High-energy Electron Diffraction Intensity Oscillations Research Articles

GaN/Al x Ga 1−x N quantum wells grown by molecular beam epitaxy with thickness control at the monolayer scale

GaN/Al x Ga 1−x N quantum wells (QWs) are grown by molecular beam epitaxy (MBE) on c-plane sapphire substrates. Both the Al composition and the well thickness are determined in situ from reflection high-energy electron diffraction intensity oscillations. It is demonstrated that MBE growth allows controlling the QW width at the monolayer (ML) scale from sample to sample. 9 K photoluminescence (PL) spectra exhibit well-resolved QW-related emission peaks (linewidths of 20–30 meV). The variation of the GaN/Al0.11Ga0.89N QW energy versus the well thickness (from 3 to 15 MLs) shows the presence of a strong built-in electric field in the quantum structure. Room-temperature PL of QWs is also presented.

Chemical beam epitaxial growth of GaAs1−xPx on GaAs (100) substrates

Phosphorus incorporation during chemical beam epitaxial (CBE) growth of GaAs1−xPx from triethylgallium, tertiarybutylarsine, and tertiarybutylphosphine is investigated. Reflection high-energy electron diffraction intensity oscillations are used to measure the phosphorus incorporation during the As-limited and the (As+P)-limited growth on a Ga-rich surface. The resulting phosphorus mole fraction is compared with the phosphorus composition measured by x-ray rocking curves on the strained GaAs1−xPx layers grown by conventional CBE with a simultaneous supply of group V and group III elements. The phosphorus incorporation rate during CBE growth is lower than that measured during the group V controlled growth but is still much higher than the incorporation rate reported for molecular beam epitaxy growth using elemental sources.

New model for reflection high-energy electron diffraction intensity oscillations

We investigate the influence of inelastic processes on reflection high-energy electron diffraction (RHEED) oscillations by recording energy filtered RHEED intensity oscillations during homoepitaxy of (001)-oriented GaAs. The results clearly show that the dominant inelastic scattering process, plasmon inelastic scattering, does not influence the phase of the oscillations. It cannot therefore account for an independent process contributing a phase to the oscillations that is different from elastic scattering. As an alternative approach, we investigate a basic coherent scattering model introduced by Horio and Ichimiya. We compare its predictions with experiments in the one-beam condition for both GaAs and AlAs(001) homoepitaxy. The average crystal potential required for the fits can be determined independently by Kikuchi line fits, yielding a value of 10.5±0.5 V for both GaAs and AlAs. This allows us to reduce the number of free parameters in the model to only the layer thickness. The theoretical fits of the model to the experimental data yield different layer thicknesses that are in good agreement with the surface reconstruction thicknesses for GaAs and AlAs. We therefore conclude that the phase of RHEED oscillations is determined by the surface reconstruction forming on top of the growing layer during crystal growth. This new model explains many experimentally observed RHEED oscillation phenomena in a unified approach.

Reflection high-energy electron diffraction intensity oscillations during Si growth on Ge(001) by solid source/molecular beam epitaxy

RHEED intensity oscillations of the specular beam are used to follow the growth of Si on Ge(001) by solid source molecular beam epitaxy (SSMBE) in a large range of substrate temperatures and Si deposition rates. From room temperature to 300°C, a kinetic governed layer-by-layer growth mode is observed, the periods of the oscillations corresponding to a single atomic layer. The slow damping of the oscillations in this regime is related to a progressive appearance of surface defects, the RHEED patterns remaining two-dimensional but becoming more diffuse after complete damping of the oscillations. The damping is faster at either higher temperature or lower deposition rates, both effects favouring the increase of the surface diffusion length. Thus, at 400°C, only two oscillations with a doubled period are observed before damping. At 500°C, no RHEED oscillations are obtained in line with the setting up of the thermodynamically expected Volmer–Weber growth mode.

Metalorganic molecular beam epitaxy and etching of AlGaSb using trisdimethylaminoantimony

Metalorganic molecular beam epitaxy and etching of AlGaSb using trisdimethylaminoantimony (TDMASb) are investigated. Etching rate of Al x Ga 1− x Sb by TDMASb rapidly decreases with an increase of x, and no etching is observed for x>0.3. Reflection high-energy electron diffraction (RHEED) intensity oscillation for the etching of AlGaSb is only observed during a few oscillations, and it attenuates rapidly showing a diffused RHEED pattern. This indicates that the etching of AlGaSb ( x≠0) stops after a few monolayers of etching due to the formation of AlN layer on the surface by the adsorption of nitrogen related species. No growth of AlSb is observed for the simultaneous supply of trimethylamine alane (TMAAl) and TDMASb. Successful growth of AlSb is achieved only by the supply of TMAAl and thermally precracked TDMASb.

Molecular-beam epitaxy of gallium nitride on (0001) sapphire substrates using ammonia

Ammonia is used for growing undoped GaN layers by gas source molecular-beam epitaxy on c-plane sapphire substrates. The growth mode is layer by layer as shown by the observation of reflection high-energy electron diffraction intensity oscillations. The structural quality is studied by x-ray diffraction, transmission electron microscopy, and Raman spectroscopy. Low-temperature photoluminescence (PL) and reflectivity demonstrate intrinsic excitonic emission. Room-temperature PL exhibits a strong band-edge intensity and a weak deep-level emission, the so-called yellow band. Finally, secondary ion mass spectroscopy is carried out to check the residual impurity levels of Si, C, and O.

V and Cr on Ru surfaces

By molecular beam epitaxy (MBE) we have prepared (0001)-hcp Ru films on SrTiO 3(111), Al 2O 3(0001) and Al 2O 3(112̄0) surfaces. Reflection high-energy electron diffraction (RHEED) intensity oscillations are observed during the growth of the Ru films, up to temperatures of 870 K. Despite that, we found that high substrate temperatures are required for the initial deposition of Ru. Scanning tunneling microscopy (STM) investigations show monoatomic terraces of up to 100 nm in width, and atomic resolution on Ru, although the imaging was performed under ambient conditions. The high crystallinity of the Ru films is proven by RHEED and by the appearance of interference fringes in the peak profiles of high-angle X-ray diffraction patterns. V and Cr grow pseudomorphic on Ru in the layer-by-layer growth mode for the first few monolayers (ML), afterwards island growth is obtained. In-plane lattice parameter measurements performed during the growth of the V films at room temperature show a rather slow relaxation within the first 10–15 ML to the bulk bcc value, in contrast to Cr films.

Is the arsenic incorporation kinetics important when growing GaAs(001), (110), and (111)A films?

The incorporation coefficients of As2 and As4, obtained from reflection high-energy electron diffraction intensity oscillations in the As-limited growth regime, are compared for the growth of GaAs on (001), (110), and (111)A surfaces by molecular beam epitaxy. The kinetic results are remarkably similar for (110) and (111)A, but very different from those obtained on (001). The incorporation coefficients decrease with increasing temperature for all three surfaces, with the effect being much more dramatic on (110) and (111) A. The low- and temperature-dependent incorporation coefficients on (110) and (111)A explain the need for high As:Ga flux ratios and low substrate temperatures in the preparation of high-quality GaAs epitaxial layers.

RHEED intensity oscillations observed during the growth of YSi 2 − x on Si(111) substrates

For the first time reflection high-energy electron diffraction intensity oscillations were observed during reactive deposition epitaxy (RDE) growth of YSi 2 − x ( x ≅0.3) on the Si(111) surface. The YSi 2 − x crystallographic structure consists of Y planes alternating with Si planes parallel to the Si(111) substrate planes. The intensity of the reflected beam is calculated by solving the one-dimensional Schrödinger equation. A birth-death model was used for the growth simulation in order to investigate fundamental behaviours of reflectivity change during the growth of YSi 2 − x on the Si(111) surface.

Study of surface terrace effect on RHEED intensity oscillations in thin-film growth by laser molecular-beam epitaxy

Morphological evolution of thin-film growth by laser molecular beam epitaxy (laser-MBE) is simulated in Monte Carlo (MC) method. Surface-terrace effect on reflection high-energy electron diffraction (RHEED) intensity oscillations is considered in our model. The validity of the model is demonstrated by comparing our MC simulation with that observed in laser-MBE experiments.

Comparison between ultraviolet-photoelectron spectroscopy and reflection high-energy electron diffraction intensity oscillations during Si epitaxial growth on Si(100)

In ultraviolet-photoelectron spectroscopy on a Si(100) surface during solid-source or gas-source molecular-beam epitaxy (MBE), intensity oscillations of photoelectrons from the valence-band surface states are observed. Recent studies by authors [Enta et al., Surf. Sci. 313, L797 (1994)] have revealed that the photoelectron intensity oscillations (PIOs) most probably originate from an alternation between the 2×1 and the 1×2 surface reconstructions during growth. This model consequently suggests that the period of the oscillations corresponds to the growth time of 2 ML. To confirm this relation, reflection high-energy electron diffraction (RHEED) intensity oscillation was measured during gas-source MBE, using Si2H6 under identical growth conditions with those used for PIO observations. The oscillation period at the half-order diffraction spots of RHEED agreed well with that of PIO at various Si2H6 pressures, providing a direct support for the above interpretation for the origin of PIO.

Computer modeling of morphological evolution and RHEED intensity oscillations of thin-film growth by laser molecular beam epitaxy

The morphological evolution of the unit cell by unit cell layer growth of thin films by laser molecular beam epitaxy (laser MBE) is simulated by the Monte Carlo (MC) method. As a more realistic simulation, the effect of the surface terrace on reflection high-energy electron diffraction (RHEED) intensity oscillations is considered in our model. The calculation shows that the surface terrace effect has an obvious influence on the intensity oscillation shapes. Moreover, we consider the growth units impinging on the surface batch by batch rather than one by one. It is found that the surface morphologies are different, with different numbers of units impinging by a laser pulse on the surface. Furthermore, the surface roughness increases with a decrease in substrate temperature and with an increase in the unit impinging rate (which is determined by the energy of each laser pulse). The validity of the growth model is demonstrated by comparing our MC-simulated RHEED intensity oscillations with those observed in laser MBE experiments.

Arsenic incorporation kinetics in GaAs(001) homoepitaxy revisited

Reflection high-energy electron diffraction (RHEED) intensity oscillations have been used to obtain the arsenic incorporation coefficients for the homoepitaxial growth of GaAs on the (001) surface at different substrate temperatures. The incorporation coefficients of As 2 and As 4 are both temperature-dependent and saturate at maximum values of 1.0 and 0.5 at low temperatures. The results have been modelled using a kinetic scheme which assumes that the incorporation process using either As 2 or As 4 occurs via the formation of a molecularly adsorbed As∗ 2 precursor. The variation of the incorporation coefficients with temperature can be attributed to the fraction of As∗ 2 which does not participate in the incorporation process as the temperature is increased. The final step leading to growth and the formation of GaAs depends only on the incorporation of arsenic from this intermediate, and the implication is that the final incorporation step is independent of the arsenic species used in growth.

Etching of GaSb with trisdimethylaminoantimony and triisopropylantimony in a metalorganic molecular beam epitaxy chamber

Abstract Metalorganic molecular beam etching of GaSb using trisdimethylaminoantimony (TDMASb) and triisopropylantimony (TIPSb) is studied. When non-precracked TDMASb or TIPSb alone is supplied, reflection high-energy electron diffraction (RHEED) intensity oscillation is observed, indicating the monolayer-by-monolayer etching. Etching rate of GaSb by TDMASb is highest compared with those of GaAs, InP and GaP, corresponding to the weakest bond strength of GaSb. When the substrate temperature is decreased, the surface reconstruction is changed from (1 × 3) pattern to (1 × 5) due to the increase in the surface coverage of Sb. Activation energy for etching rate is simultaneously increased from 1.7 to 2.1 eV for TDMASb and from 1.1 to 1.8 eV for TIPSb, indicating that the etching process is affected by the surface coverage of Sb. Quadrupole mass spectroscopy measurement shows that isopropyl radicals are stable in high vacuum condition. It is considered that isopropyl-groups released from TIPSb molecules react with surface atoms and etch them. TIPSb is completely decomposed at around 380°C, while TDMASb at around 300°C, indicating that TDMASb easily releases much Sb atoms than TIPSb does. This leads to the larger activation energy for etching by TDMASb.

Thin film growth and reflection high-energy electron diffraction intensity oscillation

Thin film growth and reflection high-energy electron diffraction intensity oscillation

RHEED intensity oscillations observed during growth of Ge on Si(111) substrates

Reflection high-energy electron diffraction (RHEED) intensity oscillations were observed during molecular beam epitaxy growth of Ge on a Si(111) surface. At 250°C the oscillations continue up to 6 ML. When Ge is grown at room temperature on the Ge Si(111) surface the oscillations continue up to 14 ML. During the growth of Ge thin films on a clean Si(111)-7 × 7 surface at room temperature the oscillations continue up to 10 ML. The intensity of the reflected beam is calculated by solving the one-dimensional Schrödinger equation. A simple birth-death model was used for the growth simulation in order to investigate fundamental behaviours of reflectivity change during the Stranski-Krastanov growth of Ge on the Si(111) surface at 250°C.

Iodine use in solid-source III–V molecular-beam epitaxy

Use of iodine for in situ etching of GaAs, AlAs, and InAs in solid-source molecular-beam epitaxy has been explored. Reflectance high-energy electron-diffraction intensity oscillations have been observed during iodine etching of GaAs and InAs, indicating a molecular layer-by-layer nature of material removal. Etch rates of GaAs and InAs determined from the period of the oscillations are comparable for a given iodine flux. Secondary-ion-mass spectroscopy depth profiles indicate that the etching of AlAs is negligible, and that iodine can be used as a selective etch which stops on AlAs or AlGaAs layers. Electrical properties of GaAs layers grown with an iodine beam impinging on the surface are comparable to those of the layers grown without iodine. Use of iodine for the surface cleaning of GaAs has also been examined. Our results show that iodine etching prior to growth reduces the level of carbon contamination at the substrate epilayer interface.

Geometry and lattice formation of surface layers of Sn growing on InSb{111}A,B.

Surfaces of Sn growing on InSb{111}A,B have been studied by using the reflection high-energy electron-diffraction intensity oscillation technique. The surfaces proceed in the formation of a bilayered lattice in the whole range of film thickness. However, the geometry of the outermost surface layer is quite different in both systems: The growing surface on InSb(111)A smoothens with the same period as the lattice formation, whereas on InSb(111)B, below and above 6 ML of Sn, smooth surfaces emerge every period of monolayer and bilayer, respectively. The monolayer-period change in surface geometry is ascribed to Sb segregation on the growing surface. \textcopyright{} 1996 The American Physical Society.

Growth study of chemical beam epitaxy of GaN xP 1 − x using NH 3 and tertiarybutylphosphine

A study in the growth of GaN x P 1 − x epilayers by chemical beam epitaxy using tertiarybutylphosphine (TBP), ammonia (NH 3), and elemental Ga or triethylgallium is reported. Monitoring reflection high-energy electron diffraction (RHEED) intensity oscillations, we observe that both group-III- and group-V-induced incorporation rates are increased when NH 3 is introduced into a single cracker with TBP. From the difference in the periods of group-V-induced RHEED intensity oscillations, a 16% N incorporation is expected, but X-ray rocking curve measurement shows only 0.08% N. Using separate TBP and NH 3 crackers results in no enhancement in incorporation rates. We conclude that the cracking efficiency of TBP is increased with NH 3 co-injection.

High-quality GaN and AlN grown by gas-source molecular beam epitaxy using ammonia as the nitrogen source

High-quality GaN and AlN have been grown by gas-source molecular beam epitaxy (MBE) using ammonia as the nitrogen source. A growth rate as high as 1 μm/h, which is an order-of-magnitude higher than previously reported for the growth of GaN by molecular beam epitaxy, has been achieved. Strong reflection high-energy electron diffraction intensity oscillations have been observed during the growth, making in situ thickness monitor and control as thin as one monolayer possible. Undoped GaN with an electron density as low as 2×1017 cm−3 and Mg-doped p-type GaN with a room temperature hole density of 4×1017 cm−3 were obtained. Low-temperature photoluminescence of as-grown materials was dominated by band-edge emissions, indicative of high-quality materials which are promising for applications to blue light emitters and high-temperature electronic devices.