Shallow Valence Band Research Articles

Opportunities in dilute nitride III–V semiconductors quantum confined p–i–n solar cells for single carrier resonant tunneling

Stunning theoretical efficiencies have been predicted for quantum-confined solar cells. However, practical realizations remain inefficient as these devices suffer from an inherent difficulty in the extraction of photo-generated carriers from the confined states. Leveraging on the shallow valence band offset of dilute nitrides and using a carefully chosen material system and device design, we show the possibility of circumventing this problem by separating the optimization of the valence and conduction band and reducing the issue to a single particle problem. Band structure calculations including strain effects, band anti-crossing models and transfer matrix methods are used to theoretically demonstrate optimum conditions for enhanced vertical transport. High-electron tunneling escape probability, together with a free movement of quasi-3D holes, is predicted to result in enhanced PV device performance. Furthermore, the increase in electron effective mass due to the incorporation of N translates in enhanced absorptive properties, ideal for PV application.

Unusual Properties of the Red and Green Luminescence Bands in Ga-rich GaN

ABSTRACTWe studied photoluminescence (PL) from deep-level defects in GaN grown under Garich conditions at relatively low temperatures (700–800°C) by molecular-beam epitaxy (MBE). The dominant features of PL spectrum are red and green bands peaking respectively at ∼1.8 and ∼2.35 eV. Both PL bands decay exponentially at low temperatures (15 – 100 K) after pulsed excitation. The characteristic lifetime for the red band decreases by almost two orders of magnitude from 110 to 2 μs with increasing temperature from 15 to 100 K, while its integrated intensity after each pulse remains nearly unchanged in the same temperature range due to an increase in the peak intensity in the time-resolved PL curve. The lifetime of the green band remained unchanged in this temperature range. We suggest that these PL bands are caused by transitions between excited and ground states of some deep defects rather than transitions involving a shallow donor, conduction or valence bands.

Photoemission spectral analysis of fullerene films polymerized by argon plasma treatment

Photoemission spectral analysis was carried out on plasma polymerized fullerene (C60) films prepared by sublimating C60 under an argon plasma. In the photoemission analysis, the overall distribution of the shallow valence band states of the plasma-treated C60 was compared with pristine C60. These changes suggest that the pi conjugation of the shallow valence band states of two C60 molecules is induced by polymerization during the plasma treatment. To investigate the changes in the onset of the shallow valence band states brought about by polymerization, the orbital energy levels in C60 dimers were examined using the semiempirical AM-1 method. Calculations showed that the highest occupied molecular orbital levels of the dimers were shallower than that of C60.

Separation dynamics of luminescence from Raman scattering in characteristic X-ray radiation processes of Y compounds

Abstract We theoretically study the separation dynamics of a luminescence from Raman scatterings in the spectrum of the X-ray radiation occurring by an electronic transition from a shallow core level to a deep one in insulating solids of Y compounds. We consider a many-body problem in a four-band model composed of dispersionless deep and shallow core bands, and conduction and valence bands. The Coulomb interactions among the conduction electrons and the valence holes are taken into account by the lowest-order perturbation theory. With this model, it is clearly shown that the radiation spectra have three components; a luminescence part and two Raman scattering parts. This result agrees with the recent experiments of YF 3 and YCl 3 . We also show the origin of the three components cannot be explained by a one-body picture.

Separation dynamics of characteristic X-ray radiation into luminescence and Raman scattering in wide gap insulators

Abstract We theoretically study the separation dynamics of luminescence from Raman scattering in the spectral shape of X-ray radiation from a shallow core level to a deep one in wide gap insulators. We employ a four-band model composed of dispersionless deep and shallow core bands, and conduction and valence bands. The effects of the conduction electron-valence hole pair, created by the original electron excited from the deep core level to the conduction band, are taken into account by the lowest order perturbation theory. When the excitation energy is above the absorption edge, the resultant radiation spectra separate into two parts: a luminescence part, and a Raman scattering part peaked in its higher energy side. The condition for luminescence is that the conduction and valence band widths are finite, and the lifetime of the deep core hole is long enough.

Electronic structure of ultrathin Fe films on TiO2(110) studied with soft-x-ray photoelectron spectroscopy and resonant photoemission.

We report on soft-x-ray photoelectron spectroscopy (SXPS) of a ${\mathrm{TiO}}_{2}$(110) surface during deposition of Fe in the monolayer regime. At low fractional monolayer coverages, the adsorbed Fe atoms are oxidized and Ti cations at the interface become reduced due to Fe adsorption. SXPS from shallow core levels and valence bands show that Fe starts to exhibit metallic character at a coverage of approximately 0.7 equivalent monolayers. Two well separated defect states appear in the band gap of ${\mathrm{TiO}}_{2}$ at iron coverages well below one monolayer. We use resonant photoemission to obtain information on the partial density of states, and we assign these defect states as being Fe-derived and Ti-derived states, located at the Fe and Ti sites, respectively. We suggest that a position change of oxygen is involved in the bonding of Fe on the ${\mathrm{TiO}}_{2}$(110) surface.

Metastable defect in amorphous silicon solar cells investigated by junction recovery technique

The effect of metastable defects caused by light soaking and carrier injection on the transport of carriers in undoped a-Si: H has been investigated by a junction recovery technique. The experiments show that after light soaking or carrier injection the product of μ p τ p decreases, but no detectable change in the distribution of shallow valence band tail states was found.

Developing trends in the application of XPS in the USA

AbstractSome recent developments in XPS in the United States are briefly described. Two new instruments have been introduced, one a ‘Small Spot ESCA’, providing analyses of <1 mm area with monochromatized x‐rays, the other a conveniently engineered hemispherical analyser with very high data gathering speed and good resolution. Studies of energies of Auger lines along with those of photoelectron lines elucidate the role of screening energy in chemical shifts. Neighboring molecules, and especially neighboring conductive surfaces can provide screening energies of more than an electron volt. Effects on thin films and highly dispersed phases are beginning to emerge. Accurate data on energies and energy differences of shallow core levels and valence bands are providing information on band bending and chemical shifts at interfaces, information of great value in the semiconductor industry. Low energy monochromatized photons from synchrotron sources have provided data on shifts in core levels between surface atoms and bulk atoms. Applications in surface science are just beginning.