Si Donors Research Articles

Double Gold/Nitrogen Nanosecond-Laser-Doping of Gold-Coated Silicon Wafer Surfaces in Liquid Nitrogen

A novel double-impurity doping process for silicon (Si) surfaces was developed, utilizing nanosecond-laser melting of an 11 nm thick gold (Au) top film and a Si wafer substrate in a laser plasma-activated liquid nitrogen (LN) environment. Scanning electron microscopy revealed a fluence- and exposure-independent surface micro-spike topography, while energy-dispersive X-ray spectroscopy identified minor Au (~0.05 at. %) and major N (~1–2 at. %) dopants localized within a 0.5 μm thick surface layer and the slight surface post-oxidation of the micro-relief (oxygen (O), ~1.5–2.5 at. %). X-ray photoelectron spectroscopy was used to identify the bound surface (SiNx) and bulk doping chemical states of the introduced nitrogen (~10 at. %) and the metallic (<0.01 at. %) and cluster (<0.1 at. %) forms of the gold dopant, and it was used to evaluate their depth distributions, which were strongly affected by the competition between gold dopants due to their marginal local concentrations and the other more abundant dopants (N, O). In this study, 532 nm Raman microspectroscopy indicated a slight reduction in the crystalline order revealed in the second-order Si phonon band; the tensile stresses or nanoscale dimensions of the resolidified Si nano-crystallites envisioned by the main Si optical–phonon peak; a negligible a-Si abundance; and a low-wavenumber peak of the Si3N4 structure. In contrast, Fourier transform infrared (FT-IR) reflectance and transmittance studies exhibited only broad structureless absorption bands in the range of 600–5500 cm−1 related to dopant absorption and light trapping in the surface micro-relief. The room-temperature electrical characteristics of the laser double-doped Si layer—a high carrier mobility of 1050 cm2/Vs and background carrier sheet concentration of ~2 × 1010 cm−2 (bulk concentration ~1014–1015 cm−3)—are superior to previously reported parameters of similar nitrogen-implanted/annealed Si samples. This novel facile double-element laser-doping procedure paves the way to local maskless on-demand introductions of multiple intra-gap intermediate donor and acceptor bands in Si, providing related multi-wavelength IR photoconductivity for optoelectronic applications.

Unlocking high-efficiency solar energy: optimizing P3HT/Si hybrid solar cells through numerical simulation

This study numerically investigates the optimization of P3HT/Si hybrid solar cells for maximum efficiency via numerical simulation using SCAPS ID. The analysis identifies P3HT layer thickness and Si donor density as critical parameters, with optimal values of 20 nm and 1017 cm−3, respectively. Exceeding these values leads to decreased efficiency due to reduced hole mobility, increased recombination, and enhanced absorption loss. Furthermore, minimizing defect density in Si is crucial for achieving high efficiency. The optical properties of the P3HT/Si hybrid thin film were investigated using the transfer matrix method (TMM), yielding exceptional transmittance (94.5%), low reflectance (4.5%), and minimal absorption (1.0%) at a wavelength of 0.5876 μm. This highlights the material’s potential for efficient and transparent photovoltaic devices. By optimizing these parameters, a maximum efficiency of ∼24% was achieved, demonstrating the potential for high-performance hybrid solar cells. This work underscores the significance of numerical analysis in designing efficient hybrid solar cells, paving the way for the development of low-cost, high-performance devices via spin coating, a groundbreaking advancement in solar energy harvesting.

Achieving n-type doped monoclinic (InxAl1-x)2O3 alloys

The monoclinic (In0.25Al0.75)2O3 alloy has been suggested as an ideal material to create monoclinic Ga2O3 heterostructures, as it provides a close lattice match to β-Ga2O3 along with a 1 eV conduction-band offset. Achieving intentional n-type doping in Ga2O3 heterostructures is important for device applications, but this may be difficult due to the high Al content of this alloy. Here, we use density functional theory with a hybrid functional to investigate common donor dopants, in particular, Si, Sn, C, and Ge substituting on cation sites, and H interstitials, in In2O3 and InAlO3. We identify Si as the optimal donor, as it is a shallow donor for In concentrations above 14%. Its formation energy is also low, indicating that these donors will incorporate during growth. For higher In concentrations, Sn (above 33% In) and Ge (above 35% In) are also promising donors, with Sn having comparable formation energies to Si.

Demonstration of near-ideal Schottky contacts to Si-doped AlN

Near-ideal behavior in Schottky contacts to Si-doped AlN was observed as evidenced by a low ideality factor of 1.5 at room temperature. A temperature-independent Schottky barrier height of 1.9 eV was extracted from temperature-dependent I–V measurements. An activation energy of ∼300 meV was observed in the series resistance, which corresponded to the ionization energy of the deep Si donor state. Both Ohmic and Schottky contacts were stable up to 650 °C, with around four orders of magnitude rectification at this elevated temperature. These results demonstrate the potential of AlN as a platform for power devices capable of operating in extreme environments.

Silicon diffusion in AlN

In this study, we investigate the diffusion of Si donors in AlN. Amorphous Si1−xNx sputtered on the surface of bulk AlN with low dislocation density is used as a Si source. The diffusion experiments are conducted through isochronal and isothermal annealing in a protective N2 atmosphere at temperatures between 1500 and 1700 °C. The Si depth profiles measured by secondary ion mass spectrometry exhibit a convex box-like shape with a steep diffusion front. These concentration profiles are best described with a diffusion coefficient that depends on the square of local Si concentration. From the characteristic box-shaped Si profiles, we conclude that diffusion of Si in AlN is mediated by singly negatively charged dopant–vacancy pairs SiAlVAl−. The strong concentration dependence of Si diffusion is due to the electric field associated with the incorporation of Si donors (SiAl+1) on substitutional Al lattice sites and reflects that Si is fully electrically active at diffusion temperature. The experimentally obtained extrinsic Si diffusion coefficient is reduced to intrinsic doping conditions. The temperature dependence of Si diffusion for intrinsic conditions is described by an activation enthalpy of (10.34±0.32)eV and a pre-exponential factor of 235−203+1485cm2s−1. The migration enthalpy of the donor–vacancy pair SiAlVAl− is estimated to be around 3.5 eV. This estimation is based on the activation enthalpy of the transport capacity of SiAlVAl− and theoretical results concerning the formation energy of negatively charged vacancies on Al-sites in AlN.

Trapping Layers Prevent Dopant Segregation and Enable Remote Doping of Templated Self-Assembled InGaAs Nanowires.

Selective area epitaxy is a promising approach to define nanowire networks for topological quantum computing. However, it is challenging to concurrently engineer nanowire morphology, for carrier confinement, and precision doping, to tune carrier density. We report a strategy to promote Si dopant incorporation and suppress dopant diffusion in remote doped InGaAs nanowires templated by GaAs nanomembrane networks. Growth of a dilute AlGaAs layer following doping of the GaAs nanomembrane induces incorporation of Si that otherwise segregates to the growth surface, enabling precise control of the spacing between the Si donors and the undoped InGaAs channel; a simple model captures the influence of Al on the Si incorporation rate. Finite element modeling confirms that a high electron density is produced in the channel.

Comprehensive Study of Electron Conduction and Its Compensation for Degenerate Si‐Doped AlN‐Rich AlGaN

The resistivity, electron concentration, mobility, and luminescence property of AlGaN epilayers with various AlN contents (x) are systematically investigated as a function of Si‐doping concentration ([Si]). In x of 59–78%, the resistivity decreases with increasing [Si] up to 4 × 1019 cm−3. The temperature dependence of the electron concentration and mobility disappears in the optimum doping range, characteristic of electric degeneracy. The ionization energy of isolated Si donor (Ed0) is slightly increased in the range of 60–80 meV with increasing x (59–78%). At higher [Si] than 4 × 1019 cm−3, self‐compensation defects such as III vacancy–Si complexes are formed, resulting in higher resistivity. When x is 89%, the resistivity is approximately one order of magnitude higher than that for lower x, and the Ed0 is drastically increased to ≈120 meV, which is a feature of a highly compensated semiconductor. Based on the increased oxygen incorporation and the distinctive defect luminescence peak for the sample with x of 89%, the formation of acceptor‐type O‐related defects such as O‐DX and VIII‐nON is indicated and can contribute to a part of the electron compensation.

Two-inch Fe-doped β-Ga2O3 (010) substrates prepared using vertical Bridgman method

The growth of large-diameter high-resistivity β-Ga2O3 (010) substrates is important for the low-cost production of lateral Ga2O3 devices. We grew a 2 inch diameter Fe-doped high-resistivity β-Ga2O3 (010) single crystal by using the vertical Bridgman (VB) method, which is expected to grow large-diameter β-Ga2O3 crystals with various crystal orientations. Two-inch substrates were prepared from the obtained crystals, and their crystallinity, concentration of Fe dopants, and electrical properties were investigated. Consequently, a 2 inch β-Ga2O3 (010) substrate, which is comparable to the largest size of (010) substrate prepared using the Czochralski method, was successfully fabricated with the VB method. The in-plane distribution of the X-ray rocking curve from 020 diffraction of the fabricated 2 inch substrate showed that the full widths at half maximums were less than 35 arcsec at almost all measurement points, indicating high crystallinity and high in-plane uniformity. In addition, the crystals contain Fe concentrations in the range of 3.5 × 1018–1.9 × 1019 cm−3, indicating that impurity Si donors are sufficiently compensated by the Fe dopants. Therefore, substrates prepared using the VB method exhibited high resistivities of 6 × 1011–9 × 1012 Ω·cm at room temperature.

Electronic structure and terahertz intersubband absorption spectra of phosphorus donor in silicon using a variationally optimized diagonalization method

The electronic structure and terahertz (THz) intersubband absorption spectra of phosphorus (P) donor in silicon (Si) are theoretically investigated using a variationally optimized diagonalization method based on the three-dimensional (3D) nonorthogonal associated Laguerre basis set, the basis set of eigenfunctions of 3D hydrogen atom and the basis set of eigenfunctions of 3D isotropic harmonic oscillator. A fully analytical expression of the matrix elements of the Hamiltonian is derived. Due to the large band anisotropy of Si, the donor electron series appear in an anomalous energy-level order and interval of the orbital angular momentum. The donor electron energy levels with even a small number of basis functions are in excellent agreement with those reported in the previous literature. Compared to two different basis sets of eigenfunctions of 3D hydrogen atom and isotropic harmonic oscillator, my proposed variationally optimized diagonalization method based on the 3D nonorthogonal associated Laguerre basis set is proven to be more efficient and reliable in calculating highly accurate impurity states of anisotropic 3D materials. The intersubband absorption spectra of P donor electron involving transitions from the 1s to excited states are strongly dependent on the orientations of the valley and the polarization vector of the incident light, which results from the fact that the large band anisotropy of Si induces strong mixing of the donor electron states. Their absorption frequencies are found to fall into the THz part of the electromagnetic spectrum. These results show that P donor in Si provides a material platform that enables intersubband devices operating in the THz frequency domain. • A fully analytical expression of the matrix elements of the donor Hamiltonian is derived. • The donor electron energy levels are in excellent agreement with those reported in the previous literature. • The intersubband absorption spectra of P donor electron are strongly dependent on the orientations of the incident light polarization. • P donor in Si provides a material platform that enables optoelectronic devices operating in the THz frequency domain. • The 3D nonorthogonal Laguerre basis is more efficient and reliable in calculating impurity states of anisotropic 3D materials.

Demonstration and operation of quantum harmonic oscillators in an AlGaAs-GaAs heterostructure

The quantum harmonic oscillator (QHO), one of the most important and ubiquitous model systems in quantum mechanics, features equally spaced energy levels or eigenstates. Here we present a new class of nearly ideal QHOs formed by hydrogenic substitutional dopants in an AlGaAs/GaAs heterostructure. On the basis of model calculations, we demonstrate that, when a δ-doping Si donor substitutes the Ga/Al lattice site close to AlGaAs/GaAs heterointerface, a hydrogenic Si QHO, characterized by a restoring Coulomb force producing square law harmonic potential, is formed. This gives rise to QHO states with energy spacing of ∼8–9 meV. We experimentally confirm this proposal by utilizing gate tuning and measuring QHO states using an aluminum single-electron transistor (SET). A sharp and fast oscillation with period of ∼7–8 mV appears in addition to the regular Coulomb blockade (CB) oscillation with much larger period, for positive gate biases above 0.5 V. The observation of fast oscillation and its behavior is quantitatively consistent with our theoretical result, manifesting the harmonic motion of electrons from the QHO. Our results might establish a general principle to design, construct and manipulate QHOs in semiconductor heterostructures, opening future possibilities for their quantum applications.

Breakdown voltage enhancement of p-GaN/AlGaN/GaN diode by controlling Mg acceptors for compensating residual Si donors

Breakdown voltage enhancement was studied for p-GaN/AlGaN/GaN heterostructure diodes, where residual Si donors during growth were compensated with Mg acceptors doped in the p-GaN layer. As the thicknesses of the p-GaN layer (T p-GaN) decreased from 140 nm, breakdown voltages were increased and maximized at 20 nm, then decreased at 0 nm. Moreover, breakdown voltages of the 20 nm T p-GaN diodes improved with the increase of the drift region lengths. This is because a uniform electric field was obtained by compensation of the residual Si donors with the Mg acceptors. In addition, for a thicker p-GaN layer, the effects of the surface states were suppressed, and injection of a large number of holes under forward bias was observed. Consequently, the thick p-GaN layer is expected to enhance forward current. Adopting the results, the potential of low on-resistance and high current for p-GaN gated anode diodes was demonstrated.

Activation of implanted Si, Ge, and Sn donors in high-resistivity halide vapor phase epitaxial β-Ga2O3:N with high mobility

Activation of implanted donors into a highly-resistive, nitrogen-doped homoepitaxial β-Ga2O3 has been investigated. Nitrogen acceptors with the concentration of ∼1017 cm−3 were incorporated during epitaxial growth yielding low-doped (net donor concentration <1014 cm−3) films subsequently implanted with Si, Ge, and Sn. Upon Ohmic contact formation to the implanted regions, sheet resistance values of 314, 926, and 1676 Ω/sq were measured at room temperature for the Si-, Ge-, and Sn-implanted samples, respectively. Room temperature Hall measurements resulted in sheet carrier concentrations and Hall mobilities of 2.13 × 1014 /93, 8.58 × 1013/78, and 5.87 × 1013/63 cm2/(V s), respectively, for these three donor species. Secondary ion mass spectroscopy showed a volumetric dopant concentration of approximately 2 × 1019 cm−3 for the three species, resulting in carrier activation efficiencies of 64.7%, 40.3%, and 28.2% for Si, Ge, and Sn, respectively. Temperature-dependent Hall effect measurements ranging from 15 to 300 K showed a nearly constant carrier concentration in the Si-implanted sample, suggesting the formation of an impurity band indicative of degenerate doping. With a bulk carrier concentration of 1.3 × 1019 cm−3 for the Si implanted sample, a room temperature mobility of 93 cm2/(V s) is among the highest reported in Ga2O3 with a similar carrier concentration. The unimplanted Ga2O3:N regions remained highly resistive after the surrounding areas received implant and activation anneal. These results open the pathway for fabricating Ga2O3 devices through the selective n-type doping in highly resistive epitaxial Ga2O3.

Controlled surface topography of nanofilm by local strain modulation in mechanical transfer process

The mechanical transfer of nanofilms has been successfully established based on interfacial delamination behaviors. However, controlling the formation of surface cracks and wrinkles in nanofilms during mechanical transfer processes remains a challenge. Here, we demonstrate the controlled surface topography of nanofilms using local strain in the mechanical transfer process. A single cantilever beam (SCB) fracture mechanics test is performed using a specimen composed of an elastomer receiver–Au nanofilm–Si donor structure to investigate the variation in the surface topography of a transferred nanofilm with respect to the receiver thickness, loading angle, and loading rate. The results indicate that owing to the reduction of the tensile strain at the crack tip, the crack density in the nanofilm decreases when a thick elastomer, high loading angle, and low loading rate are applied during the transfer process. Furthermore, beyond the reduction in the tensile strain, the use of a rigid receiver generates a compressive strain at the crack tip, resulting in wrinkles in the nanofilm without cracks. The mechanisms of surface cracking and wrinkling are elucidated by analyzing the strain distribution at the crack tip via numerical simulation. We believe that this study can potentially contribute to advancements in mechanical transfer technology.

Effects of temperature and oxygen partial pressure on electrical conductivity of Fe-doped β-Ga2O3 single crystals

In this work, we measure DC and AC conductivity and Hall voltage to determine the origin of electrical insulating properties of Fe-doped β-Ga2O3 single crystals, which are measured perpendicular to the 2¯01 crystallographic plane. We find that electrical conduction is predominantly controlled by free electrons in the temperature range 230–800 °C with the mutual compensation of the impurity donor (Si) and acceptor dopant (Fe), explaining the low concentration of free electrons and Fermi level pinning over a wide range of temperatures. Furthermore, the negative temperature-dependence of the carrier mobility indicates that it is limited by optical phonon scattering. Importantly, we find electrical conductivity to be largely independent of oxygen partial pressure (pO2) from air to 10−4 atm at 600 °C, but it becomes slightly dependent on pO2 at 800 °C, as intrinsic non-stoichiometric point defects begin to influence the charge balance.

High-Temperature Performance of AlN MESFETs With Epitaxially Grown n-Type AlN Channel Layers

We fabricated AlN metal semiconductor field effect transistors (MESFETs) with an epitaxially grown n-type AlN channel layer and characterized the temperature dependence of their device properties. As the temperature was varied from room temperature to 500°C, maximum drain current increased from 0.42 to 45 mA/mm and maximum transconductance increased from 0.05 to 4.5 mS/mm. This enhanced device performance with increasing temperature is attributed to increased carrier concentration in n-type AlN due to the large ionization energy of Si donors. Owing to a large Schottky barrier height of 2.08 eV for the Ni gate electrode/n-type AlN interface, reverse leakage current is less than <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{-{10}}$ </tex-math></inline-formula> A/mm at temperatures ranging from room temperature to 500°C. Off-state breakdown voltage is as high as 1720 V for gate-drain spacing of 16 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\mu }\text{m}$ </tex-math></inline-formula> . These results indicate that AlN MESFETs are promising for high-voltage electronic devices operating at high temperature.

Shallow donor and DX state in Si doped AlN nanowires grown by molecular beam epitaxy

Si doping of AlN nanowires (NWs) grown by plasma assisted molecular beam epitaxy was investigated with the objective of fabricating efficient AlN based deep ultra-violet light-emitting-diodes. The Si concentration ranged from 1016 to 1.8 × 1021 cm−3. Current–voltage measurements performed on nanowire ensembles revealed an Ohmic regime at low bias (below 0.1 V) and a space charge limited regime for higher bias. From temperature dependent current–voltage measurements, the presence of Si donors is evidenced in both shallow and deep DX states with an ionization energy of 75 and 270 meV, respectively. The role of Fermi level pinning on NWs sidewalls is discussed in terms of near surface depletion, inducing a favorable formation of shallow Si donors.

Incorporation of Si and Sn donors in β-Ga2O3 through surface reconstructions

Tin and silicon incorporate on gallium sites in Ga2O3 and act as shallow donors. The monoclinic structure of Ga2O3 has two inequivalent Ga sites; density functional theory calculations for bulk show that Sn prefers the octahedral site, while Si prefers the tetrahedral site. Experiments have indicated that Si and Sn can also incorporate on the thermodynamically less preferred site. We use density functional theory to study the adsorption of Si and Sn and also the co-adsorption of these impurities with Ga and O adatoms on the Ga2O3(010) surface. We identify a number of surface reconstructions in which Si adatoms prefer octahedral sites and Sn adatoms prefer tetrahedral sites. By applying the electron counting rule, we also study the mechanisms of the preferred adsorption sites for Si and Sn. We conclude that Si and Sn can also occupy the thermodynamically unfavored site due to surface reconstructions during the growth, which potentially leads to Si and Sn occupying both octahedral and tetrahedral sites in Ga2O3.

Luminescence Properties of InGaN/GaN Green Light-Emitting Diodes with Si-Doped Graded Short-Period Superlattice.

The optical properties of InGaN/GaN green light-emitting diodes (LEDs) with an undoped graded short-period superlattice (GSL) and a Si-doped GSL (SiGSL) were investigated using photoluminescence (PL) and time-resolved PL spectroscopies. For comparison, an InGaN/GaN conventional LED (CLED) without the GSL structure was also grown. The SiGSL sample showed the strongest PL intensity and the largest PL peak energy because of band-filling effect and weakened quantum- confined stark effect (QCSE). PL decay time of SiGSL sample at 10 K was shorter than those of the CLED and GSL samples. This finding was attributed to the oscillator strength enhancement by the reduced QCSE due to the Coulomb screening by Si donors. In addition, the SiGSL sample exhibited the longest decay time at 300 K, which was ascribed to the reduced defect and dislocation density. These results indicate that insertion of the Si-doped GSL structure is an effective strategy for improving the optical properties in InGaN/GaN green LEDs.

Electrical properties of heavily Si-doped GaAsN after annealing

In this study, electron traps in dilute GaAsN are investigated using the temperature dependence of electron concentration (n) and mobility (μe) for annealed heavily Si-doped GaAsN. The temperature dependence of n and μe depends on the annealing temperature, indicating that electrons are excited to the conduction band only from deep electron traps for heavily Si-doped GaAsN annealed at 580 °C. However, they are excited to the conduction band from both the deep electron traps and the shallow Si donor level for heavily Si-doped GaAsN annealed at 550 °C. The depth of the deep electron traps from the bottom of the conduction band for heavily Si-doped GaAsN annealed at 550 °C is almost equal to heavily Si-doped GaAsN annealed at 580 °C. The results demonstrate that these deep electron traps are inherent in dilute GaAsN because similar deep electron traps are also observed for the as-grown Si-doped GaAsN.

Atomic-scale study of Si-doped AlAs by cross-sectional scanning tunneling microscopy and density functional theory

Silicon (Si) donors in GaAs have been the topic of extensive studies since Si is the most common and well understood n-type dopant in III-V semiconductor devices and substrates. The indirect bandgap of AlAs compared to the direct one of GaAs leads to interesting effects when introducing Si dopants. Here we present a study of cross-sectional scanning tunneling microscopy (X-STM) and density functional theory (DFT) calculations to study Si donors in AlAs at the atomic scale. Based on their crystal symmetry and contrast strengths, we identify Si donors up to four layers below the (110) surface of AlAs. Interestingly, their short-range local density of states (LDOS) is very similar to Si atoms in the (110) surface of GaAs. Additionally we show high-resolution images of Si donors in all these layers. For empty state imaging, the experimental and simulated STM images based on DFT show excellent agreement for Si donor up to two layers below the surface.