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The thermodynamic calculations of chemical equilibrium compositions of various species were performed for silane-hydrogen system at different temperatUres [500-6000K], pressures [0.01-10 Torr], and mixing ratios [various silane fractions]. The effect of dopant impurities such as diborane and phosphine on equilibrium concentration profiles of various gaseous, tonic, and condensed species was also investigated. These data will be very useful in understanding the deposition chemistry of silane chemical systems and will be a valuable aid in optimization studies.
The semiconducting properties of a-Si are ideally suited for making a switch to control the matrix addressing of an array of liquid crystal pixels. This investigation explores the differences between a-silicon deposited by plasma enhanced chemical vapor deposition at 13.56MHz and at 60KHz. The bonded hydrogen concentration, the index of refraction and the optical band gap of a-Si have been measured. The device used to explore the field effect mobility of a-Si is an inverted staggered FET fabricated on a glass substrate with conventional photolithography.
Amorphous silicon films have been deposited by chemical vapor deposition using disilane at temperatures between 360 and 525°C at growth rates up to 50 A/s. Intrinsic films have the following properties: σp less than 5 × 10−6 (Ω-cm)−l; σd less than 5 × 10−11 (Ω-cm)−1 with Ea = 0.7 to 0.8 eV; diffusion length around 0.1 μm; Urbach energy 48 to 55 meV; and mid-gap density of states greater than 5 × 1016 cm−3 eV−1. Boron compensation improved collection efficiency by lowering the mid-gap density of states, not by improving the μτ product. Pin cells with effíciencies of 4% and Jsc = 10.9 mA/cm2 (87.5 mW ELH) were fabricated.
The thermal decomposition of disilane in a hydrogen or helium flow has been used for the deposition of hydrogenated amorphous silicon (a-Si:H) films on the surface of several substrates at 450°-500°C. The concentration of disilane in the reaction mixture has been found to affect strongly the deposition rate and the photoconductivity of a-Si:H films. The AMI conductivity of a-Si:H films increases with increasing disilane concentration and approaches lO−5(ohm-cm)−l at disilane concentrations higher than about 4%, and the conductivity ratio is about lO5. The density of gap states in CVD a-Si:H films have been determined by the photothermal deflection spectroscopy, capacitancetemperature, capacitance-frequency, and space-charged-limited current measurements with similar results.
The use of a separated chamber deposition system for the fabrication of a-Si:H solar cells from disilane at high deposition rates results in a substantial improvement in short circuit current compared to that obtained from a single-chamber system. The spectral responses of cells fabricated in the dual-chamber mode are compared to those made in the single-chamber mode. The results are interpreted by assuming that the rate of removal of boron contaminants from the chamber is independent of deposition rate.
The low temperature deposition of a-Si:H films and devices by mercurysensitized photo-decomposition of silanes is described. The problem of window deposition is ameliorated by the use of a perfluoropolyether coating to give deposition rates of 0.3 Å s−1. Highly photosensitive undoped layers can be produced. These films exhibit many properties in common with glow discharge films including thermal quenching of photoconductivity between 125-200°K and the Staebler-Wronski effect. A wide gap silicon carbide p layer is described, and is usefully employed in p-i-n structures to give 6% efficient solar cells. SIMS analysis of these devices shows that it is possible to achieve sharp p-i interfaces in a single chamber reactor. Deposition rates greater than 2 As−1 have recently been achieved.
As Amorphous silicon devices spread into more products, the advantages inherant in continuous processing utilizing a plastic roll substrate will assume greater importance. Advantages include cost, ease of handling, shipping weight, steady state processing conditions and feedback control available in continuous processing. A new set of problems and constraints also comes with plastic substrates. These problems include mechanical strength with flexing, substrate surface defects, differential expalsion coefficients, shrinkage, and contaminant control. These problems along with efforts to evaluate and overcome them are discussed.
Doped hydrogenated microcrystalline silicon (μc-Si:H) and fluorinated hydrogenated microcrystalline (μc-Si:F:H) films were prepared by the mercury photosensitized decomposition of a disilane-hydrogen or a difluorosilane-hydrogen gas mixture, respectively. The maximum dark conductivity and optical band gap of μc-Si:H films were respectively 20 S•cm−1 and ∼2.0 eV for n-type and 1 S•cm−1 and 2.3 eV for p-type. A higher dark conductivity as much as 50 S•cm−1 and a wide gap of 2.0 eV were obtained for n-type μc-Si:F:H. It is most significant that the gaseous ratio of hydrogen to disilane should be enhanced to obtain such a highly conductive and wide gap film. The crystallinity of the photo-deposited μc-Si:H films appeared to be improved in comparison with that of films by the conventional plasma glow discharge technique.
Excimer laser KrF (248 nm) annealing at 93 mj/cm2 and 175 mJ/cm2 has been found to recrystallize amorphous silicon on (100)Si. The major impurities introduced by excimer laser annealing are carbon, while surface roughness remains as a major problem. Channel mobilities measured on MOSFETs processed on epitaxially regrown silicon were 98-115 cm2/v.s. Leakage currents between recrystallized silicon regions were 1-2 uA/cm2.
In the tetrahedrally coordinated amorphous semiconductors the dominant defects deep in the gap are attributed to dangling bonds on the group IV atoms. These defects are commonly thought to have effective electronelectron correlation energies Ueff which are positive, although some tightbinding estimates suggest negative Ueff. Defect states near the band gap edges are invoked to account for many experimental results including the usual appearance of an Urbach absorption edge. These shallow defect states are usually attributed to strained bonds but two-fold-coordinated group IV atoms have also been suggested. The application of light of near-band-gap energies alters the deniity of paramagnetic dangling bonds. For large spin densities (ns ≥ 1017 cm−3) this increase is probably due to the creation of new defects, bui it is possible that at lower densities (ns ≤ cm−3) the rearrangement of electronic charge in existing defects is important. Impurities also contribute to the defects observed in tetrahedral amorphous semiconductors. Particular species include trapped atomic and molecular hydrogen, trapped N0−2 molecules, singly-coordinated oxygen atoms and E' centers.
We describe a new technique to determine the bulk density of localized states in the energy gap of amorphous silicon alloys from the temperature dependence of the low field conductance of n-i-n diodes. This new technique allows us to determine the bulk density of states in the centre of a device, and is very straightforward, involving fewer assumptions than other established techniques. Varying the intrinsic layer thickness allows us to measure the,density of states within approximately 400 meV of midgap.
We measured the temperature dependence of the low field conductance of an amorphous silicon alloy n-i-n diode with an intrinsic layer thjckness of 0.45 microns and deduced the density of localised states to be 3xlO16cm−3 eV−1 at approximately 0.5 eV below the bottom of the conduction band. We have also considered the high bias region (the space charge limited current regime) and proposed an interpolation formula which describes the current-voltage characteristics of these structures at all biases and agrees well with our computer simulation based on the solution of the complete system of transport equations.
The application of transient photoconductivity to the study of contacts and interfaces with a-Si:H is reviewed. The photocurrent is shown to contain three terms - one from the drift of photogenerated carriers, and two from contact and bulk effects due to the electric field induced by the drifting carriers. For different sample configurations, each of these terms can dominate, and each gives different information about a-Si:H bulk or surface electronic properties. The effects are illustrated with data from metal contacts, dielectric interfaces, doped layers and gap cell measurements.
The junction recovery technique is used to investigate recombination parameters and kinetics in thin film a-Si:H diodes. It is found that double injection occurs in most of the p-i-n type samples at sufficient forward bias and that the recovery currents are dominated by the extraction of trapped holes from the valence bandtail. The mobility-lifetime product of the recovered carriers is seen to be strongly dependent on the dopant concentration in the active layers of the devices.
The heterostructures obtained by growing a-Ge on a-Si:H and a-Si have been investigated by synchrotron radiation photoemission. We measured valence band and core level spectra on the heterostructures grown in situ under ultrahigh-vacuum conditions. A step-by-step monitoring of possible band-bending changes during the interface formation enabled us to determine unambiguously the band discontinuities. The measured values of the valence band discontinuity were 0.2 ± 0.1 eV for a-Si:H/a-Ge and 0.0 ± 0.1 eV for a-Si/a-Ge, respectively. Evidence was found for the formation of abrupt interfaces without interdiffusion.
The minority-carrier injection and series resistance effects on the electrical properties of a-Si:H Schottky barrier diodes are described. The conductivity modulation was observed, for the first time, in metal/HOMOCVD a-Si:H contacts. Its effect on capacitance-voltage characteristics are discussed. The minority-carrier injection ratio is estimated from current-voltage characteristics as a function of total forward current for different metals. It is shown that these effects cannot be neglected in the interpretation of the AC and DC measurements. The caution, therefore, must be taken when using a-Si:H diodes structures to obtain the fundamental physical parameters characterizing either the interface or bulk properties of amorphous semiconductors.
A glow-discharge deposited a-Si:H/insulator heterostructu re has been characterized by a range of measurements including optical absorption, internal photoemission, xerographic discharge and spectral dependence of photoconductivity. Efficient injection of photocarriers from a-Si:H into, and transport through, films of SiOx:N:H up to 10 μm thick has been achieved. Unlike the conventional thermal oxide on Si, no significant energy barrier to injection is fotInd in the plasma deposited heterostructure. The use of the structure as a potential xerographic device is demonstrated. A mobility lifetime product as high as 6 x 10-10 cm2/volt is found for electronsin the SiOx:N:H.
A number of new developments have occurred recently in research on the synthesis and properties of amorphous semiconductor multilayer structures (“amorphous superlattices”) since the discovery of this class of materials in 1983.1 This and more recentwork have shown that tetrahedrally bonded amorphous semiconductors can be fabricated in the form of multilayer structures, with highly uniform layers and atomically abrupt interfaces. The remarkably high degree of structural perfection in these materials on the length scale of the superlattice period (> 5A) has been demonstrated by transmission electron microscopy.
The metastable excess conductivity σ(E) observed in hydrogenated amorphous silicon (a-Si:H), that is alternately doped n- and p- type, is compared with the Staebler-Wronski effect and other metastable conductivity changes observed in compensated a-Si:H and in oxidized p- type a-Si:H respectively. We find that Dohler's model of electron-hole pair separation in the pn-junction fields cannot account for the long life of a(E) near and above 300ºK. A defect complex associated with boron having a large configurational relaxation after releasing an electron by photoexcitation is considered as an explanation for σ(E).
This paper discusses the bonding of hydrogen in a—Si,Ge:H alloy films prepared byreactive magnetron sputtering (RMS). We compare our results for H atom bonding with films produced by: (a) the glow discharge decomposition (GDD) of silane and germane mixtures, and (b) reactive diode sputtering (RDS). We discuss the energy states associated with Si and Ge atom nfeutrat angling bonds in the context of an empirical tight—binding modeL The model places the Ge atom dangling bond state deeper in the gap than the corresponding Si atom defect state. The differences between the electronic properties of GDDand RDS films, and R4S films are explained in terms of the degree of H compensation of Si and Ge atom dangling bonds.
We describe the preparation and electronic and optical properties of amorphous (Si, Ge) alloys. A—(Si, Ge):H alloys were prepared by glow discharge decomposition of SiH4 and GeH4. The bandgap was varied between 1.78 and 1.42 eV by changing the GeH4:SiH4 ratio in the gas phase. We find a distinct influence of growth temperature on electronic properties. Films grown at low temperatures (200–250C) tendto have much lower photo conductivity than films grown at higher temperatures (300–325C). The electron (μ τ) products of high temperature films are general> 1X10–7 cm2/V. We also obtain very sharp valence band tails in a—(Si, Ge):H alloys, with slopes of ∼ 40 meV. The hole (μ τ) product is generally ∼1–2X10–8 cm2/V. All these properties suffer a catastrophic decline when bandgap is reduced below about 1.5 eV.