High Vacuum Chemical Vapor Deposition Research Articles

High-Vacuum Chemical Vapor Deposition of Monolayer Hexagonal Boron Nitride on Ge(001) from Borazine

Hexagonal boron nitride (h-BN) has played significant roles in enhancing the optoelectronic and electronic properties of a host of two-dimensional materials. These include reducing charge scattering in field-effect transistors, increasing exciton emission efficiency in valleytronics, controlling interlayer excitons, and serving as a tunneling barrier. While the synthesis of h-BN on metallic substrates has been studied extensively, the synthesis of h-BN on CMOS-compatible substrates like Ge has not. Here, we report the growth of h-BN on Ge(001) from borazine via high-vacuum chemical vapor deposition. We find that the sublimation of Ge under high vacuum inhibits h-BN growth. To overcome this challenge, we place two Ge substrates face-to-face with a thin gap in between and tune the gap thickness to control h-BN nucleation density and island size. In addition, we systematically investigate the effects of growth temperature and precursor partial pressure on h-BN growth. Figure 1

High-Vacuum Chemical Vapor Deposition of Monolayer Hexagonal Boron Nitride on Ge(001) from Borazine

Hexagonal boron nitride (h-BN) is a promising material for next-generation electronics due to its unique optoelectronic and electronic properties. While the synthesis of h-BN on metallic substrates has been studied extensively, h-BN synthesis on CMOS-compatible substrates like Ge has not. Here, we report the growth of h-BN on Ge(001) from borazine via high-vacuum chemical vapor deposition. We find that the sublimation of Ge under high vacuum inhibits h-BN growth. To overcome this challenge, we place two Ge substrates face-to-face and achieve the growth of aligned h-BN islands and monolayer h-BN films.

Highly Sensitive and Selective Formaldehyde Gas Sensors Based on Polyvinylpyrrolidone/Nitrogen-Doped Double-Walled Carbon Nanotubes

A highly sensitive and selective formaldehyde sensor was successfully fabricated using hybrid materials of nitrogen-doped double-walled carbon nanotubes (N-DWCNTs) and polyvinylpyrrolidone (PVP). Double-walled carbon nanotubes (DWCNTs) and N-DWCNTs were produced by high-vacuum chemical vapor deposition using ethanol and benzylamine, respectively. Purified DWCNTs and N-DWCNTs were dropped separately onto the sensing substrate. PVP was then dropped onto pre-dropped DWCNT and N-DWCNTs (hereafter referred to as PVP/DWCNTs and PVP/N-DWCNTs, respectively). As-fabricated sensors were used to find 1,2-dichloroethane, dichloromethane, formaldehyde and toluene vapors in parts per million (ppm) at room temperature for detection measurement. The sensor response of N-DWCNTs, PVP/DWCNTs and PVP/N-DWCNTs sensors show a high response to formaldehyde but a low response to 1,2-dichloroethane, dichloromethane and toluene. Remarkably, PVP/N-DWCNTs sensors respond sensitively and selectively towards formaldehyde vapor, which is 15 times higher than when using DWCNTs sensors. This improvement could be attributed to the synergistic effect of the polymer swelling and nitrogen-sites in the N-DWCNTs. The limit of detection (LOD) of PVP/N-DWCNTs was 15 ppm, which is 34-fold higher than when using DWCNTs with a LOD of 506 ppm. This study demonstrated the high sensitivity and selectivity for formaldehyde-sensing applications of high-performance PVP/N-DWCNTs hybrid materials.

Towards the Growth of Hexagonal Boron Nitride on Ge(001)/Si Substrates by Chemical Vapor Deposition.

The growth of hexagonal boron nitride (hBN) on epitaxial Ge(001)/Si substrates via high-vacuum chemical vapor deposition from borazine is investigated for the first time in a systematic manner. The influences of the process pressure and growth temperature in the range of 10−7–10−3 mbar and 900–980 °C, respectively, are evaluated with respect to morphology, growth rate, and crystalline quality of the hBN films. At 900 °C, nanocrystalline hBN films with a lateral crystallite size of ~2–3 nm are obtained and confirmed by high-resolution transmission electron microscopy images. X-ray photoelectron spectroscopy confirms an atomic N:B ratio of 1 ± 0.1. A three-dimensional growth mode is observed by atomic force microscopy. Increasing the process pressure in the reactor mainly affects the growth rate, with only slight effects on crystalline quality and none on the principle growth mode. Growth of hBN at 980 °C increases the average crystallite size and leads to the formation of 3–10 well-oriented, vertically stacked layers of hBN on the Ge surface. Exploratory ab initio density functional theory simulations indicate that hBN edges are saturated by hydrogen, and it is proposed that partial de-saturation by H radicals produced on hot parts of the set-up is responsible for the growth.

Ultra High Vacuum Chemical Vapor Deposition Techniques for Economic Growth of Silicon Nanowires

Ultra High Vacuum Chemical Vapor Deposition (UHVCVD) reactor has been used to grow silicon nanowires via innovative economical approaches using chemical active materials as catalysts such as aluminum. Scanning Electron Microscopy has been used to study the success of the growth for further investigations and advanced applications such as solar cells. Solar manufacturers are looking for approaches to improve yield and cell efficiency and lower manufacturing costs overall. One of the main goals of this project is to tear down the various parameters involved in the current photovoltaic panels and improve it in one or more directions (properties, performance, costs). The current study is addressing the solar market with special concern of the efficiency goal. The mechanical flexibility of plastic materials is of high demands for all photovoltaic applications onto curved surfaces for architectural integration. Polycarbonate AND/OR poly methyl methacrylate encapsulation of photovoltaic modules and usage to fabricate advanced silicon nanowires solar cells can be emerging technologies delivering excellent performance and durability at a competitive cost. Although solar panels with glass protective facing still account for the majority of the installations of photovoltaic modules, however, it is expected that the adaptation of these new technologies will rapidly gain market share. The growth of silicon nanowires using chemical vapor deposition to fabricate advanced solar cells can be done via two innovative approaches. Alternative techniques for lithographic formation of the mask can provide advantages for low-cost processing, especially where a simple repeating pattern is required.

Low Temperature Epitaxial Barium Titanate Thin Film Growth in High Vacuum CVD

Barium titanate is a promising perovskite material, exhibiting numerous materials properties advantageous for a range of applications, such as ferroelectricity, piezoelectricity, a high dielectric constant, and large electrooptic coefficients. It represents a potential candidate for the implementation of high bandwidth and low energy consuming modulators for optical communication in future integrated systems. Integration of the optical components with electronic circuits on silicon requires, however, suitable low temperature deposition processes for the growth of high quality epitaxial BaTiO3 films, which are still missing. For integration of novel materials into the complementary metal–oxide–semiconductor platform, a variety of prerequisites must be fulfilled; among them there is a strict limitation of the thermal budget of the growth process. Furthermore, high growth rates are desirable for enhanced throughput. Many chemical vapor deposition (CVD) processes allow growing thin films at high rates, but epitaxial BaTiO3 film formation typically requires high substrate temperatures. It is demonstrated that high vacuum CVD is capable of growing epitaxial barium titanate films on magnesium oxide, strontium titanate, and strontium titanate buffered silicon substrates at process temperatures of 400 °C and growth rates of up to 100 nm h−1 without the need of an additional annealing step; this is the lowest substrate temperature reported for a CVD processes.

Low Threshold Light Emission from Reverse-Rib n+ Ge Cavity Made by P Diffusion

Strain-tensile n+ Ge is an enabler of monolithically integrated light sources on a Si chip. The challenges are (1) high lasing thresholds and (2) n+ doping methods. (1) The threshold currents reported for electrically pumped Ge lasers are 280 and 510 kA/cm2 [1, 2]; much higher than III-V lasers (less than 1 kA/cm2) and even theoretical prediction of the Ge laser (~6 kA/cm2) [3]. (2) Ge can be used as photodetectors (PDs) and modulators (MODs), which need different in-depth dopant profiles. Ge epitaxy cannot control in-plane P concentration locally unless otherwise one performs multiple epitaxy. In the present study we tried to reduce the threshold using specific epi-structures and to diffuse Phosphorus (P) to make n+ Ge, and successfully observed a threshold in light emission using P diffusion for the first time. We have reported that a reverse-rib structure can be made using the epitaxial lateral overgrowth (ELO) technique [4], and can effectively isolate light modes from Si substrate. It further reduces light scattering at the Ge/SiO2 sidewall formed by dry etching as shown in Fig. 1 [5], some of which should decrease the optical net gain. In addition, our group reported that the Ge/Si interfaces if located in the pn junction enhance significantly non-radiative recombination [6]. Thus, n+ Ge was grown on n+ Si. P diffusion was employed to achieve 1 × 1019 P/cm3 both in the reverse-rib and the blanket Ge epilayers using a well-controlled diffusion method [7]. Optical pumping was used to excite the structure. 40-nm thick SiO2 was formed by thermal oxidation on an n+ Si wafer and patterned in line-and-space shape along [110] orientation by electron beam lithography followed by a wet etching. Epitaxial Ge layers were grown on the wafer using ultra high vacuum chemical vapor deposition (UHVCVD) at 700 ˚C. The epilayers were 1 µm thick at reverse-rib areas, while 1.8 µm thick at blanket areas. P diffusion was performed at 850 ˚C for 5 minutes, which achieves uniform P concentration of 1 × 1019 /cm3 through these Ge epilayers. Schematic of the measurement system is shown in Fig. 2. A continuous wave YAG:Nd laser (wavelength: 1.07 µm, power: ~kW/cm2) was used for optical pumping and a Ge PD was used to measure the relation of light emission and optical pumping. The effective input power density should be actually smaller than the set value since the reverse-rib Ge epilayer (300 µm wide and 3 mm long) is wider than the pumping laser (30 µm wide and 4 mm long), causing out-diffusion of photo-excited minority carriers (holes) to non-pumped areas. The measurements were carried out at room temperature. Figure 3 shows a cross sectional TEM image of the as-grown reverse-rib Ge. The air semi-cylinders are observed over the SiO2 masks (invisible in this image) since Ge will not grow on SiO2 in this condition. The air semi-cylinders are 150 nm high and 450 nm wide, which is big enough to isolate light modes from the Si substrate according to a mode solver simulation. Raman scattering spectroscopy reveals 0.17 % tensile strain at the top surface of the reverse-rib Ge while there should be strain relaxation around the air semi-cylinders. The results of the optical pumping measurements are summarized in Fig. 4. Light emission from the reverse-rib Ge shows a sharp threshold at the input power density of 8 kW/cm2 (set value). The results can be understood as either amplified spontaneous emission (ASE) or lasing. We will soon report the spectra to identify the results. Assuming lasing, the threshold of 8 kW/cm2 is very small compared to the value reported (30 kW/cm2) [8].

(Invited) Development of SiGeSn Technique Towards Mid-Infrared Devices in Silicon Photonics

GeSn/SiGeSn techniques are of great interest in the area of Si photonics due to: 1) the industry scalable growth; 2) compatible with current Si complementary metal-oxide semiconductor (CMOS) process; 3) capability of monolithic integration on Si; 4) the identified group-IV based direct bandgap materials; and 5) the tunable bandgap allowing the optoelectronic devices operation features broad wavelength coverage in near- and mid-infrared ranges. In the past a few years, the GeSn/SiGeSn material growth, the materials and devices characteristics have been comprehensively studied. Plenty of promising results are reported. In this work, the following aspects regarding GeSn/SiGeSn techniques are investigated: 1) low-defect material growth technique via Ultra High Vacuum Chemical Vapor Deposition (UHVCVD) system; 2) advanced material and optical characterizations of GeSn/SiGeSn thin films, Ge/GeSn/Ge double heterostructure (DHS), and GeSn/SiGeSn quantum well samples; 3) demonstration of GeSn-based optoelectronic devices including photodetectors and emitters.

Requirement on Aromatic Precursor for Graphene Formation

We studied graphene growth from solid, aromatic precursors at low temperature (∼400 °C) on a metal surface via high vacuum (6 × 10–6 mbar) chemical vapor deposition. A set of conjugated and condensed aromatic precursor molecules, i.e., structural isomers of terphenyl and anthracene are compared. While p-terphenyl and m-terphenyl were found to be excellent precursors for the formation of graphene, no graphene was obtained from o-terphenyl or anthracene. We propose a reaction mechanism that explains the differing growth products. The key requirement for the synthesis of graphene is a three-dimensional nature and suitable molecular structure of the precursor. Its incorporation into a flat aromatic system on a metal surface has to provide sufficient energy gain for the polymerization to occur.

Surface Kinetics of Titanium Isopropoxide in High Vacuum Chemical Vapor Deposition

Understanding the surface kinetics of precursor decomposition during thin film formation represents a key aspect in the understanding and engineering of chemical vapor deposition (CVD) and atomic layer deposition (ALD) processes. The determination of activation energies of the surface reaction steps, however, is often challenging because it requires precise knowledge of precursor impinging rates. Afterward, the kinetics can be investigated by comparing the amount of deposited material with the absolute precursor flow. Ideally, the experimental equipment allows a distinction between gas phase and surface reactions. Both are difficult to achieve in conventional CVD processes. A high vacuum environment, however, enables the quantitative prediction of precursor impinging rates due to the ballistic nature of precursor transport; additionally precursor gas phase reactions do not occur. We investigated the surface reaction kinetics of titanium isopropoxide (TTIP) in the high vacuum CVD of titanium dioxide. Addit...

Combinatorial Characterization of TiO2 Chemical Vapor Deposition Utilizing Titanium Isopropoxide.

The combinatorial characterization of the growth kinetics in chemical vapor deposition processes is challenging because precise information about the local precursor flow is usually difficult to access. In consequence, combinatorial chemical vapor deposition techniques are utilized more to study functional properties of thin films as a function of chemical composition, growth rate or crystallinity than to study the growth process itself. We present an experimental procedure which allows the combinatorial study of precursor surface kinetics during the film growth using high vacuum chemical vapor deposition. As consequence of the high vacuum environment, the precursor transport takes place in the molecular flow regime, which allows predicting and modifying precursor impinging rates on the substrate with comparatively little experimental effort. In this contribution, we study the surface kinetics of titanium dioxide formation using titanium tetraisopropoxide as precursor molecule over a large parameter range. We discuss precursor flux and temperature dependent morphology, crystallinity, growth rates, and precursor deposition efficiency. We conclude that the surface reaction of the adsorbed precursor molecules comprises a higher order reaction component with respect to precursor surface coverage.

Combinatorial HV-CVD survey of barium triisopropyl cyclopentadienyl and titanium tetraisopropoxide for the deposition of BaTiO3

Barium titanate is a very promising material for the integration of optical communication into electronic integrated circuits. For a successful integration into CMOS compatible technology, a growth process has to be found which allows forming crystalline BaTiO3 at sufficiently low temperatures. We describe the combinatorial analysis of BaTiO3 formation by high vacuum chemical vapor deposition using barium triisopropyl cyclopentadienyl and titanium tetraisopropoxide as metal sources; water, oxygen, or ozone are used as oxidizing agents. We analyzed the impact of these oxidizing agents on the interaction of the precursor molecules on the substrate surface. Furthermore, we characterized the films concerning chemical composition and identified growth conditions leading to deposit composition corresponding to the stoichiometric BaTiO3 formation. XRD analysis confirms the formation of crystalline BaTiO3 phases for deposition temperatures as low as 370 °C. We report about the combinatorial characterization of barium cyclopentadienyl and titanium tetraisopropoxide precursor interaction and fast determination of parameters for the formation of crystalline barium titanate using high vacuum chemical vapor deposition.

Selective growth of titanium dioxide by low-temperature chemical vapor deposition.

A key factor in engineering integrated optical devices such as electro-optic switches or waveguides is the patterning of thin films into specific geometries. In particular for functional oxides, etching processes are usually developed to a much lower extent than for silicon or silicon dioxide; therefore, selective area deposition techniques are of high interest for these materials. We report the selective area deposition of titanium dioxide using titanium isopropoxide and water in a high-vacuum chemical vapor deposition (HV-CVD) process at a substrate temperature of 225 °C. Here—contrary to conventional thermal CVD processes—only hydrolysis of the precursor on the surface drives the film growth as the thermal energy is not sufficient to thermally decompose the precursor. Local modification of the substrate surface energy by perfluoroalkylsilanization leads to a reduced surface residence time of the precursors and, consequently, to lower reaction rate and a prolonged incubation period before nucleation occurs, hence, enabling selective area growth. We discuss the dependence of the incubation time and the selectivity of the deposition process on the presence of the perfluoroalkylsilanization layer and on the precursor impinging rates—with selectivity, we refer to the difference of desired material deposition, before nucleation occurs in the undesired regions. The highest measured selectivity reached (99 ± 5) nm, a factor of 3 superior than previously reported in an atomic layer deposition process using the same chemistry. Furthermore, resolution of the obtained patterns will be discussed and illustrated.

Low Temperature Chemical Vapor Deposition Using Atomic Layer Deposition Chemistry

Chemical vapor deposition (CVD) techniques rely on high temperatures to activate the chemical decomposition of precursors on the substrate surface. Lower temperatures are applied in atomic layer deposition (ALD), where deliberate pyrolysis of the precursor is avoided to favor self-saturating surface reactions between two or more reactive partners. In ALD the substrate is exposed sequentially to two properly separated reactive precursors that interact on the surface, which is detrimental to the growth rate but offers exceptional conformality of the obtained films. We demonstrate the adaption of an ALD type reaction to a high vacuum CVD process; i.e., we deposited homogeneous titanium dioxide thin films on flat substrates by exposing simultaneously substrates with titanium isopropoxide and water at ALD typical temperatures (175–225 °C) in a high vacuum environment obtaining growth rates of up to 2 nm·min–1. We identified different growth regimes as a function of precursor impinging rates and substrate temperature—namely, a titanium tetraisopropoxide (TTIP) flux limited and a chemical reaction limited regime. Additionally, we examined the precursor deposition efficiency, chemical composition, and surface morphology of the deposit in the different growth regimes. Our technique allows the migration of ALD reaction chemistry into HV-CVD and reveals additionally—independently of reactor geometries—new insights for the understanding of ALD, in particular in terms of surface kinetics.

C–V AND AS STUDY OF SELF-ASSEMBLED GE ISLANDS IN SI P-N JUNCTION

The electrical properties of self-assembled Ge quantum dots (QDs) and Ge quantum wells (QWs) embedded in Si p-i-n diodes were studied using capacitance-voltage(C-V) measurements and admittance spectroscopy(AS). The investigated samples were grown on Si(001) substrates by an ultra high vacuum chemical vapour deposition (UHV-CVD) system. A linear increase of the thermal activation energy observed in voltage-dependent admittance spectroscopy(from 250meV up to 380meV) for the sample with quantum dots shows that the ensemble of Ge islands has a low, continuous averaged density of states. For the sample without Boron pretreatment we found two levels with activation energies about 132meV and 202meV which were about constant in some reverse bias region.

Evaluation of niobium dimethylamino-ethoxide for chemical vapour deposition of niobium oxide thin films

Chemical vapour deposition (CVD) processes depend on the availability of suitable precursors. Precursors that deliver a stable vapour pressure are favourable in classical CVD processes, as they ensure process reproducibility. In high vacuum CVD (HV-CVD) process vapour pressure stability of the precursor is of particular importance, since no carrier gas assisted transport can be used. The dimeric Nb2(OEt)10 does not fulfil this requirement since it partially dissociates upon heating. Dimethylamino functionalization of an ethoxy ligand of Nb(OEt)5 acts as an octahedral field completing entity and leads to Nb(OEt)4(dmae). We show that Nb(OEt)4(dmae) evaporates as monomeric molecule and ensures a stable vapour pressure and, consequently, stable flow. A set of HV-CVD experiments were conducted using this precursor by projecting a graded molecular beam of the precursor onto the substrate at deposition temperatures from 320°C to 650°C. Film growth rates ranging from 8nm·h−1 to values larger than 400nm·h−1 can be obtained in this system illustrating the high level of control available over the film growth process. Classical CVD limiting conditions along with the recently reported adsorption–reaction limited conditions are observed and the chemical composition, and microstructural and optical properties of the films are related to the corresponding growth regime. Nb(OEt)4(dmae) provides a large process window of deposition temperatures and precursor fluxes over which carbon-free and polycrystalline niobium oxide films with growth rates proportional to precursor flux are obtained. This feature makes Nb(OEt)4(dmae) an attractive precursor for combinatorial CVD of niobium containing complex oxide films that are finding an increasing interest in photonics and photoelectrochemical water splitting applications. The adsorption–reaction limited conditions provide extremely small growth rates comparable to an atomic layer deposition (ALD) process indicating that HV-CVD has the potential to be an alternative to ALD for growth of ultrathin films on low aspect ratio substrates.

On the bonding environment of phosphorus in purified doped single-walled carbon nanotubes

In this work, phosphorous-doped single-walled carbon nanotubes have been synthesized by the thermal decomposition of trimethylphosphine using a high-vacuum chemical vapor deposition method. Furthermore, a modified density-gradient-ultracentrifugation process has been applied to carefully purify our doped material. The combined use of Raman and X-ray photoelectron spectroscopy allowed us to provide the first insight into the bonding environment of P incorporated into the carbon lattice, avoiding competing signals arising from synthesis byproducts. This study represents the first step toward the identification of the bonding configuration of P atoms when direct substitution takes place.

The Use of Dopants for Defect Monitoring for Silicon-Germanium Ultra-High Vaccuum Chemical Vapor Deposition

Silicon-germanium epitaxy via ultra high vacuum chemical vapor deposition has the advantage of allowing low process temperatures. Deposition processes are sensitive to substrate surface preparation and the time delay between oxide removal and epitaxial growth. A new monitoring process utilizing doped substrates and defect decoration etching is demonstrated to have controllable and unique sensitivity to interfacial contaminants. Doped substrates were prepared and subjected to various loading conditions prior to the growth of typical Si/SiGe bilayers. Two different defect etches were performed, and the defect densities counted and confirmed as originating from the substrate-epitaxy interface. The defect densities were correlated to the concentration of interfacial oxygen. The result of this study is a suggestion for a defect monitoring technique that uses already commonplace equipment and chemicals, which has a rapid turnaround, with controllable sensitivity to substrate surface contaminants.

Limitations of patterning thin films by shadow mask high vacuum chemical vapor deposition

A key factor in engineering integrated devices such as electro-optic switches or waveguides is the patterning of high quality crystalline thin films into specific geometries. In this contribution high vacuum chemical vapor deposition (HV-CVD) was employed to grow titanium dioxide (TiO2) patterns onto silicon. The directed nature of precursor transport – which originates from the high vacuum environment during the process – allows shading certain regions on the substrate by shadow masks and thus depositing patterned thin films. While the use of such masks is an emerging field in stencil or shadow mask lithography, their use for structuring thin films within HV-CVD has not been reported so far. The advantage of the employed technique is the precise control of lateral spacing and of the distance between shading mask and substrate surface which is achieved by manufacturing them directly on the substrate. As precursor transport takes place in the molecular flow regime, the precursor impinging rates (and therefore the film growth rates) on the surface can be simulated as function of the reactor and shading mask geometry using a comparatively simple mathematical model.In the current contribution such a mathematical model, which predicts impinging rates on plain or shadow mask structured substrates, is presented. Its validity is confirmed by TiO2-deposition on plain silicon substrates (450°C) using titanium tetra isopropoxide as precursor. Limitations of the patterning process are investigated by the deposition of TiO2 on structured substrates and subsequent shadow mask lift-off. The geometry of the deposits is according to the mathematical model. Shading effects due to the growing film enables to fabricate deposits with predetermined variations in topography and non-flat top deposits which are complicated to obtain by classical clean room processes.As a result of the enhanced residual pressure of decomposition products and titanium precursors and the corresponding surface coverage of them, growth conditions differ in proximity of the shadowed areas compared to the case of plain substrates and the obtained film thickness differs significantly from predictions.

Experimental Demonstration of (111)-Oriented GaAs Metal–Oxide–Semiconductor Field-Effect-Transistors with Hetero-Epitaxial Ge Source/Drain

We demonstrate source/drain (S/D) design for GaAs n-type metal-oxide-semiconductor field-effect transistor (NMOSFET) by embedding Ge into recessed S/D region to eliminate the intrinsic issues of the low solid solubility of dopants and low density of states (DOS) in GaAs material. For achieving high quality S/D epitaxy, the effects of substrate orientation and surface preparation on the quality of the epitaxial Ge film were investigated. High quality Ge film was successfully grown on the GaAs (111)A substrate by using a ultra high vacuum chemical vapor deposition (UHVCVD) tool and the significant improvement in the surface root-mean-square (RMS) roughness was observed as compared to that on the (100) substrate. The fabricated GaAs NMOSFET with hetero-Ge S/D exhibits an Ion/Ioff ratio of ∼2.5 × 102. Even though the performance can be further improved, we think our proposed scheme sheds the light on overcoming the issues of the low solid solubility of n-dopant and low DOS in III-V MOSFETs.