This paper overviews the properties and possible applications of long wavelength vertical-cavity semiconductor optical amplifiers (VCSOAs). A VCSOA operating in the 1.3-m wavelength region is presented. The device was fabricated using wafer bonding; it was optically pumped and operated in reflection mode. The reflectivity of the VCSOA top mirror was varied in the characterization of the device. Results are presented for 13 and 12 top mirror periods. By reducing the top mirror reflectivity, the amplifier gain, optical bandwidth, and saturation output power were simultaneously improved. For the case of 12 top mirror periods, we demonstrate 13-dB fiber-to-fiber gain, 0.6 nm (100 GHz) optical bandwidth, a saturation output power of 3 5 dBm and a noise figure of 8.3 dB. The switching properties of the VCSOA are also briefly investigated. By modulating the pump laser, we have obtained a 46-dB extinction ratio in the output power, with the maximum output power corresponding to 7-dB fiber-to-fiber gain. All results are for continuous wave operation at room temperature.

We have studied experimentally and theoretically the cross-plane Seebeck coefficient of short period InGaAs/ InAlAs superlattices with doping concentrations ranging from 2 ϫ 10 18 up to 3 ϫ 10 19 cm −3 . Measurements are performed with integrated thin film heaters in a wide temperature range of 10-300 K. It was interesting to find out that contrary to the behavior in bulk material the Seebeck coefficient did not decrease monotonically with the doping concentration. We did not observe a sign change in the Seebeck coefficient at dopings where the Fermi energy is just above a miniband. This is a sign that electrons' lateral momentum is conserved in the transport perpendicular to superlattice layers. A preliminary theory of thermoelectric transport in superlattices in the regime of miniband formation has been developed to fit the experimental results.

Abstract-We report on the fabrication and operation of the first electrically pumped 1.55-m vertical-cavity laser array for wavelength-division-multiplexing applications. The array consisted of four channels operating between 1509 and 1524 nm. Wafer bonding was used to integrate GaAs-AlGaAs distributed Bragg reflectors with an InP-InGaAsP active region.

We propose and demonstrate a long-wavelength vertical cavity surface emitting laser (VCSEL) which consists of a (311)B InP-based active region and (100) GaAs-based distributed Bragg reflectors (DBRs), with an aim to control the in-plane polarization of output power. Crystal growth on (311)B InP substrates was performed under low-migration conditions to achieve good crystalline quality. The VCSEL was fabricated by wafer bonding, which enables us to combine different materials regardless of their lattice and orientation mismatch without degrading their quality. The VCSEL was polarized with a power extinction ratio of 31 dB.

We present a quantitative study of the second voltage derivative (SD) of ballistic electron emission spectra of Au͞GaAs͞AlGaAs heterostructures to probe the effect of electron scattering on these spectra. Our analysis of the SD spectra shows that strong electron scattering occurs at the nonepitaxial Au͞GaAs interface, leading to an experimentally observed redistribution of current among the electron transport channels. We also show that the effects of hot-electron scattering inside the semiconductor modify the spectra and are sensitive to the heterojunction band structure, its geometry, and temperature. [S0031-9007(99)09041-9] PACS numbers: 72.10.Fk, 73.20.At, 73.40.Kp, 73.50.Gr Ballistic electron emission microscopy (BEEM), a three-terminal modification of scanning tunneling microscopy, has recently been shown to be a powerful tool for nanometer-scale characterization of the spatial and electronic properties of semiconductor structures. Since the pioneering work of Kaiser and Bell [1], the capability of BEEM to probe the electronic properties of semiconductors on the local scale has been demonstrated for several systems, including Schottky contacts [2-4] and buried heterojunctions [5][6][7].The shape of the BEEM spectrum in the threshold region has to be known in order to derive the correct Schottky or heterojunction barrier energies. Several theoretical models were developed to describe the experimental BEEM spectra. Two commonly used models, based on a planar tunneling formalism [8] and on the transverse momentum conservation at the metal-semiconductor (m-s) interface, are the Bell-Kaiser (BK) model [9] and the Ludeke-Prietsch (LP) model [10]. The LP model extends the original BK theory to include the energydependent electron mean free path (mfp) in the metal base layer and the quantum mechanical transmission at the m-s interface. Experimentally distinguishing between the BK and LP models is still difficult, because the quantitative difference between them is comparable with the experimental error, and both of them can fit experimental data reasonably well [6,11,12]. Recently, BEEM theory was extended to the case of buried heterostructures [13], where transmission at the heterojunction interfaces in addition to the m-s interface was considered.The assumption of transverse momentum conservation, made in the above models, is questionable for the case of nonepitaxial m-s interfaces, which are not atomically abrupt. A deviation from the ballistic picture was experimentally observed, e.g., for Au͞Si [14], Pd͞Si [15], and Au͞GaAs [1,6]. To consider electron scattering at the m-s interface, the m-s interface-induced scattering (MSIS) model was proposed in Ref. [16]. In the strong scattering limit, this model was found to describe the absolute magnitude of the experimentally observed BEEM current for Au͞GaAs and Au͞Si systems. However, since the observed BEEM spectra are a superposition of current contributions from several different transport channels, it is difficult to conclusively extract the different conducti...