Kingsley O. Egbo

National Renewable Energy Laboratory | NREL

Native defects in semiconductors play an important role in their optoelectronic properties. Nickel oxide (NiO) is one of the few wide-gap p-type oxide semiconductors and its conductivity is believed to be controlled primarily by Ni-vacancy acceptors. Herein, we present a systematic study comparing the optoelectronic properties of stoichiometric NiO...

Nickel oxide, NiO is an important p-type oxide semiconductor that has been studied for applications in solar cells, junction diodes, and other optoelectronic devices. In a nominally undoped NiO, depending on its oxygen stoichiometry, it only has a modest p-type conductivity of ~0.1 S/cm due to Ni vacancy acceptors. However, the overall transport ca...

In this work, we report on the realization of SnO/β-Ga2O3 heterojunction vertical diodes and lateral field-effect transistors for power electronic applications. The p-type semiconductor SnO is grown by plasma-assisted molecular beam epitaxy on n-type (100) β-Ga2O3 with donor concentrations of 3×10 17 cm-3 for the diode devices and 8.1×10 17 cm-3 fo...

Transition metal doped In2O3 with high mobility can be used as a transparent conductor with enhanced transparency spectral window. In this work, we carried out a comprehensive study on the electrical and optical properties of In2O3 doped with several transition metal (TM) species (In2O3:TM) including W, Zr, Mo, and Ti. Detailed optical properties o...

(La and Ga)-doped tin monoxide [stannous oxide, tin (II) oxide, SnO] thin films were grown by plasma-assisted and suboxide molecular beam epitaxy with dopant concentrations ranging from ~5×10^18 to 2×10^21 cm^-3. In this concentration range, the incorporation of Ga into SnO was limited by the formation of secondary phases observed at 1.2×10^21cm^3...

We have epitaxially stabilized a series of monoclinic (AlxGa1−x−yIny)2O3 alloys by careful choice of molecular beam epitaxy growth conditions, which balance alloy growth with suboxide desorption. The films are pseudomorphic to (010) β-Ga2O3 substrates at thicknesses up to 150 nm with compositions ranging from (Al0.01Ga0.83In0.16)2O3 to (Al0.24Ga0.7...

Rutile GeO$_2$ is a promising ultra-wide bandgap semiconductor for future power electronic devices whose alloy with the wide bandgap semiconductor rutile-SnO$_2$ enables bandgap engineering and the formation of heterostructure devices. The (Sn$_x$Ge$_{1-x}$)O$_2$ alloy system is in its infancy and molecular beam epitaxy (MBE) is a well-suited techn...

The potential impact of cation-substituted AlN-based materials, such as Al1-xScxN, Al1-xGaxN, and Al1-xBxN, with exceptional electronic, electromechanical, and dielectric properties has spurred research into this broad family of materials. Rare earth (RE) cations are particularly appealing as they could additionally impart optoelectronic or magneti...

Large language models (LLMs) are transforming the landscape of chemistry and materials science. Recent examples of LLM-accelerated experimental research include virtual assistants for parsing synthesis recipes from the literature, or using the extracted knowledge to guide synthesis and characterization. Despite these advancements, their application...

NiO/Ga2O3 heterojunction diodes have attracted attention for high-power applications, but their high temperature performance and reliability remain underexplored. Here, we report the time evolution of the electrical properties in the widely studied p-NiO/n-Ga2O3 heterojunction diodes and formation of NiGa2O4 interfacial layers at high temperatures....

The delivery of an elemental cation flux to the substrate surface in the oxide molecular beam epitaxy (MBE) chamber has been utilized not only for the epitaxial growth of oxide thin films in the presence of oxygen but also in the absence of oxygen for the growth temperature calibration (by determining the adsorption temperature of the elements) and...

Beta gallium oxide ( β-Ga2O3)-based semiconductor heterojunctions have recently demonstrated improved performance at high voltages and elevated temperatures and are, thus, promising for applications in power electronic devices and harsh environment sensors. However, the long-term reliability of these ultra-wideband gap (UWBG) semiconductor devices...

The delivery of an elemental cation flux to the substrate surface in the oxide molecular beam epitaxy (MBE) chamber has been utilized not only for the epitaxial growth of oxide thin films in the presence of oxygen but also in the absence of oxygen for the growth temperature calibration (by determining the adsorption temperature of the elements) and...

Because of their well-defined light-matter interaction volume, high-quality single-crystalline nature, and precise bandgap tunability, all inorganic CsPbX3 (X = Cl, Br, I) perovskite (IHP) microplates are of fundamental and technological...

Molecular beam epitaxy is one of the highest quality growth methods, capable of achieving theoretical material property limits and unprecedented device performance. However, such ultimate quality usually comes at the...

The potential impact of cation-substituted AlN-based materials, such as Al 1– x Sc x N, Al 1– x Ga x N, and Al 1– x B x N, with exceptional electronic, electromechanical, and dielectric properties has spurred research into this broad family of materials. Rare...

Large language models (LLMs) are one of the AI technologies that are transforming the landscape of chemistry and materials science. Recent examples of LLM-accelerated experimental research include virtual assistants for parsing synthesis recipes from the literature, or using the extracted knowledge to guide synthesis and characterization. However,...

The interfacial properties of a planar SnO/κ-Ga2O3 p–n heterojunction have been investigated by capacitance–voltage (C–V) measurements following a methodological approach that allows consideration of significant combined series resistance and parallel leakage effects. Single-frequency measurements were carried out in both series- and parallel-model...

Rutile GeO2 has been predicted to be an ultra-wide bandgap semiconductor suitable for future power electronic devices, while quartz-like GeO2 shows piezoelectric properties. To explore these crystalline phases for application and fundamental materials investigations, molecular beam epitaxy (MBE) is a well-suited thin film growth technique. In this...

Rutile GeO2 has been predicted to be an ultra-wide bandgap semiconductor suitable for future power electronics devices while quartz-like GeO2 shows piezoelectric properties. To explore these crystalline phases for application and fundamental materials investigations, molecular beam epitaxy (MBE) is a well-suited thin film growth technique. In this...

By employing a mixed SnO2 + Sn source, we demonstrate suboxide molecular beam epitaxy (S-MBE) growth of phase-pure single-crystalline metastable SnO (001) thin films on Y-stabilized ZrO2 (001) substrates at a growth rate of ∼1.0 nm/min without the need for additional oxygen. These films grow epitaxially across a wide substrate temperature range fro...

Lead-based organic or inorganic halide perovskites (ABX3, A= organic or inorganic cation, B= lead cation, and X= Cl, Br, I) are well-known for their excellent properties and technologically suitable for...

(La and Ga)-doped tin monoxide (stannous oxide, tin (II) oxide, SnO) thin films were grown by plasma-assisted and suboxide molecular beam epitaxy with dopant concentrations ranging from $\approx5\times10^{18}$cm$^{-3}$ to $2\times10^{21}$cm$^{-3}$. In this concentration range, the incorporation of Ga into SnO was limited by the formation of seconda...

While n-type transparent conducting oxides (TCOs) have been well developed and widely used in optoelectronic devices, the availability of high-performance p-type TCOs is still scarce which in turn hinders the further advancement of oxide based transparent optoelectronic devices. Unlike the most widely used p-type NiO, Tin monoxide (SnO) has a relat...

Tin monoxide (SnO) has attracted much attention as a p-type transparent conducting oxide (TCO) with high hole mobility, which was attributed to its relatively delocalized valence band. Nominally undoped SnO can achieve a free hole concentration of >10¹⁸cm⁻³due to the low formation energy of the Sn vacancy acceptor. Although several calculations sho...

As a promising high mobility p-type wide bandgap semiconductor, copper iodide has received increasing attention in recent years. However, the defect physics/evolution are still controversial, and particularly the ultrafast carrier and exciton dynamics in copper iodide has rarely been investigated. Here, we study these fundamental properties for cop...

By employing a mixed SnO$_2$+Sn source, we demonstrate suboxide molecular beam epitaxy growth of phase-pure single crystalline metastable SnO(001) thin films at a growth rate of ~1.0nm/min without the need for additional oxygen. These films grow epitaxially across a wide substrate temperature range from 150 to 450{\deg}C. Hence, we present an alter...

Gallium oxide (Ga2O3) has gained significant interest in recent years due to its wide bandgap and related unique properties, making it suitable for many high power and deep ultra-violet optoelectronic and photodetection devices. Nevertheless, Ga2O3 can only be doped effectively n-type, and its full potential in device applications is severely limit...

SnO/𝜀-Ga2O3 vertical p–n diodes with planar geometry have been fabricated on c-plane Al2O3 and investigated by current–voltage measurements. The effects of the in-plane conduction through the Si-doped 𝜀-Ga2O3 layer on the diode performance and their relevance have been evaluated. A significant series resistance is observed, which shows typical feat...

Reliable bipolar carrier transport remains a challenge in most wide-gap oxide semiconductors, and this limits the extensive adoption of oxides in transparent optoelectronic devices. For instance, it has been difficult to efficiently dope ZnO p-type in its hexagonal wurtzite polymorph. However, metastable octahedral rocksalt (RS) polymorph of ZnO ha...

AlMgCu alloy samples with 70 wt.% Al and varying Cu and Mg compositions have been fabricated. The fabricated samples were characterized by x-ray diffraction measurement and the diffraction patterns obtained. The samples were also subjected to hardness and tensile tests to determine the mechanical properties. A study of the diffraction patterns was...

Cu2-xS has received increasing attentions in recent years, owing to its promising applications in a broad range of optoelectronic devices. However, the phase structures and the electronic structures of Cu2-xS with varying Cu stoichiometries are surprisingly complex, and consensus on these aspects are still lacking. In this work, p-type polycrystall...

Al doped ZnO (AZO) is a promising transparent conducting oxide to replace the expensive Sn doped In2O3 (ITO). Understanding the formation and evolution of defects in AZO is essential for its further improvement. Here, we synthesize transparent conducting AZO thin films by reactive DC magnetron sputtering. The effects of oxygen flow ratio as well as...

ZnSe:x% Eu³⁺ (0 ≤ x ≤ 2.0) thin films were deposited using the photo-assisted chemical bath technique. All the films samples revealed wurtzite phase and the diffraction peak intensities decreased with increased Eu³⁺ concentration, due to loss of crystallinity. Raman spectroscopy revealed two optical phonon peaks due to first and second order longit...

NiO is one of the few transition metal oxides exhibiting p-type conductivity and visible light transparency. On the other hand, MoO3 and WO3 are high work function, wide gap metal oxides which show predominantly n-type character and have been widely applied in carrier selective contacts, heterojunction diodes, electrochromic and sensing application...

Hybrid organolead halide perovskites have attracted tremendous attention due to their recent success as high efficiency solar cell materials and their fascinating material properties uniquely suitable for optoelectronic devices. However, the poor ambient and operational stability as well as the concern of lead toxicity greatly hamper their practica...

Recognizing high thermoelectric performance in semiconducting materials is a challenging task. This is because the Seebeck coefficient and electrical conductivity which constitute the thermoelectric power factor are unfavourably coupled. This means decoupling the transport properties of thermoelectric materials to enhance the power factor without c...

Native defects in semiconductors play an important role in their optoelectronic properties. Nickel oxide (NiO) is one of the few wide-gap p-type oxide semiconductors and its conductivity is believed to be controlled primarily by Ni-vacancy acceptors. Herein, we present a systematic study comparing the optoelectronic properties of stoichiometric NiO...

Oxide based transparent p-n homojunctions are desirable for the development of transparent electronics. However, most oxide semiconductors are intrinsically n-type and to achieve p-type wide gap oxide is still challenging. Previously, we have demonstrated that alloying a high mobility n-type material such as CdO with p-type NiO can provide an avenu...

Designing highly efficient and low-cost electrocatalysts for oxygen evolution reaction is important for many renewable energy applications. In particular, strain engineering has been demonstrated as a powerful strategy to enhance the electrochemical performance of catalysts; however, the required complex catalyst preparation process restricts the i...

Zinc selenide (ZnSe) thin films have been deposited on glass substrates via photo-assisted chemical bath deposition technique. The films were annealed at different temperatures 100, 150, 200, 250, 300, 350 and 400 °C in air for 2 h and were characterized using different techniques. The glancing incidence X-ray diffraction (GIXRD) showed that the as...

Highly mismatched alloys (HMAs) provide a unique venue for controlling electronic band structures and consequently the optoelectronic properties of materials for specific device applications. In this paper, we synthesize the ZnO1-xTex HMA thin films over the entire composition range by magnetron co-sputtering. We find that the alloys with x<0.15 an...

Zinc selenide (ZnSe) thin films were deposited on non-conducting glass substrates at 80 °C for 2.0 h in an alkaline medium using photo-assisted chemical bath deposition (PCBD). The films were annealed in air at 300 °C for different time intervals ranging from 0 to 4.0 h. The annealing time had a significant effect on the structure, morphology and o...

Nickel oxide is one of the few wide gap p-type materials which has been applied in many optoelectronic devices. It is well known that the p-type conductivity in NiO arises from Ni vacancy VNi (and/or Oxygen interstitial) acceptors in Oxygen-rich NiO (NiO1+δ). However, due to the instability of these native defects, NiO1+δ is unstable, and its p-typ...

The development of transparent bipolar oxide devices is largely hampered by the lack of oxides with reliable p-type conductivity. Recent calculations show that rock salt (RS) structured ZnO alloys are promising candidates for transparent p-type oxides. Here, we synthesize wide-gap Zn1−xNixO alloy thin films over the entire composition range on glas...

ZnSe thin films were deposited using hydrazine hydrate (HH) as the only complexing agent. The deposition was carried out on glass substrates in an alkaline medium at low temperature using the chemical bath deposition method. The films were annealed at 300 °C for 2 h. The effects of varying the HH volumes on the structural and optical properties of...

Quaternary nanocrystallite (CdAl2SO3) thin films have been synthesized by the method of chemical bath deposition (CBD) technique using a reaction bath containing cadmium chloride, thiourea, and Aluminium sulphate octadecahydrate at room temperature. The thin films are white in colour and strongly adherent to the substrate. The materials were anneal...

Photo-assisted chemical bath deposition (PCBD) and non-photo-assisted chemical bath deposition (NPCBD) methods were used to deposit ZnSe thin films on non-conducting glass substrates in an alkaline medium. Zinc acetate and sodium selenosulfatewere the starting precursors. Hydrazine hydrate (HH) was used as ligand for the deposition. Different HH vo...

Cd 1-x Zn x S thin films have been grown by two-electrode electrodeposition method using an electrolytic bath containing cadmium chloride, zinc chloride and sodium thiosulphate. The deposition was carried out at three different cathodic voltages of 1695 mV, 1700 mV and 1705 mV. The characterization of the films was done using grazing incidence X-ra...

An efficient solution-processable route employing Pb(Ac) 2 as lead source and anti-solvent treatment to achieve fully covered and homogenous perovskite films is reported. The effect of different solution methods and device architectures on the morphologies of perovskite films were systematically investigated. Our results show that the planar perovs...

Wide-gap oxides exhibiting both n- and p-type bipolar conductivity are extremely desirable for the development of next generation transparent optoelectronics. While most metal oxides show a propensity for n-type conductivity, nickel oxide (NiO) is one of the few that exhibits p-type conductivity without intentional doping. We synthesize nanocrystal...

Very dense and uniform thin films of CdS were deposited on fluorine-doped tin oxide (FTO) substrates for thin film semiconductor devices application, such as in solar cells. The simplified electrochemical deposition method applied uses two-electrode system with FTO as the working electrode and high-purity graphite rod as the counter electrode. The...

The fundamental phenomenon which makes transmission unreliable is time varying fading which is the constructive/destructive interference between signals arriving at the same antenna via different paths, and hence with different delays and phases, resulting in random fluctuations of the signal level at the receiver. This paper investigates Rake rece...