Correlations between the surface morphology and electronic properties of porous multilayer structures with quantum InGaAs layers

Recently, porous materials on the base of semiconductor crystals arouse greater interest as photon crystals in terms of opto-electronic applications and formation of arrays of nano-elements with oneand zero-dimensionality. The crystals of porous silicon have been developed best to date. The obvious trends in this field now are towards using other materials for electrolytic etching, decreasing the diffraction lattice period to have photon crystals operate at smaller wavelengths, and study of porous compositions with microporosity. Besides, porous structures attract close attention as subjects for study of low-dimensional fundamental physical properties. In this work the properties of porous conducting two-dimensional InGaAs layers embedded in GaAs or InAlAs porous dielectric matrix are discussed. The influence of the electrochemical etching time of samples, their porosity and surface morphology on the optical and electrophysical characteristics of the structures with a low-dimensional electron gas has been investigated. Epitaxial multilayer GaAs/InGaAs structures of the first type have been grown on isolated GaAs substrates by the MOCVD method. These compositions contained from 1 to 11 InGaAs single quantum wells embedded into the GaAs matrix with the GaAs barrier layer thickness about 160 nm (No 30-54). The second type of the test structures is represented by the InAlAs/InGaAs/InAlAs composition with one InGaAs quantum well (No 401), grown by GSMBE. The top InAlAs layer covering a 40 nm thick InGaAs electron transport channel had the thickness of about 100 nm. The porous samples have been produced by the electrochemical etching of the structures [1] in alcohol solution of FH acid at a current density of about 0.05 А/cm. A change in the surface morphology due to the formation of a porous surface structure in the process of etching has been studied by the scanning probe microscopy (SPM). Modification of the surface structure, the change of porosity of the samples, the size of pores and nano-islands formed on the surface, the porous layer thickness can be traced with this method depending on the etching time of the samples. Initially the heteroepitaxial structures had a rough surface with the depth of surface imperfections reaching 4 nm [2,3]. Varied-depth sections of the porous layer show that the porous two-dimensional (2D) InGaAs layer in the structure with one quantum well nearest to the surface is formed in our case at the initial stage of the etching process. The change in the surface morphology of the multilayer structure during the etching process induces an inhomogeneous potential in the plane for the depth layers, that modulates the energy band edges in the quantum wells of the structure. Further etching of the sample leads to an increase in the material porosity and to formation of arrays of islands on the surface of structure (Fig.1a). Narrowing of the InGaAs layer area with an increase in the etching time leads to formation of a complex two-dimensional network of conducting quantum threading. We can see this in Fig.1 where the SPM image of the surface of sample No 401 after 10 minutes of etching is shown. We see here that the visual depth of the top layer etching up to the undoped InAlAs buffer layer is about 140 nm (Fig.1a). The thickness of the real porous layer is about 50 nm, which corresponds to the thickness of the InGaAs electron transport layer in the sample. The tomography of the SPM image at an 18 nm height from the low boundary of the InGaAs layer (it is approximately the middle of the InGaAs transport channel in the structure) is shown in Fig.1b. The geometry of the sample suggests that the electron current is directed along the InGaAs layer plane, that is why, in addition to the vertical pores, we see wide etching areas which form an irregular twodimensional net in the layer. The micro-island chains in the porous InGaAs layer with 2D