Temperature Dependence Of Hole Concentration Research Articles

Cu vacancy engineering of cage-compound BaCu2Se2: Realization of temperature-dependent hole concentration for high average thermoelectric figure-of-merit

Vacancy engineering offers an alternative route to aliovalent and interstitial doping for optimization of the carrier concentration in thermoelectric materials. For the wide-bandgap semiconductor BaCu2Se2, the Cu vacancy is dynamically stable; this feature can be rationally manipulated to maximize the thermoelectric figure of merit zT. In this work, we show that at room temperature, Cu-deficient BaCu2−xSe2 samples exhibit increased hole effective mass and mobility, attributed to the energy band modulation, which are favorable for improved electrical transport properties. More importantly, the defect energy level resulting from the Cu vacancies continually contributes holes at high temperature, thereby allowing the hole concentration to approach an optimal concentration. This effect leads to an increase of the power factors over a wide temperature range. The artificial reduction of the Cu content in BaCu2Se2 results in the strengthened point-defect scattering, suppressing the lattice thermal conductivity. This strategy allows simultaneous optimization of the electrical and thermal transport properties, with a thermoelectric figure of merit zT = 1.08 achieved for BaCu1.94Se2 at 823 K, which is 38% higher than that of stoichiometric BaCu2Se2. Within the measured temperature range, the average zT value for BaCu1.94Se2 is 0.494, which is 52.9% higher than that of BaCu2Se2.

Electrical properties of pseudo-single-crystalline Ge films grown by Au-induced layer exchange crystallization at 250 °C

We study the electrical properties of pseudo-single-crystalline Ge (PSC-Ge) films grown by a Au-induced layer exchange crystallization method at 250 °C. By inserting the SiNx layer between PSC-Ge and SiO2, we initiatively suppress the influence of the Ge/SiO2 interfacial defective layers, which have been reported in our previous works, on the electrical properties of the PSC-Ge layers. As a result, we can detect the influence of the ionized Au+ donors on the temperature-dependent hole concentration and Hall mobility. To further examine their electrical properties in detail, we also fabricate p-thin-film transistors (TFTs) with the PSC-Ge layer. Although the off-state leakage currents are suppressed by inserting the SiNx layer, the value of on/off ratio remains poor (<102). Even after the post-annealing at 400 °C for the TFTs, the on/off ratio is still poor (∼102) because of the gate-induced drain leakage current although a nominal field effect mobility is enhanced up to ∼25 cm2/V s. Considering these features, we conclude that the Au contaminations into the PSC-Ge layer can affect the electrical properties and device performances despite a low-growth temperature of 250 °C. To achieve further high-performance p-TFTs, we have to suppress the Au contaminations into PSC-Ge during the Au-induced crystallization growth.

Polarization-induced hole doping in N-polar III-nitride LED grown by metalorganic chemical vapor deposition

Polarization-induced doping has been shown to be effective for wide-bandgap III-nitrides. In this work, we demonstrated a significantly enhanced hole concentration via linearly grading an N-polar AlxGa1-xN (x = 0–0.3) layer grown by metal-organic chemical vapor deposition. The hole concentration increased by ∼17 times compared to that of N-polar p-GaN at 300 K. The fitting results of temperature-dependent hole concentration indicated that the holes in the graded p-AlGaN layer comprised both polarization-induced and thermally activated ones. By optimizing the growth conditions, the hole concentration was further increased to 9.0 × 1017 cm−3 in the graded AlGaN layer. The N-polar blue-violet light-emitting device with the graded p-AlGaN shows stronger electroluminescence than the one with the conventional p-GaN. The study indicates the potential of the polarization doping technique in high-performance N-polar light-emitting devices.

Fabrication and characterizations of nitrogen-doped BaSi2 epitaxial films grown by molecular beam epitaxy

Nitrogen doped BaSi2 layers are grown on high-resistivity n-Si (111) substrates by molecular beam epitaxy using a radio-frequency nitrogen plasma. The nitrogen concentration measured by secondary ion mass spectrometry is homogeneous throughout the grown layers. The carrier concentration is measured by Hall measurement using the van der Pauw method. Nitrogen-doped BaSi2 shows n- or p-type conductivity, depending on the intensity of nitrogen plasma. The hole concentration is of the order of 1016–1017cm−3 at room temperature. The acceptor level is estimated to be approximately 64meV from the temperature dependence of hole concentration. The temperature dependence of resistivity is explained by variable-range hopping conduction in p-BaSi2. First-principle calculation suggests that the nitrogen atoms are most likely to occupy the interstitial site in BaSi2.

Acceptor Levels due to a Complex Including the Nitrogen–Hydrogen Bond in GaAsN Films Grown by Chemical Beam Epitaxy

The acceptor levels and their concentration in GaAsN films grown by the chemical beam epitaxy technique were investigated through detailed analysis of the temperature dependence of hole concentration. Two acceptor levels A1 and A2 were found, and their energy levels were fixed at 130 ±20 and 55 ±10 meV, respectively, from the valence band maximum. Both concentrations were on the order of 1017 cm-3 and compensated by the concentration of donor on the same order. The concentration of A1 had a linear relationship with the number of N–H bonds in films grown at the same growth temperature, while the slope decreased with growth temperature. These results strongly suggested that a complex defect including the N–H bond was the origin of the A1 acceptor level.

Electrical and optical properties of p-type InGaN

Mg-doped InxGa1−xN alloys were grown by metal organic chemical vapor deposition on semi-insulating c-GaN/sapphire templates. Hall effect measurements showed that Mg-doped InxGa1−xN epilayers are p-type for x up to 0.35. Mg-acceptor levels (EA) as a function of x, (x up to 0.35), were experimentally evaluated from the temperature dependent hole concentration. The observed EA in Mg-doped In0.35Ga0.65N alloys was about 43 meV, which is roughly four times smaller than that in Mg doped GaN. A room temperature resistivity as low as 0.4 Ω cm (with a hole concentration ∼5×1018 cm−3 and hole mobility ∼3 cm2/V s) was obtained in Mg-doped In0.22Ga0.78N. It was observed that the photoluminescence (PL) intensity associated with the Mg related emission line decreases exponentially with x. The Mg energy levels in InGaN alloys obtained from PL measurements are consistent with those obtained from Hall-effect measurements.

Mechanisms of Reduction in Hole Concentration in Al-Implanted p-Type 6H-SiC by 1 MeV Electron Irradiation

The reduction in temperature-dependent hole concentration p(T) in Al-implanted p-type 6H-SiC by 1 MeV electron irradiation is investigated. By analysis of p(T), the density (NA), level position (EA) in the bandgap and nature of the acceptor are determined, and this acceptor is assigned to an Al acceptor. EA is independent of irradiation fluence (Φ), while NA is strongly dependent on Φ. We derived an analytical expression for the fluence dependence of NA and we estimated the removal coefficient (i.e., removal cross-section) of NA to be 6.4×10-18 cm2 for 1 MeV electron irradiation. The reduction in p(T) by electron irradiation is found to be mainly due to the decrease in NA, not to the increase in the density of deep-level defects in the bandgap, because the decrement in NA is much larger than the increment in the density of deep-level defects.

Optimum Rapid Thermal Activation of Mg-Doped p-Type GaN

In this paper, we describe the electrical properties of Mg-doped GaN annealed using rapid thermal annealing. Metal–organic vapor phase epitaxy (MOVPE)-grown samples with Mg concentrations ranging from 1.25 ×1019 to 1 ×1020 cm-3 were annealed at various temperatures in a nitrogen atmosphere. It was found that the maximum hole concentration achieved after rapid thermal annealing is limited to approximately 8 ×1017 cm-3 at room temperature and its optimum annealing temperature decreases with increasing Mg concentration. The temperature dependence of hole concentration suggested that the formation of donor complex defects is responsible for the suppressed electrical activation of Mg acceptors after annealing at high temperatures higher than the optimum annealing temperature.

A graphical peak analysis method for characterizing impurities in SiC, GaN and diamond from temperature-dependent majority-carrier concentration

A method for uniquely determining the densities and energy levels of impurities from the temperature dependence of the majority-carrier concentration in wide band gap semiconductors (e.g., SiC, GaN, and diamond) is discussed. It is demonstrated that the proposed graphical peak analysis method can evaluate the number of impurity species and can determine those densities and energy levels uniquely and accurately, while fitting a simulation to the experimental temperature-dependent majority-carrier concentration leads to less reliable densities and energy levels of impurities. In the case that the Fermi levels in p-type SiC, GaN and diamond are located between the acceptor level and the valence band maximum, the excited states of acceptors strongly affect the hole concentration. This indicates the distribution function including the influence of the excited states should be applied to determine the densities and energy levels of acceptors from the temperature-dependent hole concentration.

Electrical transport properties in nitrogen-doped p-type ZnO thin film

Electrical transport properties of p-type ZnO:N films grown by thermal activation of the nitrogen dopant were investigated via the temperature-dependent Hall effect. The Hall mobility increases with decreasing temperature. Varied scattering mechanisms have been analysed including lattice vibration scattering, ionized impurity scattering and dislocation scattering. A fit of the theory to temperature-dependent hole mobility experimental data in p-type ZnO:N films gives dislocation densities in the order of 1012 cm−2. The analysis shows dislocation scattering is indeed important for the p-type ZnO films grown on the mismatched substrate. The thermal ionization energy of the nitrogen acceptor is estimated to be 170 meV in terms of the temperature-dependent hole concentration. On the other hand, the emission related to the acceptors is observed in PL spectra.

Accurate Determination of Density and Energy Level of B Acceptor in Diamond from Temperature Dependence of Hole Concentration

The density (NA) and energy level (EA) of a B acceptor in B-doped p-type diamond epilayers are usually determined from the temperature dependence of hole concentration, p(T), using the Fermi–Dirac distribution function for acceptors, which does not consider the effect of the excited states of the B acceptor. However, in samples whose Fermi levels, EF(T), are located between the valence band maximum (EV) and EA in a measurement temperature range, the obtained NA is much higher than the concentration of B atoms determined by secondary ion mass spectroscopy. Because the B acceptor level in diamond is deep, the effect of the excited states of the B acceptor on p(T) should not be ignored. When EF(T) is between EV and EA, the reasonable NA and EA are obtained by fitting a curve to p(T) using the distribution function including the effect of its excited states.

Accurate Determination of Acceptor Densities and Acceptor Levels in Undoped InGaSb from Temperature Dependence of Hole Concentration

Without any assumptions regarding residual impurity species and intrinsic defects in an undoped semiconductor, it is experimentally demonstrated that the densities and energy levels of impurities and defects can be precisely determined by a graphical peak analysis method based on Hall-effect measurements, referred to as free-carrier-concentration spectroscopy (FCCS). By FCCS, the number of acceptor species in p-type undoped In0.2Ga0.8Sb epilayers is determined, and the densities and energy levels of these acceptor species are accurately estimated. Two acceptor species, whose acceptor levels are EV+25 meV and EV+86 meV, are detected, where EV is the valence band maximum. The density of the EV+25 meV acceptor increases with Sb4/(In+Ga) flux beam equivalent pressure (BEP) ratio, whereas the density of the EV+86 meV acceptor decreases with increasing BEP ratio. These observations are not consistent with the conventional assumption that these acceptor species are VSb+ and VSb2+ in GaSb-based semiconductors, where VSb is the Sb vacancy.

Carrier concentration dependence of acceptor activation energy in p-type ZnO

The characteristics of an acceptor level in Sb-doped, p-type ZnO were studied using cathodoluminescence (CL) spectroscopy as a function of hole concentration. Variable-temperature CL measurements allowed us to estimate the activation energy of an Sb-related acceptor from temperature-induced decay of CL intensity. The values of activation energy of about 212±28, 175±20, 158±22, and 135±15meV were obtained for samples with carrier concentrations of 1.3×1017, 6.0×1017, 8.2×1017, and 1.3×1018cm−3, respectively. The involvement of acceptor levels is supported by the temperature-dependent hole concentration measurements. The possible origins of the strong temperature dependence are discussed.

Impurity levels in the layered semiconductor p‐GaSe doped with group V elements As, Bi and Sb

AbstractThe radiative and non radiative recombination mechanisms in the As, Sb and Bi‐doped GaSe have been investigated on the basis of photoluminescence (PL) and Hall effect measurements. The PL features (at 77 K) related to the impurity levels coming from the As, Sb and Bi atoms are dominated by a broad emission band at about 1.7 eV. From the temperature dependences of the peak energy and PL intensity and the dependence of excitation intensity of peak energy, it was found that the 1.7 eV emission band is due to the transition from the shallow donor level at about 0.08 eV below the conduction band to the deep acceptor. In addition it was found, from the temperature dependence of hole concentration, that a deep acceptor level at about 0.6 eV above the valence band is formed by the doping atoms. It is associated with defects or defect complexes. (© 2005 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Hall-effect Characterization and Identification of Impurity Levels in Multi-Crystalline Silicon Base Material for Solar Cells

Samples of 1cm 2 sizes were made from the p-type multi-crystalline silicon boron doped 10 16 cm -3 . Aluminium contacts were made in a vacuum of 10 -6 Torr and post-annealed in Argon ambient at 400°C for 30 minutes for making the four contact electrodes. The activation energy based on low compensation was found to be 47 meV, 32 meV, and 18 meV, respectively. In order to obtain a more reliable solution, the differential evaluation on the temperature dependence of hole concentration was done. Four peaks were observed around 50, 56, 88, and 109 meV, which indicate several acceptor levels in multi-crystalline silicon (Mx-Si). Hall mobility at 300 K in this p-type sample was 189 (cm 2 /Vs) and 3231 (cm 2 /Vs) at 72 K. Based on acoustic phonon scattering the mobility decrease with temperature showed T -1.96 dependence. This behavior indicates the inter-valley scattering as well as grain boundary effects. KEY WORDS: Compensation, multi-crystalline silicon, activation energy, mobility, grain boundaries J. agric. Sci. technol. Vol.5(1) 2003: 106-115

Dependence of acceptor levels and hole mobility on acceptor density and temperature in Al-doped p-type 4H-SiC epilayers

The temperature-dependent hole concentration p(T) and hole mobility μp(T) are obtained in p-type 4H-SiC epilayers with several Al-doping densities. From p(T), the densities and energy levels of acceptors are determined by the graphical peak analysis method (free carrier concentration spectroscopy: FCCS) without any assumptions regarding the acceptor species. In the heavily Al-doped case, the excited states of acceptors affect p(T) because the Fermi level is located between the valence band maximum and the acceptor level (i.e., the ground state level of the acceptor), indicating that a distribution function for acceptors, which includes the influence of excited states of acceptors, should be required. Here, FCCS can determine acceptor densities and acceptor levels using any distribution function (e.g., the Fermi-Dirac distributing function or the distribution function including the influence of excited states). Two types of acceptor species are detected in the lightly Al-doped epilayers, while only one type of acceptor species is found in the heavily Al-doped epilayer. Some of the parameters required to simulate electric characteristics of 4H-SiC power electronic devices are obtained; (1) the dependence of each acceptor level on a total acceptor density and (2) the dependence of the hole mobility on temperature and total impurity density.

Reduction in Al Acceptor Density by Electron Irradiation in Al-Doped 4H-SiC

The influence of electron irradiation on the hole concentration in Al-doped 4H-SiC epilayers is investigated with free carrier concentration spectroscopy (FCCS) using the temperature dependent hole concentration . By 4.6-MeV electron irradiation, is reduced over the whole temperature range. Using FCCS, the densities and energy levels of acceptors or hole traps are determined. In the unirradiated and irradiated samples, ~200 meV and ~370 meV acceptor levels or hole-trap levels are detected. By irradiation, only the density of Al acceptors whose energy level is ~200 meV is reduced from 6.2×10 ) (T p ) (T p

Investigation on the P-Type Activation Mechanism in Mg-doped GaN Films Grown by Metalorganic Chemical Vapor Deposition

An investigation on the p-type activation in Mg-doped GaN epilayers has been carried out in relation to the defect structure. The samples were grown by the metalorganic chemical vapor deposition method. Sapphire with (0001) orientation (C-face) was used as the substrate. After growth, the samples were heat-treated under flowing N2, at temperatures ranging from 600 to 850°C. The p-type activation arises from the dissociation of electrically inactive Mg–H complexes and the neutralization of the dissociated H+ during the annealing process. The annealing temperature dependence of hole concentration and hole mobility was studied. The p-type activation process resulted in a different maximum hole concentration and an optimum annealing temperature. Subsequent microstructural characterization of our samples revealed that the dislocations play a key role in p-type conductivity and may explain the difference observed in the electrical properties. Indeed, the analyses of transmission electron microscopy (TEM) images and X-ray diffraction (XRD) data show that Mg-doped GaN exhibits a different X-ray rocking curve full width at half maximum (FWHM) and dislocation density. Furthermore, it was found that the higher the dislocation density, the higher the hole concentration. Therefore, we suggest that dislocations could act as a migration path or a neutralizing source for dissociated hydrogen impurities.

Electrical properties of B-doped homoepitaxial diamond (001) film

Abstract Relationship between growth condition and quality of homoepitaxially grown B-doped diamond (001) film has been studied using physical measurements of defect density as a function of doping concentration. In particular, electrical properties of the homoepitaxial diamond film were characterized using measurements of conductivity, carrier concentration and mobility. The highest mobility is found to be about 1000 cm2V−1s−1 at 293 K, indicating that the quality of the CVD diamond film is further improved through optimizing the growth condition. The density of the compensation donor was determined from the temperature-dependent hole concentration. The lowest donor density is found to be 8.4 × 1015 cm−3 in the present work. This is an order of magnitude greater than the lowest value measured in natural IIb diamond. Furthermore, it is also found that the donor density increases with increasing doping concentration during the growth. On the other hand, the mobility decreases rapidly with increasing doping concentration. From these results, we speculate that the compensation donor is an origin of an additional scattering center in diamond, and excessive B-doping makes the quality of the CVD diamond worse.

Electrical characteristics of layer semiconductor p-GaSe doped with Cd

The electrical properties of Cd-doped GaSe have been investigated by using Hall-effect and deep-level transient spectroscopy (DLTS). The temperature dependence of hole concentration shows the characteristic of a partially compensated p-type semiconductor. The moderately deep acceptor level at about 0.28 eV above the valence band is detected by using both Hall-effect and DLTS measurements. We find that the acceptor level is associated with Cd-related defects formed by the dopant atoms.