Oxide Desorption Research Articles

Growth of GaAs on hydrogen-fluoride treated Si (100) surfaces using migration-enhanced epitaxy

Heteroepitaxial growth of GaAs has been successfully achieved for the first time using migration-enhanced epitaxy (300 °C) on HF treated Si substrates which have been subjected to an initial in situ pre-heat treatment in the range of 350–620 °C and without the customary high-temperature Si oxide desorption step. High quality GaAs on Si exhibiting double-crystal x-ray rocking curve full-width at half-maximum value of 280 arcsec was obtained by using two-step growth technique MEE(300 °C)/MBE (580 °C). Secondary-ion mass spectrometry studies indicated 0.05 (monolayer) ML of oxygen at the interface of GaAs and HF treated Si substrate. The photoluminescence spectra at 10 K for GaAs grown on HF treated Si substrates was dominated by sharp excitonic related peaks and no carbon acceptor related emission could be detected.

Modeling and Simulation of a Spray Column for NO x Absorption

Absorption of nitrogen oxides was studied in a spray column of 1.25 m i.d. and 11.5 m height. Mixed-acid solutions of nitric and sulfuric acid were used as absorbents. Specific rates of absorption were measured using a stirred cell with a flat interface. The mathematical model for the adiabatic absorption included: the gas phase reaction and equilibria, gas phase mass transfer, equilibria at interface, liquid phase reactions and associated heat effects. The following outstanding features were also incorporated: heterogeneous equilibria and its role in computing the volumetric absorption rates, formation of HNO 3 , HNO 2 in the gas phase and their mass transfer to/from liquid phase, dependence of H(kD ) 1/2 on the concentration of HNO 3 with/without other electrolytes and complete energy balance. The spray column has been modeled as a countercurrently operating mass transfer equipment with known initial conditions. Concentrations and flow of the gas stream at the inlet (bottom) of the column were known. Values of the concentration and the temperature of the liquid phase being withdrawn from the bottom of the column were also specified. To begin computing a temperature and concentration profile, a differential element along the height of the column is considered. Equilibrium in the bulk of the gas phase of this differential volume were computed by the Newton-Raphson iterative method. Interfacial partial pressures of water and nitric acid were interpreted from a database by a Newton—Gregory forward-backward difference method. Volumetric mass transfer rates in the gas and the liquid phases were computed by solving two film theory model equations. These rates are used in evaluating derivatives with respect to height and then integrated numerically by using a fourth-order Runge—Kutta method, establishing mass balance across the differential height. Heat changes due to equilibria in the gas phase, absorption and desorption of nitrogen oxides and evaporation of water were computed to estimate the temperature of the incoming liquid phase. Gas phase equilibria are computed for the next element. These computations were repeated for subsequent differential elements until they added up to the desired height. A favorable agreement has been shown between the model predictions and the experimental observations.

Laser desorption on Si surfaces by use of multiple excimer laser pulses

Trains of temperature pulses generated by multiple excimer laser irradiation (KrF 248 nm; pulse length 25 ns) were used to realize the desorption of carbon and oxygen bonded on <100> Si surfaces. The effect of breaking bonds and desorption is analyzed by use of in-situ Auger electron spectroscopy (AES) as a function of the energy density and the number of applied laser pulses. Results will be presented for the laser-induced desorption of native oxides and specially prepared weak oxides on silicon surfaces.

In situ ellipsometric studies of optical and surface properties of GaAs(100) at elevated temperatures

A rotating-polarizer ellipsometer was attached to an ultrahigh vacuum (UHV) chamber. A GaAs(100) sample was introduced into the UHV chamber and heated at a number of fixed elevated temperatures, without arsenic overpressure. In situ spectroscopic ellipsometric (SE) measurements were taken, through a pair of low-strain quartz windows, to monitor the surface changes and measure the pseudodielectric functions at elevated temperatures. Real-time data from a GaAs surface covered with native oxide showed clearly the evolution of oxide desorption at approximately 580°C. In addition, surface degradation was found before and after the oxide desorption. An oxide free and smooth GaAs surface was obtained by depositing an arsenic protective coating onto a molecular beam epitaxy grown GaAs surface. The arsenic coating was evaporated immediately prior to SE measurements. A comparison showed that our room temperature data from this GaAs surface, measured in the UHV, are in good agreement with those in the literature obtained by wet-chemical etching. The surface also remained clean and smooth at higher temperatures, so that reliable temperature-dependent dielectric functions were obtained.

Arsenic pressure variations during oxide desorption from gallium arsenide prior to epitaxial deposition

Prior to epitaxial deposition of gallium arsenide by molecular beam epitaxy it is necessary to remove surface oxide films in situ. Changes in the system pressure can be measured during the desorption process. The largest pressure changes are found for ion gauges that can directly measure the flux of arsenic atoms that are reflected or scattered from the substrate surface. It is found that there is a reproducible drop in apparent pressure of ∼25% as the oxide is desorbed, which is independent of whether an As2 or As4 overpressure is used to stabilize the surface. There are, however, qualitative differences between the two arsenic species, which can be attributed to the way they interact with the GaAs surface. Nevertheless, the pressure changes correlate precisely with the oxide removal, and they provide a reproducible monitoring tool for the calibration of substrate temperatures and the establishment of appropriate epitaxial growth conditions.

Photoinduced dissociation and desorption of nitrous oxide on a platinum(111) surface

N{sub 2}O adsorbed on a Pt(111) surface was irradiated by UV light from a mercury arc lamp. The photochemistry of N{sub 2}O was studied by XPS, UPS, and TPD. Upon irradiation at 50 K, adsorbed N{sub 2}O undergoes dissociation and desorption. Photon energies exceeding 4.35 eV are required. The cross section is in the range of 10{sup {minus}19}{minus}10{sup {minus}20} cm{sup 2}. The data are adequately described with a model involving subvacuum hot electrons.

Oxide desorption from InP under stabilizing pressures of P2 or As4

The oxide desorption processes of InP have been investigated under molecular-beam epitaxy conditions, determining thickness and composition of the surface layer by x-ray photoelectron spectroscopy and surface reconstructions by reflection high-energy electron diffraction (RHEED). Under P2 stabilization the oxide desorbs within 5 min at 490 °C and a (2×4) reconstruction is observed. On the other hand, heating in a flux of As4 molecules results in the formation of an InAs layer at the top of the oxide, which strongly reduces the oxide desorption rate. The complete removal of the oxygen requires at least 520 °C and typically leaves a 1-nm-thick overlayer of InAs. In this case RHEED gives no indication whether the surface is oxide free.

370 °C clean for Si molecular beam epitaxy using a HF dip

We describe a new low-temperature clean for Si molecular beam epitaxy. Growth is carried out on Si wafers subjected to an ≊10–60 s clean in a buffered HF solution prior to insertion in the growth chamber. We demonstrate low defect densities (&amp;lt;105 cm−2) at 380 °C without either the conventional high-temperature cleaning step to desorb a chemical oxide or the use of a glovebox for chemical cleaning and transfer to the vacuum chamber. Wafers are given an ≊200 °C prebake in situ to remove hydrocarbons, and then raised to the growth temperature prior to deposition. For (100) substrates the transition from amorphous deposition to crystalline growth occurs at ≊370 °C, below the temperature at which hydrogen should desorb. On (111) the minimum temperature for epitaxy is ≊500 °C, as expected. We attribute this difference to the large number of undercoordinated Si atoms present on (100), which allows growth to take place even on the hydrogen-terminated surface. Secondary-ion mass spectrometry suggests that contamination levels at the substrate-epilayer interface using this HF dip are comparable to those for conventional (‘‘Shiraki’’) chemical oxide desorption techniques. Omission of the prebake step leads to a highly carbon-contaminated surface: this surface has the remarkable property that Si will grow epitaxially on it, but does not wet it (so that at high temperature Si/Si:C grows as islands).

Oxide thickness effect and surface roughening in the desorption of the oxide from GaAs

The temperature for thermal desorption of the gallium oxide from GaAs is shown to increase linearly with oxide thickness. In addition, we show by diffuse light scattering that highly polished GaAs substrates roughen during the oxide desorption. These results are interpreted in terms of a model in which the oxide evaporates inhomogeneously.

Carbon removal from as-received Si samples in ultrahigh vacuum using ultraviolet light and an ozone beam

A novel Si-sample cleaning method is described. This low-temperature process uses ultraviolet (UV) radiation and an ozone beam to remove carbonaceous components from the native oxide of as-received Si samples. Because the cleaning process is implemented as an ultrahigh vacuum (UHV) process in a UHV process chamber, the sample does not need to be reexposed to atmospheric conditions between the carbon removal and the desorption of the native oxide, which results in very low contamination levels. The efficiency of this carbon removal-method is determined.

GaAs wafers for direct use in epitaxy

Abstract The direct use of GaAs substrates in epitaxy requires a high standard of surface quality. Low contamination levels are necessary and the inevitable oxide and carbon layers must be removable in the thermal oxide desorption step. A new approach is described using a passivating hydrocarbon coating as a shield against undesired adsorptions during storage and transfer.

Growth of gallium arsenide on hydrogen passivated Si with low-temperature treatment (∼600 °C)

Epitaxial growth of GaAs on Si commonly employs a high-temperature (≳850 °C) oxide desorption step. In this letter, we report the first epitaxial growth of GaAs on Si without the need for this high-temperature treatment. This method utilizes a final HF treatment whereby the Si surface dangling bonds are terminated by hydrogen with a resultant (1×1) bulk-like surface structure. Upon medium temperature heat treatment (×500 °C), hydrogen leaves the surface leading to the common orthogonal 2×1 surface reconstruction. High quality GaAs epitaxial layers were successfully grown on these 2×1 reconstructed Si surfaces with the pregrowth substrate preparation temperatures of as low as 600 °C.

Scanning tunneling microscope study on mid-desorption stages of native oxides on Si(111)

Scanning tunneling microscope images lead us to the conclusion that the thermal desorption of native oxides on Si(111) does not occur layer by layer but through the formation and lateral growth of voids. This conclusion is consistent with results obtained by reflection high-energy electron diffraction, i.e., that in desorption stages, a (7×7) pattern appears with streaky diffraction spots due to small diffraction areas and with a background due to an amorphous oxide film on the surface. It also agrees with Auger electron spectroscopy results, that is, the temperature dependence of the oxide area before and during desorption. Nucleation centers for voids have not been identified.

Studies of Oxide Desorption From GaAs by Diffuse Electron Scatfering and Optical Reflectivity

ABSTRACTWe have determined that the temperature for desorption of gallium oude from GaAs increases linearly with oxide thickness, for oxide layers between about 6Å and 26Å thick. Different thicknesses of oxide layers were created by varying the exposure time of the GaAs wafers to a low pressure oxygen plasma. In addition, we show by diffuse light scattering that highly polished GaAs substrates roughen during the oxide desorption. These results are interpreted in terms of a model in which the oxide evaporates inhomogeneously. The oxide desorption was also studied by monitoring the secondary electrons produced by the high energy electrons from the RHEED gun. After the gallium oxide desorption there is a reversible, order of magnitude, increase in the number of secondary electrons produced. We interpret this result as evidence for the formation of microscopic gallium droplets on the GaAs surface.

Studies on the kinetics and mechanism of the reaction of no with carbon

The mechanism of the reaction of NO with carbon was examined, with a particular focus on the role of desorption of oxides from the surface. The carbon was an ex-phenol formaldehyde char of low impurity levels, the temperature range studied was 773–1073 K, and the partial pressure of NO was varied from 1 to 10.1 kPa. The reaction was characterized by two different mechanistic regimes. The low temperature regime (<923 K) was characterized by a non-constant activation energy in the range 63–88 kJ/mol, and the activation energy appears related to the activation energy distribution for desorption of surface oxides. The high temperature regime was governed by a product release step involving participation of NO, and exhibited by an activation energy of 180 kJ/mol. The desorption process was modeled using distributed activation energy kinetics.

In-situ Measurement of GaAs Optical Constants and Surface Quality, as Functions of Temperature

ABSTRACTIn-situ spectroscopic ellipsometry (SE) was applied to monitor GaAs (100) surface changes induced at elevated temperatures inside an ultrahigh vacuum (UHV) chamber (&lt;1×10−9 torr base pressure, without As overpressure). The real time data showed clearly the evolution of the native-oxide desorption at ∼577°C, on a molecular-beam-epitaxy (MBE)-grown GaAs (100) surface. In addition, surface degradation was found before and after the oxide desorption. A clean and smooth surface was obtained from an arsenic-capped, MBE-grown GaAs sample, after the arsenic coating was evaporated at ∼350 °C inside the UHV. Pseudodielectric functions &lt;ε&gt;GaAs, from 1.6 eV to 4.5 eV, were obtained through the SE measurements, from this oxide-free surface, at temperatures ranging from room temperature (RT) to ∼610 °C. These &lt;ε&gt; data were used as reference data to develop an algorithm for determining surface temperatures from in-situ SE measurements, thus turning the SE instrument into a sensitive optical thermometer.

Application of ellipsometry to crystal growth by organometallic molecular beam epitaxy

We report the first use of ellipsometry as a real-time monitor of III-V semiconductor crystal growth by molecular beam epitaxy, specifically growth of GaAs and AlGaAs from arsine, triethylgallium, and triethylaluminum sources. Our results provide new insight into the oxide desorption process and show a sensitivity of ±0.03 in compositions x&amp;gt;0.2 for 10 Å thickness increments of AlxGa1−xAs during initial deposition on GaAs.

Changes in surface composition and Fermi level position during thermal desorption of ultraviolet radiation/ozone formed oxides on GaAs

X-ray photoelectron spectroscopy (XPS) has been used to study the native oxide structure and surface Fermi level of GaAs exposed to ultraviolet light (UV)/ozone, as well as to monitor changes that occur during thermal desorption of the surface oxide. The XPS results confirm that the surface oxide consists of a mixture of gallium and arsenic oxide phases which desorb at two different temperature ranges: desorption of arsenic oxide phases and oxygen transfer from arsenic to gallium occur at 250–500 °C; and desorption of gallium oxide phases occurs at 550–600 °C. In addition, we found that the surface Fermi level of GaAs below the UV/ozone formed oxide was always at about 0.8 eV from the valence- band maximum (VBM) for n-type substrates and 0.5 eV for p-type substrates. Similar results were found for wet chemically treated and then air exposed GaAs surfaces. However, the surface Fermi levels of both n- and p-type substrates, during the desorption of arsenic oxide in the temperature range of 250–500 °C, moved further away from VBM by as much as 0.4 eV. When the samples were heated to higher temperatures, the surface Fermi level moved back to midgap even before complete desorption of the gallium oxide phases. Fermi level pinning on samples before and after oxide desorption has also been characterized by in situ deposition of 5 nm of silicon nitride on the sample surface in the XPS system and by monitoring the surface potential of the semiconductor with XPS as a function of charging potential on the dielectric. It was found that surfaces both before and after arsenic oxide desorption were pinned within 0.1 eV with a negative voltage of about 2 V on the thin dielectric.

Capping and decapping of InP and InGaAs surfaces

An investigation of techniques for the preparation and preservation of stoichiometric InGaAs and InP surfaces, and techniques for regenerating these surfaces in vacuum have been carried out using x-ray photoelectron spectroscopy for surface characterization. It was found that InGaAs stoichiometric surfaces grown by molecular-beam epitaxy (MBE) can be preserved from oxidation and contamination by condensation of a thin layer of arsenic in the MBE chamber. The stoichiometric surfaces can be regenerated by thermal desorption of the arsenic cap in vacuum. Although stoichiometric InP surfaces can be prepared and arsenic capped in the MBE chamber, interfacial alloying between the arsenic cap and the substrate occurred during the thermal desorption process. Stoichiometric InGaAs and InP surfaces can also be preserved by a UV/ozone-formed oxide overlayer. For the removal of ozone-formed native oxides, thermal desorption at 458 °C regenerated clean and stoichiometric InP surfaces with a band bending of less than 0.3 eV on both n- and p-type substrates. However, complete oxide desorption on InGaAs required a temperature, above 565 °C, at which the surface decomposed. It was found that the ozone-formed surface oxides on InGaAs and InP could be removed at a substrate temperature below about 250 °C using a remote hydrogen plasma. The resultant n-InGaAs substrate surface was close to flat band, whereas the surface Fermi levels of the n- and p-type InP substrates were at 0.4 and 0.7 eV from the conduction-band minimum, respectively.

GaAs(100) Surface Modifications at Elevated Temperatures, Studied By In-Situ Spectroscopic Ellipsometry

ABSTRACTSpectroscopic ellipsometric (SE) measurements of GaAs (100) were carried out in an ultrahigh vacuum (UHV) chamber, without arsenic overpressure, at temperatures ranging from room temperature (RT) to ∼610°C. Surface changes induced at elevated temperatures were monitored by in-situ spectroscopic ellipsometry. The SE data clearly displayed in real time the process of desorption of the GaAs-surface-oxide overlayer at ∼580°C. In addition, changes in the near-surface region were observed before and after the oxide desorption. The near-subsurface region (top 50–100 Å) became less optically dense after being heated to 540°C or higher. For comparison, a pre-arsenic-capped molecular-beam-epitaxy (MBE)-grown GaAs surface was also studied. After the arsenic cap was evaporated off at ∼350°C, this surface remained smooth and clean as it was heated to higher temperatures.