Heavily carbon-doped p-type (In)GaAs grown by gas-source molecular beam epitaxy using diiodomethane

Heavily carbon-doped p-type In_xGa_1−xAs (0≤x<0.49) was successfully grown by gas-source molecular beam epitaxy using diiodomethane (CH₂I₂), triethylindium (TEIn), triethylgallium (TEGa) and AsH₃. Hole concentrations as high as 2.1×10²⁰ cm⁻³ were achieved in GaAs at an electrical activation efficiency of 100%. For In_xGa_1−xAs, both the hole and the atomic carbon concentrations gradually decreased as the InAs mole fraction, x, increased from 0.41 to 0.49. Hole concentrations of 5.1×10¹⁸ and 1.5×10¹⁹ cm⁻³ for x = 0.49 and x = 0.41, respectively, were obtained by a preliminary experiment. After post-growth annealing (500°C, 5 min under As₄ pressure), the hole concentration increased to 6.2×10¹⁸ cm⁻³ for x = 0.49, probably due to the activation of hydrogen-passivated carbon accepters.     

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.     

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.     

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.     

Diode characteristics of Li3N-diffused ZnSe grown by MOVPE

We have fabricated a ZnSe diode using Li₃N diffusion technique for the purpose of forming p-type ZnSe. The maximum hole concentration in the Li₃N-diffused ZnSe layer, which has been grown on a GaAs substrate by metalorganic vapor phase epitaxy, was as high as 10¹⁸ cm⁻³. The ohmic contact to the p-type ZnSe has been demonstrated and the specific contact resistance of Au/p-ZnSe was 1 × 10⁻² Ω · cm². The Li₃N diffusion technique is useful for the bfabrication of ohmic contacts to p-ZnSe.     

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).     

Growth, morphological and structural characterization of Pb1−xSnxTe single crystals

Pb_1−xSn_xTe single crystals have been grown by a vertical Bridgman method. They have typical Hall mobilities and carrier concentration values of 10³ cm²/V · s and 10¹⁸ cm^-3, respectively, and change from p- to n-type as the Sn content increases. The ingots were single crystal with a subgrain structure that has a misorientation no higher than 2′. The segregation of Sn has been determined and it suggests that there is a convective flow in the liquid.     

The growth and physics of ultra-high-mobility two-dimensional hole gas on (311)A GaAs surface

We report on the molecular beam epitaxy growth of modulation-doped GaAs-(Ga,Al)As heterostructures on the (311)A GaAs surface using silicon as the acceptor. Two-dimensional hole gases (2DHGs) with low-temperature hole mobility exceeding 1.2×10⁶ cm² V⁻¹ s⁻¹ with carrier concentrations as low as 0.8×10¹¹ cm⁻² have been obtained. This hole mobility is the highest ever observed at such low densities by any growth technique. We also report the first observation of persistent photoconductivity in a 2DHG. An analysis of the number density and temperature dependence of the mobility leads us to conclude that the mobility is limited by phonon scattering above 4 K and interface scattering at lower temperatures.     

Blue-green laser diode grown by photo-assisted MOCVD

Operation of the first blue-green laser diode grown by metalorganic chemical vapor deposition has been demonstrated at 77 K under pulsed current injection. The precursors were dimethylzinc, dimethylcadmium, diethylsulfide, bismethyl-cyclopentadienyl-magnesium, and dimethylselenide. Diisopropylamine and ethyliodide were used for a p-type and n-type doping under irradiation with ultraviolet light generated by a high-pressure mercury lamp, respectively. A 1 × 10¹⁸ cm⁻³ nitrogen-atom concentration, which was measured by secondary ion mass spectroscopy, was obtained in the p-ZnSe contact layer. The 4.2 K photoluminescence spectrum was dominated by strong donor-acceptor pair emission and the net acceptor concentration was 1.4 × 10¹⁶ cm⁻³.     

Atomic-scale controlled incorporation of ultrahigh-density Si doping sheets in GaAs

Delta-doped GaAs:Si with doping densities up to 4×10¹⁴ cm⁻² has been grown by molecular beam epitaxy (MBE) at a substrate temperature of 590°C. To promote an ordered incorporation and thus avoid clustering of Si atoms, vicinal GaAs(001) surfaces 2° misoriented towards (111)Ga were used and Si was supplied in pulses. As evidenced by real-time reflection high-energy electron diffraction (RHEED) measurements an ordered incorporation of Si atoms on Ga sites along the step edges takes place. Although the ordered (3×2) structure degrades at high coverages, unusual high sheet carrier concentrations are obtained by pulsed delta-doping for doping concentrations >10¹³ cm⁻², as revealed by Hall measurements. The surface conditions during GaAs overgrowth have a strong influence on the free electron concentration, too. Raman scattering by local vibration modes and secondary ion mass spectrometry (SIMS) measurements are used to show that this is related to segregation effects as well as to a modification of the site occupancy.     

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.     

Effects of ion implantation on intermediate range order: IR spectra of silica

The reflectance spectra of ion implanted SiO₂ glasses has been measured from 5000 cm⁻¹ to 400 cm⁻¹. The silica was implanted with Ti, Cr, Mn, Fe, Cu and Bi to nominal doses ranging from 1×10¹⁵ ions/cm² to 1.2×10¹⁷ ions/cm² at an energy of 160 keV and currents of approximately 2.6 μA/cm². Changes in the intensity of the 1232 cm⁻¹ and 1015 cm⁻¹ vibrational modes are attributed to changes in the intermediate range order (IRO) and to changes in the concentration of non-bridging oxygen (NBO) defects in the implanted layer. These changes are ion and dose dependent. The differing effects on IRO and NBO are attributed to the chemical interaction of the implanted ions with the substrate.     

Metalorganic vapor-phase epitaxy of III/V phosphides with tertiarybutylphosphine and tertiarybutylarsine

Indium phosphide, gallium arsenide phosphide, and aluminum indium phosphide have been deposited by metalorganic vapor-phase epitaxy using tertiarybutylphosphine and tertiarybutylarsine. The effects of growth temperature and V/III ratio on the amount of silicon, sulfur, carbon, and oxygen in InP have been determined. Minimum incorporation was observed at 565 °C and a V/III ratio of 32. In this case, the material contained a background carrier concentration of 2.7×10¹⁴ cm⁻³, and the Hall mobilities were 4970 and 135,000 cm²/V s at 300 and 77 K. The oxygen contamination in AlInP was found to be only 9.0×10¹⁵ cm⁻³ for deposition at 650 °C and a V/III ratio of 35. The relative distribution of arsenic to phosphorus in GaAs_yP_1−y was determined at temperatures between 525 and 575 °C. The distribution coefficient [(N_As/N_P)_film/(P_TBAs/P_TBP)_gas] ranged from 25.4 to 8.4, and exhibited an Arrhenius relationship with an apparent activation energy of 1.2 eV.     

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.     

Tin and tellurium doping of InAs using SnTe in molecular beam epitaxy (MBE)

Tin telluride (SnTe) was utilized as an n-type dopant in the MBE growth of InAs epitaxial layers on GaAs substrates. The highest carrier concentration obtained was 2.9 × 10¹⁹ cm^-3 and the carrier density could be varied over three orders of magnitude by changing the SnTe source temperature. The highest mobilities obtained were 16,900 and 23,300 cm²/V … s at 300 and 77 K, respectively, for carrier concentration of 5 × 10¹⁶ cm^-3. Both Sn and Te were incorporated in the layers as determined by secondary ion mass spectroscopy (SIMS) analysis and the total concentration of Sn and Te were the same as the carrier density in the layer.     

CBE growth of high-quality AlGaAs/GaAs heterostructures for HEMT applications

AlGaAs/GaAs heterostructures were grown by chemical beam epitaxy using triethylgallium, triisobutylaluminium and pure arsine in flow control mode with hydrogen as carrier gas. For substrate temperatures of 580°C and V/III ratios of 10, high quality AlGaAs layers are obtained; heterostructures show abrupt and smooth interfaces. Modulation doping with silicon evaporated from a conventional effusion cell gives two-dimensional electron gases with carrier densities up to 1×10¹² cm^-2. Mobilities of 70000 cm²/V·s are obtained at 77 K for carrier densities of 4×10¹¹ cm^-2. The lateral homogeneity of the heterostructures in layer thickness, composition and doping level is excellent. Perfect morphology with defect densities of about 100 cm^-2 is observed. High electron mobility transistors (gate length 0.3 nm) fabricated from quantum well structures show a transconductance of about 380 mS/mm.     

High-quality InP grown by chemical beam epitaxy

InP films were grown by chemical beam epitaxy using trimethylindium (TMI) and pure phosphine (PH₃) in a flow control mode with hydrogen as the carrier gas, with the TMI flow rate fixed at 3 SCCM. Substrate temperatures were varied between 505 and 580°C and V/III ratios from 3 to 9. InP layers with high optical quality (intense and narrow excitonic transition lines) and high crystalline quality (narrow and symmetric X-ray diffraction peaks) could be grown only within a narrow parameter window around a substrate temperature of 545°C (δT_s ≤ 25°C) and a V/III ratio of 5.5 (δ(V/III) ≤ 2). Carrier densities of 8 × 10¹⁴ cm^-3 with mobilities of 70000 cm²/V.s measured at 77 K were obtained for growth conditions close to the edge of this parameter window towards low V/III ratios. The growth rate of inP was also clearly at its maximum in the given parameter window. Leaving the window, by changing either the growth temperature or the V/III ratio, significantly decreased the growth rate. This reduced growth rate was accompanied by a degradation in the crystalline quality. We also demonstrate that for higher TMI flow the parameter window shifts to higher growth temperatures. The InP could be doped effectively with Si in the range from 9 × 10¹⁵ to 3 × 10¹⁸ cm^-3.     

Growth and characterization of epitaxial layers on aluminum nitride substrates prepared from bulk, single crystals

A comparative study of epitaxy of AlN, GaN and their alloys, grown on c-axis and off-axis substrates of single-crystal aluminum nitride has been carried out. Growth on off-axis (>30°) substrates appears to result in rough surfaces and the absence of two-dimensional electron gas (2DEG). However, smooth morphologies were demonstrated for both homoepitaxial and heteroepitaxial growth on on-axis (<2°) substrates. On one of these oriented substrates a 2DEG, with a mobility of 1000 cm²/V s and a sheet density of 8.5×10¹² cm⁻² at room temperature, was also demonstrated for the first time.     

Photoassisted growth and nitrogen doping of ZnSe

ZnSe was grown on GaAs(100) substrates by photoassisted MOVPE using the precursors dimethylzinc-triethylamine (DMZn-TEN) and ditertiarybutylselenide (DtBSe). The optimal growth temperature was T_g = 300°C. Above 300°C no enhancement of growth rate was observed under illumination. Furthermore, nitrogen doping experiments were performed using phenylhydrazine, allylamine and tert-butylamine as nitrogen precursors. Nitrogen concentrations up to 10¹⁹ cm⁻³ (SIMS) were achieved with input flow ratios as low as [PhHz]/[Se] = 4 × 10⁻⁴. All as-grown samples were highly compensated. Successful nitrogen incorporation was also observed with allylamine. However, the use of tert-butylamine together with the adduct compound DMZn-TEN showed no incorporation of nitrogen.     

Se-doped AlGaAs grown on GaAs(111)A by molecular beam epitaxy

The electrical properties of Se-doped Al_0.3Ga_0.7As layers grown by molecular beam epitaxy (MBE) on GaAs(111)A substrates have been investigated by Hall-effect and deep level transient spectroscopy (DLTS) measurements. In Se-doped GaAs layers, the carrier concentration depends on the misorientation angle of the substrates; it decreases drastically on the exact (111)A surface due to the re-evaporation of Se atoms. By contrast, in Se-doped AlGaAs layers, the decrease is not observed even on exact oriented (111)A. This is caused by the suppression of the re-evaporation of Se atoms, by Se---Al bonds formed during the Se-doped AlGaAs growth. An AlGaAs/GaAs high electron mobility transistor (HEMT) structure has been grown. The Hall mobility of the sample on a (111)A 5° off substrate is 5.9×10⁴ cm²/V·s at 77 K. This result shows that using Se as the n-type dopant is effective in fabricating devices on GaAs(111)A.