ARTICLE IN PRESS
Journal of Crystal Growth 310 (2008) 3710– 3713

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Journal of Crystal Growthjournal homepage: www.elsevier.com/locate/jcrysgro

Growth of GaNAs ﬁlms with As2 source in atomic hydrogen-assisted
molecular beam epitaxy
Ayami Takata , Ryuji Oshima, Hidemi Shigekawa, Yoshitaka Okada
Institute of Applied Physics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japana r t i c l e in f oa b s t r a c tArticle history:
Received 17 April 2008
Received in revised form4 May 2008
Accepted 15 May 2008
Communicated by K.H. Ploog
Available online 27 May 2008

We have investigated the effect of using As2 source instead of a more commonly used As4 source on thecrystalline quality of GaNxAs1 x thin ﬁlms grown by atomic hydrogen-assisted molecular beam epitaxy.
Improved structural and optical properties of GaNAs thin ﬁlms were obtained by using As2 source. The
nitrogen atoms were incorporated into GaAs at a more stable rate under As2 ﬂux than As4 ﬂux, and a
two-dimensional nucleation growth mode was promoted for growth of GaNxAs1 x with As2 source. As aconsequence, the surface roughness measured for a 500 nm-thick Ga0.008N0.992As sample grown withAs2 ﬂux was 1–2 monolayers, which was three times more smoother than that for As4 sample. The
photoluminescense measurements showed an improved potential ﬂuctuation of 78.1 meV and twice theintensity at room temperature for As2 sample.
& 2008 Elsevier B.V. All rights reserved.

Pacs:
81.05.Ea
81.15.Hi
61.66.FnKeywords:
A1 Atomic force microscopy
A3 Molecular beam epitaxy
B1 AlloysB2 Semiconducting III–V materials

1. Introduction
Recently, dilute nitride III–V semiconductor alloys are investigated for wide applications ranging from long-wavelength
optical communication lasers [1,2] to high-efﬁciency multijunction tandem solar cells [3]. The major advantage of this alloyis that the addition of a few atomic percent of nitrogen in GaAs
leads to a gigantic reduction in the band gap energy, and can stillbe lattice-matched to GaAs. However, the inherent metastabilityof this material leads to generation of high defect densities,
compositional inhomogeneity, and rougher interfaces. Consequently, short minority carrier lifetimes [4] as well as radiativeemission dominated by localized excitons [5] have been reported.
Furthermore, both optical and electrical properties of GaInNAs
layers become increasingly degraded with increasing N composition. In order to improve the crystalline quality, there have beenreports in the literature to optimize the morphological, structural,and electronic properties of ternary GaNAs as well as quaternary
GaInNAs quantum wells (QWs). This has led to 1.3 and 1.5 mm
laser diodes on GaAs substrates with excellent device characteristics [6–9]. Furthermore, use of Sb [10] and atomic hydrogen [11]
 Corresponding author. Tel.: +81 3 5452 6501; fax: +81 3 5452 6508.

E-mail address: bk200311447@s.bk.tsukuba.ac.jp (A. Takata).
0022-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jcrysgro.2008.05.027

as surfactants in MBE growth have been reported. On the other, it
is well known that difference in the sticking coefﬁcients between
the arsenic species, As2 and As4, affects the adatom migration in
the growth of GaAs [12]. However, the difference in the growthmechanism of GaInNAs thin ﬁlms under different arsenic species
has not been fully understood at present. In this work, we studied
the effect of using As2 source instead of a more commonly used
As4 source on the material quality of GaInNAs ﬁlms grown byatomic H-assisted MBE (H-MBE).

2. Experiments
The GaNAs ﬁlms were grown on GaAs (0 0 1) substrates by HMBE using RF-plasma as a nitrogen source as reported elsewhere[11]. After the oxide removal and growth of a 250 nm-thick GaAs
buffer layer at 580 1C, a 500 nm-thick GaNxAs1 x ﬁlm was grown
at 480 1C at a growth rate of 1.0 mm/h. The RF power and nitrogenback pressure was set to 50 W and 3.0  10 6 Torr for all growths,respectively. The GaNAs layers were grown either with As2 or As4in order to compare their material qualities. The beam ﬂux of As2and As4 measured at the substrate position was varied from1.2  10 5 to 3.0  10 5 Torr. Atomic H was irradiated at a backpressure of 4.0  10 6 Torr throughout the growth. The N

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compositions in GaNAs ﬁlms were determined by using highresolution X-ray diffraction (HR-XRD), in which the relative shiftsof GaNAs Bragg peaks with respect to GaAs peak are determined.
The growth process and surface morphology were studied by insitu reﬂection high-energy electron diffraction (RHEED) and exsitu atomic force microscopy (AFM). The optical properties wereinvestigated by using photoluminescense (PL) measurements, inwhich PL spectra were recorded in the range between 30 and
300 K using the 532 nm line of a second harmonic generation(SHG) Nd:YVO4 laser as an excitation source. The PL signals weredetected by using a liquid nitrogen-cooled InGaAs photodiode anda standard lock-in technique.

3. Results and discussion
Fig. 1 shows the dependence of N compositions in GaNAs
ﬁlms measured by HR-XRD grown with As2 or As4 source as afunction of arsenic beam pressure. The average N composition of0.85% was obtained under the given arsenic pressure for both
As2 and As4 sources. However, we note that N composition insamples grown with As4 source is more sensitive to the arsenicpressure than for As2. In fact, N composition changes, using
As4 source, from 0.4% at 1.6  10 5 Torr to 0.85% at 2.3  10 5 Torr,respectively. This suggests that N atoms are incorporated
into GaAs at a more stable rate under As2 source than underAs4 ﬂux.
Next, we evaluated the surface morphology of each sample byusing RHEED and AFM. Fig. 2 shows the RHEED patterns along[11 0] azimuth after the growth of 500 nm-thick ﬁlms with (a) As2and (b) As4 source under a beam pressure of 2.0  10 5 Torr,respectively. We observe a clear (2  2) RHEED streak pattern forsample grown with As2, while a weak streak or spotty pattern isobserved for sample grown with As4. Fig. 2(c) shows the RHEED
oscillations measured for each arsenic source. A clear RHEED
oscillation with near constant amplitude is observed for growthwith As2 ﬂux, while the intensity of RHEED oscillation drops fast
as deposition proceeds for As4 sample. Therefore, the results
suggest that a 2-dimensional (2D) nucleation growth mode is
promoted by using As2 source. Fig. 3 shows the surface AFM
images and cross-sectional proﬁles of GaNAs ﬁlms grown at anarsenic beam pressure of 2.0  10 5 Torr for (a) As2 and (b) As4source, respectively. An atomically ﬂat surface and small 2D
islands with high step density can be observed for sample grownwith As2. Fig. 4 plots the root-mean-square (RMS) values ofsurface roughness measured for GaNAs ﬁlms as a function of As2
or As4 pressure as determined by AFM. The RMS value of 0.4 nm

Fig. 1. Dependence of N composition in 500 nm-thick GaNAs ﬁlms grown with As2
or As4 source as a function of arsenic pressure measured by XRD.

Fig. 2. RHEED images after the growth of 500 nm-thick GaNAs ﬁlms grown with(a) As2 and (b) As4 at a beam pressure of 2.0  10 5 Torr, respectively. (c) Shows theRHEED oscillations measured under each As source.

(less than 2 monolayers (MLs)) obtained for sample grown with
As2 is smaller by a factor of 2–4 compared to that of sample grownwith As4 under the same arsenic beam pressure.
A reduction of surface diffusion of Ga atoms under As2 source
has been reported for growth of GaAs on GaAs (0 0 1) substrates by
Sugaya et al. [12]. Since As2 is a more active species than As4, we
think that an improved surface morphology and reduced alloy
disorder in GaNAs ﬁlms can be obtained with As2 sourcecompared to As4. We thus believe that use of As2 source promotes
2D nucleation growth mode and incorporation of N atoms into thesteps of small 2D islands is facilitated thereby resulting in an
atomically smooth surface. One further point to mention is thatthe surface morphology degrades at low As2 pressures as seen inFig. 4 and this is due to an enhanced alloy disorder and/or phaseseparation caused by an increase of surface migration under a lowarsenic ﬂux.
Next, we investigated the temperature dependence of PL peakenergy in order to evaluate the degree of compositional
inhomogeneity. The PL peak energy as a function of temperaturefor each sample is plotted in Fig. 5. The excitation intensity was1 W/cm2. In general, dilute nitride ﬁlms typically show anomalous temperature dependence [5]. In the presence of potential
ﬂuctuations, the excitons can become trapped in the deepest
potential ﬂuctuation at low temperatures. As the temperature israised, some may escape out and recombine at delocalized statesleading to high band-to-band recombination efﬁciency. This leads
to a commonly observed S-shape dependence [5]. In our samples,the PL emission from localized states around 1.20 eV is observed
below 120 K and emission from delocalized states becomesdominant at higher temperatures. Here, we deﬁne DE as a degreeof potential ﬂuctuation, in which DE is the amount of emission
energy shift from the localization to a full delocalization state. DE
for the sample grown with As2 source improved to 78.1 meV from90.9 meV for sample grown with As4 source. This indicates thatuse of As2 source is effective in reducing the potential ﬂuctuationand compositional inhomogeneity.

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Fig. 3. AFM images and cross-sectional proﬁles of the GaNAs ﬁlms grown with (a) As2 and (b) As4, respectively. Scan size is 500  500 nm.

Fig. 4. Dependence of surface roughness (RMS) for 500 nm-thick GaNAs ﬁlms
grown with As2 and As4 source, respectively, as a function of arsenic pressure.
Fig. 6. PL spectra at RT for the samples grown with each As source under the same
beam pressure of 2.0  10 5 Torr.

observed around 1.26 eV, the integral PL intensity for As2 sample
is twice as large as that for As4. This suggests that the density ofdefects and non-radiative recombination centers are reduced forsamples grown with As2 source.

4. Conclusion

Fig. 5. PL peak energy as a function of temperature measured for GaNAs ﬁlms
grown with (a) As2 and (b) As4, respectively, at a beam pressure of 2.0  10 5 Torr.

Finally, Fig. 6 shows the PL spectra measured at RT for samples
grown with each arsenic source. Though PL emission from thedelocalized states, or band-to-band recombination, are clearlyWe have investigated the difference in the structural andoptical properties of Ga0.008N0.992As thin ﬁlms grown by As2 andAs4 in H-MBE. We were able to fabricate a higher crystalline
quality by using an As2 source instead of more conventionallyused As4 source. By using As2 source, we ﬁnd that 2D nucleation
growth mode is promoted, and N atoms are incorporated into thesteps of 2D islands more favorably resulting in smoother surface.
The surface roughness for a 500 nm-thick GaNAs of 0.416 nm, andthe potential ﬂuctuation of 78.1 meV has been obtained. Further,
the PL intensity at room temperature was twice as strong
compared to sample grown with As4, which thus indicates that
non-radiative recombination is reduced. Though the ﬁndings
described here might be valid just for the given growth condition,

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further experiments on the effect of arsenic species on GaInNAs
growth are currently undertaken.

Acknowledgments
This work is supported in part by the New Energy andIndustrial Technology Development Organization (NEDO) underMinistry of Economy, Trade, and Industry (METI), Japan.
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