Molecular Beam Epitaxial Growth Of ZnSe Research Articles

Improvement in the crystal quality of ZnSe films on Si(111) substrates with a nitrogen surface treatment

We have achieved a significant improvement in the crystal quality of ZnSe films grown by pulsed molecular beam epitaxy (MBE) on Si(111) by irradiating the substrates with a plasma of nitrogen (N-plasma) prior to the deposition. Reflection high-energy electron diffraction (RHEED) patterns during the pulsed MBE growth on the N-plasma-treated Si surface showed very well defined streaks with a twofold reconstruction indicating an atomically flat surface. In sharp contrast spotty RHEED patterns with a diffuse background were observed during the initial stages of the conventional MBE growth of ZnSe on untreated substrates, indicating an initial three-dimensional growth mode. Atomic force microscopy confirmed a smoother surface for the samples grown on the N-plasma-treated Si substrates. Moreover, transmission electron microscopy revealed a decrease in the density of crystal defects in the ZnSe epilayers by the use of the N-plasma treatment.

Hillocks formation during the molecular beam epitaxial growth of ZnSe on GaAs substrates

We report on the peculiar formation of hillocks during the molecular beam epitaxial (MBE) growth of ZnSe on GaAs(1 0 0) substrates. By heating the substrates up to ∼550°C prior to the growth, a clean Ga-rich surface was obtained as confirmed by high-energy electron diffraction and Auger spectroscopy. During the MBE growth of ZnSe on this surface, characteristic hilly-like features were formed with a density of the order of 10 8 cm −2, as observed by atomic force microscopy. The hillocks propagate along the growth direction following the vapor–liquid–solid (VLS) growth mechanism favored by the remanent Ga on the GaAs surface after the heating process. These hillocks could be used, after an appropriate control of their physical dimensions, to synthesize nanostructures such as quantum wires and quantum dots.

Studies in the growth of ZnSe on GaAs(001)

This paper reports a study of the molecular beam epitaxial (MBE) growth of ZnSe on GaAs substrates using elemental sources. Growth rates of ZnSe as a function of Se:Zn flux ratio for constant Zn flux were determined over a wider range of values than previously reported. Careful measurements of atomic fluxes and sample thickness lead to a determination of the sticking coefficients of Zn and Se which are at variance with many previously reported values. The temperature dependence of the sticking coefficients of Zn and Se has been measured carefully and provides evidence for a greater desorption of Zn from the growing surface than previously thought, an effect which persists at low growth temperatures. Measurements at high flux ratios support the use of a precursor model to describe MBE growth of ZnSe on GaAs substrates.

In situ, real-time diffuse optical reflectivity measurements during GaAs cleaning and subsequent ZnSe/GaAs heteroepitaxy

Real-time diffuse optical reflectivity measurements were performed during conventional in situ thermal cleaning and while employing a combined thermal/H-atom treatment of GaAs substrates, the H-atom flux being derived from a rf plasma discharge, hydrogen free-radical source. The diffuse optical reflectivity technique was subsequently employed to monitor heterointerface roughening during molecular-beam epitaxial growth of ZnSe on GaAs substrates which were cleaned in situ using the combined thermal/H-atom treatment. Atomically clean GaAs surfaces were found to be specular when prepared in situ using the combined thermal/H-atom treatment in contrast to conventional thermally treated GaAs surfaces which are considerably rough on the atomic level, surface roughening in this case being associated with the oxide desorption process. A further benefit associated with the specular nature of the thermal/H-atom treated GaAs surfaces is that heterointerface roughening occurring as a consequence of plastic deformation could be detected upon attainment of the critical thickness in the case of the ZnSe/GaAs heteroepitaxial system by monitoring diffuse optical scattering as a function of deposition time. In addition to providing critical thickness data, this real-time monitoring technique also revealed that the degree of heterointerface roughening mirrors the strain relaxation process as a function of increasing film thickness, being complete at a ZnSe film thickness around 0.5 μm.

Surface diffusion during molecular-beam epitaxy growth of ZnSe studied by reflection high-energy electron diffraction

We report the use of reflection high-energy electron diffraction (RHEED) oscillations to characterize surface diffusion during ZnSe crystal growth by molecular-beam epitaxy (MBE). For GaAs growth on vicinal (001) surfaces, RHEED oscillations disappear above a certain substrate temperature, indicating that the Ga diffusion length exceeds the step terrace width. However, for ZnSe, we observe RHEED oscillations over the entire range of substrate temperatures (250–400 °C) typically used during MBE growth. The diffusion length of Zn and/or Se is determined to be less than 40 Å. The slow diffusion of Zn and/or Se that we observe can explain the general difficulty in observing RHEED oscillations during ZnSe growth, and has implications for interface roughness in heterostructures containing ZnSe layers.

Surface Processes in ALE and MBE Growth of ZnSe: Correlation of RHEED Intensity Variation with Surface Coverage

The surface processes in ALE (atomic layer epitaxy) and MBE (molecular beam epitaxy) are investigated both by numerical calculation for the surface coverage based on the model proposed by the authors and by in situ observation of the RHEED intensity variation. The sublimation process is dominated by the Se sublimation from the surface and the adsorption process is influenced by the desorption of Se from both the surface and the precursor states in the range of practical ALE and MBE growth temperatures. To achieve the layer-by-layer growth during ALE the substrate temperature should be set below 300°C. It is also shown that the RHEED intensity variation corresponds to the change in surface coverage during ALE and MBE growth as determined via comparison of the model calculation for surface coverage with the experimental RHEED data.

Metalorganic molecular-beam epitaxy of ZnSe and ZnS

The metalorganic molecular-beam epitaxial growth of ZnSe and ZnS on (100) GaAs has been demonstrated and characterized. Diethylzinc and diethylselenide were used for the ZnSe growth, while for the ZnS growth both hydrogen sulfide and diethylsulfide (DES) were tested as sulfur sources. The dependence of the growth rate on various growth conditions was investigated to clarify the growth mechanism. When group VI alkyls were used as sources, pyrolysis in a cracking cell was required, unlike metalorganic chemical vapor deposition. Interesting differences in growth kinetics and in crystallinity between H2S-based and DES-based ZnS have been found. Photoluminescence spectra of ZnSe layers measured at 4.2 K showed resolved exciton emissions. However, donor-acceptor pair emission lines attributed to a Li acceptor (LiZn) were also observed. Smooth, monocrystalline ZnS layers could be grown at substrate temperatures above 250 °C by using cracked H2S and DES, and above 150 °C by using uncracked H2S. Photoluminescence spectra of ZnS layers at 74 K were dominated by unidentified blue emissions. These good results reflect the complete absence of a premature reaction.