Y. S. Tang

Changes in luminescence emission induced by proton irradiation: InGaAs/GaAs quantum wells and quantum dots

The photoluminescence (PL) emission from InGaAs/GaAs quantum-well and quantum-dot (QD) structures are compared after controlled irradiation with 1.5 MeV proton fluxes. Results presented here show a significant enhancement in radiation tolerance with three-dimensional quantum confinement. Some additional radiation-induced changes in photocarrier recombination from QDs, which include a slight increase in PL emission with low and intermediate proton doses, are also examined.

Optical and acoustic phonon modes in self-organized Ge quantum dot superlattices

Raman scattering measurements were carried out in self-organized Ge quantum dot superlattices. The samples consisted of 25 periods of Ge quantum dots with different dot sizes sandwiched by 20 nm Si spacers, and were grown using solid-source molecular-beam epitaxy. Optical phonon modes were found to be around 300 cm−1, and a dependence of the Raman peak frequency on the size of dots was evidenced in good agreement with a prediction based on phonon confinement and strain effects. Acoustic phonons related to the Ge quantum dots have also been observed.

RAMAN SCATTERING FROM A SELF-ORGANIZED GE DOT SUPERLATTICE

We present a Raman scattering study of a self-organized Ge dot superlattice. The structure, which consists of 20 periods of Ge quantum dots sandwiched by 6 nm Si spacers, is grown on a Si (100) substrate by solid source molecular beam epitaxy. Cross-sectional transmission electron microscopy is used to characterize the structural properties of these Ge dots. Raman spectrum shows a downward shift of the Ge–Ge mode, which is attributed to the phonon confinement in the Ge dots. From polarization dependent Raman spectra, strong inter-sub-level transition in the Ge quantum dots is observed. From a simple calculation, the observed peak at 1890 cm−1 in the polarized spectrum is attributed to the transition between the first two heavy hole states of the Ge quantum dots.

Growth and study of self-organized Ge quantum wires on Si(111) substrates

Self-organized Ge quantum wires were grown on regular atomic steps formed along [110] direction on Si(111) substrates by annealing at 870 °C in vacuum. The samples were then studied by atomic force microscopy, polarization-dependent Raman scattering, and low temperature photoluminescence spectroscopy. The results suggest that good quality Ge quantum wires were formed and clear quantum confinement-induced quantization in the wires was observed.

SiGe quantum dots prepared on an ordered mesoporous silica coated Si substrate

This letter reports a new way of preparing wafer sized SiGe quantum dots on an ordered mesoporous sol gel silica coated Si. It was found from x-ray diffraction that very good regular layers of mesoscopic sized SiGe quantum dots can be formed in the silica. Initial low temperature photoluminescence measurements show much improved light emission of the buried dots. This technique is a potential low cost method for producing quantum dot arrays.

Growth study of surfactant-mediated relaxed SiGe graded layers for 1.55-μm photodetector applications

Abstract A systematic study of Sb sufactant-mediated relaxed SiGe graded layers has been performed. The results have been compared with those of relaxed SiGe layers grown at high temperatures, showing that the Sb-mediated SiGe graded layers have been significantly improved both in surface smoothness and in threading dislocation density. We have investigated the grading rate dependence on the resulting threading dislocation density and surface smoothness of as-grown Si 0.5 Ge 0.5 buffer layer samples. With the use of Sb surfactant mediation, we have also fabricated high-quality Ge photo diodes, showing very low leaky current at the reverse bias at 1 V.

Response to “Comment on ‘Raman scattering from a self-organized Ge dot superlattice’ ” [Appl. Phys. Lett. 75, 3572 (1999)]

~301 cm in our case! does not mean that the peak mu come from Si acoustic phonons. In order to prove our ass ment, Raman scattering measurements were performe the dot sample and an identical Si substrate using the s experimental Raman system with an identical data collec time. In addition, different polarization configurations a cording to selection rules were used to better distinguish signals from the dot sample and the Si substrate. Figu shows the observed results. The spectrum from the sam ~top solid curve! was recorded in the 001(100,010)001 ̄ backscattering geometry. This configuration was chosen to m mize the acoustic phonon peak at around 303 cm 21 from Si substrate. The spectrum from the Si substrate ~bottom solid curve! was recorded in the 001(110,110)001 ̄ backscattering configuration in order to enhance the Si acoustic pho peak. A peak at 301 cm 21 from the sample in the top solid curve is about six times stronger than the Si acoustic pho peak at 303 cm from the substrate in the bottom sol curve. The strain on multilayered Si induced by the form tion of Ge dots changes the symmetry of the localized ~around the dots !. Because of this effect, the 303 cm 21 Si acoustic phonon peak may show up even though the sam is under the 001(100,010)001 ̄ configuration. Thus the ob served 301 cm Raman line from the dot sample may in clude the contribution from the Si acoustic phonons. Limit work on this issue 4 seems to indicate that the intensity of th Si acoustic phonon peak does not change significantly w and without the existence of strain. The only dominant sig in our case is from the Ge–Ge mode. In addition, the app ance of the Si–Ge mode at 403 cm 21 ~top solid curve in Fig. 1! suggests imperfect Si–Ge interfaces due to high gro temperature and/or the formation of Ge dots. Otherwise, peak should be forbidden in the 001(100,010)001 ̄ backscattering configuration. This also supports the existenc the Ge–Ge mode. In fact, the only concern here may be which parts, SiGe wetting layers or Ge dots, mostly contr ute to the Ge–Ge mode. Existing work 5 indicated that the

Direct MBE growth of SiGe dots on ordered mesoporous glass-coated Si substrate

Abstract In this paper, we propose a new method for preparing high-density nanometer-scale SiGe quantum dots on ordered mesoporous sol gel silica-coated Si substrate. It was found that a SiGe dot matrix has formed in the porous silica media, and a second layer of bigger over-grown SiGe dots was also observable. X-Ray diffraction measurements suggest the formation of the buried dots. Preliminary photoluminescence experiments show promising emission related to the buried dots.

Development and prospective of SOI-based photonic components for optical CDMA application

Optical Code-Division Multiple Access technology enables simultaneous, asynchronous, multi-rate users to transmit and receive information on a single fiber and has the full compatibility with multiple protocol network solutions. In this paper, we review our development of the technology and our effort to replace the bulky optics with Silicon-On- Insulator based photonic devices, such as arrayed waveguide gratings, thermo-optical switches as well as modulators. The future market prospective of the technology will also be discussed.

Response to “Comment on ‘Optical and acoustic phonon modes in self-organized Ge quantum dot superlattices’ ” [Appl. Phys. Lett. 78, 1160 (2001)]

. . In a comment 1 on our recent letter, 2 Yu first pointed out that there was strong alloying between the Ge dots and barrier layers because of the appearance of Si–Ge mod also observed in SiGe alloys. It is correct that the Ge samples reported in our letter have some degrees of allo due to interdiffusion. This was due to the fact that t samples were grown at a high temperature of 600 °C. reason for using this temperature came from our intention control the size uniformity. It was found that an optimu temperature occurs at around 600 °C for high-uniform mo modal Ge dots on planar Si substrate. 3,4 At lower temperatures, the uniformity becomes worse. Moreover, two kinds dots coexisted on the Si substrate ~pyramid and dome !. Figure 1~a! shows a cross-sectional TEM image of a te period Ge quantum dot sample grown at 550 °C ~sample 1!. Vertically correlated dots are evident. The size variation the first Ge dot layer, however, is determined to be 20% fr AFM measurements@Fig. 1~b!#. The nonuniformity arises from limited diffusion at the lower growth temperature 5 and becomes worse in the vertically correlated multilayers cause vertical correlation rearranges the strain distribut leading to the fact that the upperlayer dots are larger t those in the lower layers @Fig. 1~a!#. On the other hand, at higher temperature around 600 °C, the dot uniformity much improved and the size variation decreases to, for ample 7%–8% in our samples in the letter. 2 Such uniformity may be desirable, but the interdiffusion between the Ge d and Si spacers reduces quantum confinement effects a undesirable for optical applications. Thus, there is alway tradeoff between the uniformity and interdiffusion. Now let us take a look at the Raman spectrum of sample 1@Fig. 1~c!#. This figure includes a spectrum for ten-period Ge dot superlattice grown at 500 °C ~sample 2!. One can easily see Si–Ge vibration modes at around cm from both samples. The relative strength I Si–Ge/I Ge–Ge decreases as the temperature decreases. Another phe enon is that there is a weak feature ~indicated by an arrow in the figure! between Si–Ge mode and Si–Si mode for sam 1. This is due to localized Si–Si motion in the neighborho of one or more Ge atoms. 6 Such kinds of localized Si–S optical modes~Si–Siloc! are often observed in SiGe alloy