Wide Gap P-type Semiconductor Research Articles

Nickel vacancy acceptor in nickel oxide: Doping beyond thermodynamic equilibrium

This work reports on temperature-induced out-diffusion and concentration decay of the prominent intrinsic point defect VNi (nickel vacancy) in the wide-gap p-type semiconductor nickel oxide (NiO). VNi can easily be introduced into NiO thin films by offering high oxygen partial pressures during film growth, rendering nonstoichiometric semiconducting structures. However, exposure to lower oxygen supply after growth, e.g. in a standard atmosphere, usually leads to a gradual decrease of film conductivity, because the vacancy concentration equilibrates. In this study, we observe this process in situ by performing temperature-dependent measurements of the electrical conductivity on a room temperature-grown NiO film. At a temperature of 420K under exclusion of oxygen, the doping level decreases by a factor of 8 while the associated room temperature dc conductivity drops by six orders of magnitude. At the same time, out-diffusion of the mobile VNi species can be indirectly observed through the occurrence of electrode polarization characteristics.

Nickel oxide particles catalyze photochemical hydrogen evolution from water--nanoscaling promotes p-type character and minority carrier extraction.

Nickel(II) oxide (NiO) is an important wide gap p-type semiconductor used as a hole transport material for dye sensitized solar cells and as a water oxidation electrocatalyst. Here we demonstrate that nanocrystals of the material have increased p-type character and improved photocatalytic activity for hydrogen evolution from water in the presence of methanol as sacrificial electron donor. NiO nanocrystals were synthesized by hydrolysis of Ni(II) nitrate under hydrothermal conditions followed by calcination in air. The crystals have the rock salt structure type and adopt a plate-like morphology (50-90 nm × 10-15 nm). Diffuse reflectance absorbance spectra indicate a band gap of 3.45 eV, similar to bulk NiO. Photoelectrochemical measurements were performed at neutral pH with methylviologen as electron acceptor, revealing photo-onset potentials (Fermi energies) of 0.2 and 0.05 eV (NHE) for nanoscale and bulk NiO, respectively. Nano-NiO and NiO-Pt composites obtained by photodepositon of H2PtCl6 catalyze hydrogen evolution from aqueous methanol at rates of 0.8 and 4.5 μmol H2 h(-1), respectively, compared to 0.5 and 2.1 μmol H2 h(-1) for bulk-NiO and NiO-Pt (20 mg of catalyst, 300 W Xe lamp). Surface photovoltage spectroscopy of NiO and NiO-Pt films on Au substrates indicate a metal Pt-NiO junction with 30 mV photovoltage that promotes carrier separation. The increased photocatalytic and photoelectrochemical performance of nano-NiO is due to improved minority carrier extraction and increased p-type character, as deduced from Mott-Schottky plots, optical absorbance, and X-ray photoelectron spectroscopy data.

Low and small resistance hole-injection barrier for NPB realized by wide-gap p-type degenerate semiconductor, LaCuOSe:Mg

LaCuOSe:Mg is a wide-gap p-type semiconductor with a high conductivity and a large work function. Potential of LaCuOSe:Mg as a transparent hole-injection electrode of organic light-emitting diodes (OLEDs) was examined by employing N, N′-diphenyl- N, N′-bis (1,1′-biphenyl)-4,4′-diamine (NPB) for a hole transport layer. Photoemission spectroscopy revealed that an oxygen plasma treated surface of LaCuOSe:Mg formed a hole-injection barrier as low as 0.3 eV, which is approximately a half of a conventional ITO/NPB interface. Hole-only devices composed of a LaCuOSe:Mg/NPB/Al structure showed a low threshold voltage ∼0.2 V and high-density current drivability of 250 mA cm −2 at 2 V, which is larger by two orders of magnitude than that of ITO/NPB/Al devices. These results demonstrate that LaCuOSe:Mg has great potential as an efficient transparent anode for OLEDs and other organic electronic devices.

Heavy hole doping of epitaxial thin films of a wide gap p-type semiconductor, LaCuOSe, and analysis of the effective mass

The high density hole doping (1.7×1021cm−3) for a wide gap (Eg=∼2.8eV) p-type semiconductor was achieved on 40nm thick Mg-doped LaCuOSe epitaxial films. These films exhibited distinct free carrier absorption, and the effective mass and momentum relaxation time were analyzed. Its small hole mobility [∼3.5cm2∕(Vs)] compared to the electron mobilities of wide gap n-type semiconductors is attributed to a heavy effective mass of 1.6±0.2me. Regardless of the heavy hole doping, a band filling effect was not observed. These results are discussed with a rigid band model and an acceptor band model.

LEAD-BASED FERROELECTRIC COMPOUNDS: INSULATORS OR SEMICONDUCTORS?

ABSTRACT Lead zirconate-titanate (PZT) epitaxial thin films are considered wide-gap p-type semiconductors with standard Schottky contacts. Capacitance-voltage, current-voltage, as well as the dielectric hysteresis are analyzed using a model based on the conventional metal-semiconductor Schottky contact in which the effect of the ferroelectric polarization on band-bending was considered. In addition, presence of the deep traps and its effect on the measured quantities were considered and discussed. For a single-crystal like PZT films with a remnant polarization of 40 μC/cm2 the free carrier concentration is estimated to about 3× 1018 cm−3, whereas the effective density of the fixed charges in the depletion region is of about 1.8 × 1019 cm−3.

Heterojunction Type Chemical Sensors based on Wide gap p-type semiconductors

Heterojunction Type Chemical Sensors based on Wide gap p-type semiconductors

Degenerate electrical conductive and excitonic photoluminescence properties of epitaxial films of wide gap p-type layered oxychalcogenides, LnCuOCh (Ln=La, Pr and Nd; Ch=S or Se)

Electrical and photoluminescence properties were investigated for epitaxial films of layered oxychalcogenides, LnCuOCh (Ln = La, Pr, and Nd, Ch = S or Se). Epitaxial films of Mg 10 at. % doped LaCuOS1-xSex are the first demonstration of degenerate conduction with high hole concentration >1020 cm-3 in wide gap p-type semiconductors. Ion substitution varied the excitonic emission energy from 3.21 eV to 2.89 eV while lanthanide and chalcogenide ion substitutions displayed the opposite tendency against cell volume. These unique properties are discussed with respect to the electronic structure originating from the layered crystal structure.

Heteroepitaxial growth of wide gap p-type semiconductors: LnCuOCh (Lr=La, Pr and Nd; Ch=S or Se) by reactive solid-phase epitaxy

Heteroepitaxial thin films of Cu-based oxychalcogenides, LnCuOCh (Ln = La, Pr and Nd; Ch = mixture of S and Se), were fabricated and their resulting film structures were characterized. A reactive solid-phase epitaxy method successfully yielded heteroepitaxial films on MgO(001) substrates. A high-resolution electron microscopic examination of a LaCuOS film revealed a sharp film-substrate interface. Four-axes high-resolution X-ray diffraction measurements revealed that crystalline lattices in the films are fully relaxed and that the crystallographic orientation is (001)[110] LnCuOCh || (001)[110] MgO. Furthermore, systematic variations in the lattice constant by lanthanide or chalcogen ion substitutions were observed.

Heteroepitaxial growth of a wide-gap p-type semiconductor, LaCuOS

High-quality epitaxial films of LaCuOS, a wide-gap p-type semiconductor, are grown on yittria-stabilized-zirconia (001) or MgO (001) single-crystal substrates by a unique method. Postannealing of a bilayer film composed of an extremely thin metallic copper layer and an amorphous LaCuOS layer at 1000 °C results in an epitaxially grown LaCuOS thin film. This thin copper layer with high orientation, which likely acts as an epitaxial initiator, is essential for epitaxial growth. The resulting epitaxial films exhibit relatively intense ultraviolet emission associated with excitons at room temperature, confirming the high crystal quality of the films.

Heteroepitaxial Growth of a Wide Gap P-type Oxysulfide, LaCuOS

ABSTRACTEpitaxial films of LaCuOS, a wide gap p-type semiconductor, were grown on yittria- stabi-lized-zirconia (YSZ) (001) or MgO (001) substrates by a reactive solid phase epitaxy (R-SPE) method. Crystal quality, electrical and optical properties on the epitaxial films on each substrate are examined in this paper. Achievement of the heteroepitaxial growth of LaCuOS on the MgO (001) substrate improves optical properties of LaCuOS such as spectral bandwidths and emission intensity, suggesting that the MgO (001) substrate is more preferable than the YSZ (001) for epitaxial growth substrate for LaCuOS.

Wide-gap P-type Conductive Properties in Layered Oxychalcogenides

ABSTRACTSeveral Cu(I)-containing layered oxysulfides were selected as candidates for wide-gap p-type semiconductors by extending a concept of a materials design for transparent p-type conducting oxides. The electrical and optical properties of the selected oxysulfides were investigated, and their electronic structures were analyzed by energy band calculations. LaCuOS, Sr2Cu2ZnO2S2 and Sr2CuGaO3S were found to be wide-gap p-type semiconductors, and LaCuOS showed the largest energy gap (Eg=3.1eV) among these layered oxysulfides. It was also found that LaCuOS shows band edge emission under uv excitation at room temperature, which is consistent with the results of the energy band calculations that LaCuOS has a direct-allowed-type energy gap at Γ point. In further materials research, analogous layered oxychalcogenides such as LaCuOSe and LnCuOS (Ln=Pr, Nd) were found to show similar optical and electrical properties to those of LaCuOS. Therefore, it is considered that the layered crystal structure and the electronic structure are responsible for the wide-gap p-type conductive properties in these materials.

Electrical and Optical Properties of Layered Oxysulfides with CuS Layers: Sr−Cu−M−O−S System (M = Zn, Ga, In)

The electrical and optical properties of layered oxysulfides (Sr−Cu−M−O−S system (M = Zn, Ga, In)), which have common CuS layers, were studied to examine their potential as wide-gap p-type semiconductors. The attempts of electrical conductivity control by Na doping demonstrated that the conductivities of the Sr2-xNaxCu2ZnO2S2 (σ = 2.2 × 10-8 S cm-1 for x = 0.0) and Sr2-xNaxCuGaO3S (σ = 2.2 × 10-4 S cm-1 for x = 0.0) at room temperature increased up to 1.2 × 10-1 and 2.4 × 10-2 S cm-1 for x = 0.1, respectively, with an increase in Na concentration. In addition, both Hall and Seebeck measurements proved that these conductive oxysulfides were p-type semiconductors. The diffuse reflectance spectra of the materials revealed that the optical absorption edges were approximately 450 nm for Sr2Cu2ZnO2S2, 480 nm for Sr2CuGaO3S, and 540 nm for Sr2CuInO3S. It became evident from these results that the layered oxysulfides (Sr−Cu−M−O−S system (M = Zn, Ga, In)) were wide-gap p-type semiconductors utilizing the CuS layer...