The ability to detect light over a broad spectral range is central to practical optoelectronic applications and has been successfully demonstrated with photodetectors of two-dimensional layered crystals such as graphene and MoS2. However, polarization sensitivity within such a photodetector remains elusive. Here, we demonstrate a broadband photodetector using a layered black phosphorus transistor that is polarization-sensitive over a bandwidth from ∼400 nm to 3,750 nm. The polarization sensitivity is due to the strong intrinsic linear dichroism, which arises from the in-plane optical anisotropy of this material. In this transistor geometry, a perpendicular built-in electric field induced by gating can spatially separate the photogenerated electrons and holes in the channel, effectively reducing their recombination rate and thus enhancing the performance for linear dichroism photodetection. The use of anisotropic layered black phosphorus in polarization-sensitive photodetection might provide new functionalities in novel optical and optoelectronic device applications. The anisotropic optical properties of black phosphorus can be exploited to fabricate photodetectors with linear dichroism operating over a broad spectral range.

Polarization-sensitive broadband photodetector using a black phosphorus vertical p–n junction

The optical absorption due to surface states on ultrahigh vacuum cleaved Ge and Si surfaces has been directly measured. Results show an absorption extending to energies lower than the edge, which disappears when the cleaved surfaces are oxidized. Possible optical transitions giving rise to this absorption are discussed. It is concluded that the dominant processes are transitions between two bands of surface states located in the gap. Combining the present results with photoelectric data, the energy position of the surface bands in Si is given.

Optical Absorption of Surface States in Ultrahigh Vacuum Cleaved (111) Surfaces of Ge and Si

Photoelectron partial-yield and constant-initial-state (CIS) spectra of the layered-semiconductor GeS, GeSe, ${\mathrm{SnS}}_{0.9}$${\mathrm{Se}}_{0.1}$, and SnSe single crystals were investigated with use of synchrotron radiation. The cation d core (Ge 3d and Sn 4d) yield spectra show intense and sharp doublets due to cation-derived core excitons. The CIS spectra exhibit strong resonance due to an interference between the direct-recombination process of the core excitons and the direct-excitation process of the valence electrons. The strong resonance is characteristic of core excitons with a long lifetime and a fairly localized nature. The anion p and d core (S 2p and Se 3d) yield spectra are much broader than the cation d core spectra and reflect the density of states (DOS) of the conduction bands. Energies of the experimental DOS peaks compare favorably with conduction-band features derived from interband optical-absorption spectra.

/pdf/core-excitons-and-conduction-band-structures-in-orthorhombic-rno8pwlkwm.pdf

Core excitons and conduction-band structures in orthorhombic GeS, GeSe, SnS, and SnSe single crystals.

In this paper a number of optical spectroscopic methods for investigating surface electronic structure are discussed, including reflectance techniques, ellipsometry, surface photoconductivity and surface photovoltage spectroscopy. In addition to electron scattering techniques and UV-photoemission, optical spectroscopic methods have contributed much in recent times to the understanding of electronic surface states on solids. A discussion and comparison is given of the nature and significance of information obtained by these methods and exemplary experimental results are presented to illustrate the contribution of the optical techniques to the present knowledge about surface states. The relation between information obtained from optical measurements and electron spectroscopy is considered.

Optical spectroscopy of electronic surface states

We present a comprehensive study of bulk GeS optical properties and their anisotropy, by investigation of the fundamental band gap of the material and the energetically higher direct transitions. Optical absorption and photoreflectance measurements allowed us to establish the indirect gap of GeS at E0 = 1.56 eV at room temperature, and closely-lying direct transitions at E1 = 1.59 eV and E2 = 1.64 eV. Polarization-resolved measurements revealed that E1 is polarized along the armchair crystallographic direction and E2 is polarized along the zigzag direction. Finally, the experimental results are discussed in terms of first-principles calculations, which allowed us to assign all the optically active transitions in the Z–Γ–Y region of the Brillouin zone.

/pdf/anisotropic-optical-properties-of-ges-investigated-by-3invu5121s.pdf

Anisotropic optical properties of GeS investigated by optical absorption and photoreflectance

For potentials with a high surface barrier, quasi-classical quantization schemes are investigated. WKB and other previous approximations provide good results for the lower eigenvalues in special cases only. Here a modified local density approximation is used for the calculation of the density of states. From the integral density of states a quantization expression is derived where the ground state constant depends explicitly on the shape of the potential. It yields the whole eigenvalue spectrum very precisely for quite different potentials with surface barrier. The application to subband energies in inversion layers is discussed.



Quasiklassische Quantisierungsvorschriften werden fur Potentiale mit einer hohen Oberflachen-barriere untersucht. WKB und andere fruhere Naherungen liefern fur die niedrigen Eigenwertenur in Spezialfallen gute Ergebnisse. Hier wird eine modifizierte Naherung lokaler Dichte auf die Berechnung der Zustandsdichte angewandt. Aus der integralen Zustandsdichte wird eine Quantisierungsvorschrift abgeleitet, deren Grundzustandskonstante vom Potentialverlauf bestimmt ist. Diese Vorschrift liefert mit groser Genauigkeit das gesamte Eigenwertspektrum fur ganz unter-schiedliche Potentiale mit Oberfluchenbarriere. Die Anwendung auf Subbandenergien in Inversionsschichten wird diskutiert.

Quasi‐Classical Quantization for Potentials with an Infinite Surface Barrier. Subbands in Inversion Layers

In the local density approximation (LDA) the density of a many-electron system is expressed as a function of the spatially varying potential. Here a modified LDA is derived which takes into account a finite potential step at a plane interface so being applicable for semiconductor-insulator structures and, in many cases, also for semiconductor heterojunctions. Different modifications of the standard LDA are discussed. They are compared with one another and with the exact solution of the Schrodinger equation for a model system. The simplest modification is shown to be also the best one and to give very good results for inversion layers even in the case of relatively small potential jumps and up to field strengths of some 105 V cm−1.



In der Naherung der lokalen Dichte (LDN) wird die Dichte eines Vielelektronensystems als Funktion des Potentialverlaufs ausgedruckt. In der vorliegenden Arbeit wird eine modifizierte LDN abgeleitet, die eine endliche Potentialstufe an einer ebenen Grenzflache berucksichtigt und damit fur Halbleiter-Isolator-Strukturen und in vielen Fallen auch fur Halbleiter-Heteroubergange anwendbar ist. Verschiedene Modifikationen der Standard-LDN werden diskutiert. Sie werden unter-einander und mit der exakten Losung der Schrodingergleichung fur ein Modellsystem verglichen. Die einfachste Modifikation erweist sich zugleich als die beste und liefert ausgezeichnete Ergebnisse fur Inversionsschichten. Das ist selbst fur relativ flache Potentialstufen und bis hin zu Feldstarken von einigen 105 V cm−1 der Fall.

Modified Local Density Approximation for Potentials with a Finite Step

The experimental determination of the electron dispersion relation, E(k‖), for a quasi-two-dimen-sional system is performed by means of angle-resolved photoemission. For the layered single crystal GeS, the whole valence band is mapped using HeI, HeII, and NeI excitation sources. Simple interpretation of peak widths in terms of hole lifetimes, based on a three-step model of photoemission, is given.



Die Elektronendispersion E(k‖) fur das quasi-zweidimensionale System wird experimentell mit Hilfe der winkelaufgelosten Photoemission bestimmt. Fur den Schichtkristall GeS wird mit HeI-, HeII- und NeI-Anregungsquellen das gesamte Valenzband untersucht. Eine einfache, auf dem dreistufigen Modell der Photoemission begrundete Interpretation der Peakbreiten fur die Loch-lebensdauer wird angegeben.

Band Structure and Lifetime Determinations for GeS from Angle-Resolved Photoemission

The existence of bands of surface states in the energy gap of an infinite crystal gives rise to additional absorption of light. The characteristic features of behaviour of the absorption coefficient are discussed.



Die Existenz von Energiebandern der Oberflachenzustande in der verbotenen Zone eines unendlichen Kristalls verursacht zusatzliche Lichtabsorption. Die charakteristischen Eigenschaften der Absorptionskonstante werden diskutiert.

Optical Absorption at the Surface of a Semiconductor

On Inhomogeneous Damping of Electrons in Crystals

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