Laser diodes (LDs) are used in extreme conditions such as very high or low temperatures owing to their low voltage operating characteristics, high efficiency and reliability, and long lifetime^[1]. LDs have excellent performance and are widely used in various fields, including medical aesthetics, laser welding, laser guidance, laser ignition, and most importantly as a pumping source for solid-state lasers^[2]. However, as the demand for 808 nm LD applications expands, the performance requirements for LD are becoming increasingly stringent, particularly in terms of high output power, high wall-plug efficiency (WPE), superior beam quality, and reliability^[3-5].

For traditional high-power LD, there is a problem known as carrier leakage, which can increase nonradiative recombination in waveguide region, leading to decreased output power, WPE, and stability^[6]. Over the past few years, researchers have conducted numerous studies to reduce carrier leakage. Zhang B et al. inserted GaAs intermediate layer into InGaAs/AlGaAs multi-quantum wells, which reduced carrier leakage and ensured more radiative recombination in multiple quantum wells^[7]; Cao Y L et al. inserted thin GaAsP interlayer into InGaAsP/InGaP/AlGaAs LD to optimize epitaxial structure and obtain strong carrier confinement^[8]; Li X et al. added low Al content in AlGaAs interlayer between active region and n-side waveguide, and the electron leakage was remarkably depressed, owing to the reduction of injected electron energy and the improvement of quantum well capture efficiency^[9]; Asryan L V et al. adopted Ga_0.55In_0.45P and In_0.22Al_0.42Ga_0.36As as asymmetric barrier layers with electron and hole barrier heights of 78 and 240 meV, and compared to original structure, designed structure prevented hole leakage into waveguide layer resulting in low threshold current^[10]; Zubov F I et al. adopted Ga_0.83In_0.17P_0.79Sb_0.21 and Ga_0.47In_0.53P as barrier layers, which suppressed the fluxes of electron and hole for significantly improving WPE^[11]; Zhang X et al. used GaInAsP/GaAsP as asymmetric barrier with electron and hole leakage barrier heights are 2.22 and 1.76 times higher than that of conventional structure^[12], respectively; Yuan Q H et al. inhibited carrier leakage in active region by adding GaAsP material between barrier and waveguide layers for improving the temperature characteristics of LD^[13]. From previous studies, we find that researchers have significantly increased the carrier leakage barrier height by adding insertion layer or adopting asymmetric barrier, which enhances the confinement of carriers in active region, and reduces carrier leakage. Therefore, adding insertion layer or adopting asymmetric barrier is an effective way to reduce carrier leakage and improve optoelectronic performance. In this paper, we optimize the epitaxial structure by adding Ga_0.55In_0.45P and GaAs_0.6P_0.4 insertion layers. This approach reduces carrier leakage, decreases the device's series resistance, and enhances its temperature characteristics. However, the mechanism of adding insertion layer and its effect on carrier leakage need to be further studied.

A novel 808 nm LD is proposed in order to reduce carrier leakage by inserting Ga_0.55In_0.45P and GaAs_0.6P_0.4 layers between barrier and waveguide layers on both sides of active region. Ga_0.55In_0.45P and GaAs_0.6P_0.4 materials possess wide bandgaps, providing strong confinement for electron and hole. This effectively blocks carrier leakage, reduces nonradiative recombination in waveguide region, and utilizes high carrier confinement to achieve favorable temperature characteristics. Moreover, the novel LD not only can enhance optical field limiting capacity but also reduce optical loss by increasing refractive index gap between waveguide and active region. The relationship between optoelectronic performance and structure is discussed by comparing with and without insertion layer.

InAlGaAs/AlGaAs active region was used for 808 nm LD. AlGaAs, a commonly used material, was used for barrier, waveguide, and cladding layers of LD1, as shown in Figure 1. We optimized content and thickness of each layer of traditional LD1, and obtained its optimal structural parameters as 2000 nm-thick n-GaAs substrate and 350 nm-thick p-GaAs contact layer, 1200 nm-thick n-type Al_0.55Ga_0.45As cladding layer and 1000 nm-thick p-type Al_0.55Ga_0.45As cladding layer, 600 nm-thick n-type and 300-nm-thick p-type Al_0.35-0.55Ga_0.65-0.45As gradient waveguide layers, 6 nm-thick Al_0.2Ga_0.8As barrier layers and 5 nm-thick In_0.14Al_0.16Ga_0.7As quantum well (InAlGaAs has a high band step and thus will act as a good electron confinement and have a high temperature stability^{[14, 15]}). In contrast to LD1, LD2 used 8 nm-thick Ga_0.55In_0.45P as insertion layers between barrier and waveguide layers on both sides of active region. Designed structure LD3 used Ga_0.55In_0.45P and GaAs_0.6P_0.4 as insertion layers between barrier and waveguide layers on both sides of active region, and only insertion layer material was optimized, while other parameters remain unchanged. The bandgap of Ga_0.55In_0.45P and GaAs_0.6P_0.4 is larger than the bandgap of potential barrier, so that carriers in active region need high energy to cross Ga_0.55In_0.45P and GaAs_0.6P_0.4 insertion layers, thereby effectively suppresses carriers crossing barrier into cladding layer and generating nonradiative recombination. So we choose Ga_0.55In_0.45P and GaAs_0.6P_0.4 insertion layers to optimize the epitaxial structure. Figure 1 (color online) displayed the particular parameters of three LDs epitaxial structures.

In this study, LD was simulated by the simulation software SiLENSe with six-by-six k·p method. In the simulation, the device cavity length and strip width were set to be 2000 and 100 μm, and the reflectivity of the front and back cavity surfaces were 10% and 98%, respectively. The nonradiative lifetimes of electron and hole were 5.0×10⁻⁹ and 5.0×10⁻⁸ s, respectively, and the dislocation density was 100 cm⁻². The specific parameter values used for the device in the simulation were shown in Table 1.

We investigated the effect of inserting Ga_0.55In_0.45P and GaAs_0.6P_0.4 materials on refractive index and optical field distribution of three LDs, as shown in Figure 2(a) (color online). Figure 2(b) (color online) is an enlarged figure in the range of 1750−1900 nm. It is found that during photon transmission, optical loss for carrier absorption in n-side is lower than that in p-side region. Therefore, optical loss can be effectively reduced by adjusting the optical field distribution to shift toward n-side. Traditional and designed LDs have adopted an asymmetric structure for both waveguide thickness and cladding layer thickness, which is effective in reducing p-side optical loss^[16-17]. With insertions of Ga_0.55In_0.45P and GaAs_0.6P_0.4 between barrier and waveguide layers on both sides, it is found that refractive index differential between active region and waveguide layer become large so that limiting optical field enhanced and photon leakage reduced, resulting in low optical loss. As shown in Figure 2(b), the refractive index differential between barrier and waveguide layer of LD1 is 0.101, while for LD3 the refractive index differential between p-side insertion layer and the waveguide layer increases to 0.121. The asymmetric refractive index distribution keeps optical modes away from p-side, and increasing refractive index on p-side for enhancing optical field limitation, all of which lead to a decrease in free carrier induced optical absorption in high doped p-cladding layer. Therefore, designed structure LD3 increases refractive index differential between active region and waveguide layer, which reduces optical absorption in cladding layer and decreases optical loss.

Reducing optical loss in epitaxial structure is a key factor for achieving high output power^[18-19]. In order to investigate the effect of Ga_0.55In_0.45P and GaAs_0.6P_0.4 insertion layers on optical loss, we explore the variation of optical loss caused by free carrier absorption of three LDs, as shown in Figure 3 (color online). Figure 3(a) demonstrates optical loss owing to free carrier absorption outside of quantum well for three LDs. Optical loss outside (α_Out) of quantum well are 0.281, 0.262 and 0.254 cm⁻¹ for LD1, LD2 and LD3, respectively. The difference in α_Out between LD1 and LD3 is 0.027 cm⁻¹. Insertion of GaAs_0.6P_0.4 intermediate layer on p-side increases refractive index value between waveguide layer and active region, and the overlap between p-side optical field distribution and waveguide region decreases, thus reducing α_Out. Optical loss in quantum well (α_QW) of LD1, LD2 and LD3 are 0.060, 0.034 and 0.033 cm⁻¹, respectively, and the difference in α_QW between LD1 and LD3 is 0.027 cm⁻¹. Therefore, Ga_0.55In_0.45P and GaAs_0.6P_0.4 insertion layers reduce its free carrier absorption and provide sufficient energy barriers to limit carrier leakage in active region. As shown in Figure 3(c), total optical loss (α_Total) of LD1, LD2 and LD3 are 6.648, 6.603, and 6.595 cm⁻¹, respectively. The difference between α_Total of LD1 and LD3 is 0.053 cm⁻¹. This value is approximately equal to the summation of the differential values of α_Out and α_QW of LD1 and LD3. Calculation results reveal that the percentage variation of α_Out and α_QW accounts for 49.06% and 50.94% of the total variation in α_Total. Compared with LD1, quantum wells of LD2 and LD3 are shifted toward p-side by 8 nm, which solves the problem that absorption in the p-side is larger than n-side owing to the difference in the properties of electron and hole^[20]. It reduces loss of p-side photons and decreases optical absorption loss of free carrier in waveguide region, which leads to a small reduction in α_Total and thus improves conversion efficiency^[21]. Therefore p-side region is more critical for α_QW.

To investigate the effect of Ga_0.55In_0.45P and GaAs_0.6P_0.4 insertion layers on carrier leakage, energy bands of three LDs are analyzed at injection current density 5 kA/cm² (10 A), as shown in Figure 4(a) (color online). Figure 4(b) (color online) provides a magnified diagram of LD1, LD2 and LD3 in the range of 1760−1860 nm. By comparing LD1 and LD2 with LD3, it is found that the effective electron leakage height of LD1 is 338 meV and the effective hole leakage height is 405 meV. The effective electron leakage height and the effective hole leakage height of LD2 are 287 and 416 meV, respectively. LD3 has an effective electron leakage height and effective hole leakage height of 397 and 425 meV, respectively. It can be observed that the effective electron leakage height and the effective hole leakage height differences are 59 and 20 meV for LD1 and LD3, respectively. LD3 has a larger effective electron leakage height than effective hole leakage height compared to LD1, which solves the problem that electron migration rate is higher than hole migration rate at the same concentration owing to electron’s effective mass is smaller than hole’s effective mass^[22]. The effective electron leakage height and the effective hole leakage height of LD3 are 1.17 and 1.05 times higher than that of LD1, increasing the effective blocking height between waveguide layer and cladding layer in n-type region and p-type region, preventing the escape of electron (hole) to p-cladding layer (n-cladding layer) and enhancing the restriction capability of carriers. Therefore, LD3 effectively suppresses the leakage of electron and hole into cladding region by increasing the heights of effective leakage barrier of electron and hole, which reduces nonradiative recombination, improves carrier utilization efficiency, and increases radiative recombination.

We explore the effect of energy band on carrier transport in above part. It is found that increasing the effective leakage barrier heights of electron and hole is beneficial in enhancing confinement to carriers and reducing nonradiative recombination, which has an important effect on the improvement of the performance of LD^[23]. Leakage current density, Auger recombination current density, SRH recombination current density, and nonradiative recombination current density versus injection current for three LDs in Figure 5 (color online). From Figure 5(a), leakage current densities of LD1 and LD2 are 0.236 and 2.108 A/cm², respectively, leakage current density of LD3 is 0.029 A/cm², and LD3 reduces leakage current density by 87.71% compared to LD1, owing to enhanced carrier effective barrier height, leading to a reduction in carrier leakage. As shown in Figure 5(b), the Auger recombination current densities of LD1, LD2 and LD3 are 31.545, 37.796 and 30.931 A/cm², respectively, at injection current 10 A. The Auger recombination current densities of three LDs are almost constant in their values with increasing current. As shown in Figure 5(c), the SRH recombination current densities of three LDs are 20.172, 10.664 and 6.480 A/cm², respectively, at injection current 10 A, the SRH recombination current densities of LD1 and LD2 vary with injection current, and that of LD3 remains almost unchanged versus injection current. From Figure 5(d), it can be seen that the nonradiative recombination current densities of three LDs are 42.209, 57.969 and 37.411 A/cm², respectively, at injection current 10 A. Among them, the nonradiative recombination current densities of LD1 and LD2 increase with increasing injection current. While LD3 has a steady trend as current increases, and the nonradiative recombination current densities of LD1 is 1.11 times higher than that of LD3. Since LD3 optimizes carrier transport, nonradiative recombination current density and SRH recombination current density of LD3 remain almost constant as injection current increases^[24].

In order to meet high efficiency and development of device, low threshold current and operating voltage, high output power and WPE are the main objectives of epitaxial structure design^[25]. Figure 6 (color online) illustrates the relationship between threshold current, operating voltage, output power, and WPE versus injection current for three LDs. As depicted in Figure 6(a), threshold currents of three LDs are 0.511, 0.512 and 0.501 A, with LD3 showing a slightly low threshold current. When analyzing the optical performance, it is found that the optical limiting ability of LD3 is enhanced and the optical loss is reduced compared to LD1, so the reduction of threshold current is due to the decrease of carrier loss^[24]. Figure 6(b) shows the variation curve of operating voltage with injection current, and the slope of I-V curve indicates the magnitude of series resistance. Operating voltages of three LDs are 1.702, 1.744 and 1.636 V, respectively, at injection current 10 A. It is clear that the operating voltage of LD3 is very low, mainly owing to the reduction of its series resistance. According to the series resistance formula, we know that the resistance is mainly determined by the thickness, doping concentration and carrier mobility. Owing to Ga_0.55In_0.45P and GaAs_0.6P_0.4 are very thin, so their thickness is negligible. Moreover, Ga_0.55In_0.45P and GaAs_0.6P_0.4 have large mobility and high doping concentration, resulting in low series resistance, which reduces the operating voltage of LD3. As illustrated in Figure 6(c), output power are 12.68, 12.66, and 12.80 W for three LDs at injection current 10 A, with LD3 having a high output power owing to low optical loss, threshold current, and operating voltage. Figure 6(d) shows WPE of three LDs are 74.48%, 72.62%, and 78.24% at injection current 10 A. The WPE of LD3 is improved by 3.76% and 5.62% compared to LD1 and LD2, respectively. The addition of high doping and high mobility Ga_0.55In_0.45P and GaAs_0.6P_0.4 insertion layers in LD3 leads to reduction in operating voltage and series resistance, which in turn improves the WPE. The WPE of LD1 is slightly higher than that of LD3 at current of 1-2 A owing to the abrupt change of refractive index at the interface of LD1, causing increased energy loss. However, as the injection current increases, the carrier loss decreases, and the high power and high WPE are achieved by reducing optical and energy losses^[26].

The above discussion shows the optoelectronic performances of three LDs at room temperature of 25 °C. In order to evaluate the stability during operation, we simulated the variation rules of wavelength and threshold current of three LDs at different temperature. Graphs of wavelength and threshold current variation over the temperature ranged from 5 to 65 °C for three LDs are shown in Figure 7 (color online).

From Figure 7(a), it can be observed that three LDs exhibit red shift with increasing temperature owing to bandgap shrinking caused by Joule heating^[27]. The temperature drift coefficients of LD1, LD2 and LD3 are 0.210, 0.212 and 0.206 nm/°C, respectively. LD1 and LD2 have poor wavelength stability at high temperature, whereas LD3 exhibits more stable wavelength owing to the addition of Ga_0.55In_0.45P and GaAs_0.6P_0.4 insertion layers, which has a small temperature-drift coefficient at the center wavelength. Figure 7(b) reveals the graphs of threshold currents of three LDs over the range of temperature variations. Threshold current gradually increases as increasing temperature. However, LD3 exhibits a low threshold current at the same temperature, and the slopes of the fitted straight lines of threshold current versus temperature are 0.00128, 0.00118, and 0.00113 for three LDs. This indicates that LD3 has excellent threshold current stability during operation. From the above analysis, it can be seen that the center wavelength temperature drift coefficient of LD3 is smaller and the threshold current change is more stable than others. Therefore, LD3 has a slightly superior temperature dependence, which can reduce electron leakage, thereby improving the reliability^[28]. So adding Ga_0.55In_0.45P and GaAs_0.6P_0.4 insertion layers is instructive to improve the reliability.

In summary, based on InAlGaAs/AlGaAs active region, Ga_0.55In_0.45P and GaAs_0.6P_0.4 insertion layers between barrier layer and waveguide layer on both sides of active region changes energy band, which not only reduces carrier loss by resolving electron mobility larger than hole mobility but also decreases nonradiative recombination by increasing the height of effective carrier leakage barrier. Leakage current density decreases by 87.71%, and nonradiative recombination current density decreases to 37.411 A/cm². Output power and WPE reach 12.80 W and 78.24%, respectively, at injection current 10 A. In addition, temperature drift coefficient of the center wavelength is 0.206 nm/°C over the temperature variation range of 5 °C−65 °C.