Showing papers by "Stacia Keller published in 2021"

6.2 W/Mm and Record 33.8% PAE at 94 GHz From N-Polar GaN Deep Recess MIS-HEMTs With ALD Ru Gates

Abstract: This letter reports on the $W$ -band power performance of N-polar GaN deep recess MIS–high electron mobility transistors (HEMTs) using a new atomic layer deposition (ALD) ruthenium (Ru) gate metallization process. The deep recess structure is utilized to control the DC-RF dispersion and increase the conductivity in the access regions. The ALD Ru effectively fills the narrow T-gate stems aiding realization of shorter gate lengths with lower gate resistance than in prior work. In this work, the gate length was scaled down to 48 nm, resulting in the demonstration of a record high 8.1-dB linear transducer gain measured at 94 GHz by load pull. This increased gain has enabled a record 33.8% power-added efficiency (PAE) with an associated output power density ( $P_{\mathrm {O}}$ ) of 6.2 W/mm.

InGaN-Based microLED Devices Approaching 1% EQE with Red 609 nm Electroluminescence on Semi-Relaxed Substrates

Abstract: In this paper, we report the successful demonstration of bright InGaN-based microLED devices emitting in the red spectral regime grown by metal organic chemical vapor deposition (MOCVD) on c-plane semi-relaxed InGaN substrates on sapphire. Through application of an InGaN/GaN base layer scheme to ameliorate high defect density and maintain appropriate lattice constant throughout the growth, high-In quantum wells (QWs) can be grown with improved crystal quality. Improvement to the design of the growth scheme also yields higher power output resulting in an increase to the external quantum efficiency (EQE). Combined, these two improvements allow for an 80 × 80 μm2 microLED device emitting at 609 nm to achieve 0.83% EQE. Furthermore, the true In content of the QW is measured using atomic probe tomography (APT) to confirm the improved In incorporation during high temperature active region growth. These developments represent advancement toward the realization of bright, highly efficient red III-nitride LEDs to be used in RGB applications under one material system.

Evaluation of linearity at 30 GHz for N-polar GaN deep recess transistors with 10.3 W/mm of output power and 47.4% PAE

Abstract: The advantage of GaN is the capability of producing amplifiers with high output power and efficiency. At microwave frequencies, this performance has been achieved; however, when transitioning device design into mm-wave frequencies, the output power and efficiency of GaN HEMTs decrease. Traditionally, the approach taken to develop Ka-band (30–40 GHz) GaN devices has been to modify a device designed for a lower frequency. By contrast, this work modified a N-polar GaN deep recess HEMT developed for W-band power performance (94 GHz), for improved performance in the Ka-band. In this Letter, we first report on improvement in the 30 GHz continuous-wave (CW) power density through modification of the W-band device with the demonstration of 10.3 W/mm at 47.4% power-added efficiency (PAE). We then report on the two-tone linearity performance of the device when measured under the same bias and matching conditions. While the evaluation of GaN HEMTs has traditionally focused on the use of one-tone CW power measurements, with the increasing adoption of GaN transistors into communication systems, such as mm-wave 5 G cellular communication, simply demonstrating high power density and efficiency does not provide a sufficient understanding of the device as high linearity is required to transmit data using complex modulation schemes. Under two-tone stimulus, the device demonstrates an OIP3 to PDC ratio greater than 6.7 dB and a C/IM3 ratio of greater than 37 dBc under backoff conditions greater than 10 dB from the peak one-tone PAE.

Realization of III-Nitride c-Plane microLEDs Emitting from 470 to 645 nm on Semi-Relaxed Substrates Enabled by V-Defect-Free Base Layers

Abstract: We examine full InGaN-based microLEDs on c-plane semi-relaxed InGaN substrates grown by metal organic chemical vapor deposition (MOCVD) that operate across a wide range of emission wavelengths covering nearly the entire visible spectrum. By employing a periodic InGaN base layer structure with high temperature (HT) GaN interlayers on these semi-relaxed substrates, we demonstrate robust μLED devices. A broad range of emission wavelengths ranging from cyan to deep red are realized, leveraging the indium incorporation benefit of the relaxed InGaN substrate with an enlarged lattice parameter. Since a broad range of emission wavelengths can be realized, this base layer scheme allows the tailoring of the emission wavelength to a particular application, including the possibility for nitride LEDs to emit over the entire visible light spectrum. The range of emission possibilities from blue to red makes the relaxed substrate and periodic base layer scheme an attractive platform to unify the visible emission spectra under one singular material system using III-Nitride MOCVD.

2DEGs formed in AlN/GaN HEMT structures with AlN grown at low temperature

Abstract: Integration of nitrides with other material systems has recently become of interest due to the high performance of GaN-based high-electron mobility transistors. However, the elevated growth temperatures often used to grow high quality AlN pose challenges toward metalorganic chemical vapor deposition (MOCVD) on temperature sensitive substrates such as processed wafers. In this work, the growth of AlN was conducted at temperatures below 550 °C via MOCVD using a flow-modulated epitaxy scheme, and their morphological, compositional, and electronic properties of these films were investigated. Sheet charges up to 2.1 × 1013 cm−2 and mobilities on the order of 400 cm2/V s were measured for two dimensional electron gases, which formed at the interface between the low temperature grown AlN layers and the semi-insulating GaN base layers deposited at high temperatures. Despite their low growth temperatures, nominally pure AlN barrier layers exhibited measurable unintentional gallium incorporation adjacent to the GaN interface. The result sets the stage for the integration of nitride-based electronics via epitaxy-based schemes on temperature sensitive substrates.

HfO2 as gate insulator on N-polar GaN-AlGaN heterostructures

Abstract: To further increase the operation frequency of GaN high electron mobility transistors (HEMTs) with high power gain in the mm-wave band (30–300 GHz), channel thickness must be scaled. High-k dielectrics such as HfO2 can be inserted as gate insulator to reduce the leakage, while maintaining good gate control. In this work, we investigated HfO2 on N-polar HEMT structure by performing frequency dependent capacitance–voltage measurements on metal-oxide-semiconductor capacitors (MOSCAPs). The impact of annealing on the quality of HfO2 was studied. Moreover, we compared the CV characteristics of MOSCAPs with HfO2 deposited on the GaN channel with that deposited on the AlGaN cap layer. We showed that HfO2 deposition on AlGaN cap leads to Fermi level pinning, which prevents the channel being pinched off by applying gate voltage. On the other hand, well-behaved CV profiles were achieved by depositing HfO2 directly on GaN or SiN dielectric.

Metal Organic Vapor Phase Epitaxy of Thick N-Polar InGaN Films

Abstract: Hillock-free thick InGaN layers were grown on N-polar GaN on sapphire by metal organic vapor phase epitaxy using a digital growth scheme and H2 as surfactant. Introducing Mg to act as an additional surfactant and optimizing the H2 pulse time, In compositions up to 17% were obtained in 100 nm thick epilayers. Although Mg adversely affected the In incorporation, it enabled maintenance of a good surface morphology while decreasing the InGaN growth temperature, resulting in a net increase in In composition. The parameter space of growth temperature and Mg precursor flow to obtain hillock-free epilayers was mapped out.

Role of the AlGaN Cap Layer on the Trapping Behaviour of N-Polar GaN MISHEMTs

Abstract: We investigate the static and dynamic (trapping) performance of N-polar Gallium Nitride MIS-HEMT devices as a function of the aluminum concentration in the top cap layer (22%, 34% and 46%). The analysis is based on combined dc characterization, double pulse measurements, and threshold voltage transient investigation. The de results demonstrate that the use of high aluminum concentrations in the cap layer results in a lower gate leakage current (around 9 µA/mm for % Al=46, compared to 70 µA/ mm for %Al=22, measured at V Gs =-7 V and V DS =15 V). In addition, pulsed and transient investigation showed that the use of high Al concentration in the cap layer can substantially suppress the current collapse (slump ratio = 15 % for %Al=46, compared to 26 % for %AI=22). Trapping is ascribed to the presence of a defect state located at E C -0.5 eV, which is responsible for a threshold voltage shift. The results point out the key role of the AlGaN cap layer on the performance of AlGaN-based HEMTs, and give indication on how to optimize the performance of the devices.

Properties of AlN/GaN Heterostructures Grown at Low Growth Temperatures with Ammonia and Dimethylhydrazine

Abstract: The integration of different electronic materials systems together has gained increasing interest in recent years, with the III-nitrides being a favorable choice for a variety of electronic applications. To increase flexibility in integration options, growing nitrides material directly on semi-processed wafers would be advantageous, necessitating low temperature (LT) growth schemes. In this work, the growth of AlN and GaN was conducted via metalorganic chemical vapor deposition (MOCVD) using both NH3 and DMHy as N-precursors. The relationships between growth rate versus temperature were determined within the range of 300 to 550 °C. The growth of AlN/GaN heterostructures was also investigated herein, employing flow modulation epitaxy MOCVD at 550 °C. Subsequent samples were studied via atomic force microscopy, X-ray diffraction, TEM, and Hall measurements. Two-dimensional electron gases were found in samples where the LT AlN layer was grown with NH3, with one sample showing high electron mobility and sheet charge of 540 cm2/V∙s and 3.76 × 1013 cm−2, respectively. Inserting a LT GaN layer under the LT AlN layer caused the mobility and charge to marginally decrease while still maintaining sufficiently high values. This sets the groundwork towards use of LT nitrides MOCVD in future electronic devices integrating III-nitrides with other materials.

InN Quantum Dots by Metalorganic Chemical Vapor Deposition for Optoelectronic Applications

Abstract: This review will cover recent work on InN quantum dots (QDs), specifically focusing on advances in metalorganic chemical vapor deposition (MOCVD) of metal-polar InN QDs for applications in optoelectronic devices. The ability to use InN in optoelectronic devices would expand the nitrides system from current visible and ultraviolet devices into the near infrared. Although there was a significant surge in InN research after the discovery that its bandgap provided potential infrared communication band emission, those studies failed to produce an electroluminescent InN device in part due to difficulties in achieving p-type InN films. Devices utilizing InN QDs, on the other hand, were hampered by the inability to cap the InN without causing intermixing with the capping material. The recent work on InN QDs has proven that it is possible to use capping methods to bury the QDs without significantly affecting their composition or photoluminescence. Herein, we will discuss the current state of metal-polar InN QD growth by MOCVD, focusing on density and size control, composition, relaxation, capping, and photoluminescence. The outstanding challenges which remain to be solved in order to achieve InN infrared devices will be discussed.

Metalorganic chemical vapor deposition of InN quantum dots and nanostructures.

Abstract: Using one material system from the near infrared into the ultraviolet is an attractive goal, and may be achieved with (In,Al,Ga)N. This III-N material system, famous for enabling blue and white solid-state lighting, has been pushing towards longer wavelengths in more recent years. With a bandgap of about 0.7 eV, InN can emit light in the near infrared, potentially overlapping with the part of the electromagnetic spectrum currently dominated by III-As and III-P technology. As has been the case in these other III–V material systems, nanostructures such as quantum dots and quantum dashes provide additional benefits towards optoelectronic devices. In the case of InN, these nanostructures have been in the development stage for some time, with more recent developments allowing for InN quantum dots and dashes to be incorporated into larger device structures. This review will detail the current state of metalorganic chemical vapor deposition of InN nanostructures, focusing on how precursor choices, crystallographic orientation, and other growth parameters affect the deposition. The optical properties of InN nanostructures will also be assessed, with an eye towards the fabrication of optoelectronic devices such as light-emitting diodes, laser diodes, and photodetectors. This review article describes the current state of InN nanostructure and quantum dot growth by metalorganic chemical vapor deposition and the use in optoelectronic applications.

Morphological improvement and elimination of V-pits from long-wavelength all-InGaN based uLEDs grown by MOCVD on compliant substrates

Abstract: We examine the MOCVD growth conditions on c-plane semi-relaxed InGaN substrates necessary for morphological improvement, defect reduction, and elimination of V-pits during epitaxy prior to the active region growth. V-pit defects can propagate through the crystal as epitaxy continues, causing serious morphological degradation. These defects may also be a source of leakage current if they form a low-resistance path through p-n junction. By employing an InGaN/GaN periodic structure, thick base layers can be grown with the morphology improving as the epitaxy proceeds, allowing for high quality layers to be achieved. High temperature (HT) GaN interlayers in the InGaN/GaN base layer structure then continue to reduce defects significantly, notably eliminating the V-pit type defect, and significantly improving growth morphology. Resulting microLEDs on these improved base layers exhibit a nearly three order of magnitude reduction in leakage current density at 1V, far below the μLED turn-on threshold, and significantly lower dynamic resistance. This result indicates the reduction in base layer defects and layer morphology improvement results in significant improvement in electrical performance and enables production of viable LEDs.

Investigation and optimization of N-polar GaN porosification for regrowth of smooth hillocks-free GaN films

Abstract: This work investigates the process of planar electrochemical etching of pores in n-type nitrogen-polar GaN and the effect of pore morphology on regrown GaN film surface quality. An increase in the anodization voltage was found to increase the pore diameter and reduce the density of pores with inclined sidewalls near the surface of the porosified films. Simultaneously, a decrease in the hexagonal hillock size and number following GaN regrowth was observed. It is proposed that vertical pore sidewalls are essential to demonstrate high quality film coalescence. For smooth hillock-free 100 nm GaN regrowth, an optimal bias of 17 and 13 V for Ti-contacted N-polar GaN:Si with a Si doping of 4.5 × 1018 and 8 × 1018 cm−3, respectively, was found.