Optical and Electrical Properties of MBE Grown Cubic GaN/GaAs Epilayers Doped by Si
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luminescence measurements are performed at 300 K and in a He bath cryostat at 2 K. The luminescence was excited by a cw HeCd UV laser with a power of 3 mW and measured in a standard PL system. Hall-effect measurements were performed using square shaped samples at 300 K at a magnetic field of 0.3 T and with the samples in the dark. In was used for ohmic contacts. RESULTS AND DISCUSSION The incorporation of Si into c-GaN has been studied by SIMS. In Fig.1 the depth profiles of Si, GaN and As are depicted for a cubic GaN epilayer doped with elemental Si at a Si source temperature of TSi = 1075°C. A homogeneous Si distribution is measured throughout the whole c-GaN epilayer and no accumulation neither at the interface nor at the surface is observed. The Si-concentration measured at a depth of about 0.4 µ m (indicated by the arrow in 10 As O Fig.1) is 2*1019 cm-3. In Fig. 2 the Si-concentration measured by G aAs10 buffe r SIMS (full squares) and the free electron concentration measured SiO c -GaN film 10 by Hall-effect at room temperature (full triangles) are Ga N plotted versus the Si source 10 0 0.2 0.4 0.6 0.8 1 1.2 temperature. As clearly can be depth [µm] seen, both the free electron concentration and the amount of incorporated Si exactly follow Figure 1. SIMS depth profiles of a Si doped the Si-vapor pressure curve (full line in Fig.2) at TSi > 1000°C [9]. 20 -6 This indicates that nearly all Si 10 10 atoms are incorporated at Ga Si - vap or p ressure sites and act as shallow donors. 19 -7 10 10 At room temperature the maximum free electron -8 18 10 10 concentration reached so far is about 5*1019 cm-3. This clearly -9 17 10 10 demonstrates the ability of controlled n-type doping of -10 16 S IM S 10 10 cubic GaN by Si up to Hall PL concentrations which are -11 15 10 10 necessary for the fabrications of 800 900 1000 1100 laser diodes. tem perature of S i-source (°C) At room temperature the Figure 2. Si concentration measured by SIMS integrated intensity of the (full squares), free electron concentration luminescence (full dots in Fig.2) measured by Hall-effect (full triangles) and also follows the Si-vapor integrated PL intensity (full circles) at 300 K pressure curve, indicating that at versus Si source temperature. The full line 300 K the optical properties are represents the vapor pressure curve of Si [9]. also determined by the Si-doping T = 1075 °C Si
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and that even at the highest free carrier concentration of 5*1019 cm-3 no (D A ) shift E quenching of the luminescence is observed. The optical properties of Si Si-flux (c m s ) doped cubic GaN at low temperature are 5.2*10 2.5*10 shown in Fig.3. At 2 K the spectrum of 1.2*10 5.7*10 the sample grown with the lowest Si-flux 2.6*10 (8.5*105 cm-2s-1, TSi = 750°C) is 4.6*10 7.2*10 dominated either by the excitonic 9.4*10 transition X at 3.26 eV or by the donor1.0*10 8.5*10 acceptor pair transition (D°, A°) at 3.15 eV [10]. With increasing
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