Misoriented GaAs Substrates Research Articles

Molecular Beam Epitaxy and Characterization of Layered In2Se3 Films Grown on Slightly Misoriented (001)GaAs Substrates

In2Se3 epitaxial films with a layered structure were successfully grown on (001)GaAs substrates by molecular beam epitaxy. X-ray diffraction and electron diffraction studies revealed that the crystallinity of the films was improved by using slightly misoriented (001)GaAs substrates. Furthermore, electrical and optical anisotropies were observed in layered In2Se3 films. It was found that the conductivity of layered In2Se3 epitaxial films along the a-axis was much larger than that along the c-axis, and that the absorption coefficient of layered In2Se3 epitaxial films for light polarized parallel to the a-axis was much higher than that for light polarized parallel to the c-axis.

Room-Temperature Operation of In0.5Ga0.5 As Quantum Dot Lasers Grown on Misoriented GaAs Substrates by Molecular Beam Epitaxy

We have investigated the characteristics of self-assembled In0.5Ga0.5As quantum dot lasers grown on three types of (100) GaAs substrates, i.e. 0°, 4° and 15°-tilted toward the (111)Ga plane. The lasing wavelength at room temperature is 1018 nm, 1015 nm, and 1030 nm for the 0°, 4° and 15°-0ff samples, respectively. Due to the better quantum confinement and arrangement of the quantum dots, the 4o-off samples exhibit a lower threshold current of 47 mA at room temperature, compared to 73 and 65 mA for the 0° and 15°-off samples, respectively. For the same reason, the characteristic temperature obtained between 150 and 300 K is 117 K for the 4°-off samples, compared to 113 K and 100 K for the 0° and 15°-off samples, respectively.

Photoluminescence properties of Zn1-xMgxSe on misoriented GaAs substrates by molecular beam epitaxy

We have investigated the photoluminescence (PL) properties of Zn1−xMgxSe epilayers grown on GaAs substrates with different misorientation angles by molecular beam epitaxy. According to the data measured by PL and by electron-probe microanalysis, the Mg incorporation in the Zn1−xMgxSe epilayer decreases with increasing misorientation angle. In addition, the PL spectra showed that the full width at half maxima of the band-edge excitonic emission and the intensity of the defect-related donor-acceptor emission in Zn1−xMgxSe epilayers decreased appreciably when a substrate with a misorientation angle of 15° was used.

Surface characterization of ordered (GaIn)P

The knowledge of the nature of the sample surface is important to influence and optimize growth processes and to understand growth mechanisms. Here, a study of the surface of ordered (GaIn)P grown on differently misoriented (0 0 1) GaAs substrates is presented using high-resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM). Whereas the surface of GaAs consists of (0 0 1) terraces and bunched supersteps, vicinal regions appear additionally on the (GaIn)P surface. The height of supersteps increases with increasing misorientation angle and decreasing growth temperature. The surface morphology has also been investigated as a function of the phosphorus partial pressure using three different precursors (phosphine, tertiarybutylphosphine, and ditertiarybutylphosphine) and of the zinc doping. Moreover, additional characteristic facets such as (1 0 l), (0 1 l), ( h 1 l), and (1 k l ) were found.

Heteroepitaxy and Multiquantum Well Structure of Layered Compound GaSe/GaSxSe1-x on (001)GaAs Substrate

For the heterostructure with GaSe, the epitaxial growth of GaSxSe1-x films on misoriented (001)GaAs substrate was investigated. We grew c-axis inclined GaSxSe1-x/GaSe/GaSxSe1-x heterostructure films on misoriented (001)GaAs substrate. The scanning electron microscopic (SEM) images and the photoluminescence spectra confirmed the existence of GaSe and GaSxSe1-x layers. Furthermore, multiquantum well structure of the GaSe/GaSxSe1-x system were fabricated for the first time.

Optical properties of ZnSe1−xSx epilayers grown on misoriented GaAs substrates

The strain induced heavy hole and light hole exciton splitting of ZnSe1−xSx (x<0.1) epilayers grown on misoriented GaAs (001) substrates has been studied by reflectance spectroscopy. The heavy hole and light hole exciton energies are determined by the composition of the layers. It was concluded that for both thin (largely unrelaxed strain) and thick epilayers (with largely relaxed strain) misorientation of the substrate results in increase of x, i.e., in increasing incorporation of sulphur. However, the additional strain expected due to the increasing incorporation of sulphur with misorientation was not observed due to partial strain relief of the epilayers grown on misoriented substrates. The optical quality of the epilayers is tilt angle dependent and is best for about 10° tilt from the (001) direction.

Effects of GaAs substrate misorientation on strain relaxation in InxGa1−xAs films and multilayers

We have investigated the effects of GaAs substrate misorientation on strain relaxation in InxGa1−xAs films and multilayers. Our calculations of shear stresses due to misfit strain, resolved on the glide plane in the glide direction, reveal that the α and β 60° slip systems are influenced in a nearly identical fashion, for all substrate misorientation directions. Thus, classical models for nucleation and glide of 60° dislocations predict that a substrate misorientation will not influence the degree of 〈110〉 asymmetry in strain relaxation in lattice-mismatched zincblende semiconductor films. Contrary to these predictions, our experimental results reveal asymmetries in strain relaxation (for partially relaxed single layers) which favor those dislocations aligned with the offcut axis. These asymmetries depend on the substrate misorientation and growth temperature, and are not easily explained by differences in the intrinsic core properties of α and β dislocations. Furthermore, in fully relaxed multilayers (grown at lower temperatures), and single layers (grown at higher temperatures), epilayer tilt which increases the (111)B substrate miscut is observed. In the multilayers, this behavior is found to be correlated with the presence of micron-scale surface facets. We consider possible explanations for these results, including nucleation of partial dislocations, interaction of gliding threading dislocations, and strain relaxation predominated by forward and backward gliding α threading dislocation segments. Together, these results support the conclusion that local surface or interface step morphologies are more important than bulk stress effects in determining misfit dislocation formation in the InGaAs/GaAs system.

Effect of substrate misorientation on quantum-dot size distribution in the InAs/GaAs system

A study is reported of the effect of (001)GaAs substrate misorientation in the [010] direction on the distribution of MBE-grown self-assembled InAs/GaAs quantum dots in size and position in the GaAs matrix. Temperature-induced narrowing of the exciton photoluminescence line of a quantum-dot ensemble caused by redistribution of photoexcited carriers among dots of different size has been observed.

Effect of atomic bond structure on crystallographic orientation dependence of carbon doping in GaAs

The effect of atomic bond structure at the deposition surface on the crystallographic orientation dependence of carbon doping in GaAs was studied. Carbon doping into GaAs epilayers was performed by atmospheric pressure metalorganic chemical vapor deposition using extrinsic carbon sources of carbon tetrachloride (CCl4) and carbon tetrabromide (CBr4). Epitaxial growths were done on the exact (100) and four different misoriented GaAs substrates with orientations of (511)A, (311)A, (211)A, and (111)A. The electrical properties were measured by van der Pauw Hall analysis at room temperature. Electrically active concentrations in excess of 1×1019 cm−3 were obtained so that CCl4 and CBr4 were demonstrated as efficient p-type dopant sources for carbon doping into GaAs. The dependence of hole concentration on the offset angle of CCl4-doped and CBr4-doped GaAs shows the same tendency, whereas the trend of carbon doping from intrinsic carbon doping technique is different from our results. In particular, the hole concentration on the (100) surface is higher than that on the (111)A surface in the cases of CCl4 and CBr4. The result is opposite to that of the intrinsic doping case. A model based on the atomic bond structure of an adsorption site of carbon-containing species is proposed to explain the difference.

Step density dependence of acceptor concentration in Li-doped ZnSe grown on misoriented GaAs(0 0 1)

Li-doped ZnSe films have been grown on misoriented GaAs(0 0 1) substrates by molecular-beam epitaxy to investigate the effects of step density on the electrical and the optical properties. We found that the net acceptor concentration in Li-doped ZnSe has a strong relationship with the step density on the GaAs substrates and the misorientation direction. The net acceptor concentration on the off-substrate towards [1 1 0] is higher than that on the off-substrate towards [1 1 0]. The difference in Li concentration can be explained by the doping efficiency and the generation of donor-like centers.

Epitaxial growth and characterization of GaS xSe 1− x layered compound semiconductor by molecular beam epitaxy

C-axis inclined GaS x Se 1− x films were successfully grown on misoriented (001)GaAs substrate. The growth temperature ranged from 500–540°C, which is similar with the case of GaSe/GaAs system. In the Raman spectra, the shift of one-mode and two-mode phonon spectra were observed. Strong photoluminescence spectra of GaS x Se 1− x for various compositions of the film were also observed and the energy shift of the peak is consistent with the shift of the bandgap energy.

Thickness modulation in symmetrically strained III–V semiconductor superlattices grown by MOVPE

Lateral thickness modulations of epitaxial layers have been observed in symmetrically strained (GaIn)As/Ga(AsP) superlattices grown on misoriented (0 0 1) GaAs substrates by metallorganic vapour phase epitaxy. Systematic studies on the structural characteristics of the modulation as a function of the lattice mismatch of the individual layers and the misorientation of the substrates have been carried out by transmission electron microscopy (TEM) (cross-sectional and plan-view TEM, as well as high resolution TEM (HREM)). The modulation is periodic and anisotropic. The thickness maxima and minima of similar layers form a slab-like structure. The slabs are inclined from the growth direction and are parallel to the tilting axis of the misorientation of the substrates from (0 0 1), which is [1 1 0] where the misorientation is towards [1 1 0], and [1 0 0] where the misorientation is towards [0 1 0]. The onset of the modulation starts earlier during deposition in the case of higher mismatch strain. The modulation amplitudes increase as the deposition proceeds, and are larger when the mismatch strains are higher. The period of the modulation does not depend on the mismatch strain, and is larger when the width of the surface steps due to the misorientation is broader. The modulation is much less severe in (AlGaIn)As/Ga(AsP) strained layer superlattices. Surface migration of cations is considered to be the cause of redistribution of the equally spaced surface steps into step bunching, which leads to the thickness modulations. Also discussed are the roles of substrate misorientation, lattice mismatch and the cation species in the SLS.

Competitive evolution of the fine contrast modulation and CuPt ordering in InGaP/GaAs layers

We use transmission electron microscopy to characterize the morphology of InGaP epitaxial layers grown by metal-organic vapor-phase epitaxy over misoriented GaAs (001) substrates, with a cutoff angle in a range from 0° to 25°. The occurrence of phase separation and CuPt-type ordered superstructures has been observed. The most ordered configuration has been found to appear in layers grown on 2° off substrates, and the strength of order decreases with increasing the misorientation angle beyond α=2°. Conversely, whereas the phase separation is less evident in the layer grown at 2°, the sample grown with a misorientation of 25° exhibits the most phase separated configuration. The completion between these two phenomena is discussed depending on the misorientation angle.

Photoreflectance and Photoluminescence Characterization of GaAs Quantum Wells Grown by Molecular Beam Epitaxy on Flat and Misoriented Substrates

Photoreflectance (PR) and photoluminescence (PL) spectroscopies were employed in the characterization of molecular beam epitaxially grown GaAs/AlGaAs quantum wells grown on nominally flat (100) oriented and (100) misoriented GaAs substrates. Four quantum wells with nominal thickness of 100, 75, 50 and 25 Å were grown employing different temperatures between 600 and 650° C and growth rates of 0.5 and 1 µ m/h. Room temperature PR allowed us to obtain the actual aluminum composition (x=0.27), and obtain the magnitude of the interfacial electric field ( ∼106 V/cm). An evaluation of the abruptness of the quantum well interfaces was done from the analysis of PL and PR spectra in terms of the finite quantum well theory. The overall results clearly indicated that the step-flow growth mode on the misoriented substrate produced superior crystallinity of the heterostructures. In the case of growth on flat substrates higher temperatures and slower growth rates gave broader interfaces and thinner wells.

Influence of surface morphology on ordered GaInP structures

Ga0.51In0.49P layers, grown by organometallic chemical vapor deposition on differently misoriented (001) GaAs substrates exhibit CuPtB-type domains. The increase in structural homogeneity of the ordered domains, with increasing substrate misorientation, results in a very sharp distribution of the degree of ordering. The correlation between cathodoluminescence emission from ordered regions and changes in surface step distribution direction confirm the influence of the surface morphology on the distribution of ordered regions and their antiphase boundaries.

Surface Of Li-Doped ZnSe Grown On Misoriented GaAs(001) Substrates

AbstractThe surface of Li-doped ZnSe grown on misoriented GaAs(001) substrates by molecular beam epitaxy is studied by means of reflection high energy electron diffraction. Sharp and curved streaky RHEED patterns are observed for all the layers grown on the misoriented substrate towards [110], irrespective of off-angles. No curved pattern, however, is observed on ZnSe layers grown on misoriented GaAs(001) towards [110] with off-angle larger than 5°. We confirmed that the Li-array is surely formed along [110], and that the length of the Li-array is longer than 32Å to be observable as curved streaks. It is also confirmed that the growth rate of Li-doped ZnSe is proportional to the step density, and that the growth rate on the misoriented substrate towards [110] is higher than that on the misoriented substrate towards [110]. The photoluminescence spectra of Li-doped ZnSe layers grown on misoriented GaAs are reported.

Critical thickness of InAs grown on misoriented GaAs substrates

We grow InAs layers on (001) on-axis and misoriented GaAs substrates by molecular beam epitaxy, respectively. The critical thickness of each InAs layer is investigated with photoluminescence spectroscopy and transmission electron microscopy (TEM). We show that the critical thickness is significantly influenced by the substrate misorientation. The critical thickness of the InAs layer grown on the GaAs substrate misoriented toward the [11̄0] direction becomes thicker [5 monolayers (ML)] than that (3 ML) of the InAs layers grown on the GaAs substrates (001) on-axis or misoriented toward the [110] direction. The plan-view TEM images show that the islands grown coherently do not coalesce even beyond the on-axis critical thickness (3 ML). The strain energy is calculated based on valence-force-field model to investigate the interaction between dislocation and step in case on misoriented substrate. As results, we show that dislocation has the minimum length above which dislocation can stably exist and that the extra strain energy generated at the cross point of dislocation and step plays an important role in dislocation generation.

Islands and critical thickness of InAs grown by MBE on nominally- and misoriented GaAs substrates

Islands and critical thickness of InAs grown by molecular beam epitaxy are investigated. The InAs layers are grown on the (001) nominally-and misoriented GaAs substrates at 480 °C. They are characterized with atomic force microscopy and transmission electron microscopy observations and photoluminescence measurement. The InAs grows two-dimensionally at first and then begins the transition to three-dimensional growth, i.e., island formation at 1.8 mono-molecular layers (ML). The island size is not varied much, but follows a Gaussian distribution centred at 13–15 nm. We demonstrate, by observing polarized photoluminescence, that the islands can be utilized as a mesoscopic structure which has been proposed. The observations show that the critical thickness of InAs grown on the nominally-oriented substrate is 3 ML. The thickness can be increased to 5 ML with the growth on the substrate misoriented to the [1 1 0] direction by 3.5–5.0 °. The critical thickness is theoretically studied taking account of the strain energy calculated by the valence-force field method. The theory interprets well the experimental results.

Lattice relaxation in InxAl1−xAs/GaAs heterostructure grown on exact oriented and misoriented GaAs substrates

We have examined lattice relaxation properties of 2 μm thick InxAl1−xAs epitaxial layers grown simultaneously on a (100) exact oriented GaAs substrate and on a 2° misoriented GaAs substrate by low pressure metalorganic chemical vapor deposition method. Double crystal x-ray diffractometry has been used to determine the alloy compositions at various azimuthal angles. As a result of (400) x-ray rocking curve analysis, it was found that the full width at half-maximum (FWHM) of x-ray rocking curves for both samples were strongly dependent on the azimuthal angle, indicating that the lattice relaxation depended greatly on the substrate orientation. From the data of {511} asymmetric measurements for four different directions, the In contents of the epitaxial layers found were 24%. The In0.24Al0.76As epitaxial layers grown on an exact oriented substrate and on a misoriented substrate both showed 82% relaxation, which was a larger value in comparison with the similarly mismatched InGaAs/GaAs heterostructure. The periodic FWHM of the x-ray rocking curves shows that the anisotropy of residual strain was independent of the nominal substrate misoriented direction. For both types of substrate, it is most likely that the variation of the FWHM originates from asymmetric densities of misfit dislocation.

Effect of substrate misorientation on the growth of GaxIn1−x P lateral quantum wells

We report the effects of misoriented GaAs substrates and varied substrate temperatures on the formation of GaxIn1−xP lateral quantum wells (LQWs) by the strain-induced lateral-layer ordering (SILO) process. Nominally [001] GaAs on-axis substrates, [001] substrates cut 2° off toward the [110] direction, and [001] substrates cut 2° off toward the [1̄10] direction were used to simultaneously grow LQWs. The samples were characterized using plan-view and cross-sectional transmission electron microscopy and polarized photoluminescence spectroscopy. We found that regardless of the substrate misorientation or substrate growth temperature, the SILO process induced LQWs always formed along the [1̄10] direction; primarily determined by the direction of the group-V dimer bonds on the surface during growth.