Changzheng Sun

Y-branch integrated dual wavelength laser diode for microwave generation by sideband injection locking.

A Y-branch integrated dual wavelength laser diode is fabricated for optical microwave generation based on the principle of sideband injection locking. The device integrates a master laser and a slave laser with associated Y-branch coupler. By directly modulating the master laser near its relaxation resonance frequency, multiple sidebands are generated due to enhanced modulation nonlinearity. Beat signal with high spectral purity is obtained by injection locking the slave laser to one of the modulation sidebands. A millimeter-wave carrier of 42-GHz with a phase noise of -94.6 dBc/Hz at 10 kHz offset is demonstrated.

Fabrication and Packaging of 40-Gb/s AlGaInAs Multiple-Quantum-Well Electroabsorption Modulated Lasers Based on Identical Epitaxial Layer Scheme

High-speed AlGaInAs multiple-quantum-well (MQW) electroabsorption modulated lasers (EMLs) based on an identical epitaxial layer (IEL) integration scheme are developed for 40-Gb/s optical fiber communication systems. A self-aligned planarization technique has been adopted to reduce the capacitance of the electroabsorption modulator (EAM). The IEL structure EML chips exhibit a small signal modulation bandwidth around 40 GHz. The influence of residual reflection at the modulator facet on the small signal modulation response is investigated. Submount containing a grounded coplanar waveguide (GCPW) transmission line is used for packaging the EML chips into transmitter modules. The optimization of the GCPW structure to suppress resonances in frequency response due to parallel-plate modes is presented. Clear eye opening under 40-Gb/s nonreturn-to-zero (NRZ) modulation has been demonstrated for the packaged EML module.

1.55-/spl mu/m AlGaInAs-InP laterally coupled distributed feedback laser

A novel 1.55-/spl mu/m AlGaInAs-InP laterally coupled (LC) distributed feedback (DFB) semiconductor laser is presented. Longitudinal feedback and lateral optical confinement are realized simultaneously by incorporating deep gratings etched with inductively coupled plasma technique. Stable single-mode operation with a sidemode suppression ratio over 45 dB has been demonstrated for the fabricated LC-DFB laser.

40 GHz AlGaInAs multiple-quantum-well integrated electroabsorption modulator/ distributed feedback laser based on identical epitaxial layer scheme

An AlGaInAs multiple-quantum-well (MQW) distributed feedback (DFB) laser is monolithically integrated with a lumped-electrode electroabsorption (EA) modulator based on an identical epitaxial layer integration scheme. The device exhibits a threshold current of 12 mA and an extinction ratio of higher than 13 dB under a 3 V reverse bias. By adopting a dry-etched high-mesa ridge waveguide and planar electrode structures, the capacitance of the modulator is reduced to about 0.11 pF and a 3 dBe modulation bandwidth of over 40 GHz has been demonstrated. To our knowledge, this is the first report on a 40 GHz operation of AlGaInAs integrated light sources.

Highly Selective Dry Etching of GaN over AlGaN Using Inductively Coupled Cl2/N2/O2 Plasmas

A systematic study of the selective etching of GaN over Al0.28Ga0.72N was performed using Cl2/N2/O2 inductively coupled plasmas (ICP). Highly selective etching at high GaN etch rate is realized by optimizing the O2 flow rate, the ICP power and the chamber pressure. Maximum etching selectivity of about 60:1 at a GaN etch rate of 320 nm/min has been demonstrated. X-ray photoelectron spectroscopy (XPS) analysis shows that the effective oxidation of AlGaN due to the addition of a small fraction of O2 is crucial to obtain highly selective etching at high GaN etch rate.

Study on efficiency droop in InGaN/GaN light-emitting diodes based on differential carrier lifetime analysis

Efficiency droop is currently one of the most popular research problems for GaN-based light-emitting diodes (LEDs). In this work, a differential carrier lifetime measurement system is optimized to accurately determine carrier lifetimes (τ) of blue and green LEDs under different injection current (I). By fitting the τ-I curves and the efficiency droop curves of the LEDs according to the ABC carrier rate equation model, the impact of Auger recombination and carrier leakage on efficiency droop can be characterized simultaneously. For the samples used in this work, it is found that the experimental τ-I curves cannot be described by Auger recombination alone. Instead, satisfactory fitting results are obtained by taking both carrier leakage and carriers delocalization into account, which implies carrier leakage plays a more significant role in efficiency droop at high injection level.

Enhancement of electron-longitudinal optical phonon coupling in highly strained InGaN/GaN quantum well structures

An anomalously strong phonon replica is observed in the photoluminescence (PL) spectra of In0.23Ga0.77N/GaN multiple quantum well (MQW) samples at 10 K. The Huang-Rhys factor for the two-well sample reaches 0.95, in contrast to the value of 0.45 for the five-well sample obtained with the same growth conditions. Narrow linewidth and clearly resolved emission peak splitting are also exhibited in the two-well structure. The strong enhancement of electron-longitudinal optical (LO) phonon coupling is attributed to the interface features and the high residual strain in the quantum well structure. A model is proposed to account for the experimental results that takes the interface optical phonon mode as well as the impact of the residual strain into consideration.

Back-to-Back UTC-PDs With High Responsivity, High Saturation Current and Wide Bandwidth

A novel back-illuminated mesa-structure back-to-back uni-traveling-carrier photodiode (UTC-PD) is proposed and realized. By stacking two UTC-PDs to form a PINIP structure, high responsivity, high output saturation power, and wide bandwidth are achieved simultaneously. The responsivity of a 24-μm -diameter device is as high as 0.86 A/W, and a 3-dB bandwidth up to 28.3 GHz is attained. The saturation current up to 62 mA at 1-dB compression point is demonstrated at the frequency of 24 GHz.

Characteristics of n-GaN After Cl2/Ar and Cl2/N2 Inductively Coupled Plasma Etching

A systematic study on the effect of inductively coupled plasma (ICP) etching on n-type GaN is presented. The optical and electrical properties and surface stoichiometry of n-type GaN are evaluated using room-temperature photoluminescence (PL) and current–voltage (I–V) characteristic measurements, and X-ray photoelectron spectroscopy (XPS), respectively. Investigation of the effect of additive gas (N2 and Ar) and RF power on these characteristics has also been carried out. It is shown that the decrease in the O/Ga ratio after ICP etching can suppress the deterioration of the near-band-edge emission intensity. Furthermore, N vacancy (VN) with a shallow donor nature and Ga vacancy (VGa) with a deep acceptor nature are generated after ICP etching upon the addition of Ar and N2 to Cl2 plasma, respectively. Lower ohmic contact resistance could be obtained when VN or ion-bombardment-induced defect is dominant at the surface. Improved etching conditions have been obtained based on these results.

Nonselective and smooth etching of GaN/AlGaN heterostructures by Cl2/Ar/BCl3 inductively coupled plasmas

A systematic study of the nonselective and smooth etching of GaN/AlGaN heterostructures was performed using Cl2/Ar/BCl3 inductively coupled plasmas (ICPs). Nonselective etching can be realized by adjusting the BCl3 ratio in the Cl2/Ar/BCl3 mixture (20%–60%), increasing the ICP power, and decreasing the chamber pressure. Surface morphology of the etched heterostructures strongly depends on the gas chemistry and the chamber pressure, whereas selectivity and surface morphology show a slight dependence on the dc bias and total flow rate. Specifically, with the addition of 20% BCl3 to Cl2/Ar (4:1) gas mixture, nonselective etching of GaN/Al0.28Ga0.72N heterostructures at a high etch rate is maintained and the surface root-mean-square roughness is reduced from 10.6 nm to 0.5 nm, which is smoother than the as-grown sample. Auger electron spectroscopy analysis shows that the effective removal of residual oxygen from the surface of AlGaN during the etching process is crucial to the nonselective and smooth etching of GaN/AlGaN herterostructures at high etch rate.A systematic study of the nonselective and smooth etching of GaN/AlGaN heterostructures was performed using Cl2/Ar/BCl3 inductively coupled plasmas (ICPs). Nonselective etching can be realized by adjusting the BCl3 ratio in the Cl2/Ar/BCl3 mixture (20%–60%), increasing the ICP power, and decreasing the chamber pressure. Surface morphology of the etched heterostructures strongly depends on the gas chemistry and the chamber pressure, whereas selectivity and surface morphology show a slight dependence on the dc bias and total flow rate. Specifically, with the addition of 20% BCl3 to Cl2/Ar (4:1) gas mixture, nonselective etching of GaN/Al0.28Ga0.72N heterostructures at a high etch rate is maintained and the surface root-mean-square roughness is reduced from 10.6 nm to 0.5 nm, which is smoother than the as-grown sample. Auger electron spectroscopy analysis shows that the effective removal of residual oxygen from the surface of AlGaN during the etching process is crucial to the nonselective and smooth etching ...