Renate Egan

Progress in Laser-Crystallized Thin-Film Polycrystalline Silicon Solar Cells: Intermediate Layers, Light Trapping, and Metallization

Diode laser crystallization of thin silicon films on the glass has been used to form polycrystalline silicon layers for solar cells. Properties of an intermediate layer stack of sputtered SiOx/SiNx/SiOx between the glass and the silicon have been improved by reactively sputtering the SiNx layer, which result in enhanced optical and electrical performance. Light trapping is further enhanced by texturing the rear surface of the silicon prior to metallization. An initial efficiency of 11.7% with VOC of 585 mV has been achieved using this technique, which are the highest values reported for poly-Si solar cells on glass substrates. Cells suffer a short term, recoverable degradation of VOC, and fill factor. The magnitude of the degradation is reduced via the repeated thermal treatment. A selective p+ metallization scheme has been developed which eliminates the degradation altogether.

Lifetime limiting recombination pathway in thin-film polycrystalline silicon on glass solar cells

The minority carrier lifetimes of a variety of polycrystalline silicon solar cells are estimated from temperature-dependent quantum efficiency data. In most cases the lifetimes have Arrhenius temperature dependences with activation energies of 0.17–0.21 eV near room temperature. There is also a rough inverse relationship between lifetime and the base dopant concentration. Judging by this inverse law, the activation energies of the lifetimes, and the absence of plateau behavior in the lifetimes of the higher doped cells at low temperatures, it is inferred that the dominant recombination pathway involves the electronic transition between shallow states which are 0.05–0.07 eV below the conduction band and 0.06–0.09 eV above the valence band, respectively, consistent with the shallow bands in silicon dislocations. The modeled recombination behavior implies that deep levels do not significantly affect the lifetimes for most of the cells at and below room temperature.

Properties of sputtered nitride semiconductors

Abstract A comparison is reported of the properties of AlN, GaN and InN films reactively r.f. sputtered from pre-nitrided targets. All three have a densely packed (00.2) polycrystallite orientation, irrespective of the substrate used. Hydrogenic donors associated with nitrogen vacancies are found at 50 meV, 110 meV and 220 meV in InN, GaN, and AlN respectively. Compensating acceptor levels at depths between 200 and 250 meV seem to derive from M N antisite defects. Their densities, which depend on the metal ion radii, are such that they partially compensate InN, sometimes fully compensate GaN and always ovecompensate AlN.

Electron mobility in InAs1−xSbx and the effect of alloy scattering

The significance of alloy scattering on the electron mobility of InAs1−xSbx is studied in detail. Since little experimental data are available for the InAs1−xSbx compound, calculations of the electron mobility of the binary InAs and InSb semiconductors are first evaluated from the average momentum relaxation times, 〈τm〉 of the individual mechanisms (comprising ionized and neutral impurities, acoustic phonon deformation potential and piezoelectric, optical phonon deformation potential and polar) so that they can be compared to available experimental results. Having ensured that the calculated data are in good agreement with the experimental results in these binary compounds, the electron mobility of the ternary, InAs1−xSbx as a function of x at 77 K and 300 K is calculated by introducing an alloy scattering term. At 77 K, for a total doping concentration of ≂1017 cm−3, the electron mobility of InAs1−xSbx is limited by ionized impurity scattering and alloy scattering is insignificant. However, for doping concentration of ≂1015 cm−3, alloy scattering is found to be the most significant process from about x=0.1 to 0.8. At 300 K the alloy scattering reduces the total electron mobility from between a few percent at x=0.15 to a maximum of about 11% for x=0.60. It is observed that alloy scattering is greatest at x=0.15 but it still is about 11% of the total electron mobility at this composition. The alloy mobilities at 77 K for x=0.6 and x=0.15 are 1.28×106 cm2 V−1s−1 and 7.96×105 cm2 V−1s−1, respectively. The mobilities at x=0.60 are further evaluated as a function of the doping carrier concentration at 77 and 300 K. It is found that alloy scattering becomes increasingly significant for lightly doped semiconductors at low temperatures.The significance of alloy scattering on the electron mobility of InAs1−xSbx is studied in detail. Since little experimental data are available for the InAs1−xSbx compound, calculations of the electron mobility of the binary InAs and InSb semiconductors are first evaluated from the average momentum relaxation times, 〈τm〉 of the individual mechanisms (comprising ionized and neutral impurities, acoustic phonon deformation potential and piezoelectric, optical phonon deformation potential and polar) so that they can be compared to available experimental results. Having ensured that the calculated data are in good agreement with the experimental results in these binary compounds, the electron mobility of the ternary, InAs1−xSbx as a function of x at 77 K and 300 K is calculated by introducing an alloy scattering term. At 77 K, for a total doping concentration of ≂1017 cm−3, the electron mobility of InAs1−xSbx is limited by ionized impurity scattering and alloy scattering is insignificant. However, for doping co...

Defects, optical absorption and electron mobility in indium and gallium nitrides

We review the experimental evidence for the origin and location of the four native point defects in the wide gap semiconducting indium and gallium nitrides and compare then with experimental predictions. The donor triplets associated with nitrogen vacancies and the deep compensating centres ascribed to the antisite substitutional defects appear to have the greatest effect on macroscopic properties, apparently including the four luminescent bands in GaN. Calculated mobilities in InN and GaN depend principally on ionised impurity and polar-mode phonon scattering. We reconcile these results with experimental data and point out the consequences for improvements in material growth.

Dislocation scattering effects on electron mobility in InAsSb

Heteroepitaxial InAsSb is routinely prepared on InAs, InSb, or GaAs substrates under conditions favorable to dislocation formation, since the binary components are lattice mismatched by about 7.4%, and the ternary are mismatched to GaAs by between 7.2% and 14.5%, depending on composition. We here extend the description of electron scattering in InAsSb to include the effects of grain boundaries and dislocations. Comparison with experiment confirms that dislocation scattering has a strong effect on transport, while alloy scattering limits mobility in ternary samples grown with a minimum of defects.

Optical and Electronic Properties of the Nitrides of Indium, Gallium and Aluminium and the Influence of Native Defects

The nitrides of Al, Ga and In are III-V compound semiconductors with properties more closely akin to those of the II-VI system and applications problems of similar type. All three have wide, direct band gaps and relatively light, therefore mobile, electrons. Less encouragingly, native point defects appear to play a significant role in both optical and electronic properties. Both experiment and theory point to a triplet of donor states associated with the nitrogen vacancy, with deep compensating centres deriving from antisite defects. The ionic radius of the metal then seems to determine the conductivity of as-grown material, indium is reluctant to occupy nitrogen sites while aluminium does so readily and gallium is equivocal. Thus the upper donor level of InN is not depleted and n-type behaviour is always observed, the equivalent level in AIN is always overcompensated and the remaining donor levels are too deep to contribute free electrons at normal temperatures so that the material is consistently insulating. GaN may be n-type or semi-insulating since compensation ratios either side of unity appear to be possible, depending on the method of growth. In this paper we review the evidence, both optical and electrical, for the existence, nature and energetic location of the four basic point defects in each nitride, noting in particular that all four broad luminescence bands in GaN:Zn can be accounted for by the presence of nitrogen on gallium sites and of nitrogen vacancies.

Design and characterization of an adhesion strength tester for evaluating metal contacts on silicon solar cells

Strong contact adhesion is an important requirement for durable, manufacturable solar cells. Advanced contacting technologies require new methods to measure adhesion. We describe a scratch test for measuring contact adhesion that involves scanning a weighted stylus across the cell while measuring the horizontal force FD required to dislodge the contacts. FD is characteristic of the adhesive bond but independent of the contact height, stylus weight and scan speed. We observe that contact peeling depends also on the tensile strength of the metal finger. The tests provide a valuable way to assess and optimize the adhesion of metal contacts.

Hole Transport in the InSbInAs material system

Abstract We discuss the scattering mechanisms limiting hole mobility in InAs, InSb and InAsSb. Transport in the binaries is limited by ionised impurity scattering at low temperatures and phonon scattering at high temperatures and in high purity material. Dislocation scattering, at typical as-grown densities, is found to reduce mobility by only 5% in high purity InSb and to be even less significant in both InAs and the ternary where alloy scattering is the limiting mechanism.

Growth, morphology and electrical transport properties of MOCVD-grown p-InSb

p-type InSb was prepared from trimethyl indium and trimethyl antimony at temperatures between 420 and 490 degrees C. A narrow window in the growth conditions of temperature and V/III ratio was found for InSb, with non-uniform, inhomogeneous epilayer growth for conditions other than optimum. Optimum morphologies were achieved for V/III ratios between 2.2 and 2.5 and at a temperature of 450 degrees C and the use of low pressures in the growth of InSb was observed to result in an enhanced uniformity. The InSb prepared displayed type conversion over the range 70-300 K; analysis of the transport properties is then complicated by the contributions from two carriers. Calculations using Fermi-Dirac statistics were carried out, identifying the majority carrier to be holes over the entire temperature range considered. Calculations also identify an acceptor level with activation energy of 16+or-0.5 meV, an acceptor concentration of (2.65+or-0.02)*1017 cm-3, in good agreement with conductivity measurements, and a low-temperature hole mobility of 450 cm2 V-1 s-1. The detailed analysis employing Fermi-Dirac statistics is then extended to determine the temperature dependence of the Fermi level in InSb and InAs for various impurity compositions.