Surface Photoabsorption Signal Research Articles

Two-step As-desorption from (001) InP observed by surface photoabsorption

The investigation of As desorption from the (001) InP during metalorganic chemical vapor deposition was performed using surface photoabsorption (SPA). The behavior of the monochromatic SPA signal showed a clear existence of a metastable state after the AsH3 was turned off. SPA spectra at each stable surface were taken to confirm the interpretation. This result indicates that the previous criterion to measure the amount of As/P exchange reaction is not necessarily correct and that the As-desorption process should be understood as a two-step process.

In situ analysis of surface photoabsorption spectra during InP ALE in metal organic chemical vapor deposition

In situ monitoring of InP atomic layer epitaxy (ALE) by surface photoabsorption (SPA) was performed in low-pressure metal organic chemical vapor deposition. Self-limiting adsorption condition of In species was studied for various TMIn injection times. A grazing incidence X-ray study on the thickness of the film grown by ALE, however, showed that the growth rate did not reach one monolayer (ML)/cycle. The SPA signal trace measured during the InP ALE indicated incomplete PH 3 decomposition on the methyl-terminated In surface, and this observation was attributed to the cause of submonolayer growth. We also report a spectroscopic SPA study on the surface state during a cycle of ALE process. The SPA spectrum of the methyl-terminated In surface showed a different line shape at around 1.9 eV (In-dimer region) from that of an In-stabilized surface. But, during H 2 purge at 390°C, this portion of the spectrum was observed to evolve to that of an In-stabilized surface by the desorption of methyl radical.

GaAs high temperature optical constants and application to optical monitoring within the MOVPE environment

The real and imaginary components of the GaAs refractive index at temperatures between 20–700°C have been obtained. Measurements were made by comparing the variable angle reflectivity of p-polarized and s-polarized 633 nm wavelength light from a deoxidized GaAs surface. By using these temperature-dependent optical constants for GaAs, modeling has allowed the behavior of surface photoabsorption (SPA) signals with temperature and oxide layers present to be predicted for different angles of incidence. The experimentally observed SPA signals during deoxidization of GaAs show strong qualitative agreement with these calculations at each of the angles of incidence considered. The measurement of data and application to modeling provides a platform for the measurement of temperature-dependent optical data for other III–V materials and for the investigation of deoxidation mechanisms.

Analysis of P adsorption and desorption on the (001) InP surface using surface photoabsorption

We present an in situ study of P desorption and adsorption on the (001) InP surface using surface photoabsorption (SPA). The SPA spectra show three peaks at 2.1, 2.9, and 3.1 eV. Here, based on previous studies, we have identified them as a P dimer related and two surface In dimer related peaks, respectively. The initial sharp drop and the slow recovery of the SPA signal at 430 nm after PH3 is switched on implies that at least two steps are involved in the P desorption process. We also observed a structure at 3.1 eV in the spectrum of the In-stabilized surface. We have determined from temperature dependent desorption measurements that the activation energy for P desorption from this surface is 3.36 eV.

Effect of P precursor on surface structure and ordering in GaInP

Surface photo absorption (SPA) is one of the only techniques for characterization of the surface structure during organometallic vapor-phase epitaxial (OMVPE) growth. Since the ordering process leading to the formation of the CuPt structure in III/V alloys occurs completely at the surface during growth, SPA gives valuable information about the ordering process. In this study SPA was used to characterize the surface during OMVPE growth of GaInP at 670°C using several partial pressures of the phosphorus precursor. Results for the two precursors tertiarybutylphosphine (TBP) and phosphine were compared in detail. The SPA signal difference at 400 nm, attributed to the [ 1 ̄ 1 0] P dimers characteristic of the (2×4) reconstructed surface, was found to increase with increasing partial pressure with equal magnitudes, to within experimental error, for PH 3 and TBP. The degree of order in the resulting epitaxial layers was also found to increase with increasing partial pressure of the P precursor and to be virtually identical for the two precursors. A surprising difference between the two P precursors relates to the transient in the 400 nm SPA reflectivity when the flow of the P precursor is terminated. For TBP at both 620 and 670°C, the SPA reflectivity transient is rapid. For PH 3, the SPA reflectivity transient is delayed for approximately 1 min at 670°C. At 620°C the SPA response is extremely sluggish, with a time constant of approximately 6.5 min. The step structure, measured using atomic force microscopy, is also found to be different for the two P precursors. For PH 3 the steps are mainly monolayers and for TBP they are mainly bilayers. Both differences are tentatively attributed to the difference in H coverage of the surface for the two precursors.

In Situ Optical Monitoring of Hydrogen Chemisorption on the GaAs(111)B Ga Surface

The hydrogen chemisorption on the GaAs(111)B Ga surface is investigated under atmospheric pressure using the surface photoabsorption (SPA) and temperature-programmed desorption (TPD) methods. The SPA signal change shows the existence of two kinds of hydrogen-terminated surfaces, monohydride and trihydride surfaces, depending on the partial pressure of hydrogen. In both surfaces, hydrogen in the carrier gas reacts dissociatively with Ga atoms on the (111)B surface, and the desorption of hydrogen occurs via the reverse reaction in the inert carrier gas. It is shown that the surface coverages of hydrogen on the surfaces are well explained by the Langmuir equation.

Real-Time Monitoring of the Etching of GaAs(100) by Surface Photoabsorption

Surface photoabsorption (SPA) has been applied to monitor, in real time, the surface of GaAs(100) during chemical dry etching by a molecular beam of HCl. Changes in the HCl flux to the surface at a constant temperature (840 K) have been used to induce changes in the Ga:As ratio on the surface. These changes in surface stoichiometry have been detected in situ by SPA measurements of the transient fractional change in the reflectance of p-polarized, 488-nm light that is incident onto the surface near the pseudo-Brewster angle. On the basis of results from prior applications of SPA to the study of the atomic layer deposition of GaAs, the changes in the SPA signal as a function of the etching parameters can be correlated with changes in the relative surface densities of Ga and As. The findings are confirmed by independent determinations of the changes in surface stoichiometry made by measuring the time-integrated difference in the fluxes of Ga- and As-containing etching products evolved from the surface as a function of the HCl flux.

Surface photoabsorption monitoring of the growth of GaAs and InGaAs at 650°C by MOCVD

By monitoring the cyclic behavior of surface photoabsorption (SPA) reflectance changes during the growth of GaAs at 650°C and with sufficient H2 purging time between the supply of trimethylgallium and AsH3, we have been able to achieve controlled growth of GaAs down to a monolayer. Our results show, as confirmed by photoluminescence (PL) measurements, the possibility of growing highly accurate quantum well heterostructures by metalorganic chemical vapor deposition at conventional growth temperatures. We also present our PL measurements on the InGaAs single quantum wells grown at this temperature by monitoring the SPA signal.

Effects of growth temperature and [formula omitted] ratio on surface structure and ordering in Ga0.5In0.5P

Surface photoabsorption (SPA) measurements were used to clarify the CuPt ordering mechanism in Ga0.5In0.5P layers grown by organometallic vapor phase epitaxy. The CuPt ordering is known to be strongly affected by the growth temperature and the input partial pressure of the phosphorus precursor, i.e. the VIII ratio. The SPA peak at ∼ 400 nm was found to be a measure of the concentration of [1̄10]-oriented phosphorus dimers on the surface, which are characteristic of the (2 × 4) reconstruction. Both ordering, measured using the low temperature photoluminescence peak energy, and the SPA signal difference due to P dimers were studied versus the growth temperature and VIII ratio. The degree of order decreases markedly with increasing growth temperature above 620°C at a constant VIII ratio of 40. This corresponds directly to a decrease of the [1̄10]-oriented P dimer concentration on the surface determined using SPA. Below 620°C, the degree of order decreases as the growth temperature decreases, even though the concentration of P dimers increases. The presence of an isotropic “excess P” phase observed in the SPA spectrum at ∼ 480 nm might be responsible for the decrease of CuPt ordering, although it has previously been attributed to the slow rearrangement of adatoms. The degree of order is found to decrease monotonically with decreasing VIII ratio in the range from 160 to 8 at 670°C and from 40 to 8 at 620°C. This also corresponds directly to the decrease of the P dimer concentration on the surface measured using SPA. At 620°C and a VIII ratio of 160, the degree of order decreased despite an increase of the P dimer concentration. This may also be due to the formation of the isotropic “excess P” phase on the surface. The direct correlation of the [1̄10]-oriented P dimer concentration and the degree of order with changes in temperature (≥ 620°C) and VIII ratio (≤ 160 at 670°C and ≤ 40 at 620°C) suggests that, in this range of growth parameters, the (2 × 4) surface reconstruction is necessary to form the CuPt structure, in agreement with published theoretical studies.

Monolayer epitaxy of GaAs at 650 °C by metal–organic chemical-vapor deposition with surface photoabsorption monitoring

We report monolayer-by-monolayer epitaxy of GaAs at 650 °C using surface photoabsorption (SPA) to monitor growth. Our results show, as confirmed by photoluminescence measurements, the possibility of growing highly accurate quantum well heterostructures by metal–organic chemical-vapor deposition at conventional growth temperatures. In addition, we observe a continuous increase of the SPA signal during trimethylgallium exposure that cannot be explained by present growth models.

Optical in-situ analysis of GaAs/AlAs/GaAs and GaAs/(Al)GaAs/GaAs atomic layer growth using GaCl 3, AlCl 3 and AsH 3

Atomic layer growth of AlAs and AlGaAs were tried by alternate supply of GaCl 3, AlCl 3 and AsH 3, and the growth process was monitored by an optical method such as the surface photo-absorption (SPA) method and the optical interference method. Although atomic layer epitaxy (ALE) of GaAs can be performed by alternate supply of GaCl 3 and AsH 3, that of AlAs on GaAs could not be obtained, probably due to surface oxidation of AlAs, AlAs, however, could be grown by simultaneous supply of AlCl 3 and AsH 3. It was confirmed by the SPA signal that if the AlCl 3-adsorbed AlAs surface is exposed to GaCl 3, the adsorbed AlCl 3 is replaced by GaCl and/or GaCl 3 and GaAs ALE growth on AlAs can be performed. A slightly Al-containing (Al)GaAs layer could be grown by addition of AlCl 3 to GaCl 3 in GaAs ALE growth.

In situ optical investigation of the etching process in the silicon HCl system

Surface reactions in Si(100) etching with HCl gas are studied using the surface photoabsorption (SPA) method and a quadrupole mass spectrometer. The adsorption of Cl atoms on the Si surface causes the intensity of the SPA signal to increase, when HCl gas is supplied. After HCl gas injection is stopped, the signal intensity decreases due to the etching reactions. By studying the time constants and the activation energies, two types of etching reactions are found. Below 1000 K, the etching products are SiCl4, and the activation energy for this etching process is estimated at about 22 kcal/mol. On the other hand, above 1000 K, mainly SiCl2 molecules are desorbed from the Si surface, and the activation energy is evaluated to be 59 kcal/mol.

In-situ monitoring of surface reactions in Si vapor phase epitaxial growth by surface photo-absorption method

Surface reactions in Si VPE using the SiH 2Cl 2/H 2 gas system were studied using the surface photo-absorption (SPA) method. when SiH 2Cl 2 was supplied, the intensity of the SPA signal increased, due to the absorbate SiCl 2 on the Si(100) surface. After the source gas injection was stopped, the signal intensity decreased, corresponding to the CI atoms desorption from the surface. By studying the time constant and the activation energy for this desorption process are found; one is desorption of the absorbate SiCl 2 itself, and the activation energy for this reaction is estimated to be 21 kcal/mol. The other is the reaction of absorbed SiCl 2 with gaseous H 2, and its activation energy is calculated to be 73 kcal/mol.

Comparison of reflectance difference spectroscopy and surface photoabsorption used for the investigation of anisotropic surfaces

Reflectance difference spectroscopy (RDS) and surface photoabsorption (SPA) are two optical characterization techniques that are now in common use for monitoring and controlling epitaxial growth. Here we establish the connection of SPA data to the surface dielectric response, using an anisotropic three phase model (substrate, anisotropic overlayer, ambient). In contrast to the generally accepted picture, our calculations show, that the contribution to the SPA signal from the dielectric component normal to the surface is negligible, and that the major contribution arises from the projection of the dielectric tensor on the line formed by the intersection of the surface and the plane of incidence. It follows, that the difference between SPA data taken with the plane of incidence along the different principal axis is related to, and in fact can be calculated from, RDS spectra. By comparing published data to our database of RD spectra, we are able to find the surface reconstruction present during flow modulated organo metallic chemical vapour epitaxial growth of GaAs epilayers.

特集「結晶成長表面の“光プローブ”による評価」 水素雰囲気中でのＧａＡｓ表面からのＡｓ脱離過程のブリュースター角入射光反射法によるその場観察

Arsenic desorption processes from GaAs surfaces with various crystallographic orientations were studied in the MOVPE reactor under H2 atmosphere (13.3 kPa) by the surface photo-absorption (SPA) method. We found two desorption processes (fast and slow processes) on the GaAs (001) surface. The real part of the refractive index of the surface layer which is removed during the fast As desorption process, is different from that of the GaAs substrate. Both the real and the imaginary parts of the refractive index of the layer which is removed during the slow As desorption process, are different from those of the GaAs substrate. We observed single As desorption processes on the GaAs (111) A and (111) B surfaces. We also studied SPA signals while AsH3 and TMG were being alternately supplied to those surfaces. We observed the ALE process on the (001) surface around 480°C. Growth rates were less than one monolayer per cycle on (111) A within the entire temperature range of our experiments (from 420°C to 550°C).

In-situ optical monitoring of the decomposition process of gaseous sources in metal-organic chemical vapor deposition and atomic layer epitaxy

We apply surface photo-absorption (SPA) to study the heterogeneous decomposition of Group-III sources on a substrate surface during metal-organic chemical vapour deposition (MOCVD). We first show the substrate temperature ( T s) dependence of the SPA signal at the supply of a Group-III source to a Group-V stabilized surface. The decomposition onset temperature obtained from the rising of the SPA signal, agrees well with the starting temperature of growth. Triethyl Al, Ga and In sources have lower onset temperatures than the corresponding trimethyl sources. A new Al source, trimethylamine alane, decomposes below 200°C. The region of atomic layer epitaxy (ALE) appears as a shoulder or a plateau in the neighborhood of decomposition onset. In this region, the surface is saturated by adsorption of the trimethyl source, and it is concluded that the self-limiting growth rate is caused by the intermolecular repulsion between impinging metal-organic molecules and CH 3 groups metastably terminating the growth urface. SPA allows real-time control of the ALE process, because it distinguishes a metastable CH 3-terminated surface from a metal one, and the growth parameters for obtaining CH 3 termination are readily determined.