Compositional Pulling Research Articles

Spatially resolved investigation of strain and composition variations in (In,Ga)N/GaN epilayers

The strain state and composition of a 400 nm thick (In,Ga)N layer grown by metal-organic chemical vapor deposition on a GaN template are investigated by spatially integrated x-ray diffraction and cathodoluminescence (CL) spectroscopy as well as by spatially resolved CL and energy dispersive x-ray analysis. The CL investigations confirm a process of strain relaxation accompanied by an increasing indium content toward the surface of the (In,Ga)N layer, which is known as the compositional pulling effect. Moreover, we identify the strained bottom, unstrained top, and gradually relaxed intermediate region of the (In,Ga)N layer. In addition to an increase of the indium content along the growth direction, the strain relaxation leads to an enhancement of the lateral variations of the indium distribution toward the surface.

Effect of Barrier Growth Temperature on the Material Properties of InGaN/GaN Multiple Quantum Well Structures

InGaN/GaN multiple quantum well (MQW) structures with different GaN barrier growth temperatures have been grown by metallorganic chemical vapor deposition (MOCVD) and investigated. Double peak InGaN emission was observed and correlated to the barrier growth temperature. It was deduced that the extra emission peak originated from InGaN/GaN nanostructures formed around (VGa-ON)2− point defects mainly at the bottom wells of MQW stack, due to local strain relaxation in the vicinity of point defects and the compositional pulling effect on In. Increasing barrier growth temperature decreases O impurity incorporation and thus prevents the formation of an undesired radiation source in the active layer which decrease the quantum efficiency.

Phase separation in thick InGaN layers – A quantitative, nanoscale study by pulsed laser atom probe tomography

Pulsed laser atom probe tomography was employed to study 300nm thick In0.25Ga0.75N layers grown on a GaN buffer layer. The unique three-dimensional data provided by atom probe tomography revealed a complex microstructure in the InGaN formed by indium-rich columns and plates adjacent to strongly indium-depleted regions. This complex indium-related domain structure is likely to represent the early stages of spinodal decomposition occurring during layer growth. The interface with the buffer layer is characterized by the absence of this microstructure and a generally more homogeneous indium distribution. Also, a significantly reduced InN mole fraction was found in that region. Both are believed to be a result of the interfacial misfit strain, which suppresses phase separation and impedes indium incorporation during growth, which is termed the compositional pulling effect.

Growth mechanism and properties of InGaN insertions in GaN nanowires

We demonstrate the strong influence of strain on the morphology and In content of InGaN insertions in GaN nanowires, in agreement with theoretical predictions which establish that InGaN island nucleation on GaN nanowires may be energetically favorable, depending on In content and nanowire diameter. EDX analyses reveal In inhomogeneities between the successive dots but also along the growth direction within each dot, which is attributed to compositional pulling. Nanometer-resolved cathodoluminescence on single nanowires allowed us to probe the luminescence of single dots, revealing enhanced luminescence from the high In content top part with respect to the lower In content dot base.

Composition pulling effect and strain relief mechanism in AlGaN/AlN distributed Bragg reflectors

We report on the composition pulling effect and strain relief mechanism in AlGaN/AlN distributed Bragg reflectors (DBRs) grown on GaN template/α-Al2O3(0001) by metal organic chemical vapor deposition. The reciprocal space mapping contours reveal that these DBRs are coherently grown. Cross-section transmission electron microscopy image of the AlGaN/AlN DBRs and the energy-dispersive x-ray analysis indicate that an AlGaN layer with gradient Al composition is located between the Al0.4Ga0.6N and AlN layers along the [0001] direction. It is attributed to the fact that Ga atoms in AlGaN are pulled and segregated to the upper layer by the strain. The density of strain energy is estimated to reduce more than one order by forming this quasi-three-sublayer structure comparing to the designed bi-sublayer structure.

Evaluation of multiple‐quantum‐well structure on InGaN template using (11$ \bar 2 $2) facet growth and mass transport

AbstractWe succeeded in the fabrication of a thick InGaN template by (11$ \bar 2 $2) facet growth and mass transport. Thick InGaN was grown on a GaN (11$ \bar 2 $2) facet with a stable facet structure. After 20 min annealing under a flow of NH3 and N2, the InGaN with the (11$ \bar 2 $2) facet was perfectly embedded owing to mass transport. Subsequently, a multiple‐quantum‐well structure was fabricated on the InGaN template and characterized using monochromatic cathodeoluminescence measurement. As a result, we found that there were two regions with peak emission wavelengths of 420 and 500 nm because of the compositional pulling effect. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Theoretical analyses of In incorporation and compositional instability in coherently grown InGaN thin films

AbstractWe performed thermodynamic analyses to calculate the relationship between the input indium molar ratio and solid composition of a coherently grown InGaN thin film that is subjected compressive or tensile stress. The theoretical approach incorporates energy loss of a thin film system due to lattice constraint from the substrate. The results show that the indium composition x of coherently grown InGaN is lower than that of stress‐free InGaN. This represents the composition pulling effect. We also studied stable growth modes under various growth conditions. The results suggest the importance of control of partial pressure of NH3 to optimize growth conditions of InGaN with a unique composition. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth and characterization of AlGaN/GaN/AlGaN field effect transistors

AbstractField effect transistors (FETs) on both GaN and AlGaN buffers were grown and compared. Development of high quality AlGaN is the key to obtain high performance AlGaN buffer FETs. An optimized Al0.04Ga0.96N layer on SiC substrate had (002) and (201) X‐ray rocking curves with full wide at half maximum (FWHM) of 140 and 450 arc sec, respectively, indicates the comparable crystal quality of AlGaN to that of a typical GaN layer. X‐ray reciprocal space mapping and ω –2θ scans showed that the AlGaN barriers grown on GaN and Al0.04Ga0.96N buffers had different Al compositions and thickness, which was attributed to the compositional pulling effect. At room temperature, the AlGaN buffer FET had a sheet carrier concentration and electron mobility of 8.3×1012 cm‐2 and 1750 cm2/Vs, respectively. AlGaN/GaN/AlGaN FETs demonstrated a status‐of‐the‐art power added efficiency (PAE) of 53.5%, and a status‐of‐the‐art power gain of 13.4 dB at a drain bias of 20 V at 30 GHz. This device also exhibited lower output conductance and better pinch‐off due to the improved electron confinement resulting from the increase in the effective buffer energy height. The device results imply that AlGaN buffer FETs are promising devices for highly scaled transistors in RF applications in the Ka‐ band and beyond. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Microstructures produced during the epitaxial growth of InGaN alloys

Effects due to phase separation in InGaN have been identified as having major effects on the performance of devices, in particular light-emitting diodes (LEDs) and injection lasers. However, the complexity of the various materials phenomena that can occur has led to a great deal of recent confusion. Much of this confusion can be eliminated by considering the experimentally measured materials properties in the context of the set of physical phenomena occurring during epitaxial growth, including coupling that exists between the various effects. Spinodal decomposition is expected to produce phase separation due to the miscibility gap in InGaN alloys. However, the actual occurrence of this phenomenon has been disputed due to the complexity of real systems. For example, the region of solid immiscibility for InGaN is strongly dependent on elastic strain. In addition, the strain, itself, affects properties such as the bandgap energy. Complicating the analysis of these phenomena is that the solid composition can be affected by elastic strain due to the well-known thermodynamic phenomenon of “compositional pulling”. An additional factor must be considered if the experimentally observed phenomena are to be understood. Thin, lattice mismatched epitaxial layers are coherent with the substrate (or underlying layer). Thus, the actual growth process for the formation of lattice mismatched layers, namely the Stranski–Krastanov (S–K) formation of islands, must be included in any realistic growth model. By considering all the phenomena together, including the coupling between them, it becomes clear that several separate mechanisms exist for phase separation. The focus of this paper is the analysis of the thin (2–3 nm), coherent InGaN layers used in the quantum well structures used for virtually all LEDs and lasers produced by the S–K mechanism. By considering these coupled phenomena together it is possible to arrive at a coherent interpretation of the various materials properties measured using techniques such as high resolution transmission electron microscopy, X-ray diffraction, and optical techniques as well as the device characteristics.

Elimination of phase separation in metalorganic chemical vapor deposition-grown GaInP epilayers by compositionally step-graded Ga1−xInxP multilayers

In this paper, we report on the influence of compositionally step-graded Ga1−xInxP multilayers on the microstructural and optical properties of In-rich Ga0.46In0.54P film grown on a GaAs substrate. Based on the transmission electron microscope observation, the growth of Ga0.46In0.54P on GaAs was found to result in phase separation, which was due to the strain-induced composition pulling effect. This phase separation could be successfully eliminated by the step-graded Ga1−xInxP multilayers with optimized thickness. The elimination was caused by the sufficient moderation of compressive strain in the subsequently grown Ga0.46In0.54P film. The employment of step-graded Ga1−xInxP multilayers was also helpful in improving compositional uniformity and photoluminescence property of the subsequently grown film. The compositional dependence of the film structure and the ab initio elastic constants were used to show that Ga1−xInxP multilayers with a systematic increase in the In-rich compositional regime exhibit epitaxial stability.

Method for Theoretical Prediction of Indium Composition in Coherently Grown InGaN Thin Films

InxGa1-xN has attracted considerable interest as a material for multi junction solar cells. In this study, we performed thermodynamic analyses to calculate the relationship between the input In molar ratio and solid composition of a coherently grown InxGa1-xN thin film that is subjected to planar compressive or tensile stress. The theoretical approach incorporates energy loss of a thin-film system due to lattice constraint from the substrate. The results show that the indium composition x of coherently grown InxGa1-xN is lower than that of stress-free InGaN. This represents the composition pulling effect.

Growth of AlGaN/GaN/AlGaN double heterojunction field-effect transistors and the observation of a compositional pulling effect

Field-effect transistors (FETs) were grown on both GaN and AlGaN buffers. X-ray reciprocal space mapping and ω-2θ scans showed that the AlGaN barriers grown on these two buffers had different Al compositions and growth rates, which was attributed to the compositional pulling effect. AlGaN/GaN/AlGaN double heterojunction FETs exhibited lower output conductance and better pinch-off due to the improved electron confinement resulting from the increase in the effective back-side barrier height. Thus, this device is promising for highly scaled transistors. This device also demonstrated a state-of-the-art power added efficiency of 53.5% and an associated power gain of 9.1 dB at a drain bias of 20 V at 30 GHz.

Monolithic InGaN-based white light-emitting diodes with blue, green, and amber emissions

We demonstrated a monolithic white light-emitting diodes (LEDs) epitaxial structure with blue, green, and amber emissions by introducing the blue InGaN∕GaN five quantum wells (QWs) and InGaN∕GaN single quantum well (SQW) with In-phase separated green/amber emissions as an active layer. Three wavelength emissions were developed by increasing the thickness of InGaN SQW grown on blue InGaN five QWs. From high resolution transmission electron microscope, In-phase separation was clearly observed in a 3.5-nm-thick InGaN SQW. In-phase separation would be generated by the spinodal decomposition which was promoted by the composition pulling effect related to the increment of well thickness. Therefore, white lighting LEDs with three wavelengths for blue emission from InGaN∕GaN five QWs and green/amber emissions were achieved by the In-phase separation in InGaN SQW.

Dependence of In mole fraction in InGaN on GaN facets

AbstractInGaN/GaN stripe structures and hexagonal pyramid structures were grown on sapphire (0001) by low‐pressure metalorganic vapor phase epitaxy (LP‐MOVPE). It was observed that the indium composition at (0001) was greater than that at {11$ \bar 2 $2}. For InGaN/GaN stripe and pyramid structures with {10$ \bar 1 $1}, the thickness, CL peak wavelength and CL intensity gradually increased from the bottom to the top of the facet. It is considered that this behavior on the facet is due to the indium diffusion mechanism or indium composition pulling effect. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Deep level centers in InGaN/GaN heterostructure grown on sapphire and free-standing GaN

Deep level transient spectroscopy has been used to characterize the deep levels in InGaN/GaN grown on sapphire substrate as well as on free-standing GaN. The deep levels at E c − E t ∼ 0.17–0.23 eV and E c − E t ∼ 0.58–0.62 eV have been detected in our samples which are present in GaN samples reported by others. These two deep levels have been attributed by us to threading dislocations as they exhibit logarithmic capture kinetic behavior and are found to be substantially reduced in its trap concentration (∼ from 10 14 to 10 12 cm − 2 ) in GaN grown on free-standing GaN template. Other than the two deep levels, an additional level at E c − E t ∼ 0.40–0.42 eV has been identified in both samples, which is believed to be related to In segregation. AFM image shows region of pits formation in InGaN epilayer for sample grown on u-GaN using sapphire substrate while the latter gives a much smoother morphology. From the X-ray diffraction space mapping, the mosaicity of the sample structure for both samples were studied. Dislocations do not play a significant role in the structural properties of InGaN grown on free-standing GaN since the FWHM based on the Δ ω is relatively small (± 0.15°) in the case of InGaN/GaN on free-standing GaN substrate as compared to that on sapphire (± 0.35°). The wider spread in Δ ω–2 θ value for InGaN layer on free-standing GaN also suggested the effect of compositional pulling with increasing InGaN layer thickness.

Direct evidence of compositional pulling effect in Al xGa 1−xN epilayers

We report the investigation on the compositional-pulling effect of crystalline Al x Ga 1− x N/(0 0 0 1) sapphire epilayers by cross-sectional cathodoluminescence (CL) and energy-dispersive X-ray (EDX) analyses. Direct evidence for the compositional distribution was found in the Al x Ga 1− x N (0< x<0.3) layers, which accompany the different optical phenomena arising from the change of the band gap. The band gap variation of the 1.7 μm Al x Ga 1− x N epilayers with the Al content rising from x=11% to 30% was observed. It is consistent with the result of depth-resolved CL spectra acquired with increasing electron energies, which can also be used to reveal the spatial composition distribution along the growth direction. We emphasize here that the selective-wavelength CL image is a very powerful tool in exploring the correlation between the specific emission and the composition of inhomogeneous alloys.

Influence of composition pulling effect on the two-dimensional electron gas formed at AlyInxGa1−x−yN∕GaN interface

The piezoelectric field generated at the AlyInxGa1−x−yN∕GaN interface creates a two-dimensional electron gas (2DEG). The maximum 2DEG mobility in AlyInxGa1−x−yN∕GaN, for a nominal composition of x=0.01 and y=0.07, is obtained for the 40-nm-thick quaternary epilayer. With further increase in the AlyInxGa1−x−yN thickness, the mobility drops due to the generation of V defects and misfit dislocations. The 2DEG carrier concentration increases with the thickness due to the composition pulling effect observed during growth where the Al content increases toward the surface. This effect coupled with the stronger piezoelectric field results in a significant band bending and a deeper potential well is created for the sample with a thicker AlInGaN epilayer. The integrated intensity ratio of the photoluminiscence emission from the 2DEG and the AlInGaN band-edge emission, I2DEG∕IBE, is studied as a function of temperature for AlInGaN epilayers of different thickness. The effect of strain relaxation and V-pit formation on the 2DEG confinement in AlInGaN∕GaN is also discussed.

Inverted hexagonal pits formation in AlInGaN epilayer

The AlInGaN alloy has emerged as the material for potential use in UV light emitting diodes and lasers due to its higher emission quantum efficiency. However, surface defects like hexagonal pits appear on the surface of this quaternary alloy and formation of these pits can degrade the device performance. In this study, we have observed inverted hexagonal pits formation in AlInGaN epilayers grown on sapphire. The pits depth and diameter are found to increase with the epilayer thickness and pits agglomeration occurs when epilayer thickness is higher than 40 nm. Cross-sectional atomic force microscopy images of the inverted hexagonal pits show that these pits are truncated. This is probably due to the filling of pits associated with composition pulling effect in AlInGaN epilayers. An enhanced yellow luminescence emission is also detected from these inverted hexagonal pits. This is probably due to impurity decoration along the threading dislocations, which are terminated at the vertex of the pits. These impurities form deep levels and serve as effective transition levels for yellow luminescence emission.

Characterization of AlInGaN quaternary epilayers grown by metal organic chemical vapor deposition

Two sets of AlInGaN quaternary epilayers grown by metal organic chemical vapor deposition (MOCVD) were studied by atomic force microscopy (AFM), high-resolution X-ray diffraction (HR-XRD) and photoluminescence (PL) spectroscopy. In all samples a large number of pits have been observed on the surface with the density of 1×10 9 cm −2, and hexagonal opening. In one set with different thickness, it was also found that the XRD angle separation between GaN and AlInGaN diffraction peaks increased with the thickness. In PL measurements, a blue shift of the PL peak from AlInGaN was found as the thickness increased. The origin of these observations was proposed to be the Al composition pulling effect. In the other set with different Al/In ratios, domain features were observed on the surface of AlInGaN epilayers with high Al/In ratio. The surface roughness increased as Al/In ratio decreased. PL emission from AlInGaN became stronger as the Al/In ratio approached the lattice-match condition. When the Al/In ratio was below that of lattice-match condition, the emission peak was enhanced and broadened dramatically at the same time, indicating the effect of indium clustering in AlInGaN epilayers.

Probing the formation of two-dimensional electron gas in AlInGaN/ GaN heterostructures by photoluminescence spectroscopy

ABSTRACTUsing temperature-dependent photoluminescence (PL), we report a detailed study on the optical transitions in AlyInxGa1−x−yN (0.01≤ × ≤ 0.023, 0.07 ≤ y ≤ 0.14) of variable thickness (20 – 100 nm) grown on GaN by metalorganic chemical vapor deposition (MOCVD). At 100 K, highest electron mobility has been obtained for samples with 40 nm thick AlInGaN epilayer and this is due to the contribution from the two-dimensional electron gas (2DEG) in the confined two dimensional potential well. In literature, such 2DEG phenomenon is not discussed for AlInGaN quaternary alloys. In our samples, we have clearly observed such effects from low-temperature PL spectroscopy for AlInGaN epilayer of thickness ≥ 40 nm. The PL peaks observed due to the interband transitions from 2DEG sub-bands to the valence band are in the range 3.55 – 3.68 eV for the sample with an epilayer thickness of 100 nm. Due to the composition pulling effect in this alloy, there is a higher incorporation of Al towards the surface for thicker AlInGaN epilayer, which generates a stronger piezoelectric field and a deeper triangular potential for electron confinement. This is evident from the observation of higher intensity ratio for 2DEG transition compared to band-edge transitions, I2DEG/IBE in sample with thicker AlInGaN epilayer at higher temperature. The intensity ratio, I2DEG/IBE however decreases subsequently for all the samples with further increase in temperature due to thermal excitation.