Structural and Optical Investigations of Heterostructures Based on AlxGa1−xAsyP1−y:Si Solid Solutions Obtained by MOCVD

We investigated MOCVD epitaxial heterostructures based on AlxGa1−xAs ternary solid solutions, obtained in the range of compositions x ∼ 0.20–0.50 and doped with high concentrations of phosphorus and silicon atoms. Using the methods of highresolution X-ray diffraction, scanning electron microscopy, X-ray microanalysis, Raman spectroscopy, and photoluminescence spectroscopy we have shown that grown epitaxial films represent five-component (AlxGa1−xAs1−yPy)1−zSiz solid solutions. The implementation of silicon in solid solution with a concentration of ∼ 0.01 at.% leads to the formation of the structure with deep levels, DX centers, the occurrence of which fundamentally affects the energy characteristics of received materials.


Introduction
Semiconductor solid solutions based on A III B V compounds provide clear benefits compared with silicon electronics due to a number of properties as follows: the ability to control the width of the band gap by changing the composition, direct band gap, high electron mobility, and so forth.In addition, the devices based on A III B V generate less noise than the same silicon devices.Because of the higher breakdown voltage devices based on A III B V can operate at a higher capacity.All this opens broad range of applications and the effective use of these compounds, from electronics to photovoltaic devices and most forms of optoelectronic components including diodes and solid-state lasers.
The most demanded material for the quantum-well structures is Al x Ga 1−x As solid solutions, with sufficiently close lattice parameters to GaAs, which is used as a substrate; thus Al x Ga 1−x As/GaAs heteropairs possess a minimum density of mismatch dislocations near the heterojunction.Doping Al x Ga 1−x As solid solution with impurity atoms makes it easy to control the type of conductivity and electrical resistance in the heterostructure and allows one to create heterojunctions with various band gaps on the borders.However, Al x Ga 1−x As epitaxial layers have several disadvantages, which include high reaction abilities and reactivity of aluminum atoms with oxygen, which rises with an increasing concentration of aluminum atoms in the metal sublattice.In addition,  ∼ 0.30 compositions correspond to a high density of deep levels and surface states.Another very important fact is that the complete match in crystal lattices parameters is impossible in Al x Ga 1−x As /GaAs heterostructure, as the size of aluminum atoms is bigger than that of gallium and arsenic atoms.Therefore, at high  concentrations of aluminum in metal sublattice, even in such a well-coordinated heteropair, there are internal stresses which can lead to undesirable effects.
Competitive to the Al x Ga 1−x As solid solutions for design and manufacture of optoelectronic components on GaAs there are some other ternary solid solutions based on A III B V , such as Ga x In 1−x P, In x Ga 1−x As, Al x Ga 1−x P, and quaternary Ga x In 1−x As y P 1−y solid solutions [1,2].Their fundamental structural properties (type of crystal lattice, Vegard's law, Kufala equation: the dependence of the band gap on the composition, etc.) are similar to Al x Ga 1−x As solid solutions.However, the presence of incoherence areas of solid solutions and substrate lattice parameters, immiscibility regions [3], 2 Physics Research International and instability (mostly refers to Ga x In 1−x As y P 1−y ) significantly limits the range of compounds that can be used to create heterostructures based on the above mentioned systems and do not always allow one to observe quantum effects.
Another viable alternative to Al x Ga 1−x As system can be Al x Ga 1−x As 1−y P y solid solutions.Today it was shown that the implementation of low concentrations of phosphorus in the layers of Al x Ga 1−x As heterostructure allows one to receive a minimum of internal stresses in crystal lattices and provides better heat dissipation at high pump currents and, consequently, increases the power output of the laser diode based on Al x Ga 1−x As 1−y P y [4,5].An increase of phosphorus in the layers of solid solution should slow down the oxidation of the surface and increase activation energy of oxidation [6].In addition, as we have shown in previous publications, doping of Al x Ga 1−x As solid solution with high concentrations of silicon leads to the formation of quaternary solid solutions (Al x Ga 1−x As) 1−y Si y and allows one not only to control a number of electrooptical and electrical properties, but also to coordinate lattice parameters of heteropairs by substitution of the main atoms of the solid solution by smallsized atoms.In this case, silicon embeds in Al x Ga 1−x As solid solutions in the form of deep donor, known as DXcenter with special properties.At deep levels, DX-centers can accumulate a charge capable of changing a potential profile of a heterostructure.Consequently, the conductivity of the heterostructure is determined due to the effects related to the recharging process of deep levels, as well as residual effects of a positive and negative photoconductivity [7].All of this makes such solid solutions highly promising materials for optical converters, heterojunction, and detectors.
Thus, doping of Al x Ga 1−x As solid solution by smaller impurity atoms, silicon and phosphorus, should allow one not only to achieve two objectives: the controlled management of a number of electrooptical and electrical properties, but also to coordinate lattice parameters of heteropairs by the substitution of the main atoms of the solid solution by smallsized atoms.
It should be noted that the progress in technology of the epitaxial growth of heterostructures on the basis of ternary or even quaternary alloys, for example, the Ga x In 1−x As y P 1−y alloys, is already rather considerable, and a large number of publications devoted to the unique and interesting properties of these compounds are currently available.In contrast, studies of heterostructures based on quinary alloys are few and far between.At the same time, a quinary system of alloys has an extra degree of freedom, as compared to quaternary and ternary structures [8,9].In turn, this allows one to produce structures with more appropriable characteristics.It becomes possible not only to vary the band gap within the range of direct gap compositions, but, for a fairly wide range of compositions, to match the constituent layers in the lattice period and thermal expansion coefficient as well.
Therefore the purpose of our research work became the investigation of structural and optical properties of multicomponent solid solutions based on Al x Ga 1−x As, doped with high level concentration of phosphorus and silicon.

Objects and Research Methods
Epitaxial heterostructures based on Al x Ga 1−x As 1−y P y solid solutions with a thickness of ∼2 m were grown using MOCVD method on EMCORE GS 3/100 installation in a vertical reactor with a high-speed substrate holder rotation.As sources, we used trimethylgallium (Ga(CH 3 ) 3 ), trimethylaluminum Al(CH 3 ) 3 , arsine (AsH 3 ), and phosphine (PH 3 ).We used monosilane (SiH 4 ) as doping components to obtain n-type conductivity.Hydrogen was used as the carrier gas.The structures were grown on n-GaAs (100) substrates.
Processing characteristics of the samples are shown in Table 1.Table 1 also shows the flow of the dopant in reactor, which consists of 0.05% of a mixture of silane in hydrogen.The carrier concentration was determined by the Hall effect at room temperature.The estimated value of the phosphorus content in the solid solution was 1-2%.Phosphorus was implemented into a layer in order to compensate for the expected bulk stresses which are caused by slightly mismatched lattice parameter, but with a significant thickness (2 m) of Al x Ga 1−x As layers.
The structural quality of heterostructures and determination of lattice parameters of solid solutions were determined by X-ray diffractometer Seifert HR 3003 with four circuitous goniometer and monochromatized copper radiation with a wavelength of CuK 1 = 1.5405Å.The concentration of elements in solid solution has been refined by X-ray microanalysis using the attachment to an electron microscope.Raman spectra were obtained with a Raman microscope Renishaw 1000 with ×50 NPlan lens and the excitation by an argon laser with a wavelength of 514.5 nm.The laser beam did not exceed 3 mW.Photoluminescence spectra of heterostructures were obtained at room temperature from the sample surface by standard method with the use of monochromator TRIAX550 and cooled with liquid nitrogen CCD detector.Excitation photoluminescence spectra were produced by argon laser with a wavelength of 514.5 nm.In order to focus on the surface, ×10 lens were used.

Results and Discussion
At the first stage of the research using the X-ray microanalysis attachment to the electron microscope, we clarified the concentration of the elements included in solid solutions compositions.We used 20 kV electron accelerating voltage and analyzed sample area 750 × 750 m.Effective depth for microanalysis was ∼1-1.5 m.X-ray microanalysis data is presented in Table 2.As can be seen from these results, the concentrations of atoms in solid solution are different from those specified at the growth stage.The composition of the solid solution was specified on the basis of the ratio of concentrations of the elements in the gas phase using the composition and rate of growth data for standard Al x Ga 1−x As ternary solid solutions.However, the coefficients of segregation of elements included in solid solution may vary according to the overall composition of the gas phase, which, respectively, can lead to uncertainty in determination of composition of epitaxial layer.We note that according to the data (see Table 2) concentration of phosphorus and silicon atoms in epitaxial layer reach several parts of atomic percent.Also attention should be paid to the fact that the total concentration of atoms in the metal sublattice of fivecomponent layers is less than in the nonmetallic sublattice.

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This is most likely to be a consequence of the amphoteric behavior of silicon atoms as impurities.Looking back on the experience of the previous studies of growth processes when Al x Ga 1−x As solid solutions were highly doped with silicon [10,11], it is clear to see that this circumstances are sufficient for the formation of system Al x Ga 1−x As 1−y P y -Si solid solutions.
To confirm our assumptions, we used high-resolution Xray diffraction and further calculations of lattice parameters for the obtained solid solutions.Therefore, we performed the study of the structure and characteristics of grown epitaxial layers using a map of the q-space, since it allows one to receive direct information about the mismatch of lattice parameters in the epitaxial layer and substrate, disorientation or relaxation in the layer, dislocation density, its mosaic, or curvature.For each sample we produced maps of the distribution of diffracted light intensity in space around the q-symmetric (400) node and asymmetric (511) node, shown in Figures 1(a)-1(j).
The analysis of the reciprocal space maps (Figure 1) leads to the conclusion that epitaxial solid solution growth is coherent on GaAs (100) substrates, and samples have a small stress and composition gradient in the epitaxial layers, which is evidenced by the position and shape of nodes in reciprocal space for symmetric and asymmetric reflections.
Lattice constant of solid solutions  ] with cubic symmetry taking into account the elastic stresses in heteroepitaxial layer in accordance with the linear theory of elasticity can be calculated as follows [12]: where ] is Poisson's coefficients for epitaxial layers.Keeping in mind that studied samples of heterostructures were obtained with isoperiodic to GaAs compositions, we assume that the dependence of the various parameters for solid solutions will be linear.Using linear interpolation similar to which we have used in our previous work [13,14], we can write the law of Vegard for solid solutions (Al x Ga 1−x As 1−y P y ) 1−z Si z first through ternary and then through the binary compounds.
Thereby, Vegard's law for solid solutions (Al x Ga 1−x As 1−y P y ) 1−z Si z can be written in general form as follows: We used the lattice parameters values for binary compounds, presented in [15]:  AlP = 5.4635 >,  GaP = 5.4508 >, and Si = 5.431 >.
Using relations (1)-(3) as well as the analysis of the reciprocal space maps for nodes (400) and (511) (see Table 3) enables us to calculate lattice parameters of five-component solid solutions Al x Ga y In 1−x−y As z P 1−z taking into account elastic stresses.Assuming that Vegard's law is obeyed for the obtained (Al x Ga 1−x As 1−y P y ) 1−z Si z solid solutions, we specified the concentrations of the elements (see Table 3) analyzing expressions (1)-( 3) and calculation of the lattice parameters and data from microanalysis.
The strain coefficient of the alloy can be determined from XRD data as The results of calculations of the alloy allowing for elastic stresses are listed in Table 3.The calculated data show (see Table 3) that the implementation of phosphorus and silicon as an impurity of high concentrations can reduce the stress in epitaxial films.The penetration depth of laser radiation and the effective depth of analysis with Raman scattering can be determined from the ratio /2, where k is extinction coefficient.For an argon laser with  = 532 nm the analysis depth for AlGaAs is approximately 500 nm.It gives the right to say that using this laser wavelength for Raman scattering we obtain the information only from the layer of solid solution.
The selection rules derived from the analysis of the Raman scattering tensor [16] for crystals with diamond   structure with backscattering from the (100) surface allow one to observe only LO phonons, and the appearance of TO phonons is prohibited.Figure 2 presents the spectra of Raman backscattering geometry for the samples to be analyzed, which are arranged for the convenience in the subgroups: Figure 2 As can be seen from the results, spectra of heterostructures include all of the major fluctuations specific to the type of heterostructures (vibration frequencies are presented in Table 4).Thus, from Figure 2 we can see that Raman spectrum of the structure of homoepitaxial GaAs/GaAs (100)  (sample EM2350) contains high intensity longitudinal optical phonon LO(Γ) localized ∼293 cm −1 .The experimental data, including the shape of the spectrum for the homoepitaxial sample, shows the dislocation-free mechanism of growth and excellent structural properties of the layer.Raman spectra of heterostructures Al x Ga 1−x As:Si/GaAs (100) (samples EM2346, EM2438, and EM2449) contain longitudinal LO phonon modes of Ga-As and Al-As at (Γ), located in the range of ∼267 cm −1 and ∼380 cm −1 , respectively (see Figures 2(a The experiment shows that in all the studied Raman backscattering spectra in addition to main vibration modes there are a number of additional modes.In the experimental Raman spectra of all heterostructures present transverse oscillation modes TO Ga-As and Al-As (forbidden by the selection rules), which is a consequence of disorientation solid solution epitaxial growth relative to the direction given by the substrate GaAs, having already disorientation relative to the direction (100).
It should be noted that in Raman spectra of EM2449 heterostructure, a solid solution doped with silicon with high concentration, the intensity of the transverse oscillation mode TO Ga-As is higher than the allowed longitudinal LO.It is most likely to be the result of disorders in crystal lattice symmetry in epitaxial layer due to its doping.
(a): spectrum of homoepitaxial structures, Figure 2(b): spectra of heterostructures based on ternary AlGaAs and four-component AlGaAsSi solid solutions, and Figure 2(c): spectra of heterostructures based on five-component AlGaAsPSi solid solutions.
) and 2(b)).The main fluctuations presented in Raman backscattering spectra for heterostructures based on five-component (Al x Ga 1−x As 1−y P y ) 1−z Si z solid solutions are longitudinal (allowed) optical vibrations GaAs and AlAs and longitudinal mode GaP.

Table 1 :
Composition and growth condition of Al  Ga 1− As  P 1− :Si heterostructures.