Ga-stabilized Surface Research Articles

In situ optical characterization of GaAs surfaces under alternating supply of GaCl and AsH3

This letter describes the use of surface photoabsorption (SPA) measurements to characterize GaAs substrate surfaces under an alternating supply of GaCl and AsH3 in chloride atomic layer epitaxy (ALE). This characterization technique utilizes p-polarized light incident at the Brewster angle, which virtually eliminates the bulk contribution of the reflected light. It was found that the reflection intensity varied by several percent depending on the source gas supply sequence. This reflection intensity was constant during continuous GaCl supply, which corresponded to the self-limiting mechanism of chloride ALE. Optical reflection spectra were measured in the range of 300–800 nm during the flow of each source gas. The normalized spectra of reflection difference between GaCl and AsH3 supply depended on the incidence azimuth, which showed the existence of anisotropic surface bonds. By comparing these spectra to the reference data using triethylgallium as a Ga source, the GaAs surface under GaCl exposure was assumed to be a Ga-stabilized surface at 520 °C.

The adsorption and decomposition of organometallics on GaAs(001) surfaces studied with high-resolution electron energy-loss spectroscopy

Ga(CH3)3 (TMGa), Ga(C2H53 (TEGa) and Ga(C2H5)2Cl (DE-GaCl) are used in combination with arsine (AsH3) in atomic layer epitaxy (ALE) of GaAs. We stimulated this process in a UHV system by dosing a c(2×8) or (1×6) reconstructed GaAs(001) surface with AsH3 and organometallics at different temperatures. HREEL spectra were taken after annealing the sample to successively higher temperatures.Vibrations of the adsorbate and its fragments induce changes of the dipole moment at the surface. The orientation of the dipole moment is different for molecularly and dissociatively adsorbed organometallics. By considering the change of the dipole moment perpendicular to the surface we determined how the adsorbates and their fragments are adsorbed on the surface. By following the relative intensities of the energy-loss peaks of the corresponding vibrations through an annealing series, we concluded how the pyrolysis proceeds.We found that most of the TMGa and TEGa adsorbs molecularly on an As-rich cooled surface but a large fraction decomposes upon adsorption on a Ga-stabilized surface. Complete desorption occurs at a lower temperature on the (1×6) compared to the c(2×8) surface.For DEGaCl our spectra indicate that it is adsorbed dissociatively on both surfaces.

Formation of ‘‘super’’ As-rich GaAs(100) surfaces by high temperature exposure to arsine

We report that arsine exposures between 100 and 350 °C will produce ‘‘super’’ As-rich surfaces [arsenic coverages of up to ∼1.7 monolayers (ML, where 1 ML=6.26×1014 atoms cm−2)] of GaAs(100) that exhibit a c(4×4) low energy electron diffraction pattern. Temperature programmed desorption studies show that after AsD3 exposures of up to 2.6×106 L (1 L=1×10−6 Torr s) to the Ga-stabilized surface, three excess As desorption speaks are observed with maxima at 440, 480, and 570 °C. As4 desorption is detected from the lowest temperature state, while the other states desorb primarily as As2. The significance of these results for the understanding of the atomic layer epitaxy process is addressed.

Reflectance-difference probing of surface kinetics of (001) GaAs during vacuum chemical epitaxy

A reflectance-difference (RD) study of the kinetics of various surface processes involved in the growth of GaAs from triethylgallium and arsine is presented. During triethylgallium exposure of an As-stabilized (001) GaAs surface, an initial linear RD response is observed, similar to what has previously been reported for trimethylgallium. We show that in the case of triethylgallium an over-saturation of the surface occurs, which results in complex transients in the RD response and, for a critical dose, in a loss of the surface coherency as determined by the disappearance of RD growth oscillations. It is found that the arsine reactivity on the over-saturated surface is much higher that that on the Ga-stabilized surface. The arsine-induced RD transient is compared with the kinetics of the re-establishment of a perfect As-stabilized surface, using the amplitude of RD-detected growth oscillations as a probe of the status of the surface.

Hreels Study of the Adsorption of Organometallics on GaAs(001) Surfaces

ABSTRACTTrimethylgallium (TMGa), Triethylgallium (TEGa) and Diethylgalliumchloride (DEGaCl) are used in combination with arsine (AsH3) in atomic layer epitaxy of GaAs. We simulated this process in a UHV system by dosing a c(2×8) or (1×6) reconstructed GaAs(001) surface with arsine and organometallics at different temperatures. After the surface reconstruction was verfied by LEED, the sample was transferred in situ to the HREELS (high resolution electron energy loss spectroscopy) chamber. We present new studies on the adsorption and pyrolysis of organometallics on GaAs(001) surfaces.As a result we find that TMGa adsorbs molecularly on an As-rich cooled surface but decomposes upon adsorption on a Ga-stabilized (1×6) surface. These two adsorption states are identified by the geometrical position of the Gamethyl compound on the surface (Ga-C bond parallel or normal to the surface). The removal of the adsorbed species occurs on an As-rich surface at temperatures higher than 450°C and on the Ga-stabilized surface at 350°C.For TEGa and DEGaCl we found similar results.

Mass spectrometric study of the reaction of trimethylgallium and GaAs

The decomposition reaction of trimethylgallium (TMG) with a GaAs (100) surface under ultra-high vacuum was studied by mass-spectrometry and reflection high-energy electron diffraction (RHEED). The reaction depends on the substrate temperature: below 300°C, the reaction does not occur effectively; between 400 and 500°C, one monolayer adsorption of Ga-containing species is suggested. Above 500°C, the reaction occurs in two stages. The reaction rate of TMG on As-stabilized surface is faster than that on Ga-stabilized surface. The deposition of multilayers of Ga is also indicated above 500°C.

Studies of TMGa adsorption on thin GaAs and InAs (001) layers

Adsorption of TMGa on thin overlayers of GaAs/AlAs and InAs/InP structures is investigated. The number of Ga monolayers is determined precisely from such structures for various TMGa exposures. Slower reaction rates for TMGa on Ga-stabilized surfaces compared to As-stabilized surfaces result in the self limiting mechanism of atomic layer epitaxy (ALE) using TMGa. On the other hand, adsorption of TMGa on InAs (001) surfaces involves more than the very top layer atoms. Exchange of In/Ga is observed with In atoms floating on top of the surface, which enhances Ga atom formation and impedes saturation as in the case of GaAs surfaces.

Methyl exchange reaction of trimethylindium on GaAs(100) and the preferential etching of gallium

We have observed a heterogeneous methyl exchange reaction between trimethylindium (TMIn) and Ga atoms on GaAs(100) which could be important in controlling stoichiometry in III–V compound semiconductor films containing In and Ga, prepared from group-III-alkyl sources. Adsorbed layers were prepared by dosing clean GaAs (either Ga- or As-stabilized) with TMIn near room temperature, to saturated coverage (1.7 × 10 14 In atoms/cm 2). Total carbon and indium coverages were monitored by X-ray photoelectron spectroscopy (XPS). Samples were heated to ~ 500° C at a rate of 1.6° /s and desorption products were detected by a differentially pumped quadrupole mass spectrometer. TMIn decomposes to form a Ga-alkyl product (either Ga(CH 3) 2, Ga(CH 3) 3, or a mixture) which desorbs between 70 and 350 ° C, and CH 3 which is detected between 220 and 450 ° C. The Ga-alkyl yield was higher for Ga-stabilized surfaces, while the yield of CH 3 was greater for As-rich surfaces. On both Ga- and As-rich surfaces, the yields of Ga(CH 3) x and CH 3 are, within experimental error, the same as those measured for trimethylgallium decomposition. The preferential “etching” of Ga by TMIn is a likely explanation for the observed reduction in the Ga incorporation probability in InGaAs films prepared from metal-organic molecular beam epitaxy. It could also explain the In-rich surfaces left after CH 4/H 2 plasma etching of InGaAsP.

RHEED and XPS observations of trimethylgallium adsorption on GaAs (001) surfaces—Relevance to atomic layer epitaxy

A series of experiments under UHV conditions have been performed to determine the surface reacted species and surface structure that results from trimethylgallium (TMGa) adsorption on a GaAs (001) As-stabilized surface. In these experiments, the conditions were chosen to simulate typical ALE growth conditions. Thus, the substrate temperature was varied between 320 and 530° and an admittance of 10−7 to 5 × 10−6 Torr of TMGa with exposure time of 5 to 15 sec were applied. X-ray photoelectron spectroscopy (XPS) was used to identify the chemical species on the surface after TMGa adsorption. The XPS intensity associated with the Ga 2p 3/2 level was used to monitor the quantity of adsorbed Ga and RHEED was used to monitor the surface structure. Below 440°, the Ga intensity was saturated at a level close to 1 ML and no definite Ga-stabilized 4 ×X RHEED pattern was observed. At 320° and an exposure of 200 L, a 2 × 4 As-stabilized RHEED pattern still existed, which suggests that the reaction between impinging TMGa and the (001) GaAs surface is very slow at this temperature. When the substrate temperature was between 440 and 530° exposure to greater than 6 L of TMGa resulted in saturation of surface Ga atoms to one monolayer (ML) and a successive change of surface reconstruction from 2×4 As-stabilized to 4 ×X (X = 1 or 2) Ga-stabilized surface. In all runs no carbon related species were observed within the XPS detection limit. This observation suggests that adsorption and decomposition of TMGa on As sites goes to completion very rapidly in this temperature range. From these observations we conclude that the self limiting mechanism in ALE occurs because of the differential chemisorption and decomposition rates of TMGa on As and Ga sites and that the dominant surface adsorbate is atomic Ga.

Surface structures of the (Al,Ga)Sb system

We have studied the surface structures of GaSb, AlGaSb, and alternating GaSb/AlSb layers using 10-kV reflection electron diffraction. Above 540 °C, the Sb-stabilized surface (1×3) patterns change to c(8×2), a Ga-stabilized surface. Because the c(8×2) surface has been observed on all other III–V arsenides, phoshpides, and antimonides, it is now clear that the c(8×2) metal-stabilized surface is common to all III–V compounds, suggesting that bond pairing occurs on all III–V semiconductor surfaces and is a universal reconstruction mechanism. The smooth, sharp transitions observed in the growth of alternating GaSb and AlSb layers show that atomically smooth interfaces can be formed in this system. In contrast, for the AlAs overgrowth on GaAs, transient structures associated with a Ga surface layer can be observed.

In Situ Diagnostics Of Epitaxial Growth Using Reflectance-Difference

ABSTRACTOptical in situ methods are becoming of increasing importance for studies of the mechanisms of growth and for real-time control of surface processes involved in epitaxial growth. We present data from studies, using the reflectance-difference (RD) method, of different processes related to epitaxial growth of GaAs (001) from arsine (AsH3) and from triethylgallium (TEG). RD studies make it possible to follow how the near-equilibrium, Ga-rich, configuration of the GaAs (001) surface changes with temperature and we show how the transformation to this state can be followed from As-saturated as well as from Ga-saturated surfaces. From the initial slope of the RD-transient, corresponding to the AsH3 reaction with a Ga-stabilized surface, the temperature dependence of the catalytic effect of this surface on the decomposition of arsine is measured. Of special significance for an atomic layer epitaxial process (ALE) is the demonstration of a self-limiting reaction of TEG. This process can be followed as the surface modifies itself from a state obtained by exposure to excess TEG to the near-equilibrium Ga-terminated reconstruction. Finally, we demonstrate how pulsing sequences for the realization of ALE can be followed and controlled in real time using RD as a diagnostic tool.

In-situ observation of roughening process of MBE GaAs surface by scanning reflection electron microscopy

The surface roughening process of MBE-GaAs induced by excess Ga has been investigated by in-situ observation of alternate supply MBE using scanning reflection electron microscopy. It is found that a fractional layer of excess Ga on a Ga-stabilized surface cannot form a continuous thin film but forms droplets instead. Droplet formation and disappearance processes and the roughening process are discussed.

(111) CdTe epitaxy on (100) GaAs substrates

Geometrical considerations on the GaAs/CdTe interface together with chemical bonding considerations are used to predict that (i) the CdTe growth starts on the Ga-stabilized (100) GaAs surface with a Te layer, (ii) the [211]CdTe direction grows parallel to [011]GaAs and [011]CdTe‖[011]GaAs, (iii) the growth direction is a [111] direction with a Te (B) surface as the top layer, and (iv) a periodic array of lattice defects is present at the GaAs/CdTe interface with a period of 2X15.9 A along the [011] direction to accomodate the lattice mismatch. These defects are interpreted as a new type of network of “misfit dislocations“ of both edge and screw character.

Model for heteroepitaxial growth of CdTe on (100) oriented GaAs substrate

A model is described, based on chemical bonding and lattice matching considerations, to account for the heteroepitaxy of CdTe on (100) GaAs substrate. The two main features of the proposed model are that the initial growth of CdTe starts with the formation of stable clusters of chemically bond Te, and that two types of cluster configurations are obtained depending only on the atomic structure of the (100)GaAs surface: the first one, made up of tetrahedral unit cells is formed on an As-deficient surface and leads to (111) orientation, whereas the second one, formed by twin-tetrahedral structures developed on an As- or Ga-stabilized surface gives rise to (100) orientation.

Mechanisms of epitaxial GaAs crystal growth by sputter deposition: Role of ion/surface interactions

The basic mechanisms as well as the kinetics of epitaxial GaAs crystal growth by RF sputter deposition have been investigated. The film growth rate R was found to depend not only on the Ga flux, as in thermal evaporation, but also on the ratio of the incident arsenic to gallium fluxes and the film growth temperature. These additional dependences are related to the effect of the steady state As surface coverage ϑAs on the average Ga atom surface binding energy 〈UGa〉 and hence on the secondary sputtering yield due to induced low energy ion bombardment of the growing film. A model was developed to relate ϑAs to film growth parameters and thus to allow predictions of R which exhibited good agreement with experimental results. From this analysis, 〈UGa〉 was found to increase from 3.5 to 4.9 eV/atom as ϑAs was increased from 0.2 to 0.6, corresponding to Ga-stabilized and As-stabilized surfaces, respectively.

Leed, aes and photoemission measurements of epitaxially grown GaAs(001), (111)A and (1̄1̄1̄)B surfaces and their behaviour upon cs adsorption

Epitaxially grown GaAs(001), (111) and (1̄1̄1̄) surfaces and their behaviour on Cs adsorption are studied by LEED, AES and photoemission. Upon heat treatment the clean GaAs(001) surface shows all the structures of the As-stabilized to the Ga-stabilized surface. By careful annealing it is also possible to obtain the As-stabilized surface from the Ga-stabilized surface, which must be due to the diffusion of As from the bulk to the surface. The As-stabilized surface can be recovered from the Ga-stabilized surface by treating the surface at 400°C in an AsH 3 atmosphere. The Cs coverage of all these surfaces is linear with the dosage and shows a sharp breakpoint at 5.3 × 10 14 atoms cm −2. The photoemission reaches a maximum precisely at the dosage of this break point for the GaAs(001) and GaAs(1̄1̄1̄) surface, whereas for the GaAs(111) surface the maximum in the photoemission is reached at a higher dosage of 6.5 × 10 14 atoms cm −2. The maximum photoemission from all surfaces is in the order of 50μA Im −1 for white light ( T = 2850 K). LEED measurements show that Cs adsorbs as an amorphous layer on these surfaces at room temperature. Heat treatment of the Cs-activated GaAs (001) surface shows a stability region of 4.7 × 10 14 atoms cm −2 at 260dgC and one of 2.7 × 10 14 atoms cm −2 at 340°C without any ordering of the Cs atoms. Heat treatment of the Cs-activated GaAs(111) crystal shows a gradual desorption of Cs up to a coverage of 1 × 10 14 atoms cm −2, which is stable at 360°C and where LEED shows the formation of the GaAs(111) (√7 × √7)Cs structure. Heat treatment of the Cs-activated GaAs(1̄1̄1̄) crystal shows a stability region at 260°C with a coverage of 3.8 × 10 14 atoms cm −2 with ordering of the Cs atoms in a GaAs(1̄1̄1̄) (4 × 4)Cs structure and at 340°C a further stability region with a coverage of 1 × 10 14 at cm −2 with the formation of a GaAs(1̄1̄1̄) (√21 × √21)Cs structure. Possible models of the GaAs(1̄1̄1̄) (4 × 4)Cs, GaAs(1̄1̄1̄)(√21 × √21)Cs and GaAs(111) (√7 × √7)Cs structures are given.

Oriented expitaxial films of (NMP) (TCNQ) : E. E. Simonyi. J. F. Graczyk and J. B. Torrance. IBM J. Res. Dev.22, (3) 315 (May 1978)

A process for the growth of ZnCdTe(100)GaAs heteroepitaxial films using metalorganic chemical vapor deposition (MOCVD) has been developed. It is found that substrate baking pretreatment deeply affects the characteristics of ZnxCd1 − xTe(x < 0.09) on (100)GaAs substrate (such as ZnCdTe film orientations, the Zn composition in the ZnCdTe compound alloys and the quality of ZnCdTe epilayer). We compared measurements on the same set of samples by photoreflectance (PR) and photoluminescence (PL). It was found that baking temperature (or baking time) may affect the relative contribution of each transition, resulting in a shift of the transition energy. We speculate that (100)GaAs substrates may undergo decomposition at a high baking temperature, leading to the GaAs surface change from As-stabilized surface to Ga-stabilized surface, resulting in better film PL quality and different film orientation. At higher baking temperature (or baking time), the ZnCdTe epilayer tends towards a (1 1 1) orientation and the film quality is obviously improved. Up to a temperature of around 640°C, the quality begins to decline sharply due to the destruction surface.

Surface studies on clean and oxygen-exposed GaAs and Ge surfaces by low-energy electron loss spectroscopy

Energy loss spectroscopy in combination with AES and HEED is used to study and clarify the nature of intrinsic and oxygen-associated surface states. The c(2 x 8), (4 x 6), and (1 x 1) GaAs(100) surfaces obtained by in situ molecular beam evaporation exhibit different intrinsic surface states. The oxygen adsorption rates also differ, being largest for the Ga-stabilized (4 x 6) surface and smallest for the As-stabilized c(2 x 8) surface. Oxygen chemisorbs to form an unknown molecular complex common to all three surfaces. The oxidation behavior on Ge suggests two different adsorption sites, one saturating at approximately 50 percent of a monolayer. It is established that oxygen chemisorbs to form GeO molecules, which are weakly bound to the surface. 11 fig, 45 references. (auth)