Molecular Beam Flux Research Articles

Simultaneous Monitoring of Wafer- and Environment-States During Molecular Beam Epitaxy

AbstractWe report the simultaneous monitoring of the environment-state and wafer-state during epitaxial crystal growth using a single real-time measurement. Atomic absorption spectroscopy (AAS) is used to monitor the incident molecular beam flux while UV reflectance (UVR) at 396 nm with an incident angle of 78° is used to monitor growth on the wafer. We have studied the utility of AAS/UVR monitoring of AlxGa1-xAs deposition: AlAs growing on GaAs, GaAs on AlAs, and superlattice growth. Additionally, optical multichannel spectroscopy (OMS) data were acquired throughout the growth of a distributed Bragg reflector (DBR). The relationship of the structure of the real-time OMS data to absorption, optical path length variation, and differential layer thickness variations is also discussed. Numerical simulations of the real-time wafer-state monitors using pseudodielectric constants, appropriate at a growth temperature of 579 °C, show good agreement with measured spectra.

Molecular beam flux measurements and equilibria for As4, P4, As2, P2, and AsP

The dynamic equilibrium between P2, As2, and AsP at 1000 °C has been studied by mass spectrometry of fluxes from a double effusion cell. From simple ion gauge measurements of heated and unheated fluxes we determined the ionization efficiency for the dimers P2 and As2 relative to the tetramers, P4 and As4. Simultaneous mass spectrometer and ion gauge measurements of the effusion fluxes allowed the relative ionization sensitivities for P2, As2, and AsP to be determined. From the mass spectrometry data it was possible to determine the equilibrium constant 1.9±0.4 and free energy −0.55 eV molecule−1 for the process As2+P2■2 AsP.

Mean cluster size by Rayleigh scattering

Laser light scattering, associated with molecular beam flux measurements, provide an easy to perform, sensitive, non-destructive technique for the determination of relative mean sizes of molecular clusters in molecular beams formed with different expansion conditions. The results are compared to an absolute method used in the authors' laboratory and show good agreement in the range investigated (n approximately=150-4000).

Study of ALMBE growth conditions for the preparation of compositionally graded AlGaAs/GaAs structures

We have studied ALMBE growth conditions which give high quality structures with continuous or abrupt grading of Ga 1- x Al x As composition. Four 50 Å thick GaAs/Al x(i) Ga 1- x(i) As ( x( i)=0.3, 0.5, 0.7 and 1.0) quantum wells (QWs) and a direct-gap (GaAs) 9/(AlAs) 2 superlattice were stacked on the same sample by changing the supply times of cations instead of modifying molecular beam fluxes ( x resolution ∽ 1%). Linewidths of photoluminescence (PL) transitions, as narrow as 2.0–2.5 meV, suggest that interfaces grown at low temperature are pseudosmooth. We have also grown parabolically graded QWs that exhibit remarkably sharp PL linewidths and show exciton transitions, observed by optical absorption measurements, at energies very close to the expected values.

MOMBE GROWTH OF CARBON DOPED p-TYPE GaAs AND InGaAs

The growth and electrical properties of carbon doped p-type GaAs and InGaAs grown by metal-organic molecular beam epitaxy (MOMBE) using trimethylgallium (TMG), solid arsenic and solid indium as source materials have been studied systematically. The experimental results show that the growth rate and hole concentration of the samples are affected strongly by growth temperature and molecular beam fluxes,especially for InGaAs epilayers. The mechanism of carbon incorporation and its influence on the properties of the samples were also analysed based on the experimental results. It is shown that the changes in growth rate and hole concentration are mainly due to the dependence of TMG decomposition process on the growth condition.

Preparation and Characterization of CuInSe2 Thin Films by Molecular-Beam Deposition Method

Polycrystalline CuInSe2 films were prepared by coevaporation of the elements under an ultrahigh vacuum by a molecular-beam deposition method. The composition of the film was controlled by changing the In molecular-beam flux intensity while the other elements remained at a constant value. It is shown, at the substrate temperature of 500°C, that there is a critical In molecular-beam flux intensity for the fabrication of stoichiometric films. At the In molecular-beam intensities higher than the critical value, single-phase CuInSe2 films with nearly constant compositions are obtained as a result of the removal effects of excess In. It is shown that the present coevaporation process is suitable for the fabrication of stoichiometric or slightly In-rich composition films. Furthermore, the structural and electrical properties of the films were investigated and discussed in relation to film composition.

Pre- and initial stages of epitaxy in alkali halide systems: II. Interaction of molecular beams of CsCl with (100) surfaces of NaCl

The interaction of molecular beams of CsCl with (100) surfaces of NaCl is studied for crystal temperatures between 560 and 620 K and for molecular beam fluxes between 2 × 10 7 and 2 × 10 13 cm −2 s −1 by measurements of transient and steady state desorption fluxes, by static SIMS, and by electron microscopy. Also for the large Cs + ions a fast cation exchange Cs + ⇌ Na + between CsCl molecules adsorbed on the terrace and the outermost surface layer of the NaCl crystal is observed. The results for undersaturation are interpreted by incorporation of Cs + ions in the outermost surface layer of the NaCl crystal and by adsorption of CsCl molecules at the monatomic steps on the surface. The outermost surface layer proves to have a maximum capacity for Cs + of 2.4 × 10 11 cm −2. The residence time of Cs + ions in the outermost surface layer is determined as τ 0 = 1.6 × 10 −12 exp(1.49 (eV)/ kT), the time for desorption of CsCl molecules from the monatomic steps as τ 1 = 2.9 × 10 −14 exp(1.60 (eV)/ kT). By experiments with an additional NaCl flux onto the surface it is shown that τ 0 is the time for a back-exchange of Cs + ions from the outermost surface layer against Na + from NaCl admolecules. For supersaturation the growth of polymorphic CsCl islands is observed. In the first growth stages these islands show the NaCl-type structure, while for later growth stages the CsCl type structure is found. A comparative discussion of all studied alkali halide systems shows that the different results can be attributed to the different radii of guest and host cation and to lattice misfits, respectively.

Low temperature deposition of single crystalline ZnSe films on GaAs substrates by slightly ionized molecular beams

The quality of ZnSe films epitaxially grown on (100) GaAs substrates from slightly ionized molecular beams was found to strongly depend on substrate bias, substrate temperature and beam flux ratio. The GaAs substrates were chemically etched and not further treated in the growth chamber before growth. The ionization of the molecular beam fluxes caused an obvious increase of the growth rate and was effective for removing contamination of the substrate surface. A (100) ZnSe film with a streaked RHEED patterns grew with a substrate bias of 40 V, a substrate temperature of 300°C and a beam ratio of the element beam fluxes of 1.

A detailed study of the molecular beam flux distribution of MBE effusion sources

We report experimental investigations of the thickness distribution of MBE-layers using effusion sources with crucibles of cylindrical and conical shape. The influence of various parameters—the filling level of the molten materials, the evaporation rates, the inclination of the crucible, the crucible shape and, the distance between source and substrate—is studied in detail. Starting from the experimental data we demonstrate an estimation of the thickness distribution taking into account the interaction of the vapour particles.

Atomic-layer epitaxy of (100) CdTe on GaAs substrates

We present results of atomic-layer epitaxy (ALE) growth of (100) oriented CdTe on (100) GaAs substrates. It is shown that growth of exactly one monolayer per reaction cycle is possible in a temperature range between 260 and 290° C. In this range, the growth rate is independent of the substrate temperature. At higher temperatures, a second range with temperature independent growth rate exists, where the average coverage per reaction cycle is only 0.5 monolayers. A model is presented which describes this behavior and leads to the prediction that the stability range of the ALE process can be shifted in temperature by changing the molecular beam fluxes.

Interaction of molecular beams of RbCl with (100) surfaces of NaCl

The interaction of molecular beams of RbCl with (100) surfaces of NaCl is studied for crystal temperatures between 561 and 669 K and for molecular beam fluxes between 5 × 10 9 and 3.9 × 10 13 [cm −2 s −1] by measurements of transient desorption fluxes, by SIMS and by electron microscopy. A very fast ion exchange Rb + ⇋ Na + between admolecules and the crystal surface is observed. The results for undersaturation are interpreted by a model which considers this ion exchange and a maximum capacity of the surface for Rb + ions which results in 2.3 × 10 13 cm −2. Additional adsorption of RbCl at the steps is discussed. The resulting desorption times obey Arrhenius equations. At supersaturation for a saturation ratio of S = 3.5 at 595 K layer growth is found up to a deposition of ≈ 10 ML, then ramp-like crystallites with orientations parallel to the [100] directions are formed. For S = 10 the resulting surface topographies are interpreted by the Stranski-Krastanov mode. The formation of RbClNaCl solid solutions in very thin surface layers is discussed.

In situ determination of Ba, Y, Cu and O 2 flux by cold cathode discharge induced emission spectroscopy

An advanced multiple molecular beam flux monitoring system under oxygen ambient is developed by using cold cathode discharge induced photo-emission spectroscopy (CCES). In the oxygen ambient, characteristic emission peaks can be detected with Ba, Y and Cu, respectively. However, many O 2 emission peaks are observed over 300 to 600 nm, which interfere with the other metal peaks. In order to determine the actual evaporation rate of metal, the metal peak height is calibrated by substracting the increment of oxygen at the characteristic metal wavelength, which is introduced by the height of individual oxygen peak.

Angular distributions of molecular beam flux. 2nd report. Evaporation from a cylindrical crucible.

Evaporation into a vacuum from a cylindrical crucible partially filled with liquid is studied by use of the direct-simulation Monte Carlo method. The evaporation rate is obtained in the near continuum to the free molecular regime for l/a =1, 3 and 5, where a is the inner radius of the crucible and l is the distance from the crucible exit to the liquid level. When l is small, the evaporation rate does not change so much with the Knudsen number Kn. The flow structure near the crucible is similar to that of the usual free jet expansion. In the far field, the density and radial velocity of gas and the probability of finding a molecule in the unit solid angle are calculated as a function of the polar angle. There flow properties in the far field are also obtained in the case of sonic efflux from a circular orifice.

Dimers and tetramers of SiCl in a molecular beam from a heterogeneous, intermediate-pressure reactor

The molecular-beam flux from the reaction between trichlorosilane SiHCl3 and a mixture of liquid sodium and potassium has been studied by mass spectrometry. The carrier gas was He, at a pressure of 100–700 mbar. It is demonstrated that various heterogeneous reactions take place in the reactor at 400–600 K. The formation of the molecules Si2Cl2 and Si4Cl4 is observed, and the cracking pattern of the dimeric molecule is measured.

Optimum growth condition of single-crystalline undoped ZnS grown by the molecular-beam-epitaxial method using a H2S gas source

Crystal characterization by photoluminescence and x-ray diffraction has been performed to find out the optimum growth condition of ZnS grown by the gas-source molecular-beam-epitaxial (MBE) method. In this crystal growth, important parameters are the cracking temperature of H2S gas (Tcr), the film thickness (d), both molecular-beam flux intensities (JZn and JΣS or inlet H2S gas pressure Pin), and substrate temperature (Tsub). A single crystal having a fairly high quality, whose half width of x-ray diffraction FWHM is about 0.119° (the reference value of the GaAs substrate used is 0.093°), is obtained under the growth conditions Pin=1.4×10−5 Torr (corresponding flux intensity JΣS=7×1015 cm−2 s−1), JZn/JΣS=1, and Tsub=325 °C in the case of Tcr=780 °C and d≂1 μm.

ELECTRON BEAM SOURCE MOLECULAR BEAM EPITAXY OF AlxGa1-xAs GRADED BAND GAP DEVICE STRUCTURES

ABSTRACT A new method has been developed for the growth of graded AlxGai_xAs alloys by molecular beam epitaxy which is based upon electron beam evaporation of the Group III elements. The metal evaporation rates are measured real-time and feedback controlled using beam flux sensors. The system is computer controlled which allows precise programming of the Ga and Al evaporation rates. The large dynamic response of the metal sources enables for the first time the synthesis of variable Alx Ga!_xAs with arbitrary composition profiles. This new technique has been demonstrated in the growth of unipolar hot electron transistors, graded base bipolar transistors, and M- shaped barrier superlattices.The optical and electronic properties of Alx Gai_xAs/GaAs heterojunctions and quantum wells have been extensively studied in materials grown by molecular beam epitaxy (MBE). There are, however, a number of interesting structures which have been proposed which require precise alloy grading over atomic dimensions. The ability to tailor band structure to obtain novel electrical and optical properties is termed band-gap engineering11 .'1 ' Previous attempts to obtain alloy grading by varying the temperature of MBE effusion cells have been only marginally successful and have serious limitationsJ2~ 41 This is related to the large thermal inertia and restrictions in heating and cooling rates of the MBE effusion cells which leads to slow modulation of the beam flux along with a time lag in response. In addition, variations in the growth rates determined by the Group III molecular beam fluxes seriously complicates the problem of obtaining arbitrary alloy grading. Thus Group III sources with large dynamic response are required for band gap engineering of A^Ga^xAs. Several different approaches could be used which include electron beam heated sources which are typically used to evaporate Si and refractory metals,I5' gas sources,'6' and ion beam sources.'7'

Molecular Beam Flux Monitor Using Cold Cathode Discharge Induced Emission Spectroscopy

Cold Cathode discharge induced Emission Spectroscopy (CCES) was developed in order to apply a multiple molecular beam flux monitor for molecular beam epitaxy. The response characteristics of the emission intensity were in proportion to the molecular beam flux intensity under the MBE growth condition. The arsenic atomic emission intensity was satisfactorily, corresponding to the As4 molecular beam pressure. The detectable wavelengths of Al, Ga and As were found to be λ (Al)=309 nm, λ (Ga)=294, 417nm and λ (As)=194nm, which did not interfere with each other. When the growth rate of Al was at 0.1 nm/s, and the CCES had an accuracy of 0.1±0.0023 nm/s. The CCES directly controlled the Ga beam flux. The Ga beam pressure directly reflected the response time of the CCES control.

Angular distributions of molecular beam flux. (1st report. Evaporation from a two-dimensional crucible).

The angular distributions of molecular beam flux evaporated from a two-dimensional crucible partially filled with liquid are studied by use of the direct simulation Monte Carlo method. As the Knudsen number increases, the molecular flux is collimated near the center-line of the crucible. The collimation also increases as the liquid level in the crucible lowers. The thickness distributions of the film deposited on the holder surface perpendicular to the center-line of the crucible are calculated by assuming that every molecule incident on the holder is deposited on it. When the distance between the crucible exit and the holder is 50 times the crucible width, the thickness distribution can accurately be estimated by regarding the crucible as a point evaporation source.

MBE growth mechanisms of ZnSe.

The effects of molecular beam flux and substrate temperature on the MBE growth rate of ZnSe are studied. It is found that, at relatively high substrate temperatures, the growth rate is influenced by the desorption and adsorption of Se and Zn atoms. The reduction of growth rate results from Se atom desorption, Zn atom desorption, and both Zn and Se atoms desorption at flux rations, JZn/JSe>1, <1, and ≅1, respectively. The activation energy values are 1.2eV for the desorption of Zn atoms and 0.86eV for the desorption of Se atoms. Three MBE growth models were applied to explain the present results. A model considering the desorption of Zn and Se atoms from precursor states is proposed in this paper and it can best explain the relationship between the growth rate and the substrate temperature or the flux ratio.

Calculations of Molecular Beam Flux from Liquid Source

A calculation of the MBE flux from slantingly arranged crucibles containing liquid sources such as Ga and Al has been performed. The calculated and experimental results are in good agreement. The results also indicate that cylindrical crucibles are more effective in improving the uniformity of the epitaxial thickness than conical crucibles.