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

We present direct experimental evidence 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 electroluminescence spectra of InGaN/GaN quantum well structures.

Figure 1

Photoluminescence spectra of InGaN/GaN QW samples at 300 K for a $394\mathrm{nm}$ excitation wavelength. Inset: sample structure; the thickness $d$ of the p-GaN layer, below the ${\mathrm{p}}^{+}$-GaN cap, is $10\mathrm{nm}$ (resp. $90\mathrm{nm}$) in S10 (resp. S90).

Figure 2

Spectrally integrated luminescence intensity as a function of $V_b$ for a constant tunneling current of $20\mathrm{nA}$. The line is a fit by a function of ${(V_b-V_{th})}^2$ with $V_{th}=$ $3.6\mathrm{V}$. Inset: schematics of the tip-sample band diagram in real space and of the injection, transport and recombination process; the extrema of the semiconductor valence band (VB), conduction band (CB), and conduction side valley (L) are represented.

Figure 3

(a) Schematic of the STL experiment showing the electron injection from the STM tip into the QW which exhibits localization regions. (b) Narrow STL spectrum characteristic of the emission from a single localized state. (c)–(e) Evolution of the STL spectrum along $50\mathrm{nm}$ line scans measured at different locations on S10 with constant injection current and bias of $2\mathrm{nA}$ and (c) $4.1\mathrm{V}$, (d) $4.8\mathrm{V}$, (e) $5.2\mathrm{V}$.

Figure 4

Evolution of the STL spectrum measured on (a) S10 and (b) S90 along a $250\mathrm{nm}$ line scan by $5\mathrm{nm}$ steps for constant tunneling current and bias of $2\mathrm{nA}$ and $4.1\mathrm{V}$. (c)–(d) Sum of the 50 local spectra for S10 and S90, respectively.

Figure 5

(a) Calculated $30\mathrm{nm}$ $\times$ $30\mathrm{nm}$ map of the localization function $1/u$ for the electrons at the QW center plane in a structure corresponding to S10. The black lines indicate the crest lines which bound the localization regions. (b) Histogram of the local band gaps calculated from the overlap of the $1/u$ maps of electrons and holes. (c) Histograms of the STL peak energies measured along six different line scans.