Si Doping Level Research Articles

Metal‐Organic Chemical Vapor Deposition Regrowth of Highly Doped n+ (In)GaN Source/Drain Layers for Radio Frequency Transistors

Herein, epitaxially regrown n+ (In)GaN source/drain layers for radio frequency high electron mobility transistors, addressing material and electrical characterization, are reported. A range of n+ GaN and n+ InGaN layers with indium 4–12 at% and silicon 0–5.1 × 1020 cm−3 are evaluated. The active carrier concentration in n+ InGaN exceeds 2 × 1020 cm−3. The layers exhibit so‐called V‐defects, observed by atomic force microscope and transmission electron microscope (TEM), which are associated with local composition changes. In addition to the high Si doping levels, nitrogen vacancies are also considered to contribute toward their net carrier concentration. Due to its relevance for device processing, selectivity analysis is performed, and the optimal process conditions for selective regrowth are identified. Regrowth under high temperature (800 °C) is found to be conducive to improved selectivity. However, a high thermal budget negatively impacts the overall regrowth process, as reported here for In0.17Al0.83N and Al0.26Ga0.74N barriers: whereas the InAlN barrier suffers from intermixing, the AlGaN barrier demonstrates high‐temperature stability. The impact of intermixing is studied from complementary TEM and DC electrical measurements. A low overall contact resistance of 75 Ω μm is obtained with the regrown n+ InGaN layers.

Strategically constructed AlGaN doping barriers for efficient deep ultraviolet light-emitting diodes.

Here, we propose a sandwich-like Si-doping scheme (undoped/Si-doped/undoped) in Al0.6Ga0.4N quantum barriers (QBs) to simultaneously promote the optoelectronic performances and reliability of deep ultraviolet light-emitting diodes (DUV-LEDs). Through experimental and numerical analyses, in the case of DUV-LEDs with conventional uniform Si-doping QB structure, severe operation-induced reliability degradation, including the increase of reverse leakage current (IR) and reduction of light output power (LOP), will offset the enhancement of optoelectronic performances as the Si-doping levels increase to an extent, which hinders further development of DUV-LEDs. According to a transmission electron microscope characterization and a numerical simulation, an improved interfacial quality in multiple quantum wells (MQWs) and more uniform carrier distribution within MQWs are demonstrated for our proposed Si-doping structure in comparison to the uniform Si-doping structure. Consequently, the proposed DUV-LED shows superior wall-plug efficiency (4%), IR at -6 V reduced by almost one order of magnitude, and slower LOP degradation after 168-h 100 mA-current-stress operation. This feasible doping scheme provides a promising strategy for the high-efficiency and cost-competitive DUV-LEDs.

High crystalline quality homoepitaxial Si-doped β-Ga2O3(010) layers with reduced structural anisotropy grown by hot-wall MOCVD

A new growth approach, based on the hot-wall metalorganic chemical vapor deposition concept, is developed for high-quality homoepitaxial growth of Si-doped single-crystalline β-Ga2O3 layers on (010)-oriented native substrates. Substrate annealing in argon atmosphere for 1 min at temperatures below 600 °C is proposed for the formation of epi-ready surfaces as a cost-effective alternative to the traditionally employed annealing process in oxygen-containing atmosphere with a time duration of 1 h at about 1000 °C. It is shown that the on-axis rocking curve widths exhibit anisotropic dependence on the azimuth angle with minima for in-plane direction parallel to the [001] and maximum for the [100] for both substrate and layer. The homoepitaxial layers are demonstrated to have excellent structural properties with a β-Ga2O3(020) rocking curve full-widths at half-maximum as low as 11 arc sec, which is lower than the corresponding one for the substrates (19 arc sec), even for highly Si-doped (low 1019 cm−3 range) layers. Furthermore, the structural anisotropy in the layer is substantially reduced with respect to the substrate. Very smooth surface morphology of the epilayers with a root mean square roughness value of 0.6 nm over a 5 × 5 μm2 area is achieved along with a high electron mobility of 69 cm2 V−1 s−1 at a free carrier concentration n=1.9×1019 cm−3. These values compare well with state-of-the-art parameters reported in the literature for β-Ga2O3(010) homoepitaxial layers with respective Si doping levels. Thermal conductivity of 17.4 Wm−1K−1 is determined along the [010] direction for the homoepitaxial layers at 300 K, which approaches the respective value of bulk crystal (20.6 Wm−1K−1). This result is explained by a weak boundary effect and a low dislocation density in the homoepitaxial layers.

The influence of Si on the properties of MOVPE grown GaN thin films: Optical and EPR study

Metal Organic Vapour Phase epitaxy (MOVPE) grown GaN layers doped with Si were studied by means of radio-, photo- and thermally stimulated luminescence, scintillation decay kinetics, as well as electron paramagnetic resonance. Two luminescence bands were observed: the narrow and fast peaking at about 3.45 eV (ascribed to excitons) and the broad and slow one having maximum at about 2.2 eV (produced by the defect – a carbon occupying nitrogen site). The decay time and the intensity of exciton and defect-related emission exhibit dependence on the doping level of Si. Both bands are getting faster upon the increased Si content. Moreover, the presence of Si influences drastically the charge trapping processes as well – the changes in the intensity of the glow peaks within the 100–300 K temperature range were observed. Besides, the new glow peaks appear and are getting stronger in the 300–450 K region upon Si doping. Interestingly, the effect of Si on the conductivity properties of the GaN samples was observed by means of electron paramagnetic resonance.

DX center formation in highly Si doped AlN nanowires revealed by trap assisted space-charge limited current

Electrical properties of silicon doped AlN nanowires grown by plasma assisted molecular beam epitaxy were investigated by means of temperature dependent current–voltage measurements. Following an Ohmic regime for bias lower than 0.1 V, a transition to a space-charge limited regime occurred for higher bias. This transition appears to change with the doping level and is studied within the framework of the simplified theory of space-charge limited current assisted by traps. For the least doped samples, a single, doping independent trapping behavior is observed. For the most doped samples, an electron trap with an energy level around 150 meV below the conduction band is identified. The density of these traps increases with a Si doping level, consistent with a self-compensation mechanism reported in the literature. The results are in accordance with the presence of Si atoms that have three different configurations: one shallow state and two DX centers.

Nanoscale imaging of dopant incorporation in n-type and p-type GaN nanowires by scanning spreading resistance microscopy

The realization of practical semiconductor nanowire optoelectronic devices requires controlling their electrical transport properties, which are affected by their large surface/volume ratio value and potentially inhomogeneous electrical dopant distribution. In this article, the local carrier density in Si-doped and Mg-doped GaN nanowires grown catalyst-free by molecular beam epitaxy was quantitatively measured using scanning spreading resistance microscopy. A conductive shell surrounding a more resistive core was observed in Mg-doped, p-type GaN nanowires, balancing the formation of a depleted layer associated with sidewall surface states. The formation of this conductive layer is assigned to the peripheral accumulation of Mg dopants up to values in the 1020 /cm3 range, as determined by quantitative energy dispersive x ray spectroscopy measurements. By contrast, Si-doped n-type GaN nanowires exhibit a resistive shell, consistent with the formation of a depleted layer, and a conductive core exhibiting a decreasing resistivity for increasing Si doping level.

High conductivity n-Al0.6Ga0.4N by ammonia-assisted molecular beam epitaxy for buried tunnel junctions in UV emitters

Highly doped n-Al0.6Ga0.4N can be used to form tunnel junctions (TJs) on deep ultraviolet (UVC) LEDs and markedly increase the light extraction efficiency (LEE) compared to the use of p-GaN/p-AlGaN. High quality Al0.6Ga0.4N was grown by NH3-assisted molecular beam epitaxy (NH3 MBE) on top of AlN on SiC substrate. The films were crack free under scanning electron microscope (SEM) for the thickness investigated (up to 1 µm). X-ray diffraction reciprocal space map scan was used to determine the Al composition and the result is in close agreement with atom probe tomography (APT) measurements. By varying the growth parameters including growth rate, and Si cell temperature, n-Al0.6Ga0.4N with an electron density of 4×1019 /cm3 and a resistivity of 3 mΩ·cm was achieved. SIMS measurement shows that a high Si doping level up to 2×1020 /cm3 can be realized using a Si cell temperature of 1450 °C and a growth rate of 210 nm/hr. Using a vanadium-based annealed contact, ohmic contact with a specific resistance of 10−6 Ω·cm2 was achieved as determined by circular transmission line measurement (CTLM). Finally, the n-type AlGaN regrowth was done on MOCVD grown UVC LEDs to form UVC TJ LED. The sample was processed into thin film flip chip (TFFC) configuration. The emission wavelength is around 278 nm and the excess voltage of processed UV LED is around 4.1 V.

Homoepitaxial GaN micropillar array by plasma-free photo-enhanced metal-assisted chemical etching

Metal-assisted chemical etching is a plasma-free open-circuit anisotropic etching method that produces high aspect ratio structures in various semiconductors. Here, for the first time, we demonstrate the formation of ordered micropillar arrays of homoepitaxial GaN, using photo-enhanced MacEtch with patterned platinum films as the catalyst. The GaN etching rate and morphology as a function of etching chemistry, growth method, and doping conditions are investigated, and the etch mechanism is analyzed. Etch rates and surface smoothness are found to increase with the Si-doping level in GaN, approaching those achieved by reactive ion etching and photoelectrochemical etching. Spatially resolved photoluminescence shows no degradation in near band edge emission and no newly generated defect peaks, as expected due to the high energy ion free nature. This approach can also potentially be applied to InGaN and AlGaN by tuning the etch chemistry and illumination wavelength, enabling a facile and scalable processing of 3D III-nitride based electronic and optoelectronic devices such as μLEDs and finFETs.

MOCVD growth and characterization of conductive homoepitaxial Si-doped Ga2O3

Epitaxial Ga2O3 films were grown by Metal Organic Chemical Vapor Deposition (MOCVD) on native substrates at growth rate of 1 µm/hour. The electron conductivity was introduced in the films through Si doping during deposition and the growth and doping parameters were optimized to control the electrical transport properties. The films were characterized in terms of structure, surface morphology, electrical transport properties, dopant concentrations and trapping defects. It was found that electron densities are not solely dependent on dopant concentrations and the use of metal organic precursor seems to induce additional donors of carbon. The work shows that the electron density and conductivity of MOCVD Ga2O3 films are mainly governed by the interplay between dopant concentration, C concentration and the presence of trapping defects in the films, which is most likely applicable for other oxide films grown by MOCVD. Conductive films of Ga2O3 with resistivity in the order of 0.07 Ω.cm were successfully grown. The electron density in most of these films was in the range of 1019 cm−3 but the mobility was limited to 1.5 cm2/V⋅s. Higher mobility of 30 cm2/V⋅s was obtained in some films at the expense of carrier concentration by reducing Si doping level resulting in resistivity in the order of 0.3 Ω.cm. This range of conductivity and mobility is relevant for field-effect transistors (FET) and the applications of Ga2O3 as transparent FET in Deep Ultra-Violet (DUV) technology.

Metalorganic chemical vapor deposition-grown tunnel junctions for low forward voltage InGaN light-emitting diodes: epitaxy optimization and light extraction simulation

In this work, we demonstrate the detailed optimization of metalorganic chemical vapor deposition (MOCVD)-grown tunnel junctions (TJs) utilizing selective area growth (SAG) for regular size (0.1 mm2) and micro-size InGaN light-emitting diodes (LEDs and µLEDs). Finite-difference time-domain simulations show that the SAG apertures result in a more directional light emission of far-field radiation pattern for the SAG TJ LEDs grown on patterned sapphire substrate. Moreover, it is noted that the n-InGaN insertion layer and Si-doped concentration in the n+GaN TJs layer is essential to realize a low forward voltage (V f) in TJs LEDs. For both 0.1 mm2 LEDs and µLEDs, the V f is independent on the SAG aperture space varied from 3 to 8 µm when the Si-doping level in the n+GaN layer is as high as 1.7 × 1020 cm−3. The optimized TJ LEDs exhibit a comparable differential resistance of 1.0 × 10−2 Ω cm2 at 100 A cm−2 and a very small voltage penalty of 0.2–0.3 V compared to the conventional indium tin oxide contact LEDs. The low V f penalty is caused by a higher turn on voltage, which is the smallest one among the MOCVD-grown TJs LEDs and comparable to the best molecular beam epitaxy-grown TJs LEDs.

Low-Load Metal-Assisted Catalytic Etching Produces Scalable Porosity in Si Powders.

The recently discovered low-load metal-assisted catalytic etching (LL-MACE) creates nanostructured Si with controllable and variable characteristics that distinguish this technique from the conventional high-load variant. LL-MACE employs 150 times less metal catalyst and produces porous Si instead of Si nanowires. In this work, we demonstrate that some of the features of LL-MACE cannot be explained by the present understanding of MACE. With mechanistic insight derived from extensive experimentation, it is demonstrated that (1) the method allows the use of not only Ag, Pd, Pt, and Au as metal catalysts but also Cu and (2) judicious combinations of process parameters such as the type of metal, Si doping levels, and etching temperatures facilitate control over yield (0.065-88%), pore size (3-100 nm), specific surface area (20-310 m2·g-1), and specific pore volume (0.05-1.05 cm3·g-1). The porous structure of the product depends on the space-charge layer, which is controlled by the Si doping and the chemical identity of the deposited metal. The porous structure was also dependent on the dynamic structure of the deposited metal. A distinctive comet-like structure of metal nanoparticles was observed after etching with Cu, Ag, Pd, and, in some cases, Pt; this structure consisted of 10-50 nm main particles surrounded by smaller (<5 nm) nanoparticles. With good scalability and precise control of structural properties, LL-MACE facilitates Si applications in photovoltaics, energy storage, biomedicine, and water purification.

Electrical compensation and cation vacancies in Al rich Si-doped AlGaN

We report positron annihilation results on vacancy defects in Si-doped Al0.90Ga0.10N alloys grown by metalorganic vapor phase epitaxy. By combining room temperature and temperature-dependent Doppler broadening measurements, we identify negatively charged in-grown cation vacancies in the concentration range from below 1×1016 cm−3 to 2×1018 cm−3 in samples with a high C content, strongly correlated with the Si doping level in the samples ranging from 1×1017 cm−3 to 7×1018 cm−3. On the other hand, we find predominantly neutral cation vacancies with concentrations above 5×1018 cm−3 in samples with a low C content. The cation vacancies are important as compensating centers only in material with a high C content at high Si doping levels.

Temperature-dependent electrical and optical studies on nonpolar a-plane GaN thin films with various Si-doping levels

The electrical and optical properties of nonpolar a-plane GaN films with various silane flow rates were studied intensively utilizing temperature-dependent Hall-effect and photoluminescence measurements. The electron Hall mobility was found to change from increase to decrease while the silane flow beyond 16.06 nmol/min. It was inferred that the primary scattering mechanism of Si-doped GaN transferred from ionized impurity to thermal lattice vibration with the temperature over 450 K as more disordered lattice vibration was caused by Si doping. Meanwhile, the band-gap of nonpolar a-plane GaN film demonstrated a 32.56 meV narrowing owing to the Si-doping induced band-gap narrowing effect. Moreover, the basal stacking faults (BSFs) and Ga vacancy-related emission were evidently suppressed, and a decrease of 12.02% for BSFs density was achieved with Si-doping.

Strain relaxation, extended defects and doping effects in InxGa1-xN/GaN heterostructures investigated by surface photovoltage

We have analysed electrical properties of extended defects and interfaces in fully strained and partially relaxed InxGa1-xN/GaN heterostructures by means of Kelvin probe force microscopy and surface photovoltage spectroscopy. The study highlights the role of indium incorporation and Si doping levels on the charge state of extended defects including threading dislocations, V defects and misfit dislocations. Surface potential maps reveal that these defects are associated with a different local work function and thus could remarkably alter electron-hole recombination mechanisms of InxGa1-xN/GaN layers locally. Surface photovoltage spectra clearly demonstrate the role of misfit dislocations and high Si-doping level on interface recombination process. The interplay between Si doping level and In% on the electronic properties of the extended defects has been also clarified.

Direct Integration of Carbon Nanotubes in CMOS, Towards an Industrially Feasible Process: A Review

This article reviews our work on direct integration of carbon nanotubes (CNTs) in MEMS vehicles and shows initial results in realizing direct CNT integration in CMOS. Extraordinary properties of CNTs are fully exploited if they are closely integrated with signal processing electronics. An example device is a CNT-based gas sensor, where CNTs with their huge surface-to-volume ratio act as a very sensitive sensing element, and the CMOS circuits provide the necessary signal processing for a device to give a standardized, calibrated, low-noise output signal. Direct synthesis of CNTs in CMOS will enable wafer-level, low-cost fabrication of CNT-based devices. A major challenge is the 800–1000 °C CNT synthesis temperature requirement, whereas CMOS-compatible temperature is <300 °C. We synthesized CNTs on purpose-designed MEMS microheaters by local resistive heating to avoid over-heating the bulk of the chip. An applied electric field guides the CNTs to grow towards a second electrode to close electric circuits by making a Si-CNT-Si system. The process is fully controlled through electrical parameters, indicating that the process can be automated and scaled up to wafer-level for a low-cost, high-volume industrial manufacturing process. Characterization of the synthesized CNTs reveals that the hottest region (∼900 °C) ensures low-diameter, less dense and high-quality CNTs. Also, our statistical analysis shows that the electric field guiding is diameter-dependent, ensuring that only well-ordered low-diameter CNTs become part of an electric circuit. Through electrical analysis of the CNT-Si contacts, we found that the Si-CNT-Si systems show either near-ohmic or non-linear (Schottky-like) behaviours depending on the Si doping level. We also demonstrated functionalization of the locally grown CNTs. For direct CNT synthesis on CMOS chips, a high thermal gradient around the CNT growth structure is required to maintain CMOS-compatible temperature. Several promising designs of CNT growth structures and their thermomechanical simulations are presented. Partially suspended microheaters provide thermal isolation to avoid heating the active CMOS region, while keeping reliable mechanical stability. In our attempt to direct CNT-CMOS integration, a purpose-designed CMOS chip is manufactured using a standard AMS 350 nm CMOS process. Layout designs and optical characterization of several fabricated CMOS microstructures are presented.

Accurate surface band bending determination on Ga-polar n-type GaN films by fitting x-ray valence band photoemission spectrum

The surface band bending in Ga-polar n-type GaN surfaces, as well as the effect of Si doping levels and in situ Ar+ ion processing on band bending, was systematically investigated. To precisely determine the valence band maximum (VBM) of GaN beyond instrumental and material surface environments by XPS, a valence band feature fitting procedure based on photoemission spectra and theoretical densities of states has been developed. Poisson calculation with quadratic depletion approximation on surface potential has been used to model the band bending and further correct the VBM energy. Then, the actual surface band bending was correctly evaluated. Upward band bending of 1.55 ± 0.03 eV with highly Si doped n-GaN, which is about 0.88 eV higher than that of the moderately doped sample, was found. After in situ Ar+ plasma treatment, the varying degree of band bending was observed distinctly depending on the Si doping density. The surface components associated with the Ga/N ratio and Ga–O bonding concentration on the n-GaN surface have been used to evaluate the contribution to surface band bending.

Influence of Si addition on structure and properties of TiB2-Si nanocomposite coatings deposited by high-power impulse magnetron sputtering

TiB2 coatings with various Si doping levels were fabricated using magnetron sputtering from TiB2 and Si targets. The chemical and phase composition, microstructure, mechanical properties, and high-temperature oxidation resistance of the coatings were investigated using different techniques, such as electron probe microanalysis (EPMA), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), nanoindentation, and thermogravimetric analysis. The results revealed that the microstructure of the TiB2 coating with and without Si doping changed from an apparent columnar structure to a densely stacked structure, and finally to a nearly featureless planar structure. A certain amount of added Si limits the growth of TiB2 grains and favors the formation of a nanocomposite structure with nanocrystalline TiB2 grains embedded in an amorphous Si phase matrix, as shown by XPS and TEM results. The hardness reached a maximum value of 35.2 GPa at 4.5 at% Si and then linearly decreased upon further increasing the Si content, owing to the excessively amorphous phase in coatings. However, it is worth noting that the high-temperature oxidation resistance of the coatings was improved by Si addition, due to their compact homogeneous structure that hinders the diffusion of oxygen. The results indicate that the hardness and oxidation resistance of the coatings can be simultaneously improved by the addition of an appropriate Si content.

Enhanced photoelectrochemical performance of InGaN-based nanowire photoanodes by optimizing the ionized dopant concentration

InGaN-based nanowires (NWs) have been extensively studied for photoelectrochemical (PEC) water splitting devices owing to their tunable bandgap and good chemical stability. Here, we further investigated the influence of Si doping on the PEC performance of InGaN-based NW photoanodes. The Si dopant concentration was controlled by tuning the Si effusion cell temperature (TSi) during plasma-assisted molecular beam epitaxy growth and further estimated by Mott-Schottky electrochemical measurements. The highest Si dopant concentration of 2.1 × 1018 cm−3 was achieved at TSi = 1120 °C, and the concentration decreased with further increases in TSi. The flat-band potential was calculated and used to estimate the conduction and valence band edge potentials of the Si-doped InGaN-based NWs. The band edge potentials were found to seamlessly straddle the redox potentials of water splitting. The linear scan voltammetry results were consistent with the estimated carrier concentration. The InGaN-based NWs doped with Si at TSi = 1120 °C exhibited almost 9 times higher current density than that of the undoped sample and a stoichiometric evolution of hydrogen and oxygen gases. Our systematic findings suggest that the PEC performance can be significantly improved by optimizing the Si doping level of InGaN-based NW photoanodes.

820-V GaN-on-Si Quasi-Vertical p-i-n Diodes With BFOM of 2.0 GW/cm2

In this letter, we demonstrate GaN-on-Si p-i-n diodes with high breakdown voltage and state-of-the-art Baliga’s figure of merit (BFOM) among GaN-on-Si vertical devices. The growth and doping of the GaN drift layer were optimized, leading to a remarkable electron mobility of 720 cm2/Vs for a Si doping level of $\text {2} \times \text {10}^{\text {16}}$ cm−3. With a 4 $\mu \text{m}$ -thick drift layer, we achieved an excellent breakdown voltage of 820 V and ultra-low specific on resistance ( ${R}_{\text {on,sp}}$ ) of 0.33 $\text{m}\Omega $ cm2. This results in a BFOM of 2.0 GW/cm2, the highest value reported for GaN-on-Si vertical diodes. These results reveal the excellent prospect of GaN-on-Si for cost-effective vertical power devices.

Effects of Si-doping on characteristics of semi-polar [formula omitted] plane Al0.45Ga0.55N epi-layers

The semi-polar (112̅2) plane Al0.45Ga0.55N epi-layers with various Si-doping levels were successfully deposited on (10-10) m-plane sapphire substrates by metal-organic chemical vapor deposition (MOCVD). And the effects of Si-doping on the characteristics of the epi-layer samples were studied extensively by high-resolution X-ray diffraction (HR-XRD), atomic force microscopy (AFM), Raman spectroscopy, and Hall effect measurements. The characterization results showed that the compressive strain in the as-deposited Al0.45Ga0.55N films could be completely relaxed by Si-doping with appropriate silane (SiH4) mole flow rate owing to the enhanced dislocation movements induced by Si dopants. This fact implies that Si-doping to the semi-polar (112̅2) plane Al0.45Ga0.55N epi-layer is especially beneficial to the interaction and annihilation of the dislocations, resulting in a significant improvement in crystal quality. Moreover, the minimum root mean square (RMS) detected by AFM was as small as 1.47nm, and the electron concentration and mobility was determined by the Hall effect measurement to be 1.8×1019cm−3 and 36.65cm2V−1s−1 for the same sample, respectively.