Wei-I Lee

EFFECTS OF COLUMN-III ALKYL SOURCES ON DEEP LEVELS IN GAN GROWN BY ORGANOMETALLIC VAPOR-PHASE EPITAXY

Two different kinds of n‐type GaN films were prepared by organometallic vapor phase epitaxy, one by using trimethylgallium (TMGa) and another by using triethylgallium (TEGa) as the alkyl source. Schottky diodes with well‐behaved current–voltage and capacitance–voltage characteristics were fabricated. Deep‐level transient spectroscopy studies were performed on these samples. Three distinct deep levels, labeled E1, E2, and E3, were measured in the film grown with TMGa, with an activation energy of 0.14, 0.49, and 1.63±0.3 eV, respectively. Only one level, E3, was observed in the film prepared with TEGa.

Activation of p-Type GaN in a Pure Oxygen Ambient

In this study, we activated p-type GaN in a pure oxygen ambient by rapid thermal annealing. The sheet resistance of p-type GaN was greatly reduced from > 107 Ω/\Box to 7.06 ×104 Ω/\Box after annealing in oxygen ambient at 500°C. The photoluminescence intensity of blue emission increased by one order of magnitude compared to the as-grown sample. Moreover, the sheet resistance of p-type GaN annealed in pure oxygen ambient is lower than that of p-type GaN annealed in nitrogen ambient. The carrier concentrations of the samples annealed in oxygen ambient are higher than those annealed in nitrogen ambient. The better activation of p-type GaN in oxygen ambient is due to the higher activity of oxygen than that of nitrogen. Oxygen would remove hydrogen that passivates Mg atoms by forming H2O at a lower temperature.

Nonlithographic Random Masking and Regrowth of GaN Microhillocks to Improve Light-Emitting Diode Efficiency

In this study, p-GaN microhillocks are grown on the top of a standard multiple-quantum-well (MQW) light-emitting diode (LED) with novel nonlithographic random masking. Such microhillocks can dramatically increase the external efficiency of the LED because of the destroyed symmetry of LED interfaces. By controlling metalorganic chemical vapor deposition (MOCVD) growth conditions, p-GaN microhillocks of various densities and sizes can be easily grown on a standard LED structure. The use of this novel method to grow microhillocks on the top of the LED can facilitate the control of the leakage current of LED compared that of the photo enhanced chemical (PEC) wet etch and inductively coupled plasma (ICP) dry etch methods.

Using Planarized p-GaN Layer to Reduce Electrostatic Discharged Damage in Nitride-Based Light-Emitting Diode

Electrostatic discharge damage is a serious problem on nitride-based light-emitting diodes (LEDs), due to their large lattice mismatch between III–nitride material and the sapphire substrate, which induces high-density threading dislocations. In this study, GaN/GaInN-based LEDs with various thicknesses of the low-temperature planarized p-GaN layer were fabricated. We found that when the V-shaped defects were filled by the planarized p-GaN layer, the survival rate of LEDs under human-body mode -4000 V stress increases from 23 to 93% and the survival rate under machine mode -600 V stress increases from 20 to 67%. Thus the ability to endure higher electrostatic discharge stress will be greatly improved.

Wide bandwidth AlAs/AlGaAs tandem Bragg reflectors grown by organometallic vapor phase epitaxy

Wide bandwidth AlAs/Al0.6Ga0.4As tandem Bragg reflectors were grown by organometallic vapor phase epitaxy. Quarter‐wave reflector stacks designed for different wavelengths were placed in cascade in epitaxially grown structures to expand the high reflectance bands. Intermediate low‐index layers were put in between every two stacks to suppress the transmission peaks in the centers of the combined high reflectance bands. While a single‐stack structure showed a full width half‐maximum bandwidth of 500 A, the two‐stack and the three‐stack structures effectively doubled and tripled this bandwidth to approximately 1000 and 1500 A, respectively.

Majority- and minority-carrier traps in Te-doped AlInP

The properties of deep levels found in Te-doped AlInP grown by metal–organic chemical vapor deposition have been studied. By using pn-junction structure, both minority- and majority-carrier traps can be observed. Two deep levels are found in Te-doped AlInP: one majority-carrier trap and one minority-carrier trap. The activation energies of majority- and minority-carrier traps are 0.24±0.05 and 0.25±0.03 eV, respectively. The majority-carrier trap is uniformly distributed, indicating that this level belongs to some kind of bulk defect.

Influence of barrier growth temperature on the properties of InGaN/GaN quantum wells

This study invests the effect of barrier growth temperature on the properties of InGaN/GaN MQW. Increase the growth temperature will reduce the well thickness and result in the blue shift of the PL peak. This blue shift in PL peak wavelength may be resulted from the stain occur during varying barrier growth temperature rather than only the reduce the well width. Moreover, we introduce a phase separation enhance layer into InGaN/GaN MQW. This layer join with the variation of barrier growth temperature will enhance the phase separation in InGaN/GaN MQW. There are two peaks clearly revealed in RT PL spectra. The higher energy peak might originate the InGaN quasi-wetting layer on the GaN barrier surface. The other one is interpreted of localize state at potential fluctuation owning to phase separation.

Thermal Stability of Plasma-Treated Ohmic Contacts to n-GaN

In this study, the thermal stability of plasma-treated ohmic contacts by either Cl2/Ar or Ar plasma to n-GaN was investigated by high-temperature aging tests. With proper plasma treatment, ohmic contacts to n-GaN have a lower contact resistance than those without plasma treatment. High-temperature aging tests were performed at temperatures ranging from 400 to 600°C for 2 h in N2 or air ambient. No apparent electrical degradation in contact resistance was observed after aging, showing the thermal stability of plasma-treated ohmic contacts is not affected by the recovery of plasma-induced damages on the wafer surface.

Thickness dependence of current conduction and carrier distribution of GaAsN grown on GaAs

Thickness dependence of the properties of GaAsN grown on GaAs was investigated by characterizing GaAs/GaAs0.982N0.018/GaAs Schottky diodes by current–voltage (I–V), capacitance–voltage (C–V) profiling and deep-level transient spectroscopy (DLTS). I–V characteristics show a considerable increase in the saturation current when the GaAsN thickness is increased from 60 to 250 A. As GaAsN thickness is increased further, the I–V characteristic deviates from that of a normal Schottky diode with a large series resistance. These I–V characteristics correlate well with carrier distribution. In thick GaAsN samples, C–V profiling shows carrier depletion in the top GaAs layer and frequency-dispersion accumulation in the GaAsN layer. DLTS spectra show that the carrier depletion in the top GaAs layer is due to an EL2 trap and the frequency-dispersion accumulation is due to the removal of electrons from a trap at 0.35 eV in the GaAsN layer. Increasing the GaAsN thickness markedly increases the magnitude of both traps. The large series resistance in thick GaAsN samples is due to EL2 that markedly depletes the top GaAs layer.

Phosphorus Vacancy as a Deep Level in AlInP Layers

Deep levels in AlInP layers, grown by metal-organic chemical vapor deposition (MOCVD) with various V/III mole ratios, have been carefully investigated by deep-level transient spectroscopy (DLTS). A deep level originating from phosphorus vacancy was observed with the activation energy of 0.65 eV. Examining this phosphorus-vacancy-related deep level provided a relatively simple means of understanding the phosphorus vacancy in AlInP, thus allowing us to determine an appropriate V/III mole ratio for growing AlInP.