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Register a new userNonpolar m-plane GaN/AlGaN heterostructures with intersubband transitions in the 5 to 10 THz band
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This paper assesses intersubband transitions in the 1 to 10 THz frequency range in nonpolar m-plane GaN/AlGaN multi-quantum-wells deposited on free-standing semi-insulating GaN substrates. The quantum wells were designed to contain two confined electronic levels, decoupled from the neighboring wells. Structural analysis reveals flat and regular quantum wells in the two perpendicular inplane directions, with high-resolution images showing inhomogeneities of the Al composition in the barriers along the growth axis. We do not observe extended structural defects introduced by the epitaxial process. Low-temperature intersubband absorption from 1.5 to 9 THz is demonstrated, covering part of the 7 to 10 THz band forbidden to GaAs-based technologies.
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Gallium nitride (GaN) has emerged as an essential semiconductor material for energy-efficient lighting and electronic applications owing to its large direct bandgap of 3.4 eV. Present GaN/AlGaN heterostructures seemingly feature an inherently existing, highly-mobile 2-dimensional electron gas (2DEG), which results in normally-on transistor characteristics. Here we report on an ultra-pure GaN/AlGaN layer stack grown by molecular beam epitaxy, in which such a 2DEG is absent at 300 K in the dark, a property previously not demonstrated. Illumination with ultra-violet light however, generates a 2DEG at the GaN/AlGaN interface and the heterostructure becomes electrically conductive. At temperatures below 150 K this photo-conductivity is persistent with an insignificant dependence of the 2D channel density on the optical excitation power. Residual donor impurity concentrations below 10$^{17}$ cm$^{-3}$ in the GaN/AlGaN layer stack are one necessity for our observations. Fabricated transistors manifest that these characteristics enable a future generation of normally-off as well as light-sensitive GaN-based device concepts.
We have theoretically studied exciton states and photoluminescence spectra of strained wurtzite AlGaN/GaN quantum-well heterostructures. The electron and hole energy spectra are obtained by numerically solving the Schrodinger equation, both for a single-band Hamiltonian and for a non-symmetrical 6-band Hamiltonian. The deformation potential and spin-orbit interaction are taken into account. For increasing built-in field, generated by the piezoelectric polarization and by the spontaneous polarization, the energy of size quantization rises and the number of size quantized electron and hole levels in a quantum well decreases. The exciton energy spectrum is obtained using electron and hole wave functions and two-dimensional Coulomb wave functions as a basis. We have calculated the exciton oscillator strengths and identified the exciton states active in optical absorption. For different values of the Al content x, a quantitative interpretation, in a good agreement with experiment, is provided for (i) the red shift of the zero-phonon photoluminescence peaks for increasing the quantum-well width, (ii) the relative intensities of the zero-phonon and one-phonon photoluminescence peaks, found within the non-adiabatic approach, and (iii) the values of the photoluminescence decay time as a function of the quantum-well width.
Using high magnetic fields up to 60 T, we report magneto-transport and photoluminescence (PL) studies of a two-dimensional electron gas (2DEG) in a GaN/AlGaN heterojunction grown by molecular-beam epitaxy. Transport measurements demonstrate that the quantum limit can be exceeded (Landau level filling factor $ u < 1$), and show evidence for the $ u =2/3$ fractional quantum Hall state. Simultaneous optical and transport measurements reveal synchronous quantum oscillations of both the PL intensity and longitudinal resistivity in the integer quantum Hall regime. PL spectra directly reveal the dispersion of occupied Landau levels in the 2DEG and therefore the electron mass. These results demonstrate the utility of high (pulsed) magnetic fields for detailed measurements of quantum phenomena in high-density 2DEGs.
Temperature dependence of intersubband transitions in AlN/GaN multiple quantum wells grown with molecular beam epitaxy is investigated both by absorption studies at different temperatures and modeling of conduction-band electrons. For the absorption study, the sample is heated in increments up to $400^circ$C. The self-consistent Schrodinger-Poisson modeling includes temperature effects of the band-gap and the influence of thermal expansion on the piezoelectric field. We find that the intersubband absorption energy decreases only by $sim 6$ meV at $400^circ$C relative to its room temperature value.
The science and applications of electronics and optoelectronics have been driven for decades by progress in growth of semiconducting heterostructures. Many applications in the infrared and terahertz frequency range exploit transitions between quantized states in semiconductor quantum wells (intersubband transitions). However, current quantum well devices are limited in functionality and versatility by diffusive interfaces and the requirement of lattice-matched growth conditions. Here, we introduce the concept of intersubband transitions in van der Waals quantum wells and report their first experimental observation. Van der Waals quantum wells are naturally formed by two-dimensional (2D) materials and hold unexplored potential to overcome the aforementioned limitations: They form atomically sharp interfaces and can easily be combined into heterostructures without lattice-matching restrictions. We employ near-field local probing to spectrally resolve and electrostatically control the intersubband absorption with unprecedented nanometer-scale spatial resolution. This work enables exploiting intersubband transitions with unmatched design freedom and individual electronic and optical control suitable for photodetectors, LEDs and lasers.
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