Correlation between leakage current density and threading dislocation density in SiGe p-i-n diodes grown on relaxed graded buffer layers

Line, point and surface defect morphology of graded, relaxed GeSi alloys on Si substrates

Strain relaxed graded SiGe buffer layers have been grown by low energy plasma enhanced chemical vapour deposition (LEPECVD). Due to the low ion energies involved in LEPECVD, exceptionally high plasma intensities can be applied without any ion damage of the epitaxial layers. Transmission electron microscopy and X-ray reciprocal space mapping show that relaxed buffer layers grown at rates exceeding 5 nm/s are of a comparable quality to buffer layers grown by standard techniques at much lower rates. Low temperature electric transport measurements on remotely doped tensilely strained Si quantum wells synthesized by LEPECVD show that also good electrical quality is achieved.

Epitaxial lateral overgrowth (ELO) has been used to reduce threading dislocation (TD) densities in GaN. This paper reports transmission and scanning electron microscopy studies of TDs in GaN films grown by ELO on GaN nanocolumns produced either during growth by MBE (method 1), or by self‐organised patterning on a uniform layer grown by MOCVD (method 2). In method 1, isolated nanocolumns grew without TDs, but some TDs formed when nanocolumns coalesced. Although some of these TDs extended through the overlayer, extensive lateral migration of others, depending on the dislocation type, led to dislocation annihilation, lowering the overall TD density. In method 2, a similar process of lateral migration of TDs was observed during ELO, resulting in a continuous GaN overlayer with TD densities less than 108 cm–2. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Low energy plasma enhanced chemical vapour deposition (LEPECVD) is a deposition technique developed for the epitaxy of Si and SiGe at ultra-high deposition rates. Due to a high current plasma discharge composed of low energy particles, a high plasma enhancement can be obtained without any accompanying plasma induced damage of the wafer surface. The most important application of LEPECVD so far is for compositionally graded relaxed SiGe buffer layers. Such relaxed buffer layers are demonstrated with end composition up to pure Ge and with a growth time below 1 hour. A p-type hetero-MOSFET formed in a SiGe channel compressively strained to a Si0.5Ge0.5 relaxed buffer layer, is demonstrated as one example where the high growth rates of LEPECVD allows the synthesis of devices which cannot be produced with an acceptable throughput with conventional deposition methods. The room temperature effective hole mobility of 760 cm2/Vs obtained on such devices demonstrates a high structural and electrical quality of the LEPECVD material.

ABSTRACTWe have studied the possibilities of reducing the threading dislocation (TD) density in GaAs on Si using free side wall growth on patterned Si substrates and/or using a soft ZnSe interlayer in combination with post growth annealing procedures. TD densities were accurately determined using large area plan view TEM and were found to be unaffected by proximity to free side walls and by the ZnSe interlayer. Post growth heat treatments led to a factor of ∼2 reduction in TD density and to bunching of dislocations throughout the thickness of the film.

Evolution of threading dislocations in MOCVD-grown GaN films on (1 1 1) Si substrates

Strain-relaxed, compositionally graded InGaP layers grown by atmospheric-pressure metalorganic vapor phase epitaxy (APMOVPE) have previously been found to exhibit unusual contrast in transmission electron microscopy (TEM). The features that generate this contrast were termed “branch defects.” Branch defects have been shown to pin threading dislocations and are thus undesirable features for the realization of low dislocation density semiconductors. In this study, we compare the properties of branch defects formed during optimized, relaxed, graded InGaP buffer deposition in two different reactor configurations: a commercial, multiwafer, low-pressure reactor and a custom-built, atmospheric-pressure research reactor. Branch defect formation is further characterized via the introduction of in situ annealing interruptions during graded buffer deposition in the atmospheric-pressure system. Branch defects are observed in material from both reactor systems, suggesting that they are a phenomenon intrinsic to InGaP graded buffer growth. Careful TEM studies of the resulting samples reveal that the phase space for the formation of branch defects is similar in both reactor configurations. During standard optimized graded buffer growth, higher growth temperatures delay the onset of branch defect formation to higher indium fractions in the graded buffer. Low growth temperatures produce branch defects at lower indium fractions, and these defects tend to be more closely spaced. In addition, the formation of branch defects is favored by low V/III ratios and in situ growth interruption and annealing. Annealing is found to create anisotropic strain relaxation in the graded buffer, which we attribute to the blocking of gliding threading dislocations by preferentially oriented branch defects. Based on the observed properties of branch defects and the factors that affect their formation, it appears that these defects are a manifestation of local variations in indium concentration that develop on the sample surface during MOVPE and are buried in the bulk due to kinetic limitations.

This work investigates the role of threading dislocation densities (TDD) in the low density regime on the vertical transport in Si0.06Ge0.94 heterostructures integrated on Si(001). The use of unintentionally doped Si0.06Ge0.94 layers enables the study of the impact of grown-in threading dislocations (TD) without interaction with processing-induced defects originating, e.g., from dopant implantation. The studied heterolayers, while equal in composition, the degree of strain relaxation, and the thickness feature three different values for the TDD as 3 × 106, 9 × 106, and 2 × 107 cm−2. Current–voltage measurements reveal that leakage currents do not scale linearly with TDD. The temperature dependence of the leakage currents suggests a strong contribution of field-enhanced carrier generation to the current transport with the trap-assisted tunneling via TD-induced defect states identified as the dominant transport mechanism at room temperature. At lower temperatures and at high electric fields, direct band-to-band tunneling without direct interactions with defect levels becomes the dominating type of transport. Leakage currents related to emission from mid-gap traps by the Shockley–Read–Hall (SRH) generation are observed at higher temperatures (>100 °C). Here, we see a reduced contribution coming from SRH in our material, featuring the minimal TDD (3 × 106 cm−2), which we attribute to a reduction in point defect clusters trapped in the TD strain fields.

Resonant tunneling of variously strained Si/Ge xSi 1−x/Si heterostructures

A method to obtain high-quality strain–relaxed SiGe buffer layers on Si(001) substrates is presented. In this method, the strain relaxation of the SiGe layer is performed using a two-step procedure. Firstly, a low-temperature-grown SiGe layer, whose surface is covered by a thin Si cap layer, is thermally annealed. At this stage, the strain is incompletely relaxed and an atomically flat surface can be realized. Then, a second SiGe layer is grown on the first layer to achieve further strain relaxation. Transmission electron microscopy has clearly revealed that dislocations are dispersively introduced into the first SiGe/Si substrate interface and thus no pile up of dislocations occurs. The formation of a periodic undulation on the growth surface of the second SiGe layer is the key to inducing a drastic reduction in the threading dislocation density.

A method to grow a relaxed Si0.5Ge0.5 graded layer with a very smooth surface and a very low threading dislocation density using solid-source molecular-beam epitaxy is reported. This method included the use of Sb as a surfactant for the growth of a 2 μm compositionally graded SiGe buffer with the Ge concentration linearly graded from 0% to 50% followed by a 0.3 μm constant Si0.5Ge0.5 layer. The substrate temperature was kept at 510 °C during the growth. Both Raman scattering and x-ray diffraction were used to determine the Ge mole fraction and the degree of strain relaxation. Both x-ray reflectivity and atomic force microscopy measurements show a surface root mean square roughness of only 20 Å. The threading dislocation density was determined to be as low as 1.5×104 cm−2 as obtained by the Schimmel etch method. This study shows that the use of a Sb surfactant and low temperature growth is an effective method to fabricate high-quality graded buffer layers.

Device assessment of the electrical activity of threading dislocations in strained Ge epitaxial layers

We demonstrate an electrically injected semipolar (202¯1) laser diode grown on a partially relaxed AlGaN buffer layer. The coherency stresses are relaxed by misfit dislocations at the GaN/AlGaN heterointerface which form by glide of preexisting threading dislocations along the (0001) basal plane. The defects are confined to the heterointerface which allows the growth of high aluminum composition films with threading dislocation densities of less than 108 cm−2. The lasing wavelength was 384 nm with a threshold current density of 15.7 kA/cm−2. UV lasers grown on semipolar relaxed AlGaN buffers provide an alternative to devices grown on AlN or sapphire.

Low threading dislocation Ge on Si by combining deposition and etching

High germanium content SiGe virtual substrates grown at high temperatures