is moved to either the conduction or valence band edge in a ‘programmed’ intrinsic energy distribution of gap states by subjecting a metal/insulator/amorphous-silicon structure to n-type or p-type bias stress, respectively. Its effect on the energy distribution of gap states is measured by the charge version of deep level transient spectrometry. We observe that upon n-type (p-type) bias stress the energy distribution of gap states does not immediately adjust to the applied Fermi level shift, but that first an intermediate distribution is formed with a larger neutral dangling bond state contribution. In addition, it appears that the negatively charged dangling bond states are more resistant to p-type stress than the positively charged dangling bond to n-type stress.

金沢大学大学院自然科学研究科電子物性デバイス, 金沢大学工学部, Results of charge deep-level transient spectroscopy (DLTS) and electron spin resonance (ESR) measurements on undoped hydrogenated amorphous silicon (a-Si:H) clearly demonstrate that a group of gap states with a mean energy of 0.82 eV as observed in charge DLTS experiments for a-Si:H based metal/oxide/semiconductor structure is the same as the g=2.0055 ESR defect (the Dz component). This correlation provides a distinct marker for charge DLTS technique. We obtained a very good fit to spectra obtained on undoped a-Si:H in the annealed state whilst there is some discrepancy between the experimental and simulated spectra for the light-soaked state. The first quantitative comparison of defect pool model with gap states directly observed by charge DLTS offers not only additional data for more accurate identification of all the intrinsic and light-induced defects. This also renders distinct counter-evidence to recently published conjectures about the creation of another charged defect during early stage of Staebler-Wronski effect. By contrast, our presented results clearly argue for opposite process, i.e., decay of positively charged defect states Dh.

An external rf substrate bias (ERFSB) has been employed during the deposition of hydrogenated amorphous silicon (a-Si:H) by means of the expanding thermal plasma (ETP) deposition technique. The effects of the additional created ion bombardment on the a-Si:H material properties, such as the microstructure and defects, have been studied. The incorporation of nano-sized voids is reduced under ERFSB whereas the incorporation of vacancies is enhanced. The mass density increases with increasing rf power (P/sub rf/). For moderate ERFSB (P/sub rf/>12 W) the sub gap absorption of the a-Si:H material is reduced drastically. Possible plasma and ion bombardment induced surface processes responsible for these observations are discussed. Finally, it is demonstrated that the performance of the pin cells with a-Si:H films deposited with ERFSB improves.

A very thin insulating layer (VTIL) is formed by low-energy Ar-beam bombardment in the surface region of undoped hydrogenated amorphous silicon (a-Si:H). Several experimental techniques have been utilized to determine the optimal argon beam bombardment conditions to prepare electrically reliable VTIL and to investigate its physical properties (thickness, structure, nature, and density of defects). VTILs prepared under such conditions make the leakage current of a-Si:H based semiconductor structures negligible and allow bias voltages of several volts (up to 5 V).