Surface Photoabsorption Research Articles

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

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

In situ monitoring and control of atomic layer epitaxy by surface photo-absorption

Surface photo-absorption (SPA) is a real-time optical probe for examining growth and gives us a microscopic insight into atomic processes occurring on a growth surface. SPA observations during GaAs and InP atomic layer epitaxy (ALE) using trimethyl Group III sources and Group V hydrides demonstrate that the adsorption of trimethyl source saturates during the trimethyl source supply period and that the saturated surface corresponds to a metastably methyl-terminated surface with a lifetime of several tens of seconds. In contrast, when the trimethyl source is supplied onto the Group III metal surface, an increase in the SPA reflectivity from the metal surface level is observed, indicating that the trimethyl source molecule decomposes on the Group III metal surface via adsorption. These observations give rise to the conclusion that the growth rate self-limitation in ALE is caused by the inhibited adsorption of excessively supplied Group III source molecules on the methyl-terminated surface, and not by a selective adsorption mechanism. SPA observation also makes it possible to determine readily and precisely the growth parameters for ALE which strongly depend on the reactor design and the gas flow condition.

Study of photocatalytic growth-rate enhancement in MOMBE of GaAs on ZnSe by surface photoabsorption

In order to lower the epitaxy temperature of GaAs on ZnSe, the effect of photoirradiation on the growth and/or reaction mechanism in the initial growth stage in MOMBE of GaAs has been studied by utilizing the surface photoabsorption technique. It has been found that excess carriers photoinduced in ZnSe can contribute to the decomposition of TEGa on ZnSe, but those photoinduced in GaAs cannot. This has been attributed to the photocatalytic effect of the decomposition of TEGa on the ZnSe surface. It has been confirmed that photoirradiation is effective for enhancing the growth rate at low temperatures, resulting in decreased epitaxy temperature of GaAs on ZnSe.

Study of self-limiting growth mechanism in chloride ALE

The growth process of chloride atomic layer epitaxy (ALE) using GaCl is studied using the temperature programmed desorption method (TPD), the surface photo-absorption (SPA) method and ab initio molecular orbital calculations. From the TPD measurements, the GaCl adsorption energy is calculated to be 32 kcal mol -1 and 38 kcal mol -1 for Ga-terminated and As-terminated surfaces respectively. In the absence of H 2, the Ga-Cl bond appears to be very stable in the adsorption state. Process simulation using a simple cluster model of (AsH 2) 2-GaCl indicates that adsorption is completed by forming σ and π covalent bonds, and that there is hardly any self-dissociation of the Ga-Cl bond. From SPA measurements in the ALE system with ambient H 2, a strong similarity is found between the spectra of the GaCl-supplying surface and the Ga-covered surface. Based on these results, a model of the chemical process in chloride ALE is proposed where adsorbed GaCl molecules can react immediately with hydrogen and Cl is released from the surface as HCl. A self-limiting mechanism results from the very short residence time of GaCl (∼ 10 -3s at 450 °C) on the Ga-terminated surface.

Surface photo-absorption study of the laser-assisted atomic layer epitaxial growth process of GaAs

Thermal and laser-assisted decomposition of trimethylgallium (TMG) and triethylgallium (TEG) on GaAs surfaces under atomic layer epitaxy (ALE) processing conditions were monitored in real time using the (SPA) surface photo-absorption reflectivity technique to elucidate the mechanism of self-limiting deposition. When As-terminated (001) GaAs surfaces held between 340 and 390°C were exposed to TMG, the SPA reflectivity decreased to a steady state, indicative of alkylgallium on the surface. Upon laser irradiation, this surface changed to an ‘intermediate’ steady state surface that was found to be in equilibrium with gas-phase alkylgallium species. Alkyl species then desorb from this surface when the TMG flow is stopped, leaving a self-limited deposit of Ga on the surface. No self-limiting deposition of Ga could be detected using TEG in laser-assisted ALE or thermal ALE. In this case, the observation of self-limiting alkylgallum deposition could explain successful ALE growth using TEG.

In situ optical characterization of GaAs and InP surfaces during chloride atomic layer epitaxy

In this paper, the results of in situ surface photo-absorption (SPA) measurements on GaAs and InP surfaces under an alternating supply of GaCl and AsH 3 or InCl and tertialybutylphosphine (TBP) in chloride atomic layer epitaxy (ALE) are presented. It was found that the reflection intensity varied by several per cent, depending on the source gas supply sequence. This reflection intensity was constant during the GaCl and InCl supply from both the GaAs and InP surfaces, probably corresponding to the self-limiting mechanism of chloride ALE. Reflection spectra from a GaAs surface during and after GaCl supply were found to be similar. They showed anisotropic spectral structures according to the incidence azimuth of light. This anisotropy is considered to originate from the existence of surface dimers. Thus, a GaAs surface under GaCl supply is assumed to be covered by Ga. In contrast, the spectra from an InP surface during InCl supply were isotropic and different from the anisotropic spectra measured from an In-stabilized surface formed by P desorption. Accordingly, it is assumed the Cl adsorbs on an InP surface during InCl supply and prevents the formation of dimers. The different behavior of Cl on GaAs and InP may be due to a different reaction rate with H 2.

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).

特集「結晶成長表面の“光プローブ”による評価」 ＧａＡｓおよびＩｎＰクロライド原子層エピタキシーにおけるブリュースター角入射表面光吸収法（ＳＰＡ）による表面“その場”観察

特集「結晶成長表面の“光プローブ”による評価」 ＧａＡｓおよびＩｎＰクロライド原子層エピタキシーにおけるブリュースター角入射表面光吸収法（ＳＰＡ）による表面“その場”観察

Real-Time Surface and Near-Surface Optical Diagnostics for Epitaxial Growth

AbstractVarious optical diagnostics are being developed to meet new challenges in semiconductor epitaxy. New data-analysis algorithms allow near-surface alloy compositions and interface properties to be obtained by kinetic ellipsometry (KE) without any knowledge of the underlying sample structure. New information about growth is being obtained by surface-oriented probes such as laser light scattering (LLS), surface photoabsorption (SPA), anrd reflectance-difference (-anisotropy) spectroscopy (RDS/RAS). Examples are provided and likely directions of further progress discussed.

特集「結晶成長表面の“光プローブ”による評価」 ブルスター角入射光反射モニタ（Ｓｕｒｆａｃｅ Ｐｈｏｔｏ‐Ａｂｓｏｒｐｔｉｏｎ ＳＰＡ）法による有機金属気相成長表面反応とその場観察 ＡＬＥ成長機構

特集「結晶成長表面の“光プローブ”による評価」 ブルスター角入射光反射モニタ（Ｓｕｒｆａｃｅ Ｐｈｏｔｏ‐Ａｂｓｏｒｐｔｉｏｎ ＳＰＡ）法による有機金属気相成長表面反応とその場観察 ＡＬＥ成長機構

In Situ Interface Control of Pseudomorphic InAs/InP Quantum Well Structure Growth by Surface Photo-Absorption

The exchange reaction of group V atoms was monitored in situ by surface photo-absorption on a submonolayer scale during metalorganic vapor phase epitaxial growth of pseudomorphic InAs/InP heterostructure on (001) InP substrates. About a 0.4-monolayer As/P exchange reaction was observed after arsine was supplied at 0.2 µmol/s onto the In surface of InP at a substrate temperature (Ts) of 400°C. At a Ts of 350°C, this As/P exchange reaction was suppressed to less than 0.1 monolayer. In contrast, the P/As exchange reaction was less than 0.1 monolayer below 400°C. The benefits of in situ control were shown by growing pseudomorphic InAs/InP single and multiple quantum wells with 1-12 InAs well layers at Ts=350°C and characterizing them by cross sectional transmission electron microscopy, photoluminescence, and double crystal X-ray diffraction. Metallurgically abrupt and atomically flat heterostructures without dislocations were grown up to 10 monolayer (ML) of InAs well, which corresponds to the critical layer thickness.

In situ control of heterointerface quality in MOVPE by surface photo-absorption

Exchange reaction of Group V atoms is in situ monitored on a submonolayer scale during the MOVPE growth of pseudomorphic InAs/InP heterostructures on (001) InP substrate by the surface photo-absorption (SPA) method. When arsine was supplied onto the In surface of InP above 360°C, the substitution between As atoms of impinging AsH 3 molecules and P atoms of InP surface was detected as a change in surface reflectivity. The reaction proceeded to about 0.4 monolayer exchange at 400°C, and was suppressed to less than 0.1 monolayer at 350°C. In contrast, the exchange between P atoms of impinging PH 3 and As atoms of InAs surface was less than 0.1 monolayer below 400°C. To verify the formation of a metallurgically abrupt heterointerface, InAs/InP single quantum wells with 1–5 monolayers were grown at 350°C, and were characterized by transmission electron microscopy and photoluminescence. Atomic layer epitaxy was used to control precisely the InAs well thickness with an accuracy of one monolayer. A sharp and an intense quantum well photoluminescence was observed with a reasonable shift of emission energy due to the quantum size effect up to five InAs monolayers, indicating that metallurgically abrupt and atomically flat heterointerfaces were fabricated as designed.

Enhancement of AsH 3 decomposition on highly doped p-GaAs surfaces

Surface photo-absorption (SPA) was used to study the effect of semiconductor surface potential on the heterogeneous decomposition of source molecules in MOVPE. The decomposition rate of AsH 3 was enhanced on the Ga surface of p-GaAs in proportion to a square root of the carrier concentration. In contrast, an enhancement on Si doped n-GaAs was negligible up to 2 × 10 18 cm −3. The decomposition activation energy was almost independent of the conduction type and of the carrier concentration. From these results, the enhancement of AsH 3 decomposition on p-GaAs might be caused by an increase in the mean lifetime of AsH 3 adsorption on the Ga surface of p-GaAs due to an attractive force between the dipole of the AsH 3 molecule and the surface electric field. For the Group III source, a preliminary study by the scattered light method which detects the Ga droplet formation showed that the decomposition of triethylgallium was enhanced on the As surface of n +-GaAs in the kinetically controlled temperature region.

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.

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.

Dynamics and kinetics of MBE growth

The epitaxial growth of semiconductor films from molecular beams in an ultra-high-vacuum environment has made available structures of unprecedented complexity and precision. In parallel, the application of surface science techniques and computer simulation to studies of surface reaction kinetics and growth dynamics has allowed very significant advances to be made in our understanding of epitaxial film growth at the atomic level. The experimental techniques which have been used most extensively are modulated beam relaxation spectroscopy (MBRS) and reflection high energy electron diffraction (RHEED). A summary of results from solid sources for both lattice matched and strained layer systems which illustrate our present level of knowledge is related to Monte Carlo growth simulations and an analytical treatment of non-linear nucleation kinetics. Particular attention is focussed on cation effects in the growth of III-V compounds over a wide temperature range. In addition to MBRS and RHEED, optical methods such as reflectance difference spectroscopy and surface photo-absorption have recently been introduced as in-situ diagnostic probes to study surface chemistry and growth morphology. The information derived from this type of measurement is considered in the context of growth models developed from the longer-established techniques.

In-situ optical monitoring of pyrolysis process on substrate surface in GaAs MOCVD

We investigated the pyrolysis process of trimethylgallium (TMG) and As precursors of organo-arsenics and arsine on a (001) GaAs surface by the surface photo-absorption method in metalorganic chemical vapor deposition. For the pyrolysis of TMG, we directly observed the CH 3 desorption from the GaAs surface reacted with TMG. From the CH 3 desorption kinetics, we determined that the CH 3-terminated surface causes self-limitation of the growth rate in atomic layer epitaxy because the excess TMG cannot adsorb. For the pyrolysis of As precursors, we estimated the pyrolysis activation energies. We show that diethylarsenic hydride and tert-butyl arsine are promising precursors as an alternative source of AsH 3 because of their low pyrolysis activation energies.

Optical investigation of GaAs growth process in molecular beam epitaxy and migration-enhanced epitaxy

Epitaxial growth process of GaAs by molecular beam epitaxy and migration-enhanced epitaxy is investigated using a newly developed optical monitoring method called surface photo-absorption. This method is based on the reflectivity measurement of p-polarized light incident at the Brewster angle on the growing surface. This configuration minimizes the bulk GaAs contribution to the light reflection. The small signal from the growing surface is thus detected with a high signal-to-noise ratio. It was found that this optical method is useful for monitoring the surface chemical composition during molecular beam epitaxy, and the fractional layer growth during migration-enhanced epitaxy. An optimization of As 4 deposition rate during growth is also demonstrated using the recorded surface photo-absorption signal.

In-situ monitoring of GaAs growth process in MOVPE by surface photo-absorption method

Surface photo-absorption (SPA) is a newly developed in-situ optical monitoring technique for the epitaxial growth process. This method is based on the reflectivity measurement of p-polarized light incident at the Brewster angle. This configuration minimizes the bulk GaAs contribution to the total light reflection. The small change in reflected light intensity between Ga and As atomic surfaces during GaAs growth is thus detected with a high signal-to-noise ratio. By using this characteristic, GaAs growth rate can be monitored in-situ on an atomic scale. In addition to the in-situ monitoring of growth rate, the decomposition processes of Ga and As precursors in MOVPE can be studied by SPA. We demonstrate an investigation of the decomposition process of Ga organometallic, and discuss the growth mechanism of atomic layer epitaxy.

Contrast enhancement of resist images by surface cross-linkage

A general method of contrast enhancement which provides control of resist image wall profiles and is applicable to both positive and negative working resists may be obtained by surface cross‐linking. The cross‐linking is used in our process to retard dissolution at the resist surface. The creation of a slower dissolving surface layer affords greater control of wall profiles and enhanced contrast or sensitivity. One method is to use mid‐ and deep‐UV flood exposure. For a negative working electron beam resist this method is the same as surface photo absorption for contrast enhancement. With our experimental resist containing 4,4’‐diazido‐diphenylsulfide as a sensitizer, the improvement in its electron beam sensitivity was not substantial. For a positive working photoresist, the same aromatic bisazide was added to conventional positive working photoresists composed of a diazo‐naphthoquinone‐type photosensitizer and novolac resin. A short‐300‐nm UV flood exposure followed the G‐line imagewise exposures. Anoth...