Observation of an inverted band structure near the surface of InN

The dispersion of the valence band within the electron accumulation layer of n-type InN(0001̄) has been directly measured using angle-resolved photoemission spectroscopy. Intermixing between the heavy-hole and light-hole valence bands in the intrinsic quantum well potential associated with the near-surface electron accumulation layer results in an inverted band structure, with the valence band maximum lying away from the Brillouin zone center. Such an inverted band structure has not previously been observed in an intrinsic accumulation layer. Copyright c © EPLA, 2008 Electrons in the accumulation layer near the surface of InN have recently been discovered to exist in intrinsic quantum well states [1]. The confining quantum well potential is perpendicular to the sample surface, and is formed by downward band bending when the Fermi level (EF) is pinned high above the conduction band minimum near the surface of InN [2]. The fact that the quantum well exists near the surface allows angle-resolved photoemission spectroscopy (ARPES) to be used to study the detailed electronic structure of states within this semiconductor quantum well [1]. This is highly advantageous, since ARPES cannot in general be used to study the electronic structure of conventional engineered semiconductor quantum wells as these are usually buried well below the sample surface. ARPES can, of course, be used to study the electronic structure of quantum wells grown on surfaces, such as those formed by metal overlayers [3]. However, the existence of a quantum well potential near the surface of InN allows ARPES to probe the behavior of intrinsic, bulk InN states in the presence of such a potential for the first time. We report here the results of an ARPES study of the dispersion of valence band states of InN(0001̄) within the confining near-surface potential well. A minimum in the dispersion of the top of the valence band is measured at the zone center (Γ-point) in the presence of the quantized electron sub-bands in the conduction band. InN is well known to be a direct gap semiconductor in (a)E-mail: [email protected] the bulk, and our measurement of a zone center minimum for the energy of the valence band maximum (VBM), is ascribed to mixing between the heavyand light-hole valence bands within the electron accumulation layer. This phenomenon is known as an inverted band structure, and has been reported in transport and tunneling experiments from single HgTe/Hg1−xCdxTe [4] and GaAs/AlAs quantum wells [5]. An inverted band structure has also been measured in an ARPES study of an extrinsic quantum well formed by an indium overlayer on p-type Si(111) [6]. However, the fact that the existence of an electron accumulation layer and confining potential can result in an inverted band structure near the surface of a clean semiconductor has not been reported before. The experiments were undertaken on beamline 12.0.1 at the Advanced Light Source (ALS), Lawrence Berkeley National Laboratory. This beamline is equipped with 100mm hemispherical electron energy analyzer (Scienta SES100). Typical energy and full angular resolution were 35meV and 0.5◦. The InN films were grown on c-plane sapphire substrates by radio frequency plasmaassisted molecular beam epitaxy [7]. The films were auto-doped n-type with an average carrier concentration of 5× 10 cm−3 and an average electron mobility of 340 cm/V · s. The room temperature optical gap was approximately 0.77 eV as determined by the peak of the derivative of the absorption constant. This is consistent with a fundamental energy band gap of 0.65 eV [8,9]; i.e. there is a Moss-Burstein shift of the optical absorption to higher energy due to degenerate doping [8]. Samples