Thin GaN Layer Research Articles

Strongly Confined Excitons in GaN/AlN Nanostructures with Atomically Thin GaN Layers for Efficient Light Emission in Deep-Ultraviolet.

Fascinating optical properties governed by extremely confined excitons have been so far observed in 2D crystals like monolayers of transition metal dichalcogenides. These materials, however, are limited for production by epitaxial methods. Besides, they are not suitable for the development of optoelectronics for the challenging deep-ultraviolet spectral range. Here, we present a single monolayer of GaN in AlN as a heterostructure fabricated by molecular beam epitaxy, which provides extreme 2D confinement of excitons, being ideally suited for light generation in the deep-ultraviolet. Optical studies in the samples, supplemented by a group-theory analysis and first-principle calculations, make evident a giant enhancement of the splitting between the dark and bright excitons due to short-range electron-hole exchange interaction that is a fingerprint of the strongly confined excitons. The practical significance of our results is in the observation of the internal quantum yield of the room-temperature excitonic emission as high as ∼75% at 235 nm.

Study of GaN layer crystallization on GaAs(100) using electron cyclotron resonance or glow discharge N2 plasma sources for the nitriding process

Two kinds of N2 plasma source, ECR (electron cyclotron resonance) and GDS (glow discharge source) generating mostly N-radical atoms and N-cationic species respectively, were used to grow a thin nitride layer on a GaAs(100) substrate. It was found that this nitridation followed by annealing at 620 °C permits the crystallization of the nitride layer. Pyramidal Zinc Blende GaN nanostructures (zb-GaN) with four facets were obtained using GDS plasma. Surprisingly, a planar and pure wurtzite structure (w-GaN) was obtained using the ECR source. This w-GaN structure shows low photoluminescence intensity and a biaxial tensile strain due to lattice mismatch. Accordingly, the operator can select which phase is formed, simply by switching plasma source. The valence band discontinuity ΔEv has been determined to be 1.74 eV for the zb-GaN/GaAs and w-GaN/GaAs junctions by X-ray photoelectron spectroscopy. As a consequence the conduction bands of the GaAs substrate and the elaborated GaN thin layer are aligned for a zb-GaN/GaAs junction giving efficient electron transport at the zb-GaN/GaAs interface. For w-GaN/GaAs junction, the conduction band discontinuity ΔEc is 0.23 eV inducing an electron confinement in the GaAs(100) which can be an effective way to improve the electronic or optical properties of GaAs devices.

Gold Nanoclusters at the Interface Au/GaAs(001): Preparation, Characterization, and Plasmonic Spectroscopy

Local plasmons of gold nanoclusters formed at Au/GaAs interface are observed and investigated. The gold nanoclusters are prepared by thermal annealing of thin Au films deposited on GaAs(001) surface. In order to prevent chemical interaction between Au and GaAs at high temperatures, a thin layer of GaN is formed on GaAs substrate surface by chemical nitridation in hydrazine sulfide solution prior to deposition of Au. Typical shapes and sizes of Au nanoclusters are obtained from STM characterization of samples in ultrahigh vacuum followed by measuring and calculating their optical reflectivity spectra. In general, the reflectivity spectra of Au/GaAs samples reveal two resonant peaks at the energies about 1.55 and 2.15 eV. The former peak dominates for oxidized GaAs(001) substrates, while the latter peak is clearly visible for nitridized GaAs substrates. Theoretical analysis of the optical spectra of Au/GaAs interface using a model of Au nanospheroids allows us to assign the reflectance peak at 2.15 eV to local plasmons of Au nanoclusters which look like oblate islands on a Au/GaAs(001) sample. In turn, the peak at 1.55 eV is assigned to plasmons of Au nanoclusters prolate perpendicularly to the surface of GaAs crystal in whose near-surface region these Au clusters are buried.

Understanding and controlling Ga contamination in InAlN barrier layers

We have shown that in an AIXTRON Close Coupled Showerhead MOVPE system, InAlN layers are strongly contaminated with gallium, giving increasing contamination when thicker GaN layers are grown below the InAlN. We have also shown that as the gallium contamination increases, there is reduced indium in the layers and thicker layers. To explain these phenomena we propose a quantitative model for these observations, suggesting that the TMIn precursor reacts with gallium on the showerhead surface to release TMGa or MMGa, which is then incorporated into the layers.We grew InAlN layers around 14 nm thick with a 1 nm AlN spacer to form High Electron Mobility Transistor (HEMT) structures. For the InAlN barrier layers grown directly on a GaN template, the 2D electron gas (2DEG) was degraded by the regrowth interface, but this 2DEG degradation was reduced by the growth of thin GaN layers. However, for thicker layers of GaN, there was a strong gallium contamination and so a change in alloy composition, which increased the sheet resistance. Thus we need an optimum regrowth thickness to have the best sheet resistance values.

A GaN–SiC hybrid material for high-frequency and power electronics

We demonstrate that 3.5% in-plane lattice mismatch between GaN (0001) epitaxial layers and SiC (0001) substrates can be accommodated without triggering extended defects over large areas using a grain-boundary-free AlN nucleation layer (NL). Defect formation in the initial epitaxial growth phase is thus significantly alleviated, confirmed by various characterization techniques. As a result, a high-quality 0.2-μm thin GaN layer can be grown on the AlN NL and directly serve as a channel layer in power devices, like high electron mobility transistors (HEMTs). The channel electrons exhibit a state-of-the-art mobility of >2000 cm2/V-s, in the AlGaN/GaN heterostructures without a conventional thick C- or Fe-doped buffer layer. The highly scaled transistor processed on the heterostructure with a nearly perfect GaN–SiC interface shows excellent DC and microwave performances. A peak RF power density of 5.8 W/mm was obtained at VDSQ = 40 V and a fundamental frequency of 30 GHz. Moreover, an unpassivated 0.2-μm GaN/AlN/SiC stack shows lateral and vertical breakdowns at 1.5 kV. Perfecting the GaN–SiC interface enables a GaN–SiC hybrid material that combines the high-electron-velocity thin GaN with the high-breakdown bulk SiC, which promises further advances in a wide spectrum of high-frequency and power electronics.

Alleviation of efficiency droop in InGaN/GaN multiple quantum well light-emitting diodes with trapezoidal quantum barriers

We numerically investigated the effects of trapezoidal quantum barriers (QBs) on efficiency droop in InGaN/GaN multiple quantum well (MQW) light-emitting diodes (LEDs). Simulations showed that the electrostatic field in QWs of LEDs with trapezoidal barriers is reduced because of the reduced sheet charge density at the QW-QB interface caused by the thin GaN layer in trapezoidal QBs. Additionally, the InGaN grading region in trapezoidal QBs suppresses hot carrier transport and this enhances efficient carrier injection into the QWs. The electroluminescence intensity of an LED with trapezoidal QBs is increased by 10.2% and 6.7% at 245 A cm−2 when compared with the intensities of LEDs with square-type GaN barriers and multilayer barriers, respectively. The internal quantum efficiency (IQE) droop of an LED with trapezoidal QBs is 16% at 300 A cm−2, while LEDs with square-type GaN barriers and multilayer barriers have IQE droop of 31% and 24%, respectively. This IQE droop alleviation in LEDs with trapezoidal QBs is attributed to the reduced energy band bending, efficient hole injection, and more uniform hole distribution in the MQWs that results from reduction of the piezoelectric field by the trapezoidal QBs. These results indicate that the trapezoidal QB in MQWs is promising for enhanced efficiency in high-power GaN-based LEDs.

Investigation of top gate GaN thin-film transistor fabricated by DC magnetron sputtering

Top-gate thin-film transistors (TFTs) with an n-type GaN thin film active channel layer have been fabricated and characterized. At a substrate temperature of 550 °C, rather than room temperature, the GaN thin films deposited by the reactive direct current magnetron sputtering technique have an average grain size of 25 nm. Without high temperature postannealing process, the proposed GaN TFTs exhibit good electrical performance with a field effect mobility of 2.5 cm2/V s, a threshold voltage of 4.5 V, an on/off current ratio of 1.5 × 105, and a subthreshold swing of 0.7 V/decade. The proposed GaN thin film is promising for TFT active channel application in active-matrix display.

Role of the phase transition at GaN QDs formation on (0001)AlN surface by ammonia molecular beam epitaxy

We report an original method of GaN/AlN quantum dots (QDs) formation with low density by ammonia MBE on the (0001)AlN surface using a decomposition process of GaN thin layer. The QDs formation has been investigated in situ by reflection high-energy electron diffraction technique. Low density of quantum dots has been obtained in the range 107–109 cm−2. Single quantum dots photoluminescence lines corresponding to exciton and biexiton transitions were observed in micro-photoluminescence spectra. A lattice gas model was developed for correct description of the GaN QDs statistical ensemble on the surface. Effective interaction between QDs results in the discontinuous phase transitions (first-order phase transitions) from low density of QDs (gas branch) to condensed phase of QDs. The GaN QDs formation has been confirmed by high-resolution transmission electron microscopy and micro-photoluminescence of single quantum dots.

Ultraviolet-A LED Based on Quantum-Disks-In-AlGaN-Nanowires—Optimization and Device Reliability

Group-III nitride-based ultraviolet (UV) quantum-disks (Qdisks) nanowires (NWs) light-emitting diodes grown on silicon substrates offer a scalable, environment-friendly, compact, and low-cost solution for numerous applications in solid-state lighting, spectroscopy, and biomedical. However, the internal quantum efficiency, injection efficiency, and extraction efficiency need to be further improved. The focus of this paper encompasses investigations based on structural optimization, device simulation, and device reliability. To optimize a UV-A (320–400 nm) device structure, we utilize the self-assembled Qdisks NWs with varying Qdisks thickness to study carrier separation in active-region and implement an improved p-contact-layer to increase the output power. By simulation, we found a 100× improvement in the direct recombination rate for samples with thicker Qdisks thickness of ∼5 monolayers (MLs) compared to the sample with ∼2 MLs-thick Qdisks. Moreover, the sample with graded top Mg-doped AlGaN layer in conjunction with thin Mg-doped GaN layer shows 10× improvement in the output power compared to the samples with thicker top Mg-doped GaN absorbing contact layer. A fitting with ABC model revealed the increase in nonradiative recombination centers in the active region after a soft stress test. This paper aims to shed light on the research efforts required for furthering the UV NWs LED research for practical applications.

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.

Optical properties of bulk gallium nitride single crystals grown by chloride–hydride vapor-phase epitaxy

A gallium nitride crystal 5 mm in thickness was grown by chloride–hydride vapor-phase epitaxy on a sapphire substrate, from which the crystal separated during cooling. At an early stage, a three-dimensional growth mode was implemented, followed by a switch to a two-dimensional mode. Spectra of exciton reflection, exciton luminescence, and Raman scattering are studied in several regions characteristic of the sample. Analysis of these spectra and comparison with previously obtained data for thin epitaxial GaN layers with a wide range of silicon doping enabled conclusions about the quality of the crystal lattice in these characteristic regions.

Study on the band alignment of GaN/CH3NH3PbBr3 heterojunction by x-ray photoelectron spectroscopy

A GaN/CH3NH3PbBr3 heterojunction was fabricated by depositing a GaN thin layer on a CH3NH3PbBr3 single crystal by plasma enhanced atomic layer deposition. The band alignment of the GaN/CH3NH3PbBr3 heterojunction was studied by x-ray photoelectron spectroscopy. The valance band offset (VBO) is directly determined to be 0.13 ± 0.08 eV. The conduction band offset is deduced from the VBO and the band gaps, which turned out to be 1.39 ± 0.12 eV. Thus, the band alignment of the GaN/CH3NH3PbBr3 heterojunction is determined to be type-I. These results show that GaN is a promising material for carrier confinement in halide perovskite based light emitting devices.

Separation of thick HVPE-GaN films from GaN templates using nanoporous GaN layers

In this work, we have succeeded in growing an approximate 2-inch self-separated thick GaN wafer by hydride vapor phase epitaxy with an introduction of a sacrificial layer of nanoporous GaN. Such nanoporous GaN layer is invented by using the HVPE growth of thin GaN layer on the spin-coating silica nanosphere layer followed by a hydrofluoric acid etching to the silica nanosphere layer. It has been found that the nanoporous GaN layer, enabling a reduction of stickiness between thick GaN films and the substrates, plays a significant role in the self-separation of thick GaN films during the cooling process. However, the thickness of the nanoporous GaN layer is another key issue to achieve good quality self-separated GaN thick films. In our study, we suggest that the nanoporous GaN layer with a thickness of approximately 150–240 nm can best serve as the sacrificial layer in self-separation process. Raman spectroscopy also indicates the self-separated thick GaN films by using the proposed approach are virtually strain-free.

Influence of a low-temperature GaN cap layer on the electron concentration in AlGaN/GaN heterostructure

The influence of low-temperature passivating GaN cap layers on the electrophysical parameters of a 2D electron gas (2DEG) in heterostructure high-electron mobility transistors has been studied. It has been found that thin GaN layers deposited in situ at 550°C do not exhibit polar properties and do not change the carrier concentration in the 2DEG. However, GaN layers deposited at 830°C decrease the carrier concentration in the 2DEG, which is in agreement with theoretical calculations. Using the reflected high-energy electron diffraction technique, it has been established that this effect may be associated with different structures and morphologies of GaN layers deposited at different temperatures.

Birefringence in GaN epitaxial layers and GaN microrods

Light propagating through a birefringent material undergoes a change of the state of polarization. In this article, we demonstrate the application of the non‐invasive method of conoscopy to analyze the birefringence of c‐plane GaN. For thick GaN bulk layers, the difference Δn between the ordinary and extraordinary refractive index results in a modification of the state of polarization from linear to elliptical to circular and back to orthogonally linear polarization within a few degree variation of the internal angle propagation. Measurements of thin epitaxial GaN layers on c‐plane sapphire show the strong influence of the sapphire substrate to the observed changes of the state of polarization.

Band Alignment at GaN/Single-Layer WSe2 Interface.

We study the band discontinuity at the GaN/single-layer (SL) WSe2 heterointerface. The GaN thin layer is epitaxially grown by molecular beam epitaxy on chemically vapor deposited SL-WSe2/c-sapphire. We confirm that the WSe2 was formed as an SL from structural and optical analyses using atomic force microscopy, scanning transmission electron microscopy, micro-Raman, absorbance, and microphotoluminescence spectra. The determination of band offset parameters at the GaN/SL-WSe2 heterojunction is obtained by high-resolution X-ray photoelectron spectroscopy, electron affinities, and the electronic bandgap values of SL-WSe2 and GaN. The valence band and conduction band offset values are determined to be 2.25 ± 0.15 and 0.80 ± 0.15 eV, respectively, with type II band alignment. The band alignment parameters determined here provide a route toward the integration of group III nitride semiconducting materials with transition metal dichalcogenides (TMDs) for designing and modeling of their heterojunction-based electronic and optoelectronic devices.

Identifying the influence of GaN/InxGa1-xN type last quantum barrier on internal quantum efficiency for III-nitride based light-emitting diode

GaN/InxGa1-xN-type last quantum barrier (LQB) proves to be useful for Ⅲ-nitride based light-emitting diode (LED) in enhancing the internal quantum efficiency (IQE) and suppressing the efficiency droop level that often takes place especially when the injection current is high. In this work, GaN/InxGa1-xN-type LQB reported by the scientific community to enhance the IQE is first reviewed and summarized. Then, the influences of indium composition and thickness of the InxGa1-xN layer on the performance of LED incorporated with the GaN/InxGa1-xN-type LQB are studied. Through analyzing energy band diagrams calculated with APSYS, we find that the[0001] oriented LQB features an electron depletion due to the polarization induced negative charges at the GaN/InxGa1-xN interface. The electron depletion enhances the electron blocking effect and reduces the electron accumulation at the InxGa1-xN/AlGaN interface, leading to an improved IQE for the LED. In addition, increasing the indium composition of the InxGa1-xN layer will generate more negative interface charges, which result in further increased conduction band barrier height for the electrons and reduced electron leakage. On the other hand, for the GaN/InxGa1-xN-type LQB with a fixed indium composition, there exists an optimum thickness for the InxGa1-xN layer in maximizing the improvement of IQE for the LED, mainly because the interaction between two mechanisms co-exists when varying the thickness of the InxGa1-xN layer, i.e., the initial increase in the InxGa1-xN layer thickness will lead to an increased conduction band barrier height, which prevents electrons from leaking into the InxGa1-xN layer. However, further increasing the InxGa1-xN layer thickness to a certain value, tunneling effect will kick in as a result of the simultaneously reduced GaN thickness-the electrons will tunnel through the thin GaN layer in the LQB from the quantum wells to the InxGa1-xN layer. This will cause electrons to increase in the InxGa1-xN layer. Therefore, as a result of the interaction between the above-mentioned two mechanisms, there is an optimum thickness for the InxGa1-xN layer such that the electrons in the InxGa1-xN layer will reach a minimal value, which in turn will lead to a maximized conduction band barrier height for the AlGaN electron blocking layer and facilitate the performance of LEDs.

HVPE-GaN growth on GaN-based Advanced Substrates by Smart Cut™

Advanced Substrates consist of a thin GaN layer bonded to a carrier wafer. The layer is separated from starting material by Smart Cut™ technology. GaN on sapphire Advanced Substrates were successfully used as seeds for HVPE-GaN growth. Unintentionally doped and silicon-doped thick GaN layers were crystallized. The results were compared to HVPE-GaN grown on standard MOCVD-GaN/sapphire templates. Free-standing HVPE-GaN with high free carrier concentration was obtained. A laser diode was built on the n-type doped HVPE-GaN grown on an Advanced Substrate.

(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.

Polymer bonding of GaN crystal layer on silicon substrate for micro mechanical resonator applications

A polymer bonding method of GaN crystal layer on silicon substrate is proposed for the integration of GaN micro electromechanical devices on silicon substrate. A GaN crystal layer is grown on a silicon wafer epitaxially with a buffer layer. Avoiding the influence of residual stress of the buffer layer on the GaN micro electromechanical devices, the GaN layer is transferred onto another silicon wafer by the polymer bonding method and the buffer layer is removed. The bonding method consists of two-step polymer process to prevent from breaking the bonded GaN thin layer by air bubbles. The polymer is patterned to generate air channels in the first polymer process so that the air bubbles are not remained after bonding the GaN/silicon wafer. The air channels are filled with another polymer in the second polymer process. Then, the silicon substrate of the GaN/silicon wafer is removed by silicon deep reactive ion etching, and the buffer layer for GaN crystal growth is also etched by fast atom beam of SF6 plasma and Cl2 reactive ion plasma. The bonded GaN layer is used for fabricating GaN micro mechanical resonators on silicon substrate by etching the polymers as sacrificial layer. The deformation of the freestanding resonator was minimized by removing the buffer layer. A cantilever resonator fabricated from GaN layer is excited by optical means and the basic characteristics are discussed.