Improvement of Power and Efficiency of High-Mesa Semi-Insulating InP: Fe Buried Heterostructure Lasers with Wide Bandgap Layers

Abstract

High-mesa semi-insulating buried heterostructure (SIBH) lasers with InP: Fe have great potential in high-speed and high-power scenarios, but the leakage current problem under high current injections has always limited their application. In order to solve the issue of low output power and low efficiency for high-mesa SIBH lasers, the mechanism of leakage current generation in InP-based semi-insulating (SI) layers at high injection levels was analyzed through numerical simulation. The deterioration of the device performance is due to the hole current-induced electron leakage current, which results from the reduction of the potential barrier and Fe-Zn interdiffusion. Thus, lasers with wide bandgap layers of InAlAs and ZnCdSe were employed for current blocking, the power and wall-plug efficiency of which were improved by more than 36% and 5%, respectively. For the first time, a SIBH laser based on lattice-matched ZnCdSe barrier layers is proposed, which shows good output performance and high reliability. The introduction of the wide bandgap layer in the SIBH structure establishes potential barriers to confine both carrier leakages at high injection levels, which realizes the high-power and high-efficiency operation of the laser.

1. Introduction

Semi-insulating buried heterostructure (SIBH) lasers based on InP: Fe material in the optical communication band have been widely used in high-speed optical communication systems [1] and Co-packaging Optics (CPO) fields [2], profiting from their low threshold currents, high modulation bandwidth and planar structures. Further, high-mesa SIBH structures are ideal for large-scale commercial applications due to their saving of the p-capping epitaxy process and high long-term reliability [3]. Therefore, high-mesa SIBH lasers with InP: Fe are expected to be candidates for high-power, high-efficiency light sources, while challenges still remain for this scheme.

Firstly, almost all BH lasers suffer from severe leakage currents under high current injections because existing BH structures inevitably introduce leakage current channels [4,5,6] during selective area growth (SAG), together with the diffusion of Zn near an active region [7]. Secondly, Fe-doped InP will lose semi-insulating (SI) properties in the p-doped environment due to double injection [4,8,9]. Last but not the least, Fe-Zn interdiffusion could redistribute the dopant profile in the Fe-doped device, which could lead to further aggravation of leakage currents [9,10,11]. Nonetheless, the mechanism of generating leakage currents has not been quantitatively studied. At present, solutions to improve the performance of SIBH lasers include: interposing an n-doped InP layer [12,13], adopting Ru as a SI-layer dopant [13], using proton injection in a buried ridge strip (BRS) laser [14] and integrating a semiconductor optical amplifier (SOA) [2].

Wide bandgap layers are also expected to deal with the current leakage issue in BH structures. Asada et al. and Ohtoshi et al. proposed to use strained GaInP and InAlAs layers to block leakage currents in BH structures, respectively [4,15]. The pn current-blocking BH lasers with lattice-matched InAlAs thin layers have been successfully fabricated, which show an improvement in the temperature characteristics [16,17]. Moreover, thick undoped InAlAs and SI-InAlAs layers with a high resistivity have also been introduced in BH devices to prevent p-dopant diffusion [18,19]. Recently, polycrystalline ZnSe was adapted as an isolation material in mesa stripe structures with a resistivity of more than 10⁸ Ω·cm [20].

Table 1. The main wide bandgap material used in InP-based semiconductor lasers.

Ref.	Material	Bandgap	Lattice Constant	Resistivity	Crystalline	Thermal Conductivity	Thermal Expansion Coefficient c
[4]	In0.5Ga0.5P	1.87 eV	5.66 Å		Single-crystalline	15.7 W/(m·K) a	5.8 × 10−6 K−1
[21]	In0.52Al0.48As	1.47 eV	5.87 Å	2 × 108 Ω·cm	Single-crystalline	5.7 W/(m·K) a	4.1 × 10−6 K−1
[13]	In0.46Al0.54As	1.63 eV	5.84 Å		Single-crystalline	5.8 W/(m·K) a	4.0 × 10−6 K−1
[20]	ZnSe	2.7 eV	5.67 Å	108 Ω·cm	Polycrystalline	17.3 W/(m·K) b	7.7 × 10−6 K−1
[22]	SI InP: Fe	1.35 eV	5.87 Å	107 Ω·cm	Single-crystalline	68 W/(m·K)	5.5 × 10−6 K−1
	SiO2	9 eV		1014 Ω·cm	Amorphous	1.4 W/(m·K)	0.5 × 10−6 K−1
This work	Zn0.48Cd0.52Se	2.08 eV	5.87 Å		Single-crystalline	2 W/(m·K) b	7.5 × 10−6 K−1

^a Calculated by the relationship between material composition and thermal resistivity from ref. [23]. ^b From ref. [24]. ^c Calculated by the interpolation from ref. [25].

lists the main wide bandgap materials used in InP-based semiconductor lasers, as well as SI InP: Fe and SiO₂. Compared with SiO₂ insulating layers, II-VI semiconductor materials have the advantages of a single crystal, wide bandgap, high resistivity, lattice matched to InP and thermal expansion coefficient close to InP, which can provide effective current limits inside the planar BH laser. However, the above studies have mainly focused on high-speed lasers working at a small current injection. The performance of the SIBH laser with wide bandgap layers for high-power applications has not been reported.

The purpose of this paper is to analyze the influence of leakage current on the power and efficiency of high-mesa SIBH lasers with SI InP: Fe based on numerical simulation, and to realize high-power and high-efficiency light sources by using wide bandgap layers. The failure mechanism of the SIBH laser with SI InP: Fe under a large injection is that the Fe-Zn interdiffusion and reduction of the potential barrier between the p-cladding and SI-layer lead to a hole current-induced electron leakage current. Compared to InAlAs barriers inserted in InP: Fe in n-type environments to reduce leakage currents, ZnCdSe interposers maintain high-potential barriers for both electrons and holes. To our knowledge, it is the first time that a lattice-matched ZnCdSe layer has been introduced in SIBH lasers to limit current leakage, which is beneficial for the planarization and heat dissipation of the structure. The results show that the output power and wall-plug efficiency (WPE) of the laser with wide bandgap layers were improved by more than 36% and 5%, respectively. The use of a thin ZnCdSe layer inside the structure to block leakage currents provides ideas for the design of high-power SIBH lasers and expands the application of II-VI group semiconductors.

The remainder of the paper is organized as follows: Section 2 describes the deep-level trap model and the configuration of the high-mesa SIBH laser. In Section 3, we study the causes of the failure of the device under a large current injection and quantitatively analyze the effect of different wide bandgap layers on the power and WPE of the laser. In Section 4, the suitable thickness range and nonuniform growth of wide bandgap layers are discussed, as well as the impact of the mesa profile on the output performance. Finally, the summary and conclusions are given in Section 5.

2. Trap Models and Laser Configurations

2.1. Deep-Level Traps for SI Material

In this paper, SIBH lasers were studied using a two-dimensional simulation software LASTIP based on a classic deep-level trap model [4,26]. It should be pointed out that we mainly focused on the performance of lasers in the ohmic regime, ignoring the influence of the nonlinear velocity field and impact ionization on SI materials at high voltages [8,27].

Deep-level traps act as recombination centers in semiconductors. The recombination rate $R_i$ is given by:

$$R_i=c_{i}i{N}_t\left(1-f_i\right)-c_i{i}_1{N}_t{f}_i,$$

(1)

where the subscript $i$ represents electron $n$ and hole $p$, respectively; $c_i$ is the capture coefficient; $i$ represents the density of carriers; $N_t$ is the trap density; $f_i$ refers to the trap occupancy $f_t$ for subscript $n$ and $\left(1-f_t\right)$ for subscript $p$; $i_1$ is the density of carriers in the conduction or valance band when the Fermi level is located at a deep-level energy $E_t$. The capture coefficients $c_i$ can be further expressed as:

$$c_i={\sigma }_i{\upsilon }_i={\sigma }_i\sqrt{\frac{8kT}{\pi m_i}},$$

(2)

where ${\sigma }_i$ is the capture cross sections of the trap, ${\upsilon }_i$ is the thermal velocity of carriers, $m_i$ is the effective mass of carriers, $k$ is the Boltzmann’s constant, and $T$ is the temperature. The parameters for deep-level traps used in this paper are listed in

Table 2. Parameters for deep level traps.

Parameter	Fe	O
Trap type	Deep acceptor in InP	Deep acceptor in InAlAs
Trap level $E_c-E_t$(eV)	0.59 a	0.6 c
Electron capture cross section ${\sigma }_n$ (cm2)	3 × 10−15 a	5 × 10−14 c
Hole capture cross section ${\sigma }_P$ (cm2)	1 × 10−16 a	5 × 10−14 c
Trap density $N_t$ (cm−3)	8 × 1016 b	2 × 1016 c

^a From ref. [27]. ^b From ref. [22]. ^c From ref. [28].

.

The SI behavior of Fe-doped InP can be explained as follows: Fe occupies the site of In as Fe³⁺ and becomes Fe²⁺ on capturing an electron, leading to the decrease in electron density and increase in resistivity [4,29]. A typical Fe concentration of 8 × 10¹⁶ cm⁻³ is sufficient to compensate for a shallow donor background concentration of 1 × 10¹⁵ cm⁻³ in metalorganic chemical vapor deposition (MOCVD) epitaxy [30]. Nevertheless, the resistivity of InP: Fe decreases rapidly in a p-type environment [8], which causes severe leakage current for BH lasers.

2.2. Configuration for SIBH Lasers

A regular InGaAsP multiple quantum well (MQW) laser with a gain spectrum peak around 1.53 μm at 300 K was used for modeling [31]. The laser consists of an n-InP substrate, a 1.5 μm thick n-cladding layer, an undoped active region with 3 wells and 4 barriers sandwiched between a 100 nm thick undoped separate confinement heterostructure (SCH) layer, a 1.5 μm thick p-cladding layer and a 230 nm thick p-contact layer. The detailed material parameters and epitaxial structure for the SIBH laser are described in Appendix A.

The configuration of the high-mesa SIBH laser with an n-doped layer is shown in Figure 1a. The width of the mesa and substrate are 2 μm and 20 μm, respectively. Due to the symmetry of the structure, half of the laser was simulated. A 3.5 μm high-mesa with a vertical sidewall was defined [32], on which SI InP: Fe layers and an n-doped layer were selectively regrown. Since growth rates on the ($\stackrel{-}{1}10$) sidewall of the mesa and (001) substrate are different, it was inevitable to introduce a thin InP layer along the sidewall of the mesa [33]. For simplicity, we assumed a narrow width W of 0.2 μm for this thin SI-layer and an n-layer in a rectangular shape with a thickness H of 0.5 μm [13,34]. The cavity length of the laser is 1 mm, and the front and rear facets are an anti-reflection (AR) and a high-reflection (HR) coating (0.1% and 95%, reflectivity) at a conventional band range. The laser operates under pulsed conditions at 300 K.

Fe-Zn interdiffusion issues occur during regrowth and aging of the Fe-doped device, which can be explained by a kick-out mechanism [9,35]:

$${Fe}_i^{+}+{Zn}_{ln}^{-}+h\leftrightarrow {Fe}_{ln}+{Zn}_i^{+},$$

(3)

where ${Fe}_i^{+},$ ${Zn}_i^{+}$ are the mobile interstitial iron and zinc with a positive charge state, ${Fe}_{ln}$, ${Zn}_{ln}^{-}$ are the substitutional iron and zinc with neutral and negative charge states, and $h$ represents a free hole. Therefore, it is easy for Fe and Zn to exchange their lattice positions in the p-doped environment. In particular, the diffusion process is determined by equilibrium kinetics [9,35], which leads to a homogeneous distribution of the dopants. In this regard, we suggest that Fe and Zn are homogeneously distributed in the p-doped layers and SI-layer with a high concentration of 1 × 10¹⁷ cm⁻³ for both diffused dopants [9].

3. Simulation Results

3.1. Analysis of Leakage Current Mechanism

Figure 2 shows the P-I-V curves of the high-mesa SIBH laser. From the blue curves, the P-I-V characteristics of the laser without n-layers exhibit severe nonlinearity. Since the influence of thermal effects is ignored, the significant deterioration suggests that a serious leakage of current happens in the laser. The potential distribution in Figure 3a,b indicates the direction of carrier flow in the device. Due to the Fe-Zn interdiffusion, the carriers drift directly from the SI-layer to the n-substrate, as shown in Figure 3a. This corresponds to a very low forward bias of the laser, which explains the nonlinear I-V behavior of the device in Figure 2b. With the addition of an n-layer, the laser could maintain a linear output at low injection levels, as indicated by the red curve. Though the laser with n-layers can obtain a higher output power, the performance of the structure begins to worsen as the current injection increases. In the injection interval of 250 to 750 mA, the slope efficiency (SE) and the series resistance of the laser decrease from 0.64 W/A and 3 Ω to 0.26 W/A and 1.4 Ω, respectively. According to Figure 3b, there is a clear reduction of the potential barrier between the p-layer and SI-layer of the laser at 1 A, which leads to a leakage current channel. Although the n-layer raises the barrier to 0.5 V to block the hole injection at the top of the device, part of the current in the p-layer still leaks to the SI-layer below the n-doped block layer.

To further analyze the causes of leakage current, we study the hole and electron current distributions of the laser, respectively. As shown in Figure 4, the hole current (i) and electron current (j) are injected into the active region for a stimulated recombination. However, there is an obvious leakage of holes (ii) from the p-layer to the SI-layer, which recombines with the electron leakage current (jj) injected from the n-doped layers to the SI-layer for nonradiative recombination. Since InP: Fe exhibits SI properties for electrons at equilibrium, the generation of leakage currents in the SIBH laser can be explained as a hole current-induced electron leakage current behavior. On the one hand, with a carrier injection, the barrier between the p-layer and the SI-layer decreases, and the depth of hole injection into the SI-layer increases. The holes recombine with the electrons captured by Fe dopants, resulting in a lower barrier originally formed by the SI-layer against electrons in the n-layer and n-substrate. On the other hand, the diffusion of Zn dopants raises the concentration of holes in the SI material, which leads to the failure of the SI-layer. The above two reasons explain the existence of electron leakage currents in the SI InP: Fe device. To a certain degree, an n-type insertion layer is verified to work in high-SIBH lasers to alleviate the Fe-Zn interdiffusion problem with less leakage currents in the SI-layer as shown in Figure 3c,d. However, there is still room for the improvement of the SIBH lasers with n-layers due to the residual carrier leakage.

3.2. SIBH Lasers with Wide Bandgap Layers

The above analysis indicates that avoiding the reduction of the barrier between the SI-layer and the shallow donor-doped InP is an effective way to improve the performance of SIBH lasers. Therefore, wide bandgap layers can be introduced in the fabrication of BH structures. The wide bandgap material provides a larger resistance and higher potential barriers in heterostructures than the narrow bandgap material, as the intrinsic Fermi level is far from the band edge [17,18,21]. In this section, we compare the power and WPE of SIBH lasers with different wide bandgap layers, as shown in Figure 1b, including strain InAlAs and lattice-matched ZnCdSe materials. Since the growth rates on the ($01\stackrel{-}{1}$) sidewall of the mesa and (001) substrate are different during SAG, we assume a sidewall thickness T_S and bottom thickness T_B ratio of 0.4 [33]. A 6 nm thick T_S is adopted to prevent carrier tunneling, corresponding to a 15 nm thick T_B [36].

3.2.1. SIBH Lasers with InAlAs Layers

InAlAs material has a smaller bandgap shrinkage due to strain reduction, which is beneficial for the pseudopotential growth of strain InAlAs on InP [15]. According to the theoretical calculation in Figure 5, as the strain rises from 0.36% to 0.84%, the bandgap of InAlAs increases from 1.6 eV to 1.8 eV at equilibrium, whereas the critical thickness decreases from 38 nm to 8 nm [37]. Although strain InAlAs thin layers with no defects have been selectively grown on InP substrates, there is a mixture of shallow donor Si, shallow acceptor C and deep-level traps O in the InAlAs material grown by MOCVD [38]. Both of the shallow-level impurities come from the inherent metalorganic sources and dopants in MOCVD, while the deep acceptor O is introduced by the strong Al-O bond for Al-containing materials [39]. In this regard, we assume that a concentration of 5 × 10¹⁶ cm⁻³, 1 × 10¹⁶ cm⁻³ and 2 × 10¹⁶ cm⁻³ for the shallow donor, shallow acceptor and deep-level traps existing in the InAlAs barrier layer [28,38]. The trap parameters of O in InAlAs material are listed in Table 2.

We investigate the effect of InAlAs layers with different strains on the performance of the SIBH laser. As a control, the structure replacing the wide bandgap layer with an undoped InP layer is also simulated. According to Figure 6a, there is little difference in the output performance of the laser with and without InAlAs layers at small current injections, which is consistent with the previous experiments [16]. While the laser operates at a large injection of 1 A, the power and WPE of the device with a 0.84% tensile strain in the InAlAs layers improve by 36.3% and 9.7%, compared with the contrast structure. The laser with a 0.36% strain in the InAlAs layers also maintains high power and high efficiency while relieving epitaxial pressure. Nevertheless, the laser with lattice-matched InAlAs layers shows a lower maximum WPE than that based on undoped InP layers at around 130 mA. This can be attributed to the fact that the resistivity of the wide bandgap InAlAs is greater than that of InP, resulting in reduced power conversion efficiency. At the same time, the bandgap of the lattice-matched InAlAs is narrower than that of the material with strains, which is insufficient to confine carriers under high injection levels.

3.2.2. SIBH Lasers with ZnCdSe Layers

The II-VI compound semiconductor MgZnCdSe composed of ZnSe, CdSe and MgSe can be well-matched to the InP lattice, with a wide bandgap from 2.1 to 3.8 eV [40]. The epitaxy of lattice-matched Zn_0.48Cd_0.52Se on an InP substrate has already been reported [41]. To illustrate the high-resistance characteristics of ZnCdSe materials, we compare the current–voltage behavior of n-i-n and p-i-p structures with InAlAs and ZnCdSe barriers. The structure consists of a 0.5 μm thick InP substrate (shallow doping of 1 × 10¹⁸ cm⁻³), 2 μm thick SI InP: Fe (trap density of 8 × 10¹⁶ cm⁻³) sandwiched by two 20 nm thick wide bandgap layers, and 300 nm thick InP cap layer (shallow doping of 1 × 10¹⁸ cm⁻³). The parameters of the Zn_0.48Cd_0.52Se material are listed in

Table 3. Parameters for Zn_0.48Cd_0.52Se material.

Parameter	Value	Parameter	Value
Refractive index	2.48 @ 1550 nm a	Electron mobility (m2·V−1·s−1)	0.1~0.6 c
Bandgap (eV)	2.08 b	Hole mobility (m2·V−1·s−1)	0.04~0.08 c
Electron effective mass $m_e/m_0$	0.13 b	Affinity (eV)	3.82 c
Hole effective mass $m_h/m_0$	0.56 b	Absorption (m−1)	0 c

^a Calculated by single-effective-oscillator model [43]. ^b From ref. [44]. ^c Material parameters extracted from the program.

. The mobility of electrons and holes are modeled by a low field mobility model [26]. As shown in Figure 7, both interposers exhibit high-resistance behavior in the n-i-n structure, which is consistent with the study on InAlAs barriers [42]. Further, ZnCdSe barriers still work in p-doped environments, reducing the current density in p-i-p structures by about 4 orders of magnitude than that of InAlAs barriers.

Suggesting a defect-free growth of Zn_0.48Cd_0.52Se on InP, we study the effect of Zn_0.48Cd_0.52Se with different thickness T_S values on laser performance. As shown in Figure 8, an improvement of 36.8% and 7.3% for the power and WPE was obtained in the laser with 6 nm thick ZnCdSe layers at 1 A, compared with the device based on a 100 nm thick undoped-InP barrier layer. In the case of a ZnCdSe layer with a thickness of 100 nm, the power and WPE of the laser are increased by 34.4% and 5%, respectively. The slight degradation on output performance can be explained by the laser parameters extracted from the program. Owing to the large refractive index difference between ZnCdSe and the active region, the active region has a larger optical confinement factor with a thick ZnCdSe layer, increasing from 3.16% to 3.23%. As a result, the absorption loss in the active region also rises, resulting in an increase in internal loss from 9.15/m to 9.3/m. Despite a slight sacrifice in power and efficiency, the growth of thicker wide bandgap layers is conducive to improve structural reliability, which shows potential for mass production.

3.2.3. The Role of Wide Bandgap Layers

The role of wide bandgap layers in SIBH lasers can be illustrated by the energy band diagram under a large current injection. According to Figure 9a, there remains an obvious electron and hole barrier at the interface between the p-InP layer and the ZnCdSe layer at 1 A, which blocks the leakage of hole current injected into the SI-layer, as well as the electron current entering into the p-doped environment. For the device using un-doped InP as an isolation layer, the carriers pass between the p-InP and the SI material without barriers, resulting in both hole and electron current leakages.

In addition, due to the selective growth of wide bandgap layers, there are barriers of ΔE_S and ΔE_N for electrons both at the sidewall and bottom of the mesa, as shown in the conduction band profile in Figure 10a. Electrons only flow through the active region and are not re-injected into the SI-layer, eliminating the electron leakage current channel in the structure without wide bandgap layers. According to Figure 10b, the total current is still distributed in the p-n junction of the mesa under a large injection for stimulated radiation recombination, thus ensuring a high efficiency and high output power for the laser.

4. Discussion

4.1. Suitable Thickness for Wide Bandgap Layers

Despite the advantage of a lattice being matched to InP and the thermal expansion coefficient being close to InP for ZnCdSe materials, there exists challenges for the selective growth and uniform epitaxy of hundreds-of-nanometers-thick ZnCdSe layers with no defects on deep mesa strips, as well as heat dissipation due to the small thermal conductivity of ZnCdSe. Therefore, we analyze the appropriate thickness interval of ZnCdSe barriers, taking the influence of leakage current on power and the lack of growth on the mesa into account. For simplicity, the height of the missing ZnCdSe is defined as H_M, which describes the nonuniformity of the wide bandgap material epitaxy along the sidewall of the mesa. As shown in Figure 11a, the power of the laser based on the 6 nm thick barrier layers is sensitive to the nonuniform growth of the ZnCdSe materials, which maintains the same power level as the barrier-free devices. As T_S increases, the output performance of the structure becomes robust to the partial missing growth of wide bandgap layers. A reliable SIBH laser can be obtained with a ZnCdSe thickness in the range of 20 to 40 nm.

4.2. The Effect of Mesa Profiles on Output Characteristics

In mass production, the mesa stripes are usually defined by dry etching with vertical sidewalls, followed by wet modification to reduce etching damages. However, this process may result in a re-entrant mesa profile, which increases the difficulty of epitaxy along the sidewall of the mesa. A trapezoidal mesa can be etched to leave process allowance for the wet modification, promoting the uniform growth of wide bandgap materials on the sidewall. Therefore, we compare the output performance of the laser with different inclination angles of the mesa under a high current injection. As shown in

Table 4. The output characteristics of the laser with different inclination angles of the mesa at 1 A.

Inclination Angle of the Mesa	Active Region Width	Power	WPE	Horizontal Angle	Vertical Angle
90° (Vertical Sidewall)	2 μm	510.7 mW	12.4%	33°	35°
80°	2.64 μm	525.5 mW	14.0%	28°	35.5°
70°	3.35 μm	535.0 mW	15.2%	24°	36°
60°	4.14 μm	550.1 mW	16.3%	20.4°	37°

, when the slope of the mesa becomes smaller, both the power and WPE of the laser increase, which benefits from a wider active region. Meanwhile, an increase in the active region width leads to opposite changes for horizontal and vertical divergence angles, breaking the round spot of BH lasers. Of course, this problem can be solved by introducing dilute waveguide layers below the active region. Overall, a trapezoidal mesa is conducive to the reliability of epitaxy for thin wide bandgap layers.

5. Conclusions

In this study, the current leakage mechanism in high-mesa SIBH lasers with SI InP: Fe at a high current injection was analyzed based on numerical simulation. Due to the Fe-Zn interdiffusion and the reduction of potential barrier between the p-doped layer and SI-layer, a hole current-induced electron leakage current occurred in the device, causing serious degradation in the laser. We studied the effect of InAlAs and ZnCdSe wide bandgap layers on the output characteristics of SIBH lasers, with an improvement of more than 36% and 5% in power and WPE, respectively. It is the first time that we propose a SIBH laser based on lattice-matched ZnCdSe barrier layers with good output performance and high reliability. The introduction of wide bandgap layers in the laser effectively limits the leakage current for both carriers at high injection levels, which has great potential in high power and efficiency application.