A. J. Howard

Morphology and photoluminescence improvements from high-temperature rapid thermal annealing of GaN

Rapid thermal annealing of GaN in an Ar or N2 ambient up to 1100 °C is shown to improve surface morphology and photoluminescence intensity. For both ambients the average rms surface roughness as determined by atomic force microscopy decreases from ∼4 nm on the as‐grown material to ∼1 nm after a 1100 °C anneal. The band‐edge luminescence intensity was increased by a factor of 4 after a 1100 °C anneal in a N2 ambient and a factor of 2 for annealing at 1100 °C in an Ar ambient as compared to as‐grown material. The 1100 °C anneal improves the ratio of band edge to deep‐level luminescence and also reduces the electron concentration and mobility. The reduction in mobility can be explained in terms of a two‐band conduction mechanism where defect band conduction dominates at the lower carrier densities or an increase in the free‐carrier compensation ratio.

Midwave (4 μm) infrared lasers and light‐emitting diodes with biaxially compressed InAsSb active regions

Heterostructures with biaxially compressed, As‐rich InAsSb are being investigated as active regions for midwave infrared emitters. InAs1−xSbx/In1−xGaxAs (x≊0.1) strained‐layer sublattices (SLSs), nominally lattice matched to InAs, were grown using metalorganic chemical vapor deposition. An SLS light‐emitting diode was demonstrated which emitted at 3.6 μm with 0.06% efficiency at 77 K. Optically pumped laser emission at 3.9 μm was observed in a SLS/InPSb heterostructure. The laser had a maximum operating temperature of approximately 100 K.

X‐ray reciprocal‐space mapping of strain relaxation and tilting in linearly graded InAlAs buffers

The extent of relaxation and orientation of linearly graded InxAl1‐xAs (x=0.05–0.25) buffers grown on GaAs were examined using a novel x‐ray diffraction reciprocal‐space mapping technique (kmap). Samples were grown at temperatures ranging from 370 to 550 °C. The fractional relaxation of the buffers grown between 470 and 550 °C was essentially identical (77%) and symmetric in orthogonal 〈110〉 directions. These buffers are believed to be in equilibrium indicating that the incomplete relaxation is not a kinetic effect. The extent of relaxation was less than that expected for equilibrium relaxation in the absence of dislocation–dislocation interactions indicating that such interactions must be considered to accurately predict the extent of relaxation. The saturation of the relaxation as a function of temperature indicates that at the grading rate used (8% In/μm or 0.69% strain/μm), we are not working in a growth regime where the relaxation is nucleation limited. In addition, all the buffers are slightly tilte...

HIGHLY REFLECTIVE, LONG WAVELENGTH ALASSB/GAASSB DISTRIBUTED BRAGG REFLECTOR GROWN BY MOLECULAR BEAM EPITAXY ON INP SUBSTRATES

Surface normal optoelectronic devices operating at long wavelengths (≳1.3 μm), require distributed Bragg reflectors (DBRs) with a practical number (≤50) of mirror layers. This requirement implies a large refractive index difference between the mirror layers, which is difficult to achieve in the traditionally used phosphide compounds. We demonstrate a highly reflective AlAsSb/GaAsSb DBR grown nominally lattice matched to an InP substrate by molecular beam epitaxy. Reflectivity measurements indicate a stop band centered at 1.74 μm with maximum reflectivity exceeding 98%, which is well fitted by our theoretical predictions. Atomic force microscopy and transmission electron microscopy indicate reasonable crystal quality with some defects due to an unintentional lattice mismatch to the substrate.

Temperature dependent electron cyclotron resonance etching of InP, GaP, and GaAs

Electron cyclotron resonance etching of InP, GaP, and GaAs in Ar, Ar/Cl 2, Ar/Cl2/H2, and Ar/Cl2/H2/CH4 plasmas is reported for substrate temperatures from 10 to 170 °C. Etch rates increased as a function of temperature for GaP and GaAs in an Ar/Cl2 plasma. With the addition of H2 or H2/CH4 to the plasma, the GaP and GaAs etch rates decreased and were essentially temperature independent. In comparison, InP etch rates showed a strong temperature dependence regardless of plasma chemistry. At 170 °C, InP etch rates were greater than GaP and GaAs in the Ar/Cl2/H2 and Ar/Cl2/H 2/CH4 plasmas. Atomic force microscopy was used to determine the root‐mean‐square roughness of the etched surfaces. The etched surface morphology for InP was strongly dependent on temperature and plasma chemistry while smooth pattern transfer was obtained for a wide range of plasma conditions for GaAs and GaP.

High density plasma etching of III–V nitrides

Two broad classes of plasma chemistry were examined for dry etching of GaN, AlN, and InN. The etch rates for CH4/H2‐based plasmas are low (∼ 400 A/min) even under high microwave power (1000 W) electron cyclotron resonance conditions. Halogen‐based plasmas (Cl2, I2, Br2) produce rates up to ∼3000 A/min with smooth stoichiometric surfaces. Preferential sputtering of N occurs for high ion energies, leading to rough GaN surfaces at rf power above ∼200 W. Etch rates for high ion density discharges are typically an order of magnitude faster than for conventional reactive ion etching.

High rate electron cyclotron resonance etching of GaN, InN, and AlN

Electron cyclotron resonance etch rates of GaN, InN, and AlN are reported as a function of pressure, microwave power, and radio‐frequency (rf) power in a Cl2/H2/CH4/Ar plasma at 170 °C. The etch rates for GaN and InN increase as a function of rf power. At 275 W, the etch rates reach maximum values of 2850 and 3840 A/min, respectively. These are the highest etch rates reported for these materials. As a function of pressure, the etch rates reach a maximum value at 2 mTorr and then decrease as the pressure is increased to 10 mTorr. The GaN and AlN etch rates increase less than a factor of 2 as the microwave power is increased from 125 to 850 W whereas the InN etch rate increases by more than a factor of 3.5. The maximum etch rate for AlN obtained in this study is 1245 A/min at a microwave power of 850 W, 1 mTorr pressure, and 225 W rf power. Atomic force microscopy is used to determine root‐mean‐square roughness as a function of etch conditions for GaN and InN and, while very smooth pattern transfer can be o...

Fabry–Perot reflectance modulator for 1.3 μm from (InAlGa)As materials grown at low temperature

We report the first all‐semiconductor Fabry–Perot‐cavity reflectance modulators operating at wavelengths of 1.32–1.33 μm. These devices were grown on a GaAs substrate using an intermediate, linearly graded InGaAs buffer layer terminating in an In0.33Ga0.67As layer. The Bragg reflector stacks of the Fabry–Perot structure are composed of InGaAs and InAlAs layers lattice matched to the buffer, and the active cavity region is an In0.4Ga0.6As/In0.26Al0.35Ga0.39As strained‐layer superlattice. The key to obtaining device‐quality material was low temperature growth (∼400 °C) of the entire structure. For a device with a 0.38‐μm‐thick active region and a 4 dB insertion loss, we obtained a contrast ratio of ∼3:1 at 4 V bias.

Nanomechanical basis for imaging soft materials with tapping mode atomic force microscopy

The surfaces of virgin and chemically etched poly(tetrafluoroethylene) (PTFE) have been studied using scanning electron microscopy (SEM), and atomic force microscopy (AFM) in both contact and tapping modes. Contact mode AFM images of this relatively soft polymeric material are dominated by tip‐induced imaging artifacts. When subsequent, AFM imaging was performed in tapping mode these artifacts were eliminated, and comparable tapping mode AFM and SEM images were obtained for even the highly porous, unstable surface that results from sodium naphthalenide etching. Interfacial force microscopy force versus displacement, and creep experiments were performed to determine the nanomechanical nature of virgin PTFE. These experiments show that virgin PTFE is a viscoelastic material which is capable of supporting large forces on the millisecond time scale but creeps dramatically at longer times. Clearly, with scanning probe techniques which utilize constant probe force feedback, one should expect image distortions, ...

ELECTROCHEMICAL SULFUR PASSIVATION OF VISIBLE (670 NM) ALGAINP LASERS

III–V based devices such as field effect transistors, heterojunction bipolar transistors, and lasers often have surface leakage and thermal degradation problems due to surface states which pin the Fermi level to the midgap. Sulfur based passivation processes are known to improve device performance by altering surface‐state densities. We have developed a voltage‐controlled anodic sulfur passivation scheme using Na2S dissolved in ethylene glycol. Our process has repeatedly produced a ∼25% improvement in peak output power near the catastrophic damage limit in visible (λ=670 nm) AlGaInP edge‐emitting lasers. The threshold current density before and after passivation, and the I–V characteristics before and after catastrophic failure, were essentially unchanged indicating that passivation raises the threshold for facet damage.