Large Hole Concentration Research Articles

The Study of Diluted Magnetic Semiconductor: The Case of Manganese Doped Gallium Nitride

The diluted magnetic semiconductor, (Ga, Mn) N has recently attracted intense research interest for the purpose of spintronics application. The material is believed to circumvent the difficulty of combining data processing and mass storage facilities in a single crystal besides solving non-volatility problems. The concentration x, of Mn that substitutes for a fraction of Ga in the compound is thought to contribute a large concentration of magnetic moments and holes. The material studied is focused on dilute magnetic semiconductors (DMS) like <i>Ga_1-XMn<SUB>X</SUB>N</i> that play a key role in semiconductor spintronics. Due to their ferromagnetic properties they can be used in magnetic sensors and as spin injectors. The basic problems for applications are, however, the relatively low Curie temperatures of these systems. Therefore, we focus on the understanding of the magnetic properties and on a reliable calculation of Curie temperatures from first principles. We have developed a theoretical framework for calculating critical temperatures by combining first principles calculations and in terms of the Ruderman–Kittel–Kasuya–Yosida quantum spin model in Green’s function approach. Magnetic properties of the group-III nitride semiconductors are introduced here with basic material parameters (temperature, concentration, heat capacity, etc. Temperature dependencies of the spin wave specific heat, inverse magnetic susceptibility and reduced magnetization are determined. Therefore, the dependence of the Neel temperature on the manganese ion concentration is linear thus for our calculation the highest Neel temperature obtained T=146.3k within the concentration of 0.2.

How dopants limit the ultrahigh thermal conductivity of boron arsenide: a first principles study

The promise enabled by boron arsenide’s (BAs) high thermal conductivity (κ) in power electronics cannot be assessed without taking into account the reduction incurred when doping the material. Using first principles calculations, we determine the κ reduction induced by different group IV impurities in BAs as a function of concentration and charge state. We unveil a general trend, where neutral impurities scatter phonons more strongly than the charged ones. CB and GeAs impurities show by far the weakest phonon scattering and retain BAs κ values of over ~1000 W⋅K−1⋅m−1 even at high densities. Both Si and Ge achieve large hole concentrations while maintaining high κ. Furthermore, going beyond the doping compensation threshold associated to Fermi level pinning triggers observable changes in the thermal conductivity. This informs design considerations on the doping of BAs, and it also suggests a direct way to determine the onset of compensation doping in experimental samples.

Quantum Monte Carlo study of lattice polarons in the two-dimensional three-orbital Su\u2013Schrieffer\u2013Heeger model

The electron–lattice interaction gives rise to a rich set of phenomena in quantum materials. Microscopically, this interaction often arises from the modulation of orbital overlaps; however, many theoretical studies neglect such couplings. Here, we present an exact diagonalization and determinant quantum Monte Carlo study of a three-orbital Su–Schrieffer–Heeger (SSH) model, on a two-dimensional Lieb lattice and in the negative charge transfer regime. At half-filling (one hole/unit cell), we observe a bipolaron insulating phase with a bond-disproportionate lattice. This phase is robust against moderate hole doping but is suppressed at large hole concentrations, leading to a metallic polaron-liquid-like state with fluctuating patches of local distortions. We also find an s-wave superconducting state at large hole doping that primarily appears on the oxygen sublattice. Our work provides a non-perturbative view of SSH-type couplings in two dimensions with implications for materials where such couplings are dominant.

A Passive Gamma Radiation Dosimeter Using Graphene Field Effect Transistor

In this work, Graphene-based field-effect transistors (GFETs) are demonstrated as a highly sensitive dosimeter for gamma radiation. Graphene-based field-effect transistors exhibit p-type doping with the Dirac point shifting in the positive direction upon exposure to gamma radiation. Concurrently, an asymmetric degradation in the electron and hole mobility was observed with the former degrading more rapidly. It is shown that change in the Dirac voltage and carrier mobility is strongly dependent on the dose of gamma radiation. A sensitivity of ~1 V/kGy is reported. Gamma radiation causes partial aerial oxidation of graphene-channel which leads to p-doping as confirmed by the emergence of a higher binding energy peak (286.8 eV) in X-ray photoelectron spectra (XPS). The decrease in contact potential difference estimated through Kelvin probe force microscopy (KPFM) confirms this finding. The radiated devices showed a stable response for ~70 days. Our work demonstrates that gamma irradiation can also be used to induce large and stable hole concentrations in graphene. Such highly sensitive GFET can serve as real-time dosimeter operating in ambient conditions.

Super Large Sn1- xSe Single Crystals with Excellent Thermoelectric Performance.

SnSe single crystals have drawn extensive attention for their ultralow thermal conductivity and outstanding thermoelectric performance. Here, we report super large Sn1- xSe single crystals with excellent thermoelectric properties, fabricated via an advanced horizontal Bridgman technique with great yield and high reproducibility. The obtained single crystals have a super large size of ∼70 × 50 × 15 mm with a considerable weight of 148 g, which leads to a record-high mass density of >6.1 g cm-3. Extensive chemical characterization demonstrates that ∼0.3% Sn vacancies are present, which results in a large concentration of holes, ∼1.2 × 1019 cm-3, and an enhanced power factor of ∼6.1 μW cm-1 K-2 at 793 K. Simultaneously, the Sn-vacancy-induced lattice distortions result in a low thermal conductivity of ∼0.39 W m-1 K-1 at 793 K, leading to a competitive ZT of ∼1.24. This work demonstrates that large-size off-stoichiometric SnSe single crystals hold promise to achieve high thermoelectric performance.

229 nm UV LEDs on aluminum nitride single crystal substrates using p-type silicon for increased hole injection

AlGaN based 229 nm light emitting diodes (LEDs), employing p-type Si to significantly increase hole injection, were fabricated on single crystal bulk aluminum nitride (AlN) substrates. Nitride heterostructures were epitaxially deposited by organometallic vapor phase epitaxy and inherit the low dislocation density of the native substrate. Following epitaxy, a p-Si layer is bonded to the heterostructure. LEDs were characterized both electrically and optically. Owing to the low defect density films, large concentration of holes from p-Si, and efficient hole injection, no efficiency droop was observed up to a current density of 76 A/cm2 under continuous wave operation and without external thermal management. An optical output power of 160 μW was obtained with the corresponding external quantum efficiency of 0.03%. This study demonstrates that by adopting p-type Si nanomembrane contacts as a hole injector, practical levels of hole injection can be realized in UV light-emitting diodes with very high Al composition AlGaN quantum wells, enabling emission wavelengths and power levels that were previously inaccessible using traditional p-i-n structures with poor hole injection efficiency.

Ab initio study of the p-hole magnetism at polar surfaces of ZnO: the role of correlations

A standard local density approximation and its self-interaction corrected version are applied to study spontaneous magnetization, promoted by localized p electron holes, of polar oxygen-terminated ZnO surfaces. The electronic properties and magnetic exchange interactions of three different facets are calculated. It is demonstrated that partially filled oxygen p orbitals of the polar surfaces exhibit magnetic moment formation and long range magnetic order leading to the occurrence of a ferromagnetic ground state. Monte Carlo simulations predict Curie temperatures above room temperature. In contrast to isolated defects in bulk materials, applying correlation corrections to the localized p-like surface states does not lead to a collapse of magnetic interaction: as the weakening of the magnetic interaction, caused by the reduced electronic overlap, is compensated by a strengthening due to an increase of the magnetic moments, the ferromagnetism can principally persist above room temperature, provided a large hole concentration exists.

Modeling of n-InAs metal oxide semiconductor capacitors with high-κ gate dielectric

A qualitative analysis on capacitance-voltage and conductance data for high-κ/InAs capacitors is presented. Our measured data were evaluated with a full equivalent circuit model, including both majority and minority carriers, as well as interface and border traps, formulated for narrow band gap metal-oxide-semiconductor capacitors. By careful determination of interface trap densities, distribution of border traps across the oxide thickness, and taking into account the bulk semiconductor response, it is shown that the trap response has a strong effect on the measured capacitances. Due to the narrow bandgap of InAs, there can be a large surface concentration of electrons and holes even in depletion, so a full charge treatment is necessary.

Ferromagnetic-nonmagnetic and metal-insulator phase transitions at the interfaces of KTaO3 and PbTiO3

We studied the electronic and magnetic properties of hole doped KTaO3/PbTiO3 interface using density functional theory methods. Ferromagnetic-nonmagnetic phase transition and metal-insulator phase transition occur simultaneously at the interface with ferroelectric polarization reversal. Furthermore, these two transitions are coupled with each other because hole doping with large concentration of holes gives rise to ferromagnetism. The interfacial magnetization, which is proportional to hole concentration at the interface, can be tuned by ferroelectric polarization, leading to strong intrinsic magnetoelectric effect at the interface of originally nonmagnetic KTaO3 and PbTiO3.

Improvement of electrical conductivity in Pb0.96−yMn0.04SnyTe alloys for high temperature thermoelectric applications

Lead–tin–telluride is a well-known thermoelectric material in the temperature range 350–750 K. Here, this alloy doped with manganese (Pb0.96−yMn0.04SnyTe) was prepared for different amounts of tin. X-ray diffraction showed a decrease of the lattice constant with increasing tin content, which indicated solid solution formation. Microstructural analysis showed a wide distribution of grain sizes from <1 μm to 10 μm and the presence of a SnTe rich phase. All the transport properties were measured in the range of 300−720 K. The Seebeck coefficient showed that all the samples were p-type indicating holes as dominant carriers in the measurement range. The magnitude increased systematically on reduction of the Sn content due to possible decreasing hole concentration. Electrical conductivity showed the degenerate nature of the samples. Large values of the electrical conductivity could have possibly resulted from a large hole concentration due to a high Sn content and secondly, due to increased mobility by sp–d orbital interaction between the Pb1−ySnyTe sublattice and the Mn2+ ions. High thermal conductivity was observed due to higher electronic contribution, which decreased systematically with decreasing Sn content. The highest zT = 0.82 at 720 K was obtained for the alloy with the lowest Sn content (y = 0.56) due to the optimum doping level.

Magnetoelectric effects at the interfaces between nonmagnetic perovskites: Ab initio prediction

A novel mechanism of magnetoelectric coupling at the KTaO3/BaTiO3 interfaces composed of nonmagnetic ferroelectric insulators is studied by density functional theory methods. We show that holes created by electronic reconstruction concentrate near the interface, which is significantly different from the distribution of holes in the LaAlO3/SrTiO3 model system (Chen H. H. et al., Phys. Rev. B, 79 (2009) 161402(R)). A large concentration of holes induces ferromagnetism at the interface. When ferroelectric polarization reverses, the ratio of the net change in magnetization to the total is about ten percent. This ratio is improved greatly compared with typical Fe/BaTiO3 heterostructures (Duan C. G. et al., Phys. Rev. Lett., 97 (2006) 047201), where the ratio is about one percent. The results provide an opportunity for searching multiferroic materials and offer the possibility to design spintronics devices.

The marginal Fermi Liquid – An exact derivation based on Dirac's first class constraints method

Dirac's method for constraints is used for solving the problem of exclusion of double occupancy for Correlated Electrons. The constraints are enforced by the pair operator Q ( x → ) = ψ ↓ ( x → ) ψ ↑ ( x → ) which annihilates the ground state | Ψ 0 〉 . Away from half fillings the operator Q ( x → ) is replaced by a set of first class non-Abelian constraints Q α ( − ) ( x → ) restricted to negative energies. The propagator for a single hole away from half fillings is determined by modified measure which is a function of the time duration of the hole propagator. As a result: (a) The imaginary part of the self energy is linear in the frequency. At large hole concentrations a Fermi Liquid self energy is obtained. (b) For the superconducting state the constraints generate an asymmetric spectrum excitations between electrons and holes giving rise to an asymmetry tunneling density of states.

Antimony as an amphoteric dopant in lead telluride

We elucidate the amphoteric nature of antimony as a dopant in PbTe. Band-structure calculations show that Sb substituting for Pb is a donor and that Sb on the Te site is an acceptor giving rise to a large excess density of states (DOS). Experimentally, in Te-rich ${\text{Pb}}_{1\ensuremath{-}\text{x}}{\text{Sb}}_{\text{x}}\text{Te}$ samples, $^{125}\text{T}\text{e}$ NMR spectroscopy shows that Sb substitutes for Pb and transport data reveal that it then acts as a simple donor. In Pb-rich ${\text{PbSb}}_{\text{x}}{\text{Te}}_{1\ensuremath{-}\text{x}}$ samples, $^{125}\text{T}\text{e}$ NMR shows that little Sb substitutes for Te when samples are prepared above 770 K and then quenched; $^{207}\text{P}\text{b}$ NMR shows four types of charge carriers, but only a majority hole and a minority electron contribute to transport. Sb acts as an acceptor in ${\text{PbSb}}_{\text{x}}{\text{Te}}_{1\ensuremath{-}\text{x}}$, but the large DOS calculated must correspond to a large concentration of localized holes and the Seebeck coefficient is not enhanced.

Study on the origin of large thermopower in hole doped LaRhO3 based on ab-initio downfolding

We study the origin of the large thermopower and its peculiar hole concentration in the Ni-doped LaRhO3. We calculate the band structure and construct the maximally localized Wannier functions, from which a tightbinding Hamiltonian is obtained. The Seebeck coefficient is calculated within the Boltzmann's equation approach using this effective Hamiltonian. We find that the Seebeck coefficient remains nearly constant within a large hole concentration range, which is consistent with the experimental observation. The origin of this peculiar doping concentration is discussed.

First-principles study on the origin of large thermopower in hole-dopedLaRhO3 and CuRhO2

Based on first-principles calculations, we study the origin of the large thermopower in Ni-dopedLaRhO3 andMg-doped CuRhO2. We calculate the band structure and construct the maximally localized Wannier functionsfrom which a tight binding Hamiltonian is obtained. The Seebeck coefficient is calculatedwithin the Boltzmann’s equation approach using this effective Hamiltonian. ForLaRhO3, we find that the Seebeck coefficient remains nearly constant within a large holeconcentration range, which is consistent with the experimental observation. ForCuRhO2, the overall temperature dependence of the calculated Seebeck coefficient is in excellentagreement with the experiment. The origin of the large thermopower is discussed.

Reproducible increased Mg incorporation and large hole concentration in GaN using metal modulated epitaxy

The metal modulated epitaxy (MME) growth technique is reported as a reliable approach to obtain reproducible large hole concentrations in Mg-doped GaN grown by plasma-assisted molecular-beam epitaxy on c-plane sapphire substrates. An extremely Ga-rich flux was used, and modulated with the Mg source according to the MME growth technique. The shutter modulation approach of the MME technique allows optimal Mg surface coverage to build between MME cycles and Mg to incorporate at efficient levels in GaN films. The maximum sustained concentration of Mg obtained in GaN films using the MME technique was above 7×1020cm−3, leading to a hole concentration as high as 4.5×1018cm−3 at room temperature, with a mobility of 1.1cm2V−1s−1 and a resistivity of 1.3Ωcm. At 580K, the corresponding values were 2.6×1019cm−3, 1.2cm2V−1s−1, and 0.21Ωcm, respectively. Even under strong white light, the sample remained p-type with little change in the electrical parameters.

SiGeC/Si Electrooptic Modulators

The addition of carbon to silicon-germanium alloys provides the ability to lattice match thick layers with high germanium composition to silicon substrates. Thick strain-free silicon-germanium-carbon (SiGeC) layers on silicon allow the design of optical waveguides that have large optical mode overlap with the waveguide core. In addition, SiGeC/Si heterostructures enable strong confinement of large electron and hole concentrations. The combination of tightly confined carriers and photons can be used to realize high-performance broadband electrooptic modulators based on carrier density-induced refractive index changes. We show that modulators with lengths around 30 mum and turn-on times below 0.2 ns are possible with optimized designs

HIGH PERFORMANCE SIGEC/SI NEAR-IR ELECTROOPTIC MODULATORS AND PHOTODETECTORS

Numerical simulations are performed for SiGeC/Si electrooptic modulators and photodetectors operating at near-IR wavelengths. The addition of carbon provides the ability to lattice match layers with high germanium composition to silicon, which is shown to allow structures with a substantial increase in the optical confinement factor. In addition, SiGeC/Si heterostructures provide strong confinement of large electron and hole concentrations. The large optical confinement factor and strong carrier confinement enable broadband electrooptic modulators with sub-100 μm lengths and switching times below 0.5 ns with 25 mA current as well as photodetectors with quantum efficiencies as high as 90% for 300 μm length.

The growth kinetics and optical confinement studies of porous Si for application in terrestrial Si solar cells as antireflection coating

The mesoporous porous-silicon (PS) layers were grown on 〈1 0 0〉, 〈1 1 0〉, and 〈1 1 1〉 oriented wafers at constant current density of 20 mA cm −2. The pore sizes and surface morphologies were measured by atomic force and scanning electron microscopes. The thickness x of the PS formed and the refractive index were measured by an ellipsometer as a function of time duration t (in min) of anodization. The x vs. t data were fitted into a power law x= at c where c is a dimensionless constant and growth kinetics was established. The growth is practically independent of orientation. This is due the reason that the growth rate is controlled largely by the availability of holes which exchange their charge with oxidizing species and desirably large concentrations of holes were available at current density of 20 mA cm −2. For a similar reason the growth of PS layer on the front surface of the n + region of n +–p solar cells could also be done at current density of ∼20 mA cm −2 nearly at the same rate. A large concentration of holes could be injected from p region into the n + region because the positive contact was made on the p side and thus the junction was forward biased. The PS ARC of thickness 70 nm showed increase ∼26% in the short circuit current density J sc and 24% in efficiency of the cells. However, the improvement in the values of the open circuit voltage V oc were lower than the expected value indicating that the PS layers had enhanced recombination of minority carriers at the front surface or in the front emitter region immediately below the PS layer.

Light emission from Al/HfO2/silicon diodes

The metal–insulator–silicon light-emitting diode (MIS LED) using a high-dielectric-constant material (HfO2) is studied. The external quantum efficiency for light emission at room temperature from the MIS LED was observed to be 2.0×10−6, as compared to 0.5×10−6 for the metal–oxide–silicon (MOS) LED. The large hole concentration at the Si/HfO2 interface created by the high dielectric constant of HfO2 may be responsible for the enhancement. The emission line shape of the MIS LED can be fitted by the electron-hole plasma recombination model, similar to the MOS LED. The Al/HfO2/silicon LED with a high interface trap density has a continuous spectrum below the Si gap beside the electron-hole plasma emission, probably due to the radiative recombination between the electrons and holes via the interface states.