Kazuyuki Tadatomo

Analysis of deep levels in n‐type GaN by transient capacitance methods

Transient capacitance methods were used to analyze traps occurring in unintentionally doped n‐type GaN grown by hydride vapor‐phase epitaxy. Studies by deep‐level transient spectroscopy (DLTS) and isothermal capacitance transient spectroscopy indicated the presence of three majority‐carrier traps occurring at discrete energies below the conduction band with activation energies (eV) ΔE1=0.264±0.01, ΔE2=0.580±0.017, and ΔE3=0.665±0.017. The single‐crystal films of GaN were grown on GaN formed by metal‐organic chemical‐vapor deposition and on sputter‐deposited ZnO; a similar deep‐level structure was found in both types of samples. Pulse‐width modulation tests using DLTS to determine the capture rates of the traps showed that the capture process is nonexponential, perhaps due to the high trap concentration. The origins of the deep levels are discussed in light of secondary‐ion‐mass‐spectroscopy analysis and group theory results in the literature.Transient capacitance methods were used to analyze traps occurring in unintentionally doped n‐type GaN grown by hydride vapor‐phase epitaxy. Studies by deep‐level transient spectroscopy (DLTS) and isothermal capacitance transient spectroscopy indicated the presence of three majority‐carrier traps occurring at discrete energies below the conduction band with activation energies (eV) ΔE1=0.264±0.01, ΔE2=0.580±0.017, and ΔE3=0.665±0.017. The single‐crystal films of GaN were grown on GaN formed by metal‐organic chemical‐vapor deposition and on sputter‐deposited ZnO; a similar deep‐level structure was found in both types of samples. Pulse‐width modulation tests using DLTS to determine the capture rates of the traps showed that the capture process is nonexponential, perhaps due to the high trap concentration. The origins of the deep levels are discussed in light of secondary‐ion‐mass‐spectroscopy analysis and group theory results in the literature.

Internal quantum efficiency of highly-efficient InxGa1−xN-based near-ultraviolet light-emitting diodes

The internal quantum efficiency (IQE) of highly-efficient near-UV light-emitting diodes, which shows an external quantum efficiency of 43% at 406 nm, has been measured by excitation power and temperature-dependent photoluminescence (PL). Assuming peak PL quantum efficiency at 8 K is 100%, peak IQE at 300 K was measured to be as high as 63%. At the injected carrier density, which corresponds to 20 mA current injection, IQE and light extraction efficiency were estimated to be about 54% and 80%, respectively.

High Output Power InGaN Ultraviolet Light‐Emitting Diodes Fabricated on Patterned Substrates Using Metalorganic Vapor Phase Epitaxy

Ultraviolet (UV) light-emitting diodes (LEDs) with an InGaN multi-quantum-well (MQW) structure were fabricated on a patterned sapphire substrate (PSS) using a single growth process of metalorganic vapor phase epitaxy. The GaN layer grown by lateral epitaxy on a patterned substrate (LEPS) has a dislocation density of 1.5 x 10 8 cm -2 . The LEPS-UV-LED chips were mounted on the Si bases in a flip-chip bonding arrangement. When the UV-LED was operated at a forward-biased current of 20 mA at room temperature, the emission wavelength, the output power and the external quantum efficiency were estimated to be 382 nm, 15.6 mW and 24%, respectively. With increasing forward-biased current, the output power increased linearly and was estimated to be approximately 38 mW at 50 mA.

High-output power near-ultraviolet and violet light-emitting diodes fabricated on patterned sapphire substrates using metalorganic vapor phase epitaxy

The external quantum efficiency (EQE, ηe) of conventional near-ultraviolet (NUV) light-emitting diodes (LEDs) with an InGaN multi-quantum-well (MQW) structure is limited by high dislocation density and by the narrow escape cone due to total internal reflection at the GaN/air or sapphire/air interface. We have fabricated the NUV and violet InGaN-MQW-LEDs with the high EQE on the patterned-sapphire substrate (PSS) using a single growth process by metal-organic vapor phase epitaxy (MOVPE). The PSS with parallel grooves along the <11-20>GaN direction or the <1-100>GaN direction was fabricated by a standard photolithography and subsequent reactive ion etching (RIE). In this study, fabricated the LED on the PSS with parallel grooves along the <11-20>GaN direction. The GaN layer grown by lateral epitaxy on a patterned substrate (LEPS) has dislocation density of 1.5x108 cm-2. The LEPS-NUV (or violet)-LED chips were mounted on the Si bases in a flip-chip bonding arrangement. When the LEPS-NUV-LED (the emission peak wavelength λp: 382 nm) was operated at a forward-bias current of 20 mA at room temperature (RT), the output power (Po) and the EQE were 15.6 mW and 24%, respectively. When the LEPS-violet-LED (λp: 405 nm) was operated at a forward-bias current of 20 mA at RT, the Po and the EQE were 26.3 mW and 43%, respectively. Furthermore, we obtained the Po of approximately 61 mW at 50 mA and 111 mW at 100 mA, respectively. It was revealed that the PSS is very effective in reducing the dislocation density and for increasing the extraction efficiency due to the multiple scattering of the emission light at the GaN/patterned sapphire interface.

Intense Ultraviolet Electroluminescence Properties of the High-Power InGaN-Based Light-Emitting Diodes Fabricated on Patterned Sapphire Substrates.

The electroluminescence and photoluminescence characteristics of high-efficient InGaN multi-quantum-well ultraviolet light-emitting diodes have been investigated. There appeared a single emission band in the electroluminescence spectra at about 3.235 eV with a band width of 90 meV at room temperature under direct current. With increasing forward current, the luminescence intensity was not saturated, and increased linearly with increasing injection current up to 50 mA. Under pulsed current conditions at room temperature, the luminescence intensity increased linearly with increasing injection current up to 1000 mA, and a shift of the electroluminescence peak position was not observed. These results indicated that the injected carriers were confined efficiently in the active layer, and also suggested the possibility of realizing ultraviolet laser diodes. It was revealed that the forward-biased electroluminescence spectrum at 4 K reflected the distribution of hot electrons injected into the active layer. The maximum temperature of hot electrons was estimated to be about 350 K under a forward-biased pulsed current of about 500 mA, which was much higher than the lattice temperature.

Temperature dependence of Stokes shift in InxGa1−xN epitaxial layers

Optical properties of InxGa1−xN epitaxial layers with various indium compositions (x=0.02, 0.03, 0.05, 0.06, and 0.09) have been studied by means of temperature-dependent optical absorption and photoluminescence spectroscopy. A clear peak due to the absorption of InxGa1−xN ternary alloys was observed up to 300 K, which enabled us to investigate the temperature dependence of the Stokes shift. The Stokes shift at 4 K increased with an increase in the indium composition, and was estimated to be 22 and 45 meV for the samples with x=0.02 and 0.09, respectively. With an increase in temperature up to about 50 K, the Stokes shift increased slightly. With a further increase in temperature from 50 to 100 K, the Stokes shift decreased. Above 100 K, the Stokes shift was independent of the temperature and showed an almost constant value up to 300 K. The Stokes shift at 300 K was estimated to be 19 and 34 meV for the samples with x=0.02 and 0.09, respectively. This temperature dependence of the Stokes shift was characteristically common to all of the samples used in the present work, and was observed to be more prominent for the samples with higher indium compositions.Optical properties of InxGa1−xN epitaxial layers with various indium compositions (x=0.02, 0.03, 0.05, 0.06, and 0.09) have been studied by means of temperature-dependent optical absorption and photoluminescence spectroscopy. A clear peak due to the absorption of InxGa1−xN ternary alloys was observed up to 300 K, which enabled us to investigate the temperature dependence of the Stokes shift. The Stokes shift at 4 K increased with an increase in the indium composition, and was estimated to be 22 and 45 meV for the samples with x=0.02 and 0.09, respectively. With an increase in temperature up to about 50 K, the Stokes shift increased slightly. With a further increase in temperature from 50 to 100 K, the Stokes shift decreased. Above 100 K, the Stokes shift was independent of the temperature and showed an almost constant value up to 300 K. The Stokes shift at 300 K was estimated to be 19 and 34 meV for the samples with x=0.02 and 0.09, respectively. This temperature dependence of the Stokes shift was charact...

Characterization of GaN Based UV-VUV Detectors in the Range 3.4–25 eV by Using Synchrotron Radiation

The characterization of Schottky type ultraviolet (UV) detectors with transparent electrode between vacuum ultraviolet (VUV) and visible light region using synchrotron radiation is described. The responsivity spectrum of the detectors at 0 V bias was obtained in the wide range between 2 eV (563 nm) and 25 eV (50 nm). The photoemission current from Au electrode was able to be canceled by improving the measuring circuit, and thus we succeeded in operating the detectors without any photoemission current from Au and GaN. The responsivity of the detectors is about 0.15 A/W at 3.5 eV. These results show that these Schottky type detectors with the transparent electrode are effective to detect VUV-UV light (50-360 nm, 3.4-25 eV) without any photoemission.

Characterization of GaN-Based Schottky Barrier Ultraviolet (UV) Detectors in the UV and Vacuum Ultraviolet (VUV) Region Using Synchrotron Radiation

Characterization of GaN-based Schottky barrier ultraviolet (UV) detectors with a comb-shaped electrode using synchrotron radiation (hν=2.2–30 eV, λ=41–563 nm) is described. Below hν=8.0 eV (λ>155 nm), the detectors are available without any photoemission of GaN and Au electrode. Under application of reverse bias, the responsivity is increased to 0.05 A/W at -0.4 V. The photocurrent is controlled by reverse bias. On the other hand, above hν=8.0 eV (λ<155 nm), the responsivity spectra are dominated by photoemissions of Au and GaN. These results show that these Schottky type detectors with mesa structures are effective to detect vacuum ultraviolet (VUV)-UV light (155

New dopant precursors for n-type and p-type GaN

Abstract Tetraethylsilane (TeESi) and bis(ethylcyclopentadienyl)Mg (ECp 2 Mg) were employed as Si and Mg dopant precursors for MOVPE growth of n-type and p-type GaN films, respectively. In Si doping, the electron concentration was observed to increase with the increase of the TeESi flow rate. The temperature dependence of the Hall mobility showed good agreement with n-type GaN films grown using different dopant precursors (SiH 4 , GeH 4 , Si 2 H 6 ). The donor activation energy was estimated to be 27 meV, which is almost the same as the literature values. In Mg doping, we also found that the Mg concentration increases as the ECp 2 Mg flow rate increases. All of Mg-doped samples in this study showed p-type conduction after annealing. The acceptor activation energy was estimated to be 170 meV, which was close to the reported values.

Growth of GaInP thick layers by the modified yo-yo solute feeding method

Abstract A new process for the yo-yo solute feeding method has been developed to grow a thick Ga x In 1−x P alloy layer. In this process, we lowered the temperature of every cycle by 5°C. Only after three cycles was a Ga 0.69 In 0.31 P layer of about 70 μm in thickness epitaxially grown on a GaAsP(100) substrate. The thick layer shows the FWHM of PL (photoluminescence) peaks of 37.2 meV at 300 K and 13.7 meV at 4 K, which indicates high quality comparable with normal LPE-grown thin layers.