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Photoluminescence excitation spectra of high-quality GaAs-${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As quantum wells with various widths have been measured at 77 K. It has been found that the luminescent intensity for excitation photon energies exceeding the barrier band-gap energy exhibits an oscillatory behavior against the well width. The results clearly show that the trapping efficiency of the excited electrons into a well strongly depends on the well width through the resonance and off resonance of the quantum-well levels with the conduction-band bottom of the barrier.

We fabricated light-emitting diodes (LEDs) having an n+-Ge/p+-Si structure with ferromagnetic metal/insulator tunnel contact. Clear electroluminescence (EL) with peak wavelength around 1600 nm from the direct band gap of the Ge layer was successfully observed at room temperature, which indicates that tunnel contacts allow efficient direct-gap EL from the LED. Our result is an important step toward developing Ge-based spin-polarized LEDs.

SiGe-based semiconductor-on-insulator (SOI) substrates have been successfully created by the separation-by-implanted-oxygen technique. Low-energy oxygen ion implantation was performed at 25 kV on a strain-relieved Si0.82Ge0.18 virtual substrate grown on Si(001). A good SOI geometry with a 25-nm top SiGe layer was obtained over a dose window of 2–2.5×1017 cm−2, and a flat SiGe surface and sharp SiGe/SiO2 interfaces were achieved at a low substrate temperature of 550°C. Compositional analysis shows that the top SiGe layer nearly conserves the same composition as the underlying alloy substrate.

Triggered electroluminescence is demonstrated in an electrically biased strained Si1-xGex/Si single quantum well where otherwise spatially separated electron-hole pairs created through impact ionization are made to recombine upon removing the bias voltage, paving a way towards on-demand Si-based single photon emitters and optical memory application

We report a photoluminescence (PL) study of Si1−xGex/Si surface quantum wells (SFQWs). The PL peak energies are found to be affected by strain relaxation in Ge-rich SFQWs while a PL energy lowering was observed for x⩽0.47 as compared to buried quantum wells capped with Si. Exciton localization in the lateral direction is suggested to be the dominant PL mechanism in SFQWs rather than perpendicular confinement effects that are expected for SFQWs. PL degradation and a spectral dominance switch over to newly developing lower energy peaks were clearly observed after prolonged air exposure.

Three-color voltage-controlled emission wavelength switching (VCEWS) is demonstrated in a strained Si/sub 1-x/ Ge/sub x//Si triple quantum well, paving a way toward a Si-based, monolithic multi-color-selectable light emitting diode.

Fabrication technologies of SiGe-OI substrate, using SIMOX, bonding and growth of SiGe on top of Si-OI, are described. Attention was paid to the mechanism how SiGe-OI develops, and how the Ge concentration ceiling can be lifted. It is also indicated the achieved level of lattice relaxation and dislocation density of the SiGe layer in SiGe-OI. Remaining issues, which would be solved in order to develop devices based on SiGe-OI, are pointed out.

Non-linear decay of photoluminescence polarization was observed in strained Si/sub 1-x/Ge/sub x//Si quantum wells under longitudinal electric field at low temperature, which is interpreted in terms of delayed field ionization of excitons.

We have studied postgrowth annealing effects on photoluminescence from strained GaSb quantum dots grown on silicon-on-insulator substrate. A sharp line associated with {311} defects dominates the PL spectra when annealed at above 600 /spl deg/C.

Compound semiconductor quantum dots (QDs) buried in Si have been developed as a new class of band-gap engineered Si-based QDs. It is demonstrated that the excellent optical properties of GaSb/Si QDs allow gain in the near-infrared.

Carrier localization in a disordered potential was studied in type‐I strained Si1−xGex/Si superlattices (SLs) where layer widths have been artificially pseudorandomized. The photoluminescence (PL) intensity under a transverse bias voltage varies monotonically and exhibits a folded increase as the bias is increased as opposed to an ordered SL where only steady drop‐off of PL intensity, symmetric with respect to bias polarity, was observed with increasing field strength. The results indicate that electron and hole wave functions are located spatially apart at zero bias due to disorder‐induced localization. We point out that such a wave‐function displacement is even pronounced for type‐II SL potentials in the presence of monolayer thickness fluctuation.

A Si substrate purposely grown for Si‐based optoelectronic applications is described in this letter. The structure contains a built‐in Si/SiO2 Bragg reflector which is prepared by multiple separation‐by‐implanted‐oxygen technique, where in situ low energy oxygen ion implantation is performed on molecular beam deposited Si to create alternating Si/SiO2 epitaxial bilayers. The quality of the top Si layer was confirmed to be epitaxy‐ready after cross‐sectional transmission electron microscopy. Maximum reflectance at near normal incidence was over 90% for bilayer periods of 4 to 5.

Efficient carrier blocking by a single ‘‘attractive’’ potential is demonstrated using a strained Si0.85Ge0.15/Si quantum well (QW). The carrier blocking becomes pronounced at increased temperatures as most of the carriers at the QW bound state are thermalized with the barrier band edge. The carrier blocking efficiency at 100 K is almost of the order of unity as compared to insufficient carrier blocking due to a large ‘‘repulsive’’ potential associated with SiO2. The carrier blocking arises from efficient hole reemission from the QW that reflects a highly efficient carrier capture to the strained Si1−xGex/Si QWs from Si barriers. It is further shown that a single QW is the optimized geometry for efficient carrier blocking and that multiple well potentials lead only to a reduced blocking efficiency.

The origin of the no‐phonon (NP) luminescence enhancement was explored in Si‐based neighboring confinement structures (NSCs). Post growth annealing was found to lead to quenching of the NP line. Time‐resolved photoluminescence study clarified that the radiative lifetime of the annealed sample increases with increasing temperature, while that of the as‐grown sample shows no significant change at low temperatures (

Temporal decay characteristics of dislocation‐related luminescence bands (D1–D4) were explored in strain‐relieved epitaxial SiGe/Si(100). Close similarity of the decay profiles was observed not only between D1 and D2 bands but also between D3 and D4 bands. The decay transients of the D1 and D2 bands at low temperatures are characterized by long decay times, τ≳200 ns, whereas the D3 and D4 bands exhibit even sharper transients with τ

Spontaneous formation of step‐edge Ge quantum wires and lateral quantum confinement effect are clearly observed in Si submonolayer‐Ge/Si heterostructures grown on Si(100). By depositing submonolayer equivalent Ge (Q), the Ge atoms are found to line up in a wire‐like fashion at the 〈011〉 step edges as revealed by plan‐view transmission electron microscopy. Photoluminescence (PL) peak energy shift with Q due to lateral quantum confinement of holes is clearly observed, and PL features characteristic of the quasi‐one‐dimensional system are also addressed.

Influence of the surface on the radiative recombination is studied in near‐surface (NS) strained Si1−xGex/Si quantum wells (QWs). The luminescence intensity of NSQWs is clearly diminished as the Si cap thickness decreases due to carrier capture to the surface. However, the luminescence diminution discloses a saturable behavior with increasing excitation density. Time‐resolved measurements reveal selective electron capture to surface traps as being the controlling process in the luminescence attenuation, and there is no signature of quantum mechanical hole tunneling to the surface. Results of potential‐biased luminescence lend further support to the electron‐controlled picture of the luminescence attenuation in Si‐based near‐surface geometry.

SiGe quantum wire (QWR) structures grown by gas source molecular beam epitaxy have been characterized by cathodoluminescence (CL) imaging and spectroscopy. At T≊5 K, the CL spectra obtained with a focused beam contain SiGe luminescence features associated with electron‐hole plasma recombination. When the electron beam is defocused three separate SiGe no‐phonon luminescence features and their transverse optic phonon replicas are observed. These features are related to recombination in the SiGe (100) quantum wells (QWs), SiGe (111) QWs, and the SiGe QWRs. The high generation rates associated with the focused electron beam quenches the luminescence and results in electron‐hole plasma recombination in the adjacent SiGe (100) QWs, and saturation of the radiative processes. Defocusing the electron beam reduces the injection level and nonradiative processes, enabling the SiGe bound exciton features to be observed. Monochromatic CL imaging of the SiGe (100) QWs show that at low temperatures the CL image is broade...

Highly efficient photoluminescence (PL) was observed from a new class of AlP/GaP quantum‐confined geometry, neighboring confinement structure (NCS). The PL intensity of AlP/GaP NCSs was even higher than that of a 300‐period AlP/GaP superlattice (SL), and the PL of the NCS exhibited much improved immunity against thermal quenching compared to SLs. The luminescence origin of the NCS was confirmed from the well width dependence of the PL peak shift. The peak shifts were compared with the calculation within the effective mass approximation using the previously reported band parameters for AlP and GaP.

Incorporation kinetics of rare‐earth elements (Er,Pr) in Si during solid source molecular beam epitaxy (MBE) is studied using secondary ion mass spectrometry. Pronounced surface segregation is consistently observed both for Er and Pr in normal MBE growth and their segregation tendencies are even stronger than those of typical dopants. Segregant‐assisted growth (SAG) using Sb was successful in significantly reducing the surface segregation of rare‐earth atoms, thereby opening the possibility of establishing layered structures in normal MBE.

We present a photoluminescence (PL) study on the growth mode changeover during growth of Ge on Si(100) substrates. Intense PL signals originating from both the flat Ge layer and the three‐dimensional (3D) Ge islands are observed from Si/Ge/Si quantum wells with various Ge coverage. The onset of the 3D island formation is determined to be 3.7 monolayers (ML). It is also found that the 3D islands grow with only 3.0 ML of the flat Ge layer retained. This implies that only the 3.0 ML Ge is thermodynamically stable on Si(100) and hence corresponds to the ‘‘equilibrium’’ critical thickness.

Correlation between growth mode transition and photoluminescence (PL) properties of Si1−xGex/Si quantum wells (QWs) with high Ge composition grown by gas‐source molecular beam epitaxy is investigated. With increasing Ge composition in the QWs, significant deviation from the theoretical calculation is observed for x≳0.4 in the emission energy and the activation energy obtained from the temperature dependence of integrated PL intensity. Transmission electron microscopy reveals emergence of islands for x≳0.4, implying strong correlation with the anomalous PL properties. Observed PL properties are shown to be well explained by the breakdown of two‐dimensional growth. An attempt to grow luminescent QWs with flat interfaces is also presented.

Voltage-controlled emission wavelength switching (VCEWS) is demonstrated in a pseudomorphic Si1-xGex/Si double quantum well (DQW). Under cw excitation, photoluminescence (PL) of the DQW was found to exhibit a clear switchover of emission wavelengths under longitudinal electric field, where an extinction ratio of more than 100 was obtained. In the time domain, antiphase oscillations of PL decays synchronized with the polarity switch of driving voltages were observed. The unique band line-up of pseudomorphic Si1-xGex/Si QWs which makes loosely bound electrons susceptible to longitudinal electric fields underlies the operation of VCEWS.

Integration of strained Si1−xGex/Si quantum wells (QWs) in a vertical cavity is demonstrated on a Si substrate with a buried‐oxide using gas source Si molecular beam epitaxy. Spontaneous emission from the SiGe QW is found to be spectrally coupled to the longitudinal modes of a vertical cavity with buried oxide/Si and top Si/air interface mirrors, which is in excellent agreement with separate reflectance measurements. In addition, clear oscillations were observed in photoluminescence excitation spectra for photon energies even above the Si band gap, demonstrating cavity modulation of the incident light absorption.

Compositional abruptness of strained Si/Ge heterointerfaces grown by solid source Si molecular beam epitaxy under supply of atomic hydrogen (AH) was investigated using secondary ion mass spectrometry and reflection high‐energy electron diffraction. Systematic variation of growth temperature and AH exposure pressure revealed that Ge segregation length is a steadily decreasing function of AH coverage on the growth surface.

A systematic study on the optical transition energies of GaInP quantum wells with GaInP/AlInP superlattice barriers was carried out. It was found that the commonly used simple approach to regard the short‐period superlattice barrier as a bulk barrier with a height of its miniband edge brought about a substantial error in calculating the quantized levels in these structures. Instead, it was revealed that an excellent agreement between theory and experiment can be obtained when the wave function of each layer in the superlattice barriers is exactly calculated, demonstrating the validity of the effective‐mass approximation for the quantum‐well structures with superlattice barriers.

In the present work the clean Ge(001)-(2\ifmmode\times\else\texttimes\fi{}1) surface is studied with scanning tunneling microscopy. Many different dimer-vacancy (DV) defects and complexes are seen on the surface. Among them the complex consisting of a missing dimer, a twin missing-dimer, and a dimer between them, i.e., the (1+2)-DV complex, comprises a great majority. Our line scan of the (1+2)-DV complex strongly supports the structural model proposed recently by Wang, Arias, and Joannopoulos for the (1+2)-DV complex on the Si(001) surface. In addition, some information on the interactions between neighboring dimers are also obtained and discussed, and some differences between Ge(001) and Si(001) surfaces are noted.

A study of the photoluminescence properties of Si/SiGe/Si quantum wells grown on separation by oxygen implantation (SIMOX) substrate by gas source molecular beam epitaxy is presented. Intense photoluminescence and carrier confinement in the quantum well are demonstrated. It is found that buried SiO2 isolates the top silicon on insulator from the back Si of SIMOX, and alters the photoluminescence properties of SIMOX compared to those of bulk Si. The SOI layer is found to be free of any strain. A SiO2/Si/SiO2 optical cavity is proposed by depositing SiO2 on SIMOX. A substantial enhancement of the photoluminescence intensity of SiGe quantum well is found, which is attributed to the optical confinement of incident beam in the cavity.

We present the direct observation of exciton localization in a GaAs/AlGaAs quantum well. We have observed the two‐component exponential decay of photoluminescence from heavy‐hole excitons when the excitation density is very low. We have confirmed by measuring the lateral spatial motion of excitons that the fast component is attributable to the radiative recombination of free excitons while the slowly decaying component is due to the localized excitons.

Fabrication of SiGe/Si quantum wire (QWR) structures on a V-groove patterned Si substrate by gas-source Si molecular beam epitaxy (GS-SiMBE) is reported. Cross sectional image of transmission electron microscope (TEM) clarified a crescent shaped SiGe layer at the bottom of the V-groove. Existence of the quantized state in QWR was evidenced by photoluminescence (PL) spectra of the QWR grown by selective epitaxial growth (SEG) technique.

We report on the successful fabrication of SiGe quantum wire structures on a V‐groove patterned substrate by gas‐source selective epitaxial growth technique, and their optical properties. Optical anisotropy, showing the realization of luminescent SiGe layers with wire geometry, was clearly observed in electroluminescence from the SiGe layers grown inside the groove.

Summary form only given. In this paper, we present a viable approach toward SiGe-based SOI. SiGe-based SOI substrates have become more important for both electronics and optoelectronics, since record electron mobility was reported recently by utilizing a tensilely-strained Si channel grown on a strain-relaxed SiGe buffer (Ismail et al., 1995). On the other hand, a high refractive index step at the SiGe-SiO/sub 2/ interface warrants a good reflector (Fukatsu, 1995). Recently, we have reported the fabrication of SiGe-SOI using a high-quality SiGe virtual substrate grown on a step graded Si/sub 1-x/Ge/sub x/ buffer by low-energy SIMOX techniques (Ishikawa et al., 1997, and Fukatsu et al., 1998). However, throughput and production cost is limited by the availability of such virtual substrates, which require complicated epitaxy. Here, we demonstrate a simplified method for SiGe-SOI fabrication by starting with a thick pseudomorphic SiGe-Si layer, which is easily accessible and offers a greater flexibility in design.

We report successful fabrication of SIMOX-based epitaxial Bragg reflectors (BRs) to demonstrate the yet-to-be-explored potential of current SIMOX technology for conceivable optical applications.

SiGe/Si quantum wire structures were successfully fabricated on a V‐groove patterned Si substrate by using gas‐source Si molecular beam epitaxy (GS‐SiMBE). A cross sectional image of transmission electron microscope clarified a crescent‐shaped SiGe layer at the bottom of the V‐groove owing to anisotropy of the growth rate on the different crystal orientations in GS‐SiMBE.

Photoluminescence is reported in strained Si1−xGex/Si(100) coupled double quantum well (CDQW) systems. Systematic red shift of luminescence energy was found in symmetric CDQWs with decreasing width of the Si barrier centered at the DQW, the characteristics of which were used to determine the band offsets. The total and valence‐band discontinuities, ΔEc+ΔEv=156 meV and ΔEv=149±1 meV, were obtained, suggesting that the band alignment is of type I for x=0.177. Variation of the inserted Si barrier width in asymmetric CDWs revealed well‐resolved spectral evolution due to the reduction in tunneling‐controlled carrier escape competing with the radiative recombination.

An n‐type Si contact layer for an electroluminescent (EL) diode was successfully grown on a Si/Si1−xGex/Si single‐quantum‐well (SQW) structure by ‘‘hybrid’’ Si molecular beam epitaxy (MBE) for the first time. The hybrid MBE was performed by regrowing the Si contact layer in a solid‐source MBE chamber after transferring the SQW sample through air from a gas‐source (GS) MBE chamber, in which the starting SQW structure was grown. A (2×1) reconstruction was observed on a GSMBE‐prepared Si(100) surface even after the SQW sample was exposed to air for up to 15 h. An excellent quality of the EL device was evidenced by the sharpest emission lines ever reported in the EL spectra of SiGe system. The spectral features of the EL and photoluminescence were found to be almost identical, and a well‐resolved acoustic phonon replica was observed.

Optical investigation of interdiffusion at Si1−xGex/Si heterointerfaces has been performed for the first time in strained Si1−xGex/Si single quantum wells (SQWs). Photoluminescence (PL) peak energy blue shift of up to 22 meV due to interdiffusion‐induced potential profile modulation was observed after annealing in vacuum. The diffusion coefficients obtained were found to closely follow an Arrhenius behavior with an activation energy of 2.47±0.4 eV. Dramatic increase in the integrated PL intensity was observed in the annealed samples, as a result of the elimination of effective nonradiative centers. Strain relaxation was hardly observed even after 900 °C annealing, indicating the unprecedented structural stability of SQWs in contrast to rather vulnerable thick alloy layers. Anomalous peak red shift, probably due to surface oxidation, was observed by annealing in N2 ambient.

Electroluminescence (EL) was observed at temperatures up to 60 °C in p‐type strained Si0.65Ge0.35/Si multiple‐quantum‐well (MQW) diode structures grown on Si(111) substrates by Si molecular‐beam epitaxy. No‐phonon line and its transverse optical phonon replica were well‐resolved in the room‐temperature EL spectrum for the first time. EL spectrum was dominated by a broad alloy band located below the band‐edge state by ≊100 meV at lower temperatures, which was taken over by clear band‐edge emissions at elevated temperatures. The emission intensity of the alloy band exhibited a saturation behavior with increasing injection current while the band‐edge emission was found to develop with a power exponent of 1.4.

Luminescent Si0.8Ge0.2/Si single quantum well structures with abrupt interfaces were successfully grown by segregant assisted growth (SAG) using Sb adlayer. The emission energy shifted to the lower side compared to the sample grown without Sb adlayer due to the suppression of the surface segregation of Ge atoms. Improved abruptness of interfaces was confirmed by sputter depth profile measurement, where no segregation edge was seen in the sample grown by SAG technique. Light emitting p‐i‐n diode structure can be fabricated by utilizing the incorporation of Sb during SAG.

We explored optical excitation pathways in the multivalley semiconductor Ge in an attempt to expedite selective electron injection into the indirect L-band-edge. An off-peak resonant excitation route was developed, which offers the pumping efficiency outperforming the phonon-assisted near-indirect-edge absorption by more than six orders of magnitude. The valley selectivity results from the intra-valley relaxation that separates electrons and holes in momentum space following excitation. Fortuitously, the widely used green laser, 532 nm, is found to be nearly ideally suited to the efficient L-valley-selective excitation in Ge. Such valley-specific pumping may help clarify the otherwise complicated electron dynamics involving intervalley processes.

Direct band gap shrinkage in Ge-on-insulator (GOI) structures fabricated by lateral liquid-phase epitaxy (LLPE) was investigated by means of micro(μ)-photoluminescence and μ-Raman spectroscopy. The LLPE method, based on the rapid thermal process, was found to be an effective and feasible way to produce highly strained local GOI structures. We observed a significant redshift of direct gap emission amounting to 45 meV from the tensile-strained GOI layer. Strain analysis and temperature dependent PL spectra indicated that a direct band gap shrinkage was mainly due to a tensile strain of about 0.4% induced by rapid crystallization from the Ge melting point during the LLPE process. The local optical properties along the LLPE-grown GOI wire were examined and discussed on the basis of μ-PL mapping images.

Well‐resolved band edge luminescence with no deep level emissions is reported for Si0.8Ge0.2/Si single quantum well (SQW) structures grown at high substrate temperatures (Ts≊620 °C) by solid source Si molecular beam epitaxy (MBE). No‐phonon (NP) transitions due to symmetry‐breaking alloy disordering in SiGe layers and transverse optical (TO) phonon replicas were clearly identified. With decreasing well width, NP and TO emissions were found to show systematic blue shift due to quantum confinement effect. Excellent crystal quality was evidenced by total absence of defect‐related deep level emissions characteristic of photoluminescence spectra of samples grown at lower temperatures, Ts

Spectral blue shift of excitonic transition due to quantum confinement in Si0.84Ge0.16/Si strained‐layer quantum wells, grown by gas‐source Si molecular beam epitaxy, is reported. Intense photoluminescence was observed at elevated temperatures though reduced in intensity due to the thermalization of holes to the Si barriers.

Surface segregation of In atoms during molecular beam epitaxy (MBE) and its influence on the energy levels in InGaAs/GaAs quantum wells (QWs) were systematically studied using secondary‐ion mass spectroscopy (SIMS) and photoluminescence (PL). Strong dependence of In surface segregation on the growth conditions was found; when the growth temperature was raised from 370 to 520 °C, the segregation length was observed to increase from 0.8 up to 2.9 nm, accompanied by an appreciable peak energy shift in the PL spectra of the InGaAs/GaAs QWs. The correlation between In surface segregation and the energy levels in InGaAs/GaAs QWs was clarified for the first time.