Epitaxial growth of III-Nitrides

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CHAPTER 2

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grow films that are of better quality than the substrate and also to grow epilayers that have different doping levels.

Heteroepitaxy is when the epilayer and the substrate are of different materials.

This epitaxial growth technique is used to grow crystalline films of materials for which crystals cannot otherwise be obtained and also to grow integrated crystalline layers consisting of different materials. Materials like the III-Nitrides that are grown on sap- phire, SiC and other substrates are examples of heteroepitaxial growth.

In heteroepitaxy, three primary growth modes can be observed depending on the surface energy and the lattice mismatch between the epilayer and the substrate.

These are the Volmer-Weber (V-W), Frank-van der Merwe (F-M) and the Stranski- Krastanov (S-K) modes and schematically they are presented in Figure 2.1.

Figure 2.1: Primary growth modes in heteroepitaxial growth (a) Volmer-Weber, (b) Frank-van der Merwe and (c) Stranski-Krastanov modes. In each mode the evolution of growth with in- creasing coverage of the substrate in monolayers (MLs) is shown.

In the Volmer-Weber growth mode, the adatom- adatom interactions are strong- er than those of the adatom with the surface. The outcome is the formation of three- dimensional adatom clusters and/or islands. Further growth of these clusters will result to a rough film on the substrate.¹

In the Frank–van der Merwe growth, the adatoms are attached preferentially to surface sites resulting in atomically smooth and fully formed layers. This growth mode is also referred as a layer-by-layer two-dimensional growth.²

Stranski–Krastanov growth has an intermediate character of the two previous ones and is characterized by both 2-D layer and 3-D island growth. After a critical thick- ness had been obtained, transition from the layer-by-layer to island-based growth oc-

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curs. This process is highly dependent on the chemical and physical properties, such as surface energies and the lattice mismatch between the epilayer and the substrate.³

The growth techniques that are commonly used for the synthesis of the III- Nitride heterostructures and nanostructures are the Hydride Vapor Phase Epitaxy (HVPE), the Metalorganic Vapor Phase Epitaxy (MOVPE) and the Molecular Beam Epi- taxy (MBE). The last one was the growth technique used for all the samples that were studied in this thesis.

2.1.1 Molecular Beam Epitaxy (MBE)

The MBE is an epitaxial growth technique that was mostly used for the realization of semiconductor heterostructures materials since the 1970’s and especially for the growth of the III-V semiconductors such as GaAs.⁴ The MBE operation temperature is much lower than the vapour phase epitaxial growth techniques, and it is directed by the surface kinetics (adsorption, desorption, diffusion and incorporation). The growth in MBE requires ultra-high vacuum (up to 10^-11 Torr) and the elements sources are heated and evaporated onto an also heated substrate. The rate of evaporation of the elements can be controlled and the element sources are pure metal sources (which are called Knundsen effusion cells or K-cells). The dopants –if used- are provided in the same form. Regarding the nitrogen source, ammonia may be used. Instead of ammonia, an- other commonly used source is the nitrogen plasma source and then the MBE system is referred as Plasma-Assisted MBE or PAMBE in which the nitrogen gas is activated through radio-frequency (RF). In Figure 2.2 a typical MBE chamber is shown. Both the N flux ratio and the excitation power of the plasma source can be adjusted in order to con- trol the growth rate.

While the MBE is characterized by lower growth temperatures, the growth rate is also low compared to the other techniques (~1µm/ h) and this is the reason why MBE is not widely used for industrial scale growth. On the other hand, the low growth rate provides enough time for the migration of the elements on the surface, resulting to atomically sharp interfaces and flat surfaces. Additionally heterostructures consisted of different materials and compositions are also possible to be grown, since with this tech- nique very thin films can be acquired.

In heteroepitaxy between materials (substrate and epilayer) with high lattice mismatch, such the nonpolar/semipolar III-Nitrides on the r-plane sapphire, it is more energetically favourable the formation of 3D islands instead of a layer-by-layer growth, due to elastic strain. In MBE, which employs lower growth temperatures, the density of such 3D islands is increased. The strain relaxation of these islands corresponds to lattice mismatch between them, thus, when they coalesce, additional threading defects are in-

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troduced from their boundaries. In the same time though, the higher density of 3D is- lands leads to a faster coverage of the substrate surface in lower thicknesses.

In order to avoid this phenomenon, a two-step growth is employed,⁵ which con- sists of the growth of an initial nucleation layer (or buffer layer) which is usually grown on much lower temperatures than the ideal one for the epitaxial growth of the epilayer.

In this way, the defects are suppressed in some extend to the nucleation layer and at the same time we have the formation of a uniform epilayer with sharp interfaces. To achieve that, the growth of the nucleation layer must be controlled in such ways, so the final film is still of a high crystal quality. Regarding the nonpolar growth, Tsiakatouras studied the influence of the growth parameters of the nucleation layers (GaN and AlN) and the results are presented in Ref [⁶]. He reported that the use of either GaN or AlN nucleation layers improve the crystal quality of the films which have good crystal quali- ty and low surface roughness, but regarding the film mosaicity, AlN appears to be more efficient than the GaN nucleation layer.

Figure 2.2: Schematic of an MBE chamber.⁷

2.1.2 The sapphire substrate

Sapphire is the most common foreign substrate for the epitaxial growth of the III- Nitride materials. The popularity of this substrate is due to its low cost and the availabil- ity of large templates of high quality. Despite these reasons, sapphire exhibits large dif- ferences in the lattice constants with the III-Nitride materials, resulting to high mis- match, and also in the thermal expansion coefficients. Sapphire is stable at high temper- atures and also is transparent, suitable properties for the optoelectronic applications.

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Sapphire is a gemstone variety of the mineral corundum, which is an aluminium oxide (α-Al2O3). It has either rhombohedral unit cell, but also can be described by a hex- agonal cell that is larger than the basic rhombohedral unit cell.⁸ In Figure 2.3, the sur- face planes of sapphire are shown. The lattice parameters are a=4.754 Å and c=12.99 Å.

The most commonly used orientations of sapphire as substrates in the growth of III- Nitrides are the c-plane (0001), m-plane(1010) , r-plane (1102) and a-plane (1120) .

Depending on the sapphire orientation that is used for the growth, different epi- layer orientation is achieved. When the nitrides are grown on c-plane sapphire the re- sulting films are of the same orientation as well. Polar orientation of the III-Nitrides is also acquired by growth on a-plane sapphire as well.⁹ Nonpolar films with a-plane (1120) orientation are achieved if the growth is performed on r-plane sapphire. In the last case (growth on m-plane sapphire) the resulting film can be either nonpolar m- plane (1010) or semipolar with (1013) or (1122) depending on the growth conditions.

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Figure 2.3: (a) Surface planes on the rhombohedral structure of sapphire. (b) The sapphire planes viewed along the [0001] direction.