E. F. Schubert

Resonant cavity light‐emitting diode

A novel concept of a light‐emitting diode (LED) is proposed and demonstrated in which the active region of the device is placed in a resonant optical cavity. As a consequence, the optical emission from the active region is restricted to the modes of the cavity. Resonant cavity light‐emitting diodes (RCLED) have higher spectral purity and higher emission intensity as compared to conventional light emitting diodes. Results on a top‐emitting RCLED structure with AlAs/AlxGa1−xAs quarter wave mirrors grown by molecular beam epitaxy are presented. The experimental emission linewidth is 17 meV (0.65 kT) at room temperature. The top‐emission intensity is a factor of 1.7 higher as compared to conventional LEDs.

Experimental analysis and theoretical model for anomalously high ideality factors (n≫2.0) in AlGaN/GaN p-n junction diodes

Diode ideality factors much higher than the expected values of 1.0 to 2.0 have been reported in GaN-based p-n junctions. It is shown that moderately doped unipolar heterojunctions as well as metal-semiconductor junctions, in particular the metal contact to p-type GaN, can increase the ideality factor to values greater than 2.0. A relation is derived for the effective ideality factor by taking into account all junctions of the diode structure. Diodes fabricated from a bulk GaN p-n junction and a p-n junction structure with a p-type AlGaN/GaN superlattice display ideality factors of 6.9 and 4.0, respectively. These results are consistent with the theoretical model and the fact that p-type AlGaN/GaN superlattices facilitate the formation of low-resistance ohmic contacts.

Junction–temperature measurement in GaN ultraviolet light-emitting diodes using diode forward voltage method

A theoretical model for the dependence of the diode forward voltage (Vf) on junction temperature (Tj) is developed. An expression for dVf∕dT is derived that takes into account all relevant contributions to the temperature dependence of the forward voltage including the intrinsic carrier concentration, the band-gap energy, and the effective density of states. Experimental results on the junction temperature of GaN ultraviolet light-emitting diodes are presented. Excellent agreement between the theoretical and experimental temperature coefficient of the forward voltage (dVf∕dT) is found. A linear relation between the junction temperature and the forward voltage is found.

Ga2O3 films for electronic and optoelectronic applications

Properties of Ga2O3 thin films deposited by electron‐beam evaporation from a high‐purity single‐crystal Gd3Ga5O12 source are reported. As‐deposited Ga2O3 films are amorphous, stoichiometric, and homogeneous. Excellent uniformity in thickness and refractive index was obtained over a 2 in. wafer. The films maintain their integrity during annealing up to 800 and 1200 °C on GaAs and Si substrates, respectively. Optical properties including refractive index (n=1.84–1.88 at 980 nm wavelength) and band gap (4.4 eV) are close or identical, respectively, to Ga2O3 bulk properties. Reflectivities as low as 10−5 for Ga2O3/GaAs structures and a small absorption coefficient (≊100 cm−1 at 980 nm) were measured. Dielectric properties include a static dielectric constant between 9.9 and 10.2, which is identical to bulk Ga2O3, and electric breakdown fields up to 3.6 MV/cm. The Ga2O3/GaAs interface demonstrated a significantly higher photoluminescence intensity and thus a lower surface recombination velocity as compared to ...

Influence of junction temperature on chromaticity and color-rendering properties of trichromatic white-light sources based on light-emitting diodes

Trichromatic white-light sources based on light-emitting diodes (LEDs) offer a high luminous efficacy of radiation, a broad range of color temperatures and excellent color-rendering properties with color-rendering indices (CRIs) exceeding 85. An analysis of the luminous efficacy and CRI of a trichromatic light source is performed for a very broad range of wavelength combinations. The peak emission wavelength, spectral width, and the output power of LEDs strongly depend on temperature and the dependencies for red, green, and blue LEDs are established. A detailed analysis of the temperature dependence of trichromatic white LED sources reveals that the luminous efficacy decreases, the color temperature increases, the CRI decreases and the chromaticity point shifts towards the blue as the junction temperature increases. A high CRI>80 can be maintained, by adjusting the LED power ratio so that the chromaticity point is conserved.

Current crowding in GaN/InGaN light emitting diodes on insulating substrates

GaN/InGaN light emitting diodes (LEDs) grown on sapphire substrates have current transport along the lateral direction due to the insulating nature of the substrate. The finite resistance of the n-type GaN buffer layer causes the pn junction current to be nonuniform and “crowd” near the edge of the contact. The current-crowding effect is analyzed both theoretically and experimentally for p-side-up mesa structure GaN/InGaN LEDs. The calculation yields an exponential decay of the current distribution under the p-type contact with a characteristic current spreading length, Ls. It is shown that GaN/InGaN LEDs with high p-type contact resistance and p-type confinement layer resistivity have a relatively uniform current distribution. However, as the p-type GaN conductivity and p-type ohmic contact conductivity is improved, significant current crowding near the contact edge will occur. The current crowding effect is analyzed experimentally in GaN/InGaN LEDs emitting in the blue spectral range. Experimental resul...

Internal quantum efficiency and nonradiative recombination coefficient of GaInN/GaN multiple quantum wells with different dislocation densities

Room-temperature photoluminescence measurements are performed on GaInN/GaN multiple quantum wells grown on GaN-on-sapphire templates with different threading-dislocation densities. The internal quantum efficiencies as a function of carrier concentration and the non-radiative coefficients are obtained.

Highly Efficient Light-Emitting Diodes with Microcavities

One-dimensional microcavities are optical resonators with coplanar reflectors separated by a distance on the order of the optical wavelength. Such structures quantize the energy of photons propagating along the optical axis of the cavity and thereby strongly modify the spontaneous emission properties of a photon-emitting medium inside a microcavity. This report concerns semiconductor light-emitting diodes with the photon-emitting active region of the light-emitting diodes placed inside a microcavity. These devices are shown to have strongly modified emission properties including experimental emission efficiencies that are higher by more than a factor of 5 and theoretical emission efficiencies that are higher by more than a factor of 10 than the emission efficiencies in conventional light-emitting diodes.

Delta doping of III–V compound semiconductors: Fundamentals and device applications

Delta‐function‐like doping profiles can be obtained in semiconductors by growth‐interrupted impurity deposition during molecular‐beam epitaxy. The spatial localization of dopants is assessed by the capacitance–voltage profiling technique and secondary ion mass spectroscopy which yield profile widths of 20 and 37 A for Be δ‐doped GaAs grown at 500 °C, respectively. The diffusion coefficients of Si, Be, and C in GaAs and of Si in AlxGa1−xAs are determined and diffusion is shown to be negligible at low growth temperatures. At elevated growth temperatures, dopant redistribution occurs during epitaxial growth. The redistribution is shown to be due to (i) diffusion of dopants and (ii) Fermi‐level pinning induced segregation of dopants along the growth axis. Fermi‐level pinning induced segregation of dopants is a novel mechanism which results in a redistribution of dopants predominantly toward the growing surface due to electrostatic attraction of dopants and carriers localized in surface states. This mechamism is shown to be relavant at elevated growth temperatures of ≥600 °C. Electronic devices such as homostructure and heterostructure field‐effect transistors which employ the δ‐doping technique have a number of advantages including (i) high carrier density, (ii) proximity between electron channel and gate electrode, (iii) large breakdown voltage of the gate, and (iv) reduced short‐channel effects. In addition, high transconductances are obtained in such δ‐doped field‐effect transistors. The optical properties of doping superlattices are significantly improved using the δ‐doping technique. Quantum‐confined interband transitions in doping superlattices are observed for the first time in such improved doping superlattices. Furthermore, a tunable doping superlattice laser is demonstrated, which has a tuning range of 35 A. The tunable doping superlattice laser has a potential tuning range of 220 A and is a candidate for a tunable source in future optical communication systems.Delta‐function‐like doping profiles can be obtained in semiconductors by growth‐interrupted impurity deposition during molecular‐beam epitaxy. The spatial localization of dopants is assessed by the capacitance–voltage profiling technique and secondary ion mass spectroscopy which yield profile widths of 20 and 37 A for Be δ‐doped GaAs grown at 500 °C, respectively. The diffusion coefficients of Si, Be, and C in GaAs and of Si in AlxGa1−xAs are determined and diffusion is shown to be negligible at low growth temperatures. At elevated growth temperatures, dopant redistribution occurs during epitaxial growth. The redistribution is shown to be due to (i) diffusion of dopants and (ii) Fermi‐level pinning induced segregation of dopants along the growth axis. Fermi‐level pinning induced segregation of dopants is a novel mechanism which results in a redistribution of dopants predominantly toward the growing surface due to electrostatic attraction of dopants and carriers localized in surface states. This mechamism ...

Optical properties of Si-doped GaN

The optical properties of n-type GaN are investigated for Si doping concentrations ranging from 5×1016 to 7×1018 cm−3. The photoluminescence linewidth of the near-band gap optical transition increases from 47 to 78 meV as the doping concentration is increased. The broadening is modeled in terms of potential fluctuations caused by the random distribution of donor impurities. Good agreement is found between experimental and theoretical results. The intensity of the near-band-gap transition increases monotonically as the doping concentration is increased indicating that nonradiative transitions dominate at a low doping density. The comparison of absorption, luminescence, reflectance, and photoreflectance measurements reveals the absence of a Stokes shift at room temperature demonstrating the intrinsic nature of the near-band edge transition.