Design of a bandgap-engineered barrier-blocking HOT HgCdTe long-wavelength infrared avalanche photodiode

Jiale He; Jiale He; Jiale He; Qing Li; Qing Li; Qing Li; Peng Wang; Peng Wang; Peng Wang; Fang Wang; Fang Wang; Yue Gu; Yue Gu; Chuan Shen; Man Luo; Chenhui Yu; Lu Chen; Xiaoshuang Chen; Xiaoshuang Chen; Wei Lu; Wei Lu; Weida Hu; Weida Hu; Weida Hu

doi:10.1364/OE.408526

1. Introduction

APD can realize high-speed, weak signal and even single photon detection due to its signal amplification effect, and has been widely used for decades in optical fiber communication, 3D lidar, astronomical observation and atmospheric detection [1–4]. Due to the huge difference of the ionization coefficient between electron and hole, HgCdTe (MCT) has been found to be the most powerful solution of low excess noise infrared APDs [5–11]. However, the application of HgCdTe APDs are significantly limited by its high dark current [12–14]. Recent studies on HgCdTe APDs have shown that band-to-band tunneling (BBT) and trap-assisted tunneling (TAT) may be the main factors of dark current at low temperatures because of the narrow band gap and material quality [15–17]. Typically, the dark current characteristics of HgCdTe APDs are much more complex in HOT regime. With the increase of operating temperature, the bandgap of HgCdTe will have a nonnegligible broadening, and some dark current components closely related to temperature also have a sharp increase. Therefore, the analysis of the dark current mechanisms of HgCdTe APDs at high temperature and reducing its dark current will be an important way of the developing of HOT HgCdTe APDs.

A potential choice for HOT HgCdTe detection is an unipolar barrier-blocking structure, which was introduced into HgCdTe in 2011 [18–23]. The application of barrier-blocking devices on HgCdTe has been widely studied, and its inhibition effect on dark current has shown a broad prospect in the development of HOT HgCdTe detection [24–31]. By designing the energy band structure of the barrier layer, the supplement of carriers in the absorption region could be blocked and suppress the Auger current, which is the main dark current component of HgCdTe photodetector at high temperature [32–35]. However, pBp HgCdTe devices generally operate at the bias voltage below - 1 V, which is far lower than the high bias required for APD avalanche multiplication.

In this paper, a novel APD structure of pBp-APD based on barrier-blocking band engineering is proposed. By coupling barrier layer structure in APD, the carrier concentration in the absorption region can be effectively reduced. This makes all the temperature related dark current mechanisms to be less effective, allowing for a substantial increase of the operating temperature. By analyzing its physics characteristics such as the band structure and dark current, we proved that the introduction of the barrier layer does not affect the avalanche gain characteristics of the device, while the avalanche dark current component could be effectively suppressed.

2. Model and principle

In this paper, a novel pBp-APD Hg1-xCdxTe structure is designed and simulated by commercial software Sentaurus-TCAD, and x refers to the Cd fraction shown in Table 1. The drift-diffusion method is used in our calculations, thus the classical Poisson and continuity equations are self-coupled. The total dark current mechanisms consist of Shockley-Read-Hall (SRH), Auger, radiative, band-to-band tunneling, trap-assisted tunneling and impact ionization [36,37]:

{R_{SRH}} = \frac{{pn - n_i^2}}{{{\tau _p}\left[ {n + {n_i}exp \left( {\frac{{{E_t} - {E_i}}}{{kT}}} \right)} \right] + {\tau _n}\left[ {p + {n_i}exp\left( {\frac{{{E_i} - {E_t}}}{{kT}}} \right)} \right]}}

{R_{Auger}} = ({{B_1}n + {B_2}p} )({np - n_i^2} )

{R_{Rad}} = 0.58 \times {10^{ - 12}}\sqrt \varepsilon {\left( {\frac{{{m_0}}}{{m_h^{\ast } + m_e^{\ast }}}} \right)^{ - 1.5}}\left( {1 + \frac{1}{{m_h^{\ast }}} + \frac{1}{{m_e^{\ast }}}} \right){\left( {\frac{{300}}{T}} \right)^{1.5}}({E_g^2 + 3kT{E_g} + 3.75k_B^2{T^2}} )

{G_{BBT}} = \frac{{{q^2}\sqrt {2m_e^{\ast }} {E^2}({{V_{bias}} - {V_d}} )}}{{4{\pi ^3}{\hbar ^2}\sqrt {{E_g}} }}exp\left( { - \frac{{\pi \sqrt {m_e^{\ast }/2} E_g^{3/2}}}{{2qE\hbar }}} \right)

{R_{TAT}} = \frac{{pn - n_i^2}}{{\frac{{{\tau _p}}}{{1 + {\Gamma _p}}}\left[ {n + {n_i}exp \left( {\frac{{{E_t} - {E_i}}}{{kT}}} \right)} \right] + \frac{{{\tau _n}}}{{1 + {\Gamma _n}}}\left[ {p + {n_i}exp\left( {\frac{{{E_i} - {E_t}}}{{kT}}} \right)} \right]}}

where E is the electric field,${\; }{\tau _{n,p}}$is the doping-temperature-field dependent lifetime, B_1,2 is the temperature-dependent Auger coefficient, E_g is the bandgap, and E_t and N_t are the trap level and density respectively. The extended formulas for the coefficients ${\Gamma _p}$ and ${\Gamma _n}$ could be found in Ref. [16]. The impact ionization model is considered by the Okuto–Crowel model, which has been demonstrated to be in agreement with the experimental results in HgCdTe APDs, given by [16]:

{G_{avalanche}} = {\alpha _n}n{\nu _n} + {\alpha _p}p{\nu _p}

{\alpha _{n,p}} = {a_{n,p}}{E^c}exp({ - b/E} )

where the temperature-dependent coefficient ${a_{n,p}}$ and b are referred to the slope of the gain curve and the scaling of the gain curve as a function of junction width at the low bias, respectively. The fitting coefficient c equal to 0.6 is reported to give good results [38].

Table 1. Key parameters of pBp-APD and conventional mesa-APD used in the simulations.

View Table

The barrier-blocking pBp structure is coupled into APD in the improved pBp-APD, and they share the same absorption region, as shown in Fig. 1(a), and Fig. 1(b) shows the structure of conventional mesa APD for comparison.

Fig. 1. Schematic of the simulated (a) conventional APD and (b) pBp-APD, and the simulated energy bands of (c) conventional APD and (d) pBp-APD at -7V, where V denotes the reveres bias. The insert of (c) shows the comparison between the experimental results and the simulation results of the model of conventional APDs demonstrating the validity of the numerical simulations.

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Commonly, the working bias of nBn or pBp is not higher than - 1 V. Higher reverse bias will significantly reduce the conduction barrier and destroy the electron blocking effect. However, a much higher bias is required in APD to obtain a high enough built-in electric field in the multiplication layer for a high avalanche gain. The band diagrams of conventional APD and pBp-APD structure at -7 V are shown in Figs. 1(c) and 1(d). It could be seen that the conduction barrier of the barrier layer of pBp-APD is about 0.13 eV at - 7 V reverse bias, which can ensure that the structure can maintain excellent electronic blocking effect and suppress dark current even at high avalanche bias. This prevents the replenishment of electrons into the absorption region, and the electrons in the absorption region are extracted to the multiplication layer and avalanched. Also, the holes in the absorption region flow to the electrode layer under bias, and can hardly be recovered from the heavily n-doped contact region. Thus, the carrier concentration in the absorption layer could be reduced and reduce the dark current. What is shown in the insert of Fig. 1(c) is the simulation results of the model of conventional APDs built compared with our experimental results, and it shows that the simulation model fits well with our experiment. The detailed structure parameters used in the simulations are shown in Table 1.

Research shows that TAT and Shockley-Read-Hall (SRH) currents dominate the dark current of APDs at the low reverse bias at 77K, while BBT current dominates at the higher bias [39]. However, the temperature-related generation-recombination (G-R) and avalanche current should not be neglected as the operating temperature increases. The temperature-dependent dark current characteristics of the conventional APD are shown in Fig. 2(a). The dark current of APD increases monotonously with the increase of temperature at the lower bias voltage, but the trend is chaotic under the higher reverse bias. It could be easily understood because the bandgap of HgCdTe will widen with the increase of temperature, which will lead to a significant reduction of the tunneling current. At higher reverse bias, the dark temperature mechanism of APD will be the competition between the tunneling current which decreases with the increase of temperature and other dark current compositions which commonly increase at a higher temperature. However, at the operating temperature above 180K, the dark current increases steadily with the increase of temperature, and the tunneling current starts to lose its dominance. This is due to that high temperature leads to higher carrier concentration in the absorption region, and significantly increases the avalanche dark current.

Fig. 2. Dark currents of (a) conventional HgCdTe APD and (b) HgCdTe pBp-APD under different temperatures in the whole bias range. The dark currents at -0.5 V and-7 V are listed separately in (c) and (d), and the insert shows the comparison of conventional HgCdTe APD and pBp-APD at 260 K.

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However, the avalanche current is significantly suppressed, and tunneling current absolutely dominates the total dark current under high bias. At lower reverse bias, the generation-recombination current increases significantly with the increase of temperature and gradually dominate the dark current, and the dark current increases significantly with the increase of operating temperature, as shown in Fig. 2(c). However, at higher bias, the tunneling current always dominates dark current because of the suppression of generation-recombination and avalanche dark current. This is due to the electron depletion in the absorption layer, which will be shown in Fig. 4. The band gap broadening caused by increasing temperature will lead to the decrease of tunneling, so the dark current of the device exhibits a negative temperature gradient at high bias voltage.

To further understand the dark current reducing characteristics of the advanced pBp-APD structure, the vertical distribution of the tunneling generation and impact ionization rate of conventional HgCdTe APDs and pBp-APD at 260K are shown in Fig. 3. It could be seen that both the avalanches and tunneling of conventional APD and pBp-APD occur in the junction region of the multiplication layer. However, the impact ionization of pBp-APD has a significant reduction compared with the conventional APD. This further proves that the principle of pBp-APD reducing dark current is to reduce the avalanche current.

Fig. 3. Vertical distributions of tunneling generation and impact ionization of (a) conventional APD and (b) pBp-APD at -7 V.

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Previous research shows that one of the factors determining the avalanche current in APD is the number of carriers injected into the multiplication layer [40]. Therefore, an important method to suppress the avalanche dark-current of MWIR/LWIR HgCdTe APD devices is to reduce the electron density in the absorption region. Figure 4 shows the vertical distribution of electron and hole density of conventional APD and pBp-APD at - 7V. It could be obviously seen that the electrons in the absorption region of the pBp-APD structure are significantly depleted to even far lower than the intrinsic carrier concentration, thus reducing the avalanche current at high temperatures. The vertical distribution of electric field and potential are also shown in Fig. 4. It could be seen that there exists a very high electric field in the multiplication layer, which is necessary for APD devices to multiply electrons in the multiplication layer. Moreover, the potential difference is mostly distributed in the multiplication layer, and only a very small potential difference is applied to the barrier layer. Overall, it could be proved that the addition of the barrier layer can significantly reduce the dark current by depleting the electrons in the absorption region, and the high bias required by APD does not affect the barrier layer conduction barrier.

Fig. 4. Simulated vertical distributions of carrier densities, electric field, and electric potential for (a) conventional APD and (b) pBp-APD at -7 V.

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Finally, the gain M and GNDC (defined as ${I_{dark}}/M$) characteristics of the two structures are both calculated in Fig. 5. The existence of valence band-offset in the barrier layer will affect the flow of photogenerated holes, therefore reduce the gain of pBp-APD. However, the results at -7 V show that the dark current could be reduced without sacrificing the gain. This is because that the valence band-offset could be flattened at higher bias so as not to block the photogenerated holes, and the electric field concentration in the multiplication layer as shown in Fig. 3 could provide enough avalanche gain.

Fig. 5. (a) Gain and (b) GNDC characteristics of conventional APD and pBp-APD.

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3. Conclusion

In this paper, a novel pBp-APD structure is designed and numerically analyzed in detail. The results show that the dark current of conventional APD at high operating temperature could be significantly reduced by coupling barrier blocking layer. The analysis of the energy band and electric field of pBp-APD show that the conduction barrier of pBp is not destroyed by the high bias voltage, and the electric field in the multiplication layer is maintained to be large enough to produce multiplication. By discussing the mechanism of dark current of pBp-APD at different temperatures and bias voltages, and analyzing the temperature-dependent characteristics, it could be proved that the avalanche dark current is sufficiently suppressed at high temperature and reverse bias. In addition, the dark current reduction does not affect its gain performance. The results will provide a very potential solution for the development of MWIR or LWIR HOT APDs.

Funding

National Key Research and Development Program of China (2020YFB2009300, 2017YFA0205801); National Natural Science Foundation of China (61521005, 61725505, 61905266); Shanghai Sailing Program (19YF1454600); Key Research Project of Frontier Science of CAS (QYZDB-SSW-JSC031); Natural Science Foundation of Shanghai (18ZR1445800, 18ZR1445900, 19XD1404100).

Disclosures

The authors declare no conflicts of interest.

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	Region	Doping Concentration (cm^-3)	Thickness (µm)	Cd mole fraction
Conventional mesa-APD	P-Absorption layer	1×10¹⁶	7	0.23
	N-Multiplication layer	1×10¹⁶	1	0.23
	N-type contact layer	1×10¹⁸	3	0.23
pBp-APD	P-type contact layer	1×10¹⁷	3	0.33
	P-Graded layer	1×10¹⁵	0.05	0.33∼0.57
	P-Barrier layer	1×10¹⁵	0.1	0.57
	P-Graded layer	1×10¹⁵	0.2	0.57∼0.23
	P-Absorption layer	1×10¹⁶	7	0.23
	N-Multiplication layer	1×10¹⁶	1	0.23
	N-type contact layer	1×10¹⁸	3	0.23

Design of a bandgap-engineered barrier-blocking HOT HgCdTe long-wavelength infrared avalanche photodiode

Abstract

1. Introduction

2. Model and principle

3. Conclusion

Funding

Disclosures

References

Cited By

Figures (5)

Tables (1)

Equations (7)

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