Using Crystalline Metals for Solid-State Electronics


By Drs. D. Scott Katzer, David J. Meyer, and Brian P. Downey

The key to the Navy and Marine Corps’ dominance of the electromagnetic spectrum is the ongoing development of next-generation, solid-state electronic devices and circuit architectures. By improving the performance of solid-state electronics technology used in radio-frequency (RF) amplifiers and power switches, new system-level capabilities and functionality can be attained in various applications—including sensing, communications, radar, electronic warfare, efficient power transmission and power management, and new high-performance weapon systems—to ensure a technological advantage for warfighters against emerging threats. The Naval Research Laboratory (NRL) has a long history as a driving force in the basic research of solid-state electronics technology and continues to discover and develop novel electronic materials and devices.

One of the key drivers for advancing this technology’s performance is the development of the underlying materials used to fabricate core electronic devices. Since device performance is fundamentally limited by the properties of the materials used to produce them, researchers are constantly looking for new materials with exotic properties that may open the door to achieving new levels of power, gain, bandwidth, and efficiency performance. To date, nearly all high-performance, solid-state electronic devices are constructed from carefully designed multilayer composites of thin film crystalline semiconductor materials such as silicon (Si), gallium arsenide (GaAs), indium phosphide (InP), silicon carbide (SiC), aluminum nitride (AlN), and gallium nitride (GaN).

While these single crystal or “epitaxial” semiconductor thin films have traditionally been used for the active region of solid-state devices, theoretical predictions made in the 1960s have shown that extremely high-frequency performance can be achieved in materials structures that contain integrated epitaxial metals. Up until now there has been very little progress made toward incorporating epitaxial metals into multilayer device structures because of practical limitations in materials growth and thermodynamic incompatibility with existing semiconductors. A recent basic research breakthrough made at NRL, however, has led to the successful synthesis and integration of a novel epitaxial metal material, niobium nitride (Nb2N), with SiC, AlN, and GaN.

To grow Nb2N crystals that are integrated with SiC, AlN, and GaN in a controlled environment, NRL has built a customized molecular beam epitaxy system with an electron-beam evaporator source capable of evaporating transition metals with melting points greater than 2,000 degrees Celsius. Sophisticated characterization equipment is used during the growth of Nb2N to monitor film growth in real time. Since the crystal structure of the Nb2N film is very similar to that of SiC, the surface of the Nb2N film essentially mimics the SiC wafer surface, allowing it to serve as a high-quality crystalline “template” for AlN, GaN, or SiC electronic material growth. While Nb2N is just emerging from its experimental inception, initial testing indicates that its use will play an important role in advancing Navy and Defense Department system performance and reliability by enhancing current generation electronics and enabling next-generation, solid-state electronics technology.

Current Electronics

A recent example of another material that enables improved system performance is GaN, whose properties allow RF transistors to operate at 10 times the power density of previous generations on GaAs- and InP-based transistors. Consequently, the Navy and the Defense Department have invested heavily in developing this technology over the past two decades. Programs of record such as the Air and Missile Defense Radar and Next-Generation Jammer have adopted active electronically steered array systems (AESAs) based on radiating elements powered by GaN-based monolithic microwave integrated circuit (MMIC) amplifiers.

One of the primary challenges encountered in Navy AESA systems is thermal management of the MMICs. As is the case for consumer electronics such as personal computers, controlling the temperatures of components is essential to ensure highly reliable operation. Under normal RF operation, GaN MMICs experience self-heating that can cause the transistor temperatures to rise to more than 175 degrees Celsius. If the peak temperature in the transistor exceeds 225 degrees Celsius, a known failure mechanism will accelerate rapidly, causing amplifier performance and lifetime to drop off dramatically. For this reason, successfully removing the excess heat from the MMIC is extremely important and requires advanced device, circuit, and packaging design, as well as the incorporation of new materials and processing techniques. One thermal management solution could involve removing the GaN circuit from the SiC substrate (which is only used as a template for the crystal growth of GaN) and transferring it to higher thermal conductivity substrates such as water-cooled copper heat sinks or diamond. SiC’s strong chemical resistance, however, currently precludes a straightforward way to remove or etch away the substrate to leave only the GaN device layers for transferring.

The results of NRL experiments determined that Nb2N possesses a potentially useful property in that it can be selectively dissolved away in a bath of hydrofluoric and nitric acid or reactive xenon difluoride (a gas used in commercial integrated circuit processing), leaving GaN and SiC materials unaltered. By inserting an Nb2N layer between the GaN device layers and SiC substrate, full transistor processing can occur prior to release and transfer to an alternative substrate. NRL is in the midst of developing this process and has had initial success with growth and fabrication of high electrical quality GaN transistors on Nb2N/SiC. In the near future, NRL will be working to demonstrate successful release and transfer of GaN devices to other substrates to investigate potential advantages in thermal management.

Next-Generation Electronics

While epitaxial Nb2N could have a significant effect on current-generation electronics, the longer-term and potentially greater impact for this material capability may involve fundamentally higher performing next-generation, solid-state transistors. Realizing theoretical epitaxial semiconductor/metal/semiconductor devices has so far remained elusive, but the combination of metallic Nb2N and GaN-related semiconductors may provide a long-sought-after solution.

In addition to growing GaN transistors on Nb2N, NRL has been able to grow very thin, high-quality alternating layers of epitaxial Nb2N and GaN materials on SiC substrates. The Nb2N layers have been grown to thicknesses of less than five nanometers (or about 1,400 times smaller than a red blood cell) and remain electrically conductive. This thickness regime is of interest for many revolutionary RF transistor device designs. Work is under way to optimize the growth of these materials to reduce defects and begin fabricating transistors using some of these theoretical device designs. In addition to future solid-state electronics, this new material technology also can affect optical, optoelectronic, and microelectromechanical systems and plasmonic device topologies.

About the authors:

Drs. Katzer, Meyer, and Downey are with the Electronics and Science Technology Division at the Naval Research Laboratory. The authors would like to acknowledge support from the Office of Naval Research and the Defense Advanced Research Projects Agency.