The efficient room-temperature photoluminescence bands of wurtzite GaN, which are peaked in the red (1.8 eV), the yellow (2.2 eV), and the blue (2.8 eV) spectral range, have been studied as a function of doping (species and concentration) and excitation power density (PD). It is shown that the yellow and the blue band are induced by Si and Mg doping, respectively, while codoping with Si and Mg generates the red band. At high-doping levels, the yellow and the blue band reveal strong peak shifts to higher energy with increasing PD providing very strong evidence for their distant donor-acceptor (DA) pair recombination character. The deep centers involved in DA recombination having electrical activity opposite to that of the shallow level of the dopant, are suggested to arise from self-compensation and to be vacancy-dopant associates. Self-compensation is found to be weak in the case of Si doping, but significant for Mg doping. A recombination model is presented, which accounts for the ess ential properties of all three bands in deliberately doped GaN. These results also suggest that the yellow and the blue bands in nominally undoped GaN arise from distant DA pairs involving residual Si and Mg impurities, respectively, as well as their respective vacancy associates

Abstract-This paper gives an overview on the design, fabrication, and characterization of quantum cascade detectors. They are tailorable infrared photodetectors based on intersubband transitions in semiconductor quantum wells that do not require an external bias voltage due to their asymmetric conduction band profile. They thus profit from favorable noise behavior, reduced thermal load, and simpler readout circuits. This was demonstrated at wavelengths from the near infrared at 2 m to THz radiation at 87 m using different semiconductor material systems. In the NIR, fast intraband semiconductor photodetectors are only available for wavelengths up to about 1.6 m. On the other tail of optical frequencies, namely for detection of THz radiation, bolometers are widely used; however, they are not well suited for high-speed applications. For fast light detection at wavelengths above 1.6 m, ISB photodetectors are very promising candidates. As unipolar devices, their fundamental F. R. Giorgetta was with the University of Neuchatel, 2000 Neuchatel, Switzerland. He is now with the National

Alloys of scandium with AlN exhibit an enhanced piezoelectric coefficient that can boost the performance of nitride‐based electronic and optoelectronic devices such as high electron mobility transistors (HEMTs). Consequently, there is increasing interest in the epitaxial growth of high‐quality AlScN/GaN heterostructures. So far, only very recent reports on AlScN HEMT structures grown by molecular beam epitaxy (MBE) have been published. Herein, the motivation for depositing AlScN epitaxial layers by metal‐organic chemical vapor deposition (MOCVD) as well as the challenges associated with this approach are explained. For the first time, the successful deposition of epitaxial layers with a Sc content up to 30% (Al0.7Sc0.3N) is reported. It is shown that the deposited films consist of wurtzite‐type AlScN with high crystalline quality, demonstrating that MOCVD is suitable for the growth of HEMT structures with Sc‐based ternary nitrides.

A midinfrared quantum cascade detector with a spectrally broad ͑⌬E / E = 27.3% ͒ response is designed, fabricated, and tested. This detector consists of 26 differently designed active region stages in order to cover a wavelength region from 4.7 to 7.4 m. The device could be operated above room temperature and showed peak responsivities of 13 mA/ W at 10 K and 1.25 mA/ W at room temperature. A background limited detectivity of 1.55ϫ 10 10 Jones was seen up to a temperature T BLIP of 110 K.During the past couple of years, quantum cascade detectors ͑QCDs͒ have experienced a rapid development. Prototype QCDs have been demonstrated in several material systems such as GaAs/ AlGaAs, 1,2 InGaAs/ InAlAs, 3,4 and InGaAs/ AlAsSb, 5 and they cover nowadays a large range of wavelengths spanning from the far infrared ͑84 m͒ ͑Ref. 6͒ down to the near infrared at 2.1 m.5 High frequency operation of QCDs up to 23 GHz has been demonstrated in an optical heterodyne experiment using quantum cascade laser sources.3 Most recent QCDs reached also very decent performance levels, especially in terms of detectivity. 7,8 Most of this promising progress in performance seems to be the result of the optical bound-to-bound transition, which is at the origin of the detection mechanism: it leads to a very narrow spectral linewidth and thus to a substantial reduction of the blackbody background noise seen by the device. In addition, the absence of dark current and thus dark current noise has a positive effect on the noise behavior of these devices. Unfortunately, it was up to now not clear whether a device with a spectrally broad response would-at least partiallypreserve the good noise properties of the standard narrow linewidth QCD. Furthermore, it was speculated that monochromatic illumination of a spectrally broad QCD could lead to poor performance, especially at low operating temperatures. In this article, we therefore present a midinfrared QCD with a relative linewidth of 27.3%, which makes use of 26 carefully designed active region stages spanning a wavelength range between 4.7 and 7.4 m. Despite the large detection spectrum, the noise properties of this device were not too adversely affected: we achieved a background limited detectivity of 1.55ϫ 10 10 Jones up to a temperature T BLIP of 110 K.Fabrication of this device relied on molecular beam epitaxy on an Fe-doped InP substrate. The layer structure is based on 26 stages all having a similar architecture. The main well of each stage is n-doped and contains a ground state plus an excited state. The latter is in exact resonance with the uppermost state of an extraction cascade consisting itself of a series of quantum wells. These wells have thicknesses that result in a series of quantized states being separated by roughly one LO-phonon energy. The arrangement of the stages, each of which designed to detect at a different wavelength, was chosen in such a way as to guarantee a flow of electrons from higher lying toward lower lying ground states: the stage with the highest transition energy ...

The semiconductor heterostructure design and lasing characteristics of an optically in-well pumped (AlGaIn)(AsSb)-based vertical-external-cavity surface-emitting laser (VECSEL) emitting at 2.35μm are presented. The pump absorption in the active quantum wells at 1.96μm has been enhanced by a higher-order microcavity resonance. VECSEL operation in-well pumped by a thulium-doped fiber laser has been demonstrated. Compared to a VECSEL barrier pumped at 1μm, the in-well pumped device reaches a significantly higher power efficiency, and thus a higher output power at a given pump power, due to the smaller quantum deficit and hence reduced internal heat generation. Using an intracavity SiC heat spreader, a cw output power in excess of 3W has been achieved at a heat sink temperature of −15°C, and still more than 2W at +15°C.