Correlation of device performance and defects in AlGaN/GaN high-electron mobility transistors

Device performance and defects in AlGaN/GaN high-electron mobility transistors (HEMTs) have been correlated. Surface depressions and threading dislocations, revealed by optical-defect mapping and atomic force microscopy (AFM), compromised the effectiveness of the SiN_x surface-passivation effect as evidenced by the gate-lag measurements. The residual carriers in the GaN-buffer layer observed from the capacitance-voltage depth profile have been attributed to the point defects and threading dislocations either acting as donors or causing local charge accumulations. Deep-level transient-spectroscopy measurements showed the existence of several traps corresponding to surface states and bulk-dislocation defects. The formation of electron-accumulation regions on the surface or (and) in the GaN-buffer layer was confirmed by currentvoltage measurements. This second, virtual gate formed by electron accumulations can deplete the channel and cause a large-signal gain collapse leading to degraded output power. A good correlation was established between the device performance and defects in AlGaN/GaN HEMT structure.     

Electrical properties and spectra of deep centers in GaN p-i-n rectifier structures

GaN p-i-n rectifiers with 4 μm thick i-layers show typical reverse breakdown voltages of 100–600 V. We have studied the temperature dependence of current-voltage characteristics in these diodes, along with hole diffusion lengths and the deep level defects present. Generally we find that i-layer background doping varies significantly (from <10¹⁴ cm⁻³ to 2–3×10¹⁶), which influences the current conduction mechanisms. The hole diffusion lengths were in the range 0.6–0.8 μm, while deep level concentrations were ∼10¹⁶ cm⁻³.     

Effect of rapid thermal annealing on pentacene-based thin-film transistors

The bottom contact pentacene-based thin-film transistor is fabricated, and it is treated by rapid thermal annealing (RTA) with the annealed temperature up to 240 °C for 2 min in the vacuum of 1.3 × 10⁻² torr. The morphology and structure for the pentacene films of OTFTs were examined by scanning electron microscopy and X-ray diffraction technique. The thin-film phase and a very small fraction of single-crystal phase were found in the as-deposited pentacene films. While the annealing temperature increases to 60 °C, the pentacene molecular ordering was significantly improved though the grain size only slightly increased. The device annealed at temperature of 120 °C has optimal electrical properties, being consistent with the experimental results of XRD. The post-annealing treatment results in the enhancement of field-effect mobility in pentacene-based thin-film transistors. The field-effect mobility increases from 0.243 cm²/V s to 0.62 cm²/V s. Besides, the threshold voltage of device shifts from −7 V to −3.88 V and the on/off current ratio increases from 4.0 × 10³ to 8.7 × 10³.     

A deep level transient spectroscopy study of rapid thermal annealed arsenic implanted silicon

Deep level traps are observed in silicon that has been implanted with high doses of arsenic and subsequently annealed by rapid thermal annealing. The doses studied create enough damage to form a surface amorphous layer. Annealing temperatures, implant fluence, and the presence of a surface amorphous layer contribute to the type of trap observed. These results show evidence for a clustering/declustering mechanism of arsenic in silicon during rapid thermal annealing.     

Extended defects and polarity of hydride vapor phase epitaxy GaN

Hydride vapor phase epitaxy (HVPE) GaN layers on sapphire substrates and so-called free-standing platelets (layers removed from the sapphire) were studied by different transmission electron microscopy (TEM) techniques. Polarity, determined by convergent beam electron diffraction (CBED), and distribution of structural defects, determined by conventional TEM, are discussed. The HVPE layers were found to grow primarily with Ga-polarity. A few inversion domains (areas with N-polarity) were observed on the substrate side of one of the free-standing layers. The dominant structural defects in HVPE GaN layers are threading dislocations. A systematic reduction of their density with an increase in layer thickness was observed for all of the samples. The experimental results indicate that the density of dislocations is inversely proportional to the distance from the substrate, which agrees with the theoretical model.     

Study of deep level defect behavior in undoped n-InP (1 0 0) after rapid thermal annealing

The effects of rapid thermal annealing on deep level defects in the undoped n-type InP with Ru as Schottky contact metal have been characterized using deep level transient spectroscopy (DLTS). It is observed that the as-deposited sample exhibit two deep levels with activation energies of 0.66 and 0.89 eV. For the samples annealed at 300 °C and 400 °C, a deep level is identified with activation energies 0.89 and 0.70 eV, respectively below the conduction band. When the sample is annealed at 500 °C, three deep levels are observed with activation energies 0.25, 0.32 and 0.66 eV. Annealing of the sample at 300 °C, orders the lattice of as-grown material by suppressing the defect 0.66 eV (A1) which is found in the as-deposited sample. The trap concentration of the 0.89 eV deep levels is found to be increased with annealing temperature. The deep level 0.32 eV may be due to the lattice defect by thermal damage during rapid thermal annealing process such as vacancies, interstitials and its complexes, indicating the damage of the sample after annealing at 500 °C. The defects observed in all the samples are possibly due to the creation of phosphorous vacancy or phosphorous antisite.     

The behavior of Ni/Au contacts under rapid thermal annealing in GaN device structures

The effect of rapid thermal annealing (RTA) on Ni/Au contacts on P-type GaN was investigated in terms of surface morphology and diffusion depth of metallic species. Ni/Au contacts were evaporated on the P-type 0.5 μm thick top layer of a GaN P/N homojunction. Optical micrographs revealed that the contact morphology degrades when annealed above 800°C for 1 min. At the same time, both Ni and Au atoms strongly diffuse in the P-type layer and even can reach the junction for a 1 min long annealing at 900°C, therefore making the junction structure unoperable. This behavior was evidenced using the Auger voltage contrast (AVC) technique.     

A reliable guideline to maximize the detection and analysis of deep level defects: Comparison between DLTS analysis techniques

In this paper, we present reliable guidelines allowing to maximize the detection and analysis of deep defects in semiconductors by introducing a new method of analysis that we named Modified Levenberg-Marquardt Deep-Level Transient Spectroscopy (MLM-DLTS) based on the Levenberg-Marquardt algorithm that we modified deliberately and associated with two other high-resolution techniques, i.e. the Matrix Pencil algorithm and modified Prony's method. The performances of the method were first assessed with a procedure of simulation by generating multi-exponential capacitance transients with a changing signal-to-noise ratio. These different tests have demonstrated to what extent such a method of analysis is more efficient than classic correlation methods based on DLTS spectra and than three high-resolution methods already existing in the literature. Finally, when the signal-to-noise ratio is low, and in the aim of getting precise results, a few smoothing algorithms were tested. Our findings evidence how the smoothing quality can be controlled under certain conditions while avoiding both the distortions in the shape of the capacitance transients and the DLTS spectra.     

Influence of rapid thermal annealing on morphological and electrical properties of RF sputtered AlN films

Aluminum nitride films were deposited, at 200 °C, on silicon substrates by RF sputtering. Effects of rapid thermal annealing on these films, at temperatures ranging from 400 to 1000 °C, have been studied. Fourier transform infrared spectroscopy (FTIR) revealed that the characteristic absorption band of Al–N, around 684 cm⁻¹, became prominent with increased annealing temperature. X-ray diffraction (XRD) patterns exhibited a better, c-axis, (0 0 2) oriented AlN films at 800 °C. Significant rise in surface roughness, from 2.1 to 3.68 nm, was observed as annealing temperatures increased. Apart from these observations, micro-cracks were observed at 1000 °C. Insulator charge density increased from 2×10¹¹ to 7.7×10¹¹ cm⁻² at higher temperatures, whereas, the interface charge density was found minimum, 3.2×10¹¹ eV⁻¹cm⁻², at 600 °C.     

Isochronal annealing study of hydrogen interaction with implantation-induced point defects in p-type silicon

Isochronal annealing with zero and reverse bias applied to Schottky diodes was used to monitor the evolution of hydrogen interaction with point defects observed in hydrogen-implanted p-type silicon, i.e., divacancy (VV), carbon–oxygen interstitial pair (C_iO_i) and two levels at E_v+0.28 and E_v+0.50 eV. The VV and C_iO_i are passivated by hydrogen liberated from hydrogen-containing defects during annealing in the temperature range 90–150°C and reappear upon annealing above 180°C under reverse bias due to hydrogen liberation and its field drift. Two levels at E_v+0.50 and E_v+0.28 eV are ascribed to irradiation-induced and hydrogen-related defects, respectively.     

Thermodynamic modeling of native point defects and dopants of GaN semiconductors

A thermodynamic modeling of GaN was carried out to describe the thermodynamic behavior of native defects, dopants, and carriers (free electrons and holes) in GaN semiconductors. The compound energy model (CEM) was used. An unintentionally doped GaN was taken as an example. Oxygen was introduced into the model as the unintentionally doped impurity, according to the practical experimental phenomena. The energies of component compounds in the model were defined based on the results of the ab initio calculations and adjusted to fit experimental data. The thermodynamic properties of the defects and the oxygen doped were calculated to show the facility of the model.     

Low-frequency noise properties of GaN Schottky barriers deposited on intermediate temperature buffer layers

Metal–semiconductor–metal (MSM) structures were fabricated by RF-plasma-assisted MBE using different buffer layer structures. One type of buffer structure consists of an A1N high-temperature buffer layer (HTBL) and a GaN intermediate temperature buffer layer (ITBL), another buffer structure consists of just a single A1N HTBL. Systematic measurements in the flicker noise and deep level transient Fourier spectroscopy (DLTFS) measurements were used to characterize the defect properties in the films. Both the noise and DLTFS measurements indicate improved properties for devices fabricated with the use of ITBL and is attributed to the relaxation of residue strain in the epitaxial layer during growth process.     

Optically detected magnetic resonance study of defects in undoped, Be-doped, and Mg-doped GaN

Undoped, Be-doped, and Mg-doped GaN samples were investigated with photoluminescence-detected electron paramagnetic resonance (PL-EPR) and electron-nuclear double resonance (PL-ENDOR). Two types of shallow donors and a deep level Be-related complex were measured in Be-doped GaN. One type of shallow donors is probably due to Si contamination. In all investigated samples, distant gallium ENDOR lines were observed which were split by a quadrupole interaction. From this splitting the electrical field gradients (efg’s) at the Ga nuclei in all investigated samples could be determined very precisely. The efg’s were correlated with strain.     

Defect characterization of n-type Si1−xGex after 1.0 kev helium-ion etching

SiGe heterostructures with their associated geometries and properties promise a novel generation of Si-based devices. Surface processing and, in particular, dry or plasma etching of semiconductors is a key technology for producing optoelectronic integrated circuits and high speed electronic devices. We have used deep-level transient spectroscopy (DLTS) in an investigation of the electronic properties of defects introduced in n-Si ₁ −xGe_x (x = 0.00 to 0.25) during 1 keV helium-ion etching (fluence = 1 × 10¹² cm²) prior to the deposition of gold Schottky barrier diodes (SBDs). Six electron defects (EHel-EHe6) were detected after this processing stage. The defects detected after etching are compared to those introduced by 5.4 MeV alpha-particle (α-) irradiation and, also, radio frequency (rf) sputter-deposition of Au SBDs on material from the same wafer. Four of the electron defects (EHel, EHe2, EHe4, and EHe6) are detected in Si. The remaining two defects (EHe3 and EHe5) are only detected in material containing germanium. It was noted that defects introduced during the He-ion etch process have the same DLTS “signatures” as defects after the sputter deposition process, but none were the same as those introduced during the α-particle irradiation. The influence of increased Ge content on DLTS peak shape and positions is also illustrated and discussed.     

Effect of gas feeding methods on optical properties of GaN grown by rapid thermal chemical vapor deposition reactor

The origin of the radiative recombination leading to yellow luminescence (YL) has been elucidated by the study of luminescence properties of GaN films grown with two different gas feeding methods. GaN films were grown on a (0001) sapphire substrate in a rapid thermal chemical vapor deposition (RTCVD) reactor. GaN films emitted two different luminescence energies, 2.2 and 3.47 eV, depending on the introducing position of hydrogen gas in the growth reactor. The distribution of the TMGa flow and gas phase reactions in the reactor were investigated to understand the effect of the gas feeding methods on the optical properties of GaN films. The results suggest that YL is related to Ga vacancies in the grown films.     

Influence of AlGaN deep level defects on AlGaN/GaN 2-DEG carrierconfinement

We have used low energy electron-excited nanoscale luminescence spectroscopy (LEEN) to detect the defects in each layer of AlGaN/GaN HEMT device structures and to correlate their effect on two-dimensional electron gas (2-DEG) confinement. We investigated AlGaN/GaN heterostructures with different electrical properties using incident electron beam energies of 0.5 to 15 keV to probe electronic state transitions within each of the heterostructure layers. AlGaN heterostructures of 25 nm thickness and nominal 30% Al concentration grown on GaN buffer layers on sapphire substrates by plasma-assisted molecular beam epitaxy exhibited a range of polarization-induced electron densities and room temperature mobilities. In general, the spectra exhibit AlGaN band edge emission at ~3.8 eV or ~4.0 eV, GaN band edge emission at ~3.4 eV, yellow luminescence (YL) features at 2.18 eV and 2.34 eV, and a large emission in the infrared (<1.6 eV) from the GaN cap layer used to passivate the AlGaN outer surface. These heterostructures also show high strain in the 2 nm-thick GaN layer with evidence for a Franz-Keldysh red shift due to piezoelectric charging. The LEEN depth profiles reveal differences between the structures with and without 2-DEG confinement and highlight the importance of AlGaN defects in the near 2-DEG region     

Effects of vacuum annealing on electrical properties of GaN contacts

To understand formation and deterioration mechanisms of Ta/Ti ohmic contacts that were previously developed for p-GaN, the electrical properties of the Ta/Ti contacts, which were deposited on undoped GaN substrates and subsequently annealed in vacuum (where a slash (/) sign indicates the deposition sequence), were studied. The Ta/Ti contacts displayed good ohmic behavior after annealing at a temperature of 800°C for 10 min in vacuum, although the undoped GaN substrates were used. However, deterioration of the present ohmic contacts was observed during room-temperature storage. These contact properties were similar to those observed in the Ta/Ti contacts prepared on p-GaN. Hall-effect measurements revealed that thin n-type conductive layers were found to form on surfaces of both the undoped GaN and p-GaN substrates after annealing at 800°C in vacuum.     

Accurate measurement of capture cross sections in deep level transient spectroscopy: Application to EL2 in GaAs

A rigorous formulation of capacitance changes during trap filling processes is presented and used to accurately determine the electron capture cross section of EL2 in GaAs at a particular temperature, 377K, in this case. The value, σ_n (377K) = 2.7 × 10⁻¹⁶ cm², is compared with that predicted from the emission dependence.     

The effects of point defects on the electrical activation ofSi-implanted GaAs during rapid thermal annealing

The effects of point defects on the electrical activation of Si-implanted GaAs during rapid thermal annealing were investigated using slow positron beam, cross-sectional transmission electron microscopy, and Hall measurements. The increase of the Ga vacancy concentration in the GaAs substrate induced by the SiO₂ cap layer on the substrate during annealing was observed to decrease the activation efficiency and the number of extrinsic stacking faults via the recombination of interstitials with vacancies. It was found that the efficiency of the carrier creation is not dependent upon the Ga vacancy concentration during the rapid thermal annealing of Si-implanted GaAs. Hence, it is proposed that the electrical activation of Si-implanted GaAs is not due to implantation-induced vacancies but to the self-exchange of interstitial Si atoms with the host Ga substitutional atoms     

低真空退火对GaN MSM紫外探测器伏安特性的影响

利用金属有机化学气相沉积生长的非故意掺杂GaN单晶制备了金属一半导体一金属交叉指型肖特基紫外探测器。用肖特基势垒的热电子发射理论研究了低真空下不同热退火条件对器件伏安特性的影响。Au-GaN肖特基势垒由退火前的0．36eV升高到400℃0．5h的0．57eV,退火延长为1h势垒反而开始下降。分析结果表明：由工艺造成的填隙Au原子引入的缺陷是器件势垒偏低的主要原因,Au填充N空位形成了施主型杂质是退火后势垒升高的主要原因。