Continuous GaN Research Articles

A High-Efficiency Continuous Class-F GaN MMIC Power Amplifier Using a Novel Harmonic Matching Network

A High-Efficiency Continuous Class-F GaN MMIC Power Amplifier Using a Novel Harmonic Matching Network

Epitaxy of single-crystalline GaN films on novel Ca3Ta(Ga0.5Al0.5)3Si2O14 substrates by metal-organic chemical vapor deposition

The langasite (La3Ga5SiO14, LGS) is a nearly lattice-matched substrate for epitaxy of GaN films. However, it is a challenge to grow GaN films on LGS substrates by metal-organic chemical vapor deposition (MOCVD) because LGS is unstable in aggressive gas atmosphere at high temperature of ∼1000 °C. To overcome the problem, this study chooses another LGS family crystal Ca3Ta(Ga0.5Al0.5)3Si2O14 (CTAGS) which has the same crystal structure as LGS and is more stable in high temperature. The morphology of GaN films at different growth stages are characterized and the results show continuous and smooth GaN films could be obtained after growing for 3 h. Single-crystalline GaN films are determined by transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) measurements. The epitaxial relationship between GaN and CTAGS is GaN(0001)//CTAGS(0001) and GaN(101¯0)//CTAGS(213¯0), the lattice mismatch between GaN and CTAGS is 4.15%. The work is great potential in promoting the development of GaN-based devices.

Continuous Broadband GaAs and GaN MMIC Phase Shifters

We describe the design of two broadband monolithic microwave integrated circuit (MMIC) loaded-line reflective phase shifters and their performance over the 6–12-GHz band. Lange couplers with transistor-loaded transmission-line reactive loads provide variable phase shift through reverse-bias control. A GaAs phase shifter with four diode-connected transistors in each of the two reactive loads is designed with discrete inductors to provide an increasing phase shift with frequency. A GaN reflective phase shifter uses 14 transistors in each reactive load with a goal of increasing the phase shift across the band. Measurements show continuous phase shift from 40° to 70° with less than 6-dB insertion loss over the 6–10-GHz band and from 60° to 165° with less than 8-dB insertion loss across the 8–12-GHz band for the GaAs and GaN MMICs, respectively. The power performance of the MMICs is compared in terms of large-signal <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$S$ </tex-math></inline-formula> -parameters, harmonics, and IP3. The GaN phase shifter exhibits similar level of harmonic generation like the GaAs device at a 20-dB higher input power.

Growth of continuous GaN films on ZnO buffer layer by chemical vapor deposition for ultraviolet photodetector

Growth of continuous GaN films on ZnO buffer layer by chemical vapor deposition for ultraviolet photodetector

Epitaxial Growth of GaN on Magnetron Sputtered AlN/Hexagonal BN/Sapphire Substrates.

Magnetron sputtering is adopted to deposit ~25 nm thick AlN on the surface of hexagonal BN(h-BN)/sapphire substrates, followed by epitaxial GaN growth on top of the AlN/h-BN/sapphire substrate using a metal–organic chemical vapor deposition system. Compared to GaN grown on the h-BN/sapphire surface directly, this method results in a continuous and smooth GaN film with a smaller root mean square roughness. Besides, the introduction of the sputtered AlN layer reduces the dislocation density of GaN by 35.7%. We provide a pathway of GaN epitaxy on the h-BN surface, which significantly improves its surface morphology and crystal quality.

Growth of GaN on monolayer hexagonal boron nitride by chemical vapor deposition for ultraviolet photodetectors

Chemical vapor deposition (CVD) technology is a simple and flexible method used to prepare high-quality crystalline materials. Traditional CVD technology, based on pre-deposited thin catalyst metal, usually produces nanostructures instead of continuous films. In this work, a continuous GaN film on a monolayer boron nitride (h-BN) insertion layer is demonstrated using CVD technology. The experimental results and theoretical calculations indicate that abundant GaN nanocrystallites are firstly formed at the edges or grain boundaries of the monolayer h-BN by quasi-van der Waals epitaxy. Then, the vapor-solid mechanism will control further growth of the GaN nanocrystallites, causing them to merge into a continuous GaN film. Meanwhile, the CVD-grown GaN ultraviolet detector exhibits a relatively high responsivity with a value of 0.57 A W−1 at 2 V. In this paper, a simple low-cost CVD method is proposed for preparing continuous films on two-dimensional materials for electronic and optoelectronic devices.

GaN-Based LEDs Grown on Graphene-Covered SiO2/Si (100) Substrate

The growth of nitride on large-size and low-cost amorphous substrates has attracted considerable attention for applications in large-scale optoelectronic devices. In this paper, we reported the growth of GaN-based light-emitting diodes (LEDs) on amorphous SiO2 substrate with the use of nanorods and graphene buffer layers by metal organic chemical vapor deposition (MOCVD). The effect of different growth parameters on the morphology and vertical-to-lateral aspect ratio of nanorods was discussed by analyzing growth kinetics. Furthermore, we tuned nanorod coalescence to obtain continuous GaN films with a blue-LED structure by adjusting growth conditions. The GaN films exhibited a hexagonal wurtzite structure and aligned c-axis orientation demonstrated by X-ray diffractometer (XRD), Raman, and transmission electron microscopy (TEM) results. Finally, five-pair InGaN/GaN multi-quantum-wells (MQWs) were grown. The photoluminescence (PL) showed an intense emission peak at 475 nm, and the current–voltage (I-V) curve shows a rectifying behavior with a turn-on voltage of 5.7 V. This work provides a promising fabrication method for the large-area and low-cost GaN-based devices on amorphous substrates and opens up the further possibility of nitride integration with Si (100) complementary metal oxide semiconductor (CMOS) electronics.

Direct van der Waals epitaxy of stress-free GaN films on PECVD grown graphene

Residual stress is generated in GaN epitaxial layers due to the mismatch during GaN epitaxy on sapphire using the traditional method. Therefore, the use of graphene to reduce residual stress and dislocation densities in GaN epitaxy has become an important research direction. However, growing a stress-free GaN film on graphene substrate remains challenge. In this work, we directly grew graphene on sapphire via plasma enhanced chemical vapor deposition (PECVD) to obtain an epitaxial graphene with characteristic orientation, and ultra-low stress GaN films can then be obtained through metal organic chemical vapor deposition (MOCVD) assisted with the sputtering AlN buffer layer. Through this method, we successfully obtained continuous and flat GaN films with ultra-low biaxial compressive stress (0.023 GPa) without the complicated stress engineering during epitaxial growth. First principle calculation was employed to confirm that the characteristic orientation of epitaxial graphene is crucial to release the stress in GaN. The obtained GaN films can also be easily transferred because of small van der Waals force on graphene. The transferred GaN heterojunction was directly fabricated into a metal-insulator-semiconductor (MIS) device from which typical electrical properties can be obtained. Our work reveals the stress-releasing mechanism and excellent stress-releasing effect of graphene and provides a new epitaxial strategy to guide crystallographic epitaxy.

Structural and Morphological Properties of Hybrid Heterostructures Based on GaN Grown on a Compliant por-Si(111) Substrate

The possibility of synthesizing integrated GaN/por-Si heterostructures by plasma-assisted molecular beam epitaxy without an A1N/Si buffer layer is demonstrated. The beneficial effect of the high-temperature nitridation of a silicon substrate before GaN growth on the crystal quality of the GaN/Si layers is shown. It is established that, to obtain two-dimensional GaN layers on Si(111), it is reasonable to use compliant por-Si substrates and low-temperature GaN seed layers with a 3D morphology synthesized by plasma-assisted molecular beam epitaxy at relatively low substrate temperatures under stoichiometric conditions and upon enrichment with nitrogen. In this case, a self-assembled array of GaN seed nanocolumns with a fairly uniform diameter distribution forms on the por-Si substrate surface. The basic GaN layers, in turn, should be grown at a high temperature under stoichiometric conditions upon enrichment with gallium, upon which the coalescence of nucleated GaN nanocolumns and growth of a continuous two-dimensional GaN layer are observed. The use of compliant Si substrates is a relevant approach for forming GaN-based semiconductor device heterostructures by plasma-assisted molecular beam epitaxy.

Структурные и морфологичеcкие свойства гибридных гетероструктур на основе GaN, выращенного на "податливой" подложке por-Si(111)

AbstractThe possibility of synthesizing integrated GaN/por-Si heterostructures by plasma-assisted molecular beam epitaxy without an A1N/Si buffer layer is demonstrated. The beneficial effect of the high-temperature nitridation of a silicon substrate before GaN growth on the crystal quality of the GaN/Si layers is shown. It is established that, to obtain two-dimensional GaN layers on Si(111), it is reasonable to use compliant por-Si substrates and low-temperature GaN seed layers with a 3D morphology synthesized by plasma-assisted molecular beam epitaxy at relatively low substrate temperatures under stoichiometric conditions and upon enrichment with nitrogen. In this case, a self-assembled array of GaN seed nanocolumns with a fairly uniform diameter distribution forms on the por-Si substrate surface. The basic GaN layers, in turn, should be grown at a high temperature under stoichiometric conditions upon enrichment with gallium, upon which the coalescence of nucleated GaN nanocolumns and growth of a continuous two-dimensional GaN layer are observed. The use of compliant Si substrates is a relevant approach for forming GaN-based semiconductor device heterostructures by plasma-assisted molecular beam epitaxy.

Design of Highly Linear Broadband Continuous Mode GaN MMIC Power Amplifiers for 5G

In this paper, we present a design approach for broadband harmonic-tuned monolithic microwave integrated circuit (MMIC) power amplifiers (PAs). Two harmonic matching networks are proposed for the realization of continuous class-B and class-F modes in an integrated PA. A design procedure is developed for integrated PAs using these matching networks to achieve high linearity, broadband operation, and compact chip area, in the presence of parasitic components and the physical limitations of the MMIC process. Two proof-of-concept fully integrated PAs are implemented in a 0.25-μm GaN-on-SiC process. Output power of 34.2-36.4 dBm with 40% to 49% power-added efficiency (PAE) is achieved in 4.2-7.0 GHz (51.6% fractional bandwidth), from the continuous class-B PA with only 1.5-mm 2 chip area. Furthermore, the continuous class-F PA achieves 36.2 dBm output power and 52% PAE at 5.0 GHz. The PAs are also characterized using QAM signals with wide bandwidth, in order to evaluate their performance for 5G wireless applications. For a 64-QAM signal with 100-MHz bandwidth and 8 dB peak-to-average power ratio (PAPR), the continuous class-B PA achieves 29.3 dBm average output power, 28% average PAE, and -25 dB (5.5%) error vector magnitude (EVM). The continuous class-F PA, tested using a 200 MHz 256 QAM signal with 8.5 dB PAPR, provides an average output power of 28.5 dBm, average PAE of 27%, and -28 dB (4%) EVM, without any pre-distortion.

Effects of N₂ Partial Pressure on Growth, Structure, and Optical Properties of GaN Nanorods Deposited by Liquid-Target Reactive Magnetron Sputter Epitaxy.

GaN nanorods, essentially free from crystal defects and exhibiting very sharp band-edge luminescence, have been grown by reactive direct-current magnetron sputter epitaxy onto Si (111) substrates at a low working pressure of 5 mTorr. Upon diluting the reactive N2 working gas with a small amount of Ar (0.5 mTorr), we observed an increase in the nanorod aspect ratio from 8 to ~35, a decrease in the average diameter from 74 to 35 nm, and a two-fold increase in nanorod density. With further dilution (Ar = 2.5 mTorr), the aspect ratio decreased to 14, while the diameter increased to 60 nm and the nanorod density increased to a maximum of 2.4 × 109 cm−2. Yet, lower N2 partial pressures eventually led to the growth of continuous GaN films. The observed morphological dependence on N2 partial pressure is explained by a change from N-rich to Ga-rich growth conditions, combined with reduced GaN-poisoning of the Ga-target as the N2 gas pressure is reduced. Nanorods grown at 2.5 mTorr N2 partial pressure exhibited a high intensity 4 K photoluminescence neutral donor bound exciton transitions (D0XA) peak at ~3.479 eV with a full-width-at-half-maximum of 1.7 meV. High-resolution transmission electron microscopy corroborated the excellent crystalline quality of the nanorods.

The effects of GaN nanocolumn arrays and thin SixNy buffer layers on the morphology of GaN layers grown by plasma-assisted molecular beam epitaxy on Si(111) substrates

The effects of GaN nanocolumn arrays and a thin SixNy layer, used as buffer layers, on the morphology of GaN epitaxial layers are investigated. Two types of samples with different buffer layers were synthesized by PA-MBE. The morphology of the samples was characterized by SEM. The crystalline quality of the samples was assessed by XRD. The possibility of synthesis of continuous crystalline GaN layers on Si(111) substrates without the addition of other materials such as aluminum nitride was demonstrated.

Threading dislocation reduction in three-dimensionally grown GaN islands on Si (111) substrate with AlN/AlGaN buffer layers

Three-dimensional GaN island growth without any masks was first introduced under high pressure in metalorganic chemical vapor deposition after the growth of AlN and AlGaN buffer layers on Si (111) substrate, followed by two-dimensional GaN growth to form a continuous GaN film with improvement of the crystalline quality and surface smoothness. X-ray diffraction and cross-sectional scanning transmission electron microscopy analyses show that a high-quality GaN film can be achieved by bending of edge threading dislocations (TDs) and the formation of dislocation half-loops. It is observed that most of edge TDs bend 90° from the growth direction along c-axis, whereas mixed TDs bend about 30° towards the inclined sidewall facets of the islands. Consequently, a 1.2 μm thick GaN epitaxial film with a low threading dislocation density of 2.5 × 108 cm−2 and a smooth surface of 0. 38 nm roughness can be achieved on Si substrate.

Design and fabrication of enhanced lateral growth for dislocation reduction in GaN using nanodashes

The semiconductor gallium nitride is the material at the centre of energy-efficient solid-state lighting and is becoming increasingly important in high-power and high-frequency electronics. Reducing the dislocation density of gallium nitride planar layers is important for improving the performance and reliability of devices, such as light-emitting diodes and high-electron-mobility transistors. The patterning of selective growth masks is one technique for forcing a three-dimensional growth mode in order to control the propagation of threading defects to the active device layers. The morphology of the three-dimensional growth front is determined by the relative growth rates of the different facets that are formed, and for GaN is typically limited by the slow-growing {1−101} facets. We demonstrate how the introduction of nanodash growth windows can be oriented in an array to preserve fast-growing {11−22} facets at the early stage of growth to accelerate coalescence of three-dimensional structures into a continuous GaN layer. Cathodoluminescence and Electron Channelling Contrast Imaging methods, both used to measure the threading dislocation density, reveal that the dislocations are organised and form a distinctive pattern according to the underlying mask. By optimising the arrangement of nanodashes and the nanodash density, the threading dislocation density of GaN on sapphire epilayers can be reduced significantly from 109cm−2 to 3.0×107cm−2. Raman spectroscopy, used to monitor the strain in the overgrown GaN epilayers, shows that the position of the GaN E2H phonon mode peak was reduced as the dash density increases for a sample grown via pendeo-epitaxy whilst no obvious change was recorded for a sample grown via more conventional epitaxial lateral overgrowth. These results show how growth mask design can be used to circumvent limitations imposed by the growth dynamics. Moreover, they have revealed a greater understanding of the influence of the growth process on the dislocation density which will lead to higher performing electronic and optoelectronic devices as a result of the lower dislocation densities achieved.

HVPE Growth of GaN Layers on Cleaved β-Ga&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; Substrates

GaN epitaxial layers were successfully grown by hydride vapour phase epitaxy (HVPE) on β-Ga2O3 substrates produced by cleaving. The initial stages of GaN epitaxial growth on β-Ga2O3 were studied by scanning electron microscopy (SEM) and x-ray diffraction analysis (XRD). The nucleation and the transition from the nucleation layer to a continuous GaN film were studied. It was found that the growth starts with formation of small crystallites on the substrate surface. As the growth continues, crystallites transform into pyramidal islands which increase in size and merge together. It was found that the structural quality of the GaN layers rapidly improves with increasing thickness. The full width at half maximum of x-ray ω rocking curves for (0002) peak decreased from 1370 to 540 arcsec as the deposition time was increased from 30 to 120 sec. This corresponds to the variation of the nominal layer thickness from 250 nm to 1000 nm.

High quality crack-free GaN film grown on si (1 1 1) substrate without AlN interlayer

High quality crack-free GaN film has been grown on 2in. n-type Si (111) substrate without AlN interlayers by metalorganic chemical vapor deposition (MOCVD). By using a two-step-pressure growth technique for the AlN buffer layer, we have obtained crack-free 1.8μm thick continuous GaN layer. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) results indicate that the two-step-pressure growth method can improve the crystalline quality of the GaN film by prolonging the three-dimensional (3D) growth process of the GaN layer, Raman spectrum analysis shows that tensile stress in the overlaying GaN layer can be reduced obviously to 0.62GPa, which helps to reduce the possibility of cracking.

Influence of 3C–SiC/Si (111) template properties on the strain relaxation in thick GaN films

In this work, we study the influence of 3C–SiC/Si (111) template parameters (thickness, roughness and substrate miscut) on the GaN crystal quality and its strain state. For this, structures with an AlN nucleation layer and 1–4µm thick GaN layer have been grown by molecular beam epitaxy in order to select the best templates for the growth of thick GaN structures. Similar GaN structures have been grown directly on silicon for comparisons. The influence of the high silicon doping is confirmed on the enhancement of strain relaxation. Despite this limitation, a 5µm thick crack-free continuous GaN structure (with 1µm silicon doped) has been successfully grown on the best selected template. Furthermore, the growth by metal–organic chemical vapor deposition of structures with AlN and SiN inter-layers and thick continuous GaN layers on Si (111), 3C–SiC/Si (111) and SoPSiC (silicon on polycrystalline silicon carbide) has been achieved in order to show the relative benefit of each approach in terms of layer quality and strain state.

Strain states in GaN films grown on Si(111) and Si(110) substrates using a thin AlN/GaN superlattice interlayer

AbstractWe report the characterization results of the strain states in GaN films grown both on Si(111) and Si(110) substrates by applying a thin AlN/GaN superlattice structure (SLs) as an interlayer (IL). The strain states in the GaN films grown on Si(111) and Si(110) substrates are compared. It is demonstrated that the modulation effect by the SL ILs on the strain state in the GaN film is universal although different behaviours on the Si(111) and Si(110) substrate are observed due to the different lattice symmetry relationship between the substrate and the epilayer. By using the technique, we succeeded in growing thick continuous GaN films (&gt; 3 µm), which are necessary for the high‐power, high‐voltage device applications. (© 2014 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Strain evolution in GaN nanowires: From free-surface objects to coalesced templates

Top-down fabricated GaN nanowires, 250 nm in diameter and with various heights, have been used to experimentally determine the evolution of strain along the vertical direction of 1-dimensional objects. X-ray diffraction and photoluminescence techniques have been used to obtain the strain profile inside the nanowires from their base to their top facet for both initial compressive and tensile strains. The relaxation behaviors derived from optical and structural characterizations perfectly match the numerical results of calculations based on a continuous media approach. By monitoring the elastic relaxation enabled by the lateral free-surfaces, the height from which the nanowires can be considered strain-free has been estimated. Based on this result, NWs sufficiently high to be strain-free have been coalesced to form a continuous GaN layer. X-ray diffraction, photoluminescence, and cathodoluminescence clearly show that despite the initial strain-free nanowires template, the final GaN layer is strained.