RF Plasma Source Research Articles

Blue Laser Diodes by Low Temperature Plasma Assisted MBE

Recent progress in the growth of nitride based semiconductor structures made by plasma assisted MBE (PAMBE) is reported. The technology is ammonia free and the nitrogen for growth is activated by an RF plasma source from nitrogen molecules. A new approach for the growth of nitrides by PAMBE at temperature range 500 - 600°C is described. The key for this technique is to use a thin, dynamically stable metal (In or Ga) layer on the (0001) GaN surface, which enables a high quality 2D step-flow growth mode to be achieved at temperatures much lower than those determined by thermodynamic considerations. A new perspective for PAMBE in optoelectronics has been opened recently by a demonstration of continuous wave operation of InGaN blue–violet laser diodes. These laser diodes were fabricated on bulk GaN substrates with a low threading dislocation density.

POSS-Polyimide Nanocomposite Films: Simulated Hypervelocity Space Debris and Atomic Oxygen Effects

The combined effect of hypervelocity space debris impact and atomic oxygen (AO) attack on the degradation of reinforced polyhedral oligomeric silsesquioxanes (POSS)-polyimide films was studied. A laser-driven flyer (LDF) system was used to accelerate aluminum flyers to impact velocities of up to 3 km s -1. The impacted films were exposed to an RF-plasma source, which was used to simulate the effect of AO in the low Earth orbit. Scanning electron microscopy (SEM) was used to characterize the fracture morphology. The extent of damage in POSS-polyimide impacted films was found to be much smaller compared to POSS-free films, insinuating on a toughening mechanism developed due to POSS incorporation. When exposed to air RF-plasma, the impacted POSS-free film revealed a synergistic effect associated with a large increase in the erosion rate while impacted POSS-containing samples showed improved erosion resistance. The increased erosion rate of the impacted POSS-free film is explained by formation of residual stresses that affect the oxidation mainly by increasing the diffusivity of oxygen.

Cone-Shaped Hard Carbon Films Grown by Inductively Coupled RF Plasma with RF or DC Bias Voltage

A simple method capable of producing uniform, large-area cone arrays of carbon films was found in a planar inductively coupled RF plasma source. The technique employs a DC or RF bias to substrate holder. Si substrates were mechanically pretreated using diamond paste. Cone-shaped carbon crystals preferentially nucleate and grow on the scratches using relatively low bias. Variation of the depositing conditions enables control of the cone density, geometry, and height. The cone arrays are believed to can significantly improve the field emission properties and have a tempting perspective in the microelectromechanical system (MEMS).

Optimization of Well Width and N Content for Optical Properties of GaNAs/GaAs Multiple Quantum Well Grown by RF-Molecular Beam Epitaxy

We have investigated the optical properties in GaNAs/GaAs multiple quantum wells (MQWs) with various GaNAs layer widths and N contents, which were fabricated on GaAs(001) substrates by molecular beam epitaxy using an RF-plasma source. In particular, when the GaNAs layer thickness, LW, was 3 monolayers (ML), the photoluminescence (PL) peak intensity around 1.31 eV was approximately 70 times stronger than that in the case of LW=10 ML. Moreover, the full width at half maximum of the dominant PL peak became narrow as GaNAs layer thickness decreased, even though N content was increased.

Effects of Stress-relieving AlN Interlayers in GaN-on-Si Grown by Plasma-assisted Molecular Beam Epitaxy

ABSTRACTThe insertion of an AlN interlayer for tensile strain relief in GaN thin films grown on Si (111) on-axis and vicinal substrates by nitrogen rf plasma source molecular beam epitaxy has been investigated. The 15 nm AlN interlayer was inserted between the bottom 0.5 micron GaN layer and the top 1.0 micron GaN layer. The interlayer was very effective to reduce the tensile stress in the overall 1.5 micron GaN/Si film to the level required for complete avoidance of microcracks, which were present in high densities in GaN/Si heterostructures grown without an AlN interlayer. The strain of the AlN interlayer, as well as the strain in all the layers of the entire GaN/Si heterostructure was analyzed by x-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements. Reciprocal space map in XRD indicated that the 15 nm AlN interlayer was coherently strained with the GaN films. However TEM observations revealed that the AlN interlayer was partially relaxed in local regions. The AlN interlayer was also observed to interfere with the GaN growth process. In particular, above morphological features such as V-defects, GaN was overgrown with a large density of threading dislocations and inversion domain boundaries.

Effects of different plasma species (atomic N, metastable N2*, and ions) on the optical properties of dilute nitride materials grown by plasma-assisted molecular-beam epitaxy

This letter studies the effects of atomic N, metastable N2*, and ionic species on the optical properties of dilute nitride materials. Ga0.8In0.2N0.01As0.99 was grown using a 1% N2 in Ar gas mix from an Applied-Epi Unibulb™ rf plasma source. Isonitrogen samples with and without ions were studied using various plasma operating conditions. Optical emission spectrometry was used to characterize relative proportions of different active nitrogen plasma species (atomic N and metastable N2*). Samples grown without ions and with a higher proportion of atomic N resulted in the best overall material quality, although this improvement was observed at high annealing temperatures. At lower annealing temperatures, increased blueshifts were observed for samples grown with a higher proportion of atomic N; however, there was no noticeable influence of ions on blueshift regardless of whether atomic N or metastable N2* was the dominant species present in the plasma. The key implication of this work is that it helps to elucidate a possible reason for some of the contradictory reports in the literature. The ions are not solely responsible for the commonly reported “plasma damage.” Furthermore, we demonstrate herein that atomic N and metastable N2* each have different effects on the optical properties of dilute nitride materials grown by plasma-assisted molecular-beam epitaxy.

Ultra-Large Area RF Plasma Sources Employing Multiple Low-Inductance Internal-Antenna Modules for Flat Panel Display Processing

Plasma-generation and control technologies for meters-scale ultra-large-area RF plasma sources have been developed with multiple low-inductance antenna (LIA) modules, as a promising candidate of ultra-large area and high-density (1011-1012 cm-3) plasma sources for next-generation large-area processing. The present technologies are based on principle of multiple operation and integrated control of LIA modules, which enables effective control of power deposition profiles and hence the plasma uniformity over meters-scale large area for processing. This paper presents issues in designing ultra-large-area plasma sources to demonstrate feasibility of the uniform sources with 3m x 3m scales to meet requirements in the next-generation processes.

Honeycomblike large area LaB6 plasma source for Multi-Purpose Plasma facility

A Multi-Purpose Plasma (MP(2)) facility has been renovated from Hanbit mirror device [Kwon et al., Nucl. Fusion 43, 686 (2003)] by adopting the same philosophy of diversified plasma simulator (DiPS) [Chung et al., Contrib. Plasma Phys. 46, 354 (2006)] by installing two plasma sources: LaB(6) (dc) and helicon (rf) plasma sources; and making three distinct simulators: divertor plasma simulator, space propulsion simulator, and astrophysics simulator. During the first renovation stage, a honeycomblike large area LaB(6) (HLA-LaB(6)) cathode was developed for the divertor plasma simulator to improve the resistance against the thermal shock fragility for large and high density plasma generation. A HLA-LaB(6) cathode is composed of the one inner cathode with 4 in. diameter and the six outer cathodes with 2 in. diameter along with separate graphite heaters. The first plasma is generated with Ar gas and its properties are measured by the electric probes with various discharge currents and magnetic field configurations. Plasma density at the middle of central cell reaches up to 2.6 x 10(12) cm(-3), while the electron temperature remains around 3-3.5 eV at the low discharge current of less than 45 A, and the magnetic field intensity of 870 G. Unique features of electric property of heaters, plasma density profiles, is explained comparing with those of single LaB(6) cathode with 4 in. diameter in DiPS.

AlN nanoclusters formation by plasma ion immersion implantation

We report on simultaneous (Al + N) implantation of Al and N into layers of amorphous thermal silica (SiO 2) in an attempt to bond Al with N and form the binary compound AlN. As an implantation technique, plasma ion immersion implantation (PIII) is used. The energy and ion fluence were varied in order to obtain nanocluster containing films in the thickness range 10–50 nm. The elemental distribution profiles in the substrate were evaluated by means of elastic recoil detection analysis technique (ERDA). The nature of the chemical bonds was determined by X-ray photoelectron spectroscopy (XPS). The simultaneous implantation using rf plasma source for N ions and a cathodic arc to produce the Al ions provides good overlap of the elemental profiles and high retained doses of Al and N are achieved. The binding energies of the Al 2 p and N 1 s core electrons indicate that formation of near-stoichiometric aluminium nitride is achieved.

Ga 2 O 3 grown on GaAs by molecular beam epitaxy for metal oxide semiconductor field effect transistors

Molecular beam epitaxy has been used to deposit Ga2O3 onto GaAs(001) to form a III-V/oxide interface. Photoluminescence is used to monitor the interface quality and to compare these films with samples known to have a low interface state density and an unpinned Fermi level. An additional flux of molecular oxygen has been used during oxide growth, and the impact on growth rate is reported. A rf plasma source is shown to produce mainly neutral atomic oxygen. Atomic oxygen has a significant impact on the oxide growth mechanism and interface quality. The performance of metal oxide semiconductor field effect transistors fabricated from GaAs structures with their surface unpinned by Ga2O3 is discussed briefly.

III-Nitrides growth and AlGaN/GaN heterostructures on ferroelectric materials

The growth of III-nitrides on the ferroelectric materials lithium niobate (LN) and lithium tantalate (LT) via molecular beam epitaxy (MBE) using rf plasma source has been investigated. We have found that gallium nitride (GaN) epitaxial layers have a crystalline relationship with lithium niobate (tantalate) as follows: (0 0 0 1) GaN || (0 0 0 1) LN (LT) with [10−10] GaN || [11−20] LN (LT). The surface stability of LN and LT substrates has been monitored by in situ spectroscopic ellipsometry in the vacuum chamber. Three different temperature zones have been discerned; surface degas and loss of OH group (100–350 °C); surface segregation/accumulation of Li and O-species (400–700 °C); surface evaporation of O-species and Li desorption (over 750 °C). However, LT shows only surface degassing in the range of 100–800 °C. Therefore, congruent LN substrates were chemically unstable at the growth temperature of 550–650 °C, and therefore developed an additional phase of Li-deficient lithium niobate (LiNb 3O 8) along with lithium niobate (LiNbO 3), confirmed by X-ray diffraction. On the other hand, LT showed better chemical stability at these temperatures, with no additional phase development. The structural quality of GaN epitaxial layers has shown slight improvement on LT substrates over LN substrates, according to X-ray diffraction. Herein, we demonstrate AlGaN/GaN heterostructure devices on ferroelectric materials that will allow future development of multifunctional electrical and optical applications.

Effects of temperature, nitrogen ions, and antimony on wide depletion width GaInNAs

GaInNAs is a promising candidate material to increase the conversion efficiency of triple junction solar cells, but the dilute nitrides suffer from low-to-nonexistent minority-carrier diffusion lengths. The use of molecular beam epitaxy grown p-i-n structures with wide depletion widths can achieve high photocurrents in dilute nitrides, but this requires background doping below 2×1014cm−3 in the i layer. Here, the authors report on a number of factors that increase the net background acceptor concentration, hindering the effects to realize wide depletion widths, including high substrate temperatures, ions from the rf plasma source used to provide active nitrogen, and the addition of Sb. In addition, low substrate temperatures lead to an increase in n-type conductivity. Solar cell results that show the deleterious effects of Sb on GaInNAs devices are presented, including decreased open-circuit voltage and fill factor.

아르곤 이온빔을 이용한 CNT 페이스트 에미터의 표면처리에 관한 연구

In this study, a surface treatment method using accelerated Ar ions was experimented for exposing the carbon nanotubes (CNT) from the screen-printed CNT paste. After making a cathode electrode on the glass substrate, photo sensitive CNT paste was screen-printed, and then back-side was exposed by UV light. Then, the exposed CNT paste was selectively remained by development. After post-baking, the remained CNT paste was bombarded by accelerated Ar ions for removing some binders and exposing only CNTs. As results, the field emission characteristics were strongly depended on the accelerating energy, bombardment time, and the power of RF plasma ion source. When Ar ions accelerated with 100 eV energy from the 100 W RF plasma source are bombarded on the CNT paste surface for 10 min, the emission level and the uniformity were best.

Role of atomic nitrogen during GaN growth by plasma-assisted molecular beam epitaxy revealed by appearance mass spectrometry

To identify the species which contribute to GaN growth, the authors investigated the discharge parameter (0.3–4.8SCCM (SCCM denotes cubic centimeter per minute at STP), 150–400W) dependences of the atomic N flux by appearance mass spectrometry and of the incorporated nitrogen atoms into GaN layers grown by plasma-assisted molecular beam epitaxy (PAMBE) using the rf-plasma source. Ion fluxes were also evaluated by ion current measurements. A good correlation between the supplied atomic N flux and the incorporated nitrogen flux was obtained under a wide range of plasma conditions. It was clarified that the atomic N plays a dominant role in the growth of GaN by PAMBE.

Properties of Inductively-Coupled Plasmas Driven by Multiple Low-Inductance Internal-Antenna Units

We have developed cylindrical RF plasma sources sustained with multiple low-inductance antenna (LIA) units and have measured plasma parameters for argon plasmas. The plasma source could be operated at 2 000 W RF powers to attain plasma densities as high as 1012 cm−3. The plasma potential as low as 13–14 V in high-density plasmas has been achieved and the potential drop from plasma potential to floating potential was almost constant at 10 V. Kinetic energy distributions of Ar ions were measured at argon pressures around 1.3 Pa in an RF power range of 200–2 000 W, which have peaks which almost correspond to the magnitude of the plasma potential.

Large-Area and High-Speed Deposition of Microcrystalline Silicon Film by Inductive Coupled Plasma using Internal Low-Inductance Antenna

A novel inductively-coupled RF plasma source with internal low-inductance antenna (LIA) units was developed to synthesize microcrystalline silicon (µc-Si) film on a large glass substrate. A film thickness profile on a 600×720 mm2 glass substrate was achieved with high plasma uniformity and a variation of less than ±5% without a standing-wave effect. Raman and transmission electron microscope (TEM) analysis revealed that highly crystallized µc-Si films, which were directly deposited on a glass substrate, were synthesized without an amorphous-phase incubation layer at the substrate interface. A bottom-gate thin-film transistor (BG-TFT) was fabricated employing an optimized µc-Si layer and exhibited a field-effect mobility of 3 cm2/(V·s), which is one order higher than that of a typical amorphous silicon TFT.

Surface modification and contamination characterization of ion-enhanced, implanted photoresist removal

The authors study the dopant contamination removal effectiveness of blanket high-dose implanted photoresist wafers exposed to dual mode oxygen plasma, where a remote rf-plasma source is operated simultaneously with rf bias at low temperature. Based on total x-ray reflectance x-ray fluorescence spectrometry and secondary ion mass spectrometry analyses, sputtering effects are minimal during typical process times with most of the dopant remaining on the substrate after dual mode oxygen plasma processing. SC1-based (NH4OH:H2O2:H2O Standard Clean 1) wet clean treatments reduce dopant levels but 2×1013–4×1013at∕cm2 still remain unless the substrate is processed with hydrofluoric acid (HF) wet cleanings where the contamination level is less than 2×1011at∕cm2. Remote plasma low temperature fluorine post-treatments volatilize dopant residues but are less effective than HF wet cleanings. Dual mode forming gas (4%H2∕96%N2) plasma post-treatments are ineffective for dopant contamination removal. Low temperature dual mode oxygen plasma treatments followed by high temperature remote oxygen plasma treatments after wet cleaning have lower dopant levels than low temperature dual mode oxygen plasma processing alone. High temperature remote plasma forming gas post-treatments after dual temperature processing volatilize dopant residues. The increased dopant removal effectiveness with dry fluorine remote plasmas, HF wet cleanings, and dual temperature processing is consistent with the surface modification of the underlying silicon into an oxide from ion bombardment, which also drives dopant residues into the underlying oxide. The surface modification depth from exposure to dual mode oxygen plasma is about 49–53Å.

High-brightness heavy-ion injector experiments

To provide a compact high-brightness heavy-ion beam source for Heavy Ion Fusion, we have performed experiments to study a proposed merging beamlet approach for the injector. We used an RF plasma source to produce the initial beamlets. An array of converging beamlets was used to produce a beam with the envelope radius, convergence, and ellipticity matched to an electrostatic quadrupole channel. Experimental results were in good quantitative agreement with simulation and have demonstrated the feasibility of this concept. The size of a driver-scale injector system using this approach will be several times smaller than one designed using traditional single large-aperture beams. The success of this experiment has possible significant economical and technical impacts on the architecture of HIF drivers.

Effect of residual hydrogen and temperature on initial growth stages and properties of GaAs 1−xN x

In this work we investigate the evolution of the growth mode and the dependence of the structural and optical properties of III–V dilute nitrides with the growth temperature and the presence of hydrogen. GaAsN epilayers were fabricated on (0 0 1) GaAs substrates by rf nitrogen plasma-assisted chemical beam epitaxy. Various amounts of hydrogen were introduced independently in the growth chamber during the GaAsN epilayer fabrication. Reflection high-energy electron diffraction (RHEED) was used to monitor the evolution of the surface reconstruction during growth, while the interaction of the residual hydrogen with the rf plasma source was monitored, simultaneously and in real time, using optical spectroscopy. The presence of hydrogen and slight variations of the growth temperature resulted in noticeable changes in the surface reconstruction during the initial stages of GaAsN fabrication. For sufficiently high amounts of hydrogen, the RHEED image at the onset of GaAsN growth was characterized by the appearance of three-dimensional features, indicating an increased surface disorder; however, after the subsequent growth of few monolayers, two-dimensional RHEED was recovered for all samples regardless of the amount of residual hydrogen. In addition, samples fabricated under excess hydrogen exhibited an apparent nitrogen content reduction, as indicated by high-resolution X-ray diffraction analysis, and a noteworthy blue shift and signal degradation of their low-temperature photoluminescence spectra compared to samples that were fabricated under lower hydrogen partial pressures.

Residual stress effect on degradation of polyimide under simulated hypervelocity space debris and atomic oxygen

Polyimides are used as the outer layer of thermal control insulation blankets covering most of the external spacecraft surfaces that are exposed to space environment. The combined effect of ground simulated hypervelocity space debris impacts and atomic oxygen (AO) on the fracture of polyimide films was studied. A laser-driven flyer system was used to accelerate aluminum flyers to impact velocities of up to 3km/s. The impacted films were exposed to an RF plasma source, which was used to simulate the effect of AO in the low Earth orbit. Scanning electron microscopy and atomic force microscopy were used to characterize the fracture and surface morphology. When exposed to oxygen RF plasma, the impacted polyimide film revealed a large increase in the erosion rate, the damage being characterized mainly by the formation of new holes. This effect is explained by the formation of residual stresses due to the impact and enhancement of oxygen diffusivity and accumulation. A complementary experiment, in which a stressed polyimide was exposed to RF plasma, supports this model. This study demonstrates a synergistic effect of the space environment components on polymers' degradation, which is essential for understanding the potential hazards of ultrahigh velocity impacts and AO erosion for completing a successful spacecraft mission.