Si Multiple Quantum Research Articles

Observation of a large dc Kerr effect in the intervalence subband of Si1−xGex/Si multiple quantum wells

The observation of intervalence subband transitions induced third-order nonlinear effect in Si1−xGex/Si multiple quantum wells was reported. The dc Kerr in a symmetric quantum well, in which the linear-electro-optic effect is absent, was measured. The spectra of the absorption coefficient changes Δα under external field was obtained using experimental Fourier transform infrared absorption data. The resulting changes of refractive index due to quantum-confined Stark effect was obtained by Kramers–Kronig relations based on measured absorption data. Moreover, under both 45° incident angle and normal incidence, TE-active transition was excited and corresponding changes of refractive index under dc field was observed. The measured large changes of refractive index make Si1−xGex/Si multiple quantum wells quite attractive for potential application in electro-optic devices such as modulators and switches.

Valence intersubband transitions stimulated by polarized light in Si1−xGex/Si multiple quantum wells

The polarization properties of valence intersubband transitions in p-type doped Si0.75Ge0.25/Si multiple quantum wells are investigated using Fourier infrared absorption spectra. It is found that TE and TM polarized light beams both can excite valence intersubband transitions in SiGe/Si multiple quantum wells. Meanwhile, a strong dichroism is also observed for HH1→HH2 transitions because the splitting peaks between TE and TM absorption have different activity under s- and p-polarized states. However, the strong mixing of light- and heavy-hole states in split-off state SO1 weakens the dichroism of HH1→SO1 transitions.

Infrared absorption in p-type [formula omitted] quantum wells: Intersubband transition and free carrier contributions

Infrared absorption has been investigated in high quality p-type SiGe Si multiple quantum wells grown by UHV-CVD. Both intersubband and free carrier absorptions have been quantitatively analyzed using a modified Drude model, and their relative contributions to the total absorbance have been determined as a function of polarization and doping level. The absorption coefficients have been deduced for radiation electric field along the growth axis and parallel to the plane of the layers.

Doping dependence of intersubband transitions in [formula omitted] multiple quantum wells

Effect of doping on the intersubband transition of p-type SiGe Si multiple quantum wells has been studied. Three Si 0.6 Ge 0.4 Si multiple quantum well structures (10 periods each) with doping concentrations from 1 × 10 19 cm −3 to 4 × 10 20 cm −3 grown by Si molecular beam epitaxy were used in the experiment. It was found that as the doping in the quantum well is increased the peak position of the transition moves to a higher energy. This behavior is due to the formation of δ-like potential well inside the quantum well and the collective nature of the intersubband transitions. In orther to assess the experimental data, transition energies for different doping concentrations are calculated using a self-consistent scheme. The experimental data are in good agreement with the calculation if the many-body effects are incorporated. This work demonstrates the importance of doping effects in the design of quantum well IR detectors.

Si1−xGex/Si multiple quantum wells on Si(100) and Si(110) for infrared absorption

Optical emission and absorption properties of Si1−xGex/Si superlattices grown on (100) and (110) Si substrates were investigated to determine the optimal growth conditions for these structures to be used as infrared detectors. Fully strained Si1−xGex/Si superlattices were grown by molecular beam epitaxy (MBE) and examined using low-temperature photoluminescence to identify no-phonon and phonon-replica interband transitions across the alloy band gap. Phonon-resolved emission was most intense for undoped quantum wells grown at 710 °C for both silicon orientations. Room-temperature absorption measurements were conducted on (100) and (110) Si1−xGex/Si superlattices using Fourier transform spectroscopy while varying incident electric field polarization. Strong intersubband absorption was observed at 7.8 μm from a sample composed of 15 quantum wells of 40 Å Si0.8Ge0.2 separated by 300 Å of Si grown on (100) Si by MBE at 550 °C. Valence band wells were doped with a boron concentration of 5×1019 cm−3. No intersubband transitions were observed on similar (110) Si1−xGex/Si superlattices ranging in boron dopant from 1×1019 to 8×1019 cm−3, nor from any structure grown at 710 °C. This peak was most likely masked by free-carrier absorption which dominated the spectrum.

Absorption in p-Si1−xGex quantum well detectors

The normal incidence absorption between 2 and 14 μm in a pseudomorphic p-Si0.81Ge0.19/Si multiple quantum well sample with doping 5×1012 cm−2 per well is described by a Drude conductivity characteristic of free carriers, with an in-plane mobility of 32 cm2/V s and a relaxation time of 5.5 fs at 77 K. When the absorption is scaled with dopant concentration these parameters predict quantum efficiencies for quantum well infrared photodetectors in reasonable agreement with experiment.

Parametric Investigation of Si1-xGex/Si Multiple Quantum Well Growth

Si0.8Ge0.2/Si multiple quantum wells (3 nm/30 nm) have been grown by molecular beam epitaxy and have been characterized using photoluminescence (PL), secondary ion mass spectrometry, and transmission electron microscopy. A parametric investigation relating the growth conditions to the PL was carried out. The existence of phonon-resolved band-edge PL appears to be strongly related to the background impurity concentration. The connection between phonon-resolved band-edge PL and higher substrate growth temperatures is probably due to the temperature-dependent incorporation of impurities. In the as-grown samples a correlation of the broad PL with platelet density in the quantum wells was observed. The broad PL may be associated with Cr at the platelets since a high temperature ( 710° C) anneal extinguished the broad PL and caused a reduction in the Cr found in the quantum wells, but had no effect on the platelet density.

Room temperature electroabsorption in a Ge/sub x/Si/sub 1-x/ PIN photodiode

We present room temperature electroabsorption measurements in a Ge/sub 0.2/Si/sub 0.8/ pin photodiode. The results appear to be very similar to those reported earlier on Ge/sub x/Si/sub 1-x//Si multiple quantum wells. >

Room-temperature measurements of strong electroabsorption effect in GexSi1−x/Si multiple quantum wells grown by remote plasma-enhanced chemical vapor deposition

We have used photocurrent measurements to demonstrate a strong electroabsorption effect in GexSi1−x/Si multiple quantum wells grown by remote plasma-enhanced chemical vapor deposition. Large voltage-induced shifts in absorption are observed at room temperature in the wavelength range from 1.2 to 1.58 μm. We anticipate that the results can be extended to fabricate GexSi1−x optoelectronic devices operating at room temperature.

Observation of electroluminescence above room temperature in strained p-type Si 0.65Ge 0.35/Si(111) multiple quantum wells

Electroluminescence (EL) was observed above room temperature in p-type strained Si 0.65Ge 0.35/Si multiple quantum wells (MQWs) grown on Si(111) substrates by Si MBE. No-phonon band-edge luminescence and its transverse optical phonon replica were well resolved in the EL spectrum at 330 K. In contrast, a broad alloy band dominated the spectrum at lower temperatures, located approximately 100 meV below the band-edge state. Such alloy band emission was found to develop in QWs with a higher Ge content, x > 0.3. Both the emission of this alloy band and the quantized band-edge states were observed around 180 K. The emission intensity of the alloy band tended to saturate with increasing current injection, whereas the band-edge emission was found to increase superlinearly, suggesting the alloy band emission to be of localized excitonic origin.

Optical and structural investigation of SiGe/Si quantum wells

In this letter we report photoluminescence and structural results obtained on asymmetrically strained Si0.7Ge0.3/Si single and multiple quantum wells epitaxially grown by low pressure chemical vapor deposition. Well-resolved peaks were obtained which can be attributed to quantum well excitons and their transversal optical phonon replica. A good correlation between peak properties and structure results was found. From the photoluminescence peak energies a valence band offset of 0.27 eV and an effective hole mass of 0.25 were estimated.

Normal incident infrared detector using intervalence band absorption of Si/sub 1-x/Ge/sub x//Si multiple quantum wells

The authors report the demonstration of normal incident infrared detection using intervalence-band absorption in Si/sub 1-x/Ge/sub x//Si multiple quantum wells. In this case, the transition occurs between different hole bands due to the mixing of the s-like Gamma -conduction band Bloch state with p-like hole bands, particularly at large k values. This mechanism is different from the previously reported free-carrier absorption process.

Intersubband absorption in Si 1-xGe x/Si and δ-dope Si multiple quantum wells

The hole intersubband infrared absorption in SiGe/Si and δ-doped Si multiple quantum wells is reported. Typical structures consist of 10 periods of 300 Å thick undoped Si barriers and either SiGe quantum wells of 48 Å or boron doped Si quantum wells of 35 Å wide. Near 100% infrared absorption is measured by a FTIR spectrometer using waveguide structures. The polarization dependent spectra show good agreement with the intersubband selection rule. In SiGe/Si multiple quantum well structures, the transition energy depends on the Ge fraction and the well thickness and the transition between the first two heavy-hole subbands are observed. In the case of the δ-doped structures, the resonance absorption and peak energy can be turned by varying the doping concentration in the δ-doped layer. The experimentally observed transition energy positions are considerably higher than that estimated using the Hartree approximation. With the inclusion of the many-body effect, the hole-hole exchange interaction, the estimated subband energy separations are in close agreement with the experimentally observed values. This observation suggests that the multiple quantum well IR detectors may be used for Si-based optoelectronics.

Hole intersubband absorption in δ-doped multiple Si layers

The hole intersubband infrared absorption in δ-doped Si multiple quantum wells is observed for the first time. The structures used consist of ten periods of boron-doped Si quantum wells and undoped Si barriers. Near 100% infrared absorption is measured by a Fourier transform infrared spectrometer using waveguide structures. The observed absorption peaks ranging between 3 and 7 μm, which are mainly due to the transition between the first two heavy hole subbands. This absorption peak can be tuned by varying the doping concentration in the δ-doped layer. The polarization-dependent spectra show good agreement with the intersubband selection rule. The estimated peak energy positions using a self-consistency calculation with exchange effects as a perturbation agree reasonably well with the experimental observation. This observation suggests the use of multiple quantum well for IR detector application using Si technology

Observation of Intersubband Absorption in Boron δ-Doped Si Layers

ABSTRACTStrong hole intersubband infrared absorption in δ-doped Si multiple quantum wells is observed. The structures consist of 10 periods of boron doped Si quantum wells and undoped Si barriers. Near 100 % infrared absorption is measured by FTIR spectrometer using waveguide structures. Absorption peaks ranging between 3–7 μm are measured, and these peaks can be tuned by varying the doping concentration in the δ-doped layer. Polarization dependence has been verified to agree with the intersubband selection rule. The estimated peak energy positions using a self-consistency calculation are considerably lower than experimental values, probably due to a large exchange energy of many body effects. This observation suggests multiple quantum well IR detectors using Si- technology.

Punch-through currents in P + NP + and N + PN + sandwich structures—I. Introduction and basic calculations: J. Lohstroh, J. J. M. Koomen, A. T. Van Zanten and R. H. W. Salters. Solid-St. Electron.24 (9), 805 (1981)

The hole intersubband infrared absorption in SiGe/Si and δ-doped Si multiple quantum wells is reported. Typical structures consist of 10 periods of 300 Å thick undoped Si barriers and either SiGe quantum wells of 48 Å or boron doped Si quantum wells of 35 Å wide. Near 100% infrared absorption is measured by a FTIR spectrometer using waveguide structures. The polarization dependent spectra show good agreement with the intersubband selection rule. In SiGe/Si multiple quantum well structures, the transition energy depends on the Ge fraction and the well thickness and the transition between the first two heavy-hole subbands are observed. In the case of the δ-doped structures, the resonance absorption and peak energy can be turned by varying the doping concentration in the δ-doped layer. The experimentally observed transition energy positions are considerably higher than that estimated using the Hartree approximation. With the inclusion of the many-body effect, the hole-hole exchange interaction, the estimated subband energy separations are in close agreement with the experimentally observed values. This observation suggests that the multiple quantum well IR detectors may be used for Si-based optoelectronics.