p-GaN Cap Research Articles

The Effects of a Low-Temperature GaN Interlayer on the Performance of InGaN/GaN Solar Cells

An InGaN/GaN p—i—n solar cell inserted with a 5-nm low-temperature (LT) GaN interlayer between the p-GaN cap layer and the InGaN i-layer is grown on a c-plane sapphire substrate by metal organic chemical vapor deposition. The effects of the LT GaN interlayer on the performance of the InGaN/GaN solar cells are investigated. It is found that the LT-GaN interlayer prevents the extension of threading dislocations from the InGaN layer to the p-GaN layer and improves the crystal quality of both the p-GaN cap layer and the InGaN i-layer, ultimately leading to an increasing external quantum efficiency and photocurrent density of the InGaN/GaN solar cells.

Improvements in Microstructure and Leakage Current of High-In-Content InGaN p-i-n Structure by Annealing

The effects of annealing on the microstructure and electrical properties of a high-In-content InGaN p-i-n structure have been investigated. After the annealing under optimal conditions, the surface roughness of the InGaN structure reduces pronouncedly, accompanied by an improvement in the crystalline quality of the p-GaN cap layer, and an incorporation process of segregated In atoms into the p-InGaN layer occurs simultaneously and results in an increase of about 3% indium content. Furthermore, the annealing significantly improves the pn-junction behavior of the InGaN p-i-n structure by two orders of magnitude reduction in the reverse leakage current.

A new method to reduce the dark current of GaN based Schottky barrier ultraviolet photodetector

A new method to reduce the dark current of GaN based Schottky barrier ultraviolet photodetector is proposed. In comparision with conventional i-GaN/n＋-GaN structure, an additional thin p-GaN cap layer is introduced on the i-GaN(n--GaN) in the new structure. The simulation results showed that the additional layer makes the dark current to decrease in the photodetector due to the increase of the Schottky barrier height. The effects of thickness and carrier concentration of p-GaN layer on the dark current of the photodetector were also studied. It is suggested that the dark current of the new structure device could be better reduced by employing p-GaN with higher carrier concentration as the cap layer.

Inductively Coupled Plasma Mesa Etched InGaN/GaN Light Emitting Diodes Using Cl2/BCl3/Ar Plasma

InGaN/GaN multiple-quantum-well light-emitting diodes (LEDs) with 800 °C- and 1000 °C-grown p-GaN cap layers were fabricated. Inductively coupled plasma (ICP) using Cl2/BCl3/Ar was used to etch the surface of the InGaN/GaN LEDs. Different compositions of the Cl2/BCl3/Ar gas system were used to reduce surface roughness after the plasma etching process, that results from the low-temperature (800 °C)-grown p-type GaN with a higher hole concentration. The maximum etching rate (∼6000 Å/min) was attained approximately 10% BCl3 in Cl2/BCl3/Ar plasma. Sputter desorption and surface morphology are significantly improved. Furthermore, surface oxidation is suppressed.

Effects of the thickness of capping layers on electrical properties of Ni ohmic contacts on p-AlGaN and p-GaN using an ohmic recessed technique

In this study, the effects of the thickness of p-GaN (p-InGaN) capping layers grown on top of p-AlGaN (p-GaN) on electrical properties of Ni ohmic contacts on p-AlGaN (p-GaN) were investigated. The experiments and simulations indicated that the thicker p-GaN (p-InGaN) capping layer grown on p-AlGaN (p-GaN) led to the formation of the higher concentration of a two-dimensional hole gas (2DHG) at the interfaces. Consequently, we deduce that holes can be easily injected into the p-AlGaN (p-GaN) layer through recessed channels and a 2DHG channel, which results in the formation of the low contact resistivity. This provides a rational guideline for the development of new processing methodologies to enhance nitride semiconductor-based optoelectronic devices.

Origins of Parasitic Emissions from 353 nm AlGaN-based Ultraviolet Light Emitting Diodes over SiC Substrates

The effects of p-GaN capping layer and p-type carrier-blocking layer on the occurrence of parasitic emissions from 353 nm AlGaN-based light emitting diodes (LEDs) have been investigated. LEDs without a p-type Al0.25Ga0.75N carrier-blocking layer showed a shoulder peak at ∼370 nm due to electron overflow into the p-Al0.10Ga0.90N cladding layer and subsequent electron–hole recombination in the acceptor levels. Broad emission between 380 and 450 nm from LEDs having a p-GaN capping layer was caused by luminescence at 420 nm from the p-GaN capping layer, which was optically pumped by 353 nm UV emission from the quantum wells. Broad, defect-related luminescence centered at ∼520 nm was emitted from the AlGaN layers within the quantum wells.

Study of temperature dependent electroluminescence of InGaN/GaN multiple quantum wells using low temperature scanning near-field optical microscopy

Though GaN based semiconductor materials and devices have achieved giant commerc ial success, there were few reports on their electroluminescent near-field optic al studies at low temperature. In this paper we present our results of the elect roluminescent near-field images and spectra at both room temperature and liquid nitrogen temperature by using a lab-made low temperature scanning near-field opt ical microscope. We found that with the decreasing of sample temperature, the fl uctuation of electroluminescent intensity in the near-field images is reduced gr eatly and the peak photon energy of the spectra emitted from the quantum wells e xhibits a blue-shift at first and then a red-shift. A new spectral peak emerges at higher photon energy at liquid nitrogen temperature. According to our analysi s, this higher photon energy peak is attributed to the transition from the botto m of conduction band to the acceptor energy states in the p-GaN cap layer.

Origins of Parasitic Emissions from 353 nm AlGaN-based UV LEDs over SiC Substrates

AbstractThe effects of p-GaN capping layers and p-type carrier-blocking layers on the occurrence of parasitic emissions from 353 nm AlGaN-based LEDs have been investigated. LEDs without a p-type Al0.25Ga0.75N carrier-blocking layer showed a shoulder peak at ∼370 nm due to electron overflow into the p-Al0.10Ga0.90N cladding layer and subsequent electron-hole recombination in the acceptor levels. Broad emission between 380 and 450 nm from LEDs having a p-GaN capping layer was caused by 420 nm luminescence from the p-GaN capping layer, which was optically pumped by 353 nm UV emission from the quantum wells. Broad, defect-related luminescence at ∼520 nm was emitted from the AlGaN layers within the quantum wells.

Improved light output power of InGaN/GaN MQW LEDs by lower temperature p-GaN rough surface

Mg-doped p-GaN epitaxial layers grown at different temperatures were prepared and characterized. It was found that we could achieve a higher hole concentration and a rough surface by reducing the growth temperature down to 800 °C. In 0.23Ga 0.77N/GaN multiquantum well (MQW) light-emitting diodes (LEDs) with such a low 800 °C-grown p-GaN cap layer were also fabricated. It was also found that one could lower the LED operation voltage from 3.68 to 3.36 V and enhance the 20 mA LED output power by the 800 °C-grown p-GaN cap layer. However, it was also found the leakage current was larger and the lifetime was shorter for the LEDs with the 800 °C-grown p-GaN cap layer.

In 0.23Ga 0.77N/GaN MQW LEDs with a low temperature GaN cap layer

Mg-doped p-GaN epitaxial layers prepared at different temperatures were prepared and characterized. It was found that we could achieve a higher hole concentration and a rough surface by reducing the growth temperature down to 800 °C. In 0.23Ga 0.77N/GaN multiquantum well (MQW) light emitting diodes (LEDs) with such a low 800 °C-grown p-GaN cap layer were also fabricated. It was found that we could enhance the LED output intensity by more than 90% with the low 800 °C-grown p-GaN cap layer, as compared to the conventional high 1000 °C-grown p-GaN cap layer.

Electrical and Optical Characteristics of Delta Doped AlGaN Cladding Layer Materials for Highly Efficient 340nm Ultra Violet LEDs

ABSTRACTIn this paper, we report the electrical and optical characteristics of Si delta-doped AlGaN cladding layers, p-cladding structure optimization and the impact on the efficiency of 340nm AlGaN UV LEDs. Compared to the uniformly doped n-AlGaN layer, adding Si Δ-doping layers reduced the sheet resistance by improving both the Hall mobility and carrier concentration. Increasing the number of Si Δ-doped layers further lowered the sheet resistance without cracking the material. The Δ-doped layers in n-Al0.3Ga0.7N improved the optical properties by enhancing near band edge emission as much as 2-fold relative to deep level emission. Additionally, Δ-doping in n-AlGaN layers had no detrimental effect on the optical transparency of the LEDs. The p-cladding layer was found to have a strong absorption at 340nm. Reducing the p-GaN cap layer from 35nm to 10nm tripled the light emission intensity. By optimizing the n- and p-AlGaN cladding layers, a highly efficient UV LED at 340nm was achieved with 1mW output under 800mA/mm2 DC drive current.

Influence of the Layer Design on the Far Field Pattern in GaN Based Laser Structures

The waveguiding properties of nitride laser diodes are investigated for different Al concentrations in the cladding layers by calculating the optical field inside and by measuring the far field pattern behind the structures. The thicknesses of the p-cladding layer and of the electron blocking layer are also taken into account. The best optical confinement is reached for 10% and 12% Al in the n- and p-cladding layer, respectively. For these Al concentrations the full width at half maximum of the intensity distribution in the far field is increased by 15%. To prevent the leaking of the optical mode into the p-GaN cap layer the p-cladding layer should not be thinner than 270 nm.

P-capped GaN-AlGaN-GaN high-electron mobility transistors (HEMTs)

A novel p-capped GaN-AlGaN-GaN high-electron mobility transistor has been developed to minimize radio-frequency-to-dc (RF-DC) dispersion before passivation. The novel device uses a p-GaN cap layer to screen the channel from surface potential fluctuations. A low-power reactive ion etching gate recess combined with angle evaporation of the gate metal has been used to prevent gate extension and maintain breakdown voltage. Devices with gate lengths of 0.7 μm have been produced on sapphire. Current-gain cutoff frequencies (f/sub /spl tau//) of 20 GHz and maximum frequencies of oscillation (f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> ) of 38 GHz have been achieved. Unpassivated devices demonstrated a saturated output power of 3.0 W/mm and peak power-added efficiency of 40% at 4.2 GHz (V/sub DS/ = +20 V).

Ghost modes and resonant effects in AlGaN-InGaN-GaN lasers

Many diode laser structures, including those based on group-III nitride system, contain passive waveguide layers of higher refractive index than in the adjacent layers. Modes of such passive waveguides ("passive" modes) can interact with an active layer mode ("active" mode), giving rise to two kinds of normal modes or "supermodes" of a laser structure. Away from resonance, one of them is localized predominantly in the active region ("lasing" mode), while the other ones are located mostly in passive waveguides ("ghost" modes). The lasing mode is the mode at which laser generation occurs. The lossy ghost modes are parasitic modes of a laser structure that can consume energy from the active region. Resonant coupling occurs when the lasing mode and a ghost mode are in phase synchronism. In the paper, the concept of ghost modes is applied to InGaN-based diode lasers. The values for critical thickness are calculated for p-GaN cap layer and for n-GaN buffer/substrate layer for a particular multilayer laser structure. The typical thickness of 0.5 /spl mu/m of AlGaN-cladding layer is found to be insufficient to prevent rather strong coupling between the modes. Under the resonant coupling conditions, the modal gain is shown to be strongly suppressed, allowing no lasing at all.