Crack Nucleation in AlGaN/GaN Heterostructures

Parbrook, P. J.; Whitehead, M. A.; Lynch, Richard J.M.; Murray, R.

doi:10.1557/proc-743-l7.10

Cited by 4 publications

(5 citation statements)

References 9 publications

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“…2. Unlike the situation for GaN layers and low Al content AlGaN layers [5], where approximately circular pits are observed where the threading dislocations with a screw Burgers component appear to the surface, here, even for the thinnest sample examined, of 10 nm, the pits show signs of extension. Both the 20 nm and 30 nm samples show clear signs of microcracking with thin line features extending in triangular directions.…”

Section: Resultscontrasting

confidence: 66%

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Crack formation and development in AlGaN/GaN structures

Parbrook

Wang

Whitehead

et al. 2003

phys. stat. sol. (c)

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In the absence of the use of prevention methods, AlGaN layers on GaN are known to form an array of cracks if a critical thickness is exceeded. In this study growth of AlGaN-GaN structures was carried out by metalorganic vapour phase epitaxy using sapphire substrates. Under optical and atomic force microscopy two distinct crack populations have been obseved. In thin, highly strained films an initial high density population of microcracks are found propagating from threading dislocations. This crack array extends as the thickness increases, before contracting with the onset of large cracks whick extend over many mm across the wafer. The formation of the microcrack array is very dependent on the stain in the epilayer and does not follow the classic critical thickness dependence with lattice mismatch. Avoiding stresses high enough to prevent the microcracking phenomenon may be critical in the use of highly tensile strained layers in nitride devices.1 Introduction AlGaN epilayers are critical to the formation of many III-nitride devices. These include laser diodes (LDs), heterojunction field effect transistors (HFETs), ultra-violet emitters, solar blind photodetectors and also intersubband devices. However, the lattice mismatch between AlN and GaN means that AlGaN layers grown on GaN experience a high degree of tensile stress which can lead to the formation of an array cracks over the sample surface [1][2][3]. These cracks, observable in an optical microscope on a ~ mm scale, have been reported to have a potentially dual role in the strain relaxation, through the formation of misfit dislocation on the basal plane [2] in addition to the relaxation caused by the crack itself. Such crack networks can be avoided through the use of a low temperature interlayer of AlN at the AlGaN-GaN interface for example [4], but these methods are not suitable in all cases and can cause an increased threading dislocation density.The above studies have discussed the propagation and relaxation induced by cracks, but none have reported on their nucleation or on the microscopic structural properties for layers below which the gross cracks have been observed. In a previous report [5] we have suggested that the gross cracks may be formed as a result of "microcracks" observed at much lower thicknesses. These microcracks are short, sub-µm in length an hence would not be observable by optical microscopy for example. In this paper evidence is shown that this microcracking phenomenon in thin highly strained AlGaN layers grown on GaN can occur in very thin layers indeed, and that their density rapidly decreases with the onset of "gross" crack formation.

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Section: Resultscontrasting

confidence: 66%

“…In a previous report [5] we have suggested that the gross cracks may be formed as a result of "microcracks" observed at much lower thicknesses. These microcracks are short, sub-µm in length an hence would not be observable by optical microscopy for example.…”

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confidence: 87%

Crack formation and development in AlGaN/GaN structures

Parbrook

Wang

Whitehead

et al. 2003

phys. stat. sol. (c)

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“…This is therefore not contradictory to the findings from the PL and epilayer thickness measurement (figures 3 and 4). Finally, cracks may have nucleated from some layers in sample 1 which were subjected to a severe tensile stress [12]. Therefore, it is critical to locate these highly-stressed nitride layers.…”

Section: Resultsmentioning

confidence: 99%

The management of stress in MOCVD-grown InGaN/GaN LED multilayer structures on Si(1 1 1) substrates

Jiang

Allsopp

Bowen

et al. 2013

Semicond. Sci. Technol.

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The tensile stress in light-emitting diode (LED)-on-Si(1 1 1) multilayer structures must be reduced so that it does not compromise the multiple quantum well emission wavelength uniformity and structural stability. In this paper it is shown for non-optimized LED structures grown on Si(1 1 1) substrates that both emission wavelength uniformity and structural stability can be achieved within the same growth process. In order to gain a deeper understanding of the stress distribution within such a structure, cross-sectional Raman and photo-luminescence spectroscopy techniques were developed. It is observed that for a Si:GaN layer grown on a low-temperature (LT) AlN intermediate layer there is a decrease in compressive stress with increasing Si:GaN layer thickness during MOCVD growth which leads to a high level of tensile stress in the upper part of the layer. This may lead to the development of cracks during cooling to room temperature. Such a phenomenon may be associated with annihilation of defects such as dislocations. Therefore, a reduction of dislocation intensity should take place at the early stage of GaN growth on an AlN or AlGaN layer in order to reduce a build up of tensile stress with thickness. Furthermore, it is also shown that a prolonged three dimensional GaN island growth on a LT AlN interlayer for the reduction of dislocations may result in a reduction in the compressive stress in the resulting GaN layer.

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“…GaN‐based materials can experience desorption under certain conditions , which is compatible with fissure formation emanating from dislocation cores. Electron channelling contrast imaging (ECCI) measurements indicate the nanoscale fissure formation during the MOVPE cooling stage is related to threading dislocations , which are known to act as stress centres .…”

Section: Introductionmentioning

confidence: 99%

“…Beyond this, strain relaxation diminishes the piezoelectric polarisation component reducing the 2DEG carrier density , and suppresses mobility through the introduction of scattering from defect centres . Redistribution of strain has been reported to manifest as features on the wafer surface on the micro‐ and nanoscale . Nanoscale fissures may occur during MOVPE, particularly during cooling to room temperature from growth temperatures of greater than 1000 °C.…”

Section: Introductionmentioning

confidence: 99%

Nanoscale fissure formation in Al_xGa_1–xN/GaN heterostructures and their influence on Ohmic contact formation

Smith¹,

Thomson

Zubialevich³

et al. 2016

Physica Status Solidi (a)

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Nanoscale surface fissures on AlxGa1–xN/GaN (15 nm/1 µm) heterostructures grown by metalorganic vapour phase epitaxy (MOVPE) were imaged using tapping‐mode atomic force microscopy (AFM) and electron channelling contrast imaging (ECCI). Fissure formation was linked to threading dislocations, and was only observed in samples cooled under H2 and NH3, developing with increasing barrier layer Al content. No strain relaxation was detected regardless of fissure formation up to barrier layer Al composition fractions of x = 0.37. A reduction of measured channel carrier density was found in fissured samples at low temperature. This instability is attributed to shallow trap formation associated with fissure boundaries. For Ti/Al/Ni/Au Ohmic contact formation to high Al content barrier layers, fissures were found to offer conduction routes to the 2DEG that allow for low resistance contacts, with fissure‐free samples requiring additional optimisation of the metal stack and anneal conditions to achieve contact resistivity of order those measured in fissured samples. In addition, the effects of fissures were found to be detrimental to thermal stability of sheet and contact resistance, suggesting that fissure formation compromises the integrity of the 2DEG.

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Crack Nucleation in AlGaN/GaN Heterostructures

Cited by 4 publications

References 9 publications

Crack formation and development in AlGaN/GaN structures

Crack formation and development in AlGaN/GaN structures

The management of stress in MOCVD-grown InGaN/GaN LED multilayer structures on Si(1 1 1) substrates

Nanoscale fissure formation in Al_xGa_1–xN/GaN heterostructures and their influence on Ohmic contact formation

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Crack Nucleation in AlGaN/GaN Heterostructures

Cited by 4 publications

References 9 publications

Crack formation and development in AlGaN/GaN structures

Crack formation and development in AlGaN/GaN structures

The management of stress in MOCVD-grown InGaN/GaN LED multilayer structures on Si(1 1 1) substrates

Nanoscale fissure formation in AlxGa1–xN/GaN heterostructures and their influence on Ohmic contact formation

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Nanoscale fissure formation in Al_xGa_1–xN/GaN heterostructures and their influence on Ohmic contact formation