Temperature stability of intersubband transitions in AlN/GaN quantum wells

Abstract

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 • C. The self-consistent Schrödinger-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 ∼ 6 meV at 400 • C relative to its room temperature value.Intersubband transitions in nitride-based semiconductor heterostructures hold great promise for optoelectronic devices. The high band-offset between gallium nitride (GaN) and aluminum nitride (AlN) enables intersubband transitions in the near-infrared regime (λ = 1-4 µm). Thus, intersubband devices such as modulators, detectors, and quantum cascade lasers (QCLs) have the potential to operate at wavelengths useful for fiberoptic communication. QCLs also have the potential to be used when measuring characteristic absorption of small molecules. There are several studies of intersubband absorption in AlN/GaN heterostructures.[1-6] These typically show a peak at 500 to 900 meV with full width half maximum (FWHM) in the range of 60 to 200 meV. An intersubband device should operate at and above 300 K, often with the condition of a negligible change in transition energy. Temperature dependence of the transition energy, up to room temperature, has been reported for intersubband absorption in InAs/AlSb multiple quantum well (MQW) structures [7] and for interband photoluminescence in nitride-based AlN/GaN MQW structures.[8] The huge piezoelectric fields introduce an additional temperature-dependent effect in the nitride MQW structures, because the different thermal expansions of AlN and GaN induce a temperature dependent strain in the structures.In this letter, we demonstrate that the peak position of intersubband transitions red shifts only ∼ 6 meV as the temperature of the sample is increased from 25 • to 400 • C. This aspect of thermal robustness could be vital for the operation of nitride-based QCLs since their ultrafast LO-scattering rates and high intersubband energy would cause them to heat significantly. The temperature effects are studied for MQW structures both experimentally by measuring the absorption at different temperatures, and theoretically by a self-consistent solution to the Schrödinger-Poisson equations for the conductionband electrons. The nitride-semiconductor heterostructures are characterized by huge internal electric fields, which arise from the exceptionally large spontaneous and piezoelectric fields. A proper account of these fields are crucial for an accurate description and characterization of the quantum well states, and they require that the complete structure is considered in the design of heterostructures.The set of MQW structures (A -C) are grown in a Varian Gen II modular molecular beam epitaxy (MBE) system....

Polarization-balanced design of heterostructures: Application to AlN/GaN double-barrier structures

“…2,15,16,28,29 We here observe that these have a huge impact on the function and transport properties of AlN/GaN heterostructures. By calculating the conduction-band profile of a double barrier we, first, illustrate the formation of such a depletion barrier.…”

“…This demand arises because the large band offset and heavy conduction electron mass ∼(0.2-0.3) m 0 weaken the coupling between states in adjacent quantum wells. The wide depletion regions and two-dimensional electron gases (2DEGs) forming at the two sides of an active region 2,4,[15][16][17] exacerbates this challenge for devices based on perpendicular quantum transport. For instance, in resonant-tunneling diodes and QC lasers, the depletion regions make it difficult to simultaneously obtain efficient current injection and control over the alignment of levels and transitions in the structure.…”

“…26 Our calculations of conduction-band profile and quantum states presented here are based on the Schrödinger-Poisson (SP) equation 16,27 in the effective-mass, envelope-function approximation, including nonparabolicity and exchange-correlation effects as described in Ref. 16. This paper is presented as follows.…”

“…14,[18][19][20][21][22][23] A related issue exists for structures operating at zero bias (like absorption structures), especially those of only a few layers: As electrons accumulate in the 2DEG, the active region is depleted. 2,4,16,17 To tackle some of the design problems, we here introduce the two related concepts of polarization balance and polarization-balanced design. A structure is polarization balanced when the unscreened field vanishes in the cladding layers (buffer and cap) layers.…”

“…Sixth, since, in polar structures, current-driven devices such as quantum-cascade lasers can operate at the bias giving polarization balance with ensuing flat bands in the injection region, good injection can be ensured without the need of tailoring graded digital alloys in the transport region. 26 Our calculations of conduction-band profile and quantum states presented here are based on the Schrödinger-Poisson (SP) equation 16,27 in the effective-mass, envelope-function approximation, including nonparabolicity and exchange-correlation effects as described in Ref. 16.…”