Deep Level Optical Spectroscopy Research Articles

Detailed characterization of deep level defects in InGaN Schottky diodes by optical and thermal deep level spectroscopies

Schottky diode properties of semitransparent Ag(4 nm)/Au(4 nm) metal stack on In0.2Ga0.8N were investigated and defect characterization was performed using capacitance deep level transient (DLTS) and optical spectroscopy (DLOS). DLTS measurements made on the In0.2Ga0.8N Schottky diodes, which displayed a barrier height of 0.66 eV, revealed the presence of two deep levels located at Ec-0.39 eV and Ec-0.89 eV with nearly identical concentrations of ∼1.2 × 1015 cm−3. Three deeper defect levels were observed by DLOS at Ec-1.45 eV, Ec-1.76 eV, and Ec-2.50 eV with concentrations of 1.3 × 1015cm−3, 3.2 × 1015cm−3, and 6.1 × 1016 cm−3, respectively. The latter, with its high trap concentration and energy position lying 0.4 eV above the valance band, suggests a possible role in compensation of carrier concentration, whereas the mid-gap positions of the other two levels imply that they will be important recombination-generation centers

Comparison of hole traps in n-GaN grown by hydride vapor phase epitaxy, metal organic chemical vapor deposition, and epitaxial lateral overgrowth

Optical deep level spectroscopy (ODLTS) and microcathodoluminescence (MCL) spectra were measured for a large group of n-GaN samples grown via metalorganic chemical vapor deposition (MOCVD), epitaxial lateral overgrowth (ELOG), or hydride vapor phase epitaxy (HVPE). In the MOCVD and ELOG samples, the ionization energy of dominant hole traps H1 was dependent on the excitation conditions and was ∼0.9 eV for high injection levels providing saturation of the ODLTS peak magnitude. The trap concentration increased with increasing Si donor concentration and correlated with the yellow band intensity in the MCL spectra. For the HVPE samples, the hole trap spectra were radically different from the MOCVD case: four hole traps—H2, H3, H4, and H5—with activation energies of 0.55, 0.65, 0.85, and 1.2 eV, respectively, were detected. In the MCL spectra, a broad green band that peaked near 2.5 eV was observed in addition to the usual yellow luminescence near 2.3 eV. This green band was attributed to the transitions involving the H4 hole traps. Possible identities of the hole traps detected in the MOCVD/ELOG and HVPE samples are discussed.

Electrical characterization of n-type Al0.30Ga0.70N Schottky diodes

The carrier trapping properties and current transport behavior of Ni/n-Al0.30Ga0.70N Schottky diodes were quantitatively characterized by a combination of deep level optical spectroscopy (DLOS), thermally based deep level transient spectroscopy (DLTS), current-voltage-temperature (I-V-T), and internal photoemission (IPE) measurements. High quality Schottky diode behavior was observed with an IPE-determined barrier height of 1.66 eV and the observed temperature-independent reverse leakage current behavior was found to be consistent with field emission in reverse bias and thermionic-field emission in forward bias as the dominant transport mechanisms. The trap spectroscopy measurements revealed the presence of several bandgap states located at EC–0.9 eV (seen by both DLOS and DLTS), EC–1.5, EC–3.11, and EC–3.93 eV—all via DLOS. The EC–3.10 level, which is present in very high concentration, is found to correlate with the energy position expected for the cation vacancy in AlGaN, based on the vacuum referred binding energy model for the AlxGa1−xN alloy. The relatively shallow trap at EC–3.93/EV+0.15 eV, which is possibly Mg-related, is also present in significant concentration. The total observed trap concentration in this sample is in excess of the net doping extracted from capacitance-voltage, which will likely impact device behavior and is consistent with the observed I-V-T behavior.

Next generation defect characterization in nitride HEMTs

AbstractA series of constant drain current deep level transient and optical spectroscopies (CID‐DLTS/DLOS) is described, which allows characterization of deep levels with lateral spatial resolution in fully fabricated high electron mobility transistors (HEMTs). The techniques are used to examine the role of SiNx passivation in the access region of AlGaN/GaN HEMTs, in part to verify lateral resolution capabilities. Two distinct trap spectra were observed correlating to the under‐gate and access regions of the HEMT. Traps in the access region had energy levels of of EC‐0.43, EC‐1.3, EC‐2.3, and EC‐3.7 eV. The impact of each access region trap on the drain resistance before and after passivation was measured, with the total transient drain resistance change being reduced from 0.47 Ω‐mm to 0.11 Ω‐mm as a result of their partial passivation by SiNx, resulting in improved device performance. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of growth temperature on AlGaN multiquantum well point defect incorporation and photoluminescence efficiency

The dependence of (Al)GaN/AlGaN multiquantum well (MQWs) optical efficiency and defect incorporation on the growth temperature (Tg) of the optically active region was investigated. Marked increase in MQW photoluminescence (PL) intensity was observed for increasing Tg. Correspondingly, increasing Tg also significantly reduced point defect incorporation under QW growth conditions, as determined by deep level optical spectroscopy. It is suggested that enhanced MQW PL with increasing Tg resulted from improved nonradiative lifetime through reduced nonradiative defect density in the MQW region.

Determination of bandgap states in p-type In0.49Ga0.51P grown on SiGe/Si and GaAs by deep level optical spectroscopy and deep level transient spectroscopy

The presence and properties of traps in p-type In0.49Ga0.51P grown on low dislocation density, metamorphic Ge/SiGe/Si substrates and GaAs substrates were determined using deep level transient spectroscopy (DLTS) and deep level optical spectroscopy (DLOS) leading to the quantification of trap behavior throughout the entire 1.9 eV bandgap of the In0.49Ga0.51P material as a function of substrate. Thermal emission-based DLTS revealed a single hole trap at Ev + 0.71 eV for growth on both lattice matched and mismatched substrates with similar concentrations. Complementary, optical emission-based DLOS measurements detected bandgap states at Ev + 1.18 eV, Ev + 1.36 eV, and Ev + 1.78 eV for p-type In0.49Ga0.51P grown on both substrate types. The total concentration of the DLOS-detected states was found to comprise approximately 80% of the entire trap concentration in p-type In0.49Ga0.51P bandgap. This relatively high concentration of above midgap levels may be of great significance for minority carrier devices that utilize p-type In0.49Ga0.51P (such as high efficiency III–V multijunction solar cells) since their position in the bandgap and high concentrations suggest that strong minority carrier electron trapping behavior can be expected. The primary effect of substituting the GaAs substrate by Ge/SiGe/Si is to increase the concentration of these states by a factor of 2–3, with no additional levels detected due to the replacement by the Si-based substrates, indicating that all detected traps are native to the epitaxial In0.49Ga0.51P material (regardless of the substrate), but whose concentrations appear to be influenced by dislocation density.

Quantitative and Depth-Resolved Investigation of Deep-Level Defects in InGaN/GaN Heterostructures

Deep-level defects in In0.17Ga0.83N/In0.02Ga0.98N/p-GaN:Mg heterostructures were studied using deep-level optical spectroscopy (DLOS). Depth-resolved DLOS was achieved by exploiting the polarization-induced electric fields to discriminate among defects located in the In0.17Ga0.83N and the In0.02Ga0.98N regions. Growth conditions for the InxGa1� xN layers were nominally the same as those in InGaN/GaN multi-quantum-well (MQW) structures, so the defect states reported here are expected to be active in MQW regions. Thus, this work provides important insight into defects that are likely to influence MQW radiative efficiency. In0.17Ga0.83N-related bandgap states were observed at Ev + 1.60 eV and Ev + 2.59 eV, where Ev is the valence-band maximum, compared with levels at Ev + 1.85 eV, Ev + 2.51 eV, and Ev + 3.30 eV in the In0.02Ga0.98N region. A lighted capacitance–voltage technique was used to determine the areal density of deep states. The possible origins of the associated defects are considered along with their potential roles in light-emitting diodes.

Impact of N- and Ga-face polarity on the incorporation of deep levels in n-type GaN grown by molecular beam epitaxy

Deep levels in N-face and Ga-face n-type GaN grown by plasma-assisted molecular beam epitaxy were detected, analyzed and compared using deep level optical spectroscopy (DLOS) and conventional thermal deep level transient spectroscopy (DLTS), which together enable deep level detection throughout the GaN band gap. A redistribution of band gap states was observed between the two GaN crystal growth polarities but with a similar total trap density. Most significant was a tenfold concentration increase in a trap at EC-0.25 eV that is likely related to nitrogen vacancies for the N-face polarity material, with no significant change for the Ga-vacancy-related level at EC-2.60 eV. The DLOS results suggest that carbon impurities, which generate several GaN band gap states, appear to incorporate differently for both crystal polarities with the potential carbon interstitial at EC-1.28 eV being undetected for N-face material. Finally, low concentrations of several new levels in the N-face n-GaN not previously observed in Ga-face n-GaN were observed.

GaN nanowire surface state observed using deep level optical spectroscopy

Deep level defects in n-type GaN nanowires (NWs) with and without an epitaxially-grown AlGaN shell were compared using photoconductivity-mode deep level optical spectroscopy. Hole photoemission from a defect state located approximately 2.6 eV above the valence band was observed for GaN NWs but was not observed for AlGaN/GaN core-shell NWs, indicating that this deep level is associated with a GaN surface state. Identifying GaN NW surface states and developing an effective passivation mechanism is expected to aid in the understanding and improvement of GaN NW-based sensors and optoelectronics.

Effect of nitrogen plasma power on defect levels in Ni/n-GaN Schottky diodes grown by molecular beam epitaxy

The incorporation of deep level defects in n-type GaN grown by plasma assisted molecular beam epitaxy was studied via systematic adjustment of the nitrogen plasma source power from 150 to 400 W while maintaining a constant V/III beam flux ratio. Deep level optical spectroscopy and conventional thermal deep level transient spectroscopy measurements, which together enable deep level detection throughout the GaN band gap, revealed several deep level concentrations that depend strongly on rf-plasma power. The concentrations of the gallium vacancy deep level at EC-2.60 eV and carbon-related point defects with energy levels at EC-3.28 and EC-1.35 eV are found to be very sensitive to the nitrogen source power, increasing by up to 50 times for a corresponding increase in plasma power from 150 to 400 W. The relation between the concentrations of these traps and plasma power follows an Arrhenius-type behavior and is suggestive of plasma damage associated with the energetics of the constituent active nitrogen species. In contrast, two traps at EC-0.86 and EC-0.59 eV did not exhibit a systematic dependence on plasma power, with this difference a result of the dislocation-related nature of these defects.

Deep level optical spectroscopy of GaN nanorods

Deep level defects in GaN nanorods (NRs) grown by metal organic chemical vapor deposition were studied using deep level optical spectroscopy (DLOS) and microphotoluminescence (μ-PL). DLOS determines the absolute optical ionization energy, discerns majority versus minority carrier photoemission, and has sensitivity to nonradiative defect centers. These are important aspects of deep level spectroscopy for NRs that are not obtainable using luminescence techniques alone. Deep level defects were observed via DLOS at Ec−2.81 eV, Ec−1.77 eV, and Ec−3.19 eV, where Ec is the conduction band minimum. The μ-PL spectra revealed a dominant defect band peaked near 2.19 eV. The Ec−2.81 eV band gap state and the 2.19 eV PL peak can be attributed to the same defect center within a one-dimensional configuration-coordinate model. The NR DLOS spectra are compared to reports for thin film GaN, and possible physical origins of the deep level defects are discussed.

Extended defect states of Ge/Si quantum dots using optical isothermal capacitancetransient spectroscopy

We investigated the hole emission processes of optically induced charges on the defectstates and confined states of self-assembled Ge quantum dots (QDs) embedded in a p–i–nSi diode. Optical deep level transient spectroscopy (ODLTS) and optical isothermalcapacitance transient spectroscopy (OICTS) were used to study the defect states in tenstacked Ge quantum dots. Using ODLTS and OICTS for QD-embedded samples, the peaksrelated to the defect states of Ge QDs could be classified distinctly; it was about 20–50times higher in intensity than that for the bulk defect states. The charges emitted from theQD defect state were observed near 93 K, and the activation energy was calculated to beEV+177 meV. The defect state followed the logarithmic capture kinetics and the Arrhenius-determinedapparent activation energy decreased in the band gap as the optical injection widthincreased. We suggest that Ge QD defect states in Si could exist as extended states.

Band Gap States in AlGaN/GaN Hetero-Interface Studied by Deep-Level Optical Spectroscopy

AbstractPlanar Pt/AlGaN/GaN Schottky barrier diodes (SBDs) have been characterized by capacitance-voltage and capacitance deep-level optical spectroscopy measurements, compared to reference Pt/GaN:Si SBDs. Two specific deep levels are found to be located at ∼1.70 and ∼2.08 eV below the conduction band, which are clearly different from deep-level defects (Ec - 1.40, Ec - 2.64, and Ec - 2.90 eV) observed in the Pt/GaN:Si SBDs. From the diode bias dependence of the steady-state photocapacitance, these levels are believed to stem from a two-dimensional electron gas (2DEG) region at the AlGaN/GaN hetero-interface. In particular, the 1.70 eV level is likely to act as an efficient generation-recombination center of 2DEG carriers.

Intrinsic Degradation in Alq3-Based OLEDs Probed by Deep-Level Optical Spectroscopy

AbstractWe have successfully investigated degradation-induced variations in electronic band-gap states in the emissive region of the Alq3-based OLEDs by a deep-level optical spectroscopy technique. Through the intrinsic degradation, both deep-level traps and near-band-edge transitions in the Alq3 emissive zone are found to be red-shifted significantly towards their corresponding bulk levels of the Alq3 single layer. These variations in the interfacial electronic states are probably induced by the intrinsic degradation and indicate that initial molecular structures characteristic of the Alq3 emissive zone are transformed into the bulk-like relaxed ones through the degradation.

Deep level optical and thermal spectroscopy of traps in n-GaN grown by ammonia molecular beam epitaxy

The incorporation of deep level defects in n-type GaN grown by ammonia-based molecular beam epitaxy (MBE) is studied via systematic adjustment of the NH3/Ga flux ratio. Deep level optical and transient spectroscopies, which together enable deep level detection throughout the GaN bandgap, reveal defect states whose individual concentrations vary with the NH3/Ga flux ratio. A general trend of lower concentration for deep levels at EC−3.28, EC−1.28, EC−0.62, and EC−0.25 eV with higher NH3/Ga flux ratio was observed, with the strongest reduction at the EC−0.25 eV level, consistent with expectations for a VN-related defect. The known CN impurity state at EC−3.28 eV and suspected CI-related state at EC−1.28 eV also showed a moderate decrease in concentration at the higher NH3/Ga flux ratio. In contrast, the VGa-related defect at EC−2.62 eV was insensitive to the NH3/Ga flux ratio over the range studied here. Taken together, ammonia-MBE GaN has deep level defects with different sensitivities in flux ratios suggestive of independent physical sources. However, the total trap concentrations were significantly reduced for higher NH3/Ga flux ratios in n-type GaN grown by ammonia-MBE under the range of growth conditions used in this study, suggesting that higher NH3/Ga flux ratios will generate higher electronic quality GaN material when using ammonia-based MBE for device applications.

Comparison of deep level incorporation in ammonia and rf‐plasma assisted molecular beam epitaxy n‐GaN films

AbstractThe use of rf‐plasma and ammonia nitrogen sources for growth of GaN films by molecular beam epitaxy (MBE) are compared in terms of defect incorporation using deep level optical spectroscopy (DLOS) and deep level transient spectroscopy (DLTS). To better improve the understanding of ammonia‐based MBE growth of GaN and potential defect sources as opposed to the more studied plasma source‐based MBE‐grown material several V/III ratios were also investigated, which were generated via systematic adjustment of the ammonia flow rates during growth. The DLOS spectra, comparing deep traps within the n‐GaN grown using N‐plasma and ammonia sources, reveal the presence of the same deep levels due to background carbon and gallium vacancies, with energy levels at EC‐3.28, EC‐2.62, and EC‐1.28. The DLTS results of the N‐plasma and ammonia‐based MBE samples show two similarly dominant electron traps at EC‐0.60, and EC‐0.24 in each sample. Measurements made as a function of V/III flux ratio for ammonia‐based MBE growth indicate a large dependence of the EC‐0.24 eV trap concentration on growth flux ratio, which is significant for guiding continued optimization of this promising MBE growth method for GaN devices. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Characterization of majority and minority carrier deep levels in p-type GaN:Mg grown by molecular beam epitaxy using deep level optical spectroscopy

Deep level defects in p-type GaN:Mg grown by molecular beam epitaxy were characterized using steady-state photocapacitance and deep level optical spectroscopy (DLOS). Low frequency capacitance measurements were used to alleviate dispersion effects stemming from the deep Mg acceptor. Use of DLOS enabled a quantitative survey of both deep acceptor and deep donor levels, the latter being particularly important due to the limited understanding of minority carrier states for p-type GaN. Simultaneous electron and hole photoemissions resulted in a convoluted deep level spectrum that was decoupled by emphasizing either majority or minority carrier optical emission through control of the thermal filling time conditions. In this manner, DLOS was able to resolve and quantify the properties of deep levels residing near both the conduction and valence bandedges in the same sample. Bandgap states through hole photoemission were observed at Ev+3.05eV, Ev+3.22eV and Ev+3.26eV. Additionally, DLOS revealed levels at Ec−3.24eV and Ec−2.97eV through electron emission to the conduction band with the former attributed to the Mg acceptor itself. The detected deep donor concentration is less than 2% of activated [Mg] and demonstrates the excellent quality of the film.

Emissive Interface States in Organic Light-Emitting Diodes Based on Tris(8-hydroxyquinoline) Aluminum

We have investigated emissive interface states in fabricated indium–tin-oxide (ITO)/N,N'-di-1-naphthyl-N,N'-diphenyl-1,1'-biphenyl-4,4'diamine (α-NPD)/tris(8-hydroxyquinoline) aluminum (Alq3)/LiF/Al organic light-emitting diodes (OLEDs) by modified deep-level optical spectroscopy (DLOS). DLOS after hole injection into the OLEDs revealed a discrete trap level located at ∼1.77 eV below the conduction band of Alq3 in the emissive interface region, in addition to the band-to-band transitions of charge carriers from α-NPD to Alq3. This pronounced level can be activated as an efficient recombination center of the charge carriers. Also, this trap level was extensively distributed to a lower energy with increasing hole injection rate. Therefore, this level is probably attributable to intrinsic trap states at the α-NPD/Alq3 emissive interface. Thus, DLOS has been proven to be a powerful tool for the characterization of emissive interface states in the OLEDs.

Deep‐level characterization of emissive interface states in Alq3‐based OLEDs

AbstractWe have succesfully investigated emissive interface states in fabricated indium‐tin‐oxide (ITO)/N,N′‐di‐1‐naphthyl‐N,N′‐diphenyl‐1,1′‐biphenyl‐4,4′diamine (α‐NPD)/tris(8‐hydroxyquinoline) aluminum (Alq3)/LiF/Al organic light‐emitting diodes (OLEDs) by a modified deep‐level optical spectroscopy (DLOS) technique. In the vicinity of the α‐NPD/Alq3 emissive interface, a discrete trap level was found to be located at ∼1.77 eV below the conduction band of Alq3, in addition to band‐to‐band transitions of carriers from α‐NPD to Alq3. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Role of subsurface defects in metal-ZnO(0001¯) Schottky barrier formation

The authors fabricated diodes of Au, Al, Ni, Pt, Pd, Mo, Ta, and Ir on single crystal ZnO(0001¯) surfaces from different vendors and measured their Schottky barriers, idealities, and reverse currents on as-received and remote oxygen (20% O2∕80%He) plasma-treated surfaces. Using low temperature nanoscale depth-resolved cathodoluminescence spectroscopy (DRCLS) under the metal, the authors identified the presence of defect transitions at 2.1, 2.5, and 3.0eV that change dramatically depending on the process steps and choice of metal. I-V measurements exhibited transitions from Ohmic to Schottky and lower idealities for Pt, Au, Ir, and Pd with plasma treatment. ZnO with low defect densities yield lower idealities and reverse currents. Deep level optical and transient spectroscopies correlated bulk and surface defects, showing deep levels at 2.54 and 0.53eV, while DRCLS shows that these densities can increase by >100 times at the surface. The magnitude of the metal’s influence correlates directly to the relative defect concentrations near the surface and in the bulk. These results show that metals can induce defect states at the metal-semiconductor interface and impact device performance.