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

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.


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][2][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][5][6][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.
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]; 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.

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 i pn 接合フォトダイオードの作製と特性評価 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/cm 2 and 1.58 mA/cm 2 , 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.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.

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.

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
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
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.