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We report on the observation of intersubband absorption in GaN/AlN quantum well superlattices grown on [formula]-oriented GaN. The absorption is tuned in the 1.5-4.5 μm wavelength range by adjusting the well thickness. The semipolar samples are compared with polar samples with identical well thickness grown during the same run. The intersubband absorption of semipolar samples shows a significant red shift with respect to the polar ones due to the reduction of the internal electric field in the quantum wells. The experimental results are compared with simulations and confirm the reduction of the polarization discontinuity along the growth axis in the semipolar case. The absorption spectral shape depends on the sample growth direction: for polar quantum wells the intersubband spectrum is a sum of Lorentzian resonances, whereas a Gaussian shape is observed in the semipolar case. This dissimilarity is explained by different carrier localization in these two cases. [ABSTRACT FROM AUTHOR]

In this work we theoretically study the vertical transport in GaN/AlGaN resonant tunneling diodes in the ballistic regime. Heterostructures based on III-nitride compounds are characterized by a large conduction band discontinuity and a presence of an internal electric field, both of which have important effects on the electronic transport. Using the transfer matrix formalism, we investigate the effect of the energetic barrier height on the resonant current. Our calculations show an increase in the peak to valley ratio (PVR) with increasing Al content of the barriers which arises from the large decrease in the valley current. Furthermore, we show that the current resonances are different for positive and negative applied voltages. We also demonstrate that, due to the asymmetry of the conduction band profile, only one current direction leads to a significant PVR. Finally, we present an approach to achieve large PVR in both current directions by reducing the asymmetry induced by the internal field. [ABSTRACT FROM AUTHOR]

We have demonstrated a GaN/AlGaN quantum cascade detector based on a simplified design of the extractor region relying on an AlGaN thick layer. The device grown by molecular beam epitaxy exhibits both TM-polarized intersubband absorption and photocurrent at room temperature at a peak wavelength of 1.87 μm. Based on the measured absorption and responsivity, we estimate the transfer efficiency of photoelectrons to the next period to be around 62%. This simplified design is robust against thickness fluctuations in the extractor region and offers prospects for ultrafast detectors. [ABSTRACT FROM AUTHOR]

This paper reviews recent progress toward intersubband (ISB) devices based on III-nitride quantum wells (QWs). First, we discuss the specific features of ISB active region design using GaN/AlGaN materials, and show that the ISB wavelength can be tailored in a wide spectral range from near- to long infrared wavelengths by engineering the internal electric field and layer thicknesses. We then describe recent results for electro-optical waveguide modulator devices exhibiting a modulation depth as large as 14 dB at telecommunication wavelengths. Finally, we address a new concept of III-nitride QW detectors based on the quantum cascade scheme, and show that these photodetectors offer the prospect of high-speed devices at telecommunication wavelengths. [ABSTRACT FROM AUTHOR]

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The effect of the temperature on the electrical characteristics in GaN-based resonant tunneling diodes is studied both theoretically and experimentally. At room temperature, the current-voltage measurements show reproducible negative differential resistances and a current hysteresis. However these features disappear when the temperature is decreased down to 100 K. In addition, the current exhibits transients over a few tenths of seconds which effect disappears at low temperatures. Based on these results, we conclude that the observed negative differential resistance at room temperature is not due to electron resonant tunneling but to trap charging and release. [ABSTRACT FROM AUTHOR]

Room temperature intersubband electroabsorption modulation in GaN/AlN coupled quantum wells is demonstrated at short infrared wavelengths, covering the fiber-optics telecommunication wavelength range. Electroabsorption modulation with opposite sign is observed at λ=1.2–1.67 μm and λ=2.1–2.4 μm. The electromodulation originates from electron tunneling between a wide well (reservoir) and a narrow well separated by an ultrathin AlN barrier. Both the intersubband absorption and the modulation spectroscopic measurements are in good agreement with the simulations. The maximum modulation depth is ∼44% at λ=2.2 μm. The -3 dB cutoff frequency limited by the RC time constant is 11.5 MHz for 700×700 μm2 mesas. [ABSTRACT FROM AUTHOR]

We report experimental and theoretical results on interband and intersubband transitions in GaN quantum wells with strained AlN barriers. All of the samples are grown by molecular-beam epitaxy on sapphire (0001) substrates. The results show that even at room temperature, strong electron localization occurs in the plane of the quantum wells due to the combined effect of monolayer thickness fluctuations and the high internal field in the GaN layers. We also demonstrate that the intersubband absorption is systematically blueshifted in n-doped quantum wells with respect to nominally undoped samples as a result of strong many-body effects, namely the exchange interaction. The results for both undoped and doped quantum wells are in good agreement with simulations. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The intersubband absorption of cubic GaN/Al(Ga)N quantum wells is studied experimentally and theoretically over a wide spectral range. By changing the quantum well thickness it is possible to tune the intersubband absorption peak wavelength from 1.4 μm (214 THz) to 63 μm (4.76 THz). Comparing the experimental results with simulations based on the effective-mass model we demonstrate that the GaN/AlN conduction-band offset is higher than 1.2 eV. The best fit with the experimental data is achieved for a conduction-band offset of 1.4 eV and for a GaN effective mass of 0.11m0. [ABSTRACT FROM AUTHOR]