Research

Current Projects

If you're interested in our work or want to work with us, email wbgoptronix@sunypoly.edu

Novel dopant activation techniques for high voltage GaN power devices

Selective doping is required in a number of device designs including current aperture vertical electron transistors (CAVET), vertical junction FETs, and junction barrier Schottky (JBS) diodes to form junction termination extension structures. This enables high voltage, high power, and high frequency GaN power devices for applications in radar, vehicle drivetrain, power grid, and other power electronics. Implementing selective-area ion implantation to obtain p-type III-nitrides remains challenging due to the temperature required to repair defects and locate the p-type dopant, Mg, in substitutional sites. Achieving selective-area p-doping through Mg implantation requires an activation anneal at temperatures beyond the region of thermodynamic stability in GaN. This requires nonequilibrium annealing conditions to activate the material without inducing degradation.

We are developing a process to achieve acceptor activation by rapid, high temperature pulsing using a novel application of a gyrotron microwave beam. At temperatures of >1300 °C for duration of 1 min, p-type activation by gyrotron of Mg-implanted GaN has been observed by optical and electrical characterization. Photoluminescence measurements show 1.6X higher emission intensity in the Mg related UVL over VN related GL2 band, representing a regime that anneal-activated Mg acceptor concentration is beyond compensating donor defects.  Measurement of device shows characterstics of p-n diode witha turn-on voltage of 3.0 V. Gyrotron annealing also proves effective at recovering implant-induced lattice damage, increasing in effectiveness with annealing temperatures from 1150 °C to 1350 °C, as measured by XRD. These structural, optical, and electrical measurements point to the promise of the gyrotron annealing technique for selective p-doping for its implementation in power devices.

Diagram 1

digram of novel gyrotron

 

Figure 1. (left) Schematic diagram of novel gyrotron annealing system used for high temperature annealing above 1300 °C for activation of the Mg-dopant (right) heating diagram showing thermal pulsing to 1350 °C

2A GaN

Figure 2

Figure 2. (left) Diode I-V from Mg-implanted GaN annealed by gyrotron at 1350 °C showing p-n diode characteristics and (right) low-temperature (20 K) photoluminescence spectra of gyrotron annealed samples. Sample annealed at 1350 °C shows 1.6X stronger Mg related UVL over VN related GL2 band, representing a regime that anneal-activated Mg acceptor concentration is beyond compensating donor defects.


Enhanced Reliability and Performance of High Electron Mobility Transistors for Power Systems

AlGaN/GaN heterostructures possess a two-dimensional electron gas (2DEG) formed due to the presence of spontaneous and piezoelectric polarization. This two-dimensional channel can be modulated with a gate to form transistors. We have implemented dynamic body-bias technique to improve the performance of AlGaN/GaN HEMTs with the successful integration of a body-diode. In this configuration, p-GaN body-diode-based back-gate control is used to shift the threshold voltage and dynamically modulate the ON/OFF characteristics of a normally-ON AlGaN/GaN HEMT. A fourth back-gate terminal is connected to the p-GaN layer to control the depletion width of the body-diode, which in turn modulates the 2DEG density.

GAN

Figure 1. Variation of charge distribution in depletion and 2DEG channel region with the application of positive, negative, and zero bias to the back-gate terminal.
 

Enhanced Reliability and Performance of High Electron Mobility_Fig2A Enhanced Reliability and Performance of High Electron Mobility_Fig2B

Figure 2. (left) Four terminal transfer characteristics at VDS=0.5 showing the shift in the top-gate threshold voltage with the application of different back-gate voltages and (right) variation of the top-gate threshold voltage as a function of back-gate voltage.

In collaboration with Army Research Lab (ARL), we have been working to develop power HEMT devices with improved reliability and performance. We utilize passivation dielectric to reduce current collapse and gate dielectric to reduce gate-leakage current. However, the introduction of a dielectric layer leads to issues associated with the dielectric/(Al)GaN interface trap states, bulk trap states within the dielectric, and surface defect states which can negatively affect performance. We are exploring different dielectric materials including aluminum oxide, silicon dioxide, and silicon nitride deposited using atomic layer deposition. In order to decrease the density of trap states, various pre- and post-deposition treatments, such as annealing in forming gas ambient, are utilized. In order to evaluate the dielectric and dielectric/semiconductor interface, we fabricate metal-insulator-semiconductor (MIS) capacitors and measure the capacitance-voltage (C-V) behavior to determine the amount of charge trapped in the dielectric and at the interface.

Figure 3 GAN

Figure 3. C-V measurement data of Al2O3 MIS capacitors with post-dielectric deposition annealing in forming gas at (a) 350 °C for 1 min, (b) 350 °C for 10 min, (c) 350 °C for 20 min, (d) 600 °C for 1 min, (e) 475 °C for 10 min, (f) 600 °C for 20 min, and (g) as-deposited. The arrows indicate the direction of the DC bias sweep. Insets show zoomed in area of hysteresis to show ΔV from sweep up to sweep down. Reduction in hysteresis, indicating reduced trapped charge density, is observed after annealing at low temperature for extended times, while high temperature annealing results in increased trap density.


Cs-free polarization-engineered III-Nitride photocathode

III-Nitride based photocathodes have been the subject of much research in photoemissive devices for ultraviolet detection in astronomy, planetary or defense applications. In order to achieve high quantum efficiency (QE), negative electron affinity (NEA) is necessary. NEA is conventionally reached via surface cesiation which requires special in-situ fabrication steps, including cleaning and activation in vacuum and sealed-tube packaging, due to Cesium’s high chemical activity. Photocathodes using this technology have been reported to suffer from chemical instability and QE degradation over time.

Recent work has been performed to eliminate Cs-based surface treatments and improve device efficiency by taking advantage of the polarization exhibited by III-Nitrides in order to achieve NEA. Previously, we developed and reported a novel Cs-free GaN photocathode based on Si-delta-doping. While improved performance was observed with the use of Si-delta doping, the high dopant concentration results in increased scattering.

We have proposed and investigated novel Cs-free III-Nitride photocathodes that show permanent NEA without the use of delta-doping and thus show potential for higher QE. By replacing Cs-based surface treatments with polarization engineering to affect surface properties, we allow the device to be air stable. Additionally, polarization engineering is used as an alternative to impurity-based doping in order to allow high free carrier concentrations without a decrease in mobility. Devices have been studied via both simulation and photoemission measurements of devices grown via MOCVD. In support of our work to achieve high-efficiency photocathodes, we are engaged in fundamental studies on enhancement of p-type conductivity and surface engineering in N-polar III-Nitride materials.

Cs-free polarization-engineered_Fig1.  

Figure 1. Schematic band diagram of III-Nitride photocathode with photon incident on the semiconductor surface.
The photodetector absorbs photons with sufficient energy, creating photoexcited carriers which transport to the
surface aided by an intern electric field, and are emitted into vacuum and counted.

Figure 2 GAN Figure 3 GAN

Figure 2. (left) Photoemission spectroscopy shows the highest quantum efficiency for photocathode structures grown on a high hillock density film  (Right) Atom probe tomography (APT) reconstructions showing a decrease in Mg-clustering within hillock structures which allows for a greater probability for Mg-atoms to act as acceptors within the film, increasing the quantum efficiency.


(Al)GaN-based Betavoltaic Microbatteries

Conventional lithium batteries, although low in cost, expel energy relatively quickly and require replacement. Betavoltaic (BV) batteries show promise as a replacement in low power (nW-µW) applications due to high energy density, relatively low weight and long lifetime. A BV device is analogous to a solar cell, where electron hole pairs (EHPs) are created by incident radiation. The radiation source in a BV device are high energy electrons, or β-particles, emitted from a coupled radioactive isotope. The kinetic energy of these β-particles dissipates throughout a semiconductor material through creation of EHPs by impact ionization and losses due to numerous scattering mechanisms.

Using electron beam induced current (EBIC) we have reported GaN planar n-i-p devices with output powers of 70 nW and 640nW with quantum efficiencies of 1.2% and 4.0% at electron energy of 5.6 keV and 17 keV, respectively. Subsequent fabrication improvement has led to an increased efficiency of 6.96% at 16 keV beam energy.

By developing a 3D device design, a lower average defect density can be achieved, along with optimal absorption and recombination regions. We are working towards achieving higher efficiency BV devices using a hybrid 3D+2D device. A feedback loop between theory, epitaxial growth, and characterization is ongoing to achieve such efficiencies. Physics-based TCAD and Monte Carlo simulations have been employed to optimize 3D structure dimensions by simulating energy absorption for a realistic source with full emission spectrum. When coupled with a novel 63NiCl2 radioactive source, Monte Carlo N-particle Extended (MCNPX) simulations show the efficiency of energy transfer from the source material to GaN (ηsrc) to be at 2.75x that of planar device for optimized dimensions.

(AI)GaN-based Betavoltaic Microbatteries(A)
(AI) GaN-based Betavoltaic Microbatteries Figure B (B) (Al)GaN-based Betavoltaic Microbatteries_Fig1C (C)

Figure 1. (A) MCNPX simulation of 3D GaN pillars, with mesa width/mesa gap varied. Optimum condition of 2 µm width/gap is achieved, both (B) power and (C) efficiency of energy transfer are higher for all mesa gap (63Ni Gap Size) for the 2 µm mesa compared to 4 µm mesa. Self-absorption within isotope source forms a trade-off better power and transfer efficiency.

(Al)GaN-based Betavoltaic Microbatteries_Fig2A

(A)

Al)GaN-based Betavoltaic Microbatteries_Fig2B

(B)

(Al)GaN-based Betavoltaic Microbatteries_Fig2C.

(C)

Figure 2. Scanning electron microscope (SEM) images of high aspect ratio selective-area-growth GaN structures grown using bottom-up methodology. A 9:1 (vertical:lateral) growth rate is achieved under optimal V/III precursor ratio.


AlGaN based solar blind avalanche photodetector

UV photodetectors have applications in missile plume detection, UV astronomy, astrophysics and non-line of sight communication systems. Currently, microchannel plate (MCP) sealed tubes are used extensively in UV instruments for photomultiplication purposes because they can be used in photon counting mode, are solar blind and don’t require cooling. However, this technology is bulky, fragile and expensive, which restricts its use to a very specific set of applications.

Solid state UV detectors offer considerable benefits compared to MCP technology. The wide bandgap of AlGaN-based detectors can be tuned such that they are intrinsically solar blind, allowing for operation of AlGaN APDs in broad daylight without significant background radiation. AlGaN APDs also promise high efficiency, high gain and low noise characteristics which can be exploited for photon counting. Furthermore, these devices can operate at higher temperatures and are radiation hard, making AlGaN the optimal choice for UV photodetectors.

There have been significant challenges in realizing reliable, solar-blind (sub-280nm) devices based on AlGaN. A large lattice mismatch caused by heteroepitaxy results in a high defect density of the film, and poor film morphology can lead to premature and permanent breakdown. There are a number of difficulties that result from doping AlGaN in high concentrations, and Mg doping for p-type AlGaN has proven particularly difficult.

This project explores innovative methods to realize solar-blind AlGaN-based APDs, using growth, doping and processing methods developed in-house to limit premature breakdown and achieve high sensitive, high efficiency solar-blind devices. Techniques such as pulsed AlGaN, delta-doping, and novel heterostructures (SAM and nano-SAM) are being implemented in these efforts, and processes such as implantation isolation are being developed to improve performance of AlGaN APDs.


Fundamental studies into stress evolution in III-Nitrides

The heteroepitaxial nature of AlxGa1-xN films, particularly grown on Si substrates, brings about challenges like defect formation and cracking for thick films due to a large amount of stress in the films caused by lattice mismatch and thermal expansion mismatch. To reduce these effects, a number of techniques such as migration enhanced epitaxy, growth of superlattices, selective area epitaxy and lateral epitaxial overgrowth (LEO) have been adopted. To compensate for this tensile stress, intrinsic compressive stress can be built into the layers during growth that has proven to be an effective method but often involves detailed knowledge of layer stacks such as in superlattices, and interlayers. In one approach, step graded AlGaN films are grown in succession, building compressive stress in each layer. To achieve the necessary stress compensation optimization of the growth parameters, number of AlGaN layers, layer thicknesses and compositions are necessary. Such commonly used approach requires a great amount of experimentation. This is because the process of building compressive stress and stress relaxation in low mobility III-Nitride is not yet fully understood but is thought to be driven by a process known as dislocation inclination.

In order to get an depth understanding of the stress evolution and this dislocation inclination process, it is required to observe the stress in the films during the deposition. Using in-situ stress evolution measurement, the curvature of the film is measured and the stress is calculated using Stoney's equation. During the growth of AlGaN films on an AlN buffer, curvature drops initially with the deposition of each AlGaN layer, which indicates that the film is under compressive stress and over time its rate of decrease slows down, and changes into tensile stress at higher thickness. The in-situ stress evolution observation of AlGaN film growth confirms that stress relaxation is not linear and complex in nature. Work is being conducted to further understand the stress evolution process and the factors that influence it. The goal is to combine experimental data with ex-situ characterization and theoretical models to develop a predictive model for stress evolution.


III-Nitride nanostructures

Growth of III-Nitrides is typically performed along the vertical c-axis of the wurtzite crystallographic structure, which results in the material exhibiting a spontaneous and piezoelectric polarization field along that axis. This polarization field is detrimental to the performance of LEDs, for which this material system plays a critical role in home and commercial lighting. Much work over the last decade has optimized growth along alternate directions to mitigate the effect of this field on devices, however many of these techniques result in lower material quality which diminishes the benefit of this growth orientation. Another way to access these facets of the crystal is to grow the material laterally from pillars formed in conventionally grown material. These structures yield a higher surface area of the non-polar facet than a planar material would in the same footprint, while laterally-grown III-Nitrides are known to have a lower density of defects within the material. Very recently, this work has been translated to silicon, whose benefits include lower cost and compatibility with current manufacturing.

Our work in nanostructures furthers these current efforts on silicon using our pulsed MOCVD technique, which allows us to achieve precise control of the evolving planes of the nanostructures. In our lab, we have developed the proper processes to yield uniform arrays of nanostructures with non-polar planes suitable for the fabrication of LEDs and other types of devices. Utilizing these, quantum well structures on these facets have been grown yielding a 398nm violet photoluminescence using In0.08Ga0.92N.


Thermoelectric Properties of III-Nitride Materials for Energy Conversion

Thermoelectricity has emerged as a potential avenue to harness waste heat into usable form of electricity to meet the ever increasing energy demands. The quest for energy sustainability has motivated researchers to explore thermoelectric materials which can be used as sources of power. III-Nitrides in recent years have shown promise as potential candidates for thermoelectricity due to their stable chemical, mechanical, thermal characteristics and excellent electronic transport properties. These properties allow them to operate at high temperatures and in harsh environmental conditions without significant impact on their properties. Commonly used thermoelectric materials in industry, such as Bi-Te alloys, tend to break down upon reaching temperatures above 800 K. However, the wide bandgap of III – Nitrides allows them to continue operation past 800 K.

We have experimentally determined the thermoelectric properties of MOCVD grown thin film single layer and heterostructures. The thermoelectric properties of heterostructures consisting of AlGaN/GaN and AlGaN/AlN/GaN have been studied and compared with that of doped and undoped GaN thin films and (HVPE) bulk GaN samples.

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