Minority Electrons Research Articles

Specific features of the properties of half-metallic ferromagnetic Heusler alloys Fe2MnAl, Fe2MnSi, and Co2MnAl

The structural, magnetic, and electrical properties of half-metallic Heusler alloys Fe2MnAl, Fe2MnSi, and Co2MnAl have been investigated in the temperature range of 4–900 K. According to the X-ray diffraction analysis, these alloys have the B2 and L21 structures with different degrees of atomic order. The magnetic state of the alloys is considered as a two-sublattice ferrimagnet. The electrical resistivity and thermoelectric power have been discussed in the framework of the two-current conduction model taking into account the existence of an energy gap in the electronic spectrum of the alloys near the Fermi level for the subband with spin-down (minority) electrons.

The electronic, magnetic and optical properties of double perovskite A2FeReO6 (A = Sr, Ba) from first principles approach

A density functional theory (DFT) employing generalized gradient approximation (GGA) and modified Becke Johnson (TB-mBJ) potential have been used to study the electronic, magnetic and optical properties of A2FeReO6 (AFRO). The presence of indirect band gap of majority electrons at EF and the conducting behaviour of minority electrons predicts these materials to be a half metallic ferrimagnets. The GGA band gaps are enhanced with the implementation of a new electron exchange semilocal potential called TB-mBJ. The real and imaginary parts of dielectric function, refractive index, absorption coefficient and reflectivity are calculated. The inter-band transitions to the optical properties are analyzed with the band structures.

Doping dependence and anisotropy of minority electron mobility in molecular beam epitaxy-grown p type GaInP

Direct imaging of minority electron transport via the spatially resolved recombination luminescence signature has been used to determine carrier diffusion lengths in GaInP as a function of doping. Minority electron mobility values are determined by performing time resolved photoluminescence measurements of carrier lifetime on the same samples. Values at 300 K vary from ∼2000 to 400 cm2/V s and decrease with increasing doping. Anisotropic diffusion lengths and strongly polarized photoluminescence are observed, resulting from lateral composition modulation along the [110] direction. We report anisotropic mobility values associated with carrier transport parallel and perpendicular to the modulation direction.

Spin-orbit coupling effects on spin-dependent inelastic electronic lifetimes in ferromagnets

For the 3d ferromagnets iron, cobalt and nickel we compute the spin-dependent inelastic electronic lifetimes due to carrier-carrier Coulomb interaction including spin-orbit coupling. We find that the spin-dependent density-of-states at the Fermi energy does not, in general, determine the spin dependence of the lifetimes because of the effective spin-flip transitions allowed by the spin mixing. The majority and minority electron lifetimes computed including spin-orbit coupling for these three 3-d ferromagnets do not differ by more than a factor of 2, and agree with experimental results.

19.2% Efficient InP Heterojunction Solar Cell with Electron-Selective TiO2 Contact.

We demonstrate an InP heterojunction solar cell employing an ultrathin layer (∼10 nm) of amorphous TiO2 deposited at 120 °C by atomic layer deposition as the transparent electron-selective contact. The TiO2 film selectively extracts minority electrons from the conduction band of p-type InP while blocking the majority holes due to the large valence band offset, enabling a high maximum open-circuit voltage of 785 mV. A hydrogen plasma treatment of the InP surface drastically improves the long-wavelength response of the device, resulting in a high short-circuit current density of 30.5 mA/cm2 and a high power conversion efficiency of 19.2%.

Impact of solar irradiance intensity and temperature on the performance of compensated crystalline silicon solar cells

Low-cost upgraded metallurgical grade silicon (UMG-Si) with inherent boron (B) and phosphorus (P) compensation is a novel material for photovoltaic application. This paper presents the impact of solar irradiance intensity and temperature on the performance of compensated crystalline silicon solar cells. For the same rated output power, compensated crystalline silicon solar cells generate less electricity than the reference silicon solar cells at low irradiance intensity, owing to the strong injection dependence of the carrier lifetime due to high concentration of B–O complexes in compensated silicon. However, at high temperature, compensated crystalline silicon solar cells generate more electricity than the reference silicon solar cells, which mainly originates from the lower temperature-variation of the minority electron mobility in compensated silicon. It suggests that compensated silicon solar cells will be more appropriate for high irradiation application, which often contains high irradiance intensity and high temperature. These results are of great significance for understanding the actual outdoor performance of the solar cells based on the UMG-Si and their application in the photovoltaic (PV) industry.

Scanning photocurrent microscopy reveals electron-hole asymmetry in ionic liquid-gated WS2 transistors

We perform scanning photocurrent microscopy on WS2 ionic liquid-gated field effect transistors exhibiting high-quality ambipolar transport. By properly biasing the gate electrode, we can invert the sign of the photocurrent showing that the minority photocarriers are either electrons or holes. Both in the electron- and hole-doping regimes the photocurrent decays exponentially as a function of the distance between the illumination spot and the nearest contact, in agreement with a two-terminal Schottky-barrier device model. This allows us to compare the value and the doping dependence of the diffusion length of the minority electrons and holes on a same sample. Interestingly, the diffusion length of the minority carriers is several times larger in the hole accumulation regime than in the electron accumulation regime, pointing out an electron-hole asymmetry in WS2.

Spin polarized positron lifetimes in ferromagnetic metals: First-principles study

We carry out spin-polarized positron lifetime calculations for ferromagnetic metals using the electron–positron density functional theory (DFT). We investigate Fe, Co, and Ni and find that the differences between the positron lifetimes for their minority and majority spins (τ↓ − τ↑) are 11.85, 3.75, and −4.37 ps, respectively. The negative lifetime difference of Ni is presumed to originate from an unlocalized distribution of minority electrons.

Investigation of electron transmission through Co/C/Co magnetic tunnel junctions

We investigate transmission properties of electrons at the interfaces of cobalt-diamond hybrid systems. To this end, we employ a Green's function method based on spin-polarized density-functional theory calculations using a Gaussian orbital representation of the wave functions. The considered open systems are magnetic tunnel junctions consisting of semi-infinite hcp cobalt leads and $n$ layers of carbon atoms stacked in (111) direction. These systems are characterized by a very small lattice mismatch of their constituents. We have identified three types of states that give rise to an almost complete transmission. The transmission for minority electrons is considerably suppressed at the Fermi level in most parts of the interface Brillouin zone.

Spin-Dependent Electron Transfer Dynamics Probed by Resonant Photoemission Spectroscopy

Resonant photoemission spectroscopy has been used to investigate the spin-dependent electron transport from core-excited argon monolayers to ferromagnetic substrates with subfemtosecond precision. Charge transfer times as a function of the spin of the resonantly excited electron and the magnetization of the substrate have been studied for $\mathrm{Ar}/\mathrm{Fe}(110)$, $\mathrm{Ar}/\mathrm{Co}(0001)$, and $\mathrm{Ar}/\mathrm{Ni}(111)$. For argon adsorbed on iron and cobalt, a significantly faster charge transfer is found for minority electrons, whereas no such spin dependence is obtained for nickel. The results are explained considering the different empty densities of states of the metals for minority and majority electrons.

Generation-recombination and trap-assisted tunneling in long wavelength infrared minority electron unipolar photodetectors based on InAs/GaSb superlattice

A long wavelength infrared minority electron unipolar photodetector based on InAs/GaSb type-II superlattices is demonstrated. At 77 K, a dark current of 3 × 10−5 A/cm2 and a differential resistance-area of 3700 Ω.cm2 are achieved at the turn-on bias, with a 50%-cutoff of 10.0 μm and a specific detectivity of 6.2 × 1011 Jones. The dark current is fitted as a function of bias and temperature using a model combining generation-recombination and trap-assisted tunneling. Good agreement was observed between the theory and the experimental dark current.

A numerical simulation study of gallium-phosphide/silicon heterojunction passivated emitter and rear solar cells

The performance of passivated emitter and rear (PERC) solar cells made of p-type Si wafers is often limited by recombination in the phosphorus-doped emitter. To overcome this limitation, a realistic PERC solar cell is simulated, whereby the conventional phosphorus-doped emitter is replaced by a thin, crystalline gallium phosphide (GaP) layer. The resulting GaP/Si PERC cell is compared to Si PERC cells, which have (i) a standard POCl3 diffused emitter, (ii) a solid-state diffused emitter, or (iii) a high efficiency ion-implanted emitter. The maximum efficiencies for these realistic PERC cells are between 20.5% and 21.2% for the phosphorus-doped emitters (i)–(iii), and up to 21.6% for the GaP emitter. The major advantage of this GaP hetero-emitter is a significantly reduced recombination loss, resulting in a higher Voc. This is so because the high valence band offset between GaP and Si acts as a nearly ideal minority carrier blocker. This effect is comparable to amorphous Si. However, the GaP layer can be contacted with metal fingers like crystalline Si, so no conductive oxide is necessary. Compared to the conventional PERC structure, the GaP/Si PERC cell requires a lower Si base doping density, which reduces the impact of the boron-oxygen complexes. Despite the lower base doping, fewer rear local contacts are necessary. This is so because the GaP emitter shows reduced recombination, leading to a higher minority electron density in the base and, in turn, to a higher base conductivity.

Temperature dependence of diffusion length, lifetime and minority electron mobility in GaInP

The mobility of electrons in double heterostructures of p-type Ga0.50In0.50P has been determined by measuring minority carrier diffusion length and lifetime. The minority electron mobility increases monotonically from 300 K to 5 K, limited primarily by optical phonon and alloy scattering. Comparison to majority electron mobility over the same temperature range in comparably doped samples shows a significant reduction in ionized impurity scattering at lower temperatures, due to differences in interaction of repulsive versus attractive carriers with ionized dopant sites. These results should be useful in modeling and optimization for multi-junction solar cells and other optoelectronic devices.

Oxygen Nonstoichiometry and Defect Chemistry of Perovskite-Structured BaxSr1–xTi1–yFeyO3–y/2+δ Solid Solutions

The oxygen nonstoichiometry of mixed conducting perovskite-structured BaxSr1–xTi1–yFeyO3–y/2+δ (BSTF) (x = 0, 0.1, 0.5 and y = 0.05, 0.35) was measured by means of thermogravimetry as a function of oxygen partial pressure, pO2, in a temperature range typical for solid oxide fuel cell (SOFC) cathode applications. With increasing Ba content, the nonstoichiometry curve was shifted to higher pO2, indicating enhanced reducibility. Based on a defect chemical analysis of thermogravimetry derived nonstoichiometry data, the substitution of Ba for Sr on the A-site of STF was found to result in a decrease in both the reduction enthalpy and band gap energy, consistent with expectations that substitution of Sr by the larger Ba leads to a reduction in bond strength. A consequent increase in minority electron density and oxygen vacancy concentration are expected to result in enhancements in oxygen surface exchange kinetics and diffusivity and thereby cathode performance. The nonstoichiometry data obtained in this study ...

A spin-selective Kondo insulator

In our recent work, Phys. Rev. Lett. 108, 086402 (2012), we showed the intriguing properties of the ferromagnetic state in the Kondo lattice model. We showed that at zero temperature, the minority conduction electrons form an insulating state, while the majority conduction electrons form a metallic state. Furthermore, we showed that the gap width in the spectral function of the minority conduction electrons obeys a Kondo-temperature-like form, ∼ exp(−1/J), in the coupling strength. We explained that the gap is due to the commensurability between the local spins and the conduction electrons arranging themselves in a way that the number of minority electrons per lattice site adds up to unity. Here, we show how the gap and the commensurability are formed when the system enters the ferromagnetic phase when the temperature is lowered. We also clarify that this commensurable situation is not due to a special choice of system parameters or to an adjustment of the filling of the lattice, but is general for a ferromagnetic state in the Kondo lattice model.

Direct observation of minority carrier lifetime improvement in InAs/GaSb type-II superlattice photodiodes via interfacial layer control

We present improved performance in strain-balanced InAs/GaSb type-II superlattice photodetectors grown using InSb interfacial layers, measured using a cross-sectional electron beam induced current (EBIC) technique to obtain minority carrier diffusion characteristics. We detail a modified EBIC model that accounts for the long absorber regions in photodetectors and fit the experimental data. We find a significant increase in the minority hole lifetime (up to 157 ns) and increased minority electron lifetime due to the interfacial layers. Additionally, electrical characterization of the device temperature-dependent resistance-area product reveals that the interfacial treatment improves the device dark current at lower temperatures.

The New Equations of p-n Junction Carrier Injection Level

The new equations of minority carrier hole and electron injection levels kp and kn valid at high-level injection have been derived. They relate the kp and kn and the voltage drop across the p-n junction depletion region Ud. At low Ud, i.e. at low-level injection the obtained equations coincide with well known exponential equations of injection level. However, at high-level injection when Ud becomes high and is close to the potential barrier of junction, the derived equations give increased steepness of kp and kn dependence on Ud as compared with the exponential law. The dependences of kp and kn of concrete silicon p-n junctions with different impurity concentrations have been analyzed using derived equations of injection level.DOI: http://dx.doi.org/10.5755/j01.eee.19.2.3467

Vertical minority carrier electron transport in p-type InAs/GaSb type-II superlattices

Vertical minority carrier electron transport parameters in p-type InAs/GaSb type-II superlattices for long wavelength infrared (LWIR) detection have been extracted from magnetic field dependent geometrical magneto-resistance. The measurements, performed at low electric fields and at magnetic field intensities up to 12 T, exhibited multiple-carrier conduction characteristics that required mobility spectrum analysis for the extraction of individual carrier mobilities and concentrations. Within the common operating temperature range for LWIR photodiodes (80 to 150 K), the conductivity was found to be dominated by three distinct carriers, attributed to majority holes (μ=280±27 cm2/Vs), minority electrons (μ=2,460±75 cm2/Vs), and parasitic sidewall inversion layer electrons (μ=930±55 cm2/Vs). A miniband energy gap of 140 ± 15 meV for the 14/7-monolayer InAs/GaSb superlattice was estimated from the thermal activation of the minority carrier electron density.

Enhanced low-noise gain from InAs avalanche photodiodes with reduced dark current and background doping

We reduced the room temperature dark current in an InAs avalanche photodiode by increasing the p-type contact doping, resulting in an increased energetic barrier to minority electron injection into the p-region, which is a significant source of dark current at room temperature. In addition, by improving the molecular beam epitaxy growth conditions, we reduced the background doping concentration and realized depletion widths as wide as 5 μm at reverse biases as low as 1.5 V. These improvements culminated in low-noise InAs avalanche photodiodes exhibiting a room temperature multiplication gain of ∼80, at a record low reverse bias of 12 V.

Carrier trapping and recombination in TlGaSe2 layered crystals

AbstractIn nominally undoped layered TlGaSe2 crystals the trapping centers have been investigated by photo‐induced current transient spectroscopy (PICTS). Five acceptor and donor traps have been detected. Quite large magnitudes of capture cross‐sections for donor traps at 0.23 and 0.45 eV have been determined. The depth‐resolved free‐carrier absorption (FCA) technique has been applied for the investigation of the majority hole lifetime, τR, at 295 and 77 K. The pronounced τR reduction with increasing injection level is attributed to the trapping effect of the minority electrons which provides a way of estimating trap centers concentration. A moderate τR variation across layer planes is observed but no carrier diffusion related to the recombination on the external crystal surfaces is detected. Moreover, abnormally sharp τR drop is found for carrier concentration above 1017 cm−3. The possibility of excess hole and electron special separation due to compositional stacking faults and the successive their enhanced recombination at high excess carrier concentrations is discussed.