Jesper Engvall

Electroluminescence at room temperature of a SinGem strained-layer superlattice

We report for the first time on room temperature electroluminescence in the region 1.3–1.7 μm from a strain‐adjusted Si6Ge4 superlattice. These results, together with photoluminescence, short‐circuit photocurrent spectroscopy, and voltage‐intensity and current‐intensity measurements indicate that the observed electroluminescence consists of two emission bands which are believed to be caused by defect and interband recombination processes.

Room‐temperature photoluminescence of GemSinGem structures

Photoluminescence of pseudomorphic Ge wells grown by conventional molecular beam epitaxy on Si substrate is studied. The samples consist of p‐type doped Gem–Sin–Gem structures embedded in a Si1−xGex alloy. The luminescence lines shift to lower energy with increasing m, the observed band gap agrees with subband calculation based on an effective mass approximation. The temperature stability of the luminescence depends on m. In the case of m=4 the luminescence persists up to room temperature with only small reduction in intensity. The activation energies determined from the exponential drop of luminescence intensity agree with band discontinuities in the sample structures.

Photo- and electroluminescence in short-period Si/Ge superlattice structures

Interband optical transitions have been studied in a variety of short-period Si/Ge superlattice structures by means of photocurrent spectroscopy, infrared absorption, photo- and electroluminescence. Furthermore, the bandgap photoluminescence from strain-adjusted SimGen (m=9, 6, 3; n=6, 4, 2) superlattices was studied under applied hydrostatic pressure. The strain adjustment was achieved by a thick, step-graded Si1-xGex buffer layer resulting in an improved quality of the superlattice with respect to dislocation density. The hydrostatic pressure dependence was modelled using an approach based on deformation potentials and effective-mass theory. In samples annealed at 500 degrees C and higher, a systematic shift of the bandgap was observed which is discussed in terms of a process involving interdiffusion of the Si and Ge atoms. Bandgap-related electroluminescence was observed in mesa diodes at room temperature, whereas the photoluminescence disappeared at about 40 K. The electroluminescence from samples based on different buffer-layer concepts is compared. Apart from the strain-symmetrized Si/Ge superlattices, another structure that has been proposed to act as an efficient, light-emitting device in the Si-based systems is an ultrathin Ge layer (1-2 monolayers) embedded in bulk Si. We report on the electroluminescence spectra at various temperatures from a sample based on this concept, namely a layer sequence consisting of two periods of Si17Ge2 grown pseudomorphically on an n+ Si substrate. A very intensive, well resolved electroluminescence was obtained at 55 K from the QW.

Optical study of diffusion limitation in MBE growth of SiGe quantum wells

We report on detailed studies of the bandgap of Si/SixGe1-x quantum well structures grown on (001) Si by molecular beam epitaxy. Photocurrent and photoluminescence spectroscopy are used to determine the bandgap of the SiGe alloy up to x=0.67. We found that interdiffusion of the SiGe layers limited the maximum Ge content in the alloy layers at a high growth temperature (720 degrees C). At a lower growth temperature (500 degrees C) diffusion is negligible. This is verified by p-i-n structures and p-type modulation-doped quantum wells. In the modulation-doped samples the bandgap could be reduced to 1.5 mu m while still showing intense bandgap related photoluminescence. As well as an alloy-related onset the p-i-n diodes reveal a low-energy threshold, which is defect related. Low growth temperatures lead to defects located in the SiGe layers. Raising the number of quantum wells and Ge content up to almost critical thickness we found a maximum external responsivity of 4*10-4 A W-1 in normal incidence for mesa-type pin photodiodes.

Electrical characterization of SiGe heterostructure bipolar transistors

We have performed electrical measurements on Si-Ge alloy heterostructure bipolar transistors (HBTs) at various temperatures between 77 and 300 K. A current amplification of 9200 was measured at 120 K for an HBT with on Si 0.70 Ge 0.30 alloy base. Gummel plots were measured in both normal and inverse transistor operation for assessment of dopant out-diffusion during epitaxy and transistor processing. The reduction in the band gap ΔE g of the strained alloy bases was estimated for different compositions by comparing the temperature dependence of the current-voltage characteristics of HBTs with that of a reference silicon homojunction transistor. Becuase of out-diffusion, the measured values were lower than the theoretically calculated values.

Optical anisotropy of SiGe superlattices

Optical and electrical properties of SiGe strain‐adjusted superlattices have been studied. Diode structures were processed into waveguide geometries to investigate the role of optical confinement and the lowering of cubic symmetry with regards to the polarization properties of interband absorption and emission. The polarization anisotropy of the absorption coefficient suggests that the heavy‐hole band of strain‐adjusted Si6Ge4 superlattices is the top valence band.

Optical and electrical characterization of Si/Ge superlattices

Abstract Using recent studies of absorption and photoluminescence, different junction space charge techniques have been employed to separate intrinsic signals from extrinsic responses in ultrathin Si/Ge superlattices. The diodes showed good I–V characteristics with ideality factors of about 2 and a reverse current of about 10 −11 A for voltages less than 1 V. The extrinsic photoionization cross-section spectrum exhibited oscillatory properties which are discussed in terms of Wannier-Stark localization.

Optical anisotropies in strained Si/SiGe systems

Abstract Optical anisotropies were studied in two types of Si/Ge systems: strain-adjusted superlattices, and thin Ge quantum wells on Si. For the superlattices, investigations of the polarization dependence showed that the topmost valence band is of heavy-hole character. For the Ge monolayer quantum wells, the optical transitions were identified as a no-phonon transition and a TO-replica. Furthermore, the symmetry of the intermediate state over which the transitions take place at the zone-center was identified.

Interband Photo- and Electroluminescence from Short-Period Si/Ge Superlattices

We report on further investigations on the photoluminescence (PL) and electroluminescence (EL) properties of strained Si/Ge superlattices. The band-gap PL from strain-adjusted Sim Gen (m=9, 6, 3; n=6, 4, 2) superlattices has been studied as a function of applied external hydrostatic pressure. The superlattices used in these measurements are of improved quality in terms of dislocation density due to a thick, step-graded Si1-x Gex buffer layer, providing the strain symmetry. The no-phonon (NP) lines shift in all superlattices linearly to lower energies with applied hydrostatic pressure. The stress dependence was modeled using an approach based on deformation potentials and effective-mass theory. Furthermore, we report on strong EL from a novel structure consisting of only a 2-monolayer-thick Ge-layer embedded in bulk Si.

SiGe: a promise into reality?

The paper summarizes a few basic properties of SiGe showing that SiGe is an interesting material for high speed electronics. The advantage of using heterostructures in silicon-based technologies is demonstrated by taking SiGe heterojunction bipolar transistors (HBTs) as an example. First results obtained with very fast and low-noise HBTs are briefly mentioned. The paper is concluded by a short discussion of a few optoelectronic properties observed in various Si/Ge and Si/Si/sub 1-x/Ge/sub x/ strained-layer superlattices and quantum wells with particular emphasis on electroluminescence properties.