Hongyun So

Simultaneous Thermoelectric Property Measurement and Incoherent Phonon Transport in Holey Silicon

Block copolymer patterned holey silicon (HS) was successfully integrated into a microdevice for simultaneous measurements of Seebeck coefficient, electrical conductivity, and thermal conductivity of the same HS microribbon. These fully integrated HS microdevices provided excellent platforms for the systematic investigation of thermoelectric transport properties tailored by the dimensions of the periodic hole array, that is, neck and pitch size, and the doping concentrations. Specifically, thermoelectric transport properties of HS with a neck size in the range of 16-34 nm and a fixed pitch size of 60 nm were characterized, and a clear neck size dependency was shown in the doping range of 3.1 × 10(18) to 6.5 × 10(19) cm(-3). At 300 K, thermal conductivity as low as 1.8 ± 0.2 W/mK was found in HS with a neck size of 16 nm, while optimized zT values were shown in HS with a neck size of 24 nm. The controllable effects of holey array dimensions and doping concentrations on HS thermoelectric performance could aid in improving the understanding of the phonon scattering process in a holey structure and also in facilitating the development of silicon-based thermoelectric devices.

Continuous V-Grooved AlGaN/GaN Surfaces for High-Temperature Ultraviolet Photodetectors

Three-dimensional heterostructured AlGaN/GaN ultraviolet (UV) photodetectors were microfabricated using V-grooved silicon(111) surfaces and metal organic chemical vapor deposition. This novel sensor platform enabled an increase in sensitivity and operation at high temperatures (up to 200°C). More specifically, texturizing the highly conductive 2-D electron gas using the V-groove sensor surfaces, resulted in higher photodetector sensitivity (57.4% increase at room temperature and 139% at 200°C) compared with conventional designs on planar substrates due to the increased absorption of incident UV light (optical trapping). In addition, a 53% reduction in electrical resistance at room temperature and 27.3% at 200°C were observed due to the increased surface area. The decay time for the non-exponential persistent photoconductivity decreased significantly from 327 to 34 sec as the temperature increased from room temperature to 200°C as a result of the accelerated electron-hole pair recombination (generation) rate. These results support the use of textured AlGaN/GaN sensor platforms for UV detection in harsh environments (e.g., downhole, combustion, and space).

Low-resistance gateless high electron mobility transistors using three-dimensional inverted pyramidal AlGaN/GaN surfaces

In this letter, three-dimensional gateless AlGaN/GaN high electron mobility transistors (HEMTs) were demonstrated with 54% reduction in electrical resistance and 73% increase in surface area compared with conventional gateless HEMTs on planar substrates. Inverted pyramidal AlGaN/GaN surfaces were microfabricated using potassium hydroxide etched silicon with exposed (111) surfaces and metal-organic chemical vapor deposition of coherent AlGaN/GaN thin films. In addition, electrical characterization of the devices showed that a combination of series and parallel connections of the highly conductive two-dimensional electron gas along the pyramidal geometry resulted in a significant reduction in electrical resistance at both room and high temperatures (up to 300 °C). This three-dimensional HEMT architecture can be leveraged to realize low-power and reliable power electronics, as well as harsh environment sensors with increased surface area.

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.

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.

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.

Wafer-level MOCVD growth of AlGaN/GaN-on-Si HEMT structures with ultra-high room temperature 2DEG mobility

In this work, we investigate the influence of growth temperature, impurity concentration, and metal contact structure on the uniformity and two-dimensional electron gas (2DEG) properties of AlGaN/GaN high electron mobility transistor (HEMT) structure grown by metal-organic chemical vapor deposition (MOCVD) on 4-inch Si substrate. High uniformity of 2DEG mobility (standard deviation down to 0.72%) across the radius of the 4-inch wafer has been achieved, and 2DEG mobility up to 1740.3 cm2/V⋅s at room temperature has been realized at low C and O impurity concentrations due to reduced ionized impurity scattering. The 2DEG mobility is further enhanced to 2161.4 cm2/V⋅s which is comparable to the highest value reported to date when the contact structure is switched from a square to a cross pattern due to reduced piezoelectric scattering at lower residual strain. This work provides constructive insights and promising results to the field of wafer-scale fabrication of AlGaN/GaN HEMT on Si.

Effect of Frost Formation on Operation of GaN Ultraviolet Photodetectors at Low Temperatures

Effects of frost growth on the sensitivity of gallium nitride (GaN) photodetectors were investigated by characterizing electrical and optical properties under dark and 365-nm ultraviolet (UV) illumination from room temperature down to −100°C. The direct wire bonding architecture was used to create aluminum/GaN interdigitated devices for the microfabrication. As the operation temperature decreased below −5°C, the frost formed from humid air was observed on the GaN surface, and photo-to-dark current ratio (sensitivity factor) showed significant reduction (6.76 at room temperature and 2.73 at −100°C under 1 V-bias). The presence of frost on the device surface significantly reduced the absorption of incident UV light into the GaN surfaces (average 85.6% reduction from room temperature to −70°C). This paper supports the characterization of the GaN for UV detection within low-temperature environments, such as cryostats, Arctic research, and space exploration applications.

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...