We present a comprehensive review of the material properties of cadmium zinc telluride (CZT, Cd1ˇxZnxTe) with zinc content xa 0:1‐0.2. Particular emphasis is placed on those aspects of this material related to room temperature nuclear detectors. A review of the structural properties, charge transport, and contacting issues and how these are related to detector and spectrometer performance is presented. A comprehensive literature survey and bibliography are also included. # 2001 Elsevier Science B.V. All rights reserved.

Cadmium zinc telluride and its use as a nuclear radiation detector material

Trapping is one of the most deleterious effects that limit performance and reliability in GaN HEMTs. In this paper, we present a methodology to study trapping characteristics in GaN HEMTs that is based on current-transient measurements. Its uniqueness is that it is amenable to integration with electrical stress experiments in long-term reliability studies. We present the details of the measurement and analysis procedures. With this method, we have investigated the trapping and detrapping dynamics of GaN HEMTs. In particular, we examined layer location, energy level, and trapping/detrapping time constants of dominant traps. We have identified several traps inside the AlGaN barrier layer or at the surface close to the gate edge and in the GaN buffer.

http://www.mtl.mit.edu/~alamo/pdf/2011/RJ-128.pdf

A Current-Transient Methodology for Trap Analysis for GaN High Electron Mobility Transistors

A comprehensive study is reported of the photoluminescence properties of ZnS thin films between 1.6 and 320 K grown by metalorganic molecular beam epitaxy and chemical beam epitaxy on GaAs substrates. Both heavy- and light-hole free excitons were observed at low temperatures with linewidths of 7.0 and 5.3 meV, respectively, as well as donor- and acceptor-bound excitons and free-to-bound recombination along with their longitudinal optical (LO) phonon replicas. The free exciton emission was observed up to 320 K, and enabled the room temperature band gap of ZnS to be unambiguously determined as 3.723 eV. The temperature dependence of the peak position, intensity, and linewidth was well described by the conventional empirical relations and by Toyozawa’s exciton line shape theory. The bound exciton peak positions were found to follow the temperature dependence of the band gap whereas the free-to-bound recombination feature was displaced by (1/2)kT above the band gap energy. Thermal quenching of the donor-bound...

Photoluminescence properties of ZnS epilayers

Several experimental photoluminescence (PL) bands of different energies for variously prepared CdTe samples are compared. Temperature variation of the PL intensity is modeled with two nonradiative thermal activation energies, of which ET1 is dominant for about T⩽60 K, and ET2 for the upper temperature range of the measurement. The size of ET1 is invariably of the order of a few meV and, although of unclear origin, its magnitude is usually interpreted as an electronic energy level difference over which the carriers escape by thermal excitation. In CdTe the existence of such a small energy level difference ET1 is not easy to explain. On the contrary, we find clear evidence that, at low temperature, the PL intensity reduction with increasing temperature in fact results from the approximately T−2 temperature dependent capture cross sections of the carriers at the recombination centers, and not from a genuine thermal activation energy ET1.

/pdf/does-the-low-temperature-arrhenius-plot-of-the-29m142kkw1.pdf

Does the low-temperature Arrhenius plot of the photoluminescence intensity in CdTe point towards an erroneous activation energy?

GaN and AlGaN have shown great potential in next-generation high-power electronic devices; however, they are plagued by a high density of interface states that affect device reliability and performance, resulting in large leakage current and current collapse. In this review, the authors summarize the current understanding of the gate leakage current and current collapse mechanisms, where awareness of the surface defects is the key to controlling and improving device performance. With this in mind, they present the current research on surface states on GaN and AlGaN and interface states on GaN and AlGaN-based heterostructures. Since GaN and AlGaN are polar materials, both are characterized by a large bound polarization charge on the order of 1013 charges/cm2 that requires compensation. The key is therefore to control the compensation charge such that the electronic states do not serve as electron traps or affect device performance and reliability. Band alignment modeling and measurement can help to determi...

Electronic surface and dielectric interface states on GaN and AlGaN

Gate leakage/breakdown mechanism in unpassivated AlGaN∕GaN high electron mobility transistors (HEMTs) is investigated by performing temperature-dependent pulsed current–voltage (I–V) and current transient measurements of AlGaN∕GaN HEMTs without and with annealing after Schottky gate formation. After post-gate annealing, the devices exhibited significantly smaller gate leakage current and higher breakdown voltage even without any gate dielectrics or passivation layer. The temperature-dependent current transient measurements show that the current dispersion in the unannealed HEMTs is attributed to traps with an emission time constant (tE) of ∼0.5μs at 295 K and an activation energy of ∼38meV. On the contrary, the 20-min annealed devices have traps with tE of 21.6μs at 295 K and an activation energy of ∼0.31eV. The results suggest that the post-gate annealing removes shallow traps, and creates or activates deeper traps. We propose that the breakdown and gate leakage current is mainly due to the emission current from shallow traps in unpassivated AlGaN∕GaN HEMTs. The breakdown voltage improvement after the post-gate annealing is due to the removal of shallow traps near the Schottky gate metal∕AlGaN interface.

Gate current leakage and breakdown mechanism in unpassivated AlGaN∕GaN high electron mobility transistors by post-gate annealing

Ir, Ni, and Re Schottky contacts on strained Al0.25Ga0.75N/GaN heterostructures are characterized using capacitance–voltage (C–V) and I–V techniques. Based on the measured C–V characteristics, two dimensional electron gas sheet carrier concentrations at the AlGaN/GaN interface and barrier heights of Ir, Ni, and Re Schottky contacts are calculated. The barrier heights of 1.12, 1.27, and 1.68 eV are obtained for Ir, Ni, and Re Schottky contacts, respectively. The results show that the barrier heights of Schottky contacts on strained AlGaN/GaN heterostructures are strongly dependent on the metal work functions. However, contrary to Schottky contacts on bulk AlGaN or GaN, the barrier height on strained AlGaN/GaN heterostructures is lower for a Schottky contact with a higher metal work function. This is attributed to the stronger wave function coupling between electrons in the Schottky metal and surface donor electrons. The I–V characteristics for Ir, Ni, and Re Schottky contacts confirm the results obtained b...

Barrier heights of Schottky contacts on strained AlGaN/GaN heterostructures: Determination and effect of metal work functions

Room-temperature photoluminescence (PL) spectroscopy of II-VI semiconductor alloys has been proposed as a useful tool for determination of spatial variations in alloy concentration and impurity concentration. However, the nature of the PL recombination can affect the emission peak energy, as well as line shape. These effects in the band-edge emission of the compound II-VI semiconductor CdTe are reported here. PL measurements at 300 K were recorded from (1) highly doped n-type CdTe films grown by molecular-beam epitaxy, (2) undoped high-resistivity (\ensuremath{\rho}\ensuremath{\sim}${10}^{8}$--${10}^{9}$ \ensuremath{\Omega} cm) bulk CdTe, and (3) as-grown nominally p-type (\ensuremath{\rho}\ensuremath{\sim}${10}^{4}$--${10}^{5}$ \ensuremath{\Omega} cm) bulk CdTe. The dependence of PL emission intensity and line shape over a range of excitation from 0.003--70 W/${\mathrm{cm}}^{2}$ was studied. As the excitation power density was increased, a redshift in PL peak position was observed from all samples. PL peak position, intensity, and line-shape analysis show the highly excitonic nature of the radiative recombination in these materials, even though the free-exciton binding energy in CdTe is about 1/3 of kT at 300 K. In addition, the PL peak position can be more than 7 meV higher from n-type CdTe as compared to undoped CdTe.

Room-temperature band-edge photoluminescence from cadmium telluride

The thermal stability of Ni Schottky contacts on strained Al0.3Ga0.7N/GaN heterostructures and on n-type bulk GaN was investigated after various thermal stressings using capacitance–voltage and current–voltage characterization techniques. The reverse leakage current decreases after thermal treatment at up to 800 and 600 °C for Schottky contacts on the strained Al0.3Ga0.7N/GaN heterostructures and bulk n-GaN, respectively. Ni Schottky contacts on the heterostructure with 30-min thermal stressing at 700 °C exhibit lower reverse leakage current by more than three orders of magnitude lower than the control sample. However, decrease in two-dimensional electron gas sheet carrier concentration at the Al0.3Ga0.7N and GaN interface possibly due to interactions between Ni and AlGaN surface was observed with increase of stressing temperature and time. Ni Schottky contacts on bulk n-GaN layers degrade at lower annealing temperature and their rectifying property practically disappears after 700 °C annealing, while Al0...

Thermal stability of Schottky contacts on strained AlGaN/GaN heterostructures

The Ni∕AlGaN interfaces in AlGaN∕GaN Schottky diodes were investigated to explore the physical origin of postannealing effects in AlGaN∕GaN heterostructures using electron beam induced current (EBIC) and current-voltage characteristics. Black spot density in EBIC images of as-deposited Ni∕AlGaN∕GaN diodes is in the same order of 108cm−2 as the dislocation density of AlGaN∕GaN heterostructures characterized by atomic force microscopy, indicating that recombination sites near Ni∕AlGaN interface are related to dislocations. The EBIC images of the annealed diodes showed that the postannealing reduced the electrically active states at the Schottky metal/AlGaN interfaces. The thermal reaction near Ni∕AlGaN interface due to the postannealing induced passivation effect, leading to decrease in reverse leakage current density, ideality factor, and saturation current density of the diodes and increase in the Schottky barrier height. We suggest that the postannealing process is an effective way to passivate the AlGaN...

Jaesun Lee

Papers

Gate current leakage and breakdown mechanism in unpassivated AlGaN∕GaN high electron mobility transistors by post-gate annealing

Barrier heights of Schottky contacts on strained AlGaN/GaN heterostructures: Determination and effect of metal work functions

Room-temperature band-edge photoluminescence from cadmium telluride

Thermal stability of Schottky contacts on strained AlGaN/GaN heterostructures

Passivation effects in Ni∕AlGaN∕GaN Schottky diodes by annealing