Electronic states in a-Si:H/c-Si heterostructures

We have investigated PECVD-deposited ultrathin intrinsic a-Si:H layers on c-Si substrates using UV-excited photoemission spectroscopy (hν = 4–8 eV) and surface photovoltage measurements. For samples deposited at 230 °C, the Urbach energy is minimal, the Fermi level closest to midgap and the interface recombination velocity has a minimum. The a-Si:H/c-Si interface density of states is comparable to that of thermally oxidized silicon interfaces. However, the measured a-Si:H dangling bond densities are generally higher than in thick films and not correlated with the Urbach energy. This is ascribed to additional disorder induced by the proximity of the a-Si:H/c-Si interface and H-rich growth in the film/substrate interface region.     

Defects and microvoids in a-Si and a-Si:H

We report the first measurements of positron-annihilation spectra of samples of both pure and hydrogenated amorphous silicon. Comparison of these spectra with that of crystalline silicon indicates that the lowest-lifetime component can be identified as the contribution mainly from valence-band electrons. Both the pure (a-Si) and the hydrogenated (a-Si:H) samples exhibit a component with intermediate lifetime, which we attribute to small vacancies consisting of about 4 missing atoms. Finally, only a-Si:H shows a significant long-lived line (τ > 5 ns), which arises from large microvoids, with ～ 100 missing atoms. The existence of these microvoids in a-Si:H is consistent with recent reports of the presence of occluded H₂ gas under high pressure in such films.     

Back contacted a-Si:H/c-Si heterostructure solar cells

This paper shows how the amorphous/crystalline silicon technology can be implemented in the interdigitated back contact solar cell design. We have fabricated rear-junction, backside contact cells in which both the emitter and the back contact are formed by amorphous/crystalline silicon heterostructure, and the grid-less textured front surface is passivated by a double layer of amorphous silicon and silicon nitride, which also provides an anti-reflection coating. The entire self-aligned mask and photolithography-free process is performed at temperature below 300 °C with the aid of one metallic mask to create the interdigitated pattern. An open circuit voltage of 687 mV has been measured on a 0.5 Ωcm p-type monocrystalline silicon wafer. On the other hand, several technological aspects that limit the fill factor (50%) and the short circuit current density (32 mA/cm²) still need improvement. We show that the uniformity of the deposited amorphous silicon layers is not influenced by the mask-assisted deposition process and that the alignment is feasible. Moreover, this paper investigates the photocurrent limiting factors by one-dimensional modeling and quantum efficiency measurements.     

Local bonding of hydrogen in a-Si:H,a-Ge:H and a-Si,Ge:H alloy films

This paper reviews the local bonding of hydrogen (and deuterium) in a-Si:H(D), a-Ge:H(D) and a-Si, Ge:H(D) alloy films. We specify the types of atomic environments that have been identified through vibrational spectroscopy, primarily infrared (IR) absorption. We emphasize local modes and discuss the atomic motions that are responsible for the various spectral features. We discuss correlations between the occurrence of specific local bonding groups, e.g., polysilane and polygermane configurations, and the deposition techniques and parameters, including the substrate temperature (T_s), the gas mixtures and the RF power into a glow discharge, etc. We include a discussion of the theoretical approaches that have been used to treat vibrational modes in these materials. We emphasize the approximations that are valid because of the relatively light mass of the hydrogen and deuterium atoms compared with those of the silicon and germanium atoms. Finally we highlight the effects of neighboring alloy or impurity atoms on the frequencies of hydrogen vibrations in a-Si:H, and point out the differences between oxygen atom incorporation in a-Si and a-Ge alloys.     

Recombination efficacy in a-Si:H p-i-n devices

The performance of hydrogenated amorphous silicon solar cells (a-Si:H) is limited due to the relatively high defect concentration in the intrinsic layer, leading to high recombination rates. The dark current-voltage (JV) characteristics of these cells are mainly determined by recombination processes. Several parameters influence these processes: the density-of-states distribution in the mobility gap, the spatial distribution of recombination centers, and the recombination efficacy. As a result the ideality factor is non-integer and varies with voltage. The temperature dependence of the dark JV curves reveals the bias dependent activation energy, a measure for the mobility gap, and the temperature dependence of the recombination efficacy.In this contribution we present results of accurate dark JV measurements at different temperatures and a comparison to computer simulations. By analyzing the temperature dependence we study the influence of the recombination efficacy on the recombination rate and determine where in the device recombination is most effective. Simulations show that the shape of the recombination efficacy curve depends on the cross-section ratio for electron and hole trapping. Whereas the recombination rate is mainly determined by the local defect density in the device and controls the JV characteristic, the recombination efficacy controls the temperature dependence.     

Characterization and luminescence of a-Si:H:Cl films

a-Si:H:Cl films have been deposited by glow-discharge and characterized by infrared transmission, optical absorption and photoluminescence. The influence of growth parameters on the H and Cl content has been investigated. The luminescence spectra show that three different radiative transitions can occur, at 0.75, 0.95 and ～1.3 eV. These bands have been interpreted respectively in terms of the following recombinations: defect to defect, defect to band tail, band tail to band tail.     

Deep-level spectroscopy of reactively sputtered a-Si:H films

We have performed light-induced modulation of absorption spectroscopy, thermally stimulated current spectroscopy, capacitance measurements as a function of temperature and frequence, and electron spin resonance experiments on reactively sputtered a-Si:H films. We find evidence for seven different localized gap states in a range of ±0.4 eV around the midgap region. The possible origin of these states is discussed.     

Small-angle X-ray scattering studies of a-Si:C:H

Small-angle X-ray scattering studies have been performed on a series of four a-Si:C:H alloys, prepared by rf glow discharge decomposition of varying proportions of propane and silane, in an attempt to elucidate their mesoscopic structure. The observed broad scattering peak has been interpreted as originating from irregular, elongated voids with a repeat distance of about 20 Å and correlation length of about 25 Å. The implications of this result in explaining the photo-oxidation properties of the material are also discussed.     

Preparation and analysis of reactively evaporated a-Si:H

a-Si:H formed by the method of reactive evaporation of silicon with supplying hydrogen ion has much the same properties as films prepared by silane grow discharge (GD) decomposition. Growth kinetics in this method is described. Hydrogen ion suppresses the oxidation of silicon and it is incorporated as mono-hydride, whose content is proportional to hydrogen ion current. Adsorption energy of hydrogen ion is some 130 meV. Dark conductivity of a-Si:H depends mainly on energy gap associated with hydrogen content. Spin density is determined by the collision phenomenon between Si-H and spin in depositing.     

Photocapacitance measurements in irradiated a-Si:H based detectors

Photocapacitance measurements were performed on amorphous silicon p–i–n detectors before and after particle irradiation with 1.5 MeV 4 He⁺ ions. The spatial resolution across a degraded spot is similar to the one obtained in photocurrent scans and is of the order of the diameter of the scanning laser beam. We monitored the transient capacitance after applying short laser pulses to deduce trap energies of 0.64 eV. Photocapacitance measurements as a function of the applied bias, the measurement frequency up to 1 MHz, and the wavelength of laser light are discussed. The reduction in photocapacitance signal and the shift of the cut-off frequency after ion bombardment are correlated with the change in transport properties.     

Initial stage of growth of the GD a-Si:H

The time dependences of the optical emission intensities of SiH,H and He lines and also of the gas temperature have been studied. It is demonstrated that 10–100 sec are necessary for full stabilization of plasma. This can explain the different properties of the initial 30–300 Å layer of a-Si:H.     

New approach to capacitance spectroscopy for interface characterization of a-Si:H/c-Si heterojunctions

A new technique for characterization of interface defects in a-Si:H/c-Si heterostructure solar cells from capacitance spectroscopy measurements under illumination at forward bias close to open-circuit voltage is described. The proposed method allows to significantly increase the sensitivity to interface defects compared to conventional capacitance measurements at zero or small negative bias. Results of numerical modelling as well as experimental data obtained on n-type a-Si:H/p-type c-Si heterojunctions are presented. The sensitivity of the proposed method to interface states and the influence of various parameters like band mismatch, density of interface defects, recombination velocity at the back contact are discussed.     

Simulation of picosecond domain time-of-flight experiments in a-Si:H

Using a model, developed earlier, of trap controlled conduction, which includes both the Meyer–Neldel rule and field assisted detrapping, we have simulated picosecond timescale drift mobility and drift velocity measurements in a-Si:H. This model is able to duplicate both the picosecond and nanosecond data, for all values of temperature and field, using one set of fixed parameters. We observe that under certain conditions, the drift mobility is field independent. Coherence between the picosecond and nanosecond results is shown.