The III-nitride semicon-ductor light-emittingdevices that use various

combinations of gallium aluminium indium nitride(GaAlInN) have an everwidening range of wave-length applications from thevisible to deep ultraviolet.Laser diodes (LDs) havemore complex structures,where optical confinementcan be as critical as electricalperformance, than light emit-ting diodes (LEDs). However,the properties of laser lightare used widely in projectorand data storage systemsand could even be employedin general lighting in future.In particular, researchers

see potential applications forInGaN LDs in white lighting,since LDs do not suffer fromthe same efficiency droopproblems at high current asLEDs. This is because carrierconcentrations in LDs tend tobecome clamped at the valueat the laser threshold, ratherthan continuing to increasewith increased current.Here, we look at some recent

research that has tried toimprove optical confinementtechniques in blue InGaN LDs.

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Figure 1. Schematicfor LD (a) andcalculated 1Dtransverse modeprofile (b).

Mike Cooke reportson researchattempting toimprove waveguideand cladding layersfor blue 450nmwavelength devices.

Enhancing opticalconfinement for III-nitridesemiconductor laser diodes

Indium tin oxideUniversity of California Santa Barbara (UCSB)has used indium tin oxide (ITO) as part of thecladding for semipolar indium gallium nitride(InGaN) laser diodes (LDs) [A. Pourhashemi etal, Appl. Phys. Lett., vol106, p111105, 2015].The semi-polar (2021) crystal orientation

avoids quantum-confined Stark effects (QCSEs)from the large electric fields in the (0001) c-direction that arise from charge polarization ofthe III-nitride chemical bond. Devices grown onc-plane material suffer from reduced efficiencybecause these QCSE fields tend to pull apartelectrons and holes, inhibiting recombinationinto photons.The ITO cladding was used to reduce the

aluminium gallium nitride (AlGaN) content of thedevice structure. Devices with AlGaN claddingsuffer from catastrophic optical mirror damage(COMD) at lower current densities due to theconfined optical mode energy density.Al-free devices grown on gallium arsenide

(GaAs) substrates “are less prone to both suddenfailure and gradual degradation”, according tothe researchers. They add: “Although analo-gous reliability studies still need to be done forIII-nitride LDs, these studies suggest thateither reducing or eliminating the Al content inthe cladding layers could potentially improvethe manufacturability and reliability of highpower semipolar and nonpolar III-nitride LDs.”Another disadvantage of AlGaN cladding is

the low hole carrier density in p-AlGaN due tothe poor doping capability of even the bestacceptor impurity element — magnesium.Increasing Al-content further reduces holedensities. Low hole density makes for high typ-ical resistivity in p-AlGaN: 4Ω-cm, comparedwith 1x10–4Ω-cm for ITO. The resistivity in p-GaN isaround 1Ω-cm.UCSB used metal-organic chemical vapor deposition

on semipolar (2021) GaN substrates to produce theepitaxial material for the LDs (Figure 1). The 60nmupper and lower waveguides consisted of In0.06Ga0.94N.The 17nm electron blocking layer (EBL) used magnesium-doped p-Al0.15Ga0.85N. The ohmic p++-GaN contact layerwas 12nm thick. The magnesium-doping was reducedas much as possible to avoid phonon-assisted absorp-tion by acceptor-bound holes — a mechanism thoughtto be the single largest contributor to modal loss in III-nitride LDs.The material was formed into ridge waveguide

devices 8μm wide and 1200μm long. The 200nm silicon dioxide current aperture was applied with mag-netron sputtering onto the sides of the ridges and thenon-ridge areas of the device. The 180nm ITO layer

was deposited by electron-beam evaporation at 260ºCin an oxygen-rich atmosphere. The contact metalswere titanium/gold on the p-side and aluminium/goldon the n-side.The devices were separated using a dicing saw. The

mirror facets were mechanically polished. The backmirror was coated with an 8-period high reflectivityquarter-wavelength silicon dioxide/tantalum pentoxidestructure. The front mirror had a 1-period anti-reflectivestructure of the same materials.In pulsed operation at room temperature (Figure 2),

the threshold current was 509mA (5.30kA/cm2). Theslope efficiency of 1.36W/A represented a differentialefficiency of 50%. The researchers comment: “This high slope efficiency corresponded to a maximumoutput power of 2.52W and an external quantum efficiency of 39% for a drive current (current density)of 2.34A (24.4kA/cm2).”

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Figure 2. (a) Light output power, current and voltage (L–I–V)characteristics after facet coating for LD. Inset: lasing spectra.

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The team estimates that optical power density at thefront mirror with 2.52W operation was 94.43MW/cm2.The COMD power density for c-plane InGaN LDs of40–70MW/cm2 has been reported, versus 30MW/cm2

for GaAs-based devices under single-pulse excitation.The researchers point out that the peak output power

of their device was not limited by thermal roll-over orCOMD, but rather by power-supply limitations to currentsless than 2.34A. In terms of current-voltage performance,the reduction in GaN cladding enabled by use of ITOreduced the forward voltage somewhat, but the peakwall-plug efficiency was still low at 7.3%.

Waveguide confinementResearchers in Poland have been usingInGaN waveguides to enhance opticalconfinement in 450nm wavelength blue LDs, reducing threshold currents[Grzegorz Muziol et al, Appl. Phys. Expressvol8, p032103, 2015]. The team fromInstitute of High Pressure Physics andTopGaN Ltd employed low temperatureplasma-assisted molecular beam epitaxy(PAMBE) in order to achieve high material quality with indium contents upto relatively high values of 8%.The epitaxial material for the laser diodes

(Figure 3) was grown on c-plane bulkammono-GaN substrates with dislocationdensity of around 104/cm2. The layers upto and including the multiple quantum well(MQW) were constant in thickness with80nm InxGa1–xN bottom waveguide, andthree 2.6nm In0.17Ga0.83N quantum wellsseparated by 8nm InxGa1–xN barriers.The emission wavelength of all devices

was within 2nm of 450nm. The structure also included700nm silicon-doped aluminium gallium nitride(Al0.065Ga0.935N) bottom cladding, designed to compen-sate for the tendency to bow caused by strain from thedifferent lattice constants of GaN and InGaN. With theAlGaN bottom cladding the radius of curvature of the1” substrate was in the range 40–60m.The thickness D of the upper InxGa1–xN waveguide

layer was varied up to 80nm. Increasing the indiumcontent x up to 8%, and D to 80nm, reduced the laserthreshold current (Figure 4). With D at 5nm and x at8%, the threshold current was 12.5kA/cm2 — increasing

D to 80nm reduced this to 3.6kA/cm2. Changing x from 4% to 8% reduced the threshold by 50%.Despite the increased distance between the electronblocking layer (EBL) and MQW structure, theresearchers found no decrease in injection efficiency.The researchers believe that the effect of increas-

ing D is to reduce the overlap of the optical modewith the magnesium-doped layers of the device,which have larger optical absorption than undopedregions. However, increased x also increases opticalabsorption because the band-edge is closer to thatof the quantum wells and also as a result of indiumfluctuations in higher content material. At the sametime, the refraction index contrast of high-x InGaNwith the surrounding layers reduces optical leakage,enhances optical confinement and increases differ-ential gain of the light amplification. ■

The author Mike Cooke is a freelance technology journalistwho has worked in the semiconductor and advancedtechnology sectors since 1997.

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Figure 4. Threshold current density of LDs as function ofdistance D between MQW and magnesium-doped EBL.Dashed lines are guides to the eye.

Figure 3. Refractive index profile and corresponding optical modedistribution of blue LD. Solid purple and dotted green regionscorrespond to optical mode confined within Mg-doped region andInGaN waveguide, respectively.