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Influence of epitaxy on the surface conduction of diamond film
Diamond and Related Materials 13 (2004) 226–232
Influence of epitaxy on the surface conduction of diamond film M. Kasua,b,*, M. Kubovicb, A. Aleksovb,1, N. Teofilovc, Y. Taniyasua, R. Sauerc, E. Kohnb, T. Makimotoa, N. Kobayashia,2 a NTT Basic Research Laboratories, NTT Corporation, 3-1 Morinosato-Wakamiya, Atsugi 243-0198, Japan Department of Electron Devices and Circuits, University of Ulm, Albert-Einstein Allee 45, D-89081 Ulm, Germany c Department of Semiconductor Physics, University of Ulm, Albert-Einstein Allee 45, D-89069 Ulm, Germany
b
Abstract The influence of the crystalline and surface properties of diamond homoepitaxial layers on device properties in H-terminated surface-channel diamond field effect transistors (FETs) is investigated. Crystalline defects inside the layers result in gate current leakage in FET DC operation. We confirmed incorporation of boron acceptors inside the layers. Their residual acceptors result in buffer leakage in DC operation, but do not affect RF characteristics much. Adsorbates from the environment change the concentration of the surface holes generated by the H-termination. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Homoepitaxy; Surface electronic properties; High frequency electronics
1. Introduction Diamond intrinsically exhibits excellent physical properties, such as high hole saturation velocity (1=107 cmys), high electric breakdown field ()10 MVycm) and high thermal conductivity (20 Wycm K) w1x. From these values, excellent RF power performance is expected w2x, and much work has therefore been carried out in attempts to obtain diamond active devices. In the very long history of diamond research, material researchers have recognized many problems in diamond crystal, such as crystalline defects and residual impurities, and electrical engineers have encountered many difficulties, such as leakage current, gain, RF performance, reliability, in building diamond devices. However, few studies have addressed the relationship between the specific problems in diamond and electronic device properties. The purpose of this article is to clarify how epitaxy affects device properties in diamond. In this study, we focused on a surface-channel field effect transistor (FET) using a diamond homoepitaxial layer grown on a high-pressureyhigh-temperature (HPHT) synthesized diamond substrate w3x. This is because a homoepitaxial layer has no grain boundaries *Corresponding author. Tel.: q81-46-240-3451; fax: q81-46-2404729. E-mail address: [email protected] (M. Kasu). 1 Present address: North Carolina State University, USA. 2 Present address: University of Electron-Communications, Japan.
and is thus considered to have better crystalline quality than polycrystalline diamonds. Fig. 1 is a schematic view of the FET structure. The surface is H-terminated and exhibits p-type conduction w4,5x. For electric isolation of devices, surface areas around each device were oxidized by exposing the surface to oxygen plasma. The Fermi level of the oxidized surface was pinned at 1.7 eV. Ohmic contacts as source and drain were formed by evaporating gold on the H-terminated surface. The Schottky contact as a gate was formed using Al evaporation on the H-terminated surface. We can consider that this system is a three-layered one, where the three layers are the surface, the bulk and the environment as shown in Fig. 1. On the surface, the conduction for the device originates from holes generated by H-termination. In the bulk, the diamond homoepitaxial layer contains crystalline defects, which have an influence on the FET. Further in the bulk, the diamond layer contains residual impurities that result in bulk conduction. In FET operation, both the bulk and the surface conductions occur in parallel. From the environment, adsorbates adsorb on the surface and influence the surface conduction. 2. Experimental procedure 2.1. Homoepitaxial layer growth The homoepitaxial-diamond layer used in this study was grown on HPHT-synthesized Ib-type diamond
0925-9635/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2003.10.025
M. Kasu et al. / Diamond and Related Materials 13 (2004) 226–232
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Fig. 1. Schematic illustration of the surface-channel FET on a homoepitaxial layer grown on a HPHT diamond substrate, and influence of crystalline and surface properties on surface conduction.
(0 0 1) substrates by microwave plasma chemical vapor deposition. The HPHT substrate was etched using an acid mixture of HClO4:HNO3:HCls1:3:4 for 30 min at 200 8C, before it was placed in the reactor. The homoepitaxial layers were intentionally undoped. No dopant gas has ever been used in the growth system. The microwave power and frequency were 1.3 kW and 2.54 GHz, respectively. The sources were highly pure (6 N) CH4 gas and purified (better than 6 N) H2. The flow ratio of CH4 yH2 was 1%. The working pressure was 50 Torr. The growth rate was approximately 0.14 mmyh at the growth temperature of 650 8C. The growth temperature was monitored with an optical pyrometer. Sample A was grown at 770 8C and had a high defect density of 1.5=106 ycm2. Its homoepitaxial-layer thickness was 1.0 mm. As the growth temperature is lowered, the defect density can be decreased w6x. The sample B was grown at 650 8C and exhibited hole conduction in the layer (bulk conduction). Its homoepitaxial-layer thickness was 7.6 mm. 2.2. Characterization For cathodoluminescence (CL) measurements, the sample was mounted in a cold Cu finger of a continuousflow liquid He cryostat and was excited by electrons of 6-keV energy. The luminescence signals were dispersed by a monochromator (1-m focal length, grating with 1200 linesymm blazed at ls250 nm) and detected by a liquid-nitrogen-cooled ultraviolet charge-coupled device optimized for 200-nm wavelength. 3. Results and discussions 3.1. Influence of defects The defects in the homoepitaxial layer affect the bulk mobility w6x. However, the influence of defects on FET properties using a channel close to the surface is not yet clear. Fig. 2a shows drain current–drain voltage curves for different gate voltages. The FETs were fabricated on
sample A, which had a high defect density. One can see that current flows even at the zero drain bias caused by gate current leakage, indicating that there is an ohmic component between the Al gate and the channel. Most of the FETs on the sample A with high defect density exhibited gate current leakage. On the other hand, a few FETs on that same sample exhibit no gate current leakage as shown in Fig. 2b. Fig. 2c and d are photographs of FETs of (a) and (b), respectively. The separations between the source and drain contacts of Fig. 2c and d were almost the same, approximately 2.7 mm. From a comparison of these two FET devices (Fig. 2c and d), it appears that FETs exhibiting gate current leakage have more defects near the gate. Therefore, we confirmed that these defects cause the gate current leakage. 3.2. Influence of impurities Among the surface, bulk and environment, the residual impurities in the bulk affect the device properties. As shown in Fig. 3a, first we compared temperature dependence of hole sheet concentration for H-termination and O-termination of the same homoepitaxial layer (sample B). These measurements were performed in vacuum (-10y1 Torr). For the O-termination, the hole concentration changed by several orders with temperature between 100 and 500 K. The holes of the Otermination are generated by residual acceptors inside the homoepitaxial layer, i.e. the bulk. On the other hand, for the H-termination, in the higher temperature range (from room temperature (RT) to 500 K), the hole concentrations of the H-termination were the same as that of the O-termination within experimental error. But in the lower temperature range, the hole sheet concentration of the H-termination remained almost constant. Therefore, the homoepitaxial layer of the H-termination showed two parallel conduction paths: conduction of holes generated by residual acceptors in the homoepitaxial layer (bulk conduction) and conduction of holes
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Fig. 2. Comparison of drain current, ID, as a function of drain bias, VD, for different gate biases, VG , of FET (sample A) in (a) higher and (b) lower defect density regions of the sample A. Photographs (c), (d) of FET device for (a), (b), respectively. Arrows indicate twin defects. The separations between the source and drain contacts are almost the same. LG and WG are the gate length and width, respectively.
generated by the surface H-termination (surface conduction). We will explain temperature dependence of hole concentration of the H-terminated homoepitaxial layer shown in Fig. 3a. In the high-temperature range, the hole concentration inside the homoepitaxial layer (bulk-
hole concentration) is larger than that of the H-termination (surface-hole concentration), and the bulk conduction becomes dominant. In the low-temperature range, surface-hole concentration is larger than bulkhole concentration, and therefore the surface conduction becomes dominant.
Fig. 3. (a) Temperature dependence of hole sheet concentration for H-termination and O-termination of the same homoepitaxial layer (sample B). (b) Temperature dependence of hole cubic concentration for O-termination (sample B). The best-fitted relation is indicated by a solid line.
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Fig. 4. CL spectra obtained at 13 K and RT (inset).
To specify the residual acceptor in the layer, we fitted the temperature dependence of the hole cubic concentration in O-terminated homoepitaxial layers by the neutral charge relation as shown in Fig. 3b: pŽpqND.
B 1 DEA E F s NV expCy D NAyNDyp 2 kBT G
(1)
where p is the hole concentration, ND the donor concentration, NA the acceptor concentration, NV the effective density of states in the valence band, DEA the energy difference between the acceptor level to the valence band top, and kB the Boltzmann constant, T the absolute temperature. The best-fitted curve is shown by a solid line in Fig. 3b. The best-fitted values are the acceptor density of 1=1016 ycm3, the donor density of 3=1014 y cm3 and the acceptor activation energy of 0.38 eV. The best-fitted activation energy is almost the same as the value for boron acceptors w7x, which confirms that the residual acceptors in the layer are boron. Fig. 4 shows CL spectra of the homoepitaxial layer taken at RT and 13 K. The sample was grown at 650 8C and the homoepitaxial-layer thickness was 7.6 mm.
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The CL spectrum at RT exhibits a free-exciton (FE) emission. This indicates a lower density of nonradiative recombinations and is proof of the high quality of the homoepitaxial layers. In the CL spectrum at 13 K, we can see well-resolved FE transitions, such as a strong transverse-optical (TO)-phonon-assisted FE recombination and TO plus zone center optical (TOqOG)- and transverse-acoustic (TA)-phonon-assisted FE transitions. Further, we observed boron-bound exciton (BE) recombination, which supports the existence of boron in the layer. However, a boron source has never been used in our growth system, and at present, we have not determined the origin of the residual boron. CL measurement at low temperature is the more sensitive method to detect residual boron than secondary ion-mass spectroscopy measurements. We investigated a relationship between the ratio of TO-phonon-assisted BE intensity, BETO, to the TO-phonon-assisted FE intensity, FETO, in the CL spectrum at low temperature and the boron concentration obtained by fitting of Hall results. The boron concentration, NB is obtained as follows: NBs(BETO yFETO)6=1016 (1ycm3)
(2)
Roughly a prefactor of 6=1016 is about half of one Kawarada et al. obtained from their measurements of polycrystalline diamonds in a range NB)1.5=1017 y cm3 w8x. In what follows, we explain how the residual acceptors affect the FET characteristics. Fig. 5a shows drain current, ID, as a function of the drain bias, VD, for different gate biases, VG of FET (sample B). The gate length, LG, and width, WG, were 0.2 and 25 mm, respectively. Even at zero gate bias, current flows between the source and the drain. At a condition of zero gate bias the surface conduction channel should be depleted. This current flow at zero gate bias is generated in the buffer and therefore is
Fig. 5. (a) Drain current, ID , as a function of drain bias, VD , for different gate biases, VG , of FET (sample B). Buffer leakage, thus drain current at zero gate bias, is observed. (b) TLM results of sample B for the O-termination. Resistance due to the bulk leakage as a function of the distance between source and drain contacts of ungated FETs.
M. Kasu et al. / Diamond and Related Materials 13 (2004) 226–232
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Fig. 6. (a) Calculation of total mobility as a function of the layer thickness. The bulk mobility and the surface mobility were assumed to be 1200 and 150 cm2yV s, respectively. (b) A calculation of total mobility as a function of the surface mobility. The parameters are used for sample B.
buffer leakage. Fig. 5b plots transmission-line-model (TLM) measurement results of the sample B for the Otermination. Thus, the measured resistance was due to the bulk leakage as a function of the separation between the source and the drain. The resistance is proportional to the distance. This indicates that the buffer leakage originates indeed from bulk conduction of holes generated by residual acceptors in the homoepitaxial layers. We performed small signal parameter RF measurements of FET that exhibits bulk leakage (sample B) to extract h21 gain (current gain for short output circuit), MAG gain (maximum available gain) and U gain (Mason’s unilateral gain). The result will be shown in another article of this issue w9x. Surprisingly this FET which exhibited bulk leakage showed very high values of f T (transit frequency), f max (MAG) (maximum frequency of oscillation for the maximum available gain), f max (U) (maximum frequency of oscillation for Mason’s unilateral gain) cut-off frequencies of 21, 50 and 62 GHz, respectively. The reason is that the surface conduction occurs in 10 nm range from the gate, but the bulk conduction in a micron-meter range from the gate. The carrier modulation efficiency is roughly inversely proportional to the distance and, therefore, carrier modulation efficiency due to the surface conduction is larger than that due to the bulk conduction. Therefore, we conclude that the bulk conduction does not affect RF characteristics much. The next question is how high the surface mobility is. The measured hole sheet concentration, p, is the sum of holes in the layer pl and on the surface ps, i.e. ps plqps. The measured (total) hole mobility is the average of the mobility multiplied by hole concentration squared. ms
RH plml2qpsms2 s r plmlqpsms
(3)
where RH is the Hall coefficient, r the electric conduc-
tivity, and ml and ms are mobilities in the layer and on the surface, respectively. Another important thing when we consider the surface conduction is that the hole sheet concentration in the layer, pl, is proportional to the layer thickness, but the hole concentration on the surface is a sheet charge. Fig. 6a is a calculation of total mobility as a function of the thickness. We used the experimental value of bulk mobility, 1200 cm2 yV s, and we assumed surface mobility of 150 cm2 yV s as parameters. These values are roughly extracted from Hall results and FET analysis. As the thickness increases, the mobility increases slightly due to the bulk conduction. At 7.6 mm (thickness of sample B), the total mobility increases up to 500 cm2 y V s. Fig. 6b is a calculation of total mobility as a function of the surface mobility. The measured value (sample B) of total mobility was 380 cm2 yV s. But the total mobility does not change much in a surface mobility range from 200 to 400 cm2 yV s. Further, the relation bows due to the squaring of the mobility. Consequently, it is very difficult to determine the surface mobility. 3.3. Influence of adsorbates from the environment Fig. 7 shows hole sheet concentrations of H-terminated homoepitaxial diamond as a function of temperature. The sample exhibited a hole sheet concentration of 1.6=1012 ycm2 in air at RT. Then the sample was evacuated. First we explain the change of hole sheet concentration according to plot A. When the sample temperature was increased from RT to 500 K, the hole concentration decreased once and increased. Next when the sample temperature was decreased from 500 to RT, the hole sheet concentration decreased drastically. But when the sample temperature decreased from RT to 100 K, the hole concentration decreased very slowly. When the sample temperature was increased from 100 K to
M. Kasu et al. / Diamond and Related Materials 13 (2004) 226–232
Fig. 7. Hole sheet concentrations of homoepitaxial-diamond layer measured at different temperatures. Plot A: temperature increased from RT to 500 K, then decreased to 100 K, and increased to RT (bold lines with open circles). Plots B: temperature increased from RT to 400 K, then decreased to 100 K, and increased to RT (thin lines and closed circles). Plot C: temperature decreased from RT to 100 K, then increased to RT (broken lines and crosses).
RT, the hole sheet concentration increased, but was the same as that during cooling. The difference of plots A and B is only the maximum temperature, thus for plot A, 500 K, and for the plot B, 400 K. Plot C shows the case when we changed the sample temperature from RT to 100 K, then from 100 K to RT. The differences in the results of plots A, B, C in Fig. 7 clearly show that adsorbates affect the surface conduction. We will explain this by referring to plot A as shown in Fig. 8. In all measurements, the sample was
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first evacuated. In this process, the adsorbates evaporated and the hole sheet concentration decreased (Fig. 8a). During heating from RT to 500 K, the adsorbates evaporated from the surface, then the concentration of holes generated by the H-termination (surface-hole concentration) decreased, but concentration of holes generated by residual acceptors in the layer (bulk-hole concentration) increased (Fig. 8b). During cooling from 500 K to RT, the density of the adsorbates, thus the surface-hole concentration, did not change much with temperature, but the bulk-hole concentration decreased exponentially (Fig. 8c). Therefore, in the higher temperature range (between RT and 500 K), the difference in hole sheet concentrations during heating and cooling originates from evaporation of adsorbates during heating. In the low-temperature range (between 100 K and RT), surface-hole concentration is higher than bulk-hole concentration (Fig. 8d). The surface concentration in the low-temperature range is determined by the adsorbate density at the maximum temperature (500 K for plot A, 400 K for plot B, and RT for plot C). Therefore, the hole sheet concentration at 100 K in plot C is the highest, that in plot B is the second highest, and that in plot A is the lowest. The difference in hole concentration as shown by plots A, B, and C is not determined by surface hydrogen but by adsorbates. This is because, once the sample surface was exposed to air after all measurements, the hole sheet concentration recovered to the initial value before all measurements. This means that adsorbates that desorbed in the vacuum were supplied from air. Hydrogen does not exist in air and cannot bond with surface carbon atoms chemically at RT. The absorbates are likely H2O molecules. 4. Summary We investigated the influence of properties in the bulk, the surface, and the environment on the electrical properties and device properties. Our findings can be summarized as follows: 1. Defects in the bulk result in gate current leakage in FETs. 2. Residual boron acceptors in the layer result in buffer leakage in FETs, but do not affect RF characteristics much. 3. Adsorbates from the environment change the concentration of surface-holes generated by H-termination.
Fig. 8. Explanation of Fig. 7. Schematic illustrations of H-terminated layer (a) at RT, (b) during heating from RT to 500 K, (c) during cooling from 500 K to RT, and (d) during cooling from RT to 100 K and during heating from 100 K to RT.
Acknowledgments We thank Drs A. Denisenko, W. Ebert, I. Kallfass and Professor H. Schumacher of University of Ulm for their
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fruitful discussions. We also acknowledge Drs Y. Hirayama, S. Ishihara of NTT for their encouragement. References w1x D.L. Dreifus, B.A. Fox, in: M.A. Prelas (Ed.), Handbook of Industrial Diamonds and Diamond Films, Marcel Dekker, Inc, New York, 1998, p. 1043. w2x E. Kohn, W. Ebert, in: B. Dischler, C. Wild (Eds.), Lowpressure Synthetic Diamond, Manufacturing and Applications, Springer, Berlin, 1998, p. 331.
w3x A. Aleksov, A. Denisenko, U. Spitsberg, T. Jenkins, W. Ebert, E. Kohn, Diamond Relat. Mater. 11 (2002) 382. w4x M.I. Landstrass, K.V. Ravi, Appl. Phys. Lett. 55 (1989) 975. w5x M.I. Landstrass, K.V. Ravi, Appl. Phys. Lett. 55 (1989) 1391. w6x M. Kasu, N. Kobayashi, Diamond Relat. Mater. 12 (2003) 413. w7x A.T. Collins, in: G. Davies (Ed.), Properties and Growth of Diamond, INSPEC, the Institution of Electrical Engineers, London, 1994, p. 263. w8x H. Kawarada, H. Matsuyama, Y. Yokota, T. Sogi, A. Yamaguchi, A. Hiraki, Phys. Rev. B 47 (1993) 3633. w9x A. Aleksov, M. Kubovic, M. Kasu, et al., Diamond Relat. Mater. 13 (2004) 233–240.
Influence of epitaxy on the surface conduction of diamond film M. Kasua,b,*, M. Kubovicb, A. Aleksovb,1, N. Teofilovc, Y. Taniyasua, R. Sauerc, E. Kohnb, T. Makimotoa, N. Kobayashia,2 a NTT Basic Research Laboratories, NTT Corporation, 3-1 Morinosato-Wakamiya, Atsugi 243-0198, Japan Department of Electron Devices and Circuits, University of Ulm, Albert-Einstein Allee 45, D-89081 Ulm, Germany c Department of Semiconductor Physics, University of Ulm, Albert-Einstein Allee 45, D-89069 Ulm, Germany
b
Abstract The influence of the crystalline and surface properties of diamond homoepitaxial layers on device properties in H-terminated surface-channel diamond field effect transistors (FETs) is investigated. Crystalline defects inside the layers result in gate current leakage in FET DC operation. We confirmed incorporation of boron acceptors inside the layers. Their residual acceptors result in buffer leakage in DC operation, but do not affect RF characteristics much. Adsorbates from the environment change the concentration of the surface holes generated by the H-termination. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Homoepitaxy; Surface electronic properties; High frequency electronics
1. Introduction Diamond intrinsically exhibits excellent physical properties, such as high hole saturation velocity (1=107 cmys), high electric breakdown field ()10 MVycm) and high thermal conductivity (20 Wycm K) w1x. From these values, excellent RF power performance is expected w2x, and much work has therefore been carried out in attempts to obtain diamond active devices. In the very long history of diamond research, material researchers have recognized many problems in diamond crystal, such as crystalline defects and residual impurities, and electrical engineers have encountered many difficulties, such as leakage current, gain, RF performance, reliability, in building diamond devices. However, few studies have addressed the relationship between the specific problems in diamond and electronic device properties. The purpose of this article is to clarify how epitaxy affects device properties in diamond. In this study, we focused on a surface-channel field effect transistor (FET) using a diamond homoepitaxial layer grown on a high-pressureyhigh-temperature (HPHT) synthesized diamond substrate w3x. This is because a homoepitaxial layer has no grain boundaries *Corresponding author. Tel.: q81-46-240-3451; fax: q81-46-2404729. E-mail address: [email protected] (M. Kasu). 1 Present address: North Carolina State University, USA. 2 Present address: University of Electron-Communications, Japan.
and is thus considered to have better crystalline quality than polycrystalline diamonds. Fig. 1 is a schematic view of the FET structure. The surface is H-terminated and exhibits p-type conduction w4,5x. For electric isolation of devices, surface areas around each device were oxidized by exposing the surface to oxygen plasma. The Fermi level of the oxidized surface was pinned at 1.7 eV. Ohmic contacts as source and drain were formed by evaporating gold on the H-terminated surface. The Schottky contact as a gate was formed using Al evaporation on the H-terminated surface. We can consider that this system is a three-layered one, where the three layers are the surface, the bulk and the environment as shown in Fig. 1. On the surface, the conduction for the device originates from holes generated by H-termination. In the bulk, the diamond homoepitaxial layer contains crystalline defects, which have an influence on the FET. Further in the bulk, the diamond layer contains residual impurities that result in bulk conduction. In FET operation, both the bulk and the surface conductions occur in parallel. From the environment, adsorbates adsorb on the surface and influence the surface conduction. 2. Experimental procedure 2.1. Homoepitaxial layer growth The homoepitaxial-diamond layer used in this study was grown on HPHT-synthesized Ib-type diamond
0925-9635/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2003.10.025
M. Kasu et al. / Diamond and Related Materials 13 (2004) 226–232
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Fig. 1. Schematic illustration of the surface-channel FET on a homoepitaxial layer grown on a HPHT diamond substrate, and influence of crystalline and surface properties on surface conduction.
(0 0 1) substrates by microwave plasma chemical vapor deposition. The HPHT substrate was etched using an acid mixture of HClO4:HNO3:HCls1:3:4 for 30 min at 200 8C, before it was placed in the reactor. The homoepitaxial layers were intentionally undoped. No dopant gas has ever been used in the growth system. The microwave power and frequency were 1.3 kW and 2.54 GHz, respectively. The sources were highly pure (6 N) CH4 gas and purified (better than 6 N) H2. The flow ratio of CH4 yH2 was 1%. The working pressure was 50 Torr. The growth rate was approximately 0.14 mmyh at the growth temperature of 650 8C. The growth temperature was monitored with an optical pyrometer. Sample A was grown at 770 8C and had a high defect density of 1.5=106 ycm2. Its homoepitaxial-layer thickness was 1.0 mm. As the growth temperature is lowered, the defect density can be decreased w6x. The sample B was grown at 650 8C and exhibited hole conduction in the layer (bulk conduction). Its homoepitaxial-layer thickness was 7.6 mm. 2.2. Characterization For cathodoluminescence (CL) measurements, the sample was mounted in a cold Cu finger of a continuousflow liquid He cryostat and was excited by electrons of 6-keV energy. The luminescence signals were dispersed by a monochromator (1-m focal length, grating with 1200 linesymm blazed at ls250 nm) and detected by a liquid-nitrogen-cooled ultraviolet charge-coupled device optimized for 200-nm wavelength. 3. Results and discussions 3.1. Influence of defects The defects in the homoepitaxial layer affect the bulk mobility w6x. However, the influence of defects on FET properties using a channel close to the surface is not yet clear. Fig. 2a shows drain current–drain voltage curves for different gate voltages. The FETs were fabricated on
sample A, which had a high defect density. One can see that current flows even at the zero drain bias caused by gate current leakage, indicating that there is an ohmic component between the Al gate and the channel. Most of the FETs on the sample A with high defect density exhibited gate current leakage. On the other hand, a few FETs on that same sample exhibit no gate current leakage as shown in Fig. 2b. Fig. 2c and d are photographs of FETs of (a) and (b), respectively. The separations between the source and drain contacts of Fig. 2c and d were almost the same, approximately 2.7 mm. From a comparison of these two FET devices (Fig. 2c and d), it appears that FETs exhibiting gate current leakage have more defects near the gate. Therefore, we confirmed that these defects cause the gate current leakage. 3.2. Influence of impurities Among the surface, bulk and environment, the residual impurities in the bulk affect the device properties. As shown in Fig. 3a, first we compared temperature dependence of hole sheet concentration for H-termination and O-termination of the same homoepitaxial layer (sample B). These measurements were performed in vacuum (-10y1 Torr). For the O-termination, the hole concentration changed by several orders with temperature between 100 and 500 K. The holes of the Otermination are generated by residual acceptors inside the homoepitaxial layer, i.e. the bulk. On the other hand, for the H-termination, in the higher temperature range (from room temperature (RT) to 500 K), the hole concentrations of the H-termination were the same as that of the O-termination within experimental error. But in the lower temperature range, the hole sheet concentration of the H-termination remained almost constant. Therefore, the homoepitaxial layer of the H-termination showed two parallel conduction paths: conduction of holes generated by residual acceptors in the homoepitaxial layer (bulk conduction) and conduction of holes
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Fig. 2. Comparison of drain current, ID, as a function of drain bias, VD, for different gate biases, VG , of FET (sample A) in (a) higher and (b) lower defect density regions of the sample A. Photographs (c), (d) of FET device for (a), (b), respectively. Arrows indicate twin defects. The separations between the source and drain contacts are almost the same. LG and WG are the gate length and width, respectively.
generated by the surface H-termination (surface conduction). We will explain temperature dependence of hole concentration of the H-terminated homoepitaxial layer shown in Fig. 3a. In the high-temperature range, the hole concentration inside the homoepitaxial layer (bulk-
hole concentration) is larger than that of the H-termination (surface-hole concentration), and the bulk conduction becomes dominant. In the low-temperature range, surface-hole concentration is larger than bulkhole concentration, and therefore the surface conduction becomes dominant.
Fig. 3. (a) Temperature dependence of hole sheet concentration for H-termination and O-termination of the same homoepitaxial layer (sample B). (b) Temperature dependence of hole cubic concentration for O-termination (sample B). The best-fitted relation is indicated by a solid line.
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Fig. 4. CL spectra obtained at 13 K and RT (inset).
To specify the residual acceptor in the layer, we fitted the temperature dependence of the hole cubic concentration in O-terminated homoepitaxial layers by the neutral charge relation as shown in Fig. 3b: pŽpqND.
B 1 DEA E F s NV expCy D NAyNDyp 2 kBT G
(1)
where p is the hole concentration, ND the donor concentration, NA the acceptor concentration, NV the effective density of states in the valence band, DEA the energy difference between the acceptor level to the valence band top, and kB the Boltzmann constant, T the absolute temperature. The best-fitted curve is shown by a solid line in Fig. 3b. The best-fitted values are the acceptor density of 1=1016 ycm3, the donor density of 3=1014 y cm3 and the acceptor activation energy of 0.38 eV. The best-fitted activation energy is almost the same as the value for boron acceptors w7x, which confirms that the residual acceptors in the layer are boron. Fig. 4 shows CL spectra of the homoepitaxial layer taken at RT and 13 K. The sample was grown at 650 8C and the homoepitaxial-layer thickness was 7.6 mm.
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The CL spectrum at RT exhibits a free-exciton (FE) emission. This indicates a lower density of nonradiative recombinations and is proof of the high quality of the homoepitaxial layers. In the CL spectrum at 13 K, we can see well-resolved FE transitions, such as a strong transverse-optical (TO)-phonon-assisted FE recombination and TO plus zone center optical (TOqOG)- and transverse-acoustic (TA)-phonon-assisted FE transitions. Further, we observed boron-bound exciton (BE) recombination, which supports the existence of boron in the layer. However, a boron source has never been used in our growth system, and at present, we have not determined the origin of the residual boron. CL measurement at low temperature is the more sensitive method to detect residual boron than secondary ion-mass spectroscopy measurements. We investigated a relationship between the ratio of TO-phonon-assisted BE intensity, BETO, to the TO-phonon-assisted FE intensity, FETO, in the CL spectrum at low temperature and the boron concentration obtained by fitting of Hall results. The boron concentration, NB is obtained as follows: NBs(BETO yFETO)6=1016 (1ycm3)
(2)
Roughly a prefactor of 6=1016 is about half of one Kawarada et al. obtained from their measurements of polycrystalline diamonds in a range NB)1.5=1017 y cm3 w8x. In what follows, we explain how the residual acceptors affect the FET characteristics. Fig. 5a shows drain current, ID, as a function of the drain bias, VD, for different gate biases, VG of FET (sample B). The gate length, LG, and width, WG, were 0.2 and 25 mm, respectively. Even at zero gate bias, current flows between the source and the drain. At a condition of zero gate bias the surface conduction channel should be depleted. This current flow at zero gate bias is generated in the buffer and therefore is
Fig. 5. (a) Drain current, ID , as a function of drain bias, VD , for different gate biases, VG , of FET (sample B). Buffer leakage, thus drain current at zero gate bias, is observed. (b) TLM results of sample B for the O-termination. Resistance due to the bulk leakage as a function of the distance between source and drain contacts of ungated FETs.
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Fig. 6. (a) Calculation of total mobility as a function of the layer thickness. The bulk mobility and the surface mobility were assumed to be 1200 and 150 cm2yV s, respectively. (b) A calculation of total mobility as a function of the surface mobility. The parameters are used for sample B.
buffer leakage. Fig. 5b plots transmission-line-model (TLM) measurement results of the sample B for the Otermination. Thus, the measured resistance was due to the bulk leakage as a function of the separation between the source and the drain. The resistance is proportional to the distance. This indicates that the buffer leakage originates indeed from bulk conduction of holes generated by residual acceptors in the homoepitaxial layers. We performed small signal parameter RF measurements of FET that exhibits bulk leakage (sample B) to extract h21 gain (current gain for short output circuit), MAG gain (maximum available gain) and U gain (Mason’s unilateral gain). The result will be shown in another article of this issue w9x. Surprisingly this FET which exhibited bulk leakage showed very high values of f T (transit frequency), f max (MAG) (maximum frequency of oscillation for the maximum available gain), f max (U) (maximum frequency of oscillation for Mason’s unilateral gain) cut-off frequencies of 21, 50 and 62 GHz, respectively. The reason is that the surface conduction occurs in 10 nm range from the gate, but the bulk conduction in a micron-meter range from the gate. The carrier modulation efficiency is roughly inversely proportional to the distance and, therefore, carrier modulation efficiency due to the surface conduction is larger than that due to the bulk conduction. Therefore, we conclude that the bulk conduction does not affect RF characteristics much. The next question is how high the surface mobility is. The measured hole sheet concentration, p, is the sum of holes in the layer pl and on the surface ps, i.e. ps plqps. The measured (total) hole mobility is the average of the mobility multiplied by hole concentration squared. ms
RH plml2qpsms2 s r plmlqpsms
(3)
where RH is the Hall coefficient, r the electric conduc-
tivity, and ml and ms are mobilities in the layer and on the surface, respectively. Another important thing when we consider the surface conduction is that the hole sheet concentration in the layer, pl, is proportional to the layer thickness, but the hole concentration on the surface is a sheet charge. Fig. 6a is a calculation of total mobility as a function of the thickness. We used the experimental value of bulk mobility, 1200 cm2 yV s, and we assumed surface mobility of 150 cm2 yV s as parameters. These values are roughly extracted from Hall results and FET analysis. As the thickness increases, the mobility increases slightly due to the bulk conduction. At 7.6 mm (thickness of sample B), the total mobility increases up to 500 cm2 y V s. Fig. 6b is a calculation of total mobility as a function of the surface mobility. The measured value (sample B) of total mobility was 380 cm2 yV s. But the total mobility does not change much in a surface mobility range from 200 to 400 cm2 yV s. Further, the relation bows due to the squaring of the mobility. Consequently, it is very difficult to determine the surface mobility. 3.3. Influence of adsorbates from the environment Fig. 7 shows hole sheet concentrations of H-terminated homoepitaxial diamond as a function of temperature. The sample exhibited a hole sheet concentration of 1.6=1012 ycm2 in air at RT. Then the sample was evacuated. First we explain the change of hole sheet concentration according to plot A. When the sample temperature was increased from RT to 500 K, the hole concentration decreased once and increased. Next when the sample temperature was decreased from 500 to RT, the hole sheet concentration decreased drastically. But when the sample temperature decreased from RT to 100 K, the hole concentration decreased very slowly. When the sample temperature was increased from 100 K to
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Fig. 7. Hole sheet concentrations of homoepitaxial-diamond layer measured at different temperatures. Plot A: temperature increased from RT to 500 K, then decreased to 100 K, and increased to RT (bold lines with open circles). Plots B: temperature increased from RT to 400 K, then decreased to 100 K, and increased to RT (thin lines and closed circles). Plot C: temperature decreased from RT to 100 K, then increased to RT (broken lines and crosses).
RT, the hole sheet concentration increased, but was the same as that during cooling. The difference of plots A and B is only the maximum temperature, thus for plot A, 500 K, and for the plot B, 400 K. Plot C shows the case when we changed the sample temperature from RT to 100 K, then from 100 K to RT. The differences in the results of plots A, B, C in Fig. 7 clearly show that adsorbates affect the surface conduction. We will explain this by referring to plot A as shown in Fig. 8. In all measurements, the sample was
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first evacuated. In this process, the adsorbates evaporated and the hole sheet concentration decreased (Fig. 8a). During heating from RT to 500 K, the adsorbates evaporated from the surface, then the concentration of holes generated by the H-termination (surface-hole concentration) decreased, but concentration of holes generated by residual acceptors in the layer (bulk-hole concentration) increased (Fig. 8b). During cooling from 500 K to RT, the density of the adsorbates, thus the surface-hole concentration, did not change much with temperature, but the bulk-hole concentration decreased exponentially (Fig. 8c). Therefore, in the higher temperature range (between RT and 500 K), the difference in hole sheet concentrations during heating and cooling originates from evaporation of adsorbates during heating. In the low-temperature range (between 100 K and RT), surface-hole concentration is higher than bulk-hole concentration (Fig. 8d). The surface concentration in the low-temperature range is determined by the adsorbate density at the maximum temperature (500 K for plot A, 400 K for plot B, and RT for plot C). Therefore, the hole sheet concentration at 100 K in plot C is the highest, that in plot B is the second highest, and that in plot A is the lowest. The difference in hole concentration as shown by plots A, B, and C is not determined by surface hydrogen but by adsorbates. This is because, once the sample surface was exposed to air after all measurements, the hole sheet concentration recovered to the initial value before all measurements. This means that adsorbates that desorbed in the vacuum were supplied from air. Hydrogen does not exist in air and cannot bond with surface carbon atoms chemically at RT. The absorbates are likely H2O molecules. 4. Summary We investigated the influence of properties in the bulk, the surface, and the environment on the electrical properties and device properties. Our findings can be summarized as follows: 1. Defects in the bulk result in gate current leakage in FETs. 2. Residual boron acceptors in the layer result in buffer leakage in FETs, but do not affect RF characteristics much. 3. Adsorbates from the environment change the concentration of surface-holes generated by H-termination.
Fig. 8. Explanation of Fig. 7. Schematic illustrations of H-terminated layer (a) at RT, (b) during heating from RT to 500 K, (c) during cooling from 500 K to RT, and (d) during cooling from RT to 100 K and during heating from 100 K to RT.
Acknowledgments We thank Drs A. Denisenko, W. Ebert, I. Kallfass and Professor H. Schumacher of University of Ulm for their
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fruitful discussions. We also acknowledge Drs Y. Hirayama, S. Ishihara of NTT for their encouragement. References w1x D.L. Dreifus, B.A. Fox, in: M.A. Prelas (Ed.), Handbook of Industrial Diamonds and Diamond Films, Marcel Dekker, Inc, New York, 1998, p. 1043. w2x E. Kohn, W. Ebert, in: B. Dischler, C. Wild (Eds.), Lowpressure Synthetic Diamond, Manufacturing and Applications, Springer, Berlin, 1998, p. 331.
w3x A. Aleksov, A. Denisenko, U. Spitsberg, T. Jenkins, W. Ebert, E. Kohn, Diamond Relat. Mater. 11 (2002) 382. w4x M.I. Landstrass, K.V. Ravi, Appl. Phys. Lett. 55 (1989) 975. w5x M.I. Landstrass, K.V. Ravi, Appl. Phys. Lett. 55 (1989) 1391. w6x M. Kasu, N. Kobayashi, Diamond Relat. Mater. 12 (2003) 413. w7x A.T. Collins, in: G. Davies (Ed.), Properties and Growth of Diamond, INSPEC, the Institution of Electrical Engineers, London, 1994, p. 263. w8x H. Kawarada, H. Matsuyama, Y. Yokota, T. Sogi, A. Yamaguchi, A. Hiraki, Phys. Rev. B 47 (1993) 3633. w9x A. Aleksov, M. Kubovic, M. Kasu, et al., Diamond Relat. Mater. 13 (2004) 233–240.