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aPhys. Status Solidi C 7, No. 10, 2394–2397 (2010) / DOI 10.1002/pssc.200983905

Optical and structural characterization of GaN/AlGaN quantum wells for intersubband device applications Kristina Driscoll1, Yitao Liao1, Anirban Bhattacharyya1, Theodore D. Moustakas1, Roberto Paiella*,1, Lin Zhou2, and David J. Smith2 1 Department of Electrical and Computer Engineering and Photonics Center, Boston University, MA 02215 Boston, USA 2 Department of Physics, Arizona State University, AZ 85287 Tempe, USA

Received 21 September 2009, accepted 29 January 2010 Published online 21 June 2010

Keywords GaN/AlGaN, quantum wells, MBE, structure, optical properties, devices * Corresponding author: e-mail rpaiella@bu.edu

Due to their large conduction-band offsets of up to 1.75 eV, GaN/Al(Ga)N quantum wells can support intersub-band transitions at record short wavelengths, well into the low-loss transmission window of optical fibers. As a re-sult, these quantum structures are promising to enable new intersubband device applications, including all-

optical switching for ultra-broadband fiber-optic net-works and emission of short-wave infrared radiation. Here we report our work on the development of high-quality GaN/Al(Ga)N quantum well structures designed for such applications.

© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction Intersubband (ISB) transitions in semiconductor quantum wells (QWs) exhibit several desir-able features for many optoelectronic device applications. So far, they have been primarily studied in As-based III-V materials, whose relatively small conduction-band offsets limit the operating wavelengths to values greater than 3 µm. Recently, however, novel materials systems with lar-ger conduction-band offsets have also been considered for the purpose of extending the operation of ISB devices to shorter wavelengths. Particularly promising in this respect are GaN/Al(Ga)N QWs where the conduction-band offset can be as large as 1.75 eV. Consequently, these het-erostructures can accommodate ISB transitions at wave-lengths throughout the short-wave infrared spectral region [1-4]. Moreover, the strong electron-phonon interaction in these materials due to their highly polar nature yields par-ticularly fast (subpicosecond) ISB relaxation lifetimes [5]. These properties open up new opportunities for ISB device development, particularly in the area of all-optical switch-ing for ultrafast fiber-optic communications [6,7] and short-wavelength ISB lasers [8,9]. Here we present the structural and optical characterization of GaN/Al(Ga)N QW structures designed for these applications.

2 Experimental methods 2.1 Sample growth The QW samples studied in this

work were grown by rf-plasma-assisted MBE on c-plane sapphire. After nitridation of the sapphire surface, a thick AlN buffer was first deposited to serve as a template for growth of the subsequent GaN/Al(Ga)N QWs. All layers were formed under group-III-rich conditions to promote two-dimensional growth. Since Al atoms are not readily desorbed from the surface at the growth temperature of 770 °C, growth interruption under nitrogen flux was gener-ally performed after deposition of each Al(Ga)N layer to consume excess Al accumulated on the surface. While this procedure allows for improved thickness control, it may al-so result in enhanced surface roughness depending on the duration of the growth interruption. Thus, a small flux of In was simultaneously used during growth of the Al(Ga)N layers because of the beneficial role played by In as a sur-factant [10]. The In flux was chosen to guarantee sufficient surface coverage without incorporation. Finally, in order to populate the lowest electronic bound state in each QW pe-riod, the active regions were doped n type with Si to the level of a few 1019 cm−3.

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Figure 1 (a) Cross-sectional TEM image of a GaN/AlN QW sample. (b) Selected-area electron diffraction pattern in the [1120] pro-jection of the same sample. (c) Measured ISB absorption spectrum and (inset) calculated conduction-band lineup and squared bound-state wavefunctions of the same sample.

2.2 Sample characterization The structural proper-

ties of the resulting QWs were characterized by cross-sectional transmission electron microscopy (XTEM) and selected-area electron diffraction. Exemplary images are shown in Figs. 1(a) and 1(b), which reveal high structural quality with smooth interfaces and good control of the structure periodicity and are typical of all samples studied in this work. The electronic subband structure of the GaN/AlGaN QW samples was probed by means of ISB ab-sorption spectroscopy. Specifically, the ISB transmission spectra were measured at room temperature with a Fourier transform infrared spectrometer (FTIR) and a liquid nitro-gen cooled HgCdTe detector. For these measurements, the substrate side of the sample was mirror polished and two opposite facets were lapped at 45° to obtain a multipass waveguide geometry. Light from the FTIR internal source was coupled in and out at normal incidence through these facets. Due to the polarization selection rules of ISB transi-tions, only TM-polarized light is absorbed in the QWs. Therefore, the ISB absorption spectra of the QW samples were obtained from the TM-polarized transmission spectra normalized to the TE-polarized spectra to eliminate any background absorption features. An ISB absorption spec-trum typical of these high quality samples is illustrated in Fig. 1(c).

3 Coupled quantum wells As ultimately required to extend the functionality and design flexibility of nitride-based ISB devices, coupled QW (CQW) structures have also been developed. These structures can for example be used to engineer the ISB nonlinear optical properties for applications in all-optical switching [6,7]. Similarly, CQWs can be used to implement multi-levels systems with the required oscillator strengths and lifetimes for the de-velopment of optically and electrically pumped ISB lasers [11,12].

However, the study of III-nitride CQWs has been com-plicated by two fundamental material properties of these heterostructures, namely their heavy electronic effective masses and their large conduction-band offsets. As a result, inner barriers with thicknesses of only a few angstroms are

required to achieve strong interwell coupling in GaN/AlN CQW systems [13]. However, the use of such ultrathin bar-riers is undesirable in device design since it results in bound-state energies and wavefunctions that are exceed-ingly sensitive to even atomic-scale thickness fluctuations. Additionally, it leaves very little design flexibility in fine tuning the degree of interwell coupling.

To address this issue, we have developed an alternative approach for the implementation of strongly coupled III-nitride QWs still providing short-wavelength ISB absorp-tion, where an Al(Ga)N alloy is used in the coupling bar-rier [14]. Decreasing the Al content of this alloy effectively lowers the conduction-band edge, so that strong coupling can be achieved. At the same time, AlN is still used in the outer barriers to provide the large quantum confinement required for short-wavelength ISB transitions. The compo-sition of the inner barriers also provides a continuously tuneable parameter to control the coupling strength. This idea is illustrated in Fig. 2(a), where we plot the calculated conduction-band lineup and the three lowest bound-state envelope functions of an AlN/GaN/Al0.39Ga0.61N CQW structure. Even though a relatively thick inner barrier (5 monolayers) is used in this design, the ground states of the two wells (states |1⟩ and |2⟩) are coupled to each other by a sizeable electric dipole moment of about 3.6 Å.

To experimentally validate this approach, three AlN/GaN/Al(Ga)N CQW structures with essentially the same well and barrier thicknesses were grown as described. The significant difference between the three samples lies in the Al concentration of the coupling barrier which varies from 39% in sample A to 48% in B to 53% in C. The re-sulting ISB absorbance spectra are illustrated in Fig. 2(b). Each sample exhibits two pronounced absorption features attributed to the |1⟩→|2⟩ and the |1⟩→|3⟩ ISB transitions. To confirm this interpretation, the CQWs were modelled using a self-consistent Poisson and Schrödinger equations solver based on the effective mass approximation, which included the characteristic built-in electric fields of GaN/Al(Ga)N QWs [15,16]. A very good agreement be-tween theory and experiments was obtained, with all calcu-lated energies lying within 6% or less of the corresponding

2396 K. Driscoll et al.: Optical and structural characterization of GaN/AlGaN quantum wells

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measured values. Another important feature illustrated in Fig. 2(b) is the ability to fine tune the degree of interwell coupling offered by this design approach. Specifically, we observe a decrease in the relative absorption strength of the “interwell” |1⟩→|2⟩ transitions with respect to the “in-trawell” |1⟩→|3⟩ transitions in going from sample A to B to C. This trend is consistent with the expectation that an in-crease in Al content of the inner barrier results in a reduced degree of interwell coupling.

Figure 2 (a) Calculated conduction-band lineup and bound-state wavefunctions of an AlN/GaN/AlGaN coupled-QW structure. (b) Measured ISB absorption spectra of three AlN/GaN/AlGaN cou-pled-QW samples of increasing Al concentration in the inner bar-rier.

4 Optically pumped ISB light emission Despite the various nitride-based ISB laser structures that have been proposed and theoretically investigated [12,17], ex-perimental progress has so far been limited [8]. Recently, we have demonstrated ISB light emission from GaN/AlN QWs pumped with a pulsed optical parametric oscillator (OPO), yielding peak output powers of a few hundred nanowatts [9]. The measured output spectra are peaked near 2 μm, which represents a new record for the shortest ISB emission wavelength from any QW materials system.

The active region of the sample used in this study con-sists of GaN/AlN QWs designed to support three bound

states. An XTEM image of these QWs is shown in Fig. 3(a). Again, the calculated ISB transition energies are in good agreement with the measured values obtained using ISB absorption spectroscopy.

For the light emission measurements, nanosecond-scale pump pulses were used to resonantly excite electrons from the ground states to the second-excited subbands, fol-lowed by radiative relaxation into the first excited sub-bands. The OPO output consists of a train of optical pulses with 5 ns width, 20 Hz repetition rate and pulse energy of 0.5 mJ. The measurements were carried out at room tem-perature, with the sample input and output facets polished at 45° and 90°. The pump light was focused at normal in-cidence onto the 45° input facet with an estimated spot size and corresponding input power density of 300 µm and 1x108 W-cm-2, respectively. A series of broadband wave retarders and polarizers were used to select the desired state of linear polarization incident on the sample. Similar to the absorption measurements, the pump light traverses the sample through multiple total internal reflections. The emitted light, exiting through the 90° output facet, was spectrally resolved using a monochromator and detected with a room-temperature extended-range InGaAs detector. To increase the sensitivity of the measurement, gated de-tection was performed using a box-car integrator.

Figure 3(b) shows a typical measured output spectrum obtained using a TM-polarized pump at a wavelength of 910 nm (1.362 eV). The solid line in the figure is a fit to a modified Gaussian peak, and yields a photon energy of peak emission of 602 meV (2.06 µm) and a spectral full width at half maximum of 105 meV. The observed photon energy of peak emission is close to the calculated value of 633 meV for this transition. Furthermore, it is in good agreement (well within the emission linewidth) with the difference between the pump photon energy and the ex-perimental value of the |1⟩→|2⟩ ISB transition (735 meV, as measured using ISB absorption spectroscopy). These observations provide strong evidence that the observed emission is indeed due to optical pumping from the ground states to the second-excited subbands, followed by radia-tive relaxation into the first-excited subbands.

Further experimental evidence of the ISB origin of this luminescence was obtained via an extensive study of its polarization properties and pump wavelength dependence9. First, essentially no emission is detected when the pump polarization is rotated from TM to TE, consistent with the polarization selection rules of ISB transitions. Additionally, as expected for ISB light emission, the luminescence is largely TM-polarized, with a strong reduction in detected power when the output beam is passed through a polarizer with transmission axis along the TE direction. Finally, the emitted intensity is highly dependent on pump wavelength and peaked around the calculated value for the relevant transition. This resonant behavior is again consistent with optically pumped ISB light emission.

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Figure 3 (a) XTEM image of the QW sample used to demon-strate ISB light emission. (b) Measured room-temperature emis-sion spectrum.

5 Conclusions The results presented in this work show that high quality III-nitride QW structures can be de-veloped for use in new ISB devices targeting short-wavelength applications. Despite the complexities of ni-tride heterostructures, we have established that fine tuning of the interwell coupling strength is possible by varying the Al content of the inner barriers in carefully designed AlN/GaN/Al(Ga)N CQWs. Furthermore, we have demon-strated ISB light emission from AlN/GaN QWs at a record short wavelength near 2 µm. These quantum structures will be useful for the future development of advanced ISB de-vices based on nitride heterostructures.

Acknowledgements This work was supported by NSF under grant ECS-0622102.

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