To study ion beam induced modifications into MOCVD grown wurtzite AlInN layers, neon ions were im... more To study ion beam induced modifications into MOCVD grown wurtzite AlInN layers, neon ions were implanted on the samples with four doses ranging from 10 14 to 9 × 10 15 ions/cm 2 . Structural characterization was carried out by X-ray diffraction and Rutherford backscattering spectroscopy (RBS) techniques. XRD analysis revealed that GaN related peak for all samples remains at its usual Bragg position of 2θ = 34.56 • whereas a shift in AlInN peak takes place from its position of 2θ = 35.51 • for as-grown sample. Rutherford back scattering (RBS) analysis indicated that peak related to Ga atoms in capping layer provided evidence of partial sputtering of GaN cap layers. Moreover, Al peak position is shifted towards lower channel side and width of the signal is increased after implantation, which pointed to the inwards migration of Al atoms away from the AlInN surface. The results suggested that partial sputtering of cap layer has taken place without uncovering the underneath AlInN layer.
Surface Topography: Metrology and Properties, 2014
Metal organic chemical vapor deposited (MOCVD) thin films of aluminum nitride (AlN) were irradiat... more Metal organic chemical vapor deposited (MOCVD) thin films of aluminum nitride (AlN) were irradiated with 700 keV Ni ions at fluences of 1 × 10 12 , 1 × 10 13 , and 1 × 10 14 ions cm −2 . The stopping and range of ions in matter (SRIM) analysis was performed to investigate the depth distribution of the Ni ions and vacancy production in AlN film. The x-ray diffraction (XRD) patterns of the implanted samples show a shift of the AlN (0 0 2) orientation peak towards higher angles at 1 × 10 12 ions cm −2 , exhibiting the incorporation of nickel ions into the AlN phase. The XRD patterns also demonstrated a reduction in shift of the (0 0 2) orientation peak along with the formation of AlNi 3 phase with the increase of ion fluence. The AFM surface analysis of the ionirradiated AlN film exhibits a rise of film surface roughness. After ion irradiation, the samples were annealed at 900°C in a nitrogen environment. Annealing reduces the surface roughness of not only the implanted samples but also the as-grown samples.
Effects of proton irradiation on Ho doped AlN thin films are investigated. The irradiation is per... more Effects of proton irradiation on Ho doped AlN thin films are investigated. The irradiation is performed in the dose range of 1013 to 1014 ions/cm2 at room temperature. The effect of proton bombardment is studied through a systematic investigation of the structural properties using Rutherford Backscattering Spectroscopy (RBS), X-ray diffraction (XRD), and X-ray Photoemission Spectroscopy (XPS). The optical properties and the band gap change after irradiation process are studied using Defuse Reflectance Spectroscopy (DRS) technique. The electrical behavior of the material is also investigated after irradiation of AlN:Ho. The results show that high-energy protons cause a band gap change in the material, which can be exploited in developing various applications.
In this paper, an alternative method to characterize the interface between 4H polytype of Silicon... more In this paper, an alternative method to characterize the interface between 4H polytype of Silicon Carbide (4H-SiC) and passivating dielectric layers is established. The studies are made on dielectric-semiconductor test structures using Al2O3 as dielectric on 4H-SiC n-type epitaxial layers. Samples with different pre- and post-dielectric deposition preparations have been fabricated on epilayers of varying thicknesses. Effective lifetimes (τeff) of all the samples were measured by an optical pump-probe method utilizing free carrier absorption (FCA) to analyse the influence of the 4H-SiC/dielectric interface on charge carrier recombination. The relative contribution to τeff from the surfaces increases with decreasing epilayer thickness, and by analysing the data in combination with numerical modelling, it is possible to extract values of the surface recombination velocities (SRVs) for interfaces prepared in different ways. For instance, it is found that SRV for a standard cleaning procedure is 2 × 106 cm/s compared to a more elaborate RCA process, yielding a more than 50 times lower value of 3.5 × 104 cm/s. Furthermore, the density of interface traps (Dit) is extracted from capacitance-voltage (CV) measurements using the Terman method and a comparison is made between the SRV extracted from FCA measurements and Dits extracted from CV measurements on the same structures fabricated with metal contacts. It is observed that the SRV increase scales linearly with the increase in Dit. The strong qualitative correlation between FCA and CV data shows that FCA is a useful characterization technique, which can also yield more quantitative information about the charge carrier dynamics at the interface.
In this study we present the dependence of electrical properties of copper phthalocyanine (CuPc) ... more In this study we present the dependence of electrical properties of copper phthalocyanine (CuPc) and metal free phthalocyanine (H2Pc) bulk heterojunction structure under different illumination levels. To fabricate the device on ITO coated glass substrate the bulk heterojunction thin film of CuPc and H2Pc with thickness varying from 100 nm to 300 nm are deposited by thermal evaporator. Aluminum thin film was deposited by thermal evaporation as a top contact. The optical properties of the fabricated device are investigated using UV–vis spectroscopy. The current-voltage characteristics in dark and under illumination show that the device is sensitive towards visible light. The absorption spectrum describes its photo sensitivity in the range of wavelength from 200 nm to 850 nm. Simulation of current-intensity of light curve is carried out and experimental results are found in good agreement with simulated ones.
Metal organic chemical vapor deposited (MOCVD) thin films of aluminum nitride (AlN) were irradiat... more Metal organic chemical vapor deposited (MOCVD) thin films of aluminum nitride (AlN) were irradiated with 700 keV Ni ions at fluences of 1 × 1012, 1 × 1013, and 1 × 1014 ions cm−2. The stopping and range of ions in matter (SRIM) analysis was performed to investigate the depth distribution of the Ni ions and vacancy production in AlN film. The x-ray diffraction (XRD) patterns of the implanted samples show a shift of the AlN (0 0 2) orientation peak towards higher angles at 1 × 1012 ions cm−2, exhibiting the incorporation of nickel ions into the AlN phase. The XRD patterns also demonstrated a reduction in shift of the (0 0 2) orientation peak along with the formation of AlNi3 phase with the increase of ion fluence. The AFM surface analysis of the ion-irradiated AlN film exhibits a rise of film surface roughness. After ion irradiation, the samples were annealed at 900 °C in a nitrogen environment. Annealing reduces the surface roughness of not only the implanted samples but also the as-grown samples.
A steady improvement in material quality and process technology has made electronic silicon carbi... more A steady improvement in material quality and process technology has made electronic silicon carbide devices commercially available. Both rectifying and switched devices can today be purchased from several vendors. This successful SiC development over the last 25 years can also be utilized for other types of devices, such as light emitting and photovoltaic devices, however, there are still critical problems related to material properties and reliability that need to be addressed. This contribution will focus on surface passivation of SiC devices. This issue is of utmost importance for further development of SiC MOSFETs, which so far has been limited by reliability and low charge carrier surface mobilities. Also bipolar devices, such as BJTs, LEDs, or PV devices will benefit from more efficient and reliable surface passivation techniques in order to maintain long charge carrier lifetimes. Silicon carbide material enables the devices to operate at higher electric fields, higher temperatures and in more radiation dense applications than silicon devices. To be able to utilize the full potential of the SiC material, it is therefore necessary to develop passivation layers that can sustain these more demanding operation conditions. In this presentation it will also be shown that passivation layers of Al2O3 deposited by atomic layer deposition have shown superior radiation hardness properties compared to traditional SiO2-based passivation layers.
Carbon doping in hexagonal multi-walled boron nitride nanotubes (h-MWBNNTs) through ion implantat... more Carbon doping in hexagonal multi-walled boron nitride nanotubes (h-MWBNNTs) through ion implantation is successfully achieved. Nuclear reaction analysis (NRA) confirms that carbon atoms are homogeneously doped into the BNNTs, while Fourier transform infrared spectroscopy (FTIR) reveals that the C-N bonding is produced. Moreover, B-C-N phases are confirmed by XRay Diffraction (XRD) and Raman spectroscopy after C+ ion implantation. High resolution transmission electron microscopy (HRTEM) results show that BNNTs are slightly damaged by C+ ion implantation. UV absorption spectra show that carbon doping reduces the band gap of BNNTs from 5.5 eV to 4.6 eV by increasing C+ ion fluence. It is suggested that the bandgap decrease is due to carbon doping as well as defect formation in BNNTs. Carbon implantation in h-BNNTs is proposed to the knockout ejections of B and N atoms by high energy C+ ions and consequently make ternary B-C-N structure.
A theoretical design assessment is presented using two dimensional numerical computer aided desig... more A theoretical design assessment is presented using two dimensional numerical computer aided design (TCAD) tool for 15–20 kV 4H–SiC IGBTs. Physical parameters of the layer structures such as drift layer thickness, doping in the drift layer, JFET region width and interface charges underneath the gate region are varied to predict the device performance. Performance is further assessed at different temperatures and with different carrier lifetime in the drift layer. Using identical set of physical device parameters (doping, thicknesses), simulated structure was first calibrated with the experimental data. Simulations show that a minority carrier lifetime in the drift layer of 1.0–1.6 μs produces a close match with the experimental device. An on-resistance first decays with temperature (i.e., increased in ionization level, and increase in minority carrier lifetime), stays nearly constant with further increase in the temperature (may be all carriers are now fully ionized and increase in carrier lifetime is compensated with decrease in the carrier mobility) and finally increases linearly with temperature (>450 K) due to decrease in the carrier mobility. A significant increase in the forward voltage drop is observed with the presence of interface trap charges. Increasing JFET region width slightly decreases the forward on-state voltage. A drift layer of at least 175 μm thick with a doping concentration of < 2.0 × 1014 cm−3 is required to get a blocking voltage of 20 kV for 4H–SiC IGBTs assuming a minority carrier lifetime of 1 μs in the drift layer.
To study ion beam induced modifications into MOCVD grown wurtzite AlInN layers, neon ions were im... more To study ion beam induced modifications into MOCVD grown wurtzite AlInN layers, neon ions were implanted on the samples with four doses ranging from 1014 to 9×1015 ions/cm2. Structural characterization was carried out by X-ray diffraction and Rutherford backscattering spectroscopy (RBS) techniques. XRD analysis revealed that GaN related peak for all samples remains at its usual Bragg position of 2θ=34.56° whereas a shift in AlInN peak takes place from its position of 2θ=35.51° for as-grown sample. Rutherford back scattering (RBS) analysis indicated that peak related to Ga atoms in capping layer provided evidence of partial sputtering of GaN cap layers. Moreover, Al peak position is shifted towards lower channel side and width of the signal is increased after implantation, which pointed to the inwards migration of Al atoms away from the AlInN surface. The results suggested that partial sputtering of cap layer has taken place without uncovering the underneath AlInN layer.
ECS Journal of Solid State Science and Technology, Jul 2013
The electrical and chemical properties of high-k dielectric stacks consisting of Hafnium oxide (H... more The electrical and chemical properties of high-k dielectric stacks consisting of Hafnium oxide (HfO2) and Aluminum oxide (Al2O3) deposited on 4H-SiC have been investigated by preparing metal insulator semiconductor (MIS) structures of HfO2/Al2O3/SiC. The multilayer gate stack was deposited by using atomic layer deposition (ALD). The samples were also treated by rapid thermal annealing (RTA) at 970 oC for 5 mins in an inert gas atmosphere. Structural properties of the deposited films were analyzed with X-ray diffraction (XRD), atomic force microscopy (AFM) and Rutherford backscattering spectroscopy (RBS). Capacitance-voltage (CV) measurements performed on as-deposited and RTA treated structures at room temperature show that the RTA treatment increases the effective oxide charges in the whole dielectric but decreases the interface trap density. Current-voltage (IV) measurements have been performed in order to extract the leakage current density as well as the breakdown characteristics of the stack. Our results show that a combination of HfO2 and Al2O3 can be a better choice for SiC than individual Al2O3 layer because of the higher value of effective dielectric constant. It is shown, that the stacked dielectrics are stable at high temperatures and under annealing conditions up to 300 oC, which makes the layers compatible with SiC device processing and higher operation temperatures compared to silicon.
Zinc oxide (ZnO) nanowires (NWs) are exposed to energetic proton (H+), nitrogen (N+), phosphorus ... more Zinc oxide (ZnO) nanowires (NWs) are exposed to energetic proton (H+), nitrogen (N+), phosphorus (P+) and argon (Ar+) ions to understand the radiation hardness and structural changes induced by these irradiations. High-resolution transmission electron microscopy (HRTEM) is utilized to see the irradiation effects in NWs. Multiple doses and energies of radiation at different temperatures are used for different set of samples. The study reveals that wurtzite (crystalline) structured ZnO NWs experience amorphization, degradation and morphological changes after the irradiation. At room temperature, deterioration of the crystalline structure is observed under high fluence of H+, N+ and P+ ions. While for ZnO NWs, bombarded by Ar+ and P+ ions, nano-holes are produced. The ZnO NWs surfaces also show corrugated morphology full of nano-humps when irradiated by Ar+ ions at 400oC. The corrugated surface could serve as tight-holding interface when interconnects it with other NWs/nanotubes. These nano-humps may have the function of increasing the surface for surface-oriented sensing applications in the future.
This paper aims to establish a new method to characterize the interface between 4H-SiC and passiv... more This paper aims to establish a new method to characterize the interface between 4H-SiC and passivating dielectric layers. The investigations are made on MOS test structures utilizing Al2O3 and SiO2 dielectrics on 4H-SiC. These devices are then exposed to various fluences of Ar+ implantation and then measured by the new method utilizing optical free carrier absorption (FCA) technique to assess the interface traps. A program has been developed using MATLAB to extract surface recombination velocity (SRV) at the oxide/epi-layer interface from the optical data. Capacitance-voltage (CV) is done to extract the density of interface traps (Dit) and a comparison was made. It is observed that SiO2 samples show a large rise of SRVs, from 0.5×104 cm/s for a reference sample to 8×104 cm/s for a fluence of 1×1012 cm-2, whereas Al2O3 samples show more stable SRV, changing from 3×104 cm/s for the un-irradiated reference sample to 6×104 cm/s for a fluence of 1×1012 cm-2. A very similar trend is observed for Dit values extracted from CV measurements and it can therefore be concluded that the FCA method is a suitable technique for the interface characterization.
The influence of bulk traps in different regions of 4H-SiC bipolar junction transistors (BJTs) is... more The influence of bulk traps in different regions of 4H-SiC bipolar junction transistors (BJTs) is investigated. The investigation is based on experimental results obtained by implanting up to 1011 cm-2 doses of helium ions in the collector region. The results indicate that implantations, creating point defect concentrations in the range of the doping level produce a sufficiently high concentration of traps to reduce the carrier concentration in this specific region. These traps degrade the device characteristics and are irrecoverable up to 500 oC annealing. The experimental results are qualitatively analyzed by device simulations using a two dimensional numerical computer aided design tool (TCAD). Systematic simulations are then performed by introducing traps at different locations in the BJT (i.e. emitter, base and collector region). The results indicate that the device performance is highly dependent on the defect concentration in the base region. The defects at different levels inside the collector also influence the device by producing a compensated layer in the material. However, the same concentration of defects (i.e. ~1015 cm-3) has less influence in the emitter region.
Degradation of SiO2 surface passivation for 4H-SiC power BJTs as a result of ion irradiation has ... more Degradation of SiO2 surface passivation for 4H-SiC power BJTs as a result of ion irradiation has been studied to assess the radiation hardness of these devices. Fully functional BJTs with 2700 V breakdown voltage are implanted with 600 keV helium ions at fluences, ranging from 1×1012 to 1×1016 cm-2 at room temperature. These ions are estimated to reach the SiO2/SiC interface. The current-voltage characteristics before and after irradiation show that the current gain of the devices start degrading after a helium fluence of 1×1014 cm-2 and decreases up to 20% for the highest fluence of ions. Simulations show that the helium ions induce ionization inside the SiO2, which increases the interface charge and leads to a degradation of the BJT performance. Thermal annealing of the irradiated devices at 300, 420 and 500 oC further increases the amount of charge at the interface, resulting in increased base current in the low voltage range.
The radiation hardness of Al 2 O 3 as a dielectric for SiC surface passivation is studied and com... more The radiation hardness of Al 2 O 3 as a dielectric for SiC surface passivation is studied and compared to SiO 2 for potential application in radiation hard SiC devices. SiO 2 is deposited on 4H-SiC by PECVD and post annealed in N 2 O, whereas Al 2 O 3 is deposited by atomic layer deposition (ALD). The oxides are bombarded with Ar ions in an energy range to produce maximum damage near the oxide/SiC interface. Metal-insulator-semiconductor structures are prepared and their dielectric characteristics are analyzed using capacitance-voltage measurements. Additionally, the effect of the interface damage on surface recombination is studied using the optical free carrier absorption method for the same samples. The results indicate that the SiO 2 /SiC interface is significantly affected at 1×10 11 cm -2 fluence of Ar ions, however, the dielectric properties of Al 2 O 3 /SiC interface remain unaffected even for ten times higher fluences. Similar observations are made for the surface recombination measurements.
Electronic components, based on current semiconductor technologies and operating in radiation ric... more Electronic components, based on current semiconductor technologies and operating in radiation rich environments, suffer degradation of their performance as a result of radiation exposure. Silicon carbide (SiC) provides an alternate solution as a radiation hard material, because of its wide bandgap and higher atomic displacement energies, for devices intended for radiation environment applications. However, the radiation tolerance and reliability of SiC-based devices needs to be understood by testing devices under controlled radiation environments. These kinds of studies have been previously performed on diodes and MESFETs, but multilayer devices such as bipolar junction transistors (BJT) have not yet been studied.
Ion implantation effects at SiO2/SiC and Al2O3/ SiC interfaces have been investigated by implanti... more Ion implantation effects at SiO2/SiC and Al2O3/ SiC interfaces have been investigated by implanting Ar ions at the interface of oxide and SiC. Capacitance–voltage relation and breakdown properties for these dielectrics are studied before and after implantation. The results indicate that the SiO2/SiC interface is sensitive to ion fluences higher than 1 × 1011 cm−2, while Al2O3 on SiC can sustain higher fluences. In addition, the breakdown of the Al2O3 is found to be less sensitive to the ion implantation.
n- and p-type 4H-SiC epilayers were grown on heavily doped SiC substrates. The thickness of the p... more n- and p-type 4H-SiC epilayers were grown on heavily doped SiC substrates. The thickness of the p-type layer was 7 μm and the doping level around 1017 cm–3, while the n-type epilayers were 15 μm thick and had a doping concentration of 3 – 5×1015 cm–3. Several different surface treatments were then applied on the epilayers for surface passivation: SiO2 growth, Al2O3 deposited by atomic layer deposition, and Ar-ion implantation. Using collinear pump – probe technique the effective carrier lifetimes were measured from various places and statistical lifetime distributions were obtained. For surface recombination evaluation, two models are presented. One states that surface recombination velocity (SRV) is equal on both the passivation/epi layer interface (S2) and the deeper interface between the epilayer and the SiC substrate (S1), i. e. (S1 = S2). The other model is simulated assuming that SRV in the epilayer/substrate (S1) interface is constant while in the passivation layer/epilayer (S2) interface SRV can be varied S2 < S1. Empirical nomograms are presented with various parameters sets to evaluate S2 values. We found that on the investigated 4H-SiC surfaces S2 ranges from 3×104 to 5×104 assuming that the bulk lifetime is 4 μs. In Ar+ implanted surfaces S2 is between (105 – 106) cm/s.
To study ion beam induced modifications into MOCVD grown wurtzite AlInN layers, neon ions were im... more To study ion beam induced modifications into MOCVD grown wurtzite AlInN layers, neon ions were implanted on the samples with four doses ranging from 10 14 to 9 × 10 15 ions/cm 2 . Structural characterization was carried out by X-ray diffraction and Rutherford backscattering spectroscopy (RBS) techniques. XRD analysis revealed that GaN related peak for all samples remains at its usual Bragg position of 2θ = 34.56 • whereas a shift in AlInN peak takes place from its position of 2θ = 35.51 • for as-grown sample. Rutherford back scattering (RBS) analysis indicated that peak related to Ga atoms in capping layer provided evidence of partial sputtering of GaN cap layers. Moreover, Al peak position is shifted towards lower channel side and width of the signal is increased after implantation, which pointed to the inwards migration of Al atoms away from the AlInN surface. The results suggested that partial sputtering of cap layer has taken place without uncovering the underneath AlInN layer.
Surface Topography: Metrology and Properties, 2014
Metal organic chemical vapor deposited (MOCVD) thin films of aluminum nitride (AlN) were irradiat... more Metal organic chemical vapor deposited (MOCVD) thin films of aluminum nitride (AlN) were irradiated with 700 keV Ni ions at fluences of 1 × 10 12 , 1 × 10 13 , and 1 × 10 14 ions cm −2 . The stopping and range of ions in matter (SRIM) analysis was performed to investigate the depth distribution of the Ni ions and vacancy production in AlN film. The x-ray diffraction (XRD) patterns of the implanted samples show a shift of the AlN (0 0 2) orientation peak towards higher angles at 1 × 10 12 ions cm −2 , exhibiting the incorporation of nickel ions into the AlN phase. The XRD patterns also demonstrated a reduction in shift of the (0 0 2) orientation peak along with the formation of AlNi 3 phase with the increase of ion fluence. The AFM surface analysis of the ionirradiated AlN film exhibits a rise of film surface roughness. After ion irradiation, the samples were annealed at 900°C in a nitrogen environment. Annealing reduces the surface roughness of not only the implanted samples but also the as-grown samples.
Effects of proton irradiation on Ho doped AlN thin films are investigated. The irradiation is per... more Effects of proton irradiation on Ho doped AlN thin films are investigated. The irradiation is performed in the dose range of 1013 to 1014 ions/cm2 at room temperature. The effect of proton bombardment is studied through a systematic investigation of the structural properties using Rutherford Backscattering Spectroscopy (RBS), X-ray diffraction (XRD), and X-ray Photoemission Spectroscopy (XPS). The optical properties and the band gap change after irradiation process are studied using Defuse Reflectance Spectroscopy (DRS) technique. The electrical behavior of the material is also investigated after irradiation of AlN:Ho. The results show that high-energy protons cause a band gap change in the material, which can be exploited in developing various applications.
In this paper, an alternative method to characterize the interface between 4H polytype of Silicon... more In this paper, an alternative method to characterize the interface between 4H polytype of Silicon Carbide (4H-SiC) and passivating dielectric layers is established. The studies are made on dielectric-semiconductor test structures using Al2O3 as dielectric on 4H-SiC n-type epitaxial layers. Samples with different pre- and post-dielectric deposition preparations have been fabricated on epilayers of varying thicknesses. Effective lifetimes (τeff) of all the samples were measured by an optical pump-probe method utilizing free carrier absorption (FCA) to analyse the influence of the 4H-SiC/dielectric interface on charge carrier recombination. The relative contribution to τeff from the surfaces increases with decreasing epilayer thickness, and by analysing the data in combination with numerical modelling, it is possible to extract values of the surface recombination velocities (SRVs) for interfaces prepared in different ways. For instance, it is found that SRV for a standard cleaning procedure is 2 × 106 cm/s compared to a more elaborate RCA process, yielding a more than 50 times lower value of 3.5 × 104 cm/s. Furthermore, the density of interface traps (Dit) is extracted from capacitance-voltage (CV) measurements using the Terman method and a comparison is made between the SRV extracted from FCA measurements and Dits extracted from CV measurements on the same structures fabricated with metal contacts. It is observed that the SRV increase scales linearly with the increase in Dit. The strong qualitative correlation between FCA and CV data shows that FCA is a useful characterization technique, which can also yield more quantitative information about the charge carrier dynamics at the interface.
In this study we present the dependence of electrical properties of copper phthalocyanine (CuPc) ... more In this study we present the dependence of electrical properties of copper phthalocyanine (CuPc) and metal free phthalocyanine (H2Pc) bulk heterojunction structure under different illumination levels. To fabricate the device on ITO coated glass substrate the bulk heterojunction thin film of CuPc and H2Pc with thickness varying from 100 nm to 300 nm are deposited by thermal evaporator. Aluminum thin film was deposited by thermal evaporation as a top contact. The optical properties of the fabricated device are investigated using UV–vis spectroscopy. The current-voltage characteristics in dark and under illumination show that the device is sensitive towards visible light. The absorption spectrum describes its photo sensitivity in the range of wavelength from 200 nm to 850 nm. Simulation of current-intensity of light curve is carried out and experimental results are found in good agreement with simulated ones.
Metal organic chemical vapor deposited (MOCVD) thin films of aluminum nitride (AlN) were irradiat... more Metal organic chemical vapor deposited (MOCVD) thin films of aluminum nitride (AlN) were irradiated with 700 keV Ni ions at fluences of 1 × 1012, 1 × 1013, and 1 × 1014 ions cm−2. The stopping and range of ions in matter (SRIM) analysis was performed to investigate the depth distribution of the Ni ions and vacancy production in AlN film. The x-ray diffraction (XRD) patterns of the implanted samples show a shift of the AlN (0 0 2) orientation peak towards higher angles at 1 × 1012 ions cm−2, exhibiting the incorporation of nickel ions into the AlN phase. The XRD patterns also demonstrated a reduction in shift of the (0 0 2) orientation peak along with the formation of AlNi3 phase with the increase of ion fluence. The AFM surface analysis of the ion-irradiated AlN film exhibits a rise of film surface roughness. After ion irradiation, the samples were annealed at 900 °C in a nitrogen environment. Annealing reduces the surface roughness of not only the implanted samples but also the as-grown samples.
A steady improvement in material quality and process technology has made electronic silicon carbi... more A steady improvement in material quality and process technology has made electronic silicon carbide devices commercially available. Both rectifying and switched devices can today be purchased from several vendors. This successful SiC development over the last 25 years can also be utilized for other types of devices, such as light emitting and photovoltaic devices, however, there are still critical problems related to material properties and reliability that need to be addressed. This contribution will focus on surface passivation of SiC devices. This issue is of utmost importance for further development of SiC MOSFETs, which so far has been limited by reliability and low charge carrier surface mobilities. Also bipolar devices, such as BJTs, LEDs, or PV devices will benefit from more efficient and reliable surface passivation techniques in order to maintain long charge carrier lifetimes. Silicon carbide material enables the devices to operate at higher electric fields, higher temperatures and in more radiation dense applications than silicon devices. To be able to utilize the full potential of the SiC material, it is therefore necessary to develop passivation layers that can sustain these more demanding operation conditions. In this presentation it will also be shown that passivation layers of Al2O3 deposited by atomic layer deposition have shown superior radiation hardness properties compared to traditional SiO2-based passivation layers.
Carbon doping in hexagonal multi-walled boron nitride nanotubes (h-MWBNNTs) through ion implantat... more Carbon doping in hexagonal multi-walled boron nitride nanotubes (h-MWBNNTs) through ion implantation is successfully achieved. Nuclear reaction analysis (NRA) confirms that carbon atoms are homogeneously doped into the BNNTs, while Fourier transform infrared spectroscopy (FTIR) reveals that the C-N bonding is produced. Moreover, B-C-N phases are confirmed by XRay Diffraction (XRD) and Raman spectroscopy after C+ ion implantation. High resolution transmission electron microscopy (HRTEM) results show that BNNTs are slightly damaged by C+ ion implantation. UV absorption spectra show that carbon doping reduces the band gap of BNNTs from 5.5 eV to 4.6 eV by increasing C+ ion fluence. It is suggested that the bandgap decrease is due to carbon doping as well as defect formation in BNNTs. Carbon implantation in h-BNNTs is proposed to the knockout ejections of B and N atoms by high energy C+ ions and consequently make ternary B-C-N structure.
A theoretical design assessment is presented using two dimensional numerical computer aided desig... more A theoretical design assessment is presented using two dimensional numerical computer aided design (TCAD) tool for 15–20 kV 4H–SiC IGBTs. Physical parameters of the layer structures such as drift layer thickness, doping in the drift layer, JFET region width and interface charges underneath the gate region are varied to predict the device performance. Performance is further assessed at different temperatures and with different carrier lifetime in the drift layer. Using identical set of physical device parameters (doping, thicknesses), simulated structure was first calibrated with the experimental data. Simulations show that a minority carrier lifetime in the drift layer of 1.0–1.6 μs produces a close match with the experimental device. An on-resistance first decays with temperature (i.e., increased in ionization level, and increase in minority carrier lifetime), stays nearly constant with further increase in the temperature (may be all carriers are now fully ionized and increase in carrier lifetime is compensated with decrease in the carrier mobility) and finally increases linearly with temperature (>450 K) due to decrease in the carrier mobility. A significant increase in the forward voltage drop is observed with the presence of interface trap charges. Increasing JFET region width slightly decreases the forward on-state voltage. A drift layer of at least 175 μm thick with a doping concentration of < 2.0 × 1014 cm−3 is required to get a blocking voltage of 20 kV for 4H–SiC IGBTs assuming a minority carrier lifetime of 1 μs in the drift layer.
To study ion beam induced modifications into MOCVD grown wurtzite AlInN layers, neon ions were im... more To study ion beam induced modifications into MOCVD grown wurtzite AlInN layers, neon ions were implanted on the samples with four doses ranging from 1014 to 9×1015 ions/cm2. Structural characterization was carried out by X-ray diffraction and Rutherford backscattering spectroscopy (RBS) techniques. XRD analysis revealed that GaN related peak for all samples remains at its usual Bragg position of 2θ=34.56° whereas a shift in AlInN peak takes place from its position of 2θ=35.51° for as-grown sample. Rutherford back scattering (RBS) analysis indicated that peak related to Ga atoms in capping layer provided evidence of partial sputtering of GaN cap layers. Moreover, Al peak position is shifted towards lower channel side and width of the signal is increased after implantation, which pointed to the inwards migration of Al atoms away from the AlInN surface. The results suggested that partial sputtering of cap layer has taken place without uncovering the underneath AlInN layer.
ECS Journal of Solid State Science and Technology, Jul 2013
The electrical and chemical properties of high-k dielectric stacks consisting of Hafnium oxide (H... more The electrical and chemical properties of high-k dielectric stacks consisting of Hafnium oxide (HfO2) and Aluminum oxide (Al2O3) deposited on 4H-SiC have been investigated by preparing metal insulator semiconductor (MIS) structures of HfO2/Al2O3/SiC. The multilayer gate stack was deposited by using atomic layer deposition (ALD). The samples were also treated by rapid thermal annealing (RTA) at 970 oC for 5 mins in an inert gas atmosphere. Structural properties of the deposited films were analyzed with X-ray diffraction (XRD), atomic force microscopy (AFM) and Rutherford backscattering spectroscopy (RBS). Capacitance-voltage (CV) measurements performed on as-deposited and RTA treated structures at room temperature show that the RTA treatment increases the effective oxide charges in the whole dielectric but decreases the interface trap density. Current-voltage (IV) measurements have been performed in order to extract the leakage current density as well as the breakdown characteristics of the stack. Our results show that a combination of HfO2 and Al2O3 can be a better choice for SiC than individual Al2O3 layer because of the higher value of effective dielectric constant. It is shown, that the stacked dielectrics are stable at high temperatures and under annealing conditions up to 300 oC, which makes the layers compatible with SiC device processing and higher operation temperatures compared to silicon.
Zinc oxide (ZnO) nanowires (NWs) are exposed to energetic proton (H+), nitrogen (N+), phosphorus ... more Zinc oxide (ZnO) nanowires (NWs) are exposed to energetic proton (H+), nitrogen (N+), phosphorus (P+) and argon (Ar+) ions to understand the radiation hardness and structural changes induced by these irradiations. High-resolution transmission electron microscopy (HRTEM) is utilized to see the irradiation effects in NWs. Multiple doses and energies of radiation at different temperatures are used for different set of samples. The study reveals that wurtzite (crystalline) structured ZnO NWs experience amorphization, degradation and morphological changes after the irradiation. At room temperature, deterioration of the crystalline structure is observed under high fluence of H+, N+ and P+ ions. While for ZnO NWs, bombarded by Ar+ and P+ ions, nano-holes are produced. The ZnO NWs surfaces also show corrugated morphology full of nano-humps when irradiated by Ar+ ions at 400oC. The corrugated surface could serve as tight-holding interface when interconnects it with other NWs/nanotubes. These nano-humps may have the function of increasing the surface for surface-oriented sensing applications in the future.
This paper aims to establish a new method to characterize the interface between 4H-SiC and passiv... more This paper aims to establish a new method to characterize the interface between 4H-SiC and passivating dielectric layers. The investigations are made on MOS test structures utilizing Al2O3 and SiO2 dielectrics on 4H-SiC. These devices are then exposed to various fluences of Ar+ implantation and then measured by the new method utilizing optical free carrier absorption (FCA) technique to assess the interface traps. A program has been developed using MATLAB to extract surface recombination velocity (SRV) at the oxide/epi-layer interface from the optical data. Capacitance-voltage (CV) is done to extract the density of interface traps (Dit) and a comparison was made. It is observed that SiO2 samples show a large rise of SRVs, from 0.5×104 cm/s for a reference sample to 8×104 cm/s for a fluence of 1×1012 cm-2, whereas Al2O3 samples show more stable SRV, changing from 3×104 cm/s for the un-irradiated reference sample to 6×104 cm/s for a fluence of 1×1012 cm-2. A very similar trend is observed for Dit values extracted from CV measurements and it can therefore be concluded that the FCA method is a suitable technique for the interface characterization.
The influence of bulk traps in different regions of 4H-SiC bipolar junction transistors (BJTs) is... more The influence of bulk traps in different regions of 4H-SiC bipolar junction transistors (BJTs) is investigated. The investigation is based on experimental results obtained by implanting up to 1011 cm-2 doses of helium ions in the collector region. The results indicate that implantations, creating point defect concentrations in the range of the doping level produce a sufficiently high concentration of traps to reduce the carrier concentration in this specific region. These traps degrade the device characteristics and are irrecoverable up to 500 oC annealing. The experimental results are qualitatively analyzed by device simulations using a two dimensional numerical computer aided design tool (TCAD). Systematic simulations are then performed by introducing traps at different locations in the BJT (i.e. emitter, base and collector region). The results indicate that the device performance is highly dependent on the defect concentration in the base region. The defects at different levels inside the collector also influence the device by producing a compensated layer in the material. However, the same concentration of defects (i.e. ~1015 cm-3) has less influence in the emitter region.
Degradation of SiO2 surface passivation for 4H-SiC power BJTs as a result of ion irradiation has ... more Degradation of SiO2 surface passivation for 4H-SiC power BJTs as a result of ion irradiation has been studied to assess the radiation hardness of these devices. Fully functional BJTs with 2700 V breakdown voltage are implanted with 600 keV helium ions at fluences, ranging from 1×1012 to 1×1016 cm-2 at room temperature. These ions are estimated to reach the SiO2/SiC interface. The current-voltage characteristics before and after irradiation show that the current gain of the devices start degrading after a helium fluence of 1×1014 cm-2 and decreases up to 20% for the highest fluence of ions. Simulations show that the helium ions induce ionization inside the SiO2, which increases the interface charge and leads to a degradation of the BJT performance. Thermal annealing of the irradiated devices at 300, 420 and 500 oC further increases the amount of charge at the interface, resulting in increased base current in the low voltage range.
The radiation hardness of Al 2 O 3 as a dielectric for SiC surface passivation is studied and com... more The radiation hardness of Al 2 O 3 as a dielectric for SiC surface passivation is studied and compared to SiO 2 for potential application in radiation hard SiC devices. SiO 2 is deposited on 4H-SiC by PECVD and post annealed in N 2 O, whereas Al 2 O 3 is deposited by atomic layer deposition (ALD). The oxides are bombarded with Ar ions in an energy range to produce maximum damage near the oxide/SiC interface. Metal-insulator-semiconductor structures are prepared and their dielectric characteristics are analyzed using capacitance-voltage measurements. Additionally, the effect of the interface damage on surface recombination is studied using the optical free carrier absorption method for the same samples. The results indicate that the SiO 2 /SiC interface is significantly affected at 1×10 11 cm -2 fluence of Ar ions, however, the dielectric properties of Al 2 O 3 /SiC interface remain unaffected even for ten times higher fluences. Similar observations are made for the surface recombination measurements.
Electronic components, based on current semiconductor technologies and operating in radiation ric... more Electronic components, based on current semiconductor technologies and operating in radiation rich environments, suffer degradation of their performance as a result of radiation exposure. Silicon carbide (SiC) provides an alternate solution as a radiation hard material, because of its wide bandgap and higher atomic displacement energies, for devices intended for radiation environment applications. However, the radiation tolerance and reliability of SiC-based devices needs to be understood by testing devices under controlled radiation environments. These kinds of studies have been previously performed on diodes and MESFETs, but multilayer devices such as bipolar junction transistors (BJT) have not yet been studied.
Ion implantation effects at SiO2/SiC and Al2O3/ SiC interfaces have been investigated by implanti... more Ion implantation effects at SiO2/SiC and Al2O3/ SiC interfaces have been investigated by implanting Ar ions at the interface of oxide and SiC. Capacitance–voltage relation and breakdown properties for these dielectrics are studied before and after implantation. The results indicate that the SiO2/SiC interface is sensitive to ion fluences higher than 1 × 1011 cm−2, while Al2O3 on SiC can sustain higher fluences. In addition, the breakdown of the Al2O3 is found to be less sensitive to the ion implantation.
n- and p-type 4H-SiC epilayers were grown on heavily doped SiC substrates. The thickness of the p... more n- and p-type 4H-SiC epilayers were grown on heavily doped SiC substrates. The thickness of the p-type layer was 7 μm and the doping level around 1017 cm–3, while the n-type epilayers were 15 μm thick and had a doping concentration of 3 – 5×1015 cm–3. Several different surface treatments were then applied on the epilayers for surface passivation: SiO2 growth, Al2O3 deposited by atomic layer deposition, and Ar-ion implantation. Using collinear pump – probe technique the effective carrier lifetimes were measured from various places and statistical lifetime distributions were obtained. For surface recombination evaluation, two models are presented. One states that surface recombination velocity (SRV) is equal on both the passivation/epi layer interface (S2) and the deeper interface between the epilayer and the SiC substrate (S1), i. e. (S1 = S2). The other model is simulated assuming that SRV in the epilayer/substrate (S1) interface is constant while in the passivation layer/epilayer (S2) interface SRV can be varied S2 < S1. Empirical nomograms are presented with various parameters sets to evaluate S2 values. We found that on the investigated 4H-SiC surfaces S2 ranges from 3×104 to 5×104 assuming that the bulk lifetime is 4 μs. In Ar+ implanted surfaces S2 is between (105 – 106) cm/s.
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Papers by Muhammad Usman