P- and N-type Doping of Non-Polar A-plane GaN Grown by Molecular-Beam Epitaxy on R-plane Sapphire
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Internet Journal Nitride Semiconductor Research
P- and N-type Doping of Non-Polar A-plane GaN Grown by Molecular-Beam Epitaxy on R-plane Sapphire R. Armitage1, Qing Yang1 and Eicke R. Weber 1 1Materials
Science Division, Lawrence Berkeley National Laboratory, Berkeley CA 94720,
(Received Sunday, August 3, 2003; accepted Wednesday, September 3, 2003)
Non-polar a-plane GaN films doped with Si or Mg were grown by plasma-assisted molecular-beam epitaxy on r-plane sapphire substrates. The (1120) orientation of the GaN epilayers was confirmed by x-ray diffraction. The layers were further characterized by atomic force microscopy, Hall effect, and photoluminescence measurements. The Mg-doped layers showed p-type conductivity, with a maximum hole concentration of 6×1017 cm-3 (µ = 2 cm2/Vs). Comparison with Mg-doping of Npolar c-plane GaN suggests the Mg sticking coefficient may be higher on the GaN (1120) surface compared to the GaN (0001) surface. The electron mobility obtained for a-plane GaN:Si (18 cm2/ Vs for n = 1×1018 cm-3) was low compared to that of typical c-plane epilayers. The lower electron mobility is attributed to the higher density of structural defects in a-plane GaN.
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Introduction
Almost all research on GaN to date has involved c-axis oriented wurtzite epilayers. With the polar c-axis aligned perpendicular to the film surface, the spontaneous and piezoelectric polarization effects cause strong internal electric fields at nitride heterointerfaces, e.g. InGaN/GaN quantum wells used for visible light emission. Built-in electric fields perpendicular to the heterointerface are generally undesirable for optoelectronic devices, as they cause separation of the electron and hole wavefunctions. The reduced overlap between these wavefunctions increases the carrier radiative lifetime and hence reduces the internal quantum efficiency. To get around this problem, much attention has been focused on growth of the zincblende polytype of GaN, which is non-polar along the cubic [0 0 1] axis and therefore does not exhibit significant built-in electric fields at heterointerfaces. Unfortunately, the zincblende GaN polytype is metastable and it can only be grown successfully for a very narrow window of process conditions [1]. The difficulties in growth of zincblende GaN have hindered its further development for device applications. Waltereit et al. [2] proposed a different idea to eliminate the internal electric fields due to polarization effects. Using LiAlO2 substrates these authors grew wurtzite GaN epilayers with m-plane rather than the usual c-plane orientation. Wurtzite GaN epilayers with
m-plane or a-plane orientation can be called “nonpolar” since the polar c-axis is parallel to the film surface. For this crystal orientation no significant built-in electric field exists within the quantum well. The benefits of using non-polar GaN to eliminate internal electric fields was made clear by the work of Ng, [3] who compared the photoluminescence (PL) from AlGaN/GaN quantum wells grown in both the nonpolar a-plane (on r-
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