InGaN/GaN heterostructures grown by submonolayer deposition

InGaN/(Al,Ga)N heterostructures containing ultrathin InGaN layers, grown by submonolayer deposition are studied. It is shown that significant phase separation with the formation of local In-enriched regions ??3?C4 nm in height and ??5?C8 nm in lateral size is observed in InGaN layers in the case of InGaN and GaN growth by cyclic deposition to effective thicknesses of less than one monolayer. The effect of growth interruption in a hydrogen-containing atmosphere during submonolayer growth on the structural and optical properties of InGaN/(Al,Ga)N heterostructures is studied. It is shown that these interruptions stimulate phase separation. It is also shown that the formation of In-enriched regions can be controlled by varying the effective InGaN and GaN thicknesses in the submonolayer deposition cycles.     

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.     

Influence of substoichiometry,hydrogen content and crystallinity on the optical and electrical properties of HxWOy thin films

We have prepared H_xWO_y amorphous thin films both by evaporation of tungsten trioxide powder and by cathodic sputtering of a tungsten target in an argon/oxygen/hydrogen reactive gas mixture. The evaporated layers have the composition H_xWO_2.7 (0.2 < × < 0.5). Their oxygen content seems rather insensitive to the evaporation parameters. We do not observe any correlation between x and these parameters. Evaporated virgin layers are nearly transparent. Annealing,under vacuum leaves y unchanged, under oxygen increases y to 3. Annealing of the virgin layer under vacuum induces the growth of the 1.38 eV absorption band (giving blue coloration) and a decrease of the activation energy for conduction. Annealing the blue layers in oxygen destroys the 1.38 eV band and increases the activation energy for conduction. In both cases annealing at high temperature induces a microcrystalline phase with an absorption band centered about 0.72 eV (giving also a blue coloration) and a jump in electronic conductivity. As in the case of the 1.38 eV band, an increase of the intensity of the 0.72 eV band induces a decrease of the activation energy for conduction. The two bands are interpreted as polaronic like. They can be induced in transparent layers without any change in global composition by excitation of the hydrogen atoms from a “ passive” state to an “ active” state. In addition to the hydrogen content, the existence of the 1.38 eV band requires some substoichiometry. The study of the optical and electrical properties of amorphous H_xWO_y sputtered layers, supports our previous conclusions about the composition range (C) for the coloration capability of transparent thin films. In addition there is a composition range (B) where the virgin layers are blue, and a composition range (M) where they have a metallic like behavior. On the other side of (C), there is a range (C’) where uv illumination only induces a decrease in the activation energy for conduction, then a range (T) where the layers are completely nonresponsive. One can pass from (T) to (B) through (C') and (C) either at constant hydrogen content by increasing the departure from stochiometry, or at constant substoichiometry by increasing the hydrogen content. A part of this work was presented at EMC Cornell, New York July 1, 1977.     

Characteristics of Green Light-Emitting Diodes Using an InGaN:Mg/GaN:Mg Superlattice as p-Type Hole Injection and Contact Layers

Mg-doped InGaN/GaN p-type short-period superlattices (SPSLs) are developed for hole injection and contact layers of green light-emitting diodes (LEDs). V-defect-related pits, which are commonly found in an InGaN bulk layer, can be eliminated in an InGaN/GaN superlattice with thickness and average composition comparable to those of the bulk InGaN layer. Mg-doped InGaN/GaN SPSLs show significantly improved electrical properties with resistivity as low as ∼0.35 ohm-cm, which is lower than that of GaN:Mg and InGaN:Mg bulk layers grown under optimized growth conditions. Green LEDs employing Mg-doped InGaN/GaN SPSLs for hole injection and contact layers have significantly lower reverse leakage current, which is considered to be attributed to improved surface morphology. The peak electroluminescence intensity of LEDs with a SPSL is compared to that with InGaN:Mg bulk hole injection and contact layers.     

The use of short-period InGaN/GaN superlattices in blue-region light-emitting diodes

Optical and light-emitting diode structures with an active InGaN region containing short-period InGaN/GaN superlattices are studied. It is shown that short-period superlattices are thin two-dimensional layers with a relatively low In content that contain inclusions with a high In content 1–3 nm thick. Inclusions manifest themselves from the point of view of optical properties as a nonuniform array of quantum dots involved in a residual quantum well. The use of short-period superlattices in light-emitting diode structures allows one to decrease the concentration of nonradiative centers, as well as to increase the injection of carriers in the active region due to an increase in the effective height of the AlGaN barrier, which in general leads to an increase in the quantum efficiency of light-emitting diodes.     

Relaxation of InGaN thin layers observed by X-ray and transmission electron microscopy studies

Double-crystal and triple-axis x-ray diffractometry and transmission electron microscopy are used to characterize the microstructure, strain, and composition of InGaN layers grown on GaN by metalorganic chemical vapor deposition (MOCVD). Three different samples with increasing In concentration have been studied, all grown on GaN deposited on sapphire either with GaN or AlN buffer layers. It was found that InGaN layers with nominal 28% and 40% InN content consist of two sub-layers; the first sub-layer is pseudomorphic with the underlying GaN with lower In content than nominal. The top sub-layer is fully relaxed with a high density of planar defects and In content close to the nominal value. This is in contrast to a common assumption applied to InGaN quantum wells that ‘quantum-dot like areas’ are formed with different In content. The sample with the nominal indium concentration of 45% does not exhibit any intermediate strained layer, is fully relaxed and the In concentration (43.8%) agrees well with the nominal value.     

Shallow–Deep InGaN Multiple-Quantum-Well System for Dual-Wavelength Emission Grown on Semipolar ($$ 11\bar{2}2 $$) Facet GaN

We report growth and characterization of a shallow–deep InGaN/GaN multiple-quantum-well (MQW) system for dual-wavelength emission grown on semipolar (11[`2]2 11\bar{2}2 ) facet GaN. Structural and optical properties of the InGaN multiple-quantum-well system were investigated by scanning electron microscopy (SEM), cross-sectional scanning transmission electron microscopy (XSTEM), photoluminescence (PL), photoluminescence excitation (PLE), and time-resolved photoluminescence (TRPL) measurements. Cross-sectional transmission electron microscopy (XTEM) revealed that the growth rate of the InGaN well layers on the (0001) flat top microfacet (~500 nm) was about six times as fast as on the (11[`2]2 11\bar{2}2 ) inclined facet, whereas the growth rate of GaN barrier layers on the (0001) flat top facet was roughly 4.5 times as large as that on the (11[`2]2 11\bar{2}2 ) facet. A room-temperature PL spectrum showed dual-wavelength light emission of the shallow–deep InGaN multiple-quantum-well system situated at 2.720 eV (455 nm) and 2.967 eV (418 nm). The Stokes shifts between the two PL peaks and the two “effective bandgaps” were ~260 meV in energy for the deep quantum wells and ~233 meV for the shallow quantum wells. The TRPL decay demonstrated the short radiative recombination lifetime on the order of several nanoseconds in the InGaN MQW system. Realization of the shallow–deep InGaN multiple-quantum-well system with emission wavelength controllability would be useful to achieve III-nitride-based multicolor light-emitting devices for displays.     

Composite InGaN/GaN/InAlN heterostructures emitting in the yellow-red spectral region

The results of studies of the properties of composite InGaN/GaN/InAlN heterostructures are reported. It is shown that, in the InAlN layer, there is substantial phase separation that brings about the formation of three-dimensional islands consisting of AlN-InAlN-AlN regions. The dimensions of these islands depend on the thickness of the InAlN layer and the conditions of epitaxial growth. Interruptions in the growth of InAlN provide a means for influencing the structural properties of the InAlN islands. The use of composite InGaN/GaN/InAlN heterostructures, in which the InGaN layer with a high In content serves as the active region in light-emitting diode structures, makes it possible to achieve emission in the yellow-red wavelength range 560?C620 nm.     

Structural and optical investigation of InGaN/GaN multiple quantum well structures with various indium compositions

We have studied the influence of indium (In) composition on the structural and optical properties of In_x Ga_1−xN/GaN multiple quantum wells (MQWs) with In compositions of more than 25% by means of high-resolution x-ray diffraction (HRXRD), photoluminescence (PL), and transmission electron microscopy (TEM). With increasing the In composition, structural quality deterioration is observed from the broadening of the full width athalf maximum of the HRXRD superlattice peak, the broad multiple emission peaks oflow temperature PL, and the increase of defect density in GaN capping layers and InGaN/GaN MQWs. V-defects, dislocations, and two types of tetragonal shape defects are observed within the MQW with 33% In composition by high resolution TEM. In addition, we found that V-defects result in different growth rates of the GaN barriers according to the degree of the bending of InGaN well layers, which changes the period thickness of the superlattice and might be the source of the multiple emission peaks observed in the In_xGa_1−xN/GaN MQWs with high in compositions.     

Phase separation in Zn-doped InGaN grown by metalorganic chemical vapor deposition

Zn-doped InGaN thin films were deposited on GaN/sapphire by metalorganic chemical vapor deposition, and studied by a combination of high-resolution X-ray diffraction (HR-XRD), micro-photoluminescence (PL) and secondary ion mass spectrometry (SIMS). Indium phase separation is studied comparatively. HR-XRD exhibits a GaN band and a single band from InGaN for samples without phase separation, but two InGaN bands corresponding to different x(In) for samples with phase separation. PL excitation power dependence measurements reveal 2 sets of InGaN PL emissions for samples with phase separation, but only 1 set for samples without phase separation. SIMS data showed that phase separated InGaN:Zn films possess a high Zn concentration near the InGaN–GaN interface and non-uniform distributions of In and Zn contents, which are in contrast with data from InGaN:Zn films with no In-phase separation.     

MOCVD生长的InGaN薄膜中的相分离

利用X射线衍射(XRD)测量用MOCVD生长的In Ga N样品,观察到In N相.通过X射线衍射理论,计算得到In N相在In Ga N中的含量.通过退火和变化生长条件发现In N相在In Ga N薄膜中的含量与生长时N2 载气流量、反应室压力和薄膜中存在的应力有关.进一步的分析表明In Ga N合金中出现In N相的主要原因是相分离.     

Optimization of Digital Growth of Thick N-Polar InGaN by MOCVD

Smooth 200 nm thick N-polar InGaN films were grown by metal–organic chemical vapor deposition (MOCVD) on sapphire using a digital approach consisting of a constant In, Ga, and N precursor flow with pulsed injection of H2 into the N2 carrier gas. Using this growth scheme, the H2 injection time was altered and the effect on the morphology and indium incorporation in the films observed. The effect of periodic insertion of additional GaN inter-layers on the surface morphology of the InGaN layers was also studied.     

High-speed photodetector applications of GaAs and InxGa1−xAs/GaAs grown by low-temperature molecular beam epitaxy

GaAs grown by molecular beam epitaxy (MBE) at low substrate temperatures (≈200°C) exhibits the desired properties of a high-speed photoconductor: high resistivity, high mobility, high dielectric-breakdown strength, and subpicosecond carrier lifetime. The unique material properties are related to the excess arsenic content in the MBE grown epilayers. Due to the combination of the above properties, dramatically improved performance has been observed in photoconductive detectors and correlators using submicron spaced electrodes. In addition to GaAs, low-temperature growth of In_xGa_1−xAs alloys also leads to the incorporation of excess arsenic in the layers, and therefore this material system exhibits many beneficial photoconductor properties as well. In particular, the lattice-mismatched growth of LT-In_xGa_1−xAs on GaAs appears to be the most suited for high-speed detector applications in the near-infrared wavelength range used in optical communications. The material issues and the photodetector characteristics required to optimize their performance are discussed.     

Influence of Mg Doping on the Morphological, Optical, and Structural Properties of InGaN/GaN Multiple Quantum Wells

In this report, the influence of magnesium doping on the characteristics of InGaN/GaN multiple quantum wells (MQWs) was investigated by means of atomic force microscopy (AFM), photoluminescence (PL), and X-ray diffraction (XRD). Five-period InGaN/GaN MQWs with different magnesium doping levels were grown by metalorganic chemical vapor deposition. The AFM measurements indicated that magnesium doping led to a smoother surface morphology. The V-defect density was observed to decrease with increasing magnesium doping concentration from ∼10⁹ cm⁻² (no doping) to ∼10⁶ cm⁻² (Cp₂Mg: 0.04 sccm) and further to 0 (Cp₂ Mg: 0.2 sccm). The PL measurements showed that magnesium doping resulted in stronger emission, which can be attributed to the screening of the polarization-induced band bending. XRD revealed that magnesium doping had no measurable effect on the indium composition and growth rate of the MQWs. These results suggest that magnesium doping in MQWs might improve the optical properties of GaN photonic devices.     

Investigation of high quality GaAs:In layers grown by liquid phase epitaxy

Indium-doped GaAs layers are investigated by low-field Hall effect, photoluminescence, and double crystal x-ray diffraction in order to study the influence of the In concentration on the electrical, optical, and crystallographic properties. The layers were grown by liquid phase epitaxy from solution with In concentrations in the range 0–10 at.%. It was found that epitaxial growth from the melt with 7 at.% In content produces the highest quality epitaxial layers.     

Epitaxial growth of GaN/AlN/InAlN heterostructures for HEMTs in horizontal MOCVD reactors with different designs

The epitaxial growth of InAlN layers and GaN/AlN/InAlN heterostructures for HEMTs in growth systems with horizontal reactors of the sizes 1 × 2", 3 × 2", and 6 × 2" is investigated. Studies of the structural properties of the grown InAlN layers and electrophysical parameters of the GaN/AlN/InAlN heterostructures show that the optimal quality of epitaxial growth is attained upon a compromise between the growth conditions for InGaN and AlGaN. A comparison of the epitaxial growth in different reactors shows that optimal conditions are realized in small-scale reactors which make possible the suppression of parasitic reactions in the gas phase. In addition, the size of the reactor should be sufficient to provide highly homogeneous heterostructure parameters over area for the subsequent fabrication of devices. The optimal compositions and thicknesses of the InAlN layer for attaining the highest conductance in GaN/AlN/InAlN transistor heterostructures.     

Growth of InGaN HBTs by MOCVD

The design and growth of GaN/InGaN heterojunction bipolar transistors (HBTs) by metalorganic chemical vapor deposition (MOCVD) are studied. Atomic-force microscopy (AFM) images of p⁺InGaN base layers (∼100 nm) deposited under various growth conditions indicate that the optimal growth temperature is limited to the range between 810 and 830°C due to a trade-off between surface roughness and indium incorporation. At these temperatures, the growth pressure must be kept above 300 Torr in order to keep surface pit density under control. An InGaN graded-composition emitter is adopted in order to reduce the number of V-shaped defects, which appear at the interface between GaN emitter and InGaN base and render an abrupt emitter-base heterojunction nearly impossible. However, the device performance is severely limited by the high p-type base contact resistance due to surface etching damage, which resulted from the emitter mesa etch.     

A Novel Approach for Achieving High‐Efficiency Photoelectrochemical Water Oxidation in InGaN Nanorods Grown on Si System: MXene Nanosheets as Multifunctional Interfacial Modifier

MXene nanosheets with attractive electrical conductivity and tunable work function have been adopted as multifunctional interfacial modifier between InGaN nanorods and Si for photoelectrochemical water oxidation for the first time. Compared to bare InGaN/Si systems, MXene interfacial layers give rise to an ultralow onset potential of 75 mV versus reversible hydrogen electrode (RHE), which is the highest ever reported for InGaN‐ or Si‐based photoanodes by interfacial modification. Furthermore, the modified photoanode exhibits a significantly enhanced photocurrent density (7.27 mA cm^?2) at 1.23 V versus RHE, which is about 10 times higher than that achieved with the InGaN/Si photoanode. The detailed mechanism demonstrates that the formed type‐II band alignment in InGaN/MXene heterojunction and an Ohmic junction at the MXene/Si interface make MXene an ideal electron‐migration channel to enhance charge separation and transfer process. This synergetic effect of MXene can significantly decrease the charge resistance at semiconductor/Si and semiconductor/electrolyte hetero‐interfaces, eventually resulting in the fast hole injection efficiency of 82% and superior stability against photocorrosion. This work not only provides valuable guidance for designing high‐efficiency photoelectrodes through the integration of multiscale and multifunctional materials, but also presents a novel strategy for achieving high‐performance artificial photosynthesis by introducing interfacial modifier.