Ateeq J. Suria

Suppression of Persistent Photoconductivity in AlGaN/GaN Ultraviolet Photodetectors Using In Situ Heating

Photodetectors based on the AlGaN/GaN heterostructure suffer from persistent photoconductivity (PPC) in which recovery from the optical stimulus can take days. This behavior is unsuitable for many applications where reliable and consistent optical response is required. This letter presents a method for suppressing PPC in AlGaN/GaN photodetectors by employing device suspension and in situ heating. The highly conductive two-dimensional electron gas (2DEG) at the interface of AlGaN and GaN serves as both a sensor and a heater (via Joule heating). Microfabricated AlGaN/GaN-on-Si ultraviolet (UV) photodetectors (suspended and unsuspended) were exposed to UV (365 nm) for 60 s and the transient responses were measured under various in situ heating conditions. The measured transient response showed a decay time of ~39 h when the photodetector was not heated and 24 s for a suspended photodetector with in situ 2DEG heating (270°C with a power of 75 mW). This remarkable suppression of the PPC in AlGaN/GaN UV photodetectors can be attributed to the novel device architecture and in situ heating capability, which enables acceleration of the carrier capture rate during operation.

Multilayer etch masks for 3-dimensional fabrication of robust silicon carbide microstructures

This paper details the creation of 3-dimensional (3-D) microstructures in 4H-silicon carbide (4H-SiC) substrates with a plasma etch process that utilizes multilayer etch masks. An inductively coupled plasma (ICP) etch process (SF6/O2) for SiC was developed and etch rates as high as ~1 μm/min, a selectivity of 60:1 (SiC to Ni), and aspect ratio dependent etch characteristics were demonstrated. In addition, the selectivity of atomic layer deposited (ALD) Al2O3 etch masks to 4H-SiC is reported for the first time. Using this unique process, the microfabrication of complex microstructures (mechanical gears, Lego®-like bricks, and poker chips) is presented. The use of 4H-SiC as the structural material enables such microstructures to be utilized under high cycles of wear, within elevated temperatures, and within chemically corrosive environments.

DC characteristics of ALD-grown Al2O3/AlGaN/GaN MIS-HEMTs and HEMTs at 600 °C in air

To the best of our knowledge, the 600 °C device characteristics detailed here reflect the highest operation temperature reported for AlGaN/GaN metal-insulator-semiconductor high electron mobility transistors (MIS-HEMTs) in air which supports the realization of electronics for high-temperature applications (e.g., space exploration, combustion and downhole). The high-temperature response of Al2O3/AlGaN/GaN MIS-HEMTs with Al2O3 deposited by plasma-enhanced atomic layer deposition (ALD) as the gate dielectric and passivation layers was examined here. More specifically, the DC current–voltage response and the threshold voltage characteristics of the MIS-HEMTs were evaluated to temperatures up to 600 °C in air. For comparison, the response of AlGaN/GaN HEMTs without the ALD Al2O3 layer was also measured. It was observed that the HEMTs failed above 300 °C accompanied by a ~500 times increase in leakage current and observation of bubbles formed in active region of gate. On the contrary, the MIS-HEMTs continued to operate normally up to 600 °C. However, within the 30 min period exposed to 600 °C the MIS-HEMT degraded permanently. This was observed at 20 °C after return from operation at 600 °C as a change in threshold voltage and saturation drain current. The failure of the HEMTs is suggested to be due to the diffusion of gate metals (Ni and Au) into the active regions of the AlGaN/GaN heterostructure, which creates additional leakage current pathways. The impact of strain relaxation and interfacial trapped charges on threshold voltage as a function of temperature was studied using an energy band-gap model. The ALD Al2O3 gate dielectric layer acts as a diffusion barrier to the Ni and Au gate metals, thus enabling short-term operation of MIS-HEMTs to 600 °C, the highest operation temperature reported for this device architecture to date.

Inductive Coupled Plasma Etching of High Aspect Ratio Silicon Carbide Microchannels for Localized Cooling

High aspect ratio microchannels using high thermal conductivity materials such as silicon carbide (SiC) have recently been explored to locally cool micro-scale power electronics that are prone to on-chip hot spot generation. Analytical and finite element modeling shows that SiC-based microchannels used for localized cooling should have high aspect ratio features (above 8:1) to obtain heat transfer coefficients (300 to 600 kW/m2·K) required to obtain gallium nitride (GaN) device channel temperatures below 100°C. This work presents experimental results of microfabricating high aspect ratio microchannels in a 4H-SiC substrate using inductively coupled plasma (ICP) etching. Depths of 90 μm and 80 μm were achieved with a 5:1 and 12:1 aspect ratio, respectively. This microfabrication process will enable the integration of microchannels (backside features) with high-power density devices such as GaN-on-SiC based electronics, as well as other SiC-based microfluidic applications.Copyright

Effects of radiation and temperature on gallium nitride (GaN) metal-semiconductor-metal ultraviolet photodetectors

The development of radiation-hardened, temperature-tolerant materials, sensors and electronics will enable lightweight space sub-systems (reduced packaging requirements) with increased operation lifetimes in extreme harsh environments such as those encountered during space exploration. Gallium nitride (GaN) is a ceramic, semiconductor material stable within high-radiation, high-temperature and chemically corrosive environments due to its wide bandgap (3.4 eV). These material properties can be leveraged for ultraviolet (UV) wavelength photodetection. In this paper, current results of GaN metal-semiconductor-metal (MSM) UV photodetectors behavior after irradiation up to 50 krad and temperatures of 15°C to 150°C is presented. These initial results indicate that GaN-based sensors can provide robust operation within extreme harsh environments. Future directions for GaN-based photodetector technology for down-hole, automotive and space exploration applications are also discussed.

Thickness engineering of atomic layer deposited Al2O3 films to suppress interfacial reaction and diffusion of Ni/Au gate metal in AlGaN/GaN HEMTs up to 600 °C in air

In this paper, we describe the use of 50 nm atomic layer deposited (ALD) Al2O3 to suppress the interfacial reaction and inter-diffusion between the gate metal and semiconductor interface, to extend the operation limit up to 600 °C in air. Suppression of diffusion is verified through Auger electron spectroscopy (AES) depth profiling and X-ray diffraction (XRD) and is further supported with electrical characterization. An ALD Al2O3 thin film (10 nm and 50 nm), which functions as a dielectric layer, was inserted between the gate metal (Ni/Au) and heterostructure-based semiconductor material (AlGaN/GaN) to form a metal-insulator-semiconductor high electron mobility transistor (MIS-HEMT). This extended the 50 nm ALD Al2O3 MIS-HEMT (50-MIS) current-voltage (Ids-Vds) and gate leakage (Ig,leakage) characteristics up to 600 °C. Both, the 10 nm ALD Al2O3 MIS-HEMT (10-MIS) and HEMT, failed above 350 °C, as evidenced by a sudden increase of approximately 50 times and 5.3 × 106 times in Ig,leakage, respectively. AES o...

A microfabricated sun sensor using GaN-on-sapphire ultraviolet photodetector arrays

A miniature sensor for detecting the orientation of incident ultraviolet light was microfabricated using gallium nitride (GaN)-on-sapphire substrates and semi-transparent interdigitated gold electrodes for sun sensing applications. The individual metal-semiconductor-metal photodetector elements were shown to have a stable and repeatable response with a high sensitivity (photocurrent-to-dark current ratio (PDCR) = 2.4 at -1 V bias) and a high responsivity (3200 A/W at -1 V bias) under ultraviolet (365 nm) illumination. The 3 × 3 GaN-on-sapphire ultraviolet photodetector array was integrated with a gold aperture to realize a miniature sun sensor (1.35 mm × 1.35 mm) capable of determining incident light angles with a ±45° field of view. Using a simple comparative figure of merit algorithm, measurement of incident light angles of 0° and 45° was quantitatively and qualitatively (visually) demonstrated by the sun sensor, supporting the use of GaN-based sun sensors for orientation, navigation, and tracking of the sun within the harsh environment of space.

Irradiation effects of graphene-enhanced gallium nitride (GaN) metal-semiconductor-metal (MSM) ultraviolet photodetectors

Ultraviolet (UV) photodetectors are used for applications such as flame detection, space navigation, biomedical and environmental monitoring. Robust operation within large ranges of temperatures, radiation, salinity and/or corrosive chemicals require sensor materials with the ability to withstand and function reliably within these extreme harsh environments. For example, spacecraft can utilize a sun sensor (light-based sensor) to assist with determination of orientation and may be exposed to both ionizing radiation and extreme temperature swings during operation. Gallium nitride (GaN), a wide bandgap semiconductor material, has material properties enabling visible-blindness, tunable cutoff wavelength selection based on ternary alloy mole fraction, high current density, thermal/chemical stability and high radiation tolerance due to the strength of the chemical bond. Graphene, with outstanding electrical, optical and mechanical properties and a flat absorption spectrum from 300 to 2,500 nm, has potential use as a transparent conductor for GaN-based metal-semiconductor-metal (MSM) photodetectors. Here, graphene-enhanced MSM UV photodetectors are fabricated with transparent and conductive graphene interdigitated electrodes on thin film GaN-on-sapphire substrates serving as back-to-back Schottky contacts. We report on the irradiation response of graphene/GaN-based MSM UV photodetectors up to 750 krad total ionizing dose (TID) then tested under dark and UV light (365 nm) conditions. In addition, based on current-voltage measurements from 75 krad to 750 krad TID, calculated photodetector responsivity values change slightly by 25% and 11% at -5 V and -2 V, respectively. These initial findings suggest that graphene/GaN MSM UV photodetectors could potentially be engineered to reliably operate within radiation environments.

Capacitance-voltage characteristics of gamma irradiated Al2O3, HfO2, and SiO2 thin films grown by plasma-enhanced atomic layer deposition

Radiation-hardened electronics used in space, nuclear energy and radiation medicine applications require robust dielectric materials to be used as passivation layers and gate insulators. Thus, there is a need to understand the response of these materials under radiation exposure (e.g., gamma, neutron and proton) to develop radiation-tolerant and reliable electronic systems. In addition, as the size of transistors continues to scale down there is a need to have physically thicker dielectric layers with similar capacitance values to ultra-thin SiO2. High permittivity (high-k) dielectrics lend themselves well to this task as they have capacitance values similar to ultra-thin SiO2 while not facing issues of high leakage current and power dissipation as ultra-thin SiO2. Atomic layer deposition (ALD) of thin films has gained interest in the development of radiation-hardened electronics as this process results in high quality (continuous and pinhole-free) and conformal gate dielectric thin films with precise thickness control to the angstrom level. Here, we examine the impact of gamma-irradiation on plasma-enhanced ALD dielectric layers using metal-oxide semiconductor (MOS) capacitors. In this work, three ALD gate dielectric films: Al2O3, HfO2 and SiO2 (between 22 and 24 nm thick) are utilized. The capacitance-voltage (C-V) response of plasma-enhanced ALD-based MOS capacitors upon gamma irradiation (Co-60) up to 533 krad without any shielding is observed. It is shown that ALD grown HfO2 films are resistant to gamma irradiation based on the negligible shift in flat band voltage and hysteresis characteristics. Additionally, ALD grown Al2O3 films exhibited minimal generation of mobile traps but generation of trapped charges was observed. Furthermore, the flat band and hysteresis of ALD grown SiO2 films showed development of both trapped and mobile charges which may suggest that this material lends itself to radiation dosimetry applications. These initial findings support the use of plasma-enhanced ALD grown films in the development of radiation-hardened electronics and sensors.

Degradation of 2DEG transport properties in GaN-capped AlGaN/GaN heterostructures at 600 °C in oxidizing and inert environments

In this paper, the electron mobility and sheet density of the two-dimensional electron gas (2DEG) in both air and argon environments at 600 °C were measured intermittently over a 5 h duration using unpassivated and Al2O3-passivated AlGaN/GaN (with 3 nm GaN cap) van der Pauw test structures. The unpassivated AlGaN/GaN heterostructures annealed in air showed the smallest decrease (∼8%) in 2DEG electron mobility while Al2O3-passivated samples annealed in argon displayed the largest drop (∼70%) based on the Hall measurements. Photoluminescence and atomic force microscopy showed that minimal strain relaxation and surface roughness changes have occurred in the unpassivated samples annealed in air, while those with Al2O3 passivation annealed in argon showed significant microstructural degradations. This suggests that cracks developed in the samples annealed in air were healed by oxidation reactions. To further confirm this, Auger electron spectroscopy was conducted on the unpassivated samples after the anneal in...