Preferential migration of indium atoms on the (411)A plane in InGaAs grown on GaAs channeled substrates by molecular beam epitaxy

Lateral profiles of In content in a 1.5 μm thick In_xGa_1−xAs (x 0.2) layer grown on GaAs channeled substrates (CSs) with (411)A side-slopes by molecular beam epitaxy (MBE) have been investigated with the use of energy dispersive X-ray spectroscopy (EDX). The observed profiles of the In content suggested that In atoms migrate preferentially in the [1 ] direction on the (411)A plane during MBE growth. This preferential migration of In atoms along [1 ] on the (411)A plane was confirmed by comparing observed lateral profiles of In content in InGaAs layers grown on GaAs CSs and simulated In profiles which are calculated by taking into account of an additional one-way flow of In atoms along [1 ].     

Strain-compensated quantum cascade lasers operating at room temperature

Quantum cascade (QC) lasers based on strain-compensated In_xGa_(1−x)As/In_yAl_(1−y)As grown on InP substrate using molecular beam epitaxy is reported. The epitaxial quality is demonstrated by the abundant narrow satellite peaks of double-crystal X-ray diffraction and cross-section transmission electron microscopy of the QC laser wafer. Laser action in quasi-continuous wave operation is achieved at λ≈3.6–3.7μm at room temperature (34°C) for 20 μm×1.6 mm devices, with peak output powers of 10.6 mW and threshold current density of 2.7 kA/cm² at this temperature.     

The growth of InGaAsP by CBE for SCH quantum well lasers operating at 1.55 and 1.4 μm

InGaAsP has been grown by CBE at compositions of 1.1, 1.2 and 1.4 μm for the development of MQW-SCH lasers. The observed incorporation coefficients for TMI and TEG show strong temperature sensitivity while the phosphorus and arsenic incorporation behavior is constant over the substrate temperature range explored, 530 to 580°C setpoint. For higher substrate temperatures the growth rate increases with the largest growth rates occurring for the 1.4 μm quaternary. Low temperature photoluminescence indicates the possibility of compositional grading or clustering for the 1.1 μm material and also for the 1.2 μm material grown at the lowest substrate temperature. The final laser structure was grown with the InP cladding regions grown at 580°C with the inner cladding and active regions grown at 555°C. Using this approach we have successfully grown MQW-SCH lasers with the composition of the active In_xGa_1−xAs ranging from x=0.33 to x=0.73. Threshold current densities as low as 689 A/cm² have been measured for an 800 μm×90 μm broad area device with x=0.68.     

Electronic traps in mixed Si1−xGexO2 films

Thermally stimulated luminescence (TSL) and infrared (IR) spectroscopy were measured in plasma grown Si_1−xGe_xO₂ (x=0, 0.08, 0.15, 0.25, 0.5) with different thicknesses (12–40 nm). A comparison with the TSL properties of thermally grown SiO₂ and GeO₂ was also performed. A main IR absorption structure was detected, due to the superposition of the peaks related to the asymmetric O stretching modes of (i) Si–O–Si (at ≈1060 cm⁻¹) and (ii) Si–O–Ge (at 1001 cm⁻¹). Another peak at ≈860 cm⁻¹ was observed only for Ge concentrations, x>0.15, corresponding to the asymmetric O stretching mode in Ge–O–Ge bonds. A TSL peak was observed at 70°C, and a smaller structure at around 200°C. The 70°C peak was more intense in all Ge rich layers than in plasma grown SiO₂. Based on the thickness dependence of the signal intensity we propose that at Ge concentrations 0.25x0.5 TSL active defects are localised at interfacial regions (oxide/semiconductor, Ge poor/Ge rich internal interface, oxide external surface/atmosphere). Based on similarities between TSL glow curves in plasma grown Si_1−xGe_xO₂, thermally grown GeO₂ and SiO₂ we propose that oxygen vacancy related defects are trapping states in Si_1−xGe_xO₂ and GeO₂.     

Impedance spectroscopy analysis of AlGaN/GaN HFET structures

For HFET application a series of samples with 30 nm Al_xGa_1−xN (x=0.02–0.4) layers deposited at 1040°C onto optimised 2 μm thick undoped GaN buffers were fabricated. The Al_xGa_1−xN/GaN heterostructures were grown on c-plane sapphire in an atmospheric pressure, single wafer, vertical flow MOVPE system. Electrical properties of the Al_xGa_1−xN/GaN heterostructures and thick undoped GaN layers were evaluated by impedance spectroscopy method performed in the range of 80 Hz–10 MHz with an HP 4192A impedance meter using a mercury probe. The carrier concentration distribution through the layer thickness and the sheet carrier concentration were evaluated. A non-destructive, characterisation technique for verification of device heterostucture quality from the measured C–V and G–V versus frequency characteristics of the heterostructure is proposed.     

InxGa1−xAs/GaAs quantum wire structures grown on GaAs (100) patterned substrates with [001] ridges

In_xGa_1−xAs/GaAs (x = 0.12-0.23) quantum well (QW) structures were grown by molecular beam epitaxy (MBE) on [001] ridges with various widths (1.1-12 μm) of patterned GaAs (100) substrate. The smallest lateral width of the InGaAs/GaAs quantum wire (QWR) structures was estimated to be about 0.1 μm by high-resolution scanning electron microscope (SEM). The In contents of the grown InGaAs/GaAs QWs on the ridges were studied as a function of ridge top width (ridge width of the MBE grown layer) by cathodoluminescence (CL) measurements at 78 K. Compared to the InGaAs QW grown on a flat substrate, the In content of the InGaAs/GaAs QW on the ridge increases from 0.22 to 0.23 when the ridge top width decreases to about 2.9 μm, but it decreases steeply from 0.23 down to 0.12 with a further decrease of the ridge width from 2.9 to 0.05 μm. A simulation of MBE growth of InGaAs on the [001] ridges shows that this reduced In content for narrow ridges is due to a large migration of Ga atoms to the (100) ridge top region from {110} side facets.     

Seed-mediated synthesis of uniform ZnO nanorods in the presence of polyethylene glycol

Large quantities of ZnO nanorods have been synthesized by the seed-mediated method in the presence of polyethylene glycol at 90 °C. The products are characterized using X-ray diffraction, scanning electron microscopy, transmission electron microscopy and high-resolution transmission electron microscopy. The as-grown ZnO nanorods are uniform with a diameter of 40–70 nm and length about 2 μm. The nanorods grew along the [0 0 1] direction. Possible roles of ZnO seeds and polymer in the growth of ZnO nanorods are also discussed.     

Thick single crystalline vanadyl phthalocyanine film epitaxially grown on KBr crystal by molecular beam epitaxy

Structural analyses are reported of 5 μm thick VOPc films grown on a KBr(001) surface by the MBE technique. The crystal is monoclinic with lattice parameters a = 14.2 Å, b = 13.1 Å, c = 12.7 Å and β = 103.2°. The orientation of the unit cell is influenced by the crystal lattice of the substrate even in this thickness range: The projections of the a- and b-axes of VOPc onto the (001) plane of KBr coincide with the [110] directions of KBr. Four kinds of crystal orientations were observed in pole figure measurements.     

Indium content determination related with structural and optical properties of InGaN layers

We studied the structural and optical properties of a set of nominally undoped epitaxial single layers of In_xGa_1−xN (0<x0.2) grown by MOCVD on top of GaN/Al₂O₃ substrates. A comparison of composition values obtained for thin (tens of nanometers) and thick (≈0.5 μm) layers by different analytical methods was performed. It is shown that the indium mole fraction determined by X-ray diffraction, measuring only one lattice parameter strongly depend on the assumptions made about strain, usually full relaxation or pseudomorphic growth. The results attained under such approximations are compared with the value of indium content derived from Rutherford backscattering spectrometry (RBS). It is shown that significant inaccuracies may arise when strain in In_xGa_1−xN/GaN heterostructures is not properly taken into account. Interpretation of these findings, together with the different criteria used to define the optical bandgap of In_xGa_1−xN layers, may explain the wide dispersion of bowing parameters found in the literature. Our results indicate a linear, E_g(x)=3.42−3.86x eV (x0.2), “anomalous” dependence of the optical bandgap at room temperature with In content for In_xGa_1−xN single layers.     

Growth of GaN/AlN and AlGaN by MOCVD using triethylgallium and tritertiarybutylaluminium

Films of aluminium nitride (AIN) with thicknesses in the range from 200 to 3600 Å have been deposited at 1050°C by low-pressure MOCVD. Using an alternative precursor, tritertiarybutylaluminium (^tBu₃Al), and ammonia (NH₃), we have grown AlN on sapphire (c-Al₂O₃). At a growth rate of 0.35 μm/h, the FWHM of the rocking curve measured by X-ray diffraction was 150 arcsec. Therefore, we used the thin AlN films as buffer layers for the deposition of gallium nitride (GaN) at 950°C using triethylgallium (Et₃Ga). Aluminium gallium nitride (Al_xGa_1−xN) with aluminium contents x from 0 to 0.5 were grown using a mixture of Et₃Ga and ^tBu₃Al. The strctural and optical properties of GaN, AlGaN and AlN were verified by X-ray diffraction (XRD), spectrally resolved photoconductivity (SPC), photothermal deflection (PDS) and photoluminescence spectroscopies.     

Reduction of tensile stress in GaN grown on Si(1 1 1) by inserting a low-temperature AlN interlayer

We studied the selective growth behaviors of InP through narrow openings (<2 μm) by metal-organic chemical vapor deposition. The lateral overgrowth was observed to be significantly affected by both the opening width and orientation. It was found that the lateral overgrowth length reached the maximum at 60° off [0 1 1] direction. The lateral overgrowth also showed a ‘diffraction-like’ behavior, with the overgrowth length increasing with decreasing opening width. Based on these results, a novel InP/InGaAs heterojunction bipolar transistor (HBT) structure with extrinsic base laterally overgrown on SiO₂ is proposed. The device behaviors of the laterally regrown-base HBT prototypes are demonstrated.     

Compositional control of CdxZn1−xSe grown by photoassisted organometallic vapor phase epitaxy

The photoassisted OMVPE growth technique is important for the fabrication of blue/green laser diodes based on Cd_xZn_1−xSe quantum wells. Low temperature growth with photoassistance is key to the fabrication of these devices, however, the compositional control of Cd_xZn_1−xSe becomes increasingly difficult as the growth temperature is reduced. We have studied the compositional control of Cd_xZn_1−xSe using the sources DMCd, DMZn, and DMSe, with irradiation from a Hg arc lamp. We studied the dependence of the composition on the growth temperature, irradiation intensity, and source mass flows. The composition x increases with increasing temperature and decreases with increasing irradiation intensity. The solid-phase composition is a non-linear function of the gas-phase composition X. The slope of this characteristic, dx/dX, should be minimized for good compositional control. At 475°C without photoassistance, dx/dX is 1.75 near a composition of 20%, as determined from the data of Parbrook et al. Decreasing the temperature increases dx/dX. At 370°C with 12 mW/cm², dx/dX ≈ 13 and at 350°C with 58 mW/cm² dx/dX ≈ 60. We have investigated this behavior at 370°C with 12 mW/cm² irradiation by studying both the composition and the growth rate as a function of the gas-phase composition. The growth rate is non-monotonic, and is minimum for a gas-phase composition of 0.20. The behavior is quite complex, and is not fully understood at the present time. Nonetheless, our results indicate that the Cd-bearing precursor is adsorbed much more strongly than the Zn-bearing precursor. In addition to this, the introduction of the DMCd strongly inhibits the growth of ZnSe. We have achieved sufficiently good compositional control at 370°C and 12 mW/cm² to grow ZnSe/Cd_xZn_1−xSe/ZnSe multiple quantum well structures. More work is necessary in order to clarify the roles of irradiation intensity and VI/II ratio so that good compositional control can be achieved at lower growth temperatures.     

Extremely flat interfaces in GaAs/AlGaAs quantum wells with high Al content(0.7) grown on GaAs (411)A substrates by molecular beam epitaxy

Effectively atomically flat interfaces over a macroscopic area (200 μm diameter) have been achieved in GaAs/Al_0.7Ga_0.3As quantum wells (QWs) with well widths of 3.6-12 nm grown on (411)A GaAs substrates by molecular beam epitaxy (MBE) for the first time. A single and very narrow photoluminescence peak (FWHM, full width at half maximum, is 6.1 meV) was observed at 717.4 nm for the QW with a well width of 3.6 nm at 4.2 K. The linewidth is comparable to that of growth-interrupted QWs grown on (100)-oriented GaAs substrates by MBE. A 1.5 μm thick Al_0.7Ga_0.3As layer with good surface morphology also could be grown on (411)A GaAs substrates in the entire growth temperature region of 580-700°C, while rough surfaces were observed in Al_0.7Ga_0.3As layers simultaneously grown on (100) GaAs substrates at 640-700°C. These results indicate that the surface of GaAs and Al_0.7Ga_0.3As grown on the (411)A GaAs substrates are extremely flat and stable on the (411)A plane.     

Microampere laser threshold at 80°C with InGaAs/GaAs/InGaP buried heterostructre strained quantum well lasers

An extremely low CW threshold current of 670 μA and a high slope efficiency of 0.14 W/A at a high junction temperature of 80°C were obtained with a 200 μm long Al-free InGaAs/GaAs/InGaP buried heterostructure (BH) quantum well laser grown by three-step metal organic vapor phase epitaxy (MOVPE). The maximum energy conversion efficiency of a 500 μm long laser was as high as 50% at a output power level of 1 mW. Regrowth conditions of InGaP layers were found to be crucial for planarizing the grown surface to realize the high performances.     

Low-temperature growth of epitaxial layers of ZnSe1−xTex solid solutions

The layers of ZnSe_1−xTe_x (0 < x < 1.0) solid solutions have been grown by liquid-phase epitaxy in a closed tube at 620–680 °C. Zinc chloride served as a solvent. ZnTe and ZnSe crystals were used as sources and substrates with orienting surfaces (110) and (111) for ZnSe and (110) for ZnTe. The composition of the grown layer was specified by the relative content of the ZnSe and the ZnTe in the solution and was controlled by X-ray analysis. The position of the exciton bands in the photoluminescence spectra of ZnSe_1−xTe_x over the interval 0.3 < x < 1.0 is in agreement with the free exciton energies calculated for these compositions. Relatively low-ohmic (of about 10² Ω cm) epitaxial layers of ZnSe_1−xTe_x solid solutions were grown.     

Luminescence characteristics of InAlP---InGaP heterostructures having native-oxide windows

Data are presented on the luminescence characteristics of InGaP/InAlP heterostructures with oxidized InAlP cladding layers grown by metalorganic chemical vapor deposition. The structures are grown on GaAs substrates and consist of either a 20 nm thick In_0.5Ga_0.5P quantum well or a 0.75 μm InGaP layer sandwiched between two InAlP bulk barriers or between two 10-period In_0.5Al_0.5P/In_xGa_1−xP strain-modulated superlattice heterobarriers, where x varies from 0.5 to 0.45 and the period of the superlattice is 3 nm. The top InAlP cladding layer of the InAlP/InGaP heterostructures is oxidized for 2–5.5 h at 500°C in an ambient of H₂O vapor saturated in a N₂ carrier gas. Photoluminescence and time-resolved photoluminescence studies at room temperature show that, as a result of the oxidation of a portion of the top InAlP cladding layer, the photoluminescence emission intensity and lifetime from the InGaP QWs increase significantly.     

Molecular beam epitaxy of strained Si1−xGex layers on patterned substrates

Epitaxial Si_1-xGe_x layers have been grown on patterned Si(001) substrates. Mesa-like structures of 1.4 μm height on the surface limited by inclined {111} planes were used. Structure dimensions between 10 and 0.2 μm were chosen to allow the mesas to relieve elastically under the strained layer. The film growth, the crystallographic perfection and the relaxation of strained Si_1-xGe_x layers were investigated by transmission electron microscopy (TEM) including in situ annealing. The relaxation mechanism and the control of dislocation generation on top of the mesas are discussed.     

Peculiarities of liquid phase epitaxy in GaInPAs/InP lattice-matched heterostructures

The temperature phase stability of Ga_xIn_1−xP_yAs_1−y solid solution has been analyzed. A simple solution theory with the temperature-independent interaction parameters in solid and liquid phases has been used. The absence of miscibility gaps for all the compositions of the solid solution, lattice-matched to InP at a growth temperature of 640°C, has been demonstrated both theoretically and experimentally. The influence of the elastic deformations on the Ga_xIn_1−xP_yAs_1−y (λ_g = 1.4 μm) solid solution parameters has been observed assuming the model of the layer coherent conjugation in heterostructures.     

Effects of substrate misorientation on triple-period ordering in AlInAs

Substrate misorientation effects on triple-period (TP) ordering in Al_0.48In_0.52As were studied. When AlInAs was grown on a (001) InP substrate misoriented by 4° in the [ 1]A direction, TP-type ordering formed more strongly in the [ 1]A direction than in the [111]A direction. This asymmetric formation of TP-type ordering demonstrates that the step-arrays descending in the [ 0] direction play an important role in TP-type ordering formation in the [ 1]A direction. When AlInAs was grown at a higher temperature on a (001) InP substrate misoriented by 4° in the [ 11]B direction, CuPt-type ordering was formed more strongly in the [ 11]B than in the [1 1]B direction. The importance of the [ 10] step in CuPt-type ordering, which is well established in GaInP systems, was thus reconfirmed in this study on AlInAs.     

Selective area growth of GaAs using a Ga beam with a step-function lateral intensity profile

Selective area growth of GaAs has been carried out in order to investigate the surface diffusion of Ga atoms using molecular beam epitaxy (MBE) with the aid of a Ga beam with a lateral step-function intensity profile. This step-function profile was obtained using a closely fitted GaAs shadow mask. When the mask edge was parallel to [01 ], a (311)A facet was typically observed near the edge of the Ga beam, while in the case of the mask edge parallel to [011], a (111)B facet was formed. MBE growth simulation based on the diffusion model was carried out in order to understand the mechanism of this selective area growth. The calculated results were in good agreement with the experimental results, and the diffusion lengths of Ga atoms were determined to be 0.10 μm along [011] direction on the (100) GaAs surface, 0.37 μm along [233] direction on the (311)A GaAs surface and 0.17 μm along [21 ] direction on the (111)B GaAs surface during MBE growth. These diffusion lengths seem to be smaller than those previously observed, which is probably due to a large V/III ratio in the region of the substrate close to the mask edge.