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.
GaN and AlGaN based photocathodes have attracted considerable attention for their applications in image intensifiers, astronomy and UV detection and emission systems. Small electron affinity of AlGaN alloy allows for the development of high quantum efficiency negative electron affinity (NEA) photocathodes with significant advantages such as solar blindness, radiation hardness and low noise. Conventional photocathodes achieve NEA by cesiating the photocathode surface. Fabrication, optimization, and installation in vacuum is required for cesiated photocathodes due to the high chemical activity of the Cesium (Cs). Such requirement increases cost and limits the range of potential applications. Further, such photocathodes have been reported to suffer from chemical instability and degradation with time.
We have proposed and investigated novel Cs-free AlGaN based photocathode that utilizes band engineering near the photocathode surface to achieve permanent NEA.
The development of such a structure faces multiple challenges, including high Si delta doping in GaN and the presence of polarization induced negative charges on the photocathode surface. We have studied the effect of polarization induced surface charges and Si delta doping charge on the emission threshold of Cs-free GaN photocathode with a series of systematically designed device structures. We have identified significant growth parameters that dominate the overall performance of the device.
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.