Molecular Beam Epitaxy of Solid Solutions GaP_xAs_1-x: Effect of Growth Condition

on the Group V Sublattice Composition

Eugene A. Emelyanov, Mikhail A. Putyato, Boris R. Semyagin, Dmitriy F. Feklin,

Andrey V. Vasev, Valery V. Preobrazhenskii

A.V. Rzhanov Institute of Semiconductor Physics, SB RAS, Novosibirsk, Russia

Abstract – Impact of substrate temperature, density of As₂, P₂ molecule and Ga atom fluxes on the composition of layers of solid solution GaP_xAs_1-x(001) with molecular-beam epitaxy (MBE) was experimentally investigated. The experimental data obtained for a wide range of growth conditions were analyzed. The analysis results are represented in the form of the kinetic model describing the formation process for composition of solid GaP_xAs_1-x(001) solution with MBE. The model can be used for selection of growth conditions for layers of solid solution GaP_xAs_1-x(001) at a given phosphorus fraction.

Index Terms – Molecular-beam epitaxy (mbe), sublattice composition, arsenic, phosphorus, dimers, solid solution of GaPxAs1-X(001).

I. Introduction

Solid solutions (A^III)P_xAs_1-x have a unique combination of electrophysical, optical and processing properties. They can be used for solution of the so called “zonal engineering” task. For example, it is possible to control a band gap and lattice parameter for this material by varying the composition of the solid solution In_yGa_1-yP_xAs_1-x. A band gap of the four-component solution may be varied from 1.42 to 1.9 eV in case of lattice-matched In_yGa_1‑yP_xAs_1‑x/GaAs system and from 0.7 to 1.35 eV in case of In_yGa_1‑yP_xAs_1‑x/InP. In_yGa_1-yP_xAs_1-x compound is a direct-band-gap semiconductor almost over the whole range of compositions, which makes it possible to use it in development of structures for optoelectronic devices. It should be noted that layers of phosphorus-containing solid solutions are resistant to oxidation in the air and have high etch selectivity in relation to phosphorus-free layers.

High-quality heterostructures based on layers of solid (A^III)P_xAs_1-x solutions may be generated on the basis of molecular-beam epitaxy (MBE). The composition in the anionic sublattice depends on incorporation coefficient of arsenic (S_As) and phosphorus (S_P) that substantially differ from each other and depend on growth conditions. The main growth conditions include substrate temperature; size and relationship of molecular fluxes of III and V group elements; composition and condition of growth surface, molecular form of V group elements in a flux, crystal-lattice orientation of the substrate surface. Therefore, growth of solid solutions (A^III)P_xAs_1-x at a given phosphorus fraction based on MBE is a complex engineering and scientific task. The possibility to predict the composition of solid solutions on the basis of the kinetic growth model could facilitate the solution of this task

This study presents the results obtained from the experimental research of impact of growth conditions on the composition of GaP_xAs_1-x(001) layers carried out over the whole range of epitaxy conditions within the framework of the single approach to measurement of growth temperature and density of molecular fluxes. The kinetic model for formation of the composition of solid solution GaP_xAs_1-x(001) based on MBE is proposed. The obtained experimental data are well described by this model. Some data on impact of growth conditions of the composition of solid solution GaP_xAs_1-x(001) published in the literature can be analyzed with the model.

II. Model for formation of the composition of solid solution gapxas1-X(001) based on mbe

The provisions and concepts in relation to the growth process of А^IIIB^V compounds based on MBE are presented below. It is assumed that molecules of V group colliding with the substrate pass to the physically adsorbed state. This transition process is an activationless process. In this state, they migrate over the surface. Physically adsorbed particles are either desorbed or chemically adsorbed, or, coming to a break of the step and being disintegrated, they are incorporated into the crystal [1], [2].

In case of MBE of А^IIIB^V, terrace surface is known to be reconstructed. Each structure has its own degree of surface coverage with dimers of V group elements (θ). The surface structure with θ<1 can be presented as orderly arranged groups of dimmers separated with dimer-free zones. Chemical adsorption of V group molecules from the physically adsorbed state occurs on centers specified by the terrace surface structure. This process is non-dissociative [3]. Chemical adsorption results in accumulation of dimers of V group elements that are excessive for this structure.

Chemically adsorbed molecules of V group elements can be desorbed. Desorption occurs in the form of diatomic molecules. Activation energy for desorption of dimers included in the structural cell (structural dimers) is higher than activation energy for desorption of molecules chemically adsorbed on structural dimers or on gallium atoms positioned between them (off-structure dimers) [3], [4].

Steady-state concentration of excessive arsenic dimers is established at the surface in the course of time at fixed substrate temperature and density of As₂ molecule flux. If fluxes of molecules of V group are simultaneously directed to the reconstructed surface, they will compete with each other for chemical adsorption sites. If activation energy of desorption from the chemically adsorbed state of one element differs from activation energy of desorption of other element, the surface composition will change during establishment of steady-state conditions. This is associated with the fact that being desorbed molecules vacate chemical adsorption sites. However, dimers having less energy of binding with the sorption are desorbed more efficiently. This shifts the surface composition toward the element, molecules of which have a stronger binding with the surface.

This model takes into account growth rate of epitaxial layer V_g, as well as finiteness of rate the processes responsible for formation of the surface composition in the flux of V group elements. In order to take into account the impact of V_g, it is assumed that the defect-free reconstructed surface of the terrace is generated at degree of surface coverage with dimers of V group elements (θ) that is typical for this reconstruction. Chemical adsorption of V group begins at the newly formed surface, which, as mentioned above, results in formation of arsenic and phosphorus dimers that are excessive for this structure. Chemical adsorption of molecules of V group elements and desorption of off-structure dimers last until the growth front of the overlying terrace comes. When the growth front passes, all structural and off-structure dimers are incorporated into the crystal. If the steady-state concentration of off-structure arsenic and phosphorus dimers has no time to be established, change in V_g must result in changing the surface composition coming under the growth front of the upper terrace. In this case the composition of the solid solution will depend on growth rate.

The composition formation process for the solid solution can be conditionally divided into three stages within the framework of the proposed model.

1. Accumulation of off-structure arsenic and phosphorus dimers on terraces.

2. Filling of remaining vacancies with V group elements during passing of the growth front of the overlying terrace.

3. Formation of the reconstructed surface behind the growth front with surface coverage degree .