Chapter 3. Experiment
3.1 Micro-Photoluminescence
3.1.1Principles and Applications of Micro-Photoluminescence
For a perfect semiconductor crystal, a light source with photon energy higher
than the band gap of the semiconductor crystal will excite the carriers to their excited
states. As soon as the excitation occurs, all excited electrons and holes will relax to
the bottom of the conduction band and the top of valence band, respectively, and then
the radioactive recombination may occur under the condition of momentum
conservation as shown in Figure 3.1. When the maximum of the valence band and the
minimum of the conduction band occur at the same value of the wave vector k,
transitions are direct and the material is a direct-gap semiconductor (for instance,
GaAs and GaN). For a direct-gap material, the most probable transition is across the
minimum energy gap which is between the most probably filled states at the minimum
of the conduction band and the states most likely to be unoccupied at the maximum of
the valence band. If the band extreme do not occur at the same wave vector k, the
transition is indirect. To hold the condition of momentum conservation in such an
indirect-gap material (for example, Si and Ge), the participation of phonons is
required. Such a process is also called a phonon-assisted process. Therefore, the
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recombination of electron-hole pairs must be accompanied by the simultaneous
emission of a photon and a phonon. The probability of such a process is significantly
lower as compared with direct transitions. The radioactive recombination that is
caused by the incandescence coming from hot source is called photoluminescence
(PL). As shown in Figure 3.2, for example, if there is a multiplicity of excited states,
only transitions from the lowest excited state can generally be observed at low
temperatures because of rapid thermalization [2]. Figure 3.3 illustrates the process of
photoexcitation, as well as different processes that may cause light emission. If a
photon with its energy higher than the band gap of the sample, an electron in the
valence band will be excited to conduction band and soon dribbles down to the
bottom of conduction band by reaching thermal equilibrium with the lattice, i.e.,
emitting phonons. Figure 3.3(a) is an interband transition. In this case, a direct
recombination between an electron in the conduction band and a hole in the valence
band results in the emission of a photon of energy Eg =hν Although this recombination
occurs from states close to the corresponding band edges, the thermal distribution of
carriers in these states will lead, in general, to a broad emission spectrum. If the
semiconductor is pure, the recombination will be between electrons in the conduction
band and holes in the valence band. As the temperature is sufficiently low (e.g., less
than 25 K for GaAs), an electron and a hole will form a bound state due to the
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Coulomb interaction. This electron-hole pair is the so-called exciton. The
recombination of an exciton will give rise to sharp-line luminescence with energy of
the band gap minus the binding energy of the exciton. Figure 3.3(b) shows an
excitonic transition. If the semiconductor contains impurities, several new
recombination paths via the impurity states open up. Electrons from the conduction
band may recombine with neutral acceptors which become negatively charged after
recombination (as shown in Fig. 3.3(c)). Neutral donors may recombine with holes in
the valence band, becoming positively charged (as shown in Fig. 3.3(d)). At higher
impurity concentrations electrons bound to donors may recombine directly with holes
bound to acceptors, giving rise to donor–acceptor pair luminescence (as shown in Fig.
3.3(e)). A third category of impurities is isoelectronic impurities, where the impurity
has the same valency as the atom it replaces in the host lattice. The isoelectronic
impurities may bind excitons, which can give luminescence, substantially below the
energy of free excitons. Luminescence studies are, in general, a very powerful method
for obtaining information about impurities, also at low concentrations. It should also
be noted that not all recombination between electrons and holes results in light
emission, since there may also be efficient nonradiative recombination paths. PL is
one of the most useful optical methods for the semiconductor industry [2, 3], with its
powerful and sensitive ability to find impurities and defect levels in silicon and group
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III-V element semiconductors, which affect materials quality and device performance.
A given impurity produces a set of characteristic spectral features. This fingerprint
identifies the impurity type, and often several different impurities can be seen in a
single PL spectrum. In another use, the full width at half maximum (FWHM) of PL
peak is an indication of sample quality and crystallinity, although such analysis has
not yet become highly quantitative. Besides, PL is sensitive to the strain field inherent
in the semiconductor heterostructures, and can measure the magnitude and the
direction of the strain field. Photoluminescence can also determine the band gaps of
semiconductors. This is very important for binary (AxB1-x) and ternary (AxB1-xC)
alloys whose gaps vary with the compositional parameter x which must be accurately
known for applications. When the relation between gap energy and x is known, the PL
measurement of gap can be inverted to determine x. From this, a two-dimensional
map of alloy composition can be obtained as the exciting laser beam is scanned across
the surface of the sample, which is a useful tool to determine inhomogeneity.
Among the optical characterization methods, PL are probably the best developed
to carry out such spatial scanning, with commercial equipment available. An
interesting PL measurement with the aid of a polarizer can help us study the optical
anisotropy of semiconductor heterostructures. By way of this method, we are able to
investigate the microstructure of the sample and the mechanism of the radioactive
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recombination of the electrons and the holes in detail.
3.1.2 The Apparatus for Micro-Photoluminescence Measurement
The micro-PL arrangement is the most straightforward measurement in opticalsystem as shown in Figure 3.4. The excitation source can be any laser whose photon
energy is higher than the band gap of the materials to be examined, and whose power
is sufficient to excite an adequate signal. In our measurements, a laser beam with 374
nm is focused into a microscope to be the excitation source. The luminescence from
sample is again focused into the microscope and pass through optical fiber then enters
a TRIAX 320 spectrometer. Finally, a high response H5783 photomultiplier tube is
employed as the detector. The signal from detector switched by SACQ2 sends to
computer. These photons can be generated at continuous powers of watts. Usually,
tens of mW are often adequate to give good signals. The intensity of the Micro-PL
signal apparently depends on the quality of the materials to be examined, the handling
ability of the system, and the sensitivity of the detector.