Material Characterization

Atomic force microscopy (AFM) is a fast and convenient method for nano-scale surface characterization in semiconductor industry due to its operability in ambient air and minimal sample preparation for investigation. The AFM operation principle is illustrated in Fig. 2.5. The instrument consists of a cantilever with a sharp tip mounted on its end. It is the force between

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Fig. 2.5 Schematic illustration of an atomic force microscope.

the apex of the tip and the surface atoms in the sample that produces the AFM operation [2.4].

Such force depends sensitively on the distance between the tip and the sample. Considering the compromise between high image resolution and minimal damage creation for both samples and tips, the tapping mode was used throughout in this dissertation. The cantilever is excited close to its resonant frequency by a driving signal applied to the piezoelectric ceramic to which the cantilever is attached. The excited oscillation of the tip/cantilever is appropriately damped by the force between the tip and the sample due to an appropriate tip-sample distance, and the oscillation amplitude is recorded by a positional detector which detects the laser signal reflected by the end of the cantilever (see Fig. 2.5). With a feedback-control electronics and a 3-D scanning mechanics, when the oscillating tip scans over the surface of the sample, one can keep the tip-sample distance constant by keeping the oscillation amplitude constant. In this manner, the topography on the sample surface can be obtained with the resolution reaching ~1 nm. In our lab, the AFM is performed by a Veeco Digital Instrument D3100 commercial system, primarily for understanding the geometry, sheet density, and uniformity condition of the 3-D quantum objects in the samples.

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2.2-2 Photoluminescence Spectroscopy

Photoluminescence (PL) spectroscopy is a very practical and efficient material characterization method for most III-V semiconductors, in which the process of the electron-hole (e-h) recombination is mainly radiative. It serves to determine the fundamental bandgap (E0) of the sample, understand the crystalline quality of the sample, and identify the impurity species in the sample [2.5]. When a sample are excited by an optical source, typically a laser with photon energy hν > E0, e-h pairs are generated, subsequently followed by a carrier thermalization process (including diffusion, capture, and relaxation of the carriers), and then the PL signals are obtained from the spontaneous emission of the sample due to the radiative e-h recombination. In our lab, the samples grown by our MBE machines are routinely characterized by a homemade PL system as shown in Fig. 2.6. Such a system has the capability of, besides the conventional PL, also the PL-excitation (PLE), resonant PL and micro-PL measurements.

Fig. 2.6 The instrument setup for PL, PLE and micro-PL measurements in our lab.

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For conventional PL measurement, an argon ion laser is used as the excitation source and the samples for investigation are mounted in a helium close-cycled cryostat, where the sample temperature can be maintained at from ~13K to room temperature. While for PLE or resonant PL, a Ti-sapphire laser pumped by the argon laser is instead used to excite the samples. The excitation photon wavelength can be tuned from ~770 nm to ~1030 nm using different lens kits.

The emission signal of the sample was collected by a couple of plano-convex lenses, transferring through air space, dispersed by a 0.55m monochromator, and then detected by a thermal-electric cooled InGaAs photodiode. Using mechanical choppers to modulate the laser beams, the signal-to-noise ratio can be greatly improved by such a lock-in technique. Besides, a 100 times long-working-distance object lens along with an OXFORD designed liquid nitrogen/helium cooled cryostat is used for the micro-PL measurement, where the spot size of the excitation beam can be focused down to only ~1.5 μm. We take advantage of its ultra-high excitation density to obtain PL spectra revealing high-lying excited states of the InAs quantum objects that are studied in this dissertation.

2.2-3 Transmission Electron Microscopy

Transmission electron microscopy (TEM) is a powerful technique for obtaining highly magnified sample images with extremely high resolution, approaching 1-2 Å [2.6]. A very high energy electron beam, typically around 100-400 keV, is deflected and focused on the sample by electro-optic condenser lenses, and passes through the sample, forming a magnified image in the image plane which is then simply projected onto a fluorescent screen. The sample must be sufficiently thin (a few tens to a few hundred nm) to be transparent to electrons. Due to such a thin thickness, few electrons are absorbed in the sample. Thus, image contrast does not depend very much on absorption, as it does in optical transmission spectroscopy, but rather on scattering and diffraction of electrons in the sample.

Images formed using only the transmitted electrons are bright-field images and that using

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a specific diffracted beam are dark-field images. On the other hand, if large aperture is used so that one transmitted beam plus many diffracted beam are all collected in the image plane, high-resolution images are formed. The image brightness is determined by the intensity of those electrons transmitted through the sample that pass through the image forming lenses.

Therefore, heavier elements cause stronger scattering, hinder the electrons from reaching the fluorescent screen, and thus create dark image in the TEM picture. In general, for crystalline specimen, the contrast in the TEM images may arise from mass contrast, thickness contrast, diffraction contrast, etc. In this dissertation, TEM pictures are all obtained by a JEOL JEM-2100F field-emission TEM system using high-resolution image mode. Different from the AFM technique, which was routinely performed for every sample, TEM analysis was only performed for certain samples in this dissertation due to the challenging, time-consuming task on sample thinning.