Photoemission Oscillations Research Articles

Photoemission oscillation in epitaxially grown van der Waals β-In2Se3WS2 heterobilayer bubbles* *Project supported by the National Natural Science Foundation of China (Grant Nos. 51732010 and 51972280), the Natural Science Foundation of Hebei Province, China (Grant No. E2019203233), the Research Program of the College Science & Technology of Hebei Province, China (Grant No. ZD2020121).

Thin films of millimeter-scale continuous monolayer WS2 have been grown on SiO2/Si substrate, followed by the deposition of β-In2Se3 crystals on monolayer WS2 to prepare In2Se3/WS2 van de Waals heterostructures by a two-step chemical vapor deposition (CVD) method. After the growth of In2Se3 at elevated temperatures, high densities of In2Se3/WS2 heterostructure bubbles with monolayer to multilayer β-In2Se3 crystals atop are observed. Fluorescence of the resultant β-In2Se3/WS2 heterostructure is greatly enhanced in intensity upon the formation of bubbles, which are evidenced by the Newton’s rings in optical image owing to constructive and destructive interference. In photoluminescence (PL) mapping images of monolayer β-In2Se3/monolayer WS2 heterobilayer bubble, significant oscillatory behavior of emission intensity is demonstrated due to constructive and destructive interference. However, oscillatory behaviors of peak position are also observed and come from a local heating effect induced by an excitation laser beam. The oscillatory mechanism of PL is further verified by changing the exterior pressure of bubbles placed in a home-made vacuum chamber. In addition, redshifted in peak position and broadening in peak width are observed due to strain effect during decreasing the exterior pressure of bubbles.

<i>In Situ</i> Simulation by RHEED and Photoemission of GaAs (001) β<sub>2</sub>(2x4) Reconstructed Surface

In situ monitoring of surface processes and understanding of growth processes are important in achieving precise control of crystal growth. Therefore, many surface monitoring techniques are used during crystal growth by molecular beam epitaxy (MBE). The most popular is reflection high-energy electron diffraction (RHEED) and photoemission current which provides information on the morphology during the growing surface. The photoemission oscillation technique has been successfully used in situ to monitor the growth of materials and to control the thickness as well as the roughness of the deposited layer. In this paper, we report results of atomic scale simulations used to study the dynamics of homoepitaxial growth of GaAs(001) β2(2x4) reconstructed surface and, in particular, the RHEED oscillations of the photoemission current.

Evidence for the short-period oscillations in spin-resolved photoemission of thin Cr(110) films

The spin-resolved photoelectron spectroscopy study of the Cr/Fe(110) interface are presented. The system was investigated at room temperature for different thicknesses of Cr overlayer. As a result, we observe the initial fast drop, followed by a weak oscillatory behavior, of the photoelectron spin-polarization (P) at the Fermi level, EF. The observed period of weak oscillations of P give an evidence for the first time observation of short-period oscillations in Cr(110) oriented thin films.

Evidence for the short-period oscillations in spin-resolved photoemission of thin Cr(110) films

The spin-resolved electronic structure of thin Cr overlayers on top of the Fe(110) surface was investigated by means of spin- and angle-resolved photoelectron spectroscopy. The initial fast drop of photoelectron spin-polarization at the Fermi level, followed by weak oscillatory behavior with the period of about 2 ML, can give an evidence for the first time spectroscopic observation of the short period oscillations in (110)-oriented thin Cr films.

Short-period oscillations in photoemission from thin films of Cr(100)

Angle-resolved photoemission (PE) study of thin films of Cr grown on Fe(100) reveals thickness-dependent short-period oscillations of the PE intensity close to the Fermi energy at ${\mathbf{k}}_{\ensuremath{\Vert}}\ensuremath{\sim}0$. The oscillations are assigned to quantum-well states (QWS) caused by the nesting between the Fermi-surface sheets around the $\ensuremath{\Gamma}$ and the $X$ points in the Brillouin zone of antiferromagnetic Cr. The experimental data are confirmed by density-functional calculations applying a screened Korringa-Kohn-Rostoker Green's function method. The period of the experimentally observed QWS oscillations amounts to about 2.6 monolayers and is larger than the fundamental 2-monolayer period of antiferromagnetic coupling in Cr.

Oscillations in photoemission from Cr/Fe/W(100) and Cr/W(100)

The electronic structure of thin Cr films on Fe/W(1 0 0) and W(1 0 0) has been studied by angle-resolved photoemission (PE). The data have been compared with results of density functional supercell calculations. A thickness-dependent PE intensity variation at the Fermi level is observed, which is assigned to Cr quantum-well-like states.

Oscillations in photoemission from Cr/Fe/W(1 0 0) and Cr/W(1 0 0)

The electronic structure of thin Cr films on Fe/W(1 0 0) and W(1 0 0) has been studied by angle-resolved photoemission (PE). The data have been compared with results of density functional supercell calculations. A thickness-dependent PE intensity variation at the Fermi level is observed, which is assigned to Cr quantum-well-like states.

Effect of As 2 physisorbed molecules on the photoemission current during growth: simulation of GaAs and GaAlAs deposition

We report results of atomic scale simulations on the dynamics of photoemission current during growth of GaAs and AlGaAs on (001) GaAs substrate. The physisorbed nature of As 2 molecules is explicitly taken into account. The simulations show that under As-riche conditions, the maximum of photoemission current occurs at half monolayer growth as reported experimentally [H. Tsuda, J. Appl. Phys. 73 (1993) 226]. However, the oscillations show the same period reflection high-energy electron diffraction (RHEED). We also illustrate that photoemission oscillations can be used to determine the alloy composition of Al x G 1− x As in agreement with experimental results [J.J. Zinck, D.H. Chow, Appl. Phys. Lett. 66 (1995) 3524].

Closed-loop control of resonant tunneling diode barrier thickness using in situ spectroscopic ellipsometry

We examine the use of spectroscopic ellipsometry (SE) for fully automated, in situ real-time control of the barrier thickness for resonant tunneling diodes (RTDs) grown by molecular-beam epitaxy (MBE). The RTDs in this study utilize AlAs barriers, an InGaAs well with an InAs subwell, and lattice-matched InGaAs spacer layers, grown on an InP(100) substrate. Pseudodielectric functions for the strained AlAs barriers were generated from SE data acquired during MBE growth, using simultaneous photoemission oscillation measurements for in situ thickness calibration. For closed-loop control, the measured dielectric functions were utilized in conjunction with a virtual substrate model to derive the instantaneous layer thickness from real-time SE data. Repeatability was tested by growing several series of AlAs barriers and complete RTDs under fully automated control, with shutter actuation based on the real-time thickness determined from SE. The results show that by using SE for real-time control, barrier thickness repeatability on the order of ±0.1 ML can be achieved.

Monolithic integration of resonant tunneling diodes and heterojunction bipolar transistors on patterned InP substrates

We report the successful fabrication of coplanar, monolithic integrated circuits based on heterojunction bipolar transistors (HBTs) and resonant tunneling diodes (RTDs). Molecular beam epitaxy growth of the integrated device structure is performed on an InP wafer, which is patterned by photolithography and dry etching prior to growth. The substrate pattern consists of pedestals, which support RTDs after postgrowth device fabrication, and a lower level which ultimately supports HBTs. In situ sensors for layer thickness (photoemission oscillations) and substrate temperature (absorption-edge spectroscopy) play a critical role in controlling key growth and structural parameters for the HBT and RTD devices. Utilization of the photoemission oscillation sensor during deposition of the AlAs barrier layers of the RTD structure has enabled control of peak currents to a design tolerance of ±20% for devices with current densities in the mid-104 A/cm2 range. Implementation of proportional–integral–derivative control based on a transmission-mode absorption-edge spectroscopy sensor has enabled regulation of the substrate temperature to a precision of ±1 °C throughout deposition of the RTD/HBT structure.

Short-period oscillations in photoemission from Cu films on Co(100)

The electronic structure of Cu films on Co(100) in the $k$-space region associated with the short-period oscillation of the interlayer coupling in Cu/Co(100) multilayers has been examined by spin- and angle-resolved photoemission and by ab initio calculations. The photoemission intensity near the Fermi energy exhibits a rapid oscillatory behavior as a function of film thickness with a period close to that of the interlayer exchange coupling. The photoemission intensity and polarization reflect a modulation of the density of states induced by spin-polarized quantum-well states, in good agreement with ab initio calculations of quantum-well states.

Effects of morphology on photoemission oscillation measurements during growth of resonant tunneling devices

Photoemission oscillations (PEO) are measured during the growth of InAs/AlSb/GaSb resonant tunneling diodes on (0 0 1) GaAs and (0 0 1) InAs substrates. PEO of the GaSb well are unresolved for growth on GaAs substrates but clearly observable for growth on InAs. AlSb barrier oscillations are observable on either substrate. The morphology of devices grown on GaAs is characterized by micron scale roughness whereas the devices grown on InAs are featureless on the same scale. Threading dislocations which terminate at the GaAs surface with a partial step are believed to influence the growth mode of GaSb but not AlSb due to the higher mobility of Ga in this system.

Real-time Monitoring of semiconductor growth by Spectroscopic ellipsometry

ABSTRACTin situ Spectroscopic Ellipsometry (SE) is an optical technique which is well suited for the monitoring of epitaxial semiconductor growth, due to its high surface sensitivity and non-invasive nature. In this work, SE systems were installed on both MBE and MOCVD deposition systems to monitor the epitaxial growth of InxGa1−xAs and InxAl1−xAs compounds on InP substrates. The structures grown include thick lattice matched In0.53Ga0.47As buffer layers (for HBT collectors), and strained RTD structures. SE was used to monitor in real-time layer composition and thickness during growth. To enhance the precision and accuracy of the SE determined growth parameters, it was necessary to optimize the SE data analysis strategies. A methodology to determine the best spectral region for the SE data analysis in the presence of noise and systematic effects (such as angle of incidence uncertainty, detector wavelength shifts, surface roughness, uncertainty in surface temperature, non-ideal growth modes, etc.) is presented. Using the optimized data analysis strategies, long term SE-determined InxGa1−�As composition accuracy (as verified by ex situ x-ray measurements) of ±0.002 in ‘x’ was achieved. SE thickness measurements of ultra-thin (<30Å) strained AlAs barrier layers were also in excellent agreement (±0.5Å) with real-time photo-emission oscillation measurements.

Photoemission oscillations as an in situ monitor of layer thickness with monolayer resolution

Photoemission oscillations have been observed during the growth of InAs and AlSb on (001) GaAs substrates. The ability to observe photoemission oscillations is dependent on the growth conditions in a manner similar to reflection high-energy electron diffraction oscillations. Consequently, the mechanism responsible for photoemission oscillations is believed to be related to fluctuations in surface step density during deposition. We have used the photoemission oscillation technique to measure the AlSb barrier thicknesses during the growth of InAs/AlSb resonant tunneling diodes. Measured peak current densities are clearly better correlated with barrier thicknesses measured by photoemission oscillations than by barrier thicknesses assumed by time-based ‘‘dead reckoning.’’ Calculations of current/voltage characteristics of the resonant tunneling devices indicate that a substantial amount of As is incorporated in the barrier layer as a result of the growth conditions selected.

Simulation of photoemission current to monitor the thin film growth

We report results of atomic scale simulations used to study the dynamics of epitaxial growth of GaAs and, in particular, the oscillations of the photoemission current. We have simulated growth of GaAs using a monoatomic As source. The photoemission oscillations are found to depend on the growth conditions in the same manner as do reflection high-energy electron diffraction (RHEED) intensity oscillations.

Photoemission oscillation measurement of barrier thickness for InAs/AlSb resonant tunneling diodes

AlSb barrier thicknesses ranging from 5 to 12 monolayers have been measured during growth of InAs/AlSb resonant tunneling structures using the photoemission oscillation technique. A plot of peak current density as a function of both measured and estimated barrier thickness confirms that use of the photoemission oscillation technique reduces device performance variations with respect to the conventional time-based approach to layer thickness control. Our growth scheme involves a significant As background pressure during the AlSb growth which results in incorporation of As in the barrier layer. We have modeled the effect of the As incorporation on device properties and find that our measured peak current values are consistent with these calculations.

Simulation of thin film growth and in situ characterization by RHEED and photoemission

Simulations of epitaxial growth of GaAs associated with RHEED intensity and photoemission current, as in situ characterization techniques, have been performed. The nature of incoming species which can be of atomic or molecular form is taken into account. The simulations show in phase RHEED and photoemission oscillations, in agreement with experimental results.

Measurement of photoemission oscillations during molecular beam epitaxial growth of (001) GaAs, AlAs, AlGaAs, InAs, and AlSb

Photoemission oscillations are observed from (001) GaAs, AlAs, AlGaAs, InAs, and AlSb. The periodicity of the oscillations corresponds to that of reflection high-energy electron diffraction oscillations measured using the same growth conditions. For both methods, the oscillation amplitude is strongly dependent on the substrate temperature, III/V flux ratio, and the nature of the surface on which growth is nucleated for the oscillation measurement.

High-amplitude photoemission oscillation of highly As-rich GaAs (001) 2×4-γ phase during molecular beam epitaxy

Photoemission oscillation from GaAs(001) surfaces illuminated by a deuterium lamp is studied for a highly As-rich 2×4-γ phase during molecular beam epitaxy. The 2×4-γ phase shows higher oscillation amplitude than the As-poorer 2×4-α phase. Oscillation of 2×4-γ occurs as a result of two-dimensional nucleation, which is confirmed by the suppression of oscillation on highly misoriented GaAs(001) surfaces. Furthermore, the oscillation has its maxima at nearly half-monolayer growth, which correspond to maximum step-edge density. For this particular 2×4-γ phase, this implies that the surface with a higher step-edge density emits more photoelectron and has a lower ionization energy threshold.

Photoemission applied to the in situ study of epitaxial growth: Theoretical modelling

AbstractReflection high‐energy electron diffraction oscillations are commonly used to monitor the growth of materials and to control the thickness as well as the roughness of the deposited layer.A new technique based on the photoemission current has been employed by Eckstein et al.1 to study the dynamics of the epitaxial growth of GaAs by molecular beam epitaxy. In particular, they have observed oscillations in the energy‐integrated near‐threshold photo emission current that they attribute to periodic changes in the nature of the surface.Our work deals with the physical origin of the observed photoemission oscillations during growth of GaAs. We assume that the photoemission properties depend on the chemical nature of the adsorbed atoms and on their local configurations. In this way, we have developed a theoretical model that associate a simple growth model with an atomic photoemission mechanism.The photoemitted current calculated with this model exhibits the features observed experimentally which seem to be strongly dependent on the molecular As2 adsorption mechanisms and on the roughness of the growing layer.