Diamond Pressure and Temperature Sensorsfor High-Pressure High-Temperature Applications

A. M. Zaitsev1) (a), M. Burchard (a), J. Meijer (a), A. Stephan (a),

B. Burchard (b), W. R. Fahrner (c), and W. Maresch (a)

(a) Institut fur Mineralogie, Ruhr-Universitat Bochum, Gebaude NA 03/168,Universitatsstrasse 150, D-44801 Bochum, Germany

(b) Infineon AG, Munchen, Germany

(c) University of Hagen, D-59095 Hagen, Germany

(Received December 13, 2000; in revised form March 20, 2001; accepted March 20, 2001)

Subject classification: 61.72.Ww; 68.60.Bs; 68.60.Dv; 73.40.Lq; S5

A high-pressure high-temperature diamond based sensor has been developed for the use indiamond anvil cells. The electronic structure of the sensor is that of p–i–p unipolar diode withboron doped p-type regions separated by an i-region containing compensated boron acceptors. Thesensor has been fabricated by high-temperature high-energy boron ion implantation followed byhigh-energy carbon ion irradiation on the working surface of an anvil made of type IIa naturaldiamond. The performance of the sensor has been tested at pressures up to 70 kbar and tempera-tures up to 800 �C. The sensor has a resolution of 5 bar for pressure and of 0.01 �C for temperature.

1. Introduction

Precise measurements of pressure and temperature in diamond anvil cells (DAC) is asignificant problem restricting the use of this method. In particular, it concerns pressuremeasurements. Usually, pressure in diamond anvil cells is measured by the pressureinduced shift of luminescence or Raman lines of probe samples placed into the highpressure chamber [1]. A disadvantage of this spectroscopic method is that the probesample may strongly affect the impurity content of the investigated substance. Besides,the accuracy of the spectroscopic measurements is rather low because of strong broad-ening of optical lines with increasing pressure and temperature.

An effective solution of this problem could be an electronic micro-sensor depositedonto the working surface of diamond anvil and covered with a protective diamond film.To implement this idea one needs an electronic structure which must be, firstly, sensi-tive for pressure and temperature and, secondly, mechanically stable enough to standhigh pressures developing between the diamond anvils. It is also important that theprotective diamond film can survive under high pressure without being delaminatedfrom the anvil. Obviously, the most straightforward way to fabricate such a sensor is adeep boron ion implantation into the bulk of the anvil. Indeed, p-type boron-dopeddiamond is known as a good piezoresistive semiconductor [2, 3]. Boron doped p–i–pdiamond diode may exhibit even higher piezo-sensitivity than the ordinary p-type piezo-resistor [4]. An advantage of boron as an implant is its low atomic mass. Light boronions penetrate into diamond down to a depth of a few micrometers when accelerated to

1) Corresponding author; Tel.: +49 234 32 24577; Fax: +49 234 32 14433;e-mail: alexander.zaitsev@ruhr-uni-bochum.de

phys. stat. sol. (a) 185, No. 1, 59–64 (2001)

an energy of a few MeV, and they do not cause strong radiation damage. Thus, theupper layer of the boron-irradiated area remains almost intact in terms of its mechan-ical strength and serves as a perfect protective cover.

2. Experimental Details

The pressure–temperature sensor was fabricated on the working surface of an anvilmade of type IIa natural diamond (Fig. 1). The sensing region of the sensor is a buriedvertical three-layer p–i–p diode. The p-type layers were implanted with boron ions of

60 A. M. Zaitsev et al.: Diamond Pressure and Temperature Sensors

Fig. 1. Diamond anvil with pressure–temperature sensor made by ion implantation. The cross-sec-tion of the sensing area of the sensor is shown in more detail. The depth distributions of the pri-mary radiation damage and implanted boron have been calculated by TRIM-96 code. It is seenthat the two ion-doped layers are connected with a boron “tail” of a concentration above1019 cm––3. The acceptor concentration in this “tail” may be as high as 1017 cm––3. In order to com-pensate these acceptors 5 MeV carbon ion irradiation has been performed

energies 0.8 and 1.5 MeV at doses of 2 � 1016 and 4 � 1016 cm––2, respectively. To avoidion-induced graphitization and to increase the optical transparency of the implantedarea, both implantations were carried out at a temperature of 1100 �C. Ohmic contactsto the boron-doped layers were made with boron ion irradiation of the same energies,but this time at much higher doses (above 2 � 1017 cm––2 each) and intentionally atroom temperature in order to achieve graphitization. The implanted anvil was annealedin two steps at 1400 and 1500 �C for 2 h each. Further improvement of the diode per-formance has been found after additional irradiation with 5 MeV carbon ions.

3. Results and Discussion

High temperature implantation of boron ions was used to reduce the concentration ofradiation defects and to promote the electrical activation of the implants. ConfocalRaman measurements with spatial resolution of 2 mm was performed to monitor thecrystallinity and the phase content of the irradiated regions. The only Raman feature ofthe spectra was the diamond Raman line at about 1331 cm––1. The spectral position ofthe line was almost unchanged in the area irradiated with 1.5 MeV ions, whereas in the0.8 MeV area it shifted down by ––1.2 cm––1. In contrast, the line width was slightlyincreased (by +0.5 cm––1) in the 0.8 MeV area, but it was considerably broader in the1.5 MeV area (by +2.7 cm––1). Taking into account that the stress-induced shift of thediamond Raman line is of +3 cm––1/GPa [5] one can evaluate the quasi-hydrostatic andnonhomogeneous stress in the irradiated layers. It is seen that the main stress develop-ing in the 0.8 MeV layer is a tension of 0.4 GPa. In contrast, the 1.5 MeV layer ismainly under nonhomogeneous stress of 0.45 GPa. Though the internal stress in thearea of the sensor is relatively high, it has been found to cause no effect on the opera-tion of the p–i–p structure as the pressure sensor.

Electrical conductivity of the boron-implanted layers is presented in Fig. 2. It in-creases drastically with annealing. The resistances of the as-implanted layers wererather high: 60 and 120 MW for 1.5 and 0.8 MeV implantations, respectively. The as-implanted structure revealed no diode behavior at all. The Ohmic resistance of theregion between the boron-doped stripes was very low as compared with that of thestripes and it could not be reasonably evaluated. The total resistance of the sensor(between the terminals 1 and 2 in Fig. 1) in the as-implanted state was about 100 MW.After annealing at 1400 �C the resistance of the sensor was reduced down to 5 MW.The final annealing at 1500 �C resulted in a resistance as low as 100 kW (Fig. 2). Thespecific resistivity of the boron doped stripes in the latter case is evaluated to be about0.03 W cm.

However, these strong changes in the resistance of the implanted layers have notaltered the behavior of the current–voltage characteristic of the diode structure: in allthe cases it is pure Ohmic law. To account for such a behavior one has to assume thatthe distribution tails of the implanted boron is strong enough to overcompensate theradiation-induced electrically active defects (Fig. 1). Thus, a p+–p–p+ structure was ac-tually formed which is characterized, naturally, by the Ohmic conductivity.

Figure 2 shows that the subsequent carbon irradiation reduces the Ohmic conductiv-ity of the structure and results in an explicit nonlinear current–voltage characteristic.This indicates formation of an improved p–i–p diode due to compensation of theboron acceptors between the p-type layers.

phys. stat. sol. (a) 185, No. 1 (2001) 61

Performance of the structure as a pressure sensor was tested by loading the anvil in aspecially designed DAC. Figure 3a shows that the piezo-response increases strongly afterirradiation with carbon ions, implying that the p–i–p diode is much more pressure-sensi-tive than p-type diamond resistor. The calibration of the pressure sensor was performedagainst the shift of the diamond Raman line measured by a confocal spectrometer in thearea of the sensor (Fig. 3b). It is seen that the Raman method cannot provide precise pres-sure measurements because of strong broadening and distortion of the diamond Ramanline. The accuracy of the Raman method can hardly attain 10%. In contrast, the electronicsensor allows the pressure measurements with an accuracy below 0.1%. Actually the sen-sor is capable to measure a current change of 20 nA at a bias of 10 V (the range of the non-Ohmic current–voltage characteristic); that is, a pressure of 20 kbar can be measured withan accuracy of 5 bar. The gauge factor of the sensor at a pressure of 10 kbar (mechanicalstrain near the surface at 10 kbar is 10––3) is of 120. This value is an order of magnitudehigher that that expected for boron doped diamond with specific resistivity of 0.03 W cm[2, 6]. It is clear that such a high pressure sensitivity cannot be explained merely by thepiezoresistivity of p-type diamond. We believe that the main contribution to the piezosensi-tivity of the p–i–p diode is the stress-induced change of the carrier injection efficiency overthe p–i barrier [4]. Preliminary measurements of the pressure response at 600 �C revealthat the pressure sensitivity does not change noticeably at elevated temperatures.

The temperature measurements were performed when measuring the conductivity ofone of the boron-doped stripes (terminals 1 and 3 in Fig. 1). It is seen that the currentchange with temperature is high enough to ensure a measurement accuracy of 0.01 �C(Fig. 4). It is interesting that the activation energy of the conductivity over the boron-implanted stripe increases considerably with temperature (from 0.06 eV at room tem-perature to 0.2 eV at 800 �C), thus increasing the accuracy of the sensor in the high-temperature range.

62 A. M. Zaitsev et al.: Diamond Pressure and Temperature Sensors

Fig. 2. Current–voltage characteristics of the sensor (terminals 1 and 2 in Fig. 1) after implantationand annealing. It is seen that the non-Ohmic characteristic appears only after the carbon irradia-tion compensating the boron acceptors in between the p-type layers

There is an important problem to be solved before this sensor can be routinely used inDAC. This is the separation of the signals due to pressure and temperature, since all theconductive areas of the sensor are both pressure and temperature sensitive. Fortunately,the pressure response of the boron-doped stripes is very low being characterized by agauge factor of 1.1 at 10 kbar (see Fig. 3a). It means, that they can be used as pure tem-perature sensors. If so, a solution of the problem could be an appropriate subtraction ofthe temperature-induced signal from the signal of the pressure sensor. Comparing Figs. 3aand 4 it is seen that the response of the sensor exhibits an equal response for 1 kbarpressure and 1 �C temperature. Thus, in order to attain the 0.1% accuracy of the pressuremeasurements the temperature of the sensor must be kept with a precision of 0.01 �C.

phys. stat. sol. (a) 185, No. 1 (2001) 63

Fig. 3. a) Operation of the pressure sensor compared with piezoresistivity of single p-type stripe.In all cases the bias is 10 V. The piezo-response of the sensor increases considerably after carbonirradiation converting the implanted structure into a p–i–p diode. Piezo-response of the p-typestripe is negligible in comparison with that of the p–i–p diode. b) Calibration of the sensor againstthe shift of diamond Raman line in a pressure range up to 70 kbar

4. Conclusion

An integrated high-pressure high-temperature sensor has been made on diamond byhigh-temperature high-energy ion implantation. The sensor has the electronic structureof p–i–p diode with the i-region containing compensated boron acceptors. The sensorhas been fabricated on the working surface of a diamond anvil and tested in DAC atpressures up to 70 kbar and temperatures up to 850 �C. No physical limits are foreseenfor the use of this sensor in a much wider pressure–temperature range: up to 1 Mbarand 1200 �C. The obtained results show that the sensor provides a measurement accu-racy of at least 0.1% for pressure and 0.01% for temperature.

Acknowledgements The authors are grateful to Dr. S. Kubsky for the help in ionimplantation. This work was carried out under the projects DFG MA689/16-1,DFG FA219/13-1 and SFB 526-D6.

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64 A. M. Zaitsev et al.: Diamond Pressure and Temperature Sensors

Fig. 4. Operation of a boron implanted stripe as a temperature sensor. The temperature sensitivityof the stripe increases with temperature, what is a consequence of the increasing activation energyof conductivity (see insert)