Ion-Assisted Pulsed Laser Deposition of BN Films
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G. L. DOLLa, D. C. CHANCEb, AND L. SALAMANCA-RIBAc aGeneral Motors Research & Development Center, Warren, Michigan 48090 bPhysics Department, Wayne State University, Detroit, Michigan 48201 cUniversity of Maryland, College Park, Maryland 20742 US
ABSTRACT Boron nitride films grown by ion-assisted pulsed laser deposition have been characterized by infrared absorption, Auger electron spectroscopy, and transmission electron microscopy. Elemental bonding and the crystallinity of BN films grown in three nitrogen ion energy regimes: high (2500 eV), low (700 eV), and without ions (0 eV) are examined, and the results interpreted within the framework of a compressive stress mechanism for cBN film growth.
INTRODUCTION Boron nitride crystallizes in four major structures -- hexagonal (hBN), rhombohedral (rBN), wurtzitic (wBN), and cubic (cBN), all of which are isostructual to specific phases of carbon. The crystal structures of these four phases are illustrated in Figure 1 [1]. The soft hBN and rBN phases are comprised of sp2 bonds, while the hard wBN and cBN phases have sp 3 bonds. The four BN phases are useful in tribological applications, but because its properties closely resemble those of diamond, cBN is highly valued as a high temperature electronic material as well. The physical properties of the different phases of BN, as well as most other I1l-N compounds have been recently compiled in a data review by Edgar [2]. Pouch and Alterovitz [3] have compiled a review of deposition techniques used to grow boron nitride. Generally, only those deposition techniques that employ ions in the process are successful in growing cBN. In this work, we review some of our research into the growth and characterization of BN films by ion-assisted pulsed laser deposition undertaken over the past several years.
EXPERIMENTAL DETAILS BN films examined in our studies were grown by ion-assisted pulsed laser deposition. In this technique, a pulsed KrF laser (X=248 nm) is focused onto a pyrolytic hexagonal BN target. The BN from the target evaporates and subsequently condenses onto a substrate (n-type Si (001) wafers) heated to temperatures ranging from 400 to 600 °C. When cBN films are desired, nitrogen ions from a Kaufman-type broad beam ion source are utilized. Our cBN films are grown in two distinct ion energy regimes, high and low. We define the low ion energy regime as nitrogen (N2÷/N÷) ions with energies of 500 to 1000 eV, and the high ion energy regime as nitrogen ion energies with greater than 2000 eV. Whereas a Kaufman source is the sole source of the low energy regime ions, substrate bias is added to obtain the high energy ion regime. Films that are grown under conditions with no ions or with ion energies less than 500 eV, are not cBN, but are nano-crystalline sp2 bonded materials. Deposition parameters for the films discussed in 165 Mat. Res. Soc. Syrmp. Proc. Vol. 388 01995 Materials Research Society
this work are given in greater detail in earlier publications [4-6]. In this work, we examine BN films grown by pulsed laser deposi
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