M. G. Craford

High dislocation densities in high efficiency GaN‐based light‐emitting diodes

The electrical, optical, and structural properties of light emitting diodes (LEDs) fabricated from the III–V nitride material system have been studied. LEDs with external quantum efficiencies as high as 4% were characterized by transmission electron microscopy and found to contain dislocation densities in excess of 2×1010 cm−2. A comparison to other III–V arsenide and phosphide LEDs shows that minority carries in GaN‐based LEDs are remarkably insensitive to the presence of structural defects. Dislocations do not act as efficient nonradiative recombination sites in nitride materials. It is hypothesized that the benign character of dislocations arises from the ionic nature of bonding in the III–V nitrides.

High-power truncated-inverted-pyramid (AlxGa1−x)0.5In0.5P/GaP light-emitting diodes exhibiting >50% external quantum efficiency

A truncated-inverted-pyramid (TIP) chip geometry provides substantial improvement in light extraction efficiency over conventional AlGaInP/GaP chips of the same active junction area (∼0.25 mm2). The TIP geometry decreases the mean photon path-length within the crystal, and thus reduces the effects of internal loss mechanisms. By combining this improved device geometry with high-efficiency multiwell active layers, record-level performance for visible-spectrum light-emitting diodes is achieved. Peak efficiencies exceeding 100 lm/W are demonstrated (100 mA dc, 300 K) for orange-emitting (λp∼610 nm) devices, with a peak luminous flux of 60 lumens (350 mA dc, 300 K). In the red wavelength regime (λp∼650 nm), peak external quantum efficiencies of 55% and 60.9% are measured under direct current and pulsed operation, respectively (100 mA, 300 K).

Very high‐efficiency semiconductor wafer‐bonded transparent‐substrate (AlxGa1−x)0.5In0.5P/GaP light‐emitting diodes

Data are presented demonstrating the operation of transparent‐substrate (TS) (AlxGa1−x)0.5In0.5P/GaP light‐emitting diodes (LEDs) whose efficiency exceeds that afforded by all other current LED technologies in the green to red (560–630 nm) spectral regime. A maximum luminous efficiency of 41.5 lm/W (93.2 lm/A) is realized at λ∼604 nm (20 mA, direct current). The TS (AlxGa1−x)0.5In0.5P/GaP LEDs are fabricated by selectively removing the absorbing n‐type GaAs substrate of a p‐n (AlxGa1−x)0.5In0.5P double heterostructure LED and wafer bonding a ‘‘transparent’’ n‐GaP substrate in its place. The resulting TS (AlxGa1−x)0.5In0.5P/GaP LED lamps exhibit a twofold improvement in light output compared to absorbing‐substrate (AS) (AlxGa1−x)0.5In0.5P/GaAs lamps.

HIGH PERFORMANCE ALGAINP VISIBLE LIGHT-EMITTING DIODES

The performance of surface‐emitting visible AlGaInP light‐emitting diodes (LEDs) is described. The devices have external quantum efficiencies greater than 2% and luminous efficiencies of 20 lm/A in the yellow (590 nm) spectral region. This performance is roughly ten times better than existing yellow LEDs and is comparable to the highest performance red AlGaAs LEDs currently available. The devices also perform favorably compared to existing devices in the orange and green spectral regions. Low‐pressure organometallic vapor phase epitaxy (OMVPE) is used to grow the epitaxial layers. The devices consist of a double heterostructure with an AlGaInP active region grown on a GaAs substrate.

Twofold efficiency improvement in high performance AlGaInP light-emitting diodes in the 555-620 nm spectral region using a thick GaP window layer

AlGaInP light‐emitting diodes (LEDs) with external quantum efficiencies ≥6% and luminous performance of 20 lm/W have been fabricated. These LEDs are twice as efficient as previously reported AlGaInP devices throughout the spectral region from green (555 nm) to red‐orange (620 nm) owing to a thicker GaP window layer (45 vs 15 μm). Using hydride vapor phase epitaxy, thick GaP window layers were grown on top of AlGaInP double heterostructures grown by organometallic vapor phase epitaxy. The efficiency of the LEDs was found to improve as the thickness of the window layer was increased from 9 to 63 μm. This improvement is predicted by a simple model that considers the benefit of enhanced emission through the sides of the thick window. The effect of emission wavelength on quantum efficiency and luminous performance for AlGaInP LEDs with a 45 μm thick window has been studied.

A study of parasitic reactions between NH 3 and TMGa or TMAl

The growth of AlGaN using organometallic vapor phase epitaxy has been studied as a function of reactor pressure in a horizontal reactor. At atmospheric pressure, GaN with growth efficiency comparable to that of GaAs in the same reactor is obtained. In addition, the GaN growth efficiency changes little at different reactor pressures. These results indicate that the parasitic reaction between TMGa and NH3 is not substantial in the reactor used in this study. On the other hand, A1N growth at atmospheric pressure has not been possible. By lowering the reactor pressure below 250 Torr, A1N deposition is achieved. However, the growth efficiency decreases at higher reactor pressures and higher growth temperatures, indicating that a strong parasitic reaction occurs between TMAI and NH3. For the ternary AlGaN, lower pressure also leads to more Al incorporation. The results indicate that parasitic reactions are much more severe for TMAI+NH3 than for TMGa+NH3.

Recombination dynamics in InGaN quantum wells

Transient photoluminescence measurements are reported on a thin InGaN single quantum well, encompassing the high injection regime. The radiative processes that dominate the recombination dynamics, especially at low temperatures, show the impact of localized electronic states that are distributed over a large energy range (∼100 meV). We suggest that these states originate from microstructural disorder in the InGaN/GaN system.

Properties and use on In 0.5 (Al x Ga 1-x ) 0.5 P and Al x Ga 1-x As native oxides in heterostructure lasers

Data are presented demonstrating the formation of native oxides from high Al composition In0.5(AlxGa1-x)0.5P (x≳ 0.9) by simple annealing in a “wet” ambient. The oxidation occurs by reaction of the high Al composition crystal with H2O vapor (in a N2 carrier gas) at elevated temperatures (≥500° C) and results in stable transparent oxides. Secondary ion mass spectrometry (SIMS) as well as scanning and transmission electron microscopy (SEM and TEM) are employed to evaluate the oxide properties, composition, and oxide-semiconductor interface. The properties of native oxides of the In0.5(AlxGa1-x)0.5P system are compared to those of the AlxGa1-xAs system. Possible reaction mechanisms and oxidation kinetics are considered. The In0.5(AlxGa1-x)0.5P native oxide is shown to be of sufficient quality to be employed in the fabrication of stripe-geometry In0.5(AlxGa1-x)0.5P visible-spectrum laser diodes.

Impurity‐induced layer disordering of high gap Iny(AlxGa1−x)1−yP heterostructures

Data are presented showing the impurity‐induced layer disordering (IILD), via low‐temperature (600–675 °C) Zn diffusion, of In0.5(AlxGa1−x)0.5P quantum well heterostructures and an In0.5Al0.2Ga0.3P‐GaAs heterojunction grown using metalorganic chemical vapor deposition. Secondary ion mass spectroscopy, transmission electron microscopy, and photoluminescence are used to confirm IILD, which occurs via atom intermixing on the column III site aided by column‐III‐atom interstitials. In addition, high‐temperature anneals (800–850 °C) are performed on the same crystals to confirm the thermal stability of the heterointerfaces.

Short-wavelength (≤6400 Å) room-temperature continuous operation of p-n In0.5(AlxGa1−x)0.5P quantum well lasers

Data are presented demonstrating short‐wavelength (≲6400 A) continuous (cw) laser operation of p‐n diode In0.5(AlxGa1−x)0.5P multiple quantum well heterostructure (QWH) lasers grown lattice matched on GaAs substrates using metalorganic chemical vapor deposition. In the range from −30 °C to room temperature (RT≊300 K, λ≊6395 A) the threshold current density changes from 2.3×103 A/cm2 (−30 °C) to 3.7×103 A/cm2 (RT, 300 K). The cw 300 K photopumped laser operation of the same quaternary QWH crystal is an order of magnitude lower in threshold (7×103 W/cm2, Jeq∼2.9×103 A/cm2) than previously reported for this crystal system, and agrees with the successful demonstration of cw 300 K laser diodes at this short wavelength.