Contactless Electroreflectance Techniques Research Articles

Electromodulation spectroscopy of highly mismatched alloys

The electronic band structure of highly mismatched alloys (HMAs) was very successfully explored using electromodulation (EM) spectroscopy, i.e., photoreflectance (PR), electroreflectance, and contactless electroreflectance (CER). With these techniques, the optical transitions between the valence band and the E− and E+ bands, which are formed in the conduction band of dilute nitrides and dilute oxides, were observed and used to formulate the band anticrossing model, which well describes the electronic band structure of HMAs. In this tutorial, principles of EM spectroscopy are presented and shortly discussed. Special attention is focused on PR and CER techniques, which are nondestructive and have recently been widely applied to study the electronic band structure of HMAs and low dimensional heterostructures containing HMAs. For these methods, experimental setups are described, and theoretical approaches to analyze the experimental data are introduced. Finally, to show the utility of EM spectroscopy, selected examples of the application of this method to study various issues in HMAs are presented and briefly discussed.

On the deepness of contactless electroreflectance probing in semiconductor structures

AbstractThe 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)

Photoreflectance and contactless electroreflectance of an InAs quantum dash laser structure

AbstractPhotoreflectance (PR) and contactless electroreflectance (CER) spectroscopies have been used to study optical transitions in an InP‐based laser structure emitted at 1.55 μm with the active region including InAs quantum dashes (QDashes). Optical transitions related to all relevant parts of the structure, i.e. In0.53Ga0.05Al0.42As n‐ and p‐type cladding layers, quaternary In0.53Ga0.23Al0.24As QDash barrier layers, InGaAlAs‐GRINSCH region, and InAs QDashes have been clearly observed at room temperature. Moreover, it has been found that some of the transitions are not observable in CER spectra whereas they are very strong in PR spectra. This phenomenon is associated with different modulation mechanisms in the two electromodulation techniques. It means that the combination of PR and CER techniques with traditional photoluminescence measurements allows much better characterization of the electronic structure for semiconductor devices. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoreflectance and contactless electroreflectance spectroscopy of GaAs-based structures: The below band gap oscillation features

GaAs-based structures characterized below band gap oscillation features (OFs) in photoreflectance (PR) are studied in both PR and contactless electro-reflectance (CER) spectroscopies. It has been shown that the OFs are usually very strong for structures grown on n-type GaAs substrate. The origin of the OFs is the modulation of the refractive index in the sample due to a generation of additional carriers by the modulated pump beam. The presence of OFs in PR spectra complicates the analysis of PR signal related to quantum well transitions. Therefore, PR spectroscopy is often limited to samples grown on semi-insolating (SI) type substrates. However, sometimes the OFs could be observed for structures grown on SI-type GaAs substrates. In this paper we show that the OFs could be successfully eliminated by applying the CER technique instead of PR one because during CER measurements any additional carriers are not generated and hence CER spectra are free of OFs. This advantage of CER spectroscopy is very important in investigations of all structures for which OFs are present in PR spectra.