Molecular Beam Epitaxy Growth Chamber Research Articles

Reduced recombination in a surface-sulfurized Cu(InGa)Se2 thin-film solar cell

This study demonstrates surface sulfurization effects on Cu(InGa)Se2 (CIGSe) thin-film solar cells with a single back-graded band gap. Single back-graded CIGSe thin films were prepared via a three-stage process in a high-vacuum molecular beam epitaxial growth chamber and were subsequently annealed in a tube furnace under environmental conditions with H2S gas. After sulfurization, an ∼80- to ∼100-nm-thick CuIn(SSe)2 layer with significantly small Ga contents (CISSe:Ga) was formed on the CIGSe layer. The newly formed CISSe:Ga layer exhibited graded S contents from surface to bulk, thus resulting in a front-graded band gap. In addition, CISSe:Ga was covered with S-enriched CISSe region that was extended from the surface to a depth of a few nm and was depleted of Ga. A device with the sulfurized CIGSe showed reduced recombination at the buffer–absorber interface, in space-charge region and in bulk. Consequently, the open circuit voltage increased from 0.58 V (in the non-sulfurized case) to 0.66 V, and the conversion efficiency improved from 15.5 to 19.4%. This large improvement is caused by the front graded band gap at the surface and the hole-blocking barrier, which suppress recombination at the CdS/CISSe:Ga interface. In addition, sulfurization followed by KF post-deposition treatment (PDT) increased the efficiency to 20.1%. Compared to the untreated sulfurized device, the KF-PDT device delivered an increased carrier lifetime and reduced the recombination in bulk probably because the defects were passivated by the K, which penetrated into the bulk region.

Raman measurements of substrate temperature in a molecular beam epitaxy growth chamber.

A method is described for directly measuring the temperature of a substrate in a molecular-beam epitaxy (MBE) growth system. The approach relies on the establishment of the temperature dependence of Raman-active phonons of the substrate material using independently known calibration points across the range of interest. An unknown temperature in this range is then determined based on the Raman peak position with the substrate in situ the MBE chamber. The apparatus relies on conventional optics and Raman components. Shifting and broadening of the Raman spectrum are described based on the effects of thermal expansion and anharmonic decay. The choice of reference temperature is discussed. The method is qualified by examining the substrate temperature dependence, relative to that of a standard thermocouple, during a commonly used ramp procedure. Both temperature difference and time lag are obtained.

Silicon nitride thin films deposited using electron-beam evaporation in an RF plasma MBE system

Silicon nitride (SiNx) thin films were deposited on (100) Si wafers in a molecular-beam epitaxy growth chamber equipped with a customized multipocket electron-beam evaporator, a Si effusion cell, and an RF plasma source for reactive nitrogen. The films were characterized using atomic-force microscopy, spectroscopic ellipsometry, and specular x-ray reflectivity. For films deposited using an electron-beam Si source with N/Si > 1.33, the deposition temperature determined the density and refractive index. Stoichiometric Si3N4 films were produced when the deposition temperature was greater than 725 °C, in agreement with our previous results that used an effusion cell for Si. By using the electron-beam Si source, an order of magnitude increase in SiNx deposition rate was achieved over the conventional effusion cell method.

Study of site controlled quantum dot formation on focused ion beam patterned GaAs substrate

In this project, we combine Ga+ focused ion beam (FIB) patterning and self-assembly of InGaAs quantum dots in order to produce regular quantum dot (QD) arrays. The GaAs substrate is patterned before transporting into molecular beam epitaxy growth chamber to do anneal and overgrowth. The thickness of deposited InAs is precisely controlled. Quantum dots are expected to nucleate at specific locations, where the ion beam has previously implanted gallium atoms. The properties of the quantum dots formed are related to the FIB beam parameters, which include accelerating voltage, probe current, dwell time and pitch. We studied and statistically analyzed the relationship, focusing on their diameter, height and distribution. Scanning electronic microscope (SEM) and atomic force microscope (AFM) images have been used to analyze the patterns and QDs before and after overgrowth. Micro-PL study was performed to test the QD opto-electronic property. Scanning transmission electron microscope (STEM) cross sectional analysis and X-ray mapping is performed.

Design of an ultrahigh vacuum transfer mechanism to interconnect an oxide molecular beam epitaxy growth chamber and an x-ray photoemission spectroscopy analysis system

We designed a mechanism and the accompanying sample holders to transfer between a VEECO 930 oxide molecular beam epitaxy (MBE) and a PHI Versa Probe X-ray photoemission spectroscopy (XPS) chamber within a multiple station growth, processing, and analysis system through ultrahigh vacuum (UHV). The mechanism consists of four parts: (1) a platen compatible with the MBE growth stage, (2) a platen compatible with the XPS analysis stage, (3) a sample coupon that is transferred between the two platens, and (4) the accompanying UHV transfer line. The mechanism offers a robust design that enables transfer back and forth between the growth chamber and the analysis chamber, and yet is flexible enough to allow transfer between standard sample holders for thin film growth and masked sample holders for making electrical contacts and Schottky junctions, all without breaking vacuum. We used this mechanism to transfer a barium strontium titanate thin film into the XPS analysis chamber and performed XPS measurements before and after exposing the sample to the air. After air exposure, a thin overlayer of carbon was found to form and a significant shift (~1 eV) in the core level binding energies was observed.

In situ STM observation during InAs growth in nano holes at 300 °C

We have successfully confirmed that In atoms were favored to congregate inside hole structures, during In and As4 irradiations, by a STMBE system which was a scanning tunneling microscope located inside a molecular beam epitaxy growth chamber. After forming 1.5 monolayer of InAs wetting layer (WL) on a GaAs(001) surface, we applied voltage at a particular site on the WL during As4 irradiation at 300°C, creating hole structures (widths: 33–66.1nm, depths: 4.9–9.7nm). With the In and As4 irradiations, spontaneously, In atoms on the WL were congregated inside the holes, decreasing the volume of the hole structures. It was found that InAs growth rates inside the hole structures were 23.1–217 times larger than that at the WL growth region near the holes.

분자선에피택시에 의해 Si (100) 기판 위에 성장한 GaAs 에피층의 특성에 대한 기판 세척효과

분자선 에피택시 장비를 이용하여 두 단계 방법(two-step method)으로 Si (100) 기판 위에 GaAs 에피층을 성장하였다. Si 기판은 초고진공을 유지하고 있는 MBE 성장 챔버 속에서 세척 방법을 달리하여 Si 기판표면에 존재하는 불순물(산소, 탄소 등)을 제거하였다. 첫 번째는 Si 기판을 몰리브덴 히터를 사용하여 <TEX>$800^{\circ}C$</TEX>로 직접 가열하였다. 두 번째는 Si 기판 표면에 As 빔을 조사시켜 주면서 <TEX>$800^{\circ}C$</TEX>로 Si 기판을 가열하였다. 세 번째는 Si 기판 표면에 Ga을 증착한 후 Si 기판을 <TEX>$800^{\circ}C$</TEX>로 가열하였다. 이와 같은 세 가지 다른 조건으로 세척한 Si(100) 기판 위에 성장한 GaAs 에피층의 특성은 reflection high-energy electron diffraction (RHEED), atomic force microscope (AFM), double crystal x-ray diffraction (DXRD), photoluminescence (PL), photoreflectance(PR) 등으로 조사하였다. Ga 빔을 증착하여 세척한 Si 기판 위에 성장된 GaAs 에피층의 RHEED 패턴은 (<TEX>$2{\times}4$</TEX>) 구조를 가지고 있었다. Ga 빔을 증착하여 세척한 Si 기판 위에 성장된 GaAs 에피층이 가장 좋은 결정성을 가지고 있었다. The GaAs epitaxial layers were grown on Si(100) substrates by molecular beam epitaxy (MBE) using the two-step method. The Si(100) substrates were cleaned with three different surface cleaning methods of vacuum heating, As-beam exposure, and Ga-beam deposition at the substrate temperature of <TEX>$800^{\circ}C$</TEX> in the MBE growth chamber. Growth temperature and thickness of the GaAs epitaxial layer were <TEX>$800^{\circ}C$</TEX> and <TEX>$1{\mu}m$</TEX>, respectively. The surface structure and properties were investigated by reflection high-energy electron diffraction (RHEED), AFM (Atomic force microscope), DXRD (Double crystal x-ray diffraction), PL (Photoluminescence), and PR (Photoreflectance). From RHEED, the surface structure of GaAs epitaxial layer grown on Si(100) substrate with Ga-beam deposition is (<TEX>$2{\times}4$</TEX>). The GaAs epitaxial layer grown on Si(100) substrate with Ga-beam deposition has a high quality.

Growth and structural characterization of GaAs/GaAsSb axial heterostructured nanowires

GaAs nanowire (NW) heterostructures with four GaAsSb inserts were grown on GaAs(1 1 1)B substrates by Au-assisted molecular beam epitaxy (MBE). Structural characterization of the NWs showed that the transition between the GaAs and GaAsSb heterojunction is atomically abrupt and that the GaAs crystallographic phase is mainly wurtzite with some stacking faults, whereas the GaAsSb phase is zinc blende and free from twinning defects. Interestingly, the growth rates of the GaAs segments and GaAsSb inserts were observed to vary with growth time. We attribute this behavior to the effect of the inclined molecular beams with respect to the substrate surface in the MBE growth chamber. Further, the mole fraction of Sb in the GaAsSb insert was determined by energy-dispersive X-ray spectrometry and was found to increase with the inserts position along the NW.

Heteroepitaxial growth of InAs on GaAs(0 0 1) by in situ STM located inside MBE growth chamber

The growth of InAs on GaAs(001) is of great interest primarily due to the self-assembly of arrays of quantum dots (QDs) with excellent opto-electronic properties. However, a basic understanding of their spontaneous formation is lacking. Advanced experimental methods are required to probe these nanostructures dynamically in order to elucidate their growth mechanism. Scanning tunneling microscopy (STM) has been successfully applied to many GaAs-based materials grown by molecular beam epitaxy (MBE). Typical STM–MBE experiments involve quenching the sample and transferring it to a remote STM chamber under arsenic-free ultra-high vacuum. In the case of GaAs-based materials grown at substrate temperatures of 400–600°C, operating the STM at room temperature ensures that the surface is essentially static on the time scale of STM imaging. To attempt dynamic experiments requires a system in which STM and MBE are incorporated into one unit in order to scan in situ during growth. Here, we discuss in situ STM results from just such a system, covering both QDs and the dynamics of the wetting layer.

Investigation of the silicon ion density during molecular beam epitaxy growth

Ions impinging on a surface during molecular beam epitaxy influence the growth and the properties of the growing layer, for example, suppression of dopant segregation and the generation of crystal defects. The silicon electron gun in the molecular beam epitaxy (MBE) equipment is used as a source for silicon ions. To use the effect of ion bombardment the mechanism of generation and distribution of ions was investigated. A monitoring system was developed and attached at the substrate position in the MBE growth chamber to measure the ion and electron densities towards the substrate. A negative voltage was applied to the substrate to modify the ion energy and density. Furthermore the current caused by charge carriers impinging on the substrate was measured and compared with the results of the monitoring system. The electron and ion densities were measured by varying the emission current of the e-gun achieving silicon growth rates between 0.07 and 0.45 nm/s and by changing the voltage applied to the substrate ...

Low Trap Density Nonleaky SiGe Quantum Well MOS Structures:Fabrication and Characteristics

SiGe quantum well metal-oxide-semiconductor (MOS) structures were fabricated by low pressure chemical vapor deposition of the epitaxial layers, or by molecular beam epitaxy (MBE), followed by thermal oxidation of the silicon cap layer at 1100°C. Examined were the effects of the quantum layer composition, the epitaxy technique, the pre-oxidation surface preparation, the oxidation temperature, and the annealing conditions, on the device quality and the strain relaxation. Addition of Ge to the quantum well was found to increase the interface trap density, due possibly to an increase in the lattice strain, while the addition of C resulted in a reduction of the trap density, due possibly to a decrease in the lattice strain as well as passivation of the traps at the interface by C. The interface trap density was higher in the case of MBE epi layers, perhaps, due to metal contamination of the samples by the MBE growth chamber. The lowest interface trap densities and the lowest oxide leakage current densities over a range of −2.5 to 1.0 V] were obtained in the case of rapid thermal oxidation at 1100°C, without any relaxation of the strained SiGe or SiGeC layer. © 2001 The Electrochemical Society. All rights reserved.

Nitridation of the GaAs(001) surface: Thermal behavior of the (3×3) reconstruction and its evolution

The nitridation of the GaAs(001) surface using a radio frequency atomic nitrogen plasma source in a molecular beam epitaxy growth chamber has been studied. The resulting nitrogen induced GaAs(001) (3×3) reconstruction was investigated by in situ reflection high energy electron diffraction and x-ray photoemission spectroscopy (XPS). It was found that this reconstruction is only obtained in the temperature range 400–580 °C with a very low dose of atomic nitrogen. The nitrogen coverage corresponding to the (3×3) reconstruction was determined by quantitative XPS to be 0.30±0.09 ML. Below 400 °C an As–N species of disordered structure was found on the GaAs(001) surface. Subsequent annealing at about 500 °C produced the (3×3) reconstruction. Above 580 °C, nitridation lead to direct formation of β-GaN islands. In addition, the (3×3) reconstruction was found to be unstable to both exposure to atomic hydrogen and annealing. The N desorption activation energy of the (3×3) was estimated to be 2.75±0.55 eV. A surface phase diagram of the (3×3) has thus been deduced.

Focused ion molecular-beam epitaxy – a novel approach to 3D device fabrication using simultaneous p- and n-type doping

The combination of molecular-beam epitaxial (MBE) growth and in situ focused ion beam (FIB) technology, has attracted considerable research interest in recent years since early studies in this area by Hirayama and Okamoto (Jpn. J. Appl. Phys. 24 (1985) L965) and Miyauchi et al. (J. Vac. Sci. Technol. B 4 (1986) 189) on III–V semiconductor material systems. In this paper, we describe a novel fabrication technique that directly integrates a FIB column into an MBE growth chamber. This “FIMBE” system allows the generation of novel III–V device structures that would otherwise be impossible to fabricate via conventional lithographic techniques. The use of a Au 70Si 15Be 15 liquid metal ion source (LMIS) in our FIMBE system offers for the first time both n and p-type selective-area doping during MBE growth. A mass filtered 2 kV FIB column was used to selectively dope regions of a single GaAs epilayer using both Be and Si, with landing energies of 30 and 25 eV, respectively. High-quality bulk GaAs was grown with 77 K mobilities of 0.3 m 2 V −1 s −1 at a carrier concentration of 5×10 23 m −3 using the Si 2+ species, and 0.065 m 2 V −1 s −1 at a carrier concentration of 1×10 23 m −3 for Be 2+. The epilayer quality is thus comparable to that obtained from conventional thermally doped material. Low-energy ion-doped 2D hole gases (2DHGs) over selective areas have also been fabricated for the first time using 30 eV Be 2+ ions as the dopant species in a GaAs/Al x Ga 1− x As heterostructure. This resulted in a 2DHG with an unilluminated 1.7 K mobility of 8.8 m 2 V −1 s −1 at a carrier concentration of 2.6×10 15 m −2. A Si 2+ ion doped HEMT was also grown, attaining a 1.7 K 2DEG mobility of 60.7 m 2 V −1 s −1 at a carrier concentration of 1.9×10 15 m −2 after illumination. This compares favourably with a thermally doped control structure grown immediately afterwards that attained a 1.7 K 2DEG mobility of 80.7 m 2 V −1 s −1 at a similar carrier concentration after illumination.

Selective area regrowth of n-GaAs with reduced interface carrier depletion using arsenic passivation

Selective area molecular beam epitaxy (MBE) of n-GaAs through a Si 3N 4 mask has been explored. An arsenic capping layer was deposited in the MBE growth chamber prior to mask definition, in order to protect the epilayer surface during ex situ processing. After in situ thermal decapping and high-temperature (680°C) regrowth at a low growth rate (0.5 μm/h), the samples were examined with scanning electron and atomic force microscopy. The data unveil selective regrowth with lateral definition on a micrometer length scale and GaAs surfaces with a smooth mirror-like finish. Electrolytic carrier profiling and secondary ion mass spectrometry measurements of the homoepitaxial regrowth interface show a significant reduction of the interface potential barrier, from V bi=0.45 V to V bi=0.07 V, and reduced carbon impurity concentrations, compared to regrowth on non-passivated GaAs epilayers.

Patterned regrowth of n-GaAs by molecular beam epitaxy using arsenic passivation

Patterned regrowth of n-GaAs through a Si3N4 mask prepared on molecular beam epitaxy (MBE) grown n-GaAs layers has been explored. An arsenic capping layer was deposited in the MBE growth chamber prior to mask definition, in order to protect the GaAs surface during ex situ processing. Removal of this cap from the lithographically defined mask area was accomplished by hydrogen plasma etching prior to Si3N4 deposition. After in situ thermal desorption of the As cap and phase selective regrowth, the samples were inspected with scanning electron and atomic force microscopy to establish the growth selectivity and examine the surface morphology. Electrolytic carrier profiling and secondary ion mass spectrometry measurements were carried out to characterize the growth-interrupted GaAs–GaAs interface. We find that the surface morphology and defect density of regrown GaAs epilayers are comparable to those of continuously grown epilayers of the same overall thickness. Structuring of the arsenic cap with hydrogen pla...

In situ observations of nucleation and coalescence stages in Ge growth on Si surfaces using transmission electron microscope combined with molecular beam epitaxy chamber

Nucleation and defect formation processes were observed for the heteroepitaxial growth of Ge on Si(100), (110), and (111) substrates, using an ultra-high-vacuum transmission electron microscope (TEM) combined with a conventional molecular beam epitaxial growth chamber. Successive plan-view TEM images clearly showed that the changes in the radius of Ge islands as a function of film thicknesses depend on the orientations of Si substrates. The behavior of the misfit-strain relaxation in Ge islands was found to be independent of the substrate orientations. The formation of threading dislocations was shown by discontinuity in the moiré fringes between the coalesced Ge islands at 50 monolayer (ML) deposition. Twins and stacking faults, respectively, were also observed in Ge islands on (110) and (111) substrates in the early stages of 25–50 ML growth.

Atomic nitrogen doping in p-ZnSe molecular beam epitaxial growth with almost 100% activation ratio

An almost 100% activation ratio {(NA-ND)/[N]} for a nitrogen-doped ZnSe molecular beam epitaxy (MBE) layer with the highest net acceptor concentration (NA-ND) of 1.2×1018 cm−3 was obtained using a high-power rf plasma source. Even at this high NA-ND value, a 4.2 K photoluminescence spectrum shows bound exciton emission and deep donor–acceptor pair emission with well-resolved phonon replicas. The high activation in nitrogen doping could be ascribed to the generation of the predominant atomic nitrogen and to the suppressed extraction of nitrogen ions and excited neutral nitrogen molecules due to the structure of the orifice placed between the MBE growth chamber and the plasma discharge tube of the high-power plasma source.

Total in situ etching and regrowth in an MBE system: application to buried heterostructure lasers

A new chlorine-based chemical beam etching technique (CBET) has been combined with molecular beam epitaxy (MBE) technology to prepare InP-InGaAsP buried heterostructures in a total in situ etching and regrowth process for the first time. This novel processing technology, combining two techniques in the same MBE growth chamber, is very attractive for the realization of high-performance discrete or integrated optoelectronic devices. The different aspects of the etching process optimization are reviewed in the case of InP-InGaAsP heterostructures, and a first application to buried heterostructure laser fabrication is presented. The results obtained for buried ridge stripe (BRS) lasers are very promising and compare favorably to the state-of-the-art lasers. This first device application passes a milestone in the development of this new technology and demonstrates its potential for further applications to the fabrication of optoelectronic devices.

The use of atomic hydrogen for substrate cleaning for subsequent growth of II-VI semiconductors

Atomic hydrogen is shown to be particularly efficacious for the preparation of substrates for subsequent growth of II-VI compounds by molecular beam epitaxy. A commercial thermal cracker was used to produce atomic hydrogen in the molecular beam epitaxy growth chamber for in-situ cleaning. This paper discusses the use of atomic hydrogen for both oxide removal from GaAs prior to ZnSe and CdTe growth, and for low-temperature oxide removal from CdTe and HgCdTe. Reflection high energy electron diffraction, ultra violet fluorescence microscopy, Nomarski interference contrast microscopy, and atomic force microscopy were used to characterize the growths.

A soft x‐ray standing wave measurement system for analyzing compound semiconductor surfaces prepared by molecular beam epitaxy

A soft x‐ray standing wave (soft‐XSW) measurement system has been developed for analyzing III–V compound semiconductor surfaces prepared by molecular beam epitaxy (MBE). It consists of a fast entry chamber ( a load‐lock chamber), a transfer chamber, a MBE growth chamber, and an analysis chamber. It is connected to NTT’s synchrotron radiation beam line at the Photon Factory. Samples prepared by MBE can be transferred to the analysis chamber through ultrahigh vacuum. The goniometer in this analysis system was shown to be suitable for soft‐XSW triangulation studies. Both backreflection and angle‐scan soft‐XSW experiments for adsorbate/GaAs(001) systems prepared by MBE were demonstrated.