The materials used for photonic applications are traditionally semiconductor materials. The semiconductor materials according to the photonics point of view are classified into either direct band gap or indirect band gap materials as shown in the Fig. If the global minimum of the conduction band has the same wave vector ‘k’ as the global maximum of the valence band in momentum space, then it is termed to be as a direct band gap, otherwise termed as an indirect-gap semiconductor as shown in Figs. 1.1 (a) and (b), respectively. The process of electrons in the conduction band recombining with the holes in the valence band leading to light emission with energy equivalent to band gap of the semiconductor material is termed as radiative recombination process. Such radiative recombination process takes place more efficiently in direct-gap semiconductors. It is due to that these processes can happen without the need for change in its electron momentum. However, in case of indirect band gap materials, these processes involve a change in electron momentum, which occurs by absorption or emission of a phonon. Hence most of the energy in indirect band gap materials is lost in the form of phonons. Therefore, direct band gap materials such as III-V semiconductors are preferred to indirect bad gap semiconductors such as silicon in the field of photonics. In addition to the direct band gap nature, III-V materials exhibit high mobility of carriers making them useful for high speed electronic devices [].
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| Typical dispersion relation (energy vs. wavevector k) for both direct (left) and indirect (right) bandgap semiconductors [] |
Direct Band gap vs. Indirect Band gap materials
Fig. 1:
Examples: III-V , Si, Ge....
CoPolymers
Heterostructures:
III-V materials are normally deposited by the process called epitaxy. Epitaxy is a process in which the deposited film adapts the crystal phase and orientation identical to those of the substrate. The process of depositing different materials with different band gaps on each other epitaxially is called hetero-epitaxy, and such structures are called heterostructures. The main reason for interest in heterostructures is well described by Kroemer, which offers the possibility of engineering the band alignment required for confinement of carriers [24]. Two types of heterostructures classified based on the relative position (or line-up) of the conduction band and valence band, are of interest in this thesis. In type-I systems, the band gap of one material is nestled entirely within that of the wider-bandgap material as shown in Fig. 1.2 (a). The consequence of this is that both the electrons and holes are confined within the same material. The presence of both types of charge carrier in the same region of space leads to efficient (fast) recombination. In type-II systems the band gaps of the materials, are aligned such that the electrons and holes are confined in different layers of the semiconductor, as shown in Fig. 1.2 (b). The consequence of this is that the recombination times of electrons and holes are long.
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| Schematic diagram of (a) type-I and (b) type-II band alignment. The layers A and B are designated as two different semiconductor materials |
The efficiency of these devices have improved substantially in the last 5 decades by reducing the dimensions of these materials, reducing defect density and changing the design using e.g. quantum wells (QW) [34], multiple QW’s, superlattice heterostructures [35,36], quantum wires (QWR’s) [37,38] and quantum dots (QD’s) [6,7] in the active layers. QW’s, QWR’s, and QD’s are those structures in which the dimensions are reduced to few 10s of nanometers in one, two and all the three directions, respectively. QWs are double heterostructures consisting of a very thin layer of a material with a lower bandgap sandwiched between materials with a higher bandgap. Such structures not only confine the carriers in the growth direction but also passivate surface states at the lower bandgap material surface [24]. However, the choice of materials for the growth of defect free heterostructures is limited by the lattice mismatch of different materials. The interfacial strain in material systems which are closely lattice matched can be elastically accommodated. However, if the lattice mismatch is very large, then the overgrown material tend to relax irreversibly after exceeding a critical thickness, [39,40], as shown in Fig. 1.3 (a), are called misfit dislocations. This often means the development of surface roughness, coherent islands or quantum dots, and with increasing thickness, the generation of internal defects including misfit dislocations, cracks, stacking faults and point defects.
Despite of several advantages with III-V materials such as direct band gap nature and high electron mobility, many of the commercially available photonic devices such as image sensors, solar cells are still being made of Si. Some of the reasons inhibiting the usage of III-V materials for photonic applications are their high cost and the lack of its compatibility with silicon electronics. Considerable effort has been placed towards the integration of III-V materials with silicon substrates, which has long been remained as a desire [41]. The monolithic integration of III-V based optoelectronic devices with conventional electronic circuits provides the possibility to enhance the performance of high-speed devices. In addition, this enables to replace electrical interconnects with optical interconnects in integrated circuits, offering high speed of data transmission and miniaturization of devices [42]. The progress in the integration of III-V devices on Si substrates, however, has been impeded due to the formation of misfit dislocations and anti phase domains at the III-V/Si interface [43].
Light emitting diodes/Laser diodes
Fig. 2:
PhotoDetectors/Solar Cells
Fig. 3:
Characterization Techniques:
Photo-luminescence (PL):
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| Schematic illustration of the photoluminescence process. The electron excited from the valence band to the conduction band by using laser beam de-excites by first emitting an optical phonon and subsequently a photon with energy approximately equal to the band gap energy |
Optical characterization of NWs is essential to determine the optical properties of the grown structures. Optical properties are sensitive to the crystal phase, crystal defects, size effects, and the abruptness of heterojunctions. The details about the optical characterization techniques have been discussed in this chapter.
Photoluminescence (PL) spectroscopy is the technique commonly used for characterizing the optical properties of semiconductors. PL is a process in which the electrons in the valence band are excited to the conduction band with a light pulse generated by using the laser. Eventually, the excited electrons in the conduction band recombine with holes in the valence band emitting the photon with energy equivalent to band gap of the materials, as demonstrated in Fig. 3.19. This process of photon emission is called as radiative emission. The photons emitted from this process are detected by using suitable detector. The PL spectrum reveals the band gap and quality of the structures including the defect states and the discrete energy states in these structures. The schematic diagram of the experimental setup is shown in Fig. 3.20.
To determine the properties of a single quantum dot or a single NW, a smaller laser spot size is required to excite the carriers only in the region of interest. The micro-PL (μ-PL) is a technique in which the size of the laser spot is reduced to few microns, by using a microscope objective lens. Hence, this technique offers the possibility to find the region of interest and excite carriers in a small amount of material.
There are several kinds of PL-studies that can be done to determine the optical properties and optical quality of any materials. The type of studies include power dependent, temperature-dependent, polarization dependent, and time resolved measurements.
Companies related to optics:
http://www.thorlabs.de/index.cfm?
http://www.attocube.com/
