The morphology of an InP wetting layer on GaAs

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Abstract

The effects of material intermixing, group V atom desorption and exchange on the surface morphology of InP wetting layers on GaAs substrates were studied by varying the growth temperature and coverage in metalorganic vapor phase epitaxy. Tertiarybutylphosphine (TBP) was used as the phosphorus source and the surface morphology was characterized by atomic force microscopy. The observations showed that the morphology depends strongly on the growth parameters and deteriorates with increasing temperature and decreasing InP coverage. It was verified that the main reason for the morphology impairment is the strongly temperature dependent group V atom exchange and desorption within several monolayers on the sample surface during exposures of the GaAs surface to TBP. However, a smooth morphology could be obtained within a wide temperature range by depositing at least a complete monolayer of InP.

Introduction

The epitaxial growth of heterostructures is a common means of band structure tailoring in modern electronic and optoelectronic components. It is well known that the growth of epitaxial layers is categorized into different modes which describe the evolution of the layer. Under certain conditions a lattice mismatched layer can grow in the coherent Stranski–Krastanow (SK) mode in which self-assembled islands are formed on top of a wetting layer a few monolayers thick [1]. This growth mode has been observed in many systems such as InAs/GaAs [2] and InP/GaAs [3]. From energy-based considerations, the occurrence of the SK mode can be explained by the interplay of the surface free energies of the two materials and the interfacial free energy which includes the strain energy of the layer [4]. However, since the commonly used epitaxial techniques, such as molecular beam epitaxy (MBE) and metalorganic vapor phase epitaxy (MOVPE), are non-equilibrium methods, other effects must also be taken into account. For example, rather high temperatures are required during MOVPE growth to ensure proper precursor decomposition. On the other hand, at high temperatures adatom desorption [5] and material intermixing at heterointerfaces [6] are non-negligible. Therefore, the growth of materials in the SK mode is affected by the growth parameters. For instance, the abruptness of the interface between the substrate and the SK wetting layer or a quantum well can be controlled by using optimized gas switching sequences [7].

A particular material system exhibiting the aforementioned intermixing effects is InP on GaAs. As long as these effects are under control the material system has numerous applications. For example, thin InP layers have been used for passivation of GaAs surfaces [8] and near-surface AlxGa1−xAs quantum wells [9]. The InP/GaAs material system exhibits Stranski–Krastanow island growth for InP layers thicker than the critical thickness of about 2.2 ML [10]. The self-organized islands have been used as stressors to produce strain-induced quantum dots [11]. The interface quality of MOVPE-grown quantum wells has been studied and optimized in the In(Ga)P/GaAs material system by photoluminescence measurements [12], ellipsometry [13] and reflectance anisotropy spectroscopy [14].

In this work, we have investigated the effects of growth temperature and group V purging on the morphology of MOVPE-grown ultra-thin InP layers on GaAs. We show that a thin InP layer on GaAs can be grown maintaining the smooth morphology of the semiconductor surface within a wide range of temperatures.

Section snippets

Experimental procedure

The samples were grown in an atmospheric-pressure horizontal MOVPE reactor [15] using trimethylgallium (TMGa), trimethylindium (TMIn), tertiarybutylarsine (TBAs), and tertiarybutylphosphine (TBP) precursors. The substrate material was vicinal semi-insulating (1 0 0) GaAs. After deoxidizing the substrates at 700 °C for 5 min a 150 nm thick GaAs buffer layer was grown to achieve an atomically flat surface. After growing the buffer layer the growth was interrupted for 1 s under TBP flow to purge the

Results and discussion

The effect of InP coverage on the sample morphology was studied by depositing 0.3–1.8 ML of InP on GaAs at 610, 635, and 655 °C. AFM images from the samples grown at 635 °C are shown in Fig. 1. The images show that at nominal coverages above 1 ML, yet below the Stranski–Krastanow transition threshold, the sample morphology is mainly two dimensional (2D). The layers grow mainly by 2D island nucleation, lateral growth and coalescense with increasing coverage. Consequently, during the growth of 1 ML

Conclusion

Thin InP layers were grown by MOVPE on vicinal GaAs substrates and the effect of growth temperature and coverage on the sample morphology was characterized by atomic force microscopy. All InP coverages were below the critical coverage of the Stranski–Krastanow transition. At coverages above 1 ML, the surface morphology remained 2D in the growth temperature range of 610–655 °C. Below 1 ML the morphology was observed to be 2D only for samples grown at 610 °C. A 3D surface morphology with clusters of

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