UV devices emitting at wavelengths shorter than 360 nm

Diodes emitting at wavelengths less than 360 nm have AlGaN active regions or AlxGa1-xN/ AlyGa1-yN multiple-quantum well (MQW) active regions. The power efficiency of these devices is generally low, i. e. less than 1%, although substantial progress has been made in recent years

(Zhang et al., 2002a, 2003; Yasan et al., 2002; Kipshidze et al., 2003; Fischer et al., 2004; Kim et al., 2004; Oder et al., 2004; Razeghi and Henini, 2004; Shakya et al., 2004; ).

The emission spectrum of an AlGaN/AlGaN deep-UV LED with interdigitated contact geometry is shown in Fig. 13.17 for different injection currents (Fischer et al., 2004). The active region of the device is composed of three Al0.36Ga064N quantum wells with Al0.48Ga0.52N barriers for emission at 290 nm. The spectrum displays one clean emission line with a peak wavelength of 289 nm. Some sub-bandgap emission near 330 nm becomes apparent when plotting the spectrum on a logarithmic scale. The forward voltage of the 200 x 200 ^m2 and 1 x 1 mm2 devices was reported to be about 7.0 V and 6.0 V at a forward current of 20 mA, respectively.

The following issues deserve special attention in the field of AlGaN/AlGaN UV LEDs:

• Affinity of aluminum to oxygen: Al has a very high affinity to O2 making the incorporation of oxygen into AlGaN increasingly likely as the Al content increases. Oxygen forms a deep, DX-like level in Al-rich AlGaN (McCluskey et al., 1998; Wetzel et al., 2001).

• Conductivity of AlGaN: Both the p-type and n-type conductivity of AlGaN decrease as the Al mol fraction increases, particularly for Al mol fractions exceeding 30% (Katsuragawa et al., 1998; Goepfert et al., 2000; Jiang and Lin, 2002). This leads to a higher resistivity in the confinement layers and higher device series resistances. A particular problem is the p-type conductivity in AlGaN. Al^Ga^N/Al^Ga^N superlattices have been employed to alleviate the p-type doping problem.

• Lateral conductivity: In devices grown on insulating substrates, with the standard side-by - side contact configuration, the n-type AlGaN layer provides the lateral conductivity. As the resistivity of Si-doped n-type AlGaN increases with the Al content, devices generally become more resistive. To compensate for this effect, the mean distance, which the electron current flows laterally in the n-type AlGaN lateral-conduction layer, needs to be reduced. This can be accomplished by an array of micro-LEDs (Kim et al., 2003; Khan, 2004) or by closely spaced fingers in interdigitated-contact geometries.

• Contact resistance: Due to the high bandgap of AlGaN, contact-barrier heights are generally higher, which makes the attainment of low-resistance contacts increasingly difficult as the Al content increases.

• Diffusion of acceptors: During the epitaxial growth of the top cladding layer, Mg acceptors may diffuse back into the active region, thereby lowering its radiative efficiency. Acceptor diffusion and the associated decrease in radiative efficiency may impose a limit on the maximum thickness of the p-type cladding layer.

• Heterojunction barriers: Due to the larger bandgap energies, the conduction - and valence-

band discontinuities of heterojunctions are generally larger than for smaller-bandgap semiconductors. Compositional grading at the heterojunction interfaces reduces the resistance of heterojunction barriers.

• Light extraction: To reduce reabsorption effects, all device layers should have an Al content sufficiently high to be transparent to the emitted light.

• Cracking: AlGaN films grown on relaxed GaN are, due to the smaller lattice constant of AlGaN, under tensile strain. If the films are sufficiently thick, they crack. However, cracking can be strongly reduced or even eliminated by using Al-rich strain-compensating superlattices (Hearne et al., 2000; Han et al., 2001; Zhang et al., 2002b). Such strain - compensating superlattices reduce the lattice constant so that subsequent epitaxial layers have a much reduced tensile strain or are even under compressive strain. Hearne et al. (2000) provided a quantitative analysis of the maximum attainable thickness of a crack-free layer under tensile strain.

Fig. 13.18. Optical micrographs of Al0)5Ga085N layer grown (a) without and (b) with a strain - compensating AlN/Alo.45Gao.55N superlattice (SL). The SL has 10 periods and equal well and barrier thicknesses of 10 nm. Angles be­tween crack lines frequently are 60° or 120°.



/— Crack

%• >.

AI0.15Ga0.85N '


20 pm

0.9 pm thick

20 um

0.9 pm thick

1---- ----- 1

on GaN/Al203

1---------- 1

on SL/Al-,03

Exercise: Cracking. Why does cracking occur in epitaxial layers that are under biaxial tensile strain but not in epilayers that are under biaxial compressive strain?

Solution: Wafer bowing and ultimately cracking of an epitaxial film that is under biaxial tensile strain releases the strain energy stored in the film. For epitaxial layers that are under compressive strain, the strain energy can be released by wafer bowing, film buckling, and film delamination. Due to the compressive strain, there is “no room” for fissures or cracks, so that cracks generally do not form in compressively strained films.

The strain energy stored in a homo-epitaxial film, that is lattice mismatched to the substrate, is proportional to the thickness of the film. As the thickness of a strained film increases, it will at some point become energetically more favorable to reduce the strain energy by creating misfit dislocations and cracks. Thus, at a certain thickness, the film will form misfit dislocations to release the strain energy. The

Optical micrographs of a 0.9 thick Al015Ga0 85N film are shown in Fig. 13.18 (a) and (b) for growth without and with a strain-compensating superlattice, respectively. Figure 13.18 (b) shows a virtually crack-free AlGaN layer that was grown on a strain-compensating AlN/Al045Ga0 55N superlattice with 10 periods and equal well - and barrier-layer thicknesses of 100 A.

critical thickness, at which a homo-epitaxial film starts forming misfit dislocations, is given by the Matthews-Blakeslee law (Matthews and Blakeslee, 1976). As the film thickness increases further, misfit dislocations do not suffice to release the strain energy, so that at some point the film will start to crack. A formula for the critical thickness at which a film under biaxial tensile strain starts to crack was given by Hearne et al. (2000).

AlGaN UV LEDs frequently have forward voltages Vf >> hv/e. Depending on the device structure, the excess forward voltage may originate from the p-type contact, the p-type AlGaN confinement layer, the n-type AlGaN layer providing lateral conduction, or from unipolar heterojunctions.

For devices with low or moderate efficiency as well as for high-power devices, device packages with low thermal resistance are desirable. The heat resulting from an excess forward voltage and low quantum efficiency must be removed to avoid excessively high junction temperatures. Morita et al. (2004) reported a particularly well heat-sunk device, namely a structure in which the sapphire substrate was removed by laser lift-off, and, using an AuSn solder, the epilayer was directly bonded to a CuW heat sink.

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