Si Donors Research Articles

Comparison of hole traps in n-GaN grown by hydride vapor phase epitaxy, metal organic chemical vapor deposition, and epitaxial lateral overgrowth

Optical deep level spectroscopy (ODLTS) and microcathodoluminescence (MCL) spectra were measured for a large group of n-GaN samples grown via metalorganic chemical vapor deposition (MOCVD), epitaxial lateral overgrowth (ELOG), or hydride vapor phase epitaxy (HVPE). In the MOCVD and ELOG samples, the ionization energy of dominant hole traps H1 was dependent on the excitation conditions and was ∼0.9 eV for high injection levels providing saturation of the ODLTS peak magnitude. The trap concentration increased with increasing Si donor concentration and correlated with the yellow band intensity in the MCL spectra. For the HVPE samples, the hole trap spectra were radically different from the MOCVD case: four hole traps—H2, H3, H4, and H5—with activation energies of 0.55, 0.65, 0.85, and 1.2 eV, respectively, were detected. In the MCL spectra, a broad green band that peaked near 2.5 eV was observed in addition to the usual yellow luminescence near 2.3 eV. This green band was attributed to the transitions involving the H4 hole traps. Possible identities of the hole traps detected in the MOCVD/ELOG and HVPE samples are discussed.

Silicon in AlN: shallow donor and DX behaviors

AbstractIn unintentionally Si‐doped AlN bulk samples, an electron paramagnetic resonance (EPR) spectrum with characteristics of a shallow donor, previously assigned to the shallow Si donor, was observed at room temperature in darkness. Temperature dependent studies of the EPR signal showed that Si is a DX center in AlN. However, with the negatively charged DX– state determined to be only ∼78 meV below the neutral shallow donor state, Si should behave as a shallow dopant in AlN at normal device operating temperatures. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Dual temperature process for reduction in regrowth interfacial charge in AlGaN/GaN HEMTs grown on GaN substrates

AbstractThe effects of growth temperature and Mg compensation doping on the structural and electrical properties of AlGaN/AlN/GaN high electron mobility transistor (HEMTs) structures grown on low threading dislocation density bulk GaN substrates by metalorganic chemical vapor deposition were investigated. The background electron concentration in the regrown GaN was found to decrease from 1.5x1018 cm‐3 to 2x1016 cm‐3 as the growth temperature was reduced from 1100 °C to 950 °C. A dual temperature process was then employed for growth of the GaN base layer and AlGaN/AlN/GaN top heterostructure in the HEMT along with Mg doping of the GaN base to compensate residual Si donors at the regrown interface. Using this approach, AlGaN/AlN/GaN HEMTs were produced that had a sheet carrier density as high as 1.1x1013 cm‐3 with a room temperature mobility of 1600 cm2/Vs. The incorporation of a thin AlN layer at the regrown interface was found to reduce the sheet carrier density of the overall structure (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Properties of metal-insulator transition and electron spin relaxation in GaN:Si

We investigate properties of doping-induced metal-insulator transition in GaN:Si by means of electron spin resonance and Hall effect. While increasing the doping concentration, Si-related bands are formed below the bottom of the GaN conduction band. The D0 band of single-occupied Si donor sites is centered 27 meV below the bottom of the GaN conduction band, the D- band of double-occupied Si states at 2.7 meV below the bottom of the GaN conduction band. Strong damping of the magnetic moment occurs due to filling of the D- states at Si concentrations approaching the metal-insulator transition. Simultaneously, shortening of electron spin relaxation time due to limited electron lifetime in the single-occupied D0 band is observed. The metal-insulator transition occurs at the critical concentration of uncompensated donors equal to about 1.6 * 10^18 cm^-3. Electronic states in metallic samples beyond the metal-insulator transition demonstrate non-magnetic character of double-occupied states.

Atomically resolved silicon donor states of β-Ga2O3

The electronic states of silicon donors in a wide gap semiconductor, β-Ga2O3(100), have been studied using low-temperature scanning tunneling microscopy. We observe one-dimensional rows along [010], as expected from the crystal structure. In addition, substitutional Si donors are identified up to the fourth subsurface layer with clear spectroscopic features at the bottom of the conduction band. The decay length of each subsurface Si donor is systematically measured, and reasonably agrees with a picture of the Si donor in bulk β-Ga2O3. These results strongly suggest that Si impurities are shallow donors and responsible for the high electrical conductivity of β-Ga2O3.

Dielectric mismatch and central-cell corrections in doped silicon nanodots

The effect of the central-cell corrections on the shallow donor states in Si spherical quantum dot is studied within the effective mass approximation. Finite step-like spatial confining potential, Coulomb and image charge potentials arising from the dielectric mismatch at the interface of the media are taken into account. We found that it is possible to tune the impurity energies by varying the dot radius and dielectric constant of the barrier material. In the strong confinement regime, due to the enhanced weight of the donor wave functions on the impurity atoms, large values of the chemical shifts for typical donors in Si compared to the ones in bulk are obtained. The calculated size-dependence of the effective Bohr radius in donor doped nanocrystals is in reasonable accord with electron spin resonance measurements on Si quantum dots embedded in insulating glass matrices. We conclude that both the dielectric mismatch and central-cell corrections must be considered in the study of these systems in order to obtain satisfactory agreement with experimental data.

Manipulating and Imaging the Shape of an Electronic Wave Function by Magnetotunneling Spectroscopy

We measure the current due to electrons tunneling through the ground state of hydrogenic Si donors placed in a GaAs quantum well in the presence of a magnetic field tilted at an angle to the plane of the well. The component of B parallel to the direction of current compresses the donor wave function. By measuring the current as a function of the perpendicular component of B, we probe how the magnetocompression affects the spatial form of the wave function and observe directly the transition from Coulombic to magnetic confinement at high fields.

Coherent electron transport by adiabatic passage in an imperfect donor chain

Coherent Tunneling Adiabatic Passage (CTAP) has been proposed as a long-range physical qubit transport mechanism in solid-state quantum computing architectures. Although the mechanism can be implemented in either a chain of quantum dots or donors, a 1D chain of donors in Si is of particular interest due to the natural confining potential of donors that can in principle help reduce the gate densities in solid-state quantum computing architectures. Using detailed atomistic modeling, we investigate CTAP in a more realistic triple donor system in the presence of inevitable fabrication imperfections. In particular, we investigate how an adiabatic pathway for CTAP is affected by donor misplacements, and propose schemes to correct for such errors. We also investigate the sensitivity of the adiabatic path to gate voltage fluctuations. The tight-binding based atomistic treatment of straggle used here may benefit understanding of other donor nanostructures, such as donor-based charge and spin qubits. Finally, we derive an effective 3 \times 3 model of CTAP that accurately resembles the voltage tuned lowest energy states of the multi-million atom tight-binding simulations, and provides a translation between intensive atomistic Hamiltonians and simplified effective Hamiltonians while retaining the relevant atomic-scale information. This method can help characterize multi-donor experimental structures quickly and accurately even in the presence of imperfections, overcoming some of the numeric intractabilities of finding optimal eigenstates for non-ideal donor placements.

Electron Spin Coherence and Electron Nuclear Double Resonance of Bi Donors in Natural Si

Donors in silicon hold considerable promise for emerging quantum technologies, due to their uniquely long electron spin coherence times. Bismuth donors in silicon differ from more widely studied group V donors, such as phosphorous, in several significant respects: They have the strongest binding energy (70.98meV), a large nuclear spin (I=9/2), and a strong hyperfine coupling constant (A=1475.4 MHz). These larger energy scales allow us to perform a detailed test of theoretical models describing the spectral diffusion mechanism that is known to govern the electron spin decoherence of P donors in natural silicon. We report the electron-nuclear double resonance spectra of the Bi donor, across the range 200MHz to 1.4GHz, and confirm that coherence transfer is possible between electron and nuclear spin degrees of freedom at these higher frequencies.

Magnetic characterization of conductance electrons in GaN

New electron paramagnetic resonance (EPR) measurements in hexagonal and cubic GaN intentionally doped with silicon are presented. In both type of samples the well-known EPR resonance of the dominant shallow donor is observed, whereby the g-tensors are determined to g|| = 1.9512, g⟂ = 1.9485 (free-standing hexagonal GaN) and g = 1.9533 (cubic GaN layer grown on 3C-SiC substrate). The spectra show an exceptionally small line width below 0.4 mT and contain no further signature. As a result, beside the line width itself, the EPR line is characterized by its g-tensor exclusively. With the help of a qualitative analysis of the Si donor wave function within effective mass theory (EMT) and a followed up calculation of the hyperfine (HF) splittings in the framework of density functional theory (DFT) the characteristic shape of the EPR lines can be explained by an enhanced delocalization of the unpaired electrons of shallow Si donors at the gallium sublattice due to overlapping impurity and conduction bands.

Coherent control of Rydberg states in silicon

Laser cooling and electromagnetic traps have led to a revolution in atomic physics, yielding dramatic discoveries ranging from Bose-Einstein condensation to the quantum control of single atoms. Of particular interest, because they can be used in the quantum control of one atom by another, are excited Rydberg states, where wavefunctions are expanded from their ground-state extents of less than 0.1 nm to several nanometres and even beyond; this allows atoms far enough apart to be non-interacting in their ground states to strongly interact in their excited states. For eventual application of such states, a solid-state implementation is very desirable. Here we demonstrate the coherent control of impurity wavefunctions in the most ubiquitous donor in a semiconductor, namely phosphorus-doped silicon. In our experiments, we use a free-electron laser to stimulate and observe photon echoes, the orbital analogue of the Hahn spin echo, and Rabi oscillations familiar from magnetic resonance spectroscopy. As well as extending atomic physicists' explorations of quantum phenomena to the solid state, our work adds coherent terahertz radiation, as a particularly precise regulator of orbitals in solids, to the list of controls, such as pressure and chemical composition, already familiar to materials scientists.

Hyperfine Structure and Nuclear Hyperpolarization Observed in the Bound Exciton Luminescence of Bi Donors in Natural Si

As the deepest group-V donor in Si, Bi has by far the largest hyperfine interaction and also a large I = 9/2 nuclear spin. At zero field this splits the donor ground state into states having total spin 5 and 4, which are fully resolved in the photoluminescence spectrum of Bi donor bound excitons. Under a magnetic field, the 60 expected allowed transitions cannot be individually resolved, but the effects of the nuclear spin distribution, -9/2 < or = I(z) < or = 9/2, are clearly observed. A strong hyperpolarization of the nuclear spin towards I(z) = -9/2 is observed to result from the nonresonant optical excitation. This is very similar to the recently reported optical hyperpolarization of P donors observed by EPR at higher magnetic fields. We introduce a new model to explain this effect, and predict that it may be very fast.

Regional Variation in the Levels of Macular Xanthophylls and Carotenoids in Dietary Components: Comparing North and South India

Multiple epidemiological studies have emphasized the intake of dark green leafy vegetables rich in xanthophylls in reducing the risk of developing age-related macular degeneration (AMD). Therefore, the present study was undertaken to quantify the levels of major carotenoids in commonly consumed fruits and vegetables of Indian origin and of xanthophylls in the macula of Indian human donor eyes. Fresh fruits (n=20) and vegetables (n=51) collected from two zones of India were tested for the estimation of xanthophyll, lycopene and β-carotene by using HPLC with Photodiode Array Detection. Lutein and zeaxanthin were quantified from macula and in selected vegetables collected from both southern (SI) and northern (NI) regions of India. Xanthophylls, β-carotene and lycopene were found in many affordable vegetables commonly available for consumption in India. Higher content of lutein and zeaxanthin was confirmed in many economical leafy vegetables and fruits. Surprisingly, the mean macular levels of lutein and zeaxanthin of SI donor eyes (n=13) were found to be significantly (p<0.001) four times less than in NI donor eyes (n=15) and the macular levels of Northern India were comparable with reported levels in western populations. The present study showed considerable levels of xanthophylls in many of the commonly consumed fruit and vegetable sources in both parts of India. However, SI donor eyes showed lower levels as compared to NI donors and this warrants further investigation about the bioavailability of xanthophylls in their blood and food intake. The relevance of these findings with prevalence of AMD in South India needs to be explored.

Magnetodielectric coupling in nonmagnetic Au/GaAs:Si Schottky barriers

We report on a heretofore unnoted giant negative magnetocapacitance (>20%) in non-magnetic Au/GaAs:Si Schottky barriers that we attribute to a magnetic field in-duced increase in the binding energy of the shallow donor Si impurity atoms. Depletion capacitance (Cdep) dispersion identifies the impurity ionization and capture processes that give rise to a magnetic field dependent density of ionized impurities. Internal photoemission experiments confirm that the large field-induced shifts in the built-in potential, inferred from 1/Cdep^2 vs voltage measurements, are not due to a field-dependent Schottky barrier height, thus requiring a modification of the abrupt junction approximation that accounts for the observed magnetodielectric coupling.

Properties of Fast-Diffusing Oxygen Species in Silicon Deduced from the Generation Kinetics of Thermal Donors

The generation of Thermal Donors in Si is a nucleation process controlled by several mobile On clusters. The rate-limiting transitions are found to be O1  O2 and O4  O5. The individual transition rates G12 and G45, and also G23 and G34 are deduced from the experimental data. From the transient variation of the generation rate G(t), the equilibrium concentration of the dimers is found, and with it the dimeric diffusivity is also defined. In samples pre-treated at high T, the G(t) dependence has a maximum, due to quenched-in fast-diffusing oxygen monomers (FDMs). The concentration and diffusivity of FDMs were determined.

Simultaneous Subsecond Hyperpolarization of the Nuclear and Electron Spins of Phosphorus in Silicon by Optical Pumping of Exciton Transitions

We demonstrate a method which can hyperpolarize both the electron and nuclear spins of 31P donors in Si at low field, where both would be essentially unpolarized in equilibrium. It is based on the selective ionization of donors in a specific hyperfine state by optically pumping donor bound exciton hyperfine transitions, which can be spectrally resolved in 28Si. Electron and nuclear polarizations of 90% and 76%, respectively, are obtained in less than a second, providing an initialization mechanism for qubits based on these spins, and enabling further ESR and NMR studies on dilute 31P in 28Si.

Realization of Anisotropic Diamagnetic Kepler Problem in a Solid State Environment

The anisotropic diamagnetic Kepler problem (ADKP) is realized experimentally by the orbital electrons of a P donor in Si under magnetic fields. The interference of electron wave packets which leads to quasi-Landau resonances (QLR) were observed. Applying the closed-orbit theory to an anisotropic solid state environment, we have identified orbits responsible for the QLR manifesting the quantum chaotic behavior in Rydberg atoms. The excellent consistency between the measured spectra and theoretical calculation provides unambiguous evidence of quantum chaotic dynamics of electrons in the ADKP.

Tetrahedral potentials and lack-of-inversion symmetry for donors in silicon

The extensive electron-nuclear double resonance data of Hale and Mieher for P-, As-, and Sb-doped Si and the comprehensive multiband calculation of Ivey and Mieher employing only a spherical potential $U(r)$ suggests four inversion-related pairs [$(\underset{̱}{224})$, (224)], [$(\underset{̱}{444})$, (444)], [$(\underset{̱}{228})$, (228)], and [$(\underset{̱}{2212})$, (2212)] have been associated with the shells $[M,L]$, $[H,O]$, $[I,P]$, and $[S,T]$. An alternative explanation is provided resulting from tetrahedral potentials of the form ${V}_{\text{tet}}(\mathbf{r})={V}_{t}$ $xyx/{({r}^{2}+{b}^{2})}^{7/2}$ and a corresponding wave function ${\ensuremath{\psi}}_{\text{tet}}\ensuremath{\propto}\ensuremath{-}xyz$ $[{C}_{1}\text{ }\text{exp}(\ensuremath{-}r/{\ensuremath{\lambda}}_{1})+{C}_{2}\text{ }\text{exp}(\ensuremath{-}r/{\ensuremath{\lambda}}_{2})]$ with ${C}_{1}⪢{C}_{2}$ and ${\ensuremath{\lambda}}_{2}\ensuremath{\sim}4$ ${\ensuremath{\lambda}}_{1}$ (for P donor). ${\ensuremath{\psi}}_{\text{tet}}$ leads to a giant octupole moment for the donors dominated by ${C}_{2}\text{ }\text{exp}(\ensuremath{-}r/{\ensuremath{\lambda}}_{2})$. Excellent agreement is obtained for P, good agreement for As, and less satisfactory agreement for Sb. ${V}_{\text{tet}}(\mathbf{r})$ originates from the donor $\mathbf{E}$-field-induced electric dipoles ${\mathbf{p}}_{i}$ at the covalent bond sites [first nearest-neighbor (nn), second nn, and third nn] as calculated by Harrison. Reasons are provided why the inversion-related pairs cannot be explained by a spherical potential $U(r)$. These results provide strong support for the assignments by Ivey and Mieher of the four inversion-related pairs. The interest in donors in Si as possible quantum bits has led to interest in the Stark effect from external electric fields. ${V}_{\text{tet}}(\mathbf{r})$ and the associated ${\ensuremath{\psi}}_{\text{tet}}(\mathbf{r})$, resulting from an ``internal Stark effect,'' are a general feature for impurities with ${T}_{d}$ symmetry in semiconductors.

Thermal stability of semi-insulating property of Fe-doped GaN bulk films studied by photoluminescence and monoenergetic positron annihilation techniques

The thermal stability of electrical resistivity (ρ) is one of the crucial functions of semi-insulating (SI) substrates. In this paper, we describe the thermal stability of SI property in Fe-doped GaN (GaN:Fe) films grown by hydride vapor phase epitaxy, in view of point defect chemistry by means of monoenergetic positron annihilation and photoluminescence (PL) measurements. PL spectra of GaN:Fe at 8 K exhibited broad emission bands in UV, blue, and yellow spectral regions, as well as a series of characteristic infrared peaks with a sharp zero-phonon line at 1.300 eV. A ρ value higher than 108 Ω⋅cm was obtained when the doping concentration of Fe, [Fe], exceeded the major shallow donor (Si) concentration (5×1017 cm−3). For those SI samples, the relative intensity of the yellow luminescence band at 2.2 eV, of which the origin has been attributed to Ga vacancies (VGa) and/or defect complexes composed of VGa and O, over the UV/blue emission was remarkably decreased. Simultaneously, the Doppler broadening S parameter for the positron annihilation measurement, which represents the size or concentration of negatively charged vacancy type point defects such as VGa, was decreased. The results are consistent with the increase in formation energy of VGa due to the downward shift of the Fermi level by Fe doping. The values of ρ, S, and W parameters that represents the fraction of positrons annihilated with core electrons, in the bulk region did not change remarkably while the positron diffusion length was increased by the annealing in N2 between 600 and 1050 °C. Although the defect concentration in uncapped surface region was increased remarkably by annealing at 1050 °C due to the surface decomposition, the present results indicate that GaN:Fe can be used as a thermally stable SI substrate for electronic devices because the surface does not decompose during the epitaxial growths of overlayers.

Crystallographic plane dependent Fe and Si dopant incorporation and activation in InP

AbstractVarious crystallographic planes are exposed when growing along mesa sidewalls during metalorganic vapor phase epitaxy. Dopant incorporation and activation on these higher index surfaces can be quite different from that observed on (001) surfaces. Incorporation and activation of Si and Fe in MOVPE grown InP on (001), (110), and (311)A and B surfaces are reported. The Si donor concentration is higher on the P‐terminated B surface than the A surface while the opposite behavior is observed for Fe incorporation. In the fabrication of optoelectronic devices, it must be remembered that it is the carrier concentration, that is, activated atomic species, that is relevant for device performance and not the incorporation of atomic species alone. Activation of Si is &gt;1 on all surfaces; however, Fe is shown to be an effective compensating species only on the (001) and (311)A crystallographic surfaces. Data for the presence of oscillations in the iron concentration on (001) surfaces are also reported. The active Fe concentration is shown to be related to the Fe atomic concentration relative to the donor density on the different crystallographic surfaces, suggesting that Fermi level position sets a limit to the fraction of Fe centers which are electrically active as a deep trap. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)