Band Gap Of Ge Research Articles

Electrical properties of completely compensated single-crystal Ge-on-GaAs films with two-dimension Coulomb disorder

We present electrical properties of heavily doped and completely compensated Ge films grown on semiinsulating GaAs(100) substrates by vacuum evaporation. The thin (∼100 nm) Ge films are single-crystal and characterized using temperature-dependent transport measurements, with anomalously large activation energy up to half the Ge bandgap, anisotropy of the transverse magnetoresistance, high resistivity (up to 140 Ω cm), low free charge carrier mobility (∼50 cm2/V·s), and concentration (∼1014–1015 cm−3). This behaviour is attributed to a completely compensated semiconductors arising from Ga and As impurity incorporation and large-scale potential fluctuations. Analysis suggests a two-dimensional percolative transport mechanism in Ge-on-GaAs heterostructures.

Donor-induced electrically charged defect levels: examining the role of indium and n-type defect-complexes in germanium

Defect levels induced by defect-complexes in Ge play important roles in device fabrication, characterization, and processing. However, only a few defect levels induced by defect-complexes have been studied, hence limiting the knowledge of how to control the activities of numerous unknown defect-complexes in Ge. In this study, hybrid density functional theory calculations of defect-complexes involving oversize atom (indium) and n-type impurity atoms in Ge were performed. The formation energies, defect-complex stability, and electrical characteristics of induced defect levels in Ge were predicted. Under equilibrium conditions, the formation energy of the defect-complexes was predicted to be within the range of 5.90–11.38 eV. The defect-complexes formed by P and In atoms are the most stable defects with binding energy in the range of 3.31-3.33 eV. Defect levels acting as donors were induced in the band gap of the host Ge. Additionally, while shallow defect levels close to the conduction band were strongly induced by the interactions of Sb, P, and As interstitials with dopant (In), the double donors resulting from the interactions between P, As, N, and the host atoms including In atom are deep, leading to recombination centers. The results of this study could be applicable in device characterization, where the interaction of In atom and n-type impurities in Ge is essential. This report is important as it provides a theoretical understanding of the formation and control of donor-related defect-complexes in Ge.

The growth of Ge and direct bandgap Ge1-xSnx on GaAs (001) by molecular beam epitaxy.

Germanium tin (GeSn) is a tuneable narrow bandgap material, which has shown remarkable promise for the industry of near- and mid-infrared technologies for high efficiency photodetectors and laser devices. Its synthesis is challenged by the lattice mismatch between the GeSn alloy and the substrate on which it is grown, sensitively affecting its crystalline and optical qualities. In this article, we investigate the growth of Ge and GeSn on GaAs (001) substrates using two different buffer layers consisting of Ge/GaAs and Ge/AlAs via molecular beam epitaxy. The quality of the Ge layers was compared using X-ray diffraction, atomic force microscopy, reflection high-energy electron diffraction, and photoluminescence. The characterization techniques demonstrate high-quality Ge layers, including atomic steps, when grown on either GaAs or AlAs at a growth temperature between 500-600 °C. The photoluminescence from the Ge layers was similar in relative intensity and linewidth to that of bulk Ge. The Ge growth was followed by the growth of GeSn using a Sn composition gradient and substrate gradient approach to achieve GeSn films with 9 to 10% Sn composition. Characterization of the GeSn films also indicates high-quality gradients based on X-ray diffraction, photoluminescence, and energy-dispersive X-ray spectroscopy measurements. Finally, we were able to demonstrate temperature-dependent PL results showing that for the growth on Ge/GaAs buffer, the direct transition has shifted past the indirect transition to a longer wavelength/lower energy suggesting a direct bandgap GeSn material.

Comparison of optimized GeSn/Si heterojunction and GaInAsSb/GaSb thermophotovoltaic cells with similar bandgaps

The bandgap of Ge1−xSnx material can be designed within 0.5 ∼ 0.6 eV with different Sn content, and the characteristic of indirect bandgap of pure Ge will change to direct, which make Ge1−xSnx become a proper and low cost thermophotovoltaic cell material. Here we investigate direct bandgap Ge1−xSnx cells with bandgaps of 0.508 ∼ 0.548 eV. Triple antireflection layers, the surface recombination rate, p–n junction depth, impurity doping concentration, etc are optimized for cell design. The optimal cell structures are adopted for cell performance evaluation under given blackbody radiation within 1000 ∼ 2000 K. Simultaneously, the output power densities of GeSn cells are compared with those of traditional GaInAsSb cells with similar bandgap designs. GeSn cells show comparable performances with GaInAsSb cells over the temperature range of 1000 ∼ 1500 K blackbody radiation, and the efficiencies are 1.01 ∼ 2.49 times over 1500 ∼ 2000 K.

Structures, Electric Properties and STM Images of GeSe Monolayers Doped by Group IV–VI Atoms: A First-Principles Study

Doping is an important method to modulate the physical and chemical properties of two-dimensional materials. By substitutional doping, different group IV–VI atoms are doped in GeSe monolayers to compose the doped models, of which the effects are investigated using first-principles calculations. The results show that local deformations of geometrical structure can be observed around the doping atoms. According to the analysis of the formation energy and the cohesive energy, all the doped models have a strongly bonded network, and GeSe_N possesses the most stable structure. Only the bandgap of Ge_As is direct, while those of other doped models are indirect. Thus, direct and indirect bandgaps are alternative by doping different atoms. The structural and electronic properties, especially for the bond lengths variation around doping atoms, are explained by the charge density difference. Finally, the scanning tunnel microscope images of the doped models are analyzed for further experimental investigations. Our work provides a stimulating account by atom doping which could trigger the developments and applications of new two-dimensional materials for manufacturing microelectronic and optoelectronic devices.

A Ge-Channel Ferroelectric Field Effect Transistor With Logic-Compatible Write Voltage

A major roadblock for the integration of ferroelectric-field-effect transistors (FEFETs) at advanced technology nodes for embedded memory applications is their high, logic-incompatible write voltages. Herein, we explore Ge as a channel material to reduce write voltage of FEFET and report the first demonstration of p-type Ge-FEFETs with record low write voltages of ±1.4 V with a memory window (MW) of 0.6 V at DC and write voltages of ±1.4 V, ±1.8 V and ±2.4 V for MW of 0.2 V, 0.5 V and 0.8 V for a write time of 10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{s}$ </tex-math></inline-formula> , respectively. The write voltages observed in Ge-pFEFETs are ~50% lower than that of a Si-pFEFETs when compared against iso-memory window condition [±2.5 V with a MW of 0.6 V at DC, ±3.5 V for MW of 0.5 V for a write time of 10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{s}$ </tex-math></inline-formula> ]. Such dramatic reduction of write voltages in Ge-pFEFETs is achieved due to the fact that the native oxide of Ge (GeOx), formed at the Ge interface, has a larger dielectric constant and lower thickness than those for SiO2 on the Si platform. In addition, the lower bandgap and higher dielectric constant of Ge may lead to a lower surface potential for a given semiconductor charge, leading to further reduction in the write voltage. Further, our Ge-pFEFETs show write endurance of 107 cycles (the best-in-class for Ge-pFEFETs, as reported in the literature), excellent data retention, and immediate read-after-write capability. Our results indicate the attractiveness of Ge platform for FEFETs for embedded memory applications.

Electronic and optical band gap of Ge–Te–Sn nanostructure

Amorphous thin film Ge15Te85−xSnx (1 ≤ x ≤ 5) and Ge17Te83−xSnx (1 x 4) switching devices have been deposited in sandwich geometry using a flash evaporation technique, with aluminum as the top and bot- tom electrodes. Electrical switching studies indicate that these films exhibit memory type electrical switching behavior. The switching fields for both the series of samples have been found to decrease with increase in Sn con- centration, which confirms that the metallicity effect on switching fields/voltages, commonly seen in bulk glassy chalcogenides, is valid in amorphous chalcogenide thin films also. In addition, there is no manifestation of rigid- ity percolation in the composition dependence of switching fields of Ge15Te85−xSnx and Ge17Te83−xSnx amorphous thin film samples. The observed composition dependence of switching fields of amorphous Ge15Te85−xSnx and Ge17Te83−xSnx thin films has been understood on the basis of Chemically Ordered Network model. The optical band gap for these samples, calculated from the absorption spectra, has been found to exhibit a decreasing trend with in- creasing Sn concentration, which is consistent with the com- position dependence of switching fields.

Electronic properties and defect levels induced by [formula omitted]-type defect-complexes in Ge

Defect-complexes are point defects that significantly influence the geometric, optical, and electrical properties of materials. Defect-complexes are known to improve the electronic and electrical properties of Ge. Deep and shallow defect levels in Ge have been linked to defect-complexes formed by the double self-interstitials and rare earth atoms. Despite this breakthrough, several defect-complexes in Ge are not well understood; hence may pose as a challenge to the optimal performance of Ge based devices. In this study, we present the results of the hybrid density functional theory calculations of substitutional and interstitial defect-complexes (BGeNi, NGeBi, AlGePi, PGeAli, GaGeAsi, AsGeGai, InGeSbi, and SbGeIni) in Ge. Their formation energies, electronic properties, defect-complex stability and induced defect levels in Ge were predicted. While the formation energies of the defect-complexes formed by the P and Al atoms were relatively low and energetically more favourable, those defect-complexes formed by the B and N atoms were the least energetically favourable. Except for the BGeNi, all the defect-complexes significantly bound with energies lower than their formation energies. The NGeBi and PGeAli essential donor levels were in the band gap of Ge. A shallow double acceptor defect level was found to be associated with the AlGePi, while the InGeSbi induced a shallow double donor defect level. The electrical inactive defect-complexes were the BGeNi, GaGeAsi, AsGeGai and SbGeIni. The results of this report are important, as they provide theoretical insight of the prediction of the n/p-type substitutional and interstitial defect-complexes in Ge.

Design and characterization of Ge/SeO2 heterojunctions as tunneling thin film transistors

Herein stacked layers of germanium and selenium oxide are employed to fabricate heterojunction devices that can perform as a multifunctional thin film transistors (TFT). The stacked layers of Ge and SeO2 are coated onto ultrasonically cleaned glass and Ag thin film substrates using thermal deposition technique under a vacuum pressure of 1.5×10−4 mbar. The stacked layers are structurally, morphologically, optically and electrically characterized. It is observed that while amorphous germanium films exhibit two energy band gaps of values of 3.56 and 0.78 eV, amorphous SeO2 show one band gap of 3.70 eV. The conduction and valence band offsets at the Ge/SeO2 interfaces are 2.24 eV and 0.68 eV, respectively. Optically, the coating of SeO2 onto Ge films enhanced the light absorbability of SeO2 by ~ 50 times (at 2.38 eV) and ~17 times (at 1.71 eV) in the visible and infrared ranges of light, respectively. Formation of Ge/SeO2 is associated with a redshift and blue shift in the values of the energy band gaps of SeO2 and Ge, respectively. In addition, the current-voltage characteristic curve analyses have shown that Ag/Ge/SeO2/Ag TFTs is of tunneling type showing an estimated peak to valley current ratios of 2.0. The TFT displayed additional functionalities presented by PMOS characteristics, microwave resonators, negative capacitance sources and multiband band stop filters. The features of the devices nominate them for use in microwave technology as band stop filters of notch frequencies of 1.10 and 1.53 GHz.

High-performance gold/graphene/germanium photodetector based on a graphene-on-germanium wafer

The metal/germanium (Ge) photodetectors have attracted much attention for their potential applications in on-chip optoelectronics. One critical issue is the relatively large dark current due to the limited Schottky potential barrier height of the metal/germanium junction, which is mainly caused by the small bandgap of Ge and the Fermi energy level pinning effect between the metal and Ge. The main technique to solve this problem is to insert a thin interlayer between the metal and Ge. However, so far, the dark current of the photodetectors is still large when using a bulk-material insertion layer, while when using a two-dimensional insertion layer, the area of the insertion layer is too small to support a mass production. Here, we report a gold/graphene/germanium photodetector with a wafer-scale graphene insertion layer using a 4 inch graphene-on-germanium wafer. The insertion layer significantly increases the potential barrier height, leading to a dark current as low as 1.6 mA cm−2, and a responsivity of 1.82 A W−1 which are the best results for metal/Ge photodetectors reported so far. Our work contributes to the mass production of high-performance metal/Ge photodetectors.

Excitonic effects at the temperature-dependent direct bandgap of Ge

The temperature dependence of the complex dielectric function ϵ1+iϵ2 of bulk Ge near the direct bandgap was investigated with spectroscopic ellipsometry at temperatures between 10 and 710 K. Second derivatives of the dielectric function with respect to energy are obtained using a digital linear filter method. A model that incorporates excitonic effects using the Tanguy model for the Hulthén potential [C. Tanguy, Phys. Rev. B 60, 10660 (1999)] was used to fit the dielectric function and its second derivatives simultaneously. Using k⋅p theory and literature values for effective masses, reasonable agreement with the experiment is obtained for ϵ2 up to room temperature using the direct bandgap and its broadening as the only adjustable parameters.

Broadband Near-Field Near-Infrared Spectroscopy and Imaging with a Laser-Driven Light Source

The scattering-type scanning near-field optical microscope (s-SNOM) has become a powerful imaging and nano-spectroscopy tool, which is widely used in the characterization of electronic and photonic devices, two-dimensional materials and biomolecules. However, in the published literature, nano-spectroscopy is mainly employed in the mid-infrared band, and the near-infrared (NIR) nano-spectroscopy with broadband spectral range has not been well discussed. In the present paper, we introduce a home-built near-field NIR spectroscopy and imaging set-up that is based on a laser-driven light source (LDLS). By mapping the Ge-Au periodic grating sample and the photonic topology device, a ~30 nm spatial resolution and the excellent capability of characterizing complex samples are demonstrated. Spectra obtained by experiment reveal the optical band-gap of Ge with a spectral resolution of 25 cm−1, and a spectral range from 900 to 2000 nm. This technology is expected to provide a novel and unique approach for near-field NIR spectroscopy and imaging.

Charge compensation co-doping enhances the photocatalytic activity of black phosphorus

Visible light (VL) photocatalysts have received widespread attention because they can improve the utilization of sunlight and have application prospects in degrading organic pollutants. In this work, using first principles study, we systematically investigated the photocatalytic performance of metal (Ge and Sn) and nonmetal (O, S and Se) charge compensated co-doped black phosphorus (BP). The results indicate that the incorporation of the Ge and Se atom in the BP lattice will be easier in the presence of O (or S or Se) dopant. Charge compensated co-doping can eliminate the flat impurity levels (ILs) in the band gap of Ge and Sn single doped BP. More importantly, Ge-O and Sn-O co-doping can suppress the recombination rate of photogenerated carriers, increase the number of catalytic active sites, enhance the redox potential of photogenerated carriers and improve the absorption efficiency of VL. This work has guiding significance for promoting the applications of BP-based VL photocatalyst.

(Invited) Fabrication and Characterization of Ge-Based MIS Structures

Germanium (Ge) is a very attractive channel material for future high-performance metal-insulator-semiconductor (MIS) devices because it exhibits much higher low-field electron (two times higher) and hole (four times higher) mobilities than Si. In addition, the smaller band gap of Ge than that of Si should allow the further reduction of power consumption in advanced integrated circuits and the lower melting point of Ge than that of Si also enables Ge-based device integration at low processing temperatures. However, until the early years of the 2000s, the reported electrical properties of high-k/Ge interfaces had been so poor that one of the critical issues in establishing Ge-based MIS technology was the effective passivation of Ge surfaces by dielectrics with superior interface properties. Although the quality of the Ge-MIS structure has been remarkably improved in the past decade, the relationship between the processing methods and the quality has not been discussed precisely.In this paper, we report on the fabrication of Ge MIS structures with various high-k dielectric materials and interlayer materials through various methods. We also introduce the convenient method to characterize the interface trap densities (Dit's) of the MIS capacitors by spectroscopic measurement of the interface trap density in an insulator/Ge interface at room temperature [1]. The effects of the insertion of interlayers into the interface of high-k insulator/Ge and the application of heat treatment to the Ge MIS structures will be also presented.For instance, the Dit of Ta2O5/GeNx/Ge fabricated by electron-cyclotron-resonance (ECR) plasma nitridation and sputtering was estimated to be 4×1011 cm-2 · eV-1 [2], that of Si3N4/GeNx/Ge by ECR plasma nitridation to be 1×1011 cm-2 · eV-1 [3-6], and that of Al2O3/GeO2/Ge by ECR plasma oxidation and sputtering to be 4.5×1010 cm-2 · eV-1 [7-9]. These results have summarized that the adequate surface preparations of the interfaces and films by the ECR plasma process with low energy plasma irradiation have been effective to realize the low Dit of MIS structures with GeO2 and/or GeNx.These results have expanded to another MIS structures fabricated by more conventional film formation technique such as the atomic layer deposition (ALD) and the radio frequency (RF) magnetron sputtering. The Dit of Al2O3/Al germanate/Ge by ALD with trimethylaluminum (TMA) and microwave-generated atomic oxygen was estimated to be 2×1011cm-2 · eV-1 [10], and that of HfO2/Ge by RF magnetron sputtering to be 5.9×1011 cm-2 · eV-1. These results have summarized that the heat treatment after the fabrication of MIS strucures have affected interface properties of germanate/Ge and oxide/Ge.Although the mechanism by which the process conditions act on the interface state is now under estimation, it has been speculated that the fabrication method, the insertion of the interlayers, and the application of heat treatment have a great influence on the interface characteristics. The structures and the fabrication method of various MIS will be discussed.Part of this work was carried out under the Cooperative Research Project Program of the Research Institute of Electrical Communication, Tohoku University.

Gateless and Capacitorless Germanium Biristor with a Vertical Pillar Structure

For the first time, a novel germanium (Ge) bi-stable resistor (biristor) with a vertical pillar structure was implemented on a bulk substrate. The basic structure of the Ge pillar-typed biristor is a p-n-p bipolar junction transistor (BJT) with an open base (floating), which is equivalent to a gateless p-channel metal oxide semiconductor field-effect transistor (MOSFET). In the pillar formation, we adopted an amorphous carbon layer to protect the Ge surface from both physical and chemical damage by subsequent processes. A hysteric current-voltage (I-V) characteristic, which results in a sustainable binary state, i.e., high current and low current at the same voltage, can be utilized for a memory device. A lower operating voltage with high current was achieved, compared to a Si biristor, due to the low energy bandgap of pure Ge.

Light emission from direct band gap germanium containing split-interstitial defects

The lack of useful and cost-efficient group-IV direct band gap light emitters still presents the main bottleneck for complementary metal-oxide semiconductor-compatible short-distance data transmission, single-photon emission, and sensing based on silicon photonics. Germanium, a group-IV element like Si, is already widely used in silicon fabs. While the energy band gap of Ge is intrinsically indirect, we predict that the insertion of Ge-Ge split-[110] interstitials into crystalline Ge can open up a direct band gap transmission path. Here, we calculate from first principles the band structure and optical emission properties of Ge, Sb, and Sn split-[110] interstitials in bulk and low-dimensional Ge at different doping concentrations. Two types of electronic states provide the light-emission enhancement below the direct band gap of Ge: a hybridized L-$\mathrm{\ensuremath{\Gamma}}$ state at the Brillouin zone center and a conduction band of $\mathrm{\ensuremath{\Delta}}$ band character that couples to a raised valence band along the $\mathrm{\ensuremath{\Gamma}}$-X direction. Majority carrier introduced to the system through doping can enhance light emission by saturation of nonradiative paths. Ge-Sn split interstitials in Ge shift the top of the valence band towards the $\mathrm{\ensuremath{\Gamma}}$-X direction and increase the $\mathrm{\ensuremath{\Gamma}}$ character of the L-$\mathrm{\ensuremath{\Gamma}}$ state, which results in a shift to longer emission wavelengths. Key spectral regions for datacom and sensing applications can be covered by applying quantum confinement in defect-enhanced Ge quantum dots for an emission wavelength shift from the midinfrared to the telecom regime.

(Invited) Theory of Silicon-Based Light Emitting

The indirect band gap preclude the silicon from making light-emitting devices, which is, however, a quest for exploration of large-scale integration of both electronic and photonic devices on a single microchip. Why silicon and germanium are both indirect band gap and most of group III-V and all group II-VI semiconductors are direct band gap is a long standing puzzle. In this talk, I will first present our recently developed unified theory, based on occupied d orbitals in cations, for explaining why diamond, Si, Ge, and Al-containing group III-V semiconductors have indirect band gap, and the remaining group III-V and II-VI semiconductors, except GaP, have direct band gap. This theory also unravels that it is impossible to make allotropes of Si and compounds consisting by Si mixed together with other elements high efficient direct bandgap light emitting. Interface and quantum confinement in low-dimensional Si nanostructures are two knobs to relax the momentum conservation law of optical transition. Our theory confirms that the light emission efficiency of Si quantum dots increases exponentially as reducing dot size, but its highest reachable efficiently is still two orders of magnitude smaller than that of direct bandgap InP and GaAs. Fortunately, we discovered that Si/Ge magic sequence superlattices exhibit orders more efficient at emission light than their existing counterpart records and approach more than 10% brightness of real direct gap materials, such as GaAs. Furthermore, regarding the direct bandgap is only 140 meV above the indirect bandgap in Germanium (Ge), it is well established that a tiny (1-2%) extensile strain could render an indirect-to-direct bandgap transition and makes it direct bandgap. However, there is no feasible way to create such extensive strain on Ge since the lattice parameter of Ge is even 4.3% larger than Si. We propose insert external atoms into interstitial sites of Ge, which grows on Si substrate through SiGe buffer layer, to induce an effective tensile uniaxial strain on Ge, a new pathway toward direct band gap Ge. Because our proposed scheme is highly compatible with CMOS fabrication process, our presented new pathway would be a solution for on-chip lasers.

(Invited) Theory of Silicon-Based Light Emitting

Why silicon and germanium are both indirect band gap and most of group III-V and all group II-VI semiconductors are direct band gap is a long standing puzzle. In this talk, I will first present our recently developed unified theory to explain why diamond, Si, Ge, and Al-containing group III-V semiconductors have indirect band gap, and the remaining group III-V and II-VI semiconductors, except GaP, have direct band gap. We excluded Si QDs as a candidate for on-chip lasers. We also discovered that Si/Ge magic sequence superlattices exhibit orders more efficient at emission light than their existing counterpart records and approach more than 10% brightness of real direct gap materials, such as GaAs. We further propose to insert external atoms into interstitial sites of Ge to induce an effective tensile uniaxial strain on Ge toward direct band gap Ge. This scheme is highly compatible with CMOS fabrication process.

Strong enhancement of direct transition photoluminescence at room temperature for highly tensile-strained Ge decorated using 5 nm gold nanoparticles

Strain engineering of germanium has recently attracted tremendous research interest. The primary goal of this approach is to exploit mechanical strain to tune the electrical and optical properties of Ge to ultimately achieve an on-chip light source compatible with silicon technology. Additionally, this can result in enhanced electrical performance for high-speed optoelectronic applications. In this paper, we demonstrate the formation of highly tensile-strained Ge islands grown on a pre-patterned (110) GaAs substrate using a depth controlled nanoindentation process. Results show that a biaxial tensile strain, up to ∼2%, can be transferred from the mechanically stamped substrate to Ge islands by optimizing the parameters of the nanoindentation process. We verified our measurements by observing the islands’ photoluminescence (PL) emission properties. A strong emission at room-temperature was observed around the wavelength of 1.9 µm (650 meV). This strain-induced redshift of the PL spectra is consistent with theoretical predictions, revealing a direct Ge bandgap formation. Furthermore, we demonstrate a significant 6.5x enhancement in the PL emission signal of the direct-transition when the Ge islands are decorated by 5 nm gold nanoparticles. This is attributed to a longer optical path length interaction and a plasmonic induced high-field enhancement which increases the light absorption in the Ge islands. Furthermore, results show that GNPs can significantly modulate the energy band structure and the carrier’s transportation at the nanoscale metal-germanium Schottky interface. This maskless physical approach can offer a pathway towards a practical CMOS-compatible integrated laser. Additionally, it opens possibilities for designing innovative optoelectronic devices.

(Invited) Germanium Nanocrystal Luminescence: Spectral and Spatial Variations

Silicon-compatible germanium (Ge) coherent light sources have received considerable attention recently resulting in both lasing and photoluminescence (PL) in strained Ge, for which sufficient tensile strain reduces the relatively small indirect to direct bandgap (BG) difference to zero, thus making Ge a direct gap material, albeit at a relatively low energy. Previously we explained the cause of the intense, low temperature PL observed for many MBE-grown Si1-xGex epitaxial layers as being due to nm-thick Ge nanocrystals (NCs) embedded in the SiGe epilayers, as in the plan-view TEM image and schematic insets of Fig. 1. We showed that the Ge NC peak PL energy depends on several factors, namely: (a) Ge bandgap variation with vertical strain; (b) effect of the Ge-fraction in the host SiGe epilayer on the vertical lattice constant; (c) effect of strain on the SiGe bandgap energy; (d) confinement shifts in the Ge NCs; and (e) confinement shifts in the SiGe epilayers. A model incorporating these effects was developed that effectively explained the PL energy dependence for the SiGe samples comprising over 60 separate epilayer configurations (Fig. 1). The experimental data is presented in Fig. 1, which also contains on the left the PL spectra for three Ge NC samples, including the one to be discussed later in this paper. From the analysis of the data we were able to use confinement for the vertical size of the Ge NCs, which increased linearly with vertical tensile strain.Many of the Ge NC samples exhibited a very bright luminescence, with as high as 3% internal quantum efficiency. For one of the samples the PL was imaged in video format for broad band radiation from 1200 to 1900 nm. One such image is displayed in Fig. 2 for a sample at low temperature comprised of a SiGe/Si multiple quantum well (MQW) structure, with 40 epilayers of 7.6 nm thick Si0.75Ge0.25 separated by 20 nm of Si on a Si(001) substrate. In this case the excitation (100 mW at 514.5 nm in a 1 mm diameter beam) at normal incidence was at the center of the 3 mm diameter sample mask aperture. The exciting radiation was excluded from the camera aperture with a long-wave pass filter, which absorbed light with wavelengths below 1200 nm. On the right of the sample mask in the image, the sample extends with its (110) cleaved edge sticking out at a small angle (~20o) with respect to the edge of the sample mask. The PL is apparently brightest at the point of excitation, but is also quite intense at the edge of the sample, more so at the right hand edge. The trace below the image is the PL intensity profile along the thin yellow line, which indicates an intensity at the edge of the sample comparable to that at the excitation location. In this experiment the incident excitation light was polarized roughly parallel to the wafer edge.In the sample imaged, the MQW system is thick enough and has a high enough effective refractive index to act as a thin film waveguide in the infrared, which could account for much of the edge brightening in Fig. 2. In addition to the waveguiding in the thin film structure, we expect some bulk waveguiding in the silicon substrate, as the back surface is likely smooth enough and the silicon material is transparent in this wavelength range. The luminescence along the sample edge was brightest perpendicular to the excitation polarization, like fluorescence anisotropy. A broadband PL image is shown in Fig. 3 for the excitation focused to a 0.1 mm spot near the edge of the sample, where there a second bright PL spot. Two PL intensity profiles are shown, one through the excitation and edge spots and one along the wafer edge. We see that the edge brightening has also occurred, although the overall PL intensity is not so greatly increased on focusing, possible due to NC saturation effects possible when more than one exciton is captured by a Ge NC. However, we see from Fig. 3 that the edge emission is in a spot not greatly larger than the extent of the emission at the excitation spot. This observation suggests that the emission is quite collimated in the MQW waveguide, much more so than would be expected from the simple direction effects based on excitation-emission polarization that are normally seen for bulk luminescence. This lack of divergence between the excitation and the edge might be indicative of an optical amplification process occurring in the MQW waveguide. Figure 1