Molecular Beam Epitaxy System Research Articles

Formation of Ge quantum dots on Si substrate using consecutive deposition of Ge/C and in situ post annealing

Effects of intermediate carbon between amorphous Ge and Si surface on formation of Ge dots recrystallized through post-annealing were studied. The samples were prepared by solid-source molecular beam epitaxy system with electron beam gun for C sublimation and K-cell for Ge evaporation. C and Ge were deposited sequentially at 200°C and Ge/C/Si was subsequently annealed in MBE chamber. Ge dots were formed at annealing temperature (TA) of 400°C for Ge(1nm)/C(0.25ML)/Si. The dot size increased with TA, and both Ge(220) peak intensity measured by in-plane XRD and dot density were the highest at TA of 700°C. A strong correlation between dot density and surface roughness indicated recrystallization of Ge occurred during the dot formation promoted by Si–C bonds. Concerning the effect of C coverage, there was an optimum at 0.25ML which gave good crystallinity of Ge dots. Coalesced dots with bad crystallinity were observed for less or more C coverage. This was considered that dot formation combined with S–K growth mode due to large bare Si surface occurred at small C coverage and excess C incorporated into Ge dots at large C coverage. In terms of Ge thickness, there was also an optimum at 1nm to provide sufficient Ge atoms without excess.

Transition of carbon binding states on Si(1 0 0) depending on substrate temperature and its effect on Ge growth

Effect of binding states of carbon(C) with Si substrate on Ge growth was studied. The samples were prepared by solid-source molecular beam epitaxy system with electron beam gun for C sublimation and K-cell for Ge evaporation. The C binding states to Si(100) surface evaluated by XPS measurement revealed a transition of the fraction of C–C and Si–C bonds depending on carbon deposition temperature (TC). The fraction of C–C bonds was major below TC=600°C and was almost equal to that of Si–C bonds at TC=800–1000°C. Surface roughening by the c(4×4) reconstruction induced by a large amount of Si–C bonds was observed at TC=1000°C. Ge equivalent to 10-nm thick was deposited on C/Si substrate at 550°C. Amorphous Ge layers were grown for TC=200–800°C because Ge adatoms nucleated on the C–C structure dominantly. Structural transition from 2-D films to dome-shaped 3-D dots occurred for TC=1000°C due to the increase of Ge diffusion length by the Si–C structure. Concerning the dependence of C coverage, the crystallinity of Ge film deteriorated with increasing C coverage for TC=200°C due to the increase of C–C structure which enhances the formation of amorphous Ge. On the other hand, crystallinity of Ge dots were improved with C coverage and were saturated over 0.2ML for TC=1000°C. This reflects the enlargement of dots due to C-suppressed Ge growth induced by Si–C bonds at the surface.

Berkovich nanoindentation-induced dislocation energetics and pop-in effects in ZnSe thin films

The Berkovich nanoindentation-induced characteristic “pop-in” phenomena observed in ZnSe thin films are investigated in this study. The ZnSe thin films are grown on the GaAs(001) substrates by using the molecular beam epitaxy (MBE) system. No evidence of nanoindentation-induced phase transformation was revealed in ZnSe thin film by the cross-sectional transmission electron microscopy (XTEM) and selected area diffraction (SAD) analyses. Furthermore, as displayed in the SAD results, the distortion of diffraction spots did indicate severe deformation of indented ZnSe thin films resulting from the nanoindentation load. Based on this scenario, an energetic estimation of dislocation nucleation is made.

Surface nitridation induced AlN nano-columnar growth on c-sapphire

We probe the parametric dependence of the nitridation mechanism of Al2O3 surface by radio frequency nitrogen plasma in a molecular beam epitaxy system. Our quantitative analysis by XPS and RHEED shows that the chemical composition varies with exposure time and is independent of nitrogen plasma fluence and substrate temperature. Here we show that nitrogen incorporation into the host material is diffusion limited process and involves the conversion of Al2O3 into AlN, which has a higher rate initially and then saturates at 6h of nitrogen plasma exposure. We deposit a thin AlN layer on this saturated nitrided sapphire substrate, which was found to consist of nanorods with a bimodal diameter distribution and apex morphology. By RHEED, XRD and TEM studies we attribute that the faceted nanocolumns are formed at misfit-induced dislocations with the dislocations propagating axially in the nanorods, while the oval apexed features are diffusion mediated. We find that nanorods of both the morphologies are c-oriented, single crystalline and strain relaxed, but possess different in-plane orientation.

Structural transition in Ge growth on Si mediated by sub-monolayer carbon

Ge growth on Si mediated by sub-monolayer (ML) carbon (C) covered directly on Si surface was studied. C and Ge layers were grown on Si(100) substrates by using solid-source molecular beam epitaxy system. After Si surface cleaning by heating up to 900°C, C up to 0.45ML was deposited and then 10 to 15-nm-thick Ge were deposited. Reflection high energy electron diffraction patterns after sub-ML C deposition changed from streaks to halo depending on C coverage. The Ge dots were formed at low C coverage of 0.08–0.16ML. Octagonal dots had three same facet planes of (001), (111), and (113) and consisted of the mixture of single crystals with dislocations along [111]. This is due to the event that the incorporation of small amount of C into Si surface gave rise to a strain. As a result, Si surface weaved Si(100) 2×1 with SiC c(4×4) and Ge atoms adsorbed selectively on Si(100) 2×1 forming dome-shaped dots. A drastic structural transition from dots to films occurred at C coverage of 0.20ML. The Ge films, consisting of relaxed poly- and amorphous-Ge, formed at C coverage of 0.20–0.45ML. This is because a large amount of SiC bonds induced strong compressive strain and surface roughening. In consequence, the growth mode changed from three-dimensional (3D) to 2D due to the reduction of Ge diffusion length.

Antimony-assisted carbonization of Si(111) with solid source molecular beam epitaxy

The carbonization of an antimony-terminated Si (111) surface in a solid source molecular beam epitaxy system is presented. Reflection high-energy electron diffraction, atomic force microscopy, x-ray photoelectron spectroscopy, and cross-sectional transmission electron microscopy are used to characterize samples grown with and without antimony termination. It is shown that the antimony-terminated surface promotes the formation of thin, smooth and continuous SiC films at a relatively low temperature of 800 °C.

Suppression of bulk conductivity in InAs/GaSb broken gap composite quantum wells

The two-dimensional topological insulator state in InAs/GaSb quantum wells manifests itself by topologically protected helical edge channel transport relying on an insulating bulk. This work investigates a way of suppressing bulk conductivity by using gallium source materials of different degrees of impurity concentrations. While highest-purity gallium is accompanied by clear conduction through the sample bulk, intentional impurity incorporation lead to a bulk resistance over 1 M\Omega. This resistance was found to be independent of applied magnetic fields. Ultra high electron mobilities for GaAs/AlGaAs structures fabricated in a molecular beam epitaxy system used for the growth of Sb-based samples are reported.

Optimization and temperature dependence characteristics of low temperature In0.3Ga0.7As and In0.53Ga0.47As-In0.52Al0.48As semiconductor terahertz photoconductors

Recently, detailed characterisation of materials and evaluation of devices based on low temperature (LT) grown InGaAs-InAlAs and GaAs-based terahertz (THZ) photoconductors using the Molecular Beam Epitaxy (MBE) technique have been reported by our group. In this work, the characterisation is extended in order to study the growth reproducibility of the photoconductors and the temperature dependence of their transport properties. We show that the structural, optical and transport characteristics of a photoconductor can be optimised by growing the same structure under the same growing conditions but in different MBE systems. The Hall Effect measurements over the temperature range of 100 K–400 K revealed temperature independency of the mobility within a wide range, in which the concentration is changing with the temperature. The majority of carriers are found to be electrons even in the case of Be doped samples, which is attributed to the large density of excess As anti-site atoms. The transport properties of low temperature grown materials are presented for the first time and the behaviour is found to be different to those of conventional materials, which are grown under normal growth conditions.

Effect of the thickness of BiFeO3 layers on the magnetic and electric properties of BiFeO3/La0.7Sr0.3MnO3 heterostructures

BiFeO3 layers with various thicknesses were fabricated on La0.7Sr0.3MnO3 covered SrTiO3 substrates by a laser molecular-beam epitaxy system. The ferromagnetic transition temperature (Tc) and magnetic coercive field (Hc) of BiFeO3/La0.7Sr0.3MnO3 heterostructures are larger than those of the La0.7Sr0.3MnO3 film. With increasing the thickness of the BiFeO3 layer, Tc, Hc, and ferroelectric coercive field of the BiFeO3/La0.7Sr0.3MnO3 heterostructures decrease, while the dielectric permittivity, remanent polarization, and resistance ratio of the ON and OFF states increase. The variations of the magnetic and electric properties with the thickness could be due to the effects of the epitaxial strain and the interface layer.

Enhancing device characteristics of 1.3 μm emitting InAs/GaAs quantum dot lasers through dot-height uniformity study

This study systematically improves the dot-height uniformity of InAs/GaAs quantum dot (QD) active layer grown by molecular beam epitaxy system in 1.3μm-emitting QD laser diodes. The resulting self-assembled QDs with improved uniformity exhibited an elongated emission wavelength and a reduced linewidth of QD photoluminescence (PL) spectra. The 1.3μm-emitting QD lasers with an as-cleaved 5-μm ridge fabricated under optimal growth conditions exhibited a low threshold current density of 250A/cm2 and a high internal quantum efficiency of 63% under continuous-wave operation. The photoluminescent properties and characteristics of the 1.3μm-emitting QD lasers was also investigated theoretically using an effective mass model and correlated well with the experimental results, which elucidative clarifying the significant role of dot-height uniformity in achieving high performance QD lasers.

Closed cycle chiller as a low cost alternative to liquid nitrogen in molecular beam epitaxy

The high cost of cooling the cryoshroud in a molecular beam epitaxy system has been greatly reduced by replacing liquid nitrogen (LN2) as a coolant with a silicone polymer heat transfer fluid cooled to as low as −80 °C by a closed cycle chiller. Gallium arsenide epitaxial layers have been grown with two different cooling configurations of the shroud: conventional LN2 cooling and cooling to −70 °C with the chiller. The partial pressure of water in the chamber is a factor of about 2.5 higher with the closed cycle chiller operating at −70 °C than with liquid nitrogen in the shroud. No significant difference is observed in the density of deep levels in the GaAs, as determined by deep level transient spectroscopy.

Sb irradiation effect on growth of GaAs thin film on Si (111) substrate

We studied the effect of Sb4 irradiation on the growth of GaAs thin films on Si (111) substrates. GaAs thin films were grown by molecular beam epitaxy (MBE) system with As2 as a source. Sb4 was also supplied during the GaAs growth. We evaluated surface roughness with an atomic force microscope and crystallinity with an X-ray diffraction system.When GaAs is grown without Sb4 irradiation, crystallinity can be improved by subjecting it to high temperatures, although the surface becomes very rough. In contrast, GaAs grown with Sb4 irradiation showed both good crystallinity and a very flat surface. As the growth temperature increased, both crystallinity and surface flatness were improved, probably due to the enhanced migration with two-dimensional growth modes. Furthermore, plan-view observation with a transmission electron microscope (TEM) showed that Sb4 irradiation was very effective for reducing stacking faults.

(112) and (220)/(204)-oriented CuInSe2 thin films grown by co-evaporation under vacuum

CuInSe2 (CIS) layers were grown by co-evaporation in a molecular beam epitaxy system onto soda lime glass substrates by using both two-step and three step processes. The physical properties of the layers were investigated using X-ray diffraction (XRD) and optical spectroscopy. The sample atomic composition was assessed by energy dispersive analysis of X-rays. Cu-rich or In-rich CIS thin films were obtained exhibiting strong preferential (112) and (220)/(204) orientations in both cases. We performed thermal annealing at 450°C under nitrogen, keeping Se overpressure to avoid Se desorption from the layer. The annealed layers all exhibit improved crystalline quality, with reduced stoichiometric discrepancy. The secondary phases like CuxSe1−x or InxSe1−x are no more observable by XRD measurements. Regarding the preferential orientation, thermal annealing of Cu-rich CIS layers favours the (112) orientation leading to a more (112) textured layer after annealing, whatever the initial preferential growth orientation was. In opposite, thermal annealing of In-rich samples increases the (220)/(204) texture of the sample.

Structural properties of ultra-low density nanoholes for the generation of well-separated GaAs quantum dots

We apply the local droplet etching (LDE) technique in a conventional molecular beam epitaxy system for self-organized nanohole formation on AlGaAs surfaces. By optimizing the arsenic flux during the Al droplet deposition the hole density can be reduced down to the 106cm−2 regime. The nanohole shape and density are tunable in a wide range by the amount of droplet material deposited and the initial holes are customized by overgrowth with AlGaAs of varied thickness. Nanoholes with controlled depths from 30 down to a few nanometers are fabricated, while the ultra-low density is sustained. These nanoholes are filled with GaAs to form well separated GaAs QDs, with size precisely controlled by the filling level. Studies of the optical emission of single dots exhibit sharp excitonic lines at low and a clear shell structure at high excitation power. In particular, the quantization energies in dependence of the QD size are measured.

MBE growth of GaN pn-junction photodetector on AlN/Si(1 1 1) substrate with Ni/Ag as ohmic contact

In this paper, we investigated the growth of GaN pn-junction layers on silicon (111) by plasma assisted molecular beam epitaxy (PA-MBE) system and the effect of thermal annealing of Ni/Ag contacts on the sample for photodetector applications. Si and Mg were used as n- and p-dopants, respectively. The reflection high energy electron diffraction images indicated a good surface morphology of GaN pn-junction layer. The full width at half maximum (FWHM) obtained from XRD measurement was 0.34°, indicating a good quality layer of sample. The pn-junctions sample has a good optical quality which is reflected by the photoluminescence system measurement. The structural evolution and temperature dependence of the current of Ni/Ag contacts on GaN pn-junction at various annealing were investigated by scanning electron microscopy (SEM) and current–voltage (I–V) measurements. The temperature dependence of the current may be attributed to changes of the surface morphology of Ni/Ag films on the surface. SEM results indicated the degradation of Ni/Ag contacts on GaN pn-junction above 800°C.

Carbon flux assisted graphene layer growth on 6H-SiC(000-1) by thermal decomposition

Graphene layers have been grown on C-terminated 6H-SiC (000-1) substrate by thermal decomposition with the assistance of carbon flux in a solid carbon source molecular beam epitaxy system. Results indicate that the graphene films prepared directly by the reduction of SiC in vacuum chamber are defective and disorder, while appropriate carbon flux added during the graphene growth will favor the graphene layer growth and improve the graphene layer quality. While on the other hand, too much carbon flux will affect the graphene formation and deteriorate the final graphene layer. This carbon flux assisted graphene growth behavior on SiC crystal indicates that the external carbon flux should play an important role for the graphene layer growth during the thermal reduction of SiC in vacuum condition. The mechanism of the graphene layer formed on C-face SiC and the effect of the carbon flux were discussed based on the experimental results.

Structural characterization and magnetic properties of perpendicularly magnetized MnAl films grown by molecular-beam epitaxy

Perpendicularly magnetized MnAlx thin films with different Al contents have been epitaxied on GaAs (001) substrates by a molecular-beam epitaxy system. Crystalline quality of MnAlx films is closely related to Al content, and magnetic properties of MnAlx films are improved as crystalline quality of MnAlx fims increases. MnAl0.9 film shows the best crystalline quality and magnetic property among all samples. So we grew MnAl0.9 films at different growth temperatures to further optimize growth conditions. With increasing temperature, the chemical order parameter increases and the full width at half maximum of the τ(002) peak decreases, which reveal the improvement of crystalline quality. Higher perpendicular magnetization, coercivity and magnetic anisotropy are found as growth temperature increases. The best crystalline quality and perpendicularly magnetized properties are found at 350℃; the coercivity of 8.3 kOe, saturation magnetization of 265 emu/cm3, Mr/Ms of 0.933 and perpendicular magnetic anisotropy constant of 7.74 Merg/cm3 are achieved. These tunable perpendicularly magnetized properties and good compatibility associated with semiconductor materials make the noble-metal-free and rare-earth-free MnAl films attractive in the application of spintronic devices.

FABRICATION OF GaN HOMO-JUNCTION ON Si (111) SUBSTRATE FOR SENSOR APPLICATIONS

In this paper, we investigated growth of GaN pn-homo junction layers on silicon (111) by plasma-assisted molecular beam epitaxy (PA-MBE) system and its application for photo-sensors. Silicon (Si) and magnesium (Mg) were used in this study as n- and p-dopants, respectively. Structural and optical analysis of the GaN homo-junction samples were performed by using reflection high energy electron diffraction (RHEED), X-ray diffraction (XRD), Raman spectroscopy and photoluminescence (PL) to analyze the crystalline quality of the samples. Surprisingly, the Raman analysis has revealed that there is no quenching of the A1 (LO) peak, with the presence of Si - and Mg -dopants in sample. The pn-junctions sample has good optical quality as measured by the PL system. Electrical characterization of the photo-sensors was carried out by using current–voltage (I–V) measurements.

Metastable Sm(Fe,Cu)5ordered alloy films formed on Cu(111) underlayers

Sm-Fe thin films are prepared on Cu(111) underlayers hetero-epitaxially grown on MgO(111) single-crystal substrates by using an ultra-high vacuum molecular beam epitaxy system. The Sm/Fe composition is varied from Fe-rich (10 at. % Sm - Fe) to Sm-rich (30 at. % Sm - Fe) region including SmFe5 stoichiometry. The influence of film composition on the film structure is studied by in-situ reflection highenergy electron diffraction and X-ray diffraction. Metastable Sm(Fe,Cu)5 ordered phase formation is recognized in the Sm-Fe films with the investigated compositions. Cu atoms diffuse from the underlayer into the Sm-Fe film and substitute the Fe site in SmFe5 structure forming an alloy compound of Sm(Fe,Cu)5 . The Sm-Fe films with Fe-rich compositions consist of Sm(Fe,Cu)5 and bcc-Fe phases, whereas the Sm-Fe films with Sm-rich compositions consist of Sm(Fe,Cu)5 and amorphous phases. Cu atom diffusion into Sm-Fe film is assisting the formation of ordered phase.

Effects of Be doping on InP nanowire growth mechanisms

Be-doped InP nanowires were grown by the gold-assisted vapour-liquid-solid mechanism in a gas source molecular beam epitaxy system. The InP nanowire length versus diameter [L(D)] dependence revealed an unexpected transition with increasing Be dopant concentration. At Be dopant concentration below ∼1018 cm−3, nanowires exhibited the usual inverse L(D) relationship, indicating a diffusion-limited growth regime. However, as dopant concentration increased, the nanowire growth rate was suppressed for small diameters, resulting in an unusual L(D) dependence that increased before saturating in height at about 400 nm. The cause of this may be a change in the droplet chemical potential, introducing a barrier to island nucleation. We propose a model accounting for the limitations of diffusion length and monolayer nucleation to explain this behaviour.