M. L. Lee

Enhancement in output power of blue gallium nitride-based light-emitting diodes with omnidirectional metal reflector under electrode pads

In this study, we demonstrate a GaN-based light-emitting diode (LED) with nonalloyed metal contacts onto the n+-GaN surface and transparent contact layer (indium tin oxide) to serve as the n-type electrode (cathode) and the p-type electrode pad (anode), respectively. Comparing with the conventional LEDs, which the electrode pads and/or Ohmic contacts form through conventional Cr∕Au metal contacts, the nonalloyed metal contacts (Ag∕Cr∕Au or Al∕Cr∕Au) used in the present experimental blue LEDs also play the role of reflector to prevent the emitted light from absorption by the opaque electrode pads with low reflectivity (Cr∕Au). With an injection current of 20mA, the enhancement in the light output power has approximately a 14% magnitude compared to the GaN-based LEDs without Ag or Al reflectors under the Cr∕Au electrode pads.

Schottky barrier heights of metal contacts to n-type gallium nitride with low-temperature-grown cap layer

The Schottky barrier heights of metal contacts, including WSi0.8, Cr, Ti, Pt, and Ni, on n-type gallium nitride (GaN) with a GaN cap layer grown at low-temperature (LTG) were studied. Higher barriers can be formed by introducing LTG GaN on top of the conventional structures. The higher Schottky barrier observed in samples with the LTG GaN cap layer may be due to the facts that the high-resistivity LTG GaN layer may passivate the surface defects (pits) formed from threading dislocations or it may cause the Fermi-level pinning effect at the metal/semiconductor interface, revealing a weak dependence of Schottky barrier height on the metal work function. The measured barrier heights of the LTG GaN-capped samples were 1.02–1.13eV.The Schottky barrier heights of metal contacts, including WSi0.8, Cr, Ti, Pt, and Ni, on n-type gallium nitride (GaN) with a GaN cap layer grown at low-temperature (LTG) were studied. Higher barriers can be formed by introducing LTG GaN on top of the conventional structures. The higher Schottky barrier observed in samples with the LTG GaN cap layer may be due to the facts that the high-resistivity LTG GaN layer may passivate the surface defects (pits) formed from threading dislocations or it may cause the Fermi-level pinning effect at the metal/semiconductor interface, revealing a weak dependence of Schottky barrier height on the metal work function. The measured barrier heights of the LTG GaN-capped samples were 1.02–1.13eV.

Enhancement of the conversion efficiency of GaN-based photovoltaic devices with AlGaN/InGaN absorption layers

InGaN/sapphire-based p-i-n type photovoltaic (PV) devices were shown to have Al0.14Ga0.86N/In0.21Ga0.79N heterostructures that enhance the extraction of photogenerated carriers from active layers. With an appropriately increased barrier height in AlGaN/InGaN absorption layers, PV devices exhibit lower RS despite the increase in conduction-band discontinuity compared with GaN/InGaN superlattice absorption layers. This improvement can be attributed to polarization-induced electric fields enhanced by the incorporated aluminum in barrier layers. The enhancement is beneficial to increase built-in electric fields. Subsequently, the photogenerated carriers can escape more easily from recombination or scattering centers. Under 1 sun air-mass 1.5 standard testing conditions, the Al0.14Ga0.86N/In0.21Ga0.79N PV device exhibits high VOC (2.10 V) as well as an enhanced fill factor (0.66) and JSC (0.84u2002mA/cm2) corresponding to a power conversion efficiency of 1.16%.

Photodetectors formed by an indium tin oxide/zinc oxide/p-type gallium nitride heterojunction with high ultraviolet-to-visible rejection ratio

In this study, indium tin oxide (ITO) or ITO/ZnO films, which were prepared by magnetron sputtering, were deposited onto p-GaN epitaxial films to form ultraviolet photodetectors (PDs). The ITO/ZnO/p-GaN heterojunction PDs behave like the p-i-n photodiodes, which characteristically exhibit low dark current, as opposed to the ITO/p-GaN PDs, which exhibit a marked bias-dependent dark current. The zero-bias rejection ratio can be improved up to 4×105 due to a further reduction in the dark current compared to the ITO/p-GaN PDs. When the incident wavelength is 360 nm, the ITO/ZnO/p-GaN heterojunction PD exhibits a zero-bias photocurrent/dark current ratio and a responsivity of approximately 8×104 and 0.015 A/W, respectively.

Suppressed quantum-confined Stark effect in InGaN-based LEDs with nano-sized patterned sapphire substrates.

This paper demonstrates that quantum-confined Stark effect (QCSE) within the multiple quantum wells (MQWs) can be suppressed by the growths of InGaN-based light-emitting diodes (LEDs) on the nano-sized patterned c-plane sapphire substrates (PCSSs) with reducing the space. The efficiency droop is also determined by QCSE. As verified by the experimentally measured data and the ray-tracing simulation results, the suppressed efficiency droop for the InGaN-based LED having the nano-sized PCSS with a smaller space of 200 nm can be acquired due to the weaker function of the QCSE within the MQWs as a result of the smaller polarization fields coming from the lower compressive strain in the corresponding epitaxial layers.

InGaN gallium nitride light-emitting diodes with reflective electrode pads and textured gallium-doped ZnO contact layer

We demonstrate a GaN-based light-emitting diode (LED) with nonalloyed metal contacts and textured Ga-doped ZnO (GZO) contact layer to serve as the n- and p-type electrode pads, respectively. Compared with the conventional LEDs with flat surface and Cr/Au metal contacts, the nonalloyed Ag/Cr/Au contacts used in the present experimental LEDs play the role of reflector to prevent the emitted light from absorption by the opaque electrode pads. Enhancement of light output power observed from the experimental LEDs is also due to the textured GZO layer that can disperse the angular distribution of photons at the GZO/air interface. With an injection current of 20 mA, the output power of experimental LEDs can be improved markedly by a magnitude of 30% compared with conventional GaN-based LEDs.

Ultraviolet band-pass photodetectors formed by Ga-doped ZnO contacts to n-GaN

In this study, Ga-doped ZnO (GZO) films prepared by cosputtering were deposited onto n-GaN films with a low-temperature-grown GaN cap layer to form Schottky barrier photodetectors (PDs). The ultraviolet (UV) PDs exhibited a narrow band-pass spectral response ranging from 340to390nm. The short-wavelength cutoff at around 340nm can be attributed to the marked absorption of the GZO contact layer. With a zero-biased condition, the UV PDs exhibited a typical peak responsivity of around 0.10A∕W at 365nm, which corresponds to the quantum efficiency of around 34%. When the reverse biases were below 10V, the dark currents of the PDs were well below 30pA.

Hydrogen gas generation using n-GaN photoelectrodes with immersed Indium Tin Oxide ohmic contacts

An n-GaN photoelectrochemical (PEC) cell with immersed finger-type indium tin oxide (ITO) ohmic contacts was demonstrated in the present study to enhance the hydrogen generation rate. The finger-type ITO ohmic contacts were covered with SiO₂ layers to prevent the PEC cell from generating leakage current. Using a 1M NaCl electrolyte and external biases, the typical photocurrent density and gas generation rate of the n-GaN working electrodes with ITO finger contacts were found to be higher than those with Cr/Au finger contacts. The enhancement in photocurrent density or gas generation rate can be attributed to the transparent ITO contacts which allowed the introduction of relatively more photons into the GaN layer. No significant corrosion was observed in the ITO layer after the PEC process compared with the Cr/Au finger contacts which were significantly peeled from the GaN layer. These results indicate that the use of n-GaN working electrodes with finger-type ITO ohmic contacts is a promising approach for PEC cells.

Effect of Growth Pressure of Undoped GaN Layer on the ESD Characteristics of GaN-Based LEDs Grown on Patterned Sapphire

The effect of growth pressure of underlying undoped GaN(u-GaN) layer on the electrical properties of GaN-based light-emitting diodes (LEDs) grown on patterned sapphire substrates (PSS) is evaluated. The electrostatic discharge (ESD) endurance voltages could increase from 4000 to 7000 V when the growth pressure of u-GaN layers is increased from 100 to 500 torr, while the forward voltages and light output powers remain almost the same. Poor ESD endurance ability could be attributed to the underlying GaN layer grown under relative low pressure, which leads to significant surface pits. This could be further attributed to the imperfect coalescence of crystal planes above the convex sapphire patterns. The pits are associated with TDs behaving as a leakage path to degrade electrical performance.

Ultraviolet bandpass Al0.17Ga0.83N∕GaN heterojunction phototransitors with high optical gain and high rejection ratio

Ultraviolet Al0.17Ga0.83N∕GaN-based heterojunction phototransistors (HPTs) grown by metal-organic vapor-phase epitaxy were demonstrated. The HPTs showed a bandpass spectral responsivity ranging from 280to390nm. With a bias voltage of 6V, the responsivity at an incident of 340nm was as high as 1500A∕W, corresponding to a quantum gain of 5.47×103. In contrast to GaN-based photoconductors, the HPTs also featured high contrast in spectral response. With a bias voltage of 3V, the spectral response showed high rejection ratios of approximately 4×105 and 1×104 for the long-wavelength side (340∕400nm) and the short-wavelength side, respectively. The high contrast of spectral response for the long-wavelength side could be due to the long trapping time of holes blocked by the base-emitter heterojunction and the low defect-to-band response that is caused by defect-related gap states.