Amorphous Superlattices Research Articles

Optical absorption spectra of bulk and multilayer amorphous semiconductors

Abstract We introduce a two-band tight-binding Hamiltonian which describes in a unified manner the behaviour of electrons in a three-dimensional amorphous semiconductor as well as in an amorphous semiconductor superlattice. On the basis of this Hamiltonian, we calculate the density of states D(E) and the optical absorption spectra I(E). Our assertion is that the exponential of Urbach tail of I(E) corresponds to the transition region from the essential band-to-band absorption to the deep tail. As a consequence, the exponential tail is by no means an exclusive feature of the Gaussian distribution as argued before. We also evaluate the slopes of the exponential tails E O and E U of D(E) and I(E) respectively for amorphous semiconductor superlattices and discuss their dependence on the width of the well layer.

Amorphous silicon superlattices prepared by direct photochemical deposition

Abstract Hydrogenated amorphous silicon/stoichiometric silicon nitride (a-Si:H/a-Si3N4:H) superlattices have been prepared by the direct photolysis of disilane (Si2H6) and of a silane (SiH4)–ammonia (NH3) gas mixture. It is found that the structure is well defined on an atomic scale and that the optical and electrical properties are well interpreted by quantum-size effects in the ultra-thin a-Si:H layers. In particular, significant enhancement of the luminescence intensity is observed when the a-Si:H well width is less than 50 A. The extent of the luminescence enhancement is more pronounced for photochemically deposited multilayers than for films obtained by plasma-enhanced chemical vapour deposition CVD. This is attributable to the absence of ion-induced damage in superlattices obtained by photochemical vapour deposition.

Phonon softening of amorphous semiconductor superlattice films.

The Rayleigh-wave velocities in a series of superlattice films (a-Si:H/a-${\mathrm{SiN}}_{\mathrm{x}}$:H) have been measured by ultrasonic techniques. At the same time, an approximate theoretical calculation of the velocities using the long-wavelength approximation and the invariants method has also been given. The experimental results are compared quantitatively with those of the theoretical calculation. Some anomalous phenomena, so-called ``phonon softening,'' are observed. Preliminary conclusions are presented and discussed briefly.

Bias-induced metastable transverse conduction in doping-modulated amorphous silicon superlattices.

Bias-induced metastabilities in the transverse conductance of doping-modulated n-p-n-p hydrogenated amorphous silicon (a-Si:H) superlattices have been studied. The electronic transport takes place through the diffusion over the potential barrier in the transverse direction for the n-p-n-p multilayer with moderate n- and p-layer doping concentration. The tunneling current through the localized states in the gap is dominant for the multilayer with a p layer of high doping concentration. The currents through the localized gap states increase after high-voltage-bias annealing, indicating that the density of dangling bonds in p and n layers increases during the bias annealing.

Phonons in amorphous superlattices

In this contribution experimental results on phonons in amorphous superlattices are reviewed. These structures combine the amorphicity within the layers with a one-dimensional periodicity imposed by the growth process. The amorphicity governs the properties of high frequency phonons, which have a characteristic coherence length much smaller than the individual layer thickness. While these phonons are insensitive to the artificial periodicity, they can be used as efficient probe to investigate the composition of amorphous layers and interfaces. Low frequency acoustic phonons on the other hand have coherence lengths far exceeding the superlattice period. As in crystalline superlattices and dispersion relation of these phonons is folded within the mini-Brillouin-zone defined by the period. As a consequence, new lines with frequencies that depend on the acoustic properties and on the thicknesses of the layers appear in the low-frequency Raman spectrum. The dispersion also exhibits frequency gaps where no propagating phonon mode exists which appear as minima in the transmission spectrum of acoustic phonons through the superlattice. This points out the potentiality of amorphous multilayer structures for application in filters for acoustic phonons.

Bias-induced conductance change in NPNP doping-modulated amorphous silicon superlattices

Bias-induced metastabilities in the coplanar and transverse conductances have been studied for amorphous silicon npnp doping-modulated superlattices. The metastabilities, arised from the transverse electric field, are probably due to the creation of dangling bonds and the trapping of charges in p- and n-layers.

THEORETICAL ANALYSIS OF THE OPTICAL-ABSORPTION TAIL IN AMORPHOUS SEMICONDUCTOR SUPERLATTICES

We propose a two-band tight-binding model Hamiltonian to describe the behaviour of electrons in amorphous semiconductor superlattices. Our model Hamiltonian is characterized by its off-diagonal terms υz in direction z which is perpendicular to layers in a superlattice; we define υz as a parameter which represents the dimension of well layers accompanying the change in a well-layer width Lw under the condition that a barrier-layer width is fixed. On the basis of this model, we calculate the density of states D(E) and the absorption spectra I(E), from which we estimate the exponential-tail widths E0 and Eu of D(E) and I(E), respectively. We discuss the ways in which these widths E0 and Eu are influenced (i) by the dimension of the system, (ii) by the dispersion of well-layer widths and (iii) by the degree of disorder. We also propose the possibility to improve the photovoltaic efficiency in solar cells composed of amorphous semiconductor superlattices.

Structural Stability of Amorphous Semiconductor Superlattices

AbstractWe report recent results of studies of the structure and thermal stability of periodic multilayers based on hydrogenated amorphous silicon, hydrogenated amorphous germanium, silicon nitride and silicon oxide. By varying the sublayer thickness from 1 nm to 20 nm it is possible to extract information on the range and magnitude of relaxation and interdiffusion in these metastable materials. It is also possible to gain information on the influence of interfaces on crystallization and relaxation. The principal techniques discussed here are Raman scattering, optical absorption and high resolution x-ray reflectivity.

Metastable Electronic Effects in Amorphous Semiconductor Superlattices

AbstractThis paper reviews some of the novel electronic effects observed in amorphous semiconductor superlattices. Quantum size effects have been reported in compositionally modulated amorphous semiconductors based upon optical absorption and tunneling studies. However, the quantum well effects are in direct conflict with transport data estimates of the inelastic scattering length. In doping modulated amorphous semiconductors, a large persistent photoconductivity PPC is observed, which cannot be explained by the field separation model alone, but rather requires the presence of a defect which undergoes a large lattice relaxation upon trapping a charge (similar to the DX center). Studies of npnp amorphous silicon multilayers have enabled a microscopic determination of the process responsible for PPC.

Low temperature photon-controlled growth of thin films and multilayered structures

Abstract Pulsed ArF (193 nm) excimer laser photolysis of disilane, germane, and disilane-ammonia mixtures has been used to deposit amorphous superlattices containing silicon, germanium and silicon nitride layers. Transmission electron microscope cross-section views demonstrate that structures having thin (5–25 nm) layers and sharp interlayer boundaries can be deposited entirely under laser photolytic control, using low reactant gas partial pressures and with the excimer laser beam parallel to the substrate. Growth of epitaxial films and structures under similar conditions is discussed.

Carrier separation effect in single-layered-compensated amorphous silicon

In this paper we find the microscopic mechanism of persistent photoconductivity in doping-modulated amorphous silicon superlattices is the same as that of compensated amorphous silicon. The carrier separation due to a built-in n-p junction field can explain persistent photoconductivity in compensated amorphous silicon films. This new analysis is favored by experimental results on the persistent photoconductivity in compensated as well as in layered amorphous silicon films.

Vibrational properties of amorphous silicon-germanium alloys and superlattices

Densities of states and localization characteristics have been calculated for vibrational modes in a-${\mathrm{Si}}_{\mathrm{x}}$${\mathrm{Ge}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$ alloys and a-Si/a-Ge superlattices, using amorphous tetrahedral networks generated by molecular-dynamics methods. For the alloys, the calculated densities of states agree very well with Raman scattering experiments at all concentrations 0\ensuremath{\le}x\ensuremath{\le}1. We explicitly show that an x-dependent feature at 390 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ arises primarily from Si-Ge bond-stretching motions. The localization of vibrational modes is enhanced in magnitude and in frequency range for the alloys as compared with the pure amorphous materials, but extended modes are still present at all x. To account for the Raman measurements on thin a-Si/a-Ge superlattices, a mixed Si-Ge interfacial region about 5 A\r{} in thickness is indicated by our calculations.

Acoustic-wave transmission in semiconductor superlattices.

Acoustic-phonon transmission and reflection amplitudes are evaluated for semiconductor superlattices. The calculation is based on a finite application of the transfer-matrix method, which permits one to take into account both the crystal anisotropy and the coupling between excited modes of different polarizations. This method is applied to multiple-layered structures, thus allowing the study of more realistic systems, such as superlattices with disordered interfaces. The angular dependence of edge and anticrossing gaps is analyzed. The method has proved successful in explaining recently available experimental data on amorphous semiconductor superlattices.

Multilayer amorphous semiconductors

A comparison is made between periodic and quasiperiodic compositional amorphous semiconductor superlattices. We show that most of the structures have sharp and smooth interfaces to better than 5 Å. A variety of new optical and electrical phenomena similar to those of crystalline superlattices have been observed in periodic amorphous superlattices. The peculiarities of quasiperiodic amorphous semiconductor superlattices are very different from those of periodic structures. The study of these new kinds of structure provides numerous possibilities to obtain information about the properties of the effects of periodicity.

Simple photographic method for x-ray diffraction characterisation of superlattices

The authors have developed a very simple and rapid x-ray technique for the characterisation of superlattices. The method relies on continuous photographic recording of diffracted x-rays while scanning the superlattice sample through a small range of angles. They have worked up to the seventh-order diffraction spot with relatively short (1 h) exposures. A series of exposures of increasing duration permits the estimate of relative diffraction spot intensities and ultimately provides a measure of the range of interface roughness and/or intermixing between the layers forming the superlattice. The technique is demonstrated with an evaporated amorphous Ge-SiO superlattice, for which a roughness length of approximately 0.6 nm was found.

Photon-controlled fabrication of amorphous superlattice structures using ArF (193 nm) excimer laser photolysis

Pulsed ArF (193 nm) excimer laser photolysis of disilane, germane, and disilane-ammonia mixtures has been used to deposit amorphous superlattices containing silicon, germanium, and silicon nitride layers. Transmission electron microscope cross-section views demonstrate that structures having thin (5–25 nm) layers and sharp interlayer boundaries can be deposited at substrate temperatures below the pyrolytic threshold, entirely under laser photolytic control.

Optical investigation of a-Si:H/a-SiNx:H superlattices.

Optical properties of hydrogenated amorphous-silicon--amorphous-silicon-nitride (a-Si:H/a-${\mathrm{SiN}}_{\mathrm{x}}$:H) superlattices (x=1.1) with a-Si:H sublayer thicknesses ${d}_{S}$ as small as 11 A\r{} have been investigated by photothermal deflection spectroscopy combined with the transmission and photoluminescence measurements. Changes in the optical band gap, Urbach energy, and defect-state absorption were observed for ${d}_{S}$30 A\r{} and studied in detail in order to clarify the origin of the size effects in these structures. In the lowest-thickness superlattices, the room-temperature luminescence consists of two emission bands at 1.3 and 1.46 eV, which we ascribe to hydrogen-rich and nitrogen-rich interfaces, respectively. The properties of these amorphous superlattices have been compared with those of hydrogen-rich a-Si:H deposited at low temperatures and a-${\mathrm{SiN}}_{\mathrm{x}}$:H alloys (0x1.1) contaminated deliberately by nitrogen.

Band gap and activation energy in amorphous silicon doping-modulated superlattices

It has been found that both the activation energy and optical energy gap of amorphous silicon doping-modulated nipi... superlattices vary consistently with i layer thickness. There is a pronounced maximum at an i layer thickness of approximately 14 nm. The occurrence of this maximum, and the general variation, correspond exactly with the behavior of the persistent photoconductivity for the structures. The results are explained in terms of the development of deep trap boron-phosphorus complexes which are rendered shallow by hydrogen accretion.

A novel deposition concept for amorphous superlattices

Cumulative layered structures of amorphous semiconductors have attracted much interest in recent years, as they make it possible to use quantum effects to improve semiconductor properties1. There is, however, a serious disadvantage in using such superstructures in practical devices, because the conventional process requires mechanical alternation of source gases and evacuation of the system for each layer deposition. As an alternative to this inefficient physically controlled method, we have developed an efficient photochemically controlled method which facilitates the continuous deposition of various superlattices. In combination with two modes of excitation, at least one of which is operated in a pulsed mode, an appropriately selected pair of source gases accumulates continuously binary layers of amorphous semiconductors. As an example of verification of this novel process, an amorphous super-lattice composed of an amorphous silicon carbide barrier layer and an amorphous silicon well layer was prepared from a mixture of disilane and carbon tetrafluoride.

Light-induced metastable state in doping-modulated amorphous silicon superlattices

Photo-induced decrease in conductance after long light exposure has been observed for hydrogenated amorphous silicon (a-Si:H) npnp . doping-modulated superlattices showing large persistent photoconductivity (PPC). The initial increase in PPC is taken over by the decrease even to the negative PPC after long illumination. This light-induced, metastable conductance can be recovered completely by 160°C annealing, which is independent of exposure time. In particular, the metastable defects created in the p-layers are found to be annealed out at 100°C. The subband gap light exposure on the a-Si:H npnp. multilayers showing n-type conduction gives rise to the increase of conductance. On the contrary, the conductance of the multilayers having p-channel conduction decreases by IR exposure. These results strongly suggest the creation and annealing of dangling bonds by light-soaking and annealing in doping-modulated superlattices.