(Invited) Point Defect Characterization of Group-III Nitrides by Using Monoenergetic Positron Beams

The nitride semiconductor material system has played a dominant role in the current success of the solid-state lighting (SSL) industry. Because the recent progress of the SSL technology has been mainly driven by light-emitting diodes (LEDs) that use GaN and Ga-rich InGaN, many studies have been carried out on those alloys. The potential of InGaN as an SSL material comes from the ability to control band gaps from 0.65 eV to 3.43 eV, which correspond to entire wavelengths from infrared to ultraviolet regions. Because those band gap energies can cover the solar spectrum, InGaN is also recognized as having a high potential as a photovoltaic device material.

The external quantum efficiency (EQE) of an InGaN-based LED, however, is known to drop rapidly with increasing InN composition. The EQE decrease is attributed to (i) piezoelectric fields in multiplayer structures which cause a separation between electron and hole wavefunctions, and (ii) the degradation of crystal quality. For InGaN-based tandem solar cells, the degradation of conversion efficiency occurs when InN composition increases. During growth of InGaN layers, the large difference in the lattice constants of InN and GaN cause unusually high strain to be introduced into the layer, and this causes dislocations, stacking faults, misorientations, phase separations, etc. Native point-defects are also known to act as non-radiative recombination centers in group III nitrides. Thus, further knowledge regarding such defects is needed to improve the optical and electrical properties of InGaN-based LEDs and solar cells.

Positron annihilation is a powerful technique for evaluating vacancy-type defects in semiconductors [1], and the defects in group-III nitrides have been investigated using this method [2-6]. In the present study, we have used monoenergetic positron beams to probe native vacancies and deformation-induced defects in GaN and InGaN grown by MOCVD, MBE, and HVPE. For InGaN, it was found that vacancy-type defects were introduced with increasing InN composition, and the major defect species was identified as complexes between a cation vacancy and a nitrogen vacancy. The presence of the defects correlated with lattice relaxation of the InGaN layer and the increase in photon emissions from donor-acceptor-pair recombination.　The concentration of the defects, however, was found to be suppressed by Mg doping, suggesting that Mg is an excellent suppressor of cation vacancies in InGaN. The introduction mechanism of vacancy-type defects and their interactions with impurities will be discussed.

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