INFLUENCE OF THE In/Ga RELATION IN THE GAS PHASE ON THE CHARACTERISTICS OF THE InX Ga1-XP EPITAXIAL LAYERS OF CASCADE SOLAR CELLS

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Introduction. Due to the increased requirements for onboard spacecraft systems, there is a need to create solar array (SA) with high performance and energy characteris- tics and increased service life (more than 15 years). The SA will convert sunlight directly into electricity with high efficiency, creating almost constant power at low operat- ing costs [1]. The most promising elements for a modern SA for operating as a part of the spacecraft and satellites are cascade solar cells (SC) based on AIIIBV materials. Modern triple-junction CS is a complex planar device. Generating semiconductor part of such a solar cell, pro- duced by Metalorganic Chemical Vapour Deposition (MOCVD), consists of three dozen nano- and micro-sized functional layers forming triple junction InGaP / InGaAs / Ge. Due to the relatively large surface of the device - about 30 cm2 [2; 3], in order to achieve high characteris- tics of the structure, it is particularly important to comply with the requirements for uniformity of properties of all layers of the structure. To create a multi-layer SC, using different materials, a large number of technological opera- tions are necessary, each of which requires careful moni- toring of the process parameters and properties of the product, since the deviation from the norm at each stage can affect the properties of the finished structure and the output characteristics of the SC [3]. Intermediate control of properties allows detecting defects at each stage (epi- taxial growth of semiconductor structure, post-growth processes: photolithography, metallization, deposition of anti-reflection coating, the formation of overall dimen- sions, etc. [2]) and reject defective wafers. To identify the causes of possible defects, it is necessary to determine the dependences between the properties of the structure and the technological parameters of the process [3]. One of the main factors for improving the efficiency of SС is the perfection of growth of semiconductor layers, namely: matching the lattice parameters, the minimum concentration of defects in the crystal structure and achieving high homogeneity of the composition on the entire surface of the wafer. The mismatch of lattice parameters is of the first importance, as it is in itself a measure of deformation in the layer [4]. Therefore, for the development of special epitaxial layers with precise matching of the lattice parameter in the process of growth, it is essential to have full data about the ratio in the gas stream of the elements of the third group In / Ga of the periodic system of chemical elements for obtaining a target composition in the structure of the epitaxial layer InxGa1-xP (the main material not only of modern triple junction solar cell, but other promising photovoltaic devices), i. e. it is necessary to study the dependence of the ratio of the composition in the gas and solid phase. Experimental Details. For a detailed study of the epi- taxial layer of InxGa1-хP on the subject of the influence of the epitaxial growth parameters on the characteristics of the crystal structure model samples of simplified structure of more than 60 pieces with different content of indium and gallium (x = from 38 to 53 %) doped with silicon and tellurium, on the germanium substrate with a diameter of 100.0 ± 0.4 mm were manufactured. The samples were grown by the MOCVD method on the installation of an industrial type Veeco E450. Schematic images of the struc- tures assigned are shown in fig. 1, as well as the techno- logical parameters for each layer assigned in the calcula- tions, were obtained applying the program for the analysis and comparison of recipes Veeco RCPAnalysis [5; 6]. Method of gas-phase epitaxy of metal-organic and hybrid compounds. The feature of MOCVD method lies in the fact that in the epitaxial reactor creates a high tem- perature area, which receives a gas mixture containing decomposable compounds [7]. In the reactor, the release and deposition of the substance on the substrate occurs, and gaseous reaction products are carried out by the flow of hydrogen carrier gas. For the preparation of com- pounds AIIIBV, as the source of III group element is used metal-organic compounds, for example, trimethylgallium (ТМGa) and trimethylindium (TMIn) for the synthesis of InGaP. As a source of group V elements such gas as phosphine (PH3) is used. Fig. 1. Schematic representations of the structure of model samples (for epitaxial layers InxGa1-xP values of thickness are indicated) Рис. 1. Схематические изображения структуры модельных образцов (для эпитаксиальных слоев InxGa1-хP указаны значения толщины) Since the structure of the SC consists of many semi- conductor layers, with different chemical composition and different levels of alloying and doping, before the process of creating the entire semiconductor structure in one cycle of epitaxial growth it is necessary to use the software The saturated steam pressure is determined by indi- vidual equations, depending on the temperature (T) accord- ing to the tabular data [11]. For TMGa according to the expression (2), and for TMIn according to the expression (3): 1703 installation epitaxial growth recipe.The recipe is a table in which the technological parameters for each metal-organic log10 Pv(TMGa ) = 8.070 - T , (2) and hydride compound at each time of the epitaxial growth process are specified, as well as the temperature in log10 Pv(TMIn) = 10.52 - 3014 . T (3) different zones of the growth chamber, etc. [3; 8]. To obtain an epitaxial layer with specified properties, such as the crystal lattice parameter, the band gap width, it is necessary to control the chemical composition of the growing layers, determined by the composition of the gas mixture and the distribution of metal-organic compounds in the reactor. The composition of the gas mixture is set by such technological parameters as the speed of flow of sub- stances, the pressure in babbler and temperature [9; 10]. The schematic design of the bubbler for dosing liquid volatile alkyl TMGa and bubbler for TMIn into the reac- tor is shown in fig. 2. Physico-chemical calculation of the ratio of elements of the third group In / Ga in the gas phase. In order to calculate the flow of the component in the gas phase for each metal-organic compound, it is necessary to use the following formula (1): Thus, having considered the equations (1), (2), (3) it can be concluded that the velocity of the molar flow at the outlet of the bubbler can be controlled by changing the velocity of the hydrogen flow, the pressure in the bubbler and the bubbler temperature. The increase in pressure in the bubbler (Pb) reduces the velocity, temperature rise (equivalent to an increase in Pv), as well as the flow of hydrogen - increases the molar velocity. Thus, the given values are technological parameters that can be changed to achieve the required composition of the layer. Despite the perfection of the MOCVD method in the technology of semiconductor production, the periodic inspection of key parameters of epitaxial structures is required [3], in addition, such a procedure is necessary for the synthesis of new layers. One of the main methods is a method of x-ray diffraction (XRD). Methods of measurement using high-resolution two-crystal x-ray diffraction. To determine the lattice V газ TMGa,TMIn = S × Pv , (Pb - Pv ) ×V (1) parameter of separate compounds on the XRD Vector measurements were carried out, which resulted in a rock- ing curve. where S is the flow of substance through the bubbler, cm3/min; Pb - pressure in the bubbler, Pa; Pv - the partial pressure of metal-organic compounds at a given tempera- ture, Pa; V - the molar volume of ideal gas (under normal conditions), cm3/mol. It should be noticed that, the maximum contribution to the intensity, gives the thickest layer of the structure, namely Ge substrate. The presence of broadening and any other stand-alone peaks indicates the presence of layers mismatched from the lattice parameters [12]. Fig. 2. Construction of bubbler: V1, V2, V3 - pneumatic valves; MFC - device for supplying a given flow of hydrogen; MFC/PC (РС) - pressure controller - device for maintaining the preset pressure in the bubbler; Piezocon - the device that determines the molar concentration of TMIn at the outlet of the bubbler Рис. 2. Конструкция барботёров: V1, V2, V3 - пневматические клапаны; MFC - прибор подачи заданного потока водорода; MFC/PC (РС) - кон- троллер давления - прибор поддержания заданного давления в барботё- ре; Piezocon - прибор, определяющий мольную концентрацию TMIn на выходе из барботёра A classic example of the rocking curve for the epi- taxial layer of InGaP is shown in fig. 3. In structures with different number of misaligned layers, the rocking curves are characterized by the presence of one narrow peak with high intensity and spaced from it by a certain number of angular seconds of the peak with lower intensity. When the peak is located with lower intensity to the left of the main, there is an increase in the lattice parameter relative to the substrate, while the location to the right of the main one is a decrease: the broadening of the x-ray peak (secondary axis) the ra- tio of In / Ga in the epitaxial layer of InxGa1-хP was made (fig. 5). The lattice parameter increases with the increase in the percentage of indium in InxGa1-хP. Basically, all experi- mental data are well placed on the straight line, the mis- alignment of the lattice parameter is 0.1-0.26 %. In the range of 45 to 53 % monocrystalline structure is charac- terized by high perfection. When the ratio of In and Ga is 1:1 the epitaxial layer is characterized by the best prop- 2 d 2 a = × (H n 2 + K 2 + L2 ), (4) erties with the most similar value of the lattice parameter in accordance with the Ge. The deviation from the com- position for these samples ranges from 0.5 to 3 %. where a is the lattice parameter, Å; d is the inter-plane distance between the reflecting planes, Ǻ; n is an integer describing the diffraction order of the reflection; H, K, L - the indices of the interference; Additionally, studies have been carried out to deter- mine the homogeneity of the solid solution throughout the surface of the sample. Due to the center symmetry of the growth chamber, single rocking curves are measured aIn Ga P = x × aInP + (1 - x) × aGaP , (5) along the sample radius. The comparison of measurement x 1- x where x is the content of In in the solid solution. The obtained experimental data are well placed on the line based on the table data on the Vegards rule, but there are some deviations near the table value of the Ge lattice parameter. Since in this case it is difficult to separate the peaks of the substrate and the epitaxial layer, there is an error in determining the distance between them. An example of such a rocking curve is shown in fig. 4, a. For samples with a significant deviation of the lattice parameter, the rocking curves are characterized by a large value of the broadening of the peak is about 800 arcseconds (fig. 4, с). For the classical rocking curve of the faultless structure, the distance between the peaks of the order of 200-400 arcseconds and the broadening is less than 200 (fig. 4, b). The XRD survey, the data processing of the rocking curves of experimental samples of epitaxial structures, as well as data for calculating the composition of solid solu- tion from the gas phase composition, a diagram of de- pendence of the lattice parameter (the primary axis) and results shown in fig. 6 indicates a homogeneous growth of the epitaxial structure in composition according to table, which makes it reasonable to survey at the central point with multiple measurements and confirms the reliability of the data obtained. Thus, according to the results of studies on the high- resolution two-crystal XRD it is possible to conclude about the state of samples of SC, the presence of mis- aligned layers, defects and the possibility of further work with the resulting epitaxial structures, as well as if neces- sary to make adjustments to the recipe for epitaxial growth. Method for measuring the effect of photolumines- cence. To determine the composition of the solid solution by using the method of measuring the effect of photolu- minescence (PL) at 2000 RPM equipment. This method is easier to implement, the equipment allows to obtain the maps to analyze the uniformity of properties, but the method is indirect and it is necessary to take into account the contribution of other factors (alloying). Fig. 3. Experimentally obtained classical rocking curve for an epitaxial layer InGaP Рис. 3. Экспериментально полученная классическая кривая качания для эпитаксиального слоя InGaP I, relative units с 2θ, seconds of angle d2θ = 1220 seconds of angle Fig. 4. Rocking curves for a different ratio In / Ga in the solid phase Рис. 4. Кривые качания для различного соотношения In / Ga в твердой фазе The results values calculation of the of the lattice parameter and the composition of the solid solution of epitaxial layer InxGa1-хP for 5 points № survey point d2θ, arcsecond θ, º a, Å Δа/аGe, % Ga, % In, % 1 290.4 33.03 5.6518 0.108 51.87 48.13 2 295.8 33.03 5.6517 0.110 51.90 48.10 3 300.3 33.04 5.6516 0.112 51.92 48.08 4 296.1 33.03 5.6517 0.111 51.90 48.10 5 282.3 33.03 5.6519 0.105 51.83 48.17 According to the data illustrated in fig. 7, we can judge about the uniformity in thickness and composition of the epitaxial structure at the stage of growth, because h × c E = g l , (6) the PL signal depends on the width of the band gap of the material given by the chemical composition. Spectral maps show the distribution of the integral intensity of PL, wavelength peak, intensity peak and full width at half maximum of the spectrum on the planar epi- taxial structure [3; 10]. Spectral maps (fig. 7) stored in a format that allows determining the spectrum data at each point of the epitaxial structure. In particular, the data is processed by the elements of the technology support system [13; 14]. Using the results obtained, it is possible to determine the homogeneity of the studied layer and calculate the value of the band gap width using the formula (6): where Еg is photon energy, eV; λ is photon wavelength (characteristic wavelength of photoluminescence spec- trum), nm; h - Planck’s constant, eV/sec; с - is the speed of light, m/sec. The obtained results show the uniform distribution of peak spectra of PL (electron-optical properties) over the entire surface of the sample, while the average deviation of the maximum spectra of PL is 2.8 %. When comparing the data of the ratio of In / Ga in the solid phase measured by the two methods for samples dopped with Si and Te, it is seen that the composition data obtained by PL measurement should be considered as estimated. The results are presented in fig. 8. Broadening, seconds of angle In/(In+Ga ) in gas phase, % Fig. 5. The graph of the dependence of the lattice parameter and the broadening of the x-ray peak on the ratio In / Ga in the epitaxial layer InxGa1-xP: triangular points - the experimental data of the lattice parameter; points - the broadening of the diffraction peaks of some samples; the dotted line - the value of the lattice parameter Ge; direct line - data constructed according to Vegard’s rule Рис. 5. График зависимости параметра решетки и уширения рентгеновского пика от соотношения In / Ga в эпитаксиальном слое InxGa1-хP: треугольные точки - полученные экспериментальные данные параметра решетки; круглые точки - значения уширения дифракционных пиков некоторых образцов; пунктирная линия - значение параметра решетки Ge; прямая линия - данные, построенные по правилу Вегарда Fig. 6. A set of rocking curves obtained along the radius of the sample Рис. 6. Набор кривых качания, полученных вдоль радиуса образца Fig. 7. Spectral map and single photoluminescence spectrum for one sample 142 Рис. 7. Спектральная карта и единичный спектр ФЛ для одного образца Fig. 8. Graphs of the ratio In / Ga of the epitaxial layer InxGa1-xP in the solid phase, obtained by means of X-ray diffractometry and photoluminescence from the ratio in the gas phase: round shaded points - the results obtained with the X-ray diffractometry method; triangular unpainted dots - results obtained using the photoluminescence method; a straight line - the line of consistency Рис. 8. Графики зависимости соотношения In / Ga эпитаксиального слоя InxGa1-хP в твердой фазе, полученные с помощью методов РД и ФЛ от соотношения в газовой фазе: круглые закрашенные точки - результаты, полученные с помощью метода РД; треугольные незакрашеные точки - результаты, полученные с помощью метода ФЛ, прямая линия - линия соответствия данных For x < 0.63 GaxInx-1P material, the following expres- sion [15] is true to calculate the composition of the epi- taxial layer: The obtained results allow us to move to the next stage of work to determine the optimal technological parameters for the growth of perfect epitaxial layers of InxGa1-хP on a substrate of Ge in the structure of the mod- Eg = 1.34 + 0.69 × x + 0.48 × x2. (7) ern and perspective high-performance cascade SC for space applications, namely achieving the required electrical Conclusion. The research, the results of which are presented in this article, was completed as a part of the determining of the conditions of a highly homogeneous, defect-free growth of epitaxial layers. The general scheme of such studies was tested mainly on InGaAs layers [16; 17]. In the course of the work, the following important re- sults were obtained: - the optimal technological parameters of epitaxial growth in the composition of the solid solution InxGa1-хP are determined; - it is revealed that for the solid solution InxGa1-хP in the range from 45 to 53 % there is a high perfection of single crystalline structure, a slight broadening of diffrac- tional x-ray peaks (less than 200 acrsecond); - it is shown that the criterion for assessing the quality of the grown structure, along with the misalignment of diffraction maximum, can be the broadening of the dif- fraction peak of the structure. Additionally, it is shown that: - the plotted rocking curves along the radius of the sample indicate a high homogeneity of the solid solution over the entire surface of the sample (deviation of the lattice parameters 0.01 %), which makes it reasonable to detect at the central point in multiple measurements and confirms the reliability of the data obtained; - the composition of the solid solution determined us- ing the method of measuring the effect of photolumines- cence, should be considered as evaluative. characteristics using precision dopping [17; 18]. It is also possible to use the created and tested on the compound InxGa1-xP algorithm for similar studies of other materials.

About the authors

A. A. Naumova

JSC “Research-production enterprise “Kvant”

Email: otdel_17@npp.kvant.ru
16, 3-rd Mytish chinskaya, Moscow, 129626, Russian Federation

A. A. Lebedev

JSC “Research-production enterprise “Kvant”; The National University of Science and Technology “MISiS”; 4, Leninsky Av., Moscow, 119049, Russian Federation

16, 3-rd Mytish chinskaya, Moscow, 129626, Russian Federation

B. V. Zhalnin

JSC “Research-production enterprise “Kvant”

16, 3-rd Mytish chinskaya, Moscow, 129626, Russian Federation

E. V. Slyshchenko

JSC “Research-production enterprise “Kvant”; The National University of Science and Technology “MISiS”; 4, Leninsky Av., Moscow, 119049, Russian Federation

16, 3-rd Mytish chinskaya, Moscow, 129626, Russian Federation

N. T. Vagapova

JSC “Research-production enterprise “Kvant”

16, 3-rd Mytish chinskaya, Moscow, 129626, Russian Federation

References

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