Vertical Schottky Diodes Research Articles

(Invited) Strain Engineered Crack-Free GaN on Si for Integrated Vertical High Power GaN Devices with Si CMOS

The ability to grow thin GaN layers on Si substrates has led to the development of lateral high power and high-speed devices such as GaN HEMTs. These devices have already demonstrated promising performance and have been adopted for mass market. But lateral devices require large drain/gate separation to sustain high voltage impairing cost effectiveness. This highlights the need for growth of thick GaN layers to enable vertical high power device architectures and to achieve high performance and attain high breakdown voltages in small chip area. This paper presents a successful growth of over 10μm thick crack-free GaN on Si by engineering the strain induced by thermal mismatch between GaN and Si. We discuss the origin of cracking and introduce a surface strain-relief mechanism in 0.5mm diameter GaN dots to overcome thermal mismatches. The first demonstration of vertical thick GaN Schottky diodes on Si will be presented. To fully exploit these results of GaN power devices on Si, side-by-side integration of GaN and CMOS circuitry elements is necessary. We have assessed and validated the compatibility of the GaN-CMOS process. These advances can pave the way for commercialization of next generation compact and efficient power systems that are composed of monolithically integrated GaN and Si technologies.

Improved performance in vertical GaN Schottky diode assisted by AlGaN tunneling barrier

In a vertical GaN Schottky barrier diode, the free electron concentration n in the 6-μm-thick drift layer was found to greatly impact the diode reverse leakage current, which increased from 2.1 × 10−7 A to 3.9 × 10−4 A as n increased from 7.5 × 1014 cm−3 to 6.3 × 1015 cm−3 at a reverse bias of 100 V. By capping the drift layer with an ultrathin 5-nm graded AlGaN layer, reverse leakage was reduced by more than three orders of magnitude with the same n in the drift layer. We attribute this to the increased Schottky barrier height with the AlGaN at the surface. Meanwhile, the polarization field within the graded AlGaN effectively shortened the depletion depth, which led to the formation of tunneling current at a relatively small forward bias. The turn-on voltage in the vertical Schottky diodes was reduced from 0.77 V to 0.67 V—an advantage in reducing conduction loss in power switching applications.

IVT measurements of GaN power Schottky diodes with drift layers grown by HVPE on HVPE GaN substrates

To date, a majority of epitaxial layers for vertical gallium nitride (GaN) power Schottky diodes have been grown by metalorganic chemical vapor deposition. In this work, we investigate the electrical properties of vertical GaN Schottky diodes with drift layers grown by hydride vapor phase epitaxy (HVPE) on moderately-doped freestanding HVPE GaN substrates. Room temperature IV data is presented for devices tested where results for diode characteristics such as the breakdown voltage, Vb, specific on-resistance, Ron-sp, ideality factor, n, and barrier height, Φb, are measured for devices across the 1 cm × 1 cm sample. The smallest diodes, which are 30 μm in diameter, show the smallest specific on-resistance, whereas the breakdown voltage (defined as the voltage corresponding to a current of 10 mA cm−2) is independent of device size across the wafer. IV data show an average value of 1.06 ± 0.06 for n and 0.80 ± 0.04 eV for Φb with little variation across the wafer, suggesting a reasonable metal–semiconductor interface across the entire sample. For one of the 300 μm devices tested, we also examine the IV properties as function of temperature from 25 to 250 °C in increments of 25 °C and extract a zero temperature Φb of 0.908 eV and Richardson’s constant of 4.44 A cm−2 K−2 which is significantly less than the theoretical value of 26.9 A cm−2 K−2.

Improved Vertical GaN Schottky Diodes with Ion Implanted Junction Termination Extension

The realization of selectively implanted p-type regions in GaN as well as an understanding of processing effects that cause carrier type conversion are key enabling steps for vertical GaN devices. Here, GaN Schottky barrier diodes (SBDs) with edge termination formed by either a field plate or junction termination extension (JTE) achieved by ion implanted and symmetric multicycle rapid thermal annealing (SMRTA) are presented. The devices with JTE exhibited substantially reduced leakage currents and improved turn-on characteristics. This is attributed to the elimination of the plasma process steps associated with the deposition and patterning of the field oxide layer required in a field plate process. The breakdown characteristics were studied by electroluminescence imaging, and is indicative of avalanche behavior. The realization of vertical GaN devices with low reverse leakage and ion implanted termination regions represents a key step for future power electronic devices.

Si<sub>1-x</sub>C<sub>x</sub>/Si(001) Heterostructures for Use in Schottky Diode Rectifiers Demonstrating High Breakdown Voltage and Negligible Leakage Current

Vertical Schottky diodes have been fabricated on low C content Si1-xCx and 3C-SiC epilayers epitaxially grown on Si(001) substrates. Significant leakage current was observed in 3C-SiC diodes under reverse bias, masking any rectifying behavior. This issue is far less pronounced in Si1-xCx based Schottky diodes which demonstrate a clear critical breakdown. Leakage current is shown to be greater in relaxed Si1-xCx layers. While crystalline Si1-xCx is not currently a viable material for high power electronics it is useful for assessing the impact lattice mismatch and crystalline quality has on the behavior of rectifiers.

Roles of lightly doped carbon in the drift layers of vertical n-GaN Schottky diode structures on freestanding GaN substrates

We studied the roles of lightly doped carbon in a series of n-GaN Schottky diode epitaxial structures on freestanding GaN substrates, and evaluated the effects of the doping on diode performances. A large variation of compensation ratio was observed for carbon doping at (1–2) × 1016 cm−3. A model was proposed to explain this phenomenon, in which a vulnerable balance between donor-type CGa and deep acceptor CN strongly affected the free-carrier generation. Application of Norde plots and reverse biased leakage current in current–voltage measurements suggested provisional optimization for a free-carrier concentration of 8 × 1015 cm−3 to achieve a tradeoff between breakdown voltage and on-resistance of the n-GaN diodes.

Vertical semiconducting single-walled carbon nanotube Schottky diode

This paper presents a vertical semiconducting single-walled carbon nanotube (sSWCNT)-based Schottky device. For the first time, the author successfully demonstrated a vertical s-SWCNT Schottky diode on an anodized aluminum oxide (AAO) template. In the vertical pores of an AAO template s-SWCNTs were vertically grown and aligned. The vertical growth of s-SWCNTs inside the pores was achieved by successfully isolating the catalyst at the bottom of the pores by using redeposition enabled angled ion milling. The ends of the grown s-SWCNTs were coated with palladium and titanium to form Schottky and Ohmic contacts, respectively. The I-V characteristics of the vertical s-SWCNT paths engaging the Schottky and Ohmic contacts well demonstrated Schottky diode rectification.

Gigahertz Operation of a-IGZO Schottky Diodes

We present vertical Schottky diodes based on amorphous IGZO with an unprecedented cutoff frequency of 1.8 GHz at 0 V bias. These diodes have a rectification ratio of up to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> at ±1 V and a current density of 800 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at +1 V. The diodes' high performance is achieved by understanding and modeling of the two contacts, a Schottky contact using Pd at the bottom and an ohmic contact formed at the top. In particular, the choice of the latter top contact combined with an optimized IGZO layer thickness proves to be crucial: we show how the semiconductor layer thickness and the nature of the top metal modify the doping concentration profile of the IGZO film, which we fully measure and characterize, and how that affects the performance and optimization of the diodes. We measure our diodes in rectifiers, which operate up to 1.1 GHz. Finally, we show that these rectifiers can be fully modeled in SPICE using diode parameters extracted from electrical measurements.

&gt;1200 V GaN‐on‐silicon Schottky diode

AbstractM/A‐COM Technology Solutions has continuing joint development efforts sponsored by the Department of Energy with MIT main campus and MIT Lincoln Laboratory to develop GaN on silicon two and three terminal high voltage/high current switching devices. The initial developmental goals were for a Schottky diode that has a reverse breakdown blocking voltage of &gt;600 V and is capable of handling 10 A of forward current. A comparison of the M/A‐COM Technology Solutions lateral GaN Schottky diode on‐resistance as a function of reverse breakdown voltage for a number of both lateral and vertical GaN Schottky diode geometries taken from the literature is presented. The substrates employed for all of these data points are either sapphire, SiC, silicon, and even one study which utilized single crystal GaN. Also included in this plot are theoretical limits for the basic materials typically used in GaN Schottky diode construction. It can be seen that the reverse breakdown results of approximately 1500 V for M/A‐COM Technology Solutions lateral anode connected field GaN Schottky diodes on silicon substrates compare extremely favorably with the reported performance of the state‐of‐the‐art devices, regardless of substrate material or design geometry. (© 2013 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Degradation of pentacene deposited on gold, aluminum and parylene surfaces: Impact of pentacene thickness

In this work, the impact of film thickness in thermally evaporated pentacene thin films and metal electrodes is studied. In particular, we report electrical performance and stability of un-encapsulated organic Schottky diodes. The chemical composition, surface morphology and ambient stability of the resulting films were also studied. The electrical measurements were carried out in vertical Schottky diodes and its performance and stability was evaluated as function of shelf life time. Oxygen was detected at the pentacene surface. This oxygen is the main cause of the diode degradation and attributed to diffusion of oxygen into the bulk of the pentacene films. Thicker pentacene films resulted in more stable for Schottky diodes.

Temperature Dependent Capacitance–Voltage Analysis of Unintentionally Doped and Si Doped Al0.82In0.18N Grown on GaN

Unintentionally doped (UID) and Si doped Al0.82In0.18N samples were grown on Si-doped GaN by metalorganic chemical vapor deposition. The high structural quality of the Al0.82In0.18N layers was confirmed by high resolution X-ray diffraction and transmission electron microscopy. Secondary ion mass spectrometry measurement showed oxygen levels of (2–6)×1018 cm-3 regardless of the explored growth conditions. Vertical Schottky diodes were fabricated with a Pd-based Schottky contact on Al0.82In0.18N and Ti/Al/Ni/Au-based ohmic contact on n-type GaN. Capacitance–voltage (C–V) analysis showed that UID Al0.82In0.18N was an n-type with a carrier density of about 3×1017 cm-3 in the bulk region at 300 K. Based on the temperature dependent C–V analysis from 100 to 300 K, the donor activation energy was found to be 4 meV, showing very weak temperature dependence despite the large bandgap of Al0.82In0.18N. Si-doped Al0.82In0.18N ([Si] ≃2×1018 cm-3) showed almost no carrier freeze-out at carrier density of 1.0×1018 cm-3.

High-frequency polymer diode rectifiers for flexible wireless power-transmission sheets

In this work, we successfully implemented flexible high-frequency rectifying diodes on polycarbonate substrates using the polythiophene-based semiconducting polymers. We show that the appropriate conductivity of the intermediate hole-injection layer PEDOT:PSS between the anode and the semiconducting polymer is critical to simultaneously achieve enhanced forward currents and rectification ratios. Among the semiconducting polymers studied (P3HT and PQT-12), the diodes based on carefully purified PQT-12 exhibited higher forward currents, higher rectification ratios, much better environmental stability, and much higher operation frequencies. A forward current density up to ∼6 A/cm 2 at 5 V, a rectification ratio up to 2 × 10 4 at ±5 V, and an operation frequency of the diode rectifier over 14 MHz were achieved with the PQT-12 rectifying diodes. To demonstrate the use of the high-speed polymer rectifying diodes in wireless power transmission, we integrate the polymer-based vertical Schottky diode rectifier, printed antenna and capacitors on plastic substrates to demonstrate a flexible wireless power-transmission sheet that can convert a 13.56 MHz electromagnetic wave into the dc power to drive load devices.

Interdigit 4H-SiC Vertical Schottky Diode for Betavoltaic Applications

We demonstrated a 4H-SiC vertical Schottky diode for betavoltaic application using interdigit front metallization. A relevant increase in the betavoltaic short-circuit current with respect to a device with a continuous standard front electrode was achieved with this novel layout allowing to collect also low-energy electrons. In particular, by irradiating the device with a monochromatic electron beam (e-beam) of 17 keV, an internal gain that is 1.4 times higher than in conventional devices was obtained. An open-circuit voltage of ~1 V was obtained for an illumination e-beam current density of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-8</sup> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

Anomalous photoresponse of GaN x-ray Schottky detectors

GaN based materials are believed to be very stable materials, in particular, under irradiation by high energy photons such as x rays. We have studied x-ray detectors based on GaN Schottky diodes. Vertical Schottky diodes were fabricated based on a 20μm thick undoped GaN layer grown on a conductive GaN substrate. Their photoresponse to near UV light and to x rays was measured. While the response to near UV light was fast and linear as expected, anomalous behaviors were observed under x-ray illumination. The photocurrent increases as the third power of the incident x-ray flux. The photocurrent transient when the x rays is turned on are long and nonexponential (S shape) and strongly differs from the off transient which is fast and exponential. Also, a very strong quenching of the x-ray photoresponse is observed when the detector is simultaneously illuminated with visible light. All of these anomalous behaviors are explained in the frame of a complete model involving traps and tunnel currents. A reasonable quantitative agreement between the model and the experimental data is obtained.

High-resistivity GaN homoepitaxial layer studied by Schottky diode structure

An undoped GaN homoepitaxial layer has been grown by metal-organic chemical vapour deposition on a conductive free-standing bulk GaN substrate. The dislocation density of the homoepitaxial layer characterised by cathodoluminescence mapping technique is about 6×106 cm−2. By fabricating a vertical Schottky diode structure, the resistivity of the homoepitaxial layer is estimated in the order of 109 Ωcm at room temperature. This study indicates that highly-resistive GaN can be produced by a homoepitaxy approach.

DLTS study of n-type GaN grown by MOCVD on GaN substrates

Abstract Electron traps in n-type GaN layers grown homoepitaxially by MOCVD on free-standing GaN substrates have been characterized using DLTS for vertical Schottky diodes. Two free-standing HVPE GaN substrates (A and B), obtained from two different sources, are used. The Si-doped GaN layers with the thickness of 5 μm are grown on an area of 0.9×0.9 cm2 of substrate A and on an area of 1×1 cm2 of substrate B. Two traps labeled B1 ( E c – 0.23 eV ) and B2 ( E c – 0.58 eV ) are observed with trap B2 dominant in GaN on both substrates. There exist no dislocation-related traps which have been previously observed in MOCVD GaN on sapphire. This might be correlated to the reduction in dislocation density due to the homoepitaxial growth. However, it is found that there is a large variation, more than an order of magnitude, in trap B2 concentration and that the B2 spatial distributions are different between the two substrates used.

Electrical characteristics of bulk GaN-based Schottky rectifiers with ultrafast reverse recovery

A vertical Schottky diode rectifier was fabricated using a bulk n−GaN wafer. Pt Schottky contacts were prepared on the Ga face and full backside ohmic contact was prepared on the N face by using Ti∕Al. The root mean square surface roughnesses of the Ga and N faces are 0.61 and 4.7nm, respectively. A relatively high breakdown field of 5.46kV∕cm was achieved with no additional edge termination. The breakdown field decreases as the size of the device increases. The background electron concentration of the bulk GaN wafer was low (5×1015cm−3), which may lead to a relatively high breakdown field even with no special edge termination. The forward turn-on voltage was as low as 2.4V at the current density of 100A∕cm2. The device exhibited an ultrafast reverse recovery characteristics (reverse recovery time &amp;lt;20ns).

Irradiation of optically activated SI-GaAs high-voltage switches with low and high energy protons

Semi-Insulating Gallium Arsenide (SI-GaAs) devices have been tested for radiation hardness with 3-4 MeV or 24 GeV proton beams. These devices can be operated in dc mode as optically activated electrical switches up to 1 kV. Both single switches (vertical Schottky diodes) and multiple (8) switches (planar devices) have been studied, by analyzing their current-voltage (I-V) reverse characteristics in the dark and under red light illumination, both before and after irradiation. We propose to use them in the system of high-voltage (-600 V) switches for the microstrip gas chambers for the CMS experiment at CERN. Low energy protons (3-4 MeV) were used in order to produce a surface damage below the Schottky contact: their fluence (up to 2.6/spl times/10/sup 15/ p/cm/sup 2/) gives a high-dose irradiation. The high energy proton irradiation (energy: 24 GeV, fluence: 1.1/spl times/10/sup 14/ p/cm/sup 2/) reproduced a ten years long proton exposure of the devices in CMS experiment conditions. For low energy irradiation, limited changes of the I-V curves in the dark have been observed, with at most a fourfold increase of the leakage current: after exposure, however, the breakdown voltage decreases significantly. For high energy irradiation, we observed-for the vertical Schottky diodes biased at -600 V-an increase of the leakage current and a reduction of the photocurrent after irradiation, with respect to pre-irradiation conditions. For these diodes, the reduction of the photocurrent/dark current ratio was 25:1. At the same proton energy, an analogous behaviour was shown by the planar devices, but after irradiation the current gain may reduce over three orders of magnitude.

Gigahertz InGaAs-based vertical Schottky diode optical detectors by substrate removal

In this paper, we present high-performance InGaAs vertical Schottky diode optical detectors and document design criteria and fabrication processes. The photodiodes are n-type and have an asymmetric contact structure with a rectifying anode and ohmic cathode. The vertical structure is achieved through InP substrate removal which exposes the back surface of the epitaxial layers. Ohmic contact is made to the exposed back surface. High-quality Schottky diodes with /spl phi//sub Bn//spl ap/0.85 V are reproducibly achieved through an etching pretreatment, resulting in dark currents below 10 pA//spl mu/m/sup 2/. Substrate removal and mesa definition result in low parasitic capacitance without the need of air bridges and allow accurate fiber-to-detector alignment. Responsivities greater than 0.7 A/W are obtained when a transparent conductor is used as the ohmic contact. RC-limited bandwidths as high as 19 GHz are measured for 20/spl times/20 /spl mu/m/sup 2/ devices.