Chun-Min Lin

Blue light-emitting diodes with a roughened backside fabricated by wet etching

The InGaN-based light-emitting diodes (LEDs) with a roughened patterned backside on the N-face GaN surface were fabricated through a crystallographic etching process to increase light-extraction efficiency. After laser decomposition, laser scribing, and a lateral crystallographic wet etching process at the GaN/Al2O3 interface, stable crystallographic etching planes were formed as the GaN {1011¯} planes that included an angle with the top GaN (0001) plane measured at 58°. The GaN buffer layer acted as the sacrificial layer for the laser decomposition process and the lateral wet etching process with a 26 μm/min etching rate. The LED with the inverted pyramidal N-face GaN surface close to the GaN/Al2O3 interface has a larger light-scattering process than the conventional LED. The light-output power of the LED with the backside roughened surface had a 47% enhancement when measured in LED chip form.

Energy band structure of strained Si1−xCx alloys on Si (001) substrate

We report the energy band structures of strained Si1−xCx alloys on Si (001) substrates. All calculations are based on a 20×20 Hamiltonian matrix constructed from the linear combination of atomic orbital approximation with spin–orbit interaction, strain effect, and lattice disorder effect taken into account. The lattice disorder parameter is obtained from fittings with the experimental band gap of strained Si1−xCx alloy with small carbon concentration and reflects the initial reduction of band gap of relaxed Si1−xCx alloy, while simple virtual crystal approximation does not. The effect of strain on band structure is incorporated in terms of the interatomic interaction parameters, which are functions of bond length and bond angle. The strained Si1−xCx alloy becomes metallic when x=28%. All the directional effective masses are affected by the strain. Overall agreements are found between our theoretical calculations and recent experimental results.

InGaN-Based Light-Emitting Diodes with a Cone-Shaped Sidewall Structure Fabricated Through a Crystallographic Wet Etching Process

The InGaN-based light-emitting diodes (LEDs) were fabricated through a crystallographic etching process to increase their light extraction efficiency. After the laser scribing and the selective lateral wet etching processes at the LED chip edge region, the stable crystallographic etching planes were formed as the GaN {1012} planes and had an including angle with the top GaN (0001) plane measured as 40.3°. The AlN buffer layer acted as the sacrificial layer for the lateral wet process with a 27.5 μm/h etching rate. The continuous cone-shaped sidewall (CSS) structure of the treated LED has a larger light-scattering area and higher light extraction cones around the LED chips. The LED with the CSS structure around the chip edge region has a higher light output power compared to a conventional LED when measured in LED chip form.

Hole effective masses in relaxed Si1−xCx and Si1−yGey alloys

We report hole effective mass calculations of Si1−xCx and Si1−yGey alloys. All calculations are based on a 16×16 Hamiltonian matrix constructed from the linear combination of atomic orbital approximation with spin-orbit interaction taken into consideration. The 1 meV constant energy surfaces below the valence band edge are used to determine the nominal hole effective masses. The effective masses of light hole and heavy hole of Si1−yGey alloys vary as linear functions of Ge content and increase linearly as the hole energy increases from 1 to 15 meV. The heavy hole effective masses of Si1−xCx alloys, however, exhibit a totally different trend. The effective mass of Si1−xCx remains relatively unchanged from x=0.0 to x=0.9, and increases abruptly by a factor of two from x=0.9 to x=1.0. The nonparabolicity increases as the C content rises up to x=0.9, and nearly disappears when turning into pure diamond. The interaction between the split-off hole band and the heavy hole band is proposed for the anomalous behav...

InGaN light emitting diodes with a laser-treated tapered GaN structure.

InGaN light-emitting diode (LED) structures get an air-void structure and a tapered GaN structure at the GaN/sapphire interface through a laser decomposition process and a lateral wet etching process. The light output power of the treated LED structure had a 70% enhancement compared to a conventional LED structure at 20 mA. The intensities and peak wavelengths of the micro-photoluminescence spectra were varied periodically by aligning to the air-void (461.8nm) and the tapered GaN (459.5nm) structures. The slightly peak wavelength blueshift phenomenon of the EL and the PL spectra were caused by a partial compressed strain release at the GaN/sapphire interface when forming the tapered GaN structure. The relative internal quantum efficiency of the treated LED structure (70.3%) was slightly increased compared with a conventional LED (67.8%) caused by the reduction of the piezoelectric field in the InGaN active layer.

InGaN Light-Emitting Diodes With the Inverted Cone-Shaped Pillar Structures

An InGaN-based light-emitting diode (LED) with an inverted cone-shaped pillar structure was fabricated through a plasma dry etching process and a photoelectrochemical (PEC) process. The undercut structure was fabricated through a bandgap-selective PEC etching process that occurred at the InGaN active layer. Then, the inverted cone-shaped pillar structure was formed through a bottom-up crystallographic etching process in a hot potassium hydroxide solution. The light-output power of the LED with an inverted cone-shaped pillar structure had a 42% enhancement compared with the standard LED without the pillar structure at a 20-mA operating current. A higher light intensity of the PEC-treated LED was observed around the mesa-edge region and the pillar structures as a result of a higher light-scattering process occurring at the inverted cone-shaped structure.

Green Light-Emitting Diodes with a Photoelectrochemically Treated Microhole-Array Pattern

Green InGaN-based light-emitting diodes (LEDs) with roughened microhole-array (MHA) structures were fabricated through a dry etching process and a photoelectrochemical (PEC) process. The PEC process consisted of a bandgap-selective lateral etching process at the InGaN active layer, an N-face bottom-up crystallographic etching process at the bottom p-type GaN:Mg layer, and a PEC oxidation process at the n-type GaN:Si surface. The light output power of the MHA-LED and the photoelectrochemically treated microhole-array light-emitting diode (PMHA-LED) had 7 and 65% enhancement, respectively, compared to a conventional LED at a 20 mA operation current.

InGaN-Based Light-Emitting Diodes With Nanoporous Microhole Structures

InGaN-based light-emitting diodes (LEDs) with nanoporous microhole array (NMA) structures were fabricated through photoelectrochemical wet oxidation and oxide-removing processes. The average size of the nanoporous structure at the microhole regions was measured at 60-80 nm. Forward voltages were measured at 3.47 and 3.68 V for a standard LED (ST-LED) and an NMA-LED, respectively, the latter caused by the higher contact resistance at the nanoporous GaN:Mg surface. The light output power of the NMA-LED had a 40.5% enhancement compared with the ST-LED on nonencapsulated LEDs in chip form. The higher light scattering process occurred at the NMA structure on the GaN:Mg surface and at the ringlike patterns on the GaN:Si structure. The results were a higher light extraction efficiency and a larger divergent angle in the NMA-LED.

InGaN-Based Light-Emitting Diodes with a Multiple-Air-Gap Layer

InGaN/GaN multiple-quantum-well light-emitting diodes (LEDs) that have a multiple-air-gap (MAG) structure to increase light extraction efficiency and to reduce the piezoelectric field in an InGaN active layer are described. The LED structure was regrown on a GaN template layer that consists of the photoelectrochemical (PEC)-treated nanoporous GaN:Si layer with a bottom MAG structure and a top GaO x layer that acts as a self-assembled micromask for the epitaxial lateral overgrowth process. Electroluminescence (EL) spectra having a 5.0 nm blueshift phenomenon and a peak intensity of up to 52% enhancement in the MAG-LED compared to a standard light-emitting diode (ST-LED) at 20 mA operating current were obtained. The wavelength blueshift of the EL spectra, with an increasing injection current, in the MAG-LED (5.9 nm) was smaller than that in the ST-LED (8.1 nm). The wavelength blueshift of the micro-photoluminescence spectra, when varying the reverse-bias voltage from 0 to -13 V, was measured as 5.7 and 3.9 nm in the ST-LED and in the MAG-LED, respectively. A higher light extraction efficiency and a lower piezoelectric field in the InGaN wells were observed in the MAG-LED by inserting the PEC-treated MAG structure and a top GaO x layer.

Hole effective mass in strained Si1−xCx alloys

The directional, density-of-states, and carrier-concentration effective masses of light, heavy, and split-off holes have been calculated for strained Si1−xCx alloys on Si (001) substrate. The results for the directional effective mass show that the effect of strain makes the constant energy surface of light holes near the band edge more symmetric than that in pure silicon. The effect of strain on the heavy and split-off hole bands is rather regular; up to 7% of carbon concentration the strain effect monotonically reduces the density-of-states effective mass for the two bands at energy values within energy interval of 0.4eV below the valence band edge. This reduction is obtained for the carrier-concentration effective mass at temperatures from 0to600K. The strain effect on the light hole band is less trivial; at nonzero carbon concentrations the strain effect influences the density-of-states and the carrier-concentration effective mass in a similar way as it does to the heavy and split-off bands but irregu...