Fourier-transformed photoreflectance and photoluminescence have been used to study the optical transitions in type II quantum wells (QWs) ranging up to almost 5 m. High signal-to-noise ratio spectral features resulting from fundamental and excited state transitions have been detected for molecular beam epitaxially grown GaSb/AlSb/InAs/InGaSb/InAs/AlSb/GaSb ''W''-shaped QW structures designed for laser-based gas sensing applications in the mid-infrared. The spectral features' dependence on arsenic pressure during growth process and on InAs confining-layer thickness could be followed unambiguously at room temperature.

Photoreflectance ͑PR͒ and contactless electroreflectance ͑CER͒ spectroscopies have been applied to study optical transitions in undoped and Si-doped AlGaN / GaN heterostructures at room temperature. Spectral features related to excitonic and band-to-band absorptions in GaN layer and band-to-band absorption in AlGaN layer have been resolved and analyzed. In addition, a broad spectral feature related to two-dimensional electron gas has been observed for the Si-doped heterostructure. It has been found that some of the mentioned optical transitions are not observed in CER spectra whereas they are very strong in PR spectra. This phenomenon is associated with different mechanisms of the modulation of built-in electric field in the investigated structure. A combination of PR and CER gives the possibility of a richer interpretation of both PR and CER spectra.

The deepness of contactless electroreflectance (CER) probing (dCER) is investigated in this work. CER spectra have been measured for various semiconductor structures and compared with photoreflectance (PR) spectra. It has been shown that most of CER signal origins from the sample region which is very close to the sample surface. It has been found that the dCER is much smaller that the penetration depth of the probing beam (dλ ∼ 0.5–1 μm), dCER ! dλ. In addition, for samples containing a sheet of carriers (a two dimensional electron gas at an interface or a delta‐doped monolayer) the dCER is reduced to the distance from the sample surface to the carrier sheet (e.g. 20 nm). This phenomenon can be called as screening of electromodulation in CER technique. No screening of electromodulation mechanism is observed in PR technique. In addition, the deepness of PR probing (dPR) is much bigger than the dCER. It means that deeper parts of semiconductor structures (e.g. the buffer layer) can be probed in PR whereas these parts are usually not probed in CER. A different electromodulation mechanism in PR and CER techniques is the origin of various probing deepness for these two techniques. In the case of CER technique, the surface electric field is mainly modulated whereas both surface and interface electric fields can be effectively modulated in the PR technique. In the case of PR technique, the electron–hole pairs can be generated far to the sample surface and hence these carriers can modulate band bending at interfaces which are far to the sample surface, e.g. at the epilayer/buffer interface. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)