Rapid deposition of polycrystalline diamond film by DC arc plasma jet technique and its RF MESFETs

Diamond is a promising semiconductor material for high power, high frequency and high temperature electronic devices. High-purity polycrystalline diamond with large grain size has showed prominent RF properties. In this work, polycrystalline free-standing diamond film with grain size of 150 μm was grown by DC arc plasma jet technique with a growth speed as high as 20 μm/h. The prepared diamond sample showed high-purity with a (220) preferred orientation by the XRD and Raman spectra measurements. By a self-aligned process, hydrogen terminated p-type diamond MESFETs with gate length of 100 nm were fabricated on the 15 mm × 15 mm diamond film and showed good DC and RF performances with drain saturation current 225.7 mA/mm and maximum oscillation frequency (f_max) 46.8 GHz.     

AlN as passivation for surface channel FETs on H-terminated diamond

Surface Channel MESFETs (SC-FET) have suffered from instabilities and drift in the past. To overcome these effects a suitable device passivation seems to be one of the key aspects. In this investigation Atomic Layer Deposition (ALD) of AlN has been applied to Surface Channel FETs on hydrogenated diamond. Despite a deposition temperature of 370 °C, where usually the FET channel is permanently degraded, transistor operation with 65% of the initial current level could be obtained.     

Nucleation and growth surface conductivity of H-terminated diamond films prepared by DC arc jet CVD

High-quality polycrystalline diamond film has been extremely attractive to many researchers, since the maximum transition frequency (f_T) and the maximum frequency of oscillation (f_max) of polycrystalline diamond electronic devices are comparable to those of single crystalline diamond devices. Besides large deposition area, DC arc jet CVD diamond films with high deposition rate and high quality are one choice for electronic device industrialization. Four inch free-standing diamond films were obtained by DC arc jet CVD using gas recycling mode with deposition rate of 14 μm/h. After treatment in hydrogen plasma under the same conditions for both the nucleation and growth sides, the conductivity difference between them was analyzed and clarified by characterizing the grain size, surface profile, crystalline quality and impurity content. The roughness of growth surface with the grain size about 400 nm increased from 0.869 nm to 8.406 nm after hydrogen plasma etching. As for the nucleation surface, the grain size was about 100 nm and the roughness increased from 0.31 nm to 3.739 nm. The XPS results showed that H-termination had been formed and energy band bent upwards. The nucleation and growth surfaces displayed the same magnitude of square resistance (R_s). The mobility and the sheet carrier concentration of the nucleation surface were 0.898 cm/V s and 10¹³/cm² order of magnitude, respectively; while for growth surface, they were 20.2 cm/V s and 9.97 × 10¹¹/cm², respectively. The small grain size and much non-diamond carbon at grain boundary resulted in lower carrier mobility on the nucleation surface. The high concentration of impurity nitrogen may explain the low sheet carrier concentration on the growth surface. The maximum drain current density and the maximum transconductance (g_m) for MESFET with gate length L_G of 2 μm on H-terminated diamond growth surface was 22.5 mA/mm and 4 mS/mm, respectively. The device performance can be further improved by using diamond films with larger grains and optimizing device fabrication techniques.     

Electronic and surface properties of H-terminated diamond surface affected by NO2 gas

Hydrogen-terminated diamond surface exhibits p-type conductivity during its exposure to air. To investigate this phenomenon, we examined the influence of different gases on the surface conductivity. Exposure to NO₂ gas resulted in the biggest increase in conductivity, while H₂O vapor decreased the surface conductivity. Moreover, even very low concentrations of NO₂ molecules in air increased the hole sheet concentration, and with increasing NO₂ concentration, the hole sheet concentration increased up to 2.3 × 10¹⁴ cm^? 2 (at 300 ppm NO₂). This increase of hole sheet concentration was observed during exposure to NO₂ gas and simultaneous adsorption of NO₂ molecules on the diamond surface, while it decreased when the exposure stopped and NO₂ molecules desorbed from the surface. X-ray photoelectron spectroscopy investigation showed upward band bending and partial oxidation of the hydrogen-terminated surface after exposure to air and NO₂. FETs exposed to NO₂ gas exhibited lower source and drain resistances, which led to a 1.8-fold increase of maximum drain current, transconductance increased 1.5-fold and maximum frequency of oscillation increased 1.6-fold.     

Technology of passivated surface channel MESFETs with modified gate structures

Surface channel MESFETs have suffered from instabilities and drift in the past. To overcome these effects a suitable device passivation seems to be one of the key aspects. In this investigation such a passivation scheme on the basis of Atomic Layer Deposition (ALD) of Al₂O₃ has been developed and combined with advanced gate structures, like a field plate and a second MOS gate (in a dual gate configuration).     

Characterization of high-quality polycrystalline diamond and its high FET performance

We characterized high-quality polycrystalline diamond with large grains and fabricated polycrystalline diamond field effect transistors (FETs). The polycrystalline diamond had (110) preferred orientation, and its typical grain size was  100 μm. Well-resolved free exciton related emissions were observed at room temperature in cathodoluminescence. The FETs showed extremely high DC and RF performance. The cut-off frequency for current gain (f_T) and power gain (f_max) were 45 and 120 GHz, respectively. The maximum drain current (I_DS) was 550 mA/mm. These values are the highest among diamond FETs, including those fabricated from single-crystal diamond. These results suggest that high-quality polycrystalline diamond, whose maximum size is 4 in., is very promising for diamond electronic devices.     

Dispersion of surface acoustic waves in polycrystalline diamond plates

Five free-standing polycrystalline diamond plates were grown by electron-assisted hot filament CVD on molybdenum using different deposition parameters. The homogeneity of the elastic properties was studied for each side of the samples by surface acoustic waves (SAWs) using pulsed laser excitation and piezoelectric detection. From the velocities of the longitudinal acoustic bulk waves (LABWs) and SAWs, values of the Poisson ratio of approximately ν=0.14±0.06 were obtained. The Young's modulus varied between E=928±40 GPa and E=1098±43 GPa. Anomalous dispersion of the SAWs on the nucleation side was observed in some samples. In addition the mean velocity of the SAWs was generally higher on the nucleation side than on the growth side. This indicates that the elastic properties changed considerably during the growth process. Measurements of the grain size by scanning electron microscopy (SEM), of the phase purity by Raman spectroscopy, and the texture by X-ray diffraction (XRD) were performed in order to estimate the contribution of different effects to the dispersion and the side dependence of the SAW velocities. In addition Fourier transform infrared (FTIR) spectroscopy was applied to study the influence of the hydrogen content.     

镀钛金刚石制备金刚石聚晶的研究

在国产六面顶高压设备上,以镀钛金刚石为原料,镍基合金为烧结助剂,采用熔渗法成功制备了金刚石聚晶(PCD),通过扫描电镜(SEM)、X射线衍射(XRD)、拉曼光谱(Raman)等测试方法,研究了不同烧结压力和温度对镀钛金刚石聚晶组织形貌的影响,与普通金刚石聚晶进行了物相成分及残余应力的对比分析。实验结果表明:烧结条件为5.4~5.6GPa,1350℃~1450℃下的镀钛PCD具有较高的致密性和机械性能;镀钛PCD的衍射峰中有NiMnCo、碳化钛和TiMnC化合物。镀钛PCD相比普通的PCD表面残余应力略大。     

Structural and electrical properties of H-terminated diamond field-effect transistor

A dielectric barrier separating hydrogen induced p-type channel and Al gate metal contact of diamond FET has been investigated. The separation barrier is necessary to prevent tunneling current between the H-induced channel and the gate contact. In this investigation, CV measurements, fitting of forward IV characteristics, TEM and SIMS profiles have been used to obtain a more detailed picture of this barrier layer. While the composition of this layer is not clear, TEM and SIMS measurements indicate that this layer may be connected to a diamond phase or aluminium oxide. Using material properties of these materials, thickness of the separation layer extracted from the CV measurements was between 5–10 nm and the channel sheet change density was above 1 × 10¹³ cm^? 2. This thickness is in good agreement with the TEM observations. Frequency dependent CV measurements showed almost no frequency dependence, and no UV light dependence has been observed. Temperature dependent CV measurements showed a decrease of the dielectric constant at 100 °C. Fitting of the forward tunnelling current indicated a thickness of the barrier layer of about 5 nm with a barrier height of 2.4 eV.     

金刚石初始粒径对金刚石聚晶层形貌及表面残余应力的影响

在高温高压条件下(5.2~5.6 GPa,1350℃~1450℃),以镍基合金为烧结助剂采用熔渗法制备了金刚石复合片(PDC)。采用SEM考察了不同金刚石微粉粒径制备的金刚石复合片中金刚石聚晶(PCD)的组织形貌。通过R am an光谱利用静水压应力模型表征了PCD层表面应力。结果表明,PCD层形成了致密的交错生长结构;PCD层表面残余应力表现为压应力并随着初始金刚石微粉粒径的增大而增大,其值为0.12-0.22GPa。     

A MIS transistor using the nucleation surface of polycrystalline diamond

In this work, the nucleation surface of a polycrystalline diamond film was used for the first time to fabricate a MISFET structure using standard photolithographic procedures, with a channel length of 100 μm. The resulting structure works as an enhancement-type p-type MOSFET. The I_ON/I_OFF ratio is about three orders of magnitude. The saturation of the current is clearly observed, with I_DS currents of about 20 nA for V_DS of 20 V. The smoothness of the nucleation surface allows a higher control of the electrodes, as well as their size decrease. The results show that, even though in an early stage, this investigation opens the door for a new generation of devices built on free-standing diamond films.     

Negative electron affinity on polycrystalline diamond surface induced by lithium fluoride deposition

Lithium fluoride (LiF) was deposited on hydrogenated and oxygenated polycrystalline diamond surfaces. Lowering of the work function induced by the dipole effect of LiF was clearly observed using UV photoemission spectroscopy and Kelvin probe. The decrease of the work function was sufficiently large so that negative electron affinity was induced. This shows that the LiF overlayer on diamond acts as an effective dipole layer to lower the surface work function of the diamond film.     

Potential applications of surface channel diamond field-effect transistors

In order to realize high frequency and high power diamond devices, diamond FETs on the hydrogen-terminated diamond surface conductive layer have been fabricated. The fabricated diamond MESFETs show high breakdown voltage and output capability of 1 W mm⁻¹. High transconductance diamond MESFET utilizing a self-aligned gate FET fabrication process has been operated in high frequency for the first time. In the 2 μm gate MESFETs, the obtained cut off frequency f_T and maximum frequency of oscillation f_max are 2.2 and 7 GHz, respectively. It is expected that the diamond MESFET with 0.5 μm gate length fabricated by self-aligned gate process shows 8 GHz of f_T and 30 GHz of f_max.     

MESFET fabricated on deuterium-implanted polycrystalline diamond

Cr- and Al-gate MESFETs have been fabricated on deuterium-implanted polycrystalline diamond and characterized both in DC and RF regime. Their performances are compared with those of similar devices fabricated on plasma hydrogenated polycrystalline diamond, which suffer for instabilities related to the large sensitivity of the channel to surface adsorbates. It is shown that similar characteristics can be achieved both in MESFETs with deuterium-implanted and plasma hydrogenated diamond.     

Surfaces of undoped and boron doped polycrystalline diamond films influenced by negative DC bias voltage

The hydrogen ion bombardment is performed by applying a negative bias voltage to the substrate during microwave plasma chemical vapor deposition process, using only hydrogen as reactant gas. The size of (001) faces increases after hydrogen ion etching while other grains are etched off. The surfaces of [001]-oriented films after doping boron are investigated by scanning electron microscopy (SEM) and cathodoluminescent (CL) spectra. The absence of the band-A emission in the CL spectra means a low density of dislocation in the films. It is the first time that the peak at 741.5 nm and the broad peak at approximately 575 and 625 nm in the CL spectra were reduced efficiently after boron doping in (001) polycrystalline diamond films and to propose that these phenomena should be explained in simple terms with penetration of the lattice nets of the [001]-oriented faces model.     

MOSFETs on polished surfaces of polycrystalline diamond

MOSFETs on polished polycrystalline diamond surfaces have been fabricated. The best transconductance is 1.0 mS/mm. The breakdown voltages of these FETs are over 200 V, comparable to those reported in homoepitaxial diamond MESFETs. A polycrystalline diamond FET is a candidate for sensors in hard environment, due to its feasibility in large-area wafers.     

Carrier generation within the surface region of hydrogenated thin film polycrystalline diamond

Low temperature Hall effect measurements made on diamond films subjected to a hydrogenation process, such that the near surface region becomes p-type without the addition of conventional dopant atoms, are reported. The carrier concentration within the temperature range 10–300 K does not change as expected for most films, actually increasing as the temperature falls. However, polished films display more conventional behaviour in that the carrier concentration falls with falling temperature. A model involving carrier transport within both valance and impurity bands can be considered to explain these observations, leading to the suggestion that the hydrogenation process is capable of creating acceptor states with an activation energy within the range of 10–40 meV.     

n-type phosphorus-doped polycrystalline diamond on silicon substrates

The microwave plasma-assisted deposition of reproducible and homogeneously n-type phosphorus-doped polycrystalline (microcrystalline) diamond films on silicon substrates is described. The phosphorus incorporation is obtained by adding gaseous phosphine (PH₃) to the gas mixture during growth. The low CH₄/H₂ ratio (0.15%) and the use of the same growth parameters as for homoepitaxial {111} films, led to a good crystalline quality of the continuous polycrystalline diamond layers, confirmed by SEM images and Raman spectroscopy measurements.Secondary-ion mass spectrometry (SIMS) analysis measured a phosphorus concentration [P] of at least 7 × 10¹⁷ cm^− 3. Cathodoluminescence spectroscopy in our P-doped polycrystalline films shows a phosphorus bound exciton (BE^TO_P) peak between 5.142 and 5.181 eV. Cathodoluminescence and Raman-effect spectroscopy confirmed the improvement of the crystalline quality of our films as well as a decrease in the intensity of the internal strain when the grain size was decreased. Cathodoluminescence imaging and SIMS depth profile of phosphorus demonstrated a very good homogeneity of phosphorus incorporation in the films.     

热稳定金刚石聚晶的制备及表征

在高温高压条件下(5.6GPa,1200℃～1480℃),以舍硼金刚石微粉为原料,镍基合金为烧结助剂,采用熔渗法成功制备了热稳定金刚石聚晶(Thermally stable PCD).通过X射线衍射(XRD)和扫描电镜(SEM),研究了烧结温度对热稳定金刚石聚晶的物相成分、微观组织形貌的影响;并与普通金刚石聚晶进行了差热(DTA)、热重(TG)的对比分析测试,结合X射线光电子能谱(XPS)测试结果给出了相应解释.实验结果表明:在压强为5.6GPa条件下,温度在1300℃～1450℃区间内,才能实现热稳定PCD的烧结;此时形成的热稳定PCD的耐热性和抗氧化性相比普通PCD均有较大幅度提高.     

Antibacterial properties of polycrystalline diamond films

Electronic and mechanical properties, and their biocompatibility, make diamond-based materials promising biomedical applications. The cost required to produce high quality single crystalline diamond films is still a hurdle to prevent them from commercial applications, but the emergence of polycrystalline diamond (PCD) films grown by chemical vapour deposition (CVD) method has provided an affordable strategy. PCD films grown on silicon wafer have been used throughout and were fully characterised by SEM, XPS, Raman spectroscopy and FTIR. The samples contain nearly pure carbon, with impurities originated from the CVD growth and the silicon etching process. Raman spectroscopy revealed it contained tetrahedral amorphous carbon with small tensile stress. The sp² carbon content, comprised between 16.1 and 18.8%, is attributed to the diamond grain boundaries and iron-catalysed graphitisation. Antibacterial properties of PCD films were performed with two model bacteria, i.e. Escherichia coli (Gram-negative) and Staphylococcus aureus (Gram-positive) using direct contact and shaking flask methods. The samples showed strong bacteriostatic properties against S. aureus and E. coli with the direct contact method and no influence on planktonic bacterial growth. These results suggest that the bacteriostatic mechanism of PCD films is linked to their surface functional groups (carbon radicals and –NH₂ and –COOH groups) and that no diffusible molecules or components were involved.