Vapor-solid Distribution Research Articles

Interplay of Kinetic and Thermodynamic Factors in the Stationary Composition of Vapor-Liquid-Solid IIIVxV1-x Nanowires.

Compositional control over vapor-liquid-solid III-V ternary nanowires based on group V intermix (VLS IIIVxV1-x NWs) is complicated by the presence of a catalyst droplet with extremely low and hence undetectable concentrations of group V atoms. The liquid-solid and vapor-solid distributions of IIIVxV1-x NWs at a given temperature are influenced by the kinetic parameters (supersaturation and diffusion coefficients in liquid, V/III flux ratio in vapor), temperature and thermodynamic constants. We analyze the interplay of the kinetic and thermodynamic factors influencing the compositions of VLS IIIVxV1-x NWs and derive a new vapor-solid distribution that contains only one parameter of liquid, the ratio of the diffusion coefficients of dissimilar group V atoms. The unknown concentrations of group V atoms in liquid have no influence on the NW composition at high enough levels of supersaturation in liquid. The simple analytic shape of this vapor-solid distribution is regulated by the total V/III flux ratio in vapor. Calculating the temperature-dependent desorption rates, we show that the purely kinetic regime of the liquid-solid growth occurs for VLS IIIVxV1-x NWs in a wide range of conditions. The model fits the data well on the vapor-solid distributions of VLS InPxAs1-x and GaPxAs1-x NWs and can be used for understanding and controlling the compositions of any VLS IIIVxV1-x NWs, as well as modeling the compositional profiles across NW heterostructures in different material systems.

Circumventing the Uncertainties of the Liquid Phase in the Compositional Control of VLS III-V Ternary Nanowires Based on Group V Intermix.

Control over the composition of III-V ternary nanowires grown by the vapor-liquid-solid (VLS) method is essential for bandgap engineering in such nanomaterials and for the fabrication of functional nanowire heterostructures for a variety of applications. From the fundamental viewpoint, III-V ternary nanowires based on group V intermix (InSbxAs1-x, InPxAs1-x, GaPxAs1-x and many others) present the most difficult case, because the concentrations of highly volatile group V atoms in a catalyst droplet are beyond the detection limit of any characterization technique and therefore principally unknown. Here, we present a model for the vapor-solid distribution of such nanowires, which fully circumvents the uncertainties that remained in the theory so far, and we link the nanowire composition to the well-controlled parameters of vapor. The unknown concentrations of group V atoms in the droplet do not enter the distribution, despite the fact that a growing solid is surrounded by the liquid phase. The model fits satisfactorily the available data on the vapor-solid distributions of VLS InSbxAs1-x, InPxAs1-x and GaPxAs1-x nanowires grown using different catalysts. Even more importantly, it provides a basis for the compositional control of III-V ternary nanowires based on group V intermix, and it can be extended over other material systems where two highly volatile elements enter a ternary solid alloy through a liquid phase.

Composition of Vapor-Liquid-Solid III-V Ternary Nanowires Based on Group-III Intermix.

Compositional control in III-V ternary nanowires grown by the vapor-liquid-solid method is essential for bandgap engineering and the design of functional nanowire nano-heterostructures. Herein, we present rather general theoretical considerations and derive explicit forms of the stationary vapor-solid and liquid-solid distributions of vapor-liquid-solid III-V ternary nanowires based on group-III intermix. It is shown that the vapor-solid distribution of such nanowires is kinetically controlled, while the liquid-solid distribution is in equilibrium or nucleation-limited. For a more technologically important vapor-solid distribution connecting nanowire composition with vapor composition, the kinetic suppression of miscibility gaps at a growth temperature is possible, while miscibility gaps (and generally strong non-linearity of the compositional curves) always remain in the equilibrium liquid-solid distribution. We analyze the available experimental data on the compositions of the vapor-liquid-solid AlxGa1-xAs, InxGa1-xAs, InxGa1-xP, and InxGa1-xN nanowires, which are very well described within the model. Overall, the developed approach circumvents uncertainty in choosing the relevant compositional model (close-to-equilibrium or kinetic), eliminates unknown parameters in the vapor-solid distribution of vapor-liquid-solid nanowires based on group-III intermix, and should be useful for the precise compositional tuning of such nanowires.

Kinetically controlled composition of III-V ternary nanostructures

Controlling the composition of ternary III-V and III-nitride nanomaterials such as vertical nanowires, horizontal nanowires, nanosheets, and nanomembranes grown by different epitaxy techniques is essential for band gap engineering and fabrication of nanoheterostructures with tunable properties. Herein, we investigate the diffusion-induced growth process of III-V ternary materials in different geometries including planar layers, nanomembranes, and horizontal and vertical nanowires grown by selective area epitaxy or with a catalyst droplet on top and derive a rather general equation connecting the composition of ternary solid with the composition of vapor. The form of this vapor-solid distribution remains identical for a wide range of geometries, while the coefficients entering the equation contain thermodynamic factors, kinetic constants of the material transport, and geometrical parameters of the growth template. General properties of the vapor-solid distribution are investigated with respect to material constants, growth condition, and geometry, including the interplay of thermodynamics and growth kinetics leading to the suppression of the miscibility gaps in InGaAs and InGaN systems. A good correlation of the model with the data on the compositions of InGaAs, InGaP, and AlGaAs materials grown by different methods is demonstrated. Overall, these results give a simple analytical tool for understanding the compositional trends and compositional tuning of III-V ternary nanostructures, which should work equally well for Si-Ge and II-VI material systems.

Thermodynamic analysis on HVPE growth of InGaN ternary alloy

Growth of InGaN alloy using hydride vapor phase epitaxy (HVPE) was investigated. Thermodynamic analysis was performed, taking account of the source zones. It was found that InCl3 and GaCl3, which are known to be essential for appreciable InGaN growth by HVPE, could be generated preferentially at the source zone by using a group-III metal and Cl2 gas. The analysis for the growth zone revealed that a significantly large driving force for both InN and GaN deposition is possible. The calculated vapor–solid distribution was close to a linear relationship by using a high V/III ratio, inert carrier gas, and high temperature. These facts suggest that an InGaN thick layer can be grown with a high growth rate and enough controllability of solid composition by employing InCl3 and GaCl3 precursors.

Thermodynamics on tri-halide vapor-phase epitaxy of GaN and In xGa 1− xN using GaCl 3 and InCl 3

Thermodynamic analyses on tri-halide vapor-phase epitaxy (THVPE) of GaN and In x Ga 1− x N using GaCl 3 and InCl 3 are described. The partial pressures of gaseous species in equilibrium with GaN and In x Ga 1− x N and the driving force for the deposition are calculated for growth temperature and the hydrogen mole fraction in the carrier gas. It is shown that the deposition of GaN and In x Ga 1− x N is significantly influenced by the hydrogen mole fraction in the carrier gas. The vapor-solid distribution relationship of In x Ga 1− x N alloy deposition using GaCl 3 and InCl 3 is discussed in comparison with the experimental data reported in the literature. It is shown that the solid composition x in In x Ga 1− x N is thermodynamically controlled.

Thermodynamic Analysis of the MOVPE Growth of InAlN

A thermodynamic analysis is described for metalorganic vapor phase epitaxy (MOVPE) of InAlN. The equilibrium partial pressure, the vapor–solid distribution relationship and the phase diagram of deposition have been calculated. It is shown that the vapor–solid distribution relationship in InAlN depends on the growth temperature, hydrogen partial pressure in the carrier gas, input group III partial pressure and V/III ratio, as in InGaN. Based on the calculation, the growth conditions suitable for the MOVPE growth of InAlN are predicted.

Thermodynamic analysis of Ga xB 1− xN grown by MOVPE

A thermodynamic model was applied to predict the GaBN MOVPE growth such as the equilibrium partial pressures, vapor–solid distribution relation, and growth efficiency. The delta-lattice-parameter (DLP) method was employed to determine the activities of the binary compounds. The large structural dissimilarity between BN and GaN results in a high interaction parameter of 70 730 cal/mol, indicating that GaBN deviates greatly from an ideal solid solution. A V/III ratio>100 is required for the growth of GaBN at 1000°C to avoid Ga droplets on the growth front. The growth of GaBN is controlled by the much higher Ga partial pressure than that of B over the alloy. The unstable regions in which inhomogeneous compositions are formed exist in the GaBN alloy based on the vapor–solid distribution relation. The predicted growth behavior of GaBN agrees well with the experimental data.

Thermodynamic study on phase separation during MOVPE growth of InxGa1−xN

A thermodynamic study is performed to investigate the compositional unstable nature of InxGa1−xN in the MOVPE growth. It is shown that explicit evidence of phase separation is not observed in the free energy–composition curve, but the InxGa1−xN alloy is very unstable near x=0.2. The vapor–solid distribution relationship, the growth efficiency and the phase diagram for deposition are calculated. It is shown that the unstable regions in which inhomogeneous compositions are formed exist in the InxGa1−xN alloy system and that the unstable regions occur in the small driving force of the deposition.

Vapor-phase epitaxy of InxGa1−xN using chloride sources

The growth kinetics of InxGa1−xN epitaxial layer on GaN-buffer/sapphire (0001) substrate and the vapor–solid distribution relationship of the chloride source system are investigated. The growth rate decreases with increasing growth temperature. The solid composition of epitaxial layers becomes more GaN-rich with the increasing growth temperature and is very sensitive to the H2 mole reaction in the carrier gas. In the vapor–solid distribution relationship, the preferential incorporation of the GaN component is observed. The indium droplets are not observed on the surface of InxGa1−xN films in the entire composition range. Without In droplets formation, the In-rich InxGa1−xN alloys as high as 0.8 of In content are obtained.

Thermodynamic analysis of the MOVPE growth of InxGa1−xN

A chemical equilibrium model is applied to the growth of the InxGa1−xN alloy grown by metalorganic vapor-phase epitaxy (MOVPE). The equilibrium partial pressures and the phase diagram of deposition are calculated for the InxGa1−xN alloy. The vapor-solid distribution relationship is discussed in comparison with the experimental data reported in the literature. It is shown that the solid composition of the InxGa1−xN alloy grown by MOVPE is thermodynamically controlled and that the incorporation of group III elements into the solid phase deviates from a linear function of the input mole ratio of the group III metalorganic sources under the conditions of high mole fraction of decomposed NH3 (high value of α), high temperature and low input VIII ratio. The origin of the deviation of the solid composition from the linear relation is also discussed.

Vapor-solid distribution in In 1− xGa xAs and In 1− xGa xP alloys grown by atomic layer epitaxy

A chemical equilibrium model is applied to the halogen transport atomic layer epitaxy (ALE) growth of In 1− x Ga x As and In 1− x Ga x P alloys. The alloy composition is computed thermodynamically as a function of the input partial pressure ratio at various temperatures. The thermodynamically predicted composition of the alloys is compared with experimental data reported previously. It is shown that the alloy composition of layers grown by halogen transport ALE can be well explained by the equilibrium model.

Lattice-matched growth of InPSb on InAs by low-pressure plasma MOVPE

This contribution reports on results of the growth of InAs and InPSb in low-pressure (20 hPa) plasma MOVPE. Our basic aim was to study the effect of deposition temperature, vapor phase composition and precracking the group V molecules on the growth kinetics and material quality. For InAs growth (TMIn, AsH 3) on InAs substrates, material with excellent morphology was achieved in the temperature region of 670 to 770 K (best electrical and morphological quality on SI GaAs at T = 670 K: μ 300K = 19,000 cm 2/V⋯s, n 300K = 10 17 cm -3). AsH 3 precracking did not lead to a further improvement of layer quality. For the epitaxy of InPSb (TMIn, PH 3, TESb) the relative rates of incorporation of P and Sb into the solid are of major interest. Preliminary studies of the growth of InP 1− x Sb x ( x < 0.1) (with TESb plasma, PH 3 plasma) on InP were performed in order to obtain the vapor-solid distribution relation for the group V elements P and Sb under our growth conditions. For the lattice-matched growth of InP 0.69Sb 0.31 on InAs, a substantial decrease of the overall PH 3 consumption by a factor of 13 with PH 3 plasma was noticed. Due to this fact we observed a higher growth efficiency, probably caused by the suppression of prereactions. Layers with electrical properties (on SI InP: μ 300 K = 3500 cm 2/ V· s, n 300 K = 3 × 10 16 cm −3) comparable to those reported in the literature were obtained with and without plasma. In general, the plasma approach leads to an efficient, easily controllable and reproducible process for the lattice-matched growth of InPSb on InAs.

Atomic layer epitaxy of GaAsP and InAsP by halogen system

Atomic layer epitaxy (ALE) of GaAsP and InAsP is reported for the first time using the halogen transport system. It is confirmed that the grown thickness per cycle is almost equal to the monolayer thickness and independent on the composition of grown layers. The vapor-solid distribution relation for the group V elements is obtained. The preferential incorporation of arsenic is observed both in GaAsP and in InAsP growth. It is suggested that the adsorption force or the reaction rate of arsenic species with the adsorbed GaCl or InCl is greater than that of phosphorus.

Solid composition of alloy semiconductors grown by MOVPE, MBE, VPE hand ALE

An analysis of element incorporation from a thermodynamic point of view is described for the vapor phase epitaxy (MOVPE, MBE, VPE and ALE) of III–V and II–VI alloy semiconductors. The vapor-solid distribution relation of ternary and quaternary alloys is discussed in comparison with the experimental data reported in the literature. It is shown that the solid composition of alloy semiconductors grown by MOVPE, MBE and VPE is thermodynamically controlled, while that grown by halogen transport ALE is determined by the competitive adsorption of group III monochlorides on the substrate. The thermodynamically predicted order of binary compounds to incorporate into III–V alloys is very similar in all epitaxial methods and is basically governed by the Gibbs free energy of formation of binary compounds irrespective of the growth methods.

Thermodynamic analysis of the MOVPE growth of quaternary III–V alloy semiconductors

A chemical equilibrium model is applied to the MOVPE growth of quaternary III–V alloys. The equilibrium partial pressures and the diagram of solid composition versus input mole ratio are computed for INGaAsP, GaAlAsP, InGaAsSb, AlGaInP, AlGaInAs, and InAsSbP alloys. It is shown that under normal growth conditions (V/III>1), the incorporation of the group III elements into the solid phase is a linear function of the input mole ratio of the group III metalorganic sources, while that of the group V elements differs significantly from the input vapor phase content. For several alloys, the vapor-solid distribution relation is discussed in comparison with the experimental data reported in the literature.

Vapor-solid distribution relation in MOCVD GaAs xP 1− x and InAs xP 1− x

The vapor-solid distribution relation for the group V elements P and As is investigated for MOCVD GaAs x P 1- x and InAs x P 1- x . The small P deposition ratio at a lower growth temperature is caused by the small PH 3 cracking efficiency at the growing surface. The effective adsorption rate of P to As in GaAs x P 1- x is about twice higher than that in InAs x P 1- x at 600°C.