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.     

InAlAs/InGaAs/InP sub-micron HEMTs grown by CBE

The InAlAs/InGaAs/InP high electron mobility transistor (HEMT) lattice matched to InP offers excellent high frequency, low noise operation for MMICs and low-noise amplifiers. The InP channel in the InP/InAlAs HEMT offers the advantages of improved high field velocity and higher breakdown voltages (the potential for higher power applications) over InGaAs channel HEMTs. InAlAs has been grown for the first time by CBE using TMAA producing InGaAs/InAlAs and InP/InAlAs HEMTs. Sub-micron InGaAs/InAlAs HEMTs with planar Si doping have been fabricated with f_t values of 150 GHz and f_max values of 160 GHz. This device showed excellent pinch-off charateristics, with a maximum transconductance of 890 mS/mm. The planar doped InGaAs channel HEMT had a higher f_t than a similar uniformly doped device. However, the non-optimized structure of the planar doped device resulted in a large output conductance of 120 mS/mm, limiting f_max for that device. A sub-micron InP channel device was grown with a quantum well channel and double-sided planar Si doping. A sheet charge density of 4.4×10¹² cm^-2 and associated room temperature mobility of 2800 cm²/V·s were achieved; however, the saturation current was low. The most likely causes for this are diffusion of the planar doping beneath the channel and the poor quality of the InP on InAlAs interface at the bottom of the quantum well channel.     

Precise thickness measurement within a few monolayers by X-ray diffraction from InGaAs/GaAs strained-layer superlattices

We propose a technique to measure the thickness of a GaAs layer with a precision of a few monolayers (MLs) by high-resolution X-ray diffraction (HRXRD) from InGaAs/GaAs strained-layer superlattices (SLSs) on GaAs substrates. Using this technique to monitor the GaAs growth rate, we have successfully controlled the Ga beam flux within ±1% in molecular beam epitaxy (MBE) growth for continuous 40 runs during four days by increasing the Ga cell temperature to compensate the decrease of the Ga beam flux caused by the consumption of the Ga source. Precise thickness measurements are also demonstrated in the growth on InP substrates by using InAlGaAs/InGaAs SLSs and InAlGaAs/InAlAs SLSs.     

The influence of the growth temperature and interruption time on the crystal quality of InGaAs/GaAs QW structures grown by MBE and MOCVD methods

The impact of two technological parameters, i.e., the growth temperature and the interface growth interruption, on the crystal quality of strained InGaAs/GaAs quantum well (QW) structures was studied. The investigated heterostructures were grown by molecular beam epitaxy (MBE) and metalorganic chemical vapour deposition (MOCVD) under As-rich conditions. Photoluminescence (PL), reflection high-energy electron diffraction (RHEED) and atomic force microscopy (AFM) were adopted for the evaluation of specified interfaces smoothness and the quality of layers. Comparison between both epitaxial techniques allowed us to find, that the growth temperature plays more significant role in the case of structures grown by MBE technique, whereas the quality of MOCVD grown structures is more sensitive to the growth interruption. Optimum values of the investigated parameters of QW crystallization were obtained for both growth techniques.     

Growth of carbon-doped base GaAs/AlGaAs HBT by gas-source MBE using TEG, TEA, TMG, AsH3, and Si2H6

High-performance carbon-doped-base GaAs/AlGaAs heterobipolar transistors (HBTs) were grown by gas-source MBE using only gaseous sources including dopant sources. The AlGaAs emitter layer was doped with Si from uncracked SI₂H₆ (n = 9 × 10¹⁷ cm^-3), and the base layer (92.5 nm) was doped with carbon from TMG (p = 4 × 10¹⁹ cm^-3). From SIMS analysis it was confirmed that a well-defined emitter-base junction with sharp carbon profile was obtained. The base-current ideality factor from the Gummel plot was 1.47, and the emitter-base junction ideality factor was 1.12. A high DC current gain of 53 was obtained at a current density of 4 × 10⁴ A/cm². The device characteristics of our carbon-doped HBTs were found to be stable under current stress.     

Growth of heavily C-doped GaAs/AlGaAs MQW structures by MOVPE for 2–3 μm normal incidence photodetectors

The growth and intersubband optical properties of high quality heavily doped p-type GaAs/AlGaAs multiple quantum well (MQW) structures are reported. The MQWs were fabricated by the atmospheric pressure metalorganic vapor phase epitaxy process using liquid CCl₄ to dope the wells with C acceptors (N_a ≈ 2 × 10¹⁹ cm⁻³). A constant growth temperature was maintained for the entire structure while different V/III ratios were used for the well and barrier regions. By this process it is possible to achieve both high C doping densities in the wells and to simultaneously obtain good quality AlGaAs barriers. Fourier transform infrared spectroscopy measurements on heavily doped 10-period MQW structures reveal a new absorption peak at 2 μm with an effective normal incidence absorption coefficient of 4000 cm⁻¹. Photocurrent measurements on mesa-shaped diodes show a corresponding peak at 2.1 μm. The photodiodes exhibit a symmetrical current-voltage characteristic and a low dark current, which are indicative of a high quality MQW structure and a well-controlled C doping profile. The 2 μm absorption represents the shortest wavelength ever reported for any GaAs/AlGaAs or InGaAs/AlGaAs MQW structure and should be very useful for implementing multicolor infrared photodetectors.     

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.     

InGaAs/InAlAs in-plane superlattices grown on slightly misoriented (110) InP substrates by molecular beam epitaxy

InGaAs/InAlAs in-plane superlattices (IPSLs) composed of InAs/GaAs and InAs/AlAs monolayer superlattices were grown using molecular beam epitaxy. The substrates were misoriented (110) InP tilting 3° toward the [00 ] direction. We grew half monolayers of AlAs and GaAs and single monolayers of InAs alternately, keeping regular arrays of single monolayer steps. The structures were evaluated by transmission electron microscopy (TEM). In a transmission electron diffraction pattern from the ( 10) cross-section, we observed two types of superstructure spot pairs double-positioned in the [001] direction, indicating the formation of the intended IPSL structures. In a cross-sectional TEM dark-field image, we observed the InGaAs/InAlAs superlattice structures formed almost in the [001] direction. The mean period of the superlattices was approximately 4 nm, which was comparable to the terrace width expected from the substrate tilt angle. However, IPSL structures were not completely formed, i.e., the lateral interfaces meandered along the growth direction, and partial disorderings were often observed. The photoluminescence spectrum from the IPSL had a peak corresponding to the InGaAs (2 nm thick)/InAlAs (2 nm thick) superlattice in addition to a peak corresponding to the In_0.5Al_0.25Ga_0.25As alloy.     

Indium surface segregation in InGaAs-based structures prepared by molecular beam epitaxy and atomic layer molecular beam epitaxy

We present a comparative study on In surface segregation in InGaAs/GaAs structures prepared by molecular beam epitaxy (MBE) and atomic layer MBE (ALMBE) at different growth temperatures. The effect of segregation is evaluated by the energy position of exciton transitions in pseudomorphic 10 ML thick In_xGa_1−xAs/GaAs (0.15≤x≤0.30) and in 1 ML thick InAs/GaAs quantum wells. We show that: (i) In segregation decreases with the growth temperatures and is minimized at ALMBE and MBE growth temperatures lower than 260 and 340°C, respectively, and (ii) the segregation is more effective in ALMBE structures than in the MBE counterparts. The growth conditions that have been singled out allow the preparation of structures with high photoluminescence efficiencies even at the low growth temperatures required to minimize In segregation.     

Influence of initial growth parameters on the structural and optical properties of GaAs on (001) Si

The structural and optical properties of GaAs on (001) Si substrates were investigated by transmission electron microscopy (TEM) and low-temperature photoluminescence (PL). It was found that the success of the two-step growth technique is controlled by the quality (morphology and defect density) of the low-temperature grown AlGaAs nucleation layer. GaAs epilayers grown on low V/III ratio AlGaAs nucleation layers exhibit improved surface morphologies and structural properties. These results were confirmed by optical measurements where it was shown that the best PL response was obtained from GaAs epilayers in which the initial AlGaAs nucleation layers were deposited at a low V/III ratio.     

Atomic Processes during Crystal Growth Studied by Reflection High-Energy Electron Diffraction

After a short retrospect on the development of the electron diffraction techniques it is shown that the atomic-scale morphology of the crystal surface and growth processes on it can be studied in detail during molecular beam epitaxy (MBE) by reflection high-energy electron diffraction (RHEED). This is demonstrated for the evolution of the terrace-step-structure of the singular GaAs (001) surface during growth and after growth interruption and for the attachment of Si atoms at misorientation steps on vicinal GaAs (001) surfaces.     

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.     

The influence of the growth rate and V/III ratio on the crystal quality of InGaAs/GaAs QW structures grown by MBE and MOCVD methods

The influence of the growth rate and V/III ratio on the crystal quality of In_0.2GaAs/GaAs quantum well structures was examined. The investigated heterostructures were grown by molecular beam epitaxy (MBE) and metalorganic chemical vapour deposition (MOCVD). Reflection high energy electron diffraction (RHEED), photoluminescence measurements (PL), high-resolution X-ray diffraction (HRXRD) and atomic force microscopy (AFM) were applied for evaluation of the interfaces smoothness and the overall layer quality. Comprehensive characterisation of InGaAs/GaAs structures allowed us to establish optimal values of analysed technological parameters. Moreover, the comparison between the results obtained for samples grown by two different epitaxial techniques allowed us to find, which of the analysed growth parameters has the strongest influence on the quality of MBE and MOCVD grown structures. In contrast with the growth temperature and the interruption time, which in different manner impact on the crystal quality of QWs obtained by different method, the growth rate and the V/III ratio play similar role in both epitaxial techniques.     

Tilt investigation of In(Al,Ga)As metamorphic buffer layers on GaAs (001) substrate: A novel technique for tilt determination

Crystallographic tilt and Surface topography of InGaAs and InAlAs based metamorphic buffer structures on GaAs (001) substrate grown by molecular beam epitaxy (MBE) under varying growth conditions have been investigated. Compressively strained metamorphic buffer layers show anisotropic strain relaxation. A novel tilt determination technique based on X‐ray diffraction has been developed which can separate the effect of anisotropic strain. Tilt has been found to depend on compositional grading scheme, growth temperature and surface irregularities. Samples having random surfaces show smaller tilt than that of samples showing regular cross‐hatch. At higher growth temperature, reduction of tilt has been observed and correlated with thermal activation of otherwise inactive slip systems at low temperature. At low temperature and also for continuously graded samples, reduction of tilt has been observed and correlated with the slower relaxation that provide the opportunity for all the slip systems to participate and compete.     

Silicon compensation and scattering mechanisms in two-dimensional electron gases on (110)GaAs

A study of the MBE growth of (001) and (110) (Al,Ga)As is reported, and the efficiency of Si as an n-type dopant in (110)GaAs is accessed. A 40 nm spacer two-dimensional electron gas (2DEG) structure grown on (110)GaAs gives a mobility of 540,000 cm² V⁻¹ s⁻¹ at 4 K after illumination. The dominant scattering mechanisms in 2DEGs on (110) and (001)GaAs grown under the separate optimum growth conditions for the two orientations are compared.     

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.     

p-Type GaAs doped by diiodomethane (CI2H2) in molecular beam epitaxy, metalorganic molecular beam epitaxy, and chemical beam epitaxy

Highly p-type carbon-doped GaAs epitaxial layers were obtained using diiodomethane (CI₂H₂) as a carbon source. In the low 10¹⁹ cm⁻³ range, almost all carbon atoms are electrically activated and at 9×10¹⁹ cm⁻³, 91% are activated. The carbon incorporation efficiency in GaAs layers grown by metalorganic molecular beam epitaxy (MBE) and chemical beam epitaxy (CBE) is lower than that by MBE due to the site-blocking effect of the triethylgallium molecules. In addition, in CBE of GaAs using tris-dimethylaminoarsenic (TDMAAs), the carbon incorporation is further reduced, but it can be increased by cracking TDMAAs. Annealing studies indicate no hydrogenation effect.     

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.     

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.     

Fabrication of InGaAs ridge quantum wires by selective molecular beam epitaxy and their characterization

InGaAs/InAlAs ridge quantum wires were successfully fabricated by selective molecular beam epitaxy (MBE) for the first time. Prior to wire fabrication, detailed data on selective growth characteristics were taken by using test structures. Then, triangular shaped InGaAs ridge quantum wires with a width of 300 å were fabricated, using the selectivity data. Photoluminescence (PL) measurements detected a strong and narrow peak from the wires which showed a blue shift of 159 meV with respect to the InGaAs band-gap. This value agrees excellently with the calculation.