reduction of threading dislocation

8/13/2019 Reduction of threading dislocation

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Reduction of Threading Dislocations

in GaN-Based Light Emitters


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Effect of TDs

Formation of a nonradiative recombination center where carriers recombine to

produce heat rather than light [1]. Yellow luminescence (YL) band[2] possibly due to the trapping of impurities or

point defects in dislocations.

Forward [3] and reverse [4] leakage current

In case of LEDs, the abovementioned effects of TDs result in a decrease of

luminescence intensity as well as efficiency.

Fig. 2. CL and TEM images of the same area of a GaN film, showing that there is

decreased luminescence where there are TDs [5]


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Methods of reducing TDs

Heteroepitaxially grown GaN has extremely high density of TDs (108-1012cm-2) [6].

Growing thicker layer:

Interactions between TDs occurs. Dislocation density up to 107cm-2

and below can be achieved by

growing the GaN to a thickness

of ~300 m [7]

Expensive method

Fig. 3. Relationship between thickness and TD

density for GaN films [7].


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Basic GaN growth:

Nitridation of the substrate. Keller et al, reported that nitridation lowered the TDdensity from 21010cm-2 to 4108cm-2 [8].

Growing a nucleation layer (NL) followed by annealing. NL is used to achievebetter coverage of the substrate.

GaN film growth at a higher temperature than the initial NL . By varying this recipe like changing the NL (GaN or AlN ) or the growth conditions

(temperature, reactor pressure, ratio of N to Ga precursor gases etc.) differentother in situ methods are obtained.

Interlayers:

By exposing the GaN to silane and ammonia, a porous SiNx layer is formed.

Many of the TDs are annihilated at or above this layer because, (1) SiNx ILphysically block TDs, (2) 3D growth leading to bending of TDs.

TD density of mid 109 cm-2 to 9107 cm-2 can be achieved with a coalescencethickness of 6m.

Methods of reducing TDs (contd)


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SiNx Interlayers:

Fig. 4. TEM image showing 3D island growth

on a SiNx covered GaN film [9]

Fig. 5. PL showing the Effect of SiNx

Interlayers in reducing TDs [10]


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Two step ELO:

Formation of triangular pyramidal stripes by changing growth condition.

Pyramidal facets induce the TDs to bend over by 90 into the (0001) basal plane .

The growth conditions are then altered to favor lateral growth, achieving

coalescence.

The TD bending mechanism greatly decreases the dislocation density in the

coherent region as the bent-over dislocations meet and annihilate

With this technique, the average density of TDs over the entire surface has been

lowered to 1.7107 cm-2. However, in the regions between the coalescenceboundaries, a TD density of 5106cm-2has been achieved [7].

Fig. 7. Two-step ELO [7]


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Conclusion

There are some other methods like transition metal nitride ILs (TiN/ ScN), maskless

ELO, pandeoepitaxy etc used to reduce TDs .

Both in situ and ex situ methods of TDs reduction yield an improvement in

material quality.

In-situ methods are advantageous because of their shorter growth times and

easier scalability for manufacturing.

ELO techniques have been successful in reducing TD density to a great extent, but

the involved lithography steps make this method less attractive.

With further work on these reduction methods, GaN films with homogeneously

low TD densities may be produced, leading to the production of high efficiency

light emitters.


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Thank You


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[1] H. Morko, Handbook of Nitride Semiconductors and Devices, Volume1, Chapter 4. [2] F. A. Ponce et al,Appl. Phys. Lett., 1996, 68, (1), 57-59.

[3] S. W. Lee et al,Appl. Phys. Lett., 2006, 89, 132117.

[4] J. C. Moore et al,Appl. Phys. Lett., 2007, 90, 011913.

[5] T. Sugahara et al,Jpn J. Appl. Phys., 1998, 37, L398-L400.

[6] S. Nakamura, Science, 1998, 281, 956-961.

[7] P. Gibart, Rep. Prog. Phys., 2004, 67, 667-715.

[8] S. Keller et al,Appl. Phys. Lett.,1996, 68, (11), 1525-1527.

[9] M. J. Kappers et al,J. Cryst. Growth, 2007, 300, 70-74.

[10] S. E. Park et al,J. Cryst. Growth, 2003, 249, 487-491.

References

reduction of threading dislocation

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