GaN/SiC and Al0.25Ga0.75 N/SiC heterojunction diodes were fabricated using Al/Ti for p-ohmic contact to the SiC and Ti/Al/Pt/Au for n-ohmic contact to the GaN and AlGaN. Annealing at 850 °C for 20 s (GaN) or 120 s (AlGaN) was required for achieving specific contact resistances in the 10−6Ωcm2 range. The reverse breakdown voltage showed a negative temperature coefficient in both types of sample, with value ∼5.5±2.5×10−3 V/K. The I–V characteristics of both heterojunctions show evidence of tunneling via defect states.

High-resolution x-ray diffraction patterns and 90 K microcathodoluminescence (MCL) spectra were taken for undoped, symmetric AlGaN/GaN superlattices (SLs) with GaN quantum-well (QW) widths of 35 Å and 80 Å. The short-period SL spectra were blue shifted by about 60 meV compared to the GaN substrate, and the magnitude of the blue shift was increased by about 20 meV by application of a reverse bias of −3 V (electric field of about 4 · 10 5 V/cm) to a Schottky diode prepared on this SL. A small red shift of about 40 meV compared to GaN was observed for the long-period SL. The two latter observations were interpreted as manifestations of the presence of a strong built-in piezoelectric field, giving rise to the quantum-confined Stark effect (QCSE). Partial disordering of the short-period SL was observed after Ar ion implantation (energy 150 keV, dose 8·10 13 cm −2 and 80 keV, 2·10 13 cm −2 ) and subsequent annealing at 1000°C for 3 h under the protective layer of Si 3 N 4 . However, it was observed that this partial disordering was accompanied by strain relaxation via formation of misfit dislocations or cracks.

The magnetization of p-GaN or p-AlGaN/GaN superlattices was measured after implantation with high doses (3-5 x 10(16) cm(-2)) of Mn, Fe, or Ni and subsequent annealing at 700-1000 degreesC. The samples showed ferromagnetic contributions below temperatures ranging from 190-250 K for Mn to 45-185 K for Ni and 80-250 K for Fe. The use of superlattices to enhance the hole concentration did not produce any change in ferromagnetic ordering temperature. No secondary phase formation was observed by x-ray diffraction, transmission electron microscopy, or selected area diffraction pattem analysis for the doses we employed. (C) 2002 American Vacuum Society.

The magnetization of p-GaN or p-AlGaN/GaN superlattices was measured after implantation with high doses (3–5×1016 cm−2) of Mn, Fe, or Ni and subsequent annealing at 700–1000 °C. The samples showed ferromagnetic contributions below temperatures ranging from 190–250 K for Mn to 45–185 K for Ni and 80–250 K for Fe. The use of superlattices to enhance the hole concentration did not produce any change in ferromagnetic ordering temperature. No secondary phase formation was observed by x-ray diffraction, transmission electron microscopy, or selected area diffraction pattern analysis for the doses we employed.

Effects of UV/O 3 or deuterium plasma treatment, of annealing in air at 550 °C, of annealing in N 2 at 500 °C and various combinations of these treatments on leakage current and resistance in the forward direction of GaN/InGaN multiquantum-well light emitting diodes MQW LEDs were studied. It was shown that the best results are achieved with 5 minutes long UV/O 3 treatment. LED structures thus prepared showed effects of strong tunneling in their I-V characteristics. The space charge region was shown to be located in the GaN/InGaN superlattice SL. Passing of moderately high forward current through the structure for several hours enhanced the overall tunneling through the structure and facilitated faster tunneling between the layers in the GaN/InGaN SL.

Multi-quantum-well GaN/InGaN heterojunction diodes prepared by metalorganic chemical vapor deposition on sapphire showed effects of strong tunneling in their I–V characteristics. The space charge region was shown to be located in the GaN/InGaN superlattice (SL). The injection of moderately high forward currents through the structure for several hours enhanced the overall tunneling through the structure and facilitated faster tunneling between the layers in the GaN/InGaN SL. These results may have relevance to the aging characteristics of light-emitting diodes under bias.

The properties of n-GaN/p-SiC and n-AlGaN/p-SiC heterojunctions (HJ) prepared by hydride vapor phase epitaxy (HVPE) on 4H SiC substrates are reported. It is shown that the GaN/p-SiC HJ is staggered type II with the conduction bandoffset and the valence bandoffset values, respectively, DeltaE(c)=-0.49 eV and DeltaE(v)=0.65 eV. When changing GaN for AlGaN with Al mole fraction of x=0.25-0.3 the band alignment becomes normal type I with DeltaE(c)=0.2 eV and DeltaE(v)=0.6 eV. Current-voltage characteristics of both heterojunctions bear evidence of strong tunneling via defect states. The tunneling was found to be more pronounced in the AlGaN/SiC HJs even though these showed no evidence of formation of dark line defects at the interface, in contrast to GaN/SiC. (C) 2002 American Institute of Physics.

Electrical and optical properties of modulation-doped p-AlGaN/GaN superlattices are compared to those of similarly doped p-GaN films. It is shown that modulation doping increases the sheet hole concentration by several times. In p-AlGaN/GaN superlattices grown on GaN underlayers, this increase is accompanied by a significant increase in hole mobility which results in a remarkable decrease in sheet resistivity of the structure compared to p-GaN films and this decrease in sheet resistivity should hold up to temperatures exceeding 350 °C. For superlattices prepared on AlGaN underlayers, the mobility decreases compared to p-GaN. In such superlattices, one also observes a strong redshift and a strong broadening of the band edge luminescence peak coming most probably from increased mosaicity and strain which would also explain the observed deterioration of mobility. The magnitude of the redshift in the position of the band edge luminescence band slightly increased upon application of reverse bias which is interpreted as a manifestation of quantum-confined Stark effect.

Properties of n-GaN/p-SiC and n-AlGaN/p-SiC heterojunctions prepared by HVPE on 4H SiC substrates were studied by means of C-V, C/G-T, C-f, I-V and DLTS. It is shown, in agrrement with earlier publications, that the GaN/p-SiC HJ is staggered type II with ∆Ec=−0.4 eV and ∆Ev=0.6 eV. When changing GaN for AlGaN with Al mole fraction of x=0.25–0.3 the band alignment becomes normal type I with ∆Ec=0.2 eV and ∆Ev=0.6 eV. I-V characteristics of both heterojunctions bear evidence of strong tunneling via defect states, particularly centers with activation energy of 1.25 eV for GaN/4H SiC HJ. The tunneling was found to be more pronounced in the AlGaN/SiC HJs even though these HJs showed no evidence of formation of dark line defects at the interface, in contrast to GaN/SiC. DLTS measurements on both types of HJs revealed the presence of broad bands whose behavior is indicative of these bands being related to continuous states in the gap, most likely near the nitride/carbide interface.

The properties of p-AlGaN/GaN modulation doped superlattices (SLS) prepared by molecular beam epitaxy were studied by means of conductivity versus temperature, admittance spectroscopy, photoinduced current spectroscopy, microcathodoluminescence (MCL) spectra measurements, and measurements of effective diffusion lengths. It is shown that in SLs grown on GaN underlayers the sheet resistivity is about two orders of magnitude lower than for reference p-GaN films and the resistivity of SLs remains lower up to temperatures of about 350 degreesC. For SLS grown on AlGaN underlayers the gain in resistivity is much more moderate and certain advantages in using such SLs are envisaged only for temperatures below room temperature. The reason for this lower gain is a considerable decrease in hole mobility compared to p-GaN. The effect is somewhat tentatively attributed to worse crystalline perfection of these SLs. It is also shown that such SLs are characterized by a strongly broadened MCL peak and the presence of additional hole traps with activation energy of about 0.4 eV. Despite that, the photosensitivity and MCL intensity of these SLs are much better than for reference p-GaN samples. (C) 2001 American Institute of Physics.

The electronic band gap of single crystalline ZnGeN2 epitaxial layer grown on sapphire substrate by metal organic chemical vapor deposition has been measured by optical transmission and room temperature photoluminescence. The band gap energy is 2.99eV at room temperature, and the band gap is a direct transition type. The interference oscillations of the transmission spectra together with rutile prism coupling measurements have been used to determine the r fractive index and the dispersion characteristics of the single crystal ZnGeN2 below the band gap energy. The rutile prism coupling measurement displays the wave guide modes of the film at 6 2.8nm wavelength of the He-Ne laser, enabling determination of the film thickness and refractive index precisely at the wavelength. The refractive index of ZnGeN2 crystal is 2.35 at 6328Å wavelength. The measured refractive index dispersion curve can be fitted with the first-order Sellmeier equation n2(λ) = A + λ2/(λ2-B), using fitting parameters A=4.3 1, B=0.076.

Mg-doped superlattices consisting of uniformly doped AlxGa1−xN and GaN layers are analyzed by Hall-effect measurements. Acceptor activation energies of 70 meV and 58 meV are obtained for superlattice structures with an Al mole fraction ofx = 0.10 and 0.20 in the barrier layers, respectively. These energies are significantly lower than the activation energy measured for Mg-doped GaN thin films. At room temperature, the doped superlattices have free hole concentrations of 2 × 1018 cm−3 and 4 × 1018 cm−3 forx = 0.10 and 0.20, respectively. The increase in hole concentration with Al content of the superlattice is consistent with theory. The room temperature conductivity measured for the superlattice structures are 0.27 S/cm and 0.64 S/cm for an Al mole fraction ofx = 0.10 and 0.20, respectively.

Experimental and theoretical results of Mg-doped superlattices consisting of uniformly doped AlxGa1−xN, and GaN layers are presented. Acceptor activation energies of 70 and 58 meV are obtained for superlattice structures with an Al mole fraction of x=0.10 and 0.20 in the barrier layers, respectively. These energies are significantly lower than the activation energy measured for Mg-doped bulk GaN. At room temperature, the doped superlattices have free-hole concentrations of 2×1018 cm−3 and 4×1018 cm−3 for x=0.10 and 0.20, respectively. The increase in hole concentration with Al content of the superlattice is consistent with theory. The room temperature conductivity measured for the superlattice structures is 0.27 S/cm and 0.64 S/cm for an Al mole fraction of x=0.10 and 0.20, respectively. X-ray rocking curve data indicate excellent structural properties of the superlattices. We discuss the origin of the enhanced doping, including the role of the superlattice and piezoelectric effects. The transport properties of the superlattice normal and parallel to the superlattice planes are analyzed. In particular, the transition from a nonuniform to a uniform current distribution (current crowding) occurring in the vicinity of contacts is presented. This analysis provides a transition length of a few microns required to obtain a uniform current distribution within the superlattice structure.

Enhanced acceptor activation, reduced acceptor binding energy and enhanced conductivity are demonstrated in AlxGa1-xN/GaN doped superlattice structures. An acceptor activation energy of 58meV is demonstrated in an Al0.20Ga0.80N/GaN superlattice structure with a period of 200 Angstrom. This value is significantly lower than the 200meV activation energy measured in bulk GaN. The dependence of activation energy on the Al content of the superlattice is consistent with that predicted by the theoretical model. The demonstration of improved p-type doping characteristics in GaN is expected to enable the realisation of electronic and optoelectronic devices with improved properties.

Enhanced acceptor activation, reduced acceptor binding energy and enhanced conductivity are demonstrated in AlxGa1–xN/GaN doped superlattice structures. An acceptor activation energy of 58 meV is demonstrated in an Al0.20Ga0.80N/GaN superlattice structure with a period of 200 A. This value is significantly lower than the 200 meV activation energy measured in bulk GaN. The dependence of activation energy on the Al content of the superlattice is consistent with that predicted by the theoretical model. The demonstration of improved p-type doping characteristics in GaN is expected to enable the realisation of electronic and optoelectronic devices with improved properties.