Ge-on-Si Avalanche Photodiodes for LIDAR Applications

M. Wanitzek*, M. Oehme, D. Schwarz, K. Guguieva, and J. Schulze Senior Member IEEE Institute of Semiconductor Engineering (IHT), University of Stuttgart,

Pfaffenwaldring 47, 70569 Stuttgart, Germany *) Corresponding author: st149364@stud.uni-stuttgart.de

Abstract – In order to make autonomous driving in cars possible, a precise knowledge of the immediate surroundings is required. One technology for near-field detection is the LIDAR (light detection and ranging) technology. Currently available LIDAR systems operate at a wavelength of 905 nm. However, for wavelengths from 1,300 nm, a significant increase in range resolution is achieved. Here, the receiver side is typically realized as APDs (avalanche photodiodes). In this work the fabrication and characterization of APDs with an absorption region made from Germanium (Ge) are presented. The layer sequence for the APDs are grown directly on Silicon (Si) substrate using a molecular beam epitaxy system. At room temperature the Ge-on-Si-APDs achieve responsivities of up to 6 A/W at a wavelength of 1,310 nm, which corresponds to a gain of 26 compared to conventional Ge photodiodes.

Keywords – Ge-on-Si; avalanche photodiode; photodetector; lidar

I. INTRODUCTION In recent years the demand for high-sensitive, low-noise

and fast detectors has increased significantly [1] for various emerging applications as quantum key distribution [2], time-of-flight ranging [3], three-dimensional imaging [4] and time resolved spectroscopy [5]. Single photon avalan-che photodiodes (SPADs) can fulfill these requirements. SPAD detectors are avalanche photodiodes (APD), which are biased above breakdown voltage in the so-called Geiger-mode. In this mode, a self-sustaining avalanche current can be triggered by a single incident photon [6].

While SPADs made from Silicon (Si) fulfill these requirements at wavelengths below 1,000 nm in large scale complementary metal-oxide-semiconductor (CMOS) ar-rays [7], efficient detectors for wavelengths greater than this remains unsolved. Despite the fact that SPADs made from Indium gallium arsenide (InGaAs) are commercially available, detectors made from III-IV semiconductors have some drawbacks. On the one hand they are expensive and on the other hand it is hard to CMOS-integrate such detec-tors in a detector array. A solution to this problem is given by the use of Germanium (Ge), which can be fully inte-grated within existing Si technology and therefore enable integrated read-out circuits.

Applications as quantum communication in optical fibers require detectors for the wavelengths of 1,310 nm and 1,550 nm. In LIDAR technology SPAD detector arrays are already used although they are fabricated from Si and therefore operate at wavelengths around 900 nm, the so-

called infrared(IR)-A region. Using higher wavelengths in the IR-B region brings the positive effect, that significantly more optical power is permitted due to eye safety. Another problem is the glare of the cameras from the sun. This worsens the signal-to-noise ratio. One solution to this is the light irradiation from the backside of a Si wafer. Si acts as an absorber for a large part of the solar spectrum.

The usage of higher wavelengths in the IR-B region will be possible if SPAD detectors made from Ge and integrated on Si are used. Ge shows good absorption for wavelengths up to approx. 1,550 nm [8]. In Fig. 1 the spectral sensitivity of back-site illuminated Ge diodes is shown. Also the solar spectrum is plotted in this figure as the AM1.5 spectrum and idealized as the black body of a temperature of 5800 K. It shows that most of the sunlight can be filtered out by the back lighting. Using Ge as absorber material also gives the option to further increase the sensing wavelength by alloying Tin (Sn) into the active region [10]. As also shown in Fig. 1 an increasing Sn concentration increases responsivities at higher wavelengths.

Ge-on-Si-SPADs with good optical properties were already reported in literature, but they rely on very low temperatures at around T = 100 K [11]. It is the goal of this work to achieve these properties at room temperature or at least at temperatures that can be obtain with thermoelectric coolers what enables the usage in automotive applications.

In this work we show Ge-on-Si-APDs operated in linear mode with a gain of 26 compared to reference Ge-pin-photo diode with the same absorption region.

Figure 1: Responsivities of backside illuminated Ge-on-Si diodes and GexSn1-x-on-Si diodes with a Sn concentra-

tion of up to 4 %. The solar spectrum is also plotted.

300 600 900 1200 1500 1800

irrad

ianc

e [a

.u.]

Ge0.96Sn0.04

resp

onsi

vity

[a.u

.]

wavelength [nm]

Ge

Ge0.98Sn0.02

sunspectrum

MIPRO 2020/MEET 15

II. FABRICATION

A. Molecular Beam Epitaxy The presented structure for the Ge-on-Si-APDs were

grown by using molecular beam epitaxy (MBE) using a so-lid source 6”-MBE system with a base pressure of P < 10-10 mbar. The matrix material Si is evaporated by using an electron beam evaporator, whereby the matrix material Ge and the dopant materials Boron (B) and Antimony (Sb) are evaporated by Knudsen-cells, resp. For the Ge-on-Si-APDs we used an Arsenic(As)-doped n+-Si(100) substrate with a nominal sheet resistance of ρ < 0.005 Ω cm, which builds the bottom contact. The growth process starts with an in-situ thermal cleaning step at T = 900 °C for t = 5 min in order to remove the native Silicon dioxide (SiO2) [9].

The growth starts with an intrinsic Si layer with a thickness of di-Si = 500 nm at a growth temperature of T = 600 °C. On top of this a p-doped Si layer with a thickness of dp-Si = 100 nm and a nominal doping concentration of NA = 1.5 ∙ 1017 cm-3 is grown, which forms the charge layer. After that an intrinsic Ge layer with a thickness of di-Ge = 300 nm at the temperature of T = 330 °C is grown. On top of this a hetero contact layer, which consists of dp+-Ge = 100 nm thick p+-doped Ge and dp+-Si = 100 nm thick p+-doped Si is finally deposited. An illustration of this layer stack (sample A) can be seen in Fig. 2.

B. Sample variations In order to prove the concept and give further

information about the growth and fabrication process we processed two additional samples. Sample B was processed with a dGe-VS = 100 nm thick Ge virtual substrate (Ge-VS) deposited on the before grown p-Si charge layer and i-Si layer. The epitaxial growth of pure Ge on a Si substrate is technologically challenging because of the lattice mismatch of 4 % between Si and Ge. This leads to high threading dislocation densities and bad overall crystal structure if no further growth enhancements are performed. The Ge-VS reduces these threading dislocations [11] and therefore reduces the dark current. In order to create a Ge-VS, a 100 nm Ge layer has to be deposited with a temperature of T = 330 °C after which an annealing-step at Tanneal = 810 °C for t = 5 min is performed. Creating such a Ge-VS has the drawback, that the annealing-step leads to intermixing of Ge and Si at the interface. This increases the effective k-ratio of ionization coefficients of electrons and holes [12]. Afterwards another intrinsic layer with a thickness of

di-Ge = 200 nm at the temperature of T = 330 °C is grown. Therefore, the Ge absorption region has the same thickness in both samples. On top of this the same hetero contact layer as in sample A gets deposited. The structure of this sample B can be seen in Fig. 3.

In addition to this sample we grow a reference APD made from pure Si (sample C). This sample has the equivalent layer structure and nominal doping concentra-tions as sample A and was processed in a similar way. The structure is illustrated in Fig. 4.

C. Device fabrication Following the MBE growth, the 4” wafers were diced

to 4 squares with an edge length of ledge = 35 mm. A single mesa process was used to fabricate diodes on these samples. The first step in this process is a Ge cleaning. This consists of wet chemically clean with Acetone ((CH3)2CO) and Isopropyl alcohol (CH3CHOHCH3) following a plasma ashing with Oxygen (O2) to remove (in)organic remains. To etch the mesa structure the photoresist AZMIR701 was structured with a 365 nm photolithography technology. After that the mesa etching was performed by Inductive Coupled Plasma (ICP) Reactive Ion Etching (RIE) with Hydrogen bromide (HBr) with an approximated etch rate of 120 nm/s.

The photoresist was removed with the remover P1316 and a O2 plasma ashing process. Followed by the same wet chemically clean step as at the beginning. Afterwards, SiO2 (dSiO2 = 300 nm) was deposited as a passivation and isolation oxide, using Plasma Enhanced Chemical Vapor Deposition (PECVD) with Tetraethyl orthosilicate (TEOS) as a precursor at Tsub = 250 °C.

In order to make a top contact an oxide window has to be etched, which is performed using RIE with Fluoroform (CHF3) as etching gas with a subsequently wet etch step using buffered Hydrofluoric acid (bHF) for the last 30 nm of SiO2 to avoid surface roughening by RIE [12]. The metallization consists of sputtered Aluminum (Al) with a thickness of dAl = 1300 nm, which was etched using ICP-RIE dry etch step with HBr and a wet etching step with Phosphoric acid (H3PO4).

Figure 2: Schematic structure of a Ge-

on-Si-APD (sample A)

Figure 3: Schematic structure of a Ge-

on-Si-APD with Ge-VS (sample B)

Figure 4: Schematic structure of a Si-

APD (sample C)

16 MIPRO 2020/MEET

III. CHARACTERIZATION

A. DC measurements In order to investigate the electrical behavior of the

fabricated diodes, DC measurements, consisting of I(V) and transfer length method (TLM) measurements, were performed after processing at room temperature. These were accomplished on a Keithley 4200SC with a KarlSuss PA200 Manipulator.

Figure 5 shows the I(V) characteristic of the samples A to C. It can be seen that, overall current for sample A is higher than sample B. This proves the improved crystal quality because of the Ge-VS due to lower threading dislocation density. This advantage of sample B over sample A gets smaller for increasing negative applied voltages and thus making them nearly equal for the relevant voltage region. It can further be seen that the overall current of the pure-Si reference APD (sample C) is lower than both Ge samples. This is due to better overall crystal quality in Si than in Ge.

In order to compare the electrical properties, the series resistance RS and ideality factor nid are calculated using (1) and (2), respectively. These equations can be derived from the Shockley diode equation [13].

𝑅𝑆 = 𝑑

𝑑𝐼(𝐼 ∙

𝑑𝑉

𝑑𝐼)

𝑛𝑖𝑑 = 𝑉𝑇 ∙ (𝑑(ln(𝐼))

𝑑𝑉𝑖𝑑)−1

Where I is the measured current, V is the applied voltage, VT is the thermal voltage and Vid = V – RS I is the voltage applied to the diode, only. The calculated values of RS are shown in Table 1. It can be seen that the series resistance increases in both sample A and B for decreasing diode radii. This is due to smaller volume of the mesa structure. For sample C this is not the case.

Figure 5: Current voltage characteristics at room

temperature for all three samples with an adjusted voltage axis to show the forward region more clearly

TABLE I. SERIES RESISTANCES AS FUNCTION OF THE DEVICE RADIUS FOR ALL THREE SAMPLES

Diode radius

Series resistance of

sample A

Series resistance of

sample B

Series resistance of

sample C

rdiode = 80 µm

19.8 Ω 11.9 Ω 14.3 Ω

rdiode = 40 µm

28.7 Ω 18.1 Ω 12.0 Ω

rdiode = 20 µm

65.9 Ω 30.9 Ω 12.3 Ω

rdiode = 10 µm

123.4Ω 77.2 Ω 17.5Ω

Figure 6: Box-and-whisker plot of the contact resistance of all three samples

The TLM measurements were only performed on the top contact, because of the one mesa process. The contact resistance is calculated using (3).

𝑅𝐶 =𝑅𝑇𝐿𝑀(𝑙=0µ𝑚)

2

RLTM is the linear approximated resistance of the measured curve and l is the distance between two contacts. The so calculated contact resistance is shown in Fig. 6.

It can be seen that the overall contact resistance is small,

which indicates a good contact of the Al with the Si. The median contact resistance of all three samples is nearly equal at around RC ≈ 110 mΩ, even though sample C has a larger spread.

B. Optical measurements The optical parameters are the key parameters to prove

quality of the Ge-on-Si APDs. The optical measurements were performed on a Keithley 2450 with both a continuous wave (CW) laser and a supercontinuum laser (model NKT photonics SuperK).

-35 -30 -25 -20 -15 -10 -5 010-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

curr

ent [

A]

voltage [V]

sample A sample B sample C

sample A sample B sample C0

40

80

120

160

200

cont

act r

esis

tanc

e [m

W]

25%~75% min~max median mean

MIPRO 2020/MEET 17

In Fig. 7 the illuminated and dark current of the sample A is plotted. The measurement belongs to a diode with a mesa radius of rdiode = 80 µm illuminated with radiation of the wavelength λ = 1310 nm. Figure 8 shows the same for sample B.

It is seen that the illumination current and thus the photocurrent is proportional to the power of the illuminating laser.

The responsivity is defined as the optical power divided by the photocurrent as given in (5).

𝑅𝑜𝑝𝑡 =𝑑𝐼𝑝ℎ𝑜𝑡

𝑑𝑃𝑜𝑝𝑡

Figure 9 shows that the photocurrent is not linear over the optical power. Due to this we have to calculate the differential quotient.

With the calculated responsivities we can calculate the corresponding gain factor when standardizing it to a comparable Ge-on-Si pin-diode. These pin-diodes achieve a responsivity of up to 0.22 A/W at a reverse bias of Vbias = - 0.1 V [10], which leads to a gain factor of up to 26 using (5).

𝐺 =𝑅𝑜𝑝𝑡

𝑅𝑜𝑝𝑡,𝑟𝑒𝑓=

𝑅𝑜𝑝𝑡

0.22 𝐴

𝑊

The so calculated gains for different bias voltages are shown in Fig. 10. It can be seen that the gain increases significantly for decreasing optical powers. This is because the Ge-on-Si-APD generates a significant photocurrent with even low optical powers. As stated before the photocurrent is not linear to the optical power. This is due to the fact that the samples are operated near at the avalanche breakdown and therefore there is already a significant avalanche multiplication.

Additionally, Fig. 11 shows the same for sample B. It can be seen that sample B only reaches a lower gain.

Therefore, the Ge-VS decreases the optical properties due to increased k-ratio of ionization coefficients of electrons and holes which decreases multiplication factors.

IV. CONLUSION We reported the successful growth and fabrication of

Ge-on-Si APDs with responsivities of up to 6 A/W corresponding to a gain of up to factor 26. These Ge-on-Si APDs can be produced in a CMOS compatible process and therefore enable the integration in large scale detector arrays.

Figure 9: Photocurrent as function of the optical power a different working points from sample A

0 1 2 3 4 50

2

4

6

8

phot

ocur

rent

[mA

]

optical power [mW]

-24 V -24.1 V -24.2 V -24.3 V -24.4 V

Figure 7: Illuminated and dark current of sample A at a wavelength of 1310 nm as function of different optical

powers

Figure 8: Illuminated and dark current of sample B at a wavelength of 1310 nm as function of different optical

powers

-25 -20 -15 -10 -5 010-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

l = 1310 nm

curr

ent [

A]

voltage [V]

Ih,430 mW Ih,4840 mW

Ih,344 mW Ih,3880 mW

Ih,258 mW Ih,2930 mW

Ih,172 mW Ih,1990 mW

Ih,86 mW Ih,1010 mW

Idark

-25 -20 -15 -10 -5 010-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

l = 1310 nm

curr

ent [

A]

voltage [V]

Ih,440mW Ih,4920mW

Ih,354mW Ih,3950mW

Ih,266mW Ih,2990mW

Ih,178mW Ih,1990mW

Ih,88mW Ih,1000mW

Idark

18 MIPRO 2020/MEET

REFERENCES [1] G. S. Buller, R. J. Collins, “Single-photon detection and

generation”, Meas. Sci. Technol. Vol. 21, 012002, 2010. [2] H. Takesue, S. W. Nam, Q. Zhang, R. H. Hadfield, T. honjo, K.

Tamaki, Y. Yamamoto, “Quantum key distribution over a 40-dB channel loss using sperconducting single-photon detectors”, Nat. Phot., Vol. 1, No. 6. pp. 343-348, 2007.

[3] R. E. Warburton, A. McCarthy, A. M. Wallace, S. Hernandez-Marin, R. H. Hadfield, S. W. Nam, G. S. Buller, “Subcentimeter depth resolution using a single –photon counting time-of-flight laser ranging system at 1550 nm wavelength”, Opt. Lett., Vol. 32, No. 15, pp. 2266-2268, 2007.

[4] P. Yuan, R. Sudharsan, X. Bai, J. Boisvert, P. McDonald, E. Labois, M. S. Salisbury, G. M. Stuart, H. Danny, A. A. Prtillo, A. B. Roybal, S. Van Duyne, G. Pauls, S. Gaalema, “32x32 Geiger-mode LADAR cameras”, Proc. SPIE Int. Soc. Opt. Eng., Vol.7684, p. 76840C, 2010.

[5] W. Becker, A. Bergmann, G. L. Biscotti, A. Rueck, “Advanced time-correlated single photon counting techniques for spectroscopy and imaging in biomedical systems”, Proc. SPIE, Vol. 5340, pp. 104-112, 2004.

[6] F. Zappa, S. Tisa, A. Tosi, S. Cova, “Principles and features of single-photon avalanche diode arrays”, Sci. Dir, Vol. 140, No. 1, pp. 103-112, 2007.

[7] D. Bronzi, F. Villa, S. Tisa, A. Tosi, F. Zappa, D. Durini, S. Weyers, W. Brockherde, “100 000Frames/s 64 x 32 single-photon detector

array for 2-D imaging and 3-D ranging”, IEEE JSTQE, Vol. 20, No. 6, 2014.

[8] W. C. Dash, R. Newman, “Intrinsic optical absorption in singlecrystal germanium and silicon at 77 °K and 300 °K”, Phys. Rev., Vol. 99, No. 4, pp. 1151-1155, 1955.

[9] E. Kasper, M. Bauer, M. Oehme, “Quantitative secondary ion mass cleaning”, Thin Solid Films, Vol. 321, pp. 148-152, 1998.

[10] M. Oehme, M. Schmid, M. Kaschel, M. Gollhofer, D. Widmann, “GeSn p-i-n detectors integrated on Si with up to 4% Sn”, Appl. Phys. Lett. 101, 141110, 2012.

[11] P. Vines, K. Kuzmenko, J. Kirdoda, D. Dumas, M. Mirza, R. Millar, D. Paul, G. Buller, “High performance planar germanium-on-silicon single-photon avalanche diode detectors”, Nat. Comm., Vol. 10, No. 1086, 2019.

[12] M. Oehme, J. Werner, M. Kaschel, O. Kirfel, E. Kasper, “ Germanium waveguide photodetectors integrated on silicon with MBE”, Thin Solid Films, Vol. 517, pp. 137-139, 2008.

[13] Y. Kang, H. Liu, M. Morse, M. J. Paniccia, M. Zadka, S. Litski, G. Sarid, A. pauchard, Y. Kuo, H. Chen, W. S. zaoui, J. E. Bowers, A. beling, D. C. McIntosh, X. Zheng, J. C. Campbell, “Monolithic germanium/silicon avalanche photodiodes with 340 GHz gain-bandwidth product”, Nat. Phot., Vol. 3, No. 2, pp. 59-63, 2009.

[14] G. S. Oehrlein, R. G. Schad, M. A. Jaso, “Mechanism of silicon surface roughening by racive ion etching”, Surface and Interface Analysis, Vol. 8, pp. 243-246, 1986.

[15] W. Shockley, “The theory of p-n junctions in semiconductors and p-n junction transistors”, Bell System Technical Journal, Vol. 28, No. 3, pp. 435-489, 1949.

Figure 10: Gain of sample A reaching a gain of up to factor

26 at a low optical power of 86 µW and a wavelength of 1310 nm

Figure 11: Gain of sample A reaching a gain of up to factor

22 at a low optical power of 88 µW and a wavelength of 1310 nm

0 1 2 3 4 50

5

10

15

20

25

30ga

in

optical power [mW]

G-24V

G-24.1V

G-24.3V

0 1 2 3 4 50

5

10

15

20

25

30

gain

optical power [mW]

G-24.1V

G-24.3V

G-24.4V

MIPRO 2020/MEET 19