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Abstract
An aluminum-free nitride laser diode (LD) operating at a wavelength of λ = 452 nm with a threshold current density of jth = 4.2 kA/cm2 grown by plasma assisted molecular beam epitaxy is demonstrated. Aluminum is successfully eliminated from the cladding layers and the electron blocking layer. The lifetime of the devices with and without aluminum are studied. It is found that aluminum, which is highly susceptible to oxidation, has no influence on the degradation mechanism of nitride optoelectronic devices. Furthermore, comprehensive theoretical calculations are presented to show the impact of removal of aluminum from the structure on LD properties.
© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The nitride optoelectronics serves few important roles in our everyday life. The biggest is the general lighting which, due to the high efficiency of nitride light emitting diodes (LEDs), has been truly revolutionized [1–5]. It is predicted, however, that future nitride laser diodes (LDs) might have comparable or even higher efficiency than the LEDs making them a product of choice in the general lighting application [6–8]. The LDs operating above the lasing threshold at high injection currents do not suffer from the droop of internal quantum efficiency as much as LEDs do [9,10]. Furthermore, the possibility of operating the device at high current densities allows to decrease the dimensions of the device. However, the degradation rate of the devices increases with the current density [11]. The general lighting application requires the devices to have an extremely high lifetime which will became a problem to be addressed in the case of LDs. The degradation of nitride LDs has been widely studied [12–18]. The major contribution to the degradation is caused by an increase in the non-radiative recombination centers in the active region. The nature of these centers remains unknown but the cause of their appearance is electrodiffusion. Additionally, the dislocation density strongly affects the degradation rate leading to a conclusion that the diffusion takes place in the dislocation vicinity.
All of the studies on the degradation of nitride LDs were done on (Al,In,Ga)-N material system. The possibility of removal of highly susceptible to oxidation aluminum from the nitride LD design can change the degradation mechanism as in the case of infrared LDs [19,20]. In the infrared LDs the lifetime of the devices was strongly limited by the facet oxidation [21]. The removal of aluminum, highly susceptible to oxidation, from the design of LDs based on the III-As and III-P material systems had greatly improved the lifetime of the devices [19]. In particular, the change of the material system from AlGaAs/GaAs to InGaAsP/GaAs increased the lifetime of LDs operating at λ = 808 nm by few orders of magnitude [20]. The devices operated for 30 000 hours without any visible degradation. This change was solely attributed to the elimination of, highly susceptible to oxidation, aluminum from the device structure. The removal of aluminum from the nitride LD design may extend the knowledge on the mechanism of their degradation.
In case of the nitride LDs the AlGaN layers are used to serve two independent purposes:
- 1. Thick AlyGa1-yN:Si and AlyGa1-yN:Mg layers with y = 0.03-0.10, placed on the opposing sides of the active region, act as cladding layers to confine the optical mode as a result of the contrast of the refractive indices (see Fig. 1(a)).
- 2. Thin AlyGa1-yN:Mg layer, with y = 0.10-0.30, placed between the active region and the p-doped layers, acts as an energetic barrier to prevent electron overflow (see Fig. 1(a)).
To eliminate aluminum from the design of the LD both of the mentioned above functions have to be provided by other means to ensure proper operation of the device. The AlGaN claddings have been successfully removed in case of nitride LDs grown on polar, semipolar and nonpolar GaN orientations [22–25]. This was obtained by application of a thick InGaN waveguide. The high refractive index of the InGaN waveguide allowed to change the material in the cladding layers from AlGaN to GaN and maintain sufficient optical confinement. However, all of these LDs had an AlGaN:Mg EBL in the design to prevent the electron overflow. Although this layer is relatively thin (usually 10 to 30 nm), its high aluminum content and proximity to the active region might have an impact on the degradation mechanism of the LDs. In the literature there are no reports of LDs without AlGaN EBL. However, there are a few reports on LEDs without AlGaN EBL [26,27]. It is important to stress that the LEDs are operated at current densities few orders of magnitude smaller than the LDs. At high injection regime the lack of an energetic barrier for electron overflow is detrimental for carrier injection efficiency as will be shown later.
In this paper we report on fabrication of an aluminum-free LD operating in the blue regime (λ = 452 nm) grown by plasma-assisted molecular beam epitaxy (PAMBE). The parameters of the aluminum-free laser diodes are presented and compared to a LD containing AlGaN layers. The structure of Al-containing and Al-free LDs are presented in Figs. 1(a) and 1(b), respectively. The structure of Al-free LD is very simple and consists of 165 nm thick In0.08Ga0.92N waveguide and GaN claddings which provides the optical confinement and three 2.6 nm thick In0.17Ga0.83N multi quantum wells (MQW) separated by 8 nm thick In0.08Ga0.92N quantum barriers (QB). The suppression of electron overflow out of the active region is provided by an abrupt doping and composition change at the end of the InGaN waveguide and will be discussed later. Furthermore, a comprehensive theoretical study of the influence of elimination of aluminum on optical mode confinement and injection efficiency is given.
Fig. 1 Schematic design of LD (a) with and (b) without aluminum. The MQW region is composed of three 2.6 nm thick In0.17Ga0.83N quantum wells separated by 8 nm thick In0.08Ga0.92N quantum barriers.
2. Simulations
2.1 Waveguiding properties
The impact of the removal of aluminum from the design of LDs on the optical confinement will be presented. The calculations of the optical mode distribution were made using a simple one dimensional Transform Matrix method [28]. The refractive index data was taken from Ref 29. The LD structures used for the calculations are presented in Fig. 1 and the wavelength is λ = 450 nm. The effect of leakage of optical modes to GaN substrate has not been taken into account. In case of the LD with AlGaN claddings it would decrease the optical confinement factor Γ if the n-AlGaN cladding thickness would be insufficient [30–32]. However, above some certain conditions of thickness and composition of the InGaN waveguide the leakage of optical modes to GaN substrate can be fully eliminated [33]. These conditions are marked with a brown dotted line in Fig. 2(a). In case of AlGaN free LD the leakage does not occur because the effective refractive index of the propagating mode is always higher than the refractive index of GaN substrate. Figures 2(a) and 2(b) present the calculated optical confinement factor Γ as a function of InGaN waveguide thickness and composition for LDs with and without aluminum, respectively. For the case of LD with AlGaN claddings and no InGaN waveguide the Γ = 0.0204 and is sufficient to ensure lasing. When the InGaN waveguide is added the optical confinement factor rises due to the refractive index difference between AlGaN/GaN/InGaN layers. At first the increase of both thickness and composition improves Γ. The enhanced Γ results in increased optical gain and leads to a decrease in the lasing threshold [34]. However, at some point a further increase of the thickness of InGaN waveguide starts to be undesirable as it lead to a decrease of Γ. For example for composition of x = 0.08 the optimal thickness is T = 140 nm and Γ = 0.0305 and drops as the thickness is further increased.
Fig. 2 Calculated optical confinement factor Γ as a function of InGaN waveguide composition and thickness for (a) LD with AlGaN claddings, (b) Aluminum free LD. The stars indicate the compositions and thicknesses of the manufactured LDs. The brown dotted line in Fig. 2 (a) represents the boundary above which leakage of the optical mode to GaN substrate is fully suppressed.
In the case of the AlGaN cladding free design without the InGaN waveguide the optical mode is confined only by the quantum wells and due to their small thickness the Γ = 0.0002. This small number is definitely insufficient to ensure adequate optical gain and reach lasing conditions. As the thickness and composition of InGaN waveguide are increased the optical confinement factor increases and for T>150 nm and x>0.06 can reach values comparable or higher than LDs with AlGaN claddings but without InGaN waveguide (Γ>0.02). However, if the InGaN composition and thickness is similar the design incorporating AlGaN claddings is better, at least when the optical confinement and lasing threshold is taken into account. On the other hand the AlGaN cladding free design might have some beneficial factors arising from the full elimination of leakage of optical modes to GaN substrate [33,35]. The leakage of light to GaN substrate causes an additional parasitic peak in the far field pattern and thus deteriorates the optical beam quality [30,32].
2.2 Carrier injection
The influence of removal of aluminum from the electron blocking layer on carrier injection efficiency will be studied. The calculations of band profiles and carrier transport have been made with the SiLense 5.4 package [36]. Figure 3(a) presents the conduction and band profiles of the active region of LD with AlGaN EBL at high current density conditions of j = 5 kA/cm2. The energy barrier between the undoped GaN waveguide and Al0.15Ga0.85N:Mg (Mg doping set at the level of 5 × 1019 cm−3) prevents the overflow of electrons out of the MQW region and their recombination in the p-type layers. The height of the energy barrier marked in Fig. 3(a) is EB = 120 meV. Its value depends not only on the design parameters such as composition and doping of EBL but also on the injection current. At high current densities the energy barrier start to decrease and a part of the electrons start to overflow through the EBL. The ratio of the current of carriers recombining in the active region to the total current flowing through the structure is called the injection efficiency ηi and is shown in Fig. 4 as a function of current density. The injection efficiency at low current densities (j<1 kA/cm2) is reaching unity, but as the injection current is increased the ηi drops to ηi = 0.73 at j = 5 kA/cm2. All of the observed drop of ηi is due to the electron overflow to the p-type region.
Fig. 3 Calculated band profiles at a current density of j = 5 kA/cm2 for (a) LD with Al0.15Ga0.85N EBL, (b) Aluminum free LD. The height of the energetic barrier EB preventing the electron overflow is marked in the figure.
Fig. 4 Calculated injection efficiency ηi as a function of current density for different designs of waveguide and EBL.
Now, the discussion on the removal of aluminum from the EBL will be presented. If the aluminum would be simply removed from the EBL without any other change to the structure the vast majority of electrons will overflow the active region because there would be no energy barrier to prevent the overflow. The results of the calculations involving undoped GaN waveguide and GaN:Mg layer (Mg doping set at the level of 5 × 1019 cm−3) are shown in Fig. 4. As can be seen the ηi drops from ηi = 0.30 at very low current density down to ηi = 0.14 at j = 5 kA/cm2. Such a small number of carriers recombining in the active region is insufficient to build up the optical gain needed to obtain the lasing condition.
To remove aluminum from the EBL and at the same time prevent electrons from overflowing we propose to utilize the difference in the bandgap between InGaN waveguide and GaN claddings as the energetic barrier. P-type doping of the GaN cladding will ensure that the whole difference in the bandgap between InGaN and GaN will emerge in the conduction band forming an energetic barrier for electrons. The calculated band profiles showing this behavior are presented in Fig. 3(b). The injection current density is equal to j = 5kA/cm2 and is the same as in the case of LD with AlGaN EBL in Fig. 3(a). The energetic barrier which prevents the electron overflow has been indicated in the figure and is equal to EB = 250meV which is much higher than in the case of LD with GaN waveguide and AlGaN:Mg EBL. Additionally the calculated injection efficiency shown in Fig. 4 is strongly enhanced. It is equal to nearly unity in the range from j = 0 up to 5 kA/cm2. Such a behavior is well understood when the height of the energetic barrier which prevents the electron overflow is compared. In case of the LD with x = 0.08 InGaN waveguide and GaN:Mg EBL the height of the barrier is much bigger. If one would use x = 0.08 InGaN waveguide and y = 0.15 AlGaN:Mg EBL the height would also be sufficient to obtain ηi = 1 in the range from j = 0 to 5 kA/cm2 (not shown in Fig. 4)).
3. Experimental results
The experimental realization of the aluminum free LD by PAMBE will be discussed. The structure of the LD was presented in Fig. 1(b). The epitaxial growth was carried out in a VG V90 machine equipped with two Veeco RF plasma sources. The high quality Ammono-GaN substrates with threading dislocation density of 1 × 104 cm−2 were used [37]. GaN and InGaN layers were grown at gallium and indium rich conditions, respectively. The growth temperature was 730°C for GaN and 650°C for InGaN. Only the 20 nm GaN:Mg EBL was grown at 650°C to cap the InGaN surface before ramping the temperature up for the growth of the p-GaN cladding. The laser diodes were processed as oxide isolated ridge-waveguide devices. The resonator length was 1000 μm and the stripe width 3 μm. The facets of the LDs were left uncoated. Further details on the growth and processing can be found elsewhere [32,38].
Figure 5(a) presents a large area cross-sectional high-angle annular dark field scanning transmission electron microscopy (HAADF–STEM) image of the aluminum-free LD taken with a TITAN CUBED 80–300 system operating at 300 kV equipped with Cs-corrector. The total thickness of the InGaN waveguide is 165 nm. Figure 5(b) presents the LIV characteristic of the aluminum-free LD. The inset shows the high resolution lasing spectra collected slightly above the threshold. The threshold current density was jth = 4.2 kA/cm2 which is slightly higher than a LD with AlGaN layers which had jth = 3.6 kA/cm2. This 17% increase in the threshold current density can be explained by the difference in optical confinement factor Γ. In Figs. 2(a) and 2(b) the calculated Γ of Al-containing and Al-free LDs, respectively, had been marked with stars. There is a 20% decrease of the optical confinement factors from Γ = 3.05 to 2.55 for the LDs with and without aluminum, respectively. The fact that the difference in jth can be explained by the change of Γ indicates that the carrier injection is similar for both LDs. This result is consistent with the carrier injection calculations presented in Section 2.2 and shows that the removal of aluminum from the EBL design of nitride LDs is feasible.
Fig. 5 (a) Large area HAADF-STEM cross-section image of aluminum free LD. (b) LIV characteristic of aluminum free laser diode. Insert shows the high resolution lasing spectra collected slightly above threshold.
3.1 Lifetime studies
The comparison of the lifetime studies between the LDs with and without aluminum will be presented. The tests were taken in a constant optical power condition at room temperature. The operating current was constantly adjusted to obtain 15 mW of optical power. Every 2 hours the LIV characteristic was measured and the resulting threshold current is shown in Fig. 6. Surprisingly, over almost 2000 hours of tests both LDs degrade in the same manner. During the first 100 hours the threshold current decreases which is known as the initial burn-in. Afterwards, a slight degradation of the devices can be observed. A linear approximation is used to extract the degradation rate. In case of the LD with aluminum the tests have been prolonged to obtain a more reliable value of the degradation rate due to the oscillations of threshold current observed below 2000 hours. For both LDs the degradation rate is 0.004 mA/h. It is worth to note that the observed increase in the threshold current is very slow. The lifetime of the devices, defined as an increase of the threshold current by 50%, is calculated assuming a constant degradation rate to be 15 000 hours.
Fig. 6 Threshold current as a function of time collected during aging tests of LDs with and without aluminum. The trend in the change of threshold current with time is nearly identical indicating a common degradation mechanism. The tests were conducted at room temperature and the optical power was stabilized at 15 mW. The dashed lines present linear approximations of the degradation rate.
The similar behavior of the LDs shows that the degradation mechanism of nitride LDs is completely independent on the presence of aluminum in the structure. This is striking as, the highly susceptible to oxidation, aluminum is known to extremely increase the degradation rate of arsenide-based LDs. The neutral role of aluminum in the degradation rate of the studied LDs indicates that the oxygen impurity plays no role in the degradation mechanism of nitride optoelectronic devices.
4. Conclusions
Summarizing, the aluminum-free nitride laser diode grown by plasma-assisted molecular beam epitaxy was demonstrated. The removal of aluminum from cladding layers and electron blocking layer was successful thanks to the incorporation of high quality thick high indium content InGaN waveguide. A comprehensive theoretical study of the influence of aluminum removal on the optical mode distribution, band profiles and carrier transport was presented. It was found that 150 nm thick In0.06Ga0.94N is sufficient to support suitable optical confinement. Additionally, is was shown that the role of the conventional AlGaN:Mg EBL can be substituted by an abrupt change of the composition and doping at the InGaN waveguide and GaN cladding interface. The demonstrated LD had 165 nm thick In0.08Ga0.92N and its properties were compared to LD with AlGaN layers. A similar threshold current density was observed for both devices. The long term life-tests showed nearly the same behavior indicating a common degradation mechanism. This is an unexpected result as aluminum is commonly known to oxidize and strongly decrease the lifetime of arsenide LDs.
Funding
Foundation for Polish Science (TEAM TECH/2016-2/12); National Centre for Research and Development (PBS3/A3/23/2015, LIDER/29/0185/L-7/15/NCBR/2016), National Science Centre (DEC-2013/11/N/ST7/02788).
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