xPx Layers Research Articles

GaAs/GaAs 0.8 P 0.2 quantum wells grown on (n11)A GaAs substrates by molecular beam epitaxy

We investigated optical properties of GaAs/GaAs1−xPx(x≃0.2) quantum wells (QWs) with each well width (Lw) of 2.7–10.0 nm grown on (100) and (n11)A GaAs substrates (n=3, 4, 5) by molecular beam epitaxy with the use of P2 and As4 molecular beams generated from polycrystalline GaP chunk and solid As sources. It was found that the P contents (x) in GaAs1−xPx layers strongly depend on both the substrate orientation and substrate temperature (Ts). In the whole range of the Ts (535–640 °C), the largest P contents (x=0.19–0.23) were observed in the (411)A GaAs1−xPx layers, and the (100) GaAs1−xPx layers showed the smallest x (0.08–0.18). The full width at half maximum of photoluminescence (PL) peak (λ=758 nm) from the (411)A GaAs/GaAsP QW with Lw=2.7 nm was as small as 5 meV which is only 66% of that of the (100) QW probably due to improved interface flatness of the (411)A QW. Furthermore, the PL intensities of the (411)A and (311)A QWs were more than one order of magnitude larger in intensity than that of the (100) QWs.

Microstructures of GaN1−xPx layers grown on (0001) GaN substrates by gas source molecular beam epitaxy

Transmission electron microscope (TEM), transmission electron diffraction (TED), and synchrotron x-ray diffraction (XRD) studies have been performed to investigate microstructural behavior of gas source molecular beam epitaxial GaN1−xPx layers grown on (0001) GaN/sapphire at temperatures (Tg) in the range 500–760 °C. TEM, TED, and XRD results indicate that the samples grown at Tg⩽600 °C undergo phase separation resulting in a mixture of GaN-rich and GaP-rich GaNP with zinc-blende structure. However, the samples grown at Tg⩾730 °C are found to be binary zinc-blende GaN(P) single crystalline materials. As for the 500 °C layer, the two phases are randomly oriented and distributed, whereas the 600 °C layer consists of phases that are elongated and inclined by 60°–70° clockwise from the [0001]α-GaN direction. The samples grown at Tg⩾730 °C are found to consist of two types of microdomains, namely, GaN(P)I and GaN(P)II; the former having twin relation to the latter.

Chemical beam epitaxial growth of GaAs1−xPx on GaAs (100) substrates

Phosphorus incorporation during chemical beam epitaxial (CBE) growth of GaAs1−xPx from triethylgallium, tertiarybutylarsine, and tertiarybutylphosphine is investigated. Reflection high-energy electron diffraction intensity oscillations are used to measure the phosphorus incorporation during the As-limited and the (As+P)-limited growth on a Ga-rich surface. The resulting phosphorus mole fraction is compared with the phosphorus composition measured by x-ray rocking curves on the strained GaAs1−xPx layers grown by conventional CBE with a simultaneous supply of group V and group III elements. The phosphorus incorporation rate during CBE growth is lower than that measured during the group V controlled growth but is still much higher than the incorporation rate reported for molecular beam epitaxy growth using elemental sources.

Effect of strain and temperature on anomalously large interdiffusion in InAsP/InP heterostructures

We present evidence for anomalously large, strain-dependent interdiffusion in InAs1−xPx layers grown on InP(001) substrates by organometallic vapor phase epitaxy at 620 °C. Specifically, there are strong indications for the existence of a “critical strain:” if the strain is ∼1.9% or more, much P–As mixing occurs, but for smaller strain the mixing is greatly decreased. The interdiffusion is also highly sensitive to temperature. A set of samples grown at 580 °C exhibits a factor of ∼2 decrease in P–As mixing compared to a set grown at 620 °C.

Manifold arsenic and phosphorus effusion source for GaAsP alloys

We describe an independently valved solid arsenic and phosphorus effusion source for molecular beam epitaxy. A common posteffusion hot zone and orifice ensure constant flux ratios of arsenic, phosphorus, and mixed species in the beam. With this source, we have demonstrated molecular beam epitaxial growth of GaAs1−xPx layers with better than ±0.4% compositional variation over a 3-in.-diam substrate.

Atomic layer molecular beam epitaxy growth of GaAs1−xPx layers: Study of P2 incorporation by the reflectance difference technique

A study of the P2 incorporation process onto the GaAs1−xPx(001) surface by the reflectance difference technique is presented. The growth technique is atomic layer molecular beam epitaxy (ALMBE), using successive pulses of P2 and As4 on a Ga rich intermediate surface. Experimental results on the dependence of the efficiency of phosphorus incorporation on P2 dosage and Ga surface coverage show that the saturation of P2 incorporation is limited by the Ga surface coverage and that the efficiency of phosphorus incorporation is larger for a Ga surface coverage above 0.12Ns, where Ns=6.25×1014 sites cm−2 for an ideal (001) surface. These results allow one to establish the appropriate experimental conditions to grow GaAs1−xPx layers by ALMBE with accurate control of the phosphorus content.

Determination of the deformation potentials of GaAs0.80P0.20

Deformation potentials can be determined by measuring the variation of the energy of the electronic transitions with strain. In this work, the hydrostatic and shear potentials of the band-gap electronic transition (E0) and the transitions along the 〈111〉 direction (E1) of GaAs1−xPx, x≊0.20, have been determined by electroreflectance characterization of GaAs1−xPx layers with different levels of strain.

Use of surface grooves to control ordering in GaAsP

GaAs1−xPx with 0.4≤x≤0.85 forms the CuPt ordered structure during organometallic vapor phase epitaxy (OMVPE). Only the (1̄11) and (11̄1) variants are observed for growth on (001)-oriented substrates. The mechanism by which ordering occurs is only now being discovered. Total energy calculations, including the effects of surface reconstruction, indicate that the phenomenon can be explained purely on the basis of energy considerations. Indirect evidence indicates that kinetic factors, including processes occurring at steps propagating across the surface in the two-dimensional growth mode, control ordering. In this work, GaAs1−xPx layers have been grown by OMVPE on (001)-oriented GaAs0.6P0.4‘‘substrates.’’ In order to examine the effects of surface kinetic factors, the substrates were first patterned with [110]-oriented grooves 5 μm wide and a fraction of a micron deep. The groove is used to provide a source of steps moving in opposite directions from the two edges. Transmission electron diffraction reveals the formation of large domains of the two variants that meet in the center of the groove. A surprising feature is the presence of a region in the groove with absolutely no ordering. Tracing the surface shape during growth using a superlattice structure indicates that the disordered region is due to growth on {511} facets. The domains formed after the groove is filled are very large, several square microns in cross-sectional area and extending along the entire length of the groove. These results demonstrate that natural ordering in GaAsP, an alloy with mixing on the group V sublattice, can be controlled by regulating the propagation of steps during growth, exactly as for GaInP where mixing is on the group III sublattice.

Vacancy defects in as‐grown and nº‐irradiated GaP and GaAs1−xPx Studied by Positrons

AbstractMeasurements of positron lifetime τ and of the shape parameter S of the Doppler‐broadened annihilation line are used to study bulk and defect properties in GaP and GaAs1−xPx. τ and S decrease linearly with the composition x of GaAs1−xPx layers. A second lifetime component (τ2 = 290 ps) observed in as‐grown GaP is attributed to stoichiometric P vacancies. After neutron irradiation of GaP and GaAs0.13P0.87 positrons are trapped by Ga vacancies (τ2 = 250 ps). These vacancies anneal out in two stages at 200–550 °C and 550–800 °C.

Experimental Determination and Thermodynamic Calcultions of the Relationship between Vapour Phase Composition and Mixed Crystal Composition in the Ga‐HCI‐PH3‐AsH3‐(NH3)‐H2 System

AbstractThe relationship between the composition of GaAs1–xPx layers and the composition of the vapour phase has been investigated experimentally and theoretically. Using KIRWAN'Sequilibrium constants excellent agreement between calculated and experimental data is obtained over the whole composition range. Different models and parameters for the calculation of the soild solution activity coefficients result in only small differences for the calculated alloy composition so that no decision can be made from the experiments for one model to be preferred. Increasing addition of NH3 to the vapour phase up to concentrations exceeding those of the crystal forming gases by an order of magnitude has no influence on the alloy composition, although the nitrogen concentration in the layers increases as deduced from the shift of the wavelength of the maximum photoluminescence intensity.

Regular compositional steps generated in GaAs1−xPx VPE layers

For GaAs VPE layers grown on Ge substrates, it is found that a one-dimensional misfit-dislocation density 1/p is proportional to lattice constant mismatch, just as for compositionally graded GaAs1−xPx layers grown on GaAs substrates. On the basis of these experimental results, lattice constant variation due to compositional change is given in a form explicitly including a compositional step. The compositional step width, calculated from this relationship and observed dislocation density, is about 500 atomic layers. Regular compositional steps, which are unintentionally formed, are actually observed on A-B etched cleavage planes of the graded layers. The observed width of this step is about 0.3 μm. This value roughly agrees with the calculated one. Presumably, the discontinuous change in composition is caused by competition between the following two processes which alternately dominate GaAs1−x Px epitaxial growth: a free-energy increase due to a decrease in arsenic supersaturation in vapor phase and a free-energy increase due to an increase in formation enthalpy of the crystal. Furthermore, in final compositional layers equidistant striae about 3.5 μm wide are observed on A-B etched cleavage planes. The origin of these striae is considered to be misfit dislocations which are generated in graded layers and climb as a result of association with Ga vacancies. Some of the striae are regularly bent in the direction perpendicular to the growth axis by shear stress due to sample bending.

On-time determination of the composition of III-V ternary layers during VPE growth

A fast automatic ellipsometer has been used for determining the composition (and the rate of growth in favorable cases) of III-V ternary compound semiconductor layers during their growth in a VPE reactor. The technique is demonstrated on Ga1−xAlxAs and GaAs1−xPx layers grown respectively in an organometallic and a chloride system. The elllipsometric angle ψ is shown to be directly related to the composition and the rate of growth of the layer. The ellipsometric angle Δ contains additional information on the growth mechanisms at the surface of the layer.

Room temperature minority carrier lifetime and efficiency of p-type GaAs 1— xP x

Abstract Using sine wave modulated laser excitation, the optical phase shift of the luminescence of vapor grown Zn doped GaAs1−xPx layers was measured. From such measurements the minority carrier lifetime can be derived. This lifetime was measured as a function of doping level for crystals with a composition sufficiently remote from the cross-over composition (0.30 1019 cm-3) may be depressed by self-absorption of the luminescence. In addition, a few samples with a higher phosphorus content, up to x = 0.45, were measured. Their lifetimes being mainly determined by indirect transitions are also in the low nanosecond range. This stands in contrast with the longer lifetimes (up to 150 ns at p

Characterization of GaAs substrates and epitaxial GaAs1−xPx layers by divergent x-ray beam diffraction

The application of the divergent x-ray beam diffraction method was studied for characterizing lattice imperfections, lattice parameters and composition variations and lattice strain in GaAs substrates and in GaAs1−xPx epitaxial layers. Reflection conics from {117}, {026}, {155} and (004) planes predominate in pseudo-Kossel back reflection patterns obtained from samples with (001) orientations. The sensitivity of pseudo-Kossel line displacements is assessed for lattice parameter and anisotropic strain distortion measurements. The lattice parameter of GaAs was determined to be 5.6435 ∢. Moseic subgrain misorientations in GaAs1−xPx epitaxial layers were found to be relatively independent of graded layer composition gradients, whereas homogeneous stress distortion was more strongly dependent.

Isoelectronic trap transitions in GaAs1−xPx:N in the region of resonant enhancement (ENN ∼ EΓ, 0.3<x<0.4)

For GaAs1−xPx:N crystal composition x < 0.8, the distinction between NN1 and NN3 transitions becomes blurred. Photoluminescence measurements on N-doped GaAs1−xPx layers (0.3 < x < 0.4) and, for comparison, on the same vapor epitaxial crystals with the N-doped layer removed, indicate that the NN3 transition reappears (and thus also the NN1 transition) as ENN3 approaches EΓ. This is attributed to the resonant enhancement that occurs when ENN ∼ EΓ.