Brett Nener

/sup 60/Co gamma irradiation effects on n-GaN Schottky diodes

The effect of /spl gamma/-ray exposure on the electrical characteristics of nickel/n-GaN Schottky barrier diodes has been investigated using current-voltage (I-V), capacitance-voltage (C-V), and deep-level transient spectroscopy (DLTS) measurements. The results indicate that /spl gamma/-irradiation induces an increase in the effective Schottky barrier height extracted from C-V measurements. Increasing radiation dose was found to degrade the reverse leakage current, whereas its effect on the forward I-V characteristics was negligible. Low temperature (/spl les/50) post-irradiation annealing after a cumulative irradiation dose of 21 Mrad(Si) was found to restore the reverse I-V characteristics to pre-irradiation levels without significantly affecting the radiation-induced changes in C-V and forward I-V characteristics. Three shallow radiation-induced defect centers with thermal activation energies of 88 104 and 144 meV were detected by DLTS with a combined production rate of 2.12 /spl times/ 10/sup -3/ cm/sup -1/. These centers are likely to be related to nitrogen-vacancies. The effect of high-energy radiation exposure on device characteristics is discussed taking into account possible contact inhomogeneities arising from dislocations and interfacial defects. The DLTS results indicate that GaN has an intrinsically low susceptibility to radiation-induced material degradation, yet the effects observed in the Schottky diode I-V and C-V characteristics indicate that the total-dose radiation hardness of GaN devices may be limited by susceptibility of the metal-GaN interface to radiation-induced damage.

Temperature-Dependent Characterization of AlGaN/GaN HEMTs: Thermal and Source/Drain Resistances

This paper shows the application of simple dc techniques to the temperature-dependent characterization of AlGaN/ GaN HEMTs in terms of the following: 1) thermal resistance and 2) ohmic series resistance (at low drain bias). Despite their simplicity, these measurement techniques are shown to give valuable information about the device behavior over a wide range of ambient/channel temperatures. The experimental results are validated by comparison with independent measurements and numerical simulations.

Characterisation of Multiple Carrier Transport in Indium Nitride Grown by Molecular Beam Epitaxy

Transport properties of two distinct electron species in indium nitride grown by molecular beam epitaxy (MBE) have been measured. Variable field Hall and resisitivity voltages were used in a quantitative mobility spectrum analysis (QMSA) to extract the concentrations and mobilities of the two electron species, attributed to the bulk electrons and a surface accumulation layer. Single magnetic field data corresponds to neither electron species. The bulk electron distribution has an extracted average mobility of 3570 cm2/(V s) at 300 K, which rises to over 5100 cm2/(V s) at 150 K. Bulk electron concentration in the sample is 1.5 ×1017 cm-3 . The surface electrons have a higher sheet charge density and an order of magnitude lower average mobility than those in the bulk.

60Co gamma-irradiation-induced defects in n-GaN

Transient capacitance measurements of Schottky diodes fabricated on nominally undoped n-type GaN exposed to 60Co gamma irradiation indicate the introduction of two defect levels with thermal activation energies of 89±6 and 132±11 meV. While the emission characteristics of these defects manifest significant broadening, their parameters are consistent with reported electron-irradiation-induced nitrogen-vacancy related centers. Three deep-level defects present before irradiation exposure with activation energies of 265, 355, and 581 meV were found to remain unaffected for cumulative gamma-ray doses up to 21 Mrad(Si).

COMPARISON BETWEEN DEEP LEVEL DEFECTS IN GAAS INDUCED BY GAMMA, 1 MEV ELECTRON, AND NEUTRON IRRADIATION

The deep level transient spectroscopy technique has been employed to follow closely the effect of 1–300 Mrad 60Co γ irradiation on the deep electron traps in undoped vapor‐phase‐epitaxy n‐type GaAs. The 1 Mrad γ‐irradiated Schottky device was identical to the as‐grown or control device, with only two electron traps EL2 (Ec−0.820 eV) and EL3 (Ec−0.408 eV) detected. At a γ dose of 5 Mrad, two additional electron traps EL6 (Ec−0.336 eV) and E2 (Ec−0.128 eV) were observed. As the γ doses were increased to ≥10 Mrad, a third electron trap E1 (Ec−0.033 eV) was observed, and the single exponential EL2 capacitance transient became a double exponential, indicating two deep levels lying at Ec−0.820 eV (EL2/EL2‐A) and Ec−0.843 eV (EL2‐B). The trap concentration of EL2‐A remained unchanged up to a γ dose of 50 Mrad before starting to increase slowly as the γ dose was increased to ≥100 Mrad. In contrast, the EL2‐B trap concentration was found to increase by 32 times, reaching 2.6×1014 cm−3 at 300 Mrad from a low 8.0×10...

Ion versus pH sensitivity of ungated AlGaN/GaN heterostructure-based devices

We have investigated the pH and ion sensitivity of AlGaN/GaN heterostructure devices; these devices are sensitive to the ion concentration rather than to the pH of the solution. Sheet resistance as a function of pH for calibrated pH solutions and dilute NaOH, HCl, KOH, and NaCl showed an increase as a function of ionic concentration, regardless of whether the pH was acidic, basic, or neutral. An increase in resistance corresponds to accumulation of negative ions at the AlGaN surface, indicating device selectivity toward the negative ions. We attribute this to the formation of a double layer at the liquid/semiconductor interface.

Characterization of deep‐level defects in GaAs irradiated by 1 MeV electrons

Deep level transient spectroscopy has been employed to determine the defect energy levels, capture cross sections, and trap densities in Si‐doped vapor phase epitaxy GaAs both before and after irradiation by 1 MeV electrons at room temperature for electron fluence ranging from 1.1×1014 to 5.0×1015 e cm−2. The results indicate that the irradiated samples have an electron trap at Ec‐0.334 eV(EL6) in addition to the two electron traps at Ec‐0.815 eV(EL2) and Ec‐0.420 eV(EL3) which are present in the nonirradiated sample. The density of the EL6 trap increases monotonically with irradiation fluence from 6.7×1013 to 24.4×1013 cm−3 as electron fluence is increased from 1.1×1014 to 3.1×1014 e cm−2. In contrast, both the EL2 and EL3 trap densities were found to be only moderately affected by electron irradiation with trap densities slightly greater than the nonirradiated sample. These results, along with the fact that the EL6 trap was not observed in the nonirradiated sample, strongly suggest that this trap is cre...

Improved device technology for epitaxial Hg1-xCdxTe infrared photoconductor arrays

The performance of Hg1-xCdxTe infrared photoconductors is strongly dependent on the semiconductor surface conditions and, in particular, the degree to which the surface contributes to recombination of photogenerated excess carriers. Although published photoconductor fabrication processes based on bulk Hg1-xCdxTe address this issue by fully passivating both major surfaces (i.e. front and back) with anodically grown native oxide, passivation of the sidewalls is neglected. In this paper it is shown both theoretically and experimentally that leaving the sidewalls unpassivated can result in approximately a factor of two reduction in responsivity for long-wavelength infrared (LWIR) detectors used in high-resolution thermal imaging systems. Detector arrays are typically fabricated on x=0.23 Hg1-xCdxTe representing a cut-off wavelength of 9.4 mu m and use individual element sizes of approximately 50*50 mu m2. We describe in detail for the first time a device technology which enables the fabrication of Hg1-xCdxTe photoconductor arrays such that the entire surface of the semiconductor is effectively passivated, including the sidewalls. Of particular interest is the fact that this improved device technology is compatible with present-day Hg1-xCdxTe epitaxial growth processes. This is in contrast to current photoconductor technology which is primarily based on bulk Hg1-xCdxTe. Experimental results are presented which compare device performance of LWIR detectors fabricated using the improved photoconductor technology with current published photoconductor technology. These results clearly indicate that detectors fabricated on liquid phase epitaxially (LPE) grown x=0.23 Hg1-xCdxTe material using the improved photoconductor device technology achieve much higher responsivities and detectivities. Furthermore, it is shown that only a fully passivated device structure is capable of exploiting any future improvements in bulk minority carrier lifetime as it approaches the Auger recombination limit.

Temperature dependence of Hg/sub 0.68/Cd/sub 0.32/Te infrared photoconductor performance

An experimental and theoretical study has been carried out of the temperature dependent noise and responsivity performance of n-type x=0.32 Hg/sub 1-x/Cd/sub x/Te (corresponding to a 4.6 /spl mu/m cutoff wavelength at 80 K) photoconductors. The fundamental noise sources that ultimately limit the specific detectivity, D*/sub /spl lambda//, at the three main temperatures of interest (i.e., 80 K, 200 K, and 300 K) are identified and correlated with the experimental material parameters of the device. A device model is presented for the responsivity and noise voltage which takes into account surface effects such as surface recombination and accumulation layer shunting. For a given set of device and material parameters this model is well able to account for the observed experimental values of responsivity and noise voltage over the full temperature range from 80-300 K. Using a theoretical model, it is shown that under ideal conditions it is possible to achieve background limited performance at temperatures up to 210 K. Experimental results are presented for responsivity, noise voltage, semiconductor surface charge density and D*/sub /spl lambda// for a frontside-illuminated Hg/sub 1-x/Cd/sub x/ Te photoconductive detector, as a function of temperature in the range 80-300 K. The devices were fabricated on Liquid Phase Epitaxially (LPE) grown n-type Hg/sub 0.68/Cd/sub 0.32/Te, and were passivated with anodic oxide/ZnS on the front side. The detector area is 250/spl times/250 /spl mu/m/sup 2/ and has a cut-off wavelength of 4.6 /spl mu/m at 80 K. For a signal wavelength of 4 /spl mu/m and a 40/spl deg/ field of view, a background limited D*/sub /spl lambda// of 3.8/spl times/10/sup 11/ cm Hz/sup 1/2 / W/sup -1/ was obtained for temperatures up to 180 K, while D*/sub /spl lambda// of 1.4/spl times/10/sup 11/ and 2/spl times/10/sup 9/ cm Hz/sup 1/2 / W/sup -1/ were measured at 200 K and 300 K, respectively. These figures are comparable to the highest reported D*/sub /spl lambda// for n-type x=0.32 Hg/sub 1-x/Cd/sub x/Te mid-wavelength infrared photoconductive detectors operating at these temperatures. >

MOCVD-grown wider-bandgap capping layers in long-wavelength infrared photoconductors

The use of MOCVD-grown wider-bandgap as a capping layer for long-wavelength infrared (LWIR) photoconductors has been studied using both theoretical and experimental results. A device model is derived which shows that in the presence of a suitable energy barrier between the infrared absorbing layer and the overlaying passivation layer, the high surface recombination rate which is usually present at the semiconductor/passivant interface is prevented from having a significant effect on device performance. The energy barrier, which repels photogenerated minority carriers from the semiconductor surface, is introduced by employing an n-type wafer which consists of a wider-bandgap capping layer that is grown in situ by MOCVD on an LWIR absorbing layer. The derived model allows the responsivity to be calculated by taking into account surface recombination at both the front and back interfaces, thickness of capping and absorbing layers, recombination at the heterointerface, and variations in equilibrium electron concentration. Calculations show that for an absorbing layer, the optimum capping layer consists of and a thickness of the order of 0.1 to 0.2 . Experimental results are presented for x = 0.22 n-type conventional single-layer LWIR photoconductors, and for heterostructure photoconductors consisting of an LWIR absorbing layer of x = 0.22 capped by an n-type layer of x = 0.31. The model is used to extract the recombination velocities at the heterointerface and the semiconductor/substrate interface, which are determined to be and respectively. The experimental data clearly indicate that the use of a heterostructure barrier between the overlaying passivation layer and the underlying LWIR absorbing layer produces detectors that exhibit much higher performance and are insensitive to the condition of the semiconductor/passivant interface.