TECHNOLOGY AREA(S): Electronics, Ground/Sea Vehicles, Weapons

ACQUISITION PROGRAM: PMS 320 Electric Ships Office

OBJECTIVE: Develop an ultrapure, high growth-rate gallium nitride (GaN) epitaxial growth system to enable the realization of novel, vertical, high-voltage (greater than 20kV) power electronic switching and pulse power devices. After success, this material breakthrough will greatly enhance Operational Endurance, one of the key Framework Policies listed in the Naval Research and Development Framework.

DESCRIPTION: Future Navy ships will require high-power converters for applications such as rail gun, AMDR, and propulsion on DDG-51 size ship platforms. High-voltage, high-efficiency power vertical current conducting switches are required to achieve the needed power density. GaN possesses a large energy bandgap of 3.4eV, high breakdown field of 3.5 MV/cm and high bulk mobility (� 1700 cm2/v.s. These properties motivate the development of GaN for high-power, high voltage power vertical current conducting switches for megawatt compact power converters. Currently, commercial Si-based vertical bipolar devices lack sufficient performance to address this market. An important parameter in bipolar power devices, such as pin diodes, is the minority carrier lifetime. Indirect semiconductors such as Si and SiC intrinsically have a longer minority carrier lifetime compared to direct bandgap semiconductors such as GaN. There are several experimental demonstrations of SiC-based of bipolar devices that display high blocking voltages but with limited frequency response. Semiconductor diodes, used in switches or rectifiers, when forward biased (on-state) should have minimal voltage across the two terminals and the leakage current should be very low when reverse-biased (in the off-state). Schottky diodes have high switching speed but tend to have high leakage in the off-state. Increasing the thickness or decreasing the doping in the drift region increases the breakdown voltage but also increases the on-resistance which results in high power (I2R) losses. Gallium nitride-based transistors possess fundamental electronic properties that make it an ideal candidate for vertical current conducting high voltage, high-power devices [Refs 1, 2]. A number of these properties derive directly from the wide band-gap of GaN (Eg = 3.4 eV) including an exceptionally high electric breakdown field (~3.5 MV/cm). This high breakdown field allows GaN based devices to be biased at a high drain voltage with low on-resistance [Ref 2]. Furthermore, the wide band-gap of GaN allows device operation at elevated temperature (> 300 �C) without degradation. Additionally, GaN has a high-saturation electron velocity (vsat = 2 x 107 cm/s), which is partially accountable for the high current density, Imax (Imax directly proportional qnsvsat where q = 1.6 x 10^-19 coulomb, ns = sheet charge density, vsat = electron saturation velocity), and high operating frequency as ft directly proportional vsat/Leff, where Leff is the effective channel length.

Additionally, the extremely low intrinsic carrier concentration of ni = 1.8x10^-22 cm^-3 of GaN enables low generation/recombination rates and thus low leakage currents in a thick drift region. Theoretically, a vertical GaN device designed with a 100 �m thick n-type drift layer will operate with greater than 15kV breakdown. Nevertheless, the technology to produce a high-voltage GaN device structure is currently unavailable. The primary limitation is the extremely low-growth rate of current GaN epitaxy systems (e.g., metal-organic chemical vapor deposition (MOCVD)). The secondary but related limitation is the controlled n-type doping < 2x10^15 cm^-3 needed to enable high breakdown voltage. Controllable p-type doping of GaN is needed for the p-well region and junction termination region of vertical GaN power switches.

The growth of a thick (100um), low-doped GaN drift region has proven challenging with current reactor designs. Current literature on GaN epitaxy reports carbon-free halide or hydride-based epitaxy with growth rates on the order of a hundred �m/hour but with high levels of oxygen and silicon impurities; or chlorine-free metal-organic based deposition with a growth rate of a few �m/hour, which reasonably excludes the growth of thick drift layers [Refs 1, 2]. A key issue for GaN epitaxy is achieving a high growth rate while maintaining high epitaxial quality, purity, and surface morphology. A reactor technology is needed to address the specific reaction kinetics of the GaN at the gas/solid (substrate) interface as well as to minimize undesirable gas-phase nucleation that depletes the reactant supply and creates deleterious particulates. Atmospheric or greater pressure reactors enable high partial pressures of ammonia or nitrogen containing precursor. Additionally, the reactor design must enable controlled low-level (<2x10^15 cm^-3) n-type doping in this high-growth rate regime without compensating impurities. In situ measurement tools can facilitate the growth of high quality GaN.

An optimal, high-voltage, power-electronic device also necessitates a reactor design able to controllably deposit GaN at high n- and p- doping levels (>1x10^19 cm^-3). Avoiding residual impurities from p-type doping in GaN deposition systems is difficult, which, given the sticking coefficient of magnesium, the most common p-type dopant. The literature on the memory effects of p-type dopants on successive growths of n-type GaN is mixed, which again may suggest that proper design of the reaction chamber is necessary to account for the specific kinetics of deposition chamber.

Additionally, the ability to form heterostructures of GaN with AlGaN and AlN enables more sophisticated vertical power device structure such as a vertical insulated gate AlGaN/GaN heterojunction field-effect transistor [Ref 3]. In order to achieve high quality AlN films, it is advantageous to deposit films above 1250 �C [Ref 4].

Proposed growth system should meet the following thresholds:
Deliverable Design Characteristics Value
Controllable deposition with low-concentration (<2x10^15 cm^-3) n-type GaN layers without compensating (intentional or unintentional) dopants (such a carbon)
n-type GaN (<2x10^15 cm^-3)� with continuous growth rates above 10 �m/hr in Phase I and above 20 �m/hr in Phase II
nm-scale thickness uniformity at sub-nm RMS roughness levels
Smooth surfaces (<100 nm RMS) over large length scales (500 um2)
High-concentration (>1x10^19 cm^-3) n-type, thin (< 50 nm) device layers
High-concentration (>1x10^18 cm^-3) p-type, thin (< 50 nm) device layers
Deposition of a 2 �m GaN film at a reactor pressure of 200 Torr
Deposition of a 2 �m GaN film at a reactor pressure of at least 1000 Torr
Deposition of a 2 �m AlN film with a continuous growth rates above 5 �m/hr
Deposition of a 2 �m AlN film at a reactor temperature above 1300 �C

PHASE I: Establish a plan for the design and development of a reactor technology that can controllably deposit low-concentration (<2x10^15 cm^-3) n-type GaN layers enabling continuous growth rates above 10 �m/hr (>2X current state-of-the-art). Demonstrate thin (< 50nm) high-concentration (>1x10^19 cm^-3) n-type and p-type GaN layers and an appropriate ternary with nm-scale thickness uniformity at sub-nm RMS roughness levels. Produce a final report that should convince that the proposed product can be properly designed to address the desired and required features included in the Description and be achieved if Phase II is awarded. Provide a Phase II development plan addressing technical risk reduction.

PHASE II: Develop a fully-functional epitaxy system having in situ characterization tools and capable of producing a thick, controllable low-concentration (<2x10^15 cm^-3) n-type GaN drift layer (>100 �m) at a continuous growth rate above 10 �m/hr as well as continuous growth of high-concentration (>5x10^19 cm^-3) n- and p-type doped thin (sub 100 nm) device layers within the same growth run and with a smooth surface (<100nm RMS) over large length scales (500 um^2) at the completion of the epitaxial growth. The defect level as in the n-type drift layer should produce an epitaxial layer that is able to support a GaN device breakdown voltage greater than 15kV. Additional goals for the system should demonstrate GaN epitaxial growth rates of at least 15 �m/hr throughout the entire growth of a film with a thickness. Delivery to the Navy of a prototype of the fully operational system with appropriate control software is required by the end of Phase II for evaluation.

PHASE III DUAL USE APPLICATIONS: Scale up manufacturing of the system and transition the technology for Navy use. An epitaxy system of this design will enable cost-effective, semiconductor-based, high-power devices for solid-state transformers to replace electromagnetic transformers for the electric grid, rail traction, large-vehicle power systems, and wind turbines. The high-voltage power switches will meet the cost and performance goals for application to multi-megawatt power distribution on a ship.

REFERENCES:

1. Pearton, S. J. and Ren, F., �GaN Electronics�, Adv. Mater., 12 (21), 1571 (2000). DOI: 10.1002/1521-4095(200011)12:21<1571::AID-ADMA1571>3.0.CO;2-T

2. Kizilyalli, I.C., Edwards, A.P., Aktas, O., Prunty, T., and Boux, D., �Vertical Power p-n Diodes Based on Bulk GaN�, IEEE Trans on Elec. Dev. 62, 414 (2015). DOI: 10.1109/TED.2014.2360861

3. Chowdhury, S. and Mishra, U. K., �Lateral and Vertical Transistors Using the AlGaN/GaN Heterostructure�, IEEE Trans. Electron Devices, 60 (10), 3060 (2013), DOI: 10.1109/TED.2013.2277893

4. Sun, X., Li, D., Chen, Y., Song, H., Jiang, H., Li, Z., Miao, G., and Zhang, Z., �In situ observation of two-step growth of AlN on sapphire using high-temperature metal�organic chemical vapour deposition�, CrystEngComm, 15, 6066 (2013), DOI:10.1039/C3CE40755A

KEYWORDS: Gallium Nitride; Deposition System; Wide Bandgap Semiconductor; High-Power Electronics