Migration Enhanced Epitaxy Method Research Articles

Epitaxial growth of monolayer MoSe2 on GaAs

We realized 1–4 monolayer MoSe2 films on Se-terminated GaAs (111)B 2-in. substrates by molecular beam epitaxy. Atomically flat GaAs wafers were prepared by the migration-enhanced epitaxy method, and MoSe2 layers were successfully grown on Se-terminated GaAs (111)B surfaces with layer-number control over the entire wafer area. The obtained MoSe2 crystal is well aligned on the GaAs (111)B surface. The quasi-van der Waals gap formed between a Se-terminated GaAs (111)B surface and MoSe2 was directly observed with a scanning transmission electron microscope. All A1g Raman peaks from 93 points on 2-in. monolayer MoSe2 are distributed within 0.25 cm−1, indicating excellent spatial uniformity.

Effect of an InGaP spacer layer on the luminescence properties of InP/InGaP quantum structures

The optical characteristics of InP/InGaP quantum structures (QSs; quantum dots + quantum dashes) grown by using a migration enhanced epitaxy method have been investigated using temperature-dependent photoluminescence (PL) and time-resolved PL (TRPL). An InGaP spacer layer with different thicknesses (15, 30, and 50 nm) was grown on the InP QSs in order to identify the effect of the InGaP spacer layer on the luminescence and the carrier dynamics of InP QSs. As the thickness of the InGaP layer was increased from 15 to 50 nm, the PL peak exhibited a blueshift, and the PL intensity increased, which are attributed to the increased carrier confinement in the vertical direction due to the increased height and reduced width and length of QSs. The PL decay time demonstrated a strong dependence on the emission wavelength and the temperature. In addition, the decay time for InP QS samples with InGaP layers of 30 and 50 nm exhibited different emission-wavelength dependence than that for the InP QS sample with an InGaP layer of 15 nm. These results demonstrate that the optical and the structural properties of InP QSs are significantly affected by varying the thickness of the InGaP spacer layer.

Study of structural properties of cubic InN films on GaAs(001) substrates by molecular beam epitaxy and migration enhanced epitaxy

InN epitaxial films with cubic phase were grown by rf-plasma-assisted molecular beam epitaxy (RF-MBE) on GaAs(001) substrates employing two methods: migration-enhanced epitaxy (MEE) and conventional MBE technique. The films were synthesized at different growth temperatures ranging from 490 to 550 °C, and different In beam fluxes (BEPIn) ranging from 5.9 × 10−7 to 9.7 × 10−7 Torr. We found the optimum conditions for the nucleation of the cubic phase of the InN using a buffer composed of several thin layers, according to reflection high-energy electron diffraction (RHEED) patterns. Crystallographic analysis by high resolution X-ray diffraction (HR-XRD) and RHEED confirmed the growth of c-InN by the two methods. We achieved with the MEE method a higher crystal quality and higher cubic phase purity. The ratio of cubic to hexagonal components in InN films was estimated from the ratio of the integrated X-ray diffraction intensities of the cubic (002) and hexagonal (101¯1) planes measured by X-ray reciprocal space mapping (RSM). For MEE samples, the cubic phase of InN increases employing higher In beam fluxes and higher growth temperatures. We have obtained a cubic purity phase of 96.4% for a film grown at 510 °C by MEE.

MEE법으로 성장한 InAs/GaAs 양자점의 발광특성

Migration-enhanced epitaxy 성장한 InAs/GaAs 양자점(quantum dots)의 광학적 특성을 PL (photoluminescence)과 Time-resolved PL 이용하여 분석하였다. InAs 양자점은 In을 9.3초 공급하고 5초 차단한 후 As을 3초, 4초, 6초, 또는 9초 공급하고 5초 차단하는 과정을 3회 반복하여 성장하였다. As을 3초 공급한 시료의 PL 피크는 1,140 nm에서 나타나고, PL 세기는 다른 세 시료에 비해 매우 약하게 나타났다. As 공급시간을 3초에서 증가하였을 때 모든 PL 피크는 1,118 nm로 청색이동하여 나타났으며, PL 세기는 증가하였다. As을 6초 공급한 시료의 PL 세기가 가장 강하게 나타나고, 반치폭(full width at half maximum)도 가장 좁게 나타났다. 이러한 결과는 양자점의 밀도와 균일도(크기변화)로 설명된다. 또한 발광파장에 따른 PL 소멸시간은 PL 피크 근처에서 가장 길게 나타났다.

Crystalline and electrical characteristics of C60 uniformly doped GaAs layers

C60 uniformly doped GaAs and C60, Si codoped GaAs layers are grown on GaAs substrates by a migration enhanced epitaxy method. X-ray diffraction measurements indicate that the crystalline quality of the C60 doped GaAs layers is not deteriorated until the concentrations of 1×1019cm−3. In high-resolution transmission electron microscopy images, dark spots induced by the strain contrast are observed in C60 uniformly doped GaAs layer and no dislocation is confirmed around the doped C60 molecules. The electron energy-loss spectroscopy spectrum at the C60 rich region coincides well with the results of pure C60 crystals, indicating that unoccupied molecular orbitals of the C60 molecules are active even in the GaAs lattices. Raman scattering peaks from C60 molecules are obtained for the layers with high C60 concentrations. At low temperatures, the photoconductivity peak around 1000nm appears only in the C60 doped GaAs layers, implying that the resonant excitation between C60 traps and GaAs bands occur.

Electrical properties of C60 and Si codoped GaAs layers

C60 uniformly doped GaAs and C60, Si codoped GaAs layers are grown by a migration enhanced epitaxy method. C60 doped GaAs layers show a single and sharp diffraction peak in x-ray diffraction and only an LO phonon peak is confirmed, indicating that the crystalline quality is fairly good. All of the C60 doped GaAs layers have highly resistive characteristics, and C60, Si codoped GaAs layers show n-type conductivity only when the Si concentration is of the same order or greater than the total carbon concentrations. At low temperatures the conductivity of the C60, Si codoped GaAs layers increases with exposure to light whose energy is below the GaAs bandgap energy. The electron concentrations and mobilities of the layers are confirmed to be increased under illumination by wavelengths between 900 and 1100 nm. These results imply that the electron transitions from the valence band to the trap levels and from the trap levels to the conduction band occur simultaneously as if the traps act as intralevels. As a result, the carrier concentrations are enhanced in the same way they would be if the excitation was above the GaAs bandgap energy.

Self-Assembled Growth of GaAs Anti Quantum Dots in InAs Matrix by Migration Enhanced Molecular Beam Epitaxy

Self-assembled GaAs anti quantum dots (AQDs) were grown in an InAs matrix via migration enhanced molecular beam epitaxy. The transmission electron microscopy image showed that the 2D to 3D transition thickness is below 1.5 monolayers (MLs) of GaAs coverage. The average diameter and height of the GaAs AQDs for 1.5 ML GaAs coverage taken from the atomic force microscopy image were approximately 29.0 nm and 1.4 nm, respectively. The density was approximately 6.0 x 10(10) cm(-2). The size of the AQDs was enlarged in the InAs matrix compared with that on the surface. These results indicate that the GaAs AQDs in the InAs matrix under tensile strain can be effectively formed with the assistance of the migration enhanced epitaxy method.

High-power 745-nm Laser Diode Utilizing InP/InGaP Quantum Structures Grown by Using Migration Enhanced Epitaxy

We report the performance of 745-nm laser diodes (LDs) utilizing InP/InGaP quantum structures (quantum dots + quantum dashes) grown by using migration-enhanced epitaxy method. The photoluminescence of the LD structures before the fabrication of the LDs shows two peaks, around 800 nm and 750 nm, attributed to quantum dots and dashes, respectively. The lasing wavelength is thought to the result from quantum dashes instead of ground states of QDs. The pulse optical power of the LD is above 600 mW at room temperature.

Growth and characterization of C60/GaAs interfaces and C60 doped GaAs

Intensity oscillations of reflection high-energy electron diffraction are observed during epitaxial growth of a C60 layer on GaAs substrates. The frequencies of the oscillations coincide well with the growth rates of C60 layers, suggesting that C60 layers grow by layer-by-layer growth mode. Anomalous oscillations are observed in the initial stage of C60 layer growth on a GaAs (111)B surface with (2×2) structure. These oscillations indicate that the growth of the first C60 layer is completed at the point of approximately 0.5 monolayer coverage. This phenomenon is explained by a model in which C60 adsorption sites are limited by As trimers adsorbed on the GaAs (111)B surface. C60 uniformly doped and δ-doped GaAs and AlGaAs layers are grown by migration enhanced epitaxy method. Crystalline and electrical characteristics of the layers are investigated by transmission electron microscopy (TEM) and electrochemical capacitance voltage (ECV) measurements. The layers are confirmed to have no defect and well-defined δ-doped structures by TEM measurement, and ECV profiles of C60 δ-doped GaAs and AlGaAs layers suggest that C60 molecules in GaAs and AlGaAs lattice produce deep traps, which can be charged or discharged by applied electrical fields.

Electrical properties of C60 delta‐doped GaAs and AlGaAs layers grown by MBE

AbstractC60 δ‐doped GaAs and AlGaAs layers are grown by migration enhanced epitaxy method. Sticking coefficients of C60 on GaAs and AlGaAs surfaces and electrical characteristics of the layers are investigated by secondary ion mass spectroscopy (SIMS) and electrochemical capacitance voltage (ECV) measurements. SIMS profiles indicate that the layers have well‐defined δ‐doped structures, and the sticking coefficients of C60 on GaAs and AlGaAs surfaces are confirmed to be sufficiently high. ECV profiles of C60 δ‐doped GaAs and AlGaAs layers suggest that C60 molecules in GaAs and AlGaAs lattices produce electron traps which can be charged or discharged by applied electric fields. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth and Characterization of C60/GaAs Interfaces and C60 doped GaAs

Intensity oscillations of reflection high-energy electron diffraction are observed during epitaxial growth of a C60 layer on GaAs substrates. The frequencies of the oscillations coincide well with the growth rates of C60 layers, suggesting that C60 layers grow by layer-by-layer growth mode. C60 uniformly doped and delta-doped GaAs layers are grown by migration enhanced epitaxy method. C60 uniformly doped GaAs layers show highly resistive characteristics, suggesting that C60 molecules cannot be decomposed into isolated C atoms. Electrochemical capacitance-voltage profiles of C60 delta-doped GaAs layers suggest that C60 molecules in GaAs lattice produce deep electron traps which can be charged or discharged by applied electrical fields.

Effect of Growth Interruption in Migration Enhanced Epitaxy on InAs/GaAs Quantum Dots

In this study, we investigated the effect of growth interruption time (tGI) during migration enhanced epitaxy (MEE) growth of self-assembled InAs/GaAs quantum dots (QDs) to control the density of the QDs without any substrate rotation stop during QD formation. By manipulating the growth factor (tGI), the control of QD density in the range of 3.4 ×109–3.5 ×1010 dots/cm2, as well as the QD shape, was demonstrated. We concluded that three phenomena occur during growth interruption: 1) In re-evaporation, 2) In segregation, and 3) the redistribution of InAs QDs. From photoluminescence (PL), it is found that the emission wavelength of samples increased from 967.7 to 1151.7 nm as tGI increased due to redistribution. In addition, we confirmed PL peak emissions from QDs, quasi-three-dimensional (Q3D) clusters, and wetting layer. As a result, the manipulation of tGI in the MEE method can control the density, uniformity, size, and wavelength of QDs.

Crystalline and electrical characteristics of C 60-doped GaAs films

C 60 uniformly doped and δ-doped GaAs layers are grown by migration enhanced epitaxy method. Crystalline and electrical characteristics of the layers are investigated by reflection high energy electron diffraction, X-ray diffraction and capacitance–voltage ( C– V) measurements. C 60 high-concentration doping is found to introduce 2D defects, and X-ray diffraction pole-figure measurements show that the rotational domains appear predominantly on {1 1 1}A plane. This indicates that C 60 molecules are mainly incorporated on the dangling bonds of Ga atoms due to the high binding energy between C 60 molecules and Ga atoms. C 60 uniformly doped GaAs layers show highly resistive characteristics, suggesting that C 60 molecules cannot be decomposed into isolated C atoms in the GaAs lattice, and they behave as if they were electron traps or strong recombination centers. C– V profiles of Cr–Au/C 60 δ-doped GaAs Schottky diode suggest that C 60 molecules in GaAs lattice produce electron traps which can be charged or discharged by applied electrical field.

Migration-enhanced epitaxy (MEE) growth of five-layer asymmetric coupled quantum well (FACQW) and its cross-sectional STM observation

The five-layer asymmetric coupled quantum well (FACQW) is a new potential-tailored quantum well that is promising for ultra-fast and ultra-low-voltage optical modulators and switches. FACQW samples were grown by the migration-enhanced epitaxy (MEE) and the conventional molecular beam epitaxy methods with steep and flat heterointerfaces in the monolayer accuracy. They were characterized with the cross-sectional scanning tunneling microscopy (STM). In the cross-sectional STM image, double-stripe structures with different contrast were observed. The stripe area corresponds to the FACQW (about 10 nm wide), sandwiched with the AlGaAs barrier layers ( 15 nm wide). A dark line observed at the middle of the FACQW stripe area corresponds to the 3-monolayer-thick AlAs layer. The cross-sectional STM images of the high-quality heterointerface FACQW structures were successfully observed for the samples grown by the MEE method. More detailed studies of this kind of cross-sectional STM observations will be very effective to obtain the optimized growth conditions for fine and complicated ultra-thin structures.

Growth of GaAs nanostructures by area selective epitaxy by migration-enhanced epitaxy

We have carried out area selective epitaxial growth of GaAs nanostructures using solid source molecular-beam epitaxy (MBE) which makes it possible to achieve ‘damage-free’ structures. However, area selective epitaxy by MBE is very difficult unless the substrate temperature is very high. This problem has been solved by using migration-enhanced epitaxy (MEE) deposition sequence. To achieve well-defined nanostructures, lateral growth beyond the SiO 2 mask boundaries has to be strictly prohibited. By MEE method, uniform two-dimensional lattice structures with vertical sidewalls can be fabricated without shrinking holes, even though the mask diameter is as small as 30 nm with a dot density as high as 5.0×10 9 cm −2 . Also uniform one-dimensional channel structures have been successfully grown.

Investigation of a growth interruption under an As flux at AlSb/InAs interfaces with InSb bonds

We report on an optimized growth interruption time under an As flux during the AlSb/InAs interface growth of unintentionally doped InAs quantum well structures. The migration-enhanced epitaxy method is used to create InSb interface bonds. After the deposition of the first In monolayer the surface is exposed to an As flux for different time periods, before the In shutter is opened again. Time-resolved reflection high-electron-energy diffraction (RHEED) and high-resolution transmission electron microscopy are used to characterize the interface. In combination with Hall measurements of single quantum well structures the interruption time is optimized. The interface quality depends on the RHEED intensity recovering during the interruption time under an As flux. If the interruption time is sufficient to reach the same RHEED intensity level as it is for the InAs growth, the electron mobility rises from 180,000 to 255,000 cm 2/V s at 77 K. Furthermore, we observed that a disordered surface during the interface growth can be incorporated into the layer, leading to amorphous regions directly above the interface. Moreover, the disordering depends on the crystal orientation and is larger in the [1 1 ̄ : 0] azimuth.

Fabrication and Optical Characterization of Five-Layer Asymmetric Coupled Quantum Well (FACQW)

The five-layer asymmetric coupled quantum well (FACQW) is a new potential-tailored quantum well (QW) that is promising for ultrafast and ultralow-voltage optical modulators and switches. We succeeded in fabricating GaAs/AlGaAs FACQW with monolayer accuracy by the molecular beam epitaxy (MBE) method by monitoring reflection high-energy electron diffraction (RHEED) specular beam intensity oscillation. Photoabsorption current measurements of the FACQW sample showed good agreement with theoretical results, and a potential for much lower voltage operation. In addition, we studied the growth sequences of GaAs/AlGaAs QWs in the migration-enhanced epitaxy (MEE) method in order to fabricate the FACQW with steeper and flatter heterointerfaces. The sequence of supplying materials for Al0.3Ga0.7As growth, on which there is no report, was modified and optimized, and the QWs of higher quality were obtained at a growth temperature of 490°C using the optimized sequence. The results of photoluminescence measurements show that the MEE method modified as mentioned above is a promising growth technique for the fabrication of FACQWs of higher quality.

Reduction of threading dislocations in migration enhanced epitaxy grown GaN with N-polarity by use of AlN multiple interlayer

High quality GaN layer was obtained by insertion of high temperature grown AlN multiple intermediate layers with migration enhanced epitaxy method by the RF-plasma assisted molecular beam epitaxy on (0 0 01) sapphire substrates. The propagating behaviors of dislocations were studied, using a transmission electron microscope. The results show that the edge dislocations were filtered at the AlN/GaN interfaces. The bending propagation of threading dislocations in GaN above AlN interlayers was confirmed. Thereby, further reduction of dislocations was achieved. Dislocation density being reduced, the drastic increase of electron mobility to 668 cm 2/V s was obtained at the carrier density of 9.5×10 16 cm −3 in Si doped GaN layer.

Improved device performance by migration-enhanced epitaxy

Migration-enhanced epitaxy (MEE) has been employed successfully to grow high quality GaAs buffer and InGaAs channel layers at 300°C. Planar-doped GaAs MESFETs with a thin MEE GaAs buffer layer have been fabricated for comparison with devices using conventional MBE buffers. Our preliminary data indicate that the MEE buffer alternative offers reduced side-gating effect, lower output conductance, better pinchoff characteristics and higher channel current. RF measurements indicate an improvement in output power density, gain and efficiency for the MEE-buffered MESFETs. High electron mobility transistor (HEMT) devices fabricated on modulation-doped pseudomorphic AlGaAs/InGaAs/GaAs heterostructures with a thin MEE buffer layer yield higher transconductance ( g m), transistor gain, and unity current gain cutoff frequency (ƒ T) compared to the conventional MBE-buffered devices. Moreover, HEMT devices with an MEE InGaAs channel grown at 300°C achieve comparable g m and ƒ T to those with a conventional MBE channel grown at ∼500°C. Critical layer thickness of InGaAs on GaAs using the MEE method, under the growth conditions reported here, is found to exceed that of MBE.

Realization of low facet density and the growth mechanism of GaAs on GaAs(110) by migration-enhanced epitaxy

Smooth GaAs layers were successfully grown by migration-enhanced epitaxy on exactly (110) oriented substrates. The surface of layers grown by conventional molecular beam epitaxy was completely covered with facets, whose density was higher than 106 cm−2. The facet density was reduced remarkably by three orders of magnitude using the migration-enhanced epitaxy method. Observing the intensity oscillations of the specular spot of reflection high-energy electron diffraction patterns, the growth mode and the migration characteristics of surface adatoms have been investigated.