Miniband-related 1.4–1.8 μm luminescence of Ge/Si quantum dot superlattices

The luminescence properties of highly strained, Sb-doped Ge/Si multi-layer heterostructures with incorporated Ge quantum dots (QDs) are studied. Calculations of the electronic band structure and luminescence measurements prove the existence of an electron miniband within the columns of the QDs. Miniband formation results in a conversion of the indirect to a quasi-direct excitons takes place. The optical transitions between electron states within the miniband and hole states within QDs are responsible for an intense luminescence in the 1.4–1.8 µm range, which is maintained up to room temperature. At 300 K, a light emitting diode based on such Ge/Si QD superlattices demonstrates an external quantum efficiency of 0.04% at a wavelength of 1.55 µm

Voltage-tuning in multi-color quantum well infrared photodetector stacks

A systematic study of stacked quantum well infrared photodetectors is undertaken to improve the understanding of the voltage-tunable multi-color spectral response. The multi-color capability is achieved by sequentially growing conventional one color detectors, separated by conducting layers. The behavior of the stacked devices is proven to correspond to the individual detectors simply acting in series with each other. The dc resistance, photocurrent and dynamic resistance characteristics of the individual detectors are examined and correlated with the voltage-tuning in the stack.NRC publication: Ye

Mid-wavelength infrared detection with InxGa1-xAs/Al0.45Ga0.55As multiple quantum well structures

We demonstrate the detection of mid-wavelength (3-5 mu m) infrared radiation by intersubband transitions in InxGa1-xAs/Al0.45Ga0.55As multiple quantum well structures grown on GaAs substrates. The peak detector response is shifted from 4.8 to 4.3 mu m by increasing the indium fraction from x=0.05 to x=0.20, while simultaneously decreasing the well width to keep the first excited eigenstate near the top of the wells. These detectors can be combined in detector stacks with conventional long-wavelength (8-12 mu m) infrared quantum well detectors for multi-band imaging applications.NRC publication: Ye

Visible photoluminescence from biexcitons in Si1-xGex quantum wells

We have observed visible photoluminescence, at photon energies nearly twice those of the usual near-infrared excitonic emissions, from thin Si1-xGex quantum wells at liquid He temperatures. This confirms that a significant biexciton population is present in such samples under excitation conditions normally used for near-infrared photo-luminescence measurements. The intensity of the visible luminescence increases linearly with excitation density, consistent with the biexcitons being localized by fluctuations in alloy content. The biexciton lifetime is observed to vary with the Si1-xGex quantum well width indicating an enhancement of the overlap of the particle wave functions by the quantum confinement.NRC publication: Ye

Infrared detectors and arrays made of GaAs/AlGaAs quantum wells

NRC publication: Ye

Photoluminescence of thin Si1-xGex quantum wells

Well-resolved band edge photoluminescence spectra were obtained from SiGe quantum wells of various widths. In addition to the usual shallow bound exciton features, we observed a highly efficient deeper luminescence process, under conditions of low excitation density, in thick SiGe quantum wells. This luminescence band can be attributed to excitons localized by fluctuations in alloy concentration. The binding energy of the localized exciton feature is found to decrease with decreasing well width. In the thinnest quantum well samples only a single luminescence feature is observed at all power levels, while in several other thin quantum well samples having very sharp lines the localized exciton feature appears at higher energy than the bound exciton. Despite these changes in the spectra, the localized exciton luminescence could be identified in all cases by its characteristic intensity saturation at low excitation power density, as well as its slow decay time ( 3c 1 ms). The mechanism behind the changes in the localized exciton luminescence may originate from limiting the exciton motion to two dimensions in thin wells, which at low temperatures would hinder migration to the lowest energy alloy fluctuation centers.Peer reviewed: YesNRC publication: Ye

Well-resolved band-edge photoluminescence of excitons confined in strained Si1-xGex quantum wells

NRC publication: Ye

Exciton luminescence in Si1-xGex/Si heterostructures grown by molecular beam epitaxy

Coherent Si1 12xGex alloys and multilayers synthesized by molecular beam epitaxy (MBE) on Si(100) substrates have been characterized by low-temperature photoluminescence (PL) spectroscopy and transmission electron microscopy (TEM). Phonon-resolved transitions originating from excitons bound to shallow impurities were observed in addition to a broad band of intense luminescence. The broad PL band was predominant when the alloy layer thickness was greater than 40\u2013100 \uc5, depending on x and the strain energy density. The strength of the broad PL band was correlated with the areal density (up to 3c109 cm 122) of strain perturbations (local lattice dilation 3c15 \uc5 in diameter) observed in plan-view TEM. Thinner alloy layers exhibited phonon-resolved PL spectra, similar to bulk material, but shifted in energy due to strain and hole quantum confinement. Photoluminescence excitation spectroscopy, external quantum efficiency, time-resolved PL decay, together with the power and temperature dependence of luminescence intensity, have been used to characterize Si1 12xGex/Si heterostructures exhibiting both types of PL spectra. The role of MBE growth parameters in determining optical properties was investigated by changing the quantum well thickness and growth temperature. The transition from phonon-resolved, near-band-gap luminescence in thin layers to the broad PL band typical of thick layers is discussed in terms of a strain energy balance model which predicts a \u2018\u2018transition thickness\u2019\u2019 which decreases with increase in x.NRC publication: Ye

Visible photoluminescence from Si1-xGex quantum wells

We have observed photoluminescence from strained SiGe quantum well layers at energies approximately equal to twice the SiGe band-gap energy. This luminescence is caused by the simultaneous recombination of two electron hole pairs yielding a single photon. Detection of luminescence at twice the band-gap has been previously used in Si to observe luminescence originating from electron-hole droplets, biexcitons, bound multiexciton complexes and polyexcitons. Time resolved spectra at twice the band-gap have been obtained from our SiGe samples prepared by molecular beam epitaxy (MIRE) as well as rapid thermal chemical vapor deposition (RTCVD). This new luminescence clearly distinguishes multiexciton or dense e-h plasma processes from single exciton processes such as bound excitons, free excitons or localized excitons, which are difficult to separate in the usual nearinfrared luminescence.NRC publication: Ye

High quantum efficiency photoluminescence from localized excitons in Si[1-x]Ge[x]

NRC publication: Ye