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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 20, NO. 9, MAY 1, 2008
Electrooptical Modulator at Telecommunication Wavelengths Based on GaN–AlN Coupled Quantum Wells N. Kheirodin, L. Nevou, H. Machhadani, P. Crozat, L. Vivien, M. Tchernycheva, A. Lupu, F. H. Julien, G. Pozzovivo, S. Golka, G. Strasser, F. Guillot, and E. Monroy
Abstract—We report on the demonstration of an intersubband (ISB) coupled quantum-well modulator operating at room temperature and telecommunication wavelength using a GaN–AlN quantum-well structure. The optical modulation is shown to result from electron tunneling between the wells. Stark shifting of the ISB absorption is observed. The maximum modulation depth is 0.79% at = 2:3 m and 0.18% at = 1:37 m for a mesa device with only 151-nm interaction length. We show that by reducing the mesa size down to 15 2 15 m2 , optical modulation bandwidth as large as 3 GHz can be obtained. Index Terms—Electrooptic modulation, infrared spectroscopy, intersubband (ISB) transition, nitrogen compounds, optoelectronic devices, quantum-well (QW) devices.
I. INTRODUCTION IDE bandgap nitride materials are well known for their optoelectronic applications in the visible-ultraviolet spectral range. They also offer prospects for ultrafast optoelectronic devices at fiber-optics telecommunication wavelengths based on intersubband (ISB) transitions in quantum-well (QW) heterostructures. Fast multiterabit/s all-optical switches as well as GaN–AlN ISB photodetectors have been demonstrated at 1.3- to 1.55- m wavelengths [1]–[3]. One key feature is the ultrafast intrinsic speed, which stems from the extremely short ISB recovery time (0.15–0.4 ps) due to enhanced electron longitudinal optical phonon scattering in nitride materials. Nitride-based electrooptical modulators based on Stark shift of the ISB absorption with frequency response as high as 60 GHz have been proposed [4]. With respect to current technologies based on interband electroabsorption (EA), it was predicted that ISB modulators would provide better handling of chirp issues during the commutation process [4]. ISB modulators relying on the charge transfer between a two-dimensional electron gas
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Manuscript received October 29, 2007; revised January 23, 2008. This work was supported by the EU-FP6 Project “NITWAVE” (IST—004170). N. Kheirodin, L. Nevou, H. Machhadani, P. Crozat, L. Vivien, M. Tchernycheva, A. Lupu, and F. H. Julien are with the Institut d’Electronique Fondamentale, UMR 8622 CNRS, 91405 Orsay, France (e-mail:
[email protected]). G. Pozzovivo and G. Strasser are with the Zentrum für Mikro- und Nanostrukturen, TU Vienna, 1040 Vienna, Austria. S. Golka is on leave from the Zentrum für Mikro- und Nanostrukturen, TU Vienna, 1040 Vienna, Austria. F. Guillot and E. Monroy are with the Equipe Mixte CEA-CNRS-UJF Nanophysique et Semiconducteurs, DRFMC/SP2M/PSC, CEA-Grenoble, 38054 Grenoble, Cedex 9, France. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2008.919595
and a GaN–AlN superlattice have been demonstrated [5]. In a recent work, we have reported on an ISB EA modulation device operating at room temperature, which relies on electron tunneling in GaN–AlN coupled quantum wells (CQWs) [6]. The active structure consists of a wide well, which acts as an electron reservoir, and of a narrow well designed to exhibit ISB absorption at 1.3 m. By applying a bias on the CQW structure, electrons are transferred from the reservoir well to the active m. In this well, which gives rise to EA at earlier study [6], the devices were processed in the form of big mesas (0.7 0.7 mm ) resulting in a modulation bandwidth of 11 MHz. In this letter, we report on a significant improvement of the CQW modulator performances as well as the observation of new phenomena arising from the miniaturization of the devices, namely the observation of Stark shift of the ISB energy in addition to tunneling effects. With respect to the previous work on large mesas, the improved field homogeneity in the device leads to an enhancement of the modulation depth by a factor of 2.6. Optical modulation bandwidth as high as 3 GHz is measured at m for 15 15 m devices. We show that the bandwidth is limited by the parasitic capacitance and that it could be further enhanced by implementing radio-frequency (RF) contact design. II. EXPERIMENTS AND RESULTS The sample was grown at a fixed substrate temperature of 700 C on an AlN/ -sapphire template by plasma-assisted molecular beam epitaxy. This technique provides a precise control of the growth rate. The sample contains 20 periods of CQWs sandwiched between two 500- and 125-nm-thick cm . The Al Ga N contact layers Si-doped at active region consists of a 3-nm-thick GaN electron reservoir well, a 1-nm-thick AlN coupling barrier, a 1-nm-thick active cm , and a 3-nm-thick AlN GaN well Si-doped at barrier separating each period. The thickness of the coupling barrier is small enough to guarantee an efficient electron tunneling between the two GaN QWs. The sample was processed in the form of square mesas. We used reactive ion etching with chlorine gas to etch the structure down to the bottom Al Ga N Si-doped contact layer. A Ti (20 nm)/Al (150 nm)/Ti (40 nm) metallization was performed on the bottom Al Ga N layer followed by annealing at 750 C during 30 s, which results in an ohmic contact. Another Ni (25 nm)/Au (200 nm) metallization was then carried out onto the top and bottom contacts. The top metallization results in a Schottky contact. The top contacts are metallized in the
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KHEIRODIN et al.: ELECTROOPTICAL MODULATOR AT TELECOMMUNICATION WAVELENGTHS BASED ON GaN–AlN CQWs
Fig. 1. Room-temperature I –V curve for 15 a top view of a 50 50 m modulator.
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2 15 m
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mesa. The inset shows
form of hollow squares to allow testing of the devices at Brewster’s angle of incidence. For demonstrating high-speed modulators, the devices have been miniaturized and the contacts have been optimized in order to reduce the capacitance while allowing needle probe testing. The sizes of the square mesas are 270, 90, 50, 30, and 15 m. The inset of Fig. 1 presents an optical microscope image of one 50 50 m device with its contacts. Fig. 1 shows the current–voltage ( – ) characteristic at 300 K of a 15 15 m mesa, which exhibits a Schottky-like behavior arising from the top contact. The ISB absorption of the CQWs has been measured using a Fourier transform infrared (FTIR) spectrometer. Two transverse-magnetic-polarized absorption peaks are observed at m and 0.9 eV m . These 0.56 eV absorptions are attributed to the ISB transitions of the reservoir QW and the active QW, respectively. From the absorption magand nitude, we deduce the electronic population of the states of the QWs at room temperature: cm for the reservoir well and cm for the active QW [6]. We have then investigated the 270 270 m mesas in terms of their differential transmission, , where and are the transmission under bias and at zero-bias, respectra at room temperspectively. Fig. 2 shows the ature for different bias pulses, measured using the FTIR specm trometer. For positive bias, the ISB absorption at in the reservoir well decreases, while the absorption at m in the active well increases. This reflects the charge transfer from the reservoir to the active QW, as schematized in the inset of Fig. 2. An opposite behavior is observed for m and the negative bias. Both modulation peaks at m exhibit a blue shift of 30 meV when the bias is increased from 30 to 30 V. This blue shift is a consequence of the Stark shift of the ISB transition in the reservoir and active wells. Note that due to the resistivity of the contact layers, the V voltage across the active region is estimated to be V bias. for The modulation depth is defined as MD , where is the intensity of the light transmitted by the modulator in the “ON” (“OFF”) state. A good MD approximation is given by . The modulation depth is higher for positive than for negative bias. This is a consequence of the smaller electron population of the active QW
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Fig. 2. Differential transmission spectra of a 270 270 m mesa modulator at 300 K for different pulsed bias. Inset: principle of operation of the CQW modulator.
with respect to the reservoir QW. MD is 0.79% at m m for 30-V bias. These and MD is 0.18% at rather small values are due to the experimental configuration since the light passes only once through the active layer with an interaction length of only 151 nm. With respect to measurements on 700 700 m mesas [6], the improved field homogeneity in 270 270 m mesas leads to an enhancement of the modulation depth by a factor of 2.6. It should be noted that the modulation depth can be largely enhanced by making use of guided wave propagation in the plane of the layers [7]. The frequency response of the modulators was measured at m by focusing a continuous-wave Nd YVO laser at a 45 angle of incidence onto the hollow top contact. A 20-GHz bandwidth RF waveform synthesizer was used to bias the devices at 13-dBm electrical power. The transmitted light was coupled into a 10-m-long optical fiber and detected by an InGaAs photodiode with a 3-dB frequency response of 32 GHz and a spectrum analyzer with 50-GHz bandwidth. The mesas were bonded with 8-mm-long gold wires to a subminiature version A (SMA) connector. Fig. 3 shows the normalized frequency response of the modulators at room temperature. The optical modulation bandwidth, which corresponds to 6-dB electrical signal bandwidth, versus mesa size is shown in the inset of Fig. 3. The optical modulation bandwidth increases when reducing the mesa size and reaches 3 GHz for the 15 15 m mesa. The frequency response limitations of the modulator can be understood considering its electrical equivalent circuit shown in is the device capacitance and is the shunt resisFig. 4. is the access resistance, which is tance of the active region. due to the Schottky contact and resistivity of the AlGaN layers, is the parasitic capacitance between the top and bottom contacts, and is the inductance arising from the bonding wires.
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 20, NO. 9, MAY 1, 2008
time constant for mesa sizes is clearly limited by the above 30 m. This is not the case for the smaller 15- m mesas, and because the cut-off frequency due to the 50- resistor in that case is comparable to that due parasitic capacitance and filter. to the In terms of intrinsic speed, the CQW modulator is limited by the electron tunneling time through the 1-nm-thick AlN barrier which is of the order of a few picoseconds. In order to further increase the speed of the modulator, dedicated RF design is required along with a reduction of the device dimensions. The latter can be achieved by inserting the CQW active region inside a waveguide. We estimate a modulation bandwidth of the order of 16 GHz for a 2 20 m waveguide modulator. Based on recent measurements on GaN based waveguides, losses as low as 0.1 dB can be expected [7]. III. CONCLUSION Fig. 3. Measured (dots) and simulated (solid curves) frequency response for modulators of different sizes. The inset shows the optical modulation bandwidth as a function of the mesa size.
Fig. 4. Electrical equivalent circuit of CQW modulator. bias voltage and 50- line resistance.
V
and
R
We have investigated room-temperature GaN–AlN electrooptical mesa modulators based on ISB absorption and electron tunneling between two GaN QWs coupled by a thin AlN barrier. The modulation spectra for various applied biases provide evidence of electron tunneling and Stark shifting of the ISB absorptions. An optical modulation bandwidth as large as 3 GHz has been obtained for the 15 15 m modulator. Further increase of the modulation depth and speed could be achieved using waveguide modulators with RF 50- access lines. The driving voltage can also be further reduced by decreasing the access resistance using lower Al content in the contact layers.
are the
TABLE I PARAMETERS OF THE MODULATOR’S ELECTRICAL EQUIVALENT CIRCUIT
ACKNOWLEDGMENT The authors would like to thank DOWA Electronics Materials Co., Ltd. for supplying the AlN/c-sapphire templates used in this work. REFERENCES
In order to estimate the values of the equivalent circuit resistances and capacitances, a 50-GHz network analyzer was used to measure the S-parameters of the devices. We used the ADS software to estimate the parameters from the measured data. In addition, the values of the parameters were further adjusted in order to fit the measured frequency response of the modulators. The solid curves in Fig. 3 show the simulated frequency response for the various mesa sizes based on the electrical parameter values listed in Table I. The agreement between simulations and measurements is very good in the investigated frequency range. is strongly decreased by reducing the As seen in Table I, mesa size because the device capacitance is proportional to the value is in good agreement mesa surface. Interestingly, the with the capacitance value calculated using the total AlN barrier thickness (83 nm) in the 20 period active region. The access resistance and device capacitance acts as a first-order filter. As shown by the fitting curves of Fig. 3, the modulation bandwidth
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