Thin InGaN Layers Research Articles

InGaN surface morphology evolution investigated by atomic force microscope with power spectral density analysis

The morphology of thin InGaN layers grown at varying temperature by metalorganic chemical vapor deposition (MOCVD) has been studied by atomic force microscope (AFM). The surface morphology undergoes a transition process from three-dimensional (3D) island growth mode to two-dimensional (2D) nucleated mound and hillock growth mode and finally to layer-by-layer step-meandering and step flow growth mode. The changes in morphology were analyzed from a thermodynamic (mostly based on the BCF theory of crystal growth by Burton, Cabrera and Frank) and kinetic (mostly the ESB theory based on the Ehrlich-Schwöbel barrier) perspective. Power spectral density (PSD) analysis of AFM surface images is used to determine the dominant smoothing mechanisms that contribute to the feature of surface morphology of InGaN, and validate our analysis based on BCF and ESB theories.

Highly Uniform Array of Hexagonally Symmetric Micro‐Pyramid Structures for Scalable and Single Quantum Dot Emitters (Adv. Mater. Interfaces 8/2023)

Uniform Single Quantum Dot Emitters Controlling the position, size and shape of group III-nitride quantum dots is critical for the development of scalable single quantum emitters operable at room temperature. In article number 2202085, Yong-Hoon Cho and colleagues present a methodology for highly uniform, site-controlled GaN micro-pyramidal structures with a high degree of hexagonal symmetry by adjusting growth conditions under the self-limited growth mechanism. By successively growing a thin InGaN layer on the uniform pyramidal arrays with growth rate anisotropy, high-performance single photon emission from the apex of each pyramid can be achieved with suppressed background emission from side quantum wells of the pyramids.

Anisotropic Etching of InGaN Thin Films with Photoelectrochemical Etching to Form Quantum Dots

Traditional methods for synthesizing InGaN quantum dots (QDs), such as the Stranski-Krastanov growth, often result in QD ensembles with low density and non-uniform size distribution. To overcome these challenges, forming QDs using photoelectrochemical (PEC) etching with coherent light has been developed. Anisotropic etching of InGaN thin films is demonstrated here with PEC etching. InGaN films are etched in dilute H2SO4 and exposed to a pulsed 445 nm laser with a 100 mW/cm2 average power density. Two potentials (0.4 V or 0.9 V) measured with respect to an AgCl|Ag reference electrode are applied during PEC etching, resulting in different QDs. Atomic force microscope images show that while the QD density and sizes are similar for both applied potentials, the heights are more uniform and match the initial InGaN thickness at the lower applied potential. Schrodinger-Poisson simulations show that polarization-induced fields in the thin InGaN layer prevent positively charged carriers (holes) from arriving at the c-plane surface. These fields are mitigated in the less polar planes resulting in high etch selectivity for the different planes. The higher applied potential overcomes the polarization fields and breaks the anisotropic etching.

Highly Uniform Array of Hexagonally Symmetric Micro‐Pyramid Structures for Scalable and Single Quantum Dot Emitters

AbstractControlling the site, size, and shape of group III‐nitride quantum dots (QDs) is critical for the development of mass‐producible single‐photon sources for scalable quantum technologies operable at room temperature. Herein, a methodology is proposed for fabricating high‐purity single QD emitters by controlling site‐controlled GaN micro‐pyramid structures with a high degree of uniformity and symmetry. To achieve a uniformly grown, hexagonally symmetric micro‐pyramid array, the H2/N2 carrier gas ratio, growth temperature, and V/III ratio are controlled to attain self‐limited growth regime and self‐limited width at the GaN pyramid apex. A thin InGaN layer is consecutively grown on a pyramid array under the growth condition for enhancing the growth rate anisotropy to hinder the growth of InGaN quantum wells (QWs) at semi‐polar facets. As a result, single‐photon emission is observed from apex QD with suppressed background side QW emission while maintaining more than 90% high hexagonal QD symmetry over the large area of the wafer.

Deep traps in InGaN/GaN single quantum well structures grown with and without InGaN underlayers

The electrical properties and deep trap spectra were compared for near-UV GaN/InGaN quantum well (QW) structures grown on free-standing GaN substrates. The structures differed by the presence or absence of a thin (110 nm) InGaN layer inserted between the high temperature GaN buffer and the QW region. Capacitance-voltage profiling with monochromatic illumination showed that in the InGaN underlayer (UL), the density of deep traps with optical threshold near 1.5 eV was much higher than in the QW and higher than for structures without InGaN. Irradiation with 5 MeV electrons strongly increased the concentration of these 1.5 eV traps in the QWs, with the increase more pronounced for samples without InGaN ULs. The observations are interpreted using the earlier proposed model explaining the impact of In-containing underlayers by segregation of native defects formed during growth of GaN near the surface and trapping of these surface defects by In atoms of the InGaN UL, thus preventing them from infiltrating the InGaN QW region. Deep level transient spectroscopy (DLTS) also revealed major differences in deep trap spectra in the QWs and underlying layers of the samples with and without InGaN ULs. Specifically, the introduction of the InGaN UL stimulates changing the dominant type of deep traps. Irradiation increases the densities of these traps, with the increase being more pronounced for samples without the InGaN UL. It is argued that light emitting diodes (LEDs) with InGaN UL should demonstrate a higher radiation tolerance than LEDs without InGaN UL.

Gallium nitride tunneling field-effect transistors exploiting polarization fields

This report showcases a vertical tunnel field effect transistor (TFET) fabricated from a GaN/InGaN heterostructure and compares it to a gated vertical GaN p-n diode. By including a thin InGaN layer, the interband tunneling in the TFET is increased compared to the gated homojunction diode. This leads to an increased drain current of 57 μA/μm and a reduced subthreshold swing of 102 mV/dec, from 240 mV/dec. However, trap assisted tunneling prevents devices from realizing subthreshold slopes below the Boltzmann limit of 60 mV/dec. Nevertheless, this work shows the capability of tunnel field effect transistors to be realized in GaN by taking advantage of the spontaneous and piezoelectric polarization in the III-N material system.

Morphological study of InGaN on GaN substrate by supersaturation

The morphology of thin InGaN layers grown on c-plane GaN substrates by metalorganic vapor phase epitaxy (MOVPE) has been studied by atomic force microscopy. Three different morphologies appeared, a stepped surface, large flat two-dimensional (2D) islands and small high three-dimensional (3D) dots. Low growth temperature and high growth rate (i.e. increased vapor supersaturation) changed the InGaN morphology from steps to 2D islands and then 3D dots (when exceeding the critical thickness for 3D dots). Larger miscut angle of GaN substrate changed the morphology from 2D islands to step by decreasing the surface supersaturation of individual terraces. InGaN layers with a stepped morphology had the highest internal quantum efficiency at similar InN mole fraction. InGaN grown on GaN substrate more easily achieved a stepped morphology compared to InGaN on GaN/sapphire templates.

Enhanced mobility in vertically scaled N-polar high-electron-mobility transistors using GaN/InGaN composite channels

A GaN/InGaN composite channel design for vertically scaled N-polar high-electron-mobility transistor (HEMT) structures is proposed and demonstrated by metal-organic chemical vapor deposition. In a conventional N-polar HEMT structure, as the channel thickness (tch) decreases, the sheet charge density (ns) decreases, the electric field in the channel increases, and the centroid of the two-dimensional electron gas (2DEG) moves towards the back-barrier/channel interface, resulting in stronger scattering and lower electron mobility (μ). In this study, a thin InGaN layer was introduced in-between the channel and the AlGaN cap to increase the 2DEG density and reduce the electric field in the channel and therefore increase the electron mobility. The dependence of μ on the InGaN thickness (tInGaN) and the indium composition (xIn) was investigated for different channel thicknesses. With optimized tInGaN and xIn, significant improvements in electron mobility were observed. For a 6 nm channel HEMT structure, the electron mobility increased from 606 to 1141 cm2/(V·s) when the 6 nm thick pure GaN channel was replaced by the 4 nm GaN/2 nm In0.1Ga0.9N composite channel.

Quantum Confinement of Hybrid Charge Transfer Excitons in GaN/InGaN/Organic Semiconductor Quantum Wells.

We investigate hybrid charge transfer exciton (HCTE) confinement in organic-inorganic (OI) quantum wells (QWs) comprising a thin InGaN layer bound on one side by GaN and on the other by the organic semiconductors, tetraphenyldibenzoperiflanthene (DBP) or 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP). A binding energy of 10 meV is calculated for the Coulombically bound free HCTE state between a delocalized electron in GaN and a hole localized in DBP. The binding energy of the HCTE increases to 165 meV when the electron is confined to a 1.5 nm In0.21Ga0.79N QW (HCTEQW). The existence of the HCTEQW is confirmed by measuring the voltage-dependent DBP exciton dissociation yield at the OI heterojunction in the QW devices that decrease with increasing In concentration and decreasing electric field, matching the trends predicted by Poole-Frenkel emission. Combining spectroscopic measurements with optical models, we find that 14 ± 3% of the excitons that reach the GaN/DBP heterojunction form HCTEs and dissociate into free charges, while the remainder recombine. A high nonradiative recombination rate through defect states at the heterointerface account for the lack of observation of HCTEQW photoluminescence from GaN/InGaN/CBP QWs at temperatures as low as 10 K.

Quantum electronic transport in polarization-engineered GaN/InGaN/GaN tunnel junctions

We theoretically investigate GaN/InGaN/GaN tunnel junctions grown along the wurtzite c-axis. We developed a dedicated quantum electronic transport model based on an 8-band k.p Hamiltonian coupled to the non-equilibrium Green's function formalism. We first show that the transmission is dominated by quantum states localized at the heterojunction. We also confirm that, for a thin InGaN layer, current strongly increases with doping. On the other hand, for thick InGaN layers (>8 nm), our results show an unexpected low impact of doping on current. In this latter case, the spontaneous and the piezoelectric polarizations reduce the tunnel-barrier width to the InGaN layer thickness. We conclude that quantum electronic transport in such tunnel junctions is mainly controlled by interfaces with both polarizations and localized states.

Isotype InGaN/GaN heterobarrier diodes by ammonia molecular beam epitaxy

The design of isotype InGaN/GaN heterobarrier diode structures grown by ammonia molecular beam epitaxy is presented. On the (0001) Ga-polar plane, a structure consisting of a surface n+ GaN contact layer, followed by a thin InGaN layer, followed by a thick unintentionally doped (UID) GaN layer, and atop a buried n+ GaN contact layer induces a large conduction band barrier via a depleted UID GaN layer. Suppression of reverse and subthreshold current in such isotype barrier devices under applied bias depends on the quality of this composite layer polarization. Sample series were grown under fixed InGaN growth conditions that varied either the UID GaN NH3 flow rate or the UID GaN thickness, and under fixed UID GaN growth conditions that varied InGaN growth conditions. Decreases in subthreshold current and reverse bias current were measured for thicker UID GaN layers and increasing InGaN growth rates. Temperature-dependent analysis indicated that although extracted barrier heights were lower than those predicted by 1D Schrödinger Poisson simulations (0.9 eV–1.4 eV for In compositions from 10% to 15%), optimized growth conditions increased the extracted barrier height from ∼11% to nearly 85% of the simulated values. Potential subthreshold mechanisms are discussed, along with those growth factors which might affect their prevalence.

InAlN/InGaN/GaN double heterostructure with improved carrier confinement and high-temperature transport performance grown by metal-organic chemical vapor deposition

A nearly lattice-matched InAlN/InGaN/GaN double heterostructure (DH) and traditional InAlN/GaN single heterostructure (SH) were grown by metal–organic chemical vapor deposition. The InN mole fraction of InGaN channel was deduced by XRD and photoluminescence. The electrical properties were characterized by capacitance-voltage and temperature-dependent Hall measurements. Both results revealed that the InAlN/InGaN/GaN DH possessed superior carrier confinement over traditional InAlN/GaN SH owing to the back barrier formed at the InGaN/GaN interface, which prevents the spilling over of carriers and thus remarkably improves the transport performance at high temperature. Furthermore, a thin InGaN layer was preferable for carrier channel applications to a thick one.

Suppression of electron leakage by inserting a thin undoped InGaN layer prior to electron blocking layer in InGaN-based blue-violet laser diodes.

InGaN-based blue-violet laser diodes (LDs) suffer from electron leakage into the p-type regions, which could be only partially alleviated by employing the electron blocking layer (EBL). Here, a thin undoped InGaN interlayer prior to EBL is proposed to create an additional forbidden energy range above the natural conduction band edge, which further suppresses the electron leakage and thus improve the characteristics of LDs. Numerical device simulations reveal that when the proper composition and thickness of InGaN interlayer are chosen, the electron leakage could be efficiently eliminated without inducing any severe accumulation of electrons at the interlayer, resulting in a maximum output power of the device.

InGaN/GaN light-emitting diode with a polarization tunnel junction

We report InGaN/GaN light-emitting diodes (LED) comprising in situ integrated p+-GaN/InGaN/n+-GaN polarization tunnel junctions. Improved current spreading and carrier tunneling probability were obtained in the proposed device architecture, leading to the enhanced optical output power and external quantum efficiency. Compared to the reference InGaN/GaN LEDs using the conventional p+/n+ tunnel junction, these devices having the polarization tunnel junction show a reduced forward bias, which is attributed to the polarization induced electric fields resulting from the in-plane biaxial compressive strain in the thin InGaN layer sandwiched between the p+-GaN and n+-GaN layers.

Growth mode transition and relaxation of thin InGaN layers on GaN (0001)

We have investigated on the growth of InGaN layers and quantum dots (QDs) on GaN (0001) by metal-organic vapour phase epitaxy. For indium contents above 15% strained QDs form after a certain layer thickness (Stranski Krastanov mode) with typical heights of 2–3nm. The QD density increases fast during further deposition until saturation above 1010cm−2. Growing thicker layers results in larger but relaxed structures and leave only a small growth window for strained QDs. The data indicate that QDs form when the accumulated strain energy calculated from indium content and layer thickness exceeds a critical value which is independent from the indium content. The same behaviour was found for the critical thickness of relaxation, indicating that relaxation occurs via the relaxation of large QDs with heights above 3nm.

Monolithic white LEDs: Approaches, technology, design

The results of investigations of monolithic white InGaAlN LEDs with an active region containing several thin InGaN layers, emitting in the range from blue to yellow-green, and separated by short-period InGaN/GaN superlattices, are presented. The influence of the growth conditions and layer sequence in the active region on the optical properties of monolithic white LEDs was studied with the aim of controlling their color parameters.

Polarized emission and excitonic fine structure energies of InGaN quantum dots

The linear polarization of the excitonic emission from quantum dot-like potential minima formed in a thin InGaN layer is investigated. The recorded emission lines exhibit significant intensity linearly polarized along the wurtize c-axis. For many of the studied spectra, the excitonic fine-structures were resolved, revealing energy splittings in the order of ∼200μeV.

Influence of hydrogen on local phase separation in InGaN thin layers and properties of light-emitting structures based on them

Results of studies of hydrogen addition during the growth of thin (∼2–3 nm) InGaN layers on their structural properties and properties of light-emitting structures that contain InGaN/GaN heterostructures in the active region are reported. It is shown that, with the known effect of a decrease in the average content of In, hydrogen addition leads to varying the local phase separation in the InGaN layers. Hydrogen addition during the growth of the InGaN layers initially causes suppression of the local phase separation, while hydrogen addition during interruptions of the growth after deposition of the InGaN films leads to a decrease in the size of the formed local In-enriched regions and to a certain increase in the local content of the In atoms.

Formation of composite InGaN/GaN/InAlN quantum dots

Composite InGaN/GaN/InAlN quantum dots (QDs) have been formed and studied. The structural properties of thin InAlN layers overgrown with GaN have been analyzed, and it is shown that 3D islands with lateral sizes of ∼(20–30) nm are formed in structures of this kind. It is demonstrated that deposition of a thin InGaN layer onto the surface of InAlN islands overgrown with a thin GaN layer leads to transformation of the continuous InGaN layer to an array of isolated QDs with lateral sizes of 20–30 nm and heights of 2–3 nm. The position of these QDs in the growth direction correlates with that of InAlN islands.

Ohmic Contacts to Wurtzite Silicon Carbide Using Polarization Technology

AbstractWe present theoretical calculations for ohmic contact technology to wurtzite Silicon Carbide using thin Indium Gallium Nitride and Aluminium Indium Nitride cap layers. Spontaneous and piezoelectric polarization in Indium Gallium Nitride and Aluminium Indium Nitride cap layers gives rise to bound interface sheet charge density of the order of 1013 electrons per cm2, and built-in electric fields of the order of MV per cm. For Si-face p-type SiC, the large compressive strain in very thin InGaN and AlInN cap layers results in negative sheet charge densities and much lower tunneling widths for holes compared to bulk contacts. For C-face p-type SiC, pseudomorphic nitride layers yield no benefit over bulk contacts since positive interface sheet charge densities repel holes and give higher contact resistances. However, thick, relaxed cap layers lead to spontaneous polarization-based negative charge densities that attract holes, leading to lower contact resistances. The contributions of the heavy, light and split-off band holes to the total tunneling flux is taken into account in our calculations. All tunneling probabilities are calculated within the Wentzels-Kramers-Brillouin approximation. The presence of the appropriate cap layers leads to several orders of magnitude improvement in tunneling transmission probabilities, and in many cases, makes ohmic technology to SiC feasible. While the major difficulties are encountered for contacts to p-type SiC, the effect of cap-based polarization technology on contacts to n-type layers is also discussed in this paper. Our calculations have relevance to contact technology for bipolar devices built from wide band-gap wurtzite semiconductors like heterojunction bipolar transistors, light-emitting diodes and lasers.