Masao Ikeda

GaN-based green laser diodes

Recently, many groups have focused on the development of GaN-based green LDs to meet the demand for laser display. Great progresses have been achieved in the past few years even that many challenges exist. In this article, we analysis the challenges to develop GaN-based green LDs, and then the approaches to improve the green LD structure in the aspect of crystalline quality, electrical properties, and epitaxial layer structure are reviewed, especially the work we have done.

Remarkably reduced efficiency droop by using staircase thin InGaN quantum barriers in InGaN based blue light emitting diodes

The efficiency droop of InGaN/GaN(InGaN) multiple quantum well (MQW) light emitting diodes (LEDs) with thin quantum barriers (QB) is studied. With thin GaN QB (3u2009nm–6u2009nm thickness), the efficiency droop is not improved, which indicates that hole transport cannot be significantly enhanced by the thin GaN QBs. On the contrary, the efficiency droop was remarkably reduced by using a InGaN staircase QB (InGaN SC-QB) MQWs structure where InGaN SC-QBs lower the transport energy barrier of holes. The efficiency droop ratio was as low as 3.3% up to 200u2009A/cm2 for the InGaN SC-QB LED. By using monitoring QW with longer wavelength we observe a much uniform carrier distribution in the InGaN SC-QB LEDs, which reveals the mechanism of improvement in the efficiency droop.

Injection current dependences of electroluminescence transition energy in InGaN/GaN multiple quantum wells light emitting diodes under pulsed current conditions

Injection current dependences of electroluminescence transition energy in blue InGaN/GaN multiple quantum wells light emitting diodes (LEDs) with different quantum barrier thicknesses under pulsed current conditions have been analyzed taking into account the related effects including deformation caused by lattice strain, quantum confined Stark effects due to polarization field partly screened by carriers, band gap renormalization, Stokes-like shift due to compositional fluctuations which are supposed to be random alloy fluctuations in the sub-nanometer scale, band filling effect (Burstein-Moss shift), and quantum levels in finite triangular wells. The bandgap renormalization and band filling effect occurring at high concentrations oppose one another, however, the renormalization effect dominates in the concentration range studied, since the band filling effect arising from the filling in the tail states in the valence band of quantum wells is much smaller than the case in the bulk materials. In order to c...

Room-temperature GaAs/InP wafer bonding with extremely low resistance

Low-temperature direct wafer bonding is a promising technique for fabricating multijunction solar cells with more than four junctions in order to obtain high conversion efficiencies. However, it has been difficult to reduce the bond interface resistance between a GaAs-based subcell wafer and an InP-based subcell wafer. We found that a novel bonding structure comprising heavily Zn-doped (1 x 10(19) cm(-3)) p(+)-GaAs and S-doped (3 x 10(18)cm(-3)) n-InP had an interface resistance of 2.5 x 10(-5) Omega.cm(2), which is the lowest value ever reported. This result suggests that the newly developed room-temperature wafer bonding technique has high potential to realize high-efficiency multijunction solar cells

Green laser diodes with low threshold current density via interface engineering of InGaN/GaN quantum well active region

By observing the morphology evolution of green InGaN/GaN quantum well (QW) and studying the catholuminescence (CL) property, we investigate indium-segregation-related defects that are formed at green InGaN/GaN QW interfaces. Meanwhile, we also propose the approach and suggest the mechanism to remove them for green InGaN/GaN QW grown on both GaN templates and free-standing GaN substrates. By engineering the interface of green InGaN/GaN QWs, we have achieved green laser diode (LD) structure with low threshold current density of 1.85 kA cm-2. The output power of the green LD is 58 mW at a current density of 6 kA cm-2 under continuous-wave operation at room temperature.

Hole transport in c-plane InGaN-based green laser diodes

Hole transport in c-plane InGaN-based green laser diodes (LDs) has been investigated by both simulations and experiments. It is found that holes can overflow from the green double quantum wells (DQWs) at high current density, which reduces carrier injection efficiency of c-plane InGaN-based green LDs. A heavily silicon-doped layer right below the green DQWs can effectively suppress hole overflow from the green DQWs.

Conductivity enhancement in AlGaN:Mg by suppressing the incorporation of carbon impurity

Growth conditions were explored to suppress the carbon impurity incorporation in AlGaN:Mg grown at low temperatures. Electrical properties of Al0.07Ga0.93N:Mg samples with various carbon concentrations were investigated by Hall measurements. A clear correlation between carbon concentration and electrical conductivity has been found. By reducing the carbon concentration from 2 x 10(18) to 5 x 10(16) cm(-3), the resistivity of p-Al0.07Ga0.93N decreases from 7.4 to 2.2 Omega.cm. From the results of the analysis of the charge neutrality equation, we found that the carbon concentration is close to the compensating donor concentration in the AlGaN:Mg samples, which suggests that carbon acts as the main compensating donor in AlGaN:Mg

Optical characterization of InGaN/GaN quantum well active region of green laser diodes

We performed the optical characterization of InGaN/GaN quantum well (QW) active regions of green laser diodes (LDs) with different threshold current densities by temperature-dependent photoluminescence (PL) analysis. The internal quantum efficiency (IQE) was evaluated to be 39 and 59% for green LDs with threshold current densities of 8.50 and 1.85 kA cm−2, respectively. Additional nonradiative recombination centers with an activation energy of 10 meV were found in the sample with the lower IQE, which is attributed to defects located at the interface of InGaN/GaN QWs.

III–V compound semiconductor multi-junction solar cells fabricated by room-temperature wafer-bonding technique

We have developed III-V compound semiconductor multi-junction solar cells by a room-temperature wafer-bonding technique to avoid the formation of dislocations and voids due to lattice mismatch and thermal damage during a conventional high-temperature wafer-bonding process. First, we separately grew an (Al) GaAs top cell on a GaAs substrate and an InGaAs bottom cell on an InP substrate by metal solid source molecular beam epitaxy. Thereafter, we successfully bonded these sub-cells by the room-temperature wafer-bonding technique and fabricated (Al) GaAs parallel to InGaAs wafer-bonded solar cells. To the best of our knowledge, the obtained GaAs parallel to InGaAs and AlGaAs parallel to InGaAs wafer-bonded solar cells exhibited the lowest electrical and optical losses ever reported. The AlGaAs parallel to InGaAs solar cells reached the maximum efficiency of 27.7% at 120 suns. These results suggest that the room-temperature wafer-bonding technique has high potential for achieving higher conversion efficiencies

Room-temperature continuous-wave electrically pumped InGaN/GaN quantum well blue laser diode directly grown on Si

Current laser-based display and lighting applications are invariably using blue laser diodes (LDs) grown on free-standing GaN substrates, which are costly and smaller in size compared with other substrate materials.1–3 Utilizing less expensive and large-diameter Si substrates for hetero-epitaxial growth of indium gallium nitride/gallium nitride (InGaN/GaN) multiple quantum well (MQW) structure can substantially reduce the cost of blue LDs and boost their applications. To obtain a high crystalline quality crack-free GaN thin film on Si for the subsequent growth of a blue laser structure, a hand-shaking structure was formed by inserting Al-composition step down-graded AlN/AlxGa1−xN buffer layers between GaN and Si substrate. Thermal degradation in InGaN/GaN blue MQWs was successfully suppressed with indium-rich clusters eliminated by introducing hydrogen during the growth of GaN quantum barriers (QBs) and lowering the growth temperature for the p-type AlGaN/GaN superlattice optical cladding layer. A continuous-wave (CW) electrically pumped InGaN/GaN quantum well (QW) blue (450u2009nm) LD grown on Si was successfully demonstrated at room temperature (RT) with a threshold current density of 7.8u2009kA/cm2.