A. Segura

Photoconductivity and photovoltaic effect in indium selenide

Transport and phototransport properties of crystalline indium monoselenide (InSe) doped with a variety of elements are reported. Measured mobilities, lifetimes, and effective diffusion lengths of photoexcited carriers are used to interpret electrical and photovoltaic properties of several different structures. These include p‐n junctions, bismuth/p‐type InSe, platinum/n‐type InSe, and indium tin oxyde (ITO)/p‐type InSe. External solar efficiencies of the best devices are between 5% and 6%. The influence on the efficiency of the various parameters is evaluated, and ways of improvement are discussed.

Electronic structure, optical properties, and lattice dynamics in atomically thin indium selenide flakes

The progressive stacking of chalcogenide single layers gives rise to two-dimensional semiconducting materials with tunable properties that can be exploited for new field-effect transistors and photonic devices. Yet the properties of some members of the chalcogenide family remain unexplored. Indium selenide (InSe) is attractive for applications due to its direct bandgap in the near infrared, controllable p- and n-type doping and high chemical stability. Here, we reveal the lattice dynamics, optical and electronic properties of atomically thin InSe flakes prepared by micromechanical cleavage. Raman active modes stiffen or soften in the flakes depending on which electronic bonds are excited. A progressive blue-shift of the photoluminescence peaks is observed for decreasing flake thickness (as large as 0.2 eV for three single layers). First-principles calculations predict an even larger increase in the bandgap, 0.40 eV, for three single layers, and as much as 1.1 eV for a single layer. These results are promising from the point of view of the versatility of this material for optoelectronic applications at the nanometer scale and compatible with Si and III-V technologies.

Thin film growth and band lineup of In2O3 on the layered semiconductor InSe

Thin films of the transparent conducting oxide In2O3 have been prepared in ultrahigh vacuum by reactive evaporation of indium. X-ray diffraction, optical, and electrical measurements were used to characterize properties of films deposited on transparent insulating mica substrates under variation of the oxygen pressure. Photoelectron spectroscopy was used to investigate in situ the interface formation between In2O3 and the layered semiconductor InSe. For thick In2O3 films a work function of φ=4.3u200aeV and a surface Fermi level position of EF−EV=3.0u200aeV is determined, giving an ionization potential IP=7.3u200aeV and an electron affinity χ=3.7u200aeV. The interface exhibits a type I band alignment with ΔEV=2.05u200aeV, ΔEC=0.29u200aeV, and an interface dipole of δ=−0.55u200aeV.

Photovoltaic efficiency of InSe solar cells

Abstract Indium selenide n-type substrates made from ordinary-grade elements, are suitable to make heterojunctions with semitransparent platinum layers. Those low-cost devices have photovoltaic efficiencies for solar energy conversion under 6%, at the present time. They can be readily improved to reach external efficiencies in the 10% range.

Nanotexturing To Enhance Photoluminescent Response of Atomically Thin Indium Selenide with Highly Tunable Band Gap.

Manipulating properties of matter at the nanoscale is the essence of nanotechnology, which has enabled the realization of quantum dots, nanotubes, metamaterials, and two-dimensional materials with tailored electronic and optical properties. Two-dimensional semiconductors have revealed promising perspectives in nanotechnology. However, the tunability of their physical properties is challenging for semiconductors studied until now. Here we show the ability of morphological manipulation strategies, such as nanotexturing or, at the limit, important surface roughness, to enhance light absorption and the luminescent response of atomically thin indium selenide nanosheets. Besides, quantum-size confinement effects make this two-dimensional semiconductor to exhibit one of the largest band gap tunability ranges observed in a two-dimensional semiconductor: from infrared, in bulk material, to visible wavelengths, at the single layer. These results are relevant for the design of new optoelectronic devices, including heterostructures of two-dimensional materials with optimized band gap functionalities and in-plane heterojunctions with minimal junction defect density.

Investigation of conduction-band structure, electron-scattering mechanisms, and phase transitions in indium selenide by means of transport measurements under pressure

In this work we report on Hall-effect, resistivity, and thermopower measurements in n-type indium selenide at room temperature under either hydrostatic or quasihydrostatic pressure. Up to 40 kbar (=4 GPa), the decrease of carrier concentration as the pressure increases is explained through the existence of a subsidiary minimum in the conduction band. This minimum shifts towards lower energies under pressure, with a pressure coefficient of about -98 meV/GPa, and its related impurity level traps electrons as it reaches the band gap and approaches the Fermi level. The pressure value at which the electron trapping starts is shown to depend on the electron concentration at ambient pressure and the dimensionality of the electron gas. At low pressures the electron mobility increases under pressure for both three- and two-dimensional electrons, the increase rate being higher for two-dimensional electrons, which is shown to be coherent with previous scattering mechanisms models. The phase transition from the semiconductor layered phase to the metallic sodium chloride phase is observed as a drop in resistivity around 105 kbar, but above 40 kbar a sharp nonreversible increase of the carrier concentration is observed, which is attributed to the formation of donor defects as precursors of the phase transition.

Photovoltaic properties of GaSe and InSe junctions

SummaryPhotovoltaic and photoelectronic properties of GaSe and InSe metal-semiconductor contacts are reported. The shape of the spectra, at room temperature, can be interpreted with reasonable values for diffusion length and junction depth. The variation of the spectra with pressure in GaSe and the shape of the low-energy tail in InSe are in concordance with the indirect nature of the lower energy gaps in both compounds. A comparison of these materials with other semiconductors for photovoltaic conversion purposes is given.RiassuntoSi riportano le proprietà fotovoltaiche e fotoelettroniche nel GaSe e InSe utilizzando contatti metallo-semiconduttore. Si può interpretare la forma degli spettri a temperatura ambiente con valori ragionevoli per la lunghezza di diffusione e la profondità di giunzione. La variazione degli spettri con la pressione nel GaSe e la forma della coda a bassa energia nel InSe sono in accordo con la natura indiretta della gap a più bassa energia in entrambi i composti. Si fa un confronto di questi materiali con altri semiconduttori per scopi di conversione fotovoltaica.РезюмеРассматриваются фотогальванические и фотоэлектронные свойства контактов металл-полупроводник GaSe и InSe. Форма спектров при комнатной температуре может быть интерпретирована с помощью соответствующих величин для длины диффузии и глубины контактов. Изменение спектров с давлением в GaSe и форма низкоэнергетического хвоста в InSe согласуются с непрямой природой щелей при низких энергиях в обоих соединениях. Проводится сравнение этих материалов с друтими полупроводниками в целях использования фотогальва-нических преобразований.

Acceptor levels in indium selenide. An investigation by means of the Hall effect, deep-level-transient spectroscopy and photoluminescence

Acceptor levels related to I, II, IV, and V group impurities in indium selenide are studied by means of the Hall effect, deep-level-transient spectroscopy (DLTS) and photoluminescence. Activation energies for hole concentrations in the range from 200 to 300 meV have been measured. A reversible change of sign of the Hall voltage has been observed below 215 K. This behaviour can be explained through a model in which acceptor levels are assumed to be shallow and interlayer planar precipitates of ionized shallow donors create potential wells that behave as deep donors and in which a low concentration of bidimensional free electrons can exist. This model also explains the capacitance-voltage characteristics of both ITO/p-InSe and Au/p-InSe barriers. DLTS results are coherent with this model: hole traps in high concentration located about 570 meV above the valence band are detected. Photoluminescence also confirms the shallow character of acceptor levels. A broad band whose intensity is related to p conductivity appears in the PL spectra of low resistivity p-InSe. The shape and temperature dependence of this band can be explained through self-activated photoluminescence in a complex center in which the ground acceptor level must be at about 50 meV above the valence band.

The effect of quantum size confinement on the optical properties of PbSe nanocrystals as a function of temperature and hydrostatic pressure

A study based on photoluminescence and absorption measurements as a function of temperature and pressure for PbSe nanocrystals with sizes in the range 3-13 nm reveals the influence of size quantum confinement on the observed variation. In the case of the temperature variation, the effective bandgap changes from showing a positive rate of change to showing a negative one (for a quantum dot 3 nm in diameter), which can be accounted for by incorporating a linear variation of the carrier effective masses into a simple calculation of the exciton ground state in the quantum dot. In the case of the pressure variation, we observe a clear inverse correlation between the absolute value of the pressure coefficient and the nanocrystal size, a signature of quantum size confinement, with values changing from -76 to -41 meV GPa⁻¹ for quantum dots ranging from 13 to 3 nm in diameter, respectively, clearly smaller in absolute value than the rate for bulk PbSe (-84 meV GPa⁻¹). We used again the hypothesis of a linear variation of the carrier effective masses with pressure in order to fit this experimental variation quantitatively.

Synthesis of a novel zeolite through a pressure-induced reconstructive phase transition process.

The first pressure-induced solid-phase synthesis of a zeolite has been found through compression of a common zeolite, ITQ-29 (see scheme, Si yellow, O red). The new microporous structure, ITQ-50, has a unique structure and improved performance for propene/propane separation with respect the parent material ITQ-29.