V-pit Formation Research Articles

Electron Transport Properties in High Electron Mobility Transistor Structures Improved by V-Pit Formation on the AlGaN/GaN Interface

This work suggests new morphology for the AlGaN/GaN interface which enhances electron mobility in two-dimensional electron gas (2DEG) of high-electron mobility transistor (HEMT) structures. The widely used technology for the preparation of GaN channels in AlGaN/GaN HEMT transistors is growth at a high temperature of around 1000 °C in an H2 atmosphere. The main reason for these conditions is the aim to prepare an atomically flat epitaxial surface for the AlGaN/GaN interface and to achieve a layer with the lowest possible carbon concentration. In this work, we show that a smooth AlGaN/GaN interface is not necessary for high electron mobility in 2DEG. Surprisingly, when the high-temperature GaN channel layer is replaced by the layer grown at a temperature of 870 °C in an N2 atmosphere using TEGa as a precursor, the electron Hall mobility increases significantly. This unexpected behavior can be explained by a spatial separation of electrons by V-pits from the regions surrounding dislocation which contain increased concentration of point defects and impurities.

V-pits formation in InGaN/GaN: influence of threading dislocations and indium content

Two sets of InGaN/GaN MOVPE-grown samples were studied by high-resolution x-ray diffraction techniques together with statistical analysis of atomic force microscope images in order to determine the impact of In concentration and threading dislocations (TDs) density on the V-pit formation. It was shown that in our samples, the density of V-pits in the epilayer matched the TD density with a screw component in the substrate. Pure edge TDs do not affect the V-pit density. The In concentration influences the size of the V-pits, but not their density.

Suppression of V-pits formation in InGaN layer by stepped growth with annealing interval

An epitaxial method of stepped growth with additional annealing interval is proposed to suppress the V-pit formation in InGaN layer. Detailed investigation on the influences of annealing condition indicates that more cycles in an annealing interval, relatively higher annealing temperature or longer annealing time can better reduce the density and size of V-pits in InGaN, and finally obtain a InGaN layer with much less V-pits. Micro-photoluminescence mapping measurements present a better spatial distribution and a smaller full width half maximum of luminescence peaks, providing more information about the advantage of stepped growth with annealing interval. Moreover, an experiment for thick InGaN layer growth with a relatively higher indium content also proves that such stepped growth could suppress the V-pits effectively.

Effects of MOVPE Growth Conditions on GaN Layers Doped with Germanium.

The effect of growth temperature and precursor flow on the doping level and surface morphology of Ge-doped GaN layers was researched. The results show that germanium is more readily incorporated at low temperature, high growth rate and high V/III ratio, thus revealing a similar behavior to what was previously observed for indium. V-pit formation can be blocked at high temperature but also at low V/III ratio, the latter of which however causing step bunching.

MOCVD growth of thick V-pit-free InGaN films on semi-relaxed InGaN substrates

The MOCVD growth of InGaN:Si base layers on a semi-relaxed InGaN substrate, where growth is generally difficult due to the presence of V-pits, is examined. These V-pits can propagate through the crystal, causing severe morphological degradation and significantly reducing material quality for device use. Such V-pits may also be a source of leakage current if they extend from the substrate through p-n junction. A wide range of InGaN growth conditions and their impact on V-pit formation and density are investigated. The use of thin GaN interlayers, carrier gas selection, and V/III ratio are found play a critical role in managing V-pit quantity and size. Finally, high temperature GaN interlayers are implemented, fully eliminating V-pit formation in 1200 nm thick InGaN base layers grown coherently on semi-relaxed InGaN substrates.

Thermodynamics Models for V-pit Nucleation and Growth in III-Nitride on Silicon

Silicon and sapphire are common substrates for AlN, InGaN, and GaN thin films in several applications such as photovoltaic and light-embedded diodes. Threading dislocations are generated at interfaces between III-nitride (III-N) layers and these substrates because of large lattice and thermal expansion coefficient (TEC) mismatches. These dislocations penetrate the top surface of III-N layers to relax the system by forming V-pit defects. This work presents a thermodynamics-based model to study V-pit formation and growth in InGaN/GaN epilayers on either silicon or sapphire substrates. The model calculates the evolution of V-pit defects in thin films through the energy balance between the strain energy in the III-N layer, dislocation deterioration energy to form new V-pits, and V-pit facet energies that result because of facet formations. The impact of different lattice and TEC mismatches as well as a novel approach, the embedded void approach, on V-pit nucleation and growth is also investigated.

Effect of V-Shaped pits on optical properties of GaN-Based green light-emitting diodes

Abstract The optical properties of GaN-based green light-emitting diodes (LED) are investigated by transmission electron microscopy (TEM) and temperature-dependent micro-photoluminescence (μ-TDPL) in this study. Compared with the emission of normal c-plane multiple quantum wells (C-MQWs), the emission of C-MQWs close to V-shaped pits (V-pits) exhibits a shoulder peak phenomenon at cryogenic temperature, and the shoulder peak gradually disappears as the temperature increases. At room temperature, the peak of C-MQWs close to V-pits (C2) exhibits a red-shift. The formation of V-pits would induce strain relaxation. Strain relaxation can result in Indium (In) incorporation and localization, and reduced quantum-confined stark effects (QCSE). In incorporation and localization can lead to the red-shift of C2, reduced QCSE can cause the blue-shift of C2. Therefore, the red-shift of C2 arises from further In incorporation and localization of C-MQWs. It is worth noting that In incorporation and localization is dominant in the formation of V-pits for green InGaN/GaN C-MQWs. Additionally, the carriers limited in sidewall MQWs (S-MQWs) of V-pits escape into C-MQWs close to V-pits can mitigate the common phenomenon in which the emission intensity of C-MQWs decreases as the temperature increases, which helps improve the emission intensity of GaN-based green LEDs.

Suppression the formation of V-pits in InGaN/ GaN multi-quantum well growth and its effect on the performance of GaN based laser diodes

The mechanism for the formation of V-pits in InGaN/GaN multi-quantum well (MQW) growth and its effect on the performance of GaN based laser diodes (LDs) are investigated in detail. It is observed that the V-pits in InGaN/GaN MQWs begin at the GaN barrier layer rather than InGaN well layer due to the low atomic migration ability of Ga atoms at low growth temperature. In addition, it is found that the formation of V-pits can be suppressed remarkably when a small amount of In atoms is introduced during GaN barrier layer growth. This may be attributed to the decrease of the potential barrier for atom migration around the dislocation region. In this way, the thermal stability of InGaN/(In)GaN MQWs is enhanced. Therefore, the threshold current decreases abruptly when using InGaN/InGaN MQWs instead of InGaN/GaN MQWs.

Influence of GaN buffer layer under InGaN/GaN MQWs on luminescent properties

Although InGaN layers or InGaN/GaN superlattices are commonly used as efficiency improving buffers for LED structure production, there is still a controversy and active discussion about the mechanisms improving the luminescence properties of InGaN QWs grown above such buffers. In this manuscript it is shown that presence of In in the buffer layer is not the primary reason for photoluminescence improvement which can be also achieved by introduction of GaN buffer layer grown at lower temperature under nitrogen atmosphere. SIMS analysis suggests that low temperature buffer layer does not influence the impurity incorporation and hence the PL improvement is caused by suppressed contamination of MQW region grown above the low temperature buffer. AFM images for two samples that differ mostly in morphology however supports another explanation in which formation of larger V-pits is the main reason for the luminescence improvement.

The effect of nanometre-scale V-pits on electronic and optical properties and efficiency droop of GaN-based green light-emitting diodes

The development of efficient green light-emitting diodes (LEDs) is of paramount importance for the realization of colour-mixing white LEDs with a high luminous efficiency. While the insertion of an InGaN/GaN superlattice (SL) with a lower In content before the growth of InGaN/GaN multiple quantum wells (MQWs) is known to increase the efficiency of LEDs, the actual mechanism is still debated. We therefore conduct a systematic study and investigate the different mechanisms for this system. Through cathodoluminescence and Raman measurements, we clearly demonstrate that the potential barrier formed by the V-pit during the low-temperature growth of an InGaN/GaN SL dramatically increases the internal quantum efficiency (IQE) of InGaN quantum wells (QWs) by suppressing non-radiative recombination at threading dislocations (TDs). We find that the V-pit potential barrier height depends on the V-pit diameter, which plays an important role in determining the quantum efficiency, forward voltage and efficiency droop of green LEDs. Furthermore, our study reveals that the low-temperature GaN can act as an alternative to an InGaN/GaN SL structure for promoting the formation of V-pits. Our findings suggest the potential of implementing optimized V-pits embedded in an InGaN/GaN SL or low-temperature GaN structure as a beneficial underlying layer for the realization of highly efficient green LEDs.

Investigation of the properties and formation process of a peculiar V-pit in HVPE-grown GaN film

Hexagonal V-pits with an inverted pyramid shape appear on the surface of a GaN film after etching by hot H3PO4. A regular structure that is different from the V-pit morphology is observed in the cathodoluminescence image. The carrier concentration and stress distributions are nonuniform inside the V-pit. Furthermore, no dominant dislocations or defects are observed in the facets or bottom area of the V-pits. An island coalescence process is considered to contribute to the formation of such V-pits, with the coalesced island facets changing from {101m} to {112m} at a concave state for the different growth velocities.

Defects, strain relaxation, and compositional grading in high indium content InGaN epilayers grown by molecular beam epitaxy

We investigate the structural properties of a series of high alloy content InGaN epilayers grown by plasma-assisted molecular beam epitaxy, employing the deposition temperature as variable under invariant element fluxes. Using transmission electron microscopy methods, distinct strain relaxation modes were observed, depending on the indium content attained through temperature adjustment. At lower indium contents, strain relaxation by V-pit formation dominated, with concurrent formation of an indium-rich interfacial zone. With increasing indium content, this mechanism was gradually substituted by the introduction of a self-formed strained interfacial InGaN layer of lower indium content, as well as multiple intrinsic basal stacking faults and threading dislocations in the rest of the film. We show that this interfacial layer is not chemically abrupt and that major plastic strain relaxation through defect introduction commences upon reaching a critical indium concentration as a result of compositional pulling. Upon further increase of the indium content, this relaxation mode was again gradually succeeded by the increase in the density of misfit dislocations at the InGaN/GaN interface, leading eventually to the suppression of the strained InGaN layer and basal stacking faults.

Growth and Characterization of High-Quality, Relaxed In y Ga1−y N Templates for Optoelectronic Applications

Thick, high-quality InGaN layers can be used as templates for quantum well strain reduction in light-emitting diodes and as optical absorption layers in solar cell structures. Current InGaN growth technology, however, is primarily limited by V-pit formation and non-uniform indium composition. We report the growth and characterization of thick, strain-relaxed In y Ga1− y N layers, with 0.08 ≤ y ≤ 0.11, by metal organic chemical vapor deposition using the semibulk approach, which consists in periodic insertion of 2-nm GaN interlayers into the bulk In y Ga1− y N structure; these are then spike-annealed at 1000°C. Photoluminescence, x-ray diffraction, and scanning electron and atomic force microscopy revealed that the semibulk In y Ga1− y N had optical and electrical properties superior to those of conventional bulk In y Ga1− y N grown at the same temperature. Homogeneous indium content and substantial reduction of V-pit density were observed for the semibulk In y Ga1− y N films, even when grown above the critical layer thickness. Double-crystal x-ray diffraction rocking curves also revealed a one order of magnitude reduction of screw dislocation density in the semibulk In y Ga1− y N film compared with the bulk In y Ga1− y N film.

Effect of antimony on growth mode and properties of thick InGaN layers

A comprehensive study on the effect of antimony on growth mode and crystal properties of thick InGaN layers grown by metal-organic vapor phase epitaxy is presented. Two growth regimes are identified by atomic force microscopy: while already minor antimony precursor flux induces the stabilization of {101¯1} facets, the application of increased fluxes leads to the growth of nanoscopic islands which suppresses the typically observed V-pit formation and thus decreases the layer roughness. Further on, high incorporation rates of antimony cause to concentrations of up to 0.74%, at standard InGaN growth conditions, are revealed. The obtained results on the impact of antimony incorporation provide evidence that the generally assumed true surfactant behavior of antimony for InGaN growth has to be reconsidered since it significantly affects structural and luminescence properties.

Composition, structure and morphology of Al1−xInxN thin films grown on Al1−yGayN templates with different GaN contents

Four nominally Al0.85In0.15N thin films with different strains were grown simultaneously on Al1−yGayN (y = 1, 0.93, 0.87 and 0.69) templates by metal organic chemical vapour deposition. The nominal InN content of ~15% was chosen to achieve close lattice matching of Al1−xInxN with the templates of intermediate GaN molar fractions, a small tensile strain for growth on GaN, and compressive strain for the template with the lowest GaN fraction. The film deposited on GaN reveals the highest structural quality, the lowest surface roughness and a homogeneous composition with depth. For growth on the Al1−yGayN ternary templates, the film roughness and the surface pit density both increase with decreasing GaN content, in line with the roughening of the growth templates themselves. Detailed study indicates that the structural and morphological qualities of the templates influence not only the quality of the Al1−xInxN films but also their composition profile. Results suggest that surface roughness of the template and growth on the inclined facets lead to compositional gradients due to a decreased InN incorporation on these facets and to the formation of V-pits.

Effect of V-Shaped Pit Size on the Reverse Leakage Current of InGaN/GaN Light-Emitting Diodes

We report the effect of V-shaped pit size, inverted hexagonal pits embedded in InGaN multiple quantum well, on the reverse leakage current of InGaN light-emitting diodes (LEDs). It was found that the reverse leakage current of InGaN LEDs with larger V-shaped pits is significantly reduced from 1.80 mA down to 3.84 nA at -30 V by several orders of magnitude. We claim that this improvement is accounted for increased Poole-Frenkel barrier height of carrier trapped at the deep centers via enlarged V-pit formation, consequently resulting in lower reverse current of InGaN LEDs.

Optimizing the growth process of the active zone in GaN based laser structures for the long wavelength region

InxGa1−xN/GaN quantum well (QW) structures grown on c-plane surfaces for long wavelength laser structures have been investigated. We found that temperature ramping in the barriers improves the layer structure in avoiding V-pit formation and improves the homogeneity of indium incorporation. In choosing proper temperature profiles degradation of the QWs can be avoided. We demonstrate optical gain for wavelengths larger than 500nm using structures with an active zone grown in such way.

Improvement of Light Extraction Efficiency and Reduction of Leakage Current in GaN-Based LED Via V-Pit Formation

Four types of GaN-based light-emitting diodes (LEDs) with V-pits formed in different regions were grown by metal-organic chemical vapor deposition. The position of the V-pits embedded in the layers of the LED structures was controlled by varying the growth temperature. We achieved the highest output power and lowest leakage current values with the LED structures comprising V-pits embedded in active regions and the p-GaN textured surface. The V-pit formation enhances the light output power and reverse voltage values by 1.3 times the values of the conventional LED owing to the enhancement of the light scattering probability and the effective filtering of threading dislocations.

Growth of the active zone in nitride based long wavelength laser structures

In x Ga 1− x N/GaN quantum well (QW) structures grown on c-plane surfaces for long wavelength light emitters have been investigated intended. We reached indium concentrations of x In ≥ 0.35 with good optical and structural quality. For QW thicknesses d QW ≤ 2 nm a fully strained layer structure is observed. QWs of such high indium concentrations, however, are very sensitive to the growth conditions of the subsequent layers and thermal stability/degradation becomes an important issue. We modified the growth of the QWs to avoid or minimize V-pit formation without temperature ramping in the barriers and showed that their properties were unchanged when used in the active zone of a laser structure.

Erratum: “Strain relaxation due to V-pit formation in InxGa1−xN∕GaN epilayers grown on sapphire” [J. Appl. Phys. 98, 084906 (2005)

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