Atmospheric-pressure Vapor Phase Epitaxy Research Articles

A low cost and single source atmospheric pressure vapor phase epitaxy of ZnS for thin film photovoltaic applications

A novel and low cost vapor phase epitaxy (VPE) method using a single non-volatile source has been used to deposit ZnS thin films on soda lime glass substrates. Utilization of the non-volatile source eliminates the need for expensive and sophisticated reactors commonly used in conventional VPE. Instead, this experiment was carried out using inexpensive and easily attainable apparatus. The vapor phase reaction process described is also more compatible to the industry standard dry deposition processes of the other layers in the thin film solar cell stack for Cu(In,Ga)Se2 (CIGS), Cu2ZnSnS4 (CZTS) and CdTe photovoltaic (PV) devices. In this experiment, the substrate temperature was varied from 400 to 480 °C and the ZnS thin films produced were analyzed using X-ray diffractometry (XRD), optical spectroscopy, field emission scanning electron microscope (FESEM) and Hall effect measurement system. The films were found to be hexagonal structured except for the film deposited at 480 °C, where the film was found to be cubic. The thickness, bandgap and resistivity of the deposited films ranged from 54 to 351 nm, 3.18 to 3.83 eV and 2 × 103 to 1.6 × 104 Ω·cm respectively.

Strain relaxation induced surface morphology of heterogeneous GaInNAs layers grown on GaAs substrate

The partially-relaxed heterogeneous GaInNAs layers grown on GaAs substrate by atmospheric pressure vapor phase epitaxy (AP-MOVPE) were investigated by transmission electron microscopy (TEM) and atomic force microscopy (AFM). The planar-view TEM image shows a regular 2D network of misfit dislocations oriented in two orthogonal 〈110〉 crystallographic directions at the (001) layer interface. Moreover, the cross-sectional view TEM image reveals InAs-rich and V-shaped precipitates in the near surface region of the GaInNAs epitaxial layer. The resultant undulating surface morphology, known as a cross-hatch pattern, is formed as observed by AFM. The numerical analysis of the AFM image of the GaInNAs layer surface with the well-defined cross-hatch morphology enabled us to determine a lower bound of actual density of misfit dislocations. However, a close correspondence between the asymmetric distribution of interfacial misfit dislocations and undulating surface morphology is observed.

Anisotropy of strain relaxation in heterogeneous GaInNAs layers grown by AP-MOVPE

The structural investigations of thick undoped GaInNAs/GaAs heterostructures grown by atmospheric pressure vapor phase epitaxy (AP-MOVPE) have been performed by transmission electron microscopy (TEM) and X-ray diffraction (XRD) techniques. The cross-sectional view TEM images revealed plainly heterogeneous layer structure, consisting of a few sublayers with different indium and nitrogen contents. On the other hand, planar-view TEM images revealed 2D network of misfit dislocations oriented in two orthogonal 〈110〉 crystallographic directions at the (001) sublayer interface. Moreover, the XRD measurements provide information about gradient profiles of indium and nitrogen contents, as well as distinct anisotropy of strain relaxation in GaInNAs epitaxial layer, evoked by the asymmetry in formation of interfacial misfit dislocation. The obtained results were exploited to calculate the band structure diagram of the investigated GaInNAs/GaAs heterostructure and mean values of the lattice parameters of the distorted epitaxial layer unit cell, respectively.

Quartz Oscillator Films Prepared under Atmospheric Pressure

At-cut quartz thin films were prepared by an atmospheric pressure vapor-phase epitaxy. For the first time it was found that quartz thin films show oscillation characteristics at room temperature.

Growth of Fe4N thin films by atmospheric pressure vapor phase epitaxy

Growth of Fe4N thin films by atmospheric pressure vapor phase epitaxy

A comprehensive study of SiC growth processes in a VPE reactor

We performed an experimental study of the effect of the gas phase composition on the growth mechanism of 3C-SiC on Si(100) by atmospheric-pressure vapour phase epitaxy at 1350°C. Silane and propane diluted in hydrogen were used as precursors for the growth. We demonstrate the existence of an equilibrium partial pressure of carbon above the growing surface, which ensures a mirror-like morphology. For too low a carbon partial pressure (C/Si ratio in the gas phase lower than 2.7 with a growth rate of 3 μm h −1), the layer morphology and crystalline quality quickly degrade. For too high a carbon partial pressure (C/Si ratio higher than 5 with the same growth rate), SiC clusters form on the growing layers. We propose a mechanism of formation for these clusters taking into account the interactions between the C and Si species in the hot boundary layer.

Semiconductor nanostructures: a new impact on electronics

After a short introduction into the physics of semiconductor nanostructures and their eventual importance for future nano- and optoelectronics various epitaxy techniques are discussed by which self-assembly of SiGe/Si and III–V nanostructures can be achieved. Recent results are reported about the size distribution of Ge and SiGe islands on Si grown in low pressure vapor phase epitaxy (LPVPE). Furthermore correlated growth of Ge islands on adjacent stacked Si layers is described and explained quantitatively. Quantum wires (QWR) can be grown in a well defined way within lithographically pre-structured grooves. The underlying physics and some details of the growth mechanism are considered both for the SiGe/Si and for III–V material systems. Finally some examples are reported where both SiGe and III–V nanostructures are used in preliminary device structures such as optoelectronic devices with quantum dots in the active region, single electron tunneling diodes based on quantum dots as well as magnetotransport measurements on GaAs quantum wires.

Developments in GaAs pixel detectors for X-ray imaging

Position sensitive hybrid pixel-detectors have been fabricated by bump bonding silicon or bulk grown semi-insulating gallium arsenide pixel detectors to CMOS read-out chips. Their performance as X-ray imaging sensors, in the energy range of 10-70 keV, was evaluated in terms of spatial resolution. For the GaAs device a fit was made to the line spread function (LSF) obtained from the image of a narrow slit and the corresponding modulation transfer function (MTF) and noise equivalent passband (N/sub e/) evaluated. A value of 5.7 line pairs per mm (1p/mm) was found for the latter, with a modulation of 10% at the Nyquist frequency (N/sub y/). A comparison is also given of the performance of these devices with state-of-the-art scintillator on silicon CCD dental X-ray sensors. In a bid to improve detector performance, thick layers of high quality GaAs have recently been grown by low pressure vapour phase epitaxy (LP-VPE). Hall measurements on initial samples gave free carrier concentration of 1-8/spl times/10/sup 11/ cm/sup -3/ From the C-V dependence of a reverse-biased Schottky diode this material, however, a space charge density of 2/spl times/10/sup 13/ cm/sup -3/ was measured. The observed temperature and frequency dependency of the capacitance is characteristic of the presence of deep levels and so the material is believed to have a small degree of compensation due to these levels. The measured charge collection efficiency determined (c.c.e.) for 60 keV gamma rays showed an increase with reverse bias, reaching a plateau value of 93% for 100 V. The limitations of present detectors are discussed and possible future developments indicated.

Vapor Phase Epitaxy of GaN Using Gallium Tri-Chloride and Ammonia

AbstractGaN films with good crystalline quality are grown on sapphire by atmospheric pressure vapor phase epitaxy using gallium tri-chloride (GaCl3) and ammonia (NH3). Epitaxial growth is carried out over temperature and V/III-ratio ranges of 800–1000°C and 100–1000, respectively. Typical growth rate obtained is in the range of 5–20 μm/hr. The films grown below 925°C typically show three dimensional (island) growth, while above that temperature, continuous films are obtained. Films grown at 975°C with a V/III ratio > 300 exhibit a smooth surface. XRD analysis shows that the films are single crystal with hexagonal polytype. Strong band-edge photoluminescence is observed with a FWHM of 60 meV at room temperature and 25 meV at 77K. The results indicate that this simple growth technique is effective for growing high quality bulk GaN, which can be used as a substrate for subsequent epitaxy. In order to further improve the surface morphology, a preliminary experiment on GaN growth on a thin GaN buffer layer prepared by gas source MBE is also presented.

Si1?XGeX/Si quantum well infrared photodetectors

P-type Si1−XGeX/Si quantum well infrared photodetectors for thermal imaging applications have been grown by low pressure vapour phase epitaxy for the first time. Good control of periodicity in these pseudomorphic multiple quantum well structures on Si substrates was demonstrated by real-time ellipsometry during growth and subsequently confirmed by transmission electron microscopy. The photoresponse spectrum was tuned in the 8–13 Μm waveband by varying the Ge fraction in the Si1−XGeX quantum wells. Dark currents were reduced by doping only the centre regions of the quantum wells.

Selective growth of SiGe nanostructures by low pressure VPE

For the investigation of optical effects in low dimensional silicon germanium heterostructures it is necessary to transfer patterns into silicon germanium with a lateral scaling of less than 100 nm. We use electron beam lithography and a special resist system to transfer the pattern into an SiO 2 mask. By selective low pressure vapour phase epitaxy small silicon germanium structures can then be grown. By this method surface damage resulting from subsequent etching can be avoided and an immediate coverage with silicon is possible. The process and first results are presented in this paper.

Permeable-base transistors with ion-implanted CoSi 2 gate

Permeable-base transistors (PBTs) with a buried CoSi 2 gate overgrown by low pressure vapour phase epitaxy have been fabricated by high dose cobalt ion implantation through a gridded mask into n-type Si(111) and Si(100). SiO 2 and SiO 2/W/Cr were tested as implantation mask materials. For the first time different CoSi 2/Si Schottky barrier heights, deduced from I–V and C–V measurements, for the top and the bottom diodes are observed in Si(100) with values of 0.67 ± 0.03 eV and 0.78 ± 0.03 eV respectively. In Si(111) the barrier heights of the top and bottom diodes exhibit a value of 0.78 ± 0.03 eV . The ideality factor of the diodes depends strongly on implantation dose and energy. PBTs on Si(111) obtained by implantation into low doped n-type wafers show a transconductance per gate finger length of 15 mS mm −1, whereas PBTs on Si(100), obtained by implantation into a low doped epitazial silicon layer on a highly doped wafer, exceed 50 mS mm −1 for gate spacings of 1 μm. Pinch-off was achieved in these PBTs.

Formation of buried CoSi 2 layers with ion beam synthesis at low implantation energies

The formation of Si/CoSi 2/Si heterostructures by high dose Co implantation has been studied for the implantation energy range of 30–200 keV. Various implantation and post-implantation annealing procedures were investigated in order to determine the conditions for producing the thinnest possible CoSi 2 layers. Layer thicknesses ranging from 70 nm at 200 keV to as thin as 18 nm at 30 keV were produced in both (100) and (111) oriented Si. For implantation energies < 70 keV, it was necessary to keep the annealing temperatures below 950°C in (100) Si — a restriction not encountered in (111) Si — to produce a continuous layer. The mechanism for this temperature dependence is discussed. Channeled implantation was also investigated as a technique to produce a thin buried layer at low energies, but comparison of such layers with typical random implants showed no improvement in heterostructure quality nor any apparent processing advantages. Finally, the suitability of the top Si layer of the heterostructures as a seed for epitaxial Si has been investigated with low pressure vapor phase epitaxy. The heterostructures were analyzed by Rutherford backscattering spectroscopy, ion channeling, cross section transmission electron microscopy and resistivity measurements.

Influence of oxygen contamination during Si low presure vapour phase epitaxy on epitaxial layer quality

The influence of oxygen contamination on Si low pressure vapour phase epitaxy (LPVPE) at 800°C in the SiCl2H2−H2 system has been investigated. O2 was added intentionally with partial pressures between 10−8 and 2×10−4 mbar. The quality of the epitaxially grown silicon layers was determined by comparing surface morphology, defect density, Schottky diode characteristics and SIMS measurements. These four parameters are correlated and they reveal a drastic decrease in epitaxial layer quality for O2 pressures above 15×10−6 mbar. The critical oxygen pressure which has been until now considered as a limit for epitaxial growth can therefore be exceeded by one order of magnitude.

The permeable base transistor

Among the high speed electronic devices which have been developed in the last two decades the permeable base transistor (PBT) is one of the most attractive due to its conceptual simplicity. One of its prime advantages, however, is that it can be fabricated with silicon as the basic material thus pushing further the limits of Si-technology in terms of speed. We shall concentrate in this review on one particular version of the PBT employing a buried grid of CoSi2 as the gate. The implementation of this transistor rests on recent advances in Si heteroepitaxy, allowing high quality epitaxial growth of metallic silicides, of which CoSi2 is the most promising to date. Heterostructures of the kind Si/CoSi2/Si(111) are grown by molecular beam epitaxy and subsequently overgrown with additional Si by low pressure vapour phase epitaxy after the structuring of the gate metal. Strained-layer epitaxy of CoSi2 is shown to be of utmost importance for the following Si overgrowth. The lattice mismatch of 1.2% limits the growth of CoSi2 with a low density of dislocations on Si to a few nanometers. The small gate thicknesses may be shown to affect the transistor charactertistics in a significant way.

Epitaxy of metal silicides

Abstract Epitaxial growth of the silicon-rich metal silicides NiSi 2 , CoSi 2 and IrSi 3 is reviewed. Codeposition of metals and silicon at low substrate temperatures is shown to alleviate the main problem associated with solid phase epitaxy using metal deposition alone, i.e. the nucleation controlled reaction to the silicide in question. NiSi 2 is an exception since owing to an epitaxial precursor phase the interfacial energies are lowered sufficiently, so that for thin films nucleation is no longer a problem. The growth of high quality CoSi 2 /Si superlattices is shown to be feasible by combining the codeposition technique with silicon MBE. Excellent electrical properties of epitaxial CoSi 2 films is demonstrated, remaining metallic down to a thickness of 10 A. Surface scattering in Si/CoSi 2 /Si heterostructures is nearly eliminated, CoSi 2 thicker than 35 A showing bulk residual resistivities. Such heterostructures have been applied to the fabrication of the permeable base transistor (PBT), using low pressure vapour phase epitaxy for the last overgrowth step after patterning. Device performance is found to be superior to any overgrown silicon PBT reported to date.

New aspects in modern production equipment for III–V processes

The complete required range of epitaxial deposition from one monolayer to tens of microns is available by integrating low-pressure metal-organic vapor phase epitaxy and low pressure vapor phase epitaxy into wafer fabrication. Uniformity of thickness, composition, and doping of better than 2% can be obtained across 2 inch wafers. Electrical properties in the range of the 10 14 cm −3 and less for background doping, carrier mobilities of 210 000 cm 2 V −1 s −1 for AlGaAs and 190 000 cm 2 V −1 s −1 for GaInAs, both high electron mobility transistor (HEMT) structures, are achieved. Full width at half-maximum (FWHM) of 3.4 and 18.6 meV for 10 and 2 nm GaInAsP wells have been measured. GaAsP layers including a “graded” layer reveal FWHM of 15 nm. A new model for the understanding of high growth rates in the hydride system is proposed.

Submicron highly doped Si layers grown by LPVPE

By low pressure vapour phase epitaxy, epitaxial growth down to 819°C has been achieved in the SiCl 2H 2/H 2 system. High and homogeneous in situ p-type doping has been realized with boron in the range 2 × 10 18 −5 × 10 19 cm -3, with relative steep transition to be substrate. Under near-equilibrium growth conditions, selective growth results with reproducible growth rates of 8–60 nm/m. The crystalline quality of the low temperature epilayers has been investigated on a p-n junction with encouraging results.

Submicron Highly Doped Silicon Epitaxial Layers Grown by Lpvpe

AbstractBy low pressure vapour phase epitaxy (LPVPE) epitaxial growth down to 760ºC has been achieved in the SiC12H2/H2 system, as revealed by RBS and channeling measurements ( min ∼4.0%). High and relative homogeneous doping was obtained with boron resp. phosphorus in the range 1x1018 -5x1019 cm-3 for T: 800º - 900ºC. Sharp transitions in the boron doping profile to the substrate, of the order of 15mn, result only for temperatures much lower than 850ºC. Schottky barrier enhanced diodes with promising performances were formed with Ti evaporated on p+n layers grown selectively on n+ substrates.