Pseudo-direct Band Gap Research Articles

Large Number of Direct or Pseudo-Direct Band Gap Semiconductors among A3TrPn2 Compounds with A = Li, Na, K, Rb, Cs; Tr = Al, Ga, In; Pn = P, As.

Due to the high impact of semiconductors with respect to many applications for electronics and energy transformation, the search for new compounds and a deep understanding of the structure-property relationship in such materials has a high priority. Electron-precise Zintl compounds of the composition A3TrPn2 (A = Li - Cs, Tr = Al - In, Pn = P, As) have been reported for 22 possible element combinations and show a large variety of different crystal structures comprising zero-, one-, two- and three-dimensional polyanionic substructures. From Li to Cs, the compounds systematically lower the complexity of the anionic structure. For an insight into possible crystal-structure band-structure relations for all compounds (experimentally known or predicted), their band structures, density of states and crystal orbital Hamilton populations were calculated on a basis of DFT/PBE0 and SVP/TZVP basis sets. All but three (Na3AlP2, Na3GaP2 and Na3AlAs2) compounds show direct or pseudo-direct band gaps. Indirect band gaps seem to be linked to one specific structure type, but only for Al and Ga compounds. Arsenides show smaller band gaps than phosphides due to weaker Tr-As bonds. The bonding situation was confirmed by a Mullikan analysis, and most states close to the Fermi level were assigned to non-bonding orbitals.

Structural and optical characterization of GaS1−xSex layered mixed crystals grown by chemical vapor transport

Alloying is a prominent strategy for tailoring the properties of semiconductors and expanding their usability. Accordingly, an entire series of GaS1–xSex (0 ≤ x ≤ 1) alloys were grown by chemical vapor transport and their structural and optical properties were studied. It was found that, for optimal crystalline quality, the growth temperatures should decrease as x increases. The scanning electron microscopy images of the specimens showed GaS1-xSex are layered crystals. The transmission electron microscopy (TEM) images showed the GaS1−xSex crystals have hexagonal structures with high crystalline quality. Their Raman and X-ray diffraction (XRD) results indicated GaS and GaS0.8Se0.2 are in the 2 H β-phase, GaSe is in the 2 H ε-phase, while those with intermediate x are in the 2 H β-ε mixed phase. The frequencies of the Raman modes decrease continuously as x increases. This single-mode behavior differs from the dual-mode behavior reported by the literature. The lattice constants a and c of the GaS1−xSex crystals, estimated from the TEM and XRD results, increase as x increases. The variation in a with x follows linear Vegard’s law. However, the variation in c with x is quadratic. The indirect and direct bandgaps of GaS1–xSex, determined from the absorption spectra, decrease as x increases. The difference between the indirect and direct bandgaps shrinks as x increases. The minimum difference is only 20 meV, which makes GaS1−xSex with high x a pseudo-direct bandgap semiconductor. The piezoreflectance spectra of the samples showed excitonic signals, which may be attributed to the direct interband transitions. The wavelength of the samples’ photoluminescence peaks increases with x and covers the visible range. Thus, GaS1–xSex shows great promise for optoelectronic applications.

The halide ion replacement effects on the physical properties of Cs2BX6 variant perovskites

Variant perovskites of the halide family have gained great attention in the recent times due to their prominent characteristics favorable for various applications. The variant perovskite Cs2BX6 (B= Hf, Ti, Zr and X = Cl, Br, I) have been the subject of some reports for experimental growth and measurement to computational studies. However, so far no work is available for the comparative study and detail optical properties. Here in this work, the structural, electronic and optical properties of Cs2BX6 determined using FP-LAPW + lo method and compared with the existing literature. The obtained results show improvement to good comparison with the earlier reports. The optimized band structure reveals that the band gap is decreased gradually switching from Cl to Br and I. The band structures show that most of variant perovskite under consideration are pseudo direct band gap with wide energy gaps. The optical properties like dielectric constant, absorption coefficient and optical conductivity peaks are also found up to incident photon energy of 20 eV. The materials show rising peaks in the ultraviolet region showing their great potential for high frequency applications.

Lead-free Dion–Jacobson halide perovskites CsMX2Y2 (M = Sb, Bi and X, Y = Cl, Br, I) used for optoelectronic applications via first principle calculations

In this study, all-inorganic two-dimensional Dion–Jacobson halide perovskites (DJ-HPs) CsMX2Y2 (M = Sb, Bi; and X/Y= I/I, I/Br, Br/Br, I/Cl, Br/Cl, Cl/Cl) have been studied for photovoltaic, photoemission and other potential applications in optoelectronic devices using first principle calculations within the framework of density functional theory (DFT). The ground state crystal symmetry is tetragonal with a P4/mmm (No. 123) space group. In these compounds, Cs+ is a spacer cation, which separates the octahedral layers and balances the charge. The corner sharing M-centered (Sb, Bi) [MX2Y4]3– unit constructs the 2D [MX2Y2]nn– plane, where the X and Y (halogens) atoms occupy the out of plane apical and in-plane bridging sites, respectively. The DJ-HPs with P4/mmm crystal symmetry exhibit pseudo-direct band gap semiconducting nature and the spin orbit (SO) effect reduces the band gaps as well as splits the Sb/Bi-p orbital in the bottom of conduction band (CB) and X/Y-p orbital at the top of valence band (VB). Among these DJ-HPs, the band gap energies of CsSbI2Cl2 (0.87 eV), CsSbBr2Cl2 (1.00 eV), CsSbCl4 (1.40 eV), CsBiI4 (1.10 eV), CsBiI2Br2 (1.40 eV) and CsBiBr4 (1.60 eV) lie in a band gap energy range (0.9–1.6 eV) suitable for photovoltaic applications. The M-s (Sb-5s and Bi-6s) and X/Y-p (Cl-3p, Br-4p and I-5p) orbitals are responsible for the electron transition from the valence to conduction band, while the spacer cation (Cs+) has a limited effect on the charge carrier mobility. The effective masses of the charge carriers and exciton binding energies of these materials are suitable for solar cell and optoelectronic applications. From the optical coefficients it is clear that these DJ-HPs have excellent response to the incident photons in the visible and ultraviolet light region.

Structural, Electronic, Elastic, Mechanical, Optical and Thermoelectric Properties of the Chalcogenide Double Perovskites A2GaNbS6 (A = Ca, Sr and Ba): Insights From Density Functional Theory Calculations

Abstract Recently, the lead-free double perovskite compounds have been evinced to be promising candidate for thermoelectric and optoelectronic technologies. In this paper; we have probed a theoretical works on the different physical properties: Structural, electronic, elastic, optical and thermoelectrical properties of the chalcogenide double perovskites A2GaNbS6 (A=Ca, Sr and Ba) within the instructions of density functional theory. The calculations have incorporated using the full potential linearized augmented plane waves (FP-LAPW) method within gradient generalized approximation (GGA) and the modified Becke-Johnson potential (mBJ) to describe the exchange-correlation potential as embodied in the WIEN2K code. The computed structural results show that the non-magnetic structure state is energetically the most stable structure in the cubic Fm3̄m (225) configuration, also the elastic and mechanical properties indicate that A2GaNbS6 (A=Ca, Sr and Ba) have a ductile nature. According to the electronic plots the three compounds have a semiconducting behavior with indirect (pseudo-direct) band gap of 1.21, 1.28 and 1.32 eV. Important optical responses of studied chalcogenide double perovskites are found in the visible and ultraviolet energy ranges. Finally, the thermoelectric effectiveness of the three compounds have been probed by computing parameters like Seebeck coefficient, electrical conductivity, thermal conductivity and figure of merit with semi-classical Boltzmann theory and constant relaxation time approximation as implemented in BoltzTrap code, the obtained results show that the chalcogenide double perovskites could be a good candidate for thermoelectric applications.

Strong absorption and ultrafast localisation in NaBiS2 nanocrystals with slow charge-carrier recombination

I-V-VI2 ternary chalcogenides are gaining attention as earth-abundant, nontoxic, and air-stable absorbers for photovoltaic applications. However, the semiconductors explored thus far have slowly-rising absorption onsets, and their charge-carrier transport is not well understood yet. Herein, we investigate cation-disordered NaBiS2 nanocrystals, which have a steep absorption onset, with absorption coefficients reaching >105 cm−1 just above its pseudo-direct bandgap of 1.4 eV. Surprisingly, we also observe an ultrafast (picosecond-time scale) photoconductivity decay and long-lived charge-carrier population persisting for over one microsecond in NaBiS2 nanocrystals. These unusual features arise because of the localised, non-bonding S p character of the upper valence band, which leads to a high density of electronic states at the band edges, ultrafast localisation of spatially-separated electrons and holes, as well as the slow decay of trapped holes. This work reveals the critical role of cation disorder in these systems on both absorption characteristics and charge-carrier kinetics.

Photoluminescence properties of heavily Sb doped Ge1−x Sn x and heterostructure design favorable for n+-Ge1−x Sn x active layer

We investigated the photoluminescence (PL) properties of heavily Sb doped n+-Ge1−x Sn x layers and demonstrated the formation of a double heterostructure (DHS) for the n+-Ge1−x Sn x active layer. A single PL peak was observed for n+-Ge1−x Sn x layers thicker than 80 nm with increasing the Sb concentration up to 1020 cm−3, attributed to the superior crystallinity and pseudo-direct bandgap transition mechanism, while a 15 nm thick n+-Ge1−x Sn x layer did not exhibit PL signals. A favorable heterostructure for n+-Ge1−x Sn x is proposed from the viewpoint of the increased valence band offset (ΔE v) using n-Si y Ge1−y as the cladding layer. We demonstrated the formation of an n-Si y Ge1−y (15 nm)/n+-Ge1−x Sn x (15 nm)/n-Si y Ge1−y (15 nm) DHS with a superior crystallinity and high PL peak intensity comparable to that of a thick n+-Ge1−x Sn x . We discuss the reasons for the PL performance improvement by forming the DHS, including the sufficient carrier confinement and the suppression of surface recombination.

Liquid‐Phase Exfoliated Gallium Selenide for Light‐Driven Thin‐Film Transistors

AbstractGallium selenide (GaSe), a layered semiconductor of Group‐III monochalcogenides, has been recognized by the scientific community in recent years as an appealing material in the fields of photonics and (opto)electronics. Thanks to its pseudodirect bandgap and its thickness‐dependent (opto)electronic properties, GaSe has emerged as a promising candidate for the implementation of thin‐film transistors (TFTs) and photodetectors with fast response and high sensitivity. Solution processing of 2D materials provides low‐cost inks that allow the design and realization of printed electronic devices, enabling this technology to move from the laboratory to the industry. In this work, a solution‐processed GaSe‐based light‐driven transistor is presented. Liquid phase exfoliation (LPE) is used to exfoliate bulk GaSe in isopropanol, formulating a functional ink that is subsequently deposited by spray coating onto Si/SiO2 substrates. The GaSe phototransistor exhibits a p‐channel behavior with a high on/off ratio (≈103) that is gate‐voltage dependent. Moreover, the device response also depends on the illumination with a maximum responsivity of 13 A W–1 to UV–visible light and a fast response time of 35 ms. This study demonstrates that liquid phase exfoliated GaSe is a promising candidate for the design and realization of next‐generation (opto)electronic devices.

Verifying the band gap narrowing in tensile strained Ge nanowires by electrical means

Group-IV based light sources are one of the missing links towards fully CMOS compatible photonic circuits. Combining both silicon process compatibility and a pseudo-direct band gap, germanium is one of the most viable candidates. To overcome the limitation of the indirect band gap and turning germanium in an efficient light emitting material, the application of strain has been proven as a promising approach. So far the experimental verification of strain induced bandgap modifications were based on optical measurements and restricted to moderate strain levels. In this work, we demonstrate a methodology enabling to apply tunable tensile strain to intrinsic germanium nanowires and simultaneously perform in situ optical as well as electrical characterization. Combining I/V measurements and μ-Raman spectroscopy at various strain levels, we determined a decrease of the resistivity by almost three orders of magnitude for strain levels of ∼5%. Thereof, we calculated the strain induced band gap narrowing in remarkable accordance to recently published simulation results for moderate strain levels up to 3.6%. Deviations for ultrahigh strain values are discussed with respect to surface reconfiguration and reduced charge carrier scattering time.

Optical Absorption Exhibits Pseudo-Direct Band Gap of Wurtzite Gallium Phosphide

Definitive evidence for the direct band gap predicted for Wurtzite Gallium Phosphide (WZ GaP) nanowires has remained elusive due to the lack of strong band-to-band luminescence in these materials. In order to circumvent this problem, we successfully obtained large volume WZ GaP structures grown by nanoparticle-crawling assisted Vapor-Liquid-Solid method. With these structures, we were able to observe bound exciton recombination at 2.14 eV with FHWM of approximately 1 meV. In addition, we have measured the optical absorption edges using photoluminescence excitation spectroscopy. Our results show a 10 K band gap at 2.19 eV and indicate a weak oscillator strength for the lowest energy band-to-band absorption edge, which is a characteristic feature of a pseudo-direct band gap semiconductor. Furthermore, the valence band splitting energies are estimated as 110 meV and 30 meV for the three highest bands. Electronic band structure calculations using the HSE06 hybrid density functional agree qualitatively with the valence band splitting energies.

Study on Carrier Mobility Model for PD-Ge Monolithic Optoelectronic Integration Chips

Based on the difference of thermal expansion coefficient between Si and Ge, low-intensity tensile stress can be introduced into Ge epitaxial layer on Si substrate. S-Ge/Si semiconductor (as known as low tensile strained Ge grown on Si substrate) has a higher carrier mobility when compared with unstrained-Ge or Si material, so that s-Ge/Si is appropriate for the production of high-speed circuit. At the same time, transformation from indirect bandgap semiconductor Ge into Pseudo-Direct bandgap semiconductor (which is also called PD-Ge) will be happen after s-Ge/Si is heavy doped, which makes LED produced of PD-Ge material perform a higher luminous efficiency because the radiative recombining probability of carriers in PD-Ge material is greatly improved compared with unstrained one. Taking the advantages referred of s-Ge/Si into account, s-Ge/Si has the potential to PD-Ge monolithic optoelectronic integration. Carrier mobility of the semiconductor is one of the key physical parameters during the design and simulation of PD-Ge monolithic optoelectronic integrated system. While as far as the authors are aware, carrier mobility model of s-Ge/Si is still rarely reported to date. In view of that all above, based on the E-k relation in both conduction band and valence band of s-Ge/Si material, the analytical models of physical parameters in energy band are established, and the models are verified by experiments. Then the s-Ge/Si carrier models are further established based on our band structure model, and the Monte Carlo method is used to verify our s-Ge/Si carrier mobility model. The quantificational results of our paper will help understand s-Ge/Si material physics and provide an important theoretical basis for the design of PD-Ge monolithic optoelectronic integration.

Growth, structural and optical characterization of wurtzite GaP nanowires

Bulk gallium phosphide (GaP) crystallizes in the zinc-blende (ZB) structure and has an indirect bandgap. However, GaP nanowires (NWs) can be synthesized in the wurtzite (WZ) phase as well. The contradictory theoretical predictions and experimental reports on the band structure of WZ GaP suggest a direct or a pseudo-direct bandgap. There are only a few reports of the growth and luminescence from WZ and ZB GaP NWs. We first present a comprehensive study of the gold-catalyzed growth of GaP NWs via metalorganic vapor phase epitaxy on various crystalline and amorphous substrates. We optimized the growth parameters like temperature, pressure and reactant flow rates to grow WZ GaP NWs with minimal taper. These wires were characterized using electron microscopy, x-ray diffraction, Raman scattering and photoluminescence spectroscopy. The luminescence studies of bare GaP NWs and GaP/AlGaP core–shell heterostructures with WZ- and ZB-phase GaP cores suggest that the WZ-phase GaP has a pseudo-direct bandgap with weak near-band-edge luminescence intensity.

Characterization of crystalline GeSn layer on tensile-strained Ge buffer deposited by magnetron sputtering

High-quality low Sn-content GeSn film has been grown on tensile strained Ge/Si (100) substrate using a physical vapor deposition reactor and no subsequent annealing was carried out after the deposition of the film. High Resolution X-ray diffraction (HR-XRD) measurement was performed to estimate the crystallinity and the residual strain in the Ge layer. Secondary-ion mass spectrometry (SIMS) and X-ray photoelectron spectrometer (XPS) show Sn incorporation of the film is approximately 3%. High-resolution transmission electron microscopy (TEM) and atomic force microscopy (AFM) results show that the crystallinity of the film is quite good and it shows a smooth surface (RMS = 0.744 nm) in 5 µm × 5 µm scan. Raman spectroscopy and photoluminescence (PL) measurements were carried out to study the structural and optical properties of the film. Room-temperature PL of the film was observed and it results in a decrease in the difference between Γ and L valleys of tensile strained Ge (0.21%) from 87 meV to 76 meV, which makes this method promising for obtaining Pseudo-direct bandgap group IV films.

Band Structure Engineering of Cs2AgBiBr6 Perovskite through Order-Disordered Transition: A First-Principle Study.

Cs2AgBiBr6 was proposed as one of the inorganic, stable, and nontoxic replacements of the methylammonium lead halides (CH3NH3PbI3, which is currently considered as one of the most promising light-harvesting material for solar cells). However, the wide indirect band gap of Cs2AgBiBr6 suggests that its application in photovoltaics is limited. Using the first-principle calculation, we show that by controlling the ordering parameter at the mixed sublattice, the band gap of Cs2AgBiBr6 can vary continuously from a wide indirect band gap of 1.93 eV for the fully ordered double-perovskite structure to a small pseudodirect band gap of 0.44 eV for the fully random alloy. Therefore, one can achieve better light absorption simply by controlling the growth temperature and thus the ordering parameters and band gaps. We also show that controlled doping in Cs2AgBiBr6 can change the energy difference between ordered and disordered Cs2AgBiBr6, thus providing further control of the ordering parameters and the band gaps. Our study, therefore, provides a novel approach to carry out band structure engineering in the mixed perovskites for optoelectronic applications.

Electronic, optical and thermoelectric properties of SnGa2GeX6 (X = S, Se) compounds

Electronic, optical and thermo-electric properties of two quaternary chalcogenides SnGa2GeX6 (X = S, Se) have been studied by using density functional theory. Modified Becke-Johnson exchange potential (mBJ-GGA) was used to calculate the band gaps of these compounds. Both compounds show pseudo direct band gap of 1.74 and 1.24 eV for SnGa2GeS6 and SnGa2GeSe6, respectively. The band gap decreases when replacing the X cations from S to Se. The optical properties such as dielectric function, energy loss function, extension coefficient, refractive index, and reflectivity of these compounds are discussed in details. The thermo-electric properties are studied using Boltzmann statistics through BoltzTrap code. High absorption peaks and figure of merits (ZT) for both compounds reveal that they are good candidates for the optoelectronics and thermo-electric devices.

First-principles study of ternary Li-Al-Te compounds under high pressure

The ternary Li-Al-Te compounds were investigated by the first-principle evolutionary calculation based on density function theory. Apart from the known structure, I-42d LiAlTe2 and P3m1 LiAlTe2, several new structures were discovered, P-3m1 LiAlTe2, Pnma LiAlTe2, C2/c Li9AlTe2, Immm Li9AlTe2 and P4/mmm Li6AlTe. We determined that the I-42d LiAlTe2 firstly changed to P-3m1 phase at 6 GPa, and then into the Pnma structure at 65 GPa, Pnma phase was stable up at least to 120 GPa. I-42d LiAlTe2 was a pseudo-direct band gap semiconductor, but P-3m1 LiAlT2 was an indirect band gap semiconductor. This may be caused by the pressure effect. Subsequently, it was metallized under pressure. Pnma LiAlTe2 was also metallic at the pressure we studied. C2/c Li9AlTe2 was stable above 4 GPa, then turned into Immm phase at 60 GPa. C2/c Li9AlTe2 was an indirect band gap semiconductor. The results show that P4/mmm Li6AlTe was stable and metallized in the pressure range of 0.7–120 GPa. The calculations of DOS and PDOS indicate that the arrangement of electrons near Fermi energy can be affected by the increase of Li. The calculated ELF results and Bader charge analysis indicate that there was no covalent bond between Al and Te atoms for high-pressure Pnma LiAlTe2, Li9AlTe2 and Li6AlTe. For Li9AlTe2 and Li6AlTe, different from LiAlTe2, Al atoms not connect with Te atoms, but link with Li atoms. The results were further proved by Mulliken population analysis. And the weak covalent bonds between Li and Al atoms stem from the hybridization of Li s and Al p presented in PDOS diagrams. We further deduced that the pressure effect and the increase of Li content may result in the disappearance of Al-Te bonds for Li-Al-Te compound under extreme pressure.

Structural stability and electronic behaviors of Co1-xOsxSi and macroscopic magnetic susceptibilities of CoSi and OsSi: GGA-PBEsol, GW-approximation and QTAIM investigations

The present work represents a theoretical investigation based on FP-(L)APW + lo method of structural properties, mechanical stability and electronic properties of Co1-xOsxSi as well as the macroscopic magnetic susceptibilities of CoSi and OsSi. The structural properties such as cell parameter, bulk modulus, internal parameters and total energy of non-magnetic NM, ferromagnetic FM and antiferromagnetic AFM phases were predicted by GGA-PBEsol semilocal functional. The obtained results for CoSi and OsSi are in good agreement with those found previously. The spin, orbital and total macroscopic magnetic susceptibilities of CoSi and OsSi have been estimated and confirmed that these compounds are diamagnetic. The total energy of the ferromagnetic phase of Co1-xOsxSi (with x = 0.25, 0.5 and 0.75) is the lowest indicating that they are ferromagnetic materials. The generalized stability criteria indicate that Co1-xOsxSi maintain their mechanical stabilities under a hydrostatic pressure less than 10 GPa. The electronic properties calculated by GW-approximation indicate that CoSi and Co1-xOsxSi (with x = 0.25, 0.50 and 0.75) are semimetals whereas OsSi is a semiconductor with a pseudo-direct band-gap. The topological analysis by QTAIM and the charge density plots indicate that the strong covalent character is predominant for CoSi, OsSi and CoOs bonds.

Hetero-epitaxy of high quality germanium film on silicon substrate for optoelectronic integrated circuit applications

Abstract

(Invited) Synthesis, Kinetics, Characterization and Application of Sodium and Potassium (poly)Selenides for Chalcogenide Semiconductor Doping

Alkali elements are essential for batteries and solar cells. In batteries, alkali ions are typically intercalated in layered 2D structures, such as transition metal sulfides and selenides, in a reversible fashion 1–4. In solar cells based on Cu(In,Ga)Se2 and Cu2ZnSn(S,Se)4 semiconductors, Na2Sex and K2Sex compounds are sources of extrinsic doping and act as surfactants during or after the growth, ending up as segregations at the grain boundaries, where they are also thought to passivate electronic defects 5–10. Alkali (poly)chalcogenide solutions are also traditionally employed for photoelectrochemical characterization of semiconductors because they offer a wide range of redox states 11,12 and stabilize against photoanodic dissolution 13,14. More recently, they have also been adopted as electrolytes in quantum dot sensitized solar cells 15–17. However, despite their technological relevance, little is known about the single alkali selenide compounds beyond the pioneering works of Zintl and Klemm 18,19 and subsequent structural characterizations 20–24. In this presentation, our recent work on (i) synthesis, (ii) characterization (iii) reactivity to oxygen and (iv) utilization of alkali (poly)selenides is reviewed. The synthesis is performed in liquid ammonia at atmospheric pressure and -55 °C. The progress of the reaction is monitored electrochemically, in order to unveil the kinetics. The decrease of solvated electrons concentration after selenium addition is measured directly by means of cyclic voltammetry at the surface of a Pt microelectrode. The compounds are characterised by X-ray photoelectron, Raman and electronic spectroscopies. Broadening of the Se 3d XPS peaks with the increase of the number of Se atoms in the chain suggests a limited extent of negative charge delocalization among the Se atoms in the solid (poly)selenides. A direct band-gap of 1.83 eV and pseudo-direct band-gap of 2.05 eV are identified for Na2Se and K2Se, respectively. The full width at half maximum of the fluorescence emissions is on the order of 300 meV, and the emission peaks occur at more than 0.5 eV below the respected bandgaps, revealing a pronounced defect-assisted radiative recombination. The extreme sensitivity of the compounds to oxidation is studied and discussed with respect to chalcogenide grain boundary passivation. Additionally, extrinsic doping of epitaxial and polycrystalline CuInSe2 films is investigated by means of a novel gas-phase route 25 that utilizes the synthesised compounds. The resulting films are characterised by photoluminescence spectroscopy, revealing the emergence of a distinct peak at ca. 100 meV higher energy than typical 1 eV peak of Cu-poor CuInSe2, irrespective of the alkali source. Lastly, some of the species likely involved in the alkali gas-phase transport and incorporation in CuInSe2are identified for the first time by Knudsen effusion mass spectrometry.

Bandgap nature of chalcopyrite ZnXP2 (X = Si, Ge, Sn)

The bandgap nature and near-edge band structure of the chalcopyrite ZnXP2 (X=Si, Ge, Sn) are investigated using the first-principles approaches. For these materials, whereas the valence band maximums are well defined, there are several competing near-extrema at the conduction band-edge. Moreover, the energetically near-degenerate conduction states are usually derived from different electron orbitals, which results in distinct optical properties and thus different bandgap types. A pseudo-direct bandgap, which is related to a weak optical absorption onset, can also be found beside the well-known direct and indirect bandgaps. In this work, we first investigate the fundamental properties of the band-edge electronic states using the density functional theory with the generalized gradient approximation. The origins of bandgap anomaly of the ZnXP2 systems are then analyzed. Finally, we perform the many-body electronic structure calculations for more reliable band orderings. The quasiparticle calculations indicate that whereas ZnSiP2 and ZnSnP2 have a pseudo-direct and a direct bandgap, respectively. However, ZnGeP2 has a bandgap with mixed pseudo-direct and indirect characters.