If you wish to cite a paper that describes the techniques and the implementation for buffer k-d trees, please make use of the following work:

Fabian Gieseke, Justin Heinermann, Cosmin Oancea, and Christian Igel. Buffer k-d Trees: Processing Massive Nearest Neighbor Queries on GPUs. In: Proceedings of the 31st International Conference on Machine Learning (ICML) 32(1), 2014, 172-180. [pdf] [bibtex]