Ga-rich Layer Research Articles

Evolution of crystallinity of GaN layers grown at low temperature on sapphire with dimethylhydrazine and triethylgallium

The crystallinity of GaN layers grown at low temperature by organometallic vapor phase epitaxy on sapphire using dimethylhydrazine and triethylgallium has been studied with Raman spectroscopy, atomic force microscopy and Rutherford backscattering spectroscopy. The layers were grown in the temperature range from 520°C to 660°C. Amorphous, possibly non-stoichiometric Ga-rich layers were produced below 560°C. Smooth layers of crystalline GaN with a disordered structure were produced between 560°C and 600°C. Rough but crystalline layers were produced at higher temperatures. The minimum temperature for production of crystalline layers occurs at about 580°C.

Characterization of CuGaxSey/ZnO for superstrate solar cells

In the Cu(In,Ga)Se2 system the bandgap can be varied between 1.0 and 1.7 eV, which offers the possibility of tandem devices. For a development in this direction, superstrate solar cells have been fabricated with CuGaxSey absorber layers. The p–n junction is formed by vacuum evaporation of p-type CuGaxSey directly on the n-type ZnO grown by RF-magnetron sputtering. The grain size and lattice parameters of CuGaxSey depend on the Cu/Ga ratio. Optical transmission measurements have been used to determine the bandgap of CuGaxSey layers. The Ga-rich phase CuGa3Se5 has a bandgap of 1.9 eV compared to 1.7 eV of the CuGaSe2 phase. The Ga-rich layers are p-type conducting and they form a photovoltaic diode on n-ZnO. Photovoltaic properties of CuGaxSey/ZnO superstrate solar cells are correlated with the growth conditions and structural properties of the absorber layer. The formation of an interfacial layer of about 10 nm thickness and precipitates on top of the ZnO grains, have been observed with transmission electron microscopy.

Defects in CuGaSe2 thin films grown by MOCVD

Optical and electrical properties of CuGaSe2/GaAs(001) heteroepitaxial layers as a function of chemical composition are presented. The photoluminescence (PL) spectra observed for Cu-rich epitaxial layers are interpreted in a model consisting of one donor and two acceptor levels with ionization energies of 80, 50 and 10 meV, respectively. The radiative recombination of Ga-rich samples is dominated by broad donor-acceptor-pair bands that shift to lower energies with decreasing Cu/Ga-ratio. The peak energies of these broad emissions are strongly excitation power-dependent showing a blueshift of up to 17 meV per decade. The observed PL properties of Ga-rich samples are discussed in terms of strong compensation as supported by Hall measurements. Hall mobilities of above 250 cm2/V s have been found for near stoichiometric, slightly Ga-rich epitaxial layers. The electrical properties of CuGaSe2/GaAs(001) grown under Cu-rich conditions are found to be influenced by a CuxSe secondary phase as expected from the Cu-Ga-Se phase diagram.

KrF excimer laser induced ablation–planarization of GaN surface

The surface roughness of an as-grown GaN epilayer is reduced using a combination of KrF excimer laser irradiation and post chemical wet treatment. KrF excimer laser irradiation ablates GaN surfaces, resulting in the formation of a Ga-rich layer on the surface. The Ga-rich layer is etched off by hydrochloric acid treatment. X-ray photoelectron spectroscopic (XPS) analysis reveals that the chemically etched surface has similar composition and chemical bonding to those of untreated GaN. The average roughness ( R a) is improved by ∼43% compared with an untreated GaN sample with a laser fluence of over 2.0 J/cm 2.

Morphology, polarity, and lateral molecular beam epitaxy growth of GaN on sapphire

Gallium nitride was grown on sapphire (0001) substrates by radio frequency plasma assisted molecular beam epitaxy. The surface morphology was characterized during growth by reflection high energy electron diffraction, and ex situ by scanning electron microscopy (SEM), atomic force microscopy (AFM) and x-ray diffraction. It is found that surface morphological features are linked to domains of specific wurtzite crystal polarity, (0001)Ga face or (0001̄)N face, for Ga-rich growth. For growth on AlN buffer layers, we commonly observe films which consist of largely (0001)Ga polarity material, as confirmed by selective etch tests, with a varying coverage of (0001̄)N-face inversion domains threading along the growth direction. For growth near stoichiometric conditions, the growth rate of the N-face domains is slightly lower than that for the Ga-face matrix, which results in the formation of pits with inversion domains at their centers. For samples grown by first depositing GaN under N-rich conditions, followed by growth under Ga-rich conditions, a different morphology is obtained, exhibiting large hexagonal flat terraces observable by SEM and AFM. The apparent grain size of these films is increased substantially over films grown using a single step approach. The cross sectional SEM images of the two-step films show a network of voids and columns at the interface between the N-rich and the Ga-rich layers, above which micron-scale islands form and coalesce via lateral growth. Lateral growth may result in reduced defect density and improved crystal quality. The asymmetric x-ray peak (112̄4) width is reduced to approximately 280 arcsec in the two-stage GaN films.

Microstructural study of the CdS/CuGaSe2 interfacial region in CuGaSe2 thin film solar cells

The microstructure of the CdS/CuGaSe2 interface region in Cu-rich CuGaSe2-based polycrystalline thin film solar cells with KCN-treated absorber layers are characterized. Two recipes for the chemical bath deposition (CBD) of CdS with different bath temperatures (60 and 80 °C) are compared. Coherent Cu–Se precipitates are observed in both cases in the grains of the absorber layer. This precipitation cannot be avoided and seems to be a principal limitation for the performance of Cu-rich CuGaSe2-based thin film solar cells. There is a significant difference between both recipes concerning the interaction with the absorber layer surface. For bath temperatures of 80 °C the interaction is much stronger and Cu–S inclusions are found in the buffer layer. These may be responsible for shunts across the pn junction. Owing to the reduced interaction of the CdS deposited at 60 °C there are no Cu–S inclusions. For the 80 °C recipe the CdS/CuGaSe2 interface region consists of a continuous transition zone with low defect density, whereas for the 60 °C recipe the interface is sharper, but the CdS layer contains a high density of stacking faults. The structure of the CdS layer depends also on the bath temperature and the growth orientation of the CuGaSe2 grains. CdS(80 °C) crystallizes predominantly in the zincblende structure and contains less linear and planar defects than CdS(60 °C) which tends to incorporate hexagonal regions in the cubic matrix. Strains due to lattice mismatch as well as mixture between wurtzite and zincblende structures were revealed in high resolution transmission electron microscopy (HRTEM) images of the CdS(60 °C) layer. For CdS(80 °C) the strain is relaxed by twinning and small-angle grain boundaries which were imaged by HRTEM. A suitable CdS buffer layer for Cu-rich absorber layers could not be obtained by CBD because of either the low crystal quality [CdS(60 °C)] or the formation of Cu–S inclusions [CdS(80 °C)]. The enhanced interaction with the Ga-rich absorber layer and improved quality of CdS(80 °C) results in an improved device performance of Ga-rich CuGaSe2-based solar cells.

Chemistry and kinetics of the GaN formation by plasma nitridation of GaAs: Anin situreal-time ellipsometric study

The chemistry and kinetics of the nitridation of GaAs (100) surfaces by ${\mathrm{N}}_{2},$ ${\mathrm{N}}_{2}{\ensuremath{-}\mathrm{H}}_{2},$ and ${\mathrm{N}}_{2}{\ensuremath{-}\mathrm{N}\mathrm{H}}_{3}$ radio-frequency plasmas, in a remote configuration, are investigated in situ and in real time using spectroscopic ellipsometry. The effects of the surface temperature in the range 70--700 \ifmmode^\circ\else\textdegree\fi{}C and of the gas-phase chemistry on both the nitridation kinetics and the composition of the resulting GaN layer are highlighted. Pure ${\mathrm{N}}_{2}$ plasmas yield stoichiometric and smooth GaN layers with As segregation at the GaN/GaAs interface. The As segregation inhibits GaAs nitridation, because the N atoms scavenge the free As, and thereby limits the GaN thickness to a few angstroms. Thicker GaN layers (g100 \AA{}) are obtained by ${\mathrm{N}}_{2}{\ensuremath{-}\mathrm{H}}_{2}$ and ${\mathrm{N}}_{2}{\ensuremath{-}\mathrm{N}\mathrm{H}}_{3}$ plasmas, since hydrogen reduces the As segregation by the formation and desorption of ${\mathrm{AsH}}_{x}$ species. For the three plasma mixtures, the self-limiting nature of the GaAs nitridation process is revealed and explained using simple kinetic and chemical models based on the fact that the GaAs nitridation can be considered to be a topochemical reaction. Also demonstrated is the ineffectiveness of the nitridation at $Tg~600\ifmmode^\circ\else\textdegree\fi{}\mathrm{C},$ which is accompanied by the GaAs substrate decomposition and yields both a rough and Ga-rich GaN layer.

Study of the mechanisms of GaN film growth on GaAs surfaces by thermal and plasma nitridation

The kinetics of GaAs nitridation using N2 plasmas, both radio frequency and electron cyclotron resonance sources, is investigated using in situ and real time ellipsometry. A comparison of plasma nitridation with the more conventional NH3 thermal nitridation of GaAs is also reported. We report that all the GaAs nitridation processes are self-limiting yielding only very thin GaN layers. The dependence of GaN layer thickness on surface pretreatment, surface temperature and N atom density in the plasma is reported. Smooth and stoichiometric GaN layers are formed at T<600 °C, whereas nitridation at T⩾600 °C yields rough and Ga-rich GaN layers. In both cases, it is shown that As segregates at the GaAs/GaN interface, indicating that GaAs plasma nitridation kinetics is limited by outdiffusion of As and/or AsN species.

Spontaneous formation of a tilted AlGaAs/GaAs superlattice during AlGaAs growth

In transmission electron microscopical (TEM) cross-sections of AlGaAs layers grown on vicinal 111✓A facets of patterned substrates, we observe a periodic structure of Ga-rich and Al-rich layers forming angles of up to 25° with the growth front. This spontaneous tilted superlattice (TSL) formation is found in layers grown by metal-organic vapor phase epitaxy using the group III precursors dimethylethylaminealane (DMEAAl) and triethylgallium (TEGa) and in samples grown by solid source MBE. We present a growth model explaining the TSL formation in terms of step bunching and adatom diffusion. The step bunches required by the model are experimentally found by atomic force microscopy as well as high resolution TEM. Finally, we introduce a Monte Carlo simulation, which is able to reproduce the main features of the spontaneously formed TSL.

Effect of Stoichiometry on Defect Distribution in Cubic GaN Grown on GaAs by Plasma-Assisted MBE

ABSTRACTHigh resolution electron microscopy has been applied to characterize the structure of β-GaN epilayers grown on (001) GaAs substrates by plasma-assisted molecular-beam-epitaxy. The rf plasma source was used to promote chemically active nitrogen. The layer quality was shown to depend on growth conditions (Ga flux and N2 flow for fixed rf power). The best quality of GaN layers was achieved by “stoichiometric” growth; Ga-rich layers contain a certain amount of the wurtzite phase. GaN layers contain a high density of stacking faults which drastically decreases toward the GaN surface. Stacking faults are anisotropically distributed in the GaN layer; the majority intersect the interface along lines parallel to the “major flat” of the GaAs substrate. This correlates well with the observed anisotropy in the intensity distribution of x-ray reflexions. Formation of stacking faults are often associated with atomic steps at the GaN-GaAs interfaces.

Acceleration of grain boundary motion in Al by small additions of Ga

Abstract The temperature dependence of 〈111〉 tilt grain boundaries with angles of misorientation of 38·2° and 40·5° was investigated in bicrystals of both pure (99·999%) Al and the same Al doped with 10ppmGa in the temperature regime between 400 and 580°C. The grain boundary mobility for the investigated grain boundaries over the entire investigated temperature range is enhanced by minor additions of Ga. Both the activation enthalpy and the pre-exponential factor are affected by Ga doping. The orientation dependence of grain boundary mobility is strongly reduced but not completely removed. The drastic rise in grain boundary mobility by the addition of Ga to pure Al is interpreted as a consequence of a change in the boundary structure and the mechanism of boundary migration owing to a pre-wetting phase transition and formation of a liquid (or quasiliquid) Ga-rich layer on the grain boundary.

Determination of Composition and Phase Depth-Profiles in Multilayer and Gradient Solid Solution Photovoltaic Films Using Grazing Incidence X-ray Diffraction

Abstract A method for using grazing incidence x-ray diffraction (GIXD) for profiling composition changes with depth of photovoltaic quality thin films is presented. The average thickness of the first layer in a multi-layer film of CuIn2Se3.5/CuInSe2/Mo and the variation in solid solution composition of a Cu(In1-xGax)Se2 (CIGS) film with depth are solved using this method. The phase volume fraction and the phase composition profiles are developed from peak intensity and d-spacing measurements respectively at a series of fixed incident angles corresponding to a set of increasing 1/e penetration depths, τ. Inverse Laplace and numerical methods are applied to the τ profiles converting them to true depth profiles. Vegard's law is applied to the d-spacing vs z-profile to obtain x in the formula Cu(In1-xGax)Se2. The results show that an ∼1 μm thick layer of CuIn2Se3.5 is present on the surface of the multi-layer film and that the CIGS film consists of a Ga rich surface layer ∼2000 Å thick followed by a gradual decrease in Ga content with increasing depth. This gradient appears to be desirable for producing photovoltaic quality CIGS films.

Growth of InGaP by metalorganic molecular beam epitaxy using novel Ga sources

We have investigated the use of tri-isobutylgallium (TIBG) and tri-isopropylgallium (TIPG) for growth of InGaP on GaAs by metalorganic molecular beam epitaxy. In particular, we have explored the effect of growth temperature on carbon background and lattice constant. The incorporation rate of carbon was found to decrease with increasing temperature while that for Ga was found to increase. Both trends suggest enhanced alkyl-Ga decomposition with increased temperature similar to that observed for growth with triethylgallium (TEG). The variation in composition with growth temperature was, however, reduced for TIBG and TIPG relative to previous reports using TEG. Measured lattice constant varied linearly with TMI flow up to a value of ∼5.675 Å. The band gap was found to vary discontinuously with lattice constant suggesting enhanced ordering in Ga-rich layers. Finally, Sn doping from tetraethyltin was shown to vary linearly with TESn flux provided the combination of low V/III ratios and Ga-rich compositions was avoided.

Oxidation of four palladium-rich ceramic fusing alloys

Oxidation of four ceramic fusing alloys with varying amounts of palladium (35–79 wt%) and with different oxidizing elements (In, Sn, Ga, Cu, Co) was studied by scanning electron microscopy. X-ray diffraction and X-ray photoelectron spectroscopy. The alloy containing cobalt and gallium oxidized mainly externally forming a rather continuous Co-rich oxide layer on the surface and a Ga-rich layer underneath it. The other alloys were oxidized internally. The oxidation behaviour of Pd was found to be strongly dependent on the other oxidizing constituents.

X-ray photoelectron spectroscopy study of the effects of ultrapure water on GaAs

X-ray photoelectron spectroscopy has been used to investigate the effects of de-ionized water on chemical etched GaAs surfaces. When the treatment with water is performed in static conditions (stagnant water) a Ga-rich oxide layer is formed on GaAs at the rate of 10–20 Å h−1. In contrast, when the GaAs surface is treated in dynamic conditions (running water), no oxide buildup is observed. Moreover, running water can remove the oxide film formed in static conditions, as well as oxidized layers due to air exposure. These results are discussed in the framework of cleaning prior to molecular beam epitaxy.

Plasma anodization of oxygen-sputtered GaAs surfaces

A simple method of preparing plasma-grown anodic oxide on GaAs with improved physical and electrical properties is described. A GaAs surface was sputtered by positive oxygen ions prior to the plasma anodization, resulting in a Ga-rich oxide layer on the surface. The oxide formed through this Ga-rich layer had a low pin hole density and exhibited high breakdown voltages (3×106 V/cm) and less hysteresis in MOS-device C-V characteristics.

Thermal reaction of Ti evaporated on GaAs

The thermal reaction of a vacuum-evaporated Ti film with substrate GaAs has been investigated using Auger electron spectroscopy and x-ray diffractometry. As accumulation and Ga depletion at the Ti/GaAs interface and Ga pileup in front of the As-rich layer have been observed. The As-rich and the Ga-rich layers have been found to be composed of TiAs/Ti5As3 and Ti2Ga3/Ti5Ga4, respectively. It has also been found that the rate of the Ti-GaAs reaction is governed by the interdiffusion process of Ti and As at the interface with an activation energy of 1.70±0.05 eV. The low diffusivity and relatively high Schottky barrier height of the present system facilitate the formation of useful Schottky barrier junctions.

The growth and composition of anodic films on GaP

Anodic films have been grown on GaP in a variety of buffered anodizing solutions with pH ranging from 3 to 10. The films were grown at constant current densities from 1 to 100 mA/cm 2. Films with thicknesses in the range 500–1000 Å resulted from the higher current densities. The films were analyzed using the Rutherford backscattering of 1·7–2 MeV 4He ions in conjunction with ion channeling. This analysis revealed two distinct types of films; those with uniform composition as a function of depth and those with non-uniform profiles. The uniform films have Ga:P:O composition ratios typically ∼1:2:5:8 whereas the non-uniform films have typical integrated Ga:P:O ratios of ∼1:1·5:5. In the non-uniform films, which are the majority of those grown, Ga is piled up at the film-electrolyte interface; associated with this Ga excess is a corresponding P deficiency at the interface. This indicates that Ga migrates through the films during anodizing and forms a Ga-rich outer layer. In the uniform films it appears that much of the migrating Ga is continually being dissolved in the electrolyte during anodization.