Resonance Plasma-assisted Molecular Beam Epitaxy Research Articles

Study of the effect of plasma power on ZnO thin films growth using electron cyclotron resonance plasma-assisted molecular-beam epitaxy

ZnO thin films were grown on r-plane sapphire substrates using electron cyclotron resonance (ECR) plasma-assisted molecular-beam epitaxy. The effect of the oxygen ECR plasma power on the growth rate, structural, electrical, and optical properties of the ZnO thin films were studied. It was found that larger ECR power leads to higher growth rate, better crystallinity, lower electron carrier concentration, larger resistivity, and smaller density of non-radiative luminescence centers in the ZnO thin films.

Effects of nitrogen plasma irradiation during the growth process after buffer layer growth on the characteristics of hexagonal InN films grown on Si (1 1 1) by electron–cyclotron resonance plasma-assisted molecular-beam epitaxy

We report the effects of nitrogen (N) plasma irradiation during the growth process after buffer layer growth on the characteristics of hexagonal (α) InN films grown on Si(1 1 1) substrates by electron–cyclotron resonance plasma-assisted molecular-beam epitaxy. (0 0 0 1)-oriented single-crystalline α-InN films were dominantly grown independently of N plasma irradiation during the process. However, the crystalline qualities were greatly improved by irradiation in the high-growth-temperature region, effectively suppressing re-evaporation of N atoms from InN. Moreover, the n-type carrier concentrations of the N plasma-irradiated samples decreased to about 10 18–10 19 cm −3, one to two orders lower than those without irradiation. A strong infra-red PL emission of 0.75 eV was observed on the whole surface between 4 and 300 K, only from samples grown under weak N plasma conditions and irradiated by N plasma during the process. The origin of the residual donor impurities was determined to be related to N vacancy.

Influences of residual oxygen impurities, cubic indium oxide grains and indium oxy‐nitride alloy grains in hexagonal InN crystalline films grown on Si(111) substrates by electron cyclotron resonance plasma‐assisted molecular beam epitaxy

We investigated the influences of residual oxygen (O) impurities, cubic indium oxide (β-In2O3) grains and indium oxy-nitride (InON) alloy grains in 200 nm-thick hexagonal (α)-InN crystalline films grown on Si(111) substrates by electron cyclotron resonance plasma-assisted molecular beam epitaxy. Although β-In2O3 grains with wide band-gap energy were formed in In film by N2 annealing, they were not easily formed in N2-annealed InN films. Even if they were not detected in N2-annealed InN films, the as-grown films still contained residual O impurities with concentrations of less than 0.5% ([O]≦0.5%). Although [O]∼1% could be estimated by investigating In2O3 grains formed in N2-annealed InN films, [O]≦0.5% could not be measured by it. However, we found that they can be qualitatively measured by investigating In2O3 grains formed by H2 annealing with higher reactivity with InN and O2, using X-ray diffraction and PL spectroscopy. In this paper, we discuss the formation mechanism of InON alloy grains in InN films. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Visible emissions near 1.9–2.2 eV from hexagonal InN films grown by electron cyclotron resonance plasma-assisted molecular-beam epitaxy

We have investigated the influence of residual oxygen (O) atoms and In 2O 3 grains included in 200-nm-thick hexagonal (α)-InN crystalline films grown on Si (1 1 1) substrates. (1 1 1)-oriented cubic (β)-In 2O 3 crystal grains were formed from O atoms in In droplets adhering to the film surface, and from residual O atoms in the film when the samples were annealed at 500°C in a N 2 atmosphere after growth. The samples exhibited relatively strong and uniform orange photoluminescence (PL) emissions on the whole surface when using a 325 nm He–Cd laser excitation. The emission consisted of two broad bands at approximately 1.9 and 2.0 eV, however no PL emissions were observed when using a 514.5 nm Ar + ion laser. β-In 2O 3, formed from residual O atoms in the film, was also detected following post-growth annealing after In droplets were removed by HCl etching. It is shown that the residual O concentration in the as-grown film can be qualitatively estimated from the orange peaks in the PL spectra. It is suggested that the origin of the localized yellow emissions observed at 2.2 eV under a 514.5 nm Ar + ion laser excitation for as-grown InN films, as reported previously [Phys. Stat. Sol. B 228 (2001) 21; Appl. Phys. Lett. 80 (2002) 968], is not β-In 2O 3 crystal grains although it is closely related to the residual O concentration.

Effect of substrate bias voltage under growth on characteristics of InN films grown by molecular‐beam epitaxy assisted by electron cyclotron resonance plasma

We grew InN films on Si(111) substrates, to which a biased electric filed was applied positively or negatively, by electron cyclotron resonance plasma-assisted molecular-beam epitaxy (ECR plasma-assisted MBE) in order to suppress ion damage due to N2+ ions or/and impurity incorporation due to some ions of residual impurities generated by ECR plasma and have investigated the effects of bias voltage on film characteristics. The surface morphology was improved, the carrier concentration decreased, and the Hall mobility increased with increase of the bias voltage probably because of the two reduction effects of impurity incorporation and ion damage due to N2+ ions generated by ECR plasma. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Lattice vibrations ofSi1−xCxepilayers on Si(100)

Raman spectroscopy has been used to investigate the lattice vibrations of ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{C}}_{x}$ $(0lxl0.02)$ epilayers grown on Si(100) by electron cyclotron resonance plasma-assisted molecular beam epitaxy. Spectral contributions due to first- and second-order longitudinal and transverse optic phonons together with disorder-induced features have been evaluated by a detailed polarization and C concentration dependence analysis. The band parameters exhibit generally a linear dependence on $x.$ A Raman line observed near $630{\mathrm{cm}}^{\ensuremath{-}1},$ which in past work has been associated with C ordering in the Si lattice, is found not to agree with this interpretation. It is associated with interstitial C. The optical absorption coefficient near 458 nm is deduced from the Raman intensity information, and is found to increase significantly with increasing $x.$

Surface reconstruction of GaAs( [formula omitted]) nitrided under the controlled As partial pressure

Under the controlled As partial pressure, the nitridation process of GaAs(0 0 1)-(2 × 4) surface was studied using a scanning tunneling microscope (STM) combined with an electron cyclotron resonance plasma-assisted molecular beam epitaxy system. With either prolonging the nitridation time or decreasing the As partial pressure, the previously reported (3 × 3) structure with two dimers per surface cell ((3 × 3)-2D) was found to progressively convert into a new (3 × 3) structure characterized by one dimer per surface cell ((3 × 3)-1D). Reversely the exposure to arsenic transformed the structure from (3 × 3)-1D to (3 × 3)-2D, suggesting that the topmost layer is composed of As 2-dimers. Based on these STM images together with the X-ray photoelectron spectroscopy data, we propose the new As 2-dimer coverage models to explain both (3 × 3)-1D and -2D structures involving the exchange reaction of arsenic with nitrogen in the subsurface region of GaAs.

Cubic GaN formation in Mn/GaN multilayer films grown on 6H-SiC(0001)

Cubic GaN was grown on 6H-SiC(0001) by electron-cyclotron resonance plasma-assisted molecular-beam epitaxy. The growth process consisted of first depositing a 20-nm GaN buffer, followed by ten periods of alternating layers of 1 monolayer (ML) Mn and 10 ML GaN, and finally capped with 30 nm GaN. High-resolution transmission electron micrographs of film cross sections were recorded and digital diffractograms were calculated to determine the lattice structures of the different film layers. It was found that the crystal structure in the GaN buffer and capping layer matches the 2H-wurtzite GaN. However, uniform cubic zinc-blende GaN phase was observed in place of the nominal multilayer Mn/GaN region. The density of defects typically observed in GaN films is drastically reduced within the cubic and capping layer, indicating improved film quality possibly due to the surfactant effect of Mn. Based on the one-dimensional Ising model of polytype formation, a mechanism is proposed to explain the growth of cubic GaN in the Mn/GaN region.

High reflectivity and broad bandwidth AlN/GaN distributed Bragg reflectors grown by molecular-beam epitaxy

A number of distributed Bragg reflectors (DBRs) based on AlN/GaN quarterwave layers have been grown on (0001) sapphire by electron cyclotron resonance plasma-assisted molecular-beam epitaxy. The number of periods for the DBRs ranges from 20.5 to 25.5 and the thickness of the quarterwave layers were chosen such that the peak reflectance occurs from the near ultraviolet to green wavelength regions. Peak reflectance values between 97% and 99% were obtained for these DBRs. The best sample has a peak reflectance up to 99% centered at 467 nm with a bandwidth of 45 nm. The experimental reflectance data for this sample were compared with simulations using the transmission matrix method and show excellent agreement with respect to peak reflectance, bandwidth of high reflectance, and the locations of the sidelobes. The thickness of the quarterwave layers and uniform periodicity of the bilayers were confirmed by cross-section transmission electron microscopy. A network of cracks was observed in some of the samples and this is attributed to tensile stress in the AlN layers. We have grown asymmetric DBRs with thicker AlN layers and thinner GaN layers to reduce the tensile strength in the AlN layers. Such an approach resulted in samples that have significantly less cracks or even crack-free.

Distributed Bragg reflectors based on AlN/GaN multilayers

A 20.5 period distributed Bragg reflector stack based on AlN/GaN has been grown on (0001) sapphire by electron cyclotron resonance plasma-assisted molecular-beam epitaxy. Peak reflectance up to 95% was observed at a wavelength of 392.5 nm. Cross-section transmission electron microscopy studies indicate that the interface of GaN-on-AlN is always sharper and more distinct than the interface of AlN-on-GaN. This is attributed to the different growth modes of AlN and GaN. When AlN grows on GaN, it tends to grow in a two-dimensional mode (Frank–van der Merwe mode) whereas GaN grows on AlN in a three-dimensional mode (Stranski–Krastanov mode). Based on these findings, the experimental reflectance data were simulated using the transmission matrix method.

The role of dislocation scattering in n-type GaN films

The lateral transport in GaN films produced by electron cyclotron resonance plasma-assisted molecular beam epitaxy doped n type with Si to the levels of 1015–1020 cm−3 was investigated. The room temperature electron mobility versus carrier concentration was found to follow a family of bell-shaped curves consistent with a recently proposed model of scattering by charged dislocations. The mechanism of this scattering was investigated by studying the temperature dependence of the carrier concentration and electron mobility. It was found that in the low carrier concentration region (<1017 cm−3), the electron mobility is thermally activated with an activation energy half of that of carrier concentration. This is in agreement with the prediction of the dislocation model.

Atomic-scale nature of the (3×3)-ordered GaAs(001):N surface prepared by plasma-assisted molecular-beam epitaxy

In situ scanning tunneling microscopy and time-resolved reflection high-energy electron diffraction measurements were performed to study the nitridation process of the As-terminated GaAs(001)-(2×4) surface by using electron cyclotron resonance plasma-assisted molecular-beam epitaxy. We report the real-space atomic structure of the coherently strained (3×3)-ordered GaN monolayer on GaAs(001) after a limited-exposure nitridation process and the atomically smooth morphology of this nitrided surface. The unique (3×3) phase is found consisting of nitrogen dimers and a regular array of missing nitrogen rows in both [1̄10] and [110] directions.

Structure of GaN films grown by molecular beam epitaxy on (0001) sapphire

GaN films grown by electron-cyclotron resonance plasma-assisted molecular beam epitaxy were studied by transmission electron microscopy and x-ray diffraction (XRD). Two sets of films were compared that were grown under identical conditions except for the ratio of the Ga to N flux. Films with a 30% higher Ga to N ratio (A films) were found to contain inversion domains (IDs). No IDs were found in films grown with a lower Ga to N ratio (B films), but instead the zinc-blende GaN was found near the film substrate interface. A narrower XRD rocking curve width along the (0002) direction and a broader rocking curve width along the asymmetric (1102) axis were found for A films compared to B films.

Electron Mobility of N-Type GaN Films

AbstractThe lateral transport in GaN films produced by electron cyclotron resonance plasma-assisted molecular beam epitaxy (ECR-MBE) doped n-type with Si to the levels of 1015 − 1020 cm−3 was investigated. The room temperature electron mobility vs carrier concentration was found to follow a family of bell-shaped curves consistent with a recently proposed model in which dislocations with an edge component introduce acceptor centers along the dislocation lines which capture electrons from the conduction band in n-type GaN. The mechanism of scattering was investigated by studying the temperature dependence of the carrier concentration and electron mobility. We found that for the samples with high carrier concentration (> 1018cm−3), the scattering at low temperatures is dominated by impurities while for samples with low carrier concentration (<1017 cm−3), the scattering is dominated by charged dislocations.

How to induce the epitaxial growth of gallium nitride on Si(001)

Electron cyclotron resonance plasma-assisted molecular-beam epitaxy has been used to grow hexagonal and cubic GaN crystal layers on Si(100). By a combined application of in-situ reflection high-energy electron diffraction, X-ray photoelectron spectroscopy and cross-sectional transmission electron microscopy, the state of the Si(001) surface, the structure of GaN grown on this surface and the orientation relationship between substrate and layer were determined. Substrate cleaning, surface reconstruction and carbon surface contamination were found to have a strong effect on GaN growth in the early stage of epitaxy (up to 2000 Å). While polycrystalline GaN in its hexagonal phase is obtained on a clean Si(001) surface, cubic GaN grows epitaxially on a Si(001)c(4×4) surface covered by small three-dimensional cubic β-SiC crystallites.

Growth of gallium nitride by electron-cyclotron resonance plasma-assisted molecular-beam epitaxy: The role of charged species

The role of ionic and nonionic excited species of nitrogen in the growth of GaN thin films by electron-cyclotron resonance (ECR) plasma-assisted molecular-beam epitaxy has been investigated. It was found that the kinetics of film growth is significantly affected by the microwave power in the ECR discharge. Specifically, a transition from the island to a layer-by-layer and, finally, to a three-dimensional growth has been observed as a function of power. These morphological changes are accompanied by degradation of the electrical and luminescence properties, a result attributable to increased native defects and impurities. Secondary-ion-mass spectroscopic (SIMS) analysis indicates that impurity levels in the films increase with the plasma power levels used during the growth. To study the relative role of ion-induced native defects in these films, strategies for charged species extraction were developed by using an off-axis solenoid to modify the magnetic environment during growth. Films grown under a reduced ionic/excited neutral ratio environment show marked improvement in the electrical and luminescence properties. These data, together with SIMS analysis, indicate that observed improvements in these films are due to a reduction of native defects and not impurities.