Input current (mA)
Device A (without blocking layer)
Figure 5.14 Percentage of electron leakage current as a function of input current in device A when the device temperature was in a range of 25–95 ºC.
0 1 2 3 4 5 6
20 30 40 50 60 70 80 90 100 Device A (without AlGaAs)
Device B (with Al0.75Ga0.25As) Device C (with Al0.9Ga0.1As)
Per cent age of el ectr on leakage curr ent (% )
Temperature (
oC)
Figure 5.15 Percentage of electron leakage current as a function of device temperature for devices A, B, and C. The curves were obtained when the devices were biased at 10
mA.
5-5 Summary
In summary, the gain-carrier characteristics of the In0.02Ga0.98As and InAlGaAs QWs with 838 nm emission are theoretically investigated. The numerical results suggest that the incorporation of Al into InGaAs QW is found to provide higher material gain, lower transparency carrier concentration and radiative current density due to the
increment of the amount of strain and the reduced density of states. The optical properties of In0.15Al0.08Ga0.77As QW are also investigated by temperature dependent PL.
The carrier blocking effect on the output performance of 850-nm InAlGaAs/AlGaAs VCSELs are also experimentally and theoretically investigated. With the use of a high-bandgap 10-nm-thick Al0.75Ga0.25As layer in the In0.15Al0.08Ga0.77As/Al0.3Ga0.7As QW active region, the high temperature characteristics and the output performance are found experimentally improved. The results obtained theoretically also approved that the improvement in output performance is due to the reduction of the electron leakage current. Small-signal frequency response shows that these VCSELs can provide a modulation bandwidth of approximately 9.2 GHz.
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Ch C ha ap pt te er r 6 6 1. 1 .3 3- - µ µ m m I In nG G aA a As sN N/ /G G aA a A sN s N E EE EL L
Recent progress of the semiconductor lasers with emission wavelength of 1.3 µm for optical communication system has focused on using InGaAsN/GaAsN materials as quantum well active region. Laser performances of InGaAsN/GaAsN EELs have been found comparable to or superior to some of the best published results based on the conventional InP technology [1]−[11]. High-temperature operation has been anticipated in these material systems due to better electron and hole confinement by increased band offsets and a more favorable band-offset ratio. Previous research showed that for pulse operation of InGaAsN lasers, the characteristic temperature coefficient T0 value of exceeding 200 K was demonstrated [12]−[14]; while for continue-wave operation, the T0 of 70–110 K was also achieved [15]−[19]. Unfortunately, based on the predicted theoretical calculation of the maximal T0 value in InGaAsN laser structures, these high performance InGaAsN lasers showed only a slight improvement in T0 values over those achievable by the conventional InP technology. Fehse et al. found that the unexpected low T0 value of InGaAsN lasers was attributed to the existence of large Auger recombination [20]. The difficulty of nitrogen atoms incorporating into InGaAs alloys, which lead to poor crystal quality, and the hole leakage problem [21] might be the key issues that resulted in the unexpected low T0 value of InGaAsN lasers.
To achieve a more favorable T0 value and a better output performance of InGaAsN lasers, there had been several works investigating the InGaAsN lasers with strain-compensated GaAsN as direct barriers. Even GaAsN is a smaller band gap material system, using GaAsN barrier instead of GaAs barrier could reduce nitrogen outdiffusion from the well and balancing the highly compressive strain in InGaAsN QW [22]. The same phenomenon had also numerically obtained by Fan et al [12]. However, adding more nitrogen atoms into barrier may decrease barrier potential and the carrier
leakage problem follows at high device temperature, even though longer wavelength emission can be obtained. This problem was solved by Tansu et al., who utilized tensile-strain GaAs0.85P0.15 layers on both sides of the InGaAsN/GaAs active region to reduce the strain in QWs for achieving better crystal quality that in turn improved the laser performances [15], [16], [21]. Nevertheless, it could be though in the physical band alignment that the high bandgap GaAs0.85P0.15 layer on the n-side InGaAsN/GaAs active region may obstruct electrons pouring into the active region, while the GaAs0.85P0.15 layer on the p-side active region can blocking electrons overflowing to the p-side layers.
To obtain a more favorable characteristic temperature coefficient T0 in 1.3-µm InGaAsN/GaAsN lasers, in this chapter, we first investigate the material gain properties of InGaAsN/GaAs1-xNx QW lasers with various GaAs1-xNx strain compensated barriers (x=0%, 0.5%, 1%, and 2%). It is shown that, inaddition to the crystal quality concern of InGaAsN QWs duringcrystal growth, the nitrogen composition of GaAs1-xNx strain compensated barrieralso plays an important role in the confinement of carriers. Next, to better confine carriers in the InGaAsN/GaAsN active region, a high bandgap 15-nm-thick GaAs0.9P0.1 is proposed to be inserted into the active region before the growth of p-type layers of the conventional structure. The demonstrated laser characteristics and a theoretical analysis are given in the mean time. Specifically, the phenomenon of electronic leakage current is investigated.
6-1 Method and numerical parameters
Based on the k⋅ theory with valence band mixing effect, a 6×6 Hamiltonian of p the Luttinger-Kohn type matrix and an envelope function approximation were used to solve the InGaAsN/GaAsN QW subband structures. Detail illustrations could be found in Chapter 2. For this specific simulation, the ratio of conduction band to valence band
offset in InGaAsN/GaAsN band alignment was estimated to 0.7/0.3 [23]. The bandgap energy of InGaAsN material at room temperature was governed by the following bilinear terms with two bowing terms:
GaN
where x and y denoted the gallium and arsenide compositions in InGaAsN alloy, and the bandgap energies of GaAs, InAs, GaN and InN were 1.424, 0.355, 3.42 and 0.77 [24]
eV. The bowing parameters for GaAsN and InGaAs ternary alloys were -18 [25] and -0.6 eV. The temperature dependent bandgap energy was as follows:
225]
where Eg(T) was the bandgap energy of InGaAsN alloy at temperature T. Therefore, the temperature dependent bandgap energy of InGaAsN alloy was:
)
The effective mass of electrons used in simulation was as follow:
GaN
where the effective mass of electrons in GaAs, InAs, GaN and InN were 0.064×m0, 0.023×m0, 0.2×m0 and 0.11×m0 respectively. The effective masses of light holes (LH) and heavy-holes (HH) were governed by the same form in Eq. (6), and the effective masses of light holes and heavy holes for GaAs were 0.09×m0 and 0.377×m0, 0.027×m0
and 0.263×m0 for InAs, 0.9767×m0 and 1.3758 for GaN, and 0.5133×m0 and 1.5948×m0
for InN respectively. The Auger coefficients for InGaAsP and InGaAsN were 3.5×10-42
and 1.5×10-41 m2/s. Other material-dependent parameters were taken from the default database values given in the material macro file [26].
6-2 Optical gain properties of InGaAsN QW with GaAsN barriers Temperature effects on the optical gain properties of In0.4Ga0.6As0.986N0.014 and In0.8Ga0.2As0.69P0.31 QW materials were studied in the first instance. For the purpose of obtaining an emission wavelength of 1.3 µm, the nitrogen composition in InGaAsN QW was assumed 1.4% with an indium composition in InGaAsN QW of 40%. The calculated material gain of room-temperature In0.4Ga0.6As0.986N0.014 and In0.8Ga0.2As0.69P0.31 QW materials at an input carrier concentration of 2×1018 cm-3 was shown Figure 6.1. The barrier materials used under this study for In0.4Ga0.6As0.986N0.014
and In0.8Ga0.2As0.69P0.31 QWs were GaAs1-xNx with x=0%, 0.5%, 1%, 2% and In0.9Ga0.1As0.24P0.76 [27]. It was found that In0.4Ga0.6As0.986N0.014/GaAs1-xNx (x=0%, 0.5%, 1%, 2%) materials have higher maximum material gain than that of InGaAsP material. The highest maximum material gain was obtained when x=0%, i.e. GaAs barrier, and the maximum material gain was found to be red shift from 1.3 to 1.34 µm by increasing x value from 0% to 2% in GaAs1-xNx barrier. In addition, the maximum material gain decreased rapidly with increasing x value in GaAs1-xNx barrier, which was suggested as a result of the decreased conduction band carrier confinement potential.
Nevertheless, the maximum material gain of In0.4Ga0.6As0.986N0.014/GaAs1-xNx was twice approximately higher than that of In0.8Ga0.2As0.69P0.31/In0.9Ga0.1As0.24P0.76 when the input carrier concentration was 2×1018 cm-3.
The maximum material gain of using GaAs1-xNx barriers with x=0%, 0.5%, 1% and 2% as a function of temperature were shown in Figure 6.2. With increasing temperature, an almost linearly drop of maximum material gain was found. A red shift of the maximum material gain with x=0% from 1.3 to 1.35 µm and the decrease of the
maximum material gain value from 2443 to 1575 cm-1, which was due to the wider spreading of the Fermi distribution of carriers and stronger Auger recombination losses, were numerically obtained when the temperature increased from 300 to 370 K.
Manifestly, the severe decrease of the maximum material gain value caused by the linear increase of x value indicated that increasing nitrogen composition in GaAsN barrier might procure the poor laser performance as a result of the relatively low material gain.
-500 0 500 1000 1500 2000 2500 3000
1.2 1.25 1.3 1.35 1.4 1.45
x=0%
x=0.5%
x=1%
x=2%
InGaAsP
Wavelength (µm)
GaAs1-xNx barrier
Figure 6.1 Calculated material gain of room-temperature In0.4Ga0.6As0.986N0.014 and In0.8Ga0.2As0.69P0.31 QWs when the input carrier concentration is 2×1018 cm-3.
1000 1500 2000 2500
300 310 320 330 340 350 360 370
x=0%
x=0.5%
x=1%
x=2%
Temperature (K) GaAs1-xNx barrier
Figure 6.2 Maximum material gain of using GaAs1-xNx barriers with x=0%, 0.5%, 1%
and 2% as a function of temperature.
Figure 6.3 showed the transparency carrier concentration as a function of temperature when using GaAs1-xNx barriers with x=0%, 0.5%, 1% and 2%. A trend of the increased material gain with increased input carrier concentration was found. As
Maximum material gain (1/cm) Gain (1/cm)
well, the room-temperature transparency carrier concentrations of
In0.4Ga0.6As0.986N0.014/GaAs1-xNx materials were lower than that of
In0.8Ga0.2As0.69P0.31/In0.9Ga0.1As0.24P0.76 material, 1.35×1018 cm-3. The differential gains of In0.4Ga0.6As0.986N0.014/GaAs1-xNx materials were also higher than that of In0.8Ga0.2As0.69P0.31/In0.9Ga0.1As0.24P0.76 material due to the fact that In0.4Ga0.6As0.986N0.014/GaAs1-xNx based material had relatively high conduction band offset and more electrons could be confined in the active region effectively. The transparency carrier concentrations at room temperature of using GaAs1-xNx barriers with x=0% and x=2% are 9.8×1017 and 1.06×1018 cm-3 respectively. For x=0%, the transparency carrier concentration increased almost linearly to 1.25×1018 cm-3 when the temperature was 370 K. The transparency carrier concentrations of using GaAs1-xNx
barriers with x=0.5% and 1% were slightly higher than that of GaAs barrier in a temperature range of 300-370 K. However, the transparency carrier concentration increased apparently when the x value was 2% and it increased rapidly when the temperature was higher than 350 K.
0.9 1 1.1 1.2 1.3 1.4 1.5 1.6
300 310 320 330 340 350 360 370
x=0%
x=0.5%
x=1%
x=2%
Temperature (K) GaAs1-xNx barrier
Figure 6.3 Transparency carrier concentration as a function of temperature when using GaAs1-xNx barriers with x=0%, 0.5%, 1% and 2%.
From the analysis of the optical gain properties of In0.4Ga0.6As0.986N0.014 QW sandwiched between GaAs1-xNx barriers with variant x values, we find that using
Transparency carrier concentration (1018 /cm3 )
GaAs1-xNx barriers with x value ranging from zero to 1% can have better temperature dependent optical gain properties. When the x value increases to 2%, the maximum material gain and the transparency carrier concentration abate remarkably. It indicates that In0.4Ga0.6As0.986N0.014 QW sandwiched between GaAs1-xNx barriers may have better laser performance, i.e. lower threshold current density and higher slope efficiency, when the x value is zero or less than 2%. Especially, using high potential GaAs barrier provides better electron confinement and results in obtaining highest material gain and lowest transparency carrier concentration. A highest T0 value may also be obtained as a result of reducing the probability of electronic leakage current if the LD structure is under high temperature operation. Besides, after the consideration of using GaAsN barrier instead of GaAs barrier has several advantages in experiment and longer wavelength can easier be obtained, we find in this study that using GaAs1-xNx barriers with x=0.5% and 1% can also provide high material gain and low transparency carrier concentration.
6-3 Fabricated device characteristics
After numerically investigated the material gain as a function of nitrogen composition in GaAsN barrier of InGaAsN/GaAaN active region, we further tried to fabricate the 1.3-µm InGaAsN/GaAsN edge emitting lasers in this subsection. Based on the numerical results in prior subsection, a GaAsN material with 0.5% nitrogen composition was chosen as barrier for InGaAsN QW active region because of the comparable material gain properties when compared to GaAs and the help of preventing nitrogen outdiffusion from InGaAsN QW during crystal growth. As schematically plotted in Figure 6.4, the InGaAsN/GaAsN laser structure was grown on the n-type Si-GaAs substrate with (001) orientation. The laser structure under study was grown by low pressure MOCVD with group-V precursors of arsine (AsH3), phosphine (PH3), and
U-dimethylhydrazine (U–DMHy) for N-precursor. Trimethyl (TM–) sources of aluminum (Al), gallium (Ga), and indium (In) were used for group-III precursors. The dopant sources were SiH4 and CBr4. On top of the GaAs template was a 1.0-µm-thick n-type Al0.6Ga0.4As layer, followed by a 0.15-µm-thick n-type Al0.4Ga0.6As with growth temperature of 770 ºC. Then, the growth temperature was down to 530 ºC for the growth of active region, which contained two In0.41Ga0.59As0.987N0.013 wells. The V/III ratio for the growth of active region was kept to 20. The thickness of In0.41Ga0.59As0.987N0.013 well and GaAs0.995N0.005 barrier, which were determined by X-ray diffraction and growth condition, were 6 nm and 10 nm, respectively. The strains of well and barrier were 2.08% in compressive and 0.2% in tensile, respectively. After the growth of active region, a 10-nm-thick undoped GaAs layer was grown to cap the active region for maintaining better QW quality and a 15-nm-thick undoped high bandgap GaAs0.9P0.1 layer was then grown with a purpose of blocking electrons from overflowing to the p-type layers. The guiding region was formed by 0.72-µm-thick updoped GaAs with growth temperature of 530 ºC, followed by a 0.4-µm-thick p-type Al0.4Ga0.6As layer with a doping concentration of 5×1018 cm−3 and a 1.0-µm-thick p-type Al0.6Ga0.4As layer with a doping concentration of 5×1018 cm−3. Finally, a p-type 100-nm-thick GaAs with a doping concentration of 2×1019 cm−3 was grown to complete the structure. The device was proposed by photolithograthy and reactive ion-etching into narrow stripe ridge waveguide lasers with 4 µm in width and 1000 µm in length.
The end facets of the laser chips were uncoated and the laser chip was mounted p-side-down onto copper heat sinks with indium.
Figure 6.4 A schematic diagram of the double-quantum-well InGaAsN/GaAsN laser structure.
20 40 60 80 100
1.292 1.295 1.299 1.302
Intensity (a. u.)
Wavelength (µm)
Figure 6.5 Electroluminescence spectrum when the laser device was at an input current of laser threshold.
To better confine carriers in the active region, two structures that were without and with inserting an undoped high-bandgap GaAs0.9P0.1 layer into the active region before the growth of p-type layers were prepared. The double-quantum-well structure for type A was the conventional structure that was without capping the high bandgap GaAs0.9P0.1
layer on top of the QW active region. Type B was the structure with the high bandgap GaAs0.9P0.1 layer. The fabricated laser devices were tested under CW mode operation.
Figure 6.5 showed the electroluminescence spectrum when the laser device was at an input current of laser threshold. A peak emission wavelength of 1.295 µm was obtained for both type A and type B lasers.
The temperature dependent L-I characteristic of type A laser under CW mode operation in a temperature range of 25–105 ºC was shown in Figure 6.6. The threshold current and the threshold current density per QW were 84 mA and 1.05 kA/cm2 at 25 ºC.
Figure 6.7 showed the temperature dependent L–I characteristic of type B laser under CW mode operation in a temperature range of 25–105 ºC. The threshold current was 99 mA and the threshold current density per QW 1.23 kA/cm2 at 25 ºC. The room temperature slope efficiencies of type A and type B lasers were 0.09 and 0.11 W/A.
0 0.5 1 1.5 2 2.5 3
50 100 150 200 250
Type A laser (without GaAs0.9P0.1)
Figure 6.6 Temperature dependent L–I characteristic of type A laser under CW mode operation in a temperature range of 25–105 ºC.
0 1 2 3 4
50 100 150 200 250
Type B laser (with GaAs0.9P0.1)
Figure 6.7 Temperature dependent L–I characteristic of type B laser under CW mode operation in a temperature range of 25–105 ºC.
25 ºC 45 ºC 65 ºC 85 ºC 105 ºC
Output power (mW)
Current (mA)
25 ºC 45 ºC 65 ºC 85 ºC 105 ºC
Output power (mW)
Current (mA)
Despite of the high threshold current density at 25 ºC, which was caused by the
Despite of the high threshold current density at 25 ºC, which was caused by the