Poor Surface Morphology Research Articles

MBE growth of lattice-matched and mismatched films on non-(001) GaAs substrates

There has recently been great interest in growth by molecular beam epitaxy of GaAs films on non-(001) oriented GaAs substrates. This has occurred largely because of the possibility of fabricating novel device structures such as quantum wires and quantum boxes using patterned substrates. However, the growth of high quality lattice matched and mismatched structures on non-(001) GaAs substrates has had a reputation amongst many researchers of being ‘difficult’. Epitaxial layers grown on these surfaces have generally had poor surface morphology, bad crystalline quality and irreproducible dopant incorporation behaviour. In this paper we will show that the growth of these materials is not ‘difficult’, but ‘different’ and that provided suitable growth conditions are used then high quality layers may be deposited on all substrate orientations. As an illustration we will use reflection high energy electron diffraction to predict the conditions for high quality growth GaAs on the GaAs(111)A. We will then extend this to the mismatched (InGa)As:GaAs(111)A system.

Effect of methyl surface saturation during growth interruption sequences of metalorganic vapor-phase epitaxy of In0.53Ga0.47As using trimethylarsenic

Abstract We have investigated the use of trimethylarsenic (TMAs) as an alternative to arsine in the metal organic vapor-phase epitaxy (MOVPE) of heterostructures containing arsenic-based compounds on InP. We particularly focused this study on the growth interruption sequences (GIS) between two arsenic-based materials using TMAs. We show that a too long stabilization time under TMAs is not desirable. It produces a saturation of the surface with methyl radicals that hampers the growth of the second material (after the GIS). This results in a very poor surface morphology of the sample. An important improvement of the surface morphology is achieved using a long hydrogen purge after the stabilization time under TMAs. This allows the desorption of most of the methyl radicals on the surface before the growth of the second layer. However, even with this hydrogen purge, the surface morphology is not perfect and presents some defects.

MOVPE growth of thick homogeneous InGaN directly on sapphire substrate using AlN buffer layer

InGaN films have usually been grown on GaN epitaxial layers but it is difficult to grow the thick InGaN films on GaN because of strong stress induced by large lattice mismatch between InGaN and GaN. The thick InGaN films grown on GaN have poor surface morphologies. In this paper it is described that the growth of thick InGaN films of a few microns thickness has successfully been achieved directly on sapphire substrates using AlN buffer layers. On AlN buffer layers homogeneous InGaN with 2.5 μm thickness and large indium mole fraction of 0.2 could be grown. The growth rate of 2.5 μm/h is much larger than has been reported previously.

Silicon incorporation behaviour in GaAs grown on GaAs (111)A by molecular beam epitaxy

We have made a systematic study of the effect of growth conditions on Si incorporation in GaAs layers grown on GaAs (111)A substrates. We show that it is dominated by the low incorporation coefficient of As 4 on the GaAs (111)A surface. The site occupancy of the Si has been shown by local vibrational mode spectroscopy to be on Ga and As lattice sites to provide donor and acceptor character, respectively. The doping behaviour of Si for specific growth conditions may be predicted from the product of the As 4:Ga flux ratio and the As 4 incorporation coefficient at the growth temperature. The growth conditions for which Si acts as an acceptor produce films with poor surface morphology.

Effect of shroud flow on high quality InxGa1−xN deposition in a production scale multi-wafer-rotating-disc reactor

High quality InGaN thin films and InGaN/GaN double heterojunction (DH) structures have been epitaxially grown on c-sapphire substrates by MOCVD in a production scale multi-wafer-rotating-disc reactor between 770 to 840°C. We observed that shroud flow (majority carrier gas in the reaction chamber) is the key to obtaining high quality InGaN thin films. High purity H2 as the shroud flow results in poor crystal quality and surface morphology but strong photolumines-cence (PL) at room temperature. However, pure N2 as the shroud flow results in high crystal quality InGaN with an x-ray full width at half maximum (FWHM)InGaN(0002) of 7.5 min and a strong room temperature PL peaking at 400 nm. In addition, InGaN/GaN single heterojunction (SH) and DH structures both have excellent surface morphology and sharp interfaces. The full width at half maximum of PL at 300K from an InGaN/GaN DH structure is about 100 meV which is the best reported to date. A high indium mole fraction in InGaN of 60% and high quality zinc doped InGaN depositions were also achieved.

LP-MOCVD grown (lnAs)m(GaAs)m short period superlattices on InP

(InAs)m(GaAs)m (1 ≤ m ≤ 12) short period superlattices (SPSs) have been grown on semi-insulated InP substrates with a 200 nm InP cap layer using low pressure metalorganic chemical vapor deposition (MOCVD). According to double crystal x-ray diffraction and transmission electron microscopy results, the critical layer thickness of (InAs)m(GaAs)m SPS was observed to be ∼30A (m = 5). For the SPS below the critical layer thickness, mirror-like surface morphology was found without defects, and strong intensity Fourier transformed photoluminescence (FT-PL) spectra were also obtained at room temperature. The SPS with m = 4 showed a drastic improvement in photoluminescence intensity of order of two compared to an InGaAs ternary layer. However, the SPS with a large value of m (m ≥ 6), rough surface was observed with defects, with broad and weak FT-PL spectra. The surface morphology of SPS was greatly affected by the substrate orientation. The SPS with m = 5 was grown on two degree tilted substrate from (100) direction and showed poor surface morphology as compared to the one grown on (100) exact substrate Moreover, the SPS grown on a (111)B substrate showed a rough triangular pattern with Nomarski optical microscopy. In-situ thermal annealed SPS with m = 4 showed a 18 meV increase in PL peak energy compared to the as-grown sample due to phase separation resulting from thermal interdiffusion.

Electrical characterization of CVD diamond thin films grown on silicon substrates

Diamond thin films grown on high resistivity, 〈100〉 oriented silicon substrates by the hot filament chemical vapor deposition (HFCVD) method have been characterized by four-point probe and current-voltage (through film) techniques. The resistivities of the as-grown, chemically etched and annealed samples lie in the range of 10 2 Ω cm to 10 8 Ω cm. The Raman measurements on these samples indicate sp 3 bonding with a sharp peak at 1332 cm −1. The surface morphology as determined by scanning electron microscope shows polycrystalline films with (100) or (111) faceted structures with average grain size of ≈2.5 μm. The through film current-voltage characteristics obtained via indium contacts on these diamond films showed either rectifying or ohmic behavior. The difference in Schottky and ohmic behavior is explained on the basis of the high or low sheet resistivities measured by four-point probe technique. 5% methane to hydrogen concentration during film growth resulted in poor surface morphology, absence of sp 3 bonds, and low resistivity.

The growth and characterization of (211) and (133) Oriented (Hg,Cd)Te epilayers (211)B GaAs by organometallic vapor phase epitaxy

The growth of CdTe buffer layers on (211)B GaAs substrates by organometallic vapor phase epitaxy (OMVPE) was studied, and it was found that, depending on the growth conditions, either the (211) or (133) epitaxial orientation could be formed. In some cases, an epilayer showing a mixed (211) and (133) orientation was also observed. The influence of several growth parameters on the orientation of the CdTe layer was investigated, and it was found that the Te/Cd ratio, together with the growth temperature, have the most significant effect in determining the epilayer orientation. From these results, it was then possible to select nominally optimized growth conditions for CdTe buffer layers of both orientations. (Hg,Cd)Te layers of the same orientations could then be grown and characterized. Although double crystal x-ray diffraction measurements indicated a somewhat better crystalline perfection in the (133) (Hg,Cd)Te layers, these layers showed a poor surface morphology compared to the (211) orientation. Measurement of etch pit densities also indicated defect densities to be typically half an order of magnitude higher in the (133) orientation. Diodes were formed by ion implantation in both orientations and significantly better results were obtained on the (211) (Hg,Cd)Te layers.

Deposition of epitaxial β–SiC films on porous Si(100) from MTS in a hot wall LPCVD reactor

Epitaxial β-SiC thin films were grown on modified Si(100) substrates from methyltrichlorosilane (CH3SiCl3 or MTS) in a hot wall reactor by using low pressure chemical vapor deposition (LPCVD). At 1150 °C, the growth rate of the β-SiC films was 120 Å/min. Epitaxial β-SiC(100) thin films were deposited after the deposition time of 12.5 min. However, the crystallinity of the deposited films was influenced by the deposition time. For example, the occurrence of rotational β-SiC(100) crystals and polycrystalline β-SiC with a highly preferred orientation of (100) planes was obtained for the deposition time of 50 min. XRD and TEM showed the appearance of polycrystalline β-SiC films with a preferred orientation of β-SiC(111) after further increasing the deposition times (time ≥ 75 min). At 1100 °C, polycrystalline β-SiC films with poor surface morphology were observed even though the film had a preferred orientation of β-SiC(100) for short deposition time (e.g., 12.5 min). Polycrystalline β-SiC(111) film was obtained for the deposition time of 200 min at this temperature.

Double Modulation and Selective Excitation Photoreflectance for Characterizing Highly Luminescent Semiconductor Structures and Samples with Poor Surface Morphology

AbstractPhotoreflectance (PR) is a powerful, contactless, and nondestructive technique capable of probing interband electronic transitions and built-in electric-fields at the surface and in interface regions in semiconductor materials and microstructures. It has been widely used as a characterization tool. However, its application to highly luminescent systems, such as quantum-well (QW) lasers and samples with poor surface morphology has been limited because of the difficulty in minimizing the interference from the luminescence and the pump beam that is scattered from the surface. We present a double modulation procedure where both the probe and pump beams are modulated which allows the PR component to be completely separated from the luminescence and scattered contributions. The separation is achieved through the appropriate choice of modulation frequencies and specially designed tuned amplifiers. A complete PR system, along with the necessary circuits, is presented. In addition, we have also exploited the freedom provided by the system to choose any pump wavelength to selectively modulate specific regions of the multilayer structure (QW, barrier, and cladding layer) and extract detailed information regarding the properties of each layer. The applicability of the procedure is demonstrated by fully characterizing a GaInP based QW laser structure used for visible emitters and by measurements on a saw-cut and chemically etched back surface of a commercial GaAs wafer. Finally, procedures attempted in the past to overcome difficulties with the luminescence and surface scattering are discussed and compared to the double modulation PR technique.

Necessity of Ga prelayers in GaAs/Ge growth using gas-source molecular beam epitaxy

We have investigated the gas-source molecular beam epitaxy growth of GaAs on a (001) Ge surface, off-cut 6° towards the [110]. Initiation of GaAs on Ge with the typical procedure of using a self-terminating As layer invariably produces poor GaAs surface morphology and generates a plethora of antiphase boundaries, despite the large miscut of the (001) surface. However, initiation of GaAs growth with approximately 1 ML (monolayer) of Ga results in single-domain films with excellent surface morphology. We conclude that the Ga monolayer prevents the Ge surface from forming a high step density surface which is established when Ge is exposed to As2.

The growth of various buffer layer structures and their influence on the quality of (CdHg)Te epilayers

The suitability of various buffer layer structures on (100) GaAs for (CdHg)Te growth by organometallic vapour phase epitaxy (OMVPE) was investigated. The preferred epitaxial orientation of (100)GaAs (111)CdTe was found to be unsuitable due to the formation of electrically active defects in the material. An intermediate ZnTe layer was used to select the (100) orientation and (100) CdTe layers were when deposited on this ZnTe layer. The quality of the resultant CdTe buffer was found to critically depend on the thickness of this intermediate ZnTe buffer, with a ZnTe thickness of approximately 500 Å producing the best CdTe buffer. (CdHg)Te epilayers grown on these ZnTe CdTe buffers had improved electrical properties, but still suffered from a poor surface morphology. This surface morphology could be improved by using a lattice matched Cd 0.96Zn 0.04Te alloy as the final buffer layer, but the surface pyramids typical of the (100) orientation could never be completely eliminated.

Growth of Ga 0.29In 0.71As 0.61P 0.39 ( λ ≈ 1.3 μm) on InP by gas source molecular beam epitaxy

The surface morphology of GaInAsP layers for wavelengths near 1.3 μm grown on InP substrates was found to become poor under growth conditions which were only slightly different from the near optimum growth conditions. The substrate temperature was found to be a critical parameter. The quality of the films with negative lattice mismatch differed significantly from those with positive mismatch. In addition, double-crystal XRD features from the layers with poor surface morphology were considerably declined.

The effects of As overpressure and diffusion source on the diffusion of Mn in GaAs

Data are presented to show the effect of As overpressure on the diffusion of Mn in GaAs using four different Mn sources. These sources include solid Mn thin film deposited directly on the GaAs substrate and Mn vapors from pure Mn, MnAs, and Mn3As solids. In the circumstance for which a solid Mn film is used as the diffusion source, a nonuniform doping distribution and poor surface morphology is obtained due to a reaction between the Mn film and the GaAs matrix. The degraded surface consists of a layer of polycrystalline cubic alloy having a lattice constant of nearly 8.4 Å and a composition close to MnGa2 with a small amount of As. Of the remaining diffusion sources (Mn, MnAs, and Mn3As), only MnAs consistently produces a uniform doping distribution and smooth surface morphology. For diffusions at 800 °C, a uniform surface hole carrier concentration as high as 1020/cm3 can be obtained using MnAs as the source. The As overpressure is found to drastically alter the Mn diffusion profile, and Mn, like Zn, may diffuse in GaAs interstitial-substitutionally. Vapor from both the Mn and Mn3As solids degrade the GaAs surface. Mn3As, however, uncharacteristically degrades the surface more rapidly although the details of such are not well understood. With the presence of a high As overpressure, however, both surfaces of the Mn and Mn3As sources are converted to (Mn,As) compounds, the compositions being close to MnAs. High enough As overpressures are shown to completely suppress the GaAs surface degradation which is evident when Mn3As alone is used as the diffusion source.

The combination of high-temperature sputtering with excimer laser planarization for submicrometer contact via filling

We have systematically investigated the use of high-temperature sputtering in conjunction with excimer laser planarization for planarizing Al alloy surfaces and filling submicrometer, high aspect ratio (∼1–3) contact vias. In general, the process window for the complete contact filling increased significantly with the increasing sputtering temperature, and decreased with shrinking contact/via geometry. Better as-deposited metal step coverage and profile obtained at higher sputtering temperatures were identified as the key parameters contributing to this significant improvement in the process window. The process window achieved by this combined process is significantly wider than that obtained by high-temperature sputtering alone. In addition, the poor surface morphology generally observed for the Al alloy films sputtered at elevated temperatures is significantly improved due to recrystallization during laser reflow.

Cathodoluminescence and TEM characterization of defects in III-V materials grown on Ge-terminated Si substrates

The growth of III-V materials on Si has been a major focus in electronic materials research for nearly a decade. The highly complementary properties of this heteroepitaxial system include high mobility and direct band gap in III-V materials and high crystalline quality, low cost and well-established technology of Si. However, the attainment of high crystalline quality III-V materials on Si suitable for minority carrier devices has been inhibited primarily due to the large lattice mismatch (˜4%) and thermal expansion mismatch (factor of ˜3) as well as the problematic nucleation of an ordered semiconductor on an elemental semiconductor. The epitaxial layer can contain threading dislocations, microtwins, stacking faults, antiphase domain boundaries (APBs) and/or have a poor surface morphology. A buffer layer can be added to improve the III-V material quality by accommodating the lattice mismatch at one or more interfaces and/or to provide a more suitable surface for nucleation. We have used TEM in conjunction with scanning cathodoluminescence (CL) intensity mapping for complete and accurate assessment of crystalline quality and defect densities within a range of ˜ 5×102 cm-2 to > 108 cm-2.

Some properties of the first non-aqueous electro-codeposited InP, In and P thin films

A preparatory technique for InP films is described. The composition, surface feature and structural characterizations of the as-deposited films indicated the formation of InP but with poor surface morphology. Annealing appears to change the surface topography and shows no improvement in the quality of the film. The optical and electrical studies of the as-deposited films showed them to be an n-type semiconductor with a band gap of 1.35 eV.

Regrowth of a GaAs layer for n-GaAs ohmic contacts

Reaction of Au with GaAs in conventional NiAuGe/GaAs ohmic contacts results in rough interfaces, poor surface morphology, and deterioration at the edges of films. To improve these properties, ohmic contacts to GaAs were formed by heating initially discrete layers of volatile arsenic and nonvolatile gallium under a Ge–Au alloy thin film. The layer sequence beginning at the air interface was AuGe(1500 Å)/As(500 Å)/Ga(400 Å), all evaporated onto chemically cleaned (100) GaAs surfaces, followed by annealing at 400 to 650 °C in forming gas. A regrown layer of highly doped GaAs was formed between the AuGe layer and the original GaAs substrate. This layer was characterized by scanning Auger electron spectroscopy, scanning electron microscopy, and electrical measurements. The carrier concentration of the regrown GaAs layer was estimated from four-point probe data to be in the range of 1019 to 1020 cm−3. The specific contact resistivity for the AuGe/As/Ga/GaAs regrown at 550 °C for 10 min was ≊1.5×10−5 Ω cm2, which is similar to values measured for AuGe/GaAs annealed at 450 °C for 2 min. The mechanism of GaAs regrowth and ohmic contact formation are discussed.

Nucleation and Heterointerface Structure of InAs Epilayers Grown on InP Substrates

ABSTRACTHeteroepitaxial growth of indium arsenide films on indium phosphide substrates is being actively pursued since the electronic properties of these films make them promising materials for optoelectronic and other high speed devices. The various structural aspects of the film that affect their electronic properties are structural defects like dislocations, film-substrate interface roughness and chemical inhomogeneities. In InAs films, electrons accumulate at the film-air interface, making surface morphology an important factor that decides the electronic properties. The InAs films used in this study were grown on InP substrates by metal organic vapor deposition, at different temperatures. A higher growth temperature not only resulted in poor surface morphology of the film, but also created a rough film-substrate interface. However, at all deposition temperatures, the film-substrate interfaces are sharp. At lower growth temperature, the interfaces were flat. Films grown at lower temperatures had good surface morphology and a flat and shaip heterointerface.

Diffusion of manganese in GaAs and its effect on layer disordering in AlxGa1−xAs-GaAs superlattices

Several diffusion runs of Mn in GaAs are performed in sealed quartz ampoules with four different Mn-containing sources: (a) solid crystal granules of Mn, (b) Mn3As, (c) MnAs, and (d) Mn thin films coated on GaAs substrates. Among these, only MnAs results in a smooth GaAs surface and uniform doping distributions. For the others interactions between the source materials and GaAs substrates give rise to poor surface morphologies and inhomogeneous distributions of new-phase (Mn,Ga) crystals. For diffusion at 800 °C, surface p-type carrier concentrations as high as 1020/cm3 are obtained. Diffusion profiles determined by C-V techniques resemble those obtained for Zn diffusions. A substitutional-interstitial mechanism is suggested as the primary diffusion mechanism for Mn in GaAs. Data are also presented showing that layer disordering in AlGaAs-GaAs superlattices can be induced by Mn impurities.