Wurtzite Semiconductors Research Articles

Pressure dependence of elastic constants using Landau free energy and a numerical modeling analysis of W-InN

ABSTRACT In this work, we study the pressure dependence of elastic constants of wurtzite indium nitride semiconductor based on Landau free energy expansion. Elastic constants are obtained as functions pressure up to second order. We have also evaluated the bulk modulus (B) and anisotropies of compressibility and shear for wurtzite indium nitride as functions of pressure. Agreement with previously obtained results based on first-principles calculations is found to be satisfactory.

Perspectives and progress on wurtzite ferroelectrics: Synthesis, characterization, theory, and device applications

Wurtzite ferroelectrics are an emerging material class that expands the functionality and application space of wide bandgap semiconductors. Promising physical properties of binary wurtzite semiconductors include a large, reorientable spontaneous polarization, direct band gaps that span from the infrared to ultraviolet, large thermal conductivities and acoustic wave velocities, high mobility electron and hole channels, and low optical losses. The ability to reverse the polarization in ternary wurtzite semiconductors at room temperature enables memory and analog type functionality and quasi-phase matching in optical devices and boosts the ecosystem of wurtzite semiconductors, provided the appropriate combination of properties can be achieved for any given application. In this article, advances in the design, synthesis, and characterization of wurtzite ferroelectric materials and devices are discussed. Highlights include: the direct and quantitative observation of polarization reversal of ∼135 μC/cm2 charge in Al1−xBxN via electron microscopy, Al1−xBxN ferroelectric domain patterns poled down to 400 nm in width via scanning probe microscopy, and full polarization retention after over 1000 h of 200 °C baking and a 2× enhancement relative to ZnO in the nonlinear optical response of Zn1−xMgxO. The main tradeoffs, challenges, and opportunities in thin film deposition, heterostructure design and characterization, and device fabrication are overviewed.

The shift current photovoltaic effect response in wurtzite and zinc blende semiconductors via first-principles calculations.

Recently, the search for materials with high photoelectric conversion efficiency has emerged as a significant research hotspot. Unlike p-n junctions, the bulk photovoltaic effect (BPVE) can also materialize within pure crystals. Here, we propose wurtzite and zinc blende semiconductors without inversion symmetry (AgI, GaAs, CdSe, CdTe, SiGe, ZnSe, and ZnTe) as candidates for achieving the BPVE and investigate the factors that affect the shift current. GaAs with a wurtzite structure exhibits the highest shift current value of 31.8 μA V-2 when spin-orbit coupling is considered. Meanwhile, the peak position of the maximum linear optical conductivity and shift current in the wurtzite structure is lower than that in the zinc blende structure. In addition, we also found that strong covalency within the same main axis group element significantly influences the shift current, exemplified by wurtzite SiGe, which exhibits 15.8 μA V-2. Our research highlights the importance of a smaller band gap, reduced carrier effective mass, and increased covalency in achieving a substantial shift current response. Ultimately, this study provides valuable insights into the interplay of the structural and electronic properties, offering directions for the discovery and design of materials with an enhanced BPVE.

Half‐Metallic Heusler Alloy/GaN Heterostructure for Semiconductor Spintronics Devices

AbstractBecause spin‐orbit coupling in wurtzite semiconductors is relatively weak compared with that in zincblende ones, the III‐nitride semiconductor GaN is a promising material for high‐performance optical semiconductor spintronic devices such as spin lasers. For the purpose of reducing the operating power of spin lasers, it is necessary to demonstrate highly efficient electrical spin injection from a ferromagnetic material into GaN with a low‐resistance contact. Here, an epitaxial half‐metallic Heusler alloy Co2FeAlxSi1−x(CFAS)/GaN heterostructure is developed by inserting an ultrathin Co layer between the CFAS and GaN. The CFAS/n+‐GaN heterojunctions clearly show tunnel conduction with very small rectification and a low resistance‐area product of ≈3.8 kΩµm2, which is several orders of magnitude smaller than those reported in previous work, at room temperature. Nonlocal spin signals and a Hanle effect curve are observed at low temperatures using lateral spin‐valve devices with the CFAS/n+‐GaN contacts, suggesting pure spin current transport in bulk GaN. The spin transport is observed at temperatures as high as room temperature, with a high spin polarization of 0.2 at a low bias voltage less than 2.0 V. This study is expected to open a path to GaN‐based spintronic devices with highly spin‐polarized and low‐resistance contacts.

Ab initio study of the energy level alignment at semiconductor/organic interface: Coadsorption of acceptor and donor ligands with organic dyes

Conditions for ab initio evaluation of the surface band alignment in slab models are critically reviewed. Based on the employed approach, we study the relative energy levels arrangement between aromatic dye and wurtzite semiconductor surface and its dependence on the coadsorption of the representative ligands. The obtained results demonstrate the energy level shifts mediated by surface chemistry at the interface. Predictable control of the electronic properties of organically modified semiconductor surfaces can be used as a versatile adjustable instrument in the performance optimization of optoelectronic devices.

Spin polarization in quantum point contact based on wurtzite topological quantum well.

Manipulating spin polarization in wide-gap wurtzite semiconductors is crucial for the development of high-temperature spintronics applications. A topological insulator revealed recently in wurtzite quantum wells (QWs) provides a platform to mediate spin-polarized transport through the polarization field-driven topological edges and large Rashba spin-orbit coupling (SOC). Here, we propose a spin-polarized device in a quantum point contact (QPC) structure based on ZnO/CdO wurtzite topological QWs. The results show that the QPC width can sufficiently control the lateral spin-orbit coupling (SOC) as well as the band gap of the edge states through the quantum size effect. As a result, the spin-polarized conductance exhibits oscillation due to the spin precession, which can be controlled by adjusting the voltage imposed on the split gate. The QPC-induced large spin splitting is highly nonlinear and becomes strong close to the gap. The spin splitting of the edge states will be suppressed for QPC widths greater than 50 nm, and thus lead to an extremely long spin precession length. This QPC width-dependent lateral SOC effect provides an emerging electrical approach to manipulate spin-polarized electron transport in topological wurtzite systems.

Piezoelectric manipulation of spin-orbit coupling in a Wurtzite heterostructure.

The combination of piezoelectricity and spin-orbit coupling (SOC) effect makes wurtzite semiconductors attractive for the development of exotic spin-related physics as well as spintronic applications. Triggering piezoelectricity, particularly by an external stimulus, provides a new perspective for manipulating SOC, but until now, a comprehensive understanding of this mechanism is lacking. Herein, by means of self-consistent calculations and Löwdin perturbation approach, we have explored the manipulation of SOC in the wurtzite (Al, Ga)N/GaN heterostructure by external stress-induced piezoelectric polarization. The results suggest that the Rashba SOC depends weakly on stress due to the wide-gap feature of the wurtzite crystal that makes Rashba SOC predominant by a bulk term instead of the structural inversion term. The piezoelectric polarization diminishes and even turns off Dresselhaus coupling by reducing the interfacial electric field. Moreover, piezoelectricity is shown to improve the poorly gate-tunable SOC. In the heterostructure with two occupied subbands, the Dresselhaus coupling of the second subband is more sensitive than the first one in response to stress. As an extension, we further demonstrate that the correlation effect in the wurtzite heterostructure can be significantly enhanced by piezoelectric polarization. This study offers an in-depth insight into piezoelectric modulation of spin-orbit physics, which has the potential for stimulating new quantum correlation states or designing functional spintronic devices.

The spontaneous polarization in CdS to enhance the piezo-PEC performance via phase transition stress engineering

The built-in electric field caused by piezoelectric polarization has emerged as one of the most effective strategies for photoelectrochemical (PEC) water splitting, but is challenged by low piezoelectric polarization efficiency. Herein, we firstly introduce residual stress into CdS through phase transition stress engineering for improving the spontaneous polarization of CdS with a view to revealing the mechanism of the piezoelectric PEC (piezo-PEC) performance of CdS is advanced by the spontaneous polarization. The spontaneous polarization of CdS caused via phase transition residual stress generated built-in electric field to advance carriers separation. Moreover, the spontaneous polarization of CdS is motived a greater potential under external force than hexagonal wurtzite CdS due to the existence of phase transition residual stress, which effectively improves the piezo-PEC performance of CdS. The photocurrent density of miscibility of cubic sphalerite and hexagonal wurtzite CdS (M−CdS) is 0.61 mA/cm2 at 1.23 V vs RHE, which is almost 1.79 times of hexagonal wurtzite (H-CdS). In addition, the value of M−CdS photocurrent density is reached 2.12 mA/cm2 at 1.23 V vs RHE after ultrasound, which is almost 6.2 times of H-CdS. Our detail work proves that this strategy not only stimulates the piezo-PEC performance of CdS, but also can be extended to other hexagonal wurtzite semiconductor phase transition piezoelectric fields.

Formula omitted] electronic structure and strain-dependence of the crystal field splitting and inter-band transition energy of W–InN

In this paper, we study the electronic structure of wurtzite Indium Nitride (W–InN) using a k→∙π→ theory. Here π→ is the relativistic momentum operator. It is based on an eight-band model; three valence bands and one conduction band, each Kramers’ split by spin-orbit interaction. We diagonalize the k→∙π→ Hamiltonian following a two-band model, where each valence band is treated separately with the conduction band by using a suitable basis set in the presence of spin-orbit interaction. Effects of other valence bands on the dispersion of a given valence band are incorporated using second order perturbation theory. Valence band dispersions are plotted and compared with a previous calculation and our own results obtained by using a different method. Agreement is found to be satisfactory. Electronic and hole effective masses are calculated and compared with previously reported values, which are substantially different from each other. Fair agreement is noticed. Discrepancies are discussed. Effect of strain is calculated on the electronic structure by calculating the crystal field energy and transition energy for small variations of strain. Although results could not be compared with any previous calculation, due presumably to their absence in literature, trends and systematics are found to be in accord with the same found in case of other wurtzite semiconductors.

Resonant hyper-Raman scattering of light by LO phonons in wurtzite semiconductors

Theoretical treatment of resonant hyper-Raman scattering of light by LO phonons in wurtzite semiconductors is given. The hyper-Raman process was considered for the scattering geometry y(xxz)x at which it involves the two-photon transitions to the B and C excitons of the s-type. Allowance was made for different sequences of intermediate virtual states. On the example of a CdS crystal the influence of the possible dipole transitions to the deeper valence band on the frequency dependence of the scattering cross section was investigated.

Multiferroicity and giant in-plane negative Poisson\u2019s ratio in wurtzite monolayers

Monolayers of layered materials, such as graphite and molybdenum dichalcogenides, have been the focus of materials science in the last decades. Here, we reveal benign stability and intriguing physical properties in the thinnest monolayer wurtzite (wz) semiconductors, which can be exfoliated from their bulk and stacked to reform the wz crystals. The candidate ZnX and CdX (X = S, Se, Te) monolayers possess low cleavage energy and direct bandgaps, which harbor strongly coupled ferroelectricity and ferroelasticity with low transition barriers, giant in-plane negative Poisson’s ratio, as well as giant Rashba spin splitting, enabling the co-tunability of spin splitting and auxetic magnitudes via multiferroic switching. These wz monolayers can be used as building blocks of devices structures, due to their inherent “self-healable” capacity, which offer more flexibility for semiconductor fabrication and provide a natural platform to probe the interplay of multiple physical effects, bringing light into the rich physics in tetrahedral semiconductors.

Piezoelectricity in binary wurtzite semiconductors: a first-principles study

Using first-principles calculations, we investigate piezoelectricity in a wide range of binary wurtzite semiconductors. We find that piezoelectricity is intimately related to the bond character, e.g. the negative longitudinal piezoelectric effect (NLPE) tends to occur in covalent compounds. We further find a universal sign rule (negative clamped-ion term and positive internal-strain term) for piezoelectricity, and the NLPE occurs as a result of the domination of the former over the latter. Moreover, there exists an inverse linear correlation between the longitudinal and transverse piezoelectric coefficients. This work may offer a simple criterion for efficient computational screening of materials exhibiting the NLPE.

Interface optical phonons in double-channel AlGaN/GaN heterostructures: The ternary mixed crystal effect and size effect

Considering the anisotropy of wurtzite semiconductors, the interface optical phonons in double-channel AlGaN/GaN heterostructures are investigated by using a dielectric continuous model and transfer matrix method. Also, the ternary mixed crystal effect and size effect on the dispersion relations and electrostatic potentials of phonons are analyzed in detail. The results show that there are six branches of interface phonon modes in a double-channel heterostructure. For some values of Al composition, however, the phonon mode with the highest frequency may not exist, especially when the thicknesses of materials and the wave vectors of phonons are small. The ternary mixed crystal effect and size effect not only influence the values of phonon frequency and electrostatic potential, but also change the vibration mode of interface phonons. This suggests that the interface phonon vibrations can be controlled to reduce their adverse effects by changing the Al composition of AlGaN and the thickness of each layer in a double-channel heterostructure.

Stability and interaction of cation Frenkel pair in wurtzite semiconductor materials

The understanding of defects is an important issue in the development of semiconductor materials. Using ab initio calculations and ab initio molecular dynamics, the stability and interaction of cation Frenkel pairs were investigated in four wurtzite semiconductor materials, GaN, AlN, InN and ZnO. With regard to GaN material, an interesting phenomenon was found that the recombination path of Ga Frenkel pair is a kick-out mechanism which is similar to the migration of Ga interstitial. Based on the obtained path, the recombination energy barriers of different positions of Frenkel pairs were calculated respectively. The calculation results demonstrated that the recombination behavior of Ga Frenkel pair is not only determined by the distance but also the relative position of vacancy and interstitial. Meanwhile, the value of the projected interaction radius was observed to be about 7.5 Å. Compared with the small interaction range of N Frenkel pair, Ga Frenkel pair was found to be unstable, which is consistent with previous radiation experiments. Furthermore, the same phenomenon in GaN material was also found in the other three wurtzite materials and the projected interaction radius of wurtzite materials was concluded as 7/3 lattice parameter. In general, this theoretical study provides an insight of the stability of cation Frenkel pair and improves our understanding of defects in wurtzite semiconductors.

Tuning the thickness of graphitic nanofilms for wurtzite materials by vertical weak electric field under a certain pressure

Theoretical and experimental results indicate that naturally the ultrathin nanofilms of some wurtzite (WZ) semiconductors will transform into a stable nonpolar graphitelike structure from the polar surface. Normally, the thicknesses are just limited to a few atomic layers. By epitaxial tensile strain, the thickness can be extended. In this work, employing the first-principles calculations focus on WZ materials ZnO, BeO, SiC, GaN, InN and AlN, we predicted that under a certain pressure with a weak electric field, their thickness range of forming graphitic nanofilms is greatly enlarged. This research provides theoretical guidance for the synthesis of new two-dimensional (2D) structures.

External electric and magnetic field effects on the polaron in a wurtzite nitride nanowire embedded in a nonpolar matrix

We examine the Frohlich polaron problem in wurtzite nitride cylindrical nanowire embedded in a nonpolar matrix within the framework of the Lee–Low–Pines variational approach. The effects of the external electric and magnetic fields and quantum-size confinement on polaron self-energy and effective mass due to electron interactions with both quasiconfined and interface optical phonons are investigated. The numerical results obtained for wurtzite GaN material are discussed in comparison with the zinc-blende structure cylindrical nanowires. The estimation of contributions of different phonon modes reveal that interface optical phonons are much more important in narrow wires. It is found that the electric field has a relatively pronounced influence on the polaron self-energy and effective mass, while the effect of the magnetic field is small. The obtained results will be of importance in further study of the phonon-assisted electro-optical properties in wurtzite semiconductor low-dimensional structures.

Thornber–Feynman carrier-optical-phonon scattering rates in wurtzite crystals

It is well known that the carrier-optical-phonon scattering rates dominate the carrier-acoustic-phonon scattering rates in many polar materials of interest in electronic and optoelectronic applications. Furthermore, it is known that the Fröhlich coupling constants for carrier-optical-phonon in many materials is close to or great than unity, calling into question the validity of scattering rates based on the Fermi golden rule. In a celebrated paper by Thornber and Feynman it was shown that that the large Fröhlich coupling constant in polar materials does indeed lead to substantial corrections to the Fermi golden rule scattering rates. These large corrections are due to the fact that for strong coupling constants, the first-order perturbative approach underlying the Fermi golden rule does not take into account the presence of many phonons interacting simultaneous with the carrier. In this paper, the Thornber–Feymnan scattering rates for carrier-optical-phonon interactions are derived for several technologically important wurtzite semiconductors—BN, ZnO, CdS, CdSe, ZnS, InN, and SiC- and it is shown that the commonly used Fermi golden rule scattering rates must be corrected by factors ranging up to an order-of-magnitude. The corrections to the Fermi golden rule reported herein have widespread impact on carrier transport for materials with large Fröhlich coupling constants.

Dominant Polar Surfaces of Colloidal II–VI Wurtzite Semiconductor Nanocrystals Enabled by Cation Exchange

Polar surfaces of ionic crystals are of growing technological importance, with implications for the efficiency of photocatalysts, gas sensors and electronic devices. Creation of ionic nanocrystals with large percentages of polar surfaces is an option to improve their efficiency in aforementioned applications but is hard to be accomplished because they are less thermodynamically stable and prone to vanish during the growth process. Herein we developed a strategy that is capable of producing polar surface dominated II-VI semiconductor nanocrystals including ZnS and CdS, from copper sulfide hexagonal nanoplates through cation exchange reactions. The obtained hexagonal prism-shaped wurtzite ZnS hexagonal nanoplates have dominant {002} polar surfaces, occupying up to 97.8% of all surfaces. Density functional calculations reveal the polar surfaces can be stabilized by a charge transfer of 0.25 eV/formula from the anion-terminated surface to the cation-terminated surface, which also explains the presence of polar surfaces in the initial Cu1.75S hexagonal nanoplates with cation deficiency prior to cation exchange reactions. Experimental results showed that the HER activity could be boosted by the surface polarization of polar surface dominated ZnS hexagonal nanoplates. We anticipate this strategy is general and could be used to other systems to prepare nanocrystals with dominant polar surfaces. Furthermore, the availability of colloidal semiconductor nanocrystals with dominant polar surfaces produced through this strategy open a new avenue for improving their efficiency in catalysis, photocatalysis, gas sensing and other applications.

Persistent spin textures and currents in wurtzite nanowire-based quantum structures

We explore the spin and charge properties of electrons in wurtzite semiconductor nanowires where radial and axial confinement leads to tubular or ring-shaped quantum structures. Accounting for spin-orbit interaction induced by the wurtzite lattice as well as a radial potential gradient, we analytically derive the corresponding low-dimensional Hamiltonians. It is demonstrated that the resulting tubular spin-orbit Hamiltonian allows to construct spin states that are persistent in time and robust against disorder. We find that these special scenarios are characterized by distinctive features in the optical conductivity spectrum, which enable an unambiguous experimental verification. In both types of quantum structures, we discuss the dependence of the occurring persistent charge and spin currents on an axial magnetic field and Fermi energy which show clear fingerprints of the electronic subband structure. Here, the spin-preserving symmetries become manifest in the vanishing of certain spin current tensor components. Our analytic description relates the distinctive features of the optical conductivity and persistent currents to bandstructure characteristics which allows to deduce spin-orbit coefficients and other band parameters from measurements.

Why do nanowires grow with their c-axis vertically-aligned in the absence of epitaxy?

Images of uniform and upright nanowires are fascinating, but often, they are quite puzzling, when the substrate is clearly not an epitaxial template. Here, we reveal the physics underlying one such hidden growth guidance mechanism through a specific example - the case of ZnO nanowires grown on silicon oxide. We show how electric fields exerted by the insulating substrate may be manipulated through the surface charge to define the orientation and polarity of the nanowires. Surface charge is ubiquitous on the surfaces of semiconductors and insulators, and as a result, substrate electric fields need always be considered. Our results suggest a new concept, according to which the growth of wurtzite semiconductors may often be described as a process of electric-charge-induced self-assembly, wherein the internal built-in field in the polar material tends to align in parallel to an external field exerted by the substrate to minimize the interfacial energy of the system.