Search

GaN p-n diodes were formed by selective area regrowth on freestanding GaN substrates using a dry etch, followed by post-etch surface treatment to reduce etch-induced defects, and subsequent regrowth into wells. Etched-and-regrown diodes with a 150 μm diameter achieved 840 V operation at 0.5 A/cm 2 reverse current leakage and a specific on-resistance of 1.2 mΩ·cm 2 . Etched-and-regrown diodes were compared with planar, regrown diodes without etching on the same wafer. Both types of diodes exhibited similar forward and reverse electrical characteristics, which indicate that etch-induced defectivity of the junction was sufficiently mitigated so as not to be the primary cause for leakage. An area dependence for forward and reverse leakage current density was observed, suggesting that the mesa sidewall provided a leakage path.

Vertical c-plane GaN p–n diodes, where the p-GaN layer is formed by epitaxial regrowth using metal–organic chemical-vapor deposition, are reported. Current–voltage (I–V) performance similar to continuously grown p–n diodes is demonstrated, including low reverse leakage current and reverse breakdown voltage in excess of −600 V, despite the lack of field management structures to increase the reverse breakdown voltage. Secondary-ion mass spectrometry analysis of regrown interfaces reveals that the primary source of Si at the regrown interface (“Si spike”) is ex situ contamination during wafer handling prior to loading for regrowth. Continuously grown p–n diodes with intentional Si doping at the p–n junction do not show degraded I–V performance for Si sheet concentrations

The impact of dry-etch-induced defects on the electrical performance of regrown, c-plane, GaN p-n diodes where the p-GaN layer is formed by epitaxial regrowth using metal-organic, chemical-vapor deposition was investigated. Diode leakage increased significantly for etched-and-regrown diodes compared to continuously grown diodes, suggesting a defect-mediated leakage mechanism. Deep level optical spectroscopy (DLOS) techniques were used to identify energy levels and densities of defect states to understand etch-induced damage in regrown devices. DLOS results showed the creation of an emergent, mid-gap defect state at 1.90 eV below the conduction band edge for etched-and-regrown diodes. Reduction in both the reverse leakage and the concentration of the 1.90 eV mid-gap state was achieved using a wet chemical treatment on the etched surface before regrowth, suggesting that the 1.90 eV deep level contributes to increased leakage and premature breakdown but can be mitigated with proper post-etch treatments to achieve >600 V reverse breakdown operation.

AlGaN-channel high electron mobility transistors (HEMTs) were operated as visible- and solar-blind photodetectors by using GaN nanodots as an optically active floating gate. The effect of the floating gate was large enough to switch an HEMT from the off-state in the dark to an on-state under illumination. This opto-electronic response achieved responsivity >108 A/W at room temperature while allowing HEMTs to be electrically biased in the off-state for low dark current and low DC power dissipation. The influence of GaN nanodot distance from the HEMT channel on the dynamic range of the photodetector was investigated, along with the responsivity and temporal response of the floating gate HEMT as a function of optical intensity. The absorption threshold was shown to be controlled by the AlN mole fraction of the HEMT channel layer, thus enabling the same device design to be tuned for either visible- or solar-blind detection.

Enhancement-mode Al0.7Ga0.3N-channel high electron mobility transistors (HEMTs) were achieved through a combination of recessed etching and fluorine ion deposition to shift the threshold voltage (VTH) relative to depletion-mode devices by +5.6 V to VTH = +0.5 V. Accounting for the threshold voltage shift (ΔVTH), current densities of approximately 30 to 35 mA/mm and transconductance values of 13 mS/mm were achieved for both the control and enhancement mode devices at gate biases of 1 V and 6.6 V, respectively. Little hysteresis was observed for all devices, with voltage offsets of 20 mV at drain currents of 1.0 × 10−3 mA/mm. Enhancement-mode devices exhibited slightly higher turn-on voltages (+0.38 V) for forward bias gate currents. Piecewise evaluation of a threshold voltage model indicated a ΔVTH of +3.3 V due to a gate recess etching of 12 nm and an additional +2.3 V shift due to fluorine ions near the AlGaN surface.

Polarization-doped field effect transistors (PolFETs) were realized with an unintentionally doped AlxGa1-xN channel layer graded over Al compositions 0.60 ≤ x ≤ 1.0 with a maximum current density of 188 mA/mm (+10 V gate-to-source bias) and an on-resistance of 85 mΩ mm. The average mobility in the PolFET channel was estimated to be 320 cm2/V s, which exceeds that of previous AlGaN metal-semiconductor field effect transistors (MESFETs) and heterojunction field effect transistors (HFETs) of similar Al composition. The breakdown voltage was greater than 620 V, indicating an average critical electric field of >210 V/μm, which is substantially better than ∼100 V/μm that is typically achieved in GaN HFETs. These findings demonstrate that Al-rich PolFETs are attractive alternatives to MESFETs and HFETs for achieving simultaneously high channel electron density and mobility in high voltage switches.

Due to their large bandgaps (≥ 3.4 eV) compared to Si and SiC, AlGaN alloys have superior material properties that are appealing for the next generation of high-voltage power devices. Most notable is that the critical electrical field scales as the bandgap to the 2.0-2.5 power. Thus, diodes employing wide bandgap semiconductors are expected to operate at higher breakdown voltages with thinner and more heavily-doped drift regions, leading to lower resistive losses compared to Si-based devices. Additionally, the formation of AlGaN-based heterojunctions and the utilization of polarization fields offer design options not possible for devices based on materials such as SiC and diamond. Noteworthy demonstrations of high voltage (> 3 kV) PN and Schottky barrier diodes in both GaN and AlGaN alloys confirm the potential of III-Nitride materials for power devices. However, full exploitation of the efficiency gains promised by III-Nitrides for power management systems also requires vertically-conducting transistors for current switching, in addition to diodes. Analogous to power diodes, vertically-conducting power transistors such as D-MOSFETs employ a thick n-type drift layer to stand-off high reverse voltage, but unlike simple PN diodes, have the p-type material limited to spatially defined regions over the surface of the device. “Selective area p-doping” is typically accomplished in SiC and Si devices by implantation and activation of p-type dopants. However, implantation processing in III-Nitride structures has to date only yielded PN junctions that exhibit significantly higher reverse current leakage than continuously-grown PN junctions. Selective-area epitaxial regrowth of p-type material in regions etched into the drift layer offers an alternative to implantation. However, regrown pn junctions on plasma-etched III-Nitrides with low reverse leakage current have not been demonstrated. In this presentation we will discuss the advances in epitaxial p-regrowth for forming pn junctions on inductively-coupled plasma (ICP) etched n-type GaN and Al0.3Ga0.7N drift layers. The role and sources of unintentionally-introduced Si at the regrown interface will be presented. Additionally, approaches to ameliorate damage and defect states introduced by ICP etching through wet-chemical and in-situ reactor thermal treatments prior to p-regrowth will be related to current-voltage characteristics of regrown diodes. While high voltage GaN-based diodes have been fabricated on vertically-conducting GaN substrates, AlGaN-based high voltage diodes with thick drift layers (≥ 5 μm) have been fabricated on AlN and sapphire substrates. The insulating nature of these substrates necessitates planar device geometries that result in high ohmic losses, heating, and degraded device performance. However, the thick drift layer and the smaller lattice constant of AlGaN alloys present significant challenges for realizing vertically-conducting AlGaN power devices on GaN substrates. Here we report a method for fabricating vertically-conducting AlxGa1-xN PN junctions grown on conducting GaN substrates. An n-type GaN substrate is patterned with parallel, sub-micron-wide mesas separated by trenches etched > 0.5 μm deep prior to AlxGa1-xN overgrowth using metal-organic vapor phase epitaxy. We find that growth temperature, V/III ratio, and group-III molar flux can be used to manipulate the crystalline facets formed along the etched pattern prior to AlxGa1-xN coalescence. Using the described process, coalesced planar AlxGa1-xN epitaxial layers were achieved with less than 6 μm of overgrowth for Al mole fractions up to 0.45, which is the highest Al mole fraction reported to date. Cathodoluminescence measurements of an overgrown Al0.3Ga0.7N epilayer indicate that the density of threading dislocations is mid x 108 cm-2 which is similar to that measured for Al0.3Ga0.7N epilayers overgrown on patterned Al0.3Ga0.7N on sapphire. The current-voltage characteristics of vertically-conducting AlGaN PN diodes grown on GaN substrates will be presented. The information, data, or work presented herein was funded in part by the Advanced Research Projects Agency – Energy (ARPA-E), U.S. Department of Energy under the PNDIODES program managed by Dr. Isik Kizilyalli. This work was also supported, in part, by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525

Polarization-doped field effect transistors (PolFETs) were realized with an unintentionally doped Al x Ga1− x N channel layer graded over Al compositions 0.70 ≤ x ≤ 0.85 and ohmic contact formation to the Al0.85Ga0.15N surface. Current density of 24 mA/mm was attained for a modest threshold voltage of −3.95 V owing to the high sheet charge concentration (n s) of 5.4 × 1012 cm−2 and average electron mobility (μ) of 210 cm2 V−1 s−1 for the channel region along with ohmic contacts with a specific contact resistivity of 1.1 mΩ cm2 and a large Schottky barrier height of 3.0 eV for the metal–semiconductor gate. The combination of ultra-wide band gap energy, high μ, and high n s, linear ohmic contacts, and large Schottky barrier for AlGaN PolFETs makes them attractive for both high voltage switches and high power rf devices.

The near-infrared (NIR) measurement of blood pH relies on the spectral signature of histidine residing on the hemoglobin molecule. If the amount of hemoglobin in solution varies, the size of the histidine signal can vary depending on changes in either the pH or hemoglobin concentration. Multivariate calibration models developed using the NIR spectra collected from blood at a single hemoglobin concentration are shown to predict data from different hemoglobin levels with a bias and slope. A simple, scalar path length correction of the spectral data does not correct this problem. However, global partial least-square (PLS) models built with data encompassing a range of hemoglobin concentration have a cross-validated standard error of prediction (CVSEP) similar to the CVSEP of data obtained from a single hemoglobin level. It will be shown that the prediction of pH of an unknown sample using a global PLS model requires that the unknown have a hemoglobin concentration falling within the range encompassed by the global model. An alternative method for correcting the predicted pH for hemoglobin levels is also presented. The alternative method updates the single-hemoglobin-level models with slope and intercept estimates from the pH predictions of data collected at alternate hemoglobin levels. The slope and intercept correction method gave SEP values averaging to 0.034 pH units. Since both methods require some knowledge of the hemoglobin concentration in order for a pH prediction to be made, a model for hemoglobin concentration is developed using spectral data and is used for pH correction.