Proposal to Use GaAs(114) Substrates for Improvement of the Optical Transition Probability in Nitride Semiconductor Quan
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Proposal to Use GaAs(114) Substrates for Improvement of the Optical Transition Probability in Nitride Semiconductor Quantum Wells Mitsuru Funato, Yoshinobu Kawaguchi, and Shigeo Fujita Kyoto University, Department of Electronic Science and Engineering, Kyoto 615-8501, Japan ABSTRACT The dependence of the spontaneous emission lifetime of excitons in InGaN/GaN quantum disks (QDs) on the crystalline orientation is calculated. For 1-nm-thick QDs, it is found that the lifetime in the conventional c-oriented QDs is ten times as long as that in QDs tilted by 30° and 90°, and that the difference is pronounced by increasing the QDs thickness. This is totally due to the presence of the electric field in strained InGaN. Taking into account our preceding study, in which it was revealed that GaN on GaAs(114) was titled by 30°, we propose the use of GaAs(114) as a substrate for nitride light emitting devices to improve the optical transition probability. INTRODUCTION A key structure in nitride light emitters is quantum wells (QWs), and all the commercially available devices consist of QWs such as InGaN/GaN QWs [1]. Since the lattice parameters of GaN, InN, and AlN are different from each other, interesting phenomena due to the lattice mismatch are often observed in the nitride QWs. For example, the large lattice mismatch of 11% between GaN and InN makes it difficult to grow uniformly mixed InGaN alloys [2], and consequently, fluctuations in the alloy composition form quantum-dot-like regions in InGaN QWs, which have been found to work as highly efficient luminescence centers [3,4]. Also, the nitride QWs are basically c-oriented and strained, and therefore, the QWs inevitably experience a strain-induced piezoelectric polarization. The piezoelectric polarization together with the spontaneous polarization induces electric fields in the QWs, which disturb carrier recombination and, as a result, reduce the optical transition probability [5,6]. Therefore it is quite important to reduce the piezoelectric field to achieve higher emission efficiency. A clue for this can be found in Ref. [7], where the variation of the piezoelectric field as a function of the tilt of the c-axis was calculated and it is clearly shown that the piezoelectric field in strained InGaN on GaN can be zero at the tilt of 39° and 90°. This calculation suggests that InGaN/GaN QWs tilted by 39° or 90° can realize higher emission efficiency. On the other hand, we have been investigating metalorganic vapor phase epitaxy (MOVPE) of GaN on GaAs(11n) substrates (n = 1, 2, 3, 4, 8), and found that the superior structural and optical quality was obtained on the (114) substrate [8]. Much more interestingly, it was found that the c-axis of GaN grown on GaAs(114) was tilted by 30°. This tilt angle is not equal to 39°, at which the piezoelectric field is zero as described above, though is expected to bring about a weak electric field, compared with the conventional c-oriented heterostructures. The film quality on GaAs(114) is, in fact, inferior to that on sapphire(0001
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