As-grown GaN Layers Research Articles

Surface Contamination with Si and O Impurities for GaN Single-Crystalline Layers Grown by Ultra-High-Vacuum Sputter Epitaxy

Introduction GaN and related alloys have been extensively investigated for their potential applications in optoelectronic and high power electronic devices. The study on surface contamination with Si and O impurities is quite important for device fabrication, especially for Si, because Si is n-type impurity for GaN. In this work, we demonstrate the GaN surface contamination and cleaning technique focused on O and Si based compounds contaminated in air atmosphere. Experiment GaN single-crystalline layers were grown on sapphire (0001) substrates by ultra-high-vacuum radio-frequency sputter epitaxy technique. The thermal desorption spectroscopy (TDS) measurement was carried out for as-grown GaN layers at pressure of 10– 4 Pa, and the chemical etching was also carried out using buffered hydrogen fluoride (BHF) solution. The surface state of the GaN layer was analyzed by X-ray photoelectron spectroscopy (XPS) measurement. Results In the XPS measurements, O1s and Si2p XPS signals were detected from as-grown GaN layer surface. The thermal treatment was then carried out in order to remove these impurities, and the results on XPS measurements indicated the significant decrease of Ga–O bonds on the GaN layer surface. Moreover, from the TDS measurements, the signal of m/z = 70 corresponding to Ga (Figure 1(a)) and the signal of m/z = 32 corresponding to O2 (Figure 1(b)) indicated rapid increase at around 500 °C, which suggests that isolated Ga and O atoms were easily released from the GaN layer surface. On the other hand, the removal of Si compounds by the thermal treatment was quite difficult, because the thermally stable Si–N bonds which would be formed by nitrogen dangling bonds, were terminated with Si atoms during such thermal treatment. The chemical etching was very effective in order to remove the Si contamination, and after the BHF etching treatment the decrease of Si compounds on GaN layer surface was confirmed by the great decrease of Si2p/Ga3d XPS signal intensity ratio. After the chemical etching treatment, the GaN layer was then exposed in air atmosphere. Two kinds of methods were utilized for the air exposure; sample was put in open air (case 1), and also was put at the inside of closed narrow space (case 2) limiting the concentrations of impurities reaching onto the sample surface. Figure 2 shows the Si2p/Ga3d XPS signal intensity ratio depending on an elapsed time in air exposure. At the case 1, the rapid increase of the intensity ratio was observed within initial 120 minutes, and then it indicated the gradual increase. On the other hand, at the case 2, the intensity ratios were always lower than those at the case 1. Therefore, the storage of the sample in air atmosphere is the important factor for Si contamination. It seems that the grade of O contamination depends greatly on temperature and humidity in air atmosphere. The results in GaN layer grown by metal-organic chemical vapor deposition method will be also presented. Acknowledgement A part of this work was conducted at Advanced Characterization Nanotechnology Platform of the University of Tokyo, supported by "Nanotechnology Platform" of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. Figure 1

Spatially resolved cathodoluminescence of GaN nanostructures fabricated by photoelectrochemical etching

The emission properties of GaN nanostructures created by photoelectrochemical etching have been investigated by cathodoluminescence (CL) in the scanning electron microscope. Columnar structures with diameters of 150–250 nm formed near the surface of the as-grown GaN layers branch into nanowires with diameters of 20–60 nm, while islands with coral-like relief were observed at the bottom of the etched areas. CL emission of the observed nanostructures is dominated by free electron to acceptor transitions. Local CL spectra provide direct evidence of the existence of either compressive or tensile stress in different nanostructures. No free exciton luminescence was observed in GaN nanowires, supporting their relation to threading dislocations.

SAW characteristics of GaN layers with surfaces exposed by dry etching

We evaluated the possibility of monolithic integration of electron devices and surface acoustic wave (SAW) devices on GaN. We removed top n+ GaN layers of n+ GaN/unintentionally-doped GaN structures by inductively coupled plasma (ICP) etching and fabricated SAW filters on the exposed unintentionally doped GaN layers. We found that the device characteristics are almost the same as those of devices fabricated on as-grown GaN layers, although the surface morphology of GaN layers is degraded due to the ICP etching. The results indicate that SAW devices and electron devices can be monolithically integrated on GaN-based semiconductor structures.

Reactivation and passivation of theEc - 0.61 eV deep level in GaN

Deep level transient spectroscopy (DLTS) measurements on as-grown GaN layers on sapphire substrates commonly reveal three dominant traps, viz. the 0.65 eV, 0.61 eV and 0.27 eV levels below the conduction band. These deep levels are normally attributed to the native defects that are present in the as-grown layers. We have previously observed that the Ec – 0.61 eV level behaves in an almost similar manner to bistable defects during bias-on/bias-off anneals. It is proposed that this is due to the presence of a negatively charged mobile species in the material that drifts under the influence of an electric field and is able to passivated the Ec – 0.61 eV level. Defect profiles of the level under zero bias and under different reverse bias conditions showed changes in the defect profile distributions that further confirms the mobile defect/ion theory. The reactivation kinetics of the Ec – 0.61 eV defect level has also been studied and results indicate that the energy required to reactivate the level is EReac = 0.96 ± 0.03 eV with an attempt frequency ν0 = 6 × 1010±1 s–1, while the reverse transformation of this level requires EPass = 0.93 ± 0.06 eV and an attempt frequency of ν0 = 7 × 1011±1 s–1. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

P-type GaN epitaxial layers and AlGaN/GaN heterostructures with high hole concentration and mobility grown by HVPE

ABSTRACTIn this paper we report p-GaN growth by hydride vapor phase epitaxy (HVPE) on sapphire substrates. Mg or Zn impurities were used for doping. Layer thickness ranged from 2 to 5 microns. For both impurities, as-grown GaN layers had p-type conductivity. Concentration NA-ND was varied from 1016 to 1018 cm−3. An annealing procedure at 750°C in argon ambient typically increased the concentration NA-ND in 1.5–3.5 times. For Mg doped GaN layers, room temperature hole mobility of 80 cm2V−1s−1 was measured by conventional Van Der Pau Hall effect technique for material having hole concentration of about 1x1018 cm−3. Initial results on highly electrically conducting p-type AlGaN/GaN heterostructures doped with Zn are also reported.

Stability investigation of cubic GaN films grown by metalorganic chemical vapor deposition on GaAs (001)

The thermal stability of cubic-phase GaN (c-GaN) films are investigated by photoluminescence (PL) and Raman scattering spectroscopy. C-GaN films are grown on GaAs (001) substrates by metalorganic chemical vapor deposition. PL measurements show that the near-band-edge emissions in the as-grown GaN layers and thermally treated samples are mainly from c-GaN. No degradation of the optical qualities is observed after thermal annealing. Raman scattering spectroscopy shows that the intensity of the E2 peak from hexagonal GaN grains increases with annealing temperature for the samples with poor crystal quality, while thermal annealing up to 1000 °C has no obvious effect on the samples with high crystal quality.

Improved quality GaN by growth on compliant silicon-on-insulator substrates using metalorganic chemical vapor deposition

The use of compliant silicon-on-insulator (SOI) substrates instead of Si substrates is shown to improve the quality of epitaxial GaN layers by releasing the strain and absorbing the generated threading dislocations in the thin Si overlay of the SOI substrate. GaN layers have been grown on SOI substrates by low-pressure metalorganic chemical vapor deposition and various growth conditions and compared with GaN layers grown on Si substrates. Crystal uniformity, surface morphology, and number of threading dislocations of GaN layers grown on SOI substrates are improved compared to layers grown directly on Si substrates as evidenced by x-ray diffraction spectroscopy (XRD) and transmission electron microscopy. Full width at half maximum XRD values improved from 672 to 378 arcsec by growth on SOI instead of Si substrates. The GaN layers grown directly on Si substrates are highly resistive while all as-grown GaN layers on SOI substrates are unintentionally n type. For a 1–2 μm thick GaN layer grown on SOI, the electron mobility is typically in the order of 102 cm2/V s with a background carrier concentration in the range of 1.5–5.0×1017 cm−3.

GaN PN-Structures Grown by Hydride Vapor Phase Epitaxy

AbstractFor the first time, GaN pn-junctions were fabricated by hydride vapor phase epitaxy. GaN pn-structures were grown directly on 6H-SiC substrates without any buffer layer. Undoped GaN layers were n-type with Nd-Naconcentration ranged from 1×1017to 5×1018cm−3. Magnesium was used as an acceptor to grow p-type GaN layers. Mg atomic concentration determined by secondary ion mass spectroscopy ranged from 5×1019to 5×1020cm−3. As-grown GaN layers doped with Mg were p-type, and p-type conductivity was improved by post-growth anneal. Mesa diodes with a vertical current flow geometry were formed by reactive ion etching. The position of the GaN pn-junction was determined by the electron beam induced current method. The electrical characteristics of the pn diodes were studied. Electroluminescence from the pn diodes was measured.