Gas-source molecular beam epitaxial growth and characterization of InNxP1−x on InP

We report a study of N incorporation into InP using a radio frequency (rf) N plasma source. Very streaky reflection high-energy electron diffraction patterns are observed for InN_xP_1−x (x <1%) grown on InP, indicating layer-by-layer growth of the film. The sharp x-ray diffraction peak and the clear Pendelloesung fringes in the high-resolution x-ray rocking curves reveal the high crystalline quality and uniformity of the film. They also suggest the smoothness of the interface between InP and InN_xP_1−xand of the surface of the InN_xP_1−x layer. This is further confirmed by scanning electron microscopy on these samples, where featureless surface is obtained. The formation of an InNP alloy is confirmed by x-ray θ-2θ diffraction measurement where no phase separation is observed. Different ways to increase the N composition in InNP were explored. At a fixed N₂ flow-rate fraction, lowering the growth temperature increases the N composition in InNP. Raising the rf power or using a larger beam exit aperture will also increase the N incorporation as a result of the availability of more active N species.     

GaAs1−xSbx growth by OMVPE

The system GaAs_1−xSb_x has a solid phase miscibility gap ranging from x = 0.2 to x = 0.8 at the typical growth temperature of 600 C; however, metastable alloys covering this entire composition range have been grown by organometallic vapor phase epitaxy (OMVPE) using trimethylgallium, antimony and -arsenic as the source materials. The solid composition is studied for various values of growth temperature, the ratio of Sb to total group V elements in the vapor phase and the ratio of group III to group V elements in the vapor phase. The fraction of trimethylarsenic (TMAs) pyrolyzed in the vapor phase is found to be < 1 and to vary with temperature. Taking into account the incomplete pyrolysis of TMAs, solid composition is found to be controlled by thermodynamics. The effects of temperature and vapor composition are all accurately predicted using a simple thermodynamic model assuming equilibrium to be established at the solid-vapor interface. The properties of these metastable GaAs_1−x Sb_x alloys were explored using interference contrast microscopy, room temperature and 4 K photoluminescence, Hall effect and conductivity measurements. The alloy GaAs_0.5Sb_0.5 grown lattice matched to the InP substrate has excellent surface morphology, is p-type, and the photoluminescence spectrum consists of a single, intense, fairly broad (23.5 meV half width) peak at a wavelength of 1.54 Μm.     

A comparison of the bulk resistivity as a function of composition for some group lVA-boron and -silicon systems

The bulk resistivity as a function of composition has been measured for TiB_x, ZrB_x, HfB_x and TiSi _x and ZrSi _x (0 ≤x ≃ 2) samples. The results are discussed in conjunction with the metallurgical characteristics of the five systems. The diboride phases generally have low resistivities which are relatively insensitive to changes in boron composition. In the case of the Ti-B system the minimum resistivity occurs at a boron composition below that required for the diboride phase. In contrast to the boron systems, pronounced variations in electrical resistivities occur in the Ti-Si and Zr-Si systems with sharp peaks at about the compositions Ti₅Si₃ and Zr₅Si₄.     

Microstructural Aspects of Nucleation and Growth of (In,Ga)As-GaAs(001) Islands with Low Indium Content

Molecular beam epitaxy growth of multilayer In _x Ga_1-x As/GaAs(001) structures with low indium content (x = 0.20–0.35) was studied by X-ray diffraction and photoluminescence in order to understand the initial stage of strain-driven island formation. The structural properties of these superlattices were investigated using reciprocal space maps, which were obtained around the symmetric 004 and asymmetric 113 and 224 Bragg diffraction, and ω/2θ scans with a high-resolution diffractometer in the triple axis configuration. Using the information obtained from the reciprocal space maps, the 004 ω/2θ scans were simulated by dynamical diffraction theory and the in-plane strain in the dot lattice was determined. We determined the degree of vertical correlation for the dot position (“stacking”) and lateral composition modulation period (LCM) (lateral ordering of the dots). It is shown that initial stage formation of nanoislands is accompanied by LCM only for [110] direction in the plane with␣a period of about 50 to 60 nm, which is responsible for the formation of a quantum wire like structure. The role of In _x Ga_1-x As thickness and lateral composition modulation in the formation of quantum dots in strained In _x Ga_1-x As/GaAs structures is discussed.     

Growth and evaluation of lpe graded composition AlxGa1−xAs layers for high efficiency graded bandgap solar cells

A melt-mixing LPE growth technique to obtain a graded composition Al_xGa_1−xAs layer is described. The graded bandgap AlGaAs/GaAs solar cell requires the Al fraction (x) in the Al_xGa_1−xAs surface layer to increase from the junction towards the surface. The graded composition Al_xGa_1−xAs layer creates a built-in electric field for minority carriers which improves the carrier collection process. This graded bandgap solar cell should enable one to realize the full potential of GaAs as a material for solar energy conversion. Quantitative evaluation of the composition profile for these graded layers is needed to develop the proper growth technique and to understand the resulting solar cell characteristics. Rutherford backscattering analysis technique is described which has been successfully used to profile such layers.     

Composition and temperature dependence of the direct band gap of GaAs1−xNx (0≤x≤0.0232) using contactless electroreflectance

The composition and temperature dependence (20K<T<380K) of the direct gap, E₀, of a series of GaAs_1−xN_x/GaAs (0≤x≤0.0232) samples has been measured using contactless electroreflectance. Our results for the composition dependence of E₀ are different in relation to a recent experiment [W.G. Bi and C.W Tu, Appl. Phys. Lett. 70, 1608 (1997)]. In contrast to previously reported results, we find that the temperature dependence of the direct gap is in fact dependent on N composition and that the parameters which describe the temperature dependence of the band gap lie between those of GaAs and GaN.     

Characterization of high quality RTCVD relaxed Si1−xGex grown on ge graded buffer layers on Si by photoluminescence spectroscopy

Relaxed Si_1−xGe_x layers grown by rapid thermal chemical vapor deposition (RTCVD) have been characterized by photoluminescence (PL) spectroscopy. The structures consist of a Si_1−xGe_x capping layer with a 0.32 and 0.52 Ge concentration, grown on a compositionally graded Si_1−xGe_x buffer layer. The effect of the composition grading rate on the layer quality has been intensively studied. Well-resolved near band edge luminescence (excitonic lines with no-phonon and phonon replica similar as in bulk SiGe alloys) coming from the relaxed alloy capping layer and dislocation-related bands (Dl, D2, D3, D4 lines) in the graded buffer layer have been measured. The electronic quality of this relaxed capping layer, controlled by the design of the compositionally graded buffer layer, has been determined by the excitonic photoluminescence. A detailed analysis of the energy of the D4 dislocation band demonstrates that the main misfit dislocations remain confined in the first steps of the graded buffer layer. Si_1−xGe_x layers grown on these pseudo-substrates either under compressive or tensile strain and the well-defined PL results obtained are discussed on the bases of strain symmetrization and of high quality of the layers. This points out the possibility of using such high quality relaxed Si_1−xGe_x layers as substrates for the integration of new devices associated with Si technology.     

Molecular beam epitaxy of CdSe and the derivative alloys Zn1−x Cd x Se and Cd1−x M x Se

We report the epitaxial growth of CdSe, Zn_1−x Cd _x Se (0 ≤x ≤ 1) and Cd_1−x Mn _x Se (0 ≤x ≤ 0.8) on (100) GaAs. X-ray diffraction (XRD), electron diffraction and transmission electron microscopy (TEM) indicate that all the epilayers have the cubic (zinc-blende) structure of the GaAs substrate. The energy gaps of these materials were measured using reflectivity measurements. We also report the growth of ZnSe/Zn_1−x Cd _x Se superlattices. TEM and XRD measurements show that high quality modulated structures with sharp interfaces are possible.     

Growth of HgTe Quantum Wells for IR to THz Detectors

We zone-engineered HgCdTe/HgTe/HgCdTe quantum wells (QWs) using the molecular-beam epitaxy (MBE) method with in situ high-precision ellipsometric control of composition and thickness. The variations of ellipsometric parameters in the ψ–Δ plane were represented by smooth broken curves during HgTe QW growth with abrupt composition changes. The form of the spiral fragments and their extensions from fracture to fracture revealed the growing layer composition and its thickness. Single and multiple (up to 30) Cd _x Hg_1−x Te/HgTe/Cd _x Hg_1−x Te QWs with abrupt changes of composition were grown reproducibly on (013) GaAs substrates. HgTe thickness was in the range of 16 nm to 22 nm, with the central portion of Cd _x Hg_1−x Te spacers doped by In to a concentration of 10¹⁴ cm⁻³ to 10¹⁷ cm⁻³. Based on this research, high-quality (013)-grown HgTe QW structures can be used for all-electric detection of radiation ellipticity in a wide spectral range, from far-infrared (terahertz radiation) to mid-infrared wavelengths. Detection was demonstrated for various low-power continuous-wave (CW) lasers and high-power THz pulsed laser systems.     

Alternative boron precursors for BGaAs epitaxy

The organometallic vapor phase epitaxy growth of zinc-blende B_xGa_1−xAs and B_xGa_1−x−yIn_yAs with boron concentrations (x) up to 3–5% has recently been demonstrated using diborane as a boron precursor. Growth of these alloys using diborane is complicated by many factors, such as parasitic gas-phase reactions, highly temperature-dependent boron incorporation, and limited boron incorporation before the onset of structural breakdown. These factors suggest that diborane may not be the best precursor for the growth of these alloys. We compare the use of alternative boron precursors; trimethylboron (TMB), triethylboron (TEB), and boron trifluoride (BF₃), with diborane for the OMVPE growth of these boron containing III–V alloys. We find that TMB and BF₃ do not result in significant boron incorporation into GaAs. TEB does result in boron incorporation in a manner very similar to diborane. Both diborane and TEB incorporate more efficiently using triethylgallium (TEG) rather than trimethylgallium (TMG), making TEG a preferred source of gallium for BGaAs epitaxy. Using TEB together with TEG, a higher boron composition (x=4–7%) has been achieved than has been previously reported, but the complicating problems observed with diborane still exist.     

Properties and use of ln0.5(AlxGa1-x)0.5P and AlxGa1-x as native oxides in heterostructure lasers

Data are presented demonstrating the formation of native oxides from high Al composition In_0.5(Al_xGa_1-x)_0.5P (x≳ 0.9) by simple annealing in a “wet” ambient. The oxidation occurs by reaction of the high Al composition crystal with H₂O vapor (in a N₂ carrier gas) at elevated temperatures (≥500° C) and results in stable transparent oxides. Secondary ion mass spectrometry (SIMS) as well as scanning and transmission electron microscopy (SEM and TEM) are employed to evaluate the oxide properties, composition, and oxide-semiconductor interface. The properties of native oxides of the In_0.5(Al_xGa_1-x)_0.5P system are compared to those of the Al_xGa_1-xAs system. Possible reaction mechanisms and oxidation kinetics are considered. The In_0.5(Al_xGa_1-x)_0.5P native oxide is shown to be of sufficient quality to be employed in the fabrication of stripe-geometry In_0.5(Al_xGa_1-x)_0.5P visible-spectrum laser diodes.     

Acceptors in Cd1−x MnxTe (x<0.1)

Acceptor defects, which control conductivity and recombination in Cd_1−x Mn_xTe (0⩽x⩽0.1), have been observed experimentally and investigated by electric and luminescence methods. The energy levels of the defects and the composition dependence of the energy levels have been determined. The physicochemical nature of these defects is discussed. Fiz. Tekh. Poluprovodn. 31, 1017–1020 (August 1997)     

Plasma-enhanced chemical vapor deposition and structural characterization of amorphous chalcogenide films

We describe the preparation of layers of amorphous Se, As_xS_1−x , As_xSe_1−x , Ge_xS_1−x , and Ge_xSe_1−x by plasma-enhanced chemical vapor deposition using the hydrides of the elements as precursor gases. We discuss the influence of the gas ratios and the deposition conditions (pressure, rf power input) on the chemical composition and the homogeneity of the binary systems. Information concerning the structure of the films was obtained from infrared and Raman spectroscopy. Fiz. Tekh. Poluprovodn. 32, 958–963 (August 1998) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Electric and photoelectric properties of n-GaxIn1−x N/p-Si anisotypic heterojunctions

We have developed a technology for producing n-type Ga_xIn_1−x N/p-Si heterostructures by combined pyrolysis of indium and gallium monoammoniate chlorides, making it possible to obtain heterolayers with composition varying over wide limits (from GaN up to InN). The composition and basic electric and optical characteristics of nitride films were determined. The electric and photoelectric properties of the heterostructures with Ga_xIn_1−x N films of different composition were investigated. It was shown that the anisotypic heterojunction n-Ga_xIn_1−x N/p-Si is a promising photosensitive element for detecting visible-range radiation. The maximum values of the specific detectivity were D*=1.2×10¹¹ Hz^1/2·W⁻¹ at 290 K. A band diagram of the heterojunction was constructed. Fiz. Tekh. Poluprovodn. 32, 461–465 (April 1998)     

Resistivity variation of semi-insulating Cd1−xZnx Te in relationship to alloy composition

We investigated the resistivity variation of semi-insulating Cd_1−xZn_xTe used as room temperature nuclear radiation detectors, in relationship to the alloy composition. The resistivity and the zinc composition were determined using leakage current measurements and triple axis x-ray diffraction lattice parameter measurements, respectively. While the zinc content of the nominally x_Zn∼0.1 ingot varied monotonically according to the normal freezing behavior with an effective segregation coefficient of k_eff=1.15, the resistivity was found to vary non-systematically throughout the ingot. Furthermore, the “expected” relationship of higher zinc content with higher resistivity was not always observed. For example, wafer regions of x_Zn∼0.12 and x_Zn∼0.08 exhibited resistivity values of ∼10¹⁰ and ∼10¹¹ Ω·cm, respectively. In general, the experimental resistivity values can be explained by calculated values which take into account a compensating deep level defect and various electron and hole mobility values. The relative influence of the parameters that govern the resistivity (n,p, μ_e, and μ_h) are quantitatively investigated.     

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.     

Spectroscopic ellipsometry for monitoring and control of molecular beam epitaxially grown HgCdTe heterostructures

A systematic study of the effect of measurement perturbation on in situ monitoring of the composition of molecular beam epitaxially (MBE) grown Hg_1−xCd_xTe using spectroscopic ellipsometry was carried out. Of the five variables investigated, which included angle of incidence, wavelength of the light beam, modulator rotation, analyzer rotation, and modulator amplitude, the angle of incidence and the modulator rotation had the strongest effect on the in situ Hg_1−xCd_xTe composition monitoring process. A wobble-free sample manipulator was installed to reduce the impact of these two variables. With these improvements, the spectroscopic ellipsometer is now routinely used to monitor Hg _1−xCd_xTe compositions during MBE growth of heterostructures and is a useful tool in diagnosing growth-related problems. Examples are included for both application areas, that include the control of the interface between Hg_1−xCd_xTe layers of different compositions, i.e. device engineering.     

Ultra-Thin Si<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Dislocation Blocking Layers for Ge/Strained Si CMOS Devices

We demonstrate ultra-thin (<150 nm) Si_1−x Ge _x dislocation blocking layers on Si substrates used for the fabrication of tensile-strained Si N channel metal oxide semiconductor (NMOS) and Ge P channel metal oxide semiconductor (PMOS) devices. These layers were grown using ultra high vacuum chemical vapor deposition (UHVCVD). The Ge mole fraction was varied in rapid, but distinct steps during the epitaxial layer growth. This results in several Si_1−x Ge _x interfaces in the epitaxially grown material with significant strain fields at these interfaces. The strain fields enable a dislocation blocking mechanism at the Si_1−x Ge _x interfaces on which we were able to deposit very smooth, atomically flat, tensile-strained Si and relaxed Ge layers for the fabrication of high mobility N and P channel metal oxide semiconductor (MOS) devices, respectively. Both N and P channel metal oxide semiconductor field effect transister (MOSFETs) were successfully fabricated using high-k dielectric and metal gates on these layers, demonstrating that this technique of using ultra-thin dislocation blocking layers might be ideal for incorporating high mobility channel materials in a conventional CMOS process.     

Growth of GaAs1−xPx/GaAs and InAsxP1−x/InP strained quantum wells for optoelectronic devices by gas-source molecular beam epitaxy

In this paper we show that pseudomorphically strained heterostructures of InAs _x P_1−x /InP may be an alternative to lattice-matched heterostructures of In_1−x Ga _x As _y P_1−y /InP for optoelectronic applications. We first studied the group-V composition control in the gas-source molecular beam epitaxy (GSMBE) of the GaAs_1-x P _x /GaAs system. Then we studied GSMBE of strained InAs _x P_1−x /InP multiple quantum wells with the ternary well layer in the composition range 0.15 <x < 0.75. Structural and optical properties were characterized by high-resolution x-ray rocking curves, transmission electron microscopy, absorption and low-temperature photoluminescence measurements. High-quality multiple-quantum-well structures were obtained even for highly strained (up to 2.5%) samples. The achievement of sharp excitonic absorptions at 1.06, 1.3 and 1.55μm at room temperature from InAs _x P_1−x /InP quantum wells suggests the possibility of long-wavelength optoelectronic applications.     

Group-V composition control for InGaAsP grown by gas source molecular beam epitaxy

Based on our kinetics models for gas source molecular beam epitaxy of mixed group-V ternary materials, the group-V composition control in In_yGa_1−yAs_1−xP_x epilayers has been studied. The P or As composition in In_yGa_1−yAs_1−xP_x (lattice matched to InP or GaAs) can be obtained from a simple equation for substrate temperatures below 500°C. This has been verified by a series of experimental results.