Low dark current InGaAs(P)/InP p-i-n photodiodes

Low Dark Current InGaAs(P)/InP p–i–n Photodiodes

Yen-Wei C HEN , Wei-Chou H SU

, Rong-Tay H SU ¹ , Yue-Huei W U ¹ and Yeong-Jia C HEN

Institute of Microelectronics, Department of Electrical Engineering, National Cheng-Kung University, 1 University Road, Tainan, Taiwan 70101, R.O.C.

1

South Epitaxy Corporation, No. 16 Da-Shun 9th Road, Tainan Science-Based Industrial Park, Hsin-Shi, Tainan County, Taiwan 744, R.O.C.

(Received October 18, 2002; accepted for publication February 18, 2003)

Planar InGaAs(P)/InP p–i–n photodiodes have been successfully fabricated by low-pressure metalorganic chemical vapor deposition (LP-MOCVD). High-quality and uniform epitaxial layers are obtained. It is noted that the InGaAs layer background concentration is as low as 4:5  10

cm

. The dark current is significantly reduced by using a wider-band-gap material of quaternary In

Ga

As

P

as a cap layer to reduce the device surface leakage current. In addition, the device becomes highly photosensitive due to the reduction of the absorption of the radiation in the narrow-band-gap In

Ga

As

P

cap layer. The p–i–n photodiode with a wide-band-gap InP cap layer exhibits a dark current as low as 60 pA at 10 V bias, corresponding to a dark current density of 4:2  10

A/cm

. [DOI: 10.1143/JJAP.42.4249]

KEYWORDS: LP-MOCVD, PIN photodiode, In

Ga

As

P

1. Introduction

A long-wavelength optical transmission system operating at 1.0 to 1.65 mm wavelength region has attracted consider- able attention due to its low-loss and low-dispersion optical characteristics. As one of the key components, semiconduc- tor photodetectors based on InP materials, which offer the advantages of low dark current, high-efficiency and high- speed operation, have been widely studied for long-wave- length optical fiber communication applications.

There- fore, although p–i–n photodetectors do not have internal gain, a combination of a photodetector with an amplification device, such as high electron mobility transistor (HEMT) or heterojunction bipolar transistor (HBT), in optoelectronic integrated circuits (OEIC’s) has led high-sensitivity optical receivers to operate for high-rate data transmission.

In order to achieve these characteristics, it requires a precise control of the thickness and concentration of different epitaxial layers. Moreover, a reduction of InGaAs absorption layer background concentration for reducing the device dark current can bring significant improvement in receiver sensitivity. Therefore, it is necessary to produce pure epitaxial layers as well as those that have few defects.

Mixed quaternary In _x Ga _1x As _y P _1y alloys with various band-gaps (cover the energy-gap from 1.35 eV (InP) to 0.75 eV (In _0:53 Ga _0:47 As), and the corresponding wavelength range from 0.92 to 1.65 mm at 300 K) can be grown on InP substrates with perfect lattice matching for detecting various wavelength regions. Unfortunately, the resulting difficult fabrication technology of a quaternary In x Ga 1x As y P 1y

alloy, which contains both group V elements of As and P in the epitaxial layer simultaneously rather than a ternary In x Ga 1x As, is obtained.

For In x Ga 1x As material growth, lattice composition is determined solely by the ratio of TMIn/TMGa, independent of growth temperature, growth rate and V/III ratio. However, for quaternary In-

x Ga 1x As y P 1y material growth, the relative incorporation of As and P atoms strongly depends on various growth parameters. Furthermore, uniform composition is also a problem for epitaxial techniques.

In this paper, we propose planar InGaAs(P)/InP photo- diodes with various band-gaps of quaternary InGaAsP cap layers grown by low-pressure metalorganic chemical vapor

desposition (LP-MOCVD). The planar p–i–n photodiodes exhibit high-quality and uniform epitaxial layers. It is noted that the InGaAs layer background concentration is as low as 4:5  10 ¹³ cm ^3 . In addition, the quaternary In-

x Ga 1x As y P 1y alloys with various band-gaps are well lattice matched to InP with uniform composition. The p–i–n photodiode with a wider-band-gap quaternary In-

x Ga _1x As _y P _1y alloy cap layer is applied to reduce the device surface leakage current and thus to improve device dark current characteristics. The p–i–n photodiode with a wide-band-gap InP cap layer exhibits a dark current as low as 60 pA at 10 V bias (device diameter is 135 mm), corresponding to a dark current density of 4:2  10 ^7 A/

cm ² .

2. Device Fabrication

The studied structures were grown by an LP-MOCVD system on S-doped N ^þ -InP substrates. The detail epitaxial layers of the studied planar InGaAs(P)/InP p–i–n photo- diodes are shown in Table I. The deposition temperature was 640 ^ C and the reactor pressure was held at 100 mbar.

Trimethylindium (TMIn), trimethylgallium (TMGa), arsine (AsH ₃ ) and phosphine (PH ₃ ) were used as the In, Ga, As and P sources, respectively. Disilane (Si ₂ H ₆ ) was adopted as the n-type source. The uniformity in layer thickness of InP and InGaAs(P) under the above-mentioned growth conditions is

5%. The lattice mismatch of InGaA(P) is kept within

500 ppm, measured by X-ray diffraction. A summary of the quaternary In x Ga 1x As y P 1y material characteristics is shown in Table II. The quaternary In x Ga 1x As y P 1y materi- als with various compositions, which correspond to various band-gaps and wavelengths, are well controlled in the proposed structures. A schematic cross section of the studied planar InGaAs(P)/InP p–i–n photodiodes is shown in Fig. 1.

Table I. Details of epitaxial layers of the studied planar InGaAs(P)/InP p–

i–n photodiodes.

Structure Thickness A1 A2 A3 B1 B2 B3

Cap layer 1 mm InP 1.1 PQ 1.4 PQ InP 1.05 PQ 1.1 PQ Absorption

2.5 mm InGaAs InGaAs InGaAs 1.4 PQ 1.4 PQ 1.4 PQ layer

Buffer layer 0.5 mm InP InP InP InP InP InP

Substrate 350 mm InP InP InP InP InP InP

Corresponding author. E-mail address: [email protected] Jpn. J. Appl. Phys. Vol. 42 (2003) pp. 4249–4252

Part 1, No. 7A, July 2003

#2003 The Japan Society of Applied Physics

4249

2500 A dielectric layer Si 3 N 4 was deposited by plasma- enhanced chemical vapor deposition (PECVD) as a diffusion mask with local openings. A planar photodiode can provide high reliability due to the surface that is passivated during device fabrication. The p ^þ –n junction was formed in the InGaAs(P) absorption layer by diffusing Zn through the InGaAs(P) cap layer. AuGeNi/Au and AuZn/Au were adopted as the n- and p-type ohmic contact metals, respectively.

3. Experimental Results

For a reversely biased p–i–n photodiode device, it is important to minimize the device dark current and obtain high-speed response. The carrier transit time can be shortened by reducing the thickness of the absorption layer, reducing the device dark current and obtaining a high-speed response. However, reducing the absorption layer thickness usually leads to a lower quantum efficiency. Minimizing absorption layer background concentration in the depletion region is the most desirable method of increasing device performance. The intrinsic-like material ensures a higher possible mobility, thus improving carrier transit time and device operation speed. In an MOCVD process, the major growth parameters for obtaining high-quality materials include growth temperature, V/III ratio, deposition rate, source molecules, carrier gas and other considerations. Some of our studies have shown that high growth temperature (630–660 ^ C) and low V/III ratio (particularly for the In _0:53 Ga _0:47 As material, the V/III ratio is as low as 10) are necessary for the growth of high-purity InGaAs(P) materials.

Therefore, under the above-mentioned growth conditions, high-quality and low-background-concentration InGaAs(P) materials are achieved. Figures 2(a) and 2(b) show the

doping concentrations of In 0:53 Ga 0:47 As and In-

0:66 Ga 0:34 As 0:73 P 0:27 (1.4 PQ) layers of the studied In- GaAs(P)/InP p–i–n photodiodes measured based on the C–

V profile. The measured concentrations of In _0:53 Ga _0:47 As and In _0:66 Ga _0:34 As _0:73 P _0:27 absorption layers are as low as 4:5  10 ¹³ and 2:4  10 ¹⁴ cm ^3 , respectively. We believe that device performances can be significantly improved due to the excellent material quality.

Figure 3 shows the dark current characteristics of the studied In x Ga 1x As y P 1y /In 0:53 Ga 0:47 As/InP photodiode with an InP cap layer (structure A1) at various temperatures. The measured dark current at 300 K is as low as 60 pA (4:2  10 ^7 A/cm ² ) at 10 V bias. The low dark current is attributed to the excellent material quality. In general, for low fields, the dark current consists of generation–recombi- nation current and diffusion current. We observe that the dark current increases with increasing temperature. This is due to the fact that increasing temperature enhances the generation recombination current and the diffusion current, thus increasing the device dark current.

Figure 4 shows the reverse current-voltage characteristics of the studied In x Ga 1x As y P 1y /In 0:53 Ga 0:47 As/InP photo- diodes with different-band-gap InGaAsP cap layers. The wavelengths of In x Ga 1x As y P 1y cap layers of various compositions are 0.92, 1.1 and 1.4 mm. The dark currents

Table II. A summary of the quaternary In

Ga

As

P

material characteristics.

Material Composition Wavelength (mm) Energy-gap

InP InP 0.92 1.35

1.05 PQ In

Ga

As

P

1.05 1.18 1.1 PQ In

Ga

As

P

1.1 1.13 1.4 PQ In

Ga

As

P

1.4 0.89

InGaAs In

Ga

As 1.65 0.75

n

-In

Ga

As

P

p

-region Cap layer

n

-In

Ga

As

P

Absoption layer

n

-InP Buffer layer

N

-InP Substrate

AuGeNi/Au

Si

N

AuZn/Au AuZn/Au Si

N

Fig. 1. Schematic cross section of the studied planar InGaAs(P)/InP p–i–n photodiodes.

0 2

1.0E+13 1.0E+14 1.0E+15 1.0E+16 1.0E+17

Depth ( µm) Concentration (/cm

)

(a)

1 3

0 1

1.0E+14 1.0E+15 1.0E+16 1.0E+17

Concentration (/cm

)

Depth (µm) (b)

2 3

Fig. 2. Doping concentrations of (a) In

Ga

As and (b) In-

0:66

Ga

As

P

layers of the studied InGaAs(P)/InP p–i–n photo- diodes measured based on the C–V profile.

4250 Jpn. J. Appl. Phys. Vol. 42 (2003) Pt. 1, No. 7A Y.-W. C

et al.

of structures A1, A2 and A3 measured at 10 V are 60 pA, 84 pA and 6.4 nA, respectively. The results show that the dark currents of structures A1 and A2 with higher-band-gap cap layers are much lower than that of structure A3 with a lower-band-gap cap layer. We believe that the further reduction of dark current in structures A1 and A2 is related to the differences in surface effect and interface properties between dielectric and InGaAsP. The generation-recombi- nation current and the diffusion current can be expressed as

I _g{r / expðE _g =2kTÞ; ð1Þ I _diff / expðE _g =kTÞ; ð2Þ where E g is the energy-gap and kT is the Boltzman energy.

From eqs. (1) and (2), we assume that generation- recombination current and diffusion current generated by the InGaAs absorption layer of the studied structures A1, A2 and A3 are almost the same. For a higher-band-gap In x Ga 1x As y P 1y cap layer structure, the generation–recom- bination current and the diffusion current generated by the In x Ga 1x As y P 1y cap layer may be reduced due to the higher-energy-gap material. Furthermore, for the studied

In x Ga 1x As y P 1y /InGaAs/InP structure, the valence band discontinuity (E V ) at the In x Ga 1x As y P 1y /InGaAs hetero- interface also increases with the higher band-gap of the In x Ga 1x As y P 1y material. The valence band discontinuities (E V ) of InP/In 0:53 Ga 0:47 As, In 0:85 Ga 0:15 As 0:33 P 0:67 (1.1 PQ)/In 0:53 Ga 0:47 As and In 0:66 Ga 0:34 As 0:73 P 0:27 (1.4 PQ)/In-

0:53 Ga 0:47 As heterojunctions are about 0.35, 0.23 and 0.08eV, respectively. For a higher-band-gap cap layer structure (A1 or A2) under a reverse bias, it is difficult for the holes in the In _0:53 Ga _0:47 As absorption layer to have sufficient energy to surmount the barrier height and transfer to the In _x Ga _1x As _y P _1y cap layer due to the high potential barrier between In _x Ga _1x As _y P _1y /In _0:53 Ga _0:47 As heterojunc- tions, thus reducing the device reverse leakage current.

Therefore, the dark current characteristics of the studied In x Ga 1x As y P 1y /InGaAs/InP structure can be further re- duced by using a higher-band-gap In x Ga 1x As y P 1y material as the cap layer. Consequently, the lowest dark current is expected in structure A1 with an InP cap layer, which has the widest band-gap among In x Ga 1x As y P 1y material systems, compared to structures A2 and A3.

Figure 5(a) shows the dark current–voltage characteristics of the studied In x Ga 1x As y P 1y /1.4 PQ/InP photodiodes with

-20 -15 -10 -5 0

1.0E-13 1.0E-12 1.0E-11 1.0E-10 1.0E-9 1.0E-8 1.0E-7 1.0E-6 1.0E-5 1.0E-4 1.0E-3 1.0E-2

Dark Curr ent (A)

Bias Voltage (V) Structure A3 Structure A2 Structure A1

Fig. 4. Dark current–voltage characteristics of the studied In-

x

Ga

As

P

/In

Ga

As/InP photodiodes with different InGaAsP cap layers.

-20 -15 -10 -5

1.0E-13 1.0E-12 1.0E-11 1.0E-10 1.0E-9 1.0E-8 1.0E-7 1.0E-6

Bias Voltage (V)

Dark Curr ent (A)

Structure B3 Structure B2 Structure B1 (a)

0

-20 -15 -10 -5 0

1.0E-8 1.0E-7 1.0E-6 1.0E-5

PhotoCurr ent (A)

Bias Voltage (V) Structure B3 Structure B2 Structure B1 (b)

Fig. 5. (a) Dark current–voltage characteristics of the studied In-

x

Ga

As

P

/1.4 PQ/InP photodiodes with different InGaAsP cap layers. (b) Photocurrent–voltage characteristics of the studied In-

x

Ga

As

P

/1.4 PQ/InP photodiodes with different InGaAsP cap layers.

Bias Voltage (V)

-20 -15 -10 -5 0

1.0E-14 1.0E-13 1.0E-12 1.0E-11 1.0E-10 1.0E-9 1.0E-8 1.0E-7 1.0E-6 1.0E-5 1.0E-4 1.0E-3

Dark Curr ent (A)

370 K 360 K 350 K 340 K 330 K 320 K 310 K 300 K

Fig. 3. Dark current characteristics of the studied InP/In

Ga

As/InP photodiode at various temperatures.

Jpn. J. Appl. Phys. Vol. 42 (2003) Pt. 1, No. 7A Y.-W. C

et al. 4251

different InGaAsP cap layers. The wavelengths of In-

x Ga 1x As y P 1y cap layers of various compositions are 0.92, 1.05 and 1.1 mm. The dark currents of structures B1, B2 and B3 measured at 10 V are 78pA, 110 pA and 213 pA, respectively. The results also show that the lower dark current characteristics can be obtained in the structure with a higher-band-gap cap layer material due to the lower generation–recombination current and diffusion current generated by the higher bandgap cap layer. Figure 5(b) shows photocurrent–voltage characteristics (under a micro- scope) of the studied structures B1, B2 and B3. The photocurrents of structures B1, B2 and B3 measured at

10 V are 549 nA, 381 nA and 61 nA, respectively. The results show that higher photocurrent, which corresponds to higher responsivity, can be obtained in the structure with a higher-band-gap cap layer. The cap layer material requires careful attention since the incident light must traverse it and finally be absorbed in the absorption layer. Therefore, using a narrower-band-gap InGaAsP material as the cap layer may cause more incident light absorption in the cap layer, because of the reduction of the incident light traversing the cap layer into absorption lower photocurrent in the structure.

In addition, at 1.3 mm, a dc responsivity of 0.81 A/W was measured at 5 V bias in structure B1 with an InP cap layer, which is higher than those of structures B2 and B3. The p-i-n photodiode with a wide-band-gap InP cap layer in the In _x Ga _1x As _y P _1y material system can be expected to further improve device performances.

4. Conclusions

We have successfully fabricated and demonstrated a series of planar In x Ga 1x As y P 1y p–i–n photodiodes grown by MOCVD. The background concentration of the high-quality InGaAs absorption layer is as low as 4:5  10 ^13 cm ^3 . Using a wide-band-gap InP material as the cap layer, the device surface leakage current and the dark current are significantly reduced. In addition, the wide-band-gap cap layer may also reduce the incident light absorption in the cap layer, thus improving device responsivity.

Acknowledgement

Authors would like to acknowledge the support of all members of the South Epitaxy Corporation involved in this project. This work was also supported by the National Science Council of the Republic of China under contract No. NSC90-2215-E-006-014.

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