Radio Frequency Plasma Source Research Articles

High-Density Radio-Frequency Plasma Sources Produced by Capacitive Discharge with Various Methods for Thin-Film Preparation

Radio-frequency plasma produced by capacitive discharge has been widely used for microelectronic industry because it has a simple structure in its device and its easy maintenance. However, it has disadvantage such as a low plasma-density with the magnitude of 108-109 cm-3. In the paper, high-density radio-frequency capacitive-discharge plasma sources with various methods have been reviewed for thin-film preparation. These methods are the segmented electrode under high driving frequency, the high-secondary-electron-emission-electrode and the various hollow cathode electrodes. In this review, these solutions are assessed and discussed.

Photoluminescence studies of arsenic-doped Hg1−xCdxTe epilayers

Arsenic incorporation in HgCdTe epilayers has been achieved with a nonconventional radio frequency plasma source during molecular beam epitaxial growth. Photoluminescence studies were carried out on HgCdTe arsenic-doped samples. Measurements were done on the as-grown sample, after a Hg vacancy filling annealing and after a 400 °C activation annealing under Hg pressure. A comparison with extended x-ray absorption fine structure results allows us to assign the observed optical transitions to the Hg vacancies, As2Te3 glass and AsHg dopants. An optical signature of the arsenic site transfer upon activation annealing is found.

Extended x-ray absorption fine structure study of arsenic in HgCdTe: p-type doping linked to nonsubstitutional As incorporation in an unknown AsHg8 structure

An extended x-ray absorption fine structure (EXAFS) investigation has been carried out on arsenic-doped Hg70Cd30Te samples. The incorporation of atomic arsenic has been achieved using a nonconventional radio-frequency plasma source in a molecular beam epitaxy reactor. Two samples from the same epitaxial wafer have been studied. One underwent a 400°C activation annealing under Hg pressure, leading to n to p-type conversion. In the commonly admitted scenario, this conversion is associated with the annealing-induced migration of As from a Hg site to a Te site. This study shows that this is not the case. Before annealing, As is found to be involved in noncrystalline structures: 50% inside an As2Te3 chalcogenide glass and 50% inside a new AsHg8 compact structure. After annealing, the As2Te3 chalcogenide glass disappears, 31% of As occupies Hg sites and 69% incorporates inside this new AsHg8 compact structure that occupies Te sites. The EXAFS results are in excellent agreement with 77K Hall-effect measurements. The new AsHg8 structure is found to have an acceptor behavior. Overall, this study provides an entirely new vision of extrinsic p-type doping of HgCdTe as well as the first experimental evidence of As site transfer induced by annealing.

InAs/GaInAs(N) quantum dots on GaAs substrate for single photon emitters above1300 nm

We demonstrate an optimized molecular beam epitaxial growth procedure of InAs quantumdots (QDs) capped by a low nitrogen content GaInAs(N) quantum well to obtainsingle QD emission at telecommunication wavelengths. Technical separation of thenitrogen radio frequency plasma source to a second chamber does allow formation ofInAs QDs without nitrogen incorporation. Thereby, optical quality degradationis avoided and by additional careful separation of the GaInAsN cap from theInAs QD layer with a partial GaInAs cap of nominal 4 nm thickness we achievecomparatively bright single dot emission above 1300 nm at 8 K. Micro-photoluminescencespectroscopy on single QDs reveal excitonic and biexcitonic emission at 939.8 meV (∼1.319 µm) and934.6 meV (∼1.327 µm), respectively. Hence, InAs/GaAs(N) QDs can be considered as to be apromising system for use as single photon sources emitting in the 1.3 µm telecommunication band, with prospects for an extension to even longer wavelengths.

Operating a radio-frequency plasma source on water vapor

A magnetically enhanced radio-frequency (rf) plasma source operating on water vapor has an extensive list of potential applications. In this work, the use of a rf plasma source to dissociate water vapor for hydrogen production is investigated. This paper describes a rf plasma source operated on water vapor and characterizes its plasma properties using a Langmuir probe, a residual gas analyzer, and a spectrometer. The plasma source operated first on argon and then on water vapor at operating pressures just over 300 mtorr. Argon and water vapor plasma number densities differ significantly. In the electropositive argon plasma, quasineutrality requires n(i) approximately = n(e), where n(i) is the positive ion density. But in the electronegative water plasma, quasineutrality requires n(i+) = n(i-) + n(e). The positive ion density and electron density of the water vapor plasma are approximately one and two orders of magnitude lower, respectively, than those of argon plasma. These results suggest that attachment and dissociative attachment are present in electronegative water vapor plasma. The electron temperature for this water vapor plasma source is between 1.5 and 4 eV. Without an applied axial magnetic field, hydrogen production increases linearly with rf power. With an axial magnetic field, hydrogen production jumps to a maximum value at 500 W and then saturates with rf power. The presence of the applied axial magnetic field is therefore shown to enhance hydrogen production.

The adsorption of atomic N and the growth of copper nitrides on Cu(1 0 0)

We report on the adsorption of nitrogen on the Cu(1 0 0) surface when using a radio-frequency plasma source to obtain atomic nitrogen. In these conditions, more than 0.5 ML of nitrogen can be adsorbed, due to N being implanted in both the surface and subsurface layers. The N adsorption modifies drastically the Cu surface, with the formation of a number of small irregular islands and dislocations. A copper nitride Cu 3N thin film with a (1 0 0) texture can be grown by codepositing Cu and N at ∼100 °C.

HgCdTe p-on-n Focal-Plane Array Fabrication Using Arsenic Incorporation During MBE Growth

Extrinsic p-type doping during molecular-beam epitaxy (MBE) growth represents an essential generic toolbox for advanced heterostructures based on the HgCdTe material system: PiN diodes, mesa avalanche photodiodes (APD) or third-generation multispectral focal-plane arrays. Today, arsenic appears to be the best candidate to fulfill this role and our group is actively working on its incorporation during MBE growth, using an original radio frequency (RF) plasma source for arsenic. Such a cell is supposed to deliver a monatomic As flux, and as expected we observed high As electrical activation rates after annealing short-wave (SW), mid-wave (MW), and long-wave (LW) layers. At last, a couple of technological runs have been carried out in the MW range in order to validate the approach on practical devices. p-on-n focal-plane arrays (FPA) have been fabricated using a mesa delineated technology on an As-on-In doped metallurgical heterojunction layer grown on a lattice-matched CdZnTe layer (320 × 256, 30 μm pitch, 5 μm cutoff at 77 K). Observed diodes exhibit very interesting electro-optical characteristics: large shunt impedance, high quantum efficiency, and no noticeable excess noise. The resulting focal-plane arrays were observed to be very uniform, leading to high operabilities. Noise equivalent temperature difference (NETD) distributions are very similar to those observed with the As ion-implanted p-on-n technology, fabricated in our laboratory as well. In our opinion, those excellent results demonstrate the feasibility of our MBE in situ arsenic doping process. Good electrical activation rates and high-quality layers can be obtained. We believe that such an approach allows precise control of the p-doping profile in the HgCdTe layer, which is necessary for advanced structure designs.

Extended X-ray Absorption Fine Structure Investigation of Arsenic in HgCdTe: the Effect of the Activation Anneal

Extended x-ray absorption fine structure (EXAFS) investigation was carried out on two arsenic-doped HgCdTe samples. Arsenic incorporation was achieved with a non-conventional radiofrequency plasma source in a molecular beam epitaxy reactor. Both samples were taken from the same epitaxial run. One of these samples followed a 400°C activation anneal under Hg pressure. EXAFS was used here to probe the local environment around arsenic, and the experimental data were fit through first-principle calculations to extract a quantitative description of the arsenic site transfer upon annealing. Arsenic neighbors are described in terms of chemical nature, numbers and distances. Arsenic was found to be involved mostly in non-crystalline structures, either an As2Te3 glass or an AsHg compact structure. The effect of annealing is to break down the chalcogenide glass, thus favoring the compact AsHg structure. EXAFS results were compared to 77 K Hall-effect measurements, and a very good correlation was found. These findings were compared to the commonly admitted scenario for arsenic site incorporation and transfer upon annealing and provided a new picture and the first experimental evidence of the site transfer.

Plasma diagnostics by optical emission spectroscopy on argon and comparison with Thomson scattering

A novel optical emission spectroscopy (OES) technique for the determination of electron temperatures and densities in low-pressure argon discharges is compared with Thomson scattering (TS). The emission spectroscopy technique is based on the measurement of certain line ratios in argon and a collisional–radiative model (CRM) including metastable transport. The investigations are carried out in a planar inductively coupled neutral loop discharge (NLD) over a wide range of pressures, p = 0.05 Pa–5 Pa. This discharge is a weakly magnetized novel radio-frequency (rf) plasma source, proposed for plasma etching. The NLD is operated in pure argon at a frequency of f = 13.56 MHz and powers varied between P = 1 kW and 2 kW. Both diagnostics, OES and TS, are applied in parallel. The electron energy distribution functions obtained by TS are clearly Maxwellian at low pressures but exhibit a certain enhancement of the energetic tail at higher pressures. Electron densities and temperatures obtained by both diagnostic techniques are compared. Further, absolute numbers of the metastable densities derived from the measurement by the CRM are compared with earlier measurements under similar conditions. Excellent agreement is found throughout if depletion of the neutral gas density by increasing gas temperature and electron pressure is included in the CRM. Electron pressure is the dominant depletion mechanism at gas pressures p ⩽ 0.1 Pa and rf powers P > 1 kW. There, the electron pressure exceeds more than 3 times the neutral pressure and the ionization degree approaches 7% while at pressures p > 1 Pa the degree of ionization is relatively low (<10−3) and neutral gas depletion is dominated by gas heating.

Molecular beam epitaxial growth of hexagonal boron nitride on Ni(1 1 1) substrate

We demonstrate hexagonal boron nitride ( h-BN) epitaxial growth on Ni(1 1 1) substrate by molecular beam epitaxy (MBE) at 890 °C. Elemental boron evaporated by an electron-beam gun and active nitrogen generated by a radio-frequency (RF) plasma source were used as the group-III and -V sources, respectively. Reflection high-energy electron diffraction revealed a streaky (1×1) pattern, indicative of an atomically flat surface in the ongoing growth. Correspondingly, atomic force microscopy images exhibit atomically smooth surface of the resulting h-BN film. X-ray diffraction characterization confirmed the crystallinity of the epitaxial film to be h-BN, and its X-ray rocking curve has a full-width at half-maximum of 0.61°, which is the narrowest ever reported for h-BN thin film. The epitaxial alignments between the h-BN film and the Ni substrate were determined to be [0 0 0 1] h− BN∥[1 1 1] Ni, [1 1 2¯ 0] h −BN∥[1¯ 1 0] Ni, and [1 1¯ 0 0] h −BN∥[1¯ 1¯ 2] Ni.

Effect of the MgO substrate on the growth of GaN

We investigate the effect of the MgO substrate on the growth of GaN by molecular beam epitaxy with radio frequency plasma source. MgO substrate is advantageous for its closer lattice constant to GaN. However, epitaxial GaN layer grown on MgO (1 1 1) substrate is often inclined about 2 ∘ toward a particular a-axis of the GaN. This inclination is found to be caused by the domain structure of the MgO substrates. On the tilted domains of the substrate, the strain is effectively relaxed through the inclination. Another difficulty of MgO substrate is the diffusion of Mg atoms into the GaN layer. This Mg diffusion from the MgO substrate has successfully been suppressed by introducing the thin low-temperature grown AlN buffer layer. The AlN buffer also improves the crystalline quality of the GaN layer.

MBE grown GaInNAs-based multi-Watt disk lasers

We report the fabrication and characterization of high-quality GaInNAs/GaAs gain mirror as an active media for semiconductor disk lasers emitting at the 12XX nm spectral range. The structure was fabricated by molecular beam epitaxy using a radio frequency plasma source for incorporating the nitrogen. The growth parameters have been optimized to reduce detrimental effects of nitrogen on the emission efficiency. Using the gain mirror, which comprised 10 GaInNAs quantum wells with a relatively low content of nitrogen, we demonstrated semiconductor disk laser with an output power of ∼5 W and a differential efficiency of ∼20%.

Modeling of magnetically enhanced capacitively coupled plasma sources: Two frequency discharges

Magnetically enhanced, capacitively coupled radio frequency plasma sources are finding continued use for etching of materials for microelectronics fabrication at a time when multifrequency sources are also being developed. Magnetically enhanced reactive ion etching (MERIE) sources typically use magnetic fields of tens to hundreds of Gauss parallel to the substrate to either increase the plasma density at a given pressure or to lower the operating pressure. Multifrequency sources are used to separately control the magnitude of the ion and radical fluxes (typically with a high frequency source) and the ion energy distributions (typically with a low frequency) to the substrate. In this article, the properties of a two-frequency MERIE reactor are discussed using results from a computational investigation. As in single frequency sources, the reduction in transverse electron mobility as the magnetic field increases can produce a reversal of the electric field in the sheath and an increase in voltage drop across the bulk plasma. These trends decrease ion energies and increase the angular spread of ions. Similar trends are found here, including a field reversal in the sheath at the high frequency electrode. These effects produce a coupling between the high and low frequency sources that compromise the independence of ion production and ion acceleration by the two sources.

Intrinsic surface band bending inCu3N(100)ultrathin films

Highly homogeneous, ultrathin films of copper nitride $({\mathrm{Cu}}_{3}\mathrm{N})$ have been grown on Fe(001) at room temperature using a Cu evaporator and a radio-frequency plasma source to obtain atomic nitrogen in a UHV environment. ${\mathrm{Cu}}_{3}\mathrm{N}$ is a semiconductor with the valence band edge at $\ensuremath{-}0.65\ifmmode\pm\else\textpm\fi{}0.05\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ below the Fermi Level. The formation of copper nitride can be detected spectroscopically by the shape of the Cu LVV-Auger electron transition, which changes sensibly in shape and position compared to metallic Cu. ${\mathrm{Cu}}_{3}\mathrm{N}$ grows epitaxially with the substrate forming flat disklike mosaic blocks, (001) oriented. Both x-ray core level photoelectron spectroscopy and ultraviolet photoelectron spectroscopy photoemission experiments have been used to study the electronic structure. A first-principles calculation has been performed and compared with the measured spectra.

Characterisation of a nitrogen ECR plasma source for the MBE growth of the dilute nitride semiconductor GaAsN

The use of a nitrogen electron cyclotron resonance (ECR) plasma source has allowed the growth of GaAsN at GaAs substrate temperatures as high as 600 °C, unlike the case for growth using radio frequency (RF) plasma sources, for which there is significant loss of nitrogen at substrate temperatures as low as 480–520 °C. Photoluminescence (PL) intensities are significantly improved at a substrate temperature of 600 °C and are further improved slightly by using an ion trap to extract charged species from the beam. As the trap voltage is increased there is a reduction in the total nitrogen concentration, as measured by secondary ion mass spectrometry (SIMS), and a slight increase in the active nitrogen concentration, as measured by PL. These observations are consistent, for example, with charged and active nitrogen species together being involved in the formation of point defects, however more work is needed to clarify what may well prove to be a complex situation.

Piezoelectric-Resonance Effect in a Radio-Frequency-Driven Ferroelectric Plasma Source

The effect of driving frequency on the minimum voltage required to form plasma in a radio-frequency ferroelectric plasma source is examined. When the driving frequency of the plasma source is less than the translational effect radial resonant frequency, the applied electric field and background pressure have an increased effect on plasma formation. Applied-voltage frequencies that are close to the radial mechanical resonance of a ferroelectric/piezoelectric disk cause plasma formation to occur at reduced voltages. In one case, a 75% reduction in the breakdown voltage is realized by altering only the frequency of the applied voltage.

Carbon nanotubes grown by asymmetric bipolar pulsed-DC PECVD

Asymmetric bipolar pulsed-DC plasmas have been recently applied as an industrial solution to the deposition of diamond-like carbon DLC films by plasma enhanced chemical vapour deposition (PECVD) and magnetron sputtering on large areas and with a high deposition rate. New applications of vertically aligned carbon nanotubes (VACNTs) require a suitable technology compatible with the industrial needs: large area and mass production. Among carbon nanotube production techniques, chemical vapour deposition (CVD) is one of the most widely used, specially for applications in which a high control over length, diameter and positioning of the CNTs is desired, as nanoelectronics, field emission displays (FEDs), gas sensors or functionalized CNTs arrays for biomedical and electrochemical applications. Particularly, plasma enhanced CVD (PECVD) favours a parallel growth of the CNTs to the electric field of the plasma sheath, which is of main interest in most of the applications mentioned above. In the present work, a comparative study between radiofrequency (RF) and pulsed-DC plasma sources for PECVD of CNTs is described. Pulsed-DC plasma conditions offer singular characteristics, over RF plasmas, such as: higher power density and electronic temperature inducing a higher ion density and enhancing ion bombardment on the substrate. Chemical species inside the plasma bulk were determined by optical emission spectroscopy (OES) during the growing process. Their concentration showed significant changes depending on the plasma excitation. CNTs were grown on c-Si (100) using Fe nanoislands (< 50 nm) as the catalysts at 750 °C, gas flow ratio Φ NH 3 : Φ C 2H 2 = 50:25 (in sccm) and pressure of 100 Pa. Effects of the plasma conditions on the CNTs structure and morphology (in the range of 5 to 20 μm long and 10 to 50 nm thick) were determined by scanning and transmission electron microscopy (SEM and TEM).

The comparison of the growth models of silicon nitride ultrathin films fabricated using atmospheric pressure plasma and radio frequency plasma

The reaction process model during initial nitridation of Si (111) using atmospheric pressure plasma source was constructed and it was compared to that using a radio frequency plasma source. In atmospheric pressure plasma, emission lines from the N2 second positive system were dominantly observed. By exposing the atmospheric pressure plasma to Si substrate at the temperature ranging from 25to500°C, silicon nitride films with a thickness below 1.8nm were formed. In order to study the nitridation process, the changes in the film thickness against the substrate temperature and nitridation time were systematically studied at a pressure ranging from 50to700Torr. The film thickness increases with increasing the nitridation pressure below 400Torr and it saturates above 500Torr. It was completely regardless of the substrate temperature. From the time dependence of the film thickness at various nitridation pressures, it was revealed that these experimental results were well fitted to a Langmuir-type adsorption model. In the case of nitridation using atmospheric pressure (AP) plasma, molecular species play an important role for nitridation without thermal diffusion. The difference of silicon nitride films fabricated using AP plasma and rf plasma originates from the difference in the active species.

Optimization of RF plasma sources for the MBE growth of nitride and dilute nitride semiconductor material

In this paper we present an investigation of the ion currents produced by two different commercial RF (radio-frequency) plasma sources, detected at the position of a sample during growth. We have measured ion currents for a range of gas flow rates, plasma powers and Ar/N2 gas compositions and found that the two sources behave very differently under the conditions investigated. We have demonstrated that the method described in this paper is a very efficient procedure for finding the optimal conditions for the growth of a high-quality material, and that the switching of N2 into an Ar plasma is a viable technique for the growth of a dilute nitride material.

Self-assembled GaInNAs/GaAsN quantum dot lasers: solid source molecular beam epitaxy growth and high-temperature operation

Self-assembled GaInNAs quantum dots (QDs) were grown on GaAs (001) substrate using solid-source molecular-beam epitaxy (SSMBE) equipped with a radio-frequency nitrogen plasma source. The GaInNAs QD growth characteristics were extensively investigated using atomic-force microscopy (AFM), photoluminescence (PL), and transmission electron microscopy (TEM) measurements. Self-assembled GaInNAs/GaAsN single layer QD lasers grown using SSMBE have been fabricated and characterized. The laser worked under continuous wave (CW) operation at room temperature (RT) with emission wavelength of 1175.86 nm. Temperature-dependent measurements have been carried out on the GaInNAs QD lasers. The lowest obtained threshold current density in this work is ∼1.05 kA/cm2from a GaInNAs QD laser (50 × 1,700 µm2) at 10 °C. High-temperature operation up to 65 °C was demonstrated from an unbonded GaInNAs QD laser (50 × 1,060 µm2), with high characteristic temperature of 79.4 K in the temperature range of 10–60 °C.