Ga Lattice Sites Research Articles

Incorporation and Interaction of Co‐Doped Be and Mg in GaN Grown by Metal‐Organic Chemic Vapor Deposition

Despite recent advances in growth and characterization of GaN:Be, reliable conductive p‐type GaN:Be remains elusive. In this work, GaN is co‐doped with Be and Mg to improve the incorporation and ionization efficiency of both Be and Mg. Be and Mg are found to interact in complex ways rendering GaN:Be,Mg semi‐insulating. Rather than improving Be and Mg incorporation efficiency, there is an apparent mutual inhibition of incorporation when Be and Mg are co‐dopants. Furthermore, photoluminescence measurements indicate that the BeGa acceptor is reduced in particular, relative to the total [Be]. The same effect is not observed for Mg. From this, it is concluded that Mg preferentially incorporates into Ga lattice sites over Be, and excess Be that cannot incorporate into substitutional sites instead occupies interstitial sites. The interstitial Be acts as donor defect, which compensate Mg acceptors. This provides an explanation for the observed reduction in BeGa‐related luminescence intensity (without an associated decrease in [Be]) in GaN:Be with significant [Mg] content.

Material Properties ofn‐Type β‐Ga2O3Epilayers with In Situ Doping Grown on Sapphire by Metalorganic Chemical Vapor Deposition

AbstractIn this study, in situ, Si‐doped heteroepitaxial Ga2O3layers are grown onc‐plane sapphire by metalorganic chemical vapor deposition. The X‐ray diffraction peaks of the doped Ga2O3epilayers shows ß‐phase of Ga2O3,and full width at half maximum of Ga2O3crystallinity is decreased at a Tetraethoxysilane (TEOS) molar flow rate of 2.23 × 10−7 mol min−1but increased with higher flow rates. The dopant concentrations of Ga2O3grown at 825 °C with TEOS molar flows of 2.23 × 10−7, 4.47 × 10−7, and 6.69 × 10−7 mol min−1are measured to be 5.5 × 1019, 1.1 × 1020, and 1.4 × 1020 atom cm−3, respectively, using secondary ion mass spectra and Hall measurements revealn‐type nature with carrier concentrations of 6.5 × 1017, 3.2 × 1018, and 3.9 × 1018atom/cm3, respectively. To increase Si dopant activation, Ga2O3growth temperature is raised to 875 °C. The result suggests a higher growth temperature can contribute to a greater probability of Si substitution on Ga lattice sites, which further reduces the resistivity of Ga2O3epilayer. Moreover, results are compared with theoretical Density Functional Theory studies.

A combined AIMD and DFT study of the low-energy radiation responses of GaN.

Although GaN is a promising candidate for semiconductor devices, degradation of GaN-based device performance may occur when the device is bombarded by high-energy charged particles during its application in aerospace, astronomy, and nuclear-related areas. It is thus of great significance to explore the influence of irradiation on the microstructure and electronic properties of GaN and to reveal the internal relationship between the damage mechanisms and physical characteristics. Using a combined density functional theory (DFT) and ab initio molecular dynamics (AIMD) study, we explored the low-energy recoil events in GaN and the effects of point defects on GaN. The threshold displacement energies (Eds) significantly depend on the recoil directions and the primary knock-on atoms. Moreover, the Ed values for nitrogen atoms are smaller than those for gallium atoms, indicating that the displacement of nitrogen dominates under electron irradiation and the created defects are mainly nitrogen vacancies and interstitials. The formation energy of nitrogen vacancies and interstitials is smaller than that for gallium vacancies and interstitials, which is consistent with the AIMD results. Although the created defects improve the elastic compliance of GaN, these radiation damage states deteriorate its ability to resist external compression. Meanwhile, these point defects lead the Debye temperature to decrease and thus increase the thermal expansion coefficients of GaN. As for the electronic properties of defective GaN, the point defects have various effects, i.e., VN (N vacancy), Gaint (Ga interstitial), Nint (N interstitial), and GaN (Ga occupying the N lattice site) defects induce the metallicity, and NGa (N occupying the Ga lattice site) defects decrease the band gap. The presented results provide underlying mechanisms for defect generation in GaN, and advance the fundamental understanding of the radiation resistances of semiconductor materials.

Compositional Dependence of Epitaxial L 1 0 ‐Mn x Ga Magnetic Properties as Probed by 57 Mn/Fe and 119 In/Sn Emission Mössbauer Spectroscopy

The magnetic properties of Mn x Ga alloys critically depend on composition x, and the atomic-scale origin of those dependences is still not fully disclosed. Molecular beam epitaxy has been used to produce a set of Mn x Ga samples (x = 0.7 ÷ 1.9) with strong perpendicular magnetic anisotropy, and controllable saturation magnetization and coercive field depending on x. By conducting 57Mn/Fe and 119In/Sn emission Mössbauer spectroscopy at ISOLDE/CERN, the Mn and Ga site-specific chemical, structural, and magnetic properties of Mn x Ga are investigated as a function of x, and correlated with the magnetic properties as measured by superconducting quantum interference device magnetometry. Hyperfine magnetic fields of Mn/Fe (either at Mn or Ga sites) are found to be greatly influenced by the local strain induced by the implantation. However, In/Sn probes show clear angular dependence, demonstrating a huge transferred dipolar hyperfine field to the Ga sites. A clear increase of the occupancy of Ga lattice sites by Mn for x > 1 is observed, and identified as the origin for the increased antiferromagnetic coupling between Mn and Mn at Ga sites that lowers the samples’ magnetization. The results shed further light on the atomic-scale mechanisms driving the compositional dependence of magnetism in Mn x Ga.

Rare-earth defects in GaN: A systematic investigation of the lanthanide series

Rare-earth (RE) doped GaN is of interest for optoelectronics and spintronics and potentially for quantum applications. A fundamental understanding of the interaction between RE dopants and the semiconductor host is key to realizing the material's full potential. This work reports an investigation of lanthanide ($Ln$) defects in GaN using hybrid density-functional defect calculations. We find that all the $Ln$ dopants incorporated at the Ga lattice site, $Ln_{\rm Ga}$ ($Ln$ = La--Lu), are stable as trivalent ions, but Eu and Yb can also be stabilized as divalent and Ce, Pr, and Tb as tetravalent. The location of $Ln$-related defect levels and the $Ln$ $4f$ states in the energy spectrum of the host material is determined from first principles. We elucidate the interplay between defect formation and electronic structure, including the $Ln$--N interaction, and the effect of doping on the local lattice environment. Optical properties are investigated by considering possible defect-to-band and band-to-defect transitions involving $Ln_{\rm Ga}$ defects with in-gap energy levels, including broad "charge-transfer" transitions. These defects can also act as carrier traps and mediate energy transfer from the host into the $4f$-electron core of the $Ln$ ion which leads to sharp intra-$f$ luminescence.

Magnetic impurity bands in Ga1−xMnxS : Towards understanding the anomalous spin-glass transition

We report on the magnetic and electronic properties of single-crystalline ${\mathrm{Ga}}_{0.91}{\mathrm{Mn}}_{0.09}\mathrm{S}$, which is a quasi-two-dimensional diluted magnetic semiconductor (DMS). Through an analysis of magnetization data, we show the existence of an anomalously high spin-glass transition temperature at 11.2 K. Using density functional theory (DFT), we characterize the properties contributing to the spin-glass transition through an examination of the electronic and magnetic properties for ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{S}$ with $x$ varying from 0.00 to 0.18 by randomly substituting Mn atoms into the gallium (Ga) lattice sites. We show that the presence of magnetic atoms produces impurity bands in the electronic structure, where an analysis of the density of states shows an increase in magnetic impurity bands at the Fermi level that lowers the semiconducting gap and is consistent with diluted magnetic semiconductors. Furthermore, this indicates that the spin-glass transition in ${\mathrm{Ga}}_{0.91}{\mathrm{Mn}}_{0.09}\mathrm{S}$ is similar to other DMS materials, where the primary mechanism is likely through magnetic exchange. However, the increased electron density in the system with Mn doping could explain the anomalously higher spin-glass transition temperature in ${\mathrm{Ga}}_{0.91}{\mathrm{Mn}}_{0.09}\mathrm{S}$. In comparison with the substantially lower transition temperatures in related II-VI based systems (i.e., ${\mathrm{Zn}}_{1\ensuremath{-}x}\mathrm{Mn}{}_{x}\mathrm{Te}$), the high transition temperature is associated with more metallic spin-glass systems that interact through RKKY exchange, which leads to the conclusion that there may be a combination of interactions occurring in these systems. Further measurements on the other substitution percentages will hopefully clarify these interactions.

Magnetocaloric Improvements in Doped Heusler Alloys

ABSTRACTMagnetocaloric materials have gained significant interest as an environment friendly and efficient refrigeration technology. We have been working on Heusler alloys, primarily the Ni-Mn-Ga system for improved magnetocaloric effect (MCE). We have observed significant MCE increase in stress assisted thermally cycled samples and demonstrated that one primary mechanism is texture change. More recently, we have observed volumetric decrease as well as anisotropy changes due to stressed cycling. The influence of these parameters on magnetic anisotropy and MCE are discussed. We have also utilized isoelectronic Al substitution at the Ga lattice sites for optimizing atomic distances and exchange interactions in an effort to bring about magnetostructural transformation closer to room temperature (RT) while retaining high MCE. The results show that Al substitutions can bring about large decreases (50 – 70K) in both the Curie (TC) and martensite start (Ms) temperatures, and permit fine tuning the magnetostructural transformation temperature.

A first-principles surface phase diagram study for Si-adsorption processes on GaAs(1 1 1) A surfaces

The adsorption processes of an Si atom on GaAs(1 1 1) A surfaces under growth conditions are investigated on the basis of first-principles surface phase diagrams, in which adsorption–desorption behavior is described by comparing the calculated adsorption energy obtained by total-energy electronic-structure calculations with vapor-phase chemical potential estimated by quantum statistical mechanics. The calculated surface phase diagram as functions of temperature and As 2 pressure demonstrates that both Ga and As atoms are adsorbed on the Ga-vacancy site of GaAs(1 1 1) A- ( 2 × 2 ) surface under low As-pressure conditions, resulting in the formation of ( 2 × 2 ) surface with an As adatom. The surface phase diagrams as functions of temperature and Si pressure also reveal that an Si atom can be adsorbed on the ( 2 × 2 ) surface with an As adatom for temperatures less than ∼1160 K and this Si atom can occupy one of As-lattice sites after the incorporation of another As atom, leading to p-type conductivity. In contrast, the ( 2 × 2 ) surface with an As trimer is found to be stabilized under high As-pressure conditions. The surface phase diagram for Si incorporation clarify that an Si atom can be adsorbed at one of Ga-lattice sites of the ( 2 × 2 ) surface with an As trimer for temperatures less than ∼870 K. These calculated results provide one of possible explanations for the formation of p-type and n-type GaAs on GaAs(1 1 1) A surfaces under low and high As-pressure conditions, respectively.

Rare-earth defect pairs in GaN:LDA+Ucalculations

The structural and electronic properties of rare-earth REEu, Er, and Tm related defect pairs in GaN have been investigated theoretically. Based on LDA+U total-energy calculations, their possible role in the luminescence process is discussed. In all charge states, the lanthanides show a strong preference for the Ga-lattice site, either as isolated substitutional or complexed with intrinsic defects. With respect to the electronic valence, a proper description of correlation effects of the strongly localized 4f electrons is shown to be crucial, especially if the REGa is paired with donors like the Ga interstitial or the N vacancy. The pairs formed by REGa substitutionals and vacancies or interstitials lower the symmetry and are found to locally distort the environment. By this, they are quite effective in relaxing the selection rules for the luminescent intra-4f-shell transitions. While for n-type GaN, the next-nearest-neighbor pair REGaVGa pair is energetically favored, for p-type GaN, the REGaVN pair provides the most stable configuration and introduces shallow levels close to the conduction band, which can act as assistant levels in the luminescence process.

Electric field gradients at 151Eu sites in GaN

The electric field gradients at Eu sites in GaN have been investigated in conversion electron Mossbauer spectroscopy (CEMS) in which 151Eu probe ions were implanted into an undoped GaN layer grown on a sapphire substrate. The sample was implanted with 120 keV 151Eu ions to a fluence of 1 × 1015, and annealed at 1,200 K. CEMS spectra of the 151Eu 21.6 keV transition were collected, of the GaN sample as well as of a Si sample implanted with overlapping profiles of 151Eu and O. The GaN spectra were fitted with two symmetric doublets, D1 and D2, with isomer shifts and quadrupole splittings of δ = −0.27 mm/s (relative to Eu2O3), ΔE Q = 0.85 (3) mm/s; and δ = − 0.22 mm/s, ΔE Q = 2.90 (5) mm/s, respectively. D1 is attributed to Eu at substitutional Ga lattice sites; D2 to Eu at or near substitutional sites but with extensive lattice damage. The splittings of D1 and D2 correspond to quadrupole coupling frequency of 15 (2) and 50 (4) MHz, consistent with measurements of 69Ga, 71Ga and 111In in GaN.

The effect of Ga vacancies on the defect and magnetic properties of Mn-doped GaN

We perform first-principles theoretical calculations to investigate the effect of the presence of Ga vacancy on the defect and magnetic properties of Mn-doped GaN. When a Ga vacancy (VGa) is introduced to the Mn ions occupying the Ga lattice sites, a charge transfer occurs from the Mn d band to the acceptor levels of VGa, and strong Mn–N bonds are formed between the Mn ion and the N atoms in the neighborhood of VGa. The charge transfer and chemical bonding effects significantly affect the defect and magnetic properties of Mn-doped GaN. In a Mn-VGa complex, which consists of a Ga vacancy and one Mn ion, the dangling bond orbital of the N atom involved in the Mn–N bond is electrically deactivated, and the remaining dangling bond orbitals of VGa lead to the shallowness of the defect level. When a Ga vacancy forms a complex with two Mn ions located at a distance of about 6 Å, which corresponds to the percolation length in determining the Curie temperature in diluted Mn-doped GaN, the Mn d band is broadened and the density of states at the Fermi level is reduced due to two strong Mn–N bonds. Although the broadening and depopulation of the Mn d band weaken the ferromagnetic stability between the Mn ions, the ferromagnetism is still maintained because of the lack of antiferromagnetic superexchange interactions at the percolation length.

Single phase nanocrystalline GaMnN thin films with high Mn content

Ga 1 − x Mn x N thin films with a Mn content as high as x=0.18 have been grown using ion-assisted deposition and a combination of Rutherford backscattering spectroscopy and nuclear reaction analysis was used to determine their composition. The structure of the films was determined from x-ray diffraction, transmission electron microscopy, and extended x-ray absorption fine structure (EXAFS). The films are comprised of nanocrystals of random stacked GaMnN and there is no evidence of Mn-rich secondary phases or clusters. EXAFS measurements at the Mn and Ga edge are almost identical to those at the Ga edge from Mn-free nanocrystalline GaN films, showing that the Mn occupies the Ga lattice sites, and simulated radial distribution functions of possible Mn-rich impurity phases bear no resemblance to the experimental data. The results indicate that these are the most heavily Mn-doped single phase GaN films studied to date.

Ferromagnetism in cobalt-doped n-GaN

Ferromagnetic ordering is reported in the postannealed samples of Co doped n-GaN formed by Co+ implantation. A maximum Curie temperature ∼250K is recorded for the sample with 8at.% Co. Particle induced x-ray emission–channeling study confirmed the substitutional Co in Ga lattice site. Local atomic arrangement around magnetic impurities is also analyzed using Raman study. A disordered model with carrier mediated coupling of localized magnetic moments is made responsible for the observed ferromagnetic ordering.

Manganese on various lattice sites in (Ga,Mn)As investigated using electron paramagnetic resonance

The differences between variously doped and grown samples of manganese-doped GaAs have been studied and set in relation to the magnetic and electronic properties of these materials. For interion exchange and contrary to the resolved hyperfine and crystal field contributions from the ionized acceptor $\mathrm{Mn}_{\mathrm{Ga}}{}^{2+}$ in GaAs:Mn (doped at ${10}^{17}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$), the spectrum of $\mathrm{Mn}_{\mathrm{Ga}}{}^{2+}$ in (Ga,Mn)As with a dopant concentration of ${10}^{20}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ is almost unresolved. In addition, the electron paramagnetic resonance (EPR) spectra of (Ga,Mn)As exhibit no neutral manganese acceptor contribution. The latter is attributed to the strong compensation of low-temperature molecular-beam-epitaxy-grown (Ga,Mn)As, which is in part caused by interstitial manganese $({\mathrm{Mn}}_{\mathrm{I}})$. Contrary to Mn acceptors on Ga lattice sites, ${\mathrm{Mn}}_{\mathrm{I}}$ acts as a double donor. As the Curie temperature of (Ga,Mn)As films depends on the hole concentration, the compensation of these films also affects their magnetic properties. A comparison between the spectra corresponding to ${\mathrm{Mn}}_{\mathrm{I}}$ inside bulk-doped GaAs:Mn and inside epitaxial (Ga,Mn)As results in the identification of ${\mathrm{Mn}}_{\mathrm{I}}$ in (Ga,Mn)As at dopant concentrations as low as 0.5%. In this way EPR facilitates the identification of interstitial $({\mathrm{Mn}}_{\mathrm{I}})$ and the investigation of ionization and exchange of substitutional $({\mathrm{Mn}}_{\mathrm{Ga}})$ manganese in (Ga,Mn)As.

Effects of Implanted Nitrogen on the Microstructural, Optical, and Magnetic Properties of Mn-Implanted GaN

The effects of implanted nitrogen on microstructural, magnetic, and optical properties of Mn-implanted GaN were studied. N and Mn ions were coimplanted into -GaN and subsequently annealed at 973-1173 K. Compared with Mn-implanted samples, the Curie temperature and magnetic moment significantly increased. From high-resolution transmission electron microscopy, Mn nitrides such as and drastically decreased, and the concentration of N vacancies was reduced by the N implantation. This led to the increase of Mn concentration occupying Ga lattice sites, evident by the shift of Raman mode at to higher energy by about . The photoluminescence peak at 2.92 eV shifted to 2.86 eV and became strong with N implantation, indicating an increase in effective hole concentration increased due to an enhanced activation of Mn impurities in -GaN. Consequently, the enhancement of ferromagnetic property by coimplantation of Mn and N ions originated from the increase of hole concentration via the increase of Mn concentration in GaN, due to the suppression of production of Mn-N compounds.

Interstitial manganese in (Ga,Mn)As detected by electron paramagnetic resonance

Various types of Mn in epitaxially grown (Ga,Mn)As have been investigated. Ionized interstitial manganese donors Mn I 2 + have been found to occur in (Ga,Mn)As at dopant concentrations as low as 0.5%. A comparison with spectra from interstitial Mn I 2 + inside bulk-doped GaAs:Mn yields a slight decrease of 1.5% in the hyperfine splitting with increasing dopant concentration. This is attributed to an increase in the lattice constant of (Ga,Mn)As with increasing manganese concentration. Contrary to Mn interstitials, Mn dopants on Ga lattice sites acts as acceptors. It is shown that Mn on Ga lattice sites Mn Ga 2 + is non-uniformly distributed. A low percentage of isolated Mn acceptors can be distinguished from the exchange broadened Mn Ga 2 + signal. Thus, electron paramagnetic resonance is a promising tool for the investigation of (Ga,Mn)As and the classification of various types of Mn dopants responsible for the magnetic properties of (Ga,Mn)As.

Local structural change in GaCrN grown by radio frequency plasma-assisted molecular-beam epitaxy

Local structural change around Cr atoms in GaN-based diluted magnetic semiconductor GaCrN grown by radio frequency plasma-assisted molecular-beam epitaxy was studied with fluorescent X-ray absorption fine structure (XAFS) and X-ray diffraction measurements. The lattice constant of Ga 1− x Cr x N depends linearly on x up to x = 4 % and it approaches to that of non-doped GaN above x of 4.7%. It was found from our XAFS study that the majority of Cr atoms substitute the Ga lattice sites of GaN and that the critical Cr concentration of solid solution is around x = 4 % . At the critical Cr concentration, onset of the formation of NaCl-type CrN-like structure occurs, and above the critical concentration, the local structure around Cr atoms transforms to a completely CrN-like structure. Our X-ray absorption near-edge structure studies also indicate a drastic change in local electronic structure of Ga 1 −x Cr x N at x between 4% and 4.7%. This is ascribed to the fact that the atomic configuration of Cr and N undergoes a drastic transition from the substitutionally incorporated GaCrN into the CrN-like atomic configuration. The local structural change into NaCl-type CrN was also supported by the XAFS pre-edge structures and the polarized XANES spectra.

Optical Study of Plasmon–LO Phonon Modes in Ga1-xMnxAs

Ga1−xMn x As layers with Mn fractions 0 ≤ x ≤ 2.8 % grown on GaAs(001) substrates by low-temperature molecular beam epitaxy were investigated using micro-Raman spectroscopy and far-infrared (FIR) reflectance spectroscopy. The Raman and FIR spectra are strongly affected by the formation of coupled modes of longitudinal optical phonons and hole plasmons. A full line-shape analysis of the spectra was performed within a Lindhard–Mermin model for the dielectric function, including intraband and interband hole transitions. Annealing at temperatures between 250 and 500°C results in a decrease of the hole density with increasing annealing temperature and total annealing time. Simultaneously, a reduction of the number of Mn atoms on Ga lattice sites is deduced from high-resolution X-ray diffraction. After annealing at 450°C the Raman lines of elemental As are observed, which are due to the precipitation of As on the sample surface.

Effect of Be++O+ coimplantation on Be acceptors in GaN

P-type regions were produced in undoped GaN films by Be+ and Be++O+ implantation and subsequent annealing at temperatures between 1000 and 1050 °C. From thermal admittance spectroscopic measurements, the activation energy of the Be acceptor level was found to decrease from ∼240 to ∼163 meV by the implantation of additional O atoms, which is in reasonable agreement with the improvement in p-type doping characteristics determined by room-temperature Hall-effect measurements. These results indicate that Be++O+ coimplantation reduces the depth of the Be acceptor level based on a site-competition effect. Therefore, these acceptor levels are most probably attributable to Be atoms at interstitial and Ga-lattice sites.

Coupled plasmon–LO-phonon modes inGa1−xMnxAs

The vibrational and electronic properties of ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$ layers with Mn fractions $0<~x<~2.8%,$ grown on GaAs(001) substrates by low-temperature molecular-beam epitaxy, are investigated by micro-Raman spectroscopy and far-infrared (FIR) reflectance spectroscopy. The Raman and FIR spectra are strongly affected by the formation of a coupled mode of the longitudinal optical phonon and the hole plasmon. The spectral line shapes are modeled using a dielectric function where intraband and interband transitions of free holes are included. In addition to the coupled mode, the contributions of a surface depletion layer as well as a symmetry forbidden TO phonon have to be taken into account for the Raman spectra. Values for the hole densities are estimated from a full line-shape analysis of the measured spectra. Annealing at temperatures between 250 and $500\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ results in a decrease of the hole density with increasing annealing temperature and total annealing time. Simultaneously, a reduction of the fraction of Mn atoms on Ga lattice sites is deduced from high-resolution x-ray diffraction.