Continuous wave operation of quantum cascade lasers is reported up to a temperature of 312 kelvin. The devices were fabricated as buried heterostructure lasers with high-reflection coatings on both laser facets, resulting in continuous wave operation with optical output power ranging from 17 milliwatts at 292 kelvin to 3 milliwatts at 312 kelvin, at an emission wavelength of 9.1 micrometers. The results demonstrate the potential of quantum cascade lasers as continuous wave mid-infrared light sources for high-resolution spectroscopy, chemical sensing applications, and free-space optical communication systems.The mid-infrared portion of the spectrum, covering approximately the wavelength range from 3 to 12 m, is sometimes referred to as "underdeveloped" because of its lack of convenient coherent optical sources. Especially when compared to the visible or near-infrared spectral range, where interband semiconductor lasers are now produced very economically with continuous wave (CW) output power of tens of milliwatts, this assertion holds true. In the mid-infrared, a new class of semiconductor lasers-intersubband quantum cascade (QC) lasers (1)-has become a promising alternative to interband diode lasers (2, 3) in the past 7 years. In these devices, photon emission is obtained by electrons making optical transitions between confined energy lev-

We have studied the effect of growth and design parameters on the performance of Si-doped GaN/AlN multiquantum-well ͑MQW͒ structures for intersubband optoelectronics in the near infrared. The samples under study display infrared absorption in the 1.3-1.9 m wavelength range, originating from the photoexcitation of electrons from the first to the second electronic level in the QWs. A commonly observed feature is the presence of multiple peaks in both intersubband absorption and interband emission spectra, which are attributed to monolayer thickness fluctuations in the quantum wells. These thickness fluctuations are induced by dislocations and eventually by cracks or metal accumulation during growth. The best optical performance is attained in samples synthesized with a moderate Ga excess during the growth of both the GaN QWs and the AlN barriers without growth interruptions. The optical properties are degraded at high growth temperatures ͑Ͼ720°C͒ due to the thermal activation of the AlN etching of GaN. From the point of view of strain, GaN/AlN MQWs evolve rapidly to an equilibrium average lattice parameter, which is independent of the substrate. As a result, we do not observe any significant effect of the underlayers on the optical performance of the MQW structure. The average lattice parameter is different from the expected value from elastic energy minimization, which points out the presence of periodic misfit dislocations in the structure. The structural quality of the samples is independent of Si doping up to 10 20 cm −3 . By contrast, the intersubband absorption spectrum broadens and blueshifts with doping as a result of electron-electron interactions. This behavior is independent of the Si doping location in the structure, either in the QWs or in the barriers. It is found that the magnitude of the intersubband absorption is not directly determined by the Si concentration in the wells. Instead, depending on the Al mole fraction of the cap layer, the internal electric field due to piezoelectric and spontaneous polarization can deplete or induce charge accumulation in the QWs. In fact, this polarization-induced doping can result in a significant and even dominant contribution to the infrared absorption in GaN/AlN MQW structures.

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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

We demonstrated a GaAs/AlGaAs-based far-infrared quantum well infrared photodetector at a wavelength of ϭ84 m. The relevant intersubband transition is slightly diagonal with a dipole matrix element of 3.0 nm. At 10 K, a responsivity of 8.6 mA/W and a detectivity of 5ϫ10 7 cm ͱHz/W have been achieved; and successful detection up to a device temperature of 50 K has been observed. Being designed for zero bias operation, this device profits from a relatively low dark current and a good noise behavior.In recent years, there has been an increasing interest in the fabrication of so-called terahertz ͑THz͒ emitters and detectors. While electronic devices like Gunn diodes or Schottky diode frequency multipliers try to reach this range from the low frequency end, optical devices like gas or semiconductor lasers are quickly moving into the THz range from the high frequency side. [1][2][3] Since the THz region is traditionally defined as 0.1-3 THz, the currently available quantum cascade laser ͑QCL͒ sources with ϭ87 m can already be regarded as THz sources. At low temperatures, they emit several milliwatts of continuous wave output power. On the electronics side, Gunn diodes and frequency mixers have also achieved several milliwatts of radiated power at frequencies on the order of hundreds of gigahertz. 4 Once the entire THz frequency range is fully accessible by convenient radiation sources, it is obvious that the next important step towards applications is the development of suitable detectors. Like any other type of electromagnetic radiation, THz waves or pulses can be detected by coherent or incoherent means. Most coherent detection schemes utilize frequency conversion, whereas incoherent methods are based on the heat production of absorbed radiation. Typical examples of heat detectors include Si-bolometers or pyroelectric crystals like deuterated triglycerine-sulfate ͑DTGS͒; on the other hand, Schottky diode mixers, 5 nonlinear optical crystals like ͕110͖ ZnTe, 6 and gated photoconductive antennas are typical coherent detection schemes. 7 As a further incoherent solution, semiconductor-based quantum-type approaches like biased superlattices have attracted some attention.8 Bolometers are in general highly sensitive, but like all heat-based detection schemes, they are intrinsically slow and built for very low temperature operation only.9 DTGS detectors and pyroelectric crystals offer the advantage of faster detection at the prize of reduced sensitivity. Finally, extrinsic photoconductors such as doped Ge detectors are fast and sensitive, but they must be cooled to 4 K. Quite generally, coherent techniques profit from a good sensitivity, but they are experimentally more sophisticated than incoherent ones.10 Although semiconductor quantum devices might not be highly sensitive, their potential for mass fabrication and integration by means of semiconductor device technology is very appealing. This has been proven by different types of quantum well infrared photodetectors ͑QWIPs͒ which work in a variety of different wavelength...