Cr-doped GaN Research Articles

Introduction of localized spin-state transitions in the optical absorption spectrum of Cr-doped GaN

Using a hybrid density functional, we study the electronic, magnetic, and neutral excitations of the paradigmatic Cr-doped GaN solid. Our results indicate that the $\ensuremath{\approx}1$% doped wurtzite GaN crystal retains the semiconducting nature of the host, associated with a magnetic moment of 3 ${\ensuremath{\mu}}_{B}$. As a consequence of Cr doping, additional hybridized bands are formed within the intrinsic band gap of GaN, leading to a considerable band gap narrowing by 1.4 eV and low-energy optical transitions. Our results indicate dark Cr $d\text{\ensuremath{-}}d$ transitions at very low energy (0.5--0.7 eV), followed by bright transitions in the visible energy range (1.8--2.2 eV). Characterization of the electron-hole pairs suggests that the latter originates from the internal transitions among hybridized ${\mathrm{Cr}}_{d}\text{--}{\mathrm{N}}_{p}$ states. With the existence of these additional optical features within the visible energy window, one can expect an enhancement to the photoelectric conversion efficiency of GaN upon Cr doping, in addition to applications in spintronics.

Detection of SF6 decomposition components by pristine and Cr-doped GaN based on the first-principles theory

In this work, the first-principles theory is used to simulate the adsorption behavior of four SF6 characteristic decomposed gases on the pristine and Cr-doped GaN (Cr-GaN). The feasibility of the GaN for application as a gas sensor was evaluated in terms of adsorption energy, adsorption distance, transfer charge, density of state, and band structure. The results show that the adsorption energy of pristine GaN upon H2S, SOF2, and SO2F2 are all more than −0.60 eV, and both the gas molecules and pristine GaN have little geometric deformation, which is physical adsorption. The pristine GaN is more sensitive to SO2, in which the adsorption energy is less than −0.80 eV, and the deformation of GaN is obvious, which is chemical adsorption. The adsorption energies of Cr-GaN upon four gases are significantly improved over three times, which indicates the chemisorption nature. Besides, the band gap of the Cr-GaN is largely reduced, making it behave semi-metallic and magnetic properties. All the simulation studies in this paper provide a theoretical basis for the material selection of SF6 decomposition gas detection and scavenge.

Cr doped MN (M = In, Ga) monolayer: A promising candidate to detect and scavenge SF6 decomposition components

In this paper, the adsorption properties of Cr modified InN (Cr-InN) and GaN (Cr-GaN) monolayers to SF6 decomposed components were investigated based on the density functional theory (DFT). Results reveal that these two graphene-like materials exhibit strong adsorption capacity for the three decomposed species SO2, SOF2 and SO2F2. Remarkable adsorption energy, significant charge transfer and molecular deformation indicate the occurrence of chemisorption. The hybridization of atomic electron orbitals in the density of states calculation verifies the strong chemical interaction between the gases and two monolayers. According to this strong adsorption capacity, Cr-InN and Cr-GaN monolayers demonstrate their potential as gas Scavenger for removing SF6 decomposition products in the insulation equipment. Besides, the significant change in conductivity of the monolayer caused by gas adsorption makes it possible to explore gas sensors based on Cr-InN and Cr-GaN monolayer. This paper indicates that Cr-doped InN and GaN monolayers can be used as adsorbents and sensors to guard the operation status of SF6 insulation devices by detecting and scavenging SF6 decomposed components.

The contribution of Cr(III)-doping on the modulation of magnetic and luminescence properties of GaN nanowires

An external incorporation of magnetic ions into the parent compounds is often used to tune their semiconducting behavior. For example, doping of a small amount of transition metal ion in a semiconductor can lead to the formation of dilute magnetic semiconductors (DMSs) that can exhibit some novel properties related to spin interactions. Here, we report on Cr(III)-doped GaN nanowires (NWs), prepared by a thermal evaporation method. X-ray diffraction analysis indicates the single crystalline nature of as-synthesized NWs. Each NW is composed of hexagonal-facet with sharp conical shaped tip investigated by a scanning electron microscope (SEM). The appearance of 2LO mode in micro-Raman spectra indicates a strong electron-phonon coupling with the A 1 (LO) mode induced by Cr(III) ion. The occurrence of ferromagnetic domains as observed in GaN plays an important role in spin ordering which comes from mixed spin-spin, exciton-spin, and spin-carrier interaction. These findings provide a new path to realize new spin-polarized photonic devices . • Well-shaped Cr-doped GaN nanowires grown by CVD. • A large redshift effect of A 1 (LO) as compared with bulk GaN. • Ferromagnetic coupling between transition metal ions and GaN lattice. • Formation of exciton magnetic polaron (EMP) due to high exciton binding energy. • Strong interactions resulting in exciton-spin or phonon interactions.

LDA + U Study of Induced Half Metallicity in Cr-Doped GaN

Half-metallic ferromagnetism in the Ga1 − xCrxN compound at different concentrations, x = 25, 12.5 and 6.25 %, have been investigated using density functional theory as implemented in code Spanish Initiative for Electronic Simulations with Thousands of Atoms (SIESTA) using LDA + U as exchange-correlation (XC) potential, to find out the possibility of new diluted DMSs. The outcomes reveal that transition metal atom (Cr) doping in GaN induces ferromagnetism. The 3d levels of the TM ion originate a half-metallic gap at the Fermi level in the majority spin channel for all concentrations. Moreover, diluted magnetic semiconductor compounds retain the half-metallic nature at all concentrations, i.e., x = 0.25, 0.125 and 0.0625, with 100 % spin polarization at the Fermi level (EF). The total magnetic moment of these compounds is due to Cr-3d states, and the existence of a small magnetic moment on Ga and N, non-magnetic atoms, for all doping concentrations is a consequence of p-d hybridization of Cr-d and N-p states. The calculated values of s-d exchange constant N α and p-d exchange constant N βconfirm the ferromagnetic character of the Cr-doped GaN compound.

Growth of Cr-doped GaN Nanowires

Growth of Cr-doped GaN Nanowires

Growth and characterization of Cr-doped semi-insulating GaN templates prepared by radio-frequency plasma-assisted molecular beam epitaxy

Cr-doped semi-insulating GaN templates were grown using radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE). The samples were characterized by high resolution x-ray diffraction (HRXRD), atomic force microscopy (AFM) and transmission electron microscopy (TEM). The Cr incorporation was strongly dependent on the growth condition, which was primarily influenced by Cr cell temperature. The activation energy of dark conductivity was about 0.48eV which corresponds to the depth of the dominant electron traps pinning the Fermi level. The experimental results indicated that Cr doping did not affect the crystalline orientation of the GaN film but introduced more threading dislocations, and step-graded GaN/AlxGa1−xN superlattices (SLs) played an important role in hindering the penetration of the threading dislocations.

Partially filled intermediate band of Cr-doped GaN films

We investigated the band structure of sputtered Cr-doped GaN (GaCrN) films using optical absorption, photoelectron yield spectroscopy, and charge transport measurements. It was found that an additional energy band is formed in the intrinsic band gap of GaN upon Cr doping, and that charge carriers in the material move in the inserted band. Prototype solar cells showed enhanced short circuit current and open circuit voltage in the n-GaN/GaCrN/p-GaN structure compared to the GaCrN/p-GaN structure, which validates the proposed concept of an intermediate-band solar cell.

Magnetic and magnetooptical properties of ferromagnetic semiconductor GaN : Cr

Magnetic and magnetooptical properties of the Cr-doped GaN layers have been investigated in the temperature range 50–400 K. A high saturation magnetization of 25 G has been observed in the obtained material. Spectra of the magnetooptical transversal Kerr effect have revealed strong magnetic response in the energy range less than 3.0 eV due to the appearance of new spin-polarization states in the band gap of GaN upon Cr doping.

Paramagnetism and antiferromagnetic interactions in Cr-doped GaN

We report on the magnetic and structural properties of Cr-doped GaN prepared by ion implantation of epitaxial thin films. Based on a detailed analysis of the magnetometry data, we demonstrate that the magnetic interactions between Cr moments in GaN are antiferromagnetic (AFM). Increasing the Cr fractional concentration up to 0.35, we observe that strong nearest cation neighbor AFM coupling results in the reduction of the effective moment per Cr atom. The uncompensated Cr moments exhibit paramagnetic behavior and we discuss to what extent the effects of an anisotropic crystal field and AFM interactions can be inferred from the magnetization data. We discuss the observed changes in magnetic and structural properties induced by thermal annealing in terms of defect annealing and Cr aggregation.

Optical and Structural Properties of Cr-Doped GaN Grown by HVPE Method

Single crystalline Cr-doped GaN films are successfully grown by hydride vapor phase epitaxy. The structure analysis indicates that the film is uniform without detectable Cr precipitates or clusters and the Cr atoms are substituted for Ga sites. The impurity modes in the range 510–530 cm−1 are observed by the Raman spectra. The modes are assigned to the host lattice defects caused by substitutional Cr. The donor-acceptor emission is found to locate at Ec − 0.20 eV by analyzing the photoluminescence spectrum obtained at different temperatures, and the emission is attributed to the structural defects caused by CrGa-VN complex. The superconductor quantum interference device results show that the Cr-doped GaN film without detectable Cr precipitates or clusters exhibits paramagnetic properties.

Nitrogen vacancy and ferromagnetism in Cr-doped GaN: First-principles calculations

Nitrogen defects and their effect on the ferromagnetism (FM) in Cr-doped GaN have been systematically investigated by first-principles. Four considered configurations including one N vacancy ( V N), single substitutional Cr, double substitutional Cr, and complex of Cr– V N are all ferromagnetic. The lowest energy arrangements for double Cr-doped (or Cr– V N) GaN occur at the nearest Cr–Cr (or Cr– V N) distance. One Cr contributes 84.3% of the total magnetic moment (2.533 μ B), while one Cr–Cr pair leads to 5.998 μ B moment (more than twice moment of one Cr) by the strong d– d spin coupling, which is mediated by two Cr 3 d states antiferromagnetically coupling with the “commonly shared” N 2 p states, and driven by a double exchange mechanism. The V N can enhance the FM by adding about 1 μ B to the Cr moment but reduce the FM spin exchange interaction between the nearest Cr–Cr pairs, so experimentally, high-performance samples may be synthesized by controlling N pressure.

Magnetic and structural properties of Cr-based diluted magnetic semiconductors and alloys

We applied supercell approach by using local spin density functional theory for Cr-doped GaN, CrN/ScN superlattices and the linear muffin-tin orbital method to predict the structural and magnetic properties of these novel diluted magnetic semiconductors (DMS), superlattices and their Cr x Sc 1− x N alloys. The global energy minimum of CrN is obtained for rocksalt (RS) structure if the compound is expanded by 8% and the phase becomes stable in the ferromagnetic (FM) state. The global energy minimum for the stable state occurs at in-plane lattice constant of 3.9 Å. In addition, the structural and optical properties of single crystal CrN/ScN superlattices and Cr 1− x Sc x N alloys are studied in detail. We report an isostructural phase transition from wurtzite (w-CrN) to hexagonal (h-ScN) at a hydrostatic pressure of 21 GPa which is associated with anomalous optical and piezoelectric properties.

Influence of growth temperature on GaN:Cr incorporation and structural properties in MOVPE

The influence of the growth temperature in metal organic vapor-phase epitaxy (MOVPE) on intentional Cr incorporation during GaN growth is studied in the range 900–1125 °C. The structural properties are investigated and compared to those obtained from undoped layers deposited at the same growth conditions and using the same growth procedure; Whereas, the best surface morphology and the best crystal quality are found at 1125 °C for undoped GaN layers, the best structural and morphological properties are obtained for the Cr-doped GaN (GaN:Cr) layers grown at 950 °C. GaN:Cr layers deposited at this temperature additionally exhibit the highest Cr concentration in the series as well as a Cr accumulation in the beginning stage of GaN:Cr growth. The accumulation may be responsible for the changed growth mode and the improved structural characteristics as well as for the observed V-shaped defects usually associated with strain relaxation. Thermo-remanent magnetization and a hysteresis loop were observed even above room temperature. The magnetic properties correlate to the structural properties of GaN:Cr layers.

Structural properties of AlCrN, GaCrN and InCrN

III-Nitride-based diluted magnetic semiconductors, AlCrN, GaCrN and InCrN layers were grown by plasma-assisted molecular beam epitaxy and their structural properties were characterized. Relatively lower temperature growth enables the growth of AlCrN, GaCrN and InCrN alloys with higher Cr contents, while higher temperature growth induces variety of phase separations in each Cr-doped AlN, GaN and InN systems. Radial distances from Cr atoms to the first nearest neighbor N atoms are not much different, while that to the second nearest neighbor cation atoms is changed following each lattice constants in AlCrN, GaCrN and InCrN. Crystal field splitting of Cr d level in AlCrN and GaCrN was also observed in X-ray absorption near-edge structure (XANES) spectra.

Structural and optical properties of Cr-doped semi-insulating GaN epilayers

The properties of Cr-doped GaN epilayers grown by rf-plasma-assisted molecular beam epitaxy were studied. The deep acceptor nature of Cr was used to grow semi-insulating GaN epilayers on sapphire substrates for electronic device applications. The room-temperature (RT) sheet resistivity of the epilayers reached 1010 Ω/square. The activation energy of dark conductivity was about 0.48 eV. Step-graded AlxGa1−xN/GaN (x=0.3−0.2) superlattices (SLs) were designed to filter dislocations. Transmission electron microscopy images showed that the SLs can dramatically reduce dislocation density. Al0.35Ga0.65N/GaN heterostructure grown on Cr-doped semi-insulating GaN epilayer exhibited a RT mobility of 960 cm2/V s and sheet carrier density of 2.1×1013 cm−2.

Molecular-beam epitaxial growth and characterization of ferromagnetic cubic GaCrN on GaAs substrate

Epitaxial growth of transition-metal Cr-doped cubic-phase GaN (GaCrN) layers on GaAs (0 0 1) substrates by plasma-assisted molecular-beam epitaxy and the observation of ferromagnetism above room temperature are reported for the first time. Influence of V/III ratio and Cr-cell temperature on the physical properties of the grown layers is studied extensively. X-ray diffraction studies and transmission electron microscopy observations confirm the predominantly cubic-nature of GaN and GaCrN for the epitaxial layers grown under Ga-rich condition. Cr dopant atoms exhibit surfactant nature and improve the crystalline quality and also enhance the growth of cubic-phase of GaN and GaCrN. Strong and broad blue-luminescent band (centered at 2.85 eV) is induced by the Cr doping in GaN. The saturation magnetization of the grown layers initially increases with Cr concentration and then decreases for further increase of Cr concentration.

Spatial distribution and magnetism in poly-Cr-doped GaN from first principles

Large scale density-functional theory calculations have been performed to understand the spatial distribution and magnetic coupling of Cr-doped GaN, in which exhaustive structural and magnetic configurations have been investigated by doping of up to five Cr atoms in large supercells. Our results provide direct evidence that the distribution of the doped magnetic ions is neither homogeneous nor random as widely assumed previously. Rather, under both Ga-rich and N-rich growth conditions, the Cr atoms have a strong tendency to form substitutional, embedded clusters with short-range magnetic interactions maintaining the wurtzite structure. Significantly, while the ferromagnetic state is favored for pair doping, for more than two-Cr-atom clustering configurations, states containing antiferromagnetic or ferrimagnetic coupling with net spins in the range of $0.06--1.47{\ensuremath{\mu}}_{B}∕\mathrm{Cr}$ are preferred. The formation of embedded clusters leads to notable local structural distortions and considerable magnetic moments on the Cr-bonded N atoms. Also importantly, the electrical properties (metallic, half-metallic, or semiconducting) are found to strongly depend on the dopant concentration. We propose a picture where various cluster configurations coexist and the statistical distribution and associated magnetism depend sensitively on sample growth details. The results obtained are in agreement with recent experiments. Such a view can explain many hitherto puzzling experimental observations, e.g., the much lower value of the measured mean saturation magnetic moment on Cr as compared to the theoretically predicted value for the isolated dopants; the anomalous lattice constant change in relation to the dopant concentration and temperature; and the strong dependence of the magnetization on the Cr concentration, growth temperature, and annealing. We find a similar behavior for Mn in GaN and Cr and Mn in AlN and argue that such a scenario may also hold for other dilute magnetic semiconductor systems.

Ferromagnetic to ferrimagnetic crossover in Cr-doped GaN nanohole arrays

Using spin-polarized density-functional theory with exchange and correlation potential, approximated by both the generalized gradient approximation (GGA) and the $\mathrm{GGA}+\mathrm{U}$ methods, we show that the coupling between a pair of Cr atoms substituted in GaN nanoholes is ferromagnetic. The interaction between the two Cr atoms, each carrying an average magnetic moment of around $2.5{\ensuremath{\mu}}_{\mathrm{B}}$, is short ranged and mediated by the neighboring N atom. As the concentration of Cr increases, clustering ensues and the coupling changes from ferromagnetic to ferrimagnetic resulting in a significant reduction in the average magnetic moment per Cr atom.

Clustering of Cr in GaN nanotubes and the onset of ferrimagnetic order

A comprehensive theoretical study of the structure and magnetic properties of Cr-doped GaN nanotubes yields some interesting and unexpected results: (1) A single wall GaN nanotube constructed from the GaN wurtzite crystal relaxes to a carbon-like zigzag SWNT structure and remains stable at $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, while a multiwall GaN nanotube retains its original wurtzite form. (2) Cr atoms prefer to form clusters and the underlying magnetism depends on the degree of clustering. (3) The coupling between two Cr atoms mediated by the neighboring N is ferromagnetic, but changes to ferrimagnetic as the cluster grows. These results are based on spin polarized density functional theory with exchange and correlation potential approximated by both the generalized gradient approximation and the local spin density approximation $+U$ methods.