Optical investigation of as-grown NV centers in heavily nitrogen doped delta layers in CVD diamond

Journal Pre-proof Optical investigation of as-grown NV centers in heavily nitrogen doped delta layers in CVD diamond S.A. Bogdanov, S.V. Bolshedvorskii, A.I. Zeleneev, V.V. Soshenko, O.R. Rubinas, D.B. Radishev, M.A. Lobaev, A.L. Vikharev, A.M. Gorbachev, M.N. Drozdov, V.N. Sorokin, A.V. Akimov

PII:

S2352-4928(20)30654-1

DOI:

https://doi.org/10.1016/j.mtcomm.2020.101019

Reference:

MTCOMM 101019

To appear in:

Materials Today Communications

Received Date:

1 November 2019

Accepted Date:

20 February 2020

Please cite this article as: Bogdanov SA, Bolshedvorskii SV, Zeleneev AI, Soshenko VV, Rubinas OR, Radishev DB, Lobaev MA, Vikharev AL, Gorbachev AM, Drozdov MN, Sorokin VN, Akimov AV, Optical investigation of as-grown NV centers in heavily nitrogen doped delta layers in CVD diamond, Materials Today Communications (2020), doi: https://doi.org/10.1016/j.mtcomm.2020.101019

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier.

Optical investigation of as-grown NV centers in heavily nitrogen doped delta layers in CVD diamond S.A. Bogdanov1,*, S.V. Bolshedvorskii2,3,*, A.I. Zeleneev2,3,4, V.V. Soshenko2, O.R. Rubinas2,3,4, D.B. Radishev1, M.A. Lobaev1, A.L.Vikharev1, A.M. Gorbachev1, M.N. Drozdov5, V.N. Sorokin2,4, A.V. Akimov6,4,2 Institute of Applied Physics RAS, 46 Ul’yanov Street, Nizhny Novgorod 603950, Russia P.N. Lebedev Physical Institute, 53 Leninskij Prospekt, Moscow, 119991 Russia 3 Moscow Institute of Physics and Technology, 9 Institutskiy per., Dolgoprudny, Moscow Region, 141700, Russia 4 Russian Quantum Center, 100 Novaya St., Skolkovo, Moscow, 143025, Russia 5 Institute for Physics of Microstructures RAS, Academicheskaya Street 7, Afonino, Nizhny Novgorod Region, Kstovsky District, Kstovo Region 603087, Russia 6 Texas A & M University, College Station, TX 77843, USA * Corresponding authors: [email protected], [email protected] 1

ro

of

2

ur na

lP

re

-p

Graphical abstract

Highlights

3 nm thick delta layer with 1.4×1019 cm-3 nitrogen concentration and is achieved Optical and spin properties of NV centers, observed in the delta layer are studied Methods of concentration measurement of NV centers in delta layers are developed NV center surface concentration of 2.7-3.9 m-2 is obtained

Jo

   

Abstract

In this paper, we realize growth of the delta doped layer (3 nm thick) with high concentration of nitrogen atoms (about 1.4×1019 cm-3) during CVD diamond growth process. We experimentally investigate the distribution of the nitrogen inside the grown layer and formation of the NV centers during such growth. Using confocal microscopy, we analyze the spatial distribution of the NV centers and investigate the formation efficiency of the NV centers in delta doped layers. The spatial

distribution is measured by two methods, using second-order correlation function and emission volume analysis, and NV center concentration is found to be 3.9±0.6 and 2.7±0.2 µm-2 consequently. The divergence between the methods is discussed. As-grown NV centers formation efficiency was found to be 30 times lower than in the case of uniform doped diamond growth. Nevertheless, coherence time of an electron spin for a single NV center inside the delta layer was found to be around 1 μs which is quite reasonable given the concentration of the nitrogen in the sample.

of

Keywords: CVD diamond growth, delta doping, NV center, confocal microscopy

Introduction

Jo

ur na

lP

re

-p

ro

Creation of single NV centers, as well as arrays of NV centers localized in diamond with high accuracy, is now one of the challenging tasks, the solution of which will potentially enable a solidstate medium necessary for quantum information processing, as well as the creation of a roomtemperature sensor of magnetic and electric fields with extreme sensitivity. In most works on the topic of obtaining localized NV centers, the method of ion implantation has been used up to now [1– 3], which, despite the convenience of creating single NV centers, has a number of limitations, such as uncertainty of NV center depth, shorter spin coherence times due to local lattice deformation. Unlike ion implantation, NV centers localized in a thin layer of nanometer thickness can be obtained by the method of delta doping during the CVD growth of diamond [4–8]. To successfully apply delta doping for the creation of NV centers located at a given depth, known with nanometer accuracy, it is necessary to solve a number of scientific and technological problems. The first task is to learn how to create extremely thin (2-3 nm) nitrogen-doped layers with sharp boundaries during the CVD diamond growth and to control the growth rate of diamond with high accuracy to obtain repeatable results that do not vary from sample to sample. It is known, that CVD diamond growth rate is extremely sensitive not only to the synthesis conditions, such as pressure and composition of the gas mixture, microwave power, substrate temperature, but also to the surface structure of the diamond substrate - the crystallographic orientation of the surface and the misorientation angle [9,10]. The second task is the investigation of optical and spin properties of NV centers formed in delta layers. The two-dimensional structure of delta layer can potentially led to some peculiarities of NV center formation, which should be investigated in detail. For example, NV center formation efficiency (determined as the ratio of NV concentration to nitrogen concentration) has not been known, and also NV center spin properties need deeper investigation in the two-dimensional case. In this paper, we studied the creation of as-grown NV centers, which formed in the delta doped layer during CVD growth of diamond. In previous works, individual NV centers formed in delta layers with nitrogen concentration 1016 - 1017 cm-3 were studied [4]. Although low nitrogen contents are favorable for spin properties, CVD growth technique allows obtaining much higher doping concentrations in order to increase NV center concentration. Delta-doped layers with high nitrogen concentration (up to 1019 cm-3) were reported in our previous study [11]. However, the formation of as-grown NV centers within the layer was not confirmed and their concentration and spin properties

were not measured in the previous work. In the present study all experimental data on the optical and spin properties of individual NV centers, observed in the delta layer, is presented. The methods of optical investigation and concentration measurement of NV centers created in highly nitrogen doped layers are developed.

Sample preparation

ur na

lP

re

-p

ro

of

For the experiments, type IIa HPHT 3x3 mm2 diamond substrates with (100) orientation were used, supplied by New Diamond Technology. The substrate was preliminary studied by confocal microscopy and no NV centers were observed. Before the growth of CVD diamond, the substrates undergo preliminary treatment to obtain smooth and defect-free surfaces, which includes mechanical polishing and subsequent ICP plasma etching to remove polishing defects. This procedure was described in detail in the work [12]. The sample growth was carried out using a CVD reactor described in [13]. In this reactor rapid gas switching and laminar gas flow make it possible to rapidly change the gas mixture, which allows growing doped layers with sharp boundaries, including 1-2 nm thick delta layers. In our experiments, we employ delta doping of the nitrogen atoms to investigate the formation of as-grown NV centers during CVD diamond growth in the two-dimensional case. Nitrogen doped layer was grown by adding natural nitrogen gas with the ratio N/C=5.7 during CVD growth. The following growth conditions were used for the synthesis of delta layers: pressure 40 Torr, hydrogen flow 950 sccm, methane flow 1.4 sccm, microwave power was 1.3 kW at a frequency 2.45 GHz. The growth rate is about 160 nm/h at these conditions. The misorientation angle was 5.1o, surface roughness was 0.3 nm before growth and 0.35 nm after CVD growth. The total thickness of the grown epitaxial layer was about 100 nm. In the middle of the CVD diamond growth process, 4 sccm of N2 was added to the gas mixture for a short time for the synthesis of delta layer. The level of nitrogen in the gas phase was monitored by optical emission spectroscopy by measuring the CN emission, Fig. 1a. Fig. 1b shows the time-resolved CN intensity during the delta-layer growth. As a result, delta layer is located at the depth of about 50 nm below the substrate surface.

Jo

TOF·SIMS-5 by ION-TOF (Münster, Germany) mass spectrometer was used to investigate the nitrogen concentration and the doping profile (see Fig. 1c) by secondary ion mass spectrometry (SIMS). The sensitivity of SIMS in relation to nitrogen was limited to a value of ~21018 cm-3. Delta layer width was determined based on the SIMS data using the profile recovery procedure, which takes into account SIMS depth resolution function [9]. Delta layer is 3 nm thick and nitrogen concentration is about 1.4×1019 cm-3 (see green line on Fig. 1c). Analysis of the nitrogen profile of the delta-layer by SIMS was carried out in the mode which provides the best resolution in depth. It was capable to prove the localization of nitrogen atoms in a 3 nm thick layer. It should be noted, that the noise level in this mode is too high to determine the background nitrogen concentration outside the delta-layer. However, background nitrogen concentration could be estimated from the following consideration. The pure hydrogen (7 N) and methane (5.5 N) with gas purifiers for both gases were used. We estimate the upper limit of background nitrogen concentration in the gas phase as 200 ppb taking into account gas purity and leakage [20]. Very low nitrogen background level was confirmed by optical emission data obtained on CN radical emission (see Fig. 1a,b). Using the estimated value of background nitrogen concentration and assuming linear nitrogen incorporation dependence we

obtain the upper limit of nitrogen concentration outside the delta layer as 71014 cm-3. Therefore, we conclude that observed NV centers are located inside the delta layer.

(b)

-p

ro

of

(a)

Jo

ur na

lP

re

(c)

Fig. 1 (a) CN emission spectra during the delta layer growth (black line) and during the undoped layer growth (blue line); (b) Time-resolved CN emission intensity during the delta layer growth; (c) Nitrogen concentration profile determined by SIMS (blue dots); nitrogen concentration profile after recovered procedure using depth resolution function (green line); convolution of recovered profile and depth resolution function (red dashed line).

In this work, the sample was grown with extremely high concentration in order to obtain as high as possible as-grown NV concentration. Nevertheless, in our growth technique, the nitrogen concentration can be controlled within the wide limits 1015-1019 cm-3 adjusting the nitrogen flow. For any particular application of NV centers, e.g. magnetic sensing, both the sufficient fluorescence intensity and good enough spin properties are needed, and the optimal nitrogen amount in the gas during the CVD growth could be experimentally determined. The sample has not been irradiated by electron or ion beams for the creation of vacancies.

Results and discussion

ro

of

Nitrogen doped delta layers are very convenient medium for observing single and also “double” and “triple” NV centers that can be easily distinguished using confocal microscopy. All NV centers in such sample are located in a very thin layer, which makes it possible to easily distinguish single NV centers at high nitrogen concentrations (up to 1019 cm-3), while for thick uniformly nitrogen doped sample at the same concentration it is unreachable.

-p

Investigation of NV centers formation in the delta-doped layers was started with the analysis of SIMS results. According to SIMS measurements the thickness of delta layer is 3 nm and nitrogen concentration is about 1.41019 cm-3 centered at H=48.6 nm depth (see Fig. 1).

re

Direct measurements as-grown NV concentration

Jo

ur na

lP

For a direct measurement of NV centers concentration in the delta layer, a second order autocorrelation function measurements were used in each confocal spot of the investigated area on the sample containing optical active NV centers.

of ro -p re lP ur na Jo

Fig. 2 (a) Scheme of the confocal setup, inset depicts the image of the microscope calibration slide. (b) Example of the confocal map used for direct measurements of the NV centers concentration. (c)(f) g 2 (  ) for 1-4 NV centers in a confocal spot respectively.

Second order autocorrelation function measurements conducted on home built confocal microscope (see Fig. 2(a)) employing high numerical aperture oil objective (Nikon Apo TIRF 100X NA 1.49) with working distance 120 micrometers. A continuous diode laser (Coherent Compass 300, wavelength 532 nm) served as the source of NV excitation and galvo mirrors (Cambridge Technologies) were used as a scanning element, enabling a 80×80 micrometer field of view for our microscope design. We make a calibration of the confocal setup using the micrometer calibration

ro

of

slide with 10 µm step size (see inset on Fig. 2(a)). This way we can measure the dependence of the voltage on the galvo mirrors with the coordinate. For our microscope design we obtain 8  0 . 6  m V , error of estimation were taken as a half of the scale. The collected fluorescence was coupled into a Hunbury-Brown and Twiss (HBT) interferometer, which consisted of two avalanche photodiodes (PerkinElmer SPCM-AQRH-14-FC) and a 45:55 beam splitter. We used the combination of an optical notch filter with a stop-band centered at 532 nm and longpass optical filter with cut-off at 600 nm to remove the residual green excitation light and Raman signal from the collected emission. A time-correlated single photon counting module (Picoquant Picoharp 300) was used to obtain second-order photon correlation histograms from NV centers. The step size for our confocal setup is 0.04 μm, this way we ordinary investigate the 4×4 μm2 area (see Fig. 2(b)) to estimate the as-grown NV centers concentration. On the confocal map 1-4 NV centers for different bright spot containing NV centers were observed (see Fig. 2(c-f)). For each spatially resolvable bright spot we were able to measure its properties, including number of NV centers. The NV number measurement was performed by measuring the amplitude of the second order correlation function g 2 ( 0 ) . To exclude the effect of background noise on these

-p

measurements, we used a noise compensation technique [14]. The amplitude of the second order correlation function at zero time delay g 2 ( 0 ) is described by the next equation: n n  1 2

,

1

re

g2 0  

n

lP

In Eq. 2, n is the photon number. Since a bright spots can have more than one color center, g 2 ( 0 ) may differ from 0. In a bright spot possessing N NV centers, and assuming each NV center is an identical and independent emitter, the photon number state emitted by such an ensemble will be:

ur na

n 



2

N

N



k

2

CN k .

k 0

Here  is probability of photon emission in given time interval. This expression leads to a threshold of the number of NV centers for known value of g 2 ( 0 ) :

g2 0  

n n  1 n

Jo

N

2

 k k  2

N  1,

g2 0   0

N  2,

g2 0   1 / 2

N  3,

g2 0   2 / 3

N

 1CN k

k 1

 k    kC N   k 1  N

2



N 1 N

3

...

Here

k

CN

are binomial coefficients. These thresholds is a powerful tool in the case of delta

doped layers due to the fact that NV centers in <100> oriented diamond plate have the similar angle respect to the surface and the depth of the as grown NV centers can’t vary from each other no more

than delta layer thickness (3 nm in our case). The lifetime of each NV center in bulk diamond has a stable value about 12 ns [15], so therefore excitation and emission probabilities are the same for different NV’s. Nevertheless, the background subtraction method that we used to remove noise is not perfect and some residual level of g 2 ( 0 ) may be left from the substrate emission, masking actual level of second order correlation function[14]. Finally, finite time resolution and non-zero excitation power also can contribute to the measured g 2 ( 0 ) value [14]. These imperfections could overstate the number of NV center giving us an upper bound for a NV concentration. Therefore, we use solution of Eq. 3 to estimate the actual number of color centers in each bright spot on the confocal scan as: 1



N Vi

4

1  g 2 (0 )i

of

N

As an absolute error of estimation the number of NV center using we taking next assessments: N Vi



 g 2 (0 )i (1  g 2 ( 0 ) i )

ro

N

2

5

N

N V to ta l





-p

Then we find total quantity NV centers on the 3 typical confocal maps (see Fig. 2(b)) as a sum from each bright spot: N N V  1 8 6  3 0 .1 2 , i

re

i

6

To calculate the surface concentration of as-grown NV centers we use the next formula: N

N V to ta l

hS



 1   3 .9  2  48  m 

186

lP

n NV 

,

7

ur na

where S is the total area of the 3 confocal maps. To calculate the error of NV surface concentration estimation we use next expression:  n NV 

2

  N NV   S  N    2 S S   

NV

  

2

 1   0 .6  . 2   m 

8

Jo

Taking into account the total area of confocal scan about 48 µm2 we can conclude that about 3.9±0.6 as-grown NV centers per µm2 were formed in the delta layer during the growth process. In comparison, in the work [7] one NV center per 5 µm2 was found in the delta layer. To calculate NV center formation efficiency, the found value n N V = 3.9 µm-2 should be divided by the surface density of nitrogen ( n N = 3nm×1.4×1019 cm-3 = 4.2×104 µm-2). Thereby we can determine as-grown NV centers formation efficiency like: E ff 

nNV   nNV

 1 0 0 %  0 .0 0 9  0 .0 0 1 % .

9

nN

The described method is straightforward and can be used for measurement of NV centers formation efficiency in the delta layer. Nevertheless, this method is time consuming, because ordinary measurements of g 2 (  ) autocorrelation function demands around 300 second, so for the

investigation of whole map you need to spend a lot of time. Another disadvantage, which were previously discussed, is overstatement of the NV concentration due to measurements imperfections. As a limitation this method requires working with samples containing optically distinguishable spots with color centers and you should measure autocorrelation functions, therefore it can be applied to samples with relatively low concentration of color centers. In order to avoid these issues, we also developed and applied another method of calculating NV center concentration in delta layers.

NV centers concentration measurements using emission volume analysis

Jo

ur na

lP

re

-p

ro

of

Developing the delta-doping process for the creation of novel diamond based devices and technologies requires robust technique for NV concentration measurements. So, the direct measurement procedure is rather time consuming and we developed an alternative method for fast and more robust concentration measurements based on the emission volume analysis.

Fig. 3 (a) The confocal map from the random area on a sample. (b)The confocal map of single NV center. (c) Saturation curve for optical volume for single NV center. (d) Saturation curve for integral intensity of all NV centers on the confocal map. This technique based on the normalization of the whole intensity from the confocal map to the single NV center fluorescence intensity. The method of emission volume analysis required knowledge about the sample orientation which affects relative brightness of color centers. The

implementation of such a technique help to handle with concentration measurements in a fast way in the case of delta layers and <100> oriented diamond plates, which are commonly uses for a CVD growth process. Here, the confocal maps (with area of 4×4 µm2, see Fig. 3(a)) were measured at different pump powers of the laser and its optical volume was calculated, which corresponds to the integral intensity from the area of interest, using the next formula:  p h o to n s   m  I ( x, y )dxdy   , s   2

OV 

 

where

I ( x, y) 

 p h o to n s    s  

- the intensity at a point with the

10 Cartesian coordinates on the

( x, y)



O V s in g le N V  P

OV (P )  P 

– linear noise coefficient,

P

 P

2

– pump power of the laser,

,

11

 p h o to n s   m   O V s in g le N V    s   2

-

lP

where





 p h o to n s   m     s  

re

P

-p

ro

of

map,  – the square of studying area. Basis of our model is the fact that single NV center has a Gaussian irradiation pattern, so we can calculate the total optical volume from the one single NV center with different pumping conditions and with different area size. In this way, we measured the saturation curve for optical volume of the spot with single NV center (see Fig. 3(b)), to understand the saturation intensity of whole optical volume we used the following formula:

ur na

optical volume of the spot with single NV center at saturation, P  – saturation power. Then, we can calculate the saturation intensity for the different sizes map (0.200*0.200 µm2, 0.280*0.280 µm2, 0.360*0.360 µm2, 0.440*0.440 µm2, 0.520*0.520 µm2) with the same single NV center (see Fig. 3(b)). We implement this step to obtain optical volume of single center without dependence on space of the integrated area and to subtract the noise of the substrate. To analyze saturation of optical volume depending on area of integration we use the next formula: 



O V s in g le N V (  )  O V s in g le N V  e r f (

2

 p h o to n s   m   4 O V s in g le N V  ( 0 . 9 0  0 . 0 5 )  1 0   s  

Jo

where

 p h o to n s   m   )      s  

,

12

2

- desired optical volume, which correspond to

the integral radiation from the single NV center at saturation with closest neighbor far from decay of fluorescence (see Fig. 3(c)). The error estimation was taken as mean root square difference to the model function. Finally, we can measure the total radiation from the whole map (with square of 4*4 µm2) (see Fig. 3(a)) with single and multiple NV centers at different pump power of laser. The optical volume from the whole NV centers at saturation from the map can be estimated by the next equation:



O V to ta lN V P

O V to ta lN V ( P )   ' P   p h o to n s   m  (P )    s  



P'  P

 p h o to n s   m    , s   2

13

2

where P'



O V to ta lN V

 0 .2 7  0 .0 1  m W

– optical volume from the whole map at power of laser

 – the power of pumping laser at saturation,

 p h o to n s   m   4 O V to ta lN V  ( 3 8 . 7 6  0 . 5 5 )  1 0   s  

'

P

,

– linear noise coefficient,

2

– the sought-for optical volume characterizing the

N to ta lN V 

O V to ta lN V





O V s in g le N V

4

3 8 .7 6  1 0 0 .9 0  1 0

4

     num berN V  



ro

 p h o to n s   m  s   2  p h o to n s   m  s 2



of

integral luminescence from whole NV centers at saturation on the map (see Fig. 3(a)). The error estimation was taken as mean root square difference to the model function. This knowledge let us to define the number of NV centers at studying map:

43.

14

 N to ta lN V 

2





   O V s in g le N V  O V to ta lN V    2   ( O V s in g le N V )  

re

 OV  to ta lN V   O V  s in g le N V 

-p

To calculate the error of total number of NV centers on the map we use next expression:    

2

 2 .4 7 .

15

lP

To estimate the surface density of as grown NV centers we use the next formula: n NV 

N

N V to ta l

S



43

16m

2

 1   2 .7  , 2   m 

16

ur na

where S – area of the whole confocal map. To calculate the error of NV surface concentration estimation we use next expression: 2

 n NV 

  N to ta lN V    S  N to ta lN V      2 S S    

2

 1   0 .2  . 2   m 

Thereby we can determine the as-grown NV centers formation efficiency like:

Jo

E ff 

n NV   nNV

 1 0 0 %  0 .0 0 6 4  0 .0 0 0 5 % ,

17

nN

where surface density of nitrogen is nN = 3nm×1.4×1019 cm-3 = 4.2×104 µm-2. As a result, the developed technique based on the measuring of optical volume gives a bit lower NV concentration. In our opinion, this method overcome the disadvantages of the g 2 (  ) concentration measurements due to the fact that statistics exclude the possible overlapping of the different bright spots, also we believe that substrate noise compensation technique works better for a large areas of estimation and there is no problem related to the finite time resolution of the detectors and saturation influence. This lead to an important advantage of the emission volume analysis technique which is the suitability for a low and high concentration samples due to the fact there is no limitations such as optical distinguishing or that can be problem to high concentration samples. The

-p

ro

of

main error of the concentration measurement is due to the uncertainty of the confocal map sizes. Generally, to increase the speed of the concentration analysis the scanner resolution for the analysis of optical volume from a single center and whole map can be different. For a single center the scanning step should be taken to provide enough points for a saturation analysis using Eq. 11 and Eq. 12. For a whole confocal map you can take resolution twice better than the confocal spot for a given optical system. It is interesting to note, that the obtained value is 30 times lower than formation efficiency of NV centers found in the uniformly nitrogen doped sample which was grown under similar conditions. Efficiency of NV centers formation was 0.2% in that case[16]. The possible explanation of this result could be related to the differences of vacancy formation in the two-dimensional case of delta layer. In our slow growth regime, CVD diamond grows in the form of successively filling steps (terraces), and the step height is equal to the distance between atomic layers. As a result, surface roughness is very low, as well as lattice defect formation. However, this growth mode is affected by nitrogen addition, which should lead to the increased defect formation at high nitrogen flows. Thus, it is likely that vacancy concentration is much higher in the heavily nitrogen doped CVD layer, while in the case of delta layer the adjacent layers are high-quality nitrogen-free layers with lower vacancy content. The diffusion of vacancies out of the delta layer during the growth process could result to the decrease of NV center formation efficiency.

re

Electron spin properties of as grown NV centers in delta doped layers

Jo

ur na

lP

To investigate as-grown NV centers spin properties in the delta layer, the electron spin coherence times measurements were performed for single NV centers in delta doped layers. We began measurements on the spin properties of NV centers in delta-doped layers by employing an optically detected magnetic resonance (ODMR) regime. A permanent magnet in front of the diamond substrate was used to separate magnetic sublevels.

of ro -p re

lP

Fig. 4 (a) Pulse sequence for Rabi oscillation experiments with NV center. (b)Pulse sequence for Hanh echo experiments for NV center. (c) Rabi oscillation measurements for single NV center in delta layer. (d) Hanh echo measurements for NV center in delta layer.

ur na

Once the magnetic sublevel was isolated, we performed coherent manipulation of the electron spin. First, we measured Rabi oscillations of the spin under excitation by a microwave field. To measure Rabi oscillations, we varied the pulse duration and measured number of counts at two different times: right after the microwave pulse S M W and after spin polarization with green light but without the microwave pulse

S ref

(see Fig. 4(a)). Before the microwave pulse we waited for

Jo

Tdark=300 ns for relaxation of a dark state[17]. The observed spin contrast C was calculated using next equation and plotted (see Fig. 4(c)):

To obtain a

*

T2

C  2

S ref  S M W

18

S ref  S M W

time for an electron spin of the single NV the Rabi oscillation for a single NV

center were fitted by the next model: 

ae

where Ω is the Rabi frequency,

*

T2

t  t0 *

T2

 s in  ( t  t 0 )      b ,

is effective coherence time,

19 t0

is technical delay, t is time, α is

amplitude and b is a level of completely decayed Rabi oscillations. The Rabi oscillations were found to have a frequency of 4.93 MHz, corresponding to π pulse of 101 ns in duration and an effective coherence time T 2* of 360 ns for a single NV center.

From Rabi oscillation we obtained a duration of a π pulse that allowed us to measure the coherence time T 2 . We implement a Hahn echo sequence (see Fig. 4(b)). To measure spin echo signal we varied S MW

e 2

free precision time and collect the signal right after



 

2

and after polarization by the green light and



 

2

3

pulse sequence

2



pulse sequence

2 S ref

. Hanh echo signal

(see Fig. 4(d)) was fitted using the next model:

where a- signal level,

e



– free precision time,

 b,

T2

20 – coherence time, α-fit parameter,

signal. For NV centers in delta-doped layer we obtained

T2

b

– offset

of

ae

 2 e     T2 

coherence time about 920 ns. It is well

T2

n N  1 .4  1 0

19

 1  3    cm 

, so we can expect

T2

coherence time about 1.95 μs. However,

-p

concentration is

ro

known fact that NV centers in a CVD diamonds plates have limit for coherence time T 2  1 6 5  s  p p m / n N , where n N – nitrogen concentration[18]. In present experiment nitrogen

depends on the angle of the external magnetic field [19]. In our experiment angle between NV

re

centers quantization axes and magnetic field was set to about 45° thus leading to approximately 2 times reduction in T 2 time.

ur na

lP

Thus, we conclude there is no significant difference in coherence properties of the NV centers in delta doped layer in respect to the homogeneously doped diamond plates. It should be noted, that spin properties of NV centers formed in delta layers doped with lower nitrogen contents (e.g. ~ 1 ppm) could potentially differ from the uniformly doped case and require thorough future investigation.

Summary

Jo

Nitrogen delta doping during CVD diamond growth on <100> oriented diamond substrate is investigated. The thickness of the layer is 3 nm, nitrogen concentration is 1.4×1019 cm-3. The concentration of as-grown NV centers was determined by two methods. The first method is based on measurment of second order autocorrelation function. The second method utilizes confocal microscop scans based techniques, which we developed for delta doped layers. Within the delta layer, 1-4 NV centers per spot were observed, which can be optically distinguish by a confocal microscope. The estimations for NV center formation efficiency gives about 0.0064% nitrogen atoms in a layer which form an optically active NV center during growth process, which is 30 lower than for a uniform doped diamond plates. The decreased NV center formation efficiency could be explained by peculiarities of vacancy formation during delta layer growth. In the slow growth mode, the defect formation is low. Thus, in the case of delta layer the adjacent layers are high-quality nitrogen-free layers with lower vacancy content. In addition, spin properties of the single NV centers in delta layers were investigated. The measured value of T 2 was found to be about 1 μs,

which is in good agreement with the nitrogen concentration in this samples. Delta doping allows creating shallow NV centers, which are highly desirable for many modern applications, that rely on the strong dipolar coupling, afforded by NV center electron spin located in close proximity to an external spin of interest. The low conversion efficiency in delta layers and uniformly doped diamond plates requires more detailed investigation. Declaration of interests

of

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

ro

The study was performed by a grant from the Russian Science Foundation (Project no. 16-1900163).

-p

References

I. Aharonovich, C. Santori, B.A. Fairchild, J. Orwa, K. Ganesan, K.M.C. Fu, R.G. Beausoleil, A.D. Greentree, S. Prawer, Producing optimized ensembles of nitrogen-vacancy color centers for quantum information applications, J. Appl. Phys. 106 (2009) 1–7. https://doi.org/10.1063/1.3271579.

[2]

B. Naydenov, F. Reinhard, A. Lämmle, V. Richter, R. Kalish, U.F.S. D’Haenens-Johansson, M. Newton, F. Jelezko, J. Wrachtrup, Increasing the coherence time of single electron spins in diamond by high temperature annealing, Appl. Phys. Lett. 97 (2010) 10–13. https://doi.org/10.1063/1.3527975.

[3]

T. Staudacher, F. Ziem, L. Häussler, R. Stöhr, S. Steinert, F. Reinhard, J. Scharpf, A. Denisenko, J. Wrachtrup, Enhancing the spin properties of shallow implanted nitrogen vacancy centers in diamond by epitaxial overgrowth, Appl. Phys. Lett. 101 (2012). https://doi.org/10.1063/1.4767144.

[4]

K. Ohno, F. Joseph Heremans, L.C. Bassett, B.A. Myers, D.M. Toyli, A.C. Bleszynski Jayich, C.J. Palmstrøm, D.D. Awschalom, Engineering shallow spins in diamond with nitrogen delta-doping, Appl. Phys. Lett. 101 (2012). https://doi.org/10.1063/1.4748280.

Jo

ur na

lP

re

[1]

[5]

J.C. Lee, D.O. Bracher, S. Cui, K. Ohno, C.A. McLellan, X. Zhang, P. Andrich, B. Alemán, K.J. Russell, A.P. Magyar, I. Aharonovich, A. Bleszynski Jayich, D. Awschalom, E.L. Hu, Deterministic coupling of delta-doped nitrogen vacancy centers to a nanobeam photonic crystal cavity, Appl. Phys. Lett. 105 (2014). https://doi.org/10.1063/1.4904909.

[6]

K. Ohno, F. Joseph Heremans, C.F. De Las Casas, B.A. Myers, B.J. Alemán, A.C. Bleszynski Jayich, D.D. Awschalom, Three-dimensional localization of spins in diamond using 12C implantation, Appl. Phys. Lett. 105 (2014). https://doi.org/10.1063/1.4890613.

[7]

C. Osterkamp, J. Lang, J. Scharpf, C. Müller, L.P. McGuinness, T. Diemant, R.J. Behm, B. Naydenov, F. Jelezko, Stabilizing shallow color centers in diamond created by nitrogen deltadoping using SF6 plasma treatment, Appl. Phys. Lett. 106 (2015).

https://doi.org/10.1063/1.4915305. [8]

S. Cui, A.S. Greenspon, K. Ohno, B.A. Myers, A.C.B. Jayich, D.D. Awschalom, E.L. Hu, Reduced plasma-induced damage to near-surface nitrogen-vacancy centers in diamond, Nano Lett. 15 (2015) 2887–2891. https://doi.org/10.1021/acs.nanolett.5b00457.

[9]

M.A. Lobaev, A.M. Gorbachev, A.L. Vikharev, V.A. Isaev, D.B. Radishev, S.A. Bogdanov, M.N. Drozdov, P.A. Yunin, J.E. Butler, Investigation of boron incorporation in delta doped diamond layers by secondary ion mass spectrometry, Thin Solid Films. 653 (2018) 215–222. https://doi.org/10.1016/j.tsf.2017.12.008.

of

[10] A. Tallaire, M. Lesik, V. Jacques, S. Pezzagna, V. Mille, O. Brinza, J. Meijer, B. Abel, J.F. Roch, A. Gicquel, J. Achard, Temperature dependent creation of nitrogen-vacancy centers in single crystal CVD diamond layers, Diam. Relat. Mater. 51 (2015) 55–60. https://doi.org/10.1016/j.diamond.2014.11.010.

ro

[11] M.A. Lobaev, A.M. Gorbachev, S.A. Bogdanov, A.L. Vikharev, D.B. Radishev, V.A. Isaev, V. V. Chernov, M.N. Drozdov, Influence of CVD diamond growth conditions on nitrogen incorporation, Diam. Relat. Mater. 72 (2017) 1–6. https://doi.org/10.1016/j.diamond.2016.12.011.

re

-p

[12] A.B. Muchnikov, A.L. Vikharev, J.E. Butler, V. V. Chernov, V.A. Isaev, S.A. Bogdanov, A.I. Okhapkin, P.A. Yunin, Y.N. Drozdov, Homoepitaxial growth of CVD diamond after ICP pretreatment, Phys. Status Solidi Appl. Mater. Sci. 212 (2015) 2572–2577. https://doi.org/10.1002/pssa.201532171.

lP

[13] A.L. Vikharev, A.M. Gorbachev, M.A. Lobaev, A.B. Muchnikov, D.B. Radishev, V.A. Isaev, V. V. Chernov, S.A. Bogdanov, M.N. Drozdov, J.E. Butler, Novel microwave plasmaassisted CVD reactor for diamond delta doping, Phys. Status Solidi - Rapid Res. Lett. 10 (2016) 324–327. https://doi.org/10.1002/pssr.201510453.

ur na

[14] V. V. Vorobyov, A.Y. Kazakov, V. V. Soshenko, A.A. Korneev, M.Y. Shalaginov, S. V. Bolshedvorskii, V.N. Sorokin, A. V. Divochiy, Y.B. Vakhtomin, K. V. Smirnov, B.M. Voronov, V.M. Shalaev, A. V. Akimov, G.N. Goltsman, Superconducting detector for visible and near-infrared quantum emitters [Invited], Opt. Mater. Express. 7 (2017) 513–526. https://doi.org/10.1364/OME.7.000513. [15] P. Tamarat, T. Gaebel, J.R. Rabeau, M. Khan, A.D. Greentree, H. Wilson, L.C.L. Hollenberg, S. Prawer, P. Hemmer, F. Jelezko, J. Wrachtrup, Stark shift control of single optical centers in diamond, Phys. Rev. Lett. 97 (2006) 1–4. https://doi.org/10.1103/PhysRevLett.97.083002.

Jo

[16] M.A. Lobaev, A.M. Gorbachev, S.A. Bogdanov, A.L. Vikharev, D.B. Radishev, V.A. Isaev, M.N. Drozdov, NV-Center Formation in Single Crystal Diamond at Different CVD Growth Conditions, Phys. Status Solidi Appl. Mater. Sci. 215 (2018) 1–7. https://doi.org/10.1002/pssa.201800205. [17] A. Dreau, M. Lesik, L. Rondin, P. Spinicelli, O. Arcizet, V. Jacques, Avoiding power broadening in optically detected magnetic resonance of single NV defects for enhanced dc magnetic field sensitivity, Phys. Rev. B. 195204 (2011) 1–8. https://doi.org/10.1103/PhysRevB.84.195204. [18] E. Bauch, C.A. Hart, J.M. Schloss, M.J. Turner, J.F. Barry, P. Kehayias, S. Singh, R.L. Walsworth, Ultralong Dephasing Times in Solid-State Spin Ensembles via Quantum Control,

Phys. Rev. X. 8 (2018) 28–30. [19] P.L. Stanwix, L.M. Pham, J.R. Maze, D. Le Sage, T.K. Yeung, P. Cappellaro, P.R. Hemmer, A. Yacoby, M.D. Lukin, R.L. Walsworth, Coherence of nitrogen-vacancy electronic spin ensembles in diamond, Phys. Rev. B. 82 (2010) 7–10. https://doi.org/10.1103/PhysRevB.82.201201.

Jo

ur na

lP

re

-p

ro

of

[20] A.L. Vikharev, M.A. Lobaev, A.M. Gorbachev, D.B. Radishev, V.A. Isaev, S.A. Bogdanov, Investigation of homoepitaxial growth by microwave plasma CVD providing high growth rate and high quality of diamond simultaneously, Materials Today Communications 22, (2020), 100816. https://doi.org/10.1016/j.mtcomm.2019.100816