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Strain relaxation of Al Ga1−N epitaxial layers on GaN and SiC substrates
Abstract
In this article, we present a study on strain relaxation in AlxGa1−xN layers grown by molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) on the bulk GaN single crystals (lattice mismatch of –2.5% to 0%) and on the 6H SiC crystals (lattice mismatch of 1% to 3.5%). Both synchrotron and X-ray tube radiation were used for diffractometry (double- and triple-axis) and reflectivity measurements. We found that the layers on SiC relax easier than those grown on GaN. The results show that growth of AlxGa1−xN layers on GaN substrates may provide a method for producing fully strained (i.e., unrelaxed) blue-laser structures.
... AlN PVT seeded grown bulk 3.1117(5) 4.98180(2) [56] Free standing HVPE-AlN grown on PVT-AlN substrate 3.1111 4.9808 [57] Epitaxial layer on SiC 3.110 4.980 [58] First-principle-LDA 3.0698 4.9101 [59] First-principle-CGA 3.1095 4.9938 [59] Undoped Bulk Crystal (n=1x10 16 [60] bulk crystal (n=low 10 17 ) -HVPE 3.1880 (20) 5.1868 (15) [61] Non-polar (1100) bulk (n=2x10 19 ) -Ammonothermal 3.18908(10) 5.18517(10) [62] m-plan bulk crystal -Ammonothermal 3.1897 5.1861 [63] GaN substrate grown using a lateral epitaxial overgrowth (LEO) technique 3.1896(2) 5.1855(2) [64] Homoepitaxial layers (low free-electron concentration) ...
Optical, electrical and mechanical properties of group III nitrides, including of AlN, GaN, InN and their ternary and quaternary compounds are discussed. The driving force for semiconductor nitrides is device applications for emitters and detectors in the visible and ultraviolet (UV) portions of the optical spectrum and high-power amplifiers. Further advances in electronic and optoelectronic devices, which are imperative, require better understanding and precise measurements of the mechanical, thermal, electrical and optical properties of nitride semiconductors. Information available in the literature regarding many of the physical properties of nitrides, especially AlN and InN, is still in the process of evolution, and naturally in the subject of some controversy. This is, in part, a consequence of measurements having been performed on samples of widely varying quality. When possible, these spurious discrepancies have been disregarded. For other materials, too few measurements are available to yield a consensus, in which case the available data are simply reported. The aim of this work is to present the latest available data obtained by various experimental observations and theoretical calculations.
Optical, electrical, mechanical, and thermal properties of group III nitrides, inclusive of AlN, GaN, InN and their ternary and quaternary alloys, are discussed. The driving force for group III nitride semiconductors is their important applications in optoelectronics, microwave amplifiers, and high voltage power switches. Owing to the aforementioned applications, the fundamental properties of each III nitride binary, as well as the alloys that have been acquired, are discussed in this chapter. In general, an appropriate assessment of the material properties for any material is not straightforward to begin with and the nitride family is no exception, particularly considering that group III nitrides are prepared on foreign substrates as low-cost native substrates are not yet available. Understandably, precise measurements of the mechanical, thermal, electrical and optical properties of the semiconductor nitride family are imperative for further advances. Notwithstanding the great progress that has already been made to further understand and exploit group III nitrides, especially GaN, reliable data for AlN and InN are still in the state of evolution, and naturally the subject of some controversy. This is, in part, a consequence of measurements having been performed on samples of widely varying quality. When possible, the spurious discrepancies have been disregarded. For some materials, too few measurements are available to yield a consensus, in which case the available data are simply reported. Defects in group III nitrides as well as GaN-based nanostructures are also discussed.
The three volumes of this handbook treat the fundamentals, technology and nanotechnology of nitride semiconductors with an extraordinary clarity and depth. They present all the necessary basics of semiconductor and device physics and engineering together with an extensive reference section. Volume 1 deals with the properties and growth of GaN. The deposition methods considered are: hydride VPE, organometallic CVD, MBE, and liquid/high pressure growth. Additionally, extended defects and their electrical nature, point defects, and doping are reviewed.
Porous silicon carbide substrates produced by anodization that are intended for use in chemical sensing solid state devices that will operate at high temperatures (at or above 900°C) have been studied with a view to assessing the structural stability/pore modification processes occurring during thermal processing. Annealing experiments were performed using n-type porous 4H-SiC (PSiC) in the temperature range of 900°C-1700°C in Ar atmosphere. The samples were kept at the desired temperatures for 30 minutes. The morphology of the as-received samples consisted of well-oriented pores along the 〈041〉 direction. Significant pore modification occurred at 1400°C. The thickness of the porous layer was reduced following heating at 1700°C. The suitability of PSiC as a chemical sensing element is discussed.
In this paper, the p-i-n multiple-layer Al_(x)Ga_(1-x)N sample with high Al (x>0.45) content was measured by the triple-axis X-ray diffraction measurement. The strain state and screw dislocation density of each layer in Al_(x)Ga_(1-x)N epitaxial layers were determined by RSM (reciprocal space map) method. Then, the PV function was used to fit the rock curves separated from the RSM. At last, the strain and the screw dislocation density of each layer were accurately calculated by fitting these rock curves.
Laser diodes with InGaN quantum wells emitting at long wavelengths are required for the application in compact laser projection. However, quantum wells with indium contents higher than 20% show weak photoluminescence performance as a result of high defect densities. We analyzed the root causes of the low performance of such quantum wells in detail. The influence of the indium content, the annealing temperature and the barrier structure on the quantum-well stability was investigated. We found that quantum wells with indium contents higher than 20% degrade due to the diffusion of indium atoms. Vertical diffusion coefficients for different barrier materials are extracted.
Lattice parameters of gallium nitride were measured using high‐resolution x‐ray diffraction. The following samples were examined: (i) single crystals grown at pressure of about 15 kbar, (ii) homoepitaxial layers, (iii) heteroepitaxial layers (wurtzite structure) on silicon carbide, on sapphire, and on gallium arsenide, (iv) cubic gallium nitride layers on gallium arsenide. The differences between the samples are discussed in terms of their concentrations of free electrons and structural defects. © 1996 American Institute of Physics.
Synchrotron x‐ray diffraction was employed to measure the lattice constants a and c of GaN films grown with an AlN buffer layer on sapphire (0001) over a thickness range of 50 Å to 1 μm. We used multiple reflections and a least‐squares fit method for high reliability. As the thickness increased, the lattice constant a increased from 3.133 Å to 3.196 Å and c decreased from 5.226 Å to 5.183 Å. The expected trend was fitted to an equilibrium theory, allowing the critical thickness of GaN on AlN to be estimated at 29 Å ± 4 Å in good agreement with a theoretical prediction. © 1996 American Institute of Physics.
Photoluminescence (PL), Raman spectroscopy, and x-ray diffraction are employed to demonstrate the coexistence of a biaxial and a hydrostatic strain that can be present in GaN thin films. The biaxial strain originates from growth on lattice-mismatched substrates and from post-growth cooling. An additional hydrostatic strain is shown to be introduced by the presence of point defects. A consistent description of the experimental results is derived within the limits of the linear and isotropic elastic theory using a Poisson ratio {nu}=0.23{plus_minus}0.06 and a bulk modulus {ital B}=200{plus_minus}20 GPa. These isotropic elastic constants help to judge the validity of published anisotropic elastic constants that vary greatly. Calibration constants for strain-induced shifts of the near-band-edge PL lines with respect to the {ital E}{sub 2} Raman mode are given for strain-free, biaxially strained, and hydrostatically contracted or expanded thin films. They allow us to extract differences between hydrostatic and biaxial stress components if present. In particular, we determine that a biaxial stress of one GPa would shift the near-band-edge PL lines by 27{plus_minus}2 meV and the {ital E}{sub 2} Raman mode by 4.2{plus_minus}0.3 cm{sup {minus}1} by use of the listed isotropic elastic constants. It is expected from the analyses that stoichiometric variations in the GaN thin films together with the design of specific buffer layers can be utilized to strain engineer the material to an extent that greatly exceeds the possibilities known from other semiconductor systems because of the largely different covalent radii of the Ga and the N atom. {copyright} {ital 1996 The American Physical Society.}
AlN single crystal is one of the promising materials for substrates of GaN-based laser diodes. We prepared aluminum nitride single crystals by the sublimation method and characterized them. The crystals are transparent and slightly yellow. Some crystals are needle-shaped with a hexagonal cross section, diameter of 0.5 mm and length of 3 mm, grown parallel to |001|. Other crystals are plate-shaped with a maximum width of 3 mm, 5 mm length and 0.5 mm thickness, grown with a large (001) face. Also, other crystals are needle-shaped with a rectangular cross section, width of 1 mm, 7 mm length and 0.3 mm thickness, grown with a large (101) face. Their widths of X-ray rocking curves is about 39 arcsec. with a full width at half-maximum. 203 arcsec and 12 arcsec, respectively. The orientation of AlN single crystal axis is sufficient for use in substrates for GaN-based diodes.
The composition of alloys in strained ternary alloy layers, AlxGa1-xN (0 < x < 0.25) and Ga1-xInxN (0 < x < 0.20): on thick GaN was precisely determined using the high-resolution X-ray diffraction profile. The band gap of strained AlGaN is found to increase almost linearly according to the AlN molar fraction, while that of strained GaInN has a large bowing parameter of 3.2 eV.
Misfit stress in InGaAs/InP heteroepitaxial structures grown by hydride‐transport vapor phase epitaxy has been studied using an x‐ray technique to measure the wafer curvature and using an x‐ray double crystal diffractometer to measure mismatches. The stress calculated from the measured radius of curvature was, in all cases, found to be smaller than the value predicted from the lattice mismatch using the simple beam theory and assuming a coherent interface. A simple argument is used to show that the parallel mismatch is directly related to the density of misfit dislocations. By taking into account both the presence of misfit dislocations and tetragonal distortion, relationships between the curvature, density of interfacial misfit dislocations, relaxed lattice mismatch, and measured tetragonally distorted vertical and parallel mismatches are derived. Predicted values for the radius of curvature agree very well with measured values. The results also indicate that in the presence of interfacial misfit dislocations the nature of the stress in the epitaxial layer is not uniquely determined by the mismatch. The stress can change sign depending upon the density of the misfit dislocations. This effect has actually been observed. The defect structure of the epitaxial layer is analyzed by Nomarski interference contrast microscopy, transmission x‐ray topography, and transmission electron microscopy. We have found that the misfit stress can be greatly relaxed in the presence of lattice defects such as stacking faults. The theoretical calculation is further generalized from the single epilayer/substrate case to an n‐layer heterostructure. The determination of the stresses in each layer and the misfit dislocation densities are discussed.
Selectively doped Ge x Si 1 - x /Si strained layer heterostructures have been grown in a single quantum well configuration on 〈001〉‐Si substrate using molecular beam epitaxy. The modulation doping effect has been observed in p‐type structures only; although both n‐ and p‐type double heterostructures were grown. We have investigated the effects of: (i) alloy layer thickness (well width), (ii) doping setback, and (iii) cladding layer dopant concentration, on the hole mobilities. At present, optimum structures show peak hole mobilities in excess of 3300 cm<sup>2</sup>V<sup>-</sup><sup>1</sup>s<sup>-</sup><sup>1</sup> at 4.2 K, sheet charge densities of 3.0–14×10<sup>1</sup><sup>1</sup> cm<sup>-</sup><sup>2</sup>, and hole effective mass m<sup> </sup> h =0.32±0.03 m 0 . It is found that the low temperature (T∼10 K) hole mobility, in structure having x=0.2 is relatively insensitive to alloy layer (well) thickness in the range 100 Å≲L w ≲L c ; where L c is the critical thickness marking the transition from strained layer to relaxed, misfit accommodated, alloy growth. For L w ≫L c , misfit dislocation scattering dramatically decreases μ h at T≲100 K; whereas for L w ≲100 Å the hole mobility is uniformly lowered over the entire range of measurement temperatures. Doping setback and cladding layer dopant concentration affect the mobility via changes in sheet charge density, in a manner consistent with theoretical expectations. The absence of the modulation doping effect in n‐type heterostructure is taken as an indication that ΔE v ≫ΔE c .
GaN and Al <sub>x</sub> Ga <sub>1-x</sub> N alloys were grown by gas source molecular beam epitaxy using NH <sub>3</sub>. High quality GaN layers with smooth surfaces being obtained, reflection high-energy electron diffraction (RHEED) can be used to monitor the growth. The oscillations of the specular beam intensity indicate a layer-by-layer growth which allows one to precisely measure the deposition rate and the composition of Al <sub>x</sub> Ga <sub>1-x</sub> N alloys. The transition from two dimensional nucleation to step flow growth mode when increasing the substrate temperature is also evidenced. Finally, RHEED is used to investigate the relaxation processes which take place during the growth of AlN on GaN and GaN on AlN. © 1997 American Institute of Physics.
Candela‐class high‐brightness InGaN/AlGaN double‐heterostructure (DH) blue‐light‐emitting diodes (LEDs) with the luminous intensity over 1 cd were fabricated. As an active layer, a Zn‐doped InGaN layer was used for the DH LEDs. The typical output power was 1500 μW and the external quantum efficiency was as high as 2.7% at a forward current of 20 mA at room temperature. The peak wavelength and the full width at half‐maximum of the electroluminescence were 450 and 70 nm, respectively. This value of luminous intensity was the highest ever reported for blue LEDs.
La théorie présentée permet d'obtenir une formulation explicite de l'influence des rugosités ainsi que des variations locales de constante diélectrique n2 (dues par exemple à une modification de composition ou de compacité) sur la réflexion rasante d'un faisceau de rayons X monochromatique, dans la mesure où les rugosités relèvent d'une distribution gaussienne et à condition que n2 ne dépende que de la profondeur Z par rapport au plan moyen de la surface éclairée. L'analyse des verres silicatés polis mécaniquement sur polissoir en poix, à l'aide de suspensions aqueuses d'oxydes divers, révèle que la couche de polissage se compose en réalité de deux zones bien distinctes. La première, tout à fait superficielle, l'épaisseur ne dépassant pas quelques dizaines d'angströms, présente une densité toujours inférieure à celle du coeur de l'échantillon et semble imputable au fluage plastique et à l'hydrolyse de la surface pendant le polissage. La seconde, sous-jacente, s'étend au contraire sur plusieurs centaines d'angströms et met en jeu un processus soit de densification (silice pure, alumino-silicate) soit de lacunisation (verres à assez forte teneur en ions alcalins). Nous examinons également l'influence de la durée du polissage, du type d'oxyde utilisé, et (ou) des traitements thermiques effectués après polissage, sur les divers paramètres qui caractérisent ces couches.
The x-ray rocking-curve broadening accompanying the transfer to and from the metastable state of EL2 and DX centers in GaAs and AlxGa1-xAs has recently been observed experimentally [Leszczynski et al., Semicond. Sci. Technol. 6, B66 (1991)]. This paper gives a more quantitative analysis of the experimental results. Computer simulations of rocking curves based on the dynamical theory of x-ray diffraction for various models of the real crystal structure made it possible to evaluate the conditions in which the lattice relaxation could be observed in experiment. The general conclusion is that in all the materials examined, the inhomogeneities played a decisive role. The possible range of the inhomogeneities and the strains around EL2 and DX centers is discussed in relation to their microscopic models.
Amano, I. Akasaki, fact that the latter ones are under tensile stress. The main
- T Takeuchi
- H Takeuchi
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T. Takeuchi, H. Takeuchi, S. Sota, H. Sakai, H. Amano, I. Akasaki, fact that the latter ones are under tensile stress. The main Jpn. J. Appl. Phys. 36 (1997) L177. `
) 17046. The work was partially supported by the Grant of Polish
- M Leszczynski
M. Leszczynski, Phys. Rev. B 48 (1993) 17046. The work was partially supported by the Grant of Polish
J. message of the article is that the AlGaN layers (used in the Crystal Growth, to be published. blue-laser structures) grown on GaN single crystals of GaN
- G Feuillet
- B Daudin
- F Widmann
- J L Rouviere
- M Arlery
G. Feuillet, B. Daudin, F. Widmann, J.L. Rouviere and M. Arlery, J. message of the article is that the AlGaN layers (used in the Crystal Growth, to be published. blue-laser structures) grown on GaN single crystals of GaN
Committee for Scientific Research (KBN) no PBZ 28
- M Tanaka
- S Nakahata
- K Sogabe
- H Nakata
- M Tabioka
- J Jpn
M. Tanaka, S. Nakahata, K. Sogabe, H. Nakata, M. Tabioka, Jpn. J. Committee for Scientific Research (KBN) no PBZ 28.11 / Appl. Phys. 36 (1997) L1062. P3. ML is indebted to the French Ministry of Education for