fB#UUiOND RELATED MATERIALS
Diamond and Related Materials 5 (1996) 737-740
properties of polycrystalline diamond under high electric field strength
Y. Aikawa a, K. Baba a, N. Shohata a, H. Yoneda b, K. Ueda b a Fundamental
Research Laboratories, NEC Corporation, 4-l-l Miyamaeku, Miyazaki, Kawasaki, Kanagawa 216, Japan b Institute for Laser Science, University of Electra-Communications, Chofugaoka, Chofu-shi, Tokyo 182, Japan
Abstract The photoconductive properties of a diamond opto-electronic switch made by chemical vapor deposition were investigated. A new configuration of the diamond gap was proposed to reduce the surface leakage current and avoid surface flashover. This technology made it possible to apply a static electric field with strengths up to 2 x lo6 V cm-‘. The dependence of the mobilitylifetime product ~7 on the grain size was measured for a wide range of electric fields. The ~7 value was found to be linearly proportional to the electric field. Larger grain size samples had larger LIZvalues. The grain size dependence was attributed to the decrease in the mobility and lifetime inside the grain. The sensitivity to UV light was also measured and was 0.3 A W-’ at 150 nm. This indicated that diamond films were applicable to UV photosensors. Keywords: Photoconductivity; voltage
1. Introduction In recent years interest has developed in ultrafast opto-electronic switches made with a high resistivity semiconductor [ 11. The switching speed, i.e. the rise and fall time, is one of the most important parameters for such switches. The fall time tr depends mainly on carrier lifetime, while the rise time t, is determined by the propagation time over a gap distance : t, = lS&WfC
where E is the dielectric constant, d is the gap length and c is the speed of light. To reduce the rise time, the dielectric constant should be smaller. The breakdown voltage should be higher to allow a high output voltage. Among the various semiconductors, diamond has the highest breakdown voltage (about lo4 V cm-‘) and a small dielectric constant (5.7) compared with other switch materials such as Si and GaAs. With regard to the fall time, the diamond switch has been considered to be slow compared with the GaAs because the natural diamond has a longer carrier lifetime. For example, Vermeulen and Harris [ 31 demonstrated a light-sensitive switch made from natural semiconducting diamond which had a time constant of less than 300 ps. Ho et al.  have also reported a natural diamond opto0925-9635/96/$15.000 1996Elsevier Science S.A. All rights reserved SSDZO925-9635(95)00448-3
electronic switch whose response time was less than a nanosecond. However, in the chemical vapor deposited (CVD) polycrystalline diamond it is possible to decrease the carrier lifetime by controlling the grain boundary and impurity conditions. Therefore the CVD diamond optoelectronic switch is one of the best candidates for a fast high-voltage switching device. The photoconductive characteristics of CVD diamond and its switching performance under a high electric field are discussed in this paper.
2. Performance of switch 2.1. Static electric properties It is important to avoid surface flashover because the flashover voltage is less than the breakdown voltage of bulk materials. New techniques have been introduced to improve the static electrical properties. One is cleaning the surface of the diamond film after deposition with a hot-filament CVD apparatus. The surface conductive layer was removed by a three-step chemical treatment (Hz02 + NH4OH, HNOS + HCl and Cr03 + H2SO4) [ 51. The surface leakage current was reduced by four
Y. Aikawa et al.lDiamond and Related Materials 5 (1996) 737-740
UV light l-l
Fig. 1. Schematic diagram of a diamond opto-electric switch with a new gap configuration to avoid surface flashover.
Time [2OpB/div] orders of magnitude when this treatment was used. The other is a new gap configuration. Fig. 1 shows a schematic diagram of the CVD diamond opto-electronic switch. After applying Pt film electrodes by photolithography, the same quality diamond was deposited on and around the gap. The resistive properties of the dc bias field with and without the overcoated layer are shown in Fig. 2. In the uncoated sample, leakage current increased nonlinearly to induce catastrophic breakdown at lo5 Vcm- i. In contrast, the resistivity of the overcoated sample was high and breakdown was not observed up to at least 2 x lo6 V cm-‘. These techniques make it possible to fabricate a high performance diamond opto-electronic switch under a high electric field. 2.2. Dynamic properties Fig. 3 shows a typical output waveform from a CVD diamond film of grain size 0.5 urn grain under an electric field of 1.5 x lo4 V cm-‘. In this study, the gap length between the electrodes was 10 urn and a KrF laser (A= 248 nm) was used as the trigger. In Fig. 3 the rise time is 20 ps, which is equal to the temporal resolution of our sampling system and the fall time is 45 ps. The fall time of a natural single crystal of type IIa diamond was
Fig. 3. Typical output waveform for a diamond opto-electronic switch under an electric field of 1.5 x lo4 Vcm-‘. The rise time t, and the fall time tf are 20 ps and 45 ps respectively.
also measured and was 235 ps. Therefore the fall time of CVD diamond is five times faster than that for a single crystal. It is known that grain boundaries play an important role in the electrical properties of polycrystalline materials. In this study, diamond films of various grain size were prepared to clarify the grain boundary effect on the switching performance. Fig. 4 shows the dependence of switch impedance on laser intensity for samples of various grain sizes. The impedance is inversely proportional to the laser intensity in all the samples. The switch impedance Z is a function of current density J, applied voltage I/ and cross-section S (Z = v/JS). Current density is a product of the carrier number n, the electric charge e, the mobility u and the electric field E. Therefore the switch-on conductance, which is the reciprocal of the impedance, depends on I,uTE, where I and z are the laser intensity and the carrier lifetime respectively. The dependence of the switch-on conductance on grain size at constant laser intensity and electric field is shown in Fig. 5. In this case, the switch-on conductance depends
+ with overcoat
Fig. 2. Static dark electrical properties of diamond films with and without overcoated samples. The overcoated sample does not show breakdown up to at least 2 x lo6 V cm-‘.
1 / Laser Intensity [arb. inits]
Fig. 4. Dependence of switch impedance on laser intensity for various grain sizes. The impedance of all samples increases with the inverse of laser intensity.
I’. Aikawa et al./Diamond and Related Materials 5 (1996) 737-740
in the diamond films do not reach the grain boundaries, and the mobility and carrier lifetime inside the grains vary with grain size.
IO-2 & .
s mo u c lo-3 8
2.3. Spectral sensitivity measurements for UVlight
s 4 .Z 8 ----:S IO-4 IO-’
average grain size [pm] Fig. 5. Dependence of switch-on conductance at constant laser intensity on grain size. The sample with the largest grain size has the largest mobility-lifetime product.
only on the mobility-carrier lifetime product (,uz). The pr product of the sample with a grain size of 10 urn is more than 10 times larger than that of the sample of grain size 0.1 pm. The carrier lifetime, estimated from the fall time in the output waveform, increased from 40 to 80 ps with increasing grain size. Therefore the dependence of the ,N product on the grain size shown in Fig. 5 is caused by reduction in both mobility and carrier lifetime as the grain size becomes smaller. Fig. 6 shows the current densities observed in the samples with grain sizes of 10 and 0.3 pm grain size as a function of the applied field. The linear dependence of the current density on the electric field can be seen in both samples for fields ranging from 2 x lo3 to 3 x lo5 V cm-‘. If the drift distance of the carriers which is given by the product of mobility, carrier lifetime and electric field is less than the grain size, the current density does not increase at a constant ratio for variation of the electric field because the recombination ratio at the grain boundary changes nonlinearly. Therefore, under the present experimental conditions, the carriers generated
- ,I ,’ ,A
y 107 .s 5 -0 E 10s
?! 5 0
’ 1’ ““’
An ultrafast photoconductive switch made from CVD diamond was demonstrated for high electric field conditions. The new geometry of the photoconductive gap
......... grain size = 0.8pm
-d’ .. :
The diamond film opto-electronic switch can also be used as a UV sensor because of its large bandgap, high resistivity and chemical inertness. Therefore the spectral sensitivities of various CVD diamond films with grain sizes of 0.8-10 pm were measured at wavelengths from 130 to 350 nm. The configuration of the sample is the same as that of the opto-electronic switch described above. A deuterium lamp was used as the light source and monochromatic light was obtained using a diffraction grating. The power of the light incident on the sample surface was measured with a pyrometer and a photomultiplier. Fig. 7 shows the spectral sensitivities of various diamond films when an electric field of 5 x lo4 V cm-’ was applied. The result for type IIa natural diamond is also shown. The sensitivities begin to increase from the energy corresponding to the bandgap of diamond and reach more than 0.1 A W-l at wavelengths shorter than 150 nm. For example, a sensitivity of 0.3 A W-l was achieved for the sample with a grain size of 6 pm at 150 nm. These values are almost comparable with those of a silicon photodiode UV detector in the same wavelength range. Therefore these CVD diamond films are suitable for UV detectors in a harsh environment where Si photodiodes cannot be used.
L -grain size IOpm : - -o- grain:size 0.3pm:
c ’ ’ ’ 1111’
104 105 Electric field [V/cm]
3 38‘I 106
Fig. 6. Current density as a function of the applied electric field from 2x103t03x105Vcm-‘.
lO~l~~“‘~~““““““‘.l 150 200 250 300 wavelength [nm]
Fig. 7. Spectral sensitivity of diamond films of various grain sizes in the wavelength region from 130 to 350 mn.
Y. Aikawa et al/Diamond and Related Materials 5 (1996) 737-740
allowed a high static electric field up to 2 x lo6 V cm-’ to be established with a very low leakage current of less than 10e9 A with the help of chemical cleaning and overcoating. Photoconductive properties, including grain boundary effects, were analyzed using samples of various grain sizes for a wide range of electric field. The grain size dependence of the mobility-lifetime product was attributed to the decrease in the mobility and lifetime in the grain. Spectral sensitivities in the UV region (130-350 nm) were also measured. The sensitivity was more than 0.1 A W-’ at wavelengths below 150 nm.
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