The direct influence of polarity on structural and electro-optical properties of heteroepitaxial GaN

Abstract

Different polarities of heteroepitaxial GaN layers are believed to stem from different growth conditions. It follows then that the difference in structural and electro-optical properties between Ga-or N-polar samples can be assumed to have the same cause. An unintentionally n-type doped GaN layer has been grown exhibiting both polarities on the same, single two-inch sapphire substrate, which allows for a thorough study of the differences between those two polarities. In such a case the growth conditions are the same and thus the variation of all the properties must come from the sample's polarity or its influence on the growth process.Transmission electron microscopy and etching techniques confirm the large variation in morphology across the surface of the sample to be the result of the polarity difference. Wet chemical etching in a mixture of phosphoric and sulfuric acids, as well as photo-electrochemical etching in aqueous KOH solution, reveal very different defect structures for the two polarities. Hall and PL measurements show a large discrepancy in electro-optical properties suggesting preferential impurity incorporation in the N-polar GaN, which is also confirmed by results of SIMS measurements.

Optical and Microstructural Properties of N- and Ga-Polarity GaN

“…The difference in luminescence intensity can be attributed to a higher donor concentration in the N-polarity material. This is consistent with the idea that N-polarity material incorporates more impurities than Ga-polarity material [1][2][3][4][5]. It is also possible that the N-polarity material has a higher density of point defects such as Ga-or N-vacancies which contribute to the luminescence intensity.…”

“…These differences in the Ga bonding at the growth surface lead to differences in impurity incorporation, and therefore the material properties of the two polarities can be very different. N-polarity material tends to incorporate more oxygen [1][2][3][4], carbon [1][2] and aluminum [2]. Si incorporation was not found to be effected by polarity [3].…”

“…In Mg-doped GaN, it was found that no extra impurities are incorporated in the N-polarity material, but it was also found that p-type doping could not be achieved in N-polarity material [5]. By varying the nucleation conditions during growth, the polarity of GaN can be selected [3][4][5][6][7]. In this paper we report on the optical and structural properties N-and Ga-polarity GaN grown simultaneously using different nucleation layers.…”

Influence of sapphire annealing in trimethylgallium atmosphere on GaN epitaxy by MOCVD

“…This Ga-polar sample exhibits a well-defined spectrum showing a donor bound exciton peak (D 0 X) peak at 3.482 eV and the free exciton A (X A ) at 3.488 eV. On the other hand, the PL spectrum from sample #2 shows an extremely broad peak centered at 3.463 eV, which is typical for N-polar material [24]. The PL spectrum observed for sample #6 differs from the two other samples-the peak shifted to 3.495 eV suggests that the sample is under compressive strain [10,25].…”

GaN based nanorods for solid state lighting

“…In general, besides the orientation of the Ga-N bonds, there is no crystal structure difference between the Ga-polar and N-polar GaN nanorods. However, different polar surfaces induce different surface chemical reactivity, which is important, e.g., in the case of the impurity doping, 55,56 dopant incorporation, 57,58 surface reaction with gas, 59 and growth morphology of the GaN nanorods. 39,42 There are already numerous investigations reported on two-dimensional GaN layers with different polarities concerning these topics.…”