view paper - OECC/ACOFT 2014

Room-temperature low-threshold deep-ultraviolet stimulated emission
from AlGaN heterostructures grown on sapphire substrates
Xiao-Hang Li1, Theeradetch Detchprohm1, Yuh-Shiuan Liu1, Tsung-Ting Kao1, Md. Mahbub Satter1, Shyh-Chiang
Shen1, Douglas Yoder1, Russell Dupuis1, Shuo Wang2, Yong Wei2, Hongen Xie2, Alec Fischer2, and Fernando Ponce2
1
Center for Compound Semiconductors and School of Electrical and Computer Engineering,
Georgia Institute of Technology, Atlanta, Georgia 30332 USA
2
Department of Physics, Arizona State University, Tempe, Arizona 85287 USA
25000
256-nm Laser
Edge PL emission
193nm pumping
RT
20000
Intensity (a.u.)
15000
10000
5000
0
220
(a)
230
240
700000
250
260
270
Wavelength (nm)
280
290
300
10
9
600000
8
500000
7
6
400000
5
300000
4
3
200000
2
100000
1.6nm
Pth~61 kW/cm2
1
0
0
0
(b)
FWHM (nm)
Introduction
Deep-ultraviolet (DUV) emitters have numerous
applications such as optical storage and disinfection. But
most of the current DUV emitters like excimer lasers
have large size and weight and also have low reliability.
Recently, III-N semiconductor DUV emitters have
drawn great attention due to their suitable direct bandgap
which can lead to compact size and reliability. Recently,
relatively low-threshold optically-pumped DUV lasers
containing AlGaN multiple-quantum wells (MQWs)
have been demonstrated by using c-plane bulk AlN
substrates [1-2]. The bulk AlN substrates have lowdislocation densities and can reduce lattice and thermal
mismatch between the AlN substrate and high-Alcontent AlGaN epitaxial layers, thus leading to highquality active regions with low-dislocation density. But
because of small area and extremely high cost of the
bulk AlN substrates, it is desirable to grow DUV lasers
on larger and inexpensive sapphire substrates that have
been widely used to grow lower-bandgap III-N
mateirals, e.g., for visible InGaN LEDs.
In this abstract, we present stimulated emission from
239 nm to 256 nm from different optically-pumped
AlGaN-based MQW DUV lasers grown on (0001)
sapphire substrates by metalorganic chemical vapor
deposition (MOCVD). The lowest threshold were
obtained from the lasing at 249 nm (“the 249-nm laser”)
and 256 nm (“the 256-nm laser”). Atomic-force
microscopy (AFM), transmission-electron microscopy
(TEM), X-ray diffraction (XRD) and power-dependent
photoluminescence measurements were carried to
investigate material quality and lasing characteristics.
Integrated Intensity (a.u.)
Abstract Summary
Deep-ultraviolet (DUV) AlGaN heterostructure lasers
grown on c-plane sapphire substrates with record-low
thresholds were demonstrated by power-dependent
photoluminescence measurements. The results show
excellent candidacy of sapphire substrates for DUV
laser diodes.
50
100
150
200
Pumping Power Density (kW/cm 2)
14000
249-nm Laser
Edge PL emission
193nm pumping
RT
12000
Intensity (a.u.)
10000
Fig. 2: (a) Laser emission spectra and (b) spectral integrated
intensity and spectral linewidth versus pumping power
densities of the 256-nm laser.
8000
6000
4000
2000
0
220
230
240
250
(a)
260
270
280
290
300
Wavelength (nm)
10
350000
9
Integrated Intensity (a.u.)
300000
8
250000
6
5
150000
4
3
100000
2
50000
1.6nm
Pth~90 kW/cm2
1
0
0
0
(b)
FWHM (nm)
7
200000
50
100
150
200
250
300
Pumping Power Density (kW/cm2)
Fig. 1: (a) Laser emission spectra and (b) spectral integrated
intensity and spectral linewidth as a function of pumping
power densities for the 249-nm laser.
The AlGaN MQW laser structures were grown on 2inch diameter c-plane sapphire substrates in a 3×2”
MOCVD reactor. The laser structure firstly comprised a
3.5-m AlN template layers deposited directly on the
sapphire substrates. The total dislocation density of the
template layers were determined to be ~2.5×109/cm2 by
cross-sectional TEM experiments, which represented one
of the lowest dislocation densities reported for planar
AlN/sapphire templates [3-5]. The root-mean-square
(RMS) surface roughness was less than 0.10 nm and
0.12 nm as determined by 1×1 m2 and 5×5 m2 AFM
measurement. Thus the AlN template layer provided a
very smooth surface and low crystalline mosaicity for
subsequent growth of the AlGaN-based MQW
heterostructure.
Subsequently, an AlGaN waveguide layer, five to ten
periods of AlxGa1-xN / AlyGa1-yN MQWs designed for
1.12
1.02
0.92
0.82
0.72
0.62
0.52
0.42
0.32
0.22
0.12
0.02
Normalized Intensity
laser emission from 239-256 nm, and a thin AlGaN cap
layer were sequentially grown on the AlN template
layer. Growth conditions, composition and thickness of
these AlGaN-based layers were optimized to improve
optical gain and enhance the optical confinement in the
active region and thus reduce the laser threshold. All the
epitaxial layers were pseudomorphically grown as
confirmed by XRD asymmetric (105) reciprocal space
mapping. The 5×5 m2 AFM measurement show RMS
roughness of the laser surface is 0.55 nm, which was
close to that of a comparable laser structure grown on a
bulk AlN substrate [2] and thereby suggested relatively
low dislocation density. The wafers were cleaved into
laser cavities with lengths from 1.0 mm to 2.5 mm by
mechanical scribing from the back side of the substrate.
The cavities were optically pumped at room temperature
by an 193-nm ArF excimer laser.Details of optical
pumping experiment setup can be found in Ref [2].
Photoluminescence spectra of the 249-nm laser and
256-nm laser with different pumping power densities are
shown in Fig. 1(a) and Fig. 2(a), respectively. The
difference of the emission wavelengths between the two
lasers is due to normal shift of sample condition across a
two-inch diameter wafer. The cavity lengths of the 249nm laser and 256-nm laser are 1.7 mm and 1.0 mm. In
Fig. 1(b) and Fig. 2(b), the integrated spectral intensities
as a function of the pumping power density of the 249nm laser and the 256-nm laser demonstrate threshold of
~90 kW/cm2 and ~61 kW/cm2, respectively. As shown in
Fig. 1(b) and Fig. 2(b), the spectral linewidth of both the
249-nm laser and the 256-nm laser reduces with
increasing pumping power density and reaches ~1.6 nm,
indicating stimulated emission characteristics. The
thresholds are more than an-order-of-magnitude lower
than the previously-reported optically-pumped AlGaN
MQW DUV laser grown on foreign 4H-SiC substrates
[6]. In addition, these thresholds are comparable with the
reported state-of-the-art optically-pumped AlGaN MQW
DUV lasers grown on bulk AlN substrates lasing at a
longer wavelength of 266 nm [7], suggesting excellent
candidacy of sapphire substrates for III-N DUV laser
diodes.
Owing to larger size of sapphire substrates versus
current AlN substrates, dozens of laser bars of different
emission wavelengths were fabricated and measured
from wafers with different structures. Fig. 3 shows
stimulated emission spectra of some optically-pumped
AlGaN MQW DUV lasers with peak wavelengths from
239 nm to 256 nm. Notably the stimulated emission at
239 nm having a threshold of 280 kW/cm2 represents a
record-short wavelength at room temperature of AlGaN
DUV lasers grown on foreign substrates including SiC
and sapphire [6].
239nm-256nm
Edge PL emission
193nm pumping
Room temperature
220 230 240 250 260 270 280 290 300
Wavelength (nm)
Figure 3: Stimulated emission spectra at 300K of opticallypumped AlGaN MQW DUV lasers grown on sapphire
substrates with peak wavelengths from 239 nm to 256 nm.
Conclusions
Stimulated emission at wavelengths of 239-256 nm
with low thresholds were demonstrated from AlGaN
heterostructure lasers grown on sapphire substrates. The
lowest thresholds were 61 kW/cm2 and 90 kW/cm2. The
threshods were comparable with the lowest threshold of
41 kW/cm2 at 266 nm obtained from AlGaN
heterostructure laser grown on the bulk AlN substrate
but at shorter wavelengths of 256 nm and 249 nm. The
stimulated emission at 239 nm represents a record-short
wavelength of AlGaN DUV lasers grown on foreign
substrates at room temperature. The results indicate
sapphire substrates are promising for DUV laser diodes.
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