Surface Photoabsorption Research Articles

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.

Characterization of GaInP layers grown on GaAs substrates monitored by surface photo-absorption

In situ monitoring by surface photo-absorption (SPA) was used in metalorganic vapor phase epitaxy (MOVPE) growth of Ga 0.5In 0.5P ternary alloy semiconductor for the first time. Two modes were employed to supply the group III precursors: the alternate supply and the simultaneous supply. Monolayer-wise growths per supply cycle are possible for both growth modes. X-ray diffraction measurement and transmission electron microscopy (TEM) observation show that lattice mismatch of GaInP epitaxial layers can be controlled when a simultaneous supply is employed. Photoluminescene (PL) was measured for single GaInP layers and GaInP/AlGaInP double hetero (DH) structures grown at various temperatures. The results show that the PL intensity greatly depends on the growth temperature.

Effect of strain on source gas decomposition and group V desorption in metalorganic vapor phase epitaxy studied by surface photo-absorption

The decomposition rate of source gas and the desorption rate of group V atom were measured by surface photo-absorption during metalorganic vapor phase epitaxy (MOVPE). We found that the decomposition of PH 3 on GaAs and InAs surfaces, which produces a large tensile strain, is slower than that on GaP and InP surfaces. We also found that P desorption from P-stabilized GaAs and InAs surfaces, which relieves a large tensile strain, is faster than P desorption from GaP and InP surfaces, respectively. Strain-enhanced desorption was also observed for the As-stabilized InP surface. The difficulty of forming the strained surface bond evidently decreases the rate of decomposition and the ease of breaking the strained surface bond evidently increases the rate of desorption.

Real time observation of surface dielectric responses of GaAs(001) using surface photo-absorption

Surface photo-absorption is used to observe the changes in isotropic and anisotropic surface dielectric responses caused by As desorption during surface conversion from c(4 × 4) to (2 × 4)γ of As-stabilized GaAs(001) surfaces. The anisotropic part agrees with the change in the surface dielectric anisotropy spectrum observed in reflectance-difference measurements during this surface conversion. The spectral evolutions of surface dielectric response are observed in real time with a time resolution of 0.1 s during surface conversion.

Observation of a new in-situ optical monitoring signal with monolayer resolution in metalorganic molecular beam epitaxy and metalorganic vapor phase epitaxy of wide-gap II–VI compounds

A new in-situ optical probing method for studying surface reactions during heteroepitaxial growth is proposed. The new method is named surface photo-interference (SPI), because the concept of the SPI is essentially concerned with a photo-interference in the heteroepilayer. The experimental setup of the method is very similar to another optical probing method called surface photo-absorption (SPA), but the principle is quite different between the two. Unlike SPA, fairly low energy photons that are transparent for the epilayer can also be used in SPI as a probing light. Reflecting this feature, SPI is especially useful for studying growth kinetics in heteroepitaxial growth of widegap II–VI compounds, because the effect of photocatalytic growth-rate enhancement can be avoided when using such low energy photons as a probing light. A brief theoretical description of SPI signal is given, and experimental results detected in metalorganic molecular beam energy (MOMBE) of ZnSe and CdSe are mainly discussed. Further, how SPI differs from SPA will be shown.

GaAs (001) surface balanced with arsine partial pressure in metalorganic chemical vapor deposition reactor observed by surface photoabsorption

We measured surface photoabsorption signals from As adsorbed surfaces of GaAs (001) substrates in a reactor of metalorganic chemical vapor deposition (MOCVD) under various partial pressures of AsH 3. Desorption of As atoms from the surface was balanced with adsorption of As atoms under AsH 3 partial pressure. We found that the surface conditions varied as a function of the partial pressure and the temperature of the substrate. Comparing the experimental results with a multi-layer Langmuir adsorption model, we determined the phase boundaries of the surface structures under MOCVD conditions at temperatures between 500 and 700°C. One of the boundaries was assigned to the boundary between (2 × 4) and c(4 × 4), and the other to that between c(4 × 4) and a surface of excess As.

In-situ monitoring and control of surface processes in metalorganic vapor phase epitaxy by surface photo-absorption

In-situ surface photo-absorption (SPA) was applied to study the dynamic surface processes and the static surface structures during metalorganic vapor phase epitaxy (MOVPE). The static surface structure in MOVPE can be analyzed by the spectrum in near-ultra-violet and visible regions, which comprises an anisotropic dielectric response due to the surface dimer bond as well as an isotropic response probably due to the surface back-bond. Time-resolved spectra showing dynamic changes of the surface can be measured by the use of optical multichannel analyzer. The rate of surface decomposition of source molecule and the rate of desorption of surface species were measured as the change of reflectivity at a fixed wavelength. The decomposition of source molecules was characterized quantitatively, and a correlation between the rate of desorption of group V atoms and the bond strength of a surface back-bond was found. During heteroepitaxy, the surface exchange of group V atom was monitored in situ, and the growth was accordingly controlled to suppress this exchange reaction and obtain a compositionally abrupt interface.

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 observation of Ga adsorption during TMGa exposure on GaAs(001) surfaces with various As coverages

We measured transients of surface photoabsorption signals during TMGa exposure on c(4 × 4)-like, 2 × 4-like and Ga-terminated GaAs(001) surfaces in the MOCVD reactor. Ga-containing species were continuously deposited during TMGa exposure on Ga-terminated surfaces. On the other hand, self-limiting deposition was observed on c(4 × 4)-like and 2 × 4-like surfaces. The growth thickness in one cycle of ALE process on the 2 × 4-like surface was 0.7–0.8 compared to that on the c(4 × 4)-like surface. The stable duration of the SPA signal under TMGa exposure was longer than the residency time of methyl groups on the surface which was obtained from the transients during purge after TMGa exposure. Hence the methyl groups are continuously supplied to the surface by decomposition of TMGa during the exposure period under self-limiting condition.

Dielectric response of As-stabilized GaAs surfaces observed by surface photo-absorption

This letter discusses changes in the isotropic and anisotropic surface dielectric response caused by surface conversion from c(4×4) to (2×4) of a GaAs(001) As-stabilized surface, observed using surface photo-absorption. The anisotropic part quantitatively reproduces the change in the surface dielectric anisotropy spectra observed in reflectance difference measurements during this surface conversion. The isotropic part can be measured only by surface photo-absorption and comparison is made with a calculation using a three-phase model consisting of vacuum/As/GaAs.

In-Situ Control of Strained Heterostructure Growth

This paper reports the metalorganic vapor phase epitaxial growth of pseudomorphic InAs/InP single quantum wells (SQWs) on (001) InP substrates using in-situ surface photo-absorption (SPA) monitoring, and reports on the optical characterization of them. By adjusting the arsine flow rate, arsine exposure time and substrate temperature on the basis of in-situ SPA monitoring, we are able to grow pseudomorphic InAs/InP SQWs comprising up to 10-monolayer InAs wells having metallurgically abrupt and atomically flat strained heterointerfaces. Photoluminescence excitation measurement for an 8-monolayer InAs well shows energy splitting as large as 0.4 eV between heavy and light holes subbands, indicating the InAs well layer is coherently strained to a maximum extent within the critical layer thickness.

A new in-situ optical probing method for the surface reaction in photo-excited heteroepitaxy of compound semiconductors

A new in-situ optical probing method with an atomic-scale resolution in heteroepitaxial growth is proposed. The experimental set-up of the method is very similar to another optical probing method called surface photo-absorption (SPA), but the principle is quite different. The new method is named surface photo-interference (SPI) because it is essentially concerned with photo-interference in the heteroepilayer. The principle of the SPI is as follows: the complex refractive indices of the atomic or molecular layers, which alternately appear on top of the surface during epitaxial growth of compound semiconductors, greatly affect the total phase shift of probing light during propagation in the epilayer, resulting in a change in the photo-interference signal intensity. One of the features of the SPI is that, unlike in the case of SPA, fairly low energy photons which are transparent for the epilayer can also be used as probing light. This is very preferable for studying the growth kinetics and/ or surface reaction in photo-excited epitaxial growth of widegap II–VI compounds, because the effect of photocatalytic growth-rate enhancement can be avoided when using such low energy photons as probing light.

Real-time optical diagnostics for epitaxial growth

Various optical diagnostics are being developed to meet new challenges in epitaxial growth. Layer thicknesses and compositions, the primary parameters needed for growth control, can be obtained with bulk-oriented probes such as reflectometry and ellipsometry. New derivative algorithms for determining near-surface dielectric properties and compositions from kinetic ellipsometric and complex-reflectometric data have made possible closed-loop feedback control of alloy structures, including compositionally graded structures. New information about the kinetics and chemistry of growth is being obtained by surface-oriented probes such as laser light scattering (LLS), surface photoabsorption (SPA), and reflectance-difference (-anisotropy) spectroscopy (RDS/RAS). Examples are provided, and likely directions of further progress discussed.

Near-threshold enhancement in photoemission from alkali metal overlayers

The total surface photoabsorption and the emitted part of it are calculated for thin Cs layers adsorbed on a semi-infinite metallic substrate. The jellium model is used to evaluate the electronic properties of this system and the dynamical response is treated within the time-dependent density functional approach. It is shown that the self-consistently screened electronic excitations lead to an enhancement of the p-polarized emission current in the vicinity of the threshold. Photoemission measurements for Cs overlayers on Ag are carried out at various coverages. These spectra indeed show a striking increase of the p-polarized intensity within about 1 eV above threshold.

In-Situ Surface Photo-Absorption Study of Movpe Surface

ABSTRACTIn-situ surface photo-absorption (SPA) method was applied to study the dynamic surface processes and the static surface structures during metal-organic vapor phase epitaxy (MOVPE). Observed spectra in near-ultaviolet and visible regions consist of an anisotropic dielectric response due to surface dimer-bonds as well as an isotropic response probably due to surface back-bonds. Time-resolved spectra showing dynamic changes of surface can be measured by the use of optical multichannel analyzer. The rate of surface decomposition of source molecule was measured as the change of reflectivity at a fixed wavelength. The decomposition of source molecule was characterized quantitatively, and various effects on the decomposition were studied. The style of decomposition and the activation energy depended on the substituent of source molecule and the rate of decomposition was affected by the surface potential and the strain.

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.

Relationship among reflectance-difference spectroscopy, surface photoabsorption, and spectroellipsometry

From the reflectance expressions for a thin biaxial layer on an isotropic substrate we relate reflectance-difference spectroscopy (RDS), surface photoabsorption (SPA), and spectroellipsometry. Using these results and our recently acquired RD database, we determine surface reconstructions present during flow-modulated organometallic chemical vapor growth of epitaxial GaAs from SPA data that were published by others.

Spectral Observation of As-Stabilized GaAs Surfaces in Metal-Organic Chemical Vapor Deposition Using Surface Photo-Absorption

The spectral dependencies of As-stabilized (001) GaAs surfaces in metal-organic chemical vapor deposition (MOCVD) are measured using the surface photo-absorption (SPA) method and compared them with those obtained in molecular beam epitaxy (MBE). The SPA spectrum of an As-stabilized surface at 600°C is anisotropic in regards to the perpendicularly intersecting incidence azimuths, [110] and [1̄10], of the monitoring light. It is also very similar at a arsine partial pressure of 4 Pa, which is a common MOCVD growth condition, to the spectrum obtained for an MBE As-surface having a c(4×4) reconstruction pattern in reflection high-energy electron diffraction (RHEED) observation, though not to the spectrum corresponding to a(2×4) pattern. Below 500°C, an isotropic signal appears and overlaps with the anisotropic spectrum of a c(4×4)-like surface, indicating that As species adsorbs excessively on a c(4×4)-like surface, which reduces its anisotropy. It is shown that by using different wavelengths, the dynamics of As desorption from each As-stabilized surface can be measured separately.

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.