Control of ordering in GaInP and effect on bandgap energy

Ga_xIn_1-x P layers with x ≈ 0.5 have been grown by atmospheric pressure organometallic vapor phase epitaxy on GaAs substrates with 10 micron wide, [110]-oriented grooves produced photolithographically on the surface. The [110] steps and the misorientation produced at the edges of the grooves have been found to have important effects on the formation of the Cu-Pt ordered structure (ordering on {111} planes) in the GaInP layers during growth. In this work, the groove shape is demonstrated to be critically important. For the optimum groove shape, with a maximum angle to the (001) surface of between 10 and 16°, single domains of the (-111) and (1-11) variants of the Cu-Pt ordered structure are formed on the two sides of the groove. Shallow (≤0.25 μm deep) grooves, with maximum angles of <10°, are less effective. Within the large domains on each side of the groove, small domains of the other variant are observed. The boundary between the two domains is seen to wander laterally by a micron or more during growth, due to the change in shape of the groove during growth. For deep (1.5 μm) grooves, with maximum angles to the (001) plane of 35°, only a single variant is formed on each side of the groove. However, the domains are small, dispersed in a disordered matrix. For substrates with deep grooves on a GaAs substrate misoriented by 9° toward the [-110] direction, an interesting and useful pattern is produced. One half of the groove is a single domain which shrinks in size as the growth proceeds. The other half of the groove, where the misorientation is larger, is disordered. Thus, every groove contains large (>1 μm² cross-sectional area and several mm long) regions of highly ordered and completely disordered material separated by no more than a few microns. This allows a direct determination of the effect of ordering on the bandgap of the material using cathodoluminescence (CL) spectroscopy. The 10K photoluminescence (PL) consists of three distinct peaks at 1.94, 1.88, and 1.84 eV. High resolution CL images reveal that the peaks come from different regions of the sample. The high energy peak comes from the disordered material and the low energy peak comes from the large ordered domains. Electron microprobe measurements of the solid composition demonstrate that the shift in emission energy is not due to changes in solid composition. This is the firstdirect verification that ordering causes a reduction in bandgap of any III/V alloy. Decreasing the Ga_0.5In_0.5P growth rate from the normal 2.0 to 0.5 μ/h is found to enhance ordering in layers grown on planar GaAs substrates. Transmission electron diffraction results show that the domain size also increases significantly. For material grown on exactly (001)-oriented substrates, a pronounced [001] streaking of the superlattice spots is observed. This is correlated with the presence of a dense pattern of fine lines lying in the (001) plane in the transmission electron micrographs. The PL of this highly ordered material consists of a single peak that shifts to higher energy by > 110 meV as the excitation intensity is increased by several orders of magnitude.     

Ordering in InGaAs/InAlAs layers

The structure of InCaAs/InAlAs layers lattice matched to an InP substrate, grown on either (100) or on (110) with a 4° tilt toward [111] at 500 and 300°C has been investigated by transmission electron microscopy. High perfection resulted for the layers grown on [001] oriented substrates whereas growth on the near [110] substrates resulted in compositional nonuniformities, macrosteps formation, and ordering of the group III elements. This difference in structural perfection between the two sets of samples was also reflected in differences in electrical properties.     

Incorporation of Sn on GaAs (111)A substrates by molecular beam epitaty

Behavior of Sn as donor species in the MBE growth of GaAs on (111)A substrates has been investigated by varying the growth temperature from 460 to 620°C, As₄:Ga flux ratio from 4 to 25, and Sn concentration from 10¹⁶ to 10²⁰ atoms cm^-3. Secondary ion mass microscopy measurements show that Sn does not surface segregate on (111)A substrates under this growth condition, in contrast to that on (001) substrates. Sn is uniformly incorporated throughout the bulk of the grown layer for all samples, apart from the most highly doped ones. To increase the Sn carrier concentration on the (111)A substrates, the measured carrier concentration shows that doping should be carried out at a low growth temperature and/or high As₄:Ga flux ratio.     

Structural and optical properties of GaxIn1−xP layers grown by chemical beam epitaxy

Chemical beam epitaxial (CBE) Ga_xIn_1?xP layers (x≈0.5) grown on (001) GaAs substrates at temperatures ranging from 490 to 580°C have been investigated using transmission electron diffraction (TED), transmission electron microscopy, and photoluminescence (PL). TED examination revealed the presence of diffuse scattering 1/2{111}_B positions, indicating the occurrence of typical CuPt-type ordering in the GaInP CBE layers. As the growth temperature decreased from 580 to 490°C, maxima in the intensity of the diffuse scattering moved from ½{111}_B to ½{?1+δ,1?δ,0} positions, where δ is a positive value. As the growth temperature increased from 490 to 550°C, the maxima in the diffuse scattering intensity progressively approached positions of $\frac{1}{2}\{\bar 110\} $ , i.e., the value of δ decreased from 0.25 to 0.17. Bandgap reduction (～45 meV) was observed in the CBE GaInP layers and was attributed to the presence of ordered structures.     

Spontaneous lateral composition modulation in AlAs/InAs short period superlattices via the growth front

The spontaneous formation of lateral composition modulation in AlAs/lnAs short period superlattices on InP (001) substrates has been investigated. Transmission electron microscopy and x-ray diffraction reciprocal space mapping show that the lateral modulation is very regular, with a periodicity along the [110] direction on the order of 180Å. A surprising: result is that this material system also exhibits a lateral modulation along the [1–10] direction, with a periodicity of 330Å. Reflection high energy electron diffraction performed during the deposition revealed that the reconstruction changed from (2 × 1) during the InAs deposition cycle to (1 × 2) during the AlAs cycle, which may be related to the presence of the modulation in both <110> directions. High magnification transmission electron micrographs show that the surface is undulated and that these undulations correlate spatially with composition modulation. Detailed analysis of the images shows that the contrast observed is indeed due to composition modulation. Photoluminescence from the modulated layer is strongly polarized and red-shifted by 220 meV.     

Morphology and defect structures of GaSb islands on GaAs grown by metalorganic vapor phase epitaxy

The initial nucleation of GaSb on (001) GaAs substrates by metalorganic vapor phase epitaxy has been investigated using transmission electron microscopy (TEM) and high resolution electron microscopy (HREM). TEM results showed that the GaSb islands experience a morphological transition as the growth temperature increases. For growth at 520°C, the islands are longer along the [110] direction; at 540°C, they are nearly square, and at 560°C, they are longer along the direction. Possible mechanisms are proposed to describe such a transition. TEM and HREM examination showed that lattice misfit relaxation mechanisms depend on the growth temperature. For the sample grown at 520°C, the lattice mismatch strain was accommodated mainly by 90° dislocations; for the sample grown at 540°C, the misfit strain was relieved mostly by 90° dislocations with some of 60° dislocations, and for the sample grown at 560°C, the strain was accommodated mainly by 60° dislocations which caused a local tilt of the GaSb islands with respect to the GaAs substrate. The density of threading dislocations was also found to be dependent on the growth temperature. Mechanisms are proposed to explain these phenomena.     

Scanning electron microscopy and cathodoluminescence study of the epitaxial lateral overgrowth (ELO) process for gallium nitride

Traditional epitaxial growth of GaN by metalorganic vapor phase epitaxy (MOVPE) on mismatched substrates such as sapphire or SiC produces a columnar material consisting of many hexagonal grains ∼0.2–1.0 μm in diameter. The epitaxial-lateral-overgrowth (ELO) process for GaN creates a new material: single-crystal GaN. We have studied the ELO process for GaN grown by MOVPE in a vertical flow rotating substrate reactor. Characterization consisted of plan-view SEM and vertical-cross-section TEM studies, which revealed a large reduction in dislocation density in the overgrown regions of the GaN. Panchromatic and monochromatic cathodoluminescence images and spectra were used to study the spatial variation of the optical properties within the GaN ELO samples. The effects of growth temperature and stripe material on the overgrown layers were examined. Through the use of a higher substrate temperature during growth and the use of a SiN_x stripe material, the overgrown crystal shape has a smooth 2D top surface with vertical sidewalls. Applying a second ELO step, rotated by 60°, over a fully coalesced ELO layer yields a further reduction of defects in GaN overgrown surfaces.     

Characterization of the microstructure of Co thin film on silicon substrate by TEM

The microstructure of as-deposited Co thin films on silicon (001) substrate was characterized by TEM using wedge-shaped planar-view samples. Selected area electron diffraction showed that the as deposited Co thin films were composed of Co (α) and that no interfacial reaction took place between Co thin films and the Si substrate. The microstructure of Co thin films annealed at 250°C for 30 min was also investigated by using conventional planar-view samples. The analysis of selected area electron diffraction indicates that Co thin films react entirely with the Si substrate, and a silicide layer forms at the Co/Si interface. Dark field images clearly indicate that the interfacial layer consists of Co₂Si in irregular stripes and CoSi as fine particles but no CoSi₂ forms.     

Physical Properties of AlGaN/GaN Heterostructures Grown on Vicinal Substrates

We report on the growth of Al_0.25Ga_0.75N/GaN heterostructures grown on low dislocation density vicinal surfaces of semi-insulating c-axis GaN substrates. Atomic force microscopy (AFM), photoluminescence (PL), cathodoluminescence (CL), high-resolution x-ray diffraction (HRXRD), secondary-ion mass spectroscopy (SIMS), Hall effect, and Raman spectroscopy have been used to assess structural and electrical properties as a function of substrate offcut. Bulk GaN substrates with vicinal offcut between 0.5° and 1.4° are optimal with respect to surface roughness and dopant incorporation. AFM, PL, and CL show decreasing Mg incorporation with increasing offcut angle. Raman spectroscopy, used to analyze biaxial strain, confirms essentially strain-free heterostructure growth on vicinal substrates with offcut angles between 0.5° and 1.4° off [0001] toward [1[`1] 00] [1\overline{1} 00] . Aluminum (Al) incorporation in the Al _x Ga_1−x N barrier assessed by Raman vibration is in excellent agreement with trends found by HRXRD.     

Optical and crystallographic properties of high perfection InP grown on Si(111)

The growth of InP by low-pressure metalorganic chemical vapor deposition on vicinal Si(111), misoriented 3° toward [1-10], is reported. Antiphase domain-free InP is obtained without any preannealing of the Si substrate. Crystallographic, optical, and electrical properties of the layers are significantly improved as compared to the best reported InP grown on Si(001). The high structural perfection is demonstrated by a full width at half maximum (FWHM) of 121 arcs for the (111) Bragg reflex of InP (thickness = 3.4 μm) as obtained by double crystal x-ray diffraction. The low-temperature photoluminescence (PL) efficiency is 70% of that of homoepitaxially grown InP layers. The FWHM of the near-gap PL peak is only 2.7 meV as compared to 4.5 meV of the best material grown on Si(001). For the first time, InP:Fe layers with semi-insulating characteristics (ρ > 3 × 10⁷ Ω-cm) have been grown by compensating the low residual background doping using ferrocene. Semi-insulating layers are prerequisite for any device application at ultrahigh frequencies.     

Epitaxial ZnO/Pt layered structures and ZnO-Pt nanodot composites on sapphire (0001)

We report the epitaxial growth and properties of ZnO-Pt layered structures and ZnO-Pt nanodot composites on sapphire (0001) substrates fabricated by using the pulsed laser deposition (PLD) technique. Heteroepitaxial growth of these structures was accomplished by using domain-matching epitaxy. The heterostructures were characterized using x-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), optical transmittance, photoluminescence, and electrical resistivity measurements. XRD and HRTEM experiments revealed the epitaxial nature of these structures, with orientation relationship between ZnO and Pt, as [0001]_ZnO∥[111]_Pt and [ 110]_ZnO∥[011]_Pt, which is equivalent to no rotation between ZnO and Pt. For Pt epitaxy on (0001) sapphire, the epitaxial relationship was determined to be [001]_Pt∥[0001]_Sap and [110]_Pt∥[01 0]_Sap, which is equivalent to a 30° rotation in the basal plane. Electrical and optical measurements showed that these heterostructures exhibit very high electrical conductivity and at the same time possess interesting optical transmittance spectra and exhibit room temperature photoluminescence characteristics.     

Pendeo-epitaxial growth of gallium nitride on silicon substrates

Pendeo-epitaxy (PE)¹ from raised, [0001] oriented GaN stripes covered with silicon nitride masks has been employed for the growth of coalesced films of GaN(0001) with markedly reduced densities of line and planar defects on Si(111)-based substrates. Each substrate contained previously deposited 3C-SiC(111) and AlN(0001) transition layers and a GaN seed layer from which the stripes were etched. The 3C-SiC transition layer eliminated chemical reactions between the Si and the NH₃ and the Ga metal from the decomposition of triethylgallium. The 3C-SiC and the GaN seed layers, each 0.5 μm thick, were also used to minimize the cracking and warping of the GaN/SiC/silicon assembly caused primarily by the stresses generated on cooling due to the mismatches in the coefficients of thermal expansion. Tilting in the coalesced GaN epilayers of 0.2° was confined to areas of lateral overgrowth over the masks; no tilting was observed in the material suspended above the trenches. The strong, low-temperature PL band-edge peak at 3.456 eV with a FWHM of 17 meV was comparable to that observed in PE GaN films grown on AlN/6H-SiC(0001) substrates.     

The structure of the Si (211) surface

Silicon (211) has been proposed as an alternative substrate for CdTe/HgCdTe molecular beam epitaxial growth. Silicon has a clear advantage over other substrates because of its low cost, high strength, and thermal-expansion coefficient, which matches that of the silicon readout integrated circuit. The (211) orientation has been shown to yield high-quality CdTe and HgCdTe/CdTe layers over other orientations. The reconstruction and faceting of the Si (211) surface is poorly understood despite the importance of the (211) orientation. The results of low-energy electron diffraction (LEED) studies have been contradictory, and their conclusions are inconsistent with recent scanning tunneling microscopy (STM) studies. LEED and STM images were used to determine the most probable Si (211) surface facet structure as a function of annealing temperature. Samples annealed at a high temperature (i.e., >1260°C) allowed the formation of ordered LEED spot patterns as opposed to the typically reported $[\bar 111]$ streaks. The pattern in the $[0\bar 11]$ direction gave a consistent 2× (7.68 Å) reconstruction.     

Effect of Temperature on Structural and Morphologic Properties of ZnO Films Annealed in Ammonia Ambient

ZnO thin films were prepared on Si(111) substrates by pulsed laser deposition (PLD). Then, the samples were annealed at different temperatures in NH₃ ambient and their properties were investigated particularly as a function of annealing temperature. The structure, morphology, and optical properties of ZnO films were studied by x-ray diffraction (XRD), Fourier transform infrared spectroscope (FTIR), scanning electron microscope (SEM), and photoluminescence (PL). The results show that the increase of annealing temperature makes for the improvement in the crystal quality and surface morphology below the temperature of 650°C. However, when the annealing temperature is above 650°C, the ZnO films will volatilize and, especially at 750°C, ZnO will volatilize completely.     

The effects of process conditions and substrate on copper MOCVD using liquid injection of (hfac)Cu(vtmos)

Copper MOCVD (metalorganic chemical vapor deposition) using liquid injection for effective delivery of the (hfac)Cu(vtmos) [1,1,1,5,5,5-hexafluoro-2,4-pentadionato(vinyltrimethoxysilane) copper(I)] precursor has been performed to clarify growth behavior of copper films onto TiN, <100> Si, and Si₃N₄ substrates. Especially, we have studied the influences of process conditions and the substrate on growth rates, impurities, microstructures, and electrical characteristics of copper films. As the reactor pressure was increased, the growth rate was governed by a pick-up rate of (hfac)Cu(vtmos) in the vaporizer. The apparent activation energy for copper growth over the surface-reaction controlled regime from 155°C to 225°C was in the range 12.7–32.5 kcal/mol depending upon the substrate type. It revealed that H₂ addition at 225°C substrate temperature brought about a maximum increase of about 25% in the growth rate compared to pure Ar as the carrier gas. At moderate deposition temperatures, the degree of a <111> preferred orientation for the deposit was higher on the sequence of <Cu/Si<Cu/TiN<Cu/Si₃N₄. The relative impurity content within the deposit was in the range 1.1 to 1.8 at.%. The electrical resistivity for the Cu films on TiN illustrated three regions of the variation according to the substrate temperature, so the deposit at 165°C had the optimum resistivity value. However, the coarsened microstructures of Cu on TiN prepared above 275°C gave rise to higher electrical resistivities compared to those on Si and Si₃N₄ substrates.     

Structural properties of GaAsN grown on (001) GaAs by metalorganic molecular beam epitaxy

Detailed transmission electron microscopy (TEM) and transmission electron diffraction (TED) examination has been made of metalorganic molecular beam epitaxial GaAsN layers grown on (001) GaAs substrates. TEM results show that lateral composition modulation occurs in the GaAs_1−xN_x layer (x 6.75%). It is shown that increasing N composition and Se (dopant) concentration leads to poor crystallinity. It is also shown that the addition of Se increases N composition. Atomic force microscopy (AFM) results show that the surfaces of the samples experience a morphological change from faceting to islanding, as the N composition and Se concentration increase. Based on the TEM and AFM results, a simple model is given to explain the formation of the lateral composition modulation.     

Structural characterization of GaAs<Subscript>1−x</Subscript>Bi<Subscript>x</Subscript> alloy by rutherford backscattering spectrometry combined with the channeling technique

Rutherford backscattering spectrometry (RBS) combined with the channeling technique has been applied to a GaAs_1−xBi_x epilayer to investigate concentration and lattice location of Bi atoms and crystalline quality of the epilayer. The metastable GaAs_1−xBi_x alloy layer was grown on a GaAs substrate at a temperature as low as 365°C. The GaBi mole fraction obtained was 2.6 ± 0.2%. Angular scans for [100] and [111] crystal directions reveal that the incorporated Bi atoms exactly occupy substitutional sites in the GaAs crystal lattice. Crystal perfection of the GaAs_1−xBi_x metastable alloy is fairly good in spite of the low growth temperature.     

Special features of structural defects in undoped CdTe textured ingots produced by free growth from a gasdynamic vapor flow

The structural features of undoped CdTe ingots grown in a gasdynamic flow at 620°C were studied by selective etching and X-ray diffractometry. It is found that the samples grown at a deposition rate of up to 500 μm/h consist of independently growing rods with both [111] A and [111] B directions. This indicates that the vapor composition in the growth region is almost stoichiometric. Both rod types exhibited transverse striations due to rotation twins. The twin boundaries in rods with the growth direction [111] A were shown to be also small-angle boundaries with additional misorientation of separate twins of 0.2°–0.3°. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 37, No. 2, 2003, pp. 129–133. Original Russian Text Copyright ? 2003 by Klevkov, Martovitskii, Medvedev.     

Maskless selective area growth of InP on Sub-μm V-groove patterned Si(001)

Low pressure metalorganic chemical vapor deposition of InP onexactly oriented Si(OOl) substrates with a periodic V-groove pattern of periodicity ≤1.2 μm using a two temperature growth sequence (400 and 640°C) is reported. Planar InP layers with extremely low defect density of 7 × 10⁴ cm⁻² are obtained. For InP on V-grooves of width g ≤1.0μm, a planar surface is formed after less than 1 μm of growth. Formation or suppression of antiphase domains (APDs) is a function of the widths of the (OOl)-oriented ridges. For s ≤1 μm, epilayers are single domain and the direction is oriented parallel to the grooves. At 400°C, nucleation starts homogeneously on {111}-sidewallsand (001)-facets. While heating up to 640°C, InP migrates into the grooves, depleting almost completely the (001)-facets. During growth of the main layer, first the V-grooves are filled up. Subsequently (001)-ridges are overgrown laterally or voids are formed on top of them. This mechanism is responsible for both planarization and APD-suppression. The surface migration length of InP on Si(001) at 640°C is estimated to be ≈0.5 μm.     

The role of lattice mismatch in the deposition of CdTe thin films

Cadmium telluride (CdTe) is the most well-established II–VI compound largely due to its use as a photonic material. Existing applications, as well as those under consideration, are demanding increasingly stringent control of the material properties. The deposition of high-quality thin films is of utmost importance to such applications. In this regard, we present a report detailing the role of lattice mismatch in determining the film quality. Thin films were deposited on a wide variety of substrate materials using the pulsed laser deposition (PLD) technique. Common to all substrates was the strong tendency toward the preferential alignment of CdTe’s (111) planes parallel to the substrate’s surface. X-ray diffraction analysis, however, revealed that the crystalline quality varied dramatically depending upon the substrate used with the best results yielding a single-crystal film. This tendency also manifested itself in the surface morphology with higher structural perfection yielding smoother surfaces. The film quality showed a strong correlation with lattice mismatch. Texture analysis using the [111] pole figure confirmed that improvements in the lattice mismatch led to a higher degree of in-plane alignment of the (111) grains.