Study on AlGaN PININ Solar-Blind Avalanche ... - Springer Link

Electron. Mater. Lett., Vol. 11, No. 6 (2015), pp. 1053-1058 DOI: 10.1007/s13391-015-5142-6

Study on AlGaN P-I-N-I-N Solar-Blind Avalanche Photodiodes with Al0.45Ga0.55N Multiplication Layer Mengjun Hou,1 Zhixin Qin,1,* Chenguang He,1 Lise Wei,1 Fujun Xu,1 Xinqiang Wang,1,2 and Bo Shen1,2,* 1

State Key Laboratory for Artificial Microstructure and Mesoscopic Physics, School of Physics, Peking University, Beijing 100871, China 2 Collaborative Innovation Center of Quantum Matter, Beijing, China (received date: 29 March 2015 / accepted date: 5 June 2015 / published date: 10 November 2015)

This paper presents the design for a heterojunction AlGaN solarblind avalanche photodiode (APD) with improved noise performance. Increasing the Al composition of AlGaN in the multiplication layer from 0.40 to 0.45 was calculated to significantly reduce the excess noise factor of this heterojunction APD. The polarization electric field induced in the multiplication layer had the same direction as the applied reverse bias field, which helped lower the avalanche breakdown voltage. The calculated results demonstrated that the apparent spike in the electric field intensity at the i-Al Ga N/nAl Ga N interface can be effectively suppressed by inserting a grading n-AlGaN layer, which helps reduce the dark current. 0.4

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Keywords: AlGaN, avalanche photodiode, excess noise factor, dark current

1. INTRODUCTION Solar-blind APDs can detect very weak ultraviolet (UV) signals in the solar-blind range (λ < 280 nm) under intense background radiation. Owing to this advantage, they can potentially be applied to flame monitoring, missile threat warning and tracking, and bioagent detection. Wurzite AlGaN alloys are promising materials for solar-blind APDs with an Al composition of higher than 40%. AlGaN solarblind APDs have outstanding advantages such as a low operation voltage, small size, and intrinsic solar blind characteristic that make them a viable alternative to current *Corresponding author: [email protected] *Corresponding author: [email protected] ©KIM and Springer

costly, bulky, and fragile photomultiplier tubes.[1] McClintock et al.[2] were the first to report a p-i-n AlGaN solar-blind APDs with a multiplication gain of 700 at a reverse bias of 60 V, while a Schottky-type device demonstrated a higher value of 1560.[3] Subsequently, AlGaN solar-blind p-i-n APDs with a relatively high gain of 2500[4] and separate absorption and multiplication (SAM) AlGaN APDs with a gain of 3000[5] have been reported. Huang et al.[6] reported a Schottky-type AlGaN solar-blind APD with a gain of 4000. Very recently, Shao et al.[7,8] reported AlGaN solar-blind APDs with a high gain of ~104. Although great progress has been made, the development of AlGaN solar-blind APDs has lagged far behind that of GaN APDs.[9,10] Material problems such as a high dislocation density (~109 cm−2) and low p-type doping efficiency due to the large Mg acceptor ionization energy hamper the realization of high-gain APDs.

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As discussed above, most groups have focused on realizing very high-gain APDs. There have been no reports about the avalanche excess noise of AlGaN solar-blind APDs and only a few on suppressing the dark current of these devices.[7,11] GaN APDs cannot operate with low excess noise because of the comparable electron and hole ionization coefficients;[12] this is also a key problem for AlGaN solar-blind APDs. A well-designed APD should have very low excess noise even for a relatively high avalanche gain. This paper presents the design of a heterojunction AlGaN solar-blind APD to achieve very low avalanche excess noise. The effect of the polarization electric field induced by spontaneous and piezoelectric polarization at the heterointerface on the performance of the heterojunction APD is discussed in detail. An effective way to suppress the dark current was realized by inserting a grading n-AlGaN layer into the new APD structure.

2. STRUCTURES AND CALCULATIONS Figure 1 shows the schematic structures of the AlGaN solar-blind APDs used in this study. The conventional p-i-ni-n SAM structure (Fig. 1(a)) consisted of a 300-nm nAl0.5Ga0.5N window layer on an Al0.5Ga0.5N/AlN template, a 180-nm unintentionally doped i-Al0.4Ga0.6N absorption layer, a 60-nm n-Al0.4Ga0.6N charge layer, a 180-nm unintentionally doped i-Al0.4Ga0.6N multiplication layer, and a 100-nm pAl0.4Ga0.6N layer. The heterojunction APD (Fig. 1(b)) was similar to the conventional SAM structure except that its multiplication layer was i-Al0.45Ga0.55N. The electron concentrations were 2 × 1018 and 1 × 1018 cm−3 for the n-Al0.5Ga0.5N and n-Al0.4Ga0.6N layers, respectively. The hole concentration was 2 × 1018 cm−3 for the p-Al0.4Ga0.6N layer. The residual carrier concentrations of the unintentionally doped i-Al0.4Ga0.6N and i-Al0.45Ga0.55N layers were 1 × 1016 cm−3. These APDs

were back-illuminated to benefit from hole-initiated multiplication. The APSYS software was used to simulate the electric field distributions and energy band structures for these APDs. The screening factor P represents the percentage of polarization charges residing at the hetero-interface and was set to be 0.5 in our calculations.

3. RESULTS AND DISCUSSION Figures 2(a) and (b) show the calculation results for the ratio of the hole ionization coefficient to the electron ionization coefficient as a function of the electric fields for Al0.4Ga0.6N and Al0.45Ga0.55N, respectively. Here, k is defined as follows: k = β/α

(1)

where β and α are the ionization coefficients of the hole and electron, respectively. The impact ionization coefficients of AlGaN in this study were extracted from the previous report.[13] As shown in Fig. 2(a), k increased from 1.75 to 2.11 as the electric field was increased from 2.60 to 3.40 MV/cm. This result shows that the hole had a larger ionization coefficient than the electron for Al0.4Ga0.6N when the electric field was greater than 2.6 MV/cm. This is consistent with experimental reports.[6] In addition, k was indicated to be smaller than 1 in the case of a low electric field, which has already been observed by Tut et al.[14] However, for Al0.45Ga0.55N, k decreased quickly as the electric field was increased, as shown in Fig. 2(b). The value of k was much larger for Al0.45Ga0.55N than for Al0.4Ga0.6N with the same electric field. The excess noise factor F for APDs is known to fundamentally depend on the ratio of the ionization coefficients of the hole and electron. A larger value of k means a smaller noise factor F, providing the avalanche is initiated by

Fig. 1. Schematic structures of the (a) conventional p-i-n-i-n SAM APD and (b) heterojunction APD.

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Fig. 3. Calculated electric field distributions under reverse biases of (a) 0 V and (b) 90 V for the two structures. The black solid line and red dashed line are the calculation results for the conventional APD and heterojunction APD, respectively. The blue dotted line represents the calculation results for the heterojunction APD with a pAl Ga N layer. 0.25

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which are used as the multiplication layers in APDs. An analytical expression for F can be obtained in the case of a fieldindependent ratio k if the avalanche multiplication is initiated by holes:[15]

Fig. 2. Ratio (k) of the hole ionization coefficient to the electron ionization coefficient as a function of the electric field for (a) Al Ga N and (b) Al Ga N. (c) Excess noise factor F as a function of the multiplication gain M for the conventional APD and heterojunction APD at the selected value of k for the electric field of 3.0 MV/cm. 0.4

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carriers having a larger ionization rate. The avalanche multiplication for back-illuminated AlGaN solar-blind APDs is initiated by holes, and the excess noise factor F is determined by the k values of i-Al0.4Ga0.6N and i-Al0.45Ga0.55N,

1–k M–1 F = M 1 + ⎛ ----------⎞ ⎛ ------------⎞ ⎝ k ⎠⎝ M ⎠

2

(2)

Here, M is the multiplication gain. Thus, Fig. 2(c) shows the noise factor F as a function of the multiplication gain M for the conventional SAM APD and heterojunction APD. The corresponding k was chosen for an electric field of 3.0 MV/ cm, which is the critical electric field for AlGaN solar-blind APDs.[5] Obviously, the excess noise factor increased with the multiplication gain for the two structures, but the heterojunction APD showed a significantly reduced noise factor compared with the conventional one because of its larger


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value of k. Therefore, the calculation results showed that the heterojunction APD had much less avalanche excess noise than the conventional APD. In order to facilitate intuitive analysis of the properties of the heterojunction APD, Fig. 3 shows the electric field distributions of the two structures under the reverse biases of 0 and 90 V. A higher Al composition AlGaN multiplication layer was employed for the heterojunction APD, which can introduce positive polarization charges at the hetero-interface of i-Al0.45Ga0.55N/n-Al0.4Ga0.6N and negative polarization charges at the p-Al0.4Ga0.6N/i-Al0.45Ga0.55N interface because of spontaneous and piezoelectric polarization.[16] Furthermore, the direction of the polarization electric field induced in the multiplication layer is consistent with that of the applied reverse bias field. As shown in Fig. 3(a), compared with the conventional APD, the heterojunction APD clearly showed an enhanced electric field in the multiplication layer under the reverse bias of 0 V, which is consistent with the theoretical analysis. The electric field distributions at a high voltage of 90 V were also calculated. Figure 3(b) shows that an extra electric field (~0.25 MV/cm) was induced in the multiplication layer for the heterojunction APD. The enhanced electric field in the multiplication layer is believed to decrease the applied avalanche breakdown voltage for the

heterojunction APD, which can reduce the dark current resulting from the surface and sidewall leakage and also lower the risk of premature breakdown due to local high electric fields that form near dislocations.[8] However, the different critical electric fields required for avalanche ionization in i-Al0.45Ga0.55N and i-Al0.4Ga0.6N layers should be considered. Generally, a larger band gap means a higher electric field required for avalanche ionization. Fortunately, the electric field in the multiplication layer can also be enhanced by decreasing the Al composition in pAlGaN,[8,17] which can be introduced into the heterojunction APD structure. The p-Al0.4Ga0.6N layer in the heterojunction APD was replaced with a p-Al0.25Ga0.75N layer, and the corresponding electric field distribution under the reverse bias of 90 V is shown in Fig. 3(b). Because the electric field in the multiplication layer was further enhanced, the heterojunction APD may not require a higher applied reverse bias than the conventional one to achieve the same multiplication gain. At present, there are no available experimental data for the impact of the ionization characteristics of Al0.45Ga0.55N, and further studies will be necessary to clarify this issue. The extra polarization electric field in the multiplication layer for the heterojunction APD helped reduce the avalanche

Fig. 4. (a) Schematic structure, (b) energy band diagrams at equilibrium, and the electric field distributions under reverse biases of (c) 0 V and (d) 90 V for the heterojunction APD with an inserted 50-nm grading n-AlGaN layer.


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breakdown voltage as discussed above, while the negative polarization charges induced at the i-Al0.4Ga0.6N/n-Al0.5Ga0.5N interface degraded the performance of the APD. The direction of the polarization electric field in the i-Al0.4Ga0.6N absorption layer induced by these negative charges was opposite to that of the reverse bias field, while the polarization electric field in the n-Al0.5Ga0.5N window layer had the same direction as the reverse bias electric field. Thus, as shown in Fig. 3, an apparent spike in the electric field intensity at the i-Al0.4Ga0.6N/n-Al0.5Ga0.5N interface was observed for both the conventional and heterojunction APDs. Unfortunately, the dark current dominated by tunneling effects (such as band-to-band tunneling and defect-assisted tunneling) can be notably enhanced by this elevated electric field while APDs operate at low reverse bias. This has been discussed in detail by Wang et al.[18] To solve this problem, a composition-graded 50-nm n-AlGaN layer was inserted between the i-Al0.4Ga0.6N absorption layer and n-Al0.5Ga0.5N window layer, as shown in Fig. 4(a). The Al composition in this layer was linearly graded from 0.50 to 0.40, and the electron concentration was 2 × 1018 cm−3. The energy band diagrams for this structure at equilibrium were simulated in APSYS and are depicted in Fig. 4(b). Instead of the abrupt structure, a graded gap structure was achieved at the heterointerface of i-Al0.4Ga0.6N/n-Al0.5Ga0.5N owing to the inserted grading layer. This kind of energy band structure can eliminate the problem of charge accumulation at the heterojunction interface. Figures 4(c) and (d) show the electric field distributions of this APD structure under the reverse biases of 0 and 90 V, respectively. By inserting the grading nAlGaN layer, the elevated electric field at the heterointerface of i-Al0.4Ga0.6N/n-Al0.5Ga0.5N was effectively suppressed, as shown in the insets of Fig. 4(c) and (d). This is beneficial to reducing the tunneling dark current. The inserted grading layer can also reduce the lattice match, which would decrease the dislocation density at the hetero-interface. Dislocations at the hetero-interface are known to degrade the crystalline quality of the i-Al0.4Ga0.6N absorption layer grown on n-Al0.5Ga0.5N; thus, the number of bulk recombination centers in the absorption layer increased. Furthermore, the bulk Shockley-Read-Hall (SRH) recombination rate increased, and the dark current correspondingly increased. Therefore, inserting the grading layer not only decreases the tunneling current but also suppresses the dark current dominated by bulk SRH recombination.

4. CONCLUSIONS In summary, a heterojunction AlGaN solar-blind APD was designed to improve the noise performance. An i-Al0.45Ga0.55N layer was used as the multiplication region in this heterojunction APD, and the calculated results showed that the corresponding avalanche excess noise was significantly

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reduced even at high multiplication gains. The polarization effects were found to enhance the electric field in the multiplication layer, which helps lower the avalanche breakdown voltage. By inserting an n-type grading layer at the i-Al0.4Ga0.6N/n-Al0.5Ga0.5N interface, the spike of electric field at this hetero-interface was effectively suppressed. Thus, the dark current dominated by tunneling effects and bulk SRH recombination can be reduced to some extent.

ACKNOWLEDGMENTS This work was supported by National Basic Research Program of China (Nos. 2012CB619301 and 2012CB619306), National High Technology Research and Development Program of China (No. 2014AA032608), and Guangdong Innovative Research Team Program (No. 2009010044).

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