Threading Defects Research Articles

Accommodation at the interface of highly dissimilar semiconductor/oxide epitaxial systems

An equilibrium model describing accommodation at epitaxial interfaces of highly dissimilar material systems is presented. For large lattice mismatches and large interface energies, the material nucleates with its bulk lattice parameter, mismatch being accommodated by interface dislocations. No threading defects are formed, contrasting with standard plastic relaxation mechanisms. It is shown that good quality InP can be grown on ${\text{SrTiO}}_{3}$ (STO) despite the large mismatch. This opens perspectives for the monolithic integration of InP on STO/Si templates and for the widening of the application spectrum of heteroepitaxy.

Fabrication of high Ge content SiGe-on-insulator with low dislocation density by modified Ge condensation

A SiGe-on-insulator (SGOI) structure with high Ge content and low density of dislocations is fabricated by a modified Ge condensation technique. The formation and elimination of stacking faults during condensation process are analyzed by transmission electron microscopy. A Si 0.19Ge 0.81OI substrate is fabricated utilizing two steps of oxidation and intermittent annealing. The time of oxidation or annealing at 900 °C is essential for the elimination of stacking faults in high Ge content SGOI substrate. The surface morphology of SGOI is investigated by atomic force microscopy and the defect density is evaluated from wet etching method. After the final condensation, the surface root-mean-square roughness (rms) of SiGe layer is kept below 1 nm and the threading defect density is controlled around 10 4 cm −2. The smooth surface and integrated lattice structure of SiGe layer indicate that the SGOI is suitable for heteroepitaxial growth of strained Ge, GaAs and III–V compounds.

Characterizing nanometer-sized V-defects in InGaN single quantum well films by high-spatial-resolution cathodoluminescence spectroscopy

We studied cathodoluminescence (CL) spectral variations in the vicinity of the V-defects in InGaN single quantum well (SQW) films by using our newly developed SE-SEM-CL. The peak intensity and peak wavelength of the CL peaks around 448 and 400 nm were found to change significantly near the apex of a V-defect. These variations were mainly attributed to a change in the In content of the InGaN SQW layer in the sidewalls and apex of the V-defect. Furthermore, an abnormal change was observed in the CL peak at 365 nm near the apex of the V-defect; this change was mainly caused by the stress induced by a force at the film edge that results from the thermal expansion differences between the film and the GaN layer. On the basis of the obtained results, we proposed a model of the formation of V-defects by selective termination of the threading defects on the (0001) surface of pseudomorphic InGaN SQW and GaN buffer layers by In.

Effect of threading defects on InGaN∕GaN multiple quantum well light emitting diodes

Photoelectrochemical etching was used to measure the threading defect (TD) density in InGaN multiple quantum well light-emitting diodes (LEDs) fabricated from commercial quality epitaxial wafers. The TD density was measured in the LED active region and then correlated with the previously measured characteristics of these LEDs. It was found that the reverse leakage current increased exponentially with TD density. The temperature dependence of this dislocation-related leakage current was consistent with a hopping mechanism at low reverse-bias voltage and Poole-Frenkel emission at higher reverse-bias voltage. The peak intensity and spectral width of the LED electroluminescence were found to be only weakly dependent on TD density for the measured TD range of 1×107–2×108cm−2.

Non-selective thin SiGe strain-relaxed buffer layers: Growth and carbon-induced relaxation

As an economically more attractive alternative to the classical thick graded strain-relaxed SiGe Buffer layers (SRB), we have developed a thin SRB based on a 200-nm-thick homogeneous Si 0.8Ge 0.2 layer with a C-doping spike inserted in about the middle of the layer. This C-doping layer enhances strongly the relaxation of the layer to ∼ 80%, when compared to samples without C layer. The thin SRB shows threading defect densities ≤ 10 7/cm 2 and a very smooth surface (RMS < 1 nm). The relaxation mechanism appears to be very similar to the “Modified Frank Read” (MFR) mechanism as developed by Legoues et al. [F. Legoues, B. Meyerson, J. Morar and P. Kirchner; J. Appl. Phys. 71 (1992) 4230.]. In our case, the MFR source for dislocation nucleation may consist of tiny SiC precipitates.

Synthesis and Fundamental Studies of (H3Ge)xSiH4-x Molecules: Precursors to Semiconductor Hetero- and Nanostructures on Si

The synthesis of the entire silyl-germyl sequence of molecules (H(3)Ge)(x)SiH(4)(-)(x) (x = 1-4) has been demonstrated. These include the previously unknown (H(3)Ge)(2)SiH(2), (H(3)Ge)(3)SiH, and (H(3)Ge)(4)Si species as well as the H(3)GeSiH(3) analogue which is obtained in practical high-purity yields as a viable alternative to disilane and digermane for semiconductor applications. The molecules are characterized by FTIR, multinuclear NMR, mass spectrometry, and Rutherford backscattering. The structural, thermochemical, and vibrational properties are studied using density functional theory. A detailed comparison of the experimental and theoretical data is used to corroborate the synthesis of specific molecular structures. The (H(3)Ge)(x)SiH(4)(-)(x) family of compounds described here is not only of intrinsic molecular interest but also provides a unique route to a new class of Si-based semiconductors including epitaxial layers and coherent islands (quantum dots), with Ge-rich stoichiometries SiGe, SiGe(2), SiGe(3), and SiGe(4) reflecting the Si/Ge content of the corresponding precursor. The layers grow directly on Si(100) at unprecedented low temperatures of 300-450 degrees C and display homogeneous compositional and strain profiles, low threading defect densities, and atomically planar surfaces circumventing entirely the need for conventional graded compositions or lift-off technologies. The activation energies of all Si-Ge hydride reactions on Si(100) (E(a) approximately 1.5-2.0 eV) indicate high reactivity profiles with respect to H(2) desorption, consistent with the low growth temperatures of the films. The quantum dots are obtained exclusively at higher temperatures (T > 500 degrees C) and represent a new family of Ge-rich compositions with narrow size distribution, defect-free microstructures, and homogeneous, precisely tuned elemental content at the atomic level.

Zn channeled implantation in GaN: damages investigated by using high resolution XTEM and channeling RBS

Zn (140 keV) channeled (along 〈0001〉) implantations in GaN are performed at room temperature and in a dose range from 1×10 13 to 4×10 16 cm −2, respectively. Channeling RBS measurements and the high-resolution XTEM investigations show the two damage regimes after implantation: one at the surface and another in the projected range. The damage level is very small at low doses and then gradually rises with increasing dose. The backscattering yield from the near surface region reaches the random level at doses higher than 2×10 16 cm −2 and the broken crystals and the amorphous in nanometer size are formed in the top thin surface layer after implantation at a dose of 3×10 16 cm −2. In the followed defective crystalline layer, the density of defects decreases with increasing the depth. The thread defects and loops are dominant in the region close to the surface at high dose and the clustered point defects are dominant in the deeper layer.

Lattice expansion induced by Zn channeled implantation in GaN

140 keV Zn channeled implantations in GaN are performed at room temperature and in a dose range from 1×10 13 to 4×10 16/cm 2, respectively. The lattice expansion is calculated from (0 0 0 2) and (0 0 0 4) X-ray diffractions of GaN after channeled implantation. Three dose dependence regimes are observed: the perpendicular lattice parameter initially increases with dose, subsequently varies slowly in an intermediate dose range, and finally diminishes at very high dose. The high-resolution XTEM observations show the different defects, such as the clustered point defect, the thread defects and loops, the broken crystals and the amorphous in nanometer sizes. The variation of lattice expansion is attributed to the changes of density and types of defects.

Lateral Enlargement of Silicon Carbide Crystals

A new growth technique for lateral enlargement of silicon carbide crystals is presented. The technique is based on sublimation growth but modified with respect to temperature gradients and geometry as compared to conventional setup. Simulation of the temperature distribution for lateral growth as well as the growth mechanism is discussed. Synchrotron white beam x-ray topographs have been evaluated concerning threading defects along the 0001 direction. Finally, a comparison between laterally grown 4H, 6H-silicon carbide and a commercial 4H-silicon carbide wafer is demonstrated, and shows that this growth technique makes it possible to enlarge seed crystals without screw dislocations and micropipes along the 0001 direction.

A transmission electron microscopy investigation of buried defectsources within epitaxial GaN

Site-specific focused ion beam milling procedures are used to preparecross-sectional TEM sample foils through a range of epitaxial GaN thin films.The high threading defect densities associated with heteroepitaxialGaN/sapphire prohibit the unambiguous identification of hillock nucleationevents. Inversion domains are, however, identified at the core of hillocksformed on CVD grown GaN on N-polar bulk GaN substrates. It is considered thatremnant contamination from the KOH chemomechanical polishing procedure isresponsible for the inversion domain nucleation.

Defect annihilation in AlN thin films by ultrahigh temperature processing

Postgrowth thermal processing in the range of 1200–1400 °C is shown to improve markedly the quality of thin (∼200 nm) AlN films grown by molecular beam epitaxy on SiC substrates. Comparison of both on-axis (0002) and off-axis (101̄2) x-ray diffraction peaks documents this improvement. Cross-sectional transmission electron micrographs confirm the reduction in dislocations and grain boundaries, while plan-view micrographs demonstrate that threading defect densities can be reduced to ∼3×108/cm2 after annealing. The thermal treatment is particularly effective because of the unusually large temperature window between the onset of a near-zero reactant sticking coefficient at ∼1200 °C and AlN thermal decomposition at ∼1400 °C. The Al sticking coefficient and the AlN decomposition rate are also reported.

A Two-Step Method for Epitaxial Lateral Overgrowth of GaN

We report on a two-step process for the epitaxial lateral overgrowth of GaN. In the first step, the selective area epitaxy proceeds at low temperature and GaN stripes with a triangular cross section are rapidly obtained. Then, in a second step, these stripes are used as seeds for epitaxial lateral overgrowth, the growth conditions being changed either by increasing the growth temperature or by introducing a Mg precursor. Fully coalesced and planar GaN samples are thereby achieved. A comparison between samples grown without and with this two-step process evidences an additional reduction in the threading defects density with the two-step process. The dislocations emerging from the seeds are drastically reduced to 5 × 106 to 107 cm—2 between the coalescence boundaries.

Ultraviolet and violet GaN light emitting diodes on silicon

We report the fabrication and characterization of GaN-based double heterostructure light emitting diodes grown by molecular beam epitaxy on Si(111) substrates. Light emitting diode operation is achieved by using the conducting Si(111) substrate as a backside n contact and a standard transparent Ni/Au p contact. We observe electroluminescence peaked in the ultraviolet ∼360 nm, with a full width at half maximum of ∼17 nm and in the violet at ∼420 nm. Electron microscopy studies indicate a high density of threading and planar defects. Consequences of these are discussed.

Microstructure, vibrational and electronic properties of GaN grown by molecular beam epitaxy on Al2O3(0001) and 6H-SiC(0001)

GaN layers have been grown by molecular beam epitaxy with a rf plasma source on Al2O3(0001) and 6H-SiC(0001). The conductive n-SiC substrates were employed for the in situ characterization of the grown GaN layers by electron spectroscopies (HREELS and XPS). Transmission electron microscopy (TEM) in the conventional and high-resolution mode provides information regarding the structural properties. Plan-view TEM yields a threading defect density of 7×109 cm−2 in GaN/6H-SiC and 2×1010 cm−2 in GaN/Al2O3. Micro-Raman spectroscopy, by analysis of the coupled A1(LO)-phonon-plasmon mode, provides the free carrier concentrations to be n∼1–2×1017 cm−3 for GaN layers grown on both substrates. The in situ HREEL spectroscopy gives insight into the electronic and vibrational properties of the GaN surface. Deep levels at ∼900 meV above the valence band maximum induce a surface absorption structure which is reduced after heating at 600 °C and which might be assigned, according to the growth process and to the literature, to the presence of Ga vacancies in the as-grown layers, in particular in the region close to the surface. Heating the GaN/6H-SiC heterostructures at 900 °C causes complete decomposition of the GaN layer.

Nucleation of misfit and threading dislocations during epitaxial growth of GaSb on GaAs(001) substrates

The initial stages of molecular beam epitaxy of GaSb on highly mismatched GaAs(001) substrates were investigated. Transmission electron microscopy was used to analyze the defect structure in GaSb islands and at their interfaces with GaAs(001) at different stages of growth. Based on experimental observations, we propose that the semiperiodic net of 90° misfit dislocations at the GaSb/GaAs(001) interface nucleate homogeneously at the leading edges of advancing {111} planes. After nucleation, they glide inwards along the interface plane to reach their equilibrium position. Threading dislocations in GaSb layers were directly correlated with the misfit dislocation net. We demonstrate that there are no threading defects in GaSb islands when their interfaces consist solely of 90° misfit dislocations, and that threading dislocations in the GaSb epilayer are all associated with minority 60° misfit dislocations nucleated in growing islands. The number of threading dislocations per unit area of the GaSb film is found to be independent of GaSb coverage, indicating that island coalescence does not substantially increase the number of 60° dislocations.

Gallium Nitride Epitaxy on Silicon: Importance of Substrate Preparation

ABSTRACTHexagonal GaN films grown on non-isomorphic substrates are usually characterized by numerous threading defects which are essentially boundaries between wurtzite GaN domains where the stacking sequences do not align. One origin of these defects is irregularities on the substrate surface such as surface steps. Using Si &lt;111&gt; substrates and a substrate preparation procedure that makes wide atomically flat terraces, we demonstrate that reduction of these irregularities greatly improves the crystalline and luminescent quality of GaN films grown by plasma-enhanced molecular beam epitaxy. X-ray rocking curve width decreases from over 1 degree to less than 20 minutes, while PL halfwidth decreases from over 15 meV to less than 10 meV.

Growth and Processing of Relaxed-Si1-xGex/Strained-Si Structures for Metal-Oxide Semiconductor Applications

Epitaxial growth and processing issues related to strained-Si metal-oxide semiconductor field effect transistor (MOSFET) fabrication are discussed. The material quality of the graded Ge composition, relaxed- Si1-x Gex buffer layers is analyzed. The ion implants used to form complementary metal-oxide semiconductor (CMOS) “wells” prior to epitaxy are found to degrade the quality of these layers by introducing a high density of misfit dislocation nucleation sites. Rough surface morphology, short misfit dislocation line lengths, and high threading defect densities are correlated with the increased nucleation site density. In addition, the oxidation of strained Si is investigated by Rutherford backscattering and Raman spectroscopy. No measurable strain relaxation is found as a result of thermal oxidation at temperatures up to 850° C.

Relaxed, low threading defect density Si0.7Ge0.3 epitaxial layers grown on Si by rapid thermal chemical vapor deposition

Relaxed Si0.7Ge0.3 epitaxial layers that possess threading defect densities as low as 5×105 cm−2 have been grown on Si substrates by rapid thermal chemical vapor deposition. The layers were formed in a series of steps of constant Ge content, each step with either 6% or 3% more Ge than the previous one. At the low average grading rates investigated, the number of steps used to reach 30% Ge only slightly affected the density of individual threading dislocations, imaged by electron beam induced current (EBIC). However, the density of dark-line defects seen in EBIC, which are formed from pile-ups of threading dislocations, was strongly influenced only by the average grading rate. Both the dark line density and the dark spot density must be considered when assessing the surface of these relaxed layers.

Formation of polyhedral voids at surface cusps during growth of epitaxial TiN/NbN superlattice and alloy films

Epitaxial TiN/NbN(001) superlattice thin films with periods λ between 0 (alloy) and 9.6 nm have been grown by ultrahigh vacuum reactive magnetron sputter deposition on MgO(001) substrates in mixed Ar/N2 discharges. Cross-sectional transmission electron microscopy was used for characterizing layer and defect structure. Coherency strain relaxation close to the film/substrate interface resulted in nonplanar growth with surface cusps and the creation of apparent column boundaries at which TiN and NbN layers curved towards the substrate, but across which the basic lattice was ordered. Also, a fraction of the boundaries were associated with threading defects along the growth direction. The column boundaries and threading defects were decorated by voids that exhibited an orthorhombohedral shape, were elongated in the growth direction, and typically had flat surfaces on {100} planes. The origin of voids is suggested to be through self-shadowing at surface cusps in combination with limited adatom mobility. The voids subsequently attained equilibrium shape as the result of surface and bulk diffusion. The void density varied with λ and the void length in the growth direction was generally an integer number of periods, indicating that the void formation was influenced by the superlattice layers. The formation of limited voids is suggested to be characteristic of thin film growth close to the epitaxial temperature.

Strain-relief mechanism in surfactant-grown epitaxial germanium films on Si(111).

We have studied the strain-relief mechanism in Ge thin films grown on Si(111) with and without a surfactant. Films grown on bare Si tend to form islands, which results in dislocations being nucleated at numerous sites at the edges of the islands. Since the dislocations glide as Shockley partial dislocations in this system, this process generates stacking faults that thread through the film and, eventually, to the surface when the islands coalesce. When Ge is grown on an antimony-terminated Si(111) surface, the growth mode changes to a layer-by-layer mode. This forces the dislocations to nucleate at the surface. Again, Shockley partial dislocations are formed, so that, initially, a threading stacking fault extends from the surface to the interface, and then along the interface. The intersection between the surface and the threading part of the stacking fault then acts as a nucleation site for the second partial dislocation, which results in the ``self-annihilation'' of the threading defects. We describe relaxed Ge films on Si(111) grown by this method, as well as relaxed multilayers of the Si/Ge/Si(111) type.