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.     

Effects of surface reconstruction on CdTe/GaAs(001) interface structure

CdTe/GaAs(001) heterostructures were fabricated by molecular beam epitaxy on chemically etched and thermally deoxidized GaAs(001) substrates, as well as GaAs(001) (3×1) buffer layers grown in situ by molecular beam epitaxy. Different growth protocols were also explored, leading to Te-induced (6×1) or (2×1) surface reconstructions during the early growth stage. High-resolution cross-sectional transmission electron microscopy was used to examine the final interface structure resulting from the different substrate preparations, and surface reconstructions. The (2×1) surface reconstruction led to pure (001) growth, while the (6×1) reconstruction led to an interface which included small (111)-oriented inclusions. In addition, deposition on etched and deoxidized GaAs(001) wafers led to preferential CdTe growth within etch pits and resulted in a macroscopically rough interface region.     

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.     

Surface reconstruction of sulfur-terminated GaAs(001) observed during annealing process by scanning tunneling microscopy

The surface reconstruction of in-situ prepared sulfur-terminated (S-terminated) and sulfur-protected (S-protected) GaAs(001) is studied by scanning tunneling microscopy (STM) at high temperatures of up to 260°C. We demonstrate a new reordering process from the (4×6) Ga-stabilized surface to the (2×6) S-stabilized surface and a novel method of air protection using a S passivation layer. Moreover, the in-situ observation of the annealing process of this S-protection layer is performed by high-temperature STM, avoiding the adsorption from environments and verifying that the (2×6) structure still becomes dominant on the S-terminated GaAs(001) surface without the effects of As atoms.     

The characterization of the growth of sub-monolayer coverages (1/200th to 1 monolayer) of Si and Be on GaAs(001): A reflectance anisotropy spectroscopy and reflection high-energy electron diffraction study

The techniques of reflectance anisotropy spectroscopy (RAS) and reflection high-energy electron diffraction (RHEED) have been employed, for the first time in concert, to characterize the growth of sub-monolayer coverages of both Si and Be deposited onto the GaAs(001)-c(4×4) surface. The RHEED observations enabled the RAS spectra collected for a series of Si and Be coverages, in the range 0.005 to 1.000 monolayer (ML), to be interpreted in terms of changes in the sample surface structure. For the case of Si/GaAs(001) the following series of surface reconstructions were observed with increasing Si coverage: c(4×4), c(4×4)/(1×2), (1×2), (1×2)/(3×1) and, (3×1). During the deposition of Be/GaAs(001), c(4×4), c(4×4)/(1×2), c(4×4)/(1×3), (1×2)/(1×3), (1×3), and (1×2) reconstructions were noted to evolve. The fact that unique, but highly reproducible, RAS signatures were obtained for each of these surface phases demonstrates the applicability of a combined RHEED/RAS system for monitoring sub-monolayer heteroepitaxial growth with a surface sensitivity of the order of 1/200th of a monolayer.     

A scanning tunnelling microscopy study of the deposition of Si on GaAs(001); Implications for Si δ-doping

Atomic resolution scanning tunnelling microscopy (STM) has been used to study the adsorption of Si on GaAs(001) surfaces, grown in situ by molecular beam epitaxy (MBE), with a view to understanding the incorporation of Si in δ-doped GaAs structures. Under the low-temperature deposition conditions chosen, the clean GaAs surface is characterized by a well-defined c(4×4) reflection high-energy electron diffraction (RHEED) pattern, a structure involving termination with two layers of As. Filled states STM images of this surface indicate that the basic structural unit, when complete, consists of rectangular blocks of six As atoms with the As-As bond in the surface layer aligned along the [110] direction. Deposition of <0.05 ML of Si at 400°C onto this surface shows significant disruption of the underlying structure. A series of dimer rows are formed on the surface which, with increasing coverage, form anisotropic "needle-like" islands which show no tendency to coalesce even at relatively high coverages (0.5 ML). The formation of these islands accompanies the splitting of the 1/2 order rods in the RHEED pattern along [110]. As the Si is known to occupy only Ga sites, the Si atoms displace the top layer As atoms of the c(4×4) structure, with the displaced As atoms forming dimers in a new top layer. The results are consistent with a recently proposed site exchange model and subsequent island formation for surfactant mediated epitaxial growth.     

Theoretical investigation of adsorption behavior during molecular beam epitaxy growth of GaAs: Ab initio based microscopic calculation

This paper investigates, theoretically, the migration potential and adsorption energies of Ga adatoms on a reconstructed As-rich GaAs (001)-(2×4) surface by ab initio calculation. The calculated results show that migration potential depends sensitively on the number of surface Ga adatoms and that the adsorption energy oscillates with the adsorption of every other atom. The dependence on the number of surface Ga atoms is explained by the effects of the surface distortion and the electron counting model. This paper also discusses As incorporation during epitaxial growth and demonstrates the dynamical behavior of Ga adatoms at finite temperature by Monte-Carlo simulation.     

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.     

Surface diffusion kinetics of GaAs and AlAs metalorganic vapor-phase epitaxy

We have investigated the surface kinetics during metalorganic vapor-phase epitaxy (MOVPE), using high-vacuum scanning tunneling microscopy (STM) observation of two-dimensional (2D) nuclei and denuded zones. Using Monte Carlo simulations based on the solid-on-solid model, from 2D nucleus densities we estimated the surface diffusion coefficients of GaAs and AlAs to be 2 × 10⁻⁶ and 1.5 × 10⁻⁷ cm²/s at 530°C, and the energy barriers for migration to be 0.62 and 0.8 eV, respectively. The 2D nucleus size in the [110] direction was about two times larger than that in the [ 10] direction. The size anisotropy is caused primarily by a difference in the lateral sticking probability (P_s) between steps along the [ 10] direction (A steps) and steps along the [110] direction (B steps). The P_s ratio was estimated to be more than 3:1. Denuded zone widths on upper terraces were 2 ± 0.5 times wider than those on lower terraces. This showed that P_s at descending steps was 10 to 3 × 10² times larger than P_s at ascending steps.     

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.     

Shallow donors and deep levels in GaAs grown by atomic layer molecular beam epitaxy

We correlate the Si concentration measured by secondary ion mass spectrometry (SIMS) and the net donor concentration in GaAs:Si grown by atomic layer molecular beam epitaxy (ALMBE); Si was supplied during: (a) both the As and the Ga subcycles, (b) the As subcycle, and (c) the Ga subcycle; the layers were grown at temperatures in the 300-530°C range. The results show that Si incorporation and its compensation depend on the Si-supply scheme and that the extent of compensation decreases with the growth temperature. We also study the deep levels in the ALMBE GaAs grown under the above conditions. Our results show the occurrence of M1, M3 and M4 levels with concentrations that are: (i) essentially independent of both the Si supply scheme and the ALMBE growth temperature, (ii) close to those of MBE GaAs grown at 600°C, and (iii) up to 2 orders of magnitude lower than that of GaAs prepared by molecular beam epitaxy (MBE) at similar temperatures.     

The behaviour of copper on gallium arsenide

Cu has been reported to diffuse rapidly in GaAs at low temperatures (2.3×10^-5 cm² s^-1 at 600°C). The rapid diffusion is attributed to the interstitial movement of Cu atoms. The present investigation was undertaken to examine preferential diffusion of Cu⁺ along dislocations and grain boundaries in GaAs. The experiments consisted of depositing ⁶⁴Cu on a GaAs water surface, annealing in vacuum, and observing the Cu distribution by autoradiography. From these observations no preferential diffusion along dislocations or grain boundaries was detected in SI GaAs annealed between 600 and 1000°C. In the sample annealed above 800°C, the deposited Cu reacted with the GaAs forming a liquid on the sample surface, which solidified into complex Cu-Ga-As compounds. The liquid also produced Cu-rich pipes which extended through the GaAs wafer.     

RHEED studies of the growth of Si(001) by gas source MBE from disilane

The growth of Si(001) from a gas source molecular beam epitaxy system (Si-GSMBE) using disilane (Si₂H₆) was investigated using reflection high-energy electron diffraction (RHEED). The surface reconstructions occurring between 100 and 775°C were studied as a function of both substrate temperature and surface coverage. We report the first observation of (2x2) and c(4x4) reconstructions during growth at substrate temperatures near 645°C using Si₂H₆. All growth was found to be initiated by the formation of three-dimensional (3D) islands which coalesced at substrate temperatures above 600°C. The surface reconstruction was found to change from a disordered to an ordered (2x1)+(1x2) structure at 775°C via intermediate (2x2) and c(4x4) phases. Thereafter, growth was found to proceed in a 2D layer-by-layer fashion, as evidenced by the observation of RHEED intensity oscillations. This technique has been used, for the first time, to calibrate growth rates during Si-GSMBE. The intensity oscillations were measured as a function of both substrate temperature and incident beam flux. Strong and damped oscillations were observed between 610 and 680°C, in the two-dimensional growth regime. At higher temperatures, growth by step propagation dominated while at lower temperatures, growth became increasingly three-dimensional and consequently oscillations were weak or absent. Similarly, there was a minimum flux limit ( <0.16 SCCM), below which no oscillations were recorded.     

Investigation of defects in low-temperature-grown GaAs using optical transient spectroscopy

Optical transient current spectroscopy (OTCS) has been used to investigate defects in the low-temperature-grown GaAs after postgrowth rapid thermal annealing (RTA). Two samples A and B were grown at 220°C and 360°C on (0 0 1) GaAs substrates, respectively. After growth, samples were subjected to 30 s RTA in the range of 500–800°C. Before annealing, X-ray diffraction measurements show that the concentrations of the excess arsenic for samples A and B are 2.5×10¹⁹ and 1×10¹⁹ cm⁻³, respectively. It is found that there are strong negative decay signals in the optical transient current (OTC) for the annealed sample A. Due to the influence of OTC strong negative decay signals, it is impossible to identify deep levels clearly from OTCS. For a comparison, three deep levels can be identified for sample B before annealing. They are two shallower deep levels and the so-called As_Ga antisite defect. At the annealing temperature of 600°C, there are still three deep levels. However, their structures are different from those in the as-grown sample. OTC strong negative decay signals are also observed for the annealed sample B. It is argued that OTC negative decay signals are related to arsenic clusters.     

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.     

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.     

AlAs-in-AlSb digital alloy superlattice morphology versus growth temperature

We report a systematic study of how growth temperature affects the quality of AlAs-in-AlSb digital alloy superlattices grown by molecular beam epitaxy for barrier layers in type-II W-structure infrared lasers. Using cross-sectional scanning tunneling microscopy to characterize the atomic-scale structure of the material, we find substantial differences in the superlattice morphology for growth temperatures between 435 and 540 °C. At lower growth temperatures, the AlAs forms three-dimensional clusters, with continuous structures threading through multiple periods of the superlattice. With increasing temperature, the morphology of the digitally doped AlAs layers consistently improves, with nearly perfect delta doping observed at the highest temperatures studied. The changes in the superlattice structure can be attributed primarily to the known temperature dependence of the AlSb growth front morphology, with secondary effects associated with anion-exchange at the interfaces and the different surface reconstructions on the two growth surfaces.     

Low temperature growth of AlGaAs by MOMBE (CBE) using trimethylamine alane

In order to gain further insight into the surface chemistry of AlGaAs growth by metalorganic molecular beam epitaxy, we have investigated the deposition behavior and material quality of AlGaAs grown at temperatures from 350 to 500°C using trimethylamine alane (TMAA), triethylgallium (TEG) and arsine (AsH₃). Though the Al incorporation rate decreases with decreasing temperature, Ga-alkyl pyrolysis, and hence Ga incorporation rate, declines more rapidly. Thus the Al content increases from X_AlAs = 0.25 at 500°C to X_AlAs = 0.57 at 350°C. Below 450°C, the Ga incorporation rate appears to be determined by the desorption of diethylgallium species, rather than interaction with adsorbed AlH₃. Similarly, carbon incorporation is enhanced by 2 orders of magnitude over this temperature range due to the increasingly inefficient pyrolysis of the Ga-C bond in TEG. Additionally, active hydrogen from the TMAA1, which normally is thought to getter the surface alkyls, is possibly less kinetically active at lower growth temperatures. Contrary to what has been observed in other growth methods, low growth temperatures produced a slight decrease in oxygen concentration. This effect is likely due to reduced interaction between Ga alkoxides (inherent in the TEG) and the atomic hydrogen blocked Al species on the growth surface. This reduction in oxygen content and increase in carbon concentration causes the room temperature PL intensity to actually increase as the temperature is reduced from 500 to 450°C. Surprisingly, the crystalline perfection as measured by ion channeling analysis is quite good, χ_min≤5%, even at growth temperatures as low as 400°C. At 350°C, the AlGaAs layers exhibit severe disorder. This disorder is indicative of insufficient Group III surface mobility, resulting in lattice site defects. The disorder also supports our conclusions of kinetically limited surface mobility of all active surface components.     

Arsenic-free GaAs substrate preparation and direct growth of GaAs/AlGaAs multiple quantum well without buffer layer

High-temperature treatment of GaAs substrate without As flux in a preparation chamber was investigated as a substrate surface cleaning method for molecular beam epitaxial (MBE) growth. Oxide gases such as CO and CO₂ were almost completely desorbed at a temperature above which Ga and As started to evaporate from the substrate. During the cleaning at a temperature as high as 575°C for 30 min, about 100 nm thick GaAs was evaporated from the substrate, but its surface maintained mirror-like smoothness and showed streak pattern with surface reconstruction pattern in the reflection high energy electron diffraction (RHEED) observation. Direct growth of GaAs/Al GaAs quantum well (QW) structures was tried on such surfaces without introducing any buffer layers. The QW structure showed photoluminescence with both intensity and full width at half maximum comparable with those for the QW grown on the substrate cleaned by the conventional method with introducing a GaAs buffer layer.     

The electrical, optical and crystalline properties of GaAs: C grown by GSMBE using TMG and AsH3 for application to HBTs

We report the results of a comprehensive study on the electrical, optical and crystalline properties of heavily carbon doped p-type (100) GaAs epilayers (p = 6.3 × 10¹⁸−1.3 × 10²⁰ cm^-3; THICKNESS = 250−420 nm) grown by gas source molecular beam epitaxy using trimethylgallium and arsine. X-ray analysis showed epilayer lattice contraction with a mismatch of δa/a = -1.8 × 10^-3 at p = 1.3 × 10²⁰ cm^-3. Room temperature photoluminescence peak energy shifted from 1.40 eV (p = 6.3 × 10¹⁸ cm^-3) to 1.37 eV (p = 1.3 × 10²⁰ cm^-3). Stokes Raman spectra showed two modes assigned as the unscreened LO phonon (292 cm^-1) and the low frequency branch of the coupled hole-plasmon-LO-phonon (266 cm^-1). Conservation of Raman scattering rules under all incident light configurations showed that the (100) GaAs:C epilayers were of high crystalline quality without the presence of faceting or other such crystalline defects. Annealing at 900°C for between 30 s to 45 min, resulted in a significant reduction in the hole concentration, lattice contraction and photoluminescence intensity for all epilayers. The implications of these results for the development of GaAs/AlGaAs HBTs are discussed.