AlN/GaN-superlattice structures for the fabrication of intersubband detectors in the telecom wavelength range

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1 Invited Paper AlN/GaN-superlattice structures for the fabrication of intersubband detectors in the telecom wavelength range Daniel Hofstetter a*, Esther Baumann a, Fabrizio R. Giorgetta a, Manfred Maier b, Fabien Guillot c, Edith Bellet-Amalric c, and Eva Monroy c a Institute of Physics, University of Neuchatel, 1 A.-L. Breguet, CH 2000 Neuchatel b Fraunhofer Institut for Applied Solid State Physics, Tullastrasse 72, D - Freiburg i. Brsg., Germany c Equipe Mixte CEA-CNRS-UJF Nanophysique et Semiconducteurs, 17 rue des martyrs, CEA- Grenoble, Grenoble Cedex 9, France ABSTRACT We report on the fabrication and characterization of GaN/AlN based superlattice structures with intersubband transition wavelengths in the optical telecom range. The devices consist typically of 40 periods of 1.5 nm thick Si-doped GaN wells and up to 15 nm thick AlN barriers. The photovoltaic mode of operation has allowed us to test these detectors at room temperature and for frequencies ranging into the multiple GHz region. Substantial performance improvements are expected if proper high frequency mounting and processing techniques will be used in the future. Keywords: intersubband transitions, GaN/AlN superlattices, telecom wavelength, high-frequency 1. INTRODUCTION 1.1 General interest for III-nitrides In the last 15 years, research on the III-nitride material system has been very intense, and has led to the demonstration of room temperature violet-blue lasers, ultra-violet and visible light emitting diodes, and solar-blind photodetectors (1). The chemical inertness and mechanical robustness of AlN, GaN, and their alloys make them ideal materials for applications in everyday life; this is confirmed by the fact that all of the above devices are now commercially available (2). As a consequence of the large lattice mismatch between all III-nitride compounds, which renders high quality epitaxial growth difficult, another interesting aspect of these materials namely the research on intersubband (ISB) effects - has remained unexplored until The particular reason why research in this direction could potentially have a high practical payoff is the large conduction band discontinuity of nearly 2 ev between GaN and AlN, which enables the design of optical ISB transitions at near-infrared wavelengths. Gmachl et al. have first demonstrated ISB absorption down to 1.55 µm, the wavelength of low-loss optical fiber telecommunication (3), (4), (5). In 2003, the first nitride-based ISB detector working close to this technologically important wavelength was demonstrated (6). However, it took another 4 years until the difficulties of the epitaxial growth were sufficiently well mastered to allow room temperature and high frequency operation of such a device (7). Despite a substantial effort by several research groups, the exact working principle of these GaN-based ISB detectors could not immediately be clarified (8), (9). We therefore conducted recently a series of experiments aiming at a better understanding of ISB transitions in heavily Si-doped GaN/AlN superlattices. As a main result of this research, we found that the mechanism of such nitride ISB detectors is based on an optically non-linear effect just as described in the following paragraph. In contrast to other semiconductor solutions, however, GaN/AlN quantum wells offer optical non-linearities in a natural way via their strong internal polarization fields (10). These fields occur at each interface and lead to a pronounced asymmetry in the electronic potential of the quantum well (see figure 1C). * Daniel.Hofstetter@unine.ch; phone ; fax ; Gallium Nitride Materials and Devices III, edited by Hadis Morkoç, Cole W. Litton, Jen-Inn Chyi, Yasushi Nanishi, Euijoon Yoon, Proc. of SPIE Vol. 6894, 68940S, (2008) X/08/$18 doi: / Proc. of SPIE Vol S-1

2 1.2 Optical non-linearities in III-nitrides Optically non-linear materials have a long and successful history. However, in the early days of non-linear optics, only natural materials like KH 2 PO 4, LiNbO 3 or KPO 3, which offer an asymmetric electron binding potential, could be investigated. The principal driving force for this mainly applied research was the extension of the limited wavelength range of the lasers available at the time (11). Roughly 20 years ago, Rosencher et al. developed novel types of optically non-linear materials which were based on semiconductor quantum structures; and which revealed giant non-linear effects (12). All of these structures involved ISB transitions in the conduction band of carefully designed asymmetric quantum wells (13). To produce the required asymmetry of the electronic potential, they used either strongly coupled double quantum wells (figure 1A) or step-like quantum wells (figure 1B) (14). Owing to the semiconductor materials available in those days, the transition energy of the involved ISB transitions was limited to energies of mev, and therefore all experiments had to be performed in the mid-infrared; typically at the wavelength of high-power CO 2 lasers. A B E3 - E,. E, = AE3 = AE3 C AE1 = Fig. 1. Schematic conduction band diagram of step-like (A), coupled (B), and asymmetric GaN-based (C) quantum wells. In all structures, the so-called double resonance between E 1, E 2, and E 3 is drawn. With the use of III-nitride semiconductors, these limitations are no longer present: the accessible energy range is extended towards 1 ev, the wavelengths can be as short as 1.3 µm, and all experiments can now be made with low power light sources and at room temperature. In order to have a correct simulation of the involved energy levels at hand, the band structure of the superlattice samples used in this study was calculated using a self-consistent Schrodinger-Poisson equation solver. As figure 1C shows, an ISB transition between the ground state E 1 and the first excited state E 2 in such a quantum well is slightly diagonal and fulfills thus one of the main requirements for the existence of a specific nonlinear process called optical rectification. Since the excitation of an electron into the upper quantized level is accompanied by a small displacement in growth direction, an electrical dipole moment is created. For a high electron density and many quantum wells, these microscopic dipole moments add up to a macroscopic polarization of the crystal, which can be detected as an external photovoltage. By applying an AC electric field, i.e. illumination with electromagnetic radiation, one can thus produce a DC electric field. Due to this kind of optical diode behavior, the described mechanism is also known as optical rectification. Since for a certain well thickness, the energy of the optical transition energy between E 1 and E 2 happens to be exactly the same as between E 2 and E 3, we have in addition a situation referred to as double-resonance. In this case, second harmonic generation and other related two photon processes are possible. In a semiconductor system like GaAs/AlGaAs, the energy levels would not automatically have the same separation unless a more complicated design like a step quantum well is used. As the band structure simulation of a generic GaN-based quantum well shows, double resonance occurs again almost naturally because of the internal polarization induced widening of the uppermost region in each quantum well. As already mentioned above, these nonlinear effects are well visible at room temperature; this facilitates the experimental work considerably. Since the IIInitride material system is, in addition, very robust, it offers nearly ideal starting conditions as an optically non-linear material. Proc. of SPIE Vol S-2

3 2. FABRICATION In order to illustrate the excellent material quality achieved with plasma enhanced molecular beam epitaxy (PAMBE), we show here several examples of material characterization. Typical detector active regions consist of 40 periods of 1.5 nm thick Si-doped GaN quantum wells and up to 15 nm thick AlN barriers, grown by PAMBE on 1-µm-thick AlNon-sapphire templates. During growth, active nitrogen is provided by a radio-frequency plasma cell, and standard effusion cells are used for Ga, Al, Si, and In. The doping level in the GaN quantum wells is usually on the order of cm -3. In earlier experiments, we first grew a 500 nm thick Si-doped Al 0.5 Ga 0.5 N buffer layer followed by the superlattice active region. On top of the active region, we deposited a Si-doped Al 0.5 Ga 0.5 N cap layer with a thickness of 180 nm. The choice of Al 0.5 Ga 0.5 N:Si as buffer and cap layer aimed to fabricate strain-compensated devices, i.e. devices whose buffer layer is lattice matched to the average a lattice parameter of the active region. This design is advisable in order to reduce the defects along the active region, and to prevent a gradient in the internal electric field due to spontaneous and piezoelectric polarization. However, the structural quality of AlGaN alloys is often one of the limitations of the performance of nitride-based devices. For this reason, our more recent detectors are grown directly on top of the AlN buffer, use therefore a strained superlattice active region, and are covered with a strained 50 nm AlN cap. This technique has the positive effect that only binary compounds must be used and that the relatively large surface roughness of AlGaN can be avoided. Fig. 2. AFM surface scans of the SL sample E740 which was grown on a 50 % AlGaN buffer. The rms roughness on the 5 5 µm 2 shown is 1.7 nm. In the zoomed image in Fig. 2, we observe the atomic-step terraces typical for the growth of GaN, indicating a short-range roughness at the monolayer scale. Nevertheless, we have demonstrated in a previous work the capability of In as a surfactant for AlGaN growth, delimiting the range of substrate temperatures and In fluxes at which an In adlayer is dynamically-stable on Al x Ga 1-x N (0001) (15). In the present work, we have applied this growth procedure to QWIP structures. The surface morphology of the samples was analyzed by atomic force microscopy (AFM) in the tapping mode, using a Dimension 3100 system. Figure 2 presents an AFM scan of such a sample: on an area of 5 5 µm 2, it shows an rms surface roughness of about 1.7 nm. In the zoomed image in Fig. 2, we observe the atomic-step terraces typical for the growth of GaN; they indicate a shortrange roughness at the monolayer scale. Proc. of SPIE Vol S-3

4 AIG N I I Ci SB SB Diffraoiion ngie ] I 5 ID Th 20 Etch depth lom] U. Fig. 3. On the left, high-resolution X-ray diffraction θ 2θ scans of the (0002) reflection of samples E728, E739 and E740. SL satellite peaks up to the second order are clearly visible, whereas the SL zero-order reflection peak (SL 0 ) overlaps the Al 0.5 Ga 0.5 N reflection. An average period of 2.97±0.02 nm is obtained from the inter-satellite distance. The figure on the right side shows SIMS measurements on sample E740. Shown are Al, Ga, and N signals as a function of etch time. The periodicity due to the SL is clearly visible. The structural quality of the SLs has been further assessed by high-resolution x-ray diffraction (HRXRD) measurements. HRXRD (omega-2-theta) scans of the (0002) x-ray reflection of three different wafers are shown in Fig. 3 (left). As a first fact, they reveal excellent reproducibility of the growth process. Secondly, the zero-order SL peak sits right on top of the 50 % AlGaN peak, as an indication of strain compensation. And thirdly, despite of the extremely small SL period, we observe SL satellite peaks up to the second order, confirming an average period of 2.97±0.02 nm. As a last indication for the excellent material quality, we show in Fig. 3 (right) secondary ion mass spectroscopy (SIMS) measurements of one of the samples. The profiles were measured by using a Cs + primary ion beam and under a very shallow incidence angle (67.5 ). One can clearly see the periodicity of the secondary AlCs + signal, which correlates well with the secondary signal from GaCs + ions. 3. CHARACTERIZATION 3.1 ISB absorption for different samples The most basic step towards a correct characterization of ISB transitions is always an optical absorption measurement. Due to quantum mechanical polarization selection rule, only TM polarized light should be absorbed. For the direct absorption experiment, we polished sample E728 in a standard multipass geometry with a mirror-like back and two parallel 45 wedges. Absorption was then measured by passing white light from a Fourier transform infrared spectrometer through the sample. Measuring first TM- and then TE-polarized sample transmission and background spectra allowed us to normalize the signal to absolute absorbance units. The result of this procedure is shown in figure 4 (hanging from the top axis). Here, we investigate first the samples which have been previously characterized in terms of their structural quality (see figure 2 and 3). In both direct multipass absorption and electro-modulated absorption experiments, the absorbance peaks at 7280 cm -1 (910 mev, 1.4 µm), as shown in figure 4. The full width at half maximum of the direct absorption is 880 cm -1 (110 mev), whereas the electro-modulated absorption, which is suitable for lower doped samples, has a slightly smaller width of 780 cm -1 (98 mev). The relative linewidth is thus on the order of 11 %, which is an excellent value. Bandstructure simulations reveal that interface roughness on the monolayer scale results in a ±10 % change of the ISB absorption energy. We thus conclude that the interfaces, even for growth on AlGaN buffers, are atomically flat. Proc. of SPIE Vol S-4

5 , I 1.2 I 0.25 ' am E728 transte EflS trans. TM - E740 abs. TM -010 H,,. 0.0 I '/L'luI l Photon energy (on, Fig. 4. Absorption spectra for two of the three different samples whose x-ray diffraction scans were shown in the previous figure. The absorption peaks agree nicely although they were determined with different techniques (electro-modulated absorption for E740 and multipass absorption for E728) In order to learn more about the dependence of the ISB absorption on well and barrier thickness, a series of samples with different barrier but constant well thickness (t barrier = 0.75 nm, 1.5 nm, 3 nm, 7 nm, and 15 nm, t well = 1.5 nm) and varying well but constant barrier thickness (t barrier = 5 nm, d well = 1.25 nm, 1.7 nm, 2.2 nm, and 3.8 nm) has been investigated. As in the previous absorption experiments, ISB absorption spectra of these samples were measured under illumination with TM polarized light and in 45 multipass waveguide geometry, while corresponding optical response spectra were obtained by illumination through a single 45 facet. In the latter experiments, two Ti/Au contacts were evaporated on the sample surface. In order to see a signal, it was sufficient to illuminate one of the contacts while leaving the other dark. The resulting photovoltage was then amplified and fed into the external detector port of a Fourier transform infrared spectrometer. As figure 5 (top) shows, the detector performance improved when thicker barriers were used. In addition, the fact that an ISB detector with 15 nm thick and 1 ev high AlN barriers works, reveals that resonant tunnelling processes are detrimental rather than helpful for optimized detector performance. The results of absorption and photovoltage experiments for varying well thickness are presented in figure 5 (bottom); they clearly show that the ISB transition energy can be tailored by changing the thickness of the quantum well. For a 3.3 nm well, the main transition is at 4200 cm -1 ; with a 1.25 nm well, the transition energy of the fundamental transition increases to 7200 cm -1. All detectors work at slightly higher energy than the corresponding absorption peaks. This behavior is partly due to strain relaxation within the upper superlattice periods. Since the bare semiconductor surface of the absorption measurement and the metal-covered surface of the detector offer different boundary conditions for the internally reflected light, differently relaxed areas of the superlattice are probed by the two experiments. 3.2 Self-consistent band structure calculations By using a self-consistent Schrodinger-Poisson equation solver, we were able to compute the energy levels in such superlattices. The simulations are not only very useful for the prediction of the discrete energy levels in a single quantum well, but also for the general shape of the band structure. In particular, there will be always effects related to the relative strain between superlattice and its buffer or cap layer. In most general terms, one will observe two-dimensional carrier gases at the top and the bottom of the superlattice. Their size and polarity (electrons or holes) depends on the relative strain state of the different involved layers or layer stacks. Proc. of SPIE Vol S-5

6 a) Barrier thicknesses l5nm 7 nm 3nm 1.5 nm 0.75 nm Welithicknesses '' Photon energy [cm1] Fig. 5. Dependence of the detector signals as a function of barrier (top) and well thickness (bottom). Hanging from the upper x-axis in the lower figure, the absorption spectra of all samples are shown. In contrast to normal square-shaped quantum wells, not only the E 1 to E 2 transition is allowed in these asymmetric structures. For the thickest sample, we observed, for instance, not only one, but rather three optical transitions (E 1 E 4 at 8700 cm -1, E 1 E 3 at 6300 cm -1, and E 1 E 2 at 4200 cm -1 ). As will be shown below, this III-nitride specific feature allows the design of a doubly resonant structure with equal transition energies between E 1, E 2 and E 3. It turned out that the theoretical transition energies are correct to within 10 % of the experimental values. This discrepancy can be explained by the neglected depolarization shift and the insufficient knowledge of non-parabolicity effects. On the other hand, such thin quantum wells contain at the ground state level only 3-4 atomic layers. Therefore, one of the main assumptions of the envelope function approximation, namely that the envelope function varies slowly over one lattice unit cell, is no longer valid; this will result in further deviations in the simulation. As mentioned before, the changing amount of broadening present in these devices allows an estimation of the thickness uncertainty due to interfacial roughness; this procedure results in a value of about one monolayer on each interface. 3.3 Resonant optical rectification Using the optical rectification formalism as developed by Rosencher et al., we were able to compute the size of the maximal signal to be expected in our experimental configuration (16). For an input optical power of P W = 1 mw on a surface of 100 µm squared, a doping density of ρ 1 -ρ 2 = 1 x cm -3, an L = 1.5 nm thick quantum well, N = 40 active region periods, a dielectric constant of ε = 5, an electron displacement of δ 12 = <E 1 z E 1 >-<E 2 z E 2 > = 3 Å, and a dipole matrix element of z 12 = <E 1 z E 2 > = 2.5 Å, we get a result of 8 µv. This is in good agreement with the best experimental value of 10 µv. We have also investigated the temperature dependence of the optical rectification signal, and found that at present, the maximum performance of all samples lies around 200 K. Since the size of the signal is very sensitive on white light illumination, we concluded that both the quality and the thickness of the AlN barrier material are of crucial importance for the correct functioning: Even a small leakage current between adjacent superlattice periods will lead to an overall voltage loss and thus to performance degradation. χ (2) 0,max q = 3 2 ( ρ1 ρ 2 ) z12δ12 2 ε h 2Γω (1) V (2) χ 0,maxη 0P in NL = (2) ε stat z = 1/ / 2 δ = 1/ z /1 2 / / 2 (3a/b) 12 z 12 z Proc. of SPIE Vol S-6

7 In a more sophisticated experiment, we tested the speed of such a detector. For this purpose, a directly modulated diode laser beam at 1.55 µm was sent to the sample. The detector was held at room temperature to facilitate the measurement and mounted directly onto a BNC connector in order to minimize parasitic inductance effects from the bonding wires. The voltage response was amplified by two consecutive voltage amplifiers (Sonoma 317 and Miteq AFS-5) and measured in a spectrum analyzer (Agilent E4407B). As shown in figure 6, the highest frequency for which a signal was seen, was 2.37 GHz. In a more recent experiment, a frequency limit of 3 GHz was achieved. Since the mounting did not involve any optimization for high frequency testing, a substantial improvement is expected in future experiments of this type. Nevertheless, this preliminary result shows the high speed application potential of such GaN-based ISB detectors. 1oL,i#L!!I!!!!:!!epone_l Co 0) (I) Frequency [GHz] 30I io Modulation frequency [Hz] Fig. 6. Frequency response of one of the samples. The roll-off behavior was simulated with a low-pass filter characteristics having a 3 db frequency of 100 MHz. The inset shows the signal measured at the highest laser modulation frequency. 3.4 Two-photon absorption We finally investigated the non-linear behavior of such GaN/AlN superlattices in terms of a simple two-photon absorption/detection process. For this purpose, the sample with the doubly resonant energy levels was illuminated by a 1.55 µm singlemode laser diode. As seen above, electrons excited into level E 2 will lead to normal optical rectification. However, a small fraction of those excited electrons will undergo further excitation from level E 2 into level E 3, and can produce an additional photovoltaic signal at twice the frequency of the fundamental transition. While the size of the signal related to the normal E 1 E 2 process should grow linearly with incident intensity, the E 1 E 2 E 3 process should reveal a quadratic dependence on the input intensity. We first verified that the transition energy E 1 E 3 is indeed twice as large as the energy difference between E 1 and E 2. This was done first in an optical absorption experiment and second using a commercial 780 nm laser diode. Both measurements revealed the expected result: Taking into account the ratio of the dipole matrix elements, a signal of comparable size to the one at 1.55 µm could be detected. We then inspected the non-linear response of our material under illumination with the 1.55 µm laser diode and up to an input optical power of 50 mw, focused into a small spot of 10 µm diameter. Proc. of SPIE Vol S-7

8 Photon ononno Onni O.O6 JALO Photon enemy [cml Signal at Signal at Fig. 7. Spectra of the fundamental and frequency-doubled signals for a range of laser input powers (top). The figures below show the integrated signal strengths for the fundamental and the frequency-doubled signals using laser (bottom left) and white light illumination (bottom right). For the series of curves in figures 7 (top), the laser injection current was ramped in steps of 5 ma up to 100 ma. Plotted as a function of photon energy, the evolution of the two resulting peaks at 1.55 µm (6470 cm -1 ) and at 780 nm (12940 cm -1 ) is shown. Although the laser power increases linearly with injection current, the signal at the fundamental frequency grows sub-linearly with the incoming laser power. This observation is consistent with the excitation of electrons from the excited level E 2 into the even higher lying E 3 level, leading to saturation at high intensity. In figure 7 (bottom left), we therefore plot the signal at the double frequency as a function of the ground frequency signal, and find a quadratic dependence. A similar experiment was performed under illumination with a white light source (bottom right). In the latter case, one has to be somewhat careful because no optical filter was used to suppress the direct excitation from E 1 into E 3, but in this case as well, a parabolic curve is obtained. Both the quadratic dependence of the double frequency signal as a function of the fundamental frequency signal and the saturation of the fundamental signal under laser excitation are strong arguments in favor of a resonant non-coherent 2-photon-process; especially in light of the large range of input intensities used in this experiment. The spectral power density of the laser illumination was roughly a factor of larger than the illumination with the white light source. Further insight into such 2-photon-processes could be gained by performing autocorrelation measurements using ultra-short laser pulses (17). Together with recently published work on second harmonic generation in III-nitride quantum wells (18), these results clearly demonstrate a considerable application potential of these materials. 4. CONCLUSIONS In conclusion, we have presented both absorption and non-linear optical measurements on ISB transitions in GaN/AlNbased superlattices. Fast photovoltaic detection at 1.55 µm could be demonstrated using such structures. The detection mechanism, namely optical rectification, suggests that the intrinsic speed limit of this device must be at considerably higher frequencies. Because of the strong piezo- and pyro-electric polarization fields in nitride semiconductors, these effects could be observed without designing and growing intentionally asymmetric, step-like quantum wells. In addition, since the first three energy levels are doubly resonant, two-photon absorption with a quadratic dependence of the frequency doubled signal on input intensity was seen. Because of the large transition energy of 800 mev, all these effects could be observed at room temperature. These results illustrate clearly the high application potential of nitride semiconductors in areas like optical telecommunication and non-linear frequency conversion. Proc. of SPIE Vol S-8

9 5. ACKNOWLEDGEMENTS The authors acknowledge the financial support of the Professorship Program and the National Center of Competence in Research Quantum Photonics, both sponsored from the Swiss National Science Foundation, and the European STReP project NITWAVE, contract # REFERENCES 1 S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, and Y. Sugimoto, InGaN-based multi-quantum well structure laser diodes, Jap. J. Appl. Phys. Lett., vol. 35, no. 1B, pp. L74-L76, C. Gmachl, H.M. Ng, S.N.G. Chu, and A.Y. Cho, ISB absorption at lambda = 1.55 µm in well- and modulationdoped GaN/AlGaN multiple quantum wells with superlattice barriers, Appl. Phys. Lett., vol. 77, no. 23, pp , N. Suzuki, and N. Iizuka, Feasibility study on ultrafast nonlinear optical properties of 1.55-mu m ISB transition in AlGaN/GaN quantum wells Jap. J. Appl. Phys. Lett., vol. 36, no. 8A, pp. L1006-L1008, N. Iizuka, K. Kaneko, and N. Suzuki, Near-infrared ISB absorption in GaN/AlN quantum wells grown by molecular beam epitaxy, Appl. Phys. Lett., vol. 81, no. 10, pp , D. Hofstetter, S.-S. Schad, H. Wu, W.J. Schaff, and L.F. Eastman, GaN/AlN-based quantum well infrared photodetector for 1.55 µm, Appl. Phys. Lett., vol. 83, no. 3, pp , F.R. Giorgetta, E. Baumann, F. Guillot, E. Monroy, and D. Hofstetter, High frequency (f=2.37 GHz) room temperature operation of 1.55 µm AlN/GaN-based ISB detector, Electron. Lett., vol. 43, no. 2, pp , D. Hofstetter, E. Baumann, F.R. Giorgetta, M. Graf, M. Maier, F. Guillot, E. Bellet-Amalric, E. Monroy, Highquality AlN/GaN-superlattice structures for the fabrication of narrow-band 1.4 µm photovoltaic ISB detectors, Appl. Phys. Lett., vol. 88, no. 12, pp , Z. Wang, K. Reimann, M. Woerner, T. Elsaesser, D. Hofstetter, J. Hwang, W.J. Schaff, and L.F. Eastman, Optical phonon sidebands of electronic ISB absorption in strongly polar semiconductors, Phys. Rev. Lett., vol. 94, no. 4, pp , O. Ambacher, J. Smart, J.R. Shealy, N.G. Weimann, K. Chu, M Murphy, W.J. Schaff, L.F. Eastman, R. Dimitrov, L. Wittmer, M. Stutzmann, W. Rieger, and J. Hilsenbeck, Two-dimensional electron gases induced by spontaneous and piezoelectric polarization charges in N- and Ga-face AlGaN/GaN heterostructures, J. Appl. Phys., vol. 85, no. 6, pp , H.P. Weber, Method for pulsewidth measurement of ultrashort light pulses generated by phase-locked lasers using nonlinear optics, J. Appl. Phys., vol. 38, no. 5, pp , E. Rosencher, Ph. Bois, B. Vinter, J. Nagle, and D. Kaplan, Giant nonlinear optical rectification at 8-12 µm in asymmetric coupled quantum wells, Appl. Phys. Lett., vol. 56, no. 19,pp , E. Rosencher, A. Fiore, B. Vinter, V. Berger, Ph. Bois, and J. Nagle, Quantum Engineering of optical nonlinearities, Science, vol. 271, pp , E. Rosencher, P. Bois, J. Nagle, E. Costard, and S. Delaitre, Observation of nonlinear optical rectification at 10.6 µm in compositionally asymmetrical AlGaAs multiquantum wells, Appl. Phys. Lett., vol. 55, no. 16, pp , L. Doyennette, L. Nevou, M. Tchernycheva, A. Lupu, F. Guillot, E. Monroy, R. Colombelli, and F.H. Julien, GaN-based quantum dot infrared photodetector operating at 1.38 µm, Electronics Letters, vol. 41, no. 19, pp , 2005 Proc. of SPIE Vol S-9

10 16 E. Rosencher and Ph. Bois, Model system for optical nonlinearities: asymmetric quantum wells, Phys. Rev. B, vol. 44, no. 20, pp , T. Maier, H. Schneider, M. Walther, P. Koidl, and H.C. Liu, Resonant two-photon photoemission in quantumwell infrared photodetectors, Appl. Phys. Lett., vol. 84, no. 25, pp , L. Nevou, M. Tchernycheva, F.H. Julien, H. Raybout, A. Godard, E. Rosencher, F. Guillot, and E. Monroy, ISB resonsnt enhancement of second-harmonic generation in GaN/AlN quantum wells, Appl. Phys. Lett., vol. 89, no. 15, pp , 2006 Proc. of SPIE Vol S-10