Compound semiconductors based on GaN have multiple functional applications. Useful compositions include GaN, and
ternary and quaternary compositions of (AlGaIn)N. Defects arising from lattice mismatch, point defects, or impurities
may act as electrical trapping centers and degrade device efficiency. Current-voltage, capacitance-voltage, thermal
admittance spectroscopy (TAS), and deep level transient spectroscopy (DLTS) measurements are applied to characterize
the defects in Al0.40Ga0.80N and In0.18Al0.82N in this report. Broad peaks with a shoulder at high temperature dominate the
DLTS spectra in each of the materials. An acceptor trap associated with a dislocation appears at 340 K in AlGaN. The
defect has an energy of 0.2 eV and capture cross section of 10-21 cm2. A second trap at 0.35 eV, 10-14 cm2 appears in the
TAS measurements in addition to the trap at 0.2 eV. Defects in InAlN are dominated by a peak near 150 K. Two traps
appear in the TAS measurements. Both traps in the InAlN are acceptors, based on a lack of field dependent emission
rates using double pulse DLTS (DDLTS). The two energy levels in InAlN appear to be coupled, with only one state
occupied at a time.
The conduction band offset of n-ZnO/n-6H-SiC heterostructures prepared by rf-sputtered ZnO on commercial n-type
6H-SiC substrates has been measured. Temperature dependent current-voltage characteristics, photocapacitance, and
deep level transient spectroscopy measurements led to conduction band offsets of 1.25 eV, 1.1 eV, and 1.22 eV,
Electrical properties of n-ZnO/n-GaN isotype heterostructures prepared by rf-sputtering of ZnO films on GaN layers
which in turn grown by metal-organic vapour phase epitaxy are discussed. Current-voltage (I-V) characteristics of the n-
ZnO/n-GaN diodes exhibited highly rectifying characteristics with forward and reverse currents being ~1.43x10-2 A/cm2
and ~2.4x10-4 A/cm2, respectively, at ±5 V. From the Arrhenius plot built representing the temperature dependent
current-voltage characteristics (I-V-T) an activation energy 0.125 eV was derived for the reverse bias leakage current
path, and 0.62 eV for the band offset from forward bias measurements. From electron-beam induced current
measurements and depending on excitation conditions the minority carrier diffusion length in ZnO was estimated in the
range 0.125-0.175 &mgr;m. The temperature dependent EBIC measurements yielded an activation energy of 0.462 ± 0.073
The characteristics of defects that are predicted to be dominant for n-GaN or p-GaN are reviewed, and compared to
measurements. Measurements are discussed to extract the concentration, transition energy, charge state, and lattice
relaxation, each of which can be predicted from theory. Additional considerations are discussed related to the defects
that are expected to occur in highest concentration. All of the native defects with transitions in the band gap are
expected to act as minority carrier traps, in spite of the predominance of characterization using majority carrier devices.
Defects detected by deep level transient spectroscopy are also frequently cited as being associated with a dislocation
based on the capture kinetics. Possibilities for other capture mechanisms exist and are modeled along with capture at a
dislocation in order to provide a method to distinguish between the mechanisms. This work provides a framework for
systematically progressing towards identifying the composition of defects.
Surface profiles of deep levels in GaN sample grown by metal-organic chemical vapor deposition and by hydride vapor phase epitaxy are measured by differential deep level transient spectroscopy (DDLTS). The concentration of acceptor defects at the surface are expected to be lower than the bulk defect concentration because of the shift in Fermi level at the surface, based on theoretical estimates of defect formation energies and the band bending at the surface from spontaneous polarization. Similarly, donor defects are expected to increase in concentration as the surface is approached. The measured concentration profiles of various traps are found to span the range of behavior, from constant, to increasing or decreasing at the interface. Deep level profiling is therefore seen as an important tool to assist in determining defect composition. Although the behavior is as expected, the change in concentration from bulk to surface, is larger than measured values for the defects with the lowest formation energies, based on a conservative estimate of band bending. The difference may reflect a band bending that is different at the growth temperature than predicted, or a consequence of non-equilibrium growth conditions. As growth proceeds, the defects incorporated at the surface are in a non-equilibrium concentration when covered by subsequent layers, unless there is a mechanism whereby equilibrium defects can be formed, e.g. VGa by forming interstitial Ga, or there is enough energy for defect diffusion to take place. Peaks in the defect profile were measured, as would be expected for a donor defect formed at the surface, but with a non-equilibrium concentration in the bulk, driving diffusion toward the surface.
Deep levels in n-type GaN grown by molecular beam epitaxy, metalorganic chemical vapor deposition, and hydride vapor phase epitaxy were characterized for comparison between the different methods of growth. The deep level energies, capture cross sections, and concentrations were determined for each using deep level transient spectroscopy on Schottky diodes from 80 K to 700 K, to characterize traps up to ~1.2 eV. The capture kinetics and bias dependence were also measured for the main traps in each, in order to determine if they are related to threading dislocations, and if they are donor-type traps. Several traps were detected in samples from each growth method. The field dependence and the capture kinetics were not the same for peaks appearing in the same temperature in deep level spectra, associated with different growth method. Traps in HVPE GaN at 0.212 eV and 0.612 eV uniquely showed field dependence indicating singly charged donors. Overall, the thick hydride vapor phase epitaxy GaN samples showed the lowest concentration of traps.
The authors report the most recent advances in type II InAs/GaSb superlattice materials and photovoltaic detectors. Lattice mismatch between the substrate and the superlattice has been routinely achieved below 0.1%, and less than 0.0043% as the record. The FWHM of the zeroth order peak from x-ray diffraction has been decreased below 50 arcsec and a record of less than 44arcsec has been achieved. High performance detectors with 50% cutoff beyond 18 micrometers up to 26 micrometers have been successfully demonstrated. The detectors with a 50% cut-off wavelength of 18.8 micrometers showed a peak current responsivity of 4 A/W at 80K, and a peak detectivity of 4.510 cm x Hz1/2/W was achieved at 80K at a reverse bias of 110mV under 300K 2(pi) FOV background. Some detectors showed a projected 0% cutoff wavelength up to 28~30 micrometers . The peak responsivity of 3Amp/Watt and detectivity of 4.2510 cm x Hz1/2/W was achieved under -40mV reverse bias at 34K for these detectors.
One of the unique properties of GaN is the polarizability. Also, demonstration of luminescent properties in devices such as light emitting diodes and lasers has been surprising, considering the defect density. The large polarization and inactive defects may be related, as demonstrated here by the measurement of several barriers to electron capture. N-type samples grown by both MOCVD and RMBE showed two adjacent DLTS peaks at 125 - 150 K with energies of 0.190 eV and 0.253 eV and one larger peak at 300 K. The 300 K peak was resolved to two traps, one with emission energy of 0.548 eV in both samples, and one with emission energy of 0.613 eV in the GaN grown by MOCVD. Analysis of the change in amplitude of the emission transients under non-saturating filling pulse conditions gives insight to the capture behavior. The two traps at 300 K have coupled trapping and emission characteristics. Both the rate window plots and the fit of the capacitance transient amplitude showed several traps with barriers to capture of electrons at 0.1 eV, 0.04 eV, 0.14 eV, and 0.38 eV. The capture barriers may be related to the shift in core electrons on ions surrounding the defect.