Given the impact that mature silicon microelectronic and nanoelectronic industries have on our daily lives and the great potential of III-Nitride based optoelectronic devices, integration of these two platforms is one of our primary objectives. We have developed a novel method of growing GaN on Si with substrate engineering which can usher in a new era for AlInGaN optoelectronic devices.
Our substrate engineering technique consists of four major steps:
Our efforts also involve understanding the driving forces that govern the simultaneous dislocation and crack reduction that has been observed using our technique. This will enable us to further reduce the dislocations to <107 cm-2 and simultaneously obtain a crack-free surface increasing the available device area, allowing us to compete with state-of-the-art devices on more expensive substrates like SiC and bulk GaN. Besides characterizations like spectroscopic ellipsometry, Raman spectroscopy, in-situ stress measurements (in collaboration with Prof. Redwing’s research group at Pennsylvania State University), and TEM, we are also working on molecular dynamics simulations to theoretically determine the physics involved.
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 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.
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.
A hyperspectral detector is a device capable of detecting a wide range of electromagnetic radiation (typically 400 nm to 2400 nm) with high wavelength resolution (~10 nm). Hyperspectral detectors have become a key component for the development of hyperspectral imaging systems which are finding increasing applications in remote sensing, resource management, mineral exploration and environment monitoring. Different natural and manmade objects have characteristic reflection spectrum for the solar and manmade light sources. Analysis of the reflected radiation intensity from an object, as a function of wavelength, provides a wealth of information about its composition. A hyperspectral image captured by an imager contains spatial and spectral information of the incident radiation.
Present hyperspectral detectors, apart from being bulky, have limited detection range for visible and IR regions of the spectrum. Our design for a tunable hyperspectral detector has the potential to overcome both of these shortcomings providing a wider range of detection wavelengths, extending to the UV region, without use of filters and gratings which make existing hyperspectral detectors cumbersome. Detection wavelength of the tunable hyperspectral detector pixel can be dynamically changed providing a real time control over the resolution and range of the detector pixel. Such dynamic control over the detection characteristics of the imaging system will provide a versatile device that could be used for a wide range of applications. AlInGaN alloys offer the possibility of bandgap engineering between IR (0.7 eV for InN) and deep UV (6.2 eV for AlN). Due to this wide range of tunable wavelengths, III-nitrides alloy system is an excellent choice for the development of a hyperspectral detector pixel. Further, devices based on III-nitrides can potentially be used in harsh environments such as high temperature and radiation due to their high mechanical and chemical stability.
The proposed AlInGaN based detector utilizes a 5-layered epitaxially grown heterostructure of AlxGa1-xN epilayers that approximates a triangular potential barrier. Height of the barrier, which can be tuned with applied bias, provides tunability of detection wavelength.
AlGaN/GaN high electron mobility transistors (HEMTs) have attracted a lot of attention for the development of high power and high frequency transistors that find a range of applications in base station amplifiers, radars and power conditioning. Spontaneous and piezoelectric polarization present in nitride devices grown in the c-crystallographic direction induces high density of 2-dimensional electron gas (2DEG). Furthermore, high carrier saturation velocities and high breakdown voltage of the III-nitride material system allows development of the state of the art transistors.
Even though conventional AlGaN/GaN HEMTs have met ever increasing demand of high frequency and high power applications, development of III-nitride based HEMTs with enhancement mode (normally OFF) operation is still at its early stages, as is clear from very small number of reports on such devices. Normally OFF devices offer reduced power consumption, reduced circuit complexity and safer operation.
SiO2/AlGaN/GaN/AlGaN HEMT structure investigated in this research is an excellent example in order to illustrate the effects of multiple polarization-induced sheet charges on the energy band diagram and properties of III-nitride based multi-heterojunction devices. Such design provides three significant advantages over the dual heterojunction design reported previously. Additional sheet of induced negative fixed charges allows for implementation of a much thicker top AlGaN barrier layers, provides higher turn ON voltages as compared to the reported dual heterojunction design, and provides a limited control over the value of turn ON voltage. Detailed analysis has been performed to understand the interaction between the fixed sheet charges and the mobile carriers.
Properties of a material's surfaces determines its interaction dynamics with the environment. In nanostructures, surface atoms constitute a large portion of the atoms of the structure and therefore their role in determining optical, electrical, magnetic, and mechanical properties of the nanostructure are especially significant. Each nanostructure commonly possesses multiple surfaces with different crystallographic directions. Such variation in crystallographic property translates directly into a difference in the interaction of each of these surfaces with their immediate environment (e.g. heteroepitaxy).
A wide variety of faceted nanostructure shapes have been grown by MOCVD such as hexagonal prism, 6-faceted pyramid, truncated pyramids, and arrow-headed shapes (Figure 1). The Wulff plots and kinetic Wulff plots were developed for SAG of GaN, successfully predicting a variety of equilibrium shapes of GaN sub-micron structures that are observed experimentally. A classification of growth regimes was formulated to produce the respective shapes of GaN 3D structures. The equilibrium shapes in a particular growth regime were obtained by applying the Legendre transformation on its respective v-plot. Figure 2 shows an example of a calculated v-plot and the ability of the model to predict observed equilibrium shapes of GaN 3D (nano)structures grown by SAG.
Under this project, sub-micron AlGaN crystals with a size on the order of 700 nm are epitaxially grown as pyramids, truncated pyramids, or platelets. The Al percentage can be controlled such that emission wavelengths in the range of 220 nm to 365 nm are achievable. The crystals developed here form the active region of an electron-pumped deep-UV source (in collaboration with GE Global Research for the Department of Homeland Security). The applications of such a light source include biological and chemical detection.
Characteristics like high breakdown voltage and high carrier mobility of GaN has enabled development of the state of the art high power Schottky diodes and HEMTs.
High breakdown voltage GaN power diodes allow elimination of snubber circuit components that are required with Si based diodes, greatly reducing the size and complexity of the circuit. Better reverse recovery characteristics and low turn ON resistance of GaN based diodes reduce switching losses and allow high operating frequency. Although all these high performance parameters can be achieved by SiC based diodes, GaN diodes have an advantage of having much lower cost.
AlGaN/GaN HEMTs have shown high performance owing to its high density of 2DEG, high carrier mobility and high carrier saturation velocity. High operating voltage and temperature reduces circuit complexity and cooling requirements. Such characteristics allow development of compact, highly efficient, reliable and less complex power amplifier circuits.
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 mistmatch 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.