Visible and near-infrared photodetector on chemically vapor deposited diamond

Diamond & Related Materials 97 (2019) 107444

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Visible and near-infrared photodetector on chemically vapor deposited diamond☆

T

⁎

V.A. Kukushkina,b, , M.A. Lobaeva, S.A. Bogdanova, A.N. Stepanova, S.A. Kraevc, A.I. Okhapkinc, E.A. Arkhipovac, A.V. Zdoroveyshchevd, M.V. Vedb,d a

Institute of Applied Physics of the Russian Academy of Science, 46 Ulyanov str., 603950 Nizhny Novgorod, Russia Nizhny Novgorod State University named after N. I. Lobachevsky, 23 Gagarin pr., 603950 Nizhny Novgorod, Russia c Institute for Physics of Microstructures of the Russian Academy of Science, 7 Academicheskaya str., Kstovsky district, 603087 Afonino, Nizhny Novgorod region, Russia d Physical-Technical Research Institute at the Nizhny Novgorod State University named after N. I. Lobachevsky, 23 Gagarin pr., 603950 Nizhny Novgorod, Russia b

A R T I C LE I N FO

A B S T R A C T

Keywords: Chemical vapor deposition Detectors Diamond film Etching High pressure high temperature (HTHP) Ohmic contacts Optoelectronic properties P-type doping Schottky diodes Single crystal growth Surface characterization

A photodetector of the visible and near infrared wavelength ranges based on chemically vapor deposited (CVD) boron-doped diamond is fabricated and tested. The principle of its operation is based on the creation of holes by incident radiation not in a CVD diamond film itself, but in an adjoining bimetal Cr/Au layer. The latter serves simultaneously as one of the Schottky contacts with an external circuit. Due to small thickness of this layer in the order of 10 nm the generated holes do not have enough time to recombine with electrons because of diffusion in the CVD diamond film. There the holes are accelerated by the electric field of a hole depletion region and create a photocurrent. For optimized boron doping profile of the CVD diamond film the photodetector Volt-Watt sensitivity in the order of 1 mV/(W/cm2) at wavelength 445 nm and 0.1 mV/(W/cm2) at 532 nm was measured. For a constant forward bias voltage 2.5 V its Ampere-Watt sensitivity at 445 nm is 0.7 A/W. For zero bias voltage this figure is 5 ⋅ 10−5 A/W at 445 nm, 3 ⋅ 10−6 A/W at 532 nm, and 1.8 ⋅ 10−7 A/W at 1.06 μm. The photodetector time response was measured in the current mode at 532 nm. It is in the order of 600 ns without a bias voltage and 200 ns with a constant reverse bias voltage 2.5 V.

1. Introduction Diamond, due to its high chemical and radiation hardness, is a perspective basis for photodetectors to be employed in chemical and atomic industries and space missions. Thanks to a large bandgap 5.5 eV it is used, as a rule, for the creation of photodetectors of ultraviolet and soft X-ray radiation. Their principle of operation is the change of the illuminated sample resistance due to the generation of charge carriers – electrons and holes – as a result of interband photon absorption [1–8]. In the visible and near infrared ranges which are most interesting for optoelectronic and telecommunication applications this change can be attributed to other effects. They are, e.g., the generation of charge

carriers owing to impurity–conduction or impurity–valence band photon absorption [9] or radiation heating [10]. However, it is difficult to control and reproduce the impurity band parameters and thermal effects are time-persistent. Meanwhile, there is another well-known effect in optoelectronics [11] upon which a photodetector can be based. It is the creation of charge carriers by incident radiation not in a semiconductor itself, but in an adjoining metal layer. The latter serves simultaneously as the Schottky junction of a semiconductor with an external circuit (Fig. 1). Due to small thickness of such a layer which is typically in the order of 10 nm the generated charge carries do not have enough time to recombine with charge carriers of another type because of diffusion in a

☆ The declaration of the authors' individual contribution to the manuscript: V. A. Kukushkin – the idea, theory, manuscript text and figures preparation, M. A. Lobaev– the CVD diamond structure growth, the figures and manuscript text preparation, S. A. Bogdanov – the CVD diamond structure growth, the edition of English language, A. N. Stepanov – the measurement of the photodetector sensitivity and time response at 532 nm, S. A. Kraev – the Ohmic and Schottky contacts fabrication, the figure preparation, A. I. Okhapkin – the CVD diamond Inductively Coupled Plasma etching, E. A. Arkhipova – the Ohmic and Schottky contacts fabrication, A. V. Zdoroveyshchev – the measurement of the photodetector sensitivity at 445 nm, the figures preparation, M. V. Ved – the measurement of the photodetector sensitivity at 445 nm, the figures preparation. All the authors have approved the final manuscript. ⁎ Corresponding author at: Institute of Applied Physics of the Russian Academy of Science, 46 Ulyanov str., 603950 Nizhny Novgorod, Russia. E-mail addresses: [email protected] (V.A. Kukushkin), [email protected] (M.A. Lobaev), [email protected] (S.A. Bogdanov), [email protected] (A.N. Stepanov), [email protected] (S.A. Kraev), [email protected] (E.A. Arkhipova).

https://doi.org/10.1016/j.diamond.2019.107444 Received 25 March 2019; Received in revised form 1 June 2019; Accepted 4 June 2019 Available online 28 June 2019 0925-9635/ © 2019 Elsevier B.V. All rights reserved.

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in the reactor, depicted in [14]. Doping method and growth regime was described previously in [15]. The growth method we used allows obtaining high quality thick heavily boron doped layers at a growth rate of 4 μm/h. For the growth of a lightly doped layer 3 with thickness of 200 nm and boron atom concentration 1 · 1017 cm−3 and a heavily doped 3 nm-thick delta-layer 4 with boron atom concentration 5 ⋅ 1019 ÷ 1020 cm−3 we used the home-made CVD reactor. It is described in detail in [16]. One of the main features of the reactor is that the discharge is maintained in a laminar, vortex-free gas flow, and the gas mixture is switched by an electrically controlled rapid gas switch. Due to this, it is possible to obtain layers doped with boron with sharp boundaries. This reactor allows the use of slow growth regime of CVD diamond deposition [17,18]. In this regime the CVD diamond grows in form of successively filling steps, i.e. terraces. The step height is equal to the distance between atomic layers, and the width of the step is determined by the value of misorientation angle. The use of this growth mode allows obtaining atomically smooth surfaces of CVD diamond with low roughness. Then, 1 μm-deep etching of CVD diamond (PlasmaLab 80 Plus with an Inductively Coupled Plasma source) in oxygen ion plasma with a lithographical Al mask was performed (Fig. 2b). The latter allowed to uniformly etch the left half of the sample surface. In the same time on the right half of the surface it provided etching four circular grooves 5 with inner diameter 0.56 mm and width 0.09 mm thereby forming 4 mesa columns to decrease leakage current. Then, Ti/Mo/Au (20 nm/ 30 nm/100 nm from bottom to top) Ohmic contact 6 was deposited on the left uniformly etched half of the sample with the use of photolithography. Then it was burned in for 10 min at 450 °C. After that a Cr/ Au (7 nm/6 nm from bottom to top) bimetal film 7 was deposited on the tops of mesa columns with the help of photolithography. Then, Ti/Mo/ Au (20 nm/30 nm/100 nm from bottom to top) 0.1 mm wide circular contacts 8 were deposited on the periphery parts of the mesa columns covered with the Cr/Au bimetal films with the use of photolithography. Finally, Au wires were soldered to the Ohmic contact 6 and to each circular contact 8 on the four mesa columns. In Fig. 3 the surface profile of one of the four Schottky contact mesa columns taken by white-light interferometer Talysurf CCI 2000 (Taylor & Hobson, UK) with an altitude resolution 0.2 nm is given.

radiation

EF Eb

Ev

I 1 Hole energy

2

Fig. 1. The photodetector principle of operation illustrated for a case when diffusing charge carriers are holes: 1 – a semiconductor, 2 – an adjoining metal layer, hole energy is counted off from the Fermi energy downward.

semiconductor. Their acceleration by the electric field of the charge carrier depletion region in a semiconductor creates a photocurrent I. Obviously, the minimal energy of photons that can be registered by a photodetector based on this effect is in the order of Eb. The latter is the difference between the Fermi energy EF in a metal and the bottom of the conduction band Ec if diffusing charge carriers are electrons or the valence band top Ev if diffusing charge carriers are holes in a semiconductor on its interface with a metal. For contacts of typical metals with boron-doped diamond this difference can correspond to the visible and near infrared wavelength ranges [12]. So, this effect can be employed for the creation of a diamond photodetector for these spectral domains. The present article is to report the creation technology and its theoretical rationale, characterization and performance of such a device.

2. Material and methods For the creation of photodetector the structure shown in Fig. 2a was fabricated. The structure was grown on a substrate 1 with (001) orientation and of 3.0 × 3.0 × 0.5 mm3 size. It is made of type IIa HPHT diamond by New Diamond Technology. Before the growth process, the substrate was mechanically polished to a surface roughness of 0.1 nm, measured with a Zygo NewView 7300 white light interferometer on an area of 0.22 × 0.22 mm2. To remove defects, introduced by polishing from the substrate, 4–5 μm thick layer was etched away in the Inductively Coupled Plasma (Oxford Instruments, Plasmalab 80) [13]. As a result, a defect-free substrate with an atomically smooth surface was used to grow the layers of CVD diamond. Heavily doped layer 2 with thickness of 10 μm and boron concentration of 1 · 1020 cm−3 was grown

3. Theory The theoretical rationale behind this technology is as follows. The deposition of heavily-doped delta-layer 4 on low-doped layer 3 [19] narrows the upper part of the Schottky barrier for holes at the CVD diamond–Cr/Au bimetal layer interface (Fig. 1). Thereby it enhances the barrier tunneling transparency. In result holes with energies smaller than Eb can enter the CVD diamond. This obviously increases the photodetector sensitivity in the visible and near infrared wavelength ranges. At the same time the use for the barrier narrowing a thin

radiation Schottky p+

p

3

p+

2

IIa (001)

1

Ohmic

4

7 3

6 2 IIa (001)

(a)

8

4

5 1

(b)

Fig. 2. The photodetector structure frontal cross-section scheme before (a) and after (b) etching and the contact deposition. The numerical designations are explained in the text. For conciseness only one mesa column in part b is shown. 2

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5 4 3 2 1

I ( A)

0.5

0.0

-0.5 -10

heavily-doped delta-layer rather than a heavily doped thick layer [19] allows to decrease the electrical capacity of the charge carrier depletion region. This, obviously, results in the increase of the photodetector operation speed. The thickness of the bimetal layer Cr/Au was chosen from the following consideration. As shown at the end of this section, the penetration depth of the incident radiation in this layer is in the order of the hole diffusion length ld. Under this condition the layer optimal thickness is in the order of ld. This is because, on the one hand, it should not significantly exceed ld. In the opposite case the major part of holes generated in this layer by incident radiation would recombine with electrons before reaching its interface with CVD diamond. In result, the photocurrent and the photodetector sensitivity would be low. On the other hand, significant decrease of this layer thickness in comparison with ld also leads to the decrease of the photocurrent and, therefore, the photodetector sensitivity. This is because it entails the drop of the detected radiation power which is absorbed in this layer. So, the optimal bimetal layer thickness is in the order of ld. For the estimation of the latter let us neglect the bimetal layer nonhomogeneous chemical composition and consider it a pure Cr layer. The well-known formula of the Brownian movement gives

10

4. Results and discussion Firstly, the photodetector performance was studied at static illumination at wavelength 445 nm by LaserVarioRakurs MBL-445-1. The corresponding current–voltage dependences for a single mesa column at different powers of illumination are shown in Fig. 4. The laser beam cross-section was quadratic with a side of 5 mm long. The Ampere–Watt sensitivity at zero bias voltage was defined as the ratio of current at zero voltage between the Ohmic and Schottky contacts, U = 0, i.e. in the current mode, to the power of the radiation going through the free surface of the Cr/Au bimetal layer on the chosen mesa column top. It is approximately 5 ⋅ 10−5 A/W. The Volt–Watt sensitivity is defined as the ratio of the voltage U between the Ohmic and Schottky contacts at I = 0, i.e. in the voltage mode, to the intensity of the radiation going through the free surface of the Cr/Au bimetal layer on the chosen mesa column top. From the same figure it is in the order of 1 mV/(W/cm2). We also measured the Ampere–Watt sensitivity at constant forward bias voltage U = − 2.5 V. This sensitivity is defined as the ratio of the difference of a current under illumination and a “dark” current to the power of the radiation going through the free surface of the Cr/Au bimetal layer on the chosen mesa column top. From Fig. 5 it is 0.5 ÷ 0.7 A/W. So, even a moderate forward bias increases the photodetector Ampere–Watt sensitivity by 4 orders of magnitude in comparison with an unbiased situation. The measurement at wavelength 532 nm, i.e. at the second harmonic of a Nd:YAG laser, gives the following results. The unbiased Ampere–Watt sensitivity is 3 ⋅ 10−6 A/W, and Volt–Watt sensitivity is in the order of 0.1 mV/(W/cm2), i.e. approximately 10 times smaller than those at 445 nm. This drop of the sensitivities is obviously due to the decrease of the photon energy. In consequence, the generated hole energy reduces and the probability of its tunneling through the Schottky barrier at the Cr/Au bimetal layer–CVD diamond interface lowers. As to the near-infrared wavelength range, we measured the Ampere–Watt sensitivity in this spectral domain, at 1.06 μm to be concrete, for the other version of the photodetector. This is based on the same principle of operation, but differs from that considered in the present article by the CVD diamond structure parameters and the arrangements of the Ohmic and photosensitive contacts [20]. It is equal to 1.8 ⋅ 10−7 A/W. As was expected, this value is significantly smaller than that for 445 and 532 nm due to the reason pointed out at the end of the previous paragraph. But it is quite measurable and its registration

(1)

(2)

where v ~ EF/ m , v – hole velocity in Cr near the Fermi energy, EF~7 eV – Fermi energy in Cr, m – hole mass which is in the order of the free electron mass. From the data on Cr static conductivity with the use of the above found v we calculate that τp in the order of 3 fs. The recombination time τr is estimated by a formula

τr ~τp ℏω/ Ep,

5

said at the beginning of this section, an optimal thickness of the bimetal Cr/Au layer is ld~10 nm.

where lf is the hole free path length with respect to phonon radiation, τr/τp – the number of phonons emitted by a hole before its recombination with an electron, τr – a time of the hole recombination with an electron, τp – a time of the phonon emission. Value lf is estimated as

l f ~vτp,

0 U (mV)

Fig. 4. The photodetector current–voltage dependence. Voltage U is measured with respect to the Ohmic contact 6 in Fig. 2b. Structure static illumination with a laser radiation at wavelength 445 nm and powers 50 mW (curve 1), 90 mW (2), 120 mW (3), 220 mW (4), and 300 mW (5) was employed. The “dark” current–voltage dependence is undistinguishable from curve 1 in this scale.

Fig. 3. The structure profile around one of the Schottky contacts taken by Talysurf CCI 2000.

l d ~l f τr / τp ,

-5

(3)

where ℏ – Planck constant, ω – the frequency of detected radiation, Ep~25 meV – a characteristic phonon energy in Cr estimated from its Debye temperature. This formula follows from the condition that to recombine with an electron a hole has to lose energy ~ℏω gained from an absorbed detected photon. For which it has to emit ℏω/Ep phonons. The time interval between the acts of emission is~τp. In result for the visible and near infrared wavelength ranges ld~10 nm. The penetration depth of visible and near-infrared radiation in Cr is in the order of 10 nm, i.e. in the order of ld. So, according to what was 3

Diamond & Related Materials 97 (2019) 107444

U 0

R

1 2 Fig. 6.

-4 3 -1

0 U (V)

1

2

Oscillograph Voltage (mV)

-2

Fig. 5. The photodetector current–voltage dependence, extended in comparison with that in Fig. 4. “Dark” conditions (curve 1) and static illumination with a laser radiation at wavelength 445 nm and powers 120 mW (2) and 300 mW (3). The diode ideal factor is around 4 as it follows from the forward brunch of curve 1. The reverse brunch of this curve indicates a large leakage current which may be due to the surface conductivity of the mesa column side walls.

proves the possibility of the operation of the photodetector not only in the visible, but also in the near-infrared wavelength range. These Ampere-Watt and Volt–Watt sensitivities are to be compared to those of other diamond-based and usual semiconductor-based photodetectors in the visible and near-infrared ranges (please see Table 1). From it one can see that our photodetector even at smaller biases is much more sensitive or at least as sensitive as other diamond-based photodetectors in the visible and near-infrared ranges. But it is much less sensitive in these spectral domains than usual semiconductor-based photodetectors. So, further optimization of the photodetector structure is needed to improve its sensitivity. We also checked that the photodetector response is indeed due to the hole photo generation in thin bimetal film 7 in Fig. 2b and not due to the photo generation of charge carriers from defects such as B and N atoms or complexes in diamond. For this we etched away all the metal layers shown in Fig. 2b. Then we made contacts to substrate 1 and two mesa column tops by means of the thermo-sound welding of Al wires. After that we applied a voltage up to ± 8 V between the substrate and each mesa column successively and measured the corresponding currents in dark conditions and under illumination. For the latter we used a green laser pointer (wavelength 532 ± 10 nm, power around 500 mW). We focused its beam on the structure so that the beam

10 8

1

6

3

4

2

1

4

0

2 0 0

200 400 Time (ns)

600

Laser pulse (arb. un.)

-2

radiation

I (mA)

V.A. Kukushkin, et al.

800

Fig. 7. The oscillograms of the voltage on the resistor R in Fig. 6 for U = 0 V (curve 1) +2.5 V (2) and +5 V (3). The laser pulse is also shown (curve 4, right axes, arbitrary units).

diameter was in the order of the structure dimensions. We did not register any current amplification by the illumination. This means that the photo generation of charge carriers from defects such as B and N atoms or complexes in diamond is negligibly small. So, the photodetector response is indeed due to the hole photo generation in thin bimetal film 7 in Fig. 2b. To measure the time response of the photodetector a different from that used in the case of static illumination scheme was employed (Fig. 6). We shunted the photodetector with a resistor with a small resistance R = 50 Ω. The latter is significantly lower than 30 kΩ – the photodetector “dark” resistance calculated from the data in Fig. 4. So, we used a scheme very close to the current mode in Fig. 2b. It allowed the photodetector to recharge not only through itself but also through this resistor. This obviously increases the speed of its operation. For the

Table 1 Ampere-Watt and Volt-Watt sensitivities of diamond-based and usual semiconductor-based photodetectors in the visible and near-infrared ranges. Photodetector type

Bias, V

Wavelength, nm

Ampere-Watt sensitivity, A/W

A W–boron-doped homoepitaxial p-diamond epilayer Schottky photodiode [21] A p–i–n CVD diamond photodetector [22] A NiO/diamond photodetector [23] A photodetector based on a single crystal diamond substrate [24] A photodetector based on a CVD diamond single crystal layer [25]

2, reverse

A photodetector based on a CVD polycrystalline diamond on a Si substrate [26] A standard GaAs photocells [27]

12, forward

445 532 500 445 445 445 1000 445 1000 The red region of the visible range 532 1380

2 ⋅ 10−6 4 ⋅ 10−6 3 ⋅ 10−8 5 ⋅ 10−4 5 ⋅ 10−5 5 ⋅ 10−6 2 ⋅ 10−7 5 ⋅ 10−7 2 ⋅ 10−8

445 445 532 1060

5 ⋅ 10−5 0.5 ÷ 0.7 3 ⋅ 10−6 1.8 ⋅ 10−7

An InAs nanowire based photodetector [28] A photodetector based on an array of InP nanowires with embedded InAsP quantum discs [29] Our photodetector

0 30, forward 30 5, forward

0 2.5, forward 0 0

4

Volt-Watt sensitivity, mV/ (W/cm2)

103 ÷ 104 4.4 ⋅ 103 7 1 0.1

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Table 2 The time responses of diamond-based and usual semiconductor-based photodetectors in the visible and near-infrared ranges. Photodetector type A poly SiGe-based thermal photodetector [30] An amorphous Si-based thermal photodetector [31] A tungsten carbide (WC) or hafnium nitride (HfN)–a low boron-doped homoepitaxial p-diamond epilayer Schottky contact photodiode [21] A quantum cascade photodetector [32] A gold-patched graphene nano-stripes visible and infrared photodetectors with a time response of 20 ps [33] Our photodetector

photocurrent measurement 100 MHz Textronics 2014 oscilloscope was used. As the source of radiation we employed a pulsed Nd:YAG laser. We used its second harmonic with wavelength 532 nm, pulse duration at full width at half maximum 50 ns, pulse energy 25 μJ, and the beam diameter 2 mm. In Fig. 7 the oscillograms of the voltage on the resistor R in Fig. 6, which is proportional to the current through this resistor, is shown. We define the photodetector time response which determines the maximal speed of its operation as the time of the photocurrent double drop. For U = 0 it is approximately 600 ns. For U = +2.5 V this time response becomes 3 times shorter, 200 ns. This can be explained by the photodetector capacity C drop for positive U due to the closing of the Cr/Au bimetal layer–CVD diamond Schottky junction on the illuminated mesa column top. For larger U = +5 V the photocurrent drop time remains practically the same. This probably means that C does not change significantly with the growth of U above +2.5 V. This time response is to be compared with those of other diamondbased and usual semiconductor-based photodetectors in the visible and near-infrared ranges (please see Table 2). So, our photodetector is much faster than infrared thermal photodetectors based on the change of semiconductor resistance due to heating by absorbed radiation and a tungsten carbide (WC) or hafnium nitride (HfN)–a low boron-doped homoepitaxial p-diamond epilayer Schottky contact photodiodes for deep ultraviolet radiation. However, it is much slower than quantum cascade infrared photodetectors or gold-patched graphene nano-stripes visible and infrared photodetectors. So, further structure design optimization is needed to increase the speed of our CVD diamond photodetector operation.

Bias, V

Wavelength, nm

Time response, s

10, forward

Infrared Infrared 220

> 10−3 (2 ÷ 3) ⋅ 10−3 1

0 2.5, reverse 5, reverse

Infrared Visible and infrared 532 532 532

< 10−9 2 ⋅ 10−11 6 ⋅ 10−7 2 ⋅ 10−7 2 ⋅ 10−7

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5. Conclusions In conclusion, we demonstrated a visible and near-infrared photodetector based on CVD diamond. Due to the radiation and chemical endurance of the latter the photodetector can be employed in various applications. They include the engineering of communication and information systems in harsh environment in atomic and chemical industry and space missions. Further work is needed to adjust the photodetector parameters to its particular way of usage. Declaration of Competing Interest None. Acknowledgements The work was supported by the Institute of Applied Physics of the Russian Academy of Science (IAP RAS) (project no. 0035-2019-0003). The funding source had no involvement in the study design. References [1] A.T. Collins, Detectors for UV and far UV radiation, in: R.S. Sussmann (Ed.), CVD Diamond for Electronic Devices and Sensors, John Wiley & Sons Ltd, Chichester,

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