PI143

Ga(1n)AsN GROWTH BY PLASMA-ASSISTED MOLECULAR BEAM EPITAXY TOWARDS 1.3pM AND 1.55pM

S.Z.Wang(a)(b), S.F.Yoon(a), T.K.Ng(a', W.K.Loke(a), Z.Z.Sun(a', K.C.Yew(a', and W.J.Fada)

(a) School of Electrical and Electronic Engineering, Nanyang Technological Universiv, Nanyung Avenue, Singapore 639798, Republic of Singapore

Nunyang Technological University, Nanyang Avenue, Singapore 639798, Republic of Singapore

' (h) Singapore-Massachusetts Institute of Technologv(M.Ii'J Alliance,

Abstract Ga(1n)AsN is grown by molecular beam epitaxy using either a direct nitrogen beam or

dispersive nitrogen radicals generated by a radio-frequency activated nitrogen source. The nitrogen incorporation shows a linear dependence on the RF power in a dispersive growth mode, contrasted to a saturated behavior in a direct growth mode. An additional indium flux causes a decrease in the nitrogen composition, an effect resulting from the increase in the growth rate. Quality Ga(1n)AsN quantum wells were grown at 46OoC, with a photoluminescence emission close to 1.6pm. Also reported is a PL result amongst the best, as compared with the literature.

I. Introduction InGaAsF-based I .3/1 S5pm lasers have a typical

characteristic temperature(T0) of -60KI'l. Conduction- band discontinuity of more than 3OOmeV at Ga(In)AsN/GaAs interface causes the potential of creating lasers with T0>150K"'. Ga(In)AsN-based lasers operating at 1.3/1.55pm have been fabri~atedI'~[~I, and a

is reported here to extend the Ga(In)AsN materials to 1.34 55pm wavelength using a plasma-assisted MBE system. Our experiments revealed the nitrogen-indium interaction might determine the growth mode and growth dynamics of Ga(1n)AsN materials. The nitrogen incorporation behaves as somewhat different from the literature. A modified growth mode is tried for quality nitrides. Also reported is a PL result amongst the best, as compared with the literature.

11. Experimental Details InGaAsN samples were grown in a solid source

molecular beam epitaxy system, equipped with five standard effusion cells for indium, gallium, aluminum, beryllium and silicon, three cracker cells for arsenic, phosphorous and hydrogen, and one plasma source for nitrogen, respectively. The purity of all the source charges is six nines. AJJ samples were grown on (001)-oriented

semi-insulating GaAs substrates prepared using standard preparation procedures. Prior to growth, the surface oxide desorption was camed out under As, flux at a beam equivalent pressure (BEP) of 6.2X10-6 torr. A (2x4) surface reconstruction was maintained during the entire growth process. The beam equivalent pressures for Ga and As were 4.5X10-' torr and 6.2X10'6 torr, respectively. The V/III ratio was fixed at about 35 for the growth of all samples. The Ga and As fluxes were adjusted by controlling the temperature of the effusion cells. The above BEPs result in a growth rate of -1.0 p.41 verified by time resolved RHEED measurements. The nitrogen plasma source works at nitrogen background pressure of 3.6X10-6 torr in the presence of As4 (6.2X10'6 torr) and is activated by radio frequency (R.F.) power greater than 60 W to maintain the plasma in high brightness mode. Growth and measurement details have been published elsewherew.

111. Results and Discussion Figure 1 shows the relationship between the nitrogen

composition of GaAsN epilayers and RF power applied to the plasma source. The nitrogen composition of GaAsN epilayers grown with activated nitrogen beam shows a saturation behavior over the increasing RF power. While the nitrogen composition contributed by dispersive nitrogen does not show deviation from linearity. Our experiments have proved nitrides grown with dispersive

'

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nitrogen are of higher crystalline, interface and optical quality than those grown with direct nitrogen beamf6].

- - 4 5

4 0 - . E 3 5 - .- 3 0

B 2 5 :

5 . 0 2 0 - c

. GaAsN - - ~t$,=4.6XlO't01~

OJq,,=14 . P,(N)=3.4X104torr - ,TQ=4600C

A'

' ,#'

.

05 -------

+ . shutter closed

- - -+ A - - -

4 0.01 I ' I ' ' I " * ' I

150 200 250 3w 350 400 450

R.F.Power (W)

Fig. 1: Experimental results of nitrogen composition in GaAsN layer vs. RFpower of the nitrogen plasma source, under conditions indicated in the figure. [+: nitrogen composition in GaAsN grown with direct nitrogen beam, 0 : nitrogen composition in GaAsN epilayers grown with dispersive nitrogen source]

1°,t I I 1165nm I

Wavelength (nm)

Fig.2: The PL spectra from two InGuAsNlInGaAslGaAs double quantum well samples with diflerent nitrogen and

indium composition.

Figure 2 shows two PL spectra from QW samples at 4.2K with indium composition of 25% and 36% respectively. An 8nm wide InGaAsN QW is grown on a 300nm thick GaAs buffer, then a l O O n m thick GaAs is grown on the InGaAsN well layer, serving as the barrier layer of InGaAsN QW and a followed 8nm wide InGaAs QW. Finally, the InGaAs well layer is covered with a

g 0.9 K 2 0.8 .-

0.4 -5 0 5 10 15 20 25 30 35 40

Indium Composition (%)

Fig.3: The relationship between the nitrogen composition and indium composition in InCaAsN epilayers, under the growth conditions indicated in the figure.

20nm thick GaAs cap. The indium flux for InGaAs QW is controlled the same as that for InGaAsN QW. The InGaAs QW serves as a reference structure in PL measurements. The 986nm peak from the InGaAs well serves; as a reference for the llOOnm peak from the Ino.zsGaAsNo.oo~s well, similarly the 1097nm peak fiom the IriGaAs well serves as ia reference for the 1165nm peak from the Ino.36GaAsN~,~~s well. The growth conditions for these two samples are retained the same except for the indium flux. 'The nitrogen composition of the two samples is measured as 0.85% and 0.45% respectively. That implies the nitrogen incorporation could be reduced by the indium flu. This effect is testified by growing GaAsN epilayers (i.e., the indium cell closed), in which the nitrogen composition is 1 .O%.

Fig. ,4: A light microscopic picture of the InGaAsN growth surjkce.

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Figgue 3 shows the nitrogen composition as a h c t i o n of the indium composition. The squares denote the nitrogen composition of the earlier two samples, as well as that of the relevant GaAsN. The dots show the nitrogen values that are normalized to the relevant GaAsN case at the equivalent growth rate. It is clear that the nitrogen composition of InGaAsN decreases as the indium composition increases even though the growth rate is normalized to the relevant GaAsN case. This experimental result is somewhat different from Harmand and Townie's result^'^^'^^, but agree with Xin's['].

Our experiments revealed even though the reflection high-energy electron diffraction (RHEED) pattems show a sharply streaky (2x4) reconstruction during the growth of InoloGao70A~ QW, the RHEED pattems for 1% 3oGao ~ o A s I - ~ N ~ QW usually shows a partially spotty structure. That implies the nitrogen-indium interaction on the growth front might facilitate the formation of three- dimension islands though nitrogen compromises the strain of InGaAsN epilayer. That could be confirmed by transparent electron micrograph investigation, which clearly shown the gathering effect of indium and nitrogen and the formation of indium-rich and nitrogen-rich domains["'. Consequently, there should be islands presented on the growth surface(Fig.4, a light microscopic picture). The dimension of these islands is about 100nmx200nm. Kitatani have found emissions from these dot-like structures amongst InGaAsN QW["].

.

4000-

3000-

2000 -

1000-

0 -

' InGaAs 1008nm

InGaAsN 1584nm

I ,,4 1 To= 460°C

U .

@, 1 8 b . ' ' ' \ = I

GaAsN

Pb(N)= 3.6XlO'torr - $&= 6.5XlO'torr

- r.f.Power = 150W

1.2 \ 0.5 0.6 0.7 0.8 0.9 1.0

Growth Rate (pm/h)

Fig.5: The relationship between the nitrogen composition and growth rate of GaAsN epilayer, under the growth conditions indicated in the jigure.

Figure 5 shows the relationship between the nitrogen composition of GaAsN and its growth rate. It is clear the nitrogen composition reduces as the growth rate increases. For example, the nitrogen composition is 2.82% when the reduces down to 2.03%. That implies the nitrogen behaves like a dopant. The reduction of nitrogen growth

rate is O.Spm/hr. While the growth rate is increased up to 1 .Op&, the nitrogen composition composition in InGaAsN layers could be due to the growth rate increase caused by the indium flux. The dashed line indicates a theoretical result supposing the nitrogen composition decreases linearly with the growth rate. It is noticeable there is a deviation of the experimental data from the linearity assumption. Our results are somewhat different from Egorov and Spruytte's report, where they showed a linear relationshi between the nitrogen composition and the growth rate[12 P [ I3 ] .

- I

I - I ' I - ' I ' 1 ' I ' WO 1000 1100 1200 1300 1400 1

wavelength (nm)

(b) Fig.6 Two PL spectra of InGaAsN QW samples at 4.2K grown by adjusting nitrogen source condition.

DO

Our experiments revealed the nitrogen incorporation in InGaAsN could be varied by the indium, while the indium composition remained unchanged as the nitrogen radical flux varied. It is a better way to fix the indium than to fix the nitrogen for the wavelength control of InGaAsN. Figure 6 shows two PL spectra of InGaAsN samples at 4.2K grown by adjusting nitrogen source

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condition. The growth reproducibility is testified with the reference peaks at 1025nm (Fig.6(a)) and 1008nm (Fig.6(b)) respectively from the relevant InGaAs QWs. The right-hand two peaks at 1293nm (Fig.6(a)) and 1584nm (Fig.6(b)) are from InGaAsN QWs. The results indicate InGaAsN QW could reach both 1.3pm and 1.55pm. In terms of the intensity as compared with the reference InGaAs peak and the full width at half magnitude (FWHM), the optical quality of the sample shown in figure 6(a) is amongst the best as compared with literature. Fig.7 is a room temperature PL spectnun from an annealed InGaAsN QW. Also in terms of the intensity and the FWHM as compared with the reference InGaAs peak, the InGaAsN sample is of high optical quality and is ready for laser application.

600 j InGaAs - 1078nm

InGaAsN I

1000 1 rbo 1 io0 1 3 ,

Wavelength (nm)

Fig.7 A room temperature PL spectrum from un unneuled InGuAsN Q W sample

IV. Summary Ga(1n)AsN is grown using either a direct nitrogen beam

or dispersive nitrogen radicals generated by a RF activated nitrogen source. The nitrogen incorporation shows a linear dependence on the RF power in a dispersive growth mode, compared to a saturated behavior in a direct growth mode. Indium causes a decrease in nitrogen composition, an effect resulting from the increase in growth rate. Quality InGaAsN QWs were grown at 460°C, with a PL emission close to 1.6pm. Also reported is a PL result amongst the best, as compared with the literature.

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