Effective-mass Donor Research Articles

Bistable behavior of the nitrogen impurity in SiC nanoclusters.

Nitrogen doped SiC nanoclusters of various shapes and polytype structures were modeled in the framework of density functional theory using the PBE0 hybrid functional. While in the bulk crystal NC impurity may be introduced in a large concentration and shows features of the conventional shallow effective-mass donor, in the nanocluster this defect can exist in two forms with sharply different electronic structures. The first one resembles the shallow donor confined in the nanocluster with a small and nearly isotropic lattice relaxation, and spin density spread over distant shells. The second form resembles a small bound polaron structure with a large and anisotropic lattice relaxation, which leads to a strong localization of the spin density on one of the nearest silicon atoms. Relative energies of these two states depend on the location of nitrogen in the lattice as well as on the shape and polytype structure of the nanocluster. The calculations show that the position of the impurity in the vicinity of the hexagonal double layers and cluster surface facilitates the formation of small-bound-polaron-like states. As follows from the calculations, the two impurity states can be experimentally distinguished by the values of the hyperfine and quadrupole coupling parameters that can be measured in magnetic resonance experiments. They are also characterized by rather different electronic absorption spectra. We have demonstrated that for negatively charged SiC nanoclusters the polaron-like impurity states can be formed even if they do not exist in neutral clusters. This provides a reason to believe that the nitrogen impurity occupying particular sites in SiC nanoclusters is an analog of the DX centers in the bulk crystal. In addition, the formation of the polaron-like states depends on the method of the surface passivation. In particular, the substitution of the Si-H surface bond with the Si-NH3 one facilitates a bistable behavior of the NC impurity located in the central part of the nanocluster. We have also shown that the effect of the self-purification manifests itself in different ways depending on the nanocluster shape and polytype structure.

Conduction mechanism and shallow donor properties in silicon-dopedɛ-Ga2O3thin films: An electron paramagnetic resonance study

The defects in Si-doped \ensuremath{\epsilon}-${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ epitaxial layers have been investigated by electron paramagnetic resonance (EPR) spectroscopy. The results show that Si doping introduces a single, paramagnetic defect, attributed to Si incorporation on the tetrahedral gallium lattice site. It is a spin $S=1/2$ center with an axial $g$ tensor with principal values of $g//c=1.9573$ and $g\ensuremath{\perp}c=1.9591$. The temperature dependence of the EPR parameter demonstrates that it is a shallow effective mass donor, which is at the origin of the $n$-type conductivity. The EPR spectrum is modified by motional narrowing effects, the analysis of which allows one to reveal different transport regimes, including localization, hopping conductivity, and ionization in the conduction band when the temperature is raised from $T=4\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ to room temperature. Partial electrical compensation and donor clustering are equally evidenced by the EPR results, which are confirmed by correlated electrical transport measurements. Silicon is thus a promising dopant for the formation of highly conductive $n$-type \ensuremath{\epsilon}-${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$.

Electron spin resonance and Rashba field in GaN-based materials

We discuss problem of Rashba field in bulk GaN and in GaN/AlxGa1−xN two-dimensional electron gas, basing on results of X-band microwave resonance experiments. We point at large difference in spin–orbit coupling between bulk material and heterostructures. We observe coupled plasmon–cyclotron resonance from the two-dimensional electron gas, but no spin resonance, being consistent with large zero-field spin splitting due to the Rashba field reported in the literature. In contrast, small anisotropy of g-factor of GaN effective mass donors indicates rather weak Rashba spin–orbit coupling in bulk material, not exceed 400G, αBIA<4×10−13eVcm. Furthermore, we observe new kind of electron spin resonance in GaN, which we attribute to surface electron accumulation layer. We conclude that the sizable Rashba field in GaN/AlxGa1−xN heterostructures originates from properties of the interface.

Origin of the red luminescence in Mg-doped GaN

Optically detected magnetic resonance and positron annihilation spectroscopy experiments have been employed to study magnesium-doped GaN layers grown by metal-organic vapor phase epitaxy. As the Mg doping level is changed, the combined experiments reveal a strong correlation between the vacancy concentrations and the intensity of the red photoluminescence band at 1.8eV. The analysis provides strong evidence that the emission is due to recombination in which electrons both from effective-mass donors and from deeper donors recombine with deep centers, the deep centers being vacancy-related defects.

Pressure‐dependent photoluminescence study of CuGaSe2

AbstractWe present the results of a pressure‐dependent photoluminescence (PL) study on CuGaSe2 films grown on GaAs substrate by metalorganic vapor phase epitaxy. The low‐temperature PL spectra of the CuGaSe2 samples measured at atmospheric pressure are dominated by one near band edge exciton luminescence line and two strong and relatively broad emissions associated with donor acceptor pairs (DAP) transitions. All the observed luminescence emission lines shift toward higher energy with increasing pressure at the same rate. The nearly identical pressure coefficients of the two DAP emissions as compared to that of the exciton emission confirm the suggestion that the recombination processes associated with the DAPs involve one shallow effective‐mass donor and two different acceptor species with different binding energies and related to two different native defects. (© 2004 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Intrinsic defects in GaN. I. Ga sublattice defects observed by optical detection of electron paramagnetic resonance

Irradiation of GaN by 2.5-MeV electrons in situ at 4.2 K produces a broad photoluminescence (PL) band centered at 0.95 eV. Optical detection of electron paramagnetic resonance (ODEPR) in the band reveals two very similar, but distinct, signals, L5 and L6, which we identify as interstitial ${\mathrm{Ga}}^{2+}$ in two different lattice configurations. L5, present immediately after the irradiation, is seen via a spin-dependent electron transfer process from the shallow effective-mass donor (EM) which competes with the PL (negative signal). L6 emerges upon annealing at various stages starting at $\ensuremath{\sim}60\mathrm{K},$ possibly assisted by optical excitation, as a spin-feeding process (positive signal) not involving the EM donor. Both L5 and L6 disappear upon prolonged annealing at room temperature, with L6 disappearing first. Most of the 0.95-eV band $(\ensuremath{\sim}85%)$ also disappears in this anneal, the remaining fraction being stable to $\ensuremath{\sim}500\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$ Two tentative models are presented, each of which identifies L5 and L6 with ${\mathrm{Ga}}^{2+}$ in different interstitial sites near the gallium vacancy from which they were created. Both models ascribe the 0.95-eV PL band, along with an ODEPR signal observed in it L1 as arising from the Ga vacancy, which in its isolated form is therefore stable to $\ensuremath{\sim}500\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$

Magneto-optical studies of free-standing hydride-vapor-phase epitaxial GaN

Magneto-optical studies of donor excitation in hydride-vapor-phase epitaxial GaN are reported. Donor ground-to-excited state transitions are observed in the infrared for Si, O, and a third unidentified donor as a functionof magnetic field to 11 T. Transitions from the ground state to 2p γ and 3p, excited states are studied. Values for effective mass ground and excited state binding energies are determined from the observed excited-state separation. We find a value of 29.1′0.5 meV for the effective-mass donor binding energy in GaN. Ground state binding energies for Si G a and ON are 30.18′0.1 meV and 33.20′0.1 meV, respectively. Separation rates for the 2p + and 2p excited states with magnetic field are consistent with an electron effective mass of 0.22m 0 .

Magnetic resonance studies of Mg-doped GaN epitaxial layers grown by organometallic chemical vapor deposition

Abstract : Electron paramagnetic resonance (EPR) and optically detected magnetic resonance (ODMR) experiments have been performed on a set of GaN epitaxial layers doped with Mg from 2.5 x 10(exp 18) to 5.0 x 10(exp 19)/cu cm. The samples were also characterized by secondary-ion-mass spectroscopy (SIMS), temperature-dependent Hall effect, and low-temperature photoluminescence (PL) measurements. EPR at 9 GHz on the conductive films reveals a single line with g(sub parallel) approx. 2.1 and g(sub perpendicular) approx. 2 and is assigned to shallow Mg acceptors based on the similarity of the spin density with that found for the number of uncompensated Mg shallow acceptors from Hall effect and the total Mg concentration by SIMS. PL bands of different character are observed from these layers, including shallow-donor shallow-acceptor recombination at 3.27 eV from the lowest doped sample and, in most cases, broad emission bands with peak energy between 2.8 and 3.2 eV from the more heavily doped films. In addition, several of the films exhibit a weak, broad emission band between 1.4 and 1.9 eV. ODMR at 24 GHz on the blue PL bands reveals two dominant features. The first is characterized by g(sub parallel), g(sub perpendicular) approx. 1.95-1.96 GHz and is assigned to shallow effective-mass donors. The second line is described by similar g tensors as found by the EPR experiments and, thus, is also attributed to shallow Mg acceptors. Although several groups have related the 2.8 eV PL in heavily Mg-doped GaN with the formation of deep donors, no clear evidence was found from the ODMR on this emission for such centers. However, based on the near-midgap PL energy and the observation of the feature assigned to shallow Mg acceptors, the strongest case from magnetic resonance for the existence of deep donors in these films is the isotropic ODMR signal with g=2.003 found on emission <1.9 eV.

Gvalues of effective mass donors inAlxGa1−xNalloys

Electron spin resonance experiments were performed on Si-doped wurtzite and zinc-blende GaN and Si-doped wurtzite ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ alloys with $x=0.15,$ 0.32, 0.52, 0.75, and 1. For zinc-blende GaN, an isotropic g factor of 1.9475 is found. The g tensors of the silicon effective mass donor in wurtzite GaN and ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ alloys are determined experimentally, and for $x&lt;0.38$ are identical within experimental error to tensors observed in unintentionally n-type-doped material. With increasing Al content, the g values increase and the g-tensor anisotropy is reduced. The g values are calculated within a five-band $k\ensuremath{\cdot}p$ model in the cubic approximation. To account for the isotropic g factor of wurtzite AlN, a nonvanishing spin-orbit coupling of the higher ${\ensuremath{\Gamma}}_{5}$ conduction bands of the order of 50 meV is necessary. To describe the anisotropy at lower Al content, a full seven-band $k\ensuremath{\cdot}p$ model for the wurtzite crystal structure is investigated. The model shows that the anisotropy in this material system is also due to coupling to higher conduction bands rather than to valence bands. Additional estimates for the momentum matrix elements and the interband mixing coefficients are discussed.

DX-behavior of Si in AlN

In Si doped AlN, a large persistent photoconductivity is found for temperatures below 60 K after exposure to light with photon energies above 1.5 eV. Simultaneously, a persistent electron spin resonance signal is observed with an isotropic g factor of 1.9885 due to an effective mass donor state, while no spin resonance signal is detectable after cooling the sample in the dark. Both observations show that Si undergoes a $\mathrm{DX}$-like metastability in this material. Based on the experimental findings, a detailed configuration diagram is proposed.

Defect identification in homoepitaxial- and ELO-grown GaN layers using bound-exciton Zeeman spectroscopies

Two principal donor bound-exciton (BE) lines with a linewidth of 0.6 meV at 4.2 K have been observed in both the not intentionally doped homoepitaxial GaN layer and the hydride vapor phase (HVPE) grown epitaxial lateral overgrowth (ELO) GaN layer. When the external magnetic field was applied perpendicular to c-axis up to 8 T, each BE line splits into two components due to the Zeeman effects. The localization center, which binds exciton, is related to a simple donor impurity. From the observed two-electron satellite peaks, the related donor levels are estimated to be about 25.1 and 31.5 meV, which are smaller than the effective mass donor binding energy.

Effective mass donors in silicon carbide — a study with electron nuclear double resonance

With electron nuclear double resonance the superhyperfine interactions of numerous 29Si and 13C lattice neighbours of 14N donors in 3C and 4H–SiC have been measured. It was attempted to explain them using the effective mass theory (EMT). The interactions of N donors on cubic sites in 3C–SiC can be well explained with EMT assuming N to be on C sites, but not on Si sites. N in 3C–SiC represents probably the only true EMT donor, since for shallow donors in Si EMT is not sufficient as shown previously. The interactions of N on the hexagonal sites in 4H–SiC are still reasonably well explained, while the EMT approach fails to explain N on the quasi-cubic sites. Comparison between 3C–SiC and 4H–SiC shows that the “central cell corrections” due to the chemical nature of N introduced into EMT previously for silicon is very small. The major contribution comes from inter-valley interactions.

Zeeman spectroscopy of shallow donors in GaN

Gallium nitride grown by hydride vapor phase epitaxy has been studied by infrared spectroscopy in the region of donor intrasite electronic excitations as a function of magnetic field and temperature. Infrared-absorption features which are present only when the sample temperature is cooled to near 4 K have been observed. These features are identified as due to ${1s\ensuremath{-}2p}_{\ifmmode\pm\else\textpm\fi{}}$ transitions which split in a magnetic field, and a ${1s\ensuremath{-}2p}_{0}$ transition which does not split. A donor effective mass of ${0.22m}_{0}$ is calculated from the rate of splitting with field. We assume that the lower-energy transition occurs at an effective-mass donor and calculate donor binding energies of 31.1 and 33.8 meV for the two donors observed, and a static dielectric constant of 9.8.

Location Of Residual Donors In GaN Epitaxial Layers

AbstractOptically-detected electron-nuclear double resonance (ODENDOR) studies at 24 GHz on high-resistivity GaN films grown on Al2O3 have been combined with x-ray diffraction measurements to obtain information on the location of the residual shallow donors. Strong ODENDOR assigned to 69,71Ga lattice nuclei was detected on the g=1.951 effective-mass donor resonance found on the 2.2 eV emission bands. The x-ray studies reveal that the layers are under biaxial compression with high values of strain (∼ 2–3 × 10−3). The quadrupole splittings for 69Ga are smaller than those reported for strain-free samples by 15–25 %. The dominant sources of the local electric field gradient (EFG) responsible for the splittings are attributed to the wurtzite crystal structure and the strain fields that arise from the lattice constant mismatch and the difference in thermal expansion coefficients. An EFG/strain relationship of 3 × 1022 Vm−2 per unit strain at the 69,71Ga nuclei is deduced. The ODENDOR can be described with asymmetry parameter η=0. This provides evidence that the donors are in the crystallites rather than near grain boundaries.

Metal‐Insulator Transition in Degenerately Doped Si and Ge under High Uniaxial Pressure

AbstractStrain‐induced metal‐insulator (MI) transitions have been observed in degenerately doped Si:Sb and Ge:Sb crystals at T = 4.2 K and for extremely high uniaxial pressures up to 5 GPa. It is shown that in both Si and Ge the transition from a metallic‐type to an activated conductivity is determined by strain‐induced changes of the conduction band parameters. The data obtained demonstrate that the effective mass donor approach and the scaling theory of localization describe well the MI transition phenomenona observed.

Spin Resonance Investigations of GaN and AlGaN

ABSTRACTElectron Spin Resonance (ESR) and related methods have been used to study defects in GaN and in AlxGa1-xN ternary alloys. In particular, Light-induced ESR of a thick MOCVD grown layer of GaN shows that the ESR signature of the deep defect appears for excitation energies &gt; 2.6 eV and saturates above 2.7 eV. This information provides a direct measure of the energetic level position of the diamagnetic to paramagnetic transition of this center. Furthermore, standard ESR investigations of MBE-grown layers of AlxGa1-xN alloys were performed with emphasis on the effective-mass donor resonance. Composition and temperature-dependent measurements of the resonance position are presented.

Donors in cubic silicon carbide

Clearly resolved donor excitation spectra in cubic (3C) silicon carbide have been studied in infrared transmission as a function of temperature and applied magnetic field. Accurate positions for excited states of nitrogen on the carbon site have been determined and are in excellent agreement with effective mass theory using values for longitudinal and transverse effective masses from cyclotron resonance results. Photoluminescence spectra in the two-electron satellite region are consistent with the transmission results and give the nitrogen 2s state binding energy. Two other donors are observed with low concentrations: an effective mass donor at 47.7 meV and a very shallow donor with a binding energy of 34 meV. Zeeman splitting of the 2p ± excited states with B ∥ [ 111 ] for all three donors is in good agreement with the previously measured transverse effective mass. Valley-orbit splitting of the nitrogen ground state is observed but no valley-orbit splitting can be seen for the effective mass donor as expected. We suggest that the very shallow donor has a repulsive core (negative chemical shift) and inverted ordering with the 1s(A 1) ground state above the 1s(E) ground state similar to lithium in silicon.

Spin-Dependent Transport in GaN Light Emitting Diodes

ABSTRACTElectrically detected magnetic resonance (EDMR) is used to study recombination processes in two types of gallium nitride light emitting diodes: in m/i/n/n+- and InGaN/AlGaN double-heterostructure devices. In the MIS-diodes, two resonances at g=1.96 and 2.00, corresponding to the effective mass donor and a deep defect are observed at room temperature. At low temperatures, an acceptor-related resonance at g=2.06 is visible as well. After current degradation, the spectra are dominated by the defect resonance, indicating that the creation of this defect is responsible for the decreased electroluminescence efficiency. In the double-heterostrucrure devices, EDMR can only be observed below 60 K showing the g=2.00 defect resonance. The same defect resonance is also observed in conventional electron spin resonance experiments under illumination (light-induced ESR).

Photoluminescence excitation of the As iV Ga 0.8 eV emission in GaAs grown by molecular beam epitaxy at low temperatures

We report the above-bandgap photoluminescence excitation spectra obtained at 4.2K for the As i V Ga 0.8eV photoluminescence emission present in GaAs grown by molecular beam epitaxy at low temperatures 325–400°C. We observed oscillating structures due to the excitations from both the heavy- and light-hole valence bands. The oscillating structures are attributed to the energy relaxation of the conduction-band electrons by the successive emission of the zone-center longitudinal optical phonons, along with energy-dependent capture of the conduction-band electrons by As i V Ga centers via a shallow effective-mass donor, or direct capture by the shallow donor. The oscillating structure due to the light-hole excitation is attributed to the splitting of the heavy- and light-hole valence bands.

Properties Of crystalline 3C-SIC Grown From Methyl Trichlorosilane

ABSTRACTCubic silicon carbide grown by thermal decomposition of methyl trichlorosilane in hydrogen has been studied in infrared (IR) transmission, cyclotron resonance (CR), and photoluminescence (PL). Sample characteristics include: The nitrogen concentration, determined from CR and IR can be of the order of 1015 cm-3 or less. Ionized impurity concentrations determined from the linewidths of CR and IR spectra can be less than 7 × 1014 cm-3. Low temperature electron mobilities from CR are near 105 cm2 v-1 s-1. Carrier freeze-out is consistent with either uncompen-sated donors with activation energy near 50 meV or heavily compensated donors with 20 meV activation energy. The latter choice, however, is excluded by the low ionized impurity concentration. These high quality samples show little or no evidence of the apparently shallow (&lt; ∼20 meV) donor which usually dominates the low temperature conductivity of 3C-SiC grown on Si. Nitrogen donor excitation spectra are in excellent agreement with effective mass theory. Only nitrogen donors at 54.2 meV and an unidentified effective-mass donor at 47.8 meV are observed. If material with the characteristics described above can be reliably reproduced then there is substantial expectation that high quality electronic devices based on 3C-SiC can be fabricated.