Atomic Layer Epitaxy Of GaAs Research Articles

Molecular-beam epitaxy of GaAs/Si(001) structures for high-performance tandem A III B V/Si-solar energy converters on an active silicon substrate

In the present study, a method of low-temperature atomic layer epitaxy of GaAs at the initial stage of formation of a GaAs/Si heterojunction is used for growing GaAs films with a low density of threading defects. It was shown that growth of GaAs films can take place bypassing the stage of formation of islands, provided the first monolayer of GaAs is formed by atomic layer epitaxy at low temperature (200–350°C). A regime was found for growing the GaAs/Si films with a density of threading dislocations less than 106 cm–2, which corresponds to the best world achievements. In this mode, the GaAs/Si and Al0.2Ga0.8As/Si structures were grown for solar-energy converters, the devices were produced, and their characteristics were measured. It is shown that during the growth of the GaAs/Si heterojunction, a p–n-junction is formed in the near-surface layer of silicon. This allows one to produce high-performance cascade converters of solar energy based on the A III B V compounds on the active Si substrate in a single growth cycle.

Trisneopentylgallium as a precursor for atomic layer epitaxy of GaAs

We report the use of a new precursor, trisneopentylgallium (NPG) for the growth of GaAs by atomic layer epitaxy (ALE). In contrast to most other alkyl gallium precursors such as triethylgallium, which decompose via a β-hydride elimination mechanism, this compound undergoes homolysis similar to that of trimethylgallium (TMGa), the normal choice as an ALE precursor. Clear self-limiting growth behavior similar to that of TMGa was observed over a reasonably wide range of growth conditions (430–500°C). Carbon incorporation was not significantly reduced compared with TMGa suggesting that the adsorbed neopentyl radicals undergo decomposition to result in a methyl terminated surface identical to that obtained for growth with TMGa.

Effects of active hydrogen on atomic layer epitaxy of GaAs

Atomic layer epitaxy of GaAs employing thermally activated hydrogen molecules is reported. It is found that hydrogen molecules thermally activated in a high-temperature cell reduce the ALE temperature, while the activation energy of formation of the methyl–Ga surface from the As surface does not change. On the other hand, the activation energy of formation of the As surface from the methyl–Ga surface remarkably decreases in the active hydrogen flow, indicating that AsH 3 molecules dissociate into AsH x ( x=1 or 2) in the vapor phase.

In situ monitoring of the GaAs growth process in halogen transport atomic layer epitaxy

In situ monitoring of the growth process in atomic layer epitaxy (ALE) is essential to understand the growth mechanism. In this paper, our recent study of the ALE growth mechanism investigated by means of two in situ monitoring methods, a gravimetric method using a microbalance and an optical method using surface photo-absorption (SPA), is reported. The reaction mechanisms that proceed on the surface in each step of the gas sequence are discussed for GaAs ALE growth using a halogen transport system.

Atomic layer epitaxy of GaAs and GaAs xP 1 − x on nominally oriented GaAs(111) substrates with high quality surface and interfaces

Layer-by-layer growth of GaAs and GaAs x P 1 − x ( x = 0.7−0.8) on nominally oriented (111) substrates has been performed by atomic layer epitaxy (ALE). The ideal growth rate of one monolayer per cycle was obtained on a (111)B substrate, but not on a (111)A substrate. Mirror-like smooth surface morphologies can be obtained on both (111)A and (111)B substrates in a wide range of growth conditions. The X-ray rocking curve of the GaAs x P 1 − x GaAs multilayer sample grown on the (111)B substrate showing clear satellite patterns indicates abrupt interfaces that are superior to those of the (100) sample. It was also found that the disorder in growth such as the generation of hillocks can be reduced drastically by the ALE method.

Surface Chemistry for Atomic Layer Growth

Atomic layer controlled film growth is an important technological and scientific goal that is closely tied to many issues in surface chemistry. This article first reviews the basic concepts of atomic layer growth using molecular precursors and binary reaction sequence chemistry. Many examples are given for the various films that have been grown using this atomic layer growth technique. The paradigms for atomic layer epitaxy (ALE) and atomic layer processing (ALP) are then discussed in terms of self-limiting surface reactions. Recent investigations of the surface chemistry of SiO2 and Al2O3 ALP and GaAs ALE are examined and used to illustrate the possible mechanisms of atomic layer growth. Subsequently, the characteristics of film deposition using atomic layer growth techniques are explored using recent examples for Al2O3 ALP. The structure of the deposited films is also reviewed using results from previous Al2O3 deposition investigations. This article then concludes by discussing possible complications to...

Atomic layer epitaxy of GaAs from tertiarybutylarsine and triethylgallium

GaAs growth by atomic layer epitaxy (ALE) from tertiarybutylarsine (TBA) and triethylgallium (TEG) in a chemical beam epitaxy (CBE) system is reported. A stable 4×8 surface reconstruction has been observed after Ga deposition at low substrate temperatures in the absence of TBA flux. The TEG reaction rate at the sample surface has been found to be lower under ALE conditions than under CBE growth conditions. No decay in the reflection high-energy electron diffraction intensity was observed after the ALE growth of 700 monolayers of GaAs at 550 °C. Grown samples exhibit p-type doping in the range of 1015 cm−3.

In situ monitoring of the growth process in GaAs atomic layer epitaxy by gravimetric and optical methods

In situ monitoring of the growth process in atomic layer epitaxy (ALE) is essential to understand the growth mechanism. In this paper, our recent study of the ALE growth mechanism investigated by means of two in situ monitoring methods, a gravimetric method using a microbalance and an optical method using surface photo-absorption (SPA), is reviewed. In situ and real-time monitoring of the growth rate on a monolayer scale, in situ observation of the growth process during ALE optically, the determination of surface chemical species during ALE growth, and the reaction mechanism that proceeds on the substrate surface during ALE are discussed for GaAs ALE growth using the GaCl source.

Accounting for stoichiometry changes on compound semiconductor surfaces

We discuss how to self-consistently account for stoichiometry changes on surfaces with complex reconstructions, e.g. polar compound semiconductor surfaces. The key aspect of the methodology is that surface steps are allowed to act as a reservoir where atoms may be added or removed. The method is specifically applied to molecular beam epitaxy (MBE) and atomic layer epitaxy (ALE) results for GaAs(100) surfaces. The method easily demonstrates why only ∼ 12 monolayer (ML) of As is needed to convert from the Ga-rich surface to the (2 × 4) As-rich surface (as observed experimentally), even though the latter surface has an As coverage of 34 ML in the top layer. We also demonstrate how to convert from a more complex reconstruction, having two incomplete layers, to a simpler reconstruction having only one incomplete layer. The methodology also shows that ideal ALE of GaAs(100) cannot occur by cycling between the known adsorbate-free Ga-rich and As-rich surface reconstructions, because any such transition would not yield the observed 1 ML per cycle growth rate. We briefly discuss how adsorbates may stabilize ideally terminated (i.e. vacancy free) III–V surfaces. For example, methyl groups adsorbed on GaAs(100) exhibit a (1 × 2) LEED pattern, which is not seen for the clean GaAs(100) surface reconstructions. By using the electron counting model we interpret this structure as 12 ML CH3 adsorbed on a complete layer (1 ML) of dimerized Ga atoms. The ideal termination of the Ga-rich GaAs(100)-(1 × 2)-CH3 surface now allows for a plausible ALE mechanism which yields 1 ML deposition per cycle.

Role of a metalorganic As source in atomic layer epitaxy of GaAs and AlAs

Active roles of Column-V precursors in atomic layer epitaxy (ALE) of GaAs and AlAs are demonstrated using an amino-As, i.e. tris-dimethylamino-arsenic (TDMAAs). The temperature range for ALE was drastically modified by the selection of TDMAAs in place of arsine. The main factor for this modification was studied with temperature programmed desorption measurements on Ga and Al surfaces during ALE. It is shown that amine species transferred from As surfaces to Ga or Al surfaces during the alternate supplies of precursors and their desorption properties have profound effects on the ALE growth mechanism.

Kinetics and mechanism of atomic layer epitaxy of GaAs using trimethylgallium

The desorption time constant of carbon-related species from clean GaAs surfaces exposed to trimethylgallium (TMGa), measured by in situ Auger electron spectroscopy, is of the order of 100 s in the temperature range where atomic layer epitaxy of GaAs takes place. A set of rate equations based on the adsorbate-inhibition model using this desorption time constant is shown to reproduce atomic layer epitaxy growth results: the GaAs growth rate dependence on the TMGa supply duration and the growth rate of the double TMGa pulse experiments with purge time in between two TMGa pulses. The origin of the remaining discrepancies is discussed.

Carbon incorporation mechanism in atomic layer epitaxy of GaAs and AlGaAs

By an in-situ surface photo-absorption study of the ALE process and carbon atomic layer doping, an other than the so far proposed carbon incorporation mechanism via formation of a CH 2 bridge bond has been found, i.e. carbon incorporation via an exchange reaction between dissociated CH 3 radicals from TMG and As atoms of the surface onto which TMG is deposited. We also propose a method for reducing the carbon incorporation by thermal desorption of the CH 3 groups.

Surface stoichiometry and the role of adsorbates during GaAs atomic layer epitaxy

Several questions regarding the stoichiometry of atomic layer epitaxy (ALE) arise because most polar compound semiconductor surfaces reconstruct into structures terminated with less than monolayer coverage at the outermost layer. In this paper we briefly discuss how to self-consistently account for stoichiometry changes on surfaces that are not ideally terminated. The key aspect of the methodology is that surface steps are allowed to act as a reservoir where atoms may be added or removed. The methodology shows that ideal ALE of GaAs(100) cannot occur by cycling between the known adsorbate-free Ga-rich and As-rich surface reconstructions, because no such transition would yield the observed 1 monolayer (ML) per cycle growth rate. In fact, ideal ALE (1 ML/cycle) must involve at least one adsorbate induced surface reconstruction. Adsorbates may stabilize ideally terminated (i.e. vacancy free) III–V surfaces because of their ability to passivate dangling bond states. For example, methyl groups adsorbed on GaAs(100) exhibit a (1 x 2) LEED pattern, which is not seen for the clean GaAs(100) surface reconstructions. By using the electron counting model we interpret this structure as 1 2 ML CH 3 adsorbed on a complete layer (1 ML) of dimerized Ga atoms. The GaAs(100)-(1 x 2)-Ch 3 surface was also examined using surface infrared spectroscopy (SIRS) using a multiple internal reflection geometry. This surface exhibits relatively sharp infrared linewidths suggestive of a well ordered structure. The polarization dependence of the symmetric stretching and bending CH 3 modes also supports our proposed structure of the GaAs(100)-(1 x 2)-Ch 3 surface. The ideal termination of the Ga-rich GaAs(100)-(1 x 2)-CH 3 surface for a plausible ALE mechanism which yields 1 ML deposition per cycle.

Self-limiting and step-propagating nature of GaAs atomic layer epitaxy revealed by atomic force microscopy

The self-limiting and step-propagating nature of GaAs atomic layer epitaxy (ALE) is verified by using atomic force microscopy (AFM). AFM measurements reveal that the GaAs surface having a monolayer-height step structure does not change regardless of the trimethylgallium (TMG) flow rate at the ALE growth condition. This indeed confirms the self-limiting nature of ALE. Moreover, it is found that lateral growths such as step-propagation and two-dimensional nucleation take place in ALE. The TMG migration lenght of Group-III species is evaluated to be as large as several hundred nanometers in spite of the low temperature growth at 440°C. The undecomposed TMG on As-stabilized surface is responsible for the large migration length in ALE.

Surface processes of selective growth by atomic layer epitaxy

Characteristics and mechanisms, of the crystallographic selective growth by atomic layer epitaxy (ALE) and the localized ALE growth over the nanometer scale area, are discussed focusing on the surface process. The experimental results indicate that higher growth temperature, and longer hydrogen purge time after AsH 3 supply, promote the crystallographic selectivity of GaAs ALE growth. The characteristics of the ALE selective growth include high crystallographic selectivity, and high uniformity in the nanometer scale. The mechanisms of ALE selective growth seem to be caused by the separate enhancement of As desorption process and the self-limiting mechanism of Ga precursor adsorption process on the GaAs surface.

Adjusting trimethylgallium injection time to explore atomic layer epitaxy of GaAs between 425 and 500°C by organometallic vapor phase epitaxy

In experiments employing a conventional low-pressure, rotating-disk organome-tallic vapor phase epitaxy reactor, GaAs epilayers have been grown at substrate temperatures ranging from 425 to 500°C by exposing the substrate alternately to trimethylgallium (TMG) and AsH3.The GaAs growth rateR was approximately constant with TMG flow rate, but with increasing TMG injection timet, it increased to more than one monolayer per TMG/AsH3 cycle without saturating. Although growth was not self-limiting, for one specific combination of temperature andt, a value of R = 1 monolayer/cycle could be achieved by usingt values decreasing from 10.8 s at 425°C to 0.9 s at 500°C in accordance with an Arrhenius relationship between 1/t and absolute temperature.

Crystallographic selective growth of GaAs by atomic layer epitaxy

We found high crystallographic selectivity in atomic layer epitaxy (ALE) growth of GaAs, in comparison with other epitaxial growth methods. In the temperature dependence of the GaAs growth rate, no GaAs growth on the GaAs (111)A and (110) planes was observed in the high temperature range under the condition of GaAs ALE growth on GaAs (100) plane. Also we discussed the mechanism of GaAs growth selectivity, which was believed to be caused by growth limitation due to As desorption. Due to the self-limiting effect and the high selectivity of ALE growth, trapezoidal-shaped GaAs/AlGaAs quantum wire structures, with 20 nm thickness and 50 nm width, were successfully realized.

Real-time study of the reflection high energy electron diffraction specular beam intensity during atomic layer epitaxy of GaAs

The intensity behavior of the specular beam in reflection high energy electron diffraction (RHEED) from GaAs (001) is investigated during the exposure of trimethylgallium (TMGa) and tertiarybutylarsine (TBAs) in atomic layer epitaxy (ALE) of GaAs. The temporal behavior of RHEED specular beam intensity corresponding to the transient behavior of the reflectance difference (RD) at 632.8 nm reveals several phases of surface reactions in ALE using TMGa and TBAs. RHEED specular beam intensity relaxation after short exposure to TMGa shows a longer time constant than that observed by RD, suggesting that it is the result of the overall changes in the surface atomic arrangements and morphology whereas the latter is responding to the formation/annihilation of Ga dimers.

Atomic layer epitaxy of GaAs with a 2 μm/h growth rate

Atomic layer epitaxy (ALE) of GaAs with a growth rate as high as 2 μm/h was achieved in a specially designed reactor based on the rotating susceptor concept. High deposition rates are made possible by a unique partition system that permits rapid rotation of substrates between simultaneous streams of columns III and V reactant gases. Mixing of the gas streams at high rotational speeds is avoided by the gas shearing effect provided by the partition system. Background carbon levels in the GaAs films with high growth rates varied from mid 1017 cm−3 at 650 °C to mid 1018 cm−3 at 550 °C. When the growth rate was reduced to 0.3 μm/h, carbon background doping in the 1015 cm−3 was achieved.

Study of surface reactions in atomic layer epitaxy of GaAs using trimethylgallium by reflectance difference spectroscopy and mass spectroscopy

An in situ real-time study of surface reactions in atomic layer epitaxy (ALE) of GaAs using trimethylgallium (TMGa) by reflectance difference spectroscopy (RDS) and sampled beam mass spectroscopy is reported. These studies reveal several phases in atomic layer epitaxy of GaAs and the self-limiting atomic layer deposition of epilayers using TMGa. The results also lend further support to a proposed reaction kinetics model from a recent RD investigation. The self-limiting mechanism which occurs during TMGa exposure cycles is believed to result from both selective adsorption and reaction of TMGa at As atoms and Ga-vacancy-induced Ga-rich surface reconstructions. It is also shown that strategies for optimal ALE growth of GaAs can be obtained through RD monitoring.