Modification Of Growth Mode Research Articles

Growth of high-quality AlN films on sapphire substrate by introducing voids through growth-mode modification

We demonstrate the achieving of high-quality AlN films on flat sapphire substrate (FSS) by introducing voids during growth. Voids are embedded into AlN epilayers through a growth-mode transition from island growth to step flow growth. Such voids significantly facilitated the underlying dislocations annihilation as demonstrated by the transmission electron microscopy (TEM) image. For the 3 μm-thick AlN film grown on FSS, the full width at half maximum of the X-ray rocking curve was 57/260 arcsec for (0 0 2)/(1 0 2) reflection and a threading dislocation density of 1.7 × 108 cm−2 was determined from plain-view TEM image. Moreover, the voids provided an additional stress relief channel in the AlN film grown on FSS, resulting in a tensile stress comparable to that of grown on nano-patterned sapphire substrate (NPSS). The measured lattice constants and Raman shift of AlN-E2 (high) peak verified the 3 μm-thick AlN film grown on FSS is nearly stress free at room temperature. Taking advantages of the deliberately embedded voids, a crack-free and atomically flat AlN film was grown on FSS. The strategy put forward in this work to obtaining high-quality AlN films on FSS is much cost-efficient, which is believed to hold great promise for commercialization in AlN-based devices.

Defect structure of high temperature hydride vapor phase epitaxy–grown epitaxial (0 0 0 1) AlN/sapphire using growth mode modification process

Defect structures were investigated by transmission electron microscopy for AlN/sapphire (0001) epilayers grown by high temperature hydride vapor phase epitaxy using a growth mode modification process. The defect structures, including threading dislocations, inversion domains, and voids, were analyzed by diffraction contrast, high-resolution imaging, and convergent beam diffraction. AlN film growth was initiated at 1450°C with high V/III ratio for 8min. This was followed by low V/III ratio growth for 12min. The near-interfacial region shows a high density of threading dislocations and inversion domains. Most of these dislocations have Burgers vector b=1/3〈1120〉 and were reduced with the formation of dislocation loops. In the middle range 400nm<h<2μm, dislocations gradually aggregated and reduced to ∼109cm−2. The inversion domains have a shuttle-like shape with staggered boundaries that deviate by ∼±5° from the c axis.Above 2μm thickness, the film consists of isolated threading dislocations with a total density of 8×108cm−2. Most of threading dislocations are either pure edge or mixed dislocations. The threading dislocation reduction in these films is associated with dislocation loops formation and dislocation aggregation-interaction during island growth with high V/III ratio.

High-resistance GaN epilayers with low dislocation density via growth mode modification

High-resistance GaN with low dislocation density adopting growth mode modification has been investigated by metalorganic chemical vapor deposition. The sheet resistance of the order of 1016Ω/sq has been achieved at room temperature by diminishing the oxygen impurity level close to the substrate with an AlN blocking layer. Attributed to this method which offers more freedom to tailor the growth mode, a three-dimensional (3D) growth process is introduced by adjusting the growth pressure and temperature at the initial stage of the GaN epitaxy to improve the crystalline quality. The large 3D GaN grains formed during this period roughen the surface, and the following coalescence of the GaN grains causes threading dislocations bending, which finally remarkably reduces the dislocation density.

In Situ Conductance of Fe/Si and Fe/Ge Multilayers

In this paper we study Fe/Si and Fe/Ge multilayers prepared at room temperature by magnetron sputtering. In situ conductance measurements reveal the formation of interfacial Fe–Si and Fe–Ge mixtures. During the Fe deposition a modification of growth mode is noticed. Deposition of Si (or Ge) onto Fe leads to the reduction of the Fe layer thickness due to interdiffusion, and Fe–Si (or Fe–Ge) structures appear. Above about 1.3 nm of deposited Si (1.5 nm of Ge) nominally pure Si (Ge) starts growing. Surface topography of the Fe/Si multilayers is studied by atomic force microscopy.

Synchrotron white beam X-ray topography, transmission electron microscopy and high-resolution X-ray diffraction studies of defects and strain relaxation processes in wide band gap semiconductor crystals and thin films

A review is presented of recent research into imaging on different length scales of defect structures in wide band gap semiconductors. This includes: synchrotron white beam X-ray topography (SWBXT) imaging of defects in SiC wafers and epilayers; high-resolution X-ray diffraction (HRXRD) and high-resolution transmission electron microscopy (HRTEM) studies of strain relaxation in AlN and GaN epilayers on on-axis and vicinal SiC and sapphire substrates; TEM studies of fault structures in GaN/AlN films on SiC substrates; and TEM studies of threading dislocation density reduction driven by growth mode modification in AlN films grown on sapphire substrates. In detail, studies of closed-core and hollow-core screw dislocations, basal plane dislocations and stacking faults (SFs) in 4H- and 6H-SiC substrates and films are presented. The mechanism of improved mismatch strain relaxation associated with the growth of GaN (AlN) films on vicinal substrates is discussed. This is attributed to the combined effects of mutual tilt between the epilayer and the substrate, which helps to relax out-of-plane mismatch, and by the generation of geometric partial misfit dislocations (GPMDs), which serves both to relax in-plane mismatch and accommodate stacking differences between the epilayer and substrate at some proportion of the steps at the substrate/film interface. In addition, the formation mechanisms of intersecting SF structures observed in GaN/AlN/SiC epilayers are presented. These fault structures comprise SFs that fold back and forth from the basal plane (I 1 Basal Plane Faults; BSFs) to the prismatic plane (Prismatic Stacking Faults; PSFs). Finally, the threading dislocation density reduction in AlN/sapphire films is modeled based on overgrowth of the dislocation outcrops by macrosteps which appear during a transition from step flow to 2D layer-by-layer growth.

Reduction of threading dislocation densities in AlN∕sapphire epilayers driven by growth mode modification

A strategy to reduce the density of threading dislocations (TDs) in AlN epilayers grown on sapphire substrates is reported. The TDs experience a redirection of their line orientation which is found to coincide with imposed increases in both of V/III ratio and overall flux rate leading to the formation of an internal subinterface delineated by the changes in dislocation orientation. Threading dislocations either experience large kinks and then redirect into threading orientation or form dipole half loops via annihilation of redirected threading segments of opposite sign with the latter leading to a significant dislocation density reduction. These phenomena can be accounted for by a transition of growth mode from atomic step flow to two-dimensional layer-by-layer growth which accompanies the imposed changes in V/III ratio and flux. As this occurs, macrosteps (several atomic layers thick) laterally overgrow pre-existing dislocation outcrops. Image forces initiate the redirection processes and create trailing segments parallel to the interface between the advancing macrostep and the surface outcrop. This horizontal segment can be forced to redirect into threading orientation should another macrostep traveling in the opposite direction be encountered. Image forces again nucleate the redirected segment which is then replicated as the crystal grows. A dipole half loop will form if two dislocations with opposite sign are redirected so as to meet each other.

Ridge structure transformation by group-III species modification during the growth of (Al,Ga)As on patterned substrates

AlAs growth by molecular-beam epitaxy has been successfully achieved on ⟨100⟩-aligned mesas on GaAs (001) patterned substrates. The final ridge morphology was found to be radically different from that observed from the homoepitaxial growth of GaAs on similar substrates. The significant modification to the final ridge profile is related to the reduction of the adatom diffusion length when using Al as the group-III species. These results provide direct evidence of a growth mode modification with alteration of the group-III element from Ga to Al and have significant implications for controlling the ridge formation process and for fabrication of more complex structures.

Ge growth mode modification on carbon-induced Si(001)-c(4×4) surfaces

Strong Ge morphological modifications were observed upon an ordered C-pre-covered Si(001)-c(4×4) reconstructed surface used as a template as compared to the growth on bare Si(001)-(2×1) substrates. While on bare substrates, the Ge wetting layer of the Stranski–Krastanov mode has a critical thickness of approximately 3–4 monolayers (ML), with the c-(4×4) template, island nucleation already occurs after 1 Ge ML, and growth proceeds in a Volmer–Weber mode. This suggests that the C-rich surface derm associated with the c-(4×4) reconstruction is able to strongly affect the Ge wetting.

Growth of abrupt heterointerfaces by atomic hydrogen-assisted molecular beam epitaxy

High-quality epitaxy of GaAs-on-Ge system has been developed at low growth temperatures around 400°C by atomic hydrogen-assisted molecular beam epitaxy (H-MBE). The segregation of Ge is significantly reduced and abrupt GaAsGe heterointerface is successfully formed. Using reflection high-energy electron diffraction (RHEED), secondary ion mass spectroscopy (SIMS) and atomic force microscopy (AFM) as characterization tools, it is demonstrated that enhanced layer-by-layer two-dimensional growth mode is realized from the earliest stages of growth and evolution of single-domain (2 × 4) GaAs(0 0 1) is achieved on vicinal Ge substrates at 400°C by H-MBE technique. The modification of growth mode and consequent improvement in material quality are explained at the simplest level by the surfactant properties of atomic H in MBE growth of GaAs.