LED TECHNOLOGIES DEPARTMENT

Control and structure of the material

Any light emitting diode (LED) is based on a crystal which emits light due to current spreading. Structure and quality of a grown crystal influence the light output. Internal defects such as microcraks and threading dislocations decrease emission quality drastically.           

In order to achieve low defect density, it is important to accurately select growth conditions. Material widely used in LED production is Gallium nitride (GaN) films grown on a sapphire substrate with the help of epitaxial method in a special reactor. Match in film and substrate lattice parameters is necessary for a perfect defectless growth. Unfortunately, it is hardly achievable in practice due to highly expensive use of GaN as a substrate material for epitaxial growth. Sapphire is much cheaper than GaN and its lattice mismatch with GaN accounts for 14% that leads to the high density of linear defects of crystal structure, e.g. threading dislocations. Moreover, in the course of epitaxial growth in a reactor, mechanical stresses build up in a film and, if high, such stresses can lead to cracks formation.

Optogan NTL focuses its research of structure control and material engineering on determining  the defect influence on internal stresses in GaN films as well as on the search for methods of defect density reduction, e.g. via interlayers or porous structures formation. Another important issue is the reduction of mechanical stresses generated in films during growth. Regular porous structure formation as the interlayer can improve the quality of the grown structure.

Figures 1 and 2 illustrate the results obtained by Optogan NTL research department in the course of study of internal pores influence on crystal structure quality and, therefore, on the light output.

Fig. 1 illustrates an example of porous structure formed in GaN film.

Fig. 2 illustrates the influence of a pore shape on emission quality of LED chip. The 60 degree sidewall inclination angle of the pores results in 21% improvement in the light output in comparison with porousless structure. Improvement in the light output is caused by both reduction of threading dislocation density due to optimal porous structure formation and light scattering on the internal pores boundaries. 

 

Fig. 1. Porous structure formation in GaN films on a sapphire substrate. [M. Ali, A.E. Romanov, S. Suihkonen, O. Svensk, P.T. Torma, M. Sopanen, H. Lipsanen, M.A. Odnoblyudov, and V.E. Bougrov, "Void shape control in GaN re-grown on hexagonally patterned, mask-less GaN", Journal of Crystal Growth, 2011, vol.315. No 1, pp. 188-191]

a) 

b)  c) 

Fig.2. a) porousless structure; b) 85 degree pores; c) 60 degree pores.