phys. stat. sol. (a) 188, No. 1, 333–336 (2001)
High Quantum Efficiency AlGaN/GaN Solar-Blind Photodetectors Grown by Metalorganic Chemical Vapor Deposition M. M. Wong, U. Chowdhury, C. J. Collins, B. Yang, J. C. Denyszyn, K. S. Kim, J. C. Campbell, and R. D. Dupuis1) The University of Texas at Austin, Microelectronics Research Center, 10100 Burnet Road, Building 160, Austin, TX 78758, USA (Received July 10, 2001; accepted August 1, 2001) Subject classification: 73.40.Kp; 73.40.Sx; 81.05.Ea; 81.15.Gh; 85.60.Gz; S7.14 We report the growth, fabrication and characterization of high-quality AlGaN/GaN solar-blind p–i–n and MSM photodetectors by low-pressure metalorganic chemical vapor deposition (MOCVD). The epitaxial layers were grown on double-polished c-plane (0001) sapphire substrates to allow for back-side illumination. The p–i–n photodiode structures typically consist of a 0.7 mm thick Al0.58Ga0.42N “window” layer, graded to a 0.2 mm thick Al0.47Ga0.53N n layer, a 0.15 mm thick Al0.39Ga0.61N i layer, a 0.2 mm thick Al0.47Ga0.53N p layer, and capped with a 25 nm GaN : Mg contact layer. At a 0 V bias, the processed p–i–n devices exhibit a solar-blind photoresponse having a maximum responsivity of 0.058 A/W at 279 nm, corresponding to an external quantum efficiency of 26%, uncorrected for reflections, etc. The MSM devices typically consist of an AlGaN x 0.58 window layer, and an undoped AlGaN x 0.44 absorbing layer. The MSMs exhibit an external quantum efficiency as high as 47% at a bias of 15 V with a peak response at 262 nm.
Introduction Photodetectors operating in the wavelength range between 250 and 300 nm, referred to as the solar-blind regime, can be used for applications such as missile detection and tracking and for biological agent and chemical detection. The III-nitride material system is ideal for these detectors, as they can be designed to have a cutoff wavelength below 300 nm. The metal–semiconductor–metal (MSM) device is well suited for this application, as it is relatively simple to fabricate and its low capacitance can result in a fast response time. Advances in material growth have resulted in high aluminum composition and crack-free AlGaN heterostructures for use as solarblind p–i–n photodetectors [1]. The p–i–n device has the advantage of operating at zero to low bias, eliminating the requirement for a bulky power supply. We report the growth, fabrication and characterization of high-quality AlGaN/GaN heteroepitaxial back-illuminated solar-blind p–i–n and MSM photodetectors with high external quantum efficiencies. Growth The AlxGa1––xN/GaN heterostructures of this work are grown by low-pressure metalorganic chemical vapor deposition (MOCVD) in an EMCORE TurboDisc D125 UTM high-speed rotating-disk reactor on 2.0 in diameter c-plane (0001) doublepolished sapphire substrates. The AlGaN epitaxial layers are grown at pressures 50 Torr and the GaN contact layer (for p–i–n only) is grown at 200 Torr in a hydrogen ambient using adduct-purified trimethylgallium (TMGa), trimethylaluminum (TMAl), and ammonia (NH3). The n-type and p-type dopants (again, p–i–n only) used are silane 1 ) Corresponding author; Phone: +1 512 471-0537; Fax: +1 512 471-0957; e-mail:
[email protected]
# WILEY-VCH Verlag Berlin GmbH, 13086 Berlin, 2001
0031-8965/01/18811-0333 $ 17.50þ.50/0
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(SiH4) and bis(cyclopentadienyl)magnesium (Cp2Mg), respectively. A two-temperature growth process is employed with a low-temperature AlN buffer layer grown at 540 C and the high-temperature (HT) device layers grown at 1060 C. For the p–i–n structures, first a 700 nm thick lightly doped n Al0.58Ga0.42N : Si “window” layer is grown, followed by a “transition” layer with an alloy composition graded from x = 0.58 to 0.47. This layer is designed to reduce the “spikes” at the conduction and valence bands at the heterojunction interface. Next, a 200 nm thick n+ Al0.47Ga0.53N : Si n-type contact layer is grown. The p-side of the device structure typically consists of a 150 nm unintentionally-doped (ud) Al0.39Ga0.61N active layer, a 200 nm p+ Al0.47Ga0.53N : Mg p-type cladding layer, and is finally completed with another graded composition layer to the 25 nm thick p+ GaN : Mg contact layer. For the MSM structures, a 800 nm thick Al0.58Ga0.42N : ud epitaxial layer is typically grown to reduce the defect density, improving material quality for the subsequent device layers. In addition, it serves as an optical window for the solar-blind regime. Next, a “transition” layer with an alloy composition graded from x = 0.58 to 0.44 is grown, followed by the final 180 nm Al0.44Ga0.56N : ud active region. Growth conditions were optimized for layer thickness uniformity and good quality of the high Al composition epitaxial layers. To determine the presence of any strain effects, X-ray diffraction reciprocal space maps were obtained for several of the p–i–n wafers. As shown in Fig. 1(left part)2) for a typical structure, the undoped AlGaN (x = 0.39) active region is fully strained relative to the thicker AlGaN (x = 0.47) p and n cladding regions. Similar results were obtained for the MSM wafers. The material quality of the epitaxial layers was also investigated with secondary ion mass spectroscopy (SIMS) analysis. Figure 1(right part) shows the SIMS profile of a p–i–n structure. The p-region requires a high Mg doping level due to the difficulty of achieving p-type AlGaN material with a high Al content. The back-diffusion of Mg into
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) Colour figures are published online (www.physica-status-solidi.com), where indicated.
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the i-region is likely due to the high concentration of Mg atoms and the relatively high density of defects in the AlGaN epitaxial layers. In profiles for both types of structures, the different AlxGa1––xN layers can be easily distinguished. Because oxygen reacts readily with aluminum, the presence of O in these layers is attributed to the high Al composition of the films required for the solar-blind detector. This data, however, is taken using an O standard for GaN and the actual concentration may be somewhat different. Fabrication The fabrication of the p–i–n diodes employs standard processing steps that we have developed earlier [1–4]. First, the Mg acceptor atoms are activated using rapid thermal annealing in N2 at 850 ºC for 10 min. Then the diode mesas are defined by etching the AlGaN layers through window openings patterned in a 100 nm thick SiO2 mask using reactive ion etching (RIE) employing BCl3 and SiCl4. An SiO2 layer is deposited in order to passivate the devices. Through openings defined in the passivation layer, Ni/Au and Ti/Al contacts are evaporated onto the p-type GaN and n-type Al0.47Ga0.53N layers, respectively. The wafer is then annealed at 750 C for 2 min to alloy the Ohmic contacts. The MSM structures are fabricated using an interdigitated electrode mask set [5]. Mesas are defined using conventional lithography techniques and an RIE dry-etch step. Following passivation with SiO2, Schottky contacts using Ti/Pt are deposited through window openings. Results Figure 2 shows the quantum efficiency of a typical back-illuminated p–i–n device, along with transmission data taken through an unprocessed sample. Without taking into account items such as reflection, the external quantum efficiency reaches 26% at l 279 nm at zero bias. Increasing the reverse bias to 5 V, the quantum efficiency peaks at 31% at a slightly red-shifted l 280 nm. The corresponding responsivities are 0.058 A/W at zero bias and 0.070 A/W at 5 V. The transmission data correlates well with the expected epitaxial layers, with a drop around 362 nm due to the GaN contact layer and at 292 nm from the Al0.39Ga0.61N active region. The quantum efficiency of the back-illuminated MSM under reverse bias values ranging from 2 to 100 V is displayed in Fig. 3. The quantum efficiency peaks at l 262 nm, with a value reaching 48% at 100 V bias. As the bias voltage increases, the absorption region becomes fully depleted and the efficiency increases up to 15 V, where there is no significant change in the photoresponse efficiency between 15 and 100 V. Excellent device uniformity across the processed wafer lends itself well for photodetector array applications. 35
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Fig. 3 (colour). External quantum efficiency of the AlGaN MSM. The quantum efficiency at l = 262 nm is 47% at a reverse bias of 15 V
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Conclusion We have grown, fabricated and characterized high-quality AlGaN/GaN back-illuminated solar-blind p–i–n and MSM photodetectors on (0001) sapphire substrates. Uncorrected for reflections, etc., the p–i–n devices exhibit a responsivity of 0.058 A/W at l 279 nm with zero bias, corresponding to an external quantum efficiency of 26%. The MSM devices display an external quantum efficiency of 47% at l 262 nm with a 15 V bias. Both types of devices will find use in applications requiring operation in the solar-blind regime. Acknowledgements This work was partially supported by DARPA under contract N00014-9-1-0231 (E. J. Martinez) and ONR contract N00014-99-1-0304 (monitored by Y. S. Park).
References [1] D. J. H. Lambert, M. M. Wong, U. Chowdhury, C. J. Collins, T. Li, H. Kwon, B. Shelton, T. Zhu, J. C. Campbell, and R. D. Dupuis, Appl. Phys. Lett. 77, 1900 (2000). [2] C. J. Eiting, P. A. Grudowski, J. S. Park, D. Lambert, B. S. Shelton, and R. D. Dupuis, J. Electrochem. Soc. 144, L219 (1997). [3] C. J. Eiting, P. A. Grudowski, and R. D. Dupuis, J. Electron. Mater. 27, 206 (1998). [4] J. C. Carrano, T. Li, P. A. Grudowski, C. J. Eiting, R. D. Dupuis, and J. C. Campbell, J. Appl. Phys. 83, 6148 (1998). [5] J. C. Carrano, P. A. Grudowski, C. J. Eiting, R. D. Dupuis, and J. C. Campbell, Appl. Phys. Lett. 72, 1992 (1997).
