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APPLIED PHYSICS LETTERS 96, 141112 共2010兲
Low-threshold 303 nm lasing in AlGaN-based multiple-quantum well
structures with an asymmetric waveguide grown by plasma-assisted
molecular beam epitaxy on c-sapphire
V. N. Jmerik,1,a兲 A. M. Mizerov,1 A. A. Sitnikova,1 P. S. Kop’ev,1 S. V. Ivanov,1
E. V. Lutsenko,2 N. P. Tarasuk,2 N. V. Rzheutskii,2 and G. P. Yablonskii2
1
Ioffe Physical-Technical Institute, RAS, St. Petersburg 194021, Russia
Stepanov Institute of Physics of NAS Belarus, Independence Ave. 68, Minsk 220072, Belarus
2
共Received 28 January 2010; accepted 2 March 2010; published online 8 April 2010兲
We report on AlGaN multiple-quantum-well separate confinement laser heterostructures grown by
plasma-assisted molecular-beam epitaxy directly on c-sapphire at low temperatures 共⬍800 ° C兲.
Threading dislocation density was reduced down to 109 – 1010 cm−2 owing to both intentionally
introduced strained AlGaN/AlN superlattices and self-organized blocking structures in the AlGaN
step-graded buffer layers. The quantum wells were fabricated by a submonolayer digital alloying
technique. Calculations of the optical gain and confinement in the optically pumped laser structures
yielded its optimum design comprising an asymmetric waveguide. Lasing at 303 nm with the
relatively low threshold excitation density of 0.8 MW/ cm2 at 295K has been achieved. © 2010
American Institute of Physics. 关doi:10.1063/1.3373834兴
Both AlGaN-based ultraviolet 共UV兲 light emitting 共LED兲
and laser diodes 共LD兲 have numerous applications in biomedical and environmental fields, as well as UV-sensitive
material processing.1 However, poor structural quality of
high Al content AlGaN heterostructures grown on latticemismatched substrates and absence of strong enough localization effects in this material cause serious difficulties
in fabrication of UV diodes emitting in the deep UV range
共⬍300 nm兲.2,3 As a result, a minimum emission wavelength
for UV-LD is still restricted by a value of 336 nm4 whereas
optically pumped lasing with the threshold power density
above ⬃1 MW/ cm2 at room temperature 共RT兲 has been
achieved for AlGaN-based multiple-quantum-well 共MQW兲
structures emitting at wavelengths shorter than 320 nm down
to 214 nm 共AlN layer兲.5,6 To overcome this problem, different techniques have been proposed to decrease the threading
dislocation 共TD兲 density in the active region of laser
structures.6–9 Information on improving the AlGaN MQW
laser characteristics through optimization of the laser structure design is still quite scarce.10
Most of the experimental studies of UV AlGaN LEDs
and lasers are based on low-pressure metalorganic vapor
phase epitaxy 共MOVPE兲. However, such an alternative technology as molecular beam epitaxy 共MBE兲 has also succeeded in growing UV-LEDs with the minimum wavelength
of 250 nm,11 as well as MQW structures with high photoluminescence 共PL兲 efficiency within the spectral range of 220–
320 nm.12,13 Moreover, both NH3-based and plasma-assisted
共PA兲 MBE have been employed to manufacture 410 nm
InGaN-based MQW LDs and LEDs on different GaN
substrates.14,15 We have recently reported the optically
pumped lasing at 300.4 nm in an AlGaN MQW structure
grown by PA MBE on c-sapphire.16 Although the threshold
power density was rather high 共12 MW/ cm2 at RT兲, it was
in line with those of laser structures grown by MOVPE.9
a兲
Electronic mail: [email protected].
0003-6951/2010/96共14兲/141112/3/$30.00
In this paper we report on further development of the
PA MBE technology of AlxGa1-xN MQW heterostructures
共0.4⬍ x ⬍ 1.0兲 on c-sapphire as well as on application of a
special laser structure design to improve TD filtering and to
reduce the threshold power density at a wavelength around
300 nm. The main idea of our approach is the usage of an
asymmetric waveguide in a separate confinement 共SC兲 laser
heterostructure that has been successfully realized in a II-VI
active element of the optically pumped III-N/II-VI bluegreen laser converter.17 This enhances of the optical confinement factor for the fundamental mode due to optimum distribution of the internal electromagnetic field having a
maximum amplitude at the MQW active region, as well as
improves of the collection efficiency of optically excited
nonequilibrium carriers in the active region.
Figure 1共a兲 shows a schematic diagram of the AlGaN
MQW laser structure grown by PA MBE directly on a
c-Al2O3 substrate, as described in detail earlier.16,18 In particular, the growth was initiated with the deposition of a twostep AlN buffer layer consisting of 30-nm-thick lowtemperature 共570 ° C兲 and 250-nm-thick high-temperature
共800 ° C兲 layers. Then the substrate temperature was reduced
down to 700 ° C and kept constant till the end of the growth.
The 兵AlGaN/ AlN其30 superlattice 共SL兲 with a period of 10
nm and an average AlN mole fraction of 0.9 was grown atop
of the AlN layer. The structure comprises also AlxGa1-xN
buffer, cladding, and waveguide layers with x = 0.77, 0.66,
0.49, respectively, grown under the metal-rich stoichiometric
conditions. Three 3-nm-thick QWs with an average x = 0.39,
separated by 7-nm-thick barriers layers, were grown by using
a submonolayer digital alloying 共SMDA兲 technique described elsewhere.13 The composition and thickness parameters of the QW and barrier layers were chosen similar to our
previous non-optimized MQW structure16 to provide the QW
emission wavelength at around 300 nm. Other parameters
including asymmetric position of the MQW region in the
waveguide as well as the composition and thickness of cladding layers have been optimized to place the maximum of a
96, 141112-1
© 2010 American Institute of Physics
141112-2
Appl. Phys. Lett. 96, 141112 共2010兲
Jmerik et al.
3QW Al0.39Ga061N(3nm)/
Al0.49Ga0.51N(7nm)
80nm
TE-mode
Field intensity (a.u.)
Cladding
Al0.66Ga0.34N:750nm
Al0.77Ga0.23N:300nm
1.7µm
SL {AlN/AlGaN}30
HT-AlN:250nm
c-Al2O3
Distance along c (a.u.)
Waveguide Al0.49Ga0.51N
40nm
LT-AlN:30nm
(a)
(b)
1.0 1.4 1.8 2.2 2.6 3.0
1.0
0.08
0.8
0.06
0.6
0.04
0.4
0.02
0
(c)
0
5
10
Mode number
0.2
Reflection coefficient, R
Optical confinement factor, G
Refractive index
0.1
15
FIG. 1. 共Color online兲 SC heterostructure of optically pumped AlGaN
MQW laser: 共a兲 schematic diagram of the heterostructure, 共b兲 variation of
the refractive index vs thickness in the structure and a simulated near-field
distribution of the fundamental mode supported by the laser waveguide
used, 共c兲 optical confinement factors 共stars兲 and reflection coefficients
共circles兲 for the fundamental and higher order modes in the structure.
near-field distribution of the fundamental TE mode of electromagnetic field at the MQW active region, as shown in Fig.
1共b兲. The calculations using the plane wave approximation18
revealed that the fundamental mode has the maximum optical confinement factor equal to ⌫ = 0.09 关see Fig. 1共c兲兴.
Moreover, the reflection coefficient 共R兲 of laser cleaved facets for this mode, calculated in accordance with Ref. 19,
showed the value of 0.257 which exceeds the R values for
higher order modes 共up to seventh兲. Details of the calculations will be reported elsewhere.
Laser reflectometry 共532 nm兲, reflection high energy
electron diffraction 共RHEED兲 were employed for in situ determination of the growth rate, Al-content of the layers, and
their surface morphology. X-ray diffraction 共XRD兲 scans,
optical microscope observations, as well as the scanning
共SEM兲 and transmission electron microscopes 共TEM兲 were
used to characterize structural properties of the laser structure. A 4th harmonic 共266 nm兲 of the Nd-YAG laser was
employed for optical pumping of the stimulated emission.
The laser beam was focused by a cylinder lens, so the excitation stripe was normal to the cleaved facets of the laser
samples having the cavity length of about 1 mm.
During growth of the initial AlN layer RHEED demonstrated the transition from spotty pattern to streaky one, the
latter remaining unchanged during the following growth run.
This corresponds to the two-dimensional growth mode under
metal-enriched conditions at FIII / FN = 1.1 and up to 1.8 for
growth of AlN and AlGaN layers, respectively. SEM and
optical microscope revealed a droplet-free surface of the
structure despite the relatively high metal supersaturation
during AlGaN growth. XRD study confirmed the values of
FIG. 2. Cross-sectional TEM images of: the whole laser structure taken
using g = 共0002兲 共a兲 and g = 共01I0兲 共b兲 two-beam diffraction conditions, 共c兲
the AlGaN waveguide layer with MQWs 关g = 共01I0兲兴, 共d兲 the region near the
top Al0.77Ga0.23N / SL interface with the self-organized DNS marked by arrows 关g = 共0002兲兴, and 共e兲 the region near the SL/AlN interface 关g = 共01I0兲兴.
Al-content in the AlGaN layers, defined preliminary during
growth by using a simple ratio of the calibrated aluminum
flux to the measured total growth rate x = FAl / VAlGaN, the
latter being governed by the calibrated active nitrogen flux
under the metal-rich growth conditions. Moreover, clear and
rather sharp SL-0-reflection as well as multi-order satellites
共SL-2, SL-1, and SL+ 1兲 corresponding to the average AlN
content and period of the buffer AlN/AlGaN SL were observed in the ␪-2␪-scans 共not shown兲.
Figure 2 presents the cross-sectional TEM images of
different parts of the structure, which were taken using
g = 共0002兲 and 共01I0兲 diffraction conditions to evaluate the
densities of TDs with different Burgers vectors. In general,
they demonstrate a decrease of the TD density from the values above 1011 cm−2 within the AlN buffer layers toward the
109 – 1010 cm−2 range within the waveguide layer of the
MQW structure. Relatively small density ⬃1.5⫻ 109 cm−2
of the pure screw-type TD with Burgers vector b = c has been
found in this region. However, this is only 5% of the total
TD density ⬃3 ⫻ 1010 cm−2 contributed by 55% of mixed
共b = a + c兲 and 40% of vertical edge 共b = a兲 TDs. Two distinct
regions of drop of the TD density have been revealed in the
structure. Figure 2共e兲 shows the TD filtering effect within the
intentionally grown strained AlN/AlGaN SL, similar to that
observed in the SL structures grown by MOVPE.8 In addition, steep reduction of TDs was detected at the AlGaN
buffer layer exhibiting some ordering features in a dense
defect network structure 共DNS兲 which was self-organized at
a distance of 180 nm above the AlN/AlGaN SL, as seen in
Figs. 2共a兲, 2共b兲, and 2共d兲. Formation of such blocking DNS
structure can be regarded as an additional relaxation mechanism of the compressive stress in the buffer Al0.77Ga0.23N
layer. Thus heterostructure with DNS demonstrates twice as
much reduction of TD density in the top active region in
comparison with the similar MQW laser structure without
DNS described by us previously.16
The thicknesses of QWs clearly observed in Fig. 2共c兲 are
consistent well with those defined by the SMDA technique.
The high efficiency of the dominant PL emission and its
origination from the QWs have been revealed by using the
picosecond time resolved PL spectroscopy and temperature
141112-3
Appl. Phys. Lett. 96, 141112 共2010兲
Jmerik et al.
Energy (eV)
4.2
4.4
4.0
TE
Emission intensity (a.u.)
Emission intensity (a.u.)
l= 266 nm
T = 295 K
2
Pin=1070 kW/cm
3.8
2
890 kW/cm
TM
2
750 kW/cm
280 290 300 310 320 330 340
Wavelength (nm)
2
70 kW/cm
(a)
280
290
300
310 320 330 340
Wavelength (nm)
Emission intensity (a.u.)
5
4
l = 266 nm
T = 295 K
laser threshold to a quality of mirrors owing to the relatively
small value of mirror reflectivity 共R = 0.258兲 necessary for
the fundamental mode lasing; 共ii兲 increased efficiency of radiative recombination by about 70% in the studied MQW
laser structure owing to the multistage TD filtering down to
the density of 109 – 1010 cm−2 in the active region.
In summary, we have demonstrated a possibility to
grow by PA MBE on conventional c-Al2O3 substrates the
AlGaN MQW structure exhibiting optically pumped lasing at
303 nm at the TE fundamental mode. Carefully chosen laser
structure design and employing different mechanisms of
threading dislocation filtering led to the remarkably low
value of laser threshold of 0.8 MW/ cm2 at RT. This threshold value is less or comparable to those for AlGaN MQW
structures grown by MOVPE and emitting at the same
wavelengths.5,8
Authors are thankful to T.V. Shubina and M.A.
Yagovkina for TR PL and XRD studies, respectively. The
work was supported in part by RFBR 共No. 09-02-01397兲 and
BRFFR 共No. F10R-164兲, Program of RAS “Novel materials
and structures” and Program of NASB “Photonics 2.01.”
3
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1
2
1
3
(b)
200
400
600
800 1000 1200
2
Excitation density (kW/cm )
FIG. 3. 共Color online兲 共a兲 Room temperature edge PL spectra of the MQW
structure below and above the lasing threshold. Inset shows TE 共E ⬜ 共0001兲
and TM E 储 共0001兲 polarized PL spectra above the threshold. 共b兲 Room temperature emission intensity vs pumping power density.
dependences 共10–300 K兲 of the PL spectra, as we described
earlier.16
Figure 3共a兲 shows RT edge emission spectra of the AlGaN MQW laser structure versus input optical power density. The threshold power density of the stimulated emission
⬃0.8 MW/ cm2 was derived from emergence of the higher
energy narrow PL line at 303 nm with the increase in the
pumping power, followed by the fast raising of its intensity.
The polarization degree of the spectra was characterized by
P = 共ITE − ITM兲 / 共ITE + ITM兲, where ITE共ITM兲 is the PL intensity
of a transverse-electric共magnetic兲-field modes. P changes
from 0.55 for spontaneous edge PL up to 0.95 for the lasing
spectra shown in the inset in Fig. 3共a兲. This fits to the data
for AlGaN MQW structures with moderate Al content
共x = 0.36– 0.41兲 and lasing wavelengths above 280 nm reported earlier.20,6
We believe that the main factor of lowering the laser
threshold down to 0.8 MW/ cm2 at the wavelength of
303 nm is the improvement of the laser structure design
comprising the SC double heterostructure with the asymmetric waveguide, which resulted in increasing the optical confinement factor for the fundamental mode up to 0.09 instead
of 0.01 in the previous structure showing the laser threshold
of 12 MW/ cm2 at practically the same lasing wavelength.16
However, the observed improvement could also be due to
some additional factors such as 共i兲 weaker sensitivity of the