Fabrication and High Speed Optoelectronic Characterization of Semiconductor Devices

This work is an investigation on the use of high speed optoelectronic techniques for the characterization of semiconductor devices. A low-frequency electrooptic probe station was demonstrated as well as the optoelectronic sampling scheme. The optoelectronic sampling technique relies on fast photoconductive switches for its operation. The autocorrelation signal detected in optoelectronic sampling was compared with signal detection by conventional techniques employing a sampling oscilloscope and a network analyser. The optoelectronic techniques described in this work depend critically on short-pulse lasers for the measurement of high speed devices. A fibre-grating pulse compressor was set-up to shorten the 120 ps pulses produced by a mode-locked Nd:YAG laser. Compression by a factor of 40 was demonstrated and nearly transform limited pulses of 3 ps duration were obtained. However, the output of the pulse compressor is very noisy and the output power is not high enough to enable electrooptic sampling experiments, in a jitter-free scheme. The same Nd:YAG laser was frequency doubled and used to synchronously pump a rhodamine 6G dye laser. Autocorrelation measurements obtained with the dye laser are again, very noisy and with poor reproducibility. The noise problems with the pulse compressor and with the dye laser were traced back to the Nd:YAG pump laser. It is concluded that this laser should be avoided as the source of short pulses for the electrooptic and optoelectronic measurement techniques. The use of a feedback loop is likely to reduce the noise in this laser, but drift in the intensity in a long time scale would still be present. A mode-locked Ti:Sapphire laser was also used for measurements in this project. Autocorrelation measurements taken with this laser are totally reproducible and contain little or no noise. The devices measured in this project were made by a combination of electron-beam lithography and photolithography. The use of these two lithography techniques together was made possible by the design of a mask set with alignment marks which can be used for registration in a mask aligner and in the electron beam lithography machine. Discrete devices were made and characterized by electrical techniques. Fabrication procedures were developed for resistors, Metal-Insulator-Metal (MIM) capacitors and for the Optoelecttonic Sampling Device (OSD). Discrete Mesfets were fabricated on MBE grown epilayers and their I-V characteristics were measured. A simplified optoelectronic sampling device was designed and made in a single lithographic step. It provides a quick way of producing devices in which autocorrelation measurements can be performed to determine the carrier lifetime in the substrate material. The optoelectronic sampling devices were made on four different substrate materials. The first one is a high purity, MBE grown GaAs epilayer, with very long lifetime (2ns). The control samples were made on "standard" semi-insulating GaAs, whose carrier lifetime is ~200 ps. Proton implantation in some of these devices made on SI GaAs substrate was used as a means of shortening the carrier lifetime, to produce fast turn-off times in the photoconductive switches. The lifetime after implantation of 4 x 10e14 protons/cm2 was estimated from an optoelectronic sampling measurement, to be around 40 ps. This is still a very long lifetime for the photoconductive switches. It is thought that self-annealing of the deep electron traps, caused by the lack of temperature control in the implanter, prevented the achievement of short lifetime in the switches. GaAs epilayers were grown by MBE at a temperature around 25

Optical control of internal electric fields in band-gap graded InGaN nanowires

InGaN nanowires are suitable building blocks for many future optoelectronic devices. We show that a linear grading of the indium content along the nanowire axis from GaN to InN introduces an internal electric field evoking a photocurrent. Consistent with quantitative band structure simulations we observe a sign change in the measured photocurrent as a function of photon flux. This negative differential photocurrent opens the path to a new type of nanowire-based photodetector. We demonstrate that the photocurrent response of the nanowires is as fast as 1.5 ps

Photodetectors

In this book some recent advances in development of photodetectors and photodetection systems for specific applications are included. In the first section of the book nine different types of photodetectors and their characteristics are presented. Next, some theoretical aspects and simulations are discussed. The last eight chapters are devoted to the development of photodetection systems for imaging, particle size analysis, transfers of time, measurement of vibrations, magnetic field, polarization of light, and particle energy. The book is addressed to students, engineers, and researchers working in the field of photonics and advanced technologies

Exciton physics and device application of two-dimensional transition metal dichalcogenide semiconductors

Two-dimensional group-VI transition metal dichalcogenide semiconductors, such as MoS2, WSe2 and others, exhibit strong light-matter coupling and possess direct band gaps in the infrared and visible spectral regimes, making them potentially interesting candidates for various applications in optics and optoelectronics. Here, we review their optical and optoelectronic properties with emphasis on exciton physics and devices. As excitons are tightly bound in these materials and dominate the optical response even at room-temperature, their properties are examined in depth in the first part of this article. We discuss the remarkably versatile excitonic landscape, including bright, dark, localized and interlayer excitons. In the second part, we provide an overview on the progress in optoelectronic device applications, such as electrically driven light emitters, photovoltaic solar cells, photodetectors and opto-valleytronic devices, again bearing in mind the prominent role of excitonic effects. We conclude with a brief discussion on challenges that remain to be addressed to exploit the full potential of transition metal dichalcogenide semiconductors in possible exciton-based applications.Comment: 31 pages, 8 figure

STUDIES OF NEAR-FIELD NONLINEAR OPTICAL IMAGING OF THIN FILMS AND TRANSIENT PHOTOCONDUCTIVITY IN CONJUGATED POLYMERS

The dissertation is composed of two parts: 1) near-field nonlinear optical studies of ferroelectric and polymeric thin films and 2) transient phenomena in polymers. Ferroelectric thin films are imaged with both topology and surface optical second harmonic (SH) distribution. The lateral resolution is determined to be 80nm. The relation between local optical second harmonic intensity and polarization of the ferroelectric domains is explored both experimentally and theoretically. A polydomain ferroelectric film with grid domain structures is used as a standard sample to check the validity of this relationship. Second order nonlinear properties of nonlinear optical (NLO) polymers are studied both in the far-field and near-field. Transient photoconductivity in conjugated polymers is studied systematically. Picosecond electric pulses generated in a photoconductive switch fabricated on polymer are used to derive the photoconductance of the device. The pulse width of electric pulse is measured by photoconductive sampling to be around 1-2ps, which is the best result in polymers up to now. Transient photoconductivity and mobility are calculated, the transient mobility in MEH-HPPV (poly[2-methoxy-5-(28-ethylhexyloxy)-1,4-phenylenevinylene]) is estimated to be ~800 cm2/Vs. A new method of photoconductive sampling is proposed using only one photoconductive switch, and transient reflectivity change of the polymer is also observed. Optical rectification in poled nonlinear optical (NLO) polymers has been used to generate and detect THz radiation. Relation between THz radiation power and incident laser spot size are studied for the first time in polymer materials; an optimized pump laser spot size of close to THz wavelength is obtained. Application of THz to study organic samples is presented