Y. T. Moon

Effectiveness of TiN porous templates on the reduction of threading dislocations in GaN overgrowth by organometallic vapor-phase epitaxy

We report on the reduction of threading dislocations in GaN overlayers grown by organometallic vapor phase epitaxy on micro-porous TiN networks. These networks were obtained by in situ annealing of thin Ti layers deposited in a metalization chamber, on the (0001) face of GaN templates. Observations by transmission electron microscopy indicate dislocation reduction by factors of up to 10 in GaN layers grown on TiN networks compared with the control GaN. X-ray diffraction shows that GaN grown on the TiN network has a smaller (102) plane peak width (4.6 arcmin) than the control GaN (7.8 arcmin). In low temperature photoluminescence spectra, a narrow excitonic full-width-at-half-maximum of 2.4 meV was obtained, as compared to 3.0 meV for the control GaN, confirming the improved crystalline quality of the overgrown GaN layers.

Structural analysis of ferromagnetic Mn-doped ZnO thin films deposited by radio frequency magnetron sputtering

We report on the structural analysis of ferromagnetic Mn-doped ZnO thin films deposited by radio frequency magnetron sputtering, using transmission electron microscopy (TEM), high-resolution x-ray diffraction, and Rutherford backscattering spectroscopy (RBS) measurements. The ferromagnetic Mn-doped ZnO film showed magnetization hysteresis at 5 and 300K. A TEM analysis revealed that the Mn-doped ZnO included a high density of round-shaped cubic and elongated hexagonal MnZn oxide precipitates. The incorporation of Mn caused a large amount of structural disorder in the crystalline columnar ZnO lattice, although the wurtzite crystal structure was maintained. The observed ferromagnetism is discussed based on the structural characteristics indicated by TEM and the behavior of Mn when it is substituted into a ZnO lattice derived from RBS measurements.

GaN resistive hydrogen gas sensors

GaN epilayers grown by organometallic vapor phase epitaxy have been used to fabricate resistive gas sensors with a pair of planar ohmic contacts. Detectible sensitivity to H2 gas for a wide range of gas mixtures in an Ar ambient has been realized; the lowest concentration tested is ∼0.1% H2 (in Ar), well below the lower combustion limit in air. No saturation of the signal is observed up to 100% H2 flow. Real-time response to H2 shows a clear and sharp response with no memory effects during the ramping cycles of H2 concentration. The change in current at a fixed voltage to hydrogen was found to change with sensor geometry. This appears to be consistent with a surface-adsorption-induced change of conductivity; a detailed picture of the gas sensing mechanism requires further systematic studies.

Fabrication and current-voltage characterization of a ferroelectric lead zirconate titanate/AlGaN∕GaN field effect transistor

We demonstrated ferroelectric field effect transistors (FFETs) with hysteretic I-V characteristics in a modulation-doped field effect transistors (MODFET) AlGaN∕GaN platform with ferroelectric Pb(Zr,Ti)O3 between a GaN channel and a gate metal. The pinch-off voltage was about 6–7V comparable to that of conventional Schottky gate MODFET. Counterclockwise hysteresis appeared in the transfer characteristics with a drain current shift of ∼5mA for zero gate-to-source voltage. This direction is opposite and much more pronounced than the defect induced clockwise hysteresis in conventional devices, which suggests that the key factor contributing to the counterclockwise hysteresis of the FFET is the ferroelectric switching effect of the lead zirconate titanate gate.

Effect of n+-GaN subcontact layer on 4H–SiC high-power photoconductive switch

High-power photoconductive semiconductor switching devices were fabricated on 4H–SiC. In order to prevent current crowding, reduce the contact resistance, and avoid contact degradation, a highly n-doped GaN subcontact layer was inserted between the contact metal and the high resistivity SiC bulk. This method led to a two orders of magnitude reduction in the on-state resistance and, similarly, the photocurrent efficiency was increased by two orders of magnitude with the GaN subcontact layer following the initial high current operation. Both dry etching and wet etching were used to remove the GaN subcontact layer in the channel area. Wet etching was found to be more suitable than dry etching.

Dislocation reduction in GaN grown on porous TiN networks by metal-organic vapor-phase epitaxy

We report on the effectiveness of porous TiN nanonetworks on the reduction of threading dislocations (TDs) in GaN grown by metal-organic vapor-phase epitaxy (MOVPE). The porous TiN networks were formed by in situ annealing of thin-deposited Ti films deposited ex situ on GaN templates within the MOVPE growth chamber. Different annealing parameters in relation to surface porosity of TiN networks were investigated. Transmission electron micrographs indicated dislocation reduction by factors of up to 10 in GaN layers grown on the TiN nanonetwork, compared with a control sample. TiN prevented many dislocations present in the GaN templates from penetrating into the upper layer. Microscale epitaxial lateral overgrowth of GaN above TiN also contributed to TD reduction. The surface porosity of the TiN network had a strong impact on the efficiency of TD reduction. X-ray-diffraction and time-resolved photoluminescence measurements further confirmed the improved GaN quality.

Increased carrier lifetimes in GaN epitaxial films grown using SiN and TiN porous network layers

Improved structural quality and radiative efficiency were observed in GaN thin films grown by metalorganic chemical vapor deposition on SiN and TiN porous network templates. The room-temperature decay times obtained from biexponential fits to time-resolved photoluminescence data are increased with the inclusion of SiN and TiN layers. The carrier lifetime of 1.86ns measured for a TiN network sample is slightly longer than that for a 200μm-thick high-quality freestanding GaN. The linewidth of the asymmetric x-ray diffraction (XRD) (101¯2) peak decreases considerably with the use of SiN and TiN layers, indicating the reduction in threading dislocation density. However, no direct correlation is yet found between the decay times and the XRD linewidths, suggesting that point defect and impurity-related nonradiative centers are the main parameters affecting the lifetime.

Effect of hydrostatic pressure on the current-voltage characteristics of GaN∕AlGaN∕GaN heterostructure devices

The current-voltage characteristics of n-GaN∕u-AlGaN∕n-GaN heterostructure devices are investigated for potential pressure sensor applications. Model calculations suggest that the current decreases with pressure as a result of the piezoelectric effect, and this effect becomes more significant with thicker AlGaN layers and increasing AlN composition. The change in current with pressure is shown to be highly sensitive to the change in interfacial polarization charge densities. The concept is verified by measuring the current versus voltage characteristics of an n-GaN∕u-Al0.2Ga0.8N∕n-GaN device under hydrostatic pressure over the range of 0–5kbars. The measured current is found to decrease approximately linearly with applied pressure in agreement with the model results. A gauge factor, which is defined as the relative change in current divided by the in-plane strain, approaching 500 is extracted from the data, demonstrating the considerable potential of these devices for pressure sensing applications.

Quantitative mobility spectrum analysis of AlGaN∕GaN heterostructures using variable-field hall measurements

Carrier transport properties of AlGaN∕GaN heterostructures have been analyzed with the quantitative mobility spectrum analysis (QMSA) technique. The nominally undoped Al0.08Ga0.92N∕GaN sample was grown by plasma-assisted molecular beam epitaxy on a GaN/sapphire template prepared with hydride vapor phase epitaxy. Variable-magnetic-field Hall measurements were carried out in the temperature range of 5–300K and magnetic field range of 0.01–7T. QMSA was applied to the experimental variable-field data to extract the concentrations and mobilities associated with the high-mobility two-dimensional electron gas and the relatively low-mobility bulk electrons for the temperature range investigated. The mobilities at T=80K are found to be 7100 and 880cm2∕Vs, respectively, while the corresponding carrier densities are 7.0×1011 and 8×1014cm−3. Any conclusions drawn from conventional Hall measurements at a single magnetic field would have been highly misleading.

Long carrier lifetimes in GaN epitaxial layers grown using TiN porous network templates

Improved structural quality and radiative efficiency were observed in GaN thin films grown by metalorganic chemical vapor deposition on TiN porous network templates formed by in situ thermal annealing of Ti in ammonia. The room-temperature decay times obtained from biexponential fits to time-resolved photoluminescence data are longer than ever reported for GaN. The carrier lifetime of 1.86 ns measured for a TiN network sample is slightly longer than that for a 200 μm thick high-quality freestanding GaN. The linewidth of the asymmetric x-ray diffraction (XRD) (101¯2) peak decreases considerably with the use of TiN layer and with increasing in situ annealing time, indicating the reduction in threading dislocation density. However, no direct correlation is yet found between the decay times and the XRD linewidths, suggesting that point defect and impurity related nonradiative centers are the main parameters affecting the lifetime.