D. Chandrasekhar

Evolution of Ge/Si(100) islands: Island size and temperature dependence

Atomic force microscope (AFM) imaging and cross-sectional analysis were used to document the shape evolution of Ge/Si(100) islands, grown by molecular beam epitaxy, as a function of growth conditions. Growth temperatures of 450, 550, 600, and 650 °C with Ge coverages between 3.5 and 14.0 monolayers (ML) were investigated for a deposition rate of 1.4 ML/min. Low coverages produced small hut clusters which then evolved into dome clusters at higher coverages. These dome clusters eventually dislocated after further growth. Higher growth temperatures activated additional pathways for the Ge islands to relieve their strain such as Ge/Si intermixing and the formation of trenches around the islands. Our detailed AFM cross-sectional analysis indicated that dome clusters form several crystal facets in addition to those previously reported.

Lattice and energy band engineering in AlInGaN/GaN heterostructures

We report on structural, optical, and electrical properties of AlxInyGa1−x−yNGaN heterostructures grown on sapphire and 6H–SiC substrates. Our results demonstrate that incorporation of In reduces the lattice mismatch, Δa, between AlInGaN and GaN, and that an In to Al ratio of close to 1:5 results in nearly strain-free heterostructures. The observed reduction in band gap, ΔEg, determined from photoluminescence measurements, is more than 1.5 times higher than estimated from the linear dependencies of Δa and ΔEg on the In molar fraction. The incorporation of In and resulting changes in the built-in strain in AlInGaN/GaN heterostructures strongly affect the transport properties of the two-dimensional electron gas at the heterointerface. The obtained results demonstrate the potential of strain energy band engineering for GaN-based electronic applications.

Structural properties of InN films grown on GaAs substrates: observation of the zincblende polytpe

Abstract We report the first observation of the zincblende polytype of the InN semiconductor. InN films were grown on vicinal (100) GaAs substrates by plasma enhanced molecular beam epitaxy. Transmission electron microscopy showed the InN films to be highly defective with both zincblende and wurtzite domains being present. The zincblende domains were epitaxially oriented to the substrate. The wurtzite InN had its c axis normal to the 〈111〉 zincblende planes which suggests stacking faults as the nucleation mechanism of the hexagonal phase. X-ray diffractometry measured a lattice constant a = 0.498 ± 0.001 nm for the zincblende InN polytype and a = 0.36 + 0.01 nm and c = 0.574 ± 0.001 nm for the wurtzite polytype.

Characterization of structural defects in wurtzite GaN grown on 6H SiC using plasma‐enhanced molecular beam epitaxy

Thin wurtzite GaN films have been grown by plasma‐enhanced molecular beam epitaxy on the basal plane of 6H SiC, with and without AlN buffer layers. Threading defects, identified from high‐resolution electron micrographs as double‐positioning boundaries (DPBs), originate at the substrate–buffer and/or buffer–film interfaces. The density of these faults seems to be related to the smoothness of the substrate, so that their occurrence emphasizes the importance of adequate substrate preparation. Stacking faults within the GaN are often visible parallel to the SiC substrate basal plane, sometimes terminating at the DPBs. These faults are related to the particular growth conditions, with greatly decreased density obtained for lower plasma power during GaN deposition. Growth of high quality GaN without stacking faults was achieved without using AlN buffer layers by deposition directly onto a vicinal SiC surface having a miscut angle of 4°. Such stepped substrates represent a potentially useful means for controlle...

Chemical vapor deposition of heteroepitaxial Si1−x−yGexCy films on (100)Si substrates

Thin heteroepitaxial films of Si1−x−yGexCy have been grown on (100)Si substrates using atmospheric pressure chemical vapor deposition at 625 °C. The crystallinity, composition, and microstructure of the SiGeC films were characterized using Rutherford backscattering spectrometry, secondary‐ion‐mass spectrometry, and cross‐sectional transmission electron microscopy. The crystallinity of the films was very sensitive to the flow rate of C2H4 which served as the C source. Films with up to 2% C were epitaxial with good crystallinity and very few interfacial defects. Between 800 and 900 sccm of 10% C2H4 in He, the C content increased dramatically from 2% to 10% and the as‐grown films changed from crystalline to amorphous. In order to establish deposition conditions for the crystalline‐amorphous phase transformation, one SiGeC film was deposited as the 10% C2H4 flow was increased linearly from 500 to 1500 sccm during growth. When the C content reached ∼4%, the film developed considerable stacking defects and diso...

Characterization of Group III-nitride semiconductors by high-resolution electron microscopy

Abstract High-resolution electron microscopy has been used to characterize the microstructure of thin films of GaN, AlN and InN, as grown by plasma-enhanced molecular beam epitaxy. Zincblende and wurtzite polytypes were preferentially nucleated using (001) GaAs and (0001) 6H SiC substrates, respectively. Stacking faults and microtwins along 111 planes dominated the zincblende films, whereas stacking faults along 0002 planes and threading defects originating at the substrate surface were most prevalent in the wurtzite phase. Improved crystal quality was achieved by growing the films on suitable buffer layers.

Novel chemical routes to silicon‐germanium‐carbon materials

We report the use of novel molecular precursors and ultrahigh vacuum chemical vapor deposition techniques to synthesize solid solutions of cubic SiC‐GeC and diamond‐structure Si1−x−yGexCy materials. Thin films with composition Si0.37Ge0.13C0.50 were deposited on Si by thermal decomposition of Ge[Si(CH3)3]4 at 650–700 °C. Electron microscope observations showed a polycrystalline zinc‐blende‐type structure and infrared (IR) analyses revealed carbide‐type Si‐C and Ge‐C vibrations. The Si1−x−yGexCy (y≳2%) alloys were deposited at 550–600 °C on Si and SiO2 by interactions of (1) C(SiH3)4 and GeH4; (2) CH3GeH3 and SiH4; and (3) CH3GeH3 with mixtures of GeH4 and SiH4. A homogeneous alloy phase of composition Si56Ge30C14 with diamond cubic structure was obtained from reaction 1. Reactions 2 and 3 produced films with carbon compositions ranging from 2 to 27 at. %. The materials containing less than 10% carbon appeared to be exclusively diamond cubic, whereas those with greater carbon compositions showed mixtures o...

Growth and characterization of CdTe/Si heterostructures - effect of substrate orientation

Abstract Transmission electron microscopy and small-probe microanalysis have been used to compare the microstructure and compositional profiles of CdTe/Si heterostructures grown by molecular beam epitaxy on (001), (211) and (111) silicon substrates. Overall, our results have demonstrated that the final CdTe growth orientation is determined by careful preparation of the Si substrate surface, the nature of the interfacial layer, and the initial phase nucleation. Initial studies confirmed that growth on (001) was problematical, not only because of the large lattice mismatch between materials (approximately 19%), but also because the double-domain reconstruction of the Si substrate surface degraded epilayer quality. Growth of high quality, domain-free CdTe(111)B was achieved by offcutting the substrate with respect to the [110] surface direction, with an additional rotation about [110]. Alternatively, with intermediary buffer layers of Ge(001), perfect a /2〈110〉 Lomer edge dislocations accommodated the misfit at the CdTe/Ge interface, and the (001) orientation of the Si substrate was retained during CdTe growth. For (211)-oriented substrates a very thin (approximately 2 nm) buffer layer of ZnTe prior to CdTe deposition was sufficient to maintain the substrate orientation, although Zn diffusion was often observed during subsequent annealing. The growth of Cd 1−x Zn x Te(211)B (with x ∼2–4%) with intermediary CdTe buffer layers then provided substrates which were suitably lattice-matched for growth of HgCdTe. Finally, large-area, domain-free CdTe(111)B was achieved using As-passivated Si(111) substrates and thin (approximately 50 nm) ZnTe buffer layers.

Growth of heteroepitaxial Si1−x−yGexCy alloys on silicon using novel deposition chemistry

We report heteroepitaxial growth of diamond‐structured Si1−x−yGexCy alloys on (100)Si substrates. Introduction of C into the diamond lattice is kinetically favored by low‐temperature (470 °C) interactions of C(SiH3)4, a novel C–H free carbon precursor, with mixtures of SiH4 and GeH4 using ultrahigh‐vacuum chemical vapor deposition techniques. Film thicknesses of 100 to 110 nm with 4–6 at. % C as indicated by Rutherford backscattering carbon resonance spectroscopy were obtained. Cross‐sectional transmission electron microscopy and Fourier transform infrared spectroscopy showed crystalline alloy phase formation with no detectable SiC precipitation. Secondary ion mass spectrometry revealed pure and highly homogeneous films. In situ annealing at 675 °C resulted in heteroepitaxial films with relatively few defects.

Wet oxidation of amorphous and crystalline Si1-x-yGexCy alloys grown on (100)Si substrates

The oxidation of amorphous Si0.65Ge0.27C0.08 and single‐crystal Si0.63Ge0.36C0.01 in wet ambient at 700 and 900 °C has been studied using Rutherford backscattering spectrometry and transmission electron microscopy. A reference sample of Si0.63Ge0.37 was also oxidized in order to determine the influence of carbon on the oxidation behavior. The low C content alloy behaved similar to the SiGe alloy: uniform Si1‐xGexO2 was obtained at 700 °C whereas SiO2 was formed at 900 °C, and Ge piled up underneath the oxide. In both cases, carbon was not detected in the oxide layer. The amorphous Si0.65Ge0.27C0.08 alloy behaved significantly different at both oxidation temperatures in comparison with the crystalline Si0.63Ge0.36C0.01 and Si0.65Ge0.37. Negligible oxidation occurred at 700 °C whereas SiO2 was obtained at 900 °C and the rejected Ge distributed uniformly throughout the SiGeC alloy. It is proposed that fast Ge diffusion during oxidation at 900 °C resulted from diffusion at grain boundaries, since crystallizat...