Mid-infrared electro-luminescence and absorption

Mid-infrared electro-luminescence and absorption from AlGaN/GaN-based multiquantum well inter-subband structures Daniel Hofstetter, David P. Bour, and Lutz Kirste Citation: Applied Physics Letters 104, 241107 (2014); doi: 10.1063/1.4883864 View online: http://dx.doi.org/10.1063/1.4883864 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/104/24?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Systematic study of near-infrared intersubband absorption of polar and semipolar GaN/AlN quantum wells J. Appl. Phys. 113, 143109 (2013); 10.1063/1.4801528 Tunability of intersubband transition wavelength in the atmospheric window in AlGaN/GaN multi-quantum wells grown on different AlGaN templates by metalorganic chemical vapor deposition J. Appl. Phys. 112, 063526 (2012); 10.1063/1.4754543 High-quality AlGaN/GaN superlattices for near- and mid-infrared intersubband transitions J. Appl. Phys. 111, 013514 (2012); 10.1063/1.3675468 Room-temperature electroluminescence of Al Sb In As Sb single quantum wells grown by metal organic vapor phase epitaxy Appl. Phys. Lett. 88, 132102 (2006); 10.1063/1.2189572 Bright blue electroluminescence from an InGaN/GaN multiquantum-well diode on Si(111): Impact of an AlGaN/GaN multilayer Appl. Phys. Lett. 78, 2211 (2001); 10.1063/1.1362327

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APPLIED PHYSICS LETTERS 104, 241107 (2014)

Mid-infrared electro-luminescence and absorption from AlGaN/GaN-based multi-quantum well inter-subband structures Daniel Hofstetter,1,a) David P. Bour,2 and Lutz Kirste3 1

University of Neuch^ atel, Institute of Physics, 51 Avenue de Bellevaux, Neuch^ atel, CH–2009, Switzerland Avogy, Inc., 677 River Oaks Parkway, San Jose, California 95134, USA 3 Fraunhofer Institute for Applied Solid State Physics (IAF), Tullastrasse 72, D-79108 Freiburg i. Brsg., Germany 2

(Received 23 May 2014; accepted 3 June 2014; published online 16 June 2014) We present electro-modulated absorption and electro-luminescence measurements on chirped AlGaN/GaN-based multi-quantum well inter-subband structures grown by metal-organic vapour phase epitaxy. The absorption signal is a TM-polarized, 70 meV wide feature centred at 230 meV. At medium injection current, a 58 meV wide luminescence peak corresponding to an inter-subband transition at 1450 cm1 (180 meV) is observed. Under high injection current, we measured a 4 meV wide structure peaking at 92.5 meV in the luminescence spectrum. The energy location of this C 2014 AIP Publishing LLC. peak is exactly at the longitudinal optical phonon of GaN. V [http://dx.doi.org/10.1063/1.4883864] In the past couple of decades, research in the nitride material system has led to the development of short wavelength diode lasers for digital versatile disks (DVDs),1 high power electronic devices such as high electron mobility transistors (HEMTs)2 for power amplifiers, and visible light emitting diodes (LEDs) for general lighting.3 More recently, experiments dealing with optical inter-subband (ISB) transitions in this material system have been carried out and led to promising results. As first steps in this area, ISB transitions in visible LEDs have been reported by Hofstetter et al.,4 while Gmachl et al. observed optical ISB absorption at 1.7 and 1.55 lm.5,6 In 2003, 1.55 lm photo-detectors based on ISB transitions in highly doped GaN/AlN super-lattices were demonstrated.7 These detectors have since that time been greatly improved, and room temperature operation at frequencies up to 3 GHz is now possible.8,9 However, the main reason why the nitrides could eventually become a key material for ultrafast ISB transition-based semiconductor devices is the high conduction band discontinuity between AlN and GaN.10,11 Its large value around 2 eV could lead not only to multi-GHz light detection but also to high frequency light emission in the technologically important wavelength range around 1.55 lm, which is heavily used for fast optical telecommunications. In order to make progress in this direction, a thorough understanding of the material properties in the nitrides is essential. In view of these considerations, we report in this article on the measurement of several ISB absorption and luminescence phenomena in the mid-infrared wavelength range. The samples investigated in this study were grown by metal-organic vapor phase epitaxy (MOVPE) on c-face sapphire substrates. Growth started with a 5 lm thick nominally n-doped GaN layer (Si, 5 1017 cm3), followed by 5 periods of a partly n-doped GaN/Al0.2Ga0.8N-based, chirped ˚ per period (unless super-lattice with a thickness of 312 A mentioned otherwise, all layer thicknesses are exact design values). The top layer, finally, was a 200 nm thick highly a)

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n-doped GaN-layer (Si, 1 1018 cm3). The layer sequence of one super-lattice period – given in angstroms and starting from the injection barrier – is as follows: (25/33/15/22/ 12/20/14/17/15/15/15/14/18/13/21/11/22/10), where numbers in bold face indicate pure GaN quantum wells (QWs), normal face numerals refer to Al0.2Ga0.8N barriers, and underlined numbers are n-doped layers (Si, nominally 5 1017 cm3). This design enables, under the application of an electric field of 83 kV/cm perpendicularly to the layers, the observation of a vertical ISB transition. Numerical simulation of the designed structure resulted in a theoretical transition energy of 280 meV. A high resolution X-ray diffraction (HRXRD) pattern and simulation, as displayed in Figure 1, ˚ , measured: 361 6 1 A ˚) confirmed both period (design: 312 A and aluminum content (design: 20%, measured: 24.5 6 0.5%) of the structure to be correct. However, the small number of periods, the low doping level, and the relatively large interfacial roughness resulting from our specific MOVPE prevented us from spectrally seeing the ISB transition as pronounced as initially expected. The roughness was most likely the result of

FIG. 1. HRXRD H/2H-pattern around the GaN 0006 peak. Super-lattice peaks are analysed from the –10th to the þ6th order. An Al content of ˚ (design: 24.5 6 0.5% (design: 20%) and a super-lattice period of 361 6 1 A ˚ ) were found. 312 A

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the high growth rate and low sample temperature during buffer growth. Nevertheless, we observed both mid-IR absorption and luminescence signals on this structure. After epitaxial growth, sample preparation proceeded with a lift-off deposited Ti/Au layer (10/700 nm) directly on the grown surface; in order to obtain square-shaped metal pads of 200, 400, and 800 lm side length. They served both as etch masks for reactive ion etching (RIE), and afterwards as top contacts for electrical current injection. In order to reach sufficiently deep into the lower contact layer, mesa etching was carried out to a depth of 1 lm. Bottom contacts were based again on thick Ti/Au layers (10/700 nm) deposited outside of the mesas, and at sufficient separation from the previously defined top contacts. Annealing completed the processing and enabled current injection. The samples were finally polished into double-pass waveguides with a parallelogram-shaped cross-section of 5 mm length, 1 mm width, 0.4 mm thickness, and two parallel 45 angled facets on its two long sides. A schematic cross-section of such a sample is shown as an inset of Figure 2. Electro-modulated absorption measurements were then performed on the larger, 800 800 lm2 mesas, whereas the luminescence required the use of the smaller, 400 400 lm2 mesas. The available current range, voltage size, and duty cycle of the pulse generator used (Agilent 8114 A) set an upper limit for the current injection into the mesas. All investigated structures showed resistance values that are inversely proportional to the surface. They were typically 230 X for the large mesas, and 1 kX for the small ones. These are fairly typical values for ISB structures with such thicknesses, areal dimensions, and high barriers. For the absorption measurements, a Fourier transform infrared spectrometer (FTIR) was used (Nicolet Magna 800), while the samples were held at 77 K on the cold finger of a liquid nitrogen cooled flow cryostat. The radiation from the FTIR’s internal infrared light source (glow-bar) was coupled into one of the 45 facets of the sample using a beam condenser (f/1.00). As shown schematically in the inset of Figure

FIG. 2. Electro-modulated absorption spectrum of an AlGaN/GaN-based heterostructure. The absorption was measured on an 800 800 lm2 large mesa structure and at 77 K. The FWHM of this peak was 70 meV. The hatched area shows the region where the sapphire substrate is strongly absorbing. The arrows indicate fitted peaks at 1835 cm1, 2125 cm1, 2415 cm1, and 2705 cm1. The inset shows a schematic cross-section of the sample and the absorbing light beam.

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2, it then passed through the polished sapphire prism in a zigzag-mode (with the active region being on top). The transmitted radiation was finally detected by the internal liquid nitrogen-cooled HgCdTe detector. Injected current and bias voltage for this electro-modulated absorption measurement had typical values of 7 mA and 6 V, respectively; along with a repetition frequency of 10 kHz and a duty cycle of 50%. Its spectrum, DT/T ¼ [T(6 V) T(0)]/T(0) measured in step-scan mode and lock-in detection12 as a function of wavenumbers, is shown in Figure 2. Energy values lower than 1650 cm1 are dominated by the strong absorption of the sapphire substrate and are marked in the figure by hatched lines. The signal at 1835 cm1 (230 meV) corresponds therefore to differential absorption and is strongly (8:1) TM-polarized. Its transition energy is somewhat smaller than the design value; mainly because of the roughness-induced trapezoidal shape of the QWs. This effect pushes the lower energy state up, while the upper energy state moves down. The observed high polarization ratio is, however, a good indication for the ISB nature of the 230 meV peak. Nevertheless, for structures situated that close to the sample surface, a certain enhancement of the electric field in TM polarization cannot be completely ruled out. The corrected full width at half maximum (FWHM) of this electro-modulated absorption signal was about 70 meV. Given a transmission electron microscopy estimated interfacial ˚ ), the above value roughness of 1 atomic repeat distance (or 3 A of the FWHM is reasonable. The integrated area of the peaked signal, which is proportional to the oscillator strength of the involved transition, IA fij, can be used to estimate the sheet carrier density responsible for the absorption. The latter is then compared to the value from layer design. Taking into account the total thickness of the doped QWs (5.9 nm), the sheet carrier density of 3 109 cm2 obtained by the above absorption measurement results in an equivalent three-dimensional carrier density on the order of 5 1015 cm3. This experimental value is two orders of magnitude lower than the designed one. It is therefore no surprise that we observed neither an exceptionally high absorption nor a large luminescence signal. As will be shown below, the relatively high voltages necessary to obtain decent current injection indicate moderate doping in the contact layers as well. According to the conclusions of the previous paragraph, the weak electro-modulated absorption signal at 230 meV is mainly due to a generally low doping level in our sample. The reason for this low doping level is obvious: More than half of the Si donors are incorporated into the AlGaN barrier material with a high Al mole fraction. Since n-type doping of such high bandgap AlGaN compounds is rather difficult to achieve, the weak absorption and luminescence signals become understandable. In addition to the barrier material being generally difficult to dope, a considerable amount of carbon impurities being present in the GaN QW layers under the used growth conditions leads to the formation of a deep acceptor level. This has partly compensated the intentional n-type doping. In addition to the main absorption signal at 1835 cm1 (230 meV) in Figure 2, we observed additional features at 2125 cm1, 2415 cm1, and 2705 cm1. Given their regular period, they are most likely due to Fabry-Perot resonances occurring in the totally 5.5 lm thick epitaxial layer stack.


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The total layer thickness assumes nominal thicknesses of 5 lm for the GaN buffer, 300 nm for the active region, and 200 nm for the top contact layer. The large number of QWs and the energetically high barriers result already in a considerable device resistance. Together with the low doping both in the contact layers and the super-lattice, resistance values exceeding the kX limit, especially for the small quadratic devices, were seen. Due to the resulting non-negligible Joule heating of the devices under current injection, the observation of room temperature ISB luminescence became rather challenging. For this reason, the samples were cooled to 77 K. The luminescence was collected using f/1.00 optics from the 45 facet of the sample, and measured again with an external liquid nitrogencooled HgCdTe-detector. The observed luminescence spectra, measured either at 7.7 V, 95 kHz, and 50% duty cycle for low bias voltage or at 14.0 V, 95 kHz, and 50% duty cycle for medium bias voltage, are presented in Figure 3. They showed respective ISB luminescence peaks at 926 cm1 (1450 cm1) corresponding to 115 meV (180 meV). The corresponding FWHM values for low (medium) bias voltage were 38 meV (58 meV). As mentioned above, these luminescence spectra were measured on 400 400 lm2 sized mesa structures in order to avoid reaching the power limit of the voltage source. The luminescence at 1450 cm1 (180 meV) was TM-polarized, but not as strongly as the absorption signal; i.e., 4:1 instead of 8:1. Although a high TM-polarization ratio is consistent with an ISB signal, the Brewster angle and the nearby top-metallization of the sample also partly contributed to the comparably high TM-polarization of the signals. When working at smaller duty cycles (i.e., 25% instead of 50%), we observed a narrow structure in the luminescence spectrum. It was peaking at 741 cm1 (92.5 meV) and occurred exactly at the frequency of the longitudinal optical (LO) phonon in GaN.12,13 Three emission spectra under 18.5/20.0/21.5 V bias voltage, and at 25% duty cycle (95 kHz, 77 K) are displayed in Figure 4. Compared to the

FIG. 3. Typical luminescence spectra of an AlGaN/GaN-based multi-QW structure measured at 77 K under low and medium electrical pumping conditions. At low current injection (7.7 V, 50% duty cycle), the emission was centred at 926 cm1 (115 meV) and had a FWHM of 38 meV, whereas at medium injection (14.0 V, 50% duty cycle), an emission peak at 1450 cm1 (180 meV) with a FWHM of 58 meV was seen. In both cases, a frequency of 95 kHz was used.

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previously shown signals, the FWHM of this emission peak at 741 cm1 was dramatically small, namely 4 meV. Although photon emission via LO phonons seems to be an inefficient process,14 a similar phenomenon has been observed in the far IR wavelength range by Rochat et al.15 Based on the observation that the intensity of this phonon peak got larger at smaller applied bias voltage, and that the corresponding blackbody radiation spectra showed an improved overlap between blackbody emission and phonon frequency for the lowest bias voltage, we explain the origin of this peak as follows: The somewhat inefficient carrier transport in our high barrier/deep QW structure resulted in considerable device heating. This is seen best in Figure 4, where a spectral shift towards higher energies of the blackbody luminescence occurs as the bias voltage is increased. Since device heating is equivalent to the presence of lattice oscillations, there will be a higher number of phonons present at larger bias voltage. Once they exist in large enough quantity, they optically radiate their energy – for GaN at the observed LO-phonon frequency of 741 cm1. Finally, it is obvious that more phonons will be created if the spectral overlap of the phonon energy with the corresponding energy in the blackbody spectrum is higher. In our case, this happened to take place at the lowest bias voltage of 18.5 V. In conclusion, we have presented different mid-IR absorption and luminescence phenomena on AlGaN/GaN MQWs. As a first important result, an electro-modulated absorption feature at an energy of 230 meV was observed; its FWHM measured 70 meV, and its TM:TE polarization ratio was 8:1. As a second main result, we found an ISB luminescence peak around 926 cm1 (115 meV) being TM-polarized at a ratio of about 4:1. This peak shifted to 1450 cm1 (180 meV) at medium bias voltage. The FWHM of this emission in electro-luminescence configuration was 38 meV for the low injection, and 58 meV for the medium injection case. The observed FWHM values are fully consistent with the fact that an atomic step like interfacial roughness is present in this material. At high voltage bias but with a lower duty cycle, a second luminescence peak occurred exactly at the

FIG. 4. Emission spectra of the AlGaN/GaN-based MQW structure under high electrical pumping conditions resulting in a 4 meV wide peak at the GaN LO-phonon frequency of 741 cm1. These measurements were carried out at a temperature of 77 K, a duty cycle of 25%, and bias voltages of 18.5 V, 20.0 V, and 21.5 V.


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LO-phonon energy of GaN, namely, at 741 cm1. Given its spectral behavior under different current levels, it is strongly believed that this peak is due to an energy transfer from Joule’s heat, produced by current transport in the multiple barrier-well structure, towards coherent oscillations of the crystal lattice. The FWHM of this phonon emission was as narrow as 4 meV. The authors gratefully acknowledge helpful discussions with Chris G. Van de Walle (UCSB Santa Barbara, CA, USA), Matthew D. McCluskey (Washington State € ur (Virginia € Ozg€ University, Pullman, WA, USA), U. Commonwealth University, VA, USA), and Cynthia AkuLeh (ISciences, MI, USA) for careful and critical proof reading of the manuscript. This work was financially supported by the Swiss National Science Foundation. 1

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