Ion-assisted Molecular Beam Epitaxy Research Articles

Ion-beam-assisted growth of cesium-antimonide photocathodes

We report on the novel use of a Cs+ ion gun for an ion-beam-assisted molecular-beam-epitaxy (IBA-MBE) method to sequentially deposit Cs-Sb cathodes on room temperature substrates as opposed to the standard technique of thermal evaporation on elevated-temperature substrates. The details of the ultrahigh-vacuum chamber, the Cs+ ion source, and the growth technique are elaborated. The final quantum efficiency (QE) is reasonably good for Cs-Sb cathodes grown on two different substrates—Si (100) and strontium titanate—and is comparable to the QE of cathodes grown using thermal sources. This suggests that IBA-MBE could be a viable alternative to grow alkali-antimonides without substrate heating, paving the way for the growth of epitaxial alkali-antimonides in a more reproducible fashion, which may help improve the efficiency of photon detectors and accelerator applications that use alkali-antimonides as electron sources.

High-fluence hyperthermal ion irradiation of gallium nitride surfaces at elevated temperatures

Wurtzitic GaN films deposited on 6H-SiC(0001) substrates by ion-beam assisted molecular-beam epitaxy were irradiated with hyperthermal nitrogen ions with different fluences at different substrate temperatures. In situ observations with reflection high energy electron diffraction showed that during the irradiation process the surface structure of the GaN films changed from two dimensional to three dimensional at elevated temperatures, but not at room temperature. Atomic force microscopy revealed an enhancement of nanometric holes and canyons upon the ion irradiation at higher temperatures. The roughness of the irradiated and heated GaN films was clearly increased by the ion irradiation in accordance with x-ray reflectivity measurements. A sole thermal decomposition of the films at the chosen temperatures could be excluded. The results are discussed taking into account temperature dependent sputtering and surface uphill adatom diffusion as a function of temperature.

Initial stages of the ion-beam assisted epitaxial GaN film growth on 6H-SiC(0001)

Ultra-thin gallium nitride (GaN) films were deposited using the ion-beam assisted molecular-beam epitaxy technique. The influence of the nitrogen ion to gallium atom flux ratio (I/A ratio) during the early stages of GaN nucleation and thin film growth directly, without a buffer layer on super-polished 6H-SiC(0001) substrates was studied. The deposition process was performed at a constant substrate temperature of 700°C by evaporation of Ga and irradiation with hyperthermal nitrogen ions from a constricted glow-discharge ion source. The hyperthermal nitrogen ion flux was kept constant and the kinetic energy of the ions did not exceed 25eV. The selection of different I/A ratios in the range from 0.8 to 3.2 was done by varying the Ga deposition rate between 5×1013 and 2×1014at.cm−2s−1. The crystalline surface structure during the GaN growth was monitored in situ by reflection high-energy electron diffraction. The surface topography of the films as well as the morphology of separated GaN islands on the substrate surface was examined after film growth using a scanning tunneling microscope without interruption of ultra-high vacuum. The results show, that the I/A ratio has a major impact on the properties of the resulting ultra-thin GaN films. The growth mode, the surface roughness, the degree of GaN coverage of the substrate and the polytype mixture depend notably on the I/A ratio.

Epitaxial growth of Ge nanoislands on Si/Ge heterostructure by ion-assisted MBE method

The ion-assisted molecular beam epitaxy (MBE) method allows for construction of self-assembling quantum dot structures in a controllable manner. In the present work we have clearly observed the dependence of the formation behaviour of Ge nanoislands on the ion energy during ion-assisted MBE deposition of Ge on a silicon substrate. In our experiment, we have employed the partial ionization of the Ge atom flow, with the following acceleration of the ions to the desired energy. The best quantum dot structure (the smallest size and the densest population of the nanoislands) has been achieved at the ion energy of 1keV. We suggest a theoretical model, which describes the process of ion-assisted formation of Ge nanoislands on the surface. In order to estimate the effect of radiation damage on the process of nanoisland formation we performed computer simulation of the cascades in the binary collision approximation and by means of molecular dynamics technique.

Atomic assembly during ion-beam assisted growth: Kinetic modeling

The influence of an additional bombardment with low-energy ions during conventional molecular beam epitaxy deposition is studied. A model is proposed describing the initial growth stages during conventional molecular beam epitaxy and ion-beam assisted molecular beam epitaxy. The additional bombardment with low-energy ions leads to a transformation of the growth mode from island growth to layer-by-layer growth. In the first stages of film growth, the hyperthermal ion bombardment causes an increasing detachment of atoms from the tops of the growing islands. Based on the model, using simulation by the kinetic-equation method, the size distribution function of growing clusters is calculated. The theoretical results are in good agreement with experimental results obtained upon the deposition of GaN films.

Control of the crystalline quality of wurtzitic GaN films deposited on γ-LiAlO2 by ion-beam assisted molecular-beam epitaxy

Ion-beam assisted molecular-beam epitaxy was applied to deposit m-plane oriented wurtzitic gallium nitride films on γ-LiAlO2(100) substrates. The crystalline quality of the films was characterized by high resolution X-ray diffraction and transmission electron microscopy. The results show that in particular the nitrogen ion-to-gallium atom flux ratio is a crucial parameter influencing the growth direction and the crystalline quality of the GaN films on the (100) plane of γ-LiAlO2. Films deposited in the N-rich regime are exclusively m-plane oriented and exhibit a columnar morphology with a high mosaic spread and a rough surface. Films deposited in the Ga-rich regime are also exclusively m-plane oriented, but in contrast, they are of high crystalline quality and the surfaces are relatively smooth. Furthermore, extremely N-rich deposition conditions were found to lead to the formation of c-plane oriented wurtzitic GaN domains.

GaN nucleation on (0 0 0 1)-sapphire via ion-induced nitridation of gallium

The growth of epitaxial GaN films on (0 0 0 1)-sapphire has been investigated using X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED). In order to investigate the mechanism of the growth in detail, we have focused on the nitridation of pre-deposited Ga layers (droplets) using ion beam-assisted molecular beam epitaxy (IBA-MBE). Comparative analysis of XPS core-level spectra and LEED patterns reveals, that nitride films nucleate as epitaxial GaN islands. The wetting of the surface by GaN proceeds via reactive spreading of metallic Ga, supplied from the droplets. The discussed growth model confirms, that excess of metallic Ga is beneficial for GaN nucleation.

Anisotropic intrinsic and extrinsic stresses in epitaxial wurtzitic GaN thin film on γ-LiAlO 2 (1 0 0)

A wurtzitic GaN thin film with a thickness of 130 nm is heteroepitaxially deposited on γ-LiAlO 2(1 0 0) substrate by ion-beam-assisted molecular beam epitaxy (IBA-MBE). Structural properties of the film are characterized by high-temperature X-ray diffraction in the temperature range of 25–600 °C. The experimental approach demonstrates a possibility to determine residual stresses in anisotropic epitaxial thin film at high temperature using diffraction. The mechanical and thermal behavior of the film is influenced by the anisotropic nature of GaN ( 1 1 ¯ 0 0 ) crystallographic plane oriented parallel to the substrate surface. The diffraction measurements show that the specific mismatch of the in-plane thermal expansion coefficients result in anisotropic compressive in-plane residual stresses in the film, with the stress in the a-direction significantly larger than in the c-direction over the whole temperature range. Moreover, since no signs of plastic deformation are detected, the temperature dependence of residual stresses allows to extrapolate intrinsic stresses formed in the film during the deposition.

Synthesis of epitaxial SnxGe1−x alloy films by ion-assisted molecular beam epitaxy

In this letter, we report the synthesis of epitaxial SnxGe1−x/Ge/Si(001) with compositions up to x=0.34 by ion-assisted molecular beam epitaxy with 30–100 eV Ar+ ions produced by an electron cyclotron resonance ionization source with ion to atom flux ratios of the order of unity in the substrate temperature range of 120–200 °C. High flux low energy ion beam irradiation greatly inhibits Sn segregation without interrupting epitaxy.

Synthesis of epitaxial Sn xGe 1− x alloy films by ion-assisted molecular beam epitaxy

The group IV metastable Sn x Ge 1− x alloy system is an interesting semiconductor material with potential applications in the fabrication of Si-based heterojunctions and long wavelength infrared optoelectronic devices. Band structure calculations have suggested that the Sn x Ge 1− x alloys may have direct energy gaps continuously tunable from 0.55 to 0 eV for compositions x from 0.2 to 0.6 with very small electron effective masses and hence high electron mobilities. However, syntheses of Sn x Ge 1− x alloy films in the direct gap composition range by conventional epitaxial or polycrystalline thin film growth techniques have not been successful due to the severe surface segregation of Sn during the film growth. In this work, we report the synthesis of epitaxial Sn x Ge 1− x /Ge/Si(001) with compositions up to x=0.34 by ion-assisted molecular beam epitaxy with 30–100 eV Ar + ions produced by an electron cyclotron resonance ionization source with ion to atom flux ratios of the order of unity in the substrate temperature range of 120 to 200°C. High flux low energy ion beam irradiation greatly inhibits Sn segregation without interrupting epitaxy. In situ reflection high energy electron diffraction as well as X-ray rocking curve indicated epitaxial Sn x Ge 1− x alloy films, and Rutherford backscattering spectra confirmed the Sn x Ge 1− x alloy compositions and indicated an absence of Sn segregation.

A Model for Ion-Assisted Molecular Beam Epitaxy and Application to Synthesis of Epitaxial SNxGE1-x Alloy Films

ABSTRACTIn this work, we develop a model for ion-assisted molecular beam epitaxy with application to synthesis of epitaxial SnxGe1-x/Ge/Si(001) with compositions up to x=0.34. The model describes the effect of energetic beam deposition on surface segregation during thin film growth. As the value of a Péclet number expressing the ratio of surface ion-mixing rate to growth rate exceeds unity, the segregation coefficient during ion-assisted growth exceeds the value during thermal growth. A comparison of the model with experiments for SnxGei.1-x synthesis is reported. The experimental results are consistent with the model.

Suppression of strain relaxation and roughening of InGaAs on GaAs using ion-assisted molecular beam epitaxy

InxGa1−xAs films with x=0.67 and 0.75 were grown on GaAs (001) using molecular beam epitaxy (MBE) and ion-assisted MBE. The MBE films roughened and relaxed at 2–4 monolayers, after an initial coherently strained layer-by-layer growth stage, in good agreement with previous results. Ion energies <16 eV had little effect on growth. However, increasing the ion energy from 16 to 27 eV during ion-assisted MBE decreased the rate of strain relaxation and decreased the surface roughness. For x=0.75 and ion energies ≳27 eV, relaxation was eliminated to thicknesses of 50 monolayers, well beyond the Matthews–Blakeslee prediction. We infer from the results that surface roughening of large-mismatch MBE InGaAs films allows strain relaxation well before misfit dislocations are introduced.

Observation of stimulated emission in the near ultraviolet from a molecular beam epitaxy grown GaN film on sapphire in a vertical-cavity, single pass configuration

We report the first observation of stimulated emission from a GaN film grown by ion-assisted molecular beam epitaxy. The observed near-UV optical emission power was a nonlinear function of the pump power density. The characteristics of the observed stimulated emission are similar to those observed recently from films grown with low-pressure metalorganic chemical vapor deposition techniques.

Metallization of GaN Thin Films Prepared by Ion Beam Assisted Molecular Beam Epitaxy

AbstractGallium nitride has generated much interest due to its ability to emit light in the blue to UV range [1]. We have investigated the ohmic contact properties of various metals evaporated onto highly auto-doped n-type GaN thin films which were grown on basal sapphire substrates by ion-assisted molecular beam epitaxy (IAMBE). Electrical measurements of transmission line structures with the metals In, InSn and AuGeNi revealed a wide range of contact resistivity (10∼2 to 10-6 Ω-cm2) which changed with annealing.

Epitaxial Growth of Metastable SnχGe1-χ Alloy Films by Ion-Assisted Molecular Beam Epitaxy

The group IV metastable SnχGe1-χ alloy system is an interesting semiconductor material with potential applications in the fabrication of Si-based heterojunctions and long wavelength infrared optoelectronic devices. Band structure calculations have suggested that the SnχGe1-χ alloys may have direct energy gaps continuously tunable from 0.55 eV to 0 eV for compositions χ from 0.2 to 0.6 with very small electron effective masses and hence high electron mobilities. However, synthesis of SnχGeχ-1 alloy films in the direct gap composition range by conventional epitaxial or poly crystalline thin film growth techniques have not been successful due to the severe surface segregation of Sn during the film growth. In this work, we report the synthesis of epitaxial SnχGeχ-1/Ge/Si(001) with compositions up to χ = 0.34 by ion-assisted molecular beam epitaxy with 30–100 eV Ar+ ions produced by an electron cyclotron resonance ionization source with ion to atom flux ratios of the order of unity in the substrate temperature range of 120 °C to 200 °C. High flux low energy ion beam irradiation greatly inhibits Sn segregation without interrupting epitaxy. In situ reflection high energy electron diffraction as well as x-ray rocking curve indicated epitaxial SnχGeχ-1 alloy films, and Rutherford backscattering spectra confirmed the SnχGeχ-1 alloy compositions and indicated an absence of Sn segregation.

Comparison of AlN films grown by RF magnetron sputtering and ion-assisted molecular beam epitaxy

ABSTRACTCrystalline aluminum nitride (AIN) thin films were formed on various substrates by using RF magnetron sputtering of an Al target in a nitrogen plasma and also by ion-assisted molecular beam epitaxy (IAMBE). Basal-oriented AIN/(1 11) Si showed a degradation of crystallinity with increased substrate temperature from 550 to 770 °C, while the crystallinity of AIN/ (0001) A12O3 samples improved from 700 to 850 °C. The optical absorption characteristics of the AIN/(0001) A12O3 films as grown by both deposition methods revealed a decrease in subbandgap absorption with increased substrate temperature.

Elimination of planar faults in lattice-matched heteroepitaxial films using ion-assisted molecular-beam epitaxy

The nucleation and defect structure of GaAs films grown on Ge(001) by molecular-beam epitaxy (MBE) and ion-assisted MBE (IAMBE) were studied using reflection high-energy electron diffraction and transmission electron microscopy. Three-dimensional (3D) islands nucleated during the initial stages of MBE growth at 580 °C, and continued growth led to nonplanar surfaces and a high density of stacking faults and twins in the films. Irradiation by 33 eV Ar ions during IAMBE growth at 580 °C suppressed 3D island nucleation, yielding relatively flat surfaces throughout growth. Antiphase domains were the only defects observed in the IAMBE films.

Ion-assisted molecular beam epitaxy of GaAs on Si(100)

GaAs was grown by molecular beam epitaxy (MBE) and ion-assisted MBE on Si(100) substrates. Three-dimensional (3D) island nucleation, observed during MBE growth, was eliminated during ion-assisted MBE when the ion energyE was >25 eV and the product ofE and the current densityJ was ≈6-12 eV mA/cm2. IncreasingEJ to ≈15 eV mA/ cm2 resulted in excessive ion damage. Decreasing the substrate temperature from 280 to 580° C during ion-assisted MBE yielded a slight decrease in surface roughness, and flatter surfaces were obtained for lower As4/Ga flux ratios. The suppression of 3D island nucleation led to an improvement in the crystalline perfection of thicker GaAs films. For example, the x-ray diffraction rocking-curve full-width-at-half-maximum values for 0.5 μm thick films grown at 380° C decreased from 1700 arcsec to 1350 arcsec when ion irradiation was used during nucleation. IAMBE allowed nucleation of thin, relatively flat-surfaced GaAs films even at 580° C, resulting in FWHM values of 1850 arcsec for 0.14 /μm thick films.

Defect formation and diffusion mechanism in ion-assisted molecular-beam epitaxy.

A simple moving boundary diffusion model has been used to characterize defect incorporation kinetics during ion-assisted molecular-beam epitaxy. The model permits analysis of the dependence of the final defect concentration on the growth rate, defect diffusivity, defect production range, and the shape of defect depth distribution. The results indicate a linear dependence of the final defect concentration on the ion-to-atom flux ratio which is in the growth-rate-limited regime of the model. Comparison between the model and the film strains measured by x-ray rocking curve analyses has been made and reveals that the thermal spike energy deposited by the bombarding ions during epitaxial growth has a significant effect on the apparent activation energy of the defect migration. A transition temperature above which the defect migration is thermally activated and below which the defect migration is cascade assisted can be defined. The experimentally observed temperature dependence of the defect concentration can be attributed to cascade-assisted diffusion of the defects. Comparison between the model and the multisite multiply activated migration model for low-energy dopant incorporation has also been made. The results show the similarity between the defect incorporation and dopant incorporation which gives a unified view of both processes.

Amplification of Misorientation of Ge Films on Si (100) During Ion-Assisted Molecular Beam Epitaxy

ABSTRACTWe have observed significant increases in the misorientation of Ge films on Si (001) grown by ion-assisted molecular beam epitaxy. The misorientation between the Ge films and Si substrates was found to be a function of the ion-to-atom flux ratio and growth temperature. The parametric dependence of the misorientation on the growth conditions suggests that defects generated by low energy ion bombardment are responsible for the observed increase in misorientation. The amplification of misorientation produced by concurrent low energy ion bombardment during epitaxial growth was attributed to an increase in the fraction of misfit strain accommodated by threading dislocations.