J. Limb

GaN ultraviolet avalanche photodiodes with optical gain greater than 1000 grown on GaN substrates by metal-organic chemical vapor deposition

We report the performance of GaN p-i-n ultraviolet avalanche photodiodes grown on bulk GaN substrates by metal-organic chemical vapor deposition. The low dislocation density in the devices enables low reverse-bias dark currents prior to avalanche breakdown for ∼30μm diameter mesa photodetectors. The photoresponse is relatively independent of the bias voltage prior to the onset of avalanche gain which occurs at an electric field of ∼2.8MV∕cm. The magnitude of the reverse-bias breakdown voltage shows a positive temperature coefficient of ∼0.05V∕K, confirming that the avalanche breakdown mechanism dominates. With ultraviolet illumination at λ∼360nm, devices with mesa diameters of ∼50μm achieve stable maximum optical gains greater than 1000. To the best of our knowledge, this is the highest optical gain achieved for GaN-based avalanche photodiodes and the largest area III-N avalance photodetectors yet reported.

Performance of Deep Ultraviolet GaN Avalanche Photodiodes Grown by MOCVD

We report high-performance GaN ultraviolet (UV) p-i-n avalanche photodiodes (APDs) fabricated on bulk GaN substrates. The fabricated GaN p-i-n diodes demonstrated optical gains > 104 and low dark current densities operating at wavelengths from 280 to 360 nm. The result is among the highest III-N-based APD gains at the deep UV wavelength of 280 nm reported to date.

Control of quantum-confined Stark effect in InGaN∕GaN multiple quantum well active region by p-type layer for III-nitride-based visible light emitting diodes

We demonstrate the control of the quantum-confined Stark effect in InGaN∕GaN quantum wells (QWs), grown along the [0001] direction as part of the active region of visible light emitting diodes (LEDs). The effect can be altered by modifying the strain applied to the active region by the hole injection and contact layers. The optical characteristics and electrostatic potentials of the active region of the visible LEDs with different p-type layers are compared. LEDs with p-InGaN on top of the active region show a reduced blueshift in the peak wavelength with increasing injection current and a lower potential difference across the QW than those with p-GaN layers. The electrostatic potentials across the QW have estimated average values of ∼0.8 and ∼1.3MV∕cm for the active region of LEDs of current study with p-InGaN and p-GaN layers, respectively.

Graded-base InGaN∕GaN heterojunction bipolar light-emitting transistors

The authors report radiative recombination from a graded-base InGaN∕GaN heterojunction bipolar transistor (HBT) grown by metal-organic chemical vapor deposition on sapphire. For a device with a 40×40μm2 emitter area, a differential dc current gain of 15 is measured from the common-emitter current-voltage characteristics, with the HBT breakdown voltage BVCEO>65V. The heterojunction bipolar light-emitting transistor exhibits a base-region recombination radiation peak in the visible spectral range with a dominant peak at λ=385nm (blue emission).

Effect of internal electrostatic fields in InGaN quantum wells on the properties of green light emitting diodes

Variations in the strength of the piezoelectric field inside InGaN quantum wells have been observed along the growth direction in InGaN-based diodes emitting light in the green region. The internal electrostatic potential distribution across the active region consisting of five InGaN quantum wells has been determined by electron holography in a transmission electron microscope. The strength of the piezoelectric field decreases in the direction towards the p-n junction. Its effect on light emission has been evaluated by depth-profiling cathodoluminescence, where the emission from two peaks becomes increasingly distinct with increasing excitation voltage. The drop in piezoelectric field strength is proposed to be related to the neutralization of piezoelectric charges by hydrogen ions which are initially abundant in the p region and diffuse into the quantum wells during thermal annealing.

Blue light emitting diodes grown on freestanding (11-20) a -plane GaN substrates

Visible blue light emitting diodes have been produced on freestanding nonpolar GaN (11-20) a-plane substrates by metal-organic chemical vapor deposition. The growth conditions have been optimized for smooth growth morphology of GaN nonpolar homoepitaxial layers without surface features, leading to light emitting diode epitaxial structures that are free of crystalline defects such as threading dislocations and stacking faults. Electroluminescence of light emitting diodes exhibit peak wavelengths of ∼450nm and are independent of current level at low current densities before the heating effects are evidenced.

Optimization of Fe doping at the regrowth interface of GaN for applications to III-nitride-based heterostructure field-effect transistors:

The authors have studied the effects of Fe doping at the interface between GaN epitaxial layers for heterostructure field-effect transistors grown by metal-organic chemical vapor deposition and the corresponding impact on the device characteristics. The epitaxial structures were grown with different Fe-doped GaN layers at the layer-template interface. Analysis of the measured electron and interface charge distributions in the heterostructures demonstrated the important role of Fe doping at the regrowth interface. No charge at the regrowth interface was observed in transistor structures with a thick Fe-doped layer. Characterization of the electrical properties of the transistor structures revealed the presence of high sheet carrier concentrations and improved mobilities with increasing thickness of the Fe-doped GaN layer at the regrowth interface.

GaN full-vertical p-i-n rectifiers employing AlGaN:Si conducting buffer layers on n-SiC substrates

The development of a full-vertical GaN p-i-n rectifier on a 6H n-type SiC substrate by employing a conducting AlxGa1−xN:Si (x=∼0.1) buffer layer scheme is reported. In this vertical configuration, the n contact is made on the backside of the SiC substrate using a Ni∕Au metallization scheme. Epitaxial layers are grown by low-pressure metal organic chemical vapor deposition. The AlxGa1−xN:Si nucleation layer is proven to provide excellent electrical properties while also acting as a good buffer layer for subsequent GaN growth. The reverse breakdown voltage for a relatively thin 2.5μm thick i region was found to be over −330V. The devices also show a low on resistance of Ron of 7.5×10−3Ωcm2. This full-vertical configuration provides the advantage of the reduction of sidewall damage from plasma etching and lower forward resistance due to the reduction of current crowding in the bottom n-type layer.

III-nitride heterostructure field-effect transistors grown on semi-insulating GaN substrate without regrowth interface charge

Charge is observed at the regrowth interface for heterostructure field-effect transistors (HFETs) grown on semi-insulating (SI) bulk GaN substrates, even with Fe doping in the regrown buffer layer for reduction of the interface charge. Ultraviolet photoenhanced chemical (PEC) etching is used to treat the surface of SI bulk GaN substrates. Employing optimized etching conditions, a very smooth surface is achieved for the bulk GaN substrate after the etching. The charge at the regrowth interface is eliminated for HFETs grown on etched SI GaN substrates. Secondary ion mass spectrometry measurements show that the Si impurity concentration at the regrowth interface for HFETs grown on etched SI GaN substrates is much lower than that for HFETs grown on unetched SI GaN substrates, which suggests that the charge-containing layer on the SI substrate is removed by PEC etching and that the effects of the reduced charge layer near the regrowth interface can be eliminated by Fe doping for HFETs grown on etched SI substr...

Device operation of InGaN heterojunction bipolar transistors with a graded emitter-base design

The device operation of InGaN heterojunction bipolar transistors with a graded InGaN emitter-base design grown by metal organic chemical vapor deposition on sapphire substrates is demonstrated. The Gummel plot, current gain, and common-emitter current-voltage characteristics of the device are presented. The dc common-emitter current gain of a 25×25μm2 (emitter size) device increases with collector current and the current gain reaches a high value of 13 at IC=10mA with a base width of 100nm and a hole concentration of p=2×1018cm−3. The improved device performance is attributed to the graded emitter-base design as well as the high quality of the material made possible by the indium composition grading that enables the epitaxial growth of InGaN layers with a low density of pits or defects. The results demonstrate the potential of the graded InGaN emitter-base junction design in nitride heterojunction bipolar transistors.