Study of internal electric fields in AlGaAs/GaAs two-dimensional electron gas heterostructures L. Zamora-Peredo a , A. Guillen-Cervantes b , Z. Rivera-Alvarez b , M. Lo ´pez-Lo ´pez b , A.G. Rodrı ´guez-Va ´zquez a , V.H. Me ´ndez-Garcı ´a a, * a Instituto de Investigacio ´n en Comunicacio ´n O ´ ptica, Universidad Auto ´noma de San Luis Potosı ´, Av. Karakorum 1470, Lomas 4a Seccio ´n, San Luis Potosı ´, SLP 78210, Mexico b Department of Physics, Centro de Investigacio ´n y Estudios Avanzados del IPN, Apdo. Postal 14-740, Mexico DF 07000, Mexico Abstract Modulation-doped GaAs/AlGaAs heterostructures have been studied by photoreflectance spectroscopy. The spectra at room temperature show Franz–Keldysh oscillations associated to the substrate–buffer layer interface. The built-in electric field magnitude calculated from these oscillations is related with the two-dimensional electron gas (2DEG) mobility. In addition we observed two signals associated to the GaAs capping layer and to the 2DEG, respectively. q 2003 Elsevier Science Ltd. All rights reserved. Keywords: Two-dimensional electron gas; Modulation-doped GaAs/AlGaAs heterostructures; Photoreflectance 1. Introduction Modulation-doped GaAs/AlGaAs heterostructures (MDHs) have induced great interest due to important technological applications. The photoreflectance (PR) technique has been used for the study of MDHs in order to reveal in non-destructive mode different parameters of the samples [1,2]. In this work we present a detailed study on the optical properties of MDHs by PR. We found that the electric field magnitude determined by PR is related to the electronic mobility of the two-dimensional electron gas (2DEG). We also observed two signals between the GaAs and AlGaAs band gap transitions which are related to the GaAs capping-layer and the 2DEG region, respectively. 2. Experimental The MDHs in this study were grown by molecular beam epitaxy (MBE) on semi-insulating GaAs substrates. The samples structure consists of a 3-mm thick undoped GaAs buffer layer (BL), followed by an undoped Al x Ga 1-x As spacer layer with a thickness denoted by S: Next, a 1000 A ˚ - thick n-AlGaAs layer doped with Si (1 £ 10 18 cm 23 ) was grown, and finally the structure was capped with a 100 A ˚ - thick n-GaAs layer. The nominal Al concentration was 35%. The PR measurements were carried out employing a tungsten lamp as probe light, and a He–Ne Laser (543.5 nm) chopped to 200 Hz as the modulation source. Van der Pauw –Hall measurements were performed at 77 K to determine the carrier concentration and the mobility m of the samples. 3. Results and discussion In Fig. 1, we show three spectra from MDHs in the region of the GaAs band edge (, 1.42 eV). A clear signature of the existence of intense internal electric fields is the presence of damped oscillations at energies above the band gap in the PR spectra, identified as Franz–Keldysh oscillations (FKO). The strength of the internal electric field can be obtained using the asymptotic Aspnes and Studna theory. In this model the energies E j of the FKO extremes can be fitted to [3] E j ¼ "VF j þ E g ð1Þ with "V ¼ðe 2 F 2 int " 2 =8mÞ 1=3 the characteristic electro-optic energy and F j ¼½3p=2ðj 2 1=2Þ 2=3 ; where j refers to the jth extremum, E g is the energy gap of the GaAs (for this case), e is the electron charge, F int is the internal electric field strength, and m the interband reduced mass involved in 0026-2692/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0026-2692(03)00102-2 Microelectronics Journal 34 (2003) 521–523 www.elsevier.com/locate/mejo * Corresponding author. Tel.: þ52-44-48-25-01-83; fax: þ 52-44-8-25- 01-98. E-mail address: vmendez@cactus.iico.uaslp.mx (V.H. Me ´ndez- Garcı ´a).