A. Y. Polyakov

Electrical characteristics of Au and Ag Schottky contacts on n-ZnO

Au and Ag Schottky contacts on the epiready (0001)Zn surface of bulk n-ZnO crystals show Schottky barrier heights of 0.65–0.70 eV from capacitance–voltage measurements, activation energies for reverse saturation currents of 0.3–0.4 eV and saturation current densities ranging from 10−5 A cm−2 on surfaces etched in HCl to 8×10−7 A cm−2 on solvent cleaned samples. The diode ideality factors were in the range 1.6–1.8 under all conditions. The properties of both the Au and the Ag Schottky diodes were degraded by heating in vacuum to temperatures even as low as 365 K. The degradation mechanisms during annealing were different in each case, with the Au showing reaction with the ZnO surface and the Ag contacts showing localized delamination. Mechanical polishing of the ZnO surface prior to contact deposition produced a high-resistivity damaged layer with prominent deep level defects present with activation energies of 0.55 and 0.65 eV.

Lifetime-limiting defects in n− 4H-SiC epilayers

Low-injection minority carrier lifetimes (MCLs) and deep trap spectra have been investigated in n− 4H-SiC epilayers of varying layer thicknesses, in order to enable the separation of bulk lifetimes from surface recombination effects. From the linear dependence of the inverse bulk MCL on the concentration of Z1∕Z2 defects and from the behavior of the deep trap spectra in 4H-SiC p-i-n diodes under forward bias, we conclude that it is Z1∕Z2 alone that controls the MCL in this material.

Deep centers and their spatial distribution in undoped GaN films grown by organometallic vapor phase epitaxy

Deep traps in undoped n-GaN layers grown by organometallic vapor phase epitaxy on sapphire substrates were studied by temperature dependent conductivity, photoinduced current transient spectroscopy (PICTS), thermally stimulated current, electron beam induced current (EBIC), and band edge cathodoluminescence (CL) methods. Presence of electron traps with energy levels 0.1–0.2 eV below the conduction band and hole traps with energy levels of about 0.25, 0.5, and 0.85 eV above the valence band edge was detected. CL and EBIC measurements show that the deep recombination centers in GaN are distributed inhomogeneously with well defined cellular pattern. Both carrier lifetime and luminescence intensity are enhanced at cell walls indicating lower density of recombination centers. However, the density of main hole trap (0.85 eV) is enhanced in these regions as determined by local PICTS measurements. Photoconductivity in many GaN samples exhibits very long decay times at temperatures between 100 and 300 K. The effec...

Review of radiation damage in GaN-based materials and devices

A review of the effects of proton, neutron, γ-ray, and electron irradiation on GaN materials and devices is presented. Neutron irradiation tends to create disordered regions in the GaN, while the damage from the other forms of radiation is more typically point defects. In all cases, the damaged region contains carrier traps that reduce the mobility and conductivity of the GaN and at high enough doses, a significant degradation of device performance. GaN is several orders of magnitude more resistant to radiation damage than GaAs of similar doping concentrations. In terms of heterostructures, preliminary data suggests that the radiation hardness decreases in the order AlN/GaN > AlGaN/GaN > InAlN/GaN, consistent with the average bond strengths in the Al-based materials.

On the origin of electrically active defects in AlGaN alloys grown by organometallic vapor phase epitaxy

Shallow and deep centers were studied by means of temperature dependent Hall effect and photoluminescence (PL) measurements in two sets of undoped n‐AlGaN samples grown by organometallic vapor phase epitaxy. The samples of these two series were grown under different conditions and had, as a result, electron concentrations differing by several orders of magnitude. The composition dependence of ionization energies of dominant donors in these two sets of samples is very different indicating that different types of centers are involved, but in both cases they are most probably related to some native defects. These defects behave as hydrogen‐like donors for low Al compositions and become increasingly deeper with increasing Al content. The shallow‐deep transition occurs at about x=0.2 in the low conductivity AlxGa1−xN series and at about x=0.5 for the high conductivity series. Several PL bands were detected in AlGaN and it is shown that the band at 3.05 eV is due to a radiative transition between deep donors in...

Lateral AlxGa1−xN power rectifiers with 9.7 kV reverse breakdown voltage

AlxGa1−xN (x=0–0.25) Schottky rectifiers were fabricated in a lateral geometry employing p+-implanted guard rings and rectifying contact overlap onto an SiO2 passivation layer. The reverse breakdown voltage (VB) increased with the spacing between Schottky and ohmic metal contacts, reaching 9700 V for Al0.25Ga0.75N and 6350 V for GaN, respectively, for 100 μm gap spacing. Assuming lateral depletion, these values correspond to breakdown field strengths of ⩽9.67×105 V cm−1, which is roughly a factor of 20 lower than the theoretical maximum in bulk GaN. The figure of merit (VB)2/RON, where RON is the on-state resistance, was in the range 94–268 MW cm−2 for all the devices.

Properties of Si donors and persistent photoconductivity in AlGaN

The behavior of Si donors was studied in AlxGa1−xN films with composition 0<x<0.6. It is shown that the Si donors ionization energy increases from 18 meV for 0<x<0.1 to about 50 meV for x=0.4 and does not exceed 90 meV for x=0.6. Increase in Al composition is also accompanied by the growth of the density of defects with energy levels deeper than Si. Combined action of the two above effects leads to increased difficulty in n-type doping of AlGaN films with higher Al mole fractions. Persistent photoconductivity (PPC) is shown to be a characteristic feature of AlGaN samples of all compositions. It is shown that PPC in AlGaN is related to the presence of centers with a relatively high (0.1–0.2 eV) barrier for capture of electrons. In GaN and AlGaN with x<0.1 such centers are not associated with Si donors per se.

Microstructure and optical properties of epitaxial GaN on ZnO (0001) grown by reactive molecular beam epitaxy

High quality GaN epilayers have been grown on oxygen and zinc surfaces of ZnO (0001) substrates by reactive molecular beam epitaxy and the effect of the intermediate buffer layer on the structural and optical properties of the GaN films has been investigated. The optical and structural characterization of the GaN epilayers and ZnO substrates were performed using photoluminescence, reflectivity, x-ray double diffraction, atomic force microscopy, and transmission electron microscopy. The optical results indicated that GaN was grown with compressive strain due to the difference in thermal expansion coefficient between GaN and ZnO. The surface roughness has been reduced by using an intermediate low temperature GaN buffer layer. The low temperature photoluminescence spectra of GaN/ZnO epilayers did not reveal any sign of the well-known midgap yellow signal. Linear polarized reflectivity and photoluminescence indicated that GaN epilayer planes were not misoriented with respect to the ZnO substrate planes: this ...

Proton implantation effects on electrical and recombination properties of undoped ZnO

Electrical and optical properties of undoped n-ZnO crystals implanted with 50 keV protons with doses from 5×1013 to 5×1015 cm−2 are reported. Proton implantation leads to a decrease of the carrier concentration in the near-surface region, but at the end of the proton range shallow donors are observed whose concentration tracks the implant dose and that we attribute to hydrogen donors. Three deep electron traps with apparent activation energies of 0.55, 0.75, and 0.9 eV are introduced by proton implantation. The 0.9 eV traps have been observed through the increased thermal stability of the Schottky diodes prepared on heavily implanted n-ZnO compared to unimplanted or lightly implanted samples. In addition, hole traps located 0.16 eV above the valence band edge were introduced by implantation. Proton implantation also led to the formation of persistent-photocapacitance-active defects and to considerable decrease in the intensity of the band edge luminescence and in the value of the photocurrent of the Au/n-...

Electrical and optical properties of Cr and Fe implanted n-GaN

Deep levels introduced into n-GaN films by Fe and Cr implantation have been studied by means of optical absorption and microcathodoluminescence spectroscopy measurements and by deep level transient spectroscopy, admittance spectroscopy, and capacitance-voltage profiling. The results are compared with previous measurements on Mn and Co implanted GaN. It is shown that the acceptor levels of substitutional Mn, Co, Fe, and Cr in n-GaN are located, respectively, near Ev+1.6 eV, Ev+1.7 eV, Ev+1.8 eV, and Ev+2 eV, the trend being similar to that observed in GaAs, GaP, and InP. The Fermi level in the implanted region is pinned near deep electron traps at Ec−0.5 eV that are tentatively attributed to complexes between substitutional transition metal ions and native defects such as nitrogen vacancies. It is shown that for all implanted species after 700 °C annealing a damaged region with relatively high resistivity is formed down to the depth of about 1 μm much, exceeding the projected range of implanted ions. This ...