As-grown Surface Research Articles

Investigation of Defects and Polarity in GaN Using Hot Wet Etching, Atomic Force and Transmission Electron Microscopy and Convergent Beam Electron Diffraction

Availability of reliable and quick methods to investigate defects and polarity in GaN films is of great interest. We have used photo-electrochemical (PEC) and hot wet etching to determine the defect density. We found the density of whiskers formed by the PEC process to be similar to the density of hexagonal pits formed by wet etching and to the dislocation density obtained by transmission electron microscopy (TEM). Hot wet etching was used also to investigate the polarity of MBE-grown GaN films together with convergent beam electron diffraction (CBED) and atomic force microscopy (AFM). We have found that hot H3PO4 etches N-polarity GaN films very quickly resulting in the complete removal or a drastic change of surface morphology. On the contrary, the acid attacks only the defect sites in Ga-polar films leaving the defect-free GaN intact and the morphology unchanged. The polarity assignments, confirmed by CBED experiments, were related to the as-grown surface morphology and to the growth conditions.

Investigation of Defects and Polarity in GaN Using Hot Wet Etching, Atomic Force and Transmission Electron Microscopy and Convergent Beam Electron Diffraction

Availability of reliable and quick methods to investigate defects and polarity in GaN films is of great interest. We have used photo-electrochemical (PEC) and hot wet etching to determine the defect density. We found the density of whiskers formed by the PEC process to be similar to the density of hexagonal pits formed by wet etching and to the dislocation density obtained by transmission electron microscopy (TEM). Hot wet etching was used also to investigate the polarity of MBE-grown GaN films together with convergent beam electron diffraction (CBED) and atomic force microscopy (AFM). We have found that hot H 3 PO 4 etches N-polarity GaN films very quickly resulting in the complete removal or a drastic change of surface morphology. On the contrary, the acid attacks only the defect sites in Ga-polar films leaving the defect-free GaN intact and the morphology unchanged. The polarity assignments, confirmed by CBED experiments, were related to the as-grown surface morphology and to the growth conditions.

Modulated growth of thick GaN with hydride vapor phase epitaxy

High-temperature (HT) deposited buffer layer and flow modulation growth are applied as two modulated techniques to the growth of high-quality GaN films by hydride vapor phase epitaxy. The characterization results indicate that the use of HT buffer layer has improved the morphological, structural and optical properties of the GaN films. The improvement is due to the HT buffer layer that supplies both high-density nucleation centers and promotes the lateral growth of epitaxial film with a columnar structure. Moreover, the buffer layer is also helpful to relax the strain in the film. With the flow modulation growth, the dislocation density in the films are greatly reduced and the homogeneity is highly improved. We attribute this promotion to the relaxation of strain and suppression of defects in sublayers and reconstruction of the as-grown surface of sublayers due to the interruption of the growth for a certain time.

Effects of supersaturation and impurity on step advancement on TiO 2 (1 1 0) faces grown from high-temperature solution

A crucible-free crystal growth method is newly designed to investigate growth mechanisms in a high-temperature solution phase. Although the method is applicable to crystals grown either in solution or through liquid-phase epitaxy (LPE), this paper focuses on growth in solution. Changes in as-grown surface morphologies and step separation are quantitatively investigated for their dependence on supersaturation and impurities. The observed effect of impurity is that advancing steps are pinned. A reliable solubility curve is also determined by this method.

X-ray photoemission study ofMgB2

A c-axis oriented thin film and a high density sintered pellet of MgB2 have been studied by x-ray photoemission spectroscopy, and compared to measurements from MgO and MgF2 single crystals. The as-grown surface has a layer which is Mg-rich and oxidized, which is effectively removed by a nonaqueous etchant. The subsurface region of the pellet is Mg-deficient. This nonideal near-surface region may explain varied scanning tunneling spectroscopy results. The MgB2 core level and Auger signals are similar to measurements from metallic Mg and transition metal diborides, and the measured valence band is consistent with the calculated density of states.

Effect of sodium on Bridgman-grown Cu(In 1− xGa x) 3Se 5 crystalline materials

Crystalline Cu(In 1− x Ga x ) 3Se 5 ingots, corresponding to the reported ordered vacancy compound (OVC), were grown by a horizontal Bridgman method. X-ray Laue back-reflection was performed on the as-grown surface of Cu(In 1− x Ga x ) 3Se 5 ingots, showing the ingots to contain large single crystal regions. Hall effect measurements indicated that these materials have high electrical resistivity. Metallic Na was, for the first time, introduced into these materials to observe the doping effect using a sealed glass ampoule. Na can increase the carrier concentration significantly for the CuIn 3Se 5 material ( x=0) but does not have this effect on Cu(In 1− x Ga x ) 3Se 5 materials ( x>0). Furthermore, in the present work, contrary to the reported data for thin film material, Na did not change the conductivity type of the Cu(In 1− x Ga x ) 3Se 5 materials studied. Electron probe microanalysis (EPMA) showed that Na was detected only on the surface layer of the doped Cu(In 1− x Ga x ) 3Se 5 samples. The electron generation mechanism in the CuIn 3Se 5 material due to the Na diffusion could be explained by defect generation related to Se and In sites.

Nonuniformities in free-standing GaN substrates

In this study, we report that free-standing GaN substrates grown by the hydride vapor-phase epitaxy (HVPE) are found to contain nonuniform regions with low crystal and optical quality located close to the top (near as-grown surface) and bottom (near interface between GaN/sapphire) regions of substrate cross-section. We considered that the origins of these nonuniformities were surface reconstruction by undesired residual gas reaction after crystal growth on the top regions and the individual columns forming an irregular layer in the bottom regions by lattice mismatch and difference of thermal expansion coefficient between GaN films and sapphire substrate. We used cathodoluminescence imaging and spectroscopy for analyzing these nonuniform regions. The low quality regions with high electron concentration are easily visualized using cathodoluminescence (CL). The coexistence of regions with low- and high quality allows us to explain the concurrent evidence of high substrate quality in double crystal X-ray diffraction and photoluminescence (PL).

Defect investigation of GaN thin films etched by photo-electrochemical and hot wet etching by atomic force and transmission electron microscopy

ABSTRACTThe availability of reliable and quick methods to investigate defects in GaN films is of great interest. Photo-electrochemical (PEC), and hot wet etching using both H3PO4 acid and molten KOH have been used to study structural defects in GaN layers grown by hydride vapor phase epitaxy and molecular beam epitaxy. The purpose of this work is to determine whether, and under what conditions, these different methods of investigation are consistent and to get to a more accurate estimation of the defect density. As-grown and etched surfaces were investigated by atomic force microscopy (AFM), and plan-view and cross-sectional transmission electron microscopy (TEM). Free-standing whisker-like features and hexagonal etch pits were formed on the etched sample surfaces by PEC and wet etching, respectively. Using plan-view AFM, we found the density of whiskers (8x108-1×109 cm−2) to be similar to the etch pit densities when etched in both H3PO4 and molten KOH under precise etching conditions. During the wet etching process, a careful balance must be struck to ensure that every defect is delineated, but not overetched to cause merging which would lead to an underestimation of the defect density. Additionally, TEM observations confirmed the dislocation densities obtained by etching, which increased our confidence in the consistency of the methods used.

Observation of anisotropic initial growth nucleation in liquid phase epitaxy of InP

Liquid-phase epitaxial growth of InP was performed at constant growth temperature by the temperature difference method under controlled vapor pressure on InP substrates. Growth temperature was kept constant from 550°C to 280°C. The as-grown surface morphology was investigated by Nomarski interference optical microscope and electron microscope. The anisotropic initial growth nuclei are investigated on {0 0 1},{1 1 1}A,B and {1 1 0} oriented InP substrates. This anisotropic behavior is discussed in view of the anisotropy of lateral growth rate due to the preferential surface migration and sticking at specific kinks and steps on the surface.

Photoemission characteristics of diamond films

In this study, we investigate the photoelectric emission from CVD diamond films. These diamond samples present NEA properties due to their as-grown surfaces terminated with hydrogen atoms. Photocathodes are characterised by UV pulsed laser-induced photoelectric measurements and photoelectric threshold measurements because the photoelectric emission is strongly dependant on the electron affinity of the diamond surface. Photoelectric threshold measurements show the existence of a sub-bandgap signal associated to a defect-band level for both samples, with the lowest value obtained for the highest defect-density diamond film. Moreover, the quantum efficiency of undoped diamond is measured at 213nm as a function of the CH4 concentration. The highest quantum efficiency value is measured for the highest defect-density diamond film. Surface bonds modifications occurring during a prolonged laser irradiation are responsible for the decrease in the photoemissive performances of diamond films.

Scanning tunneling microscope studies on atomic structures in NdBa 2Cu 3O 7−δ single crystals II

Atomic structures of cuprate NdBa 2Cu 3O 7−δ (Nd123) processed by the oxygen-controlled-melt-growth (OCMG) method, have been studied by scanning tunneling microscope (STM). A simple free energy diagram is proposed to understand the simultaneous formation of island and striped structures observed within the same single crystal sample. Surfaces observed are the as-grown surface. Its atom images have been attributed to the metallic elements of Cu, Nd or Ba.

Precipitates in Ribbon Grown Solar Silicon

Etching experiments reveal new insights into the character of the complex interaction of structural defects and precipitates in a ribbon grown solar silicon material (RGS). Crystal defects are densely decorated with precipitates which cannot be dissolved by the etch. The etched out chains of precipitates represent their situation in the Si wafer before being etched. SEM combined with EDX are suitable means to investigate these defects. Besides the dislocations and dislocation networks in grain boundaries marked by precipitates (EDX: Si + O + C), dendrites were found which may affect the electrical properties of a solar cell. At both as-grown surfaces SiC crystallites are built in, which do not activate dislocation sources and so do not increase the dislocation density in the wafer.

Atomic structure and composition of the P-rich InP(001) surfaces

The microscopic structure of the P-rich (2×1)-like surfaces of InP(001) is investigated by soft X-ray photoemission spectroscopy (SXPS) and scanning tunneling microscopy (STM). The samples were grown by metal organic vapor phase epitaxy (MOVPE) and then transferred under ultra high vacuum (UHV) conditions to UHV analysis chambers. STM images show a P-rich as-grown surface after transfer and another less P-rich surface after prolonged annealing at 350°C. The In4d emission line is not affected by the change in surface reconstruction and shows a small surface component shifted by 0.48 eV towards higher binding energies. The line shape of the P2p core level, on the other hand, changes: On the as-grown surface, one surface component which is shifted by 0.98 eV towards higher binding energies is found after transfer, whereas two surface components shifted to higher and lower binding energies, respectively, appear after annealing at 350°C. These results are consistent with the structure models derived from STM for the different P-rich (2×1)-like surfaces.

Scanning force microscopy of ion-irradiated organic single crystals of benzoyl glycine

Single crystals of the amino acid benzoyl glycine (hippuric acid) are irradiated normal to the as-grown surface by highly charged Bi ions with a kinetic energy of 2.38 GeV and a fluence of 1×1010 ions/cm2. The projectiles create circular craters with a mean diameter of 40 (10) nm on the surface of the crystal as observed by scanning force microscopy (SFM). The mean depth amounts to 4 (1) nm, this value being considered as a lower limit due to the finite radius of curvature of the force cantilever tip. Thus, on the average, each single-ion projectile seems to eject about 104 molecules. On the surface of non-irradiated crystals, SFM reveals terraces of a few monolayers in height. In water, it was possible to visualize the lattice periodicity. Terraces were also observed on the irradiated crystal surface in the presence of the craters, indicating that the crystal is still intact at the given dose.

Surface structural and etching studies on high-temperature solution grown GdAlO3 crystals

Abstract Surface structural studies of as-grown GdAlO3 crystals revealing formation of microdisc elevations, growth fronts, solid inclusions and etch pits are reported. It is suggested that the etch patterns on as-grown different surfaces of the crystals may be due to etching during cleaning process of crystals. Etch patterns on GdAlO3 crystal surfaces are described and discussed. Defect structures on GdAlO3 crystals include impurity sites, dislocations, low-angle grain boundaries and twinning (simple as well as complex). Results of etching kinetics of H3PO4–GdAlO3 surface system are reported. H3PO4 is shown to be a suitable dislocation etchant for GdAlO3. The activation energy and Arrhenius factor for dissolution parallel, and perpendicular, to the surface for new pits (developed due to laboratory etching) are estimated for these crystals using graphical methods.

Structural improvement in sublimation epitaxy of 4H–SiC

The sublimation epitaxy growth process has been studied. The structural quality of the grown layers improves compared with the substrate mainly due to a diminished domain structure misorientation. Optical microscopy shows that the as-grown surfaces are free of typical defects appearing in silicon carbice (SiC) epitaxy, whereas atomic force microcopy measurements show macrosteps. As a possible technique to produce high-quality 4H–SiC, sublimation epitaxy was performed on substrates containing a layer grown by liquid phase epitaxy which is a growth process for closing micropipes in the initial substrate. In spite of the initial surface roughness of the liquid phase epitaxy layer, the surface morphology of the sublimation grown epilayers remained smooth and the structural quality improvement was maintained. This does not occur if the initial surfaces are too rough. A suggestion for roughness reduction is presented. The growth conditions (growth rate ramp up, growth temperature, temperature gradient, source to substrate distance, and substrate surface orientation) leading to the results are presented. A model for the mechanism for structural improvement is outlined and supporting experimental observations are given.

Characterization of Anisotropic Step-bunching on as-grown SiC Surfaces

We report the presence of anisotropic step-bunching in SiC epitaxy on off-oriented substrates. This is an effect of step-flow growth. The anisotropic step-bunching is discussed in relation to the temperature dependence of lateral growth velocities and the interface roughness. The macrostep appearance is probably related to formation of morphologically stable faces with low surface free energy.

Nucleation and growth analysis of microcrystalline silicon by scanning probe microscopy: substrate dependence, local structural and electronic properties of as-grown surfaces

Nucleation and growth of hydrogenated microcrystalline silicon films are investigated by scanning probe microscopy and spectroscopy of as-grown films. We measure highly doped films grown on oxidized silicon and discuss nucleation of crystallites in comparison with hydrogen-passivated substrates. It turns out that the oxidized substrate mediates nucleation. This mechanism is preferred over nucleation on hydrogenated amorphous silicon. For films as thin as a few nanometers we demonstrate that scanning tunneling microscopy and spectroscopy are capable of spatially resolving structural and electronic surface properties. Amorphous and microcrystalline surface elements are distinguished by different types of tunneling spectra showing by a factor of ∼5 a larger rectification for crystallites.

Investigation of the mechanism for Ti/Al ohmic contact on etched n-GaN surfaces

In this study, the mechanism for ohmic contact of Ti/Al bilayer formation on as-grown, etched and postetch annealed GaN surfaces were investigated. A nonalloyed Ti/Al ohmic contact to etched GaN surface, with postetch annealing prior to metal deposition, was obtained. The specific contact resistance of 2.3×10−4 Ω cm2 was obtained. The nonalloyed ohmic contact may be attributed to the postetch annealing which generates nitrogen vacancies that result in a heavily n-type surface thereby forming a tunneling junction. On the other hand, the nonalloyed Ti/Al contact on as-grown and as-etched GaN surfaces exhibits non-ohmic behavior. After alloying at 500 °C for 5 min, Ti/Al contacts on as-grown, as-etched and postetched annealing GaN surfaces have specific contact resistances around 9.8×10−5, 1×10−4, and 7.2×10−5 Ω cm2, respectively. Nonalloyed Ti/Al ohmic contacts would be especially useful for fabricating high breakdown, recessed-gate field effect transistors on GaN since the moderate postannealing condition converts only the near surface layer to heavily n type.

Surface reconstruction of Fe 3O 4(001)

We have investigated the surface termination, structure, morphology and composition of Fe 3O 4(001) using scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), low-energy He +-ion scattering (LEIS) and X-ray photoelectron spectroscopy (XPS). The samples consisted of ∼5000 Å thick epitaxial films of Fe 3O 4(001) grown by oxygen-plasma-assisted molecular-beam epitaxy (OPA-MBE) on MgO(001) substrates. The ( 2 × 2 ) R45° surface reconstruction that is present on the as-grown surface is recovered by heating the sample in oxygen (10 −6–10 −7 mbar) at temperatures between 420 and 770 K after a through-air transfer from the MBE chamber. STM results are best interpreted by assuming an autocompensated B-layer termination, which consists of a layer of octahedrally coordinated iron and tetrahedrally coordinated oxygen, along with one oxygen vacancy per unit cell. Evidence for a vacancy-induced lateral relaxation of the adjacent octahedral iron ions is presented. Further annealing in ultrahigh vacuum causes a transformation to either a (1× n) or a (2 2 × 2 ) R45° structure. These surfaces can be reproducibly transformed back to the ( 2 × 2 ) R45° reconstruction by annealing in oxygen. Interestingly, at no time do we observe the other autocompensated termination, which consists of one-half of a monolayer of tetrahedrally coordinated Fe(III), despite its observation on the as-grown surface. Thus, it appears that the surface termination is critically dependent on the method of surface preparation.