Extremely low-energy ARPES of quantum well states in cubic-GaN/AlN and GaAs/AlGaAs heterostructures

Mahdi Hajlaoui,Thomas Zentgraf,Michael Deppe,Tobias Henksmeier,Mirko Cinchetti,Donat Josef As,Dirk Reuter,Gunther Springholz,Stefan Cramm,Claus Michael Schneider,Stefano Ponzoni

doi:10.1038/s41598-021-98569-6

Abstract

Quantum well (QW) heterostructures have been extensively used for the realization of a wide range of optical and electronic devices. Exploiting their potential for further improvement and development requires a fundamental understanding of their electronic structure. So far, the most commonly used experimental techniques for this purpose have been all-optical spectroscopy methods that, however, are generally averaging in momentum space. Additional information can be gained by angle-resolved photoelectron spectroscopy (ARPES), which measures the electronic structure with momentum resolution. Here we report on the use of extremely low-energy ARPES (photon energy ~ 7 eV) to increase depth sensitivity and access buried QW states, located at 3 nm and 6 nm below the surface of cubic-GaN/AlN and GaAs/AlGaAs heterostructures, respectively. We find that the QW states in cubic-GaN/AlN can indeed be observed, but not their energy dispersion, because of the high surface roughness. The GaAs/AlGaAs QW states, on the other hand, are buried too deep to be detected by extremely low-energy ARPES. Since the sample surface is much flatter, the ARPES spectra of the GaAs/AlGaAs show distinct features in momentum space, which can be reconducted to the band structure of the topmost surface layer of the QW structure. Our results provide important information about the samples’ properties required to perform extremely low-energy ARPES experiments on electronic states buried in semiconductor heterostructures.

Highlights

Semiconductor heterostructures with confined electronic states constitute very intriguing systems in condensed matter physics, both from a point of view of fundamental research[1,2] as well as for device applications[3,4]
Since Angle-resolved photoemission spectroscopy (ARPES) measures occupied states, i.e. states below the Fermi level ( EF), the Quantum well (QW) structures have been designed to ensure n-doping in the QW region, so that at least one conduction subband state (CSbS) is occupied
We show the schematic energy band diagram of the QW states located at 3 nm and 6 nm below the sample surface, respectively, including the calculated Fermi level positions in the conduction band, which ensures the presence of at least one occupied QW state that we want to detect by ELE-ARPES

Summary

Introduction

Semiconductor heterostructures with confined electronic states constitute very intriguing systems in condensed matter physics, both from a point of view of fundamental research[1,2] as well as for device applications[3,4]. Time-resolved ARPES (trARPES) has been demonstrated as a unique method to measure the dispersion of e xcitons[27,28] that are more pronounced in QW structures than bulk s emiconductors[1] These peculiarities of ELE-ARPES and their advantages with respect to soft X-ray ARPES and conventional optical spectroscopies motivated us to test and assess its bulk sensitivity for measuring the buried QW states in two semiconductor heterostructures; the cubic-GaN/AlN and GaAs/AlGaAs. The bulk sensitivity will be assessed by comparing ELE-ARPES measurements to standard ARPES experiments performed with photon energy in the range where λPE has its minimum (15–21 eV, the typical photon energy range for ultraviolet photoelectron spectroscopy, UPS). The results can open a new experimental avenue for achieving detailed knowledge of the QW heterostructures using trELE-ARPES

Methods

Results

Conclusion