Evidence of nanoscale Anderson localization induced by intrinsic compositional disorder in InGaN/GaN quantum wells by scanning tunneling luminescence spectroscopy

Abstract

We present direct experimental evidences of Anderson localization induced by the intrinsic alloy compositional disorder of InGaN/GaN quantum wells. Our approach relies on the measurement of the luminescence spectrum under local injection of electrons from a scanning tunneling microscope tip into a near-surface single quantum well. Fluctuations in the emission line shape are observed on a few-nanometer scale. Narrow emission peaks characteristic of single localized states are resolved. Calculations in the framework of the localization landscape theory provide the effective confining potential map stemming from composition fluctuations. This theory explains well the observed nanometer scale carrier localization and the energies of these Anderson-type localized states. The energy spreading of the emission from localized states is consistent with the usually observed very broad photo-or electro-luminescence spectra of InGaN/GaN quantum well structures.InGaN quantum wells (QWs), the active regions of nitride-based LEDs, display broad photo-or electroluminescence spectra. The emission is assumed to be inhomogeneously broadened due to contributions from material regions with different eigenenergies associated with fluctuations of the In content. Spatial variations of emission intensities and/or energies were previously observed in nitride semiconductor structures by far field [1] and near-field [2, 3] photoluminescence microscopy or cathodoluminescence microscopy [4][5][6]. These observations have evidenced so far emitting domains of either micron or submicron (typically 100 nm) size. While significant energy shifts due to these rather large-scale fluctuations were observed, no linewidths below 100 meV at room temperature were detected. These large-scale luminescence fluctuations are usually associated with growth inhomogeneities seen for instance in AFM. However, in the case of random atom positioning in the alloy crystal [7], modeling predicts that the compositional disorder should induce Anderson localization in regions of a few nm size [8][9][10][11][12][13][14]. Moreover, emission from such single localized states is expected to exhibit a narrow linewidth of a few tens of meV at room temperature, i.e. comparable to what was observed from single GaN quantum dots [15]. Such emission features exhibiting size and spectroscopic signatures characteristic of carrier localization induced by the alloy disorder have not been evidenced up to now.Here we report on the observation of fluctuations in the luminescence spectrum on the scale of a few nm as expected for such localization effects. Our approach is based on scanning tunneling luminescence (STL) microscopy [16] which allows luminescence measurements under local carrier injection with a nm scale resolution. In order to reach the required spatial resolution, we used p-type single InGaN/GaN QW structures with a thin top GaN layer separating the QW from the injecting tip of the scanning tunneling microscope (STM) and we analyzed the spatial variations in the luminescence sp...

Plasma profiling time-of-flight mass spectrometry for fast elemental analysis of semiconductor structures with depth resolution in the nanometer range

“…The full width at half maximum (FWHM) of the (0006), (10)(11)(12)(13)(14)(15) and (-1015) reflexes were determined to be 282, 171 and 313 arcsec, respectively, confirming suitable crystal quality as already observed by STEM. Fully coherent growth of the entire epilayer stack was concluded from reciprocal space maps of asymmetric (10-15) and (-1015) reflexes.…”

Long coherent dynamics of localized excitons in (In,Ga)N/GaN quantum wells

“…Nevertheless, the Stokes shift as large as 20 meV at room temperature [17] indicates a strong lateral localization of the QW excitons. Origin and scale of the potential fluctuations in (In,Ga)N/GaN QWs were investigated by various methods, including transmission electron microscopy [5], near-field scanning optical microscopy [22], and scanning tunneling luminescence spectroscopy [23]. In particular, Hahn et al showed that micro-PL spectra display significant and chaotic potential variations on the scale of about 3 nm [23].…”

“…Origin and scale of the potential fluctuations in (In,Ga)N/GaN QWs were investigated by various methods, including transmission electron microscopy [5], near-field scanning optical microscopy [22], and scanning tunneling luminescence spectroscopy [23]. In particular, Hahn et al showed that micro-PL spectra display significant and chaotic potential variations on the scale of about 3 nm [23]. This is commensurable with the free exciton Bohr radius in GaN of about 3 nm [24] and might provide a reason for the modification of the T 1 time connected with the exciton oscillator strength.…”

Disorder effects in nitride semiconductors: impact on fundamental and device properties

“…As the scale of intrinsic disorder of alloys is a few nm (Figure 2b), it is not observed through these techniques. Only the recently developed technique of scanning tunneling luminescence (STL) provides spatial resolution at the few-nm scale and allows direct observation of localized states induced by the intrinsic disorder [19]. The indirect impact of nm-scale disorder can however be observed at the macroscopic level, through spatially averaged observations.…”

“…Such measurements are spatially averaged effects of interface disorder and one would wish more direct observations of the interface fluctuations. The STL technique provides spatial resolution at the few nm scale and could probe the interface morphology because of the very localized nature of the probing carrier density [19].…”