MBE Growth Of GaAs Research Articles

Growth of abrupt heterointerfaces by atomic hydrogen-assisted molecular beam epitaxy

High-quality epitaxy of GaAs-on-Ge system has been developed at low growth temperatures around 400°C by atomic hydrogen-assisted molecular beam epitaxy (H-MBE). The segregation of Ge is significantly reduced and abrupt GaAsGe heterointerface is successfully formed. Using reflection high-energy electron diffraction (RHEED), secondary ion mass spectroscopy (SIMS) and atomic force microscopy (AFM) as characterization tools, it is demonstrated that enhanced layer-by-layer two-dimensional growth mode is realized from the earliest stages of growth and evolution of single-domain (2 × 4) GaAs(0 0 1) is achieved on vicinal Ge substrates at 400°C by H-MBE technique. The modification of growth mode and consequent improvement in material quality are explained at the simplest level by the surfactant properties of atomic H in MBE growth of GaAs.

Simultaneous in situ measurement of substrate temperature and layer thickness using diffuse reflectance spectroscopy (DRS) during molecular beam epitaxy

Abstract Diffuse reflectance spectroscopy (DRS) is used for in situ measurements of temperature transient and temperature drift during MBE growth of AlAs GaAs and InGaAs GaAs layered structures. The experimental results are compared together with pyrometric interferometry (PI) measurement taken concurrently. Our results show that DRS is a viable method for in situ real-time monitoring of both substrate temperature and layer thickness.

Formation of multi-atomic steps and novel heterojunctions on vicinal (1 1 1)B substrates by MBE and anisotropic transport of 2D electrons

We have investigated the MBE growth of GaAs on vicinal (111)B substrates and examined in detail the multi-atomic step structures appearing at the growth front as well as at the n-AlGaAs/GaAs heterojunction. It has been found that the uniformity and periodicity of such steps depend strongly on the growth condition, especially As 4 flux, and that their morphology drastically affects transport of electrons flowing along heterojunctions. The use of high As 4 flux is effective in improving the step uniformity, and in drastically reducing the channel resistance perpendicular to the steps. Key factors for the fabrication of high-quality devices such as quantum wires on (111)B planes have been discussed.

Characteristics of lateral pn junctions grown on (100) GaAs patterned substrate

We have investigated the electrical properties of lateral pn junctions formed by Si-doped GaAs MBE growth over a patterned GaAs substrate. Current-voltage and capacitance-voltage profiling have revealed dissimilar doping profiles, dependent on the junction location. Junctions located at the lower facet-flat intersection possessed an abrupt profile and exhibited good rectifying characteristics with ideality factors (η) of approximately 2. Those situated at the upper facet-flat boundary tended towards linearly graded and displayed poor rectifying properties, with η > 2. The evolution of different minor facets at both lower and upper intersections and silicon doping behaviour on these respective surfaces is proposed as a means of explaining the differences in performance of the junction types. A dip formation present on the facet has been identified as being a cause of the poor rectification performance.

A mathematical model for epitaxial growth on nonplanar substrates

A new mathematical model devised to simulate growth on patterned substrates is described. Its aim is not to predict the evolution of the shape of any substrate during epitaxy, but to determine the value of the different kinetic parameters involved in the growth (mean lifetime before incorporation or desorption, surface diffusion length and diffusion coefficient of group III adatoms on the various crystalline facets), just by fitting the output of the program to the experimental epitaxial growth results. A simulation of GaAs MBE growth on a patterned substrate is included as an illustration of its operation.

Microstructural investigation of surface roughness in MBE-grown AlAs

Knowledge of material pseudodielectric functions at MBE growth temperatures is essential for achieving in-situ, real time growth control. This allows us to accurately monitor and control thicknesses of the layers during growth. Undesired effusion cell temperature fluctuations during growth can thus be compensated for in real-time by spectroscopic ellipsometry. The accuracy in determining pseudodielectric functions is increased if one does not require applying a structure model to correct for the presence of an unknown surface layer such as a native oxide. Performing these measurements in an MBE reactor on as-grown material gives us this advantage. Thus, a simple three phase model (vacuum/thin film/substrate) can be used to obtain thin film data without uncertainties arising from a surface oxide layer of unknown composition and temperature dependence.In this study, we obtain the pseudodielectric functions of MBE-grown AlAs from growth temperature (650°C) to room temperature (30°C). The profile of the wavelength-dependent function from the ellipsometry data indicated a rough surface after growth of 0.5 μm of AlAs at a substrate temperature of 600°C, which is typical for MBE-growth of GaAs.

Incorporation of Sn on GaAs (111)A substrates by molecular beam epitaty

Behavior of Sn as donor species in the MBE growth of GaAs on (111)A substrates has been investigated by varying the growth temperature from 460 to 620°C, As4:Ga flux ratio from 4 to 25, and Sn concentration from 1016 to 1020 atoms cm-3. Secondary ion mass microscopy measurements show that Sn does not surface segregate on (111)A substrates under this growth condition, in contrast to that on (001) substrates. Sn is uniformly incorporated throughout the bulk of the grown layer for all samples, apart from the most highly doped ones. To increase the Sn carrier concentration on the (111)A substrates, the measured carrier concentration shows that doping should be carried out at a low growth temperature and/or high As4:Ga flux ratio.

Hyperthermal (30–500 eV) C + ion-beam doping into GaAs during molecular beam epitaxy

Ion-beam doping of GaAs with hyperthermal (30–500 eV) carbon ion (C +) has been investigated using a combined ion beam and molecular beam epitaxy (CIBMBE) system. The CIBME system is a combination technique between a low-energy ion accelerator and a solid-source MBE, which are connected under ultra-high vacuum (UHV) condition. C +-doping was carried out during GaAs MBE growth at substrate temperature of 550°C with the same C + beam current density. Grown layers were characterized by low-temperature (2 K) photoluminescence and room temperature Hall effect measurements, and it was revealed that the incorporation behavior of C significantly depends on the acceleration energy of C + ( E C). The highest net hole concentration (∥ N A − N D∥) was obtained at E C + = 170 eV. In the energy range of E C + < 170 eV, ∥ N A − N D∥ slightly increased as E C + increased, and for E C + > 170 eV ∥ N A − N D∥ dramatically decreased with increasing E C + , which can be explained in terms of enhanced sputtering effect. On the other hand, ion-beam induced radiation damages were observed for E C + > 170 eV.

Molecular Beam Epitaxial Growth Mode Transitions on Vicinal Surfaces

ABSTRACTKinetic model of MBE growth on vicinal surface is investigated. The model includes step propagation, nucleation and growth of islands on the terraces and Schwoebel barrier at descending step edges as -well. By numerical solution of kinetic rate equations for growth on stepped surface, adatom and island density profiles across a terrace are obtained. With using simple criterion for growth mode transition the "phase diagram" of growth modes in parametric space γ–β is constructed, γ∼J/D and β∼tan-2φ, where J is the atomic flux, D is the surface diffusion coefficient and φ is the substrate miscut angle. The transition curve in the γ–β plane separating step flow mode region from the mixed (step-flow+nucleation) growth mode region is well describded by a simple equation γ=A/β3 where constant A=10 and 100 with and without Schwoebel effect. The relations for critical terrace width (miscut angle) and transition temperature are derived and it is shown that these relations are in fairly well agreement with available experimental data on the MBE growth of GaAs.

Ion-beam doping of GaAs with low-energy (100 eV) C+ using combined ion-beam and molecular-beam epitaxy

A combined ion-beam and molecular-beam-epitaxy (CIBMBE) system has been developed. This system consists of an ion implanter capable of producing ions in the energy range of 30 eV–30 keV and conventional solid-source MBE. As a successful application of CIBMBE, low-energy (100 eV) carbon ion (C+) irradiation during MBE growth of GaAs was carried out at substrate temperatures Tg between 500 and 590 °C. C+-doped layers were characterized by low-temperature (2 K) photoluminescence (PL), Raman scattering, and van der Pauw measurements. PL spectra of undoped GaAs grown by CIBMBE revealed that unintentional impurity incorporation into the epilayer is extremely small and precise doping effects are observable. CAs acceptor-related emissions such as ‘‘g,’’ [g-g], and [g-g]β are observed and their spectra are significantly changed with increasing C+ beam current density Ic. PL measurements showed that C atoms were efficiently incorporated during MBE growth by CIBMBE and were optically well activated as an acceptor in the as-grown condition even for Tg as low as 500 °C. Raman measurement showed negligible lattice damage of the epilayer bombarded with 100 eV C+ with no subsequent heat treatment. These results indicate that contamination- and damage-free impurity doping without postgrowth annealing can be achieved by the CIBMBE method.

Effects of ECR-Plasma Excitation in GaAs MBE Growth

Effects of ECR-Plasma Excitation in GaAs MBE Growth

Reflectance anisotropy from (001) GaAs surfaces during pseudo-ALE growth of GaAs

We demonstrate that the technique of reflectance anisotropy (RA) has now evolved to the level of sophistication required to be of use in direct MOVPE process control. Comparison with data obtained under high vacuum GaAs MBE growth and vacuum chemical epitaxy growth conditions permits the interpretation of RA changes that occur under higher pressure growth conditions. Inflections in RA responses can indicate changes in surface reconstruction that occur again both under high vacuum conditions and at higher pressures. Under pseudo-ALE growth of GaAs on (001) GaAs from triethylgallium (TEGa) and arsine in a conventional hot-wall MOVPE reactor maintained at a constant pressure of 200 mbar, the onset of Ga incorporation via TEGa decomposition has been monitored as a function of substrate temperature. For this reaction, where the rate-determining step may occur either homogeneously in the gas phase or heterogeneously on the substrate surface, changes in RA intensity have been observed at temperatures as low as 350°C. Under these conditions, while the surface changes resulting from TEGa decomposition occur quite slowly, the subsequent surface recovery upon exposure to arsine gas in the second half of the growth cycle occurs very rapidly, the data suggesting that the surface initially relaxes beyond the initial As-stabilised surface towards a very As-rich surface in response to an oversaturation of the surface with Ga atoms during the first half of the growth cycle. This surface then slowly loses As to return to the stable initial condition. In order to observe the arsine recovery kinetics in more detail, the flow rates were reduced. Under these conditions, while rapid RA variations were again observed, inflections in the RA responses became increasingly prominent, which are discussed in terms of possible surface reconstructions that may occur during recovery back to the initial surface condition.

Real-time laser-light scattering studies of surface topography development during GaAs MBE growth

In-situ laser light scattering is shown to yield valuable real-time information on the development of surface micro-topography during GaAs MBE growth. In-vacuo oxide desorption results in a large increase in scattered light intensity. Ex-situ atomic force microscopy has indicated the formation of a high density (>10 9 cm -2) of surface pits, attributed to localized GaAs consumption. Subsequent GaAs homoepitaxial growth (at 600°C) results in the development of an anisotropic ridged topography (ridge axes parallel to [110] with ridge dimensions increasing with epilayer thickness (peak-valley ≤ 12 nm for a 2.0 μm thick epilayer). Post growth in-vacuo annealing generates relatively flat surfaces, dominated by misorientation steps. For a 1 μm thick epilayer, annealing has a long time constant ( > 30 min at 600°C).

As/Ga ratio dependence of Ga adatom incorporation kinetics at steps on vicinal GaAs(001) surfaces

Systematic measurement of the growth kinetics on the transition from growth by cluster formation and coalescence to growth by step advancement on vicinal surfaces during MBE growth of GaAs(001) films has revealed that the first maximum of the RHEED intensity oscillation is gradually delayed (the period increases) as the growth mode transition temperature is approached from below. This is a direct consequence of the gradual change of the growth mode with temperature; as more atoms are involved in the step propagation mode, the number available for island nucleation decreases, with a resultant increase in the period. We have reported that detailed analysis of this delay allows us to investigate the incorporation kinetics of Ga adatoms into step edges and that the energy barrier for incorporation is considerably higher for Ga- than for As-terminated steps. Here we have examined the As/Ga ratio dependence of the Ga incorporation mechanisms using this technique and have found that the incorporation rate for Ga-terminated steps is enhanced while that for As-terminated steps is reduced as the As/Ga ratio is increased near transition temperatures. It suggests that additional arsenic enhances incorporation kinetics into the Ga-terminated steps via As-As dimers but blocks the sites on the As-terminated steps.

Incorporation of silicon during MBE growth of GaAs on (111)A substrates

Si-doped GaAs has been grown on (111)A and (111)A vicinal GaAs substrates and carrier concentrations measured for a range of Si fluxes and growth temperatures. The use of As 2 as opposed to As 4 has been examined. These results are discussed with respect to the growth mechanisms. Photoluminescence measurements have been made and compared with growth on an (001) substrate. The nature of the lattice site of incorporated Si is confirmed using local vibrational mode measurements.

MBE growth of GaAs p-n junction LEDs on (111)A GaAs substrates using only silicon dopant

GaAs LEDs were fabricated on (111)A GaAs substrates using a p-n structure grown with controlled all-Si doping. Cathodoluminescence spectra and I–V characteristics for these samples confirmed p-n structures. The peak intensity of the sample's emission spectrum was equivalent to that of the sample grown using Be as the p-type dopant. As the forward current was increased, the intensity increased without a peak shift, which was observed in the Be-doped sample. A SIMS study of Si or Be diffusion in the p-type layer using a delta-doped GaAs layer suggests that the peak shift is caused by Be diffusion.

In-situ microscopy of MBE growth of GaAs and related materials

Terraces, steps and growth islands on the surfaces of MBE heteroepitaxial AlGaAs and NiAl on GaAs are observed by an MBE-STM hybrid system. The nucleation-and-growth model was confirmed and multi-layer growth process was revealed on AlGaAs. Extended terraces and trenches between the terraces were revealed on NiAl, the latter being undetected by conventional reflection high energy electron diffraction (RHEED). A UHV high-resolution (HR) SEM was modified to perform real-time observation of GaAs MBE growth. Monolayer steps and 10 nm wide growth islands are observed during growth.

Effects of SiAsBeAs interface structure on the initial stages of GaAs MBE growth on Si(111)

We propose a new interface structure of SiAsBeAs for eliminating the charge imbalance of GaAs on Si. We studied the effects of this chalcopyrite-type interface structure on the initial stages of GaAs MBE growth on Si (111) surfaces using RHEED and UPS measurements. It is found that GaAs growth proceeds in a layer-by-layer mode at the initial stages on this charge balanced surface.

Initiation and Evolution of Epitaxial Growth of GaAs on CaF2/Si (111) Substrates

AbstractIn this work, MBE growth of GaAs on CaF2/Si (111) substrates has been studied with both Rutherford backscattering spectrometry, transmission electron microscopy and atomic force Microscopy. It has been observed that, under certain conditions, a chemical reaction between As adatoms and the CaF2 layers can be induced, by which a more stable As layer on the CaF2 surface is formed. The existence of the As layer modifies the CaF2 surface free energy, which, if properly controlled, leads to two-dimensional (2D) nucleation of GaAs on the CaF2/Si (111) surface as opposed to the more commonly observed three-dimensional (3D) growth. Artificial Modification of the CaF2 (111) surface by introducing Ca prior to GaAs growth is also discussed as a promising way to achieve 2D nucleation. In subsequent growth, two kinds of twins have been observed. All samples were observed to have Micro-twins near the GaAs/CaF2 interface. These twins can be suppressed during the first 1000Å, if the layer is grown in a narrow optimal growth window. Otherwise, the growth will be in a 3D Mode at lower temperatures, or it will suffer from the formation of large rotational twins at higher temperatures. It has been observed that growth on vicinal substrates tilted toward [112] azimuth is helpful in suppressing the development of rotational twins so that growth on these substrates have a wider optimal growth window. Surface Morphology of CaF2 epitaxial layers grown on Si (111) substrates with different vicinal angles has also been investigated. It May have significant impact on the twin development during subsequent GaAs growth.

Optical monitoring of deposition and decomposition processes in MOCVD and MBE using reflectance anisotropy

A series of experiments have been performed in order to assess the ability of reflection anisotropy (RA) to follow the MBE growth of GaAs and AlAs on (001) GaAs, combined RA/RHEED measurements reveal a layer-by-layer growth mechanism as indicated by periodic RA oscillations. For the growth of the lattice mismatched GaAs/InAs heterostructure large RA transients are observed which are found to reach their maximum during the growth of the first monolayer and before the onset of 3D growth. RA studies have also been carried out during the MOCVD decomposition of triethylgallium (TEGa) on (001) GaAs at several temperatures ranging from 300 to 425°C. Results obtained suggest that TEGa begins to fully decompose at 350°C which is found to correlate with the formation of TEGa gas phase decomposition products. Results are also presented which may indicate a change in surface reconstruction under MOCVD conditions on moving from a Ga-rich to an As-rich surface state.