Investigation of acceptor levels and hole scattering mechanisms in p-gallium selenide by means of transport measurements under pressure

The effect of pressure on acceptor levels and hole scattering mechanisms in p-GaSe is investigated through Hall effect and resistivity measurements under quasi-hydrostatic conditions up to 4 GPa. The pressure dependence of the hole concentration is interpreted through a carrier statistics equation with a single (nitrogen) or double (tin) acceptor whose ionization energies decrease under pressure due to the dielectric constant increase. The pressure effect on the hole mobility is also accounted for by considering the pressure dependencies of both the phonon frequencies and the hole-phonon coupling constants involved in the scattering rates.

High-pressure electronic structure and phase transitions in monoclinic InSe: X-ray diffraction, Raman spectroscopy, and density functional theory

We have studied the crystal and electronic structure of monoclinic (MC) InSe under pressure finding a reversible phase transition to a ${\mathrm{Hg}}_{2}{\mathrm{Cl}}_{2}$-like tetragonal phase. The pressure evolution of the crystal structure was investigated by angle-dispersive x-ray diffraction and Raman spectroscopy in a diamond-anvil cell up to $30\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. From the diffraction experiments, we deduced that MC InSe becomes gradually more symmetric under pressure, transforming the crystal structure into a tetragonal one at $19.4\ifmmode\pm\else\textpm\fi{}0.5\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. This phase transition occurs without any volume change. Ra…

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.

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 and quasi-hydrostatic 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 -105 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 an…

Excitonic absorption and Urbach's tail in bismuth sulfide single crystals

The absorption coefficient of bismuth sulfide single crystals has been measured through more than four orders of magnitude and in the range of energies from 1.25 to 1.70 eV. A detailed study as a function of temperature has been carried out from 29 to 300 K. An Urbach tail for low values of absorption has been found. This tail and its temperature evolution fit the expression for ionic materials. An excitonic region appears at low temperature and the shape of the exciton peak is Gaussian, which corresponds to a strong exciton-phonon coupling. The exciton binding energy is estimated (28±3 meV) and then the energy gap at 29 K is obtained (Eg=1.523±0.003 eV). The fundamental electronic transiti…

Photoluminescence in silicon-doped n-indium selenide

Photoluminescence results on silicon-doped indium selenide are reported. The effect of temperature and excitation intensity is studied. At low temperature, free and neutral donor bound exciton peaks are observed. Above 100 K only free exciton and band-to-band photoluminescence is detected. In order to give account of the full lineshape as a function of the absorption coefficient, the Urbach absorption tail of InSe is measured. Transmission and reflection photoluminescence spectra are also compared in order to study the effect of carrier diffusion. The shape of the photoluminescence spectrum can be accounted for through a direct gap model.

Low-temperature exciton absorption in InSe under pressure.

We have investigated the effect of pressure on the lowest direct band-edge exciton of the layered semiconductor InSe by optical-absorption measurements at 10 K and for pressures up to 4 GPa. The Elliott-Toyozawa formalism is used to analyze the line shape of the exciton absorption spectra. In this way we determine the pressure dependence of the lowest direct band gap, the exciton binding energy, and the exciton linewidth. The band gap exhibits a pronounced nonlinear shift with pressure, which is a consequence of the strong anisotropy of intralayer and interlayer bonding. The exciton binding energy decreases with pressure, mainly due to the large increase of the low-frequency dielectric cons…

Crystal symmetry and pressure effects on the valence band structure ofγ-InSe andε-GaSe: Transport measurements and electronic structure calculations

This paper reports on Hall effect and resistivity measurements under high pressure up to 3--4 GPa in $p$-type $\ensuremath{\gamma}$-indium selenide (InSe) (doped with As, Cd, or Zn) and $\ensuremath{\epsilon}$-gallium selenide (GaSe) (doped with N or Sn). The pressure behavior of the hole concentration and mobility exhibits dramatic differences between the two layered compounds. While the hole concentration and mobility increase moderately and monotonously in $\ensuremath{\epsilon}$-GaSe, a large increase of the hole concentration near 0.8 GPa and a large continuous increase of the hole mobility, which doubled its ambient pressure value by 3.2 GPa, is observed in $\ensuremath{\gamma}$-InSe.…

Shallow-donor impurities in indium selenide investigated by means of far-infrared spectroscopy.

Shallow impurities in n-type indium selenide (InSe) have been investigated by means of Fourier-transform spectroscopy in the far-infrared region. Three electric dipole transitions have been identified: 1s-2${\mathit{p}}_{\ifmmode\pm\else\textpm\fi{}}$, 1s-2${\mathit{p}}_{0}$, and 1s-3${\mathit{p}}_{\ifmmode\pm\else\textpm\fi{}}$, corresponding to electrons bound to native donors and tin-, silicon-, and chlorine-related donors, whose ionization energies (17.6, 18.1, 18.8, and 19 meV, respectively) have been determined through the Guerlach-Pollmann model. That model was also used to calculate the oscillator strengths of those dipole transitions, and then to estimate the shallow-donor concentr…

Effects of pressure and temperature on the dielectric constant of GaS, GaSe, and InSe:  Role of the electronic contribution

In this work we report on direct measurements of the temperature and pressure dependences of the low-frequency dielectric constant along c axis $({\ensuremath{\varepsilon}}_{\ensuremath{\parallel}})$ of GaS, GaSe, and InSe. The temperature dependence of both the ordinary and extraordinary refractive indexes is also presented. A large increase of ${\ensuremath{\varepsilon}}_{\ensuremath{\parallel}}$ under pressure has been observed. In the framework of a rigid ion model, the lattice contribution to ${\ensuremath{\varepsilon}}_{\ensuremath{\parallel}}$ is shown to increase slightly under pressure, due to the change of the angle between the anion-cation bond and the layer plane. Consequently, …

Electrical and photovoltaic properties of indium‐tin‐oxide/p‐InSe/Au solar cells

Conditions for efficiency improvement and optimization in indium‐tin‐oxide/p‐indium‐selenide solar cells are discussed in this paper. This aim is achieved by using low‐resistivity p‐indium‐selenide and by incorporating a back‐surface‐field contact. This contact is insured by a p‐indium selenide/gold barrier whose rectifying behavior is explained through the complex impurity structure of p‐indium‐selenide. Electrical and photovoltaic properties of the cells are also reported. The efficiency parameters under AM1 simulated conditions have been improved up to 32 mA/cm2 for the short‐circuit current density, 0.58 V for the open‐circuit voltage, and 0.63 for the filling factor. As a result, solar…

Electron scattering mechanisms inn-type indium selenide

Electron scattering mechanisms in $n$-type indium selenide are investigated by means of the temperature dependence (4-500 K) of Hall mobility and the magnetic field dependence of Hall and magnetoresistance coefficients. The Schmid model for homopolar optical-phonon scattering can explain the temperature dependence of electron mobility above 40 K. The electron-phonon coupling constant is determined, ${g}^{2}=0.054$. The optical phonon involved in the process is identified as the ${A}_{1}^{\ensuremath{'}}$ phonon with energy 14.3 meV. The magnetic field dependence of Hall and magnetoresistance coefficients is discussed in terms of the Jones-Zener expansion.

High-pressure electrical transport measurements on p-type GaSe and InSe

We performed high-pressure Hall effect and resistivity measurements in p-type GaSe and InSe up to 12 GPa. The pressure behaviour of the transport parameters shows dramatic differences between both materials. In GaSe, the hole concentration and mobility increase moderately and continuously. In InSe, the hole mobility raises rapidly and the hole concentration increases abruptly near 0.8 GPa. The observed results are attributed to the different pressure evolution of the valence-band structure in each material. In InSe a carrier-type inversion is also detected near 4.5 GPa.

Pressure Dependence of the Low-Frequency Dielectric Constant in III-VI Semiconductors

In this work we report on the pressure dependence of the low-frequency dielectric constant parallel to the c-axis (e∥) in GaS, GaSe, and InSe as obtained from direct capacitance measurements. A large increase of e∥ with pressure has been observed. The pressure change of the lattice polarizability along the c-axis is calculated in the framework of a rigid-ion model from the change of the angle of the anion–cation bond with respect to the layer plane, which results in a slight increase of the lattice contribution. Consequently, the pressure behaviour of e∥ is proposed to arise from the large increase of the electronic polarizability along the c-axis. This is explained through a decrease of th…

Investigation of the local structure of As-related acceptor centres in InSe by means of fluorescence-detected XAS

In this paper we report on fluorescence-detected XAS measurements in arsenic-doped p-type indium selenide. Absorption spectra taken at the arsenic K-edge allow for an unambiguous determination of the bond length of the As atoms with their first neighbours that turns out to be 2.56 ± 0.02 A. The XANES spectrum and the bond length (as determined from EXAFS oscillations) exhibit weak anisotropy as the angle between the crystal c-axis and the x-ray polarization vector is changed. These results are fully compatible with the model of a single substitutional acceptor (As in Se site), with no bonds along the c-axis (that would give rise to strong linear dicroism in the XANES spectrum). The In–As bo…

Indium Selenide Solar Cells

Indium selenide photovoltaic devices are described. Transport and photovoltaic properties are discussed. Efficiencies of 6% are reported and ways of improvement are proposed.

Electrical Resistivity Anisotropy of Silicon-Doped n-Indium Selenide

Optical and photovoltaic properties of indium selenide thin films prepared by van der Waals epitaxy

Indium selenide thin films have been grown on p-type gallium selenide single crystal substrates by van der Waals epitaxy. The use of two crucibles in the growth process has resulted in indium selenide films with physical properties closer to these of bulk indium selenide than those prepared by other techniques. The optical properties of the films have been studied by electroabsorption measurements. The band gap and its temperature dependence are very close to those of indium selenide single crystals. The width of the fundamental transition, even if larger than that of the pure single crystal material, decreases monotonously with temperature. Exciton peaks are not observed even at low temper…

High-pressure, high-temperature phase diagram of InSe: A comprehensive study of the electronic and structural properties of the monoclinic phase of InSe under high pressure

We report on an investigation of the high-pressure, high-temperature phase diagram of InSe. We optically observed the phase transition from the rhombohedral polytype (InSe-I) to the monoclinic phase (InSe-II) and determined the phase boundary up to $10\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. High-pressure resistivity measurements were performed to complement the optical measurements. Monoclinic and cubic InSe (InSe-III) were observed to be metastable around $14.5\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and $420\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and evidence suggesting the existence of an as yet unidentified new high-pressure and high-temperature phase was found. By means of optical ab…

High pressure and high magnetic field behaviour of free and donor-bound-exciton photoluminescence in InSe

We report here first magneto-photoluminescence investigations under high pressure up to 6 GPa on III-VI layered semiconductor InSe. Both diamagnetism and magnetic field induced gap opening driven by Landau quantization became observable by using a 60 T pulsed magnet. The pressure-induced enhancement of the diamagnetic coefficient is consistent with the increase of the dielectric constant under pressure while the evolution of the linear coefficient is consistent with a slight increase of the electron effective mass up to 4 GPa and a direct-to-indirect conduction-band crossover around that pressure.

Three-dimensional electrons and two-dimensional electric subbands in the transport properties of tin-dopedn-type indium selenide: Polar and homopolar phonon scattering

Electron-scattering mechanisms in n-type indium selenide doped with different amounts of tin are studied by means of the Hall effect (30--300 K) and photo-Hall effect (300 K). The electron mobility at room temperature is found to increase with the free-electron concentration in samples with low tin content. The same behavior is observed when the electron concentration increases due to thermal annealing or photogeneration. That is explained through the presence of two kinds of free electrons contributing to the charge transport along the layers: high-mobility three-dimensional (3D) electrons in the conduction band, and low-mobility two-dimensional electrons in the electric subbands. These 2D…

GaS and InSe equations of state from single crystal diffraction

We have performed single crystal angle dispersive X-ray diffraction at high pressure in order to investigate the GaS and InSe equations of state. We situate the transition from β-GaS to GaS-II at 2 7 0 3. ± . GPa. In the InSe experiment we locate the beginning of the phase transition at 7.6 ± 0.6 GPa. The equations of state of β-GaS ( 0 43 27 0 06V = . ± . Å 3 , 37 2 GPaB = ± , 5 2B = .¢ ), GaS-II ( 0 42 4 0 2V = . ± . Å 3 , 50 3 GPaB = ± and 4 3 0 3B = . ± .¢ ) and γ-InSe ( 0 58 4 0 2V = . ± . Å 3 , 24 3 GPaB = ± and 8 6 0 8B = . ± .¢ ) are discussed and compared with the results of an ab-initio calculation.

Tin-related double acceptors in gallium selenide single crystals

Gallium selenide single crystals doped with different amounts of tin are studied through resistivity and Hall effect measurements in the temperature range from 30 to 700 K. At low doping concentration tin is shown to behave as a double acceptor impurity in gallium selenide with ionization energies of 155 and 310 meV. At higher doping concentration tin also introduces deep donor levels, but the material remains p-type in the whole studied range of tin doping concentrations. The deep character of donors in gallium selenide is discussed by comparison of its conduction band structure to that of indium selenide under pressure. The double acceptor center is proposed to be a tin atom in interlayer…

Deep Levels in Silicon Doped n-Indium Selenide

Effect of Pressure on Direct Optical Transitions of ?-InSe

We have investigated the effect of hydrostatic pressure on direct optical transitions of the layered semiconductor γ-InSe by photoreflectance (PR) spectroscopy (T = 300 K). In addition, electroreflectance (ER) measurements were performed at ambient pressure. Six structures are resolved in the ER spectra in the energy range from 1.1 to 3.6 eV. The pressure dependence of four of these structures was determined by PR spectroscopy for pressures up to 8 GPa. In order to assign the features observed above the fundamental gap we have carried out band structure calculations for InSe at ambient pressure using a full-potential linear augmented plane wave method. Based on calculated band gap deformati…

Positrons and electron-irradiation induced defects in the layered semiconductor InSe

The positron annihilation characteristics of the layered semiconductor InSe have been investigated. No evidence for low temperature positron trapping is found in as-grown and heavily deformed InSe. The temperature dependence of the S-parameter in these sample exhibits an increase rate in good agreement with the linear expansion coefficient along the c-axis. The positron lifetime spectra of electron-irradiated 0.01% Sn-doped InSe show a long-lifetime component of 336 ps which is tentatively attributed to positrons trapped at isolated In vacancies. Isochronal annealing experiments performed on these samples show that the recovery of the positron lifetime measured at 77K is accomplished in two…

Transport properties of silicon doped n-indium selenide

Hall effect and resistivity measurements in silicon doped indium selenide (InSe), from 7K to 500K, are reported. Results are interpreted through a model, previously proposed for tin doped InSe, that takes into account the contribution of both three- and two-dimensional electrons to charge transport along the layers in InSe.

Transport measurements in InSe under high pressure and high temperature: shallow-to-deep donor transformation of Sn related donor impurities

We have investigated the temperature dependence of the transport parameters of Sn-doped InSe at different pressures, up to 2.5 GPa. A noticeable change in the temperature dependence of all the transport parameters has been observed above 1.2 GPa. This fact is explained by assuming the transformation of Sn shallow donors into deep donors at a hydrostatic pressure of 1.1 GPa, and by taking into account the transfer of electrons from the absolute minimum to higher energy minima in the conduction band. At ambient pressure, the position of the Sn deep level is estimated to lie 75 ± 20 meV above the absolute conduction-band minimum.

Transport properties of nitrogen doped p‐gallium selenide single crystals

Nitrogen doped gallium selenide single crystals are studied through Hall effect and photoluminescence measurements in the temperature ranges from 150 to 700 K and from 30 to 45 K, respectively. The doping effect of nitrogen is established and room temperature resistivities as low as 20 Ω cm are measured. The temperature dependence of the hole concentration can be explained through a single acceptor‐single donor model, the acceptor ionization energy being 210 meV, with a very low compensation rate. The high quality of nitrogen doped GaSe single crystals is confirmed by photoluminescence spectra exhibiting only exciton related peaks. Two phonon scattering mechanisms must be considered in orde…

High‐temperature behavior of impurities and dimensionality of the charge transport in unintentionally and tin‐doped indium selenide

A systematic study of the electron transport and shallow impurity distribution in indium selenide above room temperature or after an annealing process is reported by means of far‐infrared‐absorption and Hall‐effect measurements. Evidences are found for the existence of a large concentration of deep levels (1012–1013 cm−2), related to impurities adsorbed to stacking faults in this material. Above room temperature impurities can migrate from those defect zones and then become shallow in the bulk. The subsequent large increase of 3D electrons can change the dimensionality of the electron transport, which in most cases was 2D. The temperature dependence of the resistivity parallel to the c axis…

Transport measurements under pressure in III–IV layered semiconductors

PACS 61.50.Ks, 62.50.+p, 72.15.Jf, 72.80.Jc This paper reports on Hall effect, resistivity and thermopower effect measurements under high pressure up to 12 GPa in p-type γ-indium selenide (InSe) and e-gallium selenide (GaSe). The paper focuses on two applications of transport measurements under pressure: electronic structure and phase transition studies. As concerns the electronic structure, we investigate the origin of the striking differences between the pressure behaviour of transport parameters in both layered compounds. While the hole concentration and mobility increase moderately and monotonously in e-GaSe up to 10 GPa, a large increase of the hole concentration at near 0.8 GPa and a …

Effects of Conduction Band Structure and Dimensionality of the Electron Gas on Transport Properties of InSe under Pressure

We report Hall effect and resistivity measurements in InSe under pressure. The electron concentration strongly decreases under pressure in samples exhibiting 3D transport behaviour. This is explained by the existence of an excited minimum in the conduction band moving to lower energies under pressure. The related impurity level traps electrons as it reaches the band gap and approaches the Fermi level. In samples exhibiting 2D behaviour the electron concentration remains constant. This behaviour, together with the pressure dependence of the Hall mobility, is consistent with a previous model which considers high mobility 3D electrons and low mobility 2D electrons to contribute to charge trans…

Buildup and structure of theInSe∕Ptinterface studied by angle-resolved photoemission and x-ray absorption spectroscopy

The atomic structure and the electronic nature of the $\mathrm{In}\mathrm{Se}∕\mathrm{Pt}$ interface have been studied by x-ray absorption spectroscopy and angle-resolved photoemission, respectively. By these measurements, it has been found that Pt atoms equally incorporate into two trigonal-prismatic intralayer positions existing within the InSe layer, although, at low Pt coverage, Pt atoms seem to prefer one of these sites, where they have a lower interaction with Se atoms. The atomic structure of the $\mathrm{In}\mathrm{Se}∕\mathrm{Pt}$ interface appears to determine its electronic behavior as Pt deposition increases. At initial stages of Pt diffusion, isolated Pt atoms act as a surface …