We report on a quantitative identification of the shallow oxygen donor in GaN, compare the defects which are detected by optically detected magnetic resonance in the yellow and red emission bands in nominally undoped GaN, and demonstrate that independent of the doping level the microscopic structure of the Mg acceptor remains the same. There is a minor influence on the magnetic resonance parameters of Mg, which comes from a modification of the strain and changes by Mg incorporation. With increasing Al content in the Al x Ga 1− x N alloy the g -values become isotropic. The compensating centers are most likely vacancy-type complexes.

We report on temperature-dependent Hall-effect measurements and secondary ion mass spectroscopy on unintentionally doped, n-type conducting GaN epitaxial films. Over a wide range of free carrier concentrations we find a good correlation between the Hall measurements and the atomic oxygen concentration. We observe an increase of the oxygen concentration close to the interface between the film and the sapphire substrate, which is typical for the growth technique used (synthesis from galliumtrichloride and ammonia). It produces a degenerate n-type layer of ≈1.5 μm thickness and results in a temperature-independent mobility and Hall concentration at low temperatures (<50 K). The gradient in free carrier concentration can also be seen in spatially resolved Raman and cathodoluminescence experiments. Based on the temperature dependence of the Hall-effect, Fourier transform infrared absorption experiments, and photoluminescence we come to the conclusion that oxygen produces a shallow donor level with a binding energy comparable to the shallow Si donor.

Heterojunction bipolar transistors (HJBT) on the basis of GaN/SiC heterostructures have several advantages over group III nitride heterostructures grown on sapphire. For example 6H-SiC has less of a thermal and structural mismatch to GaN than sapphire has. Furthermore there is not the problem of optical recombination in a highly doped base region as there is for the group III nitrides on account of their direct bandgap.However, despite the well acknowledged potential of the GaN/SiC material system there are still unanswered questions relating to the technology used as well as to some fundamental properties of GaN/SiC heterostructures. Therefore we investigated epitaxial growth and physical properties of n-GaN/p-SiC heterojunctions with respect to their significance to n-GaN/p-SiC/n-SiC HJBT.We grew n-type GaN (n = 10(18) cm(-3)) on p-type (p = 2 x 10(18) cm(-3)) and n-type (II = 4 x 10(18) cm(-3)) 6H-SiC substrates in a horizontal hot wall reactor. This approach is very similar to the more common HVPE. Instead of synthesizing GaCl in situ from HCl and metallic Ga we used GaCl3 as the Ga precursor. All our experiments were carried out at low pressures around I mbar resulting in a good homogeneity. As it is common for the more usual HVPE we grew GaN without a buffer layer.From thermal admittance spectroscopy (TAS) as well as temperature dependent I-V characteristics we gained knowledge about deep level defects and the role of interface traps. The microstructure of the interface was investigated by transmission electron microscopy (TEM).Furthermore we present details about device processing by ion beam sputter etching with carbon dioxide (CO2) as the working gas. (C) 2000 Elsevier Science Ltd. All rights reserved.

Vibrational modes are observed at ambient pressure in O-doped GaN at 544 cm(-1) using Raman spectroscopy. Investigation of these modes with applied hydrostatic pressure reveals the existence of three closely spaced modes that shift in relative intensity with increasing pressure. Notably, transitions between the different modes occur at previously observed electronic transitions associated with the DX-like center behavior of substitutional O on the N site. A simple one-dimensional oscillator model is used to extract approximate force constants; these are consistent with the assignment of the 544 cm(-1) mode to ON and with force constants for C and B dopants in GaP. The relative intensity changes observed at 11 and 17 GPa an assigned to changes in the charge state due to the merging of the +/0 ionization level and the Fermi energy and the transition to DX- that causes a previously observed drop in free electron concentration, respectively.

Vibrational modes in O-doped GaN have been observed at 544 cm−1 in Raman spectroscopy. Under perturbation of large hydrostatic pressure the mode appears as a set of three different lines Q1⋯3 whose relative intensities change by pressure. A switching between the modes occurs near 10 and 20 GPa and is found to correlate with the electron capture process to the DX-like state of O. We employ a simple oscillator model to predict the vibrational frequencies of ON. A localization energy of 23 cm−1 with respect to the optical phonon band is predicted. This is in reasonable agreement with the observed vibrational frequencies. Therefore, we assign the Q modes to the local vibration of O on N site in GaN. Modes Q1⋯3 are tentatively assigned to three different charge states of the O defect center.

SiC/GaN p-n and n-n heterostructures grown by low pressure chemical vapor deposition were investigated using thermal admittance spectroscopy. Different kinds of defects were isolated and located. Evidence of a distribution of defects at the p-SiC/n-GaN interface is given as having thermal activation energies of (87±3) meV at 5 V and (72±4) meV at 8 V bias. Additionally, three bulk defects with activation energies between 155 and 175 meV were found. By comparison with admittance spectra of the p-type SiC substrate, one level was identified as Al acceptor in SiC, whereas the other defects are electron traps in the GaN layer.

The photocurrent decay in n-type GaN films prepared by low-pressure chemical vapor deposition (LPCVD) was measured in the ms-to-s time range using steady-state UV light and in the μs time regime using short high-power pulses from higher harmonics of a Nd:YAG laser. A power law time dependence is observed with exponents ranging from −0.1 to −0.3, which is an indication of a broad distribution of trapping states inside the band gap. Combining Hall effect results and the magnitude of the initial slope of the photocurrent decay we estimate a mobility-lifetime product of 2.1×10 −4 cm 2 /V for photogenerated electrons at times below a few μs. Slow transients might be a handicap for applications of GaN in UV detectors.

We report on the photoconductivity (PC) of AlxGa1—xN on GaN epitaxial films grown by metal organic vapor phase epitaxy (MOVPE) with Al contents between x = 0% and 18%. The concentration of the free carriers and mobilities of majority carriers are derived from Hall effect measurements. Temperature dependent photoconductivity measurements between 4.2 and 500 K have been performed. The ratio between the photoconductivity for sub-bandgap excitation and that for excitation in the excitonic region drops down with increasing temperature to a value around 105 due to a decreasing relevance of electronic traps. Time transients after sub-bandgap and UV-excitation exhibit a prolonged non-exponential decay of photoconductivity. We find a power law delay for the time transient of the photocurrent Iph(t) ∼ t—m (m = 0.27 to 0.33) which can be explained in the picture of bandtails.

We present data on the optical and electrical properties of GaN films grown by the low pressure chemical vapor deposition technique on sapphire substrates. The bound exciton line width scales with the free carrier concentration of the n-type films. SIMS analysis showed that the concentrations of oxygen and carbon are high enough to explain the transport data and compensation.

We studied the recombination at about 3.410eV in nominally undoped GaN and highly oxygen-doped GaN by photoluminescence (PL) and cathodoluminescence (CL). For this line we find an upper limit for the thermal activation energy of EA⩽21±3meV. In time-resolved PL, lifetimes of 300–480ps are observed indicating excitonic recombination. The defect giving rise to this emission can be created by Ar+ ion implantation. In conclusion, the 3.410eV luminescence is attributed to excitons bound to structural defects in hexagonal GaN.

GaN films were grown epitaxially on sapphire and 6H-SiC substrates by low-pressure chemical vapor deposition (LPCVD) using gallium(III) chloride and ammonia as gallium and nitrogen precursor, respectively. The properties of these samples were examined by X-ray diffraction, Hall effect measurements, secondary-ion mass spectroscopy (SIMS) and photoluminescence (PL). All GaN layers exhibit high free carrier concentrations between n=2×1018 and 5×1019cm−3 caused by unintentional n-type doping. We provide evidence that this high unintentionally doping is linked to the oxygen content in the films. The correlation between the optical properties with respect to position and line shape of the band-edge luminescence and the electrical properties, i.e. the free carrier concentration, is discussed.

GaN films grown on (0001) 6H–SiC and (0001) Al2O3 substrates using low-pressure chemical vapor deposition with GaCl3 and NH3 as precursors are comparatively explored by optical, scanning tunneling, and transmission electron microscopy. Independent of the substrate material used, the surface of the GaN layers is covered by hexagonally shaped islands. For GaN on 6H–SiC, the islands are larger in diameter (≈50 μm) and rather uniformly distributed. An atomically flat interface is observed for GaN on Al2O3 in contrast to GaN grown on 6H–SiC, where the interface is characterized by large steps. For both substrates, faceted holes (named as pinholes) are observed in near-surface regions of the GaN layers occurring with a density of about 7×108 cm−2. No unequivocal correlation between the density of pinholes and the density of threading dislocations (≈1.6×1010 cm−2 for GaN/Al2O3 and ≈4×109 cm−2 for GaN/6H–SiC) can be found. Rather, different types of defects are identified to be correlated with the pinholes, implying a dislocation-independent mechanism for the pinhole formation. Despite the small lattice mismatch between GaN and 6H–SiC, the pronounced original surface roughness of this substrate material is believed to account for both the marked interfacial roughness and the still existing high density of threading dislocations.

We report on the optical, electrical and structural properties of GaN films heteroepitaxially grown by low pressure chemical vapor deposition on 6H-SiC substrates. We employed photoluminescence (PL), Hall effect measurements, scanning tunneling microscopy (STM) and X-ray analysis to determine the quality of our films. Heterojunction diodes were fabricated on p-type SiC and characterized by temperature dependent current–voltage and capacitance–voltage techniques. The results are interpreted within the thermionic emission model and the barrier found is attributed to the conduction band offset between 6H-SiC and wurtzite GaN. The diodes show electroluminescence of the donor-acceptor pair recombination type of 6H-SiC at room temperature. By analysis of the injection behavior we can interpret our data, determining the high valence band offset between 6H-SiC and α-GaN to 0.67 eV. This high valence band offset favors applications for hetero-bipolar transistors (HBT).

The microstructure and morphology of hexagonal GaN crystallites grown on c-axis sapphire substrates by low pressure chemical vapor deposition is correlated with the luminescence efficiency and emission wavelength. Microscopic variation of local band gap monitored by the luminescence wavelength on the a- and c-planes of hexagonal GaN-crystallites are directly mapped by means of low temperature scanning cathodoluminescence (CL) and CL wavelength imaging (CLWI). Beside minor fluctuations from crystallite to crystallite, the a-planes show pronounced red shift of emission energy of more than 132 meV with respect to the luminescence from the c-planes. The c-plane itself shows additional inhomogeneity on a micron scale. Strongly red shifted luminescence (λ>400 nm) originates from the very center region correlated with a high dislocation density found in TEM. The CL intensity shows a reticulated structure over the c-plane visualizing the local dislocation network.

Deep defect levels and the optical as well as thermal transitions of carriers from the levels into the corresponding bands were analyzed using Thermal and Optical Admittance Spectroscopy. High resistivity GaN-layers grown by MBE and heterostructures consisting of n-type GaN-layers grown with Low Pressure Chemical Vapor Deposition on 6H-SiC substrates are investigated. In the MBE-grown GaN layers we determine deep electron traps with thermal activation energies of E A =(0.45±0.04)eV and E A =(0.65±0.03)eV. Furthermore, three different kinds of optical transitions were distinguished by Optical Admittance Spectroscopy: near band gap transitions including the transition between the valence band and a shallow donor 50meV below the conduction band, a peak at 2.1eV associated with the yellow photoluminescence band and various deep level-band transitions in the infrared region. The high sensitivity of the TAS to interface defect states was used to investigate GaN/SiC heterostructures. We found an interface defect state at 70 … 90meV. Furthermore, one level was obtained originating from the epitaxial GaN-layer having an activation energy of 63±3meV. A defect distribution was identified in the p-type SiC-substrate with activation energies between 160meV and 180meV.

We report on the heteroepitaxial growth of GaN from GaCl3 and NH3 on (0001) Al2O3 and (0001) 6H-SiC substrates. In order to enable homogeneous growth within the entire deposition zone one has to use low process pressures in the 10−1 mbar range, where still a growth rate of ≈ 2 μm/h can be achieved. We present a simple model to describe our process and explain our observations. A comparison of GaN deposited on different substrates and with GaN buffer layers is given by low temperature Photoluminescence (PL). Furthermore, impurities are traced by secondary ion mass spectroscopy (SIMS).