High-quality Epitaxial Layers Research Articles

Ion beam characterization of GaAs 1− x− yN xBi y epitaxial layers

Incorporation of Bi in GaAs 1− x N x epitaxial layers represents a significant interest as Bi compensates the lattice parameter reduction caused by the N incorporation while contributing to the reduction of the band gap energy. GaAs 1− x− y N x Bi y epitaxial layers were grown on GaAs wafers by molecular beam epitaxy. The quality of the films as well as the concentration and lattice location of Bi and N were characterized by channeling Rutherford backscattering spectrometry and nuclear reaction analysis using 2 and 3.72 MeV He beams, respectively. The amount of nitrogen in the film was measured by means of the 14N(α, p) 17O endothermic nuclear reaction and elastic recoil detection. The results indicate that high quality epitaxial layers were obtained, with y=1.8% Bi incorporated into the layer. Angular scan along the main axes showed no strain in the film and indicated that most of Bi atoms are located at substitutional sites. Nitrogen lattice incorporation is more difficult to establish because of the presence of Bi in the layer, but we estimate the substitutional fraction to be 71 ± 6%.

Epitaxy of Fe3O4 on Si(001) by pulsed laser deposition using a TiN/MgO buffer layer

Epitaxy of oxide materials on silicon (Si) substrates is of great interest for future functional devices using the large variety of physical properties of the oxides as ferroelectricity, ferromagnetism, or superconductivity. Recently, materials with high spin polarization of the charge carriers have become interesting for semiconductor-oxide hybrid devices in spin electronics. Here, we report on pulsed laser deposition of magnetite (Fe3O4) on Si(001) substrates cleaned by an in situ laser beam high temperature treatment. After depositing a double buffer layer of titanium nitride and magnesium oxide (MgO), a high quality epitaxial magnetite layer can be grown as verified by reflection high energy electron diffraction intensity oscillations and high resolution x-ray diffraction.

Low Dark Current InGaAs(P)/InP p–i–n Photodiodes

Planar InGaAs(P)/InP p–i–n photodiodes have been successfully fabricated by low-pressure metalorganic chemical vapor deposition (LP-MOCVD). High-quality and uniform epitaxial layers are obtained. It is noted that the InGaAs layer background concentration is as low as 4.5 ×1013 cm-3. The dark current is significantly reduced by using a wider-band-gap material of quaternary InxGa1-xAsyP1-y as a cap layer to reduce the device surface leakage current. In addition, the device becomes highly photosensitive due to the reduction of the absorption of the radiation in the narrow-band-gap InxGa1-xAsyP1-y cap layer. The p–i–n photodiode with a wide-band-gap InP cap layer exhibits a dark current as low as 60 pA at -10 V bias, corresponding to a dark current density of 4.2×10-7 A/cm2.

Crystallographic control in thin-layer silicon depositions by atmospheric-pressure iodine vapor transport

Silicon thin layers that are either highly textured polycrystalline, randomly oriented polycrystalline, or epitaxial have been deposited by the technique of atmospheric-pressure iodine vapor transport on silicon or non-silicon substrates. Strongly [1 1 0]-textured polycrystalline silicon films are obtained by starting with a relatively low starting substrate temperature of 700°C, to increase nucleation density, then ramping up to 900°C at the end to increase growth rate. Alternatively, if a higher constant temperature of 900°C is used, we obtain polycrystalline silicon layers with randomly oriented large grains of ∼20 μm in average size. When a still higher initial substrate temperature of 1000°C is used to eliminate condensation of silicon iodides at the interface, high-quality epitaxial silicon layers are grown on silicon substrates.

Plasma cleaning of carbon species for silicon homoepitaxial growth

As device dimensions are reduced into the sub-micron region in pursuit of higher integration density and better circuit performance, low temperature processing, including low temperature cleaning and low temperature epitaxial growth, is becoming important. In order to achieve the high-quality epitaxy with the low temperature processing, the technique of in-situ plasma cleaning has been developed [1, 2]. Although there have been some reports on the plasma cleaning of surface oxygen prior to epitaxial growth, there are rare reports on the systematic study on the removal of surface carbon. Carbon is much more tenacious and stable on the silicon surface than oxygen and temperatures over 1100 ◦C are required to remove carbon from the surface by thermal desorption process. Here, we have performed the low temperature in-situ cleaning in order to reduce the interfacial carbon concentration. Substrates were 4 inch, czochralski-grown, p-type (100) silicon with 0.5–20 -cm resistivity. The wafers were RCA cleaned and HF dipped for 20–30 s in 10:1 aqueous solutions and rinsed in DI (deionized) water and then dried by blowing nitrogen on them. All the processes were done inside the class 100 cleanroom and it took only 10 s to load the wafer into the load lock chamber of the CVD reactor after the wafer was blowdried. After the wafers were transferred and loaded onto the heater stage, the main chamber was pumped down and ultimately 1–2 × 10−8 Torr could be attained. In-situ predepositon wafer cleaning was done by using ECR hydrogen plasma. The ECR hydrogen plasma is able to deliver a higher density of low energy and light hydrogen ions to the wafer, resulting in highly efficient cleaning without substrate damage [2]. The ECR chamber was at the side of the CVD chamber. ECR was operated at the 2.45 GHz S-band microwave frequency. Depositions were done by flowing 10 sccm SiH4 without carrier gases, immediately after the plasma was extinguished. We have applied the in-situ cleaning condition with a microwave power of 300 W, DC bias of 10 V, pressure of 1 mTorr, and the cleaning temperature of 600 ◦C and subsequently have deposited the silicon epitaxial layer. Fig. 1 shows the XTEM image of silicon epilayer and the interface, revealing that the high-quality epitaxial layer is produced by applying the in-situ plasma cleaning. Fig. 2 shows the SIMS depth profile

Growth and optical properties of high-quality ZnTe homoepitaxial layers by metalorganic vapor phase epitaxy

Homoepitaxial growth of ZnTe layers have been investigated by changing the total gas flow rate in a metalorganic vapor phase epitaxial system using a horizontal reactor. Correlations between the growth process and the photoluminescence properties of the layers have been clarified. A high-quality ZnTe homoepitaxial layer with strong free excitonic emission was obtained in this experiment. The binding energy of the free exciton and the temperature dependence of the band gap has been investigated using this sample. The resistivity of a sample was estimated to be as high as 5×10 8 Ω cm supporting a high-quality epitaxial layer.

New values of the Ag-n-GaP potential barrier

Surface-barrier structures of the Ag-GaP type based on high-quality epitaxial n-GaP layers with n=(0.5–30)×1016 cm−3 were studied. It was found that the potential barrier height depends on the method of surface treatment prior to the metal deposition and correlates with the structural nonideality coefficient and the intermediate layer thickness. For high-quality structures with a reverse current below 10−14 A, the barrier height is ϕ=1.55±0.04 eV. For structures with a relatively thick intermediate layer, the barrier may reach up to ϕ=1.7±0.07 eV. The dependence of the barrier height on the method of the GaP surface treatment is related to the absence of rigid pinning of the Fermi level on the GaP surface.

High-Quality GaN Layers on c-Plane Sapphire Substrates by Plasma-Assisted Molecular-Beam Epitaxy Using Double-Step AlN Buffer Process

High-structural-quality c-axis GaN layers were grown on (0001) sapphire substrates by N2-plasma-assisted molecular-beam epitaxy (MBE) using a double-step AlN buffer layer process. The introduction of double-step AlN buffer layers grown at two different high temperatures improved the quality of the GaN layers dramatically. The full-width at half maximum (FWHM) (ω scan) of GaN layers along the symmetric (0002) and asymmetric (1011) diffraction planes were 104 and 848 arcsec, respectively, whereas they were 532 and 1335 arcsec for the GaN layers grown by a single-step AlN buffer process. Double-step AlN buffers improve the structural quality of the layers and decrease the X-ray dislocation density. We have shown that the quality of the underlying AlN buffer layer is the determining factor in the successful growth of high-quality GaN epitaxial layers.

Quantum dot nanostructures

Abstract Low dimensional structures (LDS) form a major new branch of physics research. These semiconductor structures have such a small scale in one- or two-dimensions that their electronic properties are significantly different from the same material in bulk form. These properties are changed by quantum effects. There is increasing interest in the preparation, study, and application of LDS. Their investigation has revitalized condensed matter science, in particular semiconductor materials. Complex LDS offer device engineers new design opportunities for tailor-made new generation electronic and photonic devices. New crystal growth techniques such as molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD) have made it possible to produce LDS in practice. These sophisticated technologies for the growth of high quality epitaxial layers of compound semiconductor materials on single crystal semiconductor substrates are becoming increasingly important for the development of the semiconductor electronics industry. This article is intended to convey the flavor of the subject by focusing on the technology and applications of self-assembled quantum dots (QDs) and to give an introduction to some of the essential characteristics.

Growth and characterization of ultrahigh vacuum/chemical vapor deposition SiGe epitaxial layers on bulk single-crystal SiGe and Si substrates

High-quality SiGe epitaxial layers have been grown on low-defect-density bulk single-crystal SiGe substrates using low-temperature ultrahigh vacuum chemical vapor deposition (UHV/CVD). The layers were simultaneously grown on Si substrates for comparison. Various techniques, including high-resolution x-ray diffraction (HRXRD), photoluminescence (PL), Auger electron spectroscopy, atomic force microscopy (AFM), and cross-sectional transmission electron microscopy (XTEM) were used to characterize the structural quality of the materials. There is good agreement between different techniques for the determination of the composition and thickness of the layers. HRXRD, PL, and XTEM results confirm that the materials deposited are high-quality single-crystal epilayers. The surface root-mean-square roughness measured by AFM is less than 0.4 nm, indicating very smooth surfaces. A thin undoped Si buffer layer was found to improve the structural quality of the layers. The bulk crystal SiGe substrates are useful for the growth of thick lattice-matched and strained SiGe epilayers.

ZnSTeSe metal-semiconductor-metal photodetectors

High-quality quaternary ZnSTeSe epitaxial layers were successfully grown by molecular beam epitaxy (MBE). It was found that a ZnS/sub 0.18/Se/sub 0.82/ layer was automatically formed in between the ZnSe buffer layer, and the ZnS/sub 0.17/Te/sub 0.08/Se/sub 0.75/ epitaxial layer, when we increased the ZnS cell temperature. ZnSTeSe metal-semiconductor-metal (MSM) photodetectors were also fabricated for the first time. It was found that we could achieve a photocurrent to dark current contrast higher than five orders of magnitude by applying a 10-V reverse bias. We also found that the maximum photoresponsivity is about 0.4 A/W under a 10-V reverse bias. Such a value suggests that the ZnSTeSe MSM photodetector is potentially useful in the blue-UV spectral region.

Homoepitaxial Growth of ZnO by Metalorganic Vapor Phase Epitaxy

Homoepitaxial technique of metalorganic vapor phase epitaxy (MOVPE) was used for the growth of high quality epitaxial ZnO layers. Two conditions, proper thermal treatment of substrate prior to the growth for obtaining flat surface and high flow rate ratios of source materials, nitrous oxide (N2O) and diethylzinc (DEZn), were found to be important. Surface roughness below 1 nm as well as strong free exciton emission at 15 K of an MOVPE-ZnO layer on a bulk ZnO substrate have demonstrated the high potential of MOVPE for ZnO.

Homoepitaxial Growth of ZnO by Metalorganic Vapor Phase Epitaxy

Homoepitaxial technique of metalorganic vapor phase epitaxy (MOVPE) was used for the growth of high quality epitaxial ZnO layers. Two conditions, proper thermal treatment of substrate prior to the growth for obtaining flat surface and high flow rate ratios of source materials, nitrous oxide (N 2 O) and diethylzinc (DEZn), were found to be important. Surface roughness below 1 nm as well as strong free exciton emission at 15 K of an MOVPE-ZnO layer on a bulk ZnO substrate have demonstrated the high potential of MOVPE for ZnO.

ZnTe-Based Light-Emitting-Diodes Grown on ZnTe Substrates by Molecular Beam Epitaxy

We have investigated the ZnTe-based material system for the application to light-emitting devices. To this end, ZnTe homoepitaxy techniques have been developed to grow high-quality epitaxial layers. The conductivity control of ZnTe and ZnMgSeTe layers have been investigated. High structural quality n-type ZnTe layers with high carrier concentration are achieved by aluminum doping. Ambipolar conductivity control of quaternary layers is achieved. Aluminum doped ZnMgSeTe layers show a net carrier concentration of 5 × 1016 cm—3, while a high hole concentration of 2.5 × 1019 cm—3 is achieved by p-type doping using a nitrogen plasma source. Based on those results, Zn1—xCdxTe/ZnMgSeTe triple-quantum-well(TQW) LED structures were fabricated. Bright electroluminescence was obtained at room temperature at the wavelength of 604 nm from Zn0.7Cd0.3Te and at 566 nm from Zn0.85Cd0.15Te TQW-LED.

Effect of Tantalum in Crystal Growth of Silicon Carbide by Sublimation Close Space Technique

Investigation into the effect of tantalum on silicon carbide, grown by the sublimation close space technique, was carried out. It was established that in the presence of a small piece of tantalum in the reaction space during the crystal growth process, the surface of the epitaxial layer became extremly smooth and high-crystal-quality epitaxial layers were obtained. It was also shown that the concentrations of aluminum and titanium are different in epitaxial layers grown with and without tantalum. High-quality epitaxial layers, which were comparable to epitaxial layers grown by chemical vapor deposition, could be obtained at a high growth rate by the sublimation close space technique.

Structural Characterization of High-Quality ZnS Epitaxial Layers Grown on GaAs Substrates by Low-Pressure Metalorganic Chemical Vapor Deposition

Structural properties of ZnS epitaxial layers grown on GaAs (001) substrates by low-pressure metalorganic chemical vapor deposition have been studied by high-resolution X-ray diffraction, transmission electron microscopy (TEM) and atomic force microscopy (AFM) measurements. The full width at half maximum (FWHM) of the ZnS (004) diffraction curves decreased with increasing layer thickness and the value determined for an 8-µm-thick ZnS layer was as narrow as 30 arcsec. The decrease in FWHM indicated the improvement of the crystalline uniformity of the ZnS epitaxial layer. Cross-sectional TEM measurement enabled us to observe a large number of stacking faults and microtwins in the vicinity of the interface between ZnS and GaAs. In addition, the decrease in the stacking-fault density with increasing layer thickness was observed, and the stacking-fault density for a 6-µm-thick ZnS surface was estimated to be about 5×107 cm-2. On the other hand, a large number of rhombus-shaped holes were observed on the ZnS surface. An increase in the size and a decrease in the density of these holes were observed with increasing layer thickness. Hence, the root-mean-square value of ZnS surface roughness increased with increasing layer thickness. Cross-sectional AFM measurement indicated that the rhombus-shaped holes on the ZnS surface had a reverse-pyramidal shape formed by four symmetric {113} facets.

Growth and optical properties of GaN on Si(1 1 1) substrates

We report on the growth of high-quality GaN epitaxial layers on Si(1 1 1) substrates using high-temperature-grown AlN as buffer layer by metalorganic chemical vapor deposition. It is found that the nitride compounds on the reactor wall have great influence on the initial surface condition of the Si substrates because they are dissociated from the wall and modifies the Si surface during the initial heat-up procedure. When the reactor wall is coated with the GaN after the growth of GaN/AlN/Si, the harmful Ga-containing droplets are formed on the AlN buffer layer, leading to deteriorated films. To prevent the droplets, coating AlN on the reactor wall (pre-treatment) is followed by the growth GaN/ALN/Si. This pre-treatment insures AlN environment and suppresses the formation of the droplets during initial heat-up, leading to reproducible and favorable surface condition for the growth of AlN buffer layer and subsequent GaN epitaxial films. In order to illustrate the optical properties of the GaN layer, photoluminescence is measured between 14 K and room temperature. Intense band edge emission and well-resolved excitonic transitions indicate an excellent optical quality of the epitaxial GaN films.

Investigation of the temperature profile in a hot-wall SiC chemical vapor deposition reactor

The chemical vapor deposition (CVD) technique is widely used to grow epitaxial layers of silicon carbide. To meet the demands for high quality epitaxial layers, which have good morphology and a minimum variation of the doping and thickness, a good knowledge of the CVD process is essential. The present work uses a simulation tool to investigate several parameters influencing the heating of a hot-wall CVD reactor. The simulations are set up as 2D axisymmetric problems and validation is made in a 2D horizontal hot-wall CVD reactor. By applying the knowledge achieved from the simulations, the temperature profile is optimized to give as large area as possible with homogeneous temperature. New susceptor and coil designs are tested. A very good agreement between the simulated and the measured results is obtained. The new design has a temperature variation of less than 0.5% over more than 70% of the total susceptor length at an operating temperature of 1650°C. In addition, the power input needed to reach the operating temperature is decreased by 15% compared to the original design. 3D simulations are performed to show that the changes made in the 2D case give similar results for the real 3D case.

High-quality SiC epitaxial layers and low-dimensional heteropolytypic SiC structures grown by solid-source MBE

The growth of SiC layers on hexagonal (or α-) SiC(0001) has been performed by solid-source MBE between 1300 and 1600 K. The α-SiC layers have been grown homoepitaxial via step-flow on off-axis substrates, whereas pseudomorphic cubic (or 3C-) SiC layers were obtained on α-SiC via nucleation and subsequent step-flow. Under more equilibrium-like conditions, 3C-layers nearly free of twin-boundaries were obtained. The SiC layers were of high quality and without unintentional doping, as revealed by photoluminescence investigations The controlled growth of SiC heteropolytypic structures consisting of hexagonal and cubic polytypes, such as 4H/3C/4H–SiC(0 0 0 1) and 6H/3C/6H–SiC(0 0 0 1), has also been demonstrated. Such structures were obtained by changing the growth conditions from lower temperatures (1550 K) and Si-rich Si/C ratio (3C–SiC) to higher temperatures (1600 K) and more C-rich Si/C ratio. On off-axis substrates, such heterostructures were also obtained by first nucleating selectively wire-like 3C-SiC nuclei on the terraces of well-prepared α-SiC(0 0 0 1) substrates at low T (<1500 K) and a subsequent step-flow of both the 3C wires and the surrounding α-SiC material.

GaN avalanche photodiodes operating in linear-gain mode and Geiger mode

For solar-blind ultraviolet detection, AlGaN avalanche photodiodes (APDs) that operate in Geiger mode can outperform conventional AlGaN photodiodes in sensitivity and should compare favorably to photomultiplier tubes. Toward this goal, we report GaN APDs that operate in the linear-gain mode and in the Geiger mode. The APDs were fabricated from high-quality GaN epitaxial layers grown by hydride vapor phase epitaxy. The GaN layer structure consisted of a Zn-doped /spl pi/ layer, an unintentionally doped n layer, and a Si-doped n+ layer-all on top of a thick GaN unintentionally doped n layer on a sapphire substrate. Capacitance-voltage (C-V) measurements on photodiodes fabricated from some of these layers show that field strengths between 3 and 4 MV/cm are sustainable in the depletion region at voltages slightly below the observed breakdown of /spl sim/80 V. Both mesa-etched and planar devices exhibited avalanche gains of 10 in linear-gain mode and /spl sim/10/sup 6/ in Geiger mode when top illuminated with a 325 nm HeCd laser. Raster measurements of the photoresponse show highly uniform response in gain mode that becomes slightly more inhomogeneous in Geiger mode.