Monoclinic GaTe Research Articles

Molecular beam epitaxy and polarized excitonic emission of layered GaTe/GaAs thin films

This paper reports on molecular beam epitaxy of GaTe thin films grown on GaAs(001) and GaAs(111)B substrates at temperatures of TS = 450–550 °C under weak Te-rich conditions (Te/Ga≈10–18) as well as studies of their structural and optical properties. The results obtained by transmission electron microscopy and X-ray diffraction techniques have established a correlation between the growth conditions and the GaTe polymorphic transition from hexagonal to monoclinic phase. The critical temperature of the polymorphic transition was found to be ∼540–550 °C. The monoclinic GaTe layers demonstrate an excellent excitonic emission that was confirmed by both micro-photoluminescence and time-resolved photoluminescence techniques. The strong anisotropy of the GaTe excitonic emission has been demonstrated.

Wavelength‐Dependent Anisotropic Optical Properties in Layered GaTe for Polarization‐Sensitive Applications

Anisotropic layered GaTe has emerged as a potential material for polarization‐sensitive applications. The optical anisotropy in monoclinic GaTe is triggered by its in‐plane structural anisotropy, as each layer comprises periodic chain‐like structures along its b‐axis crystal direction. However, there is ambiguity in identifying the b‐axis in GaTe in existing reports. Proper identification is vital toward the fabrication of polarization‐sensitive photodetectors and light sources. Herein, this ambiguity is addressed and a systematic structural and polarization‐resolved optical study to identify the b‐axis in GaTe is reported for a multitude of different sample thicknesses and laser excitation wavelengths. The diffraction and Kikuchi line pattern results demonstrate the relevance of the structural anisotropy on the observed anisotropic photoluminescence (PL) in GaTe. The near‐infrared PL from GaTe (≈1.66 eV) exhibits a minimum (maximum) intensity when the laser excitation is polarized parallel (perpendicular) to the b‐axis. The PL peak position behavior as a function of increasing sample temperature (77–298 K) and laser power density is characteristic of radiative recombination of free/bound excitons. Furthermore, a broadband visible excitation window is identified for which GaTe exhibits PL anisotropy. These results highlight the potential of layered GaTe in polarization‐ and wavelength‐sensitive applications.

Controlled Growth of Large-Sized and Phase-Selectivity 2D GaTe Crystals.

GaTe has recently attracted significant interest due to its direct bandgap and unique phase structure, which makes it a good candidate for optoelectronics. However, the controllable growth of large-sized monolayer and few-layer GaTe with tunable phase structures remains a great challenge. Here the controlled growth of large-sized GaTe with high quality, chemical uniformity, and good reproducibility is achieved through liquid-metal-assisted chemical vapor deposition method. By using liquid Ga, the rapid growth of 2D GaTe flakes with high phase-selectivity can be obtained due to its reduced reaction temperature. In addition, the method is used to synthesize many Ga-based 2D materials and their alloys, showing good universality. Raman spectra suggest that the as-grown GaTe own a relatively weak van der Waals interaction, where monoclinic GaTe displays highly-anisotropic optical properties. Furthermore, a p-n junction photodetector is fabricated using GaTe as a p-type semiconductor and 2D MoSe2 as a typical n-type semiconductor. The GaTe/MoSe2 heterostructure photodetector exhibits large photoresponsivity of 671.52 A W-1 and high photo-detectivity of 1.48 × 1010 Jones under illumination, owing to the enhanced light absorption and good quality of as-grown GaTe. These results indicate that 2D GaTe is a promising candidate for electronic and photoelectronic devices.

Growth and thermal stability studies of layered GaTe single crystals in inert atmospheres

In this study, we investigated the thermal stability properties in inert atmospheres of GaTe single crystals fabricated by a temperature gradient technique. The obtained crystals possess a monoclinic layered structure with high crystalline quality. To explore the thermal stability of GaTe, the differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were performed up to 1173 ​K in N2 and Ar atmospheres. GaTe crystals are thermally stable up to 700 ​K in Ar and stable up to 935 ​K in N2 due to the protective role of the physically adsorbed N2 molecules on GaTe surface. Annealing effect on the structural and optical properties of GaTe were examined after thermal treatment in N2 atmosphere at different temperatures; 573, 673, 773, and 873 ​K. Three prominent Raman modes at 97 ​cm−1 (Ag), 145 ​cm−1 (Ag), and 158 ​cm−1 (Bg) corresponding to the monoclinic GaTe phase almost remained, suggesting no structural damage at elevated temperatures. Especially, the photoluminescence (PL) intensity is improved as the annealing temperature increases and reaches a maximum at 673 ​K owing to the enhancement in the GaTe crystallinity. Above 873 ​K, we observed a significant reduction in the PL intensity, which is attributed to the vacancies induced by tellurium evaporation.

Lattice Dynamics and Thermodynamic Properties of Bulk Phases and Monolayers of GaTe and InTe: A Comparison from First‐Principles Calculations

AbstractThe hybrid density functional theory was used to study the structural, vibrational, and thermodynamic properties of stable and hypothetical bulk GaTe and InTe polymorphs, as well as their monolayer counterparts. Criteria based on the vibrational frequencies have been proposed to distinguish between different monolayer structures. Heat capacity, entropy, and Helmholtz free energy have been calculated by summing the vibrational contributions over the corresponding Brillouin zone. The relative stability of the considered systems has been estimated at different temperatures using the obtained Helmholtz free energy. Both the total energy and the Helmholtz free energy calculations confirmed that a free‐standing monolayer originated from the monoclinic GaTe phase is less stable than its hexagonal analogs. It was also found that the temperature increase favors monolayer formation in the case of GaTe, but prevents it in the case of InTe.

Dual phase two-color emission observed in van der Waals GaTe planes

After the eagerly study on two-dimensional (2D) electronic devices made by layered semiconductors, light-emitting devices based on 2D semiconductors have also flourished in recent years due to their special characters of flexibility, flatness surface, and thickness-tunable band gap modulation, etc. Among the 2D materials, III-VI layer compounds with hexagonal structure like GaSe and InSe are essentially direct semiconductors that own high band-edge luminescence efficiency and wide thickness tunable range from bulk to thin few-layer scale. In this work, dual-phase two-color emissions of multilayered gallium monotelluride positioned at 1.588 and 1.652 eV are simultaneously detected by micro-photoluminescence (μPL) measurement at 300 K. The lower-energy peak may originate from the hexagonal GaTe (H-GaTe) phase while the higher-energy luminescence might come from monoclinic GaTe (M-GaTe) phase and which are verified by micro-thermoreflectance (μTR) and high-resolution transmission electron microscopy (HRTEM). Micro-time-resolved photoluminescence (TRPL) and area fluorescence lifetime mapping (AFLM) of the multilayer GaTe indicated that the luminescence decay time constant of the H-GaTe is larger than that of the M-GaTe for verification of the dominant phase in multilayered GaTe is the monoclinic phase. In-plane structural and optical anisotropy of the multilayered GaTe were characterized by using polarized micro-Raman and polarized μPL measurements. The results clearly indicate that the monoclinic-phase GaTe possesses higher polarized extinction ratio with respect to that of the hexagonal GaTe for fully extinction of the Raman vibration mode and band-edge emission in the dual-phase gallium telluride. The existence of dual crystalline phases in multilayer GaTe may render it possible for fabrication of two-color emission device available for near-infrared (NIR) optical communication use.

Nonvariant polymorphic transition from hexagonal to monoclinic lattice in GaTe single crystal

The polymorphic transition from hexagonal to monoclinic phase was studied for the first time in melt grown GaTe crystals. The polymorphic transition goes as a non-variant process at a constant temperature and pressure without any changes in composition. Though the character of the phase transition is apparently diffusive, no interface or grain boundaries are detected. After completion of the phase transition the whole volume of the crystal remains a single crystal. The cleavage plane is preserved and proves to be a habit plane between the hexagonal and monoclinic phase. Reflections in Laue patterns are sharpened and peaks in X-ray diffraction patterns are narrowed upon the transition, which is attributed to relaxation of internal stresses. Annealing of the transformed monoclinic GaTe at 1023 K followed by quenching does not incite decomposition of the compound, which is explained by high stoichiometry of the grown crystal. Energy gap value after storage at room temperature and after the annealing is 1.65 eV, which corresponds with the data for monoclinic GaTe. The Vickers microhardness is 360 MPa.

Phase‐Engineered Synthesis of Ultrathin Hexagonal and Monoclinic GaTe Flakes and Phase Transition Study

AbstractGaTe is an important III–VI semiconductor with direct bandgap; thus, it holds great potential in the field of optoelectronics. Although it is known that GaTe can exist both in monoclinic and hexagonal phases, current studies are still exclusively restricted to the monoclinic phase of two dimensional (2D) GaTe owing to the difficulty in the fabrication of 2D hexagonal GaTe. Both monoclinic and hexagonal GaTe are demonstrated in this work, which can be selectively synthesized via a physical vapor deposition method, under precisely controlled growth temperatures. The pristine Raman and non‐linear optical properties of hexagonal GaTe has been systematically explored for the first time; moreover, a novel selected‐area phase transition from hexagonal to monoclinic of GeTe has been achieved via fs‐laser irradiation. This work may pave the way for widely use of 2D GaTe in various fields in future.

Spin splitting in 2D monochalcogenide semiconductors.

We report ab initio calculations of the spin splitting of the uppermost valence band (UVB) and the lowermost conduction band (LCB) in bulk and atomically thin GaS, GaSe, GaTe, and InSe. These layered monochalcogenides appear in four major polytypes depending on the stacking order, except for the monoclinic GaTe. Bulk and few-layer ε-and γ -type, and odd-number β-type GaS, GaSe, and InSe crystals are noncentrosymmetric. The spin splittings of the UVB and the LCB near the Γ-point in the Brillouin zone are finite, but still smaller than those in a zinc-blende semiconductor such as GaAs. On the other hand, the spin splitting is zero in centrosymmetric bulk and even-number few-layer β-type GaS, GaSe, and InSe, owing to the constraint of spatial inversion symmetry. By contrast, GaTe exhibits zero spin splitting because it is centrosymmetric down to a single layer. In these monochalcogenide semiconductors, the separation of the non-degenerate conduction and valence bands from adjacent bands results in the suppression of Elliot-Yafet spin relaxation mechanism. Therefore, the electron- and hole-spin relaxation times in these systems with zero or minimal spin splittings are expected to exceed those in GaAs when the D’yakonov-Perel’ spin relaxation mechanism is also suppressed.

Ab initio electronic band structure study of III–VI layered semiconductors

We present a total energy study of the electronic properties of the rhombohedral γ-InSe, hexagonal e-GaSe, and monoclinic GaTe layered compounds. The calculations have been done using the full potential linear augmented plane wave method, including spin-orbit interaction. The calculated valence bands of the three compounds compare well with angle resolved photoemission measurements and a discussion of the small discrepancies found has been given. The present calculations are also compared with recent and previous band structure calculations available in the literature for the three compounds. Finally, in order to improve the calculated band gap value we have used the recently proposed modified Becke-Johnson correction for the exchange-correlation potential.

Aerosol-assisted chemical vapour deposition of indium telluride thin films from {In(μ-Te)[N(iPr2PTe)2]}3

Aerosol-assisted chemical vapour deposition (AACVD) studies using {M(μ-Te)[N(iPr2PTe)2]}3 (M = In, Ga) were carried out on glass and Si(100) substrates at deposition temperatures between 325–475 °C. Black, adherent films were obtained in all cases and characterized by XRD, SEM, EDAX and Raman spectroscopy. The indium precursor gave cubic In2Te3 exclusively, whereas the gallium complex generated a mixture of cubic Ga2Te3, monoclinic GaTe and hexagonal Te. Mass spectrometric studies indicate that fragmentation of the indium precursor to give In2Te3 is accompanied by the formation of In[N(iPr2PTe)2]2+ and [N(iPr2PTe)2]2−.

On the phase diagram of the Ga-Te system in the composition range 55 at % Te

Monoclinic Ga-Te was heated in the electron microscope. At about 800° C decomposition was observed and after cooling down to room temperature, two compounds were identified: the fcc Ga2Te3 and the Ga3Te4, for which a trigonal structure was identified. A relation between the three structures of Ga-Te, Ga3Te4 and Ga2Te3 is put forward.

The far infrared complex reflectivity of GaTe

The room temperature polarized complex (amplitude and phase) infra-red reflection spectra of single crystal monoclinic GaTe have been measured between 20 and 400 cm−1 using a dispersive Fourier transform spectrometer. The measurements allow the experimental zone centre transverse and longitudinal phonon modes and optical constants to be determined without recourse to oscillator fit models or Kramers-Kronig analysis. Infrared power transmission measurements have allowed all fifteen predicted modes to be identified and the results are compared to recent Raman work.

Raman scattering from GaSe 1− xTe x

The long-wavelength optical phonons of the layer GaSe 1− x Te x have been investigated at room temperature by means of Raman scattering spectroscopy. The spectra of the Bridgman grown crystals were excited with the 1,06 μm line of the continuously operated YAG:Nd 3+ laser. Detailed study of the Raman spectra of GaSe 1− x Te x solid solutions showed that there is an abrupt change in the frequency-composition dependences for all observed modes. It is shown, that a phase transition from hexagonal ϵ-GaSe to monoclinic GaTe in GaSe 1− x Te x solid solutions takes place in the composition range 0.27 ⩽ × ⩽ 0.72. Only one mode behaviour of the optical phonons was observed in GaSe 1− x Te x system.

Long-wavelength phonons in GaTe

The long-wavelength vibrational modes of monoclinic GaTe have been investigated by Raman scattering and far-infrared absorption and reflection. The energies of 14 of the expected 18 Raman-active modes have been measured and their symmetries identified. Ten of the 15 predicted infrared-active modes have been found in the infrared spectra. A comparison of the Raman and infrared results has not provided conclusive evidence concerning the existence of conjugate modes in GaTe, and possible reasons are discussed. The results are also compared to previous work and several differences are noted and discussed in detail.