Vicinal InP Research Articles

Thermal transfer and interaction mechanisms of localized excitons in families of InAs quantum dashes grown on InP(001) vicinal substrate emitting near 1.55 μm wavelength

With the help of photoluminescence Spectroscopy (PLS), we have investigated the optoelectronic properties of two different families of InAs quantum dashes (QDashes) grown on misoriented InP(001) substrate with 2[Formula: see text]off miscut angle toward the [110] direction (2[Formula: see text]F type). The lowest full width at half maximum (FWHM) of the PL spectrum measured at 12 K indicates the good self organization of InAs QDashes. The weak ratio of the integrated PL measured in 12–300 K temperature range denotes the good spatial confinement of the photogenerated carriers in InAs QDashes. The fast redshift of the PL peaks energy and the anomalous decrease of the FWHM with the increase of the temperature are attributed to an efficient thermal relaxation process of photogenerated carriers in the vicinal sample. This result is highlighted with the help of theoretical modeling of the PL peak energy as a function of the temperature, using three models (Varshni, “Vina, Logothetidis and Cardona” and Pässler). From experimental and theoretical results, we have evidenced the contribution of longitudinal acoustic-phonons (LA-phonons) in the PL of InAs/InP QDashes, via the deformation potential, especially in high temperatures range. We have attributed this behavior to the strained InAs/InP QDashes and/or to the topography of the vicinal InP(001) substrate which favors the presence of stepped phonons polarized along the steps. These vibrational modes can further interact with the excitons at high temperatures. The measured thermal activation energies of each family of InAs QDashes demonstrate that the InAs wetting layer act as a barrier for the thermoionic emission of photogenerated carriers. This result confirms the good spatial confinement of excitons in this sample.

Formation and reduction of pyramidal hillocks on InGaAs/InP(111)A

The formation and reduction of pyramidal hillocks on InGaAs/InP(111)A grown by metal organic chemical vapor deposition were investigated. The triangular pyramidal hillocks were observed on the InGaAs surface on on-axis InP (111)A grown at 650 °C. The hillocks disappeared when the vicinal InP (111)A substrates with an off-axis angle >0.4° were applied. The step and terrace surface of InGaAs on 4.0°-off InP (111)A were obtained. InGaAs on on-axis InP (111)A exhibited a hole concentration, mobility of Hall effect measurement, and full width at half-maximum of X-ray diffraction as 1.0 × 1017 cm−3, 56 cm2/V s, and 140 arcsec, respectively. In contrast, InGaAs on 2.0°-off InP (111)A exhibited 2.3–1017 cm−3, 142 cm2/V s, and 63 arcsec, respectively. We suggest that off-axis substrates enable a reduction of stacking faults, which leads to considerable improvement of crystallinity.

Growth and kinetic Monte Carlo simulation of InAs quantum wires on vicinal substrates

Self-assembled quantum structures have been successfully grown for some time now but control over their uniformity has proven difficult due to the stochastic nature of surface diffusion. We have investigated the effect of vicinal InP(001) substrates on the uniformity of InAs quantum wires grown on InGaAlAs lattice-matched to InP using molecular beam epitaxy. Dense quantum wires were grown on both nominally flat and vicinal substrates off-cut by 0.9° toward the [110] direction for comparison. The off-cut angle was chosen to provide terraces which match the orientation and spacing of wires grown on nominally flat substrates. A modest but statistically significant improvement in the size distribution of the wires was observed on vicinal substrates through the analysis of ultrahigh resolution scanning electron micrographs. The interface between the wires and the off-cut substrate was studied using cross-sectional high resolution scanning transmission electron microscopy. In addition, a kinetic Monte Carlo model of epitaxial growth including full strain calculations was developed to further investigate the nucleation process. Using an anisotropic bond model to account for the surface energy of different crystallographic facets, our simulations produced wires similar to those observed experimentally while demonstrating the importance of anisotropic bonding compared to anisotropic diffusion. Growth on vicinal substrates is also simulated here and indicates that off-cut substrates should indeed improve the size distribution of quantum wires under proper growth conditions.

Optical investigation of InAs quantum dashes grown on InP(0 0 1) vicinal substrate

We investigate with photoluminescence (PL) measurements the optoelectronic properties of self-organized InAs quantum dots (QDs) grown on nominal InP(001) substrate. InAs/InP(001) QDs are grown by Molecular Beam Epitaxy (MBE) method with optimized conditions in Stranski-Krastanov regime. A lateral coupling behavior was shown by photoluminescence spectroscopy. This phenomena is considered as a degradation source of the optoelectronic properties of InAs/InP(001) QDs used in lasers applications. In order to overcome this disadvantage behavior, we have studied the optical properties of InAs quantum islands (QIs) grown on vicinal InP(001) with 2° off miscut angle toward the [110] direction. From Polarized Photoluminescence (PPL) measurements, we have deduced that InAs quantum nanostructures have quantum dashes (QDas) form elongated in [1–10] direction. From excitation density PL measurements, we have evidenced that the different observed PL peaks are attributed to the emission of InAs QDas of different size. The lateral coupling behavior is completely eliminated in the case of this sample. The temperature-dependent PL measurements show a good thermal stability and an emission wavelength at room temperature around 1.55μm of the vicinal sample. All these properties prove that this sample possess favorable characteristics for microlasers based devices functioning at room temperature and for optical telecommunication with long range weapon. The broad emission range observed at 300K of the vicinal sample gives the possibility to use it as an active zone in solar cells and in infrared photodectectors of high optical gain and excellent sensitivity on a wide energy range.

Formation of InAs 1−xSb x quantum dots on vicinal InP(0 0 1) for 1.55-μm DFB laser applications

We have investigated the MOVPE growth of InAs 1− x Sb x quantum dots (QDs) on vicinal InP(0 0 1) substrate for DFB laser applications. Miscut orientation was adopted toward the [1 1¯ 0] direction in which InAsSb quantum wires elongate on normal InP(0 0 1) substrate. The interval between the QDs and the lateral size of the QDs in the [1 1 0] direction decreased as the Sb fraction was increased, which brought the density to as high as 1.1×10 11 cm −2 of InAsSb QDs. The InAsSb QDs were stacked with 30-nm-thick InGaAsP spacer layers for 10 periods without correlation, and had an emission of a wavelength at around 1.55 μm although the segregation of Sb atoms was slightly observed. A pn-buried-heterostructure-type DFB laser with InAsSb-QD active layer was fabricated. The lasing wavelength was 1540.6 nm at 25 °C, and CW, single-mode lasing operation was demonstrated up to 100 °C.

Controlling shape of InAs 1−xSb x quantum structures on InP for quantum dots with 1.55-μm emission

We have investigated the effects of changing Sb fractions and misorienting InP(0 0 1) substrates on InAs 1− x Sb x quantum structures grown by metal-organic vapor-phase epitaxy (MOVPE), with the aim of obtaining small quantum dots (QDs) with 1.55-μm emission. At medium Sb content, InAsSb tended to form quantum wires (QWRs) with widths of less than 20 nm along the [ 1 1 ¯ 0 ] direction on InP(0 0 1), which was due to the surfactant effect. The [ 1 1 ¯ 0 ] size of these narrow InAsSb QWRs decreased as the miscut angles toward the [ 1 1 ¯ 0 ] direction of the vicinal InP(0 0 1) substrates was increased, and the InAsSb shapes changed from QWRs into QDs. Nearly 1.55-μm-emitting InAs 0.61Sb 0.39 QDs with lateral sizes of 20–30 nm embedded in InGaAsP barriers were fabricated on a vicinal substrate with a miscut of 5°, which will enable InP-based QD-telecom device applications.

Self-organized growth of InAs quantum wires and dots on InP(001): The role of vicinal substrates

We have studied the self-organized growth of InAs nanostructures in an InP matrix by gas-source molecular-beam epitaxy on both nominally oriented and vicinal InP(001). Atomic force microscopy and low-temperature photoluminescence demonstrate that the off-cut direction of vicinal substrates—largely independent of growth conditions—determines the morphology of nanostructures, that is, quantum dot, quantum wire, or two-dimensional growth; whereas, on nominally oriented substrates, the morphology is very strongly dependent on the growth conditions.

Atomic hydrogen cleaning of InP(100): Electron yield and surface morphology of negative electron affinity activated surfaces

Atomic hydrogen cleaning of the InP(100) surface has been investigated using quantitative reflection high-energy electron diffraction. The quantum efficiency of the surface when activated to negative electron affinity was correlated with surface morphology. The electron diffraction patterns showed that hydrogen cleaning is effective in removing surface contaminants, leaving a clean, ordered, and (2×4)-reconstructed surface. After activation to negative electron affinity, a quantum efficiency of ∼6% was produced in response to photoactivation at 632 nm. Secondary electron emission from the hydrogen-cleaned InP(100)-(2×4) surface was measured and correlated to the quantum efficiency. The morphology of the vicinal InP(100) surface was investigated using electron diffraction. The average terrace width and adatom-vacancy density were measured from the (00) specular beam at the out-of-phase condition. With hydrogen cleaning time, there was some reduction in the average terrace width. The surface quality was improved with hydrogen cleaning, as indicated by the increased (00) spot intensity-to-background ratio at the out-of-phase condition, and improved quantum efficiency after activation to negative electron affinity.

Effects of vicinal InP (001) surface on InAs dots grown by droplet hetero-epitaxy

We have systematically investigated the effects of InP substrate misorientation on the InAs dot formation by droplet hetero-epitaxy. The density and uniformity of the dots were investigated in the misorientation range between 0 and 4° from (001) toward [100]. Using 2°-off substrate, the density had the highest value of 2.2×10 10 cm −2 and the uniformity was most improved, where average dot height and base diameter were 8.1 and 51 nm. Further, we found that the total volume of the dots increased with misorientation in spite of the other growth conditions remaining unchanged.

Surface morphology of InGaAs and InP materials grown with trimethylarsenic and arsine on vicinal InP substrates

In this study, we report on an investigation using atomic force microscope (AFM) to study lattice-matched InGaAs surface layers grown by MOVPE with two different sources of group V elements: trimethylarsenic (TMAs) and arsine. The growth was performed with a home made apparatus with a T shaped reactor. The substrate temperature ranged from 570 to 630°C under pure hydrogen or a mixture of hydrogen and nitrogen as carrier gas. After thermal annealing under PH 3 and growth of InP buffer layer, we could clearly observe the step/terrace like feature of the vicinal surface morphology for 0.2° misoriented substrates. For epilayers grown under nitrogen flow, step bunching was observed with terraces as large as 170 nm which is twice larger than the nominal value (80 nm). Lattice-matched InGaAs/InP epilayers were grown with arsine and TMAs. For TMAs-grown epilayers, we observed that the roughness depends on V/III ratio and growth temperature. The comparison of the surface morphology of TMAs and arsine-based epilayers assessed by AFM characterisation will be presented and discussed in terms of As-carbon adsorbed species.

Behaviour of vicinal InP surfaces grown by MOVPE: exploitation of AFM images

InP substrates and epilayers grown by MOVPE have been studied by AFM. For different misorientation angles, we observed the surface of the substrate after annealing under phosphine (PH3) and of the epilayers under different growth conditions such as growth temperature Tg and trimethylindium (TMI) partial pressure. After annealing the terrace width corresponds to the nominal value of misorientation angle measured by X-ray diffraction. We observed different topographies and roughnesses for the grown layers corresponding to different growth modes. We propose, taking into account the roughness of the surface, a calculation of the step height and terrace width. For 2D nucleation (θ ≤ 0.2° and Tg = 500°C) and step flow mode, the roughness is low while it is increased by step bunching (θ ≥ 0.5° and Tg ≥ 580°C). Moreover we have examined the surface morphology for different misorientation angles and determined the influence of growth conditions (growth temperature, indium partial pressure) on the growth mechanism. At Tg = 580°C the increase of the TMI partial pressure in the reactor enhances the step bunching and leads to larger terraces.

Microstructural evolution during epitaxial growth of Ag on vicinal InP(100) surfaces

The initial stages of epitaxial growth of Ag on InP(100) have been studied using in situ and ex situ electron microscopy. Vicinal InP substrates were cleaned by heating to about 400 °C in ultrahigh vacuum. Silver was deposited at substrate temperatures between 350 and 500 °C. The microstructural evolution was monitored for coverage between 0.5 and 8 monolayers using in situ high spatial resolution secondary electron microscopy and ex situ plan-view transmission electron microscopy (TEM). At sub-monolayer coverage, uniform, faceted islands of Ag are formed with edges aligned along InP 〈110〉 directions. Continued deposition leads to growth of preexisting islands followed by coalescence. During coalescence, a reduction in the island height, an increase in island-substrate interfacial area, and a disappearance of faceting are observed. At high coverage, during high temperature (450 °C) growth, the formation of faceted pits are observed. For higher temperatures (500 °C) and at low coverage, islands transform from square to rectangular shapes. Plan-view TEM indicates that the smallest Ag islands (∼5 nm) are not coherent with the substrate and that there are two distinct epitaxial relationships between the Ag islands and the InP substrate. For high-temperature growth, the (110)Ag plane is parallel to InP(100) substrate and there is evidence for chemical reaction and presence of In on the surface. For lower temperature depositions (350–400 °C), the (100)Ag is parallel to (100) InP.

Microstructural Evolution During the Epitaxial Growth of Ag/InP (100)

The initial stages of epitaxial growth of Ag on InP(100) has been studied using in-situ and ex-situ electron microscopy techniques. Vicinal InP substrates were cleaned by heating to about 450°C in ultra-high vacuum. Silver was subsequently deposited at substrate temperatures between 350–500°C. The microstructural evolution was monitored for coverages between 0.5–8ML using high spatial resolution secondary electron microscopy and transmission electron microscopy. At sub-monolayer coverages, square islands of Ag are formed with edges aligned along InP directions. Subsequent deposition leads to secondary nucleation or growth of pre-existing islands, depending on the substrate temperature. Surfaces with a uniform distribution of steps have a higher island density than those without steps but the steps are not preferred sites for island nucleation. Coalescence of islands begins at a smaller coverage for lower temperatures and for stepped surfaces. At the early stages of coalescence, island shapes with low symmetry are formed, with diminished island heights. These subsequently transform into shapes of higher symmetry. Except at the highest temperatures studied (500°C), island shape transformations (e.g. from square to elongated) are not very significant. Plan view transmission electron microscopy shows that the islands are epitaxially aligned and that even the smallest islands are generally incoherent. Island size distributions are used to analyze the growth mechanisms.

Mobility anisotropy and magnetoresistance at an (InAs)1(GaAs)1–InP heterointerface grown on a (001) vicinal InP substrate

Lateral monolayer superlattices (LMSLs) of (InAs)1(GaAs)1 were successfully grown on (001) vicinal InP substrates by metal-organic chemical vapor deposition. The formation of a periodic structure in the lateral [110] direction was confirmed by x-ray diffraction. In an electronic measurement using (InAs)1(GaAs)1 LMSL–InP heterojunctions, an in-plane mobility anisotropy as well as a modified two-dimensional subband structure were observed. The latter is that at least two Shubnikov–de Haas fundamental frequencies are always observed even in the sample having a sufficiently low sheet electron density. Possible origins including a formation of a lateral periodic potential were discussed with respect to the results obtained.