This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES 1 RF Dielectric Loss Due to MOCVD Aluminum Nitride on High Resistivity Silicon Feyza Berber, Derek W. Johnson, Kyle M. Sundqvist, Edwin L. Piner, Member, IEEE, Gregory H. Huff, Senior Member, IEEE , and H. Rusty Harris, Senior Member, IEEE Abstract—The contribution of high-frequency losses from an aluminum nitride (AlN) layer on high resistivity silicon (Si) is reported. The AlN, deposited on silicon via metalorganic chemical vapor phase deposition as a nucleation layer for subsequent gallium nitride growth, is analyzed for its contribution to the dielectric losses from 6–20 GHz and differentiated from the loss due to the p-type layer formed in the silicon substrate. It is found that AlN is a stronger contributor to overall dielectric loss in comparison with the silicon substrate. Index Terms— AlGaN/GaN heterostructure field-effect tran- sistor (HFETs), aluminum nitride (AlN) dielectric loss, gallium nitride (GaN) HEMT RF loss, metalorganic chemical vapor phase deposition (MOCVD) GaN-on-Si. I. I NTRODUCTION G ALLIUM-NITRIDE (GaN) semiconductors are typically formed with heteroepitaxial growth due to the paucity and cost of high quality native substrates. While early GaN growth developments were concentrated on sapphire and sili- con carbide substrates, high quality GaN films grown on high resistivity silicon (Si) substrates have advanced significantly in recent years. Despite a greater lattice and thermal mismatch with GaN and a higher dislocation density, Si is a promising substrate due to its low cost and future ease of integration possibilities of GaN and Si-based devices [1]. It has been shown that heterostructure field-effect transis- tors (HFETs) can be realized on GaN-on-Si templates with good performance and device reliability [2], [3]. However, high power and high frequency performance of Si-based GaN devices is still limited by material quality. Several performance degradation reports in GaN HFETs have been related to threading dislocations (TDs), mainly related to a decrease in free carrier concentration, transverse mobility degradation, and leakage currents [4]–[9]. These effects become prominent when TD densities approach 10 9 cm -2 . Fig. 1 shows a cross- sectional transmission electron microscopy (TEM) image of Manuscript received April 22, 2016; revised August 23, 2016 and December 25, 2016; accepted December 26, 2016. This work was supported by the Office of Naval Research under Grant N00014-12-1-0971. F. Berber, D. W. Johnson, G. H. Huff, and H. R. Harris are with the Depart- ment of Electrical & Computer Engineering, Texas A&M University, College Station, TX 77843 USA (e-mail: feyza@tamu.edu; dwjohnson87@gmail.com; ghuff@tamu.edu; rusty.harris@tamu.edu). K. M. Sundqvist is with the Physics Department, San Diego State University, San Diego, CA 92182 USA (e-mail: ksundqvist@sdsu.edu). E. Piner is with Materials Science, Engineering and Commercialization Program and the Physics Department, Texas State University, San Marcos, TX 78666 USA (e-mail: ep26@txstate.edu). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMTT.2017.2656865 Fig. 1. Cross-sectional TEM of a GaN-on-Si used for HFET structures depicting the substrate and the epitaxial layers with the dislocations. the layers used to grow high-quality GaN-on-Si. It is clear that the dislocation density is considerably high throughout the various III–nitride layers and is greatest in the insulating AlN layer. The effect of TDs on the GaN frequency response is not well understood. While simulations of current gain as a function of frequency [5] for different dislocation densities seem to indicate a higher degradation in frequency performance, much work remains to be done. Microwave loss of commercially available GaN-on-Si device layers, where coplanar waveguide (CPW) structures are formed on the high-quality GaN with no aluminum gallium nitride (AlGaN) heterostructure, demonstrated increased RF losses compared to comparable structures on Sapphire [10]. Previous reports, assuming 10-k·cm Si substrate resistivity and insulating/semi-insulating epilayers, indicate that this extra loss is attributed to a parasitic conductive layer of ∼2 μm in the silicon at the AlN–Si interface [11]. However, the AlN layer grown on Si (111) is highly defective and has the potential to provide significant scattering and trap centers that can lead to RF loss. A comprehensive study of exactly where the losses occur is needed beyond lumping losses into a single effective layer in the substrate. In this paper, we systematically examine the contribution of the Si substrate and the AlN layer to determine and quantify their individual contributions to microwave loss mechanisms. The Si and AlN-on-Si substrates used in this paper are described in Section II. Sample preparation, including CPW fabrication and substrate treatments prior to photolithogra- phy, and transmission line parameters are also included in Section II. Section III consists of description of the experi- mental setup, summary of measurement results, and discussion of loss mechanisms. Sonnet simulations are used to determine 0018-9480 © 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.