doi:10.1016/S0749-6036(03)00030-2Superlattices and Microstructures, Vol. 32, Nos 4–6, 2002

High mobility holes on hydrogen-terminated diamond surface

H. SHINAGAWA †, G. KIDO, T. TAKAMASU

Nanomaterials Laboratory, National Institute for Materials Science, 3-13 Sakura, Tsukuba,Ibaraki 305-0003, Japan

M. N.-GAMO‡

Centre for Tsukuba Advanced Research Alliance (TARA), University of Tsukuba, 1-1-1 Tennodai, Tsukuba,Ibaraki 305-8577, Japan

T. ANDO

Advanced Materials Laboratory, National Institute for Materials Science, 1-1 Namiki, Tsukuba,Ibaraki 305-0044, Japan

(Received 24 March 2003)

Boron-doped diamond was grown by the chemical vapour deposition technique, of whichthe surface is atomically flat. We have measured cyclotron resonance of hydrogen-termi-nated boron-doped diamond and found that holeshave very high mobility. Furthermore,we have successfully fabricated nano-structures on the surface with use of the scanningprobe microscope. These properties suggest that diamond can be a candidate material forsolid state quantum devices.

c© 2003 Elsevier Ltd. All rights reserved.Key words: nano-fabrication, magnetic resonance, millimeter-wave spectroscopy.

1. Introduction

Diamond is a well-known insulator or a wide bandgap semiconductor with a 5.5 eV energy gap. Becauseof its large thermal conductivity and high breakdown field, this material has been intensively studied forapplication to high power electronic devices.

The bulk diamond is an indirect-gap semiconductor, in which the bottom of the conduction band is locatedaround (3/4, 0, 0) and the top of the valence band is located at the zone centre (the�-point) [1–3]. Thespin–orbit splitting in the valence band is very small (5∼ 7 meV) [1, 2] compared with Si (44 meV) and Ge(0.3 eV), which enables the split-off-holes to carry electronic charges in addition to the light and heavy holes.Boron-doped bulk diamond shows p-type semiconductivity with activation energy about 0.4 eV. The carriermobility has been reported larger than 104 cm2 V−1 s−1 below 100 K for low carrier concentration [4].

†Author to whom correspondence should be addressed. E-mail:shinagawa.hideyuki@nims.go.jp

‡Also at: Advanced Materials Laboratory, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki305-0044, Japan.

0749–6036/02/100289 + 06 $30.00/0 c© 2003 Elsevier Ltd. All rights reserved.

290 Superlattices and Microstructures, Vol. 32, Nos 4–6, 2002

Recently, it was found that the carrier density near the surface can be controlled by changing a surfacetermination [5, 6]. The hydrogen-terminated diamond shows p-type semiconductivity with holes stronglylimited to the surface region. On the other hand, oxygen-terminated diamond, which is the usual case ofnatural diamond, is insulating. Using such surface termination properties, fabrication of electronic devicessuch as field effect transistors are powerfully investigated in recent years [2], however, basic properties of thesurface holes are still unclear.

For the mechanism of the conductivity of hydrogen-terminated diamond, several models have beenproposed as follows: (1) surface band vending where valence band electrons transfer into an adsorbed waterlayer [7, 8], (2) shallow hydrogen-induced acceptors [9, 10] and (3) deep level passivation by hydrogen[11]. A typical carrier density and mobility of a hydrogen-terminated diamond surface was estimated as1013∼ 1014 cm−2 and 10∼ 100 cm2 V−1 s−1, respectively [12–15], by Hall effect measurements aroundroomtemperature. The temperature dependence of carrier density or activation energy and carrier mobilitydepends greatly on sample preparation. There was little reliable data below 100 K, mostly because ofdifficulties in fabricating ohmic contact for low temperatures.

Carrier mobility, which is concerned with the mean free path of the carrier and an important factor forapplications of electronic devices, is limited by the crystal quality. Especially high mobility is intrinsicallydemanded for the application ofquantum devices. It was known that high quality diamonds can be homo-epitaxially grown by the chemical vapour deposition (CVD) method, however, the crystal quality or themobility of surface holes was not sufficient for the application of quantum devices. Recently, we found thatthe crystals with extremely high quality are obtained for boron-doped diamond rather than for nondoped[16]. It was expected that the carrier density near the surface could be controlled by surface termination witha similar mechanism as the nondoped one at low temperatures where bulk carriers become thermally inactive.

We have measured cyclotron resonance (CR) of the hydrogen-terminated boron-doped diamond toinvestigate electronic properties of the system and found the carrier shows very high mobility.

It was not obvious that there exist surface holes that depend on surface termination on hydrogen-terminatedboron-doped diamond or nano-fabrication using the scanning probe microscope (SPM) could be similarlyapplicable to the case of nondoped diamond [17]. A technique to fabricate nano-structures with use of aSPM was originally developed for the silicon surface [18, 19] and found to be applicable for the surfaceof hydrogen-terminated nondoped diamond [17]. We have tried to fabricate nano-structures on the boron-doped diamond surface to check the effect of the surface termination on the electronic properties of thesurface.

2. Experimental

Samples were prepared with the microwave plasma assisted CVD technique. High pressure and hightemperature synthetic type-Ib diamond (001) crystals were used as substrates, on which CVD diamond ishomo-epitaxially grown. Commercially available synthetic diamond, which is mechanically finely polished,has surface irregularities in the order of some 10 nm. The surface irregularities were reduced to several nmby etching in hydrogen plasma at 800◦C for 30 min before CVD growth. An excess of etching does notreduce surface irregularities but creates a so-called etched pit [16]. After boron-doped diamond was homo-epitaxially grown on the substrate at 780◦C up to 0.5µm, the sample were exposed for 5 min under hydrogenplasma so that the surface is terminated by hydrogen.The surface condition was checked by an atomic forcemicroscope (AFM) and it was found that atomically flat surfaces are obtained.

For millimeter-wave magnetic resonance measurements, samples were mounted in a waveguide in Faradayconfiguration and introduced into a cryostat in a static magnetic field. The atmosphere was kept in vacuumor very low helium gas pressure less than 10−2 Torr if needed for heat exchange. A transmitted millimeterwave was detected with use of an ABmmTM (Paris) vector network analyser at constant frequencies as a

Superlattices and Microstructures, Vol. 32, Nos 4–6, 2002 291

Fig. 1. A schematic drawing of experimental setup for nano-fabrication using SPM. The bias voltage can be controlled within±10 Vwith current limit of ±100 nA. The whole apparatus shown here is enclosed ina vacuum chamber so that the atmosphere, such ashumidity, can be controlled.

function of magnetic field. The magnetic field up to 15 T was applied along the (001) crystallographic axis(perpendicular to the surface) with a superconducting coil.

Nano-fabrication was performed by electro-chemically modifying the surface using SPM at roomtemperature in air. The silicon cantilever coated with gold or rhodium is used as a conductive AFM probeto apply bias voltage and current. A schematic drawing of the experimental setup for the nano-fabrication isshown inFig. 1. During surface modification, the tip height is controlled to trace the surface by a conventionalAFM mode while the sample bias voltage can be changed from−10 to 10 V with a current limit of 100 nA.The surface modification was done by a scanning probe on a designed pattern with a positive bias voltagelarger than 5 V. The topological and current images of the surface are simultaneously obtained with smallbias voltage.

3. Results and discussion

Figure 2presents a result of magnetic resonance measurements. Transmission traces at 64.8 GHz as afunction of the magnetic field at 140 K is shown inFig. 2. A well-defined absorption peak was foundaround 1.6 T and a shoulder-like structure was found around 2.3 T. A curve fitting procedure is adoptedto obtain effective massesm∗

c and scattering timesωcτ . It is well-known that the power absorbed in amagneticfield by nth carriers from a linearly polarized electromagnetic field is proportional toσn{1/((ω −ωcn)2τ2 + 1) + 1/((ω + ωcn)2τ2 + 1)}, whereσn denotes dc conductivity for multiple carriers. By addingthe absorptions for the two types of carriers with adjustable parametersm∗

1, m∗2, τ1, τ2, σ2/σ1, we obtain

m∗1 = 0.93m0, m∗

2 = 0.70m0, ωcτ1 = 5 andωcτ2 = 7 at140 K,wherem0 denotes free-electron mass. Thesimulated curve with the obtained parameters is shown as a dashed line inFig. 2, which is well fitting in theexperimental result. Substituting to the formulaµ = eτ/m∗, carrier mobilities form∗

1 andm∗2 are obtained

as 2× 104 and 4× 104 cm2 V−1 s−1, respectively.It is not evident at the present time but very likely that the resonance came from the carriers of the surface

conductive layer because the bulk carrier is thought tobe thermally inactive and surface carriers would bedominant at low temperature. The carrier mobility obtained here is considerably larger in comparison withthe previously reported values for surface holes [12–15], suggesting the high quality of our samples. Suchimprovementof electronic properties of the diamond encourages us for developments of applications forelectronic devices including quantum devices such as single electron transistor.

292 Superlattices and Microstructures, Vol. 32, Nos 4–6, 2002

Magnetic Field (T)

heavy hole

light hole

simulated

64.8 GHz

140 K

0 2 4 6

Tra

nsm

issi

on (

arb.

u.)

Fig. 2. Transmission for64.8 GHz as a function of magnetic field at 140 K (solid line). Short dashed lines denote simulated curves forthe resonance at 1.6 and 2.3 T, respectively, and the long dashed one is the convolution of these two curves. Base lines are arbitrarilyshifted to clarify.

Nano-fabrication was successfully performed on the hydrogen-terminated boron-doped diamond surfaceby modifying the surfacetermination using SPM.Figures 3A and B shows an AFM topographical image andcurrent image, respectively, of the surface after scanning a pattern of vertical lines with a bias voltage of 10 Vand scan rate of 100 nm s−1 in air. The current image was taken with a bias voltage of−3 V. It is clear thatthe locally insulated area was formed by scanning with a positive bias voltage. In contrast, with the negativebias application, no changes were observed in the topological and current image. Less than 40 nm in linewidth are easily achieved with this technique. It is likely that the surface is oxidized and surface terminationis changed by the modification similarly to the case of nondoped diamond [17], suggesting that the electronicproperties of the surface would depend on surface termination or there would be surface holes in addition tothe bulk ones.

The nano-fabrication technique described here can be applicable for various nano-devices in the nextstep, such as a single-hole tunnelling device or metal–insulator–metal device [20] and a nano-sized fieldeffect transistor [19]. Furthermore, interesting mesoscopic effects would appear concerned with the lowdimensionality of the surface holes.

4. Summary

We have found high mobility holes are available on the boron-doped diamond surface prepared withthe CVD technique. Nano-fabrication using SPM is successfully performed on the hydrogen-terminatedboron-doped diamond surface, encouraging us for futuredevelopments for quantum devices on the diamondsurface.

Superlattices and Microstructures, Vol. 32, Nos 4–6, 2002 293

Fig. 3. A, Topological AFM image of the modified area. B, A current image of the modified area with bias voltage of−5 V. Note thatthe bright area is insulating.

294 Superlattices and Microstructures, Vol. 32, Nos 4–6, 2002

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