The use of metalorganic chemical vapor deposition to prepare device quality Ga(AsP) strained-layer superlattices

Strained-layer superlattices (SLS’s), which consist of thin (<300 Å) alternating layers of Ga(AsP) and GaAs or GaP, have been prepared by metalorganic chemical vapor deposition (MOCVD). Transmission electron microscopy and x-ray diffraction of the SLS’s indicate that the layers are coherently strained and dislocation free. The mismatch between these very thin layers is totally accommodated by strain for misfits of one percent or less. The layer thickness for the binaries and the ternary is controlled by the TMG flow while the solid composition for the ternary is determined by the arsine/phosphine ratio. The solid composition for a fixed arsine/phosphine ratio is a function of temperature and arsine partial pressure. Uniformly doped SLS’s have properties similar to the ternary of the same composition. A photodiode has been prepared from a GaAs_0.2P_0.8/ GaP SLS with a leakage current of 155 × 10^-6 A/cm² at -5V and a quantum efficiency of 40% at 420 nm.     

分子束外延生长的Ge_xSi_(1-x)/Si应变超晶格的X射线衍射研究(英文)

The structural properties of GexSi1-x/Si strained-layer superlattices grown by molecular beam epitaxy are investigated using conventional X-ray θ-2θ scans with a scintillation counter. The experimental diffraction curves for the small-angle reflection and symmetric Bragg reflection are analyzed by applying the optical mutilayer theory and a approach of the dynamical theory of X-ray diffraction for distorted crystals. The structure parameters of superlattices can be obtained without Fourier transformation or computer fitting. The theoretical fit of the experimental diffraction curves yields detailed information on the strain profile,the chemical composition, and the thickness of individual layers of superlattices, which provides a powerful tool to evaluate the structural perfection of GexSi1-x/Si strained-layer superlattices.     

Characterization of GaAs substrates and epitaxial GaAs1−xPx layers by divergent x-ray beam diffraction

The application of the divergent x-ray beam diffraction method was studied for characterizing lattice imperfections, lattice parameters and composition variations and lattice strain in GaAs substrates and in GaAs_1−xP_x epitaxial layers. Reflection conics from {117}, {026}, {155} and (004) planes predominate in pseudo-Kossel back reflection patterns obtained from samples with (001) orientations. The sensitivity of pseudo-Kossel line displacements is assessed for lattice parameter and anisotropic strain distortion measurements. The lattice parameter of GaAs was determined to be 5.6435 ∢. Moseic subgrain misorientations in GaAs_1−xP_x epitaxial layers were found to be relatively independent of graded layer composition gradients, whereas homogeneous stress distortion was more strongly dependent.     

Critical layer thickness of strained-layer InGaAs/GaAs multiple quantum wells determined by double-crystal x-ray diffraction

Double-crystal x-ray diffraction (DCXD) is shown to reveal the onset of relaxation in strained-layer InGaAs/GaAs multiple quantum well (MQW) structures. The MQW structures contain 10 nm thick In_0.16Ga_0.84As quantum wells and 55 nm thick GaAs barrier layers. As the number of periods in the structure was increased from to 3 to 15, the x-ray rocking curves were characterized by increasing distortion of superlattice interference fringes, broadening of superlattice peaks, and reduction in peak intensity. The x-ray diffraction data are correlated with an asymmetric crosshatched surface pattern as observed under Nomarski contrast microscopy. By using DCXD and Nomarski microscopy, the onset of strain relaxation in InGaAs/GaAs MQW structures was established for samples with various GaAs barrier layer thicknesses. For MQW structures in which the thickness of the barrier layers is the same or greater than that of the strained quantum wells, the critical layer thickness can be calculated according to the Matthews and Blakeslee force-balance model with dislocation formation by the single-kink mechanism.     

Low pressure MOVPE growth of ZnSe,ZnTe, and ZnSe/ZnTe strained-layer superlattices

ZnSe and ZnTe single-crystal layers have been grown onto (100) GaAs substrates by low-pressure metalorganic vapor-phase epitaxy (LP-MOVPE) using the triethylamine-dimethylzinc adduct [DMZn(NEt₃)] as the zinc precursor. The selenium and tellurium precursors were H₂Se (5% in H₂) and di-isopropyltellurium (DiPTe), respectively. These two semiconductors have been grown with different VI/II molar ratios, at different growth temperatures, and with an overall growth pressure ranging from 40 to 400 Torr. Optimal growth parameters have been determined by optical means for the two materials. This information was then used to grow ZnTe/ZnSe strained-layer superlattices. We have studied structures grown on both ZnSe and ZnTe relaxed buffer layers which display a drastic dependence of the Stokes shift between photoluminescence and the optical bandgap on the nature of the buffer layer. Growth interruptions have been used to optimize the optical properties of the superlattices. Theoretical modeling of superlattice band structures has been performed using results of optical and structural characterizations. Observations of zone center transitions as well as excitons associated with the miniband dispersion of the superlattices are reported, in agreement with the theoretical calculation.     

Vapor-grown In1−xGaxP electroluminescent junctions on GaAs

In_1?xGa_xP vapor-grown electroluminescent junctions have been deposited directly onto GaAs substrates. For these layers, an alloy composition within a few mole percent of the lattice-matching composition of 51.5 mole percent GaP has been found to be essential for high luminous efficiencies and for the avoidance of microcracks throughout the epitaxial layer. For In_1?xGa_xP alloys near this composition, the electroluminescence characteristics of the diodes have been found to be excellent, with room-temperature external quantum efficiencies as high as 0.2% attained for red emission near 6600 Å. The properties of In_.5Ga_.5P junction structures deposited directly onto GaAs ar? compared with those of In_1?xGa_xP layers previously prepared on GaP substrates.     

Cathodoluminescence assessment of GaAs1−x Px for light emitting diodes

The room temperature cathodoluminescence (CL) properties of selenium doped epitaxial layers of GaAs_1−xP_x, in the composition range 0·35 < x < 0.45, have been examined as a function of the Hall electron concentration. Material selected for this investigation had less than 2 per cent of the total CL emission in the i.r. For a fixed alloy composition the CL intensity is shown to increase with increasing electron concentration, while for a fixed electron concentration the intensity decreases with increasing GaP content. These results have been correlated with the electroluminescent efficiencies of zinc diffused diodes fabricated from the same material. It is shown that CL provides a rapid and reliable means of assessing the composition and emitting efficiency of epitaxial layers for use in the fabrication of light emitting diodes.     

Thickness dependent structural and optical properties of In/Te bilayer thin films

In_xTe_y thin films have been prepared from In/Te bilayer by sequential thermal evaporation. The samples were analyzed by x-ray diffraction (XRD), optical transmittance spectra and scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDX) in order to investigate structural, optical properties, surface morphology and elemental composition of the prepared films. XRD spectra reveal that all films exhibit mixed phases of In₂Te₃ and In₂Te₅. Increase in grain size with film increase in thickness was observed. The surface was highly porous as observed by SEM analysis. Band gap energy of In_xTe_y system is found to decrease with increase in film thickness.     

Si/Si1−x Gex epitaxial layers and superlattices. Growth and structural characteristics

Si, Ge, and Si_1−x Ge_x epitaxial layers and Si/Si_1−x Ge_x superlattices have been obtained on (100) and (111) silicon substrates by molecular-beam epitaxy. The growth processes and the structural characteristics and chemical composition of the structures were studied by x-ray diffraction and Auger spectroscopy. It is shown that under the experimental conditions for obtaining Si/Si_1−x Ge_x superlattices structurally perfect, strained superlattices with satellites up to ±5 orders can be obtained. Fiz. Tekh. Poluprovodn. 31, 922–925 (August 1997)     

Optical and structural properties of MOVPE grown GaxIn1−xAs/InP strained multiple quantum well atructures

This paper presents a study of the structural and optical properties of strained GaInAs/ InP multiple quantum well (MQW) structures fabricated by LP-MOVPE. The composition of the Ga _x In_1−x As films ranged fromx = 0.17 tox = 1.0 and was determined by sputtered neutral mass spectrometry (SNMS) on thick layers. The structures of the MQW samples with well widths from 1.5 to 5 nm were investigated by high resolution x-ray diffraction (HR-XRD). Simulations of the diffraction patterns showed that transition layers of approximately 2 monolayer (ML) thickness with high lattice mismatch exist at the interfaces. Photoluminescence (PL) measurements indicate well widths of a multiple of a monolayer with local variations of one monolayer. The PL peak energies vary smoothly with the Ga concentration. These results were confirmed by optical absorption measurements.     

Structural and optical investigation of InGaN/GaN multiple quantum well structures with various indium compositions

We have studied the influence of indium (In) composition on the structural and optical properties of In_x Ga_1−xN/GaN multiple quantum wells (MQWs) with In compositions of more than 25% by means of high-resolution x-ray diffraction (HRXRD), photoluminescence (PL), and transmission electron microscopy (TEM). With increasing the In composition, structural quality deterioration is observed from the broadening of the full width athalf maximum of the HRXRD superlattice peak, the broad multiple emission peaks oflow temperature PL, and the increase of defect density in GaN capping layers and InGaN/GaN MQWs. V-defects, dislocations, and two types of tetragonal shape defects are observed within the MQW with 33% In composition by high resolution TEM. In addition, we found that V-defects result in different growth rates of the GaN barriers according to the degree of the bending of InGaN well layers, which changes the period thickness of the superlattice and might be the source of the multiple emission peaks observed in the In_xGa_1−xN/GaN MQWs with high in compositions.     

Properties of periodic α-Si:H/a-SiNx:H structures obtained by nitridization of amorphoussilicon layers

The kinetics of nitridization of a-Si:H layers, the properties of the structures that are formed and a-Si:H in them have been investigated. The changes occurring in the resistance of the a-Si:H layers in the course of nitridization are described in terms of the competition between doping, transport, and change in the thickness of the remaining a-Si:H layer. The experimental data on the band spectrum of superlattices with a-Si:H and a-SiN_x:H layer thicknesses ～35 Å and ～5 Å, respectively, are in agreement with calculations in a model of interacting quantum wells with m*=(0.36±0.1)m ₀. Comparison of the properties of superlattices obtained by deposition of successive layers and nitridization of the a-Si:H layers showed that the latter can have a higher “structural perfection.”     

The preparation of modulation-doped GaAs/GaAs1-xPx strained-layer superlattices by metal organic chemical vapor deposition

Modulation-doped and uniformly doped GaAs/GaAs_1-xP_x strained-layer superlattices (SLS’s) have been prepared by metal organic chemical vapor deposition in a vertical, atmospheric pressure, quartz reaction chamber. The layers were doped with Se or Si by using a H₂Se or SiH₄ source and by using a vent-run valve-configuration to minimize dopant tailing. Carrier concentrations and mobilities were determined from Hall measurements at 300 and 77 K. The Se modulation-doped SLS’s with carrier concentrations in the range of 10¹⁶ to 10¹⁸ cm^-13 exhibited enhanced transport properties when compared to the properties of epitaxial GaAs prepared under the same conditions. The use of molecular sieves to purify the arsine or the use of SiH₄ resulted in lower mobilities for the SLS’s and the epitaxial GaAs layers. The presence of surface cracking or a large residual strain in the SLS’s also caused the modulation-doped SLS’s to have lower mobilities.     

Characterization of Double-Junction GaAsP Two-Color LED Structure

The structural, optical, electrical and electrical–optical properties of a double-junction GaAsP light-emitting diode (LED) structure grown on a GaP (100) substrate by using a molecular beam epitaxy technique were investigated. The p–n junction layers of GaAs_1?xP_x and GaAs_1?yP_y, which form the double-junction LED structure, were grown with two different P/As ratios. High-resolution x-ray diffraction (HRXRD), photoluminescence (PL), and current–voltage (I–V) measurements were used to investigate the structural, optical and electrical properties of the sample. Alloy composition values (x, y) and some crystal structure parameters were determined using HRXRD measurements. The phosphorus compositions of the first and second junctions were found to be 63.120% and 82.040%, respectively. Using PL emission peak positions at room temperature, the band gap energies (E_g) of the first and second junctions were found to be 1.867 eV and 2.098 eV, respectively. In addition, the alloy compositions were calculated by Vegard’s law using PL measurements. The turn-on voltage (V_on) and series resistance (R_s) of the device were obtained from the I–V measurements to be 4.548 V and 119 Ω, respectively. It was observed that the LED device emitted in the red (664.020 nm) and yellow (591.325 nm) color regions.     

Modulation spectroscopy study of a strained layer GaAs/GaAsP multiple quantum well structure

Using contactless electroreflectance (CER) and piezoreflectance at 300 K we have characterized a GaAs/GaAs_1?xP_x multiple quantum well (MQW) structure, “GaAs” (nominal) and GaAsP epilayers grown by chloride transport chemical vapor deposition on GaAs (001) substrates. From a detailed lineshape fit to the CER data from the epilayers we have determined the energies of the fundamental band gap and hence the phosphorous composition. The nominal “GaAs” epilayers were found to have phosphorous compositions of about 2.5–3.2%, a result of the phosphorous diffusion between growth chambers in the reactor. The GaAs_1?xP_x epilayer had x=0.29. For the GaAs_0.97P_0.03/GaAs_0.71P_0.29MQW comparison between the experimentally observed energies of a number of quantum transitions with a theoretical envelope function calculation, including the effects of strain in the barriers, made it possible to evaluate the unstrained conduction band offset parameter Q_c=0.50±0.05. Our value for this parameter is discussed in relation to other works. Atomic force microscopy was employed to investigate the surface morphology of the 230 Å GaAsP top layer of the MQW in addition to a 2000 Å GaAsP epilayer. From the absence of any cross-hatch pattern associated with misfit dislocations on the former we concluded that the GaAsP in the MQW is pseudomorphic. On the other hand the 2000 Å epilayer exhibited signs of strain relaxation.     

Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript> Superlattices Grown by Nanoalloying

In this work, Bi₂Te₃-Sb₂Te₃ superlattices were prepared by the nanoalloying approach. Very thin layers of Bi, Sb, and Te were deposited on cold substrates, rebuilding the crystal structure of V₂VI₃ compounds. Nanoalloyed super- lattices consisting of alternating Bi₂Te₃ and Sb₂Te₃ layers were grown with a thickness of 9 nm for the individual layers. The as-grown layers were annealed under different conditions to optimize the thermoelectric parameters. The obtained layers were investigated in their as-grown and annealed states using x-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive x-ray (EDX) spectroscopy, transmission electron microscopy (TEM), and electrical measurements. A lower limit of the elemental layer thickness was found to have c-orientation. Pure nanoalloyed Sb₂Te₃ layers were p-type as expected; however, it was impossible to synthesize p-type Bi₂Te₃ layers. Hence the Bi₂Te₃-Sb₂Te₃ superlattices consisting of alternating n- and p-type layers showed poor thermoelectric properties.     

The luminescent properties of nitrogen doped GaAsP light emitting diodes

The properties of GaAsP:N light emitting diodes, which emit throughout the range from 7000 Å to 5500Å (red to green), have been investigated. The diodes were fabricated by Zn diffusion into n-type vapor phase epitaxial layers doped with nitrogen and tellurium. The emission spectra, luminous efficiency, quantum efficiency, brightness, and decay time constant have been investigated as functions of alloy composition. Also, the luminous output of the devices has been studied as a function of drive current density and temperature. In the yellow and green spectral regions the performance of nitrogen doped diodes is more than an order of magnitude better than that of nitrogen free diodes of equivalent peak emission wavelength. The luminous efficiency of diodes in the redamber spectral region, with alloy compositions near GaAs_0.35P_0.65 is equivalent to that obtained in GaP:Zn, O red emitting diodes with 2% quantum efficiency.     

Interface strain in InGaAs-InP superlattices

Strain has been measured within (001) oriented OMVPE grown multilayer superlattices consisting of thin As-compound layers in InP and thin P-compound layers in GaAs. From the strain behavior, it is interpreted that As rapidly replaces P on an InP surface exposed to AsH₃ and P slowly replaces As on a As-terminated surface exposed to PH₃. This results in incorporation of an InAs-like strain in InP whose magnitude depends on the nature of the As-terminated surface. At growth temperatures above 600°C, the strain is equivalent to about one monolayer of InAs; while below 600°C, it is equivalent to two monolayers of InAs. PH₃ interaction with GaAs surfaces is sufficiently slow that GaP-like strain is observed only when deliberate interrupts under PH₃ are introduced. GaP grown on GaAs at 650°C is found to incorporate enough residual As to sustain a layer composition of GaAs_0.5P_0.5 over the first several monolayers.     

Transverse optical phonon splitting in GaAs/AlAs superlattices grown on the GaAs(311) surface studied by the method of Raman light scattering

GaAs_n/AlAs_m superlattices grown on the GaAs (311)A and (311)B surfaces by molecular-beam epitaxy were studied by Raman light scattering. The form of the Raman scattering tensor allowed the TO _y and TO _x modes to be separately observed using various scattering geometries (the y and x axes correspond to atomic displacements along and across facets formed on the (311)A surface, respectively). The TO1_y and TO1_x modes exhibited splitting in superlattices grown on a faceted GaAs(311)A surface. The degree of splitting increased for superlattices with an average GaAs layer thickness of 6 monoatomic layers and less. No splitting was observed for superlattices grown under the same conditions on the (311)B surface, which indicates that the splitting effect is probably due to the formation of GaAs quantum wires on the faceted (311)A surface.     

Material Characterization of Ge1−x Sn x Alloys Grown by a Commercial CVD System for Optoelectronic Device Applications

High-quality compressive-strained Ge_1?x Sn _x /Ge films have been deposited on Si(001) substrate using a mainstream commercial chemical vapor deposition reactor. The growth temperature was kept below 450°C to be compatible with Si complementary metal–oxide–semiconductor processes. Germanium tin (Ge_1?x Sn _x ) layers were grown with different Sn composition ranging from 0.9% to 7%. Material characterizations, such as secondary-ion mass spectrometry, Rutherford backscattering spectrometry, and x-ray diffraction analysis, show stable Sn incorporation in the Ge lattice. Comparison of the Sn mole fractions obtained using these methods shows that the bowing factor of 0.166 nm (in Vegard’s law) is in close agreement with other experimental data. High-resolution transmission electron microscopy and atomic force microscopy results show that the films have started to relax through the formation of misfit and threading dislocations. Raman spectroscopy, ellipsometry, and photoluminescence (PL) techniques are used to study the structural and optical properties of the films. Room-temperature PL of the films shows that 7% Sn incorporation in the Ge lattice results in a decrease in the direct bandgap of Ge from 0.8 eV to 0.56 eV.