GaSe Single Crystals Research Articles

Photoluminiscence and absorption of emission by phonons of indirect transitions in layered GaS single crystals

Absorption in E||b and E⊥c polarizations and photoluminescence at the absorption edge were studied at temperatures range 10–300 K. It was found that the smallest indirect transitions take place from the minimum of conduction band C1 (point M) to the maximum of valence band V1 in the Brillouin zone center (point Γ or nearby). A distance between these extrema is 2.4484 eV at 10 K. In addition, the magnitude of indirect transitions from the second band C2 (point M) to valence band V1 (Brillouin zone center) is 2.4912 eV at 10 K. The splitting of C1–C2 bands in M point of the Brillouin zone is equal to ∼43 meV. The indirect transitions from third conduction band C3 (point K) to valence band V1 in the nearby of Γ point are equal to 2.7683 eV at 10 K. The splitting of conduction bands C2 (point M) and C3 (point K) is higher than one for the splitting C1–C2. It was observed bands associated with self-absorption of photoluminescence emission by phonons with symmetries E1g1+A1g2, A1g2+A1g1 and A1g2+E2g2 involved in indirect transitions.

The Wannier-Mott Exciton, Bound Exciton, and Optical Phonon Replicas of Single-Crystal GaSe

We report the absorption and photoluminescence spectra of GaSe single crystals in the near-edge region. The temperatures explored the range from 17 to 300 K. Specifically, at a temperature of 17 K, the photoluminescence spectrum reveals an interesting phenomenon: the Wannier-Mott exciton separates into two states. These states are a triplet state with an energy of 2.103 eV and a singlet state with an energy of 2.109 eV. The energy difference between these two states is 6 meV. Furthermore, the bound exciton (BX) can be localized at an energy of 2.093 eV. It is worth noting that its phonon replicas (BX-nLO) can be clearly distinguished up to the fourth order. Interestingly, the energy gaps between these replicas exhibit a consistent spacing of 7 ± 0.5 meV. This intriguing finding suggests a high-quality crystalline structure as well as a strong coupling between the phonon and BX-nLO. Additionally, at low temperatures, both the ground state (n = 1) at 2.11 eV and the excited state (n = 2) at 2.127 eV of free excitons can be observed.

Composition and Surface Optical Properties of GaSe:Eu Crystals before and after Heat Treatment.

This work studies the technological preparation conditions, morphology, structural characteristics and elemental composition, and optical and photoluminescent properties of GaSe single crystals and Eu-doped β-Ga2O3 nanoformations on ε-GaSe:Eu single crystal substrate, obtained by heat treatment at 750-900 °C, with a duration from 30 min to 12 h, in water vapor-enriched atmosphere, of GaSe plates doped with 0.02-3.00 at. % Eu. The defects on the (0001) surface of GaSe:Eu plates serve as nucleation centers of β-Ga2O3:Eu crystallites. For 0.02 at. % Eu doping, the fundamental absorption edge of GaSe:Eu crystals at room temperature is formed by n = 1 direct excitons, while at 3.00 at. % doping, Eu completely shields the electron-hole bonds. The band gap of nanostructured β-Ga2O3:Eu layer, determined from diffuse reflectance spectra, depends on the dopant concentration and ranges from 4.64 eV to 4.87 eV, for 3.00 and 0.05 at. % doping, respectively. At 0.02 at. % doping level, the PL spectrum of ε-GaSe:Eu single crystals consists of the n = 1 exciton band, together with the impurity band with a maximum intensity at 800 nm. Fabry-Perrot cavities with a width of 9.3 μm are formed in these single crystals, which determine the interference structure of the impurity PL band. At 1.00-3.00 at. % Eu concentrations, the PL spectra of GaSe:Eu single crystals and β-Ga2O3:Eu nanowire/nanolamellae layers are determined by electronic transitions of Eu2+ and Eu3+ ions.

Highly Sensitive Photodetector Based on the n-Si/p-GaSe Vertical Heterojunction

Photodetectors based on two-dimensional materials and their van der Waals (vdW) heterojunctions usually have excellent optoelectronic performance and great potential applications. Herein, gallium (Aladdin, particles, 99.999% trace metal basis), selenium (Aladdin, powder, 99.999% trace metal basis), and iodine (Alfa Aesar, 99.999% trace metal basis) are the raw materials for the growth of gallium selenide (GaSe) single crystals by the chemical vapor transport method, in which iodine is the transport agent. Subsequently, GaSe nanoflakes are exfoliated from the grown bulk GaSe crystals by mechanical exfoliation. The n-Si/p-GaSe van der Waals vertical heterojunctions have been designed and fabricated for photodetection, which show good characteristics including a high responsivity of 1.74 A/W and a fast response/recovery time of 48 μs/88 μs under zero bias owing to their type-II band structure and built-in electric field. The results indicate that n-Si/p-GaSe vdW vertical heterojunctions are promising candidates in future ultrafast optoelectronic devices.

Thermoluminescence characteristics of GaSe and Ga2Se3 single crystals

GaSe and Ga2Se3 are semiconducting compounds formed from same constituent elements. These compounds have been attractive due to their optoelectronic and photovoltaic applications. Defects take remarkable attention since they affect quality of semiconductor devices. In the present paper, deep defect centers in GaSe and Ga2Se3 single crystals grown by Bridgman method were reported from the analyses of thermoluminescence measurements performed in the 350–675 K range. Experimental TL curves of GaSe and Ga2Se3 single crystals presented one and two overlapped peaks, respectively. The applied curve fitting and initial rise techniques were in good agreement about trap activation energies of 0.83 eV for GaSe, 0.96 and 1.24 eV for Ga2Se3 crystals. Crystalline structural properties of the grown single crystals were also investigated by x-ray diffraction measurements. The peaks observed in XRD patterns of the GaSe and Ga2Se3 crystals were well-consistent with hexagonal and zinc blende structures, respectively.

Biintercalate layered heterostructure: synthesis conditions and physical properties

The biintercalation of the layered GaSe semiconductor is carried out by ferroelectric and ferromagnetic guest components. Due to the separation of guest components, the GaSe <NaNO2+FeCl3> nanohybrid has a spatial-scale hybridity, which is due to the alternation of nanoscale regions of one phase with meso- or microdimensions of another. The results of electrical conductivity studies by impedance spectroscopy indicate a 250-fold increase after biintercalation of the GaSe single crystal, due to delocalized current carriers. Confirmation of a significant change in the impurity energy spectrum after biintercalation was obtained by the method of thermally stimulated discharge—GaSe nanohybrid <NaNO2+FeCl3> is characterized by a quasi-continuous spectrum in the entire temperature range of measurements and relaxation of the heterocharge. The GaSe <NaNO2+FeCl3> nano-hybrid is characterized by a high dielectric constant while a tangent of the dielectric loss angle is less than 1 in the high- frequency region of the spectrum. That opens the prospect of its use for the manufacture of high-quality radio- frequency capacitors. Changes in the impurity energy spectrum are investigated for low temperatures in the virtual crystal model, taking into account the Fivazov dispersion law both for the conductivity band and for the two impurity bands. The appearance of an additional gap in the spectrum of impurity states is established and its shift is investigated depending on the concentration of intercalants of different nature—intercalant-acceptor type and donor.

PHOTOLUMINESCENCE OF UNDOPED AND Yb-DOPED GaS SINGLE CRYSTALS IRRADIATED WITH γ-QUANTA

The infuence of γ-radiation with a dose Dγ = 140 krad on the photoluminescence of Yb-doped GaS single crystals was investigated. Based on the analysis of the experimental results, it is concluded that the doping of the rare-earth element Yb and γ-radiation leads to an increase in the intensity of photoluminescence radiation in GaS.

Impact of sulfur doping on broadband terahertz emission in gallium selenide single crystals via optical rectification

The impact of sulfur doping on broadband terahertz emission in GaSe single crystals was investigated by optical rectification with oo-o phase matching. The maximum peak value of THz radiation generated from S-doped 2.5 mass% GaSe is 28.3% larger than that of intrinsic GaSe with pump fluence of 637 mW cm−2 and azimuthal angle of 0o. The saturation of THz emission in S-doped 2.5 mass% GaSe is increased compared with that of intrinsic and S-doped 7 mass% GaSe. The results demonstrate that GaSe with appropriate S-doping concentration can effectively improve the broadband THz radiation and saturation.

Interference of exciton-polariton waves in GaSe nanocrystals

Absorption, reflection, photoluminescence and wavelength modulation transmission spectra of GaSe single crystals were investigated in temperature rage 300 – 10 K. From fitting of contours of excitonic reflection spectra parameters of excitons as energy of transversal exciton ω0 = 2.1212 eV, longitudinal transversal splitting ωLT = 2.3 meV, effective mass M = 2.5m0, background permittivity εb= 5.2 and damping constant γ = 1.4 were determined. Effective masses of electrons (mC1 = 0.37m0) and holes (mV1 = 2.13m0) in the Brillouin zone center were estimated. An interference of waves of upper and lower branches of exciton-polaritons was found out in wavelength modulation transmission spectra of GaSe nanocrystals.

Single crystal growth of GaSe via bismuth flux method and its air-stability

Gallium selenide (GaSe) shows potential applications in photoelectric fields. Herein, we successfully synthesized GaSe single crystal with large size via improved Bi-flux method. X-ray diffraction pattern and energy dispersive spectrum show the high crystallinity and high homogeneity of GaSe crystal. Optical absorption and photoluminescence measurements illustrate the band gap of the crystal is ~ 2 eV. Photocurrent behavior demonstrates the photoelectric response time is determined to be 2.08 s. The time dependent Raman spectrum shows that GaSe grown by Bi-flux method degenerates slowly under continuous air-exposure. This effective new growth method would be helpful for photoelectric device and two-dimensional materials study based on GaSe.

Investigation of traps distribution in GaS single crystals by thermally stimulated current measurements

Thermally stimulated current (TSC) investigations of p-GaS (gallium sulfide) single crystals grown by Bridgman method were achieved by virtue of consecutive experiments carried out at various heating rates in between 0.4 and 1.0 K/s in the temperature range of 10–280 K. One single TSC peak around 148 K and overlapped, incomplete peaks in the end limit temperature of the experiments were observed in the spectrum recorded at constant heating rate of 1.0 K/s. Individual peak was analyzed utilizing curve fitting method. Existence of one trapping level centered at 0.11 eV was revealed by the analyses. Heating rate dependency of obtained TSC curve was also studied and it was shown that TSC intensity decreased besides increase of peak maximum temperature with heating rate. Characteristics feature of trapping mechanism was investigated in detail by employing different stopping temperature between 50 and 110 K. Analyses on Tm–Tstop dependency resulted in a presence of quasi-continuously distributed traps with activation energies ranging from 0.11 to 0.55 eV. The revealed trap was thought to be arising from intrinsic defect possibly created by VGa or antisite SGa.

Analysis of temperature dependent electrical characteristics of Au/GaSe Schottky barrier diode improved by Ce-doping

In this paper, structural and electrical properties of p-type undoped and Ce-doped (0.1 at%) GaSe single crystals grown by using modified Bridgman method were investigated. The XRD analysis showed that both undoped and Ce-doped GaSe single crystals had hexagonal structure with preferred orientation along (004). The Schottky diodes were realized by Au contacts on GaSe wafers. The electrical characteristics of the Ce-doped GaSe device based on the current-voltage were found to be better compared to the undoped GaSe device. The electrical measurements were carried out in the temperature interval of 100-360 K. The temperature dependent ideality factor n, the barrier height Φb, the saturation current I0, the series resistance Rs and the interface states density Nss, were determined by using the thermionic emission (TE) theory. Based on this theory, it was observed that the ideality factor and series resistance decreased while the barrier height and the saturation current increased with increasing temperature. From the plot of Φb vs. 1000/T, temperature dependent I–V characteristics of Au/GaSe:Ce Schottky diode revealed the existence of a double Gaussian distribution with mean barrier height values of 1.35 and 0.83 eV and standard deviation (σ0) of 161 and 96 meV, respectively. The Norde method was used to determine the series resistance and it was observed that the series resistance decreased with increasing temperature.

Particle-Catalyst-Free Vapor-Liquid-Solid Growth of Millimeter-Scale Crystalline Compound Semiconductors on Nonepitaxial Substrates.

Direct growth of single-crystal compound semiconductors on nonepitaxial substrates is a promising route for device processing simplification in electronic and optoelectronic applications. However, the nonepitaxial growth technique for 2D single crystals is still a fundamental challenge. Here, we demonstrate that the macroscopic 2D interface of liquid metals and nonepitaxial solid substrates could be universally designed for the chemical vapor deposition growth of crystalline compound semiconductors. By adopting a sandwiched solid metal/liquid metal/solid substrate environment, millimeter-scale 2D GaS, 2D GaSe, and 1D GaTe single crystals of high quality were synthesized at the interface of liquid gallium and nonepitaxial substrates. Evidence shows that the particle-catalyst-free vapor–liquid–solid growth is driven by screw dislocations. Furthermore, we successfully extend the growth strategy to various metal chalcogenides (Sn, In, Cu, and Ag) and pnictides (Sb). Our work opens up a new route for the direct growth of single-crystalline compound semiconductors on nonepitaxial substrates.

Current-transport mechanisms in the Au/GaSe:Nd Schottky contact

Structural and electrical features of p-type neodymium-doped GaSe single crystal (0.1 at.% Nd) grown by modified Bridgman technique was investigated through X-ray diffraction (XRD) and current–voltage (I–V) measurements. The XRD spectrum reveals that GaSe:Nd single crystal investigated has hexagonal structure (a = 3.750 A, c = 15.950 A and z = 4, P63/mmc space group) with preferred orientation along (004). Ohmic contact was realized by evaporating indium (In) on one surface of the GaSe:Nd single crystal at 10–6 Torr while Shottky contact was obtained by evaporating twelve Au dot contacts with 7.85 × 10–3 cm2 area on the other surface of the crystal. The I–V characteristics of Au/GaSe:Nd/In Schottky contact was analysed in the 100–360 K temperature range. The main Schottky diode parameters such as ideality factor, barrier height and the series resistance values were determined as a function of temperature using conventional I–V method and Norde method. The ideality factor n of the Au/GaSe:Nd/In Schottky contact was observed to decrease while the barrier height $$\Phi_{\rm b}$$ increased with increasing temperature. Temperature dependence of the diode parameters were attributed to the existence of barrier height inhomogeneity by assuming a Gaussian distribution of Au/GaSe:Nd/In Schottky contact.

Charge Transport in Layer Gallium Monosulfide in Direct and Alternate Electric Fields

It was established that the variable-range-hopping mechanism of dc-conductivity takes place in the GaS single crystal at low temperatures. The parameters of localized states in the forbidden gap of studieGaS samples have been evaluated. Dielectric properties (loss tangent, real (e') and imaginary (e'') parts of complex dielectric permittivity and ac-conductivity across the layers of the GaS layered single crystals have been studied in the frequency range f = 5 × 104 to 3.5 × 107 Hz. The results demonstrate that the dielectric dispersion in the GaS single crystal has a relaxation nature. Over the studied frequency range, the ac-conductivity of the GaS crystal varies as f0.8, characteristic of hopping conduction through localized states near the Fermi level. The Fermi-level density of states, the spread of their energies, and the mean hop distance and time have been estimated.

Mechanical characteristics of gallium sulfide crystals measured using micro- and nanoindentation

Gallium sulfide single crystal is a potential material for nonlinear optics. Due to its high non-linearity it can be used as a material for IR and THz frequency converters. In the present work GaS bulk crystals of precise stoichiometry and high quality were melt grown using vertical zone melting. Elementary content of the crystals was tested using electron probe microanalysis, phase composition and unit cell parameters were studied using X-ray powder diffraction (XRD), morphology of the cleavage plane and layered structure of a cross section normal to the growth axis were studied using light microscopy, morphology of impression under an indenter tip was imaged by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Various mechanical properties, such as hardness, Young modulus of nanoindentation, residual indentation depth, contributions of elastic and plastic parts to total deformation were studied in the nanoindentation experiments. Experiments on microindentation of GaS single crystals were carried out for comparison. It was found that nanoindentation hardness and elasticity modulus depend non-monotonously on peak load. The results confirm that despite of their layered structure, GaS crystals have high micro- and nanohardness and plastic deformation ability, which provide their suitability to be processed for optical elements.

Linear and nonlinear absorption, SHG and photobleaching behaviors of Dy doped GaSe single crystal

The linear absorption, nonlinear absorption (NA), second harmonic generation (SHG) and carrier mobilities of undoped and 0.1 at% Dy doped GaSe single crystals grown by modified Bridgman method have been studied by UV–vis, open aperture Z-scan, SHG and transmission ultrafast pump probe spectroscopy experiments. Both linear absorption and SHG measurements clearly showed that the doping of GaSe crystal with Dy3+ leads to a shift in the linear absorption edge. The mechanisms contributing to the NA behavior were explained and discussed. The photobleaching behaviors of the crystals were investigated by delaying of the probe signal in transmission pump-probe spectroscopy experiments.

Nonlinear absorption, SHG behavior and carrier dynamics of Nd and Pr doped GaSe single crystals

Undoped, 0.1 at% Nd and 0.1 at% Pr doped GaSe single crystals were grown by using modified Bridgman method. The nonlinear absorption (NA), second harmonic generation (SHG) and carrier dynamics of undoped, 0.1 at% Nd and 0.1 at% Pr doped GaSe single crystals have been studied by open aperture Z-scan, SHG and transmission ultrafast pump probe spectroscopy experiments, respectively. All of the studied crystals showed nonlinear absorption under 100 fs pulse duration and 1200 nm wavelength excitations. It was found that the doping of Nd3+ and Pr3+ into GaSe single crystals caused shifting of SHG signal peaks to the shorter wavelengths. However, transmission pumb-probe experiments revealed that all of the crystals showed NA signals with long lifetime under 620 nm pump and 626 nm probe excitations.

High-Quality GaSe Single Crystal Grown by the Bridgman Method.

A high-quality GaSe single crystal was grown by the Bridgman method. The X-ray rocking curve for the studied GaSe sample is symmetric and the Full Width at Half Maximum (FWHM) is only 46 arcs, which is the smallest value ever reported for GaSe crystals. The IR-transmittance is about 66% in the range from 500 to 4000 cm−1. The photoluminescence spectrum at 9.2 K shows a symmetric and sharp excition peak in 2.1046 eV. The results indicate that the as-grown GaSe crystal is of high crystalline quality. The as-grown -GaSe crystal has a p-type conductance with the resistivity of 103 Ω/cm, and the Hall mobility is ~25 cm2 V−1 s−1. Few-layer GaSe crystals were prepared through mechanical exfoliation from this high-quality crystal sample. Few-layer GaSe-based photodetectors were fabricated, which exhibit an on/off ratio of 104, a field-effect differential mobility of 0.4 cm2 V−1 s−1, and have a fast response time less than 60 ms under light illumination.

Optical Properties of Boron-Doped Gallium Selenide

The electronic-absorption and luminescence spectra and the kinetics of relaxation of photocurrent in GaSe single crystals and thin films doped with boron atoms are experimentally investigated. Crystals are doped either in the course of synthesis or upon growing single crystals by the Bridgman method. Thin GaSe films are obtained by thermal evaporation of the compound preliminarily doped with boron. An absorption band with the maximum at λ = 925 nm, which is attributed to boron contaminations, is revealed from the comparison of the electronic-absorption spectra of a GaSe crystal and thin films doped with boron. When the crystals are excited with Nd:YAG laser pulses of a duration of 12 ns, it is established that recombination of nonequilibrium current carriers in pure crystals occurs through rapid and slow recombination channels and in crystals doped with boron only through rapid channels. In the region near λ = 932 nm, photoluminescence is observed in the crystals doped with boron, and its half-bandwidth is 15 A.