Crystal Growth and Thermal Conductivity of Boron Arsenide

With the rapid development of modern microelectronic devices, materials with high thermal conductivities are crucial to effectively cooling high-power-density electronic and optoelectronic devices due to the ever-shrinking dimensions and compacting units. In principle, heat is transferred via conduction, convection and radiation, of which directly conduction is the easiest and most efficient way. Nowadays, metal heat sinks are the most widely used in industry, due to their high thermal conductivities (200 - 500 W m-1 K-1). Meanwhile, the materials with higher thermal conductivities are so far restricted in laboratory research because of the difficulties in synthesis, high cost, or anisotropic conductivities. More related theories and designs are required for scientists and there exists a huge space for improvements. Recently, zinc-blende boron arsenide (BAs) has been predicted to have an ultra-high thermal conductivity of over 2000 W m-1 K-1 at room temperature by first-principles calculations, rendering it a close competitor for diamond which holds the highest thermal conductivity among bulk materials. Experimental demonstration, however, has proved extremely challenging, especially in the preparation of large high quality single crystals. Although BAs crystals have been previously grown by chemical vapor transport (CVT), the growth process relies on spontaneous nucleation and results in small crystals with multiple grains and various defects. Here, we report a controllable CVT synthesis of large single BAs crystals (400 - 600 um) by using carefully selected tiny BAs single crystals as seeds. We have obtained BAs single crystals with a thermal conductivity of 351 ± 21 W m-1 K-1 at room temperature, which is almost twice as conductive as previously reported BAs crystals and is comparable to most of the metal heat sinks. Furthermore, to satisfy the current research purpose of measurement and future application purpose of industry, efforts to grow BAs single crystals larger have been made. By carefully controlling the nuclei preferences, transportation rates, temperature gradients and dynamics during the CVT process, BAs single crystals of 3 mm have been obtained, which is a magnitude higher than the largest reported size (200 um). Such progress, which is transferring BAs from a microscope scale to a raw eye scale, makes BAs crystals suit more physical and chemical properties measurements, leading BAs into a substantial availability of research and a considerable possibility of applications. Further improvements along this direction seems very likely.