High Electron Concentration Research Articles

Highly doped InP as a low loss plasmonic material for mid-IR region.

We study plasmonic properties of highly doped InP in the mid-infrared (IR) range. InP was grown by metal-organic vapor phase epitaxy (MOVPE) with the growth conditions optimized to achieve high free electron concentrations by doping with silicon. The permittivity of the grown material was found by fitting the calculated infrared reflectance spectra to the measured ones. The retrieved permittivity was then used to simulate surface plasmon polaritons (SPPs) propagation on flat and structured surfaces, and the simulation results were verified in direct experiments. SPPs at the top and bottom interfaces of the grown epilayer were excited by the prism coupling. A high-index Ge hemispherical prism provides efficient coupling conditions of SPPs on flat surfaces and facilitates acquiring their dispersion diagrams. We observed diffraction into symmetry-prohibited diffraction orders stimulated by the excitation of surface plasmon-polaritons in a periodically structured epilayer. Characterization shows good agreement between the theory and experimental results and confirms that highly doped InP is an effective plasmonic material aiming it for applications in the mid-IR wavelength range.

Capping Ti-Doped GaZnO on InGaZnO Layer As the Composite-Channel Structure for Enhancing the Device Performances and Stability of Thin-Film Transistors

The device performances of an amorphous indium gallium zinc oxide transparent thin-film transistor (a-IGZO TFT) were improved by using a Ti-doped GaZnO (GTZO) as the channel capping layer in this study. The GTZO thin film exhibited a high carrier concentration of $\text{1.5} \times \text{10} ^{21} \text{cm} ^{- 3}$ and served as both a carrier supplement and passivation layer for reducing the absorption of oxygen and moisture as well as the back-channel charge trapping in the composite-channel TFT. An enhanced carrier mobility with high electron concentration in the composite GTZO/IGZO heterostructure was verified in the Hall measurement. The GTZO/IGZO composite-channel TFT was demonstrated to show an improved carrier mobility of 15.5 cm2/V-s, subthreshold swing of 0.35 V/decade, off current of $1.4 \times 10^{- 11}$ A, threshold voltage of 1.4 V, and on/off current ratio of $1.2 \times 10 ^{7}$ . Stable operation with unchanged device characteristics was observed following a bias-stress test. Thus, the fabricated GTZO/IGZO TFTs exhibited advantageous device performance and improved device operation stability.

Experimental and theoretical investigation of Cr1-xScxN solid solutions for thermoelectrics

The ScN- and CrN-based transition-metal nitrides have recently emerged as a novel and unexpected class of materials for thermoelectrics. These materials constitute well-defined model systems for investigating mixing thermodynamics, phase stability, and band structure aiming for property tailoring. Here, we demonstrate an approach to tailor their thermoelectric properties by solid solutions. The trends in mixing thermodynamics and densities-of-states (DOS) of rocksalt-Cr1-xScxN solid solutions (0 ≤ x ≤ 1) are investigated by first-principles calculations, and Cr1-xScxN thin films are synthesized by magnetron sputtering. Pure CrN exhibits a high power factor, 1.7 × 10−3 W m−1 K−2 at 720 K, enabled by a high electron concentration thermally activated from N vacancies. Disordered rocksalt-Cr1-xScxN solid solutions are thermodynamically stable, and calculated DOS suggest the possibility for power-factor improvement by Sc3d orbital delocalization on Cr3d electrons giving decreasing electrical resistivity, while localized Cr3d orbitals with a large DOS slope may yield an improved Seebeck coefficient. Sc-rich solid solutions show a large improvement in power factor compared to pure ScN, and all films have power factors above that expected from the rule-of-mixture. These results corroborate the theoretical predictions and enable tailoring and understanding of structure-transport-property correlations of Cr1-xScxN.

Surface charge transfer doping of monolayer molybdenum disulfide by black phosphorus quantum dots

Monolayer molybdenum disulfide (MoS2), consisting of covalently bonded S-Mo-S sandwiched layers, has high carrier mobility and a direct bandgap of 1.8 eV, offering properties for electronic and optoelectronic devices with high performance. Usually, it is essential to modulate the carrier concentrations and conductivities of monolayer MoS2 for practical applications. In this paper, black phosphorus (BP) quantum dots (QDs) were synthesized by a liquid exfoliation method successfully, and have a diameter of ∼5 nm as confirmed with a transmission electron microscope (TEM). BP QDs were utilized to decorate monolayer MoS2 grown by chemical vapor deposition (CVD). The Raman and PL spectra of the BP QD/MoS2 hybrid structure clearly indicate that BP QDs are an effective n-type doping scheme for monolayer MoS2. Back-gated monolayer MoS2 transistors were fabricated and show an improved source–drain current after BP QD modifications. A high electron concentration of ∼5.39 × 1012 cm−2 in monolayer MoS2 was achieved, which is beneficial for designing FETs and photodetector devices with novel functions.

Passage of the discharge current through the plasma–electrode interface in the electromagnetic rail accelerator channel

We investigate the phenomena that accompany the acceleration of a free plasma piston (without a striker) in the electromagnetic rail accelerator channel filled with different gases (argon, helium). An intense glow appears in the shock-compressed layer (SCL) in the case of strong shock waves that produce a high electron concentration (~1017–1018 cm–3) behind the front. We have proposed that explosive electron emission (EEE) ensures the high-intensity emission of electrons, the passage of a part of the discharge current through the SCL, and the glow of the SCL. The velocity of a shock wave for which the strong electric field in the Debye layer at the cathode causes EEE from its surface and the passage of the current in the SCL has been determined. It has been concluded that, for high velocities of the plasma, the EEE is a universal mechanism that ensure the passage of a strong current through the interface between the cold electrode and the plasma.

Electrical and optical properties improvement of GeSn layers formed at high temperature under well-controlled Sn migration

Electrical and optical properties of GeSn layers formed at various growth conditions under changing deposition temperature (Td) and deposition speed (vd) were systematically investigated. A high Sn content of 3.0% leads to high electron mobility and electron concentration for an as-deposited sample compared with the Sn content of 1.9%. Subsequent annealing at 550°C after the growth is effective for improving the mobility and the activated carrier concentration. Moreover, the high vd and Td growth makes it possible to get clear photoluminescence (PL) signal from the band-to-band transition of the GeSn layers. The annealing at 550°C leads to the high and sharp PL spectra compared with those for the as-deposited samples. Consequently, we found that the high vd and Td growth of the GeSn layers suppressing the Sn migration is quite important for getting electrical and optical properties to realize future electrical and optical devices.

Pulsed laser annealing of highly doped Ge:Sb layers deposited on different substrates

Germanium (Ge) is a promising material for micro- and optoelectronics to produce high speed field-effect transistors, photodetectors, light-emitting diodes and lasers. For such applications tensile-strained and/or highly n-doped Ge layers are needed. The authors have performed the formation of such layers by ion-beam sputtering of composite Sb/Ge target, deposition of thin amorphous Ge:Sb films (~200 nm thick) on different substrates (c-Si, c-Al2O3, α-SiO2) followed by pulsed laser annealing (PLA) for their crystallization and Sb dopant activation. Structural, electrical and optical characterization of Ge:Sb films was carried out using scanning electron microscopy, x-ray diffraction, micro-Raman spectroscopy, secondary ion mass spectrometry methods and by measuring sheet resistance, carrier concentration and photoluminescence. The obtained polycrystalline n-Ge:Sb layers (NSb ~ 1 at.%) are characterized by increased values of tensile strain (up to 1%) and homogenious Sb dopant distribution within layer thickness. The electrical measurements at 300 K revealed the low sheet resistance (up to 40 Ω/□) and extremely high electron concentration (up to 5.5 × 1020 cm−3) in Ge:Sb/SiO2 samples that indicated full electrical activation of Sb dopant on SiO2 substrate. The increased values of tensile strain and electron concentration of Ge:Sb films on α-SiO2 are explained by low values of thermal conductivity and thermal expansion coefficients of quartz substrate.

(Invited) In-Situ Doped Epitaxial Growth of Highly Dopant-Activated n +-Ge Layers for Reduction of Parasitic Resistance of Ge-nMISFETs

Ge has been expected as a high-performance scaled-CMOS platform due to its high carrier mobility. As for nMISFET application, fabrication of shallow n +/p junction having high electron concentration by ion-implantation method is problematic due to high diffusivity and low solubility limit of n-type dopants in Ge [1]. For a metal / n +-Ge interface, moreover, Fermi-level is pinned to near the valence band edge, resulting in high specific contact resistance (ρc) [2]. In order to reduce parasitic resistances of the Ge-nMISFETs caused by the drawbacks described above, a carrier concentration of around 1x1020 cm-3 is needed in the n +-S/D region. In this report, a shallow n +-Ge layer was grown with in-situ P doped epitaxy using conventional low-pressure (LP) CVD for reduction of ρc. Moreover, drive-current of GeOI-nMISFETs was enlarged by utilizing the n +-Ge:P layer to the S/D region. P-doped n +-Ge (Ge:P) layers were grown on p-Ge substrates with LP-CVD using SiH4, GeH4 (10% in H2), PH3 (10% in He) and H2 as a carrier gas to investigate the dopant activation rates. Growth temperature and mass-flow (F(gas)) ratio of GeH4 to H2 were fixed at 400 oC and F(GeH4)/F(H2) = 1.67 x 10-3, respectively. Growth pressure was varied from 5 to 80 Torr. Ti layers were deposited on the Ge:P layers to measure the ρc of Ti / Ge:P contact by a transmission line measurement (TLM). As a reference, Ti / Ge:P TLM patterns were also fabricated on a P ion-implanted (10 keV, 1x15cm-3) Ge layer on a p-Ge substrate after an activation anneal (600 oC, 1 min. in N2). GeOI-nMISFETs having the Ge:P S/D were fabricated to evaluate transistor characteristics. Dopant and activated carrier concentration profiles of Ge:P samples as a parameter of total growth pressure are shown in Fig.1 (a) and (b), respectively. Here, F(PH3)/F(GeH4) is fixed to 3.0 x 10-3. Both dopant and carrier concentrations exhibit almost uniform profiles throughout the Ge:P layers. From Fig.1 (a) and (b), dopant and carrier concentration were plotted as a function of growth pressure as shown in Fig.2. This result exhibits a peak dopant activation rate of 0.7 at a growth pressure of 15 Torr. Here, an electron concentration of 7x1019 cm-3 was observed. This value far exceeds the solid solubility of P in Ge of around 1x1019 cm-3 at the growth temperature. Next, I/V curves observed in the TLM for the Ti / Ge:P and the Ti / P-implanted Ge layers having the same dopant concentrations of 1 x 1020 cm-3 are shown in Fig. 3. This result indicates that ohmic contacts are formed on the P-doped epitaxial Ge layer with a carrier concentration of 7x1019 cm-3. In contrast, Schottky-like contacts are observed for the P-implanted Ge having a carrier concentration of 2x1019 cm-3 and the P-doped epitaxial Ge layer having a carrier concentration of 2x1019 cm-3. The ρc extrapolated from TLM are 1.2x10-6 Ωcm2 for the epitaxial layer having carrier concentration of 7 x 1019 cm-3 and 1.6 x 10-5 Ωcm2 for P-implanted one. Sheet resistances (Rsh) of the n +-Ge layers are plotted as a function of layer thickness as shown in Fig. 4. A low Rsh of 33 (Ω/sqr.) was shown for the 65-nm-thick P-doped Ge layer due to the high carrier concentration and the high dopant activation rate. The value of Rsh for the epi-layer agrees with the theoretically predicted value and is the lowest among the values ever reported for the Ti / Ge:P layer [3]. Finally, GeOI-nMISFETs with the Ti / Ge:P contacts were fabricated through gate-last process flow shown in Fig. 5. Here, electron concentration of the GeOI layer was 4x1017cm-3 and TaN/Al2O3 gate-stack was deposited after O3 passivation [4]. An XTEM image of the GeOI-nMISFET is shown in Fig.6. Gate-length, Lg, and GeOI thickness are 60 nm and 37 nm, respectively. Id-Vd curves are shown in Fig. 7. Id of 300 μA/mm was obtained at Vd=1V, Vg-Vth=1.5V. This is almost 2.7 times larger than the largest Id of Ge-nMISFETs ever reported [4, 5]. These results suggest that current drive of Ge-nMISFETs can be enhanced by reduction of parasitic resistances such as ρc and Rsh by utilizing the in-situ doped Ge:P layers to the S/D. This work was granted by JSPS through FIRST Program initiated by the CSTP. [1] M. Koike et al., JAP104, p.023523 (2008) [2] T. Nishimura., APEX1, p.051406 (2008) [3] B. Yang et al., Proc. of ISTDM, p.88 (2012) [4] Y. Kamimuta et al., Proc. of ISDRS, FP8-04 (2013) [5] C.T. Chung et al., Tech. Dig. IEDM, p.383 (2012) Figure 1

(Electronics and Photonics Division Award) Physics of Wide Band Gap Semiconductor Devices

Silicon carbide and nitride based devices have found important commercial applications in both optical and electronic devices. AlGaN deep ultraviolet Light Emitting Diodes (LEDs) are now used for water purification, sterilization, and prolonging food storage. Silicon carbide and gallium nitride based transistors compete with silicon for high power applications and with III-V based transistors for RF applications. The device physics of all these devices is determined by the wurtzite (hexagonal) symmetry leading to strong spontaneous and piezoelectric polarization and by a large polar optical phonon energy, which is the consequence of the strong bonding and small atomic masses of carbon and nitrogen. In AlGaN/GaN and AlGaInN/InGaN devices, the differences in the polarization at the heterointerfaces induce two-dimensional (2D) electron or hole gases at the interfaces (polarization doping) with the electron sheet concentrations up to 10 to 20 times higher than those for more conventional field effect transistors (reaching over 4.5x1013 cm-2 for AlGaN/GaN High Electron Mobility Transistors (HEMTs)). In deep UV LEDs, the polarization induced 2D hole gas is used for improved p-type contacts. A high 2D electron concentration in the III-N HEMT channels enables superior switching characteristics but makes parasitic series resistances and capacitances to be very important. The high current carrying capability and a very high breakdown voltage lead to high power densities and make the issues of heat dissipation to be a priority. The thermal conductivity of SiC (490 W/m K at room temperature) is larger than that for copper (385 W/m K) making it to be an excellent choice as a substrate for power devices. For AlGaN/GaN HEMTs, the research started from using sapphire substrates, but silicon substrates have become a commercial technology. SiC, diamond, or AlN bulk or template substrates could have significant advantages over silicon. The substrates also affect the dislocation and point defect densities in the device channel and strain induced during growth and caused by a strong piezoelectric effect. Such strain is a crucial factor affecting the device reliability (linked to the transient and permanent detects induced under the voltage stress). The large bandgap suppresses the subthreshold leakage making the surface leakage to be one of the dominant leakage mechanisms. Ballistic, overshoot, and electron runaway effects play a dominant role in short channel wide band gap devices now competing for applications in the terahertz (far infrared) range. Figure 1

Structural and electronic properties of InN epitaxial layer grown on c-plane sapphire by chemical vapor deposition technique

Growth of InN epilayers on c-plane sapphire substrate by chemical vapor deposition technique using pure indium metal and ammonia as precursors has been systematically explored. It has been found that [0001] oriented indium nitride epitaxial layers with smooth surface morphology can be grown on c-plane sapphire substrates by optimizing the growth conditions. Bandgap of the film is observed to be Burstein–Moss shifted likely to be due to high background electron concentration. It has been found that the concentration of this unintentional doping decreases with the increase in the growth temperature and the ammonia flux. Epitaxial quality on the other hand deteriorates as the growth temperature increases. Moreover, the morphology of the deposited layer has been found to change from flat top islands to faceted mounds as the flow rate of ammonia increases. This phenomenon is expected to be related to the difference in surface termination character at low and high ammonia flow rates.

Kinetics of neutral particles in HCl and HBr plasmas at low pressures and high electron concentrations

The kinetics of the creation and annihilation of neutral particles in HCl and HBr plasmas under the conditions of standard industrial etching reactors is simulated. It is established that the two systems have similar kinetic schemes and exhibit only quantitative differences in plasma compositions. It is show that the HBr plasma is characterized by higher degrees of dissociation of the initial molecules and bromine atomic concentrations in the same temperature and electron concentration ranges.

Design and characterisations of double-channel GaAs pHEMT Schottky diodes based on vertically stacked MMICs for a receiver protection limiter

A microwave receiver protection limiter circuit has been designed, fabricated and tested using vertically stacked GaAs MMIC technology. The limiter circuit with a dimension of 2.5 × 1.3 mm2 is formed by using double-channel AlGaAs/InGaAs pseudomorphic HEMT (pHEMT) Schottky diodes integrated with a low-loss V-shaped coplanar waveguide multilayer structure. The electrical parameter characteristics of the pHEMT Schottky diodes are presented including the C–V profile showing the presence of a double channel in the device layer structure. This unique feature can also be seen from the double-peak responses of the electron density as a function of the device layer width, which represent the high electron concentration at two different 2-DEG layers of the structure. An equivalent circuit model of pHEMT Schottky diodes is demonstrated showing good agreement with the measurement results. At zero-bias condition, the devices show high performance in diode detector applications with voltage sensitivities of more than 89 mV μW−1 at 10 GHz and at least 5.4 mV μW−1 at 35 GHz. The measurement results of the limiter circuit demonstrated the blocking of input power signals greater than 20 dBm input power at 3 GHz. To the best of our knowledge this is the first demonstration of the use of pHEMT Schottky diodes in microwave power limiter applications.

Harnessing Topological Band Effects in Bismuth Telluride Selenide for Large Enhancements in Thermoelectric Properties through Isovalent Doping.

Dilute isovalent sulfur doping simultaneously increases electrical conductivity and Seebeck coefficient in Bi2 Te2 Se nanoplates, and bulk pellets made from them. This unusual trend at high electron concentrations is underpinned by multifold increases in electron effective mass attributable to sulfur-induced band topology effects, providing a new way for accessing a high thermoelectric figure-of-merit in topological-insulator-based nanomaterials through doping.

Defects and properties of cadmium oxide based transparent conductors

Transparent conductors play an increasingly important role in a number of semiconductor technologies. This paper reports on the defects and properties of Cadmium Oxide, a transparent conducting oxide which can be potentially used for full spectrum photovoltaics. We carried out a systematic investigation on the effects of defects in CdO thin films undoped and intentionally doped with In and Ga under different deposition and annealing conditions. We found that at low growth temperatures (<200 °C), sputter deposition tends to trap both oxygen vacancies and compensating defects in the CdO film resulting in materials with high electron concentration of ∼2 × 1020/cm3 and mobility in the range of 40–100 cm2/V s. Thermal annealing experiments in different ambients revealed that the dominating defects in sputtered CdO films are oxygen vacancies. Oxygen rich CdO films grown by sputtering with increasing O2 partial pressure in the sputter gas mixture results in films with resistivity from ∼4 × 10−4 to >1 Ω cm due to incorporation of excess O in the form of O-related acceptor defects, likely to be O interstitials. Intentional doping with In and Ga donors leads to an increase of both the electron concentration and the mobility. With proper doping CdO films with electron concentration of more than 1021 cm−3 and electron mobility higher than 120 cm2/V s can be achieved. Thermal annealing of doped CdO films in N2 ambient can further improve the electrical properties by removing native acceptors and improving film crystallinity. Furthermore, the unique doping behavior and electrical properties of CdO were explored via simulations based on the amphoteric defect model. A comparison of the calculations and experimental results show that the formation energy of native donors and acceptors at the Fermi stabilization energy is ∼1 eV and that the mobility of sputtered deposited CdO is limited by a background acceptor concentration of ∼5–6 × 1020/cm3. The calculations offer an insight into understanding of the effects of defects on electrical properties of undoped and doped CdO and offer a potential to use similar methods to analyze doping and defect properties of other semiconductor materials.

Voltage-controlled reactive magnetron sputtering of Nb-doped TiO2 films: electrical and optical properties

ABSTRACTNiobium-doped TiO2 films as highly transparent conducting oxidic electrodes were prepared by reactive magnetron sputtering from a titanium target in an argon-oxygen gas flow. As-deposited films were amorphous and exhibited high resistivities ranging from 10 to 1×105 Ω cm in dependence on the deposition parameters. We stabilized the reactive magnetron sputtering deposition by adjusting the magnetron discharge voltage at a constant oxygen gas flow. The precise process control during the preparation of the as-deposited films was essential to gain low resistivities (10-3 Ω cm) and low optical absorption coefficients (α550nm < 2×103 cm-1) after annealing. These polycrystalline TiO2:Nb films on borosilicate glass show a quite high electron concentration > 1×1020 cm-3 and a high carrier mobility (≈ 8 cm2 V-1 s-1).

Effect of in situ Sb doping on crystalline and electrical characteristics of n-type Ge1−xSnx epitaxial layer

We examined the molecular beam epitaxy of Ge1−xSnx with in situ Sb doping on Ge substrates. The effects of Sb doping on the crystalline and electrical characteristics of Ge1−xSnx epitaxial layer were investigated in detail. We found that Sb doping with a concentration of 1020 cm−3 remarkably improves the crystallinity, and surface uniformity of the Ge1−xSnx epitaxial layer by changing the growth mode by the surfactant effect of Sb atoms. Low-temperature Ge1−xSnx growth with in situ Sb doping realizes a very high electron concentration of 1020 cm−3, which is above the thermal equilibrium solid solubility, as a result of suppressing Sb segregation and precipitation.

Electronic and optical characteristics of an m-plane GaN single crystal grown by hydride vapor phase epitaxy on a GaN seed synthesized by the ammonothermal method using an acidic mineralizer

Fundamental electronic and optical properties of a low-resistivity m-plane GaN single crystal, which was grown by hydride vapor phase epitaxy on a bulk GaN seed crystal synthesized by the ammonothermal method in supercritical ammonia using an acidic mineralizer, were investigated. The threading dislocation and basal-plane staking-fault densities of the crystal were around 104 cm−2 and less than 100 cm−1, respectively. Oxygen doping achieved a high electron concentration of 4 × 1018 cm−3 at room temperature. Accordingly, a photoluminescence (PL) band originating from the recombination of hot carriers was observed at low temperatures, even under weak excitation conditions. The simultaneous realization of low-level incorporation of Ga vacancies (VGa) less than 1016 cm−3 was confirmed by using the positron annihilation technique. Consistent with our long-standing claim that VGa complexes are the major nonradiative recombination centers in GaN, the fast-component PL lifetime of the near-band-edge emission at room temperature longer than 2 ns was achieved.

Effect of Retrapping on Thermoluminescence Peak Intensities of Small Amorphous Silicon Quantum Dots

between the edge of the conduction band and the level of the trap is called trap depth (activation energy). The shallowest trap gives the highest thermoluminescence intensity peak for first order kinetics. However, it was clearly observed that for second order and a case beyond second order kinetics, the thermoluminescence intensity peak corresponding to each trap does not depend on the trap depth. In this case, the retrapping probability coecients are taken into account and most electrons which are detrapped from the shallow trap(s) will be retrapped to the deeper trap(s) resulting in fewer electrons taking part in the recombination process. This significantly reduces the thermoluminescence intensity peaks of the shallower trap(s). It was observed that the deepest trap, with very high concentration of electrons due to the retrapping phenomenon, gives the highest thermoluminescence intensity. In addition, the variation of concentration of electrons in each trap and the intensity of the thermoluminescence are presented. Though we considered the model of three traps and one recombination center, this phenomenon is true for any multiple traps.

Conduction mechanism in highly doped β-Ga2O3 single crystals grown by edge-defined film-fed growth method and their Schottky barrier diodes

Edge-defined fed-grown β-Ga2O3 single crystals with high electron concentration of 3.9 × 1018 cm−3 at 300 K were characterized by Hall effect measurement, and Schottky barrier diodes have been demonstrated. Electron mobility was as high as 74 cm2/(V·s) at 300 K regardless of the high doping concentration. The electron concentration did not change substantially in the low temperature below 160 K. This properties can be explained by the two-band model due to the inter-band conduction. On the Schottky barrier diodes, the rectification characteristics were clearly observed, and the current density of 96.8 A/cm2 at the forward voltage of 1.6 V was obtained.

Nuclear magnetic resonance studies of DNP-ready trehalose obtained by solid state mechanochemical amorphization.

(1)H nuclear spin-lattice relaxation and Dynamic Nuclear Polarization (DNP) have been studied in amorphous samples of trehalose sugar doped with TEMPO radicals by means of mechanical milling, in the 1.6-4.2 K temperature range. The radical concentration was varied between 0.34 and 0.81%. The highest polarization of 15% at 1.6 K, observed in the sample with concentration 0.50%, is of the same order of magnitude of that reported in standard frozen solutions with TEMPO. The temperature and concentration dependence of the spin-lattice relaxation rate 1/T1, dominated by the coupling with the electron spins, were found to follow power laws with an exponent close to 3 in all samples. The observed proportionality between 1/T1 and the polarization rate 1/Tpol, with a coefficient related to the electron polarization, is consistent with the presence of Thermal Mixing (TM) and a good contact between the nuclear and the electron spins. At high electron concentration additional relaxation channels causing a decrease in the nuclear polarization must be considered. These results provide further support for a more extensive use of amorphous DNP-ready samples, obtained by means of comilling, in dissolution DNP experiments and possibly for in vivo metabolic imaging.