Tunneling effects and intersubband absorption in AlN/GaN superlattices

Abstract

We report on intersubband absorption and photovoltage measurements on regular GaN/AlN-based superlattice structures. For barrier thicknesses larger than about 25 Å, the optical intersubband absorption peaks at a considerably smaller energy than the photovoltage spectrum. A simple model taking into account the oscillator strength of the involved transitions and the corresponding tunneling probabilities agrees with the experimental findings. According to this model, the observed photovoltage is the macroscopic manifestation that the two-dimensional electron gas at the top of the superlattice changes its carrier density by a vertical transport of electrons.Thanks to their large band gap energies, the semiconductor materials GaN and AlN have attracted a lot of attention for the fabrication of long-lived visible lasers and lightemitting diodes. 1-3 More recently, the huge conduction band discontinuity of nearly 2 eV between these two semiconductors has resulted in some device proposals based on intersubband transitions. [4][5][6] Particularly interesting in this context are photodetectors, modulators, or lasers in the technologically interesting 1.55 m wavelength range. Such devices could profit from short intersubband lifetimes, which might eventually result in high operating frequencies. 7,8 Unfortunately, the heavy effective masses of 0.2 m e for GaN and 0.32 m e for AlN impose the epitaxial growth of layer thicknesses in the 15-30 Å range, which, despite substantial progress in molecular beam epitaxy, is still quite a challenging task. 9

Intersubband Transition-Based Processes and Devices in AlN/GaN-Based Heterostructures

“…13. In this experiment, three different samples with 3 QW thicknesses were grown; this resulted in a series of photodetectors being sensitive between 2.33 and 1.49 "m [47]. As mentioned in the Introduction, it was only in 2007 when the functioning of these devices could be explained in a satisfactory way.…”

Coherent vertical electron transport and interface roughness effects in AlGaN/GaN intersubband devices

“…The existence of defects such as charge traps and screw dislocations has led to the need for systematic verification of the origin of negative differential resistance (NDR) features. [13][14][15][16][17] Another important characteristic of AlGaN/GaN heterostructures is the large built-in electrostatic fields due to both spontaneous and piezoelectric polarization which alter the current-voltage (I-V) characteristics significantly. Recent advances in growth technology have reduced threading dislocation densities substantially to allow repeatable measurement of wurtzite and cubic AlGaN RTDs [18][19][20][21][22][23] and sequential tunneling devices.…”

Tunneling effects in short period strained AlN/GaN superlattices

“…As pointed out by Baumann et al, [2] in the majority of the experiments, the effects were observed purely optically [3] and no vertical current transport was demonstrated. Just a few studies to our knowledge were reported on vertical current transport through thin AlN barriers in a resonant tunneling diodes [4,5].…”