Dmitry Sizov

500-nm Optical Gain Anisotropy of Semipolar (1122) InGaN Quantum Wells

We studied the effect of carrier population on light emission polarization of green InGaN quantum wells (QWs) on the semipolar (1122) plane. The 3 nm thick QWs emitting light at about 540 nm at low pumping power have electrical field (E) component E∥[1123] stronger than that E∥[1100]. However, we found that increasing the pumping power changed the sign of the polarization ratio. Using the varied stripe length (VSL) method, we measured the optical gain for light propagating ∥[1123] direction to be ~2 times that of light propagating ∥[1100] direction. We explain this behavior by inhomogeneous QW state filling.

Gallium Indium Nitride-Based Green Lasers

In this review article, we describe group-III nitride laser diodes that emit light in the green spectral range, using epitaxial structures grown on gallium nitride (GaN) substrates with c- and semipolar-plane orientations. We address the motivation for these lasers, the challenges faced in creating them, and the progress made in this field to date. Different structural design choices are described, taking into account specific material properties and crystal growth requirements for these orientations. We review various properties of the materials involved, including optical gain, optical confinement, internal optical losses and carrier injection. We also discuss mechanical strain during the growth of active and passive regions, and the way in which it limits the structural design. Various aspects of laser chip fabrication are discussed, including self-aligned ridge waveguides and facet formation. Finally, we outline the status of green laser reliability and challenges in this area.

Carrier Transport in InGaN MQWs of Aquamarine- and Green-Laser Diodes

We studied experimentally and theoretically the substrate-orientation impact on carrier transport and capture in InGaN multiple quantum well (MQW) laser diodes (LDs) with emission in the aquamarine-green spectral range. A new simulation approach was developed to analyze this behavior of LEDs and LDs emitting at these wavelengths. We show that due to deep carrier confinement, the thermal escape from a QW in such devices is negligible. The carrier distribution among QWs is therefore determined by the carrier transport and capture rates. We also show that the ballistic transport mechanism is dominant in this type of MQW active region. In c-plane structures, this mechanism is tunneling-assisted, and therefore, the transport is much slower than in nonpolar and semipolar structures. Because of this, a strong carrier injection nonuniformity observed in c-plane LDs, causes the threshold current increase when number of QWs is >;2. This effect is not observed in semipolar LDs because the carrier transport rate is faster than the capture rate.

Impact of Carrier Transport on Aquamarine-Green Laser Performance

We studied the carrier transport phenomena of the multiple-quantum-well (MQW) active region and their impact on the performance of aquamarine and green laser diodes (LDs) grown on polar and semipolar planes. The ballistic carrier transport mechanism was found to be dominant in the MQW region. For the c-plane, because of the high hole capture probability and slow escape rate, mainly the quantum wells (QWs) positioned close to the p-side are electrically pumped. The optical loss induced by the underpumped QWs further away from the p-side leads to significantly higher laser threshold current density and a longer lasing wavelength with increased number of QWs. These effects are not significant for semipolar LD structures.

True-green (11-22) plane optically pumped laser with cleaved m-plane facets

We present a detailed experimental study of optical property of green InGaN quantum wells and optically pumped lasers, with cleaved m-plane facets and a lasing wavelength in the range of 520-530 nm, grown on semipolar (11-22) planes. Taking advantage of low transparency carrier density of the lowest-energy valence band and overcoming low differential gain by minimizing the optical loss of the laser structure, we demonstrated a low threshold pumping power of 120 mW.

60 mW Pulsed and Continuous Wave Operation of GaN-Based Semipolar Green Laser with Characteristic Temperature of 190 K

We studied characteristic temperatures (T0) of laser diodes (LDs) grown on semipolar GaN substrates and emitting in the green spectral range. For several semipolar laser designs with and without an electron blocking layer (EBL), T0 remains higher (161–246 K) than that typically reported for c-plane green LDs. The slope efficiency measured in the pulsed regime is nearly temperature independent. These observations indicate that T0 is mainly determined by intrinsic quantum well (QW) properties, such as higher differential gain. A high T0 and a sufficient injection efficiency allow the achievement of a continuous wave output power of 60 mW for an LD without an EBL.

Internal Optical Waveguide Loss and p-Type Absorption in Blue and Green InGaN Quantum Well Laser Diodes

We present a new characterization method for internal optical waveguide loss of blue, aquamarine, and green group-III–nitride laser diodes from as-grown wafers without need for further fabrication. This approach relies on excitation-position dependent polarization-resolved photoluminescence spectra collected from the edge of the planar waveguide. The high measurement accuracy of ±1 cm-1 enables for the first time determination of the mechanisms for p-layer optical loss from the waveguide loss difference before and after Mg dopant activation. Temperature-dependent measurements show that the dominant optical loss mechanism is absorption by acceptor-bound holes. This absorption mechanism does not depend significantly on light polarization.

Lasing and optical gain around 500 nm from optically pumped lasers grown on c -plane GaN substrates

Using the variable stripe length method we demonstrated positive net optical gain around 500 nm for an optically pumped laser with InGaN/GaN multiple quantum wells grown on c-plane freestanding GaN substrates. We found that owing to low optical gain, reducing optical losses is crucial to increasing lasing wavelength. We demonstrated lasing at 502 nm by using a 3-mm-long cavity with cleaved facets.

Optical absorption of Mg-doped layers and InGaN quantum wells on c-plane and semipolar GaN structures

We studied optical absorption of Mg-doped AlInGaN layers using excitation-position dependent and polarization resolved photoluminescence from the slab-waveguide edge of a laser structure. The major absorption in the Mg-doped layers was found only when p-doping is activated. It increases with the removal of residual hydrogen, which in case of Mg doping is a p-type passivation impurity, and reversibly disappears after passivation by hydrogen. This absorption is weakly wavelength and temperature dependent, and isotropic. This can be attributed to acceptor-bound hole absorption, because those holes concentration is nearly equal to that of activated acceptors and weakly temperature dependent (unlike the free hole concentration, which is much lower and is an exponential function of temperature due to high ionization energy). The cross section of photon absorption on such activated acceptor was quantified to be in the order of 10−17 cm−2. The absorption cross section of free electrons was found to be at least on...

Reliability and degradation of InAlGaN semipolar

We demonstrate >400 h continuous-wave operation with 50% output power degradation for InAlGaN ridge laser diodes with InGaN waveguide relaxed via interfacial misfit dislocations. The lasers with emission wavelength of 503 nm were grown on free-standing plane GaN substrates. After >1000 h of operation, the misfit dislocations remained localized in the waveguide/cladding interfaces and no degradation occurred via their motion or growth toward the active region. No intensity degradation was detected for spontaneous emission devices without mesa elements, which implies that point defect migration from the surfaces, induced by ridge dry etching, is responsible for the ridge laser degradation.