Dislocation mediated surface morphology of GaN
22 Apr 1999-Journal of Applied Physics (American Institute of Physics)-Vol. 85, Iss: 9, pp 6470-6476
TL;DR: In this article, the surface morphology of GaN films grown by metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) was studied using atomic force microscopy (AFM).
Abstract: The surfaces of GaN films grown by metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) were studied using atomic force microscopy (AFM). Due to the high dislocation densities in the films (108 cm−2), the typical surface morphologies of layers grown by both techniques were dominated by three dislocation mediated surface structures—pinned steps, spiral hillocks, and surface depressions. The characteristics of these surface structures were found to depend on growth technique (MOCVD vs MBE) and the group-III to group-V ratio used in the growth of MBE GaN films. Pinned steps, created by the intersections of mixed character dislocations with the free surface, were found on all GaN films. The pinned steps were observed to be predominantly straight on the MOCVD GaN and curved into spiral hillock formations on the MBE GaN. Spiral growth hillocks form when pinned steps grow outward and around the dislocation under step-flow growth conditions. The tightness of the spiral hillocks on MBE G...
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TL;DR: The surface morphologies of GaN grown by plasma-assisted molecular beam epitaxy under various growth conditions have been investigated in this article, where three growth regimes (one N stable and two Ga stable) are identified on a surface structure diagram (Ga/N ratio versus substrate temperature).
Abstract: The characteristic surface morphologies of GaN grown by plasma-assisted molecular beam epitaxy under various growth conditions have been investigated. Three growth regimes (one N stable and two Ga stable) are identified on a surface structure diagram (Ga/N ratio versus substrate temperature). The boundary between the N-stable regime (low Ga/N ratios) and the two Ga-stable regimes (high Ga/N ratios) is determined by the growth rate of the films and is constant over the range of substrate temperatures investigated. The boundary between the two Ga-stable regimes (the Ga-droplet regime and the intermediate regime) is determined by the formation of Ga droplets and has an Arrhenius dependence with substrate temperature. The characteristic morphologies of films grown within each of these regimes are investigated using atomic force microscopy and transmission electron microscopy. N-stable films have rough, heavily pitted morphologies. Films grown within the intermediate phase have areas of flat surface between la...
470 citations
TL;DR: In this article, a 2.6-μm-thick GaN film with a resistivity of 7×109 Ω/sq was attained when the first 0.3 μm of the film was Fe doped.
Abstract: Iron doped GaN layers were grown by metalorganic chemical vapor deposition (MOCVD) using ferrocene as the Fe precursor. Specular films with concentrations up to 1.7×1019 cm−3, as determined by secondary ion mass spectrometry, were grown. The Fe concentration in the film showed a linear dependence on the precursor partial pressure, and was insensitive to growth temperature, pressure, and ammonia partial pressure. Memory effects were observed, similar to Mg doping of GaN by MOCVD. The deep acceptor nature of Fe was used for growth of semi-insulating GaN films on sapphire substrates. A 2.6-μm-thick GaN film with a resistivity of 7×109 Ω/sq was attained when the first 0.3 μm of the film was Fe doped. X-ray diffraction rocking curves indicated high crystalline quality, very similar to an undoped film, showing that Fe doping did not affect the structural properties of the film. Fe doping allows for growth of semi-insulating GaN on sapphire without the high threading dislocation densities and/or high carbon leve...
414 citations
TL;DR: In this article, the authors proposed a two-step ELO (1S-ELO) technology, which significantly reduces the dislocation density to below 107 cm−2.
Abstract: Gallium nitride (GaN) is an extremely promising wide band gap semiconductor material for optoelectronics and high temperature, high power electronics. Actually, GaN is probably the most important semiconductor since silicon. However, achievement of its full potential has still been limited by a dramatic lack of suitable GaN bulk single crystals. GaN has a high melting temperature and a very high decomposition pressure; therefore it cannot be grown using conventional methods used for GaAs or Si like Czochraslski or Bridgman growths.Since there is no GaN bulk single crystal commercially available, all technological development of GaN-based devices relies on heteroepitaxy. Most of the current device structures are grown on sapphire or 6H-SiC. However, since their lattice parameters and thermal expansion coefficients are not well-matched to GaN, the epitaxial growth generates huge densities of defects, with threading dislocations (TDs) being the most prevalent (109–1011 cm−2). As a comparison, homoepitaxially grown GaAs exhibits ~102–104 dislocation cm−2, and homoepitaxial Si almost 0. Actually this large density of TDs in GaN drastically limits the performance and operating lifetime of nitride-based devices. Therefore, there is currently a tremendous technological effort to reduce these defects.Metal organic vapour phase epitaxy (MOVPE) is currently the most widely used technology. Actually, all optoelectronic commercial device structures are fabricated using MOVPE. In MOVPE, the most appropriate precursor for nitrogen is ammonia (NH3), whereas either trimethyl or triethylgallium may be used as a gallium source. MOVPE of GaN requires a high partial pressure of NH3, high growth temperatures (~1000–1100°C) and a growth chamber specially designed to avoid premature reactions between the ammonia and gallium alkyls. Since sapphire (or 6H-SiC) and GaN are highly mismatched, direct growth of GaN is impossible. Therefore, the growth of GaN on any substrate first requires the deposition of a buffer layer, which, to some extent, accommodates the mismatch. Using appropriate nucleation layers allows a reduction of the dislocation density to the low 108 cm−2 range.Though laser diodes (LDs) were demonstrated in the late 1990s with such defect layers, the real breakthrough in laser technology was the dramatic improvement of the LD lifetime at the end of 1997, with the lifetime reaching 10 000 h. This was made possible by implementation of epitaxial lateral overgrowth (ELO) technology, which significantly reduces the dislocation density to below 107 cm−2.In ELO technology, parts of the highly dislocated starting GaN are masked with a dielectric mask, after which growth is restarted. At the beginning of the second growth step, deposition only occurs within the openings, with no deposition observed on the mask. This is referred to as selective area epitaxy (SAE). The TDs are prevented from propagating into the overlayer by the dielectric mask, whereas GaN grown above the opening (coherent growth) keeps the same TD density as the template, at least during the early stages of growth.Currently, two main ELO technologies exist: the simpler one involves a single growth step on striped openings. In this one-step-ELO (1S-ELO), growth in the opening remains in registry with the GaN template underneath (coherent part), whereas the GaN over the mask extends laterally (wings). This leads to two grades, namely highly dislocated GaN, above the openings, and low dislocation density GaN, above the masks. With this technique, devices have to be fabricated on the wings. Conversely, in the two-step-ELO (2S-ELO) process, the growth conditions of the first step are monitored to obtain triangular stripes. Inside these stripes, the TDs arising from the templates are bent by 90° when they encounter the inclined lateral facet. In the second step, the growth conditions are modified to achieve full coalescence. In this 2S-ELO technology, only the coalescence boundaries are defective. ELO technology produces high quality GaN, with TD densities in the mid 106 cm−2, line widths of the low temperature photoluminescence near band gap recombination peaks below 1 meV, and deep electron trap concentration below 1014 cm−3 (compared with mid 1015 cm−3 in standard GaN). Numerous modifications of the ELO process have been proposed either to avoid technological steps (maskless ELO) or to improve it (pendeoepitaxy, PE). To further reduce the TD density, multiple-step-ELO and pendeo have also been implemented.However, even ELO quality GaN is not good enough for the next generation of LDs. ELO samples do not yet offer a full surface suitable for laser technology. What is needed for LDs with at least 30 mW output power is high quality freestanding GaN with TDs close to or even below 106 cm−2. To reach this crystalline perfection, elaborate technologies are currently being implemented. They, at some stage, involve TD reduction mechanisms occurring in the ELO process.Self-supported GaN with at least ELO quality at an affordable cost is believed to be the next breakthrough in GaN technology.
307 citations
TL;DR: In this article, the insertion of AlN/GaN superlattices was found to decrease the stress sufficiently for avoiding crack formation in an overgrown (2.5 μm) GaN layer.
Abstract: The strain in GaN epitaxial layers grown on silicon (111) substrates by metalorganic vapor phase epitaxy has been investigated. The insertion of AlN/GaN superlattices was found to decrease the stress sufficiently for avoiding crack formation in an overgrown thick (2.5 μm) GaN layer. X-ray diffraction and photoluminescence measurements are used to determine the effect of these AlN/GaN superlattices on the strain in the subsequent GaN layers. A reduction of threading dislocation density is also observed by transmission electron microscopy and atomic force microscopy when such superlattices are used. Strong band edge photoluminescence of GaN on Si(111) was observed with a full width at half maximum of the bound exciton line as low as 6 meV at 10 K. The 500 arcsec linewidth on the (002) x-ray rocking curve also attests the high crystalline quality of GaN on Si (111), when using these AlN/GaN superlattices.
275 citations
TL;DR: In this paper, a scanning currentvoltage microscope was used to map the spatial locations of leakage current on high quality GaN films under reverse bias and found that the density of reverse bias leakage spots correlates well with pure screw dislocation density, not with mixed dislocations density.
Abstract: Excess reverse-bias leakage in GaN films grown by molecular beam epitaxy on GaN templates is correlated with the presence of pure screw dislocations. A scanning current–voltage microscope was used to map the spatial locations of leakage current on high quality GaN films under reverse bias. Two samples with similar total dislocation density (∼109 cm−2) but with pure screw dislocation density differing by an order of magnitude were compared. We found that the density of reverse-bias leakage spots correlates well with pure screw dislocation density, not with mixed dislocation density. Thus, pure screw dislocations have a far more detrimental impact on gate leakage than edge or mixed dislocations.
269 citations
References
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12 Jun 1951-Philosophical transactions - Royal Society. Mathematical, physical and engineering sciences
TL;DR: In this paper, it was shown that the rate of growth of a surface containing dislocations is proportional to the square of the supersaturation for low values and to the first power for high values of the latter.
Abstract: Parts I and II deal with the theory of crystal growth, parts III and IV with the form (on the atomic scale) of a crystal surface in equilibrium with the vapour. In part I we calculate the rate of advance of monomolecular steps (i.e. the edges of incomplete monomolecular layers of the crystal) as a function of supersaturation in the vapour and the mean concentration of kinks in the steps. We show that in most cases of growth from the vapour the rate of advance of monomolecular steps will be independent of their crystallographic orientation, so that a growing closed step will be circular. We also find the rate of advance for parallel sequences of steps. In part II we find the resulting rate of growth and the steepness of the growth cones or growth pyramids when the persistence of steps is due to the presence of dislocations. The cases in which several or many dislocations are involved are analysed in some detail; it is shown that they will commonly differ little from the case of a single dislocation. The rate of growth of a surface containing dislocations is shown to be proportional to the square of the supersaturation for low values and to the first power for high values of the latter. Volmer & Schultze’s (1931) observations on the rate of growth of iodine crystals from the vapour can be explained in this way. The application of the same ideas to growth of crystals from solution is briefly discussed. Part III deals with the equilibrium structure of steps, especially the statistics of kinks in steps, as dependent on temperature, binding energy parameters, and crystallographic orientation. The shape and size of a two-dimensional nucleus (i.e. an ‘island* of new monolayer of crystal on a completed layer) in unstable equilibrium with a given supersaturation at a given temperature is obtained, whence a corrected activation energy for two-dimensional nucleation is evaluated. At moderately low supersaturations this is so large that a crystal would have no observable growth rate. For a crystal face containing two screw dislocations of opposite sense, joined by a step, the activation energy is still very large when their distance apart is less than the diameter of the corresponding critical nucleus; but for any greater separation it is zero. Part IV treats as a ‘co-operative phenomenon’ the temperature dependence of the structure of the surface of a perfect crystal, free from steps at absolute zero. It is shown that such a surface remains practically flat (save for single adsorbed molecules and vacant surface sites) until a transition temperature is reached, at which the roughness of the surface increases very rapidly (‘ surface melting ’). Assuming that the molecules in the surface are all in one or other of two levels, the results of Onsager (1944) for two-dimensional ferromagnets can be applied with little change. The transition temperature is of the order of, or higher than, the melting-point for crystal faces with nearest neighbour interactions in both directions (e.g. (100) faces of simple cubic or (111) or (100) faces of face-centred cubic crystals). When the interactions are of second nearest neighbour type in one direction (e.g. (110) faces of s.c. or f.c.c. crystals), the transition temperature is lower and corresponds to a surface melting of second nearest neighbour bonds. The error introduced by the assumed restriction to two available levels is investigated by a generalization of Bethe’s method (1935) to larger numbers of levels. This method gives an anomalous result for the two-level problem. The calculated transition temperature decreases substantially on going from two to three levels, but remains practically the same for larger numbers.
4,432 citations
Book•
11 Aug 1989
TL;DR: The OMVPE process as mentioned in this paper is a process of physical processes occurring on the surface of the Earth, and it has been used in many applications in the computer science community, e.g. superlattice structures.
Abstract: Overview of the OMVPE Process. Thermodynamics. Physical Processes Occurring on the Surface. Source Molecules. Kinetics. Hydrodynamics and Mass Transport. Design of the OMVPE Process. Specific Materials. Superlattice Structures. Devices.
1,164 citations
TL;DR: In this article, the authors demonstrate that the anomalously low (002) x-ray rocking curve widths for epitaxial hexagonal GaN films on (001) sapphire are a result of a specific threading dislocation geometry.
Abstract: In this letter we demonstrate that the anomalously low (002) x‐ray rocking curve widths for epitaxial hexagonal GaN films on (001) sapphire are a result of a specific threading dislocation (TD) geometry. Epitaxial GaN films were grown on c‐plane sapphire by atmospheric pressure metalorganic chemical vapor deposition (MOCVD) in a horizontal flow reactor. Films were grown with (002) rocking curves (ω‐scans) widths as low as 40 arcsec and threading dislocation densities of ∼2×1010 cm−2. The threading dislocations in this film lie parallel to the [001] direction and within the limit of imaging statistics, all are pure edge with Burgers vectors parallel to the film/substrate interface. These TDs will not distort the (002) planes. However, distortion of asymmetric planes, such as (102), is predicted and confirmed in (102) rocking curve widths of 740 arcsec. These results are compared with films with (002) rocking curves of ∼270 arcsec and threading dislocation densities of ∼7×108 cm−2.
811 citations
551 citations
Book•
01 Sep 1992
TL;DR: In this article, the authors present a free electron picture of Crystalline Structures and a wave diffraction from Crystals, and then show how to use it for high field transport in Semiconductors.
Abstract: Chapter 1: Free Electron PictureChapter 2: Crystalline StructuresChapter 3: Wave Diffraction From CrystalsChapter 4: Electrons in Periodic StructuresChapter 5: Semiconductor BandstructuresChapter 6: Bandstructure Modifications--Alloys and HeterostructuresChapter 7: Bandstructure Modifications: Strained StructuresChapter 8: Lattice Vibrations--PhononsChapter 9: Doped SemiconductorsChapter 10: Carrier Transport--Basic FormalismChapter 11: Carrier Scattering by DefectsChapter 12: Carrier Scattering by PhononsChapter 13: Carrier-Carrier ScatteringChapter 14: High Field TransportChapter 15: Optical Processes in SemiconductorsChapter 16: Optical Processes in Semiconductors: Excitons
544 citations