The chemical stability of a GaAs layer structure consisting of a thin (10 nm) layer of low-temperature-grown GaAs (LTG:GaAs) on a heavily n-doped GaAs layer, both grown by molecular beam epitaxy, is described. Scanning tunneling spectroscopy and X-ray photoelectron spectroscopy performed after atmospheric exposure indicate that the LTG:GaAs surface layer oxidizes much less rapidly than comparable layers of stoichiometric GaAs. There is also evidence that the terminal oxide thickness is smaller than that of stoichiometric GaAs. The spectroscopy results are used to confirm a model for conduction in low resistance, nonalloyed contacts employing comparable layer structures. The inhibited surface oxidation rate is attributed to the bulk Fermi level pinning and the low minority carrier lifetime in unannealed LTG:GaAs. Device applications including low-resistance cap layers for field-effect transistors are described.

We report several new results in hydrogen sulfide (H2S) treatment of a GaAs (001) substrate. Surface reconstruction and morphology were investigated by in situ reflection high energy electron diffraction (RHEED) and ex situ atomic force microscopy (AFM) in terms of the annealing temperature and the H2S irradiation sequence. A (4 × 3) GaAs surface was obtained by annealing the substrate under H2S irradiation (4 × 10-7 Torr). The surface was atomically flat, i.e., large terraces with monolayer steps were clearly observed. A (2 × 6) S-terminated GaAs surface was obtained by irradiation H2S at 300°C on a Ga-terminated surface, which was formed by annealing at 580°C in high vacuum. The molecular beam epitaxy (MBE) growth of ZnSSe-based semiconductors on the (4 × 3) surface results in high quality structures such as a novel ZnSSe/ZnMgSSe tensile-strained quantum well (QW).

This paper examines the conduction band offset, ∆Ec, and interface charge density, σ, of disordered InGaP and GaAs heterointerfaces by controlling the AsH 3 cover time and flow rate at the growth interval from GaAs to InGaP. Short AsH, cover time (0.05 min) creates high ΔEc of 0.2 eV and low σ of 6.3×l0 10cm −2. Extending the AsH 3 cover time by 50 times cuts ΔEc to almost 0 eV and increases σ by one order. Interface morphology for long-AsH3 -cover-time samples observed by atomic force microscopy shows a terrace structure on GaAs surface, which means the surface is As rich. These results suggest that an As-poor GaAs surface is essential to achieving high-quality InGaP/GaAs heterointerfaces.

Remote radiofrequency H2 and O2 plasma processing of InP and GaAs surfaces was investigated by in situ real time spectroscopic ellipsometry. Hydrogen plasmas were used for the native oxide removal and the defect passivation of III-V surfaces. The effect of hydrogen exposure time and of crystallographic orientation (GaAs (100), (110), (111)) on the chemistry and kinetics of oxygen removal and of phosphorus/arsenic depletion was investigated. Oxygen plasma anodization was used to grow oxide films on GaAs (100), (110) and (111) substrates. The effect of bias voltage and UV-light irradiation on the chemistry and kinetics of oxidation process and on the oxide properties was studied. The composition and morphology of the InP and GaAs surfaces resulting from these plasma treatments was described.

High density plasma etching of III-V compound semiconductors is critically important to the development of advanced optoelectronic and high frequency devices. Unfortunately, the surface chemistry of these processes is not well understood. In an effort to monitor surface processes and their dependence on process conditions in a realistic etching environment, we have applied mass spectroscopic techniques for the study of GaAs etching in Cl2/Ar chemistry. Etch product chlorides were monitored, together with optical measurement of the surface temperature by diffuse reflectance spectroscopy, as pressure (neutral flux), microwave power (ion flux) and rf bias of the substrate (ion energy) were varied. Observations from the spectroscopic techniques were correlated with ex situ surface damage assessments of unpassivated surfaces by photoreflectance spectroscopy. As a result, insights are made into regions of process conditions that are well suited to anisotropic, low damage etching.

The interaction of a Cl2 plasma with a Si(100) surface has been investigated by angle resolved x-ray photoelectron spectroscopy (XPS) and spectroscopic ellipsometry. From XPS, it was found that the amount of chlorine incorporated at the Si surface increases with ion energy. Chlorine is present as SiClx (x = 1-3) with average relative coverages (integrated over depth) of [SiCl]:[SiCl2]:[SiCl3] ≅ 1:0.33:0.1. These relative coverages don’t depend strongly on ion energy between 40 and 280 eV. Real-time spectroscopic ellipsometry measurements showed that the layer present during etching is stable when the plasma is extinguished and the gas pumped away. In addition, the equivalent thickness of damaged silicon and silicon-chloride within the surface layer increases with ion energy.

Density functional calculations on cluster models of Si(100)-2xl have been used to predict reaction mechanisms and energetics for several reactive adsorption processes. Dissociative adsorption of H2, H20, BH3, and SiH4 will be described and compared to available experimental data. Based on these examples, a qualitative theory of mechanisms for dissociative adsorption of hydrides on silicon surfaces will be proposed. These reactions can largely be understood in terms of the electron density distributions in the molecule and surface dimer. On a buckled dimer, there are both electron-rich and electron-deficient sites, which have different chemical interactions with adsorbates. The role of this difference is illustrated in a novel surface Diels-Alder reaction, for which symmetric addition to an unbuckled surface dimer is allowed by orbital symmetry. This reaction creates new reactive surface sites that may be useful for subsequent chemical surface modification.

The anodic oxidation of p-Si(100) in aqueous NH4F solution is investigated in-situ by photoluminescence (PL) with short N2-laser pulses as a function of pH at constant potential. The PL intensity depends sensitively on the modification of the Si surface and on the change of the oxidation rate during current oscillations. The interruption of the anodic oxidation at the maximum of a current oscillation peak leads to a current transient which shows typical features of oxide removal and hydrogenation for two different oxide thicknesses. This makes it possible to realize an intermediate state of the Si surface with a coexisting hydrophilic (oxide) and hydrophobic (hydrogenated) part.

A novel method for generating lateral features by patterning the naturally forming surface hydride layer on Si is described. Because of the relatively strong chemical bonding between silicon and hydrogen, the hydride layer acts as a robust passivation layer with essentially zero surface mobility at ordinary temperatures. A focused electron beam from a scanning electron microscope was used for patterning. Upon losing the hydrogen passivation the silicon surface sites become highly reactive. Ideally, the lifetime of such a pattern in a clean environment should be infinite. Deliberate exposure of the entire wafer to a suitable gas phase precursor results in selective area film growth on the depassivated pattern. Linewidths and feature sizes of silicon dioxide on silicon below 100 nm were achieved upon exposure to air. The silicon dioxide is robust and allows effective pattern transfer by anisotropic wet-chemical etching. In this paper, the mechanism of hydrogen desorption and subsequent pattern formation, and the factors that govern the ultimate pattern resolution will be discussed.

Fourier transform infrared attenuated total reflection method was employed to observe Si surfaces during wet chemical treatments. We observed the time evolution of the surface chemical structure during the oxidizing of hydrogenated Si surfaces in such oxidants as ozonized water and hydrogen peroxide using (100) and (111) surfaces. We also examined the adsorption of surfactants which were introduced in an HF solution. The interaction of adsorbates at the interface and with molecules in a liquid phase was discussed based on our in situ observations.

The use of thiourea/ammonia pre-treatments on (100) InP, followed by chemical bath deposition (CBD) of CdS thin films (∼ 30 Å), with low-temperature, low-pressure chemical vapor deposited SiO2 has been shown to produce metal-insulator-semiconductor (MIS) samples with near-ideal capacitance-voltage (C-V) response. Here, we report on x-ray photoelectron spectroscopy (XPS) analysis of the near-surface of InP following pre-treatment and CdS deposition. The pre-treatment was shown by XPS to form an indium sulfide layer and effectively remove native oxides from the InP surface. The subsequent deposition of CdS on a sulfur-passivated surface forms a stable layer which protects the substrate from oxidation during SiO2 chemical vapor deposition. MIS samples prepared using the pre-treatment without CdS deposition showed improved C- V response, while samples prepared with both the pre-treatment and CdS deposition showed a dramatic reduction in the density of interface states.

A surface study of Si-doped GaAs (100) oriented wafers treated with NH4OH and HC1 following exposure to fluorine containing plasma was conducted using fourier transform infrared spectroscopy (FTIR). These treatments were observed to produce various oxidation products, such as AS2O5 and GaO. Though inorganic salts, such as (NH4)3GaF6, can be formed on the Si-doped GaAs wafers during cleaning with hydrofluoric acid buffered with ammonium fluoride, the applied cleaning method which consisted of NH4OH and HCl treatments subsequent to exposure to fluorine containing plasma did not induce formation of any inorganic salts. A small amount of hydroxide group was also presented in the samples. Water molecules and ammonium hydroxide can be sources of OH which can then be incorporated interstitially into the wafer surfaces.

Nanosized powders exhibit high specific surface areas resulting in enhanced reactivities. Surface tailoring by controlled adsorption of molecules can thus be conveniently performed and more easily monitored by surface-sensitive techniques. In situ and ex situ grafting procedures of hexamethyldisilazane (HMDS) on nanosized titania (n-TiO2) powder were carried out and studied by Fourier transform infrared spectrometry (FT-IR). In addition to a decrease of the hydrophilic OH groups, the vibration analysis revealed hydrophobic CH3 groups on the grafted samples. Co-adsorption of CO and H2O on the differently grafted samples showed a large reduction of water effect compared to the as-received n-TiO2 powder. Modulation of infrared transmitted energy by controlled adsorption of O2 and CO made it possible to qualitatively compare the electronic properties of the surface-tailored samples.

We have studied the etching effect of AlxGa1-xAs (0≤ x ≤ 0.5) by trisdimethylaminoarsenic (TDMAAs) at different substrate temperatures, and the quality of the resulting etched/regrown GaAs interface. We find that the etching rate of AlxGa1-x As decreases with increasing Al composition, and the interface trap density of the TDMAAs etched/regrown interface can be reduced by about a factor of 10 as deduced from capacitance-voltage carrier profiles. A smooth surface morphology of GaAs with an interface state density of 1.4×l011 cm−2 can be obtained at a lower in-situ etching temperature of 550°C. Moreover, by using this in-situ etching the I-V characteristics of regrown p-n junctions of Al0.35Ga0.65As/Al0.25Ga0.75As and Al0.35Ga0.65As/GaAs can be improved.

We observe the atomic structures at the multilayer step region on MOVPE-grown GaAs (001) vicinal surface using ultra high vacuum scanning tunneling microscopy (UHV-STM), and clarify that (4×2) or (4×3) like reconstruction units are dominant. Oxide free AlAs surfaces grown on GaAs vicinal surface are also successfully observed by UHV-STM. The reconstruction units at the multilayer step region on AlAs surface have the same units on GaAs vicinal surface. GaAs surface has the lack of dimmer rows on the terrace region just below the multilayer step region, while AlAs surface has dimmer rows even on the terrace just below the multilayer step region. GaAs layer growth leads tothe step bunching phenomenon and AlAs surface leads to the step debunching phenomenon.

We investigated the electron beam induced surface modification of CaF2(1 11) and initial stage of GaAs growth on the modified CaF2 surface by means of the surface photoabsorption technique and atomic force microscopy (AFM). The CaF2 surface was modified by 300 eV electron beam irradiation at 200°C in a As4 molecular beam. The amount of adsorbed As atoms increased with electron dose and it follows Langmuir adsorption principle and saturated at a value equivalent to I monolayer adsorption. In situ observation of GaAs growth on this modified surface clarified that the sticking coefficient of GaAs on CaF2 surface was drastically improved by the surface modification. AFM observation revealed that the surface roughness of initial growth of GaAs on modified CaF2 was improved at the growth temperature of 550°C

ZnSe/GaAs (001) heterovalent heteгostructures are fabricated by metalorganic vapor phase epitaxy. During the growth, both GaAs and ZnSe surfaces are kept atomically flat to achieve precise control of the interface formation. Interface composition, Ga/As, are controlled by means of either Zn or Se treatment of a GaAs surface, and then ZnSe growth follows. Consequently, it is revealed by X-ray photoemission spectroscopy (XPS) that artificial control of Ga/As from 1.0 to 2.8 leads to the variation of valence band offsets from 0.6 to 1.1 eV. Based on the electron counting model and layer-attenuation model, it is proposed that the As plane just below the interface consists of As, anti-site Ga and As vacancy.

A study of the interface chemical and physical abruptness of Si-Ge heterostructures grown on (001) Si by molecular beam epitaxy under atomic hydrogen exposure is reported. Atomic hydrogen (AH) was produced by the dissociation of molecular hydrogen interacting with a hot tungsten filament. Secondary-ion mass spectroscopy (SIMS) of structures made of alternating Ge (0.5 nm)/Si (40 nm) layers demonstrated that AH can effectively suppress Ge surface segregation. The segregation length was reduced from 1.5 nm to about 0.5 nm in films grown at a hydrogen partial pressure of −5 × 10-3 Pa and cell temperature of 2140 °C with an estimated cracking efficiency of ~5%. However, the high hydrogen background pressure had detrimental effects on the physical sharpness of the interfaces. This was evidenced by comparing the interface quality of Si/Ge atomic layer superlattices grown with and without AH exposure. X-ray reflectivity and Raman spectroscopy revealed a significant increase of the interface roughness, although the periodic character and the good crystallinity of the structures were preserved.

The adsorption and decomposition of diethylgermane, triethylgermane, and digermane on the Ge(100) surface are investigated with the intent of elucidating the surface processes leading to the deposition of epitaxial Ge films. Room temperature adsorption of diethylgermane or triethylgermane leads to the formation of surface germanium hydrides and ethyl groups. The ethyl groups decompose at higher temperatures and form ethylene via a β-hydride elimination reaction. Isotopic labeling experiments are used to confirm this reaction step. This is in contrast to the Si(100) surface where both α- and β-hydride elimination is observed for the decomposition of surface ethyl groups. The adsorption and reaction of digermane with the Ge surface is also determined to help provide a comparison with the ethylgermanes. Low energy electron diffraction is used to evaluate the quality of the deposited germanium films.

We describe physics and control of Si/SiGe heterointerfaces. A clear distinction will be made between the vertical and lateral effects of the Si/SiGe interface from the viewpoint of interface engineering. Ge surface segregation during nonequilibrium MBE growth and surfactant-mediated-growth are highlighted as prominent examples for the vertical effects while interface microroughness is addressed for the lateral effects. The influence of the interface effects on radiative recombination of indirect excitons is described in the context of SiGe-based optoelectronic applications.