Realization of wide electron slabs by polarization bulk doping in graded III–V nitride semiconductor alloys

Abstract

We present the concept and experimental realization of polarization-induced bulk electron doping in III-V nitride semiconductors. By exploiting the large polarization charges in the III-V nitrides, we are able to create wide slabs of high density mobile electrons without introducing shallow donors. Transport measurements reveal the superior properties of the polarization doped electron distributions than comparable shallow donor doped structures. The technique is readily employed for creating highly conductive layers in many device structures. PACS numbers: 61.72.Vv,72.20.-i, 73.40.-c Doping in semiconductors has been a much researched topic. The traditional shallow 'hydrogenic' doping technique is very well understood and gainfully employed. A good understanding of the role of ionized dopant atoms on carrier scattering in semiconductors led to the concept of modulation doping, which improved low temperature carrier mobilities in quantum-confined structures by many orders of magnitude [1]. The last decade witnessed the emergence of the III-V nitrides as a wide bandgap semiconductor with the property of large embedded electronic polarization fields owing to the lack of inversion symmetry in the crystal structure [2],[3]. This property has been widely exploited to make nominally undoped two-dimensional electron gases (2DEGs) in AlGaN/GaN heterostructures, which had led to high-electron mobility transistors (HEMTs) with record high performance characteristics[4]. The 2DEG at the AlGaN/GaN interface of a III-V nitride heterostructure is formed to screen the polar-

Ultrawide‐Bandgap Semiconductors: Research Opportunities and Challenges

“…This technique has in fact been demonstrated for AlGaN graded from 0-30% Al. [333] Another approach is to utilize a superlattice in which the polarization-induced electric fields cause ionization of the impurity dopants due to extreme band-bending. [334] Ohmic Contacts: Ohmic contacts are another significant challenge for UWBG materials.…”

AlGaN/GaN polarization-doped field-effect transistor for microwave power applications

“…To screen the net positive charge at the AlGaN/GaN junction, a 2DEG forms. Jena et al 11 have shown that the same effect can also be used to create a bulk three-dimensional electron slab. This is achieved by grading from GaN to AlGaN, thus spreading the polarization-induced charge over the graded region.…”

“…As in a HEMT, the performance of the PolFET improves at lower temperatures since phonon scattering is reduced, and the mobility is higher. 11 For comparison, two samples, a conventional impuritydoped MESFET and a polarization-doped FET, or PolFET, were studied ͑Fig. 1͒.…”

Controlling a three dimensional electron slab of graded AlxGa1−xN

“…3 (b) we observe for samples S1-S4 carrier mobilities which are comparable [36] or slightly lower than those observed in 3DES fabricated by MBE [18,19]. However,the Al concentration in the above mentioned works had a gradient in the range x: 0%→20%, while all the samples considered in this work have a Al gradient x: 0%→37%.…”

“…Specifically, a compositional grading in the Al x Ga 1−x N layer results in a non-vanishing divergence of the polarization, leading to the formation of a bulk three-dimensional (3D) polarization charge background, i.e. to a polarization induced 3D electron slab (3DES) [18][19][20]. An analogous approach can also be exploited in a polarization doped hole system, where the grading may be designed to span from an Al rich to an Al depleted alloy [21].…”