Vicinal GaAs Substrates Research Articles

Arrangement of atoms and strain distribution in CdSe/ZnSe strained single quantum well on vicinal GaAs substrate

Strain distribution in a ZnSe/CdSe/ZnSe strained single quantum well structure grown on a vicinal GaAs substrate is investigated by calculating the positions of all the atoms in the structure. The strain distribution around atomic step edges is inhomogeneous, while that in terrace regions is the same as that of quantum wells grown on just-oriented substrates. The inhomogeneity extends for only 2 or 3 ML along the vicinal direction from the step edge. Microscopic lattice-planetilting of CdSe and ZnSe epitaxial layers becomes the largest at the step edge. CdSe expands and ZnSe shrinks in the growth plane at the lateral interface of CdSe and ZnSe to minimize the total strain energy. Cd atoms at the step edge have the largest strain energy due to shear deformation. Conduction and valence band potential profiles calculated by using the strain distribution and deformation potentials are found to enhance carrier confinement into the terrace region of the quantum well, although the confinement effect is not enough to explain the experimentally observed blueshift in photoluminescence.

Growth of InNAs by low-pressure metalorganic chemical vapor deposition employing microwave-cracked nitrogen and in situ generated arsine radicals

InNAs has been grown by low-pressure metalorganic chemical vapor deposition on vicinal GaAs(1 0 0) substrates misoriented by 2° towards [0 0 1]. In situ plasma-generated arsine radicals were employed for the first time to achieve high source flux ratio of N/As, with the aim of synthesizing InNAs with a high solid-phase nitrogen composition. The in situ generated arsine is produced by placing solid arsenic downstream of a microwave hydrogen plasma. Nitrogen cracked by microwave excitation, and trimethylindium feedstock carried by hydrogen gas were used as the other sources. The grown InNAs is always accompanied by hexagonal InN, while the nitrogen composition of the InNAs varies with changing growth conditions. A small amount of cubic InN also occurs in the grown films at low nitrogen radical flow. The growth mechanism and the key pathways for synthesis of InNAs single crystal films are discussed.

Anisotropic surface segregation of indium atoms in molecular beam epitaxy of InGaAs/GaAs quantum wells

In order to investigate the dependence of indium surface segregation on step structures, InGaAs (InAs)/GaAs quantum wells (QWs) were grown by molecular beam epitaxy on vicinal GaAs(001) substrates, misoriented toward [011] and [01̄1] directions. High energy shift and narrow linewidth of photoluminescence spectra were observed for InAs QWs prepared on the [011]-stepped surface. By cross-sectional transmission electron microscope observation, it was found that broadening of InGaAs QWs was enhanced on the [011]-stepped surface. These phenomena were explained by the anisotropic segregation effect due to the difference in the step structure between the [011] step and the [01̄1] step.

AFM study and optical properties of GaAsN/GaAs epilayers grown by MOVPE

We report a study on surface morphology and optical properties of GaAsN epilayers grown on GaAs (1 0 0) vicinal substrates by metalorganic vapor-phase epitaxy (MOVPE), using dimethylhydrazine as the N source. The surface morphology was investigated by atomic force microscopy (AFM) as a function of N composition and growth temperature ( T g). At T g=530–550°C, we observed a transition from a bunched step/terrace structure towards a surface with indistinct step/terrace structure when increasing the N content. We attribute this evolution to a transition from step-bunching towards 2D-nucleation growth, related to decreased Ga surface-diffusion length. The optical properties were studied using room-temperature (RT) and low-temperature photoluminescence (PL), and photoreflectance (PR). Post-growth thermal annealing were carried out on GaAs-capped layers. A drastic increase of the RT PL intensity and a narrowing of the linewidth were observed under optimum annealing conditions. The influence of annealing temperature and duration were also examined.

Heteroepitaxial growth of InAs by low-pressure metalorganic chemical vapor deposition employing in situ generated arsine radicals

Single-crystal InAs films are grown by metalorganic chemical vapor deposition on vicinal GaAs(1 0 0) substrates in the temperature range of 350–550°C. We employed for the first time in InAs growth in situ plasma-generated arsine radicals as the group V source, trimethylindium as the group III source, and hydrogen as the carrier gas. The in situ generated arsine is produced by placing solid arsenic downstream of a microwave hydrogen plasma. The Arrhenius plot of InAs growth rate vs. reciprocal growth temperature shows an activation energy of 10 kcal/mol in the 350–450°C temperature range, and a rapidly decreasing growth rate at temperatures above 450°C, neither of which has been reported to our knowledge in the literature.

Reduction of indium segregation in InGaAs/GaAs quantum wells grown by molecular beam epitaxy on vicinal GaAs(001) substrates

Low-temperature photoluminescence (PL) was used to investigate the optical properties of In0.10Ga0.90As/GaAs quantum wells grown on GaAs(001) substrates with a miscut angle of 0° (nominal), 2°, 4° and 6° towards [110]. The luminescence from the quantum wells grown on a vicinal surface exhibited a blueshift compared to the nominal case. An extra emission at low energy was only observed for the nominal sample and was related to In segregation. Its absence from the PL spectrum of quantum wells grown on vicinal surfaces is a strong indication that In segregation is modified on this type of surface. Theoretical calculations confirmed our experimental data.

Strain distribution around the step edge of ZnSe/CdSe/ZnSe strained quantum well grown on vicinal GaAs substrate

The strain energy and strain element distribution of ZnSe/CdSe/ZnSe strained quantum well grown on vicinal GaAs(0 0 1) substrate are investigated by an atomic scale calculation based on valence-force field model. The strain distribution around the step edge is inhomogeneous, while that on the terrace is uniform and not different from just-oriented substrate. The largest strain energy is found at Cd atom at the step edge. The origin of the largest strain energy is shear deformation. The strain inhomogeneity extends for only 2 ML from the step edge in the vicinal direction. The lattice deformation extends to longer distance along the 〈1 1 1〉 direction than the 〈1 1 0〉 direction. Considerably large lattice-plane-tilting is caused by the shear strain in (1 1 0) plane. The tilt angle in CdSe lattice is more than 10 000 arcsec at the step edge, while that at terrace is less than 1000 arcsec. The lattice-plane-tilting also occurs on ZnSe layer top and below the step edge of CdSe, though the tilt angle is opposite to CdSe.

RHEED study of atomic steps on ZnSe surface growing on vicinal GaAs substrates

Atomic steps of ZnSe surface growing on vicinal GaAs(1 0 0) substrate are investigated with in-situ reflection high-energy electron beam diffraction. The diffraction patterns indicating 1 ML-height step with uniform terrace width are clearly observed as the growth proceeds. The uniformity of step array on growing surface becomes stable by ZnSe growth of 500 Å. The terrace width and substrate vicinal angle measured by the diffraction patterns agree well with the substrate nominal values. The observed diffraction patterns are compared with simulation results based on kinematical diffraction theory to discuss the fluctuation of surface step structure. The improvement of uniformity of step array is anisotropic with regard to the substrate vicinal direction. The insertion of CdSe layer does not cause broadening of full width at half maximum of reciprocal lattice rod, which indicates that the uniformity of step array is unchanged.

Potential profile and quantum energy level in ZnSe/CdSe/ZnSe strained quantum well grown on vicinal GaAs substrate

The potential profile and quantum energy level in ZnSe/CdSe/ZnSe strained quantum well grown on vicinal GaAs(0 0 1) substrate are calculated. The potential in CdSe at the step edge is increased due to the strain inhomogeneity for both electron and heavy hole. As a result, the lateral length of CdSe well decreased compared with the terrace width. The strain-induced potential modulation raises the quantum energy level. The shift of quantum energy level in conduction band increases with substrate vicinal angle whereas no significant shift is found for valence band because of large effective mass of heavy hole. The shift of quantum energy level increases with increasing well width for thin well, however, it decreases for wide well. Carrier confinement in the terrace region is hardly found since the wave function shows undulation of only 2% in the substrate vicinal direction.

Growth of (Zn)CdSe quantum structures on vicinal GaAs(0 0 1) substrates: step flow growth versus strain effects

Single ZnCdSe quantum wells (QWs) as well as stacks of CdSe sub-monolayer (SML) insertions embedded in ZnSe have been grown on GaAs(0 0 1) substrates misoriented up to 10° towards the (1 1 1)A or the (0 1 ̄ 1) direction, respectively. The influence of the substrate misorientation on the structural and optical properties has been studied by high-resolution X-ray diffraction (HRXRD) and photoluminescence spectroscopy. HRXRD measurements revealed a strain-induced tilting of the II–VI layers with respect to the GaAs substrate in excellent agreement with theoretical predictions. A decreased Cd incorporation with increasing misorientation angle was found for the ZnCdSe QWs due to the competition process between Zn and Cd. In case of the CdSe SMLs, the additional strain perpendicular to the surface overcompensates the advantage of more binding partners at the step edges for Cd atoms leading to a possible formation of quantum structures only for the highest step density.

Isoelectronic dopant induced ordering transition in GaInP grown by organometallic vapour phase epitaxy

Transmission electron microscope and transmission electron diffraction examination (TED) has been performed to investigate effects of Sb doping on ordering in organometallic vapour phase epitaxial (OMVPE) Ga 0.5In 0.5P layers grown on (001) GaAs singular and vicinal substrates at 620°C. The TED results show that adding Sb during the OMVPE growth of GaInP leads to a transition in ordering from CuPt type to triple period type (TPO). It is shown that the intensity of the CuPt superspots decreases with increasing Sb/P ratio in the vapour and is absent at a ratio of 8.0×10 – 4. A further increase in the ratio leads to the formation of TPO that is also reduced with increasing Sb concentration. Addition of [110] or [1̄10] steps by using misoriented substrates has virtually no effects on the TPO.

Single-crystal hexagonal and cubic GaN growth directly on vicinal (001) GaAs substrates by molecular-beam epitaxy

Single-crystal hexagonal and cubic GaN thin films have been grown by radio-frequency nitrogen plasma source molecular beam epitaxy directly on vicinal (001) GaAs substrates, misoriented by 2° toward [100], without using an incident As beam during oxide desorption or the following stages of growth. Both the GaAs nitridation and GaN growth conditions were found to control the structure of the layers. Cubic layers could be grown only without nitridation and under stoichiometric N/Ga flux ratio conditions. N-rich conditions favored the growth of hexagonal layers, which exhibited significantly higher photoluminescence intensities compared to cubic ones. Hexagonal single crystalline GaN films were grown with (101̄2) planes and presented characteristic surface roughness striations along a 〈110〉 substrate direction. On the contrary, a stepped surface morphology was observed for cubic GaN.

Effects of Si doping on ordering and domain structures in GaInP

The effects of Si doping on ordering and domain structures in GaInP grown by organometallic vapour phase epitaxy on (001) GaAs singular and vicinal substrates at 620°C have been investigated by transmission electron microscopy (TEM) and transmission electron diffraction (TED). TEM results show that the behaviour of antiphase boundaries (APBs) in the singular samples differs from that of the vicinal samples. As the carrier concentration increases, the density of APBs in the vicinal samples increases slightly, whilst that of the singular samples varies insignificantly. APBs are inclined some degrees from the [001] growth surface. TEM results show that for the highly doped samples, the ordered domains contain dark mottled contrast corresponding to either much less ordered regions or disordered regions. TED results show that the ordering is present in layers exceeding a concentration of ∼2.0×1018 cm−3.

Effects of Te Doping on Ordering and Antiphase Boundaries in GaInP

Transmission electron microscope (TEM) and transmission electron diffraction studies have been performed to investigate the effects of Te doping on ordering and antiphase boundaries (APBs) in organometallic vapour phase epitaxial Ga0.5In0.5P layers grown on (001) GaAs singular and vicinal substrates at 670°C. TEM results show that the behaviour of APBs for the singular samples differs from that of the vicinal samples. The density of APBs in the vicinal samples is increased by roughly a factor of 2, whilst that of the singular samples is slightly increased, as the Te concentration increases. APBs are inclined 10–57° from the (001) growth surface. As for the singular samples, the angle seems to remain virtually unchanged with increasing doping level. However, for the vicinal samples, the angle decreases significantly with increasing concentration. A simple model is presented to explain the dopant concentration dependence of the behaviour of APBs in the ordered GaInP.

Extension mechanism of antiphase-boundaries in CuPt B-type ordered GaInP 2 and (Al,Ga)InP 2 epitaxial layers

We observe the antiphase boundaries (APBs) in the ordered CuPt B-type GaInP 2 and (Al,Ga)InP 2 layers grown on a vicinal GaAs(0 0 1) substrate by means of postgrowth transmission electron microscopy. We conclude that APBs are active sites for nucleation of two-dimensional (2D) islands on the terraces of vicinal surfaces. Growth on these surface is otherwise dominated by step flow growth. The growth (by step flow) and collision of this 2D island with a normal step extend the APBs in a direction inclined to the normal growth direction.

Transmission Electron Microscopy and Photoluminescence Characterization of InAs Quantum Wires on Vicinal GaAs(110) Substrates by Molecular Beam Epitaxy

InAs quantum wires have been naturally formed on vicinal GaAs(110) surfaces with giant steps by molecular beam epitaxy. Transmission electron microscope observations reveal the high density of stacking faults and the moire fringe for thick wires (>4 ML) due to the large InAs–GaAs lattice mismatch. From the moire fringe separation, thick InAs wires are found to be completely relaxed. No defect is observed for thin wires (<4 ML). Strongly polarized photoluminescence (PL) observed for InAs wires confirms the carrier confinement to the quantum wire structures and the strain effect induced by the large lattice mismatch. A considerable energy shift is observed in the excitation intensity dependence of PL, indicating large nonuniformity.

Influence of (001) vicinal GaAs substrates on the optical properties of defects in low-temperature grown GaAs/AlGaAs multiple quantum wells

Photoluminescence (PL) spectroscopy and carrier lifetime measurement has been used to characterize optical properties of defects in the low-temperature (LT) grown GaAs/AlGaAs multiple quantum well structures. Two sets of samples were grown at 400 °C by molecular beam epitaxy on nominal (001) and miscut [4° off (001) towards (111) A] GaAs substrates, respectively. After growth, samples were subjected to 30 s rapid thermal annealing at 600–800 °C. It is found that after annealing, two defect-related PL features appear in the samples grown on nominal (001) GaAs substrates, but not in those grown on miscut (001) GaAs substrates. The carrier lifetimes are about 31 and 5 ps in as-grown samples grown on nominal and miscut (001) GaAs substrates, respectively. The different PL spectra and carrier lifetimes in two sets of samples are attributed to different structures of the AsGa-like defects formed during LT growth.

Photoluminescence of modulation doped GaAs quantum wires on vicinal GaAs (110) substrates

Modulation-doped GaAs quantum wires are grown on the surfaces with giant steps which are naturally formed on vicinal GaAs (110) substrates by molecular beam epitaxy (MBE). It is found that photoluminescence (PL) peaks shift to lower energy with increasing doping level and to higher energy with increasing excitation intensity. These results indicate that the GaAs quantum wires are successfully modulation-doped.

Self-organization of quantum dots in multilayer InAs/GaAs and InGaAs/GaAs structures in submonolayer epitaxy

The experimental results of RHEED and scanning tunneling microscopy investigations of multilayer structures of InGaAs/GaAs quantum dots, obtained by submonolayer epitaxy on singular and vicinal GaAS (100) substrates, are reported. The results presented show that spatial ordering of nano-objects exists in multilayer structures for InAs and heteroepitaxial InGaAs layers.

Formation of the charge balanced ZnSe/GaAs(1 1 0) interfaces by molecular beam epitaxy

We have investigated ZnSe layers grown by molecular beam epitaxy on vicinal GaAs(1 1 0) substrates misoriented 6° towards (1 1 1)B using reflection high-energy electron diffraction (RHEED), transmission electron microscopy, and high-resolution X-ray diffraction. The RHEED observations reveal that ZnSe growth proceeds in a layer-by-layer fashion on the vicinal GaAs(1 1 0) surfaces and the surfaces consist of regular arrays of monoatomic steps. However, characteristic structures of stacking faults and high density of threading dislocations were observed. We successfully decreased these stacking faults and threading dislocations lower than two or three orders by incorporating GaAs buffer layers. ZnSe(1 1 0) films with high-quality and flat interfaces are coherently grown on vicinal GaAs(1 1 0) surfaces misoriented 6° towards (1 1 1)B with GaAs buffer layers.