ABSTRACT Multiple modulation-doped Ge-dot/SiGe-QW stack structures were grown using MBE, and proc... more ABSTRACT Multiple modulation-doped Ge-dot/SiGe-QW stack structures were grown using MBE, and processed as FET devices for mid/far infrared detection. From a non-optimized device, a broadband photoresponse has been observed in the mid-infrared range of 3-15 μm. A peak responsivity was estimated to be as high as 100 mA/W at T= 20 K. This work indicates that SiGE QD/QW structures using the lateral transport geometry can be a potential candidate for photodetectors operating in far-infrared range.
The optical properties of Ge quantum dots embedded in Si were investigated by means of photolumin... more The optical properties of Ge quantum dots embedded in Si were investigated by means of photoluminescence, with temperature and excitation power density as variable parameters. Two different types of recombination processes related to the Ge quantum dots were observed. A transfer from the spatially indirect to the spatially direct recombination in the type-II band lineup was observed with increasing temperature. A blueshift of the spatially indirect Ge quantum-dot-emission energy with increasing excitation power is ascribed to band bending at the type-II Si/Ge interface for high carrier densities. Comparative studies were performed on uncapped Ge dot structures.
A single-grid UIIV-compatible ion source was used to provide partially-ionized accelerated In+ do... more A single-grid UIIV-compatible ion source was used to provide partially-ionized accelerated In+ dopant beams during Si growth by molecular beam epitaxy (MBE). Indium incorporation probabilities in 800 *C MBE Si(lO0). as measured by secondary ion mass spectrometry, ranged from < 10-5 (the detection limit) for thermal In to values of 0.02-0.7 for In+ acceleration energies El, between 50 and 400 eV. Temperature-dependent ltall-effect and resistivity measurements were carried out on Si films grown at 800 C with Eil = 200 eV. Indium was incorporated substitutionally in electrically active sites over the entire concentration range examined. 1 0 16-1 0 19 cm-3, with an acceptor level ionization energy of 165 meV. The 11I meV level associated with In-C complexes and the 18 meV "supershallow" level reported for In ion-implanted Si were not observed. Roomtemperature hole mobilities u2 were higher than both annealed In-ion-implanted Si and Irvin's values for bulk Si. Phonon scattering was found to dominate at temperatures between 100 and 330 K and /i varied as T-2 2 .
ABSTRACTBroad photoluminescence (PL) bands with energy position ranging from 0.70 to 1.03 eV, are... more ABSTRACTBroad photoluminescence (PL) bands with energy position ranging from 0.70 to 1.03 eV, are shown to appear in Si epilayers and Sil−xGex/Si heterostructures grown by molecular beam epitaxy under non-optimized conditions. The presence of a particular broad band (BB) is found to be determined by the growth conditions, in particular by the bias applied to the substrate during the growth. Ion bombardment during the growth is shown to be a main factor enhancing the formation of BB-related defects. To clarify their origin, the effects of post-growth hydrogen treatments and of an external magnetic field are investigated. The BBs are shown to be related to various types of radiative centres, either point or extended defects. A correlation between X-ray diffraction and PL measurements is also observed. The spatial location of the BB-related defects is inferred by results from optically detected cyclotron resonance (ODCR) and PL excitation studies.
The radiative recombination processes of the two-dimensional hole gas formed in SiGe quantum well... more The radiative recombination processes of the two-dimensional hole gas formed in SiGe quantum wells (QWs) due to modulation doping are studied in details by photoluminescence (PL) spectroscopy. Boron-modulation-doped Si/Sil−xGex/Si heterostructures grown by molecular beam epitaxy (MBE) are studied. It is shown that charge transfer of holes from the doped Si layers causes the filling of the SiGe QWs leading to an appearance of a broad asymmetric PL band with a characteristic sharp high energy cut-off and enhanced recombination near the Fermi edge. A reduction of this PL enhancement is observed with an increase of measuring temperature. The PL bandwidth and the high energy cut-off are found to vary with either the doping level or the spatial separation between the delta-doped layers and the QWs. This PL band is argued to arise from the recombination of the holes in the QWs and electrons confined near the QWs as a result of the band bending induced by the delta-doping. The shape of the PL band with enhanced intensity near the Fermi edge are discussed in terms of the phase space filling and many-body effects.
Ge dots embedded in Si offer the possibility of Si-based light detection at 1.3-1.55 μm. In this ... more Ge dots embedded in Si offer the possibility of Si-based light detection at 1.3-1.55 μm. In this communication, we report a very efficient photo-detector based on a Si/SiGe heterojunction bipolar transistor structure with 10 Ge dot layers (8 ML Ge each) incorporated in the basecollector junction. The device structures were grown using low-temperature molecular beam epitaxy, and fabricated for both normal and edge incidence with no electrical contact to the base. The processed Ge-dot transistor detectors revealed a rather low dark current density, 0.01 mA/cm2 at -2 V. Photoconductivity measurements were performed at room temperature. At 1.31 μm, responsivity values of 50 mA/W at normal incidence have been directly measured at Vce = -4 V, without involving any rescaling factor due to light coupling. This value is a ∼250-fold increase compared to a reference p-i-n diode with the same dot layer structure, due to the current amplification function of the transistor. For a rib waveguide device, a very high responsivity value of about 470 mA/W (Vce = -4V) has been obtained at 1.31 μm. Measurements were also performed at 1.55 μm, and the photo-response of the waveguide phototransistor was 25 mA/W, which is again a large improvement compared with the reference waveguide photodiode (∼1 mA/W). Moreover, time-resolved photoconductivity measurements have been carried out. The results have indicated that the device frequency performance is primarily limited by the emitterbase junction capacitance.
We report on the quantum-confined Stark effect for spatially indirect transitions in Stranski-Kra... more We report on the quantum-confined Stark effect for spatially indirect transitions in Stranski-Krastanov grown type-II Si/ Ge quantum dots. A linear blueshift of the spatially indirect transition is observed at increasing electric field in contrast to the commonly observed redshift for type-I transitions. A shift of the emission-peak position and different quenching rates of the photoluminescence for p-in and n-i-p diodes at increased electric field and temperature indicate a deeper notch potential for electrons above the dot than below due to a strain-induced asymmetry in the band alignment.
Influence of postgrowth hydrogen treatments on nonradiative recombination centers in undoped and ... more Influence of postgrowth hydrogen treatments on nonradiative recombination centers in undoped and B-doped Si epilayers, grown by molecular beam epitaxy at low temperatures, are studied by optical detection of magnetic resonance. Hydrogen passivation of the dominant nonradiative defects in undoped Si is shown to be rather effective, whereas in the B-doped Si epilayers the effects of the hydrogen treatment of the same defects are found to be only marginal. Possible mechanisms for this are discussed. Information on two new nonradiative defects is provided.
Dopant surface segregation during molecular beam epitaxy (MBE) growth is a serious problem for co... more Dopant surface segregation during molecular beam epitaxy (MBE) growth is a serious problem for controlling the doping profiles. To understand the segregation mechanism is essential. In this study, we report the B segregation ratio values, determined using concentration transient analysis based on secondary ion mass spectrometry (SIMS) measurements, for Si and SiGe, respectively. For a comparison, segregation ratio calculations based on a simplified two-site exchange model were performed. It is found that the surface segregation effects of B and Ge during MBE Si growth are interconnected, where the lattice strain likely plays an important role.
ABSTRACT Different aspects of boron doping have been studied for silicon and Si1−xGex layers grow... more ABSTRACT Different aspects of boron doping have been studied for silicon and Si1−xGex layers grown by molecular beam epitaxy. Boron segregation at high doping levels was measured in situ in both cases and it was found to be relatively suppressed in the alloy samples. Some possible mechanisms for this effect are discussed. Germanium segregation was also measured during the growth of the first monolayers in Si1−xGex structures. The limits for boron incorporation and activation as dopant in such structures are discussed on the basis of electrical measurements.
Ge quantum dots embedded in Si are studied by means of photoluminescence (PL). The temperature de... more Ge quantum dots embedded in Si are studied by means of photoluminescence (PL). The temperature dependent PL measurements show two different types of recombination processes related to the quantum dots. We ascribe a peak near 0.80 eV to the spatially indirect recombination in the type-II band lineup where the electron is located in the surrounding Si close to the interface and the hole in the Ge dot. Furthermore, a peak near 0.85 eV is attributed to the spatially direct recombination. We observe a transition from the spatially indirect to the spatially direct recombination as the temperature is increased. The measurements also show an up-shift of the Ge quantum dot emission energy with increasing excitation power density. The blueshift is primarily ascribed to an enhanced confinement of the electron associated with the increased band bending at the type-II Si/Ge interface at high carrier densities. Comparison is made with results, derived from measurements on uncapped samples. For these uncapped samples, no energy shifts due to excitation power or temperatures are observed in contrast to the capped samples.
The fabrication and room temperature light emission of a series of 50 nm Si-SiGe quantum dot diod... more The fabrication and room temperature light emission of a series of 50 nm Si-SiGe quantum dot diodes emitting light at about 1.3 µm are reported. These diodes are made from molecular beam epitaxially (MBE) grown p-in structures with the active region being either a superlattice or a single heterolayer. It was found that the electrical-to-optical power conversion efficiency of the diodes was 0.14%. Possible light emission mechanisms and technological aspects of the work are discussed.
Physica E-low-dimensional Systems & Nanostructures, Mar 1, 2003
ABSTRACT The aim of this work is to develop a Si/SiGe HBT-type phototransistor with several Ge do... more ABSTRACT The aim of this work is to develop a Si/SiGe HBT-type phototransistor with several Ge dot layers incorporated in the collector, in order to obtain improved light detectivity at 1.3–. The MBE grown HBT detectors are of n–p–n type and based on a multilayer structure containing 10 Ge-dot layers (8 ML in each layer, separated by Si spacer) in the base-collector junction. The transistors were processed for normal incidence or with waveguide geometry where the light is coupled through the edge of the sample. The measured breakdown voltage, BVceo, was about . Compared to a p–i–n reference photodiode with the same dot layer structure, photoconductivity measurements show that the responsivity is improved by a factor of 60 for normal incidence at . When the light is coupled through the edge of the device, the detectivity is even further enhanced. The measured photo-responsivity is more than 100 and at 1.3 and , respectively.
ABSTRACT Multiple modulation-doped Ge-dot/SiGe-QW stack structures were grown using MBE, and proc... more ABSTRACT Multiple modulation-doped Ge-dot/SiGe-QW stack structures were grown using MBE, and processed as FET devices for mid/far infrared detection. From a non-optimized device, a broadband photoresponse has been observed in the mid-infrared range of 3-15 μm. A peak responsivity was estimated to be as high as 100 mA/W at T= 20 K. This work indicates that SiGE QD/QW structures using the lateral transport geometry can be a potential candidate for photodetectors operating in far-infrared range.
The optical properties of Ge quantum dots embedded in Si were investigated by means of photolumin... more The optical properties of Ge quantum dots embedded in Si were investigated by means of photoluminescence, with temperature and excitation power density as variable parameters. Two different types of recombination processes related to the Ge quantum dots were observed. A transfer from the spatially indirect to the spatially direct recombination in the type-II band lineup was observed with increasing temperature. A blueshift of the spatially indirect Ge quantum-dot-emission energy with increasing excitation power is ascribed to band bending at the type-II Si/Ge interface for high carrier densities. Comparative studies were performed on uncapped Ge dot structures.
A single-grid UIIV-compatible ion source was used to provide partially-ionized accelerated In+ do... more A single-grid UIIV-compatible ion source was used to provide partially-ionized accelerated In+ dopant beams during Si growth by molecular beam epitaxy (MBE). Indium incorporation probabilities in 800 *C MBE Si(lO0). as measured by secondary ion mass spectrometry, ranged from < 10-5 (the detection limit) for thermal In to values of 0.02-0.7 for In+ acceleration energies El, between 50 and 400 eV. Temperature-dependent ltall-effect and resistivity measurements were carried out on Si films grown at 800 C with Eil = 200 eV. Indium was incorporated substitutionally in electrically active sites over the entire concentration range examined. 1 0 16-1 0 19 cm-3, with an acceptor level ionization energy of 165 meV. The 11I meV level associated with In-C complexes and the 18 meV "supershallow" level reported for In ion-implanted Si were not observed. Roomtemperature hole mobilities u2 were higher than both annealed In-ion-implanted Si and Irvin's values for bulk Si. Phonon scattering was found to dominate at temperatures between 100 and 330 K and /i varied as T-2 2 .
ABSTRACTBroad photoluminescence (PL) bands with energy position ranging from 0.70 to 1.03 eV, are... more ABSTRACTBroad photoluminescence (PL) bands with energy position ranging from 0.70 to 1.03 eV, are shown to appear in Si epilayers and Sil−xGex/Si heterostructures grown by molecular beam epitaxy under non-optimized conditions. The presence of a particular broad band (BB) is found to be determined by the growth conditions, in particular by the bias applied to the substrate during the growth. Ion bombardment during the growth is shown to be a main factor enhancing the formation of BB-related defects. To clarify their origin, the effects of post-growth hydrogen treatments and of an external magnetic field are investigated. The BBs are shown to be related to various types of radiative centres, either point or extended defects. A correlation between X-ray diffraction and PL measurements is also observed. The spatial location of the BB-related defects is inferred by results from optically detected cyclotron resonance (ODCR) and PL excitation studies.
The radiative recombination processes of the two-dimensional hole gas formed in SiGe quantum well... more The radiative recombination processes of the two-dimensional hole gas formed in SiGe quantum wells (QWs) due to modulation doping are studied in details by photoluminescence (PL) spectroscopy. Boron-modulation-doped Si/Sil−xGex/Si heterostructures grown by molecular beam epitaxy (MBE) are studied. It is shown that charge transfer of holes from the doped Si layers causes the filling of the SiGe QWs leading to an appearance of a broad asymmetric PL band with a characteristic sharp high energy cut-off and enhanced recombination near the Fermi edge. A reduction of this PL enhancement is observed with an increase of measuring temperature. The PL bandwidth and the high energy cut-off are found to vary with either the doping level or the spatial separation between the delta-doped layers and the QWs. This PL band is argued to arise from the recombination of the holes in the QWs and electrons confined near the QWs as a result of the band bending induced by the delta-doping. The shape of the PL band with enhanced intensity near the Fermi edge are discussed in terms of the phase space filling and many-body effects.
Ge dots embedded in Si offer the possibility of Si-based light detection at 1.3-1.55 μm. In this ... more Ge dots embedded in Si offer the possibility of Si-based light detection at 1.3-1.55 μm. In this communication, we report a very efficient photo-detector based on a Si/SiGe heterojunction bipolar transistor structure with 10 Ge dot layers (8 ML Ge each) incorporated in the basecollector junction. The device structures were grown using low-temperature molecular beam epitaxy, and fabricated for both normal and edge incidence with no electrical contact to the base. The processed Ge-dot transistor detectors revealed a rather low dark current density, 0.01 mA/cm2 at -2 V. Photoconductivity measurements were performed at room temperature. At 1.31 μm, responsivity values of 50 mA/W at normal incidence have been directly measured at Vce = -4 V, without involving any rescaling factor due to light coupling. This value is a ∼250-fold increase compared to a reference p-i-n diode with the same dot layer structure, due to the current amplification function of the transistor. For a rib waveguide device, a very high responsivity value of about 470 mA/W (Vce = -4V) has been obtained at 1.31 μm. Measurements were also performed at 1.55 μm, and the photo-response of the waveguide phototransistor was 25 mA/W, which is again a large improvement compared with the reference waveguide photodiode (∼1 mA/W). Moreover, time-resolved photoconductivity measurements have been carried out. The results have indicated that the device frequency performance is primarily limited by the emitterbase junction capacitance.
We report on the quantum-confined Stark effect for spatially indirect transitions in Stranski-Kra... more We report on the quantum-confined Stark effect for spatially indirect transitions in Stranski-Krastanov grown type-II Si/ Ge quantum dots. A linear blueshift of the spatially indirect transition is observed at increasing electric field in contrast to the commonly observed redshift for type-I transitions. A shift of the emission-peak position and different quenching rates of the photoluminescence for p-in and n-i-p diodes at increased electric field and temperature indicate a deeper notch potential for electrons above the dot than below due to a strain-induced asymmetry in the band alignment.
Influence of postgrowth hydrogen treatments on nonradiative recombination centers in undoped and ... more Influence of postgrowth hydrogen treatments on nonradiative recombination centers in undoped and B-doped Si epilayers, grown by molecular beam epitaxy at low temperatures, are studied by optical detection of magnetic resonance. Hydrogen passivation of the dominant nonradiative defects in undoped Si is shown to be rather effective, whereas in the B-doped Si epilayers the effects of the hydrogen treatment of the same defects are found to be only marginal. Possible mechanisms for this are discussed. Information on two new nonradiative defects is provided.
Dopant surface segregation during molecular beam epitaxy (MBE) growth is a serious problem for co... more Dopant surface segregation during molecular beam epitaxy (MBE) growth is a serious problem for controlling the doping profiles. To understand the segregation mechanism is essential. In this study, we report the B segregation ratio values, determined using concentration transient analysis based on secondary ion mass spectrometry (SIMS) measurements, for Si and SiGe, respectively. For a comparison, segregation ratio calculations based on a simplified two-site exchange model were performed. It is found that the surface segregation effects of B and Ge during MBE Si growth are interconnected, where the lattice strain likely plays an important role.
ABSTRACT Different aspects of boron doping have been studied for silicon and Si1−xGex layers grow... more ABSTRACT Different aspects of boron doping have been studied for silicon and Si1−xGex layers grown by molecular beam epitaxy. Boron segregation at high doping levels was measured in situ in both cases and it was found to be relatively suppressed in the alloy samples. Some possible mechanisms for this effect are discussed. Germanium segregation was also measured during the growth of the first monolayers in Si1−xGex structures. The limits for boron incorporation and activation as dopant in such structures are discussed on the basis of electrical measurements.
Ge quantum dots embedded in Si are studied by means of photoluminescence (PL). The temperature de... more Ge quantum dots embedded in Si are studied by means of photoluminescence (PL). The temperature dependent PL measurements show two different types of recombination processes related to the quantum dots. We ascribe a peak near 0.80 eV to the spatially indirect recombination in the type-II band lineup where the electron is located in the surrounding Si close to the interface and the hole in the Ge dot. Furthermore, a peak near 0.85 eV is attributed to the spatially direct recombination. We observe a transition from the spatially indirect to the spatially direct recombination as the temperature is increased. The measurements also show an up-shift of the Ge quantum dot emission energy with increasing excitation power density. The blueshift is primarily ascribed to an enhanced confinement of the electron associated with the increased band bending at the type-II Si/Ge interface at high carrier densities. Comparison is made with results, derived from measurements on uncapped samples. For these uncapped samples, no energy shifts due to excitation power or temperatures are observed in contrast to the capped samples.
The fabrication and room temperature light emission of a series of 50 nm Si-SiGe quantum dot diod... more The fabrication and room temperature light emission of a series of 50 nm Si-SiGe quantum dot diodes emitting light at about 1.3 µm are reported. These diodes are made from molecular beam epitaxially (MBE) grown p-in structures with the active region being either a superlattice or a single heterolayer. It was found that the electrical-to-optical power conversion efficiency of the diodes was 0.14%. Possible light emission mechanisms and technological aspects of the work are discussed.
Physica E-low-dimensional Systems & Nanostructures, Mar 1, 2003
ABSTRACT The aim of this work is to develop a Si/SiGe HBT-type phototransistor with several Ge do... more ABSTRACT The aim of this work is to develop a Si/SiGe HBT-type phototransistor with several Ge dot layers incorporated in the collector, in order to obtain improved light detectivity at 1.3–. The MBE grown HBT detectors are of n–p–n type and based on a multilayer structure containing 10 Ge-dot layers (8 ML in each layer, separated by Si spacer) in the base-collector junction. The transistors were processed for normal incidence or with waveguide geometry where the light is coupled through the edge of the sample. The measured breakdown voltage, BVceo, was about . Compared to a p–i–n reference photodiode with the same dot layer structure, photoconductivity measurements show that the responsivity is improved by a factor of 60 for normal incidence at . When the light is coupled through the edge of the device, the detectivity is even further enhanced. The measured photo-responsivity is more than 100 and at 1.3 and , respectively.
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Papers by Wei-Xin Ni