M. Topf

A comparison of the Hall-effect and secondary ion mass spectroscopy on the shallow oxygen donor in unintentionally doped GaN films

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 en...

Electrical and optical properties of p-SiC/n-GaN heterostructures

Abstract 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).

Strong morphological dependence of luminescence efficiency and emission wavelength in hexagonal GaN crystallites directly imaged by scanning cathodoluminescence microscopy

Abstract 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.

Low-pressure chemical vapor deposition of GaN epitaxial films

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 x 10 18 and 5 x 10 19 cm - 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.

Interface and bulk defects in SiC/GaN heterostructures characterized using thermal admittance spectroscopy

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.

Photoconductivity in AlxGa1—xN with Different Al Contents

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.

Low Pressure CVD of GaN from GaCl 3 and NH 3

We report on the heteroepitaxial growth of GaN from GaCl 3 and NH 3 on (0001) A1 2 O 3 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).

Ion beam sputter etching of galliumnitride grown by chloride transport LP–CVD

Abstract Galliumnitrid (GaN) layers, grown by chloride transport LP–CVD, were etched by ion beam sputtering with carbon dioxide (CO2). Before etching all samples were masked by electron beam evaporated titanium. We report on the dependence of the etch rate on the angle of incidence of the ion beam. Furthermore we present structural examinations of the surface before and after ion etching as well as an analysis of masking effects. Surface roughening and structural defects were investigated by optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

Defects in undoped and Mg-doped GaN and AlxGa1-xN

Abstract 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.

GaN/SiC heterojunctions grown by LP–CVD

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 =1 0 18 cm ˇ3 )o np-type (p =2 10 18 cm ˇ3 ) and n-type (n =4 10 18 cm ˇ3 ) 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 1 mbar resulting in a good homogeneity. As it is common for the more usual HVPE we grew GaN without a buAer 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 )a s the working gas. # 2000 Elsevier Science Ltd. All rights reserved.