Basal Plane Stacking Faults Research Articles

Emission properties of a-plane GaN grown by metal-organic chemical-vapor deposition

We report on the emission properties of nonpolar a-plane GaN layers grown on r-plane sapphire. Temperature-, excitation-density-, and polarization-dependent photoluminescences and spatially resolved microphotoluminescence and cathodoluminescence are employed in order to clarify the nature of the different emission bands in the 3.0–3.5eV spectral range. In the near band-edge region the emission lines of the donor-bound excitons (3.472eV) and free excitons (3.478eV) are resolved in the polarized low-temperature spectra, indicating a good quality of the layers. At low energies two other emissions bands with intensity and shape varying with the excited area are observed. The 3.42eV emission commonly attributed to the excitons bound to basal plane stacking faults shows thermal quenching with two activation energies (7 and 30meV) and an S-shaped temperature dependence of the peak position. This behavior is analyzed in terms of hole localization in the vicinity of the stacking faults. The emission band that peaked at 3.29eV is found to blueshift and saturate with increasing excitation intensity. The spatially resolved cathodoluminesence measurements show that the emission is asymmetrically distributed around the triangular-shaped pits occurring at the surface. The 3.29eV emission is suggested to involve impurities, which decorate the partial dislocation terminating the basal stacking faults.

Intersecting basal plane and prismatic stacking fault structures and their formation mechanisms in GaN

A systematic study of intersecting planar boundary structures observed in a GaN epifilm grown on a vicinal 6H-SiC substrate (offcut towards [12¯10]) with an AlN buffer is presented. These structures are shown to comprise stacking faults that fold back and forth from the basal plane [I1 basal plane stacking faults (BSFs)] to the prismatic plane [prismatic stacking faults (PSFs)]. The PSFs, with fault vector 12⟨101¯1⟩ nucleate at steps on the substrate surface as a consequence of the different stacking sequences exposed on either side of the step. Once nucleated, PSFs intersecting the vertical step risers in the AlN buffer and eventually in the GaN film are replicated during the predominantly step-flow growth and propagate into the growing crystal. As a consequence of the different growth rates experienced on either side of the intersection of a PSF with a vertical step riser, the PSF may be redirected onto an equivalent {112¯0} plane, leaving an I1 BSF between the bottom of the redirected section of PSF and the top of that portion of the original PSF which was below the terrace. This leads to the formation of folded PSF/BSF fault structures which exhibit various configurations. Such folded stacking fault configurations form walls which enclose domains of different stacking sequence. Stair-rod dislocations (SRDs) are observed to form at the intersections of these various faults and the Burgers vectors of some of these are discussed. In some cases, reconstruction of these SRDs occurs through the formation of partner dislocations which serve to minimize the energy associated with the lattice disconnections located at the cores of the SRDs.

Structural defects and luminescence features in heteroepitaxial GaN grown on on-axis and misoriented substrates

The microstructure and luminescence properties of a series of GaN epilayers grown on sapphire and SiC substrates with various misorientations have been correlated to assess the origins of the luminescence features in the misoriented samples. Samples grown on 3.5° offcut SiC and 5° and 9° offcut sapphire substrates both exhibit photoluminescence peaks near ∼3.2 and ∼3.4eV, which are absent in the on-axis SiC and sapphire cases. Transmission electron microscopy shows that the misoriented samples have configurations of I1 basal plane stacking faults, which fold into prismatic stacking faults with stair-rod dislocations at their intersections. The luminescence features are proposed to possibly arise from transitions involving the prismatic stacking faults and∕or the stair-rod dislocations associated with their mutual intersections and their intersections with the basal plane stacking faults.

Fabrication and analysis of GaN nanorods grown by MBE

GaN nanorods were grown on c-plane sapphire substrates under N-rich conditions by plasma-assisted molecular-beam epitaxy. Scanning electron microsopy revealed densely packed nanorods of hexagonal cross section with diameters ranging from roughly 40 to 100 nm. Atomic force microscopy indicated that the rods protruded 50 to 75 nm above the average height of the surface. Transmission electron microscopy (TEM) showed that the nanorods were approximately 1.4 micrometers in length but an accurate measurement of the rod separation was difficult to assess. Contrary to expectations for GaN grown under N-rich conditions, a high density of basal plane stacking faults were not revealed in TEM under typical imaging conditions. X-ray diffraction using the (0002), (0004), (0006), (104), and (105) reflections yielded c = 0.5188 ± 0.0002 nm and a = 0.3188 ± 0.0004 nm. Low temperature photoluminescence and cathodoluminescence showed broad near-bandgap emission around 3.4 eV that shifted to the blue with reduced temperature in the usual manner, and the presence of a similarly blue-shifting peak near 3.2 eV. The spectra were deconvolved using nine lineshape functions revealing 2 phonon replicas asssociated with the peak near 3.2 eV. Room temperature spectroscopic reflection fited to the standard Aspnes third-derivative lineshape function yielded a transition energy of 3.407 eV for the A exciton and 3.490 eV for the B+C excitons (not spectrally resolved). Both the x-ray and photoreflectance results indicate that the nanorods are fairly relaxed. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Microstructure and enhanced morphology of planar nonpolar m-plane GaN grown by hydride vapor phase epitaxy

Nonpolar (\(1\bar 100\)) m-plane gallium nitride has been grown heteroepitaxially on (100) γ-LiAlO2 by several groups. Previous attempts to grow m-plane GaN by hydride vapor phase epitaxy (HVPE) yielded films unsuitable for subsequent device regrowth because of the high densities of faceted voids intersecting the films’ free surfaces. We report here on the growth of planar m-plane GaN films on (100) γ-LiAlO2 and elimination of bulk and surface defects. The morphology achieved is smooth enough to allow for fabrication of m-plane GaN templates and free-standing substrates for nonpolar device regrowth. The GaN films were grown in a horizontal HVPE reactor at 860–890°C. Growth rates ranged from 30 µm/h to 240 µm/h, yielding free-standing films up to 250-µm thickness. The m-plane GaN films were optically specular and mirror-like, with undulations having 50–200-nm peak-to-valley heights over millimeter length scales. Atomic force microscopy revealed a striated surface morphology, similar to that observed in m-plane GaN films grown by molecular beam epitaxy (MBE). Root-mean-square (RMS) roughness was 0.636 nm over 25-µm2 areas. Transmission electron microscopy (TEM) was performed on the m-plane GaN films to quantify microstructural defect densities. Basal-plane stacking faults of 1×105 cm−1 were observed, while 4×109 cm−2 threading dislocations were observed in the g=0002 diffraction condition.

Defect reduction in (11¯00) m-plane gallium nitride via lateral epitaxial overgrowth by hydride vapor phase epitaxy

This letter reports on extended defect density reduction in m-plane (11¯00) GaN films achieved via lateral epitaxial overgrowth (LEO) by hydride vapor phase epitaxy. Several dielectric mask patterns were used to produce 10 to 100 μm-thick, partially and fully coalesced nonpolar GaN films. X-ray rocking curves indicated the films were free of wing tilt. Transmission electron microscopy showed that basal plane stacking fault (SF) and threading dislocation (TD) densities decreased from 105cm−1 and 109cm−2, respectively, less than 3×103cm−1 and ∼5×106cm−2, respectively, in the Ga-face (0001) wing of the LEO films. SFs persisted in ⟨0001⟩-oriented stripe LEO films, though TD reduction was observed in the windows and wings. Band-edge cathodoluminescence intensity increased 2 to 5 times in the wings compared to the windows depending on the stripe orientation. SFs in the low TD density wings of ⟨0001⟩-stripe films did not appear to act as nonradiative recombination centers.

Effect of off-orientation of seed crystal on silicon carbide (SiC) single-crystal growth on the (11 $$\bar 2$$ 0) surface0) surface

The effect of off-orientation growth has been investigated in terms of stacking fault formation during physical vapor transport (PVT) growth of silicon carbide (SiC) single crystals on the (11\(\bar 2\)0) seed crystal surface. Occurrence of stacking fault formation is largely dependent on the direction of off-orientation, and basal plane stacking fault density is significantly reduced by growing the crystals on a (11\(\bar 2\)0) seed crystal off-oriented toward 〈0001〉. The density of the basal plane stacking faults rapidly decreases from 100–150 cm−1 to ∼10 cm−1 as the degree of off-orientation is increased from 0 to 10 deg. The results are interpreted in the framework of microscopic facet formation during PVT growth, and the introduction of off-orientation of seed crystal is assumed to prevent (01\(\bar 1\)0) and (10\(\bar 1\)0) microfacet formation on the (11\(\bar 2\)0) growing surface through modification of the surface growth kinetics and to suppress the stacking fault formation.

Intersecting basal plane and prismatic stacking fault structures and their formation mechanisms in GaN

AbstractComparative TEM studies have been carried out on GaN/AlN epifilms grown on both on-axis and off-cut 6H-SiC substrates to study the defects formed in the GaN/AlN films and the state of strain relaxation at the interface. Prismatic Stacking Faults (PSFs) are observed to form at I1 type substrate steps in both the on-axis and vicinal samples. In the vicinal samples, the PSFs expand into GaN/AlN film forming intersecting stacking fault configurations comprising faults that fold back and forth from the basal plane (I1 Basal-Plane Stacking Faults; BSFs) to the prismatic plane (PSFs). On the other hand, in the on-axis sample the PSFs are observed to mostly annihilate each other to form enclosed domains confined to the near-interface region. In addition, HRTEM studies suggest the existence of Geometric Partial Misfit Dislocations (GPMDs) at the SiC/AlN interface of the vicinal sample, which form at I2 type substrate steps. These GPMDs simultaneously accommodate the lattice mismatch and stacking sequence mismatch present at the SiC/AlN interface. This provides explanation of the improved strain relaxation observed in the vicinal versus the on-axis sample.

Energetics and electronic structure of stacking faults in ZnO

The energetics and electronic structures of basal-plane stacking faults in wurtzite (WZ) ZnO are studied using first-principles density-functional total energy calculations. All the basal-plane stacking faults are found to have very low formations energies. They also introduce a downward shift at the conduction-band minimum (CBM). However, plane-averaged charge densities of the CBM state reveal that the CBM states are not very localized, indicating that these stacking faults should be electronically inert. The high concentration of these stacking faults can result in embedded zinc-blende (ZB) ZnO surrounded by WZ materials. The WZ/ZB interface exhibits a type-II lineup with $\ensuremath{\Delta}{E}_{V}\ensuremath{\approx}0.037\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and $\ensuremath{\Delta}{E}_{C}\ensuremath{\approx}0.147\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$.

Microstructure of M-plane GaN epilayers grown on γ-LiAlO2 by plasma-assisted molecular beam epitaxy

We report on the defect structure of M-plane GaN( ) epilayers grown on γ-LiAlO2(100) substrates by plasma-assisted molecular beam epitaxy. Our transmission electron microscopy studies reveal that the M-plane layers mainly contain intrinsic and extrinsic basal plane stacking faults. Beyond this, a complex type of planar defect is detected in the ( ) prism plane which is inclined with respect to the interface. This prism plane boundary is connected to stacking faults intersecting the whole sample. Its displacement vector is along the c axis and thus not able to relieve the epitaxial strain. Threading dislocations are dissociated into Shockley partials. The origin of the defect microstructure is discussed with respect to the misfit strain and the substrate surface morphology.

Correlated structural and optical characterization of ammonothermally grown bulk GaN

A series of ammonothermally grown bulk GaN crystals containing stacking faults has been characterized using structural [transmission electron microscopy (TEM) and synchrotron white-beam x-ray topography (SWBXT)] and optical [low-temperature photoluminescence (PL)] methods. Strong correlations are found between structural and optical properties. In particular, the occurrence of low-temperature PL peaks observed in the 3.30–3.45 eV range correlates with the observation of basal plane stacking faults by TEM (all of which were bounded by Shockley partial dislocations). In addition, the full width at half-maximum of the neutral donor-bound exciton PL peak correlates with the extent of mosaicity revealed on SWBXT Laue patterns recorded from the same crystals.

Relationship of basal plane and prismatic stacking faults in GaN to low temperature photoluminescence peaks at ∼3.4 eV and ∼3.2 eV

ABSTRACTThe relationship between the optical properties and microstructure of GaN is of great interest due to the important optical and electronic applications of this material. Several different studies have been reported attempting to link the low temperature photoluminescence (PL) peak at ∼3.4 eV to the presence of various microstructural defects. However, no clear systematic studies have been reported establishing such a link for the PL peak observed at ∼3.2 eV. In this paper, we present evidence linking the ∼3.4 eV PL peak to the presence of a thin layer of cubic phase associated with basal plane stacking faults (BSF). This relationship is mainly established by studying a series of ammonothermally-grown GaN bulk crystals. The existence and strength of the ∼3.4 eV peak are found to be related to the I2 type BSF (RI2=1/3<1>) observed in these samples. To investigate the relationship between the ∼3.2 eV peak and structural defects, a series of GaN epilayers grown on either SiC or sapphire (of various off-cut angles) was investigated by TEM and PL spectroscopy. Samples grown on 3.5° off-cut SiC and 5° and 9° off-cut sapphire substrates exhibit PL peaks near ∼3.2 and ∼3.4 eV, which are absent in the on-axis SiC and sapphire cases. TEM shows that the former group of samples has defect configurations consisting of prismatic stacking faults (PSFs) folding back and forth between two different {1120} planes connected by stair rod dislocations, which in turn fold onto to I1 type BSFs again with stair rod dislocations at the fault intersections. The ∼3.2 eV PL peaks are proposed to possibly arise from transitions involving the PSFs and the stair rods associated with their mutual intersections and their intersections with the BSFs. The ∼3.4 eV peak is again attributed to the thin layer of cubic phase associated with the I1 type BSF (three bilayers as opposed to four bilayers for the I2 type BSF).

Arsenic incorporation and its influence on microstructure of wurtzite GaN grown by molecular-beam epitaxy

We present a comprehensive study on the incorporation of As in a local area and its influence on the microstructures of wurtzite GaN grown by rf-plasma-assisted molecular-beam epitaxy. Using transmission electron microscopy (TEM) and secondary ion mass spectrometry (SIMS), we found that the incorporation of As in GaN is correlated with the formation of intrinsic basal plane stacking faults. The stacking fault, analyzed by high-resolution TEM, can be regarded as a stacking sequence of cubic GaAs or Ga(N,As) inserted into hexagonal GaN. Indeed, it may be energetically favorable for the As incorporation to form a thin cubic layer in hexagonal GaN, resembling the thermodynamically stable cubic phase for GaAs. We found experimental evidence of As surface segregation and analyzed it by fitting the As SIMS profiles with a one-dimensional empirical surface segregation model. This result declares a significantly large tendency of As surface segregation with the surface segregation coefficient R∼0.99, which indicates the difficulties to incorporate As into GaN, and thus to grow the ternary GaNAs alloys.

Structural and morphological characteristics of planar (112̄0) a-plane gallium nitride grown by hydride vapor phase epitaxy

This letter discusses the structural and morphological characteristics of planar, nonpolar (112̄0) a-plane GaN films grown on (11̄02) r-plane sapphire by hydride vapor phase epitaxy. Specular films with thicknesses over 50 μm were grown, eliminating the severely faceted surfaces that have previously been observed for hydride vapor phase epitaxy-grown a-plane films. Internal cracks and crack healing, similar to that in c-plane GaN films, were observed. Atomic force microscopy revealed nanometer-scale pitting and steps on the film surfaces, with rms roughness of ∼2 nm. X-ray diffraction confirmed the films are solely a-plane oriented with on-axis (112̄0) and 30° off-axis (101̄0) rocking curve peak widths of 1040 and 3000 arcsec, respectively. Transmission electron microscopy revealed a typical basal plane stacking fault density of 4×105 cm−1. The dislocation content of the films consisted of predominately edge component (bedge=±[0001]) threading dislocations with a density of 2×1010 cm−2, and mixed-character Shockley partial dislocations (b=13〈11̄00〉) with a density of 7×109 cm−2.

Structural Defect-Related Photoluminescence in GaN

ABSTRACTBroad, low temperature photoluminescence (PL) peaks near 3.4–3.42 eV in GaN have previously been associated with basal-plane stacking faults. Recently, we observed unusually sharp and highly structured PL peaks in this region in high quality bulk GaN grown from a Na/Ga flux, some of which display characteristic shifts and narrowing as a function of excitation power. Here, we study these peaks as a function of excitation intensity and crystal polarity, and compare them to those observed in GaN grown on off-axis SiC or sapphire (0001) substrates by metalorganic chemical vapor deposition (MOCVD). In the off-axis material on either substrate, similar peaks are observed to those in the bulk samples. In addition, a low energy peak near 3.21 eV is observed, which does not occur in the bulk material.

Interaction of twin boundaries with stacking faults in 2H martensite: A high-resolution electron microscopy study

Twinned 2H martensite is observed in copper-based shape memory alloys together with basal and non-basal plane stacking faults. Knowledge of the twin-boundary structure and its interaction with the faults is important to understand the deformation mechanism by twin coalescence. High-resolution electron microscopy coupled with image simulations have been used for these studies. The interface of the type I twinning in 2H martensite shows an atomic configuration with mirror antisymmetry. An atomic plane is shared by the twinned variants in such a way that the distance between the planes, parallel to the interface, is unchanged at the boundary and the mean atomic volume is conserved. The interaction of the boundary with a basal plane fault generates a shift in the interface. A mirror antisymmetrical boundary is maintained by introducing an imperfect interface dislocation.

Defect clustering in GaN irradiated with O+ ions

Transmission electron microscopy (TEM) was used to study microstructures formed in GaN irradiated with 600-keV O+ ions at room temperature. Three types of defect clusters were identified in the irradiated GaN: (i) basal-plane stacking faults with dimensions ranging from 5 to 30 nm, (ii) pyramidal dislocation loops, and (iii) local regions of highly disordered material. High-resolution TEM imaging clearly revealed that one type of the basal-plane stacking faults corresponded to insertion of one extra Ga–N basal plane in the otherwise perfect GaN lattice. The interpretation of these results indicated that interstitials of both Ga and N preferentially condensed on the basal plane to form a new layer of Ga–N under these irradiation conditions. The formation of these extended defects and their interactions with the point defects produced during irradiation contributed to a dramatic increase in the dynamic recovery of point defects in GaN at room temperature.

As-mediated stacking fault in wurtzite GaN epilayers

Growth of a thin GaAs layer embedded in wurtzite GaN was performed on sapphire (0001) by rf-plasma assisted molecular-beam epitaxy. A well-confined As-rich layer with a tail in the cap layer was measured by secondary ion mass spectrometry. Transmission electron microscopy studies demonstrated the formation of basal plane stacking faults, which were correlated with the presence of As in the layer. High-resolution microscopy revealed the stacking fault as a thin platelike cubic inclusion in the hexagonal GaN.

Process Induced Extended Defects in SiC Grown via Sublimation

ABSTRACTThree possible situations of extended defect formation during sublimation growth of SiC have been investigated: a-plane growth, occurrence and effect of irregular growth interfaces, and development of elongated open volume morphological defects. While in the first case the most preferred defects are basal plane stacking faults (SFs), in other studied samples threading dislocations and high energy SFs have been revealed, these suggesting stress accumulated in the course of the growth process. Formation of morphological defects of the above character has been observed in the presence of micropipes and step flow growth mode on vicinal surfaces. Growth on atomically flat surfaces can eliminate this type of defects but the polytype uniformity is more difficult to maintain.

Ab initio study of the effect of doping on stacking faults in GaN

The influence of impurities on the basal plane stacking fault energy in GaN is investigated using density functional theory. It is found that silicon, indium, magnesium and carbon impurities each reduce the stacking fault energy by introducing changes to the bonding properties of the material. These bonding properties are analysed in terms of Mulliken charges and bond populations. It is found that the reduction in stacking fault energy correlates both with a reduction in the average anion charge and with an increase in the overlap population.