Fabrication and Characterization of Mg2Si pn-junction Photodiode with a Ring Electrode

Kenta Daitoku1*, Masaaki Takezaki1, Shuntaro Tanigawa2, Daiju Tsuya2, and Haruhiko Udono1

1Ibaraki University, 4-12-1 Nakanarusawa, Hitachi, Ibaraki 316-8511, Japan 2National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki

305-0047, Japan

E-mail: udono@mx.ibaraki.ac.jp

(Received July 31, 2014)

We have fabricated Mg2Si pn-junction photodiodes with an Au-ring electrode and a SiO2

passivation layer by means of a lift-off photolithography process. Current-voltage (I - V)

characteristics of the photodiodes showed obvious rectifying behavior at room temperature. The

ideality factor of n determined from the slope of I - V characteristics was 1.76 - 1.92. The

photodiode showed a photoresponse with threshold energy of approximately 0.6 eV under a

zero-bias condition. The intensity of peak photoresponse was improved approximately three

times compared with the opaque Au-circular electrode type Mg2Si photodiode previously

reported.

1. Introduction

Infrared (IR) photodetectors are attracting much attention for applications in a filed of security

and safety such as assist facility of automobile, molecular sensing and green energy like

thermo-photovoltaic systems [1-3]. Magnesium half silicide, Mg2Si, is one of the candidate

materials for Si-based IR-photodetectors operating at wavelengths below about 2.1μm, because it has

an energy gap of Eg = 0.6 eV, a small lattice mismatch with Si (< 2%), an abundance of resources, and

a potential for band gap engineering in the narrow band gap range (0.3–0.6 eV) in the form of alloy

compounds with Mg2Ge and Mg2Sn [4-7]. The first photorespose of Mg2Si was observed by Stella

and Lynch in their photoconductivity measurement of bulk Mg2Si and Mg2Ge at low temperature

(< 90K) [8]. They reported that an energy gap of Mg2Si is approximately 0.6 eV. Kato et al. also

observed the change in conductivity of their sputter deposited Mg2Si layer on a Si substrate under

simulated AM1.5G illumination [9]. Recently, we have reported the infrared photoresponsivity

from Mg2Si pn-junction diode under a zero-bias condition at room temperature [10]. The diode was

fabricated by rapid thermal diffusion of Ag dopant into a high purity n-type Mg2Si substrate.

Although the diode showed a photoresponsivity with threshold photon energy at approximately 0.6

eV, its intensity was pretty weak, since the diode had an opaque Au-circular electrode over the

pn-junction region and most of the incident light was blocked by the electrod [11, 10]. In this study,

we report the fabrication and characterization of Mg2Si pn-junction photodiode with a ring

electrode and a SiO2 passivation layer by means of a lift-off photolithography process.

2. Experimental produre

Singe-crystalline n-type Mg2Si substrates (nsub = 6.8 x 1015 cm-3) were prepared from Mg2Si

ingots grown by modified vertical Bridgman method using high-purity source materials of Si

(10N-grade, Furuuchi Chemical. Co.) and Mg (5N-grade, Osaka Asahi Co., Ltd.) and purified

JJAP Conf. Proc. (2015) 011103©2015 The Japan Society of Applied Physics

3Proc. Int. Conf. and Summer School on Advanced Silicide Technology 2014

011103-1

pyrolytic graphite (PG) crucibles (Ibiden Co.). Mg2Si substrates of 3 x 3 mm2, 1-mm-thick were

polished on both sides by fumed silica (AKASEL, water-free, 0.2 μm).

The pn-junction photodiodes with the Au-ring electrode were fabricated by the conventional

photolithography process. The p-type region with the diameter (D) of 150 μm, 200 μm, and 300 μm

was made by the rapid thermal diffusion of Ag metal layer at 550 °C for 10 min [11, 10]. The

diffusion source of Ag metal with the appropriate diameter was formed on the Mg2Si substrate with

the Au capping metal by the lift-off process using a conventional heat-resistive evaporator. After

the diffusion of the Ag metal source, the surface of the substrate was polished to flat, and then 100

nm-thick SiO2 passivation layer was deposited on the top surface by chemical vapor deposition

method. After that, the Au-ring electrodes with the inner diameter (ID) of 50 μm, 100 μm and 200

μm were formed on top of the p-type region with the D of 150 μm, 200 μm, and 300 μm,

respectively. Fig.1(a) and (b) show the top view of the fabricated pn-junction photodiode with the

ID = 100 μm and the cross sectional device structure of the photodiode, respectively.

Photoresponse properties of the photodiode were measured under a zero bias condition at room

temperature using a halogen lamp chopped and passed through an IR filter (cut-off = 1.2 m) and a

single monochromator (JASCO CT-50) with a focal length of 500 mm [10]. We also evaluated the

ideality factor n and surface leakage current density Jsurf from the current-voltage (I-V) characteristics of the photodiode by the following equations [12];

𝐽 ∝ exp (𝑞𝑉

𝑛𝑘𝑇) (1),

𝐽 = 𝐽𝑏𝑢𝑙𝑘 + 4𝐽𝑠𝑢𝑟𝑓

𝐷 (2),

where k is Boltmann's constant, J is the total current density under the dark condition, D is the

junction diameter, and Jbulk and Jsurf are current density in the bulk and surface leakage current

density, respectively.

3. Results and discussion

Three pairs of photodiodes with different pn-junction areas and Au-ring diameters were

fabricated on the Mg2Si substrate (Fig.1 (a) and (b)). The ID of the ring electrode and the D of

pn-junction area were varied between 50 μm and 200 μm and 150 μm and 300 μm, respectively.

Fig.2 (a) shows I-V characteristics of the diode with various pn-junction diameter, measured under

the dark condition at room temperature. All diodes had rectifying behavior, indicating that the

depletion layer and potential barrier are formed at pn-junction of Mg2Si. In the forward bias, the

current increased exponentially at lower voltage below about 0.5 V, while at higher voltage above

about 0.5 V, the current increased linearly due to the large series resistance. In the reverse bias, the

リング状電極Mg2Si pn接合フォトダイオードの作製と特性評価

Fabrication and characterization of Mg2Si pn-junction photodiode with circular

electrode

茨城大院 1，NIMS2 竹崎 誠朗 1, ○大徳 健太 1, 鵜殿 治彦 1, 谷川俊太郎 2津谷大樹 2

Ibaraki Univ. 1

, NIMS2 M. Takezaki

1, ○K. Daitoku

1, H. Udono

1,*, S. Tanigawa

2, D. Tsuya

2

*E-mail: udono@mx.ibaraki.ac.jp

【はじめに】Mg2Si は資源量が豊富な Si 系の

赤外受光素子として注目される[1-4]。これまで

に、我々はMg2Siショットキーダイオード[2]、

および Mg2Si pn接合ダイオードの試作し、接

合特性と波長 2 m からの光応答を報告してい

る[3,4]。従来構造では、図 1(a)に示すように pn

接合上部に金電極がある構造のため、光が十分

に pn 接合に届かなかった。そこで今回、Ag

ドープ p+-Mg2Si上に SiO2パッシベーション膜

およびフォトリソグラフィでのリング状電極

の形成によってデバイス構造の改善に取り組

んだ。今回その J-V特性及び分光特性を評価し

たので報告する。

【実験方法】基板には、高純度 Mg2Si(電子濃

度=7×1015)を 3mm角、厚さ~1mmに切りだし、

鏡面状に研磨して使用した。pn接合は Agの熱

拡散により形成した。熱拡散条件は、Ar 雰囲

気中で 550℃10分間である。その後、プラズマ

CVD法により SiO2膜、フォトリソグラフィに

よりリング状の電極を形成した。図 1(b)が今回

作製したデバイスの断面構造である。J-V特性

及び分光特性の評価は室温で行った。

【結果と考察】図 2は試作した pn接合フォト

ダイオードの表面写真である。通常の Si プロ

セスを利用してリング状電極のフォトダイオ

ード構造が形成できている。図 3に直径 300 m

のリング構造の J-V特性の測定結果を示す。明

確な整流性が得られており pn接合特性が得ら

れている。更に、分光特性を測定したところ零

バイアスにおいて波長 2 m 近傍からの光電流

が得られ、pn 接合フォトダイオードが形成で

きていることが判った。

図 1.Mg2Si pn接合デバイス構造

図 2.pn接合表面写真

図 3.室温での J-V特性

【謝辞】本研究の一部は、ナノテクノロジープ

ラットフォーム事業試行的利用課題の採択を

得て行った

【参考文献】

[1] D. Tamura et al., Thin Solid Films 515 (2007) 8272.

[2] K.Sekino et al., Physics Procedia, 11(2011)171.

[3] H. Udono et al., J.Phys.Chem.Sol., 74(2013)311.

[4] M.Takezaki et al., Phys. Status. Solidi C, in press.

Fig. 1 (a) Top view of a pn-junction Mg2Si photodiode with ID = 100μm. (b) Cross sectional drawing of

the photodiode structure.

(a) (b)

011103-2JJAP Conf. Proc. (2015) 0111033

saturation of current reached to about 2.5 x 10-4 A at -3 V was observed. We estimated ideality factor

n from the slope at the lower voltage region in forward bias and obtained n=1.76 for D = 150 μm, n

= 1.92 for D = 200 μm, and n = 1.91 for D = 300 μm. These values are better than that of the

previous pn-junction Mg2Si diodes with opaque Au-circular electrode (n = 2.8) [10], although

recombination current still dominates in our photodiodes. Fig.2 (b) plots the relationship between

the dark current density in the reverse bias and the inverse diameter (1/D) of the pn-junction area.

As expressed in Eq.(2), the intercept and the gradient of the least square fitted line correspond to

Jbulk and Jsurf, respectively. The value of Jbulk and Jsurf are 5.90 mA/cm2 and 1.58 mA/cm2,

respectively. The large value of Jsurf indicates that the improvement of surface passivation structure

would be needed to reduce the total dark current of our Mg2Si photodiode.

Fig. 3 (a) shows photoresponse spectrum of the ring electrode type (ID = 100 μm) and the

opaque circular electrode type (D = 800 μm) Mg2Si photodiodes measured under a zero-bias

condition at room temperature.

Fig. 2 (a) Current-voltage characteristics of pn-junction Mg2Si diodes at room temperature.(b)

Relationship between the dark current density at reverse bias and inverse junction diameter. Dashed

line is the least square fitted line.

Fig. 3 (a) Spectral photoresponsivity of pn-junction Mg2Si photodiode with ring electrode and opaque

circular electrode. (b) Specific detectivity of the Mg2Si photodiode. The data of PbS infrared detector is

also plotted as a reference.

(a) (b)

(a) (b)

011103-3JJAP Conf. Proc. (2015) 0111033

The photon energy threshold at approximately 0.6eV is as same as previous report [10]. The

intensity of peak photoresponsivity of the ring electrode type photodiode is approximately three

times higher than that of the circular electrode type one, suggesting the increase of incident light to

the pn-junction for the ring electrode type one. Specific detectivity (D*) calculated using Eq. (2) is

plotted in Fig.3 (b). In the figure, D* of commercial PbS (HAMAMATSU) is also plotted as a

reference. The D* of Mg2Si is quite low compared with that of PbS, because of its high dark

current density and relatively weak photoresponsivity. One of the considerable reasons of the weak

photoresponsivity is the deep junction depth (~ 75 μm) of the measured photodiode [10, 11].

Takezaki et al. reported that photoresponsivity of Mg2Si photodiode is significantly affected by the

junction depth due to the absorption loss of the incident light [11]. Furthermore, improvement of

the Ag diffusion profile and surface passivation structure is also very important to reduce the Jbulk

and Jsurf. Therefore, we believe that the specific detectivity of our Mg2Si pn-junction photodiode

would be improved significantly by optimizing those device parameters.

4. Conclusion

We have fabricated Mg2Si pn-junction photodiode with an Au-ring electrode of the inner

diameter of 50 μm, 100 μm and 200 μm using the conventional life-off photolithography process.

All photodiodes showed a clear rectifying behavior in the current-voltage characteristics at room

temperature. The ideality factor was n = 1.76 - 1.92 at the lower voltage region in forward bias.

The photoresponse was observed in the infrared wavelength below 2.1 μm under the zero-bias

condition. The peak photoresponsivity of the ring electrode type photodiode was improved

approximately three times compared with the opaque circular electrode type ones.

Acknowledgment

This study was partially supported by NIMS Nanofabrication Platform in Nanotechnology

Platform Project sponsored by the Ministry of Education, Culture, Sports, Science and Technology

(MEXT), Japan.

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