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.     

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.     

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.     

Transmission electron microscopy observation of GaAs/Al0.3Ga0.7As T-shaped quantum well structure fabricated by glancing angle molecular beam epitaxy on GaAs(100) reverse-mesa etched substrates

GaAs/Al_0.3Ga_0.7As multi-layer structures were grown on GaAs (100) reverse-mesa etched substrates by glancing angle molecular beam epitaxy (GA-MBE). A(111)B facet was formed as a side-facet. Surface migration of Ga and Al atoms from the (100) flat region to the (111)B side-facet region has been investigated to fabricate T-shaped GaAs/AlGaAs quantum wells (QWs) under the condition that Ga and Al atoms impinge only an the (100) flat region and do not impinge on the (111)B side-facet. Observation of T-shaped GaAs/AlGaAs quantum wires (QWRs) by cross-sectional transmission electron microscopy (TEM) revealed that there is no migration of Al atoms from the (100) to the (111)B facet region at a substrate temperature (T_s) as high as 630°C, under a V/III ratio of 28 (in pressure ratio). On the other hand, very thin GaAs epitaxial layers grown on the (111)B side-facet region owing to the Ga migration were observed for substrate temperatures of 600 and 630°C. It was found that the mass flow of Ga atoms from the (100) region to the (111)B side-facet region increases, with the thermal activation energy of 2.0 eV, as the substrate temperature increases from 570 to 630°C. The GA-MBE growth on a reverse-mesa etched GaAs substrate at a low temperature 570°C or lower is desirable to fabricate a nm-scale GaAs/AlGaAs QWR structure with nm-scale precision.     

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.     

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.     

MOCVD growth of high-quality AlGaAs on Si substrates for high-efficiency solar cells

The crystallinity and solar cell efficiency of Al_0.22Ga_0.78As layers grown on Si substrates have been studied by varying the thermal cycle annealing (TCA) temperature. The optimum TCA temperature to obtain an Al_0.22Ga_0.78As layer with long minority carrier lifetime and high conversion efficiency has been presented. The active-area conversion efficiency of an Al_0.22Ga_0.78As solar cell on a Si substrate as high as 10.2% has been obtained under AM0 and 1 sun conditions.     

Strain relaxation in InGaAs/GaAs quantum wells grown on GaAs (111)A substrates

We have grown In_0.2Ga_0.8As strained quantum wells (SQWs) on GaAs (111)A just and off-angled substrates by molecular beam epitaxy (MBE). The photoluminescence (PL) peak energy of SQWs grown on (111)A related substrates shows a large redshift as compared with the calculated values. The red-shift observed in SQWs grown on a (111)A 5° off toward [001] substrate can be explained by the presence of a built-in electric field E = 154 kV/cm due to piezoelectric effect. The larger red-shift observed in samples grown on the other substrates is partially due to strain relaxation. A strain relaxation mechanism that consists of coherently grown islands when InGaAs growth begins and the generation of misfit dislocations when these islands coalesce, gives a qualitative explanation of the observed results.     

Heterointerface study of perturbed LPE growth of AlGaAs/InGaP single heterostructure

This paper presents the perturbed growth of Al_0.7Ga_0.3As/In_0.5Ga_0.5P single heterostructure on a GaAs substrate by liquid-phase epitaxy. The AlGaAs-InGaP heterointerface was characterized by scanning electron microscopy, photoluminescence, Auger electron spectroscopy, and transmission electron microscopy. Evidence is provided showing that a small amount of droplets, after the slider operation of the In_0.5Ga_0.5P epitaxial growth, mixed with the Ga-rich AlGaAs melt, is sufficient to attack the In_0.5Ga_0.5P underlying layer. Even with complete melt removal, there is still a partial dissolution at the “flat” Al_0.7Ga_0.3As-In_0.5Ga_0.5P heterojunction. The Auger depth profiles reveal the composition-depth transition width at this interface to be 560Å from the 90%-10% of Al (or Ga, As, and In) Auger profile; however, the P atoms penetrate deeply into the Al_0.7Ga_0.3As layer due to the partial dissolution of In_0.5Ga_0.5P layer. By high-resolution electron-micrograph analysis, some dislocations are observed at the heterojunction leading to nonradiative recombination and to poor optical device performance, even though the heterointerface observed by scanning electron microscopy is very flat.     

Highly uniform growth in a low-pressure MOVPE multiple wafer system

A novel horizontal metal organic vapor phase epitaxy (MOVPE) system, which is capable of handling six 3 inch wafers or eighteen 2 inch wafers mounted on a 10 inch diameter susceptor, has been developed for the growth of III–V compound semiconductors. The characteristic features in this system are “triple flow channel” gas injection and “face-down” wafer setting configuration. The inlet for the source gas flow is divided into three zones (upper, middle and lower flows for hydrides, organometals and hydrogen, respectively) to control the concentration boundary layer and the growth area. The wafers are placed inversely to prevent thermal convection and particles on the growing surface. The independent controlled three-part heating system is also adopted to achieve a uniform temperature distribution over an 8 inch growing surface. The thickness and the doping of GaAs, Al_0.3Ga_0.7As, In_0.48Ga_0.52P and In_0.2Ga_0.8As grown by this system are uniform within ± 2% over all 3 inch wafers.     

Relaxation mechanism of GaP/InGaAlP/GaAs heterostructure

Transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM) were carried out to investigate the structural properties of the GaP/In_0.48(Al_0.7Ga_0.3)_0.52P heterostructures grown on GaAs (0 0 1) substrates. The lattice-matched In_0.48(Al_0.7Ga_0.3)_0.52P/GaAs material system could be used as a defect-free substrate because no lattice misfit exists between the In(AlGa)P layer and the GaAs substrate. Both TEM and HREM measurements indicated that there were not only misfit dislocations, but also microtwins at the GaP/In(AlGa)P heterointerface. The mechanism of the microtwins formation is elucidated.     

Experimental evidence of difference in surface and bulk compositions of AlxGa1-xAs, AlxIn1-x As and GaxIn1-x As epitaxial layers grown by molecular beam epitaxy

The surface composition of Al_xGa_1-xAs, Al_xIn_1-xAs and Ga_xIn_1-x As epitaxial layers grown by moleculer beam epitaxy (MBE) has been determined in situ by X-ray photoelectron spectroscopy (XPS). The comparison of the values deduced from XPS with bulk compositions resulting from X-ray diffraction and RHEED oscillations leads to the conclusion that the most weakly bound group III element (i.e. Ga in Al_xGa_1-xAs, In in Al_xIn_1-xAs and Ga_xIn_1-xAs) segregates at the alloy surface. The key rôle of of the surface temperature is illustrated in the case of Al_xGa_1-xAs alloys for which a large range of growth temperatures are possible: a significant difference between surface and bulk compositions is observed between 620 and 680°C reaching a maximum near 650°C. This influence of the growth temperature is discussed in the framework of the previously suggested correlation between Ga segregation and the so-called forbidden temperature zone in MBE growth of Al_xGa_1-xAs.     

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.     

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.     

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.     

Structural and photoluminescence characteristics of molecular beam epitaxy-grown vertically aligned In0.33Ga0.67As/GaAs quantum dots

In this paper, we present the results of structural and photoluminescence (PL) studies on vertically aligned, 20-period In_0.33Ga_0.67As/GaAs quantum dot stacks, grown by molecular beam epitaxy (MBE). Two different In_0.33Ga_0.67As/GaAs quantum dot stacks were compared. In one case, the In_0.33Ga_0.67As layer thickness was chosen to be just above its transition thickness, and in the other case, the In_0.33Ga_0.67As layer thickness was chosen to be 30% larger than its transition thickness. The 2D–3D growth mode transition time was determined using reflection high-energy electron diffraction (RHEED). Structural studies were done on these samples using high-resolution X-ray diffraction (HRXRD) and cross-sectional transmission electron microscopy (XTEM). A careful analysis showed that the satellite peaks recorded in X-ray rocking curve show two types of periodicities in one sample. We attribute this additional periodicity to the presence of an aligned vertical stack of quantum dots. We also show that the additional periodicity is not significant in a sample in which the quantum dots are not prominently formed. By analyzing the X-ray rocking curve in conjunction with RHEED and PL, we have estimated the structural parameters of the quantum dot stack. These parameters agree well with those obtained from XTEM measurements.     

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.     

Self-formation of 100 nm scale wire structures during molecular beam epitaxial growth of AlGaAs on patterned substrates

A wire structure with 100 nm scale buried in AlGaAs is shown to be formed spontaneously during the molecular beam epitaxial (MBE) growth of AlGaAs on a pre-patterned substrate. Scanning electron microscope (SEM) and photoluminescence (PL) study revealed that a triangular-shaped wire region with Al content of 0.12 was embedded by Al_0.3Ga_0.7As with fairly sharp boundaries. The cross-sectional dimensions and the Al molar fraction of the wire are shown to be independent of the patterned mesa width on which the wire structure is grown.     

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.     

90° double reflection high-energy electron diffraction experiments on vicinal surfaces of GaAs

Reflection high-energy electron diffraction (RHEED) experiments recording the intensity of the specular beam in two azimuths have been carried out simultaneously. The critical temperature for the transition from 2D nucleation to step-flow growth was simultaneously estimated in the [ 10] and [110] directions by observing the disappearance of RHEED intensity oscillations on GaAs(001) substrates 2° misoriented towards GaAs(111)A. It was found that the oscillations disappear at a lower substrate temperature in the [110] azimuth. The difference in the transition temperatures was 25°C.