Nm For GaN Research Articles

Ballistic Thermal Transport at Sub‐10 nm Laser‐Induced Hot Spots in GaN Crystal (Adv. Sci. 2/2023)

Ballistic Thermal Transport In this article number 2204777, Yanan Yue and co-workers develop a tip-enhanced Raman thermometry to study ballistic thermal transport within 10 nm in GaN, simultaneously achieving local laser heating and temperature probing. The phonon mean free path is predicted which can find wide application in the thermal management of GaN-based electronics.

Observation of self-assembled InGaN/GaN superlattice structure grown on N-polar GaN by plasma-assisted molecular beam epitaxy

In this paper, we report on the observation of self-assembled InGaN/(In)GaN superlattice (SL) structure in a nominal “InGaN” film grown on N-polar GaN substrate. 350 nm thick InGaN films were grown at different temperatures ranging from 600 to 690 °C. Structural characterization was conducted via atomic force microscopy, scanning transmission electron microscopy, high-resolution x-ray diffraction, and XRD reciprocal space map. A SL structure was unexpectedly observed on all samples. However, the In content in each layer varied depending on growth temperature. By increasing the substrate temperature to 670 °C, a periodic structure composed of 3 nm In0.26Ga0.74N and 3 nm of GaN with a surface roughness of ∼0.7 nm was achieved. This work establishes a method for the growth of InGaN films with high structural quality on N-polar GaN and opens a new pathway for the design and fabrication of various electronic and optoelectronic devices with enhanced performance.

Temperature Dependence of Dark Spot Diameters in GaN and AlGaN

Threading dislocations in c‐plane (Al,Ga)N layers are surrounded by areas with reduced light generation efficiency, called “dark spots.” These areas are observable in luminescence measurements with spatial resolution in the submicrometer range. Dark spots reduce the internal quantum efficiency in single layers and light‐emitting devices. In cathodoluminescence measurements, the diameter of dark spots (full width at half maximum [FWHM]) is observed to be 200–250 nm for GaN. It decreases by 30–60% for AlxGa1−xN with x ≈ 0.5. Furthermore, the dark spot diameter increases with increasing temperature from 83 to 300 K in AlGaN, whereas it decreases in GaN. Emission energy mappings around dark spots become less smooth and show sharper features on submicrometer scales at low temperature for AlGaN and, on the contrary, at high temperature for GaN. It is concluded that charge carrier localization dominates the temperature dependence of dark spot diameters and of the emission energy distribution around threading dislocations in AlGaN, whereas the temperature‐dependent excitation volume in cathodoluminescence and charge carrier diffusion limited by phonon scattering are the dominant effects in GaN. Consequently, with increasing temperature, nonradiative recombination related to threading dislocations extends to wider regions in AlGaN, whereas it becomes spatially limited in GaN.

Growth by MOCVD and photoluminescence of semipolar [formula omitted] InN quantum dashes

Semipolar (202¯1¯) InN quantum dash growth by metalorganic chemical vapor deposition on GaN was explored as a function of growth time and temperature. Near-infrared photoluminescence was observed at room temperature from the uncapped InN quantum dashes with peak emission wavelengths from 1350 to 1500 nm. After capping with 30 nm of GaN, room temperature photoluminescence remained with an increase in intensity and slight blue-shifting of the emission. The photoluminescence was shown to be sensitive to the growth temperature and time of the GaN capping layer.

Ultraviolet electroluminescence from flowers-like n-ZnO nanorods/p-GaN light-emitting diode fabricated by modified chemical bath deposition

In this work, an ultraviolet (UV) light-emitting diode (LED) based on flowers-like n-ZnO nanorods (NFs)/p-GaN was fabricated using direct heat substrate-modified chemical bath deposition (DHS-MCBD) method. Initially, the ZnO nanoparticles were grown on the top of a p-type GaN substrate as a seed layer which facilitated the growth of high-density of ZnO nanorods in various directions. The current-voltage (I–V) characteristics of the fabricated LED device showed a rectifying behavior of ZnO NFs/p-GaN heterojunction whereas the breakdown mechanism for the device was generated by the tunneling effect. The luminescence properties of the device were studied at room temperature (RT) by using electroluminescence (EL) technique. The back side EL spectrum of the device displayed a sharp UV emission peaks at 367 and 378 nm for GaN and ZnO respectively. On the other hand, the EL spectrum taken from the front side was composed of UV emission, near blue emission along with a broadband green emission. The EL emission for the device was also observed with the naked eye under normal light from both of its sides. The light emission mechanism and the waveguiding effect of the ZnO NRs have been discussed in this work.

Self-Assembled Single-Crystalline GaN Having a Bimodal Meso/Macropore Structure To Enhance Photoabsorption and Photocatalytic Reactions.

This paper describes the self-assembled fabrication of single-crystal GaN with a bimodal pore (meso/macropore) size distribution (BiPS-GaN). A 4.7 μm-thick BiPS-GaN layer was grown spontaneously using halogen-free vapor phase epitaxy in conjunction with boron impurity doping (>1 × 1019 atoms/cm3) on a GaN template fabricated via metalorganic chemical vapor deposition (MOCVD-GaN). The boron impurity acted as a surfactant, and its segregation generated a dense (>1 × 1010 cm-2), homogeneous distribution of mesopores with sizes of 30-40 nm in GaN during growth. In addition, macropores with sizes of 0.1-2 μm were produced by the fusion of mesopores in close proximity to one another. As a result, BiPS-GaN exhibited a high density of both meso- and macropores, all aligned in the vertical direction (that is, along the c axis). BiPS-GaN showed good electroconductivity and almost the same high degree of crystallinity as the MOCVD-GaN template. Furthermore, the hybrid meso/macropore structure of BiPS-GaN imparted excellent photoabsorption properties and allowed this material to work as an efficient support for a nanosized IrO x catalyst. The photocurrent density in BiPS-GaN was enhanced by as much as a factor of 5 compared to planar GaN by effective absorption due to the hybrid meso/macropore structure of BiPS-GaN. Moreover, the oxygen generation efficiency of BiPS-GaN with the IrO x catalyst was approximately doubled, compared to that of BiPS-GaN without IrO x, while maintaining long-term stability. These results demonstrate that BiPS-GaN fabricated in this facile manner has significant potential in applications such as photoelectrochemical reactions and catalysis.

Alloy disorder limited mobility of InGaN two-dimensional electron gas

The mobility of an InGaN based two-dimensional electron gas is determined for an indium content ranging from 0 to 20%. While the electron density remains constant at ∼2.5 × 1013 cm−2, the room-temperature mobility drastically decreases from 1340 to 173 cm2 V−1 s−1 as the In content increases. In fact, the mobility already drops below 600 cm2 V−1 s−1 for an In content as low as 3%. A theoretical model including random alloy fluctuations reproduces well the experimental data confirming that alloy disorder is the main scattering mechanism. With the aim of probing how sensitive the electron mobility is to the InGaN channel/barrier interface, a very thin GaN interlayer was inserted. A dramatic increase in the mobility is observed even for 2 nm of GaN, shedding light on the impact of unintentional GaN interlayers, which may form upon growth conditions or reactor-associated parasitic deposition.

Modeling and Simulation of Capacitance–Voltage Characteristics of a Nitride GaAs Schottky Diode

A nitride GaAs Schottky diode has been fabricated by the nitridation of GaAs substrates using a radio frequency discharge nitrogen plasma source with a layer thickness of approximately 0.7 nm of GaN. The capacitance–voltage (C–V) characteristics of the Au/GaN/GaAs structure were investigated at room temperature for different frequencies, ranging from 1 kHz to 1 MHz. The C–V measurements for the Au/GaN/GaAs Schottky diode were found to be strongly dependent on the bias voltage and the frequency. The capacitance curves depict an anomalous peak and a negative capacitance phenomenon, indicating the presence of continuous interface state density behavior. A numerical drift–diffusion model based on the Scharfetter–Gummel algorithm was elaborated to solve a system composed of the Poisson and continuities equations. In this model, we take into account the continuous interface state density, and we have considered exponential and Gaussian distributions of trap states in the band gap. The effects of the GaAs doping concentration and the trap state density are discussed. We deduce the shape and values of the trap states, then we validate the developed model by fitting the computed C–V curves with experimental measurements at low frequency.

Burying non-radiative defects in InGaN underlayer to increase InGaN/GaN quantum well efficiency

The insertion of an InGaN underlayer (UL) is known to strongly improve the performance of InGaN/GaN quantum well (QW) based blue light emitting diodes (LEDs). However, the actual physical mechanism responsible for it is still unclear. We thus conduct a systematic study and investigate different hypotheses. To this aim, InGaN/GaN single (S) QWs are grown on sapphire and GaN free-standing substrates with or without InGaN UL. This allows us to conclude that (i) improvement of LED performance is due to a higher internal quantum efficiency of the InGaN/GaN SQW and (ii) reduction of structural defects is not at play. Furthermore, we show that neither the surface morphology nor the strain of the top GaN layer before the growth of the QW is affected by the InGaN UL. Finally, we find that the beneficial effect of the InGaN UL is still present after 100 nm of GaN. This result combined with band structure modelling rules out the hypothesis of higher QW oscillator strength induced by a reduction of the internal electric field due to band bending. In conclusion, we demonstrate that the increase in InGaN/GaN QW efficiency is the consequence of a reduction of non-radiative recombination centers in the QW itself, independent of the dislocation density.

Ambient-temperature diffusion and gettering of Pt atoms in GaN with surface defect region under 60Co gamma or MeV electron irradiation

Abstract Generally, the diffusion and gettering of impurities in GaN needs high temperature. Calculated with the ambient-temperature extrapolation value of the high temperature diffusivity of Pt atoms in GaN reported in literature, the time required for Pt atoms diffusing 1 nm in GaN at ambient temperature is about 19 years. Therefore, the ambient-temperature diffusion and gettering of Pt atoms in GaN can hardly be observed. In this work, the ambient-temperature diffusion and gettering of Pt atoms in GaN is reported for the first time. It is demonstrated by use of secondary ion mass spectroscopy that in the condition of introducing a defect region on the GaN film surface by plasma, and subsequently, irradiated by 60Co gamma-ray or 3 MeV electrons, the ambient-temperature diffusion and gettering of Pt atoms in GaN can be detected. It is more obvious with larger irradiation dose and higher plasma power. With a similar surface defect region, the ambient-temperature diffusion and gettering of Pt atoms in GaN stimulated by 3 MeV electron irradiation is more marked than that stimulated by gamma irradiation. The physical mechanism of ambient-temperature diffusion and gettering of Pt atoms in a GaN film with a surface defect region stimulated by gamma or MeV electron irradiation is discussed.

Indium-Rich InGaN Films Grown on Ge Substrate by Plasma-Assisted Molecular Beam Epitaxy for Solar Water Splitting

Indium-rich InGaN films were grown on Ge(111) substrate by plasma-assisted molecular beam epitaxy (PAMBE). The influence of the indium flux on the structural properties, surface morphology, and photocurrent for water splitting has been investigated. Before the InGaN growth, 20 nm of GaN was deposited as a buffer layer. A streaky reflection high-energy electron diffraction (RHEED) pattern was observed for the GaN buffer growth. At the onset of InGaN growth, the streaks became spotty, indicating roughening of the surface and three-dimensional (3D) growth due to the lattice mismatch between GaN and InN. The indium composition in the InGaN structure was roughly fit to be around 50% from x-ray diffraction (XRD) ω–2θ measurements. Growth with excess indium supply led to segregation of metal indium on the surface. During cooling down, this metal indium transformed partially into InN. The crystal quality of the InGaN film decreased with increase of the indium flux. The photocurrent of the InGaN films used as photoelectrodes for water splitting also decreased with increase of the indium flux.

Low-density nanoporous phases of group-III nitrides built from sodalite cage clusters

We report a new family of M12N12 (M = Al and Ga) cluster-assembled low-density materials with distinguished structures and properties based on state-of-the-art first-principles calculations. Specifically, the thermodynamic stability of the sodalite cage M12N12, with Th symmetry and a large HOMO-LUMO gap, is firstly proved using a first-principles molecular dynamics (FPMD) study. We consider this novel structure as a building block to construct new cluster-assembled materials. On the basis of the interaction of the cages with each other, eight new low-density nanoporous phases have been characterized, some of which with high stability are even more stable than experimentally synthesized MN phases. The intrinsic higher flexibilities (lower bulk moduli) and porous characteristics (the pore size: from 0.360 to 0.952 nm for AlN, 0.381 to 0.982 nm for GaN) of these phases should make them extremely promising for molecular sieving, gas storage, and particularly, atomic transport, control and purification applications. Furthermore, these new materials can not only retain the structural characteristics of the building block, but also preserve its electronic properties of wide-energy gap, with an indirect or a direct band gap of 1.038-2.640 eV. Our results may be feasible for extending the range of properties and applications of the corresponding MN compound.

Thermo-Optic Coefficients of 4H-SiC, GaN, and AlN for Ultraviolet to Infrared Regions up to 500 °C

The temperature dependence of the refractive indices of 4H-SiC, GaN, and AlN were determined in a wavelength range from the near band edge (392 nm for 4H-SiC, 367 nm for GaN, and 217 nm for AlN) to infrared (1700 nm) and a temperature range from room temperature to 500 °C. Optical interference measurements with vertical incidence along a c-axis configuration were employed to precisely evaluate ordinary refractive indices. Near the band-edge region, the temperature dependence of the refractive index mainly originates from the temperature change of the bandgap. At 450 nm, the thermo-optic coefficients of 4H-SiC, GaN, and AlN were measured to be 7.8 ×10-5, 1.6 ×10-4, and 3.6 ×10-5 K-1, respectively.

Thermo-Optic Coefficients of 4H-SiC, GaN, and AlN for Ultraviolet to Infrared Regions up to 500 °C

The temperature dependence of the refractive indices of 4H-SiC, GaN, and AlN were determined in a wavelength range from the near band edge (392 nm for 4H-SiC, 367 nm for GaN, and 217 nm for AlN) to infrared (1700 nm) and a temperature range from room temperature to 500 °C. Optical interference measurements with vertical incidence along a c-axis configuration were employed to precisely evaluate ordinary refractive indices. Near the band-edge region, the temperature dependence of the refractive index mainly originates from the temperature change of the bandgap. At 450 nm, the thermo-optic coefficients of 4H-SiC, GaN, and AlN were measured to be 7.8 ×10-5, 1.6 ×10-4, and 3.6 ×10-5 K-1, respectively.

Characterization of GaN/AlGaN epitaxial layers grown by metalorganic chemical vapour deposition for high electron mobility transistor applications

GaN and AlGaN epitaxial layers are grown by a metalorganic chemical vapour deposition (MOCVD) system. The crystalline quality of these epitaxially grown layers is studied by different characterization techniques. PL measurements indicate band edge emission peak at 363.8 nm and 312 nm for GaN and AlGaN layers respectively. High resolution XRD (HRXRD) peaks show FWHM of 272 and 296 arcsec for the (0 0 0 2) plane of GaN and GaN in GaN/AlGaN respectively. For GaN buffer layer, the Hall mobility is 346 cm2/V-s and carrier concentration is 4.5 × 1016/cm3. AFM studies on GaN buffer layer show a dislocation density of 2 × 108/cm2 by wet etching in hot phosphoric acid. The refractive indices of GaN buffer layer on sapphire at 633 nm are 2.3544 and 2.1515 for TE and TM modes respectively.

Suppression of domain formation in GaN layers grown on Ge(1 1 1)

Heteroepitaxial growth of GaN on Ge(1 1 1) by molecular beam epitaxy has previously been reported to be feasible. However, structural characterization revealed that these GaN layers consisted of misoriented domains. In this work, it is shown that the formation of domains can be suppressed by increasing the substrate temperature, decreasing the nitrogen flux or increasing the surface step density by using off-oriented substrates. Hereby, the step flow growth is enhanced with respect to 2D nucleation. The suppression of domains significantly improves the crystal quality of the GaN layer. For 47 nm of GaN a (0 0 0 2) and (1 0 1¯ 2) ω FWHM value of, respectively, 408 and 935 arcsec is obtained.

The temperature dependence of the refractive indices of GaN and AlN from room temperature up to 515 °C

The temperature dependence of the refractive indices of GaN and AlN was investigated in the wavelength range from the near band edge (367 nm for GaN and 217 nm for AlN) to 1000 nm and the temperature range from room temperature to 515 °C. Optical interference measurements with vertical incident configuration were employed to precisely evaluate the ordinary refractive indices.

Gallium nitride based micro-cavity light emitting diodes emitting at 498 nm

A gallium nitride based micro-cavity light emitting diode emitting at a peak wavelength of 498 nm has been fabricated. The epitaxial structure was grown by metalorganic chemical vapor deposition, and the device was fabricated using a laser lift-off process. Cavity thinning was carried out using reactive ion etching until a cavity length of roughly 830 nm (corresponding to a cavity order of ∼8 for λ = 498 nm in GaN) was achieved. Electroluminescence measurements of the micro-cavity device show a narrowed spectral width compared to the un-thinned device, and angularly resolved spectral measurements yielded a spectral shift, evincive of micro-cavity effects.

Surface morphology and strain of GaN layers grown using 6H-SiC(0 0 0 1) substrates with different buffer layers

The evolution of both the surface morphology and the strain in GaN layers grown by metalorganic vapor phase epitaxy on either AlN or Al x Ga 1− x N ( x≈0.15) buffer layers previously deposited on 6H-SiC(0 0 0 1) substrates has been investigated by varying the layer thickness from one GaN bilayer to 4 μ m. The GaN surfaces initially contained pits and undulations; the latter became smoother with continued growth. The growth of a 1–1.5 nm thick and continuous wetting layer and the subsequent formation of islands within the undulations are characteristic of the Stranski–Krastanov growth mode observed for the growth of GaN on AlN. The islands coalesced within the first 10 nm. Instead, a step-flow growth mode was observed for the growth of GaN on AlGaN. Increasing the thickness of the GaN grown on either AlN or AlGaN caused these biaxially stressed layers to gradually change their state of stress from compression to tension with regard to both their average strain and their local strain along the growth direction. The compressive and tensile stress components are attributed to the mismatch in lattice parameters between the GaN and the buffer layer and the mismatch in the coefficients of thermal expansion between GaN and SiC, respectively. A portion of the compressive stress is relieved within the first 20 nm of GaN grown on AlN buffer layer. The relief of the remaining stress follows an exponential dependence on the thickness of the GaN layer with values for the characteristic decay length of 0.24 and 0.64 μ m for the AlN and AlGaN buffer layer, respectively. The relaxation mechanism is discussed in terms of the formation of misfit dislocations via surface undulations.

Growth and Characterisation of AlGaN/GaN HEMT on Silicon Substrates

In order to analyse and to compare the properties of AlGaN/GaN HEMT on silicon and on sapphire substrates, studies on both layers and device types have been performed. Besides the substantially lower substrate costs compared to SiC, the use of silicon as substrate provides the advantage of a higher thermal conductivity compared to sapphire allowing a more efficient heat removal from the device and thus higher RF power densities. On silicon, up to 900 nm of GaN as well as HEMT structures have been deposited and characterised regarding their structural, optical and electrical properties. HEMT devices with various gate lengths were processed and measured on-wafer under continuous and pulsed operation conditions. The properties of the layers and devices on silicon substrates are developing to become comparable to those based on sapphire and silicon carbide.