Heavily carbon-doped p-type InGaAs by MOMBE

Carbon-doped In_xGa_1−xAs layers (x=0−0.96) were grown by metalorganic molecular beam epitaxy (MOMBE) using trimethylgallium (TMG), solid arsenic (As₄) and solid indium (In) as sources of Ga, As and In, respectively. The carrier concentration is strongly affected by growth temperature and indium beam flux. Heavy p-type doping is obtained for smaller In compositions. The hole concentration decreases with the indium composition from 0 to 0.8, and then the conductivity type changes from p to n at x=0.8. Hole concentrations of 1.0×10¹⁹ and 1.2×10¹⁸ cm^-3 are obtained for x=0.3 and 0.54, respectively. These values are significantly higher than those reported on carbon-doped In_xGa_1−xAs by MBE. Preliminary results on carbon-doped GaAs/In_xGa_1−xAs strained layer superlattices are also discussed.     

Chemical beam epitaxy of AlGaAs and AlInAs using trimethylamine alane precursor

Al_xGa_1−xAs and Al_xIn_1−xAs alloys were grown on GaAs and InP, respectively, by chemical beam epitaxy, using trimethylamine alane (TMAA) as the source of aluminium. TMAA could be used properly only after some problems had been solved. Low carbon and oxygen concentrations were obtained in both alloys, leading to residual hole concentrations of 2 × 10¹⁶ cm^-3 in Al_0.3Ga_0.7As. The abruptness of the AlGaAs/GaAs interface proved the absence of TMAA memory effect. The control of Al_xIn_1−xAs solid composition was more difficult than for Ga_xIn_1−xAs, but was less sensitive to growth temperature. Photoluminescence intensities of Al_0.3Ga_0.7As and Al_0.48In_0.52As grown at 510°C were similar to those of MBE grown materials.     

High carbon doping of Ga1−xInxAs (x≈0.01) grown by molecular beam epitaxy

We have grown layers of Ga_1−xIn_xAs:C (x ≈ 0.01) on (100) GaAs by molecular beam epitaxy. As C source a graphite filament was used. Structures coherent with the substrate were obtained by adjusting properly the In and C concentrations. With simultaneous incorporation of In and C the strain is compensated and, consequently, the defect density is reduced. A maximum hole concentration value of p = 6×10¹⁹ cm⁻³ was achieved, which is twice higher than the saturation value of C doping of GaAs produced under the same conditions. There is evidence that this value is not in the saturation limit. The product of the hole density times the mobility increases, so the resistance decreases with higher C doping. Raman spectra show that the C_As peak broadens and shifts to lower frequencies for increasing concentration of indium. In H-passivated samples, Raman spectroscopy shows that C_As is surrounded by Ga atoms only. Indium atoms are thus present only in the second group III shell.     

Structural analysis of AlGaAs quantum wires on vicinal (110)GaAs by transmission electron microscopy and energy dispersive X-ray spectroscopy

Giant step structures consisting of coherently aligned multi-atomic steps were naturally formed during the molecular beam epitaxy growth of Al_0.5Ga_0.5As/GaAs superlattices (SLs) on vicinal (110)GaAs surfaces misoriented 6° toward (111)A. The growth of AlAs/Al_xGa_1−xAs/AlAs quantum wells (QWs) on the giant step structures realized Al_x0Ga_1−x0As (x₀<x) quantum wires (QWRs). We studied the giant step structures and the QWRs by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX). TEM observations revealed that the QWRs were formed at the step edges. The cross sections of the QWRs were as small as 10 nm×20 nm and the lateral distances between them were about 0.15 μm. We clarified the roles of the SLs to form the coherent giant step structures. From EDX analysis, it was estimated that the AlAs composition in the Al_0.5Ga_0.5As layers varied from 0.5 (terrace) to 0.41 (step edge). In the AlAs/Al_xGa_1−xAs/AlAs QWs, the AlAs compositional modulation and the confinement by the AlAs barriers led to the embedded Al_x0Ga_1−x0As regions. These results supported the existence of the Al_x0Ga_1−x0As QWRs on the giant step structures.     

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.     

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.     

Quantum transport measurements on Si δ- and slab-doped In0.53Ga0.47As grown by molecular beam epitaxy

A series of high quality δ-doped In_0.53Ga_0.47As samples have been grown lattice matched to InP with design doping densities in the range 2×10¹² to 5×10¹² cm⁻². Analysis of the individual sub-band densities deduced from the Shubnikov-De Haas effect shows that both spreading and amphoteric behaviour increase with doping density.     

Structural and electrical properties of GaSb, AlGaSb and their heterostructures grown on GaAs by metalorganic chemical vapor deposition

A systematic study of structural and electrical properties of GaSb and AlGaSb grown on GaAs by metalorganic chemical vapor deposition is reported. In general, the results obtained from surface morphologies, X-ray linewidths and Hall properties are consistent with each other and indicate that the optimal growth conditions for GaSb are at 525°C around V/III = 1. A highest hole mobility of 652 cm²/V · s at RT (3208 cm²/V · s at 77 K) and a lowest concentration of 2.8 × 10¹⁶ cm⁻³ (1.2 × 10¹⁵ cm⁻³ at 77 K) were obtained for GaSb grown under this optimal condition. Compared to the GaSb growth, a smaller V/III ratio is needed for the AlGaSb growth to protect the surface morphology. When Al was incorporated into GaSb growth, mobility decreased and carrier concentration increased sharply. The AlGaSb grown at 600°C had a background concentration about one order of magnitude lower than the AlGaSb grown at 680°C. Room-temperature current-voltage characteristics of GaSb/Al_xGa_{1 − x}Sb/GaSb show a rectifying feature when Al composition x is higher than 0.3, suggesting a valence-band discontinuity at the AlGaSb/GaSb interface. A leakage current much higher than the value predicted by the thermionic emission theory is observed at 77 K, presumably due to a large number of dislocations generated by the huge lattice mismatch between GaSb and GaAs.     

New aluminium precursors for MOMBE (CBE): a comparative study

Recently different new Al precursors have been developed to improve the electrical and optical quality of AlGaAs layers grown by MOMBE (CBE), since AlGaAs layers still suffer from the high incorporation of oxygen and carbon. Three approaches are introduced and results obtained from Al_xGa_1−xAs layers (0 < x ≤ 1) are discussed. APAH, a double ring structure molecule, was found to yield AlGaAs layers with high contents of carbon and nitrogen. The use of an Alane-adduct decreases impurity concentrations and improves optical properties. However, TIBAl is superior and provides highest PL response together with carrier concentrations below p = 10¹⁶ cm^-3. Even though the concept of coordinative saturation is promising, results achieved by TIBAl showed that trialkyls could also be well suited for AlGaAs, assuming that they are properly synthesized.     

Structure and optical properties of self-assembled InAs/GaAs quantum dots with In0.15Ga0.85As underlying layer

A high density of 1.02×10¹¹ cm⁻² of InAs islands with In_0.15Ga_0.85As underlying layer has been achieved on GaAs (1 0 0) substrate by solid source molecular beam epitaxy. Atomic force microscopy and PL spectra show the size evolution of InAs islands. A 1.3 μm photoluminescence (PL) from InAs islands with In_0.15Ga_0.85As underlying layer and InGaAs strain-reduced layer has been obtained. Our results provide important information for optimizing the epitaxial structures of 1.3 μm wavelength quantum dots devices.     

Computer simulation for stability of quartenary solid solutions

A thermodynamic analysis of stability for quarternary alloys of the type A_xB_1−xC_yD_1−y has been developed. The necessary and sufficient stability conditions of the four-component system with respect to diffusional processes were obtained. The calculation of the thermodynamic properties of In_xGa_1−xAs_yP_1−y solid solutions has been based on their pair approximation. Numerical simulation results shown that In_xGa_1−xAs_yP_1−y alloys can be only conditionally unstable in (T,x) space and are applicable to explain the composition modulation in In_xGa_1−xAs_yP_1−y epitaxial layers.     

Thermodynamic analysis of the MOVPE growth of InxGa1−xN

A chemical equilibrium model is applied to the growth of the In_xGa_1−xN alloy grown by metalorganic vapor-phase epitaxy (MOVPE). The equilibrium partial pressures and the phase diagram of deposition are calculated for the In_xGa_1−xN alloy. The vapor-solid distribution relationship is discussed in comparison with the experimental data reported in the literature. It is shown that the solid composition of the In_xGa_1−xN alloy grown by MOVPE is thermodynamically controlled and that the incorporation of group III elements into the solid phase deviates from a linear function of the input mole ratio of the group III metalorganic sources under the conditions of high mole fraction of decomposed NH₃ (high value of ), high temperature and low input V/III ratio. The origin of the deviation of the solid composition from the linear relation is also discussed.     

Ionic transport and defect structure of vitreous beryllium fluoride

Vitreous BeF₂ was prepared by two techniques; (1) remelting of a technical grade material, and (2) vacuum distillation/fluoridation. Infrared spectroscopy studies have established that the first material contains about 0.5 wt.% hydroxyl, predicted to be coherently incorporated into the vitreous network as edge-linked [Be(OH)₄]²⁻ units. The distilled BeF₂ is water-free. The dc electrical conductivity of the remelted BeF₂ was measured as σ = (7.9 × 10³/T) exp(−24500 cal/mol/RT) ω⁻¹ cm⁻¹ and for the distilled BeF₂ as σ = (3.0 × 10⁵/T) exp(−36700 cal/mol/RT ω⁻¹ cm⁻¹ at temperatures to 280°C. Ionic transport studies utilizing a dc electrolysis polarization technique with N₂−F₂ and H₂−HF gas electrodes have demonstrated that the fluorine ion is the transport species. A general model for fluorine transport is proposed based upon a modified anti-Frenkel defect model. The difference in the fluorine transport process for the undistilled grade of BeF₂ is seen as a consequence of the anti-Frenkel defect pair interaction with the [Be(OH)₄[²⁻ groupings.     

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.     

Comparative analysis of characteristics of Si, Mg, and undoped GaN

We have grown undoped, Si- and Mg-doped GaN epilayers using metalorganic chemical vapor deposition. The grown samples have electron Hall mobilities (carrier concentrations) of 798 cm²/V s (7×10¹⁶ cm⁻³) for undoped GaN and 287 cm²/V s (2.2×10¹⁸ cm⁻³) for Si-doped GaN. Mg-doped GaN shows a high hole concentration of 8×10¹⁷ cm⁻³ and a low resistivity of 0.8 Ω cm. When compared with undoped GaN, Si and Mg dopings increase the threading dislocation density in GaN films by one order and two orders, respectively. Besides, it was observed that the Mg doping causes an additional biaxial compressive stress of 0.095 GPa compared with both undoped and Si-doped GaN layers, which is due to the incorporation of large amount of Mg atoms (4–5×10¹⁹ cm⁻³).     

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.     

Phonon behavior and interfacial stress in the strained (InAs)m/(GaAs)n ultrathin superlattices

Phonon behavior in the strained (InAs)_m/(GaAs)_n ultrathin superlattices grown by molecular beam epitaxy has been investigated by means of Raman scattering spectroscopy. The phonon frequency in the GaAs layers shifts toward lower energy with increasing InAs layer thickness under fixed thickness of GaAs layers. The frequency in the InAs layers does not change significantly, as deduced from the behavior of the InAs-like mode in In_xGa_1−xAs alloys. These observed results are phenomenologically discussed on the basis of the combined effect of phonon confinement in the respective layer and stress at the alternating interfaces. Furthermore, a large softening of the phonon confined in the GaAs layers decreases the frequency gap, resulting in traveling of the optical phonons confined in both layers. The strain at the interfaces is estimated by an empirical method, i.e., by comparing the frequency in the superlattice and in the alloy of equivalent composition. In the In_xGa_1−xAs alloys, the composition dependence of the mode frequency is considered as being due to the local strain. The group III elements are considered to be in the local strain state from an extended X-ray absorption fine structure (EXAFS) analysis.     

Anomalous behavior of excess energy curves of InxGa1−xN grown on GaN and InN

We worked out the excess energies for bulk In_xGa_1−xN and In_xGa_1−xN thin films on GaN and InN in order to investigate their thermodynamic stabilities. It has been found that the excess energy maximum shifted toward x0.80 for InGaN/GaN and x0.10 for InGaN/InN due to the lattice constraint in contrast with x0.50 for bulk. Moreover, it has been revealed that the excess energy for InGaN/GaN is larger than that for bulk at x>0.65. This suggests that In-rich films are less stable on GaN than bulk state. These results indicate that the lattice constraint has a significant influence on thermodynamic stabilities of thin films.     

SiC epitaxial layer growth in a novel multi-wafer vapor-phase epitaxial (VPE) reactor

Experimental results are presented for SiC epitaxial layer growths employing a unique planetary SiC-VPE reactor. The high-throughput, multi-wafer (7×2″) reactor, was designed for atmospheric and reduced pressure operation at temperatures up to and exceeding 1600°C. Specular epitaxial layers have been grown in the reactor at growth rates ranging from 3–5 μm/h. The thickest layer grown to date is 42 μm thick. The layers exhibit minimum unintentional n-type doping of 1×10¹⁵ cm⁻³, and room temperature mobilities of 1000 cm²/V s. Intentional n-type doping from 5×10¹⁵ cm⁻³ to >1×10¹⁹ cm⁻³ has been achieved. Intrawafer layer thickness and doping uniformities (standard deviation/mean at 1×10¹⁶ cm⁻³) are typically 4 and 7%, respectively, on 35 mm diameter substrates. Moderately doped, 4×10¹⁷ cm⁻³, layers, exhibit 3% doping uniformity. Recently, 3% thickness and 10% doping uniformity (at 1×10¹⁶ cm⁻³) has been demonstrated on 50 mm substrates. Within a run, wafer-to-wafer thickness deviation averages 9%. Doping variation, initially ranging as much as a factor of two from the highest to the lowest doped wafer, has been reduced to 13% at 1×10¹⁶ cm⁻³, by reducing susceptor temperature nonuniformity and eliminating exposed susceptor graphite. Ongoing developments intended to further improve layer uniformity and run-to-run reproducibility, are also presented.     

Growth of InGaAs/GaAs heterostructures with abrupt interfaces on the monolayer scale

Segregation processes entail severe deviations from the nominal composition profiles of heterostructures grown by molecular beam epitaxy for most semiconductor systems. It is, however, possible to compensate exactly these effects, as shown here for InGaAs/GaAs. The deposition of a one-monolayer-thick indium-rich prelayer of InGaAs (or of a sub-monolayer amount of InAs) prior to growth of In_xGa_1−xAs allows forming a perfectly abrupt In_xGa_1−xAs-on-GaAs interface. Thermal annealing can furthermore be performed at the GaAs-on-InGaAs inter face, so as to desorb surface indium atoms and suppress In incorporation in the GaAs overlayer. This powerful approach has been validated from a detailed study of the surface composition at various stages of the growth of InGaAs/GaAs quantum wells, as well as from high-resolution transmission electron microscopy and photoluminescence investigations.