xP Layers Research Articles

Effect of alloy composition on structural, optical and morphological properties and electrical characteristics of GaxIn1−xP/GaAs structure

The structural, optical and morphological properties of Ga-rich GaxIn1−xP layers with various gallium compositions grown on epi-ready semi-insulating (100)-oriented GaAs substrates by using Molecular Beam Epitaxy technique are presented in this study. The GaxIn1−xP/GaAs structures (S1, S2 and S3) have been evaluated by means of high resolution X-ray diffraction, photoluminescence (PL) and atomic force microscopy measurements at room temperature. Experimental forward and reverse bias current–voltage (I–V) characteristics of structure S3 was investigated at room temperature due to its better characteristics when compared to the other two samples. The main electrical parameters such as ideality factor (n), barrier height (Φ b ) and series resistance (R s ) were extracted from forward bias I–V characteristics and Cheung’s function. In addition, Hall measurements were carried out as a function of temperature (30–300 K) and at a magnetic field of 0.4 T were presented for structure S3.

Band structure properties of novel BxGa1−xP alloys for silicon integration

We have grown and investigated the band-structure properties of novel III-V alloys based upon BxGa1−xP. These layers are utilized as strain-compensating layers for the lattice-matched integration of novel direct bandgap Ga(NAsP) quantum well lasers on silicon. Experimental and theoretical studies reveal the dependence of the direct and indirect band gaps for strained BxGa1−xP layers grown on silicon as a function of Boron composition from which we derive the properties of free-standing BxGa1−xP. For Boron fractions up to 6%, we find that the bowing parameter for the lowest (indirect) band gap is − 6.2 ± 0.2 eV. High crystalline quality and promising optical material properties are demonstrated and applied to monolithically integrated Ga(NAsP)/(BGa)P multi-quantum well heterostructures on (001) silicon substrates. Our results show that novel (BGa)P layers are suitable for strain compensation purposes, which pave the way towards a commercial solution for the monolithic integration of long term stable laser diodes on silicon substrates.

Vibrational mode and dielectric function spectra of BGaP probed by Raman scattering and spectroscopic ellipsometry

The influence of boron incorporation in BxGa1−xP (0 < x ≤ 6%) layers on the dielectric function spectrum and on the vibrational mode spectrum has been studied. BGaP layers were grown by metal-organic vapor phase epitaxy, using a GaP interlayer, on Si and, for reference purposes, also on GaP substrates. The boron content of the layers was determined by high-resolution x-ray diffraction. Two vibrational modes arising from the two boron isotopes 10BGa and 11BGa were observed in the Raman spectrum, increasing in scattering strength and shifting to higher frequencies with increasing boron content at a rate of (1.40 ± 0.20) cm−1/% and (1.41 ± 0.14) cm−1/% for the 10BGa and 11BGa modes, respectively. Spectroscopic ellipsometry was used to asses the pseudodielectric function of the BxGa1−xP layers. The main effect of boron incorporation is a strong broadening of the E1 interband transition with increasing boron content due to alloy disorder and inhomogeneous random strains introduced by the large size mismatch of boron. However, upon the addition of arsenic to BGaP, i.e., forming BxGa1−xAsyP1−y with y ≈ 10%, a clear low-energy shift of the E1 interband transition is resolved. A small low-energy shift (≈ 20 meV) of the lowest direct bandgap has also been observed for BxGa1−xP layers with x ≤ 0.7% grown on GaP substrates.

Composition modulation analysis of In xGa 1− xP layers grown on (0 0 1) germanium substrates

The development of new photovoltaic approach to improve costs and efficiencies is focused on the new materials and new technologies. InGaP is, in this sense, a key material for solar conversion. In particular, in the solar concentration approach, this material is part of multiple junction solar cells. Its low lattice mismatch with germanium and its adequate bandgap make it very promising. This paper shows how compositional modulation can affect the InGaP emitter and the AlGaAs tunnel junctions. The influence of the growth conditions, on the compositional modulation and misfit and threading dislocations, in In 0.49Ga 0.51P layers is demonstrated by TEM on purposely grown single InGaP layers. High resolution electron microscopy (HREM) intensity profiles showed no elastic lattice related modulation.

CuPt ordering in high bandgap GaxIn1−xP alloys on relaxed GaAsP step grades

We have fabricated a series of GaxIn1−xP samples over the compositional range 0.51<x<0.76 on GaAs substrates. The samples were prepared by first growing a thick step-graded layer of GaAs1−yPy to bridge the lattice misfit between the GaxIn1−xP layers and the GaAs substrate. The order parameter was tuned using a dilute antimony surfactant during growth. The composition, strain, and order parameter of each sample were characterized by x-ray diffraction, and the bandgap was measured by photoluminescence. We find good agreement between the experimentally measured bandgaps and theoretically modeled curves.

Investigation of graded InxGa1−xP buffer by Raman scattering method

The compositional changes of InxGa1−xP graded buffer inserted between GaP substrate and subsequently grown In0.36Ga0.64P homojunction LED structure were investigated by Raman spectroscopy. The indium content of InxGa1−xP interlayers was increased in eight steps with thickness of 300nm and constant compositional change ΔxIn between the steps. The properties of InxGa1−xP graded buffer along the structure cross-section have been studied by Raman back scattering method and the changes in GaP LO and TO phonons were investigated. Raman shift of 13cm−1 in GaP-like LO1 phonon was measured on beveled [100]surface for compositional change of InxGa1−xP layer in the range of 0<xIn<0.32. The measurements on the cleaved edge of the sample in [011] direction revealed a strong TO phonon at 366cm−1 and weak LO phonon peak at 405cm−1 in GaP substrate. By reaching the graded InxGa1−xP region the intensity of TO phonon decreases and appearance of considerable TO1 phonon shift up to 350cm−1 for In content xIn=0.16was observed. For upper graded layers with xIn from 0.16 to 0.24 the position of GaP-like TO1 was constant and can be ascribed to relaxation of lattice mismatched thin InxGa1−xP graded upper layers in the structure.

Effect of rapid thermal annealing on the ordering of AlInP grown by metal-organic vapor-phase epitaxy

Spontaneous CuPt ordering is observed in AlxIn1−xP layers grown by metal-organic vapor-phase epitaxy in pure nitrogen ambient with tertiarybutylphosphine as phosphorus precursor. Changes of the degree of ordering of the AlxIn1−xP epilayer versus annealing temperature have been investigated by using photoluminescence and transmission electron diffraction. The degree of ordering of the AlxIn1−xP layers is reduced after annealing the sample at a temperature higher than the sample growth temperature. It becomes completely disordered when the annealing temperature reaches 900°C and above.

Effect of strain-compensation in stacked 1.3μm InAs∕GaAs quantum dot active regions grown by metalorganic chemical vapor deposition

We have introduced tensile layers embedded in a GaAs matrix to compensate compressive strain in stacked 1.3μm InAs quantum dot (QD) active regions. The effects of the strain compensation are systematically investigated in five-stack and ten-stack QD structures where we have inserted InxGa1−xP (x=0.30 or 0.36) layers. High-resolution x-ray diffraction spectra quantify the overall strain in each sample and indicate &amp;gt;35% strain reduction can be accomplished. Both atomic force and transmission electron microscope images confirm that strain compensation improves material crystallinity and QD uniformity. With aggressive strain compensation, room temperature QD photoluminescence intensity is significantly increased demonstrating a reduced defect density.

Ferromagnetic semiconductors based upon AlGaP

Ion implantation of Mn or Cr at concentrations of 1–5 at. % were performed in AlxGa1−xP (x=0.24,0.38) epilayers grown by gas source molecular beam epitaxy. Ferromagnetic-like ordering above 100 K for Cr and 300 K for Mn was observed in superconducting quantum interference device measurements. Structural characterization revealed no second phases that could influence the measured magnetic properties. As the AIP mole fraction in the AlxGa1−xP layers increased, the magnetic ordering temperatures were generally observed to increase, while the calculated magnetic moment decreased. Mn appears to be a more promising choice than Cr for high temperature ferromagnetism in AlGaP.

Some evidences of ordering in InGaP layers grown by liquid phase epitaxy

We make a comparative study of the optical and structural characteristics of two InxGa1−xP (x≈0.5) films nearly lattice matched to GaAs, here referred to as sample S1 and sample S2. The films were grown by liquid phase epitaxy (LPE). Photoluminescence (PL) measurements were performed in a wide temperature and exciting power density range for different polarizations (parallel to the [011] and [01̄1] directions) of the emitted radiation. Observations suggest that the InxGa1−xP layer in one of the samples has the usual characteristics commonly obtained in an LPE grown material, while in the other sample, an anomalous growth took place. Our experimental observations show differences in the layer thickness, the surface morphology, as well as in the structural and optical characteristics such as the 4K PL energy peak position.

Strain relief by long line defects in tensile GaxIn1−xP layers grown on InP substrates

Fully strained layers of GaInP on InP significantly thicker than the theoretical critical thickness were grown by metalorganic molecular beam epitaxy. The excess strain energy of these layers is most probably accommodated by long line defects observed by atomic force microscopy. The thickness at which the long line defects appear is about four times the Matthews and Blakeslee critical thickness [J. W. Matthews and A. E. Blakeslee, J. Cryst. Growth 27, 118 (1974)]. Partial relaxation is measured by x-ray at about 14 times the theoretical critical thickness.

Growth of strained Ga 1− xIn xP layers on GaP (0 0 1) by gas source molecular beam epitaxy: similarities and differences with the growth of strained arsenides

Using reflection high energy electron diffraction and atomic force microscopy, the growth of Ga 1− x In x P alloys on GaP (0 0 1) with x varying from 0.2 to 1 (InP) is investigated and compared to that of arsenides on GaAs (0 0 1) or InP (0 0 1). At 520°C, the evolution of the critical thickness for 3D growth versus In content is rather similar to that observed in the GaInAs/GaAs system. For x⩽0.5, 3D growth leads to the development of wire-like structures along the [1 1 0] direction which can be related to recent results on the phosphide surface reconstructions. Finally, for the growth of InP on GaP at 520°C, the critical thickness is 2.1 MLs and we observe a small density of very large islands, in contrast to the InAs/GaAs case. At 400°C, the critical thickness decreases (1.7 MLs) as well as the island mean size whereas the density increases. We discuss this behavior in terms of surface energy.

Micro-Raman Studies of AlxGa1—xP/GaP Graded Structures

We report applications of the micro-Raman technique to study the compositional dependence of phonon modes in graded AlxGa1—xP layers. The dependence of the phonon frequencies on the Al content was monitored in a single sample for two different crystallographic orientations. The measured compositional dependence of the LO and TO phonon frequencies are in good agreement with results of calculations based on the Modified Random Element Isodisplacement (MREI) model. The Raman spectra of the samples reveal also the existence of other features due to disorder.

Electrical and morphological properties of ordered In xGa 1−xP

The electrical uniformity of ordered In xGa 1−xP epitaxial layers prepared by a low-pressure MOCVD technique was studied. Resistivity measurements using a four-point-probe method showed that samples with a low misfit value (0 up to−1.5(10 −3) were electrically uniform. For samples with higher misfit the anisotropy of resistivity markedly increased up to a maximum of 460. Disordered samples prepared at a growth temperature outside of the ‘ordering’ interval of growth temperatures were found be electrically uniform with respect to lattice mismatch. Comparing the results obtained from X-ray diffraction, low temperature photoluminescence, and atomic force microscopy experiments, we have shown that lattice mismatch can support the evolution and extension of the ordering effect in the In xGa 1−xP layers.

A gas-source MBE growth study of strained Ga 1− xIn xP layers on GaAs

The growth of strained Ga 1− x In x P alloys on GaAs substrate with the indium composition varying from 0 (GaP) to 1 (InP) was studied by means of reflection high-energy electron diffraction (RHEED). Compressive layers ( x>0.48) exhibit an InGaAs-like behavior. Below 2% misfit strain ( x⩽0.75) plastic relaxation occurs before the 2D–3D growth mode transition for which the lattice parameter relaxation is observed directly on the RHEED pattern. However, growth temperature has not a strong influence on the relaxation behavior as in the InGaAs/InP system. On the other hand, if layers under tensile strain have a normal behavior at low temperature, the critical thickness is found to be independent of the composition for x<0.35 at higher temperature (520°C). This unusual behavior, as compared to strained InGaAs on InP layers for instance, could be associated to a spinodal decomposition of the GaInP alloy.

Diffuse diffracted features and ordered domain structures in GalnP layers grown by organometallic vapor phase epitaxy

Transmission electron diffraction (TED) and transmission electron microscope (TEM) studies have been made of organometallic vapor phase epitaxial GaxIn1−xP layers (x ≈ 0.5) grown at temperatures in the range 570–690°C to investigate ordering and ordered domain structures. TED and TEM examination shows that the size and morphology of ordered domains depend on the growth temperature. The ordered domains change from a fine rod-like shape to a plate-like shape as the growth temperature increases. The domains are of width 0.6∼2 nm and of length 1∼10 nm. Characteristic diffuse features observed in TED patterns are found to depend on the growth temperature. Extensive computer simulations show a direct correlation between the ordered domain structures and such diffuse features. A possible model is suggested to describe the temperature dependence of the ordered domain structure.

Resistivity anisotropy in ordered InxGa1−xP grown at 640 °C

The anisotropy of electrical properties in ordered InxGa1−xP epitaxial layers was studied. These samples were prepared by a low-pressure metalorganic chemical vapor phase epitaxy technique at the growth temperature of 640 °C. Resistivity measurements using a four-point-probe method have shown that samples with a low misfit value (0–1.5×10−3) are electrically uniform. For samples with higher misfit the anisotropy of resistivity markedly increases up to a maximum of 950. Comparing the results obtained from x-ray diffraction, low temperature photoluminescence, and atomic force microscopy experiments, we have shown that lattice mismatch can support the evolution and extension of the ordering effect in the InxGa1−xP layers.

Optical characterization of disordered InxGa1−xP alloys

We present results on the optical characterization of InxGa1−xP layers grown by atomic layer molecular beam epitaxy on GaAs (001) substrates at a growth temperature of 420 °C. Our results show that the optical characteristics of these layers, which do not show ordering effects, are strongly dependent on surface stoichiometry during growth. In this way, we can obtain either highly homogeneous alloys with a predictable band-gap energy or layers with optical properties indicative of spatial localization effects, like an anomalous behavior of photoluminescence peak energy with temperature and a large shift between the emission energy and absorption edge.

Polarization anisotropy of sub-band gap oscillatory features in contactless electroreflectance spectrum of InxGa1−xP layers grown on GaAs (001) substrates

We report the observation of strongly polarization sensitive sub-band gap oscillatory features in the contactless electroreflectance spectrum of InxGa1−xP layers grown on GaAs (001) substrates. At a given energy in the sub-band gap region, the peak strength of these oscillatory features decreases from a positive maximum to a negative minimum passing through zero as the polarization of the incident probe beam is rotated by 90° from [11̄0] direction to [110] direction in the (001) plane. The origin of this phenomenon is explained on the basis of optical interference coupled with linear electro-optic effect induced changes in the sub-band gap refractive index of the InxGa1−xP layers. Numerical simulations based on the above mechanism are shown to reproduce the polarization dependent observations quite well.

Structural and optical properties of GaxIn1−xP layers grown by chemical beam epitaxy

Chemical beam epitaxial (CBE) GaxIn1−xP layers (x≈0.5) grown on (001) GaAs substrates at temperatures ranging from 490 to 580°C have been investigated using transmission electron diffraction (TED), transmission electron microscopy, and photoluminescence (PL). TED examination revealed the presence of diffuse scattering 1/2{111}B positions, indicating the occurrence of typical CuPt-type ordering in the GaInP CBE layers. As the growth temperature decreased from 580 to 490°C, maxima in the intensity of the diffuse scattering moved from ½{111}B to ½{−1+δ,1−δ,0} positions, where δ is a positive value. As the growth temperature increased from 490 to 550°C, the maxima in the diffuse scattering intensity progressively approached positions of \(\frac{1}{2}\{\bar 110\} \), i.e., the value of δ decreased from 0.25 to 0.17. Bandgap reduction (∼45 meV) was observed in the CBE GaInP layers and was attributed to the presence of ordered structures.