Bulk Aluminum Nitride Crystal Growth

Introduction
Light emitting diodes (LEDs) are ubiquitous in modern society: they are in traffic lights, automobile interiors, backlights in cell phones, and many other applications. Their growing popularity comes from their many advantages over incandescent and fluorescent lamps including a high energy efficiency, long lifetimes, compact size, and shock resistance. Furthermore, they can emit light of a precise color, which is useful for many applications. Currently, commercial LEDs are available that emit light over the entire visible range - from red to blue, plus infrared light. What is lacking are LEDs that emit ultraviolet light. Efficient UVLEDs would have many applications including water purification, sterilization of heat sensitive materials such as plastics, biochemical sensors, and polymer curing. To date, making UV-LEDs has proven difficult. One of the main problems is the poor quality of suitable materials available for device fabrication. To address this problem we are studying the growth of large aluminum nitride (AlN) single crystals. These are being developed as substrates (a structural template) for aluminum gallium nitride (AlxGa1-xN) layers, the base semiconductor for fabricating UV-LEDs and laser diodes (LDs). Aluminum nitride has several physical and chemical properties that make it a superior material as a substrate: a better match of the crystal structure and dimensions (lattice constants); high electrical resistivity; ultraviolet light transparency; high thermal conductivity; and good chemical compatibility with AlxGa1-xN. As a consequence of these properties, AlN substrates should reduce the defect density in AlxGa1-xN layers, thereby increasing device performance (reducing turn-on voltage and increasing power output) and efficiency (power output per power input).

Crystal Growth Process
Because the melting temperature for AlN exceeds 2750 °C at a nitrogen pressure of 100 atmosphere, it is impractical to grow crystals from the melt, as is usually done with the more established semiconductors silicon and gallium arsenide. It can not be grown by precipitation from solution either, as the solubility of nitrogen in most solvents is low (less than 10-3 at% in liquid aluminum at 1000 °C, for example). Chemical vapor deposition may be a suitable route to producing bulk AlN single crystals, but to date, growth rates are low and the feasibility of this technique has not been verified. The most successful method of producing large, bulk AlN single crystals - and the method employed at KSU- is the sublimation-recondensation method. In this process, the overall reaction AlN(s) = Al(v) + ½N2 is run in the forward (sublimation or decomposition) direction at the source, and in the reverse (recondensation) direction in the crystal growth zone. The source is maintained at a higher temperature than the crystal growth zone, causing the direction of the reaction to change with position in the crucible.

While the process is simple to describe, successful implementation is challenging. Extreme temperatures (greater than 2000 °C) are required for a significant aluminum partial pressure and to increase the reactivity of N2 (normally an inert gas), so reasonable growth rates (>100 μm/h) are attained. Aluminum has a tremendous affinity for oxygen, so all oxygen must be eliminatedfrom the crystal growth zone. Consequently, specially designed high temperature furnaces are required. A photograph of the tungsten heating element furnace used for some of our crystal growth studies is shown in Figure 2.

Crucible Materials
At such extreme temperatures, there are relatively few materials thermally and chemically stable for fabricating the crucible and other furnace fixtures. If elements from the crucible volatilize, they can incorporate into the AlN crystals, and degrade its properties. Even in small concentrations, such impurities can increase the crystal’s defect density, change the crystal habit, or cause the AlN to absorb visible wavelengths, changing the color of the crystals. Examples of these effects are shown in Figures 3-5. Identifying how the predominate AlN crystal habits change with the crucible material is one aspect of research at KSU. In addition, the fabrication and performance (durability and failure modes) of different materials crucibles themselves is being studied. Methods of converting refractory metals to nitrides and carbide have been developed. Thermodynamic calculations are employed to determine what reactions specific materials might undergo that would impact their crucible durability. The vapor pressures of the elements from the crucible are predicted at the crystal growth temperature by a free energy minimization. This is helpful to understanding the stability of the crucible material, and its propensity to incorporate elements into the crystal. Currently, the most suitable materials for AlN crystal growth appear to be tungsten and tantalum carbide, as they are inert, stable and durable at the crystal growth conditions. Yet these materials are not perfect, as their coefficients of thermal expansion are much higher than for AlN, putting the crystal under stress as it is cooled from the growth temperature. We are continuing to examine new crucible materials and are developing better crucible designs.

Crystal Growth Procedures
The best method of obtaining a single crystal is to initiate the growth with a single crystal seed. A seed crystal can be used to control the crystal orientation and polarity. Unfortunately, only small AlN crystals are available, limiting the ultimate size of the single crystal. Instead, at KSU single crystals of 6H-silicon carbide are studied as potential seed crystals for initiating AlN crystal growth. 6H-silicon carbide wafers up to 75 mm in diameter are commercially available, which may make it possible to produce large AlN crystals. Its thermal stability is reasonably good at AlN crystal growth temperatures.

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Characterization of the Crystal Quality
The application of AlN single crystals as a substrate requires that it enables better quality epitaxial growth of AlxGa1-xN thin films than is possible on currently favored substrates such as sapphire and silicon carbide. The bulk AlN crystals must have a high degree of crystal perfection (low density of defects), good optical transparency down to wavelengths of 200 nm, and a smooth surface finish suitable for supporting high quality epitaxial thin films. Thus the characterization of the quality and purity of the AlN crystals is an ongoing aspect of research at KSU. The defect types and densities are studied by x-ray diffraction and defect selective etching at KSU. The former method is a general assessment of the crystal quality over relatively large areas. With the later technique, the exact location of the defects can be determined.

Bárbara Scarlett Betancourt Morales

CAF