Letter pubs.acs.org/NanoLett
Self-Assembled GaN Nanowires on Diamond Fabian Schuster,*,† Florian Furtmayr,† Reza Zamani,‡,∥ Cesar Magén,§ Joan R. Morante,∥,⊥ Jordi Arbiol,‡,¶ Jose A. Garrido,† and Martin Stutzmann† †
Walter Schottky Institut, Technische Universität München, Am Coulombwall 4, 85748 Garching, Germany Institut de Ciència de Materials de Barcelona, ICMAB-CSIC, Campus de la UAB, 08193 Bellaterra, Spain § Laboratorio de Microscopías Avanzadas, Instituto de Nanociencia de Aragon-ARAID and Departamento de Física de la Materia Condensada, Universidad de Zaragoza, 50018 Zaragoza, Spain ∥ Catalonia Institute for Energy Research, IREC, 08930 Sant Adrià del Besòs, Spain ⊥ Department d’Electrònica, Universitat de Barcelona, 08028 Barcelona, Spain ¶ Institució Catalana de Recerca i Estudis Avançats (ICREA), 08010 Barcelona, Spain ‡
ABSTRACT: We demonstrate the nucleation of selfassembled, epitaxial GaN nanowires (NWs) on (111) singlecrystalline diamond without using a catalyst or buffer layer. The NWs show an excellent crystalline quality of the wurtzite crystal structure with m-plane faceting, a low defect density, and axial growth along the c-axis with N-face polarity, as shown by aberration corrected annular bright-field scanning transmission electron microscopy. X-ray diffraction confirms single domain growth with an in-plane epitaxial relationship of (101̅0)GaN ∥ (011̅)Diamond as well as some biaxial tensile strain induced by thermal expansion mismatch. In photoluminescence, a strong and sharp excitonic emission reveals excellent optical properties superior to state-of-the-art GaN NWs on silicon substrates. In combination with the high-quality diamond/NW interface, confirmed by high-resolution transmission electron microscopy measurements, these results underline the potential of p-type diamond/n-type nitride heterojunctions for efficient UV optoelectronic devices. KEYWORDS: Nanowires, GaN, nitrides, diamond, molecular beam epitaxy, heteroepitaxy
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instance, AlGaN/GaN high-electron-mobility transistors (HEMTs) show an improved performance when prepared on diamond substrates.7−11 A new approach for defect-free AlGaN on diamond is the growth of nanowires, which are expected to yield much better structural quality and do not suffer from thermal mismatch. This nucleation of high-quality nitrides directly on a diamond substrate represents a major step forward toward the fabrication of efficient p-diamond/n-nitride heterostructures. We currently explore two possible applications of nitride NWs on diamond. First, efficient optoelectronic devices operating in the UV spectral range, benefiting from the effective p-type doping as well as the high thermal conductivity of diamond. The NW configuration does not only provide defect-free AlGaN with excellent luminescence properties but can also be used as intrinsic cavities for strong light-matter coupling. Second, we target the field of quantum computation, where nitrogenvacancy (NV) centers in diamond currently attract much attention as potential qubits.12,13 To electrically control single NV centers, nitride NWs are ideal local contacts for applying
t is now almost ten years since the idea of combining the IIInitride material system with diamond substrates was investigated, mainly motivated by the lack of efficient p-type doping in the nitrides, especially with high aluminum content, and n-type doping in diamond, respectively. The complementary configuration of these wide bandgap materials, namely p-type boron-doped diamond and n-type silicon-doped AlGaN, is promising to form efficient heterodiodes with high carrier mobilities. Driven by this motivation, the growth of heteroepitaxial AlN films on diamond substrates was reported,1,2 resulting in the fabrication of the first heterobipolar diode emitting in the UV spectral range.2 However, the performance of these light-emitting diodes stayed behind the expectations due to numerous structural imperfections in the nitride films, especially in the interface region. Up to now, direct nucleation of GaN and AlN films on diamond remains challenging due to the high difference in the thermal expansion coefficients of GaN and diamond,3,4 which leads to cracking of the epitaxial film during cooling down of the samples from growth to room temperature. To circumvent this problem, an AlN nucleation layer is most commonly used,5,6 sometimes even followed by a strain engineering stack of AlN/GaN multilayers.7,8 As a consequence, the use of diamond substrates is currently limited to applications that do not benefit from diamond doping, but only use its high thermal conductivity. For © 2012 American Chemical Society
Received: November 3, 2011 Revised: April 16, 2012 Published: April 16, 2012 2199
dx.doi.org/10.1021/nl203872q | Nano Lett. 2012, 12, 2199−2204
Nano Letters
Letter
Figure 1. Tilted-view and top-view scanning electron microscopy images of as-grown GaN NWs on diamond after a growth duration of (a) 20, (b) 40, and (c) 180 min.
electric fields or for electrical read-out. To this end, the same lithography mask can be used for both position-controlled implantation of NV centers and self-aligned selective area epitaxy of single NWs. This can provide an interesting all solidstate alternative to the recently demonstrated electrical control of NV centers by an electrolytic contact.14 In this publication, we demonstrate that the controlled nucleation of self-assembled GaN NWs on diamond is possible. The structural and optical properties of these NWs and their heterointerface toward the diamond substrate are discussed. Experimental Section. Gallium nitride nanowires (NWs) were grown catalyst-free by plasma-assisted molecular beam epitaxy (PAMBE) on diamond substrates that have been purchased from Element Six. These type Ib single crystal diamonds were grown by high-pressure/high-temperature (HPHT) synthesis. They had a (111) surface orientation and an offcut angle of about 5°. The surface was polished to an atomically flat level (rms
