HVPE magnifies perfect crystals

Ideally, GaN devices would be grown on native substrates. However, the large, high-quality crystals that are needed to form them cannot be synthesized by the stoichiometric methods used to produce GaP and GaAs crystal growth because to melt GaN requires extreme conditions of 2225 °C and 6 GPa. So instead devices are usually fabricated on foreign substrates, such as sapphire, which have different lattice constants and thermal conductivities. The differences ultimately limit device performance because they lead to high defect densities in the epilayers – even when advanced techniques such as epitaxial layer overgrowth or low temperature buffer growth are employed.

The desire for low defect density material has prompted the recent launch of free-standing 2 inch GaN wafers by several companies, including Technologies and Devices International, Lumilog and Kyma Technologies. Hydride Vapor Phase Epitaxy (HVPE) is used to deposit GaN on a foreign substrate, which is subsequently removed to leave a free-standing GaN layer with a typical dislocation density of 106 cm-2. This platform is suitable for LEDs and lower-power lasers diodes, but restricts the performance of high-power lasers that would benefit from dislocation densities of less than 105 cm-2.

Much lower defect densities can be produced by forming crystals from a gallium-based solution in a high-pressure nitrogen environment, which is an approach that we have developed at TopGaN and the Institute of High Pressure Physics in Warsaw, Poland. This method can deliver dislocation densities below 100 cm-2, but it is restricted to a maximum temperature of 2333 °C. This corresponds to nitrogen gas pressure of 2 GPa and a nitrogen concentration of less than 1 atomic percent, and leads to very low growth rates that limit crystal dimensions to 1-2 cm.

Most of the crystals produced with our high-pressure technique are thin, hexagonal platelets with major surfaces along {0001} polar crystallographic planes of the wurzite structure (see figure 1a). These platelets take between 100 and 150 h to grow and are 10–20 mm long and 80–120 μm thick. At very high super-saturation conditions partially hollow needles with elongated c-directions are also formed, which have well developed {10  1bar0} faces (see figure 1b).

We have used these platelets to grow high-power laser diodes and GaN-based structures with record carrier mobilities (see "Lasers grown on GaN crystals"). However, their limited size is a major drawback, which has led us to develop a HVPE process for extending the size of our platelet and needle-shaped crystals (see figure 2). HVPE offers growth rates of 100–200 μm/h in both polar and non-polar directions, but can suffer from a parasitic deposition that involves the formation of GaN particles that can then fall onto the crystal. This unwanted process significantly alters the local growth conditions and makes it difficult to grow large bulk GaN single crystals by HVPE.

We have used HVPE to grow flat and uniform layers on the pressure-grown platelets at a rate of 100 μm/h (see figure 3). Non-polar cross-sections of these extended crystals reveal that the material is macroscopically continuous and that growth proceeds in both the polar (0001) direction and other directions perpendicular to the crystal s c-axis. The electrical and optical properties of these crystals are direction-dependent, according to Jan Weyher, a member of our research group who also holds a position at Radboud University Nijmegen, the Netherlands.

Complementary photo-etching, micro-Raman spectroscopy and photoluminescence measurements show that the free electron concentration is often less than 1017 cm3 for the growth in (0001) direction, but is about 1019 cm-3 for lateral directions such as and the (000 1bar) N-polar direction. This dependence of the material properties on the growth direction impacts crystal enlargement by HVPE. At high growth rates, local changes in growth direction can take place at the edges of 3D islands or columns, macrosteps and pinholes, and lead to inhomogeneities in physical properties that ultimately produce strain and defect generation.

We have found that our bulk GaN crystals, which are grown directly on almost dislocation-free platelets have dislocation densities of up to 106 cm-2 and low-angle grain boundaries. The defects were generated in n-type and highly resistive magnesium-doped substrate material after exceeding critical thicknesses of 30-50 mm and typically 100 mm, respectively.

However, these defects can be overcome by reducing the growth rate to 20 μm/hr and with this approach we have grown almost dislocation-free 100 mm thick films on n-type substrates. The improvement results from the suppression of growth mechanisms that can generate strain, and further studies of this approach may enable routine production of these very high-quality hybrid crystals.

The high-pressure technique can also form highly crystalline GaN needles. These needles suffer from an unstable morphology, but HVPE growth on these seeds stabilizes the shape and produces a transparent and colorless or slightly yellow top layer (see figure 4). The free electron concentration in these crystals, which are grown entirely in directions, is 5–8 × 1018 cm-3.

Using the same growth conditions and rates used for platelet growth leads to larger needles with a crystallization front that shows no deterioration with additional growth. This allows their size to be bolstered with subsequent HVPE growth runs (see figure 5a). With this approach we have increased the size of our needle-shaped crystals through a series of four separate HVPE runs, but it is obviously preferable to enlarge crystals by the same amount in a single, longer run. However, this requires a new HVPE reactor that is capable of continuous operation for more than 100 h.

We have analyzed the defects in these crystals by etching the material in a molten potassium hydroxide and sodium hydroxide eutectic, and then examining polar and non-polar cross-sections. Our investigation reveals no deterioration in material quality with subsequent HVPE growth runs. More than half of the crystal usually has a defect density below 104 cm-2 and the seed crystal is usually free of dislocations. On areas of the (0001) surfaces there are areas with non-uniformly distributed hexagonal etch pits that spread from the morphological imperfections in the seed. Etching of non-polar [11 2bar0] surfaces reveals lines that are perpendicular to the crystal s c-axis, which have been identified with transmission electron microscopy (TEM) basal stacking faults.

We have grown heterostructures on 300 μm-thick substrates with lengths of 5–9 mm and widths of 2–3 mm, which we formed by slicing our HVPE-grown platelet crystals in the (11 2bar0) direction. These substrates are non-polar and provide a platform for making light-emitting devices that emit polarized light and are free from piezoelectric fields, which can reduce emission intensity.

We have deposited a tenfold GaN/AlGaN sequence of 2, 3, and 4 nm-thick GaN quantum wells separated by 7 nm Al0.11Ga0.89N barriers onto these non-polar substrates with plasma-assisted MBE. The quality of this heterostructure, according to TEM images and X-ray diffraction plots, is equivalent to that of our control structure, which was grown on a polar GaN substrate for comparison.

The quantum well structure grown on the non-polar substrate produces an emission intensity that is typically 100 times brighter than the equivalent structure on polar GaN. Photoluminescence measurements from these samples at low temperatures reveal a series of well-resolved excitonic lines. The position of these spectral features reveals that there is no built-in polarization field in the multi-quantum-well samples.

Non-polar substrates can also be fabricated from HVPE-extended high-pressure needles. This material has a high electrical conductivity, making it particularly attractive for laser fabrication. The HVPE method is well suited to enlarging high-quality pressure-grown crystals, but we need to carry out further studies regarding the influence of growth mechanisms on defect generation to improve the yield of our extended platelet crystals. Fabrication of extended needle-shaped crystals by HVPE is a much more stable process and more extended growth times are the only barrier to the production of a larger form of crystal production. Improvements to our extended crystal production process can also be made through more careful and thorough selection and preparation of seeds, which would eliminate parasitic 3D nucleation that takes place on seed imperfections.

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